Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2000-06-16
2002-05-28
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S135000
Reexamination Certificate
active
06395663
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a Bi
2
O
3
—ZnO—Nb
2
O
5
(BZN) ceramic material, and especially to a BZN ceramic material having a low sintering temperature by adding a flux therein.
BACKGROUND OF THE INVENTION
Microwave ceramic materials in the microwave frequency, f, ranged from 1 to 12 GHz are used as dielectric resonators, e.g. bandpass or bandstop filters and oscillator stabilizer devices. These devices are becoming increasingly important with the continued development of microwave integrated circuitry, microwave telecommunications, and satellite broadcasting systems.
BaO—RE
2
O
3
—TiO
2
is one system of ceramic materials being developed for microwave applications, wherein RE
2
O
3
is a rare earth oxide, e.g. La
2
O
3
, Nd
2
O
3
, Sm
2
O
3
, etc. See for example: (1) Journal of the American Ceramic Society, K. Wakino, et al., 67(4), 1984, pp.278-281 and (2) Am. Ceram. Soc. Bull., S. Nishigaki, 66(9), 1987, pp.1405-1410. These ceramic materials have high dielectric constant (∈
r
≈70~90) and low temperature resonant coefficient (&tgr;
f
<10 ppm/° C.). BaO—MgO—Ta
2
O
5
is another ceramic material system being developed. This ceramic material has high dielectric loss quality (Q·f≈150000). See for example: J. Appl. Phys., M. Furuya, 85(2), 1084 (1999). Although these ceramic materials have excellent microwave dielectric characteristics, they need to be densified at a higher sintering temperature above 1300° C. Therefore, the processes for manufacturing the electronic devices need to be further modified. In order to avoid resonance interference and dielectric loss, the electrodes used in the multilayer structure should exhibit a high conductivity. Although Ag-based internal electrode is the best choice, the melting point of Ag is quite low (T
m
=961° C.). If the sintering temperature of the ceramic materials is too high, Ag-based internal electrode can not be utilized. Therefore, the point of present invention is to provide a low-temperature cofired ceramic material with good microwave properties.
PRIOR ARTS
The reports of recent researches for lowering the sintering temperature of the ceramics can be divided as follows:
(I) A first method to lower the sintering temperature of microwave ceramics such as BaO—RE
2
O
3
—TiO
2
from 1300° C. to 950° C. is done by adding a lot of glass phases (10 wt %~30 wt %) inside. However, adding glass phases will lower the dielectric constant from 90 to 20. Moreover, the sintering temperature (950° C.) is not low enough for the low-temperature cofired ceramic (LTCC) applications. See for example: (1) U.S. Pat. No. 4,504,339 and (2) Journal of The American Ceramic Society, T. Takada, 77(7), 1994, pp.1909-1916.
(II) A second method to lower the sintering temperature of microwave ceramics is made by using a glassy-phased dielectric ceramics, which is formed by melting all oxides and then quick cooling the mixture. The sintering temperature can be lowered to 850° C. but the dielectric constant is only 7~13. See for example: (1) U.S. Pat. No. 3,656,984 and (2) “Materials Compatibility and Cosintering Aspects of Shrinkage Control in Low-Temperature Cofired Ceramic Packages, H. Sawhill, pp.307-319, Advanced in Ceramics, Vol. 26, “
Ceramic Substrates and Packages for Electronic Applications
” (American Ceramic Society, Westerville, Ohio, 1990).
(III) As disclosed in U.S. Pat. No. 5,449,652, a new dielectric ceramic material, Bi
2
O
3
—ZnO—Nb
2
O
5
(or Bi
2−x
(Zn
2/3
Nb
4/3
)O
7−3x/2
, 0≦x≦0.67, BZN), is evolved. By improving the composition and forming method, the sintering temperature of BZN ceramics is lowered to 920° C. and the microwave dielectric properties is excellent [∈
r
≈75·140, Q·f>5000 (at 1~5 GHz)]. See for example: (1) “Phase Structure and Dielectric Properties of Bi
2
O
3
—ZnO—Nb
2
O
5
-based Dielectric Ceramics”, D. Liu, et al., J. Am. Ceram. Soc., 76(8), 1995,pp.2129-2132, and (2) J. Am. Ceram. Soc., M. F. Yan, et al., 73(4), 1987, pp.1106-1107. (3) J. Mater. Res. 5, 1990, pp. 1752-1762. Although there are many developments done to modify BZN, the sintering temperatures of BZN ceramics in the conventional researches are all above 900° C. and most of the dielectric properties were measured at a frequency below 10 MHz.
The reports for the dielectric properties of BZN ceramics at a high frequency in the GHz range are still not too much. European Patent No.558319A discloses that after the xBiO
1.5−y
ZnO
z
NbO
2.5
, wherein 0.41<x<0.51, 0.19<y<0.3, and 0.25<z<0.345, is sintered at 925° C., the dielectric properties at 2~4 GHz are ∈
r
=89~133, Q=40~310, and &tgr;
f
=110~120 ppm/° C. Others try to improve the sintering temperature of BZN by replacing a small amount of Bi, Zn, and Nb elements (usually less than 20%) with Ca, Mg, Co, Sn, Ti, etc. or changing the manufacturing method of BZN. As an example disclosed in U.S. Pat. No. 5,449,652, by mixing ZnO and Nb
2
O
5
to form ZnNb
2
O
6
at first and sintering ZnNb
2
O
6
with Bi
2
O
3
, ZnO, and CaCO
3
at 950~1100° C., BZN ceramics can be formed with the dielectric properties of ∈
r
=90~110, f
f
<100 ppm/° C., tan &dgr;(at 100 KHz)<0.0002, Q·f(at 1·5 GHz)>5000. Although BZN is an excellent low-temperature microwave ceramic material, the sintering temperature still has to be over 925° C.
In order to lower sintering temperature, many fluxes, such as Li
2
CO
3
, B
2
O
3
, LiF, PbO and CuO, are added. See for example: U.S. Pat. No. 5,433,917 “PZT Ceramic compositions having reduced sintering temperatures and process for producing same”. Unfortunately, although these fluxes can be added in BaTiO
3
or piezoelectric ceramics (PZT), they can not be added in BZN systems since the dielectric properties will be significantly affected.
It is therefore attempted by the applicant to develop a novel fluxed BZN dielectric ceramic materials having sintering temperature below 900 to overcome the aforementioned limitations and difficulties encountered with the prior art. Additionally, excellent microwave properties, i.e., Q (quality factor) and ∈ (dielectric constant) can be obtained at such a low sintering temperature as 800~850° C.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a dielectric ceramic material which is mainly made by Bi
2
O
3
—ZnO—Nb
2
O
5
(BZN). By adding a special flux in BZN, the sintering temperature is lowered and the dielectric properties still maintain excellent.
It is another object of the present invention to provide a method for forming a dielectric ceramic material by adding a flux. The flux is formed by mixing and melting the mixture oxides according to the eutectic composition of the oxides.
It is a further object of the present invention to provide a dielectric ceramic material which is formed by directly mixing, calcining and sintering Bi
2
O
3
, ZnO and Nb
2
O
5
or by directly mixing, calcining and sintering Bi(CH
3
COO)
3
, Zn(CH
3
COO)
2
and Nb(OC
2
H
5
)
5
.
The dielectric ceramic material of the present invention includes a mixture represented by the formula of Bi
2−x
(Zn
2/3
Nb
4/3
)O
7−3x/2
(BZN), 0≦x≦0.67, and a flux having an eutectic composition for lowering the sintering temperature of the dielectric ceramics from 950~1100° C. to 800~850° C.
According to the present invention, the dielectric material has a dielectric constant higher than 45 (∈
r
>45) and a quality factor larger than 1200 (Q·f>4500) at 3.5 GHz.
The fluxes used in the present invention include BaCO
3
—2.5CuO powder and 0.15CuO—0.85MoO
3
powders melted at about 1200° C., 900° C.
According to the present invention, the flux is added into the mixture with the addition of 0.1·10 wt %, preferably 0.5·5 wt %.
According to the present invention, BZN is formed from Bi
2
O
3
, ZnO, Nb
2
O
5
calcined at 850° C., or from Bi(CH
3
COO)
3
, Zn(CH
3
COO)
2
, and Nb(OC
2
H
5
)
5
by a sol-gel method.
The method of forming a dielectric ceramic material includes (a) preparing a mixture represen
Chen San-Yuan
Lin Yih-Jaw
Burr Matthew E.
Gardere Wynne & Sewell LLP
Group Karl
National Science Council
Warren, Jr. Sanford E.
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