Large-area atomic-smooth polyvinylidene fluoride Langmuir-Blodgett film exhibiting significantly improved ferroelectric and piezoelectric responses.

Large roughness and structure disorder in ferroelectric ultrathin Langmuir-Blodgett (LB) film results in severe space scatter in electrical, ferroelectric and piezoelectric characteristics, thus limiting the nanoscale research and reliability of nano-devices. However, no effective method aiming at large-area uniform organic ferroelectric LB film has ever been reported to date. Herein, we present a facile hot-pressing strategy to prepare relatively large-area poly(vinylidene fluoride) (PVDF) LB film with ultra-smooth surface root mean square (RMS) roughness is 0.3 nm in a 30 µm × 30 µm area comparable to that of metal substrate, which maximized the potential of LB technique to control thickness distribution. More importantly, compared with traditionally annealed LB film, the hot-pressed LB film manifests significantly improved structure uniformity, less fluctuation in ferroelectric characteristics and higher dielectric and piezoelectric responses, owing to the uniform dipole orientation and higher crystalline quality. Besides, different surface charge relaxation behaviors are investigated and the underlying mechanisms are explained in the light of the interplay of surface charge and polarization charge in the case of nanoscale non-uniform switching. We believe that our work not only presents a novel strategy to endow PVDF LB film with unprecedented reliability and improved performance as a competitive candidate for future ferroelectric tunnel junctions (FTJs) and nano electro mechanical systems (NEMS), but also reveals an attracting coupling effect between the surface potential distribution and nanoscale non-uniform switching behavior, which is crucial for the understanding of local transport characterization modulated by band structure, bit signal stability for data-storage application and the related surface charge research, such as charge gradient microscopy (CGM) based on the collection of surface charge on the biased ferroelectric domains.

An alternating multilayer architecture boosts ultrahigh energy density and high discharge efficiency in polymer composites.

Poly(vinylidene fluoride) (PVDF)-based polymers with excellent flexibility and relatively high permittivity are desirable compared to the traditional bulk ceramic in dielectric material applications. However, the low discharge efficiency (<70%) caused by the severe intrinsic dielectric loss of these polymers result in a decrease in their breakdown strength and other problems, which limit their widespread applications. To address these outstanding issues, herein, we used a stacking method to combine poly(methyl methacrylate) (PMMA) with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) for the synthesis of a series of alternating multilayer films with different layers. Benefitting from the blocking effect of the multilayer structure and excellent insulation performance of PMMA, simultaneous improvements in the breakdown strength and discharge efficiency of the multilayer films were achieved. Compared with the pure polymer films and other multilayer films with different layers, the film with a 9-layer structure exhibited the highest energy storage density of 25.3 J cm-3 and extremely high discharge efficiency of 84% at 728 MV m-1. Moreover, the charge and discharge performance of the other multilayer films were also better than that of P(VDF-HFP). In addition, it was also found that for the multilayer composite films with the same components, the blocking effect was reinforced with an increase in the number of layers, which led to a significant improvement in the breakdown strength. We consider that the multilayer structure can correlate with the dielectric properties of different polymer materials to enhance the energy storage of composite materials, and will provide a promising route to design high dielectric performance devices.

High electrocaloric cooling power of relaxor ferroelectric BaZrxTi1-xO3 ceramics within broad temperature range.

Electrocaloric effect (ECE) is much promising to realize high efficiency and environment friendly solution in solid cooling devices. Relaxor ferroelectrics are good candidates for the materials with high electrocaloric cooling power. In this paper, relaxor ferroelectric Ba(ZrxTi1-x)O3 (BZT, x = 0.2, 0.21, 0.22, 0.23) ceramics were prepared with their temperature change (ΔT) induced by the ECE and electrocaloric strength (ΔT/E) measured within broad temperature range. It is found that the BZT21 (x = 0.21) exhibits the largest ΔT of ∼4.67 K and a high ΔT/E value of ∼0.46 km/MV at 9.9 MV/m and 25 °C. BZT21 also exhibits apparent relaxor ferroelectric response, showing a very broad EC peak in the temperature interval between 15 °C and 50 °C. Moreover, the relationship between EC properties and relaxor features was analyzed by piezoresponse force microscopy test. The results reveal that more dispersed phase structures induce additional configurational entropy, which is in favor for the enhanced EC performance. The interplay and compromise between the kinetic and thermodynamic mechanisms of domain switching determines the optimal composition for the EC performances of the BZT ceramics.

Polymer Nanocomposites with Ultrahigh Energy Density and High Discharge Efficiency by Modulating their Nanostructures in Three Dimensions.

Manipulating microstructures of composites in three dimensions has been a long standing challenge. An approach is proposed and demonstrated to fabricate artificial nanocomposites by controlling the 3D distribution and orientation of oxide nanoparticles in a polymer matrix. In addition to possessing much enhanced mechanical properties, these nanocomposites can sustain extremely high voltages up to ≈10 kV, exhibiting high dielectric breakdown strength and low leakage current. These nanocomposites show great promise in resolving the paradox between dielectric constant and breakdown strength, leading to ultrahigh electrical energy density (over 2000% higher than that of the bench-mark polymer dielectrics) and discharge efficiency. This approach opens up a new avenue for the design and modulation of nanocomposites. It is adaptable to the roll-to-roll fabrication process and could be employed as a general technique for the mass production of composites with intricate nanostructures, which is otherwise not possible using conventional polymer processing techniques.

Tuning Phase Composition of Polymer Nanocomposites toward High Energy Density and High Discharge Efficiency by Nonequilibrium Processing.

Polymer nanocomposite dielectrics with high energy density and low loss are major enablers for a number of applications in modern electronic and electrical industry. Conventional fabrication of nanocomposites by solution routes involves equilibrium process, which is slow and results in structural imperfections, hence high leakage current and compromised reliability of the nanocomposites. We propose and demonstrate that a nonequilibrium process, which synergistically integrates electrospinning, hot-pressing and thermal quenching, is capable of yielding nanocomposites of very high quality. In the nonequilibrium nanocomposites of poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and BaTiO3 nanoparticles (BTO_nps), an ultrahigh Weibull modulus ß of ∼30 is achieved, which is comparable to the quality of the bench-mark biaxially oriented polypropylene (BOPP) fabricated with melt-extrusion process by much more sophisticated and expensive industrial apparatus. Favorable phase composition and small crystalline size are also induced by the nonequilibrium process, which leads to concomitant enhancement of electric displacement and breakdown strength of the nanocomposite hence a high energy density of ∼21 J/cm3. Study on the polarization behavior and phase transformation at high electric field indicates that BTO_nps could facilitate the phase transformation from α- to ß-polymorph at low electric field.