Studies on the Al2OC mesophase in synthesized AlN powder and its effects on properties of AlN ceramic substrates

A mesophase of Al₂OC was first determined in AlN powder synthesized in batch quantities via a carbothermal reduction nitridation (CRN) process. The formation and elimination mechanisms of the mesophase were investigated. Effects of the mesophase on properties of the AlN ceramic substrates were evaluated via bending strength and thermal conductivity tests of the substrates fabricated with AlN powder of different O contents. At the conditions of the synthetic furnace, i.e. T = 1700 °C, P_N2 = 10⁻⁵ kPa, and P_CO = 10^−0.008–10^0.973 kPa, the formation of Al₂OC is thermodynamically favorable. By increasing the flow rate of N₂ in the synthetic furnace, the formed Al₂OC was unstable and decomposed into AlN. The properties of the AlN substrates depend on the O content of the AlN powder. The thermal conductivity/bending strength of the AlN substrates increase or decrease, accordingly, based the O content of the reduced AlN powder. AlN substrates made of AlN powder with 0.84 wt% oxygen content show a thermal conductivity and bending strength of 176.3 W/(m·K) and 421.3 MPa, respectively.     

Ignition and chemical mechanisms of volume combustion synthesis of titanium diboride

Reaction ignition and chemical mechanisms in volume combustion synthesis of TiB₂ via TiO₂–B₂O₃–Mg precursors were studied using in-situ differential thermal analysis, X-ray diffraction, scanning electron microscopy and thermochemical modeling. Mg–TiO₂ samples ignited at 607 °C through a sudden single step solid-solid reaction while Mg–B₂O₃ samples ignited at 810 °C after melting of magnesium. X-ray diffraction analysis revealed that reduction of TiO₂ occurs in multiple steps and forms intermediate compounds. Results showed that heat released from the first reaction between TiO₂ and Mg ignites the reactions between Mg, Ti and B₂O₃ resulting in the formation of TiB₂. Samples with larger TiO₂ particle size or a higher sample surface to volume ratio showed a two-step reaction behavior and the released heat in the first solid state reaction was insufficient for the propagation of the reaction throughout the sample. In addition, Mg₃B₂O₆ undesired by-product was formed as a result of this two-step reaction.     

Synthesis of MgSiN2 under 0.1 MPa of nitrogen pressure via combustion

In this study, MgSiN₂ is prepared via combustion synthesis under low-pressure nitrogen (0.1 MPa), and the effects of the mixture composition and nitrogen pressure on the obtained products are investigated. The combustion temperature increases with the amount of silicon, and β-Si₃N₄ is detected as the main impurity when a significant amount of silicon is incorporated. In addition, the combustion temperature increases with the nitrogen pressure, and the purity of MgSiN₂ increases because the high pressure restricts the vaporization of magnesium. These results suggest that increasing magnesium content is efficient for the synthesis of single-phase MgSiN₂ at 0.1 MPa of nitrogen because magnesium is volatilized from the reaction system under the low nitrogen pressure. Hence, single-phase MgSiN₂ can be synthesized under 0.1 MPa of nitrogen using a mixture containing a significant amount of magnesium.     

Effect of vibro-milling time and calcination condition on phase formation and particle size of nickel niobate nanopowders

The effect of milling time on the phase formation and particle size of nickel niobate, NiNb₂O₆, powder synthesized by a solid-state reaction via a vibro-milling technique was investigated. Powder samples were characterized using DTA, XRD, SEM and laser diffraction techniques. It was found that the smallest particle size of 32 nm was achieved at 25 h of vibro-milling after which a higher degree of particle agglomeration was observed on continuation of milling to 35 h. In addition, by employing an appropriate choice of the milling time, a narrow particle size distribution curve was also observed.     

Synthesis and characterization of MgAl2O4 micro-rods by a molten salt method

Large amounts of MgAl₂O₄ micro-rods were successfully synthesized using the molten-salt technology. The effect of KCl contents on the formation of MgAl₂O₄ micro-rods was investigated. The structure and morphology of MgAl₂O₄ were investigated by means of powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy, respectively. The experimental results showed that the contents of KCl significantly influenced the formation of MgAl₂O₄ micro-rods. MgAl₂O₄ micro-rods could be prepared at 1150 °C with a weight ratio of 100:1 between the salt and the starting materials. The formation of MgAl₂O₄ micro-rods could be suggested to be due to the inhomogeneous nucleation and orientated growth perpendicularly to the surfaces of Al₂O₃ grains. An impedance-type humidity sensor was finally fabricated based on the as-prepared MgAl₂O₄ micro-rods. According to tests of the humidity performance, MgAl₂O₄ micro-rods might be suitable for high-performance humidity sensors.     

Ferro-/pyroelectric response of 0.57BF-0.31PMN-0.12PT ternary ceramic far away from morphotropic phase boundaries

A ternary BiFeO₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (BF-PMN-PT) solid solution with a composition far away from morphotropic phase boundaries (MPB) has been synthesized by solid-state reaction method. XRD analysis revealed a pure perovskite phase without any trace of pyrochlore phase. The system exhibited a broad phase transition with transition temperature T_c ~ 285 °C. The hysteresis loop of the ceramics displayed high remanent polarization (Pr ~ 41.23 μC/cm²) and coercive field (E_c ~ 35.41 kV/cm). The true-remanent polarization after removing the non-remanent components was evaluated by remanent hysteresis and PUND tasks, which revealed the actual usable polarization component for memory devices. Time-dependent compensated plots confirmed the resistive-leakage free characteristic of the BF-PMN-PT ceramics which makes them useful for application in devices like actuators where the electric field is switched at different rates. Fatigue test showed the ferroelectric parameters (switching and non-switching polarization values) do not vary over 10⁷ cycles which is important especially in electromechanical transducers and memory devices. A large piezoelectric charge coefficient (d₃₃ ~ 115 pC/N) was observed for the investigated ceramics.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

Cordierite ceramics from silicone resins containing nano-sized oxide particle fillers

Many silicates and alumino-silicates feature remarkable mechanical properties at high temperatures, low thermal expansion and high thermal shock resistance, optimum dielectric properties, etc. The poor interdiffusion, due to their characteristic partially covalent bonding however, greatly complicates the obtainment of dense and/or phase pure articles, by conventional sintering. The present paper concerns the realization of high-purity cordierite (2MgO·2Al₂O₃·5SiO₂) components by direct thermal treatment in air of preceramic polymers embedding suitable nano-sized oxide particles. More precisely, a selection of silicone resins allowed the obtainment of both dense and highly porous bodies.     

Characterization of SiO2-encapsulated WSi2/W5Si3 nanoparticles synthesized by a mechanochemical route

This study reports on the mechanochemical synthesis (MCS) of SiO₂-encapsulated WSi₂/W₅Si₃ nanoparticles starting from tungsten oxide (WO₃), silicon dioxide (SiO₂) and magnesium (Mg) powder blends. MCS process, carried out in a high-energy ball mill, was evaluated in terms of various milling time and initial composition of WO₃-SiO₂-Mg powders. A subsequent purification step using aqueous HCl solution was conducted on the as-synthesized powders. Compositional, microstructural and thermal properties of the powders were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimeter (DSC). Based on the adiabatic temperature values, SiO₂-encapsulated WSi₂/W₅Si₃ nanoparticles exhibit a high potential as a result of a mechanically induced self-sustaining reaction. The exothermic reaction took place after 20?min of milling, and WSi₂, W₅Si₃, W, MgO, Mg₂SiO₄ and residual Mg phases were detected. The utilization of an optimized amount of excess Mg (3.5?mol) resulted in the elimination of unwanted Mg₂SiO₄ and W phases, leaving behind WSi₂, W₅Si₃, Mg₂Si, MgO and Mg. After removal of Mg-based by-products by leaching process, WSi₂, W₅Si₃ and a very small amount of SiO₂ phases were obtained. TEM analysis revealed that W silicide nanoparticles with an average size of 97?nm were encapsulated by SiO₂ layers with an average thickness of 15?nm.     

Effect of vibro-milling time on phase formation and particle size of barium titanate nanopowders

Barium titanate (BT) nanopowder was synthesized by a solid state reaction via a rapid vibro-milling technique. The effect of milling time on phase formation and particle size of BT powder was investigated. Powder samples were characterized using XRD (X-ray diffraction) and SEM techniques. It was found that the resulting BT powders have a range of particle size depending on milling times. Production of a single-phase BT nanopowder can be successfully achieved by employing a combination of 30 h milling time and calcination conditions of 1200 °C for 2 h.     

Solid-phase synthesis of wollastonite in natural and technogenic siliceous stock mixtures with varying levels of calcium carbonate component

Solid-phase synthesis of wollastonite has been determined to be primarily affected by the structural and mineralogical properties of the siliceous stock – its chemico-mineralogical composition (purity and presence of amorphous component) and granularity, as well as variations in phase change behaviour at high temperatures. Wollastonite synthesis occurs most completely in the mixtures of calcium carbonate with amorphous (microsilica) or semi-crystalline (gaize or diatomite) siliceous stock, yielding 92–96% wollastonite at 1200 °С. For natural quartz-based stock (marshallite), wollastonite yield does not exceed 60–80%. Solid-phase wollastonite synthesis in mixtures of reactive amorphous or semi-crystalline siliceous stock with varying quantities of calciferous stock (10–50 wt% CaO) produces wollastonitic ceramics chemically resistant to molten aluminium with the specific gravity of 1.1–1.7 g/cm³ and the compressive strength of 28–76 MPa, which exceeds the requirements for ceramics in foundry equipment by a factor of 3–3.5.     

Fabrication of TaB2/mullite composites by combustion synthesis with excess silicon and B2O3 additions

Preparation of TaB₂/mullite composites from a cost- and energy-effective mixture of reactants was conducted by self-propagating high-temperature synthesis (SHS). The sample stoichiometry of 1.18Ta₂O₅+(2.36x)B₂O₃+6Al+(2y)Si was formulated with x=1.0–1.3 and y=1.0–2.0 for the study of the effects of excess B₂O₃ (x>1.0) and Si (y>1.0) on combustion characteristics and product compositions. The synthesis reaction involved coreduction of Ta₂O₅ and B₂O₃ by Al and Si. The reaction exothermicity and combustion velocity increased slightly with increasing B₂O₃ but decreased considerably with Si. Formation of TaB₂ and mullite was improved by adopting excess B₂O₃ and Si to compensate for their evaporation loss at high combustion temperatures up to about 1600 °C. The sample with x=1.3 and y=1.5 was shown to yield the optimum formation of TaB₂ and mullite. Mullite grains with a tubing-like shape were produced from in situ formed SiO₂ and Al₂O₃ and had an atomic composition close to that of 3:2 mullite (3Al₂O₃·2SiO₂).     

Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15

Pure and lanthanum doped barium bismuth titanate BaBi_4−xLa_xTi₄O₁₅ (BBLT, x=0, 0.05, 0.15, 0.30) ceramics were prepared utilizing solid state method. The X-ray diffraction (XRD) data confirmed formation of single-phase Aurivillius compounds while SEM micrographs did not show evident grain size change of doped ceramics. Dielectric properties were investigated in 1.21 kHz to 1 MHz frequency range and in the temperature range of 20 to 727 °C. When Bi³⁺ is substituted with La³⁺, a significant disorder was induced and the material exhibited broadening of the phase transition. Impedance analysis confirmed the presence of two semicircular arcs in doped samples suggesting the existence of grain and grain-boundary conduction. The dc-conductivity and activation energies were evaluated for all compositions.     

Fabrication and characterization of amorphous SiBCN powders

A new approach in the synthesis of amorphous SiBCN has been suggested using high-energy shaker ball mill. Hexagonal boron nitride (h-BN), graphite (C) and amorphous silicon (Si) were blended according to the mole ratio of 1:1:1, and then ball milled by different ball-to-powder mass ratio, diameter of ball and milling time. The structural evolution at different stages of milling has been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and electron energy-loss spectroscopy (EELS). The results showed that the SiBCN powders were mainly amorphous with some nanocrystlline phases. The thermal stability of SiBCN powders has been analyzed by thermogravimetry in argon. Mass loss occurred at high temperature, especially above 1000 °C.     

RF thermal plasma-assisted metallothermic synthesis of ultrafine ZrB2 powders

Ultrafine ZrB₂ powders were synthesized in radio-frequency atmospheric-pressure thermal plasma using ZrCl₄, B, and Mg as raw materials. The metallothermic reaction was ignited by the high temperature flame when the mixed precursors were carried into thermal plasma, and then propagated with the exothermic reaction enthalpies and the external energy supplied by plasma flame. Both thermodynamic analysis and experiments were conducted. The as-prepared samples were characterized by X-ray Diffraction, Transmission Electron Microscopy, and particle size analyzer. Results showed that the obtained ZrB₂ powders had particle size of about 100 nm and surface area of 30.75 m²/g. The laboratory production rate was about 300 g/h.     

Structural and Raman spectroscopic studies of poled lead-free piezoelectric sodium bismuth titanate ceramics

Lead-free piezoelectric sodium bismuth titanate (NBT) ceramics are synthesized by a solid state reaction method. Poled NBT ceramics possess rhombohedral R3c structure. Vibrational phonon frequencies associated to Bi–O, Na–O, TiO₆ octahedra and oxygen vibrations are observed. Experimentally unreported lowest frequency region peak centered at 111 cm⁻¹ is assigned to E(TO1) mode related to Bi–O vibrations. Appearance of lowest frequency mode may be due to well-polished surfaces with reduced Rayleigh scattering background and well-defined ordering of Bi and Na ions in the lattice.     

Electronic structure and electrical conduction by polaron hopping mechanism in A2LuTaO6 (A= Ba,Sr, Ca) double perovskite oxides

Impedance spectroscopy has been applied to explore electrical properties of polycrystalline double perovskite oxides A₂LuTaO₆ (A = Ba, Sr, Ca; ALT). The phase purity and microstructural analysis of the samples are obtained from XRD and SEM. Ba₂LuTaO₆ (BLT) crystallizes in a single phase whereas in Sr₂LuTaO₆ (SLT) and Ca₂LuTaO₆ (CLT) a small secondary phase of Lu₂O₃ was detected. The Nyquist plots for ALT reveals the rising dominance of the grain boundary contribution to the conduction process with increasing temperature. The complex modulus plots confirm the presence of both the grain and grain boundary contributions to the relaxation process at low temperatures. The relaxation mechanism is seen to be shifted from its ideal character as observed in the Nyquist plots. The frequency dependent conductivity spectra follow the power law behavior. Small polaron hopping is responsible for the electrical conduction in these materials in the entire temperature range. Density functional theory calculations are performed to understand the electronic structure of the materials. The density of states (DOS) for BLT, SLT and CLT establishes the semiconducting ground state of the materials. The conductivity mechanism is analysed on the basis of the calculated DOS.     

New scheelite-type Cd1−3x⌷xGd2x(MoO4)1−3x(WO4)3x ceramics – their structure,thermal and magnetic properties

Polycrystalline samples of scheelite-type Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x solid solution with limited homogeneity (0<x≤0.25) and cationic vacancies (denoted as ⌷) have successfully prepared by a high-temperature annealing of CdMoO₄/Gd₂(WO₄)₃ mixtures composed of 50.00 mol% and less of gadolinium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(WO₄)₃ system was constructed. The eutectic point corresponds to 1404±5 K and ~70.00 mol% of gadolinium tungstate in an initial CdMoO₄/Gd₂(WO₄)₃ mixture. With decreasing of Gd³⁺ content in a CdMoO₄ framework, the melting point of Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x increases from 1406 (x=0.25) to 1419 K (x=0.0833), and next decreases to 1408 K (x=0). EPR method was used to identify paramagnetic Gd³⁺ centers in Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x for different values of x parameter as well as to select biphasic samples containing both Cd_0.25⌷_0.25Gd_0.50(MoO₄)_0.25(WO₄)_0.75 and Gd₂(WO₄)₃.     

Detonation synthesis of nanoscale silicon carbide from elemental silicon

Direct reaction of precursors with the products of detonation remains an underexplored area in the ever-growing body of detonation synthesis literature. This study demonstrated the synthesis of silicon carbide during detonation by reaction of elemental silicon with carbon products formed from detonation of RDX/TNT mixtures. Continuum scale simulation of the detonation showed that energy transfer by the detonation wave was completed within 2–9 μs depending on location of measurement within the detonating explosive charge. The simulated environment in the detonation product flow beyond the Chapman-Jouguet condition where pressure approaches 27 GPa and temperatures reach 3300 K was thermodynamically suitable for cubic silicon carbide formation. Carbon and added elemental silicon in the detonation products remained chemically reactive up to 500 ns after the detonation wave passage, which indicated that the carbon-containing products of detonation could participate in silicon carbide synthesis provided sufficient carbon-silicon interaction. Controlled detonation of an RDX/TNT charge loaded with 3.2 wt% elemental silicon conducted in argon environment lead to formation of ～3.1 wt% β-SiC in the condensed detonation products. Other condensed detonation products included primarily amorphous silica and carbon in addition to residual silicon. These results show that the energized detonation products of conventional high explosives can be used as precursors in detonation synthesis of ceramic nanomaterials.     

Structural,morphological and electrical properties of Ce1−xRuxO2−δ (x=0.005–0.02) solid solutions

Nanoparticles of ceria–ruthenium oxide solid solutions with composition Ce_1−xRu_xO_2−δ (x=0.005–0.02) were successfully produced by self–propagating room temperature synthesis using reaction between metal nitrates and sodium hydroxide. These compositions were characterized by X–ray powder diffraction (XRD), Raman spectroscopy, specific surface area, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X–ray spectroscopy (EDX). The experimental measurements were complemented by calculations based on the ion–packing model. XRD analysis revealed the presence of single–phase solid solutions with CeO₂ fluorite structure (regardless of dopants concentration) and Raman spectroscopy confirmed the presence of the RuO₂ phase. Electrochemical impedance spectroscopy (EIS) measurements of sintered samples at different temperatures showed that the small ionic radius dopant reduces oxygen vacancies mobility that is responsible for the conductivity of these ceramics.