Dielectric material for high frequencies

There is disclosed a dielectric material for high frequencies comprising a composition system represented by the following formula: EQU x(Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 --yCaTiO.sub.3 --zBa(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 wherein PA1 0.60.ltoreq.x.ltoreq.0.8 PA1 0.04.ltoreq.y.ltoreq.0.38 and PA1 0.02.ltoreq.z.ltoreq.0.16.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates, in general, to a dielectric material for 
high frequencies and, more particularly, to a dielectric material for high 
frequencies which possesses high quality factor Q and exhibits superior 
temperature coefficient of resonant frequency even in high frequency 
regions. 
2. Description of the Prior Art 
In recent years, there have been rapidly developed communication systems 
using microwaves (frequency band ranging from 300 MHz to 300 GHz), such as 
wireless telephones, car phones, cellular phones, satellite broadcasting 
systems, and the like, and there is an increasing demand the dielectric 
ceramics with better electrical properties, which are extensively used in 
resonator devices, band pass filters, microwave integrated circuits and 
other parts of communication systems. 
For application for communication systems using microwaves, a dielectric 
material for high frequencies needs to satisfy the following conditions: 
1. A large dielectric constant for the miniaturization of parts of 
communication systems because the wave length of the microwaves in a 
dielectric material is inversely proportional to the square root of the 
dielectric constant; 
2. A high Q value (reciprocal of dielectric loss) for high performance 
because dielectric loss is proportional to frequency; 
3. A small temperature coefficient of resonant frequency, so as to obtain 
desired resonant characteristics which are stable to temperature change. 
In addition, it is required that the dielectric material for high 
frequencies is resistant to ageing, large in thermal conductivity and high 
in mechanical strength. 
Well known representative dielectric materials for high frequencies will be 
exemplified. 
On the one hand, the following are known as dielectric materials which are 
low in dielectric loss but have a dielectric constant of not more than 40: 
i. A Ba(M.sup.+2.sub.1/3 M.sup.+5.sub.2/3)O.sub.3 system wherein M.sup.+2 
.dbd.Mg or Zn, M.sup.+5 .dbd.Ta or Nb [reference: K. Matsumoto, T. Hiuga, 
K. Takada and H. Ichimura, "Ba(Mg.sub.1//3 Ta.sub.2/3)O.sub.3 Ceramics 
with Ultra-low Loss at Microwave Frequencies" In Proce. of the Sixth IEEE 
International Symposium on Applications of Ferroelectrics, pp. 
118.about.121, (1986)]. 
ii. A Ba.sub.2 Ti.sub.9 O.sub.20 system [reference: S. Nisikaki et al., 
"BaO--TiO.sub.2 --WO.sub.3 Microwave Ceramics and Crystalline BaWO.sub.4 " 
J. Am. Ceram. Soc., 71(1), C-11-C-17 (1988)]. 
iii. A (Zr,Sn)TiO.sub.4 system [reference: K. Wakino et al., "Microwave 
Characteristics of (Zr, Sn)TiO.sub.4 and BaO--PbO--Nd.sub.2 O.sub.3 
--TiO.sub.2 Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278.about.281 
(1983)]. 
On the other hand, the following are known as dielectric materials which 
have a dielectric constant of not less than 80 yet are relatively high in 
dielectric loss (Q.times.fo (GHz)&lt;10,000): 
i. A BaO--Sm.sub.2 O.sub.3 --TiO.sub.2 system [reference: J. M. Wu and M. 
C. Chang, "Reaction Sequence and Effects of Calcination and Sintering on 
Microwave Properties of (Ba,Sr)O--Sm.sub.2 O.sub.3 --TiO.sub.2 Ceramics" 
J. Am. Ceram. Soc., 73(6), 1599.about.1605 (1990)]. 
ii. A (Ba,Pb)O--Nd.sub.2 O.sub.3 --TiO.sub.2 system [reference: K. Wakino 
et al., "Microwave Characteristics of (Zr,Sn)TiO.sub.4 and 
BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 Dielectric Resonators" J. Am. 
Ceram. Soc. 67(4), 278.about.281 (1983)]. 
iii. A (Pb,Ca)ZrO.sub.3 system [reference: J. Kato, "Material Produces 
Small Resonators with High Dielectric Constant" JEE, Sep., 114.about.118 
(1991)]. 
Dielectric ceramics for high frequencies having high dielectric constants 
are suitable as materials for microwave devices using electric waves of 
long wavelength and are in great demand in devices for communication 
systems requiring miniaturization. 
However, it is very difficult to develop dielectric materials having stable 
temperature coefficients of resonant frequency as well as high dielectric 
constants and high Q values. Generally, dielectric losses and temperature 
coefficients of resonant frequency in dielectric materials having large 
dielectric constants, both increase because of dipole coupling therein. 
That is to say, since materials with high dielectric constants are 
inclined to have low Q values and large temperature coefficients of 
resonant frequency, it is very difficult to satisfy the three desired 
electrical properties in a single material system. 
For application in communication systems, however, dielectric materials for 
high frequencies, first of all, must have stable temperature coefficients 
of resonant frequency. 
Of the known conventional dielectric compositions, in fact, a material 
satisfying the three desired electrical properties at same time has not 
been found. For example, a SrTiO.sub.3 system exhibits satisfactory 
dielectric constant and quality factor (Q.times.fo) which are in a range 
of 110 to 151 and 8,900 to 36,000 GHz, respectively but is problematic in 
that the temperature coefficient of resonant frequency is too large, 
ranging from 505 to 7,500 ppm/.degree. C. On the other hand, a (Li.sub.1/2 
Nd.sub.1/2)TiO.sub.3 system exhibits a low dielectric constant of about 
75, and a temperature coefficient of resonant frequency of about -274 
ppm/.degree. C. yet a small dielectric loss. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to make the best of the 
(Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, CaTiO.sub.3 and Ba(Zn.sub.1/3 
Nb.sub.2/3)O.sub.3 substances as well as to overcome the above problems 
encountered in prior arts and to provide a novel dielectric material, 
capable of exhibiting a dielectric constant of not less than 80 yet a 
small dielectric loss and easily controlling a temperature coefficient of 
resonant frequency into positive and negative values according to demand. 
Based on the intensive and thorough study by the present inventors, the 
above object could be accomplished by a provision of a dielectric material 
for high frequencies comprising a composition system represented by the 
following formula: 
EQU x(Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 --yCaTiO.sub.3 --zBa(Zn.sub.1/3 
Nb.sub.2/3)O.sub.3 
wherein 
0.60.ltoreq.X.ltoreq.0.8 
0.04.ltoreq.Y.ltoreq.0.38 and 
0.02.ltoreq.Z.ltoreq.0.16.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a dielectric material comprising a 
composition system which comprises (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, 
CaTiO.sub.3 and Ba(Zn.sub.1/3 Nb.sub.2/ 3)O.sub.3 with the amounts of the 
components being in the ranges shown in FIG. 1. 
Accordingly, the dielectric material for high frequencies is prepared with 
a composition comprising about 0.6 to about 0.9 mole fraction of 
(Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, about 0.09 to about 0.28 mole fraction 
of CaTiO.sub.3, and about 0.02 to about 0.16 mole fraction of 
Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. 
The presence of three components (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, 
CaTiO.sub.3 and Ba(Zn.sub.1/3 N b.sub.2/3)O.sub.3 allows dielectric 
materials to be superior in the dielectric properties including dielectric 
constant, dielectric loss and temperature coefficient of resonant 
frequency. Especially, the amount of (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 has 
an important influence on the dielectric properties. When (Li.sub.1/2 
Nd.sub.1/2)TiO.sub.3 is contained in an amount of around 0.7 mole 
fraction, excellent dielectric material is prepared with a dielectric 
constant of 90 to 102.5 and a temperature coefficient of resonant 
frequency of not more than 10 ppm/.degree. C. In addition, the value of 
the temperature coefficient of resonant frequency in the dielectric 
material can be easily either positive or negative by controlling small 
amount of (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, according to the present 
invention. 
The preferred embodiments of the present invention will now be further 
described with reference to specific examples. 
Other features, advantages and embodiments of the invention disclosed 
herein will be readily apparent to those exercising ordinary skill after 
reading the foregoing disclosures. 
EXAMPLE 
Powdery Li.sub.2 CO.sub.3, Nd.sub.2 O.sub.3, CaCO.sub.3, TiO.sub.2, ZnO, 
Nb.sub.2 O.sub.3 and BaCO.sub.3, all having a purity of above 99%, were so 
weighed in a balance as to give compositions shown in the following Table 
1 and mixed. The powder mixtures were calcined at 1,050.degree. C. for 4 
hours in air, to give three systems (Li.sub.1/2 Nd.sub.1/2)TiO.sub.3, 
CaTiO.sub.3, and Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 which were then 
pulverized and calcined at a temperature of 1,200.degree. to 1,300.degree. 
C. for 4 hours, again, to produce perovskite type solid solutions. 
Following pulverization, each of the solid solutions was molded under 
pressure into a disk type specimen with a diameter of 10 mm and a 
thickness of 4 to 5 mm which was subsequently sintered at a temperature of 
1,400.degree. to 1,500.degree. C. for a period of 2 to 12 hours in air. 
The sintering temperature was raised as the content of Ba(Zn.sub.1/3 
Nb.sub.2/3)O.sub.3 was increased. After the sintering, the specimens 
exhibited contraction rates ranging from 12 to 20%. 
Opposite faces of the specimens were well polished with an abrasive paper 
(up to #3000) and then, they were tested by a Hakki-Coleman method for 
dielectric constant, Q value and temperature coefficient of resonant 
frequency at a frequency of 2 to 4 GHz and at a temperature of 20.degree. 
to 30.degree. C. The results are given as shown in the following Table 1. 
TABLE 1 
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Dielectric Properties of 
X(Li.sub.1/2 Nd.sub.1/2)TiO.sub.3 --YCaTiO.sub.3 --ZBa(Zn.sub.1/3 
Nb.sub.2/3)O.sub.3 system 
Specimen 
Composition Dielectric 
Q .times. fo 
TCF 
No. X Y Z Const. (.epsilon..sub.r) 
(GHz) (ppm/.degree.C.) 
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1 0 0 0 40 5600 28 
2 0 1 1 170 6200 756 
3 1 0 0 75 1800 -274 
4 0 0.9 0.1 151.3 8900 750 
5 0 0.8 0.2 138.5 12000 655 
6 0 0.7 0.3 125 29000 580 
7 0 0.6 0.4 110.5 36000 505 
8 0.9 0.09 0.01 82.5 2500 -150 
9 0.8 0.18 0.02 92.1 3500 -80 
10 0.7 0.27 0.03 102.5 4300 10 
11 0.6 0.36 0.04 106.6 5900 112 
12 0.9 0.08 0.02 82.6 3020 -178 
13 0.8 0.16 0.04 90.2 4100 -83.8 
14 0.7 0.24 0.06 97.8 4900 8 
15 0.6 0.32 0.08 105.4 6200 106.4 
16 0.9 0.07 0.03 81.3 3150 - 190 
17 0.8 0.14 0.06 87.6 4750 -90 
18 0.7 0.21 0.09 93.9 5600 5 
19 0.6 0.28 0.12 101.4 6900 98 
20 0.9 0.06 0.04 80 5500 -195 
21 0.8 0.12 0.08 85.5 5900 -100 
22 0.7 0.18 0.12 90.5 6900 3 
23 0.6 0.24 0.16 95.5 9500 105 
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