Novel predictive model for metallic structure corrosion status in presence of stray curre

ARTICLEOPENReceived8Apr2015|Accepted25Aug2015|Published29Sep2015Entropy-stabilized oxidesChristina M.Rost1,Edward Sachet1,Trent Borman1,Ali Moballegh1,Elizabeth C.Dickey1,Dong Hou1,Jacob L.Jones1,Stefano Curtarolo2&Jon-Paul Maria1Configurational disorder can be compositionally engineered into mixed oxide by populating asingle sublattice with many distinct cations.The formulations promote novel and entropy-stabilized forms of crystalline matter where metal cations are incorporated in new ways.Here,through rigorous experiments,a simple thermodynamic model,and afive-componentoxide formulation,we demonstrate beyond reasonable doubt that entropy predominatesthe thermodynamic landscape,and drives a reversible solid-state transformation betweena multiphase and single-phase state.In the latter,cation distributions are proven tobe random and homogeneous.Thefindings validate the hypothesis that deliberateconfigurational disorder provides an orthogonal strategy to imagine and discover new phasesof crystalline matter and untapped opportunities for property engineering.1Department of Materials Science and Engineering,North Carolina State University,Raleigh,North Carolina27695,USA.2Department of Mechanical Engineering and Materials Science,Center for Materials Genomics,Duke University,Durham,North Carolina27708,USA.Correspondence and requests for materials should be addressed to J.-P.M.(email:jpmaria@)or to S.C.(email:stefano@).A grand challenge facing materials science is the continuoushunt for advanced materials with properties that satisfythe demands of rapidly evolving technology needs.The materials research community has been addressing this problem since the early1900s when Goldschmidt reported the‘the method of chemical substitution’1that combined a tabulation of cationic and anionic radii with geometric principles of ion packing and ion radius ratios.Despite its simplicity,this model enabled a surprising capability to predict stable phases and structures.As early as1926many of the technologically important materials that remain subjects of contemporary research were identified (though their properties were not known);BaTiO3,AlN,GaP, ZnO and GaAs are among that list.These methods are based on overarching natural tendencies for binary,ternary and quaternary structures to minimize polyhedral distortions,maximize spacefilling and adopt polyhedral linkages that preserve electroneutrality1–3.The structure-field maps compiled by Muller and Roy catalogue the crystallographic diversity in the context of these largely geometry-based predictions4.There are,however,limitations to the predictive power,particularly when factors like partial covalency and heterodesmic bonding are considered.To further expand the library of advanced materials and property opportunities,our community explores possibilities based on mechanical strain5,artificial layering6,external fields7,combinatorial screening8,interface engineering9,10and structuring at the nanoscale6,11.In many of these efforts, computation and experiment are important companions.Most recently,high-throughput methods emerged as a power-ful engine to assess huge sections of composition space12–17and identified rapidly new Heusler alloys,extensive ion substitution schemes18,19,new18-electron ABX compounds20and new ferroic semiconductors21.While these methods offer tremendous predictive power and an assessment of composition space intractable to experiment, they often utilize density functional theory calculations conducted at0K.Consequently,the predicted stabilities are based on enthalpies of formation.As such,there remains a potential section of discovery space at elevated temperatures where entropy predominates the free-energy landscape.This landscape was explored recently by incorporating deliberatelyfive or more elemental species into a single lattice with random occupancy.In such crystals,entropic contributions to the free energy,rather than the cohesive energy, promote thermodynamic stability atfinite temperatures.The approach is being explored within the high-entropy-alloy family of materials(HEAs)22,in which extremely attractive properties continue to be found23,24.In HEAs,however,discussion remains regarding the true role of configurational entropy25–28, as samples often contain second phases,and there are uncertainties regarding short-range order.In response to these open discussions,HEAs have been referred to recently as multiple-principle-element alloys29.It is compelling to consider similar phenomena in non-metallic systems,particularly considering existing information from entropy studies in mixed oxides.In1967Navrotsky and Kleppa showed how configurational entropy regulates the normal-to-inverse transformation in spinels,where cations transition between ordered and disordered site occupancy among the available sublattices30,31.These fundamental thermodynamic studies lead one to hypothesize that in principle,sufficient temperature would promote an additional transition to a structure containing only one sublattice with random cation occupancy.From experiment we know that before such transitions,normal materials melt,however,it is conceivable that synthetic formulations exist,which exhibit them.Inspired by research activities in the metal alloy communities and fundamental principles of thermodynamics we extend the entropy concept tofive-component oxides.With unambiguous experiments we demonstrate the existence of a new class of mixed oxides that not only contains high configurational entropy but also is indeed truly entropy stabilized.In addition,we present a hypothesis suggesting that entropy stabilization is particularly effective in a compound with ionic character.ResultsChoosing an appropriate experimental candidate.The candi-date system is an equimolar mixture of MgO,CoO,NiO,CuO and ZnO,(which we label as‘E1’)so chosen to provide the appropriate diversity in structures,coordination and cationic radii to test directly the entropic ansatz.The rationale for selection is as fol-lows:the ensemble of binary oxides should not exhibit uniform crystal structure,electronegativity or cation coordination,and there should exist pairs,for example,MgO–ZnO and CuO–NiO, that do not exhibit extensive solubility.Furthermore,the entire collection should be isovalent such that relative cation ratios can be varied continuously with electroneutrality preserved at the net cation to anion ration of unity.Tabulated reference data for each component,including structure and ionic radius,can be found in Supplementary Table1.Testing reversibility.In thefirst experiment,ceramic pellets of E1are equilibrated in an air furnace and quenched to room temperature.The temperature spanned a range from700to 1,100°C,in50-°C increments.X-ray diffraction patterns showing the phase evolution are depicted in Fig.1.After700°C,two prominent phases are observed,rocksalt and tenorite.The tenorite phase fraction reduces with increasing equilibration temperature.Full conversion to single-phase rocksalt occurs between850and900°C,after which there are no additional peaks,the background is low andflat,and peak widths are narrow in two-theta(2y)space.Reversibility is a requirement of entropy-driven transitions. Consequently,low-temperature equilibration should transform homogeneous1,000°C-equilibrated E1back to its multiphase state(and vice versa on heating).Figure1also shows a sequence of X-ray diffraction patterns for such a thermal excursion;initial equilibration at1,000°C,a second anneal at750°C,andfinally a return to1,000°C.The transformation from single phase,to multiphase,to single phase is evident by the X-ray patterns and demonstrates an enantiotropic(that is,reversible with tempera-ture32)phase transition.Testing entropy though composition variation.A composition experiment is conducted to further characterize this phase tran-sition to the random solid solution state.If the driving force is entropy,altering the relative cation ratios will influence the transition temperature.Any deviation from equimolarity will reduce the number of possible configurations O(S c¼k B log(O)), thus increasing the transition temperature.Because S c(x i)is logarithmically linked to mole fraction via B x i log(x i),the com-positional dependence is substantial.This dependency underpins our gedankenexperiment where the role of entropy can be tested by measuring the dependency of transition temperature as a function of the total number of components present,and of the composition of a single component about the equimolar formulation.The calculated entropy trends for an ideal mixture are illustrated in Fig.2b,which plots configurational entropy for a set of mixtures having N species where the composition of an individual species is changed and the others(NÀ1)are keptequimolar.Two dependencies become apparent:the entropy increases as new species are added and the maximum entropy is achieved when all the species have the same fraction.Both dependencies assume ideal random mixing.Two series of composition-varying experiments investigate the existence of these trends in formulation E1.The first experiment monitors phase evolution in five compounds,each related to the parent E1by the extraction of a single component.The sets are equilibrated at 875°C (the threshold temperature for complete solubility)for 12h.The diffraction patterns in Fig.2a show that removing any component oxide results in material with multiple phases.A four-species set equilibrated under these conditions never yields a single-phase material.The second experiment uses five individual phase diagrams to explore the configurational entropy versus composition trend.In each,the composition of a single component is varied by ±2,±6and ±10%increments about the equimolar composition while the others are kept even.Since any departure from equimolarity reduces the configurational entropy,it should increase transition temperatures to single phase,if thattransitionI n t e n s i t y2030405060702 (°)801.81,100N =5No ZnONo MgON =4No CuON =3No NiONo CoON =21,0501,000950T e m p e r a t u r e (°C )T e m p e r a t u r e (°C )T e m p e r a t u r e (°C )T e m p e r a t u r e (°C )T e m p e r a t u r e (°C )S /k B9008501,1001,0501,0009509008501,1001,0501,0009509008501,1001,0501,0009509008501,1001,0501,0009509008500.0X NX NiOX CuOX ZnOX MgoX CoO0.5 1.00.10.20.30.10.20.30.10.20.30.10.20.30.10.20.31.62223112202001111.41.21.00.80.60.40.20.0J14**********Figure 2|Compositional analysis.(a )X-ray diffraction analysis for a composition series where individual components are removed from the parent composition E1and heat treated to the conditions that would otherwise produce full solid solution.Asterisks identify peaks from rocksalt while carrots identify peaks from other crystal structures.(b )Calculated configurational entropy in an N -component solid solutions as a function of mol%of the N th component,and (c –g )partial phase diagrams showing the transition temperature to single phase as a function of composition (solvus )in the vicinity of the equimolar composition where maximum configurational entropy is expected.Error bars account for uncertainty between temperature intervals.Each phase diagram varies systematically the concentration of one element.L o g i n t e n s i t y750 °C750 °C800 °C850 °C900 °C1,000 °C2001111,000 °C 2 (°)T (200)T (002)T (110)T (200)T (002)T (110)Figure 1|X-ray diffraction patterns for entropy-stabilized oxide formulation E1.E1consists of an equimolar mixture of MgO,NiO,ZnO,CuO and CoO.The patterns were collected from a single pellet.The pellet was equilibrated for 2h at each temperature in air,then air quenched to room temperature by direct extraction from the furnace.X-ray intensity is plotted on a logarthimic scale and arrows indicate peaks associated with non-rocksalt phases,peaks indexed with (T)and with (RS)correspond to tenorite and rocksalt phases,respectively.The two X-ray patterns for 1,000°C annealed samples are offset in 2y for clarity.is in fact entropy driven.The specific formulations used are given in Supplementary Table 2.Figure 2c–g are phase diagrams of composition versus transformation temperature for the five sample sets that varied mole fraction of a single component.The diagrams were produced by equilibrating and quenching individual samples in 25°C intervals between 825and 1,125°C to obtain the T trans -composition solvus .In all cases equimolarity always leads to the lowest transformation temperatures.This is in agreement with entropic promotion,and consistent with the ideal model shown in Fig.2b.One set of raw X-ray patterns used to identify T trans for 10%MgO is given as an example in Supplementary Fig.1.Testing endothermicity .Reversibility and compositionally dependent solvus lines indicate an entropy-driven process.As such,the excursion from polyphase to single phase should be endothermic.An entropy-driven solid–solid transformation is similar to melting,thus requires heat from an external source 33.To test this possibility,the phase transformation in formulation E1can be co-analysed with differential scanning calorimetry and in situ temperature-dependent X-ray diffraction using identical heating rates.The data for both measurements are shown in Fig.3.Figure 3a is a map of diffracted intensity versus diffraction angle (abscissa)as a function of temperature.It covers B 4°of 2y space centred about the 111reflection for E1.At a temperature interval between 825and 875°C,there is a distinct transition to single-phase rocksalt structure—all diffraction events in that range collapse into an intense o 1114rocksalt peak.Figure 3b contains the companion calorimetric result where one finds a pronounced endotherm in the identical temperature window.The endothermic response only occurs when the system adds heat to the sample,uniquely consistent with an entropy-driven transformation 33.We note the small mass loss (B 1.5%)at the endothermic transition.This mass loss results from the conversion of some spinel (an intermediate phase seen by X-ray diffraction)to rocksalt,which requires reduction of 3þto 2þcations and release of oxygen to maintain stoichiometry.To address concerns regarding CuO reduction,Supplementary Fig.2shows a differential scanning calorimetry and thermal gravimetric analysis curve for pure CuO collected under the same conditions.There is no oxygen loss in the vicinity of 875°C.Testing homogeneity .All experimental results shown so far support the entropic stabilization hypothesis.However,all assume that homogeneous cation mixing occurs above the tran-sition temperature.It is conceivable that local composition fluc-tuations produce coherent clustering or phase separation events that are difficult to discern by diffraction using a laboratory sealed tube diffractometer.The solvus lines of Fig.2c–g support random mixing,as the most stable composition is equimolar (a condition only expected for ideal/regular solutions),but it is appropriate to ensure self-consistency with direct measurements.To characterize the cation distributions,extended X-ray absorption fine structure (EXAFS)and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM EDS)is used to analyse structure and chemistry on the local scale.EXAFS data were collected for Zn,Ni,Cu and Co at the Advanced Photon Source 12-BM-B 34,35.The fitted data are shown in Fig.4,the raw data are given in Supplementary Fig.3.The fitted data for each element provide two conclusions:the cation-to-anion first-near-neighbour distances are identical (within experimental error of ±0.01Å)and the local structures for each element to approximately seven near-neighbour distances are similar.Both observations are only consistent with a random cation distribution.As a corroborating measure of local homogeneity,chemical analysis was conducted using a probe-corrected FEI Titan STEM with EDS detection.Thin film samples of E1,prepared by pulsed laser deposition,are the most suitable samples to make the assessment.Details of preparation are given in the methods,and X-ray and electron diffraction analysis for the film are provided in Supplementary Figs 4and 5.The sample was thinned by mechanical polishing and ion milling.Figure 5shows a collection of images including Fig.5a,the high-angle annular dark-field signal (HAADF).In Fig.5b–f,the EDS signals for the K a emission energies of Mg,Co,Ni,Cu and Zn are shown (additional lower magnification images are included in Supplementary Fig.6).All magnifications reveal chemically and structurally homogeneous material.1,100R 111R 111Mass change (%)510151,000900800700600500400300200DSC –30–20Endo DSC (mW) Exo35.536.537.52θ (°)–10010Mass100T e m p e r a t u r e (°C )T e m p e r a t u r e (°C )Figure 3|Demonstrating endothermicity.(a )In situ X-ray diffraction intensity map as a function of 2y and temperature;and (b )differential scanning calorimetry trace for formulation ‘E1’.Note that the conversion to single phase is accompanied by an endotherm.Both experiments were conducted at a heating rate of 5°C min À1.04k (Å–1)(k )×k 2 (Å–2)2ZnNiCuCo681012Figure 4|Extended X-ray absorption fine structure.EXAFS measured at Advanced Photon Source beamlime 12-BM after energy normalization and fitting.Note that the oscillations for each element occur with similar relative intensity and at similar reciprocal spacing.This suggests a similar local structural and chemical environment for each.X-ray diffraction,EXAFS and STEM–EDS probes are sensitive to 10s of nm,10s of Åand 1Ålength scales,respectively.While any single technique could be misinterpreted to conclude homogenous mixing,the combination of X-ray diffraction,EXAFS and STEM–EDS provide very strong evidence.We note,in particular,the similarity in EXAFS oscillations (both in amplitude and position)out to 12inverse angstroms.This similarly would be lost if local ordering or clustering were present.Consequently,we conclude with certainty that the cations are uniformly dispersed.DiscussionThe set of experimental outcomes show that the transition from multiple-phase to single phase in E1is driven by configurational entropy.To complete our thermodynamic understanding of this system,it is important to understand and appreciate the enthalpic penalties that establish the transition temperature.In so doing,the data set can be tested for self-consistency,and the present data are brought into the context of prior research on oxide solubility.First,we consider an equation relating the initial and final states of the proposed phase transition:MgO ðRS ÞþNiO ðRS ÞþCoO ðRS ÞþCuO ðT ÞþZnO ðW Þ¼Mg ;Ni ;Co ;Cu ;Zn ðÞO ðRS ÞFor MgO,NiO and CoO,the crystal structures of the initial and final states are identical.If we assume that solution of each into the E1rocksalt phase is ideal,the enthalpy for mixing is zero.For CuO and ZnO,there must be a structural transition to rocksalt on dissolution from tenorite and wurtzite,respectively.If we again assume (for simplicity)that the solution is ideal,the mixing energy is zero,but there is an enthalpic penalty associated with the structure transition.From Davies et al.and Bularzik et al.,we know the reference chemical potential changes for the wurtzite-to-rocksalt and the tenorite-to-rocksalt transitions of ZnO and CuO;they are 25and 22kJ mol À1,respectively 36,37.If we make the assumption that the transition enthalpies of ZnO(wurtzite)to ZnO(rocksalt E1)and CuO(tenorite)to CuO(rocksalt E1)are comparable,then the enthalpic penalty for solution into E1can be estimated.For ZnO and CuO,the transition to solid solution in a rocksalt structure involves an enthalpy change of (0.2)Á(25kJ mol À1)þ(0.2)Á(22kJ mol À1),a total of þ10kJ mol À1.This calculation is based on the productof the mol fraction of each multiplied by the reference transition enthalpy.This assumption is consistent with the report of Davies et al.who showed that the chemical potential of a particular cation in a particular structure is associated with the molar volume of that structure 36.Since the rocksalt phases of ZnO and CuO have molar volumes comparable to E1,their reference transition enthalpy values are considered suitable proxies.In comparison,the maximum theoretically expected config-urational entropy difference at 875°C (the temperature were we observe the transition experimentally)between the single species and the random five-species solid solution is B 15kJ mol À1,5kJ mol À1larger than the calculated enthalpy of transition.It is possible that the origins of this difference are related to mixing energy as the reference energy values for structural transitions to rocksalt do not capture that aspect.While the present phase diagrams that monitor T trans as a function of composition demonstrate rather symmetric behaviour about the temperature minima,it is unlikely that mixing enthalpies are zero for all constituents.Indeed,literature reports show that enthalpies of mixing between the constituent oxides in E1are finite and of mixed sign,and their magnitudes are on the same order as the 5kJ mol À1difference between our calculated predictions 36.This energy difference may be accounted for by finite and positive mixing enthalpies.Following this argument,we can achieve a self-consistent appreciation for the entropic driving force and the enthalpic penalties for solution formation in E1by considering enthalpies of the associated structural transitions and expected entropy values for ideal cation mixing.As a final test,these predictions can be compared with experiment,specifically by calculating the magnitude of the endotherm observed by DSC at the transition from multiple-phase to single-phase states.Doing so we find a value B 12kJ mol À1(with an uncertainty of ±2kJ mol À1).While we acknowledge the challenge of quantitative calorimetry,we note that this experimental result is intermediate to and in close agreement with the predicted values.Compared with metallic alloys,the pronounced impact of entropy in oxides may be surprising given that on a per-atom basis the total disorder per volume of an oxide seems be lower than in a high-entropy alloy,as the anion sublattice is ordered (apart from point defects).The chemically uniform sublattice is perhaps the key factor that retains cation configurational entropy.As an illustration,consider a comparison between random metal alloys and random metal oxide alloys.Begin by reviewing the case of a two-component metallic mixture A–B.If the mixture is ideal,the energy of interaction E A–B ¼(E A–A þE B–B )/2,there is no enthalpic preference for bonding,and entropy regulates solution formation.In this scenario,all lattice sites are equivalent and configurational entropy is maximized.This situation,however,never occurs as no two elements have identical electronegativity and radii values.Figure 6a illustrates a two-component alloy scenario A–B where species B is more electronegative than A.Consequently,the interaction energies E A–A ,E B–B and E A–B will be different.A random mixture of A–B will produce lattice sites with a distribution of first near neighbours,that is,species A coordinated to 4-B atoms,2-A and 2-B atoms,etc y Different coordinations will have different energy values and the sites are no longer indistinguishable.Reducing the number of equivalent sites reduces the number of possible configurations and S .Now consider the same two metallic ions co-populating a cation sublattice,as in Fig.6b.In this case,there is always an intermediate anion separating neighbouring cation lattice sites.Again,in the limiting case where only first near neighboursareFigure 5|STEM–EDS analysis of E1.(a )HAADF image.Panels labelled as Zn,Ni,Cu,Mg and Co are intensity maps for the respective characteristic X-rays.The individual EDS maps show uniform spatial distributions for each element and are atomically resolved.considered,every cation lattice site is ‘identical’because each has the same immediate surroundings:the interior of an oxygen octahedron.Differentiation between sites is only apparent when the second near neighbours are considered.From the configura-tional disorder perspective,if each cation lattice site is identical,and thus energetically similar to all others,the number of microstates possible within the macrostate will approach the maximum value.This crystallographic argument is based on the limiting case where first-near-neighbour interactions predominate the energy landscape,which is an imperfect approximation.Second and third near neighbours will influence the distribution of lattice site energies and the number of equivalent microstates—but the impact will be the same in both scenarios.A larger number of equivalent sites in a crystal with an intermediate sublattice will increase S and expand the elemental diversity containable in a single solid solution and to lower the temperature at which the transition to entropic stabilization occurs.We acknowledge the hypothesis nature of this model at this time,and the need for a rigorous theoretical exploration.It is presented currently as a possibility and suggestion for future consideration and testing.We demonstrate that configurational disorder can promote reversible transformations between a poly-phase mixture and a homogeneous solid solution of five binary oxides,which do not form solid solutions when any of the constituents are removed provided the same thermal budget.The outcome is representative of a new class of materials called ‘entropy-stabilized oxides’.While entropic effects are known for oxide systems,for example,random cation occupancy in spinels 30,order–disorder transfor-mations in feldspar 38,and oxygen nonstoichiometry in layered perovskites 39,the capacity to actively engineer configurational entropy by composition,to stabilize a quinternary oxide with a single cation sublattice,and to stabilize unusual cation coordination values is new.Furthermore,these systems provide a unique opportunity to explore the thermodynamics and structure–property relationships in systems with extreme configurational disorder.Experimental efforts exploring this composition space are important considering that such compounds will be challenging to characterize with computational approaches minimizing formation energy (for example,genetic algorithms)or with adhoc thermodynamic models (for example,CALPHAD,cluster expansion)6.We expect entropic stabilization in systems where near-neighbour cations are interrupted by a common intermediateanion (or vice versa),which includes broad classes of chalcogenides,nitrides and halides;particularly when covalent character is modest.The entropic driving force—engineered by cation composition—provides a departure from traditional crystal-chemical principles that elegantly predict structural trends in the major ternary and quaternary systems.A companion set of structure–property relationships that predict new entropy-stabilized structures with novel cation incorporation await discovery and exploitation.MethodsSolid-state synthesis of bulk materials .MgO (Alfa Aesar,99.99%),NiO (Sigma Aldrich,99%),CuO (Alfa Aesar,99.9%),CoO (Alfa Aesar,99%)and ZnO (Alfa Aesar 99.9%)are massed and combined using a shaker mill and 3-mm diameter yttrium-stabilized zirconia milling media.To ensure adequate mixing,all batches are milled for at least 2h.Mixed powders are then separated into 0.500-g samples and pressed into 1.27-cm diameter pellets using a uniaxial hydraulic press at 31,000N.The pellets are fired in air using a Protherm PC442tube furnace.Temperature evolution of phases .Ceramic pellets of E1are equilibrated in an air furnace and quenched to room temperature by direct extraction from the hot zone.Phase analysis is monitored by X-ray diffraction using a PANalytical Empyrean X-ray diffractometer with Bragg-Brentano optics including programmable diver-gence and receiving slits to ensure constant illumination area,a Ni filter,and a 1-D 128element strip detector.The equivalent counting time for a conventional point detector would be 30s per point at 0.01°2y increments.Note that all X-ray are collected using substantial counting times and are plotted on a logarithmic scale.To the extent knowable using a laboratory diffractometer,the high-temperature samples are homogeneous and single phase:there are no additional minor peaks,the background is low and flat,and peak widths are sharp in two-theta (2y )space.Temperature-dependent diffraction data are collected with PANalytical Empyrean X-ray diffractometer with Bragg-Brentano optics includingprogrammable divergence and receiving slits to ensure constant illumination area,a Ni filter,and a 1-D 256element strip detector.The samples are placed in a resistively heated HTK-1200N hot stage in air.The samples are ramped at a constant rate of 5°C min À1with a theta–two theta pattern captured every 1.5min.Calorimetry data are collected using a Netzsch STA 449F1Jupiter system in a Pt crucible at 5°C min À1in flowing air.Determining solvus lines .Five series of powders are mixed where the amount of one constituent oxide is varied from the parent mixture E1.Supplementary Table 2lists the full set of samples synthesized for this experiment.Each individual sample is cycled through a heat-soak-quench sequence at 25°C increments from 850°C up to 1,150°C.The soak time for each cycle is 2h,and samples are then quenched to room temperature in o 1min.After the quenching step for each cycle,samples are immediately analysed for phase identification using a PANalytical Empyrean X-ray diffractometer using the conditions identified above.If more than one phase is present,the sample would be put through the next temperature cycle.The temperature at which the structure is determined to be pure rocksalt,with no discernable evidence of peak splitting or secondary phases,is deemed the transition temperature as a function of composition.Supplementary Fig.1shows an example of the collected X-raypatterns after each cycle using the E1L series with þ10%MgO.Once single phase is achieved,the sample is removed from the sequence.Note that this entire experiment is conducted two times.Initially in 50°C increments and longer anneals,and to ensure accuracy of temperature values and reproducibility,a second time using shorter increments and 25°C anneals.Findings in both sets are identical to within experimental error bar values.In the latter case,error bars correspond to the annealing interval value of 25°C.In the main text relating to Fig.2a we note that in addition to small peaks from second phases,X-ray spectra for N ¼4samples with either NiO or MgO removed show anisotropic peak broadening in 2y and skewed relative intensities where I (200)/I (111)is less than unity.This ratio is not possible for the rocksalt structure.Supplementary Table 3shows the result of calculations of structure factors for a random equimolar rocksalt oxide with composition E1.Calculations show that the 200reflection is the strongest,and that the experimentally measured relative intensities of 111/200are consistent with calculations.We use this information as a means too best assess when the transition to single phase occurs since the most likely reason for the skewed relative intensity is an incomplete conversion to the single-phase state.This dependency is highlighted in Supplementary Fig.1.X-ray absorption fine structure .X-ray absorption fine structure (XAFS)is made possible through the general user programme at the Advanced Photon Source in Lemont,IL (GUP-38672).This technique provides a unique way to probe the local environment of a specific element based on the interference between an emitted core electron and the backscattering from surrounding species.XAFS makes no assumption of structure symmetry or elemental periodicity,making it an ideal means to study disordered materials.During the absorption process,coreelectronsBFigure 6|Binary metallic compared with a ternary oxide.A schematic representation of two lattices illustrating how the first-near-neighbour environments between species having different electronegativity (the darker the more negative charge localized)for (a )a random binary metal alloy and (b )a random pseudo-binary mixed oxide.In the latter,near-neighbour cations are interrupted by intermediate common anions.。

表面技术第53卷第4期金属材料表面纳米化研究与进展杨庆，徐文文，周伟，刘璐华，赖朝彬*（江西理工大学 材料冶金化学学部，江西 赣州 341000）摘要：大多数金属材料的失效都是从其表面开始的，进而影响整个材料的整体性能。

研究表明，在金属材料表面制备纳米晶，实现表面纳米化，可以提升材料的表面性能，延长其使用寿命。

金属材料表面纳米化是指利用反复剧烈塑性变形让表层粗晶粒逐步得到细化，材料中形成晶粒沿厚度方向呈梯度变化的纳米结构层，分别为表面无织构纳米晶层、亚微米细晶层、粗晶变形层和基体层，这种独特的梯度纳米结构对金属材料表面性能的大幅度提升效果显著。

根据国内外表面纳米化的研究成果，首先对表面涂层或沉积、表面自纳米化以及混合纳米化3种金属表面纳米化方法进行了简要概述，阐述了各自优缺点，总结了表面自纳米化技术的优势，在此基础上重点分析了位错和孪晶在金属材料表面自纳米化过程中所起的关键作用，提出了金属材料表面自纳米化机制与材料结构、层错能大小有着密不可分的联系，对金属材料表面自纳米化机制的研究现状进行了归纳；阐明了表面纳米化技术在金属材料性能提升上的巨大优势，主要包括对硬度、强度、腐蚀、耐磨、疲劳等性能的改善。

最后总结了现有表面强化工艺需要克服的关键技术，对未来的研究工作进行了展望，并提出将表面纳米化技术与电镀、气相沉积、粘涂、喷涂、化学热处理等现有的一些表面处理技术相结合，取代高成本的制造技术，制备出价格低廉、性能更加优异的复相表层。

关键词：金属材料；表面纳米化；梯度纳米结构；纳米化机理；表面性能中图分类号：TG178 文献标志码：A 文章编号：1001-3660(2024)04-0020-14DOI：10.16490/ki.issn.1001-3660.2024.04.002Research and Progress on Surface Nanocrystallizationof Metallic MaterialsYANG Qing, XU Wenwen, ZHOU Wei, LIU Luhua, LAI Chaobin*(Department of Materials Metallurgy and Chemistry, Jiangxi University ofTechnology, Jiangxi Ganzhou 341000, China)ABSTRACT: It is well known that the failure of most metallic materials starts from their surfaces, which in turn affects the overall performance of the whole material. Numerous studies have shown that the preparation of nanocrystals on the surface of metallic materials, i.e., surface nanosizing, can enhance the surface properties of materials and extend their service life. Surface nanosizing of metallic materials makes use of repeated violent plastic deformation to make the surface coarse grains gradually收稿日期：2023-02-23；修订日期：2023-06-29Received：2023-02-23；Revised：2023-06-29基金项目：国家自然科学基金项目（52174316，51974139）；国家重点研发计划项目（2022YFC2905200，2022YFC2905205）；江西省自然科学基金项目（20212ACB204008）Fund：National Natural Science Foundation of China(52174316, 51974139); National Key Research and Development Program of China (2022YFC2905200, 2022YFC2905205); Natural Science Foundation of Jiangxi Province (20212ACB204008)引文格式：杨庆, 徐文文, 周伟, 等. 金属材料表面纳米化研究与进展[J]. 表面技术, 2024, 53(4): 20-33.YANG Qing, XU Wenwen, ZHOU Wei, et al. Research and Progress on Surface Nanocrystallization of Metallic Materials[J]. Surface Technology, 2024, 53(4): 20-33.*通信作者（Corresponding author）第53卷第4期杨庆，等：金属材料表面纳米化研究与进展·21·refine to the nanometer level, forming nanostructured layers with gradient changes of grains along the thickness direction, including surface non-woven nanocrystalline layer, submicron fine crystal layer, coarse crystal deformation layer and matrix layer, and this unique gradient nanostructure is effective for the significant improvement of surface properties of metallic materials. The process technology and related applications of nanocrystalline layers on the surface of metallic materials in China and abroad are introduced, and the research progress of high-performance gradient nanostructured materials is discussed.Starting from the classification of the preparation process of gradient nanostructured materials and combining with the research results of surface nanosizing in China and abroad, a brief overview of three methods of metal surface nanosizing, namely, surface coating or deposition, surface self-nanosizing and hybrid nanosizing, was given, the advantages and disadvantages of each were discussed and the advantages of surface self-nanosizing technology were summarized. On the basis of this, the key role of dislocations and twins in the process of surface self-nanitrification of metallic materials was analyzed, and the mechanism of surface self-nanitrification of metallic materials was inextricably linked to the material structure and the size of layer dislocation energy, and the current research status of the mechanism of surface self-nanitrification of metallic materials was summarized. Finally, the key technologies required to be overcome in the existing surface strengthening process were summarized, and future research work was prospected. It was proposed to combine surface nanosizing technology with some existing surface treatment technologies such as electroplating, vapor deposition, tack coating, spraying, chemical heat treatment, etc., to replace the high-cost manufacturing technologies and prepare inexpensive complex-phase surface layers with more excellent performance.Techniques for the preparation of gradient nanostructured materials include surface coating or deposition, surface self-nanosizing, and hybrid surface nanosizing. Surface coating or deposition technology has the advantages of precise control of grain size and chemical composition, and relatively mature process optimization, etc. However, because the coating or deposition technology adds a cover layer on the material surface, the overall size of the material increases slightly, and there is a certain boundary between the coating and the material, and there will be defects in the specific input of production applications.In addition, the thickness of the gradient layer prepared by this technology is related to the deposition rate, which takes several hours to prepare a sample. The surface self-nanitrification technique, which generates intense plastic deformation on the surface of metal materials, has the advantages of simple operation, low cost and wide application, low investment in equipment and easy realization of unique advantages. The nanocrystalline layer prepared on the surface of metal materials with the surface self-nanitrification technique has a dense structure and no chemical composition difference from the substrate, and no surface defects such as pitting and pores, but the thickness of the gradient layers and nanolayers prepared by this technique as well as the surface quality of the material vary greatly depending on the process. Hybrid surface nanosizing is a combination of the first two techniques, in which a nanocrystalline layer is firstly prepared on the surface of a metallic material by surface nanosizing technology, and then a compound with a different composition from the base layer is formed on its surface by means of chemical treatment.To realize the modern industrial application of this new surface strengthening technology, it is still necessary to clarify the strengthening mechanism and formation kinetics of surface nanosizing technology as well as the effect of process parameters, microstructure, structure and properties on the nanosizing behavior of the material. For different nanosizing technologies, the precise numerical models for nanosizing technologies need to be established and improved, and the surface self-nanosizing equipment suitable for industrial scale production needs to be developed. In the future, surface nanosizing technology will be combined with some existing surface treatment technologies (e.g. electroplating, vapor deposition, adhesion coating, spraying, chemical heat treatment, etc.) to prepare a complex phase surface layer with more excellent performance, which is expected to achieve a greater comprehensive performance improvement of the surface layer of metal materials.KEY WORDS: metal material; surface nanocrystallization; gradient nanostructures; nanocrystallization mechanism; surface properties金属材料在基建工程、航空航天中扮演着重要角色，随着当今科学技术的高速发展，传统金属材料的局限性日趋明显，开发一种综合性能优异的金属材料迫在眉睫。

第39卷第3期2021年6月Vol.39No.3Jun.2021 Journal of Shaanxi University of Science&Technology文章编号：2096-398X(2021)03-0075-06三金属NiFeGa水滑石材料的制备及其电解水析氧性能杨阳，王雯洁，郭鹏飞，朱兵，杨骞楠，冯婉欣(陕西科技大学化学与化工学院，陕西西安710021)摘要：NiFe水滑石材料(NiFe-LDH)是非贵金属基析氧催化剂的基准催化剂.如何进一步提升该催化剂的本征活性是一个巨大的挑战.通过水热法将G#+嵌入NiFe-LDH成功的制备了三金属NiFeGa-LDH电催化剂.通过X射线衍射图、拉曼谱、傅里叶转换红外光谱以及扫描电子显微镜确定GF+成功的嵌入了NiFe-LDH结构中.电化学性能研究表明三金属NiFeGa-LDH具有良好的析氧活性和稳定性.在1.0M KOH溶液中，当电流密度达到10mA•cm"2时，该催化剂的过电势只有265mV,塔菲尔斜率为70mV•dec"1.这也许是因为Ga”促进了Ni与Fe电子之间的相互作用从而提升了该催化剂的本征活性.为未来设计和制备新型三金属基电催化剂提供了普适性策略.关键词：电催化；析氧反应；三金属NiFeGa水滑石材料中图分类号：0643.36文献标志码：APreparation of NiFeGa-LDH and its properties ofelectrolytic water oxygen evolvingYANG Yang,WANG Wen-jie,GUO Peng-fei,ZHU Bing,YANG Qian-nan,FENG Wan-xin(College of Chemistry and Chemical Engineering,Shaanxi University of Science&Technology,Xi'an710021 China)Abstract•NiFe layered double hydroxides(NiFe-LDH)is a benchmark catalyst among the non-pre-cious metal-based catalyst toward oxygen evolution reaction.How to further improve the intrinsic activity of NiFe-LDH is a huge challenge.In this paper,we successfully prepared the NiFeGa-LDH by hydrothermal method.Ga3+was incorporated into NiFe-LDH matrix,which was confirmed by X-ray diffraction,Raman,Fourier transform infrared spectroscopy?and scanning electron microscopy.Electrochemical measurements demonstrate that trimetallic NiFeGa-LDH has a highly intrinsic ac­tivity and long-term stability.The NiFeGa-LDH required an overpotential of265mV at the current density of10mA・cm-2and a low lafel slope of70mV・dec-1in1.0M KOH solution.This may because Ga3+can promote the electronic interaction between Ni and Fe in trimetallic NiFeGa-LDH.This work provides a general strategy for designing and synthesizing novel trimetallic-based electro­catalysts in the future.Key words:electrocatalysis;oxygen evolution reaction；trimetallic NiFeGa-LDH*收稿日期：2021-01-23基金项目：陕西省科技厅自然科学基金项目2020JQ-706)；陕西科技大学博士科研启动基金项目(2019QNBJ-04)作者简介：杨阳(1989-),男，甘肃镇原人，副教授，博士，研究方向：电催化剂•76•陕西科蛊尢曆層报第39卷0引言氢能由于其高能量比、零碳排放和来源广泛等特点被认为是本世纪最具应用潜力的清洁能源口②.电解水制氢是一种清洁绿色的制氢手段冏.电解水制氢过程中涉及两个半反应，即在阴极部分发生析氢反应，在阳极部分发生析氧反应.然而，析氧反应不仅具有缓慢的反应动力学，而且需要较高的过电势来克服反应能垒，这严重阻碍了电解水产氢技术的发展⑷.因此，急需发展一种能够加快析氧反应速率、降低反应所需过电势且具有良好稳定性的电催化剂.贵金属氧化物IrO2和R u O2具有良好的析氧反应性能，但是其昂贵的成本以及储量稀少等缺点限制了这些催化剂的大规模使用.因此，科研工作者致力于发展含有地球富集元素的高活性电催化剂.近年来，人们发现了一系列含有地球富集元素Ni和Fe的新型析氧电催化剂®门.如戴宏杰等刀发现在轻度氧化的多孔碳纳米管上生长超薄的镰铁水滑石(NiFe-LDH)纳米片具有很高的析氧活性.然而，如何进一步提高NiFe-LDH的析氧活性是一个新的挑战.由于金属元素的协同效应对提升电催化剂的本征活性具有良好的效果，而且此效应并不局限于两种金属元素之间.因此，科研工作者尝试在NiFe-LDH中嵌入第三种金属元素以期提升该材料本征催化活性.例如，孙晓明等固在NiFe-LDH中掺入V提升了该催化剂的本征催化活性.这是因为V的嵌入促进了Ni与Fe之间的协同效应并且调整了整个催化剂的电子结构•但是这些元素对该材料本征活性的提升依旧有限，这就要求科研工作者探索新的金属元素，以期能够大幅度提升NiFe-LDH的本征催化活性.地球富集元素Ga具有多种氧化态.最常见的GF+是路易斯酸阳离子，能够改变析氧反应过程中水分子的亲核能力和质子转移能力，从而促进多电子氧化还原阳离子Ni和Fe电子之间的相互作用,最终提升催化剂的本征活性•同时，氢氧化傢属于两性氢氧化物.这种性质有可能导致含有Ga元素的LDH基催化剂在析氧反应过程中由于傢的易刻蚀性而在催化剂中原位产生缺陷和空位，从而提升催化剂的本征活性.此外，Fe元素对NiFe-LDH的催化性能有很大的提升能力，而GF+与FM+具有类似的电子结构.因此6云+对催化剂的本征活性也许有提升作用.本文采用水热法成功的把GF+嵌入NiFe-LDH合成了三金属NiFeGa-LDH电催化剂.从电化学表征数据分析，三金属NiFeGa-LDH比双金属NiFe-LDH具有更好的析氧性能和优异的稳定性.此工作为未来设计和合成更高催化性能的金属基电催化剂提供了普适性策略.1实验部分1.1药品与仪器1.1.1实验药品实验所用到的药品及规格如表1所示.表1药品、规格及生产厂家药品名称规格生产厂家六水合硝酸镰(II)99.99%上海阿拉丁生化科技股份有限公司水合硝酸傢(IH)99.9%上海阿拉丁生化科技股份有限公司氟化钱98%上海阿拉丁生化科技股份有限公司尿素99%上海阿拉丁生化科技股份有限公司四水合氯化亚铁(II)98%上海麦克林生化科技有限公司氢氧化钾90%上海麦克林生化科技有限公司Nafion溶液5wt%西格玛奥德里奇(上海)贸易有限公司乙醇99.7%国药集团化学试剂有限公司1.1.2仪器X射线衍射仪(Bruker D8,德国)；拉曼光谱仪(DXR,美国)；傅里叶变换红外光谱仪(Bruker VECTOR-22,德国)；场发射扫描电子显微镜(Zeiss Sigma300,德国)；纯水机(UlupureUPR-III-10T,中国).1.2NiFeGa-LDH和NiFe-LDH的制备将0.1745g Ni(NO3)2•6H2O,0.0298g FeCl2•4H2O,0.0384g Ga(NO3)3•丄巴0,和0.1480g NH4F溶于20mL的超纯水中，再加入0.2402g(NH2)2CO充分溶解，配成前驱体溶液.取15mL前驱体溶液转移到含聚四氟乙烯内衬(25mL)的不锈钢高压反应釜中，在120°C的鼓风干燥箱中保温6h.自然冷却至室温后，依次用超纯水、乙醇在4000rpm的转速下离心洗涤三次，每次离心5min,随后在60°C的真空干燥箱中干燥过夜，得到的产物即为NiFeGa-LDH.NiFe-LDH的合成过程与上述流程类似，区别在于制备前驱体溶液时不加入傢源.1.3材料电化学性能表征1.3.1制备工作电极称取5mg催化剂置于600“L超纯水、400//L 乙醇和20“L5wt%Nafion的混合溶液中，超声至少30分钟至分散均匀.用移液枪吸取5“L悬浮液滴在已抛光好的玻碳电极上，在红外灯下烤干.在制备工作电极过程中，使用的玻碳电极均使用低速抛光机在0.05/zm的A12O3浆体下抛光5min.1.3.2电化学性能测试第3期杨阳等:三金属NiFeGa水滑石材料的制备及其电解水析氧性能•77•电化学性能测试选用CHI760E型电化学工作站.实验采用三电极系统进行测试，氧化汞电极(Hg/HgO)为参比电极,钳丝为对电极，负载了催化剂的玻碳电极作为工作电极*电解液为 1.0M KOH溶液，测试温度为室温.所测数据的电势均转换为可逆氢电极(RHE)，具体计算公式如式(1)所示；ERHE=EHg/Hgo+°・059XpH+0.098V(1)式(1)中：1.0M KOH溶液的pH为13.71,即：E rhe=EHg/Hgo+0*907V在电化学测试过程中，全程通入高纯氧气.循环伏安曲线(CV)的电势扫描范围为1.1〜1.7V (w.RHE),扫速为5mV・s3所有测得的CV曲线均经过溶液电阻校正，校正水平为95%.通过CV 法测得双电层电容(CR,测试条件如下:扫描范围为0.2〜0.3V(vs.Hg/HgO);扫速分别为20mV・s-1、40mV•s_1、60mV・s_1、80mV•s_1400mV-s_1和120mV・s'1.文中所有的电流密度均依据工作电极的几何面积(0.196cm")计算.最后,在电流密度为10mA・cnT?时，使用计时电位法对NiFeGa-LDH进行时长为6h的稳定性测试.2结果与讨论2,1LDH的表征通过X射线衍射仪对样品进行物相和晶体结构分析，其结果如图1所示.NiFeGa-LDH粉末样品的衍射峰与LDH材料的特征峰吻合(JCPDS No.40-0215)3门，表明成功制备了NiFeGa-LDH 且具有较高的结晶度，但随着前驱体溶液中Ga离子的掺入，位于23。

科技论文标题中“以……为例”英译用词探讨杨廷君;李跃平【摘要】Based on statics and analysis, we think the priority of the English word for Chinese “yi. . . weili” is “case”, then “example”. ln English, when it comes to the clusters with the word case, “the case of” and “a case study of” are preferable. lf the word “example” is to be used in the English version, “an example from” and “the example of” are better choices than other clusters.%探讨如何准确、恰当地英译科技论文的程式化用语“以……为例”。

统计和分析表明,英译“以……为例”的主要用词为case和example,优先考虑的单词是case,然后才是example。

单词case组成的词簇中,the case of和a case study of是最佳选择；而单词example组成的词簇中, an example from和the example of是最佳选择。

【期刊名称】《中国科技术语》【年(卷),期】2015(000)006【总页数】7页(P30-36)【关键词】论文标题;程式化用语;英译;“以……为例”【作者】杨廷君;李跃平【作者单位】宁波大学外语学院，浙江宁波 315211;西南民族大学外语学院，四川成都 610041【正文语种】中文【中图分类】N04;H315.9此次所提取的474篇学术论文，从时间分布上看，出现逐年递增的趋势，见表1。

由表1不难发现，2001年及之前包含“以……为例”的年均论文量没有超过10篇，2002—2008年徘徊在20至30篇之间，2009年之后迅速攀升，2012年接近70篇。

The Allure of Chinese Ink Painting: AnAssessment of Its Unique PerspectiveInk painting, a quintessential form of Chinese art, has captivated the world with its profound aesthetic and philosophical depth. From the delicate brush strokes that paint landscapes, flowers, and figures to the ink washes that create an atmosphere of tranquility, this artistic medium embodies the essence of Chinese culture and aesthetics.The allure of ink painting lies in its simplicity and complexity. On the one hand, the basic tools are minimal: ink, brush, paper, and water. Yet, these tools allow for an infinite array of expressions and interpretations. Theartist's mastery over the brush and ink is what transforms these basic elements into works of art that speak volumes about life, nature, and the universe.The perspective adopted by ink painting is unique. Unlike Western art, which often emphasizes realism and three-dimensionality, ink painting focuses on capturing the essence of a subject. It is less about reproducing reality and more about expressing the artist's feelings andunderstanding of the world. This perspective is reflected in the way ink painting emphasizes the harmony between man and nature, as well as the transient and impermanent nature of life.One of the most striking features of ink painting isits use of negative space. This artistic device allows the viewer to imagine and complete the picture, creating a sense of engagement and participation. The blank areas of the canvas are not just empty spaces; they are extensions of the painted elements, complementing them and adding depth to the composition.The color palette of ink painting is also noteworthy. Black ink, the primary color, represents infinity, emptiness, and the void. By manipulating the concentration and application of ink, the artist can create a range of effects, from deep, rich blacks to subtle gradations of gray. This limited color palette, coupled with the artist's skill in brushwork, creates a world of visual interest and emotional resonance.The assessment of ink painting as a perspective art type is complex yet rewarding. It requires a deepunderstanding of Chinese culture, history, and aesthetics. However, the rewards are immense. Ink painting offers a window into the soul of China, revealing its wisdom, philosophy, and aesthetic sensibilities. It is a form ofart that transcends boundaries and connects people across cultures and languages.Ink painting's评估价值 lies not only in its aesthetic beauty but also in its ability to evoke emotions and启发思考. It encourages us to look beyond the surface and appreciate the deeper meanings hidden within. In an increasingly globalized world, ink painting remains a powerful testament to the enduring charm and relevance of Chinese culture and art.**中国水墨画的魅力：对其独特视角的评估**水墨画，这种典型的中国艺术形式，以其深刻的美学和哲学内涵吸引了世界。

第 1 期第 101-107 页材料工程Vol.52Jan. 2024Journal of Materials EngineeringNo.1pp.101-107第 52 卷2024 年 1 月TiVNbTa难熔高熵合金的吸放氢动力学Hydrogen absorption-desorption kinetics ofTiVNbTa refractory high-entropy alloy龙雁1，2，张李敬1，2，杨继荣1，2，王芬1，2*（1 广东省金属新材料制备与成形重点实验室，广州 510640；2 华南理工大学机械与汽车工程学院，广州 510640）LONG　Yan1，2，ZHANG　Lijing1，2，YANG　Jirong1，2，WANG　Fen1，2*（1 Guangdong Provincial Key Laboratory for Processing and Forming ofAdvanced Metallic Materials，Guangzhou 510640，China；2 School ofMechanical and Automotive Engineering，South China University ofTechnology，Guangzhou 510640，China）摘要：通过真空电磁感应悬浮熔炼技术制备TiVNbTa难熔高熵合金试样，采用多通道储氢性能测试仪测试合金的吸放氢性能，并研究该合金的吸（放）氢行为及其动力学机制。

结果表明：单相BCC结构的TiVNbTa难熔高熵合金吸氢后生成TiH1.971，Nb0.696V0.304H和Nb0.498V0.502H2 3种氢化物新相。

氢化高熵合金粉末在 519 ，593 K和640 K 分别发生氢化物的分解反应，放氢后恢复单相BCC结构，因此TiVNbTa合金的吸氢反应属于可逆反应。

该合金在423~723 K温度区间具有较高的吸（放）氢速率，其吸（放）氢动力学模型分别符合Johnson-Mehl-Avrami （JMA）方程和二级速率方程，吸（放）氢的表观活化能E a分别为-21.87 J/mol和8.67 J/mol。

Structured catalysts for non-adiabatic applicationsEnrico Tronconi,Gianpiero Groppi and Carlo Giorgio ViscontiAlthough a number of advantages of structured catalysts over randomly packed catalyst pellets have been well known for many years,there is now growing interest in their potential for engineering the heat management in strongly exothermic or endothermic catalytic processes,primarily via an efficient heat conductive mechanism in their thermally connected structure.Herein we review the recent literature in this emerging field,addressing first conductive honeycomb monolith catalysts,and then metallic open-cell foams as well as metallic microfibrous entrapped catalysts.AddressesLaboratory of Catalysis and Catalytic Processes,Dipartimento di Energia,Politecnico di Milano,Piazza Leonardo da Vinci,32-20133Milano,ItalyCorresponding author:Tronconi,Enrico (enrico.tronconi@polimi.it )Current Opinion in Chemical Engineering 2014,5:55–67This review comes from a themed issue on Reaction engineering and catalysisEdited by Marc-Olivier Coppens and Theodore T.Tsotsis/10.1016/j.coche.2014.04.0032211-3398/#2014Elsevier Ltd.All rights reserved.IntroductionStructured catalysts consist in ceramic (e.g.Al 2O 3,cordierite,SiC)or metallic (e.g.stainless steel,Al,Cu)substrates,pre-shaped in the form of a single continuous structure with stable geometry (often a monolithic honeycomb matrix including many small parallel channels with openings in the order of one to few millimeters),over which the catalytically active sites are properly dispersed.It is well recognized that the high void fractions of such substrates,combined with the laminar flow prevailing in the channels,enable substantial reduction of pressure drop with respect to conventional packed beds of catalyst pellets.Also,the large geometrical surface areas and the thin catalyst layers in coated monoliths may diminish mass transfer limitations [1 ].Finally,the well-defined regular geometry,as well as the laminar flow conditions,enable accurate theoretical predictive modeling of mass,heat and momentum transport in honeycomb monolith catalysts,at least for fluid-solid systems [2 ].After their extremely successful commercial application to the control of automotive emissions and to the reduction of nitrogen oxides from power stations in the 1980s,honeycomb monoliths have become the standard catalyst shape in most applications of environmental catalysis [1 ].This motivated the study of the adoption of structured catalysts in other areas of heterogeneous catalysis.Particularly attractive were the expectedly lower pressure drop and the potentially smaller size of the reactor as compared to conventional pelletized cata-lysts in many gas-phase chemical processes.Early studies in this field,using methanation and hydrogenation as model reactions,pointed out however additional prospec-tive benefits.In a pioneering piece of work,for example,Tucci and Thomson [3]carried out a comparative study of methanation over ruthenium catalysts both in pellet and in honeycomb form:in addition to pressure drops lower by two orders of magnitude they found also significantly higher selectivities over the monolith catalyst,likely resulting from lower internal diffusional resistances.Par-maliana and coworkers [4–7]investigated the hydrogen-ation of benzene and dehydrogenation of cyclohexane in ceramic monoliths washcoated with alumina impregnated with either Ni or Pt:again,the low diffusional resistances in monolith catalysts enabled the authors to determine intrinsic Eley-Rideal rate expressions.In spite of the initial promising indications,however,over three decades later the use of monoliths as catalysts or catalyst supports in the processes of the chemical industry is still very limited.Two statements have so far mostly discouraged the extensive use of monolithic catalysts outside the well-known environmental applications [8 ]:a)the overall load of catalytically active phase in a washcoated monolith catalyst is less than the amount of catalyst in a bed of bulk pellets of comparable volume:this is not important for the fast,diffusion-limited reactions of environmental catalysis,but represents a clear disadvantage for the reactions under kinetic control usually met in the chemical process industry;b)conventional parallel channel monoliths are virtually adiabatic:this is compatible with the processes for the abatement of pollutants (e.g.NOx,VOCs)in diluted streams,but would severely limit the control of temperature in many endothermic and exothermic chemical processes,wherein heat exchange is often a crucial issue.In reality,both such concerns can be overcome by dedicated designs of structured catalysts,addressing the specific requirements of chemical applications.Wash-coat catalyst loadings in excess of 25%(v/v)are withinAvailable online at ScienceDirectthe range of what is feasible with monoliths nowadays: in combination with the enhanced effectiveness factors in the washcoat layers,this can be enough for several industrial processes.As presented in the following, conductive heat exchange in monolith structures can be even more effective than convective heat transfer in packed beds.Furthermore,new structured supports, like open-cell foams,are now being considered,which also show promising heat transfer properties.There remain,however,several more practical reasons which hinder the application of structured catalysts and supports to chemical syntheses,as well summar-ized in[9]:a)the many different pelletized catalysts operating inthe many processes of the chemical industry are often the result of long and costly development work,their properties are well tailored to the specific process needs and their performances are typically quite satisfactory:accordingly,replacement of the conven-tional catalyst technology with monolith catalysts requires very significant and proved benefits;b)the production volumes of industrial catalysts arelower by orders of magnitude as compared to the volumes of catalysts for the environment:thus,it is difficult to justify dedicated research efforts as well as capital investment to develop monolithic systems with intrinsic catalytic properties similar to those of conventional systems;c)the methods for loading,packaging,sealing andunloading structured catalysts in the synthesis reactors are different from those well established for pellet catalysts,and cannot be directly derived from the experience made in stationary environmental installa-tions:additional developments in this area are required,too;d)structured catalysts are intrinsically more expensivethan pellet catalysts.In essence,it appears that substantial improvements are required in order to motivate such a significant change of the catalyst technology.Notwithstanding such difficul-ties,however,there is a steadily increasing number of research activities concerning the use of monolithic and other structured catalysts/reactors in chemicals pro-duction[10 ].In fact,after the early phase when only sparse attempts were reported,multiple application areas have been now identified and rationalized in which monolithic catalysts may have intrinsically superior per-formance characteristics.One topic receiving great attention nowadays in view of its large industrial potential is the development of novel catalytic oxidation processes using structured reactors with extremely short contact times,whose largeflow rates would generate unacceptable pressure drops in packed-bed reactors.Manufacture of olefins via catalytic oxidative dehydrogenation of light paraffins and catalytic partial oxidation of hydrocarbons for syngas production are two important examples of processes in this area for which applications of monolithic catalysts have been envisaged, being facilitated by low pressure drops at highflow rates [11].Again in view of their reduced pressure drop,it has been recognized that monolith structures hold also a good potential for applications as pre-reactors and post-reactors of selective oxidation processes[8].Such applications still involve adiabatic operation of the structured reactors.A novel,innovative area of development is represented by the use of structured catalysts in chemical processes under non-adiabatic conditions.As mentioned above,the global heat transfer properties of honeycomb monoliths have been traditionally regarded as very poor,but recently monolithic structures and configurations have appeared with interesting characteristics for heat exchange:a new promising area is,for example,the use of honeycomb catalysts with high thermal conductivity in exothermal selective oxidation processes where multitubular reactors are employed;along similar lines,there is growing interest in the potential of open-celled foam(or sponges)as novel structured catalyst supports with enhanced heat exchange properties.Such an emerging application of structured catalysts/reactors is the focus of the present contribution: it is reviewed and discussed in the following(Figure1).56Reaction engineering and catalysisFigure 1(a)(b)(c)Current Opinion in Chemical EngineeringExamples of conductive structured substrates for exothermic orendothermic applications:(a)aluminum honeycomb monoliths,(b)aluminum open-cell foam,(c)copper open-cell foam.Honeycomb monoliths as non-adiabatic catalystsFor quite some time the use of monolithic catalysts in non-adiabatic reactors has been regarded as unfeasible due to poor radial heat transfer:indeed,ceramic honey-combs are made of essentially insulating materials;a pioneering theoretical analysis of Cybulski and Moulijn [12 ]evidenced that commercial monolith structures consisting of corrugated metal sheets exhibit modest heat transfer performances,too.Nonetheless,the thermally connected nature of the monolith supports provides in principle for an alternative mechanism of radial and axial heat transport,namely heat conduction,which is essentially not available for random packings of catalyst pellets.The conduction within the solid phase of the pellets in fact is almost negligible,since only point contacts exist between the pellets,and con-vection in the gas phase dominates as the primary mech-anism for heat exchange.Accordingly,the only practical way of enhancing heat transfer is to increase the flow velocity,but this is limited by the pressure drop,which grows more than linearly with flow rate.By using mono-lith honeycomb structures with parallel channels as cat-alyst elements no radial transfer of gas may exist,but the contribution of thermal conduction through the solid phase (i.e.the monolith matrix)can become quite sig-nificant if suitable materials and geometries are adopted.The effective axial heat conductivity of monolith sub-strates k e,a is readily estimated ask e ;a ¼k s ð1Àe Þ(1)with k s is the intrinsic thermal conductivity of the support material and e is the monolith void fraction (or open frontal area,OFA).Early attempts to model radial heat conduction in mono-liths,also including comparison with experimental data,were published in [13–15].Based on a simple analysis of heat conduction in the unit cell of a honeycomb monolith with square channels according to an electrical network analogy,Groppi and Tronconi [16 ,17]derived approxi-mate predictive equations for the effective radial thermal conductivity in washcoated monoliths,with square and equilateral triangular channels.Hayes and coworkers [18]validated such equations against numerical solutions of the temperature field in honeycomb structures,finding maximum deviations of less than 20%for a typical mono-lith void fraction of 75%.They derived also an alternative equation,based on a different (parallel)arrangement of the resistance network,which improved somewhat the prediction accuracy.Recently,Visconti et al .[19 ]have published a simple ‘symmetric’model,which provides the best match with the exact T-field:neglecting the contribution of the catalytic washcoat,as well as the minorcontribution of heat conduction in the gas phase,the symmetric model yields.k e ;r ¼k s1Àe 1þe(2)As Eq.(2)shows,the effective radial conductivity k e,r is directly proportional to the intrinsic thermal conductivity of the support material,k s :thus,the adoption of highly conductive materials is expectedly very beneficial for the enhancement of radial heat transfer in monoliths.In Figure 2estimates of k e,r according to Eq.(2)are plotted versus the monolith open frontal area e for honeycomb structures made of metallic and non-metallic materials with different intrinsic thermal conductivity.It should be emphasized that when highly conductive materials (Cu,Al,SiC)are used the estimates of k e,r in Figure 2become one order of magnitude greater than the effective radial thermal conductivities in packed beds of catalyst pellets,which under typical conditions of selective oxi-dations in externally cooled multitubular reactors are typically in the range 2–5W/m/K [20,21].The plot shows also that the radial effective conductivity is adversely affected by large monolith void fractions e .These evaluations point out that heat exchange in mono-lithic structures can be made more efficient than in pellets,but monolith supports with specific designs must be adopted,based on a dedicated selection both of the monolith geometry and of the fabrication material aimed at minimizing resistances to conductive heat transfer.Notably,the existing commercial monolith substrates,developed for the adiabatic applications of environmentalStructured catalysts for non-adiabatic applications Tronconi,Groppi and Visconti 57Figure 20.50.60.70.80.91.0020406080100AISI 304SiCAlN AlCuMonolith void fractionR a d i a l e f f e c t i v e c o n d u c t i v i t y , W /(m K )Current Opinion in Chemical EngineeringEffect of material properties and monolith void fraction on the radial effective thermal conductivity of honeycomb monoliths with square channels.Based on Eq.(2).catalysis,were not originally designed for that purpose: neither the construction material nor the geometry of such supports is optimized for heat conduction.In fact, the intrinsic conductivity of ceramic honeycombs is very low,whereas the available metallic monolith structures are made of poorly conductive alloys(e.g.FeCrAlloy)and are assembled by piling up and rolling corrugated sheets which result in poor thermal contact.Finally,in com-mercial monoliths the open frontal area is kept as high as possible,typically0.7–0.8for ceramic monoliths and 0.85–0.95for metallic ones,so as to match the severe pressure drop constraints of environmental processes:this is also negative for radial heat transfer.Based on the above considerations,heat conduction in the walls of monolith structures can be exploited in principle as an effective heat transfer mechanism for exothermic or endothermic catalytic processes:published studies con-cerning such applications are summarized in the following. Groppi and Tronconi have systematically investigated the potential of novel monolithic catalyst supports with high thermal conductivity in view of replacing conven-tional packed beds of catalyst pellets in multitubular reactors for gas/solid selective oxidations[16 ,17,22–25].Starting from the evaluation of effective radial ther-mal conductivities in monolith structures(Figure2),they predicted that in principle the radial heat transfer infixed-bed gas/solid reactors could be substantially enhanced when changing the dominating heat transfer mechanism from convection to conduction.This would be a very important result,since both the design and the operation of technical packed-bed reactors are limited at present by the removal of the reaction heat,which occurs by con-vective transport from the randomly packed catalyst pel-lets to the reactor tube walls:therefore limits on the reactor tube diameter of1–1.5inches as well as very high gasflow rates are typically required to prevent unaccep-table hot spots.Significantly improved radial heat trans-fer,on the other hand,would bring about reduced risks of thermal runaway,better thermal stability of the catalyst, improved selectivity,as well as potential for novel designs of industrial reactors with incremented throughputs and/ or enlarged tube diameters,corresponding to reduced investment costs.In order to assess such prospective advantages,the ther-mal behavior of‘high conductivity’monolith catalysts in exothermic reactions was investigated both by simu-lations and by experiments.This early work,focused on selective oxidation processes,is reviewed in[26 ]. As an important result,it was recognized that,while the early models assumed no heat transfer resistance between the monolith catalyst and the coolant,actually a thermal contact resistance can be expected at the inter-face between the monolith and the inner reactor tube wall (‘gap’resistance),as detected also in the experimental investigations[25,27].Calculations predict that such a resistance becomes crucial for the onset of hot spots in the ethylene oxide reactor whenever the corresponding‘wall’heat transfer coefficient is less than about500W/(m2K) [17].Accordingly,solutions aimed at achieving effective thermal contact between the honeycombs and the reactor tubes(‘packaging’methods)represent an important de-velopment goal,which must be necessarily pursued in connection both with the manufacturing technologies of monolithic catalysts and with the specific features of the individual catalytic processes.The‘gap’resistance was further rationalized in sub-sequent work[26 ,28 ].It was shown that the associated heat transfer mechanism relies primarily on heat conduc-tion across the stagnant gasfilm trapped in the gap between the monolith and the reactor tube.In fact,the gap resistance was inversely proportional to the gap size d evaluated at the reaction conditions(so differential ther-mal expansion of the monolith and tube materials should be considered)[28 ],and directly proportional to the gas-phase conductivity k g,as evidenced by heat transfer measurements over Cu monoliths using N2–He mixtures of different compositions[26 ]:h w%k gd(3)h w estimates in excess of700W/(m2K)were obtained when using pure He[26 ].Although the experimental results reported above were all collected at the laboratory scale,afirst proof-of-con-cept at an industrial scale has been recently reported in the open literature[29 ],involving a campaign of o-xylene oxidation runs in a tubular pilot reactor loaded with washcoated conductive(aluminium)honeycomb catalysts and operated under representative conditions for the industrial production of phthalic anhydride(PA). In a preliminary exploratory phase structured supports(Al slabs and honeycombs)were washcoatedfirst with a primer(dispersible boehmite)and then with a V2O5/ TiO2-based precursor powder for industrial o-xylene oxi-dation catalysts(Polynt).The coating procedure wasfirst validated by isothermal kinetic runs in a lab-scale micro-reactor(i.d.=12.6mm)loaded with a washcoated Al slab shaped in the form of a spiral(3cmÂ15cm,total active catalyst mass=400mg),covering a range of representa-tive temperatures(320–4008C)and o-xylene feed con-tents(1–3%,v/v).For the pilot reactor runs,sixteen Al honeycombs supplied by Corning Inc.(26cpsi, o.d.=24.4mm,length=10cm)washcoated with a total catalyst mass of46g were loaded in the upper part of an industrial pilot reactor(Polynt)consisting of a single jacketed tube(length=3m,i.d.=24.6mm)cooled with molten salts.The tube loading was completed with inert rings.Axial T-profiles were recorded by a thermocouple sliding in a2mm o.d.thermowell inserted in the central58Reaction engineering and catalysischannel of the honeycombs.The pilot reactor was oper-ated continuously for over1600hours.After startup,the airflow rate was kept at4Nm3/h and the o-xylene feed load was progressively increased from120to400g/h while adjusting the salt bath temperature to keep the measured hot spot temperature around4408C.Figure3a compares the axial T-profile from one of such runs at reference industrial conditions for PA production(o-xylene feed concentration=80g/Nm3)with a T-profile measured in the same pilot reactor loaded with conven-tional catalyst pellets(rings)and operated at the same conditions with a similar hot spot temperature.The Al honeycomb supports afforded substantially reduced axial T-gradients,and enabled operation of the reactor with a much higher salt bath temperature(3928C vs.3598C in the case of rings):the maximum T-difference with the salt bath was halved(Figure3b)and the average bed temperature was therefore about208C higher.T-gradi-ents were still moderate at o-xylene loads close to100g/ Nm3,an upper limit for the current industrial PA packed-bed reactors technology.The Al honeycombs were suc-cessfully unloaded at the end of the pilot reactor runs.A strong enhancement of radial heat transfer rates(%2x) associated with the use of novel monolithic catalysts with high thermal conductivity has been thus demonstrated at an industrial scale for thefirst time.Such a unique im-provement can be exploited for intensification of the PA process in a number of ways,for example,to increase the o-xylene feed load>100g/Nm3(and the PA productivity accordingly)in existing technical reactors,or to design new reactors with larger tube diameters.In more general terms,the results herein presented,being obtained with substrates and under conditions representative of real applications,appear quite encouraging in view of practical implementations of‘high conductivity’monolith catalysts in selective oxidation processes.Another area of growing interest for conductive monolith catalysts is represented by exothermic CO hydrogenation processes(syngas chemistry).In the well known low-T,low-P exothermic methanol synthesis over Cu-based catalysts,temperature control is a crucial aspect in order to optimize selectivity and catalyst lifetime.Work from Holmen and coworkers [30 ,31]showed improved performances over coated metallic structured substrates,and ascribed them to the better thermal properties of the structured systems, resulting in a more uniform temperature distribution. The application of conductive(copper)monolith(and also open-cell foam)catalysts to the methanol synthesis has been recently investigated in our labs in collabor-ation with Total Petrochemicals[32–34].Specifically, simulation results have pointed out that radial heat transfer in technical Lurgi-type multitubular packed-bed converters is very efficient due to highflow velocities,which require however long tubes(e.g. 8m).Reduction of the tube length in view of compact methanol synthesis units for exploitation of small natural gas reservoirs or biomass would be therefore unfeasible because of the dramatic loss of convective heat transfer performance.On the other hand the conductive heat transfer mechanism of structured systems is essentiallyStructured catalysts for non-adiabatic applications Tronconi,Groppi and Visconti59 Figure3Temperature profiles of the gas phase(left)and maximum temperature difference(right)in o-xylene oxidation pilot runs with ring pellets or Al honeycombs.The maximum temperature difference is defined as the difference between the hot-spot temperature and the temperature of the cooling salt bath.Source:Reprinted with permission from[29 ].Copyright2014American Chemical Society.flow independent,so that shortening of the tube length would bring about in this case no significant loss in radial heat transfer efficiency.The low-temperature Fischer–Tropsch(FT)synthesis for the production of synthetic fuels is a strongly exother-mic process of growing strategic importance,wherein temperature control is a crucial issue.The adoption of honeycomb monoliths as supports for cobalt based FT catalysts was proposed at the beginning of this century by the research groups of Holmen at NTNU Trondehim [35 ]and of Moulijn at TU Delft[36],and considered a few years later also by the group of Turek at Clausthal University of Technology[37,38].Their works refer however to conventional ceramic monoliths,similar to those widely adopted for environmental applications, which are essentially made of thermally insulating materials.FT Reactors are therefore operated adiabati-cally and the heat removal issue is solved by recycling to the reactor a fraction of the liquid products pre-cooled in an external heat exchanger.For this reason such units are referred to as monolith loop reactors(MLR).Co-feed of a liquid phase is managed so as to operate the reactor in the slugflow or Taylorflow regime(i.e.the liquid phase is recirculated at high liquidflow rates),where high mass transfer coefficients can be achieved[38].Such reactors have been successfully used in experimental studies at the lab scale[35 ,37,39–41]and their performances at the industrial scale have been simulated by mathematical models[38,40],but this solution has received scarce attention by the industrial world so far.Upon attempting the development of a mass-transfer and heat-transfer enhanced catalytic systems,the use of home-made or commercial monolith structures consisting of corrugated metal sheets has been also proposed[42–44]as an alternative to ceramic honeycomb monoliths for the FTS.However such structures exhibit modest heat transfer performances even assuming an infinitely high wall heat transfer coefficient,as evidenced by the theor-etical analysis of Cybulski and Moulijn[12 ]and by the experimental work of Boger and Heibel[28].A related technology,named monolith loop catalytic membrane reactor(ML-CMR),was proposed in2005by the US company CeraMem[45]mainly to overcome the limita-tions related to the segregatedflow typical of classical honeycomb monoliths.The potential of conductive honeycomb catalysts for the FTS in multitubular reactors is actively investigated in our group at Politecnico di Milano.Preliminary results concerning both washcoating of aluminum structured supports with increasing geometrical complexity and lab scale FT tests over such catalysts were published in[46].Aluminum was selected as the material for the structured supports because of(i)the excellent intrinsic thermal conductivity(200W mÀ1KÀ1[47]),(ii)the chemical inertia at the actual FTS process conditions,(iii)the low density,(iv)the chemical affinity with the morphological support of the adopted catalyst and(v)the reasonable volumetric cost.Well-adherent washcoat layers with thickness less than45m m were obtained by dip-blowing aqueous slurries of a representative Co/g-Al2O3catalyst onto a27cpsi extruded aluminum honey-comb monolith supplied by Corning Inc.The thickness of the washcoat layer was chosen considering both the need of maximizing the catalyst loading in the reactor and the threshold limit of40–50m m reported in the literature for the onset of intraporous mass-transfer restrictions[48]. Comparative tests with the original powdered catalyst and the washcoated monoliths,carried out in a lab-scale tubular reactor at industrial process conditions,confirmed the adequacy for the FTS of such structured catalysts, which resulted stable with time on stream while showing activities and selectivities similar to those of the original catalyst powders[46].In a subsequent paper[49]the adoption of conductive honeycomb catalysts in tubular reactors for the FT was investigated by means of a pseudo-continuous,hetero-geneous,two-dimensional mathematical model of a single reactor tube.Simulation results indicate that extruded aluminum honeycomb monoliths,wash-coated with a Co/g-Al2O3catalyst,are promising for the application at the industrial scale,in particular when adopting supports with high cell densities and catalysts with high activity.Limited temperature gra-dients are in fact predicted even at extreme process conditions,thus leading to interesting volumetric reac-tor yields with negligible pressure drop.Flat axial and radial temperature profiles have been simulated along the catalytic bed,showing the unique ability of the adopted structured catalysts to manage the heat removal issue and to guarantee an excellent tempera-ture control,that is crucial in FTS(see Figure4). Notably,this result can be achieved without recycling to the reactor large amounts of liquid hydrocarbon products in order to remove the reaction heat,as opposite to existing industrial Fischer–Tropsch packed-bed tubular reactors.At the present stage of development the crucially T-sensitive Fischer Tropsch synthesis appears as one of the most promising areas of application of conductive structured catalysts.Further developments are therefore expected in the near future.In the samefield,a compet-ing solution,though based on somewhat similar prin-ciples,is represented by the so called microchannel reactor technology[44,50–57],whose distinguishing feature is also the very high efficiency of reaction heat removal.A major difference with the conductive struc-tured catalysts concept is that microchannel systems introduce a totally new reactor technology,which is intrinsically more complex and expensive and still needs to be fully proven at the industrial scale.60Reaction engineering and catalysis。