Topotactically Oriented α-Si3N4 Cores in Sintered β-Si3N4

α-Si₃N₄ core structures within β-Si₃N₄ grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si₃N₄ crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism.     

Impurity Phases in Hot-Pressed Si3N4

Impurity phases in commercial hot-pressed Si₃N₄ were investigated using transmission electron microscopy. In addition to the dominant, β-Si₃N₄ phase, small amounts of Si₂N₂O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si₃N₄ grains and ∼ 30 Å thick between Si₂N₂O and β-Si₃N₄ grains.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Phase Equilibria in the System Si3N4-SiO2-BeO-Be3N2

The 1780°C isothermal section of the reciprocal quasiternary system Si₃N₄-SiO₂-BeO-Be₃N₂ was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be₆Si³N₈, Be₁₁Si₅N₁₄, Be₅Si²N₆, Be₉Si₃N₁₀, Be₈SiO₄N₄, Be₆O₃N₂, Be₈O₅N₂, and Be₉O₆N₂. Large solid solubility occurs in β-Si₃N₄, BeSiN₂, Be₉Si₃N₁₀, Be₄SiN₄, and β-Be₃N₂. Solid solubility in β-Si₃N₄ extends toward Be₂SiO₄ and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be₂SiO₄ at 1880°C. A 4-phase isotherm, liquid +β-Si₃N₄ ( ss )Si₂ON₂+ BeO, exists at 1770°C.     

Fabrication of Si3N4/SiC Composite by Reaction-Bonding and Gas-Pressure Sintering

Si₃N₄/SiC composite materials have been fabricated by reaction-sintering and postsintering steps. The green body containing Si metal and SiC particles was reaction-sintered at 1370°C in a flowing N₂/H₂ gas mixture. The initial reaction product was dominated by alpha-Si₃N₄. However, as the reaction processed there was a gradual increase in the proportion of β-Si₃N₄. The reaction-bonded composite consisting of alpha-Si₃N₄, β-Si₃N₄, and SiC was heat-treated again at 2000°C for 150 min under 7-MPa N₂ gas pressure. The addition of SiC enhanced the reaction-sintering process and resulted in a fine microstructure, which in turn improved fracture strength to as high as 1220 MPa. The high value in flexural strength is attributed to the formation of uniformly elongated β-Si₃N₄ grains as well as small size of the grains (length = 2 μm, thickness = 0.5 μm). The reaction mechanism of the reaction sintering and the mechanical properties of the composite are discussed in terms of the development of microstructures.     

Calculation of XANES/ELNES Spectra of All Edges in Si3N4 and Si2N2O

Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si- L _2,3, Si- K , and N- K edges in α-Si₃N₄, β-Si₃N₄, spinel c -Si₃N₄, and Si₂N₂O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β-Si₃N₄ and c -Si₃N₄. The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ELNES spectral calculation.     

Low-Temperature Synthesis of Nanocrystalline α-Si3N4 Powders by the Reaction of Mg2Si with NH4Cl

Nanocrystalline α-Si₃N₄ powders have been prepared with a yield of 93% by the reaction of Mg₂Si with NH₄Cl in the temperature range of 450° to 600°C in an autoclave. X-ray diffraction patterns of the products can be indexed as the α-Si₃N₄ with the lattice constants a = 7.770 and c = 5.627 Å. X-ray photoelectron spectroscopy analysis indicates that the composition of the α-Si₃N₄ samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the α-Si₃N₄ crystallites prepared at 450°, 500°, and 550°C are particles of about 20, 40, and 70 nm in average, respectively.     

Influence of Phase Formation on Dielectric Properties of Si3N4 Ceramics

The influence of phase formation on the dielectric properties of silicon nitride (Si₃N₄) ceramics, which were produced by pressureless sintering with additives in MgO–Al₂O₃–SiO₂ system, was investigated. It seems that the difference in the dielectric properties of Si₃N₄ ceramics sintered at different temperatures was mainly due to the difference of the relative content of α-Si₃N₄, β-Si₃N₄, and the intermediate product (Si₂N₂O) in the samples. Compared with α-Si₃N₄ and Si₂N₂O, β-Si₃N₄ is believed to be a major factor influencing the dielectric constant. The high-dielectric constant of β-Si₃N₄ could be attributed to the ionic relaxation polarization.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

Dissolution Kinetics of β-Si3N4 in an Mg-Si-O-N Glass

The rate of dissolution of β-Si₃N₄ into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si₃N₄-SiO₂-MgO such that Si₂N₂O rather than β-Si₃N₄ was the equilibrium phase. Dissolution was driven by the chemical reaction Si₃N₄(c)+SiO₂( l )→Si₂N₂O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si₃N₄) and the precipitating phase (Si₂N₂O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si₃N₄, crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol⁻¹, at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si₃N₄ hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si₃N₄.     

Microstructural Observations on Hot-Pressed Si3N4

The development of microstructure in hot-pressed Si_aN₄ was studiehd for a typical Si₃N₄ powder with and without BeSiN₂ as a densification aid. The effect of hot-pressing temperature on density, α- to β-Si₃N₄ conversion and specific surface area showed that BeSiN₂ appears to increase the mobility of the system by enhancing densification, α- to β-Si₃N₄ transformation, and grain growth at temperatures between 1450° and 1800°. These processes appear to occur in the presence of a liquid phase.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Additive-Assisted Nitridation to Synthesize Si3N4 Nanomaterials at a Low Temperature

Starting from Si powder, NaN₃ and different additives such as N -aminothiourea, iodine, or both, Si₃N₄ nanomaterials were synthesized through the nitridation of silicon powder in autoclaves at 60°–190°C. As the additive was only N -aminothiourea, β-Si₃N₄ nanorods and α-Si₃N₄ nanoparticles were prepared at 170°C. If the additive was only iodine, α-Si₃N₄ dendrites with β-Si₃N₄ nanorods were obtained at 190°C. However, when both N -aminothiourea and iodine were added to the system of Si and NaN₃, the products composed of β-Si₃N₄ nanorods and α, β-Si₃N₄ nanoparticles could be prepared at 60°C.     

Hot-Pressing of Si3N4 with Y2O3 and Li2O as Additives

The rates of densification and phase transformation undergone by α-Si₃N₄ during hot-pressing in the presence of Y₂O₃, Y₂O₃−2SiO₂, and Li₂0−2Si0₂ as additives were studied. Although these systems behave less simply than MgO-doped Si₃N₄, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si₃N₄ phase.     

Solvothermal Synthesis of Si3N4 Nanomaterials at a Low Temperature

Silicon nitride nanowires or nanorods have been synthesized from SiCl₄, NaN₃, and metallic Mg at temperatures ranging from 200° to 300°C. X-ray powder diffraction patterns indicated that the as-obtained products were mainly β-Si₃N₄. Scanning electron microscope and high-resolution transmission electronic microscopy showed that the samples mostly consisted of Si₃N₄ nanowires or nanorods. As metallic iron powder was used, α-Si₃N₄ was mainly formed at 250°C.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Combustion Synthesis of Si3N4 by Selective Reaction of Silicon with Nitrogen in Air

Silicon nitride (Si₃N₄) was synthesized by a selective combustion reaction of silicon powder with nitrogen in air. The α/β-Si₃N₄ ratio of the interior product could be tailored by adjusting the Si₃N₄-diluent content in the reactant mixtures. The synthetic β-Si₃N₄ showed a well-crystallized rod-like morphology. Mechanical activation greatly enhanced the reactivity of silicon powder, and the slow oxidation of silicon at the sample surface promoted the combustion reaction in air. The formation mechanism of Si₃N₄ was analyzed based on a proposed N₂/O₂ diffusion kinetic model, and the calculated result is in good agreement with the experimental phenomenon.     

Effective Doping in Cubic Si3N4 and Ge3N4: A First-Principles Study

First-principles calculations have been conducted to investigate impurities in cubic Si₃N₄ and Ge₃N₄. Impurity species suitable for n - and p -type doping are suggested, in terms of the formation and ionization energies. The suggested species are P and O as n -type dopants and Al as a p -type dopant for c -Si₃N₄, and Sb and O as n -type dopants and Al as a p -type dopant for c -Ge₃N₄. The dependence of the formation energies on the chemical potentials indicates that a proper choice of growth conditions is mandatory for suppressing the incorporation of these impurities into anti and interstitial sites, where the impurities can be charged to compensate carriers.     

Hot-Pressing Behavior of Si3N4

The densification behavior of Si₃N₄ containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si₃N₄. Evidence of a second-phase material between the Si₃N₄ grains was obtained through etching studies. Transformation of α- to β-Si₃N₄ during hot-pressing is not necessary for densification.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.