Characterization of Microstructure in Experimental Triaxial Ceramic Body July 2008 | Volume 4Page 1 of 8DOI: 10.2240/azojomo0270 Characterization of Microstructure in Experimental Triaxial Ceramic Body Simn Y. Reyes Lpez and Juan Serrato Rodrguez Copyright AD-TECH; licensee AZoM.com Pty Ltd. This is an AZo Open Access Rewards System (AZo-OARS) article distributed under the terms of the AZoOARS http://www.azom.com/oars.asp which permits unrestricted use provided the original work is properly cited but is limited to non-commercial distribution and reproduction. AZojomo (ISSN 1833-122X) Volume 4 July 2008 DOI: 10.2240/azojomo0262 Topics CoveredAbstract Keywords Introduction Experimental Procedure Results and Discussion Phase Evolution and Densification of Triaxial Body Conclusions Acknowledgements References Contact Details AbstractThe microstructural evolution in a triaxial body containing kaolin, quartz and anorthoclase has been examined.DTA reveals that kaolin dehydroxylates at 4940C and mullitization starts at 9900C. Decreased intensity of the -quartz peaks both in XRD and IR spectra up to 13000C indicated the onset of partial dissolution of -quartz at temperatures higher than 12000C. Evidence of kaolinite relicts being engulfed and dissolved by liquid glass was seen by SEM. Nanometric mullite formed on a pure clay relict was detected by XRD at 10500C. Apparent porosity and linear shrinkage data show the onset of densification at 10000C and the appearance of bloating at temperatures higher than 12500C. During the last stage of sintering at temperatures higher than 11000C porosity is highly reduced and the body densifies abruptly as feldspar melts.Keywords Riaxial Ceramics, Microstructure, Quartz Dissolution, Mullite, Bloating. Simn Y. Reyes Lpez and Juan Serrato Rodrguez July 2008 | Volume 4Page 2 of 8DOI: 10.2240/azojomo0270 Introduction Triaxial ceramics have been widely studied due to their diverse applications. Studies over several decades, confirmed the need for a higher volume of data, particularly regarding microstructural development to assist in the interpretation of triaxial systems [1, 2]. The microstructure of a triaxial porcelain typically consists of residual quartz, and mullite in a vitreous matrix [3, 4]. Lundin [5] and Schuller [6] reported the formation of two types of mullite in porcelain bodies. Crystalline mullite in the form of clusters that show up in clay residuals, are considered as primary mullite, while long needle habit mullite is referred to as secondary mullite. Schuller [6] reports that to 14000C as primary mullite transforms to secondary mullite; the mullite forms in the presence of a silica-rich glass liquid phase derived from the melting of feldspar. Iqbal and Lee [2], reported the microstructural evolution in a model triaxial porcelain in samples fired for 3 h at 60015000C. They found that the clay component dehydroxylated to metakaolin at 5500C and metastable sanidine formed from decomposition of the feldspar at about 6000C and dissolved at about 9000C. Liquid formation at 10000C was associated with melting of feldspar and silica discarded from metakaolin formation via the K2OAl2O3SiO2 eutectic. Fine mullite and -alumina crystals precipitated in pure clay relicts and larger mullite crystals in mixed clay-feldspar relicts at 10000C.Feldspars are low-melting mineral alkaline aluminosilicates and in triaxial bodies serve to lower the temperature at which viscous liquid forms. The liquid phase reacts with other body constituents and gradually permeates the microstructure, leading to its densification. The group of feldspar minerals consists of three silicates: a potassium-aluminum silicate (the orthoclase feldspars), a sodium-aluminium silicate, and a calcium-aluminium silicate (the plagioclase feldspars) and their isomorphous mixtures. Typical final microstructures of fired porcelain bodies consist of 10%25% mullite, with composition ranging from 2Al2O3.SiO2 to 3Al2O3.2SiO2, 5 25% -quartz (SiO2), and 0 8% pores dispersed in 65 80% potassium aluminosilicate glass. Bodies with a high percentage of quartz also may contain cristobalite [2]. A soda microcline named anorthoclase frequently used as a flux in triaxial body compositions is an isomorphous mixture of KAlSi3O8 and NaAlSi3O8, the sodium-aluminium silicate being in larger proportion. The purpose of this study is to investigate the microstructure-densification relationships in an experimental stoneware obtained by slip casting at temperatures up to 13000C Experimental Procedure Homogeneous batches of powders, comprising kaolin (50 wt%), anorthoclase feldspar (40 wt%), and quartz sand (10 wt%) were made into a slurry by mixing in a porcelain jar in which sodium silicate was added as deflocculating additive. Slips were cast into gypsum moulds by the standard technique and dried at 1000C followed by firing up to 13000C for 0.5 hr in an electrical furnace. Apparent density was measured via the Archimedess method and water absorption according to ASTM Designation: C 373-88 (i.e. weight gain of dried bulk samples after immersion in boiling water for 5 h and 24 h in cool water). Linear shrinkage during sintering was calculated from the dimensions of the green and the sintered samples. The shrinkage behaviour was also assessed by in-situ measurements in a Theta thermodilatometer. Scanning electron microscopy, (Jeol JSM-6400, 25 kV acceleration voltage), was done in secondary and backscattered electron modes in mirror-polished surfaces etched by immersing in 2 vol% HF solution for 4 min.X-ray diffraction analysis used a Siemens D5000 Cu K radiation 1.54 at 20 kV. DTA measurements were done in a DSC (Model Q600 Instruments, New Castle, DE). Particle size measurements were done in a Horiba Capa 300. Attenuated Total Reflection (ATR) (ZnSe crystal) technique used an IR Spectrometer fitted with a Fourier transform (TENSOR 37 series FT-IR, Bruker Optics Inc), while Raman spectra were obtained by a LabRam Analytical Raman Microscope spectrometric (HORIBA Jobin Yvon). Characterization of Microstructure in Experimental Triaxial Ceramic Body July 2008 | Volume 4Page 3 of 8DOI: 10.2240/azojomo0270 K = Kaolinite Q Results and Discussion Phase Evolution and Densification of Triaxial Body DTA of triaxial composition (Figure 1) reveals endotherms with maxima at 31.71C for dehydration. The peak at 494.560C, corresponds to dehydroxylation of kaolin to form metakaolin. The exotherm at 990.360C corresponds to the onset of crystallization of mullite. TGA profile (Figure 1), shows mass loss of 5.9 wt%. Figures 2 and 3 show XRD and IR phase evolution data for the triaxial body fired up to 13000C.31.71C494.56C990.36C94.00%94.01%419.83C98.25%-0.2-0.10.00.10.2Temperature Difference (C/mg)92949698100102Weight (%)0 200 400 600 800 1000 1200 1400Temperature(C)ExoUp Universal V3.8BTAInstruments Figure1. DTA and DTG Data of triaxial body. Dehydroxylation occurs at 4940C bringing about a weight loss of 5.9 wt %.Note Mullitization at 9900C. Figure 2. X Ray Diffraction of triaxial body showing partial dissolution of quartz and the appearance of mullite at about 10500C. Simn Y. Reyes Lpez and Juan Serrato Rodrguez July 2008 | Volume 4Page 4 of 8DOI: 10.2240/azojomo0270 Figure 3. IR Spectra at various temperatures. Note dehydroxylation of Si-O-Al band at 3600 and 1000 cm-1 at 6000C. XRD peaks show the absence of kaolinite at 6000C brought about from the loss of hydroxylic groups from the structure at 4940C. Kaolinite and anorthoclase peaks are hardly noticeable at 6000C. Decreased intensity of the -quartz peaks both in XRD and IR spectra up to 13000C showed the onset of partial dissolution of -quartz at temperatures higher than about 12000C.SEM (Figure 4) shows crystalline quartz dissolving in a glassy matrix.Mullite was first detected by XRD at 10500C and its amount increased at higher temperatures. Such primary mullite as disclosed by SEM (Figure 5) is a nanometric mullite formed on a pure clay relict. On the other hand, Figure 6 shows the physical process of engulfing and dissolving clay relicts, presumably feldspar initiated liquid is first formed and then dissolves clay relicts from which mullite crystals may eventually crystallize. Figure 7 shows bubble formation caused by gas elimination after heating to temperatures higher than 12000C. Characterization of Microstructure in Experimental Triaxial Ceramic Body July 2008 | Volume 4Page 5 of 8DOI: 10.2240/azojomo0270 Figure 4. SEM micrograph showing a large quartz crystal in glassy matrix in a sample etched with HF. Figure 5. SEM micrograph showing detail of primary mullite forming out of kaolinite. Figure 6. SEM micrograph depicting submicronic clay relicts being engulfed and dissolved into the glass. 1 m Simn Y. Reyes Lpez and Juan Serrato Rodrguez July 2008 | Volume 4Page 6 of 8DOI: 10.2240/azojomo0270 Figure 7. SEM micrograph showing large bubbles due to gas elimination. Finally, apparent porosity and linear shrinkage were plotted at temperatures up to 13000C, in Figure 8. both curves show the onset of densification at 10000C and the appearance ofbloating at temperatures higher than 12500C. Figure 8.In situ densification correlated to decreasing porosity. Figure 9 shows DTA (a) and thermodilatometry (b) data, by comparing both curves it can be seen that after dehydroxylation, the slope of the shrinkage curve changes as it does after mullitization and again after feldspar melting during the last sintering stage. This suggests a dependence of the aforementioned physical chemical changes and the sintering process. Figure 10 correlates the shrinkage behaviour and the measured apparent porosity of the body. As might be expected at temperatures higher than 11000C the higher rate of decreasing porosity brings about the abrupt last stage of shrinkage of the body. 100 Characterization of Microstructure in Experimental Triaxial Ceramic Body July 2008 | Volume 4Page 7 of 8DOI: 10.2240/azojomo0270 Figure 9. In situ firing shrinkage of triaxial bodyfrom thermodilatometry (b) and DTA (a). Note shrinkage changes coincident with dehydroxilation and mullitization temperature. Conclusions A microstructure-densification relationship was found. Changes in the rate of shrinkage from dilatometric in situ measurements were sensitive to phase transitions as detected by DTA. The overriding rate of shrinkage took place after most of the liquid phase was formed during the last stage of sintering within the 1100- 12000C temperature range.Heating beyond 12000C led to body bloating. Acknowledgements Authors acknowledge financial support of the Coordination de la Investigacin Cientifica de la Universidad Michoacana de San Nicols de Hidalgo. Assistance of Dr. Satoshi Sugita Seuyoshi is gratefully appreciated.References 1.Y. Iqbal and W. E. Lee, Fired Porcelain Microstructure Revisited, J. Am. Ceram. Soc., 82 [12] (1999) 35843590. 2.Y. Iqbal and W. E. Lee, Microstructural Evolution in Triaxial Porcelain, J. Am. Ceram. Soc., 83 [12] (2000) 31213127. 3.A. Klein, Constitution and Microstructure of Porcelain, Natl. Bur. Stand, Tech. Pap., 3-38 (1916-1917). 4.W. D. Kingery, Introduction to Ceramics, Second Edition. John Wiley & Sons, New York. 1976 pp.448-514. 5.S. T. Lundin, Electron Microscopy of Whiteware Bodies, Trans. Int. Ceram. Congr., 4, 383-390 (1954). 6.K. H. Schuller, Reactions Between Mullite and Glassy Phase in Porcelains, Trans. Br. Ceram. Soc., 63 [2] (1964) 103-117. Contact Details Simn Y. Reyes Lpez and Juan Serrato Rodrguez Universidad Michoacana de San Nicols de Hidalgo Instituto de Investigaciones Metalrgicas, Santiago Tapia 403,Morelia, Mich. Mxico. C.P. 58000 E-mail: yobannyr@yahoo.com.mx E-mail: jserrato@zeus.umich.mx ThispaperwasalsopublishedinAdvancesinTechnologyofMaterialsandMaterialsProcessingJournal,9[2] (2007) 173-178.