CA2267881A1 - Method of making fast solid state gas sensors - Google Patents

Abstract

A process is disclosed for forming finely porous solid layers used in fabricating gas and humidity sensors, which comprises electrophoretically depositing submicron-sized particles of the said solid onto a conductive substrate from a stable or semi-stable suspension and a subsequent heat treatment. The process readily affords making of a fast resistive gas sensor comprising a finely textured porous semiconductor film and at least two ohmic contacts. In a preferred embodiment the said porous layer is deposited onto a shaped article of an alloy containing substantial amounts of aluminum, chromium and iron, where a native aluminum oxide film insulates it from the said semiconductor layer after the deposition. The process also facilitates the making of finely porous multi-layer structures for more sophisticated gas sensors.

Description

PATENT APPLICATION
INVENTION: METHOD OF MAKING FAST SOLID STATE GAS
SENSORS
INVENTOR: FARA,~~IARZ HOSSEINBABAEI
APPLICANT: FAR.~~MARZ HOSSEINBABAEI
~~-~~: dc~4-~~~_o~o'~

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 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:

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 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.

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:

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.

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.

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.).

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.

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.

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 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:

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 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 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 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 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.

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 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.

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 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.

Claims (27)

1. A process for forming finely porous solid layers, required in fabricating different types of both gas sensors and humidity sensors, which comprises:
A. Providing a stable or semistable thin suspension containing submicron-sized particles of the said solid in a liquid.
B. Electrophoretically depositing the said submicron-sized particles from the said suspension onto an appropriately shaped conductive or semiconductive substrate, to form the layer required.
C. Drying the layer and the substrate.
D. Heat treatment of the said layer at an elevated temperature.

2. The process of Claim 1 in which binding and charging agents are added to the said suspension upon its preparation.

3. The process of Claim 2 in which the drying (C) process is extended gradually to higher temperatures to remove the said agent materials by an evaporation or decomposition, process.

4. Any of the processes of Claims 1-3 in which the heat treatment (D) of the formed layers is carried out for any sintering, solid-state reaction or diffusion purposes.

5. Any of the processes of Claims 1-4 in which the heat treatment (D) of the formed layer is carried out in a controlled atmosphere for any protection, oxidation, nitridation, reduction or doping purposes, regarding either the substrate or the layer.

6. Processes of Claims 1-5, in which the said suspension contains submicron-sized particles of a pure or doped semiconductor.

7. Process of Claim 6 in which the said semiconductor is either a metal oxide or an organic semiconductor.

8. Any of the processes of Claims 1-5 in which the suspension contains submicron-sized particles of an electrically insulating material.

9. Any of the processes of Claims 1-5 in which the suspension contains submicron-sized particles of a metal.

10. The processes of Claims 1-9 in which submicron-sized particles of two or more materials are present in the suspension and they are co-deposited electrophoretically, where homogeneity is subsequently obtained upon the said heat treatment (D).

11. Any of the processes of Claims 1-10 in which the suspension contains, solely or in addition to the main solid particles, submicron-sized particles of a binder or a sintering aid, for either reducing the temperature needed in the said heat treatment (D) or to enhance the adherence of the porous layer to the substrate.

12. A process for forming finely porous multi-layers for fabricating more sophisticated gas or humidity sensors consisting two or more porous layers of either different materials or the same material with or without different dopings. The process comprises any of the processes of Claims 1-11 in which the said electrophoretic deposition (B) is repeated for each layer, wet-in-wet, and the final multilayer structure is then dried and heat treated (D).
Alternatively, the process comprises any of the processes of Claims 1-11 carried out for the material of highest heat treatment (D) temperature followed by repeating one or more of the processes of Claims 1-11 for other materials in descending order of their heat treatment temperatures.

13. Any of the processes of Claims 1-12 in which the conductive substrate used has the property of acquiring a stable oxide or nitride insulator film on its surface, which can electrically insulate the said substrate from the said porous layer after the said EPD.

14. The process of Claim 13 in which the said insulating oxide or nitride film is formed during the said heat treatment (D).

15. The process of Claim 13 in which the said insulating oxide film is formed by an anodization process either before or after the said heat treatment (D).

16. Any of the processes of Claims 1-15 in which the said conductives substrate is an article made of either silicon or silicon carbide.

17. Any of the processes of Claims 1-15 in which the said substrate is an article made of an alloy containing substantial amounts of either aluminum or vanadium.

18. Any of the processes of Claims 1-17 in which the substrate used is an article made of any alloy containing substantial amounts of aluminum, chromium and iron.

19. Any of the processes of Claims 1-18 in which the conductive substrate is appropriately shaped so that it can be locally heated by passing an electric current through the said substrate.

20. Any of the processes of Claims 1-12 in which the substrate is an insulative article on which a conductive film is provided.

21. The process of Claim 20 in which the said conductive film is patterned prior to the said electrophoretic deposition.

22. Either of the processes of Claims 20 or 21 in which the said metal film is converted to an oxide or nitride after the said electrophoretic deposition.

23. Either of the processes of Claims 20 or 21 in which the said insulative substrate is a ceramic, glass or glass ceramic article.

24 24. Either of the processes of Claims 20 or 21 in which the said conductive film comprises a precious metal film.

25. Any of the processes of Claims 1-12 in which at least two metal wires, secured at a distance from each other, constitute the said conductive substrate, where the said distance is bridged by the electrophoretically deposited layer.

26. The process of Claim 25 in which the said wires also provide the electrical contacts to the layer deposited.

27. A self heating substrate, made of an alloy containing substantial amounts of aluminum, chromium and iron, for the deposition of electroceramic materials.