Laser synthesis of silicon carbonitride nanopowders; structure and thermal stability

Si₃N₄/SiC nanocomposite materials are of great interest for structural applications at high temperature. In silicon nitride based ceramics, the small size and the spherical shape of the grains constituting the material are two important parameters in favour of high temperature deformation. Therefore, SiCN nano-sized powders are real candidates as starting materials to elaborate dense Si₃N₄/SiC nanocomposites exhibiting the microstructure required for ductility at high temperature. SiCN nanopowders with different chemical compositions and characteristics can be prepared by CO₂ laser pyrolysis of organosilicon precursors. Laser pyrolysis of gaseous precursors is able to produce partly crystallised SiCN nanoparticles exhibiting a reasonable thermal stability and suitable for elaboration of ceramic materials. In order to reduce the cost and to improve the safety of the process, an aerosol generated from a liquid precursor, hexamethyldisilazane (HMDS), has also been used to synthesised SiCN nanopowders. However, in this latter case the powders obtained exhibit a high weight loss during heat treatment at high temperature. Therefore, in this study the effects of various synthesis parameters (chemical nature of the precursor and laser power) on the degree of crystallisation and on the thermal stability of nanopowders are investigated. Characteristics of powders such as chemical composition, morphology, structure and thermal stability are reported. A correlation between the synthesis conditions of powders and their thermal stability is established, and the synthesis parameters enabling improvement of thermal stability are determined.     

Laser induced oxidation processes in iron, copper and nickel nanopowders

The present paper is devoted to the study of oxidation processes occurring in iron, copper and nickel nanopowders under the action of CO₂ laser radiation. It was established that two different phases of iron oxide (magnetite and hematite) are stabilized in the oxidation products of the iron nanopowder suggested to laser radiation whereas the non-irradiated nanopowder contains the hematite phase only. The phase composition of the oxidation products are the same for both the irradiated and non-irradiated copper nanopowders, but their morphology is different. The thermal behavior of the Ni irradiated nanopowder differs from that of the non- irradiated one: it is characterized by an additional exothermal peak at 276 °C.     

Intrinsic and Ce-related luminescence of YAG and YAG:Ce single crystals, single crystalline films and nanopowders

A comparative analysis of the luminescent properties of YAG and YAG:Ce nanopowders (NP) in comparison with single crystalline film (SCF) and single crystal (SC) analogues was performed under excitation by a pulsed synchrotron and X-ray radiation. It was shown that the natural defects concentration in NP was between the SC with a large (0.18–0.19 at.%) concentration of Y_Al antisite defects (AD) and SCF of these garnets where Y_Al AD were completely absent. At the same time, Ce³⁺ doped YAG NP showed luminescent properties close to those of YAG:Ce SCF.     

Ox/Red-controllable combustion synthesis of foam-like PrFeO3 nanopowders for effective photo-Fenton degradation of methyl violet

Praseodymium orthoferrite nanopowders were synthesized by Red/Ox-controlled glycine-nitrate combustion followed by heat treatment. It was found that the temperature of PrFeO₃ formation from amorphous combustion products varied from 612 °C to 647 °C, depending on the initial reaction solution. The purity, crystallite size, morphology and specific surface area of ??the samples were affected by the glycine-nitrate ratio (G/N). At nonstoichiometric G/N (more or less than 0.6), amorphous or low crystalline phases were formed. Also, at these ratios, nanopowders had the smallest crystallite size before (17.4–36.2 nm) and after (45.3–49.2 nm) heat treatment. All powders were characterized by developed porous morphology with a specific surface area of ??2.9–11.9 m²/g and a band gap of 2.07–2.13 eV. The results of ⁵⁷Fe Mössbauer spectroscopy confirmed magnetically ordered state of PrFeO₃ nanocrystals with 1 mol % of amorphous component, associated with the combustion mode of a glycine-nitrate solution. The photocatalytic activity of PrFeO₃ was evaluated by the rate of decomposition of methyl violet (MV) in the presence of peroxide under the action of visible light.     

Upconversion and infrared luminescences in Er/Yb codoped Y2O3 and (Y0.9La0.1)2O3 transparent ceramics

Er³⁺ and Yb³⁺ codoped Y₂O₃ and (Y_0.9La_0.1)₂O₃ transparent ceramics were fabricated by the conventional ceramics processing with nanopowders. Compared to Er/Yb:Y₂O₃, Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics had higher transmittance. Intense upconversion (UC) and infrared emission (1543 nm) were observed under excitation of 980 nm. According to three intensity parameters Ω₂, Ω₄, and Ω₆ fitted by the Judd-Ofelt theory, the spectroscopic quality parameters (X), radiative lifetimes (τ_rad), and emission cross-sections (α_em) were determined. Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics owned broader peaks and longer lifetime (12.3 ms) at 1548 nm due to the glass-like structure of (Y_0.9La_0.1)₂O₃ ceramics. The results showed Y₂O₃ and Y_1.8La_0.2O₃ transparent ceramics are promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable UC lasers.     

A novel economical grain boundary engineered ultra-high performance ZnO varistor with lesser dopants

A varistor having ultra-high performance was developed from doped ZnO nanopowders using a novel composition consisting of only three (Bi, Ca and Co oxides) dopants. Improved varistor properties were obtained (breakdown field (E_b) 27.5?±?5?kVcm^?1, coefficient of nonlinearity (α) 72?±?3 and leakage current density (L_c) 1.5?±?0.06?μAcm^?2) which are attributed to the small grain size and grain boundary engineering by phases such as Ca₄Bi₆O₁₃ and Ca_0.89Bi_3.11O_5.56 along with Co⁺² doping in the ZnO lattice. Complex impedance data indicated three relaxations at 25?°C and two relaxations at high temperature (>100?°C). The complex impedance data were fitted into two parallel RC model to extract electrical properties. Two stages of activation energy for DC conductivity were observed in these varistor samples where region I (<150?°C) is found to be due to shallow traps and region II (<225?°C) is due to deep traps. The novel composition is useful for commercial exploitation in wide range of surge protection applications.     

Microstructure and properties of BN/Si3N4 composites with addition of microwave synthesized Y2O3-MgO nanopowders

Y₂O₃-MgO nanocomposite powder was synthesized by a microwave technique and doped into gas-pressure sintered BN/Si₃N₄ composites to acquire a highly wave-transparent material that could be utilized in spacecrafts. The XRD and SEM analyses indicated that the main phases were rod-like β-Si₃N₄ particles and MgSiO₃. In this case, a BN/Si₃N₄ ceramic with 8?wt% Y₂O₃-MgO nanopowder displayed a density of 2.5?g/cm³. The permittivity and transmission efficiency were approximately 4.6% and 71.4%, respectively, at frequencies of 8.2–12.4?GHz, which achieved the best overall wave-transparent performance. The porosity of the material in combination with the inclusion of the boron nitride component creates a synergistic effect that appears to contribute to wave-transparent efficiency.     

Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Hydrogen storage of the Mg–C composites

The effect of different kinds of carbon on the hydrogen sorption kinetics by magnesium–carbon composites was analyzed. To prepare magnesium-based composites by ball milling, graphite and carbon nanomaterials (hereinafter CNM) obtained by the electroexplosion technique were used. Phase composition and structure state of the as-milled and hydrogenated magnesium–carbon and magnesium–nickel–carbon composites have been investigated. It was found the crystallite size in the Mg–CNM composite is smaller in comparison with the magnesium–graphite and magnesium–graphite–nickel mixtures. The CNM additives to magnesium essentially improve the hydrogen sorption kinetics. It results in a reduction of hydrogen sorption temperature. The noticeable hydrogen absorption took place already at a temperature of 363 K. The hydrogen capacity was about 5 wt% for magnesium ball milled with CNM additives.     

Influence of process parameters in gel casting of a pure yttria nanopowder to fabricate transparent ceramics

The goal of this research is to fabricate pure transparent yttria ceramics through gel casting and vacuum sintering. A specific processing method has been used and optimized for this purpose. A pure yttria nanopowder was synthesized as the starting material to produce pure transparent ceramics through a low-temperature sintering process. It was attempted to minimize the undesirable nanopowder hydration by using the as-synthesized yttria nanopowder and a rapid deagglomeration and slurry preparation process. The synthesized nanopowders were deagglomerated to enhance the efficiency of both powder shaping and sintering stages. Carrageenan was used as the gelling agent because it is a low-cost and abundant material, and because the temperature is the only catalyst needed for its gelation; therefore, it is possible to control its gelation to obtain high-density and pure optical ceramics. The effect of the deagglomeration method and the processing parameters, including the amounts of dispersant, gelling agent, solid loading, pH, and deagglomeration time, on the rheology of slurry, density, and microstructure of the obtained green yttria ceramics was examined and optimized in order to obtain high solid loading nanoyttria suspensions of 38 vol%, which is more than those obtained in many of the previous investigations. The precise gelling temperature and time were measured, and green gel cast ceramics with a density of 63 % of the theoretical density were produced. A rapid deagglomeration and slurry preparation method was used instead of using a conventional planetary ball-milling approach to minimize the risk of the hydrolysis of yttria nanopowder. No sintering aid was necessary, and transparent yttria ceramics with 99 % of the theoretical density were produced after vacuum-furnace sintering at 10^-2 mbar and 1715 °C.