β-SiAlON Phase Research Articles

Ceramic substrates of β-SiC/SiAlON composite from preceramic polymers and Al–Si fillers

Commercial polysilsesquioxanes filled with silicon and aluminum particles were prepared in the form of a thin sheet by tape casting and pyrolyzed in nitrogen atmosphere at various temperatures up to 1500°C. Two silicone resins with different carbon content were used to prepare slurries containing 30vol% of Si and Al as filler with volume ratios of 1:1 and 1:3. The crystalline phases that formed in the polymer derived ceramic (PDC) substrates pyrolyzed at different temperatures were identified using x-ray diffraction patterns. Substrates pyrolyzed at 1500°C were characterized with respect to their density, total open porosity and thermal expansion coefficient. The resulting microstructures were examined using scanning and transmission electron microscopy. The main crystalline phases in substrates pyrolyzed at 1000°C were identified as Si, β-SiC and AlN. These gradually converted to β-SiC/SiAlON composites at 1500°C. The higher carbon content favored formation of the phases β-SiC and AlN. The β-SiAlON phase predominated in substrates with higher Si content while SiALON polytypoids were more significant in substrates with higher Al content. The 21R (SiAl6O2N6), 12H (SiAl5O2N5) and 15R (SiAl4O2N4) SiAlON polytypoids were observed in all the samples and were identified by high resolution electron microscopy (HREM) in samples pyrolyzed at 1500°C.

Action Mechanism of β-Si3N4 in Bauxite Sintering

In this paper, we added silicon nitride in the samples of common bauxite and the bauxite with high potassium and sodium oxide content, treated them at the high temperature of 1,400°Cand 1,500°C, and then conducted analysis with XRD, SEM and EDS. The results are as follows: After adding β-Si3N4 in sintered bauxite, the partial pressure of oxygen in the composite materials will be reduced, the decomposition and volatilization of compounds with high partial pressure of oxygen (such as potassium oxide and sodium oxide) will be promoted, the content of these compounds in the bauxite will be lowered, and the hazards of the compounds with low melting point will be weakened; meanwhile, with the reduction of potassium, sodium and iron oxide, the lattice of alumina will be activated, the β-Sialon phase will be easily formed and the high temperature properties of sintered bauxite materials will be strengthened.

The origin of bimodal luminescence of β-SiAlON:Eu2+ phosphors as revealed by fluorescence microscopy and cathodoluminescence analysis

Eu2+-doped SiAlON phosphors with the composition of EuxSi6−zAlzOzN8−z (0.5≤z≤3) at a fixed x=0.01 were synthesized by the gas pressure sintering method. Dependence of luminescence properties on the phase compositions in β-SiAlON:Eu2+ phosphors has been examined via fluorescence microscope and scanning electron microscope equipped with a cathodoluminescence spectrometer and an energy dispersive spectrometer. Bimodal emission (green and violet) from β-SiAlON phase is observed in the samples with z≥2, indicating co-existence of two different kinds of coordination for Eu2+ ions in the host lattice.

Influencing Factors of Coal Gangue Carbothermal Reduction and Nitridation Reaction

nfluencing factors of target products such as X phase, β-SiAlON phase and O-SiAlON phase of Inner Mongolia coal gangue carbothermal reduction and nitridation were researched by calculating the loss rate on ignition of specimens, and by means of XRD and SEM. During the carbothermal reduction and nitridation reaction of coal gangue, the loss rate on ignition of specimens rises with carbon reducer increasing, and keeping time has a little influence on the loss rate on ignition of specimens. If β-SiAlON is target phase, the yield from corundum is much higher than that from special grade bauxite. Corundum or bauxite is used as starting material, the yield of X phase is low and the highest yield is only 12.88%. For the carbothermal reduction and nitridation reaction of coal gangue, the appropriate addition of reducer carbon is 10%-16%, and temperature influence is larger. The reaction temperature over 1420°C and keeping time of 6h are beneficial to the formation of X phase, β-SiAlON and O-SiAlON.

Study on Phase Stability during Pressure-Less Sintering Process of Slag α-Sialon Powder and Properties of As-Sintered Materials

The pressure-less sintering of slag α-Sialon powder, derived from slag wastes by self-propagating high-temperature synthesis technology, was carried out and the phase assemblages of slag α-Sialon sintered at different temperatures were studied. The results showed that the formation of β-Sialon phase did occur during the pressure-less sintering and was restrained to some extent via mass diffusion of Ca2+/Mg2+ added in embedded powder in samples. α-Sialon phase with two different unit cells was detected in samples sintered at 1700°C and above, and the formation mechanism was also discussed.

Reaction sintered Fe–Sialon ceramic composite: Processing, characterization and high temperature erosion wear behavior

Fe–Sialon ceramic matrix composite has been newly developed from ferro-silicon alloy and commercial-grade industrial alumina powders by reaction sintering under a nitrogen atmosphere. The phase composition, mechanical properties and impact erosion wear behavior were investigated. The solid particle erosion tests have been conducted at elevated temperatures ranging from 25 °C to 1200 °C. Sharp SiC particles between 325 and 830 μm in diameter were employed as impact abrasives. The results showed that Fe–Sialon ceramic consisted of β-Sialon and Fe3Si phases. The Z value of the as-formed β-Sialon varied from 0 to 3.2 with increasing the alumina content in the starting powders. The bending strength and Rockwell hardness gradually increased with raising the alumina addition. The erosion rate of Fe–Sialon ceramic is highly dependent on the testing temperature. The minor erosion took place at room temperature or 1200 °C, while the major erosion occurred at 600–1000 °C. Fe–Sialon composites showed better erosion wear resistance than the control material of alumina ceramic at 1200 °C, although having much lower density and slightly lower bending strength.

Effect of wear debris of Al2O3, ZrO2 and WC balls on Y-α/β-SiAlON ceramics consolidated by spark plasma sintering

Three types of Y-α/β-SiAlON powders were prepared by mixing and milling the raw materials for 20 h using Al2O3, ZrO2, and WC balls, thereafter denoted as SNA, SNZ and SNW, respectively. The three types of specimens were sintered using spark plasma sintering (SPS) at 1510 °C for 5 min under 30 MPa in a vacuum. α-phases (SiAlON and Si3N4) and β-SiAlON phase were observed in the SNA and SNZ specimens, but only the β-SiAlON phase existed in the SNW specimen. The wear debris of the WC balls affected the grain boundary properties and eventually promoted the full phase transformation of α-Si3N4 into α- and β-SiAlON phases. However, SNA and SNZ were partially transformed into α- and β-SiAlON phases. The Vickers hardness of SNA was the highest (18.7 GPa), due to its having the highest content of α-phase, but its fracture toughness was the lowest (4.09 MPam1/2) due to its having the lowest content of β-SiAlON phase. The wear debris and secondary phases existed at the grain boundary, mostly at the triple junction, and also affected the color. The color of the sintered specimen was quite different depending on the milling media.

Thermal and mechanical properties of hot pressed translucent Y2O3 doped Mg–α/β-Sialon ceramics

Different Sialon phases, intergranular phase and sinterbility of additives play an important role on thermal, mechanical and optical properties of Sialon ceramics. Hot pressed translucent Mg–α/β-Sialon composite was fabricated with MgO as a sintering additive. But with MgO, complete densification was not achieved because of its higher viscosity and higher melting point. Y2O3 is one of the best sintering additives to achieve complete densification and good mechanical properties in silicon nitride based ceramics. Therefore, different amount of Y2O3 was added to Mg–α/β-Sialon composite and effect of Y2O3 addition on microstructure, thermal, mechanical and optical properties was investigated. α-Sialon phase was increased with increasing the amount of Y2O3 up to 1wt.%, but addition of 2wt.% Y2O3, most β-Sialon phase was observed. Thermal conductivity and coefficient of thermal expansion values were increased with increasing the amount of Y2O3. However, translucency was decreased in higher Y2O3 addition.

Influence of nitrogen overpressure on the nitridation, densification and formation of β-SiAlONs produced by silicothermal reduction

Abstract β-SiAlON materials (with z = 1) with 2 mol% DyAG have been synthesised by silicothermal reduction under different nitrogen pressures. The possibility of rapidly nitriding silicon and forming the SiAlON phase has been investigated through its reaction sequence of formation under different pressures. β-SiAlONs were fully nitrided (degree of nitridation greater than 93%) and formed after a 1-h hold at 1400 °C, or after a straight ramp to 1500 °C under 0.7 MPa of nitrogen. This was not achievable under static nitrogen at atmospheric pressure where the degree of nitridation was only 43% and β-SiAlON phase represented only about 60% of the crystalline phase assemblage at the same temperature. The formation of β-SiAlON depended on the formation of Si 3 N 4 whose reaction rate was enhanced by nitrogen overpressures.

Effect of composition variation on phases and photoluminescence properties of β-SiAlON:Ce3+ phosphor

Abstract Ce 3+ -doped β-SiAlON blue phosphors with composition of Ce x Si 6− z Al z O z N 8− z (0.2≤ z ≤4) at a fixed x =0.01 were synthesized by the gas pressure sintering method. The phases and photoluminescence properties were found to be directly correlated to the z value. Higher z value would result in the formation of AlN polytypoid impurity phases. The optimal z value resides in the range 0.3≤ z ≤0.5, by which high purity of β-SiAlON phase, good crystallinity and fine grain size were obtained, leading to the best luminescence properties. These phosphors could be a good candidate for application in white light-emitting diodes using InGaN-based near–ultraviolet chips.

Preparation of Ca-α SiAlON powder by reduction-nitridation in a gas mixture of C<sub>3</sub>H<sub>8</sub> and NH<sub>3</sub>

Ca-α SiAlON powders were synthesized from a SiO2–Al2O3–CaCO3 powder mixture by reduction-nitridation with C3H8–NH3 gas. Ca-α SiAlON with small amounts of AlN and β-SiAlON phases was produced at 1450°C for 120 min. Angular particles and agglomerated and bonded particles with irregular shapes were formed. In the product prepared at 1450°C without any holding time, Si2N2O was detected as an intermediate phase, in addition to the Ca-α SiAlON, β-SiAlON and AlN phases. A large number of agglomerated and bonded particles were observed.

Synthesis of Monolithic β-Sialon Powders (Si6−zAlzOzN8−z, Z = 2−4) through Controlling the Combustion Reaction Temperature

Pure β-sialon powders with the controlled z values (z = 2− 4) were successfully prepared from stoichiometric mixtures of constituent elements (Si, Al and SiO2) under a nitrogen pressure of 1 MPa by the combustion synthesis (CS) method. The raw materials were combusted with different ratios of a commercial sialon powder of z = 1 as a diluent (0, 10, 20, 30, 40, 50 wt %). Without diluent, the reactions temperatures were very high (> 2000 °C) and the product contained metal Si residue besides Sialon products. The combustion temperatures of nitridation reactions were controlled by using Sialon (z = 1) as a diluent. The combustion reactions completed at 50 wt% dilution, and pure β-sialon phases were synthesized. However, the individual peaks of β-Sialon (z = 1) diluent were also detected. Single-phase β-sialon powders were obtained by using the products as diluents and repeating the combustion for few times.

Synthesis of single phase β-SiAlON ceramics by reaction-bonded sintering using Si and Al 2O 3 as raw materials

Single-phase β-SiAlON compacts with z = 1 have been synthesized by reaction-bonded sintering using Si and Al2O3 as raw materials. The nitrided samples consisted of fine equiaxed particles and continuous fibers. β-SiAlON fibers with a diameter approximately 50 nm were obtained through the vapor–solid mechanism. After post-sintering at 1650–1750 °C for 2 h, only β-SiAlON phase with z = 1 was obtained. α-Si3N4 completely changed into β-SiAlON through the solution-precipitation mechanism. β-SiAlON shows different microstructures at different post-sintering temperature.

The effect of precursor composition and sintering additives on the formation of β-sialon from Al, Si and Al 2O 3 powders

A study was performed to investigate the effect of increasing the Al or Al 2O 3 precursor content, above the stoichiometric amount, on the formation of β-sialon by pressureless sintering of Al, Si and Al 2O 3 powders in flowing nitrogen gas. The effect of adding Y 2O 3 or Fe to the precursor mixture, on the β-sialon formation, was also studied. The phase morphology and yield produced by the various compositions were examined using X-ray diffraction (XRD). Additional Al 2O 3 decreases the β-sialon phase yield and results in a greater amount of Al 2O 3 in the final sintered material. Additional Al improved the conversion to β-sialon up to a maximum of 4 wt% Al beyond which the β-sialon:15R sialon ratio in the sintered material decreases. 1 wt% Y 2O 3 was determined to be the optimum sintering additive content, as yttrium aluminium garnet (YAG) was found to be present in materials formed from higher Y 2O 3 containing precursors. The presence of Fe in the precursor powder retards the formation of β-sialon by preferentially forming Fe silicides at low temperatures, thus depleting the reaction system of elemental Si, favouring the formation of 15R sialon.

Incorporation of the transition metals (Cr and Fe) into β-SiAlON crystal structure

Outstanding properties for SiAlON ceramics can be obtained by tailoring the microstructure through α-SiAlON and β-SiAlON phase content as well as a type of secondary phases. It is so far well known that while some of the elements could be accommodated in α-SiAlON structure, β-SiAlON does not easily accommodate different elements in its structure. In this work, SiAlON ceramics were produced by using β-Si 3N 4 starting powder containing small amount of iron and chromium and the possible incorporation of iron and chromium into β-SiAlON structure was investigated by using analytical transmission electron microscopy (TEM) techniques. As a result of analytical TEM analysis, it is found that transition metals (Cr and Fe) can enter into the β-SiAlON crystal structure.

Combustion synthesis of single-phase β-sialons ( z = 2–4)

Single-phase β-sialon powders ( z = 2–4) have been prepared with homogeneous compositions by the combustion synthesis. The raw materials (Si, Al and SiO 2) were combusted under N 2 pressure of 1 MPa. Without using a diluent, the reaction temperatures were very high (>2000 °C) and the combustion products contained Si and Al residues. With addition of commercial β-sialon ( z = 1) as a diluent (up to 50 wt%), both the reaction temperatures and amount of residual Si and Al significantly decreased. The combustion reactions completed at 50 wt% dilution, and pure β-sialon phases were synthesized. When the combustion product itself is used instead of the commercial diluent, the phase content of desired z value increased with the repetition times until a single-phase powder is produced. The sinterability of the synthesized powders was then tested using 5 wt% Y 2O 3 as a sintering aid by the spark plasma sintering (SPS).

Luminescence properties of Eu 2+-doped β-Si 6−zAl zO zN 8−z microcrystals fabricated by gas pressured reaction

Eu 2+-doped β-SiAlON microcrystals with compositions of Eu x Si 6− z Al z O z N 8− z ( x=0.016, z=0.1–2.0) were successfully fabricated by gas pressured reaction. The phase purity, microstructure, crystallinity and luminescent properties of Eu 2+-doped β-SiAlON microcrystals were investigated in detail. The prepared β-SiAlON:Eu 2+ phosphor crystals with Al-doping concentration of z<1.0 showed pure single β-SiAlON phase without secondary polytypoid phases. The prepared β-SiAlON:Eu 2+ microcrystals absorbed a UV-visible spectral region, and showed a single intense emission peak in a spectral range from 528 to 536 nm. The optimal conditions of Al 3+-doping concentration ( z value) for emission intensity, peak wavelength and bandwidth for application to light emitting diodes (LEDs) were discussed.

Luminescent properties of .BETA.-SiAlON:Eu2+ green phosphors synthesized by gas pressured sintering

β-SiAlON: Eu2+ oxynitride green phosphors with compositions of EuxSi6-zAlzOzN8-z (x=0.0065 -0.0390, z=0.231) were successfully prepared using GPS (gas pressured sintering) method. The phase purity, microstructure, luminescent and thermal quenching properties for the prepared β-SiAlON:Eu2+ phosphors were investigated in detail. The prepared β-SiAlON:Eu2+ phosphor samples with Eu2+ doping concentration of x<0.0238 showed pure single β-SiAlON phase. The prepared β-SiAlON: Eu2+ phosphors absorbed UV-visible spectral region, and showed a single intense broadband emission in a range from 525 to 540 nm. The effects of the Eu2+ doping concentration on the optical properties for the β-SiAlON: Eu2+ phosphors were discussed under consideration of concentration quenching. The temperature dependence of photoluminescence (PL) properties was investigated from 25 to 300°C, and the activation energies (ΔE) for thermal quenching of the prepared β-SiAlON:Eu2+ phosphors were determined by Arrhenius fitting. The experimental results clearly indicates that the prepared β-SiAlON:Eu2+ has great potentials as a down-conversion green phosphor for white light emitting diodes (LEDs) utilizing near UV or blue LEDs as the primary light source.

Influence of Chemical Composition and Y2O3 on Sinterability, Dielectric Constant, and CTE of β‐SiAlON

Different types of dense β‐Si6−zAlzOzN8−z (z=1, 1.5, 2, 2.5, 3, 3.5, and 4) ceramics have been prepared following a conventional reaction sintering process at 1675°–1700°C using α‐Si3N4, α‐Al2O3, AlN, and 3–7‐wt% Y2O3 as raw materials. Sintered materials were thoroughly characterized for bulk density (BD), apparent porosity (AP), water absorption (WA) capacity, phase formation, microstructure, coefficient of thermal expansion (CTE), hardness, fracture toughness, three‐point bent strength, and dielectric constant at 16–18 GHz frequency. Characterization results suggest that an increase in z value, Y2O3 concentration, and sintering temperature leads to an increase in β‐SiAlON phase formation, BD, grain size, fracture toughness, and dielectric constant, and as a consequence, AP, WA capacity, hardness, and three‐point bend strength of the materials decrease. These materials also exhibited stable and low dielectric constants (5.67–7.67) between 16 and 18 GHz frequency. The β‐Si4Al2O2N6 exhibited a BD of ∼3.06 g/mL, AP of ∼0.01%, WA capacity of ∼0.01%, ∼94.43%β‐SiAlON phase, a hardness of ∼1317 kg/mm2, a fracture toughness of ∼3.30 MPa·m1/2, a three‐point bend strength of ∼226 MPa, a CTE of 3.628 × 10−6°C−1 between 30° and 700°C, and a dielectric constant of ∼7.206 at 17 GHz after sintering at 1675°C for 4 h with 7‐wt% Y2O3.

Preparation and Characterization of β-SiAlON Ceramics from High Aluminium Fly Ash via Carbothermal Reduction-Nitridation

In this work, β-sialon ceramics were prepared from high-aluminium fly ash via carbothermal reduction-nitridation (CRN) and the physicochemical properties of the materials such as bulk density, apparent porosity, water absorption and flexural strength were also discussed. The results showed that the percentage of β-sialon phase in the product decreases as the temperature increases from 1400°C and the weight of the sintered specimen experienced an increase during 1350°C~1450°C due to the nitridation reactions, and followed by a gradual decrease till 1550°C for the decomposition of β-sialon. It is indicated that the optimum sintering temperature to obtain the highest yield of β-sialon ~93% lies in 1400°C~1450°C. The SEM images revealed that the prepared β-sialon sintered at 1400°C were mainly in shape of elongated prisms, typically ~5μm in length and 0.5~1μm in width. As the temperature increased to 1500°C and above, β-sialon decomposed and the new phases of SiC and AlN were formed at 1550°C as confirmed by XRD.