Abstract:
Disclosed herein is a microwave dielectric ceramic composition which comprises a compound represented by the formula BaO·xTiO 2  (where 3.7≦x≦4.5) which is incorporated with 1.0-10 wt. % Ba 3  Ti 12  Zn 7  O 34  and 2-8 wt. % Ta 2  O 5 . It has well-balanced performance owing to a Q value not less than 8000 (at 4.5 GHz), a relative permittivity ε not less than 31, and a practical temperature coefficient in the range of -15 to +10 ppm/°C. It may be further incorporated with 0.1-0.4 wt. % MnO 2  for improved sinterability in addition to the above-mentioned performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of industrial application 
     The present invention relates to a microwave dielectric ceramic composition, more particularly, to a microwave dielectric ceramic composition which has a high Q value and relative permittivity and also has a temperature coefficient within the range of practical use. The present invention is also concerned with a microwave dielectric ceramic composition which can be produced easily by firing. The microwave dielectric ceramic composition of the present invention will be used for dielectric resonators in the microwave frequency region, microwave IC substrates, and impedance matching for microwave circuits. 
     2. Prior art 
     A microwave dielectric ceramic composition (simply referred to as a dielectric ceramic composition hereinafter) tends to increase in dielectric loss with the increasing frequency. Therefore, there is a demand for a dielectric ceramic composition which has a high Q value in the frequency region of microwave. 
     Known dielectric ceramic compositions include those of ZrO 2  --SnO 2  --TiO 2  system and BaO--TiO 2  system, sometimes with a part thereof replaced by any other element. (See Japanese Patent Publication No. 37807/1989.) These known products, however, suffer from a disadvantage of low Q and relative permittivity, undesirable temperature coefficient, and poor sinterability. In addition, there is a demand for a new dielectric ceramic composition which is different in components from conventional ones and can be easily produced with good sinterability. 
     SUMMARY OF THE INVENTION 
     Object of the invention 
     The present invention was completed with the foregoing in mind. It is an object of the present invention to provide a dielectric ceramic composition which has a high Q value and relative permittivity and also has a temperature coefficient of resonance frequency within the range of practical use. It is another object of the present invention to provide a dielectric ceramic composition which can be produced easily by firing. 
     Features of the invention 
     The present inventors investigated the dielectric ceramic compositions of Ba--Zn--Ta--Ti--O system which satisfy all of the following requirements, and the results of the investigation led to the present invention. 
     (1) Q value: generally not less than 5000, particularly not less than 8000 (at 4.5 GHz) 
     (2) Relative permittivity εr: not less than 30 
     (3) Temperature coefficient τf: -20 to +20 ppm/°C. particularly -15 to +10 ppm/°C. 
     (4) Sinterability: good; to be capable of firing satisfactorily at 1300° C. or below. 
     The first invention is embodied in a dielectric ceramic composition which comprises a compound represented by the formula BaO·xTiO 2  (where 3.7≦x≦4.5) which is incorporated with 1.0-10 wt % Ba 3  Ti 12  Zn 7  O 34  and 2-8 wt % Ta 2  O 5 . The third invention is embodied in a dielectric ceramic composition as defined in the first invention, which is further incorporated with 0.1-0.4 wt % MnO 2 . 
     With a value of x smaller than 3.7, the dielectric ceramic composition has an undesirably low relative permittivity εr; with a value of x greater than 4.5, the dielectric ceramic composition has an undesirably low Q value. 
     Ba 3  Ti 12  Zn 7  O 34  is added to shift the temperature coefficient τf of resonance frequency to the plus side. Without this compound, the dielectric ceramic composition would have a temperature coefficient τf of resonance frequency which is high in the minus side. An excess amount of this compound more than 10 wt % will cause the temperature coefficient τf to increase in the plus side and also cause both the Q value and relative permittivity εr to decrease. 
     Ta 2  O 5  is added to increase the Q value. It takes effect when added in an amount more than 2 wt %. However, it lowers the Q value when added in an excess amount more than 8 wt %. 
     MnO 2  is added to improve the sinterability. An amount of 0.1-0.4 wt % permits sintering at low temperatures (1250°-1300° C.). With any amount outside the specified range, the ceramic composition needs high temperatures (above 1300° C.) for sintering. 
     In the second, fourth, and sixth inventions, the dielectric ceramic composition comprises BaO·xTiO 2  in which 4.0≦x≦4.4, so that it has a temperature coefficientof -5 to +5 ppm/°C. (or close to zero). 
     In the fifth invention, the dielectric ceramic composition is incorporated with 4.0-10 wt % Ba 3  Ti 12  Zn 7  O 34 , so that it has a temperature coefficient of -10 to +9 ppm/°C. (or close to zero). 
     The dielectric ceramic composition of the present invention may be produced by firing the raw materials mixed as prescribed. For example, the production process starts with the synthesis of BaO·xTiO 2  powder having a desired value of x from a substance which becomes BaO on firing and TiO 2  powder, and the BaO·xTiO 2  powder is incorporated with Ba 3  Ti 12  Zn 7  O 34 , MnO 2 , and Ta 2  O 5  powder in prescribed amounts. The mixture is finally sintered. 
     Effect of the invention 
     As mentioned above, the dielectric ceramic composition of the present invention has a high Q value and relative permittivity and a temperature coefficient τf in the range of practical use. Because of this balanced performance, it is of great practical use. 
     Moreover, the dielectric ceramic composition incorporated with 0.1-0.4 wt % MnO 2  has good sinterability on producing it in addition to the above-mentioned balanced performance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples 
     The invention will be described in more detail with reference to the following examples. 
     (1) Preparation of samples 
     As much BaCO 3  powder (99.9% pure) and TiO 2  powder as required were crushed and mixed using a dry mixer, and the mixture was heated at 1000° C. for 6 hours in an atmosphere of oxygen to give BaO·xTiO 2  powder (where 3.7≦x≦4.5). 
     Similarly BaCO 3  powder (99.9% pure), TiO 2  powder, and ZnO powder were crushed and mixed in a molar ratio of Ba:Ti:Zn=3:12:7 using a dry mixer, and the mixture was heated at 1250° C. for 10 hours in an atmosphere of oxygen to give Ba 3  Ti 12  Zn 7  O 34  powder. 
     The thus obtained BaO·xTiO 2  powder was incorporated with Ba 3  Ti 12  Zn 7  O 34  powder, MnO 2  powder, and Ta 2  O 5  powder according to the formulations shown in Tables 1 to 4. The resulting mixture was comminuted and mixed using a dry mixer and further comminuted together with an adequate amount of organic binder and water using a ball mill containing alumina balls 30 mm in diameter. The mixture was granulated by spray drying. The granules were formed into disks (18 mm in diameter and 15 mm thick) by pressing under a pressure of 1500 kg/cm 2 . 
     The green compacts were fired at 1250°-1350° C. for 10 hours in the atmosphere. The fired products were finally finished by grinding into disks (12 mm in diameter and 10 mm thick) as dielectric samples. 
     The samples, which were fired at sintering temperature where relative permittivity εr was lowest, were tested for Q value (1/tan δ), relative permittivity εr, and temperature coefficient τf of resonance frequency. Q and εr were measured by the dielectric resonator method, and τf was calculated from the equation τf=-1/2τε-α in the range from -30° C. to 80° C. The resonance frequency is about 4.5 GHz. The coefficient of linear expansion (α) is 9.5-11 ppm/°C. The τε is the temperature coefficient of εr. 
     (2) Evaluation of performance 
     The characteristic properties of the dielectric samples are shown in Tables 1 to 4, in which unsatisfactory results (with the Q value lower than 5000 and the temperature coefficient smaller than -20) are marked with * and slightly poor results (although practical) are marked with #. Those samples which had to be fired at a temperature higher than 1300° C. for satisfactory sintering are marked with x. Those samples which are fired at a temperature not more than 1300° C. for satisfactory sintering are marked with ◯. Those samples which are outside the scope of the present invention are marked with x in the column of &#34;Overall rating&#34;, and those samples which are either inside or outside of the scope of the present invention or poor in sinterability are marked with Δ. 
     
                                           TABLE 1__________________________________________________________________________                   Firing                        Relative                              Q value (=                                    TemperatureSample    Value   Additives (wt %)                   tempera-                        permittivity                              1/tan δ) at                                    coefficient                                           Sinter-                                               OverallNo. of x   Ba.sub.3 Ti.sub.12 Zn.sub.7 O.sub.34           MnO.sub.2               Ta.sub.2 O.sub.5                   ture (° C.)                        (ε.sub.r)                              4.5 GHz                                    (τ.sub.f) ppm/°C.                                           ability                                               rating__________________________________________________________________________1   *3.6   5       0.3 4   1250 31.0  7800  -12    ◯                                               X2   3.7 5       0.3 4   1250 34.2  9600  -10    ◯                                               ◯3   3.8 5       0.3 4   1250 34.6  9500  -8     ◯                                               ◯4   3.9 5       0.3 4   1250 35.1  9200  -5     ◯                                               ◯5   4.0 5       0.3 4   1250 35.5  9000  -3     ◯                                               ◯6   4.1 5       0.3 4   1250 36.0  9100   1     ◯                                               ◯7   4.2 5       0.3 4   1250 36.7  9800   1     ◯                                               ◯8   4.3 5       0.3 4   1250 37.1  9000   2     ◯                                               ◯9   4.4 5       0.3 4   1250 37.5  9300   3     ◯                                               ◯10  4.5 5       0.3 4   1250 38.2  8600   5     ◯                                               ◯11  *4.6   5       0.3 4   1250 34.7  *4200  3     X   X12  3.8 *0      0.3 4   1260 37.2  8400  *-24   ◯                                               X__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________                   Firing                        Relative                              Q value (=                                    TemperatureSample    Value   Additives (wt %)                   tempera-                        permittivity                              1/tan δ) at                                    coefficient                                           Sinter-                                               OverallNo. of x   Ba.sub.3 Ti.sub.12 Zn.sub.7 O.sub.34           MnO.sub.2               Ta.sub.2 O.sub.5                   ture (°C.)                        (ε.sub.r)                              4.5 GHz                                    (τ.sub.f) ppm/°C.                                           ability                                               rating__________________________________________________________________________13  3.8 1       0.3 4   1260 36.5  8900  -15    ◯                                               ◯14  3.8 2       0.3 4   1260 35.8  9200  -12    ◯                                               ◯15  3.8 4       0.3 4   1260 35.0  9100  -10    ◯                                               ◯16  3.8 6       0.3 4   1260 34.0  9400   -5    ◯                                               ◯17  3.8 8       0.3 4   1260 33.5  8200    0    ◯                                               ◯18  3.8 10      0.3 4   1260 33.0  #7800.sup. .sup.                                      6    Δ                                               ◯19  3.8 *11     0.3 4   1260 31.7  *4900  12    Δ                                               X20  4.2 *0      0.3 4   1290 39.1  10200 *-21   ◯                                               X21  4.2 1       0.3 4   1290 39.0  11000 -16    ◯                                               ◯22  4.2 2       0.3 4   1290 38.8  11000 -12    ◯                                               ◯23  4.2 4       0.3 4   1290 38.5  11000  -6    ◯                                               ◯24  4.2 6       0.3 4   1290 38.2  10300  -2    ◯                                               ◯__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________                   Firing                        Relative                              Q value (=                                    TemperatureSample    Value   Additives (wt %)                   tempera-                        permittivity                              1/tan δ) at                                    coefficient                                           Sinter-                                               OverallNo. of x   Ba.sub.3 Ti.sub.12 Zn.sub.7 O.sub.34           MnO.sub.2               Ta.sub.2 O.sub.5                   ture (°C.)                        (ε.sub.r)                              4.5 GHz                                    (τ.sub.f) ppm/°C.                                           ability                                               rating__________________________________________________________________________25  4.2 8       0.3 4   1290 37.7  9400   2     ◯                                               ◯26  4.2 10      0.3 4   1290 37.4  9200   9     ◯                                               ◯27  4.2 11      0.3 4   1290 37.0  9100  #11    ◯                                               Δ28  3.8 5       *0  4   1310 34.5  9200  -5     X   Δ29  3.8 5       0.1 4   1260 34.5  9200  -4     ◯                                               ◯30  4.2 5       *0  4   1330 38.7  11000 -4     X   Δ31  4.2 5       0.1 4   1280 38.6  10500 -4     ◯                                               ◯32  3.8 5       0.4 4   1240 34.4  9200  -5     ◯                                               ◯33  3.8 5       *0.5               4   1310 34.1  #6800.sup. .sup.                                    -7     X   Δ34  4.2 5       0.4 4   1290 38.5  10200 -4     ◯                                               ◯35  4.2 5       *0.5               4   1310 38.4  9400  -4     X   Δ36  3.8 5       0.3 *1  1250 34.3  *4800 -5     ◯                                               X__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________                   Firing                        Relative                              Q value (=                                    TemperatureSample    Value   Additives (wt %)                   tempera-                        permittivity                              1/tan δ) at                                    coefficient                                           Sinter-                                               OverallNo. of x   Ba.sub.3 Ti.sub.12 Zn.sub.7 O.sub.34           MnO.sub.2               Ta.sub.2 O.sub.5                   ture (°C.)                        (ε.sub.r)                              4.5 GHZ                                    (τ.sub.f) ppm/°C.                                           ability                                               rating__________________________________________________________________________37  3.8 5       0.3 2   1250 34.3  #5800.sup. .sup.                                    -4     ◯                                               Δ38  3.8 5       0.3 6   1250 34.2  9400  -5     ◯                                               ◯39  3.8 5       0.3 8   1250 34.1  8400  -5     ◯                                               ◯40  3.8 5       0.3 *9  1250 33.9  *4800 -5     ◯                                               X41  4.2 5       0.3 *1  1290 38.4  *4200 -3     ◯                                               X42  4.2 5       0.3 2   1290 38.5  #6200.sup. .sup.                                    -4     ◯                                               Δ43  4.2 5       0.3 6   1290 38.4  11200 -3     ◯                                               ◯44  4.2 5       0.3 8   1290 38.4  9400  -4     ◯                                               ◯45  4.2 5       0.3 *9  1290 38.5  *4600 -7     ◯                                               X46  4.4 5       *0  4   1330 37.8  9700   0     Δ                                               Δ47  *4.6   5       *0  4   1330 36.7  *4600  5     X   X48  *3.6   5       *0  4   1310 30.7  6000  -15    X   X__________________________________________________________________________ 
    
     Tables 1 to 4 show the following. Sample No. 1 prepared from BaO·xTiO 2  in which x is 3.6 has a low εr value of 31.0. Sample No. 11 prepared from BaO·xTiO 2  in which x is as great as 4.6 has a low Q value of 4200. Samples Nos. 2 to 10 prepared from BaO·xTiO 2  in which x is 3.7-4.5 have adequate εr, Q, and τf values which are well balanced. 
     Samples vary in τf values depending on x of BaO·xTiO 2 . Samples Nos. 5-9 (x; 4.0-4.4) of samples Nos. 1˜10 (x; 3.6-4.6, containing MnO 2 ), have τf values of from -3 to +3 ppm/°C., and Samples Nos. 28, 30, 46-48 (x; 3.6-4.6, not containing MnO 2 ) prepared from BaO·xTiO 2  in which x is 4.0-4.4 have τf values of from -4.5 to 0 ppm/°C. In other words, BaO·xTiO 2  in which x is 4.0-4.4 gives rise to preferred samples having τf values close to 0 ppm/°C. 
     Samples Nos. 12 and 20 (not containing Ba 3  Ti 12  Zn 7  O 34 ) have great negative τf values of -24 and -21 ppm/°C. respectively. On the other hand, sample No. 19 (containing 11 wt % Ba 3  Ti 12  Zn 7  O 34 ) has a Q value of 4900, a slightly low εr value of 31.7, and a great positive τf value of +12 ppm/°C. They all have nonlinear τf values. Thus it is impossible for them to have a properly controlled τf value. 
     The better performance of samples is achieved with Ba 3  Ti 12  Zn 7  O 34  in an amount from 4.0 to 10 wt %. For example, samples Nos. 5-18 have τf values of -10 to +6 ppm/°C. (which are close to zero), and samples Nos. 23-26 have τf values of -6 to +9 ppm/°C. (which are close to zero). In the case where the amount of Ba 3  Ti 12  Zn 7  O 34  is outside the specified range, as in samples Nos. 13, 14, 19, 21, 22, and 27, the desired performance is not achieved. 
     The content of Ta 2  O 5  affects Q values. Samples Nos. 36 and 41, containing 1 wt % Ta 2  O 5 , gave a small Q value of 4800 and 4200, respectively. Samples Nos. 40 and 45, containing 9 wt % Ta 2  O 5 , also gave a small Q value of 4800 and 4600, respectively. 
     The content of MnO 2  affects sinterability. Samples Nos. 28 and 30, not containing MnO 2 , need firing at temperatures higher than 1300° C. for complete sintering (production of high-density sintering body). Samples Nos. 33 and 35, containing 0.5 wt % MnO 2 , are poor in sinter-ability. 
     Except for the above-mentioned samples, samples Nos. 2-10, 13-18, 21-26, 29, 31, 32, 34, 37-39, 42-44, which contain BaO·xTiO 2  wherein 3.7≦x≦4.5, 1.0-10 wt % Ba 3  Ti 12  Zn 7  O 34 , 2-8 wt % Ta 2  O 5 , and 0.2-0.4 wt % MnO 2 , are superior in all of relative permittivity, Q value, temperature coefficient, and sinterability and hence have well-balanced performance. Samples Nos. 28 and 30, which contain all the components in amounts as specified above except MnO 2 , which contain BaO·xTiO 2  wherein 3.7≦x≦4.5, 1.0-10 wt % Ba 3  Ti 12  Zn 7  O 34  and 2-8 wt % Ta 2  O 5 , have good performance for practical use except sinterability. 
     The above-mentioned embodiments may be modified within the scope of the invention according to the intended object and application. In other words, the calcining and firing temperatures may be properly changed. In addition, BaCO 3  as the starting material for BaO may be replace by a peroxide, hydroxide, or nitrate.