A new millisecond high-temperature pulse calorimeter

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1 High Temperatures ^ High Pressures, 2001, volume 33, pages 505 ^ ECTP Proceedings pages 1041 DOI: /htwu155 A new millisecond high-temperature pulse calorimeter Peter Reiter, Erhard Kaschnitz Oë sterreichisches GieÞerei-Institut, ParkstraÞe 21, A-8700 Leoben, Austria; fax: ; kaschnitz.ogi@unileoben.ac.at Presented at the 15th European Conference on Thermophysical Properties, Wu«rzburg, Germany, 5 ^ 9 September 1999 Abstract. A new pulse-heating system for resistive self-heating of metallic specimens has been constructed for the accurate measurement of thermophysical properties. A controlled high current pulse is used to heat alloys in a time interval of approximately 1 s from room temperature into the melting range. From time-resolved measurements of current and voltage drop across the specimen, enthalpy and electrical resistivity are determined. A high-speed pyrometer is used to measure the (time-dependent) temperature of the specimen. From these measured quantities, specific enthalpy, heat capacity, and electrical resistivity as a function of temperature are obtained. Measurements of these thermophysical properties of a pure element have been performed and compared with results of another research group in order to check the accuracy of the system. Results of measurements of a selected technical alloy are reported. 1 Introduction Measurements of the solidification range, heat capacity, electrical resistivity, and other thermophysical properties of metals and alloys are usually limited to temperatures below about 2000 K. The reasons for this limit are problems with heat radiation, contamination, chemical reactions, evaporation, diffusion, etc. A special technique of thermophysical measurements, so-called pulse calorimetry, has been developed by Cezairliyan (1971) to overcome these experimental difficulties. A metallic specimen is resistively self-heated by a controlled high current pulse (subsecond duration), and time-resolved measurements of current, voltage drop across the specimen, and radiance temperature are performed. Because of the short duration of the experiment, a high-speed pyrometer is used for the temperature measurement. From the measured quantities, specific enthalpy, heat capacity, and electrical resistivity as a function of temperature can be calculated. In industrial applications thermophysical data of technical alloys are of special interest. Numerical simulation of casting or welding processes requires accurate data of heat capacity, thermal conductivity, and density in the vicinity of melting in order to obtain realistic results. Unfortunately, knowledge about thermophysical properties of technical alloys is quite poor. A new pulse-heating system, designed to investigate commercially used alloys, has been developed. Measurements of electrical resistivity as a function of temperature of molybdenum are presented and compared with results of another research group as an accuracy check of the system. The influence of the microstructure on the measured electrical resistivity of TiAl6V4 at elevated temperature is demonstrated. 2 Experimental 2.1 Experimental setup The experimental setup consists of the electrical pulse-heating system including a current shunt and voltage probes, the pyrometer, the measurement and control system, and the mechanical setup with the vacuum and gas system. Figure 1 shows a functional diagram of the entire system.

2 506 P Reiter, E Kaschnitz 15 ECTP Proceedings page Figure 1. Functional diagram of the entire experimental setup: 1, specimen; 2, voltage probes; 3, shunt; 4, adjustable resistor; 5, fast-acting MOSFET switch; 6, safety relay; 7, battery bank; 8, high-speed pyrometer; 9, measurement PC; 10, control PC; 11, experimental chamber; 12, vacuum pumping station; 13, inert gas; 14, vacuum measurement; 15, battery charger. 2.2 Pulse-heating system The specimen is connected in series with a precision shunt, a current-limiting resistor, fast-acting power-mosfet modules, and safety relays. The current of up to 3000 A is supplied by a battery bank consisting of eight heavy truck batteries (Exide) with a voltage of 12 V and 230 A h nominal capacity each. The batteries are connected to four parallel strings, giving a supply voltage of 24 V. Because of the constant operating voltage, a current-limiting resistor is used to control the heating current and heating rate, respectively. This resistor is made from a Kanthal-strip (30 1:6 mm); different lengths are used to adapt for adequate resistance. The current measurement is performed by a 40 mo shunt (Siemens AG Oë sterreich, Niederlassung Graz, StraÞganger StraÞe 315, A-8054 Graz, Austria), which is connected to a differential input of the sample-and-hold amplifier of data acquisition. The voltage probes, defining an effective length of the specimen, are knife-edges made of tungsten. They are mounted to the clamps via a lever mechanism that allows the probes to move in the axial direction to follow the thermal expansion of the specimen during the experiment.the distance between the knife-edges can be adjusted between about 40 mm and 70 mm, depending on specimen length, diameter, and heat conductivity. The knife-edges are pressed against the specimen by a second lever mechanism which is softly preloaded by a spring. The voltage probes are connected to another differential input of the sample-and-hold amplifier of data acquisition. The current is switched by 16 parallel fast-acting MOSFET modules (Siemens Simopac BSM 111 AR), also suitable for analogue current control, which keep the specimen at a chosen temperature. Four MOSFET modules are connected together in one independent switch unit for each of the four supply strings. They are controlled by a driver stage consisting of a buffer and a protecting isolation amplifier. The system is controlled by a personal computer which is equipped with a multiinput ^ output board (National Instruments AT-MIO-16 E-2 National Instruments Austria, PlainbachstraÞe 12, A-5101 Salzburg-Bergheim, Austria). This computer drives the MOSFET power switches, the safety relays, produces a trigger signal for the measurement, and takes current measurements as feedback for the MOSFET control. In

3 A new millisecond high-temperature pulse calorimeter ECTP Proceedings page 1043 order to avoid electrical shorts or if a MOSFET module fails, additional safety relays cut the battery from the circuit in non-operating condition. Data consisting of current, voltage, and radiance temperature are measured every 0.5 ms by a second personal computer equipped with a 16-bit data acquisition board (National Instruments AT-AI-16 XE-10) and a sample-and-hold amplifier (National Instruments SC-2040). Enthalpy of the specimen as a function of time is obtained by integrating the electrical power over time, and dividing by the specimen mass. Electrical resistivity is calculated by Ohm's law; all resistivity values are related to room temperature dimensions. 2.3 Experimental chamber Figure 2 shows the experimental chamber. The experiment is carried out in an inert gas atmosphere or under high-vacuum conditions to prevent chemical reactions with the specimen. The chamber contains several vacuum ports for pumping, gas inlet, and vacuum measurement, electrical feed-throughs for the heating current, and four windows for pyrometry and visual observation. The specimen is mounted vertically with two brass clamps. To allow for axial expansion of the specimen during heating, the lower clamp runs in Teflon bearings and is connected to an expansion joint. The experimental chamber is evacuated by a turbomolecular vacuum pumping station which is connected to the chamber via an angle valve. A needle valve allows filling of the chamber with inert gas (argon). The specimen is either a rod or a tube made of the investigated metal or alloy. The total length of the specimen is about 70 mm. A length of 5 mm at each end of the specimen is needed for clamping, and the distance between the clamps and voltage probes is about 5 mm to 15 mm which leaves an effective length of about 40 5 mm between the voltage probes. Rod-shaped specimens have a diameter of 1.5 to 3.5 mm; tube-shaped specimens have outer diameters in the range of 6 mm, and a wall thickness of about 0.5 mm. specimen upper part window specimen holder lower part Teflon bearing expansion joint vacuum flanges coaxial feed-through Figure 2. Cross-section of the experimental chamber.

4 508 P Reiter, E Kaschnitz 15 ECTP Proceedings page Pyrometer A high-speed pyrometer is used to obtain the temperature of the specimen during the heating and cooling process. The pyrometer includes an optical system consisting of an achromatic lens pair with a focal length of 150 mm, a pinhole with a diameter of 0.2 or 0.4 mm, an interference filter and a PIN photodiode (figure 3). A collapsible mirror and a viewing microscope enable aligning of the pyrometer which is mounted on a positioning stage. The interference filter has a nominal centre wavelength of 650, 900, or 1500 nm, and a bandwidth of approximately 20 nm (FWHM). Depending on the chosen wavelength, the photodiode used is a silicon type (for 650 or 900 nm) or an InGaAs type for use with the 1500 nm filter. For minimum noise the photodiode amplifier is included in the pyrometer housing. The amplifier consists of three stages with low-noise operational amplifiers. Figure 4 shows a schematic diagram of the photodiode ^ amplifier. For simplicity the voltage-supply connections are not represented. With the given values for resistance, R 1, and capacitance, C 1, the limiting frequency is f 3dB 1000 Hz. In the near infrared (1500 nm) and the visible range, the pyrometer has lower temperature levels of 750 K and 1150 K, respectively. The pyrometer is calibrated to a tungsten-strip lamp, which in turn has been calibrated by the German Calibration Service (DKD, Institut fu«r Kernenergetik und Energiesysteme, Abteilung Energiewandlung und Wa«rmetechnik, Universita«t Stuttgart, Pfaffenwaldring 31, D Stuttgart, Germany). All measurements are related to the ITS-90 scale (Preston-Thomas 1990a, 1990b) Figure 3. Pyrometer: 1, achromatic lens; 2, pinhole; 3, interference filter; 4, photodiode; 5, collapsible mirror; 6, viewing microscope; 7, X^Ypositioning stage.

5 A new millisecond high-temperature pulse calorimeter ECTP Proceedings page 1045 Out 10 Out 100 C 1 D 1 R 1 OP 1 R 3 R 4 OP 2 R 6 R 1 OP 3 R 2 R 5 R 8 Figure 4. Photodiode amplifier: D 1, photodiode (either EG&G UV-100 BQ or EG&G C G); C 1 ˆ 10 pf; R 1 ˆ 20 MO; R 2, R 5, R 8 ˆ 100 O; R 3, R 6 ˆ 1 ko; R 4, R 7 ˆ 10 ko; OP 1,OP 2,OP 3, operational amplifiers, AD OP 07 EZ. 2.5 Measurement checks Resistivity is calculated from voltage, current, effective length, and cross-section, which is calculated from total length, total mass, and density. The density of the specimen is measured by an Archimedean balance. Enthalpy is calculated from voltage, current, time, effective length, density, and diameter. Molybdenum specimens were provided by Plansee AG (Reutte, Austria) with a purity better than 99.97%. The tubular specimens had an outer diameter of 6 mm, and a wall thickness of 0.5 mm. The effective length was approximately 40 mm, and a 0.5 mm slit was machined into the wall to provide nearly blackbody conditions to the pyrometer. The pyrometer has a spot size of 0.2 mm and emissivity is assumed to be unity. A total of 15 single measurements with 3 different specimens was made. The reproducibility between individual measurements is 0.2%, and between different specimens 0.5%. Eventually all specimens were heated up to melting, where they collapsed. The results obtained for electrical resistivity agree within the measurement uncertainties with results of Cezairliyan (1980), which are considered to be very accurate. Titanium alloy (TiAl6V4) specimens were provided by the National Institute of Standards and Technology (Gaithersburg, MD, USA), where measurements on the same batch of material were performed. The rod-shaped specimens had a diameter of 1.6 mm and an effective length of 50 mm. Figure 5 shows the electrical resistivity of TiAl6V4 as a function of enthalpy (relative to 298 K), compared to the results of Cezairliyan (1994). The second heating shows a small but noticeable change in resistivity in the (a b) phase. This is due to the relatively 1.95 Electrical resistivity mo m :5% Enthalpy Jkg 1 Figure 5. Electrical resistivity of TiAl6V4 as a function of enthalpy ( ö, first heating; heating) compared with measurements (*) performed by Cezairliyan (1994). = *, second

6 510 P Reiter, E Kaschnitz 15 ECTP Proceedings page 1046 high cooling rate after the first heating, which leads to the formation of a martensitic phase. 2.6 Estimate of uncertainties An analysis of possible errors is given in table 1. The uncertainty is calculated as recommended in EA-4/02 (European Cooperation for Accreditation 1996, 1997, 1998) and is 1.5% for electrical resistivity and 1.5% for specific enthalpy. The reported uncertainties are based on a standard uncertainty multiplied by a coverage factor of 2, providing a level of confidence of approximately 95%. Table 1. Sources of uncertainty in resistivity measurements. Source of Estimated Standard Sensitivity Contribution to uncertainty value uncertainty coefficient standard uncertainty Shunt resistor 40.0 mo 0.2 mo m mo m Voltage divider mo m mo m Voltage V V 0.18 mo mv mo m Shunt voltage mv 0.02 mv mo mv mo m Effective length m m 45 mo mo m Total length m m 25 mo mo m Total mass g g mo mg mo m Density 4415 kg m 3 5kgm mo m 4 kg mo m Total uncertainty mo m Because the pulse-heating system is brand-new, the pyrometer has not been tested extensively for stability and linearity. Nevertheless, the accuracy of the obtained results for molybdenum is very encouraging. The uncertainty of the strip lamp, the alignment of the pyrometer, the mean effective wavelength of the filter, the emissivity of the slitshaped specimen and the window of the experimental chamber are the main contributions to the uncertainty of the temperature measurement. From our experience, an overall uncertainty of 7 K at 1300 K and 12 K at 2500 K is estimated. 3 Conclusions A new pulse calorimeter for resistive self-heating of metallic specimens has been set up. Measurements of electrical resistivity of molybdenum were made to check the accuracy of the system. The sensitivity of the system is demonstrated by the measured dependence of the electrical resistivity on the microstructure of TiAl6V4 at elevated temperatures. Acknowledgements. This project is supported by the Fonds zur Fo«rderung der wissenschaftlichen Forschung (FWF), Vienna, Austria under contract No. P12804-PHY. References Cezairliyan A, 1971 J. Res. Natl. Bur. Stand. C, Eng. Instrum. 75 7^18 Cezairliyan A, 1980 Int. J. Thermophys ^427 Cezairliyan A, 1994, personal communication European Cooperation for Accreditation, 1996, EA-4/02, European Cooperation for Accreditation, 1997, EA-4/02-S1, European Cooperation for Accreditation, 1998, EA-4/02-S2, Preston-Thomas H, 1990a Metrologia 27 3^10 Preston-Thomas H, 1990b Metrologia ß 2001 a Pion publication printed in Great Britain