黄敏章 男 教授 中原大学物理系 Dynamical Stability and Intrin.

2A review of research progress on CO capture,storage,and utilization in 34Q156789111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576with 58%of the sources being located within the east and south 77central regions.The contributions of large point sources in each 78sector to total CO 2emissions in China are listed in Fig.2[13].With 79rapid development of energy technologies in the 21st century,fos-80sil fuels,especially coal,will still remain the dominant energy0016-2361/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.fuel.2011.08.022⇑Corresponding authors.Q2Address:State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China (Y.Sun).Tel.:+863514049612;fax:+863514041153.E-mail addresses:zhaoning@ (N.Zhao),weiwei@ (W.Wei),yhsun@ (Y.Sun).Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022source in China for decades to come.Chinese government recognized the huge challenge of CO 2abatement while satisfying ever-increasing energy demand.In the light of this situation,November 26,2009,China officially announced action to control emissions per unit of GDP by 40–45%by 2020,based on 86levels [14].To address this,China is undertaking a range of techni-87cal research and development projects on CCSU,including the na-88tional fundamental research and high-tech programs,as well as a 89large number of international programs.The CCS projects,fun-90dings,and research institutes in China is shown in Table 1.91Since 1990,China had carried out a series of climate change 92projects under framework of national programs,such as China’s 93National Climate Change Program (CNCCP),National Hi-tech R&D949596979899100101102103104105106107108109110111112113114115116117118119120121cooperation with international on CCSU.On the basis of the finical 122support of both Chinese government and CAS,a lot of progresses 123were obtained in several academic institutes in CAS including 124CO 2capture;enhanced oil recovery (EOR)and enhanced coal bed 125methane (ECBM)projects as well as CO 2chemical utilizations.This 126brief review has covered the research progress in CO 2capture,stor-127age,and utilization in CAS.2.The contributions of large point sources in each sector to overall total emissions in China [12].Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022128129130131132133134135136137aration,and cryogenic fractionation.138 2.1.Amine-based scrubbing solvent139Amine scrubbing is a well known technology for capturing CO 2140from flue gas,which has been widely deployed on a large scale 141across several industries [25–28].The industrially most important142143144145146147148149150151152the environment [30].1532.2.Ionic liquids154Therefore,a nonvolatile solvent that could facilitate CO 2capture 155without the loss of solvent into the gas stream would be advanta-156geous.Ionic liquids (ILs)are commonly defined as liquids whichTable 1CCS projects,fundings,and research institutes in China.China and international cooperation on CCS projects and fundingsResearch institutes aBNLMS–CAS IET–CAS RCEES–CAS ICC–CAS IPE–CAS LICP–CAS CIAC–CAS National High Technology Research and Development Program of China (863)SIC–CAS National Key Basic Research and Development Program of China (973)IGG–CAS China’s National Climate Change Program (CNCCP)IAP–CAS L.L Q1i et al./Fuel xxx (2011)xxx–xxx3Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022157are composed entirely of ions with a melting point of less than 158100°C.ILs have many unique properties in comparison to other 159solvents as extremely low volatility,broad range of liquid temper-160ature,high thermal and chemical stability,and tunable physico-161chemical characteristics and as a result,ILs have been considered 162as a potential substitute of aqueous amine solutions for CO 2cap-163ture [31–34].164In Changchun Institute of Applied Chemistry,Chinese Academy 165of Sciences (CIAC–CAS),a novel dissolving process for chitin and 166chitosan was developed by using the ionic liquid 1-butyl-3-167methyl-imidazolium chloride ([Bmim]Cl)as a solvent for capturing 168and releasing CO 2.The results showed that the chitin/IL and chito-169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201tant to consider the maximum mass loading when considering the 202support of ILs on inert substrates –yes,these can enhance the ILs’203ability to take up CO 2,but at the expense of cycling an inert sorbent 204round a thermal cycle.The ILs are also monstrously expensive,the 205complex structure,and high cost for preparation,compared to sim-206pler solvents such as MEA or ammonia.Thus,the potential for 207improving CO 2solubilities and reducing cost of the ILs still needs 208to be studied for future applications.In 2010,in Beijing National 209Laboratory for Molecular,Chinese Academy of Sciences (BNLMS–210CAS),Zhang and co-workers first reported on CO 2capture by 211hydrocarbon surfactant liquids.It also found that CO 2had high sol-212ubility in low-cost hydrocarbon surfactant liquids,and the ab-213and the 214215216such as corrosion,at 217regenerable solid sor-218concept for CO 2recov-219into amine-based and 220221222with various so-223as silica gels,activated 224have been shown to phys-225enhance the sorp-226of many amine-based 227In Dalian Institute of 228(DICP–CAS),Zhang 229silica foam (MCF)materi-230with polyethyl-231The results showed that 232having large window 2333.45mmol CO 2/g sorbent 234In Institute of Coal Chem-235Zhao et al.studied 236materials derived 237sorption capabili-238°C and 1bar.The as-pre-239selectivity for CO 2over 240prepared a series of CO 2241pentamine (TEPA)was 242(PMHS)based mesopor-243The highest absorption 24475°C with the 10vol.%245was higher than most 246Desorption could 247in 1h [46].248of amine-based sorbent 249capacity of solid sorbent.250have poor mechanical 251amine-based sorbents 252and require signifi-253processes.254255sorbents for CO 2capture 256Alkali earth metal,such 257form alkali earth metal-258vapor at high tempera-259and post-combus-260simplified process flow 261the calcium looping 262vessel (the carbonator)4L.L Q1i et al./Fuel xxx (2011)xxx–xxxPlease cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022the carbonation reaction between CO 2and solid CaO separates CO from coal-combustion flue gas at a temperature between 600°and 650°C.The CaCO 3formed is then passed to another vessel (the calciner),where it is heated to reverse the reaction (900–950°C),releasing the CO 2suitable for sequestration,and regener-ating the CaO-sorbent which is then return to the carbonator.The carbonation process is exothermic,which is matched with the temperature of a steam cycle,allowing recuperation of the heat.In IPE–CAS,the decomposition conditions of CaCO 3particles for CO 2capture in a steam dilution atmosphere (20–100%steam 307308309310311312313314315316317318319320321322323324325326327328329330332333334335336337338339340341342343344345346347348349350351352353354355Fig.3.The process flow diagram of post-combustion capture using the calcium looping cycle [47,48].Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022strength (998N/cm 2)and exhibited good stability in multiple cy-cles [74,75].Furthermore,the application of a conceptual CO 2cap-ture process using this sorbent was proposed for an existing coal fired power plant [75].However,to optimize CO 2sorption capacity,understand of the interaction between CO 2and the sorbent need to be studied in the further work.Moreover,much work remains before the technology fluidized bed CO 2capture can be commercialized.Simulta-neously,the numerical simulation based on the computational fluid 365dynamics (CFD)method will become a research focus in the future.366 3.CO 2storage367Following the capture and transport process,CO 2can be dis-368posed of in natural sites such as deep geological sequestration,369mineral carbonation,or ocean storage [76].There are three geolog-370ical formations that have also been recognized as major potential 371CO 2sinks:deep saline-filled sedimentary (DSFs),depleted oil 372natural gas reservoirs,and unmineable coal-seams.The geology 373also suggests possibilities for CO 2enhanced oil recovery (CO 374EOR),CO 2enhanced gas recovery (CO 2–EGR)and CO 2enhanced 375coal-bed methane recovery (CO 2–ECBM)projects [12].3763.1.Geological sequestration377Geological storage involves injecting CO 2at depths greater than 3781000m into porous sedimentary formations using technologies 379derived from the oil and gas industry [77].CO 2can be stored in 380supercritical state at depth below 800–1000m,which provides 381the potential for efficient utilization of the space,due to the li-382quid-like density of supercritical CO 2.The point at which CO 2Table 2Performance summary of K-based sorbents capturing CO 2.Material Temperature (°C)CO 2partial pressure (bar)e Total capacity (mmol CO 2/g sorbent)Method f Regenerature temperature (°C)Ref.K 2CO 3/AC a 600.01 1.95TCD g 150[62]K 2CO 3/SiO 2600.010.23TCD g –[62]K 2CO 3/USY 600.010.43TCD g –[62]K 2CO 3/CsNaX 600.01 1.35TCD g –[62]K 2CO 3/Al 2O 3600.01 1.93TCD g 350[62]K 2CO 3/CaO 600.01 1.11TCD g –[62]K 2CO 3/MgO 600.01 2.70TCD g 400[62]K 2CO 3/TiO 2600.01 1.89TCD g 150[62]K 2CO 3/Al 2O 3600.01 1.96TCD g >300[63]Re-KAl(I)30b 600.01 1.86TCD g <200[63]g Fig.5.The schematic diagram of experimental apparatus for the fluidized bed [74,75].6L.L Q1i et al./Fuel xxx (2011)xxx–xxxPlease cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022383transforms from critical to supercritical point is 31.1°C and 3847.38MPa [78].CO 2is injected usually in the supercritical form into 385the saline aquifer or depleted oil or gas reservoir.Four major clas-386ses of deep geologic reservoirs present within China have been 387identified and evaluated as candidates for the long-term storage 388of anthropogenic CO 2:deep saline-filled sedimentary (DSFs)for-389mations,depleted gas basins,depleted oil basins with potential 390for CO 2–EOR,and deep unmineable coal seams with potential for 391CO 2–ECBM.Fig.6shows the map of the combined location and ex-392tent of candidate geologic CO 2storage formations in China [13].393Because the CO 2industry is not mature,there are few active CO 2394storage projects which can provide site specific information;hence417China are also potential storage candidates.Recently,other re-418search has also focused on estimating the distance between CO 2419sources and potential sinks.Zheng et al.superimposed the loca-420tions of these 27facilities onto maps of sedimentary basins in each 421of the five regions of China (Huabei,Ordos,Dongbei,Yuwan,and 422Xinjiang).The majority of the candidate CO 2sources are found in 423the Ordos,Huabei and Dongbei regions [85].424The China–UK Near Zero Emissions Coal (NZEC)Initiative exam-425ined options for carbon (CO 2)capture,transport and geological 426storage in China,which was developed under the 2005EU–China 427NZEC Agreement that aims to demonstrate CCS in China and the 428EU [16,86–88].The NZEC Initiative has evaluated the potential to 429430431432433434435436437438439440441442443444445446447448449450451L.L Q1i et al./Fuel xxx (2011)xxx–xxx7Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022452Kailuan mining area (Hebei Province)and deep saline aquifers in 453the Jiyang Depression (Shandong province)[89,90].The results 454show that the Dagang oilfield is not suitable for large-scale storage,455though could be considered for EOR pilots.The Shengli oilfield was 456considered more promising for storage (472Mt in eight selected 457fields).Storage potential in the Kailuan mining area is 504,000458Mt adsorbed onto the coal and 38,100Mt void storage capacity.459However,the coals have low porosity and permeability that will af-460fect future energy resources [90].The Institute of Geology and Geo-461physics,Chinese Academy of Sciences (IGG–CAS)studied the 462potential for storage in the Jiyang Depression.The results revealed 463that Guantao Formation in the Jiyang Depression has good porosity 464and permeability 465areas was 4662010,in South China 467of Sciences 468storage capacity in 469the Pearl River 470CCS-related 471China [78].There 472saline formations 473tive storage 474including 60Mt in 475large for storaging 476in Guangdong in 477In a word,these 478age of CO 2in deep 479Although this is 480countries,it will 481and there was 482characteristics.483 3.1.2.CO 2–EOR484Although CO 2485oil recovery (EOR)486this process can be 487oil,the cost of CO 2,488the CO 2source [92].489the production of 490be an ideal option 49184commercial or 492tion worldwide [1].493been implemented 494Oil Corporation 495in the Daqing,496ernments of Japan 497out a project to 498plant in China into a 499duced from the 500that between 270501ered by using CO 2502including IGG–CAS 503three large oil fields 504oil reservoirs in the 505were suitable both 506found suitable for 507showed that the 508CO 2storage 509the oil recovery by steam injection has been already applied at Lia-510ohe oil field.Each single well,in average,had conducted 7.6times 511of steam injection-oil recovery processing for EOR propose.The to-512tal recovered oil amounts were 12.06Mt [92].Active oil producing 513fields where CO 2–EOR is technically possible provide credible 514opportunities to initiate CO 2storage demonstration projects.How-515ever,significant further investigations,including detailed site516appraisals would be necessary before such fields can be considered 517as technically and economically suitable for CO 2storage.5183.1.3.CO 2–ECBM519In a similar manner,ECBM recovery can be used to store CO 2520while improving methane recovery.A bright prospect of gas injec-521tion technology for ECBM production has been suggested by Chi-522nese engineers since the late 1990s [94].More recently,a joint 523venture was formed between the China United Coal Bed Methane 524Corporation and the Alberta Research Council of Canada to develop 525a project entitled ‘‘Development of China’s coalbed methane tech-526nology/CO 2sequestration’’[12].This project was initiated in March 527project was performed 528in the anthracitic coals of 529China [95],which is 530in China up to now 531at ICC–CAS in 2005532were investigated based 533An equipment simulated 534middle pressures was 535in coal seam,536behaviors were studied.537coal mine and salt-water 538four coals of various rank 539China were tested for 540one of the most impor-541process.The result 542capacities for methane 543>Bulianta coal >Zhangji 544adsorption isotherms 545lattice model [100].546given to estimate the 547[101],which was 548underground stress is so 5492and CH 4respectively 550the mechanical sta-551of the impact factors of 552stress could be obviously 553of the casing or by using 554and Young’s modulus 555China was also estimated 556prospecting data of coal 557and the replacement ratio 558different ranks,it is esti-559methane resources will 560technology is uti-561in coalbeds is about 562as the total CO 2emission 563also developed 564simulation of the CO 2–565is a lack of knowledge 566due to the complexity 567fluid transport processes.568will be the next 569570571Large amounts of CO 2can also be fixed by a process called min-572eral carbonation,which is natural or artificial fixation of CO 2into 573carbonates.It has been proposed as a promising CO 2sequestration 574technology e.g.the silicate rocks (calcium or magnesium)could be 575turned into carbonates by reacting with CO 2following this mech-576anism [8,105]:577ðMg ;Ca Þx Si y O x þ2y þx CO 2!x ðMg ;Ca ÞCO 3þy SiO 25798L.L Q1i et al./Fuel xxx (2011)xxx–xxxPlease cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639hanced by the fact that this method of storage is highly verifiable 640and unquestionably permanent,the grinding energy required to 641produce particles of the size required to react rapidly with the 642acids is large,and the residence times on the order of hours re-643quired to allow carbonation of the solids,via either route,is so long 644that immense reactors would be required,associating environmen-645tal concerns.Furthermore,mineral carbonation will always be646expensive than most applications of geological storage 647important gap in mineral carbonation is the lack of 648onstration plant.649Ocean storage650Captured CO 2also could help reduce the atmospheric 6516526536546556566576586596606616626636646656666676686696706716726736746756766776786796806816826836842685carbonic acid,which would be likely harmful to ocean organisms 686and ecosystems [17].Additionally,it is not known whether the 687public will accept the deliberate storage of CO 2in the ocean as part 688of a climate change mitigation strategy.The development of ocean 689storage technology is generally at a conceptual stage;thus,further 690research and development would be needed to make technologies 691available.3.Reaction mechanism for enhanced carbonation crystallization Q1Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022692693694695696697698699700701702703704705706707708709710711712713714715716717718719721722723the reaction [131–135],such as in ICC–CAS,Zhao et al.had reported 724a catalyst system composed of KI supported on metal oxides for 725cycloaddition of propylene oxide with CO 2.It was found that the 726activity of KI for cycloaddition was greatly enhanced by ZnO as both 727support and promoter,resulting in a high yield of propylene 728carbonate within a short reaction time.The mechanism is also pro-729posed (Scheme 4)[133].Recently,a large number of catalytic730systems,such as metal oxides,transition metal,ammonium 731well as main group complexes,were reported to be active 732reactions [136–139].In ICC–CAS,the efficient ultrasonic tech-733nique was used for the preparation of amine-functionalized porous 734catalysts for CO 2coupling with epoxide.According to 735study by Zhang and co-workers [140],the reaction conditions 736great influence on the performance and the silanols on the surface 737played an important role in the chemical fixation of CO 2.In addi-738they also proposed the possible reaction mechanism for 739coupling with epoxide over such type of catalysts (Scheme 5).740In recent years,ionic liquids as environmentally benign media 741organic synthesis and catalytic reaction significant progress 7427437447457467477487492750catalyst system without using additional organic solvents was 751achieved in excellent selectivity and TOF (5410h À1)[144].In 752IPE–CAS,an efficient Lewis acid/base catalyst composed of ZnCl 2/753PPh 3C 6H 13Br was developed and showed high activity and selectiv-754ity for the coupling reaction of CO 2and epoxide under the mild 755conditions [145].Sun et al.prepared a series of hydroxyl-function-756alized ionic liquids (HFILs)which showed efficient reactivity andScheme 4.The proposed diagram of reaction mechanism [133].Q1Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022780781782783784785786787788789790791792793794795796797798799800802803804805806807Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022CO 2þCH 4¼2CO þ2H 2841842In the past decade,a lot of researches have been devoted to the 843catalytic performance of noble metals,including Pt,Ru,Rh,Pd,844and Ir for this reaction [165–169].It showed that Rh and Ru845exhibited both high activity and stability in CH 4dry reforming,846while Pd,Pt and Ir were less active and prone to deactivation.847Nevertheless,considering the aspects of high cost and limitedPlease cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022848availability of noble metals,it is more practical to develop non-849noble metal catalysts which exhibited both high activity and pared with noble metals,Ni-based catalysts have been 851widely investigated because of their high activity and relatively 852low price [170–172].Nevertheless,application of Ni-based cata-853lysts in a large scale process is not so straightforward due to rapid 854carbon deposition,resulting in the deactivation of the catalyst 855[173].It was found that when Ni is supported on a alkaline earth 856metal oxide such as MgO,CaO,and BaO with strong Lewis basi-857city,carbon deposition can be attenuated or even suppressed 858[174]which is because that the support could promote chemi-859sorption of CO 2and thus,accelerated the reaction of CO 2and C 8608618628638648658668678688698708718728738748758768778788798808818828838848858868878888898908918928938948958968974.4.Reaction of CO 2with ethane and propane898Ethylene and propylene are basic raw material in the petrol-899chemical industry.Thermal cracking of hydrocarbons (such as eth-900ane)in the presence of steam is currently the main source of eth-901ylene [181,182].Nevertheless,steam cracking of ethane to 902ethylene is a highly endothermic process that must be performed 903at high temperatures,which means the consumption of a large 904amount of energy.The introduction of CO 2could reduce the extent 905of deep oxidation which results in many byproducts whereas eth-906ylene selectivity drops when oxygen is used as oxidant [183].907Thermodynamics analysis and experimental results have indi-908909910911912913915916917918919920921922923924925926927928929930931932933934935936938939940941L.L Q1i et al./Fuel xxx (2011)xxx–xxx13Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.022969970971972973974975976977978979980981982983984985986987988990991992993994995996997998999100010011002100310041005100610071008100910101011101210131014Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.0221015DMC in supercritical phase was proposed in Scheme 11.Recently,developed a supported Cu-Ni/V 2O 5-SiO 2heterogeneous because the reaction can be carried out in a fixed-bed the side production of water molecules 10341035103610371038103910401041104210431044and theoretical approaches,which enable the development of 1045CO 2selective sorbents.Besides,the sorbent performance,lifetime,10461047104810491050105110521053105410551056105710581059106010611062106310641065Scheme 11.The proposed catalytic reaction mechanism Please cite this article in press as:L Q1i L et al.A review of research progress on CO 2capture,storage,and utilization in Chinese Academy of Sciences.Fuel(2011),doi:10.1016/j.fuel.2011.08.0221066of component costs,specific Chinese market conditions,and other 1067factors impacting costs of deployment in China will be important 1068to consider in greater detail.To propose the storage mechanism,1069monitoring,simulation,risk assessment,control methods as well 1070as engineering design will be studied in future.1071The utilization of CO 2to chemicals has attracted considerable 1072attention as a possible way to manufacture useful commercial 1073chemicals from CO 2in some specific locations (Scheme 12)[222].1074The utilization of CO 2as a raw material in the synthesis of chemi-1075cals was also conducted by CAS,including synthesis of cyclic car-1076bonate from CO 2and epoxide,reaction of CO 2and propylene 1077glycol (PG),CO 2reforming of CH 4,reaction of CO 2and ethane 1078and propane,CO 21079methyl carbonate 1080amount of CO 2can 1081to the order of 1082ent the typical 1083cations is only 1084laminates are used 1085of the materials can 1086and overall net 1087ation.1088and fundamental 1089lue-added chemicals 1090tive net carbon 1091moderate reaction 1092the energy or 1093clear,wind,1094also important to 1095 6.Uncited 1096[201].Q31097Acknowledgments1098This work was 1099vation Programme 1100323);the Ministry of 1101lic of China 1102Climate Change:1103Academy of 1104the 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Stabilized high-power laser system forthe gravitational wave detector advancedLIGOP.Kwee,1,∗C.Bogan,2K.Danzmann,1,2M.Frede,4H.Kim,1P.King,5J.P¨o ld,1O.Puncken,3R.L.Savage,5F.Seifert,5P.Wessels,3L.Winkelmann,3and B.Willke21Max-Planck-Institut f¨u r Gravitationsphysik(Albert-Einstein-Institut),Hannover,Germany2Leibniz Universit¨a t Hannover,Hannover,Germany3Laser Zentrum Hannover e.V.,Hannover,Germany4neoLASE GmbH,Hannover,Germany5LIGO Laboratory,California Institute of Technology,Pasadena,California,USA*patrick.kwee@aei.mpg.deAbstract:An ultra-stable,high-power cw Nd:Y AG laser system,devel-oped for the ground-based gravitational wave detector Advanced LIGO(Laser Interferometer Gravitational-Wave Observatory),was comprehen-sively ser power,frequency,beam pointing and beamquality were simultaneously stabilized using different active and passiveschemes.The output beam,the performance of the stabilization,and thecross-coupling between different stabilization feedback control loops werecharacterized and found to fulfill most design requirements.The employedstabilization schemes and the achieved performance are of relevance tomany high-precision optical experiments.©2012Optical Society of AmericaOCIS codes:(140.3425)Laser stabilization;(120.3180)Interferometry.References and links1.S.Rowan and J.Hough,“Gravitational wave detection by interferometry(ground and space),”Living Rev.Rel-ativity3,1–3(2000).2.P.R.Saulson,Fundamentals of Interferometric Gravitational Wave Detectors(World Scientific,1994).3.G.M.Harry,“Advanced LIGO:the next generation of gravitational wave detectors,”Class.Quantum Grav.27,084006(2010).4. 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A.Araya,N.Mio,K.Tsubono,K.Suehiro,S.Telada,M.Ohashi,and M.Fujimoto,“Optical mode cleaner withsuspended mirrors,”Appl.Opt.36,1446–1453(1997).27.P.Kwee,B.Willke,and K.Danzmann,“Shot-noise-limited laser power stabilization with a high-power photodi-ode array,”Opt.Lett.34,2912–2914(2009).28. ntz,P.Fritschel,H.Rong,E.Daw,and G.Gonz´a lez,“Quantum-limited optical phase detection at the10−10rad level,”J.Opt.Soc.Am.A19,91–100(2002).1.IntroductionInterferometric gravitational wave detectors[1,2]perform one of the most precise differential length measurements ever.Their goal is to directly detect the faint signals of gravitational waves emitted by astrophysical sources.The Advanced LIGO(Laser Interferometer Gravitational-Wave Observatory)[3]project is currently installing three second-generation,ground-based detectors at two observatory sites in the USA.The4kilometer-long baseline Michelson inter-ferometers have an anticipated tenfold better sensitivity than theirfirst-generation counterparts (Inital LIGO)and will presumably reach a strain sensitivity between10−24and10−23Hz−1/2.One key technology necessary to reach this extreme sensitivity are ultra-stable high-power laser systems[4,5].A high laser output power is required to reach a high signal-to-quantum-noise ratio,since the effect of quantum noise at high frequencies in the gravitational wave readout is reduced with increasing circulating laser power in the interferometer.In addition to quantum noise,technical laser noise coupling to the gravitational wave channel is a major noise source[6].Thus it is important to reduce the coupling of laser noise,e.g.by optical design or by exploiting symmetries,and to reduce laser noise itself by various active and passive stabilization schemes.In this article,we report on the pre-stabilized laser(PSL)of the Advanced LIGO detector. The PSL is based on a high-power solid-state laser that is comprehensively stabilized.One laser system was set up at the Albert-Einstein-Institute(AEI)in Hannover,Germany,the so called PSL reference system.Another identical PSL has already been installed at one Advanced LIGO site,the one near Livingston,LA,USA,and two more PSLs will be installed at the second #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10618site at Hanford,WA,USA.We have characterized the reference PSL and thefirst observatory PSL.For this we measured various beam parameters and noise levels of the output beam in the gravitational wave detection frequency band from about10Hz to10kHz,measured the performance of the active and passive stabilization schemes,and determined upper bounds for the cross coupling between different control loops.At the time of writing the PSL reference system has been operated continuously for more than18months,and continues to operate reliably.The reference system delivered a continuous-wave,single-frequency laser beam at1064nm wavelength with a maximum power of150W with99.5%in the TEM00mode.The active and passive stabilization schemes efficiently re-duced the technical laser noise by several orders of magnitude such that most design require-ments[5,7]were fulfilled.In the gravitational wave detection frequency band the relative power noise was as low as2×10−8Hz−1/2,relative beam pointingfluctuations were as low as1×10−7Hz−1/2,and an in-loop measurement of the frequency noise was consistent with the maximum acceptable frequency noise of about0.1HzHz−1/2.The cross couplings between the control loops were,in general,rather small or at the expected levels.Thus we were able to optimize each loop individually and observed no instabilities due to cross couplings.This stabilized laser system is an indispensable part of Advanced LIGO and fulfilled nearly all design goals concerning the maximum acceptable noise levels of the different beam pa-rameters right after installation.Furthermore all or a subset of the implemented stabilization schemes might be of interest for many other high-precision optical experiments that are limited by laser noise.Besides gravitational wave detectors,stabilized laser systems are used e.g.in the field of optical frequency standards,macroscopic quantum objects,precision spectroscopy and optical traps.In the following section the laser system,the stabilization scheme and the characterization methods are described(Section2).Then,the results of the characterization(Section3)and the conclusions(Section4)are presented.ser system and stabilizationThe PSL consists of the laser,developed and fabricated by Laser Zentrum Hannover e.V.(LZH) and neoLASE,and the stabilization,developed and integrated by AEI.The optical components of the PSL are on a commercial optical table,occupying a space of about1.5×3.5m2,in a clean,dust-free environment.At the observatory sites the optical table is located in an acoustically isolated cleanroom.Most of the required electronics,the laser diodes for pumping the laser,and water chillers for cooling components on the optical table are placed outside of this cleanroom.The laser itself consists of three stages(Fig.1).An almostfinal version of the laser,the so-called engineering prototype,is described in detail in[8].The primary focus of this article is the stabilization and characterization of the PSL.Thus only a rough overview of the laser and the minor modifications implemented between engineering prototype and reference system are given in the following.Thefirst stage,the master laser,is a commercial non-planar ring-oscillator[9,10](NPRO) manufactured by InnoLight GmbH in Hannover,Germany.This solid-state laser uses a Nd:Y AG crystal as the laser medium and resonator at the same time.The NPRO is pumped by laser diodes at808nm and delivers an output power of2W.An internal power stabilization,called the noise eater,suppresses the relaxation oscillation at around1MHz.Due to its monolithic res-onator,the laser has exceptional intrinsic frequency stability.The two subsequent laser stages, used for power scaling,inherit the frequency stability of the master laser.The second stage(medium-power amplifier)is a single-pass amplifier[11]with an output power of35W.The seed laser beam from the NPRO stage passes through four Nd:YVO4crys-#161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10619power stabilizationFig.1.Pre-stabilized laser system of Advanced LIGO.The three-staged laser(NPRO,medium power amplifier,high power oscillator)and the stabilization scheme(pre-mode-cleaner,power and frequency stabilization)are shown.The input-mode-cleaner is not partof the PSL but closely related.NPRO,non-planar ring oscillator;EOM,electro-optic mod-ulator;FI,Faraday isolator;AOM,acousto-optic modulator.tals which are longitudinally pumped byfiber-coupled laser diodes at808nm.The third stage is an injection-locked ring oscillator[8]with an output power of about220W, called the high-power oscillator(HPO).Four Nd:Y AG crystals are used as the active media. Each is longitudinally pumped by sevenfiber-coupled laser diodes at808nm.The oscillator is injection-locked[12]to the previous laser stage using a feedback control loop.A broadband EOM(electro-optic modulator)placed between the NPRO and the medium-power amplifier is used to generate the required phase modulation sidebands at35.5MHz.Thus the high output power and good beam quality of this last stage is combined with the good frequency stability of the previous stages.The reference system features some minor modifications compared to the engineering proto-type[8]concerning the optics:The external halo aperture was integrated into the laser system permanently improving the beam quality.Additionally,a few minor designflaws related to the mechanical structure and the optical layout were engineered out.This did not degrade the output performance,nor the characteristics of the locked laser.In general the PSL is designed to be operated in two different power modes.In high-power mode all three laser stages are engaged with a power of about160W at the PSL output.In low-power mode the high-power oscillator is turned off and a shutter inside the laser resonator is closed.The beam of the medium-power stage is reflected at the output coupler of the high power stage leaving a residual power of about13W at the PSL output.This low-power mode will be used in the early commissioning phase and in the low-frequency-optimized operation mode of Advanced LIGO and is not discussed further in this article.The stabilization has three sections(Fig.1:PMC,PD2,reference cavity):A passive resonator, the so called pre-mode-cleaner(PMC),is used tofilter the laser beam spatially and temporally (see subsection2.1).Two pick-off beams at the PMC are used for the active power stabilization (see subsection2.2)and the active frequency pre-stabilization,respectively(see subsection2.3).In general most stabilization feedback control loops of the PSL are implemented using analog electronics.A real-time computer system(Control and Data Acquisition Systems,CDS,[13]) which is common to many other subsystems of Advanced LIGO,is utilized to control and mon-itor important parameters of the analog electronics.The lock acquisition of various loops,a few #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10620slow digital control loops,and the data acquisition are implemented using this computer sys-tem.Many signals are recorded at different sampling rates ranging from16Hz to33kHz for diagnostics,monitoring and vetoing of gravitational wave signals.In total four real-time pro-cesses are used to control different aspects of the laser system.The Experimental Physics and Industrial Control System(EPICS)[14]and its associated user tools are used to communicate with the real-time software modules.The PSL contains a permanent,dedicated diagnostic instrument,the so called diagnostic breadboard(DBB,not shown in Fig.1)[15].This instrument is used to analyze two different beams,pick-off beams of the medium power stage and of the HPO.Two shutters are used to multiplex these to the DBB.We are able to measurefluctuations in power,frequency and beam pointing in an automated way with this instrument.In addition the beam quality quantified by the higher order mode content of the beam was measured using a modescan technique[16].The DBB is controlled by one real-time process of the CDS.In contrast to most of the other control loops in the PSL,all DBB control loops were implemented digitally.We used this instrument during the characterization of the laser system to measure the mentioned laser beam parameters of the HPO.In addition we temporarily placed an identical copy of the DBB downstream of the PMC to characterize the output beam of the PSL reference system.2.1.Pre-mode-cleanerA key component of the stabilization scheme is the passive ring resonator,called the pre-mode-cleaner(PMC)[17,18].It functions to suppress higher-order transverse modes,to improve the beam quality and the pointing stability of the laser beam,and tofilter powerfluctuations at radio frequencies.The beam transmitted through this resonator is the output beam of the PSL, and it is delivered to the subsequent subsystems of the gravitational wave detector.We developed and used a computer program[19]to model thefilter effects of the PMC as a function of various resonator parameters in order to aid its design.This led to a resonator with a bow-tie configuration consisting of four low-loss mirrors glued to an aluminum spacer. The optical round-trip length is2m with a free spectral range(FSR)of150MHz.The inci-dence angle of the horizontally polarized laser beam is6◦.Theflat input and output coupling mirrors have a power transmission of2.4%and the two concave high reflectivity mirrors(3m radius of curvature)have a transmission of68ppm.The measured bandwidth was,as expected, 560kHz which corresponds to afinesse of133and a power build-up factor of42.The Gaussian input/output beam had a waist radius of about568µm and the measured acquired round-trip Gouy phase was about1.7rad which is equivalent to0.27FSR.One TEM00resonance frequency of the PMC is stabilized to the laser frequency.The Pound-Drever-Hall(PDH)[20,21]sensing scheme is used to generate error signals,reusing the phase modulation sidebands at35.5MHz created between NPRO and medium power amplifier for the injection locking.The signal of the photodetector PD1,placed in reflection of the PMC, is demodulated at35.5MHz.This photodetector consists of a1mm InGaAs photodiode and a transimpedance amplifier.A piezo-electric element(PZT)between one of the curved mirrors and the spacer is used as a fast actuator to control the round-trip length and thereby the reso-nance frequencies of the PMC.With a maximum voltage of382V we were able to change the round-trip length by about2.4µm.An analog feedback control loop with a bandwidth of about 7kHz is used to stabilize the PMC resonance frequency to the laser frequency.In addition,the electronics is able to automatically bring the PMC into resonance with the laser(lock acquisition).For this process a125ms period ramp signal with an amplitude cor-responding to about one FSR is applied to the PZT of the PMC.The average power on pho-todetector PD1is monitored and as soon as the power drops below a given threshold the logic considers the PMC as resonant and closes the analog control loop.This lock acquisition proce-#161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10621dure took an average of about65ms and is automatically repeated as soon as the PMC goes off resonance.One real-time process of CDS is dedicated to control the PMC electronics.This includes parameters such as the proportional gain of the loop or lock acquisition parameters.In addition to the PZT actuator,two heating foils,delivering a maximum total heating power of14W,are attached to the aluminum spacer to control its temperature and thereby the roundtrip length on timescales longer than3s.We measured a heating and cooling1/e time constant of about2h with a range of4.5K which corresponds to about197FSR.During maintenance periods we heat the spacer with7W to reach a spacer temperature of about2.3K above room temperature in order to optimize the dynamic range of this actuator.A digital control loop uses this heater as an actuator to off-load the PZT actuator allowing compensation for slow room temperature and laser frequency drifts.The PMC is placed inside a pressure-tight tank at atmospheric pressure for acoustic shield-ing,to avoid contamination of the resonator mirrors and to minimize optical path length changes induced by atmospheric pressure variations.We used only low-outgassing materials and fabri-cated the PMC in a cleanroom in order to keep the initial mirror contamination to a minimum and to sustain a high long-term throughput.The PMCfilters the laser beam and improves the beam quality of the laser by suppress-ing higher order transverse modes[17].The acquired round-trip Gouy phase of the PMC was chosen in such a way that the resonance frequencies of higher order TEM modes are clearly separated from the TEM00resonance frequency.Thus these modes are not resonant and are mainly reflected by the PMC,whereas the TEM00mode is transmitted.However,during the design phase we underestimated the thermal effects in the PMC such that at nominal circu-lating power the round-trip Gouy-phase is close to0.25FSR and the resonance of the TEM40 mode is close to that of the TEM00mode.To characterize the mode-cleaning performance we measured the beam quality upstream and downstream of the PMC with the two independent DBBs.At150W in the transmitted beam,the circulating power in the PMC is about6.4kW and the intensity at the mirror surface can be as high as1.8×1010W m−2.At these power levels even small absorptions in the mirror coatings cause thermal effects which slightly change the mirror curvature[22].To estimate these thermal effects we analyzed the transmitted beam as a function of the circulating power using the DBB.In particular we measured the mode content of the LG10and TEM40mode.Changes of the PMC eigenmode waist size showed up as variations of the LG10mode content.A power dependence of the round-trip Gouy phase caused a variation of the power within the TEM40mode since its resonance frequency is close to a TEM00mode resonance and thus the suppression of this mode depends strongly on the Gouy phase.We adjusted the input power to the PMC such that the transmitted power ranged from100W to 150W corresponding to a circulating power between4.2kW and6.4kW.We used our PMC computer simulation to deduce the power dependence of the eigenmode waist size and the round-trip Gouy phase.The results are given in section3.1.At all circulating power levels,however,the TEM10and TEM01modes are strongly sup-pressed by the PMC and thus beam pointingfluctuations are reduced.Pointingfluctuations can be expressed tofirst order as powerfluctuations of the TEM10and TEM01modes[23,24].The PMC reduces thefield amplitude of these modes and thus the pointingfluctuations by a factor of about61according to the measuredfinesse and round-trip Gouy phase.To keep beam point-ingfluctuations small is important since they couple to the gravitational wave channel by small differential misalignments of the interferometer optics.Thus stringent design requirements,at the10−6Hz−1/2level for relative pointing,were set.To verify the pointing suppression effect of the PMC we used DBBs to measure the beam pointingfluctuations upstream and downstream #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10622Fig.2.Detailed schematic of the power noise sensor setup for thefirst power stabilizationloop.This setup corresponds to PD2in the overview in Fig.1.λ/2,waveplate;PBS,polar-izing beam splitter;BD,glassfilters used as beam dump;PD,single element photodetector;QPD,quadrant photodetector.of the PMC.The resonator design has an even number of nearly normal-incidence reflections.Thus the resonance frequencies of horizontal and vertical polarized light are almost identical and the PMC does not act as polarizer.Therefore we use a thin-film polarizer upstream of the PMC to reach the required purity of larger than100:1in horizontal polarization.Finally the PMC reduces technical powerfluctuations at radio frequencies(RF).A good power stability between9MHz and100MHz is necessary as the phase modulated light in-jected into the interferometer is used to sense several degrees of freedom of the interferometer that need to be controlled.Power noise around these phase modulation sidebands would be a noise source for the respective stabilization loop.The PMC has a bandwidth(HWHM)of about 560kHz and acts tofirst order as a low-passfilter for powerfluctuations with a-3dB corner frequency at this frequency.To verify that the suppression of RF powerfluctuations is suffi-cient to fulfill the design requirements,we measured the relative power noise up to100MHz downstream of the PMC with a dedicated experiment involving the optical ac coupling tech-nique[25].In addition the PMC serves the very important purpose of defining the spatial laser mode for the downstream subsystem,namely the input optics(IO)subsystem.The IO subsystem is responsible,among other things,to further stabilize the laser beam with the suspended input mode cleaner[26]before the beam will be injected into the interferometer.Modifications of beam alignment or beam size of the laser system,which were and might be unavoidable,e.g., due to maintenance,do not propagate downstream of the PMC tofirst order due to its mode-cleaning effect.Furthermore we benefit from a similar isolating effect for the active power and frequency stabilization by using the beams transmitted through the curved high-reflectivity mirrors of the PMC.2.2.Power stabilizationThe passivefiltering effect of the PMC reduces powerfluctuations significantly only above the PMC bandwidth.In the detection band from about10Hz to10kHz good power stability is required sincefluctuations couple via the radiation pressure imbalance and the dark-fringe offset to the gravitational wave channel.Thus two cascaded active control loops,thefirst and second power stabilization loop,are used to reduce powerfluctuations which are mainly caused by the HPO stage.Thefirst loop uses a low-noise photodetector(PD2,see Figs.1and2)at one pick-off port #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10623of the PMC to measure the powerfluctuations downstream of the PMC.An analog electronics feedback control loop and an AOM(acousto-optic modulator)as actuator,located upstream of the PMC,are used to stabilize the power.Scattered light turned out to be a critical noise source for thisfirst loop.Thus we placed all required optical and opto-electronic components into a box to shield from scattered light(see Fig.2).The beam transmitted by the curved PMC mirror has a power of about360mW.This beam isfirst attenuated in the box using aλ/2waveplate and a thin-film polarizer,such that we are able to adjust the power on the photodetectors to the optimal operation point.Afterwards the beam is split by a50:50beam splitter.The beams are directed to two identical photode-tectors,one for the control loop(PD2a,in-loop detector)and one for independent out-of-loop measurements to verify the achieved power stability(PD2b,out-of-loop detector).These pho-todetectors consist of a2mm InGaAs photodiode(PerkinElmer C30642GH),a transimpedance amplifier and an integrated signal-conditioningfilter.At the chosen operation point a power of about4mW illuminates each photodetector generating a photocurrent of about3mA.Thus the shot noise is at a relative power noise of10−8Hz−1/2.The signal conditioningfilter has a gain of0.2at very low frequencies(<70mHz)and amplifies the photodetector signal in the im-portant frequency range between3.3Hz and120Hz by about52dB.This signal conditioning filter reduces the electronics noise requirements on all subsequent stages,but has the drawback that the range between3.3Hz and120Hz is limited to maximum peak-to-peak relative power fluctuations of5×10−3.Thus the signal-conditioned channel is in its designed operation range only when the power stabilization loop is closed and therefore it is not possible to measure the free running power noise using this channel due to saturation.The uncoated glass windows of the photodiodes were removed and the laser beam hits the photodiodes at an incidence angle of45◦.The residual reflection from the photodiode surface is dumped into a glassfilter(Schott BG39)at the Brewster angle.Beam positionfluctuations in combination with spatial inhomogeneities in the photodiode responsivity is another noise source for the power stabilization.We placed a silicon quadrant photodetector(QPD)in the box to measure the beam positionfluctuations of a low-power beam picked off the main beam in the box.The beam parameters,in particular the Gouy phase,at the QPD are the same as on the power sensing detectors.Thus the beam positionfluctuations measured with the QPD are the same as the ones on the power sensing photodetectors,assuming that the positionfluctuations are caused upstream of the QPD pick-off point.We used the QPD to measure beam positionfluctuations only for diagnostic and noise projection purposes.In a slightly modified experiment,we replaced one turning mirror in the path to the power sta-bilization box by a mirror attached to a tip/tilt PZT element.We measured the typical coupling between beam positionfluctuations generated by the PZT and the residual relative photocurrent fluctuations measured with the out-of-the-loop photodetector.This coupling was between1m−1 and10m−1which is a typical value observed in different power stabilization experiments as well.We measured this coupling factor to be able to calculate the noise contribution in the out-of-the-loop photodetector signal due to beam positionfluctuations(see Subsection3.3).Since this tip/tilt actuator was only temporarily in the setup,we are not able to measure the coupling on a regular basis.Both power sensing photodetectors are connected to analog feedback control electronics.A low-pass(100mHz corner frequency)filtered reference value is subtracted from one signal which is subsequently passed through several control loopfilter stages.With power stabilization activated,we are able to control the power on the photodetectors and thereby the PSL output power via the reference level on time scales longer than10s.The reference level and other important parameters of these electronics are controlled by one dedicated real-time process of the CDS.The actuation or control signal of the electronics is passed to an AOM driver #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10624。

DOI: 10.1126/science.1220854, 773 (2013);339 Science et al.Marc André Meyers ConnectionsStructural Biological Materials: Critical Mechanics-MaterialsThis copy is for your personal, non-commercial use only.clicking here.colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to othershere.following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles): May 1, 2013 (this information is current as of The following resources related to this article are available online at/content/339/6121/773.full.html version of this article at:including high-resolution figures, can be found in the online Updated information and services, /content/339/6121/773.full.html#ref-list-1, 12 of which can be accessed free:cites 51 articles This article/cgi/collection/mat_sci Materials Sciencesubject collections:This article appears in the following registered trademark of AAAS.is a Science 2013 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n M a y 1, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o mStructural BiologicalMaterials:CriticalMechanics-Materials ConnectionsMarc AndréMeyers,1,2*Joanna McKittrick,1Po-Yu Chen3Spider silk is extraordinarily strong,mollusk shells and bone are tough,and porcupine quills and feathers resist buckling.How are these notable properties achieved?The building blocks of the materials listed above are primarily minerals and biopolymers,mostly in combination;the first weak in tension and the second weak in compression.The intricate and ingenious hierarchical structures are responsible for the outstanding performance of each material.Toughness is conferred by the presence of controlled interfacial features(friction,hydrogen bonds,chain straightening and stretching);buckling resistance can be achieved by filling a slender column with a lightweight foam.Here,we present and interpret selected examples of these and other biological materials.Structural bio-inspired materials design makes use of the biological structures by inserting synthetic materials and processes that augment the structures’capability while retaining their essential features.In this Review,we explain this idea through some unusual concepts.M aterials science is a vibrant field of in-tellectual endeavor and research.Thisfield applies physics and chemistry, melding them in the process,to the interrela-tionship between structure,properties,and perform-ance of complex materials with technological applications.Thus,materials science extends these rigorous scientific disciplines into complex ma-terials that have structures providing properties and synergies beyond those of pure and simple solids.Initially geared at synthetic materials,ma-terials science has recently extended its reach into biology,especially into the extracellular matrix, whose mechanical properties are of utmost im-portance in living organisms.Some of the semi-nal work and important contributions in this field are either presented or reviewed in(1–5).There are a number of interrelated features that define biological materials and distinguish them from their synthetic counterparts[inspired by Arzt(6)]: (i)Self-assembly.In contrast to many synthetic processes to produce materials,the structures are assembled from the bottom up,rather than from the top down.(ii)Multi-functionality.Many com-ponents serve more than one purpose.For exam-ple,feathers provide flight capability,camouflage, and insulation,whereas bones provide structural framework,promote the growth of red blood cells, and provide protection to the internal organs.(iii) Hierarchy.Different,organized scale levels(nano-to ultrascale)confer distinct and translatable prop-erties from one level to the next.We are starting to develop a systematic and quantitative understandingof this hierarchy by distinguishing the character-istic levels,developing constitutive descriptionsof each level,and linking them through appro-priate and physically based equations,enabling afull predictive understanding.(iv)Hydration.Theproperties are highly dependent on the level ofwater in the structure.There are some exceptions,such as enamel,but this rule applies to mostbiological materials and is of importance to me-chanical properties such as strength(which isdecreased by hydration)and toughness(which isincreased).(v)Mild synthesis conditions.Themajority of biological materials are fabricated atambient temperature and pressure as well as in anaqueous environment,a notable difference fromsynthetic materials fabrication.(vi)Evolution andenvironmental constraints.The limited availabil-ity of useful elements dictates the morphologyand resultant properties.The structures are notnecessarily optimized for all properties but arethe result of an evolutionary process leading tosatisfactory and robust solutions.(vii)Self-healingcapability.Whereas synthetic materials undergodamage and failure in an irreversible manner,biological materials often have the capability,due to the vascularity and cells embedded in thestructure,to reverse the effects of damage byhealing.The seven characteristics listed above arepresent in a vast number of structures.Nevertheless,the structures of biological materials can bedivided into two broad classes:(i)non-mineralized(“soft”)structures,which are composed of fibrousconstituents(collagen,keratin,elastin,chitin,lignin,and other biopolymers)that display widelyvarying mechanical properties and anisotropiesdepending on the function,and(ii)mineralized(“hard”)structures,consisting of hierarchicallyassembled composites of minerals(mainly,butnot solely,hydroxyapatite,calcium carbonate,and amorphous silica)and organic fibrous com-ponents(primarily collagen and chitin).The mechanical behavior of biological con-stituents and composites is quite diverse.Bio-minerals exhibit linear elastic stress-strain plots,whereas the biopolymer constituents are non-linear,demonstrating either a J shape or a curvewith an inflection point.Foams are characterizedby a compressive response containing a plastic orcrushing plateau in which the porosity is elim-inated.Many biological materials are compositeswith many components that are hierarchicallystructured and can have a broad variety of con-stitutive responses.Below,we present some of thestructures and functionalities of biological ma-terials with examples from current research.Here,we focus on three points:(i)How high tensilestrength is achieved(biopolymers),(ii)how hightoughness is attained(composite structures),and(iii)how bending resistance is achieved in light-weight structures(shells with an interior foam).Structures in Tension:Importance of BiopolymersThe ability to sustain tensile forces requires aspecific set of molecular and configurational con-formations.The initial work performed on exten-sion should be small,to reduce energy expenditure,whereas the material should stiffen close to thebreaking point,to resist failure.Thus,biopolymers,such as collagen and viscid(catching spiral)spidersilk,have a J-shaped stress-strain curve where mo-lecular uncoiling and unkinking occur with con-siderable deformation under low stress.This stiffening as the chains unfurl,straighten,stretch,and slide past each other can be repre-sented analytically in one,two,and three dimen-sions.Examples are constitutive equations initiallydeveloped for polymers by Ogden(7)and Arrudaand Boyce(8).An equation specifically proposedfor tissues is given by Fung(3).A simpler for-mulation is given here;the slope of the stress-strain(s-e)curve increases monotonically with strain.Thus,one considers two regimes:(i)unfurlingand straightening of polymer chainsd sd eºe nðn>1Þð1Þand(ii)stretching of the polymer chain backbonesd sd eºEð2Þwhere E is the elastic modulus of the chains.Thecombined equation,after integrating Eqs.1and2,iss=k1e n+1+H(e c)E(e–e c)(3)Here k1is a parameter,and H is the Heavisidefunction,which activates the second term at e=e c,where e c is a characteristic strain at whichcollagen fibers are fully extended.Subsequent straingradually becomes dominated by chain stretch-ing.The computational results by Gautieri et al.(9)on collagen fibrils corroborate Eq.3for n=1.This corresponds to a quadratic relation between1Department of Mechanical and Aerospace Engineering andMaterials Science and Engineering Program,University ofCalifornia,San Diego,La Jolla,CA92093,USA.2Department ofNanoengineering,University of California,San Diego,La Jolla,CA92093,USA.3Department of Materials Science and En-gineering,National Tsing Hua University,Hsinchu30013,Taiwan,Republic of China.*To whom correspondence should be addressed.E-mail:mameyers@ SCIENCE VOL33915FEBRUARY2013773o n M a y 1 , 2 0 1 3 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o mstress and strain (s ºe 2),which has the char-acteristic J shape.Collagen is the most important structural bio-logical polymer,as it is the key component in many tissues (tendon,ligaments,skin,and bone),as well as in the extracellular matrix.The de-formation process is intimately connected to the different hierarchical levels,starting with the poly-peptides (0.5-nm diameter)to the tropocollagen molecules (1.5-nm diameter),then to the fibrils (~40-to 100-nm diameter),and finally to fibers (~1-to 10-m m diameter)and fascicles (>10-m m diameter).Molecular dynamics computations (9)of entire fibrils show the J -curve response;these computational predictions are well matched to atomic force microscopy (AFM)(10),small-angle x-ray scattering (SAXS)(11),and experiments by Fratzl et al .(12),as shown in Fig.1A.The effect of hydration is also seen and is of great impor-tance.The calculated density of collagen de-creases from 1.34to 1.19g/cm 3with hydration and is accompanied by a decrease in the Young ’s modulus from 3.26to 0.6GPa.The response of silk and spider thread is fascinating.As one of the toughest known ma-terials,silk also has high tensile strength and extensibility.It is composed of b sheet (10to 15volume %)nanocrystals [which consist of highly conserved poly-(Gly-Ala)and poly-Ala domains]embedded in a disordered matrix (13).Figure 1B shows the J -shape stress-strain curve and molecular configurations for the crystalline domains in silkworm (Bombyx mori )silk (14).Similar to collagen,the low-stress region corre-sponds to uncoiling and straightening of the pro-tein strands.This region is followed by entropic unfolding of the amorphous strands and then stiffening due to load transfer to the crystalline b sheets.Despite the high strength,the major mo-lecular interactions in the b sheets are weak hy-drogen bonds.Molecular dynamics simulations,Fig.1.Tensile stress-strain relationships in bio-polymers.(A )J -shaped curve for hydrated and dry collagen fibrils obtained from molecular dynamics (MD)simulations and AFM and SAXS studies.At low stress levels,considerable stretching occurs due to the uncrimping and unfolding of molecules;at higher stress levels,the polymer backbone stretches.Adapted from (9,12).(B )Stretching of dragline spider silk and molecular schematic of the protein fibroin.At low stress levels,entropic effects domi-nate (straightening of amorphous strands);at higher levels,the crystalline parts sustain the load.(C )Mo-lecular dynamics simulation of silk:(i)short stack and (ii)long stack of b -sheet crystals,showing that a higher pullout force is required in the short stack;for the long stack,bending stresses become im-portant.Hydrogen bonds connect b -sheet crystals.Adapted from (14).(D )Egg whelk case (bioelastomer)showing three regions:straightening of the a helices,the a helix –to –b sheet transformation,and b -sheet extension.A molecular schematic is shown.Adapted from (18).300.000.2Yield pointEntropic unfoldingMD simulationsStick slipStiffening β-crystal123456700012345670102030405050010001500200025050075010001250150017500.40.60.80.010.020.030.040.05MD wet (Gautieri et al)SAXS (Sasaki and Odajima)AFM (Aladin et al)MD dry (Gautieri et al)2520151050S t r e s s (M P a )S t r e s s(M P a )StrainABCDStrain (m/m)Length (nm)Length (nm)Stick-slip deformation (robust)"brittle" fracture (fragile)i iiP u l l -o u t f o r c e (p N )00.20.4Native state Unloading: reformation of α-helicesDomain 4: Extension andalignmentof β-sheets0.60.8ε=0ε4ε=01.0012345StrainS t r e s s (M P a )E n e r g y /v o l u m e (k c a l /m o l /n m 3)L e n g t hI I II II III IIIIVIVFDomain 3: Formation of β-sheetsfrom random coilsε3Domain 2: Extension of random coilsε2Domain 1: Unraveling of α-helicesinto random coilsε1Toughness (MD)Resilience (MD)T=-1°C T=20°C T=40°C T=60°C T=80°C15FEBRUARY 2013VOL 339SCIENCE 774REVIEWo n M a y 1, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o mshown in Fig.1C,illustrate an energy dissipative stick-slip shearing of the hydrogen bonds during failure of the b sheets (14).For a stack with a height L ≤3nm (left-hand side of Fig.1C),the shear stresses are more substantial than the flex-ure stresses,and the hydrogen bonds contribute to the high strength obtained (1.5GPa).How-ever,if the stack of b sheets is too high (right-hand side of Fig.1C),it undergoes bending with tensile separation between adjacent sheets.The nanoscale dimension of the b sheets allows for a ductile instead of brittle failure,resulting in high toughness values of silk.Thus,size affects the mechanical response considerably,changing the deformation characteristics of the weak hydro-gen bonds.This has also been demonstrated in bone (15–17),where sacrificial hydrogen bonds between mineralized collagen fibrils contribute to the excellent fracture resistance.Other biological soft materials have more complex responses,marked by discontinuities in d s /d e .This is the case for wool,whelk eggs,silks,and spider webs.Several mechanisms are responsible for this change in slope;for instance,the transition from a -to b -keratin,entropic changes with strain (such as those prevalent in rubber,where chain stretching and alignment decrease entropy),and others.The example of egg whelk is shown in Fig.1D (18).In this case,there is a specific stress at which a -keratin heli-ces transform to b sheets,with an associated change in length.Upon unloading,the reverse occurs,and the total reversible strain is,therefore,extensive.This stress-induced phase transforma-tion is similar to what occurs in shape-memory alloys.Thus,this material can experience sub-stantial reversible deformation (up to 80%)in a reversible fashion,when the stress is raised from 2to 5MPa,ensuring the survival of whelk eggs,which are continually swept by waves.These examples demonstrate the distinct properties of biopolymers that allow these ma-terials to be strong and highly extensible with distinctive molecular deformation characteristics.However,many interesting biological materials are composites of flexible biopolymers and stiff minerals.The combination of these two constit-uents leads to the creation of a tough material.Imparting Toughness:Importance of Interfaces One hallmark property of most biological com-posites is that they are tough.Toughness is defined as the amount of energy a material ab-sorbs before it fails,expressed asU ¼∫e fs d eð4Þwhere U is the energy per volume absorbed,s is the stress,e is the strain,and e f is the failure strain.Tough materials show considerable plastic deformation (or permanent damage)coupled with considerable strength.This maximizes the integral expression in Eq.4.Biological com-posite materials (for example,crystalline and noncrystalline components)have a plethora oftoughening mechanisms,many of which depend on the presence of interfaces.As a crack im-pinges on an interface or discontinuity in the material,the crack can be deflected around the interface (requiring more energy to propagate than a straight crack)or can drive through it.The strength of biopolymer fibers in tension im-pedes crack opening;bridges between micro-cracks are another mechanism.The toughening mechanisms have been divided into intrinsic (ex-isting in the material ahead of crack)and extrinsic (generated during the progression of failure)cat-egories (19).Thus,toughening is accomplished by a wide variety of stratagems.We illustrate this concept for four biological materials,shown in Fig.2.All inorganic materials contain flaws and cracks,which reduce the strength from the theo-retical value (~E /10to E /30).The maximum stress (s max )a material can sustain when a preexisting crack of length a is present is given by the Griffith equations max ¼ffiffiffiffiffiffiffiffiffiffi2g s E p a r ¼YK Icffiffiffiffiffip ap ð5Þwhere E is the Young ’s modulus,g s is the sur-face (or damage)energy,and Y is a geometric parameter.K Ic ¼Y −1ffiffiffiffiffiffiffiffiffiffi2g s E p is the fracture toughness,a materials property that expresses the ability to resist crack propagation.Abalone (Haliotis rufescens )nacre has a fracture tough-ness that is vastly superior to that of its major constituent,monolithic calcium carbonate,due to an ordered assembly consisting of mineral tiles with an approximate thickness of 0.5m m and a diameter of ~10m m (Fig.2A).Additionally,this material contains organic mesolayers (separated by ~300m m)that are thought to be seasonal growth bands.The tiles are connected by mineral bridges with ~50-nm diameter and are separated by organic layers,consisting of a chitin network and acidic proteins,which,when combined,have a similar thickness to the mineral bridge diame-ters.The Griffith fracture criterion (Eq.5)can be applied to predict the flaw size (a cr )at which the theoretical strength s th is achieved.With typical values for the fracture toughness (K Ic ),s th ,and E ,the critical flaw size is in the range of tens of nanometers.This led Gao et al .(20)to propose that at sufficiently small dimensions (less than the critical flaw size),materials become insensitive to flaws,and the theoretical strength (~E /30)should be achieved at the nanoscale.However,the strength of the material will be determined by fracture mechanisms operating at all hierar-chical levels.The central micrograph in Fig.2A shows how failure occurs by tile pullout.The interdigitated structure deflects cracks around the tiles instead of through them,thereby increasing the total length of the crack and the energy needed to fracture (increasing the toughness).Thus,we must de-termine how effectively the tiles resist pullout.Three contributions have been identified and are believed to operate synergistically (21).First,themineral bridges are thought to approach thetheoretical strength (10GPa),thereby strongly attaching the tiles together (22).Second,the tile surfaces have asperities that are produced during growth (23)and could produce frictional resist-ance and strain hardening (24).Third,energy is required for viscoelastic deformation (stretching and shearing)of the organic layer (25).One important aspect on the mechanical prop-erties is the effect of alignment of the mineral crystals.The oriented tiles in nacre result in an-isotropic properties with the strength and modulus higher in the longitudinal (parallel to the organic layers)than in the transverse direction.For a composite with a dispersed mineral m of volume fraction V m embedded in a biopolymer (bp)matrix that has a much lower strength and Young ’s modulus than the mineral,the ratio of the lon-gitudinal (L)and transverse (T)properties P (such as elastic modulus)can be expressed,in simpli-fied form,asP L P T ¼P mP bpV m ð1−V m Þð6ÞThus,the longitudinal properties are much higher than the transverse properties.This aniso-tropic response is also observed in other oriented mineralized materials,such as bone and teeth.Another tough biological material is the exo-skeleton of an arthropod.In the case of marine animals [for instance,lobsters (26,27)and crabs (28)],the exoskeleton structure consists of layers of mineralized chitin in a Bouligand arrange-ment (successive layers at the same angle to each other,resulting in a helicoidal stacking sequence and in-plane isotropy).These layers can be en-visaged as being stitched together with ductile tubules that also perform other functions,such as fluid transport and moisture regulation.The cross-ply Bouligand arrangement is effective in crack stopping;the crack cannot follow a straight path,thereby increasing the materials ’toughness.Upon being stressed,the mineral components frac-ture,but the chitin fibers can absorb the strain.Thus,the fractured region does not undergo physical separation with dispersal of fragments,and self-healing can take place (29).Figure 2B shows the structure of the lobster (Homarus americanus )exoskeleton with the Bouligand ar-rangement of the fibers.Bone is another example of a biological ma-terial that demonstrates high toughness.Skeletal mammalian bone is a composite of hydroxyapatite-type minerals,collagen and water.On a volu-metric basis,bone consists of ~33to 43volume %minerals,32to 44volume %organics,and 15to 25volume %water.The Young ’s modulus and strength increase,but the toughness decreases with increasing mineral volume fraction (30).Cortical (dense)mammalian bone has blood ves-sels extending along the long axis of the limbs.In animals larger than rats,the vessel is encased in a circumferentially laminated structure called the osteon.Primary osteons are surrounded by hypermineralized regions,whereas secondary SCIENCEVOL 33915FEBRUARY 2013775REVIEWo n M a y 1, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m(remodeled)osteons are surrounded by a cement line (also of high mineral content)(31).In mam-malian cortical bone,the following intrinsic toughening mechanisms have been identified:molecular uncoiling and intermolecular sliding of collagen,fibrillar sliding of collagen bonds,and microcracking of the mineral matrix (19).Extrinsic mechanisms are collagen fibril bridging,uncracked ligament bridging,and crack deflec-tion and twisting (19).Rarely does a limb bone snap in two with smooth fracture surfaces;the crack is often deflected orthogonal to the crack front direction.In the case of (rehydrated)elk (Cervus elaphus )antler bone (shown in Fig.2C)(32),which has the highest toughness of any bone type by far (33),the hypermineralized re-gions around the primary osteons lead to crackdeflection,and the high amount of collagen (~60volume %)adds mechanisms of crack re-tardation and creates crack bridges behind the crack front.The toughening effect in antlers has been estimated as:crack deflection,60%;un-cracked ligament bridges,35%;and collagen as well as fibril bridging,5%(33).A particu-larly important feature in bone is that the fracture toughness increases as the crack propagates,as shown in the plot.This plot demonstrates the crack extension resistance curve,or R -curve,behavior,which is the rate of the total energy dissipated as a function of the crack size.This occurs by the activation of the extrinsic tough-ening mechanisms.In this manner,it becomes gradually more difficult to advance the crack.In human bone,the cracks are deflected and/ortwisted around the cement lines surrounding the secondary osteons and also demonstrate R -curve behavior (34).The final example illustrating how the presence of interfaces is used to retard crack propagation is the glass sea sponge (Euplectella aspergillum ).The entire structure of the V enus ’flower basket is shown in Fig.2D.Biological silica is amorphous and,within the spicules,consists of concentric layers,separated by an organic material,silicatein (35,36).The flexure strength of the spicule notably exceeds (by approximately fivefold)that of monolithic glass (37).The principal reason is the presence of interfaces,which can arrest and/or deflect the crack.Biological materials use ingenious meth-ods to retard the progression of cracks,therebyAbalone shell: NacreMineral bridgesLobsterDeer antlerChitin fibril networkHuman cortical boneMineral crystallitesPrimary osteonsSubvelvet/compact Subvelvet/cCompact Comp p actTransition zoneCancellousCollagen fibrilsDeep sea spongeSkeletonSpicules20 mm1 cmHuman cortical boneElk antlerTransverseIn-plane longitudinalASTM validASTM invalid Mesolayers ABCD0.1 mm500 nm500 nm ˜1 nm˜3 nm˜20 nmCrack extension, ⌬a (mm)T o u g h n e s s , J (k J m -2)50 nm200 nm 10 ␮m500 nm2 ␮m1 ␮m200 ␮m300 ␮m˜10 ␮m0.010.11101000.20.40.6500 00 nm50 nmFig.2.Hierarchical structures of tough biological materials demonstrating the heterogeneous interfaces that provide crack deflection.(A )Abalone nacre showing growth layers (mesolayers),mineral bridges between mineral tiles and asperities on the surface,the fibrous chitin network that forms the backbone of the inorganic layer,and an example of crack tortuosity in which the crack must travel around the tiles instead of through them [adapted from (4,21)].(B )Lobster exoskeleton showing the twisted plywood structure of the chitin (next to the shell)and the tubules that extend from the chitin layers to the animal [adapted from (27)].(C )Antler bone image showing the hard outer sheath (cortical bone)surrounding the porous bone.The collagen fibrils are highly aligned in the growth direction,with nanocrystalline minerals dispersed in and around them.The osteonal structure in a cross section of cortical bone illustrates the boundaries where cracks perpendicular to the osteons can be directed [adapted from (33)].ASTM,American Society for Testing and Mate-rials.(D )Silica sponge and the intricate scaffold of spicules.Each spicule is a circumferentially layered rod:The interfaces between the layers assist in ar-resting crack anic silicate in bridging adjacent silica layers is observed at higher magnification (red arrow)(36).15FEBRUARY 2013VOL 339SCIENCE776REVIEWo n M a y 1, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o mincreasing toughness.These methods operate at levels ranging from the nanoscale to the structur-al scale and involve interfaces to deflect cracks,bridging by ductile phases (e.g.,collagen or chitin),microcracks forming ahead of the crack,delocal-ization of damage,and others.Lightweight Structures Resistant to Bending,Torsion,and Buckling —Shells and FoamsResistance to flexural and torsional tractions with a prescribed deflection is a major attribute of many biological structures.The fundamental mechanics of elastic (recoverable)deflection,as it relates to the geometrical characteristics of beams and plates,is given by two equations:The first relates the bending moment,M ,to the curvature of the beam,d 2y /dx 2(y is the deflection)d 2y dx 2¼MEIð7Þwhere I is the area moment of inertia,which de-pends on the geometry of the cross section (I =p R 4/4,for circular sections,where R is the ra-dius).Importantly,the curvature of a solid beam,and therefore its deflection,is inversely propor-tional to the fourth power of the radius.The sec-ond equation,commonly referred to as Euler ’s buckling equation,calculates the compressive load at which global buckling of a column takes place (P cr )P cr ¼p 2EI ðkL Þ2ð8Þwhere k is a constant dependent on the column-end conditions (pinned,fixed,or free),and L is the length of the column.Resistance to buck-ing can also be accomplished by increasing I .Both Eqs.7and 8predict the principal designLongitudinal sectionToucan beak Keratin layers(i) Fibers(circumferential)Megafibrils and fibrilsBarbsBarbulesCortexCortical ridgesFoamRachisNodes(iii) Medulloidpith(ii) Fibers (longitudinal)Feather rachisPlant-Bird of ParadisePorcupine quillsNodesRebarClosed-cell foamTransverseLongitudinalCross sectionABCD5 mm 1 mm1 cm 0.1 mm5m 5 m m1c 1 c m1 mm100 ␮m500 ␮mFig.3.Low-density and stiff biological materials.The theme is a dense outer layer and a low-density core,which provides a high bending strength –to –weight ratio.(A )Giant bird of paradise plant stem showing the cellular core with porous walls.(B )Porcupine quill exhibiting the dense outer cortex surrounding a uniform,closed-cell foam.Taken from (42).(C )Toucan beak showing the porousinterior (bone)with a central void region [adapted from (43)].(D )Schematic view of the three major structural components of the feather rachis:(i)superficial layers of fibers,wound circumferentially around the rachis;(ii)the majority of the fibers extending parallel to the rachidial axis and through the depth of the cortex;and (iii)foam comprising gas-filled polyhedral structures.Taken from (45)SCIENCEVOL 33915FEBRUARY 2013777REVIEWo n M a y 1, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m。