Applications of Microwave Heating to Ceramics

© G. Whittaker, 1994 & 2007. This work, or extracts from this work may be reproduced only with the written permission of the author.

Ceramic Materials

 As with the microwave processing of polymers, the microwave processing of ceramic materials has reached a high degree of maturity. Early work in the field was principally concerned with the removal of solvents from the solid samples,^93-96 making use of their dielectric loss properties. It is estimated that below ~5% water content, the microwave drying of materials is more energy efficient than conventional drying methods. Above this level it seems that a hybrid system, incorporating both methods at different stages in the process is most efficient.⁹⁷ In many cases, microwaves offer superior drying properties to those of thermal techniques, since the power may be more evenly distributed through the sample, leading to more even drying profiles;

 Once dry, however, it was apparent that many of the compounds were themselves capable of effective dielectric heating.⁹⁸ Furthermore, rapid progress was made and the strong dielectric loss properties of a large number of oxides,^99,100 sulphides,¹⁰¹ and glass ceramics¹⁰² was soon recognised. By the use of a screened thermocouple (see Chapter 3) the heating characteristics of over one hundred and fifty compounds were measured,^{103, 104} representative examples of these data¹⁰⁵ are shown in Table 1. It is of particular interest to the author that the metal samples - aluminium and nickel - heat rapidly under these conditions. The rate of heating in these samples is the consequence of a number of factors, and has been discussed in a number of his publications

A* B* Material Temp/oC Time/Min Material Temp/oC Time/Min Al 577 6 CaO 83 30 C 1283 1 CeO2 99 30 Co2O3 1290 3 CuO 701 0.5 CuCl 619 13 Fe2O3 88 30 FeCl3 41 4 Fe3O4 510 2 MnCl2 53 1.75 La2O3 107 30 NaCl 83 7 MnO2 321 30 Ni 384 1 PbO2 182 7 NiO 1305 6.25 Pb3O4 122 30 SbCl3 224 1.75 SnO 102 30 SnCl2 476 2 TiO2 122 30 SnCl4 49 8 V2O5 701 9 ZnCl2 609 7 WO3 532 0.5

^*A - 25g samples , 1kW irradiation with a 1L vented water load ; B - 5-6g samples , 500W irradiation

 Table 2Heating characteristics of some solids under microwave irradiation

 The single most important and widely investigated application of microwaves in ceramic processing has been in the area of sintering. Initial studies¹⁰⁶ were carried out using a 400W microwave tuned waveguide applicator to effect the sintering of alumina and silica rods at >1700^oC. Since then, a wide range of materials have been processed in this way, Table 2 summarises a selection of the work which has been published.

Material Dimensions Methodc Temp/oC Time/sec Density/% Ref. Al2O3 pellet M 1500 - 95 107 Al2O3+ 0.1 MgO 100g pellet M28 1200 3600 98.2 108 Al2O3+ 0.1 MgO 10mm pellet M60 1450 320 77 109 2ZrO2 + Y2O3 pellet M - - 93.3 110 SrTiO3 pellet M 1300 430 98 107 B4C 10mm cylinder M 2000 450 95 111 SiC+0.6B+4C 4.5mmrod M 2200 - - 112 Al2O3+ 20%YSZa 12mm pellet M 1650 3200 >99 113 Ba ferrites 13mm discs M 1230 2700 - 114 YBa2Cu3Ox pellets M 960 36000 >95 115 TiB2+ 3%CrB2 pellet M 2100 1800 98 116 Y-TZPb rod S 1450 ~1000 98.5 117

^a Yttria stabilised zirconia; ^b Yttria stabilised tetragonal zirconia polycrystals; ^c M =multimode cavity, S=single mode cavity, Superscript indicates frequency (GHz) if other than 2.45GHz.

 Table 2 Sintering of ceramics under microwave irradiation

 Heating at 2.45GHz may lead to inhomogeneous heating in some volumes of the material during the sintering process. In the microwave sintering of calcium vanadium garnet ferrites, for example, temperatures 100&#176C above the surface temperature were registered in the sample core.¹¹⁸ A similar result in the microwave treatment of Y-TZP/Y-TZP - Al₂O₃ samples led to the over-sintering of the sample core.¹¹⁷ When sample homogeneity is of paramount importance, this is clearly undesirable. For this reason, higher frequencies of 28GHz, may be used to smooth out these inhomogeneities.¹¹⁹ Because the wavelength ([lambda]~1cm) in such work is smaller than the sample, it was reasoned that the high field regions leading to inhomogeneous heating would be closer to one another. Ideally, heat conduction from these points through the sample would be rapid enough to give more even sintering.

 Often, it is not sufficient to heat only the sample, and the importance of using 'caskets' has been studied.¹²⁰ Containing the sample in a close-fitting material (the casket) with suitable properties allows high temperatures to be reached rapidly, and maintained, and improves the quality of the sintered product. The casket materials must be chemically inert, and may be either microwave absorbing or transparent. The role of transparent materials, for example boron nitride, is simply to act as insulation, retaining the microwave energy which has been converted to heat by the sample. Where the sample has poor dielectric properties at room temperature, such as are observed with alumina, a microwave absorbent material such as zirconia or hafnia may be used to initiate the heating. The advantage of zirconia in particular is that its dielectric heating properties fall off at >=1000^oC and allow direct microwave heating of the sample.

 It has been noted that materials sintered by microwave heating display markedly higher densities for a given reaction temperature and time than do conventionally treated samples, the effect being most evident at low temperatures/sintering densities.¹¹³ For a sample of alumina under 700W irradiation, a particular density may be reached at 100^oC lower than that required by conventional heating (Figure 1). In other systems, differences as high as 300-400^oC have been reported.¹²¹ From studies of the rate of densification, it has been suggested that these differences arise from a reduction in the activation energy required for grain boundary diffusion under microwave heating, although a precise mechanism has not yet been fully developed.

 Despite the technological success of microwave sintering, economic analyses suggest that it does not yet offer energy savings.¹²² Given that the energy efficiency for conversion of natural gas to microwave power may be as low as 15%, unless microwave sintering can offer improved products or process simplification, it may remain uneconomic for most sintering applications.

Figure 1 Microwave and conventional sintering densities as a function of temperature¹¹³

 Going beyond sintering, ceramic materials have been successfully melted using microwave heating. During the microwave drying of oxide samples, it was noted that UO₂ could be successfully heated to its melting point of >2600&#176C using microwaves.⁹⁹ Many other examples of melting have been reported.^{98, 123-125}

 Microwave joining has been achieved using a number of ceramic materials. Using a single mode cavity, Palaith et al .¹²⁶ butt welded two mullite rods together at 1300&#176C in approximately 10 minutes. Similarly, joining has been reported using either Al₂O₃ or Si₃N₄, the latter in an inert atmosphere.¹²⁷ Interestingly, although it was possible join impure samples (92% and 96% purity respectively) of the ceramics to themselves directly, the pure materials could not be joined without the presence of a thin layer of impure material in the join. This illustrates the importance of impurities in the low temperature microwave heating properties of these compounds (cf. Tan d for 97% pure and 99+% pure alumina in Figure 1.5).

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