Synthesis of single-crystal barium titanate nanorods transformed from potassium titanate nanostructures

Single-crystal barium titanate (BaTiO₃) nanorods were synthesized by a hydrothermal reaction using precursors of potassium titanate (K₂O·nTiO₂, n = 4 or 6) nanostructures. The precursors of potassium tetratitanate (K₂Ti₄O₉) and potassium hexatitanate (K₂Ti₆O₁₃) nanostructures were prepared by a sol-gel method in which a growth of K₂O·nTiO₂ (n = 4 or 6) nanorods was induced by a role of pre-crystallized K₂O phase at defined heating temperatures. The specific morphologies of BaTiO₃ nanorods featured with flat or stepped surfaces and rectangular or polygonal cross-section, were obtainable by selecting the structure of precursors.     

Crystallization of nanosized Aurivillius phase Bi2W2O9 from amorphous precursor

Nanoparticles of bismuth layered Aurivillius phase, Bi₂W₂O₉, have been synthesized by annealing of precursor prepared by high energy milling in ball mill. The obtained powders have been investigated using the XRD, TEM, SEM, diffuse reflectivity, Raman and infrared spectroscopy. The results show that mechanochemical activation allows obtaining Bi₂W₂O₉ at much lower temperatures than those required in a conventional solid state reaction or synthesis through a complex organic precursor. As a result, material with smaller grain size can be obtained. Therefore this synthesis method is the best route to enhance photocatalytic activity of Bi₂W₂O₉. Our results also show that milling time has great impact on the crystallization mechanism. Bi₂W₂O₉ crystallizes easily already at 600 °C from precursor milled by 8 h. However, prolong milling time results in stabilization of an unknown phase or phases, which crystallize below 700 °C, and transform into the well-known Bi₂W₂O₉ phase after annealing at 750 °C.     

化学沉淀法制备的α-Fe2O3非晶纳米线的晶化研究

以AAO为模板,采用化学沉淀法制备α-Fe2O3纳米线,经TEM和XRD分析表明,所制备的α-Fe2O3纳米线长度约为5 μ m,直径约为40～50nm,且为非晶态,该非晶纳米线的晶化温度约为850℃,明显高于α-Fe2O3纳米粉体的晶化温度.     

Crystallization of amorphous Al-Cu films

Quench-condensed (- 160°C) Al-Cu films in the composition range from 19% to 50% Cu were heated to room temperature and annealed up to 300°C. The temperature dependence of the resistivity and the temperature coefficient of the resistivity were observed during various cycles of temperature changes. The films are amorphous at the condensation temperature. Phase transitions which occur as the temperature is increased initiate sharp decreases in the resistance. Investigations of the film structure and composition were carried out with an electron microscope (transmission electron microscopy, electron diffraction, scanning transmission electron microscopy and X-ray microanalysis). The results could be clearly related to the phase transitions as observed via the resistance changes during annealing. For film compositions below 37.4% Cu the phase transition is determined by a eutectic crystallization into two phases. For larger copper contents, an intermediate single- phase state is produced first during the transition by polymorphous crystallization and is succeeded by a multiphase stable state.     

Crystallization of amorphous antimony films

Diffusion of the divalent impurities Mn²⁺, Ca²⁺, Cu²⁺, Fe²⁺, Co²⁺, Pb²⁺, Zn²⁺, Eu²⁺ and S^2- in sodium chloride and other alkali halide crystals has been studied by electron microscopy and diffraction. At temperatures in the range 100°–700°C the impurities diffuse to the surface and form oxides which in some cases are epitaxial. This distribution of the impurities on the surface is studied and is found to have a strong influence on the overgrowth of other epitaxial deposits.     

Anomalous dielectric properties of amorphous lead titanate

We studied the effect of a thermal treatment below the crystallization temperature on the dielectric properties of amorphous lead titanate obtained by melt quenching and containing silicon impurity. Anomalies in the permittivity and dielectric losses were observed at 200°C and at a temperature (463°C) corresponding to the ferroelectric phase transition in the crystallized material. The magnitude and shape of these anomalies depend on the thermal prehistory of a sample. The features in the dielectric properties are explained by the presence of a relict polar phase region interacting with the mobile charge carriers.     

Hydrothermal synthesis of barium titanate fine particles from amorphous and crystalline titania

Barium titanate fine particles of cubic system were synthesized hydrothermally from aqueous barium hydroxide solutions with fine particles of either amorphous or crystalline (rutile) titania in suspension. The mean size of barium titanate particles prepared from amorphous titania ranged from 0.03 m to 0.11 m, depending on hydrothermal conditions. The particle size approximately agreed with the crystallite size (0.04–0.09 m). On the other hand, the mean size of barium titanate particles prepared from rutile titania ranged from 0.2 to 0.7 m, which was about six times as large as the crystallite size. The difference of sizes of barium titanate particles prepared might be ascribed to the difference in dissolution rates of amorphous or crystalline titania particles.Nomenclature mean particle size, m - BT barium to titanium molar ratio - d crystallite size, m - K shape factor appearing in Equation 3 - half value breadth of diffraction peak - lattice strain - Bragg diffraction angle, degree - wave length of X-ray, urn - _g geometric standard deviation defined by 84.1%-size/50%-size in a log normal size distribution     

Crystallization of sputter deposited amorphous MoNi

A crystallization temperature of 1000 K was observed for an amorphous alloy of MoNi. Recovery of the amorphous matrix prior to crystallization was noted, however. During crystallization, a Mo-rich phase and a Ni-rich phase first precipitated, then reacted to form the equilibrium -phase. Exaggerated grain growth or secondary crystallization of the -phase occurred upon extensive annealing. This multi-step mechanism contrasts with that in the amorphous -phase. The multi-step crystallization process is similar to, though not as complex as, that observed in many liquid-quenched alloys. In particular, no incipient stage of crystallization was noted.     

Crystallization characteristics of amorphous Fe-Si-B alloys

Crystallization of soft magnetic Fe-Si-B glasses was investigated by transmission electron microscopy and X-ray diffraction. Electron micrographs of the successive crystallization stages of some Fe-Si-B glasses were obtained and the morphology variations for different alloy compositions were determined. The compositional dependence of the crystallization mode exhibited by the Fe-Si-B glasses was analysed, and on this basis some suggestions about crystallization rules in these glasses are proposed. Two kinds of iron—silicon phases occurred, depending on the Fe-Si-B alloy composition: bcc Fe(Si) solid solution and an ordered solid solution on the structural basis of Fe₃Si iron suicide. It is suggested that the metastable Fe₃B phase (observed during crystallization of only few glasses) was not a simple consequence of boron content. Crystallization of the Fe₃B phase was related to the formation of the iron—silicon phase which was produced during the first crystallization stage.     

The origin of photoluminescence in amorphous lead titanate

We discuss the nature of visible photoluminescence at room temperature in amorphous lead titanate in the light of the results of recent experimental and theoretical calculations. Experimental results obtained by XANES and EXAFS revealed that amorphous lead titanate is composed of a Ti-O network having fivefold Ti coordination and NBO-type (non-bridging oxygen) defects. These defects can modify the electronic structure of amorphous compounds. Our calculation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the lead titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous lead titanate. The results of our theoretical calculations of amorphous lead titanate indicate that the formation of fivefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). A comparison of the experimental and theoretical results of amorphous compounds suggests the possibility of a radiative recombination (electron-hole pairs), which may be responsible for the emission of photoluminescence.     

Crystallization of amorphous complex oxides: New geometries and new compositions via solid phase epitaxy

The crystallization of amorphous complex oxides via solid phase epitaxy enables a wide range of opportunities in the formation of oxide materials in new geometries and with previously inaccessible compositions. Emerging methods for controlling crystallization from the amorphous form arise from recent advances in the deposition of amorphous oxides, the formation and placement of crystalline seeds, and have built on an expanded understanding of the kinetics of nucleation and crystal growth. Key discoveries include methods for the creation of epitaxial layers in perovskite, spinel, and pyrochlore complex oxides. The creation of nanoscale homoepitaxial and heteroepitaxial seeds has the potential to enable new directions in the integration of complex oxides with semiconductors and in devices based on oxygen ion transport. Future opportunities include the creation of complex oxides in morphologies and compositions exhibiting electronic, thermal, and magnetic phenomena enabling a variety of applications.     

Crystallization process on amorphous Mg-Ga-Sn system

The crystallization processes on Mg_81.09Ga_18.91 and Mg_80.50Ga_17.76Sn_1.67 amorphous samples were studied by means of X-ray diffraction and electrical resistivity isothermal measurements. A small amount of Sn added to the binary eutectic was found to modify the crystallization products and the evolution time. Crystallization activation energies were estimated.     

Crystallization behavior of bulk amorphous Se-Sb-In system

In the present work crystallization behavior of the amorphous Se_90–x Sb₁₀In _x (0 x 15) system has been investigated using Differential Scanning Calorimetry (DSC). The DSC data was taken at different heating rates varying from 1°C/min to 90°C/min. From the heating rate dependence of the crystallization temperature, the activation energy for crystallization has been determined using the Kissinger equation and Matusitas equation for non-isothermal crystallization of materials. The effect of addition of In to the Se-Sb system on the dimensionality of crystal growth has been investigated.     

Crystallization of amorphous Ni60Nb40

The crystallization of Ni₆₀Nb₄₀ metallic glass during continuous heating and isothermal annealing at temperatures from 845 to 904 K, was studied by differential scanning calorimetry (DSC), electrical resistance measurements, X-ray diffraction and transmission electron microscopy. Crystallization occurred in four clearly defined stages. In the initial stage a metastable phase, with structure similar to the M-phase in the Ni-Nb-Al ternary system, forms in the amorphous matrix. In the two subsequent stages the remaining glass crystallizes to the Ni₃Nb- and-phases found in the equilibrium Ni-Nb phase diagram. The M-phase transforms into the equilibrium Ni₃Nb- and-phases only at high temperature. The crystallization of the M-phase could be described by Johnson-Mehl-Avrami kinetics with the time exponentn=1.3 and activation energyE ₁=628 kJ mol^–1. M-phase crystal growth was apparently diffusion controlled and the diffusion coefficient was estimated to be 4.2×10^–20 m² sec^–1. Activation energies for the second and third stages of crystallization were found to beE ₂=446 kJ mol^–1 andE ₃ = 430 kJ mol^–1.     

Crystallization kinetics of bulk amorphous Se-Te-Sn system

In the present work crystallization kinetics of the amorphous Se_80–xTe₂₀Sn_x (0 x 9) system have been investigated using Differential Scanning Calorimetry. From the heating rate dependence of the glass transition temperature and the crystallization temperature the activation energy for the glass transition and that for crystallization have been determined using the Kissingers equation and Matusitas equation for non-isothermal crystallization of materials. The effect of addition of Sn to the Se-Te system on the dimentionality of crystal growth has been investigated. An increase in the glass transition temperature with increase in Sn content suggests that Sn plays a role in cross-linking the already existing Se-Te chains which causes an increase in the thermal stability of the material.