Technique: High-Field Dielectrics |
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Dielectric relaxation measurements at high electric ac fields (up to 650 kV/cm) combine energy absorption from the field (as in microwave heating) with the dielectric response acting as sensitive indicator of configurational temperatures. Non-linear dielectric behavior, configurational heat capacities, and the heterogeneity of thermal relaxation times are the main topics addressed with this technique [147, 157, 168]. The alternative case of using a dc type field eliminates energy absorption from the field, and this technique is used to assess nonlinear behavior based upon dielectric saturation, chemical effects, as well as effects associated field induced entropy reduction [224, 231]. |
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Schematic representation of the impedance measurement system based upon the Solartron
SI-1260 gain/phase analyzer. The amplifier Trek PZD-700 or PZD-350 acts as voltage booster,
the current is measured as voltage drop across a 100Ω or 1 kΩ resistor.
The buffer amplifier prevents damages to the SI-1260 in the case of sample failures
and provides a 800 kHz low-pass filter. The setup facilitates impedance measurements
at high ac or dc electric fields of up to 650 kV/cm and for frequencies between 0.1 Hz
and 350 kHz. (The blocking capacitor is used only for dc field experiments.) [151, 157, 162, 168, 171, 172, 185, 193, 215, 224, 231, 237, 240] |
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Time dependent high electric (ac) fields lead to the absorption of energy of those modes that overlap with the frequency of the applied signal (as in dielectric hole-burning). The energy is absorbed by slow degrees of freedom and remains decoupled from the phonon bath for the relaxation time of the mode in question. This allows us to study the effect of the resulting increase of the configurational temperature before the energy is surrendered to the phonons. In terms of a dielectric relaxation measurement, the net effect is a reduced relaxation time resulting in a considerable increase in the loss of up to 20%. In the case of high static (dc) fields, the dominant effect on the relaxation time is associated with the field induced reduction of entropy, leading to longer relaxation time constants. |
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Time resolved voltage and current signals for a given frequency and for a low-to-high-to-low
transition of the peak ac voltage by a factor of 5 (left) or of the dc volrage (right).
The graphs show only few cycles compared to the total number of cycles acquired in an
actual experiment (e.g., 1000 cycles measured at a sampling rate of 1 MS/s). The system is capable
of averaging over 5000 such traces for noise reduction. The signals allow for a resolution of
5×10-5 regarding tanδ for a particular period after changing the field amplitude. [157, 162, 224, 231] |
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Time resolved dielectric measurements at high fields are performed by generating a sinusoidal signal using a Stanford Research DS-345 synthesized function generator (1 μHz to 30 MHz) with significant changes in the peak amplitude. Voltage and current are recorded with a Nicolet Sigma 100 four-channel storage scope (resolution 12 bit at 100 MS/s, 14 bit at 10 MS/s, 106 points record length). Signals are then evaluated via period-by-period Fourier analysis in order to obtain the time resolved change in tanδ after a low-to-high or high-to-low peak amplitude transition, or to evaluate the amplitudes at various harmonics, that is at 1ω, 2ω, 3ω etc. [162, 224]. |
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Reference numbers refer to the list of publications |
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Experimental techniques:
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Selected projects:
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