Gas detecting element and method of making it

(74) Agent: Armengaud Afné, 21, boulevard Poissonnière, Paris (2).

© Gas detection element and its manufacturing process.

72) Invention of:

33) (32) (31

Conventional priority: Patent application filed in Japan on December 19, 1969 n. 102.827 / 1969 in the name of the applicant.

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The present invention relates to a detection element, in particular for detecting the presence of gases and fumes, comprising a semiconductor body whose conductivity changes when it adsorbs a gas or a smoke, as well as to a method for manufacturing a such element _e

Elements for detecting gases and fumes consisting of a metal-oxide semiconductor whose conductivity changes when it adsorbs a gas or a smoke are well known in the art and can be divided into two groups, one of which is that of the elements reducing agents comprising SnOg, ZnOg, etc., while the other comprises oxidizing elements such as NiO, etc. most of these elements are produced by sintering at very high temperatures in order to give them the desired mechanical resistance. However, sintering these elements at a high temperature makes them lose a large part of their adsorption surface and often considerably reduces their sensitivity. Although the above-mentioned semiconductor materials can be produced and used in the form of thin films, the resulting detection elements are inferior to the sintered detection elements in terms of their gas detection sensitivity and therefore do not are hardly usable.

Consequently, one of the nozzles of the invention is to provide a gas detection element constituted by a metal-oxide semiconductor which has a high mechanical resistance joined to a high sensitivity.

Another object of the invention is to provide an improved process for the manufacture of such gas detection elements.

According to the invention, a gas detection element comprises a metal-oxide semiconductor body and silica or a silica gel formed therein. To manufacture the gas detection element according to the invention, one or more substances which can be transformed into silica or silica gel by a subsequent treatment are incorporated into the metal-oxide semiconductor material.

It is well known that silica and silica compounds are easily transformed into

45319 silicon SiO ^ by heating in air. It is thus, in particular, that organic silicon compounds, such as alkyl silicides, silanol and silane diol, which come from silicon chloride SiCl 4, silane SiH 4, etc.

readily hydrolyze, producing silica gel when heated in air. This substance has a high permeability to gases and serves, at the same time, to improve the mechanical resistance of the element and to decrease its variation in sensitivity over time. According to one embodiment of the invention, a sintered metal-oxide semiconductor element is impregnated with a hydrated silicate, then it is heated to transform this silicate into silica gel. The resulting element is rigid, is hard and is able to withstand vibrations, shocks, scratches and other relatively harsh external conditions.

Other characteristics and advantages of the invention will emerge from the description which follows, given solely by way of nonlimiting example, with reference to the appended drawing, in which:

- Fig.1 is a schematic sectional view of a gas detection element according to the invention;

- Fig.2 is a schematic section of a second embodiment of the invention; and,

- Fig.3 is a block diagram of an alarm device comprising the gas detection element of Fig.1.

In all the figures of the drawing, the same elements have been designated by the same references.

Referring to Fig.1, there is seen a gas detection element according to the invention which comprises a semiconductor body 1, two electrodes 2 and 3 embedded therein and a spacer 4 placed between the two electrodes. The body 1 se. consists of a sensitive metal-oxide semiconductor 5 and particles 6 intended to increase its cohesion, such as an alumina or quartz powder. The electrodes 2 and 3 are formed by rectilinear or helical wires of gold, platinum, iridium, palladium, nickel, chromium or an alloy of these. Although the electrodes 2 and 3 may be different, both; in their composition than in their form, it is more practical for them to be identical. To give an example, electrodes 2 and 3 are identical

45319 and are respectively constituted by a wire of 0.09 mm in diameter of an alloy of palladium and iridium comprising 80% of Pd and 20% of Ir comprising a helical part with an internal diameter of 0.6 mm and having 12 turns. The resistance of this electrode is 2 ohms. The spacer 4 is made of ceramic or glass and, in the present example, has 1 S. shape of a rectangular parallelepiped of 1.5 x 2 x 3mm

To manufacture the detection element shown above, the two electrodes 2 and 3 are placed on the two faces of the spacer 4 so as to be in contact with them, then they are heated to about 1000 ° in passing a current through it, in order to weld the two faces of the spacer to the parts of the helical electrodes with which they are in contact.

Mixing a tin oxide powder SnO ^ whose particles have about 1 micron and which contains 0.3% by weight of palladium with an equal weight of alumina or quartz powder passing the sieve No. 100 (mesh 0.15 mm) and then knead this mixture with water. The resulting composition is applied to the structure prepared above, as shown in Fig.1, to form the body 1. Then, this body is dried in the air, then it is heated to about 700 ° C by passing a current through the electrodes 2 and 3 ·

75 ml of tetraethyl silicate diluted with

25 0.3 ml of water, 0.3 ml of hydrochloric acid is added, then the mixture is stirred for 30 minutes to obtain a transparent solution. This hydrosol transforms into silica gel when it dries.

Then, the semiconductor body 1, prepared as explained above, is soaked in this solution for about 10 seconds, then it is slowly heated to

600 ° 0 as shown. After cooling in air, the body 1 has a very high mechanical resistance and an improved detection sensitivity.

Fig. 2 shows a small gas detection element which is particularly useful as an alarm device or as a portable detector powered by a battery. The spacer 4 in Fig. 1 has been deleted in this element. Since the body 1 shrinks freely when it is agglomerated due to the removal of the spacer 4, the particles 6

45319 intended to prevent cracks, which can be seen in Fig. 1, can be removed in Fig. 2. In this embodiment, the electrodes are preferably formed by fine twisted gold wires. This embodiment of the invention makes it possible to easily manufacture elements having less than a millimeter in diameter.

The sensing element of Fig. 1, manufactured as described above, was tested against an identical element, but which had not been soaked in silicate hydrosol, using for comparison the alarm circuit shown in Fig. 3. In this figure, the primary of a transformer 7 is connected to an alternative electrical energy source of 100 volts. The ends of the electrode 2 of the detection element 1 are respectively connected to one of the ends and to a socket of the secondary of the transformer 7 ", the other end of this secondary being connected, through a buzzer 8, to one end of the other electrode 3, the opposite end of which remains in the air.

The buzzer 8 used in the test had an impedance of 4 k / ohms and its threshold voltage was set at 40 V. The transformer 7 was calculated to apply a volt to the two ends of the electrode 2 and so that the difference in voltage between the two ends of the secondary is 100 V. It has been measured that the voltage across the buzzer 8 is 10 V in pure air and 70 V in air containing 0.1% isobutane. On the other hand, with an identical element, but which has not been soaked in the silicate hydrosol, we measure 8 V in pure air and 24 V in the same air containing isobutane. Since the voltage across the buzzer is proportional to the sensitivity of the gas detection element, it is obvious that the element according to the invention is much more sensitive than that of the prior art. In addition, this voltage was measured with the present element as being 70 i 2 V in the above air containing isobutane after continuous service for 100 days. As a result, it is also evident that the variation in sensitivity over time is very small.

As described above, according to the invention, it is possible to remarkably increase the mechanical resistance of the element, to increase its sensitivity of

45319 sensing and minimizing variations in sensitivity over time. This greatly increases the reliability and usefulness of such gas detection elements.

It goes without saying that numerous modifications can be made to the example shown and described, without however departing from the scope of the invention. Thus, the silicate hydrosol could be hydrolyzed with concentrated sulfuric acid, instead of heating, if there is no objection to it taking a relatively long time. Although the element was impregnated with a silicate hydrosol in the above example, the semiconductor material could also be first kneaded with one. hydro silicate soil. However_t in this case, it is better to prepare only the necessary amount each time, because gelation occurs automatically.

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