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Avhandling Och Då Speciellt Till PREPARATION AND CHARACTERISATION OF REFRACTORY WHISKERS AND SELECTED ALUMINA COMPOSITES Mats Carlsson Department of Inorganic Chemistry Stockholm University 2004 Doctoral Dissertation 2004 Department of Inorganic Chemistry Stockholm University 106 91 Stockholm Sweden © Mats Carlsson ISBN 91-7265-866-5 Printed in Sweden by Intellecta DocuSys AB ABSTRACT A whisker is a common name of single crystalline inorganic fibre of small dimensions, typically 0.5−1 µm in diameter and 20−50 µm in length. Whiskers are mainly used as reinforcement of ceramics. This work describes the synthesis and characterisation of new whisker types. Ti0.33Ta0.33Nb0.33CxN1-x, TiB2, B4C, and LaxCe1-xB6 have been prepared by carbothermal vapour–liquid–solid (CTR-VLS) growth mechanisms in the temperature range 900–1800°C, in argon or nitrogen. Generally, carbon and different suitable oxides were used as whisker precursors. The oxides reacted via a carbothermal reduction process. A halogenide salt was added to form gaseous metal halogenides or oxohalogenides and small amount of a transition metal was added to catalyse the whisker growth. In this mechanism, the whisker constituents are dissolved into the catalyst, in liquid phase, which becomes supersaturated. Then a whisker could nucleate and grow out under continuous feed of constituents. The syntheses of TiC, TiB2, and B4C were followed at ordinary synthesis conditions by means of mass spectrometry (MS), thermogravimetry (TG), differential thermal analysis (DTA) and quenching. The main reaction starting temperatures and reaction time for the different mixtures was revealed, and it was found that the temperature inside the crucible during the reactions was up to 100°C below the furnace set-point, due to endothermic nature of the reactions. Quench experiments showed that whiskers were formed already when reaching the temperature plateau, but the yield increased fast with the holding time and reached a maximum after about 20−30 minutes. Growth models for whisker formation have been proposed. Alumina based composites reinforced by (2−5 vol.%) TiCnano and TiNnano and 25 vol.% of carbide, and boride phases (whiskers and particulates of TiC, TiN, TaC, NbC, (Ti,Ta)C, (Ti,Ta,Nb)C, SiC, TiB2 and B4C) have been prepared by a developed aqueous colloidal processing route followed by hot pressing for 90 min at 1700°C, 28 MPa or SPS sintering for 5 minutes at 1200−1600°C and 75 MPa. Vickers indentation measurements showed that the lowest possible sintering temperature is to prefer from mechanical properties point of view. In the TiNnano composites the fracture mode was typically intergranular, while it was transgranular in the SiCnano composites. The whisker and particulate composites have been compared in terms of e.g. microstructure and mechanical properties. Generally, additions of whiskers yielded higher fracture toughness compared to particulates. Composites of commercially available SiC whiskers showed best mechanical properties with a low spread but all the other whisker phases, especially TiB2, exhibited a great potential as reinforcement materials. LIST OF PAPERS THIS THESIS IS BASED ON THE FOLLOWING PAPERS I. M. Carlsson, M. Johnsson, and M. Nygren: “Synthesis and characterisation of Ti0.33Ta0.33Nb0.33C and Ta0.33Ti0.33Nb0.33CxN1-x whiskers”, J. Am. Ceram. Soc., 82, 1969-76 (1999) II. M. Carlsson, P. Alberius-Henning and M. Johnsson:”Vapour-Liquid-Solid growth of TiB2 whiskers”, J. Mat. Sci., 37, 2917-2925 (2002) III. M. Carlsson, J. G. Garcia and M. Johnsson: “Synthesis and characterisation of boron carbide whiskers and thin elongated platelets”, J. Cryst. Growth, 236, 466-476 (2002) IV. E. Laarz, M. Carlsson, B. Vivien, M. Johnsson, M. Nygren and L. Bergström: “Colloidal processing of Al2O3-based composites reinforced with TiN and TiC particulates, whiskers and nanoparticles”, J. Eur. Cer. Soc. 21, 1027-1035, (2001) V. M. Carlsson, M. Johnsson and A. Pohl: “Preparation and Characterization of Alumina based TiNn and SiCn Composites” Materials Research Society Symposium Proceedings, Vol 740 (Nanomaterials for Structural Applications), 173-178 (2002). VI. M. Carlsson, M. Johnsson, J. G. Garcia, A. Gulian: Synthesis of LaxCe1-xB6 whiskers, submitted to J. Mat. Sci. Lett. (2004) ADDITIONAL PAPERS, NOT DISCUSSED VII. M. Johnsson, M. Carlsson, N. Ahlén, and M. Nygren: “Synthesis of (Ta0.5Ti0.5)C and (Ta0.33Ti0.33Nb0.33)C and whiskers and fibers”, Innovative Processing and Synthesis of Ceramics, Glasses, and Composites II, Eds. P.Bansal and J.P. Singh, Ceramic Transactions, Vol. 94, The American Ceramic Society, 473-479 (1999). VIII. M. Carlsson and M. Johnsson: “Synthesis of TiB2 whiskers”, Ceramic Engineering and Science Proceedings Vol 21(4), The American Ceramic Society, 375-382. (2000) IX. M. Johnsson, M. Carlsson, M. Nygren: “Synthesis of transition metal carbide, carbonitride and boride whiskers”,Key Engineering Materials, 247 (Advanced Ceramics and Composites), 145- 148 (2003). X. A. Gulian, K. Wood, D. Van Vechten, G. Fritz, H. -D. Wu, S. Bounnak, K. Bussman, K. Winzer, S. Kunii, V. Gurin, M. Morsukova, C. Mitterer, M. Carlsson, F. Golf, A. Kuzanyan, G. Badalyan, S. Harutyunyan, S. Petrosyan, V. Vardanyan, T. Paronyan, V. Nikoghosyan: “Current developmental status of thermoelectric (QVD) detectors”, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 520(1-3), 36-40 (2004) XI. K. Wood, D. Van Vechten, G. Fritz, H. -D. Wu, S. Bounnak, K. Bussman, K. Winzer, S. Kunii, V. Gurin, M. Korsukova, C. Mitterer, M. Carlsson, F. Golf, A. Kuzanyan, G. Badalyan, S. Harutyunyan, S. Petrosyan, V. Vardanyan, T. Paronyan, V. Nikoghosyanet A. Gulian: “Toward ultimate performance limits of thermoelectric (QVD) detectors”, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 520(1-3), 56-59 (2004). TABLE OF CONTENTS 1. INTRODUCTION.......................................................................................................................... 3 1.1. Ceramics .................................................................................................................................... 3 1.2. Manufacturing of ceramics ....................................................................................................... 5 1.2.1. Powder processing ...................................................................................................................................5 1.2.2. Sintering.....................................................................................................................................................6 1.3. Mechanical properties............................................................................................................... 7 1.3.1. Toughening by reinforcement materials...............................................................................10 1.3.2. Reinforcement materials and mechanisms..........................................................................................10 1.4. Whiskers ...................................................................................................................................13 1.4.1. Whisker synthesis methods...................................................................................................................14 1.4.1.1. Liquid-Solid (LS).............................................................................................................................14 1.4.1.2. Vapour-Solid (VS)...........................................................................................................................14 1.4.1.3. Vapour-Liquid-Solid (VLS) ...........................................................................................................15 1.4.2. Health hazards concerning whiskers....................................................................................................16 1.5. The aim of the present work ....................................................................................................17 1.5.1. Whisker synthesis ...................................................................................................................................17 1.5.1.1. Background to the synthesised whiskers .....................................................................................17 1.5.1.2. CTR-VLS Synthesis parameters....................................................................................................20 1.5.2. Ceramic composites ...............................................................................................................................21 1.5.2.1. Powder processing..........................................................................................................................21 1.5.2.2. Preparation and characterisation of nano-composites...............................................................21 1.5.2.3. Preparation and characterisation of whisker and particulate composites................................22 2. EXPERIMENTAL.........................................................................................................................25 2.1. Thermodynamic predictions for whisker synthesis ................................................................25 2.2. Whisker synthesis procedure ...................................................................................................25 2.2.1. Starting materials ....................................................................................................................................25
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