Microwave hybrid fast sintering of porcelain bodies

Journal of Materials Processing Technology 190 (2007) 223–229

Microwave hybrid fast sintering of porcelain bodies Romualdo R. Menezes a,∗ , Pollyane M. Souto b , Ruth. H.G.A. Kiminami b a

Laboratory of Materials Engineering, Department of Materials Engineering, Federal University of Campina Grande, 58109-970 Campina Grande, PB, Brazil b Ceramic Materials Synthesis and Processing Laboratory, Department of Materials Engineering, Federal University of S˜ao Carlos, 13565-905 S˜ao Carlos, SP, Brazil Received 29 November 2006; received in revised form 8 February 2007; accepted 22 February 2007

Abstract Microwave heating offers many advantages over conventional heating methods, such as saving energy, very rapid heating rates and considerably reduced processing times. However, few studies have used microwave energy to sinter traditional ceramics. Thus, the aim of this work is microwave hybrid fast sintering of porcelain bodies. Bodies of sanitary ware, dental and electrical porcelain were sintered quickly. The control of the heating cycle was the main factor in achieving success in microwave hybrid fast sintering of porcelain bodies. Heating cycles lower than 60 min with soaking times ranging from 8 to 19 min were used for the sintering of the porcelain bodies. The modulus of rupture of the microwave-sintered sanitary ware and electrical porcelain bodies were similar to those of conventionally sintered samples, despite the short sintering cycle. The modulus of rupture of the microwave-sintered dental porcelain bodies was higher than those of the conventional sintered samples. © 2007 Elsevier B.V. All rights reserved. Keywords: Microwave; Porcelain; Sintering; Ceramic; Processing

1. Introduction Processing ceramic materials using microwave heating is a relatively new and exciting technological development. Microwave heating has been used successfully to sinter a wide variety of oxide and non-oxide ceramics, composites and glasses [1]. It differs from the conventional heating processes because the use of microwaves facilitates the transfer of energy directly into the materials, providing volumetric heating [2]. The direct deposition of energy in the bulk of a material eliminates the need for wasting energy by simultaneous heating of furnace or reactor walls or of other massive components and heat carriers and also offers the possibility of applying high heat-up rates [3]. Hence, microwave methods drastically reduce the energy consumption, particularly when compared with high temperature processes where heat losses increase drastically with rising process temperatures, and it is possible to reduce the time required to complete a process.

∗

Corresponding author. Tel.: +55 83 3310 1183; fax: +55 83 3310 1178. E-mail addresses: [email protected] (R.R. Menezes), [email protected] (P.M. Souto), [email protected] (Ruth.H.G.A. Kiminami). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.02.041

The particular requirements of sintering ceramic powders make this process one of the most challenging applications for microwave processing. These requirements often include some or all of the following: high temperature, high heating rates, uniform temperature, and equivalent thermal history throughout the specimen. When sintering ceramic materials by direct microwave heating, various fundamental problems are usually encountered [4]. Firstly, most research on ceramic processing by microwaves to date is based on conventional low-frequency (2.45 GHz) microwave applicators; however, such applicators do not couple microwave power efficiently to many ceramics at room temperature and poor microwave absorption characteristics make initial heating difficult [3,4]. Secondly, thermal instabilities may occur, which can lead to the phenomenon of temperature runaway [4–6]; i.e., the specimen overheats catastrophically. This occurs for a wide variety of ceramic materials, including Al2 O3 , SiO2 , Fe3 O4 , ␤-alumina, ZrO2 , etc. [4,7–9]. Finally, the inherent temperature gradients present during volumetric heating can lead to severe temperature non-uniformities, which at high heating-up rates may cause non-uniform properties and cracking [10,11]. In many sintering experiments insulation has been used to minimize these gradients. However, the use of insulation can seriously aggravate temperature runaway [4]. These problems have led researchers [8,12–14]

224

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229

Table 1 Composition of the formulations used in this work (wt.%) Raw materials

Dental porcelain

Electrical porcelain

Sanitary ware

Quartz Ball clay Feldspar

10.8 9.7 79.5

21.5 38.5 40.0

10.8 49.2 40.0

to develop hybrid heating techniques, which combine direct microwave heating with infrared heat sources. The hybrid heating system will heat the sample more readily at low temperatures and at high temperatures will flatten out the temperature profile inside the ceramic body [7,15]. Various studies [1,16–24] have demonstrated the efficiency of the hybrid heating for sintering a wide variety of ceramic materials, however no work on the microwave hybrid sintering of porcelain bodies has been observed. Some authors [25–27] have analyzed microwave sintering of porcelain bodies, but they applied higher frequencies (84 GHz) or special blanket arrangements to achieve total control of the sintering process and, thus, obtain perfectly sintered porcelain bodies. However, optimizing the sintering system involves a complicated process and the final heating cycles used were longer than three hours. Thus, the aim of this work is microwave hybrid fast sintering of porcelain bodies.

Fig. 1. Assembly to sinter the porcelain bodies: (1) mode stirrer, (2) insulator, (3) susceptor material, (4) samples, (5) alumina support, (6) turntable, (7) thermocouple.

2. Experimental Three formulations of the clay-quartz-feldspar system were used. They were obtained from the literature [28] and were designated as sanitary ware, dental and electrical. Table 1 presents the proportions of starting materials in the investigated formulations. The commercial raw materials used in this investigation were: ball clay (Tyle Mesh 200, Armil Minerios, Brazil—D50 ≈ 1.5 ␮m, D10 ≈ 0.8 ␮m, D90 ≈ 9.5 ␮m), quartz (Tyle Mesh 200, Armil Minerios, Brazil—D50 ≈ 17.3 ␮m, D10 ≈ 2.1 ␮m, D90 ≈ 53.4 ␮m) and feldspar (Tyle Mesh 200, Armil Minerios, Brazil—D50 ≈ 16.5 ␮m, D10 ≈ 1.8 ␮m, D90 ≈ 54.1 ␮m). Table 2 presents the chemical composition of the raw materials, determined by wet process. The raw materials were mixed by ball milling for 4 h in an aqueous medium and then spray dried. Samples of 35 mm × 5 mm and ≈5 mm thickness were uniaxially pressed under 65 MPa and sintered in a conventional furnace and in a microwave furnace (multimode cavity) at 2.45 GHz (Cober Electronics, MS6K). To initiate the microwave heating and minimize thermal gradients, susceptor materials, developed in Ceramic Materials Synthesis and Processing Laboratory of the Federal University of S˜ao Carlos [29], were used as auxiliary heating elements (hybrid heating). Fig. 1 shows the assembly to sinter the porcelain bodies. A heating rate of 5 ◦ C/min and soaking times of 30 and 60 min were applied for the conventional sintering. Sintering cycles with two heating rates were used for the microwave sintering, the initial heating of approximately 65 ◦ C/min up to 1000 ◦ C, and the second of approximately 25 ◦ C/min up to the sintering temperature. The decrease of the heating rate at high temperatures aims to avoid

Fig. 2. Sintering curves of microwave and conventional sintered porcelain bodies (electrical porcelain had sintering curve similar to that of the sanitary ware but with 10 min of soaking time). uncontrolled thermal runaway. Soaking times of 6, 8 and 10 min were used in the microwave hybrid sintering of the dental, sanitary ware and electrical formulation respectively. The dental composition was also microwave-sintered at 1200 ◦ C for 7 and 8 min. The cooling cycle was not controlled. Fig. 2 illustrates the microwave sintering cycle, including the monitored cooling step in comparison with the conventional cycle. In the microwave sintering process, the temperature was measured using an Inconel-sheathed K-type thermocouple. The sheaths were grounded to the cavity.

Table 2 Chemical composition of the raw materials used in this work (wt.%) Raw materials

LOIa

SiO2

Al2 O3

Fe2 O3

CaO

MgO

Na2 O

K2 O

Quartz Ball clay Feldspar

0.2 13.0 0.3

97.8 49.4 66.2

0.9 35.7 17.5

0.1 1.1 0.5

0.1 0.2 0.1

0.0 0.0 0.2

0.1 0.0 4.0

0.2 0.3 11.2

a

LOI, loss on ignition.

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229 The water absorption and apparent density of the sintered bodies were determined by the Archimedes’ method with water-immersion. The flexural strength of the sintered bars was measured using the three-point-bend test, with a cross-head speed of 0.5 mm/min. The sintered samples were characterized by X-ray diffraction (Siemens, D500), and their microstructures were observed by scanning electron microscopy, SEM (Philips, XL30 FEG), after polishing and chemical etching with HF (10%) for the removal of the siliceous vitreous phase of the porcelain bodies and observation of the mullite phase.

3. Results and discussion Fig. 3 shows the water absorption of the microwave and conventionally sintered samples. As can be seen, despite the short heating cycle the microwave heated samples reached water absorptions similar to those of the conventional fired samples. To achieve the same degree of water absorption, the temperatures applied to microwave hybrid sintering were slightly higher (approximately 20 ◦ C) than those used in conventional firing. The main conditions required for fast firing are the formulation of more reactive bodies, higher firing temperatures and reduction in the particle size to improve the kinetics of the reactions and transformations in the firing [29]. Thus, when a formulation applicable to conventional sintering is used, these higher temperatures are associated with the fast firing technique itself [29–32] and are not a consequence of the microwave sintering process.

225

A soaking time of 8 min was sufficient to obtain low water absorption on the sanitary ware bodies, while the electrical porcelain required a soaking time of 10 min to achieve low water absorption. Only 6 min were necessary for the sintered samples of the dental composition attained water absorptions lower than those obtained with the conventional fire (the samples of dental formulation were fired for just 30 min at 1200 ◦ C because soaking times of 60 min produced an incipient fusion). The water absorption decreased with temperature rise, which is a consequence of the vitrification process. However, the sanitary ware samples and the electrical porcelain bodies presented an increase in the water absorption when sintered at 1250 ◦ C and 1225 ◦ C, respectively. When two materials of different expansion coefficients are heated, stresses are set up between the two phases when the material are cooled, sometimes causing cracking and separation at the grain boundary. This is very observed in whiteware compositions (materials formed by clay, feldspar and quartz), where the cracks (also called microstress cracks) is caused by the greater contraction of the quartz grain compared with that of the surrounding vitreous matrix [33]. The development of microstress cracks, which associated with the fast firing cycle of the microwave hybrid heating may be the responsible for the water absorption increase of the sanitary ware and electrical porcelain bodies when sintered at 1250 and 1225 ◦ C.

Fig. 3. Water absorption vs. the sintering temperature of: (a) sanitary ware, (b) dental, (c) electrical samples.

226

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229

The microwave-sintered bodies were devoid of cracks, which is a reliable indicator of homogeneous distribution of temperature in the bodies. Incipient fusion was also absent from the microwave-sintered bodies. Some samples of the dental formulation presented incipient fusion and pyroplastic deformation at the beginning of the study, however the use of two heating rates on the firing cycle avoided this phenomenon. The high amount of fluxes (feldspar) in dental formulation was supposed to make its sintering process very sensible to heating rates and soaking times. As the amount of fluxes in the formulations increase, the heating cycle becomes more difficult to control. The development of a high amount of liquid phase at high temperatures increases the microwave absorption characteristic of the body abruptly, which may lead to uncontrolled thermal runaway. This increase in the dielectric losses make the decreasing of the heating rates necessary to avoid the incipient fusion of the dental samples. These findings indicate that adequate firing schedules make the microwave hybrid sintering of porcelain bodies possible, despite their compositions, in cycles lower than 60 min, without the need of special blanket arrangements or use of high frequencies. Table 3 presents the apparent density of the microwave and conventionally fired samples. It can be observed that the apparent density values of the microwave and conventionally sintered samples are similar. The microwave-sintered density values did not varied significantly with the increase of temperature and the decrease of the water absorption. The sanitary ware and elec-

Table 3 Apparent density of the fired porcelain bodies Formulation

Fire conditions

Density (g/cm3 )

Sanitary ware

Microwave 1175 ◦ C/8 min 1200 ◦ C/8 min 1225 ◦ C/8 min 1250 ◦ C/8 min

2.37 2.38 2.44 2.41

Conventional 1100 ◦ C/60 min 1150 ◦ C/60 min 1200 ◦ C/60 min

2.25 2.40 2.44

Microwave 1150 ◦ C/6 min 1200 ◦ C/6 min 1200 ◦ C/7 min 1200 ◦ C/8 min

2.36 2.37 2.35 2.35

Dental

Electrical

Conventional 1100 ◦ C/60 min 1150 ◦ C/60 min 1200 ◦ C/30 min

2.14 2.34 2.31

Microwave 1175 ◦ C/10 min 1200 ◦ C/10 min 1225 ◦ C/10 min

2.40 2.43 2.31

Conventional 1100 ◦ C/60 min 1150 ◦ C/60 min 1200 ◦ C/60 min

2.21 2.41 2.42

Fig. 4. Modulus of rupture vs. the sintering temperature of: (a) sanitary ware, (b) dental, (c) electrical samples.

trical porcelain density values decreased when the bodies were sintered at 1250 and 1225 ◦ C, respectively. Fig. 4 shows the modulus of rupture of the microwave and conventionally sintered samples. As can be seen, despite the short heating cycle the sanitary ware and electrical microwaveheated samples attained modulus of rupture similar to those of the conventional fired samples. The dental microwave-heated samples attained modulus higher than those of the conventionally fired samples.

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229

It can be observed that the modulus of rupture of the sanitary ware and electrical microwave-sintered samples did not vary with the firing temperature, despite the variation in the apparent porosity—open porosity (decrease and increase in water absorption). This behavior may be a consequence of the characteristics of the microwave heating (transfer of energy directly into the materials, providing volumetric and internal heating, which make possible retain a high level of open porosity during the final stages of sintering and a low amount of total porosity after sintering [7]) or of the fast firing technique itself. The modulus of rupture of the microwave-heated dental samples increased with the sintering time. This parameter varied from approximately 78 to 98 MPa, while the water absorption practically did not varied (0.08% for 7 min and 0.02% for 8 min). The similar water absorption of the bodies fired for 6, 7 and 8 min indicates that the increase in the modulus of rupture may be associated with any increase in the mullite content, as appeared from the XRD patterns, or with a reduction in the closed porosity (total porosity) of the bodies. Comparing the obtained values of the modulus of rupture of the microwave-heated samples with values reported in literature for triaxial porcelain bodies [34–36], it can be observed that they are very similar despite the short heating cycle used in the microwave sintering process. Fig. 5 depicts X-ray diffraction patterns of microwave and conventionally sintered samples. The crystalline phases of the conventionally sintered bodies were mullite and quartz. The conventionally sintered electrical porcelain also presented cristobalite when fired at 1220 ◦ C. The microwave-sintered sanitary ware and electrical porcelain presented quartz and mullite as crystalline phases, while the microwave-sintered dental samples presented quartz and feldspar as crystalline phase. The presence of feldspar after sintering at 1200 ◦ C in the microwaveheated dental bodies are probable related to the high amount of feldspar in the formulation and to the short sintering cycle, which did not provide sufficient time for the complete fusion of the feldspar. The X-ray diffraction patterns of the microwave-sintered dental samples heated for 7 and 8 min at 1200 ◦ C does not present any difference of the pattern present at Fig. 5 for the sample sintered for 6 min. The presence of only quartz and feldspar in the X-ray patterns eliminated the assumption that an increase in

227

Fig. 5. X-ray diffraction patterns of the microwave and conventionally sintered porcelain bodies.

the amount of mullite may be the responsible for the increase in the modulus of rupture with the sintering time. Fig. 6 shows SEM micrographs of the microwave sintered porcelain bodies. The microstructures of the microwave

Fig. 6. SEM micrographs of microwave fired samples: (a) sanitary ware: 1250 ◦ C/8 min, (b) electrical: 1200 ◦ C/10 min.

228

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229

hybrid sintered porcelains consist of fine-grained primary mullite smaller than 1 ␮m, undissolved quartz particles with sizes ranging from few microns to higher than 10 ␮m and also, the presence of secondary mullite. Primary mullite originates from clay minerals in raw materials, and secondary mullite is precipitated in feldspar relics. Based on Fig. 6, the presence of a microcrack around a quartz particle on the electrical porcelain can be observed. The microcrak is probably associated with the highest amount of quartz in the electrical porcelain and with the fast cooling step of the sintering cycle (Fig. 2). According to the monitored cooling step, in just ≈13 min the temperature drops from the soaking temperature to ≈500 ◦ C, (below the ␣ ↔ ␤ quartz inversion temperature). The presence of microcraks around quartz particles is not uncommon in fast heating of traditional ceramics, but may be diminished using a controlled cooling cycle that decreases the cooling rate around the quartz inversion temperature, 573 ◦ C. As mentioned earlier, no cracking or incipient fusion of the bodies during microwave fast firing was observed, despite the rapid formation of a high level of liquid phase as the feldspar fused. At lower temperatures (without liquid phase), the possible thermal stress caused by the selective heating of some of the composition’s constituents is relaxed by the porous structure [25]. At higher temperatures, the body begins to display a liquid phase, which is heated selectively due to its higher dielectric loss. However, thermal runaway in the liquid phase temperature is suppressed by the conduction of heat through the solid particles wetted by the liquid. Moreover, the fluidity of the liquid phase prevents the creation of thermal stress [25]. Thus, with the correct heating procedure porcelain bodies may be fast sintered using the hybrid heating without the generation of stresses and developing of cracks. 4. Conclusion In this study it was demonstrated that microwave hybrid fast sintering of porcelain bodies is possible. On the basis of the reported results the following conclusions can be drawn: • The control of the heating schedule makes fast firing of porcelain bodies by microwave hybrid heating possible, without stress-related cracking, incipient fusion or pyroplastic deformation of the bodies; • Heating cycles lower than 60 min can be used for the sintering of porcelain bodies; • To achieve the same degree of water absorption, the temperatures applied in microwave hybrid fast sintering were slightly higher (approximately 20 ◦ C) than those used in conventional firing; • The modulus of rupture of the microwave sintered bodies were similar to those of the conventionally sintered samples. References [1] D.D. Upadhyaya, A. Ghosh, G.K. Dey, R. Prasad, A.K. Suri, Microwave sintering of zirconia ceramics, J. Mater. Sci. 36 (2001) 4010–4707.

[2] D.E. Clark, W.H. Sutton, Microwave processing of materials, Annu. Rev. Mater. Sci. 26 (1996) 299–331. [3] G. Link, L. Feher, M. Thumm, H.J. Ritzhaupt-Kleissl, R. B¨ohme, A. Weisenburger, Sintering of advanced ceramics using a 30-GHz, 10kW, CW industrial gyrotron, IEEE Trans. Plasma Sci. 27 (2) (1999) 547–554. [4] M.S. Spotz, D.J. Skamser, D.L. Johnson, Thermal stability of ceramics materials in microwave heating, J. Am. Ceram. Soc. 78 (4) (1995) 1041–1048. [5] A.J. Berteaud, J.C. Badot, High temperature microwave heating in refractory materials, J. Microwave Power 11 (4) (1976) 315–320. [6] G. Roussy, A. Bennani, J.M. Thiebaut, Temperature runaway of microwave irradiated materials, J. Appl. Phys. 62 (4) (1987) 1167–1170. [7] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (2) (1989) 376–386. [8] M.A. Janney, C.L. Calhoun, H.D. Kimrey, Microwave sintering of solid oxide fuel cell materials: I, zirconia-8 mol% yttria, J. Am. Ceram. Soc. 75 (2) (1992) 341–346. [9] R.R. Menezes, P.M. Souto, E. Fagury-Neto, R.H.G.A. Kiminami, Microwave sintering of ceramics, in: R.L. Shulz, D.C. Folz (Eds.), Proceedings of the Fourth World Congress on Microwave and Radio Frequency Application, The Microwave Working Group, Arnold, MD, 2004, pp. 118–128. [10] A.W. Fliflet, R.W. Bruce, R.P. Fischer, D. Lewis, L.K. Kurihara, B.A. Bender, G.M. Chow, R.J. Rayne, A study of millimeter-wave sintering of fine-grained alumina compacts, IEEE Trans. Plasma Sci. 28 (3) (2000) 924–935. [11] A. Birnboim, D. Gershon, J. Calame, A. Birman, Y. Carmel, J. Rodgers, B. Levush, Y.V. Bykov, A.G. Eremeev, V.V. Holoptsev, V.E. Semenov, D. Dadon, P.L. Martin, M. Rosen, R. Hutcheon, Comparative study of microwave sintering of zinc oxide at 2.45, 30 and 83 GHz, J. Am. Ceram. Soc. 81 (6) (1998) 1493–1501. [12] M.A. Janney, C.L. Calhon, H.D. Kimrey, Microwave sintering of zirconia8 mol % ytrria, in: D.E. Clark, F.D. Gac, W.H. Sutton (Eds.), Ceramic Transactions, Microwaves: Theory and Applications in Materials Processing, vol. 21, American Ceramic Society, Westerville, 1991, pp. 311–318. [13] A. D´e, I. Ahmad, D. Whitney And, D.E. Clark, Microwave (hybrid) heating of alumina at 2,45 GHz: II. Effect of processing variables, heating rates and particle size, in: D.E. Clark, F.D. Gac, W.H. Sutton (Eds.), Ceramic Transactions, Microwaves: Theory and Applications in Materials processing, vol. 21, American Ceramic Soc., Westerville, 1991, pp. 329–337. [14] A. D´e, I. Ahmad, D. Whitney And, D.E. Clark, Microwave (hybrid) heating of alumina at 2,45 GHz: I. Microstructural uniformity and homogeneity, in: D.E. Clark, F.D. Gac, W.H. Sutton (Eds.), Ceramic transactions, Microwaves: Theory and applications in materials processing, vol. 21, American Ceramic Society, Westerville, 1991, pp. 319–328. [15] K.H. Brosnan, G.L. Messing, D.K. Agrawal, Microwave sintering of alumina at 2.45 GHz, J. Am. Ceram. Soc. 86 (8) (2003) 1307–1312. [16] P.D. Ramesh, D. Brandon, L. Sch¨achter, Use of partially oxidize SiC particles bed for microwave sintering of low loss ceramics, Mater. Sci. Eng. A 266 (1999) 211–230. [17] C. Zhao, J. Vleugels, C. Groffils, P.J. Luypaert, O. Van Der Biest, Hybrid sintering with a tubular susceptor in a cylindrical single-mode microwave furnace, Acta Mater. 48 (2000) 3795–3801. [18] A. Kishimoto, M. Ito, S. Fujitsu, Microwave sintering of ion conductive zirconia based composite dispersed with alumina, J. Mater. Sci. Lett. 20 (2001) 943–945. [19] S. Marinel, M. Pollet, G. Desgardin, Sintering of CaZrO3 -based ceramic using mixed conventional and microwave heating, Mater. Electron. J. Mater. Sci. 13 (2002) 149–155. [20] B. Vaidhyanathan, D.K. Agrawal, R. Roy, Microwave-assisted synthesis and sintering of NZP compounds, J. Am. Ceram. Soc. 87 (5) (2004) 834–839. [21] R. Peelamedu, A. Badzian, R. Roy, R.P. Martukanitz, Sintering of zirconia nanopowder by microwave-laser hybrid process, J. Am. Ceram. Soc. 87 (9) (2004) 1806–1809. [22] K.S. Kumar, T. Mathews, Sol–gel synthesis and microwave assisted sintering of zirconia–ceria solid solution, J. Alloy. Compd. 391 (2005) 177–180.

R.R. Menezes et al. / Journal of Materials Processing Technology 190 (2007) 223–229 [23] W.L.E. Wong, S. Karthik, M. Gupta, Development of hybrid Mg/Al2 O3 composites with improved properties using microwave assisted rapid sintering route, J. Mater. Sci. 40 (2005) 3395–3402. [24] N. Azurmendi, I. Caro, A.C. Caballero, T. Jardiel, M. Villegas, Microwaveassisted reaction sintering of bismuth titanate-based ceramics, J. Am. Ceram. Soc. 89 (4) (2006) 1232–1236. [25] M. Sato, T. Mutoh, T. Shimotuma, M. Mizuno, S. Ito, T. Inoue, K. Esaki, O. Motojima, M. Fujiwara, S. Takayama, M. Mizuno, S. Obata, T. Shimada, K. Satake, Insulation blankets for microwave sintering of traditional Ceramics, in: D.E. Clark, J.G.P. Binner, D.A. Lewis (Eds.), Ceramic transactions, Microwaves: Theory and Applications in Materials Processing V, vol. 111, American Ceramic Society, Westville, Ohio, 2001, p. 277. [26] S. Takayama, M. Mizuno, S. Obata, T. Shimada, K. Satake, M. Sato, T. Mutoh, T. Shimotuma, S. Ito, K. Ida, T. Inoue, K. Esaki, O. Motojima, M. Fujiwara, Sintering of traditional ceramics by microwaves (84 GHz and 2.45 GHz), in: D.E. Clark, J.G.P. Binner, D.A. Lewis (Eds.), Ceramic transactions, Microwaves: Theory and Applications in Materials Processing V, vol. 111, American Ceramic Society, Westville, Ohio, 2001, p. 305. [27] M. Mizuno, T. Takayma, S. Obata, T. Hirai, T. Shimada, K. Satake, M. Sato, T. Mutoh, T. Shimatsuma, S. Ito, T. Inoue, K. Esaki, O. Motojima, Analysis of microwave sintered porcelain, in: D.E. Clark, J.G.P. Binner, D.A. Lewis (Eds.), Ceramic transactions, Microwaves: Theory and Applications in Materials Processing V, vol. 111, American Ceramic Society, Westville, Ohio, 2001, p. 313.

229

[28] F.H. Norton, Introduc¸a˜ o a` Tecnologia Cerˆamica (J.V. Souza, Trans.). Edgard Blucher, S˜ao Paulo, Brazil, 1973. [29] R.R. Menezes, Development of susceptor systems for the microwave sintering of ceramics, Ph.D Thesis (in portuguese), Federal Univerty of S˜ao Carlos, Brazil, 2005. [30] T. Manfredini, L. Pennisi, Recent innovations in fast firing process, in: V.E. Henkes, G.Y. Onoda, W.M. Carty (Eds.), Ceramic Transactions, Science of Whitewares, American Ceramic Society, Westville, Ohio, 1996, pp. 213–219. [31] M.P. Harmer, E.W. Roberts, R.J. Brook, Rapid sintering of pure and doped ␣-Al2 O3 , Trans. J. Br. Ceram. Soc. 78 (1979) 22–25. [32] M.P. Harmer, R.J. Brook, Fast firing—microstructural benefits, Trans. Br. Ceram. Soc. 80 (5) (1981) 147–148. [33] D.W. Kingery, H.K. Bowen, D.R. Ulhmann, Introduction to Ceramics, second ed., John Wiley & Sons Inc., New York, 1976. [34] W.P. Tai, K. Kimura, K. Jinnai, A new approach to anorthite porcelain bodies using nonplastic raw materials, J. Eur. Ceram. Soc. 22 (2002) 463–470. [35] S.R. Braganc¸a, C.P. Bergmann, Traditional and glass powder porcelain: technical and microstructure analysis, J. Eur. Ceram. Soc. 24 (2004) 2383–2388. [36] G. Stathis, A. Ekonomakou, C.J. Stournaras, C. Ftikos, Effect of firing conditions, filler grain size and quartz content on bending strength and physical properties of sanitaryware porcelain, J. Eur. Ceram. Soc. 24 (2004) 2357–2366.