Pressureless spark plasma–sintered Bioglass® 45S5 with enhanced mechanical properties and stress–induced new phase formation

Commercial Bioglass^® 45S5 powder was sintered using spark plasma sintering (SPS) technique without the assistance of mechanical pressure with heating and cooling rate of 100 °C/min, dwell temperature of 1050 °C and dwell time of 30 min. Such route enabled the production of samples exhibiting superior mechanical properties in comparison with Bioglass^® sintered in furnace. In particular, flexural strength and fracture toughness reached values close to those of apatite-wollastonite bioceramics, already widely used in clinical applications. The residual stresses implemented by indentation promoted the formation of a new phase in samples sintered by SPS. Complementary use of Raman and energy dispersive spectroscopy (EDS) indicated the phase as sodium carbide and a formation mechanism was proposed.     

Spark plasma sintered BaTiO3/graphene composites for thermoelectric applications

The viability of spark plasma sintered graphene/barium titanate ceramic matrix composites as thermoelectric materials is investigated. The temperature dependence of electrical conductivity, thermal conductivity and Seebeck coefficient was analyzed. The addition of low amounts of graphene oxide combined with the spark plasma sintering process increases electrical conductivity of pure BaTiO₃ several orders of magnitude, whereas the thermal conductivity shows only a moderate enhancement. The composites display a semiconducting behaviour, with the resistivity decreasing with increasing temperature and following a thermally activated temperature dependence at high T. A strong dependence of ZT figure of merit with the graphene concentration and the measurement temperature was found. Optimal values are found for 1.7 wt% graphene oxide at the maximum experimental temperature (600 K).     

Spark plasma assisted carbothermal reduction-nitridation synthesis of (Ti,W, Mo)(C,N)-(Ni,Co) powders

Ultrafine (Ti, W, Mo)(C, N)-(Ni, Co) cermet powders were rapidly synthesized from various metal oxides, mainly anatase-TiO₂, by spark plasma assisted carbothermal reduction-nitridation (SPCRN) at low temperature. The phase evolution of the SPCRN reaction was investigated using X-ray diffraction (XRD) and the microstructure of the product powders was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). NiO, Co₃O₄ and MoO₃ were converted to Ni, Co and Mo₂C by CR reaction at temperatures below 900 °C. WO₃ was successively transformed from W₂C to WC by CR reaction up to 1100 °C. Finally, at up to 1350 °C, (Ti, W, Mo)(C, N) formed into the sequence of TiO₂, Ti₄O₇, Ti₃O₅, Ti(O, N), Ti(C, N), (Ti, W)(C, N) and (Ti, W, Mo)(C, N). The crystal structure of (Ti, W, Mo)(C, N)-(Ni, Co) cermet powders was analyzed by the Rietveld method and transmission electron microscopy (TEM). The findings demonstrated that the pure (Ti, W, Mo)(C, N)-(Ni, Co) cermet powders with grain size of below 0.5 µm were synthesized from metal oxides by SPCRN reaction at 1400 °C for 10 min.     

Oxidation resistance of tantalum carbide-hafnium carbide solid solutions under the extreme conditions of a plasma jet

The oxidation behaviors of tantalum carbide (TaC)- hafnium carbide (HfC) solid solutions with five different compositions, pure HfC, HfC-20 vol% TaC (T20H80), HfC- 50 vol% TaC (T50H50), HfC- 80 vol% TaC (T80H20), and pure TaC have been investigated by exposing to a plasma torch which has a temperature of approximately 2800 °C with a gas flow speed greater than 300 m/s for 60 s, 180 s, and 300 s, respectively. The solid solution samples showed significantly improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 300 s oxidation tests. A new Ta₂Hf₆O₁₇ phase was found to be responsible for the improved oxidation performance exhibited by solid solutions. The oxide scale constitutes of a scaffold-like structure consisting of HfO₂ and Ta₂Hf₆O₁₇ filled with Ta₂O₅ which was beneficial to the oxidation resistance by limiting the availability of oxygen.     

Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Spark plasma sintering of Sm3+ doped Y2O3 transparent ceramics for visible light lasers

Transparent Sm:Y₂O₃ ceramics were fabricated by spark plasma sintering (SPS). The effects of LiF additive and sintering temperature on the microstructure and optical transmittance of the Sm:Y₂O₃ ceramics were investigated. The optimal content of LiF additive and sintering temperature was found to be 0.3 wt% and 1500 ℃. The transmittance of Sm:Y₂O₃ ceramics with a thickness of 1.7 mm reached 75.3% at 609 nm, which is about 94% of the theoretical value. The average grain size of the sample was about 50 µm.     

Wear and friction properties of spark plasma sintered SiC/Cu nanocomposites

In this study, in order to determine the effect of SiC nanoparticles on tribological properties of nanostructured copper, the dry sliding wear and friction behaviors of nanostructured copper and copper reinforced with silicon carbide nanoparticles, produced by high energy ball milling and spark plasma sintering, were investigated by using an oscillating friction and wear tester under different normal loads. To determine the dominant wear mechanism, the worn surfaces and obtained debris after wear tests were analyzed by scanning electron microscope (SEM). The results showed that the addition of 4 vol% silicon carbide to copper matrix reduced the wear track depth and the coefficient of friction. Investigation of the worn surfaces revealed that SiC nanoparticles on the top of worn surface decreases the plastic deformation in subsurface region and alleviate severe wear. Lower plastic deformation during dry sliding wear test was attributed to high hardness of the nanocomposite that has been resulted from grain growth inhibiting and reinforcing effects of the nanoparticles. Plastic deformation and delamination were determined as major wear mechanisms in both materials.     

Synthesis of n-type Bi2Te1-xSex compounds through oxide reduction process and related thermoelectric properties

We propose a new process for the fabrication of n-type Bi₂Te_3-xSe_x (x = 0, 0.25, 0.4, 0.7) compounds. The compounds could be synthesized successfully using only oxide powders as the starting materials via the mechanical milling, oxidation, reduction, and spark plasma sintering processes. The controllability of the Se content could be ascertained by structural, electrical, and thermal characterizations, and the highest thermoelectric figure of merit (ZT) of 0.84 was achieved in Bi₂Te_2.6Se_0.4 compound at 423 K without any intentional doping. This process provides a new route to fabricate n-type Bi₂Te_3-xSe_x compounds with competitive ZTs using all oxide starting materials.     

适用于海量负荷数据分类的高性能反向传播神经网络算法

负荷分类对于指导电网发用电规划与保证电网可靠运行具有重要意义。面向负荷数据海量化与复杂化趋势,传统负荷分类方法已无法满足用电大数据分析要求。首先,针对用户侧数据体量大、类型多、速度快等特点,在Spark平台上将反向传播神经网络(BPNN)算法并行化,实现对海量负荷数据的高效分类。然后,通过对训练样本抽样分块以降低各网络学习时间,针对分布式后BPNN基分类器由于学习样本缺失潜在的准确度下降问题,采用集成学习予以改善。并通过BPNN学习不同训练样本块构建差异化基分类器,对基分类结果多数投票得到最终分类结果。另外,提供了一种基于K-means和K-medoids聚类的负荷数据训练样本选取方法。算例表明所提方法既能对负荷曲线有效分类,又能大幅提高海量数据的处理效率。     

并行计算框架Spark的自适应缓存管理策略

并行计算框架Spark缺乏有效缓存选择机制,不能自动识别并缓存高重用度数据;缓存替换算法采用LRU,度量方法不够细致,影响任务的执行效率.本文提出一种Spark框架自适应缓存管理策略（Self-Adaptive Cache Management,SACM）,包括缓存自动选择算法（Selection）、并行缓存清理算法（Parallel Cache Cleanup,PCC）和权重缓存替换算法（Lowest Weight Replacement,LWR）.其中,缓存自动选择算法通过分析任务的DAG（Directed Acyclic Graph）结构,识别重用的RDD并自动缓存.并行缓存清理算法异步清理无价值的RDD,提高集群内存利用率.权重替换算法通过权重值判定替换目标,避免重新计算复杂RDD产生的任务延时,保障资源瓶颈下的计算效率.实验表明：我们的策略提高了Spark的任务执行效率,并使内存资源得到有效利用.