Dielectric Ceramic Composition

1. Field of the Invention

The present invention relates to a dielectric composition, especially suitable for microwave applications, which contains three components of BaO, TiO.sub.2 and WO.sub.3 as main components.

2. Description of the Prior Art

In recent years, dielectrics having a high dielectric constant, a low dielectric loss and a low temperature coefficient of resonant frequency have been used in the microwave region, for example, resonators and have been expanding their practical utility in microwave communication or broadcasting apparatus, for example, in car telephone systems or satellite broadcasting receivers.

Heretofore, in such applications, there have been known BaO-TiO.sub.2 system dielectric ceramics. For instance, dielectric ceramics having a composition of Ba.sub.2 Ti.sub.9 O.sub.20 are described in detail in Japanese Patent Publication No. 58-20 905. Dielectric ceramics with a composition of BaTi.sub.4 O.sub.9 have been also reported.

Further, there has been known dielectric ceramics of the composite perovskite type and Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 proposed in Japanese Patent Application Laid-Open No. 53-35 454 may be mentioned as a representative example of such a type.

However, the aforesaid known dielectric ceramic materials have problems in the microwave applications. For example, Ba.sub.2 Ti.sub.9 O.sub.20 can not be satisfactorily used without complicated special treatments. In the preparation of the dielectric ceramics with the Ba.sub.2 Ti.sub.9 O.sub.20 composition, calcined materials should be treated with an acid solution or after firing, the fired bodies should be treated over long periods of time in an oxygen gas atmosphere, in order to improve the dielectric properties. The Q values of the thus obtained dielectric ceramics have been reported to be on the order of 4200 at 10 GHz, wherein Q is the reciprocal of dielectric loss tan .delta., i.e., Q=1/tan .delta. [refer to "Ba.sub.2 Ti.sub.9 O.sub.20 as a Microwave Dielectric Resonator" J. K. Plourde, D. F. Linn, H. M. O'Bryan, John Thomson Jr., J. Am. Ceram. Soc., 58 (9-10) pp. 418-420 (1975) and "A New BaO-TiO.sub.2 compound with Temperature-Stable High Permittivity and Low Microwave Loss," H. M. O'Bryan, John Thomson Jr., and J. K. Plourde, J. Am. Ceram. Soc., 57 ( 10) pp. 450-53(1974)]. However, for practical use of the Ba.sub.2 Ti.sub.9 O.sub.20 dielectric ceramics as resonators, further improvements have been required in the Q values.

Further, with respect to the dielectric ceramic with the BaTi.sub.4 O.sub.9 composition, it has been reported that it has a Q value of 2500 at 11 GHz ["New Low Loss High-K Temperature-Compensated Dielectric for Microwave Application", D. J. Masse, et al., Proceedings of the IEEE 59 (11), pp. 1628-29 (1971)] and a further improved Q value has been required. Further, in the use for dielectric resonators, the BaTi.sub.4 O.sub.9 dielectric ceramic has an unfavorably large temperature coefficient of resonant frequency, .tau..sub.f, of +15 to 20 ppm/.degree.C.

The perovskite type dielectric ceramics need the use of large quantities of expensive Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 and thereby the production cost is increased.

It is therefore an object of the present invention to overcome the foregoing problems encountered in the prior art and, more specifically, to provide a dielectric ceramic composition having a high Q and a small temperature coefficient of resonant frequency, .tau..sub.f.

In our previous U.S. patent application Ser. No. 860,617, filed May 7, 1986 (based on Japanese Patent Application No. 60-237 457), we now U.S. Pat. No. 4,749,669 have proposed a dielectric ceramic having a superior microwave properties of a high Q and a low .tau..sub.f, the ceramic consisting essentially of a BaO-TiO.sub.2 -WO.sub.3 ternary system composition and, optionally, ZrO.sub.2 and/or MnO. The present Inventors have made a further investigation on the BaO-TiO.sub.2 -WO.sub.3 ternary system dielectric ceramic composition and found in strictly controlled ceramic compositions that dielectric ceramics with further improved dielectric properties can be achieved by a special compositional range which is based on the BaO-TiO.sub.2 -WO.sub.3 ternary system, as referred to in the foregoing our Patent Application but has molar ratio ranges of the constituents different from those of the U.S. Application.

In the new compositional range, Q values are at the same level as those of the U.S. patent application Ser. No. 860,617 now U.S. Pat. No. 4,749,669 but, in comparison with other known BaO-TiO.sub.2 system ceramic compositions, the Q values are high. Further, the new compositional range provides a properly low dielectric constant (.epsilon.r=20-35). In applications at high frequencies in the microwave range, when the dielectric constant is too high, the size of resonators become too small and difficulties are presented in handling them. For such a reason, dielectric ceramics having a low dielectric constant (e.g., 20-35) are demanded. In addition, the new compositions have sufficiently small .tau..sub.f values and the .tau..sub.f values can be discretionally adjusted in the vicinity of 0 ppm/.degree.C.

The present invention is to provide a dielectric ceramic consisting mainly of three components of BaO, TiO.sub.2 and WO.sub.3 and, if necessary, containing MnO as an additive. More specifically, the dielectric ceramic according to present invention is so designed that the composition consists basically of a combination of crystallized phases selected from BaTi.sub.4 O.sub.9, Ba.sub.2 Ti.sub.9 O.sub.20, BaWO.sub.4 and TiO.sub.2 and, as examples of such there may be obtained BaTi.sub.4 O.sub.9 -BaWO.sub.4, BaTi.sub.4 O.sub.9 -Ba.sub.2 Ti.sub.9 O.sub.20 -BaWO.sub.4, Ba.sub.2 Ti.sub.9 O.sub.20 -BaWO.sub.4, Ba.sub.2 Ti.sub.9 O.sub.20 -BaWO.sub.4 -TiO.sub.2. According to the present invention, the dielectric ceramic has a ternary system composition represented by the formula:

wherein a, b and C are molar proportions and 20.ltoreq.a.ltoreq.40, 30.ltoreq.b.ltoreq.78.5 and 0<c<33 and, more specifically, the composition of the present invention is within the range enclosed by a line joining the points A, B, C, D, E and A of a composition diagram shown in the attached drawing in which compositions lying on a line joining A, E and D are excluded from range, because they are claimed in our previous application above mentioned. In other words, the dielectric ceramic of the present invention has a BaO-TiO.sub.2 -WO.sub.3 ternary system composition consisting essentially of BaO, TiO.sub.2 and WO.sub.3 within the molar percent range joining the points of A(21.5, 78.5, 0), B(40.0, 30.0, 30.0), C(37.0, 30.0, 33.0), D(20.0, 75.0, 5.0), E(21.5, 75.0, 3.5) and A shown in FIGURE, in which compositions laying on the line joining A, E, D are excluded. In such a specified compositional range, it is preferable that WO.sub.3 be contained at least 0.1 mole %. As necessary, the dielectric ceramic composition may further include up to 3 mole % MnO with respect to 100 mole % of the sum of BaO, TiO.sub.2 and WO.sub.3. Further, 10 mol % or less of TiO.sub.2 may be substituted by tetravalent metal oxide(s) which can dissolve as a solid solution when used as a substitute for the TiO.sub.2 component and examples of such a tetravalent oxide include ZrO.sub.2 and SnO.sub.2. Since the area defined by lines joining the points A, E, D, F (17.0, 78.5, 5.0), G(17.0, 83.0, 0) and A on the left side of a line connecting the points A, E and D is the region claimed in our previous patent application Ser. No. 860,617, this region is excluded from the range claimed in the present application in order to avoid overlapping.

The drawing is a BaO-TiO.sub.2 -WO.sub.3 ternary composition diagram in mole % and the area enclosed by a line connecting the points of A, B, C, D, E and A, but excluding compositions lying on the line joining A, E and D, is the compositional range claimed in claim the present invention.

In the preparation of the dielectric ceramic composition of the present invention, the starting materials for the components of BaO and TiO.sub.2 may be any materials which have been widely used in capacitors for use at lower frequencies than microwave frequencies and those materials are inexpensive. The WO.sub.3 component is added to these two components to provide the composition as specified above and, thereby, high Q dielectric ceramics can be obtained by a simple process and in a low production cost. In the composition of the present invention, the WO.sub.3 component is preferably contained in amounts of at least 0.1 mole %.

The reason why the dielectric ceramic composition of the present invention is limited as specified above is that in the region outside a line joining the points A and B and a line joining the points B and C, shown in the drawing, high Q can not be achieved and dense ceramics can not be successfully obtained after firing. Further, the range outside a line joining the points C and D results in a reduction of Q values.

MnO promotes the densification of the ceramic material during firing and, as effects on the properties, provides improved Q values and makes possible slight adjustments for .tau..sub.f. The reason why the addition of MnO is limited up to 3 mole % is that MnO addition exceeding 3 mole % reduces Q values.

The ceramics of the present invention can be obtained in a conventional production manner and any extra or special step or operation is not required. The starting materials of the ceramics are not limited only to oxides. Any known materials, for example, carbonates such as BaCO.sub.3 and MnCO.sub.3, may be usually used, as long as they can ultimately yield oxide sintered bodies.

The starting materials are weighed corresponding to the intended composition, wet-mixed together with pure water in a polyethylene pot containing alumina balls, and then dried. The dried materials are calcined in an alumina crucible at a temperature of 1000.degree. to 1200.degree. C. for 1 to 6 hours. The calcination process is carried out in an oxidizing atmosphere, such as air or oxygen. The calcined materials are wet-milled in pure water using a polyethylene pot containing alumina balls, dried and molded under pressure. The molded samples are sintered on a setter made of zirconia or platinum at 1350.degree. to 1420.degree. C. for 2 to 6 hours in an oxidizing atmosphere such as oxygen or air.

This invention will now be illustrated in more detail with reference to the following Examples.

BaCO.sub.3, TiO.sub.2, WO.sub.3 and MnCO.sub.3 were used as raw materials and dielectric ceramics were prepared in the same procedure as described above. The dielectric ceramics thus obtained were measured for the dielectric constant, .epsilon.r, and Q values at 5.5-6.5 GHz and .tau..sub.f over the temperature range of -25.degree. C. to +75.degree. C.

Table 1 shows the compositional proportions (by mole %) and the results of the measurements.

Example Nos. 1 through 6 have compositions lying on the line connecting BaTi.sub.4 O.sub.9 and BaWO.sub.4 shown in the drawing and, as noted from the following Table 1, the .tau..sub.f decreases as composition closes toward the BaWO.sub.4 composition, and .tau..sub.f near 0 ppm/.degree.C. can be obtained. Further, the Q values are on the order of at least 5000 at 6 GHz and higher than prior art BaO-TiO.sub.2 system compositions. For example, the Q of the foregoing Ba.sub.2 Ti.sub.9 O.sub.20 reported by O'Bryan, et al. is on the order of 8000 at 4 GHz and this Q value is equivalent to 5000 at 6 GHz.

Further, the .epsilon.r values have been found to be of the order of 22 to 33 and small. Comparative Example No. 3 shows that when composition is above the line connecting the points A and B, shown in the drawing, the Q value is decreased.

Comparative Example No. 4 shows that fully densified fired bodies can not be obtained when ceramic compositions are brought toward the BaWO.sub.4 composition and lie beyond the line joining the points B and C. Example Nos. 7 to 9 have compositions lying on the line joining between Ba.sub.2 Ti.sub.9 O.sub.20 and BaWO.sub.4. From the experimental data of these specimens, as compositions are brought closer to the BaWO.sub.4 composition, the temperature characteristic, .tau..sub.f, becomes smaller. Consequently, negative .tau..sub.f values which can not be obtained in the conventional compositions of BaO-TiO.sub.2 system may be achieved.

Further, the dielectric ceramics of the present invention exhibit high Q values of at least 5000 at 6 GHz as compared to those of conventional compositions. It can be seen that the .epsilon.r is at low levels of 29 to 33. Example No. 10 not containing MnO is inferior in Q as compared to Example No. 1 containing MnO in an amount of 0.1 mole %. However, an excess addition of MnO will produced deterious effects. For example, in Example No. 11 with MnO addition of 2 mole %, Q was reduced and, in Comparative Example No. 1 with MnO addition of 4 mole %, Q excessively lowered below the levels contemplated by the present invention. Comparative Example No. 2 indicates that compositions beyond the line connecting the points C and D result in reduction of the Q values.

The dielectric ceramic compositions of the present invention were examimed by X-ray diffraction analysis and it has been found that compositions on the line joining the compositions of BaTi.sub.4 O.sub.9 and BaWO.sub.4 consist basically of BaTi.sub.4 O.sub.9 and BaWO.sub.4 and, on the other hand compositions on the line joining Ba.sub.2 Ti.sub.9 O.sub.20 and BaWO.sub.4 consist basically of Ba.sub.2 Ti.sub.9 O.sub.20 and BaWO.sub.4. Compositions between these two lines, namely, in the hatched zone shown in the drawing consist basically of crystallized phases of BaTi.sub.4 O.sub.9, Ba.sub.2 Ti.sub.9 O.sub.20 and BaWO.sub.4. Further, compositions closer to TiO.sub.2 than the line joining Ba.sub.2 Ti.sub.9 O.sub.20 and BaWO.sub.4 consist basically of crystallized phases of Ba.sub.2 Ti.sub.9 O.sub.20, BaWO.sub.4 and TiO.sub.2.

Further dielectric ceramics, Example Nos. 12-14 and Comparative Example No. 5 were obtained in the same procedure set forth above in which TiO.sub.2 was partially replaced with ZrO.sub.2 or SnO.sub.2. For the thus obtained ceramics, dielectric properties were examined and the results are given in Table 2 below. According to the experimental data, when this partial replacement with the tetravalent metal oxide exceeds 10 mole % of TiO.sub.2, such an excessive replacement results in deterioration of Q values and is unfavorable. Further, microstructural studies indicated that Zr ions or Sn ions dissolve as a solid solution in sites normally occupied by Ti ions and any new crystallized phase is not formed due to the partial replacement of TiO.sub.2 with ZrO.sub.2 and SnO.sub.2.

As described above, the dielectric ceramic composition according to the present invention can be obtained without requiring any special step and has great merits that high Q, properly small dielectric constant and sufficiently reduced temperature coefficient of resonant frequency, .tau..sub.f, can be simultaneously obtained and the .tau..sub.f can be adjusted to a desired value in the vicinity of 0 ppm/.degree.C. Further, since the dielectric ceramics of the present invention also have satisfactory electrical properties for use at low frequencies, it can be provide usefulness for temperature-compensating capacitors.