Nano-sized AlN Powders Research Articles

Preparation of single phase nano-sized AlN powder by carbothermal reduction and nitridation of hydrothermal synthesized carbon coated γ-Al2O3 from glucose via fast heating

By twice pre-milling γ-Al2O3 in absolute ethyl alcohol plus in glucose aqueous solution, γ-Al2O3 was first coated by carbon via hydrothermal treatment technique. Then, at a fast heating speed of 50 °C/min, nano-sized pure AlN powders were synthesized through carbothermal reduction and nitridation (CRN) at 1550–1600 °C for 60–120 min in N2. The pre-milling in absolute ethyl alcohol effectively broke the initial aggregation of γ-Al2O3, and the subsequent milling in glucose aqueous solution led to well contact between γ-Al2O3 and glucose, which contributed to the formation of intact carbon coating on the surface of γ-Al2O3 during hydrothermal treatment. The coated carbon served as one reactant and inhibitor of particle aggregation for CRN, which also led to retention of γ-Al2O3 and its morphology during heating with the help of high heating speed. As a result, fine AlN of ∼100 nm was directly formed by nitridation of γ-Al2O3. The synthesis mechanism of nano-sized AlN powder by the carbon coated Al2O3/C precursor was discussed.

Reaction mechanisms of nano-sized AlN powders synthesized from dicyandiamide and its optical property

Nano-sized AlN powders were successfully synthesized using a wet chemical route, with dicyandiamide as the nitrogen source reagent. The influence of dicyandiamide/metal precursor molar ratio on the purity, crystallization, microstructure, and optical properties of the calcined powders were studied. The underlying reaction mechanisms, along with the intermediates formed during the reactions, were also investigated. The compositions and morphologies of the as-received and prepared powders were subsequently characterized using XPS, FTIR, XRD, SEM, and TEM. The optical properties of the prepared powders were measured by ultraviolet–visible spectroscopy (UV–Vis). The precursor components decompose, and follow a complex reaction sequence as a function of temperature, finally leading to AlN formation at 800 °C. Increasing AlN content is achieved between 800 and 890 °C. Essentially only AlN arises following calcination at 900 °C. The AlN crystallite size and degree of crystallization further increase when calcining at 1000 °C. This work demonstrates a promising route to produce nano-sized AlN powders with favorable microstructures and optical properties.

Investigation on low‐temperature sintered AlN nanoceramics with high thermal conductivity

AbstractThe aim of this research was to investigate the effect of sintering additive and relatively low‐sintering temperature on the thermal conductivity of aluminum nitride nanoceramic. While using nanosized AlN powder and liquid‐phase sintering additives, the various sintering processes were performed at temperatures 1400 and 1500°C for several hours. In the analysis methods, thermal conductivity (K) and thermal diffusivity (α) were measured using thermal conductivity analyzer (Hot Disk), scanning electron microscope (SEM) was used to observe the surface morphology of the microstructure, x‐ray diffraction analyzer (XRD) to analyze the grain size and crystal structure, Raman spectroscopy (Raman) emission spectrum was analyzed to identify the material microstructure and the densities of AlN specimens were measured by Archimedes method. It was found that the thermal conductivity is related to the densification of nanosize low‐temperature sintered material, which can be controlled by additives and sintering temperature. With Y2O3 sintering add, the densification of AlN for low‐temperature sintering increased by the factor of ～5% to ～12%, and the thermal conductivity was enhanced by 25%. The relative density observed in this research is about 78%‐84%, and the thermal conductivity measured is in the range of 9‐14 W/mK.

Polymer Network Gel Method for Surperfine AlN Powder Preparation

To research mixing uniformity of precursor was helpful to improve the quality of the powder late processing. Layered problems of the precursor comprised of aluminium source and of carbon source were well solved by the polymer network method. High quality raw precursor was synthesized by new process which contained polyacrylamide-gel method and carbothermal process. The synthesized precursor was investigated to obtain porous structure precursor. The results showed that sucrose was more suitable for the preparation of AlN precursor. The effect of reaction temperature and heating time on the phase composition of AlN powder were also studied. In this work, ultrafine nano-sized AlN powder was prepared at low temperature.

Fabrication of Al/AlN nano-composite layers by friction stir processing of 6061 Al-T6 substrate

Friction stir processing was employed for the production of Al/AlN nano-composite layers on a 6061 Al-T6 substrate. Nano-sized AlN powder was inserted in a groove in the middle length of the substrate. Defect-free layers were achieved using tool rotation and substrate advancing speeds in the range of 900–1400 rpm and 63–310 mm/s, respectively. Subsequent passes were conducted to break-up AlN clusters that formed in a non-uniform fashion after initial pass. The grain size of aluminum matrix was found to decrease by the introduction of AlN powder. A nano-composite layer with near uniform dispersion of nano-sized AlN reinforcements with a ~9.6% volume fraction was achieved in a matrix of fine dynamically restorated Al grains with a mean size of ~2.5 µm after three subsequent passes. This layer showed an average micro hardness value of ~164 HV (much greater than ~103 HV of the underlying substrate). In addition, the nano-composite layer exhibited superior dry sliding wear performance against hardened steel compared to that of 6061-T6 substrate. Increasing tool rotation and substrate advancing speeds were found to decrease the AlN content of the processed layer possibly due to increasing in powder scattering by the pin tool. This was associated with a decrease and increase in hardness values and wear-loss data, respectively. Copyright © 2014 John Wiley & Sons, Ltd.

Preparation and Light Transmission Properties of AlON Ceramics by the Two-Step Method with Nanosized Al2O3 and AlN

The single-phase aluminum oxynitride (AlON) powder used in this experiment was obtained at relatively low temperature by the solid-state-reaction method with nanosized AlN powder and nanosized alumina powder. The obtained powder was ground by ball milling and doped with Y2O3; then, it was shaped into pellets. Transparent ceramics sintering was carried out at 2153 K (1880 °C) for 5, 10, and 20 hours. Obtained samples for three duration times are all are transparent, and the light transmittance of samples increases with increasing the holding time at the sintering temperature. The transmittance of the sample with 20 hours holding time is 55 pct at near 5 μm. X-ray diffraction (XRD) measurement shows that all samples were single-phase AlON. The fracture surfaces of samples from scanning electron microscopy (SEM) analysis shows that with increasing the holding time, the grain size grows slightly, while the pore size and porosity decrease apparently, which bring about the transmittance increase. The pore size and the porosity are the main factors affecting the light transmitting properties of samples.

Large-scale synthesis of AlN nanofibers by direct nitridation of aluminum

AlN nanopowders and nanofibers were synthesized by direct nitridation of Al and rice bran mixture compacts in a tube furnace up to 1300 °C in a nitrogen flow without addition of extra catalyst. The effect of the compaction pressure applied onto the green bodies on the morphology of the final AlN products was investigated. A green body compacting pressure in between 320 MPa and 480 MPa was found to be favorable for the synthesis of AlN fibers with aspect ratio up to 400, diameter in the range of 50–500 nm, and length up to tens of micrometers; for a lower pressure of 160 MPa and a higher pressure of 640 MPa, nano-sized AlN powders were the primary morphology in the final product. The AlN products were characterized by several techniques and the VLS growth mechanism was proposed as the main reason for the AlN fibers formation.

Preparation of Transparent AlON Ceramics Using Powder Synthesized from Nano-Sized Al<sub>2</sub>O<sub>3</sub> and AlN

The single-phase AlON powder used to prepare transparent ceramics was synthesized from solid-state-reaction using nano-sized Al2O3 and AlN powders with high purity. An appropriate amount of the powder was molded into pellets and isostatically pressed using polyvinyl alcohol as the binder. The AlON ceramic samples were obtained by sintering the pellets at 1880°C for 10 h, 20 h and 40 h, respectively. The transparency of the samples increased as the temperature holding time was prolonged. The density, phase compositions and microstructures of the sintered samples were discussed in detail.

Preparation of Aluminum Oxynitride Powder by Solid State Reaction Method

Single-phase aluminum oxynitride powders were obtained by varying the holding temperature (T) and the weight percentage of α-Al2O3 in Al2O3 (X) with nano-sized Al2O3 and AlN powders as raw materials. Influences of T and X on the phase composition of obtained powders were studied. The results showed that the content of aluminum oxynitride increased with increasing T or X. When X was 50% and T was 1750°C, obtained powders had better properties than those of others.

Superplastic Extrusion of Nano-Structured Si<sub>2</sub>N<sub>2</sub>O-Sialon

Nano-structured Si2N2O-Sialon composites were prepared by liquid phase sintering method with the amorphous nano-sized Si3N4 and nano-sized AlN powders. Nano-sized Al2O3 and Y2O3 powders synthesized by polyacrylamide gel method were introduced as additives. SEM examination shows the grain size of sintered body to be less than 80nm. Superplastic extrusion can be undertaken at 1550°C with a high velocity of 0.5mm/min and a big extrusion ratio of 3.57. There are a lot ring-shaped structure just like “annual ring” of trees in radial fracture and clear flow trace in axial fracture, all which is very similar to deformed metal materials and approve good superplastic deformation capability of Si2N2O-Sialon composites.

The role of chemisorbed anions in the aqueous processing of AlN powder

Abstract In the production of ceramics containing AlN it is necessary to prevent the AlN powder from reacting with water. To do that, non-aqueous powder processing is required or water-resistant AlN powders must be used. Alternatively, as in the Hydrolysis Assisted Solidification process, which exploits the hydrolysis of AlN for the solidification of the ceramic suspensions, the hydrolysis has to be prevented at room temperature but initiated at elevated temperatures. In this study the role of the chemisorbed anions in the hydrolysis of the AlN was investigated in order to make the control of the hydrolysis reactions possible. The method for preparing water-resistant, hydrophilic AlN powder using the dispersion of the powder in a solution of aluminium dihydrogen phosphate is also presented. The mechanism leading to the formation of the protective surface layer is proposed and explained. The method was also successfully tested on nanosized AlN powder. This powder was processed in water and sintered to high density.

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y 2O 3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y 2O 3 and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO 2 beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.

Rheological Properties of Nanosized AlN Powder Suspensions for Advanced Ceramics

The rheological properties (flow curves and viscoelastic behavior) of injection molding suspensions of a plasma-processed AlN nanosized powder (nanopowder) in paraffin are investigated over a broad range of shear rates (0.07–1350 s−1). Two viscosity plateaux are observed on the flow curves and two values of the yield stress are obtained. The lower value of the strain amplitude (0.66%), exceeding the linearity limit of periodic shear, is restricted by the rheometer resolution. The ultrasound treatment and shear deformation of suspensions affect the structure of particle packing, which is responsible for the dependence of their rheological properties on the prehistory of mechanical actions.