FIELD Of THE INVENTION
This Invention relates to a method for uniform sintering of ceramic bodies using microwave field.
This invention further relates to a method of uniform sintering of ceramic bodies using a special casket arrangement.
BACKGROUND OF THE INVENTION
Microwave energy 915 and 2450 MHz, among other frequencies is used for industrial applications, At present, most microwave ovens in use operate at 2450 MHz, which is a wavelength of 4.8" in air. Materials differ in their reaction to microwave field. Polar molecules in receptive materials respond to these fields by oscillating in rotary motion, The energy generated by this motion causes these substances to heat.
Microwave energy has been in use for over 50 years in a variety of applications, such as communication, food processing, rubber vulcanization, textile and wood products, and drying of ceramic powders. The application of microwaves in the sintering of ceramics and metals is relatively new. Although many potential advantages of using microwaves to process ceramics have been long recognized, it is only now that this field has finally shown to be at the take off stage, especially for the commercialization of some speciality ceramics, including composites, Sintering of metals is still in its evolution stage and even basic small sample sintering facilities are not available in the market which will go to establish the efficacy and reliability of sintering of metallic system.

Firing or sintering, one of the most critical stages of ceramic manufacture, must be precisely controlled to avoid thermal stresses developing in the product. If !h!s is not achieved it could result in failure of the piece or batch being fired. Microwave sintering employs microwaves to fuse powders into solids, which produce dense products with better mechanical properties. The Electromagnetic energy of the wave is efficiently converted into thermal energy helping to produce a finer grain size in the finished product than Is produced through traditional sintering.
PRIOR ART
Patent 6172346 discloses a microwave furnace comprising a microwave source coupled to an enclosure for the confinement of microwaves and for containing an object to be heated with an independently controllable alternate heating disposed in relation to the enclosure to provide at least one of radiant and convective heating within the enclosure. The method comprises the steps of energizing the alternate heater so as to generate heat substantially throughout the heating cycle of the furnace and controlling the quantity of heat generated in the object by one or both of the microwaves and the alternate heater so as to provide a desired thermal profile in the object.
in the patent 5,736,092 to Apte,et al is described a Microwave sintering process using a microwave susceptor bed useful for sintering ceramics, ceramic composites and metal powders.The susceptor bed includes granules of a major amount of a microwave susceptor material, and a minor amount of a refractory parting agent, either dispersed in the susceptor material, or as a coating on the susceptor material.

in the patent, 6,066,290, Dennis et al describes a Method and apparatus for transporting green work pieces through a microwave sintering system. individual green work pieces are formed in a small mold cavtty cruclble, and individual crucibles are then indexed into and out of a tube for a controlled transit time along the tube. The tube extends in one embodiment through a preheater and then into the microwave cavity, the preheater providing an initial heating step to change the rate of absorption of microwave energy so that microwave sintering is accomplished in the cavity,
in the patent 6,344,635, Brennan describes a Hybrid method for firing of ceramics. That involving places the ceramic material in a microwave heating apparatus having a microwave cavity and subjecting the ceramic material to combination of microwave radtation and conventional heat energy according to predetermined time-temperature profile.
Various pubiications disciose details of system used as well as results obtained in sintering varied materials, explaining hybrid arrangements using SIC rods, plates with fiber insulation as well as using SiC powders surrounding the objects. it is further disclosed that SIC susceptor Is a normal means in microwave sintering. Now SiC susceptors are available commercially and its effectiveness in sintering large components is reported by the group in its publication.
However, most of the publications or patents use inductive heating / electric heating arrangement as a hybrid arrangement and the sample is generally preheated to a threshold temperature from which it can absorb microwave effectively.

OBJECTS OF THE INVENTION
It is therefore an object of this Invention to propose a method of sintering of ceramic bodies using microwave field, which leads to an uniform sintering.
It is a further object of this Invention to propose a method of sintering of ceramic bodies using microwave field, which eliminates the step of preheating the body.
Another object of this invention to propose a method of sintering of ceramic bodies using microwave field, which is simple, easy to operate and is cost effective.
Yet another object of this invention is to propose a method of sintering of ceramic bodies using microwave field, which establishes equilibrium temperature quickly and creates and isothermal region round the body to avoid heat loss.
These and other objects of the invention will be apparent from the ensuing description when read in conjunction with the accompanying drawings.
DESCRIPTION OF THE INVENTION
Thus according to this invention is provided a method of sintering ceramic bodies
using microwave field.
According to this invention is further provided a system for sintering of ceramic bodies, using a special casket arrangement.

In accordance with this invention the microwave sintering system comprises a cavity connected to a microwave source through wave guides. The cavity is the work-space, which is accessible through a door, gasketed to prevent microwave leakage, The cavity contains; a mode stirrer which improves the uniformity of the field distribution within the cavity and a turn table. The component to be sintered is placed in a casket arrangement, specially constructed to achieve the unique features of the invention. The casket in turn is placed on the turn fable of the cavity which rotates uniformly to ensure uniform absorption of microwave power by the component. The temperature is monitored by a thermometer. The output from the thermometer is fed into a programmer to control the power output of the magnetron for precise control of the heat sintering cycle,
The microwave heating system consists of a microwave generator connected to the cavity by a wave guide.
The novel features of the invention can be arrived at by the use of a special
casket arrangement. The casket arrangement for sample holder has partially
absorbing boundary made of a low cost material, which is flexible enough to be
formed to a desired shape based on the component requirement. The partially
absorbing boundary is wrapped with low density alumina fiber mat 50 mm thick.
It is made of low density alumina castable which is mixed with silicon carbide
(SiC) grits. The wet mix is cast in to a cylinder using simple fixtures made of
PVC pipes. Because of the coarse bubbles present In alumina castables no
shrinkage is associated with heating even to 1750 C, After 24 hrs the cast
sample holders become strong and are ready for use. After casting in
cylinders the caskets are wrapped with low density alumina fiber mats to a thickness in the range of 50 to 70 mm.

The invention will now be explained in greater detail with the help of the ensuing description when read In conjunction with the accompanying drawings.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1: Conventional casket with SIC susceptor plates and tile components
Fig 2: The Microwave sintering system
Fig 3: Cast casket used in 1 kW system
Fig 4: Isothermal casket according to this Invention
Fig 5: Isothermal casket in the cavity
Fig 6: Isothermal casket kept outside after the sintering operation
Fig 7: Large tile in isothermal casket after sintering
The macrowave sintering system according to the prior art is depicted in Fig 1, in which the casket which has an outer dimension 250mm(L) x 250 mm(W) x 150 mm(H) including lid, Is made from 50 mm thick, 1750 Grade of Vacuum formed Fiber boards from RATH and has a lid with a hole of 12 mm for Temperature measurement through IR thermometer. To effectively heat the components In the lower temperature region, where the microwave absorption Is poor, SIC plates of 150 mm [L] 50 mm [w] x 10 mm [t] are used. The susceptor plates can be slipped between the fiber wall and the sample and can be either on the ends or at the sides, in case of tile components, it was observed that slipping In the sides resulted in uniform sintering without cracks.
In this arrangement a maximum of 3 samples could be loaded and typical dimensions of alumina tiles investigated were 133 x 100 x 30 mm Straight tiles and 138/141 X 82/84 X 15,5/120/127 X 58/62 X 20 curved tiles.

Fig 2 shows a preferred embodiment of the microwave sintering system according to the invention, The system comprises a cavity (t)Vhich is connected to a 6 kw, 2.45 GHz (2) microwave source through wave guide (3). The components to be sintered are placed in a casket as shown in Figs 3 & 4 in the present invention. The casket is placed in the turntable of the cavity which rotates at a speed of 4rpm to ensure homogenous absorption of nfterowave power by the component. The temperature is monltored by Infra red thermometer (4). The output from lR Thermometer is fed into a PID programmer to control the power output of the magnetron for precise control of the heat / sintering cycle.
The microwave heating system consists of two main parts - 6 kilowatt microwave generator and a 2' x 2' x 2' cavity, to which it is connected via wave guide The generator produces microwave power at 2450 MHz and is controllable from 600 to 6000 watts. The casket in the form of a cylindrical vessel is cast to a shape of 220 mm [ID] x 20 mm [H] and a wall thickness of 7 mm using a mixture of alumina castabfe and mixed with SIC grits in the ratio of for e.g. 2:1. The top cover is a disk of 5 mm thickness and has a 12 mm hole fro temperature measurement and control. The sample holding cylinder is wrapped with low density alumina fiber to a thickness in the range of 50 to 70 mm. This enables The vessel to be taken out of cavtty on completion of sintering cycle. Typical casket for a 1 kW system is shown in fig. 3 and a typical arrangement for a 6 kW system in fig. 4 casket is taken out from the cavity is as per figure 5 and casket in cavity In fig. 6, Fig.7 shows sintered large alumina tile in casket. The large tile dimensions are 210 mm [L] x 200 mm [H] x 45 mm [t].

Sintering of large tile by conventional casket arrangement was not possible and resulted in cracks or non uniform densification.
SINTERING CHARACTERISTICS
The sintering uniformity of large tiles were evaluated based on the variation in porosity and water absorption at different segment of the component. For this the component was cut into 8 segments and analyzed for porosity and water absorption. In case of conventional casket arrangement for optimum processing condition the results are as given in table 1. During sintering the temperature fluctuation during soaking was to the tune of ± 3°. Table 1
8 samples, 4 in each layer was successfully sintered in isothermal caskets with results as shown in table 2. During the soaking period of sintering cycle, a temperature fluctuation of 0.1 degree was observed indicating an isothermal situation facilitating negligible radiation from the component surface, The uniformity of sintering of ail the 8 samples at different sections were measured and high degree of uniformity in sintering characteristics is easily achieved. In addition, the large tile sintering high lights the enhancements of sintering volume for the 6 kW, 600 mm x 600 mm cavity by using newly developed casketlng arrangement. This provides a method of uniform sintering of large volume components.


Tables 2a and 2b
In addition, the temperature fluctuation with in 0.2 degree was very easily achieved in 6 kW system during soaking period of sintering cycle. The number of standard ceramic tiles of size 133 x 100 x 30 mm has been increased to two layers of 4 numbers each layer as against conventional casket arrangement which can accommodate only 3 components. The sintering uniformity is extremely consistent.

The method allows sintering of objects which is more uniform and also provides an equilibrium temperature in the entire sintering volume defined by the inner surfaces of the cast casket. The casket also facilitates usage of low temperature low density fiber mats or fiber boards.
The casket as a whole is suitable for uniform sintering of small volume to large volume components.

WE CLAIM
1. A method of uniform sintering of ceramic bodies using a microwave
field comprising:-
- a microwave heating apparatus (2) through wave guide (3) and a means for monitoring and measuring temperature (4);
- a holding means (1) for holding the metallic sample (6) wherein said holding means (1) is housed in a cavity configured in the heating apparatus (2);
- the said holding means (1) comprises a casket made of low density alumina castable mixed with silicon carbide grits (7) and cast into a casket configuration to define a space therein and
- the said configuration being wrapped on the outer surface with low density alumina fibra material to provide the casket;
Characterised in that the compound to be sintered is placed in a casket and the casket in turn is placed on the turn table of cavity which may or may not rotate and achieve uniform temperature in the entire volume of the casket and hence uniform sintering of components.
2. The method as claimed in claim 1, wherein said alumina castable and silicon carbide grits are present in a ratio of 2:1.
3. The method as claimed in claim 1, wherein said casket configuration is made by mixing silicon carbide grits with low density alumina castable and cast into a defined shape and heated up to 1750°C.
4. The method as claimed in claim 1, wherein the casket configuration is wrapped on the outer surface with low density alumina fibre mats to a thickness of 50 to 70 mm.

5. The method as claimed in claim 1, wherein the casket is provided with a top cover with hole for temperature measurement and monitoring.
6. The method as claimed in claim 1, wherein said holding means is placed on turn table in the cavity.
7. The method as claimed in claim 1, wherein means for monitoring and measuring temperature comprises an infra red thermometer, the output of which is fed to a progammer.
8. The method as claimed in claim 1, wherein said microwave heating apparatus is a microwave generator and a controller.
9. The method as claimed in claim 1, wherein said holding means houses the sample to be subjected to heat treatment, in the cavity.
10. The method as claimed in claim 1, wherein the fiber material is 1450°C grade low-density fiber material.
11. The method as claimed in claim 1, which ensures minimal temperature gradient in the component as well as surface of the component to the partially microwave absorbing walls of the cylinder surrounding the component ensuring uniform temperature I its volume.
12. The method as claimed in claim 1, which due to uniform temperature in the entire component and hence can achieve uniform sintering in large volume.

The present invention relates to a method of uniform sintering of ceramic bodies using a microwave field comprising a microwave heating apparatus (2) through wave guide (3) and a means for monitoring and measuring temperature (4) and a holding means (1) for holding the metallic sample (6) wherein said holding means (1) is housed in a cavity configured in the heating apparatus (2) and the said holding means (1) comprises a casket made of low density alumina castable mixed with silicon carbide grits (7) and cast into a casket configuration to define a space therein and the said configuration being wrapped on the outer surface with low density alumina fibra material to provide the casket.
Characterised in that the compound to be sintered is placed in a casket and the casket in turn is placed on the turn table of cavity which rotates uniformly to ensure uniform absorption of microwave power by the component.