Reaction spark plasma sintering of niobium diboride

Niobium diboride (NbB₂) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200 °C for synthesis and followed by densification at the temperatures in the range of 1700 °C to 1900 °C. High dense NbB₂ (~ 97.7%ρ_th) is obtained at 1900 °C after 15 min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB₂ are measured as 20.25 GPa, 539 GPa and 4 MPa m^1/2 respectively.     

Elaboration and characterization of nanocrystalline TiO2 thin films prepared by sol–gel dip-coating

Nanocrystalline TiO₂ thin films were deposited on a ITO coated glass substrate by sol–gel dip coating technique, the layers undergo a heat treatment at temperatures varying from 300 to 450 °C. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, Atomic Force Microscopy (AFM), visible, (Fourier-Transform) infrared and ultraviolet spectroscopy, Fluorescence and spectroscopic ellipsometry. The results indicate that an anatase phase structure TiO₂ thin film with nanocrystallite size of about 15 nm can be obtained at the heat treatment temperature of 350 °C or above, that is to say, at the heat treatment temperature below 300 °C, the thin films grow in amorphous phase; while the heat treatment temperature is increased up to 400 °C or above, the thin film develops a crystalline phase corresponding to the titanium oxide anatase phase. We have accurately determined the layer thickness, refractive index and extinction coefficient of the TiO₂ thin films by the ellipsometric analysis. The optical gap decreases from 3.9 to 3.5 eV when the annealing temperature increases. Photocatalytic activity of the TiO₂ films was studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 350 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance

Fe₂B coating was prepared on low-carbon steel by surface alloying. A series of experiments were carried out to examine some surface properties of boride coating. The surface heat treatment of coated low-carbon steel was performed at 700 °C, 800 °C and 900 °C for 2 h, 4 h, 6 h and 8 h under hydrogen atmosphere. The boride coating was revealed by XRD analysis and the microstructure of the boride coating was analyzed by scanning electron microscopy (SEM). Depending on the temperature and time of the process, the hardness of the borided low-carbon steel ranged from 99 to 1100 HV. The hardness showed a maximum (about 1100 HV) at 900 °C for 8 h. The corrosion resistance of the borided samples was evaluated by the Tafel polarization and electrochemical impedance spectroscopy (EIS). Shift in the corrosion potential (E_corr) towards the noble direction was observed, together with decrease in the corrosion current density (I_corr), increase in the charge transfer resistance (R_ct) and decrease in the capacitance (C_c), which indicated an improvement in corrosion resistance with increasing temperature and time of the treatment.     

Effect of heat treatment on the structure,piezoelectricity and actuation behavior of a cellulose electroactive-paper actuator

The effect of heat treatment on the structure, piezoelectricity and actuation behavior of cellulose electroactive-paper actuators was studied by infrared spectroscopy, piezoelectricity measurement and the tip displacement test. After heat treatment at 60 °C for 2 h, the piezoelectric coefficient (d₃₁) was enhanced nearly 10-fold as compared to that of non-heat-treated films. However, when the treatment temperature was raised above 60 °C the piezoelectric coefficients were higher than the non-treated film but lower than that of film treated at 60 °C. Infrared and UV–visible spectroscopy suggested that there were changes in chemical structure at higher treatment temperatures. Furthermore, the tip displacement tests performed on the actuators also showed almost same trend as that of the piezoelectric constant.     

A novel approach to in situ synthesis of WC-Al2O3 composite by high energy reactive milling

In-situ synthesis of WC-Al₂O₃ composite by milling and its subsequent heat treatment were investigated. Mixtures of Al, W, and C with stoichiometric ratio of W₃AlC₂ were ball milled up to 20 h. Then, the 20-hour ball milled powder was heat treated at different temperatures of 900 and 1200 °C. The reaction path was investigated by X-ray diffractometry (XRD). The particle size and microstructure of powders after milling was investigated by field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Also, in order to analyze the heating behavior of 20 h ball milled powder mixture during heat treatment, simultaneous thermal analysis (STA) was used. The results showed that after milling for 20 h, the reactants reacted together and new phases including W₂C and (W,Al)C_1 − x were formed. After heat treatment, the semi-stable compounds synthesized at the milling stage, were transferred to more stable compounds including WC and Al₂O₃.     

The effect of annealing on the structure,magnetic properties and AC heating of CoFe2O4 for biomedical applications

Single and nanosized spinel CoFe₂O₄ phase has been prepared successfully by a simple combination of mechanical milling from a mixture of Fe₂O₃ and Co₃O₄ powder precursors followed by a subsequent annealing. X-ray diffraction analysis reveals that the estimated crystallite size of CoFe₂O₄ increases with increasing temperature but remains at the nanoscale, i.e. 85 nm at 900 °C. Moreover, magnetic measurements show that a great enhancement in the saturation magnetization was achieved whereas a large hysteresis loop was observed (i.e.72 emu/g at 900 °C). Evaluation and applicability of CoFe₂O₄ nanoparticles under high frequency AC magnetic field for heating in biomedical applications were examined. It was found that under fixed amplitude (516 Oe) and frequency (229 kHz), the prepared nanoparticles generate significant heat: after 5 s the temperature was around 97 °C for the as-milled powder and reached almost 178 °C for the powder annealed at 900 °C.     

Properties of composite films of titania nanofibers and Safranin O dye

Titania (TiO₂) nanofibers and composite thin films of titania nanofibers and Safranin O dye (SAF) were studied. TiO₂ nanofibers were prepared by electrospinning technique from titanium tetra-isopropoxide precursor solution in ethanol. Surface topology of the nanofibers was observed using scanning electron microscopy (SEM), their crystal structure was studied by X-ray diffraction (XRD) and the chemical composition by X-ray photoelectron spectroscopy (XPS). Properties of the TiO₂ nanofibers were studied in dependence on the values of relative air humidity in the range from 15% to 55%. It was necessary to maintain the relative humidity lower than 30% during electrospinning in order to obtain high quality nanofiber films. The average minimum diameter of the as-prepared TiO₂ nanofibers was found to be around 100 nm. Nanofiber diameter diminishes to about 50 nm after annealing at 420 °C for 1 h. The as-prepared titania nanofiber films were completely amorphous while anatase crystal phase was detected in the films after annealing. In order to prepare the composite films, solution of SAF dye with poly(vinylpyrrolidone) in ethanol/water was dropped off on the prepared titania nanofibers surface. Opto-electrical properties of SAF dye and the resulting nanocomposite films were studied by UV–Vis spectroscopy and current–voltage characteristics. Safranin O is characterized by two strong absorption peaks; one at 274 nm and a wide band with splitting between 420 nm and 600 nm. The optical energy band gap of titania nanofibers was estimated from the UV–Vis measurements to be 3.4 eV. The charge transport in the composite films is influenced by the space charge limited currents due to the very high resistance of the materials.     

Processing of ultra-high strength SiCp/Al–Zn–Mg–Cu composites

The ultra-high strength SiC_p/Al–10%Zn–3.6%Mg–1.8%Cu–0.36%Zr–0.15%Ni composite was prepared by spray co-deposition followed by extrusion process. The heat treatment processing, microstructures and mechanical properties of the as-processed composite were investigated. The well-bonded SiC/Al interfaces and fine grains of matrix alloy were obtained in the as-extruded composite. The precipitated phase MgZn₂ dissolved during solid solution treatment at 490 °C for 1 h, but the Cu-rich phase was residual in the matrix. Comparatively, the Cu-rich phase dissolved into the matrix alloy exposed at 470 °C for 1 h and then at 490 °C for 1 h. The composite heat-treated with 470 °C/1 h + 490 °C/1 h + 120 °C/28 h exhibited high modulus above 100 GPa and ultra-high strength about 785 MPa, which was 30 MPa higher than that of the same composite treated with 490 °C/1 h + 120 °C/28 h processing. The low elongation of the composite can be attributed to the breakage of SiC particulates and interfacial debonding of SiC/Al.     

A facile synthesis of TiB2 nano-particles via mechano-thermal route

A mixture of TiO₂, B₂O₃ and Si powders was milled up to 50 h using a high energy planetary ball mill. The milling products were heat treated at 850, 1200 and 1300 °C. Effects of Si content and addition of catalyst on the phase evolutions and morphology of the products were investigated. XRD results showed that phase composition of the 50 h milled sample after heat treatment at 1300 °C consists of nano-crystalline TiB₂ with mean crystallite size of 50 nm together with some Ti₂O₃, Si and SiO₂ phases. Addition of 2 mol of NaCl in the same sample facilitates the process and resulted in the single phase TiB₂with a mean crystallite size of 30 nm. SEM and TEM micrographs exhibited the nano-sized particles of TiB₂.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Polyaniline nanofibers synthesized in compressed CO2

Polyaniline (PANI) nanofibers were synthesized in compressed liquid carbon dioxide without any template or surfactant. The polymerization of aniline took place at the interface between CO₂ and aqueous solution in a high-pressure stirred reactor. The prepared PANI nanofibers were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), electrical conductivity (EC), Fourier-transform infrared (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses. The yield of polymerization was high enough to reach 63.04% while maintaining small diameters of the PANI nanofibers. This result is very important for the preparation of the PANI nanofibers because no other previous investigations have achieved both high yield and small diameter of fibers at the same time. Through SEM and TEM analyses, we observed that the PANI nanofibers had diameter range of 30–70 nm and a length range of 0.3–1 μm, which caused them to disperse well in various solvents such as water, ethanol, 2-propanol, m-cresol and toluene. The electrical conductivity of the PANI nanofibers was 4.34 S/cm at 20 °C. The XRD diffraction pattern showed that the PANI nanofibers had crystalline one-dimensional structures, which gave high thermal stabilities as confirmed by TGA.     

Control of carbon content in WC-Co hardmetal by heat treatment in reducing atmospheres containing methane

Pressed WC-Co hardmetal compacts of two different compositions, 6 and 10 wt% Co, were heat treated under flowing atmospheres of nitrogen, hydrogen and methane at temperatures from 500 to 900 °C prior to sintering under argon. Microstructural examination showed excessive carburisation up to 2.5 mm into the compacts with regions most exposed to heat treatment atmospheres showing greatest carburisation. η-Phase was present in the 6 wt% Co samples heat treated at low temperatures without methane but was not present with heat treatment temperatures of 700 °C or above with methane present. The hardness of both materials was significantly lower in highly carburised regions, highlighting the need for careful control of heat treatment parameters.     

Swaging and heat treatment studies on sintered 90W–6Ni–2Fe–2Co tungsten heavy alloy

The effect of swaging deformation and low temperature aging on mechanical properties of tungsten heavy alloy (90W–6Ni–2Fe–2Co) was studied. The alloy was prepared by conventional hydrogen sintering followed by vacuum heat treatment and swaging with varying reductions in area (ranging from 10 to 75%). The swaged samples were aged in nitrogen atmosphere at temperatures ranging from 300 to 1100 °C followed by microstructure and mechanical property evaluation. Both microstructure and mechanical properties were sensitively dependent on the amount of deformation imparted during swaging. Subsequently, aging also influenced the properties depending upon the aging temperature. Detailed fractographic study revealed that the fracture mode of the alloy changed from intergranular to transgranular mode depending upon thermo-mechanical processing. Aging treatment after swaging affected the fracture morphology of alloy considerably. Samples aged at 500 °C showed predominant transgranular failure of tungsten grains, whereas the one heat treated at 700 °C showed increasing evidence ductile tear. Based on the results, an approach for optimizing mechanical properties that involves heat treatment and swaging operation was proposed.     

Preparation of helical carbon and graphite films using morphology-retaining carbonization

Helical carbon and graphite films were prepared from iodine-doped helical polyacetylene (H-PA) film using currently developing morphology-retaining carbonization. It was found from scanning electron microscopy (SEM) observations that the hierarchical helical morphology of the H-PA film remains unchanged even after the carbonization at 800 °C. Besides, the weight loss of the film due to the carbonization was very small, which was only a few percent to the weight of the film before doping. Furthermore, the graphite film prepared by the subsequent heating at 2600 °C still retained the same morphology as those in the original H-PA film and in the helical carbon film prepared at 800 °C. X-ray diffraction (XRD) and Raman scattering measurements were then pursued. The results showed that graphitic crystallization proceeds in the carbon film through the heat treatment at 2600 °C. Transmission electron microscopy (TEM) image of a single helical graphitic fibril was also observed by ultrasonicating the graphite film in ethanol. Carbonization of the H-PA films by way of iodine doping was found to afford helical carbon and graphite films, where spiral morphologies and even helical fibril structures were completely preserved.     

A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction

Silicon carbide nanoparticles were produced by a carbothermic reduction of nano and micro size silica with graphite at 1450 °C for 1 h. The SiC nanoparticles were characterized by XRD, SEM and TEM. The results showed that in the case of nano silica, milling up to 20 h could develop SiC particles of 5–25 nm with some residual SiO₂ particles. By extending milling time to 40 h, more energy was provided and produced Fe contamination, which could act as catalyst and increased SiC yield as well as Fe₂Si phase formation after heat treatment. Coarser particles of micro silica caused higher Fe erosion, more SiC formation with 20–70 nm size and presence of Fe₂Si phase at shorter milling times after heat treatment. Leaching treatment could purify SiC nanoparticles. Increase of milling from 20 to 40 h changed the morphology from polygonal shape to spherical with some reduction in the particle size.     

Oxidation behavior,microstructure evolution and thermal stability in nanostructured CrN/AlN multilayer hard coatings

CrN/AlN multilayer coatings with modulation period of 4 and 12.3 nm were manufactured by RF magnetron sputtering. The films were annealed at temperatures 800–950 °C for 1 h in air environment. The microstructure evolution and chemical composition of the formed oxides after heat treatment were identified by transmission electron microscope (TEM) and energy dispersive spectrometer (EDS). After heat treatment at 800 °C for 1 h, three regions which formed on the surface of CrN/AlN coatings with 12.3 nm modulation was observed, including the Al-rich layer covered at the topmost surface, the mixed nano-crystalline Al₂O₃ and Cr₂O₃ film and the spherical nano-voids embedded in between. On the contrary, for CrN/AlN coating with a modulation period of 4 nm, the dense oxide layer around 37 nm formed on the top of the un-reacted film was much thinner than that of CrN/AlN coating with 12.3 nm. Besides, no nano-voids was detected which implied that the outward diffusion of atom species was suppressed as compared to that in the film with 12.3 nm. The presence of the topmost Al-rich layer protected the multilayer coating from the outward diffusion of Cr and Al, as well as the inward diffusion of oxygen. After 950 °C for 1 h and 800 °C for 16 h, the grain growth of surface oxides occurred and non-uniform interface between oxide and coating was also determined by TEM in the CrN/AlN multilayer coating with 12.3 nm. However, no substantial oxidations were detected in the coating with modulation period of 4 nm. It was evident that the CrN/AlN multilayer with smaller modulation period exhibited better oxidation resistance.     

Structure and electronic properties of Grafoil heated with transition metals

We investigated the effects of iron, cobalt and nickel on the crystallinity and electronic properties of Grafoil (a polycrystalline graphite foil) by heating it with these metals at temperatures between 1000 and 2000°C and at 2800°C. Interlayer spacing d_0 0 2 decreased with increasing heat treatment temperature (HTT) irrespective of transition metals used, indicating the catalytic effect of these metals on improvement of graphite structure. Transverse magnetoresistance at 77 K increased with increasing HTT, passed through a maximum around 1500–1600°C, decreased at 2000°C, and then increased markedly at 2800°C for Grafoil heated with each metal. The effect of metal atoms as impurities introduced in Grafoil appears at 2000°C. Shifts of Hall coefficient toward the positive side were observed at 77 K for Grafoil heated with each metal, and the positive Hall coefficient was observed for each of the 2000 and 2800°C-treated ones. The results suggest that very small amounts of metal atoms were trapped in the crystal grains of Grafoil during the heat treatment and functioned as acceptors.     

An investigation on the in situ synthesis–sintering and mechanical properties of MoSi2–xSiC composites prepared by spark plasma sintering

Composites of MoSi₂–x wt.% SiC (x = 5, 10, 15, 20) prepared using spark plasma sintering. The effect of temperature on the in-situ synthesis–sintering was investigated between 1100 °C and 1500 °C. X-ray diffraction patterns showed that at 1100 °C the reactions were incomplete and elementary diffraction peaks of Mo, Si and C still exist. With an increase in temperature from 1100 to 1300 °C the reactions were performed completely. The study showed that the sintering ability at higher temperature at the presence of enough mechanical pressure was better because the heat released from the reaction between Mo, Si and C causes higher temperature than the melting point of Si (1410 °C). Consequently the silicon would melt during the heating process. The molten Si can strengthen the interconnections and it has higher diffusion rate. Therefore, due to the liquid phase sintering and at the presence of mechanical pressure, the sintering ability at higher temperature is better than lower temperature. Scanning electron microscopy showed that with the addition of carbon, there was no silica phase in the microstructure of the synthesized samples, due to the formation of SiC. Therefore, it can be noted that the addition of carbon leads to better mechanical properties due to elimination of silica phase.     

Effect of heat treatment on the microstructure and mechanical properties of the consolidated Mg alloy AZ91D machined chips

Influence of heat treatment on the properties of the consolidated AZ91D Mg alloy chips was performed in this study. The chips were pressed into a die to form a compact with a green density of 1.6 g/cm³. The 50-mm diameter green compact was then extruded into a 20-mm rod at 350 °C. The extruded rod was solution treated at a temperature of 415 °C for 24 h; the solution treated specimens were then aged at two temperatures: 170 and 215 °C. Heat treatments were conducted to explore the microstructure evolution and mechanical properties of the extruded rod. Heat treatments revealed that the age hardening effect was related to the transformation of the microstructure. Over aging during age heat treatment was believed to be caused by the formation of a lamellar structure composed of alternating layers of Mg₁₇Al₁₂ phase and magnesium matrix.     

Equilibrium phase of high-entropy FeCoNiCrCu0.5 alloy at elevated temperature

The phase transformations of FeCoNiCrCu_0.5 alloy with the as-cast structure and heat-treated structures were studied. The as-cast alloy specimens were first heated at 1050 °C with a holding time of 1 h. Serial heat-treatment processes at 350 °C, 500 °C, 650 °C, 800 °C, 950 °C, 1100 °C, 1250 °C and 1350 °C with a holding time of 24 h were then carried out to understand the phase evolution and the relationship between the microstructure and the hardness of the specimens. The microstructures were investigated and chemical analyses performed by optical microscopy (OM), scanning elector microscopy (SEM), X-ray diffractometer (XRD) and transmission elector microscopy (TEM). The results show that FCC peaks were observed from the X-ray diffraction of the as-cast specimens and a precipitate phase was present in the specimens that had been heated to 950 °C. The hardness of the FeCoNiCrCu_0.5 alloy remained unchanged in the specimens that underwent various heat treatments that were applied in this study.