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US-12617043-B2 - Bottom ohmic silver paste for strontium titanate ring varistor, preparation method and use thereof

US12617043B2US 12617043 B2US12617043 B2US 12617043B2US-12617043-B2

Abstract

The present invention provides a bottom ohmic silver paste for strontium titanate ring varistor including silver powder, doped SnO 2 micro powder, glass powder, organic solvent and organic binder, and the mass ratio of the silver powder, the doped SnO 2 micro powder, the glass powder, the organic solvent and the organic binder is 65-85:0.9-4.3:0.5-5:10-20:10-15. The present invention also provides a preparation method and use of the bottom ohmic silver paste for strontium titanate ring varistor of the present invention.

Inventors

  • Zhinan Chai
  • Shuiming Zhou
  • Keqiang WANG
  • Xuezhao Wang

Assignees

  • Guangzhou Newlife New Material Co., Ltd.

Dates

Publication Date
20260505
Application Date
20230407
Priority Date
20220407

Claims (16)

  1. 1 . A bottom ohmic silver paste for strontium titanate ring varistor, wherein the bottom ohmic silver paste comprises silver powder, doped tin dioxide SnO 2 micro powder, glass powder, organic solvent and organic binder, and a mass ratio of the silver powder, the doped tin dioxide SnO 2 micro powder, the glass powder, the organic solvent and the organic binder is 65-85:0.9-4.76:0.5-5:10-20:5-15.
  2. 2 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the doped tin dioxide SnO 2 micro powder is tin dioxide SnO 2 micro powder doped with one or two of antimony, fluorine, phosphorus and tungsten ions.
  3. 3 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the doped tin dioxide SnO 2 micro powder is doped with Sb 2 O 3 .
  4. 4 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein a mass ratio of tin dioxide SnO 2 to dopant in the doped tin dioxide SnO 2 micro powder is 89-99:1-11, in which sum of mass ratio of the tin dioxide SnO 2 micro powder and the dopant is 100.
  5. 5 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the doped tin dioxide SnO 2 micro powder is spherical and has a median granularity D50 of 0.5-3 μm.
  6. 6 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the doped tin dioxide SnO 2 micro powder has a specific surface area of 2-50 m2/g and a resistivity of 0.1-1 Ω·cm at room temperature.
  7. 7 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein a method for preparing the glass powder comprises the steps of: mixing H 3 BO 3 , SiO 2 , BaCo 3 , ZnO, Al 2 O 3 , Na 2 CO 3 , K 2 CO 3 at a mass ratio of 20-60:15-25:10--20:15-20:1-10:5-20:1-10 via ball milling and obtaining a mixture; melting the mixture at 1250° C. for 2-3 h and obtaining glass slag via water-quenched after melting; placing the glass slag into a ball milling tank and carrying out ball milling for 4-24 h; and obtaining the glass powder with softening point of 635-700° C. and median granularity of D50≤3 μm after sieving and drying.
  8. 8 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the silver powder comprises a spherical silver powder A having a particle size of 1.0-2.5 μm, a spherical silver powder B having a particle size of 0.8-1.0 μm and a spherical silver powder C having a particle size of 0.2-0.84 μm, and a mass ratio of spherical silver powder A:spherical silver powder B:spherical silver powder C is 30-60:20-30:5-104.
  9. 9 . A bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein a method for preparing the organic binder comprises the steps of: accurately weighing at least one of 80-90 mass parts terpineol and butyl carbitol and adding the terpineol and/or butyl carbitol into a glue making machine, and heating the terpineol and/or butyl carbitol to 80° C.; adding 10-20 mass parts of ethyl cellulose, and controlling the temperature at 90-95° C.; fully stirring the mixture to form a transparent gelatinous mixture; and cooling the mixture naturally to room temperature in a plastic or stainless steel drum, to obtain the organic binder having a viscosity of 400-600 dPa·S.
  10. 10 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the organic solvent is one or two of terpineol, butyl carbitol, dibutyl phthalate.
  11. 11 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein a method for preparing the bottom ohmic silver paste comprises the steps of: mixing the silver powder, the doped tin dioxide SnO 2 micro powder, the glass powder, the organic solvent and the organic binder at a mass ratio of 65-85:0.9-4.76:0.5-5:10-20:5-15 to obtain a mixture; ball milling the mixture and rolling the mixture to obtain the bottom ohmic silver paste.
  12. 12 . The bottom ohmic silver paste for strontium titanate ring varistor of claim 1 , wherein the bottom ohmic silver paste is printed on a surface of a high resistance layer of a strontium titanate substrate having nonlinear volt-ampere characteristics, and the bottom ohmic silver paste is sintered in air at a sintering temperature of 800-860° C.
  13. 13 . A method for improving easy welding and enduring welding performances of strontium titanate ring varistor comprising the steps of: screen printing a bottom ohmic silver paste on a substrate of strontium titanate ring varistor having volt-ampere characteristics and drying, screen printing surface layer of silver paste in the same position again and drying; sintering the substrate with bottom ohmic silver paste and surface layer of silver paste in the air to form silver electrode, wherein the bottom ohmic silver paste is the bottom ohmic silver paste for strontium titanate ring varistor of claim 1 .
  14. 14 . The method for improving easy welding and enduring welding performances of strontium titanate ring varistor of claim 13 , wherein a method for preparing the strontium titanate ring varistor substrate comprises the steps of: mixing SrCO 3 , BaCO 3 , CaCO 3 and TiO 2 proportionally proportioned to obtain a mixture; ball-milling and drying the mixture, to solid-state synthesize strontium barium calcium titanate powder; doping one or more of Nb 2 O 5 , La 2 O 3 , Ta 2 O 5 and SiO 2 , MnCO 3 to prepare ceramic body of strontium barium calcium titanate ring varistor by ball milling, granulation and molding; and obtaining strontium titanate substrate having nonlinear volt-ampere characteristics by discharging glue, reduction sintering and oxidation sintering.
  15. 15 . The method for improving easy welding and enduring welding performances of strontium titanate ring varistor of claim 13 , wherein the surface layer silver paste is a silver conductive paste having a silver content of 70-80 percentages.
  16. 16 . A ring varistor with easy welding and enduring welding performances prepared by the method for improving easy welding and enduring welding performances of strontium titanate ring of claim 13 .

Description

CROSS-REFERENCE TO RELATED APPLICATION The present application claims priority to Chinese patent application number CN 202210353067.8 filed on Apr. 7, 2022, the entire contents of which are hereby incorporated by reference. FIELD OF THE INVENTION The present invention generally relates to the field of semiconductor functional ceramics and, more particularly, relates to bottom ohmic silver paste for strontium titanate ring varistor, preparation method and use thereof. BACKGROUND OF THE INVENTION Strontium titanate varistor is made from strontium titanate. After molding and discharging glue, due to the combined action of donor doping and reduction atmosphere sintering, the grain and the semifinished product are semi-conductive. Due to the combined action of donee doping and high temperature oxidation, the high resistance insulation of the surface layer of the grain boundary and the substrate is realized, to obtain a kind of surface layer semiconductor substrate. After screen printing and electrode sintering, a semiconductor ceramic sensor with both nonlinear volt-ampere characteristics and capacitive characteristics is finally fabricated. After high temperature oxidation treatment, the surface of strontium titanate ceramics has an oxygen adsorption layer, which is a high resistance insulating layer. Silver is a stable noble metal. The electrode formed by sintering and infiltration of the pure silver paste and the high resistance layer on the surface of the strontium titanate semiconductor ceramic substrate cannot form ohmic contact. To form ohmic contact, the preparation of the silver electrode for the strontium titanate ring varistor includes the printing of the bottom layer of ohmic paste and the printing of the surface layer of the silver paste. The ohm paste is the silver paste containing metals having strong reducibility, for example, metal powders such as zinc, aluminum and nickel, in which the content of the reductive metal powder is about 15-40%. The surface layer of the silver paste is a pure silver paste having 70%-80% silver. Although the addition of strong reducibility metal powder in the silver paste can form desirable ohmic contact between the electrode and the substrate, the micron-sized metals zinc, aluminum and nickel are very easy to be oxidized in high temperature air. Therefore, the sintering temperature of the strontium titanate ring varistor electrode can only be controlled in the range of 550° C.-620° C. Higher sintering temperature may cause the reductive metal in ohmic silver paste be oxidized and the ohmic characteristics will be lost. In addition, the nonlinear volt-ampere characteristics of the strontium titanate ring varistor cannot play the role while paralleled with the DC micro motor. Therefore, the surface layer silver electrode cannot form a dense silver layer because the sintering temperature cannot be raised, and the electrode welding performance of the strontium titanate ring varistor is not ideal. In the assembly of micro-motor welding, the problem of burning the electrode or unable to coating tin onto the electrode often occurs. With the rapid development of modern, efficient and rigorous welding technology, such as high frequency welding and laser welding, the requirement of high temperature resistance for the weldability of varistor electrode is more and more strict. Many manufacturers of strontium titanate ring varistor increases the thickness of silver layer by printing multi-layer silver paste to solve the problem of poor welding of silver electrode of varistor, which will increases the production and material cost of varistor. CN102855960B discloses a method for preparing ohmic silver paste for use in SrTiO3 varistor, including the steps of: mixing silver powder, aluminum powder or zinc powder, glass powder, organic binder and grinding; adding diluent and stirring evenly to obtain the ohmic silver paste for SrTiO3 varistor. The method of CN102855960B can overcome the problems of poor ohmic contact and poor sintering performance of the domestic paste at present. The comprehensive performances of the ohmic silver paste according to CN102855960B are almost the same as those of foreign products. However, the method of CN102855960B cannot meet the requirement of high temperature welding. CN1629986B discloses a strontium titanate ring varistor having copper electrode, which can overcome the volatilization phenomenon of silver and has better welding performance. However, the strontium titanate ring varistor having copper electrode of CN1629986B cannot withstand the laser welding. One object of the present invention is to improve the high temperature resistance welding of the strontium titanate ring varistor electrode under the condition that the good ohmic contact and the resistivity of the silver electrode itself must be as small as possible, to solve the problem of burning silver electrode and unable to coating tin on the electrode in welding of SrTiO3 varistor. The add