Corrosion of β-sialon-based ceramics by molten steel

The corrosion resistance of sialons made from commercial powders (AlN, Al₂O₃ and Si₃N₄) and from powder precursor produced by carbothermal reduction and nitridation of raw aluminosilicate (pyrophyllite) in molten steel were investigated. The corroded zone in sialon made from raw pyrophyllite (P1) is more then two times deeper compared to the corroded zone of sialon made from commercial powders (C1). The corrosion zone of sample P1 is on the average 610 μm deep, while in sample C1 it is only 260 μm. The main corrosion products are γ-Al₂O₃ and iron silicides. The phase compositions were estimated by neutron Rietveld refinement.     

Quaternary Ti–Zr–Be–Ni bulk metallic glasses with large glass-forming ability

The glass-forming ability (GFA) of Ti–Zr–Be ternary alloys is dramatically improved by partially replacing Be with Ni. Centimeter-scale fully amorphous samples can be obtained in a wide Ni content range of 4 at.%–12 at.%. In particular, some of the developed Ti–Zr–Be–Ni alloys exhibit a critical diameter up to 20 mm, which is larger than that of other quaternary Ti-based bulk metallic glasses (BMGs). Moreover, Ni addition also enhances the yield strength and compressive plastic strain of Ti–Zr–Be alloys obviously. Based on the experimental results, the effect of substituting elements addition on the glass-forming ability of Ti–Zr–Be alloys has been systematically investigated and an empirical composition design method for the development of novel Ti-based BMGs with large GFA has been proposed.     

Enhanced thermoelectric cooling properties of Bi2Te3−xSex alloys fabricated by combining casting,milling and spark plasma sintering

Bi₂Te_3−xSe_x alloys are extensively used for thermoelectric cooling around room temperature, but, previous studies have reported peak thermoelectric efficiency of the material at higher temperature around 450 K. This study presents the casting followed by high energy ball milling and spark plasma sintering as a thriving methodology to produce efficient and well-built Bi₂Te_3−xSe_x material for the thermoelectric cooling around room temperature. In addition, changes in electrical and thermal transport properties brought up by amount of Se in the Bi₂Te_3−xSe_x material for this methodology are measured and discussed. Although Seebeck coefficient and electrical conductivity showed irregular trend, power factor, thermal conductivity and figure of merit ZT gradually decreased with the increase in amount of Se. A maximum ZT value of 0.875 at 323 K was obtained for x = 0.15 sample owing to its higher power factor. This value is 17% and 38% greater than for x = 0.3 and x = 0.6 samples respectively. At 323 K, herein reported ZT value of 0.875 is higher than the state of art n-type Bi₂Te₃ based thermoelectric materials produced by the time consuming and expensive methodologies.     

Mechanical properties and microstructures of hot-pressed MgAlON–BN composites

The relationship between the mechanical properties and microstructure of hot-pressed MgAlON–BN composite materials was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The phase compositions of hot-pressed samples prepared from starting mixtures of Al₂O₃, AlN, MgO and h–BN consisted of MgAlON phases as a matrix and BN phases as the second phase. The density, bending strength at room temperature, fracture toughness and Vickers hardness were measured. The results indicated that the density, strength and Vickers hardness decrease with increasing h–BN content due to the non-reactive nature and layered structure of h–BN. The fracture toughness, however increased with increasing h–BN addition, reaching a maximum of 3.64 MPa m^0.5; it decreased with further increase of BN content. The increase of fracture toughness was attributed to the presence of microcracks and the decrease was considered to be the discontinuous microstructure of the MgAlON phases. Temperature dependence of bending strength remained constant at low temperature, followed by an increase at 800 °C and then, dropped quickly. The increase in the bending strength of the composite was attributed to the decrease of residual stress and to the interwoven microstructure of the composites which prevented grain boundary slip and reduced the attenuation rate of high temperature strength. The machinability of the composites was examined. The results indicate that the composite materials with BN content more than 15 vol.% exhibit excellent machinability and could be drilled using conventional hard metal alloy drills.     

Degradation behavior of Ta2O5 stacks and its dependence on the gate electrode

Response of 8 nm Ta₂O₅ stacks with Al and Au gate electrodes to voltage stress at room temperature and at 100 °C is investigated. Stress-induced leakage current (SILC) reveals significant gate dependence and distinct difference to SILC in SiO₂. The mechanisms for SILC generation and stress degradation are discussed. Unlike SiO₂, pre-existing traps and positive charge build-up are recognized as a key factor for generation of SILC in Ta₂O₅ stacks.