Patent Publication Number: US-5840642-A

Title: Dielectric material for microwave of CaTIO3 -La(Mg1/2 Ti1/2) O3 -LaAlO3 group

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dielectric material for a microwave of a CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  group, and in particular to an improved dielectric material for a microwave of a CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  group which has a low dielectric loss and an excellent temperature coefficient of a resonant frequency. 
     2. Description of the Conventional Art 
     Recently, as a communication system which uses a microwave (a frequency band: 300MHz˜300GHz) for a satellite broadcast including a mobile communication such as a wireless telephone set or a vehicle telephone set advances, the applications of a high frequency dielectric ceramic to a resonator device, a band pass(or blocking) filter, a microwave integrated circuit (MIC), etc. is sharply increased. 
     For the high frequency dielectric used for a communication system using a microwave, 
     1) Since the wavelength of a microwave is inversely proportional to square root of an dielectric constant, the dielectric constant must be high for the compactness of the product, 
     2) Since the dielectric loss is increased proportionally to the frequency, the value of Q (the reverse value of the dielectric loss) must be high for a high performance, and 
     3) The temperature coefficient of the resonant frequency of a dielectric resonator must be small. 
     Additionally, the time lapse-based variation must be small, and the thermal conductivity and mechanical strength must be high. 
     As a microwave dielectric material, there are known a solid solution having a combined perovskite structure such as Ba(Mg 1/3  Ta 2/3 )O 3  having an dielectric constant of 20˜30 and a quality coefficient of Qxf 0  =200,000, a simple solid solution such as (Zr, Sn)TiO 4  having an dielectric constant of 30˜40 and a quality coefficient of Qxf 0  =50,000, a combined phase material such as (Ba, Pb)O-Nd 2  O 3  -TiO 2  having an dielectric constant of 80˜90 and a quality coefficient of Qxf 0  =6,000. The above-described materials will be explained, in more detail, based on the articles by the dielectric constant. 
     First, as materials having a lower dielectric loss while the dielectric constant is below 40, there are known: 
     1) Ba(M +2   1/3  M +5   2/3 )O 3  (M +2  =Mg, Zn, M +5  =Ta, Nb) group  Reference article: K. Matsumoto, T. Hiuga, K. Takada and H. I Chimura, &#34;Ba(Mg 1/3  Ta 2/3 )O 3  ceramics with ultra-low loss at microwave frequencies&#34; In proce, of the sixth IEEE International symposium on Applications of Ferroelectrics, pp. 118-121 (1986)!, 
     2) Ba 2  Ti 9  O 20  group  Reference article: S. Nisikaki et al., &#34;BaO-TiO 2  -WO 3  Microwave Ceramics and Crystalline BaWO 4  &#34;, J. Am. Ceram. Soc., 71(1), C-11˜C-17(1988)!, and 
     3) (Zr, Sn)TiO 4  group  Reference article: K. Wakino et al., &#34;Microwave characteristics of (Zr, Sn)TiO 4  and BaO-PbO-Nd 2  O 3  -TiO 2  Dielectric resonators,&#34; J. Am. Ceram. Soc., 67(4), 278˜281 (1983)!. 
     In addition, as materials having an dielectric constant of above 80 where the dielectric loss is relatively high, and  Qxf 0  (GHz)!&lt;10,000, there are known: 
     1) BaO-Sm 2  O 3  -TiO 2  group  Reference article: J. M. wu and M. C. chang, &#34;Reaction Sequence and Effects of calcination and sintering on Microwave Properties of (Ba, Sr)O-Sm 2  O 3  -TiO 2  ceramics,&#34; J. Am. Ceram. Soc., 73(6), 1599˜1605 (1990)!, 2) (Ba, Pb)O-Nd 2  O 3  -TiO 2  group  Reference article: K. Wakino et al., &#34;microwave characteristics of (Zr, Sn)TiO 4  and BaO-PbO-Nd 2  O 3  -TiO 2  Dielectric Resonators,&#34; J. Am. Ceram. Soc., 67(4), 278˜281 (1983)!, and 
     3) (Pb, Ca)ZrO 3  group  Reference article: J. Kato, &#34;Material Produces Small Resonators with High Dielectric Constant,&#34; JEE, Sep., 114˜118 (1991)!. 
     Generally, in the material having a high dielectric constant, a dielectric loss and the temperature coefficient of a resonant frequency are increased due to the bipolar of a dielectric and the internal defects thereof. In the high frequency dielectric, when the temperature coefficient of a resonant frequency is ±10 ppm/°C., the application thereof becomes available. 
     In the case of CaTiO 3 , the dielectric constant is 170˜180 at 3 GHz, namely, the same is relatively high. However, there is a problems in that the temperature coefficient of the resonant frequency is +800ppm/°C., namely, the same is greatly high. In the case of La(Mg 1/2  Ti 1/2 )O 3 , the dielectric constant is about 28, namely, the same is low. However, the quality coefficient  Qxf 0 , (GHz)! is about 55,000, namely, the same is greatly high, and the temperature coefficient of a resonant frequency is -70 ppm/°C. In addition, in the case of LaAlO 3 , the dielectric constant is about 26˜27, and the temperature coefficient is about +90 ppm/°C. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a dielectric material for a microwave of CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  group which overcomes the aforementioned problems encountered in the conventional art. 
     It is another object of the present invention to provide a dielectric material for a microwave of CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  group which has an dielectric constant of below 40, a low dielectric loss, and a temperature coefficient of a resonant frequency which is variable from a positive (+) value to a negative (-) value with respect to the value of 0 ppm/°C. by using the advantages of the characteristics of CaTiO 3 , La(Mg 1/2  Ti 1/2 )O 3  and LaAlO 3  and improving the problems encountered in the conventional art. 
     To achieve the above objects, there is provided a dielectric material for a microwave of CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  group which includes a composition condition of 0.3≦x≦0.6 and 0≦y≦1.0 wherein (1-x)CaTiO 3  -x (1-y)La(Mg 1/2  Ti 1/2 )0 3  -yLaAlO 3 . 
     Additional advantages, objects and features of the invention will become more apparent from the description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a CaTiO 3  -La(Mg 1/2  Ti 1/2 )O 3  -LaAlO 3  composition diagram illustrating a composition range of a dielectric material according to the present invention; and 
     FIG. 2 is a graph illustrating a temperature coefficient of a resonant frequency in accordance with a composition variation of a dielectric material according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a dielectric material for a microwave, a perovskite type solid solution made of CaTiO 3  and La(Mg 1/2  Ti 1/2 )O 3  as a main composition is used. The composition range thereof is shown in FIG. 1. 
     The range of the composition is limited because since the temperature coefficient of the resonant frequency of a dielectric is +50˜-40ppm/°C., it is possible to fabricate a dielectric for a microwave having a low dielectric loss and a better temperature characteristic by using itself or by adding an additive thereto by a small amount. 
     The characteristic of a dielectric material for a microwave and a detailed fabrication method thereof according to the present invention will now be explained with reference to the following example. 
     EXAMPLE 
     As a source material powder, CaCO 3 , La 2  O3, Al 2  O3, TiO2 and MgO were used. The above-described material powders were accurately measured and mixed with each other based on the composition range shown in Table 1. The mixed material powders were calcinated in the air at a temperature of 1,250° C. for 4 hours, thus fabricating a solid solution having a perovskite structure. 
     The mixed powders were well ground, and then a circular test sample having a diameter of 15 mm and a thickness of 6˜7 mm was fabricated by a pressure and molding method. Thereafter, the resultant structure was sintered in the air at a temperature 1,450°˜1,550° C. for 2˜6 hours. After the sintering was performed with respect thereto, the contraction rate of the test sample was 15˜20%. 
     Both the side surfaces of the sintered test sample were well polished by using a polishing paper of #3000, and the dielectric constant, the value of Q, and the temperature coefficient of the resonant frequency were measured by a dielectric resonant method after inserting the polished test sample into an optical waveguide tube. At this time, the measuring frequency was 4˜6GHz, and the measuring temperature range was -30°˜80° C. The microwave dielectric characteristic of the test sample is shown in Table 1 by the composition rate. 
     As shown in Table 1, there was not a great variation in the high frequency is dielectric characteristic in accordance with the added amount of La(Mg 1/2  Ti 1/2 )O 3 , but the high frequency dielectric characteristic was greatly varied in accordance with the added amount of CaTiO 3 . Namely, as the added amount of CaTiO 3  is increased, the dielectric constant was steadily increased within a range of 30˜50. However, the value of  Qxf 0  (GHz)! was greatly decreased, and the temperature coefficient of the resonant frequency was gradually varied within a range from a negative value to a positive value as shown in FIG. 2. Therefore, it is possible to fabricate a better microwave dielectric which contains 0.45˜0.55 mol of CaTiO 3 , having an dielectric constant of 42˜48 and the value of Qxf 0  (GHz) of above 45,000, and a temperature coefficient of a resonant frequency of ±10 ppm/°C. In addition, it is possible to obtain a desired temperature coefficient of a resonant frequency which is variable from a positive (+) value to a negative value (-) under the same condition by adjusting the added amount of CaTiO 3 . 
     
                       TABLE 1                                                     
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The high frequency dielectric characteristic of                           
(1 - x)CaTiO.sub.3 -x (1 - y)La(Mg.sub.1/2 Ti.sub.1/2)O.sub.3 -yLaAlO.sub.
3 !                                                                       
Test    Composition (mol)                                                 
Sample               Dielectric                                           
No.     x      y     Constant                                             
                             Q × f.sub.o (GHz)                      
                                     TCF (ppm/°C.)                 
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1       0.3    0     44.2    47842   +47                                  
2       0.3    0.2   44.4    46437   +45                                  
3       0.3    0.4   45.0    46743   +40                                  
4       0.3    0.5   45.9    44372   +35                                  
5       0.3    0.6   47.1    41748   +31                                  
6       0.3    0.8   47.6    40093   +26                                  
7       0.4    0     39.86   52247   +15                                  
8       0.4    0.2   40.02   51072   +13                                  
9       0.4    0.4   41.73   48092   +12                                  
10      0.4    0.5   42.36   52183   +10                                  
11      0.4    0.6   41.3    49979   +9                                   
12      0.4    0.8   40.26   49963   +5                                   
13      0.5    0     37.8    53323   -13                                  
14      0.5    0.2   37.6    52718   -16                                  
15      0.5    0.4   38.9    52643   -17                                  
16      0.5    0.5   40.5    52983   -19                                  
17      0.5    0.6   39.2    53040   -20                                  
18      0.5    0.8   37.3    53469   -22                                  
19      0.6    0     35.5    55443   -33                                  
20      0.6    0.2   36.4    54939   -34                                  
21      0.6    0.4   37.1    56843   -35                                  
22      0.6    0.5   38.8    57934   -36                                  
23      0.6    0.6   37.4    58326   -38                                  
24      0.6    0.8   33.6    58949   -39                                  
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     Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the accompanying claims.