Volume 111, Issue 7

By calculating the Gibbs free energy of a magnesium aluminium oxynitride (MgAlON) system, pure MgAlON was obtained by pressureless sintering at 1 570 °C in a nitrogen atmosphere using Al, Al 2 O 3 , and MgO as raw materials. The formation mechanism of MgAlON and phase evolution of the Al-Al 2 O 3 -MgO-N 2 system were investigated. The aluminium was melted at approximately 660 °C and formed AlN in the range 740- 1 250 °C. The reaction of AlN, Al 2 O 3 , and MgO started to form a primary crystalline MgAlON phase at 950 °C, and as the temperature increased, the minerals continuously dissolved in the primary crystalline phase.

This manuscript highlights the structural, electrical and optical properties of Bi 2 Mo 1-x W x O 6 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds. These compounds were prepared employing the citrate sol-gel method. These compositions were characterized using thermo-gravimetric and differential scanning calorimetry, X-ray diffraction and infrared techniques. The Rietveld refinements of X-ray powder diffraction data determined and confirmed the crystal structure of the compositions. DC electrical conductivity indicated the conduction behaviour and diffuse reflectance spectroscopy confirmed the semiconducting nature of the compounds. Room temperature photoluminescence results exhibited two type of emission namely blue-green and green emission in the visible region.

Charge transport measurements in compressed bulk graphene oxide (GO) have been studied within the temperature range 15-450 K. Structural properties and surface morphologies of the bulk compressed GO were studied using X-ray diffraction and transmission electron microscopy. Raman and X-ray photoelectron spectroscopies were also used to confirm the presence of graphitic phases and the various functional groups in the GO, respectively. Current-voltage characteristics of the GO measured with gold (Au) electrodes at different temperatures showed no Schottky barrier at the Au/GO interface. At low temperatures and low bias voltages, the electron transport through the compressed GO sample showed no significant voltage dependence, which is consistent with a direct tunneling mechanism at all the bias voltages (0.01 -1.0 V). It was also observed that no Fowler- Nordheim transport mechanism occurred within this bias voltage range.

Yttrium oxide (Y 2 O 3 ) and ytterbium (Yb)-doped Y 2 O 3 particles were synthesized via the sol-gel method from the aqueous solution of their nitrate salts. The synthesized powders were then deposited on Corning glass substrates using an electron beam evaporation technique to examine the growth morphology of the thin films. The effect of the Yb dopant concentration on the microstructure and morphology of the Y 2 O 3 powders and thin films was investigated using various characterization techniques. The synthesized powders and fabricated thin films have body-centered cubic structures with space group Ia-3. Based on the X-ray peak broadening, crystallite size and lattice parameters were evaluated with the Williamsom-Hall and Cohen-Wagner methods. The lattice parameter and crystallite size decrease with increasing Yb concentration. The intensities of the Raman peaks decrease due to microstructural disorder caused by the increase in the Yb dopant concentration. The band gap values of the powders also decrease depending on the dopant concentration, similar to the lattice parameter and crystallite size. While the synthesized powders have a sponge-like morphology, they exhibit different morphological structures depending on the dopant concentration when converted into thin films.

In the current study, different types of thermal barrier coatings with various mass fractions were investigated in terms of oxidation, hot corrosion, and thermal shock resistance. The thermal barrier coatings consisted of six different samples, which included the usual YSZ, Al 2 O 3 , hybrid composites with 65 wt.% YSZ and 35 wt.% Al 2 O 3 , 50 wt.% YSZ and 50 wt.% Al 2 O 3 , 35 wt.% YSZ and 65 wt.% Al 2 O 3 and the final one, which was a double layer composite (Al 2 O 3 and YSZ). High temperature isothermal oxidation behavior of the coatings was tested at 1050 °C, using an air furnace for 48 h, 80 h, and 120 h respectively. Hot corrosion tests were applied at 1050 °C using a 45 wt.% Na 2 SO 4 and 55 wt.% V 2 O 5 mixture of salts. The microstructure and phase stability of coatings were evaluated by means of scanning electron microscopy and X-ray diffraction techniques. The usual YSZ showed better hot corrosion and thermal shock resistance, while Al 2 O 3 showed the lowest hot corrosion and oxidation resistance. Thermally grown oxide formation, thermal expansion coefficient mismatch and phase transformation in the thermal barrier coatings could be the main causes of degradation after thermal shock testing.

ZrSi/ZrC nanocomposites have stable high-temperature properties, where conventional materials cannot meet increasingly demanding high-temperature environments. In this paper, the microstructure and electrochemical reduction mechanism of ZrSi/ZrC nanocomposites have been studied. A mixture of ZrSiO 4 and carbon black powder was processed using ball grinding, sheet pressing, and sintering, and cylindrically-sintered sheet was prepared as the cathode for the electrolytic work. A high purity graphite rod was utilized as the anode.The microstructure of the electrolytic product was characterized and analyzed using X-ray diffraction, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The experimental results showed that the diameter of the as-synthesized ZrSi/ ZrC fibers typically range between 100-400 nm when produced by the electrolysis of sintered pellets in equimolar CaCl 2 -NaCl molten salt at 850°C with a cell voltage of 2.8 V for 20 h under an argon atmosphere. The nanofibers were formed in core-shell microstructures that overlap and grow.

Copper-chrome-niobium alloys exhibit excellent thermal and electrical properties combined with high strength at elevated temperatures. Additive manufacturing techniques such as laser metal deposition using powder as raw material offer the potential for rapid solidification as well as a high freedom of design to manufacture parts layer by layer. Powder samples of copper- chrome-niobium alloys were produced by gas atomization. Via laser metal deposition, bulk volumes without cracks and with a very low porosity can be built up. Rapid solidification leads to the formation of fine precipitates which are likely to be (Cr,Fe) 2 Nb. The precipitates are distributed homogeneously in the copper matrix. The copper crystals grow across the layers due to epitaxial nucleation on the preceding layer.

In this study, the experimental setup of a constrainedstudded- pressing method (CSP) as a severe plastic deformation process was investigated to produce nano-crystalline copper sheets. The sheets were CSPed up to 35 passes. The crystallite size and the dislocation density of CSPed samples were calculated by X-ray-diffraction pattern analysis. Also, tensile tests were used to investigate the mechanical properties of the CSPed samples. The results revealed that increasing the number of CSP passes augmented the density of dislocations, reduced the crystallite size to 124 nm (while for the as-received sample it was about 7.8 lm), enhanced the yield-strength and the ultimate-tensile- strength, and lowered the elongation of samples. This study for the first time has challenged the ability of the CSP method to produce nanocrystalline materials.

The glass-forming ability (GFA), nanocrystallisation, electrical resistivity and soft magnetic properties of (Fe 1-x Co x ) 79 Nb 3 B 18 (x = 0, 0.15, 0.3, 0.45, 0.6, 0.75) glassy alloys were investigated. We found that the substitution of Fe by Co is beneficial for improving the GFA. As-spun (Fe 0.55 Co 0.45 ) 79 Nb 3 B 18 glassy alloys show the best GFA, along with excellent soft magnetic properties in the supercooled liquid region, with a saturation magnetisation and coercivity of 140 A · m 2 · kg -1 and 19.9 A · m -1 , respectively, at 60 K. With increasing Co content, the electrical resistivity initially decreases rapidly, and then fluctuates around approximately 70 ± 4 μΩ cm. With increasing the annealing temperature, the saturation magnetisation improves initially, but then decreases for the (Fe 0.4 Co 0.6 ) 79 Nb 3 B 18 alloy, and the coercivity does not significantly improve. These newly developed FeCoNbB multicomponent alloys exhibit appreciable GFA, good magnetic properties and low material cost, and they can serve as a promising soft magnetic material for use in industrial applications.

The microstructures of an Al-Bi immiscible alloy and the corresponding composites containing TiC (1 wt.% and 2 wt.%) were explored for melt temperatures of 800 °C, 850 °C, and 900°C. It was demonstrated that serious coarsening and macrosegregation of Bi-rich minority phase particles occurred, which was slightly alleviated by increasing the melt temperature from 800 °C to 900 °C. By adding TiC particles, the coarsening and macrosegregation of Bi-rich minority phase particles were significantly impeded. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that TiC particles were located inside and on the surface of Bi-rich minority phase particles, exhibiting heterogeneous nucleation and self-assembly behaviour. By properly increasing the holding time of the melt, finer and more uniform Bi-rich minority phase particles were obtained.