Dissociation of LiAlO2 and LiGaO2

The thermodynamic calculations of dissociation of LiAlO₂ and LiGaO₂ as function of temperature and oxygen pressure are presented. It has been found that LiGaO₂ evaporates congruently at the melting point under the oxygen pressure equal to 1 · 10^—5 atm. LiAlO₂ evaporates incongruently, the equilibrium oxygen partial pressure at the melting point is equal to 9.6 · 10^—5 atm and the partial pressure of Li and Al oxides decrease with increasing of oxygen pressure.     

Slow dynamic properties of molten silver halides

The sound absorption has been measured for molten AgCl–AgI mixtures to investigate the relation between the dynamic properties and chemical bonding. The sound absorption for molten AgI shows a large value just above the melting temperature, and decreases with increasing temperature. The sound absorption for molten mixtures around the eutectic composition increases with temperature from the melting point, showing maximum value around 580 °C, and decreases with increasing temperature. The frequency dependence of the sound absorption for molten AgI and mixtures are also measured, and the relaxation time and relaxation strength are evaluated from the observed data by assuming single Debye-type relaxation process. The evaluated relaxation time for molten AgI is about 9.0 ns just above melting temperature and decreases slightly with temperature to about 6.4 ns at 750 °C. This relaxation time corresponds to a quite slow motion compared with the ordinary atomic one, and may suggest a structural relaxation of atomic association between ionic and covalent character. In the mixture around the eutectic region, evaluated relaxation time shows a rather complicated temperature dependence. Those values decreases slightly with temperature from the melting temperature, and have a minimum value around 600 °C, and then slightly increase with the temperature.     

Crystallization of blast furnace slag glass melted in SnO2 crucible

The scope of this work was to study the possibility of obtaining glass–ceramics from Brazilian blast furnace slag. Glass melting on laboratory scale is usually carried out in platinum crucibles, which are normally not attacked by molten silicates and can be easily cleaned and reused. However, the use of platinum-coated furnaces is not economically viable for producing these materials on industrial scale. As an alternative, SnO₂ crucibles were used to melt glasses. Crucible corrosion was observed and thus the effect of SnO₂ on the crystallization behavior of the resulting glass was studied. For this purpose differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, scanning electron microscopy and micro-Raman spectroscopy were used. Besides merwinite, melilte, larnite and wollastonite, a crystalline phase containing tin was only possible to be determined by Raman spectroscopy, due to the presence of a band at 570 cm⁻¹, attributed to a calcium tin silicate.     

Molecular dynamics simulation of premelting effect in AgBr

The effect of premelting in silver bromide crystals has been simulated for the first time. It is shown that at the temperature about 150°C lower than the melting point of silver bromide, a considerable increase in the mobility in the cationic sublattice is observed, whereas the (self-)diffusion coefficient of silver ions attains values exceeding 10^?6 cm²/s. The assumption about the superionic nature of conductivity in the region of premelting is confirmed by the break of the long-range order in the cationic subsystem, which, in turn, is confirmed by the comparison of the pair cation-cation correlation functions far from and in the vicinity of the melting point. It is established that the premelting effect correlates with the experimentally observed effect of a considerable increase in ionic conductivity in the vicinity of the melting point. It is shown that the premelting effect in AgBr is similar to the diffuse superionic phase transition in anionic conductors of the MF₂ family (M = Ca, Ba, Sr, and Pb).     

Impact of chlorine dissociation for modified chemical vapor deposition

Modified chemical vapor deposition (MCVD) is the platform technology used to create a wide range of silica-based optical fibers. This paper reports on the extension of the reaction scheme embedded within a computational fluid dynamics model of the MCVD process to include chlorine dissociation and recombination. Simulations employing this modified kinetic scheme indicate that chlorine dissociation acts primarily as a ‘heat sink’ in cases where the operating conditions promote a high peak temperature in the narrow reaction zone where most of the SiCl₄ oxidation occurs. The extended model allows a wider range of operating parameters to be examined in terms of the deposition profile of silica ‘soot’ particles on the substrate tube wall.     

Estimation of point defect parameters of solids on the basis of a defect formation model of melting (I)

It is proposed that the melting process is governed by the creation of lattice defects followed by a relaxation of the lattice in the vicinity of the defects. At the melting point T_m the mole fraction x_L of defects within the melt is given by x_L = L/h_o (L heat of fusion; h_o formation enthalpy of defects within the solid).     

Density fluctuations in silicate glasses

The potential use of inorganic glasses as waveguides at optical wavelengths has precipitated the need to understand the relationship between intrinsic optical loss and glass composition. The contribution to optical loss due to scattering from density fluctuations, is analyzed and discussed for various homogeneous (non-phase-separated) silicate glasses. The results show that binary alkali silicates in certain composition ranges form homogeneous glasses with density fluctuations less intense than that of pure silica glass. As third components added to these binary compositions alkali oxides (different from the one already present in the base glass), alkaline earth oxide (CaO) and Al₂O₃ cause further reductions in the magnitude of the density fluctuations.     

An alumina mat with a nano microstructure prepared by centrifugal spinning method

Ceramic fiber products specially alumina mat because of low thermal conductivity and high melting point are used as high temperature insulating materials. Alumina has so high melting point (T_m > 2040 °C) that its mat can be produced through sol–gel method. In this research alumina mat has been manufactured by sol–gel spinning method using our laboratory-designed centrifugal spinneret. The desired viscosity of sol for spinning is 150 P. Phase transformation of the product begins at 600 °C and there is not any amorphous phase at 800 °C and theta alumina (θ-Al₂O₃) is the main phase. In this work, transformation of transitional phase to alpha alumina (α-Al₂O₃) takes place from 1000 °C to 1200 °C. The optimum percent of silica in alumina mat is 4 wt%. Fibers constitute network structure that their average diameter is about 10 μm and contains very fine grains (100 nm). The silica percent concerning the limits of this study (<10 wt%) does not effect on fiber diameter, but grain size decreases from about 200 nm to less than 100 nm while increasing silica percent.     

Thermodynamics of (Ga,In)‐Sb‐O‐Si and impact on dewetting process

A thermodynamic study is performed for the systems (Ga or In)‐Sb‐O‐Si in order to better understand the difference observed during dewetting experiments of GaSb and InSb in silica ampoules. Results show that the melts can be considered as non reactive toward silica. When the atmosphere is clean (≤ 1 ppm O₂), no oxide is formed, while, under oxidising atmosphere, oxides exist above the melting point of the antimonide and are known to increase the wetting angle of the melt on the crucible. However the temperature range for oxide stability is smaller in the case of InSb and this may explain why dewetting is easy for GaSb in presence of oxygen, while it is difficult for InSb. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Corrosion of electrocast AZS refractories by CAS glass–ceramics melting

Extensive corrosion experiments on electrocast alumina–zirconia–silica (AZS) refractories by molten CaO–Al₂O₃–SiO₂ (CAS) and Na₂O–CaO–SiO₂ (NCS) glasses were carried out at various temperatures under static condition. The features and mechanism of the corrosion were compared and analyzed. The changes of microstructure and phase composition of refractories in the course of the melt corrosion were also studied. X-ray diffraction (XRD), scanning electron microscope (SEM) and chemical analysis were used to characterize the corroded refractory materials and reacted melts. The reasons of alumina–zirconia–silica bricks corroded are the meltdown of their own composition, penetration or permeation of alkali oxide in the glass melt and scouring of the glass melt. The results show that the refractories resistance against corrosion of the oxides like Na₂O, K₂O or CaO is weak, and that the corrosion mechanism of NCS/AZS is different from that of CAS/AZS. In a static condition, CaO–Al₂O₃–SiO₂ melts corroded alumina–zirconia–silica brick more severely than Na₂O–CaO–SiO₂. The result provides useful reference to a prospective selection of refractory materials in glass and glass–ceramics manufacture.     

Formation Volume of Vacancies in Elements and the Defect Formation Mechanism of Melting

The formation volume V_v of vacancies is given by V_v = (h_v/L) ΔV_f with h_v = 8L (formation enthalpy h_v of vacancies and heat L of fusion given in same units; ΔV_f = change of volume due to melting). If there are phase transitions within the solid, L and ΔV_f must be replaced by (L + Δ H_t) and by (ΔV_f + ΔV_t), respectively (Δ H_t and Δ V_t refer to the heat (s) of transition (s) and to the volume change(s) due to transition(s), resp.). The pressure dependence of the melting point is dT_m/dp = (T_mV_v)/h_v. Independent of the sign of V_v any increase of the vacancy concentration above the maximum concentration possible within the solid decreases the melting point thus resulting in the observed surface melting. The melting point is fixed by the characteristics of vacancy formation (h_v, S_v, V_v) and by the bulk modulus of the solid (S_v = formation entropy of vacancies).     

Conversion of tin and alkali chlorides to phosphates and vitrification into silicate glasses

Mixtures of tin(II) chloride and tin(II)-sodium–potassium chlorides were converted into phosphate salts by reaction with ammonium dihydrogen-phosphate at 400 °C. Element analyses showed that no loss occurred during the treatment, and Mössbauer and wet analyses indicated no change in tin valence. The converted tin phosphate and tin–sodium–potassium phosphates were then dissolved into silicate glasses at 1350 °C without loss. Structural analyses were realized with ³¹P, ²⁹Si, ¹¹⁹Sn NMR and ¹¹⁹Sn Mössbauer, which revealed a complete dissociation of tin phosphate into the silicates. Phosphate units consisted in ortho- and pyro-phosphates. Tin(II) was partially oxidized into tin(IV), but there was no evidence for a phase separation into SnO₂ or SnP₂O₇, tin being mainly bonded to silicate units. These results are discussed in terms of O²⁻ exchange between phosphate and silicate units during the melting process.     

Estimation of point defect parameters of solids on the basis of a defect formation model of melting (II). Vacancy formation enthalpy in relation to the heat of fusion and the increase of volume due to melting

The mole fraction of vacancies within melts at their melting points is found to be x_L = = 0.125 in the case of elements (with exception of inert gases), alkali halides, binary oxides and organic compounds. As to inert gases it is x_L = 0.194. The formation enthalpy of vacancies (Schottky defects) is given by h_S = L/x_L (L heat of fusion) in every case. The increase of volume due to melting can be explained well by the vacancy model of melting: ΔV = V_Sx_L (V_S formation volume of Schottky defects).     

Control of alkali-metal oxide activity in molten silicates

A new thermochemical reactor is designed to control alkali-metal oxide activity simultaneously in several molten silicates at temperature up to 1400 °C. The method consists in imposing an alkali metal vapor pressure in a closed system by Na_(g) evaporation from Na₂O–xSiO₂ melt and equilibrating this vapor with the molten silicate samples. By comparison of experiments carried out in regular furnaces, the drastic reduction of the working volume ensures a better control of sodium metal vapor pressure for durations of the order of a hundred of hours. This device has been applied to measuring the sodium solubility and sodium-metal oxide activity at 1350 °C in the anorthite–diopside eutectic melt composition of the CaO–MgO–Al₂O₃–SiO₂ system.     

Molecular dynamics simulation of anhydrous lithium acetate: crystalline and molten phases

The results of molecular dynamics simulations of the crystalline and molten phase of anhydrous lithium acetate are presented. The potential parameters were obtained from empirical fitting to the crystalline phases of the material. The simulations were carried out for 216 molecules in an NPT ensemble using the DLPOLY program. A structural model is proposed for both the crystalline and molten phases of lithium acetate. Calculated values of the melting point, diffusion coefficient and structural parameters of lithium acetate are in reasonable agreement with experimental results.     

Estimation of point defect parameters of solids on the basis of a defect formation model of melting (III). Formation entropy and concentration of schottky defects (vacancies)

The formation entropy of Schottky defects (vacancies) is calculated from the entropy of fusion and the concentration x_L = 0.125 of vacancies within the melt at melting point T_m. The formation of vacancies is connected with a decrease of the vibrational frequency of those lattice forming particles (atoms, ions, molecules) being neighbours of the vacancies. Theoretical values of the vacancy concentrations x_s agree with those x_s obtained with the help of experimentally determined free formation enthalpies of the defects.     

The structures and vibrational spectra of crystals and glasses in the silica-alumina system

Solar furnace melting and fast-quench techniques have been used to prepare SiO₂Al₂O₃ glasses to high alumina content (near 60 mol% Al₂O₃), which have been studied by Raman spectroscopy. These spectra may not be simply interpreted. The structures of crystalline compounds in the SiO₂Al₂O₃ system are discussed in relation to their vibrational spectra. On the basis of this discussion and other considerations, a structural model for the silica-alumina glass system is proposed, which is consistent with the stable or metastable immiscibility suggested along this join. The essential features of this model include a modified silica structure at low alumina content, and “structure-broken” regions at high alumina compositions, with silicon in tetrahedral coordination, but aluminium assuming a variety of bonding geometries. These are proposed to include aluminate tetrahedra with higher polymerization than simple corner-sharing, and less well-defined polyhedra of higher average coordination number.     

固态材料形成过程中的晶体生长机理探讨

同样由熔体凝固、溶液沉淀或者气相沉积出来,为什么有的材料呈晶相,有的呈非晶相? 晶体生长是由自组装形成的还是由外界条件决定的?是哪项因素决定了晶体生长时原子的有序排列?本文根据实际现象,用晶体生长的热力学理论分析了逆向离子解离对晶体成核及生长的作用机理,并对固态材料的形成与晶体成核生长之间的关系也作了进一步的阐述和分析,由此得出结论认为,由化学键结合的材料在晶体生长时必须伴随着逆向离子解离平衡,正是固态材料形成过程中的逆向离子解离过程,如同时伴随着电离的溶解、熔化及升华过程,决定了晶体生长时原子的有序排列.     

Physicochemical model for predicting molten glass density

Models have been developed since the 1970s to predict the composition- and temperature-dependent density of silicate melts in which the molten glass is considered as a mixture of virtually ideal solutions. Published data were compiled to set bounds on the partial volumes of each constituent oxide of radioactive waste containment glass. A model based on the data is proposed to predict the density of complex molten borosilicate glass formulations between 900 and 1300 °C. The model is limited to compositions with silica concentrations between 40 and 80 mol%, alkali oxide concentrations between 5 and 50 mol%, and SiO₂/B₂O₃ molar ratios exceeding 0.9. Within this composition range the model is capable of reproducing the experimental data within 4%. One application of this approach is to construct an interpolation equation suitable for use in models simulating the thermal, rheological, electrical, and magnetic conditions of vitrification processes.