Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Patent
1993-10-07
1995-06-27
McMahon, Timothy M.
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
422 94, 422 98, 436144, 436127, 436138, 7333505, 419 19, 437188, 437205, G01N 2712
Patent
active
054277400
DESCRIPTION:
BRIEF SUMMARY
This invention relates to gas sensors of the type in which the resistance, or other electrical property, of a sample of tin (IV) oxide (SnO.sub.2) is measured, the resistance, or other electrical property, being dependent on the concentration of the gas in the surrounding medium. In the following reference is made to measurement of resistivity but it should be understood that the invention is not restricted to such measurement. At the rear of this description is a list of prior art relating to tin oxide sensors and reference numerals in the description refers to this list of prior art.
Tin (IV) oxide (SnO.sub.2) is widely used as the basis of solid state sensors capable of detecting a variety of toxic and flammable gases [1-7]. Tin (IV) oxide is an n-type semiconductor in which electrical conductivity occurs through negative charge carriers.
The active sensing element usually consists of a sintered polycrystalline mass of the oxide as exemplified by the many forms of the Figaro gas sensor produced in Japan. The fabrication procedure adopted involved heat treatment of the SnO.sub.2 along with any other additives such as PdCI.sub.2 or ThO.sub.2 [2, 5, 6], which are initially dispersed in an aqueous slurry. This sintering process yields a sensor body of suitable mechanical strength and also confers thermal stability, which is essential considering the elevated temperatures (300.degree. C.-400.degree. C.) at which these devices are operated. In order to achieve the desired thermal stability, the sintering temperature (T.sub.S) used must be significantly higher than the sensor operating temperature (T.sub.o). Typical values of T.sub.S quoted in the literature for commercially available devices lie in the range 500.degree.-700.degree. C. [2, 6, 7]. However, it is widely known that tin oxide alone sinters poorly at these temperatures. Values of T.sub.S exceeding 1100.degree. C., which marks the Tamman temperature of the material [8], are required to achieve accelerated adhesion between neighbouring crystallites. To improve low temperature inter-granular cementing, binders such as tetraethyl ortho-silicate (TEOS) [9], MgO [6]or alumina [10] are often incorporated prior to heat treatment. These binders may significantly alter the gas sensing characteristics of the material, for example, in the case of TEOS, which decomposes at elevated temperatures forming Si--O bridges between the SnO.sub.2 grains, the presence of the binder confers a marked increase in sensitivity to flammable gases [9].
Considering the importance of the heat treatment step in the overall fabrication procedure, comparatively few studies have been performed on the influence of sintering temperature on the characteristics of SnO.sub.2 based gas sensors. Research carried out by Borand [11] on pressed pellets of polycrystalline SnO.sub.2 annealed in the 400.degree. C.-900.degree. C. range showed that maximum CO sensitivity along with the shortest response time occurred at a sintering temperature of 700.degree. C. However, for a tin oxide combustion monitoring device Sasaki and his co-workers [12] found that a sintering temperature of 1300.degree. C. gave the most desirable characteristics.
Such sensors have been widely described and are usually in the form of a thin or thick film deposit of the tin oxide on an alumina or other insulating ceramic substrate. Platinum paste contacts are used to connect the tin oxide to wires for resistance measurement and an electrical resistance heating element may be provided on the substrate.
Tin oxide sensors suffer a major drawback in that they are sensitive to many gases and worse there are also some cross-sensitivities, i.e. the presence of one gas will alter the sensitivity of the sensor to the presence of a second gas.
A notable cross-sensitivity is the influence of oxygen at low oxygen partial pressures. It is found that an undoped SnO.sub.2 sensor experiences large changes in resistance (greater than three orders of magnitude) upon exposure to gases such as CO or H.sub.2 under conditions of reduced oxygen par
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Keizo Uematsu, Nobuyasu Mizutani, Masanor
Coles Gary S. V.
Smith Brian M.
Williams Geraint
Bhat N.
British Gas plc
Hinds William R.
McMahon Timothy M.
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