Structural Transformation and Pressure-Induced Phase Transition in PZT

Angle-dispersive X-ray diffraction and Raman scattering measurements were carried out to follow the crystal-to-crystal transformation occurring in PbZr 0.52 Ti 0.48 O 3 ceramic powders with either increasing hydrostatic pressure or decreasing temperature. A low-temperature monoclinic phase was confirmed to exist below 298 K. External pressures of several GPa were found to fundamentally alter the structural and polar properties of PZT near the MPB and novel phase transitions were found to occur at close to 5GPa at 298 K, 7.5 GPa at 154 K and 9 GPa at 44 K. Whereas X-ray diffraction data indicated no deviation from the cubic symmetry above these pressures at each given temperatures, the unusually strong and persistent Raman signals, similar to relaxor-specific spectral signatures, indicated the presence of symmetry-breaking disorder in this phase. Based on these results, an updated version of the Pressure-Temperature phase diagram is proposed.     

Effect of zirconia particle size on the properties of alumina-spinel castables

The industrial application of alumina-spinel refractory castables has crucial requirements on the service performance. Thus, the effects of different sized desilicated zirconia particles on the castables microstructure, thermal-mechanical properties and high temperature elastic modulus have been investigated. The zirconia particle sizes were varied from 1000 µm to 2.5 µm (d₅₀). It was observed that the finer (below 88 µm) zirconia particles were beneficial to improve the cold modulus of rupture (CMOR) and the hot modulus of rupture (HMOR), but could not effectively enhance the thermal shock resistance. Fine zirconia particles can homogeneously disperse in the matrix and significantly promote the sintering process. Accompanied with the phase transformation of zirconia, both the high density of matrix cracks and the strong ceramic bonding (between the coarse grains and the matrix) were found in the refractory castables, which was responsible for an increase of CMOR. However, the binding characteristic could also give rise to the high stored elastic energy that was adverse to the thermal shock resistance, and the excessive amount of preexisting matrix cracks could induce more microdamage during the thermal shock. Additionally, it was proposed that the second-phase dispersion reinforcement and the highly ceramics bonding resulted in the superior HMOR when introducing fine ZrO₂ particles.     

Competition between ferromagnetic and antiferromagnetic interaction in monoclinic LiMnO2

The electronic and magnetic structures of monoclinic LiMnO₂ are investigated by first-principles calculations with spin-polarized generalized gradient approximation. It is found that the Mn³⁺ ion is in the high-spin state. The ground state is found to be an insulating state with competing ferromagnetic and antiferromagnetic interaction, and it is this competition that results in the spin-glass behavior observed in monoclinic LiMnO₂.     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.     

Formation of thaumasite in synthetic cement mineral slurries

In order to investigate the effect of alite polymorphism on the susceptibility of cements to thaumasite formation, slurries were prepared from magnesium sulphate solution and mixtures of calcium carbonate with synthetic cement preparations plus additions of 0%, 1% or 2% w/w alumina. Two synthetic cement preparations were used; a co-prepared mixture of monoclinic alite (C₅₄S₁₆AM) with β-C₂S (designated A) and a 99% pure, primarily triclinic C₃S (designated B). The slurries were prepared after the manner of Gaze [Cement Concrete Res. 27(2) (1997) 259] and maintained in a state of continual agitation at 5 °C for 100 days. After this period, the slurries were dried and characterised using X-ray diffraction. In slurries made with A, both ettringite and thaumasite (or a solid solution thereof) were detected, the apparent ratio of the former to the latter increasing with alumina addition. No gypsum was detected. In slurries made with B, the only sulphate phase detected was gypsum.

This interesting observation leads to a number of avenues for discussion;

• that alumina must be present in sufficient quantities within the clinker or gel structure, rather than as an addition, to initiate TF;

• that TF is only initiated by C₂S, not C₃S;

• that TF cannot be initiated by triclinic C₃S.

     

Electrosynthesis and characterization of iron selenide thin films

Iron selenide thin films have been deposited onto stainless steel and fluorine-doped tin oxide (FTO) glass substrates by the electrodeposition process, in potentiostatic mode using ferric chloride (FeCl₃) and selenium dioxide (SeO₂) salts. The deposition mechanism and growth of the films were investigated by cyclic voltammetry and chronoamperometry. The structural, morphological, compositional and optical properties of the deposited films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques, respectively. XRD studies reveal that the films are polycrystalline with monoclinic crystal structure. The surface morphology study shows that grains are uniformly distributed over the entire surface of the substrate. EDAX study reveals that the iron selenide films are rich in iron. Optical absorption study shows the presence of direct transition with bandgap energy of 1.23 eV.     

Effect of some nucleating agents on crystallizing phases and microstructure in Li2O–BaO–Al2O3–SiO2 system

Different concentrations of LiF and Cr₂O₃ were incorporated in Li, Ba aluminosilicate glass to establish their effects on the crystallization process. The kinetics of phase transformations, the final crystalline phase assemblages and the microstructures formed were found to be dependent on the types and concentration of the nucleant involved. Cr₂O₃ was found to increase the melting temperature and favor crystallization of β-spodumene ss hexacelsian and traces of monoclinic celsian. It also favors volume crystallization of finer grained microstructure. LiF was found to decrease the melting temperature and favor crystallization of β-spodumene ss and monoclinic celsian. LiF in low concentrations greatly facilitates the crystallization process, the β-eucryptite ss/β-spodumene ss transformation and hexacelsian/monoclinic celsian transformation. It also stimulates surface crystallization with holocrystalline coarse non-uniform textures. The effects of various Cr₂O₃ concentrations were discussed on the basis of the increased viscosity and separation of Cr₂O₃ and/or chromium spinel phases. The role of LiF was attributed to the role of fluorine ions in reducing the viscosity of the glasses, consequently facilitating crystallization of the structurally more complex silicate in addition to favoring reaching thermodynamic equilibrium.     

Electron transport mechanism of tungsten trioxide powder thin film studied by investigating effect of annealing on resistivity

We report a new approach for improving the recharging and discharging speed of lithium ion batteries based on understanding of the electron conduction mechanism of tungsten trioxide (WO₃) powder thin films fabricated from nanoparticles and used in lithium ion battery electrodes. Resistivity measurements are carried out after annealing in N₂ or 5% O₂ + 95% N₂ ambient. Annealing in N₂ ambient decreases the resistivity owing to the increased number of oxygen vacancies in the WO₃ thin film. Fitting results obtained from the resistivity are used to propose the simultaneous existence of two types of electron conduction mechanism, band conduction and nearest-neighbor hopping (NNH) conduction, contributing to electron conduction in WO₃ thin films.     

First-principles calculations of electronic and optical properties of Ti-doped monoclinic HfO2

Based on the first-principles density-functional theory, the electronic structures and optical properties of monoclinic HfO₂ and Ti-doped m-HfO₂ are comparatively investigated. The calculated lattice parameters of m-HfO₂ are in good agreement with the experimental values and the previous works, and the incorporation of Ti into HfO₂ induces a decrease in the lattice parameters. Electronic structures of m-HfO₂ and Ti-doped HfO₂ are studied through the densities of states (DOS) and band structures. The results indicate that the Ti substitution of Hf sites modifies the conduction band structure of HfO₂, which leads to a reduction of the band gap of HfO₂. The complex dielectric function and refractive index are calculated and the peak position distributions of imaginary parts of the complex dielectric function have been explained. The calculated optical properties are consistent with the experimental measurements for m-HfO₂.