In this paper, α-Si 3 N 4 nanobelts assembled by Si 3 N 4 microcrystals on the nanowires were synthesized from amorphous Si 3 N 4 powders that were prepared by thermolysis of silicon diimide powders. By prolonging the heating time at 1550 °C, the amorphous Si 3 N 4 powders were converted to hexagonal phase α-Si 3 N 4 microcrystals which were adhered to the surface of nanowires to obtain α-Si 3 N 4 nanobelts. The obtained α-Si 3 N 4 nanobelts were hollow in the centre with a width of ∼850 nm and a thickness of ∼150 nm. Microcrystals were deposited on the surface of 2–3 parallel nanowires. The diameter of the Si 3 N 4 microcrystals was 30–230 nm, and the diameter of the nanowires was 50–100 nm. Evaporation of the amorphous Si 3 N 4 powders released Si vapour and N vapour, which led to the formation of the Si 3 N 4 microcrystals. The nanowires grew on the surface of the amorphous Si 3 N 4 powders due to the nucleation and diffusion of Si and N atoms of amorphous Si 3 N 4 powders. The growth mechanism of the products with unique morphology can be considered as a combination of vapour-solid mechanism and secondary nucleation process.
β-Si 3 N 4 powder with well-dispersed spherical particles was synthesized by liquid-phase modified carbothermal reduction and nitridation. The influence of the type and content of additives on the particle morphology was investigated. The liquid phase formed at the reaction temperature modifies the morphology of the Si 3 N 4 powder. The Si 3 N 4 products precipitated along the liquid-solid interface, thus forming a spherical morphology and promoting the transformation from α-Si 3 N 4 to β-Si 3 N 4 . The type of additives has a strong influence on the formation temperature and viscosity of the liquid phase and hence affects the product morphology. In the samples with CaF 2 and CaO additives, sufficient liquid phase occurred and spherical particles were obtained. In the sample with Y 2 O 3 , however, the liquid phase was insufficient due to the higher formation temperature, thus resulting in an irregular particle morphology.
Silicon nitride (Si 3 N 4 ) particles with different morphologies have been used in many fields. In this work, α-Si 3 N 4 whiskers and granular particles with high-phase purity were successfully tailored by the controllable crystallisation process of amorphous Si 3 N 4 powders under different N 2 pressure. Impressively, α-Si 3 N 4 whiskers were prepared by simply heat treating amorphous Si 3 N 4 powders at 1550 °C for 2 h under the low N 2 pressure of 0.2 MPa, whereas equiaxed α-Si 3 N 4 particles with uniform size of ~280 nm were obtained under an elevated N 2 pressure of 2.0 MPa. With the evaluated N 2 pressures and temperatures, large scale α-Si 3 N 4 whiskers or equiaxed α-Si 3 N 4 particles could be produced. The growth mechanisms of the α-Si 3 N 4 particles with distinct morphologies were rationally proposed, and these consist of two main growth processes. First, amorphous Si 3 N 4 powders decomposed into Si(g) and N 2 (g) under high-temperature treatment. Subsequently, N 2 (g) dominated the recombination of the evaporated chemical with the Si 3 N 4 molecule. The initial N 2 concentration, which plays a key role in tailoring the shape and size of products, was controlled by the N 2 pressure.
A facile procedure was provided to obtain amorphous Si 3 N 4 powder via a liquid-phase method, followed by a heat treatment process to realize tailorable shapes for α-Si 3 N 4 crystal, while Si 3 N 4 granules were synthesized and whiskers were inhibited. The influence of the temperature on the particle morphology and growth mechanisms were investigated. Characterizations indicated that the crystallization temperature played an important role in controlling the morphology of α-Si 3 N 4 powder, while the relatively lowly temperature (below 1519 °C) was beneficial to forming whiskers. The growth of Si 3 N 4 whiskers, derive from the reaction between SiO(g) and N 2 , follows the vapor–solid (VS) mechanism, which is inhibited by the increase in temperature. Instead, higher temperatures were conducive to forming granular structures. Highly crystalline α-Si 3 N 4 granules were prepared at 1550 °C due to Si*(g) and N*(g) recombining to Si 3 N 4 nuclei via the vapor–solid-solid (VSS) mechanism providing a theoretical basis for inhibiting the production of the whiskers.
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