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Priority is claimed to Japanese Patent Application No. 2004-312454 filed on Oct. 27, 2004, and Japanese Patent Application No.
2004-340347 filed on Nov. 26, 2004, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramics, a multilayer ceramic capacitor using the same and a method for manufacturing them.
2. Description of Related Art
In recent years, as mobile devices such as cellular phones are more widely used and semiconductor devices that are the main part of a personal computer have higher speed and higher frequency, a multilayer ceramic capacitor for these electronics
is increasingly required to have small size and large capacity.
To meet such requirement, in a multilayer ceramic capacitor (MLC), efforts have been made for smaller size and larger capacity, by making dielectric layers thinner to enhance electrostatic capacity and increasing the number of lamination.
Therefore, to meet the demand for thinner multilayered dielectric layers as above, in a dielectric layer constituting a multilayer ceramic capacitor, attempts have been made for atomization and better relative dielectric constant in dielectric powder
constituting a dielectric ceramics (For example, Japanese Unexamined Patent Publication No. 2004-210636).
For example, the above publication mentions that barium titanate powder that is typical dielectric powder is obtained in the form of fine grains by mixing barium hydroxide aqueous solution and Ti alkoxide solution and ripening the mixture in a
vessel for a predetermined time followed by dehydration and drying.
However, barium titanate powder obtained through the above-mentioned liquid phase method is dried under the condition of 110.degree. C. and three hours after mixing and ripening. This condition is employed to only remove moisture contained in
the mixture. The barium titanate powder has much crystal water and impurities such as hydroxide. The barium titanate powder so obtained is made so small as to have a mean grain size of 0.05 .mu.m (50 nm), while having a lattice constant larger than the
value figured out from a single crystal (a=0.4032 nm, V=0.065548 nm.sup.3) and, in terms of crystal structure, mainly having cubic crystals. Therefore, there has been a problem that a dielectric ceramics prepared by using the dielectric powder so
obtained has a low relative dielectric constant.
Meanwhile, grains have been made finer to produce a flat dielectric layer that can meet the demand for thinner layers and to prevent reliability from being lowered by increasing an applied electric field to a multilayer ceramic capacitor due to
thinner layers. For example, Japanese Unexamined Patent Publication No. 2003-309036 mentions that a dielectric layer is formed so that the thickness of dielectric layer t and the maximum diameter of glass grain D can satisfy the relation of
D/t.ltoreq.0.5, thereby attaining high insulation and improving reliability in high temperature load test. In addition, Japanese Unexamined Patent Publication No. 2003-40671 describes thinner dielectric layers and the use of barium titanate powder
having a mean grain size of 0.4 .mu.m to prevent a decrease in relative dielectric constant that occurs when applying DC bias.
According to Ferroelectrics, 1998, Vols. 206-207, pp 337-353, M. H. FREY, Z. XU, P. HAN and D. A. PAYNE, for example, barium titanate used mainly for a dielectric material of the above-mentioned multilayer ceramic capacitor has perovskite
crystal structure, and it is known to show a very high relative dielectric constant of about 4800.
However, if fine barium titanate powder according to Japanese Unexamined Patent Publication No. 2003-309036, for example, is used to make dielectric layers thinner in manufacturing a multilayer ceramic capacitor, abnormal grain growth occurs
during firing at atmospheric pressure. Therefore, crystal grains constituting a dielectric layer cannot have uniform grain size, and large crystal grains resulting from grain growth are present in part. There has been a problem that a multilayer
ceramic capacitor having such crystal grains has larger temperature characteristic of relative dielectric constant and lower insulation, in particular, lower reliability in high temperature load test.
SUMMARY OF THE INVENTION
The main advantage of the present invention is to obtain a dielectric ceramics that is composed of crystal grains having a high relative dielectric constant while being atomized.
The other advantage of the present invention is to obtain a multilayer ceramic capacitor that has high relative dielectric constant, stable temperature characteristic and insulation as well as high reliability, even if dielectric layers have
smaller thickness.
The dielectric ceramics of the present invention has a mean grain size of not more than 0.2 .mu.m, preferably not more than 0.15 .mu.m, and contains crystal grains mainly composed of barium titanate. The crystal grains have a volume per unit
cell V of not more than 0.0643 nm.sup.3. V is represented by a product of lattice constant (a, b, c) figured out from the X-ray diffraction pattern of the crystal grains. Thereby, the dielectric ceramics of the present invention can attain a high
relative dielectric constant.
The method for manufacturing such a dielectric ceramics comprises the steps of: (a) obtaining dielectric primary powder whose mean grain size is not more than 0.1 .mu.m through any one liquid phase method selected from oxalic acid method, sol-gel
method and hydrothermal synthesis method; (b) obtaining dielectric powder by drying and heating the dielectric primary powder with a zeolitic drying agent in the atmosphere of a temperature 300 to 500.degree. C. at atmospheric pressure; and (c) firing a
forming body that is molded into a predetermined shape with the dielectric powder.
The first multilayer ceramic capacitor of the present invention comprises dielectric layers composed of the above-mentioned dielectric ceramics and internal electrode layers. The dielectric layers and the internal electrode layers are
alternately laminated.
The second multilayer ceramic capacitor of the present invention comprises a capacitor body wherein dielectric layers and internal electrode layers are alternately laminated. The dielectric layers have crystal grains sintered through a grain
boundary layer. (a) Crystal grains constituting the dielectric layer have a mean grain size of not more than 0.2 .mu.m. (b) The crystal grains are mainly composed of barium titanate. (c) When the volume per unit cell V.sub.bulk is represented by a
product of lattice constant (a, b, c) figured out from the X-ray diffraction pattern of the dielectric layer, and the volume per unit cell V.sub.powder is represented by a product of lattice constant (a, b, c) figured out from the X-ray diffraction
pattern of crystal grains obtained by grinding the dielectric layer, V.sub.bulk and V.sub.powder satisfy the relation Of V.sub.bulk/V.sub.powder.gtoreq.1.005.
This makes it possible to obtain a multilayer ceramic capacitor that has a high relative dielectric constant, excellent temperature characteristic and insulation as well as high reliability, even if a dielectric layer has smaller thickness.
The multilayer ceramic capacitor of the present invention is manufactured by firing a capacitor body forming body wherein green sheets and internal electrode patterns are alternately laminated. The green sheets contain a mixture of dielectric
powder mainly composed of barium titanate and glass powder. According to this method for manufacturing a multilayer ceramic capacitor, the second multilayer ceramic capacitor can be easily manufactured under the following conditions: (A) the dielectric
powder has a mean grain size of not more than 0.2 .mu.m; (B) glass powder has a softening point of not less than 650.degree. C. and a thermal expansion coefficient of not more than 9.5.times.10.sup.-6/.degree. C.
BRIEF DESCRIPTION OF THE
DRAWINGS
FIG. 1 is a vertical sectional view of the multilayer ceramic capacitor of the present invention.
FIG. 2 is a pattern diagram showing the way to evaluate the resistance of grain boundary in a dielectric layer, using alternating-current impedance measurement.
FIG. 3(a) is a graph showing one example of evaluations on the resistance of grain boundary in a dielectric layer, using alternating-current impedance measurement. FIG. 3(b) is a circuit diagram showing an equivalent circuit to analyze the
resistance of grain boundary in a dielectric layer.
FIG. 4 is a flow chart showing the method for manufacturing a multilayer ceramic capacitor of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
(Dielectric Ceramics)
In the dielectric ceramic of the present invention, crystal grains mainly composed of barium titanate have a mean grain size of not more than 0.2 .mu.m, preferably not more than 0.15 .mu.m, and the volume per unit cell V that is represented by a
product of lattice constant (a, b, c) figured out from the X-ray diffraction pattern of the crystal grains is not more than 0.0643 nm.sup.3.
The volume V is preferably not less than 0.062 nm.sup.3 in terms of forming perovskite crystal structure. The volume per unit cell V that is represented by a product of lattice constant (a, b, c) is more preferably in the range from 0.063 to
0.064 nm.sup.3.
The mean grain size of crystal grains is more preferably not less than 0.03 .mu.m in terms of attaining a high relative dielectric constant. The ceramics density of the dielectric ceramics of the present invention is 5.8 to 5.9 g/cm.sup.3. In
addition, cubic crystals and tetragonal crystals coexist in crystal grains. Therefore, it is desirable in terms of higher dielectric constant that the ratio of lattice constant c/a is 1.005 to 1.01, especially 1.006 to 1.009.
When crystal grains have a mean grain size of more than 0.2 .mu.m, particularly more than 0.15 .mu.m, the dielectric layer of a multilayer ceramic capacitor has a smaller number of grain boundaries per unit thickness, making it impossible to
attain high insulation. Moreover, when the volume per unit cell V that is represented by a product of lattice constant (a, b, c) is more than 0.0643 nm.sup.3, relative dielectric constant becomes lower.
In the dielectric ceramics of the present invention, it is preferable in terms of higher dielectric constant that the stress figured out from the gap between the peak positions when comparing the X-ray diffraction pattern of dielectric ceramics
surface with that of barium titanate single crystal is not less than 1 MPa in absolute value. Particularly when not less than 100 layers are laminated in a multilayer ceramic capacitor, compressive stress that influences a dielectric layer is added due
to the difference in thermal expansion coefficient with an internal electrode layer mainly composed of nickel, and the stress is more preferably not less than 5 MPa in absolute value.
(Method for Manufacturing a Dielectric Ceramics)
The method for manufacturing a dielectric ceramics in the present invention will be now described. Through any one liquid phase method selected from oxalic acid method, sol-gel method and hydrothermal synthesis method, dielectric primary powder
having a mean grain size of not more than 0.1 .mu.m is obtained. Among the above-mentioned methods, sol-gel method is particularly preferable in terms of high monodispersibility. In this case, Ba(OH).sub.2 is used as a Ba source, and TiO.sub.2 is used
as a Ti source. The ratio of Ba/Ti is preferably in the range from 0.995 to 1.005 in terms of improved relative dielectric constant and sinterability. Slurry obtained by mixing Ba and Ti sources undergoes preliminary drying under the condition of
atmospheric pressure and 200.degree. C.
The dielectric primary powder so obtained is dried and heated with a zeolitic drying agent in the atmosphere of a temperature 300 to 500.degree. C., in particular 350 to 450.degree. C. at atmospheric pressure to obtain dielectric powder.
Preferably, the dielectric powder so obtained has a grain size variation (CV value) of not more than 50%.
As a zeolitic drying agent, molecular sieve, metallosilicate and cloverite are suitable. Especially, in terms of heat resistance, molecular sieve is more preferable. The specific surface of a zeolitic drying agent is preferably not less than
400 m.sup.2/g and, in terms of drying efficiency and durability, more preferably 500 to 700 m.sup.2/g.
The amount of a zeolitic drying agent is preferably 5 to 20 parts by weight to 100 parts by weight of dielectric primary powder. It is preferable that the temperature is below 600.degree. C. in order to maintain the specific surface of a
zeolitic drying agent. When the temperature is higher than the above, a zeolitic drying agent changes its nature, reducing its specific surface.
Next, using the dielectric powder so obtained as a main component, the forming body molded in a predetermined shape is prepared and then fired. The above dielectric powder is molded together with a binder, forming a predetermined shape of (for
example, discoidal) forming body to be a single layer capacitor.
To form a multilayer ceramic capacitor, the above dielectric powder is mixed with a binder and a solvent to obtain slurry. This slurry goes through sheet forming method such as doctor blade method to form a sheet-like forming body having a
thickness of 1 .mu.m, for example. Next, a conductor pattern is printed on the sheet-like forming body, thereby forming a sheet in which a conductor pattern is formed. A plurality of these sheets are laminated to form a multilayer forming body. Then,
through firing at a temperature nearly equal to the sintering temperature of conductor pattern, a multilayer ceramic capacitor is obtained.
When dielectric primary powder obtained through liquid phase method has a mean grain size of not less than 0.1 .mu.m, crystal grains obtained after sintering becomes larger, lowering insulation resistance. When the drying temperature of
dielectric primary powder is not more than 300.degree. C., drying is not enough, making it difficult to eliminate such impurities as hydroxide in the powder that come from liquid phase method and to induce grain growth. Meanwhile, when the temperature
is over 600.degree. C., the mean grain size of dielectric primary powder becomes too large to obtain a desired size of dielectric powder and thinner green sheets. When pressure is below or over atmospheric pressure, an industrially high-cost
decompression device or pressure device is needed, making it difficult to manufacture dielectric powder as raw material powder at a low cost.
In other words, compared to conventional dielectric powder that is only dried, impurities are eliminated in the surface and the interior of dielectric powder obtained through the above process. Therefore, more defects are produced and lattices
in the surface portion are more easily shrunk, thereby adding compressive stress to the interior of powder and making the lattice constant smaller across the board. This leads to a decrease in the volume per unit cell.
(Multilayer Ceramic Capacitor)
The multilayer ceramic capacitor of the present invention will be described in detail, referring to the schematic sectional view of FIG. 1. FIG. 1 is a schematic sectional view showing the multilayer ceramic capacitor of the present invention.
The partially enlarged view in FIG. 1 is a pattern diagram showing crystal grains 9 and a grain boundary layer 11 that constitute a dielectric layer. In the multilayer ceramic capacitor of the present invention, an external electrode 3 is formed at both
ends of a capacitor body 1. The external electrode 3 is formed by baking Cu, or an alloy paste of Cu and Ni. The capacitor body 1 comprises a dielectric layer 5 and an internal electrode layer 7 that are alternately laminated. The dielectric layer 5
is composed of crystal grains 9 and a grain boundary layer 11.
Thickness of the dielectric layer 5 is preferably not more than 1.6 .mu.m for a multilayer ceramic capacitor to have smaller size and larger capacity. When the dielectric layer 5 has small thickness, the effectiveness of the structure including
dielectric crystal grains is increased.
Moreover, in the present invention, to stabilize variations in electrostatic capacity and capacity-temperature characteristic, thickness variation in the dielectric layer 5 is more desirably within 10%.
The internal electrode layer 7 is preferably a base metal such as nickel (Ni) and copper (Cu) in terms of curbing manufacturing cost even if a larger number of layers are laminated. In particular, nickel (Ni) is more desirable in order to
co-fire with the dielectric layer 5 according to the present invention.
The crystal grains 9 constituting the dielectric layer 5 mainly comprise perovskite barium titanate crystal grains. That is, the crystal grains 9 of the present invention are mainly composed of barium titanate. For this reason, high relative
dielectric constant is shown as above. Since the crystal grains 9 constituting the dielectric layer 5 according to the present invention have high insulation and reliability in high temperature load in the above dielectric layer 5, it is important that
its mean grain size is not more than 0.2 .mu.m. When the mean grain size is larger than 0.2 .mu.m, high insulation and reliability in high temperature load cannot be attained. The mean grain size is represented by D50 that is calculated from a
volume-integrated value in grain size distribution.
On the other hand, the grain size lower limit of the crystal grains 9 is preferably not less than 0.05 .mu.m in order to improve the relative dielectric constant of the dielectric layer 5 and decrease the temperature dependence of relative
dielectric constant.
It is desirable that the crystal grains 9 contain Mg, a rare earth element and Mn. Preferably, to 100 parts by weight of barium titanate component, 0.04 to 0.3 parts by weight of Mg, 0.5 to 2 parts by weight of a rare earth element and 0.04 to
0.3 parts by weight of Mn are contained in the crystal grains 9. Since Mg, a rare earth element and Mn are derived from a sintering additive, some of these elements go into solid solution in the crystal grains 9 while many are present in the grain
boundary layer 11.
In the dielectric layer 5, Mg and a rare earth element are a component constituting the core-shell structure in a crystal grain. Meanwhile, Mn can compensate for oxygen defect in the crystal grains 9 produced by firing in the reducing atmosphere
and increase insulation and high temperature load lifetime.
In the dielectric layer 5 of the present invention, it is desirable that a rare earth element is contained with the highest concentration in the grain boundary layer 11 that is the grain surface, and has a concentration gradient of not less than
0.05 atomic %/nm from the surface to the interior of the crystal grain 9. When a rare earth element has such concentration gradient, relative dielectric constant and high temperature load lifetime can be improved and also capacity-temperature
characteristic can satisfy X5R standard. As a rare earth element in the present invention, it is preferable to use at least one selected from La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, Lu, and Sc. Particularly, Y is preferable for the crystal
grains 9 to attain a higher dielectric constant and higher insulation.
Since, in the dielectric layer 5, the relative dielectric constant can keep high and the resistance in acceleration test can be increased, it is desirable that the amount of alumina impurities contained in a dielectric ceramics is not more than
1% by weight.
As above, the crystal grain 9 constituting the dielectric layer 5 has core-shell structure where Mg and a rare earth element derived from a sintering additive are more distributed in the grain surface rather than in the grain center. As a
result, a high dielectric constant is attained and the grain surface has a characteristic of high insulation. The relative dielectric constant of the dielectric layer in the present invention is preferably not less than 2000, in particular, not less
than 2500.
In the dielectric layer 5 of the present invention, when the volume per unit cell V.sub.bulk is represented by a product of lattice constant (a, b, c) figured out from X-ray diffraction pattern and the volume per unit cell V.sub.powder is
represented by a product of lattice constant (a, b, c) figured out from the X-ray diffraction pattern of crystal grains that are obtained by grinding the dielectric layer, V.sub.bulk and V.sub.powder satisfy the relation of
V.sub.bulk/V.sub.powder.gtoreq.1.005. This relation comes from residual stress that the grain boundary layer 11 puts on the crystal grains 9 in a dielectric ceramics. When the difference in thermal expansion coefficient between the crystal grain 9 and
the grain boundary layer 11 is large, the relative dielectric constant becomes large. In other words, as glass powder that is an additive has a smaller thermal expansion coefficient, greater effect is shown. On the other hand, when
V.sub.bulk/V.sub.powder is less than 1.005, the relative dielectric constant is prevented from improving. In finding out the relation of V.sub.bulk/V.sub.powder, the indices (h k l) from X-ray diffraction pattern reach a peak in the range of 1 to 4, for
example, h: (1 0 0), (2 0 0), (4 0 0). The same is true of the other "k" and "l".
In the crystal grain 9 constituting the dielectric layer 5, when a lattice constant ratio satisfies the relation of c/a.gtoreq.1, a higher relative dielectric constant can be attained. The lattice constant ratio c/a is preferably not less than
1.005 in order to improve the relative dielectric constant of the dielectric layer 5.
Furthermore, in the present invention, the mole ratio of A site for barium and B site for titanate in barium titanate constituting the crystal grains 9 satisfies the relation of A site/B site.gtoreq.1, preferably, A site/B site.gtoreq.1.003 in
order to inhibit grain growth. The above definition of the ratio A site/B site makes it possible to inhibit grain growth of the crystal grains 9 and stabilize the temperature characteristic of relative dielectric constant.
In the present invention, the relation of V.sub.bulk/V.sub.powder.gtoreq.1.005 is satisfied. This is because the crystal grains 9 originally have a larger thermal expansion coefficient than the grain boundary layer 11, and once the sintered body
is cooled down after sintering, the crystal grains 9 are drawn to the grain boundary layer 11. Therefore, when the sintered body is ground to release the crystal grains 9 from the grain boundary layer 11, the volume turns larger.
FIG. 2 is a pattern diagram showing the way to evaluate the resistance of grain boundary in a dielectric layer, using alternating-current impedance measurement. In FIG. 2, 20a is a thermostatic bath in which a sample multilayer ceramic capacitor
is placed and a temperature is controlled. 20b is a HALT (Highly Accelerated Life Test) device to add direct-current voltage to the sample. 20c is an impedance measuring device having alternating-current power supply. FIG. 3(a) is a graph showing the
evaluations on resistance of grain boundary in a dielectric layer, using alternating-current impedance measurement. FIG. 3(b) is a circuit diagram showing an equivalent circuit for analysis.
In the present invention, a multilayer ceramic capacitor is exposed to the high temperature loading atmosphere of a higher temperature than the Curie temperature shown by perovskite barium titanate crystal grains that constitute the dielectric
layer 5, and a voltage as high as not less than one third of the rated voltage of the multilayer ceramic capacitor. Before and after exposing the multilayer ceramic capacitor to this high temperature loading atmosphere, under the same conditions, the
rate of resistance decrease of the grain boundary layer 11 in the dielectric layer 5 is measured through alternating-current impedance measurement. FIG. 3(a) is a graph (Cole-Cole plot) showing impedance change in the core (central portion) of the
crystal grain 9 in the multilayer ceramic capacitor of the present invention, its shell (peripheral portion), the grain boundary layer and the interface between the internal electrode layer 7 and the dielectric layer 5. As shown in the equivalent
circuit of FIG. 3(b), in this evaluation, the dielectric layer 5 is divided into four elements: core (central portion), shell (peripheral portion), the grain boundary layer 11 and the interface between the internal electrode layer 7 and the dielectric
layer 5. The horizontal axis of the graph indicates the real part of impedance signal and the vertical axis indicates the imaginary part. The graph showing impedance change is obtained by fitting of the difference between before and after Highly
Accelerated Life Test (HALT), and simulation. The present invention especially focuses on resistance change in the grain boundary layer 11. The rate of change of the real part (the rate of change per load time), that is, the rate of resistance decrease
of grain boundary in a dielectric layer is desirably not more than 0.5%/min.
This evaluation can be made by dividing the Cole-Cole plot before and after Highly Accelerated Life Test (HALT) in FIG. 3(a) into the above four elements with a special software. Comparing before and after high temperature load process, ion
diffusion and electron transfer become significant in the dielectric layer 5, and the rate of resistance decrease of the grain boundary layer 11 can be clearly observed. For this reason, preferably, the temperature is 1.5 times as high as the Curie
temperature and the voltage is not less than two fifth V of the rated voltage.
(Method for Manufacturing a Multilayer Ceramic Capacitor)
Next, the method for manufacturing a multilayer ceramic capacitor according to the present invention will be described in detail. FIG. 4 is a flow chart showing the method for manufacturing a multilayer ceramic capacitor of the present
invention.
The multilayer ceramic capacitor of the present invention is manufactured by firing a capacitor body forming body wherein green sheets and internal electrode patterns are alternately laminated. The green sheets contain a mixture of dielectric
powder mainly composed of barium titanate and glass powder. In the present invention, it is preferable that the dielectric powder has a mean grain size of not more than 0.2 .mu.m while glass powder has a softening point of not less than 650.degree. C.
and a thermal expansion coefficient of not more than 9.5.times.10.sup.-6/.degree. C.
In the above method for manufacturing a multilayer ceramic capacitor, preferably, dielectric powder is plated with an oxide of Mg, a rare earth element and Mn, and when barium site is A and titanate site is B in barium titanate dielectric powder,
the relation of A/B.gtoreq.1 in mole ratio is satisfied and glass powder has a mean grain size of not more than 0.3 .mu.m.
The manufacturing method of the present invention will be described as follows, in reference to each step shown in FIG. 4.
Step (a): First, raw material powder shown below is mixed with organic resin such as polyvinyl butyral resin or solvent such as toluene and alcohol, using a ball mill, to prepare ceramic slurry. Next, through sheet forming method such as doctor
blade method or die coating method, a ceramic green sheet 21 is formed on a carrier film 22. It is preferable that the ceramic green sheet 21 is 1 to 2 .mu.m thick in order to make a dielectric layer thinner for larger capacity and maintain high
insulation.
Barium titanate powder (BT powder) is dielectric powder used in the manufacturing method of the present invention, and raw material powder represented as BaTiO.sub.3. In the BT powder, preferably, its components A site (barium) and B site
(titanate) satisfy the relation of A/B.gtoreq.1 in mole ratio, especially, A/B.gtoreq.1.003 in terms of inhibiting grain growth during firing. This dielectric powder is obtained through one synthetic method selected from solid phase method, liquid phase
method (including a producing method through oxalate), hydrothermal synthesis method and the like. Among these, dielectric powder obtained through hydrothermal synthesis method is desirable because the dielectric powder so obtained has narrow grain size
distribution and high crystallinity.
It is important that the grain size distribution of BT powder is not more than 0.2 .mu.m so that the dielectric layer 5 can be easily made thinner, and desirably, 0.05 to 0.2 .mu.m so as to improve the relative dielectric constant by making the
c/a ratio higher and attain higher insulation.
Regarding dielectric powder having a high relative dielectric constant, powder wherein a peak indicating a cubic crystal is larger than a peak indicating a tetragonal crystal in the evaluation on crystallinity by X-ray diffraction, can make the
lattice constant ratio c/a higher. Preferably, 0.04 to 0.3 parts by weight of Mg, 0.5 to 2 parts by weight of a rare earth element and 0.04 to 0.3 parts by weight of Mn respectively to 100 parts by weight of BT powder are added to plate the
above-mentioned dielectric powder.
Glass powder to be added to the dielectric powder may have a softening point of not less than 650.degree. C. A softening point of lower than 650.degree. C. allows glass softening and flow to occur for long hours during firing, and the grain
growth of barium titanate to be easily caused. For the above reason and in terms of suppressing aggregation caused by softened glass component itself and increasing dispersibility in a dielectric ceramics, the softening point is preferably not less than
690.degree. C.
It is preferable that the glass powder according to the present invention has a thermal expansion coefficient of not more than 9.5.times.10.sup.-6/.degree. C. at room temperature to 300.degree. C. Effects are shown when the thermal expansion
coefficient of glass component is not more than 9.5.times.10.sup.-6/.degree. C., and greater effects are shown in improving a dielectric constant when it is not more than 9.times.10.sup.-6/.degree. C. Moreover, when the softening point of the above
glass is higher, for example, not less than 700.degree. C. or not less than 800.degree. C., more stress is added between the crystal grains 9 and the grain boundary layer 11 in cooling process and dielectric characteristic is more effectively
controlled.
On the contrary, when the thermal expansion coefficient is larger than 9.5.times.10.sup.-6/.degree. C. (the temperature range of room temperature to 300.degree. C.), the difference with the thermal expansion coefficient
(12.5.times.10.sup.-6/.degree. C.) of dielectric powder becomes smaller, which makes the stress to dielectric crystal grains smaller, leading to a decrease in relative dielectric constant.
In terms of reducing the grain size difference with barium titanate powder increasing dispersibility, it is desirable that the above glass powder has a mean grain size of not more than 0.3 .mu.m.
It is preferable that glass powder is mainly composed of SiO.sub.2, BaO, CaO and B.sub.2O.sub.3. The preferable composition is 40 to 70 mole % of SiO.sub.2, 5 to 40 mole % of BaO, 5 to 40 mole % of CaO and 1 to 30 mole % of B.sub.2O.sub.3. In
order to keep the softening point high, it is preferable not to contain Li component.
In the present invention, besides the above composition, glass powder containing no Si component, 10 to | | |
