Elsevier

Ceramics International

Volume 26, Issue 6, 17 July 2000, Pages 605-608
Ceramics International

The effect of ZnO addition on the densification and properties of magnesium aluminate spinel

https://doi.org/10.1016/S0272-8842(99)00104-2Get rights and content

Abstract

Stoichiometric magnesium aluminate spinel can be developed by solid oxide reactions of calcined magnesia and calcined alumina. The raw materials were mixed; attrition milled, compacted under a uniaxial pressure of 100 MPa and finally fired in the temperature range of 1500 to 1650°C. Up to 2 wt% ZnO was incorporated as an additive. In this investigation the effect of ZnO on the densification and properties of the magnesium aluminate spinel has been studied. It was found that 99% of theoretical density was achieved on firing at 1550°C with the addition of 0.5 wt% ZnO. The optimum properties in terms of bulk density, hot strength and thermal shock resistance was obtained with 1 wt% ZnO. All the ZnO containing samples retained their strength up to 6–8th cycle on thermal shock. ZnO containing samples are comparatively more resistant to thermal shock than ZnO free samples.

Introduction

The MgO–Al2O3 system includes highly refractory materials, which have a wide range of applications in steel, cement and glass industries [1], [2]. MgO–Al2O3 spinel has a high melting point, a low thermal expansion, good chemical stability, resistant to thermal spalling and corrosion [3]. Various methods can be adopted to produce Mag–Al spinel, e.g. coprecipitation, alkoxide route, spark discharge process, freeze drying, plasma discharge process, etc. But all these processes are not cost effective for commercial production. The solid state reaction of alumina and magnesia is a simple approach for spinel formation, however the densification temperatures are relatively high. The effect of various additives on the spinelisation and densification of magnesium aluminate were studied previously [4], [5].

The present investigation was undertaken to develop the stoichiometric Mag–Al spinel from calcined magnesia and calcined alumina by solid oxide reaction route. Attempts have been made to study the effect of ZnO additive in the densification and properties of the magnesium aluminate spinel.

Section snippets

Experimental

The raw materials selected for the study were high pure calcined alumina and calcined magnesia. Alumina was obtained from Indian Aluminium Co., India and magnesia was obtained from M/s. Ned Mag Industries, The Netherlands. Raw materials were characterised in terms of chemical analysis, phase identification by X-ray diffraction study and surface area measurement. Chemical analysis of the raw materials was done by standard wet chemical method and surface area was measured by BET method in a BET

Characterisation of raw materials

The two basic raw materials used in this study, i.e. calcined magnesia and calcined alumina, were very pure in nature. Chemical analyses of the raw materials used in this investigation are given in Table 2. The magnesia which was originated from brine solution consists MgO level of 97 wt% and 1 wt% impurity (on dry basis), while the calcined alumina contain 99.3 wt% Al2O3. Both the raw materials were obtained in powder form. The surface area of alumina was 5.7 m2/gm and that of magnesia was 0.7

Conclusions

Magnesium aluminate spinel can be developed by solid oxide reaction of calcined alumina and calcined magnesia in the temperature range of 1550–1600°C. ZnO incorporation in magnesium aluminate spinel favours the densification and up to 99% of theoretical density can be achieved at 1550°C. Samples containing 1 wt% ZnO show highest flexural strength at all temperatures (except 500°C). ZnO goes into the spinel structure by solid solution formation and the maximum solid solubility of ZnO in Mag–Al

Acknowledgements

The authors wish to thank the Director, Central Glass & Ceramic Research Institute for his keen interest and kind permission to publish this paper. Valuable suggestion and support received from Dr. B. Mukherjee, Scientist, CGCRI during this project work are gratefully acknowledged.

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