Electronic Structures of Ln3+α-SiAIONs with Correlations to Solubility and Solution Effects

First-principles molecular-orbital calculations using the discrete-variational Xα method have been made on model clusters of α-Si₃N₄ and its solid solutions with lanthanide elements, which occupy interstitial sites in the structure. The formula is Ln_χSi_12–4.5χAl_4.5χO_1.5χN16–1.5χ (Ln = La, Nd, Gd, Dy, Ho, Er, Tm, Yb), i.e., a Ln-α-SiAION solid solution. Covalent bond strength between Si and N, evaluated by overlap population, increases because of the presence of trivalent charges at the interstitial sites. When a Ln³⁺ ion is present, antibondings occur between Ln orbitals and N/Si orbitals, and they depend significantly on the ionic radius of Ln³⁺. The total overlap population for the whole cluster is determined by the balance of Si-N bond reinforcement and Ln-N/Si antibonding. Although no lattice relaxation around the Ln³⁺ ion is included in the present calculation, good correlation between maximum solubility and the total overlap population for the whole cluster is demonstrated for the first time.     

Calculation of Grain-Boundary Bonding in Rare-Earth-Doped β-Si3N4

First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si₃N₄ and the interface between prismatic planes of β-Si₃N₄ and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln³⁺ ions at the grain boundary and no solubility of Ln³⁺ into bulk β-Si₃N₄. Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si₃N₄ during liquid-phase sintering.     

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.     

Effect of Grain Size on the Thermal Conductivity of Si3N4

Polycrystalline Si₃N₄ samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si₃N₄ resulted in the formation of a large amount of ß-Si₃N₄ grains ∼10 µm in diameter and ∼50 µm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si₃N₄ materials were 80 and 102 Wm⁻¹K⁻¹, respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 Wm⁻¹K⁻¹). The calculated phonon mean free path of sintered Si₃N₄ was ∼20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si₃N₄ at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.     

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.     

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.     

Synthesis and Luminescence Properties of Orange–Red-Emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) Light-Emitting Diode Conversion Phosphors by a Simple Nitridation of MSi2

Eu²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) orange–red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi₂, Eu₂O₃, and α-Si₃N₄ as the starting materials under N₂–H₂ (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M₂Si₅N₈ for M=Ca, Sr, and a little amount of BaSi₇N₁₀ impurity phase for M=Ba. Under the blue light excitation, M₂Si₅N₈:Eu²⁺ shows a typical broad band emission of Eu²⁺ ranging from orange to red (585–620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca2Si₅N₈:Eu²⁺ has the highest application potential as a red conversion phosphor for white light-emitting diodes.     

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.     

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.     

Change of Crystal Phases and Microstructure of Amorphous Si-C-N Powder by Hot Pressing

An amorphous Si-C-N powder with Y₂O₃ and Al₂O₃ powder as sintering additives was hot-pressed at 1900°C for 120 min in a nitrogen atmosphere. Changes in the crystalline phases and microstructure of the amorphous Si-C-N powder during sintering were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The defects at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). XRD showed that alpha-Si₃N₄ was formed initially from the amorphous Si-C-N by 1530°C, which then transformed to ß-Si₃N₄ at 1600°C. Also, a slight formation of crystalline SiC occurred during the transformation from alpha- to ß-Si₃N₄, and it increased after the transformation was completed at 1900°C. TEM revealed that many SiC nanoparticles were incorporated into ß-Si₃N₄ grains after the transformation from alpha- to ß-Si₃N₄ at 1600°C. They were located at the triple points of the grain boundaries of ß-Si₃N₄ after continued Si₃N₄ grain growth at 1900°C. Besides the SiC nanoparticles, large agglomerations of carbon or SiC particles of 20-60 µm size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests.     

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.     

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.     

Synthesis and Photoluminescence of Eu2+-Doped α-Silicon Nitride Nanowires Coated with Thin BN Film

A feasible doping strategy is introduced to synthesize Eu²⁺-doped α-Si₃N₄ nanowires coated with a thin BN film. The nanowires were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and a fluorescence spectrophotometer. The Eu²⁺-doped α-Si₃N₄ nanowires emitted strong yellow light, which is related to the 4 f ⁶5 d –4 f ⁷ transition of Eu²⁺, upon a broad excitation wavelength range between 250 and 450 nm. The obtained nanowires provided a potential candidate for application in optical nanodevices, as well as in white LEDs.     

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.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.     

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.     

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.     

Microstructure and Mechanical Properties of the in situβ-Si3N4/α'-SiAlON Composite

The in situ β-Si₃N₄/α'-SiAlON composite was studied along the Si₃N₄–Y₂O₃: 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si₃N₄/α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si₃N₄ during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si₃N₄ fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m^1/2. α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions.     

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.     

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

The kinetics of anisotropic β-Si₃N₄ grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si₃N₄ grains follows the empirical growth law Dⁿ– D₀ⁿ = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si₃N₄ growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.