The phase equilibrium diagram of the ternary subsystem CaO-CaO·Al2O3-11 CaO·7 Al2O3·CaF2

On the basis of the experimental results obtained from the study of high-temperature equilibrium relations of seven pertinent joins the phase diagram of the subsystem CaO-CaO·Al₂O₃-11 CaO·7 Al₂O₃·CaF₂ has been constructed. In this diagram the delineation of the boundary curves of the primary fields of CaO, 3 CaO·Al₂O₃, CaO·Al₂O₃ and the 11 CaO·7 Al₂O₃·CaF₂ solid solution has been improved over our previously published phase diagram of the system CaO-Al₂O₃-CaF₂ [1]. The isotherms have also been drawn more precisely to give a better idea about the topography of the portion investigated.     

Dip coated 12CaO·7Al2O3 thin films through sol-gel process using metal alkoxide

Transparent nanostructured 12CaO·7Al₂O₃ thin films with cubic structure have been prepared on soda lime glass substrates via the sol-gel dip coating using the precursor sol solution at low temperature. The structural, compositional, morphological and optical properties of the 12CaO·7Al₂O₃ films and powder were studied using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy. Optical properties of 12CaO·7Al₂O₃ films have been investigated using UV-visible spectroscopy. Two different precursor sols were prepared using calcium-2-ethyl hexonate and aluminium isopropoxide as precursor materials in isopropanol and ethylene glycol monomethyl ether solvents. Dip coated gel like films were dried at 120 °C for 15 min and subsequently heat-treated at 450 °C for 1 h in air atmosphere. The influence of films thickness and optical transparency with use of different solvent and sol concentration on microstructure of the films were established. In addition, XRD patterns revealed that 12CaO·7Al₂O₃ films have been composed of cubic phase. SEM observations exhibited that the films structure becomes more homogeneous using isopropanol as compared to ethylene glycol monomethyl ether solvent. The 12CaO·7Al₂O₃ films prepared using 2 (wt.%) sol in isopropanol had high transparency nearly 88% in wide visible range with maximum of 90% at 600 nm wavelength.     

Phase equilibria in the CaF2-Al2O3-CaO system

Computations of phase equilibria in the CaF₂-Al₂O₃-CaO system have been carried out on the basis of experimentally found thermodynamic properties of all intermediate phases and melts. Coordinates of the phase equilibrium boundaries were determined by solving a system of equations expressing equality of chemical potentials of the components in coexisting phases. The nature and quantity of the coexisting phases were established by a search for the Gibbs energy minimum of the system. All the phases of the CaF₂-Al₂O₃-CaO system were taken into consideration. Calculated phase diagrams of the CaO-CaF₂, CaO-Al₂O₃ and CaF₂-Al₂O₃ binary subsystems are in good agreement with the data available in the literature. Isotherms of the CaF₂-Al₂O₃-CaO system were calculated at 1600, 1650, 1723 and 1773 K. A wide region of liquid separation into two phases is observed in the system. One phase is composed of practically pure CaF₂ with additions of several mol% of CaO and Al₂O₃, and the other consists of 50 to 65 mol% of CaF₂ only. Eleven invariant points of the CaF₂-Al₂O₃-CaO system include seven ternary eutectics, two ternary peritectics and two points of four-phase monotectic transition. The primary fields of crystallization of all the phases are alongated toward the CaF₂ apex, the CaO field being the widest and the 3CaO·Al₂O₃ field the narrowest. Seven junctions of the CaF₂-Al₂O₃-CaO phase diagram were represented. Computed saturation lines of CaF₂-Al₂O₃-CaO melt with CaO, Al₂O₃, CaO·6Al₂O₃ and CaO·2Al₂O₃, and also the positions of a number of characteristic points, agree well with the experimental data available. The present calculations reveal a number of details and peculiarities of the constitution of the CaF₂-Al₂O₃-CaO phase diagram.     

The friction and wear characteristics of plasma-sprayed ZrO2-Cr2O3-CaF2 from room temperature to 800°C

The friction and wear characteristics of low-pressure plasma-sprayed (LPPS) ZrO₂-Cr₂O₃-CaF₂ from room temperature to 800°C were studied by using a high-temperature reciprocating wear tester. At room temperature, friction of ZrO₂-Cr₂O₃-CaF₂ composite coating against Al₂O₃ sphere was quite high, and exhibited a decrease trend with the increase of load from 30 N to 80 N. At 700°C, the composite exhibited the lowest friction and wear among all selected temperature conditions. Brittle fracture and delamination with large wear sheets were considered as the dominant wear mechanism at room temperature. At 400°C, microfracture dropping become more dominated. However, plastic deformation, formation of CaF₂ lubrication films and particle removal of Cr₂O₃ appeared as the main wear mechanisms at elevated temperatures. CaF₂ acting as an effective lubricant at 600°C and 700°C reduced the friction and wear of the composite. Cr₂O₃ particles played a very important role on controlling the size and type of microcracks and preventing or deflecting the microcrack propagation during 700°C test. Cr₂O₃ particles also acted as hard barriers to resist the scratching and high-temperature deformation of ZrO₂-CaF₂ matrix without increasing the friction. To a great extent, wear mainly depended on the degree of debonding and removal of Cr₂O₃ particles at 700°C.     

Phase transformation of calcia-alumina-magnesia fibres produced by inviscid melt spinning

CaO-Al₂O₃-MgO (CAM) ceramic fibre produced via inviscid melt spinning (IMS) was investigated for phase transformation. Differential thermal analysis (DTA) on the as-spun CAM fibre gave two transformation peaks, one for exothermic peak at around 927°C and the other for endothermic one at around 1100°C. In order to identify each phase transformation x-ray diffraction (XRD) analysis was performed on the CAM fibres heat-treated to each phase transformation completion temperature. The exothermic peak was determined to represent crystallization of remaining amorphous phase in the as-spun CAM fibre. The endothermic peak was determined to correspond to transformation of non-equilibrium CaO · Al₂O₃ phase to equilibrium 3CaO· 5Al₂O₃ phase.     

Comparison of some properties of neodymium-doped silica glasses prepared by sol-gel and melting

Samples with 10Nd₂O₃·5CaO·17Al₂O₃·68SiO₂ composition (mol %) were prepared by the sol-gel method and by melting. The structure of the samples was investigated by X-ray diffraction, scanning electron microscopy, infrared spectroscopy, a.c. susceptibility measurements and electron spin resonance. The gel-derived sample heat-treated between 250 and 700°C is amorphous and has a granular microstructure. In the samples heat-treated at 800, 900 and 1000°C we detected by X-ray diffraction AlNdO₃ crystalline particles. The melted sample was amorphous and the micrograph shows a glass matrix containing particles. The infrared measurements show structural changes produced by the heat-treatment of the gel samples. The magnetic susceptibility data indicates that neodymium oxide is present in these samples as Nd³⁺-O-Nd³⁺ aggregates with an antiferromagnetic interaction.     

Nucleation and crystal growth in a fly ash derived glass

The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al₂O₃·2SiO₂) and anorthite (CaO·Al₂O₃·2SiO₂). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol^–1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.     

The phase equilibrium diagram of the system CaO-Al2O3-CaF2

The phase equilibria of the system CaO-Al₂O₃-CaF₂ have been studied by quenching in sealed platinum capsules followed by microscopic and X-ray examination of quenched products. The established phase diagram contains five ternary eutectics, two ternary peritectics and two invariant points of four-phase monotectic transformations. The system Al₂O₃-CaF₂ has been established to be the stable diagonal of the reversible reciprocal system Al₂O₃ + 3CaF₂ = 3CaO + 2AlF₃. In the high-fluoride region a wide zone of liquid immiscibility has been found. The system also shows such rarely noticed but theoretically possible phenomena as the transformation of peritecticals into eutecticals and occurrence of three different primary fields under single continuous zone of liquid immiscibility.     

Aluminium-27 solid state NMR spectroscopic studies of chloride binding in Portland cement and blends

Alumina-rich pozzolanic and latent hydraulic binders such as pulverised fuel ash, metakaolin, and ground granulated blast furnace slag, together with silica fume, are frequently added to Portland cement concrete to improve performance and to retard chloride ingress and thereby inhibit chloride-induced corrosion of the carbon steel reinforcement. ²⁷Al{¹H} MAS and CP/MAS NMR spectroscopies have been used to follow both the hydration processes of the cement blends and the interactions of chloride ion with the hydrated aluminium species. The spectra of the hydrated aluminate phases were interpretable on the basis that the AFt (Aluminate Ferrite tri-) phase ettringite, C ₆ A H ₃₂(3CaO·Al₂O₃·3CaSO₄·32H₂O, or C ₃ A·3CaSO₄·32H₂O), and the AFm (Aluminate Ferrite mono-) phases calcium mono-sulphoaluminate, C ₄ A H ₁₂ (3CaO·Al₂O₃·CaSO₄·12H₂O, or C ₃ A·CaSO₄·12H₂O), and the lamellar tetracalcium aluminate hydrate, C ₄ AH ₁₃ (3CaO·Al₂O₃·Ca(OH)₂·12H₂O, or C ₃ A·Ca(OH)₂·xH₂O) were present as the only hydrated species containing octahedrally-coordinated aluminium. In all cases, only the AFm phase Friedel's salt (3CaO·Al₂O₃·CaCl₂·10H₂O, or C ₃ A·CaCl₂·10H₂O) could be identified as the major chloroaluminate phase produced by the interactions of the cement pastes with chloride ion.     

Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y₂O₃, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO₃ and Al₂O₃ will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y₂O₃-77 mol% Al₂O₃ and 1702±7° C. YAlO₃ (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO₃ formed during metastable solidification transforms to YAG in the presence of Al₂O₃ at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.     

Sintering and crystallization of (SrO·SiO2)-(SrO·Al2O3·2SiO2) glass-ceramics

In the ternary SrO-Al₂O₃-SiO₂ system the pseudobinary join composition of 50 wt % SrO·SiO₂–50 wt % SrO·Al₂O₃·2SiO₂ (SS-SA2S) showed a glass melting temperature of 1500 °C and a crystallization peak temperature of 1100 °C. The (SS-SA2S) glass-ceramic pellets prepared by cold pressing and pressureless sintering, showed very low porosity. The (SS-SA2S) glass-ceramics containing B₂O₃ and those containing B₂O₃ and TiO₂ revealed crystallization peak temperatures of 1000 °C and unexpectedly high porosity. By applying Kissinger analyses to the DTA data the activation energy values for crystallization of the three glass-ceramics were determined to range from 196 to 255 kJ/mol. The Ozawa analyses on the DTA data gave the Avrami parameter values at 3.69 to 3.95. The X-ray diffraction (XRD) patterns from the three glass-ceramics revealed formation of the equilibrium crystalline phases of SrO·SiO₂ and SrO·Al₂O₃·2SiO₂ (monocelsian).     

Thin film fabrication of nano-porous 12CaO·7Al2O3 crystal and its conversion into transparent conductive films by light illumination

Thin film fabrication of crystalline 12CaO·7Al₂O₃ (C12A7) with zeolitic structure was examined, and their electrical and optical properties were measured. Polycrystalline thin films were prepared by post-annealing of amorphous films in oxygen atmosphere at temperatures above 800 °C. Choice of substrates was crucial for obtaining single-phase thin films. Although various oxide substrates (single crystals of Al₂O₃, Y-stabilized ZrO₂, MgO and silica glass) were examined, single-phase films were obtained only for MgO substrates and the other substrates reacted with the CaO component in the films during post-annealing. The optical band gap of C12A7 was evaluated to be 5.9 eV. Hydride ions were incorporated into the film by a thermal treatment in a hydrogen atmosphere at 1200 °C. The resulting transparent thin films were converted into transparent persistent electronic conductors exhibiting an electrical conductivity 6.2×10⁻¹ S cm⁻¹ at 300 K by ultraviolet light illumination. This is the first example of transparent conductive thin film in which conductive areas can be patterned directly by light.     

Low-temperature synthesis of AlN powder with multicomponent additive systems by carbothermal reduction-nitridation method

AlN powders were synthesized at low temperatures (1300 and 1400 °C) by the carbothermal reduction-nitridation (CRN) method using multicomponent additive systems. The synthesis treatments were conducted in a graphite furnace with flowing nitrogen gas between 1200 and 1500 °C using powder mixtures with Al₂O₃:C molar ratio of 1:3 and 0.5-3 wt% of CaF₂, Y₂O₃, Li₂CO₃ and/or SrCO₃ as additives. In relation to the conventional CRN process, the use of multicomponent additive systems reduced the synthesis temperature in 200 °C (CaF₂-SrCO₃), 100 °C (CaF₂-Li₂CO₃ and CaF₂-Y₂O₃-Li₂CO₃) or <100 °C (CaF₂-Y₂O₃ and CaF₂-Y₂O₃-Li₂CO₃-SrCO₃). X-ray diffraction patterns showed that the additives reacted with the alumina powder forming aluminate phases, which vaporized with the increase of synthesis temperature. The enhanced AlN conversion rate was discussed in terms of the vaporization of aluminates in the reducing atmosphere.     

Synthesis and characteristics of the C12A7-O nanoparticles by citric acid sol-gel combustion method

In this study, the O⁻-containing nanocrystal 12CaO·7Al₂O₃ (C12A7-O⁻) was synthesized by the citric acid sol-gel combustion method. The formation of the C12A7-O⁻ material was investigated via X-ray diffraction, thermogravimetric analysis, differential thermal analysis, electron paramagnetic resonance, field emission-scanning electron microscopy, and time of flight mass spectroscopy. The C12A7-O⁻ material was formed at a lower synthesis temperature (900-1150 °C), with a narrower particle size (33-74 nm). The anionic species stored in the C12A7 material was dominated by the active atomic oxygen anions (O⁻) with a concentration of (1.2 ± 0.3) × 10²⁰ cm^− 3, and the emission current density of O⁻ was about 0.89 ± 0.15 μA/cm² at T_{(sample surface)} = 800 °C and E_{(extraction field)} = 800 V/cm.     

High-temperature glass-ceramics of the BaO-Al2O3-SiO2-Ta2O5 system and molybdenum metal composites

Glass-ceramics which consist of tantalum pentoxide (Ta₂O₅), hexacelsian (BaO·Al₂O₃· 2SiO₂), and aluminium tantalate (Al₂O₃·Ta₂O₃) are described. These glass-ceramics can form refractory composites up to 1400° C with molybdenum metal. The glass-ceramics and metal have compatible physical and chemical properties which allow close thermal expansion and excellent bonding.     

Enhancement of ionic conduction in CaF2 and BaF2 by dispersion of Al2O3

Ionic conductivity measurements were performed on polycrystalline CaF₂, BaF₂ and those dispersed with Al₂O₃ particles. The ionic conductivity of both CaF₂ and BaF₂ increased by about 1 to 2 orders of magnitude by dispersion of Al₂O₃ particles, while X-ray diffraction measurements showed there were no other phases present other than fluoride and Al₂O₃. The conductivity of the dispersed system strongly depended on the particle size and the concentration of Al₂O₃, which suggested the high ionic-conductivity layers were formed at the interface between the ionic conductor matrix and the Al₂O₃ particles. The effective thickness and electrical conductivity of the interface layer at 500° C were calculated, using a simple mixing model, to be 0.3 to ~ 0.6m and ~ 10^–3 S cm^–1, respectively.     

Glass formation in the system CaO-Al2O3-CaF2

The formation of glass in the system CaO-Al₂O₃-CaF₂ has been investigated in sealed platinum capsules having about one atmosphere pressure of AlF₃ vapour. Transparent colourless glass could be obtained in the low-fluoride moderate-alumina region of the system (Al₂O₃ 35 to 60%, CaF₂ 0 to 20%). With the concentration of CaF₂ exceeding 20% considerable amount of quench crystals of CaF₂ appeared in the glass. Moderate-alumina low-lime melts containing more than 35% CaF₂ occur in an immiscibility zone. At the low-fluoride periphery of the liquid immiscibility zone a small zone of metastable liquid immiscibility has been found. The results of electron microscopic and infra-red spectroscopic studies of a few selected glasses have been analysed in combination with the molar refractivity data to reflect upon the co-ordination characteristics of aluminium in these glasses.     

Oriented eutectic microstructures in the system Al2O3/ZrO2

Oriented eutectic microstructures have been produced in the system Al₂O₃/ZrO₂ using a Bridgman-type crystal-growing furnace. Ingots consisted of elongated columnar grains or colonies. Inside the colonies a rod-type eutectic microstructure consisting of rods of ZrO₂ surrounded by an Al₂O₃ matrix was observed. The eutectic point was re-established at 63 mol % Al₂O₃/37.0 mol % ZrO₂ and 1870±5° C. Al₂O₃ is the first phase to nucleate when eutectic growth occurs.     

Sintering and crystallization of off-stoichiometric SrO·Al2O3·2SiO2 glasses

Glass-ceramics with the celsian-corundum binary join composition of 88.8 wt% SrO·Al₂O₃·2SiO₂ – 11.2 wt% Al₂O₃, (SA2S-A), were fabricated by pressureless sintering and investigated for their sintering and crystallization behaviors. The (SA2S-A) glass powder showed crystallization peak and melting temperatures of 1059 and 1550 °C, respectively and high sintering ability. The (SA2S-A) glass powders containing B₂O₃, (SA2S-A)B and those containing B₂O₃ and TiO₂, (SA2S-A)BT showed lowered crystallization peak temperatures of 1033 and 997 °C, respectively. By applying Kissiger analyses to the DTA data of the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders, the activation energy values for crystallization were determined as 488, 370 and 333 kJ/mol, respectively. The Ozawa analyses on the DTA data gave the Avrami parameter values at 1.2, 1.1 and 1.9, respectively for the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders. The x-ray diffraction (XRD) patterns of the (SA2S-A) glass-ceramics, crystallized at 1100 °C for 4 h, showed formation of both the monocelsian and hexacelsian phases. The (SA2S-A)B and (SA2S-A)BT glass-ceramics crystallized at 1100 °C for 1 h, showed formation of the phase-pure monocelsian and did not show any evidence of the hexacelsian formation prior to the monocelsian formation.     

Preparation, sintering and fracture behavior of Al2O3/LaAl11O18 ceramic composites

Al₂O₃/25 vol% LaAl₁₁O₁₈ composites were prepared by pressureless sintering at 1550°C with composite powders obtained by copercipiated method using La(NO₃) · 6H₂O and Al(NO₃)₃ · 9H₂O as starting materials. The enhanced reactive activity of Al₂O₃ and chemically homogeneous mixing of the constituents made LaAl₁₁O₁₈ phase to be formed at low temperature in composite powders. AlF₃ additive was used to reduce the transformation temperature of transition alumina. The LaAl₁₁O₁₈ grains in the composite powder obtained at 1500°C showed rodlike morphology distributed homogeneously in Al₂O₃ powder. The samples sintered at 1550°C for 4 h with CAS (CaO-Al₂O₃-SiO₂) sintering aid can obtain a high relative density. The effects of the sintering time on the grain growth of Al₂O₃ and the fracture toughness of the composites were studied and the results showed that LaAl₁₁O₁₈ grains reduced the growth of Al₂O₃ grains and the rodlike grains increased the fracture toughness. The improvement in fracture toughness of the composites was mainly attributed to the mechanism of crack deflection.