Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Phase Equilibria in the System Si3N4-SiO2-BeO-Be3N2

The 1780°C isothermal section of the reciprocal quasiternary system Si₃N₄-SiO₂-BeO-Be₃N₂ was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be₆Si³N₈, Be₁₁Si₅N₁₄, Be₅Si²N₆, Be₉Si₃N₁₀, Be₈SiO₄N₄, Be₆O₃N₂, Be₈O₅N₂, and Be₉O₆N₂. Large solid solubility occurs in β-Si₃N₄, BeSiN₂, Be₉Si₃N₁₀, Be₄SiN₄, and β-Be₃N₂. Solid solubility in β-Si₃N₄ extends toward Be₂SiO₄ and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be₂SiO₄ at 1880°C. A 4-phase isotherm, liquid +β-Si₃N₄ ( ss )Si₂ON₂+ BeO, exists at 1770°C.     

Thermodynamics of the Si-O-N System: II, Stability of Si2N2O(s) by High-Temperature Mass Spectrometric Vaporization

The Si₂N₂O(s) compound is vaporized in a multiple Knudsen-cell connected to a mass spectrometer. Comparison of SiO(g) and N₂(g) pressures between different samples in the Si-N-O system, as triphasic or diphasic mixtures from Si(s), Si₃N₄(s), and Si₂N₂O(s), allows the determination of the SiO(g) evaporation coefficient and consequently the calculation of the equilibrium pressure of SiO(g). Then the standard enthalpy of formation of Si₂N₂O(s) is determined: A_fH°_m(Si₂N₂O, s, 298.15 K) =−887.5 ± 10 kj.mol⁻¹.     

Thermal Diffusivity of Be4B, Be2B, and BeB6

The thermal diffusivities of polycrystalline Be₄B, Be₂B, and BeB₆ were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be₄B, Be₂B, and BeB₆ varied from 0.13 to 0.072 to 0.031 cm²/s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm²/s at 1000°C. Heat transport in BeB₆ is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be₄B and Be₂B. Analytical expressions for the thermal diffusivities (α) of Be₄B and Be₂B at 200° to 1000°C and of BeB₆ at 25° to 1500°C are: α(Be₄B)=1/(5.83+9.05×10 3 T ), α(Be₂B)=1/(10.92+1.40×10 2 T ), and α(BeB₆)=5.60×10 4+5.72/ T +77.3/T²-4.09×10⁴/T³ cm²/s.     

Heat of Vaporization of EuN and Its Standard Heat of Formation

The vaporization of EuN was studied by Knudsen effusion mass spectrometry with Mo Knudsen cells of 0.010 and 0.020 in. orifice diameter. Europium nitride has a narrow N-deficient homogeneity range and vaporizes congruently by dissociation into Eu vapor and N₂. No gaseous EuN was observed. The second-law enthalpy change for the reaction EuN_{1- x} ( s ) = Eu(g) + [(1- x )/2]N₂(g), where x is very small, was Δ H ⁰₂₉₈= 94±6 kcal (393.3±25.1 kJ). In conjunction with the heat of vaporization of Eu, the standard heat of formation of EuN, Δ H ⁰_f.298=–52±6 kcal mol⁻¹ (217.6±25.1 kJ mol⁻¹), was obtained.     

Vacuum Vaporization in the System MgO-Cr2O3

The vaporization of the system MgO-Cr₂O₃ was studied in a vacuum of 10⁻⁵ torr (10⁻³N/m²) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr₂O₄ increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr₂O₃. The enthalpies of vaporization of MgCr₂O₄ were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

Vaporization Thermodynamics and Enthalpy of Formation of Aluminum Silicon Carbide

The vaporization thermodynamics of aluminum silicon carbide was investigated using Knudsen effusion mass spectrometry. Vaporization occurred incongruently to give Al( g ), SiC( s ), and graphite as reaction products. The vapor pressure of aluminum above (Al₄SiC₄+ SiC + C) was measured using graphite effusion cells with orifice areas between 1.1 × 10⁻²and 3.9X10⁻⁴ cm². The vapor pressure of aluminum obtained between 1427 and 1784 K using an effusion cell with the smallest orifice area, 3.9X10⁻⁴ cm², is expressed as
log p (Pa) =−(18567 ± 86) ( K/T ) + (12.143 ± 0.054)
The third-law calculation of the enthalpy change for the reaction Al₄SiC₄( s ) = 4Al( g ) + SiC( hex ) + 3C( s ) using the present aluminum pressures gives Δ H °(298.15 K) = (1455 ± 79) kJ·mol⁻¹. The corresponding second-law result is Δ H °(298.15 K) = (1456 ± 47) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from the elements calculated from the present vaporization enthalpy (third-law calculation) and the enthalpies of formation of Al( g ) and hexagonal SiC is Δ H °_f= -(221 ± 85) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from its constituent carbides Al₄C₃( s ) and SiC( c, hex ) is calculated to be Δ H °(298.15 K) = (38 ± 92) KJ·mol⁻¹.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

Kinetics of the UO2-C-N2 Reaction at 1700°C

The mechanism of the reaction of UO₂ with carbon in the presence of N₂ at 1700°C and the rate of formation of the carbonitride product were determined. Uranium carbonitride forms at specific O₂ and N₂ chemical potentials by reactions such as (1) UO₂( s ) + 0.67HCN( g )→UO_1.33N_0.45( s ) + 0.67CO( g ) + 0.11N₂( g ) + 0.335H₂( g ) and (2) UO_1.33N_0.45( s ) + 1.58HCN( g )→UO_0.25N_0.75( s ) + 1.33CO( g ) + 0.79H₂( g ) + 0.64N₂( g ). At P _H2=2×10^-5 atm, HCN formed, permitting a gas-phase transport of reactions not observed in the UO₂-C reaction. Reaction (1) is completed in 0.01 to 0.1 of the time for complete conversion to carbonitride; reaction (2), which proceeds as soon as oxynitride is available, is controlled by solid-state diffusion across the carbonitride layers after they become continuous on the entire specimen. The reaction rates and product compositions depend on the P _N2 and P_CO in the system.     

Synthesis and Luminescence Properties of Orange–Red-Emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) Light-Emitting Diode Conversion Phosphors by a Simple Nitridation of MSi2

Eu²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) orange–red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi₂, Eu₂O₃, and α-Si₃N₄ as the starting materials under N₂–H₂ (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M₂Si₅N₈ for M=Ca, Sr, and a little amount of BaSi₇N₁₀ impurity phase for M=Ba. Under the blue light excitation, M₂Si₅N₈:Eu²⁺ shows a typical broad band emission of Eu²⁺ ranging from orange to red (585–620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca2Si₅N₈:Eu²⁺ has the highest application potential as a red conversion phosphor for white light-emitting diodes.     

Thermodynamic Properties of RuO2

The free energy change for the reaction RuO₂( s )+4Cu( s ) = 2Cu₂O( s )+Ru( s ) was determined from 600° to 1000°C from emf measurements on a solid oxide galvanic cell using a stabilized ZrO₂ electrolyte. The cell was designed to minimize the reduction of RuO₂ by the gas phase. The results were used to develop an equation for the standard molar free energy of formation of RuO₂:
The standard molar enthalpy and entropy of formation of RuO₂ at 298°K were calculated to be −72,430 ±200 cal/mol and –40.44±0.2 eu, respectively, using the available heat capacity data. The absolute entropy of RuO₂ at 298°K was calculated to be 15.46±0.2 eu.     

Synthesis, Characterization, and Consolidation of Si3N4 Obtained from Ammonolysis of SiCl4

Thermal decomposition of silicon diimide, Si(NH)₂, in vacuum resulted in very-high-purity, fine-particle-size, amorphous Si₃N₄ powders. The amorphous powder was isothermally aged at 50° to 100° intervals from 1000° to 1500°C for phase identification. Examination of ir spectra and X-ray diffraction patterns indicated a slow and gradual transition from an amorphous material to a crystalline α-phase occurring at 1200°C for >4 h and/or 1300° to 1400°C for 2 h. As the temperature was increased to ≥1450°C for 2 h, the crystalline β-phase was observed. Phase nucleation and crystallite morphology in this system were studied by electron microscopy and electron diffraction combined with TG as functions of temperature for the inorganic polymer starting materials. Powders prepared in this manner with 4 wt% Mg₃N₂ added as a sintering aid were hot-pressed to high-density fine-grained bodies with uniform microstructures. The optimum hot-pressing condition was 1650°C for 1 h. Silicon concentration steadily increased as the hot-pressing temperature or time was increased. A method for chemical etching for high-density fine-grained Si₃N₄ is described. Electrical measurements between room temperature and ∼500°C indicated dielectric constant and tan δ values of 8.3±0.03 and 0.65±0.05×10⁻², respectively.     

Reaction-Bonding Preparation of Si3N4/MoSi2 and Si3N4/WSi2 Composites from Elemental Powders

Si₃N₄/MoSi₂ and Si₃N₄/WSi₂ composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi₂ and WSi₂; the nitridation in a N₂-base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si₃N₄ and MoSi₂ or WSi₂; no free elemental Si and Mo or W were detected by X-ray diffraction.     

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Vapor Pressure of Gd2O3 from 2350° to 2590°K

Knudsen evaporation experiments were made on gadolinium sesquioxide from 2350° to 2590°K. Two sets of Knudsen cell data yielded average values of 468.7 ± 0.8 and 471.0 ± 0.5 kcal/mole for the assumed evaporation reaction

The results indicate that the evaporation coefficient may be much less than one.     

Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si₃N₄ compacts, containing ≅7 wt% of both BeSiN₂ and SiO₂ as densification aids, can be reproducibly sintered to relative densities >99% by a gas-pressure sintering process. Nearly all densification takes place via liquid-phase sintering of transformed β-Si₃N₄ grains at T =1800° to 2000°C. Compacts with high density are produced by first sintering to the closed-pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N₂ pressure at 2000°C and then increasing the N₂ pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T = 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas-filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.