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.     

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.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Phase Equilibria in the System MgO-B2O3

Phase equilibria in the system MgO-B₂O₃ were investigated using DTA and quenching techniques. The system contains 4 invariant points. The compounds MgO·2B₂O₃ and 2MgO·B₂O₃ melt incongruently at 995° and 1312°C, respectively, whereas 3MgO·B₂O₃ melts congruently at 1410°C. A eutectic occurs at 1333°C and 71% MgO.     

Preparation and Characterization of Aluminum Borate

Aluminum borate, 9Al₂O₃·2B₂O₃ or Al₁₈B₄O₃₃, was synthesized by the reaction of stoichiometric amounts of α-Al₂O₃ and B₂O₃. The Al₁₈B₄O₃₃ material was formed into a dense ceramic by pressureless sintering with CaO, MgO, or CaAl₂B₂O₇ additives. The material was characterized by low bulk density, moderate coefficient of thermal expansion (3 × 10⁻⁶/°C to 5 × 10⁻⁶/°C), moderate strength (210 to 324 MPa), and low dielectric constant.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

Lowering Crystallization Temperatures by Seeding in Structurally Diphasic Al2O3-MgO Xerogels

Thermal reactions in 93% Al₂O₃-7% MgO and 95.8% Al₂O₃-4.2% MgO gels seeded with α-Al₂O₃, MgAl₂O₄, α-Fe₂O₃, and SiO₂, sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al₂O₃ lowered the α-Al₂O₃ crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe₂O₃ and SiO₂ seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Experimental Establishment of the CaAl2O4–MgO and CaAl4O7–MgO Isoplethal Sections within the Al2O3–MgO–CaO Ternary System

The isoplethal sections CaAl₂O₄–MgO and CaAl₄O₇–MgO of the Al₂O₃–MgO–CaO ternary system have been experimentally established at 1 bar total pressure and air of normal humidity. The sections obtained provide new data and information that are in disagreement with thermodynamic evaluations and optimizations of the Al₂O₃–MgO–CaO ternary system published to date. These differences arise mainly from the inclusion, or exclusion, of the binary compound Ca₁₂Al₁₄O₃₃, mayenite, as a stable phase in the reported studies of the system. The presence or absence of this compound within the system has an important impact on the solid state and melting relationships of the whole ternary system. The present study confirms the solid-state compatibility CaAl₂O₄–MgO and CaAl₂O₄–MgO–MgAl₂O₄ up to 1372°± 2°C, the peritectic melting point of the later mentioned subsystem.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Diffusion of 51Cr in Surface Layers of Magnesia, Alumina, and Spinel

Diffusion of ⁵¹Cr isotope on MgO (100), Al₂O₃ (0001, and MgAl₂O₄ (111) surfaces was investigated by using the edgesource method. The surface diffusion parameter, αD,δ, where α is the segregation factor, D , the surface diffusion coefficient, and δ the thickness of the high-diffusivity layer, was determined for the temperature region 650° to 1250°C. For calculation of experimental results Whipple's solution was used. Arrhenius plots show a break at ∼1050°C for MgO and at ∼900°C for MgAl₂O₄. Above these temperatures vapor transport seems to be the overriding diffusion mechanism. Below these temperatures ionic transport predominates. Ionic transport is the predominant mechanism for surface diffusion of ⁵¹Cr on Al₂0₃ over the entire investigated temperature region. This can be explained by the weak bond between Cr-vapor species and Al₂0₃ surface. The apparent activation energies for ionic transport of 51Cr are 110 ± 12, 121 ± 12, and 119 ± 12 kJ/mol on MgO, Al₂O₃, and MgAl₂O₄ surfaces, respectively. They include enthalpy of motion and binding enthalpy and are 2.5 to 2.8 times smaller than the activation energies for volume diffusion. Investigations of surfaces by LEED, Auger, and SIMS indicated structural nonperiodicity and surface segregation of impurities.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel Powder

MgAl₂O₄ (MA) spinel powder was synthesized by heating an equimolar composition of MgO and Al₂O₃ in LiCl, KCl, or NaCl. The synthesis temperature can be decreased from >1300°C (required by the conventional solid–solid reaction process) to ∼1100°C in LiCl, or to ∼1150°C in KCl or NaCl. The molten salt synthesized MA powder was pseudomorphic and retained, to a large extent, the size and morphology of the original Al₂O₃ raw material, indicating that a "template formation mechanism" plays an important role in the synthesis process.     

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.     

Interdiffusion in the System MgO-MgAl2O4

Interdiffusion coefficients in single-crystal MgO were determined using an MgO-MgAl₂O₄ diffusion couple. For a concentration of 1 mol% Al₂O₃ in MgO, the interdiffusion coefficient can be expressed as D =2.0±0.2 exp (−76,000±3,000/ RT ) for the MgO-MgAl₂O₄ couple. This relation compares well with previous measurements in the MgO-Al₂O₃ system. The interdiffusion coefficients, which increased with the mol fraction of cation vacancies, were in the range of 10⁻⁸ to 10⁻¹⁰ cm²s⁻¹ for the concentrations and temperatures studied. Diffusion was enhanced below 1640°C if powdered MgAl₂O₄ was used. Self-diffusion coefficients for Al³⁺ ions in MgO were calculated; Al³⁺ diffuses faster than Cr³⁺ in MgO.     

Modification of MgAl2O4 Microwave Dielectric Ceramics by Zn Substitution

MgAl₂O₄ microwave dielectric ceramics were modified by Zn substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg_{1− x} Zn _x )Al₂O₄ ceramics were obtained by sintering at 1550°–1650°C in air for 3 h, and the (Mg_{1− x} Zn _x )Al₂O₄ solid solution was determined in the entire composition range. With Zn substitution for Mg, the dielectric constant ɛ of MgAl₂O₄ just varied from 7.90 to 8.56, while the Q × f value had significantly improved up to a maximal value of 106 000 GHz at x =1.0. Moreover, the τ_f of MgAl₂O₄ ceramics had declined from −73 to −63 ppm/°C.     

Mullite-Type Structures in the Systems Al2O3–Me2O (Me = Na, K) and Al2O3–B2O3

A morphous solids belonging to the systems Al₂O₃–Me₂O (Me = Na, K) and Al₂O₃–B₂O₃ were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850° to 900°C for the compositions Al₆Me_xO_{(9+0.5 x )} ( x ≅ 1; Me = Na, K) and Al_{6- x} B _x O₉ (1 x 3). The resultant solids were stable only within a difinite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X-ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in the Pbam (no. 55) space group. The present results indicate that these silica-free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na⁺, K⁺) or (2) substitution of B for Al in some of the tetrahedral positions.     

Synthesis, Microstructure, and Mechanical Properties of Al3BC3

In situ synthesis of bulk Al₃BC₃ was achieved via a reactive hot-pressing method using Al, B₄C, and graphite powders at 1800°C for 2 h. The reaction path for synthesizing Al₃BC₃ was investigated. It was found that Al₃BC₃ formed via the reaction of C, B₄C, and Al₄C₃ above 1180°C. Dense Al₃BC₃ was prepared with a little B₄C and graphite remained. Microstructure observations revealed the plate-like morphology of Al₃BC₃ grains. Moreover, the mechanical properties of Al₃BC₃ were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa·m^1/2, and Young's modulus of 163 GPa). Young's modulus decreased slowly with increasing temperature, and at 1600°C remained 79% of that at ambient temperature. These results show that Al₃BC₃ is a promising lightweight high temperature structural material.     

Coherent Precipitation in the System MgO-Al2O3-Cr2O3

An isothermal section of the ternary system MgO–Al₂O₃-Cr₂O₃ was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl₂O₄–Cr₂O₃ and MgCr₂O₄-Al₂O₃, and the binary join MgAl₂O₄-MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al_1-xCr_x)₂O₃, with the corundum structure or through a metastable phase of the probable composition Mg(Al_1-xC_r)₂₆O₄₀. The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.