Relation between microstructure,properties and spark plasma sintering (SPS) parameters of pure ultrafine WC powder

A combined experimental/numerical methodology is developed to fully consolidate pure ultrafine WC powder under a current-control mode. Three applied currents, 1900, 2100 and 2700 A, and a constant pressure of 20 MPa were employed as process conditions. The developed spark plasma sintering (SPS) finite-element model includes a moving-mesh technique to account for the contact resistance change due to sintering shrinkage and punch sliding. The effects of the heating rate on the microstructure and hardness were investigated in detail along the sample radius from both experimental and modeling points of view. The maximum hardness (2700 HV10) was achieved for a current of 1900 A at the core sample, while the maximum densification was achieved for 2100 and 2700 A. A direct relationship between the compact microstructure and both the sintering temperature and the heating rate was established.     

稀土钼SPS烧结体阴极的次级发射性能和微观结构研究

利用湿法冶金的方法制备了稀土-钼粉末，然后用SPS快速烧结的方法制备了稀土-钼烧结体阴极材料，并测试了材料的发射性能。通过XRD和SEM对材料进行了物相、元素分布与含量的研究。测试结果表明用SPS烧结方法制备的稀土-钼阴极材料的次级发射系数达到3．84，比常规烧结方法制备的阴极材料的次级发射系数(2．92)大很多。用SPS烧结的方法可得到晶粒比较细小且分布比较均匀的烧结体，并发现材料经过1600℃的高温激活后稀土元素将富集于材料的表面，在表面形成一厚为5μm左右的稀土氧化物薄膜。     

A comparative study of nickel and alumina sintering using spark plasma sintering (SPS)

Nickel and alumina powders with particle sizes of 4.3 μm and 108 nm, respectively, have been sintered in moulds of different sizes. Only the diameter (inner and/or outer) of moulds has been varied. The dimension modification is responsible for sample microstructure variation because set temperature, heating rate, dwell time and pressure were identical in all experiments. The influence of the die (or sample) dimension on the sample microstructure appears to depend strongly on the electric characteristics of the powder. The present paper is an attempt to correlate the bulk microstructure evolution with the die and sample size and the electrical current distribution within the system.     

Fabrication of ultrafine powder using processing control agent,and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi,Sb)2Te3 alloys

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)₂Te₃ alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ～800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.     

Pore properties,inner chemical environment,and microstructure of nano-modified CFA-WBP (class C fly ash-waste brick powder) based geopolymers

In order to study the influence of waste brick powder (WBP) and nano-modification on class C fly ash (CFA) based geopolymer, pore properties (i.e. bulk density, apparent porosity, and true porosity, and closed porosity) were tested according to the related Chinese National Standard, and the pore structure was also explored by mercury intrusion porosimetry (MIP); Water leaching procedure were performed to study the inner chemical environment; X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) were measured to the microstructure and modification mechanism. The results show that WBP as additive results in a denser structure of geopolymer; nano-modification improves the pore structure, ameliorates the anti-leaching ability of geopolymer by constraining the soluble ions in gels, makes the geopolymerization more complete, and results in a denser paste structure by filling gaps between the particles with amounts of reaction products.