Electrical resistors – Resistance value responsive to a condition – Gas – vapor – or moisture absorbing or collecting
Patent
1989-09-28
1991-01-29
Reynolds, Bruce A.
Electrical resistors
Resistance value responsive to a condition
Gas, vapor, or moisture absorbing or collecting
H01C 700
Patent
active
049889702
DESCRIPTION:
BRIEF SUMMARY
The invention relates to semiconductors for a resistive gas sensor or, more precisely, a resistive semiconductor gas sensor having a high response speed as defined in the preambles of claims 1, 2 and 3, respectively.
To measure the partial pressure of oxygen and reducing gases (such as, for example, NO, CO, C.sub.3 H.sub.8, etc.), specific mixed oxides, the so-called perovskites, are particularly suitable semiconductors.
Perovskite semiconductors for gas sensors and their production by sintering was disclosed by Obayashi et al in U.S. Pat. No. 3,951,603 and in U.S. Pat. No. 3,953,173, by Sakurai et al in U.S. Pat. No. 4,044,601 and by Perry et al in U.S. Pat. No. 4,221,827.
In contrast to semiconductors made of tin oxide or similar materials in which the surface resistance changes as a function of the partial pressure of the gas being measured, the change in resistance in perovskite semiconductors is based on a volume effect; i.e. the semiconductor resistance changes as a function of the concentration of the gas being measured in that oxygen is diffused in the semiconductor.
The addition of reducing gases to the gas being measured causes the oxygen concentration in the semiconductor to be reduced due to the diffusion of oxygen to the surface of the solid body; the addition of oxygen to the gas being measured causes a corresponding increase in the oxygen concentration in the semiconductor due to the diffusion of oxygen into the solid body.
Since, however, after a change in the partial pressure of the gas being measured, oxygen must be able to diffuse into or out of the semiconductor material such semiconductors, in the interest of a high response speed, should be as thin as possible as the diffusion time for the oxygen is proportional to the square of the layer thickness. Such semiconductors are substantially insensitive to surface soiling and can also be used in rougher environments.
Sintering permits the production of semiconductors with minimum layer thicknesses between 20 and 50 .mu.m, usually, however, 500 .mu.m. The sintering method is frequently employed as a simple manufacturing process for gas sensor semiconductors which are produced in small numbers.
For mass production, the high energy costs caused by the long duration of the sintering process and the additional costs for the required subsequent treatment of sawing, grinding and polishing become a weighty factor.
For higher requirements with respect to response speed, the semiconductors could be produced in a thin-film process. In the past, dielectric materials have been applied to capacitor plates in this way. In this case, the substances constituting the semiconductor would be applied to a substrate by sputtering. This would permit the realization of very small layer thicknesses.
However--as a result of the high vacuum procedure this process is very complicated and thus expensive so that it is hardly suitable for the mass production of semiconductor sensors.
Due to the short free path lengths of the particles to be sputtered on and the collision processes resulting therefrom, no accurate geometric structures with sharply defined edge zones can be produced even if templates are employed.
Another serious drawback is that the number of substances to be sputtered on is limited; it is not possible to produce repeatedly doped perovskites in this manner.
As tests have shown, the perovskite semiconductors described in the literature so far do not exhibit an unequivocal characteristic (change in resistance as a function of the concentration of the gas being measured) over a greater range of measuring gas partial pressures of several orders of magnitude. Usually, the characteristic passes through a minimum so that two different measuring gas concentration ranges exist in which the semiconductor has the same electrical resistance.
Such semiconductors can be employed only in a measuring gas concentration range in which the characteristic is not ambiguous.
It is the object of the invention to produce resistive semiconductors for gas sensors which have a hig
REFERENCES:
patent: 3951603 (1976-04-01), Obayashi et al.
patent: 3952567 (1976-04-01), Shinagawa et al.
patent: 4507643 (1985-03-01), Sunano et al.
patent: 4677414 (1987-06-01), Yates
Hafele Edelbert
Hardtl Karl-Heinz
Muller Andreas
Schonauer Ulrich
Kernforschungszentrum Karlsruhe GmbH
Lateef Marvin
Reynolds Bruce A.
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