Investigation of the dielectric fatigue on the example of lead titanate films PbTiO3

The phenomenon of dielectric fatigue of active dielectrics, which consists in a decrease in the residual polarization depending on the number of switching cycles, is researched. A model of the dependence of the residual polarization of ferroelectric materials on the number of switching cycles is proposed. The model is based on piecewise - linear approximation of the results of measurements of the hysteresis loops of thin films PbTiO3 at a temperature T = 470 (°C), the electric field strength E = 100 (kV/cm). The developed model was used in the development of a technique for studying dielectric fatigue, depending on different modes of material switching.

Исследования проводящих и сегнетоэлектрических свойств BZT-пленок

The local polarization processes in thin BaTi1-xZrxO3 films were investigated by contact conducting scanning probe microscopy and piezoresponse microscopy. A relationship between the direction of the created domains and the magnitude of the flowing currents is established. The value of the residual polarization is found, and the hysteresis loop is measured using scanning probe microscopy as well as the values ​​of the piezomodule dzz and the value of the coercive field Ec for these films are determined.

Study on Temperature Characteristics of Ceramic Materials for Piezoelectric Actuators and Model Modification

The performance of piezoelectric actuator ceramics under strong electric field(20 kV/cm) and variable temperature (30~150℃) were tested on a piezoelectric ceramic thermo-electro-mechanical multi-field loading test bench. The variation of hysteresis loop, strain loop, free capacitor and dielectric loss tangent with temperature was analyzed. A mathematical model of displacement characteristics of ceramic materials considering temperature effect is established, and the accuracy of the model is verified. The results show that the hysteresis loops become slender with the increasing of temperature, while the residual polarization, maximum polarization and coercive field decrease. The effect of the temperature on the residual polarization and coercive field is stronger than that on maximum polarization. The strain loop presents a typical butterfly curve, and the negative strain decreases gradually to 0.12% with the increasing of temperature. In the unipolar electric field, the residual polarization varies slightly with the increasing of temperature, and the maximum polarization increases about 40%. The piezoelectric constant of the material increases linearly. The free capacitor and dielectric loss tangent increases continuously. The higher the temperature, the greater the increase.

Магнитоэлектрический эффект в парамагнитной области в Gd-=SUB=-0.15-=/SUB=-Mn-=SUB=-0.85-=/SUB=-Sе

In the region of the percolation concentration in the GdxMn1-xSe solid solution, the electric polarization without a field and in 12 kOe magnetic field in the temperature range of 80–380 K was measured . For the composition with x = 0.15, the polarization hysteresis and the dependence of the residual polarization on the magnetic field and temperature were found. The hysteresis is explained in the model of migration polarization and the magnetoelectric effect in the Maxwell-Wagner model.

Detection of 40–48 GHz dust continuum linear polarization towards the Class 0 young stellar object IRAS 16293–2422

Aims. The aims of this work are to test the feasibility of observing dust polarization at frequencies lower than 50 GHz, which is the optically thinner part of the modified black body spectrum, and to clarify whether or not the polarization mechanism is identical or similar to that for (sub)millimeter observations. Methods. We performed the new Karl G. Jansky Very Large Array (JVLA) full polarization observations at 40–48 GHz (6.3–7.5 mm) towards the nearby (d= 147 ± 3.4 pc) Class 0 young stellar object (YSO) IRAS 16293–2422, and compared these with the previous Submillimeter Array (SMA) observations. We observed the quasar J1407+2827, which is weakly polarized and can be used as a leakage term calibrator for <9 GHz observations, to gauge the potential residual polarization leakage after calibration. Results. We did not detect Stokes Q, U, and V intensities from the observations of J1407+2827, and constrain (3σ) the residual polarization leakage after calibration to be ≲0.3%. Limited by thermal noise, we only detected linear polarization from one of the two binary components of our target source, IRAS 16293–2422 B. The measured polarization percentages range from less than one percent to a few tens of percent. The derived polarization position angles from our observations are in excellent agreement with those detected from the previous observations of the SMA, implying that on the spatial scale we are probing (~50–1000 au), the physical mechanisms for polarizing the continuum emission do not vary significantly over the wavelength range of ~0.88–7.5 mm. Conclusions. We hypothesize that the observed polarization position angles trace the magnetic field, which converges from large scale to an approximately face-on rotating accretion flow. In this scenario, magnetic field is predominantly poloidal on >100 au scales, and becomes toroidal on smaller scales. However, this interpretation remains uncertain due to the high dust optical depths at the central region of IRAS 16293–2422 B and the uncertain temperature profile. We suggest that dust polarization at wavelengths comparable or longer than 7 mm may still trace interstellar magnetic field. Future sensitive observations of dust polarization in the fully optically thin regime will have paramount importance for unambiguously resolving the magnetic field configuration.

Experimental study of electrode effects of resistance type electrodes for monitoring wood drying process above fibre saturation point / Elektroodefektide uurimine puidu kuivatamisel niiskussisaldustel üle kiu küllastuspunkti

Due to practical need to improve the accuracy of measurement of wood electrical resistance via creating multivariate calibration models the study of electrode effects was carried out. The experiments were made in DC mode and five types of electrode effects were measured: electrical resistance of the galvanic contact between the measuring electrode and wood, corrosion of measuring electrodes in wet wood, polarization and depolarization of the double layer forming on the surface of the measuring electrodes, residual polarization voltage between measuring electrodes and effect of repeated use of measuring electrodes. For multi-channel measurement mode effect of neighbouring electrodes was measured. Behind each of these factors is a complex transfer mechanism of free and bounded charge carriers from wood to measuring electrodes. Among the factors, polarization and depolarization have an immediate effect (duration of the processes in seconds) while resistance of electrode/wood contact, corrosion of electrodes and residual polarization have a slow effect (duration of processes in hours). For pine sapwood, coefficients of the Stamm formula were found in moisture contents (MC) above the fibre saturation point. It was determined that wood polarization and depolarization indicators are dependent on wood moisture content as is the case with wood electrical resistance. Compared to wood electrical resistance, wood polarization proved approximately four times and depolarization approximately eight times less sensitive to moisture content variation.