Dielectric properties of solution-cast poly(vinylidene fluoride) films

Dielectric parameters have been measured for solution-cast films of poly(vinylidene fluoride) (PVDF) in the temperature range 40 to 120°C and frequency range 50 Hz to 1 MHz. The infrared absorption spectrum shows these films to be predominantly of Phase III crystal structure. The _c relaxation, which has generally been reported in the literature for Phase II PVDF, does not appear in the dielectric loss spectra. A small loss peak is observed which could probably be assigned to relaxation arising from molecular motions in the folds on the surface of the crystallites. Only a part of the _a relaxation is observed due to the limitations of our experimental range of temperature and frequency. Trapped charge carriers make a large contribution to the dielectric parameters at lower frequencies.     

Performance enhancements in poly(vinylidene fluoride)-based piezoelectric films prepared by the extrusion-casting process

The extrusion-casting process can realize large-area and continuous preparation of polymer-based films. In this paper, five different types of polyvinylidene fluoride (PVDF)-based piezoelectric films: PVDF, PVDF/PZT, PVDF/PZT@105, PVDF/PZT/BNNS and PVDF/PZT@105/BNNS were prepared by the extrusion-casting process. The mechanical, dielectric, thermal conductivity and piezoelectric properties were studied. It is found that PZT particles can well improve the dielectric performance and also the mechanical stability under variable temperature conditions. PZT powders modified by titanate coupling reagent (UP-105) can further improve the performance of the PVDF/PZT@105 films by improving the combination and dispersion of organic and inorganic phases. The addition of boron nitride nanosheets (BNNS) can improve the thermal conductivity of the films and the breakdown strength. The piezoelectric coefficient (d₃₃) of PVDF/PZT@105/BNNS composite film can reach 21pC/N, compared with the neat PVDF film (4pC/N) and PZT/PVDF (9pC/N) film realizing great improvement.

     

Inverse piezoelectric effect and electrostrictive effect in polarized poly(vinylidene fluoride) films

The piezoelectric stress coefficient and electrostriction coefficient for unpoled and poled films of polyvinylidene fluoride have been determined by applying a sinusoidal electric field and detecting stresses with the same and double frequency as the applied field, respectively. The piezoelectric coefficient shows a hysteresis with the cyclic change of the d.c. bias field. Under the poling field, the dipoles in polymers are preferentially oriented in the direction of the field, thus producing a residual polarization associated with a residual stress. Both coefficients increase with increasing temperature. A phenomenological interpretation for the piezoelectricity in poled polymers is given in terms of electrostriction and residual stress.     

纳米BaTiO3-SiCw/聚偏氟乙烯三元复合薄膜的制备及其介电性能

以15wt%十六烷基三甲基溴化铵改性碳化硅晶须(CTAB-SiC_w)和KH550改性纳米BaTiO₃(BT)为填料,聚偏氟乙烯(PVDF)为成膜物质,通过溶液流延法制备了BT-SiC_w/PVDF三元复合薄膜,利用FTIR、XRD、SEM和LCR介电温谱仪-高温测试系统联用装置对产物进行结构表征和介电性能测试。结果表明:KH550可以成功改性BT粒子且不会改变BT晶体结构,SiC_w和BT能够较好地分散在PVDF基体中;随着BT引入量的增加,复合薄膜的介电常数先增加后减小,其中当引入10wt%BT时介电性能最优,即频率f=500 Hz、介电常数ε_rmax=33、介电损耗tanδ_max=0.154。随着温度的升高,该试样的介电常数和介电损耗也逐渐增加,并在120℃达到最大值(f=500 Hz、ε_rmax=110、tanδ_max=1.3)。结果对于研究具有高介电常数的三元复合电介质材料为在埋入式电容器中获得应用提供了一种策略。      

Fabrication and dielectric properties of Na0.5Bi0.5Cu3Ti4O12/poly(vinylidene fluoride) composites

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO)/poly(vinylidene fluoride) (PVDF) composites with various NBCTO volume fractions were prepared via solution mixing and hot pressing process. The structure, morphology, and dielectric properties of the composites were characterized with X-ray diffraction (XRD), thermal-gravimetric analysis (TGA), scanning electron microscope (SEM), and broadband dielectric spectrometer. The dielectric constant (ε) and dielectric loss (tan δ) of the composites were both found to increase with increasing NBCTO volume fraction within the frequency range of 1–10⁶ Hz at room temperature. Relatively high dielectric constant of 79.8 and low loss of 0.21 at 1 kHz were obtained for the NBCTO/PVDF composite with 50 vol% NBCTO. Additionally, theoretical models like Logarithmic mixture rule, Maxwell–Garnet, Effective medium theory, and Yamada model were also employed to predict the dielectric constant of these composites. The values obtained by the EMT model are in close agreement with the experimental values.     

聚偏氟乙烯电极化的动力学相变和铁电性

对聚偏氟乙烯进行高温电极化处理以改善半晶聚合物的铁电性,研究了其铁电极化值随着极化电场的连续非线性变化,实现了结晶相中非铁电相向铁电相的转变.用动力学平衡方法模拟了聚偏氟乙烯样品的电场极化相变过程,结果表明:聚偏氟乙烯中结晶区域的有效极化电场约为50 MV/m,与铁电聚合物的矫顽电场吻合;非晶无定型区域的等效电场与实验的相变开启电场接近,并得到了理论与实验符合较好的结果,从而证明动力学平衡方法模拟聚偏氟乙烯铁电相变的可行性.     

Double beam pulsed laser deposition of composite films of poly(methyl methacrylate) and rare earth fluoride upconversion phosphors

The objective of the work is to demonstrate feasibility of producing functional polymer nanocomposite films for light emitting applications using the new double beam pulsed laser deposition (DPLD) technique. The existing pulsed laser deposition vacuum chamber has been modified to accommodate two laser beams of different wavelengths for the in situ ablation of two targets: a polymer host and a rare earth based highly efficient upconversion emitting inorganic dopant. Special provisions were made for cooling the target to control the ablation of the polymer without interrupting the process. Nanocomposite films of acrylic polymer and nanoparticles of the compounds of the rare earth elements were fabricated by the proposed method with near-infra-red laser radiation (1064-nm wavelength) ablating the polymer targets and visible radiation (532 nm) ablating the inorganic targets. The fabricated nanocomposite films were characterized using atomic force microscopy, X-ray diffraction, optical fluorescence spectroscopy, and visual observation of the fluorescence. It was discovered that the produced polymer nanocomposite films retained the crystalline structure and the upconversion fluorescence properties of the initial rare earth compounds mainly due to the better control of the deposition process of the materials with substantially different properties. The proposed method can be potentially used for making a wide variety of composite films.     

Effect of annealing on phase transition in poly(vinylidene fluoride) films prepared using polar solvent

The γ-phase poly (vinylidene fluoride) (PVDF) films are usually prepared using dimethyl sulfoxide (DMSO) solvent, regardless of preparation temperature. Here we report the crystallization of both α and γ-phase PVDF films by varying preparation temperature using DMSO solvent. The γ-phase PVDF films were annealed at 70, 90, 110, 130 and 160°C for five hours. The changes in the phase contents in the PVDF at different annealing conditions have been described. When thin films were annealed at 90°C for 5 h, maximum percentage of β-phase appears in PVDF thin films. The γ-phase PVDF films completely converted to α-phase when they were annealed at 160°C for 5 h. From X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), differential scanning calorimetry (DSC) and Raman studies, it is confirmed that the PVDF thin films, cast from solution and annealed at 90°C for 5 h, have maximum percentage of β-phase. The β-phase PVDF shows a remnant polarization of 4.9 μC/cm² at 1400 kV/cm at 1 Hz.     

Enhanced dielectric and ferroelectric properties induced by TiO2@MWCNTs nanoparticles in flexible poly(vinylidene fluoride) composites

Flexible polymer based composites containing multi-walled carbon nanotubes (MWCNTs) have been reported to present high dielectric constant. However, the composites generally exhibit high dielectric loss and low dielectric breakdown strength, which prohibits their practical use in electronic and electric industry. MWCNTs were coated with a continuous layer of TiO₂ nanoparticles (TiO₂@MWCNTs) by a simple hydrothermal process and TiO₂@MWCNTs/poly(vinylidene fluoride) (PVDF) composites were prepared by a solution casting method. Compared to the pristine MWCNTs/PVDF composites, the TiO₂@MWCNTs/PVDF composites presented enhanced dielectric constant and lower dielectric loss. Additionally, the breakdown strength of the TiO₂@MWCNTs/PVDF composites was also improved, which is favorable for enhanced ferroelectric properties in the composites.     

Synthesis and luminescence properties of rare earth doped gamma-ray-irradiated GdCa4O(BO3)3 phosphors

Undoped and rare earth (RE = Ce, Dy, and Eu) doped GdCa₄O(BO₃)₃ phosphors were synthesized by solid-state diffusion technique. Formation of the sample was confirmed by taking X-ray diffraction (XRD) pattern of the sample. Photoluminescence (PL) emission spectrum showed characteristic emission of RE doped in the GdCa₄O(BO₃)₃ sample. It is observed that doping of RE ion initially enhanced the TL yield, attained an optimum TL for a particular concentration of dopant (i.e., 0.5 mol%) then decreased with further increase in dopant concentration for all the samples. We found that Ce is the best activator for enhancing the TL yield in GdCa₄O(BO₃)₃ system. Fading of TL intensity of the sample was studied and it is found that fading of the TL is about 5% over the period of 15 days. The simple glow curve, linear response to γ-ray dose and less fading; makes the GdCa₄O(BO₃)₃:Ce(0.5 mol%) phosphors a suitable candidate for TL dosimetry.     

Insight into the nucleating and reinforcing efficiencies of carbon nanofillers in poly(vinylidene fluoride): a comparison between carbon nanotubes and carbon black

Poly(vinylidene fluoride) (PVDF) nanocomposites containing homogeneously dispersed multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) were fabricated by a small melt mixer. The uniform dispersion of the nanofillers in PVDF was confirmed by both scanning electron microscopy and transmission electron microscopy. Both the heterogeneous nucleation efficiency and crystallization half-time show that MWCNTs exhibit higher nucleation efficiency than CB for the crystallization of PVDF. Meanwhile, MWCNTs show greater contribution to the reinforcement of the storage modulus of PVDF as revealed by dynamic mechanical analysis, especially at low temperatures. However, the enhancement of the storage modulus in the melt state is reversed due to the network formed by serious agglomeration of CB. This study provides some insights into the nucleating and reinforcing efficiency of MWCNTs and CB in polymers.     

Magnetic properties of gamma-Fe2O3/poly(ether-ester) nanocomposites

The magnetic properties of gamma-Fe2O3 nanoparticles embedded in a thermoplastic elastomer poly(ether-ester) copolymer by the in situ polycondensation reaction process have been investigated by means of magnetization and ferromagnetic resonance (FMR) measurements at low filler concentrations of 0.1 and 0.3 wt% with the magnetic additive introduced in the polymer matrix in powder and solution form. The magnetic behavior of the magnetopolymeric nanocomposites indicates significant interparticle interaction effects that depend mainly on the dispersion state of the magnetic nanoparticles as well as their concentration, consistent with the variation of the particle microstructure characterized by magnetic aggregates in the nanometer and micron scale for the solution and powder dispersions, respectively. The magnetization and FMR results at different filler concentrations and dispersions show a close correspondence to the relaxation processes of the copolymer, implying the coupling of polymeric and magnetic properties.     

Preparation of aligned Fe3O4@Ag-nanowire/poly(vinyl alcohol) nanocomposite films via a low magnetic field

Aligned Fe₃O₄@Ag-nanowire (Ag-NW)/poly(vinyl alcohol) (PVA) nanocomposite films are prepared via a magnetic field-assisted method under a low magnetic field (B < 0.1 T) induction. The effects of the mass ratio (MR) of Fe₃O₄ to Ag-NWs and the Ag-NW content are systematically studied on the composite electrical conductivity (EC). The preferential alignment of Ag-NWs brings about a significant increase in the EC of the oriented composite in the parallel direction along the magnetic field. The optimal MR is determined to be equal to 0.15 at which the random composite has a good EC meanwhile the oriented composite shows a good response to the applied magnetic field. The oriented composite with the 20 wt% Ag-NWs shows a high EC anisotropy of ca. 6.6 and a very high EC of 4500 S/cm via the external magnetic field. In addition, the introduction of Ag-NWs leads to an obvious improvement in the thermal stability of PVA composites.     

Hybrid network structure and thermal conductive properties in poly(vinylidene fluoride) composites based on carbon nanotubes and graphene nanoplatelets

A small quantity of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were introduced into the poly(vinylidene fluoride) (PVDF)/GNP and PVDF/CNT composites, respectively, to prepare the corresponding ternary PVDF/CNT/GNP and PVDF/GNP/CNT composites. The results demonstrated that adding CNTs into the PVDF/GNP composites greatly promoted the formation of the hybrid network structure of fillers. This was much different from the scenario that adding GNPs into the PVDF/CNT composites. GNPs and CNTs exhibited excellent nucleation effects for the crystallization of PVDF matrix; however, the variation of the PVDF crystallinity was small. Adding CNTs into the PVDF/GNP composites greatly enhanced the electrical conductivity of the PVDF/CNT/GNP composites. This was also different from the scenario of the PVDF/GNP/CNT composites. Furthermore, the PVDF/CNT/GNP composites exhibit higher thermal conductivity and higher synergistic efficiency compared with the PVDF/GNP/CNT composites. The conductive mechanisms and the synergistic effects of the ternary composites were then analyzed.     

Porous yolk-shell structured Na3(VO)2(PO4)2F microspheres with enhanced Na-ion storage properties

Na3(VO)2(PO4)2F(NVPOF)has been considered as one potential candidate for sodium-ion batteries because of its high operating voltage and theoretical capacity.However,the poor intrinsic electronic conductivity significantly restricts its widespread application.In response to this drawback,we adopt the optimization strategy of tuning the morphology and structure to boost the electrical conductiv-ity and mitigate the capacity fading.In this paper,NVPOF microspheres with unique porous yolk-shell structure were fabricated via a facile one-step solvothermal method for the first time.By monitoring the morphological evolution with time-dependent experiments,the self-sacrifice and Ostwald ripening mechanism from rough spheres to yolk-shell structure was revealed.Benefited from the favorable inter-woven nanosheets shell,inner cavity and porous core structure,the resulting NVPOF electrode exhibits superior rate capability of 63 mA h g-1 at 20 C as well as outstanding long-cycling performance with the capacity retention up to 92.1％over 1000 cycles at 5 C.     

The electrochemical energy storage and photocatalytic performances analysis of rare earth metal (Tb and Y) doped SnO2@CuS composites

The rational design of composite electrode and catalyst materials is critical for increasing high energy density and structural stability, electron-hole pair production in supercapacitors, and photocatalytic applications. By employing a simple hydrothermal technique, we were able to produce SnO₂, CuS, SnO₂@CuS, Tb doped SnO₂@CuS, and Y doped SnO₂@CuS samples. Various analytical techniques such as PXRD, FTIR, UV, SEM, EDS, and XPS examine the synthesized materials' structural, optical, purity, morphology, and binding energy. The fabricated asymmetric Y doped SnO₂@CuS/AC device revealed the energy and power densities of 16.2 W h kg^?1 and 740 W kg^?1 at a current density of 1 A/g, with outstanding cycle stability of 80.9% retention capacitance after 5000 charge–discharge cycles. Furthermore, when exposed to visible light, the Y doped SnO₂@CuS catalyst degrades the methylene blue dye with a maximum degradation efficiency of 97%. All of the above results suggested that the obtained Y doped SnO₂@CuS material can be utilized in supercapacitor and photocatalytic applications.     

凝固浴强度对原位聚合改性聚偏氟乙烯膜结构与性能的影响

采用原位聚合的方法得到聚偏氟乙烯(PVDF)与聚甲基丙烯酸羟乙酯(PHEMA)的均相共混溶液,以不同配比的水和溶剂磷酸三乙酯(TEP)的混合溶液作为凝固浴,通过非溶剂复合热诱导相分离方法,制备了系列具有互穿网络结构孔的亲水性PVDF微孔膜.X射线光电子能谱(XPS)分析表明,随着凝固浴中水含量的增加,PHEMA在PVDF微孔膜表面富集度增加.原子力显微镜(AFM)和扫描电镜(SEM)发现,随着凝固浴中水含量的增加,PVDF微孔膜上表面由开孔结构逐渐发展为致密结构,且粗糙度随之减小.通过动态接触角测试发现,随着凝固浴中水含量的增加,改性PVDF微孔膜初始接触角由139.7°降低到64.7°,水滴在膜表面的浸润时间由313s减小到16s.水通量测试表明,随着凝固浴中水含量的增加,PVDF微孔膜的纯水通量由2 300L/(m2.h)减小到400L/(m2.h),但由于具有良好的可润湿性,改性干膜仍然保持与湿膜相当的水通量.