Klaus Schaefers

Enthalpy measurements of undercooled melts by levitation calorimetry: the pure metals nickel, iron, vanadium and niobium

A circulating gas cooling (CGC) system is added to a combination of an electromagnetic levitation apparatus and a drop calorimeter to measure the enthalpies of the pure metals nickel, iron, vanadium and niobium in the undercooled temperature range. The CGC system extends the experimental temperature range to lower temperatures. The effect of the CGC system on the heat losses during the drop of the sample is discussed and the evaluation procedure presented. The measured enthalpies of the metals in the undercooled range confirm the temperature dependences of the enthalpies of the liquid phase above their melting points.

Measurements of dendritic growth and recalescence rates in undercooled melts of cobalt

Abstract Electromagnetic levitated liquid cobalt was undercooled with the purpose to trigger solidification at desired temperatures. Triggering was performed at the underside of the sample. The solidification velocity was determined by detecting the trigger signal and the temperature rise at the top of the sample by a pyrometer. Measurements were carried out up to 102 K below the melting point. The values of the dendritic growth rate can be well described by the theory of Lipton, Kurz and Trivedi (LKT theory) and follow v = A ·Δ T α , where Δ T is the undercooling, A is a pre-exponential factor which was found to be 1.146×10 −4 m s −1 K − α and α =2.48. The recalescence rate, T , is generally given by T =B· Δ T β . The result obtained for cobalt is B =2.79×10 2 K 1− β s −1 and β =1.066.

Concentration dependence of the spectral emissivity of liquid binary metallic alloys

A review is given on the literature data on the concentration dependence or the emissivity of liquid binary metallic alloys (Ni-Fe, Ce-Cu, Ni-Cr, Ti-Al). Our measurements at the liquidus temperatures are presented for the systems Ni-V, Fe-V, and Fe-Nb and the pure components atλ=547 nm andλ=650 nm. All available results are interpolated in comparison to the phase diagrams of the systems. This comparison indicates that the nonvariant liquidus temperatures have the highest deviation from a linear interpolation between the emissivity of the pure components. The corresponding concentration ranges are characterized by stronger atomic interactions in the melt. Therefore errors in noncontact temperature measurements occur if the concentration dependence is neglected or estimated from the pure components.

A new variant for measuring the surface tension of liquid metals and alloys by the oscillating drop method

The theoretical background of the oscillating drop technique for measuring surface tension is briefly presented and the different analysis procedures are cited. A new method is described for obtaining oscillation frequencies by fast fourier transformation (FFT) of the pyrometer voltage signals from temperature measurements at the top of the levitated sample. The results on the first experiments on liquid nickel are in a good agreement with the hterature data.

Electromagnetic levitation and its application in high temperature calorimetry

Abstract The electromagnetic levitation technique is presented. It is a useful method to measure the thermophysical properties of high melting metals. The levitation alloying calorimetry (LAC) is described. Using the associate model and the TAP series, the excess heat capacities of the systems FeV and FeTi are calculated on the basis of the concentration and temperature dependent descriptions of their mixing enthalpies. The relation between the levitation alloying and drop calorimetry is discussed. Using the temperature dependent description of the mixing enthalpy given by the associate model, the enthalpy of the intermetallic compound Fe 2 Zr is calculated. The calculated and measured enthalpies were in perfect accordance.

Description of the concentration and temperature-dependent mixing enthalpy of binary metallic melts

Abstract An introduction is given to different thermodynamic models for representing mixing enthalpy measurements. The TAP series and the associate model are applied and compared on the basis of the concentration and temperature-dependent mixing enthalpy of the system MgPb. A new appoximation method is introduced to calculate the parameters of the associate model. The number of available enthalpy values for the approximation process is systematically varied in order to simulate the flexibility of the models. From this point of view the associate model is a useful model for the representation of the mixing enthalpy and the prediction of the excess heat capacity.