SE432097B - SINTERED BODY OF SEMI-CONDUCTIVE CERAMIC MATERIAL, PROCEDURE FOR MANUFACTURING THE BODY AND ANY USE OF THE BODY - Google Patents

Description

790 ^ 212lr2 '2 - characterized in that the doped strontium titanate contains 0.1-2% by weight of silica and 0.1-2% by weight of alumina both calculated on the amount of strontium-titanate. It has been established that the growth of certain crystal formations is strongly promoted during sintering through the presence of silica, which means that the sintered product contains larger crystal formations in connection with smaller crystal formations.

It has been shown that the presence of both silica and alumina during sintering in the above amounts, results in a regulated and homogeneous growth of the crystal formations, which results in a more even grain distribution. The presence of Al2 O3 stimulates grain growth as a result of the presence of silica, which is more uniform. By varying the relative amounts of silica and alumina, it is possible to achieve a desired, uniform grain size in the product as a whole. A large dielectric constant range (about 5000 to about 50,000) can be covered with the ceramic material according to the invention.

The insulating layers at the grain boundaries are achieved in the usual way by indifusion of metal oxides, e.g. bismuth oxide or bismuth oxide containing metal oxide mixtures. Insulating vitreous layers are formed at the grain boundaries, which layers are wholly or partly glass crystallized by the simultaneous presence of titanium oxide, silica and possibly alumina in addition to the above-mentioned metal oxides which are diffused into the grain boundaries. The reduced temperature sensitivity of the material according to the invention seems to be attributable to this fact. When using amounts of silica and alumina which are less than 0.1% by weight, the said effect on grain growth and the lower temperature sensitivity may prove to be insufficient for technical purposes. When using quantities greater than 2% by weight, no further effect on the mentioned parameters can be ascertained. When producing the semiconducting ceramic material according to the invention, it is preferable to start from such amounts of strontium compound (oxide, carbonate), titanium oxide and oxides of niobium or tantalum, that in gram atoms of the final product the ratio of Ti + dopant (Nb or Ta) and Sr is between 0.98 and 1.04. The mixture of starting materials, which consists of, for example, strontium carbonate, titanium dioxide, niobium bentoxide (Nb205), silica and alumina, is subjected to pre-sintering for 2-20 hours at a temperature between 1000 and 1200 ° C, and thereby, for example, in air. The material is then pulverized and mixed with an organic binder, which disappears completely during the sintering process, e.g. polyvinyl alcohol. The mixture thus obtained is formed, e.g. by pressing or spraying onto bodies and then the resulting bodies are sintered at a temperature between 1350 and 1500 ° C in a reducing atmosphere. Thereafter, the bodies are coated with a metal oxide or mixture of metal oxides which form an insulating layer upon indiffusion in the grain boundaries. Suitable metal oxides are, for example, CuO, MnO2, Bi2O3. However, a mixture of lead oxide, bismuth oxide and boron oxide is preferably used, which mixture contains, for example, 50% by weight of lead oxide, 45% by weight of bismuth oxide and 5% by weight of boron oxide.

The formation of the insulating layers takes place by heating metal oxide-coated bodies of semiconducting ceramic material to a temperature of 1000 - 1300 ° in an oxidizing atmosphere, during which process the semiconducting ceramic material absorbs the metal oxide.

Then metal layers are applied, e.g. copper or silver, which act as an electrode, on two opposite surfaces.

The invention will be described in more detail below in connection with some embodiments.

Embodiment 1.

Manufacture of a sintered body of semiconductor ceramics with an effect equal to 6000. The ceramic material is prepared as follows: 183.5 g SrTiO3 2.0 g Nb205 0.8 g SiO2 $: 0.4% by weight} calculated in relation to the amount of 1.1 g Al2O3 :: 0 .6% by weight formed strontium titanate niobate. The substances were mixed thoroughly and then pre-sintered for 15 hours at 1150 ° C in air. The product obtained was pulverized and pressed, after mixing with polyvinyl alcohol, into plates with a diameter of 6 mm and a thickness of 0.6 mm. The plates were sintered at 1450 ° C for 4 hours in a reducing atmosphere consisting of a mixture of 80% by volume of nitrogen and 20% by volume of hydrogen, which was passed through water at a temperature of 25 ° C. The ratio (Ti + Nb) / Sr in gram atoms was 1,015. Semiconductor plates with a diameter of 5.0 nm and a thickness of 0.5 mm were manufactured. The plates were coated on one side with a 10 mg of a metal oxide mixture (composition: 50% by weight Pb0 + 45% by weight Bi2 O3 + 5% by weight B20) and heated for half an hour in air to a temperature of 100 ° C after which time the metal oxide mixture was diffused into the plates. Thereafter, copper electrodes were applied to both sides of the plates by evaporation deposition in vacuo. The plates obtained showed the following well-producible properties: c / czo = _ »¿1% mean. ~ 2s and + ss ° c 5 e ”= sooo (1 kHz) Tg § = 0.4% (1 kHz) ß-sov = 101m. m. 790212li-2 q 4 Most characteristic is that the capacitance has an extremely low temperature dependence, which means that the material is particularly suitable for use in capacitors that require low temperature sensitivity.

Embodiment II.

Manufacture from a sintered body of a semiconducting ceramic material (¿feff equal to 5000-15000). The ceramic material is prepared as follows: From a mixture containing 147.6 g SrCO 3 79.5 g TiO 2 2.0 g Nb 2 O 5 and varying amounts of SiO 2 and Al 2 O 3, plates were made in the same manner as described in working procedure I except that the sintering, except from the previously described process, was carried out for 4 hours at the temperature of 1100 ° C in air. Expressed in gram atoms, the ratio (Ti + Nb) / Sr was equal to 1.01.

The plates were fitted with silver electrodes. The table below shows the electrical properties of the barrier capacitors manufactured (the amounts of SiO2 and Al2O3 are calculated on the amount of strontium titanate niobate formed during sintering).

Chart. 5l ° 2 i ^ 12 ° 3 ”C20 feff '195 fsov viktz vfktz -25 tm + s5 ° c vid 1 | zz 0.5 1.5 i 1 5000 0.4 44011 1.0 1.0 3 0.5 5000 0.5 4.10 ”0.5 1.0 3 2 9000 0.6 44011 0.5 0.5 + 1 15000 0.7 64010 0.2 0 + 2 10000 0.5 1.10 s 79Ü212í + ~ 2 The low temperature dependence of the capacitance is also clear from the materials reported in the table.

Embodiment Example III.

Manufacture of a sintered body of semiconductor ceramic material (efëff = 8000). The ceramic material was prepared as follows: 183.5 g SrTiO 3 0-3 9 5lÛ2? = 0.4 V1Kt% :) calculated on the amount of strontium formed 1.1 g Al 2 O 3 Q: 0.6 wt% titanium tantalate. 3.3 g Ta2O5 'These substances were mixed thoroughly and then pre-sintered for 15 hours at 1150 ° C in air. The resulting product was then ground. The resulting powder was mixed with polyvinyl alcohol and compression molded into plates 6 mm in diameter and 0.6 mm thick. The plates were sintered for 4 hours at the temperature of 1450 ° C in a reducing atmosphere according to working procedure I. In gram atoms, the ratio (Ti + Ta) / Sr was equal to 1.015. Semiconductor plates with a diameter of 5 mm and a thickness of 0.5 mn were manufactured. The plates were coated on one side with 10 mg of a metal oxide compound of the same kind as in Embodiment I and heated for half an hour in air to a temperature of 1100 ° C. Then, electrodes were applied to both sides of the plate as described in Example I. The following electrical properties could be measured: áeff = sooo (1 kHz) A c / czo = <¿1% between -2s ° c and + ss ° c tg <§ = 0 , 41, (1 kHz) 350V = s.1o1 ° .n cm.

The sintered bodies can be given any desired phono, e.g. disc mold, cylinder shape or tubular shape.

Claims (6)

1. 79Û212ë-2 i 6 Claims 1. Sintered body of semiconducting ceramic material built on the basis of niobium- or tantalum-doped strontium titanate, the body having an electrically insulating layer at the grain boundaries, characterized in that the doped strontium titanate contains 0.1 -2% by weight of silica and 0.1-2% by weight of alumina, both calculated on the amount of doped strontium titanate. Sintered body of semiconducting ceramic material according to Claim 1, characterized in that in the doped strontium titanate, in gram atoms, the ratio (titanium + doping metal Nb or Ta) / strontium is between 0.98 and 1.04. Process for the manufacture of a sintered body according to claim 1, characterized in that a mixture of strontium oxide or a strontium oxide-forming compound, for example strontium carbonate, titanium dioxide, niobium or tantalum dioxide, silica and alumina or a mixture of strontium titanate, nobium pentoxide or tantalum pentoxide and silica and alumina are sintered in the desired form in a reducing atmosphere at a temperature between 1350 ° C and 1500 ° C and then a metal oxide or mixture of metal oxides forming an insulating layer at the grain boundaries is diffused at a temperature between 1000 and 1300 ° C. lay. 4. A process according to claim 3, characterized in that the starting mixture contains 0.1-2% by weight of silica based on the amount of doped strontium titanate formed, 5. A process according to claim 3, characterized in that the starting mixture contains 0.1-2% by weight of alumina, calculated on the amount of doped strontium titanate formed, Use of a sintered body according to claim 1 in a barrier layer capacitor, wherein the body is provided with electrodes on opposite side surfaces. 7902124-2 Abstract. Sintered body of conductive strontium titanate, doped with niobium or tantric oxide and containing between 0.1 and 2% by weight of SiO2 and, where appropriate, 0.1 to 2% by weight of Al2 O3. After indiffusion of a metal oxide mixture which forms an insulating layer at the grain boundaries, and application of electrodes, a barrier layer capacitor is obtained.