Gas sensor

A gas sensor comprises a layered structure with an ionic conductive film and a high gas-permeability interlayer film, a first catalyst electrode and a second catalyst electrode, a conductivity promotion structure, a high-k layer and a current detecting unit. The ionic conductive film includes a material with ionic conductivity ranging from 0.02 to 1000 S/cm. The first catalyst electrode and second catalyst electrode are located on the layered structure and spaced by a predetermined distance for ionizing a gas and reducing the ionized gas, respectively. The conductivity promotion structure includes a material with electronic conductivity ranging from 10−5 to 105 S/cm, and provides wanted electrons for a reduction reaction. The high-k layer is interposed between the conductivity promotion structure and layered structure. The current detecting unit is coupled to the first catalyst electrode and second catalyst electrode to sense a detecting current with respect to the ionized gas.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a gas sensor, particularly for a current-type gas sensor.

(2) Description of the Prior Art

Basically, current popular conventional gas sensors according to the operation mechanism can be categorized into following types: catalytic combustion type, oxide semiconductor type, solid state electrolyte type (using sensing electric voltage, current or resistance as signal), field-effect transistor type (FET), infrared type and so on. By electrode configuration of the conventional technology, the popular conventional gas sensors can further be classified into electrode coplanar configuration and electrode un-coplanar configuration. In the early phases, the conventional gas sensors of electrode un-coplanar configuration take the dominance, which are further sorted into tubular profile, planar profile and compound profile. Whereas, the conventional gas sensors of electrode coplanar configuration are apt to be adopted in popular manner because the thin-film technology is gradually improved and advanced. Moreover, the gas sensors can be grouped into electric voltage signal kind and electric current signal kind in accordance with the sensing method and detecting signal. Nowadays, a combinational gas sensor in merging the electric voltage signal kind and electric current signal kind emerges from the automobile industry.

FIG. 1is an illustrative schematic view for a conventional current-type oxygen sensor10. As shown in the figure, the oxygen sensor10comprises a first catalyst electrode11, a second catalyst electrode12, an electrolyte layer13, a gas diffusion cavity14with a gas diffusion opening17, a power supply15and a galvanometer or current meter16. The power supply15, which is usually a battery, is electrically connected to both of the first catalyst electrode11and the second catalyst electrode12. The current meter16is parallel connected to the power supply15. The first catalyst electrode11is located in the gas diffusion cavity14. The gas diffusion opening17is bored at the top surface of the gas diffusion cavity14.

Upon performing detecting operation, the power supply15will provide a voltage to both of the first catalyst electrode11and second catalyst electrode12to initiate of the gas sensor10so the oxygen gas is enabled to enter into the gas diffusion cavity14via the gas diffusion opening17. In the gas diffusion cavity14, the oxygen gas is chemically is ionized into oxygen ion by the first catalyst electrode11so a limiting current is generated by the flow of gas ions and/or electrons from the first catalyst electrode11to the second catalyst electrode12by the oxygen vacancies in the electrolyte layer13. Thereby, by measuring the magnitude of the limiting current via the current meter16, the ambient oxygen concentration can be determined. Normally, the measured limiting current signal value of the current meter16is direct proportional to the partial oxygen pressure in the ambient atmosphere.

Regarding the conventional current-type gas sensor, the thickness of the electrolyte layer13will affect the sensitivity of the gas sensor10, which means the thinner for the thickness of the electrolyte layer13, the better for the sensitivity of the gas sensor10is. However, subjecting to the material feature of the electrolyte layer13, the thickness reducing of the electrolyte layer13has its critical limit otherwise it is susceptible to break if it exceeds its critical limit. Moreover, the conventional current-type gas sensor requires operating in higher working temperature for keeping stability because the conductor material of able ionization adopted by the conventional electrolyte layer13is almost solid electrolyte. Accordingly, an extra conventional heating accessory is needed to maintain the higher working temperature. However, if sudden fluctuation happens in the ambient temperature, the conventional heating accessory is usually unable to adequately response in adjustment for the suitably corresponding working temperature with result that the detecting accuracy of the conventional current-type gas sensor is harmfully affected.

SUMMARY OF THE INVENTION

Having realized foregoing issue and demand, the inventor of the present invention elaborately performs long term research and development on the basis of personal experience accumulated from practical application of many years. Eventually, a brand-new gas sensor of the present invention is worked out. The primary object of the present invention is to provide a gas sensor with features of high stability, comprehensive applicability and capability for solving existing drawbacks in the conventional gas sensor.

The present invention provides a current-type gas sensor comprising a layered structure, a first catalyst electrode, a second catalyst electrode, a conductivity promotion structure, a high-k layer (k denotes dielectric constant) and a current detecting unit. The layered structure includes an ionic conductive film and a high gas-permeability interlayer film stacked in an alternative manner. The thickness of the ionic conductive film is greater than or equivalent to that of the high gas-permeability interlayer film. The ionic conductive film is made of ionic material with thickness in range of 1 to 500 nanometers, and ionic conductivity in range of 0.02 to 1,000 S/cm. The first catalyst electrode and the second catalyst electrode are disposed on the layered structure or at a lateral side of the layered structure with a gap or an interspace therebetween. A gas is ionized at the first catalyst electrode into gaseous ions, and the gaseous ions move to the second catalyst electrode via the high gas-permeability interlayer film of the layered structure such that the gaseous ions can be reduced by an reduction reaction at the second catalyst electrode. A voltage required to generate a detecting current is also provided by the first catalyst electrode and the second catalyst electrode. The conductivity promotion structure is made of a material with electronic conductivity in range of 10−5to 105S/cm for serving as a electron sink to is provide free electrons to enhance foregoing dissociating and reduction reactions. The high-k layer is sandwiched between the layered structure and the conductivity promotion structure, wherein the k denotes dielectric constant. The current detecting unit is electrically connected to the first catalyst electrode and the second catalyst electrode to detect and measure detecting current.

In an exemplary embodiment, the ionic conductive film for the current-type gas sensor in the present invention is made of ionic material with thickness in range of 1 to 500 nanometers, and the thickness of the ionic conductive film is greater than or equivalent to that of the high gas-permeability interlayer film.

In another exemplary embodiment, the current-type gas sensor in the present invention further comprises an active thermal control module including a heating unit and a temperature control unit, and the heating unit is used to heat the layered structure while the temperature control unit is used to monitor and control the heating unit for the purpose of controlling the power output of the heating unit.

In the other exemplary embodiment, the current detecting unit can be replaced by a voltage detecting unit to form a voltage-type gas sensor.

The other objects and features of the present invention can be further understood from the disclosure in the specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2is an illustrative schematic view for a current-type gas sensor100in the first preferred exemplary embodiment of the present invention.

As shown in theFIG. 2, the gas sensor100comprises a layered structure130, a first catalyst electrode110, a second catalyst electrode120, a gas diffusion layer170, a power supply180, a conductivity promotion structure140, a high-k layer (k denotes dielectric constant)150and a current detecting unit160.

The layered structure130, which is preferably disposed on the high-k layer150, includes an ionic conductive film131and a high gas-permeability interlayer film132stacked in an alternative manner. The size of the ionic conductive film131is bigger than that of the high gas-permeability interlayer film132. The ionic conductive film131is made of ionic material with thickness in range of 1 to 500 nanometers, and ionic conductivity in range of 0.02 to 1,000 S/cm.

The first catalyst electrode110and the second catalyst electrode120, which are disposed on an upper surface of the layered structure130, are preferably interdigitated with mutually interspaced (as shown inFIG. 3) to increase the sensing area thereof and shorten the mutual interspace or the gap therebetween.

The gas diffusion layer170fully covers the first catalyst electrode110and second catalyst electrode120as well as the layered structure130.

The power supply180, which is electrically connected to the first catalyst electrode110and the second catalyst electrode120preferably, provides potential energy to the first catalyst electrode110and the second catalyst electrode120so the natural gas to be sensed at the first catalyst electrode110is ionized into gaseous ions status while the ionized gaseous ions at the second catalyst electrode120are reduced back to non-ionic natural gas status.

The conductivity promotion structure140is made of a material with electronic conductivity in range of 10−5to 105S/cm to provide free electrons to enhance foregoing dissociating and reduction reactions.

The high-k layer150is sandwiched between the layered structure130and the conductivity promotion structure140to isolate both of which.

The current detecting unit160such as galvanometer or current meter, which is electrically connected to the first catalyst electrode110and the second catalyst electrode120in parallel with the power supply180, to preferably detect and measure an electric current interflowing between the first catalyst electrode110and the second catalyst electrode120. In an embodiment, the electric current is a limiting current.

FIG. 2Atakes oxygen gas as example to show three paths for generated a detecting electric current by the current-type gas sensor in the first preferred exemplary embodiment of the present invention. When the power supply180provides potential energy to the first catalyst electrode110and the second catalyst electrode120, the natural oxygen gas molecule (O2) to be sensed at the first catalyst electrode110is ionized into gaseous oxygen ions (O2−) status while the ionized gaseous oxygen ions (O2−) at the second catalyst electrode120are reduced back to non-ionic natural oxygen gas molecule (O2) status.

In path (1), partial oxygen ions (O2−) ionized at the first catalyst electrode110are permeated into the ionic conductive film131and moved to the second catalyst electrode120, where the ionized gaseous oxygen ions (O2−) are reduced back to non-ionic natural oxygen gas molecule (O2) status for releasing out. Normally, a higher working temperature in operating the gas sensor100is required for the path (1) because it purely relies on the ionic conductive film131due to completely being proceeded therein.

In path (2), the high gas-permeability interlayer film132serves as another path for generated a detecting electric current as the high gas-permeability interlayer film132is full of oxygen vacancies. By these oxygen vacancies, a lot of oxygen ions (O2) are moved from the first catalyst electrode110to the second catalyst electrode120for reducing back to non-ionic natural oxygen gas molecule (O2) status upon potential difference applied between the first catalyst electrode110and the second catalyst electrode120by the power supply180.

In path (3), because all thicknesses of the ionic conductive film131and the high gas-permeability interlayer film132are in nanometer scale, a lot of free electrons generated in the conductivity promotion structure140are penetrated through the layered structure130to the first catalyst electrode110by the tunneling effect, and vice versa. The conductivity promotion structure140serves as a electron sink for provide more free electrons interflowing between the first catalyst electrode110and second catalyst electrode120, so more oxygen ions (O2−) are created. With these interflowing free electrons by the tunneling effect, the gas sensor100in the present invention can even operate under low temperature as in room temperature. Contrastively, please refer to a non-conductivity promotion structure140′ of the gas sensor100′ shown in theFIG. 2B, which lacks of the conductivity promotion structure140shown in theFIG. 2orFIG. 2A. Without free electrons generated by the conductivity promotion structure140, all the detecting electric currents solely rely on the oxygen ions (O2−) created in path (1) and path (2), a higher working temperature in operating the gas sensor100′ is required even the existing of the high gas-permeability interlayer film132.

The reaction rate of abovementioned reaction increases when the concentration of oxygen contained in the ambient atmosphere. Therefore, more carriers flow from the first catalyst electrode110to the second catalyst electrode120, and the larger limiting current is measured by the current detecting unit. The partial concentration of the reacting oxygen contained in the ambient atmosphere can be effectively detected by the gas sensor100of the present invention.

In addition, the measuring sensibility of the gas sensor100in the present invention is adjusted by adjusting the transverse displacement of the reacting gaseous ion and/or electron. For the gas sensor100of the present invention, the transverse displacement is normally the distance between the first catalyst electrode110and the second catalyst electrode120, and the transverse displacement is controllable by the ordinary semiconductor process. Therefore, the gas sensor100of the present invention has potential in suitably application in the sensing environment, which requires high sensibility.

Regarding the gas sensor100in the present invention, the material of the first catalyst electrode110and the second catalyst electrode120can select from metals such as platinum, gold, palladium, rhodium, Iridium, ruthenium, osmium, nickel, cobalt, aluminum and iron etc., each of which is easy to form electrochemical reaction with gaseous oxygen, or the perovskite family of ceramic materials such as LaSrMnO3or LaSrCoFeO3, each of which is easy to form electrochemical reaction with gaseous oxygen, or the composites formed by zirconia with foregoing metals or ceramic materials to provide conductivity for both of free ions and electrons. Moreover, for the composing materials in the first catalyst electrode110and the second catalyst electrode120, an extra second material of property-modifying additive such as copper, cerium oxide etc. can be added to enhance anti-carbon, antitoxic and anti-sulfuring capabilities. The material of the gas diffusion layer170can select from serial materials of aluminum spinel, magnesium spinel, lanthanum aluminate, or the composites formed with foregoing aluminum spinel, magnesium spinel and lanthanum aluminate. Similarly, for the composing materials of the gas diffusion layer170, an extra second material of property-modifying additive such as copper, cerium oxide etc. can also be added to enhance anti-carbon, antitoxic and anti-sulfuring capabilities. The material of the high-k layer150can select from serial materials of silicon oxide (SiOx), zirconia and cerium oxide etc. each of which has high dielectric constant (k) and fixed oxygen content.

Besides, certain operating statuses such as working temperature, externally applied voltage and feedback current signal etc. will be affected by the parameters such as thickness of the ionic conductive film131, the thickness of the high gas-permeability interlayer film132and the match with the conductivity promotion structure140. Moreover, the thickness of the high-k layer150is also an important parameter for affecting behavior of electron in tunneling effect. For the exemplary preferred embodiment of the present invention, the thickness range of the ionic conductive film131is in scale of 1 to 500 nanometers, the thickness range of the high gas-permeability interlayer film132is in scale of 1 to 50 nanometers while the thickness range of the high-k layer150is in scale of 1 to 500 nanometers. All the foregoing film layers of the ionic conductive film131, the high gas-permeability interlayer film132and the high-k layer150can be fabricated by the micro electro-mechanical systems (MEMS) such as screen printing process, electroplating process, sputtering process or evaporation process etc.

FIG. 3shows a typical configuration of the first catalyst electrode110and the second catalyst electrode120for a current-type gas sensor in the first preferred exemplary embodiment of the present invention. In the preferred exemplary embodiment of the present invention, the first catalyst electrode110and the second catalyst electrode120are in mutual coplanar interdigitated configuration to minimize the interspace or gap and to maximize the sensing area of the first catalyst electrode110and the second catalyst electrode120so the sensibility of the gas sensor100can be enhanced due to increasing of the detecting electric current. Foregoing first catalyst electrode110and second catalyst electrode120can be fabricated by any kind of thick film process such as screen printing process, inkjet technology, coating technology etc, or any kind of thin film process such as lift-off process in micro-structuring technology. For example, the line width between the first catalyst electrode110and the second catalyst electrode120can be reduced to 0.03 mm scale if the line width is fabricated by the automatic screen printing machine while the line width can be miniaturized to 7 μm-20 nm if it is fabricated by the lift-off process in micro-structuring technology. Because the first catalyst electrode110and the second catalyst electrode120are fabricated in a coplanar configuration, it is beneficial to reducing manufacturing cost, labor hours and processing difficulty.

FIG. 4Ais an illustrative schematic view for a current-type gas sensor in the second preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting toFIG. 2, the first catalyst electrode210is disposed on the layered structure230while the second catalyst electrode220is disposed under the layered structure230. Moreover, the layered structure230is vertically laminated by multiple pairs of ionic conductive film231and high gas-permeability interlayer film232. The orientation for all pairs of ionic conductive film231and high gas-permeability interlayer film232is almost perpendicular to boundaries formed by the layered structure230with the first catalyst electrode210and the second catalyst electrode220respectively. The high-k layer250is sandwiched between the layered structure230and conductivity promotion structure240. The second catalyst electrode220is embedded in the high-k layer250with resultant manner that the second catalyst electrode220is enveloped by the high-k layer250. Here, the high-k layer250has fixed content of lattice oxygen so it can be functioned as a referential gaseous layer. Besides, if the power supply180and the current detecting unit160inFIG. 4Aare replaced by a voltage detecting unit (not shown), a voltage-type gas sensor is formed.

In this exemplary embodiment, the gap of the first catalyst electrode210and the second catalyst electrode220(namely the moving displacement of the free gas ions or free electrons) equals the thickness of the layered structure230. The rest components with features and functions thereof in this exemplary embodiment are the same as those in the exemplary embodiment shown inFIG. 2, which are unnecessary to disclose here in redundant manner.

FIG. 4Bis an illustrative schematic view for a current-type gas sensor in the third preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting to the layered structure230having a plurality of ionic conductive film231made by the same material inFIG. 4A, the corresponding layered structure230′ here is vertically laminated by multiple pairs of ionic conductive film231a,231bin respective different material and high gas-permeability interlayer film232in same material. Although the ionic conductive films231aand231bare formed in different material respectively in the third preferred exemplary embodiment, it is not limited to this status. In one embodiment, the multiple high gas-permeability interlayer films232can be also formed into respective different material.

FIG. 4Cis an illustrative schematic view for a current-type gas sensor in the fourth preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting toFIG. 2, the layered structure330is horizontally laminated by multiple pairs of ionic conductive film331and high gas-permeability interlayer film332. The first catalyst electrode210and the second catalyst electrode220are disposed on the upper surface of the layered structure330. The orientation for all pairs of ionic conductive film331and high gas-permeability interlayer film332is almost parallel to the first catalyst electrode210and second catalyst electrode220. The rest components with features and functions thereof in this exemplary embodiment are the same as those in the exemplary embodiment shown inFIG. 2, which are unnecessary to disclose here in redundant manner.

FIG. 4Dis an illustrative schematic view for a current-type gas sensor in the fifth preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting to the plurality of ionic conductive films331being made by same material inFIG. 4C, the corresponding layered structure330′ here is horizontally laminated by multiple pairs of ionic conductive film331a,331bin respective different material.

FIG. 5is an illustrative schematic view for a current-type gas sensor in the sixth preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting to the gas sensor100inFIG. 2, the corresponding gas sensor here is additionally provided an active thermal control module400. The active thermal control module400comprises a heating unit410and a temperature control unit420. The heating unit410is used to heat the layered structure130while the temperature control unit420is used to monitor and control the heating unit410so the working temperature for the ionic conductive film131of the layered structure130can be constantly kept in a preset range.

Moreover, the heating unit410includes a heating filament412sheathed in an insulating coat layer414such that the overall heating unit410is closely attached beneath the bottom surface of the conductivity promotion structure140. The material of the heating filament412is selected from metal with excellent electric properties such as nickel, gold, silver, platinum etc, while the material of the insulating coat layer414is selected insulating material such as alumina, zirconia, cerium oxide, magnesia, strontium titanate, barium titanate, lanthanum aluminate, lithium niobate etc. Besides, the disposing location of the heating filament412is not limited in the foregoing status as long as it can effectively heat the layered structure130without harmful side-effect. Accordingly, the heating filament412can be not only disposed in any location of non-sensing surface of the gas sensor but also disposed in the internal location of the gas sensor. The non-sensing surface of the gas sensor means that surface of the gas sensor excluding the sensing surface formed by the first catalyst electrode110and the second catalyst electrode120.

The temperature control unit420further includes a current meter422and a logic circuit424. The current meter422serves to detecting the heating current flowing through the heating filament412. The logic circuit424is used to control the power output of the heating filament412according to the heating current therein. In physics, the resistance of the heating filament412is a function of the heating temperature, which means that the heating current of the heating filament412changes with heating temperature even external applied voltage is kept in constant. Accordingly, via measuring the fluctuation of the heating current by the current meter422, the actual heating temperature of the heating filament412can be computed so the logic circuit424can precisely the power output of the heating filament412other than intelligently provides adequate voltage to the heating filament412for heating requirement to further constantly maintain the suitable working temperature for the gas sensor100.

FIG. 6is an illustrative schematic view for a current-type gas sensor equipped with an active thermal control module in the seventh preferred exemplary embodiment of the present invention. In this preferred exemplary embodiment, contrasting to the additional heating filament412for the gas sensor inFIG. 5, the corresponding gas sensor here is additionally provided a conducting lamina513connecting to an electric couple including a N-type semiconductor texture512aand a P-type semiconductor texture512b. The conducting lamina513, which is embedded in an insulating coat layer514, is disposed under the bottom surface of the conductivity promotion structure140. Via changing the flowing direction of the electric current passing the electric couple of N-type semiconductor texture512aand P-type semiconductor texture512b, the heating or cooling purposes of the gas sensor can be achieved by regulating the conducting lamina513into heating mode or cooling mode. The material of the N-type semiconductor texture512aand P-type semiconductor texture512bcan selects from bismuth telluride, telluride selenide or tellurium, bismuth selenide etc., or any kind of combination from foregoing bismuth telluride, telluride selenide or tellurium, bismuth selenide etc.

As mentioned above, by taking advantage of semiconductor feature, an electric couple including the N-type semiconductor texture512aand P-type semiconductor texture512bis created, which is not the only possibility for the present invention. There are some more possibilities to create useful parts or components for the present invention by taking advantage of semiconductor feature. For example, anyone of foregoing bismuth telluride, telluride selenide or tellurium, bismuth selenide etc. in making the N-type semiconductor texture512a(or P-type semiconductor texture512b) can be selected to fabricate the conductivity promotion structure140,240such that an additional P-type semiconductor texture512bis fabricated on the non-sensing surface of the conductivity promotion structure140,240via thick-film process/thin-film process or micro-electro-mechanical systems (MEMS). By this way, one semiconductor texture in the electric couple is directly replaced by the conductivity promotion structure140,240to simplify fabricating process and to reduce manufacturing cost.

In a preferred exemplary embodiment, the constructing material for the ionic conductive film131can select from base material of zirconia, cerium oxide and bismuth oxide, which are doped by bi-valence and tri-valence cations in single/common mode, or can select from material of lanthanum molybdate (LaMo2O9) and perovskite ((La1-xSrx)(Ga1-yMgy)O3-δ). The high gas-permeability interlayer film132of the present invention can be formed via directly employing interface reaction of cladding material between two different materials. For example, the high gas-permeability interlayer film132,232inFIGS. 2,4C and4D of the present invention can be fabricated by firstly constructing a substrate by insulating material containing low-valence ions such as magnesia, strontium titanate, lanthanum aluminate, barium titanate and lithium niobate etc, then plating the ionic conductive film131by tetra-valence material of zirconia or cerium oxide etc. Moreover, the vertical orientated ionic conductive film231,231a,231binFIGS. 4A and 4Bof the present invention can be fabricated by directly employing sputtering process while the high gas-permeability interlayer film232,332can be interposed into transition boundaries among columnar crystal structures, which are formed by the target material of alumina and zirconia.

Following factors should be considered in the material selections for the foregoing ionic conductive film131,231and high gas-permeability interlayer film132,232. First factor group includes the matching status between the ionic conductive film131,231and the high gas-permeability interlayer film132,232such as thermal expansion coefficient, matching property of lattice and interface stress created in the process etc. Second factor group includes the chemical element difference and element valence difference in respective layers to prevent generating a chemical compound with bad gas permeability. Third factor group includes anti-reducing ability in respective layers. In considering this factor, a multi-layer design is adopted to enhance overall anti-reducing ability.

Besides, the constructing materials of the conductivity promotion structure140,240can be categorized into insulation material, metallic alloy and semiconductor material. The insulation material includes magnesia, strontium titanate, lanthanum aluminate, lithium niobate etc. The metallic alloy includes stainless steel 17-4PH. The semiconductor material includes composite material of boron silicon and borosilicate group.