Aluminium nitride probes for application in iron melts

Review on high-temperature properties of aluminium nitride AIN. Investigations on aluminium nitride as a possible solid electrolyte at temperatures around 1600°C. Presentation of available data on the electrical conductivity, the thermal conductivity and the thermal expansion coefficient of AIN at elevated temperatures. Experimental study on the oxidation behaviour of AIN in oxidizing gases and oxygen-containing iron melts. EMF measurements on electrochemical cells of the type iron melt ‖ AIN (Al₂O₃) solid electrolyte ‖ reference representing oxygen and aluminium probes in iron melts under long-term conditions.     

New reference materials for electrochemical on-line sensors to measure steel bath composition

Further efforts have to be made to develop reliable sensors for on-line measurements in steel melts. Accurate performance of conventional sensors with a Cr-Cr₂O₃ reference over extended periods is disturbed by polarization effects mainly at the solid electrolyte/oxygen reference interface. A promising way to minimize or eliminate this problem is to reduce the flux of ionic oxygen across the solid electrolyte by applying oxygen references featuring oxygen potentials close to the actual oxygen potential of the steel melt. In search of oxygen references with 's lower than that of Cr-Cr₂O₃ mixtures, EMF measurements were made to determine the equilibrium oxygen pressures of the systems Cr-CaO-CaCr₂O₄, Cr-MgO-MgCr₂O₄, Cr-Y₂O₃-Y₂Cr₂O₆, and Nb-NbO. It is shown that the use of these reference materials enables correct on-line measurements in fully-killed steel baths over periods of 4 hours at least.     

Thermodynamic properties of oxygen in yttrium-oxygen solid solutions

The oxygen potential in yttrium-oxygen (Y-O) solid solutions was measured by equilibration with titanium-oxygen (Ti-O) solid solutions. Yttrium and titanium samples were immersed in calcium-saturated CaCl₂ melts at temperatures between 1108 and 1438 K, and oxygen levels in the two metals were measured. With the Ti-O system acting as a reference, oxygen potentials in Y-O solid solutions were determined. By this technique, it was possible to make reliable measurements of extremely low oxygen potentials (as low as 10^?44 atm at 1273 K), far beyond the range of solid oxide electrolyte sensors.     

Nitrogen solubility in metallurgical slags equilibrated with Fe–Al or Fe–Ca melts

Considerations are directed to the denitrogenation potential of metallurgical slags with respect to steel melts under reducing conditions. Experiments were made to determine partition ratios of nitrogen between molten slag and iron. The investigated systems were aluminate-based slags, containing CaO, MgO, SrO, BaO, CaF₂ or ZrO₂, in equilibrium with Fe–AI melts and Ca–CaO–CaF₂ slags equilibrated with Fe–Ca melts. Denitrogenation efficiency of aluminate-based slags is comparatively low and essentially determined by oxygen potential and basicity of the slag. Denitrogenation efficiency of Ca–CaO–CaF₂ slags is much higher and is dependent on calcium activity.     

Low oxygen activities in steel melts – Possibilities and limits of the solid electrolyte measuring technique

Review on the application of solid oxide electrolyte cells for the determination of low oxygen activities at about 0.0001 to 0.01 in steel melts. Investigation and discussion of possible error sources, such as partial electronic conductivity and oxygen ion transport in the electrolyte, reaction layers on the electrolyte surface, chemical reduction of the electrolyte and the use of inaccurate thermochemical data for the evaluation of oxygen activities, in the EMF measurement and interpretation of results. Comparison of properties of ZrO₂-and ThO₂-based electrolytes.     

A high-accuracy, calibration-free technique for measuring the electrical conductivity of molten oxides

A high-accuracy, calibration-free technique to measure the electrical conductivity of molten oxides has been developed—the coaxial cylinders technique. Because the melt under investigation comes in contact only with metal and not with anything dielectric, the new technique enables the measurement of the electrical properties of liquids inaccessible by classical high-accuracy techniques. Two coaxial cylindrical electrodes are immersed in the melt to an arbitrary initial depth, and ac impedance is measured over a wide range of frequency. The electrodes are then immersed deeper, and ac impedance is again measured over the same range of frequency. This process is repeated at multiple immersions. The electrical conductivity is calculated from the change in measured conductance with immersion. The temperature dependence of electrical conductivity has been measured for two oxide melts: (M) 50.95 pct CaO, 12.51 pct MgO, 36.54 pct SiO₂, 1733 K<T<1843 K; and (S) 24.59 pct CaO, 26.15 pct MgO, 49.26 pct SiO₂, 1763 K<T<1903 K, where concentration is expressed in mole percent. Both melts exhibited behavior characteristic of ionic liquids.     

Thermodynamic conditions for inclusions modification in calcium treated steel

The presented paper discussed the fundamental or common thermodynamical relations between calcium-treated aluminium-killed molten steel and non-metallic inclusions. The phase and chemical analyses of inclusions have proven that the correctness of calcium addition can be confirmed and that the analysis of those phenomena can show the effects of previous calcium treatment of aluminium-killed steel. To make the process of manufacturing quality steel successful the factor affecting the necessary calcium addition should be taken into consideration already during the process. Steel, containing too much calcium could have CaS inclusions with a high melting point, while too low contents of calcium cause unsatisfactory modification of solid alumina inclusions to complex liquid calcium-aluminate inclusions. This research included the examination of thermodynamic relations in calcium addition and its reactions with solid Al₂O₃ inclusions. A detailed analysis of the CaO–Al₂O₃ binary system established the modification of solid alumina inclusions via the following intermediate phases: CaO · 6Al₂O₃, CaO · 2Al₂O₃, CaO · Al₂O₃ and liquid phase 12CaO · 7Al₂O₃ and finally again solid CaO, at 1873 K (1600°C). The investigation discusses the further research engaged in consideration of CaO- and Al₂O₃-activities change in each of the quoted intermediate phases. The system as a whole includes details of oxygen activities.     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_? ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_? ^b+ is the þ-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the þ-@#@ type electronic conductivity and the ionic conductivity are equal.     

Oxygen and carbon sensing in Fe—O—C and Fe—O—C—Xn melts at elevated carbon contents

In the present study on solid electrolyte probes, the attempt was made to measure accurate and reproducible EMF values representing the extremely low oxygen activities in Fe—O—C melts at varying C contents. Various types of sensors were designed and successfully tested in laboratory experiments. Reliable oxygen activities were measured in Fe—O—C melts up to 4 wt. % C under pure CO gas, and f_o and f_c values were derived as a function of C content at 1 400 to 1 600°C. Further measurements were made in Fe—O—4 wt. % C—X_n melts at various contents of X_n (S_n = Si, Mn, Cr). Moreover, a solid oxide electrolyte probe with a CO gas channel to measure oxygen and carbon activities was developed and successfully tested in high-carbon iron melts.     

A thermodynamic study on cobalt containing calcium ferrite and calcium iron silicate slags at 1573 K

Redox equilibria, activities of cobalt, iron and their oxides in calcium ferrite and calcium ironsilicate slags, were measured through metal-slag-gas equilibrium experiments under controlled oxygen potentials (10⁻⁷ to 3 × 10⁻⁷ atm) at 1573 K. Results on the redox equilibria show that addition of CoO to calcium ferrite slag increases the equilibrium Fe³⁺/Fe²⁺ ratio in these melts. Measured activities of CoO and FeO showed positive deviations from ideal behavior, while that of Fe₂O₃ showed negative deviation. Partial substitution of CaO by SiO₂, by up to 4 wt pct SiO₂ in the calcium ferrite based melts, resulted in increases in the activity coefficients of CoO and Fe₂O₃. Phase equilibria studies on the cobalt containing CaO-FeO-Fe₂O₃-SiO₂ slags were also carried out using the drop-quench technique. Good agreement between the activity data and the liquidus temperature with respect to magnetite solid solution containing CoO was observed.     

Development of electrochemical silicon sensors for iron and steel melts

Based on the commonly used oxygen sensors with O^2- ion-conducting ZrO₂ electrolytes and solid metal-metal oxide references, new sensors have been developed for direct measurements of Si content in iron and steel melts. The sensors are designed in such a way that local chemical equilibrium of the reaction ZrO₂ + [Si] + 2[O] = ZrSiO₄ is established at the surface of the ZrO₂ electrolyte tube or inside a cavity adjacent to the ZrO₂ electrolyte sensor tip using a saturating ZrO₂-ZrSiO₄ mixture. Concepts and dimensions of the sensors were optimized in view of a fast EMF response upon immersion and the maintenance of a stable EMF recording over extended periods. Reliable measurements were performed in Fe-Si melts at 0.05 to 5 wt.-% Si.     

Electrochemical determination of thermodynamic properties and x-ray diffraction investigation of the Fe3O4-ZnFe2O4 system

The Bureau of Mines investigated the thermodynamic properties of the Fe₃O₄-ZnFe₂O₄ solid solutions. Activities of Fe₃O₄ (magnetite) in solid solutions were determined with high-temperature electromotive force (emf) cells using stabilized ZrO₂ (zirconia) as the solid electrolyte. Potential measurements were obtained from the cell Pt, Fe₃O₄, Fe₂O₃ || ZrO₂ || Fe₂O₃, (Fe₃O₄)_x(ZnFe₂O₄)_1−x, Pt with the overall cell reaction Fe₃O₄ (pure solid) = Fe₃O₄ (in solid solution). The activity of Fe₃O₄ in solid solutions coexisting with Fe₂O₃ (hematite) exhibits a positive deviation from ideal solution behavior for the entire system. Equilibrium oxygen pressures for zinc oxyferrite solid solutions coexisting with Fe₂O₃ were calculated from the potential measurements. Partial and integral molar properties were derived for the Fe₃O₄-ZnFe₂O₄ system. Lattice parameter measurements of the Fe₃O₄-ZnFe₂O₄ solid solutions show a positive deviation from Vegard’s law.     

The activity of calcium in calcium-metal-fluoride fluxes

The standard Gibbs energy of reaction Ca (1) +O (mass pct, in Zr) = CaO (s) has been determined as follows by equilibrating molten calcium with solid zirconium in a CaO crucible: ΔG° = -64,300(±700) + 19.8(±3.5)T J/mol (1373 to 1623 K) The activities of calcium in the CaO_satd-Ca-MF₂ (M: Ca, Ba, Mg) and CaO_satd-Ca-NaF systems were measured as a function of calcium composition at high calcium contents at 1473 K on the basis of the standard Gibbs energy. The activities of calcium increase in the order of CaF₂, BaF₂, and MgF₂ at the same calcium fraction of these fluxes. The observed activities are compared with those estimated by using the Temkin model for ionic solutions. Furthermore, the possibility of the removal of tramp elements such as tin, arsenic, antimony, bismuth, and lead from carbon-saturated iron by using calcium-metal-fluoride fluxes is discussed. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo.     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_‡ ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_‡ ^b+ is the t-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the t-@#@ type electronic conductivity and the ionic conductivity are equal.     

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.     

Phase relations in the system CaO-SiO2-ZrO2 at 1573 K

Zirconia-based solid electrolytes with zircon (ZrSiC₄) as the auxiliary electrode have been suggested of sensing silicon concentrations in iron and steel melts. A knowledge of phase relations in the ternary system MO-SiO₂-ZrO₂ (M = Ca, Mg) is useful for selecting an appropriate auxiliary electrode. In this investigation, an isothermal section for the phase diagram of the system CaO-SiO₂ZrO₂ at 1573 K has been established by equilibrating mixtures of component oxides in air, followed by quenching and phase identification by optical miroscopy, energy disperse analysis of X-rays (EDAX) and X-ray diffraction analysis (XRD). The equilibrium phase relations have also been confirmed by computation using the available thermodynamic data on condensed phases in the system. The results indicate that zircon is not in thermodynamic equilibrium with calcia-stabilized zirconia or calcium zirconate. The silica containing phase in equilibrium with stabilized zirconia is Ca₃ZrSi₂O₉. Calcium zirconate can coexist with Ca₃ZrSi₂Og and Ca₂SiO₄.     

Thermodynamics of antimony,arsenic, and tin in CaO—CaF2 melts

In view of the importance of tramp element contamination of steel products through a large volume of scrap consumption in the near future, the thermodynamic behaviour of Sb, As, Sn and Cu in CaO—CaF₂ melts under reducing conditions was studied by examining the dependences of distribution of each element between Cu, Ag or Sn alloys and CaO—CaF₂ melts on the CaO activity, oxygen partial pressure and temperature around 1 500°C. As a result, the reaction products on treatment by Ca compounds were demonstrated to be Ca₃Sb₂, Ca₃As₂, Ca₂Sn and CaCu. Experimentally obtained distributions were extrapolated to lower oxygen partial pressures less than 10^?18 atm to estimate the feasibility of removing those tramp elements from molten iron. It is thermodynamically indicated that if the prevailing oxygen partial pressure of the environment is below 10^?23 atm, these impurities, except Cu, would be substantially removed from molten steel.     

Electrochemical Deoxidation of Solid Zirconium Dioxide in Molten Calcium Chloride

The reduction of zirconium dioxide pellets by electro-deoxidation in molten calcium chloride-calcium oxide (900 °C) has been studied. In this technique, the solid oxide is cathodically polarized against a graphite counter electrode under a constant applied potential. Unlike other metal oxides that have been reduced by this technique, only a small area around the cathodic current-collector wire was reduced to zirconium metal with zirconia pellets sintered at ~1100 °C; the rest of the sample was largely calcium zirconate. Pellets sintered above 1200 °C showed better reduction near the cathode wire and the reduction extended to the entire surface of the pellet with the passage of time. However, reduction of the inner core was found to be increasingly difficult, because the surface metal layer thickened on continuous electro-deoxidation. An analysis of the experimental results showed that the poor electrical conductivity of the intermediate compound, CaZrO₃ and its blocky morphology inhibited the electro-deoxidation process. The increase in the sintering temperature of the pellet made it better conducting. However, the pores formed in the thick zirconium metal layer in such samples were too small for an ideal contact between the inner core and the molten electrolyte and hence the reduction of the inner core remained incomplete. Within the scope of this study, it is concluded that preforms with good grain growth and porosity are necessary for the electro-deoxidation of solid zirconium oxide.     

Electrical and electronic conductivity of CaO-SiO2-FeO x slags at various oxygen potentials: Part I. Experimental results

The oxygen potential dependences of total electrical and partial electronic/ionic conductivities for ‘FeO’-CaO-SiO₂ slags have been studied both experimentally and theoretically in the present work. In the first part of this two-part article, the experimental results are presented for slags with 30 wt pct ‘FeO’ and CaO/SiO₂ wt ratios of 0.5, 1.0, and 2.0. For each slag composition, measurements of total electrical conductivity and electronic transference were made over a range of oxygen potentials and temperatures. The results were used to calculate the partial conductivities. A maximum was achieved in total and electronic conductances as a function of equilibrium CO₂/CO. The CO₂/CO corresponding to the maximum was shifted to lower values with increasing slag basicity (CaO/SiO₂ ratio). The other effect of basicity was increasing total and partial conductivities, with a magnitude that depends on oxygen potential and temperature. The activation energies for ionic and electronic conductances were in similar ranges and decreased with the basicity.