Advances in Science and Technology Vol. 45 (2006) pp. 442-446 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland
Roll Pressed LZSA Glass-Ceramics
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G. M. Reitz1,4,a, O. R. K. Montedo4,b, O. E. Alarcon1,2,c, D. Hotza1,3,d, A. P. Novaes de Oliveira1,2,e
Graduate Program in Materials Science and Engineering, PGMAT/UFSC 2 Department of Mechanical Engineering, EMC/UFSC 3 Department of Chemical Engineering, EQA/UFSC 4 Center of Technology in Materials (CTCmat) Federal University of Santa Catarina (UFSC) PO Box 476, 88040-900, Florianópolis, SC, Brazil a
[email protected];
[email protected];
[email protected];
[email protected]; e
[email protected] Keywords: roll pressing, glass-ceramics, ceramics, sintering and crystallization.
Abstract This work reports some experimental results regarding to a Li2O-ZrO2-SiO2-Al2O3 (LZSA) sintered glass-ceramic material obtained by roll pressing of glass powders (mean particle size ≈ 5 µm) with an added (7 wt.%) inorganic material (bentonite) as binder. The composition was characterized using chemical analysis, laser-scattering particle size analysis, DTA, XRD, thermal expansion, modulus of rupture (MOR) and deep abrasion (DA) measurements as well as density measurements and SEM observations. From the results it was verified that the glass-ceramic materials obtained by sintering and controlled crystallization, in the 850-1030°C temperature range, of glass powders, have properties and characteristics decisively better than those of other traditionally used materials. It is concluded that roll pressing technology is a potential candidate to produce sintered glassceramics for many applications, such as, for example, large sheets panels for electrical and thermal insulation. Introduction The usual technology for fabrication of glass-ceramics materials consists on preparation of monolithic glass articles by the application of glass technology and subsequently crystallization [1,2]. However, this technology requires great investments and can be justified only for high production volumes [3]. On the other hand, the production of glass-ceramic materials processed from glass powders and consolidate by sintering and crystallization seems to be a valid alternative since is possible to use the same equipments of a ceramic plant for the production of components with very complicated shapes [3-6]. The process involves the following steps: i) glass melting and its cooling (parent glass frit); ii) pulverization; forming by ceramic technology (axial pressing [7], extrusion [8], slip casting [9], powder injection moulding [10-11], ceramic foams [12], roll pressing [13], etc.); and iii) sintering for consolidation and crystallization. Among the forming technologies roll pressing of binder added ceramic powders in the gap between two rollers is an alternative method of producing semi finished products such as sheets and strips. In this work glass-ceramic materials were selected because they have several important properties such as low coefficient of thermal expansion, high abrasion and scratch resistances, and good chemical and thermal shock resistances. Therefore, glass-ceramic materials have found applications in various fields of society and industry [9]. Considering the specific properties and the relative stability of zirconia and alumina giving place to zircon and β-spudomene, lithium-based glass according to preliminary studies [14], enables homogenous glasses belonging to the Li2O-ZrO2SiO2-Al2O3 (LZSA) system to be obtained as precursors materials (parent glass) for glass-ceramics for a number of applications with optimised properties. In this context, this article reports results regarding to a LZSA glass-ceramic material obtained by the roll pressing or roll compaction process with an added inorganic material (bentonite) as binder. 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 the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 150.162.70.208-08/08/07,04:38:40)
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Experimental Procedures A glass composition belonging to the system Li2O-ZrO2-SiO2-Al2O3 (LZSA) was prepared from commercially available raw materials, whose chemical analysis, obtained by atomic absorption spectroscopy, AA (Unicam 969) and by X-ray fluorescence spectroscopy, XRF (Philips PW 2400), is presented in Table 1. A batch to produce ~150 kg of glass was placed in a mullite crucible and melted at ~1500±5°C for 7 h in a gas furnace. The melt was quenched in water and dried. The glass frit was milled for 20 h in an aluminous porcelain mill containing alumina grinding media and water. Subsequently, the milled glass powder suspension was again milled in a continuous mill containing zirconia micro spheres so that the average particle size was found to be about 5 µm using laser-scattering particle-size analysis (Model 1064L, Cilas). In order to obtain compacts by extrusion and roll pressing a commercial bentonite (average particle size of about 4.5 µm), supplied by Colorminas, was added (7 wt%) to the glass powder and mixed and humidified with water (18 wt%). Chemical analysis is shows in Table 1. Table 1 Chemical composition of LZSA parent glass and bentonite. Oxide (wt.%) SiO2 Al2O3 ZrO2 Li2O K2O Na2O TiO2 Fe2O3 CaO MnO MgO P2O5 59.4 13.6 15.6 8.6 0.3 0.7 0.1 0.2 0.6 0.02 0.82 LZSA glass 62.8 20.3 0.5 2.4 0.1 3.8 1.2 < 0.1 2.3 0.2 Bentonite Subsequently, the mixture was stored for 12 h to homogenise and then it was extruded in a Netzsch extruder MA 01 and then roll pressed in SUPER/SC Machine ( Model CS-400 LIEME). Thermal linear shrinkage of compacted samples was measured using a dilatometer (Netzsch dilatometer Model DIL 402PC) at 10°C/min in air, using alumina as reference material, for cylindrical samples of 10 mm length and 5 mm diameter. The crystallization temperature of the glass powder was measured using DTA (Netzsch, STA EP 409) in air at a heating rate of 10°C/min using powdered specimens of about 30 mg in an alumina sample holder with an empty alumina crucible as reference material. The extruded samples with nominal dimensions of 150 x 25 x 9 mm were then roll pressed so that samples with thickness in the 4-5 mm range were obtained. The roll pressed samples were then dried in an electrical drier at 60ºC for 2 h and heat treated (sintered and crystallized) in an electrical furnace with a temperature control within 2ºC at selected temperatures (from 850 to 1030ºC) with a holding time of 10 min. The theoretical density (ρt) of the sintered samples was measured by using a picnometer and the apparent density (ρap) was measured by the Archimedes principle by mercury immersion at 20oC. Taking in account the apparent density and theoretical density measurements, the relative density (ρr) was calculated. After sintering, samples were transversally cut, grounded and polished with 1 µm alumina paste and then etched in 0.5% HF for 25 s. Subsequently, all the samples were coated with a thin Au film for Scanning Electron Microscopy (SEM) observations (Model Philips XL-30). To investigate the crystalline phases formed during heat-treatments, powdered samples were analysed with a Philips PW 3710 computerassisted X-ray (Cu Kα) powder diffractometer (XRD). Mechanical strength of sintered samples was performed in an EMIC test machine (Model DL 2000) according to ISO 10545-4. At last, linear thermal expansion of the sintered samples were measured on bars with dimensions of 3 × 5 × 15 mm, using a automated recording dilatometer (Netzsch. DIL 402PC) at 10°C min-1 and deep abrasion in a Gabrielli test machine (Model CAP) according to ISO 10545-6. Results and Discussion The linear thermal shrinkage curve is shown in Figure 1. According to this Figure, densification starts at ~850°C and is completed up to 950°C. Crystallization starts soon after the completion of densification. A slight increase in volume is observed from ~950°C, associated to the melting of crystalline phases according to DTA analysis.
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1.00
-7.00 0.95 Densidade Relativa
-8.00
Relative density (-)
Thermal linear shrinkage(%) (%) Retração Sinterização
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-9.00 -10.00 -11.00 -12.00 -13.00 -14.00 -15.00 800
850
900 950 Te mperatura Sinterizaç Temperature (ºC) ão
1000
1050
Fig. 1 Thermal linear shrinkage curve
0.90 0.85 0.80 0.75 0.70 800
850
900 950 Temperature (ºC) Te mperatura Sinterização
1000
1050
Fig. 2 Relative density vs. temperature
In fact, these observations are in good agreement with density measurements (Figure 2) since relative densities values (ρr) increased from about 77% at 850°C to about 96% at 950°C. In this case, the number and size of pore are affected by the amount of crystals and their sizes, since, for samples sintered in 1000-1030°C temperature range for 10 min, crystal growing probably took place according to DTA and SEM analysis (Figure3).
(a) (b) Fig. 3 SEM photographs of samples sintered at (a) 950°C/10 min and (b) 1000°C/10 min. According to XRD analysis (Fig. 4) the reflections associated with the sintered samples were assigned to the crystalline phases of zirconium silicate, JCPDS 6-266, lithium metasilicate, JCPDS 29-828 and β-spodumene, JCPDS 21-503. Taking in account the sintering and crystallization behaviour of the glass considered in this research work, samples were prepared and sintered in the 850-1030°C temperature range for 10 min respectively by applying a one-stage cycle. The samples were then subjected to some measurements so that typical properties of ceramic materials were determined as shows Table 2. Table 2 Properties related to samples sintered in the 850-1030°C temperature range for 10 min PROPERTY MOR (MPa) DA (mm3) CTE x 106 (ºC-1) (25-325ºC)
850ºC 65.2 ± 5.1 222 ± 15 4.05 ± 0.10
900ºC 79.6 ± 2.9 147 ± 13 3.97 ± 0.11
950ºC 109.8 ± 0.5 108 ± 10 4.00 ± 0.08
1000°C 88.2 ± 3.9 94 ± 12 3.99 ± 0.12
1030°C 83.6 ± 5.3 100 ± 13 3.93 ± 0.11
From Table 2 it can be seen that results close agree with the sintering and crystallization behaviour observed, i. e., the sintering temperature increasing (850-1030°C) produces, according to XRD results, a higher crystallinity and consequently, for temperatures higher than 950°C, a decreasing on densification which can be observed from results of relative densities. This behaviour directly
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reflects on mechanical properties, especially on the modulus of rupture (MOR). In this case, porosity and crystallinity play fundamental roles. The increasing of crystallinity give rise to higher mechanical property values with respect to MOR and deep abrasion (DA). On the other hand, even considering that higher crystallinities can improve mechanical properties by crystallisation controls, the porosity (1-ρr) was increased and this causes a decreasing of the mechanical properties. In fact, crystallinity and especially the porosity have a very strong effect on mechanical properties as well evidenced in the case of samples sintered at 850 and 1030°C for 10 min where MOR and DA are drastically affected.
E
ZR
(e)
SL
E ZR
SL
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E
ZR SL
(c) E ZR SL
(b) E
ZR SL 10
14
18
(a) 22
26
30
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Ângulo 2Te ta (Graus )
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Fig. 4 XRD patterns of samples. (a) Powdered sample sintered at 850°C for 10 min; (b) Powdered sample sintered at 900°C for 10 min; (c) Powdered sample sintered at 950°C for 10 min; (d) Powdered sample sintered at 1000°C for 10 min; Powdered sample sintered at 1030°C for 10 min. ZR - Zircon (ZrSiO4), SL - Lithium metasilicate (Li2SiO3) and E - β-spodumene. Note that low DA values means high deep abrasion resistance since the volume of material removed in test is smaller. In this context, a relationship between crystallinity and porosity must be established in order to obtain glass ceramic materials with optimised properties for a given application. The relatively constant and low CTE can be explained since β-spodumene (0.9 x 106 °C-1) and zircon (4 x 10-6°C-1) have a low CTE compared to lithium disilicate (11 x 10-6°C-1).
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Conclusions A Li2O-ZrO2-SiO2-Al2O3 (LZSA) sintered glass-ceramics was obtained from roll pressed glass powders. Densification was found to start at about 850°C and was completed in a very short temperature interval (100°C) and crystallization took place just after densification and was almost completed at about 950°C (according to DTA curves) for a heating time of 10 min. On heating, the glass powder compacts crystallize into zircon, lithium metasilicate and β-spodumene. The microstructure consisted of fine crystals uniformly distributed and randomly oriented through out the glassy phase as well as a residual porosity. The results related to properties demonstrated that the studied glass-ceramic is a potential candidate to produce sintered glass-ceramics for many applications, such as, for example, large sheets panels for electrical and thermal insulation. Acknowledgements The authors are grateful to CAPES and CNPq/Brazil for funding this work. References [1] P. W. Mc MILLAN. Glass Ceramics, 2nd Edn, Academic Press, New York, 1979. [2] J. H. SIMMONS et al. Advances in Ceramics, Vol. 4: Nucleation and Crystallization in Glasses (Edited by American Ceramic Society), 1982. [3] E. M. RABINOVICH. Review. Preparation of glass by sintering. Journal of Materials Science, 20, 4259-97, 1985. [4] D. M. MILLER. US Patent 3926648 (1975). [5] TAKHER et al. Glass and Ceramics 34 (7), 1977, 445. [6] C. I. HELGESSON. In “Science of Ceramics, vol. 8 (British Ceramic Society), 347, 1976. [7] A. P. NOVAES DE OLIVEIRA. Materiales Vitrocerámicos: Características, Propriedades y Aplicaciones Industriales. In: Introducción a los Esmaltes Cerámicos”. Editora: Faenza Editrice Ibérica, ISBN: 84-87683-23-1, Castellón – Espanha, 2002. [8] A. P. NOVAES DE OLIVEIRA; G. REITZ; O. R. K. MONTEDO; D. HOTZA. Extruded LZS Glass-Ceramics. American Ceramic Society Bulletin. Westerville, Ohio - USA, v.83, n. 8, p.9201 9206, 2004. [9] Z. STRNAD. ‘Glass Science and Technology’; 9, 1996, New York, Elsevier. [10] A. P. NOVAES DE OLIVEIRA; L. GIASSI; D. HOTZA; O. E. ALARCON; M. C. FREDEL. Sintering and Crystallization of LZSA Glass Powder Compacts Formed by Injection Moulding. American Ceramic Society Bulletin, Westerville, Ohio - USA, v. 84, n. 6, p. 9301-9306, 2005. [11] A. P. NOVAES DE OLIVEIRA; L. GIASSI; O. R. K. MONTEDO; M. C. FREDEL; D. HOTZA. Injection Moulding of Li2O-ZrO2-SiO2-Al2O3 (LZSA) Glass Ceramics. Glass Technology, Sheffield, v. 46, n. 3, p. 277-280, 2005. [12] A. P. NOVAES DE OLIVEIRA; E. SOUZA; C. B. SILVEIRA; T. FEY; P. GREIL; D. HOTZA. LZSA Glass Ceramic Foams Prepared by Replication Process. Advances in Applied Ceramics - Structural, Functional and Bioceramics, Inglaterra - Londres, v. 104, n. 1, p. 22-29, 2005. [13] F. THÜMMLER and R. OBERACKER. An introduction to Powder Metallurgy. London: The Institute of Materials, 1993. [14] A. P. NOVAES DE OLIVEIRA; T. MANFREDINI, G. C. PELLACANI; A. B. CORRADI; L. Di LANDRO. CIMTEC’98 – 9th Int. Conf. on Modern Materials and Technologies, Florence, Italy, June 1998.
