Wet electrolytic capacitor for an implantable medical device
A wet electrolytic capacitor containing a cathode, fluidic working electrolyte, and planar anode formed from an anodically oxidized sintered porous pellet is provided. The pellet may be formed from a pressed valve metal powder, which in turn, is formed by reacting an oxide of a valve metal compound (e.g., tantalum pentoxide) with a reducing agent that contains a metal having an oxidation state of 2 or more (e.g., magnesium). Through the use of such a powder, the present inventors have discovered that higher capacitance levels can be achieved than previously thought possible for the high voltage capacitors employed in implantable medical devices.
1. A wet electrolytic capacitor comprising:
a planar anode that comprises an anodically oxidized pellet formed from a pressed and sintered tantalum powder, wherein the powder contains primary particles having an aspect ratio of about 4 or less, wherein the powder has a specific charge of from about 15,000 μF*V/g to about 45,000 μF*V/g;
a leadwire that extends from the planar anode;
a cathode that comprises a metal substrate coated with a conductive coating; and
a fluidic working electrolyte in communication with the anode and the cathode.
2. The capacitor of claim 1 , wherein the primary particles are agglomerated.
3. The capacitor of claim 2 , wherein sinter necks are formed between agglomerated particles that have a size of about 200 nanometers or more.
4. The capacitor of claim 1 , wherein the electrolyte has a pH of from about 5.0 to about 7.5.
5. The capacitor of claim 4 , wherein the first casing member contains a face wall and a surrounding sidewall that extends to an edge, and further wherein the second casing member is in the form of a lid that is sealed to the edge of the sidewall.
6. The capacitor of claim 1 , wherein the tantalum powder is formed by reacting an oxide of tantalum with a reducing agent that contains magnesium, strontium, barium, cesium, calcium, aluminum, or a combination thereof.
7. The capacitor of claim 1 , wherein the powder has a specific surface area of from about 4 to about 30 meters squared per gram.
8. The capacitor of claim 1 , wherein the powder has no more than about 50 ppm of alkali metals.
9. The capacitor of claim 1 , wherein the powder is nodular or angular.
10. The capacitor of claim 1 , wherein the primary particles have a median size of from about 5 to about 1000 nanometers.
11. The capacitor of claim 1 , wherein the powder has a specific surface area of about 1 square meter per gram or more.
12. The capacitor of claim 1 , wherein the anode has a thickness of about 5 millimeters or less.
13. The capacitor of claim 1 , wherein the anode has a D-shape.
14. The capacitor of claim 1 , wherein the metal substrate includes titanium or stainless steel.
15. The capacitor of claim 1 , wherein the conductive coating includes a substituted polythiophene.
16. The capacitor of claim 1 , wherein a separator is positioned between the anode and cathode.
17. The capacitor of claim 1 , wherein the capacitor contains a casing that contains a first casing member and a second casing member between which the anode and the fluid working electrolyte are disposed, wherein the metal substrate forms at least a portion of the first casing member, the second casing member, or both.
18. An implantable medical device comprising the capacitor of claim 1 .
19. A method for forming a wet electrolytic capacitor, the method comprising:
pressing a tantalum powder into the form of a pellet, wherein the powder is formed by reacting tantalum pentoxide with a reducing agent that contains magnesium, calcium, strontium, barium, cesium, aluminum, or a combination thereof, wherein the powder contains primary particles having an aspect ratio of about 4 or less, and wherein the powder has a specific charge of from about 15,000 μF*V/g to about 45,000 μF*V/g;
sintering the pellet;
anodically oxidizing the sintered pellet to form a dielectric layer that overlies the anode; and
positioning the anode and a fluidic working electrolyte within a casing.
20. The method of claim 19 , wherein the pellet is anodically oxidized at a voltage of from about 100 to about 300 volts.
21. The method of claim 19 , wherein the powder is nodular or angular.
22. The method of claim 19 , wherein the primary particles have a median size of from about 5 to about 1000 nanometers.
23. The method of claim 19 , wherein the powder has a specific surface area of about 1 square meter per gram or more.
24. The method of claim 19 , wherein the powder has no more than about 50 ppm of alkali metals.
25. The method of claim 19 , wherein the anode has a thickness of about 5 millimeters or less.