FIELD OF THE INVENTION
The present invention relates to a method of developing a pink alumina component which is wear resistant and can be processed by a very simple fabrication method and can be fired in large scale using microwave sintering technique. More particularly, the invention relates to a method of producing microwave processed pink alumina component which forms a part of electrode for water level indicator for Boilers in power plants.
BACKGROUND OF THE INVENTION
The failure to detect low water levels in boilers in a thermal power plant may lead to a potentially disastrous consequence. Reliable water level detection is vital to prevent damage to the plant and personnel. The electrode for water level indicator requires a small insulating component which is made up of alumina typically pink in color. The main properties of such component is high insulation resistance, inert to corrosion and wear resistance in the hot boiler environment. The components known in the art for such applications do not always satisfy the requirement of long life in boiler environment. Hence, a need exists to develop an appropriate material and component using a simple process.

Alumina ceramic is most matured amongst the engineering ceramics, offering excellent electrical insulation properties together with high hardness and good wear resistance but relatively low strength and fracture toughness. Alumina Ceramics are generally white but can be made to many colors by addition of transition metal compounds in appropriate quantity. One such example is a known product which is Brown in color due to addition of Manganese. An international publication by BHEL in 1999 described the development of different colors in alumina with addition of different coloring oxides.
Alumina is an electrically insulating material, with a high electrical resistivity, increasing with purity. Good chemical stability of alumina leads to high corrosion resistance. It is insoluble in water and only slightly soluble in strong acid and alkaline solutions. Testing has shown that the low chemical solubility of alumina ceramics components makes them highly resistant to chemical corrosion. This is what makes high purity alumina the material of choice for components in a wide range of industrial applications.
Pink Fused Aluminum Oxide contains chromium oxide, which gives the material its pink color. The incorporation of the Cr203 into the Al203 crystal lattice produces a slight increase in toughness and a reduced friability compared with white fused Alumina.

Compared to Brown Regular Aluminum Oxide the Pink material is harder, more aggressive and has better cutting ability. The grain shape of Pink Aluminum Oxide is sharp and angular. The amount of chromium oxide added affects the colour of the final product, as well as the toughness of the grain. With increased addition of chromium oxide up to a limit, the toughness of the grain increases. However, the use of pink alumina is mostly limited to applications like grinding wheels, coated abrasives, blasting abrasives etc. The use of such materials for advanced ceramic applications are not very common. This may be due to the additional processing steps involved and use of advanced fabrication techniques like isostatic pressing and sintering the components at very high temperature.
United States Patent 5256611 described an abrasion resistant a-AI2 O3 -based colored sintered corundum with an essentially transparent corundum matrix containing metal oxides integrally formed therewith in the sinter mass and homogeneously distributed therein and imparting intense color thereto, the metal oxides being inorganic colored pigments and the quantity of said metal oxides, expressed as metal ions, being 0.1 to 30% by weight based on the Al2 03 matrix. A colored corundum has been thus obtained by the sol-gel process. This process is however difficult to upscale for large batch production.
Indian Patent 217563 (2003) described an improved process for hydrothermal synthesis of boehmite, a-alumina or their mixtures in presence of aqueous solution salts Cr203 for chromium doping, MnC03 for manganese doping and magnesium in the presence of magnesium formate or acetate salts or in the form of Al - Mg alloy for Mg doping in the range from 1-30 wt.% of alumina in an autoclave at temperatures in the range of 300-550°, pressure in the range of

15-20 MPa, cooling an autoclave to room temperature, separating the unreacted solid from the product powders by removing physically, washing and drying the said powders in air or in vacuum to obtain free flowing powders of boehmite„a-alumina or their mixtures; heating the said free flowing powders in the temperature range of 1250-1450°C for 2 to 4 hrs. for homogeneous distribution of dopants to obtain desired products. This process very complex and scale up is difficult to produce colored alumina.
US 4204874 A described a process of manufacturing light-transmitting polycrystalline alumina body using chrome oxide, magnesium oxide and strontium oxide in high-purity alumina powder. The materials are mixed in a ball mill or, if the amount is small, in a beaker to provide a uniform composition. The uniform composition can be more easily prepared is a small amount of the raw material is pulverized before hand in a mortar together with a portion of the alumina, followed by blending in and mixing the remaining portion of the alumina. This is followed by pressing the mixture to form a compact, prefiring the compact at a temperature at about 1100°C in an oxygen containing atmosphere, and firing the prefired compact at a temperature of about 1650 deg °C consisting of vacuum and hydrogen. This method requires vacuum and hydrogen based sintering which is very expensive.
An alumina-based opaque ceramic is also known which is similar to ruby and having a high toughness. This ceramic comprises, by weight: 0.4% to 5% of at least from one oxide of a metal chosen from chromium, cobalt, nickel, manganese, vanadium, titanium and iron; 0.00080 to 0.5% of magnesium oxide;

and 0.05% to 6% of at least one oxide of an element of the group of rare earths. The ceramic is applicable in particular in jewellery, fine jewellery and watch making. This invention is limited to applications in jewellery which emphasizes mainly on optical properties.
Ruby is the red variety of the corundum family which consists of alumina having a particular crystalline structure. The red coloration of ruby is due to the presence of chromium in the corundum. Polycrystalline rubies exist commercially which are obtained by mixing alumina and chromium oxide and a small amount of MgO as a sintering additive, by subsequently forming the mixture and by sintering it under a hydrogen atmosphere or under a vacuum of at least 10-1 Torr. The polycrystalline rubies obtained by these routes have, however unsatisfactory mechanical characteristics due to a low homogeneity of the microstructures and to a grain size that is much too high since it often exceeds 10 microns. Furthermore, their translucent appearance is not desirable in certain scenarios where it would be preferable to be able to use a bulk-colored and opaque ceramic of high toughness.
Japanese Patent Application No. JP 56-140071 describes a process for manufacturing alumina-based reddish-purple ceramics. These ceramics are also translucent, they contain chromium oxide, magnesium oxide, lanthanum oxide and yttrium oxide. The preparation thereof comprises sintering under a hydrogen atmosphere. This application does not include any exemplary embodiment.

Japanese Patent Application No. JP 04-193760 describes colored alumina-based materials. These materials are translucent. These materials may contain up to 2% of chromium oxide, cobalt oxide, nickel oxide, vanadium oxide, manganese oxide, iron oxide or titanium oxide, and also at least one rare-earth oxide chosen from praseodymium, neodymium and erbium oxides. Magnesium oxide may also be added as an agent that inhibits the growth of particles during the sintering. However, the amount of this oxide to be added is not indicated. The preparation of these materials includes a sintering step which may take place in air, at a temperature between 1300 and 1800°C. This high-temperature sintering step must necessarily be followed by hot isostatic pressing under a pressure between 500 and 2000 atmospheres and at a temperature of 1400°C. The examples from this patent application describe the preparation of light green, light pink, light blue, very light green and very light blue materials. These three patents used expensive methods like Hydrogen atmosphere sintering or HIP, which are difficult to upscale.
Thus, none of the prior art documents relates to the manufacture of a pink alumina-based ceramic which will be dense, wear resistant and can be used as an engineering component. This invention thus will focus on the development of a dense pink alumina processed by a very simple process and fired using cost effective microwave energy.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of producing a pink alumina composition which can be made by a simple fabrication process and made dense for use as an engineering component.

Another object of the present invention is to demonstrate the sinterability of the said component by the cost effective Microwave assisted firing method in large scale.
A still another object of the present invention is to characterize the component for material properties and use as a component in the Electrode for water level indicators in boilers.
SUMMARY OF THE INVENTION
According to the present invention, a method is disclosed for producing a pink alumina based composition which can be fabricated as a component by simple die pressing method eliminating costly and complicated fabrication methods like isostatic pressing, sol gel technology etc. The present invention also relates to the development of a microwave assisted firing method for reducing cycle time compared to the conventional process and making the compound dense without having significant water absorption. The final embodiment of the present invention is to study in detail the properties in general and abrasion and erosion properties in particular of such developed product and use the component of the electrode for water level indicators in Boilers.

BRIEF DESCRIPTION OF THF ACCOMPANYING DRAWINGS
Figure 1 depicts the X-ray diffraction patterns of Pink alumina of different compositions. Major phase is alpha alumina with small amount of second phase
mullite due to the reaction between alumina and silica from the glassy phase of sintering aid. No phase related to Chromium compound was noticed.
Figure 2 is the scanning electron micrograph of Pink 2.0 demonstrating liquid phase sintering during microwave processing.
Figure 3 demonstrates thermal expansion behavior of pink alumina over a temperature range of RT-1000 °C.
Figure 4 depicts a view of large numbers of pink alumina tubes after firing in a microwave furnace of 1600 °C.
Figure 5 demonstrates the application of pink alumina component in the electrode for water level indicator for Industrial Boilers in thermal Power plants.
Table 1 demonstrates the different properties of the final product obtained in this invention.


B.D Bulk Density, WA: Water absorption. HV5: Vicker's harness @5 Kg load.MOE:
Modulus of Elasticity.
AVL: Adjusted Volume loss following ASTM G 65, AEV: Average Erosion volume
following ASTM G 76.
Table 1: Summary of properties of Pink alumina developed in this study.

DETAILED DESCRIPTION OF THE INVENTION
The present invention is focused on developing a cost effective pink alumina composition with high alumina content and wear resistant in nature. The invention also focused on the development of a simple fabrication technique to form component required for electrode sensor in boilers. A cost effective microwave processing method was also developed to heat treat these components in bulk to achieve the desired properties.
The selection of raw material plays an important role in deciding the final properties of the component. The particle size, purity of material, green density after fabrication into suitable shapes etc. are important factors for achieving full density of the component with good properties. In the present invention, a high alumina composition was developed by using a commercial spray dried alumina powder and added with different concentrations of a transition metal oxide additive such as a chromium compound. The mixed powder was used for fabricating a simple cylindrical component using uniaxial pressing. This method has resulted in very good green density of the component thus avoiding complicated and costly fabrication technique like cold isostatic pressing, which is generally used for fabricating such components.
The sintering of such components is generally carried out at a temperature of 1600 deg C in a conventional heating. The process takes long time and not energy efficient. In this invention, a microwave assisted firing has been established which brings down the heating cycle time from 16h to 6h and can

produce in large numbers. It has been demonstrated that in a small 6 kW microwave furnace, more than 200 such components can be sintered in a heating cycle of 6h. This results in energy conservation and due to volumetric heating nature of microwave sintering, the components are fully dense and attains required mechanical properties.
In a typical process, chromium oxide or any chromium water soluble compound is added to alumina with normal alpha alumina content exceeding 95 % and the additive content not exceeding 5 %. Historically, chromia-alumina refractories with high chromia content have relatively low strength. The strength of alumina-chromia system reduces with increase in chromia content. Chromia forms a complete solid solution with alumina. In the present work, the chromia has been limited to 5 wt. % in the alumina. The typical initial particle size of the alumina is 2-2.5 micron and the chromium compound is of lab reagent grade. The composition is dry mixed in a pot mill and sieved. The powder is then used to make cylinders of approx. 25 mm long and 10 mm dia by simple uniaxial pressing using a hardened metal die. The pressed component could achieve green density in the range of 55-58 % of the theoretical density. The pressed components were dried for 4-6h in an electrical oven at 110 deg C followed by sintering in batches of 150-200 numbers in a 6kW microwave furnace. The insulation assembly is planned in such a manner that, isothermal heating of all components simultaneously can be achieved during microwave sintering, this method eliminates temperature gradient and results in fully dense compound.

The pale green color of the component after pressing changed to pink color after sintering. The varied content of the additive in alumina changes the intensity of pink color in the sintered alumina body. The physical properties like bulk density, water absorption, thermal property like Thermal expansion, microstructure by scanning electron micrograph and mechanical properties like hardness and modulus of elasticity (MOE), abrasion volume loss in mm3 following ASTM G 65 standard and erosion resistance followed by ASTM G 76 standard and reported as average erosion volume (AEV) expressed as mm3/g were measured for all the compositions. Finally, a suitable composition was finalized for making component which yielded best results.
The present invention can better be explained with few suitable examples.
Example 1:
A batch consisting of alumina of 99.5 g of a composition containing approximately 96 % alpha alumina with other alumino silicate additives and 0.5 g of chromium oxide were mixed in a pot mill for 4-6h followed by addition of organic binder and mixed again for 15 min. The mixed powder was used for fabrication of cylindrical components using uniaxial pressing with a suitable die and sintered in a microwave furnace at a temperature of 1600 °C for 2h. The sintered product yielded results as follows: bulk density: 3.82 g/cc, water absorption: 0.06 % and adjusted abrasion volume loss (AVL) following ASTM G 65 standard of 4.5 mm3, average erosion volume loss (AEV) following ASTM G 76 standard of 0.012 mm3/g , Vickers hardness in the range of 1600 Kg/mm2 and MOE of 413 GPa.

Example 2
A batch consisting of alumina of 99 g of a composition containing approximately 96 % alpha alumina with other alumino silicate additives and 1.0 g of chromium oxide were mixed in a pot mill for 4-6h followed by addition of organic binder and mixed again for 15 min. The mixed powder was used for fabrication of cylindrical components using uniaxial pressing with a suitable die and sintered in a microwave furnace at a temperature of 1600 °C for 2h. The sintered product yielded results as follows: bulk density: 3.82 g/cc, water absorption 0.05 % and AVL OF 2.9 mm3, AEV of 0.011 mm3/g, Vickers hardness of 1520 Kg/mm2 and MOE of 368 GPa.
Example 3
A batch consisting of alumina of 98 g of a composition containing approximately 96 % alpha alumina with other alumino silicate additives and 2.0 g of chromium oxide were mixed in a pot mill for 4-6h followed by addition of organic binder and mixed again for 15 min. The mixed powder was used for fabrication of cylindrical components using uniaxial pressing with a suitable die and sintered in a microwave furnace at a temperature pf 1600 °C for 2h. The sintered product yielded results as follows: bulk density: 3.83 g/cc, water absorption: 0.08 % and AVL of 4.7 mm3, AEV of 0.010 mm3/g, Vickers hardness of 1510 Kg/mm2 and MOE of 360 GPa.

Example 4
A batch consisting of alumina of 95 g of a composition containing approximately 96 % alpha alumina with other alumino silicate additives and 5.0 g each of chromium oxide were mixed in a pot mill for 4-6h followed by addition of organic binder and mixed again for 15 min. The mixed powder was used for fabrication of cylindrical components using uniaxial pressing with a suitable die and sintered in a microwave furnace at a temperature of 1600 °C for 2h. The sintered product yielded results as follows: bulk density: 3.62 g/cc, water absorption: 01 % and AVL of 7.3 mm3, AEV of 0.032 mm3/g Vickers hardness of 1340 Kg/mm2 and MOE of 337 GPa.

WE CLAIM :
1. A method of producing a pink alumina component for electrode for Electronic water level Indicator used in Boilers in thermal Power plants, comprising :
a) producing a composition with alumina as the main raw material of average particle size 2 + 0.2 un;
b) mixing the said ingredients in a pot mill;
c) pressing the moist powder in the form of hollow cylinders by a hydraulic press followed by oven drying;
d) sintering the tiles in a microwave furnace to obtain a pink alumina component with suitable properties required for the application.

2. A method of producing pink alumina component as claimed in claim 1, wherein the component is made fully dense at 1600 °C with bulk density >3.8 g/cc and water absorption < 0.08 %.
3. A method of producing pink alumina component wherein the micro-wave assisted firing is used to heat treat the composition.

4. A method of producing pink alumina component as claimed in claim 1, wherein the heating cycle of the component can be maintained at 6h compared to > 15h used in conventional firing.
5. A method of producing pink alumina component as claimed in claim 1, wherein, the least sand abrasion volume loss due to abrasion is 2.9 mm3 for 1% additive as measured following ASTM G 65 standard.
6. A method of producing pink alumina component as claimed in claim 1,
wherein, the least average erosion volume loss is 0.011 mm3/g for 1%
additive as measured following ASTM G 76 standard.
7. A method of producing pink alumina component as claimed in claim 1, wherein, the Vicker's hardness of the best composition is 1600 Kgs/mm3 and the elastic modulus is 413 GPa.
8. A method of producing pink alumina component as claimed in claim 1, wherein the permissible range of additive content is 0.5-2 %.

9. A method as claimed in claim 1, wherein the main raw material is a composition with alumina content > 95 % and a transition metal oxide additive such as chromium oxide in different proportion