A comparison of bulk metal nitride catalysts for pyridine hydrodenitrogenation

A comparison of various group IV–VIII bulk metal nitride catalysts identified Co₄N and Fe₃N as having higher pyridine hydrodenitrogenation activity per unit area than Mo₂N. Formation of the metal nitrides was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy, and all the nitrides were prepared from metal oxide precursors using the same temperature-programmed reaction technique. In general, the specific activity of the metal nitrides decreased with increased heat of formation of the metal nitride. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ultrasonic-assisted preparation of AlON from alumina/carbon core-shell nanoparticle

An aluminium oxynitride (AlON) powder was synthesized by carbothermal reduction nitridation (CRN) method. For this purpose, first Al₂O₃/C core-shell nanoparticles were prepared by the pyrolysis of Al₂O₃/polyacrylonitrile (PAN) nanocomposite precursor at 800?°C for 2?h in an argon atmosphere. Alumina/PAN precursor was prepared by ultrasonic method at room temperature. Then, by two-step thermal treatment of Al₂O₃/C core-shell nanoparticles at 1500–1600?°C for 2?h, followed by subsequent heating at 1750?°C for 1?h in N₂ flow, AlON powder was synthesized. The sample was investigated via Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and CHNS elemental analysis.     

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Synthesis of carbon nitrides with graphite-like or onion-like lamellar structures via a solvent-free route at low temperatures

Carbon nitrides with graphite-like or onion-like lamellar structures were synthesized at low temperatures by the reactions of cyanuric chloride (C₃N₃Cl₃) with NaNH₂, K, or NaN₃. The synthesized samples were investigated by powder X-ray diffraction, elemental analysis (from C-N combustion), X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, mass spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and high-resolution electron microscopy. The synthesized carbon nitride with a graphite-like lamellar structure was obtained and observed for the first time. The formation mechanism of the carbon nitride was discussed.     

Preparation of GdBa2Cu3O6+x (GdBCO) precursor powder by spray-drying a nitrate solution containing PEG

Several precursor powders, obtained after precipitation from metal nitrate solution containing polyethylene glycol (PEG) (inside a Pyrex glass reactor or by spray-drying), and their thermal evolution to GdBa₂Cu₃O_6+x (GdBCO) were analyzed by thermogravimetry, differential thermal analysis, Fourier-transform infrared spectroscopy, and X-ray diffraction. The amount of PEG had a crucial role in the BaCO₃ content of the “Kjeldahl precursors,” but a minor effect on the degree of transformation to GdBCO at 900°C, which did not reach completion after 1 hour. In contrast, a low-PEG spray-dried powder led to almost 100% GdBCO in only 5 minutes. The high degree of cation dispersion reached by spray-drying and the coexistence with a liquid phase can explain this short reaction time. The spray-dried powder compares favorably with the mechanical mix of metal oxides and Ba carbonate that is commonly used as precursor powder for the synthesis following a solid-state reaction.     

CCl4-Assisted Preparation of Highly Dispersed Molybdenum and Tungsten Nitrides

Nanoscopic transition metal nitrides were prepared by means of a newly developed technique, using nanoparticles of scheelites (AMO₄, A?=?Ca, Sr, Ba; M?=?Mo, W) as precursors. After treatment with CCl₄?CNH₃ gas mixtures the precursor salts were transformed to mixtures of alkali earth chlorides (ACl₂) and transition metal nitrides (M₂N). The thermodynamic driving force of the process is the high free enthalpy of the simultaneous formation of carbon dioxide and an electropositive chloride, from the interaction between CCl₄ and AMO₄. As a result, the gas?Csolid reaction occurs at relatively low temperature (500?C550?°C). After water extraction of soluble chlorides, finely dispersed solids Mo₂N _x C _y and W₂N _x C _y (x?=?0.79?C0.95, y?=?0.03?C0.15) were obtained, which demonstrated high specific surface areas and well developed mesoporosity. Their catalytic activity was compared in the thiophene hydrodesulfurization reaction, where the active phase is provided by MS₂ sulfides formed in situ under the reaction conditions, as confirmed by temperature-programmed reduction. Due to their open porosity, the solids as obtained demonstrated high activity and increased stability as compared to the same nitrides prepared by means of other techniques. The catalytic activity of the newly prepared nitrides was evaluated in the guaiacol hydrodeoxygenation (HDO) reaction. Molybdenum nitride showed promising HDO performance and unusual selectivity, whereas the tungsten homolog demonstrated poor activity.     

Manufacturing of ceramic matrix composite using a hybrid process combining TiSi2 active filler infiltration and preceramic impregnation and pyrolysis

The manufacturing of silicon carbide reinforced ceramic matrix composites by a hybrid process is explored. Fibre preforms are infiltrated with TiSi₂ powders using the slurry method. Using TiSi₂ active filler leads to reduce the porosity by the subsequent formation of nitride phases after treatment under N₂ atmosphere at low temperatures (≤1100 °C). Taking into account the influence of the specific surface area of the powder on the nitridation rate, it is shown that it is possible to produce nitrides TiN and Si₃N₄ at 1100 °C with an interesting volume expansion inside the composite. To complete the densification of the composite, a polymer impregnation and pyrolysis (PIP) process are performed with a liquid polymeric precursor. Characterizations of the composites show that mechanical properties are improved with the presence of the TiN and Si₃N₄ phases, and the number of PIP cycles.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

The Denitridation of Nitrides of Iron, Cobalt and Rhenium Under Hydrogen

The denitridation behaviour of binary iron, cobalt and rehnium nitrides under H₂ /Ar has been investigated. The iron nitride was found to lose over 70 % of its as prepared nitrogen content at 400 °C. The cobalt nitride was completely denitrided at 250 °C. Rhenium nitride lost close to 90 % of its nitrogen at 350 °C. In addition, Co-Re₄ prepared by ammonolyis was investigated, whilst only traces of NH₃ were lost from this material under H₂/Ar at 400 °C, with H₂/N₂ it proved to be an active ambient pressure ammonia synthesis catalyst in accordance with previous literature.     

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.     

Formation of HxN-rich graphitic carbon nitride network from guanidine carbonate salt by pyrolysis

Hydrogenated nitrogen-rich graphitic carbon nitride material was successfully synthesized by pyrolysis at 550 °C of an unusual organic precursor molecule, namely guanidine carbonate. The product was characterized in detail using X-ray diffraction, chemical analysis, diverse spectroscopy techniques (Fourier transform infrared, X-ray photoelectron, ultra-violet visible absorption), nitrogen adsorption, and scanning electron microscopy. The results of characterization clearly confirmed the synthesis of a graphitic carbon nitride material (stacking of tri-s-triazine planes) composed of C_3.00N_4.29H_1.59O_0.77. In order to understand the mechanism of material formation, products obtained at different temperatures between 200 and 500 °C were systematically investigated. The guanidine carbonate salt precursor is presented to be a promising alternative source for the elaboration of the g-C₃N₄ material in relation to the classically used cyanamide, melamine or cyanuric-based molecules. Since it is more environmentally friendly and highly soluble in aqueous solvents this new method is definitely strengthening the possibility of process industrialization.     

Preparation of silicon carbide-silicon nitride fibers by the pyrolysis of polycarbosilazane precursors: A review

The development of silicon carbide-silicon nitride (SiC-Si₃N₄) fibers by the pyrolysis of polycarbosilazane precursors that was carried out in this laboratory is reviewed. Precursor resin, which was prepared by heating tris(N-methylamino)methylsilane or tris(N-methylamino)phenyl-silane to about 520°C, was drawn into fibers from the melt and then made unmeltable by humidity conditioning at 100°C and 95 percent relative humidity. The humidity-treated precursor fibers were pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity. For example, SiC? Si₃N₄ fibers derived from tris(N-methyl-amino)-methylsilane had a tensile rupture modulus of 29 × 10⁶ psi and electrical resistivity of 6.9 × 10⁸ Ω-cm, which is 10¹² times greater than a value obtained for graphite fibers.     

Microstructural evolution of Si(HfxTa1−x)(C)N polymer-derived ceramics upon high-temperature anneal

Ultra-high temperature ceramic nanocomposites (UHTC-NC) within the Si(Hf_xTa_1?x)(C)N system were synthesized via the polymer-derived ceramics (PDC) synthesis route. The microstructure evolution of the materials was investigated upon pyrolysis and subsequent heat treatment. The crystallization behavior and phase composition were studied utilizing X-ray diffraction, scanning- and transmission electron microscopy. Single-source-precursors were converted into amorphous single-phase ceramics, with the exception of surface crystallization effects, at 1000 °C in NH₃. Annealing in N₂ at 1600 °C resulted in fully crystalline UHTCs. The powder samples revealed microstructures consisting of two characteristic regions, bulk and surface; displaying intrinsic microstructure and phase composition differences. Instead of the expected nitrides, transition metal carbides (TMC) were detected upon high-temperature anneal. The residual carbon available in the system triggered a decomposition reaction, resulting in the formation of TMCs plus gaseous nitrogen and SiC. Experimental data underline that N-containing PDCs are prone to phase separation accompanied by thermal decomposition and diffusion-controlled coarsening.     

Effect of the residual phases in β-Si3N4 seed on the mechanical properties of self-reinforced Si3N4 ceramics

In this work, the self-reinforced silicon nitride ceramics with crystal seed of β-Si₃N₄ particles were investigated. Firstly, the seeds were prepared by heating of α-Si₃N₄ powder with Yb₂O₃ and MgO, respectively. Then the self-reinforced silicon nitride ceramics were obtained by HP-sintering of α-Si₃N₄ powder, Yb₂O₃ and the as-prepared seeds which were not treated with acid and/or alkali solution. The results indicated that the introduction of seed with Yb₂O₃ could obviously increase the toughness and room temperature strength of the ceramics. Furthermore, its high temperature strength (1200 °C) could nearly keep higher value as the one of room temperature measured from unreinforced ceramic. However, the seed with MgO abruptly decrease the high temperature strength of the ceramics. The SEM and TEM characterization showed that the rod-like seed particle could favor the toughness and the presence of the Mg promote the formation of crystalline secondary phase.     

CRN synthesis of β-SiAlON from a nanocomposite precursor of mesoporous silica–alumina (Al-SBA-15) with poly 4-vinyl pyridine

This paper reports the use of a nanocomposite precursor of mesoporous silica?Calumina (Al-SBA-15) with poly 4-vinyl pyridine (P4VP) to synthesise ??-SiAlON by carbothermal reduction and nitridation under N₂ at 1,450?°C. Small-angle XRD, SEM/TEM, ²⁹Si and ²⁷Al MAS-NMR and BET specific surface area measurements of the Al-SBA-15 and P4VP/Al-SBA-15 composite confirmed the mesoporous structure and the presence of Al in the precursors, while surface polymerization of the Al-SBA-15 was indicated by ²⁹Si-NMR. Firing the nanocomposite in a N₂ atmosphere forms the nitride phase, depending on the amount and type of carbon present, on the surface area of the precursor. Even with these precursors of high surface area, the present result was obtained using approximately twice the stochiometric amount of carbon.     

Green emitting β$\ubeta$-SiAlON:Eu2+ phosphors derived from chemically modified perhydropolysilazanes

We report the first synthesis of β-SiAlON:Eu²⁺ phosphors from single-source precursors, perhydropolysilazane (PHPS), chemically modified with Al(OCH(CH₃)₂)₃, and EuCl₂. The reactions occurring during the precursor synthesis and the subsequent thermal conversion of polymeric precursors into β-SiAlON:Eu²⁺ phosphors have been studied by a complementary set of analytical techniques, including infrared spectroscopy, gas chromatography–mass spectrometry, thermogravimetry–mass spectrometry, X-ray diffraction (XRD), photoluminescence spectroscopy, and scanning electron microscopy. It has been clearly established that Al(OCH(CH₃)₂)₃ immediately reacted with PHPS to afford N–Al bonds at room temperature, whereas N–Eu bond formation was suggested to proceed above 600°C accompanied by the elimination of HCl up to 1000°C in flowing N₂. The subsequent 1800°C-heat treatment for 1 h under an N₂ gas pressure at 980 kPa allowed converting the single-source precursors into fine-grained β-SiAlON:Eu²⁺ phosphors. XRD analysis revealed that the Al/Si of .09 was the critical atomic ratio in the precursor synthesis to afford single-phase β-SiAlON (z = .55). Moreover, Eu²⁺-doping was found to efficiently reduce the carbon impurity in the host β-SiAlON. The polymer-derived β-SiAlON:Eu²⁺ phosphors exhibited green emission under excitation at 460 nm and achieved the highest green emission intensity at the critical dopant Eu²⁺ concentration at 1.48 at%.     

Synthesis of molybdenum nitrides nanosheets by nitriding 2H‐MoS2 with ammonia

Metal nitrides nanosheets possess remarkable physical and chemical properties such as high electrical conductivities, catalytic properties, energy storage, and conversion efficiency. In this paper, molybdenum nitride (Mo₅N₆, MoN, and Mo₂N) nanosheets were synthesized by nitriding and exfoliating the bulk 2H‐MoS₂ via dropping N from ammonia at high temperature. Molybdenum nitride nanosheets with the thickness of dozens of nanometers were prepared successfully under different conditions. It was found that the reaction between MoS₂ and NH₃ began from about 696°C, and reduction products and reaction mechanisms were strongly dependent on the temperature. When there was MoS₂, the generated Mo₅N₆, MoN, and Mo₂N can exist stably at even 820, 1020, and 1120°C, respectively. However, they will decompose progressively after MoS₂ was consumed completely: at 820°C, Mo₅N₆ started to decompose to δ‐MoN; at 1020°C, the phase evolution process of MoN can be described as follows: δ‐MoN→ γ‐Mo₂N→ β‐Mo₂N→ Mo, while at 1120°C, the β‐Mo₂N will transform to Mo.     

Synthesis of nanocrystalline boron nitride by combustion process

Nanocrystalline boron nitride powders were synthesized by combustion process using urea as a fuel. Experiments were carried out by heating boric acid and urea in an N₂ atmosphere at 850°C. Boric acid was used as a source of boron while urea, as a source of nitrogen. The reactions were carried out in an autoclave with provisions for purging with nitrogen gas. The samples were characterized by powder X-ray diffraction, Fourier transform IR spectroscopy (FT-IR), FT-Raman spectroscopy, UV-VIS spectroscopy, and SEM. The article is published in the original.     

Joining of Silicon Nitride by Local Heating for Fabrication of Long Ceramic Pipes

This study demonstrated that a long silicon nitride pipe of several meters with adequately strong joints can be fabricated by a local‐heating joining technique. Commercially available silicon nitride ceramic pipes sintered with Y₂O₃ and Al₂O₃ additives were used for parent material, and powder slurry of Si₃N₄‐Y₂O₃‐Al₂O₃‐SiO₂ system was brush‐coated on the rough or uneven end faces of the pipes. Joining was carried out by locally heating the joint region at different temperatures from 1500°C to 1650°C for 1 h with a mechanical pressure of 5 MPa in N₂ flow; using a horizontal electrical furnace specially designed for this experiment. The silicon nitride pipe 3‐m long was successfully fabricated without voids or cracks in the joint region, and the microstructure of the joint region was similar to that of the parent one. The joint strength was examined in flexure using specimens cut from the joined pipes, and those joined at 1600°C and 1650°C indicated the highest strength of about 680 MPa, which was almost the same as that of the parent material. This study also indicated that the slurry brush‐coating technique is advantageous to easily joining ceramic pipes with rough or uneven end faces, which is essentially important for practical use.     

A 3D oxalate-bridged [CuIIFeII] coordination polymer as molecular precursor for CuFe2O4 spinel—photocatalytic features

A 3D heterometallic oxalate-bridged coordination polymer [Cu^IIFe^II₂(H₂O)(terpy)(C₂O₄)₃]_n (terpy = 2,2′:6′,2″-terpyridine) ( 1 ) was investigated both as photocatalyst for the organic dye removal and as a single-source precursor for the preparation of the copper ferrite (CuFe₂O₄) nanocrystals by thermal processing. The dual functionality of 1 was supported by the degradation of aqueous solutions of rhodamine B (RhB) and methylene blue (MB) solutions under visible (Vis) and ultraviolet (UV) light irradiation, powder X-ray diffraction data collection at room temperature, and the optical and scanning electron microscopy analyses. A close inspection of the X-ray diffraction patterns unveiled qualitative and quantitative information on the phase composition obtained after the single-source molecular precursor route to spinel oxide. By optimizing the temperature levels and setting the controlled heating rate at 6 h of holding time, the phase composition of thermal processing of 1 was evaluated—thermal treatment of 1 at 950°C for 6 h and a heating/cooling rate of 10°C min⁻¹ resulted in the formation of solely tetragonal spinel phase of CuFe₂O₄, whereas the formation of both tetragonal and cubic CuFe₂O₄ phases was observed at 950°C by the heating rate of 30°C min⁻¹. To obtain the high-temperature cubic CuFe₂O₄ oxide, compound 1 was heated and then quenched at 925°C, which led to the formation of the cubic spinel ferrite as the main crystalline oxide phase. Moreover, the photocatalytic properties of the t-CuFe₂O₄ spinel were investigated under the same conditions as for 1 . The optical bandgap energies were estimated from UV–Vis absorption spectra for both metal oxide and precursor powder.