Effect of Milling Treatment on the Particle Size in the Preparation of AIN Powder from Aluminum Polynuclear Complexes

Aluminum nitride powder was synthesized from aluminum polynuclear complexes. Basic aluminum chloride (BAC) provided the aluminum polynuclear complexes. The effect of the milling treatment for the precursor made from basic aluminum chloride and glucose on the particle size was investigated. BAC and glucose were dissolved in water and mixed homogeneously. AIN powder was obtained by calcining under a nitrogen gas flow after drying, milling by vibration mill, and precalcining at 800°C. Then excess carbon was removed by firing in air. It was found that the milling treatment affected the particle size of AIN powder and nitridation mechanism. Without the milling treatment, AIN powder was synthesized directly from the γ-alumina of an intermediate product. In using a milled precursor, however, α-alumina was formed during the calcining, and the particle size of the AIN powder obtained was about five times larger than that of AIN powder synthesized from a raw precursor. The formation of α-alumina phase began at the rather low temperature of 800°C. These results suggest that the mechanochemical effects added by the milling promotes the formation of α-alumina during calcining, and the α-alumina phase formed accelerates the particle growth. AIN powders obtained were very uniform. The oxygen content and the surface area of AIN powders synthesized from the raw and milled precursors were 2.9 wt% and 17.5 m²/g, and 1.1 wt% and 3.6 m²/g, respectively.     

Effect of Polynuclear Complex Content on the Synthesis of Aluminum Nitride from Aluminum Polynuclear Complex

Aluminum nitride powders were synthesized from an aluminum polynuclear complex and glucose. Basic aluminum chloride (BAC) was used as the aluminum polynuclear complex. The effect of the polynuclear complex content on the nitridation reaction and particle size of the AIN powder synthesized was investigated. BAC solutions with various polynuclear complex contents were synthesized using an aqueous solution prepared from AlCl₃ and Al metal with various compositions. In this system, AlN was synthesized through γ-Al₂O₃ as an intermediate regardless of the polynuclear complex content. Polynuclear complex content affects the reactivity of nitridation and the particle size of the synthesized AIN powder. The reactivity of nitridation and specific surface area of the products increased with the polynuclear complex content. When the precursor with a polynuclear complex content of 94% was calcined at 1400°C, the AlN content reached 95%. Crystallite size by X-ray diffraction measurement and particle size calculated from the specific surface area for the products were in good agreement, indicating that AlN particles formed in the synthesis process are in the form of single crystals. AlN powder synthesized from the precursor with a polynuclear complex content above 80% had a fine particle size and narrow size distribution.     

Sintering Behavior of Fine Aluminum Nitride Powder Synthesized from Aluminum Polynuclear Complexes

The sintering behavior of fine AIN powder synthesized from an aluminum polynuclear complex was investigated. The focus of this work was to investigate the densification behavior of the AIN powder with different particle sizes (specific surface area: 3.2–22.8 m²/g). The AIN powder was synthesized from basic aluminum chloride and glucose mixed in a water solution. This powder was divided into two groups: one with 2 wt% Y₂O₃ added as the sintering aid and the other without such an additive. The AIN powder investigated possessed favorable densification potential. The density of the AIN powder with a surface area of 16.6 m²/g and without additives attained theoretical density at 1700°C. Adding Y₂O₃ further decreased the sintering temperature required for full densification to 1600°C. It is speculated that low-temperature sintering of our fine AIN powder with Y₂O₃ proceeds in two steps: in the initial stage, sintering proceeds predominantly through interdiffusion between yttrium aluminates formed on the AIN powder surface; in the second stage, the densification may occur by the interdiffusion between solid phases formed by a reaction between the yttrium aluminates and AIN. To investigate the effect of oxygen on sintering, the content of oxygen in AIN powder was varied while the particle size was kept constant. In this study, the difference in surface oxygen content scarcely affected the sintering behavior of fine AIN powder.     

Carbothermal Synthesis of Aluminum Nitride Using Sucrose

Several aluminum oxides (α-Al₂O₃, θ-Al₂O₃, and AIOOH) were examined to study the differences in reaction behavior and powder characteristics during carbothermal nitrida-tion to AIN using sucrose and carbon black. The reaction conditions investigated were carbon-to-alumina ratio, reaction temperature, and time. Carburized sucrose resulted in Full conversion to AIN and produced a uniform powder morphology using a near-istoichiometric ratio of C:Al₂O₃ while carbon black required higher C:Al₂O₃ ratios (i.e., >4:1) for full conversion and led to agglomeration of the AIN powder. The most favorable reaction temperature was 1600°C, with the reaction time to full conversion being dependent on the type of Al₂O₃. The particle and agglomerate size of the AIN powders did not change significantly with reaction time. However, the particle size and morphology were strongly dependent on that of the initial AI₂O₃ with sucrose, whereas agglomeration of the AIN occurs when using carbon black. A solid–solid reaction mechanism is proposed.     

Reaction Synthesis of Aluminum Nitride–Boron Nitride Composites Based on the Nitridation of Aluminum Boride

Aluminum nitride–boron nitride (AlN–BN) composites were prepared based on the nitridation of aluminum boride (AlB₂). AlN powder was added to change the BN volume fraction in the obtained composites. Thermogravimetry–differential thermal analysis (TG-DTA), X-ray diffractometry, and the nitridation ratio were used to investigate the nitridation process of AlB₂. At ∼1000°C, a sharp exothermic peak occurred in the DTA curve, corresponding to the rapid nitridation of aluminum in AlB₂. On the other hand, the nitridation of the transient phase, Al_1.67B₂₂, was very slow when the temperature was <1400°C. However, the nitridation speed obviously accelerated at temperatures >1600°C. The pressure of the nitrogen atmosphere was also an important factor; high nitrogen pressure remarkably promoted nitridation. Treatment at 2000°C was disadvantageous for nitridation, because of the rapid formation of a dense surface layer that inhibited nitrogen diffusion into the specimen interior. Three specimens, with 5 wt% Y₂O₃ additive and different BN contents, were prepared by pressureless reactive sintering, according to the determined sintering schedule. Electron microscopy (scanning and transmission) observations revealed that the in-situ -formed BN flakes were homogeneously and isotropically distributed in the AlN matrix. A schematic mechanism for microstructural formation was developed, based on the results of nitridation and the microstructural features of the obtained composites. The obtained composites, with a low BN content, exhibited a high bending strength, comparable to that of reported hot-pressed AlN–BN composites.     

Controlling Thermophoresis in the Exothermic Nitridation of an Aluminum Aerosol

Thermophoretic deposition in an exothermic aluminum (AI) nitridation aerosol process was redirected from the reactor wall to a flowing inert powder when the inert powder was admixed with the feed atomized AI. The inert powder can be a compatible (aluminum nitride) or removable (carbon) powder, which provides for a lower-temperature surface to be in close proximity to the exothermic reaction sites. Aluminum nitride manufactured by this process was pressurelessly sintered with 3 wt% Y₂O₃ to fabricate dense AIN parts exhibiting high thermal conductivity (141 W/m – K).     

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β-SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self-sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al-Si-C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.     

Auger Electron Spectroscopy of Aluminum Nitride Powder Surfaces

The chemical states of powder surfaces depend on the manufacturing processes of the powders. The surface chemistry of three different commercial AIN powders, which are processed by carbothermal nitridation of Al₂O₃, chemical vapor deposition (CVD), and direct nitridation of aluminum, were evaluated by using Auger electron spectroscopy (AES). In order to obtain reference AES spectra of aluminum compounds, α-, γ-, θ-Al₂O₃, γ-AIOOH, γ-AION, and sintered AIN were also examined. Line shapes of aluminum LVV ; Al( LVV ), nitrogen KLL ; N( KLL ) and oxygen KLL ; O( KLL ) are discussed for the AIN powders and all the other aluminum compounds. The differential Auger electron spectra, i.e., E dn /AE were obtained directly, where n is the number of Auger electrons, and E is the kinetic energy of the electron. Their integrated spectra, i.e., n ( E ) are also employed for analysis. The results confirm the conclusions of our previous temperature-programmed desorption work. The AES line shape analysis implies the presence of an oxide-like θ -Al₂O₃ containing AION phase on the carbothermal nitride AIN powder surfaces. The surfaces of CVD and direct-nitrided AIN powders are covered by an oxide–like γ-Al₂O₃ with an oxygen diffusion layer and does not have AION phase.     

Effect of Thermal History on the Thermal Diffusivity and Thermal Expansion of an Alumina–Aluminum Titanate Composite

A study was conducted of the temperature dependence of the thermal diffusivity and thermal expansion of an alumina–aluminum titanate composite heated to a range of maximum temperatures followed by cooling to room temperature. Heating to temperatures above about 600°C resulted in a hysteresis behavior in which the data on cooling differed from the data obtained during heating. For both the thermal diffusivity and thermal expansion, the degree of hysteresis increased with increasing maximum temperature. On return to room temperature, following heating to a temperature of about 1200°C, the thermal diffusivity exhibited a significant decrease, with a corresponding increase in specimen size. This effect was attributed to an increase in microcrack density over the corresponding value prior to heating. On subsequent cycles of heating and cooling for a maximum temperature of 1200°C this decrease in thermal diffusivity was partially recovered, indicative of the structural integrity of the alumina–aluminum titanate composite of this study in practical applications involving temperatures of at least 1200°C.     

Effects of Additive on the Microwave Synthesis of AlN Powder

In this paper, a series of aluminum nitride powders were synthesized using a microwave synthesis method with various additives. The effects of additives on the nitridation of aluminum powder by microwave processing were studied. The results showed that the pure AlN powder with regular and fine grains could be obtained at a temperature of 1050°C for 30 min by the mixed additive under atmospheric pressure of nitrogen. The mixed additive composed by ammonium fluoride and zinc granules played a vital role in the microwave synthesis process.     

Synthesis and Characterization of Nanocrystalline Niobium Nitride Powders

Nanocrystalline NbN powder was synthesized by the direct nitridation of amorphous Nb₂O₅ powder with high BET surface area. X-ray diffractometry analysis indicated that the cubic-phase NbN powder could be obtained by nitridation at 650°–800°C for 3–8 h. Transmission electron microscopy images showed that the particle sizes were in the range of 15–40 nm. The effect of the nitridation temperature and holding time on the powder properties was discussed.     

铝源对碳热还原氮化法制备AIN粉末的影响

本文用无机铝盐借助溶胶-凝胶工艺和表面活性剂的作用制得了一种铝碳良好结合、均匀、无(或弱)团聚的混合凝胶细粉(文中简称“均质混料”),以此为原料氮化合成了纯度达98%的超细AIN粉末,文中着重分析了使用这种均质混料能降低合成条件、提高粉末性能的热力学机制,探讨了影响合成过程的诸因素,最后得出结论认为:铝凝胶均质混料的制备和使用是改进成热还原氮化工艺的最有效途径。     

Synthesis of Nanocrystalline Chromium Nitride Powders by Direct Nitridation of Chromium Oxide

Nanocrystalline CrN powder was synthesized by the direct nitridation of nanosized Cr₂O₃ powder. Powder X-ray diffractometry patterns indicated that pure cubic-phase CrN powder could be obtained by nitridation at 800°C for 8 h. Transmission electron microscopy images showed that the particle sizes were 40–80 nm. The effect of the nitridation temperature and holding time on the powder properties was studied.     

Thermal Oxidation of Aluminum Nitride Powder

The oxidation kinetics, morphology, and crystallinity of aluminum nitride (AlN) powder thermally oxidized in flowing oxygen were determined from 800° to 1150°C. At 800°C the oxidation became detectable with weight change. AlN powder was almost completely oxidized at 1050°C after only 0.5 h. Amorphous aluminum oxide formed at relatively low temperatures (800°–1000°C), with a linear oxidation rate governed by the oxygen–nitride interfacial reaction. Transmission electron microscopy displayed individual aluminum oxide grains which formed a discontinuous oxide layer at this temperature range. The aluminum oxide was crystalline at higher temperatures (>1000°C), as detected by X-ray diffraction, and the density of oxide grains increased with temperature.     

Synthesis of Nanocrystalline Aluminum Nitride by Nitridation of δ-Al2O3 Nanoparticles in Flowing Ammonia

Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m²/g was synthesized by nitridation of nanosized δ-Al₂O₃ particles using NH₃ as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. It was found that nanocrystalline δ-Al₂O₃ was converted into AlN completely (by XRD) at 1350°–1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al₂O₃ powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al₂O₃ was hard to convert to AlN under the same conditions.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

Low-Temperature Fine-Grained Alumina–Aluminum Titanate Composites Produced from a Titania-Coated Alumina Precursor

A method for the preparation of alumina–aluminum titanate (AT) composites, which can be sintered to high density with a fine-grained microstructure at <1450°C, is reported. The composite precursor is alumina particles coated by sol–gel-derived titania, which reacts during sintering to form AT in situ at temperature as low as 1300°C. The composite can be sintered at 1350°C to 98% density with 1.5–2.0 μm grain size. Other composites containing 5–50 wt% AT are also investigated.     

Investigation of Phase Stability in the System Sic-AlN

Dense samples of several compositions in the system SiC-AIN were fabricated by hot-pressing. The SiC-AIN powder was prepared by carbothermal reduction of an intimate mixture of alumina, silica, and carbon in a nitrogen atmosphere. X-ray diffraction and electron and optical microscopy were used to determine the chemical and microstructural characteristics of the hot-pressed specimens. Materials with bulk compositions between 15 and 75 wt% AIN were found to be nonhomo-geneous when hot-pressed below 2100°C. These materials were determined to be a mixture of SiC-AIN solid solutions with different compositions. The observed compositional variations were distinctly bimodal. The source of the in-homogeneity was the starting SiC-AIN powder. The powders, as well as the hot-pressed samples, consisted of a mixture of small crystals rich in SiC and large AIN-rich crystals. Compositions outside the 15 to 75 wt% AIN region were found to be single phase and to have the wurtzite structure. Hot-pressing SiC-AIN in the intermediate composition range at 2300°C produced an optically and chemically homogeneous material. The precipitation of an SiC-rich phase from a 75 wt% AIN solid solution and the precipitation of an AIN-rich phase from a 47 wt% AIN alloy when annealed at 1700°C are strong indications that a miscibility gas exists in the system SiC-AIN.     

Synthesis of Nanocrystalline Titanium Nitride Powders by Direct Nitridation of Titanium Oxide

Nanocrystalline TiN powder has been synthesized by the direct nitridation of nanocrystalline TiO₂ powder. Powder XRD patterns indicated that the TiN nanocrystalline powder could be obtained by nitridation at 800°C for 5 h. TEM micrographs showed that the synthesized TiN powders consisted of uniform spherical particles with an average diameter of ∼20 nm. The effect of the nitridation temperature and holding time on the powder properties is discussed.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.