Varistor for high temperature applications
The present invention is directed to a varistor comprising a dielectric material comprising a sintered ceramic composed of zinc oxide grains and a grain boundary layer between the zinc oxide grains. The grain boundary layer contains a positive temperature coefficient thermistor material in an amount of less than 10 mol % based on the grain boundary layer.
1. A varistor comprising:
a dielectric material comprising a sintered ceramic composed of zinc oxide grains and a grain boundary layer between the zinc oxide grains, wherein the grain boundary layer contains a positive temperature coefficient thermistor material in an amount of less than 10 mol % based on the grain boundary layer.
2. The varistor according to claim 1 , wherein the grain boundary layer contains a positive temperature coefficient thermistor material in an amount of 5 mol % or less based on the grain boundary layer.
3. The varistor according to claim 1 , wherein the grain boundary layer contains a positive temperature coefficient thermistor material in an amount of from 0.1 mol % to 8 mol % based on the grain boundary layer.
4. The varistor according to claim 1 , wherein the grain boundary layer contains a positive temperature coefficient thermistor material in an amount of from 4 mol % to 6 mol % based on the grain boundary layer.
5. The varistor according to claim 1 , wherein the positive temperature coefficient thermistor material includes a titanate.
6. The varistor according to claim 5 , wherein the titanate includes a barium titanate.
7. The varistor according to claim 1 , wherein the positive temperature coefficient thermistor material includes an alkaline earth metal carbonate.
8. The varistor according to claim 7 , wherein the alkaline earth metal carbonate includes a calcium carbonate.
9. The varistor according to claim 1 , wherein the positive temperature coefficient thermistor material includes a rare earth metal oxide.
10. The varistor according to claim 9 , wherein the rare earth metal oxide includes a lanthanum oxide.
11. The varistor according to claim 1 , wherein the dielectric material includes a boron containing compound.
12. The varistor according to claim 11 , wherein the boron containing compound includes a boron containing acid.
13. The varistor according to claim 12 , wherein the boron containing acid includes boric acid.
14. The varistor according to claim 1 , wherein the varistor has a maximum operating temperature of from greater than 125° C. to 300° C.
15. The varistor according to claim 1 , wherein the varistor has a maximum operating temperature of from 150° C. to 250° C.
16. The varistor according to claim 1 , wherein the varistor has a maximum operating temperature of from 160° C. to 200° C.
17. The varistor according to claim 1 , wherein the varistor has a clamping voltage of from about 10 volts to about 200 volts.
18. The varistor according to claim 1 , wherein the varistor has a breakdown voltage of from about 10 volts to about 150 volts.
19. The varistor according to claim 1 , wherein the varistor has a leakage current of about 1 μA or less at an operating voltage of 18 volts.
20. The varistor according to claim 1 , wherein the varistor has a leakage current of from about 0.1 μA to about 0.6 μA at an operating voltage of 18 volts.
21. The varistor according to claim 1 , wherein the varistor has a capacitance of from about 0.1 pF to about 50,000 pF.
22. The varistor according to claim 1 , wherein the varistor has a capacitance of from about 250 pF to about 750 pF.
23. A method for forming the varistor of claim 1 , the method comprising
forming the dielectric material by calcining a zinc oxide, and
then mixing the calcined zinc oxide with the positive temperature coefficient thermistor material.
24. The method according to claim 23 , further comprising
mixing a bismuth oxide after the calcining step.