Patent Publication Number: US-6984355-B2

Title: Semiconducting ceramic material, process for producing the ceramic material, and thermistor

Description:
This is a divisional of U.S. patent application Ser. No. 09/705,049, filed Nov. 2, 2000 now abandoned. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconducting ceramic material, a process for producing the ceramic material and a thermistor comprising the ceramic material. More particularly, the present invention relates to a BaTiO 3 -type semiconducting ceramic material which exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics), to a process for producing the ceramic material and to a thermistor comprising the ceramic material. 
   2. Background Art 
   Conventionally, a BaTiO 3 -type semiconducting ceramic material is widely employed in the manufacture of PTC thermistors since the ceramic material exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics). The PTC thermistor is widely employed for the demagnetization of a cathode-ray tube or in a heater. 
   In order to reduce resistance, there has been keen demand for a PTC thermistor comprising a laminate which includes semiconducting ceramic materials and internal electrodes. Since a base metal such as Ni is employed to form an internal electrode of the PTC thermistor, the semiconducting ceramic material must be re-oxidized after the material is fired in a reducing atmosphere. The re-oxidation of the semiconducting ceramic material is carried out in order to obtain the PTC characteristics of the material through the re-oxidation of grain boundaries of the material. 
   However, it is difficult to completely re-oxidize the semiconducting ceramic material at a temperature sufficiently low to prevent oxidation of the internal electrode formed of a base metal. 
   In order to solve this problem, for example, Japanese Patent Application Laid-Open (kokai) No. 8-153604 discloses a process in which a material having a low firing temperature is employed as a semiconducting ceramic material. However, such a process is not necessarily satisfactory. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an object of the present invention is to provide a BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation. 
   Another object of the present invention is to provide a process for producing a BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, which process comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material. 
   Another object of the present invention is to provide a thermistor comprising a BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation. 
   Accordingly, in a first aspect of the present invention, there is provided a BaTiO 3 -type semiconducting ceramic material which has undergone firing in a reducing atmosphere and re-oxidation, wherein the relative density of the ceramic material after sintering is about 85–90%. 
   Relative density is the ratio of the density of a sintered ceramic to the ideal density of the ceramic which is calculated under an assumption that the ceramic consists of a perfect crystal lattice. The relative density is usually expressed in percentage. 
   Preferably, the size of grains constituting the matrix of a semiconducting ceramic material of the present invention is about 0.5–2 μm. 
   In a second aspect of the present invention, there is provided a process for producing a BaTiO 3 -type semiconducting ceramic material, which comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material, wherein the ceramic material is fired at a temperature about 25° C. or more lower than a sintering completion temperature of the ceramic material. 
   Preferably, a BaTiO 3 -type semiconducting ceramic material having a sintering completion temperature of about 1,275° C. or higher is fired at about 1,250° C. or lower. 
   In a third aspect of the present invention, there is provided a thermistor comprising a laminate in which a semiconducting ceramic material of the present invention and an electrode are alternately laminated. 
   When the semiconducting ceramic material is fired at low temperature, the density of the material after sintering can be appropriately reduced and pores through which oxygen passes during re-oxidation can be established. As a result, the semiconducting ceramic material exhibits excellent PTC characteristics. 
   Specifically, when the semiconducting ceramic material is fired at a temperature about 25° C. or more lower than the sintering completion temperature, the sintering completion temperature being the temperature at which the density of the material is maximized, the semiconducting ceramic material exhibits excellent PTC characteristics. 
   When the relative density of the semiconducting ceramic material after sintering is about 85–90%, the ceramic material exhibits excellent PTC characteristics. 
   The size of grains constituting the matrix of the semiconducting ceramic material is preferably about 0.5–2 μm, more preferably about 0.7–1.5 μm. In addition, the relative density of the semiconducting ceramic material after sintering is preferably about 87–89%. 
   In the present invention, the semiconducting ceramic can be produced from an inexpensive material used into a solid-state process instead of an expensive wet process. 
   The lower limit of the firing temperature of the semiconducting ceramic material is not particularly limited. However, when the firing temperature is excessively low, the resistance of the ceramic material becomes high. Therefore, in general, it is not preferable that the ceramic material is fired at a temperature about 150° C. or more lower than the sintering completion temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with an accompanying drawing, in which: 
       FIG. 1  is a schematic representation of an embodiment of the monolithic PTC thermistor of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Example 
   BaCO 3 , TiO 2 , Sm 2 O 3 , and SiO 2 , serving as raw materials, were mixed in the compositional proportion to form
 
(Ba 0.998 Sm 0.002 ) 1.002 TiO 3 +0.001SiO 2 .
 
   The resultant powder was pulverized by use of a zirconia ball for five hours, and the resultant powder was calcined at 1,100° C. for two hours. The thus-calcined product was mixed with an organic binder, and the mixture was shaped into a sheet. Ni serving as an internal electrode was printed on the sheet. The resultant sheets were laminated with one another, and the thus-obtained laminate was fired in a reducing atmosphere (H 2 +N 2 ). Thereafter, Ni external electrodes were formed on the laminate through baking at 500–800° C. in air, simultaneously with re-oxidation of the semiconducting ceramic material, to thereby produce a monolithic PTC thermistor  10  as shown in  FIG. 1 . The monolithic PTC thermistor  10  shown in  FIG. 1  comprises a laminate  12 . The laminate  12  comprises semiconducting ceramic materials  14  formed of the aforementioned raw materials, and internal electrodes  16 , the materials  14  and the electrodes  16  being alternately laminated. As shown in  FIG. 1 , the internal electrodes  16  extend to first and second side surfaces of the laminate  12  on which are formed external electrodes  18   a  and  18   b,  respectively, and the electrodes  16  are electrically connected to the external electrodes  18   a  and  18   b.    
   The sintering completion temperature of the above semiconducting ceramic material is 1,300° C. In Example 1, Example 2, Example 3 and Comparative Example, the firing temperatures of the ceramic material are 1,150° C., 1,200° C., 1,250° C. and 1,300° C., respectively. Table 1 shows firing temperature, re-oxidation temperature, mean grain size, and relative density for each ceramic material. Table 1 also shows the resistance of the monolithic PTC thermistor comprising the ceramic material at room temperature; the logarithm of the maximum resistance of the PTC thermistor (R max ) to the resistance thereof at 25° C. (R 25 ); i.e., log (R max /R 25 ); and the withstand voltage of the PTC thermistor. The re-oxidation temperature of each semiconducting ceramic material was optimized on the basis of the firing temperature thereof. In the Comparative Example, the semiconducting ceramic material was re-oxidized at 800° C., which is the upper limit for preventing oxidation of Ni. 
   
     
       
         
             
             
             
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Re- 
               Mean 
                 
               Resistance 
                 
                 
             
             
                 
               Firing 
               oxidation 
               grain 
               Relative 
               at room 
                 
               Withstand 
             
             
                 
               temperature 
               temperature 
               size 
               density 
               temperature 
               log 
               voltage 
             
             
                 
               (° C.) 
               (° C.) 
               (μm) 
               (%) 
               (Ω) 
               (R max /R 25 ) 
               (V) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
               Example 1 
               1150 
               550 
               0.8 
               85 
               0.15 
               3.5 
               15 
             
             
               Example 2 
               1200 
               680 
               1.0 
               88 
               0.12 
               3.4 
               14 
             
             
               Example 3 
               1250 
               750 
               1.5 
               90 
               0.11 
               3.0 
               12 
             
             
               Comparative 
               1300 
               800 
               8 
               94 
               0.10 
               1.0 
               2.5 
             
             
               Example 
             
             
                 
             
          
         
       
     
   
   As is apparent from Table 1, when the semiconducting ceramic material is fired at a temperature about 25° C. or more lower than the sintering completion temperature, the ceramic material exhibits excellent PTC characteristics. Particularly when the semiconducting ceramic material is fired at about 1,250° C. or lower, the ceramic material exhibits remarkably excellent PTC characteristics. 
   The present invention provides the BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation. 
   The present invention also provides the process for producing the BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, which comprises firing the ceramic material in a reducing atmosphere and re-oxidizing the ceramic material. 
   The present invention also provides a thermistor comprising the BaTiO 3 -type semiconducting ceramic material which exhibits excellent PTC characteristics, the ceramic material having undergone firing in a reducing atmosphere and re-oxidation.