Intergranular insulation type semiconductive ceramic and method of producing same
BACKGROUND OF THE INVENTION 
     The invention relates to ceramic compositions having high effective dielectric constants for use in capacitors. More particularly, the invention relates to dielectric bodies of the intergranular insulation type and to methods of producing such dielectric bodies. A large effective dielectric constant is achieved in this type of structure by electrically insulating the grain boundaries of a conventional ceramic semiconductor with a material having a high dielectric constant. 
     The invention also relates to the production of semiconductive dielectric bodies, having electrically insulating dielectric materials at their grain boundaries, by coating the body with the insulating material and heating the coated body to diffuse the insulating material along the grain boundaries. 
     U.S. Pat. No. 3,933,668 discloses intergranular insulation type ceramic semiconductor compositions comprising a major portion of SrTiO 3 , minor amounts of Nb 2  O 5  or Ta 2  O 5 , and minor amounts of GeO 2  or ZrO. The ceramic has either (i) Bi 2  O 3  or (ii) Bi 2  O 3 , PbO, and B 2  O 3  diffused therein at the grain boundaries. The first ingredients are mixed, shaped into discs, and fired in a reducing atmosphere at, for example, 1350° C. to 1480° C. The fired discs are then coated with Bi 2  O 3 , alone or in combination with PbO and B 2  O 3 , and then sintered at, for example, 1300° C. for approximately two hours in an oxidizing atmosphere to diffuse the coating into the grain boundaries. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide new materials, for intergranular insulation of semiconductive dielectrics, in order to produce compositions having very high effective dielectric constants, low dissipation factors, and high resistivities over a wide temperature range. 
     The present invention provides an intergranular insulation type dielectric body based on a polycrystalline, semiconductive ceramic which is made of an alkaline-earth metal titanate, zirconate, or combination thereof. The alkaline-earth metal is one or more of barium, strontium, and calcium. An electrically insulating dielectric layer is provided at the grain boundaries of the ceramic body. The electrically insulating dielectric is made of a combination of Bi 2  O 3  and one or more metal oxides. The metal oxides are NiO, Al 2  O 3 , and Cu 2  O. Where the electrically insulating dielectric layer consists solely of Bi 2  O 3  and Cu 2  O, the ratio of Cu 2  O to Bi 2  O 3  is less than 1.5% by weight but greater than 0%. 
     The present invention also provides a method of manufacturing an intergranular insulation type dielectric body from a polycrystalline, semiconductive ceramic body as described above. According to the method of the invention, at least a part of the surface of the ceramic body is coated with a mixture of Bi 2  O 3  and one or more metal oxides. The metal oxides are NiO, Al 2  O 3  and Cu 2  O. Where the coating consists solely of Bi 2  O 3  and Cu 2  O, the ratio of Cu 2  O to Bi 2  O 3  is less than 1.5% by weight but greater than 0%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING p FIG. 1 is a graph showing the change in capacitance of samples 1, 6, 23, 30, 36 and 43 as a function of temperature. 
     FIG. 2 is a graph showing the loss factor of samples 1, 6, 23, 30, 36, and 43 as a function of temperature. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An intergranular insulation type dielectric body according to the invention includes a polycrystalline, semiconductive ceramic body and an electrically insulating dielectric layer situated in the grain boundaries of the ceramic. For the purposes of the present invention, suitable ceramics include alkaline-earth metal titanates, zirconates or combinations thereof. Suitable alkaline-earth metals include barium, strontium, and calcium. The preferred ceramic material is, however, strontium titanate (SrTiO 3 ). 
     Although suitable ceramics may consist solely of strontium titanate, silicon dioxide (SiO 2 ) and strontium carbonate (SrCO 3 ) can be added to the ceramic in order to control the sintering behavior of the ceramic and the grain size and size distribution of the ceramic. Other permissible additions to the ceramic include niobium pentoxide (Nb 2  O 5 ) and tantalum pentoxide (Ta 2  O 5 ), which are n-type dopants, for increasing the conductivity of the ceramic. These latter two additions also affect the sintering behavior of the ceramic. 
     Although the present invention can be used in connection with all of the ceramic materials described above, in the following examples the ceramic material consists of strontium titanate, strontium carbonate, niobium pentoxide, and silicon dioxide. 
     More particularly, in the following examples the ceramic material was made from: 
     1382 grams SrTiO 3  (98.67 weight percent); 
     0.79 grams SrCO 3  (0.056 weight percent); 
     15.14 grams Nb 2  O 5  (1.081 weight percent); 
     2.71 grams SiO 2  (0.193 weight percent); 
     The ceramic material was prepared from these compounds by mixing them thoroughly into slurry form. Sheets were cast from the slurry and pressed into discs having a diameter of 10 millimeters and having a thickness of 0.64 millimeters. The discs were then sintered at 1440° C. for four hours in a reducing atmosphere consisting of 6% H 2  and 94% N 2 . Semiconductive ceramic discs having a diameter of 8.6 millimeters and having a thickness of 0.55 millimeters were obtained. 
     EXAMPLE 1 
     Different quantities (0 to 4 grams) of cuprous oxide (Cu 2  O) were mixed with 20 grams of bismuth oxide (Bi 2  O 3 ) in order to obtain Cu 2  O/Bi 2  O 3  weight ratios of between 0% and 20%. These mixtures were each fired at 850° C. for two hours. Frits having different ratios of Cu 2  O/Bi 2  O 3  were thus obtained. 
     The semiconductive ceramic discs described above were then coated with 10 weight % of Cu 2  O-Bi 2  O 3  frit on one side. The coated discs were heat treated at 1150° C. for 21/2 hours in air to cause the frit material to diffuse along the grain boundaries throughout the discs. Silver electrodes were then applied to both sides of the discs. The dielectric properties of these discs are shown in Table I. In Table I, and Tables II-V, all measurements were made at 1 kilohertz, except for the resistivity which was measured with a direct current. In addition, all measurements were made at 25° C. unless otherwise indicated. 
     
                       TABLE I                                                     
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Sample                                                                    
      Cu.sub.2 O/Bi.sub.2 O.sub.3   ρ50V, 2 min                       
No.   (wt %)       ε.sub.eff                                      
                           tanδ (%)                                 
                                    (ohm-cm)                              
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1     0            26,100  0.4      4.0 × 10.sup.10                 
2     .005         42,800  0.3      6.6 × 10.sup.9                  
3     .01          36,400  0.4      8.6 × 10.sup.9                  
4     .05          50,500  0.3      4.4 × 10.sup.9                  
5     .1           48,500  0.4      9.6 × 10.sup.9                  
6     .5           54,900  0.1      4.8 × 10.sup.10                 
7     1.0          38,400  0.6      2.7 × 10.sup.10                 
8     2.0          41,100  0.2      1.3 × 10.sup.11                 
9     3.0          35,400  1.5      1.8 × 10.sup.10                 
10    5.0          26,800  3.9      1.7 × 10.sup.9                  
11    10.0         22,900  8.2      1.2 × 10.sup.8                  
12    20.0         25,100  12.1     2.2 × 10.sup.7                  
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     EXAMPLE 2 
     Frits having Cu 2  O/Bi 2  O 3  weight ratios of between 0% and 1.5% were prepared the same way as in Example 1. Ceramic discs having the composition described above were then coated with 10 weight % of Cu 2  O-Bi 2  O 3  frit on one side thereof. The coated discs were heat treated at 1100° C. for 21/2 hours in air. Silver electrodes were then applied to both sides of the discs. The dielectric properties of the discs are shown in Table II. 
     
                       TABLE II                                                    
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Sample                                                                    
      Cu.sub.2 O/Bi.sub.2 O.sub.3   ρ50V, 2 min                       
No.   (wt %)       ε.sub.eff                                      
                           tanδ (%)                                 
                                    (ohm-cm)                              
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13    0            32,200  0.4      3.3 × 10.sup.8                  
14    .005         36,600  0.1      4.9 × 10.sup.8                  
15    .01          37,400  0.2      9.1 × 10.sup.8                  
16    .05          35,600  0.1      4.6 × 10.sup.9                  
17    .1           34,700  0.6      4.6 × 10.sup.9                  
18    .5           36,200  0.2      4.6 × 10.sup.9                  
19    1.0          36,900  0.3      9.2 × 10.sup.8                  
20    1.5          30,500  0.4      7.6 × 10.sup.9                  
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     EXAMPLE 3 
     Different quantities (0 to 1.2 grams) of nickel oxide (NiO) were mixed with 20 grams of bismuth oxide in order to obtain NiO/Bi 2  O 3  weight ratios of between 0% and 6%. These mixtures were fired at 850° C. for two hours. Frits having different ratios of NiO/Bi 2  O 3  were thus obtained. 
     Ceramic discs, having the composition described above, were then coated with 10 weight % of the NiO-Bi 2  O 3  frit on one side. The coated discs were heat treated at 1150° C. for 21/2 hours in air. Silver electrodes were then applied to both sides of the discs. The dielectric properties of these discs are shown in Table III. 
     
                       TABLE III                                                   
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Sample                              ρ50V, 2 min                       
No.   NiO/Bi.sub.2 O.sub.3 (wt %)                                         
                    ε.sub.eff                                     
                             tanδ (%)                               
                                    (ohm-cm)                              
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 1    0             26,100   0.4    4.0 × 10.sup.10                 
21    .05           22,700   0.3    6.5 × 10.sup.10                 
22    .1            30,700   0.2    3.1 × 10.sup.10                 
23    .5            33,600   0.2    6.6 × 10.sup.10                 
24    1             32,600   0.2    1.3 × 10.sup.11                 
25    2             28,700   1.0    1.2 × 10.sup.9                  
26    3             26,900   3.0    3.3 × 10.sup.7                  
27    6             27,500   2.5    3.3 × 10.sup.8                  
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     EXAMPLE 4 
     Different quantities (0 to 0.6 grams) of alumina (Al 2  O 3 ) were mixed with 20 grams of bismuth oxide in order to obtain Al 2  O 3  /Bi 2  O 3  weight ratios of between 0% and 3%. These mixtures were then fired at 850° C. for two hours. Frits having different ratios of Al 2  O 3  /Bi 2  O 3  were thus obtained. 
     Ceramic discs having the composition described above were then coated with 10 weight % of the Al 2  O 3  -Bi 2  O 3  frit on one side. The coated discs were heat treated at 1150° C. for 21/2  hours in air. Silver electrodes were then applied to both sides of the discs. The dielectric properties obtained are shown in Table IV. 
     
                       TABLE IV                                                    
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Sample                               ρ50V, 2 min                      
No.   Al.sub.2 O.sub.3 /Bi.sub. 2 O.sub.3 (wt %)                          
                     ε.sub.eff                                    
                             tanδ (%)                               
                                    (ohm-cm)                              
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 1    0              26,100  0.4    4.0 × 10.sup.10                 
28    .05            20,100  0.4    8.0 × 10.sup.10                 
29    .1             20,900  0.5    1.0 × 10.sup.11                 
30    .5             25,600  0.5    4.9 × 10.sup.10                 
31    1.0            20,500  0.8    7.4 × 10.sup.10                 
32    1.5            15,500  2.5    5.0 × 10.sup.10                 
33    3.0            12,100  2.0    5.1 × 10.sup.10                 
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     EXAMPLE 5 
     Different quantities of Cu 2  O, Al 2  O 3  and NiO were mixed with 20 grams of bismuth oxide in order to obtain (Al 2  O 3  +Cu 2  O+NiO)/Bi 2  O 3  weight ratios of between 0% and 1.5%. These mixtures were fired at 850° C. for two hours to obtain frits. 
     Ceramic discs having the composition described above were then coated with 10 weight % of the frit on one side. The coated discs were heat treated at 1150° C. for 21/2 hours in air. Silver electrodes were then applied to both sides of the discs. The dielectric properties obtained are shown in Table V. 
     
                                           TABLE V                                 
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Sample No.                                                                
      Cu.sub.2 O/Bi.sub.2 O.sub.3 (wt %)                                  
                Al.sub.2 O.sub.3 /Bi.sub.2 O.sub.3 wt %)                  
                          NiO/Bi.sub.2 O.sub.3 (wt %)                     
                                   ε.sub.eff                      
                                       tanδ (%)                     
                                            ρ50V, 2                   
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                                            min(ohm-cm)                   
 1    0         0         0        26,100                                 
                                       0.4  4.0 × 10.sup.10         
34    0.25      0         0.05     30,000                                 
                                       0.3  1.8 × 10.sup.11         
35    0.25      0         0.5      29,000                                 
                                       0.3  1.1 × 10.sup.11         
36    0.05      0         0.05     33,000                                 
                                       0.4  1.2 × 10.sup.11         
37    0.05      0         0.5      27,000                                 
                                       0.5  1.0 × 10.sup.11         
38    0.005     0         0.05     29,000                                 
                                       0.4  6.0 × 10.sup.10         
39    0.005     0         0.5      31,000                                 
                                       0.5  5.0 × 10.sup.10         
40    1.0       0         0.05     29,000                                 
                                       0.4    9 × 10.sup.10         
41    1.0       0         0.5      28,000                                 
                                       0.5    9 × 10.sup.10         
42    0.25      0.025     0        25,000                                 
                                       0.4  1.5 × 10.sup.11         
43    0.25      0.05      0        35,000                                 
                                       0.4  3.8 × 10.sup.10         
44    0.25      0.25      0        26,000                                 
                                       0.4  1.0 × 10.sup.11         
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     In the above examples, acceptable results are achieved for the Cu 2  O-Bi 2  O 3  frit where the weight ratio of Cu 2  O/Bi 2  O 3  is between 0 and 1.5%. For the NiO-Bi 2  O 3  and Al 2  O-Bi 2  O 3  systems the preferred weight ratios are 0-2 and 0-1.5 weight %, respectively. For mixtures of frits, the preferred weight ratio is between 0 and 1.5 weight %. 
     Table VI shows the frequency dependence of both the capacitance and the loss factor for Sample Number 6. These dielectric properties are clearly shown to be very stable with respect to frequency changes. 
     
                       TABLE VI                                                    
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Capacitance                                                               
(nF)         (ΔC/C)                                                 
                      tanδ (%)                                      
                                  Frequency                               
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38.2         +0.5%   0.6          120 Hz                                  
38.0                 0.3           1 KHz                                  
37.8         -0.5%   0.3           10 KHz                                 
37.8         -0.5%   0.3           20 KHz                                 
 37.75       -0.7%   0.3           40 KHz                                 
37.7         -0.8%   0.3          100 KHz                                 
 37.65       -0.9%   0.3          200 KHz                                 
37.6         -1.0%   0.4          400 KHz                                 
37.8         -0.5%   0.4          1.0 MHz                                 
39.2         +3.2%   0.7          2.0 MHz                                 
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     FIG. 1 is a graph of the percent change in capacitance as a function of temperature for Sample Nos. 1, 6, 23, 30, 36, and 43. By inspection of this graph it is apparent that Samples Nos. 6 and 30 meet or exceed EIA (Electronic Industries Association) type designation code Y7R. All of the samples exceed EIA type designation codes Z5U and Y7S. 
     FIG. 2 is a graph showing the temperature dependence of the loss factor of Samples Nos. 1, 6, 23, 30, 36, and 43. All of the samples show acceptably low loss over the entire temperature range of -55° to +125° C.