FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)
TITLE OF INVENTION
A modified tray for sintering of ZnO based Varistor
APPLICANTS
Name : CROMPTON GREAVES LIMITED
Nationality: Indian Company
Address : CG House, Dr Annie Besant Road, Worli, Mumbai 400030, Maharashtra, India
INVENTORS:
Roy Pradip, Dr Singal Vivek, Advanced Materials and Process Centre, Crompton Greaves Ltd, 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:

FIELD OF THE INVENTION
This invention relates to a method of designing a sintering tray for ZnO based Varistors. This invention also relates to a sintering tray prepared according to the above method. This invention also relates to modified sintering cycle by using the above sintering tray. This invention also relates to modified varistors having the above sintering tray.
PRIOR ART AND BACKGROUND DESCRIPTION
Varistors are electrical devices which are used as surge arresters or lightening arresters to protect transmission and distribution lines and peripheral devices thereof from the destructive voltage levels induced or caused by lightening impulses or switching surges. Zinc oxide varistors have lately replaced the silicon carbide varistors because of the several advantages they posses like cost effectiveness, flexibility of being fabricated into a variety of shapes, excellent non- linearity with respect to voltage, discharge withstand current rating properties or life characteristics under voltage. In Zinc oxide varistor the composition after sintering is difficult to handle because several oxides with high vapor pressure are used in typical ZnO varistor formulation used as arresters in power transmission. The composition is likely to change after sintering because of component vaporization. Beyond the control of electrical properties of varistors there is also the problem in production of furnace life-expectancy and toxicity of vaporized oxide such as antimony and bismuth. US4767582 describes evaporation of Boron from ZnO varistor, which partially effect on grain size. This difficulty overcome by a method, in which in order to prevent the evaporation of Boron, the mold is enclosed in a sheath box, i.e. is baked in a closed container. Holes are made on the trays so that the oxygen is made available to the blocks and the B2O3 evaporation is kept to a minimum. R. Metz et al. said in Materials Chemistry and Physics 63 (2000) 157-162, the evaporation of Bi203 is prevented in practice by applying a mix onto the inside surface of the furnaces. Such a practice is not possible in a tunnel type of furnace because they are continually held at high temperature. There is, therefore, scope for development of such method for sintering. The present practice of sintering involves closed box type of trays. Such a practice is known to prevent the vaporization of volatile compounds like Bi203 and Sb203. This practice has a disadvantage in that it restricts the amount of oxygen that is made available to varistors during sintering. It is well known that oxygen is responsible for varistor


action. There are patents trying to address the issue of vaporization by modifying the degree of openness (Number of pores) and location of those pores. There are references in literature suggesting that the inside surface of the furnace walls be covered (coated) with a mix of those volatile compounds. However, coating on the inside surface of furnaces is difficult in the case of a tunnel furnace.
An object of the invention is to provide designing the sintering trays and sintering cycle in order to prevent the vaporization of volatile compounds like Bi2O3, Sb2O3 in tunnel kiln. Another object of the invention is to provide the sintering trays and sintering cycle by means of making a number of pores or using a window type tray in order to give sufficient access to air flow.
Another object of the invention is to provide the sintering trays coated with the slurry of mixture of volatile components such as B12O3 and Sb2O3 to eliminate the problem of vaporization of additives.
DETAILED DESCRIPTION
According to the invention there is provided a method of designing sintering trays; the method comprising molding a rectangular tray having at least six pores on the long sides of the rectangular tray.
The method further comprises coating the rectangular tray having at least six pores with a slurry of Bi2O3 and Sb2O3.
According to the invention there is also provided a sintering tray having at least six pores on long side of the rectangular tray and coated with a slurry of Bi2O3 and Sb2O3 prepared according to the above mentioned method.
Process for the manufacture of a varistor comprises mixing an additive mix with water and milling it to form a homogenous mixture. The homogenous mixture is dried at about 100-120°C followed by Calcination in a furnace at 850-900°C. The lumps formed are cooled to ambient temperature and milled. The resulting powder is formed into slurry with water, aluminum nitrate, zinc oxide, dispersing agent(s) such as isopropyl alcohol or


Decylamine acetate and binder(s) such as polyvinyl alcohol. The slurry is spray dried into granules which are sieved, mixed with water and allowed to soak in air tight containers. The wet granules are compacted into varistor blocks and are subjected to de-bindering in a furnace at 390-410°C.The green varistors are calcined in a furnace at 870-900°C by placing them in open trays for formation of the various phases like spinal or pyrochlore. The varistors are cooled to ambient temperature, coated with insulation material and sintered in closed trays in a furnace at 1150-1170°C. The varistors are cooled to ambient temperature and lapped or ground so as to even out the surfaces thereof. The varistors are then annealed in a furnace by heating to 490-510°C cooling them down to ambient temperature.
Electrical properties of the varistors like breakdown voltage, residual discharge, nonlinear co-efficient, watt leakage or energy capability are formed during sintering. As the varistors are placed in closed trays during sintering, oxygen which is required for the development of the electrical properties is not available to the varistors and their electrical properties are not well developed. During sintering the varistors are required to be placed in air tight trays as antimony and bismuth in the varistors which are also required for the development of the electrical properties would otherwise evaporate at the sintering temperature.
In order to obviate the above problems, it is known to carry out sintering process in open trays by providing a coating of antimony and bismuth mixture on the inner walls of the furnace so as to minimize evaporation of the antimony and bismuth present in the varistors during sintering and to allow the electrical properties to be properly developed. The coating of the entire interior of the furnace with such a mixture, however, renders the process expensive. Also, as the coating deteriorates during the continued operations of the furnace the interior of the furnace needs to be recoated at regular intervals. In order to carry out the re-coating, the furnace has to be shut down and allowed to cool down. This reduces productivity of varistors. Also, re-coating of the furnace at regular intervals is time consuming, requires labour and further adds to the cost of the process.
According to the process of the invention sintering of the varistors is carried out by placing them in open trays coated with antimony and bismuth coating on the inner walls thereof. The electrical properties of the blocks are properly developed as oxygen required


for development of the properties is freely available to the varistors. Due to the antimony and bismuth coating on the inner walls of the trays, evaporation of the antimony and bismuth present in the varistors required for the development of their electrical properties is minimized. Therefore, the varistors manufactured by the process of the invention have improved electrical properties. The process of the invention is economical as only the inner walls of the trays are to be coated with the antimony and bismuth mixture as against the entire interior of the furnace. The productivity is increased as the furnace need not be shut down and cooled. Time, cost and labour for the coating of the inner walls of the trays are reduced as compared to coating of the inner walls of the furnace.
As it is known to carry out the sintering in open trays in the presence of antimony and bismuth by coating the inner walls of the furnace in an oxygen atmosphere and as the process of the invention also comprises carrying out the sintering in open trays in the presence of antimony and bismuth by coating the inner walls of the trays in an oxygen atmosphere, it may be alleged that the process of the invention is obvious to a person skilled in the art even though it is novel. It should be possible for us to overcome this allegation by stating that the process of the invention is cost effective as the coating and re-coating of the trays is less expensive and time consuming and involves minimal labour as compared to the coating of the entire interior of the furnace. Also, the productivity is increased as the furnace interior is not required to be shut down and cooled.
A method of designing a sintering tray in which there is a greater flexibility: The size of the opening can be bigger as the evaporation can not happen. The tray is made to give sufficient access to air flow by means of number openings. The vaporization problem is eliminated with the application of slurry of easy vaporization components like Bi2O3 and Sb2O3. The tray is coated and hence coating is not required on the inside surface of furnaces. This process of the invention is feasible for a tunnel kiln which operates continuously for months. This way, we can increase the quality of varistors without evaporation problems. The ultimate advantage is a reduction in the process cycle time as the present invention eliminates the vaporization issue. By providing more oxygen and controlling the vaporization, the conventional firing which involves a provisional sintering and a later sintering can be combined into a single cycle. The first stage Provisional sintering or Calcination must be done in oxidizing atmosphere because of the requirements on the stoichiometry of the phases formed.


Because the tray is coated with the mixture, there is an equilibrium partial pressure created between the Bi2O3, SbiO3 in the blocks and the external area so no evaporation from the blocks takes place. So sintering can be done in a more "open" tray so that more oxygen is available to the block and so better non-linear characteristics. Because of this, it eliminates evaporation of Bi203, and thus the Calcination (Provisional Sintering) and sintering is carried out in a single cycle which means a lot of cost saving, time saving and increase in productivity.
Dated this 31st day of March 2008