Dielectric Ceramic Composition for Microwave Application

The present invention relates to dielectric ceramic compositions adapted for use at microwave which is especially, but not exclusively, useful for devices or equipment for communication or broadcasting.

In recent years, there have been used dielectric materials having a high dielectric constant, a low dielectric loss and a small temperature coefficient of resonance frequency in resonators. Hereinafter, temperature coefficient of resonance frequency is referred to as ".tau.f". Especially, such materials have been now brought into practical use in communication or broadcasting equipments, such as receivers for a satellite broadcasting or automobile telephone.

BaO-TiO.sub.2 or MgO-CaO-TiO.sub.2 system dielectric ceramics have heretofore been known in such applications. However, since those heretofore known BaO-TiO.sub.2 system or MgO-CaO-TiO.sub.2 system dielectrics have a low dielectric constant of the order of 20 to 40, they are disadvantageous in that large dimensions are required because of their low dielectric constant when those dielectrics are employed as resonators at a relatively low frequency region of about 1 GHz. As represented by automobile telephones, currently, there is an increasing demand for dielectric resonators for use in frequencies near 1 GHz and, with this need, much effort or attempts have been made to provide miniaturized resonators. In order to make possible the miniaturization, dielectrics having a high dielectric constant .epsilon., a low dielectric loss and a small .tau.f are required. Particularly, in the case of a coaxial resonator used for automobile telephones, the dielectric constant of a dielectric ceramic used for the coaxial resonator should be sufficiently high without decreasing the Q of the resonator to unacceptable levels (wherein Q is the reciprocal of dielectric loss tan .delta., i.e., Q=1/tan .delta.). More specifically, an increase in the dielectric constant of the dielectric ceramic permits the reduction in the height of the resonator required for the use at the desired resonance frequencies, thereby making possible the miniaturization of the resonator. While in order to minimize deleterious effects due to harmonics, the diameter of the resonator should be reduced, the reduction in the diameter leads to an unfavorable decrease in the Q value of the resonator. Therefore, it is essential that the dielectric ceramic used for the resonator has the highest possible dielectric constant, while the Q value of the resonator has to be kept at the acceptable level from the matter of the design of a resonator, in addition to the requirement that the dielectric ceramic used in the resonator should have a small dielectric loss and a small .tau.f. Such requirements can be satisfied with dielectric materials which make it possible to obtain various desired dielectric constant values with a small dielectric loss and a small .tau.f.

Heretofore, there has been known BaO-TiO.sub.2 -R.sub.2 O.sub.3 system (wherein R.sub.2 O.sub.3 represents lanthanide oxide) as high dielectric constant materials but among them materials having a sufficiently small .tau.f exhibit a small Q of 2000 to 3000 at 3 GHz and thus result in a large dielectric loss.

Taking as a representative example a BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 system (Japanese Patent Publication No. 59-37 526), materials of fully small .tau.f possess a dielectric constant .epsilon. of 75 to 92 and a Q value of 2300 to 2790 at 2.2 to 2.3 GHz ("Dielectric Properties at Microwave Frequencies of the Ceramics in BaO-Sm.sub.2 O.sub.3 -TiO.sub.2 System" presented by S. Kawashima et al., at Am. Ceram. Soc., 87th, Annual Meeting) and, particularly, at 3 GHz their Q value is reduced to an impractical level of 3000 or less and an improved Q has been required.

Therefore, an object of the present invention is to improve the foregoing disadvantages and thereby to provide improved dielectric ceramic compositions having various desired dielectric constant values in a wide range while maintaining a large Q and small .tau.f.

According to a first feature of the present invention, there are provided dielectric ceramic compositions for microwave application having the formula ##EQU2## wherein x, y and z are molar percentages and 10.ltoreq.x.ltoreq.22, 58.ltoreq.y.ltoreq.80 and 8.ltoreq.z.ltoreq.22, 0<n<25 and x+y+z=100;

M is at least one selected from the group consisting of Sr, Ca and Mg; and

R is at least one selected from the group consisting of Nd, Sm and La.

A further feature of the present invention resides in dielectric ceramic compositions for microwave applications, the composition comprising

(a) a compound having the formula ##EQU3## wherein x, y and z are molar percentage and 10.ltoreq.x.ltoreq.22, 58.ltoreq.y.ltoreq.80 and 8.ltoreq.z.ltoreq.22, 0<n<25, and x+y+z=100;

M is at least one selected from the group consisting of Sr, Ca and Mg; and

R is at least one selected from the group consisting of Nd, Sm and La; and

(b) at least one oxide selected from the group consisting of Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, WO.sub.3, SnO.sub.2 and ZrO.sub.2 in the range of more than zero to 5 mole percent based on the total molar amount of x, y and z.

In the drawing, FIG. 1 is a graph showing the relationship between the substituted percentage (n) of SrO and .tau.f.

As previously described, the present invention is directed to the dielectric ceramic composition having the following formula: ##EQU4## wherein x, y, z, n, M and R are defined previously.

In the foregoing formula, x exceeding 22 mole percent results in an excessively increased .tau.f and a reduction in dielectric constant. On the other hand, an insufficient x below 10 mole percent excessively increases .tau.f.

When the molar percentage, y, is more than 80 mole percent, .tau.f becomes too large, while the y value less than 58 mole percent makes sintering difficult and results in an unacceptably large .tau.f and the reduction of Q.

When z is more than 22 mole percent, Q is reduced and .tau.f is increased, while small z values less than 8 mole percent increases .tau.f to an unacceptable level.

Substitution of SrO and/or CaO for BaO within the range above defined enables the controlled change in .tau.f and dielectric constant without accompanying an undesirable reduction in Q and thereby can provide various desired dielectric constant values in a wide range while maintaining a small f and a high Q.

Substitution of MgO for BaO is effective to improve Q values when the substitution is in small amounts as above specified. The reason why the amount of the substitution, which is represented by n in the foregoing formula, is limited to a maximum of 25 mole percent is that when the substitution is more than 25 mole percent, .tau.f value becomes too large of Q value is greatly reduced.

In the further feature of the present invention, the dielectric ceramic composition further contains at least one oxide selected from the group consisting of Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, WO.sub.3, SnO.sub.3 and ZrO.sub.2 in amounts of more than zero to 5 mole percent based on the total molar percentage of x, y and z.

The reason of the addition for Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, WO.sub.3, SnO.sub.2 and/or ZrO.sub.2 is that these additive oxides provide an improved sinterability and further, in a small addition amount, given a favorable effect on Q. As a further beneficial effect, even when the addition amount is increased within the specified range, dielectric constant and .tau.f can be adjusted without any detrimental reduction in Q. Thus, the additive oxides help to obtain materials having various desired dielectric constant values over a wide range with while maintaining small .tau.f and a high Q.

In the present invention, the most preferable ranges of the x, y and z are 12.ltoreq.x.ltoreq.20, 65.ltoreq.y.ltoreq.75 and 10.ltoreq.z.ltoreq.20.

Now, the present invention will be described in detail with reference to Examples.

As starting materials BaCO.sub.3, SrCO.sub.3, CaCO.sub.3, MgO, TiO.sub.2, La.sub.2 O.sub.3, Nd.sub.2 O.sub.3 and Sm.sub.2 O.sub.3 of high purity were weighed to give the desired compositions, and were wet blended thoroughly in pure water in a polyethylene pot containing alumina balls. The resulting slurry was filtered by means of suction, dried and then calcined in air at temperatures of 850.degree. to 1000.degree. C. for a period of 1 to 6 hours. The calcined material was wet milled with pure water in a polyethylene pot containing alumina balls, filtered by means of suction and then dried. After adding binder to the dried material, the resulting mixture was granulated into a particle size passable through a 70 mesh sieve and then shaped by a compression molding at a pressure of 1,000 kg/cm.sup.2. The shaped material was fired on a setter, made of zirconia or platinum, at a temperature of 1300.degree. to 1550.degree. C. for a period of 1 to 6 hours in air.

At a frequency of 3 GHz, dielectric constant, reciprocal value Q of dielectric loss tan .delta. (i.e., Q=1/tan .delta.), and .tau.f (temperature coefficient of resonance frequency) at temperatures of -25.degree. C. to +75.degree. C. were measured for the dielectric ceramic products thus obtained.

Starting materials are not limited only to oxides. Carbonates, such as BaCO.sub.3 or SrCO.sub.3, and hydroxide, such as Mg(OH).sub.2 may be also used, as long as they are chemically stable and eventually yield oxides.

The compositions and the properties of the above various dielectric ceramic products are listed in Table 1 below. For comparison purposes, the compositions and the properties of Comparative Examples 1 and 2 are also given in Table 1.

As seen from Table 1, substitution of SrO, MgO and/or CaO for part of BaO has been found to change .tau.f values and dielectric constant without the reduction of Q. FIG. 1 illustrates the relationship of the percentage (n) of the substitution of SrO to .tau.f in which numerals, 1, 2, 3 and 4 correspond to the numbers of samples, respectively. From the foregoing experimental data, it was found that the present invention can provide dielectric ceramics having a high dielectric constant, a small .tau.f and a high Q. The values of Q shown in Table 1 are measured at 3 GHz and the results of the measurements measurement results show that Examples of the present invention have Q values of not less than 3000. Such high Q values at 3 GHZ are equivalent to Q values of the order of 1000 or more at 1 GHZ and are sufficient use as resonators.

In Table 1, Comparative Example 1 in which BaO was replaced by a large amount of SrO beyond the range limited in the present invention has a excessively reduced Q. Further, Comparative Example 2 exhibits the properties of BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 ternary system dielectric ceramic with .tau.f=0 in which the Q of this ceramic was 3000 at 3 GHz.

Further dielectric ceramics having the compositions given in Table 2 were prepared in the same manner as Example 1 using high pure BaCO.sub.3, SrCO.sub.3, CaCO.sub.3, Sm.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, WO.sub.3, SnO.sub.2 and ZrO.sub.2 as starting materials and their properties are also shown in Table 2. In Table 2, x, y, Z and n are the same as defined in Table 1.

As shown in Table 2, addition of Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, WO.sub.3, SnO.sub.2 and/or ZrO.sub.2 was found to cause change in dielectric constant and a slight change of .tau.f. The obtained ceramics have proved to have a high dielectric constant, a low .tau.f and a high Q. The percentages of additives shown in Table 2 are represented by mole percent based on the total molar amount of BaO, MO, TiO.sub.2 and R.sub.2 O.sub.3.

As described previously, the present invention can provide dielectric ceramic compositions having a wide range of dielectric constant with a high Q (namely, a satisfactorily reduced dielectric loss), and a substantially reduced .tau.f, by changing the mixing ratio of each component within the range defined in the present invention. Those beneficial properties make the dielectric ceramics of the present invention useful as materials adapted to applications at microwave, such as receiver for a satellite broadcasting, automobile telephone or other communication or broadcasting apparatus at microwave.

Although the dielectric ceramic compositions of the present invention are described and illustrated in reference with microwave applications, they also give good utility in temperature-compensating capacitors or other applications because of its good electrical properties in a low frequency region.