An Exploration In LaCoO₃ Crystal Growth Using Optical Floating Zone Method

At present, almost all the natural crystals can be produced manually, however, it does not mean that the methods and techniques of crystal growth have been improved to the extent. In another words, there is a long way to go yet. Especially, the demands of crystal materials with good quality and high-tech are increasing, while the varieties are included. The methods and techniques of crystal growth which we have are either obsolete or incapable of new materials. In addition, it is necessary to increase the quality, utilization and to reduce cost in the production of artificial crystals. Therefore, the crystal growth methods and techniques are still required to deeply study, in order to the adaptation to the use of crystal materials in the ever-increasing trend.Pfann announced the Melting Zone method for the first time in his paper "Principles of Zone-Melting", published in 1952. Followed by his principle, a new method for crystal growth was carried out by the latter scientists, which called"Floating Zone Method"or"Vertical Zone Melting Method". This method can deal with refractory metals, oxides, halides, carbide and other materials which have a high melting point. All these advantages are due to its uniquely crucible-free technique.LaCoO₃ is the inimitable perovskite oxide because of its character that with the rise of temperature, it will change from non-magnetic state (the total spin is zero) to the paramagnetic state. It is an important for its theoreticalsignificance. LaCoO₃ can be used as catalytic materials, high-temperature superconducting materials, GMR materials etc..Therefore, we have chosen LaCoO₃ to carry out the crystal investigation. Four ellipsoidal mirrors infrared furnace (FZ-T-10000-H-VI-VP) is used to make the crystal growth happen. This paper aims to explore some of the key issues in the growth process via optical Floating Zone method.The ExperimentWe choose La2O3 and Co3O4 powders as initial raw materials to prepare LaCoO₃ perovskite oxides using a solid-phase sintering method. The equipment used in Crystal Growth is Four ellipsoidal mirrors infrared furnace (FZ-T-10000-H-VI-VP) imported from Japan. The structure, morphology and micro-components of the samples are analyzed using XRD, SEM and micro-Raman spectroscopy.The Results and Analysis:In the preparation process, we choose the excessive ratio of Co3O4 to make sure of completely reaction of La2O3 due to the volatilization of Co. In our experiments, we investigate different ratios (7.5%¹5% excessive atomic ratio of Co), the sintering temperature and the reaction time on the impact of the final products. The results show that the excessive ratio at 12.5% Co makes the products contain only a small amount of CoO, without residual La2O3 at 1050℃. The increase of sintering time and temperature can not reduce the miscellaneous phase as CoO. Thus considered the entire growth process, the best way to synthesize the initial raw LaCoO₃ powder issintering 15% excessive atomic ratio of Co at 1050℃.In Floating Zone Method, the initial raw LaCoO₃ powders are packed into the rubber tube of appropriate diameter and length to make a rod. To obtain a dense rod, we investigate the influence of the sintering time and temperature. Structural analysis indicates that LaCoO₃ is still the main phase in the rod, and the crystallization has not changed under conditions of 1200℃, sintering 5h and 10h respectively. The diffraction peaks which indicate the existence of Co can be observed using XRD spectra. Compared with the original powder, the contents of the rod do not change obviously. Morphology and composition analysis shows that the rod's grain size is small and uniform, 1-3μm in size around. The presence of porosity. could still be seen When the conditions change to 1510℃, 6 hours, the diffraction peaks show that sintering temperature can not reduce the miscellaneous phase. Morphology and composition analysis shows that the rod's small grains assemble and become larger; the largest grain size is 15-20μm. Some large stomata appear. Therefore, the rod sintered at 1200℃is suitable for further crystal growth.In crystal growth processes, we adopt two modes: reverse rotation mode and no rotation mode. We investigate the stability of the melting zone in the vertical moving process and the sample's homogeneity in the two modes. The suitable heating rate and power control are taken in order to determine the best conditions for melting within a lot of works. The uniformity of the pre-rods can stabilize the melting zone. SEM and micro-Raman analysisshow that the internal samples which are gained in two different modes are uniform LaCoO₃ phase, i.e., the influences of the two modes are no obvious. There is a very small amount of CoO particles on the surface of the sample. The concretion of the molten LaCoO₃ is composed of small grains aggregation. The cross-section shows that the gap is very small, and the density of the grains is high. Bigger single-crystal grains had not been gained in the experiments due to the lack of seeds up to now. However, XRD patterns of the samples show that crystalline state of solidified molten LaCoO₃ has been improved, and an oriented growth trend has been found.Discussion:In this paper, some key problems in crystal growth process using Four ellipsoidal mirrors infrared furnace (FZ-T-10000-H-VI-VP) are discussed and some valuable experimental results are gained. Crystal Growth is a difficult work because some practical techniques and experience that can not be neither explained theoretically nor denoted through measuring characterization, but they are very important. A lot of experience about preparing rods, the control of the Stability and the speed of the melting zone has been accumulated in present work. They are very useful and build a foundation for the future .