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.     

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.     

Effect of grain-boundary crystallization on the high-temperature strength of silicon nitride

Si₃N₄ specimens having the composition 88.7 wt% Si₃N₄-4.9wt% SiO₂-6.4wt% Y₂O₃ (85.1 mol% Si₃N₄-11.1 mol% SiO₂-3.8mol% Y₂O₃) were sintered at 2140° C under 25 atm N₂ for 1 h and then subjected to a 5 h anneal at 1500° C. Crystallization of an amorphous grainboundary phase resulted in the formation of Y₂Si₂O₇. The short-time 1370° C strength of this material was compared with that of material of the same composition having no annealing treatment. No change in strength was noted. This is attributed to the refractory nature of the yttrium-rich grain-boundary phase (apparently identical in both glassy and crystalline phases) and the subsequent domination of the failure process by common processing flaws.Chemical analysis of the medium indicated 5.25 wt% O₂, 0.46 wt% C, 0.8 wt% Al, and expressed in p.p.m. 670 Ca, 30 Cu, 2000 Fe, <2 Ti, 370 Cr, 130 Mg, 90 Mn, <10 V, <20 Zr, 2000 Mo, 240 Ni, 130 Zn, <30 Pb, <60 Sn.     

Effect of oxide addition on the sintering and high-temperature strength of Si3N4 containing Y2O3

The effect of oxide addition on the sintering behaviour and high-temperature strength of Si₃N₄ containing Y₂O₃ was studied at 0.1 to 30 MPa N₂ at 1600 to 2000° C. The addition of oxide, i.e. MgO, Al₂O₃, La₂O₃, or Nd₂O₃, was found to lower the densification temperature and increase the densification rate. The addition of Al₂O₃ or MgO reduced the strength of sintered materials at >1350° C. The addition of La₂O₃ or Nd₂O₃, on the other hand, did not affect high-temperature strength which remained equivalent to that of the material containing only Y₂O₃. These results indicate that the glassy phases in these systems are as refractory as that in the Si₃N₄-Y₂O₃.     

Mechanochemically induced formation of La2SiO5

A mechanochemical technique was applied to prepare La₂SiO₅ under conditions where the conventional solid-state synthesis shows unsatisfactory results. The effects of the mechanochemical treatment of a mixture of lanthana and silica gel (in molar ratio La₂O₃/SiO₂ = 4:3 has been studied by X-ray diffractometry (XRD), infrared spectroscopy (IR) and using a scanning electron microscope (SEM). Differential thermal analysis (DTA) and thermogravimetry (TG) have been used to follow the thermal behaviour of initial and milled samples. It was found that the amorphous silicate precursor of La_4.67(SiO₄)₃O is formed as a result of a mechanochemical solid-state reaction. The crystallization of the latter silicate occurs at 880°C during the subsequent heat treatment of the milled samples. The formation of La₂SiO₅ without any XRD—detectable traces of La_4.67(SiO₄)₃O takes place after heating at 1100°C for 2 h. The rate of conversion increases with increasing the milling time, reaching 96% after mechanochemical treatment for 3 h and subsequent heating at 1100°C.     

Crystallization, microstructure development and dielectric behaviour of glass ceramics in the system [SrO · TiO2]-[2SiO2 · B2O3]-La2O3

Various glasses in the system (65 – x)[SrO · TiO₂]-(35)[2SiO₂ · B₂2O₃]-(x)La₂O₃, where x = 1,5,10 (wt%) were prepared by melting in alumina crucible (1375–1575 K). Heat treatment schedules were selected from DTA plots of respective glasses. X-ray diffraction studies of glass ceramic samples containing different concentrations of La₂O₃ revealed the formation of Sr₂B₂O₅, Sr₃Ti₂O₇ and TiO₂ (rutile) phases. The addition of La₂O₃ results in the development of well formed, elongated crystallites of different phases. Results of the dielectric behaviour demonstrate higher values of dielectric constant for some of the glass ceramic samples. This can be ascribed to the relaxation polarization at the crystal-glass interface due to conductivity differences between crystalline and glassy phases.     

High-temperature glass-ceramics of the BaO-Al2O3-SiO2-Ta2O5 system and molybdenum metal composites

Glass-ceramics which consist of tantalum pentoxide (Ta₂O₅), hexacelsian (BaO·Al₂O₃· 2SiO₂), and aluminium tantalate (Al₂O₃·Ta₂O₃) are described. These glass-ceramics can form refractory composites up to 1400° C with molybdenum metal. The glass-ceramics and metal have compatible physical and chemical properties which allow close thermal expansion and excellent bonding.     

Single-crystal growth of La4Si2O7N2 by the floating zone method

Single crystals of La₄Si₂O₇N₂ were prepared successfully by the floating zone method using a radiation convergence type apparatus with a halogen lamp as the heat source. Crystals were grown under a nitrogen pressure of 11 atm and at a rate of 2 mm h^–1. The addition of 2.6 wt% (3SiO₂+Si₃N₄) to the stoichiometric raw mixture was used as the starting material in order to compensate for volatilization during growth. The crystals obtained were colourless, transparent and free from inclusions. Homogeneity was confirmed by means of EPMA and the chemical composition of La₄Si₂O₇N₂ was assured by wet analysis and EPMA. The space group and cell constants were determined by the X-ray single-crystal diffraction method as P2₁/m, a=8.03 Å, b=10.96 Å, c=11.57 Å and =111.58°, respectively.     

The silicon lanthanide oxynitrides

A new group of materials called the silicon lanthanide oxynitrides has been prepared by the reaction between Si₃N₄ and several oxides of the lanthanide series (La₂O₃, Sm₂O₃, Dy₂O₃, Er₂O₃, and Yb₂O₃). These oxides formed compounds of the type Si₃N₄ · R₂O₃ and R₄Si₂O₇N₂ (R being lanthanide). In addition La₂O₃ and Yb₂O₃ formed the compounds 2Si₃N₄ · La₂O₃ and Yb₂Si₃O₅N₂, respectively. Certain similarities in the unit cells of these compounds have been noted, and their structures are discussed in terms of similarity to known minerals. It is suggested that this group of materials contains a large number of compounds.     

Structural properties of Al2O3-La2O3 binary oxides prepared by sol-gel

The structural properties of La₂O₃ and Al₂O₃-La₂O₃ binary oxides prepared by sol-gel were studied by XRD, HRTEM and UV-vis. The binary oxides with high lanthana contents show an amorphous structure after calcination at 650 °C. At calcination temperatures higher than 1000 °C there is a phase transformation from the amorphous state to the crystalline LaAlO₃ with a perovskite structure. The structure of La₂O₃ is consistent with the hexagonal system; however, some crystalline microdomains with a monoclinic structure were detected by HRTEM. Islands of La₂O₃ and LaAl₁₁O₁₈ phases were detected at high lanthana concentration in the binary oxide. The modification in the coordination shell of the Al³⁺ cations due to the interaction with La³⁺ cations confirms the formation of phases with a perovskite structure and the presence of islands of the LaAl₁₁O₁₈ phase.     

Phase Relations in the LiOH–TiO2–SiO2–H2O System at 500°C and 0.1 GPa

The compounds crystallizing in the LiOH–TiO₂–SiO₂–H₂O system at 500°C and 0.1 GPa are Li₂SiO₃, Li₂Si₂O₅, and Li₂SiO₃. At a TiO₂ : SiO₂ molar ratio of 6.4 : 1, the dominant phase is Li₂TiSiO₅. The crystallization fields of Si-containing phases at TiO₂ : SiO₂ ratios from 1 : 1 to 1 : 4 are (in order of increasing LiOH concentration) SiO₂ (quartz), Li₂Ti^[5]Si^[4]O₅ + Li₂Si₂O₅, and Li₂Ti^[5]Si^[4]O₅ + Li₂Si₂O₅ + Li₂Si^[4]O₃. The observed crystallization behaviors of Li₂TiSiO₅, Li₂Si₂O₅, and Li₂SiO₃ are interpreted in terms of the matrix assembly of the structure from cyclic subpolyhedral structural components.     

Surface oxidation kinetics of Si3N4-4%Y2O3 powders studied by Bremsstrahlung-excited Auger spectroscopy

Samples of silicon nitride powder containing 4.0% Y₂O₃ in weight were heated in air at temperatures between 900 and 1000 °C. The average SiO₂ layer thickness on the Si₃N₄ powder particles, as a function of time at a particular temperature, was measured by Bremsstrahlung-excited Auger electron spectroscopy. Oxidation was found to follow a linear rate law with an activation energy of 56±1.5 kcal mol^–1. The yttrium level measured by X-ray photoelectron spectroscopy was also found to decrease as a function of the oxide layer thickness. This suggests that there is a reaction between the Si₃N₄ and Y₂O₃ particles which results in the formation of an yttrium-rich phase at the interface between the surface SiO₂ layer and the underlying Si₃N₄ particle.     

Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate Part II Thermo-oxidative stability of Cs2MoOS3/Si3N4mixtures

Cesium oxythiomolybdate (Cs₂MoOS₃) may be an excellent high temperature lubricant, providing a friction coefficient below 0.2 at 650°C. However, oxidation products provide the lubrication above 400°C. Lubricant effectiveness depends strongly on the composition of the substrate materials in contact, such as Si₃N₄, suggesting that tribochemical and/or thermal reactions at the interface produce new compounds. The thermo-oxidative stability of Cs₂MoOS₃/Si₃N₄and Cs₂MoOS₃/SiO₂mixtures have been evaluated between room temperature and 1000°C in air. The transition temperatures and oxidation products were identified. The thermal chemistry of Cs₂MoOS₃/Si₃N₄mixtures was significantly different than that of Cs₂MoOS₃alone, largely due to the oxidation of Si₃N₄to glassy SiO₂. Cesium oxythiomolybdate formed cesium oxides, which melted below 600°C. As SiO₂is formed, the cesium oxides diffused into it, creating a cesium silicate glass. Also, Cs₂MoO₄was preferentially formed over complex cesium molybdates and molybdenum oxides. In a tribological application, Cs₂MoO₄, oxides, and cesium silicate glass may be formed at contacting interfaces from Cs₂MoOS₃films deposited on Si₃N₄substrates. Lubrication would be provided as the shear strength of these compounds decreases with increasing temperature.     

Thermal Transformations of the Nd2P6O18 · 12H2O and Li3Nd3(P6O18)2 · 26H2O Cyclohexaphosphates into Polyphosphates

The thermal dehydration and transformations of the cyclohexaphosphates Nd₂P₆O₁₈ · 12H₂O and Li₃Nd₃(P₆O₁₈)₂ · 26H₂O are investigated in the range 20–1000°C using differential thermal analysis and high-temperature x-ray diffraction, in conjunction with chromatographic determination of the anion composition of reaction products. The results demonstrate that, during heating, the cyclohexaphosphates convert into the polyphosphates Nd(PO₃)₃ (monoclinic and orthorhombic forms) and LiNd(PO₃)₄ (monoclinic form). These processes are not accompanied by P₂O₅ removal. The temperature stability limits of the polyphosphates are determined, and the possible mechanisms of the cyclohexaphosphate–polyphosphate transformations are discussed.     

Phase relations in the system CaO-B2O3-SiO2

Phase relations in the lime-rich portion of the system CaO-B₂O₃-SiO₂ have been studied by microscopy, infrared spectroscopy and X-ray powder diffraction of heated mixtures and quenched charges. Extensive solid solution of B₂O₃ in Ca₂SiO₄ occurs along the Ca₂SiO₄-Ca₃B₂O₆ boundary, which has been studied in detail. It contains a ternary compound, Ca₁₁B₂Si₄O₂₂, which is stable to liquidus temperatures, melting incongruently to Ca₂SiO₄ and liquid at 1420 °C. Ca₁₁B₂Si₄O₂₂forms a eutectic with Ca₃B₂O₆ at 1400 °C and, in the ternary system, with CaO and Ca₃B₂O₆ at 1390 °C.     

Control of particle size in Si3N4 powders prepared by high-pressure carbothermal nitridation

The grain size variation in “unseeded” Si₃N₄ powders, prepared by high-pressure carbothermal nitridation of SiO₂ (in stoichiometric 1∶2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and “Sedigraph” measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO₂ areaA(SiO₂) ≈ 50 m² g^?1 in the reaction mixture, the resulting mean Si₃N₄ particle diameter,d(Si₃N₄), is very sensitive to the carbon surface area,A(C), such that the minimumd(Si₃N₄) ≈ 1 μm was obtained withA(C)=30m²g^?1 and the maximumd(Si₃N₄) ≈ 7μm withA(C)=115m²g^?1. Using mixtures withA(SiO₂)=50m²g^?1 andA(C)=115m²g^?1, a slight dependence ofd(Si₃N₄) on the furnace heating rate was also observed; larger grains (≈ 7 μm) were obtained with 20deg min^?1 than with 2deg min^?1 (≈5μm). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3–6.5 MPa), annealing temperature (1470–1830°C) and gas flow rate (2–20 l_(stp) min^?1).     

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.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.     

Sintering and crystallisation of a P2O5-added Li2O-ZrO2-SiO2 glass powder system

Sintering and crystallisation of a 11.5 wt % Li₂O, 22.8 wt % ZrO₂, 65.7 wt % SiO₂ glass powder with P₂O₅ added were investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and was completed in a very short temperature interval (T 100°C) in less than 30 min. Crystallisation took place just after completion of densification and was almost completed at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallisation stage. The glass powder compacts first crystallised into lithium metasilicate (Li₂SiO₃) and/or zircon (ZrSiO₄) and tridymite (SiO₂) which transformed and/or grew into lithium disilicate (Li₂Si₂O₅), zircon and tridymite after the crystallisation process was essentially complete, so that, a crystallinity degree between 52.4 ± 2.0 and 68.5 ± 3.2 wt % was obtained. P₂O₅ doping little affected the densification. However, adding P₂O₅ remarkably enhanced the zircon and tridymite crystallisation while delaying the Li₂SiO₃ to Li₂Si₂O₅ transformation. The microstructure is characterised by fine crystals uniformly distributed arbitrarily oriented throughout the residual glass phase.     

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.