U.S. patent application number 10/760924 was filed with the patent office on 2007-08-23 for thin film mixed potential sensors.
Invention is credited to Eric L. Brosha, Fernando H. Garzon, Rangachary Mukundan.
Application Number | 20070193883 10/760924 |
Document ID | / |
Family ID | 34807530 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070193883 |
Kind Code |
A1 |
Garzon; Fernando H. ; et
al. |
August 23, 2007 |
THIN FILM MIXED POTENTIAL SENSORS
Abstract
A mixed potential sensor for oxidizable or reducible gases and a
method of making. A substrate is provided and two electrodes are
formed on a first surface of the substrate, each electrode being
formed of a different catalytic material selected to produce a
differential voltage between the electrodes from electrochemical
reactions of the gases catalyzed by the electrode materials. An
electrolytic layer of an electrolyte is formed over the electrodes
to cover a first portion of the electrodes from direct exposure to
the gases with a second portion of the electrodes uncovered for
direct exposure to the gases.
Inventors: |
Garzon; Fernando H.; (Santa
Fe, NM) ; Brosha; Eric L.; (Los Alamos, NM) ;
Mukundan; Rangachary; (Santa Fe, NM) |
Correspondence
Address: |
LOS ALAMOS NATIONAL SECURITY, LLC
LOS ALAMOS NATIONAL LABORATORY
PPO. BOX 1663, LC/IP, MS A187
LOS ALAMOS
NM
87545
US
|
Family ID: |
34807530 |
Appl. No.: |
10/760924 |
Filed: |
January 20, 2004 |
Current U.S.
Class: |
204/426 |
Current CPC
Class: |
G01N 27/4074
20130101 |
Class at
Publication: |
204/426 |
International
Class: |
G01N 27/26 20060101
G01N027/26 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0001] This invention was made with government support under
Contract No. W-7405-ENG-36 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. A mixed potential sensor for oxidizable or reducible gases
comprising: a substrate; two electrodes formed on a first surface
of the substrate, each electrode being formed of a different
catalytic material selected to produce a differential voltage
between the electrodes from electrochemical reactions of the gases
catalyzed by the electrode materials; and an electrolytic layer of
an oxygen ion conducting metal oxide electrolyte formed over the
electrodes to cover a first portion of the electrodes from direct
exposure to the gases, with a second portion of the electrodes
uncovered for direct exposure to the gases.
2. The mixed potential sensor of claim 1, wherein the substrate is
an inert, gas impermeable material.
3. The mixed potential sensor of claim 1, wherein the electrodes
have a density characteristic of materials deposited by RF
magnetron or DC sputter vapor deposition processes.
4. The mixed potential sensor of claim 2, wherein the electrodes
have a density characteristic of materials deposited by RF
magnetron or DC sputter vapor deposition processes.
5. The mixed potential sensor of any one of claims 1-4, wherein the
electrolytic layer is formed to a thickness of 5 .mu.m to 10 .mu.m
with a density characteristic of materials deposited by an electron
beam evaporation process.
6. A method for making a mixed potential sensor for oxidizable or
reducible gases comprising: providing a gas impermeable substrate;
forming electrode strips of two different materials effective to
produce a differential potential therebetween in electrochemical
reactions with one of the oxidizable or reducible gases; depositing
a thin, dense layer of an oxygen ion conducting metal oxide
electrolyte to cover a first portion of the electrodes from direct
exposure to the gases while leaving a second portion of the
electrodes uncovered for direct exposure to the gases.
7. The method of claim 6, including forming the electrodes by a
deposition process selected from the group consisting of RF
magnetron vapor deposition or DC sputter vapor deposition.
8. The method of claim 6, including depositing said layer of said
oxygen ion conducting metal oxide electrolyte by an electron beam
evaporation process.
9. The mixed potential sensor of claim 1 wherein said oxygen ion
conducting metal oxide electrolyte is selected from yttria
stabilized-zirconia, zirconia-based electrolytes, ceria-based
electrolytes, or bismuth-based electrolytes.
10. The mixed potential sensor of claim 9 wherein said oxygen ion
conducting metal oxide electrolyte is yttria
stabilized-zirconia.
11. The method of claim 6 wherein said oxygen ion conducting metal
oxide electrolyte is selected from yttria stabilized-zirconia,
zirconia-based electrolytes, ceria-based electrolytes, or
bismuth-based electrolytes.
12. The method of claim 11 wherein said oxygen ion conducting metal
oxide electrolyte is yttria stabilized-zirconia.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to gas sensors, and,
more particularly, to mixed-potential sensors for the detection of
various oxidizable and reducible gases.
BACKGROUND OF THE INVENTION
[0003] Mixed potential sensors are used for the detection of
various oxidizable and reducible gases. Oxidizable gases include
hydrogen, hydrocarbons, carbon monoxide, nitric oxide, ethanol, and
the like, while reducible gases include oxygen, nitrogen dioxide,
and the like. Typical sensors utilize an ionic conducting
electrolyte, such as yttria stabilized-zirconia (YSZ), and thin
film metal and/or metal oxide electrodes, such as platinum (Pt) and
perovskite-type oxides. Multiple reduction/oxidation reactions
occurring between the gas phase and the electrode/electrolyte
interface cause mixed potentials of differing magnitude to develop
at the dissimilar electrodes. The selectivity of such sensors is
achieved by the proper selection of the metal and metal oxide
electrodes, while the stability of the sensor is achieved by the
precise control of the surface area of the electrode and the
3-phase interface region (gas-electrolyte-electrode) of the
sensor.
[0004] The mixed-potential that is developed at an
electrode-electrolyte interface in the presence of oxidizable
gases, such as CO or hydrocarbons, is fixed by the rates of
reduction and oxidation of the oxygen and the oxidizable gas,
respectively (Reactions 1 and 2). Carbon monoxide is used here as
an example but the theory is also applicable to other
oxidizable/reducible gases:
1/2O.sub.2+V.sub.o+2e.revreaction.O.sub.0 Reduction (1)
CO+O.sub.0CO.sub.2+V.sub.o+2e.sup.- Oxidation (2)
[0005] When the kinetic rate for the reduction reaction equals that
of the oxidation reaction a stable potential is established.
Similar reactions with different kinetics occur on the other
electrode triple-phase boundary area. The difference in potential
between the electrodes is the device output voltage. The preferred
mixed potential CO sensing device consists of an electrode that
kinetically inhibits the oxygen reduction reaction yet is fast at
CO electro-oxidation and a second electrode that exhibits fast
oxygen reduction kinetics yet is poor at CO electrochemical
oxidation.
[0006] Since the mixed-potential is controlled by the kinetics of
various reactions, control of the 3-phase
(electrolyte/electrode/gas) area is of importance. Moreover, since
the response time of these sensors is fixed by the speed of various
reactions, the sensor design is preferably optimized to maximize
the rates of reactions 1 and 2.
[0007] In the prior art, a dense YSZ electrolyte is used as the
substrate and a metal or metal oxide electrode is deposited on top
of this electrolyte. The length of the active 3-phase interface is
controlled by the morphology of the electrode. A highly porous
electrode results in better gas access and greater 3-phase
interface, whereas a denser electrode leads to poorer gas access
and less 3-phase interface. One drawback of this type of
arrangement is that the gas has to meander through the pores of a
catalytically active material (the electrode) before reaching the
3-phase interface where the reduction and oxidation reactions
occur. Hence, the hydrocarbons (or other reducing gases) are
heterogeneously oxidized at the metal (or metal oxide) electrode
before they reach the 3-phase interface with the electrolyte, with
a concomitant loss in sensor sensitivity and increase in response
time.
[0008] Mukundan et al. (U.S. Pat. No. 6,656,336, issued Dec. 2,
2003) disclose a non-methane hydrocarbon sensor that measures the
amount of hydrocarbons present in an exhaust stream containing
oxygen. The selectivity of the device is achieved by the proper
selection of the oxide electrode, while the stability of the device
is achieved by the precise control of the surface area (SA) of the
electrode and the 3-phase interface region (3PA)
(gas-electrolyte-electrode) of the sensor. By controlling the ratio
of the SA to the 3PA, the rates of the heterogeneous catalysis and
electrochemical catalysis are controlled for any particular
electrode used. Thus, by proper selection of the electrode material
and electrode dimensions, the magnitude of sensor response to any
particular gas species can be amplified (selectivity).
[0009] Mukundan et al. (U.S. Pat. No. 6,605,202, issued Aug. 12,
2003) disclose a mixed-potential electrochemical sensor for the
detection of gases, such as CO, NO, and non-methane hydrocarbons,
in room air. The sensor utilizes a ceria-based electrolyte, and
metal wire electrodes. The stability and reproducibility of the
sensor is achieved by using wire electrodes instead of the usual
thin or thick film electrodes that are currently employed. The
metal wire-electrodes are directly embedded into the electrolyte
and co-sintered with the electrolyte in order to produce a stable
metal/electrolyte interface.
[0010] The present invention uses thin film technology to produce a
mixed-potential electrochemical sensor for the detection of gases,
such as CO, NO, and non-methane hydrocarbons, where the sensor
exhibits fast response time, good reproducibility, and can be
produced using inexpensive thin film technology.
[0011] Bloemer et al. (U.S. Pat. No. 6,352,631, issued Mar. 5,
2002) teach a mixed-potential sensor formed by depositing
electrodes on a dense electrolyte, where the electrodes are exposed
directly to the gas stream to be sampled. Muller et al. (U.S. Pat.
No. 4,277,323) teach an oxygen sensor where two electrodes are put
on a porous substrate and completely covered with a thin YSZ film.
While this works well for an O.sub.2 sensor and improves
performance, this will not provide a mixed potential sensor.
Further, Mueller et al. provide gas access only through a porous
substrate onto an electrode embedded in an electrolyte where the
3-phase interface region is not well defined.
[0012] Various objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
[0013] In accordance with the purposes of the present invention, as
embodied and broadly described herein, the present invention
includes a mixed potential sensor for oxidizable or reducible gases
and a method of making the mixed potential sensor. A substrate is
provided and two electrodes are formed on a first surface of the
substrate, each electrode being formed of a different catalytic
material selected to produce a differential voltage between the
electrodes from electrochemical reactions of the gases catalyzed by
the electrode materials. An electrolytic layer of an electrolyte is
formed over the electrodes to cover a first portion of the
electrodes from direct exposure to the gases with a second portion
of the electrodes uncovered for direct exposure to the gases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0015] FIGS. 1A and 1B are a partial top view and a side view of
one embodiment of a sensor according to the present invention.
[0016] FIG. 2 is a pictorial illustration of a sensor shown in
FIGS. 1A and 1B.
[0017] FIG. 3 graphically compares sensor performance without and
with a thin electrolyte cover for the electrodes.
[0018] FIG. 4 graphically depicts sensor performance with
electrodes on various substrates.
[0019] FIG. 5 graphically depicts sensor performance in various
oxidizable and reducible gases.
DETAILED DESCRIPTION
[0020] The sensor design of the present invention provides dense
thin film electrodes deposited on an inert substrate in a
non-intersecting, preferably parallel, geometry with a second layer
of thin film electrolyte that partially covers the two electrode
strips. The fabrication of sensors in this manner produces a device
with a reproducible triple phase boundary. The triple phase
boundary is the region where the gases come in contact with the
electrode/electrolyte interface. If the YSZ electrolyte film were
100% dense, then the region is defined by the line at which the YSZ
ends and the gas/electrode interface begins. If the YSZ is not
completely dense, there is then gas diffusion through the
electrolyte to the surface contact between the electrodes and the
overlying electrolyte, where the electrode/electrolyte interface
forms a triple phase boundary.
[0021] The fabrication process also produces a device that does not
rely on gas diffusion through porous electrode materials and
consequent changes in gas composition from reactions within the
electrodes. The end result is a higher signal response and improved
device-to-device response reproducibility. Another advantage is
that the need for sintering of the electrodes is greatly reduced,
as the materials are already dense.
[0022] The present devices also offer an advantage over dense
electrode structures produced on solid electrolyte substrates.
Surprisingly, we have found that dense films deposited onto
zirconia electrolyte substrates do not perform as well, nor is the
device response as reproducible, as devices where the electrolyte
is deposited on top of the electrode material.
[0023] A schematic of a sensor configuration according to one
embodiment of the present invention is illustrated in FIGS. 1A, 1B,
and 2. In the depicted sensor configuration, thin (e.g., 0.5 .mu.m)
electrode (metal or metal oxide) films 12, 14 are deposited on a
relatively thick (e.g., 0.5 mm) dense substrate 16. Suitable dense
substrates include sapphire, YSZ, alumina, MgO, and the like.
Electrodes 12, 14 are then partially covered with a thin (e.g.,
5-10 .mu.m above substrate 16) electrolyte layer 18. Suitable
electrolytes include YSZ, or other zirconia-based or ceria- or
bismuth-based electrolytes, or other oxygen ion conducting
oxide.
[0024] To achieve thin dense films, deposition techniques such as
RF magnetron or DC sputter vapor deposition process (for electrodes
12, 14) and electron beam evaporation process (for the
yttrium-zirconium oxide solid electrolyte 18) have been used. These
processes typically produce thin films having theoretical densities
>70% of theoretical density. A well defined 3-phase interface
region 20 is formed along the edge of electrolyte 18 where
electrodes 12, 14 emerge from beneath electrolyte layer 18 and
along the interface surface between electrolyte 18 and electrodes
12, 14.
[0025] Exemplary deposition processes are as follows:
RF Magnetron Sputtering
[0026] Substrates (either YSZ, sapphire, or alumina) were
ultrasonically cleaned in isopropanol or acetone and then dried and
fired in air at 1100.degree. C. for several hours. The substrates
were then mounted onto a Ni faceplate that was subsequently placed
into contact with a boron nitride heater specifically designed for
vacuum operation. The substrate was typically held to the Ni
faceplate by an alumina mask held by a metal clip or the substrate
could be glued to the faceplate using a water-based, silver epoxy
(AREMCO). The substrate temperature was monitored using a type K
thermocouple embedded within the Ni faceplate. To measure thickness
and deposition rates, a masked piece of polished sapphire was
mounted next to the substrate. The step created on this witness
sample from the shadow mask was then used to measure the film
thickness produced in the PVD run using a DEKTAK profilimeter.
[0027] Substrates were mounted to a heater box using silver paint;
the nickel heater faceplate temperature was monitored using a
thermocouple and/or an IR camera. For the film depositions used in
this work, the temperature was maintained at 700.degree. C. The
heater assembly was placed into an ultra-high vacuum sputter system
with two R.F. magnetron guns. An off-axis source sample geometry
was used in the sputtering process. The target material consisted
of a mixture of Cr metal, LaF.sub.3, and SrF.sub.2 or MgF.sub.2.
The mixture amounts are determined by the desired stoichiometery of
the electrode, e.g., La.sub.0.8Sr.sub.0.2CrO.sub.3, or
La.sub.0.8Mg.sub.0.2CrO.sub.3. Suitable metal electrode targets
include Pt, Pd, and Au. These exemplary electrode configurations
are not meant to be limiting, since a person skilled in the art can
select any number of electrode configurations based on the sensor
environment, desired sensitivity, and the like.
[0028] All of the sputter depositions were carried out at a RF
power level of 125 W. Typical sputter pressures were between 40 and
45 mTorr of Argon. The films were post-annealed at 1000.degree. C.
in a water vapor and argon atmosphere to convert the fluorides to a
perovskite oxide. Other known target materials are used as a target
material to obtain the metal electrode. X-ray diffraction and
electron microprobe analysis confirmed that the films were the
desired phase and composition. Film thicknesses were determined by
sputtering onto shadow-masked polished sapphire single crystal
substrates using identical sputter conditions. The film thickness
was measured using a stylus profilometer DEKTAK.
Electron Beam Evaporation
[0029] An electrolyte is then deposited over the electrodes by
electron beam evaporation. For an exemplary YSZ file, the YSZ
source consisted of sheets of Cera-Flex.TM. brand yttria-stabilized
zirconia (YSZ) obtained from Marketech International in
0.5.times.100.times.100 mm sheets that were cut into smaller pieces
and arranged evenly to fill the electron beam hearth. A series of
calibration runs were required first in order to find deposition
rates that produced films with the desired thickness. The typical
substrate temperature was maintained at 800.degree. C. throughout
the run. The heater faceplate (and affixed substrate) was
positioned on-axis for the electron beam depositions, 6.5 inches
away from the source. A quartz crystal rate monitor was used to
control the material deposition rate. Other electrolytes can be
deposited by selecting a suitable material source, the selection of
which is well known to persons skilled in electron beam
evaporation.
[0030] The sensor (FIGS. 1A, 1B, and 2) is formed by only partially
coating electrodes 12, 14 with a thin film of electrolyte 18, i.e.,
a first portion of electrodes 12, 14, is covered by electrolyte 18
and not directly exposed to the gas or gases being detected, and a
second portion of the electrodes 12, 14 is masked during the
deposition of electrolyte 18 and is not covered by electrolyte 18,
for direct exposure to the gas or gases. The performance of a
sensor with partially coated electrodes is compared to that of a
sensor with uncoated electrodes, as shown in FIG. 3. The circles
show the response of a sensor with Pt and
La.sub.0.8Sr.sub.0.2CrO.sub.3 electrodes sputtered onto a 0.5 mm
thick YSZ substrate, while the squares show the response of the
same sensor that has now been partially coated with a layer of the
electrolyte. The response of the uncoated sensor is very slow and
shows some hysterisis (e.g., response to 300 ppm propylene is
.apprxeq.30 mV when the concentration is increasing and is
.apprxeq.38 mV when the gas concentration is decreasing). On the
other hand, the response of the sensor is significantly improved by
the partial YSZ over-coat (8.9 .mu.m). The over-coated sensor shows
much faster response times and the hysterisis is completely
eliminated. This enhancement in sensor performance may be due to
the higher quality (less interfacial reaction) of the triple phase
interface produced by the over-coating method.
[0031] The sensor described herein can be made with most commonly
available dense substrates including alumina, sapphire and YSZ. A
dense substrate is impermeable to gases (i.e., no open, through
porosity) generally where the density is >92-95% of theoretical
density. FIG. 4 graphically depicts the propylene response of three
sensors having different substrates of YSZ, alumina, and sapphire.
The response curves of all these sensors are excellent and the
sensor sensitivity is increased either by using more inert or
better quality substrates. Sapphire and alumina have the same
catalytic activity, but the sapphire substrate provides a smoother
surface for higher sensitivity. YSZ and alumina have approximately
the same surface quality, but YSZ is more catalytic, so alumina
provides better sensitivity. Persons of ordinary skill in the art
can make these sensors on other inert and gas impermeable
substrates.
[0032] FIG. 5 illustrates the response of a thin film sensor using
Pt (0.5 .mu.m) and La.sub.0.8Sr.sub.0.2CrO.sub.3 (2.1 .mu.m thick)
electrodes and a 8YSZ electrolyte (ZrO.sub.2 doped with 8 mole %
Y.sub.3O.sub.3, where a suitable doping is in the range 2-10 mole
%), 10.6 .mu.m thick, on an alumina substrate. The response is
shown for propylene, methane, carbon monoxide and nitrogen dioxide.
Note the negative response for the nitrogen dioxide, a reducible
gas.
[0033] Thus, this configuration has several advantages over common
geometries such as parallel electrode on electrolyte and symmetric
electrode-electrolyte-electrode devices described in the
literature: [0034] a) The triple phase interface area may be
precisely controlled by lithographic techniques. [0035] b) The
electrolyte film can be made thin and porous to minimize the
diffusion distances for the gas species. [0036] c) Because the
diffusion distances are short, the non-electrochemical
heterogeneous reactions are minimized. The non-electrochemical
catalysis by the sensor materials reduces the concentration of
gases that can reach the electrochemical interface and thus lowers
the mixed potential. [0037] d) The sensors may be produced on
common substrates that are not electrolytes such as aluminum oxide
or magnesium oxide. [0038] e) The sensors may be miniaturized via
photolithographic methods. Multiple sensors that utilize different
electrode materials and electrolyte may be patterned on a single
wafer.
[0039] The foregoing description of the invention has been
presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching. The embodiments were
chosen and described in order to best explain the principles of the
invention and its practical application to thereby enable others
skilled in the art to best utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto.
* * * * *