Abstract:
There is disclosed a high dielectric ceramic composition for capacitors, which comprises (1-x)Pb(Fe 1/2  Ta 1/2 )O 3  xPb(Fe 1/2  Nb 1/2 )O 3  wherein 0.35≦x≦0.65, and 0.01-0.5 % by weight of manganese, chromium or cobalt. The composition can be sintered at a temperature of 1,000° C. with a high dielectric constant and a low dielectric loss coefficient.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates, in general, to dielectric ceramic capacitor compositions and, more particularly, to high dielectric ceramic capacitor compositions which can be sintered at low temperatures with low temperature-dependent dielectric loss coefficients. 
     2. Description of the Prior Art 
     Ceramic compositions which comprise BaTiO 3  in combination with CaZrO 3  and/or CaSnO 3  have been widely used as dielectrics for capacitors. The high sintering temperatures (1,200-1,400° C.) for these ceramics, however, compel layer-built condensers made from these ceramics to use internal electrodes made of precious metals, such as platinum (Pt), gold (Au), palladium (Pa) and the like. Thus, in order to utilize relatively low-priced metals, such as silver (Ag) and nickel (Ni), there is a need to develop dielectric ceramic compositions which can be sintered at a temperature lower than 1,000° C. 
     Japanese Pat. Laid-Open Publication No. 52-87700 discloses Pb(Fe 1/2  Nb 1/2 )O 3  and Pb(Fe 1/2  W 1/2 )O 3  compositions, which are asserted to meet the low temperature sintering condition. Japanese Pat. Laid-Open Publication No. 53-110500 suggests a modified composition which is improved in physical properties by adding silicon (Si) and manganese (Mn) to the composition of the supra patent. The layer-built condensers employing these dielectric ceramic compositions, however, are found to be problematic in practice because the ceramic compositions are poor in temperature-dependent dielectric loss coefficient. 
     There have been developed Pb(B 1 , B 2 )O 3  -based dielectric relaxor compositions (T. R. Shrout and A. Halliyal, &#34;Preparation of Lead based Ferroelectric Relaxors for Capacitors&#34;, Am. Cerm. Soc. Bull., 66[4], 704 919870; M. T. Lanagan, N. Yang, D. C. Dube and S. J. Jang, &#34;Dielectric Behavior of the Relaxor Pb[Mg 1/3  Nb 2/3  ]O 3  Solid-Solution System in the Microwave Region&#34;, J. Am. Ceram. Soc., 72[3], 481, 83 (1989)). They can be sintered at low temperatures and show stable temperature coefficients in a range of operation temperatures with a broad range of dielectric constants. They are quite different from the present invention in technical constitution. 
     SUMMARY OF THE INVENTION 
     Accordingly, the art has long sought a dielectric ceramic composition for capacitors, capable of being low-temperature sintered and showing high dielectric constants and low dielectric loss coefficients. 
     The intensive and thorough research on the dielectrics suitable for capacitors required by contemporary industry standards, repeated by the present inventors, resulted in the finding that, in a two-component system consisting of Pb(Fe 1/2  Ta 1/2 )O 3  and Pb(Fe 1/2  Nb 1/2 )O 3 , the molar ratio of the two components has a great influence of sintering temperature while a certain metal plays an important role in determining the dielectric loss coefficient. 
     Therefore, it is an object of the present invention to provide a dielectric ceramic composition for capacitors, which can be sintered at a temperature as low as or lower than 1,000° C. 
     It is another object of the present invention to provide a dielectric ceramic composition for capacitors, which allows silver as a main material for the internal electrodes of the capacitors. 
     It is a further object of the present invention to provide a dielectric ceramic composition for capacitors, which is superior in dielectric constant and dielectric loss coefficient, both. 
     These and other objects are attained in a dielectric ceramic composition comprising (1-x) Pb(Fe 1/2  Ta 1/2 )O 3  -xPb(Fe 1/2  Nb 1/2 )O 3  wherein 0.35≦x≦0.65, and 0.01-0.5% by weight of manganese, chromium or cobalt. 
     DESCRIPTION OF THE INVENTION 
     The dielectric ceramic composition of the present invention is based on Pb (Fe 1/2  Ta 1/2 )O 3  and Pb (Fe 1/2  Nb 1/2 )O 3  in combination with a manganese oxide. On the whole, the sintering temperature of the composition is determined by the molar ratio between Pb(Fe 1/2  Ta 1/2 )O 3  and Pb(Fe 1/2  Nb 1/2 )O 3 . The presence of manganese in the composition enhances the dielectric properties, including dielectric constant and dielectric loss coefficient. 
     For best results, the molar ratio between Pb(Fe 1/2  Ta 1/2 )O 3  and Pb(Fe 1/2  Nb 1/2 )O 3  resides in a range of 0.35:0.65 to 0.65:0.35 and manganese is present at an amount of 0.01-0.5% by weight based on the total weight of the composition. 
     A similar advantage can be obtained by using a transition metal, such as chromium or cobalt, instead of manganese. 
     The following examples are set forth to illustrate more clearly the principles and practice of this invention to one skilled in the art. As such, they are not intended to limit the invention, but are illustrative of certain preferred embodiments. 
    
    
     EXAMPLES I TO XXIX 
     99.5% pure PbO, Fe 2  O 3 , Ta 2  O 5 , Bb 2  O 5 , manganese oxide, chrome and cobalt were powdered and weighed as indicated in Table 1, below. The weighed powder was wet-mixed by a ball milling method, followed by calcination (ignition to eliminate volatile contents) at 800° C. for 1 hour. The calcined materials were pulverized to give fine powder which was, then, molded into circular plates, 0.6-0.8 mm in thickness and 12 mm in diameter, under a pressure of 0.7 ton/cm 2 . After being sintered at 900-1,000° C. for 1-10 hours, the circular plates were heated at 600-700° C. to attach Ag electrodes thereto. The electrical properties of the plate samples thus prepared were measured and the results are given in Table 1. 
     
                                           TABLE 1__________________________________________________________________________Electrical Properties of Dielectric Ceramic CompositionsMain component       Sub   Sinter                          Dielec.                              Dielec. loss                                    Specific(mol %)              component                      Temp.                          const.                              Coeffi. tanδ                                    resist.EXAMP.Pb(Fe.sub.1/2 Ta.sub.1/2)O.sub.3        Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3                (wt %)                      (° C.)                          (K) (%)   (Ω · cm)__________________________________________________________________________I*   0.2     0.8     Mn 0.05                      1030                           8250                              3.5   2 × 10.sup.11II*  0.2     0.8     Mn 0.2                      1030                           7540                              3.1   7 × 10.sup.11III* 0.3     0.7     Mn 0.05                      1000                           8750                              2.5   5 × 10.sup.11IV*  0.3     0.7     Mn 0.2                      1000                           7800                              5.5   6 × 10.sup.8V.sup.#0.4     0.6     --    1050                          21000                              6.2   3 × 10.sup.8VI.sup.+0.4     0.6     Mn 0.05                       980                          19100                              0.4   1 × 10.sup.11VII.sup.+0.38    0.62    Mn 0.2                       980                          14500                              1.1   1.5 × 10.sup.11VIII.sup.+0.43    0.57    .sup. Cr 0.05                       980                          18900                              0.5   3 × 10.sup.11IX.sup.+0.37    0.63    .sup. Cr 0.2                       980                          15000                              1.2   1 × 10.sup.11X.sup.+0.4     0.6     .sup. Co 0.05                       980                          19200                              0.4   5 × 10.sup.11XI.sup.+0.41    0.59    .sup. Co 0.2                       980                          14000                              1.3   7 × 10.sup.11XII.sup.#0.5     0.5     --    1030                          20000                              5.7   4 × 10.sup.8XIII.sup.+0.5     0.5     Mn 0.05                       980                          18900                              0.9   6 × 10.sup.11XIV.sup.+0.48    0.52    Mn 0.2                       980                          17800                              1.4   6.5 × 10.sup.11XV.sup.+0.5     0.5     .sup. Cr 0.05                       980                          18700                              0.9   5.4 × 10.sup.11XVI.sup.+0.5     0.5     .sup. Cr 0.2                       980                          17500                              1.3   7.6 × 10.sup.11XVII.sup.+0.52    0.48    .sup. Co 0.05                       980                          18800                              0.8   8 × 10.sup.11XVIII.sup.+0.47    0.53    .sup. Co 0.2                       980                          17300                              1.2   2 × 10.sup.11XIX.sup.#0.6     0.4     --    1050                          22000                              6.5   5 × 10.sup.8XX.sup.+0.62    0.38    Mn 0.05                       980                          14200                              0.8   2 × 10.sup.11XXI.sup.+0.63    0.37    Mn 0.2                       980                          13900                              1.9   2.4 × 10.sup.11XXII.sup.+0.58    0.42    .sup. Cr 0.05                       980                          14500                              1.0   3.5 × 10.sup.11XXIII.sup.+0.57    0.43    .sup. Cr 0.2                       980                          13500                              1.5   6 × 10.sup.11XXIV.sup.+0.6     0.4     .sup. Co 0.05                       980                          14300                              1.1   4 × 10.sup.11XXV.sup.+0.6     0.4     .sup. Co 0.2                       980                          13300                              1.4   3.8 × 10.sup.11XXVI*0.7     0.3     Mn 0.05                      1000                           9500                              3.7   4 × 10.sup.11XXVII*0.7     0.3     Mn 0.2                      1000                           8550                              4.2   5 × 10.sup.11XXVIII*0.8     0.2     Mn 0.05                      1000                           8400                              4.1   1 × 10.sup.11XXIX*0.8     0.2     Mn 0.2                      1000                           7750                              4.7   1.5 × 10.sup.11__________________________________________________________________________ *Comparative samples .sup.+ Samples of the invention .sup.# Absent of Mn 
    
     In Table 1, the dielectric constants (K) and dielectric loss coefficients (tanδ) were measured at 25° C. at 1 kHz by use of an LCR meter while the specific resistance was determined at a direct current voltage of 100 V by use of a high resistance meter. 
     As shown in Table 1, the compositions of Examples V, XII and XIX, which contained no manganese oxides, are improper to be used for ceramic condensers because of their low specific resistance. When manganese (Mn) was complemented at an amount of 0.05 wt % and 0.2 wt % to the compositions of Example V, XII and XIX, the specific resistance of the resulting compositions was increased by 10 3  factor. In the high dielectric ceramic composition according to the present invention, manganese oxide is preferably present at an amount of 0.01 to 0.5% by weight. For example, if manganese oxide is present at an amount less than 0.01% by weight, a significant increase in specific resistance is not attained. On the other hand, excess manganese oxide weakens the mechanical strength of the resulting dielectrics. 
     In the presence of manganese at an amount of 0.01 to 0.5 % by weight, when the mol % of Pb(Fe 1/2  Ta 1/2 )O 3  to Pb(Fe 1/2  Nb 1/2 )O 3  ranged from 0.2:0.8 to 0.8:0.2, a notable change was not detected in specific resistance with a dielectric constant being in a range of about 7,500 to 9,500, as in Examples I to IV and XXVI to XXIX. Under this condition, the samples of these Examples were 2.5 to 4.7% in dielectric loss coefficient (tanδ). These temperature coefficients are better than those obtained in the absence of manganese oxides. 
     On the other hand, when the mol % of Pb(Fe 1/2  Ta 1/2 )O 3  to Pb(Fe 1/2  Nb 1/2 )O 3  was about 0.4:0.6 to 0.6:0.4 in the presence of 0.2 or 0.05% by weight of manganese, the dielectric constant was at least 13,900 while the dielectric loss coefficient was below 2% at 25° C., as shown in Examples 6, 7, 13, 14, 20 and 21. Thus, it is apparent that the dielectric compositions of these examples are far better in temperature-dependent dielectric loss coefficient than other compositions. In detail, the dielectric compositions which comprise Pb(Fe 1/2  Ta 1/2 )O 3  and Pb(Fe 1/2  Nb 1/2 )O 3  in a molar ratio of 0.35:0.65 to 0.65:0.35 in combination with Mn, Co or Cr have high dielectric constants and low dielectric loss coefficients (below 2%) while the compositions which comprise Pb(Fe 1/2  Ta 1/2 )O 3  and Pb(Fe 1/2  Nb 1/2 )O 3  in a molar ratio exceeding the above range are relatively low in dielectric constant and high in dielectric loss coefficient and the dielectric compositions which comprise no sub-components, such as Mn, Co and Cr, show very high dielectric loss coefficients. Also, the data of Table 1 demonstrate that Co or Cr can be used instead of Mn without deteriorating the temperature-dependent dielectric loss coefficient. 
     As described hereinbefore, the dielectric ceramic compositions of the present invention can be sintered at a temperature less than 1,000° C., so that they are suitable for ceramic condensers and Ag-based internal electrodes can be employed in the layer-built ceramic condensers made of the compositions, providing still more economical favors than precious metals. What is better, the dielectric ceramic compositions of the present invention are superior in dielectric properties because they have a dielectric constant ranging from about 15,000 to 20,000 and show a dielectric loss coefficient less than 2% in a temperature of -55 to 125° C. 
     Although the invention has been described in detail by referring to certain preferred embodiments, it will be understood that various modifications can be made within the spirit and scope of the invention. The invention is not to be limited except as set forth in the following claims.