Abstract:
Disclosed is an SrTiO 3 -based grain boundary barrier layer capacitor which is superb in dielectric constant and temperature characteristics. It is prepared by infiltrating a liquid-phase oxide mixture into a donor-doped SrTiO 3 matrix to form second-phase dielectric layers at the grain boundaries of the matrix. The liquid-phase oxide mixture comprises CaO and BaO in a particular molar ratio. The SrTiO 3 -based grain boundary barrier capacitor exhibits a high dielectric constant and a low dieletric loss with stable temperature characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an SrTiO 3 -based capacitor which has a high dielectric constant with stable temperature characteristics. More particularly, the present invention relates to the use of BaO and CaO as an oxide liquid infiltrant for the formation of second-phase dielectric layers at the grain boundaries of an SrTiO 3 -based grain boundary barrier layer capacitor (hereinafter referred to as “GBBLC”). 
     2. Description of the Prior Art 
     GBBLCs exhibit by high dielectric constants which result from their specific microstructure of thin dielectric layers between semiconducting grains. The preparation of GBBLCs is well known, as reported by M. Fujimoto and W. D. Kingery (“Microstructure of SrTiO 3  internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties”, J. Am. Ceram. Soc., 68[4] 169-173 (1985)), and G. Goodman (“Capacitors Based on Ceramic Grain Boundary Barrier Layer—a Review”, Advanced in Ceramics, vol 1, Grain Boundary Phenomena in Electronic Ceramics, 215-231 (1981)). According to these reports, GBBLCs are prepared via two process steps: i) preparation of semiconducting sintered bodies and ii) infiltration of an oxide liquid into the sintered bodies to form dielectric layers at grain boundaries. For the preparation of semiconducting sintered bodies, donor dopants are usually added to SrTiO3 powder compacts and the donor-added SrTiO 3  compacts are sintered in a reducing atmosphere. The infiltration of an oxide melt in an oxidizing atmosphere then produces dielectric layers at grain boundaries of the semiconducting SrTiO 3  grains. The infiltrants for GBBLCs in current use include mostly low melting point oxides, such as PbO, B 2 O 3 , Bi 2 O 3 , CuO, etc [Takahashi Yasunori, Yamaoka Nobutatsu, Yamaoka Yoshifumi, Kakubari; Shuichi, “Intergranular insulation type polycrystalline ceramic semiconductive composition”, U.S. Pat. No. 3,933,668, Nobutatsu Yamaoka, “SrTiO 3 -Based Boundary-Layer Capacitors”, Am. Ceram. Soc. Bull., 62[8], 1149-1152(1986)]. 
     The dielectric property of the thus prepared SrTiO 3 -based GBBLCs is known to be determined by the infiltrant chemistry and the infiltrated microstructure. According to R. Wernicke (“Two-layer Model explaining the Properties of SrTiO 3  Boundary layer Capacitors”, Advances in Ceramics, vol. 1, Grain Boundary Phenomena in Electronic Ceramics, 272-281, 1981), the dielectric constant of SrTiO 3 -based GBBLCs increases with an increase in the SrTiO 3  grain size and with a decrease in the thicknesses of an infiltrated oxide liquid phase and an oxidized layer formed during the liquid infiltration. In addition, the higher dielectric constant of the dielectric layer formed between SrTiO 3  grains, the higher is the dielectric constant of the fabricated SrTiO 3 -based GBBLC. 
     SUMMARY OF INVENTION 
     With this background in mind, the present inventors repeated intensive and thorough research on SrTiO 3 -GBBLCs and found that BaO and CaO are effective in increasing the dielectric constant of the SrTiO 3 -based GBBLCs. According to the present invention, the infiltration of a conventional oxide liquid phase in combination with a mixture of BaO and CaO into the boundaries of SrTiO 3  grains results in a higher dielectric constant of the SrTiO 3 -based GBBLC with stable temperature characteristics than the infiltration of a conventional oxide liquid phase without BaO and CaO. 
     Therefore, it is an object of the present invention to provide an SrTiO 3 -based GBBLC which is superior in both dielectric constant and temperature characteristics. 
     In accordance with the present invention, the above and other objects could be accomplished by infiltrating an oxide melt, which comprises BaO and CaO, into an SrTiO 3 sintered body. The BaO and CaO may be provided from a BaO precursor and a CaO precursor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 a  is a graph in which the dielectric constant values of the SrTiO 3 -GBBLCs are plotted against temperature for various molar ratios of BaO and CaO; 
     FIG. 1 b  is a graph in which the dielectric loss values of the SrTiO 3 -GBBLCs are plotted against temperature for various molar ratios of BaO and CaO; 
     FIG. 2 a  is a graph which shows the variations of dielectric constant values with temperature for SrTiO 3 -GBBLCs infiltrated with Bi 2 O 3  along and with Bi 2 O 3  together with BaO and CaO of equal molar ratio; 
     FIG. 2 b  is a graph showing the relative change in dielectric constant values of the SrTiO 3 -based GBBLCs of FIG. 2 a , based on the values at 0° C.; 
     FIG. 3 a  is a graph in which the dielectric constant values of the SrTiO 3 -based GBBLCs are plotted against temperature for various molar ratios of BaO and CaO when the infiltration treatment is conducted at 1,300° C. for 8 hours; and 
     FIG. 3 b  is a graph in which the dielectric constant values of the SrTiO 3 -based GBBLCs are plotted against temperature for various molar ratios of BaO and CaO when the infiltration treatment is conducted at 1,300° C. for 12 hours. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, SrTiO 3 -based GBBLCs are prepared by producing sintered SrTiO 3  bodies; producing oxide mixtures comprising BaO, CaO or a combination thereof, which liquefy at high temperatures; and infiltrating the liquid phase-formative oxide mixtures into the sintered SrTiO 3  bodies. Compared with the conventional technique, the present invention is characterized by the existence of BaO and CaO in an oxide mixture for infiltration. 
     In detail, in order to prepare the SrTiO 3 -based GBBLC of the present invention, first, an SrTiO 3  powder is mixed with a donor powder, such as Nb 2 O 5  or La 2 O 3 , and this mixture is subjected to cold pressing to form a predetermined shape. Then, the powder compact is sintered at 1,400° C. or higher for several hours in a reducing atmosphere, for example 95N 2-5 H 2 . 
     Separately, precursor materials for BaO and CaO, which can be converted into BaO and CaO at high temperatures, for example BaCO 3  and CaCO 3 , are mixed at a predetermined molar ratio with ordinary liquid phase-formative oxide materials. 
     Finally, this liquid phase-formative oxide mixture thus prepared is infiltrated into the grain boundaries of the sintered SrTiO 3  bodies. In contrast to the sintering process, this infiltrating process is conducted in an oxidizing atmosphere. As aforementioned, the present invention is characterized by use of BaO and CaO together with an ordinary oxide mixture for infiltration in forming a dielectric layer. The use of BaO and CaO in forming the dielectric layer enjoys the following advantages: 
     First, upon infiltration, the BaO in the liquid phase-formative oxide mixture is expected to react with TiO 2 , which exists at the grain boundaries of the sintered body, to form a BaTiO 3  phase, having a high dielectric constant, at the grain boundaries. 
     Second, BaTiO 3 , which is ferroelectric at room temperature, and exhibits an increase in dielectric constant with temperature increase, can serve to compensate the temperature characteristics of SrTiO 3 , whose dielectric constant decreases with a temperature increase. Therefore, the resultant GBBLC can have stable temperature characteristics, showing that its dielectric constant is not greatly modulated with temperature. 
     Third, CaO in the liquid phase-formative oxide mixture buffers the microstructure change which may be caused by the presence of BaO. Hence, the addition of CaO together with BaO is essential in the present invention. 
     A better understanding of the present invention may be obtained in light of the following examples. In the examples, in order to confirm the improvement in dielectric constant and temperature characteristics by virtue of the existence of BaO and CaO in the infiltrant liquid phase, BaCO 3  and CaCO 3  were used as precursors for BaO and CaO, respectively, while Bi 2 O 3  was selected as the base of the infiltrant liquid phase. However, other oxides which are usually used for conventional SrTiO 3 -based GBBLCs, such as PbO, CuO and B 2 O 3  may be used together with any precursors which provide CaO and BaO at liquid phase infiltrating temperatures. Therefore, the examples are set forth to illustrate but not limit the present invention. 
     EXAMPLE I 
     To SrTiO 3  powder was added 0.2 mol % of Nb 2 O 5 , a donor, and the mixture was sintered at 1,480° C. for 5 hours. To Bi 2 O 3 , which is a liquid phase-formative oxide at an infiltrating temperature, was added BaCO 3  and CaCO 3  at molar ratios as indicated in Table 1, below. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 CaO:BaO 
                   
               
               
                   
                 X* 
                 (mole ratio) 
                 Composition (mole ratio) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                 0:0 
                 1Bi 2 O 3   
               
               
                   
                 0 
                 0:1 
                 0.8Bi 2 O 3 -0.2CaO 
               
               
                   
                 0.2 
                 0.2:0.8 
                 0.8Bi 2 O 3 -0.04CaO-0.16BaO 
               
               
                   
                 0.5 
                 0.5:0.5 
                 0.8Bi 2 O 3 -0.1CaO-0.1BaO 
               
               
                   
                 0.7 
                 0.7:0.3 
                 0.8Bi 2 O 3 -0.14CaO-0.06BaO 
               
               
                   
                 1.0 
                 1:0 
                 0.8Bi 2 O 3 -0.2BaO 
               
               
                   
                   
               
               
                   
                 *mole fraction of CaO in a mixture of CaO and BaO  
               
             
          
         
       
     
     The mixed powders thus obtained were slurried, uniformly coated on the polished surfaces of the sintered specimens, and infiltrated into the sintered specimens for 4hours at 1,300° C., a temperature at which the oxides are a liquid phase. After completion of the infiltration, the opposite surfaces of the specimens were polished to the extent that they had a final thickness of 500 m. 
     An examination was made for the dielectric properties and the temperature dependency of the dielectric constant of the SrTiO 3 -based GBBLCs thus obtained. For this, silver was thinly coated on both surfaces of the infiltrated specimens. The silver-coated specimens were then heat-treated at 600° C. for 10 min. After cooling the specimens with liquid nitrogen, their dielectric constant and its variation with temperature were measured while increasing measurement temperature. The results are shown in FIGS. 1 and 2. 
     FIGS. 1 a  and  1   b  show the change of the dielectric constant and dielectric loss as a function of temperature. As shown in FIGS. 1 a  and  1   b , the highest dielectric constant and the lowest dielectric loss are obtained when BaO and CaO exist at an equal mole (x=0.5) in the liquid phase-formative oxide mixture. On the other hand, when x=0 and 1.0, the lowest dielectric constant and the highest dielectric loss are detected. Numerically, at room temperature, the GBBLC has a dielectric constant of 2.8×10 4  for x=0.5 and a dielectric constant of 1.0×10 4  for x=1.0 and 0. As for the dielectric loss above room temperature, it is as low as 1% or less for x=0.5, but 3 to 5% for x=0 and 1.0. Consequently, better dielectric and dielectric loss values can be obtained when using both CaO and BaO than when using only CaO or BaO. 
     With reference to FIG. 2, the changes of dielectric constant with temperature are compared between SrTiO 3 -based GBBLCs infiltrated with Bi 2 O 3  alone and with equal moles of BaO and CaO together with Bi 2 O 3 . As plotted in FIG. 2 a , higher dielectric constant values are obtained when BaO and CaO are used together with Bi 2 O 3  than when only Bi 2 O 3  is used. The data in FIG. 2 b  demonstrate that the infiltration of BaO and CaO into the SrTiO 3  grain boundaries allows high stability of dielectric constant of the GBBLC with respect to temperature. When x=0.5, the dielectric constant varies only within ±10% over the tested temperature range between −60° C. and 60° C. This variation is about a half of the variation of the specimens infiltrated with Bi 2 O 3  only. 
     EXAMPLE II 
     The same procedure as in Example I was repeated, except an increase in infiltration treatment time, from 4 hours to 8 hours and 12 hours. The resulting GBBLCs were measured for the dielectric constant change with temperature. The results are shown in FIG.  3 . As shown in FIG. 3, the highest dielectric constant value is obtained when equal moles of BaO and CaO were infiltrated into the grain boundary. The dielectric constant is 2.2×10 4  when the infiltration treatment was carried out for 8 hours (FIG. 3 a ) and 1.8×10 4  when the infiltration treatment was carried out for 12 hours. 
     As described hereinbefore, the use of BaO and CaO in the infiltrating oxide liquid phase allows the SrTiO 3 -based GBBLCs to have stable temperature characteristics with high dielectric constants. These GBBLCs fabricated according to the present invention can be widely used in the electronic component industry. 
     The present invention has been describe in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.