JPS621574B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing fine tin oxide powder with low electrical resistance.

Tin oxide powder is used as a catalyst, gas sensor, and conductive powder. As a catalyst, it is used as a catalyst for the allylic oxidation of olefins, for example, for the synthesis of acrolein from propene, butadiene from n-butene, and isoprene from isopentene.

Gas sensors utilize the fact that electrical resistance changes when a flammable gas such as city gas, propane gas, hydrogen gas, etc. is adsorbed.
As a conductive powder, SnO ₂ doped with Sb is known to have low electrical resistance, but there is a problem with the toxicity of Sb, so a material that does not contain Sb is desired. Products that do not contain Sb have higher resistance, but can be used as antistatic agents, and their color is close to white, so they can be used as antistatic agents for paper, resin, and fibers. Furthermore, transparency is achieved by making the particles smaller than the wavelength of visible light (0.4 to 0.8 μm), so they can be mixed into paints and used as antistatic paints. A film coated with this paint not only becomes a transparent antistatic film, but also a film for photosensitive machines to control charging voltage. In either case,
It is desirable that the powder be fine. That is, since the surface of the powder is used in catalysts and sensors, the finer the powder, the higher the activity, and the finer the powder, the better the transparency. Furthermore, it is desired that the powder has high dispersibility.

The following methods are known as methods for producing tin oxide powder.

(1) Oxidize metal tin by heating it in air at high temperatures.

(2) The white precipitate obtained by treating tin with concentrated nitric acid is calcined at high temperature.

(3) Neutralize an aqueous solution of a tetravalent tin salt with an alkali to obtain a white precipitate, which is then calcined at a high temperature.

In methods (1) and (2), the specific surface area is at most 10 m ² /g.
Only the following powders can be obtained, and even with method (3), sintering tends to occur during firing, and although a specific surface area of up to 50 m ² /g is possible, the dispersibility is poor and it is not desirable for the purpose of achieving transparency.

The present inventors have previously discovered that extremely fine tin oxide powder can be obtained when method (3) is carried out under specific conditions (Japanese Patent Application No. 58-52439).

In this method, tin chloride () aqueous solution is added to an alkaline aqueous solution kept at 65°C or above, and the pH is finally adjusted.
to 5 or less. When tin chloride is added to alkaline water, the initial pH is naturally close to 14, and the pH is 10.
Even if an aqueous tin chloride solution is added until the amount of Precipitate rapidly forms when the pH is below 10. In other words, tin chloride, which was dissolved at a pH of 10 or higher, precipitates out as tin hydroxide at once as the pH decreases, becoming a fine product. Furthermore, dispersibility is improved by adding tin chloride and ultimately maintaining the pH at 5 to 1. The precipitate thus generated is
In the subsequent firing process of the powder, sintering is reduced and the powder particles can be kept fine. If the reaction is terminated outside this range, the powder will easily sinter during firing. As a result of the above, fine particles of 50 m ² /g or more and good dispersibility can be obtained. On the other hand, with the method of neutralizing an aqueous tin chloride solution with an alkali, a precipitate begins to form as soon as the alkali is added, and as the reaction progresses, particles grow and a fine powder is obtained. do not have.

Examples of alkaline aqueous solutions include sodium hydroxide,
A solution of ammonia, potassium hydroxide, and sodium carbonate in water can be used. As the tin chloride solution, a solution in which tetravalent tin chloride is dissolved in water, in an aqueous hydrochloric acid solution, or in alcohol can be used.

The concentration of the tin chloride solution injected into the alkaline aqueous solution is not particularly limited, but the preferred concentration is 10 to 60.
%. There is also no particular limitation on the injection rate. It was found that the critical temperature for precipitation was approximately 65°C.
If it is lower than this, particles tend to sinter during firing. Tin dioxide powder can be obtained by washing and drying the generated precipitate and further calcination, but if the calcination temperature is below 350°C, the crystals will not develop well and it will be amorphous, and if the temperature is above 700°C, it will become amorphous. A conclusion occurs. For this reason, the firing temperature is 350℃ to 700℃ in air.
And so.

The method of the present invention is an improvement of the above method,
The dried precipitate obtained in the precipitate generation step of the above method,
A method is provided comprising reduction calcination at 350°C to 700°C. As a result, the electrical resistance measured in the form of a green compact pressurized to 100 kg/cm ² is 10 ¹ to ^{10 5} Ω.
cm of low electrical resistance tin oxide powder is obtained. On the other hand, the electrical resistance of the product of Japanese Patent Application No. 58-52439 mentioned above, measured in the same manner, is on the order of 10 ⁶ Ω·cm or more.

By calcination in a reducing or inert atmosphere, a small portion of _SnO2 is reduced to metallic tin.
It is thought that it is in a state that could be described as Sn-doped SnO ₂ or SnO _2-x , but this has not been confirmed yet. (Even when heated in an inert atmosphere, oxygen in SnO ₂ is removed and reduction occurs.) If the firing temperature is lower than 350°C, the crystals will not develop well and it will be amorphous, resulting in high resistance, and if it exceeds 700°C. When the reduction progresses too much and it comes into contact with air, rapid oxidation occurs, causing fire and sintering.

The low electrical resistance tin oxide fine powder obtained by the method of the present invention has a low electrical resistance value, is extremely fine, and exhibits good dispersibility.

Examples of the present invention will be shown below. As a measure of the fineness and dispersibility of the powder, the specific surface area or the percentage of particles of 0.3 μm or less after dispersing the powder in an aqueous solution with a pH of 10 was used.

Example 1 335g of sodium hydroxide was added to 10ml of water and heated to 80°C. Add to this 500 g of raw material with 60% tin chloride dissolved in water, and 610 ml of a 1:1 solution of hydrochloric acid and water.
The added material was added dropwise using a metering pump to carry out the reaction, and the final pH was set to 2. The resulting precipitate was washed with water, dried, replaced with nitrogen in an atmosphere furnace, and then hydrogen was poured at 1/min for 400 min.
It was baked at ℃ for 1 hour. After pulverizing this powder with an atomizer, the specific surface area was measured using Model SA-1000 manufactured by Shibata Kagaku Kiki Kogyo Co., Ltd. and found to be 68.9 m ² /g. This powder was pressurized at 100Kg/cm ² and the specific resistance was measured, and the result was 95 ⁶
It was Ω・cm.

Example 2 The dried precipitate produced in the first step of Example 1 was calcined in vacuo at 500° C. for 1 hour. This powder is 1.2×10 ⁵
It was Ω・cm. This powder was placed in a dryer at 110°C.
When measured immediately after drying for a while, it was 3.7×10 ⁵ Ω・cm
It was hot. 2.6×10 ⁶ fired in air at 500℃ for 1 hour
Ω・cm powder is 1.7× immediately after drying at 110℃ for 8 hours.
It was 10 ⁷ Ω・cm.

Example 3 The dry precipitated powder produced in the first step of Example 1 was heated to 400 ml of nitrogen in a fluidized bed with a diameter of 10 cm with a nitrogen flow of 1/2.
It was baked at ℃ for 1 hour. The specific resistance of this powder is 1.8×
It was 10 ⁴ Ω・cm. 40 ml of water was added to 5 g of this powder, the pH was adjusted to 10 with aqueous ammonia, and after dispersion in a ball mill for 17 hours, the content of particles below 0.3 μm was measured by a centrifugal sedimentation method and found to be 56.2%.

Example 4 The dry precipitated powder produced in the first step of Example 1 was purged with nitrogen and then flushed with nitrogen and hydrogen at 1/min each at 500°C for 2 hours.
Baked for an hour. The specific resistance of this powder is 1.2×10 ⁴ Ω・
It was cm.

Claims (1)

[Claims] 1. While keeping an alkaline aqueous solution with a pH of 10 or higher at 65°C or higher, a tin chloride solution is added to this solution to form a precipitate, and finally by keeping the pH between 5 and 1, fine particles can be formed. A method for producing a low electrical resistance tin oxide fine powder, which comprises obtaining a precipitate, washing and drying the precipitate, and then calcining the precipitate at a temperature of 350°C to 700°C in a reducing atmosphere or an inert atmosphere. 2. A method for producing a fine powder of low electrical resistance tin oxide according to claim 1, wherein the calcination is performed using H ₂ ,
A method carried out in He, N ₂ , Ar, or a combination thereof. 3. A method for producing a fine powder of low electrical resistance tin oxide according to claim 1, wherein the firing is performed in a vacuum.