Zinc oxide nanostructure decorated amorphous carbon nanotubes: An improved field emitter

Amorphous carbon nanotubes (aCNTs), synthesized through a low-temperature simple method have been decorated with chemically synthesized different zinc oxide (ZnO) nanostructures like nanocones and nanoflowers. The as-prepared samples have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM). It has been found that attachment of ZnO nanostructure can effectively enhance the field electron emission properties of the aCNTs. Modification with ZnO nanoflower-like structure can reduce the turn-on field of aCNTs from 8.91 to 3.64 V/μm. The results have been explained in terms of increased roughness which in-turn led to large enhancement of the local electric field and thus facilitated electron emission from this hybrid emitter.     

Synthesis and characterization of UV-curable nanocellulose/ZnO/AlN acrylic flexible films: Thermal,dynamic mechanical and piezoelectric response

This work is aimed at fabricating nanocomposites based on zinc oxide (ZnO) nanostructures and nanocellulose dispersed in a UV-cured acrylic matrix (EC) for application as functional coatings for self-powered applications. Morphological, thermal, and dynamic mechanical properties of the nanocomposites were characterized by X-Ray diffractometry (XRD), scanning electron microscopy, and differential scanning calorimetry. The piezoelectric behavior was evaluated in terms of root mean square (RMS) open circuit voltage, at different accelerations applied to cantilever beams. The generated voltage was correlated with ZnO nanostructures morphology, aluminum nitride film integration on the beam and proof mass insertion at the tip. Nitride layer increased the RMS voltage from 1 to 2.4 mV up to 3.9 mV (using ZnO nanoflowers). As confirmed by XRD analyses, the incorporation of ZnO nanostructures into the acrylic matrix favored an ordered structural arrangement of the deposited AlN layer, hence improving the piezoelectric response of the resulting nanocomposites. With proof mass insertion, the output voltage was further increased, reaching 4.5 mV for the AlN-coated system containing ZnO nanoflowers.     

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al₂O₃). The effects of Al₂O₃ particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al₂O₃ powder than with coarse Al₂O₃ powder. The cubic AlN may be the product of Al₂₃O₂₇N₅ synthesized from Al₂O₃ and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.     

Simultaneous growth of well-aligned diamond and graphitic carbon nanostructures through graphite etching

A hot filament chemical vapor deposition process based on hydrogen etching of graphite has been developed to synthesize diamond and graphitic carbon nanostructures. Well-aligned diamond and graphitic carbon nanostructured thin films have been synthesized simultaneously on differently pretreated silicon substrates in a pure hydrogen plasma. Graphitic nanocones, diamond nanocones and carbon nanotubes were selectively grown on uncoated, diamond and nickel pre-coated silicon substrates, respectively, in a single deposition process. The nanocones are solid cones with submicron scale roots and nanometer-size sharp tips. The nanotubes are hollow tubes with outer diameter of approximately 50 nm. The orientation of the well-aligned carbon nanostructures depends on the direction of the electric field at the samples surface. Nucleation and growth of diamond on the nanocones were further investigated under similar conditions without plasma. Diamond nanocomposite films have been obtained by depositing a nanocrystalline diamond film on the layer of diamond nanocones.     

铝源孔容和焙烧升温过程对碳热还原法制备氮化铝粉体的影响

碳热还原氮化法是大规模制备高纯度氮化铝（AlN）粉体的主要方法,通过微反应器制备不同孔容的铝源,系统探究了前体孔容和微观形貌对AlN粉体产物的影响,并通过动力学模拟验证了筛选出的前体的活性。同时对氮化反应升温过程的影响也做了探究,最终通过对前体和焙烧升温过程的优化,得到纯度99%以上的AlN粉体,其平均粒径约为150 nm,O元素含量为0.55%。     

Synthesis and growth mechanism of aluminum nitride nanowires via a chloride-assisted chemical vapor reaction method

We report a large scaled fabrication of AlN nanowires via a chloride-assisted chemical vapor reaction technique at 1100?°C in flowing N₂ atmosphere using aluminum powders as starting materials. The as-obtained hexagonal AlN nanowires had the length of hundreds of microns and diameter of 20–100?nm, and indicated a single crystalline characteristic. The yield production was significantly increased with ammonium chloride and aluminum chloride addition because of the formation of intermediate gaseous AlCl and HCl. The addition of ammonium chloride and aluminum chloride also promoted the formation of ferric chloride, which served as the catalyst and further facilitated the growth of AlN nanowires. The vapor–liquid–solid and vapor–solid growth mechanism are proposed and discussed in details.     

Radial AlN nanotips on carbon fibers as flexible electron emitters

A facile catalyst-free approach using a simple thermal transport method has been developed to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3 V μm⁻¹, a low threshold electric field of 1.5–2.5 V μm⁻¹, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.     

有机羧酸改性氮化铝粉体的抗水解性能

采用有机羧酸和PEG作为表面活性剂对工业AlN粉体进行表面改性处理,研究了改性前后AlN粉体的抗水解特性以及表面改性机制。研究结果表明,AlN表面与水分子发生化学反应,导致溶液的pH值迅速提高,表面包覆有机羧酸可有效地改善AlN粉体的抗水解性能;AlN粉体在水中浸泡48h后,氮含量基本保持不变,除了AlN晶相外,没有其他晶相出现,且改性粉体在高剪切应力的水基球磨介质中也保持较强的稳定性。     

Shaping potentialities of aluminum nitride polymeric precursors: Preparation of thin coatings and 1D nanostructures in liquid phase

The present paper is focused on the synthesis of a series of poly[N-(alkylimino)alanes] of the type [HAlNR]_n as preceramic polymers for the preparation of aluminum nitride (AlN). Polymers were characterized by means of Fourier transform infrared spectra (FT-IR), liquid-state ¹H and ¹³C NMR spectrometry and elemental analyses. The polymers were prepared in different physical states going from viscous liquid to solid (soluble and/or fusible) compounds with the decrease of the carbon content in the polymer chain. Such properties offer potentialities in the preparation of complex forms of ceramics including thin coatings and 1D nanostructures. AlN thin coatings and 1D nanostructures were obtained from a solution of poly[N-(isopropylimino)alane] in toluene followed by heat-treatment in flowing ammonia up to 1000 °C resulting in a ceramic yield of 50.6%. Subsequent heat-treatment to 1800 °C in flowing nitrogen allowed the production of crystalline AlN coatings and nanorods identified by Raman spectrometry and X-ray diffraction.     

磷酸二氢铝和磷酸球磨改性氮化铝粉末工艺研究

采用机械球磨法,以磷酸二氢铝和磷酸为改性剂,制备了抗水解氮化铝粉末,并研究了改性氮化铝粉末在水基球磨过程中的稳定性。通过X射线衍射(XRD)和氮含量测定对改性前后氮化铝粉末进行了表征,并讨论了磷酸二氢铝和磷酸的加入量、球磨时间和球料质量比对改性效果的影响。结果表明:在磷酸二氢铝和磷酸的添加质量分别为氮化铝质量的1%和2.5%、球磨时间为2 h、球料质量比为3∶1的条件下,氮化铝的改性效果最佳;改性氮化铝粉末在60℃水中浸泡24 h后,其氮质量分数为32.97%,且其X射线衍射谱图中未发现氢氧化铝相,说明其抗水解能力得到显著提高;改性氮化铝粉末在水中高速球磨16 h后,其氮质量分数约为32%,氮化铝悬浮液的pH约为6,说明其在水基球磨过程中具有较好的稳定性。     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.     

Self-Propagating High-Temperature Synthesis of Aluminum Nitride under Lower Nitrogen Pressures

The synthesis of aluminum nitride (AlN) via self-propagating high-temperature synthesis (SHS) was attempted, using aluminum powder that was mixed with AlN powder as a diluent. The AlN content in the reactant was varied over a range of 30%–70%, and the nitrogen pressure was varied over a range of 0.1–1.0 MPa. The SHS reaction that was performed using a reactant that contained 50% AlN diluent, under a nitrogen-gas pressure of 0.8 MPa, yielded the highest conversion ratio of aluminum powder to AlN powder. A mechanism for the reaction of aluminum with nitrogen gas during the SHS process was discussed, based on observations of the microstructures of the reaction zone and products.     

Sintering and mechanical properties of titanium diboride with aluminum nitride as a sintering aid

Titanium diboride (TiB₂) was hot-pressed at 1800 °C with aluminum nitride (AlN) as a sintering aid. The presence AlN had a strong influence on the sinterability and mechanical properties of the TiB₂. When a small amount (⩽5 wt.%) of AlN was added to the TiB₂ the titania (TiO₂) present on the surface of the TiB₂ powder was eliminated by a reaction with AlN to form TiN and Al₂O₃. The elimination of TiO₂ markedly improved the sinterability and consequently the mechanical properties of TiB₂. However, when too much AlN was added (⩾10 wt.%), the sinterability and the mechanical properties decreased, apparently due to the remaining unreacted AlN.     

Synthesis and photoluminescence characteristics of blue‐green luminescent aluminum oxynitride

Aluminum oxynitride (AlON), which can be regarded a nitrogen‐stabilized cubic γ‐Al₂O₃, has attracted attention in terms of its good mechanical, chemical, and optical stability. Because of its optical inertness, however, photoluminescence (PL) emission from nominally pure AlON has not been carefully investigated and evaluated. In this work, we prepared visibly luminescent AlON by nitridation of γ‐Al₂O₃ under N₂ atmosphere without adding aluminum nitride (AlN) using a high‐frequency induction heating unit. The resulting AlON exhibits a broad PL emission in the blue/green spectral region under excitation with light of ~260 nm. In the luminescent AlON sample, the excitation and emission events will occur at different sites; the electron transfer from the excitation site to the emission site is preceded by the radiative recombination process. It has also been found that the PL peak wavelength shows an anomalous blue shift by ~50 nm with increasing temperature from 78 to 500 K. The observed temperature dependent PL characteristics are governed by thermalization among multiple emitting levels. Aluminum vacancies and oxygen vacancies, both of which are introduced into the crystalline lattice during nitridation without the presence of AlN, are very likely candidates for the excitation and emission centers, respectively. Hence, the present direct nitridation method provides a simple and effective way to add an additional optical functionality to otherwise optically inactive AlON.     

Facile synthesis of powder-based processing of porous aluminum nitride

The use of aluminum nitride (AlN) has recently begun to be explored for advanced functional applications. For some particular applications, it is more advantageous to use porous-structured AlN, simply because it can provide a greater surface area and higher permeability. However, porous or bulk AlN is very difficult to achieve due mainly to its high melting point (2200 °C). This study proposes a new novel processing method to synthesize porous AlN through a complete transformation from porous aluminum (Al) using a remarkably low nitriding/sintering temperature (620 °C) as opposed to only the surface nitride AlN-Al core composite systems reportedly at or above 1000 °C in the literature. A uniform microporous bead structure of porous AlN with a mean pore size of 74.0 ± 27.7 μm was obtained that also contained nanoparticles ranging from 80 to 230 nm that covered the surface.     

Dynamic mechanical properties of polystyrene–aluminum nitride composite

A composite of aluminum nitride (AlN) particles dispersed around polystyrene matrix particles was synthesized in this study. The purpose of using this microstructure is to improve the thermal properties of a polymer at a low filler content with a minimal increase in the dielectric constant of the polymer composite to meet the material requirements for electronic packaging. The dynamic mechanical properties of this type of polystyrene–AlN composite were investigated here. The experimental results indicate that the dynamic mechanical property of the polystyrene–AlN composite is a function of the polystyrene particle size, AlN filler concentration, and temperature under this dispersion state. The addition of an AlN concentration into polystyrene increases both the storage modulus and the α‐transition temperature. The smaller polystyrene particle size gives a higher storage modulus and damping peak. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1348–1353, 2000     

A Versatile Route for the Synthesis of Nickel Oxide Nanostructures Without Organics at Low Temperature

Nickel oxide nanoparticles and nanoflowers have been synthesized by a soft reaction of nickel powder and water without organics at 100 °C. The mechanism for the formation of nanostructures is briefly described in accordance with decomposition of metal with water giving out hydrogen. The structure, morphology, and the crystalline phase of resulting nanostructures have been characterized by various techniques. Compared with other methods, the present method is simple, fast, economical, template-free, and without organics. In addition, the approach is nontoxic without producing hazardous waste and could be expanded to provide a general and convenient strategy for the synthesis of nanostructures to other functional nanomaterials.     

Hydrolysis-induced simultaneous foaming and coagulation casting of secondary aluminum dross aqueous suspension

The reactive AlN, heavy metal ions, and salts in secondary aluminum dross make them harmful to the environment and difficult to handle, and therefore categorized as hazardous solid waste. A novel, simple, and versatile technique has been proposed here to prepare hierarchically porous ceramics using secondary aluminum dross as the sole raw material, namely, the hydrolysis-induced simultaneous foaming and coagulation casting method. As the main component, Al and AlN could be hydrolyzed to generate aluminum hydroxide sol, which can form three-dimensional network and lead to the outstanding compressive strength of green body (7.03 MPa). Gas release during hydrolysis, high-temperature oxidation of AlN, and metallic Al based on Kirkendall effect could account for the final multilevel pores. The utilization of harmful components in secondary aluminum dross, supplys an alternative colloidal forming way for porous green body with complex shape when combined with other ceramic powders or inorganic solid waste. Low sintering shrinkage of 1.38%–3.60% for hierarchically porous ceramics offers this highly effective and low-cost method to be easily promoted for industrial production.     

Fabrication and characterization of aluminum nitride thick film coated on aluminum substrate for heat dissipation

For effective heat dissipation in high-power LED applications, aluminum nitride (AlN) thick films as thermally conductive dielectric layers were developed, which were deposited on an Al substrate by aerosol deposition (AD). The aerosol-deposited AlN thick films on Al substrates have advantages over conventional polymer-based dielectric substrates or ceramic substrate mounted heatsink systems including an epoxy adhesive, such as excellent heat dissipation capacity and low thermal resistance. AD is an effective method to fabricate high-quality AlN thick film without the Al₂O₃ phase because the film is formed at room temperature. Highly dense and well-adhered, pure AlN thick films with thicknesses up to 30 µm were deposited on an Al substrate. AlN-Al₂O₃ and AlN-polyvinylidene fluoride (PVDF) composite films were also deposited on an Al substrate in order to investigate the effect of Al₂O₃ and polymer on the microstructure and thermal properties. Among the films, pure AlN thick film exhibited the highest dielectric strength, the highest thermal conductivity, and the lowest thermal resistance. Therefore, it can be expected that the aerosol-deposited AlN thick film on Al substrate could be used as a powerful heatsink.     

The effects of aluminum on structural development of a carbon derived from phenolic resin

The catalytic graphitization of a phenolic resin carbon by aluminum powder was carried out under an N₂ atmosphere. The following catalytic mechanism is proposed: In the heating process up to 2000°C, AlN is formed at first by the reaction of the aluminum with the N₂ gas. At approximately 2000°C, the AlN begins to react with the carbon matrix to form an intermediate, A₁₄C₃. The Al₄C₃ penetrates into the surrounding carbon matrix through an Al₄C₃ formation-decomposition sequence, leaving a graphite crystal behind. As the penetration proceeds, smaller graphite crystals are formed because of the division of the Al₄C₃ particles. When a most finely divided Al₄C₃ is produced by the further penetration, instead of the graphite, only turbostratic carbon is formed by this catalytic process.