Method of preparing dielectric ceramic material and dielectric resonator

Abstract: There is provided a dielectric ceramic composition wherein a main crystal phase is a perovskite-type crystal phase, comprising a complex oxide which contains, as a metal element, at least a rare earth element, Al, M (M is Sr, alternatively, Sr and Ca) and Ti, and is represented by the following composition formula: aLn 2 O x bAl 2 O 3 cMO dBaO eTiO 2 wherein Ln is a rare earth element; and a, b, c, d and e is a mole ratio in a predetermined range, and more specifically comprising a solid solution of LnAlO( X+3)/2 (3≦x≦4) and RTiO 3 (R is an alkaline earth metal containing at least Sr). This composition provides a large dielectric constant ε r and a high Q value in a high frequency region, and also lessens the variation in dielectric constant ε r, Q value, and resonance frequency temperature coefficient τ f.

Description: FIELD OF THE INVENTION The present invention relates to a method of preparing a dielectric ceramic composition and dielectric resonator which stably control dielectric constant ε r, resonator acutance Q value and temperature coefficient τ f in a high frequency region such as of microwave and millimeter-wave, and which have less variation in these characteristic values. More particularly, the invention relates to a method for preparing a dielectric ceramic material and a dielectric resonator, which are applied for high frequency regions in various resonator materials, dielectric substrate materials for MIC (monolithic IC), dielectric waveguide materials, multilayer ceramic capacitors, and the like. BACKGROUND OF THE INVENTION Dielectric ceramic materials are widely utilized in resonator materials, MIC dielectric substrate materials, dielectric waveguides, dielectric antennas, and any of various electronic components, which are employed for microwave and millimeter-wave regions in various equipment such as car telephones, cordless telephones, personal wireless units, and satellite broadcasting receivers. The dielectric ceramic materials are generally required to have the following three principal characteristics: (i) A high dielectric constant to meet a demand for size reduction of devices, because the wavelength of propagating electromagnetic wave is reduced to 1/ ε r 1/2 in dielectrics; (ii) A small dielectric loss in high frequency regions, that is, a high Q value; and (iii) A small change in resonant frequency with respect to a temperature change, that is, a stably low temperature dependency of dielectric constant ε r. Exemplary dielectric ceramic materials hitherto known are oxide ceramic materials such as BaO-TiO 2 based materials, BaO-REO-TiO 2 (wherein REO is an oxide of a rare earth element) based materials, and MgTiO 3 -CaTiO 3 based materials (see Japanese Unexamined Patent Publication Nos. 61-10806/1986 and 63-100058/1988). Although the BaO-TiO 2 based materials have a high dielectric constant ε r of 37 to 40, and a high Q value on the order of 40,000, it is difficult to attain a resonance frequency temperature coefficient τ f of zero in a single phase, and these materials suffer from a great change in the dielectric constant to a composition change, and in the temperature dependency of dielectric constant. Therefore, it is difficult to stably reduce resonance frequency temperature coefficient τ f, while maintaining a high dielectric constant and a low dielectric loss. Known as the BaO-REO-TiO 2 based materials are BaO-Nd 2 O 3 -TiO 2 based materials and BaO-Sm 2 O 3 -TiO 2 based materials. These materials, however, have an extremely high dielectric constant r of 50 to 80. Some of them have a resonance frequency temperature coefficient τ f of zero, however, they have a low Q value on the order of 5,000. As to the MgTiO 3 -CaTiO 3 based materials, Q value is large on the order of 30,000. Some of them have a resonance frequency temperature coefficient τ f of zero, however, their dielectric constants are small of 16 to 25. Accordingly, any of the above materials does not satisfy the three characteristics (i) to (iii) at the same time, which are required of dielectric materials for high frequency application. Although these materials provide a high Q value in a high frequency region, Q value at a high temperature (120 °C) is reduced considerably than Q value at room temperature (25 °C). Hence, they fail to satisfactorily enjoy the merit of high Q value, thus causing the problem of increasing the insertion loss (IL) of a resonator. On the other hand, the present applicant has proposed a dielectric ceramic composition which comprises a complex oxide containing a rare earth element, Al, Ca and Ti (see Japanese Unexamined Publication No. 6-76633/1994). This dielectric ceramic composition has excellent features that it has a high dielectric constant of 34 to 46, and a high Q value of over 20,000. On the contrary, during its manufacturing process there is a considerable variation in dielectric constant ε r, Q value, and resonance frequency temperature coefficient τ f. This makes it difficult to stably control these characteristics. The above dielectric ceramic composition also has the problem that the rate of retention of Q value at a high temperature (120°C) to Q value at room temperature (25°C) is insufficient. With the development of communication industry, there is a demand for materials having a higher Q value. Sun et al (JAPANESE JOURNAL OF APPLIED PHYSICS, vol. 37, no. 10, 1998, pages 5625-5629) discloses a solid solution system (1-x)LaAlO 3 xSrTiO 3 with perovskite structure which has dielectric behavior. Nakamura et al (FERROELECTRICS, vol. 196, 1997, pages 205-209) discloses that a xSrTiO 3 -(1-x)LaAlO 3 solid solution system with perovskite structure has dielectric behavior. Vanderah et al (JOURNAL OF THE AMERICAN CERAMIC SOCIETY, vol. 77, No. 12, 1994, pages 3125-3130) discloses that SrTiO 3 is highly soluble in LaAlO 3 and form in cubic perovskite. WO 98 43924 A (MURAKAWA SHUNICHI; KYOCERA CORP (JP)) discloses a dielectric ceramic composition which contains a certain amount of Mn for the main elements having a composition expressed by aLn 2 O x , bAl 2 O 3 , cSrO, dTiO 2 where Ln shows lanthanide series, and a, b, c and d show designated molar ratios, 3≤x≤4. JP 08 077829 A (KYOCERA CORP) discloses a dielectric ceramic composition which contains SrO and/or BaO for the main elements having a composition expressed by aLn 2 O x , bAl 2 O 3 , cCaO, dTiO 2 where Ln is the same as the above-mentioned, and a, b, c and d show certain molar ratios, 3≤x≤4. JP 11 106255 A (KYOCERA CORP) discloses a dielectric ceramic composition which contains α-Al 2 O) as crystallized phase showing a composition formula of aLn 2 O x bAl 2 O 3 , cMO, dBaO, eTiO 2 where Ln is the same as the above-mentioned, and M is the one at least selected in Ca and Sr, and a, b, c and d show certain molar ratios, 3≤x≤9. JP 11 071171 A (KYOCERA CORP) discloses a dielectric ceramic composition which shows a composition formula of aLn 2 O x , bAl 2 O 3 , cSrO, dTiO 2 where Ln is the same as the above-mentioned, and a, b, c and d show certain molar ratios, 3≤x≤4. JP 11 130528 A (KYOCERA CORP) discloses a dielectric ceramic composition which contains at least one selected from Ce, Nb, Ta, Y, V, Cr, Mo, W, Co, Ni, Cu, Zn, Sn, Bi, B and Si in a certain amount for the main elements having a composition formula expressed by aLn 2 O 3 bAl 2 O 3 cSrO, dTiO 2 . SUMMARY OF THE INVENTION According to the invention, there is provided a method of preparing a dielectric ceramic material wherein a main crystal phase is a perovskite-type crystal phase comprising a solid solution of LnAlO (x+3)/2 (Ln is a rare earth metal, and 3≤x≤4) and RTiO 3 (R is Sr and Ba or Sr, Ba and Ca) which is represented at a mole ratio of the metal element by the following composition formula: aLn 2 O x bAl 2 O 3 cMO dBaO eTiO 2 wherein M is Sr, alternatively Sr and Ca, a, b, c, d and e satisfying the following equations: 0.056≤a≤0.450; 0.056≤b≤0.450; 0.100≤c≤0.500; 0.0001≤d≤0.100; 0.100≤e≤0.470; 0.75≤b/a≤1.25; 0.75≤e/(c+d)≤1.25; a+b+c+d+e=1; the method comprising the steps of: blending a calcined powder of which the main crystal phase is LnAlO (x+3)/2 and a calcined powder of which the main crystal phase is RTiO 3 ; molding the resultant mixture; and firing the resultant molding body. This method enables to obtain a large dielectric constant ε r and high Q value, and to lessen variations in dielectric constant ε r, Q value and resonance frequency temperature coefficient τ f, thereby to provide a dielectric ceramic material capable of stably controlling these characteristic values. That is, the composition of the invention has accomplished from the point of view of that control of the main crystal phase in a dielectric ceramic composition is important in order to reduce and stably control the variation of dielectric characteristics. The present invention can thus provide a method for preparing a dielectric ceramic material, and a dielectric resonator, which have a large dielectric constant ε r, high Q value and low resonance frequency temperature dependency, and which have less variation in dielectric constant ε r, Q value and resonance frequency temperature coefficient τ f, thereby to stably control these characteristic values. The invention can also provide a method of preparing a dielectric ceramic material for high frequency application which can maintain a high retention of Q value at 120 °C, to Q value at room temperature (25 °C), as well as a dielectric resonator using this composition. The method of preparing a dielectric ceramic material of the invention is characterized in that a calcined powder of which main crystal phase is LnAlO (x+3)/2 (3≦x≦4), and a calcined powder of which main crystal phase is RTiO 3 (R is an Sr and Ba or Sr, Ba and Ca) are blended, molded and fired. With this method, the solid dissolution between the two calcined powders is facilitated, thereby to obtain a dielectric ceramic material of a single crystal phase which is substantially composed of a complex oxide crystal phase of a perovskite-type (ABO 3 type). According to the present invention, a dielectric resonator be operated by electromagnetic coupling comprises a dielectric ceramic material and a pair of input and output terminals disposed on the sides of the dielectric ceramic material, and the dielectric material comprising the dielectric ceramic composition as described. This enables to provide a resonator having excellent stability of no-load Q and resonance frequency. Other objects and advantages of the present invention will become apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram illustrating a dielectric resonator which is an embodiment of the invention; Fig. 2 is an X-ray diffraction chart of a calcined powder of LaAlO 3 in an example of the invention; Fig. 3 is an X-ray diffraction chart of a calcined powder of SrTiO 3 in the example; Fig. 4 is an X-ray diffraction chart of a mixed powder of a calcined powder of LaAlO 3 and a calcined powder of SrTiO 3 in the example (Sample No. 1); Fig. 5 is an X-ray diffraction chart of a dielectric ceramic material in the example (Sample No. 1); and DETAILED DESCRIPTION OF THE INVENTION A dielectric ceramic composition prepared according to the invention comprises a complex oxide which contains at least a rare earth element (Ln) ; Al; Sr; alternatively, Sr and Ca; Ba and Ti, and which has a perovskite-type crystal phase as a main crystal phase. The ceramic composition may be in powder, alternatively, a mass body (ceramic body) which is obtained by molding the composition into a specific shape, followed by firing. The main crystal phase can be examined by analyzing a dielectric ceramic composition with X-ray diffraction. In the invention the feature that a perovskite-type crystal phase is the main crystal phase means that based on the X-ray diffraction, the main peak of a crystal phase comprising the solid solution is higher than that of other crystal phase. Especially, it is desirable that the ceramic composition is substantially composed of the perovskite-type crystal phase alone. It is further desirable in reducing the variations of dielectric characteristic values that the perovskite-type crystal phase is composed of a solid solution represented by (Ln, R) (Al, Ti)O 3 which comprises at least LnAlO (x+3)/2 (3≦x≦4) and RTiO 3 (R is Sr and Ba or Sr, Ba and Ca). When an oxide of the respective metal elements is represented by the following composition formula: aLn 2 O x bAl 2 O 3 cMO dBaO eTiO 2 wherein Ln, M and x are as defined above, it is essential that the mole ratios, a, b, c, d and e, satisfy the following equations: 0.056≦a≦0.450; 0.056≦b≦0.450; 0.100≦c≦0.500; 0.0001≦d≦0.100 ; 0.100<e<0.470; 0.75≦b/a≦1.25; 0.75≦e/(c+d)≦1.25; and a+b+c+d+e=1 The reason why the quantity of each metal element is limited to the above range is as follows. The definition of 0.056≦a≦0.450 is due to the facts that when a<0.056, τ f increases toward the positive side, and Q value decreases; and that when a>0.450, dielectric constant ε r decreases, Q value decreases, and τ f increases toward the negative side. Most preferred is 0.078≦a≦0.325. The definition of 0.056≦b≦0.450 is due to the facts that when b<0.056, τ f increases toward the positive side, and Q value decreases; and that when b>0.450, Q value decreases. Most preferred is 0.078≦b≦0.325. The definition of 0.100≦c≦0.500 is due to the facts that when c<0.0100, Q value decreases, and τ f increases toward the negative side; and that when c>0.500, τ f increases toward the positive side, and Q value decreases. Most preferred is 0.250 ≦a≦0.47. The definition of 0.0001≦d≦0.100 is due to the fact that when d>0.100, Q value decreases. The definition of 0.100<e<0.470 is due to the facts that when e≦0.100, τ f increases toward the negative side, and Q value decreases; and that when e≧0.470, τ f increases toward the positive side, and Q value decreases. Most preferred is 0.250 ≦e≦0.422. The definition of 0.75≦b/a≦1.25 is due to the facts that when b/a<0.75, Q value decreases; and that when b/a>1.25, Q value decreases. Most preferred is 0.80≦b/a≦1.20. The definition of 0.75≦e/(c+d)≦1.25 is due to the facts that when e/(c+d)<0.75, Q value decreases; and that when e/(c+d)>1.25, Q value decreases. Most preferred is 0.80≦ e/ (c+d) ≦1.20. Consequently, the dielectric ceramic composition prepared by the method of the invention has a large dielectric constant ε r and a high Q, and causes less variation in dielectric constant ε r, Q value and resonance frequency temperature coefficient r f, thereby to allow for their stable control. In the preparation method of the invention, a calcined powder of which main crystal phase is LnAlO (x+3)/2 (3≦x≦4) which contains a rare earth element (Ln) and Al, and a calcined powder of which main crystal phase is RTiO 3 (R is Sr and Ba or Sr, Ba and Ca, and this definition shall apply hereinafter) are blended, molded and fired. Specifically, a calcined powder obtainable by calcing and milling a mixture of a rare earth element oxide and Al oxide; and a calcined powder obtainable by calcing and milling a mixture of an alkaline earth metal oxide containing at least SrO, BaO and TiO 2 are prepared; and both powder are blended, molded into a predetermined shape, and fired. This method enables to provide a dielectric ceramic material, the main crystal phase of which is a solid solution of the LnAlO (x+3)/2 and RTiO 3 , that is, a perovskite-type crystal phase represented by (Ln, R) (Al, Ti)O 3 . This dielectric ceramic material has the advantage to reduce the variation in the characteristic values of ε r, Q value and τ f. The reason why the variation in the characteristic values is reduced by the preparation method seems to be due to the following facts. In general, the raw material of a dielectric ceramic composition contains impurities, water and the like. In addition, during the manufacturing process, the impurities may be eluted in the form of ions into a solvent such as water, or precipitated in a slurry, alternatively, a heavy element may be exhausted during spray drying, thus causing a composition change. Therefore, even with a high accurate compounding, it is difficult to precisely control the ratio of the respective constituents of Ln, Al, Sr, Ca, Ba and Ti. This results in variation in characteristic values. Whereas in the preparation method, the blending of the calcined powder in the above combination allows for precise control of the ratio of the respective constituents, thus enabling to reduce the variation in characteristic values. More specifically, the preparation method may comprise the following steps (1) to (7): (1) At least a rare earth element (Ln) oxide (Ln 2 O 3 ) powder and an aluminum oxide (Al 2 O 3 ) powder, each having a high purity, are used as a starting material. The starting material is weighed so as to be a predetermined ratio, particularly, a mole ratio of 0.75≦ Al 2 O 3 /Ln 2 O 3 ≦1.25. A pure water is added thereto, and then blended and milled in wet process by means of a ball-milling treatment using zirconia balls, etc. for 1 to 100 hours, until the mixture has a mean particle diameter of 2.0 µm or smaller; (2) This mixture is dried, calcined at 1000 to 1300 °C for 1 to 10 hours, and milled, resulting in a calcined powder of which main crystal phase is LnAlO (x+3)/2 (wherein Ln is a rare earth element, and 3≦x≦4); (3) Separately, as a starting material, powder of carbonates of Sr and Ba or Sr, Ba and Ca, which are capable of generating an oxide (RO) (R) by heat treatment (e.g., calcium carbonate, strontium carbonate or barium carbonate), and powder of titanium oxide (TiO 2 ) are weighed so as to have a predetermined ratio, particularly, a mole ratio of 0.75 ≦ TiO 2 /RO≦1.25. A pure water is added thereto, and then blended and milled in wet process by means of a ball-milling treatment using zirconia balls, etc. for 1 to 100 hours, until the mixture has a mean particle diameter of 2.0 µm or smaller ; (4) This mixture is dried, calcined at 1000 to 1300 °C for 1 to 10 hours, and milled, resulting in a calcined powder of which main crystal phase is RTiO 3 ; (5) Subsequently, the resultant calcined powder of which main crystal phase of which is LnAlO (x+3)/2 , and the resultant calcined powder of which main crystal phase is RTiO 3 are blended at a predetermined ratio. The obtained mixture is blended and milled in wet process by means of a ball-milling treatment using zirconia balls, etc. for 1 to 100 hours, until the mixture has a mean particle diameter of 2.0 µm or smaller; (6) To the resultant powder, an organic binder for molding is added in an amount of 3 to 10 % by weight, which is then dehydrated and granulated by means of spray drying or the like. The granulated body is molded into a given shape by any known molding means, such as mold pressing, cold isostatic pressing or extrusion molding; and (7) The resultant molding body thus obtained is fired in the air atmosphere at a temperature of 1400 to 1700 °C for 1 to 10 hours, thereby to prepare a dielectric ceramic material of which main crystal phase is a solid solution of LnAlO (x+3)/2 and RTiO 3 . With the preparation method as described, the crystal phase is substantially composed of a solid solution of LnAlO (x+3)/2 and RTiO 3 , that is, a single phase of a perovskite-type crystal phase represented by (Ln, R) (Al, Ti)O 3 . This enables to prepare a dielectric ceramic material having excellent stability of dielectric constant ε r, Q value and τ f. Examples of the rare earth element (Ln) are Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er, Yb, and Nd. The oxide of such a rare earth element (Ln) is in the form of, for example, Ln 2 O x (3≦x≦4). Of these rare earth elements, Y, La, Sm, Gd, Dy, Er, Yb and Nd are preferred, and La and Nd are especially preferred. Using the above dielectric ceramic composition as a main constituent, ZnO, NiO, SnO 2 , CO 3 O 4 , ZrO 2 , WO 3 , LiCO 3 , Rb 2 CO 3 , Sc 2 O 3 , V 2 O 5 , CuO, SiO 2 , MgCO 3 , Cr 2 O 3 , B 2 O 3 , GeO 2 , Sb 2 O 5 , Nb 2 O 5 , or Ta 2 O 5 may be added thereto. These can be added, depending on the kind, in an amount of not more than 6 parts by weight to 100 parts by weight of the main constituent. The dielectric ceramic composition prepared by the method of the invention is most suitably used as a dielectric ceramic material of a dielectric resonator. A schematic diagram of a TE mode type dielectric resonator is shown in Fig. 1. The dielectric resonator of Fig. 1 comprises an input terminal 2 and output terminal 3 that are disposed on opposite sides in a metal case 1, and a dielectric ceramic material 4 that is composed of the above dielectric ceramic composition and disposed between the input and output terminals 2 and 3. In this TE mode type dielectric resonator, microwaves are inputted from the input terminal 2, and the microwaves are confined within the dielectric ceramic material 4, due to the reflection of the boundary between the dielectric ceramic material 4 and a free space, thereby causing resonance at a specific frequency. Signals generated at this time are electromagnetically coupled to the output terminal 3, and then outputted. Alternatively, a dielectric ceramic composition prepared by the method of the invention is, of course, applicable to TEM-mode coaxial resonators and strip line resonators, TM-mode dielectric ceramic resonators, and other resonators, all of which are not shown. Alternatively, a dielectric resonator may be constructed by directly attaching the input and output terminals 2 and 3 to the dielectric ceramic material 4. The dielectric ceramic material 4, which is a resonator medium composed of a dielectric ceramic compositions prepared by the method of the invention, is in a predetermined shape, for example, rectangular solid, cube, plate, disk, cylinder, polygonal column, or any other solid shape which allows for resonance. It is desirable in practice that the frequency of a high frequency signal inputted is from about 1 GHz to about 300 GHz, and the resonance frequency is from about 10 GHz to about 100 GHz. Thus, the use of the dielectric ceramic composition in the dielectric resonator of the invention enables to improve the stability of non-load Q and resonance frequency, as wells as the temperature stability of resonance frequency. EXAMPLES The preparation of a dielectric ceramic composition of the present invention will hereinafter be described in detail by way of examples thereof. It should be understood that the invention is not limited to these examples. Example 1 Based on the preparation method as previously described, dielectric ceramic materials were prepared through the following steps (1) to (7): (1) Powder of a rare earth element (Ln) oxide Ln 2 O x (3 ≦x≦4) and powder of aluminum oxide (Al 2 O 3 ), each having a 99 % purity or higher, were used as a starting material. These starting materials were weighed so as to have the mole ratio shown in Table 1. A pure water was added thereto and blended, which was further blended and milled in wet process by means of a ball-milling treatment until the mixture had a mean particle diameter of 2.0 µm or smaller. As an oxide Ln 2 O x of a rare earth element, Y 2 O 3 , La 2 O 3 , CeO 2 , Pr 6 O 11 , Sm 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Er 2 O 3 , Yb 2 O 3 and Nd 2 O 3 were previously prepared. (2) The resultant milled body was dried, calcined at 1200 °C for two hours, and milled, thereby to obtain a calcined powder A, the main crystal phase of which was LnAlO (x+3)/2 (3≦x ≦4), having a mean particle diameter of 0.4 to 1.5 µm. (3) Separately, the respective powder of calcium carbonate (CaCO 3 ), strontium carbonate (SrCO 3 ), barium carbonate (BaCO 3 ) and titanium oxide (TiO 2 ), as a starting material, were weighed so as to have the mole ratio shown in Table 1. A pure water was added thereto and blended. The mixture was then blended and milled in wet process by means of a ball-milling treatment until the mixture had a mean particle diameter of 2.0 µm or smaller. (4) The milled body obtained in the step (3) was calcined at 1200 °C for two hours, and then milled, thereby to obtain a calcined powder B, the main crystal phase of which was RTiO 3 , having a mean particle diameter of 0.4 to 1.5 µm. (5) The calcined powder A obtained in the step (2) and calcined powder B obtained in the step (4) were blended together with a pure water. This mixture was blended and milled in wet process by means of a ball-milling treatment until it had a mean particle diameter of 2.0 µm or smaller. (6) To the resultant slurry, a 5 % by weight of an organic binder for molding was added and granulated by means of spray drying. (7) The resultant granulated body was press-molded into a disk, at a pressure of about 1 ton/cm 2 , and then fired in the air atmosphere at a temperature of 1400 to 1700 °C for two hours, thereby to prepare a dielectric ceramic material. Each of the dielectric ceramic materials was subjected to an X-ray diffraction, showing that every sample was one which has, as a main crystal phase, a perovskite-type crystal phase composed of a solid solution of LnAlO (x-3)/2 and RTiO 3 . Each disk part (the main surface) of the resultant ceramic materials was subjected to a surface grinding and to an ultrasonic cleaning in acetone, followed by drying at 150 °C for one hour. At a measuring frequency of 3.5 to 4.5 GHz in a cylinder resonator method, the dielectric constant ε r, Q value and resonance frequency temperature coefficient τ f were measured with respect to 30 samples, and each mean value was calculated. The Q value was converted to Q value at 1 GHz, on the basis of the following relation: (Q value) × (Measuring frequency f) = Constant which has generally been established for microwave dielectric materials. The resonance frequency temperature coefficient τ f was measured in the temperature range of -40 to 85 °C. The same test as above was carried out 30 times by using the same starting materials as described. There were prepared 30 samples per test, which were regarded as one lot. The mean value of the dielectric constant ε r, Q value and resonance frequency temperature coefficient τ f were calculated per lot. By using 30 lots, the standard deviation σ of each characteristic value was calculated from the following equation: σ={Σ (w-y) 2 / (n-1) } 1/2 wherein w is the mean value of the ε r, Q value or τ f, with respect to 30 samples of each lot; y is the value obtained by dividing the sum of the ε r, Q value or τ f of each lot by 30; and n=30. The results are shown in Tables 1 to 3. The mean value of the ε r, Q value or τ f indicated in Tables 1 and 2, corresponds to the value of y. Separately, Comparative Examples were prepared as follows. That is, the respective powder of a rare earth element oxide Ln 2 O x (3≦x≦4), aluminum oxide (Al 2 O 3 ), calcium carbonate (CaCO 3 ), strontium carbonate (SrCO 3 ), barium carbonate (BaCO 3 ) and titanium oxide (TiO 2 ) were blended at the same time, which was then milled, dried, calcined, granulated, fired and ground, thereby to prepare dielectric ceramic materials. The same test as above was conducted for the resultant samples. The results are indicated by Sample Nos. 40 to 55 in Table 2. The variation in the X-ray diffraction measurement during the above process was examined. As an example thereof, the results as to Sample No. 1 is shown in Figures 2 to 5. Specifically, Figures 2 and 3 show the X-ray diffraction chart of the calcined powder A and B prepared by calcination, respectively. It can be seen from Figure 2 that the calcined powder A is powder of which main crystal phase is LaAlO 3 . It can be seen from Figure 3 that the calcined powder B is powder of which main crystal phase is SrTiO 3 . Figure 4 shows the X-ray diffraction charts of mixed powder which was obtained by blending the calcined powder A and B with the ratio of Sample No. 1. Figure 5 shows the X-ray diffraction charts of a dielectric ceramic material which was obtained by firing these mixed powder shown in Figs. 4. As seen from the results in Figure 5, between the peak location of LaAlO 3 and that of SrTiO 3 , a peak of the solid solution composed of both is detected, which shows a single crystal phase formed by a perovskite-type crystal phase (La, Sr) (Al, Ti) O 3 . As seen from the results in Tables 1 and 2, the dielectric ceramic compositions of which constitution ratio was within the scope of the invention (Sample Nos. 1 to 28), attained excellent dielectric characteristics. That is, the dielectric constant ε r was more than 30, the Q value converted to 1 GHz was more than 20,000, resonance frequency temperature coefficient τ f was within ±30 (ppm/°C), and the standard deviations of ε r, Q value and τ f were within 0.3, 3000 and 0.7 ppm/°C, respectively. On the other hand, as to the dielectric ceramic compositions of which constitution ratio was outside of the scope of the invention (Sample Nos. 29-55), the ε r or Q value was low, or the absolute value of τ f exceeded 30. In Sample Nos. 40 to 55 which were prepared by the simultaneous blending, each mean value of ε r, Q value and τ f was equivalent to that of the invention. However, the standard deviation σ of ε r was more than 1, and the standard deviation σ of τ f was more than 2 ppm/°C, resulting in a large variation. In some Comparative Examples the standard deviation σ of Q value exceeded 3000, resulting in a large variation. Example 2 Each of the calcined powder of ceramic compositions indicated in Tables 1 and 2 of Example 1 was blended in the weight percentage indicated in Table 4, followed by molding, firing, grinding in the same manner as Example 1. Measurement of the dielectric characteristics was conducted with respect to 30 samples, and the same test as Example 1 was conducted 30 times, and the standard deviation σ of dielectric characteristics was calculated. As a result, in all Samples 88 to 99, the dielectric constant ε r was more than 30, and Q value when it was converted into 1 GHz, was more than 20,000. As seen from Table 4, the standard deviations of ε r, Q value and τ f were within 0.2, 1000 and 0.2 ppm/°C, respectively. This permitted a smaller variation than Example 1. All the dielectric ceramic materials obtained in Example 2 were subjected to an X-ray diffraction measurement, which showed that they were all composed of a single crystal phase formed by a perovskite-type crystal phase.

Claims: A method of preparing a dielectric ceramic material wherein a main crystal phase is a perovskite-type crystal phase comprising a solid solution of LnAlO (x+3/2 (Ln is a rare earth metal, and 3≤x≤4) and RTiO 3 (R is Sr and Ba or Sr, Ba and Ca) which is represented at a mole ratio of the metal element, by the following composition formula: aLn 2 O x bAl 2 O 3 cMO dBaO eTiO 2 wherein M is Sr, alternatively Sr and Ca, a, b, c, d and e satisfying the following equations: 0.056≤a≤0.450; 0.056≤b≤0.450; 0.100≤c≤0.500; 0.0001≤d≤0,100; 0.100≤e≤0.470; 0.75≤b/a≤1.25; 0.75≤e/(c+d)≤1.25; a+b+c+d+e=1; the method comprising the steps of: blending a calcined powder of which the main crystal phase is LnAlO (x+3)/2 and a calcined powder of which the main crystal phase is RTiO 3 ; molding the resultant mixture; and firing the resultant molding body. A dielectric resonator to be operated by electromagnetic coupling, which comprises a dielectric ceramic material comprising a composition obtainable by the process of claim 1, and a pair of input and output terminals disposed on the sides of the dielectric ceramic material.

Inventors: MURAKAWA S, OKAWA Y, MURAKAWA, S., OKAWA, Y., Murakawa, S., Kyocera Corp. Kagoshima-Kokubu Fty., Okawa, Y., Kyocera Corp. Kagoshima-Kokubu Factory
Applicants: KYOCERA CORP, KYOCERA CORPORATION
Application year: 2000
Main classification: H01P7
IPC codes: H01P7, C04B35