Materials Letters 27 (1996) 155-159
Microwave sintering of flyash Y. Fang
Intercollege Materials Research Laboratory,
Chen, M.R. Silsbee, D.M. Roy
Received 31 October
State Unirersi@, Unilwsity
1995; accepted 4 November
Class F flyash has been sintered by microwave and conventional processes. The sintering was carried out in a temperature range of 800 to 1000°C. Densities up to 2.2 g/cm’, and diametral tensile strength up to 26 MPa (corresponding diametral compressive strength 78 MPa) were achieved after sintering for IO-20 min. The microwave sintered samples were denser and thus stronger than the samples conventionally sintered at the same temperature for the same time. The sintered bodies are glass-ceramic material, with mullite (AI,Si,O,,) as the major crystalline phase. Kqw~rcis: Coal; Combustion; Flyash; Sintering; Density glass-ceramic composite; Mullite
When coal is burned, 4-25 wt% of it is converted to ash [ 11. This ash, or so-called flyash, is a huge amount waste product of the coal-burning electric power plants. In the United States, for example, more than 70 million tons of flyash is being produced each year. Disposal of flyash takes a lot of space and causes soil contamination, thus is of great concern environmentally. Utilization of such a waste is obviously a significant subject. In chemical composition, flyash is composed of silica, alumina, iron oxide, calcium oxide, etc., mostly in glass state. In morphology, flyash is composed of smooth tiny spherical particles, either solid or hollow. Due to the complex composition, the melting temperature of flyash is relatively low, and the sintering temperature range is narrow. In conventional sintering of the large-size flyash workpiece at high
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heating rates, due to the low thermal conductivity, significant temperature gradient could exist between the surface and the interior during heating, leading to a nonuniform structure and poor quality of the sintered products. Microwave processing has been investigated for sintering of various ceramic materials . Since the heat in the microwave sintering is generated by a microwave-material interaction, volumetric heating can be achieved if the workpiece is within the penetration depth of the microwaves. For the materials of fair dielectric losses, high heating rates, short processing time, and uniform structure are the common features in the microwave heating. Efficient microwave absorption of silica and alumina in amorphous state has been reported [3,4]. It is expected that the existence of SiO,, Al,O,, and Fe,O, et al. in glass state will allow flyash to absorb microwave energy very well, thus enhancing sintering. This paper reports the preliminary results of our study on the microwave sintering of class F flyash.
0 1996 Elsevier Science B.V. All rights reserved
2. Experimental 2. I. Starting material Class F flyash (supplied by the American Fly Ash Co., Des Plaines, IL) as defined in ASTM C 618 was used in this study. The chemical composition data of the flyash are listed in Table I. In sample preparation, a small amount of water was used to wet the flyash before compaction. 1.5 g of flyash was mixed in a mortar with water at the water/solid ratio of 0.1 for each pellet. The circular pellets of 12.7 mm diameter were uniaxially pressed at 350 MPa to achieve a reasonable green strength. The average green density was 1.82 g/cm3 (calculated based on the weight of the ignited sample). The use of water will improve microwave absorption of the samples at low temperatures. 2.2. Sintering Microwave sintering of flyash was carried out in a multimode microwave cavity at 2.45 GHz, which was modified from a 900 W commercial microwave oven (Panasonic) with a turntable. The sintering packet is schematically shown in Fig. 1. Three pellets were placed in the center of a sintering packet in each run. The specimens were vertically surrounded by a porous zirconia cylinder (Zircar Products, Inc., Florida, NY), which was used as a microwave absorber to preheat the specimens, and as a thermal insulator to minimize heat dissipation. The sintering packet was placed on the turntable. The rotation of the turntable during processing ensures all the samples to be subjected to the same irradiation conditions and provides uniform heating. The microwave power input was controlled using a variac controlling system, so that the radiation is continuous and temperature can be well controlled. Temperature in the microwave sintering was measured with a platinum-sheathed S-type (Pt-Pt 1ORh)
Table I Chemical
Fig. I. Configuration of the microwave sintering packet: turntable. (2) rirconia cylinder, (3) sample, (4) thermocouple, Fihermax insulator. and (6) MoSi, heating inserts.
thermocouple, which was inserted from the top of the microwave cavity and properly grounded to the wall of the metallic cavity. In this way, microwave interference to the temperature measurement was completely avoided during the processing, and the temperature display was stable. Conventional sintering was carried out in a lab-assembled fast-heating electric furnace, using molybdenum silicide as heating elements. The heating rates were kept the same as in the corresponding microwave processing (= lOO”C/min) by adjusting the input power. Temperature was also measured with the same S-type thermocouple as used in the microwave sintering, but without platinum sheath. The sample pellets were placed in a platinum crucible which was in contact with the tip of the thermocouple. 2.3. Characterization Density of weighing and tensile strength on a Universal
the sintered pellets was calculated by dimensional measurements. Diametral  of the sintered samples was tested Testing Instrument (Model TTBML,
of starting class F flyash (wt%)
Y. Fang et al./Materials
Letters 27 (1996) 155-159
Instron Corp., Canton, Mass., USA) at a loading rate of 0.2 mm/min, and calculated using 6 = 2 P/TDn, where 6 is the diametral tensile strength, P the applied load at failure, T the thickness of the sample, and D the diameter of the sample. Microstructures of the sintered flyash were studied on a scanning electron microscope (ISI-DS 130 SEM, Akashi Beam Technology Corp.). Phase composition study was performed by powder X-ray diffraction (XRD) using Cu Ko radiation on a Scintag diffractometer (Scintag Inc., Sunnyvale, CA).
3. Results After sintering, the pellets processed at 800°C were earth yellowish, and could be scratched by finger nail, thus were under-sintered. Those processed at 900°C were darker and obviously harder. The pellets processed at 950- 1000°C were wellsintered, reddish-brown, hard, and difficult to scratch even with a steel nail. The trial samples processed at 1100°C were deformed due to large amount of the liquid phase. The densities of the sintered pellets are listed in Table 2. Microwave processing at 800-1000°C for 10 to 20 min produced densities ranging from 1.83 to 2.18, while conventional sintering at the same temperature range from 10 min to 2 h achieved 1.82 to 2.15. The sintered density slightly increased as increasing time and temperature. The densification kinetics was sluggish in the temperature range up to 1000°C because of the low green density and the hollow structure of the large part of the flyash particles. However, higher densities were constantly achieved by microwave processing, indicating that
Table 2 Densities of the flyash compacts Temperature
Fig. 2. Comparison of the diametral tensile strength pellets sintered at 800, 900 and 1000°C for 20 min.
there was certain microwave enhancing effect on the sintering of flyash. That the sample microwave sintered for 20 min was even denser than that achieved by conventional sintering for 2 h is a good example. Although the difference in density between the microwave and conventionally sintered samples seems slight (within 2.5%), that in diametral tensile strength is obvious. For the samples sintered for 10 min, the diametral tensile strength of the microwave sintered sample is 37% and 45% higher than the conventionally sintered samples after sintering at 800 and 900°C respectively. When sintered for 20 min, the enhancement at 900°C was 61%, although the difference at 1000°C was not as significant. Fig. 2 shows the diametral tensile strength of the samples sintered for 20 min at 800, 900 and IOOO”C, respectively. Noticing that the diametral compressive strength will be triple of the diametral tensile strength, the sintered flyash is a reasonably strong light-weight
sintered under different conditions Conventional
I.917 2.01 I
I .895 I.991
I .887 _
Fig. 3. Microstructures of the flyash samples sintered for IO min at 950°C by (A) microwave by (C) microwave and (D) conventional processing.
material. Fig. 3 shows the fracture surface microstructures of the samples sintered for 10 min at 950°C (A, B) and 1000°C (C, D), respectively. The microwave sample sintered better than the conventional sample at 950°C but the difference is not significant at 1000°C. Fig. 4 shows the XRD patterns of the flyash samples sintered at various temperatures. The only detectable crystalline phase in both starting material and the sintered samples was mullite of low crystallinity. There was not significant change in crystallinity, and no new phase appeared, either, in the samples sintered at temperatures up to 1000°C for 20 min. However. the relative diffraction intensity of mullite at 33” 20 (220) increased as increasing temperature, while that at 36” 28 (210)
and (B) conventional
and at IOOO”C
FinC’ 4. X-ray diffraction patterns of flyash sintered by (A) microwave and (B) conventional method at various temperatures. showing peak\ of mullite.
Y. Fang et al./Muterials
slightly decreased. This indicates that a limited preferential crystal growth of mullite took place during sintering. Anorthite appeared in the sample conventionally sintered at 1000°C for 2 h. The broad bump in the background implies that large amount of glass existed, thus the sintered product of flyash was a glass-ceramic material.
4. Summary Experiments have demonstrated that flyash can be sintered into a reasonably strong and relatively porous glass-ceramic material in a short period of time by microwave processing. The microwave sintered samples show higher density and strength than the con-
Letters 27 (19961 155-159
ventionally sintered ones, indicating that the sintering process was enhanced during microwave processing.
References [l] B. Cumpston, F. Shadman and S. Risbud, J. Mater. Sci. 27 (1992) 1781.  W.H. Sutton, Am. Ceram. Sot. Bull. 68 (1989) 376.  R. Roy, S. Komarneni and L.J. Yang, J. Am. Ceram. Sot. 68 (1985) 392.  Y. Fang, J. Cheng, D.K. Agrawal, D.M. Roy and R. Roy, Microwave Processing of Diphasic Aluminosilicate Gel, J. Am. Ceram. Sot., submitted for publication.  A. Rudamich, A.R. Hunter and F.C. Holder, Mater. Res. Stand. April, 1963, p. 283