AlN Powder Research Articles

Synthesis of aluminum nitride powder by carbothermal reduction of a combustion synthesis precursor

A new homogeneous mixing precursor of alumina and carbon (Al 2O 3 + C) was prepared by low temperature combustion synthesis (LCS) with aluminum nitrate (Al(NO 3) 3·9H 2O), urea (CO(NH 2) 2), and glucose (C 6H 12O 6·H 2O) as starting materials. The precursor prepared was a foamy and porous mass due to the large amount of gases liberated in the combustion reaction and composed of nanosize particles. XRD analysis showed that this precursor consisted of amorphous alumina and carbon. The amorphous alumina in the precursor first transformed into γ-Al 2O 3, and then γ-Al 2O 3 was directly nitrided to yield AlN during the calcination process. The reaction temperature needed for a complete conversion for the precursor was about 1400 °C, which is much lower than that when using alumina and carbon black as starting materials. The synthesized AlN powder was composed of very fine particles and had good dispersability.

The influence of temperature and time on the AlN powder hydrolysis reaction products

The influence of hydration temperature and ageing time on the formation of crystalline phases found after the AlN powder hydrolysis was investigated. The AlN hydrolysis behavior was observed by measuring the pH of the suspension, whereas for the characterization of the reaction products XRD, SEM and TEM analyses were employed. The starting temperature, from RT up to 90 °C and the ageing time, from 10 min to 24 h, have an impact on the reaction products and their morphology. After prolonged time of the AlN reaction with water, the only crystalline product is bayerite at RT. At elevated temperatures the very first solid reaction product is pseudoboehmite, transforming to crystalline boehmite and part of it dissolving and recrystallizing to bayerite with ageing time. At temperatures above 80 °C, boehmite is the predominant crystalline phase, nevertheless, after long time of ageing in the mother liquor, even at these temperatures the boehmite → bayerite conversion was observed.

Oxidation kinetics of aluminum nitride at different oxidizing atmosphere

Abstract In the present work, the oxidation kinetics of AlN powder was investigated by using thermogravimetric analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experiments were carried out both in isothermal as well as non-isothermal modes under two different oxidizing atmospheres. The results showed that the oxidation reaction started at around 1100 K and the rate increased significantly beyond 1273 K forming porous aluminum oxide as the reaction product. The oxidation rate was affected by temperature and oxygen partial pressure. A distinct change in the oxidation mechanism was noticed in the temperature range 1533–1543 K which is attributed to the phase transformation in oxidation product, viz . alumina. Diffusion is the controlling step during the oxidation process. Based on the experimental data, a new model for predicting the oxidation process of AlN powder had been developed, which offered an analytic form expressing the oxidation weight increment as a function of time, temperature and oxygen partial pressure. The application of this new model to this system demonstrated that this model could be used to describe the oxidation behavior of AlN powder.

Characterization of plasma processed ultra-fine Ni–Al powder

Thermal plasma processing utilizes the exotic properties of the plasma medium to effect physical, chemical or metallurgical changes/reactions in materials. Commercially available nickel and aluminium powders were mixed in the appropriate amount, ball milled and fed into a dc plasma jet produced by an atmospheric plasma torch. The particles are molten and leave the flame and falls through the atmospheric air inside a plasma reaction chamber. The particles are quenched, solidified and collected in a collection chamber. During in-flight if the particles are overheated, depending on the size, may be partially or fully vaporize. The smaller particles fly in all directions and get deposited on the walls of the reaction chamber. Vapors also condense on the cooled walls. The particles are collected. During in-flight the exothermic reaction to form nickel aluminide may also take place. The particles on the walls of the reaction chamber were collected and characterized using XRD, SEM with EDX and TG\DTA analysis. The results are reported and discussed.

Fabrication and Electrical Property of SiC-AlN Composites

SiC and AlN form a solid solution in the wide compositional range, expectantly leading to control of the semiconductive property. In the present work, the SiC-AlN composites were fabricated by sintering process, and evaluated with emphasis on the distribution of SiC and AlN and electrical property. SiC and AlN powders were mixed at a molar ratio between 90:10 and 10:90, and sintered at 1900-2100 °C for 30 min under 50 MPa in Ar atmosphere by spark plasma sintering technique. The sintered bodies reached high densities over 95 % of theoretical, and the grain size increased with an increase in the sintering temperature and the AlN content. The SiC-AlN composites had 3C and 2H phases in SiC-rich composition, while 2H phase only in AlN-rich composition, and the mutual dissolution between SiC and AlN was enhanced at high temperatures. The electrical conductivity decreased with dissolution of AlN into SiC because of the increase in band gap.

NiAl Oxidation and Corrosion Resistant Coatings Obtained by Thermal Spraying

There are investigated the possibilities to avoid or at least to reduce the Al2O3 scales formation on NiAl powder particles at its plasma spray deposition on steel substrates. The optimum processing parameters and the necessity to surround the plasma jet by an inert gas have been established. In appropriate processing conditions, the obtained coating layer is formed by flattened particles, welded together and to the substrate, proving their melting during spraying. It is dense and adherent, consisting of NiAl with only small Al2O3 inclusions, proving the NiAl stability preserving without decomposition or a notable oxidation, as premises of its desired functionality achievement.

Low wet etching rates of ZnO films prepared by sputtering of mixed ZnO and AlN powder targets

ZnO films codoped with Al and N have been prepared by radio frequency magnetron sputtering in an Ar atmosphere, using targets of mixtures of ZnO and AlN powders. The Al-doped ZnO films are transparent, whereas the films codoped with Al and N are colored. The Al- and N-concentrations in the colored films are estimated to be 4–7 at.% and 1–2 at.%, respectively. No enhancement of the carrier density is seen in the colored ZnO films, whereas the colored films exhibit lower etching rates of 3–5 nm/s in a 0.1 M HCl solution, in comparison with the Al-doped ZnO films. For the colored film, the anisotropic grain growth occurs, and cubic grains are produced after etching. The low etching rates of the colored films are ascribed to the epitaxial growth of AlN films on the surfaces of ZnO grains, rather than the incorporation of Al–N and Al–O bonds into the ZnO lattice.

AlN ceramics prepared by high-pressure sintering with La2O3 as a sintering aid

Abstract Aluminum nitride ceramics with La 2 O 3 as a sintering aid were obtained under high pressure (5.0 GPa) in a cubic anvil high-pressure and high-temperature apparatus. The results showed that the method could effectively lower the sintering temperature and shorten the sintering time. Under 5.0 GPa, the lowest sintering temperature was 1300 °C, which was above 300 °C lower than that for conventional liquid-phase sintering. The lattice parameters of the specimens sintered at high pressure were found to decrease by about 0.09% than that of the AlN powder, and residual compressive stress was found, while hexagonal lattice of AlN was not changed.

Assessment of tensile behaviour of an Al–Mg alloy composite reinforced with NiAl and oxidized NiAl powder particles helped by nanoindentation

Abstract AA5056 matrix composites have been reinforced with as-received and oxidized NiAl particles and their nanohardness investigated as a function of distance to reinforcement. Results indicate that a non-heat treatable aluminium matrix, as is the present case, does not require that the intermetallic particles are surrounding by a protective Al2O3 layer to avoid reactions at matrix-reinforcement interfaces. On the other hand, the quality of the matrix-reinforcement bonding has been quantified by the reinforcement influence distance, defined as the distance from the particle at which the nanohardness of the matrix drops to its asymptotic value.

Aqueous Processing of AlN Powder

For the production of ceramics containing AlN as a major or minor constituent it is necessary to avoid hydrolysis. To do that, non-aqueous powder processing is required or waterresistant AlN powders must be used. Alternatively, as in the Hydrolysis Assisted Solidification (HAS) process, which exploits the hydrolysis of the AlN for the solidification of the ceramic suspensions, the hydrolysis has to be prevented at room temperature but initiated at elevated temperatures. In this work a systematic study of AlN powder reactivity in water and other aqueous environments is presented. The AlN hydrolysis was investigated by measuring the pH of diluted suspensions and by analysis of the reaction products. The results indicate possible solutions for control of the reaction with water in order to exploit it or to prevent it to enable aqueous AlN powder processing.

Improving the Microstructure of Si<sub>3</sub>N<sub>4</sub>-TiN Composites Using Ammonium Citrate to Disperse Raw TiO<sub>2</sub> Powders

Dense and homogenous Si3N4-TiN composites (5 vol% TiN) were prepared by using in situ synthesis method from Si3N4, AlN and TiO2 mixtures, containing Y2O3 and Al2O3 as sintering aids. In the prepared Si3N4-TiN composites, TiN grains were formed from TiO2 and AlN powders during the sintering process, in which ammonium citrate was used as a dispersant for raw TiO2 powders. The microstructures of the Si3N4-TiN composites with the increase of ammonium citrate were investigated by scanning electron microscopy (SEM). Citrate ions modified on the surface of TiO2 particles and protected the TiO2 particles in the mixed slurry to reduce the aggregations of TiO2 powders, and homogenous Si3N4-TiO2-AlN composite powder was prepared for sintering. The microstuctures of Si3N4-TiN were developed after sintering with the uniform distribution of TiN grains in the Si3N4 ceramics. It was found that the microstructure of Si3N4-TiN composite was improved significantly with 0.20 g ammonium citrate in the system, TiN grains with 0.2-0.3 μm in diameter distributed throughout Si3N4 matrix. It was a practical and useful way to improve the microstructure of Si3N4-TiN composite without the alteration of the preparation procedure.

Characteristics of mechanical alloyed Ni-Al powder for sintering

Mechanical alloying was employed to obtain high-activity Ni-Al powder. The effects of mechanical alloying on the microstructure and characteristics of milled powder with a normal composition of Ni-22.89 at.% Al-0.5 at % B were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that a solution Ni (Al) was obtained after milling. During mechanical alloying, the milled powder obtains extra surface energy and crystalline energy because the crystallite size becomes fine and the lattice strain increases with the milling time prolonging. Furthermore, about 0.5 mol% oxide in the particles were formed after milling, and this kind of dispersed oxide is effective to improve the properties of the sintered alloy by dispersion strengthening. It is confirmed that Ni 3Al alloy with outstanding properties has been prepared with mechanical alloyed powders.

Preparation and Properties of Spark Plasma Sintered AlN/BN Nano-Composites

Nano-sized turbostritic-BN (t-BN) was fabricated through chemical process using boric acid and urea in this work. By the same method, the AlN powders coated with nano-BN were prepared too. The results of X-ray diffraction (XRD) and transmission electron microscope (TEM) revealed that nano-sized t-BN was synthesized at about 600°C in nitrogen gas and it surrounded the surface of AlN particles. High-density AlN/BN nano-composites were fabricated spark plasma sintering (SPS). Microstructure and properties of AlN/BN nano-composites (5~30vol% BN) were investigated. The h-BN flake particles were homogenously dispersed at AlN grain boundaries and within grains in the AlN/BN composites. A little nano-BN additions significantly improved the bending strength of the nano-composites. However, the bending strength was decreased with the BN content increasing. The thermal conductivity of AlN/BN nano-composites was investigated too.

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.

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.

Preparation and properties of HVOF NiAl nanostructured coatings

Nanostructured NiAl powder has been prepared by mechanical alloying. A high velocity, oxy-fuel (HVOF) thermal spray process has been used to deposit the agglomerated NiAl nanopowders. The coatings were characterized by means of X-ray diffraction, and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry analysis (EDS). The results showed that it was possible to deposit high quality NiAl nanostructured intermetallic compound coatings. Properties of the coating regarding hardness, elastic modulus, dynamic hardness, high temperature hardness, and wear behavior were also reported.

Preparation of Functionally Graded Cu/AlN/Cu Electrode Materials for Thermoelectric Devices

The Cu/AlN/Cu functionally graded materials (FGMs) were successfully fabricated using the spark plasma sintering (SPS) method, and a two-step process was used. First, a symmetrical porositygraded AlN plate was prepared using AlN powder consisting of particles of varying sizes. Afterwards, graded Cu/AlN/Cu samples were made by introducing Cu into the pores of the external, porous AlN layer.

Phase formation regularities of ultrafine TiAl, NiAl and FeAl intermetallic compound powders during solid–liquid reaction milling

TiAl, NiAl and FeAl system intermetallic compound nanometer powders were prepared by a novel patent solid–liquid reaction milling technology. TiAl 3, Ni 2Al 3, NiAl 3, Fe 2Al 5, FeAl 2, Fe 3Al, FeAl powders with particle size less than 100 nm were obtained, respectively. The phase formation regularities of these alloy systems during milling under various conditions were investigated. The influence of PCAs on the solid–liquid reaction milling process was examined. The results indicate that the added PCAs can reduce the bonding between the balls and the melt, and restrain the powder from agglomeration during milling. Moreover, the results of solid–liquid reaction milling were compared with that obtained by mechanical alloying processing under the similar conditions. The results show that solid–liquid milling is an effective method to prepare ultrafine intermetallic compound powders.

AlN ceramics synthesized by carbothermal reduction of mechanical activated Al 2O 3

AlN ceramics synthesized by carbothermal reduction using the mechanical activated Al 2O 3 were investigated. The results show that, using the commercial Al 2O 3 not milled, the temperature was higher than 1600 °C when the carbothermal reduction reaction was completed, but, if the Al 2O 3 was milled by high-energy ball mill for 20 h, the carbothermal reduction reaction occurred below 1100 °C and completed thoroughly at 1250 °C, and all of Al 2O 3 transformed to AlN. These AlN powders can be sintered at about 1600 °C, and the mechanical properties of these AlN ceramics are as good as that of commercial AlN powders sintered at 1800 °C.

Dilution rate and microstructure of TIG arc Ni-Al powder surfacing layer

Surfacing beads are prepared by a direct current tungsten inert gas arc nickel-aluminum (Ni-Al) powder surfacing process. With the aim of controlling the dilution rate and obtaining surfacing beads rich in intermetallic compounds, the effects of surfacing parameters on geometric parameters, dilution rate, composition, and microstructure of the bead are investigated. An assistant cooler, which can potentially reduce the temperature of the base metal, is used in the surfacing process and its effect on dilution rate and microstructure is studied. The result indicates that with the surfacing parameter combination of low current and speed, the width and penetration of the bead decrease, reinforcement increases, and dilution rate drops markedly. With the reduction of the parameter combination, the intergranular phase γ-(Fe, Ni) is formed in the grain boundaries of Ni-Al intermetallic matrix instead of the intergranular phase α-Fe, and large amount of intermetallics are obtained. With the use of an assistant cooler on a selected operation condition during the surfacing process, the reinforcement of the bead increases, penetration decreases, and dilution rate declines. The use of an assistant cooler helps obtain a surfacing bead composed of only intermetallics.