Optical Lfoating Zone Method Growth And Photoelectric Properties Of CaNb₂O₆ And BiNbO₄ Crystals

The beauty of the single crystals has been treasured since prehistoric times. Thenatures of many materials can only be discovered and studied on the crystal structure.Most of materials in nature are existed in the form of polycrystalline ormicrocrystalline, and the single-crystal with large size and complete structure is few.Since the late 19^thcentury, with the development of scientific technology, people haverealized the importance of the role that the crystal plays in terms of developinghigh-technology. Therefore, the history of applying the crystal materials is muchshorter than that of other microcrystalline or polycrystalline materials. From then on,scientists have been making great efforts to make artificial crystals. After nearly acentury of research, great achievements have been made both in terms of technologyand methods of making crystal and applying it. At present, nearly all the naturalmineral crystals could be synthesized or grown. In addition, great amount of newcrystals have been made with artificial methods. Various growth technologies andmethods of crystals have rapidly flourished. However, this does not mean that theartificial synthesis method has been perfect and mature. There are still many difficultproblems to be further explored and studied in the aspect of the crystal growth theoryand the operation process of crystal growth. The optical float region method, which isone of melt growth methods, refers to that the infrared light from halogen lamp orxenon lamp focuses through the elliptic mirror, and the melt maintain its shape relyingon its own surface tension and the crystals grow in the direction of the vertical upward.Due to its advantages such as its fast growth and it does not cause contact pollutionwith clamp pans etc, it has been widely applied to the growth of diverse crystals,especially, to the growth of oxides possessing strong response or higher melting pointand single metal compounds. CaNb₂O₆has the columbite structure. Due to its advantages such as excellentdielectric performance, unique optical and magnetism performance, CaNb₂O₆hasbecome a crystal matrix material which has been widely studied in recent years. Atpresent, the nature of CaNb₂O₆is mainly understood by means of polycrystalline andnanocrystalline samples, and there are still contradictions between the results of somebasic experiments. Many fundamental properties of materials can be confirmed onlyafter the experimental measurements of crystals are conducted. However, there aresome certain difficulties in the growth of CaNb₂O₆with its big size and high quality.One possible reason is that CaNb₂O₆ has been in the boundary of the stable structureof columbite, of which larger AO6octahedra could increase the chance of distortedarrangement of NbO6octahedra. At the time of the liquid-solid changes, it is difficultfor the atom to make orderly arrangement and to form large-sized single crystal.BiNbO₄ceramic, which is a kind of microwave dielectric ceramics, has becomea focus of concern in recent years. Nowadays, understanding the nature of BiNbO₄ismainly available through polycrystalline and nanocrystalline samples. There still existcertain discrepancies between the results of some basic experiments. Many elementalproperties of materials can be further confirmed only after the experimentalmeasurements of crystals. However, there are some certain difficulties in the growthof BiNbO₄crystal with its large size and high quality, which may be due to the highermelting point of BiNbO₄ than that of Bi₂O₃, and the element Bi₂O₃will be missing inthe process of BiNbO₄ growth so that the mixed crystal consisted of Bi5Nb₃O₁5andβ-BiNbO₄is formed.This research has successfully developed the single crystals, CaNb₂O₆andBiNbO₄ with FZ-T-10000-H-VI-VP single crystal furnace; characterized their growthdirection and crystal quality, calculates many kinds of optical parameters combinedwith the test results and does a research of its the photoluminescence.1. Several CaNb₂O₆single crystals with about 7 mm in diameter and 8 mm in lengthhave been grown by optical floating zone method. The as-grown crystal are colorlessand transparence with the orthorhombic columbite structure. A crystal wafer was cutperpendicular to the growth direction. Transmission polarized light microscopy measurements show that the crystal wafer is free of low-angle grain boundaries andinclusions. The measured transmission and absorption spectra of the as-grown crystalat room-temperature, which is transparent (from 50 to 62%) in the visible to infraredregion (400-1000 nm) and has a low absorption coefficient (α¹.56). Based on thetheory of band to band transitions, the types of transition and the direct energy gapwere determined as Eg=4.28 eV and direct transitions, respectively. The wavelengthdependent refractive index and extinction coefficient the CaNb2O6crystal have beenderived from the measured T andαspectra. In addition, the photoluminescencespectra exhibit two broad emission band centered at 428 and 465 nm.2.β-BiNbsingle crystals are grown by the optical floating zone method. Theas-grown crystals are lemon yellow in color, and the dimensions areФ7 mm×L 30mm, with the largest crystal domain beingФ6-7 mm×L 6 mm. The powder X-raydiffraction analysis shows that the crystals areβ-BiNbO₄. The crystal grows along thea-axis and the cleavage plane is the (2 0 0) plane. Based on the theory of band to bandtransitions, the types of transition and the direct energy gap were determined asEg=3.16 eV and direct transitions, respectively. The wavelength dependent refractiveindex and extinction coefficient theβ-BiNbO₄crystal have been derived from themeasured T andαspectra. We firstly investigate the optical property ofβ-BiNbO₄crystals. It can be observed that the emission spectrum ofβ-BiNbO₄crystals showtwo broad peaks centered at 423 nm and 470 nm, respectively. the excitation spectrumofβ-BiNbO₄crystals have three excitation peaks located at 210 nm, 268 nm and370nm, respectively. The luminescence mechanism of theβ-BiNbO₄crystals arestudied byβ-BiNbO₄structure and Bi3+energy level diagram.Another research system is sol–gel synthesis, solid sintering, and thermalstability of single-phase YCoO₃. So far, the research on the preparation methods ofnanometer materials is a very important research area. For it has the element Co oftransition family, YCoO₃possesses special electromagnetic properties. And in recentyears, it has been widely applied to the areas of gas sensitive materials, thermoelectricmaterials and the purification of car exhaust etc. There are certain difficulties in thepreparation of the YCoO₃pure phase, and the YCoO₃samples reported all have raw material phase of Y2O₃. The thesis has successfully obtained the single-phasenanometer powder of YCoO₃at a relatively low temperature with Sol-Gel method,and has obtained the optimum synthesis condition under air and oxygen atmosphere.Besides, the research makes an analysis of its thermal stability.3. Using Y(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O as the starting materials, thesingle-phase YCoO₃has been synthesized by a two-step process involving a sol–geltechnique and a sintering method. The structure, electromagnetic properties, andthermal stability of the synthesized samples were measured by XRD,thermogravimmetry and differential thermal analysis (TG–DTA), TEM, and vibrationsample magnetometer (VSM). The experimental results show that the synthesisconditions of the single-phase YCoO₃are 900–950℃for 10 h in air. And thesynthesis temperature is extended to 900–1000℃and the sintering time is shortenedto 5 h in an oxygen atmosphere. The synthesized powders have orthorhombicstructure, with a diameter about 30 nm, which is stable in air below1050℃and inoxygen atmosphere below 1100℃. Above those temperatures, YCoO₃decomposesinto Y2O₃and Co₃O₄.