Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2002-03-20
2004-10-05
Brunsman, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C361S321500, C333S219100, C343S785000, C343S907000
Reexamination Certificate
active
06800577
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a set of novel microwave dielectric ceramic compositions xMO-yLa
2
O
3
-zTiO
2
(M=Sr, Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9) and devices comprising the same.
BACKGROUND OF THE INVENTION
The dramatic developments in the microwave integrated circuit technology have revolutionized the field of telecommunications. Dielectric resonators (DRs) are key components in MIC technology that increasingly replace the conventional metallic cavity resonators and microstrip circuits. The size of the microwave circuit is inversely proportional to the square root of its dielectric constant. A DR should have high dielectric constant (for miniaturization), high quality factor (for frequency selectivity) and low temperature coefficient of resonant frequency (for frequency stability). These constraints usually limit the DR applicability to only those ceramics with &egr;
r
=20-100, Q>2000 and &tgr;
f
<±20 ppm/° C. Compared to the use of alumina substrates, low loss high dielectric constant materials decrease the size not only for strip line resonators and filters but also for all microwave circuits. It is also possible to use these dielectric materials in the fabrication of devices such as circulators, phase shifters etc. for impedance matching. The DRs are used for the manufacture of microwave oscillators, filters and dielectric resonator antennas for satellite and personnel communication applications. They are also used for applications such as telemetry and tracking.
Conventional microwave ceramics fall into categories. (1) Ceramics with low permittivity (20<&egr;
r
<40) with high quality factor (Qxf>50000 GHz) such as Ba(Mg,Ta)O
3
, Ba(Zn,Ta)O
3
, Ba(Mg,Nb)O
3
, Ba(Zn,Nb)O
3
, their solid solution modifications, Ba(Mg,Sn,Ta)O
3
, Ba
2
Ti
9
O
20
and (Zr,Sn)TiO
4
. (2) Ceramics with high permittivity (&egr;
r
>75) and low Q factor (Qxf<10000 GHz) such as tungsten bronze type BaO—RE
2
O
3
—TiO
2
(1:1:4 and 1:1:5) and Ba
6−3x
Ln
8+2x
Ti
18
O
54
where Ln is La
3+
, Nd
3+
, Sm
3+
and Gd
3+
. The first group of ceramics are usually employed at frequencies >1.5 GHz where as the second group at frequencies <2 GHz. For applications at <2 GHz, though ceramics with &egr;
r
≧70 can provide greater miniaturization, due to the requirements of narrow bandwidth and extremely low insertion loss (<0.3 dB) ceramics with even &egr;
r
=38 are used by compromising size. Hence further miniaturization of devices require low &tgr;
f
ceramics with &egr;
r
>45 and Qxf>45000 GHz. [B. Jancar, D Suvorov, M. Valent, “Microwave dielectric properties of CaTiO
3
—NdAlO
3
ceramics”, J. Mater. Sci. Letters 20 (2001), 71-72]. Two different methods are used in this regard. (1) The formation of solid solutions between high Q ceramics with &egr;
r
in the range 20 to 40 and having reasonably high &tgr;
f
with ceramics of opposite &tgr;
f
(usually positive), high &egr;
r
(>100) and low loss such as CaTiO
3
, TiO
2
, SrTiO
3
, BaTiO
3
etc. (2) Explore the microwave dielectric properties of Ba
5
Nb
4
O
15
type cation deficient hexagonal perovskites. [H. Sreemoolanadhan, M. T. Sebastian and P. Mohanan, Mater. Res. Bull, 30(6) 1995, pp 653-658, C. Veneis, P. K. Davies, T. Negas and S. Bell, Mater. Res. Bull 31(5) 1996 pp 431-437 and S. Kamba, J. Petzelt, E. Buixaderas, D. Haubrich and P. Vanek, P. Kuzel, I. N. Jawahar, M. T. Sebastian and P. Mohanan, J. Appl. Phys 89(7) 2001, pp 3900-3906]. The cation deficient provskites reported such as Ba
5
Nb
4
O
15
and Ba
5−x
Sr
x
Nb
4
O
15
, are having dielectric constant between 40 and 50 and high quality factor, but their high &tgr;
f
make them not suitable for practical applications. The isostructural BaLa
4
Ti
4
O
15
and Ba
2
La
4
Ti
5
O
18
have high dielectric constant (43 and 46), high quality factor with low temperature coefficient of resonant frequency. [C. Veneis, P. K. Davies. T. Negas and S. Bell, Mater. Res. Bull 31(5) 1996 pp 431-437] The MO—La
2
O
3
—TiO
2
(M=Sr, Ca) ceramics covering the present patent consist of cation deficient perovskites MLa
4
Ti
4
O
15
(M=Sr, Ca) and Ca
2
La4Ti
5
O
18
belonging to the homologous series A
n
B
n−1
O
3n
(5, 6 or 8) [V. A. Saltykova, O. V. Mehikova, N. F. Fedorov, Russian J. Inorg. Chem. 34(5) 1989, p 758-759] and orthorhombic structured compounds CaLa
4
Ti
5
O
17
and CaLa
8
Ti
9
O
31
[see JCPDS files 27-1057, 27-1058, 27-1059]. The dielectric properties of these materials are being investigated for the first time.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide a set of novel microwave dielectric ceramic compositions xMO-yLa
2
O
3
-zTiO
2
(M=Sr, Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9) and devices comprising the same which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a set of novel dielectric ceramic compositions having high dielectric constant and low &tgr;
&egr;
for capacitor applications.
Yet another object of the present invention is to provide a set of novel microwave dielectric ceramics having high dielectric constant and low &tgr;
f
for microwave substrate applications.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a novel microwave dielectric ceramic composition of the general formula xMO-yLa
2
O
3
-zTiO
2
wherein M is Sr or Ca; x:y:z=1:2:4, 2:2:5, 1:2:5 or 1:4:9. ps and devices comprising the same which comprises the manufacture of the inventive perovskites in the powder form, moulding of the powder in the suitable shape, drying, sintering and the final treatment.
In an embodiment of the present invention the dielectric ceramic compositions xCaO-yLa
2
O
3
-zTiO
2
(x:y:z=1:2:4 and 2:2:5) are prepared in cylindrical pellet shape through the solid state ceramic route by taking high purity CaCO
3
, La
2
O
3
, and TiO
2
in the molar ratios 1:2:4 and 2:2:5, the pellets are polished, physical, structural and microwave dielectric properties are characterized. The initial firing of the mixed powders is carried out at sufficiently higher temperatures and for sufficient durations such that single-phase polycrystalline ceramics CaLa
4
Ti
4
O
15
and Ca
2
La
4
Ti
5
O
18
are obtained.
In another embodiment of the present invention the ceramic compositions SrO—2La
2
O
3
—4TiO
2
are prepared in cylindrical pellet shape through the solid state ceramic route from high purity SrCO
3
, La
2
O
3
and TiO
2
in the molar ratio 1:2:4, the mixed powders are fired at different temperatures above 1200° C., the pellets are polished, physical, structural and microwave dielectric properties are characterized. The firing of the mixed oxide powders is carried out at sufficiently high temperatures for sufficient duration for getting single-phase polycrystalline ceramics of SrLa
4
Ti
4
O
15
.
In yet another embodiment of the present invention dielectric ceramic compositions xCaO-yLa
2
O
3
-zTiO
2
(x:y:z=1:2:5 and 1:4:9) are prepared in cylindrical pellet shape through the solid state ceramic route, from a mixture of high purity CaCO
3
, La
2
O
3
, and TiO
2
taken in the molar ratios 1:2:5 and 1:4:9 and by firing at temperatures greater than 1200° C. The pellets are polished and physical, structural and microwave dielectric properties are characterized. The firing of the mixed oxide powders is carried out at sufficiently higher temperatures for sufficient duration such that single-phase polycrystalline CaLa
4
Ti
5
O
17
and CaLa
8
Ti
9
O
31
ceramics are obtained.
The first two embodiments comprise materials with the cation deficient hexagonal perovskite structure whereas the third one comprises materials with orthorhombic structure. The AO—La
2
O
3
—TiO
2
(A=Ca, Sr) system provides ceramic materials with high dielectric constant (>40), high quality factor (>6800 at 4-5 GHz) and low temperature coefficient of resonant frequency (<±25 ppm/° C.)
Iyer Santha Narayana
Jawahar Isuhak Naseemabeevi
Mailadil Thomas Sebastian
Brunsman David
Council of Scientific and Industrial Research
Nixon & Vanderhye P.C.
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