Dielectric ceramic composition for high frequency purposes
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a dielectric ceramic composition, which has purposes for a microwave frequency range, particularly a frequency range above the X-band and has a high dielectric constant, a high unloaded Q and stable temperature characteristics in such high frequency ranges. 
     2. Description of the Prior Art 
     With recent development of communication networks the operating frequency range is increased up to the microwave range. In this connection, a dielectric ceramic is finding such applications as impedance matching in dielectric resonators, microwave integrated circuit substrates and various microwave circuits in the microwave frequency range. Particularly, its demand is recently increasing for the purposes of frequency stabilization of filters, guns and FET microwave oscillators. In the meantime, there is a recent trend for smaller sizes of microwave circuits. The size of the microwave circuit depends on the wavelength of a propagating electromagnetic wave. The wavelength of the electromagnetic wave propagating through a microwave circuit utilizing a dielectric is given as λ 0  /√ε where λ 0  is the wavelength of the wave propagating through vacuum and ε is the relative dielectric constant. This means that the higher ε is, the smaller circuit element can be realized. For this reason, there is a strong demand for a dielectric ceramic composition, which causes low loss, has stable temperature characteristics and has a high dielectric constant. 
     Prior art dielectric ceramic materials include ZrO 2  -SnO 2  -TiO 2 , BaO-TiO 2 , partly substituted for by different elements, dielectric ceramic having positive temperature coefficient of the relative dielectric constant and mixtures of glass and TiO 2 , with the temperature coefficient of the relative dielectric constant thereof being negative, for providing controlled temperature coefficients of the relative dielectric constant. These materials, however, have various problems, e.g., low relative dielectric constant, low unloaded Q and incapability of providing a desired temperature coefficient. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide a dielectric ceramic composition, which is free from the above difficiencies and has a high dielectric constant, low loss and stable temperature characteristics suited for the microwave frequency range. The dielectric ceramic composition according to the present invention is expressed as (Ba X  Sr 1-x )(Ni 1/3  Nb 2/3 )O 3  and mole fraction range of 0≦x&lt;1. 
     The foregoing information and sample are presented herein for illustrative purpose only and are not intended to unduly limit the scope of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Test values obtained with the composition according to the present invention are given below. 
     SAMPLES 
     BaCO 3 , SrCO 3 , Nb 2  O 5  and NiO were weighed in accordance with the composition noted above and were mixed together with added water using a ball mill, prereacted at 1,000° C. for 2 hours, and milled with balls before formation into disks, which were then sintered in a temperature range of 1,400° to 1,600° C. for 2 hours. The sintered disks were then polished to a predetermined shape. 
     TEST VALUES 
     Parameters as listed in a Table below were measured with x in the formula (Ba x  Sr 1-x ) (Ni 1/3  Nb 2/3 )O 3  in a range of 0 to 1.00. The temperature coefficient of resonant frequency was measured in a temperature range of +25° C. to +85° C. and is given as 
     
         τ.sub.f =-1/2τ.sub.ε -α 
    
     where τ.sub.ε  is the temperature coefficient of the dielectric constant, and α is the linear thermal expansion coefficient of the sample of ceramic. 
     
                       TABLE                                                       
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                                       Linear                             
               Relative       Temperature                                 
                                       thermal                            
               dielectric     coefficient                                 
                                       expansion                          
    Sintering  constant Un-   τ.sub. f of resonant                    
                                       coefficient                        
    temperature                                                           
               Σ  loaded                                            
                              frequence                                   
                                       α                            
x   °C. at 9GHz  Q     (ppm/°C.)                            
                                       (ppm/°C.)                   
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0   1600       31.17    3180  -30      9.9                                
0.1 1550       32.56    3350  -18      11.7                               
0.2 1550       32.60    3900  -14      10.2                               
0.3 1550       33.71    5600  +1       9.1                                
0.4 1550       35.35    9300  +3       9.0                                
0.5 1550       38.26    7630  +6       9.1                                
0.6 1500       38.53    6300  +9       8.9                                
0.7 1500       34.70    3600  +12      9.4                                
0.8 1500       31.76    3300  +14      9.5                                
0.9 1500       31.62    3300  +21      9.3                                
1.0 1450       30.88    4000  +35      9.6                                
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     RESULTS 
     As is obvious from the Table above, the temperature coefficient of the resonant frequency lies in a range of 0±30 ppm/°C. with x in a range of 0≦x&lt;1. Besides, the temperature coefficient τ f  of resonant frequency can be controlled continuously. For x=1, however, the value of τ f  is too high for practical purposes. From the results shown in the Table, it was confirmed that the ceramic composition according to the present invention has low temperature dependency and is stable. It was also confirmed that the relative dielectric constant ε at 9 GHz is above 30 with any value of x, meeting the requirement for use in the microwave frequency range. The unloaded Q was above 3,000 with x in a range of 0≦x≦1. 
     No particular effects could be obtained by increasing the sintering time. Rather, a higher bulk density could be obtained by setting the sintering time to about 2 hours. 
     As has been made obvious from the above test results, with the dielectric ceramic composition according to the present invention, the temperature dependency is low as is seen from the values of the temperature coefficient of resonant frequency as given above, and also the relative dielectric constant ε at 9 GHz is higher than 30 with any value of x. Further, the unloaded Q is suitable for practical purposes. It is thus obvious that the dielectric composition material according to the present invention has a low temperature coefficient, a high dielectric constant and a high unloaded Q and is thus suitable for use in the microwave frequency range, particularly in a frequency range above the x-band.