Abstract:
The present invention provides dielectric ceramics with less variation of temperature coefficient at a resonant frequency due to heating history during the sintering of ZrTiO 4  and ZrO 2  --SnO 2  --TiO 2  ceramics, and having a high unloaded Q value and a high dielectric constant, and a temperature coefficient at a resonant frequency which is widely variable as desired, and dielectric resonators using the dielectric ceramics which have a high unloaded Q value and a strong electrode layer. The main components of the dielectric ceramics may be expressed by the formula: xZrO 2  --yTiO 2  --zA.sub.(1+u)/3 B.sub.(2-u)/3 O 2  wherein A denotes at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn, B denotes at least one component selected from the group (B) consisting of Nb and Ta, and x, y, z and u (x, y and z are molar fractions and u is a value within the range from 0 to 1.90) have the relation of x+y+z=1.0, 0.10≦x≦0.60, 0.20≦y≦0.60, 0.01≦z≦0.70. The main component may include, as accessory components, at least one component selected from the group (C) consisting of Ba, Sr, Ca, Cu, Bi and W at 0.005 to 7.000% by weight of the entire weight of the ceramic.

Description:
FIELD OF THE INVENTION 
     This invention relates to dielectric ceramics and dielectric resonators for use in such high-frequency ranges as microwave and millimeter-wave frequencies. 
     Recently, dielectric ceramics have been widely used in dielectric resonators and filters in microwave and millimeter-wave frequencies at wavelengths of several centimeters or less (hereinafter referred to as microwave in general). It is required that a dielectric material for use in such applications have a high unloaded Q (Qu) value and dielectric constant .di-elect cons. r , and that the temperature coefficient at a resonant frequency τ f  be variable as desired. The Q value is the inverse of an inductive loss tan δ. 
     Various materials appropriate for use in such applications have been conventionally reported, among which ZrTiO 4  ceramics are included. Also included in such materials are ZrO 2  --SnO 2  --TiO 2  ceramics, the ZrO 2  --SnO 2  --TiO 2  --MgO ceramic suggested in Japanese Laid-Open Patent No. 62-132769 and the ZrO 2  --SnO 2  --TiO 2  --CoO-Nb 2  O 5  ceramic in Japanese Laid-Open Patent No. 2-192460, for example. 
     However, although the ZrTiO 4  ceramics according to the prior art have a high dielectric constant of 45, the temperature coefficient at a resonant frequency is high in the positive side at 54ppm/° C., and the temperature coefficient at a resonant frequency is significantly varied by the heating history during sintering. The ZrO 2  --SnO 2  --TiO 2  ceramic has achieved a low temperature coefficient of about Oppm/°C. at a resonant frequency, but the variation of temperature coefficient at the resonant frequency caused by the heating history causes problems. In addition, conventional materials have such problems that the dielectric constant and unloaded Q value are low, and that the temperature coefficient at a resonant frequency cannot be varied as desired. Further, in the case of application to a coaxial resonator having electrodes formed on the surface of the dielectric ceramic, a planar filter and the like, there are problems in that the resonant frequency is easily deviated and the unloaded Q value is decreased when the bond strength of the electrode layer to the dielectric ceramic is low. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide ZrTiO 4  and ZrO 2  --SnO 2  --TiO 2  ceramics with less variation of temperature coefficient at a resonant frequency due to heating history during sintering. 
     It is another object of the present invention to provide dielectric ceramics that have a high unloaded Q value and a high dielectric constant, and have a temperature coefficient at a resonant frequency which is variable as desired. 
     It is still another object of the present invention to provide dielectric resonators which comprise strong electrode layers having a high unloaded Q value. The subject of the present invention is to achieve one of these objects or to achieve more than two objects at the same time. 
     In order to achieve the objects described above, the present invention provides a dielectric ceramic comprising as the main component a complex oxide formed of both Zr and Ti, at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta, and as the accessory components at least one component selected from the group (C) consisting of Ba, Sr, Ca, Cu, Bi and W. 
     For the present dielectric ceramic, it is preferred that the main component of the dielectric ceramic is expressed by the Formula: xZrO 2  --yTiO 2  --zA.sub.(1+u)/3 B.sub.(2-u)/3 O 2 , wherein A denotes at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Pin, B denotes at least one component selected from the group (B) consisting of Nb and Ta, and x, y, z and u are present within the range expressed by Formula 1, wherein x, y and z denote molar fractions and u denotes a value expressed by the Formula 1. 
     
         ______________________________________x + y + z = 1               Formula 10.10 ≦ x ≦ 0.600.20 ≦ y ≦ 0.600.01 ≦ z ≦ 0.700 ≦ u ≦ 1.90______________________________________ 
    
     According to the dielectric ceramic, it is preferred that the accessory components of the dielectric ceramic are present within the range of 0.005 to 7.000% by weight based on the entire weight of the ceramic. The amount of the accessory component is the weight of an oxide form in which the component is present in the ceramic, and is a value which is changed into {BaO, SrO, CaO, CuO, Bi 2  O 3 , WO 3  }. 
     Further, it is preferred that the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta. 
     Preferably, the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta, and a/b (a and b denote the total of molar fractions of the components A and B) ranges from 0.5 to 1.9. 
     For the present dielectric ceramic, it is preferred that the main component further comprises a complex oxide formed of at least one component selected from the group (D) consisting of Sn, Hf and Ge. 
     Preferably, the main component of the dielectric ceramic is expressed by the Formula: xZrO 2  --yTiO 2  --zA.sub.(1+u)/3 B.sub.(2-u)/3 O 2  --vDO 2 , wherein A denotes at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn, B denotes at least one component selected from the group (B) consisting of Nb and Ta, and D denotes at least one component selected from the group (D) consisting of Sn, Hf and Ge, and x, y, z, v, and u are present within the range expressed by Formula 2, wherein x, y, z and v denote molar fractions and u denotes a value expressed by the Formula 2. 
     
         ______________________________________x + y + z + v = 1           Formula 20.10 ≦ x ≦ 0.600.20 ≦ y ≦ 0.600.01 ≦ z ≦ 0.500.001 ≦ v ≦ 0.200 ≦ u ≦ 1.90______________________________________ 
    
     Further, it is preferred that the accessory components of the dielectric ceramic comprise at least one component selected from the group (C) consisting of Ba, Sr, Ca, Cu, Bi and W within the range from 0.005 to 7.000% by weight based on the entire weight of the ceramic. 
     For the present dielectric ceramic, it is preferred that the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of M g, Co, Zn, Ni and Mn, at least one component selected from the group (B) consisting of Nb and Ta, and at least one component selected from the group (D) consisting of Sn, Hf and Ge. 
     Preferably, the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn, at least one component selected from the group (B) consisting of Nb and Ta, and at least one component selected from the group (D) consisting of Sn, Hf and Ge, and a/b (a and b denote the total of molar fractions of the components A and B) ranges from 0.5 to 1.9. 
     The dielectric resonator of the present invention is characterized by a dielectric ceramic having the structure described above, and an electrode formed on the surface of the dielectric ceramic. 
     For the present dielectric resonator, it is preferred that the electrode is copper or silver. 
     In the structure of the present invention, the main component includes a complex oxide formed of both Zr and Ti, at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta, and as the accessory components at least one component selected from the group (C) consisting of Ba, Sr, Ca, Cu, Bi and W. Consequently, the present invention provides dielectric ceramics with less variation of temperature coefficient at a resonant frequency due to heating history during the sintering of the ZrTiO 4  ceramic. 
     According to the preferred structure of the composition formula and the Formula 1, the present invention provides dielectric ceramics that have a high unloaded Q value and a high dielectric constant, and have a temperature coefficient at a resonant frequency which is variable as desired. 
     According to the dielectric ceramic expressed by the composition formula and the Formula 1, the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta. Thus, the present invention provides dielectric ceramics having a higher unloaded Q value and a high dielectric constant, and is superior in thermo-stability at a resonant frequency. 
     According to the dielectric ceramic expressed by the composition formula and the Formula 1, the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn and at least one component selected from the group (B) consisting of Nb and Ta, and a/b (a and b denote the total of molar fractions of the components A and B) ranges from 0.5 to 1.9. Thus, the present invention provides dielectric ceramics having a much higher unloaded Q value and a high dielectric constant, and is superior in thermo-stability at a resonant frequency. 
     For the present dielectric ceramics, it is preferred that the main component further comprises a complex oxide formed of at least one component selected from the group (D) consisting of Sn, Hf and Ge. Thus, the present invention provides dielectric ceramics with less variation of temperature coefficient at a resonant frequency due to heating history during the sintering of ZrO 2  --SnO 2  --TiO 2  ceramics. 
     For the present dielectric ceramic expressed by the composition formula and the Formula 2, it is preferred that the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn, at least one component selected from the group (B) consisting of Nb and Ta, and at least one component selected from the group (D) consisting of Sn, Hf and Ge. Thus, the present invention provides dielectric ceramics having a higher unloaded Q value and a high dielectric constant, and is superior in thermo-stability at a resonant frequency. 
     According to the dielectric ceramic expressed by the composition formula and the Formula 2, the main component comprises a ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component selected from the group (A) consisting of Mg, Co, Zn, Ni and Mn, at least one component selected from the group (B) consisting of Nb and Ta, and at least one component selected from the group (D) consisting of Sn, Hf and Ge, and a/b (a and b denote the total of molar fractions of the components A and B) ranges from 0.5 to 1.9. Thus, the present invention provides dielectric ceramics having a much higher unloaded Q value and a high dielectric constant, and is superior in thermo-stability at a resonant frequency. 
     The dielectric resonator of the present invention has a high unloaded Q value and a strong electrode layer. 
     In the foregoing, the ZrTiO 4  or crystallographical ZrTiO 4  phase substituted with at least one component of each of the groups A, B and D is a phase in which both or either of Zr and Ti sites are (is) substituted with at least one of each of the groups A, B and D. Basically, the ZrTiO 4  phase has an element ratio Zr/Ti=1. Actually, also in a composition area where the element ratio is slightly shifted to the direction which is higher or lower than 1 (for example, in case Zr and Ti are soluble in the ZrTiO 4  phase), an area which can be regarded as the ZrTiO 4  constitution is present. Such an area is crystallographically referred to as a ZrTiO 4  phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a sectional view of a cylindrical coaxial dielectric resonator, which is axially cut off, according to an embodiment of the present invention. 
     FIG. 1B is a sectional view taken out along the line I--I shown in FIG. 1A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Any compound such as oxide, carbonate, hydroxide, chloride, alkoxide of the component elements described above may be used as an initial material of the dielectric ceramic according to the present invention. As blending methods of powdery raw materials, wet blending for mixing the materials with water or organic solvent in a ball mill and dry blending for mixing them by a mixer or a ball mill, etc. without using any solvent are generally known and any of these methods maybe employed. Alternatively, the alkoxide method and coprecipitation method may be used depending on the initial materials. Because the process is thus comparatively uncomplicated, and a homogeneous mixture can be easily obtained, it is desirable to employ the wet blending method for mixing them in a ball mill by using a solvent, and a dispersing agent may be additionally used for increasing the dispersing property of powders, or pH adjustment may be performed. 
     Although calcination of the mixture is not required, the sintering time can be reduced by calcination. Although the calcination temperature depends on the particular compositions, it is generally in the order of 1 to 8 hrs at about 700° to 1200° C. 
     As the milling method for the calcined material or mixture, any suitable method such as using a ball mill, high-speed rotor mill, media agitating mill and jet mill may be employed. 
     For molding, press molding is generally carried out to obtain a desired shape. Although not specifically limited, the pressure used in the press molding is generally in a range of approximately 0.5 to 2 tone/cm 2 . A binder used for molding may be a binder used for molding ceramics, such as a polyvinyl alcohol binder, a polyvinyl butylal binder, an acrylic resin binder or a wax binder. Although not specifically limited, the amount of the binder to be used is generally in a range of approximately 0.5 to 1% by weight by solid matter conversion. 
     Although sintering is not specifically limited, as it depends on the particular compositions and dimensions of the moldings, it is generally desirable to perform firing at a temperature of approximately 400° to 700° C. for about 1 to 24 hrs in order to remove binders, and then, at approximately 1100° to 1650° C. for about 2 to 100 hrs. 
     An example of a cylindrical coaxial dielectric resonator will be described with reference to the drawings. FIG. 1A is a sectional view of the cylindrical coaxial dielectric resonator which is axially cut off, in which electrodes 1 and 4 are continuously formed on the surface of a cylindrical dielectric ceramic 2. The electrode is not formed on the surface of an opening end 3. FIG. 1B is a sectional view taken along the line I--I shown in FIG. 1A. According to the cylindrical coaxial dielectric resonator, electromagnetic waves are incident in the direction of the opening end 3, and the resonance (TEM mode) of the electromagnetic waves in a specific frequency area is utilized to obtain the necessary output from the electrodes 1 and 4. 
     The present invention will be described with reference to the following examples. 
     EXAMPLE 1 
     As initial materials, ZrO 2 , TiO 2 , MgO, CoO, ZnO, NiO, Nb 2  O 5 , Ta 2  O 5 , MnCO 3 , BaCO 3 , SrCO 3 , CaCO 3 , CuO, Bi 2  O 3 , WO 3 , SnO 2 , HfO 2  and GeO 2  of high chemical purity (more than 98 wt. %) were used, weighed so as to make predetermined compositions and wet-blended with ethanol by using a ball mill. The volume ratio between the powder and ethanol was approximately 2:3. 
     The mixture was removed from the ball mill, dried, and calcined for 2 hrs at 1000° C. in the air. The calcined product was wet-milled in the ball mill with ethanol. After the milled slurry was removed from the ball mill and dried, the powder was mixed with 8% by weight of polyvinyl alcohol solution of 6 vol. % in concentration as a binder, homogenized, and granulated by sieving through a 32 mesh screen. The granulated powder was molded into a disk of 7 mm in diameter and approximately 3 mm in thickness by using molds and an oil hydraulic press at a molding pressure of 1.3 ton/cm 2 . 
     The molding was placed in a magnesia vessel of high purity, kept in the air at a temperature of 600° C. for 4 hrs to remove binders, then retained in the air at 1200° to 1500° C. for 24 hrs for sintering, and quenched (taken out of a furnace and air-cooled) or slowly cooled down to 1000° C. (at a cooling rate of 20° C./hr) thereafter, and dielectric ceramics were obtained. 
     The resonant frequency was obtained from measurement by the dielectric rod resonator method. The temperature coefficient at a resonant frequency τ f  was obtained in a range between -25° C. and 85° C. 
     The compositions of main components and the amount of accessory components, which are to be added, of the dielectric ceramics thus produced are shown in Tables 1 and 2, respectively. Cooling conditions after sintering and a temperature coefficient at a resonant frequency (ppm/° C.) are shown in Table 3. In Tables 1 to 3, comparative examples have an asterisk. 
     
                                           TABLE 1__________________________________________________________________________Sample    Composition (molar fraction)No. Zr Ti Mg Co Zn Ni Mn Nb Ta Sn Hf Ge__________________________________________________________________________*1,*2    0.50  0.50     0  0  0  0  0  0  0  0  0  03,4 0.35  0.50     0.05        0  0  0  0  0.10                       0  0  0  05,6 0.35  0.50     0  0.05           0  0  0  0.10                       0  0  0  07,8 0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.10                       0  0  0  0 9,10    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.10                       0  0  0  011,12    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.10                       0  0  0  013,14    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.05                       0.05                          0  0  0*15*16    0.40  0.50     0  0  0  0  0  0  0  0.10                             0  0*17*18    0.32  0.50     0  0.03           0  0  0  0.05                       0  0.10                             0  019,20    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.05                       0.05                          0.010                             0  021,22    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.05                       0.05                          0  0.01                                023,24    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.05                       0.05                          0  0  0.0125,26    0.35  0.50     0.01        0.01           0.01              0.01                 0.01                    0.05                       0.05                          0.01                             0.01                                0.01__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________Sample    Amount of additive (wt. %)No.       Ba    Sr        Ca  Cu     Bi  W______________________________________*1,*2     0     0.1       0   0      0   03,4       0     0.1       0   0      0   05,6       0.1   0         0   0      0   07,8       0     0         0   0      0.1 0 9,10     0     0         0   0.1    0   011,12     0     0         0   0      0   0.113,14     0.1   0.1       0.1 0.1    0   0*15*16    0     0         0   0      0   0*17*18    0     0         0   0      0   019,20     0.1   0.1       0.1 0.1    0   021,22     0.1   0         0   0      0   023,24     0     0.1       0   0      0   025,26     0.1   0.1       0.1 0.1    0   0______________________________________ 
    
     
                       TABLE 3______________________________________Sample No. Cooling condition after sintering                        τ.sub.f (ppm/°C.)______________________________________*1         Quenching         58.9*2         Slow cooling      69.13          Quenching         8.24          Slow cooling      8.95          Quenching         5.56          Slow cooling      4.87          Quenching         9.48          Slow cooling      9.39          Quenching         9.710         Slow cooling      10.111         Quenching         9.812         Slow cooling      9.613         Quenching         9.814         Slow cooling      9.7*15        Quenching         -5.7*16        Slow cooling      1.2*17        Quenching         -22.5*18        Slow cooling      -16.319         Quenching         5.820         Slow cooling      4.221         Quenching         0.922         Slow cooling      1.223         Quenching         9.524         Slow cooling      9.925         Quenching         8.526         Slow cooling      9.7______________________________________ 
    
     As recognized from the results shown in Table 3, in dielectric ceramics of sample Nos. 3 to 14 and 19 to 26 of the present example, the variation of temperature coefficient at a resonant frequency due to the heating history during sintering of ZrTiO 4  and ZrO 2  --SnO 2  --TiO 2  ceramics is reduced. 
     EXAMPLE 2 
     As initial materials, ZrO 2 , TiO 2 , MgO, CoO, ZnO, NiO, MnCO 3 , Nb 2  O 5 , Ta 2  O 5 , BaCO 3 , SrCO 3 , CaCO 3 , CuO, Bi 2  O 3 , and WO 3  of high chemical purity which are the same as in Example 1 were used, weighed so as to make a predetermined compositions and wet-blended with ethanol by using a ball mill. The volume ratio between the powder and ethanol was approximately 2:3. 
     The mixture was removed from the ball mill, dried, and calcined for 2 to 8 hrs at 800° to 1250° C. in the air. The calcined product was wet-milled in the ball mill with ethanol. After the milled slurry was removed from the ball mill and dried, the powder was mixed with 8% by weight of polyvinyl alcohol solution of 6 vol. % in concentration added thereto as a binder, homogenized, and granulated by sieving through 32 mesh screen. The granulated powder was molded into a disk of 7 mm in diameter and approximately 3 mm in thickness by using molds and an oil hydraulic press at a molding pressure of 1.3 ton/cm 2 . 
     The molding was placed in a magnesia vessel of high purity, kept in the air at a temperature of 400° to 700° C. for 4 to 8 hrs to remove binders, retained in the air at 1200° C. to 1650° C. for 1 to 100 hrs for sintering, and dielectric ceramics were obtained. 
     The resonant frequency. unloaded Q (Qu) value and dielectric constant .di-elect cons. r  were obtained from measurement by the dielectric rod resonator method. The temperature coefficient at a resonant frequency τ f  was obtained in a range between -25° C. and 85° C. The resonant frequency was obtained in a range between 4 GHz and 12 GHz. 
     The compositions of main components and the amount of accessory components, which are to be added, of the dielectric ceramics thus produced are shown in Table 4. The dielectric constant thus obtained, the temperature at a resonant frequency τ f  (ppm/° C.) and the unloaded Q value are shown in Table 5. In Tables 4 and 5, comparative examples have an asterisk. 
     
                       TABLE 4______________________________________             CompositionSam-              of main component                              Accessoryple                   (molar fraction)                            (value)                                  componentNo.  A         B      x    y    z    u     (wt. %)______________________________________*27  Mg        Nb     0.400                      0.150                           0.450                                0     Sr   0.005*28  Co        Nb     0.400                      0.150                           0.450                                0     Sr   0.005*29  Zn        Nb     0.400                      0.150                           0.450                                0     Ba   0.005*30  Ni        Nb     0.400                      0.150                           0.450                                0     Ca   0.00531   Mg        Nb     0.200                      0.200                           0.600                                0     Sr   1.00032   Co        Nb     0.200                      0.200                           0.600                                0     Sr   1.00033   Ni        Nb     0.200                      0.200                           0.600                                0     Sr   1.00034   Mg        Nb     0.450                      0.200                           0.350                                0     Sr   0.50035   Co        Nb     0.450                      0.200                           0.350                                0     Sr   0.50036   Mg        Nb     0.200                      0.600                           0.200                                0     Ba   0.50037   Zn        Nb     0.200                      0.600                           0.200                                0     Ba   0.500*38  Mg        Nb     0.150                      0.700                           0.150                                0     Sr   1.000*39  Mg        Nb     0.150                      0.120                           0.730                                0     Ba   0.50040   Mg        Nb     0.100                      0.200                           0.700                                0     Ba   0.50041   Ni        Nb     0.100                      0.200                           0.700                                0     Ba   0.500*42  --        --     0.550                      0.450                           0    0     --   043   Mg        Nb     0.490                      0.500                           0.010                                0     Ba   0.00544   Co        Nb     0.490                      0.500                           0.010                                0     Ba   0.00545   Zn        Nb     0.490                      0.500                           0.010                                0     Ba   0.00546   Ni        Nb     0.490                      0.500                           0.010                                0     Ba   0.00547   Mg        Nb     0.300                      0.300                           0.400                                0     Sr   1.00048   Mg        Nb     0.400                      0.400                           0.200                                0     Sr   0.00549   Co        Nb     0.400                      0.400                           0.200                                0     Sr   0.00550   Zn        Nb     0.400                      0.400                           0.200                                0     Sr   0.00551   Mg        Nb     0.340                      0.520                           0.140                                0     Sr   0.10052   Co        Nb     0.340                      0.520                           0.140                                0     Sr   0.10053   Zn        Nb     0.340                      0.520                           0.140                                0     Sr   0.10054   Ni        Nb     0.340                      0.520                           0.140                                0     Sr   0.10055   Mg        Nb     0.450                      0.450                           0.100                                0     Sr   0.100*56  Mg        Nb     0.650                      0.200                           0.100                                0     Ca   0.100*57  Co        Nb     0.650                      0.200                           0.100                                0     Ca   0.10058   Mg        Nb     0.600                      0.300                           0.100                                0     Ca   0.10059   Mg        Nb     0.100                      0.400                           0.500                                0     Bi   0.10060   Ni        Nb     0.100                      0.400                           0.500                                0     Bi   0.100*61  Mg        Nb     0.050                      0.500                           0.450                                0     Ca   0.100*62  Co        Nb     0.050                      0.500                           0.450                                0     Bi   0.10063   Mg        Nb     0.450                      0.350                           0.200                                0     Sr   1.00064   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      Cu   0.00565   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      Cu   0.10066   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      W    0.00567   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      W    0.10068   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      W    1.00069   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.000                                      W    2.00070   Mg        Nb     0.350                      0.450                           0.200                                0     Sr   1.00071   Mg        Nb     0.350                      0.450                           0.200                                0.01  Sr   1.00072   Co        Nb     0.350                      0.450                           0.200                                0.01  Sr   1.00073   Zn        Nb     0.350                      0.450                           0.200                                0.01  Sr   1.00074   Ni        Nb     0.350                      0.450                           0.200                                0.01  Sr   1.00075   Mg        Nb     0.350                      0.450                           0.200                                0.05  Sr   1.00076   Mg        Nb     0.350                      0.450                           0.200                                0.20  Sr   1.00077   Co        Nb     0.350                      0.450                           0.200                                0.20  Sr   1.00078   Mg        Nb     0.350                      0.450                           0.200                                0.50  Sr   1.00079   Mg        Nb     0.350                      0.450                           0.200                                1.00  Sr   1.00080   Mg        Nb     0.350                      0.450                           0.200                                1.90  Sr   1.00081   Co        Nb     0.350                      0.450                           0.200                                1.90  Sr   1.00082   Mg        Nb     0.350                      0.450                           0.200                                2.00  Sr   1.00083   Mg.sub.1/4 Co.sub.1/4          Nb     0.340                      0.520                           0.140                                0     Sr   0.100Zn.sub.1/4 Ni.sub.1/484   Mg.sub.1/2 Co.sub.1/2          Nb     0.340                      0.520                           0.140                                1.00  Sr   0.10085   Mg.sub.1/3 Co.sub.1/3          Ta     0.340                      0.520                           0.140                                1.00  Sr   0.100Ni.sub.1/386   Mg.sub.1/4 Co.sub.1/4          Nb.sub.1/2                 0.340                      0.520                           0.140                                1.00  Sr   0.100Zn.sub.1/4 Ni.sub.1/4          Ta.sub.1/287   Mg.sub.39/40          Nb.sub.1/2                 0.340                      0.520                           0.140                                0.02  Bi   0.100Mn.sub.1/40          Ta.sub.1/288   Mg.sub.113/200          Nb     0.328                      0.502                           0.170                                0.41  Bi   0.100Mn.sub.87/20089   Mn        Nb     0.200                      0.600                           0.200                                0     Ca   0.10090   Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr   0.50091   Mg        Nb     0.300                      0.400                           0.300                                1.00  Ba   0.50092   Mg        Nb     0.300                      0.400                           0.300                                1.00  Ca   0.50093   Mg        Nb     0.300                      0.400                           0.300                                1.00  Bi   0.50094   Mg        Nb     0.300                      0.400                           0.300                                1.00  Ba   0.500                                      Sr   0.500                                      Ca   0.500                                      Bi   0.50095   Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr   1.50096   Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr   3.00097   Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr   7.000*98  Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr   8.000______________________________________ 
    
     
                       TABLE 5______________________________________Sample No. ε.sub.r                    τ.sub.f                           Qu______________________________________*27        30.1          -84.9  950*28        29.9          -70.5  850*29        30.0          -88.7  970*30        30.8          -69.4  83031         32.5          -10.8  800032         31.3          -8.9   900033         30.9          -7.8   810034         32.3          -32.1  430035         33.1          -19.5  480036         57.5          47.4   530037         55.5          40.2   6100*38        75.8          225.2  370*39        28.0          -84.8  98040         33.8          -8.5   960041         26.8          -5.4   9600*42        45.8          250.8  180043         44.5          42.3   390044         44.3          42.0   360045         43.8          45.9   310046         44.5          46.8   310047         39.3          15.3   600048         42.0          0.3    970049         43.1          12.1   850050         42.2          -5.3   830051         43.1          9.3    790052         45.1          13.4   510053         44.1          1.3    790054         42.9          16.2   510055         41.5          10.3   5500*56        35.7          60.5   870*57        29.5          21.3   62058         40.2          45.9   400059         65.5          48.9   330060         55.8          48.1   4100*61        82.9          151.8  830*62        86.8          213.3  42063         42.8          14.3   480064         45.6          20.0   470065         43.7          28.7   400066         45.7          19.5   490067         42.9          16.3   460068         40.2          7.3    420069         37.1          4.2    400070         45.8          19.7   470071         44.5          14.1   490072         47.0          35.4   470073         43.7          19.8   510074         44.2          40.2   470075         42.7          14.0   510076         43.8          10.5   670077         45.2          30.4   710078         42.9          9.3    720079         41.5          8.5    750080         35.2          0.1    600081         32.3          -9.8   520082         34.2          -5.3   430083         45.1          17.3   570084         43.8          6.7    750085         40.1          9.8    850086         41.8          4.9    730087         43.4          9.8    720088         42.9          8.7    680089         61.2          48.5   350090         41.2          -25.8  850091         40.3          -35.7  920092         40.7          -36.4  630093         44.3          -24.1  580094         42.5          -34.2  490095         42.8          -17.5  810096         44.5          -3.2   700097         52.1          38.5   3800*98        59.3          58.3   910______________________________________ 
    
     As is apparent from the results shown in Table 5, in dielectric ceramic compositions within a composition range of the invention, the dielectric constant is kept at a high value at microwave frequencies, while a high unloaded Q value is provided. 
     On the contrary, when X is higher than 0.60, the unloaded Q value is significantly reduced as observed in sample Nos. 56 and 57. The objects of the invention cannot readily be achieved. When x is lower than 0.10, the unloaded Q value is reduced as observed in Example Nos. 61 and 62. 
     When y is higher than 0.60, the unloaded Q value is significantly reduced as observed in sample No. 38. Further, when y is lower than 0.20, the unloaded Q value is significantly reduced as observed in sample Nos. 27 to 30 and No. 39. Consequently, the objects of the invention cannot readily be achieved. 
     When z is higher than 0.70, the unloaded Q value is reduced as observed in sample No. 39. Further, when z is lower than 0.01, the unloaded Q value is reduced as observed in sample No. 42. Consequently, the objects of the invention cannot readily be achieved. 
     Additionally, the unloaded Q value can be improved by increasing u to a higher value than 0. However, when u exceeds 1.90, the unloaded Q value is reduced as observed in sample No. 82. Also in the case of sample No. 82, the properties were better than those of conventional dielectric ceramics. 
     When the amount of accessory component to be added is higher than 7.000% by weight, the unloaded Q value is significantly reduced as observed in sample No. 98. Consequently, the objects of the invention cannot readily be achieved. 
     It was confirmed within the composition range of the example that the unloaded Q value was improved by using A, which is at least one element selected from Mg, Co, Zn, Ni, and Mn and B, which is at least one element selected from Nb and Ta, oxides were calcined in advance at a temperature of 800° to 1200° C. 
     Additionally, a ZrTiO 4  phase or one recognized as being a crystallographical ZrTiO 4  phase was confirmed by powder X-ray diffraction of a dielectric ceramic within the composition range of Examples 1 and 2 of the invention. It was further confirmed in composition analysis by a local X-ray diffractometer of a fracture surface and polished surface of the dielectric ceramic having, as the main component, ZrTiO 4  phase or crystallographical ZrTiO 4  phase that all components of Zr, Ti, A and B (wherein A is at least one component selected from Mg, Co, Zn, Ni and Mn, and B is at least one component selected from Nb and Ta) were present in a single grain, and their composition ratio was consistent with the composition ratio between other grains that constitute the main phase in the same dielectric ceramic. It was also confirmed that all components A and B blended were present in a single grain. Moreover, it was confirmed that a dielectric ceramic with components Zr, Ti, A and B present in a grain which constitutes the main phase showed a higher lattice constant in comparison with ZrTiO 4  ceramic obtained under the same sintering conditions. Accordingly, it was confirmed that components A and B are substituted in the ZrTiO 4  phase or the crystallographical ZrTiO 4  phase. 
     Such dielectric ceramics specifically showed a high unloaded Q value and a high dielectric constant, and were superior in thermo-stability at a resonant frequency. The unloaded Q value was even higher when the molar ratio a/b of the component A to the component B was 0.5 or more and 1.9 or less. 
     As is apparent from the results described above, it was confirmed that the dielectric ceramics of the example are capable of maintaining the dielectric constant at a high value at microwave frequencies, while providing a high unloaded Q value, and are superior in thermo-stability at a resonant frequency. 
     EXAMPLE 3 
     As initial materials, ZrO 2 , TiO 2 , MgO, CoO, ZnO, NiO, MnCO 3 , Nb 2  O 5 , Ta 2  O 5 , SnO 2 , HfO 2 , GeO 2 , BaCO 3 , SrCO 3 , CaCO 3 , CuO, Bi 2  O 3 , and WO 3  of high chemical purity which are the same as in Example 1 were used, weighed so as to make predetermined compositions and wet-blended with ethanol by using a ball mill. The volume ratio between the powder and ethanol was approximately 2:3. 
     The mixture was removed from the ball mill, dried, and calcined for 2 to 8 hrs at 800° C. to 1250° C. in the air. The calcined product was wet-milled in the ball mill with ethanol. After the milled slurry was removed from the ball mill and dried, the powder was mixed with 8% by weight of polyvinyl alcohol solution of 6 vol. % in concentration added thereto as a binder, homogenized, and granulated by sieving through a 32 mesh screen. The granulated powder was molded into a disk of 7 mm in diameter and approximately 3 mm in thickness by using molds and an oil hydraulic press at a molding pressure of 1.3 ton/cm 2 . 
     The molding was placed in a magnesia vessel of high purity, kept in the air at a temperature of 400° C. to 700° C. for 4 to 8 hrs to remove binders, retained in the air at 1200° C. to 1650° C. for 1 to 100 hrs for sintering, and dielectric ceramics were obtained. 
     The resonant frequency, unloaded Q (Qu) value and dielectric constant .di-elect cons. r  were obtained from measurement by the dielectric rod resonator method. The temperature coefficient at a resonant frequency τ f  was obtained in a range between -25° C. and 85° C. The resonant frequency was obtained in a range between 4 GHz and 12 GHz. 
     The compositions of main components and the amount of accessory components, which are to be added, of the dielectric ceramics thus produced are shown in Table 6. The dielectric constant thus obtained, the temperature at a resonant frequency τ f  (ppm/° C.) and the unloaded Q value are shown in Table 7. In Tables 6 and 7, comparative examples have an asterisk. 
     
                                           TABLE 6__________________________________________________________________________          Composition of main component                            AccessorySample         (molar fraction)                     (value)                            componentNo. A   B  D   x   y   z  v   u  (wt. %)__________________________________________________________________________*99 Mg  Nb Sn  0.400              0.150                  0.400                     0.050                         0  Sr 0.005100 Mg  Nb Sn  0.200              0.200                  0.400                     0.200                         0  Sr 1.000101 Co  Nb Sn  0.200              0.200                  0.400                     0.200                         0  Sr 1.000102 Ni  Nb Sn  0.200              0.200                  0.400                     0.200                         0  Sr 1.000103 Mg  Nb Sn  0.450              0.200                  0.300                     0.050                         0  Sr 0.500104 Co  Nb Sn  0.450              0.200                  0.300                     0.050                         0  Sr 0.500105 Mg  Nb Sn  0.200              0.600                  0.100                     0.100                         0  Ba 0.500106 Zn  Nb Sn  0.200              0.600                  0.150                     0.050                         0  Ba 0.500*107    Mg  Nb Sn  0.150              0.700                  0.100                     0.050                         0  Sr 1.000*108    Mg  Nb Sn  0.150              0.120                  0.530                     0.200                         0  Ba 0.500109 Mg  Nb Sn  0.100              0.200                  0.500                     0.200                         0  Ba 0.500*110    Mg  Nb Sn  0.100              0.200                  0.300                     0.400                         0  Ba 0.500*111    --  -- --  0.550              0.450                  0  0   0  -- 0112 Mg  Nb Sn  0.490              0.450                  0.010                     0.050                         0  Ba 0.005113 Mg  Nb Sn  0.300              0.300                  0.300                     0.100                         0  Sr 1.000114 Mg  Nb Sn  0.400              0.400                  0.199                     0.001                         0  Sr 0.005115 Co  Nb Sn  0.400              0.400                  0.199                     0.001                         0  Sr 0.005116 Zn  Nb Sn  0.400              0.400                  0.199                     0.001                         0  Sr 0.005117 Mg  Nb Sn  0.450              0.450                  0.050                     0.050                         0  Sr 0.100*118    Mg  Nb Sn  0.650              0.200                  0.050                     0.050                         0  Ca 0.100119 Mg  Nb Sn  0.600              0.300                  0.050                     0.050                         0  Ca 0.100*120    Mg  Nb Sn  0.050              0.500                  0.400                     0.050                         0  Ca 0.100121 Mg  Nb Sn  0.450              0.350                  0.150                     0.050                         0  Sr 1.000122 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            Cu 0.005123 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            Cu 0.100124 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            W 0.005125 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            W 0.100126 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            W 1.000127 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000                            W 2.000128 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0  Sr 1.000129 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0.01                            Sr 1.000130 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0.05                            Sr 1.000131 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0.20                            Sr 1.000132 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         0.50                            Sr 1.000133 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         1.00                            Sr 1.000134 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         1.90                            Sr 1.000135 Co  Nb Sn  0.350              0.450                  0.150                     0.050                         1.90                            Sr 1.000136 Mg  Nb Sn  0.350              0.450                  0.150                     0.050                         2.00                            Sr 1.000137 Mg.sub.1/4   Nb Sn  0.340              0.520                  0.130                     0.010                         0  Sr 0.100    Co.sub.1/4    Zn.sub.1/4    Ni.sub.1/4138 Mg.sub.1/3   Ta Sn  0.340              0.520                  0.130                     0.010                         1.00                            Sr 0.100    Co.sub.1/3    Ni.sub.1/3139 Mg.sub.1/4   Nb.sub.1/2      Sn  0.340              0.520                  0.130                     0.010                         1.00                            Sr 0.100    Co.sub.1/4   Ta.sub.1/2    Zn.sub.1/4    Ni.sub.1/4140 Mn  Nb Sn  0.200              0.600                  0.190                     0.010                         0  Ca 0.100141 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Sr 0.500142 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Ba 0.500143 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Ca 0.500144 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Bi 0.500145 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Ba 0.500                            Sr 0.500                            Ca 0.500                            Bi 0.500146 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Sr 1.500147 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Sr 3.000148 Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Sr 7.000*149    Mg  Nb Sn  0.300              0.400                  0.200                     0.100                         1.00                            Sr 8.000150 Mg  Nb Hf  0.350              0.450                  0.150                     0.050                         1.90                            Sr 1.000151 Mg  Nb Ge  0.350              0.450                  0.150                     0.050                         1.90                            Sr 1.000__________________________________________________________________________ 
    
     
                       TABLE 7______________________________________Sample No. ε.sub.r                    τ.sub.f                           Qu______________________________________*99        29.8          -85.3  1200100        27.6          -20.4  9600101        26.7          -17.5  7500102        28.3          -15.4  6900103        31.7          -34.0  4000104        32.9          -22.5  4100105        55.4          28.3   5500106        53.1          15.3   5800*107       74.3          225.0  480*108       21.3          -91.2  1200109        30.2          -14.5  8600*110       24.3          -60.5  400*111       45.8          250.8  1800112        42.5          23.2   4800113        37.2          7.8    6500114        41.5          -0.2   8300115        43.0          11.5   7800116        42.0          -5.8   6800117        40.1          4.2    5100*118       33.0          60.0   320119        40.1          42.3   3100*120       82.3          147.2  200121        42.1          10.3   4100122        43.1          15.5   4000123        41.0          25.7   3800124        43.1          15.1   4200125        40.9          11.9   4000126        38.0          3.1    3700127        35.4          -1.0   3500128        43.2          15.3   4000129        42.9          14.9   4500130        41.2          3.2    5000131        40.5          0.2    6100132        37.2          -4.5   6300133        34.3          -15.5  7000134        30.1          -32.5  7100135        30.9          -35.4  5800136        28.9          -37.4  3900137        44.7          15.3   5100138        39.7          9.3    8100139        40.9          4.5    6500140        60.3          44.9   3800141        37.9          -30.3  7800142        36.1          -35.4  8100143        36.9          -35.1  5400144        42.3          -25.9  3800145        40.1          -33.2  3500146        39.3          -20.1  6900147        44.5          1.2    5800148        50.2          39.5   4100*149       61.5          65.4   320150        45.3          -28.5  5300151        30.9          -34.5  4900______________________________________ 
    
     As is apparent from the results shown in Table 7, it was confirmed that, in dielectric ceramic compositions within a composition range of the present invention, the dielectric constant is kept at a high value at microwave frequencies, while a high unloaded Q value is provided. 
     On the contrary, when X is higher than 0.60, the unloaded Q value is significantly reduced as observed in sample No. 118. Consequently, the objects of the invention cannot readily be achieved. When x is lower than 0.10, the unloaded Q value is reduced as observed in sample No. 120. Consequently, the objects of the invention cannot readily be achieved. 
     When y is higher than 0.60, the unloaded Q value is significantly reduced as observed in sample No. 107. Further, when y is lower than 0.20, the unloaded Q value is significantly reduced as observed in sample Nos. 99 and 108. Consequently, the objects of the invention cannot readily be achieved. 
     When z is higher than 0.50, the unloaded Q value is reduced as observed in sample No. 108. When z is lower than 0.01, the unloaded Q value is reduced as observed in sample No. 111. Consequently, the objects of the invention cannot readily be achieved. 
     Additionally, the unloaded Q value can be improved by increasing w to a higher value than 0. However, when w exceeds 1.90, the unloaded Q value is reduced as shown in sample No. 136. Also in the case of sample No. 136, the properties were better than those of conventional dielectric ceramics. 
     When the amount of accessory component to be added is higher than 7.000% by weight, the unloaded Q value is significantly reduced as observed in sample No. 149. Consequently, the objects of the invention cannot readily be achieved. 
     It was confirmed within the composition range of the example that the unloaded Q value was improved by using A, which is at least one element selected from Mg, Co, Zn, Ni, and Mn and B, which is at least one element selected from Nb and Ta, oxides that were calcined in advance at a temperature of 800° C. to 1200° C. 
     Additionally, a ZrTiO 4  phase or one recognized as being a crystallographical ZrTiO 4  phase was confirmed by powder X-ray diffraction of a dielectric ceramic within the composition range of Examples 1 and 3 of the invention. It was further confirmed in composition analysis by a local X-ray diffractometer of a fracture surface and polished surface of the dielectric ceramic having as the main component ZrTiO 4  phase or crystallographical ZrTiO 4  phase, that all components of Zr, Ti, A, B and D (wherein A is at least one component selected from Mg, Co, Zn, Ni and Mn, B is at least one component selected from Nb and Ta, and D is at least one component selected from Sn, Hf and Ge) were present in a single grain, and their composition ratio was consistent with the composition ratio between other grains that constitute the main phase in the same dielectric ceramic. It was also confirmed that all components A, B and D blended were present in a single grain. 
     Moreover, it was confirmed that a dielectric ceramic with components Zr, Ti, A, B and D present in a grain which constitutes the main phase showed a higher lattice constant in comparison with ZrTiO 4  ceramic obtained under the same sintering conditions. Accordingly, it was confirmed that components A, B and D are substituted in the ZrTiO 4  phase or the crystallographical ZrTiO 4  phase. 
     Such dielectric ceramics specifically showed a high unloaded Q value and a high dielectric constant, and were superior in thermo-stability at a resonant frequency. The unloaded Q value was even higher when the molar ratio a/b of the component A to the component B was 0.5 or more and 1.9 or less 
     As is obvious from the results described above, it was confirmed that the dielectric ceramics of the example are capable of maintaining the dielectric constant at a high value at microwave frequencies, while providing a high unloaded Q value, and are superior in thermo-stability at a resonant frequency. 
     EXAMPLE 4 
     As initial materials, ZrO 2 , TiO 2 , MgO, CoO, ZnO, NiO, MnCO 3 , Nb 2  O 5 , Ta 2  O 5 , BaCO 3 , SrCO 3 , CaCO 3 , CuO, Bi 2  O 3 , and WO 3  of high chemical purity were used, weighed so as to make predetermined compositions and wet-blended with ethanol by using a ball mill. The volume ratio between the powder and ethanol was approximately 2:3. 
     The mixture was removed from the ball mill, dried, and calcined for 2 to 8 hrs at 800° C. to 1250° C. in the air. The calcined product was wet-milled in the ball mill with ethanol. After the milled slurry was removed from the ball mill and dried, the powder was mixed with 8% by weight of polyvinyl alcohol solution of 6 vol. % in concentration added thereto as a binder, homogenized, and granulated by sieving through 32 mesh screen. The granulated powder was molded into a cylindrical coaxial shape by using molds and an oil hydraulic press at a molding pressure of 1.3 ton/cm 2 . 
     The molding was placed in a magnesia vessel of high purity, kept in the air at a temperature of 400° C. to 700° C. for 4 to 8 hrs to remove binders, retained in the air at 1200° C. to 1650° C. for 1 to 100 hrs for sintering, and cylindrical coaxial dielectric ceramics having an outer diameter of 7.2 mm and an inner diameter of 3.6 mm were obtained. 
     When copper was used for an electrode material, a copper coat having a thickness of about 3.5 μm was formed on the dielectric ceramic surface by the electroless plating method. When silver was used for the electrode material, silver paste which is on the market was burned to form a silver coat. In both cases, one of two end faces of the coaxial type device ground the electrode material, so that a TEM mode resonator was obtained. 
     The compositions of main components and the amount of accessory components, which are to be added, of the dielectric resonator thus produced are shown in Table 8, the electrode material which was used, the unloaded Q value and the bond strength are shown in Table 9. The resonant frequency was 1.3 to 1.7 GHz. In Tables 8 and 9, comparative examples have an asterisk. 
     
                       TABLE 8______________________________________             Composition ofSam-              main component   Accessoryple               (molar fraction)                          (value)                                componentNo.  A         B      x    y    z    u     (wt. %)______________________________________*152 Mg        Nb     0.340                      0.520                           0.140                                0     -- 0*153 --        --     0.550                      0.450                           0    0     -- 0154  Mg        Nb     0.340                      0.520                           0.140                                0     Sr 0.100155  Co        Nb     0.340                      0.520                           0.140                                0     Sr 0.100156  Zn        Nb     0.340                      0.520                           0.140                                0     Sr 0.100157  Ni        Nb     0.340                      0.520                           0.140                                0     Sr 0.100158  Mg        Nb     0.100                      0.400                           0.500                                0     Bi 0.100159  Ni        Nb     0.100                      0.400                           0.500                                0     Bi 0.100160  Mg        Nb     0.450                      0.350                           0.200                                0     Sr 1.000161  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      Cu 0.005162  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      Cu 0.100163  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      W 0.005164  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      W 0.100165  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      W 1.000166  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000                                      W 2.000167  Mg        Nb     0.350                      0.450                           0.200                                0     Sr 1.000168  Mg        Nb     0.350                      0.450                           0.200                                0.05  Sr 1.000169  Mg        Nb     0.350                      0.450                           0.200                                0.20  Sr 1.000170  Co        Nb     0.350                      0.450                           0.200                                0.20  Sr 1.000171  Mg        Nb     0.350                      0.450                           0.200                                0.50  Sr 1.000172  Mg        Nb     0.350                      0.450                           0.200                                1.00  Sr 1.000173  Mg        Nb     0.350                      0.450                           0.200                                1.90  Sr 1.000174  Co        Nb     0.350                      0.450                           0.200                                1.90  Sr 1.000175  Mg.sub.1/3 Co.sub.1/3          Ta     0.340                      0.520                           0.140                                1.00  Sr 0.100Ni.sub.1/3176  Mg.sub.1/4 Co.sub.1/4          Nb.sub.1/2                 0.340                      0.520                           0.140                                1.00  Sr 0.100Zn.sub.1/4 Ni.sub.1/4          Ta.sub.1/2177  Mg.sub.39/40          Nb.sub.1/2                 0.340                      0.520                           0.140                                0.02  Bi 0.100Mn.sub.1/40          Ta.sub.1/2178  Mg.sub.113/200          Nb     0.328                      0.502                           0.170                                0.41  Bi 0.100Mn.sub.87/200179  Mg        Nb     0.300                      0.400                           0.300                                1.00  Ba 0.500180  Mg        Nb     0.300                      0.400                           0.300                                1.00  Ca 0.500181  Mg        Nb     0.300                      0.400                           0.300                                1.00  Bi 0.500182  Mg        Nb     0.300                      0.400                           0.300                                1.00  Ba 0.500                                      Sr 0.500                                      Ca 0.500                                      Bi 0.500183  Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr 1.500184  Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr 1.500185  Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr 3.000186  Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr 7.000*187 Mg        Nb     0.300                      0.400                           0.300                                1.00  Sr 8.000______________________________________ 
    
     
                       TABLE 9______________________________________Sample  Electrode            Bond strengthNo.     material      Qu     (kg/4 mm.sup.2)______________________________________*152    Cu            150    0.4*153    Cu            Unmeasurable due to                 electrode peeling154     Cu            550    6.8155     Cu            520    6.4156     Cu            500    6.3157     Cu            510    6.0158     Cu            530    6.1159     Cu            500    6.3160     Cu            630    9.7161     Cu            670    12.0162     Cu            650    12.0163     Cu            660    11.5164     Cu            650    10.9165     Cu            620    10.7166     Cu            600    11.0167     Cu            650    9.3168     Cu            670    9.1169     Cu            690    9.5170     Cu            670    9.1171     Cu            710    8.8172     Cu            740    9.6173     Cu            750    9.0174     Cu            740    8.9175     Cu            580    7.3176     Cu            620    9.1177     Cu            640    7.8178     Cu            630    7.5179     Cu            640    9.7180     Cu            530    6.1181     Cu            610    9.3182     Cu            670    11.5183     Cu            750    10.4184     Ag            730    12.0185     Cu            740    11.5186     Cu            680    12.0*187    Cu            210    10.5______________________________________ 
    
     As is apparent from the results shown in Table 9, the dielectric resonators of the embodiment had a high bond strength also when an electroless copper electrode is used in the same way as a silver electrode. For this reason, the unloaded Q value is high in a microwave frequency band and the resonant frequency can be prevented from deviating due to electrode peeling so that electric characteristics are stable. In addition, the dielectric resonators of the invention are suitable for forming electrodes by copper plating, so that they are suitable for large scale production and manufacturing costs can be reduced. 
     Additionally, a ZrTiO 4  phase or one recognized as being a crystallographical ZrTiO 4  phase was confirmed by powder X-ray diffraction of a dielectric ceramic within the composition range of Example 4 of the invention. It was further confirmed in composition analysis by a local X-ray diffractometer of a fracture surface and polished surface of the dielectric ceramic having as the main component ZrTiO 4  phase or crystallographical ZrTiO 4  phase, that all components of Zr, Ti, A and B (wherein A is at least one component selected from Mg, Co, Zn, Ni and Mn, and B is at least one component selected from Nb and Ta) were present in a single grain, and their composition ratio was consistent with the composition ratio between other grains that constitute the main phase in the same dielectric ceramic. It was also confirmed that all components A and B blended were present in a single grain. Moreover, it was confirmed that a dielectric ceramic with components Zr, Ti, A and B present in a grain which constitutes the main phase showed a higher lattice constant in comparison with ZrTiO 4  ceramic obtained under the same sintering conditions. Accordingly, it was confirmed that components A and B are substituted in the ZrTiO 4  phase or the crystallographical ZrTiO 4  phase. 
     Such dielectric ceramics specifically showed a high unloaded Q value and a high dielectric constant, and were superior in thermo-stability at a resonant frequency. The unloaded Q value was even higher when the molar ratio a/b of the component A to the component B was 0.5 or more and 1.9 or less. The dielectric resonators having such dielectric ceramics had an unloaded Q value which is specially high, and a high electrode bond strength. 
     As is obvious from the results described above, it was confirmed that the dielectric resonators of the example have the unloaded Q value which is specially high at a microwave frequency band. Moreover, the resonant frequency can be prevented from deviating due to electrode peeling. In addition, the dielectric resonators of the invention are suitable for forming electrodes by copper plating, so that they can be produced on a large scale and manufacturing costs can be reduced. 
     EXAMPLE 5 
     As initial materials, ZrO 2 , TiO 2 , MgO, CoO, ZnO, NiO, MnCO 3 , Nb 2  O 5 , Ta 2  O 5 , SnO 2 , HfO 2 , GeO 2 , BaCO 3 , SrCO 3 , CaCO 3 , CuO, Bi 2  O 3 , and WO 3  of high chemical purity which are the same as in Example 1 were used, weighed so as to make predetermined compositions and wet-blended with ethanol by using a ball mill. The volume ratio between the powder and ethanol was approximately 2:3. 
     The mixture was removed from the ball mill, dried, and calcined for 2 to 8 hrs at 800° C. to 1250° C. in the air. The calcined product was wet-milled in the ball mill with ethanol. After the milled slurry was removed from the ball mill and dried, the powder was mixed with 8% by weight of polyvinyl alcohol solution of 6 vol. % in concentration added thereto as a binder, homogenized, and granulated by sieving through 32 mesh screen. The granulated powder was molded into a cylindrical coaxial shape by using molds and an oil hydraulic press at a molding pressure of 1.3 ton/cm 2 . 
     The molding was placed in a magnesia vessel of high purity, kept in the air at a temperature of 400° C. to 700° C. for 4 to 8 hrs to remove binders, retained in the air at 1200° C. to 1650° C. for 1 to 100 hrs for sintering, and cylindrical coaxial dielectric ceramics having an outer diameter of 7.2 mm and an inner diameter of 3.6 mm were obtained. 
     When copper was used for an electrode material, a copper coat having a thickness of about 3.5 μm was formed on the dielectric ceramic surface by the electroless plating method. When silver was used for the electrode material, silver paste which is on the market was burned to form a silver coat. In both cases, one of two end faces of the coaxial type device ground the electrode material, so that a TEM mode resonator was obtained. 
     The compositions of main components and the amount of accessory components, which are to be added, of the dielectric resonator thus produced are shown in Table 10, the electrode material which was used, the unloaded Q value and the bond strength are shown in Table 11. The resonant frequency is 1.3 to 1.7 GHz. 
     
                       TABLE 10______________________________________      Composition of main component                       AccessorySam-                          (val-     com-ple              (molar fraction)                         ue)       ponentNo.  A      B      D   x    y    z    v    u    (wt. %)______________________________________188  Mg.sub.1/4       Nb     Sn  0.340                       0.520                            0.130                                 0.010                                      0    Sr  0.100Co.sub.1/4Zn.sub.1/4Ni.sub.1/4189  Mg.sub.1/3       Ta     Sn  0.340                       0.520                            0.130                                 0.010                                      1.00 Sr  0.100Co.sub.1/3Ni.sub.1/3190  Mg.sub.1/4       Nb.sub.1/2              Sn  0.340                       0.520                            0.130                                 0.010                                      1.00 Sr  0.100Co.sub.1/4       Ta.sub.1/2Zn.sub.1/4Ni.sub.1/4191  Mn     Nb     Sn  0.200                       0.600                            0.190                                 0.010                                      0    Ca  0.100192  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  0.500193  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Ba  0.500194  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Ca  0.500195  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Bi  0.500196  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Ba  0.500                                           Sr  0.500                                           Ca  0.500                                           Bi  0.500197  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           Cu  0.005198  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           Cu  0.100199  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           W   0.005200  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           W   0.100201  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           W   1.000202  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.000                                           W   2.000203  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.500204  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  1.500205  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  3.000206  Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  7.000*207 Mg     Nb     Sn  0.300                       0.400                            0.200                                 0.100                                      1.00 Sr  8.000208  Mg     Nb     Hf  0.350                       0.450                            0.150                                 0.050                                      1.90 Sr  1.000209  Mg     Nb     Ge  0.350                       0.450                            0.150                                 0.050                                      1.90 Sr  1.000______________________________________ 
    
     
                       TABLE 11______________________________________Sample   Electrode           Bond strengthNo.      material     Qu     (kg/4 mm.sup.2)______________________________________188      Cu           580    8.8189      Cu           540    6.1190      Cu           590    8.3191      Cu           480    5.1192      Cu           670    9.3193      Cu           600    9.1194      Cu           480    5.7195      Cu           540    8.7196      Cu           720    10.8197      Cu           760    12.0198      Cu           740    12.0199      Cu           730    10.9200      Cu           740    11.4201      Cu           700    9.7202      Cu           650    8.9203      Cu           760    10.8204      Ag           720    12.0205      Cu           720    11.1206      Cu           700    11.3*207     Cu           180    12.0208      Cu           630    10.1209      Cu           540    8.9______________________________________ 
    
     As is apparent from the results shown in Table 11, the dielectric resonators of the embodiment had a high bond strength also when an electroless copper electrode is used in the same way as a silver electrode. For this reason, the unloaded Q value is high in a microwave frequency band and the resonant frequency can be prevented from deviating due to electrode peeling so that electric characteristics are stable. In addition, the dielectric resonators of the invention are suitable for forming electrodes by copper plating, so that they are suitable for large scale production and manufacturing costs can be reduced. 
     Additionally, a ZrTiO 4  phase or one recognized as being a crystallographical ZrTiO 4  phase was confirmed by powder X-ray diffraction of a dielectric ceramic within the composition range of Example 5. It was further confirmed in composition analysis by a local X-ray diffractometer of a fracture surface and polished surface of the dielectric ceramic having as the main component ZrTiO 4  phase or crystallographical ZrTiO 4  phase that all components of Zr, Ti, A, B and D (wherein A is at least one component selected from Mg, Co, Zn, Ni and Mn, B is at least one component selected from Nb and Ta, and D is at least one component selected from Sn, Hf and Ge) were present in a single grain, and their composition ratio was consistent with the composition ratio between other grains that constitute the main phase in the same dielectric ceramic. It was also confirmed that all components A, B and D blended were present in a single grain. Moreover, it was confirmed that a dielectric ceramic with components Zr, Ti, A, B and D present in a grain which constitutes the main phase showed a higher lattice constant in comparison with ZrTiO 4  ceramics obtained under the same sintering conditions. Accordingly, it was confirmed that components A, B and D are substituted in the ZrTiO 4  phase or the crystallographical ZrTiO 4  phase. 
     Such dielectric ceramics specifically showed a high unloaded Q value and a high dielectric constant, and were superior in thermo-stability at a resonant frequency. The unloaded Q value was even higher when the molar ratio a/b of the component A to the component B was 0.5 or more and 1.9 or less. The dielectric resonators having such dielectric ceramics had an unloaded Q value which is specially high, and a high electrode bond strength. 
     As is apparent from the results described above, it was confirmed that the dielectric resonators of the example have the unloaded Q value which is especially high at a microwave frequency band. Moreover, the resonant frequency can be prevented from deviating due to electrode peeling. In addition, the dielectric resonators of the invention are suitable for forming electrodes by copper plating, so that they can be produced on a large scale and manufacturing costs can be reduced. 
     Although a dielectric ceramic having a cylindrical coaxial shape was used in Examples 4 and 5 of the invention, it is not limited to such a shape. For example, in the case of a TEM mode resonator using a dielectric ceramic having a prismatic coaxial or stepped coaxial shape, a microstrip line resonator using a dielectric resonator which has a planar shape, or a triplate resonator, an unloaded Q value which is equivalent to or more than the conventional ones can be obtained. Thus, a dielectric resonator in which stability is high and manufacturing costs are reduced can be obtained. 
     According to the structure of the dielectric ceramic of the invention, the variation in temperature coefficient at a resonant frequency due to heat history during sintering of ZrTiO 4  and ZrO 2  --SnO 2  --TiO 2  ceramics can be reduced. In addition, a high unloaded Q value is provided, and the temperature coefficient at a resonant frequency can be changed as desired without reducing the dielectric constant. 
     Furthermore, according to the structure of the dielectric resonator of the invention, a dielectric resonator having a high unloaded Q value and a strong electrode layer can be obtained. 
     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not restrictive, the scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.