First found widespread thermal relaxation hysteresis effect in ferroelectrics
I. Background
Kano effect is reversible thermal change material under adiabatic or isothermal conditions for the magnetic field, electric field, pressure, mechanical stress and external stimuli. Recent studies have found that, in the vicinity of phase change materials discovered enormous thermal effects, phase change materials are considered solid state refrigeration to achieve clean and efficient the most promising candidates. In order to develop a solid-state phase change material for cooling, the past 20 years, people use of direct and indirect methods of thermal effects tephigram adiabatic temperature change and change, etc. intensive studies. However, the material Another important parameter – thermal hysteresis, the parameters for performance and thermal cycling applications is critical materials, but drew widespread attention. In fact, the thermal hysteresis may lead to significantly reduce the efficiency of the thermal effects of materials and the corresponding equipment, in practical applications should be minimized or even completely eliminate the effects of thermal cycling thermal hysteresis properties of the material.
Second, the research results
Recently, Professor Wang University of Pennsylvania Task Force In Chi significant progress Yu ferroelectric polymer phase transition mechanism made. In this study, the thermal hysteresis of ferroelectric behavior of the polymer was fully characterized. Polyvinylidene fluoride (PVDF) and its copolymer as the matrix, by measuring the polarization dependence of the heating and cooling process, with the decrease of the content of VDF ferroelectric copolymer P (VDF-TrFE) ferroelectrics typical from a phase transition to a second order phase transition. In addition, the authors found relaxation ferroelectrics exhibit unusual negative thermal hysteresis during thermal scan. A blend yuan / terpolymer study, according to a change terpolymer content reproduced from the ordinary thermal hysteresis change process to the ferroelectric relaxor ferroelectric body. Meanwhile, the author of the negative thermal hysteresis clarify the relationship between the electrical line and the thermal effect . The work \”Observation of a Negative Thermal Hysteresis in Relaxor Ferroelectric Polymers\” was published in top international academic journals Advanced Functional Materials on. 
III. Highlights herein:
1: This is the first discovery relaxation ferroelectric prevalent thermal hysteresis. 2: Discovering thermal hysteresis provides guidance for the development of efficient solid state refrigeration materials and equipment.
Fourth, the research results discussed ideas and specific
Comparative P (VDF-TrFE) 65/35 mol% copolymer of heating and cooling loop of the ferroelectric understood, in the heating process, the remnant polarization as the temperature drops fast, a ferroelectric transition occurs near 90 ℃, corresponding to the result of XRD and DSC results consistent. In contrast, in the reverse process of its cooling, the residual polarization value which is decreased as the temperature rises rapidly, and the ferroelectric transition point in the vicinity of 70 ℃. In summary, through the heating and cooling loops ferroelectric comparison result found that the presence of the polymer significant thermal hysteresis. In contrast to the P (VDF-TrFE) 65/35 mol% results, P (VDF-TrFE) 50 / 50mol% of the thermal hysteresis almost disappeared. Is well known, the leakage current may be significantly change the shape of the hysteresis loop. For the polymer ferroelectric, the electric field induced electron or ionic charge redistribution results in the Curie temperature (≈65 ° C) or more rapid increase in the rate of remanent polarization, it is not an intrinsic property of the polymer. In particular, at high temperatures, P (VDF-Tr-FE) of 50 / 50mol% no negative thermal hysteresis, so that the leading role exclude leakage current. Here we increase the negative effects due to thermal hysteresis fieldLocal enhancement ferroelectric electrically induced distortion domains, and this change in the heating and cooling process. In this regard, two scans (heating and cooling) of the field effect is expected than one scan (heating) is strong, this results in having a higher phase transition temperature Tc> Th.
Negative thermal hysteresis occurs not only in the binary copolymer, the same authors also found that a similar phenomenon also occurs in the ternary copolymer P (VDF-TrFE-CTFE) in. In the same field, the residual polarization value P (VDF-TrFE-CFE) is generated to be less than the P (VDF-TrFE-CTFE), Moreover, P (VDF-TrFE-CFE) thermal hysteresis up to -10 K effect, the P (VDF-TrFE-CTFE) -5 K produce only heat hysteresis value. This indicates that the negative thermal hysteresis effect may be relaxation ferroelectrics unique physical effect, the first time that a relaxation of the ferroelectric prevalent negative thermal hysteresis.
Negative thermal hysteresis relaxor ferroelectric polymers, may help to distinguish it from the ordinary ferroelectric region. To prove this point, of a binary copolymer / terpolymer blend for the study, with the increase of terpolymer component, the phase transition from the ferroelectric to the normal of the relaxor ferroelectric.The results showed that the binary copolymer / terpolymer mixture ratio is 3/7, a blend of long-range order ferroelectric phase disappears, which is consistent with the results of the dielectric. Notably, increasing the electric field intensity leads to Th and Tc shifts to lower temperature, while the negative thermal hysteresis maintained at almost 0 K. At the same time, the authors found that the change in frequency has little effect on the negative thermal hysteresis.
On the ferroelectric phase transition behavior of polymers and blends were quantitatively analyzed. To do this, we use the sixth-order polynomial fit polarization – temperature curve, and the minimum value is determined by a sign change of the derivative thereof, corresponding to Th and Tc. Can be clearly seen from the figure, when the VDF content is 49 mol% when the critical point of transition to a ferroelectric, this critical point corresponds to the disappearance of the ferroelectric behavior, which is consistent with previous analyzes. On the right side of the critical point (VDF> 49 mol%), the phase transition temperature decreases as the VDF content is lowered, the thermal hysteresis △ T = Th-Tc <1 K。对于弛豫区（VDF<49 mol%），由于Tc大于Th导致聚合物的负热滞后（△T<0 K）。
