Composition of dielectric ceramics

Abstract: The dielectric composition according to the present invention is used in a microwave band and is characterized in having a low sintering temperature and a property to be controllable independently in the respect of a dielectric constant and a temperature coefficient of the resonant frequency. The dielectric composition is represented by the formula: A 1+a B O 3+a (a≧ 0) wherein A site mainly contains at least Pb atom and Ca atom which is more than 20wt% of A and further may contains Sr or Ba atom, and B site mainly contains Fe and Nb atoms and further contains at least one atom selected from the group comprising Ti and W and may contains at least one atom selected from the group comprising Cr, Mn, Co, Ni, Cu, Zn, Si, Al, Mg, Bi and Sb.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a dielectric ceramic composition for use in the microwave band range. 2. Description of the Prior Art In recent years, much attention has been paid to miniaturization of movable communication apparatuses using microwave band radio waves such as car telephones and portable telephones in association with the development of satellite broadcast. In order to satisfy this requirement, it is necessary to miniaturize each of the components constituting the communication apparatus. A dielectric body is assembled as a resonator in an oscillator or a filter device of the communication apparatus. The size of the resonator is in reverse proportion to the root of the dielectric constant of the dielectric body as long as the same resonating mode is used. Therefore, the miniaturisation of the dielectric resonator requires a dielectric body having a high dielectric constant. In addition, in order to put the dielectric resonator into practical use, it is necessary to obtain a low loss, that is, a high Q value in the microwave band and further a low temperature coefficient of the resonant frequency. There have been recently developed various dielectric bodies for use in a resonator. For example, U.S. Patent Publication No. 4,330,631 disclosed a dielectric body of a BaO-TiO 2 -Sm 2 O 3 system having a high dielectric constant. The dielectric body of this system has a relative dielectric constant of about 80, a Q value of about 3000 at a frequency of 2 to 4 GHz and a low temperature coefficient of the resonant frequency. A further miniaturization of the resonator requires a dielectric body having a higher relative dielectric constant in the micro wave band, a higher no-load Q and a lower temperature coefficient of the resonant frequency. There have been proposed dielectric bodies having such higher dielectric constant. Among them, Japanese Patent Tokkaihei 1993-20925 discloses a dielectric body represented by the formula: (Pb 1-x Ca x ) 1+a (Fe 1/2 Nb 1/2 ) O 3+a having a dielectric constant as high as 90 or more and a practical Q value and temperature characteristics. The sintering temperature is as low as about 1150°C. However, in the case of the dielectric body represented by the above formula: (Pb 1-x Ca x ) 1+a (Fe 1/2 Nb 1/2 ) O 3+a selection of x causes determination of the dielectric constant, and the temperature coefficient of the resonant frequency and the dielectric constant at τ f =0 cannot be varied, so that such the dielectric body is not practical for use. Fig.1 shows the variation of the dielectric constant corresponding to the temperature coefficient of the resonant frequency when varying x in the (Pb 1-x Ca x ) 1+a (Fe 1/2 N 1/2 ) O 3+a system. In the case of the above system, the dielectric constant is fixed to 90 at τ f =0, which value cannot be adjusted to be lower or higher. Accordingly, in order to make the system practical there is required a dielectric composition whose dielectric constant and temperature coefficient τ f can be controlled independently. That is, a dielectric composition in which the variation curve of Fig.1 can be shifted to the side of a higher or a lower dielectric constant. In addition, in order to achieve a manufacturing process at low cost, it is necessary to obtain a dielectric body which can be sintered at low temperatures. EP-A1-0 460 231 discloses a Pb(B 1 ,B 2 )O 3 ceramic dielectric for use in electronic components which can be sintered even at a temperature as low as 1,100 °C or below, has a high permittivity and excellent mechanical strength and is formulated to have a crystal structure represented by [Pb 1-x A x ] [A] (B 1 ,B 2 ) [B] O 3 , wherein A is at least one element selected from among Ca, Sr, Ba, Ag, La and Nd; B 1 is at least one element selected from among Mg, Zn, Ni, Co, Fe, Mn and In; B 2 is at least one element selected from among Nb, W, Ta and Sb; x is 0 to 0.5; and the molar ratio [A]/[B] ranges from 0.95 to 0.99. EP-A1-0 121 161 discloses a constant type dielectric ceramic composition which comprises a composite material comprising at least one of BaO and CaO, PbO, Fe 2 O 3 , Nb 2 O 5 , WO 3 and CuO, wherein said composite material is represented by the general formula xPb(Fe 1/2 Nb 1/2 )O 3-y M(Cu 1/2 W 1/2 )O 3-z Pb(Fe 2/3 W 1/3 )O 3 wherein M is at least one of Ba and Ca, and 0 to 1.0 % by weight of manganese oxide (MnO) with respect to the composite material. SUMMARY OF THE INVENTION It is the object of the present invention to provide a ceramic composition for a dielectric body allowing independent control of the dielectric constant and the temperature coefficient of the body, which leads to the result that the dielectric constant at τ f =0 can be shifted to higher or lower values. A further object of the invention is to provide a ceramic composition for a dielectric body that can be sintered at lower temperatures compared to conventional dielectric bodies and which has superior dielectric properties in the microwave band. These objects are attained by a dielectric ceramic composition as defined in claim 1. Suitable and preferred embodiments of the dielectric ceramic composition are defined in the subclaims. According to the first aspect of the present invention, there is provided a composition of dielectric ceramics represented by the formula: A 1+a B O 3+a (0 ≤ a ≤ 0.08) wherein A and B site are as defined in claim 1. B site contains at least one atom selected from the group comprising Ti and W to render the composition independently controllable in respect of the dielectric constant and the temperature coefficient of the resonant frequency. Fig.2 shows the substitution effect at B site in the above generic formula A 1+a BO 3+a . As apparent therefrom, B site substitution by Ti causes a lower dielectric constant at τ f =0, while B site substitution by (Fe 2/3 W 1/3 ) causes a higher dielectric constant at τ f =0. Accordingly, B site substitution by Ti or W renders the composition independently controllable in respect of the dielectric constant and the temperature coefficient of the resonant frequency depending to the sustitution amount. In the above composition represented by the formula A 1+a BO 3+a , A site may contain Sr or Ba atoms and B site may contain at least one of Cr, Mn, Co, Ni, Cu, Zn, Si, Al, Mg, Bi and Sb. Therefore, in the first embodiment according to the present invention, there is provided a dielectric ceramic composition which comprises lead oxide, calcium oxide, iron oxide, niobium oxide and titanium oxide and is represented by the following formula: (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a wherein x and y are within a phase diagram square area enclosed by the following four points A, B, C and D: A:(x,y)= (0.43, 0.0); B:(x,y)= (0.63, 0.0); C:(x,y)= (0.74, 0.25); D:(x,y)= (0.57, 0.25); and a is defined by the following relation 0.0 ≤ a ≤ 0.08 with the proviso that y = 0 is excluded. In the second embodiment of the present invention, there is provided a dielectric ceramic composition which comprises lead oxide, calcium oxide, iron oxide, niobium oxide and tungsten oxide and is represented by the following formula: (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-z (Fe 2/3 W 1/3 ) z }O 3+a wherein x, y and a are defined by the following relations; 0.43 ≤ x ≤ 0.63 0.0 ≤ z ≤ 0.5 and 0.0 ≤ a ≤ 0.08. Further, it is found that A site substitution by Sr causes a lowering of the sintering temperature. Therefore, in the preferred third embodiment, there is provided a dielectric ceramic composition which comprises lead oxide, calcium oxide, iron oxide, niobium oxide, titanium oxide and strontium oxide and being represented by the following formula: {(Pb 1-x Ca x ) 1-z Sr z } 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a wherein x and y are within a phase diagram square area without y=0, enclosed by the following four points A, B, C and D: A:(x,y)= (0.43, 0.0); B:(x,y)= (0.63, 0.0); C:(x,y)= (0.74, 0.25); D:(x,y)= (0.57, 0.25); and a and z are defined by the following relations 0.0 ≤ a ≤ 0.08 0.0 ≤ z ≤ 0.4 with the proviso that y = 0 is excluded. In the preferred fourth embodiment of the present invention, there is provided a dielectric ceramic composition which comprises lead oxide, calcium oxide, strontium oxide, iron oxide, niobium oxide and tungsten oxide and is represented by the following formula: (Pb 1-x-y Ca x Sr y ) 1+a {(Fe 1/2 Nb 1/2 ) 1-z (Fe 2/3 W 1/3 ) z }O 3+a wherein x and y are within a phase diagram square area without y=0, enclosed by the following four points A, B, C and D: A:(x,y)= (0.43, 0.0); B:(x,y)= (0.3, 0.13); C:(x,y)= (0.3, 0.4); D:(x,y)= (0.7, 0.0); and a and z are defined by the following relations 0.0 ≤ a ≤ 0.08 0.0 ≤ z ≤ 0.7 with the proviso that y=0 is excluded. In the above preferred embodiments, a part or all of Nb atoms of the composition may be substituted by Ta atom or atoms in order to lower the sintering temperature. Therefore, there are provided compositions according to the first and the third embodiment and represented by the following formula: ({Pb 1-x Ca x } 1-z Sr z ) 1+a {Fe 1/2 (Ta 1-m Nb m ) 1/2 } 1-y Ti y O 3+a wherein m is defined by the following relation 0.0 ≤ m ≤ 1.0. or compositions according to the second and the fourth embodiment and represented by the following formula: (Pb 1-x-y Ca x Sr y ) 1+a {Fe 1/2 (Ta 1-m Nb m ) 1/2 } 1-z (Fe 2/3 W 1/3 ) z O 3+a wherein m is defined by the following relation 0.0 ≤ m ≤ 1.0. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a graph showing a relation between the dielectric constant and the temperature coefficient of resonant frequency depending on x variation of the system represented by the formula: (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y M y } O 3+a Fig. 2 is a graph showing a B site substitution effect by Ti or (Fe 2/3 W 1/3 ) in the relation between the dielectric constant and the temperature coefficient of resonant frequency depending on x variation of the system represented by the formula: (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y M y } O 3+a . Fig.3 shows a preferred zone of x and y in the system represented by the formulas: (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a or {(Pb 1-x Ca x )} 1-z Sr z } 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a Fig.4 shows a in a ternary phase diagram of Pb, Ca and Sr of the system represented by the formula: (Pb 1-x-y Ca x Sr y ) 1+a {(Fe 1/2 Nb 1/2 ) 1-z (Fe 2/3 W 1/3 ) z }O 3+a DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following examples, compositions Nos. 1 to 4, 12, 16, 20 and 24 of table 1 and Nos. 21 and 44 of table 3 are outside the invention. (EXAMPLE 1) This example is correspoding to the embodiment defined by the claim 2 of the present invention. Starting material comprise PbO, TiO 2 , CaCO 3 , Fe 2 O 3 , and Nb 2 O 5 of a high purity. After the purity compensation of each of the starting materials, a given amount of each of the starting materials is weighed to give a composition (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a having x, y and a shown in the following Table 1. Numbers in Fig.3 indicate the Sample numbers listed in the Table 1. The starting materials are mixed for 17 hours in a ball mill using balls made of stabilized zirconia and pure water as a solvent. The pure water is separated from the starting materials by a filtering process after the ball mill operation. The starting materials are dried, put in an alumina crucible and then preheated for two hours at a temperature of 750 to 850°C. The pre-heated materials are crushed into rough particles in a alumina mortar and are crushed for 17 hours in a ball mill. The crushed materials are dried and then further admixed with an aqueous solution of 5 % of polyvinyl alcohol. The added aqueous solution is adjusted to be 6 wt.% of the crushed materials. The materials with the aqueous solution are granulated through 30 mesh. The granulated materials are then pressed into a circular disc having a diameter of 13 mm and a height of 5 mm with a pressure of 100 MPa. The pressed disc is heated in air at 600°C for 2 hours in order to burn out the polyvinyl alcohol and then put in a magnesia crucible having a magnesia cap with the pre-heated same composition powder in order to prevent evaporation of PbO. The disc in the magnesia crucible is heated at a temperature of 950 to 1200°C for 2 hours and then cooled to give a sintered body having a highest apparent density. The sintered body is subjected to a test for measuring a resonant frequency and a Q value through a dielectric resonating method. It is possible to calculate a relative dielectric constant(εr) from the size of the sintered body and the resonant frequency. The resonant frequency ranges from 2 to 5 GHz. The resonant frequency is measured at temperatures of -25°C, 20°C and 85°C and the temperature coefficient (τ f ) thereof is calculated by the least square method. The results are lasted in Table 1. It is to be noted that the compositions shown by a mark (#) show a composition outside of the composition range according to the present example. No. Composition Dielectric constant Q value τ f ppm/°C X Y a 1# 0.43 0 0 146 810 +99 2# 0.50 0 0 113 1230 +38 3# 0.57 0 0 90 1510 +2 4# 0.63 0 0 70 2090 -14 5# 0.43 0.02 0 151 840 +110 6 0.50 0.02 0 117 1310 +40 7 0.57 0.02 0 94 1640 +4 8 0.63 0.02 0 73 2190 -11 9# 0.65 0.02 0 66 2200 -2100 10# 0.47 0.10 0 147 1220 +109 11 0.50 0.10 0 132 1410 +90 12# 0.57 0.10 -0.02 99 310 +20 13 0.57 0.10 0 97 1830 +18 14 0.57 0.10 0.03 96 1890 +16 15 0.57 0.10 0.08 92 1320 +12 16# 0.57 0.10 0.10 90 470 +10 17 0.65 0.10 0 77 2120 -7 18# 0.70 0.10 0 61 2250 -29 19# 0.53 0.20 0 143 1190 +112 20# 0.57 0.20 -0.02 122 220 +92 21 0.57 0.20 0 119 1510 +81 22 0.57 0.20 0.03 117 1560 +76 23 0.57 0.20 0.08 111 1020 +72 24# 0.57 0.20 0.10 107 480 +69 25 0.70 0.20 0 75 2090 -1 26# 0.74 0.20 0 62 2210 -23 27 0.57 0.25 0 131 940 +98 28 0.74 0.25 0 69 2100 +1 29# 0.65 0.27 0 96 1200 +62 From the Table 1, the composition not containing Ti, that is, the composition at y=0 has Q value =1510 at the dielectric constant of 90 as shown sample No. #3 while the composition at y=0.1 has Q value =1830 even at the dielectric constant of 97 as shown Sample No. 13. This means introduction of Ti at B site causes Q value higher. The compositions belonging to the scope of the claim 2 indicate the dielectric constant of more than 70, the Q value of more than 500, the absolute value of the temperature coefficient of less than 100 ppm/°C. On the other hand, the composition outside the scope of the claim 2 having x more than line BC of Fig.3 comes to indicate the dielectric constant of less than 70, while the composition having y more than line CD indicates a lower Q value when the dielectric constant has same value at y=0 and the composition having x less than line AD indicates the temperature coefficient of + 100 ppm/°C. Further, the composition having (a) value less than 0 or more than 0.8 indicates a suddenly lowered Q value. Accordingly, such compositions are not practical to the dielectric body used for a microwave. It is noted that there is present the composition zone neighboring Sample No.13 indicating such a superior dielectric characteristics as the dielectric constant of more than 95, the Q value of more than 1800, the absolute temperature coefficient of less than 20ppm/°C. (Example 2) This example is corresponding to the compositions claimed in the claims 3 and 4. The methods for preparing the sintered body and evaluating the dielectric characteristics are same as Example 1. The test composition is represented by the formula: {(Pb 0.43 Ca 0.57 ) 1-z Sr z } 1.0 {(Fe 1/2 (Nb 1-b Ta b ) 1/2 ) 0.9 Ti 0.1 }O 3 As the starting materials of the components Sr and Ta, SrC03 and Ta2O5 are used. The results are listed in the following Table 2. The sintering temperature in Table 2 is the temperature at which a highest apparent density is given by sintering. From the Table 2, introduction of Sr at A site causes lowering the sintering temperature. The composition having z more than 0.4 indicates a lowered Q value and a positive larger temperature coefficient which is out of the scope claimed. Especially, as shown in Sample No.3, Sr introduction at z= about 2 causes the sintering temperature lowered without substantially degradation of the microwave dielectric characteristics. Further, substitution of Nb by Ta also causes lowering of the sintering temperature, which results in lowering of the manufacturing cost and depressing of characteristics dispersion No. Composition Sintering temp. °C Dielectric constant Q value τ f ppm/°C Z b 1 0 0 1150 97 1830 +18 2 0.05 0 1100 98 1850 +17 3 0.20 0 1050 99 1900 +18 4 0.4 0 1050 99 1700 +24 5# 0.45 0 1050 100 870 +36 6 0 0.1 1100 96 1800 +16 7 0 0.3 1050 93 1740 +10 8 0 0.7 1050 76 1300 -30 9 0 1.0 1050 57 930 -52 (Example 3) This Example is corresponding to the compositions claimed in the claims 5, 6 and 7. The methods for preparing the sintered body and evaluating the characteristics are the same as Example 1. The test sample is represented by the formula: (Pb 1-x-y Ca x Sr y ) 1+a {Fe 1/2 (Ta 1-m Nb m ) 1/2 } 1-z (Fe 2/3 W 1/3 ) z O 3+a Sample Nos. 1 to 22 are the compositions containing no Sr (y=0) which correspond to that claimed in the claims 5 and 7. Sample Nos. 23 to 45 are the compositions containing Sr which correspond to the compositions claimed in the claims 6 and 7. As the starting materials, there are used SrO, Ta 2 O 5 and WO 3 . The results are shown in the following Table 3. As shown in Table 3, the compositions claimed in the claims 5, 6 and 7, achieve a dielectric constant higher than 50, a Q value higher than 500 and the temperature coefficient of resonant frequency τ f less than 100 ppm/°C in an absolute value. An increase of (z) in the formula, that is, an increase of W of the composition results in a somewhat decrease in the Q value but results in a higher dielectric constant at the same temperature coefficient. Addition of W more than the scope of the invention causes remarkable lowering of the Q value. All the composition claimed in the claims are sintered at a temperature of less than 1100 °C. As shown above, the composition of dielectric ceramics are provided with excellent dielectric characteristics at the microwave band. It is possible to make a resonator in a small size superior in the temperature characteristics when the resonator is composed of a dielectric body having a dielectric constant higher than 100, a practical Q value and a small temperature coefficient. Such a resonator is very useful for the miniaturization of the car telephone and a portable telephone. The compositions according to the present invention can be sintered at a temperature lower than the temperature used in the conventional dielectric body. This results in a decrease in the manufacturing cost. The dielectric compositions according to the present invention can applicable for use to the small, coaxial dielectric resonator: The composition according to the present invention can be sintered at a low temperature at which an electric conductor such as copper or silver paste can be sintered and accordingly can be used for formation of a circuit substrate for the microwave band and an integrated device. It can be said that the composition according to the present invention makes a big contribution to the electronic industry. It is possible to add any oxide materials such as Ba, Cr, Mn, Co, Ni, Cu, Zn, Si, Al, Mg, Bi and Sb other than the oxides defined by the Examples so long as the other oxides have no bad effect on the dielectric characteristics of the resultant dielectric body.

Claims: A dielectric ceramic composition represented by the general formula A 1+a BO 3+a wherein A contains Pb and Ca, with the proviso that Ca is more than 20 wt% of A, and A optionally contains Sr or Ba, and B contains Fe and Nb and Ti or W, and a is defined by the relation 0 ≤ a ≤ 0.08 The composition of claim 1, represented by the formula (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a wherein x and y are within a phase diagram area enclosed by the points A, B, C and D: A:(x,y) = (0.43, 0.0); B:(x,y) = (0.63, 0.0); C:(x,y) = (0.74, 0.25); D;(x,y) = (0.57, 0.25); and a is defined by the relation 0.0 ≤ a ≤ 0.08 with the proviso that y = 0 is excluded. The composition of claim 1, represented by the formula {(Pb 1-x Ca x ) 1-z Sr z } 1+a {(Fe 1/2 Nb 1/2 ) 1-y Ti y }O 3+a wherein x and y are within a phase diagram area enclosed by the points A, B, C and D: A:(x,y) = (0.43, 0.0); B:(x,y) = (0.63, 0.0); C:(x,y) = (0.74, 0.25); D:(x,y) = (0.57, 0.25); and a and z are defined by the relations 0.0 ≤ a ≤ 0.08 and 0.0 ≤ z ≤ 0.4 with the proviso that y = 0 is excluded. The composition of claim 2 or 3, wherein a part or all of Nb is substituted by Ta and wherein the composition is represented by the formula ({Pb 1-x Ca x } 1-z Sr z ) 1+a {Fe 1/2 (Ta 1-m Nb m ) 1/2 } 1-y Ti y O 3+a and wherein m is defined by the relation 0.0 ≤ m ≤ 1.0. The composition of claim 1, represented by the formula (Pb 1-x Ca x ) 1+a {(Fe 1/2 Nb 1/2 ) 1-z (Fe 2/3 W 1/3 ) z }O 3+a wherein x, y and a are defined by the relations 0.43 ≤ x ≤ 0.63 0.0 ≤ z ≤ 0.5 and 0.0 ≤ a ≤ 0.08. The composition of claim 1, represented by the general formula (Pb 1-x-y Ca x Sr y ) 1+a {(Fe 1/2 Nb 1/2 ) 1-z (Fe 2/3 W 1/3 ) z }O 3+a wherein x and y are within a phase diagram area enclosed by the points A, B, C and D: A:(x,y) = (0.43, 0.0); B:(x,y)= (0.3, 0.13); C:(x,y) = (0.3, 0.4); D:(x,y) = (0.7, 0.0); and a and z are defined by the relations 0.0 ≤ a ≤ 0.08 and 0.0 ≤ z ≤ 0.7 with the proviso that y = 0 is excluded. The composition of claim 5 or 6, wherein a part or all of Nb is substituted by Ta and wherein the composition is represented by the formula (Pb 1-x-y Ca x Sr y ) 1+a {Fe 1/2 (Ta 1-m Nb m ) 1/2 } 1-z (Fe 2/3 W 1/3 ) z O 3+a and wherein m is defined by the relation 0.0 ≤ m ≤ 1.0. The composition of claim 1, wherein B, in addition to Fe and Nb and Ti or W, contains at least one of Cr, Mn, Co, Ni, Cu, Zn, Si, Al, Mg, Bi and Sb.

Inventors: INOUE TATSUYA, KAGATA HIROSHI, KAMEYAMA ICHIRO, KATO JUNICHI, NISHIMOTO KEIJI, INOUE, TATSUYA, KAGATA, HIROSHI, KAMEYAMA, ICHIRO, KATO, JUNICHI, NISHIMOTO, KEIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: C04B35
IPC codes: C04B35, H01B3, H01G4