Solid electrolytic capacitor for use in a humid atmosphere

1. A solid electrolytic capacitor comprising:

a sintered porous anode body;

a dielectric that overlies the anode body;

a solid electrolyte that overlies the dielectric, wherein the solid electrolyte includes a plurality of conductive polymer particles; and

a cathode coating that contains a metal particle layer that overlies the solid electrolyte, wherein the metal particle layer includes a plurality of conductive metal particles dispersed within a resinous polymer matrix, where the particles constitute from about 50 wt. % to about 99 wt. % of the layer and the resinous polymer matrix constitutes from about 1 wt. % to about 50 wt. % of the layer, wherein the resinous polymer matrix includes a vinyl acetal polymer.

2. The solid electrolytic capacitor of claim 1 , wherein the metal particles include silver.

3. The solid electrolytic capacitor of claim 1 , wherein the vinyl acetal polymer is polyvinyl butyral.

4. The solid electrolytic capacitor of claim 1 , wherein the vinyl acetal polymer has a residual hydroxyl content of about 35 mol. % or less.

5. The solid electrolytic capacitor of claim 1 , wherein the anode body includes tantalum.

6. The solid electrolytic capacitor of claim 1 , wherein the conductive polymer particles contain an extrinsically conductive polymer having repeating units of the following formula (III):

wherein,

R 7 is a linear or branched, C 1 to C 18 alkyl radical, C 5 to C 12 cycloalkyl radical, C 6 to C 14 aryl radical, C 7 to C 18 aralkyl radical, or a combination thereof; and

q is an integer from 0 to 8.

7. The solid electrolytic capacitor of claim 6 , wherein the extrinsically conductive polymer is poly(3,4-ethylenedioxythiophene).

8. The solid electrolytic capacitor of claim 6 , wherein the particles also contain a polymeric counterion.

9. The solid electrolytic capacitor of claim 1 , wherein the conductive polymer particles contain an intrinsically conductive polymer having repeating units of the following formula (IV):

wherein,

R is (CH 2 ) a —O—(CH 2 ) b ;

a is from 0 to 10;

b is from 1 to 18;

Z is an anion;

X is a cation.

10. The solid electrolytic capacitor of claim 1 , further comprising an external polymer coating that overlies the solid electrolyte and contains pre-polymerized conductive polymer particles and a cross-linking agent.

11. The solid electrolytic capacitor of claim 1 , further comprising:

an anode termination that is in electrical connection with the anode body;

a cathode termination that is in electrical connection with the solid electrolyte; and

a casing that encloses the anode, dielectric, solid electrolyte, and silver layer and leaves exposed at least a portion of the anode termination and the cathode termination.

12. The solid electrolytic capacitor of claim 1 , wherein the capacitor is in contact with an atmosphere having a relative humidity of about 40% or more.

13. The solid electrolytic capacitor of claim 1 , wherein the cathode coating exhibits a moisture content of about 10 wt. % or less when exposed to an atmosphere having an 85% relative humidity in accordance with ASTM D6869-03 (2011).

14. A solid electrolytic capacitor comprising:

a sintered porous anode body that includes tantalum;

a dielectric that overlies the anode body, wherein the dielectric includes an oxide of tantalum;

a solid electrolyte that overlies the dielectric, wherein the solid electrolyte includes a plurality of conductive polymer particles, wherein the conductive polymer particles include a thiophene polymer; and

a cathode coating that contains a metal particle layer that overlies the solid electrolyte, wherein the metal particle layer includes a plurality of silver particles dispersed within a resinous polymer matrix, where the particles constitute from about 50 wt. % to about 99 wt. % of the layer and the resinous polymer matrix constitutes from about 1 wt. % to about 50 wt. % of the layer, wherein the cathode coating exhibits a moisture content of about 10 wt. % or less when exposed to an atmosphere having an 85% relative humidity in accordance with ASTM D6869-03 (2011).

15. The solid electrolytic capacitor of claim 14 , wherein the resinous polymer matrix includes a polyvinyl butyral having a residual hydroxyl content of about 35 mol. % or less.

16. The solid electrolytic capacitor of claim 14 , wherein the thiophene polymer is poly(3,4-ethylenedioxythiophene).

17. The solid electrolytic capacitor of claim 14 , wherein the particles also contain a polymeric counterion.

18. The solid electrolytic capacitor of claim 14 , further comprising an external polymer coating that overlies the solid electrolyte and contains pre-polymerized conductive polymer particles and a cross-linking agent.

19. The solid electrolytic capacitor of claim 14 , wherein the capacitor is in contact with an atmosphere having a relative humidity of about 40% or more.

20. A method for forming a capacitor, the method comprising:

anodically oxidizing a sintered porous anode body to form a dielectric that includes an oxide of a valve metal compound;

applying a dispersion that contains conductive polymer particles to form a solid electrolyte layer; and

applying a metal paste over the solid electrolyte layer, wherein the paste contains a plurality of conductive metal particles, a vinyl acetal polymer, and an organic solvent; and

drying the metal paste.

21. The method of claim 20 , wherein the metal particles include silver.

22. The method of claim 20 , wherein the paste has a viscosity of from about 200 to about 1,000 centipoise at a temperature of 25° C.

23. The method of claim 20 , wherein the metal particles constitute from about 25 wt. % to about 40 wt. % of the paste.