Electrochemical devices based on single-component solid oxide bodies

Single-component bodies useful in fuel cells and other electrochemical devices are provided. In preferred embodiments, the single-component bodies comprise an anodic region at a first side, a cathodic region at a second, non-adjacent side, and an oxygen ion-conducting region substantially free from anodic or cathodic character disposed between said anodic and cathodic regions. The single-component bodies comprise either transition metal perovskites or oxide electrolytes such as yttria-stabilized zirconia doped with multivalent cations.

FIELD OF THE INVENTION 
This invention relates to electrochemical devices incorporating solid oxide 
bodies, to solid oxide fuel cells useful in the generation of electric 
current, and to solid oxide fuel cells fabricated from single-component 
solid oxide bodies. 
BACKGROUND OF THE INVENTION 
Solid oxide fuel cells (SOFCs) operate by converting chemical energy 
directly to electricity by way of an isothermal electrochemical oxidation 
process that is not governed by the Carnot cycle thermodynamics 
characteristic of other energy conversion devices. SOFCs typically possess 
efficiencies approaching 90 percent and therefore require lower energy 
input to produce a specific amount of power. Additionally, SOFC 
performance is relatively independent of the power plant size. 
During operation of a SOFC, an oxidant such as air or some other 
oxygen-containing medium typically is introduced at a cathodic portion of 
the fuel cell, and a fuel such as hydrogen, carbon monoxide, natural gas, 
or coal-derived gas is introduced at an anodic portion. Upon application 
of an external load, oxygen at the cathode (air electrode) reacts with 
incoming electrons from an external circuit to generate oxygen ions, which 
then migrate to the anode (fuel electrode) through an oxygen 
ion-conducting electrolyte within the body of the fuel cell. At the anode, 
the fuel is electrochemically oxidized with these oxygen ions to liberate 
electrons to an external circuit. The oxidation occurring at the fuel 
electrode causes current to flow through the external circuit, returning 
electrons to the air electrode to form more oxygen ions. 
Traditionally, solid oxide fuel cells have been fabricated as 
multiple-component assemblages such as laminates which during operation 
suffer from a variety of problems attributable to thermal, mechanical, and 
chemical incompatibilities between each component. These problems have 
included poor fuel tolerance, limited chemical and thermal endurance, 
complex and expensive fabrication techniques, and poor mechanic 
durability. For example, many multiple-component fuel cells can only be 
energized and de-energized a few times before the component layers 
de-laminate due to differential thermal expansion and contraction. 
Accordingly, there exists a need for new fuel cells which eliminate or at 
least minimize the problems associated with multiple component structures. 
OBJECTS OF THE INVENTION 
It is therefore one object of the invention to provide electrochemical 
devices such as solid oxide fuel cells having simpler designs, fewer 
fabrication problems, higher durability, and lower processing costs than 
those known in the art. 
It is a further object of the invention to provide such features in 
single-component solid oxide fuel cells (SCOFCs) having the required 
electrical, electrochemical, chemical, and catalytic properties. 
It is a still further object to provide SCOFCs capable of operating in a 
moderate temperature range (800.degree. C. or less), to provide additional 
advantages such as reduced materials costs and improved cell performance. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by the present invention, which 
provides electrochemical devices such as fuel cells comprising 
multi-sided, single-component bodies which, in turn, comprise an anodic 
region at a first side, a cathodic region at a second side not adjacent 
the first side, and an oxygen ion-conducting region substantially free 
from anodic or cathodic character disposed between the anodic and cathodic 
regions. In certain embodiments, the single-component bodies comprise 
oxide electrolytes doped with multivalent cations. In other embodiments, 
the single-component bodies comprise transition metal perovskites Also 
provided are multi-sided, single-component, bodies having ether an anodic 
region or a cathodic region at a single side. 
The present invention also provides methods for fabricating 
single-component solid oxide fuel cells. In certain embodiments, these 
methods comprise the steps of providing a single-component body comprising 
an oxide electrolyte having the formula: 
EQU R.sub.x M.sub.1-x O.sub.2-x/2 ( 1) 
wherein R is at least one rare earth element such as yttrium, samarium, 
ytterbium, calcium, or strontium; M is at least one metal selected from 
the group consisting of zirconium, cerium, and bismuth; and x is from 
about 0.05 to about 0.25. The processes further comprise contacting a 
first side of such a body with a first dopant to form an anodic region at 
the first side, and/or contacting a second, non-adjacent side of the body 
with a second dopant to form a cathodic region at the second side. Said 
contacting is effected such that the anodic region is separated from the 
cathodic region by an oxygen ion-conducting region substantially free from 
dopant. In certain embodiments, only one side is contacted with dopant to 
form either an anodic or a cathodic region. 
In another aspect, the invention provides a process for fabricating 
single-component, multi-sided bodies comprising transition metal 
perovskites having the formula: 
EQU ABO.sub.3-z ( 2) 
wherein A is Ln.sub.1-x D.sup.2+.sub.x ; Ln is at least one lanthanide such 
as lanthanum, yttrium, or strontium; D is at least one divalent metal such 
as calcium, barium, or strontium; B is N.sub.1-y Q.sub.y where N and Q are 
transition metals such as chromium, manganese, iron, cobalt, nickel, 
copper, or vanadium; x is 0 to about 1; y is 0 to about 1; and z is less 
than 3. In fuel cells comprising such bodies, anodic and cathodic regions 
are established by an oxygen pressure gradient which results from exposure 
of the first side of the body to fuel and the second side of the body to 
air or other oxygen-containing gas mixtures. In certain embodiments, 
multi-sided bodies comprising transition metal perovskites have either an 
anodic or a cathodic region at only one side. 
The single-component oxide fuel cells (SCOFCs) of the present invention are 
provided using a new materials approach to the development of efficient, 
fuel-tolerant fuel cells. Single-component cells eliminate the mechanical 
and chemical compatibility problems associated with current designs, and 
also provide significant technological advantages, including simplified 
cell design, fewer fabrication problems, and lower processing costs as 
compared with solid oxide fuel cells of the prior art. 
The catalytic, electrochemical, and chemical characteristics of the 
provided novel fuel cell systems are ideally suited for the development of 
high efficiency, thin-film, SCOFCs operating in the temperature range of 
approximately 600.degree. C. to 800.degree. C. An SCOFC operating in the 
moderate temperature range of 600.degree. C. to 800.degree. C. offers 
additional advantages in reducing materials costs and improved cell 
performance. These advantages include the elimination of the high 
temperature corrosion of system components, ease of multiple-cell stack 
sealing, and increased options for interconnection materials including 
metals and alloys.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention applies a new materials approach to produce 
electrochemical devices containing single-component bodies. Devices such 
as fuel cells are based on a single oxide component in which the 
appropriate conduction characteristics are produced either by chemically 
modifying oxide electrolytes to produce n-type mixed-conducting anode 
regions and/or p-type mixed-conducting cathode regions, or by chemically 
tailoring electronically conducting perovskites to produce an oxygen-ion 
conduction region between the n- and p-type mixed conducting regions. 
Other electrochemical devices such as oxygen sensors are based on a single 
component in which an oxide electrolyte or transition metal perovskite is 
modified to produce either an n-type mixed-conducting anode region or a 
p-type mixed-conducting cathode region. It will be understood that "mixed 
conducting" as employed herein denotes a region exhibiting both oxygen-ion 
and electronic (n- or p-type) conductivity. 
The single-component bodies of the present invention can be used in 
virtually any known solid oxide fuel cell design. Preferred, non-limiting, 
fuel cell designs are shown in FIGS. 1-4, wherein first sides 12a,b of 
single-component bodies 10a,b are in contact with fuel provided via means 
6a,b and second, non-adjacent sides 16a,b are in contact with oxidant 
provided by conduits 8a,b or some other suitable means. The bodies 
individually comprise n-type mixed conducting anodic regions (14a,b) at 
the first sides, p-type mixed conducting cathodic regions (18a,b) at the 
second sides, and oxygen ion-conducting regions (20a,b) disposed between 
the n-type and p-type regions. A tubular fuel cell design is indicated by 
FIGS. 3 and 4, wherein the first side (12b) and the second side (16b) are 
somewhat concentric. 
As will be recognized, the single-component bodies of the invention also 
can be used a wide variety of other electrochemical devices. For example, 
a multi-sided, single-component body having either an n-type mixed 
conducting anodic region or a p-type mixed conducting cathodic region at 
one side can be used in fabricating oxygen sensors of the type generally 
disclosed by, for example, Logothetis, Adv. in Ceramics, 1981, 3, 388-405; 
Haaland, J. Electrochem., 1980, 127, 796-804; Worrell, Proc. of Symp. on 
Electrochemistry and Solid State Science Education, ed. by Smyrl and 
Mclarnon, pp. 98-109, The Electrochemical Society, Pennington, N.J. 
(1987); Kleitz, et al., Proceed of the Int. Meeting on Chem. Sensors, Vol. 
17, Anal. Chem. Symp. Series, pp. 262-272, Elsevier, N.Y.C. (1983); and 
Lukaszewicz, et al., Sensors and Actuators, 1990, B1, 195-198. 
In certain embodiments, the single-component bodies of the present 
invention are fabricated from oxide electrolytes containing multivalent 
ions. In preferred embodiments, oxide electrolytes have formula (1): 
EQU R.sub.x M.sub.1-x O.sub.2-x/2 (1) 
wherein R is a rare earth element such as yttrium, samarium, ytterbium, 
calcium, or strontium; M is a metal such as zirconium, cerium, or bismuth; 
and x is from about 0.05 to about 0.25. Preferred oxide electrolytes have 
the formulas Y.sub.x Zr.sub.1-x O.sub.2-x/2 (yttria-stabilized zirconia; 
YSZ) and Sa.sub.1-x Ce.sub.1-x O.sub.2-x/2 (samaria-stabilized ceria; 
SSC). YSZ is particularly preferred. The selected oxide electrolyte should 
possess good chemical stability and high oxygen ion conductivity at the 
fuel cell operating temperatures. 
The chemistry of these oxides is modified in accordance with the present 
invention to provide either n- and p-type mixed conduction, respectively, 
at the opposing sides or n- or p-type conduction at a single side. For 
example, catalytically active, low resistance, n- and p-type mixed 
conducting yttria-stabilized zirconia (YSZ) can be produced by doping YSZ 
with, for example, titania or terbia, respectively. In one preferred 
embodiment, titania (titanium oxide) is introduced into one side of the 
oxide electrolyte having formula (1) to produce n-type conductivity and a 
mixed-conducting anode. One unique property of these YSZ-titania regions 
is that they exhibit mixed oxygen-ion and n-type electronic conductivity 
at oxygen pressures below 10.sup.-12 atmospheres. This is due to the 
presence of both Ti.sup.4+ and Ti.sup.3+, and the associated electron 
hopping which occurs in low oxygen pressure environments. See, e.g., 
Worrell, Proceedings of the EPRI/GRI Workshop on Ceramic Conductors for 
Solid State Electrochemical Devices, Snowbird, Utah, May 12-15, 1991; 
Worrell, et al., Proceedings of the First International Symposium on Ionic 
and Mixed Conducting Ceramics, ed. by Ramanarayanan and Tuller, 
Proceedings Volume 91-12, Worrell, et al., Solid Oxide Fuel Cells, ed. by 
Singhal, Proceedings Volume 89-11, pp. 81-89, The Electrochemical Society, 
Pennington, N.J. (1989). 
The percentage of n-type electronic conductivity in, for example, 
YSZ-titania solutions of the invention can be varied from 0 to 100 percent 
by changing the amount of the titania addition. One procedure for 
dissolving titania into YSZ is disclosed by Worrell, et al., Solid Oxide 
Fuel Cells, ed. by Singhal, Proceedings Volume 89-11, pp. 81-89, The 
Electrochemical Society, Pennington, N.J. (1989). A 5 to 10 mole percent 
variation in the titania concentration increases the percentage of 
electronic conductivity from 10 to 85 percent at 800.degree. C. Typical 
oxygen pressures at the fuel-gas electrode (anode) are 10.sup.-18 to 
10.sup.-22 atm. Under these conditions a surface region of 
yttria-stabilized zirconia containing titania would have excellent n-type, 
mixed conductivity. U.S. Pat. Nos. 4,791,079 and 4,827,071, both in the 
name of Hazbun, confirmed that yttria-stabilized zirconia-titania is also 
an effective catalytic membrane for hydrocarbon conversion. 
Other oxides can be employed in accordance with the present invention to 
produce n-type electronic conductivity in oxide-electrolyte surface 
regions at low oxygen pressures. The most likely candidates are oxides 
that show a significant range of solid solubility in the fluorite lattice 
of the oxide electrolyte and can be stabilized in a mixed valence state 
under the reducing conditions experienced at the anode. 
N-type mixed conducting surface regions can be established in oxide 
electrolytes by at least two techniques. The first technique involves 
substitution for tetravalent M using cations with accessible trivalent 
states. In addition to titanium, other systems include Ce.sup.4+/3+, 
Pr.sup.4+/3+, coupled (R).sup.3+ -Nb.sup.5+ substitutions leading to 
Nb.sup.4' formation under reducing conditions, and vanadium oxides. The 
second technique involves substitution for R with multivalent ions having 
accessible divalent states. Useful oxides of this first technique include 
rare earth oxides such as Yb.sub.2 O.sub.3 (Yb.sup.3+ /Yb.sup.2+) and 
Eu.sub.2 O.sub.3 (Eu.sup.3+ /Eu.sup.2+), transition metal oxides such as 
Fe.sub.2 O.sub.3 (Fe.sup.3+ /Fe.sup.2+) and Mn.sub.2 O.sub.3 (Mn.sup.3+ 
/Mn.sup.2+), and reduced vanadium oxides. It will be recognized that the 
term "rare earth" is descriptive of elements having atomic numbers 21, 39, 
and 57, that the term "lanthanide" is descriptive of elements having 
atomic numbers 58-71, and that the term "transition metal" is descriptive 
of elements having atomic numbers 21-31, 39-49, and 71-81. The procedures 
for introducing these oxides would be similar to those used for titania 
dissolution. 
Mixed-conducting p-type cathodic regions can be established in oxide 
electrolytes having formula (1) by, for example, substitution of trivalent 
R-stabilizing cations by ions with stable 3.sup.+ /4.sup.+ mixed valences 
under high oxygen pressure, or by substitution of tetravalent M by cations 
with stable 4.sup.+ /5.sup.+ states. M can also be substituted with 
Bi.sup.3+/5+ or the higher oxidation states of transition metals such as 
iron, manganese, and chromium. 
Multivalent cations that produce significant p-type conductivity at 
stabilized oxide electrolyte surfaces have stable mixed-valences under 
oxidizing conditions. For example, terbium (Tb) substitutions in both 
zirconia and ceria (cerium oxide) can lead to the formation of outstanding 
p-type u mixed conductors with conductivities exceeding 10.sup.-2 
S.cm.sup.-1 at 700.degree. C. See, e.g., Burgraaf, et al., Solid State 
Ionics, 1986, 18/19, 807 and Van Dijk, et al., Solid State Ionics, 1983, 
9/10, 913. 
As will be recognized, the oxide electrolytes R.sub.x Zr.sub.1-x 
O.sub.2-x/2 and R.sub.1-x Ce.sub.1-x O.sub.2-x/2 generally crystallize in 
fluorite-related structures for x less than 0.5. When x equals 0.5 they 
can be stabilized in the pyrochlore structure which is an ordered variant 
of fluorite. P-type mixed conduction can be introduced into both structure 
types using cations with variable valency, in particular by 
Tb-substitutions. The stabilization of Tb.sup.4+/3+ mixed valences in air 
at temperatures between approximately 600.degree. C. to 900.degree. C. 
leads to outstanding p-type electronic conductivities. By controlling the 
total Tb content, and by using different thermal treatments to produce 
fluorite, pyrochlore and fluorite-pyrochlore nano-composite structures, 
the percentage of the electronic contribution to the total conductivity of 
both ceria and zirconia oxides can be varied from 0 to 100 percent. For 
excellent cathodic behavior, the optimum multivalent-cation ratio that 
produces significant p-type conductivity in yttria-stabilized zirconia 
must be stable in an oxidizing environment such as air. Thus, 
terbia-substituted YSZ is a preferred cathode surface. Other oxides which 
can be employed in accordance with the present invention to produce p-type 
mixed-conducting regions in oxide electrolytes of formula (1) at high 
oxygen pressures include the oxides of bismuth and those of transition 
metals such as iron, manganese, and vanadium which exhibit multiple 
oxidation states in air. 
Certain single-component bodies according to the present invention comprise 
mixed-conducting transition metal perovskites having formula (2): 
EQU ABO.sub.3-z (2) 
wherein A is Ln.sub.1-x D.sup.2+.sub.x ; Ln is a lanthanide such as 
lanthanum, yttrium, or strontium; D is a metal such as calcium, barium, or 
strontium; B is N.sub.1-y Q.sub.y where N and Q are transition metals such 
as chromium, manganese, iron, cobalt, nickel, copper, or vanadium; x is 0 
to about 1; y is 0 to about 1; and z is less than 3. 
The oxygen-ion conductivities of certain transition metal perovskites 
having formula (2) can exceed those of many solid oxide electrolytes. For 
example, Teraoka, et al., Mat. Res. Bull, 1988, 23, 51, disclosed that at 
800.degree. C. the ionic conductivities of La.sub.1-x Sr.sub.x Co.sub.1-y 
Fe.sub.y O.sub.3-z perovskites are at least two orders of magnitude higher 
than that of YSZ. The major obstacle to the formation of a 
single-component fuel cell from these transition metal perovskites is the 
establishment of an oxygen ion-conducting region between n- and p-type 
mixed conducting electrodes. In accordance with the present invention, the 
chemistry of selected perovskites is controlled by an appropriate divalent 
cation substitution to maximize the oxygen vacancy concentration such that 
the oxygen-pressure gradient typically present under fuel cell operating 
conditions produces an efficient SCOFC in which the p- and n-type 
mixed-conduction regions are separated by an electrolyte region exhibiting 
high oxygen ion conductivity. 
For a perovskite to sustain intrinsic n- and p-type conductivity, it must 
contain ions that can be stabilized in three different oxidation states. 
There are several transition metal ions that can meet these criteria. 
However, for a single-component oxide fuel cell, it is essential that 
these three oxidation states can be stabilized in the same structural host 
over the wide range of oxygen pressures experienced between the anode and 
cathode sides. 
The electronic properties and oxygen stoichiometry of the transition metal 
perovskites having formula (2), which are dependent upon oxygen pressure, 
can be controlled by changes in temperature or oxygen pressure and the 
extent of divalent-cation (D) substitutions on the A sites of the 
perovskite lattice. FIG. 5 shows an idealized representation of the oxygen 
content and electronic characteristics of a Ln.sub.1-x D.sup.2+.sub.x 
BO.sub.3-z perovskite as a function of oxygen pressure. Ideally, the 
stoichiometry of these systems shows two well-defined plateaus, the first 
corresponding to the lattice stoichiometry where z=0 and the second to the 
electronic stoichiometry where y=x/2 and the average valence of B is 
3.sup.+. At high oxygen pressures, electronic conduction, typically in the 
range of 1 to 100 S.cm.sup.-1, results from the introduction of holes into 
the system either by the incorporation of excess oxygen into the lattice 
(i.e., Mn perovskites), or more generally from the divalent substitutions 
on the A-site (typically D.sup.2+ =Ca, Sr, or Ba). The conduction in these 
p-type regions can be metallic or activated generally via a B.sup.3+ 
-B.sup.4+ hopping mechanism. P-type conductivity in the substituted 
perovskites is observed for oxygen stoichiometries with z less than x/2. 
When z is greater than O for these compositions, the oxygen vacancies in 
the perovskite lattice also lead to significant ionic contributions to the 
total conductivity and, therefore, result in mixed-conduction. As the 
oxygen pressure decreases, the number of p-type carriers decrease as the 
number of vacancies increase. For z equal to x/2, the electrical 
conductivity of the "electronically stoichiometric" oxide is a minimum. 
Because of the intrinsic stability of the 3.sup.+ state, this 
stoichiometry can be maintained over a wide range of oxygen pressure. It 
is in this region that the oxygen-ion contribution to the total 
conductivity can be maximized. At even lower oxygen pressures when z is 
greater than x/2, the introduction of n-type carriers leads to a rapid 
increase in the electronic conduction due to a D.sup.2+ -D.sup.3+ electron 
hopping mechanism. 
For an ideal transition metal perovskite, the oxygen-pressure gradient 
present in a fuel cell can lead to the oxygen stoichiometry profile shown 
in FIG. 5 and, thus, to the direct formation of a single-component (e.g., 
n-type anode/oxygen-ion conducting region/p-type cathode) cell. In fuel 
cells of the present invention, the transition metal perovskite is 
chemically modified to exhibit this idealized behavior over the typical 
oxygen pressure range experienced during the operation of the fuel cell. 
The perovskites of the invention should possess good chemical stability at 
low oxygen pressure. They should also exhibit a minimum in their 
electronic conductivity in the potential electrolyte region over a wide 
range of oxygen pressures, as well as relatively low electronic 
contribution to the total conductivity. Iron-based perovskites are 
preferred in accordance with the present invention. In general, iron-based 
perovskites closely approximate the idealized behavior shown in FIG. 5. 
For example, Mizusaki, et al., J. Solid State Chem., 1985, 58, 257 
disclosed that the La.sub.1-x Sr.sub.x FeO.sub.3-z perovskite is stable 
under oxygen pressures as low as 10.sup.-18 atm. at 900.degree. C. and 
10.sup.-22 atm. at 700.degree. C. Variations of the conductivity of 
La.sub.1-x Sr.sub.x FeO.sub.3-z with oxygen pressure, temperature and x 
from 0 to 0.25 have been reported by Mizusaki, J. Amer. Ceram. Soc., 1983, 
66, 247. The p- and n-type conductivities are typically in the range of 1 
to 100 S.cm.sup.-1, which is more than adequate for fuel cell operation. 
The ability of the perovskite lattice to sustain considerable oxygen 
non-stoichiometry is believed to result in the wide range of electronic 
conductivities described above. It also is believed to result in high 
oxygen mobilities and, therefore, high oxygen-ion conductivities. Direct 
measurement of selected compositions in the (La.sub.1-x Sr.sub.x) 
(Co.sub.1-y Fe.sub.y)O.sub.3-z perovskites show conductivities of the 
perovskites can surpass those of many solid electrolytes. See, e.g., 
Teraoka, et al., Mat. Res. Bull, 1988, 23, 51. 
Conductivity data reported by Teraoka, et al. for La.sub.0.6 Sr.sub.0.4 
FeO.sub.3-z can be used to estimate the oxygen-transference numbers for 
iron perovskites in the region of electronic stoichiometry shown in FIG. 
5. Combining this data with electronic conductivity data [See, e.g., 
Mizusaki, J. Amer. Ceram. Soc., 1983, 66, 247] the expected oxygen-ion 
contribution to the conductivity of an electronically stoichiometric iron 
perovskite with (La.sub.0.33 D.sub..66)FeO.sub.2.67 is greater than 91.5 
percent at 700.degree. C. and greater than 84 percent at 800.degree. C. 
The oxygen ion contribution in the La.sub.1-x Ba.sub.x or Sr.sub.1-x 
X.sub.x iron perovskites (X=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, 
Tm, Yb, and Lu) should be well in excess of 0.91. Thus, other iron-based 
perovskites which can be employed in accordance with the present invention 
include the La.sub.1-x Ba.sub.x Fe.sub.y Q.sub.1-y O.sub.3 perovskites 
(Q=Cu, Co, Cr, Mn, Ni, V) and the Sr.sub.1-x X.sub.x Fe.sub.y Q.sub.1-y 
O.sub.3 perovskites (X=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, 
and Lu) where x and y are, independently, 0 to about 1. 
Many of the oxide electrolytes having formula (1) and the iron-based 
perovskites having formula (2) exhibit low oxygen-ion conductivities in 
the 600.degree. C.-800.degree. C. temperature range. They usually require 
the use of thin films to achieve useful current densities, particularly at 
the lower temperatures. The oxide electrolyte can be fabricated into the 
form of a dense, crack- and pore-free film about 1 to about 10 microns 
thick using a number of thin-film technologies such as magnetron 
sputtering. See, e.g., Barnett, Energy, 1990, 5, 1 and Barnett, et al., 
Proceedings of the EPRI/GRI Workshop on Ceramic Conductors for Solid State 
Electrochemical Devices, Snowbird, Utah, May 12-15, 1991. The use of 
magnetron sputtering, where the sputtered fluxes from metal targets are 
reacted with oxygen gas at the depositing film, has been shown to provide 
much higher deposition rates than sputtering from ceramic targets. Another 
attractive technology is electrochemical vapor deposition (EVD), which has 
been used to prepare thin films of yttria-stabilized zirconia and 
yttria-stabilized titania. see, e.g., U.S. Pat. Nos. 4,791,079 and 
4,827,071, both in the name of Hazbun. 
Those skilled in the art will appreciate that numerous changes and 
modifications may be made to the preferred embodiments of the invention 
and that such changes and modifications may be made without departing from 
the spirit of the invention. It is therefore intended that the appended 
claims cover all such equivalent variations as fall within the true spirit 
and scope of the invention.