Zinc oxide varistor ceramics
Provided according to embodiments of the invention are varistor ceramic formulations that include zinc oxide (ZnO). In particular, varistor ceramic formulations of the invention may include dopants including an alkali metal compound, an alkaline earth compound, an oxide of boron, an oxide of aluminum, or a combination thereof. Varistor ceramic formulations may also include other metal oxides. Also provided according to embodiments of the invention are varistor ceramic materials formed by sintering a varistor ceramic formulation according to an embodiment of the invention. Further provided are varistors formed from such ceramic materials and methods of making such materials.
1. A method of preparing a ceramic material, the method comprising:
providing a formulation comprising:
Bi 2 O 3 at a concentration in a range of about 0.1 mol % to about 1.5 mol %;
Sb 2 O 3 at a concentration in a range of about 0.01 mol % to about 2.0 mol %;
Co 3 O 4 at a concentration in a range of about 0.01 mol % to about 1.0 mol %;
Mn 3 O 4 at a concentration in a range of about 0.01 mol % to about 1.0 mol %;
NiO at a concentration in a range of about 0.1 mol % to about 1.0 mol %;
Cr 2 O 3 at a concentration in a range of about 0.1 mol % to about 1.0 mol %;
B 2 O 3 or H 3 BO 3 at a concentration in a range of about 0.01 mol % to about 0.2 mol %;
Na 2 CO 3 at a concentration in a range of about 20 ppm to about 1000 ppm; and
a remainder of ZnO to 100 mol % of metal and metalloid compounds; and
heating the formulation at a temperature in a range of about 1100° C. to about 1200° C. in an atmosphere including oxygen to produce the ceramic material.
2. The method of claim 1 , further comprising, after heating the formulation, cooling the ceramic material to a temperature of about 850° C. at a first cooling rate of at least about 15° C./min.
3. The method of claim 1 , further comprising cooling the ceramic material at a second cooling rate of less than about 3° C./min, wherein the second cooling rate is applied after the first cooling rate.
4. The method of claim 1 , wherein the formulation is homogenized as a slurry.
5. The method of claim 1 , further comprising, prior to heating the formulation, pressing the formulation into a formed object, and wherein heating the formulation comprises heating the formed object.
6. The method of claim 1 , further comprising metallizing the ceramic material with at least one metal electrode.
7. The method of claim 1 , wherein the ZnO in the formulation has an average particle size of about 1 μm or less and one or more selected from Bi 2 O 3 , Sb 2 O 3 , Co 3 O 4 , Mn 3 O 4 , NiO, Cr 2 O 3 , B 2 O 3 , and H 3 BO 3 have an average particle size in a range of about 0.5 to about 10 μm.
8. The method of claim 7 , wherein the one or more selected from Bi 2 O 3 , Sb 2 O 3 , Co 3 O 4 , Mn 3 O 4 , NiO, Cr 2 O 3 , B 2 O 3 , and H 3 BO 3 is present in the formulation as a powder and has a particle size that is less than the particle size of the ZnO.
9. The method of claim 1 , wherein the formulation further comprises a binder and/or plasticizer in an amount of up to about 2% by weight of the formulation.