The thermal expansion of slags used in Electroslag Remelting

Measurements of thermal expansion of ESR slags for the systems CaF₂ + Al₂O₃ + CaO and CaF₂ + MgO. Hysteresis in thermal expansion data for some slags indicates the formation of microcracks in the slag during cooling. A simple model for the estimation of thermal contraction values for ESR slags is proposed.     

The electrical conductivity of some liquids in the system CaF2+CaO+Al2O3

The conductivity of slags in the binary systems CaF₂+Al₂O₃, CaF₂+CaO and the ternary system CaF₂+CaO+Al₂O₃ has been measured, using a four-lead electrode technique at a frequency of 1 kHz. The cell design used ensured that only molybdenum metal was in contact with the slag at high temperature and that the slag was wholly contained in molybdenum. No frequency dispersion could be detected at frequencies between 0.8 to 10 kHz. It is suggested that the formation of complex ions (e.g. AlO₂F₂ ³⁻ and AlOF ₂ ⁻ ) might account for the observed effects in CaF₂+Al₂O₃, CaF₂+CaO+Al₂O₃ liquids as has been previously suggested. an oxide ion clustering mechanism may explain the conductivities found in CaF₂+CaO.     

Electroslag remelting with all-fluoride low conductivity slags

The electrical conductivity of liquids in the composition ranges CaF₂ + 0 to 12 wt pct AIF₃; CaF₂ + 0 to 20 wt pct LaF₃; and CaF₂ + 0 to 30 wt pct YF₃ has been determined at 1500° and 1600°C. It is deduced from the conductivities and the form of the phase diagrams of these systems that CaF₂ + 20 wt pct YF₃ is the optimum all-fluoride composition for electroslag melting or welding high melting point materials. It is demonstrated that pure iron, AISI 4340, AISI 321, and Hastelloy-X may all be electroslag melted without arcing through this slag using 60 Hz power. However, the initial postulate is confirmed in that only those materials with liquidus temperatures below that of the phase precipitated on freezing the slag can be made into ingots with good surface quality. The use of this slag in electroslag welding pure iron is investigated. It is inferred from the results that the slag composition chosen could probably be used to electroslag weld thick sections of titanium. Formerly Student in the Department of Metallurgy, University of British Columbia.     

Effects of Transition Metal Oxides on Thermal Conductivity of Mould Fluxes

The thermal conductivity of the mould fluxes containing transition metal oxides was measured by hotline method at different temperatures. The relationship between the thermal conductivity of mold fluxes and the contents of transition metal oxides was discussed. The synthetic slags were composed of 30.0% — 35.4% CaO, 34.7% — 38.6% SiO₂, 6% Al₂O₃, 9% Na₂O, 14.4% CaF₂, 0–4% Cr₂O₃ and 0–8% MnO in mass percent. The results indicated that Cr₂O₃ and MnO had a negative effect on thermal conductivity of mold fluxes. The thermal conductivity of mold fluxes was about 0.25 — 0.55 W/(m K) when the temperature reached 1300 °C, and it increased sharply to about 1.32–1.99 W/(m K) when the temperature reduced from 1300 to 1000 °C. The thermal conductivity of mold fluxes containing Cr₂O₃ and MnO was 10%—25% lower than those of original fluxes. The decrease in thermal conductivity was attributed to the change of molecular structure of mold fluxes. In addition, the poor integrity and regulation of polycrystal structure, complexity of crystal structure, and effects of impurities in the boundary and lattice distortion leaded to the reduction in the thermal conductivity. Na₂CrO₄, Mn₂SiO₄ and other minor phases were also found in the samples containing Cr₂O₃ and MnO, respectively.     

Electrical conductivity of niobium-containing oxide-fluoride melts

The effect of Nb₂O₅ on electrical conductivity of metals based on CaF₂ and CaF₂-30% Al₂O₃ at T = 1500–1900 K in an oxidizing atmosphere is investigated using an ac bridge at the frequency 5 kHz. It is found that, as the Nb₂O₅ concentration increases from 0 to 25%, the electrical conductivity of the melts decreases, while the activation energy of electrical conductivity increases monotonically. The data obtained agree with viscosity investigations and indicate that niobium is the complexing agent in oxide-fluoride melts.     

Thermal Conductivity of Molten Silicate of Al2O3-CaO-Na2O-SiO2 Measured by Means of a Front Heating-Front Detection Laser Flash Method

Thermal conductivity values have been systematically obtained for molten silicates containing Al₂O₃, CaO, Na₂O, and SiO₂ by means of a front heating-front detection laser flash method. The measurements were made for 13 samples in the temperature range between 1073?K and 1823?K (800?°C and 1550?°C), depending on the composition. Thermal conductivities of the silicate melts are found to be relatively insensitive to the variation of temperature, but they depend on the composition ratio, particularly the ratio of Non-Bridging Oxygen ions per Tetrahedrally coordinated cation??NBO/T. The thermal conductivity values decrease from 2.8?W/mK to 1.5?W/mK with the NBO/T value until it reaches about 1. Thermal conductivity values become constant for silicate melts with a higher value of NBO/T. It is known that the length of the silicate chain decreases with disconnection by the addition of alkaline earth cation or alkaline cation. The strong correlation between thermal conductivity and NBO/T is quite likely to suggest that silicate chain is a preferential path for heat transport in silicate melts.     

Effect of niobium-containing additions on the viscosity and electrical conductivity of oxide-fluoride melts

An oscillatory viscometer and ac 5-kHz bridge are used to measure the viscosity and electrical conductivity (at 1450–1600°C) of two types of slag melts based on CaF₂(ANF-1P flux) and on the CaF₂-30% Al₂O₃ system (ANF-6 flux) with additions (to 35%) of one of the following three types of niobium-containing compounds: Nb₂O₅ (extrapure grade), Nb₂O₅ (commercial purity), and a niobium concentrate (about 70% Nb₂O₅). An addition of the niobium-containing compounds to the fluxes is found to increase the viscosity, to increase the solidification range, and to decrease the electrical conductivities of the melts. In this case, the activation energies of viscous flow (34–148 kJ/mol) and conduction (22–100 kJ/mol) increase monotonically, which indicates polymerization of the melts.     

Computer study of structure, thermodynamic, and electrical transport properties of Na3AlF6-Al2O3 and CaF2-Al2O3 melts

Structure, thermodynamic, and electrical transport properties of Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts were examined by molecular dynamics. Ionic models were constructed for Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts at 1283 and 2000 K, respectively. It was found that in the Na₃AlF₆-Al₂O₃ melts, stable aluminum-fluorine-oxygen groups are formed. Although bonds between F⁻ and Al³⁺ ions in the first coordination shell are weaker than between O²⁻ and Al³⁺ ions, very stable negatively charged AlF ₆ ³⁻ groups are formed at low oxygen concentrations in the Na₃AlF₆-Al₂O₃. This results in migration of aluminum to the anode in an external electric field. In the CaF₂-Al₂O₃ melts, positively charged aluminum-oxygen groups dominate. This results in migration of aluminum to the cathode at almost all Al₂O₃ concentrations. Therefore, in Na₃AlF₆-Al₂O₃ melts, the Al³⁺ ion as a component of the complex anion has a negative partial conductivity and the O²⁻ ion has positive partial conductivity; in CaF₂-Al₂O₃ melts, Al³⁺ has a positive transport number while O²⁻ has a negative transport number.     

Thermal Conductivity of the Molten CaO-SiO2-FeO x System

Thermal conductivity measurements were carried out on synthetic steelmaking slag using the hot-wire method. Furthermore, local structure analysis in the melts was carried out in order to investigate the relationship with the composition dependence. The thermal conductivity of the CaO-SiO₂-FeO _x melts significantly decreased as the content of FeO _x increases, particularly at lower basicity. Both chemical analysis and the observation show that the amount of Fe²⁺ increases when CaO/SiO₂ is smaller, implying more basic behavior of FeO than FeO_1.5. According to further analyses by M?ssbauer spectroscopy, the degree of basicity of FeO_1.5 remains virtually unchanged in the composition range of interest. From the experimental results, it could be concluded that the thermal conductivity of the silicate melt containing iron oxide is highly dependent on the valence of the Fe ion and comparatively independent of the amphoteric behavior of FeO_1.5.     

Preparation and luminescent properties of CaF2:Ln3+ (Ln:Er,Er/Yb)/Nafion composite films

In this work, CaF_2:Ln~(3+)(Ln:Er,Er/Yb)/Nafion composite films were prepared using Nafion as modifications and matrices by dripping method. The composite films were characterized by Fourier transform infrared spectroscopy(FT-IR), X-ray diffraction(XRD) and scanning electron microscopy(SEM). Composite films are transparent and CaF_2:Ln~(3+)(Ln:Er,Er/Yb) nanoparticles are well dispersed in Nafion films.The thicknesses of CaF_2:Er~(3+)/Nafion and CaF_2:Er~(3+),Yb~(3+)/Nafion composite film are about 77 and 73 μm,respectively. The nanoparticles in composite film possess cubic phase. CaF_2:Er~(3+),Yb~(3+)/Nafion composite film has stronger characteristic emission of Er~(3+) around 1530 nm with full width at half-maximum(FWHM) of 73 nm and longer luminescence lifetimes of 22.04 μs(25.03%) and 100.77 μs(74.97%).     

Growth and optical properties of ytterbium and rare earth ions codoped CaF_2-SrF_2 eutectic solid-solution(RE=Y~(3+),Gd~(3+),La~(3+))

Ytterbium and rare earth ions(RE=Y,Gd,La)codoped CaF₂-SrF₂single crystals(3 at%Yb,6 at%RE:CaF₂-SrF₂)were fabricated by temperature gradient technology(TGT).All the space groups remain the same Fm3m as that of Yb:CaF₂-SrF₂.The lattice parameter a,unit cell volume V,as well as bond length of Ca/Sr-F and F-F increase in the sequence of rare-earth ions radius Y³⁺3+3+.The segregation coefficients of Yb ions are 0.87 in Yb,La:CaF₂-SrF₂and Yb,Gd:CaF₂-SrF₂,which are larger than 0.85 in Yb,Y:CaF₂-SrF₂and 0.80 in Yb:CaF₂-SrF₂.Absorption spectra in the range of 200 and 400 nm were analysed with Yb²⁺contents.The absorption and emission cross-sections in the range of 900-1100 nm were determined together with fluorescence lifetime.The saturation pump density/Sat,minimum pump density/m in and gain cross-section were analysed.Yb,La:CaF₂-SrF₂has a relatively higher optical parameter(δem×t,0.52×10²⁰cm²·ms),lower Isat(3.68 kW/cm²)and^min(0.50 kW/cm²)at 1038 nm indicating the potential application in high power laser.Low phonon energy of CaF₂-SrF₂is 302 cm^-1which is located between those of CaF₂and SrF₂as measured by Raman spectra.It is believed that ytterbium and rare earth ions(RE=Y³⁺,Cd³⁺,La³⁺)codoped CaF₂-SrF₂eutectic solid-solution is promising for high-power and wavelength-tunable solid-state lasers.     

Electrical and electronic conductivity of CaO-SiO2-FeOx slags at various oxygen potentials: Part II. Mechanism and a model of electronic conduction

The experimental results obtained for ionic and electronic conductivity of ‘FeO’-CaO-SiO₂ melts have been analyzed considering the mechanism of each conduction process. The Nernst-Einstein equation was employed to calculate diffusion coefficients of Fe²⁺ and Ca²⁺ cations from ionic conductance. A “diffusion-assisted charge transfer” model was developed to explain the dependence of the electronic conductivity on the oxidation state of iron in the slag. The model considers the electronic conduction as a two-step process: in one step, ferrous ions diffuse from their initial position to a proper distance from ferric ions; in the next step, an electron is transferred between Fe²⁺ and Fe³⁺. The optimum distance of the iron ions for electron hopping was found to be approximately 4 Å, in great consistency with the values reported for electron transfer between Fe²⁺ and Fe³⁺ in aqueous solutions and solid glasses.     

Activities of FexO in complex slags used during the final stages of external dephosphorization of hot metal

Measurements of the activities of Fe_xO in complex slags during the final stages of external dephosphorization by using a disposable electrochemical oxygen probe. Positive deviations of the Fe_xO activities from the Raoult law. Iso-activity curves drawn on the “ternary” diagram, (CaO + MgO + MnO + CaF₂) + Fe_xO + (SiO₂ + P₂O₅) at 1673 K. Calculations of the activities of P₂O₅ in complex slags with an assumption of thermodynamic equilibrium between slags and hot metal. Beneficial effect of CaF₂ in the slags with respect to lowering of the P₂O₅ activity.     

The solution rate of alumina in CaF2-Al2 O3 slags

The rate of solution of A1₂O₃ in CaF₂ + 30 wt pct A1₂O₃ (at 1518° and 1509°C) and CaF₂ + 20 wt pct A1₂O₃ (at 1500°C) liquids has been determined. The operative process is diffusion-controlled, with an interdiffusion coefficient,D for the process varying between 8.5 and 8.1 x 10^-5 sq cms ^- 1 in the CaF₂ + 30 wt pct A1₂O₃ slags, and 4.0 × 10^-5 sq cms ^- 1 in the CaF₂ + 20 wt pct A1₂O₃ slag. Estimations of the rate at which alumina inclusions would react with these slag during the electroslag processing of steels, indicate that electrode inclusions approaching 100 μ in diam will be dissolved.     

Electrode polarization in the DC electroslag melting of pure iron

It is evident from the known ionic properties of the slags used in electroslag melting, that the dc melting process must be accompanied by Faradaic reactions on the slag/ingot and slag/electrode interfaces. The present work has determined the magnitude of the overpotentials resulting from concentration polarization at these interfaces, in the case of pure iron/CaF₂+Al₂O₃, CaF₂+CaO slags using a galvanostatic pulsing technique in an electrolytic cell. The polarization overpotential existing on an electrode in an operating ESR unit has been measured by the same technique. It is found that the potentials observed on the ESR electrode agree well with the results from the electrolytic cell. The primary anodic process is postulated to be the corrosion of iron, leading to an Fe²⁺-saturated layer on the anode surface at sufficiently high current densities. The cathodic process is suggested to be the Faradaic reduction of Al³⁺ or Ca²⁺, to give a concentration of [Al]_Fe or (Ca)_slag in the cathode interface region. This observation is supported by the fact that the cathodic potentials with respect to a C/CO reference electrode are close to those predicted from the reactions: (Al₂O₃)+3C=3CO(g)+2Al(l) or (CaO)+C=CO(g)+Ca(g) At very high current densities both the anodic and cathodic processes may convert to arcs, leading to process instability. The chemical and thermal effects of the overpotentials are briefly discussed and compared with the present results on ESR ingots of pure iron.     

Phase diagram cuspidine (3CaO · 2SiO2 · CaF2)-CaF2

Equilibrium phase diagram for the system cuspidine (3CaO · 2SiO₂ · CaF₂)-CaF₂ has been studied by the quenching method and differential thermal analysis (DTA). Hermetically closed platinum capsules were used in both methods to prevent fluorine loss in the form of HF and SiF₄ by reaction of the CaF₂ with water vapor or SiO₂. Cuspidine congruently melts at 1680±2 K. The cuspidine-CaF₂ system presents a simple eutectic type of phase diagram. The eutectic composition and temperature are 46 mass pct 3CaO · 2SiO₂−54 mass pct CaF₂ and 1515±3 K, respectively.     

Mechanical and Thermal Properties of Hot-Pressed ZrB2-SiC Composites

ZrB₂-SiC composites were hot pressed at 2473 K (2200 °C) with graded amounts (5 to 20 wt pct) of SiC and the effect of the SiC addition on mechanical properties like hardness, fracture toughness, scratch and wear resistances, and thermal conductivity were studied. Addition of submicron-sized SiC particles in ZrB₂ matrices enhanced mechanical properties like hardness (15.6 to 19.1 GPa at 1 kgf), fracture toughness (2 to 3.6 MPa(m)^1/2) by second phase dispersion toughening mechanism, and also improved scratch and wear resistances. Thermal conductivity of ZrB₂-SiC (5 wt pct) composite was higher [121 to 93 W/m K from 373 K to 1273 K (100 °C to 1000 °C)] and decreased slowly upto 1273 K (1000 °C) in comparison to monolithic ZrB₂ providing better resistance to thermal fluctuation of the composite and improved service life in UHTC applications. At higher loading of SiC (15 wt pct and above), increased thermal barrier at the grain boundaries probably reduced the thermal conductivity of the composite.     

Electrical conductivities of M3P2O8 (M = Ca,Ba)

Two-probe conductivity measurements made for M₃P₂O₈ (M = Ca, Ba) suggested that the electrical conduction of these phosphates would primarily be due to the migration of Ca²⁺ and Ba²⁺ ions. At relatively low temperatures and high oxygen partial pressures, in contrast to Ca₃P₂O₈, however, Ba₃P₂O₈ shows partial electronic conduction.     

Preparation and luminescence properties of Eu3+ doped oxyfluoride borosilicate glass ceramics

Oxyfluoride borosilicate glass with the molar composition of 60SiO2-15B2O3-15Na2O-8CaF2-2NaF-0.25Eu2O3 was synthesized by a traditional glass melting method. Glass ceramics containing CaF2 nanocrystals were prepared by heat treating the glass samples at a tem-perature in the range of 620-680 °C. The results of X-ray diffraction (XRD) indicated that the average crystallite size and the lattice constant of CaF2 nanocrystals increased with the heat treatment temperature increasing. The luminescence spectra showed that the emission intensity of Eu3+ doped glass ceramics was stronger than that of the glass matrix, and increased with the heat treatment temperature increasing. The left edge of excitation band shifted to shorter wavelength in the glass ceramics. The local environments of Eu3+ ions in the glass and glass ceram-ics were different.     

The heat capacity and enthalpy of fusion of slags used in electroslag remelting

Measurements of the heat capacity, the enthalpy (H_T–H₂₉₈) and the enthalpy of fusion of slags from the CaF₂ + Al₂O₃ + CaO and CaF₂ + MgO systems for temperatures between 298 and 1900 K. Thermodynamic properties and recommended c_p(T) relationships. Check of estimation methods for the calculation of c_p and (H_T–H₂₉₈) of ESR slags against experimental data.