Liquid Al2O3 Research Articles

Neutron diffraction of calcium aluminosilicate glasses and melts

The combination of neutron diffraction with aerodynamic levitation and laser heating, pioneered by Neville Greaves and co-workers about 15years ago, is an important tool for studying the structure of liquid melts. Since the first work on liquid Al2O3 published in 2001, the technique has been largely improved and experiments are now routinely performed at neutron sources, providing interesting structural information on various materials.In this paper, the structure of glass-forming compounds in the system CaO-Al2O3-SiO2 was measured by applying neutron diffraction with aerodynamic levitation. Results obtained in the liquid state above the melting point and from the glass at room temperatures are presented. Various compositions were studied by increasing the silica content and by changing the ratio CaO/Al2O3. As observed using other methods, the main structural changes relate to modification of the Al-O short range order.

Factors Affecting the Separation of Ti-Al Alloy in the TiPro Process

Abstract This paper presents and discusses the factors influencing the yield of Ti-Al alloy in the TiPro process which is a process developed at the University of Waikato for producing titanium alloy powders by mechanically activating Al/TiO2 powder mixtures and subsequently preheating the resultant composite powder in order to ignite a combustion synthesis reaction and separate the liquid Ti-Al alloy by extrusion. In this study, TiO2/Al composite powders with different powder particle microstructures have been produced and used to study the effects of starting composite powder particle microstructure on the solid/liquid separation of TiAl from solid Al2O3 by extrusion. Results obtained so far indicate that maximizing the time the Ti-Al alloy phase is maintained in the liquid state after the reaction between TiO2 and Al is one of the critical factors to increase the yield of Ti-Al alloy produced through the separation of liquid Ti-Al from the solid Al2O3 phase by extruding the mixture of liquid Ti-Al and Al2O3 formed through reactions and heating.

Ablation mechanisms of interpenetrating Al20Si/graphite composites

In order to utilize the latent heat of metal fusion and evaporation, Al20Si/graphite was added into graphite by squeeze casting infiltration. Its ablation properties were tested by oxy-acetylene flame. Compared with graphite, linear ablation rates of Al20Si/graphite decrease nearly by one order of magnitude. However, linear ablation rate of Al20Si/graphite increases with ablation time, which is caused by the change of ablation mechanism. At the beginning of ablation process, thermal chemical erosion plays an important part and thermal protective effect comes from the melting of Al20Si, the alleviation of liquid Al2O3 towards flame scouring and the decrease of porosities compared with graphite, while with ablation processing thermal mechanical erosion turns to dominate and thermal protective effect is mainly presented on the prevention of oxygen diffusion by the molten Al4SiC4 covered on ablation surface and the oxygen diffusion barrier effect of Al4SiC4 stuffed into graphite pores.

Metallic liquid hydrogen and likely Al2O3 metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al2O3 might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H2 and in sapphire (single-crystal Al2O3), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al2O3. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al2O3, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.

Molecular dynamic simulation of liquid Al2O3 under densification

We show first that when the densification of liquid Al2O3 occurs, the structure consists of identical units AlOx and blocks OAly. Here x = 4, 5 and 6; y = 2, 3 and 4. Al2O3 liquids with different densities and the same chemical composition differ from each other only in the relative number of these units and blocks. Taking this into consideration, we examine seven MD models with the density ranging from 2.7 to 3.3 g cm−3. The simulation shows that the volume of all constructed models is a linear function of the fraction of AlOx units. Furthermore, we observe the first sharp diffraction peak (FSDP) in the scattering structure factor S(q) located at q1 = 1.85–2.15 Å−1. The microscopic mechanism of densification and the origin of FSDP have been studied and discussed here.

Thermodynamic Modeling of the Al2O3–Ti2O3–TiO2 System and Its Applications to the Fe–Al–Ti–O Inclusion Diagram

All available thermodynamic and phase diagram data have been critically evaluated and optimized for the liquid slag phase and for all solid phases at 1 bar pressure from 298 K to above the liquidus temperatures for the systems Al2O3–TiO2, Al2O3–Ti2O3, and Al2O3–Ti2O3–TiO2, and a database of model parameters has been prepared. The Modified Quasichemical Model was employed for the molten oxide phase. The thermodynamic modeling predicts the existence of a liquid oxide Al2O3–Ti2O3–TiO2 phase at secondary steelmaking conditions. The database was used to calculate the inclusion diagram for Al–Ti deoxidation at 1600°C.

IN SITUSTRUCTURAL STUDIES OF ALUMINA DURING MELTING AND FREEZING

Modern levitation furnaces are enabling melting and freezing of refractory materials like Al2O3 to be studied in depth with synchrotron radiation techniques. Whilst α-Al2O3 is a close packed Debye-like solid, liquid Al2O3 has smaller coordination numbers and the structure has network-like characteristics. Under contactless conditions, substantial under cooling can be achieved. Melting involves a significant decrease in density which is approximately recovered on recrystallization, both of which can be followed with high speed video imaging. Freezing occurs with a burst of energy — recalescence — which substantially raises the temperature momentarily. Using Rietveld refinement the density of α-Al2O3 and the mean square displacement of the average atom 〈μ2〉 can be followed up to the melting point and upon freezing using in situ X-ray diffraction. As melting is approached 〈μ2〉 for α-Al2O3 exceeds the harmonic approximation of the Lindemann–Galvarry law and for liquid Al2O3 above the melting point 〈μ2〉 appears to be greater still. On freezing from the undercooled state α-Al2O3 is initially decompressed with the T - P point falling on the melting curve extrapolated to negative pressures.

In situ studies of agglomeration between Al2O3–CaO inclusions at metal/gas, metal/slag interfaces and in slag

Studies of inclusion behaviour at the metal/slag interface is of great importance for the steel industry in order to achieve better control of both the size and amount of the inclusions, as well as improving the steel quality and the casting process. In this work agglomeration of liquid Al2O3–CaO particles at both steel/argon gas and steel/slag interfaces was studied with a confocal scanning laser microscope. In addition, agglomeration of liquid Al2O3–CaO–SiO2 inclusions present in the slag was investigated. The results showed that liquid inclusions more easily agglomerated to semiliquid inclusions than to liquid inclusions. Moreover, the agglomeration of liquid particles was found to be improved remarkably when the particles were present in the slag compared to when they were in the steel/slag interface.

Local environments of oxygen in Al2O3SiO2 melts

Abstract Local environments of oxygen in Al2O3 SiO2 melts have been investigated via Molecular Dynamics (MD) simulation in a model containing about 3000 atoms with the Born–Mayer type pair potentials under periodic boundary conditions. The study has been carried out via determination of temperature and composition dependences of fraction of [n]O at temperatures ranged from 2100 to 7000 K in the system containing 33.33, 50.00 and 66.67 mol% Al2O3. In addition, fraction of [n]O in liquid Al2O3⋅2SiO2 at the density ranged from 2.60 to 4.20 g/cm3 has been estimated in order to observe the pressure dependence. The label [n]O refers to the number ( n ) of T atoms that are nearest neighbors to a given O (i.e. T = Al , Si and n = 0 , 1 , 2 , etc.). The possible relation between local environments of oxygen and dynamics of the system has been discussed. In addition, evolution of triclusters (O atoms surrounded by three cations) and their composition caused by varying temperature, composition and density of liquids has been found and presented.

Liquid–liquid phase transition in simulated liquid Al2O3·2SiO2

Liquid–liquid phase transition in liquid Al2O3·2SiO2 (AS2) has been studied at 4200 K in a model containing 3025 particles under periodic boundary conditions with Born–Mayer type pair potentials via molecular dynamics (MD) simulation. Structure and dynamics of models at densities ranging from 2.60 to 4.20 g cm−3 at a temperature of 4200 K have been analysed in detail through the partial radial distribution functions (PRDF), coordination number distribution, bond-angle distribution, interatomic distances and diffusion constant. We found clear evidence of transition from a tetrahedral to an octahedral network structure in the model upon compression, like that observed in simple oxide systems. Moreover, the transition is accompanied by an anomalous diffusion of components in the system.

Local microstructure of liquid and amorphous Al2O3

We present a local microstructure study of liquid and amorphous alumina by means of a molecular dynamic method. The simulation was performed on systems prepared at five different densities and at temperatures from 400 to 800K for amorphous glass and at 3000K for liquid. The local microstructure has been analyzed through the pair radial distribution function, bond-angle distribution and the characteristics of voids and void aggregations. We focused on two kinds of void aggregations: void clusters and void tubes. The results showed that many voids clustered nearby and created a very large cavity with volume five times bigger than volume of the aluminum atom. In addition, we found a void tube in low-density systems, which consisted of 75% of all voids and spread over the entire system. The diffusion mechanism in low-density and high-density systems has also been discussed and presented here.

Computer simulation of liquid Al2O3

Molecular dynamic simulation has been done to determine the dynamic and local structureof liquid alumina at 3000 K. Fourteen different systems at densities ranging from 2.5 to4.5 g cm−3 was prepared by compressing the low-density melt. Two kinds of pore aggregation, porecluster and pore tube, were examined. Clear evidence was found of structuraltransformation from a tetrahedral to an octahedral network. For a low-density system therewas a large pore tube, which involved 93% of oxygen-vacancy-like pores and spread overthe whole simulation cell. Conversely, in a high-density system the largest pore tubecontained less than 1% of all oxygen-vacancy-like pores. A similar trend also was observedfor other pore kinds such as aluminium-vacancy-like pores and large pore clusters. Thediffusion constants significantly decreased in the region of the structural transformation.The diffusion mechanism in low- and high-density systems was examined and is discussedhere.

Molecular dynamics study on local structure of amorphous and liquid Al2O3

Microscopic features of liquid and amorphous alumina (Al2O3) were simulated by molecular dynamics calculations. The simulations were performed in an orthorhombic cell with 3000 particles using the Born-Mayer potential at temperatures of 0, 2500, 2700, and 3000 K under constant pressure. It was found that a large cluster of pores contained several thousand spherical pores, which were formed with radii larger than 0.73 A. The observed variations of Al2O3 structures with atomic arrangements of AlOx (x=3, 4, 5, and 6) are discussed.

Temperature‐induced phase transition in simulated amorphous Al2O3

AbstractOur previous simulation studies indicated the existence of pressure‐induced phase transition in liquid and amorphous Al2O3. In the current paper, we present the structural transformation of simulated amorphous Al2O3 from an octahedral to a tetrahedral network structure by heating from the high‐density amorphous (hda) model at constant pressure P = 0 GPa. The structure of the models was analyzed through the partial radial distribution functions (PRDFs), coordination number distributions, bond‐angle distributions and interatomic distances. Furthermore, significant differences in structural characteristics of low‐density amorphous (lda) models obtained by heating from the hda one and by cooling from the low‐density melt have been found and are presented. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heating rate effects in simulated liquid Al2O3

The heating rate effects in simulated liquid Al 2 O 3 have been investigated by Molecular Dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born-Mayer type pair potentials. The temperature of the system was increasing linearly in time from the zero temperature as $T(t)=T_0 +\gamma t$ , where $\gamma $ is the heating rate. The heating rate dependence of density and enthalpy of the system was found. Calculations show that static properties of the system such as the coordination number distributions and bond-angle distributions slightly depend on $\gamma $ . Structure of simulated amorphous Al 2 O 3 model with the real density at the ambient pressure is in good agreement with Lamparter's experimental data. The heating rate dependence of dynamics of the system has been studied through the diffusion constant, mean-squared atomic displacement and comparison of partial radial distribution functions (PRDFs) for 10% most mobile and immobile particles with the corresponding mean ones. Finally, the evolution of diffusion constant of Al and O particles and structure of the system upon heating for the smallest heating rate was studied and presented. And we find that the temperature dependence of self-diffusion constant in the high temperature region shows a crossover to one which can be described well by a power law, $D\propto (T-T_c )^\gamma $ . The critical temperature T c is about 3500 K and the exponent $\gamma $ is close to 0.941 for Al and to 0.925 for O particles. The glass phase transition temperature T g for the Al 2 O 3 system is at anywhere around 2000 K.

Thermodynamic Assessment of the CaO–Al2O3–SiO2 System

The CaO–Al2O3–SiO2 system has been assessed with the CALPHAD technique, based on recent assessments of its binary systems. A new species AlO2−1 was introduced for modeling liquid Al2O3. The ternary liquid phase was described using the ionic two‐sublattice model as . The available experimental data were critically examined, and a self‐consistent set of thermodynamic descriptions was obtained. Various phase diagrams and property diagrams, including isothermal sections, isoactivity lines, and a projection of the liquidus surface, are presented. Information on viscosity seems to support the use of the AlO2−1 species.

Structure and diffusion simulation of liquid Al2O3

Structure and diffusion of liquid Al2O3 have been investigated by molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with the BKS pair potentials. Structure of liquid models agrees reasonably with experiment. The microstructure of systems has been analyzed through partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Temperature dependence of these distributions was obtained. Calculations show that in the liquid Al2O3 model with real density at 2.80g/cm3 there is a short-range order dominated by distorted AlO4 tetrahedron, in agreement with Landron's experiment. Self-diffusion constants have been calculated. The temperature dependence of diffusion constants D in liquid Al2O3 shows an Arrhenius law with activation energy which is close to experimental one for liquid SiO2 and close to the calculated data for diffusion in liquid aluminum silicate. With increasing temperature we find that this dependence shows a crossover to one which can be described well by a power law, D∝(T-Tc)γ. The critical temperature Tc is about 3500K and the exponent γ is close to 0.50. We also present the effects of MD run time on the structure of models. The phase transition temperature Tg for the Al2O3 system is anywhere around 2100K.

Characterisation of Interfaces Between Liquid Tin and Alumina in the Presence of Titanium Alloy Additions

Neutron reflection spectroscopy has been used to characterise the composition of interfaces between liquid Sn-Ti alloys and Al2O3. The reflectivity profiles for both 1% and 3% Ti alloys are consistent with the presence of a layer approximately 2 nm thick at the liquid/solid interface, showing extensive segregation of Ti. The composition of this layer is not pure Ti but shows a greater Ti content than any known Ti oxide composition. In all interfaces exposed to liquid metals at elevated temperature (including pure Sn) there is a layer of thickness 20–100 nm on the alumina surface with slightly lower neutron scattering length density than pure alumina. This is interpreted as evidence for surface roughening of the Al2O3 surface during exposure to the liquid metal.

The phase stability of solid LiAlO2 used for the electrolyte matrix of molten carbonate fuel cells

LiAlO2 is used as a solid matrix for molten carbonate fuel cell (MCFC) electrolyte tiles. These devices operate within the temperature range of 870–970 K. The γ-crystallographic form of this compound is commonly used for fabrication of these matrix tiles. The thermodynamic stability of this phase within the above temperature range is not clear and it is reasonable to consider the γ→α transformation to take place in solid LiAlO2 in presence of molten alkali carbonates. In order to establish the crystallographic form of the compound that forms as a product of reaction between the liquid Li2CO3 and solid Al2O3, the kinetics of LiAlO2 formation has been investigated. Values of conversion factor as a function of the reaction time have been determined both for a stoichiometric reaction mixture and for mixtures with an excess of the liquid 0.53 Li2CO3 + 0.47 Na2CO3 eutectic. The simultaneous determination of the reaction conversion factor and the relative intensity of the characteristic x-ray peak for the α − LiAlO2 form in the reaction mixture have shown that, within 843–973 K, in experiments of ca 100–10 hours, respectively, only this phasev of LiAlO2 is formed. These results may explain the presence of some α − LiAlO2 amount in the MCFC electrolyte tiles after long run tests of these devices.

The electrical conductivity of levitated liquids

The electrical conductivities of aerodynamically levitated liquid spheres have been determined by an electrodeless method. It is shown that this technique is reliable over a wide range of temperatures; results are presented for a variety of systems including metals, semiconductors at room temperature and at their melting points, and solid and liquid Al2O3.