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.     

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 mechanism of AlN–SiC solid solution reinforced Al2O3 composite by two-step nitriding of Al–Si3N4–Al2O3 compact at 1500 °C

The in-situ controllable synthesis of AlN–SiC solid solution reinforcement in large-sized Al–Si₃N₄–Al₂O₃ composite refractory by two-steps nitriding sintering was examined. In the first step, a dynamic Al@AlN structure was constructed in the composite by pre-nitriding at 580 °C. During the subsequent sintering process, it cracked above ∼900 °C, and micronized Al cluster (mixture of droplets and vapor) was extracted out gradually. As a result, multiple AlN mesophases were formed through different reaction paths, including i) initial AlN shell formed by solid Al with N₂, ii) reaction of Al cluster with N₂, and iii) reaction of Al cluster with Si₃N₄ from 900 °C to 1500 °C. The Si₃N₄ precursor serves as both a solid nitrogen source and an active Si source, and the controllable reaction between Al and Si₃N₄ leading to uniformly distributed AlN and Si mesophases. AlN–SiC solid solution is significantly formed when liquid Si appears. The shell, granule and whisker SiC–AlN solid solution were observed mainly depending on the dynamic AlN mesophase. The SiC–AlN solid solution reinforced Al₂O₃ materials is a novel promising refractory for large-scale blast furnace lining.     

Carbon-containing droplet combustion–carbothermal synthesis of well-distributed AlN nanometer powders

A novel three-step technique was employed to synthesize the well-distributed AlN nanopowders. First, the hollow spherical precursor particles with an average diameter of 2–5 μm, consisting of an amorphous structure mixture of Al₂O₃ and C, was prepared by carbon-containing droplet combustion method by using glucose, urea, and aluminum nitrate as starting materials. The carbothermal reduction and nitridation (CRN) was carried out at 1500°C under N₂ flow for 2 h and subsequently the CRN product was calcined at 700°C in air for 1 h to remove residual carbon and transform the CRN product to high-purity AlN powders consisting of nanostructured hollow spheres. The formation mechanism of precursor and AlN hollow spheres was discussed in detail. The AlN powder exhibited well-distributed spherical particles with a size of 30–50 nm and good sinterability. After additive-free and pressureless sintering at 1800°C for 2 h, the relative density of the sintered AlN sample was measured to be 99.02%.     

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb₂O₃) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb₂O₃ were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200 °C–1450 °C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb₂O₃ clearly promoted the nitridation of Si compacts at 1200 °C compared with the pure Si compact containing no additives. The critical role of Yb₂O₃ on the nitridation of Si, was elucidated that Yb₂O₃ promotes the loss of initial SiO₂ of the raw Si powder via the measurement of the weight changes at low temperature (1100 °C) and thermogravimetric analysis under N₂ atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200 °C and 1300 °C.     

Formation of aluminium nitride whiskers by direct nitridation

This work describes novel results on the growth of aluminium nitride (AlN) whiskers by direct nitridation of Al–NH₄Cl starting mixtures. The nitridation experiments were carried out in a horizontal tube furnace at 1000 °C for 1 h in 1 l/min N₂ gas flow. It is found that the growth of AlN whiskers was principally promoted by NH₄Cl which provided a different reaction pathway depends on vapor-phase reactions mechanism instead of normal liquid–gas mechanism. The thermodynamic analysis of possible intermediate reactions in the operating temperatures range confirmed that the AlN whiskers could be grown through spontaneous vapor-phase chlorination–nitridation sequences. The SEM observation revealed that depending on NH₄Cl concentration homogeneous AlN nanowhiskers of <150 nm in diameters can be obtained as well as composites of particles-whiskers of AlN which may be potential for preparing useful sintered AlN materials.     

New Method of Synthesizing Aluminum Oxynitride Spinel Powders

Aluminum oxynitride spinel (AlON) powders were synthesized by aluminothermic reaction in a reducing N₂‐CO atmosphere. Low cost and easily available aluminum and γ‐Al₂O₃ alumina micrometer‐sized powders were employed as starting materials. Mixed powders consisting of 75 wt% Al and 25 wt% Al₂O₃ were milled together and pressed into billets with diameter of 20 mm and height of 15 mm. Green‐body billets were then calcined in charcoal‐protected condition (namely in a N₂‐CO atmosphere) at 1600°C. Phase composition and microstructure of final sintered products were analyzed. The results showed that AlON phase with AlN as a minor phase was formed at 1600°C for 3 h. At the same time, grains of AlON were tabular in shape and whiskers can be found in samples after being sintered at 1600°C.     

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.     

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.     

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.     

Effect of Y2O3 additive on nitridation of diamond wire silicon cutting waste

The effect of Y₂O₃ additive on the nitridation of diamond wire silicon cutting waste (DWSCW) was studied by using X-ray diffraction, thermo gravimetry, differential thermal analysis, scanning electron microscope equipped with energy-dispersive spectrometry, and an equivalent alternative method, and the individual particles of DWSCW were simulated using cubic polycrystalline silicon blocks. The results showed that the native SiO₂ film on the surface of DWSCW can be disrupted at low temperature (1300°C) by adding Y₂O₃ additive, which provide good channels for the diffusion of SiO and N₂ and improve the overall conversion of DWSCW. Y₂O₃ additive can also reduce the initial nitriding temperature of cutting waste, change the nitriding kinetic behavior, and promote the formation of β-Si₃N₄ through accelerating the nitridation of cutting waste at high temperature (≥1500°C). In addition, when 8 wt% Y₂O₃ additive is added to the cutting waste, the complete nitridation is achieved, at 1350°C, and ω_α + ω_β reaches a maximum of 83.6 wt%.     

Conversion of Alumina to Aluminum Nitride Using Polymeric Carbon Nitride as a Nitridation Reagent

Polymeric carbon nitride, which was synthesized by polymerization of dicyandiamide at 500°C, was used as a nitridation reagent in the conversion of δ‐alumina (δ‐Al₂O₃) to aluminum nitride (AlN). The products obtained at various reaction temperatures were characterized by powder X‐ray diffraction, ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). δ‐Al₂O₃ began to convert to AlN at 900°C, which is the lowest temperature reported for the formation of AlN from Al₂O₃, and completely converted to AlN at 1400°C. The occurrence of reaction intermediates during nitridation was confirmed by ²⁷Al MAS NMR and XPS. The change in Raman spectra with reaction temperatures indicated that lattice defects in AlN were reduced by calcining at higher reaction temperatures.     

Preparation of AlN powder of low oxygen content via carbothermal reduction and nitridation by active gas exchange technique

By generating a periodic impulse-like pressure (2.0–4.4 kPa, 84 s) to actively exchange the gas in synthesis furnace, pure AlN powder of low oxygen content was synthesized via additive free carbothermal reduction and nitridation (CRN) of Al₂O₃ powder. Compared with the conventional CRN method, the proposed extra gas exhaust process can more effectively remove the side-produced CO from the reaction sites to accelerate nitridation process and decrease the residual oxygen content in the obtained AlN powder. For example, with 39 wt% activated carbon loaded in the raw material at 1650 °C for 4 h, the prepared AlN powder by the proposed synthesis scheme has only 0.68 wt% residual oxygen. The effects of carbon content, synthesis temperature and holding time on the residual oxygen content in AlN powder by the proposed synthesis scheme were also studied. The ball-milled as-prepared AlN powder was pressureless sintered at 1880 °C for 2.5 h to obtain a translucent AlN ceramics (37.6% at ~5700 nm), which demonstrates the excellent sinterability of the as-prepared AlN powder.     

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.     

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.     

Studies on the Al2OC mesophase in synthesized AlN powder and its effects on properties of AlN ceramic substrates

A mesophase of Al₂OC was first determined in AlN powder synthesized in batch quantities via a carbothermal reduction nitridation (CRN) process. The formation and elimination mechanisms of the mesophase were investigated. Effects of the mesophase on properties of the AlN ceramic substrates were evaluated via bending strength and thermal conductivity tests of the substrates fabricated with AlN powder of different O contents. At the conditions of the synthetic furnace, i.e. T = 1700 °C, P_N2 = 10⁻⁵ kPa, and P_CO = 10^−0.008–10^0.973 kPa, the formation of Al₂OC is thermodynamically favorable. By increasing the flow rate of N₂ in the synthetic furnace, the formed Al₂OC was unstable and decomposed into AlN. The properties of the AlN substrates depend on the O content of the AlN powder. The thermal conductivity/bending strength of the AlN substrates increase or decrease, accordingly, based the O content of the reduced AlN powder. AlN substrates made of AlN powder with 0.84 wt% oxygen content show a thermal conductivity and bending strength of 176.3 W/(m·K) and 421.3 MPa, respectively.     

Synthesis of highly sinterable AlN nanopowders through sol-gel route by reduction-nitridation in ammonia

Aluminum nitride (AlN) nanopowders were synthesized through sol-gel route by reduction-nitridation reaction in a temperature range of 1000 °C-1300 °C in NH₃ for 2 h. Effects of combustion atmosphere on characteristics and nitridation reaction of the precursor were systematically investigated. It was found that the precursor combusted in argon, with the features of higher carbon content and smaller specific surface area, was completely converted to AlN after being calcined at 1300 °C without additional carbon removal process. The TEM and FESEM images showed that the completed nitridation product was primarily composed of nearly spherical AlN granules with sizes of 20–30 nm. In addition, the formation process and potential mechanism of AlN low-temperature synthesis were proposed on the basis of thermodynamics calculations. The AlN ceramic was prepared by calcining the AlN green body at 1600 °C for 4 h with pressureless and additive free sintering. The relative density and Vickers hardness of the sintered sample were calculated to be 98.5% and 11.97 GPa, respectively, which attested to excellent sinterability of the synthesized AlN nanopowders.     

Highly conductive mesoporous hierarchical carbon hollow fibers fabricated via confinement of TiO2 coating by ALD

A polysulfone (PSF) hollow fiber composed of interconnected nanofibers within its wall was employed as a template to deposit with a layer of TiO₂ by atomic layer deposition. Direct nitridation of the TiO₂-coated PSF hollow fiber at 800 and 1000°C was conducted, and a new hierarchical structure of TiO_xN_1−x and TiN@nitrogen-doped carbon hollow fibers, respectively, was formed. The PSF fiber served as the source of carbon and was directly transformed to a nitrogen-doped carbon fiber because the shape change was confined by the TiO₂ coating. In the meantime, TiO_xN_1−x or TiN was formed after the nitridation of TiO₂. X-ray photoelectron spectrometric analysis indicated that there was no chemical bonding between the nitridized coating and the carbon nanofibers. It implies that the nitridation of TiO₂ and carbonization of PSF proceed independently and simultaneously in the nitridation process. Raman spectroscopic analysis also confirmed the formation of graphitic lattice and Ti–N bonding. Electrical measurement indicated that both fibers were highly conductive, with the electrical resistivity in the order of 10⁻⁵ Ω m, which is lower than those of amorphous carbon and graphite along the direction perpendicular to the basal plane.     

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.     

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.