Low-Loss Microwave Dielectric Material

1. Field of the Invention

The present invention relates to a low-loss microwave dielectric material suited for use in a circuit processing electric signals of microwaves, for example, in an SHF band, and to a process for preparing the same.

2. Prior Art of the Invention

A low-loss dielectric material is used for a dielectric resonator in an SHF band circuit. More specifically, the dielectric resonator is coupled with a filter, a Gunn oscillator or an FET oscillator. Thus, it is an indispensable element for stabilizing frequency. The dielectric resonator is used in a receiver for broadcasting via a broadcasting satellite or SHF broadcasting on ground. For improving the performance of the receiver, there has been a serious demand for an improvement in Q factor of the dielectric resonator. For the purpose, the dielectric material of the resonator is required to have a relative dielectric constant of 25 to 40, an unloaded Q factor of 10.sup.4 or more and a temperature coefficient of resonant frequency of within .+-.10 ppm/.degree.C. in a 10 GHz band and at about room temperature.

Examples of conventional dielectric materials practically used as a resonator satisfying the above-mentioned requirements include ceramics of, for example, Ba.sub.2 Ti.sub.9 O.sub.20, and ceramics of (Zn.sub.0.8 Sn.sub.0.2)TiO.sub.4. These materials satisfy the above-mentioned conditions as regards relative dielectric constant and the temperature coefficient of resonant frequency but not unloaded Q factor. The unloaded Q factors of the Ba.sub.2 Ti.sub.9 O.sub.20 and (Zn.sub.0.8 Sn.sub.0.2)TiO.sub.4 ceramics with the addition of 0.5-2.0 mol % Mn are about 5000 in 7-10 GHz band (see "Effect of Mn Doping on Ceramics with a Low Loss and a High Dielectric Constant in X band" by Shoichiro Nomura et al., Japanese Applied Physics Association-Conference Paper 2a-I-4 (p. 763), April, 1982). Thus, no material has been found that satisfies all the above-mentioned three conditions.

In this age expecting practical broadcasting via a broadcasting satellite in the near future, there has been an ever-increasing necessity for a dielectric material meeting all the above-mentioned conditions.

It is, therefore, an object of the present invention to provide a low-loss microwave dielectric material having not only required relative dielectric constant and temperature coefficient thereof but also a high unloaded Q factor of 10.sup.4 or more.

Another object of the present invention to provide a low-loss microwave dielectric material which is formed at a relatively lower sintering temperature so as to obtain ceramics having a high unloaded Q factor.

Further object of the present invention is to provide a process for preparing a low-loss microwave dielectric material of the kind as described above.

In accordance with one aspect of the present invention, there is provided a low-loss microwave dielectric material comprising a sinter of a solid solution of a perovskite type structure compound oxide comprising singly or mainly at least one member selected from the group consisting of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 with a small amount of Mn, less than 5 mol %, preferably in the range of 0.5-2.5 mol %.

The perovskite type structure compound oxide preferably includes Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and/or Ba(Mg.sub.1/3 Nb.sub.2/3))O.sub.3 in addition to the main component, Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and/or Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, the amount of which is preferably within 25 mol %. The amount of Mn is preferably in the range of 0.5-2.5 mol %, more preferably 1.0 mol %.

In accordance with another aspect of the present invention, there is provided a process for preparing a low-loss microwave dielectric material comprising the steps of:

(1) adding a small amount of Mn to a perovskite type structure compound oxide comprising singly or mainly at least one member selected from the group consisting of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, and

(2) sintering the resulting mixture.

The preferred sintering atmosphere is nitrogen and/or air. Mn is added preferably in the form of manganese carbonate or manganese oxide to the compound oxide. Alternatively, Mn may be added to the compound oxide by treating a powder of calcined perovskite type structure compound oxide with an aqueous solution of MnSO.sub.4.4H.sub.2 O or MnSO.sub.4. The sintering temperature is preferably 1550.degree. C. to 1600.degree. C., depending upon the composition. The sintering period of time is preferably 2 to 3 hours.

Since the low-loss microwave dielectric material of the present invention is in the form of a porcelain, the process for preparing the same may involve the two steps of calcination and sintering.

It has been recognized that a porosity of ceramic affects an unloaded Q factor. If a porosity is more than 3%, an unloaded Q factor is less than 9000. A preferable porosity is substantially within the range of 1-3% in order to obtain a high unloaded Q factor.

FIG. 1 illustrates the change of unloaded Q in accordance with the change of an amount of Mn addition;

FIG. 2 illustrates the relationship between porosity(%) and unloaded Q;

FIG. 3 illustrates the relationship of a temperature coefficient of resonant frequency and a mol % ratio of the compositions of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 with an addition of 1 mol % of Mn in 9-11 GHz band; and

FIG. 4 illustrates the dependency of a temperature coefficient of resonant frequency upon an amount of Mn addition.

The following Examples illustrate the present invention in more detail but should not be construed as limiting the scope of the invention.

In Example 1, a low-loss microwave dielectric material was prepared which is a sinter of a mixture including Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 as the main component.

BaCO.sub.3, ZnO and Ta.sub.2 O.sub.5, which are all above 99.9 wt. % in purity, and which were weighed to be in a molar oxide ratio of 1:1/3:2/3, were mixed with ethanol and ball-milled using agate balls for 5 to 10 hours, followed by drying. The resulting mixture was calcined twice or three times in a porcelain alumina crucible in an atmosphere of air at 1,000.degree. to 1,400.degree. C. for 10 hours twice or three times. The calcined oxide powder was Mn-treated with an aqueous solution of MnSO.sub.4.4H.sub.2 O. The oxide powder thus Mn-treated was again calcined in the porcelain alumina crucible in an atmosphere of air at 1200.degree. C. to 1400.degree. C. for 10 hours.

Subsequently, the resulting powder was pressure-molded under 400 kg/cm.sup.2 and sintered in an atmosphere of air at 1550.degree. C. to 1600.degree. C. for 1 to 2 hours. A dense ceramic having a porosity within 3% was obtained.

A Mn-treated ceramic of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 prepared in Example 1 according to the process of this invention was 30 in relative dielectric constant, 15,000 in unloaded Q factor and within 0.5 ppm/.degree.C. in temperature coefficient of resonant frequency in 11 GHz band.

A Mn-treated ceramic of each of the following compounds in Examples 2-4 had the characteristics listed in Table 1.

A 1 mol % Mn-added sinter of Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 prepared in Example 2 was 26 in relative dielectric constant, 17,000 in unloaded Q factor and within 5 ppm/.degree.C. in temperature coefficient of resonant frequency in 11 GHz band.

FIG. 1 illustrates the change of unloaded Q in accordance with the change of an amount of Mn addition in Examples 1 and 2. The result illustrated in FIG. 1 shows that the amount was preferably within a range of 0.5-5 mol %, and more preferably in the range of 0.5-2.5 mol % and that the most preferable unloaded Q was obtained at 1 mol %.

FIG. 2 illustrates the relationship between porosity(%) and unloaded Q in 10-11 GHz band in Example 2. The result in FIG. 2 shows that a preferable porosity was within the range of 1-3% in order to obtain an unloaded Q of more than 9000.

In Example 5, a low-loss microwave dielectric material was prepared which is a sinter of a mixture including Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 as the main component and Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3.

Substantially the same procedures as in Example 1 except that BaCO.sub.3, MgO, ZnO, Ta.sub.2 O.sub.3 and Nb.sub.2 O.sub.3 were used as the raw materials in a molar proportion of 1:0.8/3:0.2/3:0.8/3:0.2/3were repeated to prepare a dense porcelain having a porosity of 2%. The low-loss microwave dielectric material of the present invention can be in the form of a ceramic having a high Q factor by the addition of a small amount of Mn (1-2 mol %)to the raw materials or by the treatment of the calcined powder of the raw materials with a Mn solution. Without Mn addition, a dense ceramic cannot be obtained even if sintering is carried out at 1600.degree. C. for any long period of time, and the Q factor attainable is at most 6,000.

A sinter prepared in the form of a ceramic according to the process of this invention can be fabricated into a desired shape by means of a diamond cutter.

Dielectric materials prepared from Mn-added mixtures of compound oxide such as a combination of (1-x)Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and xBa(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, a combination of (1-x)Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and xBa(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, a combination of (1-x)Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and xBa(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, a combination of (1-x)Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and xBa(Zn.sub.1/3 Ta.sub.2/3)O.sub.3, or a combination of (1-x)Ba(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and xBa(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, wherein 0.ltoreq.x.ltoreq.1, are, in general, low in temperature coefficient of relative dielective constant.

Examples 6-9 in Table 2 show these dielectric materials. Further, in Table 2, the characteristics in Example 5 are also shown.

Especially, ceramics prepared from Mn-added mixtures of compound oxide having a composition close to Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 (Example 6) or Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 (Example 8) are within +5 ppm/.degree.C. in temperature coefficient of resonant frequency.

FIG. 3 illustrates the relationship of a temperature coefficient of resonant frequency and a mol % ratio of the compositions of Ba(Zn.sub.1/3 Ta2/3)O.sub.3 and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 with an addition of 1 mol % of Mn in 9-11 GHz band. FIG. 3 shows that the temperature coefficient was within the range of 0-4 ppm/.degree.C.

A Q-factor improvement by Mn addition applies to Ba.sub.2 Ti.sub.9 O.sub.20 and Zn.sub.0.8 Sn.sub.0.2 TiO.sub.4 ceramics which have conventionally been employed as resonator material. 1,400.degree. C.-sintered ceramics respectively prepared from a 1 mol % Mn-added mixture of Ba.sub.2 Ti.sub.9 O.sub.20 and a 0.5 mol % Mn-added mixture of Zn.sub.0.8 Sn.sub.0.2 TiO.sub.4 are 5,000 and 6,000, respectively, in unloaded Q factor in 10 GHz band, which are higher than those of the corresponding conventional ceramics.

Mn addition in the preparation of oxide ceramics not only favorably affects sinterability to comparatively lower the required sintering temperature, but also advantageously heightens the unloaded Q factor in an SHF band.

Furthermore, a temperature coefficient of resonant frequency can finely be adjusted by adequately controlling an amount of Mn addition. FIG. 4 illustrates the dependency of a temperature coefficient of resonant frequency upon an amount of Mn addition in 9-11 GHz band.

The dielectric material of the present invention is simple in preparation as compared with those materials of single crystal, and, hence, is adapted to mass production.

The dielectric material of the present invention is quite stable in air and satisfactory in mechanical strength inherently in high temperature sintered-ceramics. Further to the advantage of preparation of the dielectric material of this invention, Mn addition enables the sintering temperature to be lowered, so that various defects conventionally involved in sintering above 1,600.degree. C. can be obviated such as considerable consumption of a furnace material, a raw material container and a heating body and possible infiltration and diffusion of impurities into the desired product during the course of sintering.