Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-12-14
2004-08-24
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S593100, C423S335000, C423S336000, C423S339000, C423S605000, C423S608000, C423S607000, C423S632000, C423S633000, C423S594190, C423S618000, C423S622000, C423S624000, C423S625000, C423S263000, C423S636000, C423S637000, C423S638000, C423S639000, C423S610000, C423S611000, C423S612000, C423S599000, C423S594120, C423S595000, C423S594100, C423S594300, C423S594900, C423S594140, C423S600000, C423S598000
Reexamination Certificate
active
06780393
ABSTRACT:
This application is based on Japanese Patent Application Nos. 2000-399199 filed Dec. 27, 2000, and 2001-290782 filed Sep. 25, 2001, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a method of producing fine particles of a metal oxide having a nanoscale grain size.
2. Discussion of Related Art
In the field of electronic ceramics, it is desired to reduce the grain size of the materials of the electronic ceramics. In the field of a gas sensor, for instance, it is expected to improve its sensitivity and lower its operating temperature, by reducing the grain size of the material down to the order of several tens of nanometers (nm) for thereby reducing the crystal grain size of the material. In the electronic ceramics such as a varistor and a thermistor which utilize the characteristics of the crystal grain boundaries, the grain boundaries are increased and the characteristics of the electronic ceramics are improved by reducing the crystal grain size and densely sintering the crystal grains. Based on a fact that the reduction of the crystal grain size of a metal oxide results in a quantum effect, it is expected that the finely grained material whose grain size is reduced is used to provide new-type functional ceramics utilizing the quantum effect. Since the above-described effects will appreciably increase with a decrease of the crystal grain size of a sintered body of the metal oxide, it is desirable that the crystal grain size is reduced down to the nanometer scale, in other words, the sintered body of the metal oxide is constituted by the nanocrystal grains. The metal oxide is used as a material of a grinding stone, for instance. The sharpness of cutting edges provided by the abrasive grains increases with a decrease of the crystal grain size of the grinding stone, in other words, with a decrease of the size of the abrasive grains which constitute the grinding stone. When the metal oxide is used as free abrasive grains, the quality of the surface which is subjected to a finish grinding by the free abrasive grains can be increasingly enhanced by reducing the grain size of the metal oxide. It is noted that the term “nanocrystal grains” refers to the crystal grains having a grain size of not greater than about 100 nm. A sintered body in which the nanocrystal grains are densely bonded together is refereed to as “a dense sintered body of nanocrystal grains”.
As a method of producing a fine powder as described above, a chemical process using an inorganic salt is known, for instance. This method comprises the steps of: adding an alkaline solution to an aqueous solution of a metal chloride; precipitating a hydroxide of the metal; and heat-treating (calcining) the precipitated hydroxide in a non-reducing atmosphere after it has been dried. This method is described, for instance, in “The Stannic Oxide Gas Sensor” (pp. 11-47) published by CRC Press in 1994. For obtaining a powder of a stannic oxide (tin oxide) used for fabricating a gas sensor, tin tetrachloride (SnCl
4
) is used as a starting material and an ammonia water is used as the alkaline solution.
FIG. 1
shows the conventional process steps for producing the tin oxide. In this method, however, the precipitated hydroxide having a nanoscale grain size is aggregated into an agglomerate after drying, and the nanometer-sized primary grains are fixedly bonded together to form coarse grains (secondary grains) after calcining. Accordingly, even if the product is mechanically pulverized into a powder each time after drying and calcining, the size of the secondary grains obtained by the mechanical pulverization is inevitably greater than about several microns (&mgr;m). Further, the purity of the pulverized powder is undesirably low since impurities get into the powder during the pulverizing operation.
For the purpose of obtaining the fine and highly pure powder of a metal oxide, various methods are proposed. For instance, JP-A-7-187668 proposes a method of oxidizing a graphite intercalation compound obtained by reacting a compound of a specific metal (such as nitrate or oxynitrate) with a graphitic carbon modification. The fine powder of the metal oxide having a high degree of purity is also produced by hydrolysis of an alkoxide material. Further, J. Aerosol Sci., Vol 24 (pp.315-338) published in 1993 describes nanostructured oxides synthesized by thermal vaporization/magnetron sputtering and gas condensation, wherein a metal is subjected to an oxidizing treatment after it has been evaporated and deposited in a vacuum. All of these methods, however, have disadvantages described below. In the method using the graphitic carbon modification, the metal to be used is limited to the one which is capable of forming the graphite intercalation compound. Further, it takes a relatively long period of time to form the graphite intercalation compound, deteriorating the production efficiency. In the method using the alkoxide, the material (alkoxide) is rather expensive for mass-production of the fine powder of the metal oxide. In the gas-phase evaporation method, the rate of formation of the fine power of the metal oxide is considerably low, deteriorating the production efficiency.
Accordingly, the conventional methods described above experience difficulty in mass-producing a fine powder of a metal oxide, making it difficult to use the fine powder of the metal oxide for various applications which require the fine powder. Further, it is difficult to mass-produce a sintered body formed of considerably fine crystals whose grain size is not greater than several tens of nanometers (nm).
SUMMARY OF THE INVENTION
The present invention was developed in the light of the background art described above. It is therefore an object of the invention to provide a method of producing fine particles of a metal oxide having a grain size on the order of nanometer (nm).
The object indicated above may be achieved according to an aspect of the present invention, which provides a method of producing fine particles of an oxide of a metal, comprising the steps of: preparing an acidic solution which contains ions of the metal; precipitating fine particles of a hydroxide of the metal by adding an alkaline solution to the acidic solution; collecting the fine particles of the hydroxide of the metal precipitated in a mixed solution of the acidic solution and the alkaline solution; mixing fine particles of a carbon with the collected fine particles of the hydroxide of the metal; and heat-treating a mixture of the fine particles of the hydroxide of the metal and the fine particles of the carbon at a predetermined temperature in a non-reducing atmosphere, whereby the fine particles of the oxide of the metal are produced.
According to the present method described above, the fine particles of the hydroxide of the metal (the metal hydroxide) which have been precipitated in the mixed solution of the acidic solution and the alkaline solution and collected therefrom are mixed with the fine particles of the carbon, and heat-treated in the non-reducing atmosphere, so that the fine particles of the oxide of the metal (the metal oxide) are produced. In the present method, the fine particles of the carbon are mixed with the fine particles of the metal hydroxide prior to the heat-treatment including the drying and calcining steps in the synthesis of the fine particles of the metal oxide according to the above-described chemical process using the inorganic salt. According to the present method, the fine particles of the carbon which are mixed with the fine particles of the metal hydroxide are effective to prevent the formation of the coarse secondary grains in which the fine particles of the metal hydroxide are bonded together. Accordingly, the present method provides the fine particles of the metal oxide having a nanoscale grain size owing to or derived from the nanoscale grain size of the precipitated metal hydroxide. In the present method described above, the process steps for producin
Hayashi Makiko
Murayama Norimitsu
Sago Sumihito
Shin Woosuck
Bos Steven
National Institute of Advanced Industrial Science and Technology
Oliff & Berridg,e PLC
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