Dielectric ceramic composition

Details Dielectric ceramic composition Dielectric ceramic composition

1999-09-21

2001-06-05

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Titanate, zirconate, stannate, niobate, or tantalate or...

C501S127000, C501S136000

Reexamination Certificate

active

06242376

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to alumina based dielectric ceramic compositions. Specifically, the invention is an alumina based dielectric ceramic composition exhibiting a low dielectric constant, a high Q factor, a low temperature coefficient, and a high percent fired density. The composition requires a relatively lower peak soak temperature during sintering (as compared to conventional alumina ceramics) and is particularly suited for use in high frequency applications.
BACKGROUND OF THE INVENTION
Dielectric ceramic compositions have found use in the field of electronic communications in such components as filters and resonators. In recent years, the range of frequencies used in electronic communications has expanded so that higher frequencies, i.e., those in the microwave range, are increasingly utilized. A filter may be employed to select a signal within a specific frequency range. The frequency range selected by the filter is referred to as the resonant frequency. Such filters may be based upon a block of dielectric material, often a ceramic material. The resonant frequency of the filter is determined by the dielectric properties of that material and by the dimensions of the block. In general, a dielectric material is required which has a low dielectric loss (indicated by a low dielectric loss factor) in order to minimize energy absorption by the dielectric material that would otherwise reduce resonant signal intensity. The Q factor is defined as the inverse of the dielectric loss factor. Therefore, a relatively lower loss factor results in a relatively higher Q factor. In general, a higher dielectric constant allows the design of a filter with reduced dimensions. For resonant frequencies above about 2 GHz, however, it becomes more difficult to obtain a functional filter because of the small dimensions necessitated by the shorter wavelengths. Thus, a material with a lower dielectric constant, and lower dielectric loss factor (high Q factor), is needed in order to maintain the dimensions of the filter in a range conducive to manufacturing limitations. A percentage fired density approaching 100 also is conducive to achieving a high Q factor.
Conventional dielectric ceramic materials made of alumina or modified alumina do not exhibit sufficiently high Q factor values along with sufficiently low temperature coefficients for satisfactory use as filters and resonators in the microwave frequency band. Additionally, these conventional materials are limited in that they require sintering at relatively high peak soak temperatures of about 1550° C. The peak soak temperature is the maximum (peak) temperature achieved during sintering; it is at this temperature that the material remains (soaks) for a period of time.
Furthermore, under normal operating conditions, a filter is typically subjected to a range of temperatures. As temperature changes, the filter's dimensions are altered by thermal expansion or contraction of the filter material. This results in a shift in resonant frequency. Dielectric properties are affected by a change in temperature, also tending to shift the resonant frequency. The change in resonant frequency caused by a change in temperature is termed the temperature dependence of the resonant frequency. The temperature coefficient (T
f
) expresses the frequency shift caused by a change in temperature of 1° C. For example, a T
f
of +5 means that the resonant frequency shifts upward by five ppm with a temperature change of 1° C. A T
f
of −5 means that the resonant frequency shifts downward by five ppm with a temperature change of 1° C. A temperature coefficient approaching zero (0) is preferred to minimize the shift in resonant frequency due to variations in operating temperature.
SUMMARY OF THE INVENTION
The present invention is directed to a dielectric ceramic compound comprising a base material and an additive material. The base material is represented by the general formula: (x) Al
2
O
3
+(y) TiO
2
, wherein x and y are percentages of the total weight of said base material. The value of x is in the range of about 60 to about 96, with y being in the range of about 4 to about 40. The values of x and y are more preferably in the range of about 94 to about 96 and about 4 to about 6, respectively.
In one embodiment of the invention the additive material comprises Nb
2
O
5
in an amount from about 0.1 to about 3 weight percent of the total weight of the base material, and is more preferably in an amount from about 0.1 to about 1.
In a second embodiment, the additive material further comprises at least one component selected from the group consisting of BaCO
3
, SnO
2
, Mn
2
O
3
, MnCO
3
, Mg(OH)
2
, and Y
2
O
3
. When present, the BaCO
3
is in an amount from about 0.25 to about 1 weight percent of the total weight of the base material, more preferably in an amount from about 0.25 to about 0.5. When present, the SnO
2
is in an amount from about 0.5 to about 1.25 weight percent of the total weight of the base material. When present, the Mn
2
O
3
is in an amount from about 0.075 to about 0.115 weight percent of the total weight of the base material. When present, the MnCO
3
is in an amount from about 0.02 to about 0.1 weight percent of the total weight of the base material. When present, the Mg(OH)
2
is in an amount from about 0.005 to about 0.075 weight percent of the total weight of the base material. When present, the Y
2
O
3
is in an amount from about 0.004 to about 0.03 weight percent of the total weight of the base material, more preferably from about 0.004 to about 0.01.
The dielectric ceramic compositions may be sintered at a peak soak temperature of about 1320 to about 1400° C., more preferably from about 1350 to about 1370° C., for about 4 hours. The resulting sintered compositions exhibit the following improved electrical properties: a dielectric constant (K) from about 10.00 to about 12.00, more preferably from about 11.50 to about 12.00; a percent fired density of about 95 to about 100, more preferably from about 98.33 to about 99.20; a Q factor of about 10,000 to about 55,000, more preferably from about 30,000 to about 50,000; and a temperature dependence (T
f
) from about −30 to about 30 ppm/° C., more preferably from about −3 to about 1 ppm/° C., within a high frequency range greater than about 2 GHz.


REFERENCES:
patent: 4866016 (1989-09-01), Ando et al.
patent: 5024980 (1991-06-01), Negas et al.
patent: 5147835 (1992-09-01), Franzak et al.
patent: 5792379 (1998-08-01), Dai et al.
patent: 5830819 (1998-11-01), Shikata et al.
patent: 09052762 (1997-02-01), None
patent: 10-120460 (1998-12-01), None
Gui Zhilun et al., “Low-Temperature Sintering Of Lead-Based Piezoelectric Ceramics”American Ceramic Society, Jul. 11, 1988, vol. 72, No. 3, pp. 486-491.

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