Alkali-Resistant Magnesium Aluminosilicate Oxynitride Glasses

Glasses containing up to 3.3 wt% nitrogen were prepared in the system MgO─Al₂O₃─SiO₂─AIN─Si₃N₄. GlasGlass transition temperature, density, and elastic properties increased with increasing nitrogen content. Resistance against aqueous alkaline solutions was determined by weight loss measurements, solution and surface analysis. The good durability of both oxide and oxynitride glasses is caused by a protective surface layer. The introduction of nitrogen decreases the alkaline attack. The data suggest that nitrogen is incorporated as Si─N boxhs.     

Viscosity of Molten Na2O─B2O3 Slags

The viscosity of sodium borate slags at high Na₂O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na₂O/mass% B₂O₃= 0.86) containing CaO and CaF₂ for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na₂O₃mass% B₂O = 0.53 to 0.86) with increase in Na₂O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na₂O/mass% B₂O₃= 0.86), the viscosity increased considerably, while an addition of CaF₂ (S to 15 mass%) to the slag (30.9 mass% Na₂O₃ 35.8 mass% B₂O₃, 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na₂O─B₂O₃, Na₂O─B₂O₃─CaO₃ and Na₂O─B₂O₃─CaO─CaF₂ slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range.     

Electrical Joining of Silicon Nitride Ceramics

The electrical joining of sintered Si₃N₄ ceramics by Joule heating was studied. A mixture of CaF₂/kaolinite (70/30 wt%) with excellent electroheating characteristics and reactivity with Si₃N₄ ceramics was selected as a joining agent. The optimum conditions for electrical joining were determined using this joining agent. Analysis of the joint obtained under optimum conditions revealed that joining was accomplished by the formation of reaction zones and a joining layer through the mutual diffusion of the components in the joining agent and the sintering aids in the Si₃N₄. The joint layer was composed of a glassy substance consisting of Ca─Al─Si─Y─O─(F)─(N) and contained a few particles of β─Si₃N₄. Four-point bend tests indicated that joined bodies could be obtained which maintained a strength of about 300 MPa up to 800°C. Finally, a comparative study was made with a joint obtained using furnace heating. These results indicated that the joints obtained using electrical joining were superior to those produced in the furnace.     

Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty-nine compatibility tetrahedra were established in the region Si₃N₄─AIN─Al₂O₃─Y₂O₃. The subsolidus phase relationships in the region Si₃N₄─AIN─YN─Y₂O₃ have also been studied. Only one compound, 2YN:Si₃N₄, was confirmed in the binary system Si₃N₄─YN. The solubility limits of the α'─SiAION on the Si₃N₄─YN:3AIN join were determined to range from m = 1.3 to m = 2.4 in the formula Y _{m /3}Si_{12- m} Al _m N₁₆. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si₃N₄─AIN─YN─Y₂O₃.     

Synthesis and Characterization of Amorphous Si2N2O

Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si₂ p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si₂ p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The ²⁹Si NMR analysis showed the formation of (amorphous) Si₃N₄ (Si–N₄) and possibly two oxynitride phases (Si–N₃O, Si–N₂O₂). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si₂N₂O) phase, the formation of phase–pure, amorphous Si₂N₂O is extremely difficult via this route.     

Synthesis of Silicon Carbide–Silicon Nitride Composite Ultrafine Particles Using a Carbon Dioxide Laser

The synthesis and the structure of silicon carbide-silicon nitride (SiC─Si₃N₄) composite ultrafine particles have been studied. SiC─Si₃N₄ composite ultrafine particles were prepared by irradiating a SiH₄, C₂H₄, and NH₃ gas mixture with a CO₂ laser at atmospheric pressure. The composition of composite powders changed with the reactant gas flow rate. The carbon and nitrogen content of the powder could be controlled in a wide range from 0 to 30 wt%. The composite powder, which contained 25.3 wt%. carbon and 5.8 wt% nitrogen, had a (β-SiC structure. As the nitrogen con- tent increased, SiC decreased and amorphous phase, Si₃N₄, Si appeared. The results of XPS and lattice constant measurements suggested that Si, C, and N atoms were intimately mixed in the composite particles.     

Nitrogen Dissolution in Sodium Alkaline-Earth Metaphosphate Melts

Phosphorus oxynitride glasses were prepared by remelting (5O – X )Na₂O · X BaO · 50P₂O₅, X = 0, 10, 20, 25, and 30, and 30Na₂O · 20MO · 50P₂O₅, M = Mg, Ca, Sr, and Ba', glasses in anhydrous ammonia. The nitrogen content depends upon the temperature and time of remelting in ammonia and the concentration and size of the alkaline-earth ion. Nitriding the starting glass decreased the dissolution rate in deionized water and thermal expansion coefficient and increased the dilatometric softening temperature and refractive index. The dissolution rate of the base glass in deionized water depended upon the concentration and size of the alkaline-earth ion, but for nitrided glasses, it was essentially independent of the alkaline-earth cation. The thermal expansion coefficient for all the oxynitride glasses decreased with increasing alkaline-earth concentration and cation field strength.     

Solubility Limits of α'-SiAION Solid Solutions in the System Si,Al,Y/N,O

The solubility limit of α'-SiAION solid solutions on the Si₃N₄─YN:3AIN composition join in the system Si₃N₄─YN─AIN has been determined at 1800°C. The end members of these solid solutions are Y_0.43Si_10.7Al_1.3N₁₆ and Y_0.8Si_9.6Al_2.4N₁₆. Unit-cell dimensions of the α'-SiAION solid solutions in the system Si,Al,Y/N,O can be expressed as follows: a _o(Å) = 7.752 + 0.045 m + 0.009 n , c _o(Å) = 5.620 + 0.048 m + 0.009 n , where the α'-SiAION solid solution has the formula Y _x Si_{12-( m+n )}Al _m+n N_{16- n} O _n . The single-phase boundary of the solid solution α'-SiAION on the composition triangle Si₃N₄─YN:3AIN─AIN:Al₂O₃ is delineated. The present paper also reports the phase relationships involving α'-SiAION.     

Problems Associated with the Melting of Oxynitride Glasses

Carbon dioxide from carbonate batch materials oxidizes Si₃N₄, causing N₂ loss from oxynitride glass melts. Lithia is the only alkali which is compatible with oxynitride melts; all other alkalis are reduced and volatilized during melting. A high partial pressure of oxygen (P₀₂ over oxynitride melts oxidizes Si₃N₄, leading to N₂ loss, whereas a low P_o2, results in vaporization of SiO.     

Influence of Thermal History on the Crystallization Behavior and Hardness of a Glass-Ceramic

The glass system SiO₂─Al₂O₃─Li₂O─TiO₂ was examined to determine the dependence of hardness on heat-treatment temperature. In addition to hardness measurements, X-ray diffraction (XRD) was used to determine the type and amount of phases present and to relate this to the hardness results. Hardness increased with heat-treatment temperature. This correlated with the amounts of crystals present, as determined by XRD. For a particular heat-treatment temperature, when hardness was plotted versus load on the indenter, there was a load dependence. There was at first a rapid decrease as the load on the indenter increased, with a constant value being approached at higher loads.     

Crystal Structure of Lu4Si2O7N2 Analyzed by the Rietveld Method Using the Time-of-Flight Neutron Powder Diffraction Pattern

The crystal structure of a lutetium silicon oxynitride (Lu₄Si₂O₇N₂) was analyzed by the Rietveld method using time-of-flight (TOF) neutron powder diffraction data. The compound crystallizes in a monoclinic cell, space group P 2₁/ c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1) Å, and β= 109.773(1)° at 297 K. One nitrogen atom in Lu₄Si₂O₇N₂ occupies the bridging site between the two Si atoms, and the other one is statistically situated at the terminal sites of Si₂O₅N₂ ditetrahedra. In the local structure, Si₂O₅N₂ ditetrahedra consist of SiO₃N and SiO₂N₂ tetrahedral units sharing the N atom. Lu atoms are in sixfold, sevenfold (×2) and eightfold coordinations of O/N atoms. X-ray powder diffraction data were also analyzed with the model obtained by the neutron diffraction.     

Silicon Nitride Oxidation Based on Oxynitride Interlayers with Graded Stoichiometry

The oxidation kinetics of Si₃N₄ were modeled by describing the simultaneous diffusion and reaction of interstitial oxygen that is believed to occur inside of the silicon oxynitride interlayer. The oxynitride was assumed to have a variable composition, and oxidation was described as a reaction where interstitial oxygen is incorporated into the network structure of the oxynitride and nitrogen is removed. It was assumed that both the diffusion coefficient and the solubility of interstitial oxygen decrease as the nitrogen content of the network structure increases. The results accurately describe both the formation of an oxynitride layer during oxidation, and the relatively slow oxidation kinetics of Si₃N₄ (compared to Si and SiC).     

Molten-Salt Corrosion of Silicon Nitride: II, Sodium Sulfate

The corrosion mechanism of Si₃N₄+ Na₂SO₄/O₂ at 1000°C was investigated. Corrosion of both pure and additive-containing Si₃N₄ was studied. The reaction of Si₃N₄+ Na₂SO₄ consists of an initial period of slow weight loss due to Na₂SO₄ vaporization and oxidation-dissolution. This initial period was the same for all forms of Si₃N₄. A second region consisted of further oxidation or near termination of reaction, depending on the additive in the Si₃N₄. A 5- to 10-μm yttrium depletion zone was found after corrosion for the Si₃N₄ with yttria additives. For comparison, Si and SiC were corroded under similar conditions. These materials corroded substantially faster than all forms of Si₃N₄.     

Molecular Dynamics Study of Na-Si-O-N Oxynitride Glasses

The structure and properties of Na-Si-O-N oxynitride glasses have been studied by molecular dynamics calculations using a pair potential of the Busing approximation of the Born-Mayer-Huggins type. Nitrogen atoms bonded to one, two, and three silicon atoms coexist in the glass structure. The mean of the number of silicon atoms bonded to a nitrogen atom ranges from 2.4 to 2.1, decreasing with increasing Na₂O content from 15 to 30 mol%. It has been assumed that nitrogen atoms bonded to two or fewer silicon atoms are formed when nitrogen atoms substitute for non-bridging atoms. The bond angle ∠Si-N-Si exhibits a bimodal distribution around 105–135° and 140–170°, roughly corresponding to the nitrogen atoms bonded to three and two silicon atoms, respectively. The dependences of the density, the bulk thermal expansion, and the bulk modulus on the nitrogen content are consistent with those observed in real systems.     

Formation and Stability of β-Quartz Solid-Solution Phase in the Li-Si-Al-O-N System

The development of crystalline phases in lithium oxynitride glass-ceramics was examined, with particular emphasis placed on the effect of the nitrogen source (AlN or Si₃N₄) on the formation and stability of a β-quartz solid-solution ( ss ) phase. Oxynitride glasses derived from the Li-Si-Al-O-N system were heat-treated at temperatures up to 1200°C to yield glass-ceramics in which β-quartz( ss ) and β-spodumene( ss ) of approximate composition Li₂OAl₂O₃4SiO₂ formed as major phases and in which X-phase (Si₃Al₆O₁₂N₂) and silicon oxynitride (Si₂N₂O) were present as minor phases. The nitrogen-containing β-quartz( ss ) phase that was prepared with AlN was stable at 1200°C; however, the use of Si₃N₄ as the nitrogen source was significantly less effective in promoting such thermal stabilization. Lattice parameter measurements revealed that AlN and Si₃N₄ had different effects on the crystalline structures, and it was proposed that the enhanced thermal stability of the β-quartz( ss ) phase that was prepared with AlN was due to both the replacement of oxygen by nitrogen and the positioning of excess Al³⁺ ions into interstitial sites within the β-quartz( ss ) crystal lattice.     

Formation of Frozen α-Agl in Twin-Roller-Quenched Agl─Ag2 O─MxOy (MxOy= WO3, V2O5) Glasses at Ambient Temperature

Superionic conductor α-AgI, which is stable only above 147°C, was successfully frozen at ambient temperature in AgI─Ag₂O─M_xO_y (M_xO_y= WO₃, V₂O₅) glass matrices by a twin-roller-quenching technique. The system with WO₃, provided the larger composition regions where α-AgI was frozen at ambient temperature, compared to the system with V₂O₅. The matrix glasses with higher glass-transition temperatures had a stronger effect in depressing the α–β transformation of AgI. The α-AgI-frozen samples exhibited extremely large conductivities of 3 × 10⁻²-5 × 10⁻²S.cm⁻¹at 25°C.     

Molten-Salt Corrosion of Silicon Nitride: I, Sodium Carbonate

Corrosion of Si₃N₄ under thin films of Na₂CO₃ was investigated at 1000°C. Pure Si₃N₄ and Si₃N₄ with various additives were examined. Thermogravimetric analysis and morphology observations lead to the following detailed reaction mechanism: (I) decomposition of Na₂CO₃ and formation of Na₂SiO₃, (II) rapid oxidation, and (III) formation of a protective silica layer below the silicate and a slowing of the reaction. For Si₃N₄ with Y₂O₃ additions, preferential attack of the grain-boundary phase occurred. The corrosion of pure Si and SiC was also studied for comparison to Si₃N₄. The corrosion mechanism generally applies to all three materials. Silicon reacted substantially faster than Si₃N₄ and SiC.     

Preparation and Properties of Glass-Ceramics Derived from Nitrogen-Containing β-Spodumene Glasses

Crystallization of Li-Al-Si-O-N oxynitride glasses based on the β-spodumene composition and the properties of the resultant glass-ceramics have been studied. The onset of the precipitation of metastable high-quartz solid solution and its transformation to β-spodumene shift to higher temperatures with increasing nitrogen content of the oxynitride glasses. Nitrided glass-ceramics crystallized at 1200°C have negative thermal expansion coefficients, since high-quartz structure is maintained up to 1000° and 1200°C. Knoop hardness and density of the glass-ceramics increase with increasing nitrogen content. There was evidence that part of the nitrogen atoms were incorporated into the high-quartz solid-solution structure and that a small amount of the minor phase of Si₂N₂O was precipitated in highly nitrided glass-ceramics.     

Microstructure and Properties of Self-Reinforced Silicon Nitride

Problems associated with manufacturing Si₃N₄/SiC-whisker composites have been overcome by developing selfreinforced Si₃N₄ with elongated β-Si₃N₄ grains formed in situ from oxynitride glass. This Si₃N₄–Y₂O₃–MgO–SiO₂–CaO-based material has a flexure strength >1000 MPa and fracture toughness >8 MPa·m^½. The optimum combination of mechanical properties has been obtained with Y₂O₃:MgO ratios ranging from 3:1 to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si₃N₄ contents between 90 and 96 wt%.     

Theoretical Study on the Chemistry of Intergranular Glassy Film in Si3N4–SiO2 Ceramics

The intergranular glassy film (IGF) composed of silicon oxynitride in a Si₃N₄ ceramic material has been studied by molecular dynamics calculations. Structural analyses showed that the presence of an IGF having both nitrogen and oxygen reduces the number of dangling bonds at the junction between the IGF and adjacent Si₃N₄ grains, which reduces the interface energy at the grain boundary. More dangling bonds were generated at the junction when the N/(N + O) ratio of the IGF was decreased due to the larger chemical and structural mismatch between the IGF and the adjacent grain. On the other hand, the increase in the N/(N + O) ratio of the IGF caused a greater energy penalty in the IGF. The balance of these two contributions should determine the chemistry of the IGF.