Drying and Sintering of Ceramic Based Parts Using Microwave Heating

Ceramic slurries contain water, fine clay and processing additives. Water must be removed from the body of the clay piece by heating before sintering. Differences have been observed when using convection and microwave heating to raise the temperature for drying and sintering of ceramic parts. In the present work, production of ceramic based parts by microwave drying and sintering have been investigated. Ceramic parts with a body formulation of kaolin, ball clay, feldspar and quartz are dried by microwave heating. Glaze is applied on the parts and then they are sintered using a commercial microwave oven at 2450 MHz. The effect of body materials on drying and sintering time of the products are analyzed with the effect of sintering time on the properties using wet to dry contraction, total contraction and modulus of rupture. Results of both conventional and microwave processing are investigated. Introduction The use of microwave energy to process a wide variety of ceramic materials offers many new and exciting opportunities. The reasons for the growing interest in microwave processing over conventional processing methods are the potential for significant reduction in manufacturing costs due to energy savings and shorter processing times, improved product uniformity and yields, improved or unique microstructure and properties, and synthesis of new materials [1]. Water strongly absorbs 915 and 2450 MHz microwaves, thus is very efficiently heated at these frequencies. Microwave energy may be used to heat and evaporate liquid in large cross sections relatively rapidly, independently of the thermal conductivity of the solid [2]. The microwave sintered samples are densified much more rapidly and at much shorter time and lower temperature than the conventionally sintered samples. One of the most important reasons for developing microwave energy for producing materials is the evident rapid and internal heating. Enhanced grain boundary diffusion appeared to be responsible for the accelerated densification rates in the microwave processing of ceramics [3,4]. The fundamental principles of microwave sintering and different equipment configurations for industrial purposes were investigated elsewhere [5,6]. The heating efficiency of the microwave oven was increased by placing the articles in a susceptor container [7-9]. Susceptors are materials that couple well with microwaves and thus generate heat when irradiated. The container surrounds and intimately contacts the article inside the container. In early studies of microwave sintering, samples of boron oxide and hydroxyapatite ceramics were sintered in Boğaziçi University [10-12]. In this paper, we examine the microwave heating properties of whiteware ceramic bodies prepared with different compositions and compare with conventional heating data. Experimental Procedure Preparation of the Samples. Drying and sintering of the samples were performed at a Moulinex B7495 model commercial type microwave oven with a maximum heating power of 1100 W and 1400 W of convective heating power adjustable to 50-250 °C with turbo circulation [13]. The clay slurries used in the work were commercial whiteware ceramic body formulation, produced within Vitra Seramik Grubu Kartal Tesisleri. The body compositions are shown in Table 1. Composition A is also the commercially used body formulation of the factory. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 731-734 doi:10.4028/www.scientific.net/KEM.264-268.731 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:13:43) Table 1. Body compositions [wt %] Composition A Composition B Composition C China clay 20 25 15 Ball clay 32 27 37 Quartz 16 16 16 Feldspar 32 32 32 For a 10 kg clay slurry, the necessary amount of materials were mixed in a ball mill and three liters of water was added in the mixture. The samples were prepared by solid casting. Two different geometry of the same material were cast. The dimensions of modulus of rupture samples were 12 25 150 × × mm and wet to dry contraction samples were 100 50 10 × × mm. Different drying methods and cycles were used to dry the samples without any defect. The use of microwave heating together with conventional heating was unsuccessful and the samples were damaged. Microwave heating of the samples by operating the oven manually in 10 and 20 seconds intervals were also unsuccessful. The cycle with five seconds of heating and 25 seconds of waiting was perfectly successful to dry the samples of composition A in 120 minutes, where the total microwave heating time was only 20 minutes. These times were 135, 105 minutes for total drying and 22.5, 17.5 minutes for microwave heating time for composition B and C respectively. Modulus of Rupture Test. Five modulus of rupture samples were prepared by solid casting for each composition. Samples were dried in the microwave oven. After drying the samples were placed in a desicator for two hours to cool without humidification. The modulus of rupture values of the samples were measured with a Netzch model modulus of rupture apparatus. The change of modulus of rupture values of the samples with the average values are shown in Fig. 1. The modulus of rupture values of the samples with body composition A are less than the values of conventional dried samples. The high drying time of eight hours can be the reason for higher modulus of rupture values for the conventional dried samples. Wet to Dry Contraction Test. Five samples are prepared for each composition. The contraction samples were marked two times diagonally at a length of 100 mm with a micrometer after solid casting. They were placed on a smooth surface and left there for one day and finally they were dried by microwave oven. The samples were left in the furnace to cool down to the room temperature before the distance between the marks was measured. The wet to dry contraction was found by subtracting the length after drying from the initial length and dividing by the initial length. The changes of wet to dry contractions of the five samples with average values are shown in Fig 2. Although the microwave drying time is very short, the wet to dry contraction values of compositions A and C are in the standard range of the commercial clay, which is 0.3 3.0 ± mm. But the values of composition B are less than the standard values. Sintering Experiments. The temperatures of the samples were measured by an S type (Pt-10%RhPt) alumina-sheathed tip open thermocouple and a K type (Cr-Ni/Al-Ni) thermocouple. The thermocouples were insulated by fibers and arranged so that the tip is kept three to four millimeters away from the surface of the sample. K type thermocouples were used with a multimeter, but the S type thermocouple was used with a calibrated (for 600-1600°C) M1D00T digital temperature indicator from Metronik AŞ. Temperatures lower than 600°C were measured with the K type thermocouple. It was not possible to heat the samples to the sintering temperatures without insulation. Samples were placed in six SiC plates arranged like a box, and heated without any damage. SiC is a microwave absorber and can act as a susceptor. Therefore it was used in order to heat and insulate the materials. By using SiC slabs, sintering was achieved as a result of both indirect and direct heating. SiC plates were placed on a thick fiber cushion and then covered all around with a layer of fiber insulation. 732 Euro Ceramics VIII