Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
1999-04-23
2001-04-24
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S138000, C501S139000
Reexamination Certificate
active
06221800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a PTC semiconducting ceramic (PTC thermistor) which has a positive temperature coefficient (PTC) and is used in, for example, a low-temperature heating element and an automatic demagnetizer for a color TV and, in particular, to a method of producing a PTC semiconducting ceramic whose characteristic dispersion is small relative to a fluctuation in the producing conditions.
DESCRIPTION OF BACKGROUND
It is known that barium titanate, BaTiO
3
, which has a perovskite type crystal structure is transformed by the addition of a minute amount of a semiconductor-forming agent, such as a rare earth metal, niobium or antimony into a semiconductor, so as to manifest the PTC (positive temperature coefficient) phenomenon in which the magnitude of resistance abruptly increases at a temperature no less than the Curie point.
In recent years, studies have been actively carried out for elucidating this peculiar phenomenon and finding practical applications, and the semiconducting ceramic compositions of the barium titanate type have been finding actual utility in various heating elements, switching elements, sensors, and automatic demagnetizers of color TV.
Methods which are available for producing such semiconducting ceramic compositions of the barium titanate type include, for example, (1) a solid-phase reaction method which entails using oxides and carbonates as raw materials, compounding them at a given ratio, and then firing them; and (2) a liquid-phase reaction method which entails using a metal alkoxide as a raw material and synthesizing the composition from a liquid phase. The liquid-phase reaction method is advantageous over the solid-phase reaction method in that a homogeneous and stable ceramic composition can be easily obtained, but its production processes are complicated resulting in a high production cost. Consequently, the solid-phase reaction method is generally used.
In the solid-phase reaction method, however, the conditions for producing the ceramic composition by solid-phase reaction, such as the state of mixture of the raw materials, the temperature of firing and the condition of ambience, largely influence the electrical characteristic of the product. Thus, for ensuring a uniform quality over many production lots, the production conditions in the respective producing processes must be strictly managed. It has, thus, been difficult to prepare products of high quality in an inexpensive manner.
Conventionally, the following methods are known:
(1) a method of producing a semiconducting ceramic article which entails compounding a barium titanate type semiconducting material containing a semiconductor-forming agent with a compound,
(Ba
(2-x)
A
x
)TiSi
2
O
8
(wherein A is at least one element selected from the group consisting of Li, Na and K), and then filing them (JP A-4-311002); and
(2) a method of producing a thermistor which entails adding to a composition A using barium titanate as a main component and having a minute amount of a semiconductor-forming element or Si, Mn, and Al, a composition B represented by the formula, BaTi
n
O
n+1
(n=2, 3, 4) in an amount of 0.1 to 4.0 mol % relative to 1 mol of the compound A (JP-A-7-297009). The products obtained by these methods, however, still suffer from the disadvantage that their characteristics are liable to be affected by a fluctuation of the conditions of production and, as a result, are liable to be dispersed. Particularly, Li, Na and K which are added in the above technique (1) tend to promote the dispersion of the characteristics. Further, Al which is added in the above technique (2) tends to lower the temperature coefficient.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide, a method of producing a PTC semiconducting ceramic, which entails preparing a calcine of a main composition of a barium titanate-based semiconductor containing substantially no Si and having BaTiO
3
as a main component thereof, preparing additive compositions, Ba
2
TiSi
2
O
8
and Ba
n
Ti
m
O
n
+
2
m (1≦n≦4, 2≦13, n≦m), respectively; and compounding the calcine of the main composition and the additive compositions and mixing them, and then subjecting them to formal firing.
REFERENCES:
patent: 6002578 (1999-12-01), Kumagai et al.
patent: 6071842 (2000-06-01), Tahahashi et al.
patent: 0 694 930 (1996-01-01), None
patent: 57-109301 (1982-07-01), None
patent: 62-296401 (1987-12-01), None
patent: 04338601 (1992-11-01), None
patent: 04311002 (1992-11-01), None
patent: 06224006 (1994-08-01), None
patent: 06340472 (1994-12-01), None
patent: 07297009 (1995-11-01), None
patent: 07335404 (1995-12-01), None
patent: 09100157 (1997-04-01), None
Derwent Publication, AN 1988-038600, JP 62 296401, Dec. 23, 1987.
Sato Shigeki
Takahashi Chihiro
Group Karl
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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