JPH0212883B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing conductive zinc oxide. Conductive zinc oxide has excellent advantages such as its conductivity, oil odor resistance, heat resistance, and light resistance, so it is used for facsimile recording paper and other information recording paper for electrostatic recording, current recording, and discharge recording, and for industrial use. In addition to being used as a coating agent in the production of recording paper, it is also widely used as a conductive agent in synthetic resin films and synthetic fibers. Various methods for producing such conductive zinc oxide are already known. A typical example of these is as disclosed in U.S. Pat. This method involves heating and firing at a temperature of 600 to 900°C. However, since this method uses hydrogen or carbon monoxide as the reducing gas, there is a risk of explosion. To avoid this, the reducing gas must be mixed with an inert gas such as nitrogen and fired in a reducing atmosphere with a predetermined non-explosive composition, which makes process management complicated. In addition, the equipment becomes complicated and the product ends up being expensive. Furthermore, in all conventional methods, zinc oxide and an activator are uniformly kneaded with water to form a slurry or paste, and then this is dried, pulverized, and heated and calcined. Since the raw material tends to sinter and become granular or agglomerated during heating and firing, it is necessary to crush it again after heating and firing, but the resulting zinc oxide powder has a significantly large average particle size. As mentioned above, the conventional method requires complicated process control and a large number of steps, but the conductivity is still not sufficient, and furthermore, due to the large average particle size, it is subject to various restrictions in terms of application. Of course, it is also extremely unsatisfactory economically. In order to solve these various problems, the present inventors have already dry-mixed zinc oxide powder and aluminum oxide powder and heated and fired them in the presence of solid carbon to create an oxide with improved conductivity. We are proposing a method to produce zinc at low cost. (Tokuko Showa 55
−19897). According to this method, the raw material oxides are dry mixed and heated and fired, so the raw material powders do not significantly adhere to each other and agglomerate during the heating and firing stage, and relatively fine conductive zinc oxide powder can be obtained. There is a strong demand for a smaller diameter.
For example, in order to impart conductivity to synthetic fibers, fine powder with an average particle size of 1 μm or less is preferred from the viewpoint of spinnability. however,
In the method of heating zinc oxide and aluminum oxide in the presence of solid carbon, the raw materials must be Mixed powder 900~
It requires firing at a high temperature of 1100℃, and as a result, even if fine aluminum oxide with an average particle size of about 20μ is used, the raw material powder tends to adhere to each other and agglomerate, resulting in an average particle size of 1μ or less. It is difficult to obtain conductive zinc oxide fine powder. On the other hand, the conductivity is also 1Ω. Even improved zinc oxide with a volume resistivity of less than cm is also required, but this appears to be difficult under normal processing conditions as long as aluminum oxide is used. The present invention has been made in order to solve the above-mentioned problems in the conventional method, and has an average particle diameter of less than 1 μm and/or further improved conductivity to achieve a volume resistivity of 1Ω. The purpose of the present invention is to provide a method for producing zinc oxide smaller than cm. The method for producing conductive zinc oxide according to the present invention uses zinc oxide powder and a
It is characterized in that 5 moles of gallium oxide powder are dry mixed and then heated and fired at a temperature of 700 to 1100°C in the presence of solid carbon. In the present invention, the mixing ratio of gallium oxide to zinc oxide is 0.05 to 100 moles of zinc oxide.
-5 mol, preferably 0.1-2.5 mol. When the mixing ratio of gallium oxide is less than the above range, the zinc oxide obtained by heating and firing will not have high conductivity, and on the other hand, it is economically disadvantageous to use a larger amount than the above range. The method of the invention has two aspects. One aspect relates to the average particle size of the resulting zinc oxide, and the other aspect relates to the volume resistivity of the resulting zinc oxide.
Although the present invention is not limited to this, as the heating and firing temperature of the raw material mixed powder of zinc oxide powder and gallium oxide powder increases, the volume resistivity of the resulting conductive zinc oxide decreases, but on the other hand, the average A tendency for the particle size to increase is observed. Furthermore, if the heating and firing temperature of the raw material mixture is fixed, the volume resistivity of the resulting conductive zinc oxide tends to decrease as the ratio of gallium oxide to zinc oxide increases. Therefore, according to the method of the present invention, in a preferred case, the average particle size is smaller than 1μ and the volume resistivity is 1Ω. Highly conductive zinc oxide fine powder smaller than cm is obtained. Furthermore, if necessary, by heating and firing the raw material powder at a high temperature, a highly conductive zinc oxide powder having a significantly small volume resistivity can be obtained at the expense of some average particle size. In the present invention, in order to obtain conductive zinc oxide having an average particle size of less than 1 μm, the average particle size of the zinc oxide used must be less than 1 μm, and preferably
It is 0.1-0.8μ. Since the amount of gallium oxide powder mixed with zinc oxide is small, the average particle size is a few microns.
It may be less than or equal to 2 μm, and gallium oxide powder of 2 μ or less is usually preferably used. The heating and firing temperature must be relatively low, especially at temperatures above 700°C and 900°C.
A temperature range below 0.degree. C. is preferred. When zinc oxide and gallium oxide are heated and calcined in the presence of solid carbon under these conditions, a conductive zinc oxide powder with an average particle size of less than 1μ is obtained, preferably an average particle size of 0.7-0.8μ. Moreover, the volume resistivity is also 1Ω. Highly conductive zinc oxide fine powder smaller than cm is obtained. When fired at a high temperature of 900° C. or higher, the raw material powders tend to adhere to each other and agglomerate, so the resulting conductive zinc oxide has an average particle size of 2 to 3 μm. However, regardless of the average particle size of the conductive zinc oxide powder obtained, if the purpose is to improve the conductivity, the average particle size of the zinc oxide powder and gallium oxide powder used is not particularly limited, and the raw material mixture powder
It is preferable to heat and bake at a high temperature in the range of 850 to 1100°C, preferably 900 to 1100°C in view of the amount of gallium oxide used. By treatment under such conditions, highly conductive zinc oxide powder having a volume resistivity of less than 1 Ω·cm can be obtained. As mentioned above, conductive zinc oxide fine powder with an average particle size of less than 1 μm can be obtained by heating and firing at a low temperature of 800°C or less, but on the other hand, its volume resistivity is several Ω·cm to several Ω·cm. Ten ohms. cm and 1Ω.
This is because it is difficult to make the size smaller than cm. In both aspects of the present invention,
Heating to a high temperature exceeding 1100° C. is not preferable because the formation of metallic zinc becomes noticeable and the yield of the desired conductive zinc oxide decreases. Further, in the present invention, a gallium compound capable of producing gallium oxide under the processing conditions can be used in place of or together with gallium oxide, if necessary. To dry mix zinc oxide powder and gallium oxide powder, for example, a rotating container type mixer such as a V-type mixer, a ribbon type, screw type, rotary blade type, or other fixed container type mixer is used as appropriate. be able to. The mixing method is not particularly limited as long as the zinc oxide powder and the gallium oxide powder can be dry mixed uniformly. Further, the heating and baking time is usually about 15 minutes to 5 hours. As the solid carbon, for example, powdered, granular or lump charcoal is suitable. Such solid carbon is separated from or mixed with the raw material mixed powder of zinc oxide and gallium oxide, and heated together with the mixed powder in a container. When solid carbon is mixed with the raw material mixed powder and heated, it is sufficient that the carbon has a particle size large enough to be easily separated from the fired product using a screen or the like after heating the raw material mixed powder. however,
In particular, fine powder solid carbon is rapidly oxidized to carbon monoxide and carbon dioxide under the heat treatment conditions of the present invention, so if the amount is oxidized in this way under the treatment conditions, it can be added to the raw material mixed powder. Uniform mixing may be possible and may be desirable. In carrying out the method of the present invention, it is convenient to use a closed container equipped with an inlet pipe for a non-oxidizing gas such as nitrogen and an exhaust pipe as the container for heating and firing the raw material mixed powder. That is, one specific method is to lay a layer of charcoal on the bottom of a container, place a crucible containing zinc oxide powder and gallium oxide powder on the charcoal layer in an air atmosphere, and then pour a non-oxidizing gas The inlet pipe is closed, the exhaust pipe is opened, and the container is heated to a predetermined temperature for a predetermined time in an electric furnace. In the present invention, even if heating and baking of the raw material powder mixture is started in an air atmosphere, as time passes,
Since solid carbon reacts with the air in the atmosphere to naturally form a reducing atmosphere, there is no need to force hydrogen, carbon monoxide, or a mixed gas of these and nitrogen into the container during heating and firing. . In the present invention, reducing gas is generated during heating and firing, so as mentioned above, this reducing gas is naturally discharged from the exhaust pipe along with the air to create a reducing atmosphere in the container at an approximately constant pressure. It is desirable to keep it. When cooling the raw material mixed powder after heating and firing for a predetermined period of time, it is desirable to isolate the inside of the container from the atmosphere or to circulate nitrogen gas in the container to place it in a non-oxidizing atmosphere. This is because if the fired product comes into contact with an oxidizing gas such as air during the cooling process, the conductivity of the resulting fired product may decrease. In the present invention, up to 200°C or lower, preferably at room temperature,
It is desirable to cool the heated and fired product under a non-oxidizing atmosphere in this manner. However, the cooling method may be natural cooling or forced cooling. In addition, in the present invention, the raw material mixed powder may contain one or more kinds of oxides of metals such as aluminum, indium, tin, nickel, and molybdenum, if necessary, and in this case, the blended amount is preferably 10% by weight or less based on zinc oxide. As described above, the method of the present invention is a method for producing conductive zinc oxide, in which gallium oxide is used as an activator to impart conductivity to zinc oxide, and a mixed powder of zinc oxide and gallium oxide is used. By heating and firing in the presence of solid carbon,
We have succeeded in obtaining conductive zinc oxide fine powder with an average particle size of less than 1μ, which was previously difficult, and in favorable cases, the resulting zinc oxide has significantly improved conductivity, reaching 1Ω. It has a volume resistivity smaller than cm. Of course, according to the method of the present invention, solid carbon is heated in an air atmosphere to generate a reducing gas, so there is no need to circulate hydrogen or carbon monoxide gas into the container. Examples of the present invention are listed below, but the present invention is not limited to these Examples. Example 1 100 parts by weight of zinc oxide powder with an average particle size of 0.6μ and a predetermined amount of gallium oxide with an average particle size of 1.4μ were mixed using a small mixing machine "Micro Speed Mixer" (manufactured by Takara Koki) for testing.
The mixture was dry mixed for 5 minutes to prepare a raw material powder. 8 parts by weight of coarsely crushed charcoal was spread in a layer at the bottom of a heat-resistant stainless steel container, and a crucible containing 30 parts by weight of the above raw material mixed powder was placed on the charcoal layer, then heated in an electric furnace at the temperature shown in the table for 3 hours. It was heated inside. During this time, the reducing gas generated was allowed to vent naturally from the container. Next, the heated and fired product was cooled to room temperature while nitrogen gas was slowly passed through the container. Volume resistivity (Ω・cm) and average of the obtained zinc oxide powder

[Table] The average particle diameter (μ) is shown in the table. In addition, the whiteness of the above conductive zinc oxide powder JIS
8123) was 80, and it was recognized that the whiteness was also improved compared to the zinc oxide powder obtained in the following comparative example. Comparative Example 100 parts by weight of the same zinc oxide powder as in the example and aluminum oxide having an average particle size of 20 mμ were dry mixed in the same manner as in the example, and then fired at a temperature of 1010° C. for 2.5 hours. The conductive zinc oxide thus obtained had a volume resistivity of 60 Ω·cm and an average particle size of 14.3 μm.
In addition, the Hunter whiteness was 78.8, which was slightly grayish.

Claims (1)

[Claims] 1. Zinc oxide powder and 100 moles of zinc oxide
Dry mixing 0.05 to 5 moles of gallium oxide powder,
A method for producing conductive zinc oxide, which comprises heating and firing at a temperature of 700 to 1100°C in the presence of solid carbon. 2. Production of conductive zinc oxide according to claim 1, characterized in that heating and firing is performed at a temperature of 700°C or higher and lower than 900°C to obtain zinc oxide powder having an average particle size of less than 1μ. Method. 3. The method for producing conductive zinc according to claim 1, which comprises cooling to a temperature of 200° C. or lower in a non-oxidizing atmosphere after heating and baking.