Recent Progress on Fabrication of Thermal Conductive Aluminum Nitride Fibers

Among nitride fibers,aluminum nitride(AlN)fibers have been developed for various advanced applications due to their mechanical flexibility,high thermal conductivity,and excellent electrical insulation and chemical stability.This article presents an overview on the recent progress of AlN fibers.The properties of AlN,particularly the thermal conductivity of AlN in polymer matrix composites are introduced.Afterward,two major approaches,carbothermal reduction and nitriding polycrystalline alumina fiber,for the preparation of AlN fibers are discussed.The carbothermal reduction includes electrospinning,solution blow spinning,and chemical vapor deposition.Furthermore,some perspectives on the future directions for the preparation and application of fibrous AlN are highlighted.This review is expected to provide readers with valuable guidance on the preparation of AlN fibers and inspire researchers to explore more potential applications.

Synthesis of aluminum nitride in a coke – calcium reduction bed using nitrogen in air

An experimental study on the heating of a mixture of aluminum and lithium hydroxide （LiOH） powders in a reductive bed under air atmosphere is reported. The formation of aluminum nitride （A1N） during this process was the focus of this study. The formation of A1N was achieved using LiOH as an additive and heating the sample in a resistance furnace in a specially designed double crucible within a bed of a mixture of coke and filamentous calcium. The temperature range of the reaction was between 700℃ and 1100℃. The optimum temperature of 1100℃ and the optimum LiOH amount （Swt%） required to achieve maximum yield were determined by powder X-ray diffraction （XRD） analysis. Scanning electron microscopy （SEM） micrographs clearly indicated the transformation of grain structures from rods （700℃） to cauliflower shapes （1100℃）.

Morphology control of aluminum nitride(AlN)for a novel high-temperature hydrogen sensor

Hydrogen is a promising renewable energy source for fossil-free transportation and electrical energy generation.However,leaking hydrogen in high-temperature production processes can cause an explosion,which endangers production workers and surrounding areas.To detect leaks early,we used a sensor material based on a wide bandgap aluminum nitride(AlN)that can withstand a high-temperature environment.Three unique AlN morphologies(rod-like,nest-like,and hexagonal plate-like)were synthesized by a direct nitridation method at 1400℃usingγ-AlOOH as a precursor.The gas-sensing performance shows that a hexagonal plate-like morphology exhibited p-type sensing behavior and showed good repeatability as well as the highest response(S=58.7)toward a 750 ppm leak of H2 gas at high temperature(500°C)compared with the rod-like and nest-like morphologies.Furthermore,the hexagonal plate-like morphology showed fast response and recovery times of 40 and 82 s,respectively.The surface facet of the hexagonal morphology of AlN might be energetically favorable for gas adsorption–desorption for enhanced hydrogen detection.

Chloride Assisted Growth of Aluminum Nitride Nanobelts and Their Enhanced Dielectric Responses

Aluminum nitride （AlN） nanobelts were successfully synthesized in high yield through a chloride assisted vapor-solid process. X-ray diffraction, transmission electron microscopy, and selected area electronic diffraction demonstrate that the as-prepared nanobelts are pure, structurally uniform and single crystalline, and can be indexed to hexagonal wurtzite structure. The micro observations show that there exist no defects in the obtained nanobelts. The growth direction of the nanobelts is along [0001]. The frequency spectra of the relative dielectric constant and of the dielectric loss were measured in the frequency range of 50 Hz to 5 MHz. Analysis of these spectra indicates that the interface in samples has great influence on the dielectric behavior of samples. As compared with AlN micropowders, AlN nanobelts have much higher relative dielectric constant, especially at low frequencies at room temperature.

Preparation of Nanometric Particles of Aluminum Nitride by Pyrolysis of Diethylaluminun Azide

Thermal stabilities of diethylaluminum azide were studied by means of theoretical analysis and expriments.The results have shown that diethylaluminum and azid be heated to vigorous refluxing under vacuum(400 Pa) at temperature as high as 420℃ without incident of explosion and decomposes smoothly during 460 ～580℃ to form nanometric particles of aluminum nitride in the aerosol synthesis reactor. A new way of preparing nanometric parties of aluminum nitrideis found.

Preparation of Ultra-fine Aluminum Nitride in Thermal Plasma

Ultra-fine aluminum nitride has been synthesized by the evaporation of aluminum powder at atmospheric-pressure nitrogen plasma in a hot-wall reactor. The average size of aluminum nitride particle is 0.11 μm measured by scanning electric mirror (SEM), and the purity is at least over 90% evaluated by X-Ray diffraction (XRD). The conversion of Al powder to aluminum nitride is strongly depended on the injection of NH3. Typical experimental parameters such as the feed rate of raw material, the flow rate of ammonia and the position of injecting aluminum powder into the reactor are given.

Synthesis of aluminum nitride nano-powder by decomposed diethylaluminum azide in aerosols

An aerosol process for making aluminum nitride nano-powder by decompositionof single compound diethylalumimm az-ide was described. X-ray diffraction (XRD) and transmissionelectron microscopy (TEM) were used to study characters of the A1N powder. It is shown that theprocess can produce spherical A1N powder with mean particle diameters ranging from 10 to 50 nm at500-800°C. The generated amorphous A1N powder is characterized by a BET (Brunauer-Emmett-Teller)surface area of 103 m2/g and is very reactive to moisture.

Low temperature sintering and performance of aluminum nitride/borosilicate glass

Aluminum nitride （AlN）/borosilicate glass composites were prepared by the tape casting process and hot-press sintered at 950 ℃ with AIN and SiO2-B203-ZnO-Al2O3-Li2O glass as starting materials. We characterized and analyzed the variation of the microstructure, bulk density, porosity, dielectric constant, thermal conductivity and thermal expansion coefficient （TEC） of the ceramic samples as a function of AIN content. Results show that AIN and SiO2-B2O3-ZnO-Al2O3-Li2O glass can be sintered at 950 ℃, and ZnAI204 and Zn2SiO4 phase precipitated to form glass-ceramic. The performance of the ceramic samples was determined by the composition and bulk density of the composites. Lower AlN content was found redounding to liquid phase sintering, and higher bulk density of composites can be accordingly obtained. With the increase of porosity, corresponding decreases were located in the dielectric constant, thermal conductivity and TEC of the ceramic samples. When the mass fraction of AlN was 40%, the ceramic samples possessed a low dielectric constant （4.5-5.0）, high thermal conductivity （11.6 W/（m.K）） and a proper TEC （3.0× 10^-6 K^-1 which matched that of silicon）. The excellent performance makes this kind of low temperature co-fired ceramic a promising candidate for application in the micro-electronics packaging industry.

Graphene-aluminum nitride NEMS resonant infrared detector

The use of micro-/nanoelectromechanical resonators for the room temperature detection of electromagnetic radiation at infrared frequencies has recently been investigated,showing thermal detection capabilities that could potentially outperform conventional microbolometers.The scaling of the device thickness in the nanometer range and the achievement of high infrared absorption in such a subwavelength thickness,without sacrificing the electromechanical performance,are the two key challenges for the implementation of fast,high-resolution micro-/nanoelectromechanical resonant infrared detectors.In this paper,we show that by using a virtually massless,high-electrical-conductivity,and transparent graphene electrode,floating at the van der Waals separation of a few angstroms from a piezoelectric aluminum nitride nanoplate,it is possible to implement ultrathin(460 nm)piezoelectric nanomechanical resonant structures with improved electromechanical performance(450% improved frequency×quality factor)and infrared detection capabilities(4100×improved infrared absorptance)compared with metal-electrode counterparts,despite their reduced volumes.The intrinsic infrared absorption capabilities of a submicron thin graphene–aluminum nitride plate backed with a metal electrode are investigated for the first time and exploited for the first experimental demonstration of a piezoelectric nanoelectromechanical resonant thermal detector with enhanced infrared absorptance in a reduced volume.Moreover,the combination of electromagnetic and piezoelectric resonances provided by the same graphene–aluminum nitride-metal stack allows the proposed device to selectively detect short-wavelength infrared radiation(by tailoring the thickness of aluminum nitride)with unprecedented electromechanical performance and thermal capabilities.These attributes potentially lead to the development of uncooled infrared detectors suitable for the implementation of high performance,miniaturized and power-efficient multispectral infrared imaging systems.

Oxidation mechanism of aluminum nitride revisited

Different from the oxidation kinetics of other nitrides,the oxide layer on AlN can easily reach tens of micrometers at a temperature above 1200 ℃.In the present study,the oxidation mechanism of AlN is investigated through microstructure observation.The analysis indicates that the oxide layer is full of small pores.The formation of pores generates additional surface area to induce further reaction.The reaction thus controls the oxidation in the temperature range from 1050 to 1350 ℃.The oxidation rate becomes slow as the oxide layer reaches a critical thickness.

Octave-spanning visible supercontinuum generation from an aluminum nitride single crystal pumped by a 355 nm nanosecond pulse

Supercontinuum generation（SC） of more than one octave spectrum spanning covering from 400 nm to 820 nm was achieved by pumping a piece of aluminum nitride（AIN） single crystal using a nanosecond 355 nm ultraviolet laser. The AlN with a thickness of ~0.8 mm was grown by an optimized physical vapor transport technique and polished with solidification technology. Compared to previously reported ones, the achieved visible SC exhibited the broadest spectrum spanning from bulk materials pumped by a nanosecond pulse laser. The visible supercontinuum in Al N presents new opportunities for bulk material-based white light SC and may find more potential applications beyond typical applications in integrated semiconductive photoelectronic devices.

Potential acute effects of suspended aluminum nitride (AlN) nanoparticles on soluble microbial products (SMP) of activated sludge

The study aims to identify the potential acute effects of suspended aluminum nitride（Al N）nanoparticles（NPs） on soluble microbial products（SMP） of activated sludge.Cultured activated sludge loaded with 1,10,50,100,150 and 200 mg/L of Al N NPs were carried out in this study.As results showed,Al N NPs had a highly inverse proportionality to bacterial dehydrogenase and OUR,indicating its direct toxicity to the activated sludge viability.The toxicity of Al N NPs was mainly due to the nano-scale of Al N NPs.In SMP,Al N NPs led to the decrease of polysaccharide and humic compounds,but had slight effects on protein.The decrease of tryptophan-like substances in SMP indicated the inhibition of Al N NPs on the bacterial metabolism.Additionally,Al N NPs reduced obviously the molecular weight of SMP,which might be due to the nano-scale of Al N.

In situ fabrication and properties of Al N dispersion strengthened 2024 aluminum alloy

Nanoscaled aluminum nitride （AlN） dispersion strengthened 2024 aluminum alloy was fabricated using a novel approach in which Al-Mg-Cu compacts were partially nitrided in flowing nitrogen gas. The compacts were subsequently consolidated by sintering and hot extrusion. The microstructure and mechanical properties of the material were preliminarily investigated. Transmission electron microscopy and X-ray diffraction results revealed that AlN particles were generated by the nitridation of Al-Mg-Cu compacts. The material exhibited excellent mechanical properties after hot extrusion and heat treatment. The ultimate tensile and yield strengths of the extruded samples containing 8.92vol% AlN with the T6 heat treatment were 675 and 573 MPa, respectively.

Salt-Assisted SHS Synthesis of Aluminium Nitride Powders for Refractory Applications

Powders of aluminum nitride can be prepared by self-sustain high-temperature synthesis (SHS) between aluminum and nitrogen but its high exothermic effect causes melting and evaporation of aluminum and low efficiency of such reaction. A presence of inorganic salt in the starting powder mixture can decrease a heat evolved in the SHS reaction, hinders melting and coalescence of aluminum, and facilitates penetration of nitrogen into interior of a powder bed. Mixtures of alumina powders with different grain sizes and different amounts of aluminum carbonate were subjected to the SHS reaction under 0.05, 0.1 or 1 MPa nitrogen. The powders were composed of aluminum nitride, unreacted aluminum, aluminum oxynitride and in some cases corundum and aluminum oxycarbonate. The finale effects are strongly dependent on the amount of the salt, a grain size of aluminum and a nitrogen pressure.

Aluminum Reduction and Nitridation of Bauxite

The application of bauxite with low Al2O3 content has been studied in this paper and β-SiAlON has been obtained from two kinds of bauxites （Al2O3 content 68.08 mass% and 46.30 mass% respectively） by aluminum reduction and nitridation method. The sequence of reactions has been studied using thermal analysis （TG-DTA）, X-ray diffraction （XRD） analysis and scanning electron microscopy （SEM） with EDS. Compared with carbon thermal reduction and nitridation of aluminosilicates employed presently, the reaction in the system of bauxite-Al-N2 occurs at lower temperature. β-SiAlON appears as one of the main products from 1573K and exists stably in the range of the present experimental temperature. The microstructure of β-SiAlON obtained at 1773 K is short column with 5-10μm observed by SEM.

Chair-like N_(6)^(6-) in AlN_(3) with high-energy density

The search for stable novel polynitrogen clusters has garnered significant attention in the field of energetic materials due to their potential applications as high-energy-density materials.In this study,a chair-like N_(6)^(6-) ring with N-N single bonds in the AlN_(3) compound is theoretically predicted through first-principles calculations in conjunction with an unbiased structure searching method.The predicted AlN_(3) phase exhibits high kinetic and thermodynamic stability,along with a high energy density of 5.04 kJ/g relative to AlN and N_(2) gas.Additionally,its detonation velocity and pressure are estimated to reach 12.93 km/s and 1009.63 kbar,respectively.These values are greater than those of TNT and HMX,positioning it as a promising candidate for high-energy-density materials in the field of explosive combustion.The analysis of electronic properties and the related chemical bonding patterns indicates that the compounds are stabilized by both Coulomb interactions and covalent bonds.More importantly,the calculated formation of enthalpy indicates that the N_(6)^(6-)anions within AlN_(3) can be synthesized by compressing AlN and N_(2) at a moderate pressure(46 GPa).These findings present a viable approach for synthesizing and stabilizing the all-nitrogen N_(6)^(6-)anions.

Low-Temperature Synthesis of Nano-AlN Based on Solid Nitrogen Source by Plasma-Assisted Ball Milling

Plasma-assisted ball milling was carried out on the Al+C3H6N6 system and Al+C_(4)H_(4)N_(4) system,respectively.The phase structure,functional groups and synthesis mechanism were analyzed by XRD and FT-IR,and the differences in the synthesis process of nano-AlN with different solid nitrogen sources were discussed.The results show that C3H6N6 has a stable triazine ring structure,and its chemical bond is firm and difficult to break,so AlN cannot be synthesized directly by solid-solid reaction at room temperature.However,there are a large number of nitrile groups(-CN)and amino groups(-NH_(2))in C_(4)H_(4)N_(4) molecules.Under the combined action of plasma bombardment and mechanical energy activation,C_(4)H_(4)N_(4) molecules undergo polycondensation and deamination,so that the ball milling tank is filled with a large number of active nitrogen-containing groups such as N=,≡N,etc.These groups and ball milling activated Al can synthesize nano-AlN at room temperature,with a conversion rate of 92%.SEM,DSC/TG analysis showed that the powder obtained by ball milling was formed by soft agglomeration of many fine primary particles about 50–80 nm.The surface morphology of the powder was loose and porous,and it had strong activity.After annealing at 800℃,the conversion rate of the Al+C_(4)H_(4)N_(4) system reached 99%.

Boron nitride microribbons strengthened and toughened alumina composite ceramics with excellent mechanical,dielectric,and thermal conductivity properties

Aluminum oxide(Al_(2)O_(3))ceramics have been widely utilized as circuit substrates owing to their exceptional performance.In this study,boron nitride microribbon(BNMR)/Al_(2)O_(3)composite ceramics are prepared using spark plasma sintering(SPS).This study examines the effect of varying the amount of toughened phase BNMR on the density,mechanical properties,dielectric constant,and thermal conductivity of BNMR/Al_(2)O_(3)composite ceramics while also exploring the mechanisms behind the toughening and increased thermal conductivity of the fabricated ceramics.The results showed that for a BNMR content of 5 wt%,BNMR/Al_(2)O_(3)composite ceramics displayed more enhanced characteristics than pure Al_(2)O_(3)ceramics.In particular,the relative density,hardness,fracture toughness,and bending strength were 99.95%±0.025%,34.11±1.5 GPa,5.42±0.21 MPa·m^(1/2),and 375±2.5 MPa,respectively.These values represent increases of 0.76%,70%,35%,and 25%,respectively,compared with the corresponding values for pure Al_(2)O_(3)ceramics.Furthermore,during the SPS process,BNMRs are subjected to high temperatures and pressures,resulting in the bending and deformation of the Al_(2)O_(3)matrix;this leads to the formation of special thermal pathways within it.The dielectric constant of the composite ceramics decreased by 25.6%,whereas the thermal conductivity increased by 45.6%compared with that of the pure Al_(2)O_(3)ceramics.The results of this study provide valuable insights into ways of enhancing the performance of Al_(2)O_(3)-based ceramic substrates by incorporating novel BNMRs as a second phase.These improvements are significant for potential applications in circuit substrates and related fields that require high-performance materials with improved mechanical properties and thermal conductivities.

Characteristics of inclusions in high-Al steel during electroslag remelting process

The characteristics of inclusions in high-A1 steel refmed by electroslag remelting （ESR） were investigated by image analysis, scanning electron microscopy （SEM）, and energy-dispersive spectrometry （EDS）. The results show that the size of almost all the inclusions observed in ESR ingots is less than 5 μm. Inclusions smaller than 3 μm take nearly 75% of the total inclusions observed in each ingot. Inclu- sions observed in ESR ingots are pure AIN as dominating precipitates and some fine spherical Al2O3 inclusions with a size of 1 μm or less. It is also found that protective gas operation and slag deoxidation treatment during ESR process have significant effects on the number of inclusions smaller than 2μm but little effects on that of inclusions larger than 2 μm. Thermodynamic calculations show that AIN inclusions are unable to precipitate in the liquid metal pool under the present experimental conditions, while the precipitation of AlN inclusions could take place at the solidifying front due to the microsegregation orAl and N in liquid steel during solidification.

Thermal conductivity behavior of SPS consolidated AlN/Al composites for thermal management applications

A1N/A1 composites are a potentially new kind of thermal management material for electronic packaging and heat sink applications. The spark plasma sintering （SPS） technique was used for the first time to prepare the A1N/A1 composites, and attention was focused on the effects of sintefing parameters on the relative density, microstructure and, in particular, thermal conductivity behavior of the composites. The results showed that the relative density and thermal conductivity of the composites increased with increasing sintering temperature and pressure. The composites sintered at 1550℃ for 5 min under 70 MPa showed the maximum relative density and thermal conductivity, corresponding to 99% and 97.5 W.m-1.K-1, respectively. However, the thermal conductivity of present A1N/A1 composites is still far below the theoretical value. Possible reasons for this deviation were discussed.