Gas detector

Abstract: A gas detector for detecting the decomposed SF₆ gas produced by discharge in gas-insulated equipment. The gas detector operates as a cell generating voltage in pro­ portion to amount of the decomposed SF₆ gas wherein the voltage is generated between the detection electrode in­ cluding Ag, reacting upon contact by the decomposed gas and the opposing electrode including Ag also, both elec­ trodes sandwiching the ionic conductive solid electrolyte layer including Ag ion therebetween.

Description: The present invention relates generally to a gas detector for detecting the decomposed SF₆ gas produced by electric discharching in a gas-insulated equipment. A wet method or a dry method is known as a conventional method for detecting the decomposed SF₆ gas produced by discharge in gas-insulated equipment. In the wet method, decomposed SF₆ gas produced by discharge such as SF₄ absorbed in alkali-absorbing solution is detected as ions of fluorine by an absorptiometric method (JAPAN ANALYST Vol. 16, P44 (1967)). In another wet method, acid and acid producing constituents in the sample which contains decomposed SF₆ gas are absorbed in a standard alkali solution and the excess alkali is back-titrated with a standard sulphuric acid solution (IEC (INTERNATIONAL ELECTROTECHNICAL COMMISSION) RECOMMENDATION Publication 376 "Specification and acceptance of new sulphur hexafluoride"). Although the wet method needs many equipment such as a gas-liquid contact equipment for absorbing decomposed SF₆ gas in the absorbing solution and an absorptiometer for measuring fluorine ion or a titrator (cf. a buret) for measuring component in the absorbing solution. Thus, there are shortcomings such that many necessary equipments and much complicated measurement are required in the wet method. As a dry method, a gas detecting tube which encloses an element showing coloration by reaction with the integrated SF₆ gas is shown in Japanese examined publication Tokko sho 57-38091. The gas detecting tube of the dry method is small-sized and light weight and enables easy measurement. Although, the dry method is easy to carry out the measurement, it has been necessary a man who observes the coloration, since the gas detecting tube has no conversion function from change of the coloration to an electric signal. Thus it is not suitable for use of unmanned continuous measurement. According to FR-A-2526999 a film permeable to the decomposed SF₆ gas is used for introducing the decomposed SF₆ gas. And introduced decomposed SF₆ gas is detected as an electric signal. FR-A-2272395 discloses detection of gas content utilizing an oxygen ion conductive solid electrolyte oparable at temperatures of 400 - 900 °C. It is difficult to obtain high sensitivity (e.g. about 0.03 ppm as a HF (gas)) which is required in detection of decomposed SF₆ gas. US-A-4661211 discloses a gas sensor utilizing hydrogen ion conductive solid electrolyte. In contrast, in detection of decomposed gas of SF₆, a reaction of a halogen or halogen compound is to be used. The object of the present invention is to provide a gas detector which is small-sized and light weighted and enabling easy measurement of decomposed SF₆ gas amount by an electric signal. This is achieved by a gas detector in accordance with the present invention comprising; a detecting electrode having a surface exposed to objective gas and containing at least a metal element, an ionic conductive solid electrolyte layer which is formed on said detecting electrode and contains ions of said metal element, an opposing electrode which is formed on said ionic conductive solid electrolyte layer and contains said metal element, an insulative support means supporting said detecting electrode, said ionic conductive solid electrolyte layer and said opposing electrode, and isolating said ionic conductive solid electrolyte layer from said opposing electrode from gas, a first electric terminal connected with said detecting electrode and, a second electric terminal connected with said opposing electrode. In the gas detector of the present invention, the gas detector operates as a cell for generating voltage in proportion to amount of the decomposed SF₆ gas. The voltage is generated between the detecting electrode for reacting with the integrated gas and the opposing electrode wherein both electrodes are sandwiching the ionic conductive electrolyte layer therebetween. Thus the small-sized and light-weighted gas detector which needs no external electric power source and enables unmanned continuous measurement is obtained. While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a sectional view showing a gas detector embodying the present invention. FIG.2 is a sectional view showing a gas detector integration embodying the present invention. FIG.3 is a graph showing a relation of the output voltage of the gas detector integration and the number of cells therein. It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereafter the present invention is elucidated in detail with reference to the accompanying figures of FIG.1 through FIG.3 whereby the preferred embodiments are shown. [First embodiment] A first preferred embodiment of the present invention is elucidated hereafter with reference to FIG.1. In FIG.1, a detecting electrode 1 which is made of deposition layer of Ag for reacting with the decomposed SF₆ gas and an opposing electrode 3 which is also made of deposition layer of Ag, and an ionic conductive solid electrolyte layer 2 such as Ag₃SI including Ag ion sandwiched between the each other opposing electrodes 1 and 3. When there is no object gas which is decomposed SF₆ gas, namely before a gas detector detects the object gas, there exists no electric potential difference between the detecting electrode 1 and the opposing electrode 3, since the both electrodes 1 and 3 are made of same material (metal) Ag. Since the opposing electrode 3 and the ionic conductive solid electrolyte layer 2 are formed on the substrate 7 which is made of alumina, and further the opposing electrode 3 and the layer 2 are surrounded with the insulator 6 and/or the detecting electrode 1, there is no exposed surface of the opposing electrode 3 to SF₆ gas atmosphere. Thereby, only the outside surface of the detecting electrode 1 is exposed to SF₆ gas atmosphere. When the object gas namely decomposed SF₆ gas is produced by discharge in SF₆ gas, SF₆ gas is decomposed into SF₄ gas, SF₂ gas, F(fluorine) and/or S(sulfur). Some Ag in the detecting electrode 1 is converted to AgF (Silver Fluoride) through the following reaction with F produced in the decomposed SF₆ gas. 1 / 2 F₂(produced in SF₆ gas) + Ag → AgF (1) Then a galvanic cell comprising AgF on the detecting electrode 1 as cathode active material, Ag in the opposing electrode 3 as anode active material and the solid electrolyte layer 2 as what is called electrolytic solid solution is formed as shown in following reaction formulas. Thus electric potential difference between the detecting electrode 1 and the opposing electrode 3 occurs in accordance with the amount of AgF converted on the detecting electrode 1. The electric potential difference is measured by a voltmeter 8 through terminals 5,5 and leads 4,4. The following equation (6) between measured voltage V (volt) and concentration of the decomposed gas L (%) bold as known. V = A + BlogL (6) wherein A, B are constant. In the equation (6), constants A and B are obtained experimentally. Thus the amount of decomposed SF₆ gas can be estimated from the measured electric potential difference. Our experiment shows that a voltage of several µV is measured from a concentration of several ppm of the decomposed SF₆ gas. As to the above-mentioned gas detector, the method for making the gas detector is elucidated hereafter briefly. The opposing electrode 3 about 3µm thick Ag layer 3 is formed on the substrate 7 made of alumina by sputtering or deposition. Next, in an electric heater, the Ag layer on the substrates is reacted with mixed gas of hydrogen sulfide and air in volume ratio of about 1 : 3 at about 200°C. In this heat reaction, the surface of the Ag layer is converted to silver sulfide. Then the substrate 7 is put in a closed vessel together with iodine. Reaction period with iodine is controlled so that iodine as silver iodine is contained in the ratio of 1 : 1 on silver sulfide, by measuring the weight increase of the substrate 7. Next, the substrate 7 is heated in N₂ gas at a temperature of 300°C ∼ 400°C. Through the above-mentioned reactions, the surface of the Ag layer is converted finally to the Ag₃SI layer for the solid electrolyte layer 2. The depth of the Ag composed layer produced by the reaction, namely the thickness of the Ag₃SI layer is controlled to be about 2µm changing condition such as period and temperature of the above-mentioned reactions on the basis of data given by experiments. And Ag layer of about 1µm thickness for detection electrode 1 is formed layer by sputtering or deposition on the Ag₃SI. Then the substrate 7 is cut to obtain desired size as a gas detector. After boning Au wires as leads 4, 4 and terminals 5, 5 for both electrodes 1 and 3, alumina layer as insulator 6 is formed by sputtering while masking the surface of the detecting electrode 1. [Second embodiment] A second preferred embodiment of the present invention is elucidated hereafter with reference to FIG.2 and FIG.3. In FIG. 2, a grouped gas detector integration comprising at two or more gas detectors is shown. Corresponding parts and components to the first embodiment are shown by the same numerals and marks, and the description thereon made in the first embodiment similarly apply. Differences and features of this second embodiment from the first embodiment are as follows. The gas detector integration has a constitution of a row of cells of FIG. 1, and electric serial connection is made by connecting detection electrodes 1, 1, --- with respective opposing electrodes 3, 3 --- of next cells, like a series connected accumulated battery. An output voltage of the gas detector integration is multiplied by the number of cells therein. A relation of the output voltage of the gas detector integration and the numbers of the series connected cells therein is shown in FIG.3. Thus, the gas detector integration produced a high output voltage unable as a high accurate gas detector. The method for making the gas detector integration is substantially the same as the above-mentioned method of the first embodiment. In the embodiment of Fig. 1 and Fig. 2, ions of Ag are carriers of electric charge, since both electrodes comprises Ag and solid electrolyte is made of Ag₃SI. An electric conductor of mixed metal ion and electron such as Ag₂S or Ag x Mo₈S₈ wich is an electric conductor of mixed Ag ions and electrons can be used for material of electrodes 1 and 3. Also other Ag ion conductive solid electrolyte, such as Ag₄RbI₅ or Ag₆IWO₄ can be used for the ionic conductive solid electrolyte layer 2. Instead of Ag ions, Cu ions can be used as carrier of electric charge, and in this case both electrodes are made of a compound of Cu. As an instance, the detecting electrode 1 is made of Cu, the opposing electrode 3 is made of copper sulfide (Cu₂S) and the ionic conductive solid electrolyte layer 2 is made of Rb₄Cu₁₆I₇CI₁₃. The embodiments for objective gas of decomposed SF₆ gas has been described; however other gases which make reaction with Ag or Cu such as gas of H₂S, F₂, Br₂, Cl₂ and so on can be detected by the present gas detector.

Claims: A gas detector comprising; a detecting electrode (1) having a surface exposed to objective gas and containing at least a metal element, an ionic conductive solid electrolyte layer (2) formed on said detecting electrode (1) and containing ions of said metal element, an opposing electrode (3) which is formed on said ionic conductive solid electrolyte layer (2) and contains said metal element, an insulative support means (7) supporting said detecting electrode (1), said ionic conductive solid electrolyte layer (2) and said opposing electrode (3) and isolating said ionic conductive solid electrolyte layer from said opposing electrode from gas, a first electric terminal (5) connected with said detecting electrode (1) and, a second electric terminal (5) connected with said opposing electrode (3). A gas detector in accordance with claim 1 which further comprises at least two or more of said gas detectors as claimed in claim 1 wherein said detecting electrode (1) of one gas detector is connected to said opposing electrode (3) of a next gas detector for series connection. A gas detector in accordance with claim 1 or 2 wherein said detecting electrode (1) is an electric conductor of mixed metal ion and electron. A gas detector in accordance with claim 1, 2 or 3 wherein said metal element is Ag. A gas detector in accordance with claim 1, 2 or 3 wherein said metal element is Cu. A method for making a gas detector comprising the steps of: forming a first Ag layer (3) on a substrate (7); converting surface of said Ag layer to AgS; containing I in said Ag and AgS layer as AgI; converting said AgI and AgS to Ag₃SI; forming a second Ag layer (1) on said Ag₃SI layer (2); bonding leads (4) on each said first and second Ag layer; and forming an alumina layer (6) except on the surface of said second Ag layer (1).

Inventors: IZUMO MASAO ITAMI SEISAKUSHO M, TADA SHOJI ITAMI SEISAKUSHO MI, YAMAUCHI SHIRO ITAMI SEISAKUSH, IZUMO, MASAO ITAMI SEISAKUSHO MITSUBISHI, TADA, SHOJI ITAMI SEISAKUSHO MITSUBISHI, YAMAUCHI, SHIRO ITAMI SEISAKUSHO MITSUBISHI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01H33
IPC codes: H01H33, G01N27