JPS5990040A - Detector for gaseous carbon monoxide - Google Patents

Abstract

PURPOSE:To improve the sensitivity of a gaseous CO detector using a film-like tin oxide as a gaseous carbon monoxide CO sensitive body by regulating the structure of the tin oxide film. CONSTITUTION:A film-like tin oxide having <=120 deg.crystal particle size determined by a Debye-Scherrer formula from the half-amplitude level of the diffraction peak corresponding to the 110 face appearing near 2theta=26.6 deg. (theta is a diffraction angle) in the X-ray diffraction chart of an SnO2 crystal is used as an adequate sensitive material of tin oxide. The gas detector using the tin oxide film selected in such a way exhibits the value of >=(1/3-2/3)order of reaction derived from the reaction considered for the reaction between gaseous CO and the surface of tin oxide with respect to gaseous CO thus enabling detection of gaseous CO with high sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a CO gas detector using a film of tin oxide as a carbon monoxide (CO) gas sensor.

Conventional structure and its problems A CO gas detector that uses tin oxide as a gas sensitive material has an electron-donating CO gas adsorbed to tin oxide, which is an n-type semiconductor, so that the electrical resistance of tin oxide can be changed by CO gas. This takes advantage of a phenomenon that has long been well known in the field of physics and chemistry, in which the concentration decreases in proportion to the concentration.

This so-called semiconductor gas detector consists of a sensitive body whose electricity increases or decreases in proportion to the gas concentration, a pair of electrodes for extracting this electrical resistance change as an external signal, and a sensor for operating the sensitive body at an appropriate temperature. It has an extremely simple structure. One of the factors that determines the sensitivity, response speed, and other performance of a semiconductor gas detector, which has an extremely simple structure, is the physical properties of the materials that make up the sensitive body.

In other words, even if the same material were used as a sensor, there would be a difference in material properties in the sense of a difference in reactivity to gas on the surface defined by its shape or internal structure, and a gas detector. This becomes noticeable as a difference in the characteristics of the two. It is not difficult to understand this, considering that the gas detection mechanism involves a chemical interaction between the sensitive body and the gas to be detected, as is generally said.

When trying to obtain a highly sensitive gas detector, it is important to consider the conductivity in tin oxide! The reaction with co on the surface of tin oxide is more effective if the number of electrons present in the surface layer of tin oxide is larger than the number of electrons present inside the tin oxide ( In recent years, attempts have been made to use tin oxide in the form of a film with a relatively large luminous area in place of the tin oxide sintered body with a small specific surface area, but it has not been possible to reproduce the detection characteristics. Hateful. In other words, the relationship between the physical properties of the film and its gas sensitivity has not been clarified, and the high sensitivity that is expected by making it into a film is not clear! 14 properties cannot necessarily be obtained, which makes it difficult to use it as a practical gas detector instead of a detector using a sintered body.

This is due to the complicated reaction between the Co gas and the tin oxide surface. The reactions currently being considered are shown below.

Co-+CO+e 2CO+O (oxygen in the lattice of tin oxide) →CO2+CO+e 2CO+02- (oxygen molecules adsorbed on the surface of tin oxide) →
2 CO2+ e CO+o- (adsorbed on the surface of tin oxide/arsenic oxygen atom) → CO
2+ e co+o2- (oxygen atom adsorbed on the surface of tin oxide) → C
02+ 2 e In this way, the reaction between CO and the surface of tin oxide changes greatly depending on the surface condition of tin oxide, that is, the ease with which oxygen is removed from the lattice, and the adsorption state of oxygen on the surface. The gas sensitivity of the tin oxide sensitizers is particularly dependent on this influence.

OBJECTS OF THE INVENTION The present invention provides a CO gas detector that improves the surface state by defining the structure of the tin oxide film and has the high sensitivity inherent to the film-form tin oxide sensor. The purpose is to

Structure of the Invention The present invention corresponds to the (110) plane that appears around 20 = = 26.60 (θ is the diffraction angle) in the X-ray diffraction diagram of Sn○2 crystal as a suitable tin oxide sintered body core. Based on the half-value width of the diffraction peak, it is characterized by using a film-like tin oxide having a crystal grain size of 120 Å or less as determined by a Debye-Scherer set (not shown below).

D=λ/β cos θ where D is the crystal particle diameter, K is a coefficient close to 1, λ is the X-ray wavelength used for diffraction measurement, and β is B-b.

B is the half value of the specimen [IJlb is a value specific to the diffraction grating;
This is half the value of the diffraction peak of SiO2 single crystal powder.

A gas detector using a tin oxide film selected as described in Section 2 is capable of reacting to Co gas based on the reaction that is considered to occur between Co gas and the surface of tin oxide, as described above. It shows a value of % to % or more of the reaction order, and highly sensitive Co gas detection becomes possible. Party B's reasons will be discussed later.

DESCRIPTION OF EMBODIMENTS FIG. 1 shows one of the embodiments of the present invention, in which the crystal grain size is 1.
Figure 2 shows a CO gas detector with a tin oxide sensitizer that is less than 20 Å. In the figure, 1 has a surface roughness of approximately 2.5 μm, a thickness of 0 mm, a length of 5 mm, a width of 5 + nm, and a purity of 96 mm.
This is the related alumina substrate. 2 is a planar heater made of a resistor obtained by applying a conductive paste mainly composed of ruthenium oxide to one side of this alumina substrate and baking it;
This maintains the detector at a defined constant operating temperature.

3 is a glass layer coated and baked to cover the surface of the sheet heater 2. 4 is a lead wire made of platinum wire with a wire diameter of 0.5 mm for supplying electric power to the planar heater 2. 6 is a conductive paste mainly composed of platinum on the other side of the alumina substrate 1, and the central part of the alumina substrate is
This is an electrode for taking out the electrical signal of the gas sensitive body, which is printed and printed in a shape exposed by a +n+n width groove. Reference numeral 6 designates a thermocouple for measuring the temperature of the gas sensitive body, which is made of a chromel-alumel wire and has a fixed seizure together with the electrode 6.

7 is an IJ made of platinum wire with a wire diameter of 0.6 mm for electrode 6.
- line. 8 has a crystal grain size of 120 Å according to the present invention.
This is a Co gas sensitive body made of a tin oxide film as shown below.

The tin oxide film J1 is, for example, a 125 mm diameter targlow material made of a metal tin plate with a purity of 99.99, an electrode distance of 60 mm, an argon gas pressure of 1.25 Pa,
A film thickness of 600 mm obtained by performing DC sputtering for 60 minutes at an oxygen gas pressure of 1.25 Pa and a current of 100 mA.
A thin film of tin oxide with a size of about 0A is heated at 600° in air.
4 can be obtained by heating and baking at C for 2 hours.

FIG. 2 is a diffraction pattern of 20=20 to 36° obtained by X-ray diffraction of the tin oxide film obtained in this manner at a tube current of 20 mA and a tube voltage of 60 KV using copper as an anticathode. The diffraction peak corresponding to the (110) crystal plane peculiar to SnO2 is 141 degrees at 20-26.6°.

Note that the sharp diffraction peak near 2θ=25.60 is a peak attributed to alumina of the substrate. The crystal grain size of the tin oxide film is determined using the Debye-Scherer set described above.

1 17J) In L, K-; 1.0. λ=1.5
406 people.

θ=26.6/2, B-1,10 (2θ shown in Figure 2
= 26.6° (during the half value of the diffraction peak), and substitute b-0,13. It can be seen that the crystal grain size of the tin oxide film is 105. The crystal grain size of the tin oxide film is determined by the above method. In order to see the effect of the present invention, the cross-sectional area is 2.6c.
The gas detector shown in FIG. 1 was placed in the gas flow path d, and the sample gas containing CO gas was completely flowed at a rate of 21 per minute to evaluate the characteristics.

The sample gas composition has a CO concentration of O to 1500 ppm.
.

O2 5.4%, balance N2. The gas detector is heated to 330'C or 390'C by the planar heater 2.
The O gas concentration was varied between 1500 ppm.

To measure the change in electrical resistance of the gas detector at this time, connect a constant resistor of 150Ω to the gas detector in series, and apply 100mV to both ends.
It was observed by applying a DC voltage of 160Ω and measuring the voltage change across a constant resistance of 160Ω.

Figures 3 and 4 show these results, and ΔlogR/
The vertical axis represents Δlol/Cco, and the horizontal axis represents crystal grains. Note that R represents the electrical resistance value of the gas detector, and CCO represents the CO concentration in the sample gas.

Δlo,! The larger the value of 7R/Δdogcco, the more CO
can be detected with high sensitivity. FIG. 3 shows the operating temperature at 330''C, and FIG. 4 shows the operating temperature at 390°C.

As is clear from these figures, according to the present invention, particle size 1
A CO gas detector with a tin oxide film of 20 Å or less is Δl
The ogR/ΔlogCco value was as high as about 0.75, indicating that CO gas could be detected with high sensitivity.

Regarding the crystal grain size dependence of tin oxide sintered bodies, see, for example, Electrochemistry Vol. 750. g1 (1982
As stated in pages 99 to 104 of 2007), it has been shown that Co sensitivity is hardly affected by the crystallite size within the grain size range of 50 to 600.
The inventors fabricated an element similar to that described in the above-mentioned literature for a sintered body, and obtained ΔlogR/Δ1o9cc.
When the value of o was investigated, the value showed a value of % to only, regardless of the crystal grain size. This value corresponds to C according to the present invention.
This is much smaller than the 0.76 of an O gas detector, which shows how superior the detector of the present invention is.
.

In order to see the effects of the present invention, tin oxide films with different crystal grain sizes were formed by forming a tin oxide film by the sputtering method using the metal tin plate as a target, and then heating and baking it in the atmosphere. The results were obtained by selecting the alumina substrate temperature during sputtering and the temperature during heating and baking as shown in the following table.

The thickness of each film is 5,000 to 5,500 people.

￥1: / As a tin oxide film with a crystal grain size of 120 Å or less, the film thickness I by the sputtering method described above is about 1
In addition to the 000-layer film, there is also a film obtained by the so-called in-gas evaporation method, in which metallic tin is heated and evaporated in an oxygen gas atmosphere. This product has the same effect as the present invention, or CO
It is thermally unstable at the operating temperature of 300 to 400°C, which is required for a gas detector, and particles recombine over time, and the crystal grain size can be maintained at 120 Å or less.
Extremely 4 (l<, Q for the present invention), not applicable.

Effects of the Invention As described above, according to the present invention, a CO gas detector with CJl and high I 6 degrees can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a vertical cross-sectional view of a CO gas detector according to an embodiment of the present invention, Fig. 2 is an X-ray diffraction diagram of a tin oxide film, and Figs. 3 and 4 are graphs showing the crystal grain size of tin oxide and CO gas. FIG. 3 is a diagram showing the relationship with sensitivity. 1... Substrate, 2... Planar heater, 5...
·····electrode. Figure 1 @ Figure 2? θ Kujun) 3 Figure d /# t5θ 2ρρ? Figure 4

Claims (1)

[Claims] (1) SnO2 crystals appearing in X-ray diffraction measurements (1
1o) From the half value l of the diffraction peak attributed to the surface, Debye
The crystal grain size calculated using the Scherer set is 1
A carbon monoxide gas detector characterized in that a film-form tin oxide having a temperature below 20 AIJ is used as a sensitive material. (Rumor) The carbon monoxide gas detector according to claim 1, wherein the tin oxide film is provided by sputtering.