GAS DETECTOR

Abstract

A gas detection unit is accommodated within a housing of a gas sensor and the outside atmosphere of the housing is introduced through the filter to the gas detection unit. The filter comprises an organic polymer gas-permeable filter removing siloxanes and an inorganic filter removing alcohols and passing gases to be detected.

Claims

1: A gas detector comprising: a gas detection unit; and a filter introducing surrounding atmosphere to the gas detection unit, wherein the filter comprises an organic polymer filter comprising a gas-permeable organic polymer membrane, removing siloxanes, and passing gases to be detected and an inorganic filter removing alcohols and passing the gases to be detected.

2: The gas detector according to claim 1, wherein the gas detector is a gas sensor, and wherein the gas sensor further comprises a housing accommodating the gas detection unit and wherein said filter is attached to the housing.

3: The gas detector according to claim 2, wherein said organic polymer filter is allocated towards outside atmosphere of the housing and said inorganic filter is allocated towards the gas detection unit both in the housing.

4: The gas detector according to claim 1, wherein said inorganic filter includes oxidation catalyst.

5: The gas detector according to claim 1, wherein said gas-permeable organic polymer membrane includes an acidic group or a basic group.

6: The gas detector according to claim 5, wherein said gas-permeable organic polymer membrane comprises one of the group consisting of carboxy-methyl-cellulose including carboxyl group, cellulose sulfate, fucoidan, chitosan including amino group.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a cross-sectional view of a gas sensor according to the embodiment.

[0020] FIG. 2 is a cross-sectional view of a laminated membrane according to the embodiment.

[0021] FIG. 3 is a plan view of a chip according to the embodiment.

[0022] FIG. 4 indicates a driving pattern for the gas sensor according to the embodiment.

[0023] FIG. 5 is a characteristic view revealing the durability of the embodiment and that of a comparative example, against siloxane (D5×100 ppm).

[0024] FIG. 6 is a characteristic view revealing the sensitivities of the embodiment and a comparative example to 200 ppm ethanol.

FEATURES FOR CARRYING OUT THE INVENTION

[0025] The best embodiment for carrying out the invention will be described.

Embodiment

[0026] FIGS. 1 to 4 indicate a gas sensor 2 according to the embodiment, and FIGS. 5 and 6 indicate test results. The gas sensor 2 is, for example, provided with a Si chip 4, which is an example of a gas detection unit. The Si chip 4 is accommodated in a housing 5, such as a ceramic housing, and is fixed in the housing 5, for example, by die bonding. The opening of the housing 5 is covered by a ceramic lid 6 so that atmosphere outside the housing 5 is supplied through plural openings 7 to a filter 8. On the inside surface of the lid 6 (the surface towards the Si chip 4), the membranous filter 8 is attached. The filter 8 comprises the two layers of an organic polymer filter 10 and an active charcoal filter 14; the organic polymer filter 10 is positioned towards the outside of the housing 5 and the active charcoal filter 14 is towards the Si chip 4. The active charcoal filter 14 is an example of the inorganic filter and may be replaced by an oxidation catalyst filter, or the like. it is preferable to allocate the organic polymer filters 10 at both sides of the inorganic filter. Further, the inorganic filter may comprise laminated distinct inorganic filters. The species of gas detection unit and structure of the housing are arbitrary. In addition, the pads of the Si chip 4 are connected to terminals 17 on the housing 5 via leads 16.

[0027] FIG. 2 indicates the construction of the filter 8. The organic polymer filter 10 comprises a porous support membrane 11 and a gas-permeable organic membrane 12 overlayed on the support membrane, but the support membrane 11 may be omitted. The support membrane 11 is a synthetic resin membrane or a polysaccharide membrane having continuous pores and has a thickness of, for example, 1 micro-meter to 100 micro-meter. The gas-permeable organic polymer membrane 12 has a thickness of, for example, 0.1 micro-meter to 5 micro-meter. The active charcoal filter 14 in the embodiment comprises an active charcoal fiber sheet having a thickness of about 1 mm, but granular active charcoal, silica gel, meso-porous silica, zeolite, aluminum-silicate may be used. The shape and the material for the inorganic filter are arbitrary. Further, in place of the active charcoal filter 14, an oxidation catalyst filter may be usable. The oxidation catalyst filter comprises the above adsorbents and an oxidation catalyst such as a noble metal supported on the adsorbents or polymer fibers and an oxidation catalyst such as a noble metal supported on the fibers.

[0028] The gas-permeable organic polymer membrane 12 comprises a polysaccharide membrane or a gas-selective permeable membrane made from a synthetic polymer. Since long-chain molecules in polysaccharide membranes tend to be regularly ordered, continuous micro-pores are easily generated. These micro-pores work as the gas diffusion path and the sizes of the micro-pores determine the sizes of molecules that can permeate the membrane. Namely, siloxanes having larger molecular diameters can not permeate but hydrogen, methane, LPG, CO, ethanol, and so on, can permeate the membrane. Gas-selective permeable membranes comprising synthetic polymers are generally highly gas-permeable, their gas permeability depends upon the molecular sizes. They permeate hydrogen, methane, CO, LPG, ethanol, and so on, but do not permeate siloxanes. The polymer membrane 12 may be formed by casting; spin-coating; spray-coating, roll-coating; and so on.

[0029] When an acidic group, such as carboxyl group, sulfo group, or phosphoric acid group, or a basic group, such as amino group or basic hydroxyl group is introduced into the gas-permeable organic polymer membrane 12 (hereinafter, simply “polymer membrane 12”), these groups bind with the —(O—Si—O)— portion in the siloxane molecules. When the siloxane concentration in the membrane increases, the siloxane molecules are hydrolyzed at the —(O—Si—O)— portion and are polymerized such that they are completely fixed in the membrane. Therefore, even when exposed to high concentration siloxanes for a long while, the siloxanes do not permeate the organic polymer filter 12.

[0030] Preferable polymer membrane 12 comprises a polysaccharide, such as carboxy-methyl-cellulose; cellulose-sulfate; fucoidan; and chitosan, and an acidic group, such as carboxyl group (carboxy-methyl-cellulose) and sulfo group (cellulose sulfate and fucoidan), or a basic group such as amino group (chitosan). Instead of using the polysaccharide membrane, an acidic or a basic group may be introduced into synthetic resin gas-selectively permeable membranes. For example, a protonic-conductive polymer such as Nafion or a hydroxyl ion-conductive polymer may be blended with gas-selectively permeable membranes comprising fluoropolymers.

[0031] FIG. 3 indicates the Si chip 4; the Si chip 4 is provided with a micro-hotplate 20 with electrodes and a heater, on a cavity 26. The hotplate 20 is supported by beams 24 and has a metal oxide semiconductor 22 on it. Indicated by 28 are pads.

[0032] Other gas detection units than the Si chip 4 are usable, and other gas detection materials than the metal oxide semiconductors are usable. For example, a contact combustion catalyst is usable as the gas detection material and, in this case, is supported on the hotplate 20 or by a heater coil not shown in the drawings. In addition, the metal oxide semiconductor 22 may be supported by other means than the hotplate 20. Further, electrochemical gas sensors that have a liquid or solid electrolyte and detection and counter electrodes connected to the electrolyte or further have a reference electrode connected to the electrolyte are usable as the gas detection unit.

[0033] FIG. 4 indicates the operational pattern of the gas sensor 2. The gas sensor 2 is operated with a period P, is heated to an operational temperature of 250 degree Celsius to 450 degree Celsius (200 degree Celsius in the embodiment) for a period T1 for each period P, and gases are detected based upon the resistance of the metal oxide semiconductor when heated.

[0034] FIG. 5 shows the results of a durability test to siloxane (exposure for ten days in 20 ppm D5). The tested gas sensors are shown in FIGS. 1 to 3; the embodiment shown by solid lines was provided with the polymer membrane (carboxy-methyl-cellulose) and sheet-like active charcoal, and the comparative example was provided with only the sheet-like active charcoal. The outputs in 100 ppm hydrogen were measured and they were converted into hydrogen concentrations by the initial dependence of the outputs on hydrogen concentrations. The shifts of the concentrations from 100 ppm indicate the degree of being poisoned, and, when poisoned by siloxanes, the outputs converted into concentrations generally increase. Similarly, the outputs in 1000 ppm methane were measured for the estimation of siloxane durability. The organic polymer filter 10 reduces the influence of siloxane. As a remark, as is obvious from the above, the organic polymer filter 10 prevents the oxidation catalyst filter from being poisoned when the oxidation catalyst filter is used in place of the active charcoal filter 14.

[0035] FIG. 6 indicates the methane concentrations that give the same outputs to 200 ppm ethanol; A indicates a comparative example without any filter, B a comparative example with only a polymer membrane (carboxy-methyl-cellulose), and C the embodiment with the polymer membrane (carboxy-methyl-cellulose) and sheet-like active charcoal. It is indicated that ethanol can not be removed by the polymer membrane but removed by the active charcoal.

LIST OF SYMBOLS

[0036] 2 gas sensor [0037] 4 Si chip (gas detection unit) [0038] 5 housing [0039] 6 lid [0040] 7 opening [0041] 8 filter [0042] 10 organic polymer membrane [0043] 11 support membrane [0044] 12 gas-permeable organic membrane [0045] 14 active charcoal filter (inorganic filter) [0046] 16 lead [0047] 17 terminal [0048] 20 micro-hotplate [0049] 22 metal oxide semiconductor [0050] 24 beam [0051] 26 cavity [0052] 28 pad