FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A method for the manufacture of a varistor ceramic having a high breakdown voltage and varistor ceramic thereof.
APPLICANTS
Company Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Singal Vivek and Roy Pradip Kumar of Crompton Greaves Ltd, AMPTC, CG Global R&D Centre, Kanjur (E), Mumbai 400042, Maharashtra, India; all Indian Nationals.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

ECHNICAL FIELD OF THE INVENTIONAL:
The present invention relates to a method for the manufacture of a varistor ceramic having a high breakdown voltage and varistor ceramic thereof.
BACKGROUND AND PRIOR ART:
A varistor is an electronic component with a significant nonlinear current-voltage characteristic. It is often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor (VDR). A varistor's function is to conduct significantly increased current when voltage is excessive.
The most common type of varistor is the Metal Oxide Varistor (MOV). Metal oxide varistors, consisting of a ceramic material, sintered at a high temperature, containing primarily one electrically conductive metallic oxide such as ZnO, Ti02, Sn02, or Sr02 with small amounts of other selected metal oxides or fluorides, are well known to the art. For example, US 3,953,373, describes various compositions of metal oxide varistors in which the major conductive component is zinc oxide. US 3,953,375 discloses various compositions of metal oxide varistors in which the major conductive component is titanium oxide, and US 3,899,451 describes various compositions of metallic oxide varistors in which the main conductive components are select mixtures of ZnO with Ti02, Sn02, or Zr02. Additives which may be used in these metal oxide varistors include the oxides or fluorides of bismuth, cobalt, manganese, barium, boron, berylium, magnesium, calcium, strontium, titanium, antimony, germanium, chromium, nickel, lithium, indium, cerium, aluminum, tin, molydenum, vandium, tantalum and iron.
The manufacturing process for all such metal oxide varistors is similar. Accurately weighed quantities of metal oxides and additives, having predetermined composition ratio, are powdered and mixed together, generally by a ball mill. The mixture may be preliminarily calcined at a relatively low temperature in the range of 400° C to 800° C and again pulverized in a ball mill. The powder thus obtained is mixed with a suitable binder such as polyvinyl alcohol, etc and mixed with water to

form slurry. The slurry is sprayed dried to form granules and the granules are compacted under a pressure of about 50 to 1000 kg/cm2, into a disc or block having very smooth, planar, parallel top and bottom surfaces. These blocks are then sintered at a high temperature, in the range of 1000° C to 1450° C, for about 1 to 20 hours, then furnace-cooled to room temperature. The sides of the blocks are coated with a high resistive coating composition comprising glass. The sintered blocks are provided at their respective top and bottom surfaces with ohmic electrodes applied by a suitable method such as silver painting, vacuum evaporation, or flame spraying of metals such as Al, Zn, Sn, etc. The top and bottom surfaces of the block may be lapped before the electrodes are applied thereon to assure a uniform thickness of the block. The sintering temperature used is the range of 1000°C to 1450°C which increases the energy consumption and thus increases the cost of the production of varistor.
This varistor contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.
In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress. Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike, as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.

It is recognized that the electric properties of a varistor originate from its grain boundaries. A ZnO varistor generally contains ZnO grains around which a highly resistant boundary layer is located and bound thereto. Thus breakdown voltage depends on the grain size of the ZnO. Smaller grain size increases the porosity and hence increases the breakdown voltage of the varistor. One of the ways to achieve the smaller grain size is employing the additives such as aluminum oxide as growth inihibitor in the varistor composition. Thus, breakdown voltage depends on the efficiency of the growth inhibitor.
There is therefore a need for cost-effective varistor ceramic having a very high breakdown voltage.
Objects of the invention
An object of the invention is to provide a method for the manufacture of a varistor ceramic having a high breakdown voltage.
Another object of the invention is to provide a method for the manufacture of a varistor ceramic having a high breakdown voltage where sintering is carried out at lower temperature than the conventional temperature thereby reducing the energy consumption and making the process cost-effective.
Another object of the invention is to provide a method for the manufacture of a varistor ceramic having a high breakdown voltage where the grain size of varistore ceramic is reduced to 4 u.m which increases the high breakdown voltage.
Yet another object of the invention is to provide a varistor ceramic having a high breakdown voltage.
Yet another object of the invention is to provide a varistor ceramic having a high breakdown voltage where sintering is carried out at lower temperature ie 800 to 900°C than the conventional temperature of 1100 o 1200°C thereby reducing the energy consumption and making the varistor cost-effective.

Yet another object of the invention is to provide a varistor ceramic having a high breakdown voltage where the grain size of varistor ceramics is reduced to 4μm.
Detailed description:
According to the invention, there is provided a method for the manufacture of a varistor ceramic having a high breakdown voltage, comprising: (a) mixing nano powder of zinc oxide (ZnO) with bismuth oxide (Bi203), antimony oxide (Sb203), manganese oxide (Mn02), silica (Si02), cobalt oxide (CoO), cromium oxide (Cr203) and binder like polyvinyl alcohol, said nano powder of zinc oxide is at least 90 mol % of the resulting mixture;, (b) subjecting said mixture to milling and calcinations to obtain precursor; and (c) subjecting the precursor to a consolidation treatment wherein said consolidation treatment includes a sintering which is carried out at 800 to 900° C to keep a grain size of varistor ceramic at least 4 urn.
The nano zinc oxide powder has an average crystallite size 3.42 nm, average particle size 40-50nm, average powder density 3.01 gm/cc, average surface area 60m2 /gm and average spherical shape.
In the above-mentioned method, before carrying out step (c) the powders or their mixture are calcinated at a temperature in the range of 400 to 800°C.
The consolidation treatment of step (c) given to precursor comprises pressing of the precursor to get the block; followed by burning out the binder from the block; subjecting the block to calcinations followed by sintering at 800 to 900°C; subjecting the block to lapping treatment followed by heat treatment.
The consolidation treatment of step (c) includes or is followed by a heating. The heat treatment is selected from the group consisting of convection heating, induction heating, microwave heating, laser heating and electric discharge heating. The heating is carried out for one or several short periods of time.

The sintering of step (c) is carried out at a temperature lower than 800°C for a period of time equal to or less than 4 hours. Preferably, the sintering is carried out at a temperature of about 800 °C. The sintering is carried out for a period of time equal to or less than 4 hours. The sintering is carried out with a heating rate comprised between 2 and 3°C./min.
The mixture prepared during step (a) comprises: from 90 to 98 % zinc oxide (ZnO), from 0 to 3 % bismuth oxide (Bi203), from 0 to 3 % Antimony oxide (Sb203), from 0 to 2 % manganese oxide (Mn02), from 0 to 1 % silica (Si02), from 0 to 2 % cobalt oxide (CoO), from 0 to 2 % Cromium oxide (Cr2O3).
The said nano powder of Zinc oxide (ZnO) is used to keep the grain size of varistor ceramic upto 4 μm. The grain size of Varistor ceramic is in the range of 4 to 8 μm.
According to the invention there is provided Varistor ceramic having grain size upto 4 μm manufactured according to the method of the invention and having high breakdown voltage in the range of 230 to 250 V/nm.
In the present invention, we have reduced the grain size of varistor ceramic to 4 to 8 μm by using nano powder of ZnO, which results in-ncrease in the break down voltage of the varistor ceramic. The increased quantity of nano powder of ZnO in the composition, reduces the grain size of ZnO and increases the breakdown voltage.
Thus, the sintering in the present invention is carried out at lower temperature i.e. 800 to 900°C than the conventional sintering temperature i.e. 1100 to 1200°C thus reducing the energy consumption which makes the process and product cost-effective. The use of nano powder ZnO in the present invention reduces the grain size of varistor ceramic to 4 μm. This leads to increase in the high breakdown voltage in the range of 230 to 250 V/nm, which makes the product highly rated varistor block.
The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1:
96.5 gm of nano powder of ZnO (Nano zinc oxide powder having average crystallite size 3.42nm, average particle size 40-50nm, average powder density 3.01 gm/cc, average surface area 60m2 /gm and average spherical shape) was mixed with 0.5 gm of Bi2O3, 1 gm Sb2O3 0.5 gm CoO, 0.5 gm of Mn02, 0.5 gm of Cr203 and 0.5 gm of Si02and milled together. The polyvinyl alcohol (3 wt %) was added to the mixture to form a slurry. The slurry was sprayed dried to form granules and the granules were compacted into blocks. The blocks were heated to burn out the binder and calcined. The calcined blocks were sintered 800° C for 4 hours and the sides of the blocks were polished and coated with a high resistive coating composition comprising glass. The top and bottom surfaces of the blocks were metallised to form the electrodes. It was tested for the grain size of ZnO and Break down voltage. It was found that the grain Size of ZnO is 6.4 urn and Break down Voltage is 249 V/mm.
Example 2:
93 gm of nano powder of ZnO (Nano zinc oxide powder having average crystallite size 3.42nm, average particle size 40-50nm, average powder density 3.01 gm/cc, average surface area 60m /gm and average spherical shape) was mixed with 1 gm of Bi203, 2 gm Sb203, 1 gm CoO, 1 gm of Mn02, 1 gm of Cr203 and 1 gm of Si02 and milled together. The polyvinyl alcohol (3 wt %) was added to the mixture to form a slurry. The slurry was sprayed dried to form granules and the granules were compacted into blocks. The blocks were heated to burn out the binder and calcined. The calcined blocks were sintered 800° C for 4 hours and the sides of the blocks were polished and coated with a high resistive coating composition comprising glass. The top and bottom surfaces of the blocks were metallised to form the electrodes. It was tested for the grain size of ZnO and Break down voltage. It was found that the grain Size of ZnO is 7.6μm and Break down Voltage is 237 V/mm. Form the above two examples it was observed that the grain size of varistor ceramic is reduced by using nano powder of ZnO, which results in an increase in the break down voltage of the varistor ceramic.

We claim :
1. A method for the manufacture of a varistor ceramic having a high breakdown voltage, comprising: (a) mixing nano powder of zinc oxide (ZnO) with bismuth oxide (Bi203), Antimony oxide (Sb203), manganese oxide (MnO2), silica (SiO2), cobalt oxide (CoO), Cromium oxide (Cr2O3) and binder like polyvinyl alcohol, said nano powder of zinc oxide is at least 90 mol % of the resulting mixture; (b) subjecting said mixture to milling and calcinations to obtain precursor; and (c) subjecting the precursor to a consolidation treatment wherein said consolidation treatment includes a sintering which is carried out at 800 to 900° C to keep a grain size of varistor ceramic at least 4um.
2. The method as claimed in claim 1, wherein the nano zinc oxide powder having average crystallite size 3.42 ran, average particle size 40-50nm, average powder density 3,01 gm/cc, average surface area 60m2 /gm and average spherical shape.
3. The method according to claim 1, wherein the consolidated treatment step (c) given to precursor comprises pressing of the precursor to get the block followed by burning out the binder from the block; subjecting the block to calcinations followed by sintering at a temperature of about_800 to 900°C; subjecting the block to lapping treatment followed by heat treatment.
4. The method according to claim 1, characterized in that the mixture prepared during step (a)
comprises: from 90 to 98 % zinc oxide (ZnO), from 0 to 3 % bismuth oxide (Bi2O3)} from 0 to 3 % Antimony oxide (Sb2O3), from 0 to 2 % manganese oxide (Mn02), from 0 to 1 % silica (Si02), from 0 to 2 % cobalt oxide (CoO), from 0 to 2 % Cromium oxide (Cr203).
5. The method according to claim 1, wherein said nano powder of Zinc oxide (ZnO) is used to keep
the grain size of varistor ceramic upto 4 μrn.

6. A varistor ceramic having Zinc oxide grain size of upto 4 urn manufactured according to the method as claimed in any of the preceding claims and having high breakdown voltage in the range of 230 to 250 V/nm.