Chemistry of inorganic compounds – Rare earth compound
Reexamination Certificate
1997-03-11
2001-08-14
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Rare earth compound
C423S625000
Reexamination Certificate
active
06274110
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to complex oxides used as raw materials for fine ceramic materials, and more particularly to complex oxide precursors in the form of a fine powder which are readily converted to complex oxides with low energy and also a process for preparing the same.
DESCRIPTION OF THE RELATED ART
With rapid development of science and industry nowadays, many new materials of various functions have been required, and they are used in our daily lives and industry. In particular, a complex oxide is an oxide including one or more metal oxides having unique physical and chemical functions. Such complex oxides have been widely used in various fields such as mechanics, electronics, and optics for high temperature uses, capacitors, varistors, solid electrolytes, fluorescent materials, heating elements, piezoelectric materials and structural uses.
Methods to produce complex oxides include the solid phase method, the liquid phase method and the combustion method. The solid phase method is a process to produce a complex oxide by mechanical mixing with a ball mill charged with respective component oxides or in the form of carbonate salts and followed by sintering. However, the solid phase method requires many hours for mixing and also requires a relatively long time for sintering even at a high temperature. Because the mixing and the sintering costs are a large portion of the production costs, the method disadvantageously consumes a lot of energy and is inefficient. It is also difficult to mix homogeneously so that inevitably poor quality products are produced. With respect to quality management of the products, there are significant problems with impurities during the mixing step. To solve the problems of the solid phase method, the liquid phase method has been developed, which involves obtaining an intermediate product in solid form from a homogeneous solution mixture of soluble components followed by heat treatment. Examples of liquid phase methods include the thermal decomposition method, the co-precipitation method and the metal alkoxide method.
The thermal decomposition method involves obtaining a decomposed solid product by heating steadily to the decomposition temperature after mixing the metal salts of desired oxides. This method has the advantage that it is possible to synthesize at a relatively low temperature (around 1000° C.). However, it has the disadvantages that the process is complex, that longer time is required, and that there is a high possibility of contamination of impurities because raw material should be heated slowly for minimizing segregation while attaining complete decomposition.
The co-precipitation method is a method for obtaining a complex oxide by precipitating an oxide of a single element, such as yttrium or aluminium, from a salt solution of hydrochloric acid, nitric acid or sulfuric acid containing a precipitation agent, such as oxalic acid or aqueous ammonia, to produce an insoluble metal material followed by filtering, drying and calcination. However, the co-precipitation method has a drawback when preparing complex oxides comprising two or more components. It is normally difficult to determine the precipitation condition because the possibility of fractional precipitation is high due to the difference in the solubilities or precipitation velocities of the elements. It also has potential contamination problems and is difficult in mass production and continuous operations.
The alkoxide method is a method for obtaining a complex oxide by reacting metals with alcohol to give alkoxide formation and then hydrolyzing to produce a complex hydroxide in the form of a gel, followed by heating. However, the alkoxide method is restricted to certain metals and is disadvantageous because the manufacturing process for the intermediate material is not simple and the raw materials are expensive.
Additionally, there are the sol-gel method, the colloidal precipitation method where homogeneous precipitation is attained, and the spray drying method. However, the sol-gel method and homogeneous precipitation method are not suitable for mass production. The spray drying method has the problem of poor quality due to the void formation inside the granule powder.
Therefore, a process for preparing a powder by a simple method at a comparatively low temperature is focused on a process where the merits of both the solid phase method and the liquid phase method are adopted.
The combustion method is a method for obtaining the powder of a complex oxide quickly and in a simple manner by mixing a metal nitrate and urea, heating the mixture in a reactor at a comparatively low temperature (around 500° C.), and igniting the decomposed gas. However, in this method there are still the problems in obtaining quantitative synthesizing conditions, because the combustion condition can be changed according to the heating speed of the charge mix, the right mixing ratio of the materials, and also the shape and volume of the reactor. Moreover, the quality of the product is not constant because uniform combustion is not always maintained. Accordingly, there are problems remaining for industrialization. So far, only small scale synthesis (10 g) batch laboratory test results have been known. No work for large production or methods for continuous production have yet been published.
SUMMARY OF THE INVENTION
As discussed above, various processes for preparing a complex oxide, including the combustion method, are known, but only the solid phase method and alkoxide method are used in industry. The factors important in industrial application are the manufacturing cost and the product quality. In other words, if these two parameters are improved, it can be said to be an ideal industrial method. The factors of the process cost are (1) the price of material, (2) the energy cost, and (3) the price of processing, all of which should be minimized. Further, product quality should be guaranteed, not only because high recovery of the final product can be obtained, but also because reliability of the final product is assured.
It is one object of the present invention to lower the production cost while maintaining quality assurance and offering convenience for the product user, and also to reduce the plant investment and process operation costs.
It is another object of this invention to provide a complex oxide precursor capable of being used to produce a high quality complex oxide crystalline product with low energy cost.
It is a further object of the invention to provide a process for preparing a complex oxide of high quality using a complex oxide precursor in an economic and effective way. To achieve the foregoing objects of the invention, the present invention provides a process for preparing a complex oxide precursor comprising the steps of mixing a metal salt solution or the crystal thereof with urea or a carbohydrazide to produce a mixture and heating the mixture without ignition. The mixture can further include metal chloride.
In accordance with this invention, it is preferred that the rate of temperature increase in the heating step is 1° C./min to 100° C./min. If the rate of temperature increase is below 1° C./min, the heating is insufficient to produce the complex oxide precursor; if the rate is above 100° C./min, the complex oxide precursor can not be formed because of combustion action.
Metals useful for this invention include one or more metals selected from the group consisting of Ag, Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Na, Nb, Ni, Pb, Si, Sn, Sr, Th, Ti, U, Zn, Zr and rare earth metals.
Further, the present invention provides a process for preparing a complex oxide comprising the steps of mixing a metal salt solution or the crystal thereof with urea or a carbohydrazide to produce a mixture, heating the mixture without ignition to produce a complex oxide precursor, and heating the complex oxide precursor to produce a crystallized complex oxide.
In accordance with the invention, the mixing ratio of metal nitrate to urea may be chan
Jo Min Young
Kim Geon Bae
Kim Yeun Shik
Bos Steven
Christie Parker & Hale LLP
Kim Yuen Shik
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