Dielectric ceramic and a capacitor using the same

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dielectric ceramic and a capacitor using the same. More particularly, the present invention relates to a pulse-generating capacitor which is used as a starter of a discharge lamp, inter alia, a high-pressure vapor discharge lamp which houses a starter inside an outer bulb of the lamp.

2. Background Art

An ordinary commercial power supply is insufficient for starting a high-pressure vapor discharge lamp such as a high-pressure sodium lamp, and therefore, a high voltage pulse must be applied to the lamp. There has come into wide use a high-pressure vapor discharge lamp in which a starter for generating a high voltage pulse is built into an outer bulb and which is used in combination with a ballast for ordinary high-pressure mercury lamps. Such a high-pressure vapor discharge lamp has a basic structure such that a capacitor formed from a non-linear dielectric ceramic is connected in parallel to an arc tube, into which a semiconductor switch (SSS) is incorporated in order to generate a high voltage pulse. The thus generated high voltage pulse is applied to the arc tube together with a power source voltage in order to start the discharge lamp.

As a capacitor serving as means for stably generating such a high voltage pulse, a non-linear dielectric ceramic capacitor made of a barium titanate polycrystalline substance has been used.

As shown in FIG. 1, such a non-linear dielectric ceramic capacitor exhibits a D-E hysteresis in which dielectric displacement (D) sharply changes with voltage (E), and when a voltage greater than the coercive field of the dielectric ceramic capacitor is applied to the capacitor, the amount of charge quickly reaches a saturation level in the vicinity of the polarization reversal voltage. Variation in current at this time causes a change in the ballast, so that a pulse of a high voltage corresponding to -L.multidot.di/dt is generated due to the inductance of the ballast.

A non-linear dielectric ceramic capacitor used for a high-pressure vapor discharge lamp such as a high-pressure sodium lamp is required to have a steep D-E hysteresis which is stable over a wide temperature range. Non-linear dielectric ceramic capacitors that meet such a requirement are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 63-221504, 63- 221505, 1-136323, and 1-136324.

In general, the interior of the outer bulb of a high-pressure vapor discharge lamp such as a high pressure sodium lamp is maintained at a high vacuum of 1.times.10.sup.-5 torr, and during a period in which light is on, the interior of the outer bulb is exposed to high temperature (300.degree. C.) and high vacuum (1.times.10.sup.-5 torr). Further, a barium getter is disposed inside the outer bulb in order to cause adsorption of oxygen generated when the discharge lamp is operated so that the degree of vacuum inside the outer bulb is maintained. However, when the discharge lamp is operated for a long period of time, the interior of the outer bulb becomes a reducing atmosphere due to hydrogen adsorbed onto the arc tube, a metal support for the arc tube, the glass or a like member that constitutes the outer bulb, or due to hydrogen generated as a result of decomposition of adsorbed water. Therefore, when a high-pressure vapor discharge lamp is used for a long period of time in a state in which a non-linear dielectric ceramic capacitor disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 63-221504, 63-221505, 1-136323, or 1-136324 is built into the outer bulb to be used for pulse generation, the dielectric ceramic is reduced, and thus the insulating resistance decreases resulting in the problem that the voltage of generated pulses decreases, or no pulse is generated, with the result that the discharge lamp does not start.

In order to solve this problem, there have been proposed the measure disclosed in Japanese Patent Application Laid-Open (kokai) No. 60-52006 in which a dielectric ceramic capacitor is completely coated with inorganic glass except for its electricity supply portions, and a measure disclosed in Japanese Patent Application Laid-Open (kokai) No. 4-34832 in which a hydrogen adsorption getter is disposed inside the outer bulb. However, these measures cannot suppress deterioration of the non-linear dielectric ceramic capacitor. In addition, the structure of the capacitor for pulse generation and the discharge lamp becomes complex, resulting in increased cost. Further, if the dielectric ceramic capacitor is completely coated with inorganic glass as in the invention of Japanese Patent Application Laid-Open (kokai) No. 60-52006, the D-E hysteresis characteristics of the dielectric ceramic capacitor deteriorate due to the glass so that high voltage pluses cannot be generated in some cases.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is to provide an inexpensive capacitor for pulse generation (hereinafter may be referred to as a pulse-generating capacitor) whose characteristics do not deteriorate even when the capacitor is used in a high temperature, high vacuum, reducing atmosphere, and which enables generation of high-voltage pulses over a wide temperature range.

In one aspect of the present invention, there is provided a pulse-generating capacitor which is used inside an outer bulb of a high-pressure vapor discharge lamp, said capacitor comprising a dielectric element constituted of a non-linear dielectric ceramic which exhibits resistance against reduction.

Preferably, the non-linear dielectric ceramic which exhibits resistance against reduction comprises a polycrystalline substance containing barium titanate as a primary component, and when the polycrystalline substance is represented by the following formula:

(1-a-b)ABO.sub.3 +aM+bR

wherein ABO.sub.3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions,

A, B, a, and b satisfy the following relationships:

1.000.ltoreq.A/B.ltoreq.1.006 (mole ratio)

0.3.ltoreq.b/a.ltoreq.3

0.0015.ltoreq.a.ltoreq.0.0050

and

0.0015.ltoreq.b.ltoreq.0.0050

and a total content of M and R represented by Ad satisfies

0.3<Ad.ltoreq.1.0 (wt. %).

These ranges will be collectively referred to as a first preferable range.

Also preferably, the non-linear dielectric ceramic further comprises an oxide containing Si as a main component in an amount of about 0.005-0.1 parts by weight with respect to 100 parts by weight of the polycrystalline substance. This range for the Si-containing oxide and the aforementioned ranges, i.e., 1.000.ltoreq.A/B.ltoreq.1.006 (mole ratio), 0.3.ltoreq.b/a.ltoreq.3, 0.0015.ltoreq.a.ltoreq.0.0050, 0.0015.ltoreq.b.ltoreq.0.0050, 0.3<Ad.ltoreq.1.0 (wt. %), will hereafter be collectively referred to as a second preferable range.

Also preferably, the above-described ABO.sub.3 when represented by {(Ba.sub.1-x-y-z Sr.sub.x Ca.sub.y Mg.sub.z)O}.sub.m (Ti.sub.1-.phi.-p Zr.sub..phi. Hf.sub.p)O.sub.2 is such that x, y, z, .phi., p, and m satisfy the following relationships:

0.ltoreq.x.ltoreq.0.05

0.ltoreq.y.ltoreq.0.02

0.ltoreq.z.ltoreq.0.005

0.ltoreq..phi..ltoreq.0.12

0.ltoreq.p.ltoreq.0.12

0.0035.ltoreq..phi.+p.ltoreq.0.12

1.000.ltoreq.m.ltoreq.1.006.

These ranges will be collectively referred to as a third preferable range.

In another aspect of the present invention, there is provided a dielectric ceramic which comprises a polycrystalline substance containing barium titanate as a primary component, wherein when the polycrystalline substance is represented by the following formula:

(1-a-b)ABO.sub.3 +aM+bR

wherein ABO.sub.3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions, A, B, a, and b satisfy the following relationships:

1.000.ltoreq.A/B.ltoreq.1.006 (mole ratio)

0.3.ltoreq.b/a.ltoreq.3

0.0015.ltoreq.a.ltoreq.0.0050

and

0.0015.ltoreq.b.ltoreq.0.0050

and a total content of M and R represented by Ad satisfies

0.3<Ad.ltoreq.1.0 (wt. %).

In yet another aspect of the present invention, there is provided a dielectric ceramic which comprises a polycrystalline substance containing barium titanate as a primary component, and when the polycrystalline substance is represented by the following formula:

(1-a-b)ABO.sub.3 +aM+bR

wherein ABO.sub.3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions, A, B, a, and b satisfy the following relationships:

1.000.ltoreq.A/B.ltoreq.1.006 (mole ratio)

0.3.ltoreq.b/a.ltoreq.3

0.0015.ltoreq.a.ltoreq.0.0050

and

0.0015.ltoreq.b.ltoreq.0.0050

and a total content of M and R represented by Ad satisfies

0.3<Ad.ltoreq.1.0 (wt. %),

and the dielectric ceramic further comprises an oxide containing Si as a main component in an amount of about 0.005-0.1 parts by weight with respect to 100 parts by weight of the polycrystalline substance.

Preferably, the above-described ABO.sub.3 when represented by {(Ba.sub.1-x-y-z Sr.sub.x Ca.sub.y Mg.sub.z)O}.sub.m (Ti.sub.1-.phi.-p Zr.sub..phi. Hf.sub.p)O.sub.2 that x, y, z, .phi., p, and m satisfy the following relationships:

0.ltoreq.x.ltoreq.0.05

0.ltoreq.y.ltoreq.0.02

0.ltoreq.z.ltoreq.0.005

0.ltoreq..phi..ltoreq.0.12

0.ltoreq.p.ltoreq.0.12

0.0035.ltoreq..phi.+p.ltoreq.0.12

1.000.ltoreq.m.ltoreq.1.006.

In a still further aspect of the present invention, there is provided a high-pressure metal vapor discharge lamp using as a starter a capacitor of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in conjunction with accompanying drawings, in which:

FIG. 1 is an exemplary D-E hysteresis characteristic profile of a non-linear dielectric ceramic which is incorporated into a capacitor of the present invention;

FIG. 2 is a cross-sectional view of an exemplary pulse-generating capacitor of the present invention;

FIG. 3 (PRIOR ART) is a cross-sectional view of a conventional pulse-generating capacitor;

FIG. 4 is a diagram showing a pulse-generating circuit and a pulse-measuring circuit;

FIG. 5 is a graph showing the relationship between generated pulse voltage and operation time of the lamp, obtained from a pulse-generating capacitor;

FIG. 6 is a sketch showing the structure of an example high-pressure sodium lamp incorporating a capacitor of the present invention; and

FIG. 7 is a circuit diagram for the lamp shown in FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The pulse-generating capacitor of the present invention will be described with reference to the drawings.

FIG. 2 is a cross-sectional view of a pulse-generating capacitor A according to one embodiment of the present invention. Pulse-generating capacitor A is fabricated as follows. First, a starting material ceramic powder composed of various components at predetermined proportions is mixed with a binder. The resultant material is mixed, dried, granulated and then formed into a disk-shaped molded product by, for example, press molding. The molded product is fired so as to obtain a non-linear dielectric ceramic 1 which is to serve as a dielectric element. Subsequently, electrodes 2 are formed on the two major surfaces of the non-linear dielectric ceramic 1 by, for example, burning. Thereafter, rings made of insulation glass 3 are provided for insulation. Lead terminals 5 are connected to the electrodes by use of a conductive adhesive 6 so as to establish electrical connection to thereby obtain a pulse-generating capacitor A.

The non-linear dielectric ceramic is constructed of a polycrystalline substance containing barium titanate as a primary component. The barium titanate is expressed by ABO.sub.3, which is a formula representing a perovskite structure. In the present invention, the A/B mole ratio is controlled and moreover, an oxide of at least one element selected from the group consisting of Mn, Ni and Co, and an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb are added and incorporated in specific amounts so as to secure resistance to reduction and to obtain a steep D-E hysteresis. Therefore, even when the capacitor is exposed to a high-temperature, high-vacuum, reducing atmosphere, a high pulse voltage can be obtained while not permitting a decrease in insulation resistance.

The proper selection of A/B ratio and addition of an oxide of Mn, Ni or Co are effective in improving resistance to reduction. However, it should be noted that these two alone cannot provide a steep D-E hysteresis and cannot ensure a high pulse voltage. Therefore, in a characteristic feature of the present invention, an oxide of an element selected from among La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb is also added, and the ratio of the amount thereof to that of the oxide of Mn, Ni or Co is adjusted to thereby secure both sufficient resistance to reduction and satisfactory pulse voltage characteristics.

Through a further incorporation of an oxide containing Si as a main component, excellent reproducibility of sintering-related properties of the non-linear dielectric ceramic can be obtained, resulting in ceramics whose constitutional grains have a size of small variation. This provides a high voltage of generated pulses and an increase in breakdown voltage.

EXAMPLES

The present invention will next be described in detail by way of examples. However, the invention is not limited only to these examples.

Example 1

BaCO.sub.3, CaCO.sub.3, SrCO.sub.3, MgCO.sub.3, TiO.sub.2, ZrO.sub.2 and Hf.sub.2 O.sub.3 each having a purity of 99% or more were provided as starting materials.

These starting materials were mixed to produce compositions represented by

{(Ba.sub.1-x-y-z Sr.sub.x Ca.sub.y Mg.sub.z)O}.sub.m (Ti.sub.1-.phi.-p Zr.sub..phi. Hf.sub.p)O.sub.2

in which x, y, z, .phi., p and m assume the values shown in Table 1. Each composition was wet-blended by use of a ball mill, crushed, dried and calcined in air at 1120.degree. C. for 2 hours to obtain a compact. The resultant compact was crushed by use of a dry-crusher to yield a crushed material having a particle diameter of 1 .mu.m or less.

TABLE 1 Sample {(Ba.sub.1-x-y Sr.sub.x Ca.sub.y Mg.sub.z)O}.sub.m (Ti.sub.1-.phi.-p Zr.sub..phi. Hf.sub.p)O.sub.2 No. x y z m .phi. p .phi. + p *1 0.10 0.01 0.003 1.004 0.03 0.04 0.07 *2 0.01 0.05 0.001 1.003 0.04 0.01 0.05 *3 0.03 0.005 0.010 1.001 0.02 0.02 0.04 *4 0.02 0.015 0.004 1.002 0.005 0.005 0.01 *5 0.04 0.01 0.003 1.003 0.18 0.02 0.2 *6 0.04 0.015 0.002 0.995 0.03 0.03 0.06 *7 0.01 0.015 0.003 1.010 0.04 0.06 0.1 *8 0.03 0.01 0.002 1.003 0.05 0.04 0.09 *9 0.02 0.005 0.002 1.004 0.01 0.05 0.06 *10 0.03 0.01 0.004 1.003 0.06 0.05 0.11 *11 0.04 0.01 0.001 1.005 0.02 0.03 0.05 *l2 0.02 0.005 0.001 1.001 0.07 0.02 0.09 *l3 0.01 0.005 0.002 1.005 0.03 0.04 0.07 *14 0.03 0.015 0.003 1.002 0.01 0.03 0.04 *15 0.04 0.005 0.004 1.005 0.02 0.01 0.03 *16 0.04 0.015 0.002 1.002 0.02 0.05 0.07 *17 0.02 0.01 0.002 1.002 0.04 0.03 0.07 18 0.03 0.01 0.004 1.000 0.07 0.01 0.08 19 0.02 0.015 0.001 1.006 0.05 0.02 0.07 20 0.01 0.005 0.004 1.001 0.01 0.09 0.1 21 0.03 0.005 0.003 1.005 0.06 0.02 0.08 22 0.04 0.01 0.004 1.003 0.08 0.03 0.11 23 0.03 0.005 0.002 1.003 0.09 0.01 0.1 24 0.01 0.015 0.003 1.004 0.05 0.04 0.09 25 0.04 0.015 0.001 1.002 0.10 0.01 0.11 26 0.02 0.01 0.003 1.002 0.02 0.02 0.04 27 0.01 0.01 0.004 1.003 0.04 0.03 0.07 28 0.01 0.015 0.002 1.001 0.07 0.01 0.08 29 0.03 0.01 0.001 1.005 0.09 0.02 0.11 30 0 0.005 0.002 1.005 0.03 0.06 0.09 31 0.05 0.01 0.004 1.004 0.08 0.03 0.11 32 0.03 0 0.004 1.002 0.01 0.05 0.06 33 0.02 0.02 0.003 1.003 0.06 0.02 0.08 34 0.03 0.015 0 1.001 0.04 0.01 0.05 35 0.03 0.01 0.005 1.003 0.05 0.04 0.09 36 0.01 0.015 0.001 1.005 0.005 0.03 0.035 37 0.04 0.015 0.004 1.001 0.10 0.02 0.12 38 0.01 0.005 0.001 1.004 0 0.12 0.12 39 0.03 0.005 0.003 1.005 0.12 0 0.12 40 0.02 0.015 0.002 1.002 0 0.05 0.05 41 0.04 0.01 0.004 1.003 0.05 0 0.05

To the crushed material were added MnCO.sub.3, NiO, CoO, La.sub.2 O.sub.3, CeO.sub.2, Nd.sub.2 O.sub.3, Pr.sub.6 O.sub.11, Sm.sub.2 O.sub.3, EU.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Er.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and SiO.sub.2 so as to obtain compositions as shown in Tables 2 and 3. Further, polyvinyl alcohol (3 wt. %) and pure water were added thereto, and each of the resultant mixtures was wet-blended by use of a ball mill, dried, subjected to granulation and molded under a pressure of 2 tons/cm.sup.2 to thereby obtain disk-shaped molded products.

TABLE 2 aM bR Sample No. MnO NiO CoO Total of "a" La.sub.2 O.sub.3 CeO.sub.2 Nd.sub.2 O.sub.3 Pr.sub.6 O.sub.11 Sm.sub.2 O.sub.3 Eu.sub.2 O.sub.3 *1 0.002 0.001 0 0.003 0 0 0.002 0 0 0 *2 0.002 0 0.001 0.003 0.002 0 0 0 0 0 *3 0.002 0.001 0 0.003 0 0 0.002 0 0 0 *4 0.001 0.001 0.002 0.004 0 0.002 0 0 0 0 *5 0.002 0 0 0.002 0 0.002 0 0 0 0 *6 0.002 0 0.001 0.003 0.002 0 0 0 0 0 *7 0.001 0.001 0.001 0.003 0.002 0 0 0 0 0.001 *8 0.001 0 0 0.001 0 0 0.002 0 0 0 *9 0.004 0.002 0.002 0.008 0 0.002 0 0 0 0 *10 0.001 0.001 0 0.002 0 0 0.0005 0.0005 0 0 *11 0.002 0 0.001 0.003 0.001 0.003 0 0 0.001 0 *12 0.002 0.002 0.002 0.006 0 0 0.0005 0 0 0 *13 0.001 0.0005 0 0.0015 0 0.0005 0.002 0.0005 0 0 *14 0 0.001 0 0.001 0.0005 0.001 0 0 0 0 *15 0.001 0 0.004 0.005 0 0 0 0 0 0 *16 0.002 0 0 0.002 0 0.002 0 0 0 0 *17 0.001 0.001 0 0.002 0 0.002 0 0 0 0 18 0.002 0 0.001 0.003 0 0 0.002 0 0 0 19 0.002 0.001 0 0.003 0 0 0.002 0 0 0 20 0.001 0 0.0005 0.0015 0 0 0.002 0.0005 0 0 SiO.sub.2 (Parts bR M + R by Sample No. Gd.sub.2 O.sub.3 Tb.sub.2 O.sub.3 Dy.sub.2 O.sub.3 Ho.sub.2 O.sub.3 Er.sub.2 O.sub.3 Yb.sub.2 O.sub.3 Total of "b" b/a (wt %) weight) *1 0 0 0 0 0 0 0.002 0.7 0.5 0.05 *2 0 0 0 0 0.001 0 0.003 1 0.67 0.04 *3 0 0 0 0 0 0.002 0.004 1.3 0.84 0.06 *4 0 0 0 0 0 0 0.002 0.5 0.44 0.02 *5 0 0.001 0 0.001 0 0 0.004 2 0.6 0.04 *6 0 0 0 0 0 0 0.002 0.7 0.5 0.01 *7 0 0 0 0 0 0 0.003 1 0.64 0.03 *8 0 0 0 0 0 0 0.002 2 0.35 0.07 *9 0 0 0 0 0 0.001 0.003 0.4 0.89 0.04 *10 0 0 0 0 0 0 0.001 0.5 0.43 0.02 *11 0.001 0 0 0 0 0 0.006 2 0.88 0.01 *12 0 0 0.0005 0 0 0 0.001 0.2 0.59 0.02 *13 0 0.001 0 0.0005 0.0005 0 0.005 3.3 0.96 0.03 *14 0 0 0 0 0.0005 0 0.002 2 0.3 0.07 *15 0 0 0 0 0.005 0 0.005 1 1.2 0.01 *16 0.001 0 0 0 0 0 0.003 1.5 0.44 0 *17 0 0 0 0.001 0 0 0.003 1.5 0.45 0.2 18 0 0 0 0 0 0 0.002 0.7 0.5 0.01 19 0 0 0 0 0 0 0.002 0.7 0.5 0.09 20 0.001 0 0 0 0.001 0 0.0045 3 0.9 0.02

TABLE 3 aM bR Sample No. MnO NiO CoO Total of "a" La.sub.2 O.sub.3 CeO.sub.2 Nd.sub.2 O.sub.3 Pr.sub.6 O.sub.11 Sm.sub.2 O.sub.3 Eu.sub.2 O.sub.3 21 0.002 0.001 0.002 0.005 0 0 0.001 0 0 0 22 0.001 0.0005 0 0.0015 0.002 0 0 0 0.001 0 23 0.003 0.001 0.001 0.005 0.002 0 0 0 0 0 24 0.002 0 0.001 0.003 0.001 0 0 0 0 0 25 0.002 0 0.001 0.003 0.003 0.002 0 0 0 0 26 0.002 0 0 0.002 0.0005 0.0015 0 0 0 0 27 0.002 0 0.002 0.004 0.003 0 0.001 0 0 0.001 28 0.002 0 0 0.002 0 0 0.002 0 0 0 29 0.001 0.001 0 0.002 0 0 0.002 0 0 0 30 0.002 0 0.001 0.003 0 0.001 0.001 0.001 0 0 31 0.002 0.001 0 0.003 0 0.001 0.001 0 0 0 32 0.002 0 0.002 0.004 0 0.001 0.001 0 0 0.001 33 0.002 0.002 0 0.004 0 0.002 0 0 0 0 34 0.001 0.002 0 0.003 0 0.002 0 0 0 0 35 0.002 0.001 0 0.003 0 0.002 0 0 0 0 36 0.002 0 0.002 0.004 0 0 0.001 0.001 0 0 37 0.002 0.001 0 0.003 0 0 0.002 0 0.001 0 38 0.002 0 0.002 0.004 0.001 0.001 0 0 0 0 39 0.003 0 0.001 0.004 0.001 0.001 0 0 0 0 40 0.001 0.001 0.001 0.003 0.001 0 0.001 0 0 0 41 0.002 0 0 0.002 0.001 0 0.001 0 0 0.001 SiO.sub.2 (Parts bR M + R by Sample No. Gd.sub.2 O.sub.3 Tb.sub.2 O.sub.3 Dy.sub.2 O.sub.3 Ho.sub.2 O.sub.3 Er.sub.2 O.sub.3 Yb.sub.2 O.sub.3 Total of "b" b/a (wt %) weight) 21 0 0.0005 0 0 0 0 0.00015 0.3 0.59 0.07 22 0 0 0.001 0 0 0 0.004 2.7 0.69 0.05 23 0 0 0 0 0 0 0.002 0.4 0.64 0.02 24 0 0.0003 0 0 0.0002 0 0.0015 0.5 0.43 0.03 25 0 0 0 0 0 0 0.005 1.7 0.78 0.08 26 0 0 0 0 0 0 0.002 1 0.32 .02 27 0 0 0 0 0 0 0.005 1.3 1 0.06 28 0 0 0 0.001 0 0 0.003 1.5 59 0.005 29 0.001 0 0 0 0 0 0.003 1.5 0.58 0.1 30 0 0 0.0005 0 0 0 0.0035 1.2 0.93 0.04 31 0 0 0 0 0 0 0.002 0.7 0.43 0.06 32 0 0 0 0 0 0.001 0.004 1 0.83 0.01 33 0.001 0 0 0 0 0 0.003 0.8 0.59 0.04 34 0 0.001 0 0 0 0 0.003 1 0.53 0.02 35 0 0 0 0.001 0 0 0.003 1 0.52 0.05 36 0 0 0 0 0 0.005 0.0025 0.6 0.95 0.03 37 0 0 0 0 0 0 0.003 1 0.65 0.07 38 0 0 0 0 0.001 0 0.003 0.8 0.65 0.04 39 0.001 0 0 0 0 0 0.003 0.8 0.65 0.06 40 0 0 0.001 0 0 0 0.003 1 0.66 0.03 41 0 0 0 0 0 0 0.003 1.5 0.58 0.07

Subsequently, the resultant molded products were fired at temperatures shown in Table 4 for 2 hours to obtain non-linear dielectric ceramics each having a diameter of 18 mm and a thickness of 0.6 mm.

As shown in FIG. 2, electrodes made of silver and having a diameter of 16 mm were formed on the two major surfaces of each dielectric ceramic by burning. Further, insulation glass rings made of glass ceramic and having an outer diameter of 17 mm and an inner diameter of 14 mm were provided. Subsequently, lead terminals made of nickel were connected to the above-described electrodes by use of a conductive adhesive to thereby obtain a pulse-generating capacitor A.

The voltage of generated pulses was measured for the resultant capacitor at -40.degree. C., at room temperature (20.degree. C.) and at 50.degree. C. by use of a pulse generating circuit and a pulse-measuring circuit shown in FIG. 4. The pulse-generating capacitor A fabricated as described above was placed within a thermo-vessel. A semiconductor switch C having a breakover voltage of 150 V and a ballast D for use with a 400 watt high-pressure mercury lamp (power supply: 220 V, 60 Hz) were connected in series to the capacitor A. The thus-formed series circuit was connected to an AC power supply E (100-Vac, 60-Hz) to thereby complete the pulse-generating circuit. The voltage of generated pulses (hereinafter simply referred to as pulse voltage) was measured through use of an oscilloscope F connected between the opposite ends of a series circuit including the capacitor A and the semiconductor C.

Then, the insulation resistance was measured by applying 100 Vdc for 2 minutes by use of an insulation resistance meter, and volume resistivity (.rho.) was calculated.

In order to measure time-course change of properties under high temperature and in a reducing atmosphere, each of the capacitors was allowed to stand for 1000 hours in a vacuum chamber (400.degree. C., 1.times.10.sup.-5 torr, hydrogen concentration: 0.5%). The pulse voltage of the capacitors after being allowed to so stand was measured at room temperature (20.degree. C.) by use of the pulse generating circuit and the pulse measuring circuit shown in FIG. 4. The insulation resistance was measured and the volume resistivity (.rho.) was calculated.

AC breakdown voltage was measured as follows. Silver electrodes having a diameter of 14 mm were formed, by burning, on the two major surfaces of each non-linear dielectric ceramic having a diameter of 18 mm and a thickness of 0.6 mm which had been obtained through firing at temperatures shown in Table 4 to thereby produce sample capacitors. The voltage of each sample was measured upon breakage of the sample after being subjected to rising voltage applied at 60 Hz, 100 Vrms/second in a silicone-oil bath.

The complete results of the tests are shown in Table 4. Sample Nos. 6 to 15 in Tables 1 to 4 fall outside the first preferable range. Sample Nos. 6 to 17 fall outside the second preferable range. Sample Nos. 1 to 17 fall outside the third preferable range, and are marked with "*." That is, Sample Nos. 18 to 41 fall within the most preferred ranges according to the present invention.

TABLE 4 400.degree. C./1 .times. 10.sup.-5 Torr/0.5% H.sub.2, Voltage of generated After 1000 hr AC Firing pulses (kV) Volume Voltage of Volume breakdown Sample temp. Room resistivity .rho. generated resistivity .rho. voltage No. .degree. C. -40.degree. C. temp. 50.degree. C. (.OMEGA.-m) pulses (kV) (.OMEGA.-m) (kV/mm) *1 1340 1.91 1.84 1.35 3.51 .times. 10.sup.12 1.77 1.24 .times. 10.sup.12 6.24 *2 1320 1.59 1.48 1.37 2.96 .times. 10.sup.12 1.42 9.52 .times. 10.sup.10 6.08 *3 1300 1.57 1.42 1.31 1.36 .times. 10.sup.12 1.37 9.87 .times. 10.sup.10 4.92 *4 1320 1.59 1.45 1.34 6.52 .times. 10.sup.11 1.55 8.51 .times. 10.sup.11 6.25 *5 1340 1.82 1.59 1.36 3.05 .times. 10.sup.12 1.64 1.62 .times. 10.sup.12 6.49 *6 1300 2.00 1.82 1.71 4.34 .times. 10.sup.12 1.18 5.60 .times. 10.sup.7 4.15 *7 1380 1.48 1.33 1.22 2.46 .times. 10.sup.12 1.33 2.46 .times. 10.sup.12 9.52 *8 1320 2.03 1.85 1.74 6.42 .times. 10.sup.11 1.11 4.43 .times. 10.sup.7 6.18 *9 1340 1.58 1.43 1.35 2.87 .times. 10.sup.12 1.53 1.09 .times. 10.sup.12 6.41 *10 1320 1.57 1.48 1.37 8.27 .times. 10.sup.11 1.68 5.61 .times. 10.sup.11 6.54 *11 1360 1.58 1.45 1.34 2.87 .times. 10.sup.11 1.39 5.05 .times. 10.sup.10 5.55 *12 1320 1.74 1.60 1.49 2.21 .times. 10.sup.12 1.50 1.03 .times. 10.sup.12 6.13 *13 1360 1.96 1.81 1.70 4.18 .times. 10.sup.11 1.29 3.71 .times. 10.sup.7 4.66 *14 1300 2.03 1.88 1.77 8