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, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION High energy varistor
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Roy Pradip Kumar and Saravanan Seman; both of Crompton Greaves Ltd, AMPTC, CG Global R&D Centre, Kanjur (E), Mumbai 400042, Maharashtra, India; both Indian Nationals.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

Technical Field:
The present invention relates to field of Varistor.
Particularly, the present invention relates to high energy varistor, class 5 Varistor.
More particularly, the present invention relates to a composition of a class 5 Varistor.
More particularly, the present invention relates to a method for the manufacture of a class 5 varistor using the said composition.
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, TiO2, SnO2, or SrO2 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 TiO2, SnO2, or ZrO2. 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 900° 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 along with grain growth inhibitor and antifoam agent. The slurry is sprayed dried to form granules and the granules are compacted under a pressure of about 50 to 1000 kg/cm , 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 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 which is 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 large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the varistor 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 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 and its effect on the 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 decreases 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 inhibitor in the varistor composition. Thus, breakdown voltage of the varistor will depend on the efficiency of the growth inhibitor. The energy handling capability of varistor block will depend on area and watt loss will depend on internal grain microstructure.
Objects of the invention
An object of the invention is to provide a composition for a class 5 varistor ceramic,
Another object of the invention is to provide a composition for a class 5 varistor ceramic; said varistor ceramic having a high energy handling capability.
Still another object of the invention is to provide a composition for a class 5 varistor ceramic, said varistor ceramic having low watt losses.
Yet another object of the invention is to provide a method for the manufacture of a class 5 varistor ceramic by using the said composition.
Yet another object of the invention is to provide a method for the manufacture of a class 5 varistor ceramic by using the said composition; said varistor ceramic having a high energy handling capability.
Yet another object of the invention is to provide a method for the manufacture of a class 5 varistor ceramic by using the said composition; said varistor ceramic having low watt losses.

Detailed description:
According to one of the embodiment of the invention, there is provided a composition for a varistor ceramic having a high energy handling capability and low losses, said composition comprising zinc oxide (ZnO); additives comprising bismuth oxide (Bi203), antimony oxide (Sb203), manganese oxide (MnO2), silica (SiO2), cobalt oxide (CoO); and cromium oxide (Cr2O3); binder; grain growth inhibitor; and antifoam agent.
According to preferred embodiment of the invention, there is provided a composition for a varistor ceramic having a high energy handling capability and low losses, said composition comprising about 90 to 98 % zinc oxide (ZnO) ); additives comprising about 0 to 3 % bismuth oxide (Bi203), about 0 to 3 % antimony oxide (Sb2O3), about 0 to 2 % manganese oxide (MnO2), about 0 to 1 % silica (Si02), about 0 to 2 % cobalt oxide (CoO), and about 0 to 2 % cromium oxide (Cr203); about 1 to 5 wt% binder; about 1 to 3 wt% grain growth inhibitor; and about 1 to 4 wt % antifoam agent.
According to another embodiment of the invention, there is provided a method for the manufacture of a varistor ceramic having a high energy handling capability and low losses, said method comprising:
(a) dispersing zinc oxide (ZnO) with additives comprising bismuth oxide (Bi203), antimony oxide (Sb203), manganese oxide (Mn02), silica (Si02), cobalt oxide (CoO) and cromium oxide (Cr203); binder; grain growth inhibitor; and antifoam agent;
(b) subjecting said dispersion 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 1000 to 1450° C to keep a varistor ceramic grain size at least 12 μm.
The binder is poly vinyl alcohol.
The grain growth inhibitors are selected from AI2O3, Sb2O3, CaO, etc.
The antifoaming agents are but not limited to Silicon based antifoaming agents.
The zinc oxide (ZnO) powder used in the invention is preferably in the form of but not limited to nano particles.
In another embodiment of the invention, before carrying out step (c) of the said method, the powders or their mixture are calcined at a temperature in the range of 400 to 800°C.
In another embodiment of the invention, the consolidation treatment of step (c) given to precursor comprises pressing of the precursor to get the block (OD- 115mm, ID-39mm, Thickness- 26mm); followed by burning out at 400°C the binder from the block; subjecting the block to calcination followed by sintering at 800 to 900°C; subjecting the block to lapping treatment followed by heat treatment.
In yet another embodiment of the invention, 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.
In yet another embodiment of the invention, the sintering of step (c) is carried out at a temperature 1450°C for a period of time equal to or less than 4 hours. Preferably, the sintering is carried out at a temperature of about 1200°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.
In yet another embodiment of the invention, the dispersion prepared during step (a) comprises about 90 to 98 % zinc oxide (ZnO); additives comprises about 0 to 3 % bismuth oxide (Bi2O3), about 0 to 3 % Antimony oxide (Sb2O3), about 0 to 2 % manganese oxide (MnO2), about 0 to 1 % silica (SiO2), about 0 to 2 % cobalt oxide (CoO), and about 0 to 2 % Cromium oxide (Cr2O3); about 1 to 5 wt% binder; about 1 to 3 wt% grain growth inhibitor and about 1 to 4 wt % antifoam agent.
In yet another embodiment of the invention, each additive are subjected for milling, followed by mixing the milled additives, calcinating the mixture and milling the calcined additive mixture.
In yet another embodiment of the invention, varistor ceramic is subjected to lapping, ultrasonication and metalizing.
According to yet another embodiment of the invention, there is provided varistor ceramic block having high energy handling capability in the range of 14 to 19 kJ/kV and watt loss at rated voltage in the range of 2.5 to 3.6W

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:
Example 1:
96.5 gm of ZnO was mixed with 0.5 gm of Bi2O3, 1 gm Sb2O3, 0.5 gm CoO, 0.5 gm of MnO2, 0.5 gm of Cr2O3 and 0.5 gm of SiO2 and milled together. The polyvinyl alcohol
3 wt%, 0.5% grain growth inhibitor and 1% antifoaming agent was added to the mixture to form 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 1200° 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 found that the grain Size of ZnO is 14 u.m and energy handling capability of the varistor is in the range of 14 kJ/kV and watt loss at rated voltage is 3.4W.
Example 2:
93 gm of ZnO 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 5 wt%, 2% grain growth inhibitor and 2% antifoaming agent 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 1200° 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 found that the grain Size of ZnO is 12 μm and energy handling capability of the varistor is in the range of 18 kJ/kV and watt loss at rated voltage is 2.8W.

The present invention as described above, it is to be understood that this invention is not limited to particular methodologies and materials described, as these may vary as per the person skilled in the art. It is also to be understood that the terminology used in the description is for the purpose of describing the particular embodiments only, and is not intended to limit the scope of the present invention.

We claim:
1. A composition a varistor ceramic having a high energy handling capability and low losses,
said composition comprising zinc oxide (ZnO); additives comprising bismuth oxide (Bi203), Antimony oxide (Sb2O3), manganese oxide (Mn02), silica (Si02), cobalt oxide (CoO), and Cromium oxide (Cr203); binder; grain growth inhibitor; and antifoam agent.
2. The composition as claimed in claim 1, wherein the said composition comprising about 90 to 98 % zinc oxide (ZnO); additives comprising about 0 to 3 % bismuth oxide (Bi203), about 0 to 3 % antimony oxide (Sb203), about 0 to 2 % manganese oxide (MnO2), about 0 to 1 % silica (Si02), about 0 to 2 % cobalt oxide (CoO), and about 0 to 2 % cromium oxide (Cr203); about 1 to 5 wt% binder; about 1 to 3 wt% grain growth inhibitor; and about 1 to 4 wt % antifoam agent.
3. A method for the manufacture of a class 5 varistor ceramic having a high energy handling capability and low watt loss,
said method comprising:
(a) dispersing zinc oxide (ZnO) with additives comprising bismuth oxide (Bi203), antimony oxide (Sb203), manganese oxide (Mn02), silica (Si02), cobalt oxide (CoO), and cromium oxide (Cr203); binder; grain growth inhibitor; and antifoaming agent;
(b) subjecting said dispersion 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 1200-
1450° C to keep a varistor ceramic.

4. The method as claimed in claim 3, wherein before carrying out step (c) of the said method, the powders or their mixture are calcined at a temperature in the range of 400 - 800°C.
5. The method as claimed in claim 3, wherein 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.
6. The method according to claims 3 to 5, wherein the said composition of step (a) comprises: about 90 to 98 % zinc oxide (ZnO), about 0 to 3 % bismuth oxide (Bi203), about 0 to 3 % Antimony oxide (Sb203), about 0 to 2 % manganese oxide (Mn02), about 0 to 1 % silica (Si02), about 0 to 2 % cobalt oxide (CoO), about 0 to 2 % Cromium oxide (Cr203), about 1 to 5 wt% binder, about 1 to 3 wt% grain growth inhibitor and about 1 to 4 wt % antifoam agent.
7. A varistor ceramic block prepared according to the compositions as claimed in claims 1 to 2 and the method as claimed in claims 3 to 6, wherein the varistor block having energy handling capability in the range of 14 to 19 kJ/kV and watt loss at rated voltage in the range of 2.5 to 3.6W.