Abstract:
Disclosed herein is a microwave dielectric ceramic composition which comprises as the main ingredient, a composition represented by xMgTiO 3  ·(1-x)CaTiO 3  (0.93≦x ≦0.95), to which 3-9 wt % of ZnO, 2-4 wt % of Ta 2  O 5 , 1-6 wt % of Nb 2  O 5 , 0.1-6 wt % of MnO 2 , 3-12 wt % of Al 2  O 3  or 3-12 wt % of Co 2  O 3  to xMgTiO 3  ·(1-x)CaTiO 3 . And the microwave dielectric ceramic compositions have a Qu value in the range of 3000 to 4600 (at 6 GHz), a temperature coefficient of the resonance frequency (τf) in the range of -10 to +10 ppm/°C. and relative dielectric constant (ε r ) in the range of 18 to 22. The dielectric ceramic compositions are capable of reducing τf approximately to zero or optionally and stably controlling τf to a desired value both to positive and negative sides around zero as the center while maintaining ε r  and Qu within practical characteristic ranges by controlling the addition (constitution) ratio of the specified oxides. Further, even if the sintering temperature is changed within a wide range, the sintering products of high density and high quality can be attained due to the addition of the specified oxides.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a microwave dielectric ceramic composition and, more specifically, it relates to a microwave dielectric ceramic composition capable of reducing a temperature coefficient of a resonance frequency (hereinafter simply referred to as τf) while maintaining non-load Q (hereinafter simply referred to as Qu) and, further, capable of optionally controlling τf to positive and negative sides around zero as the center by controlling the mixing ratio of CaTiO 3  or a specified oxide. The present invention also relates to a microwave dielectric ceramic composition and having high quality even if it is sintered at a low temperature due to the addition of a specified oxide or with less scattering in the quality even if it is sintered in a wide temperature range. 
     The present invention is applicable to impedance matching or the like of dielectric resonators, microwave integrated circuit boards and various kinds of microwave circuits in a microwave region. 
     Since the dielectric loss of a microwave dielectric ceramic composition (hereinafter simply referred to as a dielectric ceramic composition) tends to increase as the working frequency goes higher, a dielectric ceramic composition of high Qu in a microwave frequency region has been demanded. 
     As conventional dielectric ceramic materials, a dielectric ceramic composition of a crystal structure containing two phases, that is, a perovskite phase and an ilmenite phase (Japanese Patent Laid-Open Hei 2-129065) and a dielectric ceramic composition containing a predetermined amount of CaTiO 3  in MgTiO 3  and TiO 2  (Japanese Patent Laid-Open Sho 52-118599) have been known. 
     However, the former dielectric ceramic composition obtained other ingredients such as Nd 2  O 3 , La 2  O 3 , PbO, ZnO in a great amount and, in addition, the value Qu is not always high. The latter dielectric ceramic composition contains TiO 2  as the essential ingredient and τf changes greatly as from +87 to -100 within a range for the mixing amount of CaTiO 3  from 3 to 10% by weight to bring about a problem such as a difficulty for the control to a small value around zero. 
     The present invention intends to solve the foregoing problem and it is an object of the present invention to provide a dielectric ceramic composition capable of reducing τf approximately to zero or optionally and stably controlling τf to a desired value both to positive and negative sides around zero as the center while maintaining ε r  (relative dielectric constant) and Qu within practical characteristic ranges by controlling the addition (constitution) ratio of specified oxides (ZnO, Ta 2  O 5 , Nb 2  O 5 , MnO 2 , Al 2  O 3  or Co 2  O 3 ) and, further, having a high quality even if it is sintered at a low temperature or even if it is sintered in a wide temperature range due to the addition of a specified oxide. 
     SUMMARY OF THE INVENTION 
     The present inventor has made various studies on a composition of a dielectric ceramic composition capable of approaching τf to zero while maintaining a high Qu value and having stable quality even if sintering temperature varies and, as a result, has accomplished the present invention based on the finding that the foregoing drawback can be overcome by adjusting the addition ratio of CaTiO 3  and specified oxide. 
     Namely, the dielectric ceramic composition according to the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x)CaTiO 3  (0.925≦x≦0.950), to which 3 to 9 parts by weight (hereinafter referred to as % by weight) of ZnO is added and incorporated based on 100 parts by weight of xMgTiOa·(1-x)CaTiO 3 . 
     If x is smaller than 0.925, τf takes a large positive value and Qu is lowered, whereas τf takes a large negative value if x exceeds 0.950, which are not preferred. In particular, it is preferred that x is from 0.935 to 0.945 and the addition amount of ZnO is 6% by weight, since Qu is from 3900 to 4100, τf is -5 to +5 (ppm/°C.) and ε r  is about 21, high quality can be ensured even by sintering at a low temperature and stability to the sintering temperature is obtained. 
     As the mixing ratio of CaTiO 3  increases, τf increases from negative to positive side (FIG. 3) and ε r  increases (FIG. 2), while Qu tends to decrease (FIG. 1). Further, as shown in FIG. 7, a sintering product of high density can be produced even by sintering at a low temperature (for example, at 1325° to 1350° C.) and, accordingly, stable quality can be attained even if the sintering temperature changes variously (or even if sintering is conducted at a low temperature) (FIGS. 4-11). 
     In view of the above, within an appropriate mixing range for CaTiO 3  and ZnO described above, a balance is attained for the characteristics and stable quality can be obtained. 
     The dielectric ceramic composition according to another aspect of the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x) CaTiO 3  (in which 0.923≦x≦0.940), to which 2 to 4 parts by weight (hereinafter referred to as % by weight) of Ta 2  O 5  is added and incorporated based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 . 
     If x is smaller than 0.923, τf takes a large positive value and Qu decreases, whereas if x exceeds 0.940, τf takes a large negative value, which are not preferred. Particularly, it is preferred that x is from 0.927 to 0.935 and the addition amount of Ta 2  O 5  is 3% by weight, since Qu is from 3920 to 4260, τf is from -8.5 to +2.3 (ppm/°C.), ε r  is from 19.7 to 21.6 and the scattering in the characteristics is extremely small even if sintering temperature is varied. 
     As the mixing ratio of CaTiO 3  increases, τf increases from negative to positive value (FIG. 16), ε r  increases (FIG. 15), while Qu tends to reduce (FIG. 14). Further, as shown in FIG. 23, sintering products at high density can be produced even if sintering is applied at a low temperature (for example, at 1300°-1325° C.) due to the addition of Ta 2  O 5 . Further, products of stable quality can be obtained if the sintering temperature is varied within a range of 1300° to 1400° C. (FIGS. 17-23). 
     In view of the above, within the appropriate mixing range of CaTiO 3  and Ta 2  O 5  described above, the characteristics are well-balanced and products of stable quality can be obtained. 
     The dielectric ceramic composition according to other aspect of the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x) CaTiO 3  (in which 0.925≦x≦0.940), to which 1 to 6 parts by weight (hereinafter referred to as % by weight) of Nb 2  O 5  is added and incorporated based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 . 
     If x is smaller than 0.925, τf takes a large positive value and Qu decreases, whereas τf takes a large negative value if x exceeds 0.940, which are not preferred. Particularly, it is preferred that x is 0.930 and the addition amount of Nb 2  O 5  is 2% by weight, since Qu is from 3980 to 4320 τf is from -3.69 to +0.52 ppm/°C. and ε r  is from 19.05 to 21.5 when sintering is applied at 1275°-1400° C., and products of high quality can be obtained stably even if sintering is applied within a wide temperature range. 
     As the mixing ratio of CaTiO 3  increases, τf increases from negative to positive value (FIG. 28), ε r  increases (FIG. 27), while Qu tends to decrease (FIG. 26). Further, as shown in FIGS. 29-31 although Qu and τf do not vary so much but τf tend to decrease due to the addition of Nb 2  O 5 . Particularly, at the mixing ratio of CaTiO 3  of 0.07, the addition amount of Nb 2  O 5  of about 2% by weight is extremely preferred since τf is about 0 ppm/°C. Further, as shown in FIGS. 32-36 and Tables 5 and 6, sintering products of stable performance and high sintering density can be produced even if sintering is applied within a wide temperature range. Accordingly, addition amount of Nb 2  O 5 , particularly, of 1 to 4% by weight, (more particularly, about 2% by weight) is excellent in view of further excellent balance of performance and stable sintering product. 
     In view of the above, within the appropriate mixing range of CaTiO 3  and Nb 2  O 5  described above, products of excellent performances with excellent balance and of stable quality can be obtained. 
     The dielectric ceramic composition according to other aspect of the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x)CaTiO 3  (in which 0.93≦x≦0.95), to which 0.1 to 6 parts by weight (hereinafter referred to as % by weight) of MnO 2  is added and incorporated based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 . 
     If x is smaller than 0.93, τf takes a large positive value and Qu decreases, whereas τf takes a large negative value if x exceeds 0.95, which are not preferred. Particularly, if x is 0.94 and the addition amount of MnO 2  is 3 to 6% by weight, Qu is from 3080 to 3890, τf is from -2.60 to +1.78 ppm/°C., and ε r  is from 19.04 to 20.95 when sintering is applied at 1350°-1425° C. as shown in Table 8, and products of high quality can be obtained even if sintering is applied within a wide temperature range. As the mixing ratio of CaTiO 3  increases, τf increases from negative to positive value (Table 7, FIGS. 40, 44, 48), ε r  increases (Table 7, FIGS. 39, 43, 47), while Qu tends to decrease (Table 7, FIGS. 38, 42 and 46). Further, in a case of adding MnO 2 , Qu tends to decrease (FIG. 38), ε r  (within a range of 1375° to 1425° C.) shows no substantial change (FIG. 39), τf tends to decrease (FIGS. 40, 48) and the sintering density tends to increase (FIGS. 41, 49). Particularly, at the mixing ratio of CaTiO 3  is 0.06, the addition amount of MnO 2  of about 3% by weight is extremely preferred since τf is about 0 ppm/°C. even if the sintering temperature is greatly varied as 1325° to 1425° C. as shown in FIG. 40. Further, as shown in FIGS. 38-49 and Tables 8-10, sintering products with stable performance and high sintering density can be produced even if sintering is applied within a wide temperature range (particularly from 1375° to 1425° C.). 
     Further, in a case where the mixing ratio of CaTiO 3  is 0.05, τf is as small as about -10 ppm/°C. even within the range for the addition amount of MnO 2  from 0.1 to 0.6. On the other hand, in a case where the mixing ratio of CaTiO 3  is 0.07, τf is as large as +10 ppm/°C. within the addition amount range of MnO 2  of 3 to 6% by weight. Accordingly, the mixing ratio of CaTiO 3  of about 0.06 and the addition amount of MnO 2 , particularly, of 1 to 6% by weight (more particularly, about 3% by weight) are more excellent in view of further excellent balance of performance and stable sinterability. 
     In view of the above, within the appropriate mixing range of CaTiO 3  and MnO 2  described above, products of excellent performance with well-balancing thereof and of stable quality can be obtained. 
     The dielectric ceramic composition according to other aspect of the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x)CaTiO 3  (in which 0.93≦x≦0.95), to which 3 to 12 parts by weight (hereinafter referred to as % by weight) of Al 2  O 3  is added and incorporated based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 . 
     If x is smaller than 0.93, τf takes a large positive value and Qu decreases, whereas τf takes a large negative value if x exceeds 0.95, which are not preferred. 
     As the addition amount of Al 2  O 3  increases Qu tends to increase (FIG. 51), and ε r , τf and the sintering density tend to decrease (FIGS. 52-54). Particularly, if the addition amount of Al 2  O 3  is 3 to 9% by weight and the sintering temperature is from 1300° to 1375° C., Qu is 3360 to 3730, ε r  is 18.4 to 20.6, τf is -2.2 to +2.6 ppm/°C., the sintering density is 3.57 to 3.82 g/cm 3  as shown in Table 11 and FIGS. 51-54, and products of high quality can be obtained stably even if sintering is applied within a wide temperature range. Particularly, τf scarcely fractuates (namely, concentrating to -2-+3 ppm/°C.) and shows a value around 0 in each of the cases. 
     In view of the above, products of excellent performance described above and excellent balance thereof and of stable quality can be obtained by the addition amount of Al 2  O 3  (3-9% by weight) and in a wide and appropriate sintering temperature range (1300°-1375° C.). 
     The dielectric ceramic composition according to other aspect of the present invention comprises, as the main ingredient, a composition represented by xMgTiO 3  ·(1-x)CaTiO 3  (in which 0.93≦x≦0.95), to which 3 to 12 parts by weight (hereinafter referred to as % by weight) of Co 2  O 3  is added and incorporated based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 . 
     If x is smaller than 0.93, τf takes a large positive value and Qu decreases, whereas τf takes a large negative value if x exceeds 0.95, which are not preferred. 
     If the addition amount of Co 2  O 3  is less than 3% by weight, τf increases toward the positive side and Qu decreases if the addition amount exceeds 12% by weight, which are not preferred. On the other hand, if the addition amount is from 3 to 12% by weight, τf can be reduced to a value around zero while concentrating Qu and ε r  within practical ranges with no substantial reduction. 
     Then, as the mixing ratio of CaTiO 3  increases, it tends to decrease Qu, increase ε r  and turn τf from negative to positive value (Table 7). Further, as the addition amount of Co 2  O 3  increases, Qu tends to decrease (FIG. 64), ε r  tends to decrease slightly although shows no substantial change (FIG. 65), τf tends to turn to negative side (FIG. 66) and the sintering density tend to increase (FIG. 67). 
     Particularly, if the addition amount of Co 2  O 3  is 6 wt % and sintering is applied at 1350° to 1450° C., Qu is 3780  to 3970, τf is -0.74 to -0.03 ppm/°C., ε r  is 20.4 to 20.8 and the sintering density is 3.83 to 3.89 g/cm 3  as shown in Table 12 and FIGS. 64-71, and each of the performances less scatters and excellent performances is exhibited even sintering is applied within a wide temperature range of 100° C. In particular, τf takes an extremely small absolute value around 0 ppm/°C. and shows a value of about 0.5 (ppm/°C.)/1 wt % of Co 2  O 3 , and fine control for τf can be done extremely easily. 
     In view of the above, products having such excellent performance and well balance thereof, as well as capable of easy fine control for τf and of stable quality can be attained by the addition amount of Co 2  O 3  and in a wide and appropriate sintering temperature range. 
     As has been described above, in the dielectric ceramic composition according to the present invention, τf can be approached to zero or optionally controlled to a desired value between both of positive and negative sides around 0 (zero) as a center by controlling the blending ratio of CaTiO 3  while maintaining Qu and ε r  within practical (high) ranges of characteristics, as well as τf can be controlled stably around 0 (zero). Further, even if the sintering temperature is changed within a wide range, sintering products of high density and high quality can be attained due to the addition of ZnO 2 , Ta 2  O 5 , Nb 2  O 5 , MnO 2 , Al 2  O 3  or Co 2  O 3 . In the case of adding ZnO, sintering products of high quality can be obtained even if sintering is applied at a relatively low temperature. Further, in a case of adding Co 2  O 3 , τf can be finely controlled easily. 
     Accordingly, the mixing ratio of CaTiO 3  and specific oxides (ZnO and so on) can be varied depending on the purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing a relation between x and Qu in a (xMgTiO 3  ·(1-x)CaTiO 3  +6 wt % ZnO) ceramic composition produced at a sintering temperature of 1350° C. 
     FIG. 2 is a graph showing a relation between x and ε r  in the ceramic composition shown in FIG. 1. 
     FIG. 3 is a graph showing a relation between x and τf in the ceramic composition shown in FIG. 1. 
     FIG. 4 is a graph showing a relation between the amount of ZnO and Qu in a (0.94MgTiO 3  ·0.06CaTiO 3  +(0-9) wt % ZnO) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 5 is a graph showing a relation between the amount of ZnO and ε r  in the ceramic composition shown in FIG. 4. 
     FIG. 6 is a graph showing a relation between the amount of ZnO and τf in the ceramic composition shown in FIG. 4. 
     FIG. 7 is a graph showing a relation between the amount of ZnO and the sintering density in the ceramic composition shown in FIG. 4. 
     FIG. 8 is a graph showing a relation between the sintering temperature and Qu in the (0.94MgTiO 3  ·0.06CaTiO 3  +(0-9) wt % ZnO) ceramic composition. 
     FIG. 9 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 8. 
     FIG. 10 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 8. 
     FIG. 11 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 8. 
     FIG. 12 is a graph showing the result of X-ray diffraction of a (0.95MgTiO 3  ·0.05CaTiO 3  +6 wt % ZnO) ceramic composition. 
     FIG. 13 is a graph showing the result of X-ray diffraction of a (0.95MgTiO 3  ·0.05CaTiO 3 ) ceramic composition. 
     FIG. 14 is a graph showing a relation between x and Qu in a (xMgTiO 3  ·(1-x)CaTiO 3  +3 wt % Ta 2  O 5 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 15 is a graph showing a relation between x and ε r  in the ceramic composition shown in FIG. 14. 
     FIG. 16 is a graph showing a relation between x and τf in the ceramic composition shown in FIG. 14. 
     FIG. 17 is a graph showing a relation between the amount of Ta 2  O 5  and Qu in a (0.93MgTiO 3  ·0.07CaTiO 3  +(2-4) wt % Ta 2  O 5 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 18 is a graph showing a relation between the amount of Ta 2  O 5  and ε r  in the ceramic composition shown in FIG. 17. 
     FIG. 19 is a graph showing a relation between the amount of Ta 2  O 5  and τf in the ceramic composition shown in FIG. 17. 
     FIG. 20 is a graph showing a relation between the sintering temperature and Qu in the (0.93MgTiO 3  ·0.07CaTiO 3  +(0-4) wt % Ta 2  O 5 ) ceramic composition. 
     FIG. 21 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 20. 
     FIG. 22 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 20. 
     FIG. 23 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 20. 
     FIG. 24 is a graph showing the result of X-ray diffraction of a xMgTiO 3  ·(1-x)CaTiO 3  +3 wt % Ta 2  O 5 , the sintering temperature: 1300° C.) ceramic composition. 
     FIG. 25 is a graph showing the result of X-ray diffraction of a xMgTiO 3  ·(1-x)CaTiO 3  +3 wt % Ta 2  O 5 , the sintering temperature: 1350° C.) ceramic composition. 
     FIG. 26 is a graph showing a relation between x and Qu in a (xMgTiO 3  ·(1-x)CaTiO 3  +2 wt % Nb 2  O 5 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 27 is a graph showing a relation between x and ε r  in the ceramic composition shown in FIG. 26. 
     FIG. 28 is a graph showing a relation between x and τf in the ceramic composition shown in FIG. 26. 
     FIG. 29 is a graph showing a relation between the amount of Nb 2  O 5  and Qu in a (0.93MgTiO 3  ·0.07CaTiO 3  +(0-6) wt % Nb 2  O 5 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 30 is a graph showing a relation between the amount of Nb 2  O 5  and ε r  in the ceramic composition shown in FIG. 29. 
     FIG. 31 is a graph showing a relation between the amount of Nb 2  O 5  and τf in the ceramic composition shown in FIG. 29. 
     FIG. 32 is a graph showing a relation between the amount of (0-6 wt %) Nb 2  O 5  and the sintering density in the ceramic composition shown in FIG. 29. 
     FIG. 33 is a graph showing a relation between the sintering temperature and Qu in the (0.93MgTiO 3  ·0.07CaTiO 3  +(1-6) wt % Nb 2  O 5 ) ceramic composition. 
     FIG. 34 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 33. 
     FIG. 35 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 33. 
     FIG. 36 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 33. 
     FIG. 37 is a graph showing the result of X-ray diffraction of a (0.93MgTiO 3  ·0.07CaTiO 3  +(2, 4 or 6 wt %) Nb 2  O 5 ) ceramic composition. 
     FIG. 38 is a graph showing a relation between the amount of MnO 2  and Qu in a (0.94MgTiO 3  ·0.06CaTiO 3  +0-6 wt % MnO 2 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 39 is a graph showing a relation between the amount of MnO 2  and ε r  in the ceramic composition shown in FIG. 38. 
     FIG. 40 is a graph showing a relation between the amount of MnO 2  and τf in the ceramic composition shown in FIG. 38. 
     FIG. 41 is a graph showing a relation between the amount of 0-6 wt % MnO 2  and the sintering density in the ceramic composition shown in FIG. 38. 
     FIG. 42 is a graph showing a relation between the sintering temperature and Qu in the (0.95MgTiO 3  ·0.05CaTiO 3  +(0-0.6) wt % MnO 2 ) ceramic composition. 
     FIG. 43 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 42. 
     FIG. 44 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 42. 
     FIG. 45 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 42. 
     FIG. 46 is a graph showing a relation between the sintering temperature and Qu in the (0.93MgTiO 3  ·0.07CaTiO 3  +(0-6) wt % MnO 2 ) ceramic composition. 
     FIG. 47 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 46. 
     FIG. 48 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 46. 
     FIG. 49 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 46. 
     FIG. 50 is a graph showing the result of X-ray diffraction of a (0.94MgTiO 3  ·0.06CaTiO 3  +(0, 3 or 6 wt %) MnO 2 ) ceramic composition. 
     FIG. 51 is a graph showing a relation between the amount of Al 2  O 3  and Qu in a (0.94MgTiO 3  ·0.06CaTiO 3  +0-12 wt % Al 2  O 3 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 52 is a graph showing a relation between the amount of Al 2  O 3  and ε r  in the ceramic composition shown in FIG. 51. 
     FIG. 53 is a graph showing a relation between the amount of Al 2  O 3  and τf in the ceramic composition shown in FIG. 51. 
     FIG. 54 is a graph showing a relation between the amount of 3-12 wt % Al 2  O 3  and the sintering density in the ceramic composition shown in FIG. 51. 
     FIG. 55 is a graph showing a relation between the sintering temperature and Qu in the (0.94MgTiO 3  ·0.06CaTiO 3  +(3-12) wt % Al 2  O 3 ) ceramic composition. 
     FIG. 56 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 55. 
     FIG. 57 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 55. 
     FIG. 58 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 55. 
     FIG. 59 is a graph showing a relation between the amount of Al 2  O 3  and the average grain size of sintering product in the (0.94MgTiO 3  ·0.06CaTiO 3  +3-12 wt % Al 2  O 3 ) ceramic composition produced at a sintering temperature of 1350° C. 
     FIG. 60 is a graph showing a relation between the sintering temperature and the average grain size of sintering product in the (0.94MgTiO 3  ·0.06CaTiO 3  +12 wt % Al 2  O 3 ) ceramic composition produced at a sintering temperature of 1350° C. 
     FIG. 61 is the electron microscopic photograph (1000 magnification ratio) which explains the structure of crystals of the (0.94MgTiO 3  ·0.06CaTiO 3  +6 wt % Al 2  O 3 ) ceramic composition. 
     FIG. 62 is the electron microscopic photograph (1000 magnification ratio) which explains the structure of crystals of the (0.94MgTiO 3  ·0.06CaTiO 3  +12 wt % Al 2  O 3 ) ceramic composition. 
     FIG. 63 is a graph showing the result of X-ray diffraction of a (0.94MgTiO 3  ·0.06CaTiO 3  +3, 6, 9 or 12 wt % Al 2  O 3 ) ceramic composition. 
     FIG. 64 is a graph showing a relation between the amount of Al 2  O 3  and Qu in a (0.94MgTiO 3  ·0.06CaTiO 3  +3-12 wt % Co 2  O 3 ) ceramic composition produced by sintering at each of sintering temperatures. 
     FIG. 65 is a graph showing a relation between the amount of 3-12 wt % Co 2  O 3  and ε r  in the ceramic composition shown in FIG. 64. 
     FIG. 66 is a graph showing a relation between the amount of 3-12 wt % Co 2  O 3  and τf in the ceramic composition shown in FIG. 64. 
     FIG. 67 is a graph showing a relation between the amount of 3-12 wt % Co 2  O 3  and the sintering density in the ceramic composition shown in FIG. 64. 
     FIG. 68 is a graph showing a relation between the sintering temperature and Qu in the (0.94MgTiO 3  ·0.06CaTiO 3  +3-12 wt % Co 2  O 3 ) ceramic composition. 
     FIG. 69 is a graph showing a relation between the sintering temperature and ε r  in the ceramic composition shown in FIG. 68. 
     FIG. 70 is a graph showing a relation between the sintering temperature and τf in the ceramic composition shown in FIG. 68. 
     FIG. 71 is a graph showing a relation between the sintering temperature and the sintering density in the ceramic composition shown in FIG. 68. 
     FIG. 72 is a graph showing a relation between the sintering temperature and the average grain size in the ceramic composition shown in FIG. 68. 
     FIG. 73 is a graph showing the result of X-ray diffraction of a (0.94MgTiO 3  ·0.06CaTiO 3  +3, 6 or 12 wt % Co 2  O 3 ) ceramic composition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be explained specifically by way of examples. 
     EXAMPLE 1 
     MgO powder (99.4% purity), CaCO 3  powder for CaO (99% purity), TiO 2  powder (99.98% purity) and ZnO powder (99.5% purity) as the starting material were weighted each by a predetermined amount (about 600 g in total) and mixed so as to provide a composition in which each of x and y varies in the formula xMgTiO 3  ·(1-x)CaTiO 3  +y wt % ZnO (in which y represents parts by weight of ZnO based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 ) as shown in Table 1 and FIGS. 1-4. Then, after dry mixing in a mixer (20 to 30 min.) and primary pulverization, they were calcined in an atmospheric air at a temperature of 1100° C. for 2 hours. Subsequently, an appropriate amount of an organic binder (29 g) and water (300-400 g) was added to the calcined powder, which was pulverized together with alumina balls of 20 mmΦ at 90 rpm for 23 hours. Then, the pulverizates were pelletized in vacuum freeze drying (about 0.4 Torr, at 40°-50° C. for about 20 hours), and the thus pelletized raw material was molded into a cylindrical shape of 19 mm Φ×11 mmt (thickness) under a pressing pressure of 1,000 kg/cm 2 . 
     Subsequently, the molding product was degreased in an atmospheric air at 500° C. for 3 hours and then sintered at each of temperatures with a range of 1300° to 1425° C. 
     
                                           TABLE 1__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 + 0˜9 wt % ZnO] ceramiccomposition                      Rel.   Amount        Calcining             Sinter.  dielect.                          τ f                               Sint.   of ZnO        temp.             temp.                 Qu   const.                          (ppm/                               densityNo.   x    (wt %)        (°C.)             (°C.)                 (6 GHz)                      ε r                          °C.)                               (g/cm.sup.3)__________________________________________________________________________1  0.950   0    1100 1350                 3540 19.00                          -21.1                               --2  0.950   3    1100 1350                 4030 20.48                          -10.4                               3.8633  0.950   6    1100 1350                 4090 20.39                          -14.1                               3.9114  0.950   9    1100 1350                 --   --  --   --5  0.9475   0    1100 1350                 2880 16.80                          -16.9                               3.3866  0.9475   3    1100 1350                 4270 20.05                          -5.07                               3.7927  0.9475   6    1100 1350                 4300 20.13                          -9.71                               3.8478  0.9475   9    1100 1350                 4360 20.05                          -14.7                               3.8959  0.943   0    1100 1350                 3030 17.16                          -11.4                               3.39410 0.943   3    1100 1350                 4370 19.49                          -7.26                               3.71011 0.943   6    1100 1350                 4140 20.38                          -5.01                               3.84912 0.943   9    1100 1350                 4210 20.47                          -8.50                               3.90613 0.940   0    1100 1350                 3820 18.27                          0.19 3.48014 0.940   3    1100 1350                 3720 21.52                          19.0 3.83415 0.940   6    1100 1350                 4000 20.94                          0.50 3.88816 0.940   9    1100 1350                 3680 21.56                          15.3 3.94417 0.935   0    1100 1350                 3890 18.39                          7.85 3.46718 0.935   3    1100 1350                 3530 21.39                          21.9 3.79819 0.935   6    1100 1350                 3900 21.25                          5.32 3.89320 0.935   9    1100 1350                 3640 21.83                          17.2 3.95421 0.930   0    1100 1350                 3820 19.54                          15.1 3.57722 0.930   3    1100 1350                 3610 21.02                          17.2 3.78923 0.930   6    1100 1350                 3690 21.64                          16.6 3.89624 0.930   9    1100 1350                 3780 21.55                          8.86 3.94225 0.925   0    1100 1350                 3110 18.82                          17.3 3.46726 0.925   3    1100 1350                 3600 21.13                          21.6 3.75827 0.925   6    1100 1350                 3670 21.92                          21.1 3.89728 0.925   9    1100 1350                 3660 21.89                          16.6 3.948__________________________________________________________________________ 
    
     shown in each of the figures of 4 hours and, finally, both end faces were polished to form cylindrical members each of about 16 mmΦ×8 mmt (thickness) as dielectric specimens. 
     Then, measurement was conducted for each of the specimens, regarding ε r  (relative dielectric constant), Qu and τf by a parallel plate dielectric cylindrical resonator method (TE 011  MODE) or the like and, further, sintering density by an Archimedes method. τf was measured in a temperature region of 20° C. to 80° C. and calculated as τf=((f(80° C.)-f(20° C.))/(f(20° C.)×ΔT)·ΔT:80° C.-20° C.=60° C. The resonance frequency is 6 GHz. The results are shown in FIGS. 1-11. As an example, results of X-ray diffraction is shown for a case of 0.95MgTiO 3  ·0.05CaTiO 3  in FIG. 12 (containing 6% by weight of ZnO based on 0.95 MgTiO 3  ·0.05CaTiO 3 , sintered at 1300° C. for 4 hours) and in FIG. 13 (not containing ZnO, sintered at 1360° C. for 4 hours). 
     As can be seen from the results, the value Qu tends to decrease if x in xMgTiO 3  ·(1-x)CaTiO 3  is small, whereas τf and ε r  tend to increase toward the positive side. The sintering density tends to increase as the sintering temperature is higher, whereas it becomes flat relative to the sintering temperature as the addition amount of ZnO is increased. 
     At a sintering temperature of 1350° to 1425° C., a preferred range for x is from 0.925 to 0.950, since practical characteristic ranges are shown for τf as +23 to -17 ppm/°C., for ε r  as 19 to 22 and for Qu as 3330 to 4390. Particularly, if x=0.940 and the addition amount of ZnO is 6% by weight, at a sintering temperature, for example, of 1350° C. and 1375° C., a particularly excellent balance of characteristics is shown as shown in FIG. 1 to FIG. 6, in which τf is from -1 to +1 ppm/°C., ε r  is about 2.1 and Qu is about 4000. Further, with regard to τf, since the variation coefficient relative to the sintering temperature is low, a small value around 0 can be adjusted easily. On the other hand, it is not preferred that CaTiO 3  is not contained since the value Qu is large but ε r  is small and τf remarkably decreases toward the negative side as -25  to -44 ppm/° C. 
     Further, the analysis method based on the absence or presence of X-ray diffraction peak in FIG. 12, shows that the structure of products according to the present invention contains MgTiO 3  (◯), partially containing ZnTiO 3 ) and CaTiO 3  () and, as other peaks, Mg 2  TiO 4  (Δ partially containing ZnTiO 4 ) and not containing MgO, CaO and TiO 2 . FIG. 13 shows a case not containing ZnO, which shows that the structure comprises MgTiO 3  (◯) and CaTiO 3  (), also shows that MgO, CaO, TiO 2  are not contained. 
     Further, although not illustrated, results of electron microscopic photograph show that the grain size increases along with elevation of the sintering temperature (1300° C.: 3.1 μm, 1350° C.: 7.2 μm, 1400° C.: 12.8 μm, each measured by Intercept method), pores decrease and densification is completed at 1400° C. The rapture cross section showed grain boundary destruction at 1300° C. and intergrain destruction above 1350° C. 
     EXAMPLE 2 
     Using Ta 2  O 5  powder (99.9% purity) instead of the ZnO powder used in Example 1, a predetermined amount of the starting material (about 500 g in total) was weighted and mixed so as to provide a composition in which x and y vary in the formula: x MgTiO 3  ·(1-x)CaTiO 3  +y wt % Ta 2  O 5  in which y means % be weight of Ta 2  O 5  based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3 ) as shown in Tables 2 and 3 and FIGS. 14 and 17. Subsequently, molding products of the same shape were produced, sintered in the same manner, and then polished to prepare dielectric specimens of the same shape as in Example 1. Further, characteristics were measured by the same method and the results are shown in Tables 2-3 and FIGS. 14-23. Further, as an example, results of X-ray diffraction of 0.93MgTiO 3  ·0.07CaTiO 3  3 wt % in a case of Ta 2  O 5  are shown in FIG. 24 (sintered at 1300° C. for 4 hours) and in FIG. 25 (sintered at 1350° C. for 4 hours). 
     From the foregoing results, the value Qu tends to decrease if x is small in x MgTiO 3  ·(1-x)CaTiO 3  but τf and ε r  tend to increase toward the positive side. Further, the sintering density tends to increase along with elevation of 
     
                                           TABLE 2__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 + 3 wt % Ta.sub.2 O.sub.5 ] ceramiccomposition                      Rel.   Amount        Calcining             Sinter.  dielect.                          τ f                               Sint.   of Ta.sub.2 O.sub.5        temp.             temp.                 Qu   const.                          (ppm/                               densityNo.   x    (wt %)        (°C.)             (°C.)                 (6 GHz)                      ε r                          °C.)                               (g/cm.sup.3)__________________________________________________________________________1  0.940   3    1100 1400                 4350 20.68                          -12.48                               3.8702  0.9375   3    1100 1400                 4310 20.82                          -11.0                               3.8723  0.935   3    1100 1400                 4260 20.97                          -8.50                               3.8754  0.930   3    1100 1400                 4140 21.28                          -1.47                               3.8685  0.927   3    1100 1400                 3940 21.50                          1.02 3.8756  0.923   3    1100 1400                 3850 21.72                          5.10 3.8757  0.940   3    1100 1375                 4380 20.76                          -11.4                               3.8758  0.9375   3    1100 1375                 4330 20.88                          -9.40                               3.8789  0.935   3    1100 1375                 4260 21.07                          -6.58                               3.88110 0.930   3    1100 1375                 4130 21.39                          0.09 3.87611 0.927   3    1100 1375                 3890 21.60                          1.70 3.88412 0.923   3    1100 1375                 3850 21.81                          6.20 3.88213 0.940   3    1100 1350                 4230 20.77                          -11.4                               3.87414 0.9375   3    1100 1350                 4190 20.90                          -9.12                               3.87415 0.935   3    1100 1350                 4180 21.08                          -6.40                               3.88116 0.930   3    1100 1350                 4030 21.28                          0.65 3.87417 0.927   3    1100 1350                 3920 21.64                          2.26 3.88418 0.923   3    1100 1350                 3900 21.87                          6.87 3.88219 0.940   3    1100 1325                 4230 20.49                          -11.3                               3.83520 0.9375   3    1100 1325                 4220 20.62                          -8.94                               3.83521 0.935   3    1100 1325                 4190 20.90                          -5.78                               3.85022 0.930   3    1100 1325                 4070 21.15                          -0.11                               3.83423 0.927   3    1100 1325                 3980 21.46                          3.09 3.85524 0.923   3    1100 1325                 3890 21.61                          7.16 3.84425 0.940   3    1100 1300                 4130 19.06                          -11.4                               3.64026 0.9375   3    1100 1300                 4150 19.26                          -9.73                               3.65027 0.935   3    1100 1300                 4180 19.71                          -6.75                               3.68728 0.930   3    1100 1300                 4120 20.04                          -0.53                               3.68629 0.927   3    1100 1300                 4070 20.70                          1.82 3.75330 0.923   3    1100 1300                 3980 20.59                          6.8  3.706__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 + 2˜4 wt % Ta.sub.2 O.sub.5 ]ceramiccomposition                      Rel.   Amount        Calcining             Sinter.  dielect.                          τ f                               Sint.   of Ta.sub.2 O.sub.5        temp.             temp.                 Qu   const.                          (ppm/                               densityNo.   x    (wt %)        (°C.)             (°C.)                 (6 GHz)                      ε r                          °C.)                               (g/cm.sup.3)__________________________________________________________________________1  0.923   3    1100 1425                 3840 21.59                          3.92 3.8632  0.927   3    1100 1425                 3900 21.34                          0.   3.8623  0.930   2    1100 1425                 4060 21.26                          0.31 3.8484  0.930   4    1100 1425                 4000 21.05                          -6.27                               3.8725  0.930   2    1100 1400                 4000 21.37                          0.37 3.8616  0.930   3    1100 1400                 4140 21.28                          -1.47                               3.8687  0.930   4    1100 1400                 4000 21.25                          -5.08                               3.8898  0.930   2    1100 1375                 4000 21.44                          0.81 3.8639  0.930   3    1100 1375                 4130 21.39                          0.09 3.87610 0.930   4    1100 1375                 3960 21.31                          -3.52                               3.89511 0.930   2    1100 1350                 4050 21.47                          1.10 3.86512 0.930   3    1100 1350                 4030 21.28                          0.65 3.87413 0.930   4    1100 1350                 3980 21.36                          -2.33                               3.89414 0.930   2    1100 1325                 4100 21.23                          0.42 3.82915 0.930   3    1100 1325                 4070 21.15                          -0.11                               3.83416 0.930   4    1100 1325                 4020 21.10                          -0.76                               3.84817 0.930   2    1100 1300                 4120 20.27                          0.68 3.69818 0.930   3    1100 1300                 4120 20.04                          -0.53                               3.68619 0.930   4    1100 1300                 4020 19.90                          -0.53                               3.686__________________________________________________________________________ 
    
     the sintering temperature but it becomes flat relative to the sintering temperature as the addition amount of Ta 2  O 5  increases. 
     At a sintering temperature from 1300° C., a preferred range of x is from 0.923 to 0.940, since practical characteristic ranges are shown for τf as -13 to +8 ppm/° C., for ε r  as 19 to 22 and for Qu as 3850 to 4380. Particularly, if x is 0.930 and the addition amount of Ta 2  O 5  is 3% by weight, at a sintering temperature, for example, of 1325° C. to 1375° C., τf is from -0.1 to +0.65 ppm/° C., ε r  is from 21.1 to 21.4 and Qu is from 4030 to 4230, as shown in FIG. 14 to FIG. 19 and Tables 2 and 3, showing particularly excellent balance of the characteristics and the scattering thereof is extremely small. On the other hand, it is not preferred that CaTiO 3  is not contained, since the Qu value is large, ε r  is small and, in addition, τf extremely decreases toward the negative side as -25 to -44 ppm/° C. 
     Further, the analysis method based on the absence or presence of X-ray diffraction peaks shown in FIGS. 24 and 25 shows that the structure of products according to the present invention contains MgTiO 3  (◯) and CaTiO 3  () and MgTi205 (∇) as other peaks and that MgO, CaO and TiO2 are not contained. 
     Further, although not illustrated, results of electron microscopic photograph show that the grain size increases along with the elevation of the sintering temperature (1300° C.:2.5 μm, 1350° C.:3.7 μm, 1400° C.:6.5 μm, each measured by Intercept method), pores decrease and densification is completed at 1350° C. The rapture cross-section shows grain boundary destruction at 1300° C. and intergrain destruction above 1350° C. Further, the surface tissue was classified into two kinds of grains, namely, huge grains (usual grains, 10-50 μm of grain size) and fine square grains (less than 1-10 μm). As a result of EDS (Energy Dispersive Spectrum) analysis, Mg, Ca, Ti and Si were detected from the usual grains while Mg, Ca, Ti and Ta were detected from the square grains. It is considered that the square grains are in the form of Ta compounds. Further, it is considered that the Si ingredient was mixed from the starting material. 
     EXAMPLE 3 
     Using Nb 2  O 5  powder (100% purity) instead of the ZnO powder used in Example 1, a predetermined amount of the starting material (about 500 g in total) was weighted and mixed so as to provide a composition in which x and y vary in the formula: xMgTiO 3  ·(1-x)CaTiO 3  +y wt % Nb 2  O 5  in which y means % by weight of Nb 2  O 5  based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3  as shown in Tables 4-6 and FIGS. 26-36. 
     Table 4 shows the change of the addition amount of MgTiO 3  and Tables 5 and 6 show the change of the addition amount of Nb 2  O 5 . Subsequently, molding products of the same shape were produced, sintered in the same manner, and then polished to prepare dielectric specimens of the same shape as in Example 1 (Nos. 1-24 in Table 4 and Nos. 1-36 in Tables 5 and 6). Further, characteristics were measured by the same method and the results are shown in Tables 4-6 and FIGS. 26-36. Further, as an example, results of X-ray diffraction of 0.93MgTiO 3  ·0.07CaTiO 3  are shown in FIG. 37 (containing 2%, 4% or 6% by weight of Nb 2  O 5  to 0.93MgTiO 3  ·0.07CaTiO 3  sintered at 1350° C. for 4 hours). 
     From the foregoing results, the value Qu tends to decrease if x is small in xMgTiO 3  ·(1-x)CaTiO 3  but τf and ε r  tend to increase toward the positive side. Further, the sintering density tends to increase along with elevation of the sintering temperature but it shows no substantial change to the change of the sintering temperature as the addition amount of Nb 2  O 5  increases. 
     Further, although Qu and ε r  do not change so much by the addition of Nb 2  O 5 , τf tends to decrease. Accordingly, addition of Nb 2  O 5  is suitable to the control of τf without reducing the performance of Qu and ε r . Further, if Nb 2  O 5  is not added, characteristics can not be measured when the sintering temperature is low (1275°-1325° C.) (Table 6). Even upon sintering at 1350°-1400° C., although characteristics can be measured τf is remarkably large (Table 5 and FIG. 31; 19.7 (No. 1) and 17.5 (No. 7), 15.1 ppm/°C. (No. 13)). The sintering density is also low upon 
     
                                           TABLE 4__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 + 2 wt % Nb.sub.2 O.sub.5 ] ceramiccomposition                      Rel.   Amount        Calcining             Sinter.  dielect.                          τ f                               Sint.   of Nb.sub.2 O.sub.5        temp.             temp.                 Qu   const.                          (ppm/                               densityNo.   x    (wt %)        (°C.)             (°C.)                 (6 GHz)                      ε r                          °C.)                               (g/cm.sup.3)__________________________________________________________________________1  0.940   2    1100 1400                 4380 20.77                          -11.0                               3.8162  0.935   2    1100 1400                 4250 21.08                          -6.50                               3.8213  0.930   2    1100 1400                 4100 21.36                          -0.76                               3.8214  0.925   2    1100 1400                 3930 21.71                          4.95 3.8245  0.940   2    1100 1375                 4220 20.87                          -10.8                               3.8286  0.935   2    1100 1375                 4130 21.19                          -6.26                               3.8327  0.930   2    1100 1375                 3980 21.48                          -0.16                               3.8308  0.925   2    1100 1375                 3900 21.82                          5.71 3.8319  0.940   2    1100 1350                 4230 20.92                          -10.1                               3.82810 0.935   2    1100 1350                 4150 21.23                          -5.55                               3.83411 0.930   2    1100 1350                 4030 21.51                          0.51 3.83112 0.925   2    1100 1350                 3920 21.86                          8.01 3.83413 0.940   2    1100 1325                 4260 20.77                          -11.0                               3.80514 0.935   2    1100 1325                 4160 21.16                          -5.58                               3.82115 0.930   2    1100 1325                 4050 21.44                          0.52 3.81716 0.925   2    1100 1325                 3940 21.73                          6.40 3.81517 0.940   2    1100 1300                 4540 20.07                          -11.72                               3.71618 0.935   2    1100 1300                 4320 20.73                          -6.21                               3.75719 0.930   2    1100 1300                 4250 20.95                          -0.92                               3.75520 0.925   2    1100 1300                 4160 21.15                          7.49 3.73921 0.940   2    1100 1275                 4010 17.87                          -13.0                               3.44422 0.935   2    1100 1275                 4470 18.97                          -8.20                               3.54323 0.930   2    1100 1275                 4320 19.05                          -3.69                               3.53224 0.925   2    1100 1275                 4040 18.99                          1.16 3.487__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________[0.93 MgTiO.sub.3.0.07 CaTiO.sub.3 + 0˜6 wt % Nb.sub.2 O.sub.5 ]ceramiccomposition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of Nb.sub.2 O.sub.5   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  0    1100 1400            3490 21.78  19.7 3.8342  1    1100 1400            2740 21.10  -8.19                             3.8203  2    1100 1400            4100 21.36  -0.76                             3.8214  3    1100 1400            4140 21.34  -3.64                             3.8175  4    1100 1400            4170 21.33  -7.27                             3.8196  6    1100 1400            4130 21.26  -11.6                             3.8127  0    1100 1375            3650 21.22  17.5 3.7698  1    1100 1375            2260 21.16  -7.63                             3.8239  2    1100 1375            3980 21.48  -0.16                             3.83010 3    1100 1375            4120 21.55  -2.47                             3.82911 4    1100 1375            4160 21.55  -6.14                             3.83312 6    1100 1375            4110 21.52  -9.93                             3.83013 0    1100 1350            3820 19.54  15.1 3.57714 1    1100 1350            4190 21.49  1.71 3.82915 2    1100 1350            4030 21.51  0.51 3.83116 3    1100 1350            4120 21.55  -1.57                             3.83317 4    1100 1350            4130 21.59  -4.63                             3.83718 6    1100 1350            4110 21.56  -9.00                             3.837__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________[0.93 MgTiO.sub.3.0.07 CaTiO.sub.3 + 0˜6 wt % Nb.sub.2 O.sub.5 ]ceramiccomposition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of Nb.sub.2 O.sub.5   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________19 0    1100 1325            Unmeasurable     3.13120 1    1100 1325            4250 21.25  0.53 3.79921 2    1100 1325            4050 21.44  0.52 3.81722 3    1100 1325            4140 21.49  -2.04                             3.81623 4    1100 1325            4080 21.57  -3.86                             3.83124 6    1100 1325            4040 21.58  -8.16                             3.82925 0    1100 1300            Unmeasurable26 1    1100 1300            4310 20.53  1.59 3.70427 2    1100 1300            4250 20.95  -0.92                             3.75528 3    1100 1300            4200 21.03  -2.49                             3.76229 4    1100 1300            4070 21.34  -3.18                             3.79230 6    1100 1300            4070 21.39  - 8.24                             3.80131 0    1100 1275            Unmeasurable32 1    1100 1275            Unmeasurable     3.39033 2    1100 1275            4320 19.05  -3.69                             3.53234 3    1100 1275            3740 18.92  -5.80                             3.50835 4    1100 1275            4230 19.97  -4.93                             3.62136 6    1100 1275            4190 19.90  -9.14                             3.624__________________________________________________________________________ 
    
     sintering at 1375° C. or 1350° C. (No. 7 and No. 13 in Table 5). 
     At a sintering temperature from 1300° to 1400° C. and the addition amount of Nb 2  O 5  of 2% by weight, a preferred range of x is from 0.925 to 0.940, since practical characteristic ranges are shown for τf as -11.72 to +8.01 ppm/°C. for ε r  as 20.07 to 21.86 and for Qu as 3900 to 4540. Particularly, if x is 0.930 and the addition amount of Nb 2  O 5  is 2% by weight at a sintering temperature, for example, of 1350° C. and 1375° C., τf is -0.16 to +0.51 ppm/°C, ε r  is 21.48 to 21.51 and Qu is 3980 to 4030, showing particularly excellent performance balance with scarce scattering thereof. Further, regarding τf, a small value around 0 ppm/°C. can be controlled easily since the variation coefficient to the sintering temperature is low. On the other hand, it is not preferred that CaTiO 3  is not contained, since the Qu value is large, but ε r  is small and, in addition, τf extremely decreases toward the negative side as -25 to -44 ppm/°C. 
     Further, the analysis method based on the absence or presence of X-ray diffraction peaks in FIG. 37 shows that the structure of products according to the present invention contains MgTiO 3  (◯)) and CaTiO 3  () and MgTi 2  O 5  (Δ) as other peaks and that MgO, CaO and TiO2 are not contained. Further, peaks for Nb 2  O 5  compounds were not detected. However, although a diffraction pattern is substantially the same for the addition amount of Nb 2  O 5 , peaks for MgTiO 3  and MgTi 2  O 5  shifted slightly to the lower angle side of 2θ--along with increase of the addition amount from 2 to 6% by weight (FIG. 37). Accordingly, it is considered that Nb 5+  ions are solid solubilized in MgTiO 3  and MgTi 2  O 5 . 
     Further, although not illustrated, results of electron microscopic photograph show that the grain size increases along with the elevation of the sintering temperature (1275° C.:2.6 μm, 1300° C.:3.1 μm, 1350° C.:4.9 μm and 1400° C.:6.1 μm, each measured by an Intercept method), and pores decrease, showing that densification is completed at 1350° C. The rapture cross-section shows inter-grain destruction for each of the cases. 
     EXAMPLE 4 
     Using MnO 2  powder (96% purity) instead of the ZnO powder used in Example 1, a predetermined amount of the starting material (about 500 g in total) was weighted and mixed so as to provide a composition in which x and y vary in the formula: xMgTiO 3  ·(1-x)CaTiO 3  +y wt % MnO 2  in which y means % by weight of MnO 2  based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3  as shown in Tables 7-10 and FIGS. 38 and 42. Table 7 shows the change of the addition amount (constitutional amount of CaTiO 3  and Tables 8-10 show the change of the addition amount of MnO 2 . Subsequently, molding products of the same shape were produced, sintered in the same manner, and then polished to prepare dielectric 
     
                                           TABLE 7__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 ] ceramic composition (x= 0.97˜0.93)  Calcining       Sinter.  Rel. dielect.                       τ f                            Sint.  temp.       temp.           Qu   const. (ppm/                            densityNo.   1 - x  (°C.)       (°C.)           (6 GHz)                ε r                       °C.)                            (g/cm.sup.3)__________________________________________________________________________1  0.03  1100 1425           4020 18.60  -49.6                            3.7652  0.05  1100 1425           2840 19.82  -33.8                            3.8033  0.07  1100 1425           2390 21.17  -4.47                            3.8194  0.03  1100 1400           3800 18.58  -47.8                            3.7585  0.05  1100 1400           2900 19.79  -28.6                            3.8036  0.07  1100 1400           3490 21.78  19.72                            3.8347  0.03  1100 1375           4820 18.61  -33.3                            3.7168  0.05  1100 1375           4190 19.81  -9.63                            3.7469  0.07  1100 1375           3650 21.22  17.5 3.76910 0.03  1100 1350           4700 17.06  -34.4                            3.52911 0.05  1100 1350           4230 18.22  -11.7                            3.54312 0.07  1100 1350           3820 19.54  15.1 3.57713 0.03  1100 1325           1150 13.56  -42.8                            3.05714 0.05  1100 1325           Unmeasurable     3.07215 0.07  1100 1325           Unmeasurable     3.131__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________[0.94 MgTiO.sub.3.0.06 CaTiO.sub.3 + 0˜6 wt % MnO.sub.2 ] ceramiccomposition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of MnO.sub.2   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  3    1100 1425            3570 20.86  0.56 3.8412  6    1100 1425            3090 20.68  -2.38                             3.8543  3    1100 1400            3640 20.95  1.78 3.8474  6    1100 1400            3080 20.79  -1.21                             3.8615  3    1100 1375            3700 20.68  -0.09                             3.8156  6    1100 1375            3080 20.84  0.20 3.8627  3    1100 1350            3890 19.04  -2.60                             3.6168  6    1100 1350            3150 20.45  -1.23                             3.8079  3    1100 1325             660 17.00  -3.26                             3.38410 6    1100 1325            3360 18.52  -4.36                             3.57811 3    1100 1300            Unmeasurable     3.25512 6    1100 1300            Unmeasurable     3.41813 0    1100 1425            2480 20.46   -23.0                             3.81514 0    1100 1400            3360 20.51  -18.0                             3.81215 0    1100 1375            3940 20.83  4.84 3.77816 0    1100 1350            3820 18.27  0.19 3.48017 0    1100 1325             430 16.28  3.72 3.26618 0    1100 1300            Unmeasurable     3.121__________________________________________________________________________ 
    
     
                                           TABLE 9__________________________________________________________________________[0.95 MgTiO.sub.3.0.05 CaTiO.sub.3 + 0.1˜0.6 wt % MnO.sub.2 ]ceramic composition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of MnO.sub.2   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  0.1  1100 1400            3790 19.80  -31.4                             3.8042  0.2  1100 1400            3810 19.83  -31.3                             3.8073  0.3  1100 1400            3660 19.85  -30.7                             3.8144  0.6  1100 1400            3850 19.83  -29.6                             3.8185  0.1  1100 1375            3970 20.21  -11.8                             3.8056  0.2  1100 1375            4030 20.21  -10.7                             3.8047  0.3  1100 1375            4100 20.27  -9.31                             3.8078  0.6  1100 1375            4060 20.25  -9.87                             3.8109  0.1  1100 1350            4220 19.50  -10.4                             3.71310 0.2  1100 1350            4270 19.30  -10.7                             3.69311 0.3  1100 1350            4280 19.20  -11.6                             3.68312 0.6  1100 1350            4250 19.34  -11.1                             3.70313 0.1  1100 1325            3950 17.42  -13.4                             3.44914 0.2  1100 1325            3970 17.41  -12.7                             3.46215 0.3  1100 1325            3550 17.05  -14.2                             3.42216 0.6  1100 1325            3650 17.10  -14.1                             3.42617 0.1  1100 1300            1090 15.43  -26.4                             3.23018 0.2  1100 1300            1010 15.56  -30.2                             3.25119 0.3  1100 1300            1450 15.41  -21.5                             3.23220 0.6  1100 1300             950 15.54  -23.7                             3.245__________________________________________________________________________ 
    
     
                                           TABLE 10__________________________________________________________________________[0.93 MgTiO.sub.3.0.07 CaTiO.sub.3 + 0˜6 wt % MnO.sub.2 ] ceramiccomposition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of MnO.sub.2   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  3    1100 1425            3310 21.64  14.3 3.8442  6    1100 1425            2900 21.38  12.8 3.8553  3    1100 1400            3330 21.60  15.0 3.8514  6    1100 1400            2880 21.42  12.3 3.8645  3    1100 1375            3400 21.46  13.8 3.8336  6    1100 1375            2890 21.44  11.8 3.8657  3    1100 1350            3560 20.06  11.8 3.6688  6    1100 1350            2960 21.00  12.0 3.8039  3    1100 1325            1390 17.71  6.92 3.41610 6    1100 1325            3140 18.90  7.07 3.56311 3    1100 1300            Unmeasurable     3.26812 6    1100 1300            Unmeasurable     3.39513 0    1100 1425            2390 21.17  -4.47                             3.81914 0    1100 1400            3490 21.78  19.7 3.83415 0    1100 1375            3650 21.22  17.5 3.76916 0    1100 1350            3820 19.54  15.1 3.57717 0    1100 1325            Unmeasurable     3.131__________________________________________________________________________ 
    
     specimens of the same shape as in Example 1 (Nos. 1-18 in Table 8, Nos. 1-20 in Table 9 and Nos. 1-17 in Table 10). Further, characteristics were measured by the same method. The results are shown in Tables 8-10 and FIGS. 38-49. Further, as an example, results of X-ray diffraction of 0.94MgTiO 3  ·0.06CaTiO 3  are shown in FIG. 50 (containing 0%, 3% or 6% by weight of MnO 2  to 0.94MgTiO 3  ·0.06 CaTiO 3  sintered at 1300° C. for 4 hours). 
     From the foregoing results, the value Qu tends to decrease if (1-x) is large in xMgTiO 3  ·(1-x)CaTiO 3 , whereas τf and ε r  tend to increase toward the positive side. Further, the sintering density tends to increase along with elevation of the sintering temperature but it shows no substantial change relative to the change of the sintering temperature as the addition amount of MnO 2  increases. 
     Further, although ε r  shows no substantial change, Qu and τf tend to decrease due to the addition of MnO 2 . Accordingly, addition of MnO 2  is suitable to the control of τf with no reduction of the performance of ε r . Further, as shown in Table 8, in a case if MnO 2  is not added, the sintering density is small if the sintering temperature is low (1325° to 1350° C.) and a temperature higher than 1375° C. is necessary for ensuring a sufficient sinterability. On the other hand, in a case of adding MnO 2  (3 to 6% by weight), a sufficient sinterability is shown even at a sintering temperature of 1350° C. 
     At a sintering temperature from 1350° to 1425° C. and with the addition amount of MnO 2  of 3% by weight, a preferred range of (1-x) is from 0.06 to 0.07, since practical characteristic ranges are shown for τf as -2.60 to +15.0 ppm/°C., for ε r  as 19.04 to 21.64 and for Qu as 3310 to 3890. Particularly, if (1-x) is 0.06 and the addition amount of MnO 2  is 3% by weight, at a sintering temperature, for example, of 1375° C., τf is -0.09 ppm/°C., ε r  is 20.7 and Qu is 3700, showing particularly excellent balance of performance and the scattering thereof is extremely small. Further, regarding τf, since the variation coefficient to the sintering temperature is low, a small value around zero ppm/°C. can be controlled easily. On the other hand, it is not preferred that CaTiO 3  is not contained, since the Qu value is large, but ε r  is small and, in addition, τf extremely decreases toward the negative side as -25 to -44 ppm/°C. 
     Further, the analysis method based on the absence or presence of X-ray diffraction peaks shown in FIG. 50 shows that the structure of products according to the present invention contains MgTiO 3  (◯) and CaTiO 3  () and Mg 6  MnO 6  (Δ) and Mn 3  O 4  (∇) as other peaks and that MgO, CaO and TiO2 are not contained. Namely, the relative peak intensity of MgTiO 3  is reduced along with the increase of the addition amount of MnO 2 , while Mg 6  MnO 8 , Mn 3  O 4 , etc., are increased. It is considered that Qu decreases due to the decrease in the amount of MgTiO 3 . 
     Further, although not illustrated, results of electron microscopic photograph show that the grain size increases along with the elevation of the sintering temperature (1300° C.:2.7 μm, 1350° C.:8.7 μm, 1400° C.:9.2 μm, each measured by an Intercept method). The rapture cross-section shows inter-grain destruction in each of the cases. As a result of EDS analysis, only Mg, Ca, Ti and Mn were detected. 
     EXAMPLE 5 
     Using Al 2  O 3  powder (99.9% purity) instead of the ZnO powder used in Example 1, a predetermined amount of the starting material (about 500 g in total) was weighted and mixed so as to provide a composition in which x and y vary in the formula: xMgTiO 3  ·(1-x)CaTiO 3  +y wt % Al 2  O 3  in which y means % by weight of Al 2  O 3  based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3  in Tables 7 and 11 and FIGS. 51-58. Table 7 shows the change of the addition amount (constitutional amount) of CaTiO 3  (1-x) and Tables 11 shows the change of the addition amount of Al 2  O 3 . Subsequently, molding products of the same shape were produced, sintered in the same manner at a predetermined temperature and then polished to prepare dielectric specimens of the same shape as in Example 1 (Nos. 1-12 in Table 7  and Nos. 1-24 in Table 11). Further, characteristics were measured by the same method. The results are shown in Table 7 and 11 and FIGS. 
     
                                           TABLE 11__________________________________________________________________________[0.94 MgTiO.sub.3.0.06 CaTiO.sub.3 + 3˜12 wt % Al.sub.2 O.sub.3 ]ceramic composition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of Al.sub.2 O.sub.3   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  12   1100 1425            4040 18.90  -13.2                             3.7252  9    1100 1400            3940 19.47  -11.3                             3.7563  12   1100 1400            4000 19.10  -13.2                             3.7394  3    1100 1375            3550 20.54  -2.21                             3.8115  6    1100 1375            3530 20.07  -3.85                             3.7816  9    1100 1375            3700 19.70  -5.50                             3.7627  12   1100 1375            3860 19.34  -5.77                             3.7398  3    1100 1350            3560 20.60  -0.70                             3.8169  6    1100 1350            3390 20.13  -0.74                             3.78110 9    1100 1350            3640 19.68  -1.53                             3.74711 12   1100 1350            3750 19.09  -2.06                             3.69812 3    1100 1325            3530 20.56  0.97 3.80213 6    1100 1325            3360 20.00  1.91 3.75914 9    1100 1325            3600 19.29  0    3.69615 12   1100 1325            3850 18.20  -1.99                             3.59316 3    1100 1300            3470 20.24  2.25 3.76217 6    1100 1300            3370 19.50  2.59 3.69518 9    1100 1300            3730 18.40  0.12 3.56819 12   1100 1300            3870 16.39  -5.22                             3.37620 3    1100 1275            3590 19.20  2.37 3.60721 6    1100 1275            3470 18.05  1.76 3.51722 9    1100 1275            2640 15.77  -6.50                             3.29823 3    1100 1250            2870 16.18  -2.39                             3.28724 6    1100 1250            2810 15.15  -2.87                             3.203__________________________________________________________________________ 
    
     51-58. The resonance frequency used for the measurement of Qu is 5.7 to 6.3 GHz. Further, FIG. 29 shows a relation between the addition amount of Al 2  O 3  and the average grain size of the sintering product and FIG. 60 shows a relation between the sintering temperature and the average grain size of the sintering product. Further, as an example, FIG. 63 shows results of X-ray diffraction of 0.94MgTiO 3  ·0.06CaTiO 3  ·3.12 wt % Al 2  O 3  (sintered at 1350° C. for 4 hours). 
     From the foregoing results, the value Qu tends to decrease if (1-x) is large in xMgTiO 3  ·(1-x)CaTiO 3  but τf and ε r  tend to increase toward the positive side. Further, the sintering density tends to increase along with elevation of the sintering temperature. 
     As the addition of Al 2  O 3 , increases, Qu tends to increase, and ε r , τf and the sintering density tend to decrease. Then, at a sintering temperature of 1275° C., Qu, ε r  and the sintering density tend to decrease and their scattering is large, whereas highly stable performances are shown at 1300° to 1375° C. Further, in a case where the addition amount of Al 2  O 3  is 3 to 9% by weight and the sintering temperature is 1300° to 1375° C., well balanced performance and physical property are shown, as Qu is 3360 to 3730, ε r  is 18.4 to 20.6, τf is -2.2 to +2.6 ppm/°C. and the sintering density is 3.568 to 3.816 g/cm 3  as described above. 
     In view of the above, appropriate addition of Al 2  O 3  can improve the performance of Qu while maintaining ε r  and further it is suitable to the control of τf. Particularly, regarding τf, since the variation coefficient relative to the addition of Al 2  O 3  and the sintering temperature is low, a small value around 0 ppm/°C. can be controlled easily. 
     Further, as shown in FIG. 60, the grain size of crystal grains increases along with the elevation of the sintering temperature (1300° C.:1.61 μm, 1350° C.:2.19 μm, and 1400° C.:4.46 μm, each measured by an Intercept method), whereas the grain size decreased along with the increase of the addition amount of Al 2  O 3  as shown in FIG. 59 (3 wt %: 3.97, 6 wt %:3.76, 12 wt %:2.19). Further, FIG. 61 is a view explaining the structure of crystals by electron microscopic photograph (1000 magnification ratio) for a composition containing 6 wt % Al 2  O 3  added (sintered at 1350° C. for 4 hours) and FIG. 62 shows a view explaining the structure of crystals by electron microscopic photograph (1000 magnification ratio) for a composition containing 12 wt % Al 2  O 3  added (sintered at 1350° C. for 4 hours). According to the figures, each of them shows substantially uniform grain size distribution. Further, rapture cross-section tissues inter-grain destruction for each of 3-12 wt % Al 2  O 3  addition systems. 
     Further, the analysis method based on the presence or absence of X-ray diffraction peak in FIG. 63 shows that the structure of products according to the present invention contains MgTiO 3  (◯) and CaTiO 3  () and MgTi 2  O 5  (Δ) as other peaks and that reaction products with Al 2  O 3  are not contained. 
     EXAMPLE 6 
     Using Co 2  O 3  powder (99.1% purity) instead of the ZnO powder used in Example 1, a predetermined amount of the starting material (about 500 g in total) was weighted and mixed so as to provide a composition in which x and y vary in the formula: x MgTiO 3  ·(1-x)CaTiO 3  +y wt % Co 2  O 3  in which y means % by weight of Co 2  O 3  based on 100 parts by weight of xMgTiO 3  ·(1-x)CaTiO 3  as shown in Tables 7 and 12-13 and FIGS. 64-71. Subsequently, molding products of the same shape were produced, sintered in the same manner, and then polished to prepare dielectric specimens of the same shape as in Example 1 (Nos. 1-12 in Table 7, in Nos. 1-24 in Table 12 and Nos. 25 and 26 in Table 13). Further, characteristics were measured by the same method. The results are shown in Tables 7 and 12-13 and FIGS. 64-71. Further, as an example, results of X-ray diffraction of 0.94MgTiO 3  ·0.06CaTiO 3  ·3 wt %, 6 wt % or 12 wt % Co 2  O 3  ceramic composition (sintered at 1300° C. for 4 hours) are shown in FIG. 73. 
     From the foregoing results, the value Qu tends to decrease, whereas τf and ε r  tend to increase toward the 
     
                                           TABLE 12__________________________________________________________________________[0.94 MgTiO.sub.3.0.06 CaTiO.sub.3 + 3˜12 wt % Co.sub.2 O.sub.3 ]ceramic composition   Amount   Calcining        Sinter.  Rel. dielect.                        τ f                             Sint.   of Co.sub.2 O.sub.3   temp.        temp.            Qu   const. (ppm/                             densityNo.   (wt %)   (°C.)        (°C.)            (6 GHz)                 ε r                        °C.)                             (g/cm.sup.3)__________________________________________________________________________1  3    1100 1450            3920 20.77  -0.26                             3.8492  6    1100 1450            3780 20.58  -0.40                             3.8513  9    1100 1450            3710 20.37  -2.14                             3.8844  12   1100 1450            3640 20.17  -0.99                             3.8935  3    1100 1425            3920 20.91  1.36 3.8546  6    1100 1425            3840 20.72  -0.74                             3.8787  9    1100 1425            3750 20.54  -1.79                             3.9038  12   1100 1425            3680 20.32  -3.29                             3.9219  3    1100 1400            3900 21.00  1.64 3.86410 6    1100 1400            3840 20.78  -0.05                             3.88911 9    1100 1400            3790 20.60  -1.92                             3.91412 12   1100 1400            3700 20.43  -3.13                             3.93513 3    1100 1375            4070 20.80  1.11 3.84314 6    1100 1375            3870 20.82  -0.10                             3.89415 9    1100 1375            3840 20.68  -1.66                             3.92116 12   1100 1375            3740 20.49  -2.26                             3.94317 3    1100 1350            4180 19.36  1.65 3.66918 6    1100 1350            3970 20.35  -0.03                             3.83219 9    1100 1350            3860 20.48  -1.12                             3.89320 12   1100 1350            3760 20.46  -1.87                             3.93421 3    1100 1325            2400 17.38  -4.95                             3.44022 6    1100 1325            4160 18.38  -3.02                             3.59623 9    1100 1325            4020 18.82  -3.42                             3.68824 12   1100 1325            3860 19.34  -3.02                             3.792__________________________________________________________________________ 
    
     
                                           TABLE 13__________________________________________________________________________[xMgTiO.sub.3.(1 - x)CaTiO.sub.3 + 6 wt % Co.sub.2 O.sub.3 ] ceramiccomposition                      Rel.   Amount        Calcining             Sinter.  dielect.                          τ f                               Sint.   of Co.sub.2 O.sub.3        temp.             temp.                 Qu   const.                          (ppm/                               densityNo.   1 - x   (wt %)        (°C.)             (°C.)                 (6 GHz)                      ε r                          °C.)                               (g/cm.sup.3)__________________________________________________________________________25 0.05 6    1100 1375                 4100 20.10                          -11.2                               3.89014 0.06 6    1100 1375                 3870 20.82                          -0.10                               3.89426 0.07 6    1100 1375                 3650 21.63                          10.8 3.892__________________________________________________________________________ 
    
     positive side if (1-x) is large in xMgTiO 3  ·(1-x)CaTiO 3 . Further, the sintering density tends to increase along with the elevation of the sintering temperature. 
     Further, although ε r  shows no substantial change Qu and τf tend to decrease and the sintering density tends to increase by the addition of Co 2  O 3 . Particularly, is (1-x) is 0.06 and the addition amount of Co 2  O 3 . Particularly, if (1-x) is 0.06 and the addition amount of Co 2  O 3  is 6% by weight, τf is -0.10 ppm/°C., ε r  is 20.82, Qu is 3870 and the sintering density is 3.894 at a sintering temperature, for example, of 1375° C., showing particularly excellent performance balance. Further, at a sintering temperature of 1350° to 1450° C., each of Qu, ε r  and τf scarcely scatters (for example, in case of Co 2  O 3  6 wt %, 3780 to 3970, 20.35 to 20.82, -0.03 to -0.74 ppm/°C., respectively) and products of desired quality can be obtained stably. Further, even if addition amount of Co 2  O 3  changed, each of Qu, ε r  and τf scarcely scatters (for instance at a sintering temperature of 1400° C., 3700 to 3900, 20.43 to 21.00, -0.05 to -3.13 ppm/°C., successively). Accordingly, even if the addition amount of Co 2  O 3  and the sintering temperature are changed, products of extremely stable quality can be obtained. Particularly, fine control for τf can be conducted extremely easily. 
     Further, the analysis method based on the absence or presence of X-ray diffraction peaks in FIG. 73 shows that the structure of products according to the present invention contains MgTiO 3  (◯) and CaTiO 3  () and MgTi204 () as other peaks and that reaction products with Co 2  O 3  are not contained. Further, the relative peak intensity of MgTiO 3  is reduced along with the increase of the addition amount of Co 2  O 3  while the intensity of MgTi 2  O 4  is increased. It is considered that Qu decreases due to the decrease in the amount of MgTiO 3 . 
     Further, results of electron microscopic photograph (not illustrated) show that the grain size increases along with the elevation of the sintering temperature (1350° C.:4.30 μm, 1400° C.:5.32 μm, 1450° C.:7.20 μm, each measured by an Intercept method) as showned in Table 72 . The rapture cross-section tissue shows inter-grain destruction in each of the cases. 
     The present invention is not restricted to specific embodiments described above but can be modified into various other embodiments depending on the purpose and the application use within the scope of the invention. That is, the calcining conditions such as calcining temperature and sintering conditions such as sintering temperature can be selected variously. Further, also for the starting material for CaO, peroxides, hydroxides and nitrates and so on can also be used in addition to CaCO 3  described above. Also for other oxides, other compounds that can form oxides by heating may also be used.