Yttrium chromite chromia thermistors

Details Yttrium chromite chromia thermistors Yttrium chromite chromia thermistors

2000-03-27

2001-03-20

Donovan, Lincoln (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C252S519100

Reexamination Certificate

active

06204748

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the fields of ceramics, electrical equipment and temperature sensing and in particular provides ceramic compositions and thermistors made therefrom, methods of making ceramic compositions and thermistors and methods of using same.
BACKGROUND OF THE INVENTION
A thermistor is a device in which the resistance or resistivity changes in a predictable and repeatable manner when exposed to a specified range of temperatures. A thermistor can be used to measure a set temperature or to monitor temperature variation. The potential usefulness of a particular material for producing a thermistor is judged on a number of factors including the rate of change in the material's electrical properties with a change in temperature; the consistency or linearity of that change throughout a given range of temperatures; and the ability of the material to be used over a wide range of temperatures. Other factors include ruggedness, ease of manufacture, cost, useful life and the like. A thermistor made from a particular ceramic material may be ideal when used at temperatures of between 800° C. and 900° C. However, despite the obvious value of such a thermistor, its relatively narrow temperature window would limit its overall utility to applications which constantly required the use of temperatures between 800 and 900° C. Accordingly, it has always been desirable to discover materials which can be used to produce thermistors having as wide a useful temperature range possible.
Thermistors based on lanthanum have been used in the past because of their relatively broad range of applicability. Lanthanum chromium thermistors, for example, can be used at temperatures as low as −50° C. and as high as about 900° C. Unfortunately, these LaCrO
3
based thermistors must be glazed with a glass or glass-like coating because LaCrO
3
undergoes a structural transition at 270° C. which causes a resistance change. This transition is prevented by glassing or glazing. Unfortunately, most of the glazes are only useful at temperatures up to about 900° C. Above about 900° C., they melt thereby exposing the lanthanum based thermistor to the undesirable structural transition.
Iwaya et al., U.S. Pat. Nos. 5,476,822, 5,610,111 and 5,637,543 each disclose thermistors made from various ceramic compositions. The '543 patent describes a ceramic composition for a thermistor represented by the formula (YCrO
3
)
1−x
(YAl
3
)
x
where 0.8 >X>0. The '822 patent contains similar claims although much broader in scope. Again, however, an aluminum oxide based phase is required. The '111 patent claims a ceramic composition for a thermistor represented by the formula: (Y
1−x
Sr
x
) (Cr
1−y−x
Fe
y
Ti
z
)O
3
where X, Y and Z are specifically defined.
All three of these patents describe the resulting ceramic composition as being useful over a range of from room or ambient temperature up to about 1100° C. However, a review of their complete disclosures suggest applicability only between about 300° C. and 1100° C. Indeed, a review of table 1 of the '822 patent demonstrates that the Beta values of the disclosed materials was higher at lower temperatures than at higher temperatures. Moreover, the aluminum oxide phase is a high resistance insulator type material. As a result, these formulations are not generally suitable for use at lower temperatures.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a ceramic composition which, when used in a thermistor, provides a thermistor which is capable of use over a broad range of temperatures, including high temperature applications such as 1100° C. as well as generally lower temperature applications at 0° C. and below. The ceramic compositions in accordance with the present invention include at least one rare earth element, preferably Y, and a stoichiometric excess of Cr. The ceramic compositions include at least a first phase having the formula XCrO
3
where “X” is a rare earth element, and preferably the first phase has the formula YCrO
3
(yttrium chromite). These compositions also have at least a second phase having the formula Cr
2
O
3
(chromia).
More preferably, the ceramic compositions of the present invention may include: Ti provided in an amount of between about 0 and about 5% by weight based on the weight of the ceramic composition, Si provided in an amount of between about 0 and about 5% by weight based on the weight of the ceramic composition and Ca provided in an amount of between about 0 and about 5% by weight based on the weight of the ceramic composition are also presented. Generally, the total mole % of such additional components provided ranges as high as about 20% and most often up to about 15% mole %. These ceramic compositions are often sintered to make them hard and cohesive.
Preferably, these compositions are used in the thermistors. They are sized, shaped and provided with electrical contacts suitable for placing the thermistor into an electrical connection or electrical contact with a circuit or another device.
It has been discovered that unlike the suggestions of the '822 and '543 patents previously mentioned, it is possible to make rare earth and chromium based thermistors without the use of an aluminum oxide phase. The result is a thermistor capable of measuring at low temperatures comparable to that achieved through the use of prior known lanthanum based ceramic compositions as well as at high temperatures comparable to those achieved using yttrium and chromium thermistors incorporating a yttrium aluminum oxide (YAlO
3
) phase.
The ceramic compositions in accordance with the present invention include at least two phases, both of which are based on chromium oxide. The first phase has a formula XCrO
3
and the second phase has a formula Cr
2
O
3
. The chromia (Cr
2
O
3
) phase, unlike the aluminum based phases previously discussed, is a good conductor at high temperatures. In fact, chromia based thermistors which include small amounts of titanium, calcium and silicon oxides, have been used in high temperature thermistors which are useful at above 350° C. This material is not insulating as is the case of YAlO
3
.
In addition, it has been found that thermistors made from the ceramic compositions of the present invention exhibit Beta values which are lower at lower temperatures and increase as temperatures rise. A thermistor produced from a typical yttrium composition in accordance with the present invention gave Beta values of 2819K between room temperature (about 25° C.) and 150° C., 3059K between 150° C. and 300° C., 3400K between 300° C. and 450° C. and 4072K between 450° C. and 750° C. Thus the ceramic compositions of the present invention are useful at lower temperatures.
While the prior art yttrium based materials do not appear to be able to provide resistance values below room temperature, if even that low, materials in accordance with the present invention gave readings of 2.91 MegaOhm at −50° C.


REFERENCES:
patent: 4013592 (1977-03-01), Matsuoka et al.
patent: 4110260 (1978-08-01), Yamamoto et al.
patent: 4252768 (1981-02-01), Perkins et al.
patent: 5084426 (1992-01-01), Iwaya et al.
patent: 5476822 (1995-12-01), Iwaya et al.
patent: 5610111 (1997-03-01), Iwaya et al.
patent: 5637543 (1997-06-01), Iwaya et al.
patent: 5879750 (1999-03-01), Kuzuoka

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