Title of Invention


Abstract An energy efficient process for sintering commercial grade alumina composition comprising the steps of : - mixing alumina, calcified clay and manganese oxide with total alumina content of about 85% with water in ballmills and spray drying the mixture to form dried powder; - pressing the powder in metal die to form green tiles; and - sintering said green tiles in a microwave furnace, characterized in that the sintering temperature is reduced by 20°C and the total duration of sintering is reduced by 27 hrs as compared to the processes in the prior art. Fig.3
Full Text This invention relates to an energy efficient process for sintering commercial grade alumina composition,
Background Art/Prior Art
Microwaves are a form of electromagnetic energy characterized by mutually perpendicular electrical "E" and magnetic "H" fields. The FCC has reserved 915 MHz and 2456 MHz, among other frequencies, for industrial applications. At present, most microwave differ in their reaction to microwave fields. 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 communications, 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 specialty ceramics, including composites. [H.S. Shulman, M.L. Fall, W.J. Walker, Jr., T.A. Treado, S.J. Evans, M. Marks and M.L. Tracy "Sintering Uniformity and Reproducibility with 2.45 GHz, Microwaves in an Industrial Sized Chamber", presented at the 104 Annual Meeting of the American Ceramic Society' St. Louis, MO, April 28 - May 1, 2002. D. Agarwal, J. Cheng and R. Roy, "Microwave Sintering of Ceramic, Composites and Metal. W.H. Sutton, "Microwave Processing of Ceramic Materials", Am. Cer.Soc. Bull, 67[2] 376-86(1989). J.D. Katz and R.D. Blake, "Microwave Sintering of Multiple Alumina and Composite Components, Am.Ceram. Soc.Bull, 70[8] 1304-08(1991). 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 systems.
Microwave Sintering
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 this 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 electro-magnetic 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. Microwave sintering can be successfully used on range of materials including metal powders. Advantages of the technique include volumetric heating, significantly faster heating rates, lower sintering temperatures, enhanced densification, and smaller average grain sizes basic interaction mechanisms are shown to depend strongly on the dielectric and magnetic properties of the process material.
In the Patent 6172346, a microwave fiimace 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 is disposed in relation to the enclosure to provide at least one of radiant and convective heating within the enclosure is described. 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 to provide a desired thermal profile in the object.
In the Patent 5,736,092, Apte, et al. Microwave Sintering process use 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 refractory parting agent, either dispersed in the susceptor material, or as a coating on the susceptor material.
US 6,172,346, discloses a Method of processing ceramic materials and a microwave furnace, January 9, 2001, Wroe; Fiona Catherine Ruth, EA Technology Limited (Chester, GB).
US 5,736,092 described a Microwave sintering process Apte; Prasad Shrikrishna (St. Albert, CA); Morris; Larry Roy (Yarker, CA). Microwear Corporation (Fort Saskatchewan, CA) June 17, 1996.
Field of Invention
The need to move large volumers of highly abrasive materials, such as coal, grain, sand, and various waste products, place great demands on the piping systems through which they are conveyed. Excessive wear is a common problem, especially evident in the sections of pipe, which change the direction of flow of the abrasive material. Due to their excellent wear characteristics, ceramic materials have become increasingly popular for use in lining pipe casing subjected to high abrasive wear conditions.
Presently, product of high alumina are manufactured by our industry an applied in large pipes in thermal power plant, cement, fertilizer, coal washer etc. They are made by conventional process, which takes nearly 30 hrs through a kiln. In addition the kilns are oil fired, necessitating a relook at an alternate process, which is not oil dependent, and less polluting and yielding products of better quality.
Objective of the Invention
In this study, feasibility of microwave sintering of high alumina body [ R.F. Schiffman, "Commercializing microwave systems: Paths to success of failure", Ceram Trans, 59,7-17(1965)] to realize the short
process time and energy efficiency advantage and improved properties of the body were established. Studies were carried out on 85% commercial grade abrasion resistant alumina body. The effect of sintering temperature on bulk density, water absorption bending strength (MOR), cold compressive strength, hardness, microstructure have been investigated. The effect of sintering temperature on different size and shaped alumina body carried out and have shown comparable with conventional process. The results also generated inputs required for scale-up of microwave processes for production such as the uniformity and reproducibility of material properties for given processing parameters.
The raw materials used in this experiment were alumina (d50 ~ 4 jam), calcified clay and manganese oxide with total alumina content of ~ 85%. The raw materials are mixed in 2 tons ballmills in water and spray dried to from dried powders. The powders were used for pressing of different sizes and shapes in a metal die prior to drying. In conventional processing, a total load of nearly 4 metric ton (MT) body of different sizes were staked and fired in an oil fired kiln. The total process duration is 30 hrs. For microwave processing experiments, green tiles from the production lot were used for sintering in a 6 kW microwave furnace.
Detailed investigations were carried out on bodies of sizes varying up to 160mm (L) x 90 mm (W) x 20 mm (t) using a 6 kW batch processing Microwave sintering oven. Initially 25 mm squares with 5 mm thickness were sintered in "sinterwave" a modified kitchen microwave system (designed and developed by us) to estimate the sintering temperature and densification. The 6 kW batch system fi"om M/s COBER Inc., USA was used for sintering of bodies and test samples which are larger in size. The Microwave system was integrated with a micron 680 IR (infra red) temperature measurement system of 0.68 mm spectral responses and a Eurotherm PID controller 2404 to control the process parameters including ramp rate. The critical components for microwave sintering include an insulation box and susceptors. The insulation box consists of a small chamber fabricated from RATH 17/400 grade vacuum formed low density fiber insulation board of 25 and 50 mm thickness. The caskets were made with an inner diameter (ID) of 170 mm X 150 mm deep and a top cover with a 12 mm hole for IR temperature measurement. The joints were fixed using high temperature cement and finally wrapped with a layer of 1450°C alumina fiber mat and tied with glass fiber tape. SiC plates of size 150 mm length x 100 mm wide and 10 mm thick were slipped on either side of the stack. The impact of susceptor position and the relationship was also significant in getting dimensionally stable and crack fee sintered product.
Different arrangements of susceptors and mass of susceptors in relation to the component were tried to arrive at a combination to avoid warping or cracking of component.
The process was optimized obviating the need for any intermediate soaking for binder burnout. A maximum of 5.5 kW microwave field was used in the test conditions and the time to reach 1550-1600°C was set for 30 minutes. During soaking, the turntable was stopped and temperature variation could be limited to less than 1 degree. Number of test pieces for batch experiments were also repeatedly sintered under different conditions to estimate the optimum sintering parameters, uniformity of sintering and reproducibility of material properties such as hardness, water absoption, Cold Crushing Strength (CCS), Modulus of Rupture (MOR) and Relative Abrasion Resistance (RAJ) to evaluate improvement in properties. X-ray diffraction analysis was also carried out to evaluate the formation of alumina phase. The density and water absorption studies were carried out by water displacement method. The MOR or bending strength was measured on test samples of 12mm diameter and 100 mm length using a Lloyd's universal testing machine and the CCS was measured using the same machine with a 25 mm diameter cylindrical samples.
The microstructural analysis and compositional analysis was carried out on polished and thermally etched samples using a Jeol Scanning Electron Microscope (SEM). The relative abrasion index was measured by comparing the adjusted volume loss of the material during sand abrasion test with that of mild steel using the following formula. The detailed experimental procedure for sand abrasion test following ASTM G65 is described elsewhere [L.N. Satapathy, Development and characterization of a high abrasion resistant alumina-zirconia composite material, Mat.Res.Bull., 34(8), 1233-1241 (1999)].
Results and Discussion
The present invention is described hereinbelow in detail with reference to the accompanying drawings, wherein
Figure 1 shows known 6 kW batch system used for sintering of bodies and test samples which are larger in size;
Figure 2 shows the photograph of typical components in green and fired conditions and demonstrates the arrangement of bodies with susceptor in the insulation box, which is used for sintering inside the microwave fiimace;
Figure 3 shows the variation of bulk density with sintering temperature;
Figure 4 indicates the MOR and CCS values of test samples;
Figure 5 indicates the required Relative Abrasion Index (RAI) of Ceralin test sample;
Figure 6 shows further sintered flat cut pieces; and
Figure 7 shows the scanning electron micrographs of the sintered alumina body.
The results of microwave sintered body are very Interesting considering that the basic raw materials are commercial grade materials and not as experimented usually in laboratories with laboratory grade materials.
The variation of bulk density with sintering temperature is given in Fig.3. It was observed that a minimum sintering temperature of 1550°C is required to achieve the comparable bulk density of 3.2 g/cc as obtained by conventional methods by sintering at 1580°C for four hours. Further increase in temperature improved the density and remained almost constant afterwards. However, the water absorption was slightly on higher side and could be reduced to less than 0.5% when sintered in microwave at 1560°C for one hour. This result indicated that using microwave firing,
the sintering temperature can be reduced by 20°C and soaking period by 3 h compared to conventional processing. Further, the total duration of the microwave experiment was ~ 3 h and thus is highly beneficial since this is nearly one order of magnitude lower than that used in a conventional firing.
The MOR and CCS values of test samples (Fig.4) indicated that both the parameters were increased with increase in sintering temperature and as observed earlier for density, a sintering temperature of 1560°C was found to be optimum. The mechanical properties were better for the materials sintered at a higher temperature of 1580°C. However, the Vicker's hardness experiment using 49 N load indicated that the hardness was highest for the 1560°C sintered sample.
The required Relative Abrasion Index (RAI) of the Ceralin test sample of > 4 could even be achieved for the samples sintered at 1500°C (Fig.5). This result indicates that uniform heating of the material has taken place during microwave sintering resulting in uniform hardness (78 mon) throughout the material when sintered at particular temperature. This results in low mass loss during sand abrasion resulting in increase in RAI value.
This explanation is in agreement with the inside out heating phenomenon during microwave treatment in contrast to the outside in heating during conventional process wherein during intermediate sintering like at 1500°C, the surface is harder than inside resulting in more mass loss during sand abrasion test. The RAI further increased with increase in sintering temperature and reached a maximum for the sample sintered at 1580°C. Table 1 compares the results of test samples, which were sintered
(Table Removed)

Table 1
Comparison of properties of microwave processed samples with those obtained by conventional processing.
In the second part of the experiment, the actual sizes of the product were sintered in a 6 kW furnace. Both rectangular flat and rectangular curved product were used in the sintering experiments. The density and water absorption of the product sintered at different conditions are presented in Table 2. While the sintering sample was carried out, it was realized that the samples are not getting sintered properly at a temperature lower than 1580°C. This is an important observation and related to the size effect of the components. Due to this reason, all further experiments were carried out at a sintering temperature of either at 1580°C or at 1600°C. However, soaking at different temperatures has been performed to check the densification of the components.

(Table Removed)
Rect Curve Tiles, int. soak to avoid cracks
(Table Removed)
Table 2
Effect of sintering condition on bulk density and absorption on component.
The bulk density of two different shaped products of three different dimensions could be maintained at > 3.3 g/cc and there was no significant effect observed with intermediate soaking of the components. However, the water absorption could be reduced significantly for the material sintered at 1580°C to the material sintered at 1600°C. Similar to bulk density, no appreciable improvement was observed in water absorption value with intermediate soaking except reduction in variation of the values. Further one particular sintered flat tile was cut into six pieces (Fig.6) and the density of each pieces indicated that fairly uniform density could be obtained throughout the tile during microwave heating (Table 3). This observation confirms the homogeneity of properties in a large component during microwave heating. The variation of temperature on the surface
and interior has been measured recently by Mizuno et al. [ M. Mizuno, S. Obata, S. Takayama, S. Ito, N. Kato, T. Hirai and M. Sato, Sintering of alumina by 2.45 GHz microwave heating, j.Eur. Ceram. Soc, 24(2), 387-391 (2004) ] using the isothermal barrier concept for microwave sintering alumina. The authors noted that though large temperature difference existed at temperatures near 1500°C, it become smaller as the temperature reached 1600°C.
(Table Removed)
Table 3
Bulk density and water absorption values of all the six segments of the same body.
The scanning electron micrographs (Fig.7) of the alumina body which was sintered at 1580°C for one hour under microwave field is compared with that of conventionally processed 1580°C/4 h. There was no appreciable difference in the microstructure except the grain sizes. Both the microstructures revealed a mixture of elongated and round grains. Both the microstructures revealed no porosity in the thermally etched polished surface.
Summary of present Invention
The microwave assisted densification studies of a commercial grade alumina composition has been studied and the properties of the test pieces have been compared with those of actual product and the conventionally processed samples. It was observed that though there is no reduction in sintering temperature in both microwave and conventional methods, the soaking period can be deduced by 3 h. Further, significant energy can be saved by reducing the total duration of the sintering by an order of magnitude fi-om 30 h to ~ 3h. The relative abrasion index of the material sintered in microwave field could be achieved even at a lower temperature of 1500°C. The density variation with in an actual product could be
maintained uniformly during microwave processing. The hardness was highest for the 1560°C sintered sample the mechanical properties were at 1580°C; over all the properties are comparable in the conventional products. Further work is in progress to establish the optimum parameters for microwave sintering these product which can be translated in a commercial production.

1. An energy efficient process for sintering commercial grade alumina
composition comprising the steps of:
- mixing alumina, calcified clay and manganese oxide with total alumina content of about 85% with water in ballmills and spray drying the mixture to form dried powder;
- pressing the powder in metal die to form green tiles; and
- sintering said green tiles in a microwave furnace,
characterized in that the sintering temperature is reduced by 20°C and the total duration of sintering is reduced by 27 hrs as compared to the processes in the prior art.
2. The process as claimed in claim 1, wherein the microwave furnace is rated at 6 kw.
3. The process as claimed in claim 1, wherein the microwave furnace is set at a maximum of 5.5 kw of microwave field.
4. The process as claimed in claim l,wherein the soaking period is reduced by 3hrs as compared to the processes in the prior art.






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Patent Number 239723
Indian Patent Application Number 259/DEL/2004
PG Journal Number 15/2010
Publication Date 09-Apr-2010
Grant Date 30-Mar-2010
Date of Filing 23-Feb-2004
Applicant Address BHEL HOUSE, SIRI FORT, NEW DELHI-110 049, INDIA
# Inventor's Name Inventor's Address
PCT International Classification Number F27B 21/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA