1. Field of the Invention
The present invention relates to a semiconducting ceramic material, a process for producing the ceramic material and a thermistor comprising the ceramic material. More particularly, the present invention relates to a BaTiO3-type semiconducting ceramic material which exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics), to a process for producing the ceramic material and to a thermistor comprising the ceramic material.
2. Background Art
Conventionally, a BaTiO3-type semiconducting ceramic material is widely employed in the manufacture of PTC thermistors since the ceramic material exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics). The PTC thermistor is widely employed for the demagnetization of a cathode-ray tube or in a heater.
In order to reduce resistance, there has been keen demand for a PTC thermistor comprising a laminate which includes semiconducting ceramic materials and internal electrodes. Since a base metal such as Ni is employed to form an internal electrode of the PTC thermistor, the semiconducting ceramic material must be re-oxidized after the material is fired in a reducing atmosphere. The re-oxidation of the semiconducting ceramic material is carried out in order to obtain the PTC characteristics of the material through the re-oxidation of grain boundaries of the material.
However, it is difficult to completely re-oxidize the semiconducting ceramic material at a temperature sufficiently low to prevent oxidation of the internal electrode formed of a base metal.
In order to solve this problem, for example, Japanese Patent Application Laid-Open (kokai) No. 8-153604 discloses a process in which a material having a low firing temperature is employed as a semiconducting ceramic material. However, such a process is not necessarily satisfactory.
In view of the foregoing, an object of the present invention is to provide a BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation.
Another object of the present invention is to provide a process for producing a BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, which process comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material.
Another object of the present invention is to provide a thermistor comprising a BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation.
Accordingly, in a first aspect of the present invention, there is provided a BaTiO3-type semiconducting ceramic material which has undergone firing in a reducing atmosphere and re-oxidation, wherein the relative density of the ceramic material after sintering is about 85–90%.
Relative density is the ratio of the density of a sintered ceramic to the ideal density of the ceramic which is calculated under an assumption that the ceramic consists of a perfect crystal lattice. The relative density is usually expressed in percentage.
Preferably, the size of grains constituting the matrix of a semiconducting ceramic material of the present invention is about 0.5–2 μm.
In a second aspect of the present invention, there is provided a process for producing a BaTiO3-type semiconducting ceramic material, which comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material, wherein the ceramic material is fired at a temperature about 25° C. or more lower than a sintering completion temperature of the ceramic material.
Preferably, a BaTiO3-type semiconducting ceramic material having a sintering completion temperature of about 1,275° C. or higher is fired at about 1,250° C. or lower.
In a third aspect of the present invention, there is provided a thermistor comprising a laminate in which a semiconducting ceramic material of the present invention and an electrode are alternately laminated.
When the semiconducting ceramic material is fired at low temperature, the density of the material after sintering can be appropriately reduced and pores through which oxygen passes during re-oxidation can be established. As a result, the semiconducting ceramic material exhibits excellent PTC characteristics.
Specifically, when the semiconducting ceramic material is fired at a temperature about 25° C. or more lower than the sintering completion temperature, the sintering completion temperature being the temperature at which the density of the material is maximized, the semiconducting ceramic material exhibits excellent PTC characteristics.
When the relative density of the semiconducting ceramic material after sintering is about 85–90%, the ceramic material exhibits excellent PTC characteristics.
The size of grains constituting the matrix of the semiconducting ceramic material is preferably about 0.5–2 μm, more preferably about 0.7–1.5 μm. In addition, the relative density of the semiconducting ceramic material after sintering is preferably about 87–89%.
In the present invention, the semiconducting ceramic can be produced from an inexpensive material used into a solid-state process instead of an expensive wet process.
The lower limit of the firing temperature of the semiconducting ceramic material is not particularly limited. However, when the firing temperature is excessively low, the resistance of the ceramic material becomes high. Therefore, in general, it is not preferable that the ceramic material is fired at a temperature about 150° C. or more lower than the sintering completion temperature.
The above and other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with an accompanying drawing, in which:
BaCO3, TiO2, Sm2O3, and SiO2, serving as raw materials, were mixed in the compositional proportion to form
(Ba0.998Sm0.002)1.002TiO3+0.001SiO2.
The resultant powder was pulverized by use of a zirconia ball for five hours, and the resultant powder was calcined at 1,100° C. for two hours. The thus-calcined product was mixed with an organic binder, and the mixture was shaped into a sheet. Ni serving as an internal electrode was printed on the sheet. The resultant sheets were laminated with one another, and the thus-obtained laminate was fired in a reducing atmosphere (H2+N2). Thereafter, Ni external electrodes were formed on the laminate through baking at 500–800° C. in air, simultaneously with re-oxidation of the semiconducting ceramic material, to thereby produce a monolithic PTC thermistor 10 as shown in
The sintering completion temperature of the above semiconducting ceramic material is 1,300° C. In Example 1, Example 2, Example 3 and Comparative Example, the firing temperatures of the ceramic material are 1,150° C., 1,200° C., 1,250° C. and 1,300° C., respectively. Table 1 shows firing temperature, re-oxidation temperature, mean grain size, and relative density for each ceramic material. Table 1 also shows the resistance of the monolithic PTC thermistor comprising the ceramic material at room temperature; the logarithm of the maximum resistance of the PTC thermistor (Rmax) to the resistance thereof at 25° C. (R25); i.e., log (Rmax/R25); and the withstand voltage of the PTC thermistor. The re-oxidation temperature of each semiconducting ceramic material was optimized on the basis of the firing temperature thereof. In the Comparative Example, the semiconducting ceramic material was re-oxidized at 800° C., which is the upper limit for preventing oxidation of Ni.
As is apparent from Table 1, when the semiconducting ceramic material is fired at a temperature about 25° C. or more lower than the sintering completion temperature, the ceramic material exhibits excellent PTC characteristics. Particularly when the semiconducting ceramic material is fired at about 1,250° C. or lower, the ceramic material exhibits remarkably excellent PTC characteristics.
The present invention provides the BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation.
The present invention also provides the process for producing the BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, which comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material.
The present invention also provides a thermistor comprising the BaTiO3-type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation.
Number | Date | Country | Kind |
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11-312532 | Nov 1999 | JP | national |
This is a divisional of U.S. patent application Ser. No. 09/705,049, filed Nov. 2, 2000 now abandoned.
Number | Name | Date | Kind |
---|---|---|---|
3474043 | Andersen et al. | Oct 1969 | A |
3673119 | Ueoka et al. | Jun 1972 | A |
4384989 | Kamigaito et al. | May 1983 | A |
4535064 | Yoneda | Aug 1985 | A |
4571276 | Akse | Feb 1986 | A |
H415 | Newnham et al. | Jan 1988 | H |
5683790 | Suzuki et al. | Nov 1997 | A |
6346496 | Nabika et al. | Feb 2002 | B2 |
6359327 | Niimi et al. | Mar 2002 | B1 |
6362521 | Nabika et al. | Mar 2002 | B1 |
6432558 | Nabika et al. | Aug 2002 | B1 |
6544443 | Niimi et al. | Apr 2003 | B2 |
20010003361 | Niimi et al. | Jun 2001 | A1 |
Number | Date | Country |
---|---|---|
1-238101 | Sep 1989 | JP |
1-239829 | Sep 1989 | JP |
1-239830 | Sep 1989 | JP |
1-305855 | Dec 1989 | JP |
2-174201 | Jul 1990 | JP |
2-260510 | Oct 1990 | JP |
3-35501 | Feb 1991 | JP |
3-35503 | Feb 1991 | JP |
3-38802 | Feb 1991 | JP |
3-38803 | Feb 1991 | JP |
4-23401 | Jan 1992 | JP |
4-115502 | Apr 1992 | JP |
4-154661 | May 1992 | JP |
4-154664 | May 1992 | JP |
4-245401 | Sep 1992 | JP |
5-251207 | Sep 1993 | JP |
6-144926 | May 1994 | JP |
6-181108 | Jun 1994 | JP |
7-14702 | Jan 1995 | JP |
8-153604 | Jun 1996 | JP |
8-213205 | Aug 1996 | JP |
10-70007 | Mar 1998 | JP |
10-294203 | Nov 1998 | JP |
Number | Date | Country | |
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20030030192 A1 | Feb 2003 | US |
Number | Date | Country | |
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Parent | 09705049 | Nov 2000 | US |
Child | 10246008 | US |