Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.     

Oxidation Protection of MgO–C Refractories by Means of Al8B4C7

The effect of Al₈B₄C₇ used as an antioxidant in MgO–C refractories and the behavior of Al₈B₄C₇ in CO gas were investigated in the present study. Al₈B₄C₇ was found to react with CO gas, to form Al₂O₃( s ), B₂O₃( l ), and C( s ), at temperatures >1100°C. The Al₂O₃ reacts with MgO to form MgAl₂O₄ near the surface of the material. At the same time, B₂O₃( l ) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B₂O₃ and MgO. The coexistence of the liquid and MgAl₂O₄ makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C.     

Thermal Stability of Al3BC3

The thermal stability of Al₃BC₃ powder was analyzed. Nearly X-ray-pure Al₃BC₃ powder was obtained through the calcination of the aluminum, B₄C, and carbon mixture at 1800°C in Ar. In contrast to the former investigations, which reported the melting of so called "Al₈B₄C₇" at 1800°C, Al₃BC₃ did not melt up to 2100°C. Instead, it decomposed by the vaporization of aluminum. The decomposition occurred distinctly at 1400° and 1900°C in flowing Ar and a sealed carbon crucible, respectively. The results indicated that the decomposition temperature depended on the partial pressure of aluminum vapour in the atmosphere.     

Crystalline Compounds and Glasses in the System B2O3-NaF-NaBF4

The system B₂O₃-NaF-NaBF, has been studied by subjecting selected compositions to thermal treatment in the range 400° to 600°C. Weight losses, chemical analyses, ir, Raman, and X-ray diffraction techniques were used to define the composition of the crystalline phases and the structural units being formed in the system. The stoichiometry of the BF₃ evolved from NaBF₄-B₂O₃ mixtures indicated that a composition corresponding to Na₂B₃F₅O₃ was formed in mixtures containing up to 33.3 mol% B₂O₃. At higher boron oxide concentrations, Na₂B₃F₅O₃ was consumed, yielding 2NaF.3B₂O₃. The crystalline compounds Na₃B₃F₆O₃, 2NaF.3B₂O₃, and phase B (apparently NaF.B₂O₃) were formed in the system. The compound Na₃B₃F₆O₃ appeared as the stable oxygen-containing species in NaBF₄-NaF mixtures of low oxide content. The main fluorine-containing structural units of the system are BF₄, (–O)₃BF, (–O)₂BF₂, (–O)₂BF, whereas the main structure for binary NaF-B₂O₃ mixtures is (–O)₃BF.     

Subsolidus Phase Equilibria of the CaO-B2O3-BaO System

The "subsolidus" phase relations at room temperature in the system CaO-B₂O₃-BaO are investigated. Specimens of various compositions were prepared from appropriate ratios of CaCO₃, B₂O₃, and BaCO₃, and fired from 780° to 1040°C according to their melting points. There are three ternary compounds in this system. The crystal structures of these compounds were determined by X-ray diffraction (XRD). CaBa₂(BO₃)₂ and Ca₅Ba₂B₁₀O₂₂ are monoclinic structures. The lattice constants a = 14.221 Å, b = 4.569 Å, c = 11.926 A, β= 99.947°, and V = 763.4 å³ for CaBa₂(BO₃)₂ and a = 15.714 å, b = 6.184 å, c = 10.204 å, β= 93.954°, and V = 989.29 å₃ for Ca₅Ba₂B₁₀O₂₂ are obtained. The third compound, CaBa₂(B₃O₆)₂, is isostructural with the high form of BaB₂O₄ with lattice constants a = 7.167 å and c = 35.298 å. Powder second harmonic generation efficiencies of these ternary compounds were measured using a homemade apparatus.     

Microwave Dielectric Properties of a New A5B4O15-Type Cation-Deficient Perovskite Ba2La3Ti3TaO15

High dielectric constant and low loss ceramics with composition Ba₂La₃Ti₃TaO₁₅ have been prepared by a conventional solid-state ceramic route. This compound adopts A₅B₄O₁₅ cation-deficient hexagonal perovskite structure. The dielectric properties of dense ceramics sintered in air at 1520°C have been characterized at microwave frequencies. It shows a relative dielectric constant of ∼45, quality factor Q _u× f of ∼26 828 GHz and temperature variation of resonant frequency of −0.97 ppm/°C.     

Sr3LaNb3O12: A New Low Loss and Temperature Stable A4B3O12-Type Microwave Dielectric Ceramic

The microwave dielectric properties of dense ceramics of a new A₄B₃O₁₂ type cation-deficient hexagonal perovskite Sr₃LaNb₃O₁₂ are reported. Single-phase powders can be obtained from the mixed-oxide route at 1320°C and dense ceramics (>96% of the theoretical X-ray density) with uniform microstructures (5–12 um) can be obtained by sintering in air at 1430°C. The ceramic exhibits a moderate dielectric constant ɛ_r∼36, a high quality factor Q × f ∼45 327 GHz, and a low temperature coefficient of resonant frequency τ _f of −9 ppm/°C.     

Growth of β-Alumina (Na2O · 11Al2O3) Single Crystals by the Flux Method

The crystal-growth process and growth conditions of β-alumina (Na₂O · Al₂O₃) were investigated using the Na₂B₄O₇-Na₃AlF₆ flux method. β-Alumina (electric fusion brick) was used as both nutrient and seed. Weight loss of the flux varied widely for various runs: ≅ 10 wt% of flux evaporated at 100 h, ≅ 17 wt% at 150 h, and 43 wt% at 600 h. When β-alumina crystal was grown, only 20 wt% Na₂B₄O₇ was added to the Na₃AlF₆ flux. The linear growth rates of the β-alumina single crystal grown by an Na₃AlF₆-20 wt% Na₂B₄O₇ flux method at 1040°C and Δ t = 18°C were ≅ 1.0 × 10⁻³ mm/h ( a face) and ≅0.3 × 10⁻³ mm/h ( c face). The β-alumina single crystals grown were bounded by only c [001] and a [100] and were colorless and transparent.     

High-Temperature Phase Relations in the Sc2O3-Ta2O5 System

The phase relations for the Sc₂O₃-Ta₂O₅ system in the composition range of 50-100 mol% Sc₂O₃ have been studied by using solid-state reactions at 1350°, 1500°, or 1700°C and by using thermal analyses up to the melting temperatures. The Sc_5.5Ta_1.5O₁₂ phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO₄, and Sc₂O₃ were found in the system. The Sc_5.5Ta_1.5O₁₂ phase formed in 78 mol% Sc₂O₃ at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc₂O₃ and decomposed to Sc_5.5Ta_1.5O₁₂ and ScTaO₄ at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc₂O₃. The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc₂O₃. A phase diagram that includes the four above-described phases has been proposed, instead of the previous diagram in which those phases were not identified.     

Subsolidus Phase Relations in the Ga2O3-In2O3-SnO2 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃–SnO₂ system were studied by X-ray diffraction over the temperature range 1250–1400°C. At 1250°C, several phases are stable in the ternary system, including Ga₂O₃( ss ), In₂O₃( ss ), SnO₂, Ga_{3− x} In_{5+ x} Sn₂O₁₆, and several intergrowth phases that can be expressed as Ga_{4−4 x} In_{4 x} Sn _{n −4}O_{2 n −2} where n is an integer. An In₂O₃–SnO₂ phase and Ga₄SnO₈ form at 1375°C but are not stable at 1250°C. GaInO₃ did not form over the temperature range 1000–1400°C.     

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.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

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.     

A New Low-Loss Microwave Dielectric Ceramic Sr4LaTiNb3O15

A high dielectric constant and low-loss ceramic with composition Sr₄LaTiNb₃O₁₅ has been prepared by the conventional solid-state ceramic route. This compound adopts an A₅B₄O₁₅ cation-deficient hexagonal perovskite structure and crystallizes in the trigonal system with unit cell parameters a =5.6307(2), c =11.3692(3) Å, V =312.16(2) ų, and Z =1. The dielectric properties of dense ceramics sintered in air at 1460°C have been characterized at microwave frequencies. The results show that the material affords a relatively high dielectric constant ɛ_r∼43, a high quality factor Q × f ∼44 718 GHz, and a low temperature coefficient of resonant frequency TC_f∼13 ppm/°C.     

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.     

Crystallization of Na2B4O7 from Its Melt

The temperature dependence of the rate of growth of Na₂B₄O₇ from its melt was determined between 1° and 192°C of undercooling. The maximum rate of growth was 2000 μm/min at 57°C of undercooling. Analysis of the growth data indicated that growth could occur by a screw dislocation mechanism over the entire range of undercooling. When this mechanism was assumed, there was good correlation between the experimental data and the predictions of the Turnbull and Cohen equation.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

Low-Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System

Low-temperature phase equilibria ranging from 1000° to 1200°C in the ZrO₂–CeO₂ system were investigated by annealing compositionally homogeneous ZrO₂–CeO₂ solid solutions in a Na₂B₂O₇.1 NaF flux. The 5 mol% CeO₂ samples decomposed into monoclinic ( m ) and tetragonal ( t ) phases during annealing at 1100°2 and 1120°C, and the t -phase transformed diffusionlessly into monoclinic ( m ') symmetry during quenching. A eutectoid reaction, t → ( m + c ), was confirmed to occur at 1055°± 10°C, where the equilibrium compositions of the t -, m -, and c -phases were 11.2 ± 2.8, 0.9 ± 0.9, and 84 ± 1 mol% CeO₂, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate withouts affecting the equilibration. The previous data are discussed using metastable–stable phase diagrams. The discrepancies of the low-temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggish kinetics.     

Microwave Dielectric Properties of A6B5O18-Type Perovskites

Cation-deficient perovskites with the general formula A₆B₅O₁₈ (A=Ba, Sr, La; B=Nb, Ta, Ti, Zr, Mg, and Zn) have been synthesized and their microwave dielectric properties have been investigated. X-ray diffraction studies indicate the formation of monophase materials. The structures of Ba₆Ta₄TiO₁₈ and Ba₅SrTa₄TiO₁₈ are different from that of Sr₆Ta₄TiO₁₈. The A₆B₅O₁₈ have Q × f in the range 5600–51 000 GHz, dielectric constants 26–48, and the temperature coefficient of resonant frequency varies from −39 to +83 ppm/°C, depending on the composition. Scanning electron microscopy studies show that the grain size decreases with an increase in the Sr content.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.