Low-Temperature Synthesis of BaAl2O4/Aluminum-Bearing Composites by the Oxidation of Solid Metal-Bearing Precursors

BaAl₂O₄/aluminum-bearing composites have been synthesized via the low-temperature oxidation of Ba-Al precursors. Ba-Al powder mixtures that were prepared via high-energy vibratory milling were uniaxially pressed into bar-shaped specimens that were then exposed to a series of heat treatments in pure, flowing oxygen at temperatures up to 640°C. Oxidation at a temperature of 300°C resulted in the formation of barium peroxide (BaO₂). Additional heat treatment at a temperature of 550°C resulted in the consumption of BaO₂ and some aluminum to yield BaAl₂O₄ and Al₄Ba. The oxidation of Al₄Ba at a temperature of 640°C yielded additional BaAl₂O₄. Microstructural analyses revealed that a well-dispersed, co-continuous mixture of Al₂O₃-excess BaAl₂O₄ and 99.5% pure aluminum was produced.     

The System Si3N4-SiO2-Y2O3

Subsolidus phase relations were established in the system Si₃N₄-SiO₂-Y₂O₃. Four ternary compounds were confirmed, with compositions of Y₄Si₂O₇N₂, Y₂Si₃O₃N₄, YSiO₂N, and Y₁₀(SiO₄)₆N₂. The eutectic in the triangle Si₃N₄-Y₂Si₂O₇-Y₁₀(SiO₄)₆N₂ melts at 1500°C and that in the triangle Si₂N₂O-SiO₂-Y₂Si₂O₇ at 1550°C. The eutectic temperature of the Si₃N₄-Y₂Si₂O₇ join was ∼ 1520°C.     

Hot-Pressed Silicon Nitride with Lu2O3 Additives: Oxidation and Its Effect on Strength

The oxidation behavior and effect of oxidation on room-temperature flexural strength were investigated for hot-pressed Si₃N₄ ceramics, with 3.33 and 12.51 wt% Lu₂O₃ additives, exposed to air at 1400° and 1500°C for up to 200 h. Parabolic oxidation behavior was observed for both compositions. The oxidation products consisted of Lu₂Si₂O₇ and SiO₂. The Lu₂Si₂O₇ grew out of the surface silicate in preferred orientations. The morphology of oxidized surfaces was dependent on the amount of additive; Lu₂Si₂O₇ grains in the 3.33 wt% composition appeared partially in a needlelike type, compared with a more equiaxed type exhibited in the 12.51 wt% case. The high resistance to oxidation shown for both compositions was attributed to the extensive amounts of crystalline, refractory secondary phases formed during the sintering process. Moreover, after 200 h of oxidation at 1400° and 1500°C, the strength retention displayed by the two compositions was 93%–95% and 85%–87%, respectively. The strength decrease was associated with the formation of new defects at the interface between the oxide layer and the Si₃N₄ bulk.     

Reaction-Formed Porous Yb4Si2N2O7 Materials with Uniform Open-Cell Network Structure

A novel porous Yb₄Si₂O₇N₂ material with uniform open-cell network structure was fabricated from the reaction between Si₃N₄, Yb₂O₃, and SiO₂. The formation of Yb₄Si₂O₇N₂ during heating was studied using X-ray diffractometry. The porous structure was characterized using scanning electron microscopy and mercury porosimeter. It is shown that the formation of Yb₄Si₂O₇N₂ phase starts at ∼1150°C and completes at 1350°C for 4 h, accompanied by the development of open-cell network structure. The necks between Yb₄Si₂O₇N₂ particles become much thicker with increasing temperature because of the coarsening of Yb₄Si₂O₇N₂ particles, thus leading to a uniform three-dimensional network structure. Furthermore, the pore size can be well controlled by adjusting reacting temperature and altering atmosphere.     

Crystallization and Seeding Effect in BaAl2Si2O8 Gels

Two sol-gel routes have been used to prepare celsian (BaAl₂Si₂O₈) precursors. The crystallization behaviors of unseeded gels and the gels seeded with 5 wt% feldspar, rutile, LiAISi₂O₂, and ZrO₂ crystals as well as SrAl₂Si₂O₈ gel have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric and differential thermal analyses (TGA and DTA). It was found that metastable hexacelsian, rather than the preferred monoclinic celsian, was the major crystallization product in the two unseeded gels. Seeding largely enhanced the crystallization of monoclinic celsian, but the mechanisms involved were different for seeding with different crystals. SrAl₂Si₂O₈ gel and ZrO₂ crystals acted as normal heterogeneous centers and had little catalytic effect on monoclinic celsian crystallization. Feldspar and rutile crystals worked as epitaxial substrates for monoclinic celsian and produced pure monoclinic celsian in some temperature regions. LiAISi₃O₈ seeds lowered the glass transition temperature of the immediately adjacent gel matrix and led to the crystallization of pure monoclinic celsian.     

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.     

Formation and Transformation of δ-Ta2O5 Solid Solution in the System Ta2O5-Al2O3

In the system Ta₂O₃-Al₂O₅ solid solutions of metastable δ-Ta₂O₅ (hexagonal) are formed up to 50 mol% Al₂O₃ from amorphous materials prepared by the simultaneous hydrolysis of tantalum and aluminum alkoxides. The values of the lattice parameters decrease linearly with increasing Al₂O₃, content. The to β-Ta₂O₅ (orthorhombic, low-temperature form) transformation occurs at ∼950°C. The solid solution containing 50 mol% Al₂O₃ transforms at 1040° to 1100°C to orthorhombic TaAlO₄. Orthorhombic TaAlO₄ contains octahedral TaO₆ groups in the structure.     

Phase Relations in the System NaF-CaF2-NaAlSiO4-CaAl2Si2O8

The phase diagram of the pseudoternary reciprocal system NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ is reported in this paper. The phase relations in the system have been investigated by differential thermal analysis, quenching melts, X-ray diffractometry, and optical and electron microscopies. The stable diagonal CaF₂–NaAlSiO₄ divides the system in two pseudoternary systems. The solidus temperatures in the two subsystems NaF–CaF₂–NaAlSiO₄ and CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ are 805°± 2°C and 1095°± 4°C, respectively. An extensive region of liquid–liquid immiscibility is evident in the NaF–CaF₂–NaAlSiO₄ subsystem. The compositions of the two liquids fall outside the compositional surface NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈, but only a small deviation from the ternary behavior is observed.     

Phase Relations in the System Al2O3─Ce2Si2O7 in the Temperature Range 900° to 1925°C in Inert Atmosphere

Equilibrium relationships in the system Al₂O₃-Ce₂Si₂O₇ in inert atmosphere have been investigated in the temperature range 900° to 1925°C. A simple eutectic reaction was found at 1375°C and 51 mol% Ce₂Si₂O₇. A high-low polymorphic transformation in Ce₂Si₂O₇ was observed at 1274°C. New XRD patterns are suggested for both polymorphs of cerium pyrosilicate. The melting point of Ce₂Si₂O₇ was found to be 1788°C. A value for ΔH°_m,Ce2Si2O7 of 36.81 kJ/mol was calculated from the initial slope of the experimentally determined liquidus in equilibrium with the pyrosilicate phase.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.     

Phase Equilibria in the Ga2O3In2O3 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃ system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In₂O₃ in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga₂O₃ in cubic In₂O₃ increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO₃, which is not stable, but is likely the In-doped β-Ga₂O₃ solid solution.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

Phase Formation in the System Na2O · Al2O3-CaO · A12O3-Al2O3 at 1200°C in Air

Phase relations in the system Na₂O_· Al₂O₃-CaO· Al₂O₃-Al₂O₃ at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na₂O · 3CaO · 5Al₂O₃, known from the literature, was reformulated as Na₂O · CaO · 2Al₂O₃. A new compound with the probable composition Na₂O · 3CaO · 8Al₂O₃ was found. Cell parameters of both compounds were determined. The compound Na₂O · CaO-2Al₂O₃ is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na₂O · Al₂O₃ up to 38 mol% Na₂O at 1200°C. The compound Na₂O · 3CaO · 8Al₂O₃ is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al₂O₃ is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al₂O₃ make it very difficult to determine equilibrium phase relations in the high-Al₂O₃ part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram.     

Formation of α-Si3N4 Solid Solutions in the System Si3N4-A1N-Y2O3

The subsolidus phase diagram of the quasiternary system Si₃N₄-AlN-Y₂O₃ was established. In this system α-Si₃N₄ forms a solid solution with 0.1Y₂O₃: 0.9 AIN. The solubility limits are represented by Y_0.33Si_10.5Al_1.5O_0.5N_15.5 and Y_0.67Si₉A1₃ON₁₅. At 1700°C an equilibrium exists between β-Si₃N₄ and this solid solution.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.