JP3218821B2 - Gas detection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detecting element. More specifically, the present invention relates to a gas detection element in which a tin oxide semiconductor thin film as a gas sensitive body is formed on an insulating substrate.

[0002]

2. Description of the Related Art Conventionally, a gas detecting element using a tin oxide semiconductor as a gas sensing element has been known. In recent years, this tin oxide semiconductor has been shifted from a sintered type to a thin film type. The tin oxide semiconductor thin film has a feature that power consumption for heating the gas sensitive body is small and that even a very low concentration of flammable gas can be detected.

[0003] However, the tin oxide semiconductor thin film has a drawback that the resistance value as a gas sensible body varies greatly with time, and stable element characteristics cannot be maintained. It is pointed out that crystal grains grow inside the thin film as one of the causes. That is, since the gas detecting element for detecting a flammable gas or the like is generally used under a high temperature condition of about 300 to 500 ° C. and used for a long time, the crystal grain growth proceeds, and accordingly, the resistance of the thin film is increased. Since the value changes, stability over time cannot be secured.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a gas detecting element in which a tin oxide semiconductor thin film is formed as a gas sensitizer on an insulating substrate. It is to provide a product with a small change.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
Tin tetraalkoxide and germanium tetraalkoxy
Germanium-containing tin oxide semiconductor, which is a fired product mixed with iron
Do not form a thin film on an insulating substrate as a gas sensitive material.
This is achieved by a gas detection element .

As the insulating substrate, a heat-resistant glass substrate,
Approx. 300 to 500 ° C for alumina substrate, aluminum nitride substrate, etc.
Those having a heat-resistant temperature of are generally used.

On these insulating substrates, a set of electrodes and a germanium-containing tin oxide semiconductor thin film are formed in any order. The formation of the germanium-containing tin oxide semiconductor thin film is performed by forming about 0.1-1 mol% of tin tetra lower alkoxide, for example, triethanolamine of tin tetraisopropoxide.
After adding a germanium lower alkoxide, such as germanium isopropoxide, to the isopropanol solution,
Further, this is carried out by using an immersion solution in which dehydrated isopropanol and triethanolamine are added to adjust the total metal concentration to about 0.5 mol%. Here, triethanolamine is used for stabilizing the alkoxide solution, and the amount of triethanolamine added is adjusted to be equal to the total metal concentration.

[0008] In this immersion solution, the insulating substrate is put in about 10 to 60
After immersion for about 2 seconds, pull up at a constant lifting speed,
After drying at about 100-200 ° C, use an electric furnace, etc.
Bake at 0-800 ° C for about 30-60 minutes. Such immersion
-By repeating the series of steps of drying and baking several times, the tin oxide thin film with germanium
It is formed on an insulating substrate to a thickness of about 500 nm.

The amount of germanium added is about 1 to 5 in all metals.
It is used in such a proportion that it occupies 0 mol%, preferably about 10 to 40 mol%. The resistivity of the tin oxide semiconductor thin film to which germanium is not added is about 0.06 Ωcm, and the resistivity when such a ratio of germanium is added is 0.1 to 0.4.
Although the resistivity remains within the range of Ω · cm, the addition of 50 mol% causes the resistivity to rapidly rise to about 2 Ω · cm, so it is not preferable to add at a higher ratio.

[0010]

As described above, in a gas detection element in which a tin oxide semiconductor thin film is formed as a gas sensitive substance on an insulating substrate, germanium (mostly GeO ₂ , but a part of ₍ It is thought that some are dissolved in ₂ ), and by suppressing the growth of tin oxide crystal grains, it is possible to reduce the change over time of the resistance value as a gas sensitizer, and to stabilize The device characteristics can be maintained.

That is, although the addition of various Group IV elements (Ge, Zn, Ti or Si) which are considered to have no influence on the valence control can suppress the grain growth of tin oxide and the change in resistivity, particularly, The effect of suppressing the grain growth by adding germanium is the highest, and the response speed to the measurement gas is also improved.
Also, by adding germanium, for example, 800
The amount of decrease in resistivity due to heat treatment at a high temperature such as ° C. can be reduced.

[0012]

Next, the present invention will be described by way of examples.

EXAMPLE To 17.73 g (0.05 mol) of tin tetraisopropoxide, 50 ml of dehydrated isopropanol is added and stirred. To this solution, 7.46 g (0.05 mol) of triethanolamine was added, and dehydrated isopropanol was further added to make a total volume of 100 ml. This 0.5
In a molar concentration of tin tetraisopropoxide in triethanolamine-isopropanol, germanium tetraisopropoxide was added to tin tetraisopropoxide.
After addition of 10 mol%, dehydrated isopropanol and triethanolamine were further added to adjust the total metal concentration to 0.5 mol, and used as an immersion solution.

In this immersion solution, a heat-resistant glass substrate is
After immersion for 0 second, it was pulled up at a constant pulling rate of 6 cm / min, dried at 110 ° C. for 15 minutes, and then baked at 600 ° C. for 30 minutes in an electric furnace. Such a series of steps of immersion-drying-baking treatment was repeated 5 cycles to form a tin oxide thin film (250 nm in thickness) to which 10 mol% germanium was added, on a heat-resistant glass substrate.

Comparative Example 1 In the example, a solution of tin tetraisopropoxide in a mixed solvent of triethanolamine and isopropanol was used as it was as an immersion solution.

(Grain growth suppression effect by germanium addition)
In Examples and Comparative Example 1, each of the immersion solutions was gradually heated, and the solvent was distilled off.
When the crystal grain size (unit: nm) after heat treatment at each temperature of 00, 800 or 900 ° C. for 3 hours was measured by the X-ray diffraction method, the following results were obtained. Heat treatment temperature (° C) Example Comparative Example 1 600 20.4 28.1 700 30.2 37.2 800 28.7 39.9 900 32.9 78.9

From these results, it can be seen that the particle size of the comparative example 1 increases remarkably when the heat treatment temperature is increased, whereas the particle size increase of the example is small, that is, germanium is added to the tin oxide thin film. Thus, it is considered that the growth of the particles was suppressed.

(Stabilization of Tin Oxide Thin Film Resistivity by Addition of Germanium) In Example 1 and Comparative Example 1, each of the germanium-added tin oxide thin films formed on a heat-resistant glass substrate was measured for the above crystal grain size. With reference to the heat treatment temperature (800 ° C. and 600 ° C.) where the difference in crystal grain size between those of Comparative Example 1 and Comparative Example 1 is large, a tin oxide thin film at a heat treatment temperature of 800 ° C. and a tin oxide thin film at a heat treatment temperature of 600 ° C. about,
The resistivity (unit: Ω · cm) at room temperature was measured by the four probe method, and the ratio R ₈₀₀ / R ₆₀₀ was calculated. Example Comparative example Heat treatment temperature 600 ° C 0.21 0.10 Heat treatment temperature 800 ° C 0.10 0.034 R ₈₀₀ / R ₆₀₀ 0.48 0.34

From these results, it was found that the resistivity ratio (R ₈₀₀ / R ₆₀₀ ) was about 1/2 in the working example, but about 1/3 in the comparative example 1. It can be seen that the resistivity of the tin oxide thin film tends to decrease due to the heat treatment in the above, but by adding germanium to the tin oxide thin film, the decrease rate can be reduced and the change with time in the resistivity can be suppressed.

Comparative Examples 2 to 4 In Examples, zirconium tetra n-butoxide (Comparative Example 2), titanium tetraisopropoxide (Comparative Example 3) or silicon tetraethoxide (Comparative Example) was used instead of germanium tetraisopropoxide. 4) was used in an amount of 10 mol% based on tin tetraisopropoxide.

The germanium, zirconium or silicon-added tin oxide thin films obtained in Example (■), Comparative Example 2 (▲) and Comparative Example 4 (▼) were each subjected to a heat treatment at 600 ° C. The change in resistivity with the amount of silicon added is shown in the graph of FIG.
Looking at the addition amount of 10 mol%, zirconium has 24 Ω
· Cm, and the addition of titanium under the same conditions (Comparative Example 3) results in a film with a high resistance exceeding the measurement limit that cannot be displayed in this graph, whereas germanium and silicon maintain a low resistivity of 1 Ω · cm or less. I was Above all, in the film to which germanium was added, the value was 2.1 Ω · cm even at the addition amount of 50 mol%.

Further, regarding the above-mentioned grain growth suppressing effect,
Regarding the crystal grain size when heat-treated at a temperature of 600 to 1000 ° C., in Comparative Example 1 (●) in which nothing was added, remarkable grain growth started from about 800 ° C., but in all cases, 10 mol% of germanium was added. In Example (■), Comparative Example 2 (▲) in which zirconium was added, and Comparative Example 3 (◆) in which titanium was added, grain growth was suppressed, and the effect of addition of germanium was particularly high. This is shown in the graph of FIG. Such a grain growth suppressing effect is observed not only for powder but also for thin films.

Further, the change of the X-ray diffraction pattern when the tin oxide powder to which 50 mol% of germanium is added is heat-treated at each temperature of 600 to 1000 ° C. is shown in the graph of FIG. At 600 ° C, the (110) peak of SnO ₂ (○) is seen,
From 700 ° C., the (101) peak of hexagonal GeO ₂ (●) is observed, and the SnO ₂ (110) peak becomes broader as the heat treatment temperature increases. When analyzing this in detail, under the influence of tetragonal (rutile) tin oxide, GeO ₂ crystallizes into a tetragonal form which is originally unstable in this temperature range, and its (110) peak is (110) of SnO ₂ . ) It is considered to appear near the peak. The crystallization of tetragonal GeO ₂ consists of SnO ₂ and GeO ₂
Since it is considered that this does not occur if they are independently present, germanium-added tin oxide firstly crystallizes SnO ₂ ,
It is considered that GeO ₂ is generated on the surface of the crystal grains to suppress the growth of crystal grains.

In connection with this, germanium 10 mol
% -Added tin oxide thin film is composed of particles with a particle size of about 10 nm, and it can be seen from the results of TEM observation that the particle sizes are relatively uniform, which is influenced by the crystallization process as described above. It is thought that it is. On the other hand, in the tin oxide thin film to which germanium was not added, a wide distribution of the particle diameter was observed.

(Gas sensor characteristics) 2-methyl- having a concentration of 5240 ppm
The response behavior of the thin film at 350 ° C. in air containing 2-butene is shown in the graph of FIG. When the atmosphere is switched from 2-methyl-2-butene-containing air (gas) to air (air), the response speed of the tin oxide thin film (solid line) to which 10 mol% of germanium is added does not include germanium It turns out that it is faster than the tin oxide thin film (dotted line).
This may be related to the fact that the diameter of the particles forming the germanium-added tin oxide thin film is more uniform than the diameter of the particles of the tin oxide thin film to which germanium is not added.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the added amount of germanium, zirconium or silicon and the resistivity.

FIG. 2 is a graph showing a relationship between a heat treatment temperature and a crystal grain size in Examples and Comparative Examples 1 to 3.

FIG. 3 is an X-ray diffraction pattern of a tin oxide powder to which 50 mol% of germanium is added, which is heat-treated at a temperature of 600 to 1000 ° C.

FIG. 4 10 mol% based on air containing 2-methyl-2-butene
4 is a graph showing the response behavior of a germanium-added tin oxide thin film and a tin oxide thin film not containing germanium.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-51-65997 (JP, A) JP-A-59-26043 (JP, A) JP-A-59-94050 (JP, A) JP-A Sho 60- JP-A-1-942952 (JP, A) JP-A-58-44339 (JP, A) JP-A-59-90037 (JP, A) JP-A-2-132361 (JP, A) JitsuHiraku Akira 59-134054 (JP, U) (58 ) investigated the field (Int.Cl. ^7, DB name) G01N 27/12 JICST file (JOIS)

Claims (1)

(57) [Claims] (1) tin tetraalkoxide and germanium te
Contains germanium, which is a mixed fired product with traalkoxide
Tin Oxide Semiconductor Thin Film as Gas Sensitive Body on Insulating Substrate
A gas detection element formed .