Technique: High-Field Dielectrics |
|
Dielectric relaxation measurements at high electric fields (up to 450 kV/cm) combines 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]. |
|
Schematic representation of the impedance measurement system based upon the Solartron
SI-1260 gain/phase analyzer. The amplifier Trek PZD-700 acts as voltage booster, the
current is measured as voltage drop across a 100 W or
1 kW resistor. The buffer amplifier prevents damages to the
SI-1260 in the case of sample failures and provides a 50 kHz low-pass filter. The setup
facilitates impedance measurements at fields up to 450 kV/cm and for frequencies between
0.1 Hz and 70 kHz. [151] |
|
Time dependent high electric 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 considerable increase in the loss of up to 20%. |
|
Time resolved voltage and current signals for a frequncy of 1 kHz and for a low-to-high
transition of the peak voltage by a factor of 5, equivalent to an energy increase by a
a factor of 52 = 25. The graph shows 12 cycles out of the total number of
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 tand.
[157, 162] |
|
Time resolved dielectric measurements at high fields are performed by generating a sinusoidal signal using a Stanford Research DS-345 synthesized function generator (1 m Hz 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 tand after a low-to-high or high-to-low peak amplitude transition, or to evaluate the amplitudes at various harmonics, that is at 1w and 3w [162]. |
|
Reference numbers refer to the list of publications |
|
Experimental techniques:
|
Selected projects:
|