Piezoelectric oxide materials
BACKGROUND OF THE INVENTION 
     This invention relates to piezoelectric oxide materials, particularly, to those having a binary metal oxide system of PbTiO 3  -Pb(Co 1/2  W 1/2 )O 3  in which Pb is partly replaced by at least one element selected from the group consisting of Ba, Sr and Ca. 
     As is well known, piezoelectric materials are used in wide fields including ultrasonic vibration elements, transducer elements of, for example, mechanical filters, elements of ceramic filters and ceramic vibrators, and elements of vibrometers and accelerometers. Various efforts are being made to provide improved piezoelectric materials based on a binary metal oxide system of PbTiO 3  -PbZrO 3  for use in such fields. For example, it has been proposed to mix additives such as Bi 2  O 3 , Cr 2  O 3 , MnO 2  and ZnO into a binary system of PbTiO 3  -PbZrO 3  in order to provide materials having an improved piezoelectric property. Further, piezoelectric materials based on a ternary system of PbTiO 3  -PbZrO 3  -(Mg 1/3  Nb 2/3 )O 3  are also under development. However, these piezoelectric materials have a curie point of about 300° C. and, thus, can not be used under temperatures higher than 300° C. Also, these piezoelectric materials have a dielectric constant as large as about 1,000 and, thus, are unsuitable for use in a high frequency region. 
     Also reported are PbTiO 3  type piezoelectric materials having a dielectric constant as small as 150 to 250. However, these materials tend to bear cracks after subjected to a sintering treatment, rendering it impossible to prepare a large sintered mass having a diameter of 20 mm or more. Naturally, the PbTiO 3  type material is unsuitable for use as a piezoelectric material generating a strong ultrasonic wave and for forming transducing elements of probes for linear scan type ultrasonic diagnosis apparatus. An additional difficulty inherent in the PbTiO 3  type material is a severe polarizing condition. Specifically, it is necessary to apply an electric field of as high as 60 KV/cm at 200° C. in order to polarize the PbTiO 3  type piezoelectric material, leading to a low yield of the product. Further, the conventional PbTiO 3  type material has a mechanical quality factor Qm of about 1,000 at most. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide PbTiO 3  type piezoelectric oxide materials having a high Curie point, a low dielectric constant and is easy to be polarized. The piezoelectric material of this invention can be used stably under temperatures higher than 300° C. and in frequency region higher than several MHz. In addition, the polarizing condition of the material of this invention is prominently mild, compared with the conventional PbTiO 3  type piezoelectric material. 
     According to this invention, there is provided a piezoelectric oxide material of a binary metal oxide system having a general formula; 
     
         (1-x)PbTiO.sub.3 -xPb(Co.sub.1/2 W.sub.1/2)O.sub.3 
    
     where, &#34;x&#34; ranges between 0.01 and 0.20, preferably, between 0.01 and 0.10, more preferably, between 0.01 and 0.05, and 0.5 to 20 atomic %, preferably, 0.5 to 10 atomic % of Pb is replaced by at least one element selected from the group consisting of Ba, Sr and Ca. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between the polarization temperature and the electro-mechanical coupling coefficient for a piezoelectric material according to this invention and a conventional piezoelectric material, 
     FIG. 2 is a graph showing the relationship between the temperature and the dielectric constant for piezoelectric materials according to this invention and a conventional piezoelectric material, 
     FIG. 3 is a graph showing the relationship between the temperature and the electro-mechanical coupling coefficient for the samples of the piezoelectric materials shown in FIG. 2, 
     FIG. 4 shows changes with time in frequency constant of piezoelectric materials according to this invention, and 
     FIG. 5 shows the relationship between the mechanical quality factor Qm and the amount of additives consisting of MnO and NiO. 
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
     The piezoelectric oxide material of this invention has a general formula 
     
         Pb.sub.1-a Me.sub.a [(Co.sub.1/2 W.sub.1/2).sub.x Ti.sub.1-x ]O.sub.3 
    
     where, &#34;Me&#34; is at least one metal element selected from the group consisting of Ba, Sr and Ca, 
     &#34;x&#34; ranges between 0.01 and 0.20, preferably, between 0.01 and 0.10, more preferably between 0.01 and 0.05, and 
     &#34;a&#34; ranges between 0.005 and 0.20, preferably, between 0.005 and 0.10. 
     It is possibly to enable the piezoelectric oxide material of the above general formula to exhibit more improved temperature characteristic, time variability and mechanical quality factor by adding at least one additive selected from the group consisting of MnO, NiO and Fe 2  O 3  in an amount of 0.05 to 2.0 wt%, preferably, 0.05 to 1.0 wt%, more preferably, 0.05 to 0.5 wt%. 
     The piezoelectric material of this invention can be prepared without difficulty by employing a conventional powder metallurgical process. For example, predetermined amounts of raw metal oxides including PbO, TiO 2 , CoO, WO 3  and MeO (&#34;Me&#34; representing Ba, Sr or Ca) are sufficiently mixed first at a predetermined ratio by using, for example, a ball mill. Incidentally, metal compounds capable of conversion to metal oxides upon heating such as hydroxides, carbonates and oxalates of metals can also be used as the raw materials in place of the metal oxides. 
     The metal oxide mixture is presintered at, for example, about 600° C. to 900° C., followed by grinding the presintered mass by using, for example, a ball mill so as to obtain a powder of a desired particle size, for example, a powder having a particle size of about 1 to 2μ. A binder such as water or polyvinyl alcohol is added to the powder and then, the mixture is shaped by pressure molding under a pressure of about 0.5 to 2 tons/cm 2 , followed by sintering the shaped mass at about 1,000° C. to 1,250° C. It is important to carry out the sintering treatment within a closed furnace because PbO tends to escape partly by evaporation. In general, it suffices to maintain the highest temperature in the sintering step for about. 
     The sintered mass of metal oxides thus obtained is polarized by a known method. For example, a pair of electrodes are baked to both sides of the sintered mass and a D.C. voltage of an electric field intensity of 40 to 60 KV/cm is applied across the electrodes for 1 to 30 minutes within a silicone oil maintained at 80° to 150° C. so as to polarize the sintered mass. 
     As described previously, the piezoelectric oxide material of this invention is represented by a general formula 
     
         (1-x)PbTiO.sub.3 -xPb(Co.sub.1/2 W.sub.1/2)O.sub.3 
    
     where, &#34;x&#34; ranges between 0.01 and 0.20. 
     If the value of &#34;x&#34; is greater than 0.20, the piezoelectric material has a Curie point of at most 350° C. and, thus, can not be used stably at temperatures higher than 300° C. Further, the value of &#34;x&#34; greater than 0.20 causes the piezoelectric material to have a dielectric constant of at least 250, resulting in difficulty to use the piezoelectric material in a high frequency region. It is important to note that the value of &#34;x&#34; ranging, particularly, between 0.01 and 0.10 enables the piezoelectric material to exhibit a Curie point higher than 400° C., rendering the piezoelectric material particularly suitable for use at high temperatures. 
     In this invention, 0.5 to 20 atomic % of Pb is replaced by at least one element selected from Ba, Sr and Ca so as to facilitate the sintering and polarizing treatments referred to previously. The amount of replacement smaller than 0.5 atomic % fails to produce the particular effect mentioned. On the other hand, the value greater than 20 atomic % causes the piezoelectric material to have a Curie point not higher than 350° C., resulting in difficulty to use the piezoelectric material at temperatures higher than 300° C. It is important to note that the piezoelectric material is enabled to exhibit a Curie point higher than 450° C. particularly where the value of &#34;x&#34; ranges between 0.05 and 0.10 and the amount of Pb replacement ranges between 0.5 and 10 atomic %. Naturally, such a material is particularly suitable for use at high temperatures. 
     Further, at least one additive selected from MnO, NiO and Fe 2  O 3  is used in this invention in an amount of 0.05 to 2.0 wt % so as to improve the temperature characteristic, time variability and mechanical quality factor of the piezoelectric material. If the amount of the additive is smaller than 0.05 wt %, the particular effects mentioned can not be produced. On the other hand, the value greater than 2.0 wt % leads to an unsatisfactory sintering property of the metal oxide mixture. If the amount of the additive ranges between 0.05 and 2.0 wt %, the piezoelectric material is enabled to exhibit a mechanical quality factor of 500 to 2,000 and thus, is rendered suitable for forming various vibration elements. Where the value ranges, particularly, between 0.5 and 1.0 wt %, the material is enabled to exhibit a Curie point higher than 450° C. and a mechanical quality factor exceeding 1,000. In this case, the piezoelectric material is suitable for use as the material of vibration elements exposed to high temperatures. 
     It is convenient to summarize in the following the advantages of this invention over the prior art. 
     (1) PbTiO 3  has a Curie point as high as about 500° C. But, this material is unsatisfactory in sintering property, rendering it difficult to put the material to practical uses. In the present invention, however, Pb(Co 1/2  W 1/2 )O 3  is used as one component of the binary system and Pb is partly replaced by Ba, Sr and Ca. What should be noted is that Co, W, Ba, Sr and Ca act as a mineralizer, thereby facilitating the sintering treatment of the metal oxide mixture. Naturally, the mixture has a low sintering temperature and, thus, the PbO contained in the mixture is prevented from evaporation in the sintering step, thereby permitting producing a dense piezoelectric material without difficulty. 
     It is also important to note that the conventional PbTiO 3  type ceramic must be sintered in a manner to suppress the growth of grains to, for example, 3μ or less in order to prevent the sintered mass from bearing cracks. In the present invention, however, Ba, Sr or Ca partly replacing Pb serves to suppress the grain growth, leading to an improved sintering property of the metal oxide mixture. 
     (2) The component of Ba, Sr or Ca partly replacing Pb also serves to facilitate the polarization of the PbTiO 3  type ceramic. Specifically, it is necessary to apply a D.C. electric field of 60 to 80 KV/cm at such a high temperature as 180° to 200° C. for polarizing the conventional PbTiO 3  type ceramic. In contrast, the piezoelectric oxide material of the present invention can be polarized sufficiently by applying a D.C. electric field of 40 to 60 KV/cm at 80° to 150° C. 
     (3) The use of at least one additive selected from the group of MnO, NiO and Fe 2  O 3  renders the piezoelectric oxide material of the present invention prominently advantageous over the conventional PbTiO 3  type ceramic in temperature characteristic, time variability and mechanical quality factor. For example, the particular additive permits readily preparing a sintered mass of excellent time variability in various forms including a disc having 50 mm of diameter and 1 mm of thickness and a rectangular plate sized 100 mm×20 mm×1 mm. Described in the following are Examples of this invention. 
     EXAMPLES 
     Raw materials of PbO, TiO 2 , CoO, WO 3  and MeO (&#34;Me&#34; representing at least one of Ba, Sr and Ca) were sufficiently mixed at various mixing ratios by a ball mill. Then, each of the mixtures was presintered at 800° C., followed by grinding the presintered mass by a ball mill into a powder having a particle size of 1 to 2μ. The powder sample thus obtained was mixed with polyvinyl alcohol and shaped under a pressure of 1 ton/cm 2 , followed by sintering the shaped mass at 1080° C. to 1200° C. The resultant sintered mass was abraded into a disc 20 mm in diameter and 1.0 mm in thickness and a pair of silver electrodes were baked to both sides of the disc. Then, a voltage generating 60 KV/cm of D. C. field intensity was applied across the electrodes within a silicone oil maintained at 100° C. so as to polarize the disc. 
     Piezoelectric properties of the polarized disc were measured by a standard process described in, for example, &#34;The proceedings of I.R.E., Vol. 137, pp 1378  to 1395, 1949&#34;, the results being shown in Table 1. The symbols used in Table 1 represent: 
     F.T.--Sintering temperature (°C.) 
     D--Specific gravity at 23° C., 
     ε--Dielectric constant measured at 1 KHz and 23° C., 
     Kt--Electro-mechanical coupling coefficient (%), 
     Qm--Mechanical quality factor, and 
     Tc--Curie point. 
     
                                           Table 1                                 
__________________________________________________________________________
        Composition                                                       
         ##STR1##                                                         
                                    F.T D          Kt  T.sub.c            
        a(×10.sup.-2)                                               
                x(×10.sup.-2)                                       
                      1-x(×10.sup.-2)                               
                             b(wt.%)                                      
                                    (°C.)                          
                                        (g/cm.sup.3)                      
                                             ε                    
                                                   (%) (°C.)       
                                                           Q.sub.m        
__________________________________________________________________________
Example 1                                                                 
        Me = Ba 0.5                                                       
                1.0   99.0   --     1200                                  
                                        7.68 163   43.7                   
                                                       510 402            
Example 2                                                                 
        Me = Sr 0.5                                                       
                1.0   99.0   --     1200                                  
                                        7.66 161   44.2                   
                                                       507 396            
Example 3                                                                 
        Me = Ca 0.5                                                       
                1.0   99.0   --     1200                                  
                                        7.63 159   43.9                   
                                                       502 389            
Example 4                                                                 
        Me = Ba 5.0                                                       
                1.0   99.0   --     1200                                  
                                        7.69 169   44.9                   
                                                       493 354            
Example 5                                                                 
        Me =  Sr 5.0                                                      
                1.0   99.0   --     1200                                  
                                        7.67 167   44.9                   
                                                       490 372            
Example 6                                                                 
        Me = Ba 10.0                                                      
                1.0   99.0   --     1200                                  
                                        7.60 173   45.8                   
                                                       483 302            
Example 7                                                                 
        Me = Sr 10.0                                                      
                1.0   99.0   --     1200                                  
                                        7.59 170   46.2                   
                                                       481 333            
Example 8                                                                 
        Me = Sr 20.0                                                      
                1.0   99.0   --     1200                                  
                                        7.54 199   45.3                   
                                                       462 299            
Control 1                                                                 
           --   1.0   99.0   --     1200                                  
                                        7.17 157   27.2                   
                                                       514 344            
Control 2                                                                 
        Me = Sr 0.3                                                       
                1.0   99.0   --     1200                                  
                                        7.40 150   32.1                   
                                                       512 349            
Example 9                                                                 
        Me = Ca 20.0                                                      
                1.0   99.0   --     1200                                  
                                        7.52 196   45.5                   
                                                       443 283            
        Me = Ba 0.3                                                       
Example 10      1.0   99.0   --     1200                                  
                                        7.67 161   45.9                   
                                                       500 377            
        Me = Sr 0.2                                                       
        Me = Sr 3.0                                                       
Example 11      1.0   99.0   --     1200                                  
                                        7.68 174   45.4                   
                                                       483 355            
        Me = Ca 2.0                                                       
        Me = Ba 5.0                                                       
Example 12      1.0   99.0   --     1200                                  
                                        7.60 179   46.2                   
                                                       477 353            
        Me = Ca 5.0                                                       
        Me = Sr 10.0                                                      
Example 13      1.0   99.0   --     1200                                  
                                        7.54 196   45.7                   
                                                       449 311            
        Me = Ca 10.0                                                      
        Me = Ba 0.2                                                       
Example 14                                                                
        Me = Sr 0.2                                                       
                1.0   99.0   --     1200                                  
                                        7.67 162   44.8                   
                                                       498 334            
        Me = Ca 0.1                                                       
        Me = Ba 2.0                                                       
Example 15                                                                
        Me = Sr 2.0                                                       
                1.0   99.0   --     1200                                  
                                        7.66 169   45.5                   
                                                       480 376            
        Me = Ca 1.0                                                       
Control 3                                                                 
        Me = Sr 25.0                                                      
                1.0   99.0   --     1200                                  
                                        7.49 252   43.7                   
                                                       399 277            
        Me = Ba 15.0                                                      
Control 4       1.0   99.0   --     1200                                  
                                        7.40 294   40.0                   
                                                       360 295            
        Me = Sr 15.0                                                      
Example 17                                                                
        Me = Ba 0.5                                                       
                5.0   95.0   --     1140                                  
                                        7.79 174   46.2                   
                                                       490 292            
        Me = Sr 0.3                                                       
Example 18      5.0   95.0   --     1140                                  
                                        7.80 176   46.4                   
                                                       482 283            
        Me = Ca 0.2                                                       
Example 19                                                                
        Me = Ca 5.0                                                       
                5.0   95.0   --     1140                                  
                                        7.73 193   47.0                   
                                                       477 302            
        Me = Ba 2.5                                                       
Example 20      5.0   95.0   --     1140                                  
                                        7.74 190   47.3                   
                                                       479 345            
        Me = Sr 2.5                                                       
Example 21                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   --     1140                                  
                                        7.66 202   48.4                   
                                                       452 314            
        Me = Ba 5.0                                                       
Example 22                                                                
        Me = Sr 5.0                                                       
                5.0   95.0   --     1140                                  
                                        7.66 211   48.0                   
                                                       423 297            
        Me = Ca 5.0                                                       
Example 23                                                                
        Me = Ba 20.0                                                      
                5.0   95.0   --     1140                                  
                                        7.61 220   47.3                   
                                                       411 365            
        Me = Ba 10.0                                                      
Example 24      5.0   95.0   --     1140                                  
                                        7.60 217   47.0                   
                                                       403 372            
        Me = Ca 10.0                                                      
Control 5                                                                 
        Me = Sr 0.3                                                       
                5.0   95.0   --     1140                                  
                                        7.47 166   40.0                   
                                                       493 392            
Control 6                                                                 
        Me = Ca 25.0                                                      
                5.0   95.0   --     1140                                  
                                        7.41 254   30.4                   
                                                       372 300            
Example 25                                                                
        Me = Ba 0.5                                                       
                10.0  90.0   --     1120                                  
                                        7.82 201   44.2                   
                                                       449 420            
        Me = Sr 0.3                                                       
Example 26      10.0  90.0   --     1120                                  
                                        7.81 203   44.4                   
                                                       447 377            
        Me = Ca 0.2                                                       
Example 27                                                                
        Me = Ca 0.5                                                       
                10.0  90.0   --     1120                                  
                                        7.80 197   45.0                   
                                                       444 353            
        Me = Ba 2.5                                                       
Example 28      10.0  90.0   --     1120                                  
                                        7.76 211   47.3                   
                                                       432 333            
        Me =  Sr 2.5                                                      
Example 29                                                                
        Me = Sr 10.0                                                      
                10.0  90.0   --     1120                                  
                                        7.74 225   47.6                   
                                                       421 353            
        Me = Ba 5.0                                                       
Example 30                                                                
        Me = Sr 5.0                                                       
                10.0  90.0   --     1120                                  
                                        7.71 236   47.1                   
                                                       411 383            
        Me = Ca 5.0                                                       
Example 31                                                                
        Me = Ba 20.0                                                      
                10.0  90.0   --     1120                                  
                                        7.67 272   46.0                   
                                                       400 327            
        Me = Ba 10.0                                                      
Example 32      10.0  90.0   --     1120                                  
                                        7.66 254   45.7                   
                                                       400 299            
        Me = Ca 10.0                                                      
Control 7                                                                 
        Me = Ba 0.3                                                       
                10.0  90.0   --     1120                                  
                                        7.50 191   39.9                   
                                                       472 349            
Control 8                                                                 
        Me = Sr 25.0                                                      
                10.0  90.0   --     1120                                  
                                        7.49 305   42.3                   
                                                       372 348            
Example 33                                                                
        Me = Ba 0.5                                                       
                20.0  80.0   --     1080                                  
                                        7.86 215   41.1                   
                                                       391 416            
        Me = Sr 0.3                                                       
Example 34      20.0  80.0   --     1080                                  
                                        7.84 210   41.4                   
                                                       390 422            
        Me = Ca 0.2                                                       
Example 35                                                                
        Me = Ca 5.0                                                       
                20.0  80.0   --     1080                                  
                                        7.79 233   40.9                   
                                                       395 414            
        Me = Ba 2.5                                                       
Example 36      20.0  80.0   --     1080                                  
                                        7.81 229   41.8                   
                                                       390 395            
        Me = Sr 2.5                                                       
Example 37                                                                
        Me = Sr 10.0                                                      
                20.0  80.0   --     1080                                  
                                        7.74 254   41.7                   
                                                       385 437            
        Me = Ba 5.0                                                       
Example 38                                                                
        Me = Sr 5.0                                                       
                20.0  80.0   --     1080                                  
                                        7.70 272   41.0                   
                                                       363 445            
        Me = Ca 5.0                                                       
Example 39                                                                
        Me = Ba 20.0                                                      
                20.0  80.0   --     1100                                  
                                        7.69 299   40.3                   
                                                       357 383            
        Me = Ba 10.0                                                      
Example 40      20.0  80.0   --     1100                                  
                                        7.71 287   40.4                   
                                                       352 393            
        Me = Ca 10.0                                                      
Control 9                                                                 
        Me = Ca 0.3                                                       
                20.0  80.0   --     1080                                  
                                        7.61 210   37.7                   
                                                       396 407            
Control 10                                                                
        Me = Ba 25.0                                                      
                20.0  80.0   --     1080                                  
                                        7.49 344   39.3                   
                                                       301 445            
Example 41                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   MnO 0.05                                     
                                    1150                                  
                                        7.71 200   45.2                   
                                                       455 975            
Example 42                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   MnO 0.5                                      
                                    1150                                  
                                        7.76 196   45.9                   
                                                       453 1314           
Example 43                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   Fe.sub.2 O.sub.3 1.0                         
                                    1150                                  
                                        7.75 206   45.8                   
                                                       451 1640           
Example 44                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   MnO 2.0                                      
                                    1150                                  
                                        7.74 210   45.0                   
                                                       456 990            
Example 45                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   NiO 0.5                                      
                                    1150                                  
                                        7.78 214   46.9                   
                                                       459 1015           
Example 46                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   NiO 2.0                                      
                                    1150                                  
                                        7.72 233   46.3                   
                                                       453 808            
                             NiO 0.25                                     
Example 47                                                                
        Me = Sr 10.0                                                      
                5.0   95.0          1150                                  
                                        7.80 220   48.2                   
                                                       457 1631           
                             MnO 0.25                                     
                             Fe.sub.2 O.sub.3 1.0                         
Example 48                                                                
        Me = Sr 10.0                                                      
                5.0   95.0          1150                                  
                                        7.76 227   47.3                   
                                                       452 900            
                             NiO1.0                                       
Control 11                                                                
        Me = Sr 10.0                                                      
                5.0   95.0   MnO 0.02                                     
                                    1150                                  
                                        7.55 199   41.0                   
                                                       455 490            
                             MnO 1.5         no pola-                     
Control 12                                                                
        Me = Sr 10.0                                                      
                5.0   95.0          1150                                  
                                        7.02 rization                     
                                                   --  --   --            
                             NiO 1.5                                      
Example 49                                                                
        Me = Sr 5.0                                                       
                5.0   95.0   --     1150                                  
                                        7.75 192   46.2                   
                                                       477 401            
                             NiO 0.1                                      
Example 50                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.79 199   46.5                   
                                                       470 650            
                             MnO 0.1                                      
                             NiO 0.2                                      
Example 51                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.82 200   46.6                   
                                                       463 980            
                             MnO 0.2                                      
                             NiO 0.3                                      
Example 52                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.88 205   46.0                   
                                                       461 2010           
                             MnO 0.3                                      
                             NiO 0.4                                      
Example 53                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.90 211   45.9                   
                                                       461 2008           
                             MnO 0.4                                      
                             NiO 0.5                                      
Example 54                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.84 210   45.9                   
                                                       459 2000           
                             MnO 0.5                                      
                             NiO 0.6                                      
Example 55                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.83 214   45.6                   
                                                       455 996            
                             MnO 0.6                                      
                             NiO 0.7                                      
Example 56                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.80 210   45.1                   
                                                       456 200            
                             MnO 0.7                                      
                             NiO 1.0                                      
Example 57                                                                
        Me = Sr 5.0                                                       
                5.0   95.0          1150                                  
                                        7.77 218   45.0                   
                                                       455 550            
                             MnO 1.0                                      
__________________________________________________________________________
 
    
     FIG. 1 shows the relationship between the electromechanical coupling coefficient Kt and the polarization temperature obtained from measurments applied to the materials of Example 21 and Control 1 shown in Table 1. Curves (a) and (b) shown in FIG. 1 represent Example 21 and Control 1, respectively. It is clearly seen that the piezoelectric material of this invention is markedly advantageous over the conventional PbTiO 3  type piezoelectric material in plarization property. 
     FIG. 2 shows the relationship between the dielectric constant and the temperature obtained from measurements applied to the materials of Examples 10, 13 and Control 6. Curves (a), (b), (c) shown in FIG. 2 represent Examples 10, 13 and Control 6, respectively. It is clearly seen that the piezoelectric material of this invention has a Curie point high enough to permit a stable use thereof at temperatures higher than 300° C. 
     FIG. 3 shows the relationship between the temperature and the electro-mechanical coupling coefficient Kt for the material of Examples 10, 13 and Control 6 as in FIG. 2. Curves (a), (b), (c) shown in FIG. 3 represent Examples 10, 13 and Control 6, respectively. It is clearly seen that the piezoelectric material of this invention, having a high Curie point, exhibits a substantially constant value of Kt over such a wide temperature range as between the room temperature and about 400° C. Further, FIGS. 2 and 3 collectively indicate that the piezoelectric material of this invention can be used at the highest temperature in this field. 
     FIG. 4 shows the change with time in frequency constant Nt(Hz·m) for each of the ceramic filters made of the materials of Examples 21, 46, 42 and 47 shown in Table 1. Curves (a), (b), (c) and (d) shown in FIG. 4 represent Examples 21, 46, 42 and 47, respectively. It is important to note that these Examples are substantially the same in composition except that an additive of MnO and/or NiO is contained in Examples 46, 42 and 47. FIG. 4 shows that Examples 46, 42 and 47 are superior in time variability to Example 21 (Curve &#34;a&#34;) which does not contain the particular additive. In other words, FIG. 4 substantiates that the additive used in this invention is very effective for improving the time variability of the piezoelectric material. 
     Further, the ceramic filters mentioned above were subjected to tests for measuring the temperature characteristic and time variability of the resonance frequency, obtaining the results as shown in Table 2. 
     
                       Table 2                                                     
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           Example                                                        
                  Example  Example  Example                               
           21     46       42       47                                    
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Temperature coef-                                                         
ficient of                                                                
resonance    77       30       25     21                                  
frequency (-40 to                                                         
80° C.) p.p.m./°C.                                          
Change with time                                                          
in resonance 0.24     0.12     0.10   0.09                                
frequency (in                                                             
one year) percent                                                         
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     Table 2 shows that the addition of MnO and/or NiO enables the piezoelectric material of this invention to exhibit further improved temperature characteristic and time variability. Incidentally, similar experiments were conducted by using ceramic filters made of the materials of Examples 43 and 48 each containing Fe 2  O 3  as the additive, with the result that Fe 2  O 3  was found to be substantially equivalent to MnO in the effects produced. 
     FIG. 5, shows the relationship between the mechanical quality factor Qm and the amount of NiO+MnO for the materials of Examples 49 to 57 shown in Table 1. Points (a) to (i) plotted in FIG. 5 represent Examples 49 to 57, respectively. It is important to note that these Examples are substantially the same in composition except that each of Examples 50 to 57 contains both NiO and MnO, the amount of NiO being equal to that of MnO. It is clearly seen that the piezoelectric material of this invention containing the particular additive exhibits a very large value of Qm, compared with Example 49 (point &#34;a&#34;) which does not contain the additive. As seen from FIG. 5, the value of Qm is increased to exceed, for example, 2,000 in some cases by the use of the particular additive. 
     As described in detail, the piezoelectric material of this invention has a high Curie point and, thus, can be used stably at temperatures higher than 300° C. Further, the invented piezoelectric material is suitable for use in such a high frequency region as more than several MHz and is prominently advantageous over the conventional PbTiO 3  type piezoelectric material in polarizing property. Still further, the piezoelectric material of this invention exhibits excellent temperature characteristic and time variability. Naturally, the piezoelectric material of this invention can be used as the material of various transducing elements, with quite satisfactory effects. 
     The piezoelectric material of the present invention exhibiting such excellent properties as mentioned above can be used extensively as exemplified in the following: (1) Determination of vibration, acceleration and pressure applied to high temperature objects approaching 500° C. or other objects subjected to sharply changing temperature and also determination of presssure prevailing in the interior of such high temperature objects. 
     (2) Application of ultrasonic waves to high temperature objects 
     The piezoelectric material of this invention can be used as a source of ultrasonic waves in working high temperature objects using ultrasonic waves or as an element for examining said objects using ultrasonic waves. 
     (3) Generation of strong ultrasonic waves 
     When subjected to vigorous vibrations, ordinary piezoelectric materials fail to be useful due to high heat build-up. However, those of the invention withstand application at a temperature exceeding 300° C., offering great advantage in generating strong ultrasonic waves by heavy vibrations. 
     (4) Application in high frequency regions 
     Conventional piezoelectric materials have the drawback that they have a large dielectric constant e.g., 1000, and are unadapted for use in high frequency regions. Generally, impedance Z is expressed by an equation 
     
         Z=d(2πf·ε·S) 
    
     (where d represents the thickness of a sample, S its cross sectional area, f the frequency used and ε a dielectric constant). Therefore, d should be reduced in inverse proportion to f, so that the impedance may be eventually indicated as Z∝1(f 2  ·ε·S). If f increases, Z will sharply fall because f 2  is a large factor. Though the matching of Z requires S or ε to be reduced, it is more advantageous to decrease ε, because S is subject to certain limitation from the standpoint of working. The piezoelectric materials of this invention have a dielectric constant ε of about 200, that is, as low as 1/5 to 1/10 of that of the prior art product. Accordingly, in the case where the prior art piezoelectric materials are available up to the frequency of 10 mHz those of this invention permit application at a frequency as 50 mHz. 
     (5) Preparation of probes for linear scan type ultrasonic diagnosis apparatus 
     It was customary in the past to use PZT type piezoelectric materials for forming sonic transducing elements used in probes of ultrasonic diagnosis apparatus. It should be noted in this connection that such a probe tends to be required to generate higher and higher frequency waves, rendering it necessary to provide a thin plate of piezoelectric member sized as large as, for example, 50 to 100 mm length and 15 to 20 mm width. However, the conventional piezoelectric material mentioned above fails to meet this requirement. In contrast, the piezoelectric material of this invention is excellent in sintering property and mechanical strength, rendering it possible to produce without difficulty a large thin plate having a thickness of, for example, 200μ. It follows that the piezoelectric material of this invention is suitable for forming a probe generating a high frequency wave and having a high sensitivity. 
     As described above in detail, it will be understood that the piezoelectric materials of this invention can be used in the applications which have been impossible with those of the prior art. A problem is caused when a high frequency filter or resonator changes frequency with time and temperature. Since, however, the present product varies minimally in frequency characteristics, it has several practical applications.