Abstract:
Disclosed herein is a ceramic composition which gives a microwave dielectric having a permittivity and unloaded Q in a practical range and a small temperature coefficient (τf) in the neighborhood of zero of resonance frequency owing to the controlled amount of ZnO and Ta 2  O 5  added. The τf value may be positive or negative in the neighborhood of zero according to need. The ceramic composition comprises a principal component represented by (BaO-3.7TiO 2 ).xZnO, where x is in the range of 15≦x≦17 wt % and a secondary component containing 2-6 wt % Ta 2  O 5  and not more than 1 wt % MnO 2  based on the amount of the principal component. The ceramic composition may also contain BaO-3.8TiO 2 , 12-16 wt % ZnO, and 2-6 wt % Ta 2  O 5 , or contain BaO-3.6TiO 2 , 17-19 wt % ZnO, and 2-6 wt % Ta 2  O 5 .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ceramic composition for a microwave dielectric (simply referred to as a dielectric ceramic composition), said microwave dielectric having a temperature coefficient of resonance frequency (simply referred to as τf hereinafter) which is close to zero or has any controlled positive or negative value depending on the amount of ZnO added. 
     The microwave dielectric pertaining to the present invention will be used for microwave dielectric resonators, microwave IC substrates, and impedance matching in microwave circuits etc. 
     2. Description of the Prior Art 
     There are several known dielectric ceramic compositions which are represented by BaO.3.9-4.1TiO 2  . xZnO (Japanese Patent Publication No. 37807/1989), BaO.4TiO 2 .x ZnO.yTa 2  O 5  (Japanese Patent Laid-open No. 10806/1986), BaO.4TiO 2 .x ZnO.yWO3 (Japanese Patent Laid-Open No. 10807/1986), and BaO.wTiO 2 .xBa(Zr 1/3  Ta 2/3 )O 3 .yTa 2  O 5 .zMnO 2  (Japanese Patent Laid-open No. 117957/1988). 
     Of the above-mentioned four dielectric ceramic compositions, the first three vary in τf (which ranges from positive to negative) depending on the amount of additives. However, they are often poor in sinterability, and there has been a demand for a new dielectric ceramic composition of different make-up which is comparable or superior to the conventional ones in performance. And the last one does not have negative τf values. 
     SUMMARY OF THE INVENTION 
     The present invention was completed in view of the foregoing. It is an object of the present invention to provide a dielectric ceramic composition for a microwave dielectric which has a small τf value close to zero, or any desired positive or negative τf value depending on the amount of ZnO and Ta 2  O 5  added, while maintaining the permittivity (simply referred to as εr hereinafter) and unloaded Q (simply referred to as Qu hereinafter) in a practical range. 
     The present inventors carried out a series of researches on a variety of dielectric ceramic compositions of (BaO-xTiO 2 ).yZnO.zTa 2  O 5  systems which has a small τf value close to zero, while maintaining εr and Qu in a practical range. As the result, the present invention was accomplished. 
     In a first preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO 2  and ZnO, and a secondary component, said principal component being represented by (BaO-3.8TiO 2 ).xZnO, where x is in the range of 12≦x≦16 wt % based on the amount of BaO-3.8TiO 2 , said secondary component containing 2-6 wt % Ta 2  O 5  based on the amount of the principal component. 
     In a second preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO 2  and ZnO, and a secondary component, said principal component being represented by (BaO-3.7TiO 2 ).xZnO, where x is in the range of 15≦x≦17 wt % based on the amount of BaO-3.7TiO 2 , said secondary component containing 2-6 wt % Ta 2  O 5  based on the amount of the principal component. 
     In a third preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO 2  and ZnO, and a secondary component, said principal component being represented by (BaO-3.6TiO 2 ).xZnO, where x is in the range of 17≦x≦19 wt % based on the amount of BaO-3.6TiO 2 , said secondary component containing 2-6 wt % Ta 2  O 5  based on the amount of the principal component. 
     In a fourth preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO 2  and ZnO, and a secondary component, said principal component being represented by (BaO-3.7˜3.8TiO 2 ).xZnO, where x is in the range of 12≦x≦17 wt % based on the amount of BaO-3.7˜3.8TiO 2 , said secondary component containing 2-6 wt % Ta 2  O 5  based on the amount of the principal component, and τf values is in the range of -5 to +5 ppm/°C. 
     In a fifth preferred embodiment of the invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO 2 , and ZnO, and a secondary component, said principal component being represented by (BaO-3.6˜3.7TiO 2 ).xZnO, where x is in the range of 15≦x≦19 wt % based on the amount of BaO-3.6˜3.7TiO 2 , said secondary component containing 2-6 wt % Ta 2  O 5  based on the amount of the principal component, and a temperature coefficient of resonance frequency τf values is in the range of -5 to +5 ppm/°C. 
     In a sixth preferred embodiment of the invention, a ceramic composition for microwave dielectric as defined in any of the first to fifth embodiments, further comprises MnO 2  in an amount not more than 1 wt % of the amount of the ceramic composition. 
     The dielectric ceramic composition of the present invention is represented by [(BaO.xTiO 2 ).yZnO].zTa 2  O 5 , with x, y, and z having specific values as defined in the first to fifth embodiments, so that the resulting microwave dielectric has the τf value close to zero, while maintaining the Qu and εr values in a practical range (Qu≧2200, εr≧26). The τf value ranges from +4.9 to -1.7  in the first embodiment, from +2.5 to -1.4 in the second embodiment, from +4.1 to -1.0 in the third embodiment and from +5 to -5 in the fourth and fifth embodiments. 
     The composition contains ZnO which improves Qu and εr and also contains Ta 2  O 5  which improves Qu and permits the control of τf. 
     The composition may optionally contain MnO 2  which permits the composition to be sintered stably at a comparatively low temperature. The amount of MnO 2  in not more than 1 wt % has effect enough. The amount of MnO 2  in excess of 1 wt % has an adverse effect on the dielectric properties. 
     According to the present invention, the dielectric ceramic composition is prepared by mixing BaO, TiO 2 , ZnO, Ta 2  O 5 , and MnO 2  (all in powder form) according to the formula and calcining the mixture. The calcined powder is formed into a desired shape, which is subsequently sintered. 
     According to the present invention, the dielectric ceramic composition is represented by any one of the following formulas: 
     1[BaO-3.8 (mol) TiO 2 .x (wt %) ZnO (12≦x≦16)].2-6 (wt %) Ta 2  O 5 .0.2 (wt %) MnO 2 , or 2[BaO-3.7 (mol) TiO 2 .x (wt %) ZnO (15≦x≦17)].2-6 (wt %) Ta 2  O 5 .0.2 (wt %) MnO 2 , or 3[BaO-3.6 (mol) TiO 2 .x (wt %) ZnO (17≦x≦19)].2-6 (wt %) Ta 2  O 5 .0.2 (wt %) MnO 2 , 
     It gives rise to a microwave dielectric which has a τf value close to zero while maintaining εr and Qu in a practical range. It also has a desired positive or negative τf value depending on the amount of ZnO or Ta 2  O 5  added. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.8TiO 2  -based dielectric ceramic composition. 
     FIG. 2 is a graph showing the relationship between the τf value and the amount of Ta 2  O 5  at different sintering temperatures for the BaO-3.8TiO 2  -based dielectric ceramic composition. 
     FIG. 3 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.7TiO 2  -based dielectric ceramic composition. 
     FIG. 4 is a graph showing the relationship between the τf value and the amount of Ta 2  O 5  at different sintering temperatures for the BaO-3.7TiO 2  -based dielectric ceramic composition. 
     FIG. 5 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.6TiO 2  -based dielectric ceramic composition. 
     FIG. 6 is a graph showing the relationship between the τf value and the amount of Ta 2  O 5  at different sintering temperatures for the BaO-3.6TiO 2  -based dielectric ceramic composition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described in more detail with reference to the following example. 
     (1) Preparation of samples 
     Five starting materials, BaCO 3 , TiO 2 , ZnO, Ta 2  O 5 , and MnO 2 , all in powder form and 99.9% pure, were weighed according to the formula below. 
     [BaO-y (mol) TiO 2 .x (wt %) ZnO].z (wt %) Ta 2  O 5 .0.2 (wt %) MnO 2  where: 
     when y is 3.8, x is 12, 14, or 16, and z is 4, as shown in Table 1; 
     when y is 3.8 and x is 14, z is 2, 4, or 6, as shown in Table 2; 
     when y is 3.7, x is 15, 16, or 17, and z is 4, as shown in Table 3; 
     when y is 3.7 and x is 17, z is 2, 4, or 6, as shown in Table 4; 
     when y is 3.6, x is 17, 18, or 19, and z is 4, as shown in Table 5; and 
     when y is 3.6 and x is 19, z is 2, 4, or 6, as shown in Table 6. 
     Incidentally, each amount of ZnO, Ta 2  O 5 , and MnO 2  is expressed in terms of wt % (parts by weight) of the total amount (100 parts by weight) of BaCO 3  -yTiO 2 . They were mixed and crushed by dry process using a mixer, and the mixture was calcined at 900°-1100° C. for 2 hours. The calcined product was crushed together with an adequate amount of organic binder and 260-500 g of deionized water in a ball mill containing alumina balls (20 mm in diameter). The crushed product was granulated by spray drying. The granules were formed into a cylinder, 19.35 mm in diameter and 8 mm high, by pressing at 1000 kg/cm 2 . 
     
                                           TABLE 1__________________________________________________________________________(BaO-3.8TiO.sub.2 -based dielectric ceramic composition)        Firing             Firing                  Relative                        Qu value                             TemperatureSample    Additives (wt %)        tempera-             density                  permittivity                        (at  coefficientNo. ZnO Ta.sub.2 O.sub.5        ture (° C.)             (g/cm.sup.3)                  (ε.sub.r)                        4.5 GHz)                             (τf) ppm/°C.__________________________________________________________________________1   12  4    1175 3.81 23.1   700 7.802   14  4    1175 4.54 30.9  3270 -0.573   16  4    1175 3.94 23.8  1350 -1.694   12  4    1200 4.45 30.4  3040 3.855   14  4    1200 4.68 32.3  3250 0.006   16  4    1200 4.51 30.1  3200 -0.747   12  4    1225 4.54 31.3  3120 2.898   14  4    1225 4.77 33.1  3050 0.709   16  4    1225 4.58 31.2  2860 -0.5010  12  4    1250 4.71 33.0  2840 2.5711  14  4    1250 4.56 33.4  2840 0.2112  16  4    1250 4.74 32.4  2670 -0.46__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________(BaO-3.8TiO.sub.2 -based dielectric ceramic composition)    Additives      Firing           Firing                Relative        TemperatureSample    (wt %) tempera-           density                permittivity                        Qu value                                coefficientNo. ZnO  Ta.sub.2 O.sub.5      ture (° C.)           (g/cm.sup.3)                (ε.sub.r)                        (at 4.5 GHz)                                (τf) ppm/°C.__________________________________________________________________________13  14 2   1175 3.99 25.1     530    7.3014  14 4   1175 4.54 30.9    3270    -0.5715  14 6   1175 3.89 Unmeasurable                        Unmeasurable                                Unmeasurable16  14 2   1200 4.55 30.8    3190    1.3417  14 4   1200 4.68 32.3    3250    0.0018  14 6   1200 4.43 29.6    3140    -0.0919  14 2   1225 4.64 31.7    2700    1.1320  14 4   1225 4.77 33.1    3050    0.7021  14 6   1225 4.60 31.1    3020    -1.0922  14 2   1250 4.72 32.6    2330    1.3723  14 4   1250 4.56 33.4    2840    0.2124  14 6   1250 4.73 32.6    2550    -0.09__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________(BaO-3.7TiO.sub.2 -based dielectric ceramic composition)    Additives      Firing           Firing                Relative        TemperatureSample    (wt %) tempera-           density                permittivity                        Qu value                                coefficientNo. ZnO  Ta.sub.2 O.sub.5      ture (° C.)           (g/cm.sup.3)                (ε.sub.r)                        (at 4.5 GHz)                                (τf) ppm/°C.__________________________________________________________________________25  15 4   1250 4.85 33.7    2800    2.3426  16 4   1250 4.81 33.0    2550    0.7727  17 4   1250 4.84 32.8    2600    -0.2728  15 4   1225 4.82 33.6    2780    2.4429  16 4   1225 4.73 32.2    2600    1.3130  17 4   1225 4.83 32.7    2750    -0.1631  15 4   1275 4.83 33.7    2680    1.3532  16 4   1275 4.82 33.2    2760    1.1833  17 4   1275 4.86 33.0    2510    -1.3434  15 4   1200 4.68 32.2    3040    1.6935  16 4   1200 4.30 27.7    2950    0.0036  17 4   1200 4.77 32.0    2960    -0.8037  15 4   1175 4.54 30.6    2780    1.4638  16 4   1175 4.11 Unmeasurable                        Unmeasurable                                Unmeasurable39  17 4   1175 4.62 30.6    3210     -1.16__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________(BaO-3.7TiO.sub.2 -based dielectric ceramic composition)    Additives       Firing            Firing                 Relative                       Qu value                            TemperatureSample    (wt %)  tempera-            density                 permittivity                       (at  coefficientNo. ZnO Ta.sub.2 O.sub.5       ture(° C.)            (g/cm.sup.3)                 (ε.sub.r)                       4.5 GHz)                            (τf) ppm/°C.__________________________________________________________________________40  17  2   1175 4.13 25.9  2210 0.2541  17  4   1175 4.62 30.6  3210 -1.1642  17  6   1175 4.00 22.7  1420 -7.8443  17  2   1200 4.30 30.5  2690 1.0244  17  4   1200 4.77 32.0  2960 -0.8045  17  6   1200 4.44 28.3  2720 -2.3746  17  2   1225 4.62 31.3  2830 0.9947  17  4   1225 4.83 32.7  2750 -0.1648  17  6   1225 4.64 30.4  3230 -1.5149  17  2   1250 4.75 32.7  2540 3.3650  17  4   1250 4.84 32.8  2600 -0.2751  17  6   1250 4.84 32.4  2560 -1.60__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________(BaO-3.6TiO.sub.2 -based dielectric ceramic composition)    Additives      Firing           Firing                Relative        TemperatureSample    (wt %) tempera-           density                permittivity                        Qu value                                coefficientNo. ZnO  Ta.sub.2 O.sub.5      ture (°C.)           (g/cm.sup.3)                (ε.sub.r)                        (at 4.5 GHz)                                (τf) ppm/°C.__________________________________________________________________________52  17 4   1250 4.84 33.0    2600    3.5453  18 4   1250 4.84 32.6    2590    1.7154  19 4   1250 4.85 32.2    2490    -0.6155  17 4   1225 4.77 32.4    2780    4.0756  18 4   1225 4.79 32.1    2300    0.5557  19 4   1225 4.79 31.6    2530    -0.9858  17 4   1275 4.85 33.2    2460    3.3959  18 4   1275 4.85 32.9    2690    2.1260  19 4   1275 4.86 32.4    2560    -0.3961  17 4   1200 4.59 30.5    2990    2.6762  18 4   1200 4.65 30.8    2920    0.9563  19 4   1200 4.54 29.3    2900    -0.3564  17 4   1175 4.32 Unmeasurable                        Unmeasurable                                Unmeasurable65  18 4   1175 4.50 29.2    1700    2.4466  19 4   1175 4.30 Unmeasurable                        Unmeasurable                                 Unmeasurable__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________(BaO-3.6TiO.sub.2 -based dielectric ceramic composition)    Additives      Firing           Firing                Relative        TemperatureSample    (wt %) tempera-           density                permittivity                        Qu value                                coefficientNo. ZnO  Ta.sub.2 O.sub.5      ture (° C.)           (g/cm.sup.3)                (ε.sub.r)                        (at 4.5 GHz)                                (τf) ppm/°C.__________________________________________________________________________67  19 2   1175 4.36 27.8    1540    0.7868  19 4   1175 4.30 Unmeasurable                        Unmeasurable                                Unmeasurable69  19 6   1175 4.10 23.7    1740    -2.4170  19 2   1200 4.68 31.0    2650    0.7271  19 4   1200 4.54 29.3    2900    -1.3572  19 6   1200 4.61 29.4    3090    -1.4973  19 2   1225 4.72 31.4    2810    0.5974  19 4   1225 4.79 31.6    2530    -0.9875  19 6   1225 4.73 30.7    2940    -0.7276  19 2   1250 4.78 32.1    2480    0.4677  19 4   1250 4.85 32.2    2490    -0.6178  19 6   1250 4.86 32.0    2900    -0.78__________________________________________________________________________ 
    
     The molded article was fired in the air at 1175°-1275° C. for 4 hours. Finally, the fired article was polished to give a cylindrical article, 16 mm in diameter and 6 mm in height. Thus there were obtained dielectric samples Nos. 1 to 78. 
     The samples were tested for εr, Qu and τf by the parallel conductor plate type dielectric resonator method (TE 011  MODE) at a resonant frequency of 4.5 GHz. They were also tested for sintering density. 
     (2) Evaluation of Performance 
     The characteristic properties of the samples are shown in Tables 1 to 6 and FIGS. 1 to 6. Incidentally, Tables 1 and 2 and FIGS. 1 and 2 are concerned with BaO-3.8TiO 2  -based Tables 3 and 4 and FIGS. 3 and 4 are concerned with BaO-3.7TiO 2  -based compositions; and Tables 5 and 6 and FIGS. 5 and 6 are concerned with BaO-3.6TiO 2   compositions. 
     These results indicate the following. 
     The BaO-3.8TiO 2  -based samples (in Tables 1 and 2 and FIGS. 1 and 2) gave τf values in the range of -1.09 to +3.85 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta 2  O 5  increases. It is a positive value close to zero when the amount of Ta 2  O 5  is 4 wt % and the amount of ZnO is 14 wt %. In other words, it is close to zero when the amount of ZnO is 14-16 wt % and the amount of Ta 2  O 5  is 4-6 wt %. 
     The BaO-3.7TiO 2  -based samples (in Tables 3 and 4 and FIGS. 3 and 4) gave τf values in the range of -2.37 to +3.36 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta 2  O 5  increases. It is a negative value close to zero when the amount of Ta 2  O 5  is 4 wt % and the amount of ZnO is 17 wt %. In other words, it is close to zero when the amount of ZnO is 16-17 wt % and the amount of Ta 2  O 5  is 2-4 wt %. 
     The BaO-3.6TiO 2  -based samples (in Tables 5 and 6 and FIGS. 5 and 6) gave τf values in the range of -1.49 to +3.54 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta 2  O 5  increases. It is a negative value close to zero when the amount of Ta 2  O 5  is 4 wt % and the amount of ZnO is 19 wt %. In other words, it is close to zero when the amount of ZnO is 18-19 wt % and the amount of Ta 2  O 5  is 2-4 wt %. 
     Moreover, it is noted that Qu tends to decrease and εr tends to increase with the increasing firing temperature. The BaO-3.8TiO 2  -based samples gave Qu values in the measurable range of 2330-3250, εr values in the range of 29.55-33.41, and firing densities in the range of 4.43-4.77 g/cm 3  (high enough for practical use). 
     The BaO-3.7TiO 2  -based samples gave Qu values in the measurable range of 2510-3210, εr values in the range of 27.7-33.7, and firing densities in the range of 4.30-4.86 g/cm 3  (except sample No. 38, high enough for practical use). In addition, all the samples (except No. 38) gave sufficiently compact sintered bodies even at comparatively low firing temperatures, 1175° C. and 1200° C. Incidentally, εr values tend to be low when the firing density is low. Presumably, this is due to incomplete densification. 
     The BaO-3.6TiO 2  -based samples gave Qu values in the measurable range of 2300-2990 (except sample No. 65 fired at 1175° C.), εr values in the measurable range of 29.2-33.2, and firing densities in the measurable range of 4.50-4.86 g/cm 3  (except samples Nos. 64 and 66, high enough for practical use). In addition, all the samples (except Nos. 64 and 66) gave sufficiently compact sintered bodies even at comparatively low firing temperatures, 1175° C. and 1200° C. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. In other words, the calcination and firing may be carried out under various conditions, and the BaCO 3  as a raw material of BaO may be replaced by a peroxide, hydroxide, or nitrate.