Low frequency behaviour of thin-film multijunction thermal converters

Thermal converters are the most accurate standards for the transfer of alternating voltage and current to the equivalent dc quantities. Today, thin-film multijunction thermal converters are used in the frequency range from 10 Hz up to 1 MHz with measurement uncertainties below 10"-"6 in the audio-frequency range. For frequencies below 100 Hz, it is known that the ac-dc transfer differences of thin-film multijunction thermal converters as well as of all other types of thermal converters increase; their origin is believed to be in the temperature oscillation in the converter due to the lack of integration of the oscillating Joule heat at decreasing frequencies. To explore the low-frequency behaviour of thermal converters, the electric-to-thermal conversion in the heater, the heat transport mechanisms in the thin films and the thermal-to-electric conversion in the thermocouples of the thin-film or planar multijunction thermal converter (PMJTC) have been analysed. The dynamic nonlinear electro-thermal differential problem was solved using the finite element method. The knowledge of the temperature distribution with ac and dc input voltages allows the ac-dc transfer difference #delta#_L_F at low frequencies to be calculated. The temperature dependence of the heater resistance, of the heat transport mechanisms, i.e. radiation and conduction through layer materials and through the air, and of the Seebeck effect was included in the nonlinear model. The accuracy of the model was tested comparing the results of the simulation with those of measurements. Parameters in the simulation were adjusted by the measurement of the sensitivity and the thermal time constant of the manufactured PMJTC. A digital method was used as a reference for the measurements. The uncertainty of this measurement method was investigated. (orig./GL)SIGLEAvailable from TIB Hannover: RN 3295(63) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman