Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – By extrusion spraying or gravity fall through orifice
Reexamination Certificate
2002-08-27
2004-11-30
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
By extrusion spraying or gravity fall through orifice
C264S040100, C264S616000
Reexamination Certificate
active
06824713
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of producing a thermistor element, formed mainly of a metal oxide sintered body, and a production apparatus for producing raw materials for such a thermistor element. The thermistor element can be appropriately used for a thermistor element of a temperature sensor, for an automobile exhaust gas, etc, capable of detecting a temperature from room temperature to a high temperature in the range of 1,000° C. or above.
2. Description of the Related Art
Thermistor elements of this kind, and formed mainly of a metal oxide sintered body, have been used in the past for temperature sensors for measuring temperatures from a medium temperature range to a high temperature range of 400 to 1,300° C. such as an automobile exhaust gas temperature, a gas flame temperature of gas fed water heaters, a temperature of a heating furnace, and so forth.
Metal oxide sintered bodies made of a perovskite type material, a corundum type material, etc, have been mainly used for the thermistor elements of this kind. A thermistor element using the perovskite type material, for example, is described in Japanese Unexamined Patent Publication (Kokai) No. 7-201528.
To produce a thermistor element that can be used in a broad temperature range, the thermistor element in this reference is obtained by a so-called “solid phase method” that mixes, pulverizes, granulates and sinters a plurality of oxide materials, e.g. Y, Sr, Cr, Fe and Ti, in a predetermined composition ratio.
In the preparation of the raw materials of the thermistor element in the solid phase method described above, mixing and pulverization of a plurality of oxide raw materials are carried out by use of a medium stirring mill, for example. However, mechanical pulverization using the medium stirring mill is essentially not free from the limit of the pulverization capacity, and the mean particle size of the thermistor raw materials after mixing and pulverization is 0.3 &mgr;m, as a limit.
Since the particle size of the pulverized starting materials has a limit when pulverization and mixing of the raw materials are simultaneously carried out, uniformity of the composition is not sufficient to obtain a thermistor element having a higher level of accuracy. Therefore, the resulting thermistor element has large variance of resistance, and this variance invites deterioration of temperature accuracy of the temperature sensors using this thermistor element. Temperature accuracy of temperature sensors using the thermistor element according to the prior art is at most ±15° C. (from room temperature to 800° C.).
In the mixing-pulverization operation by use of the medium stirring mill, components of zirconia balls as a pulverization medium mix as impurities into the thermistor raw materials and result in variance of the resistance or invites deviation of a composition from a target composition.
In the temperature sensors of the automobile exhaust gas, there is a great need for a system for detecting exhaust gas temperatures before and after a catalyst for purifying the exhaust gas of gasoline-engine cars to detect deterioration of the catalyst, and for a system for detecting the exhaust temperatures before and after the catalyst to control the temperature of the catalyst for controlling the exhaust gas, particularly a NOx gas, of diesel engines.
However, the temperature accuracy of the temperature sensors using the thermistor element according to the prior art cannot establish this system, and expensive thermocouples or platinum resistors have been used for the temperature sensors. In other words, no temperature sensors are available, to this date, that have temperature accuracy adaptable to the system described above.
In view of the problems described above, the present invention contemplates to reduce variance of the resistance value of the thermistor element when producing the thermistor element formed mainly of the metal oxide sintered boy, and to make further uniform the composition of the thermistor raw materials to obtain a higher level of temperature accuracy.
SUMMARY OF THE INVENTION
(I) To begin with, a solution means for obtaining excellent temperature accuracy by forming micro-particles of a thermistor raw material and making uniform the composition will be explained.
To accomplish the object, a first aspect of the invention provides a method of producing a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising the steps of mixing a precursor of a metal oxide in a liquid phase and preparing a precursor solution; spraying the precursor solution and obtaining droplet particles; heat-treating the droplet particles and obtaining thermistor raw material powder; and molding and sintering the thermistor raw material powder into a predetermined shape, and obtaining the metal oxide sintered body.
According to this method, mixing of the raw materials can be conducted under the state of the precursor solution. In other words, the composition for obtaining the final metal oxide sintered body can be uniformly regulated in the liquid phase state in which the particles are finer than in the solid phase method according to the prior art. Consequently, the composition of the resulting thermistor raw material powder can be made move uniform. This method is free from mixing of a pulverization medium as an impurity that has been observed in the solid phase method.
The metal oxide sintered body obtained by molding and sintering this raw material powder, that is, the thermistor element, has reduced variance of the resistance value and can provide a higher temperature accuracy than the prior art.
Here, the precursor solution preferably contains at least one kind of metal ion complex.
Water or an organic solvent, or a mixed solution of water and the organic solvent, can be used as the solvent of the precursor solution.
According to a second aspect of the invention, there is provided a method of producing a thermistor element consisting of a metal oxide sintered body as a principal component thereof, comprising the steps of preparing a slurry solution dispersing particles of a metal or a metal oxide; spraying the slurry solution and obtaining droplet particles; heat-treating the droplet particles and obtaining thermistor raw material powder; and molding and sintering the thermistor raw material powder into a predetermined shape, and obtaining the metal oxide sintered body.
According to this method, mixing of the raw materials can be conducted in the form of the slurry solution. In other words, the composition for obtaining the final metal oxide sintered body can be regulated to a uniform composition under the liquid phase state where the particles are finer than in the solid phase method according to the prior art, in the same way as in the first aspect of the invention. Therefore, the composition of the resulting thermistor raw material powder can be made move uniform. This method is free from mixing of the pulverization medium as the impurity as has been the case with the solid phase method.
The metal oxide sintered body formed and sintered by use of this raw material powder, that is, the thermistor element, exhibits reduced variance of the resistance value, and can provide a higher temperature accuracy than the prior art.
To uniformly mix the raw materials, the particle size of the particles of the metal or metal oxide in the slurry solution is preferably 100 nm or below.
The solvent of the slurry solution is preferably water or an organic solvent, or a mixed solution of water and the organic solvent.
The precursor solution or the slurry solution preferably uses a solution to which an inflammable solvent is added and mixed.
In this case, because the inflammable solvent is added and mixed, thermal decomposition and combustion of the droplet particles proceeds rapidly during heat-treatment of the droplet particles sprayed, and the thermistor raw material powder can be obtained with a more uniform composition.
The inflammable solvent
Kuzuoka Kaoru
Makino Daisuke
Ogata Itsuhei
Yorinaga Muneo
Denso Corporation
Fiorilla Christopher A.
Nixon & Vanderhye P.C.
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