NEI-NO--965 RECEIVED NO9805458 DEC ? 2 egg Os Tl Giinnar Pettersen Development of microstructure during sintering and aluminium exposure of titanium diboride ceramics QSTRBUTDNmaw WnGBXWMBKG as mow® UWWgg* 0 DEVELOPMENT OF MICROSTRUCTURE DURING SINTERING AND ALUMINIUM EXPOSURE OF TITANIUM DIBORIDE CERAMICS by Gunnar Pettersen Thesis submitted in partial fulfilment of the requirements for the degree Doktor Ingenipr Norwegian University of Science and Technology Department of Physics April 1997 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. ACKNOWLEDGEMENTS The work presented in this thesis was carried out at the Department of Physics, Norwegian University of Science and Technology (NTNU) in the period March 1993 - April 1997. Financial support was provided by the Norwegian Research Council who paid my salary and Norsk Hydro who helped paying for the experiments. Their support is gratefully acknowledged. My supervisor has been Professor Ragnvald Hpier. Thanks a lot for the encouragement and guidance during these years Ragnvald! Important parts of the work were carried out during a four months stay at the Max- Planck-Institut fur Metallforschung in Stuttgart in 1995. I am indebted to Professor Manfred Ruble for the invitation to visit Stuttgart and to Dr. Joachim Mayer for his hospitality and valuable discussions and suggestions to my work. Jurgen Plitzko and Jorg Thomas are acknowledged for their assistance with the Zeiss and VG microscopes. Thanks also to the rest of the staff and students at MPI for making my stay a pleasant and instructive one. Furthermore the helpful discussions and comments of colleagues at SINTEF (Foundation of Scientific and Industrial Research) Materials Technology and NTNU are gratefully acknowledged. In particular I would like to thank Dr. Katrin Nord- Varhaug and Dr. Jon Herman Ulvenspen whose works with titanium diboride became the starting point of mine, and Dr. Asgeir Bardal and Associate Professor Mari-Ann Einarsrud who also reviewed parts of this manuscript. I would also like to express my gratitude for the valuable discussions and the interest in my work shown by Dr. Dag 0vreb0 at Norsk Hydro Research Centre in Porsgrunn. Some people deserve an extra thanks for their kindness and practical help: * Geir Sagmo at SINTEF taught me how to use the hot press * Egil Skybakmoen at SINTEF did the aluminium exposure experiments * Kjell Ramspskar at the Physics Department cut my hard samples * Kjell Muller at SINTEF did the elemental mapping with the SEM * everybody in the Electron Microscopy Group at NTNU for making Trondheim an interesting and inspiring place to work. Finally, thanks to my family for care and support and to God for always looking after me. Trondheim, April 1997 Gunnar Pettersen ii Ill CONTENTS ACKNOWLEDGEMENTS i CONTENTS iii SUMMARY v 1. INTRODUCTION 1 2. THEORETICAL BACKGROUND - REVIEW OF PREVIOUS WORK 3 2.1 Aluminium electrolysis 3 2.1.1 The Hall-Heroult process 3 2.1.2 Inert, wetted cathodes 5 2.1.3. Energy consumption 6 2.1.4 Demands for a new cathode material 7 2.2 Titanium diboride - properties, production and use 7 2.2.1 Properties of TiB2 7 2.2.2 Production of TiB2 materials 12 2.2.3 Uses of TiB2 14 2.3 Sintering of TiB2 15 2.3.1 Introduction to sintering 16 2.3.2 Literature review 18 2.3.3 Concluding remarks 22 2.4 Degradation of TiB2 cathodes during aluminium electrolysis 23 2.4.1 Literature review 24 2.4.2 Possible mechanisms of aluminium penetration 28 2.4.3 Concluding remarks 33 3. EXPERIMENTAL 35 3.1 Materials selection and preparation 35 3.2 Microstructural characterisation 38 IV 4. RESULTS 41 4.1 The TiB2 powder 41 4.2 Hot-pressing 42 4.3 Microstructure of hot-pressed samples 44 4.3.1 Pure TiB2 44 4.3.2 TiBa with boron addition 52 4.3.3 TiB2 with titanium addition 57 4.4 Microstructure after exposure to liquid aluminium 70 4.4.1 Pure TiB2 70 4.4.2 TiB2 with boron addition 79 4.4.3 TiB2 with titanium addition 81 5. DISCUSSION 87 5.1 The TiB2 powder 87 5.2 Hot-pressing 90 5.2.1 Pure TiB2 90 5.2.2 TiB2 with boron addition 93 5.2.3 TiB2 with titanium addition 95 5.3 Aluminium exposure 100 5.3.1 Penetration of aluminium 100 5.3.2 Reactions with secondary phases 105 5.3.3 Degradation mechanisms 109 5.4 Further challenges 113 6. CONCLUSIONS 117 REFERENCES 121 Appendix 1 Hot pressing parameters Appendix 2 Thermodynamical data and calculations SUMMARY The introduction of an inert, wetted cathode in the aluminium electrolysis cell makes possible large savings in energy consumption during aluminium production. Titanium diboride (TiB2) holds special promise as a cathode material thanks to its low solubility in and high wettability by liquid aluminium. However, a limited lifetime during electrolysis has so far prevented industrial use of TiB2 cathodes. Degradation of the material is believed to be caused by penetration of aluminium into the cathode during electrolysis, and is closely linked to the impurities present in the TiB2 material after sintering. The aim of this work has been to achieve a better understanding of the sintering process and the mechanisms of aluminium penetration in TiB2. Detailed investigations of the microstructural development during sintering and penetration have provided the basis for such an understanding, using transmission electron microscopy (TEM) as the main characterisation tool. Hot pressing at 5.0 MPa in an argon atmosphere at temperatures in the range 1790 - 1960 °C was employed to achieve high purity and high density TiB2 compacts. Increasing the sintering temperature increased the density from 90 to 96% in the samples sintered with no additives, but grain growth caused extensive microcracking at the highest temperature. In some samples 2 wt% of elemental boron or titanium was added to investigate the effect on the sintering behaviour. These elements did not dissolve in the TiB2, but caused formation of secondary phases in the grain junctions. Adding boron resulted in a slight increase in density and in formation of B4C. The reducing atmosphere set up by the graphite elements of the hot press is the origin of the carbon, and also removed the surface oxides present on the TiB2 powder. Evaporation of boron oxide is another possible mechanism of oxygen removal, both mechanisms depend upon an open network of pores to exist for gas transport to be possible. The absence of any grain boundary phase after sintering was proven by high resolution lattice imaging. The addition of titanium resulted in high densities, 99%, to be reached even at the lowest sintering temperature thanks to the introduction of liquid phase sintering. Due to the fast densification and closure of the pores the surface oxides have not been reduced by the sintering atmosphere, but has instead formed a Ti-O-rich liquid. Upon cooling this wetting liquid phase has crystallised into four different oxide phases located in the grain junctions of the sintered material. In VI addition the liquid sintering has resulted in a strong segregation of iron to the grain boundaries. The equivalent of one monolayer of iron was found in a two nanometer wide region at the grain boundary, but lattice imaging still showed the grain boundary to be clean. To investigate the aluminium penetration into the various TiB2 materials they were exposed to liquid aluminium at 980 °C for 24 hours. All the materials were penetrated in this test, but the amount and appearance of the penetrating aluminium depend on the sintering aid used. Due to the open porosity and microcracking the pure (pure = no sintering additives) TiB2 materials were easily penetrated and the pores were filled up with metallic aluminium. The increased density of the samples with boron addition prevented this rapid penetration. However, grain boundary penetration took place in both kind of materials and resulted in thin, typically 10 nm, wetting films of metallic aluminium at some grain boundaries. Other boundaries were clean, as judged from high resolution lattice images, or contained small lens-shaped aluminium particles. The difference in penetration behaviour between individual grain boundaries is believed to be caused by differences in grain boundary energy. No relation to segregants or grain boundary misorientation could be found, however. The orientation of the grain boundary plane and dewetting of thin films upon cooling of the samples are suggested as explanations for the observed microstructural development, but this needs further investigation for confirmation. The samples sintered with titanium addition suffered from extensive penetration in spite of their high density. Un like the other materials the grain boundaries became faceted and contained thicker films of metallic aluminium. This is believed to be caused by to the iron segregations increasing the solubility, and thus the rate of dissolution, of TiB2 in an iron-aluminium grain boundary phase. Finally all the secondary phases present in the grain junctions after sintering, except from the B4C phase, were found to react with the penetrating alu minium. The reactions were not, however, found to cause swelling and cracking of the materials as has been suggested by other authors. 1 1. INTRODUCTION The present work is motivated by the request for a wettable, inert material to replace the carbon cathodes currently used in aluminium electrolysis cells. This will in turn allow considerable improvements in terms of lowered energy consumption. Thanks to its inertness and excellent wettability by liquid aluminium, titanium diboride (TiB2) has for many years been considered the most promising choice of material for a new cathode. Despite the development work and many test runs performed by the aluminium industry, it has not yet been implemented in large scale production of aluminium, however.