Kinetics of α- and β-Si3N4 formation from oxide-free high-purity Si powder

Available kinetic data for the nitridation of high-purity oxide-free Si powder are analysed. The analysis suggests that the - and -phases of Si₃N₄ are formed by separate and parallel reaction paths, and kinetic expressions for their formation are reported. The formation of the -phase follows first-order kinetics, while the -phase is formed by a phase-boundary-controlled rate law. These conclusions are consistent with other kinetic and micrographic analyses reported in the literature.     

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.     

Hot-pressing and the α-β phase transformation in silicon nitride

The kinetics of densification and the kinetics of the- phase transformation have been measured during the hot-pressing of silicon nitride ceramics using magnesia as additive. Two mechanisms of densification have been identified. The first is a very rapid particle rearrangement, liquid-enhanced above 1550° C, which operates up to relative densities of about 0.65. The kinetics of the much slower decelerating second stage obey the Coble hot-pressing equation and the rate of densification is found to be proportional to the amount of additive. The controlling mechanism is believed to be diffusion in a boundary second phase, and values for the diffusion coefficient,D _b, of the rate-controlling species in the boundary phase for temperatures above and below 1550° C are given. The kinetics of the to transformation, the greater part of which occurs after densification is complete, are described by a first order reaction; the dependence of rate on the quantity of additive and on temperature is similar to that found for densification, and a similar controlling mechanism is believed to be responsible for the two processes.     

The α/β silicon nitride phase transformation

The literature on the / silicon nitride transformation is reviewed briefly. Data are presented on the kinetics of the tranformation of 1600° C on low and high purity silicon nitride powders. The addition of magnesia increased the rate of transformation while the addition of yttria had no effect. Scanning electron photomicrographs show clearly the morphology changes that accompany the transformation. It is concluded that the transformation occurs via a solution-precipitation mechanism and that and are probably low and high temperature forms of silicon nitride.     

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.     

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.     

The improved mechanical properties of Al matrix composites reinforced with oriented β-Si3N4 whisker

The β-Si₃N₄ whiskers (β-Si₃N₄w) reinforced Al matrix composites were first fabricated by hot pressing, then treated through hot extrusion. The microstructure characterization demonstrated the preferred orientations of both β-Si₃N₄w and Al grains in the as-extruded composites. It indicated that β-Si₃N₄w were aligned along the extrusion direction and Al grains exhibited a distinct <111>_Al texture. The interface between β-Si₃N₄w and Al was in a good bonding status without voids and reaction products. Effects of extrusion process on the mechanical properties of composites were also investigated. The results indicated the extrusion process had a prominent strengthening effect on the mechanical properties of composites. The maximum yield strength and ultimate tensile strength of composites reached up to 170 and 289 MPa, respectively, accompanied by a 12.3% elongation at fracture when the whisker fraction was 15 vol.%. This improvement was collectively attributed to the densification of composites, the strong interface, and the preferred orientation inside composites. The yield strength of the composites reinforced with 5 vol.% β-Si₃N₄w corresponded well with the theoretical value from different strengthening mechanisms.     

The characterization and polymorphism of α-Glycine in the presence of butyric acid

The present study aims to investigate the effect of butyric acid used as an additive and its concentration ranging between 50?ppm and 250?ppm during the polymorphic phase transformation of β-glycine to α-glycine. Analysis includes a continuous measurement of the ultrasonic velocity and periodically the X-ray diffraction (XRD) pattern. Morphological characterization shows that the α-glycine crystals obtained in pure media are smooth, prismatic shapes, and that butyric acid plays a major role in crystal shape change. SEM imaging and morphology analysis indicates that the presence of butyric acid to media results in shorter, rounded and aggregated crystals. Further analyses of Fourier Transform Infrared Spectroscopy (FTIR), zeta potential measurements, and elemental analysis reveals that butyric acid adsorbs on the crystals’ surface and changes both the surface charge and the elemental composition of the crystals obtained. In addition, thermal decomposition behaviors of the crystals are investigated and the obtained data is modeled using the thermal decomposition kinetic models of FWO, KAS, and Tang. Based on the data of the FWO kinetic model, the average activation energy of the crystals obtained in butyric acid media is calculated as 118.8?±?17.0?kJ/mol, which is higher than that of crystals obtained in pure media.     

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.     

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.     

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.     

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.     

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from to -SiC reveals a dependency on the initial -content and the sintering atmosphere. The transformation rate decreases with an increasing -content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure -SiC starting powders when the additive system consists of 10.34 wt% Y2O3 and 2.95 wt% AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial /-ratio (>1:9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.     

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.     

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.     

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).     

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.     

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.     

Characterization of the transformation from calcium-deficient apatite to β-tricalcium phosphate

The structural changes that occur during the transformation of a Ca-deficient apatite, prepared by a wet chemical method, to -TCp were investigated. X-ray diffraction (XRD) analysis of as-prepared samples and samples calcined at temperatures between 500 and 1100 °C showed that the transformation occurs over the temperature range 710–740 °C, under non-equilibrium conditions. The change in crystallite size with increasing calcination/sintering temperature was studied by XRD using the Scherrer formula. Fourier transform infra-red (FTIR) analysis indicated considerable structural change in samples above and below this temperature range. Changes were observed in the hydroxyl, carbonate and phosphate bands as the calcination temperature was increased from 500 to 1100 °C. Even once a single -TCP phase is obtained at 740 °C there remains a considerable amount of structural change at temperatures between 740 and 1100 °C. This effect was illustrated by an unusual change in the lattice parameters of the -TCP structure and significant changes in the phosphate bands of FTIR spectra as the calcination temperature was increased. The results obtained in this study show that the combined experimental techniques of XRD and FTIR are excellent complimentary methods for characterizing structural changes that occur during phase transformations.