Effects of Nitridation on Properties of SiC Fiber and Interface of Ti Matrix Composite

Prenitridation of the TiBx coating surface of the Sigma SM1240 SiC fiber can from more stable compounds at the surface and obstruct the release of boron atoms into the Ti-based alloy matrix.The effect of nitridation on the tensile strength of the fiber was investigated in this work.Nitridation could degrade the tensile strength of the SiC if the treating temperature and time are not optimized.The chemical reaction between the W core and SiC and the modification of fiber microstructure during the nitridation are respornsible for the degradation in strength.The strength can be maintained by further optimization of the treating temperature and time.Therefore stabilizing the surface of TiBx coating and hence the interface of the SiCf/Ti composite by the nitridation of the SiC fiber is a feasible technique for practical application.     

Low temperature matrix synthesis by activated nitridation of a TiSi2 powder

This paper describes a new method to prepare a matrix for Ceramic Matrix Composites (CMC) at low temperatures from a solid/gas chemical reaction. Contrary to previous works, a TiSi₂ powder is fully nitrided at 1100 °C leading to the formation of Si₃N₄ and TiN phases while avoiding the presence of free silicon. This new result can be obtained by the addition of a chemical element promoting a full chemical conversion. In the present case, thermochemical computations led to select nickel (Ni) as this chemical element.     

Synthesis and characterization of mesostructured tungsten nitride by using tungstic acid as the precursor

Mesostructured tungsten nitride was firstly prepared from tungstic acid via the temperature programmed reaction with ammonia. The N₂ adsorption isotherm of as-synthesized tungsten nitride was of type IV with a type H-3 hysteresis loop. BJH pore size distribution was of bimodal distribution (2.5 and 3.5 nm), among which the latter was the main channel of tungsten nitride. The surface area of as-synthesized tungsten nitride was up to 89 m² g⁻¹. XRD pattern showed that the crystal phase of the product was β-W₂N. The effect of synthesis parameters on the surface area of tungsten nitride was investigated extensively. The nitridation mechanism was investigated by in-situ XRD and N₂ adsorption analysis. It was found that H₂WO₄ was initially transformed into WO₃ by eliminating the axial water molecules, and WO₃ retained the layered and porous structure of H₂WO₄. Below 773 K, WO₃ was just partially reduced to W₂₀O₅₈ and W₂₀O₄₀. Above 773 K, β-W₂N phase could be detected. It indicated that during nitridation, WO₃ was gradually reduced and then the homogeneous substitution of oxygen vacancies in the reduced oxides with nitrogen atoms occurred. Based on the experimental results, a reduction-nitridation mechanism was firstly proposed.     

α″-Fe16N2 phase formation of plasma-synthesized core–shell type α-Fe nanoparticles under various conditions

Four kinds of plasma-synthesized core–shell type α-Fe nanoparticles with various particle diameters, input composition of shell’s raw materials, and shell compounds were used to investigate dependence of these nanoparticle parameters on nitridation and magnetic performance. Effects of hydrogen-gas reduction conditions (i.e., temperature and reduction time) prior to nitridation treatment were also investigated in detail. Experimental result showed that the nanoparticle parameters and the hydrogen reduction treatment influenced yield of α″-Fe₁₆N₂. Increases in particle diameter and shell amount resulted in the more difficulties in nitridation reaction because of the limitation in nitrogen diffusion phenomena. Changes in shell compound from Al₂O₃ to SiO₂ resulted in the more difficulties in α″-Fe₁₆N₂ phase formation. We also found that modification of reduction conditions affect the final product quality. We obtained that by optimization the nanoparticle parameters and the reduction process, the formation of nanoparticles with high yield of α″-Fe₁₆N₂ (up to 99%) can be achieved. Finally, we found that for 43-nm core–shell Fe/Al₂O₃ magnetic nanoparticles containing 10 wt% of Al₂O₃, the combination of 1.5-h reduction at 300 °C and 10-h nitridation at 145 °C gave the highest yield of α″-Fe₁₆N₂. The best saturation magnetization of 190 emu/g was achieved when using the amount of Al₂O₃ of 20 wt%.     

Influence of electronic and optical properties of GaN nanoparticles as potential electrocatalyst in hydrogen production

Due the high electron density of the anion lattice found in nitrides, these compounds have been used for such applications as high power devices, photonics and, more recently, hydrogen production. Among nitride compounds, GaN is well known for its thermal stability, while the different physical techniques used for its synthesis have been reported to obtain different physicochemical properties. However, these techniques are not designed for the mass production of GaN. For these reasons, two different methods for both the chemical synthesis of GaN and its physical chemical characterization are reported in this study, with both synthesis methods presenting wurtzite phase crystallization. Furthermore, both samples presented agglomerates formed by similar sized nanoparticles (~20 nm). Despite the similarity in the particle size, higher pore volume and surface area as well as carbon traces are promoted by the solvothermal route. In contrast, structural and photoluminescence (PL) characterization revealed that Ga vacancies and interstitial N formed in those samples synthesized by the nitridation method, promoting higher intensity in ~2.8 eV PL band. Finally, based on cathodic linear sweep voltammetry, the samples prepared in this study can be considered good candidates for use in hydrogen production.     

Internal Oxidation–Nitridation of Ferritic Fe(Al) Alloys in Air

Exposure of undoped Fe(Al) and Fe(Al)+Cr ferritic alloys in laboratory air at 900–1,000 °C resulted in significant internal attack after 5,000 h, including oxides and underlying nitrides. In the most severely attacked alloys, kinetics based on mass gain and maximum penetration depth were linear; also, the deepest penetrations were a significant fraction of the specimen thickness, and were thickness-dependent. Little internal attack was observed at 700–800 °C where these compositions may be used as coatings. The extent of internal attack did not decrease with increasing Al or Cr content which may indicate that rather than classical internal oxidation this attack is related to the permeation of N through a defective external scale. No internal attack was observed in alloys doped with Y, Zr, Hf or Ti where the substrate-alumina scale interface was flatter.     

Processing,characterization and properties of novel gradient Si3N4/SiC fibers derived from polycarbosilanes

In this work, we report a novel kind of Si₃N₄/SiC composite fibers, which exhibit a controlled gradient Si₃N₄(shell)/SiC(core) structure. These composite fibers are fabricated through a controlled nitridation and pyrolysis process on electron irradiation-cured polycarbosilane fibers. Structural and chemical analysis based on Elemental Analyzer, FT-IR, Raman spectroscopy, electron probe micro-analyzer, X-ray photoelectron spectroscopy, and X-ray diffraction confirm the gradient structure of obtained fibers, which consist a shell with high Si₃N₄ content and a SiC core. The as-fabricated fibers exhibit dense and smooth surfaces, and no microscopic holes or defects were observed. The effects of nitridation temperature on mechanical properties and electrical resistivity were also investigated. Combined with high mechanical properties and lightweight, the present gradient Si₃N₄/SiC fibers open a new strategy to fabricate multifunctional and electromagnetic wave absorbing materials.     

Effects of nitrogen incorporation by plasma immersion ion implantation on electrical characteristics of high-k gated MOS devices

Nitridation treatments are generally used to enhance the thermal stability and reliability of high-k dielectric. It is observed in this work that, the electrical characteristics of high-k gated MOS devices can be significantly improved by a nitridation treatment using plasma immersion ion implantation (PIII). Equivalent oxide thickness, (EOT) and interface trap density of MOS devices are reduced by a proper PIII treatment. At an identical EOT, the leakage current of devices with PIII nitridation can be reduced by about three orders of magnitude. The optimal process conditions for PIII treatment include nitrogen incorporation through metal gate, ion energy of 2.5 keV, and implantation time of 15 min.     

Effects of low-temperature nitridation on the electrical conductivity and corrosion resistance of 446M stainless steel as bipolar plates for proton exchange membrane fuel cell

Low-temperature nitridation was used to form a protective and conductive layer on stainless steel. The surface characterization reveals that a continuous and protective Cr-nitride/oxide layer (CrN and Cr₂O₃) forms on the 446M stainless steel surface after low-temperature nitridation. The electrical conductivity of the sample is investigated in terms of the interfacial contact resistance. This value for nitrided 446M at low temperature is 6 mΩ cm², which is much lower than that of the bare 446M stainless steel (about 77 mΩ cm²) at a compaction force of 140 N/cm². The corrosion resistance of low-temperature nitrided 446M stainless steel is examined in potentiodynamic and potentiostatic tests under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions with pH 3 H₂SO₄ at 80 °C. In a simulated anode condition, the current density is −1 × 10⁻⁶ A/cm². In a simulated cathode condition, the current density is 1 × 10⁻⁷ A/cm². Low-temperature nitrided 446M stainless steel shows superior electrical conductivity and corrosion resistance than bare 446M stainless steel.     

Nitridation of MgO-loaded MCM-41 and its beneficial applications in base-catalyzed reactions

Nitrogen-containing MgO-MCM-41 solid base material is prepared by nitridation of MgO-loaded mesoporous MCM-41. The ordered mesoporous structure and high surface area of MCM-41 are well preserved after MgO impregnation and nitridation, as proved by the XRD, TEM and nitrogen adsorption/desorption analysis. FTIR spectra the bridging -NH- groups and terminal -NH₂ groups are incorporated into the framework of MgO-MCM-41 by nitridation, and the base strength is expected to be enhanced due to the replacement of oxygen by nitrogen with lower electronegativity. FTIR spectra with the adsorption of different probe molecules, e.g. CO, CD₃CN and ¹³CO₂, are employed for the characterization of surface acidic-basic properties of MgO-MCM-41 before and after nitridation. It is revealed that the acidic hydroxyls in MgO-MCM-41 are greatly reduced through nitridation process. Compared with MgO-MCM-41, the nitridized MgO-MCM-41 material exhibits improved basic catalytic performances in Knoevenagel condensation reaction, Claisen-Schmidt reaction and dehydrogenation reaction of 2-propanol.