Structural and optical properties of magnesium nitride formed by a novel electrochemical process

The electrochemical formation of magnesium nitride (Mg₃N₂) films in LiCl-KCl containing Li₃N at 723 K was investigated. From a thermodynamic point of view, a potential-pN³⁻ diagram was constructed for the Mg-N system in an analogous fashion to Pourbaix diagrams for aqueous solutions. As a result, the thermodynamically stable region of Mg₃N₂ in LiCl-KCl-Li₃N was identified. XRD analysis revealed that Mg₃N₂ film was obtained by potentiostatic electrolysis of a magnesium electrode between 0.4 and 0.8 V (versus Li⁺/Li), and the structure of obtained Mg₃N₂ was anti-bixbyite (a = 1.001 nm). Reflectance measurements clarified that assuming direct transition, the bandgap energy was 3.15 eV and assuming indirect transition, the bandgap energy was 2.85 eV.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.     

Synthesis of magnesium silicon nitride nanopowder by employing two-step method

In this work, a new method for the preparation of magnesium silicon nitride (MgSiN₂) nanopowder was studied using a two-step process in nitrogen gas from Mg/Si starting mixtures. This method is known as mechanical alloying followed by heat treatment. The results showed that the compositions of the combustion products depended on the starting mixtures. In addition, the content of magnesium and silicon in the starting powder should fulfil the condition Mg/Si?=?2 to obtain single phase MgSiN₂.Single phase MgSiN₂ nanopowder can be fabricated from Mg and Si powders with mole ratio of 2:1 at 1350°C in N₂ atmosphere. Composition and structure of reactants and products were examined by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy.     

Preparation and characterization of magnesium diboride superconductor by melting process

The recent discovery of the binary metallic magnesium diboride (MgB₂) superconductor having a remarkably high transition temperature (T_c) of 39 K has generated excitement among the scientist worldwide and gained great scientific interest. Various methods (viz. PLD, solid state reaction etc.) are reported for the preparation of this material in different forms (bulk, wire, thin film) which require a high processing temperature (750 to 950 °C). In this paper, we report a new method of processing MgB₂ superconductor that meets all the properties when compared with other processes. In this work, polycrystalline MgB₂ was prepared by using melting process at low temperature (660 °C). The stoichiometric mixture of Mg-rich and B-rich was pressed into pellets and piled to form Mg-rich/B-rich/Mg-rich system. The piled specimen was then heated up to 800 °C for four hrs with a heating rate of 5 °C/min. The sample was then kept at 660 °C for 12 hrs after cooling from 800 to 660 °C in 30 min. For comparison, the sample was also sintered at 660 °C for 24 hrs. The samples were characterized by using XRD, EDX, SEM, four probe AC methods and magnetization measurements using SQUID magnetometer. The critical temperature was found to be 39 K which shifts towards lower temperature with increasing applied field (0 to 9T). The critical current density, according to Bean’s critical state model was estimated and found to be ∼10₅ A/cm₂, which is comparable to the reported data.     

Preparation and characterization of multicomponent porous materials prepared by the sol-gel process

An experimental strategy was developed to obtain transparent Si-Al-Ti-Ni-Mo and Si-Zr-Ti-Ni-Mo sols via the sol-gel process. The sol was prepared from Si(OEt)₄ (TEOS), Al(OBu^s)₃ (OBu^s: C₂H₅CH(CH₃)O), Ti(OEt)₄ (OEt: OCH₂CH₃), Zr(OPrⁿ)₄ (OPrⁿ: OCH₂CH₂CH₃). In both cases nickel nitrate hexahydrate (Ni(NO₃)₂ · 6H₂O) and ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄ · 4H₂O) were the Ni and Mo sources, respectively. The sols were characterized by Fourier Transform Infrared Spectroscopy (FTIR). Assignments of the simultaneous formation of the Si-O-Al, Si-O-Ti, Si-O-Ni, and Si-O-Zr bonds were done. The sols were polymerized at room temperature (293 K) to obtain gels, and these were dried at 423 K and calcined at 573, 853 and 893 K in air. The characterization techniques used were small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The density of the solids was measured following ASTM method D-4892 and the porosity and surface area were determined by N₂ adsorption/desorption isotherms. The corresponding average pore diameters were evaluated using the BJH, HK, and DA methods.     

Fabrication and characteristic of non-oxide fiber tow reinforced silicon nitride matrix composites by low temperature CVI process

Non-oxide fiber tow reinforced silicon nitride matrix composite was fabricated by low temperature CVI process with PyC as interphase. The tensile strength of the C and SiC fiber tow composites were 547 MPa and 740 MPa, respectively. The difference in tensile strength was analyzed based on the length, amount of pull-out fiber and also interface bonding. The infiltration uniformity of CVI silicon nitride (SiN) matrix within SiC fiber tow was comparable with that of CVI SiC matrix. These results suggested that the low temperature CVI process is suitable for the fabrication of fiber reinforced SiN matrix composites with proper interface bonding and high strength.     

Low temperature densification of silicon nitride using Li2O-based surface coatings

For better control of the mechanical properties of Si₃N₄ ceramics, it is necessary to generate homogeneous microstructures, and for this purpose, chemical heterogeneities must be minimised, by careful control of powder processing and the subsequent consolidation steps. Coating of the starting silicon nitride powder is a convenient way of incorporating a liquid forming sintering aid more homogeneously than can be achieved by current commercial methods such as ball-milling.Thin layers of oxides, corresponding to additions of up to 5 w/o Li₂O have been deposited on the surface of grains of a commercial silicon nitride powder using alcoholic solutions containing appropriate amounts of the metal alkoxide. The resulting powders have been densified by pressurelesss sintering techniques, and their sintering characteristics identified in comparison with equivalent materials produced by adding the oxide in particulate form. In every case, a better sintering performance was observed at lower temperatures for the oxide-coated materials, with fully dense pressureless-sintered materials being obtained at temperatures as low as 1250 °C. An added observation was that for the coated samples, the final microstructure was more uniform, and showed an absence of large pores.     

Decomposition of MgSiN2 in nitrogen atmosphere

Magnesium silicon nitride MgSiN₂ was prepared by direct nitridation of Si/Mg₂Si/Mg/Si₃N₄ powder compact in a temperature range 1350-1420 °C. The thermal stability examination showed that MgSiN₂ is stable up to 1400 °C at 0.1 MPa N₂ pressure. The activation energy of decomposition of MgSiN₂ calculated from the temperature dependence of mass loss in the range of 1400-1650 °C is ΔH = 501 kJ mol⁻¹. The time dependence and nitrogen pressure dependence of MgSiN₂ decomposition was also investigated at constant temperature. MgSiN₂ is stable at 1560 °C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs energy of formation of MgSiN₂ was calculated in a temperature range 25-2200 °C.     

Preparation of tungsten borides by combustion synthesis involving borothermic reduction of WO3

An experimental study on the preparation of two tungsten borides, WB and W₂B₅, was conducted by self-propagating high-temperature synthesis (SHS), during which borothermic reduction of WO₃ and elemental interaction of W with boron proceeded concurrently. Powder mixtures with two series of molar proportions of WO₃:B:W = 1:5.5:x (with x = 1.16–2.5) and 1:7.5:y (with y = 0.5–1.33) were adopted to fabricate WB and W₂B₅, respectively. The starting stoichiometry of the reactant compact substantially affected the combustion behavior and the phase composition of the final product. The increase of metallic tungsten and boron reduced the overall reaction exothermicity, leading to a decrease in both combustion temperature and reaction front velocity. The initial composition of the reactant compact was optimized for the synthesis of WB and W₂B₅. In addition to small amounts of W₂B and W₂B₅, the powder compact of WO₃ + 5.5B + 2 W produced WB dominantly. Optimum formation of W₂B₅ was observed in the sample of WO₃ + 7.5B + 0.85W. Experimental evidence indicates that an excess amount of boron about 10–13% is favorable for the formation of WB and W₂B₅.     

Production of titania nanoparticles by a green process route

The manufacture of heterogeneous catalysts and catalyst supports produces substantial amounts of nitrate containing aqueous effluent. The use of nitrate free precursors and an environmentally friendly process would change the manufacture so that the entire process of catalyst synthesis and use can be considered green. In this work the precipitation of titania acetylacetonate nanoparticles for use as catalytic supports using a supercritical carbon dioxide anti-solvent process was investigated over a range of conditions. The effects of 1) pressure, 2) temperature, 3) solution flowrate, 4) solution concentration of TiO(acac)₂ in methanol, 5) nozzle diameter and 6) CO₂/methanol flow ratio on the mean particle size and morphology were studied. Particle sizes between 27 and 78 nm were obtained and were generally string and branch-like with an amorphous nature. Pressure and temperature had little effect on the mean particle size. A decrease in the velocity of the solution flow rate led to an increase in mean particle size and to particles that exhibited greater interconnectivity. It was also observed that an increase in concentration of TiO(acac)₂ in methanol led to an increase in mean particle size. The process shows promise for the production of catalysts by an environmentally acceptable route.     

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 1: Factors influencing the reaction-bonding process

In order to establish a process for the manufacturing of injection moulded micro-components of sintered reaction-bonded silicon nitride (SRBSN) several process parameters were investigated with regard to their influence on the reaction-bonding step. One question to be answered was how the sintering aids affect the nitridation behaviour of a silicon green body. For the processing of micro-components it was of special interest to study, how a decreasing sample size and wall thickness would influence the rate of Si₃N₄ formation. By varying the added amounts of the sintering aids, it was found that increasing the Y₂O₃ and MgO contents both improved the nitridation rate, whereas an increase of Al₂O₃ content resulted in reduced nitridation rates. Within the investigated range of sample dimensions (0.2–4.0 g) the unexpected observation was made, that with decreasing sample weight the nitridation rate also decreased. This was explained by the exothermic nature of the reaction between Si and N₂ and the fact that small samples with a large surface-to-volume ratio attain thermal equilibrium with their environment better than large samples which may be subject to local overheating.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

Preparation and sintering pure nanocrystalline α-alumina powder

Seeding of aluminum nitrate solution with ≥0.5 wt.% of the 25 nm α-Al₂O₃ powder markedly accelerates the kinetics of α-alumina crystallization from the gel, the latter being formed in course of addition of ammonia to the solution. Depending on the amount of seeds and the way of their incorporation to the solution the complete transformation occurs from 800 to 930 °C. The resulting powder consists of 50-60 nm crystallites which, in turn, are agglomerated in 10-20 μm species, the size of crystallites being independent of the amount of seeds added. Grinding under mild conditions destroys agglomerates and leads to a nano-powder of which 95-99% dense bodies have been sintered at 1300-1400 °C.     

Effects of diazenedicarboxamide additive on the content of α-Si3N4 synthesized by combustion method

Combustion synthesis (CS) of high content of α-Si₃N₄ powders was carried out using Si and Si₃N₄ powders as reactants with the addition of diazenedicarboxamide (AC) at a relatively low N₂ pressure of 3 MPa. Effects of diazenedicarboxamide contents on the phase compositions and Si₃N₄ particle morphologies were studied. In addition, the reaction mechanisms were discussed. The results indicated that the additive diazenedicarboxamide promoted the nitridation of Si. The α-Si₃N₄ content in the combustion-synthesized products showed great dependence on the additive contents, which reached 85.21 wt% with 24 wt% diazenedicarboxamide added. N₂, CO and NH₃ produced by the decomposition of diazenedicarboxamide leaded to a change of compact porosity and the formation of micro-pores in the reactive area, which was responsible for the increasing contents of α-Si₃N₄ and the discrepancy morphology of the products.     

Indentation fracture resistance test round robin on silicon nitride ceramics

Reproducibility of indentation fracture resistance of three commercial silicon nitrides including bearing balls was evaluated by an international round robin with six laboratories. The between-laboratory standard deviations for indentations at 196 N on the perfectly mirror-finished surfaces were in the range of 0.2–0.5 MPa m^1/2, demonstrating an excellent precession of the test results. The scatter in the fracture resistance increased as the indentation load decreased from 196 to 98 N. The errors in measuring crack lengths deduced from the deviation of each laboratory's readings from author's reading for the same indentations tended to increase with a decrease in the magnification of the lab's microscope, which suggested that finding exact crack tips with lower magnification was difficult especially for those samples with insufficiently mirror-finished surfaces indented at 98 N. Observation of indentations at the load of 196 N with powerful optics was advised to ensure the validity of the indentation technique which is used as the quality assessment of Si₃N₄ bearing balls.     

Nanoscale observation of surface potential and carrier transport in Cu2ZnSn(S,Se)4 thin films grown by sputtering-based two-step process

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu₂ZnSn(S,Se)₄ (CZTSSe). The samples have different absorber layer thickness of 1 to 2 μm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se₂.PACS number: 08.37.-d; 61.72.Mm; 71.35.-y     

Alkoxysilane layers deposited by SC CO2 process on silicon oxide for microelectronics applications

Self-assembled monolayers are studied to replace metallic films used as barrier/adhesion layers between dielectric material and copper used for interconnects in semiconductor devices. In this study, alkoxysilane layers were deposited by SC CO₂ processes on oxidized Si substrates using mainly two silanes: mercapto and aminopropyltrimethoxysilane. In order to obtain various layers, several process conditions were tuned such the process mode, static and dynamic, the temperature and the solvent. X-ray reflectometry was the main technique used to probe the structural and morphological characteristics of the films. All of these results show that the structure of the resulting organic layer depends on the molecule and on the process conditions. Aminoalkyltrimethoxysilanes lead to polycondensed layers formed with a thermally activated reaction while self-assembled monolayers are obtained with mercaptopropyltrimethoxysilane whatever the process condition.     

An algorithm for the generation of silicon nitride structures

Silicon nitride is used for demanding tasks due to its high stiffness, strength and, especially, its high fracture toughness. Examples include cutting tools, forming rolls and ball bearings. The microstructure is characterized by elongated β-Si₃N₄ grains of different size and shape, which lead to the increased fracture toughness. Consequently, the paper will present an algorithm for the generation of three-dimensional and periodic silicon nitride-like microstructures, which will be used for micromechanical finite element simulations. The structure generation algorithm enhances the sequential adsorption technique with growth of particles and steric hindrance, which are motivated by experimental results. Results of the structure generator, such as the pseudo-time evolution and its statistical geometric distributions are presented and compared to literature data. With the finite element simulations, using a periodic unit cell, a validation of the model with literature values for Young's modulus and Poisson's ratio was possible.     

Morphology control of a silicon nitride thick film derived from polysilazane precursor using UV curing and IR heat treatment

Silicon nitride (Si₃N₄) has excellent thermo-mechanical properties, and can be used as heat dissipation substrate for various devices. Si₃N₄ thin films are generally synthesised by chemical vapour deposition (CVD) or plasma-enhanced CVD. The use of polysilazanes (PSZs) as a precursor to the synthesis of Si₃N₄ has attracted significant attention because of their high mouldability and processability. In this study, Si₃N₄ thick films were prepared on silicon wafers or aluminium substrates by a spin- or dip-coating liquid PSZ, followed by UV curing and IR heat treatment under various conditions. The effects of the heat treatment conditions on the Si₃N₄ thick film surface were analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. An almost single phase of Si₃N₄ was synthesised successfully on the single crystalline silicon with UV curing at 400°C for 30 min and IR heating at 800°C in N₂ atmosphere.     

Preparation of ceramic nanospheres by CO2 laser vaporization (LAVA)

In order to prepare ceramic nanoparticles by CO₂ laser vaporization (LAVA) coarse ceramic powders (e.g. ZrO₂, Al₂O₃, and TiO₂) were used as raw materials. The raw powder was vaporized into a flowing process gas under normal pressure. Expanding into the gas, the vapor instantly cools down. Gas-phase condensation leads to the formation of nanoscale particles with a composition that corresponds to that of the raw powder. LAVA nanoparticles are chemically pure, spherical, crystalline, exhibit a narrow size distribution, and form merely soft agglomerates. The co-laser vaporization of mixtures of ceramic raw powders allows for the preparation of nanoparticles of multi-phase (e.g. Fe₂O₃-SiO₂) or single-phase (e.g. CaTiO₃) mixed-oxides and dispersion ceramics (e.g. ZrO₂-Al₂O₃). In order to modify the surface of the nanoparticles they can be coated in-process with an organic additive. Thus, the LAVA method allows for the targeted development of a wide range of ceramic nanopowders with tailored properties.