Kinetic interpretation ofα- andβ-Si3N4 formation from oxide-free high-purity silicon powder

A kinetic analysis of the isothermal nitridation of high-purity oxide-free silicon powder is described. The kinetic analysis suggests that the and polymorphs of Si₃N₄ are formed by separate and parallel reaction paths. This analysis provides for the decoupling and quantitative kinetic interpretation of- and-Si₃N₄ formation reactions. Consistent with existing microstructural and thermodynamic evidence, the-forming reaction is shown to obey a first-order rate law, whereas a phase-boundary controlled rate law describes the-forming reaction. A kinetic model employing these rate laws is developed and is used to predict the/ phase ratio as a function of isothermal reaction temperature and extent of reaction. The/ phase ratios so obtained are shown to be in good agreement with experimental observations made under a variety of reaction conditions.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.     

Carbothermal nitridation synthesis of α-Si3N4 powder from pyrolysed rice hulls

The carbothermal nitridation synthesis of α-Si₃N₄ was studied using a high-temperature tube furnace to react a precursor, comprised of pyrolysed rice hulls (C/SiO₂) and additive “seed” Si₃N₄, with N₂. The experimental design for synthesis was a three-level factorial surface response design for determining the effect of temperature (1300–1380°C) and reaction time (1–5 h) on kinetics. In addition, all precursors were reacted at 1460, 1480 and 1500°G for 5 h in order to ensure high conversion suitable for product powder evaluation (composition and morphology). Following excess carbon removal, the product Si₃N₄ was >95% α-phase and had a surface area of 7.7 m²g^?1 with an oxygen content of 3.6 wt% O. The powder was comprised of a bimodal size distribution of submicrometre solid α-Si₃N₄ crystallites centred at 0.03 and 0.22 μm. No whiskers or high aspect ratio elongated crystallites were found in the powder. The addition of carbon black to the seeded pyrolysed rice hull C/SiO₂ mixture had no significant impact on the reaction rate or product powder properties. The reaction was modelled using a nuclei-growth rate expression as $$\begin{gathered} (kt)^{0.58} = - ln(1 --- X) \hfill \\ k = 1.09 \times 10^{10} exp (--- 50502/T) \hfill \\ \end{gathered} $$ k=1.09×10¹⁰ exp (?50502/T) where (1573 K<T<1653 K), (3600<t<18000 s), (0<X<1), andk=rate in s^?1.     

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Analysis of Si3N4+β-Si3N4 whisker ceramics

Dense Si₃N₄+-Si₃N₄ whisker composite ceramics were fabricated by hot pressing powder-whisker mixtures. Addition of -Si₃N₄ whiskers had no significant influence on the densification behaviour for up to 20 wt% addition. Light microscopy and scanning and transmission electron microscopy were used to study their microstructure and fracture behaviour. An increase in fracture toughness was observed for -Si₃N₄ whisker additions of up to 10 %. The main toughening mechanisms observed were crack deflection, crack branching, whisker-matrix debonding and whisker pull-out.     

The α- to β-Si3N4 transformation in the presence of liquid silicon

The compacts consisted of , -Si₃N₄ and free silicon are heat treated in the range 1650° C to 1750° C in an argon atmosphere in order to observe the following behaviours; the to phase transformation and variations of the microstructure during heat treatment in silicon nitride. For the microstructural observation of the heat treated specimens, the same grains in the polished surface were investigated before and after eliminating the retained silicon by etching. The to phase transformation, in this case, occurs via silicon melts irrespective of added -Si₃N₄. Both and phases are soluted and precipitated into molten silicon and their morphology are changed from an equiaxed shape to prismatic one. Although elongated grains are precipitated at low temperature or in the early stage of heat treatment, fine precipitated grains are mainly observed with increasing heat treating temperature.     

Mechanical properties of β-Si3N4 whisker reinforced α′-SiAlON ceramics

The effects of -Si₃N₄ whisker additions on the mechanical properties of -SiAlON ceramics were studied. The room temperature fracture toughness and fracture strength of the composites increased with increasing whisker content, and were 6.5 MPa m^1/2 and 900 MPa, respectively, for the addition of 30 vol% whiskers. Although creep resistance of the composites was not enhanced at 1200°C, the whisker additions were observed to be beneficial in reducing the oxidation induced slow crack growth of -SiAlON that occurred at 1300 °C, and thereby, improved the creep resistance of the composites at 1300°C.ORNL Postdoctoral Fellow, Oak Ridge Institute of Science and Technology, Oak Ridge Associated University.     

Phase quantification of β-Si3N4/β-SiC mixtures by X-ray powder diffraction analysis

X-ray powder diffraction methods of phase quantification were adapted and compared to mixtures of -Si₃N₄ and -SiC. Multiline mean-normalized-intensity methods and whole pattern analysis (Rietveld) both have advantages and disadvantages over each other. Satisfactory results (less than 3% absolute deviation) can be achieved in minimal time using intensity normalization methods. Phase quantification using the Rietveld method requires significantly longer measuring time, evaluation time and expertise to obtain the same results.     

Effect of grain size distribution on the strength of porous Si3N4 ceramics composed of elongated β-Si3N4 grains

The grain size distributions (diameter and aspect ratio) of porous Si₃N₄ ceramics composed of elongated -Si₃N₄ grains were evaluated statistically, and their effect on the pore size distribution and the flexural strength of the porous Si₃N₄ was investigated. Porous Si₃N₄ ceramics having porosities of 27 to 43% and median pore diameters of 0.56 to 0.96 m were used as specimens. The grain diameter distribution was well correlated to the pore size distribution of the porous Si₃N₄ ceramics. We concluded that the strength of the porous Si₃N₄ ceramics increased with increasing grain length of -Si₃N₄ as well as with decreasing porosity.     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

Microstructure characterization of hot-pressed β-silicon nitride containing β-Si3N4 seeds

Self-reinforced silicon nitride ceramics were synthesized by introducing 5 wt% previously prepared β-Si₃N₄ seeds into a mixture of α-Si₃N₄ and 8 wt.% of sintering additive. The microscopic evidence indicates that β-Si₃N₄ seeds play an important role in microstructural development and mechanical properties for silicon nitride ceramics. The growth of β-Si₃N₄ grains is initiated from the seeds, resulting in a core/shell microstructure observed in the abnormal grain growth. The internal dislocation, π-boundary, and Moire fringes are present in Si₃N₄ grains, which were formed to compensate the mismatch and compositional differences between the two grains. Coalescence can occur at the final stage of sintering.     

Separation–permeation performance of porous Si3N4 ceramics composed of columnar β-Si3N4 grains as membrane filters for microfiltration

The separation–permeation performance of porous silicon nitride (Si₃N₄) ceramics (consisting of columnar grains connected at random in three dimensions) as membrane filters was evaluated, and compared with commercial Al₂O₃ membranes having a three-layer structure. Si₃N₄ membranes separate particles with diameters much less than their pore diameters. The permeability of Si₃N₄ membranes with separability values the same as those of the Al₂O₃ membranes was about 1.3–2.4 times as large as the Al₂O₃ membranes. Dead-end filtration examination, using Al₂O₃ particles with a particle size distribution, indicated that the Si₃N₄ membrane filtration mechanism obeyed the cake filtration mechanism although the particle size was smaller than the pore size of the Si₃N₄ membranes.     

Strength of β-sialon/Si3N4 layered composites

The strength of layered composites consisting of -sialon and Si3N4 layers, which were prepared by hot pressing, was investigated. The strength increased as the thickness of the sialon (outer layer) decreased, and reached almost the same level of Si3N4 (inner layer) when the sialon thickness was 250–300 m. No specific fracture morphologies were recognized around the interface of sialon and Si3N4. The aluminium concentration changed sharply around the interface, while the yttrium tended to diffuse deeper than aluminium. This tendency was remarkable in the samples hot-pressed at higher temperature (1900°C). The existence of compressive residual stress in the surface sialon layer was revealed and the residual stress increased as the sialon thickness decreased down to 250–300 m. The increase of strength with the decrease of sialon thickness was discussed based on the mechanical calculations in which the residual stress was considered. This calculation approximately agreed with the results of the samples hot-pressed at lower temperature (1800°C). However, the strength of the samples hot-pressed at 1900°C was much higher than the prediction in the thin range of the sialon thickness. The deep diffusion of yttrium into the sialon layers was thought to be one of the causes of this unpredictable effect.     

Preparation and Properties of Sintering Additives Coated Si3N4 from Heterogeneous Nucleation Processing

The sintering additives such as Al2O3 and /or Y2O3 were coated on the surfaces of Si3N4 particles via heterogeneous nucleation processing using a buffered pH solution as the precipitation reagent .They nucleated and grew only on the surfaces of Si3N4 and did not form sol particles in solution by TEM observation .The isoelectric point(IEP) of coated Si3N4 was different from that of as-received Si3N4.The IEP of Al(OH)3-coated Si3N4 occurred at pH8.4, which is close to that of alumina .When Al(OH)3-coated Si3N4 particles were coated with Y(OH)3,the IEP of coated Si3N4 powder shifted from pH8.4 to pH9.2 ,similar to that of yttria.In addition ,the rheological data showed that Al2O3 and /or Y2O3 coated Si3N4 suspension is nearly Newtonian and that added Si3N4 suspension shows a shear rate thinning behavior.     

Aligned ultra-long single-crystalline α-Si(3)N(4) nanowires

We report the synthesis of aligned ultra-long single-crystalline α-Si(3)N(4) nanowires by pyrolysis of a polymeric precursor without any template. The length of the wires is up to several centimeters, which is significantly longer than that of any Si(3)N(4) wires reported previously. Microscopy characterization reveals that the wires are single crystals, with a uniform diameter of ～200?nm. Intense visible photoluminescence was observed between 1.3 and 3.7?eV. The wires could be useful in the fabrication of optoelectronic nanodevices and nanocomposites.     

Enhanced thermal conductivity of Cu matrix composites reinforced with Ag-coated β-Si3N4 whiskers

Cu matrix composites reinforced with 10 vol.% Ag-coated β-Si₃N₄ whiskers (ASCMMCs) were prepared by powder metallurgy method. With the aim of improving the thermal conductivity of the composites, a quite thin Ag layer was deposited on the surface of β-Si₃N₄ whiskers. The results indicated that thermal conductivity of ASCMMCs with 0.30 vol.% Ag (0.30ASCMMCs) reached up to 273 W m⁻¹ K⁻¹ at 25 °C, which was 98 W m⁻¹ K⁻¹ higher than that of Cu matrix composites reinforced with uncoated β-Si₃N₄ whiskers (USCMMCs). The Ag coating could promote the densification of composites, reduce the aggregation of β-Si₃N₄ whiskers and enhance the Cu/Si₃N₄ interfacial bonding, therefore it could efficiently enhance the thermal conductivity of Cu matrix composites reinforced with β-Si₃N₄ whiskers (SCMMCs).     

Microstructure and room-temperature mechanical properties of Si3N4 with various α/β phase ratios

After polycrystalline silicon nitride samples with various / phase ratios were fabricated by uniaxial hot-pressing under vacuum, their microstructures, -to- transformations, and grain-growth mechanisms were then characterized by electron microscopy. Room-temperature fracture toughness (KIC) increased with increasing -phase content, and a value of 7.0 ± 0.5 MPa m1/2 was obtained for a sample that contained 60 vol% phase. The increase in KIC is the result of an increase in elongated-grain fractions with increasing content, which promotes energy absorption by crack deflection, as well as by grain debonding, pullout, and bridging mechanisms.     

Effect of rare-earth oxide additives on the morphology of combustion synthesized rod-like β-Si3N4 crystals

This paper presents the results of synthesizing rod-like β-Si₃N₄ crystals by combustion synthesis (CS) with six rare-earth oxides (Y₂O₃, La₂O₃, CeO₂, Sm₂O₃, Gd₂O₃ and Yb₂O₃) as additives under high nitrogen pressure. The effect of rare-earth oxide additives on the final morphology of rod-like β-Si₃N₄ was investigated and the crystal growth mechanism was discussed in detail. The results reveal that the final morphology of combustion synthesized Si₃N₄ is strongly dependent on the viscosity of liquid formed by the rare-earth oxide and the SiO₂ on the Si powder surface. With the increasing of atomic number of the rare-earth elements, the final morphology is from elongated rod-like crystals to short columnar crystals.