Thin-film Thermal Converters Research Articles

Comparison of a Thermal AC Voltage Standard in the 1–30-MHz Frequency Range

This article presents results of a comparison of a traveling ac voltage standard, which was a fused-silica planar multijunction thin-film thermal converter (PMJTC) developed at the National Institute of Standards and Technology (NIST). The ac–dc voltage-transfer difference of the standard was measured at 2 V and selected frequencies from 1–30 MHz against primary thermal ac voltage standards at the Silesian University of Technology, Istituto Nazionale di Ricerca Metrologica, RISE Research Institutes of Sweden, and Trescal A/S Silkeborg.

A Novel Film Thermal Converter Based on an Electrothermally Excited/Piezoresistively Detected Microbridge Resonator

This paper presents the design, fabrication, and preliminary results of a resonant thermal converter consisting of an electrothermally excited/piezoresistively detected SiO2/Si3N4/Si x N y microbridge resonator and a Ni24.9Cr72.5Si2.6 heating resistor placed on another beam parallel to the microbridge. The noncontact temperature sensing technique reduces the diversion of thermal energy from the temperature sensors, as well as the ac–dc transfer differences due to the capacitive coupling between the heater and the temperature sensor. The semidigital output of the resonant thin-film thermal converter eliminates problems such as intensity fluctuations associated with analog signals. The temperature coefficient of resistance of the Ni24.9Cr72.5Si2.6 film with a thickness of 89 nm is only $-0.86 \times 10^{-6}$ /K. The responsivity of the resonant thermal converter is $9.21~\text {kHz}\cdot \text {W}^{-1}$ in air. The thermoelectric transfer difference measured by the fast-reversed dc method at 1 kHz is $2.8 \times 10^{-6}$ . It indicates that the resonant thermal converter is a promising thermal converter.

Comparison of a Planar Thin-Film Thermal AC Voltage Standard up to 1 MHz

This paper presents results of a trilateral comparison with a traveling ac voltage standard comprising a planar thin-film thermal converter. The ac-dc voltage transfer difference of the standard was measured at 1 V and at selected frequencies from 20 Hz to 1 MHz against primary thermal ac voltage standards at the Silesian University of Technology, Istituto Nazionale di Ricerca Metrologica, and Trescal.

Fabrication of Low-Frequency AC-DC Thermal Converter with Thermal Mass

The use of a thin film or a planar multijunction thermal converter (PMJTC) for the ac-dc transfer of voltage and current is well established. In general, a three dimensional multijunction thermal converter (MJTC) has the advantage of accuracy and a single junction thermal converter (SJTC) has that of low cost. PMJTCs provide a long-term stability, a high sensitivity and a high-dynamic-frequency range. It is well known that PMJTCs show small ac-dc voltage transfer differences in the audiofrequency range. However, it is difficult to obtain a small ac-dc transfer difference at low frequencies, particularly with thin-film-based thermal converters. The small ac-dc transfer difference at low frequencies is caused by nonlinearities in the heat transport mechanism (in thin films) and in the thermal-to-electric conversion process (in the thermocouples of thin films). This study has been conducteded to achieve a smaller ac-dc transfer difference on thin-film chromel-alumel thermal converters operating at low frequencies. An optional silicon obelisk is formed underneath the thin membrane to minimize ac-dc transfer differences at lower frequencies. The floating obelisk structure enhances the isothermal operation and slows thermal transfer compared with the standard configuration without the obelisk, resulting in a smaller ac-dc transfer difference at low frequencies. Also, it is possible that thin-film thermal converters can be used as the flow sensor and infrared sensor.

New thin-film multijunction thermal converters with negligible low frequency AC-DC transfer differences

The AC-DC transfer differences of thin-film thermal converters at low frequencies are caused by nonlinearities in the heat transport mechanisms and in the thermal-to-electric conversion process. The resulting power coefficient of the sensitivity has been compensated by adding another nonlinear temperature sensitive component to the thermal-to-electric conversion process. This is a thin-film Ni resistor mounted under the thermocouples and connected to the output of the thermocouples. The Ni resistor together with the internal resistance of the thermocouples forms a resistive divider for the output voltage. The positive temperature coefficient of the Ni resistor in the divider compensates for the negative power coefficient of the sensitivity. As a result, converters used at a rated voltage of 1.5 V or a rated current of 7 mA have an AC-DC transfer difference below 0.3 /spl mu/V/V and 0.3 /spl mu/A/A respectively with a standard uncertainty of 0.3 /spl mu/V/V and 0.3 /spl mu/A/A respectively at a frequency of 10 Hz.

Dynamic non-linear electro-thermal simulation of a thin-film thermal converter

This article describes the electro-thermal simulation of the ac–dc transfer differences at low frequencies of a thin-film thermal converter. The dynamic non-linear model includes the temperature dependence of all the material parameters, and the radiation losses. It is used to optimise the performance of the device at low frequencies, where temperature oscillations are present due to the lack of integration of the oscillating Joule heat. The results of the simulation are compared with those of the measurements using a digital method.