Effect of NiS addition on nitridation of alumina to aluminum nitride using polymeric carbon nitride as a nitridation reagent

We investigated the effect of adding nickel(II) sulfide (NiS) on nitridation of alumina (Al₂O₃) to aluminum nitride (AlN) using polymeric carbon nitride (PCN), which was synthesized by polymerization of dicyandiamide at 500 °C. The product powders obtained from nitridation of a mixture of δ-Al₂O₃ and NiS powders (mole ratio of 1:0.01) at various reaction temperatures were characterized by powder X-ray diffraction, ²⁷Al magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. δ-Al₂O₃ began to convert to AlN at 900 °C and completely converted to AlN at 1300 °C. The as-synthesized sample powders contained nitrogen-doped carbon microtubes (N-doped CMTs) with a length of several tens of mm and thickness of ca. 3 µm. The addition of NiS to δ-Al₂O₃ resulted in the enhancement of the amount of N-doped CMTs and nitridation rate, which might be due to the catalytic action of Ni particles on the thermal decomposition of vaporized PCN. The change in Raman spectra with reaction temperatures indicated that the crystallinity of N-doped CMTs was increased by calcining at higher reaction temperatures.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al₂O₃). The effects of Al₂O₃ particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al₂O₃ powder than with coarse Al₂O₃ powder. The cubic AlN may be the product of Al₂₃O₂₇N₅ synthesized from Al₂O₃ and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.     

Kinetics of microwave synthesis of AlN by carbothermal‐reduction‐nitridation at low temperature

Aluminum nitride (AlN) was synthesized at 1000‐1400°C from a mixture of alumina and carbon powders using 2.45 GHz microwaves in a N₂ atmosphere. High nitridation ratios (>0.90) were obtained in the temperature range 1200‐1400°C. The apparent activation energy of the carbothermal reduction and nitridation (CRN) reaction using 2.45 GHz microwave irradiation was calculated from the nitridation ratio. The value obtained, 79.9 kJ/mol, is 11% of the energy reported for conventional synthesis using α‐Al₂O₃ as the raw material. This result indicates that 2.45 GHz microwave irradiation could promote the kinetics of the CRN reaction, and AlN could be effectively synthesized at low temperature.     

Is it possible to synthesize cubic aluminum nitride by the carbothermal reduction and nitridation method?

In order to investigate whether it is possible to synthesize cubic AlN (c-AlN) by the carbothermal reduction and nitridation method, the products obtained by calcining a (hydroxo)(suberato)Al(III) complex under a flow of nitrogen in the temperature range of 1200–1600 °C were characterized by XRD and ²⁷Al magic-angle spinning (MAS) NMR spectroscopy. The products consisted of wurtzite AlN (w-AlN), γ-Al₂O₃, α-Al₂O₃ and γ-aluminum oxynitride (γ-AlON). The two materials γ-AlON and c-AlN, which have very similar XRD patterns each other, were differentiated by their ²⁷Al MAS NMR spectra. The ²⁷Al MAS NMR spectra of the products showed no peak at the chemical shift of c-AlN, which was estimated by the correlation between the ²⁷Al chemical shifts of AlX (X = P, As and Sb) in the cubic phase with the reciprocal of their band gaps. These results indicate that it is impossible to synthesize metastable c-AlN by the CRN method because of very high reaction temperature.     

Hydrothermal‐carbothermal synthesis of highly sinterable AlN nanopowders

Aluminum nitride (AlN) nanopowders have been synthesized by a novel method which combines the hydrothermal and carbothermal techniques. The phase of the nitridation products can be controlled through adjusting the carbon content of precursor by adding different amount of urea (U). The nitridation product synthesized from the precursor (U/Al=2) was comprised of spherical and well‐distributed particles with sizes ranging from 50 to 100 nm. The nitridation procedure was investigated in detail with TG/DSC, XRD, and TEM measurements. It was found that the core‐matrix structured Al₂O₃/C mixture presents after pyrolysis, favoring the nitridation completed at low temperature, and brings in temperature independency for AlN grain size. Finally, the additive‐free AlN ceramic with 99.08% relative density has been achieved by pressureless sintering at 1850°C.     

Novel nanoceramics from in situ made nanocrystalline powders of pure nitrides and their composites in the system aluminum nitride AlN/gallium nitride GaN/aluminum gallium nitride Al0.5Ga0.5N

Presented are investigations of in situ preparation of composite nanopowders of AlN, GaN, and, optionally, solid solution Al_0.5Ga_0.5N, which were used in no-additive, high-pressure/high-temperature sintering. Two precursor synthesis pathways and two nitridation temperatures afforded nanopowders containing both (i) mixed AlN and GaN and (ii) such AlN and GaN admixed with Al_0.5Ga_0.5N as well as (iii) reference individual AlN and GaN. The applied sintering temperatures were either to preserve powder nanocrystallinity (650 °C) or promote crystallite growth and sintering-mediated Al_0.5Ga_0.5N formation (1000 °C). One specific route led to the novel nanoceramics of Al_0.5Ga_0.5N. The powders and nanoceramics were characterized by XRD, FT-IR, SEM/EDX, ²⁷Al/⁷¹Ga MAS NMR, BET/BJH surface areas, and helium densities. Vicker’s hardness tests confirmed many of the sintered composites and individual nitrides having high hardness comparable with monocrystalline AlN and GaN. Formation of pure Al_0.5Ga_0.5N nanoceramics was associated with closed pore evolution and had a detrimental effect on hardness.     

Effect of substituting CaO with BaO on the viscosity and structure of CaO‐BaO‐SiO2‐MgO‐Al2O3 slags

In this study, the effect of CaO and BaO substitution on the viscosity and structure of CaO‐BaO‐SiO₂‐MgO‐Al₂O₃ slags was investigated. The results showed that the viscosity increased with an increase in the BaO substitution concentration, which was correlated to an increase in the degree of polymerization (DOP) of the slag structural units as the activation energy increased from 207.9 to 263.8 kJ/mol for viscous flow. Deconvolution and area integration of the Raman spectrum of the slag revealed that the ratio of Q³/Q² (Qⁱ, i is the number of O⁰ in a [SiO₄]‐tetrahedral unit) increased and NBO/Si (nonbridging oxygen per unit silicon atom) decreased with higher BaO content. It was also observed from the ²⁷Al magic angles pinning nuclear magnetic resonance (²⁷Al MAS‐NMR) spectrum that the relative proportion of Al^IV increased, while that of Al^V decreased because of the decrease in the percentage of nonbridging oxygen (O^?), indicating the polymerization of the slag. O_1s X‐ray photoelectron spectroscopy (XPS) was also carried out to semi‐quantitatively analyze the various types of oxygen anions present in the slag. The XPS results correlated well with the results obtained from the analysis of the Raman and ²⁷Al MAS‐NMR spectra of the slags and its viscous behavior.     

Preparation of Nitrogen‐Doped Mullite Fiber by Nitridation of Silica–Alumina Gel Fiber in Ammonia

Nitrogen‐doped mullite fibers were first synthesized through the nitridation of Al₂O₃–SiO₂ gel fibers in NH₃. The results showed that nitrogen take‐up began at 800°C, reached the maximum at 900°C, and then decreased with increasing temperature. The ceramic fibers nitridated at 900°C were essentially amorphous, but contained a small amount of nano‐sized Al–Si spinel crystals. Mullite was formed after nitridation at 1200°C, accompanied by crystallization of χ‐SiAlON and δ‐Al₂O₃. The incorporation of nitrogen resulted in the formation of a variety of nitrogen‐containing crystalline phases. The grain size of the mullite fibers can be adjusted by changing of the nitrogen content.     

AC Impedance Spectroscopy of CaF2‐doped AlN Ceramics

The electrical conductivity of CaF₂‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF₂‐doped AlN ceramics were sintered with a second additive, Al₂O₃, added to control the electrical conductivity. The effects of calcium fluoride (CaF₂) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF₂ additive have no secondary phases at grain boundaries. Addition of Al₂O₃ caused the formation of amorphous phases at grain boundaries. Addition of Al₂O₃ to CaF₂‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al₂O_3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.     

High Fraction of Penta‐Coordinated Aluminum and Gallium in Lanthanum–Aluminum–Gallium Borates

This study is focused on structural changes induced by increasing treatment temperature of sol‐gel–derived La₂O₃?Al₂O₃?Ga₂O₃?5B₂O₃ system. The structure of samples heated for 30 min up to 900°C was investigated by X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) analysis of ²⁷Al, ¹¹B, and ⁷¹Ga nuclei. The vitreous structure is preserved inclusively after 800°C treatment, and starting with 850°C the only crystalline phase evidenced in XRD patterns is of LaAl_2.03B₄O_10.54 type, of La(Al,Ga)_2.03B₄O_10.54 composition. The FTIR results point out the presence of BO₃, AlO₄, and AlO₆, and starting with 800°C treatment also of BO₄ and AlO₅ structural units, but more detailed information related to boron, aluminum, and gallium environments is obtained from the analysis of MAS‐NMR data. These data evidenced in both amorphous xerogels and in crystallized samples a high fraction of penta‐coordinated aluminum and gallium.     

Gas‐Phase and Solid‐State Simultaneous Mechanism for Two‐Step Carbothermal AlON Formation

AlON powders were synthesized by two‐step carbothermal reduction nitridation method, which includes thermal treatment of Al₂O₃/C mixtures at 1200°C–1600°C for 2 h, followed by subsequent heating at 1750°C for 1.5 h in N₂ flow. The effects of soaking temperatures of the first step on phase compositions and morphologies of the final products were investigated. It is found that the variation in precalcination does not have impact on phase compositions of the final products, which are all single‐phase AlON. However, it impacts the AlON morphology significantly. Lower precalcining temperature results in severer agglomeration of AlON powder. Obvious terrace surface morphology was also observed on AlON particles with lower precalcination. Both the agglomeration and terrace‐like morphology are attributed to the gas‐phase reaction induced by the residual carbon in the AlON formation process. An AlON formation mechanism including simultaneous solid‐state reaction between Al₂O₃ and AlN, and gas‐phase reaction among Al (g), O₂ (g), and N₂ (g) with the presence of residual carbon is proposed based on the experiment, kinetics, and thermodynamics. The mechanism was further examined by carefully designed control experiments, which was confirmed to be both experimentally and theoretically valid.     

Effect of nitriding atmosphere on the morphology of AlN nanofibers from solution blow spinning

High purity AlN fiber is a promising thermal conductive material. In this work, AlN fibers were prepared using solution blow spinning followed by nitridation under N₂ or NH₃ atmosphere. Soluble polymer, such as polyaluminoxane, and allyl-functional novolac resin were adopted as raw materials to form homogeneous distribution of Al₂O₃ and C nanoparticles within the fibers, which could inhibit the growth of alumina crystal and promote their nitridation process. The effect of nitriding atmosphere on the fiber morphology was investigated. XRD results showed that complete nitridation was achieved at 1300 °C in the NH₃ or at 1500 °C in the N₂ atmosphere. Hollowed fiber structure was observed when fiber was nitrided in N₂ at high temperature, which was caused by gaseous Al gas diffusion, and this phenomenon was eliminated in NH₃ atmosphere. The nitridation mechanisms in different atmosphere were analyzed in detail. It was demonstrated that the nitridation of Al₂O₃ fibers in the NH₃ atmosphere offered the favored AlN morphology and chemical quality. Flexible AlN fiber with O content of 0.7 wt% was achieved after nitriding in NH₃ at 1400 °C. The high quality AlN can be used in thermal conductive composite materials.     

Characterization and Catalytic Activities of Al2O3-Promoted Sulfated Tin Oxides

A series of Al₂O₃-doped (0.5–3 mol%) sulfated tin oxides have been prepared by a co-precipitation method, followed by sulfation and calcination. Textural and structural characterizations of these samples were performed by means of XRD, N₂ adsorption, XPS, DTG, Raman spectroscopy, diffuse reflectance UV-vis spectroscopy and ²⁷Al MAS NMR. FT-IR spectra of adsorbed pyridine were used to determine the acid properties. The addition of small amounts of Al₂O₃ (0.2–1.5 mol%) to sulfated tin oxide brings about a dramatic improvement of catalytic activity from 36.1 to 52.4–58.0% for acylation of 2-methoxynaphthalene with acetic anhydride.     

Ethanol‐Water‐Derived Sucrose‐Coated‐Al2O3 for Submicrometer AlON Powder Synthesis

Fluffy and homogenous sucrose‐coated‐γ‐Al₂O₃ structured precursor was prepared by drying ethanol‐water sucrose/Al₂O₃ suspension, in which the ethanol content of 85 vol% was optimized. Using the C/Al₂O₃ mixture pyrolyzed from such precursor with 23.2 wt% sucrose, single‐phase AlON powder was synthesized by two‐step carbothermal reduction and nitridation method at 1550°C for 2 h and 1700°C for another 1.5 h. The particle size of the AlON powder was around 0.6–1.0 μm. Compared with those synthesized by the traditional approaches with mechanical C/Al₂O₃, Al/Al₂O₃, or AlN/Al₂O₃ mixtures, the synthesis temperature was reduced about 50°C, and the AlON powder was fine and exhibited good dispersity. Such superiority of this method was attributed to that the pyrolyzed carbon film on Al₂O₃ particle greatly restrained Al₂O₃ coalescence during the thermal treatment.     

Carbothermal/aluminothermic reduction nitridation synthesis of ZrN–SiAlON refractory composites from zircon and bauxite: a comparative study of the reduction effect of reducers

The ZrN–SiAlON composite refractory powder was successfully synthesised from zircon and bauxite minerals by the carbothermal reduction nitridation or the aluminothermic reduction nitridation method with three typical reducers, including carbon coke, carbon black and aluminium powder. The effect of reaction temperatures on phase composition and microstructure was investigated using X-ray diffraction and a scanning electron microscopy (SEM), respectively. The thermodynamics equilibrium relationships of the condensed phases were analysed as well. The results showed that carbon coke was the most optimum reducer and when it was used as the reducer, the main final products were granular ZrN with a little β-SiAlON. Nevertheless, ZrO₂ was produced during reduction at 1600°C when the reducer was carbon black, because the activity of carbon black was the poorest. Additionally, more byproducts were produced in the case of Al powder used as reducer at 1600°C, such as AlN polytype and Al₂O₃.     

In-situ synthesis of 15R-Sialon from Al-Si3N4-Al2O3 composite at 1500°C via liquid-phase sintering

In this study, alumina-based composite with 12 wt% Al and 16 wt% Si₃N₄ was designed to achieve the synthesis of 15R-Sialon reinforced alumina composite. To investigate the reaction mechanism, two-step sintered Al-Si₃N₄-Al₂O₃ samples at different temperatures ranging from 600°C to 1500°C were prepared and characterized via X-ray diffraction and scanning electron microscope (SEM). The results revealed that 15R-Sialon was synthesized at 1500°C through a novel liquid Si phase sintering and Si₃N₄ played as a precursor and a reactant. First, Si₃N₄ precursor reacted with Al to form intermediate phases AlN and Si, which were not further transformed below 1400°C. When the sintering temperature was 1500°C, the formed Si presented as a liquid phase, under the influence of which plate-like15R-Sialon was generated from Al₂O₃, residual Si₃N₄, and derived AlN. The obtained Si was also involved in the synthesis of 15R-Sialon and completely transformed. In addition to the AlN from Si₃N₄, the AlN deriving from the nitridation of Al may not react with liquid Si. Compared to 15R-Sialon from liquid Si, plate-like 15R-Sialon with smaller size was generated from AlN, SiO, and O₂.     

Crystallization of fibre-coating compounds of potential use in fibre-reinforced oxide ceramics

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, BaAl₂Si₂O₈ and lanthanum hexaluminate, LaAl₁₁O₁₈) were prepared by hybrid sol–gel synthesis and their thermal crystallisation was monitored by thermal analysis, X-ray diffraction and multinuclear solid state MAS NMR. Both the gels convert to the crystalline phase below about 1200°C, via amorphous intermediates in which the Al shows an NMR resonance at 36–38 ppm sometimes ascribed to Al in five-fold coordination. Additional information about the structural changes during thermal treatment was provided by ²⁹Si and ¹³⁷Ba MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about 500°C but the Ba is still moving into its polyhedral lattice sites about 400°C after the sluggish onset of crystallization. Lanthanum hexaluminate crystallises sharply at 1230°C via γ-Al₂O₃.     

Formation of (Al2OC)1−x(AlN)x solid solution starting from Al–Si–Al2O3 powder matrix at 1300°C in flowing nitrogen

(Al₂OC)_1−x(AlN)_x solid solution-reinforced Si–Al₂O₃ composite was successfully synthesized by designed heating of the Al–Si–Al₂O₃ composite to 580°C and held for 8 hours, followed by heating to 1300°C at a rate of 12°C/h in flowing nitrogen. The reaction mechanism is as follows: after the Al–Si–Al₂O₃ composite is heated to 580°C and held for 8 hours, an AlN cladding is formed on the surface of the Al powder, thus the composite is preconverted into (Al–AlN cladding structure)–Si–Al₂O₃ system. With increasing temperature, the AlN cladding ruptures and the reactive Al(l) flows out. The Al(l) preferentially undergoes active oxidation to form metastable Al₂O(g), which lowers P_O2 inside the composite and inhibits the active oxidation of Si. Moreover, ultrafine carbon is produced by the pyrolysis of the phenolic resin binder. Both metastable Al₂O(g) and ultrafine carbon are highly reactive. Therefore, under the induction of AlN and N₂, (Al₂OC)_1−x(AlN)_x solid solution is formed by the reaction which easily occurs at a relatively low temperature. In the presence of a large amount of Al₂O(g), the P_O2 in the composite does not satisfy the condition required for both Si nitridation and active oxidation, so the free Si remains stable in the composite, forming a metal-non-oxide-oxide composite. The cold crushing strength of the composites is up to 305 MPa, and the composites do not show hydration after 20 months of storage in the environment.     

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.