Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Long wavelength Ce emission in Y6Si3O9N4 phosphors for white-emitting diodes

Y₆Si₃O₉N₄:Ce³⁺ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y₆Si₃O₉N₄:Ce³⁺. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce³⁺-doped Y₄Si₂O₇N₂ phosphors, Ce³⁺-doped Y₆Si₃O₉N₄ phosphors showed longer wavelengths of both excitation and emission. The Y₆Si₃O₉N₄:Ce³⁺ is a potential green-emitting phosphor for white LEDs.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

Fabrication and characterization of porous unidirectional Si2N2O-Si3N4 composite

Porous unidirectional Si₂N₂O-Si₃N₄ composite was fabricated by in-situ nitriding of a porous unidirectional Si substrate. The porous unidirectional Si substrate having a diameter of 450 μm, was prepared by forming ethanol bubbles in a slurry which contained Si, Y₂O₃, Al₂O₃ and methylcellulose powder. After nitridation at 1400 °C, the Si substrate was transformed into Si₂N₂O-Si₃N₄ composite and the pore surface of the unidirectional Si₂N₂O-Si₃N₄ composite was covered throughout with Si₂N₂O fibers, which had a diameter of about 55 nm. The Si₂N₂O fibers were orthorhombic single-crystals with an amorphous layer having a thickness of about 1 nm. The compressive strength of the in-situ synthesized Si₂N₂O-Si₃N₄ composite was about 30 MPa.     

Sintering process of Y2O3-added Si3N4

The sintering process of Y₂O₃-added Si₃N₄ has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y₂O₃-SiO₂-Si₃N₄ triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol^–1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol^–1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si₃N₄ starting powder promoted the reaction of Si₃N₄ with eutectic melts. After a period of initial-stage sintering, the formation of fibrous -Si₃N₄ grains resulted in interlocked structures to interrupt the densification.     

Pressureless sintering of Si3N4 with Y2O2 and Al2O3

Pressureless sintering of Si₃N₄ with Y₂O₃ and Al₂O₃ as additives was carried out at 1750°C in N₂ atmosphere. Si₃N₄ materials which had more than 92% relative density were obtained with 20wt% addition of additives. The flexural strength of as-sintered materials containing 5 to 8.6wt% Al₂O₃ and 15 to 11.4wt% Y₂O₃ was in the range of 480 to 560 MPa at room temperature. The glassy grain-boundary phase of as-sintered materials crystallized to 3Y₂O₃ · 5Al₂O₃ (YAG), Y₂O₃ · SiO₂ (YS), Y₂O₃ · 2SiO₂ (Y2S) and 10Y₂O₃ · 9SiO₂ sd Si₃N₄ (NA) by heat-treatment at 1250° C for 3 days. A specimen containing 15wt% Y₂O₃ and 5wt% Al₂O₃ sintered at 1750° C for 4 h was heat-treated at 1250° C for 3 days to precipitate YAG and YS. The nitrogen concentration of the grain-boundary glassy phase of the specimen was found to be very high, and therefore the flexural strength of the crystallized specimen scarcely decreased at elevated temperatures (the flexural strength of this specimen is 390 MPa at room temperature and 360 MPa at 1300° C). Resistance to oxidation at 1200° C of the specimen was good as well as the flexural strength, compared with that of as-sintered materials.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.     

Formation Mechanism of Microstructure of Fe2(MoO4)3/Si3N4 Composite Powder by Hydrogen Reduction

观测Fe₂(MoO₄)₃和Fe₂(MoO₄)₃/Si₃N₄粉末H₂还原后的微结构特征, 研究了其微观组织结构的演变。 结果表明: Fe₂(MoO₄)₃还原后转变为20 nm厚的Fe薄层包覆Mo颗粒的微结构; Fe₂(MoO₄)₃/Si₃N₄粉末被还原后转变为两种结构形式颗粒粉末, 一种为3--5 nm的薄层Fe包覆在Mo颗粒表面粉末, 一种为粘附有纳米Fe--Mo氮化物、Si、Mo等颗粒的Si₃N₄粉末。Fe₂(MoO₄)₃/Si₃N₄粉末还原后形成这种微结构的原因是, 在还原过程中同时发生了两种反应: 一种是Fe₂(MoO₄)₃自身发生分解还原反应, 另一种是Fe₂(MoO₄)₃与Si₃N₄颗粒表面发生反应。     

Characterisation of binary (Fe3O4/SiO2) biocompatible nanocomposites as magnetic fluid

This article describes coating of magnetite nanoparticles (NPs) with amorphous silica shells. Controlled co-precipitation technique under N₂ gas was used to prevent undesirable critical oxidation of Fe²⁺. The synthesised Fe₃O₄ NPs were first coated with trisodium citrate to achieve solution stability and then covered by SiO₂ layer using Stober method. For uncoated Fe₃O₄ NPs, the results showed an octahedral geometry with saturation magnetisation range of 82–96?emu/g and coercivity of 85–120?Oe for particles between 35 and 96?nm, respectively. The best value of specific surface area (41?m²/g) for Fe₃O₄ alone was obtained at 0.9?M NaOH at 750?rpm and it increased to about 81?m²/g for Fe₃O₄/SiO₂ combination. The total thickness and the structure of core–shell was measured and studied by transmission electron microscopy. The average particles size was about 50?nm, indicating the presence of about 15?nm SiO₂ layer. Finally, the stable magnetic fluid contained well-dispersed magnetite-silica nanocomposites which showed monodispersity and fast magnetic response.     

Hot-pressed β-Si3N4 containing small amounts of Be and O in solid solution

Single phase, hot-pressed Si₃N₄ ceramics with relative densities >95% and equiaxed grain structures have been prepared from high purity Si₃N₄ powders having specific surface areas of 8 to 20 m² g⁻¹ and oxygen contents ⩾2 wt % using a small amount of Be₃N₂ or BeSiN₂ as a densification aid. Densification depended sensitively on the concentration of Be and O in a given Si₃N₄ powder and on the usual hot-pressing parameters of pressure, temperature and time. A close association was found between densification and the conversion ofα- toβ-Si₃N₄ during hot-pressing. Based on the data presented, chemical reactions that occur during hot-pressing involve: (1) reaction of the densification aid with SiO₂ on the Si₃N₄ particle surfaces to form BeO and Si₂N₂O; (2) the further reaction of these two reaction products to give probable formation of a transient liquid phase (TLP); and (3) the reaction between TLP andα-Si₃N₄ particles to cause densification, probably by a solution-reprecipitation process, and conversion ofα-Si₃N₄ into aβ-Si₃N₄ solid solution. The chemical composition of a single phaseβ-Si₃N₄ solid solution prepared in this study by hot-pressing was approximately Si_2.9Be_0.1N_3.8O_0.2.     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.     

High-temperature oxidation of silicon nitride-based ceramics by water vapour

Hot-pressed Si₃N₄, sintered Si₃N₄ and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO₂, Y₂Si₂O₇ and Y4A1209. The oxidation rate was in the order sialon > sintered Si₃N₄ > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y₂O₃ content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol^–1.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase

The comparison of microstructure and fracture characteristics of gas-pressure-sintered (GPS) Si₃N₄ ceramics with and without second crystalline phase at the junction regions was investigated by a combination of SEM, HRTEM and micro-indentation techniques. The bimodal microstructure composed of many large and fine rod-like Si₃N₄ grains was made by carbothermal reduction treatment (CRT). Most of triple regions were comprised of Y₃AlSi₂O₇N₂ as a second crystalline phase while all of grain boundaries and interfaces between Si₃N₄ and Y₃AlSi₂O₇N₂ phase were an amorphous layer of about 2 nm in thickness. Although a few large, rod-like Si₃N₄ grains were also observed in the sample without CRT, but most of junction regions existed with an amorphous phase. The value of fracture toughness in the sample with second crystalline phase at the junction regions by CRT was 6.6 MPa m^1/2. This was slightly higher than the value without second crystalline phase (5.1 MPa m^1/2). The main reason for the slight increase of the fracture toughness, even though showing well-developed bimodal microstructure, is the formation of the Y₃AlSi₂O₇N₂ phase at junction regions, which led the transgranular fracture of Si₃N₄ grains at local regions.     

Plasmons in double interfaces system of Si3N4/SiO2/Si irradiated by Co

The first level plasmons of Si in the pure Si state, in the SiO₂ state and in the Si₃N₄ state (corresponding to bonding energy 116.95, 122.0 and 127.0 eV) were investigated directly with X-ray photoelectron spectroscopy before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of plasmons of Si in the Si₃N₄ and SiO₂ states, while another was made of plasmons of Si in the pure Si state and in the SiO₂ state. When the Si₃N₄-SiO₂-Si samples were irradiated by ⁶⁰Co, the interface at Si₃N₄/SiO₂ was extended and at the same time the center of this interface moved towards the surface of Si₃N₄. The concentration of plasmon for silicon in the SiO₂ state is decreased at the SiO₂-Si interface, and the effects of radiation bias field on plasmons in the SiO₂-Si interface are observable. Finally, the mechanism of experimental results is analyzed by the quantum effect of plasmon excited by the photoelectron.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

BN/Si3N4 nanocomposite with high strength and good machinability

BN/Si₃N₄ nanocomposite was prepared using BN/Si₃N₄ powder obtained by nitriding Si₃N₄/NH₄HB₄O₇ mixture in ammonia gas as the starting powder. Microstructural investigations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that BN particles were homogeneously distributed within the matrix grains as well as at the matrix grain boundaries, and the growth of Si₃N₄ matrix grain was significantly retarded by BN particles. The BN/Si₃N₄ nanocomposite showed a higher strength than the conventional BN/Si₃N₄ microcomposite due to the formation of fine and homogeneous microstructure in it. BN/Si₃N₄ nanocomposite with a BN content of 20 vol% and above showed excellent machinability, because of the formation of weak BN/Si₃N₄ interfaces and the cleavage behavior of BN particles.