Note: Descriptions are shown in the official language in which they were submitted.
CA 02267881 1999-03-25
PATENT APPLICATION
INVENTION: METHOD OF MAKING FAST SOLID STATE GAS
SENSORS
INVENTOR: FARA,~~IARZ HOSSEINBABAEI
APPLICANT: FAR.~~MARZ HOSSEINBABAEI
~~-~~: dc~4-~~~_o~o'~
CA 02267881 1999-03-25
Method of Making Fast Solid State Gas Sensors
1. Introduction and Review of the Prior Art
This invention relates to improvements in solid-state gas sensors and
particularly to
improvements in the method of fabricating these sensors.
Gas sensors are widely used for atmospheric monitoring in general, e.g. in
coal mines,
offshore installations and industrial production facilities. Gas sensors are
also used for
controlling combustion processes in engine exhaust systems, etc., for both
economical and
environmental reasons. In simple terms, a gas sensor performs as the nose of a
robot or
an electronic control system. Moreover, nearly in all modern buildings, use of
smoke and
carbon monoxide detectors has become a common practice.
Solid-state gas sensors, are simple and economically attractive, yet they
present solutions
for many of the said problems. Different members of this family of devices
operate on
different physiochemical bases. For example, a resistive type gas sensor
employs the
semiconducting properties of some polycrystalline (usually oxide) materials
for their
change of electrical conductivity by atmospheric changes; in catalytic type
gas sensors, the
flammable gases they come into contact with, are oxidized on the surface of
the sensor,
raising the device temperature detected by various techniques; in solid
electrolyte
(electrochemical) gas sensors and also in metal-semiconductor junction gas
sensors,
different material or junction properties are employed. A comprehensive review
of
solid-state gas sensors and their applications is presented by Madelis and
Christofides in
"Physics, Chemistry and Theory of Solid-State Gas Sensor Devices", John Wiley
and Sons
Inc. (1993).
All solid-state gas sensors rely upon an interaction with the target gas which
takes place
at the surface of the device. It is well known that increasing the effective
surface area of
the sensing element would enhance the sensitivity and response time of the
sensor. In
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general, the effective surface area of the sensing element is increased by
employing the
sensing element in a porous form. Apart from increasing the solid-gas
interface for
interactions, the said porosity facilitates penetration of the target gas into
the body of the
sensing element and increases the speed of detection. Therefore, porosity of
the sensing
body is of great importance in both static and dynamic performance of the
sensor.
Thus, making of, at least, one porous layer or body is a basic step in the
fabrication
process of these gas sensors. Only five out of many examples, from the prior
art, is given
below:
1. In patent CA 02188771 (1995), page 6, the pore size distribution in a
semiconducting
oxide body has been declared as the first of the main parameters controlling
the
performance of a resistive type gas sensor.
2. In patent US 5814281 (1998), a resistive gas sensor comprising a porous
oxide body
is described in which, pore surfaces decorated with a precious metal, control
the device
performance.
3. Patent CA 02204413 (1995), describes the forming of porous planar
electrodes on a
porous substrate for fabricating an electrochemical gas sensor.
4. In US 4355056 (1982), a differential thermocouple gas sensor is described
which
comprises a porous ceramic layer, on a part of which, pores have catalytic
decorations.
5. In US 54455796 (1995), a porous ceramic layer has been employed to cover
the
solid electrolyte or oxide semiconductor layer used in an oxygen concentration
sensor.
The said porous elements of gas sensors are conventionally fabricated by
various
techniques. Only a few examples, out of many, are presented from the prior
art:
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1. In patent US 4536241 (1985), a slip casting technique is employed to form a
porous
titanium dioxide layer on a pare of conductive leads, for fabricating an
oxygen sensing
element.
2. In patent CA 02188771 (1995), a porous tin oxide body is formed by pallet
pressing,
for fabricating a resistive gas sensor.
3. In "A Novel PVD Technique for the Preparation of Sn02 Thin Films as C2HSOH
Sensors", Sensors and Actuators, vol. 7 (1992), pp 721-726, the authors have
reported
using a physical vapor deposition technique for forming a porous tin dioxide
layer.
4. In patent CA 02226056 ( 1997), a porous layer of alumina- glass composite
has been
formed on a substrate by various techniques, i.e. screening, brushing and
"doctor blade"
techniques, for fabricating a catalytic gas sensor.
5. In patent CA 02172515 (1995), a porous tin oxide layer is formed on a
substrate by a
mufti-step organometalic chemical process.
6. In patent US 5576067 (1996), a porous Zn0 pallet is formed by powder
pressing for
fabricating a carbon monoxide sensor.
7. In patent US 4359709 (1982), a porous iron oxide body is formed by a
pressing
technique for making a combustible gas sensor.
A particular problem which this invention addresses is that the above
mentioned
conventional forming techniques do not render the optimum porous structure
needed.
Although the importance of porosity in device performance is clearly
emphasized in the
prior art, the background literature is, however, virtually silent about the
importance of
pore size distribution and pore morphology on the device performance. A
proposed
porous microstructure is schematically illustrated in Figure 1; in which a and
b present the
perpendicular and horizontal cut views respectively. In this structure, only a
small portion
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of the surface area of each particle is covered by the nearest neighbors but
rest of the said
surface is in contact with open micropores and ready for interaction with the
surrounding
atmosphere. Moreover, the existence of, perpendicular to the surface,
macropores
facilitates a rapid penetration of the gas into the body of the layer. In a
way, the proposed
microstructure is an irregular conical honey come-like porous microstructure
which
affords both a high effective surface area and a rapid gas penetration.
I have discovered that electophoretic deposition (EPD) process applied for a
powder of
suitable particle size distribution, at optimum deposition conditions, renders
porous solid
bodies of fine pore microstructures desired for fabricating gas sensors and
practically,
resembling the above proposed microstructure.
Electrophoresis is an electrokinetic phenomenon in which, charged solid
particles
suspended in a liquid medium, are transported due to an externaly applied
electric field. In
an EPD process, the said transportation is employed for the deposition of the
said particles
onto an oppositely charged electrode (substrate). Although EPD is a widely
used
technique and of a vast background literature in the fields of biochemistry,
analytical
chemistry, chemical engineering and ceramics, it has not yet been used for the
fabrication
of porous solid structures for gas or humidity sensing devices. On the
contrary, many
workers have reported their efforts to obtain flawless and pore-free layers of
ceramic
materials by EPD. In fact, the author has also been working for four years on
minimization, if not elimination, of the porosity in the electrophoretically
deposited
ceramic layers.
EPD of many metallic and ceramic powders onto electrically conductive
substrates has
been carried out for various applications:
1. In patent US 4482447 (1983), a method is taught for EPD of powders from
non aqueous suspensions containing nitrocellulose.
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2. In patent US 5415748 (1995), EPD has been employed for a continuous
deposition of
dense and uniform oxide coatings on electrically conductive substrates.
3. In patent US 464170 (1987), EPD is used for deposition of polymer coatings
on
conductive substrates.
4. In patent US 579456 ( 1998), EPD is used for fabricating polyfunctional
catalysts for
catalytic converters.
5. In patent US 5472583 (1995), dense ceramic bodies containing conical pores
are
produced with an EPD process for filters, burners and catalyst support
applications.
6. In numerous patents granted to Copytele, Inc., e.g. US 05707738 (1998), a
temporary
and selective EPD of colored particles has been employed as a base for
fabricating an
alternative information display device.
7. In patent US 5246916 (1993), EPD of superconductor ceramic materials on
conductive substrates is reported.
A comprehensive review of the background literature on EPD is presented by
Sarkar and
Nickelson in their feature paper: "Electrophoretic Deposition, Mechanisms,
Kinetics, and
Application to Ceramics", J. Am. Ceram. Soc., Vol. 79, No. 8, p.p. 1987
(1996).
2. Summary of the Invention
The first aspect of the present invention is directed to porous solid layers,
deposited on
different substrates by EPD technique for fabricating gas sensor heads:
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a. It is an object of this invention to provide a method for the forming of a
porous layer
of a known ceramic semiconductor such as zinc oxide, tin oxide, titanium
oxide, silicon,
titanates, ferrates, etc., doped or pure, by an EPD of submicron-sized
particles of the said
material onto a substrate and a subsequent controlled sintering of the said
deposite for
fabricating (or as a step in the fabrication process of) a resistive gas or
humidity sensor.
b. It is another object of the present invention to form a porous doped
semiconductor
layer, e.g. copper doped zinc oxide, by a co-EPD of the related powders, e.g.
copper oxide
and zinc oxide, onto a suitable substrate followed by a sintering and
diffusion process, for
fabricating (or as a step in the fabrication process of) a gas or humidity
sensor.
c. It is another object of the present invention to provide a method for the
forming of a
porous layer of a known compound ceramic, e.g. Srl_X BaX Ti03, by a co-EPD of
submicron sized powders of the constituting chemicals, e.g. SrC03 + BaC03 +
Ti02,
followed by a solid state reaction and sintering at an elevated temperature,
for fabricating
(or as step in the fabrication process ofJ a gas or humidity sensor.
d. It is another object of this invention to provide a method for the forming
of a finely
porous layer of a ceramic refractory material, such as alumina, zirconia,
magnesia, etc., by
an EPD of submicron-sized particles of the said material onto a substrate and
a subsequent
controlled sintering of the said deposit followed by a selective area
decoration of the pores
with a precious metal, for fabricating (or as step in the fabrication process
of) a catalytic
gas sensor.
e. It is another object of the present invention to provide a method for the
forming of a
finely porous dielectric, e.g. alumina, E-glass, etc., layer by an EPD of
submicron-sized
particles of the said dielectric onto a solid-state gas sensor element, e.g. a
solid electrolyte
gas sensor head, to protect the said sensor body from environmental hazards.
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f. It is another object of the present invention to provide a method for the
forming of a
solid electrolyte, e.g. (~-alumina, layer by an EPD of submicron-sized
particles of the said
material onto a substrate, for fabricating (or as a step in the fabrication
process of) an
electrochemical gas sensor.
g. It is another object of the present invention to provide a method for the
forming of a
finely porous metal, e.g. Pt, Ni, etc., layer by an EPD of submicron-sized
metal particles
onto a suitable substrate as a step in the fabrication of a catalytic gas
sensor.
h. It is another object of this invention to provide a method for the forming
of a finely
porous metal layer on a semiconductor or solid electrolyte by an EPD of the
submicron-sized particles of the said metal onto the said semiconductor or
solid
electrolyte to provide either an ohmic contact or an electrode on them, while
not
obstructing the penetration of the atmospheric gas molecules into the said
semiconductor
or solid electrolyte body, as a step in the fabrication process of a gas or
humidity sensor.
i. It is another object of this invention to provide a method for the forming
of a finely
porous metal, e.g. Pt., Rh., etc., layer by an EPD of submicron-sized
particles of the said
metal onto a semiconductors e.g. Si, SiC, ZnO, etc., body and a subsequent
controlled
sintering of the said deposit for fabricating a metal-semiconductor junction
in which, the
porosity of the metal affords a quicker and higher level of interaction
between a target gas
and the device.
j. It is yet another object of this invention to provide a method for the
forming of a
mufti-layer structure comprising at least two of the above described layers by
successive
EPDs of the relevant sub micron-sized particles on a single substrate. The
multilayer
structure is sintered either at a single sintering process, if all the layers
have nearly the
same sintering temperature; or layer by layer, assuming that each layer has a
higher
sintering temperature than those above.
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The second aspect of the present invention is directed to providing suitable
substrates for
deposition of the above described layers. The use of different substrates
result in gas
sensors of different embodiments appropriate for different applications.
k. In a preferred embodiment of the present invention, said porous layer is
electrophoretically deposited onto a shaped article made of an
aluminoferrochrome alloy
(an alloy containing substantial amount of aluminum, chromium and iron), the
surface of
which acquires an adherent native aluminum oxide film upon the sintering
process of the
deposit or a subsequent oxidation process, due to which the layer becomes
electrically
insulated from the said metal substrate.
1. In another embodiment, said porous layer is electrophoretically deposited
onto a
shaped segment of an aluminoferrochrome alloy wire the surface of which
acquires an
adherent alumina film as described above, due to which, said porous layer is
electrically
insulated from the said wire and the said wire acts as an electric miniature
heater upon
passing an electric current through. This embodiment is more appropriate for
devices
which operate at elevated temperatures.
m. In another embodiment, the porous layer is electophoretically deposited
onto, a
single crystalline, polycrystalline or amorphous, silicon or silicon carbide
article, the
surface of which acquires a silicon dioxide film upon the said sintering
process or a
subsequent oxidation process, due to which, the said layer is electrically
insulated from
the substrate.
n. In another embodiment, the porous layer is electrophoretically deposited
onto an
insulative ceramic substrate the surface of which is coated with a metal (e.g.
aluminum)
thin film, where the said metal is oxidized upon a subsequent controlled
oxidization (e.g.
heating in air, anodization etc.).
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o. In another embodiment the porous layer is electrophoretically deposited
onto a
ceramic substrate provided with, at least, two precious metal lines printed at
a short
distance from each other (e.g. ~ 0.5 mm) on the surface. The deposit bridges
the gap
between the lines, i.e. it covers the said lines as well as the said gap.
After a subsequent
sintering or heat treatment process, the said metal lines can be used as the
contact
electrodes or ohmic contacts to the device fabricated. The embodiment
described is more
compatible with the present configuration of hybrid electronic circuits.
p. In yet another embodiment of the invention the porous layer is deposited by
an
electrophoretic deposition onto two metal wires secured firmly at a short
distance
(0. S mm) from each other. During the EPD process, the deposit bridges the gap
between
the said wires so that the said wires can be benefited from as the electric
connections or
the lead wires of the device. This embodiment is more favorable for humidity
sensors and
other room temperature operating gas sensing devices.
3. Brief Description of the Drawings
The above objects of the invention will become more clear by reference to the
attached
drawings in which:
Figures la and lb schematically illustrate the perpendicular and horizontal
cross sectional
views of a porous microstructure, proposed for fabricating solid-state gas
sensor heads,
respectively.
Figures 2a, 2b and 2c schematically illustrate the structures of three
different
electrophoretic deposition cells employed.
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Figure 3 graphically illustrates the variations of temperature vs. time in a
typical sintering
process carried out on Zn0 layers.
Figure 4 is a SEM micrograph of an electrophoretically deposited zinc oxide
layer after
drying.
Figures Sa, Sb, Sc and Sd are SEM micrographs of electrophoretically deposited
zinc
oxide layers after sintering for 20 minutes at 800° C (a), 900°
C (b), 1000° C (c) and 1050
oC (d).
Figure 6 schematically illustrates the structure of the zinc oxide resistive
gas sensor
fabricated; a and b are side and top views respectively.
Figure 7 graphically shows the variations of the device resistance vs. the
smoke
concentration in air, at 300° C.
Figure 8 graphically shows the relationship between the device sensitivity and
smoke
concentration, at 300° C.
Figure 9 shows a plot of the device conductance vs. time when it is inserted
to and
extracted from an air chamber containing 1000 ppm smoke, as fast as possible
by hand, at
300° C.
Figure 10 schematically illustrates a zinc oxide resistive gas sensor
fabricated on a
self heating aluminoferrochrome alloy substrate, where (a) and (b) are the top
and side
views respectively, the mounted sensor head is shown in (c).
Figure 11 graphically illustrates the variations of the device impedance vs.
humidity of the
surrounding atmosphere, at room temperature.
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Figure 12 schematically illustrates a zinc oxide resistive gas sensor head
fabricated on a
pair of Pt electrodes printed on a commercially available alumina substrate.
Figure 13 schematically illustrates a porous zinc oxide body
electrophoretically formed on
a pair of platinum wires (a), and a mounted humidity sensor head (b).
Figure 14 schematically illustrates the mufti-layer Zn0/Mg0/Zn0 resistive gas
sensor,
fabricated according to the invention.
4. Detailed Description of the Invention
It has been discovered that an electrophoretically deposited solid layer has a
porous
microstructure which is valuable in fabricating gas and humidity sensors. It
was also
discovered that the dynamic performance of a gas sensor fabricated based on
the said
porous layer is superior to that of a similar but conventionally fabricated
device. The
technique is virtually applicable for preparing all the porous layers required
in fabricating
various solid-state gas sensor types. Moreover, the introduction of EPD
technique to the
field of gas sensor fabrication brings in many economical and technical
facilities which
are difficult or expensive to acquire by using conventional techniques. An
important
example is the fabrication of finely porous mufti-layer structures for
multifunctional gas
sensors. Another important feature is that, EPD affords layer deposition on
differently
shaped conductive articles, and its application to gas sensor fabrication adds
a "substrate
shape freedom" to the art related. Moreover, selective deposition for
preparing patterned
layers (arrays, etc.) is also readily possible by masking or conventional
lithography prior to
the EPD process.
However, the discovery is of particular significance in connection with the
resistive type
gas and humidity sensors, for which it readily facilitates the deposition of
finely porous
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oxide semiconductor layers such as zinc oxide, tin oxide, titanium oxide iron
oxide,
ferrates (e.g., Srl_X BaX Fe03), titanates (e.g. Srl_X BaX Ti03), etc. The
technique can also
be used for preparing porous layers of organic semiconductors, which are
materials of
great potentials in the future of gas sensors. The technique is also of
significance in
making porous solid layers required in fabricating humidity sensors, as the
dynamic
performance of the accordingly fabricated humidity sensors is superior to
those fabricated
by conventional techniques.
Another aspect of the invention is directed to a basic technical problem
encountered with
the electrophoretically deposited semiconductor layers, The problem arises
from the fact
that EPD, similar to all other electro-deposition techniques, requires an
electrically
conductive substrate. However, substrate conductivity interferes with any
electrical
measurement which is to be carried out on the said semiconductor layer. For
example, in
case of a resistive gas sensor, a conductive substrate would short circuit the
varying
resistance of the device and considerably reduce the device sensitivity. This
technical
problem was solved by using a substrate made of a material which can acquire a
native,
electrically insulating film on its surface after the EPD process. Silicone
and silicone
carbide are both quite well known for readily acquiring a native silicon
dioxide film, both
of which were successfully employed as substrates. However, aluminoferrochrome
alloys
were discovered to be more acceptable EPD substrates. This alloy material not
only
solves the above described problem by acquiring an aluminum oxide film on its
surface,
but also presents versatilieties absent in other substrates. Particularly,
their high
temperature stability, suitable resistivity and ease of shaping afforded many
technical
advantages. This aspect of the present invention is of general significance in
the field of
electroceramics, and its applications are not limited to gas sensor
fabrication or the
electrophoretic deposition described. To the best knowledge of the author,
such alloys
have never been considered as the substrate for any electroceramic deposition,
though they
present many interesting features specially in conjunction with high
temperature
electronic components and circuits.
The invention will now be discussed in greater details for a few non-limiting
example
embodiments:
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Example 1
In accordance with the invention, a resistive polycrystalline zinc oxide gas
sensor was
fabricated; details follow:
Zinc oxide powder was prepared by oxidation of zinc vapour obtained by melting
a
commercially available zinc ingot in an alumina crucible and heating the said
melt to
800° C . About S.0 gr. Zn0 powder was added to 200 cc analytical grade
acetone
(MERCK # 13) and agitated in an ultrasonic bath for 15 min. The slurry was
left
unagitated for 30 min. for sedimentation of larger particles and aggregates.
Some of the
thin suspension at the top was then poured into a second beaker. This
suspension was
stable and usually had a solid concentration of ~ 0.03 w%. The solid
concentration was
measured by weighing of the residues obtained from drying of 10 cc samples.
Exposure
of all materials and suspensions to the open air was minimized and experiments
were
carned out in clean conditions. Addition of ~ 40 ppm of HCl to the acetone,
upon
preparation of the suspension as a charging agent, enhanced both the stability
of the
suspension and the deposit / substrate adherence.
The solid phase of a sample suspension, separated totally by an
electrophoretic deposition
on a Pt substrate, was investigated for impurities present by a plasma
spectroscopy
technique, revealing the major impurities to be Pb, Ca & Al with
concentrations of 1400,
180 and 180 ppm respectively; other impurities were below 100 ppm. The average
particle size of the said phase, measured by a line averaging method on
scanning electron
micrographs, was ~ 0.1 Vim.
The schematics of the electrophoresis cell employed is shown in Figure 2a. It
consists of a
borosilicate glass beaker (1) containing ~ 100 cc suspension (2) of a known
concentration
(~ 0.02 w%) and two circular, perforated Pt foil electrodes (3 and 4). The
electrodes are
secured at a distance of 40 mm from each other by three alumina rods (5).
Platinum wire
(6) was used for actuation of all electrical and mechanical connections. The
alloy
substrates (described below), polished and thoroughly washed with pure
acetone, were
placed on the lower electrode. The electrode system was then immersed in the
glass
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beaker. A D.C. voltage of 1000 volts was applied between the electrodes; while
the
lower electrode constituted the cathode. Deposition started upon applying the
said
voltage. After a deposition time of ~ S minutes, nearly all the solid
particles of the
suspension had been deposited, and the slurry was totally clear. The voltage
was then
disconnected and the electrode system was extracted from the cell. The
substrates were
taken off the cathode and placed on a SiC slab and left for drying in air. The
said slab was
then placed in a laboratory electric muffle furnace. The heating elements and
the muffle
of the furnace were both made of SiC. A typical temperature profile used for
the
sintering of the deposits is shown in Figure 3. Various sintering temperatures
were
employed. For a soaking time of 20 minutes, the optimum sintering temperature
was
about 1030° C.
The layers deposited appeared uniform when observed with naked eye or under a
magnifying glass. They were examined for uniformity and thickness measurement
by
optical microscope. The layer thickness was controlled by controlling the
concentration
of the suspension used. Uniform layers in the thickness range of 10-50 '.,tm
were
reproducibly obtained. For the measurements reported below, a ~ 25 ~..l,m
thick layer has
been employed. The density of the layer was estimated to be less than 20% of
the
theoretical density of ZnO.
The microstructures of the porous zinc oxide films obtained, were studied by a
scanning
electron microscope. Figure 4 shows the micrograph of an as deposited zinc
oxide layer,
which illustrates the morphology and size of the zinc oxide particles present
in the
suspension. Figures Sa to 5d show the micrographs of the samples sintered at
800, 900,
1000 and 1050° C respectively. At higher temperatures, however, grain
growth decreased
the porosity of the layer. Best results regarding sensitivity, response time
and stability of
the device were obtained from the layers sintered at 1030° C.
It is important to mention that the microstructures of Figures 5 c and d are
in many ways
similar to that schematically shown in Figure 1. In fact, my experimental work
was not
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limited to Zn0 and the results of similar observations on electrophoretically
deposited
layers of other materials e.g. MgO, CdS, Si, Ti02, etc. in respective
conditions followed
the same pattern. Furthermore, SEM studies on deposits obtained from
suspensions of
very low solid concentrations and short deposition times proved that the fine
porous
structure obtained is a result of particle by particle deposition mechanism
encountered
with EPD process. This effect was more profoundly observed in EPD from thin
suspensions (concentrations <0.1 w%). The particle size distribution and the
average
particle size of the powder used were also, obviously, very important factors
in
determining the porosity and the pore size distribution of the layer
deposited. A narrower
particle size distribution and a smaller average particle size were both
advantageous in
resulting for finer porous structures suitable for fabricating faster gas
sensors.
Ohmic contacts were constructed, as shown in Figure 6, by silver paste
printing and a
subsequent heat treatment at 250° C . Alternatively the same can be
done by gold
sputtering; alternatively the same can be done by aluminum evaporation,
alternatively it
can be done by other conventional metallization techniques. The I-V, i.e.
current vs.
voltage, curve of the device fabricated was linear, indicating that the said
contacts were
substantially ohmic.
The alloy substrates used, were polished 1 x 6 x 15 mm3 slabs of
aluminoferrochrome
alloy (# Al, obtained from Kanthal, Sweden), containing about 5 w% of
aluminum. This
alloy has the ability of acquiring an adherent aluminum oxide layer, on its
surface at
elevated temperatures, which protects it from further destructive oxidation.
In fact, this
property is the main reason for their extensive use in the field of high
temperature
technology, specially as electric heating elements. During the above mentioned
sintering
process the surface of the substrate acquired a film of aluminum oxide which
electrically
insulated the metal substrate from the zinc oxide layer. The said aluminum
oxide film
also enhanced the mechanical stability of the zinc oxide layer on the
substrate. In case a
thicker layer was needed (e.g. when the final device is to operate with a
higher applied
voltage) or the sintering temperature was not high enough to result the oxide
of sufficient
CA 02267881 1999-03-25
thickness, oxidation was further proceeded by a conventional anodization at
room
temperature, after sintering. The porous zinc oxide layer did not interfere
with the
oxidation of the substrate surface during the anodization due to its high
porosity. The
configuration of the sensor head fabricated is schematically shown in Figure
6.
The performance of the device was tested in an air tight chamber containing
air with
known amounts of wood smoke (the target gas). The device was placed on a
controlled
miniature hot plate, the temperature of which was adjusted at 300° C .
The device
resistance was measured at various smoke concentrations. Results are
graphically shown
in Figure 7. Also in Figure 8 the sensitivity of the device, defined as (Cg-
Ca)/Ca, where
Cg and Ca are the conductances of the device at the presence of the target gas
and in clean
air respectively, is drawn vs. the target gas concentration. The dynamic
response of the
device was measured at 300° C by suddenly entering the device into a
chamber with a
target gas concentration of 300 ppm. The response time, defined as the time
necessary for
the device to acquire 70% of the conductivity gain expected, was estimated to
be less
than 0.2 sec. Equipment available did not allow more accurate measurements. A
plot of
the conductance vs. time when the device is inserted into a chamber containing
1000 ppm
of the target gas and then extracted as fast as possible by hand, is given in
Figure 9. The
recovery time, defined as the time needed for the device to lose 70% of its
conductivity
gain when returned into clean air, was estimated to be shorter than 3 seconds.
Example 2
According to another aspect of the invention, a 70 mm long segment of an
aluminoferrochrome wire of ~ 1.2 mm diameter (# Al, Kanthal, Sweden) was
hammered
at the center to form a flat and thin platform as shown in Fig. 10a. The
platform was
polished and washed. Then the alloy segment was placed on the cathode and was
inserted
in the EPD cell for deposition, as described in Example 1. Other fabrication
steps of
Example 1 were followed. The schematic of the device obtained is shown in Fig.
lOb.
The device performance was similar to that described in Example 1 but it
needed no
external heating, because passing an electric current through the said wire,
as shown in
Figure lOb, controllably heated the platform. The temperature of the said
platform was
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CA 02267881 1999-03-25
controlled, by attaching a temperature sensor to the base of the said platform
as shown
in Figure l Ob.
Due to the excellent high temperature durability of the aluminoferrochrome
wires, the
invented integrated heating system described is reliable, less power consuming
ana
inexpensive. The device was mounted on a ceramic stand, e.g. steatite,
alumina, etc., to
form a complete detector head. The device fabricated is schematically shown in
Figure lOc. It is important to mention that the substrate material and the
substrate
configuration described is of a general technical significance in fabrication
of all
elecroceramic devices specially if the said device is to operate at an
elevated temperature.
The invented substrate eliminates the need for cumbersome and expensive
conventional
miniature substrate heaters.
Example 3
The experiment of Example 1 was repeated while the zinc oxide powder used
contained
~1.0 w% of copper oxide. The dopant powder (Cu0) was mixed with the Zn0 powder
using a fast mill. The mixture was co-deposited by EPD from a thin acetone
suspension
as described in Example 1. The copper ions diffused uniformly into the deposit
during the
sintering process and the product was a porous copper doped zinc oxide layer.
Alternatively the same doping was achieved by the immersion of the device,
fabricated as
in Example 1, in a copper nitrate solution and heat treating it at about
700° C for nitrate
decomposition and dopant diffusion. Copper doping enhances the device
sensitivity to
hydrogen, as described in the prior art (e.g. see US 5576067, 1996).
Example 4
A device fabricated according to Example 1 but operated at room temperature,
was
humidity sensitive. Its A.C. resistance, measured at a frequency of 420 Hz,
varied
with the humidity of a closed chamber, measured by a standard electronic
humidity
sensor. The relationship between humidity and device impedance is graphically
shown in
Figure 11. The device can be used as a humidity sensor head, characterized in
both a
linear operation curve and a fast response.
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CA 02267881 1999-03-25
A similar humidity sensor can be made according to Example 2, where heating of
the
sensitive layer is carried out occasionally for eliminating surface poisons
and recalibration
purposes. Similar devices can also be fabricated using other semiconductors or
dielectric
materials, zinc oxide was employed for demonstration purpose only. The device
can also
be altered by introducing dopants as described in Example 3.
Example 5
In a different embodiment of the invention, a commercially available ceramic
(e.g.
alumina) substrate was used as a substrate for the EPD of zinc oxide powder.
At least,
two parallel platinum lines, at a short distance (~ 0.5 mm) from each other,
had been
printed on the surface of the said substrate. Said lines could also be of any
other
refractory and oxidation resistant metal such as platinum alloys, gold,
nickel, etc. The
substrate surface was roughened by an abrasive for a better adherence of the
deposit to
the substrate. The schematic of the substrate is shown in Figure 12. Aluminum
foil with a
rectangular slot, defining the deposition area, was rapped around the
substrate as a
deposition mask, which was also the means of connection to the cathode. The
substrate
was then placed in the EPD cell as described in Example 1. The deposit bridged
the gap
between the two metal lines due to an electrostatic edge effect. After the
sintering
process, the said platinum lines constituted the ohmic contacts needed. The
configuration
of the device is schematically shown in Figure 12. The device is compatible
with the
prevailing hybrid circuit technology, and can be operated in conjunction with
all types of
miniature heaters to provide the elevated temperature necessary. Both gas
sensors and
humidity sensors can similarly be fabricated on substrates which also support
the hybrid
back-up circuitry. By providing more than two conductor lines on the
substrate, a linear
array of gas sensors can be obtained.
In another embodiment, the surface of the said ceramic substrate had been
coated with a
metal (e.g. aluminum) thin film by a conventional technique (e.g. vacuum
evaporation).
EPD of the porous layer was then carried out, as described above, onto the
metal thin film.
The metal thin film was later oxidized upon a heat treatment in air or during
the sintering
process, or by an anodization process after sintering, leaving the porous zinc
oxide layer
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CA 02267881 1999-03-25
on an insulative background. The said metal can also be chosen so that its
oxide is of a
second use or significance, such as enhancing the layer/substrate adherence or
providing a
dopant source. For example using a copper thin film in the experiment
described would
finally provide the copper oxide source needed for fabricating a copper doped
zinc oxide
layer. The ohmic contacts of the device were provided as described in Example
1.
Example 6
In a different embodiment, the alumina substrate used was cylindrical in
shape, while 12
metal lines, printed parallel to its axis, had covered the outer surface. An
EPD cell of
cylindrical geometry, schematic of which is shown in Figure 2b, was employed
for the
electrophoretic deposition of the porous zinc oxide layer. The deposit bridged
the gap
between the metal lines as described in Example 5. A miniature electric heater
and a
temperature sensor were placed inside the cylinder to provide the controlled
elevated
temperature needed. T'he multi-contact cylindrical head afforded a two
dimensional,
directional gas sensing ability.
Example 7
In another embodiment of the invention, EPD of zinc oxide powder was carried
out onto,
at least, two segments of Pt, or any oxidation resistant metal wires. The wire
segments
were secured at a short axial distance (~ 0.5 mm) from each other, so that the
said distance
was bridgeable by the EPD deposit. A cylindrical EPD cell (Figure 2b) was
employed for
the deposition, in which the diameter of the cylindrical anode was 50 mm while
the
voltage applied was 100 volts. The. deposit was sintered as described in
Example 1. The
said pair of wires constituted the ohmic contacts and lead wires of the device
and no
further metallization was required. Other wires or stand rods, to provide
further facilities
such as other ohmic contacts, mechanical stability, temperature measurement,
etc. can
also be added to the structure described. The schematic of the device is
presented in
Figure 13.
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CA 02267881 1999-03-25
Example 8
To produce a different embodiment of the invention, the experiment of Example
1 was
repeated for a polished substrate of single or poly-crystalline silicon (or
silicon carbide).
The substrate surface acquired a layer of Si02 due to a controlled oxidation
during the
sintering process, by which the deposit was electrically insulated from the
substrate. The
partial pressure of oxygen in the sintering chamber was reduced (in case of
silicon only) to
avoid excessive oxidation. Contact electrodes could be deposited by
conventional
metallization techniques, e.g. aluminum evaporation.
The same experiment can be repeated for a similar deposition on a preoxidized
silicon or
silicon carbide wafer, on the surface of which at least two windows are opened
by
lithography, at an EPD bridgeable distances from each other. Sintering is
carried out at a
controlled atmosphere. The windows should connect the sensor to an integrated
circuit
fabricated on the same chip or wafer.
Example 9
According to another aspect of the invention and to demonstrate the
possibility of
1 S fabricating porous mufti-layer structures, the invented process was
employed for making a
porous Zn0/Mg0/Zn0 mufti-layer resistive gas sensor. The substrate used and
the first
layer deposition were both as described in Example 1. The electrode system was
then
extracted from the Zn0 deposition cell and immediately immersed in a second
similar cell
but containing a thin suspension of magnesia. The magnesia powder used was
"light extra
pure" obtained from MERCK (# 105862). The preparation of Mg0 suspension was
carried out as was described in Example I. The electrophoretic mobility of Mg0
particles
in acetone is positive i.e. Mg0 particles, similar to Zn0 particles, move
towards the
cathode. The second layer (Mg0) deposition was carried out by an applied
voltage of
1000 volts. Thickness of the layer deposited was estimated by optical
microscope
examinations to be about 15 p.m. The electrode system was then reimmersed in
the first
cell for the third layer deposition and a layer of Zn0 similar to the first
layer was
deposited. In all of the depositions simple masks were employed to delineate
the areas of
each layer. Ohmic contacts were made to both of the Zn0 layers (i.e. the first
and the
CA 02267881 1999-03-25
third layers), after a sintering process at 1050° C. Finally both
layers were tested for
smoke detection, while different measurement frequencies were employed for
each layer
to prevent them interfering with each other.
Different mufti-layers could be resulted by starting from any of the
substrates and
deposition techniques described in Examples 2-8. My experiments on this more
sophisticated gas sensor is not yet complete, observation of a sensing delay
between the
third and first layers indicates great potentials for future applications of
mufti-layer
structures. In Figure 14 the structure of the above described mufti-layer is
schematically
illustrated.
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