Solid solution directionally solidified eutectic oxide composites: Part II. Co 1-x Ni x O single-crystal growth and char
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M. Vela´zquez and A. Revcolevschi Laboratoire de Physico-chimie de l’Etat Solide, Universite´ de Paris—Sud Centre d’Orsay, France (Received 20 July 2001; accepted 10 January 2002)
The growth and structure of the Co1−x NixO series of single crystals with 0 艋 x 艋 1 were investigated with the goal of achieving a single-phase crystal over several centimeters of bulk growth. The single crystals were grown via the floating zone method controlling the partial pressure of oxygen to prevent secondary phase precipitation. The resulting crystals were single phase for all compositions. CoO was a single-domain crystal but contained microvoids. The lattice parameters followed a rule-of-mixtures trend, but the coefficient of thermal expansion exhibited a maximum in the middle of the solid solution, which was attributed to enhanced vacancy formation.
I. INTRODUCTION
This paper was written in tandem with the preceding paper in this issue on directionally solidified eutectics (DSEs) Part I. The purpose of this work was to examine the growth, structure, and thermal expansion behavior of the solid solution (Co1−x Nix)O in single-crystal form for comparison with the corresponding (Co1−x NixO)/ ZrO2(CaO) quaternary eutectic. NiO and CoO single crystals have been grown from the melt by a variety of methods including flame fusion (Verneuil) and the optical floating zone method.1– 4 A challenge in the growth of these otherwise simple oxides is to avoid the occurrence of porosity, precipitates, and multiple domains or grains.5–7 Single crystals of the solid solution (Co1−x Nix)O have been grown previously for 0 艋 x 艋 1 by the flame fusion method9 and by the optical floating zone method in air, but with the notable formation of Co3O4 precipitates at mole fractions of CoO above x ⳱ 0.8.9,10 Further studies of the intrinsic properties of this solid solution series require sizeable single crystals free from grain boundaries, secondary phases, or porosity. In this paper, we describe growth of single domain crystals along the solid solution (Co1−x Nix)O for 0 艋 x 艋 1 using reducing atmospheres to block the formation of Co3O4 precipitates at high CoO fractions. The growth crystallography, composition, and thermal expansion behavior of these crystals will be discussed. II. EXPERIMENTAL METHODS
All of the crystals in this study were grown via the floating zone method using an optical image furnace.11 The optical floating zone method has the advantage over 768
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J. Mater. Res., Vol. 17, No. 4, Apr 2002 Downloaded: 03 Apr 2015
other crystal growth techniques of being crucible-free and of allowing controlled atmospheres during growth. Feed rods were prepared by high-temperature solid-state reaction from mechanically mixed commercial powders of Co3O4 and NiO (>99.9% purity). These powders were isostatically cold pressed under 2 kbar pressure into cylindrical rods and then sintered in air at 1200 °C for 24 h. Seed crystals of 〈100〉-oriented NiO were used to initiate the growing crystal. Crystal growth was performed in a c
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