Effects of particle size of BaTiO3 powder on the dielectric properties of BaTiO3/polyvinylidene fluoride composites

The effects of particle size of fine BaTiO₃powder on dielectric properties of BaTiO₃/polyvinylidene fluoride (PVDF) composites were investigated. When the frequency of the applied field was less than 100 kHz, the dielectric constant and loss for BaTiO₃/PVDF composites decreased with increasing BaTiO₃particle size. When the frequency was greater than 100 kHz, the opposite results were obtained. The resistivity increased with increasing BaTiO₃particle size, whereas the open areas of hysteresis loops decreased gradually. The degree of poling efficiency for BaTiO₃/PVDF composites increased with increasing BaTiO₃particle size. The BaTiO₃particle size dependence of the dielectric properties of BaTiO₃/PVDF composites is explained by space charge effects at the interface between BaTiO₃and PVDF, and domain configurations (single or multi-domain) of the BaTiO₃powders.     

Fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composite ceramics synthesized by novel powder-in-sol precursor hybrid processing route

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via sol-gel wet chemistry process. Uniformly distributed slurry is obtained after ball milling and used in the fabrication of the ceramics with low sintering temperatures. The ceramics show coexistence of ferromagnetic and ferroelectric phases with obvious ferromagnetic and ferroelectric hysteresis loops at room temperature, besides exhibiting excellent magnetic and dielectric properties in a wide range of frequency. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6,6 fibers

The versatile electrospinning technique was used to successfully align and disperse multiwalled carbon nanotubes (MWCNT) in nylon 6,6 matrix to obtain composite fibers. The morphology of the composite fibers and the dispersion of the CNTs within the fibers were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. TEM analysis revealed that the CNTs were well-dispersed, separated and aligned along the fiber axis. The thermal and mechanical properties of the composite fibers were characterized as a function of weight fraction of the CNTs. Incorporation of the CNTs in the fibers resulted in an increase in glass-transition temperature (T_g) by ∼7 °C, indicating that the addition of CNTs has restricted the mobility of the polymer chains and provided confinement to neighboring molecular chains. Tensile and nanoindentation experiments were performed to investigate the mechanical deformation behavior of the composite fibers. The results suggested that incorporation of high strength and high aspect ratio CNTs into the fiber matrix enhanced significantly the stiffness and strength of nylon 6,6 fibers. An understanding of the structure–property relationships can provide fruitful insights to develop electrospun fibers with superior properties for miniaturized and load-bearing applications.     

Ternary nano-CaCO3/poly(ethylene terephthalate) fiber/polypropylene composites: Increased impact strength and reinforcing mechanism

The binary nano-CaCO₃/polypropylene (PP), poly(ethylene terephthalate) (PET) fibers/PP and ternary nano-CaCO₃/PET fibers/polypropylene composites were prepared by melt blending method, and their structure and mechanical properties were investigated. The results show that the ternary nano-CaCO₃/PET fibers/PP composite displays significantly enhanced mechanical properties compared with the binary PET fibers/PP and nano-CaCO₃/PP composites, and neat PP. The X-ray diffraction, dynamic mechanical analysis, scanning electron microscopy and analysis of the non-isothermal crystallization kinetics were used to investigate the reinforcement mechanism of composites. The results indicate that the interfacial action and compatibility between PET fiber and PP are obviously enhanced by the addition of modified nano-CaCO₃ particles in the ternary composites and the mechanical property enhancement in the ternary system may be mainly originated from the formation of β-form crystallites of PP induced by the synergistic effect between PET fibers and nano-CaCO_3.     

One-dimensional multiferroic bismuth ferrite fibers obtained by electrospinning techniques

We report the fabrication of novel multiferroic nanostructured bismuth ferrite (BiFeO(3)) fibers using the sol-gel based electrospinning technique. Phase pure BiFeO(3) fibers were prepared by thermally annealing the electrospun BiFeO(3)/polyvinylpyrrolidone composite fibers in air for 1 h at 600?°C. The x-ray diffraction pattern of the fibers (BiFeO(3)) obtained showed that their crystalline structures were rhombohedral perovskite structures. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that the BiFeO(3) fibers were composed of fine grained microstructures. The grains were self-assembled and self-organized to yield dense and continuous fibrous structures. The magnetic hysteresis loops of these nanostructured fibers displayed the expected ferromagnetic behavior, whereby a coercivity of ～ 250 Oe and a saturation magnetization of ～ 1.34 emu g(-1) were obtained. The ferroelectricity and ferroelectric domain structures of the fibers were confirmed using piezoresponse force microscopy (PFM). The piezoelectric hysteresis loops and polar domain switching behavior of the fibers were examined. Such multiferroic fibers are significant for electroactive applications and nano-scale devices.     

Experimental evidence of enhanced ferroelectricity in Ca doped BiFeO3

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), polarization and piezoelectric measurements. Structural studies by XRD and TEM reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ where enhanced ferroelectric and piezoelectric properties are produced by internal strain. Resistive switching is observed in BFO and Ca-doped BFO which are affected by the barrier contact and work function of multiferroic materials and Pt electrodes. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Piezoelectric properties are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain. This observation introduces magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom which are already present in the multiferroic BiFeO₃.     

Comparative study of dielectric properties of the PVDF composites filled with spherical and rod-like BaTiO<Subscript>3</Subscript> derived by molten salt synthesis method

In the molten salt environment, the BaTiO₃ spherical nanoparticles (BTNPs) and BaTiO₃ nanorods (BTNRs) have been synthesized, respectively, in which spherical TiO₂ and rod-like BaTi₂O₅ are precursors. The dissolution–precipitation is the main dominated mechanism in the formation of BTNPs, while the dissolution–diffusion is the main mechanism responsible for the formation of BTNRs. The latter is also called as topochemical mechanism, which is associated with the assembly of [TiO₆] octahedron units in the transformation from BaTi₂O₅ to BaTiO₃. By using these two kinds of BT as fillers, polyvinylidene fluoride (PVDF)-based composites, BTNPs/PVDF and BTNRs/PVDF, have been constructed and their dielectric properties have been investigated. It was found that there were three main factors related to filler morphology affecting the dielectric properties of the composites, i.e., intrinsic polarization of filler, the interface polarization and electric field distribution between the filler and the matrix. Though the spontaneous polarization of 600-nm-sized BTNPs is larger, the interfacial area of BTNRs/PVDF composite is larger than that of BTNPs (600 nm)/PVDF composite, which is advantageous to enhance the interface polarization. Moreover, the analysis through Potential-Across model revealed that BTNRs/PVDF composite has stronger electric field intensity distribution across BTNRs filler in comparison with BTNPs/PVDF, which plays the key role in improving the dielectric properties of composites. This work not only presents the BTNRs/PVDF composite with good dielectric performance, the related design and the theory analysis also facilitate the development of more new high dielectric composites based on morphology control of ferroelectric filler.     

The effect of interfacial state on the thermal conductivity of functionalized Al2O3 filled glass fibers reinforced polymer composites

Rapidly increasing packaging density of electronic devices puts forward higher requirements for thermal conductivity of glass fibers reinforced polymer (GFRP) composites, which are commonly used as substrates in printed circuit board. Interface between fillers and polymer matrix has long been playing an important role in affecting thermal conductivity. In this paper, the effect of interfacial state on the thermal conductivity of functionalized Al₂O₃ filled GFRP composites was evaluated. The results indicated that amino groups-Al₂O₃ was demonstrated to be effective filler to fabricate thermally conductive GFPR composite (1.07 W/m K), compared with epoxy group and graphene oxide functionalized Al₂O₃. It was determined that the strong adhesion at the interface and homogeneous dispersion of filler particles were the key factors. Moreover, the effect of interfacial state on dielectric and thermomechanical properties of GFRP composites was also discussed. This research provides an efficient way to develop high-performance GFRP composites with high thermal conductivity for integrated circuit packaging applications.     

Cosmic radiation shielding tests for UHMWPE fiber/nano-epoxy composites

Cosmic radiation shielding properties are important for spacecraft, and hydrogenous materials such as polyethylene have been shown to be effective in shielding against galactic cosmic rays and solar energetic particles. Ultrahigh molecular weight polyethylene (UHMWPE) fibers, which are effective in such shielding, also have advanced mechanical and physical properties, which potentially are very valuable for NASA space missions both as a radiation shield and as vehicle structure. In our previous studies, we fabricated a nano-epoxy matrix with reactive graphitic nanofibers that showed enhanced mechanical (including strength, modulus and toughness) and thermal properties (higher T_g, stable CTE, and higher ageing resistance), as well as wetting and adhesion ability to UHMWPE fibers. In this work, the radiation shielding performance of the UHMWPE fiber reinforced nano-epoxy composite was characterized by radiation tests at the NASA Space Radiation Laboratory at Brookhaven National Laboratory. The results showed that the high radiation shielding performance associated with UHMWPE was not degraded by the addition of graphitic nanofibers in the matrix. Together with the previous studies showing higher mechanical properties, these new studies validate the importance of the UHMWPE fiber/nano-epoxy composite for potential applications in more durable space composites and structures, and offer reduced manufacturing costs and wider design applications through avoidance of specialized and in some cases ineffective UHMWPE fiber surface treatment processes.     

Effect of silver incorporation into PVDF-barium titanate composites for EMI shielding applications

Composites of polyvinylidene fluoride (PVDF) with micron and nano sized BaTiO₃ powders were developed for electromagnetic interference (EMI) shielding applications in the X band. PVDF-nano BaTiO₃ composites show better shielding property compared to PVDF-micron sized BaTiO₃ composites. The composite of PVDF with 40 vol% of nano BaTiO₃ showed the best EMI shielding effectiveness and is about 9 dB. The contributions from reflection and absorption to the total EMI shielding effectiveness is same for the PVDF-BaTiO₃ composites. Addition of small amount of silver particles improved the shielding properties of these composites due to the increased conductivity. An EMI shielding effectiveness of about 26 dB is obtained in the measured frequency range for the PVDF-20 vol% nano BaTiO₃-10 vol% Ag composite of thickness 1.2 mm. Novel three phase composite combining the advantages of metal, nano ceramic and polymer is obtained with the potential for effective EMI shielding applications.     

Mechanical response of barium-titanate/polymer 0–3 ferroelectric nano-composite film under uniaxial tension

Ferroelectric polymer based 0–3 composite films are attractive for applications such as capacitors and electric energy storage devices. In this paper, deformation and fracture behavior under uniaxial tension is characterized for BaTiO₃/poly(vinyledene fluoride-trifluoroethylene) (abbreviated as BT/P(VDF-TrFE)) ferroelectric composite film. Compared with the pure P(VDF-TrFE) copolymer film, the composite film with a small volume fraction of BT powders shows an enhanced ductility in accompany with reduced stiffness and fracture strength. Scanning electron microscope (SEM) observation and X-ray diffraction (XRD) analysis are carried out to examine the morphology and microstructure change during uniaxial tension. It is demonstrated that addition of a small amount of BaTiO₃ powders into the copolymer matrix inhibits the growth of the crystallite size, causes reduction in the crystalline content and a loosely packed molecular chain structure. Consequently, the fracture strain increases while the stiffness and fracture strength decreases for the composite films.     

Poling effects and piezoelectric properties of PVDF-modified 0–3 connectivity cement-based/lead-free 0.94(Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript>–0.06BaTiO<Subscript>3</Subscript> piezoelectric ceramic composites

PVDF-modified 0–3 connectivity cement-based/lead-free 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ piezoelectric ceramic composites were fabricated using 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNBT), Portland cement, and polyvinylidene fluoride (PVDF). The microstructure, acoustic impedance (Z _c), dielectric properties, and influence of poling temperature and electrical poling field on the piezoelectric coefficient (d ₃₃) and the total period of the poling process of composites with 50 vol% BNBT and 1–10 vol% PVDF were investigated. The results indicated that Z _c, the dielectric constant, and the dielectric loss of the composites decrease as the PVDF content increases. The d ₃₃ of the composites was found to enhance more clearly when the content of PVDF is more than 2 vol%. The d ₃₃ results of the composites showed an optimum increase of 45% when 5 vol% PVDF was used (under an electrical poling field of 1.5 kV/mm and a poling temperature of 80°C). Moreover, these composites with PVDF were found to exhibit enhanced poling behavior in that the PVDF was able to reduce the total period of the poling process. Interestingly, the piezoelectric voltage coefficient (g ₃₃) of the composite with 5 vol% PVDF content had the highest value of 33.59 mV·m/N. Therefore, it can be safely concluded that this new kind of PVDF-modified 0–3 connectivity cement-based/lead-free 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ piezoelectric ceramic composite has the potential to be used in concrete as a sensor for structural health monitoring applications.     

Photoelectret and electrophotographic properties of barium titanate-polyvinylidene fluoride composite film

Photoferroelectric barium titanate (BaTiO₃)-polyvinylidene fluoride (PVDF) composite films in the ratio 7030 are found to possess good photosensitivity in the visible region of the spectrum. Results on investigations of charge generation, trapping, recombination, discharge and retention properties of the composite film are presented in this paper. Photoinduced polarization is obtained in a BaTiO₃-PVDF composite film by the simultaneous application of a d.c. field and illumination from an argon-ion laser or by white light. The dark (photo) polarization and the dark (photo) depolarization current decay characteristics of the polarized films have been studied under different experimental conditions. The studies show that the composite film retains good polarization for a fairly long time in the dark condition, and the magnitude of the depolarization current is comparable with that of BaTiO₃ single crystal. Under illumination, the decay is fast enough to make use of the film for electro-photographic applications.     

Dielectric and ferroelectric properties of (1-x)(Na0.5K0.5)NbO3-xBaTiO3 ceramics

(1-x)(Na_0.5K_0.5)NbO₃-xBaTiO₃ ceramics were prepared by a solid state reaction approach, and their dielectric and ferroelectric properties were evaluated together with the crystal structure. Three phase transitions at T_t1, T_t2 and T_t3 were observed by the combination of DTA analysis and dielectric characterization. These phase transitions corresponded to those of (Na_0.5K_0.5)NbO₃, and they were greatly pulled down by forming solid solution with BaTiO₃. The phase transition around T_t1 was incompletely diffusive and the appearance of diffusiveness of non ferro-paraelectric phase transition was an exception. The hysteresis loops changed their shapes from “square” into “thin square” with increasing x.     

Multi‐Nonvolatile State Resistive Switching Arising from Ferroelectricity and Oxygen Vacancy Migration

Resistive switching phenomena form the basis of competing memory technologies. Among them, resistive switching, originating from oxygen vacancy migration (OVM), and ferroelectric switching offer two promising approaches. OVM in oxide films/heterostructures can exhibit high/low resistive state via conducting filament forming/deforming, while the resistive switching of ferroelectric tunnel junctions (FTJs) arises from barrier height or width variation while ferroelectric polarization reverses between asymmetric electrodes. Here the authors demonstrate a coexistence of OVM and ferroelectric induced resistive switching in a BaTiO₃ FTJ by comparing BaTiO₃ with SrTiO₃ based tunnel junctions. This coexistence results in two distinguishable loops with multi‐nonvolatile resistive states. The primary loop originates from the ferroelectric switching. The second loop emerges at a voltage close to the SrTiO₃ switching voltage, showing OVM being its origin. BaTiO₃ based devices with controlled oxygen vacancies enable us to combine the benefits of both OVM and ferroelectric tunneling to produce multistate nonvolatile memory devices.     

Tunable BT@SiO2 core@shell filler reinforced polymer composite with high breakdown strength and release energy density

Barium titanate@silicon dioxide (BT@SiO₂) core@shell fillers with an average diameter of 100 nm were prepared by a facile sol–gel synthesis. The thickness of SiO₂ shell can be easily tuned by varying different mass ratio of BT to tetraethyl orthosilicate (TEOS). Polyvinylidene fluoride (PVDF) based composite films reinforced by BT and BT@SiO₂ were fabricated via a solution casting method. The effects of SiO₂ shell on morphology structure, wettability, interfacial adhesion, dielectric, electrical and energy performances of composites were investigated. Compared with BT/PVDF, BT@SiO₂/PVDF composites show significantly increased breakdown strength due to enhanced interfacial adhesion and suppressed charge carrier conduction. Benefiting from enhanced breakdown strength and reduced remnant polarization induced by SiO₂ shell, BT@SiO₂/PVDF shows increased release energy density (energy density which can be fully discharged and applicable). Especially, BT@SiO₂/PVDF with SiO₂ thickness of 4 nm exhibits the highest release energy density of 1.08 J/cm³ under applied electric field of 145 kV/mm.     

Electrical and optical modulation on ferroelectric properties of P(VDF-TrFE) thin film capacitors

Modulating the ferroelectric properties of P(VDF-TrFE) polymers both electrically and optically could open up new opportunities for their applications in non-volatile memories and sensors. Here by using the Nb:SrTiO₃ semiconductor as electrode compared with metal Au electrode, we report on the modulation of ferroelectric properties of P(VDF-TrFE) thin film capacitors both by electric field and UV light. A ferroelectric hysteresis loop shift together with the asymmetric switching behavior has been observed when using semiconducting electrode, which could be explained by the band alignment model based on interfacial charge screening. On the basis of band bending near the ferroelectric/semiconductor interface, we could further modulate the ferroelectric switching behaviors reversibly by UV light illumination. Our research provides a new route to engineer the ferroelectric properties of P(VDF-TrFE) polymer thin film capacitors, promising their applications in optoelectronic devices.     

Mechanical and electrical switching of local ferroelectric domains of K<Subscript>0.5</Subscript>Bi<Subscript>4.5</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> film

Ferroelectric/Piezoelectric K_0.5Bi_4.5Ti₄O₁₅ (KBT) film was fabricated by pulsed laser deposition method and confirmed by ferroelectric, dielectric measurements and local butterfly-type piezoresponse hysteresis loops. Importantly, ferroelectric domain switching by both electrical field and mechanical force in KBT film was demonstrated. The dark and bright contrast represents the PFM response of the up and down polarized domains, which can be written by a dc bias of ±12 V or a mechanical force of 40–50 nN. The successful demonstration of mechanical force switching of ferroelectric domain in KBT film other than electric field provides a novel mean for information storage and sensors.     

Simulation of curing process of carbon/epoxy composite during autoclave degassing molding by considering phase changes of epoxy resin

Strain monitoring of a carbon/epoxy composite cross-ply laminate ([0₅/90₅]_s) during thermoforming was conducted by using fiber Bragg grating (FBG) sensors. The entire process was simulated by employing finite element analysis (FEA) by taking into consideration the phase changes of the epoxy resin. For the precise simulation of the curing process, a dielectrometry sensor was used to detect the epoxy-resin dissipation factor, which in turn was used to identify the curing point. To investigate the phase changes and consolidation of the composite laminate by employing FEA, modulus changes with temperature were measured by dynamic mechanical analysis (DMA), and the permeability was estimated by measuring the fiber volume fraction according to the curing temperature. As the epoxy resin changed from a liquid to solid phase, the strain generated along the carbon fibers dynamically changed, and the analysis results generally predicted the strain variation quite well. To apply this simulation technique to practical structures, a composite-aluminum hybrid wheel was analyzed and experimentally verified.     

Hierarchical poly(p-phenylene benzobisoxazole)/graphene oxide reinforcement with multifunctional and biomimic middle layer

A new hierarchical reinforcement developed by coating biomimic polydopamine (PDA) on the surface of poly(p-phenylene benzobisoxazole) (PBO) fibers, which served as a platform for the graphene oxide (GO) grafting, using branched polyethyleneimine (b-PEI) as a bridging agent. The surface morphologies and chemical structures of PBO fibers were characterized for confirming the formation of covalent bond between GO and PBO fibers. The surface roughness (R_a) and wettability of the obtained fibers, denoted as PBO@PDA-PEI-GO, were obviously increased in comparison with those of untreated one. The reinforcement offered a 68.8% enhancement in the interfacial shear strength (IFSS) without degrading the base fiber. The PDA layer on the PBO fiber surface led to improved UV resistance. The hydrothermal aging resistance of PBO/epoxy composite was also greatly improved. This biomimic surface modification approach is facile to prepare, highly efficient to enhance interface, adaptable to all high-performance fibers, and meaningful in multifunctional applications.