Preparation of nano-and submicrosized aluminum oxide fibers by the dehydration of a hybrid gel

The spontaneous growth of hybrid (organoinorganic) microfibers on the surface of a gel prepared by dispersing an organoinorganic nanocomposite in water and subjected to dehydration in air, has been investigated. Aluminum oxide nano-and submicrofibers have been prepared by the thermal treatment of the microfibers.     

Preparation and lithium doping of gallium oxynitride by ammonia nitridation via a citrate precursor route

Gallium oxynitride, isostructural to hexagonal gallium nitride (h-GaN), was obtained by ammonia nitridation of a precursor prepared from the addition of citric acid to an aqueous solution of gallium nitrate. Gallium oxynitride produced at 750 °C had a small amount of gallium vacancies, and was formulated as (Ga_0.89□_0.11) (N_0.66O_0.34) where the symbol □ stands for gallium vacancy. Both the gallium vacancies and oxygen substituted for nitrogen were randomly distributed within the structure. The amount of vacancies decreased with nitridation temperatures in the range of 750-850 °C. Approximately, 10 at% Li⁺ was doped into the gallium oxynitride, using a similar preparation with the additional presence of lithium nitrate, resulted in the random substitution of Ga³⁺ in an atomic ratio of Li/Ga<1 at 750 °C. Oxygen was codoped with lithium and substituted nitrogen in the wurtzite-type crystal lattice. These substitutions reduced the electrical conductivity in the gallium oxynitride semiconductor. A new oxynitride, Li₂Ga₃NO₄, was also obtained with Li₂CN₂ impurity using similar preparations from a mixture of Li/Ga?1. The crystal structure was isostructural with h-GaN, and was refined as P6₃mc with a=0.31674(1) nm, and c=0.50854(2) nm. The Ga and Li occupancies at the 2b site were refined to be 0.6085 and 0.3915, respectively, assuming that the other 2b site was randomly occupied with 1/5O and 4/5N. When the new compound was washed for over 1 min for the removal of Li₂CN₂ impurities, it was decomposed to a mixture of α-GaOOH and α-LiGaO₂. The as-prepared product with Li/Ga=1 showed the highest intensity in yellow luminescence among the products under excitation at 254 nm.     

Preparation of an aluminum oxo-hydroxide gel in organic medium

A new method has been developed to prepare aluminum oxohydroxide containing spinnable material and gel. Partial hydrolysis of Al(NO₃)₃·9H₂O in 1-propanol at 78°C produces a spinnable, viscous mixture. The role of the propanol in the hydrolysis proved to be to decrease the polarity of the solvent. In this medium the dissociation of nitric acid is driven back and it decomposes to nitrous gases resulting in the increase of pH in the solution. The conditions have been optimized to obtain the highest hydrolysis degree and to avoid precipitation of basic aluminum nitrate. The resulting optimal temperature is 76–80°C, the time needed is at least 15 h in the case of a laboratory scale preparation. Increasing the ratio of propanol: water and the concentration of Al(III) to the maximum value, leads to the decomposition of 54% of the initial amount NO ₃ ^– ion. By careful drying, the decomposition continues to about 70% and a solid foam comes into existence from the viscous mixture. This foam is able to swell in water, the degree of swelling in mass is about 10. The drying of swollen gel was examined. The spinnable mixture most likely contains polymer chains built up by H-bonds, the foam and the gel probably contain platelets.     

Preparation and characterization of alumina supported silicalite membranes by sol–gel hydrothermal method

Two types of unsupported zeolites (silicalite-1 and silicalite-2) and porous alumina discs supports were prepared by the hydrothermal sol–gel synthesis method. The influence of the raw materials used as SiO₂ source, the temperature of the thermal treatment and the presence of the ceramic support on the crystallization of zeolites were studied. The reaction products were characterized by X-ray diffraction (XRD), IR spectroscopy (IR) and scanning electron microscopy (SEM) studies. The SiO₂ source had a significant effect on the final zeolite obtained: the use of colloidal silica sol (ZCS) as SiO₂ source in the synthesis led to ZSM-11 (silicalite-2) crystals, while the sodium silicate solution (ZSS) produced the ZSM-5 (silicalite-1) type. The presence of the alumina support influences the crystallization process of ZSM-5, as it improves nucleation and the ordering of the crystals.     

Promotion of hydrogen release from ammonia borane with magnesium nitride

Hydrogen release from ammonia borane (NH(3)BH(3), AB) can be greatly promoted by mechanical milling with magnesium nitride (Mg(3)N(2)). For example, a post-milled 6AB/Mg(3)N(2) sample started to release hydrogen from ～65 °C and gave a material-based hydrogen capacity of ～11 wt% upon heating to 300 °C. In addition to the improved dehydrogenation kinetics, the 6AB/Mg(3)N(2) sample also showed satisfactory performance in suppressing the volatile byproducts. X-ray diffraction, Fourier transform infrared spectroscopy and solid-state (11)B MAS NMR, as well as a series of designed experiments, were carried out to gain mechanistic understanding of the property improvements that arise from addition of Mg(3)N(2). Our study found that the formation of 3Mg(NH(2)BH(3))(2)·2NH(3), which is in single or mixed amidoborane ammoniate phases in nature, is an important mechanistic step in the dehydrogenation process of the 6AB/Mg(3)N(2) sample.     

Preparation of films from aluminum acetylacetonate by plasma sputtering

Aluminum acetylacetonate has been reported as a precursor for the deposition of alumina films using different approaches. In this work, alumina‐containing films were prepared by plasma sputtering this compound, spread directly on the powered lowermost electrode of a reactor, while grounding the substrates mounted on the topmost electrode. Radiofrequency power (13.56 MHz) was used to excite the plasma from argon atmosphere at a working pressure of 11 Pa. The effect of the plasma excitation power on the properties of the resulting films was studied. Film thickness and hardness were measured by profilometry and nanoindentation, respectively. The molecular structure and chemical composition of the layers were analyzed by Fourier transform infrared spectroscopy and energy dispersive spectroscopy. Surface micrographs, obtained by scanning electron microscopy, allowed the determination of the sample morphology. Grazing incidence X‐ray diffraction was employed to determine the structure of the films. Amorphous organic layers were deposited with thicknesses of up to 7 µm and hardness of around 1.0 GPa. The films were composed by aluminum, carbon, oxygen and hydrogen, their proportions being strongly dependent on the power used to excite the plasma. A uniform surface was obtained for low‐power depositions, but particulates and cracks appeared in the high‐power prepared materials. The presence of different proportions of aluminum oxide in the coatings is ascribed to the different activations promoted in the metalorganic molecule once in the plasma phase. Copyright © 2013 John Wiley & Sons, Ltd.     

Initial nitridation of the Ge(100)-2 x 1 surface by ammonia

The reaction of ammonia (NH(3)) on the Ge(100)-2 x 1 surface is investigated using density functional theory (DFT). We find that NH(3) adsorbs molecularly onto Ge(100)-2 x 1 via the formation of a dative bond. The calculations also show that, unlike Si(100)-2 x 1, the activation barrier for subsequent dissociation of NH(3) adsorbed on Ge(100)-2 x 1 is higher than that of reversible desorption, which indicates that NH(3) has a low reactive sticking probability on the Ge(100)-2 x 1 surface. We also predict that nitrogen insertion into the Ge-Ge dimer requires NH(3) overexposure because the activation barrier for NH(2) insertion into the Ge-Ge dimer is significantly above the entrance channel. The nitridation reaction pathway results in the N-H bridge-bonded state, which is found to be 17.4 kcal/mol more stable than the reactants. We find that the reactions of NH(3) on the Ge(100)-2 x 1 surface generally involve higher activation barriers and less stable intermediates than the analogous reactions on the Si(100)-2 x 1 surface.     

氮化铝的研究进展

AIN在热、电、光和机械等方面具有非常良好的综合性能。本文综述了近年来AIN在导热机理、粉末其及薄膜制备等领域的研究进展，展望了AIN未来的发展趋势。     

Polymer precursor synthesis of alumina fibers

High strength alumina fibers were fabricated with a polymer precursor process which was monitored with ¹³C-NMR, elemental analysis, gas evolution, SEM, and X-ray diffraction. Methylaluminoxane was chain extended by reaction with H₂O, the unreacted methyl groups were converted to propanoyloxyl and i-propanoyl groups in molar ratios of 0.2 : 0.8 to 0.4 : 0.6 to afford spinning dope of adequate fluidity which can be spun into filaments with suitable stability. These precursor fibers were hydrolyzed in two stages at ambient temperatures and at 320–350°C, and then calcined at 950°C. The resulting alumina fibers have average tensile strengths up to 1.1 GPa and modulus of greater than 140 GPa.     

Pyrolytic decomposition of ammonia borane to boron nitride

The thermal decomposition of ammonia borane was studied using a variety of methods to qualitatively identify gas and remnant solid phase species after thermal treatments up to 1500 °C. At about 110 °C, ammonia borane begins to decompose yielding H(2) as the major gas phase product. A two step decomposition process leading to a polymeric -[NH═BH](n)- species above 130 °C is generally accepted. In this comprehensive study of decomposition pathways, we confirm the first two decomposition steps and identify a third process initiating at 1170 °C which leads to a semicrystalline hexagonal phase boron nitride. Thermogravimetric analysis (TGA) was used to identify the onset of the third step. Temperature programmed desorption-mass spectroscopy (TPD-MS) and vacuum line methods identify molecular aminoborane (H(2)N═BH(2)) as a species that can be released in appreciable quantities with the other major impurity, borazine. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify the chemical states present in the solid phase material after each stage of decomposition. The boron nitride product was examined for composition, structure, and morphology using scanning Auger microscopy (SAM), powder X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Thermogravimetric Analysis-Mass Spectroscopy (TGA-MS) and Differential Scanning Calorimetry (DSC) were used to identify the onset temperature of the first two mass loss events.     

Preparation of unsupported alumina membrane by sol-gel techniques

Unsupported alumina membranes were prepared by sol-gel technique using aluminum isoproxide. The influence of the hydrolysis conditions, the type and concentration of peptizant acid on the boehmite sols has been studied. The suitable hydrolysis temperature for the aluminum isoproxide was above 50°C. Crack-free unsupported alumina membranes were obtained by rapid gelation processing of sols. The boehmite gel membrane and γ-Al₂O₃ membrane formed exhibited (020) and (440) preferred orientation.     

Deposition of latex particles on alumina fibers

The deposition of sub-micron latex particles during flow through beds of fine alumina fibers has been studied as a function of pH and ionic strength in the vicinity of the fiber isoelectric point (i.e.p.). Conditions were chosen so that the predominant capture mechanism was diffusion. Results have been compared with the “classical” theory of convective diffusion to cylinders and with modern theory, which takes into account colloidal and hydrodynamic interactions between particles and collector.Initial fiber collection efficiencies, determined at the i.e.p. where the fibers bear no surface charge, are considerably lower than those predicted by classical theory and are insensitive to ionic strength. This lack of dependence on ionic strength suggests that neither constant potential nor constant charge conditions are maintained during particle-fiber encounters and that some intermediate condition is more appropriate. Particle capture results obtained at pH values above and below the fiber i.e.p. agree qualitatively with the predictions of the Spielman and Friedlander “surface reaction” model, although this is not strictly valid below the i.e.p. where the fibers are positively charged and there is no repulsion barrier. Under these conditions a significant enhancement of capture efficiency is observed at low ionic strength, as a result of double layer attraction. Above the i.e.p., where the fibers and particles are both negatively charged, double layer repulsion causes a large reduction in capture efficiency, although the predicted is even larger.The saturation coverage of the fibers by deposited particles was found to decrease strongly as the ionic strength was decreased, indicating the importance of lateral interactions between particles.     

Dehydrogenation of ammonia by early transition metals: formation of metal nitride clusters

The dehydrogenation of NH₃ by Ti, Zr, and Nb cations, produced in a laser-induced plasma reactor, is found to form both neutral and cationic clusters. Depending on the conditions, either the production of neutral metal nitride clusters is favored or, the formation of cationic `precursor' metal nitride clusters containing additional intact NH₃ is observed. The neutral clusters are formed when full dehydrogenation of the NH₃ is achieved, while the cationic precursor species are formed under conditions where a large fraction of the NH₃ remains intact. Metastable dissociation studies show that intact NH₃ molecules are bound to the developing transition metal nitride clusters.     

Synthesis of two-dimensional gallium nitride via spin coating method: influences of nitridation temperatures

This paper reports the synthesis and characterization of gallium nitride (GaN) thin films deposited on p-type silicon (100) substrates by using low cost spin coating method under various nitridation temperatures. This work demonstrated that spin coating with the new prepared precursor solution can be used as a versatile method for the successfully growth of GaN thin films. Furthermore, the influence of varying nitridation temperatures on the structural, morphological, and optical properties of synthesized GaN thin films were studied in this work. The GaN thin films were characterized by X-ray diffraction, field-emission scanning electron microscopy, atomic force microscopy, photoluminescence and Raman spectroscopy. All the characteristics of the GaN thin films were effectively improved with the increasing of the nitridation temperatures from 750 to 950 °C and degraded at temperature of 1,050 °C. The measured results show that nitridation temperature plays an important role in improving the crystalline quality of the GaN thin films and the most efficient nitridation temperature was at 950 °C.     

Synthesis of aluminum nitride in transferred arc plasma furnaces

Ultrafine particles of aluminum tnitride (AIN) arc produced by a transferred an plasma. Two devices are used: a transferred arc plasma on aluminum natal in nitrogen or nitrogenlammonia atmospheres, and a item concept of transferred arc plasma when, DC anode and cathode ares are coupled together above an alumintun melt. Equilibrium chemical compositions mere calculated. The temperature distributions in the plasma are measured hr emission spectroscopy Flit, powder, made from 99.8%, aluminum ingot, it as analyzed and confirmed to be 99.3%, of hexagonal phase aluminum nitride. In othertests, from 99.99% aluminum ingot, a translucent AIN vinter was obtained. The densification behavior was assessed by hot pressing and by pressureless sintering, with and without additives. The thermal conductivities are given.     

Carbon black/alumina gel composite: Preparation by sol-gel process in the presence of polymer-grafted carbon black and its electric properties

The preparation of a novel carbon black/alumina gel composite by sol-gel reaction of aluminum isopropoxide (AIP) was investigated. When sol-gel reaction of AIP was carried out in the presence of untreated carbon black, a thin film of alumina gel was hardly obtained, because of the presence of aggregated carbon blacks. On the contrary, in the presence of poly(N-vinyl-2-pyrrolidone)-grafted, poly(2-methyl-2-oxazoline)-grafted, and poly(N,N-diethylacrylamide)-grafted carbon black, a deep-black, thin film of alumina gel, in which carbon blacks were uniformly incorporated, was obtained. Carbon black was incorporated into an alumina gel matrix by the hydrogen bond between carbonyl groups of the grafted polymer and the residual hydroxyl groups in the alumina gel. Electric resistance of the polymer-grafted carbon black/alumina gel composite was very sensitive to the vapor of solvents of the grafted polymer chains on carbon black surface: the electric resistance of the composite drastically decreased in humidity and N,N-dimethylformamide vapor, but not in n-hexane vapor. The logarithm of electric resistance of polymer-grafted carbon black/alumina gel composite linearly decreased in proportion to relative humidity. In addition, the electric resistance of poly(N,N-diethylacrylamide)-grafted carbon black/alumina gel composite under humidity decreased drastically at 32°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3591–3597, 1999