当前位置：文档之家 › Study of porous glass doped with quantum dots or laser dyes under alpha irradiation

Study of porous glass doped with quantum dots or laser dyes under alpha irradiation

  • 格式：pdf
  • 大小：994.39 KB
  • 文档页数：4

Study of porous glass doped with quantum dots or laser dyes under alpha irradiationS. E. Létant and T.-F. WangCitation: Applied Physics Letters 88, 103110 (2006); doi: 10.1063/1.2182072View online: /10.1063/1.2182072View Table of Contents: /content/aip/journal/apl/88/10?ver=pdfcovPublished by the AIP PublishingArticles you may be interested inNon-degenerate two photon absorption enhancement for laser dyes by precise lock-in detectionAIP Advances 5, 127138 (2015); 10.1063/1.4939568CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imagingAppl. Phys. Lett. 98, 181914 (2011); 10.1063/1.3589366Quantum dot solar cell tolerance to alpha-particle irradiationAppl. Phys. Lett. 91, 183108 (2007); 10.1063/1.2803854Scintillators based on aromatic dye molecules doped in a sol-gel glass hostAppl. Phys. Lett. 86, 101914 (2005); 10.1063/1.1882758Water-soluble silica-overcoated CdS:Mn/ZnS semiconductor quantum dotsJ. Chem. Phys. 121, 7421 (2004); 10.1063/1.1797071Reuse of AIP Publishing content is subject to the terms at: https:///authors/rights-and-permissions. Download to IP: 202.195.129.247 On: Tue, 19 Apr2016 13:32:35Study of porous glass doped with quantum dots or laser dyes under alpha irradiationS.E.Létant and T.-F.Wang a ͒Chemistry and Materials Science Directorate,Lawrence Livermore National Laboratory,7000East Avenue,Livermore,California 94550͑Received 20June 2005;accepted 24January 2006;published online 8March 2006͒We demonstrate that nanocomposite materials based on semiconductor quantum dots have potential for radiation detection via scintillation.While quantum dots and laser dyes both emit in the visible range at room temperature,the Stokes shift of the dyes is significantly larger.The scintillation output of both systems was studied under alpha irradiation and interpreted using a combination of energy loss and photon transport Monte Carlo simulation models.The comparison of the two systems,which allows the quantification of the role played by the Stokes shift in the scintillation output,opens up exciting possibilities for a new class of scintillators that would take advantage of the limitless assembly of nanocrystals in large,transparent,and sturdy matrices.©2006American Institute of Physics .͓DOI:10.1063/1.2182072͔Although both the synthesis and characterization of quantum dots have been developed for more than a decade,1,2the applications explored to date seem to have focused on tagging,3chemical and biological sensing,4and lasing applications.5The interest generated by quantum dots in both academic and industrial research communities comes from the fact that the optical properties of these materials are di-rectly tied to their composition,size,and geometry,therefore allowing the engineering of key parameters,such as emission wavelength and quantum efficiency.Standard gamma-ray detection technology relies on cooled germanium detectors ͑0.2%energy resolution at 1.33MeV ͒6–8and on scintillating crystals such as sodium iodide ͑7%energy resolution at 662keV ͒.9The main prob-lem associated with the former is the necessity to cool and stabilize the detector at a temperature near liquid nitrogen to reduce thermal noise.The main problem associated with the latter is its poor energy resolution.The use of nanocomposite materials could potentially lead to a new class of scintillators that would operate at room temperature,and more importantly,that would not rely on crystal growth,but on the assembly of nanometer-sized crys-tals in a transparent and sturdy matrix.Moreover,scintillator materials have output wavelengths in the ultraviolet ͑UV ͒and blue ͑the most commonly used scintillator,sodium io-dide,emits at 460nm ͒,wavelengths at which the quantum efficiency of photomultiplier tubes ͑PMTs ͒is below 25%.10This means that,even in an ideal situation,only 14of the photons produced in the scintillator material are detected.The use of quantum dots as a scintillator medium would allow fine tuning of the output wavelength in the visible range and therefore,the use of avalanche photodiodes ͑APD ͒with quantum efficiencies up to 70%.11The visible band gap of quantum dots would ensure both high photon output and efficient photon counting.Only one preliminary example of a nanocrystal-based scintillator has been reported to date.12In this work,core/shell ͑CdSe ͒ZnS quantum dots were embedded into a thinlithiated sol-gel matrix,characterized optically,and tested with an alpha source.Unfortunately,no conclusion can be drawn from this publication since the only scintillation spec-trum presented was not corrected from the background radia-tion.In this letter,we demonstrate the ability of quantum dots to convert alpha radiation into visible photons,compare the scintillation output of quantum dots and laser dye mol-ecules,and interpret the results using a combination of energy-loss and photon transport models.This set of data allows us to discuss the limitations of basic quantum dots for gamma-radiation detection applications that require large volumes of scintillator material,and to suggest a new route to solve these issues.Porous VYCOR®was purchased from Advanced Glass and Ceramics ͑Holden,MA ͒in 1/16in.thick sheets.As re-ceived,the material is constituted of an array of intercon-nected pores with a diameter of 4nm and is opalescent.The porous glass matrix was slowly dissolved for 4days in an aqueous solution containing 1%of hydrofluoric acid and 20%of ethanol per volume,rinsed in ethanol,and dried in air.The purpose of this step was to slightly enhance the pore size and to obtain a clear matrix.Scanning electron micro-scope ͑SEM ͒top views of the material recorded after etch-ing,cleaning,and drying without applying any conductive coating on the sample surface ͓see Fig.1͑a ͔͒revealed an average pore diameter in the 10–20nm range.The absorp-tion curve of the same sample shows very good transparency in the visible range ͓see Fig.1͑b ͔͒.Porous glass constitutes a test matrix of choice for scintillation applications because it is made of a succession of nanometer-sized cavities that can hold guest molecules while separating them from each other,therefore preventing self-quenching effects.In addition,it is sturdy,inert,and transparent.CdSe/ZnS core-shell quantum dots with a luminescence output at 540nm were purchased from Evident Technologies ͑Troy,NY ͒and suspended in toluene at a concentration of 10mg/mL.Rhodamine B laser dye was purchased from Sigma-Aldrich ͑Milwaukee,WI ͒and suspended in methanol at a concentration of 0.5mM.The dry “thirsty”porous glass samples were immersed in the solutions of dots or dyes for 48h with continuous stirring in order to allow homogeneousa ͒Author to whom correspondence should be addressed;electronic mail:wang6@APPLIED PHYSICS LETTERS 88,103110͑2006͒0003-6951/2006/88͑10͒/103110/3/$23.00©2006American Institute of Physics88,103110-1 Reuse of AIP Publishing content is subject to the terms at: https:///authors/rights-and-permissions. Download to IP: 202.195.129.247 On: Tue, 19 Apr2016 13:32:35diffusion of the guest molecules into the nanoporous hostmatrix.They were then let to dry in order to evaporate the solvent.Absorbance spectra of the samples were recorded using a UV-visible spectrometer ͑Cary 100,Varian Inc.͒and emis-sion spectra were recorded using a fluorimeter fitted with an optic fiber ͑Cary Eclipse,Varian Inc.͒.The spectra,as well as pictures of the devices,are presented in Fig.2.Although both quantum dots and laser dyes present similarities,such as their tunable absorption and emission spectra in the visiblerange,one noticeable difference is that laser dye molecules have an inherent Stokes shift ͑shift between absorption and emission spectra ͒due to vibrational energy levels,while quantum dots do not.An alpha source ͑243–244Cm,0.2␮C ͒was placed on one side of the 1/16in.thick nanocomposite sample and a PMT ͑model R1924A from Hamamatsu ͒probing a 1.5cm diam-eter area rested directly on the other side to measure the scintillation output.The PMT output was amplified,digitized by the multichannel analyzer and collected for 10h in order to obtain a histogram.Figure 3shows the scintillation histo-gram recorded with the 243–244Cm source,through the dot and dye samples.The histograms are plotted in counts per channel,where each channel represents the scintillation en-ergy digitized by the multichannel analyzer.The data pre-sented in Fig.3shows that the quantum dot system does convert the alpha-radiation into visible photons,but that the scintillation output of the laser dye system is significantly higher,showing partial energy resolution of the 243–244Cm source.In order to specifically understand the role played by the Stokes shift on the energy resolution for both dye and dot systems,as well as to account for the wavelength mismatch between the emission of the sample and the optimal detec-tion of the PMT,we performed simulations using a combi-nation of TRIM and DETECT 2000programs in history mode.13,14Photons generated at the source/sample interface traveled to the PMT surface which was located on the oppo-site side of the sample.The history was analyzed and the energy deposition and light transport were simulated while accounting for properties of the scintillator media such as the wavelength-dependent absorption and emission coefficients of the emitters,the nature of the matrix ͑reflectivity,indexof FIG.1.͑Color ͒Characterization of the etched,cleaned,and dried porous glass matrix:͑a ͒SEM top views,without any conductive coating.͑b ͒Ab-sorbance spectrum of a 1/16in.thickslab.FIG.2.͑Color ͒Absorbance and emission spectra of ͑a ͒porous glass im-pregnated with quantum dots,and ͑b ͒porous glass impregnated with Rhodamine B laser dye.Pictures of the devices are presented in theinset.FIG.3.͑Color ͒Background-subtracted PMT output ͑energy response ͒upon exposure to a 243–244Cm source located 1mm away from the surface,for ͑a ͒porous glass impregnated with quantum dots,and ͑b ͒porous glass impreg-nated with Rhodamine B dye.The peak recorded for the dye sample was fitted with a Gaussian curve ͑black line ͒.The spectrum of the alpha source recorded by a Si ͑SB ͒detector in the ideal case where both source and detector are in a vacuum is presented in the inset for reference.Reuse of AIP Publishing content is subject to the terms at: https:///authors/rights-and-permissions. Download to IP: 202.195.129.247 On: Tue, 19 Apr 2016 13:32:35refraction,scattering length,and absorption length͒,and thescintillation decay times.The wavelength-dependent quantum efficiency of thePMT was entered into the simulation code.We found that inour unoptimized experimental conditions,4.1%of the pho-tons generated in the dye system were amplified by the PMT,and that0.4%were amplified for the dot system.Replacingthe PMT,which only has a quantum efficiency of4%in thegreen,by an APD,which has a maximum quantum efficiencyof70%,would lead to an improvement of these values by anorder of magnitude͑38%of the photons amplified for thedye system and3.6%for the dot system͒.Many other factors contribute to the observed energyresolution,15such as the intrinsic properties of the scintillatorand the transport efficiency of the photons.For an idealscintillator,the energy resolution R,is given by R=͑⌬E/E͒FWHM=2.35͓͑1+v͑M͒͒/͑Np͔͒1/2,where N is the average number of photons generated at a given energy E, v͑M͒is the variance in the multiplication factor of the PMT, and p is the average transport efficiency.15For a typical10stage PMT͑Ref.16͒with a gain of2.106,v͑M͒is approxi-mately0.08.For the dye system,wefitted the experimentalpeak with a Gaussian͓see Fig.3͑b͒,inset͔and obtained anenergy resolution R of0.64.The p value calculated with DETECT2000being0.041,an estimation of the number of photons N generated at5MeV in the dye nanocomposite is400.For the dot nanocomposite,no⌬E/E can be determinedexperimentally since no peak was detected.This result is inagreement with the simulations,which indicate that if anequivalent photon number N is inputted in the formula tocalculate R,no energy resolution is expected.The compari-son of the dot and dye systems constitutes an important steptoward the understanding of the role played by the spectralproperties of the dye and dot emitters in the scintillationoutput of these nanocomposite systems.After inserting thewavelength-dependent PMT efficiency,and the lifetimes,theonly major difference left between the two systems is theStokes shift,which is significantly smaller for the dots.Wetherefore attribute the output difference of these two systemsto the Stokes shift,which governs the photon transport effi-ciency in the material.Although the number of photons generated in the presentnanocomposite systems is low,this number can change dra-matically for thicker samples studied with penetrablegamma-rays.Moreover,the present systems have neitherbeen optimized for loss minimization͑reflective coating onall surfaces͒nor for detection efficiency͑APD instead ofPMT͒.Ultimately,simulations show that an energy resolutionof2%could be achieved for quantum dot nanocompositematerials due to their visible band gap assuming that the system is linear.This approach would provide gamma-ray detectors with an energy resolution between the cooled semi-conductor detectors and the inorganic scintillator crystals, without limitation on the volume of the detector.For future applications,increased dot density and optimized surface treatments should improve the light output by at least a fac-tor of20,and efficient photon collection using APDs should increase the number of photons counted by a factor of10. However,in order to accommodate the increased dot density, the Stokes shift will have to be increased to limit reabsorp-tion losses.Our next step is therefore to engineer the Stokes shift of semiconductor quantum dots.We will do this by anchoring chemical groups at the surface of the quantum dots,17or by using different semiconductor materials such as PbS.This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No.W-7405-ENG-48.It was also supported by a Laboratory Directed Research and Development grant͑Grant No.04-ERD-107͒. 1C.B.Murray,D.J.Norris,and M.G.Bawendi,J.Am.Chem.Soc.115, 8706͑1993͒.2X.Peng,M.C.Schlamp,A.V.Kadavanich,and A.P.Alivisatos,J.Am. Chem.Soc.119,7019͑1997͒.3S.Kim,Y.T.Lim,E.G.Soltesz,A.M.De Grand,J.Lee,A.Nakayama, J.A.Parker,T.Mihaljevic,urence,D.M.Dor,L.H.Cohn,M.G. Bawendi,and J.V.Frangioni,Nat.Biotechnol.22,93͑2004͒.4I.L.Medintz,A.R.Clapp,H.Mattoussi,E.R.Goldman,B.Fisher,and J. M.Mauro,Nat.Mater.2,630͑2003͒.5Y.Chan,J.S.Steckel,P.T.Snee,J.-M.Caruge,J.M.Hodgkiss,D.G. Nocera,and M.G.Bawendi,Appl.Phys.Lett.86,73102͑2005͒.6D.E.Persyk,M.A.Schardt,T.E.Moi,K.A.Ritter,and G.Muehllehner, IEEE Trans.Nucl.Sci.27,168͑1980͒.7L. A.Andryushchenko, B.V.Grinev, A.M.Litichevskii,and L.V. Udovichenko,Instrum.Exp.Tech.40,59͑1997͒.8M.Moszynski,M.Balcerzyk,W.Czarnacki,M.Kapusta,W.Klamra,P. Schotanus,A.Syntfeld,and M.Szawlowski,IEEE Trans.Nucl.Sci.50, 767͑2003͒.9W.H.Berninge,IEEE Trans.Nucl.Sci.21,374͑1974͒.10M.Itaya,T.Inagaki,T.Iwata,G.I.Lim,H.Oishi,H.Okuno,Y.Tajima, H.Y.Yoshida,and Y.Yoshimura,Nucl.Instrum.Methods Phys.Res.A 522,477͑2004͒.11M.Akiba,M.Fujiwara,and M.Sasaki,Opt.Lett.30,123͑2005͒.12S.Dai,S.Saengkerdsub,H.-J.Im,A.C.Stephan,and S.M.Mahurin,AIP Conf.Proc.632,220͑2002͒.13J.F.Ziegler,J.P.Biersack,and U.Littmark,The Stopping and Range of Ions in Solids͑Pergamon,New York,1985͒.14G.F.Knoll,T.F.Knoll,and T.M.Henderson,IEEE Trans.Nucl.Sci.35, 872͑1988͒.15J.B.Birks,The Theory and Practice of Scintillation Counting͑Pergamon, New York,1967͒.16P.Dorenbos,J.T.M.de Haas,and C.W.E.van Eijk,IEEE Trans.Nucl. Sci.42,2190͑1995͒.17A.Puzder,A.J.Williamson,J.C.Grossman,and G.Galli,J.Am.Chem. Soc.125,2786͑2003͒.Reuse of AIP Publishing content is subject to the terms at: https:///authors/rights-and-permissions. Download to IP: 202.195.129.247 On: Tue, 19 Apr2016 13:32:35。

合集下载

Preparation of sintered foam glass with high fly ash content

Preparation of sintered foam glass with high fly ash content

Preparation of sintered foam glass with high fly ash contentBo Chen,Zhiwei Luo,Anxian Lu ⁎School of Materials Science and Engineering,Central South University,Changsha 410083,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 30May 2011Accepted 12July 2011Available online xxxx Keywords:Porous materials MicrostructureMechanical strength SinteringFoam glasses with 50wt.%,60wt.%and 70wt.%fly ash are prepared using fly ash and flux agent sodium borate with sodium silicate as foaming agent at the temperature of 800°C.Heat treatment at 800°C for 1h of pellets having different fly ash mass percentages results in well-sintered bodies of dark gray color featuring a vitreous aspect with smooth surface.The effect of amount of fly ash on the density,porosity,mechanical strength and microstructure is evaluated.The experimental results show that the foam glass with 70wt.%fly ash exhibits the excellent comprehensive properties and the suitable sintered temperature is 800°C.©2011Elsevier B.V.All rights reserved.1.IntroductionThe ef ficient utilization of fly ash which is produced as a by-product of the coal combustion in power stations has been a worldwide issue.The production of foam glass with fly ash is a useful way of recycling large mounts of fly ash.Foam glass is a heterophase system consisting of the solid and gaseous phases.The solid phase is glass that forms thin walls of single cells several micrometers thick and the cells are filled with the gaseous phase.Heat-insulating and soundproof foam glass can have predominantly closed or intercommunicating pores,respectively.Because of a complex of properties including low thermal conductivity,low density,incombustibility,etc.foam glass has attracted great interest and been applied in many areas such as building,chemistry and defense fields.The way of utilization of fly ash in foam glass at present is to prepare foam glass from fly ash and other components including kinds of glass.Fernandes [1]has prepared porous foam glass using the fly ash and sheet glass as the main raw materials.Mangutova has produced glass –ceramics foams by utilization of fly ash and waste glass [2].In this paper,foam glass is successfully prepared only from fly ash as the main raw material.However,only a minor part of fly ash is utilized in the preparation of foam glass and in the conventionally prepared foam glass the most of fly ash mass percentage is only 40%[3],which could not recycle large mounts of ash.In this paper,the foam glass with 50,60and 70wt.%fly ash by introducing flux agent and properties of prepared foam glass are reported.The relation between fly ash mass percentage and property of foam glass is studied.2.Experimental2.1.Materials and processingIn this paper,sodium silicate (Na 2O·2.3SiO 2)and Na 2HPO 4were chosen as foaming agent and foaming stabilizer,respectively.Na 2B 4O 7was added as the flux agent to lower the softening temperature point of compounds and make the composition be close to formation range of glass.Fly ash was sieved through 300sieve mesh.Table 1shows composition of three compounds.The compounds were combined and prepared by uniaxial dry-pressing into disk shapes with a diameter of 20mm,using a pressure of 15MPa.Then the mixtures would be used in further sintering process.Sintering was realized in air atmosphere at 800°C for 1h in an electrical furnace,using heating rate of 3.5°C/min.One of the key points of preparing foam glass is the accordance of sintering temperature with softening temperature of compounds [4]and the softening temperatures of compounds in the experiment get close to 800°C.Too high sintering temperature and too rapid and slow heating rate would affect the formation of foam glass.2.2.Microstructural characterizationThe crystalline phases precipitated in the prepared samples are investigated by Rigaku D/max 2550PC X-ray.(CuK α,scanning rate 8°/min,Materials Letters 65(2011)3555–3558⁎Corresponding author.Tel.:+8673188830351;fax:+8673188877057.E-mail addresses:chencsu2009@ (B.Chen),axlu@ (A.Lu).Table 1Compositions of the samples.Sample Fly ash (wt.%)Na 2B 4O 7(wt.%)Na 2O·2.3SiO 2(wt.%)Na 2HPO 4(wt.%)A15033.316.75A26026.713.35A370201050167-577X/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.matlet.2011.07.042Contents lists available at ScienceDirectMaterials Lettersj o u r na l ho m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /m a t l e tscanning range 5°–80°).The pore surface and pore morphologies of samples are investigated by scanning electronic microscopy using a Quanta200.2.3.Density and porosityThe bulk density is measured with helium pycnometry.The total porosity is calculated from the bulk density and powder density using the following equation [5]:%Porosity =1−bulkdensity =powderdensity ðÞ×1002.4.Mechanical strengthThe specimens used in bending strength test are machined into 20mm ×6mm ×7mm test bars.Square foam glass samples of 12.5mm length and 5mm×5mm section area are subjected to uniaxial compressive loading.The bending strength and compressive strength are measured for specimen bars with a span of 10mm at a cross-head speed of 1mm/min,using a CSS44100testing machine.Each result is the average of five measurements.3.Results and discussion 3.1.MicrostructureFig.1represents the XRD patterns of the three samples.As shown in Fig.1,the main crystalline phase precipitated in all sintered samples is Na 2Al 2Si 5O 14.SiO 2and Al 2O 3which are the main constituents of fly ash still exist during the sintering process and form Na 2Al 2Si 5O 14with Na 2O introduced from sodium silicate and Na 2B 4O 7.The small number of micro-crystals has densi fied the porous structure [6]and improved the mechanical performances.It results in mechanically stronger glass foams.As can be convinced from Fig.1samples are mostly consisted of glassy phase which is generated during the sintering process.It is proposed that all B 2O 3which could not be detected from XRD patterns have become glass network former and formed the glassy phase.It is easy for B \O bonds with much lower binding energy to destroy the network structure of mullite as the main crystalline phase of fly ash.Fig.2is the SEM micro-structure graphs of three samples.There exist a lot of pores with different diameters in all three samples.The decomposition of sodium silicate with water leads to great volume expansion.After bodies begin to soften under the expansion pressure the foaming process occurs which brings large mounts of pores in samples.The softening temperatures of samples increase by the order of A1,A2and A3.Obviously the soften temperatures of samples A1and A2are lower than 800°C,so at 800°C the viscous flow sintering occurs as a result of the generation of a large amount of liquid phase with lower high-temperature viscosity [7],which enhances the densi fication of samples [8].Particles of samples are bonded together leading to formation of cell walls as can be seen from Fig.2(a)and (b)and small pores disappear or develop to large pores gradually with sintering sustaining.The sintering degree of A1is higher than A2because of lower softening temperature and the amount of existing pores is much less.For sample A3,pores are uniformly distributed as can be seen from Fig.2(c).The softening temperature is closer to 800°C than A1and A2and litter sintering process has occurred,so there preserve most of pores.Besides the largest amount of fly ash helps pores distributed uniformly.3.2.Density and porosityTable 2illustrates the densities and porosities of samples.The density decreases and porosity increases by the order of A1and A2.The result of porosity is in good agreement with that of density.The existence of B 2O 3in compounds is bene ficial to the formation of melting liquid.At 800°C the low-viscosity liquid flows to fill the pores and consequently helps the process of densi fication [9].With B 2O 3percentage increasing the sintering degrees increase and sintering process leads to densi fication.So sample A1with the least fly ash percentage appears to have the most densi fication which makes the density be the highest of three samples.Porosity directly related to the amount of pores decreases by the order of A3,A2and A1.Some pores in sample A1and A2gradually disappear during the sintering process but for sample A3there preserve most of pores.3.3.Mechanical strengthAs can be seen from Table 2,fly ash mass percentage affects the mechanical strength of samples obviously.Both flexural strength and compressive strength decrease with fly ash percentage at first and increase later.The flexural strengths of the three samples are 6.58MPa,1.42MPa and 4.59MPa,respectively.And the correspond-ing compressive strengths vary from 10.51MPa, 2.62MPa to 3.44MPa which are superior to glass –ceramics foams from waste glass and fly ash [10].The type of porous structure and cell walls play an important role in the resulting mechanical strengths [11].The mechanical strength of sample A1with the least pores is much higher than another two samples attributed to dense structure.For sample A2it has the lowest mechanical strength due to large pores in the sample so that it is easy to crack where the large pores are distributed.And sample A3with uniform structure exhibits adequate strength.4.ConclusionIn this study,foam glasses with 50,60,70wt.%fly ash are successfully prepared at 800°C.The foam glass with 70wt.%fly ash which has the characters of low density (0.542g/cm 3),high porosity (68.85%)and adequate mechanical strength(flexible strength 4.59MPa and compressive strength 3.44MPa,respectively)could allow high incorporation percentage of fly ash.The mechanical strengths of foam glass sintered at 800°C decrease at first and later increase with fly ash percentage.The sintering temperature of 800°CFig.1.XRD patterns of the samples:sample A1(a),A2(b)and A3(c).3556 B.Chen et al./Materials Letters 65(2011)3555–3558is suitable for foam glass with 70wt.%fly ash but not for glass with lower fly ash percentages.References[1]Fernandes HR,Tulyaganov DU,Ferreira JMF.Ceram Int 2009;35:229–35.[2]Mangutova B,Angju ševa B,Milo ševski D,Fidan čevska E,Bossert J,Milo ševski M.Bull Chem Technol Macedonia 2004;32(2):157–62.[3]Feng XP.Coal Ash China 2004;16(3):24–6.Table 2Densities and porosities of the sintered samples.SampleA1A2A3Bulk density (g/cm 3)0.9170.7260.542Powder density (g/cm 3) 2.05 1.99 1.74Porosity (%)55.2763.5268.85Flexural strength (MPa)6.58 1.42 4.59Compressive strength (MPa)10.512.623.44Fig.2.SEM micrographs of sintered foam glasses with different fly ash percentages:(a).50wt.%Fly ash;(b).60wt.%Fly ash;and (c).70wt.%Fly ash.3557B.Chen et al./Materials Letters 65(2011)3555–3558[4]Fang RL,Liu M,Zhou YL.New Build Mater2003;6:38–41.[5]Francois M,Pascal Y,Romain V,Michel R.Ceram Int2007;33:543–50.[6]Guo HW,Gong YX,Gao SY.Mater Lett2010;64:997–9.[7]Mackenzie J,Shuttleworth R.Proc Phys Soc1949;62:633–52.[8]He F,Wang J,Deng DW,Cheng JS.J Chinese Ceram Soc2009;37(10):1791–5.[9]Guo A,Liu J,Xu R,Xu H,Wang C.Fuel2010;89:3630–6.[10]Wu JP,Boccaccini AR,Lee PD,Kershaw MJ,Rawlings RD.Adv Appl Ceram2006;105:32–9.[11]Bernardo E,Albertini F.Ceram Int2006;32:603–8.3558 B.Chen et al./Materials Letters65(2011)3555–3558。

The study of the properties of metal complexes

The study of the properties of metal complexes

The study of the properties of metalcomplexesMetal complexes are compounds formed by metal ions and ligands. They have unique properties that make them important in various fields like medicine, material science, nanotechnology, and environmental studies. Understanding the properties of metal complexes is crucial for designing new compounds with specific functions. In this article, we will discuss the important properties of metal complexes and their applications.Ligand Exchange ReactionsLigand exchange reactions are the most important properties of metal complexes. In these reactions, a ligand replaces another ligand from the metal ion, which results in the formation of a new complex. The rate of the ligand exchange reaction depends on the steric and electronic factors. Steric factors such as the size of the ligand and the geometry of the metal complex affect the rate of the reaction. Electronic factors such as the charge and the electronegativity of the ligand also play a crucial role.Applications: Ligand exchange reactions are important in catalysis and bioinorganic chemistry. Many catalysts use metal complexes because of their ability to undergo ligand exchange reactions. In bioinorganic chemistry, metal complexes play a crucial role in the transport and storage of metals in the body.Redox PropertiesRedox properties refer to the ability of a metal complex to undergo oxidation-reduction reactions. In these reactions, the metal ion changes its oxidation state, resulting in the formation of a new complex. The redox potential of a metal complex depends on the ligands and the metal ion. The presence of strong field ligands like cyanide and carbon monoxide increases the redox potential of the metal complex.Applications: Redox properties of metal complexes are important in electrochemistry and catalysis. In electrochemistry, metal complexes are used as mediators in the redoxreactions. In catalysis, many reactions are driven by the redox properties of metal complexes.Optical PropertiesOptical properties refer to the ability of a metal complex to exhibit color and luminescence. The color of a metal complex depends on the nature of the ligands and the metal ion. The presence of d-orbitals in the metal ion gives rise to the color of the complex. The luminescence of a metal complex depends on the energy gap between the ground and excited states.Applications: Optical properties of metal complexes are important in materials science, biology, and medicine. In materials science, metal complexes are used as dyes, pigments, and sensors. In biology, metal complexes are used as probes to study biological processes. In medicine, metal complexes are used as imaging agents and anticancer drugs.Structural PropertiesStructural properties refer to the geometry and bonding of a metal complex. The geometry of a metal complex is determined by the coordination number, the ligands, and the metal ion. The bonding in a metal complex can be classified as covalent, ionic, and dative.Applications: Structural properties of metal complexes are important in catalysis, material science, and environmental studies. In catalysis, the geometry and bonding of metal complexes determine their catalytic activity. In material science, the structure of metal complexes determines their thermal stability and mechanical properties. In environmental studies, the structure of metal complexes plays a crucial role in their toxicology and biodegradation.In conclusion, the study of the properties of metal complexes is crucial for their understanding and applications. The properties of metal complexes like ligand exchange reactions, redox properties, optical properties, and structural properties make them important in various fields like medicine, material science, nanotechnology, andenvironmental studies. Further research in this field will lead to the development of new compounds with specific functions.。

gas inject

gas inject

Transp Porous Med(2008)72:351–367DOI10.1007/s11242-007-9154-zMicroscopic Mechanisms of Oil RecoveryBy Near-Miscible Gas InjectionMehran Sohrabi·Ali Danesh·Dabir H.Tehrani·Mahmoud JamiolahmadyReceived:3October2006/Accepted:1July2007/Published online:20July2007©Springer Science+Business Media B.V.2007Abstract As gasflooding becomes a more viable means of enhanced oil recovery,it is important to identify and understand the pore-scaleflow mechanisms,both for the develop-ment of improved gasflooding applications and for the predicting phase mobilisation under secondary and tertiary gasflooding.The purpose of this study was to visually investigate the pore-level mechanisms of oil recovery by near-miscible secondary and tertiary gasfloods. High-pressure glass micromodels and modelfluids representing a near-misciblefluid system were used for theflow experiments.A new pore-scale recovery mechanism was identified which significantly contributed to oil recovery through enhancedflow and cross-flow between the bypassed pores and the injected gas.This mechanism is strongly related to a very low gas/oil interfacial tension(IFT),perfect wetting conditions and simultaneousflow of gas and oil in the same pore,all of which occur as the gas/oil critical point is approached.The results of this study helps us to better understand the pore-scale mechanisms of oil recovery in very low-IFT(near-miscible)systems.In particular we show that in near-miscible gas floods,behind the main gas front,the recovery of the oil continues by cross-flow from the bypassed pores into the mainflow stream and as a result almost all of the oil,which has been contacted by the gas,could be recovered.Our observations in high-pressure micro-model experiments have demonstrated that this mechanism can only occur in near-miscible processes(as opposed to immiscible and completely miscible processes),which makes oil displacement by near-miscible gasfloods a very effective process.Keywords Gas injection·Near-miscible·Micromodel·Pore-scale mechanisms·Three-phaseflowM.Sohrabi(B)·A.Danesh·D.H.Tehrani·M.JamiolahmadyInstitute of Petroleum Engineering,Heriot-watt University,Riccarton,Edinburgh EH144AS,UKe-mail:mehran.sohrabi@352M.Sohrabi et al. 1IntroductionInjection of enriched natural gas is proven to increase crude oil recovery from partially or completely water-flooded reservoirs(McGuire and Moritz1992).This often involves blend-ing liquefied petroleum gas(LPG)or natural gas liquid(NGL)with dry-gas(mainly methane) to obtain the desired injectant composition.Conventionally,this process is operated so as to maintain the pressures above the minimum miscibility pressure(MMP),which is determined for each crude and gas combination,to achieve a miscibleflood process.Near-miscible gasflood refers to injection of gases that do not quite develop complete miscibility with the oil,but come close.For instance,condensing–vaporising gas drives at enrichment slightly below minimum miscibility enrichment or at pressures slightly below MMP are near miscible processes.Near-miscible gas drives appear attractive from both eco-nomic and operational standpoints.A leaner injectant(lower LPG or NGL enrichment in dry gas)is less expensive than a richer injectant.A lower-pressure process also reduces cost because the injectant density is lower and costs of compression are reduced at near-misci-bility.Some reservoirs are at pressures lower than MMP,while other miscible-flood projects are experiencing pressure decline.In these cases,a near-miscible operation may be more feasible than a miscible one.2D simulation studies(Burger et al.1994;Thomas et al.1994)have demonstrated that injectant with less enrichment than that required forfirst-contact miscible with the oil often yields optimum oil recoveries owing to better sweep efficiencies in heterogeneous reser-voir models.Field-scale simulations show that near-miscible solvents are attractive because of improved sweep over miscible solvents(Pande1992).Burger et al.(1994)showed that economically optimum enrichment in high-viscosity-ratio secondary gasfloods could be operated at pressures below the MMP.Thomas et al.(1994)reported that gas–oil IFT can be reduced adequately under near-miscible conditions and that zero IFT is unnecessary for effective displacement processes in realistic porous media.Shyeh-Yung(1991)demonstrated that tertiary gasflood recoveries below MMP do not decrease as severely as predicted by slim-tube tests for CO2.Bypassing of large quantities of oil can occur during gas injection.The degree to which bypassing occurs in a reservoir depends on many factors including heterogeneity,wettability of the medium and enrichment of the injectionfluids.Subsequent mass transfer(cross-flow) from the bypassed regions can recover a significant fraction of this oil.Four mechanisms have been found to contribute to this mass transfer:dispersion/diffusion-drive(Hara and Christman 1990),gravity-drive(Firoozabadi and Markeset1994),pressure-drive(Pande and Orr1990) and capillarity-drive(Lee and Fayers1992).Burger et al.(1996)and Burger and Mohanty (1997)have found that mass transfer increases with solvent enrichment and that horizontal mass transfer provides the most efficient oil recovery as a result of gravity-driven cross-flow.The inverted,or positive gravity orientation,exhibits counter current gravity-driven cross-flow that inhibits mass transfer somewhat.The vertical,or negative gravity orientation, yielded the lowest recovery,as diffusion was the only significant mass-transfer mechanism for their particularfluid system.Wylie and Mohanty(1997)have investigated the effect of water saturation on bypassing and mass transfer,concluding that mass transfer,between oil and gas,is decreased in the presence of water,but the capillary forces become more dominant as enrichment decreases.Less bypassing,due to gravity override,was observed in horizontal gasflood experiments in the presence of water;however it was conjectured that bypassing was still present as a result offluid redistribution and water shielding.Dumore and Schols(1974)showed that gravity drainage could lead to high oil recoveries during gas injection in water-wet rock.Subsequent studies on sandstone cores,bead packsMicroscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection353 and sand columns confirmed these results(Chatzis et al.1988;Kantzas et al.1988a,b;Vizika 1993).The high oil recovery and the resultant low oil saturation in gravity drainage were attributed to drainage through thin layers of oil that lie between the water and the gas in the pore space.It was hypothesised by Oren(1992),Vizika(1993)and Kalaydjian(1992) that systems with a positive spreading coefficient,which means that the oil spontaneously spread over aflat water/gas interface,would experiencefilm drainage and high oil recoveries whereas non-spreading systems would see lower oil recoveries.Blunt et al.(1994)showed that although most oils will spread over water,they would eventually form afilm that is only a few molecules thick.Both spreading and non-spreading oils can formfilms and thefilm thickness is determined by the capillary pressure and by whether or not thefilm was formed from thinning a thicker layer or by encroaching over a surface.Drainage through thisfilm would be far too slow to account for the recoveries seen experimentally.They then presented results from drainage experiments in capillary tubes with a square cross-section.The tube had water-wetting the corners and contained oil ganglia separated by air bubbles along its length. The gravitational driving force in the experiments allowed oil ganglia to drain by swelling the oilfilm and eventually all the oil was recovered.Soroush and Saidi(1999)reported results of a series of vertical gas–oil displacements in a low permeability oil-wet core at high-pressures below MMP.They reported oil recoveries as high as75%,which was attributed partly due to the thinning of the adsorbedfilms and thus the shrinkage of pendular rings as well as the possibility that the displacing phase starts wetting the rock surface.The limited experimental evidence available in the literature of near-miscible gas injec-tion is based on corefloods.It should be realised that macroscopic observations made at the core scale invariably reflect the collective outcome of the displacement mechanisms over an ensemble of pores.Pore-scale information is therefore essential for describing the displace-ment processes at a larger scale.Etched-glass micromodels are useful for viewing pore level events because of their visual clarity.Micromodels were used as early as1960forfluid displacement studies(Mattax and Kyte1961).The ability to see the movement offluid interfaces makes it possible to distin-guish between a variety of mechanisms that may take place in a porous medium when more than one phase is present.Chatzis and Dullien(1983)presented an excellent example of the use of the micromodel to evaluate existing theories of two-phaseflow in a simple geometry. Lenormand and Zarcone(1984)have taken advantage of the well-defined shapes of capillary tubes in their moulded resin models to compare the results of calculations of two-phaseflow in both drainage and imbibition processes with observation results.Micromodel observations have played a significant role in development of network models for application to multi-phaseflow(Lenormand et al.1983;Oren and Pinczewski1991;Oren et al.1992;Bakke and Oren1997;Oren et al.1998;Sohrabi et al.2000;Oren and Bakke2002;Sohrabi et al.2004).Pore-scale mechanisms of displacements during high-IFT(immiscible)gas injections have been extensively studied(Kantzas et al.1988a,b;Chatzis et al.1988;Oren et al.1992;Dong et al.1995;Sohrabi et al.2000,2004).However,knowledge of the pore-scale mechanisms during low-IFT(near-miscible)gas injections is considerably less-developed than for immis-cible gas injection.This is partly because performing visual observation of the pore-scale events during gas injection under high-pressure conditions(low-IFT)is by far more compli-cated and requires a more advanced technology than under low-pressure(high-IFT).To our knowledge we are thefirst to directly visualise the pore-scale events during near-miscible gas injection in a high-pressure transparent porous medium.In this study,multiphaseflow visualisations at high-pressure conditions were performed to explore pore-scale events during near-miscible oil displacement with hydrocarbon gases. The main purpose of the study was to gain a better understanding of the process and shed354M.Sohrabi et al. light on some of the conclusions reached by other researchers(mainly drawn from corefloods and simulations),which are sometimes contradictory.The experiments were conducted in a high-pressure water-wet glass micromodel under negligible gravity effects.The results show that behind the main gas front,the recovery of the oil continues,by cross-flow from the bypassed pores into the main gasflow stream,and almost all of the oil that has been contacted by the gas will be produced.This is due to a mechanism that can only occur in near-miscible process as opposed to immiscible or completely miscible processes.2Experimental FacilitiesFor the displacement experiments a high-pressure glass micromodel rig has been used.The rig has been designed to operate at pressures of up to41.37Mpa(6000psia)with high qual-ity images offluids distributions and pore-scale mechanisms operating during displacement experiments being recorded.The experiments reported here were carried out at35.16Mpa (5100psia)pressure and37.8◦C(100◦F)temperature and thefluid system represented a near-misciblefluid system.2.1MicromodelA2D pore structure is etched onto the surface of a glass plate,which is otherwise completely flat.A second,opticallyflat,glass plate is then placed over thefirst,covering the etched pattern and thus creating an enclosed pore space.This second plate,the cover plate,has an inlet hole and an outlet hole drilled at either end,allowingfluids to be displaced through the network of pores.Since the structure is only one pore deep,and the containing solid walls are all glass,it is possible to observe thefluids as theyflow along the pore channels and interact with each other.It is also possible to observe how the geometry of the pore network affects the patterns offlow and trapping of thefluids.Figure1(a)shows a magnified picture of the pore pattern of the glass micromodel used for this study.This pattern has been repeated a few times to make the full length of the micromodel,which is shown in Fig.1(b).The porous pattern has a width of6mm and a length of38mm.The pore pattern was etched on the glass surface using acid etching technique.The depth of the pores was between 35and45µm with their width being between35and300µm.Details of the experimental set up have been described by the authors elsewhere(Sohrabi et al.2000,2004).2.2Fluid SystemFor the experiments reported here a three-component,three-phasefluid system that exhib-ited a near-miscible(near-critical point)gas/oil(equilibrium mixture of C1/n-C10)system at37.8◦C(100◦F)and35.16Mpa(5100psia)was used.The aqueous phase was prepared by adding0.1%(wt/wt)blue dye to distilled water.The hydrocarbon components were n−decane(n-C10)and methane(C1).To avoid mass transfer during the experiments,theflu-ids(C1,n-C10and water)were pre-equilibrated at the conditions of the experiments,(100◦F and5100psia).At37.8◦C(100◦F),the presence of water does not significantly affect the hydrocarbon phase behaviour.Figures2and3show the weight fraction(solubility)data and density values of n-decane(n-C10)and methane(C1)mixture at37.8◦C(100◦F)respectively (Reamer et al.1942).As it can be seen,at37.8◦C(100◦F),the critical pressure of the C1-n C10 system is around36.54Mpa(5300psia)so at35.16Mpa(5100psia),which is the pressure of the experiments,the gas–oil system is close to its critical point and hence,nearly miscible.Microscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection355Fig.2Weight fraction of(C1/n-C10)mixture at37.8◦C(100◦F)and different pressures.Shown on the graph is also thecritical point pressure(5300psia)of C1/n-C10mixture at37.8◦C(100◦F)and the pressure of ournear-miscible gas injection(5100psia)The measured equilibrium values of interfacial tensions(IFT)of thefluid system and also the estimated viscosity of the hydrocarbon vapour(gas)and that of hydrocarbon liquid(oil), at the prevailing conditions of the experiments,are given in Table1.It should be pointed out that although pressure and interfacial tension(IFT)are related, however,the distinction between immiscibility,miscibility and near-miscibility should be356M.Sohrabi et al. Fig.3Density of methane inmethane/n-decane(C1/n-C10)mixture at37.8◦C(100◦F)anddifferent pressuresTable1The equilibrium properties of the n-decane, methane system at37.8◦C (100◦F)and35.16Mpa(5100 psia)Viscosity(mPa s)Gas0.0378Oil0.1085 Water0.67 Interfacial Tension(m Nm−1)Gas/oil0.08±0.01 Gas/water41±2Oil/water41±2made on the basis of the value of the IFT between the oil and gas.Previous works includ-ing our own experimental and theoretical work(e.g.,Bardon and Longeron1980;Asar and Andy1988;Henderson et al.1996,2001)have shown that theflow behaviour at near-miscible conditions is fundamentally different from conventional(high-IFT immiscible) gas oilflow and for the oil and gas to be considered as near-miscible the gas–oil IFT should be under1m Nm−1with the effects being more pronounced with further reduction of IFT.However,at complete miscibility(IFT equal to zero),two-phaseflow can no longer exist and henceflow mechanisms would be different compared to near-miscible systems. Jamiolahmady et al.(2000,2003)studied theflow of low IFT gas–oil systems mechanis-tically.Capturing the competition of viscous and capillary forces at the pore level,they captured the simultaneousflow of both gas and oil within the same pore.They have dem-onstrated that as IFT approaches zero the coupledflow of the two phases inside each single pore diminishes but the effect of multiple pore interaction promotes thisflow behaviour. However,when IFT is above a certain value theflow behaviour changes to that of conven-tional(high-IFT immiscible)gas–oilflow described by Darcy typeflow mechanism used in the channelflow concept.In another publication,Jamiolahmady et al.(2006)have correlated relative permeability data measured at different IFT values and have identified the gas–oil IFT of around0.85m Nm−1as the threshold under which the conventional Darcy typeflow do not prevail.Microscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection357 3Experimental ResultsUsing the abovefluid system and micromodel,a number of secondary and tertiary gasfloodswere carried out.While water was present in tertiary gasfloods,it was eliminated from sec-ondaryfloods.All of the displacements were performed at35.16Mpa(5100psia)and37.8◦C(100◦F)with the micromodel mounted horizontally to minimise the gravity effects.In secondaryfloods,first the micromodel was fully saturated with oil(equilibrated n-C10),which was then displaced by near-miscible gas(equilibrated C1)injection.In tertiaryfloods,the micromodel was fully saturated with waterfirst.Then it was subjected to an oil(equil-ibrated n-C10)injection followed by a waterflood.Finally,a near-miscible gasflood wascarried out to recover some of the residual oil,which had been left behind after the waterflood.A number of experiments were carried out.Details of the experimental procedure anddiscussion of the results for a typical secondary and tertiary gasflood experiment are givenin the following sections.3.1Secondary Gas FloodsInitially,the micromodel was fully saturated with n-Decane(n-C10)pressurised to35.16Mpa(5100psia)and subsequently displaced by oil(equilibrated n-C10).Then the near-misciblegas(equilibrated n-C10)was injected into the micromodel.As the gasflowed through themicromodel,a distinct piston type frontal advance,at the pore level,was observed for theincoming gas and equally important was the drainage of oil through the wetting layers inthe angular corners.Figure4shows a part of the micromodel during near-miscible gas injec-tion where the main gas front has just passed.The yellow colour represents the pores thathave been invaded by the gas.As it is shown,a significant part of the original oil in place(S oi=100%)has been recovered at this stage in this low-IFT gas injection.Some of the oil(shown in white colour,which is distinct to the white glass)has been by-passed by themain gas front.This pore-level bypassing is always present in oil recovery by gasfloods evenat zero IFT(complete miscibility).In general,bypassing of the oil could happen as a resultof;unfavourable mobility ratio,gravity override,heterogeneities,dead-end pores,water(ifpresent)and viscous instability at the pore level.We observed that,as the near-miscible gasinjection continued,recovery of the bypassed oil continued too.This extra oil recovery,innear-miscible gas injection,had not been observed,in high-IFT(low-pressure immiscible)gas injection that the authors had performed using the same experimental facility(Sohrabiet al.2000,2004).As we will see later,in near-miscible gas injection the displacement andrecovery of the by-passed oil continues behind the main gas front and can eventually causetotal recovery of the contacted oil.Magnified pictures shown in Fig.5(a)through Fig.5(d)demonstrate the recovery of someof the bypassed oil in a part of the micromodel during this near-miscible gas injection.Thesepictures were taken during30min of near-miscible gas injection at velocity of about1.2m/d.Figure5(a)shows that a relatively large volume of oil has been by-passed by the main gasfront(that had just passed this spot of the micromodel)and is apparently trapped betweensome gas-filled pores.As the gas injection continued,the volume of this by-passed oil shrank,Fig.5(b)and5(c),andfinally it was completely recovered,Fig.5(d).Figure6shows a section of the micromodel after2h of near-miscible gas -parison of Fig.6with Fig.4shows that the recovery of the by-passed oil has continuedbehind the main gas front and complete oil recovery has been achieved by near-miscible gasinjection.The prevailing mechanisms for the displacement and recovery of this bypassed oilis described in Sect.4(Discussion).358M.Sohrabi et al.Fig.4A section of thehorizontal micromodel duringnear-miscible gas(a )(b)(c)(d )oilg as oil g asoil g as g asFig.5(a )A part of the horizontal micromodel during near-miscible gas injection after the breakthrough of the gas where some oil has been left behind and surrounded by several gas–oil interfaces.The oil volume shrank as the gas injection continued,(b )As the gas injection continued,more of the trapped oil was recovered (c )and finally (after 2h of near-miscible gas injection)the oil patch was completely transported and recovered through the oil layers,(d )3.2Tertiary Gas Floods (Gas Injection after Water Flooding)In oil reservoirs,there is always some interstitial water present,which plays an important role in displacement process and recovery efficiency.Naturally,in the absence of water,oil is always wetting phase and gas is non-wetting phase.Whereas if water is present,depending on rock wettability,oil could be wetting,non-wetting or partially wetting phase and also itMicroscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection359 Fig.6A section of thehorizontal micromodel after twohours of near-miscible gasparison with Fig.3shows that complete oil recoveryhas been achievedmay or may not form spreading layers.It is well-known that every one of these conditions could have an important impact onflow and displacement process.In tertiary gasfloods,initially the water-wet micromodel was fully saturated with clear distilled water and pressurised to35.16Mpa(5100psia)and subsequently displaced with blue live water,equilibrated with gas and oil at35.16Mpa(5100psia)and37.8◦C(100◦F). To simulate primary drainage of water(initial migration of oil into the water bearing porous media),the equilibrated oil was injected from one end of the horizontal micromodel.The injection of the oil was carried out at a low velocity of1.2m/d(capillary number,Nc≈1E-7) and continued until oil reached the other end of the micromodel.Figure7is a magnified image of a section of the micromodel at the end of primary drainage of water(oil injection through water saturated micromodel).It shows the relative positions of the wetting phase(blue water)and non-wetting phase(white oil),in a water-wet micromodel. The direction and shape of the water-oil interfaces are good indications of water-wet condi-tions.As well as occupying some of the smaller and dead-end pores water is also present as a wetting layer over the pore surface,even in oil-filled pores,as expected.unetchedFig.7A magnified section of the water-wet micromodel at the end of primary drainage of water demonstrating shapes of water(blue)/oil(white)interfaces360M.Sohrabi et al.(a) (b)Fig.8A magnified picture of the micromodel before(a)and after(b)waterfloodAfter this initial oil injection stage(establishment of‘irreducible’water),waterflooding began.Water was injected into the micromodel at a low rate of0.01cm3/h corresponding to a capillary number of1E-7and a velocity of1.2m/d.Due to a very low rate of water injec-tion,high-water/oil IFT(41mN/m)and the micromodel being strongly water-wet,water was observed to enter the model by layerflow(as opposed to piston-wise displacement)with no distinct waterfront.The water layers,surrounding the oil,were seen to thicken progressively. Also oil was displaced mostly from the sides and corners of the pores,with residual oil remaining in the middle of the pores in the shape of narrow oilfilaments.Figure8compares a section of the micromodel before and after waterfl-parison of Fig.8(a)and8(b)highlights the fact that during waterflooding the dominant mechanism has been layerflow as opposed to piston-type.The slow thickening of water films at the sides and corners of the oilfilled pores was a direct consequence of a capillary dominatedflow regime.Video recording of the process was carried out,that shows theflow process vividly.These were presented at previous SPE and UK Department of Trade and Industry seminars and meetings(Sohrabi et al.2000,2004;Tehrani et al.2001).Waterflooding continued until no more oil production or changes influids distribution was observed.At the end of this waterflood,near-miscible gas injection commenced with the same slow rate as the previous stages of the experiment,i.e.,at1.2m/d.Gas was injected from the same end as water was.During gas injection,as soon as the invading gas came in contact with the residual oil,the gas front was covered with a small amount of oil.As the gas advanced further,an oil bank was formed and moved ahead of the gas front.The oil bank,moving ahead of the gas front,supplied oil to pores where residual oil was present.This resulted in thickening of those oilfilaments,which came in contact with the invading gas or the oil front ahead of it.Figure9shows a part of the model during gas injection.As it can be noticed,the oil shown in this picture can be divided in two types.The first type is oil in the form of somefilaments surrounded with thick water layers.This is the oil that has not been contacted by the incoming gas.The second type is thick oil ganglia in contact with the gas channel.These have been formed byfluid redistribution or by local oil transfer brought about by the gas injection.Figure10depicts thefluid distribution of near miscible gas injection in a part of the micromodel immediately after the main gas front has passed.As it can be seen,despite a very low gas–oil IFT,the gas has made only a single channel through the network of pores. Compared to secondary gasfloods(Fig.5),the efficiency of this tertiary gasflood has been rather poor with a significant amount of oil being bypassed by the gas front.There are two main reasons for this.First,in waterflooded micromodel the water saturation is high,whichgasFig.9Fluid distribution in the micromodel during gas injection.Thickening of oilfilaments during gas injec-tion is evident.During gas injection a small bank of oil develops in front of the gas,feeding oil into narrow oilfilaments that were formed during the previous waterfloodFig.10Fluid distribution withinnear-miscible gas injection atcm3/h.the main gas front has justleft the micromodel(breakthrough)tends to restrict the movement of the gas and shields the residual oil from being contacted by the gas.The second reason is the topology of the porous medium,which causes some of the oil that has been contacted by gas or its associated oil bank,to become trapped in dead-end or semi dead-end pores.Dead-end pores are those that are physically closed by rock surfaces and semi dead-end refers to those pores,which are not physically closed but theflow of oil is restricted by water at one or more ends(water/oil IFT is by far larger than gas/oil IFT in near-miscible gas injections).Figure11shows the distribution of thefluids within the same part of the micromodel as was shown in Fig.10after1h of near-miscible gas parison of Figs.11and 10and also video clips recorded during this gas injection reveals that the saturation of the by-passed oil,behind the main gas front,kept reducing and eventually led to a complete recovery of all the contacted oil.。

材料导论中英文讲稿 (4)

材料导论中英文讲稿 (4)

6.2.2 Porosity and DensityHello, everybody, in this Section, we are going to talk about porosity and density.译文：在这一节，我们将讨论气孔率和密度。

When referring to a solid material such as a part made from copper or stainless steel, the word density takes into consideration the microstructures that contains no porosity. By which term we do not mean the voids or vacancies in the atomic structure. In speaking of a solid material we mean the material’s theoretical density or mass density, it is the mass of a material divided by its volume.我们通常说到密度，指的是理论密度，表示结构中不包含气孔。

通过质量除以体积得到。

译文：当提到一种固体材料，尤其是由铜或不锈钢制成的材料时，密度这个词通常会考虑到一个没有气孔率的微观结构，在这个词中，我们指的不是原子结构中的孔洞或空位。

我们通常说到密度，指的是理论密度，表示结构中不包含气孔。

通过质量除以体积得到。

There are two factors influence the density of material. Atomic weight is a major factor in determining the density of the materials. Low-atomic-weight elements have low densities. 影响材料密度的因素有两个。