Sialon Phase Research Articles

Zol-gel synthesis of SiAlON materials dopped by rare-earth elements

The phase composition and morphological features of си-sialons doped with REE (Eu, Tb, Ce) were studied by nitriding the mixed sol prepared on the basis of silicon and aluminum alkoxides without introducing the corresponding nitrides. It was established that as a result of nitriding of a doped REE mixed Si-Al sol at 1650 °C, the β-SiAlON phases with a hexagonal crystal structure corresponding to β-Si3Al3O3N5 are formed. At a firing temperature of 1700 °C crystalline sialon types corresponding to the χ-phase with a monoclinic structure (Si2Al3O7N and Si6Al10O21N4) are formed. The effect of the addition of red-earth elements on the morphology of crystalline particles of the resulting sialon phases is shown.

Isotropic Si3N4 ceramics fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting

Isotropic Si3N4 ceramics were fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting (DCC/LP-SPS). A novel phenomenon is observed that hardness and fracture toughness of Si3N4 lift simultaneously as temperature rises. It can be attributed to the enhanced densification and formation of β′-sialon. Rapid coagulation of the submicron ceramic suspension prepares green body with high relative density and offers the sufficient driving force for densification, while electric field facilitates particles rearrangement, thus Si3N4 ceramics can obtain the relative density above 97% under the axial pressure of 5 MPa at 1550 °C. As SPS temperature rises, there is the formation of β′-sialon phase in Si3N4 ceramics without AlN as sintering aid. This is most likely due to electric field promoting the motion and uniform distribution of charged species in liquid phase, which facilitates the replacement of Si4+ and N3− by Al3+ and O2− during the process of dissolution-precipitation. Additionally, compared with Si3N4 prepared by hot pressure sintering, there is no obvious directional alignment of grains in the samples that are spark plasma sintered under a low applied pressure. As a consequence, Si3N4 ceramics prepared by DCC/LP-SPS acquire complete transformation of α→β phase and full densification without abnormal grain growth, leading to ultimate formation of an isotropic self-reinforced bimodal structure containing β-Si3N4 and β′-sialon. Therefore, Si3N4 ceramics, coagulated by 45 vol% suspension and sintered at 1650 °C for 20 min with a uniaxial pressure of 5 MPa, simultaneously achieve the high Vickers hardness and fracture toughness as follows: 17.5 ± 1.0 GPa and 8.5 MPa m1/2.

Effects of Nitridation Time and Precursor Particle Size on Yield of β-SiAlON Synthesized from Calcined Kaolinite

Carbothermal reduction and simultaneous nitriding (CTRN) of natural aluminosilicates is a method used to obtain phases of Si–Al–O–N system. SiAlON phases have important applications, such as high technology ceramics, for wear parts, cutting tools or special purpose refractory materials. In this study carbothermal reduction and nitriding reactions of kaolinite obtained from Balikesir-Duvertepe area of Turkey were studied. The mean particle sizes of kaolinite used for CTRN process were 6.4, 4.67, and 2.29 μm, respectively. The reactions of kaolinite and carbon black mixture carried out at 1400°C for 2, 4, and 8 h. CTRN process was conducted in an atmosphere controlled tube furnace in nitrogen flow of 4 cm3/min. All products were examined by XRD and SEM to determine the transformation, morphology and chemical composition. The results showed that the most of SiAlON transformations occurred at 1400°C for 8 h. It was observed that when decreasing the mean particle size of the starting material the transformation of kaolinite to SiAlON was increased.

Formation mechanism of Sialon in alumina-ferro-silicon-nitride composite under nitrogen atmosphere at high temperatures

Sialon bonded Al2O3 composites were successfully synthesized using ferro-silicon nitride and different alumina sources at 1500 °C and 1600 °C under N2 atmosphere. Fused corundum, sintered alumina and the mixture of both were used as different alumina sources to evaluate their effects on the formation of Sialon phases. The samples were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDAS). The results show that the Sialons (β-Sialon (Si2Al4O4N4) and 15R-Sialon (SiAl4O2N4)) contents are dependent on the sintering temperature and alumina sources. Formation mechanism of Sialon in samples prepared with different alumina sources is different. Sintered alumina can react with Si3N4 directly to form Sialon. In sample prepared with fused corundum, AlON is formed first and then it reacts with Si to form Sialon. Sintered alumina exhibits better reactivity than fused corundum. Sialon is more stable than AlON under N2 atmosphere at high temperature with the existence of carbon.

Effect of Artificial Graphite and Nickel Nitrate on the Microstructure and Properties of Carbon Blocks for Blast Furnace

Improving thermal conductivity of carbon blocks is one of the most important developing trends for carbon blocks. In this work, artificial graphite and nickel nitrate catalyst were introduced into carbon blocks with an attempt to improve thermal conductivity of carbon blocks and their effect on microstructure and properties of carbon blocks were systematically studied by means of X ray diffraction (XRD), scanning electron microscopy (SEM) and laser thermal conductivity meter. The results revealed that artificial graphite possessed lower oxidation activation energy than electrically calcined anthracite, and thus, higher reactivity, which could accelerate the formation of SiC whiskers in carbon blocks and have a positive effect on thermal conductivity of carbon blocks. Moreover, it was interestingly noted that one dimensional nano-carbon was catalytically formed at 1000 °C and lots of Sialon phases were formed at 1400 °C when nickel nitrate catalyst was further added in carbon blocks containing artificial graphite. These in-situ ceramic phases formed in carbon blocks constructed high thermal conductive network and reduced the interface thermal resistance, thus improving the thermal conductivity of carbon blocks significantly.

Synthesis and characterization of β-Sialon powders from Si, halloysite clay and AlN powders

Two [Formula: see text]-Sialons, with [Formula: see text]-values of 1 and 4, respectively, were successfully synthesized by silicothermal reduction and nitridation method under 0.4 MPa nitrogen pressure. The effect of firing temperatures on the phase transformations and morphologies of [Formula: see text]-Sialons were analyzed by XRD and SEM. For [Formula: see text]-Sialons ([Formula: see text]), the product was finally composed of targeted [Formula: see text]-Sialon ([Formula: see text]) and secondary phase [Formula: see text]-Si3N4; for [Formula: see text], [Formula: see text]-Sialon ([Formula: see text]) was the main phase, and 15R-Sialon and [Formula: see text]-Al2O3 co-existed as secondary phases. A higher firing temperature is more beneficial for the phase transformations and crystal growth of [Formula: see text]-Sialons, however, the most suitable firing temperature was 1400[Formula: see text]C.

Effect of SiO2 addition on Si3N4 ceramics prepared by rapid nitridation and post-sintering route

Abstract Effects of SiO 2 content and Y 2 O 3 :Al 2 O 3 mass ratio on phase compositions, densification, microstructures and mechanical properties of sintered reaction-bonded Si 3 N 4 ceramics prepared by rapid nitridation of Si powders using TiO 2 as catalyst and subsequent gas pressure sintering were investigated. The low Y 2 O 3 :Al 2 O 3 ratio could reduce the adverse effect of high SiO 2 content on densification and microstructure by the formation of O′-Sialon phase. The bulk density of the ceramics with low Y 2 O 3 :Al 2 O 3 ratio was higher than that with high Y 2 O 3 :Al 2 O 3 ratio, especially at high SiO 2 content. Irrespective of Y 2 O 3 :Al 2 O 3 ratio, the increasing SiO 2 content led to microstructural coarsening, but the ceramics with high Y 2 O 3 :Al 2 O 3 ratio and SiO 2 content contained more large-elongated grains. The hardness and toughness of the ceramics with low Y 2 O 3 :Al 2 O 3 ratio slightly decreased with increasing SiO 2 content, and the strength basically had no change. Whereas the mechanical properties of the ceramics with high Y 2 O 3 :Al 2 O 3 ratio obviously decreased with increasing SiO 2 content due to the markedly decreased bulk density and increased grain size. The present results suggested the low Y 2 O 3 :Al 2 O 3 ratio would contribute to the improvement of mechanical properties of SRBSN ceramics prepared from low-grade Si powers containing high content of SiO 2 .

The effect of silicon nitride powder characteristics on SiAlON microstructures, densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si3N4 powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si3N4 powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si3N4 powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si3N4 powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si3N4 powders were coarser, but the fine β-Si3N4 powders yielded a bimodal microstructure. The critical particle diameter of the β-Si3N4 powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.

The Fluorine Effect on the Grain-Growth of -SiAlON Ceramics

SiAlON ceramics are of major interest for industrial applications due to their excellent mechanical properties. The starting Si 3 N 4 powder (α or β), the type of sintering additive and sintering conditions govern the microstructural evolution of SiAlON ceramics. In this study, α ı :β ı SiAlON ceramics with an equal mole intergranular phase content, different type cation systems (Y:Sm:Ca and Ca:Ce) with different molar ratio (30:64:6 and 33:67) were produced by using coarse (d 50 =1 µm) β-Si 3 N 4 powder. CaF 2 and CaCO 3 were utilized as Ca sources. An increase in α ı -SiAlON amount in the case of employing CaF 2 as opposed to CaCO 3 in Y:Sm:Ca cation system was observed. Despite the fact that α ı -SiAlON phase cannot be stabilized in Ca:Ce (CaCO 3 as a Ca source) system, α ı -SiAlON phase stability and unpredictable α ı -SiAlON grain-growth in both a and c directions was easily achieved when use of CaF 2 . Furthermore, using CaF 2 instead of CaCO 3 gives rise to an increase in densities. Therefore, the impressive effect of CaF 2 on abnormal grain growth and α ı -SiAlON phase stabilization was discussed.

Phase and morphological transformation stages during carbothermal reduction nitridation process: From coal gasification slag wastes to Ca-α-SiAlON powders

Effective utilization of industrial solid wastes is of great benefit for environmental and economic needs. In order to clarify the theoretical issues and pave the way to synthesize high quality Ca-α-SiAlON powder from coal gasification slag by carbothermal reduction nitridation (CRN), in this study, the theoretical issues including the network structure of the slag as well as the phase and morphological transformation stages of raw slag during CRN process were comprehensively studied at different temperatures (1350, 1400, 1450 and 1500°C). It was found that the CRN process was dominated by nitridation of the Ca-Si-Al-O glass component in slag. With the increase of processing temperature, spherical Ca-Si-Al-O particles were firstly nitrided into porous Ca-Si-Al-O-N spheres, plate-like O-SiAlON grains, and then nitrogen-rich SiAlONs (β-SiAlON, 15R polytypoid and α-SiAlON). At the end of CRN, Ca-α-SiAlON was found to be the only SiAlON phase presenting elongated prismatic morphology. In addition, high concentration Ca-α-SiAlON powder was obtained through a two-step purification process with a conversion rate as high as 45wt%. The obtained results shed light on the preparation of Ca-α-SiAlON products from coal gasification slag and related wastes.

Effect of holding time on microstructure and properties of dense α-Si3N4/Sialon composite coating on porous Si3N4 ceramic

A dense α-Si3N4/Sialon composite coating was fabricated on porous Si3N4 ceramic using the liquid infiltration and filling method in order to improve mechanical properties and waterproof. The effect of holding time on microstructure, water absorption and mechanical properties of the coating was investigated in detail. The results showed that all the coatings mainly consisted of a dominant α-Si3N4 phase and minor O′-Sialon, β-Sialon and Y-Si-Al-O-N glass phase. The content of O′-Sialon phase increased gradually as the holding time increased from 0 to 3h, however it started to decrease while the content of β-Sialon phase increased abruptly when the holding time reached 4h. As-prepared coatings exhibited good waterproof ability, and water absorption of porous Si3N4 ceramic decreased obviously after fabricating the coatings. Vickers hardness and fracture toughness of the coated porous Si3N4 ceramic increased gradually as the holding time increased from 0 to 3h, but they decreased obviously when the holding time was 4h because large amount of pores were formed within the coating.

Effects of Si<sub>3</sub>N<sub>4</sub> Additions on Creep Resistance of Al-Al<sub>2</sub>O<sub>3</sub> Composites

Si3N4 (0 and 6 mass%) were added to Al-Al2O3 composite respectively, with phenolic resin as binder, to make Al-Si3N4-Al2O3 unfired composites, which were dried at 200°C for 24h.The creep properties of composites were tested researched under 0.2MPa×50h at 1300°C. The results indicated that the samples put up the features of steady-state creep stage from 25h to 50h; Al4O4C appeared in the corundum matrix with the addition of Al,which improve its creep resistance; when Al and Si3N4 were both existed, sialon phase was formed while Al4O4C was disappeared.

Photoluminescence and thermal quenching properties of tunable green-emitting oxynitride (CaxSr1–x)6Si25.6Al6.4N41.6O4.4:Eu2+ phosphors

Green emitting Eu2+ doped (CaxSr1–x)6Si25.6Al6.4N41.6O4.4 phosphors with x value ranging from 0 to 0.1 were synthesized by the solid state reaction method under nitrogen atmosphere. The X-ray diffraction (XRD) patterns of the phosphors with different Ca2+ concentrations indicated that pure sialon phases were obtained. Crystal structure of these sialon phases was estimated to be a commensurate composite network stacking by two different types of layers. Intense and tunable green emissions with a slight red shift from 515 to 520 nm were observed with varying Ca/Sr ratios. The emission intensity decreased gradually because of the increase of the crystal splitting effect. Thermal quenching properties of the phosphors with different Ca2+ saturation were also discussed. The thermal stability became worse as more Ca2+ ions substituted for Sr2+ ions according to a larger Stokes shift. The solid solution phosphors could be a promising candidate for white LEDs for their interesting photoluminescence properties when the thermal stability would be improved.

IR to visible upconversion luminescence of transparent (Mg,Er)–α-Sialon ceramics

A detailed study of upconversion (UC) process was carried out in Er2O3 doped Mg–α-Sialon ceramics. IR to visible upconverting transparent Sialon ceramics has been fabricated by hot press sintering method. Different Sialon phases, grain morphology, and chemical compositions were analyzed by X-ray diffraction and Transmission Electron Microscopy. With increasing the amount of Er2O3 in Mg–Sialon system, the α-Sialon phase was also increased. The UC emissions were observed around 527, 547 and 660nm wavelength corresponding to the 2H11/2, 4S3/2 and 4F9/2 transitions, respectively under 980nm continuous wave (CW) laser diode excitation. UC process in Er3+ is governed by two photonic processes. Increasing the doping concentration (Er2O3), dipole–dipole energy transfer (ET) mechanisms between nearby Er3+ ions become more efficient than excited state absorption (ESA). The intense UC was observed in the green region due to low phonon energy of Sialon system as revealed by Raman spectroscopy. The red UC luminescence was also observed, but the emission intensity was less as compared to the green emission. Moreover, the down-conversion luminescence was observed around 1530nm corresponding to 4I13/2 transition.

Upconversion luminescence properties of Er3+/Yb3+ in transparent α-Sialon ceramics

Er3+ stabilized transparent α-Sialon ceramics with upconversion (UC) luminescence properties were fabricated for the first time by hot press sintering method. The fabricated samples show better transparency as well as UC luminescence properties. Moreover, the effect of Yb2O3 as a sensitizer on UC properties was analyzed in Er-α-Sialon system. The change in Sialon phases and crystal structure were analyzed by X-ray diffraction. The chemical compositions and morphology of grains and intergranular phase were analyzed by EDS and Transmission Electron Microscopy, respectively. The UC emissions were observed around 527, 547 and 670nm corresponding to the 2H11/2, 4S3/2 and 4F9/2 transitions, respectively when excited by 980nm continuous wave (CW) laser diode. UC process in Er3+ is found to be two photon processes. Intense green UC luminescence was observed with Er3+ and low amount of Yb3+ addition but higher amount of Yb3+ addition shows higher UC luminescence in red region. The transparency was higher in higher Er2O3 and low Yb2O3 content Sialon that was around 60% in visible light region. The chromaticity was also calculated for different samples and high color purity above 90% was obtained for green and yellow emissions. The mechanical properties were also investigated in this study.

Fabrication of Porous SiAlON Using Fe<sub>2</sub>O<sub>3</sub> as Pore Former

Porous SiAlON ceramics were fabricated by carbo-thermal reduction nitridation method using Fe2O3 as pore former. Particle size effects of Fe2O3 were reported in this paper. The results showed that composites were composed by SiAlON, AlN and Iron Silicon phases. The median pore diameter of Sialon was affected by the composition and particle size of Fe2O3. The fracture mode of this material was intergranular. With the increase of Fe2O3 additions, the porosity of this materials increased. The bending strength of this material was reversely proportional to Fe2O3 particle size. The maximum bending strength of Porous materials with 30wt.% Fe2O3 additions (with a porosity about 65% and the pore size is about 1μm) could reach 22 MPa. The porous Sialon ceramics with a smaller pore size exhibited a higher bending strength.

Properties of amorphous SiAlON thin films grown by RF magnetron co-sputtering

SiAlON thin films were deposited by RF magnetron co-sputtering of Al and Si targets in Ar/O2/N2 mixtures to a thickness of ~200nm onto both bare and Pt-coated r-cut sapphire substrates. Films deposited at 200°C are amorphous as determined from X-ray diffraction (XRD) and have <1nm RMS roughness as measured by X-ray reflectivity (XRR). In situ X-ray photoelectron spectroscopy (XPS) measurements from films grown over a wide range of SixAlyOzN100−x−y−z stoichiometries indicate that a homogenous amorphous phase is formed over all compositional regions of the quaternary thin film phase diagram. After annealing at 1000°C for 10h in vacuum, the film stoichiometries remained nearly unchanged and the films retained an amorphous structure, as verified by XRD. The films lost nitrogen during air exposure at 1000°C, leading to the formation of an amorphous aluminum silicate layer at the surface. No crystalline SiAlON phases, which have been reported for bulk SiAlON materials, were observed in films even after heating at 1500°C for 10days. Pin-on-disk measurements showed that SiAlON films have negligible wear up to 80gram loads, while significant wear occurs on the sapphire pin in sliding contact, indicating that the SiAlON films have excellent wear resistance.

Effects of a carbothermal reduction reaction on the microstructure of nano-silicon nitride ceramics

In this study, using amorphous nano-Si3N4 as the starting powder, the oxides on the Si3N4 powder surface were removed by carbothermal reduction to prevent the formation of the O′-sialon phase in the sintered product. The results indicate that carbothermal reduction can be effectively induced at 1250°C and produce single-phase Si3N4. However, as the temperature increased, the liquid phase that formed during Si3N4 sintering began to impede the carbothermal reduction. Analysis revealed that when the temperature exceeded 1350°C, the reaction gradually shifted from that of carbothermal reduction to that of decomposition, and residual carbon and O′-sialon phase were observed in the sintered product.

IR transparent hot pressed Mg-α/β-Sialon:Ba2+ ceramics

IR transparent Mg-α/β-Sialon:Ba2+ ceramics were fabricated by the hot press sintering method. The influence of BaCO3 addition to Si3N4–MgO–AlN system on transparency, microstructures and mechanical properties were investigated. Reactions between the mixed powders were analyzed from differential thermogravimetric (DTG) curve and formation of Celsian (BaAl2Si2O8) was observed at 1175°C. The α-Sialon phase in sintered bodies was increased with BaCO3 addition due to the excess liquid phase that allows Mg2+ diffusion in α-Si3N4 lattice to give higher α-Sialon phase as revealed from XRD patterns. To minimize the light scattering by mismatch of refractive indices between the Sialon phase and glassy phase, barium ion was introduced to match the refractive index of glassy phase to Sialon phase. The study of electron spin resonance (ESR) spectra shows that unpaired electrons affect the IR transmittance in Sialon ceramics. Weak ESR spectrum was shown for the IR transparent BaCO3 added Sialon ceramics. The highest light transmission was observed around 78% in IR region (2500nm), and had better transparency than previously reported Sialon ceramics. The fabricated Sialon ceramics had high hardness of 21.4GPa and fracture toughness of 5.4MPam1/2.

Analytical transmission electron microscopy observations on the stability of TiCN in electrically conductive α-β SiAlON/TiCN composites

Abstract Coating of α-β SiAlON granules with TiCN powder led to a continuous 3D electrically conductive network formation. Here, the stability of TiCN grains following gas pressure sintering (GPS) and its effect on the electrical properties of an α-β SiAlON/TiCN composite were analytically investigated by using transmission electron microscopy (TEM) based techniques. In situ formation of nano-sized SiC grains adjacent to TiCN and α-β SiAlON grains were observed. These SiC grains may play a role on the high electrical conductivity of α-β SiAlON/TiCN composite. Ti:C:N ratios from TiCN grains along the network showed that TiCN grains did not preserve their initial composition after sintering. Finally, Ti diffusion from TiCN grains into α-β SiAlON and triple junction phases was observed. The incorporation of Ti into the SiAlON crystal lattices contributes to the high electrical conductivity of α-β SiAlON/TiCN composite.