Yijian Jiang

Raman scattering study of a phase transition in tantalum pentoxide

Ta2O5 ceramics and crystals were prepared and analyzed using Raman spectroscopy in the temperature range 25–873 K. The low-energy phonon bands (ν < 100 cm−1) of Ta2O5 are assigned to external modes originating from the interaction between different Ta polyhedra and TaOn5 − 2n Ta6O126+ clusters. The temperature and polarization dependence of external modes suggests the existence of clusters in the material. A softening of the low-energy bands was observed with increasing temperature. The spectral evolutions show a soft mode-derived second-order monoclinic to orthorhombic phase transition at about 600 K in Ta2O5. Copyright

Micro-Raman scattering and x-ray diffraction studies of (Ta2O5)1-x(TiO2)x ceramics

High dielectric constant (Ta2O5)1−x(TiO2)x ceramics for x=0.00, 0.05, 0.08, and 0.11 were prepared and studied utilizing micro-Raman scattering and x-ray diffraction techniques. The modification in the lowest frequency phonon mode of pure Ta2O5 with increasing TiO2 content in these ceramics was assigned as Ta–Ti vibrations originating from the interaction between TiO6 octahedra and TaOn5-2n or Ta6O12+6 clusters. Raman spectra, in the range −200–900 °C, reveal a softening of this mode with increasing temperature followed by a structural transformation. A triclinic to monoclinic phase (Hmon′) transition was observed at about 300, 360, 450, and 540 °C for x=0.00, 0.05, 0.08, and 0.11, respectively. It was also found that reannealing at 1150 °C for 12 h transforms these ceramics from triclinic to a metastable monoclinic phase Hmon, which was found to be an irreversible process.

Processing and annealing conditions on the dielectric properties of (Ta2O5)0.92(TiO2)0.08 ceramics

Abstract Dielectric properties of (Ta2O5)0.92(TiO2)0.08 polycrystalline ceramics prepared under different processing and annealing conditions are investigated at 1 MHz between −100 and 100 °C. The dielectric permittivity can be significantly enhanced (up to ∼80%) by increasing calcination time and sintering temperature. Annealing at higher temperature in air or in oxygen caused a moderate enhancement of the dielectric permittivity with only slight influence on dielectric loss. Microstructure of (Ta2O5)0.92(TiO2)0.08 ceramics was studied and reported in this paper; along with the discussions on its relation with processing conditions and resulted dielectric properties.

Structural modifications in titania-doped tantalum pentoxide crystals: a Raman scattering study

Abstract Tantalum pentoxide (Ta 2 O 5 ) is promising for coating and piezoelectric applications and has been considered as the dielectric gate material for the next generation of memory devices. In this work, TiO 2 -doped Ta 2 O 5 crystals were prepared using the laser-heated pedestal growth technique, as grown specimens were found to crystallize in monoclinic phase at room temperature. The structural modifications in these crystals resulting from variation of TiO 2 composition (0–11%) and temperature (−248–900°C) were studied using Raman spectroscopy. The low frequency external modes ( v −1 ) that originate from the interaction between TaO 5−2 n n /Ta 6 O +6 12 clusters/polyhedra exhibit a strong compositional and temperature dependence in terms of their intensity and frequency variations. The Raman spectral evolutions suggested a monoclinic to orthorhombic structural phase transition at about 327, 397, 487, and 577°C for 0, 5, 8, and 11% TiO 2 -doped Ta 2 O 5 , respectively.

Structure and magnetic properties of magnetically isotropic and anisotropic Nd–Fe–B permanent magnets prepared by spark plasma sintering technology

Spark plasma sintering technique had been applied to prepare bulk isotropic and anisotropic nanostructured Nd–Fe–B permanent magnets via hot pressing and subsequent hot deformation process. Influences of processing conditions and deformation height reduction on the structure and magnetic properties of the magnets were investigated. For the hot deformed magnet with 80% height reduction, XRD patterns of the anisotropic magnets show dominant (00l) diffraction peaks indicating evident c-axis crystallographic alignment in the magnet. Under the optimal processing conditions, the anisotropic magnet with 80% height reduction exhibits excellent magnetic properties as remanence (Br) of 1.492 T, coercive force (Hci) of 1004 kA/m, and the maximum energy product [(BH)max] of 400 kJ/m3, which are among the highest reported magnetic properties of nanostructured Nd–Fe–B permanent magnets.

Growth and Properties of CaTiO3 Single Crystal Fibers

Single crystal fibers of CaTiO3 have been grown by the Laser Heated Pedestal Growth (LHPG) method up to several cm in length with diameters of 500–700 μm. The crystals, characterized by X-ray diffraction analysis, have the distorted orthorhombic perovskite structure. Dielectric constant, dielectric loss and temperature coefficient of resonant frequency along the [101]o and [010]o (o means orthorhombic unit cell) direction of the crystal were investigated over the temperature range from −250°C to 100°C. The results of thermal expansion measurements up to 850°C reveal no unusual behavior associated with phase transitions. One of the modes of twinning reported by Bowman from optical analysis is confirmed by optical microscopy.

Single crystal fibers of rare earth ortho-niobate: Growth, ferroelastic property, and domain studies

One dimensional single crystals of LaNbO/sub 4/, NdNbO/sub 4/ and CeNbO/sub 4/ have been grown successfully by a laser heated pedestal growth technique. Some physical properties and the ferroic phase transitions as a function of temperature are studied over the range room temperature to 800/spl deg/C by various thermal, electric and optical analytical techniques. Twin walls are observed, and their relations with the ferroelastic phase transition are discussed.