Normal spectral emissivity of liquid Al-Ti binary alloys

The normal spectral emissivity ε of four compositions in the Al-Ti binary liquid system was measured in dependence of the wavelength and temperature. It was found that all compositions show negligible temperature dependence. At a wavelength of 940 nm, the emissivity amounts to 0.37, 0.40, 0.32, and 0.31 for Ti, Al20Ti80, Al50Ti50, and Al70Ti30, respectively. The dependence of the emissivity on composition is in good agreement with literature data of binary and multi-component Al-Ti-based alloys. Using the classical Drude model, electrical resistivities are predicted for the Al-Ti system from the measured emissivities. Comparison with existing data from literature for Al show reasonable agreement.

Identification of wavelength regions for non-contact temperature measurement of combustion gases at high temperatures and high pressures

Stationary gas turbines are still an important part of today’s power supply. With increasing temperature of the hot combustion gas inside a gas turbine, the efficiency factor of the turbine increases. For this reason, it is intended to operate turbines at the highest possible gas temperature. Therefore, in the combustion chamber and especially at the position of the first stage guide vanes the gas temperature needs to be measured reliably. To determine the gas temperature, one promising approach is the application of a non-contact measurement method using a radiation thermometer. A radiation thermometer can measure the gas temperature remotely from outside of the harsh environment. At ZAE Bayern, a high temperature and high-pressure gas cell has been developed for this purpose in order to investigate gases and gas mixtures under defined conditions at high pressures and high temperatures. This gas cell can be placed in a FTIR-spectrometer in order to characterize the infrared-optical properties of the gases. In this work the measurement setup is introduced and gas mixtures, which are relevant for gas turbine applications are analyzed thoroughly. The derived results are presented and discussed in detail. To identify suitable wavelength regions for non-contact gas temperature measurements, first tests have been performed. Based on these tests, an appropriate wavelength region could be chosen, where future gas temperature measurements can be carried out.

Surface tension of liquid nickel: Re-evaluated and revised data

Nickel is an important component in many alloys, so reliable surface tension data in the liquid phase are essential for simulation processes in the metal industry. First results for surface tension of liquid nickel from our working group by Aziz et al. [1], which led to the first publication on the topic of our Electromagnetic Levitation (EML) setup, delivered unusual high values compared to the literature, which itself covers a wide range. To find the reason for this behaviour the aim of this work was to investigate the surface tension of nickel samples from different suppliers at similar purity grades by the Oscillating Drop (OD) technique using the EML setup of the Thermophysics and Metalphysics Group at Graz University of Technology. Since no significant deviations between samples from different suppliers have been found, an extensive literature research according to various experimental and evaluation parameters has been performed. In the course of this investigation, the earlier obtained experimental data of Aziz et al. were re-evaluated. Due to gained awareness in evaluating the translational frequency in vertical direction, the mystery of these elevated surface tension results could be solved, so that in the end the originally obtained results of Aziz have been drastically decreased through re-evaluation.

Surface tension of liquid Ti with adsorbed oxygen and a simple associate model for its prediction

Surface tensions of electromagnetically levitated liquid Ti-samples were measured under the influence of oxygen. For this purpose, Ti-O samples were prepared by adding different amounts of TiO2 powder to pure Ti. The surface tension was found to strongly depend on the bulk oxygen mole fraction determined by chemical analysis. The results could be described by a simple model presented in the present work. In this model the Butler equation is applied and the formation of TiO2 – associates are taken into account. Non-ideal interactions ΔH≠0 between titanium and the associates also need to be taken into account. Good agreement with the experimental data is evident and also with a different model developed earlier by us.

Non-linear effects in the oscillating drop method for viscosity measurements

The contribution of non-linear fluid flow effects to the damping of surface oscillations in the oscillation drop method was investigated in a series of experiments in an electromagnetic levitation device installed on the International Space station, ISS-EML. In order to correctly evaluate the damping time constant from measured surface oscillation decays the effect of a modulated signal response on measured surface oscillation decay curves was investigated. It could be shown that various experimentally observed signal patterns could be well represented by a modulated response. The physical origin of such modulations is seen in rotation and precession. Over a temperature range of 220 K covered by different surface oscillation excitation pulses with an initial sample shape deformation of 5 – 10% the amplitude of surface oscillations as a function of time could be very well represented by a Lamb type damping with a temperature dependent viscosity. A direct comparison of surface oscillation decay times measured in the same temperature range but for different oscillation amplitudes showed no non-linear contribution to the damping time constant with a confidence level better 10%.

Erratum

Erratum to the paper “The heat capacity at constant pressure in the nearly-free-electron approximation for binary liquid alloys of alkali metals” (HTHP, 2018, v. 47, p. 205-211)

Editorial

Since 2007 High Temperatures-High Pressures regularly publishes invited articles selected from presentations given during the International Workshop on Subsecond Thermophysics (IWSSTP). These well-established workshops take place every three years at various locations. They are aimed to provide a forum for exchange of knowledge, experience and ideas on the latest developments and trends in the field of thermophysical property measurement. A special focus is given to measurements under extreme conditions, at high temperatures or pressures, and/or on the study of the behavior of matter under conditions close by or far away from thermodynamic equilibrium. Timescales range from milliseconds to picoseconds, necessitating rapid resistive or inductive (volume) heating, laser pulse heating, levitation techniques on ground and under microgravity, as well as other recently developed fast investigation techniques. The 12th IWSSTP, which this present special issue is devoted to, took place from June, 3rd to June, 6th, 2019. It was a pleasure and a great privilege for me and the Institute of Materials Physics in Space of the German Aerospace Center (DLR), Cologne to organize the current event and welcome the international community as hosts. This time, the 12th IWSSTP coincided with the 50th anniversary since the launch of High Temperatures – High Pressures which gave reason to celebrate.

Density and thermal expansion of the nickel-based superalloy INCONEL 625 in the solid and liquid states

Density and thermal expansion of the nickel-based superalloy INCONEL 625 were measured in the temperature range 150 °C to 1400 °C using pushrod and piston dilatometry. Commercial pushrod-dilatometers were used for the measurements. The specimens are cooled and heated slowly at controlled rates in a furnace; the expansion is transferred by a long thin rod to displacement sensors. In the high temperature range an alumina tubular body with two alumina pistons of just sufficient clearance was used to contain the specimen in the mushy region and in the liquid state The investigated material was primary heat treated at 930 °C for 1 hour. As INCONEL 625 is an age-hardening alloy, the thermophysical properties including density at elevated temperature depend slightly on heat treatment conditions. Therefore, different measurement runs with a variation of the maximum temperature in the solid state (from room temperature to 1000 °C, 1100 °C and 1250 °C) were performed to cover different heat treatments (product grades) of INCONEL 625. Due to the lack of density and thermal expansion data of INCONEL 625 in the solid and liquid states in the literature, the measured density is compared to published density data of INCONEL 718 and INCONEL 738. A detailed uncertainty analysis of the measured data in the solid and liquid state of the alloy is provided.

Electrical resistivity measured by millisecond pulse-heating in comparison to thermal conductivity of the hot work tool steel AISI H11 (1.2343) at elevated temperature

Selected thermophysical properties of the hot work tool steel AISI H11 (1.2343) were measured in the temperature range from room temperature to the melting temperature. Thermal diffusivity was measured by the laser-flash method; heat capacity by differential scanning calorimetry; linear thermal expansion by push-rod dilatometry; and density at room temperature by an Archimedean balance. From these experimentally obtained data, thermal conductivity was calculated. Additionally, electrical resistivity of AISI H11 (1.2343) was measured by millisecond pulse-heating in the above mentioned temperature range. The measurement results of electrical resistivity as a function of specific enthalpy was combined with results of specific heat capacity measurements by differential-scanning calorimetry to obtain the relation between resistivity and temperature. Based on measured electrical resistivity and thermal conductivity, a Smith-Palmer-plot for the hot work tool steel AISI H11 (1.2343) is obtained for the ferritic and austenitic phases. No linear behaviour – as expected by the Wiedemann-Franz law – is observed in the ferritic phase region. In the high temperature austenitic region, the thermal conductivity can be computed from electrical resistivity using empirical constants of similar austenitic steels or superalloys.

Densities and electrochemical potential window of the BaCl2-NaCl-NaF-AlF3 electrolyte

The effect of melt composition (BaCl2-NaCl-NaF-AlF3) on its density, resistivity and redox potentials of the main electrode processes is reported. A three-factor two-level experimental design was used to study the effects of cryolite ratio (1.1-1.6) and BaCl2 (0-60wt%) or NaCl (0-10wt%) content on the melt density. The obtained equation satisfactorily describes the measured melt densities. The observed non-linear behavior of melt density with BaCl2 additions is assigned to the formation of BaClF complex. Unlike BaCl2, additions of 0-10 wt% NaCl have little effect on the melt density, however, affect the melt conductivity significantly.