GAS SENSOR

Abstract: A gas sensor comprising a heat resisting electric resistor of a noble metal, and a semiconductor of sintered metal oxide covering the resistor, characterized in that the semiconductor comprises a sintered piece formed of metal oxide powder hav­ ing a specific electric resistance such that the ratio (Ro/ro) of a resistance (ro) of the resistor with respect to a composite re­ sistance (Ro) of the resistor and the semiconductor of metal oxide in the ambient air is in a range defined by an inequality 0.3 < Ro/ro <0.98. The above semiconductor of metal oxide may have oxidation activity restrained by a compound of bismuth, phosphor, lead, titanium, silicon or aluminum, and/or may be coated with a prorous layer of silica, alumina or silica-alumina, and/or may contain a lanthanide oxide and a IVa group element oxide, and/or may be sintered by using super fine powder of tin oxide as a binder compound.

Description: GAS SENSOR This invention relates to a gas sensor formed by applying a semiconductor of metal oxide to an electric resistor, which is aimed at a simple construction and stable operation, requiring a single electric source as distinct from conventional sensors requiring two kinds of electric source. Most of the gas sensors utilizing a semiconductor of metal oxide such as tin oxide (SnO2), zinc oxide, ferric oxide or the like, which are on the market or in the process of research and development at present, require two kinds of electrodes, namely a sensor electrode and a heater electrode, whether the sensors are the thick layer type, the film type or the sintered chip type. These sensors must be formed with film-like electrodes for sensing and heating, one on a front face of a base plate and the other on a back face thereof, and include four lead wires,which require a complicated manufacturing process and are costly. Known gas sensors also include the type comprising a quartz pipe, an electrode of platinum film trimmed in a spiral form on the quartz pipe, a gas sensing layer consisting of a semiconductor of metal oxide and covering the entire electrode, and heating means mounted in the quartz pipe. This type too has the disadvantage of requiring our lead wires and two kinds of electric source for the electrode and the heating means. On the other hand, some known composite sensors which have only two lead wires include the type comprising two gas sensing units having a semiconductor adhered to a coiled resistor, the two gas sensing units being assembled in a bridge. One of the units has its resistance decrease and the other unit conversely has its resistance increase as gas adheres thereto, which are interconnected in series. Therefore, as the resistance of one of the units is reduced through contact with gas, an increased voltage acts on the other unit, raising the temperature of the latter. Gas sensing units utilizing a semiconductor of metal oxide depend largely on temperature for its gas sensitivity. Thus, changes in the resistance of the two units each bring about complex temperature changes on the sensor, rendering the gas sensitivity of the sensor unstable particularly in the presence of two or more kinds of gas. Furthermore, the two gas sensing units constituting the sensor have different response speeds upon contact with gas, which may result in abnormalities in output of the sensor. In addition, known gas sensors generally have a disadvantage of depending largely on humidity for their sensitivity. This invention has been made having regard to the above state of the art, and intends to provide a two terminals gas sensor comprising a semiconductor of metal oxide applied and sintered to a single electric resistor in order for the resistor to act as heating means and also as a sensing electrode with its resistance changeable by gas adhesion. In other words, the object of this invention is to provide a gas sensor which is simple of construction but has a high degree of sensitivity and reliability. To achieve the above object, a gas sensor according to this invention comprises an electric resistor of a film-like or coiled noble metal formed on an electricity insulating base plate, and a semiconductor of metal oxide applied to the resistor, wherein the semiconductor comprises a sintered piece formed of metal oxide powder having an electric resistance such that the electric resistor and the semiconductor of metal oxide have a composite resistance Ro with respect to a resistance ro of the electric resistor in the ambient air in a range of 0.3 < Ro/ro < 0.98, and preferably in a range of 0.45 < Ro/ro < 0.95. This invention includes varied embodiments which all share the basic features set out above. Thus all the embodiments invariably constitute a gas sensor having two Lead wires and operable by a single electric source to provide data with a high degree of sensitivity and reliability. This invention is hereinafter described in detail with reference to the drawings showing varied embodiments, in which: Fig. 1 is a plan view of a gas sensor according to one embodiment of the invention, Fig. 2 is a section taken on line A-A in Fig. 1, Fig. 3 is a front view of a modified example including a coiled platinum wire, Fig. 4 is a view showing an example of measuring electric circuit including the sensor of Fig. l, Fig. 5 is a graph showing a relationship of gas sensitivity with respect to Ro/ro, Fig. 6 is a graph showing response characteristics of a known gas sensor with respect to ethanol vapor, Fig. 7 is a graph showing response characteristics of the known gas sensor with respect to hydrogen gas, Fig. 8 is a graph showing response characteristics of a gas sensor according-to a second embodiment of this invention, Fig. 9 is a schematic view in section of a third embodiment of this invention, Fig. 10 is a view showing a reaction between surface hydroxyl groups and organic silicone of the sensor according to the third embodiment, Fig. 11 is a graph comparing sensitivity changes with the passage of time between the sensor according to the third embodiment and a known sensor, Fig. 12 is a graph showing hydrogen gas sensitivity changes with the passage of time of a sensor according to a fourth embodiment of the invention, and Fig. 13 is a graph showing humidity dependency in detectibn of methane gas by a sensor according to a fifth embodiment of the invention. FIRST EMBODIMENT Referring to Figs. 1 and 2, the gas sensor 4 shown comprises an alumina base plate 1, a platinum film electric resistor 2 formed by the vapor deposition method in a winding shape on the alumina base plate 1, and a sintered piece of metal oxide 3 mounted on the alumina base plate 1 to cover the resistor 2. The resistor 2 may consist of a coiled platinum wire extending centrally of the sintered piece 3, as shown in Fig. 3. According to this invention, the platinum film resistor 2 acts as gas detection electrode and heating means, and specific electric resistance of powder employed for the sintered piece of metal oxide 3 is selected so as to provide an optimal relationship between the resistance of the resistor 2 and that of the sintered piece 3. This diminishes temperature changes of the gas sensor 4 during its contact with gas and permits its output to be obtained with a high degree of sensitivity. More particularly, powder having such a specific electric resistance is selected that provides a relationship, 0.3 < Ro/ro < o.98, wherein ro is the resistance of the platinum film resistor 2 and Ro is a composite resistance of the platinum film resistor 2 and the sintered piece of metal oxide 3. The above specific resistance is achieved, for example, by adding a very small amount of antimony pentaoxide. Fig. 4 shows an example of drive circuit for the described gas sensor, in which the gas sensor 4 is connected to a constant current electric source 5 and voltage detector terminals 6 are provided at respective ends of the gas sensor 4. Fig. 5 shows relationship of gas sensitivity with respect to Ro/ro, and the gas sensitivity here is represented by apparent changes in the resistance of the gas sensor 4 when the resistance of the sintered piece of metal oxide 3 falls by bne figure in response to the gas. The gas sensitivity has a value derived from the following AR value divided by ro: AR = Ro - Rg wherein Ro and Rg are resistance values of the gas sensor 4 in the air and in the gas, respectively. As seen, a desired gas sensitivity is achieved in the case of 0.3 < Ro/or < o.98, and even a better gas sensitivity is achieved in the case of 0.45 < Ro/ro < 0.95. In other words, AR/ro must at least be greater than 0.15, and preferably greater than 0.3. While the platinum film resistor 2 is used in this and subsequent embodiments, a film of a noble metal other than platinum or a coiled wire of any suitable noble metal may also be employed. Other noble metals useful herein include gold, silver, rhodium, palladium, indium, osmium and ruthenium. The sintered piece 3 of metal oxide acting as semiconductor may be a sintered piece of a metal oxide other than tin oxide. Similarly, the alumina base plate 1 may be replaced by another kind of heat resisting electricity insulating base plate. The two terminals type gas sensor having the described construction is operable by a single electric source and by a simple drive circuit. Therefore, such a sensor as a whole is not only economical but highly reliable. SECOND EMBODIMENT The sintered piece of metal oxide 3 shown in Fig. 1 comprises tin oxide, ferric oxide, zinc oxide or the like. Since this sintered piece is active as an oxidation catalyzer, oxidation takes place on the surface of the gas sensor 4 when in contact with a combustible gas. Therefore, when the gas sensor 4 is placed in a gas of certain concentration, its output signal changes with the passage of time. For example, where the gas sensor employing zinc oxide for the sintered piece 3 is operated at 4000C ih the air containing a 0.1 vol. percent of ethanol, its output attenuates with the passage of time, without keeping steady, as seen in Fig. 6. Further, where the gas sensor 4 having the sintered piece 3 whose main component is tin oxide is operated at about 350 C, an overshoot appears on a graph of the output value responsive to a gas such as hydrogen (density 0.1 %) or ethanol, as seen in Fig. 7. Such a characteristic has been a disadvantage to designing of a circuit for setting an alarm level and the like of a gas alarm. The secpnd embodiment eliminates the above disadvantage. One of the causes of response characteristic abnormality is reduction in the real density reaching the inside of the sintered layer which is due to oxidation and consumption of the gas on the surface ofthe sintered piece of metal oxide 3. Another cause is changes in the gas sensitivity with the passage of time which is due to a rise in the temperature at gas sensing parts because of oxidation heat. Therefore, the gas sensor according to the second embodiment includes a sintered layer having a restrained oxidation activity. More particularly, the sintered layer has a surface adhered with a material inactive to oxidize gases thereby to kill activity points of the layer and to restrain oxidation of gases. For the second embodiment, a method is adopted to have the sintered piece of metal oxide 3 soaked with a diluted aqueous solution of at least one type of salt of metal selected, for example, from metals such as bismuth (Bi), phosphor (P), lead (Pb), titanium (Ti), silicon (ski), aluminum (Al) and the like, and provided with a heat treatment after drying. Alternatively, the sintered piece of metal oxide 3 is caused to carry a very small amount of oxidation preventing material on the surface by the chemical deposition method utilizing organometallic compounds of the foregoing metals. For specific examples of the foregoing, the sintered piece of metal oxide 3-is soaked with a 0.01 mol % aqueous solution of compounds such as H3PO4, Pb(NO3)2J Tic3, Bi(NO3)3.5H2O or the like, and is put to a thermal decomposition process at 6000C for one hour after drying. Fig. 8 shows gas response characteristics of the gas sensor 4 formed as above. As seen, it takes about a minute before the gas sensor 4 provides a steady output in the air containing a 0.1 percent of hydrogen gas, the output value being invariable thereafter. THIRD EMBODIMENT In recent years organic silicone materials such as silicone putty, silicone rubber, silicone oil and so forth have become widely used in construction. However, such organic silicone materials contain volatile oligomer which vaporizes into the ambient air. This silicone vapor adheres to the gas sensor surface and causes catalytic poisoning which results in deterioration of the gas sensor with the passage of time. A gas alarm comprising the sintered piece such as of tin oxide will, when organic silicone adheres thereto, have an increased sensitivity to miscellaneous gases such as of alcohol, as shown by a dotted line B of Fig. 11, and give false alarms. However, the sensitivity lowers where the sensor comprises an unsintered semiconductor film. For example, some gas alarms have nil sensitivity in one month and in the case of catalytic oxidation type gas alarms the sensitivity sometimes is lost in one week. This is considered to be caused by organic silicone molecules having strong adhering activity which cover the activity points on the surface of the gas sensor and thus prevent the gas from coming into contact with the activity points. The third embodiment provides a gas sensor which is free from the above disadvantage. This gas sensor has an outer periphery formed with a porous layer of silica, alumina or silica-alumina including a high concentration of hydroxyl groups on the surface thereof which readily react with organic silicone molecules. Thus, organic silicone molecules are unable to reach the gas sensor surface per se thereby to restrain changes with the passage of time in the sensitivity of the sensor caused by poisoning. Therefore, sensor durability is not affected even in bad surroundings. Fig. 9 schematically illustrates the gas sensor according to the third embodiment, in which number 4 represents the gas sensor having the sintered piece of metal oxide 3 peripherally coated with a porous layer 9. This porous layer 9 comprises silica, alumina or silicaalumina which is prepared by the wet method, calcinated under low temperature and left standing in the ambient air, and therefore includes more hydroxyle groups on its surface than oxides prepared by other methods such as the dry method. The alumina base plate 1 and the platinum film resistor 2 are anifted from Fig. 9 for siEplicity. When organic silicone such as trimethylsilanol, i.e. (CH3)3SiOH, adheres to the surface of the porous layer 9 such as of silica, it readily reacts with the hydroxyl groups on the surface of silica as shown in Fig. 10. Alumina too has the surface hydroxyl groups likewise reactive with silanol group. Organic silicone molecules are taken into the porous layer surface and thermally decompose into silica at once, without reaching the inner sintered piece of metal oxide 3. On the other hand, H2, CO, CH4 and other gases do not react at the porous layer 9 but reach the sintered piece of metal oxide 3 to be detected. The formation of the porous layer 3 is described next. Ammoniac water is dripped into an aqueous solution of aluminum nitrate to precipitate aluminum hydroxide. After treating this precipitation to produce boehmite, it is washed, dried and calcinated at 550 0C for five hours to obtain alumina powder. This alumina powder is kneaded with colloidal alumina and water into a mud-like paste. Then the paste is applied in a thickness of about 0.1 mm to the periphery of the sintered piece of metal oxide 3 as shown in Fig. 9, which is calcinated after drying to obtain the poison resisting gas sensor 4. The porous layer 9 such as of alumina may be provided also on the sensor comprising the alumina base plate 1 including the platinum film resistor 2, and the sintered piece 3 of metal oxide such as tin oxide applied in a 0.4 mm thickness to the alumina base plate 1, as shown in Fig. 2. In this case, even if the sensor is contacted by the air containing a 0.4 percent of hydrogen gas and a 10 ppm of trimethylsilanol, there occur little changes with the passage of time in the output of the sensor 4 as shown by a solid line A in Fig. 11. It has been found that, apart from the above alumina powder, like effect is produced by a sintered layer of silica or silica-alumina prepared by treating an aqueous solution of sodium silicate with a cation-exchange resin. FOURTH EMBODIMENT The gas sensor provided with a semiconductor generally is in use constantly energized to heat the sintered semiconductor portion to 300-4500C. Therefore, the continuing high temperature, especially at times of high humidity, brings about changes in a surfacial fine structure of the sintered semiconductor and an increase in the electroconductivity of the sintered semiconductor. Depending on the lapse of use period and on use surroundings, an alarm may be given when gas concentration is below a predetermined concentration. The fourth embodiment provides a gas sensor free from the above disadvantage. The sintered piece used with this gas sensor is composed of tin oxide, an oxide of any lanthanide element in a 0.01-20 mol % of the tin oxide, and an oxide of any element included in the periodic group IVa, i.e. titanium (Ti), zirconium (Zr), hafnium (Hf) or thorium (Th), and therefore keeps suitable electric conductance. Its electric conductance and gas sensitivity are little affected by a long period of use at high temperatures or in a highly humid atmosphere. In other words, this gas sensor has a significantly improved stability relative to the passage of time. One example of manufacturing method of the above sintered piece is as follows: A predetermined amount of tin toxide (SnO2) is measured, to which the oxide of lanthanide element and the oxide of an element included in the periodic group IVa are added in a predetermined ratio. The three components are then mixed one another with water, crushed and kneaded into a paste. Next, the paste is applied to the platinum film resistor, dried under 800C for two hours, and is then calcinated under 8000C for two hours (This method is hereinafter referred to as the kneading method). In another method, tin oxide powder is first kneaded with water to form a paste which is applied to the platinum film resistor and allowed to dry as in the foregoing method, and is then ',presintered" under 4000C for one hour. The "presintered" piece is caused to soak an aqueous solution of a lanthanide element compound thermally decomposable intoan oxide and a compound of element of the group IVa, to cause the latter two to be contained in the former in predetermined amounts. The resulting product is dried and is thereafter calcinated under 8000C for two hours (This method is hereinafter referred to as the soaking method). According to a further example, an aqueous solution of a lanthanide element compound and a compound of element of the group IVa is first added in a predetermined amount to - an aqueous solution of tin (Sn) compound. Next, when the tin compound is caused to precipitate as hydroxide by adjusting-the pH, these additives too are caused to coprecipitate as hydroxide at the same time. Thereafter, the precipitation is separated, dried, and turned into a uniform paste, which is applied to the platinum film resistor and, after drying, is calcinated under 8000C for two hours (This method is hereinafter referred to as the coprecipitation method). Gas sensors comprising the sintered semiconductors prepared by the kneading method, soaking method and coprecipitation method were maintained in energized, use state for 200 hours, and were made to contact the gases of 1,000 ppm hydrogen, 1,000 ppm methane and 1,000 ppm ethanol to measure the electric resistance in the respective cases, which was regarded as the initial resistance. Thereafter, changes in the gas sensitivity with the passage of time were measured by maintaining the gas sensors in the use state. Measuring results of each gas sensor are shown in Table 1, and sensitivity change characteristics (relative to the initial resistance) with the passage of time with respect to hydrogen in the case of Type 2 in Table 1 are shown in Fig. 12. As seen from Table 1 and Fig. 12, stability in relation to the passage of time is markedly improved by the addition of lanthanide oxide and the like. It has been confirmed through tests that good results are obtained from lanthanide oxide and the group IVa element oxide in the range of 0.01-20 mol % with respect to tin oxide. Table 1 EMI18.1 <tb> <SEP> Amounts <SEP> of <SEP> addivies <SEP> in <SEP> R <SEP> (resistance <SEP> after <SEP> 3000 <SEP> Hrs) <tb> Sample <SEP> SnO2 <SEP> R'o <SEP> (initial <SEP> resistance) <SEP> Method <tb> of <SEP> <tb> <SEP> Lanthanide <SEP> IVa <SEP> element <SEP> In <SEP> air <SEP> In <SEP> air <SEP> containing <SEP> a <tb> <SEP> No. <SEP> oxide <SEP> oxide <SEP> contain- <SEP> gas <SEP> of <SEP> 1000 <SEP> ppm <SEP> addition <tb> <SEP> -ing <SEP> no <tb> <SEP> - <SEP> mol <SEP> % <SEP> mol <SEP> % <SEP> gas <SEP> H2 <SEP> CH4 <SEP> C2H5OH <SEP> <tb> <SEP> La2 3 <SEP> 0.5 <SEP> ZnO2 <SEP> 1 <SEP> 0.88 <SEP> 0.78 <SEP> 0.92 <SEP> 0.76 <SEP> (A) <tb> <SEP> 2 <SEP> La2O3 <SEP> 0.5 <SEP> HfO2 <SEP> 1 <SEP> 0.92 <SEP> 0.81 <SEP> 0.92 <SEP> 0.78 <SEP> (A) <tb> <SEP> 3 <SEP> CeO2 <SEP> 0.5 <SEP> ZnO2 <SEP> 1 <SEP> 0.98 <SEP> 0.94 <SEP> 0.99 <SEP> 0.89 <SEP> (C) <tb> <SEP> 4 <SEP> CeO2 <SEP> 0.5 <SEP> Th02 <SEP> 1 <SEP> 0.97 <SEP> 0.92 <SEP> 0.98 <SEP> 0.92 <SEP> (C) <SEP> <tb> <SEP> 5 <SEP> Nd2O3 <SEP> HfO2 <SEP> 2 <SEP> 0.95 <SEP> 0.84 <SEP> 0.96 <SEP> 0.84 <SEP> (B) <tb> <SEP> 6 <SEP> Sm2O3 <SEP> 1 <SEP> -Tho2 <SEP> 2 <SEP> 0.94 <SEP> 0.86 <SEP> 0.98 <SEP> 9.82. <SEP> <tb> <SEP> 7 <SEP> Dy2O3 <SEP> 1 <SEP> ThO2 <SEP> 2 <SEP> 0.96 <SEP> 0.90 <SEP> 0.98 <SEP> 0.86 <SEP> (B) <tb> <SEP> TiO2 <SEP> 2 <tb> <SEP> 8 <SEP> La2O3 <SEP> 2 <SEP> 0.98 <SEP> 0.94 <SEP> 0.98 <SEP> 0.90 <SEP> (B) <tb> <SEP> ThO2 <SEP> 2 <tb> <SEP> La2O3 <SEP> 2 <tb> <SEP> 9 <SEP> Zr02 <SEP> 5 <SEP> 0. <tb> <SEP> Sm2O3 <tb> <SEP> Dy2O31 <SEP> <tb> <SEP> Blank <SEP> (without <SEP> any <SEP> additive) <SEP> 0.74 <SEP> 0.56 <SEP> 0.92 <SEP> 0.52 <tb> Notes: The symbol (A), (B) and (C) respectively indicate the kneading, the soaking and the coprecipitation methods. FIFTH EMBODIMENT In gas sensors having a sintered piece of metal oxide such as tin oxide or zinc oxide, it has heretofore been necessary to sinter the metal oxide at a low temperature below 800 C in order to maintain gas sensing activity. Sintering at 1,0000C or higher is impossible although it is suited to manufacture of a strong sintered piece. Therefore, the strength of sintered pieces has been markedly inferior to ordinary ceramics, and it has been impossible to achieve a mechanical strength enough to withstand impacts and vibrations occuring during ordinary use. Sintered pieces which are prepared with use of a binder compound such as silica made of an organosilicic compound, silica sol or the like, retain silica or alumina therein. These impurities are hygroscopic and therefore the sensitivity of the gas sensor is very dependent upon humidity, which constitutes a disadvantage of undergoing changes in a highly humid atmosphere with the- passage of time. The fifth embodiment provides a gas sensor free from the above disadvantage. This sensor comprises a semiconductor manufactured by a method utilizing super fine powder of tin oxide as binding agent to sinter ordinary particle size powder of tin oxide. The fifth embodiment makes use of the nature of super fine powder of tin oxide which is an excellent binder in low temperature sintering. In other words, a mechanical strength adequate for practical purposes is achieved by sintering a mixture of conventionally C used powder of tin oxide having a particle size 200A or more and super fine powder of tin oxide having an o average particle size 50A, for example. Hydroxide obtainable from an aqueous solution of stannic chloride (SnC14) by the precipitation method 0 has a primary particle size 20A or less. This hydroxide is dehydrated and oxidized in air at 200-3000C, and is then crushed into super fine powder having an average C particle size SQA. This super fine powder is mixed with SnO2 powder having ordinary particle sizes, in a proportion about 30 percent of the total powder weight, the mixture being kneaded with water into a paste. Next, the paste is applied tó the platinum film resistor, dried and sintered under8000C for five hours, to finally arrive at the gas sensor 4. Fig. 13 is a graph showing humidity dependency of the gas sensor 4 in methane gas, in which the curve C represents dependence on absolute humidity (mmHg) of the sensor according to the fifth embodiment and the curve D of a sensor employing silica sol as binding agent, and methane concentratin in the gas is 500 ppm. As seen from Fig. 13, the output of the gas sensor 4 according to this embodiment shows very little changes even below 20 mmHg absolute humidity.

Claims: What is claimed is: 1. A gas sensor comprising an electric resistor of a noble metal, and a semiconductor of sintered metal oxide covering the resistor, characterized in that the semiconductor comprises a sintered piece formed of metal oxide powder having a specific electric resistance such that the ratio (Ro/ro) of a resistance (ro) of the resistor with respect to a composite resistance (Ro) of the resistor and the semiconductor of metal oxide in the ambient air is in a range defined by an inequality 0.3 < Ro/ro < 0.98. 2. A gas sensor as claimed in claim 1 characterized in that the semiconductor comprises a sintered piece formed of metal oxide powder having a specific electric resistance such that the ratio.'(Ro/ro) of a resistance (ro) of the resistor with respect to a composite resistance (Ro) of the resistor and the semiconductor of metal oxide in the ambient air. is in a range defined by an inequality 0.45 < Ro/ro < 0.95. 3. A gas sensor as claimed in claim 1 or 2 characterized in that the semiconductor of metal oxide carries on a surface thereof at least one type of compound composed of at least one element selected from bismuth, phosphor, lead, titanium, silicon and aluminum. 4. A gas sensor as claimed in any one of claims 1 to 3 characterized in that the semiconductor of metal oxide is peripherally coated with a porous layer of at least one substance selected from the group consisting of silica, alumina and silica-alumina. 5. A gas sensor as claimed in any one of claims 1 to 4 characterized in that the semiconductor comprises a sintered piece of tin oxide containing at least one of lanthanide oxides in 0;01-20 mol % of the tin oxide and at least one of IVa group element oxide in 0.01-20 mol % of the tin oxide. 6. A gas sensor as claimed in any one of claims 1 to 5 characterized in that the semiconductor comprises a sintered piece formed by using a binder compound including super fine powder of tin oxide as a main component thereof.

Inventors: FUKUI, KIYOSHI, KOMATSU, KOJI, SAKAI, SAI
Applicants: NEW COSMOS ELECTRIC CO., LTD.
Application year: 1984
Main classification: G01N27
IPC codes: G01N27