A METHOD AND APPARATUS FOR HIGH TEMPERATURE GLAZING USING MICROWAVE HEATING

FIELD OF INVENTION

The invention relates to a method of high temperature glazing. Particularly, the invention relates to high temperature glazing using non-conventional heating method such as microwave heating.

BACKGROUND OF THE INVENTION

Dense sintered alumina is one of the most widely used ceramic materials due to its excellent wear resistance, corrosion resistance and electrical resistance properties. It has applications in areas ranging from thermal power plants, cement or steel industries to electrical and electronic industries. For many of its industrial applications, there is a need for its surface to be impervious in order to apply it in electrical and other critical applications. This is achieved by applying a ceramic glaze layer on the external part of the product. The glaze applied on the ceramic product is required to be of high temperature type when it is going to be exposed to high temperatures during subsequent manufacturing process step (e.g. ceramic to metal sealing) or in the end use of the product. The specific demands on the properties of the glazed layer for each of its applications make the process very intricate and a specific firing schedule needs to be followed.

Glaze is a vitreous ceramic coating on second ceramic surface. The chemical composition of the glaze material and the way it is heated through different stages of its firing cycle are critical to its fired properties as the formation of the glass network or embedding of the modifiers is dependent on it. The glass is formed by combination of network formers (such as Si+4, B+3), modifiers (such as Na+, K+, Ca+2, Mg+2) and intermediates (such as AI+3).

Intermediates can act sometimes as glass network former and sometimes as modifier. The starting material of the glaze consists of these elemental oxides-based compounds and minerals. The method of heating used at the firing stage plays an important role for formation of quality glaze-product. The glaze is also classified as low-temperature, medium-temperature and high-temperature based on peak-temperature of firing to develop the glaze coating. The high temperature type glazing involving temperatures, say, above 1200 °C is employed for the glaze to be compatible with some subsequent heating step such as metallization or with the end use temperature or other requirements of the product. The former case applies to, say, for example, the processing of alumina products used in electrical insulation applications which often have metallized parts. Therefore, the glaze is exposed to higher temperatures during subsequent ceramic metallization firing to bond metal to ceramic product.
The alumina products that are used in high quality electrical insulation applications have to be impervious to moisture or other gaseous contaminants from severe ambient environments. A high temperature glazing with certain structural features is particularly suitable for such products.

For any industrial application of a glazed material, the processing time and the cost involved are vital to its implementation. The microwave heating of ceramic glaze is one such alternative which can be utilized for reduction in processing time and savings in manufacturing cost. The microwave heating involves application of high frequency (in GHz range) electromagnetic field across the material with the bulk of the material getting heated almost instantaneously and throughout the material with its extent dependent on the absorption properties of the particular material. As a result the heating rates can be very high. Very importantly it is also a clean and environment friendly way of processing of the materials, very much in tune with the current industrial requirements.

The article 'Microwave glazing of alumina-titania ceramic composite coatings' A K Sharma et al. Material Letters Vol. 50 (2001) pgs. 295-301 shows how microwave heating is being pursued for some industrial processes such as post-deposition annealing of Alumina-Titania coatings. 'Comparison of porcelain surface and flexural strength obtained by microwave and conventional oven glazing' S Prasad et al. J. Prosthet. Dent Vol. 101 (2009) 20-8 shows the study of moderate temperature glazing of dental ceramics with porcelain surfaces by microwaves. However, this way of heating is not used in the process for glazing alumina bodies.

The glazing of alumina bodies using conventional heating was studied in late fifties leading by now to a well optimized industrial process. The drawback of this prior art is that of very long preheating, soaking and cooling times. In addition, there is an undue heating of the full system surrounding the part to be heated unlike the very local heating in the case of microwaves.

US patent No. 6, 812, 441 (Cheng et al.) describes sintering of a work piece of alumina powder compact using a single mode microwave system and hydrogen atmosphere to convert the work piece into transparent alumina and sapphire. The alumina work piece contained a large percentage of up to 50 % magnesium oxide which is a good absorber of microwaves thus enabling the compact to be heated to the required high temperatures. In general, alumina starts absorbing the microwave radiation of frequency 2.45 GHz only above 800 °C. Hence below that temperature, the work piece is heated through microwave absorption in magnesium oxide. The process is particularly suitable for producing transparent polycrystalline alumina. In this case the microwave absorbing material is incorporated as an inherent part of the work piece material.

An apparatus for microwave firing of a material containing organic binders is described in US publication No. 2003205573 A1 (Okumura et al.) The publication teaches a provision to appropriately control burning of the organic binders by providing a gas flow with oxygen percentage in the gas much lower than that in ambient atmosphere. This prevents localized temperature gradients and resultant cracking of the material at debinding stage. The inner faces of the furnace wall have a surface layer of microwave absorbing material to avoid undue temperature differences.

US Patent No. 4,880,578 (Holcombe et al.) describes a method to sinter metal oxides which do not absorb microwaves at room temperature or several hundred degrees above. The material to be heated was enclosed in a housing made up of a compact mainly of Zirconia, and Hafnia which absorbed microwaves in the lower temperature region in turn heating the inside metal oxide by radiation. Once the metal oxides started absorbing microwaves and were self-heated, the role of the housing was to thermally insulate the metal oxide until its sintering temperature. This is another way of heating a non-microwave absorbing material which is indirect and uses a susceptor externally.

Another type of apparatus using a hybrid method of firing ceramics by mix of radiative/convective conventional heating and microwave heating is also disclosed in US Patent No. 6,172,346 (Wroe) in 2001 and in US Patent No. 7,112,769 (Regano et al.) By monitoring both core and the surface temperature of the work piece and using the difference in the two to control the heating rate in various heating segments, the work piece could be fired uniformly. US Patent No. 6,537,481 (Brennan) discloses a scheme to achieve uniform microwave heating of a large volume work piece.

The microwave heating many times brings down the process temperature considerably as described in US Patent No. 5,223,186 (Eastman et al.) related to the sintering of nanophase ceramics. The material could be sintered to 95% density while heated to only 70% of its melting point in degrees using microwaves. Another important point was related to its average grain size which remained less than three times that before sintering. A similar better performance has been observed in glazes prepared by microwaves as compared to conventionally heated porcelain dental samples as described in 'Comparison of porcelain surface and flexural strength obtained by microwave and conventional oven glazing' S. Prasad et al. J. Prosthet. Dent Vol. 101 (2009) 20-8.

Although there is a definite amount of electrical energy expended in generating high frequency microwaves, its application is still commercially viable and significantly advantageous in a wide range of applications.

OBJECTS OF THE INVENTION

The objective of the present invention is to achieve firing of high temperature glaze material on a work piece using microwave heating technique.

Another objective of the present invention is to use microwave heating technique for firing of high temperature glaze material in a controlled way such that the processing time is significantly less than that required for firing of the said glaze using conventional heating method.

Yet another objective of the present invention is to use microwave heating technique for firing of glaze material at relatively high temperatures in a controlled way such that the glazed material is fully mature and as good as the conventionally fired glaze.

Yet another objective of the present invention is to use microwave heating technique for firing of glaze material at relatively high temperatures in a controlled way such that the process is environment friendly and saves energy.

Yet another object of the present invention is to exemplify the high temperature glazing method on an already sintered alumina ceramic body.

SUMMARY OF THE INVENTION

The present invention relates to high temperature microwave glazing to obtain impervious surface layer as good as that obtained using conventional heating in a significantly shorter time.

In an exemplary embodiment, the present invention provides a method of firing of high temperature glazes containing substantial refractory material portion on sintered work piece by microwave heating to reduce the firing time and cost of firing.

The glaze-forming component can be prior-milled and calcined to remove the loss-on-ignition to facilitate fast-firing process by microwave heating technique.

The glaze-applied work-piece can be indirectly heated by first absorbing the microwaves in a susceptor material kept in close proximity to the work piece up to temperatures around 800°C at which the work piece also can start absorbing microwaves. Thereafter, the work piece is self-heated and is exposed to the different stages of the firing of the glaze followed in terms of temperatures with the conventional firing cycle but with much reduced time scale and also with the peak glaze maturing temperature being in the range of 1000 to 1500°C, which is less than that of the conventional temperature by 50 to 200°C and with the reduction in overall heating time as compared to conventional heating cycle. The microwave-fired glazed surface shows comparable or better properties in terms of surface roughness, porosity or defect density.

The susceptor material can be oxide-bonded SiC, nitride-bonded SiC, recrystallized SiC, reaction-bonded SiC, sintered SiC or any combination thereof.

The susceptor material can also be pre-oxidized surface of SiC to improve the surface oxidation resistance and hence maintaining the quality as well as life of the susceptor.

The susceptor containing the work-piece to be glaze-fired can be placed within a ceramic insulation cavity. The ceramic insulation can be made with the ceramic fibre/board/shape materials that are relatively transparent to microwave and which also has reasonably good thermal insulation properties.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein the showings are for the purpose of illustrating a possible embodiment of the invention only, and not for the purpose of limiting the same,

Figure 1 shows the schematic diagram of the microwave heating apparatus for glaze firing according to this invention;

Figure 2 shows the schematic of the work piece holder arrangement placed centrally in the microwave oven;

Figure 3 shows the temperature profile of the central hot zone as measured by an optical pyrometer depicting the zone temperature vs. microwave exposure time.

Figure 4 shows the schematic flow chart of the microwave glaze firing process;

DETAILED DESCRIPTION OF THE INVENTION

The method of glaze preparation and microwave firing of the same high temperature glazes on alumina bodies will be explained with reference to figure 1.

The glaze forming raw materials are admixed and milled in known mixing & milling equipment, such as ball mill, with the milling media made of one or combination of the glaze-forming compound(s), such that it will not disturb the glaze-forming composition. The glaze-forming compound can be optionally pre-calcined to remove the loss-on-ignition to facilitate the fast-firing process with the help of microwave. Then a known dispersant and organic bonding material is added along with the glaze-forming raw material and requisite water in the mixing & milling equipment and milled to achieve the require particle size & particle size distribution. The green glaze thus prepared is applied on the sintered alumina/pre-heated sintered alumina work-piece by known method such as dipping, spraying etc.

Thus, the glazed work-piece is kept within a microwave oven (1) working at 2.45 GHz frequency employed with suitable provision for microwave compatible thermocouple arrangement. The work piece (3) to be glazed-fired is placed in a sample housing made using microwave transparent special thermal insulation fibre/board/shape (2), preferably surrounded by insulation wool (11). A digital temperature indicator (4) with a module (5) for PC operated data acquisition is connectively provided in the set-up to obtain the various outputs and analysed results of the product obtained from the glazing process according to this invention. The work piece (3) is surrounded by microwave susceptor (7) that acts as a main microwave absorber material indirectly heating the alumina/glaze. Preferably, the microwave susceptor (7) is silicon carbide. The susceptor material can be oxide-bonded SiC, nitride-bonded SiC, recrystallized SiC, reaction-bonded SiC, sintered SiC or any combination thereof.

The susceptor material can also be pre-oxidized surface of SiC or oxide ceramics layer coated on such SiC to improve the surface oxidation resistance and hence maintaining the quality as well as life of the susceptor.

According to an exemplary embodiment of the invention presented here, the work piece is ceramic and more preferably an already sintered alumina ceramic.

Initial tests involved identification of the microwave power levels and exposure times required for reaching the maximum temperature corresponding to conventional heating cycle recommended for the particular work piece while keeping the oven conditions within safe operation limits. The various test samples were microwave heated working in that power-time range. After microwave glaze firing, the sample surface was analyzed for porosity, defect density and surface roughness.

As mentioned above, the microwave glaze firing is carried out in a microwave oven (1) with various magnetron (6) output. The majority of the experimental runs were carried out using variable power level. The microwave power output from the magnetron (6) was suitably fed to the cavity generating multimode microwave field (8) in the chamber (9). The work piece holder arrangement consisting of a housing made up of thermal insulation material (2) with a central space for holding the microwave susceptor (7) and the work piece (3) to be fired is placed centrally in the chamber (9). The housing may be additionally covered by glass wool (11 in fig. 2). As the base plate (15) on which this work piece holder arrangement is kept rotates, the inside microwave susceptor get exposed to microwave input port turn by turn. Hence the radiation heat from them to the work-piece (3) is also uniform.

A customized thermocouple (10) with a stainless steel outer tube (not shown) for microwave-shielding can be used for measurement and controlling the temperature. The outer stainless steel tube is electrically isolated from the inner thermocouple (10) while keeping its tip in thermal contact. The thermocouple output can be monitored using the temperature indicator (4) with a retransmit output. Using this indicator (4) along with a data acquisition module (5), the temperature as measured by the thermocouple (10) during the test run could be recorded on-line by a computer (12) and displayed on a monitor (14).

Figure 2 shows a schematic of the work piece holder arrangement (20) having a housing placed centrally in the microwave oven (not shown) depicting the microwave absorber plates (7) heating the central work piece (3) indirectly by radiation and the surrounding thermal insulation boards (2) with insulating wool (11) wrapped on the outside. The tip of the microwave compatible thermocouple (10) at entrance point is shown to be at a reference point. As illustrated in figure 2, in the present set up, the reference point for temperature measurement was at the entrance of the thermal insulator housing (2). A correspondence with the hot zone temperature was set by suitable experimentation using measurements by optical pyrometer and the thermal properties of the silicon carbide susceptor (7). The repeatability of the zone temperature for similar microwave power and other conditions made this way was found to be good.

For optimum utilization of the energy absorbed by the susceptor (7) surrounding the work piece (3), it is necessary that the heated susceptor (7) and the work piece (3) are wrapped by a thermally insulating material (2). This material should be such that while providing adequate thermal insulation, it allows microwave radiation to pass through it adequately and reach the inside susceptor material (7). Finally this thermal insulator (2) should be chemically stable at the working temperature to avoid contamination as also remain physically intact to avoid handling problem or problem of dust formation. Over the years, a number of thermal insulation boards made of alumina-silicate or zirconia based fibres have been developed which are microwave transparent and thermally stable up to 1400°C or 1600°C or 1800°C depending on the composition. In addition, the cube made of these boards was surrounded by glass wool (11) that is 1000°C compatible for additional thermal insulation reducing still further the amount of heat reaching the oven walls.

The heating zone of the set-up is a hollow central part obtained by using the thermal insulation boards (2a, 2b, 2c) placing them one above the other and cutting out the central portion of a middle board (2b). A small hole (13) was provided at the centre in the upper board (2a) for inserting the thermocouple (7) as illustrated in figure 2. Figure 3 shows a typical temperature profile of the central hot zone as measured by an optical pyrometer depicting zone temperature vs. microwave exposure time.

The sequential steps in the method for high temperature glaze firing process are illustrated in figure 4.

The glaze material or the glaze forming components are mixed and milled by wet/dry milling process. The glass network (glaze) is formed by combination of network formers (such as Si+4, B+3), modifiers (such as Na+, K+, Ca+2, Mg+2) and intermediates (such as AI+3). Intermediates can act sometimes as glass network former and sometimes as modifier. The glaze forming components are preferably calcined to remove the loss-on-ignition to facilitate fast-firing of glazes by microwave heating

A sprayable suspension of slurry is formed with addition of requisite water, binder component and a deflocculating agent. The slurry is applied on the work piece by spraying, dipping etc. The work piece is then placed inside the work piece housing and surrounded by microwave susceptor plates and outer microwave transparent thermal insulation. In a preferred embodiment the susceptor may be Silicon Carbide (SiC).

The work piece housing with the work piece is kept on a rotatable glass plate (15) in the microwave oven chamber and a thermocouple is lowered from a top opening. The work piece is then microwave heated through the radiation from the susceptors such that the glaze material slurry applied on the work piece is heated along the prescribed temperature profile with set final soaking temperature and time interval.
The microwave fired glazed work piece (3) inside the housing is allowed to be naturally air-cooled and kept undisturbed in the microwave oven (1) after switching it off. Unlike in the conventional heating set up, the system cooling is much faster since the bulk of the work-piece was heated directly and begins to cool immediately. Thus the overall process time is also comparatively less.

Subsequently, the characterization of the microwave fired glaze surface is performed for reflectance, surface roughness or other defects.

EXPERIMENTAL RESULTS

Table 1 shows the microwave glaze firing tests on sintered alumina cylinders for typical temperature and microwave power ranges.

TABLE 1

Experiment 1
The minerals and compounds equivalent to the following oxide composition on loss-on-ignition (LOI) free basis are dry milled and calcined at 1025°C to remove the LOI.

66.80% of Si02
19.30% of Al203
11.0%ofK2O
2.5% of Na20
0.25%ofCaO And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 40% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ frequency. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1400°C in 2 hours and soaked at 1400°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.02 - 0.04 pm. The glaze alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.01 - 0.02 urn was observed.

Experiment 2
The minerals & compounds equivalent to the following oxide composition on loss-on-ignition (LOI) free basis are dry milled and calcined at 950°C to remove the LOI.
64.80% of Si02 23.30% of Al203 9.0% of K20 2.0% of Na20 0.25% of CaO
And balance minor amounts of Ti02) MgO, Fe203, BaO and other oxides.

The above mix is mixed with 45% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1450°C in 3 hrs and soaked at 1450°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.03 - 0.05 urn. The glaze alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.02 - 0.03 urn was observed.

Experiment 3
The minerals & compounds equivalent to the following oxide composition on loss-on-ignition (LOI) free basis are dry milled and calcined at 1150 deg C to remove the LOI.
62.20% of Si02 28.40% of Al203 7.0% of K20 1.5%ofNa20 0.20% of CaO
And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 38% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1500°C in 3 hrs and soaked at 1500°C for a period of 1 hour and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.1 - 0.2 urn. The glazed alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.1 urn was observed.

Experiment 4
The minerals & compounds equivalent to the following oxide composition are taken in a ball mill.
65.90% of Si02 21.10% of Al203 10.0%ofK2O 2.3% of Na20 0.25% of CaO
And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 42% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1450°C in 3 hrs and soaked at 1450°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.02 - 0.04 urn. The glazed alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.02 - 0.03 urn was observed.

Experiment 5
The minerals & compounds equivalent to the following oxide composition are taken in a ball mill.
65.60% of Si02
21.70% of Al203
9.9% of K20
2.2% of Na20
0.25% of CaO And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 45% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1450°C in 3 hrs and soaked at 1450°C for a period of 1 hour and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.03 - 0.04 urn. The glazed alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics rather an improvement in surface roughness value of 0.02 - 0.03 urn.

Experiment 6

The minerals and compounds equivalent to the following oxide composition was taken.
66.80% of Si02
19.30% of Al203
11.0%ofK2O
2.5% of Na20
0.25%ofCaO And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 40% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ frequency. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1400°C in 2 hours and soaked at 1400°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.02 - 0.04 urn. The glaze alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.02 - 0.03 urn was observed.

Experiment 7
The minerals and compounds equivalent to the following oxide composition was taken.
64.80% of Si02
23.30% of Al203
9.0% of K20
2.0% of Na20
0.25%ofCaO And balance minor amounts of Ti02, MgO, Fe203) BaO and other oxides.

The above mix is mixed with 45% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1450°C in 3 hrs and soaked at 1450°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.03 - 0.05 urn. The glaze alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.03 - 0.04 urn was observed.

Experiment 8
The minerals and compounds equivalent to the following oxide composition was taken.
62.20% of Si02 28.40% of Al203 7.0% of K20 1.5%ofNa20 0.20% of CaO
And balance minor amounts of Ti02, MgO, Fe203l BaO and other oxides.

The above mix is mixed with 38% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on
firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1500°C in 3 hrs and soaked at 1500°C for a period of 1 hour and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.1 - 0.2 urn. The glazed alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.2 urn was observed.

Experiment 9
The minerals & compounds equivalent to the following oxide composition on loss-on-ignition (LOI) free basis are dry milled and calcined at 1150 deg C to remove the LOI.
65.90% of Si02 21.10% of Al203 10.0%ofK2O 2.3% of Na20
0.25%ofCaO
And balance minor amounts of Ti02, MgO, Fe203, BaO and other oxides.

The above mix is mixed with 42% water and ball milled along with the minor amount of binder and deflocculating agent to disperse it uniformly to make the sprayable suspension. The suspension is sprayed on a sintered alumina ceramic to make the green ceramic coating to be converted into glaze on firing. The aforesaid product is placed inside a microwave applicator of 2.45 GHZ inside. The aforesaid product inside the microwave applicator is placed within a susceptor wrapped within an insulated ceramic fibre cavity with an opening to measure and control the temperature.

The temperature is raised to 1450°C in 3 hrs and soaked at 1450°C for a period of 0.5 hrs and was allowed to cool naturally.

The glazed alumina was found to be defect-free, impervious with a surface roughness (Ra) value in the range of 0.02 - 0.04 urn. The glazed alumina was further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing. The glaze surface was found to retain its defect-free characteristics. Rather an improvement in surface roughness value of 0.02 urn was observed.
General Characteristics of Microwave-fired Glaze Sample:

The microwave fired glazed surface was found to be fully matured with its microstructure having glossy features without any glaze defects that would affect its reflectance, with a low surface Roughness (Ra) value. The average roughness of the microwave fired glazed surface was found to be less than 0.1um.
Further, the microwave fired glazed alumina surface had no pits or bubbles or other defects. The glazed surface layer has a thickness of up to 1000um.

The various experiments of the microwave firing method described herewith is generally applicable to high temperature glaze and not limited to the particular glaze described above nor is the work piece material restricted to alumina.

Many modifications and other embodiments of the invention may come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

WE CLAIM

1. A method of high temperature glaze-firing comprising the steps of:

(a) coating a glaze composition on a work piece;
(b) microwave heating such that the work piece reaches a temperature in the range of 1000 - 1500°C in a period of 2 - 3 hours;
(c) soaking the coated work piece at a temperature of 1000 -1500°C for a period of 0.5 - 2 hours; and
(d) naturally air cooling the microwave fired glazed work piece inside the housing to form a glaze surface layer on the said work piece.

2. The method as claimed in claim 1, comprising the step of indirect initial microwave heating of the glazed work piece by convection and radiation using microwave susceptor material wherein the susceptor facilitates the initial heating by interacting with the microwave from low temperature.

3. The method as claimed in claim 2, wherein the susceptor material can be oxide-bonded SiC, nitride-bonded SiC, recrystallized SiC, reaction-bonded SiC, sintered SiC or any combination thereof.

4. The method as claimed in claim 3, wherein the SiC-based susceptor is optionally pre-oxidized or coated with an oxide ceramic layer.

5. The method as claimed in claim 2, wherein the microwave susceptor and the work piece are covered by a thermally insulating housing (2) such that it allows microwave radiation to pass through and reach the microwave susceptor inside.

6. The method as claimed in claim 2, wherein the holder arrangement is rotated in front of a microwave source for uniform exposure of the microwave susceptor and, by radiation, uniform heating of the work piece.

7. The method as claimed in claim 1, wherein the glaze composition comprises:

10%to30%ofAI2O3 55% to 75% of Si02 0%to15%ofK2O 0%to10%ofN2O and balance minor amounts of CaO, Ti02 and MgO as glaze forming materials.

8. The method as claimed in claim 7, wherein the glaze composition is mixed with 38% to 45% water and ball milled along with a minor amount of binder and deflocculating agent to disperse the glaze
uniformly.

9. The method as claimed in claim 8, wherein the glaze is coated on the work piece by dipping or spray coating.

10. The method as claimed in claim 7, wherein the glaze forming materials are mixed, milled and calcined to remove the loss-on-ignition to facilitate fast-firing of glazes by microwave heating.

11. The method as claimed in claim 1, wherein the work piece is ceramic.

12. The method as claimed in claim 11, wherein the ceramic is sintered alumina ceramic.

13. The method as claimed in claim 12, wherein the glazed alumina ceramic is further heat-treated at about 1500°C in an inert atmosphere upon metallization for ceramic to metal sealing.

14.The method as claimed in claim 1, wherein the glazed surface layer has a thickness of up to 1000 urn.

15. An apparatus for performing the method as claimed in the preceding claims comprising:

a holder arrangement (20) for holding a work piece (3) surrounded by a microwave susceptor (7), said holder arrangement (20) capable of rotating in front of the microwave input port for uniform exposure to a microwave source, and

a housing made of thermal insulation material (2) surrounding the work piece and the microwave susceptor;

16. The apparatus as claimed in claim 15, wherein it comprises a microwave compatible thermocouple (10) having a tip located at a suitable reference point in the thermal insulation housing (2) and whose output is monitored using a temperature indicator (4) connected to a data acquisition module (5).

17. The apparatus as claimed in claim 16, wherein the thermocouple (10) is covered by a metal outer tube for shielding of microwaves.

18. The apparatus as claimed in claim 15, wherein the microwave source is an oven (1) working at 2.45 GHz or 0.915 GHz frequency.

19. The apparatus as claimed in claim 15, wherein the housing may be additionally covered by glass wool (11).

20. A glazed alumina ceramic obtained by the process claimed in claims 1 to 14 and by the apparatus claimed in claims 15 to 19, said glazed alumina ceramic having a surface roughness less than 0.1 µm.