Growth of nitride crystals,BN, AlN and GaN by using a Na flux

Bulk crystals of BN, AlN and GaN were grown by means of Na flux. All these crystals were grown at a temperature of 800°C and a nitrogen pressure of 100 atm, relatively lower than that required by many flux and melt growth methods. High-quality GaN single crystals as large as 0.5 mm were obtained. Furthermore, the oriented GaN crystals were obtained by means of the seeded Na flux method with the addition of oriented AlN (0001) film in the growth ambient. The nucleation of bulk GaN was spatially confined on top of the AlN film and grown with the GaN [0001] axis parallel to the AlN [0001] axis. In addition, the h-BN polycrystals were confirmed by the h-BN (0002) peak of X-ray diffraction (XRD) at 2θ=26.700. A hexagonal grain with a size as large as 2 μm was observed by scanning electron microscopy (SEM). Likewise, AlN crystals were also obtained from Al wires.     

Epitaxial aluminum nitride thin films grown by pulsed laser deposition in various nitrogen ambients

The effect of nitrogen ambient pressure on growth of AlN films has been examined. High-quality epitaxial AlN films were grown on (0001) sapphire substrates using pulsed laser deposition from a sintered AlN target in low nitrogen ambient of 9.0×10⁻⁵ Torr. The orientation of AlN films can be controlled by nitrogen pressure. AlN films are c-axis oriented when grown in a nitrogen pressure of 9.0×10⁻⁵ to 4.0×10⁻² Torr. Film orientation converted to a-axis as nitrogen pressure increased to 4.0×10⁻¹ Torr. The X-ray rocking curves of the AlN (0002) peak became narrower with decreasing ambient pressure and yielded a full width at half-maximum of 0.078°. The N/Al composition ratio increases with nitrogen pressure.     

The influence of crystallization temperature and boron concentration in growth environment on its distribution in growth sectors of type IIb diamond

Type IIb diamond single crystals of size up to 8 mm were grown using the temperature-gradient method at high static pressure and temperature in Fe–Al–C system with added boron. The influence of crystallization temperature and boron concentration in growth environment on its distribution in growth sectors has been investigated using optical techniques. Variation from cubo-octahedral to octahedral habit with an increasing added amount of boron has been noted and explained.     

In-situ observation of AlN formation from Ni-Al solution using an electromagnetic levitation technique

Aluminum nitride is a promising substrate material for AlGaN-based UV-LED. In order to develop a robust growth processing route for AlN single crystals, fundamental studies of solution growth experiments using Ni-Al alloy melts as a new solution system were performed. Al can be stably kept in solution the Ni-Al liquid even at high temperature; in addition, the driving force of the AlN formation reaction from solution can be controlled by solution composition and temperature. To investigate AlN crystal growth behavior we developed an in situ observation system using an electromagnetic levitation technique. AlN formation behavior, including nucleation and growth, was quantitatively analyzed by an image processing pipeline. The nucleation rate of AlN decreased with increasing growth temperature and decreasing aluminum composition. In addition, hexagonal c-axis oriented AlN crystal successfully grew on the levitated Ni-40 mol%Al droplet reacted at low driving force (1960 K), on the other hand, AlN crystal with dendritic morphology appeared on the sample with higher driving force (Ni-50 mol%Al, 1960 K). Thus, the nucleation rate and crystal morphology were dominated by the driving force of the AlN formation reaction.     

High pressure sintering of cubic boron nitride compacts with Al and AlN

Cubic boron nitride (cBN) compacts, using 15 wt.% Al and 20 wt.% AlN respectively as additives, were sintered in the temperature range of 1300–1700 °C for 20 min under high pressure of 5.0 GPa. The hardness, microstructure, phase composition and cutting performance of the high pressure sintered samples were investigated. A liquid phase sintering and reaction process was observed in the cBN–Al system, which leads to the formation of AlN and AlB₂ as confirmed by X-ray diffraction (XRD) in the sintered compacts. Scanning electron microscopy (SEM) analysis shows that the samples have a homogeneous microstructure. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. While in the cBN–AlN system, AlN grains agglomerate heavily at temperature below ~ 1500 °C. As the sintering temperature increasing, Al₂O₃ appeared and the AlN agglomeration disappeared gradually. A highest cBN–AlN composite hardness of 29 GPa was achieved when sintered at 1600 °C. Turning tests showed that cBN compacts with 15 wt.% Al as the additive has a longer tool life as compared to that with 20 wt.% AlN. Our results indicated that cBN–Al system is more favourable to obtain well-sintered cBN compacts comparing with the cBN–AlN system.     

Morphology of cubic boron nitride crystals synthesized using (Fe,Co, Ni)–(Cr,Mo)–Al alloy solvents under pressure

The growth region of the cubic boron nitride (cBN) using (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents were presented in the pressure range of about 4–6 GPa and the temperature range between 1200 and 1700 °C. The minimum pressure for cBN formation was confirmed at about 4–4.1 GPa for both (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents. Based upon this pressure–temperature condition of the cBN growth region, the morphology of cubic boron nitride crystals was examined under various compositions of the solvents. The morphology of cBN crystals was affected by not only the reaction pressure and but also the composition of the solvents. It was found that the variation of alloy composition provides various morphologies and grain sizes of cBN crystals.     

A Novel Method of Synthesis of Nanostructured Aluminum Nitride Through Sol‐Gel Route by In Situ Generation of Nitrogen

Nanostructured Aluminum Nitride (AlN) has been prepared by carbothermal reduction followed by nitridation (CTRN) of alumina gel over a temperature range 1200°C–1350°C and time period of 30 min to 3 h. Before heat treatment the gel is repeatedly evacuated and purged with ammonia. The nanopores of the gel are filled with ammonia which acts as a source of in situ nitrogen at heat‐treatment temperature. Dextrose also decomposes at the reduction temperature and generates ultrafine carbon. The stability diagram of the carbon saturated Al–N–O system is constructed and it shows that extremely low partial pressure of oxygen is required for the stability of AlN. The ultrafine carbon as well as hydrogen from the cracking of ammonia is not sufficient to create the extremely low partial pressure of oxygen required for the stabilization of AlN. So the sample is heat treated in charcoal boat in nitrogen atmosphere to achieve an extremely low partial pressure of oxygen required for the formation of AlN. The material has been characterized through XRD, FESEM, and HRTEM analyses. The spherical particle size of AlN is obtained ～21 nm.     

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.     

Multiple twinning and nitrogen defect center in chemical vapor deposited homoepitaxial diamond

Homoepitaxial diamond films were grown on polished {100} faces of single crystal type IIa diamond substrates using microwave plasma assisted chemical vapor deposition system. 14 homoepitaxial diamond films were grown under a variety of substrate temperatures (1000–2000°C), methane concentration (1–6% in hydrogen gas) and processing pressure (60–200 Torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is incorporated as a singly substitutional impurity (P1-defect center) and the nitrogen concentration is in the range 10–100 parts per million (ppm). The substitutional nitrogen concentration in homoepitaxial diamond was observed to decrease with increasing substrate temperature. Multitwin percentages of all grown diamonds derived from EPR spectra are correlated with the growth parameter α, which is simply the growth velocity along the 〈100〉 direction divided by the growth velocity along the 〈111〉 direction. With the aid of multitwin morphology and multitwin percentages derived from EPR, we describe conditions under which a twin-free and low defect single crystal diamond can be grown from the vapor phase on the {100} oriented substrates.     

Effect of nitrogen pressure on the microstructure,mechanical and electrochemical properties of CrAlN coatings deposited by filter cathode vacuum arc

CrAlN coatings were prepared on Al–Si alloys using filter cathode vacuum arc deposition technique with nitrogen as the reactive gas and Cr₂₅Al₇₅ alloy target as the arc source. The effect of nitrogen pressure on the microstructure, mechanical properties and electrochemical properties of the coatings had been systematically studied. The results showed that the composition, structure and performance of the CrAlN coating depended on the nitrogen pressure. As the nitrogen pressure increased, the Al and Cr content decreased while the N content increased slowly in the coating. Meanwhile, the phase structure gradually changed from AlN phase to CrN phase. The hardness of the CrAlN coating increased significantly with the increase of nitrogen pressure from 0.04 to 0.06 Pa due to the formation of CrN phase and grain refinement. However, further increasing the nitrogen pressure to 0.07 Pa, the hardness was reduced owing to the deterioration of the surface quality caused by target poisoning. Moreover, the adhesion strength of the coating gradually decreases, and the corrosion resistance of the CrAlN coating first increased and then decreased with increasing the nitrogen pressure. The CrAlN coating deposited at a nitrogen pressure of 0.05 Pa had the best corrosion resistance, with the highest polarization resistance, charge transfer resistance and pore resistance, which was related to the combined effect of great compactness and AlN-dominant phase structure in the coating.     

Effect of Al addition on the single and cyclic ablation properties of C/C–SiC composites

This article focuses on the damage behavior and mechanism of aluminum addition on reactive melt infiltrated C/C–SiC composites in single and cyclic ablation environments. Plasma ablation tests were performed on C/C–SiC composites containing 20 wt % and 40 wt % aluminum respectively. Coupled with TMA, XRD, SEM and EDS, the results showed that composites with 40 wt % Al had better ablation resistance during the cyclic ablation, while the composites with 20 wt % Al had excellent ablation damage resistance during a single ablation. This difference was due to higher number of microcracks formed inside the composites containing 40 wt % Al than 20 wt % Al, the lower specimen surface temperature during ablation, and the thermal stresses can be released by pore crack expansion during gas reciprocal loading. While in the single continuous loading of gas, the 20 wt % Al composite formed a protective oxide layer with smaller pores and fewer gas and oxygen entry channels, resulting in good resistance to ablation.     

Verneuil growth of TiO2 (rutile) crystals of large size and low dislocation density at low gas flow rate

Large (30 mm in diameter) TiO₂ (rutile) single crystals with low dislocation density were grown by the Verneuil method using a single crystal furnace designed and improved by the authors. The structure of the burner was optimized by numerical simulation analysis so that the crystal could be grown at a low gas flow rate. The investigation of the growth process parameters (i.e., growth rate, outer flow rate of O₂, inner flow rate of O₂, and the increment of H₂ flow rate) shows that the inner flow rate of O₂ and the increment of H₂ flow rate have the strongest influence on the crystal growth process. On this basis, other growth parameters (growth rate, outer flow rate of O₂) were also optimized. Compared with the conventional Verneuil method, the crystal can be grown at a considerably low gas flow rate (40–50% lower) with the method in this work, which reduces the melt turbulence impacted by gas flow, enabling a steady and clear solid-liquid interface and improving the crystal quality. The optimum growth conditions are for the growth rate of 6 mm/h, O₂ outer flow rate of 3.5 L/min, O₂ inner flow rate of 5.5 L/min, and increment of H₂ flow rate of 0.1 L/4 min. The etch pit density of the rutile crystals is 3.29 × 10⁴ cm^?2, an order of magnitude lower than that of the crystals grown by the conventional Verneuil method. The optical properties of the crystal are comparable to those grown by the floating zone method. Especially, it is easier to obtain a larger crystal size with lower production costs. Our results provide a possible route for industrializing the Verneuil production of large, high-quality and low-cost rutile single crystals.     

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.     

Growth of diamond with Zr-containing molten metal solvents and metal elements as incorporated impurities

Single crystals of diamond were grown using high pressure–high temperature conditions in alloy solvents with Zr added as a nitrogen getter. Problems in the growth process caused by the Zr addition were observed. With a reaction container made of SiO₂ or SiO₂-based material, its nitrogen-gettering ability was markedly decreased. MgO and Al₂O₃ were found to be suitable materials for the container to avoid this problem. Diamonds grown from Zr-containing solvents always had highly developed {111} facets. Zr of 3.8 wt.% in the solvent is sufficient to grow type 2-a diamond. Concentrations of Zr as impurity in single crystals of diamond grown in the Zr-containing solvents have been evaluated as of the order of 0.1 ppm.     

Ultrasonic-assisted wetting and soldering of AlN ceramic by using a nonactive solder (Sn9Zn) in air

AlN ceramic was successfully wetted and then joined with nonactive Sn9Zn eutectic solder assisted by ultrasonication in air. The effect of ultrasonic time on the formation of joint was studied. Results indicated that the defect-free joint can be obtained at an ultrasonic time of 5 s. Two regions, namely, AlN/Sn (s,_s) and AlN/Zn (s,_s), were found in the bonding interface. Zn and O accumulated in the AlN/Sn (s,_s) interface. An amorphous and nanocrystalline layer of ZnO formed in the hard-wet AlN surface. And Zn (s,_s) directly bonded with AlN. The low temperature and fast bonding of the AlN was attributed to the high pressure and temperature caused by cavitation effect. The shear strength of the joint increased from 10.6 MPa to 30.7 MPa when the ultrasonic treatment time increased from 5 s to 150 s. With the prolongation of ultrasonic time, more AlN ceramic particles entered the solder and acted as the reinforcing phase.     

Reaction sintering of AlN–AlON composites

Sintering behavior of three different compositions in the AlN–Al₂O₃ system using Y₂O₃ as a sintering aid was investigated. Samples with various ratios of AlN/Al₂O₃ were sintered in nitrogen atmosphere using a gas pressure furnace in the temperature range 1750–1950 °C. The densification of the samples was studied by shrinkage and relative density measurements. Results showed that samples containing 1 and 70 wt.% alumina were sintered to near theoretical density at 1800 °C; whereas the sample with 20 wt.% alumina never reached densities higher than 93% in the temperature range considered. It was found that the AlN/Al₂O₃ ratio and the sintering temperature had a great influence on the microstructure and crystalline phases present in the samples, namely, AlN, γ-AlON, 27R, and YAG. In the sample with 20 wt.% alumina, porosity formation prevented further densification. These porosities were probably due to the release of oxygen during sintering.     

Effects of additive Al on the HPHT diamond synthesis in an Fe–Mn–C system

In this paper, diamond crystals with perfect octahedron shape were synthesized in an Fe–Mn–C system with and without additive Al at 5.6–5.8 GPa and 1600–1800 K. The color, morphology, nitrogen impurity concentration and inclusions of synthetic diamond crystals were characterized by optical microscope, scanning electron microscope, microinfra-red spectrometer, and Mössbauer spectrometer, respectively. The results have shown that the additive Al has a significant influence on the color of crystals, growth rate, and inclusions etc. The color of diamond crystals synthesized without additive Al is yellow, while that with additive Al becomes light and nearly disappears. The growth rate of synthetic diamond crystals without additive Al is higher than that with additive Al. The concentration of nitrogen impurity in diamond crystals without additive Al is higher than that with additive Al. The inclusions in diamond crystals synthesized without additive Al is mainly Fe and Fe₃C, while that with additive Al is Fe–Al alloy.     

Two‐Step Carbothermal Synthesis of AlN–SiC Solid Solution Powder Using Combustion Synthesized Precursor

In the present work, a two‐step carbothermal reduction method is employed to prepare the AlN–SiC solid solution (AlN–SiCss) powders by using a combustion synthesized precursor. The precursor is prepared by low‐temperature combustion synthesis (LCS) method using a mixed solution of aluminum nitrate, silicic acid, polyacrylamide, glucose, and urea. The synthesized LCS precursor exhibits a porous and foamy uniform mixture of Al₂O₃ + SiO₂ + C consisting of flaky particles. The carbothermal reduction in the LCS precursor is carried out in two steps. First, the precursors are calcined at 1600°C in argon for 3 h. Subsequently, the precursors are further calcined at 1600°C–1900°C in nitrogen for 3 h. The results indicate that the precursor calcined at and above 1850°C in nitrogen for 3 h yields the single‐phase AlN–SiCss powders. The synthesized AlN–SiCss powder exhibits near‐spherical particles with diameter of 200–500 nm. The experimental and thermodynamical results reveal that the formation of AlN–SiCss occurs via the diffusion of AlN into SiC by virtue of formation of a highly defective β′ intermediate during the second step reaction.     

Combustion Based Synthesis of AlN Nanoparticles Using a Solid Nitrogen Promotion Reaction

Combustion based synthesis of AlN nanoparticles using the “solid nitrogen” promotion reaction was investigated in Al₂O₃ + 3Mg system in nitrogen atmosphere. A controlled amount of Mg + 0.5NH₄Cl mixture as a solid source of nitrogen was blended with the Al₂O₃ + 3Mg starting system and the synthesis reaction of AlN nanoparticles was conducted using the exothermic heat of the entire reaction system. The resulting AlN nanoparticles were characterized by X‐ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR), PL spectroscopy, field‐emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. The analysis results confirmed that single phase and crystalline AlN nanoparticles with an average size of 50–500 nm were obtained from the developed approach. Photoluminescence spectra of AlN nanopowders under the excitation of 230–270 nm UV light revealed that AlN emits yellow‐red light having a wavelength near to 590 nm. The chemistry of the combustion process is discussed and the basic reactions that led to the formation of AlN are presented.     

Effects of Al particle size and nitrogen pressure on AlN combustion synthesis

This study investigates the combustion synthesis of AlN fibers using an NH₄Cl additive and reports the effects of Al particle size (3, 30, and 180 µm) and N₂ pressure (0.10, 0.25, and 0.50 MPa) on the purity and morphology of AlN fibers. The combustion temperature was directly measured during the synthesis to elucidate the formation mechanism of the AlN fibers. The phase purity and morphology of the products were studied using X-ray diffraction and scanning electron microscopy, respectively. When the particle size of Al was reduced from 180 to 3 µm, the purity of the AlN product increased significantly owing to the large reaction area, which increased the combustion temperature. Furthermore, lower N₂ pressures enhanced the formation of AlN nanofibers due to the accelerated gasification of Al. The optimum values of the particle size of Al and the N₂ pressure for the formation of high-purity AlN nanofibers were found to be 3 µm and 0.10 MPa, respectively.