BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a dielectric ceramic material. More particularly, it relates to a dielectric ceramic material which exhibits excellent dielectric characteristics, i.e., a high relative dielectric constant (hereinafter represented by .di-elect cons..sub.r) in a high frequency region, a high unloaded quality coefficient (hereinafter represented by Q.sub.u), and a small absolute value of the temperature coefficient of resonance frequency (hereinafter resonance frequency is represented by f.sub.0, and the temperature coefficient thereof is represented by .tau..sub.f). The dielectric ceramic material of the present invention is suited for use in multilayer circuit boards, resonators and filters particularly for use in a high frequency region, an impedance matching element for various microwave circuits, and the like.

2. Description of the Related Art

With the recent increase of communication information, rapid progress is being made in various communication systems utilizing the microwave region, such as mobile telecommunication systems, satellite communication systems, positioning systems using communication data, and satellite broadcasting. Use of the communication systems in a submillimeter wave region has been demanded. Many dielectric materials have been developed with the rapid progress. These dielectric materials are required to have (1) a high relative dielectric constant .di-elect cons..sub.r, (2) a high unloaded quality coefficient Q.sub.u (i.e., a small dielectric loss 1/Q.sub.u), and (3) a small absolute value of .tau..sub.f (i.e., small temperature dependence of f.sub.0).

In particular, a dielectric material used in a submillimeter wave region is required to have an especially high Q.sub.u, and it is desirable that .tau..sub.f be controllable freely around 0 ppm/.degree. C.

Microwave dielectric porcelain compositions based on Li.sub.2 O--CaO--Sm.sub.2 O.sub.3 --TiO.sub.2 are disclosed in JP-A-5-211007 and JP-A-5-211009. These materials have a particularly excellent .di-elect cons..sub.r value but a relatively small Q.sub.u. While the .tau..sub.f is relatively controlled by the composition, the precise control has been difficult, and control to nearly 0 ppm/.degree. C. has not been realized as yet.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a dielectric ceramic material which has a high Q.sub.u value even in a submillimeter wave region, can have its .tau..sub.f controlled around 0 ppm/.degree. C., and exhibits a high .di-elect cons..sub.r value.

The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a dielectric filter utilizing the dielectric material of the present invention, and

FIG. 1B is a front view taken from the open end surface of FIG. 1A.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a dielectric ceramic material represented by composition formula: (uLi.sub.2 O--vNa.sub.2 O)--wSm.sub.2 O.sub.3 --xCaO--yTiO.sub.2, wherein u+v+w+x+y=100 mol %, 0<u<40, 0<v<40, 0<u+v.ltoreq.40, 0<w.ltoreq.30, 0<x.ltoreq.40, and 0<y.ltoreq.80 (hereinafter referred to as dielectric ceramic material A).

The present invention also provides a dielectric ceramic material represented by composition formula: (uLi.sub.2 O--vNa.sub.2 O)--wSm.sub.2 O.sub.3 --xCaO--(yTiO.sub.2 --zM.sub.2 O.sub.3), wherein M represents Ga or Al; u+v+w+x+y+z=100 mol %, 0.ltoreq.u.ltoreq.40, 0.ltoreq.v.ltoreq.40, 0<u+v.ltoreq.40, 0<w.ltoreq.30, and 0<x.ltoreq.40, 0<y<80, 0<z<80, and 0<y+z.ltoreq.80 (hereinafter referred to as dielectric ceramic material B).

In dielectric ceramic material A, the molar ratio of the Li oxide (u) and that of the Na oxide (v) satisfy 0<u<40 and 0<v<40, respectively. That is, dielectric ceramic material A contains both Li oxide and Na oxide. The total molar ratio of the Na oxide and the Li oxide (i.e., u+v) satisfies 0<u+v.ltoreq.40, preferably 3.ltoreq.u+v.ltoreq.15, still preferably 5.ltoreq.u+v.ltoreq.8.

It is preferred that u and v satisfy 3.ltoreq.u<40 and 0<v.ltoreq.30. When u and v satisfy this ranges, it is preferred in the point that .tau..sub.f is not greatly shifted to the plus side.

In dielectric ceramic material B, the molar ratio of Li oxide (u) and that of Na oxide (v) satisfy 0.ltoreq.u.ltoreq.40 and 0.ltoreq.v.ltoreq.40, respectively, and the total molar ratio of the Li oxide and the Na oxide (u+v) satisfies 0<u+v.ltoreq.40. That is, dielectric ceramic material B contains at least one of Li oxide and Na oxide. The total molar ratio u+v is preferably equal to or greater than 3 and equal to or smaller than 15, still preferably equal to or greater than 5 and equal to or smaller than 8.

It is preferred that u and v satisfy 3.ltoreq.u.ltoreq.40 and 0.ltoreq.v.ltoreq.30. When u and v satisfy this ranges, it is preferred in the point that .tau..sub.f is not greatly shifted to the plus side.

According to the desired dielectric characteristics, Li oxide may be excluded from the composition in dielectric ceramic material B.

If dielectric ceramic materials A and B contain excessive Li oxide and Na oxide, Q.sub.u xf.sub.0 tends to be small.

The molar ratio of Sm oxide (w) in dielectric ceramic materials A and B satisfies 0<w.ltoreq.30, preferably 8.ltoreq.w.ltoreq.20, still preferably 10<w.ltoreq.16. Where Sm oxide is present in excess, .di-elect cons..sub.r tends to be reduced.

The molar ratio of Ca oxide (x) in dielectric ceramic materials A and B satisfies 0<x.ltoreq.40, preferably 5<x.ltoreq.30, still preferably 9<x.ltoreq.18. Where Ca oxide is present in excess, .tau..sub.f tends to be greatly shifted to the plus side.

The molar ratio of Ti oxide (y) in dielectric ceramic material A satisfies 0<y.ltoreq.80, preferably 30.ltoreq.y.ltoreq.77, still preferably 50.ltoreq.y.ltoreq.75.

The molar ratio of Ti oxide (y) and that of at least one of Ga and Al oxides (z) in dielectric ceramic material B satisfy 0<y<80 and 0<z<80. That is, dielectric ceramic material B has part of Ti displaced with at least one of Ga and Al to contain at least one of Ga oxide and Al oxide. The total molar ratio of Ti oxide and at least one of Ga and Al oxides (i.e., y+z) satisfies 0<y+z.ltoreq.80, preferably 30.ltoreq.y+z.ltoreq.75, still preferably 60.ltoreq.y+z.ltoreq.70.

If the dielectric ceramic material B contains Ti oxide and Ga or Al oxide in excessive amounts, .di-elect cons..sub.r tend to be low.

Q.sub.u xf.sub.0 increases with the ratio of z to y in (yTiO.sub.2 --zM.sub.2 O.sub.3), i.e., the degree of substitution of Ti with at least one of Ga and Al. However, if the degree of substitution exceeds 50 mol % based on the Ti oxide, .di-elect cons..sub.r tends to decrease due to phase separation or change of the crystal structure. Accordingly, it is preferred that 40.ltoreq.y<80 and 0.ltoreq.z<40, particularly 30.ltoreq.y.ltoreq.60 and 3.ltoreq.z.ltoreq.35.

In the present invention, where 5.ltoreq.u+v.ltoreq.20, a .di-elect cons..sub.r value of 75 or greater and Q.sub.u xf.sub.0 of 6000 GHz or greater can be secured. In particular where u, v, w, x, and y satisfy 5.ltoreq.u.ltoreq.7, 1.ltoreq.v.ltoreq.3, 12.ltoreq.w.ltoreq.14, 12.ltoreq.x.ltoreq.18, and 60.ltoreq.y.ltoreq.65, .di-elect cons..sub.r of 99 or greater, Q.sub.u xf.sub.0 of 6100 GHz or greater, and .tau..sub.f of -9 to 12 ppm/.degree. C. can be secured.

Further, where 4.ltoreq.u.ltoreq.8, 0.ltoreq.v.ltoreq.3, 12.ltoreq.w.ltoreq.15, 16.ltoreq.x.ltoreq.19, 32.ltoreq.y.ltoreq.35, and 28.ltoreq.z.ltoreq.32, it is possible to obtain .di-elect cons..sub.r of 58 or greater and Q.sub.u xf.sub.0 of 10080 GHz or greater.

It has been known that many dielectric materials having a perovskite structure exhibit excellent dielectric characteristics. The dielectric ceramic material according to the present invention is considered to be a solid solution structure made up of Ca.sub.1-x Sm.sub.2x/3 TiO.sub.3 that has a perovskite structure and exhibits a very high .di-elect cons..sub.r value, a large Q.sub.u xf.sub.0 value, and a positive .tau..sub.f value; Li.sub.1/2 Sm.sub.1/2 TiO.sub.3 that has a perovskite structure and exhibits a high .di-elect cons..sub.r value and a large .tau..sub.f value in the minus side; and Na.sub.1/2 Sm.sub.1/2 TiO.sub.3 that has a perovskite structure and exhibits a large Q.sub.u xf.sub.0 value and a large .tau..sub.f value in the plus side. The excellent dielectric characteristics of the ceramic material of the invention seems attributed to this structure. Where the dielectric ceramic material of the invention has such a solid solution composition or a nearly solid solution composition, it is possible to obtain .di-elect cons..sub.r of nearly 100, Q.sub.u xf.sub.0 of about 6000 GHz, and .tau..sub.f of around 0 ppm/.degree. C. Where part of Ti.sup.4+ contained in the structure is replaced with at least one of Ga.sup.3+ and Al.sup.3+, whose ionic radius is relatively close to that of Ti.sup.4+ and whose valence is different from that of Ti by +1, Q.sub.u xf.sub.0 is improved. Further, the value of .tau..sub.f can be controlled by varying the proportions of Li oxide and Na oxide, which exhibit .tau..sub.f values of opposite signs.

EXAMPLES

The present invention will now be illustrated in greater detail by way of Examples.

Predetermined amounts of commercially available powders of Li.sub.2 CO.sub.3 (purity: 99%), Na.sub.2 CO.sub.3 (purity: 99%), Sm.sub.2 O.sub.3 (purity: 99.9%, D50: 1.9 .mu.m), CaCO.sub.3 (purity: 99.9%, D50: 2.6 .mu.m), TiO.sub.2 (purity: 99%, Average particle size: 0.25 .mu.m) , Al.sub.2 O.sub.3 (purity: 99.9%, Particle size: 2 to 3 .mu.m) and Ga.sub.2 O.sub.3 (purity: 99.9%) were weighed out (100 g in total) to give the final composition shown in Tables 1 and 2 below in terms of the respective oxides.

The powders were wet mixed in a ball mill for 15 hours using ethanol as a medium, and the resulting slurry was dried on a hot water bath and calcined in the air atmosphere at 1000.degree. C. for 2 hours. The calcined product was wet ground for 15 hours in a ball mill together with a wax binder, a dispersant, and ethanol. The resulting slurry was dried on a hot water bath, granulated and compacted under a pressure of 10 MPa into a rod form of 20 mm in diameter and 12 mm in thickness. The rod compact was subjected to cold isostatic pressing (CIP) under a pressure of 150 MPa, and then sintered by firing at 1300.degree. C. for 5 hours in the air atmosphere to obtain a sintered body.

After surface of the resulting sintered body, i.e., a dielectric material, was ground with a #200 diamond, .di-elect cons..sub.r, Q.sub.u, and .tau..sub.f were measured by the Hakki and Coleman's method in a measuring frequency range of from 1 to 3 GHz at a measuring temperature of from 25 to 80.degree. C. The .tau..sub.f value was calculated according to equation: .tau..sub.f =(f.sub.80 -f.sub.25)/{(f.sub.25.times.(80-25)}.times.10.sup.6, wherein f.sub.25 is a resonance frequency at 25.degree. C., and f.sub.80 is a resonance frequency at 80.degree. C.

The results obtained are shown in Tables 1 and 2.

                              TABLE 1
                              Dielectric Characteristics
    Run    Composition               Q.sub.u xf.sub.0  .tau..sub.f
    No.    u    v      w    x    y    .epsilon..sub.r  (GHZ)   (ppm/.degree.
     C.) Remark
    1      3    3      33   6    56   31   4890    91        comparison
    2      3    3      15   10   69   79   6200    1         invention
    3      4    2      15   10   69   78   6180    -10 "
    4      3    4      15   10   69   79   6270    8         "
    5      7    1      13   16   63   99   6110    -9  "
    6      6    2      13   16   63   99   6140    2         "
    7      5    3      13   16   63   99   6180    12        "
    8      3    12     11   16   59   88   6050    112       "
    9      25   20     15   15   20   85   3070    66        comparison


       TABLE 2
           Composition                      Dielectric Characteristics
    Run                       z                     Q.sub.u xf.sub.0
     .tau..sub.f
    No.    u    v    w    x    y    Ga.sub.2 O.sub.3  Al.sub.2 O.sub.3
     .epsilon..sub.r  (GHZ)   (ppm/.degree. C.) Remark
    10     7    -- 13   17   58   5       -- 89     7200    -11 invention
    11     6    1    13   17   53   10      -- 80     7940    0         "
    12     7    -- 13   17   53   10      -- 80     7810    -16 "
    13     7    -- 13   17   43   20      -- 71     9340    -12 "
    14     6    -- 14   16   44   20      -- 70     9760    -22 "
    15     6    1    13   17   33   30      -- 59     12030   1         "
    16     7    -- 13   17   33   30      -- 58     11700   -10 "
    17     6    2    13   16   3    60      -- 37     8750    65        "
    18     3    2    2    3    5    85      -- 22     3420    72
     comparison
    19     6    1    15   15   58   -- 5       78     6900    2
     invention
    20     6    1    15   15   53   -- 10      75     7350    1         "
    21     7    -- 13   17   43   -- 20      69     8060    20        "
    22     5    -- 14   18   34   -- 30      59     10080   30        "
    23     6    1    10   10   13   -- 60      35     8340    61        "
    24     3    2    2    3    5    -- 85      21     3190    69
     comparison