Monolithic semiconductor ceramic capacitor having varistor function and method for manufacturing monolithic semiconductor ceramic capacitor

• US 7,583,493 B2
• Filed: 02/01/2008
• Issued: 09/01/2009
• Est. Priority Date: 07/03/2006
• Status: Expired due to Fees

• Current Assignee: Murata Manufacturing Co Limited
• Original Assignee: Murata Manufacturing Co Limited
• Inventors: Kawamoto, Mitsutoshi, Tani, Shinsuke
• Primary Examiner(s): THOMAS, ERIC W
• Application Number:
  US12/024,730
• Publication Number:
  US 20080117561A1
• Time in Patent Office: 578 Days
• Field of Search: 3613211-3215, 361/311, 361/303, 29/254.2
• US Class Current: 361/321.4
• CPC Class Codes: B32B 18/00 Layered products essentially comprising ceramics, e.g. refractory products
• CPC Class Codes: B82Y 30/00 Nanotechnology for materials or surface science, e.g. nanocomposites
• CPC Class Codes: C04B 2235/3224 Rare earth oxide or oxide forming salts thereof, e.g. scandium oxide
• CPC Class Codes: C04B 2235/3225 Yttrium oxide or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3227 Lanthanum oxide or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3229 Cerium oxides or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3241 Chromium oxides, chromates, or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3251 Niobium oxides, niobates, tantalum oxides, tantalates, or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3258 Tungsten oxides, tungstates, or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/326 Tungstates, e.g. scheelite
• CPC Class Codes: C04B 2235/3262 Manganese oxides, manganates, rhenium oxides or oxide-forming salts thereof, e.g. MnO
• CPC Class Codes: C04B 2235/3275 Cobalt oxides, cobaltates or cobaltites or oxide forming salts thereof, e.g. bismuth cobaltate, zinc cobaltite
• CPC Class Codes: C04B 2235/3279 Nickel oxides, nickalates, or oxide-forming salts thereof
• CPC Class Codes: C04B 2235/3281 Copper oxides, cuprates or oxide-forming salts thereof, e.g. CuO or Cu2O
• CPC Class Codes: C04B 2235/3418 Silicon oxide, silicic acids, or oxide forming salts thereof, e.g. silica sol, fused silica, silica fume, cristobalite, quartz or flint glass constituents C04B2235/36
• CPC Class Codes: C04B 2235/36 Glass starting materials for making ceramics, e.g. silica glass
• CPC Class Codes: C04B 2235/441 Alkoxides, e.g. methoxide, tert-butoxide
• CPC Class Codes: C04B 2235/5409 expressed by specific surface values
• CPC Class Codes: C04B 2235/5454 nanometer sized, i.e. below 100 nm
• CPC Class Codes: C04B 2235/6025 Tape casting, e.g. with a doctor blade
• CPC Class Codes: C04B 2235/6582 : Hydrogen containing atmosphere
• CPC Class Codes: C04B 2235/663 : Oxidative annealing
• CPC Class Codes: C04B 2235/785 : Submicron sized grains, i.e. from 0,1 to 1 micron
• CPC Class Codes: C04B 2235/786 : Micrometer sized grains, i.e. from 1 to 100 micron
• CPC Class Codes: C04B 2235/79 : Non-stoichiometric products, e.g. perovskites (ABO3) with an A/B-ratio other than 1
• CPC Class Codes: C04B 2235/85 : Intergranular or grain boundary phases
• CPC Class Codes: C04B 35/47 : based on strontium titanates
• CPC Class Codes: H01C 7/115 : Titanium dioxide- or titanate type
• CPC Class Codes: H01C 7/18 : comprising a plurality of layers stacked between terminals
• CPC Class Codes: H01G 4/1227 : based on alkaline earth titanates
• CPC Class Codes: H01G 4/1281 : with grain boundary layer
• CPC Class Codes: H01G 4/30 : Stacked capacitors H01G4/33 takes precedence
• CPC Class Codes: Y10T 29/435 : Solid dielectric type

Abstract

A monolithic semiconductor ceramic capacitor includes semiconductor ceramic layers made of a semiconductor ceramic having a Sr site and a Ti site. The semiconductor ceramic satisfies the inequality 1.000<m≦1.020, wherein m represents the molar ratio of the Sr site to the Ti site. The semiconductor ceramic contains crystal grains and has grain boundary layers. The crystal grains contain a donor element such as La or Sm in the form of a solid solution. The grain boundary layers contain an acceptor element such as Mn, Co, Ni or Cr. The amount of the acceptor element therein is equal to or less than 0.5 mol (preferably 0.3 to 0.5 mol) per 100 mol of Ti. The crystal grains have an average size of 1.0 μm or less (preferably 0.5 to 0.8 μm). Therefore, the monolithic semiconductor ceramic capacitor has good electrical properties, good resistivity, good dielectric strength, and high reliability and is suitable for thin or compact apparatuses.

Claims

1. A monolithic semiconductor ceramic capacitor having a varistor function, comprising a monolithic sintered body and external electrodes located on end surfaces of the monolithic sintered body, the monolithic sintered body comprising a plurality of semiconductor ceramic layers and a plurality of internal electrodes each of which is arranged between adjacent semiconductor ceramic layers, the semiconductor ceramic layers made of a semiconductor ceramic which is of a boundary insulative layer-type comprising SrTiO₃, each of the external electrodes being electrically connected to different ones of the internal electrodes,

wherein the semiconductor ceramic has a Sr site, a Ti site, crystal grains and grain boundary layers and satisfies the inequality 1.000<

m≦

1.020 where m represents the molar ratio of the Sr site to the Ti site;

the crystal grains having an average size of 1.0 μ

m or less and comprising a solid solution containing a donor element;

the grain boundary layers containing an acceptor element in an amount greater than 0 mol and equal to or less than 0.5 mol per 100 mol of Ti.

2. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the amount of the acceptor element is 0.3 to 0.5 mol per 100 mol of Ti.

3. The monolithic semiconductor ceramic capacitor according to claim 2, wherein the acceptor element is at least one member selected from the group consisting of Mn, Co, Ni and Cr.

4. The monolithic semiconductor ceramic capacitor according to any claim 3, wherein the donor element is at least one member selected from the group consisting of La, Sm, Dy, Ho, Y, Nd, Ce, Nb, Ta and W.

5. The monolithic semiconductor ceramic capacitor according to claim 4, wherein the crystal grains have an average size of 0.5 to 0.8 μ

m.

6. The monolithic semiconductor ceramic capacitor according to claim 5, wherein the ceramic layers contain a low-melting point oxide in an amount equal to or less than 0.1 mol per 100 mol of Ti.

7. The monolithic semiconductor ceramic capacitor according to claim 6, wherein the low-melting point oxide is SiO₂.

8. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the acceptor element is at least one member selected from the group consisting of Mn, Co, Ni and Cr.

9. The monolithic semiconductor ceramic capacitor according to any claim 1, wherein the donor element is at least one member selected from the group consisting of La, Sm, Dy, Ho, Y, Nd, Ce, Nb, Ta and W.

10. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the crystal grains have an average size of 0.5 to 0.8 μ

m.

11. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the ceramic layers contain a low-melting point oxide in an amount equal to or less than 0.1 mol per 100 mol of Ti.

12. The monolithic semiconductor ceramic capacitor according to claim 11, wherein the low-melting point oxide is SiO₂.

13. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the acceptor element is Mn and the amount of the donor element is not more than 0.8 mol per 100 mols of Ti.

14. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the internal electrodes comprise a base metal.

15. The monolithic semiconductor ceramic capacitor according to claim 1, wherein the external electrodes comprise at least one of Ga, In, Ni and Cu.

16. A method for manufacturing monolithic semiconductor ceramic capacitor having a varistor function and containing a semiconductor ceramic which is of a boundary insulative layer-type and which contains SrTiO₃, the method comprising:

providing a heat treated mixture of an acceptor element and a pre-fired powder of ceramic raw materials for the ceramic layers including a donor compound in amounts such that the inequality 1.000<

m≦

1.020 is realized, where m represents the molar ratio of a Sr site to a Ti site in the ceramic and wherein the acceptor element amount is greater than 0 mol and equal to or less than 0.5 mol per 100 mol of Ti in the ceramic layers,

forming a laminate of alternately stacked green ceramic sheets comprising the heat-treated mixture and green internal electrode layers; and

firing the laminate in a reducing atmosphere and then re-oxidizing the laminate by firing the resulting laminate under oxidizing condition.

17. The method according to claim 16, wherein the crystal grains in the pre-fired powder have an average size of 1.0 μ

m or less.

18. The method according to claim 17, wherein the temperature at which the pre-fired powder was prepared is higher than the reduction firing temperature of the laminate.

19. The method according to claim 16, wherein the temperature at which the pre-fired powder was prepared is higher than the reduction firing temperature of the laminate.

20. The method according to claim 16, further comprising forming external electrodes electrically connected to different ones of the internal electrodes on different surfaces of the laminate.