Fabrication of polymer derived ceramic parts by selective laser curing

Polymer derived ceramic parts of complex shape were fabricated by selective laser curing (SLC). The ceramic parts were built similar to the selective laser sintering process by sequentially irradiating layers consisting of SiC loaded polysiloxane powder with a CO₂-laser beam (λ = 10.6 μm), which locally induces curing reaction of the polymer phase at moderate temperatures around 400 °C. The laser-cured bodies were converted to SiOC/SiC ceramic parts in a subsequent pyrolysis treatment at 1200 °C in argon atmosphere. The scanning speed and the power of the CO₂-laser beam were varied, leading to pronounced differences in material properties. Laser irradiating a powder mixture containing 50 vol.% polymer/50 vol.% SiC resulted in relative green densities between 38 and 60%. Relative densities decreased to 32–50% after pyrolysis due to the polymer shrinkage. A post-infiltration with liquid silicon was carried out in order to produce dense parts. While before infiltration the bending strength only attained 17 MPa as a result of both microcracks in the SiOC matrix and a pronounced porosity, an average value of 220 MPa was achieved after post-infiltrating with Si. In the case of 50 vol.% SiC content the linear shrinkage after pyrolysis was only 3%. Thus, the SLC approach can be considered as a near-net-shape forming process of ceramic components with complex geometries.     

Properties of hybrid fibre reinforced shell for investment casting

In order to improve the properties of silicon sol shell for investment casting process, a varying content of hybrid fibres (aluminium silicate and polypropylene) was introduced into slurry for preparation of fibre-reinforced shell in the present work. The bending strength, self-load deformation at elevated temperature, and the permeability of fibre-reinforced shell specimens were investigated and the fracture surfaces of shell specimens were observed by SEM. The results show that the bending strength of green shell increases with content of fibres in it. The maximum bending strength of 4.96 MPa was obtained in the fired shell with 0.6 wt% hybrid fibres addition. The high temperature self-loaded deformation of specimens of shell reinforced with a hybrid fibre addition above 0.6 wt% is higher than that of the unreinforced. However, the shell with a hybrid fibre addition up to 0.4 wt% exhibits the lower self-loaded deformation at high temperature compared to the unreinforced. It is also found that the permeability of shell specimens can be improved by hybrid fibres addition. Based on the fracture surfaces observation using a scanning electron microscope (SEM), the failure mode of the green shell reinforced with hybrid fibres is identified as fibre rupturing from the substrate of shell specimens, and/ or debonding from adhesive film surrounding it in shell. Even though the specimens of shell being fired at 900 °C for 2 h, the same failure features also exist in the fracture surfaces of specimens. This indicates that the specimens of shell can still be reinforced with aluminium silica fibres (residue of hybrid fibres) for their interpenetrating fibres network structure although go through firing.     

Sintering behaviour of silicon carbide matrix composites reinforced with multilayer graphene

The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1], [2], [3], [4], [5], [6], [7], [8], [9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.     

Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young's modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ_compression = 531.3 MPa and K_IC = 1.12 MPa m^1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications.     

Actively Q-switched fiber lasers: Switching dynamics and nonlinear processes

The Q-switching characteristics of actively Q-switched fiber lasers, especially the inconsistency of experimental observations with traditional Q-switching theory, are reviewed first in this paper. Based on a few typical high-power Q-switched rare-earth-doped fiber lasers, the switching dynamics in the linear and ring fiber cavities is then systematically illustrated under different fiber, pumping and switching conditions by using the traveling wave method, which leads to a new understanding of Q-switched fiber lasers. In particular, the switching-induced perturbation and its influence on the Q-switched outputs are emphasized, and some experimental demonstrations are shown for the purpose of comparison. Nonlinearity is an important concern in these lasers due to the high-power optical pulses confined in small fiber cores. In the second part of this paper, some nonlinear effects, such as self-phase modulation (SPM), four-wave mixing (FWM), stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), in Q-switched fiber lasers are discussed. In particular, their temporal and spectral influences on the Q-switched outputs are analyzed experimentally and theoretically. These theoretical analyses and experimental demonstrations provide a complete picture of the switching dynamics and optical nonlinear processes in Q-switched fiber lasers, and are hence important for laser optimization and control.     

Preparation and characterization of low-cost ceramic microfiltration membranes for the removal of chromate from aqueous solutions

Low-cost ceramic microfiltration membranes were prepared using clay of IIT Guwahati. Two membranes were prepared by paste casting followed by sintering at different temperatures, the first one from clay only (membrane A) and the second one from clay with small amounts of sodium carbonate, sodium metasilicate and boric acid (membrane B). Both the membranes were characterized by TGA, SEM, XRD, water permeability test and acid–base treatment. With the increase of sintering temperature, pore size as well as permeability and flexural strength were increasing while porosity and pore density were decreasing. The overall performance of membrane B was better than membrane A. The average pore size, porosity, pore density and flexural strength of membrane B sintered at 1000 °C were 4.58 μm, 0.42, 2.06 × 10¹⁰ m^− 2 and 11.55 MPa respectively. This membrane was used for the removal of chromate from aqueous solutions by micellar enhanced microfiltration (MEMF) using cetylpyridinium chloride (CPC). 100% rejection of chromate ions were obtained at a feed ratio (CPC/chromate) of 10. Based on raw material prices, the membrane cost was estimated to be $19/m². The prepared low-cost membrane showed good promise for the treatment of wastewater containing such heavy metals.     

Physicochemical and biological properties of composite bone cement based on octacalcium phosphate and tricalcium silicate

Biocompatible tricalcium silicate (C₃S) bone cement is widely used as dental and bone repair material; however, its long setting time, poor injectability and low initial mechanical properties limit clinical applications. In order to improve C₃S silicate bone cement and its derivatives to play a more important role in tooth restoration, bone defect repair, implant coating and tissue engineering scaffolds, a novel C₃S and octacalcium phosphate (OCP) composite bone cement (OCP/C₃S) was prepared and evaluated for setting time, injectability, anti-flocculation, pH, microstructure, bioactivity and cytotoxicity. The setting time of the OCP/C₃S composite bone cement was controlled within the clinically operable time range (8.3–13.7 min); the cement exhibited good compressive strength, injectability (93.54%), and anti-collapse performance. The 20% OCP/C₃S composite bone cement had a compressive strength of 28.94 MPa, 93% stronger than pure C₃S (14.98 MPa). An in vitro immersion test showed that the composite bone cement had excellent hydroxyapatite forming ability, proper degradation rate, and a low pH value. Cellular experiments confirmed the low cytotoxicity of the composite bone cement and its great capacity for cell proliferation. These results indicate that 20% OCP/C₃S composite bone cement is a promising biomaterial.     

Li2O含量对碲系玻璃温度及析晶性能的影响

通过DTA研究了TeO2-PbO-B2O3-Bi2O3-Li2O系统玻璃中TeO2、PbO物质的量比变化对玻璃转变温度(Tg)的影响,重点研究了n(TeO2)∶n(PbO)=2∶1条件下Li2O含量对玻璃转变温度的影响,并采用IR和XRD对玻璃的结构和析晶性进行了分析。结果表明:随着Li2O含量(2.7%~6.6%,摩尔分数)的增加,Tg呈N型变化趋势,当Li2O为2.7%(摩尔分数)时,Tg为最低值(300℃)。碲系统玻璃中形成了[TeO4]三角双锥体结构,它与[BO4]、[BiO6]等多面体共同构成了玻璃的三维网络结构,该玻璃经热处理后易形成Bi2TeO5晶体而会影响Tg。     

防眩显示屏闪耀点的研究进展

闪耀点(Sparkle)是影响防眩屏幕显示效果的重要因素之一,为满足人们对显示屏分辨率日益提高的要求,闪耀点越来越受到关注。但目前对闪耀点的确切定义,产生原因,表征手段及评价指标等尚无统一的认识和标准。本文从闪耀点现象入手,介绍了目前闪耀点主要的研究表征方法,例如Gollier和Becker各自提出了表征闪耀点的评价测试体系并给出了一系列相关参数;Evans等人在Gollier和Becker工作的基础上进行了一系列心理物理学实验来评价闪耀点现象的实际效果等等。本文就闪耀点的研究现状进行了详尽的综述,为后续屏幕闪耀点研究,建立统一的评价方法与标准提供了宝贵的参考资料。     

Microresonator effects in optically and electrically pumped thin-film light-emitting diodes

We describe the alteration of spontaneous emission of conjugated polymers as emitting materials in optical Fabry-Perot microcavities. We report on detailed studies of the transmission as well as the emission behavior. Particular emphasis is placed on transmission and emission measurements with variable Q factor. Our devices show strongly enhanced and spectrally narrowed emission at the cavity resonance. It is shown that the microcavity effects occur in both photoluminescence and electroluminescence.