Non-isothermal crystallization kinetics of La2CaB10O19 from glass

The non-isothermal crystallization kinetics of La₂CaB₁₀O₁₉ (LCB) from a La₂O₃-CaO-B₂O₃ glass was studied. Differential thermal analysis methods were performed on three glass powders to obtain the kinetic parameters of LCB crystallization mechanism. The activation energies for overall crystallization (E), obtained by the methods of Kissinger and Ozawa, were in the range of 479-569 kJ/mol. Multiple (five) analysis methods were used to estimate the Avrami exponent (n), which could consequently be reduced into the single value of n = 3.1 ± 0.3. The growth morphology index (m) of LCB was corroborated by microscopy (optical and electron) images, which revealed a three dimensional growth. Energy dispersive spectroscopy confirmed that LCB is the crystallizing phase from the glass by an interface controlled mechanism. The parameters of the Johnson-Mehl-Avrami kinetic model for the analysis of LCB crystallization from glass were found to be n = m = 3.     

Effects of Nb2O5 Replacement by Er2O3 on elastic and structural properties of 75TeO2-(10 − x)Nb2O5-15ZnO-(x)Er2O3 glass

Tellurite 75TeO₂-(10 − x)Nb₂O₅-15ZnO-(x)Er₂O₃; (x = 0.0-2.5 mol%) glass system with concurrent reduction of Nb₂O₅ and Er₂O₃ addition have been prepared by melt-quenching method. Elastic properties together with structural properties of the glasses were investigated by measuring both longitudinal and shear velocities using the pulse-echo-overlap technique at 5 MHz and Fourier Transform Infrared (FTIR) spectroscopy, respectively. Shear velocity, shear modulus, Young's modulus and Debye temperature were observed to initially decrease at x = 0.5 mol% but remained constant between x = 1.0 mol% to x = 2.0 mol%, before increasing back with Er₂O₃ addition at x = 2.5 mol%. The initial drop in shear velocity and related elastic moduli observed at x = 0.5 mol% were suggested to be due to weakening of glass network rigidity as a result of increase in non-bridging oxygen (NBO) ions as a consequence of Nb₂O₅ reduction. The near constant values of shear velocity, elastic moduli, Debye temperature, hardness and Poisson's ratio between x = 0.5 mol% to x = 2.0 mol% were suggested to be due to competition between bridging oxygen (BO) and NBO ions in the glass network as Er₂O₃ gradually compensated for Nb₂O₅. Further addition of Er₂O₃ (x > 2.0 mol%) seems to further reduce NBO leading to improved rigidity of the glass network causing a large increase of ultrasonic velocity (v_L and v_S) and related elastic moduli at x = 2.5 mol%. FTIR analysis on NbO₆ octahedral, TeO₄ trigonal bipyramid (tbp) and TeO₃ trigonal pyramid (tp) absorption peaks confirmed the initial formation of NBO ions at x = 0.5 mol% followed by NBO/BO competition at x = 0.5-2.0 mol%. Appearance of ZnO₄ tetrahedra and increase in intensity of TeO₄ tbp absorption peaks at x = 2.0 mol% and x = 2.5 mol% indicate increase in formation of BO.     

One-step fabrication of Fe2O3/Ag core-shell composite nanoparticles at low temperature

The Fe₂O₃/Ag core-shell composite nanoparticles were successfully prepared via a simple method at low temperature. X-ray diffraction data revealed the formation of core-shell composite nanoparticles, with Fe₂O₃ as the core and silver as the shell. The results from the transmission electron microscopy and scan electron microscopy further indicated that the composite nanoparticles were spherical with a core diameter and shell thickness of 26.0 nm and 13.5 nm, respectively. Magnetic measurements showed that the composite nanoparticles exhibited a typical ferromagnetic behavior, a specific saturation magnetization of 0.95 emu/g and an intrinsic coercivity of 104.0 Oe at room temperature. For a standard two-probe analysis at room temperature, the composite nanoparticles showed a typical conductive behavior and its conductivity was about 3.41 S/m. Moreover, this present synthesis method of Fe₂O₃/Ag core-shell composite nanoparticles shows an easy processing and does not need high-temperature calcining to attain the final product, which can be applied in a variety of areas, including catalysis, medicine, photonics, and new functional device assemblies.     

Investigation of the structure evolution process in sol-gel derived CaO-B2O3-SiO2 glass ceramics

A sol-gel method was used to prepare CaO-B₂O₃-SiO₂ (CBS) glass powder for making low-temperature cofired ceramics. This paper was focused on the mechanism of hydrolysis and polymerization and also on the structural evolution of xerogel at various temperatures. The xerogel was transformed into glass ceramics containing CaSiO₃ and CaB₂O₄ crystalline phases through nucleation and crystallization processes. The results indicated that the xerogel exhibits [BO₄] or [SiO₄] based three-dimensional network structure whose interstices Ca ions fill in, which becomes more orderly and stable after heat treatments. The CBS glass ceramics through controlled crystallization have a potential as electronic packaging materials.     

Dielectric relaxation study of hydrogen exposure as a source of two-level systems in Al2O3

An important question in the manufacture of superconducting electronics is how to control the two-level systems found in amorphous insulators. The present article shows that hydrogen has a marked impact on the two-level systems in thin films of reactively sputtered Al₂O₃, a standard tunnel oxide for Josephson junctions. The magnitude of dielectric relaxation current in Al₂O₃ films, believed to be caused by two-level systems, is shown to increase monotonically with the flow rate of H₂ into the chamber during deposition. This points toward a potential need for controlling hydrogen during the manufacture of superconducting electronics utilizing Al₂O₃.     

Structure and growth morphology of Gd2O3-doped CeO2 thin films

Gd₂O₃-doped CeO₂ (Gd_0.1Ce_0.9O_1.95, GDC) thin films were synthesized on (1 0 0) Si single crystal substrates by a reactive radio frequency magnetron sputtering technique. Structures and surface morphologies were characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and one-dimensional power spectral density (1DPSD) analysis. The XRD patterns indicated that, in the temperature range of 200–700 °C, f.c.c. structured GDC thin films were formed with growth orientations varying with temperature—random growth at 200 °C, (2 2 0) textures at 300–600 °C and (1 1 1) texture at 700 °C. GDC film synthesized at 200 °C had the smoothest surface with roughness of R_rms=0.973 nm. Its 1DPSD plot was characterized with a constant part at the low frequencies and a part at the high frequencies that could be fitted by the f^−2.4 power law decay. Such surface feature and scaling behavior were probably caused by the high deposition rate and random growth in the GDC film at this temperature. At higher temperatures (300–700 °C), however, an intermediate frequency slope (−γ₂≈−2) appeared in the 1DPSD plots between the low frequency constant part and the high frequency part fitted by f⁻⁴ power law decay, which indicated a roughing mechanism dominated by crystallographic orientation growth that caused much rougher surfaces in GDC films (R_rms>4 nm).