Ohmic Pulseheating

Using subsecond pulse heating techniques it is possible to reach temperature regions that are not accessible to steady state experiments, which are limited to temperatures below about 2000 K. This limitation is a result of chemical interaction of the specimens with the containers, the loss of mechanical strength, problems with heat transfer, evaporation and electrical insulation while the sample and its environment are kept at such high temperatures. Fast dynamic methods have been developed to avoid these difficulties by heating up the specimen in a very short time to the liquid state while the sample keeps its cylindrical shape because of inertia. In this way it is possible to extend the measurements to higher temperatures and determine thermophysical properties of metals from temperatures of about 1500 K up to more than 5000 K.

Wire shaped specimens (0.5 mm diameter, 50 mm length) were resistively self-heated by a nearly rectangular current pulse of 10 kA supplied by a high voltage capacitor discharge circuit. Melting is reached in less than 30 µs and heating continues up into the liquid phase. Due to the short timescale, gravitational forces do not distort the geometry of the specimen even in the liquid phase. Time resolved, with sub-µs resolution, the following quantities have been measured: voltage drop along a defined inner part of the specimen as well as current through it, and the surface temperature radiation. Additionally pictures of the specimen have been taken at various instants by a high speed CCD framing camera. These measurements allow the determination of heat capacity and the mutual dependencies between enthalpy, electrical resistivity, temperature, and density in the high-temperature solid and in the liquid phase. Thermal conductivity can be estimated from resistivity data using the Wiedemann - Franz - law.

Measurements of properties of matter at high temperatures are useful for many reasons:
They are required for high temperature technologies (i) to model liquid metal processing operations such as casting and welding e. g. input data for numerical simulations, (ii) to understand, simulate and design new processing equipment such as facilities for the growth of silicon single crystals from the melt, (iii) for obtaining phase diagrams, (iv) for obtaining temperature reference points, (v) for accurate assessment of potential accidents in the design of safe nuclear reactors, (vi) for aerospace techniques, and (vii) for basic therory such as critical points of metals, which require high temperature and high pressure conditions.


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Polarimetry

Knowledge of emissivity and its behaviour is of great importance, espacially if you are dealing with optical (pyrometrical) temperature determination by means of Planck's law of radiation.

The µs-DOAP (Division-Of-Amplitude-Polarimeter), which is embedded in the pulse-heating setup, enables us to determine normal spectral emissivity as well as the optical constants refractive index and extinction coefficient at a wavelength of 684.5 nm as a fucntion of temperature from melting up into the liquid state of the specimen material.

What makes this DOAP unique is the fact that it comes without any rotation devices (as can be found in standard polarimeters) to keep up with the average experimental duration of about 50 µs. Its completely optical measurement principle is based on the STOKES formalism of polarized light.

General principle of measurement:

An initially polarized laser beam (of defined polarization) is focused onto the surface of the specimen and the change in polarization of the reflected beam during pulse-heating is recorded and optically analyzed. All mentioned optical properties can be calculated from this change in polarization as long as the sample is opaque at the laser wavelength.


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Contact
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Institut of Experimental Physics
Graz University of Technology
Petersgasse 16
8010 Graz
Austria

Group Leader
Gernot POTTLACHER,
ao. Univ.-Prof. Dr.

Tel.: +43 (0) 316 / 873 - 8149
Fax: +43 (0) 316 / 873 - 8655

pottlachernoSpam@tugraz.at

All images © TU Graz/Institute of Experimental Physics