DOI : 10.2240/azojomo0270

Characterization of Microstructure in Experimental Triaxial Ceramic Body

Jul 10 2008

Simón Y. Reyes López and Juan Serrato Rodríguez

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AZojomo (ISSN 1833-122X) Volume 4 July 2008

Topics Covered

Abstract
Introduction
Experimental Procedure
Results and Discussion
Phase Evolution and Densification of Triaxial Body
Conclusions
Acknowledgements
References
Contact Details

Abstract

The microstructural evolution in a triaxial body containing kaolin, quartz and anorthoclase has been examined. DTA reveals that kaolin dehydroxylates at 4940°C and mullitization starts at 990°C. Decreased intensity of the α-quartz peaks both in XRD and IR spectra up to 1300°C indicated the onset of partial dissolution of α-quartz at temperatures higher than 1200°C. 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 1050°C. Apparent porosity and linear shrinkage data show the onset of densification at 1000°C and the appearance of bloating at temperatures higher than 1250°C. During the last stage of sintering at temperatures higher than 1100°C porosity is highly reduced and the body densifies abruptly as feldspar melts.

Keywords
Triaxial Ceramics, Microstructure, Quartz Dissolution, Mullite, Bloating.

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 1400°C 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 600–1500°C. They found that the clay component dehydroxylated to metakaolin at 550°C and metastable sanidine formed from decomposition of the feldspar at about 600°C and dissolved at about 900°C. Liquid formation at 1000°C was associated with melting of feldspar and silica discarded from metakaolin formation via the K₂O–Al₂O₃–SiO₂ eutectic. Fine mullite and γ-alumina crystals precipitated in pure clay relicts and larger mullite crystals in mixed clay-feldspar relicts at 1000°C.

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% α-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 1300°C

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 1300°C for 0.5 hr in an electrical furnace. Apparent density was measured via the Archimedes’s 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 Kα 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).

Results and Discussion

Phase Evolution and Densification of Triaxial Body

DTA of triaxial composition (Figure 1) reveals endotherms with maxima at 31.71°C for dehydration. The peak at 494.56°C, corresponds to dehydroxylation of kaolin to form metakaolin. The exotherm at 990.36°C 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 1300°C.

Figure 1. DTA and DTG Data of triaxial body. Dehydroxylation occurs at 494°C bringing about a weight loss of 5.9 wt %. Note Mullitization at 990°C.

Figure 2. X Ray Diffraction of triaxial body showing partial dissolution of quartz and the appearance of mullite at about 1050°C.

Figure 3.IR Spectra at various temperatures. Note dehydroxylation of Si-O-Al band at 3600 and 1000 cm^-1 at 600°C.

XRD peaks show the absence of kaolinite at 600°C brought about from the loss of hydroxylic groups from the structure at 494°C. Kaolinite and anorthoclase peaks are hardly noticeable at 600°C. Decreased intensity of the α-quartz peaks both in XRD and IR spectra up to 1300°C showed the onset of partial dissolution of α-quartz at temperatures higher than about 1200°C.

SEM (Figure 4) shows crystalline quartz dissolving in a glassy matrix. Mullite was first detected by XRD at 1050°C 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 1200°C.

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.

Figure 7. SEM micrograph showing large bubbles due to gas elimination.

Finally, apparent porosity and linear shrinkage were plotted at temperatures up to 1300°C, in Figure 8. both curves show the onset of densification at 1000°C and the appearance of bloating at temperatures higher than 1250°C.

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 1100°C the higher rate of decreasing porosity brings about the abrupt last stage of shrinkage of the body.

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- 1200°C temperature range. Heating beyond 1200°C led to body bloating.

Acknowledgements

Authors acknowledge financial support of the “Coordination de la Investigación Cientifica de la Universidad Michoacana de San Nicolás 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) 3584–3590.
2. Y. Iqbal and W. E. Lee, “Microstructural Evolution in Triaxial Porcelain”, J. Am. Ceram. Soc., 83 [12] (2000) 3121–3127.
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.