GB2027008A

GB2027008A - Ceramic Dielectrics

Info

Publication number: GB2027008A
Application number: GB7925661A
Authority: GB
Original assignee: Matsushita Electronics Corp
Current assignee: Panasonic Holdings Corp
Priority date: 1978-07-25
Filing date: 1979-07-24
Publication date: 1980-02-13
Also published as: NL7905587A; DE2929764A1; DE2929764C2; FR2433818A1; JPS5517965A; FR2433818B1; JPS623569B2; GB2027008B; CA1140741A

Abstract

A ceramic dielectric capacitor material is comprised of fifteen elements, namely titanium, strontium, barium, calcium, niobium, tantalum, iron, aluminum, sodium, silicon, phosphorus, bismuth, lead, copper and oxygen. The material comprises mainly SrO and TiO2 where the atomic number ratio of Ti/Sr is 0.99 to 1.02; the material further comprises Bi2O3 and Ta2O5 where the atomic number ratio of Bi/Sr is 0.02 to 0.06 and the atomic number ratio of Ta/Sr is 0.002 to 0.006. The other elements as oxides (or precursors) are present in very small amounts.

Description

SPECIFICATION Ceramic Dielectrics and Process for Production Thereof The present invention relates to a ceramic dielectric and a process for production thereof.

More specifically, the invention relates to a ceramic dielectric comprising of fifteen kinds of elements: titanium, strontium, barium, calcium, niobium, tantalum, iron, aluminum, sodium, silicon, phosphorus, bismuth, lead, copper and oxygen, and to a process for production thereof.

Heretofore, among ceramic dielectrics, those which utilized barium titanate type ceramic semiconductor have been known as so-called "grain boundary" type ceramic dielectrics, obtained by providing a high insulation layer in the grain boundary of semiconductive ceramics.

However, although in the barium titanate type ceramic semiconductive capacitor, a capacitor having a very large value in insulation resistance, such as 10" S--cm and with an effective dielectric constant within the range of 50,000 to 70,000 can be obtained, it has been known that in a temperature range of-300C to +850C it has the defect of showing a variation in its electrostatic capacity up to a degree of +40% based on its measured electrostatic capacity at 200C and to show a large dielectric loss (tan S) of about 5 to 10%.

Therefore, in the recent years, a ceramic semiconductive capacitor with strontium titanate as its main constituent having a low temperature variation ratio of dielectrostatic capacity has been developed to replace the above barium titanate type ceramic semiconductive capacitor. The ceramic semiconductive capacitor with strontium titanate as its main constituent has been produced by re-oxydizing the grain boundary by simply heat treating the semiconductive ceramic obtained by adding small amounts of manganese oxide (MnO2), silicon dioxide (SiO2) etc. to strontium titanate (SrTiO3) and sinterring it under a reductive atmosphere or by thermal-diffusing manganese oxide (MnO2), bismuth oxide etc. into the grain boundary.

Such strontium titanate type ceramic semiconductive capacitor has its distinctive feature in its small temperature deviation ratio of electrostatic capacity and its small dielectric loss (tan .5) value, as compared with the barium titanate type seimiconductive capacitor. On the other hand, it has the defect of having a small effective dielectric ratio, as compared with the barium titanate type. Thereupon, for the purpose of improving the effective dielectric ratio thereof, the addition of some new additives to the strontium have been proposed.For example, by adding singly or in combination zinc oxide (ZnO), rare earth metal oxide etc.; besides the materials necessary for semiconducting strontium titanate such as tantalum oxide (Ta205), niobium oxide (Nb205), tangsten oxide (W03) etc., a ceramic semiconductive capacitor having an effective dielectric constant of about 40,000 to 50,000 and a dielectric loss equal to or less than 1% has been obtained.

At the same time there have been plans to obtain a capacitor even smaller in size and higher in efficiency. However, such a small sized and highly effective element has a problem caused by high efficiency itself. Namely, in such an element, there is a problem of long-term reliability, that is, the insurance that it will maintain such feature as high efficiency for long time under varying conditions. However, so far no element has yet been found which would adequately solve such a problem. Further although from the viewpoint of production it is necessary to prepare and provide in large quantitives elements having such properties, there has not yet been sufficient thorough research to obtain elements having small deviation between and inside production lot.Fundamentarily, the grain boundary type semiconductive capacitor has a microstructure consisting of semiconductor grain having a very low resistance and resistance layer surrounding the same grain. The said microstructure, of course, is determined not only by the blending formulation of the raw materials, but also by the factors in the individual steps in the manufacturing process. Accordingly, it is now possible to obtain an element having highly excellent properties, at the same time having high reproducibility of those properties and further having a high degree of reliability by effectively controlling these factors. Therefore the formulation of the raw materials of the capacitor and the method for production thereof cannot be discussed apart from each other.Especially in the case of a ceramic dielectric having a high dielectric constant discussing hereto, the correlations between those factors mentioned above should be considered together.

The present=inventors have conducted extensive research to solve the above-mentioned problems accompanying the conventional ceramic semiconductive dielectrics from the both the points of the raw material formulation and manufacturing process. As the result of the research they have succeeded in producing a ceramic dielectric having a high dielectric constant.

An object of the present invention is to provide a ceramic dielectric having a high dielectric constant which has a significantly excellent properties, has a good reproducibility of these properties and has a high degree of reliability in maintaining these properties in use under various conditions.

Another object of the invention is to provide a process for preparing the said ceramic dielectric.

Fig. 1 is a diagram showing the relations between atomic number ratio of titanium and strontium and the properties of the present element.

Fig. 2 to 6 are diagrams showing the relations between sintering temperatures and the properties of the present element.

Fig. 7 is a diagram showing a necessary condition for producing the present element having E of 25,000 and the accompanying conditions thereto.

Fig. 8 to 13 are diagrams showing the results of studies of the various properties necessary for a capacitor.

In the following, the invention will be discussed in detail by way of example. "Parts" in the example mean "parts by weight" unless otherwise specifically indicated.

Example A powder mixture containing titanium oxide as the main component was prepared, the said mixture containing at least 98.0 to 99.0% of titanium oxide (TiO2), 0.001-0.005% of ferric oxide (Fe203), 0.2-0.5% of aluminum oxide (AI203), 0.2-0.5% of niobium oxide (nub205) and 0.10.2% of phosphorus pentaoxide. On the one hand, another powder mixture containing strontium carbonate as the main component was prepared, the mixture containing at least 96.0- 99.0% of strontium carbonate (SrCO3), 0.1- 1.2% of sodium carbonate (Na2CO3), 0.001- 0.004% of ferric oxide (Fe203) and 0.0050.02% of silicon dioxide (SiO2).Then the two powder mixtures were mixed together to obtain several kinds of blends having ratios of atomic number of Ti to Sr(N/Ns) in a range of 0.95 to 1.05.

To the said blended mixtures were added respectively a mixture containing bismuth oxide as its main component which is comprised of at least 99.099.9% of bismuth oxide (Bi2O3), 0.0050.02% of silicon dioxide (SiO2), 0.001- 0.003% of ferric oxide (Fe2O3), 0.002-0.01% of lead oxide (PbO), 0.005-0.02% of copper oxide (CuO) and 0.0010.01% of sodium oxide (Na2O), so as to obtain the mixtures having ratios of atomic number of Bi to Sr (NB,/Nsr) in a range of 0.02 to 0.06.

Each of the obtained mixtures was further blended with a mixture containing tantalum oxide as the main component which comprises of at least 99.099.9% of tantalum pentoxide (Ta2O5, 0.0005-0.002% of ferric oxide (Fe2O3) and 0.01 to 0.04% of silicon dioxide (SiO2), so as to obtain mixtures having ratios of atomic numbers of Ta to Sr (NTa/NSr) in a range of 0.002 to 0.006.

Then these obtained powder mixtures were shaped into discs each having a diameter of 75 mm and thickness of 1 5 to 20 mm, and then the discs were calcinated at a temperature in a range of 1,1600 to 1 ,2400C. The calcinated discs were then pulverized in a ball-mill. After drying the powder was press-molded into discs each having a diameter of 1 5 mm and thickness of 0.7 mm.

The discs were sintered for 2 to 4 hours at a temperature ranging from 1,3500 to 1 ,4500C under a reducing atmosphere consisting of 1 to 10% of hydrogen and 99 to 90% of nitrogen gas.

Then cuprous oxide (Cu2O) in an amount of 0.1 to 0.5 mg/cm2 of the discs adhered to the surface of sintered discs, and was heat-treated for 1 to 2 hours at a-temperature of 1,0500 to 1 ,2000C.

After that, electrodes were provided on the both surfaces of the elements obtained.

Fig. 1 is a diagram showing the mean values of the electrical properties of 20 element specimens obtained by varying the ratio of atomic numbers of Ti to Sr, Nr/Nsr, in the range of 0.95 to 1.05.

As is evident from the diagram, an element having a large dielectric constant E and a high insulation resistance can be obtained with a NT/Nsr value in a range of 0.99 to 1.02.

Fig. 2 to 6 show the results of electrical properties of 20 element specimens obtained by varying both the calcinating temperature and the sintering temperature with the fixed NT,/Nsr value of 1.004. The results are summarized as follows: (A) Dielectric constant (see Fig. 2): The higher the calcinating temperature is, the dielectric constant of the element is, and the dielectric constant becomes higher as the sintering temperature becomes higher, but no significant change is observed at a sintering temperature equal to or over 1,42000.

(B) Insulation resistance (S2) (see Fig. 3):The higher the calcinating temperature is, the smaller the insulation resistance of the element is, and the insulation resistance tends to decrease as the sintering temperature becomes higher, but no significant variation can be observed at a sintering temperature over 1,4200C.

(C) C.R. product (M.yF) (see Fig. 4): C.R.

product shows the same tendency as that shows by the insulation resistance value of the element.

(D) Dielectric loss tan S (see Fig. 5): No significant influence on tan S of calcinating or sintering temperature can be observed.

(E) Change of electric capacity with temperature (Ac, %), (see Fig. 6): No significant change in the ratio between the change of electric capacity with the change of temperature can be observed at a temperature over 1,4100C. The influence on the change of electric capacity of calcinating temperature is large and the higher the calcinating temperature is, the smaller the change is.

Fig. 7 shows the set conditions of calcinating temperature and sintering temperature necessary for obtaining an element having an E of, for example, 25,000. Furthermore, the C.R. product of the elements obtained under the said conditions are also shown in Fig. 7. Also, the relation between shrinkage factor of calcinated bodies and particle size of the pulverized particles is shown in Fig. 7. As can be seen from Fig. 7, the shrinkage factor of the calcinated bodies becomes larger as the calcinating temperature becomes higher, and the particle size of the particles obtained by grinding the calcinating body with the ball-mill becomes larger as the calcinating temperature becomes higher. Therefore it is evident from this that the electrical properties of the sintered body are related to the said particle size.

Therefore if a higher calcinating temperature is adopted as the set condition to obtain an element having E of, for example, 25,000, the sintering temperature must be set at a lower temperature.

Then if an additional requirement to obtain an element having a C.R. product over 300 MQ.,uF is added to this set condition, this will be attained by setting the following condition: (1) calcinating temperature: 1.1900C+10"C (2) sintering temperature: 1 ,4350C+50C After providing electrodes to both surfaces of thus obtained elements and soldering a lead wire to the electrodes, the several properties required thereof as a capacitor were examined. The results are shown in Fig. 8 to 13.

The results manifest that the dielectric having high dielectric constant of the invention has a significantly stable and excellent properties in all respects: temperature property (Fig. 8), frequency property (Fig. 9), dielectric voltage property (Fig.

1 0), alternating voltage property (Fig. 11), moisture resistance (Fig. 12) and high temperature loading test (Fig. 13).

As aforementioned, the ceramic dielectric of the invention is an epoch-making element having excellent high reliability and suitable for long term use under all kinds of circumstances as compared with the conventional grain boundary type ceramic semi-conductive capacitor. The present capacitor can take over the field heretofore occupied by the existing organic film capacitor, and can be widely applied to uses as a by-pass-, coupling-, filter-capacitor etc. and has been a claimed for destroying the conventional concepts of a "capacitor". Its wide and varied applicability can bring significant benefits to industry.

The elements of the invention as mentioned above can be prepared by the present process for production thereof.

Although detail explanation of the electrodes used in the above example has not been given, an electrode material containing silver in an amount over 60% was used. Furthermore it has been confirmed that the same effect can be obtained by metalizing either aluminum or copper in place of silver in electrode mentioned above. The particle size of the pulverized particle shown in the example has mean values measured by a sedimentation method using 0.2% aqueous solution of sodium hexametaphosphate as dispersing agent. The particle size obtained by this method has a tolerance limit of 2.0-4.0 from.

Moreover although the powder press molding method to form discs is mentioned in the example, the extruding forming method or sheet forming method can, of course, be used for mass- production scale.

The crystal particles of the sintered body shown in the example have the specific characteristics that more than 90% of the particles are distributed in a range of 5 to 100 E"m

Claims

1. A ceramic dielectric which comprises of at least all of the fifteen elements: titanium, strontium, barium, calcium, niobium, tantalum, iron, aluminum, sodium, silicon, phosphorus, bismuth, lead, copper and oxygen, as the components of said ceramic.

2. A ceramic dielectric according to claim 1 which used as a component of ceramic, a mixture composition obtained by mixing a powdered mixture containing as its main component titanium dioxide which comprises of 98.0 to 99.0% by weight of TiO2,0.001 to 0.005% by weight of Fe203, 0.2 to 0.5% by weight of Awl203, 0.2 to 0.5% by weight of Nb2O3, and 0.1 to 0.2% by weight of P > Os with a powdered mixture containing as its main component strontium carbonate which comprises of 96.0 to 99.0% by weight of SrCO3, 1.00 to 3.00% by weight of BaCO3, 0.01 to 0.5% by weight of CaCO3, 0.1 to

1.2% by weight of Na2CO3,0.001 to 0.004% by weight of Fe203, and 0.005 to 0.02% by weight of SiO2, so as to obtain a mixture having an atomic number ratio of Ti to Sr of 0.99 to 1.02; further mixing the obtained mixture with a powdered mixture containing as its main component bismuth trioxide which comprises of 99.0 to 99.9% by weight of Bi2O3, 0.005 to 0.02% by weight of SiO2, 0.001 to 0.003% by weight of Fe203, 0.002 to 0.01% by weight of PbO, 0.005 to 0.02% by weight of CuO, and 0.001 to 0.01% by weight of Na20,so as to obtain a mixture having an atomic number ratio of Bi to Sr of 0.02 to 0.06;; further mixing the obtained mixture with a mixture containing as its main component, tantalum pentoxide which comprises of 99.0 to 99.9% by weight of Ta205, 0.0005 to 0.002% by weight of Fe203, and 0.01 to 0.04% by weight of SiO2, so as to obtain a mixture having an atomic number ratio of Ta to Sr of 0.002 to 0.006.

3. A ceramic dielectric according to claim 2 wherein a powder is used, the powder being obtained by shaping and calcinating said powder mixture composition to obtain a solid solution and pulverizing the said solid solution.

4. A ceramic dielectric according to claim 1 wherein the constituent particles of said ceramic are semi-conductor and an insulating layer which isolates particles from each other.

5. A ceramic dielectric according to claim 1 wherein at least two electrodes are provided with the surfaces of the element to be used as a ceramic capacitor.

6. A ceramic dielectric according to claim 5 wherein a composition containing silver in an amount over 60% is baked on the surfaces as an electrode material.

7. A ceramic dielectric according to claim 5 wherein at least either aluminum or copper is metallized as an electrode.

8. A process for producing a ceramic dielectric which comprises using a mixture composition obtained by mixing a powder mixture containing titanium dioxide as its main component which contains 98.0 to 99.0% by weight of TiO2, 0.001 to 0.005% by weight of Fe203, 0.2 to 0.5% by weight of Awl203,0.2 to 0.5% by weight of Nb2O5, 0.1 to 0.2% by weight of P2O5 with a powder mixture containing strontium carbonate as its main component which contains 96.0 to 99.0% by weight of SrCO3, 1.00 to 3.00% by weight of BaCO3, 0.01 to 0.5% by weight of CaCO3, 0.1 to

1.2% by weight of Na2CO3, 0.001 to 0.004% by weight of Fe2O3, and 0.005 to 0.02% by weight of Six2, so as to obtain a mixture having an atomic number ratio of Ti to Si of 0.99 to 1.02; further mixing the obtained mixture with a powder mixture containing bismuth trioxide as its main component which contains 99.0 to 99.9% by weight of Bi2O3, 0.005 to 0.02% by weight of SiO2, 0.001 to 0.003% by weight of Fe2O3, 0.002 to 0.01% by weight of PbO, 0.005 to 0.02% by weight of CuO and 0.001 to 0.01% by weight of No20, so as to obtain a mixture having an atomic number ratio of Ti to Sr of 0.02 to 0.06;; further mixing the obtained mixture with a mixture containing tantalum pentoxide as its main component which contains 99.0 to 99.9% by weight of Ta2O5, 0.0005 to 0.002% by weight of Fe2O3, and 0.01 to 0.04% by weight of SiO2, so as to obtain a mixture having an atomic number ratio of Ta to Sr of 0.002 to 0.006.

9. A process for producing a ceramic dielectric according to claim 8, which uses a powder obtained by shaping and calcinating said mixture composition to form a solid solution containing SrTiO2 as its main component and then pulverizing said solid solution.

1 0. A process for producing ceramic dielectric according to claim 9, wherein a pulverized powder is used, the particle size of said pulverized powder being in a range of 2.0 to 4.0 m in mean value thereof as measured by a sedimentation method using as a dispersant an aqueous 0.2% solution of sodium hexametaphosphate.

11. A process for producing a ceramic dielectric according to claim 9, wherein a semiconductive ceramic is used, the semiconductive ceramic being obtained by forming the pulverized powder in a desired form by the powder press-molding process, the extruding forming process, or the sheet forming process, then calcinating the formed powder whereby the crystal particles are distributed in a range between 5 to 100,us.

1 2. A process for producing a ceramic dielectric according to claim 11 wherein said semiconductive ceramic is coated thereon Cu2O in a range of 0.1 to 0.5 mg/cm2 and then is heattreated for 1 to 2 hours in air at a temperature in a range of 1,050 to 1 ,2000C.

13. A ceramic dielectric substantially as hereinbefore described with reference to the Example and the drawings.

14. A process for producing a ceramic dielectric substantially as hereinbefore described with reference to the Example and the drawings.