Download - spr06_p48-51
-
8/13/2019 spr06_p48-51
1/4
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are no longer valid.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
-
8/13/2019 spr06_p48-51
2/4
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)
-
8/13/2019 spr06_p48-51
3/4
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.)
-
8/13/2019 spr06_p48-51
4/4
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 [email protected].
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 [email protected].
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 [email protected].