Aluminum nitride formation

Nickel, K., Riedel, R., and Petzow, G., Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition, /. Amer. Ceram. Soc., 72(10) 1804-1810 (1989)... 

Aluminum nitride from bauxite, carbon and nitrogen (17), and the formation of silica-aluminum-nitride mixtures from various clays at about 1500°C. and higher temperatures. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

Table 1 summarizes some of the important properties of the carbon isotopes. Note that only the rare ( 1%), naturally occurring, stable carbon isotope, namely, C, has a nuclear spin and is observable by NMR. The organic chemist is fortunate that 99% of natural carbon is the isotope C with no nuclear spin, so that proton and carbon-13 NMR spectra of organic compounds are not complicated by spin - spin splitting arising fi om adjacent carbon atoms. The radioisotope C is made by thermal neutron irradiation of lithium or aluminum nitride (equation 1). It decays back to stable yN by jS emission, with a half-life of 5570 years (equation 2). Cosmic rays generate thermal neutrons, which leads to the formation of C02 in the atmosphere (equation 1). Metabolism of... 

The radioisotope 14C (/ , 5570 y), which is widely used as a tracer, is made by thermal neutron irradiation of lithium or aluminum nitride, 14N( ,p)14C. It is available not only as C02 or carbonates but also in numerous labeled organic compounds. Its formation in the atmosphere and absorption of C02 by living organisms provide the basis of radiocarbon dating. 

The production of silicon nitride [16,17] and aluminum nitride [18-21] has been extensively studied plasma reactions [21-23] were also reported. Instead of ammonia nitrogen may be used [24,25]. Hydrocarbons and metal halides for the formation of carbides are worth mentioning [26,27]. 

Aluminum nitride powder synthesized by high-temperature routes can be sintered to a density of more than 97% of the theoretical density by adding calcium or yttrium compounds as sintering aids [52-54]. Therefore, new processing routes to aluminum nitride predominantly aim at powders with lower quantities of cationic impurities. In addition, the formation of aluminum nitride coatings [55-58] or fibers [59] has become subject of extensive research. 

The first low-temperature route for the formation of aluminum nitride was discovered in the 1950s by Wiberg with Amberger and May [60,61]. Three-di-mensionally polycondensed iminoalanes [e.g., Al(NR)i.5] are formed using aluminum hydride and ammonia or primary organic amines as starting materials. It should be added that aluminum hydride is extremely difficult to handle... 

Calcining polyiminoalanes usually results in the formation of aluminum nitride at comparably low temperatures. If the calcination is carried out in ammonia, carbon contamination is easier to avoid, calcining in argon usually results in the formation of black carbonaceous residues in the range of 10-20 wt% carbon. In general, crystallization is promoted by using ammonia and is prevented if argon is used. 

In analogy to the route of Wiberg for the formation of polyiminoalanes and subsequently of aluminum nitride [61], NaBH4, BCI3, and ammonia may be used as starting materials. The intermediate formation of diborane, B2H6, was assumed [81] in this case. 

The method was first reported by Seibold and Russel [92] for the formation of aluminum nitride, who used an electrolyte consisting of propylamine, acetonitrile, and tetrabutylammonium bromide as a supporting electrolyte to achieve sufficient electrical conductivity. The electrodes were formed by sheets of various metals. The assumed chemical reactions at the electrodes are as follows ... 

All precursors are amorphous up to calcination temperatures of around 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 20-40 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth. With aluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96] the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt% ammonia, 5.1 wt%). 

The formation of aluminum nitride is illustrated by the following stoichiometric reaction equation ... 

The mechanism of azide SHS systems reactions is quite complex when using halides of elements to be nitrided. The final nitride formation in this case goes through a number of intermediate species such as NajAlFg with their following reducing or decomposition and simultaneous aluminum nitriding. 

Structure formation of nitrides synthesized in the SHS-Az systems is investigated. During the combustion process, the use of SHS-Az systems sodium azide-halide of azotized element in some cases allowed to get nanostruc-tured powders of titanium, borium, silicium, aluminum nitrides with particles of nanofibrous structure with fiber diameter of 50-100nm and nanocrys-talline structure with crystal average-sized of 100-200 nm. 

The plasma technique allows also production of nanosized nitride powder doped with sintering additives, for example, aluminum nitride doped with yttria. Such composite powders are characterized with high degree of homogeneity because the formation of powder particles occurs by simultaneous condensation from gaseous-vapor phase [13]. 

The possibility of using aluminum nitride (AIN) nanopowders as flame retardant additive was studied [25]. Concentration of the nanopowder AIN incorporated in a polyethylene matrix was 0.1 0.25 0.75 1.5 and 3 wt%. The incorporation of 1.5 wt% AIN in a polyethylene matrix caused the significant increase in the temperature of the beginning of oxidation of 33 °C (to 183 °C) in comparison with pure polyethylene (150 °C) and in the onset temperature of the intensive weight loss of 15 °C (to 375 °C) against 360 °C for pure polyethylene. The resulting effect is explained by the influence of nanoparticles on the microstructural characteristics of polyethylene. Nanoparticles are the crystallization centers and participate in the formation of fine-grained structure. 

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