11B and 15N solid state NMR investigation of a boron nitride preceramic polymer prepared by ammonolysis of borazine

A poly(aminoborazine), precursor for hexagonal boron nitride (h-BN) obtained by reaction of borazine B₃N₃H₆ with ammonia, and its pyrolysis derivatives have been extensively characterised by ¹⁵N and ¹¹B MAS NMR. The various B and N sites have been identified according to their first neighbouring atoms, as well as to the second ones in the case of ¹⁵N, and have also been quantified. This study demonstrates that a suitable choice of NMR techniques together with the use of isotopic enrichment can lead to a large improvement in spectral resolution, which allows a better understanding of such complex BN preceramic polymer structures and permits to follow the polymer-to-ceramic transformation.     

Improved Repeatability of Pulsed Field Magnetometry Measurements Through Peltier Effect Temperature Control of Measurement Coils

Pulsed Field Magnetometers (PFMs) offer a method for high speed full loop characterisation of hard magnetic materials. As there is no dependency on iron to close a circuit, the repeatability of the technique is good. By utilising Peltier effect techniques to control the temperature of the measurement coil systems employed in a Pulsed Field Magnetometer, the level of repeatability of measurements can be significantly improved.     

不同添加剂对SrAl2O4:(Eu,Dy)磷光体发光性能的影响

在较低的温度下用燃烧合成法快速合成了 Eu2 ,Dy3 掺杂的SrAl2O4长余辉磷光体发光材料.研究了P2O5,CaF2,H3BO3,NaF 几种添加剂对SrAl2O4:(Eu,Dy)粉体发光性能的影响.结果发现:H3BO3和P2O5的添加有利于改善磷光体的发光性能,而在配料中加入CaF2和NaF,磷光体发光效率降低.随着这几种添加剂的加入,SrAl2O4:(Eu,Dy)磷光体材料发射光谱的主发射峰不同程度的出现蓝移.根据实验结果分析了添加剂的作用机理.     

Biomimetic Mg- and Mg,CO3-substituted hydroxyapatites: synthesis characterization and in vitro behaviour

The doping of the apatite with carbonate or/and Mg ions in biologically-like amounts (6 and 1 wt.%, respectively) was performed. Chemico-physical characterizations and cell culture tests were carried out onto the synthetic Mg- and Mg,CO₃-substituted (∼30–40 nm particle size) powders in comparison with stoichiometric HA (∼160 nm particle size) to determine as mesenchymal stem cells (MSCs) can directly use the mineral microenvironment to stimulate their own proliferation and differentiation activities. At the same time the growth of human osteoblast like cells (MG-63) was evaluated to determine the compatibility of the synthetic doped apatites for bone substitution. Cell morphology analysis by SEM as well as MTT and ALP tests were performed.The peculiar chemico-physical properties of the doped (Mg- and Mg,CO₃-substituted) materials improved the behaviours of MSC and MG-63 cells in term of adhesion, proliferation and metabolic activation compared to stoichiometric HA.     

Thermal expansion of some pre-stressed nuclear grade graphites

Thermal expansion coefficients (CTE) of some nuclear grade polycrystalline graphites were measured in the range from room temperature to 1173 K after pre-stressing to levels comparable with their fracture strength. The CTEs has been found to increase with residual strain independently of the samples as well as their manufacturing processes. Analytical discussions are given on the effects of both pre-stressing and/or thermal annealing. Phenomenological formulae appropriately expressing CTEs in heating and cooling processes have been derived.     

Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900 °C for 1 h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni₃Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10 nm to 50 nm were precipitated and uniformly distributed in the Ni₃Ti phase to form a thin granular layer after heat treatment at 700 °C. The granular layer spread from the edge toward the center of the Ni₃Ti phase with increasing temperature. A large number of fine equiaxed Cr₂₃C₆ particles with 0.2–0.5 μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900 °C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.     

On structural model of AB5-type multi-element hydrogen storage alloy

The present study is focused on the development of a structural model for multi−element AB₅−type alloy which can correlate the hydrogenation characteristics with structural parameters. At present no such model is available, which can predict the trend of hydrogenation properties like hydrogen storage capacity, heat of formation and plateau pressure of multi−element hydrogen storage material. It is done by trial and error method. In present investigation efforts are made to correlate atomic radius of substituted elements with heat of formation of hydride, plateau pressure and hydrogen storage capacity by calculating equivalent radius of B (r_B*) in multi−element composition, contraction in A−B bond, radius of voids and ratio of r_A with r_B*. The heat of formation decreases and hydride stability increases with increasing value of r_B* and contraction in A−B bond. While hydrogen storage capacity decreases with increasing value of r_B* and contraction in A−B bond. Based on this, correlation between thermodynamic and structural properties has been established and structural model has been developed.