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48 The Electrochemical Society Interface Spring 2006

High TemperatureMaterialsby Karl E. Spear, Steve Visco,Eric J. Wuchina, and Eric D. Wachsman

The interdisciplinary nature of ECS is uniquely reflectedwithin the High Temperature Materials (HTM) Division. Here,scientists and engineers are concerned with the chemicaland physical characterization of materials, the kinetics ofreactions, the thermodynamic properties and phase equilibriaof systems, the development of new processing methods,and ultimately, the use of materials in advanced technologyapplications at high temperatures. The mission of the HTMDivision is to stimulate education, research, publication,and exchange of information related to both the science andtechnology of high temperature materials, which include

ceramics, metals, alloys, and composites. While seeking tofulfill its mission, a continuing goal of the Division is to helpensure the development of new materials and processes toovercome the limitations that currently hold back advancesin technology. These advances include more efficient andcleaner energy sources and storage systems; smaller and morereliable electronic, magnetic, optical, and mechanical devices;a wider variety of technologically useful chemical sensors andmembranes; lightweight, corrosion-resistant structural materialsfor use at elevated temperatures in extreme environments; andeconomic methods for recycling and safely disposing of our

waste materials.

What is High Temperature?

A definition of high temperature canhe confusing. One often used definition

in materials science and technologyis that it is a temperature equal to,or greater than, about two-thirds ofthe melting point of a solid. Anotherdefinition, attributed to Leo Brewer,is that high temperatures are those atwhich extrapolations of a materialsproperties, kinetics, and chemical

behavior from near ambient temperatures

are no longer valid. For example,chemical reactions not favorable

at room temperature may becomeimportant at high temperaturesthermodynamic properties rather thankinetics tend to determine the hightemperature reactivity of a material.Vaporization processes and speciesbecome increasingly important at hightemperatures. Unusual compounds andvapor species, which do not conformto the familiar oxidation states of the

elements, may form. For example, in

the vaporization of Al2O

3(s), common

high temperature gas species can include

Al2O, AlO, and AlO

2. The complexity

of the vapor phase also increases withtemperature; BeO(s) vapor speciesinclude not only the elemental vapors,but at high temperatures, also significant(BeO)

nspecies, with n= 1 to 6. While

the vaporization of BeO(s) in air to theelements is suppressed by the oxygen, the

(BeO)nvapor pressures are independentof air, and can produce much largeractive corrosion rates than those

calculated using only the elemental gasspecies. With increasing temperatures,ordered defect structures becomedisordered, and solid solution rangesincrease significantly. For example,stoichiometric solids such as MgAl

2O

4

may develop significant compositionranges at high temperatures. Physicalproperties of materials that correlate withthe above high temperature chemical

behavior are also unpredictable fromextrapolations of low temperatureproperties.

Examples of High TemperatureActivities and Materials

High temperature materials providethe basis for a wide variety of technologyareas, including energy, electronic,

photonic, and chemical applications.While some applications involve the useof these materials at high temperatures,others require materials processed athigh temperatures for room temperatureuses. In electrochemistry, the interactionof these materials with each other,

the atmosphere, and the movement ofelectrons are of high importance. Thehigh value of a cross-cutting technologysuch as high temperature materials to

a wide variety of technical arenas isreflected by the number of science andengineering disciplines involved inthe study of processing and propertiesof these materials, including ceramicscience, chemistry, chemical engineering,electrical engineering, mechanicalengineering, metallurgy, and physics.The diversity of interests ranges fromexperimental observations to predicting

behavior, from scientific principles to

engineering design, from atomic scalemodels to performance while in use. Afew examples of the diversity of hightemperature materials and applicationsare summarized below.

High Temperature Materials Processing.High temperature materials of interestinclude not only advanced alloys, butalso oxide and non-oxide ceramics andvarious composite materials. In additionto the chemical and physical propertiesthat make a material important fortechnology, the ability to synthesize the

material in physical forms ranging from

powders to thin films to bulk pieces of

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The Electrochemical Society Interface Spring 2006 49

varying macroscopic sizes and shapes iscrucial to their applications. Particle size,

grain size, surface structure, chemicalpurity, crystalline perfection, and degreeof crystallinity and homogeneity are allimportant parameters that may criticallyinfluence the end-use properties of amaterial. The ability to tailor-make anengineered material to within precisespecifications requires an enhanced

scientific understanding of mechanisticprocesses that convert startingmaterials to end product. In many

cases this involves high temperatureprocesses, such as sintering for particledensification, or chemical vapordeposition for film growth. The resultingmaterial or device may then utilizefrozen-in chemistry and microstructurefor operation at room temperature, orfor certain devices such as fuel cellsoperating at high temperature.

High Temperature Fuel Cells.International concerns regarding the

emission of greenhouse gases andthe trend toward distributed power

generation are of current interest tothe technical community. Accordingto the most recent U.S. Department ofEnergy reports, an extensive expansionof installed generating capacity will berequired to meet projected electricitydemand. U.S. electricity generatingcapacity is expected to grow from920 GW in 2003 to 1186 GW in2030. Worldwide installed electricity

generating capacity is expected to growfrom 3315 GW in 2002 to 5495 GW in2025. Clearly, the utility market will

respond to the increased demand. Theimportant question is how this demandcan be satisfied without simultaneouslyincreasing greenhouse gases and otherharmful emissions. Among the variousfuel cell technologies, solid oxide fuelcells (SOFCs) and molten carbonate fuelcells (MCFCs) are among the best suited

for distributed power generation due totheir high system efficiency and abilityto reform natural gas internally.

SOFC configurations, as illustratedin Fig. 1a and b, are an example of acomplex electrochemical system with

challenging high temperature materialsproblems. The electrolyte must conductions (such as O2), but not electrons,while the electrodes must conduct theelectrons generated by the electrodereactions. In addition, the tubes instructural components must be gastight

and mechanically stable at hightemperatures. This requires minimizingthermal expansion differences amongthe components, and developinggastight seals for the high temperatureuse. Developing the technology forproducing components that meet thesestringent property requirements requires

processing schemes that produce specific

types of micro- and macrostructures.In addition, the composite componentsmust be chemically compatible with eachother and with the fuel. These majorhigh temperature materials challengesappear overwhelming, but they havebeen met; and improved materials

and processes are continually beingdeveloped.

Recent advances in materials selectionand microstructure, combined with

fabrication of electrode-supportedthin-electrolyte planar geometries, hasresulted in tremendous performancegains. Current advanced planar SOFCshave demonstrated ~2 W/cm2at the celllevel, at 700C. These power densitiesare greater than previous generationcells at 1000C, thus, providing theopportunity to utilize less expensive

metal interconnects. However, the use ofmetal interconnects brings with it newchallenges in high temperature corrosionprevention.

High Temperature Corrosion. In addition

to oxide ceramics, which include notonly the SOFC materials along with

new sensors and high temperaturesuperconducting materials, silicon-basedceramics such as SiC, Si

3N

4, and sialons

along with other borides, carbides,nitrides, silicides, and diamond anddiamond-like materials are now commonhigh temperature materials of scientific

and technological interest in bothbulk and coating configurations. SiCand Si

3N

4have properties of value for

advanced microelectronic applications as

well as for use as lightweight structuralcomponents at high temperatures.Single crystal SiC can be used as ahigh temperature semiconductor, andSi

3N

4and its oxynitride Si

2N

2O provide

excellent insulating coatings in device

production. As bulk ceramics, bothmaterials are lightweight and can beused in structural applications at highertemperatures than is possible for metalalloy systems. Also, SiC particles, fibers,and weaves have been used extensivelyin composite materials developedfor lightweight, high temperaturestructural applications. However, a

Fuel Flow

Interconnection

Electrolyte

AirElectrode

AirFlow

Fuel Electrode

FIG.la.Schematic diagram of a state-of-the-art air electrode support (AES) cell for SOFCs.The porous air electrode tube is doped LaMnO

3[2.2 mm thick, extrusion sintered]; the

electrolyte is ZrO2(Y

2O

3) [40 m, electrochemical vapor deposited] ; the inte rconnection is

doped LaCrO3[85 m, plasma sprayed]; and the fuel electrode is NiZrO

2(Y

2O

3) [100 m,

slurry spray-electrochemical vapor deposited].

FIG.lb.Schematic diagram of a SOFC bundle configuration. (Fig. la and

b courtesy of Siemens Westinghouse Power Generation.)

(continued on next page)

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50 The Electrochemical Society Interface Spring 2006

major problem of corrosive oxidation

at high temperatures must not only beminimized, but as important, it must beunderstood so component lifetimes areavailable for design engineers. At lowoxygen partial pressures, these silicon-based materials actively oxidize to SiO(g),but at higher pressures, a protective SiO

2

glassy coating forms on the surfaces ofthe component, dramatically slowingthe oxidation rates. These rates can be

predicted reliably. However, complexcombustion gases contain active speciesin addition to oxygen which greatlyenhance degradation rates. More recently,the enhanced water vapor corrosion ofsilicon-based ceramics in hydrocarbon-fuel combustion environments has ledto an entirely new research area. Theformation of volatile Si(OH)

4in gas-

turbine engines for both electricitygeneration and propulsion applications

was not considered a major materialdegradation mechanism a few years ago.However, the shortened service livesof combustion liners, turbine blades,and vanes has led to the developmentof environmental barrier coatings(EBCs) to protect the base load-bearingmaterial, either SiC/SiC

fcomposites or

monolithic Si3N

4. These coatings are

tailored to be thermodynamically stable

in water vapor at very high combustiontemperatures, protecting the structuralintegrity of the components. Much ofthis work has focused on celsian-based(BaOAl

2O

32SiO

2) and rare-earth (RE)

silicate (RESiO5) coatings.The high temperature oxidation of SiC

in combustion atmospheres is of highinterest to the technical community.An example of the materials problemsencountered in high temperatureapplications is the effects of alkali amidsulfur impurities in the combustion gasesthat lead particularly to shortened service

life and system failure. Figures 2 and 3illustrate the effects of alkali (Na) duringthe oxidation of SiC in a combustionatmosphere. The Na

2CO

3forming on the

surface of the SiC from the combustion

gas components reacts with SiO2whichis produced in the oxidation of SiC.The sodium silicate glass that forms notonly releases CO

2, but also eliminates

the protective SiO2layer which forms in

a clean oxygen environment. The highrates of oxidation can then continue,causing a pressure buildup at the SiCinterface which produces gas bubbles,as shown schematically in Fig. 2. Figure2 shows a mechanism that allowsa SiO

2-rich layer to form at the SiC

interface at longer times, again providingsome protection and slowing the rate ofoxidation to a point that further bubble

formation is eliminated. The effects on

the SiC component are shown in the

micrographs in Fig. 3.High temperature materials research

in the metals and alloys area is still anextremely important field, and new alloyand composite systems are continuallybeing developed for new applications.The ferrous metallurgy of early dayshas also broadened to include a broadspectrum of high temperature tungsten,tantalum, titanium, nickel, and NiCrAl-based alloys for various applications such

as in high temperature gas turbines.

Brief History of HTM Division

The roots of the High TemperatureMaterials Division can be tracedback to 1921 when it was foundedas the Electrothermics Division,the first formalized Division of TheElectrochemical Society. Some Society

technical committees that mergedin 1921 to form the ElectrothermicsDivision were Electrodes, Carbons;Carbides, Abrasives, Refractories;FerroAlloys; and Electrometallurgy. In1954, in recognition of the importanceof high temperature alloys in futuretechnological developments, theElectrothermics Division changed

its name to the Electrothermics and

Metallurgy Division. However, the

broadening diversity of materials andtheir synthesis and/or applications athigh temperatures continued duringthe next twenty-five years, and wereaccompanied by a broadening of themembership and activities of theDivision. The result was a change in 1982to the Divisions current name, High

Temperature Materials. The focus of theDivision has expanded significantly fromits origins in high temperature materialschemistry and corrosion to encompasshigh temperature electrochemicalsystems such as SOFCs, ionic membranes,sensors, and the science and technology

of chemical vapor deposition andrelated processes. The High TemperatureMaterials Divisions activities andinterests cut across many scientific and

technical disciplines, and serve the needsand interests of Society members inseveral divisions.

HTM Division Activities

The above-stated mission ofthe High Temperature MaterialsDivisionto stimulate education,research, publication, and exchange

of information related to the science

and technology of high temperaturematerialshas been most actively

High Temperature Materials(continued from previous page)

FIG.2.Schematic of pitting forming via bubbles in the oxidation of SiCin combustion gasses containing alkali metal salts. (1) Initial stages, (2)intermediate stages, and (3) later stages. (Courtesy of N. Jacobsen and J.Smialek at NASA Glenn Research Center.)

FIG.3.Photomicrographs illustrating pit formation during oxidation ofsintered SiC in a combustion atmosphere. A mechanism explaining theformation is given in Fig. 2 (Courtesy of N. Jacobsen and J. Smialek atNASA Glenn Research Center.)

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The Electrochemical Society Interface Spring 2006 51

pursued by sponsoring and cosponsoringinternational symposia, many of which

produce proceedings volumes. Theobvious overlap of interests between theHTM Division and those of other ECSDivisions has resulted in many symposiabeing cosponsored by HTM with otherDivisions. Major symposia sponsored orcosponsored by the Division include

Chemical Vapor Deposition.This

ongoing series of symposia started in1967, and has continued with meetingstypically held every two to three years.The extended lifetime of this series isa good measure of the ever increasing

importance of thin films and coatingsin modem technology. A diverse arrayof topics in CVD including fundamentalprinciples of gas phase and surfacechemistry, kinetics and mechanisms,thermochemistry, mass and energytransport, fluid dynamics, and precursordesign and synthesis, as well as topics inmodeling and experimental verification

of CVD processes. These symposia alsocover applications in optical materials,

semiconductors, superconductors,insulators, and metals. Further topicsinclude dielectrics, ferroelectrics,magnetic materials, nuclear materials,hard coatings, refractories, organicmaterials, thermal and environmentalbarrier coatings, as well as multilayers,and solid lubricants.

Solid Oxide Fuel Cells and MoltenCarbonate Fuel Cells.Increasing world-wide interest in fuel cells for cleanand efficient electrochemical powergeneration has resulted in a large

international research and developmenteffort. The HTM Division has provided afocal point for the technical communityto present and discuss its findings byorganizing successful symposia on bothSOFCs and MCFCs. The highly successfulbiennial symposia on high temperatureSOFCs developed a particularlystrong international following, withmeetings in this series also held inGreece, Japan, and Germany. Topics

in the area of solid-state ionic devicesinclude modeling and characterizationof defect equilibria, theories for ionic

and electronic transport, studies ofinterfacial and electrocatalytic propertiesof ion conducting ceramics, and novelsynthesis and processing of thin filmsand membranes.

High Temperature Corrosion andMaterials Chemistry. Extensive work hasbeen carried out since the pioneeringwork of Carl Wagner to understandand reduce the deleterious effects on

materials exposed to harsh environmentsat high temperatures. The HTMDivision remains a focal point forscientists and engineers to discuss new

measurements and understanding inthis technologically important high

temperature field. Measurements andpredictions of the high temperature

chemistry related to processing,fabrication, behavior, and properties ofmaterials systems in both reactive andnonreactive environments are topicsof general interest. Session topics haveincluded fundamental aspects of hightemperature oxidation, high temperaturecorrosion, and other chemical reactions

involving inorganic materials athigh temperatures. One objective ofthese symposia is to encourage the

development of theoretical models, basedon experimental results, which allow theprediction of high temperature reactionsand the rates at which they occur.Real-world problems such as alloy andceramic oxidation, molten salt corrosion,volatilization reactions, and coatingdurability in complex environmentsare all addressed in these symposia.These issues are of interest for such

diverse applications as power generation,aerospace propulsion, metal halide lampdesign, and waste incineration.

Ionic and Mixed Conducting Ceramics.This field has attracted a large body ofresearchers worldwide and has grownrapidly in the past decade. Topicscovered in recent symposia include ionictransport in solid electrolytes, mixedconduction in ceramics, electrocatalyticphenomena, electrochemical processesfor hydrocarbon conversion, electrodereactions in solid-state electrochemical

cells, batteries and fuel cells involvingceramic components, thin-film ceramicmembranes, and applications in ceramic

sensors.

Solid-State Ionic Devices. Solid-stateelectrochemical devices, such as batteries,fuel cells, membranes, and sensors, arecritical components of technologicallyadvanced societies in the 21st centuryand beyond. The development of thesedevices involves common researchthemes such as ion transport, interfacialphenomena, and device design andperformance, regardless of the class of

materials or whether the solid state isamorphous or crystalline. The intent ofthis international symposia series is to

provide a forum for recent advances insolid-state ion conducting materials andthe design, fabrication, and performanceof devices that utilize them.

Conclusion

As stated previously, it is primarilythe ECS High Temperature MaterialsDivision that is concerned with thechemical and physical characterizationof materials, the kinetics of reactions,the thermodynamic properties andphase equilibria of systems, thedevelopment of new processing methods,and ultimately, the use of materials inadvanced technology applications at

high temperatures. As more complex anddiverse problems are encountered, the

mission of the HTM Division becomesincreasingly important to stimulateeducation, research, publication, andexchange of information related to boththe science and technology of hightemperature materials. Efforts to fulfillthis mission will continue through theDivisions major activity of sponsoring

and cosponsoring international symposiato promote communication among theinterdisciplinary groups investigating

high temperature materials.

References

1. U.S. Department of Energy, EnergyInformation Administration, InternationalEnergy Outlook 2005, DOE/EIA-0484 (2005),

July 2005. www.eia.doe.gov/oiaf/ieo/index.html

2. U.S. Department of Energy, EnergyInformation Administration, Annual EnergyOutlook 2006 with Projections to 2030(Early Release), Dec 2005. www.eia.doe.gov/oiaf/aeo/index.html

3. Solid Oxide Fuel Cells IX, S. C. Singhaland J. Mizusaki, Editors, PV 2005-07, TheElectrochemical Society Proceedings Series,Pennington, NJ (2005).

4. Solid State Ionic Devices III, E. D. Wachsman,K. S. Lyons, M. Carolyn, F. Garzon, M. Liu,and J. Stetter, Editors, PV 2002-26, TheElectrochemical Society Proceedings Series,Pennington, NJ (2003).

About the Authors

KARLSPEARis a professor emeritus of materialsscience and engineering at The Pennsylvania StateUniversity (Penn State). He is a Fellow of The

Electrochemical Soc iety and the American CeramicSociety. He received the ECS Solid State Scienceand Technology Award in 1997, the HTM Division

Outstanding Achievement Award in 2004, andwas President of the Society in 2002-2003. Hehas been a member of the executive committeeof the High Temperature Materials Division fornineteen years, serving all of the off icer positions,and has co-organized and co-edited the proceedingsof a number of Societ y symposia and proceedings.

A common thread in his research has beenthe application of high temperature chemistry

principles, phase equilibria, and thermodynamicsto predict and understand the chemical behavior ofmaterials at high temperatures. He may be reachedat kes@psu.edu.

ERICWUCHINAhas worked in the Ceramic ScienceGroup at the Naval Surface Warfare Center since1988. Dr. Wuchinas research interests includeultra-high temperature ceramics, oxidation

behavior in extreme environments, chemical vapordeposition, and thermomechanical properties ofnon-oxide ceramics. He is currently the SeniorVice-Chair of the High Temperature Materials

Division of ECS, and may be reached at er ic.wuchina@navy.mil.

ERICD. WACHSMANis a professor in theDepartment of Mate rials Sc ience and Engineeringand Director of the DOE High Temperature

Electrochemistr y Center at the Universityof Florida. He is the chair of the ECS HighTemperature Materials Division, and organizesbiennial ECS symposia on ionic conductors, SolidState Ionic Devices. Dr. Wachsmans researchinterests are in ionic and electronic conductingceramics, the development of moderate temperatureSOFCs, gas separation membranes, and solid-statesensors. He may be reached at ewach@mse.ufl.edu.