Modulation of electromagnetic wave absorption by carbon shell thickness in carbon encapsulated magnetite nanospindles–poly(vinylidene fluoride) composites

A series of Fe₃O₄/C core–shell nanospindles with different shell thickness have been synthesized by a wet chemical method and subsequent high-temperature carbonization. The thickness of carbon shell can be well adjusted from 9 to 32 nm by changing the addition amounts of resorcinol and formaldehyde precursors during the coating process. Structure and morphology characterizations reveal that the carbon shell is amorphous structure and uniformly encapsulates on porous Fe₃O₄ nanospindles. For the first time, a flexible Fe₃O₄/C/poly(vinylidene fluoride) (PVDF) composite absorber was prepared by embedding the core–shell Fe₃O₄/C nanospindles in PVDF matrix. The electromagnetic properties of the composite show strong dependence on the carbon-shell thickness. The impedance matching for electromagnetic absorption is improved by the synergy effect between Fe₃O₄ nanospindles and encapsulated carbon shell. The Fe₃O₄/C/PVDF composite with thick carbon shell exhibits strong electromagnetic wave absorbing ability with thin absorber thickness. The minimum reflection loss for the absorber with thickness of 2.1 mm can reach −38.8 dB.     

COOH?MWCNTs‐induced BiFeO3‐poly(vinylidene fluoride) (PVDF) composites with enhanced dielectric and ferroelectric properties

In this work, flexible three phase composite films were prepared with surface functionalized multi‐walled carbon nanotubes (f‐MWCNTs) and bismuth ferrite (BiFeO₃;BFO) particles embedded into the poly(vinylidene fluoride) (PVDF) matrix via solution casting technique. The properties and the microstructure of prepared composites were investigated using an impedance analyzer and field emission scanning electron microscope. The micro‐structural study showed that the f‐MWCNTs and BFO particles were dispersed homogeneously within the PVDF matrix, nicely seated on the floor of the f‐MWCNTs separately. The dielectric measurement result shows that the resultant composites with excellent dielectric constant (≈96) and relatively lower dielectric loss (<0.23 at 100 Hz). Furthermore, the percolation theory is explored to explain the dielectric properties of the resultant composites. It says that the percolation threshold of f_MWCNTs = 0.9 wt % and the enhancement of the dielectric constant of the composite was also discussed. In addition, the remnant polarization of the un‐poled PVDF‐BFO‐f‐MWCNTs composites (2P_r ～1.34 µC/cm² for 1.1 wt % of f‐MWCNTs) is also improved. These three phase composites provide a new insight to fabricate flexible and enhanced dielectric properties as a promising application in modern electrical and electronic devices. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46002.     

Improved tribological properties of polyimide composites by micro–nano reinforcement

The hydroxylate carbon nanotubes (CNTs) were grafted by chemical method on the surface of the oxidized carbon fibers (CF) to improve the mechanical and tribological properties of polyimide (PI). The microstructure and fracture surface of the polyimide composites indicated that CF–CNTs hybrid as a multiscale reinforcement can distribute into the PI matrix homogeneously. Tribo-tests further showed that CF–CNTs hybrid had a better effect on hardness increment, impact strength enhancement, friction reduction, and wear resistance. Compared to the neat PI, the friction coefficient and wear rate of CF–CNTs/PI composite deceased by 23.2 and 55.9%, respectively. In particular, the loading capacity and high speed resistance of CF–CNTs/PI composite were greatly improved. The corresponding wear mechanisms were also discussed by observing the worn surface of the PI composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47900.     

Crystallization kinetics of γ phase poly(vinylidene fluoride)(PVDF) induecd by tetrabutylammonium bisulfate

The phase transition behavior of poly(2-ethyl-2-oxazoline) (PEtOx) under complexation with star-shaped poly(acrylic acid) (PAA) having various arm numbers (two, three, four, and six) has been studied by turbidity and laser light scattering measurements. The change in cloud point temperature (T _cp) of PEtOx was monitored as a function of pH, ionic strength, and arm number of the star polyelectrolyte. The shift in T _cp to a lower value than that of pure PEtOx was more pronounced at pH 4.2 (pH?a), when the carboxylic acid groups are protonated as compared to pH 7.0 (pH?>?pK_a ), when the acid moieties are partially ionized. Dynamic light scattering showed that these complexes may have micellar core-shell type structure with a mean hydrodynamic radius (R _h) ranging from 12 nm to ～200 nm depending upon the temperature. Significant shift in T _cp was observed for six-arm star poly(acrylic acid) complexes at both pH values. This change in the T _cp is accredited to the differences in the driving forces of phase transition, including hydrogen bonding between carboxylic acid groups of PAA and the carbonyl moiety of PEtOx as well as the hydrophobic interactions.     

Electrical properties of low-temperature-fired ferrite–dielectric composites

The effects of the BaO·(Nd_0.8Bi_0.2)₂O₃·4TiO₂ (BNBT) to NiCuZn ferrite ratio and addition of Bi₂O₃–B₂O₃–SiO₂–ZnO (BBSZ) glass on the sintering behavior, microstructure evolution, dielectric and magnetic properties of BNBT–NiCuZn ferrite composites were investigated in developing low-temperature-fired composites for high frequency electromagnetic interference (EMI) devices. The results indicate that these composites can be densified at 900 °C and exhibit superior dielectric and magnetic properties with the addition of BBSZ glass. The dielectric system used in the ferrite–dielectric composites reported in the previous studies mostly belong to the ferroelectricity group, which are not suitable for use in the high frequency range (>800 MHz) due to the selfresonance frequency limit. In this study, the dielectric constant remains nearly a constant over a wide range of frequencies (100 MHz to 1 GHz) and the magnetic resonance frequencies are larger than 100 MHz for the BNBT + BBSZ glass–NiCuZn ferrite composites. Therefore, the BNBT + BBSZ glass–NiCuZn ferrite composites can be a good candidate material for high frequency EMI device applications.     

Effect of polarization switching on piezoelectric and dielectric performance of electrospun nanofabrics of poly(vinylidene fluoride)/Ca–Al LDH nanocomposite

At present, highly flexible, durable, and lightweight piezoelectric nanogenerators with high-power density and energy conversion efficiency are of great interest. The present study reports a new synthetic route for Ca–Al layered double hydroxide (LDH) nanosheets and incorporation of these two-dimensional nanosheets as filler material into poly(vinylidene fluoride) (PVDF) to produce composite nanofabrics by electrospinning. The polymorphism, crystallinity, and the interaction between PVDF and LDH were studied by Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry techniques. The synergetic effect of PVDF–LDH interaction and in situ stretching due to electrospinning facilitates the nucleation of electroactive β phase up to 82.79%, which makes it a suitable material for piezoelectric-based nanogenerators. The piezoelectric performance of PVDF/Ca–Al LDH composite nanofabrics was demonstrated by hand slapping and frequency-dependent mechanical vibration mode, which delivered a maximum open circuit output voltage of 4.1 and 5.72 V, respectively. Moreover, the composite nanofabrics exhibited a high dielectric constant and low dielectric loss due to superior interfacial polarization at low-frequency region with LDH loading, promising its potential applications in electronic devices. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48697.     

Non-isothermal crystallization behavior of poly(vinylidene fluoride)/ethylene–vinyl acetate copolymer blends

Non-isothermal crystallization behavior of poly(vinylidene fluoride) (PVDF) and ethylene–vinyl acetate (EVA) copolymer and their binary blends with different blending ratios were investigated by the use of differential scanning calorimetry (DSC). With the increasing cooling rates, PVDF, EVA and their binary blends showed wide crystallization temperature range and high crystalline enthalpy. Jeziorny and Mo’s models were applied to calculate non-isothermal crystallization kinetics parameters of neat PVDF, EVA and their binary blends. By Jeziorny method, the crystallization process of neat PVDF, EVA and PVDF/EVA = 7/3 blend can be divided into two parts: primary and secondary crystallization processes. The Avrami exponent n ₁ indicated that the primary crystallization process was a mixture model of three-dimensional and two-dimensional space extensions. In comparison, PVDF/EVA = 5/5 and PVDF/EVA = 3/7 blends showed a single crystallization process. Through Mo’s analysis, faster cooling rate was demanded to reach higher relative crystallinity. Crystallization rate coefficient (CRC) was used to describe the effect of crystallization rates on the interaction between PVDF and EVA. CRC reached a maximum value when the mass ratio of PVDF and EVA was 7/3. The maximum CRC values of PVDF system and EVA system were 98.1 and 179.9 h^?1, respectively. The activation energy was closely related to the extent of conversion and the neat samples had a maximum value of crystallization activation energy. This was consistent with the observation for the parameters from Jeziorny analysis and could be correlated to the heterogeneous nucleation.     

Remarkable change in the broadband electrical behavior of poly(vinylidene fluoride)–multiwalled carbon nanotube nanocomposites with the use of different processing routes

Poly(vinylidene fluoride) (PVDF) nanocomposites have plenty of applications in the electronic realm. In this study, we produced nanocomposites based on PVDF and multiwalled carbon nanotubes (MWCNTs), with various MWCNT loadings, using three different processing routes: solution mixing, melt mixing, and electrospinning. The broadband electrical behavior of these nanocomposites was studied and compared via impedance spectroscopy. The morphologies of the nanocomposites were characterized by transmission electron microscopy and scanning electron microscopy. The results reveal that the electrical behaviors of the samples were completely different according to the processing route used. Solution mixing was the most suitable method for producing nanocomposites with the highest conductivities, at low MWCNT loadings, whereas electrospinning was the most suitable method for producing nanocomposites with the lowest dielectric permittivity. These differences were attributed to the different arrangements of the MWCNTs caused by the different processes. Although the solution-mixed samples exhibited long and twisted MWCNTs, the melt-mixed samples had shorter MWCNTs, and the electrospun samples had MWCNTs embedded and aligned inside the insulating polymer nanofibers. Thus, these results project a vast horizon for tailoring the structure and thereby the broadband electrical behavior of PVDF–MWCNT nanocomposites for different types of applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47409.     

Thermally-stable high dielectric properties of (1–x)(0.65Bi1.05FeO3–0.35BaTiO3)–xBiGaO3 piezoceramics

The thermal stability of the dielectric behavior and the crystal structure, surface morphology, polarization, and piezoelectric properties of lead-free BiGaO₃ (BG)-modified 0.65Bi_1.05FeO₃–0.35BaTiO₃ (abbreviated as BF–BT–xBG with 0 ≤ x ≤ 0.03) ceramics were investigated. XRD analysis of BF–BT–xBG dielectric ceramics revealed no remarkable change in the crystal structure within the studied composition range. Around the critical composition (BF–BT–0.02BG), the piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) reached maximum values of ⁓205 pC/N and 34.5%, respectively. The BF–BT–BG dielectric system also exhibited a thermally-stable ε_r (801–902, at 30 °C – 500 °C), high T_max (395 °C – 416 °C), colossal ε_rmax (46,363–76,303), and ε_rmid (2241 ± 15%–3678 ± 15%, with tanδ ≤ 0.08) across a wide temperature range of 198 °C–332 °C. This improvement in the dielectric properties and high T_max of the optimum specimen can be attributed to the BiGaO₃-modification and quenching process, which made the system viable for applications that require high-temperature dielectric stability.     

Degradation of the dielectric and piezoelectric response of β-poly(vinylidene fluoride) after temperature annealing

The effect of annealing temperature and time on the dielectric and piezoelectric response of poly(vinylidene fluoride), PVDF, was studied. The observed decrease in the value of the dielectric, ε′, and piezoelectric, d ₃₃, constants is related to depoling of the material and not to variations of the degree of crystallinity or the electroactive β-phase content. In a general way, the dielectric and piezoelectric responses decrease strongly in the first 4 h at a given temperature, in particular for temperatures higher that 80 °C, reaching stable values for longer annealing times. For most applications, the temperature of 100 °C will set the limit of suitable performance. Nevertheless, the material still retains a stable piezoelectric response of ca. 4 pC/N after reaching temperatures of 140 °C. The mechanisms behind the observed behavior are discussed.     

Mechanical properties of poly(ε-caprolactone) composites with electrospun cellulose nanofibers surface modified by 3-aminopropyltriethoxysilane

To enhance the reinforcement effects of regenerated cellulose nanofibers (RC-NF) in poly(ε-caprolactone) (PCL), we synthesized RC-NF-3-aminopropyltriethoxysilane (APS), the surface-modified RC-NF by APS. The RC-NF were fabricated by the saponification of electrospun cellulose–acetate nanofibers. The surface modification by APS was confirmed by the X-ray photoelectron spectroscopy (XPS). To enhance the mechanical property of PCL, the RC-NF and the RC-NF-APS were separately compounded into PCL by compression molding. It was found that, when the fiber concentration of RC-NF-APS was 17 wt %, the Young's modulus at room temperature increased from 698.0 to 744.7 MPa, whereas the storage modulus at 55 °C almost increased from 180 to 220 MPa. The micrographs of the fracture surface of the composites revealed that the surface modification prevented the pull-out of RC-NF from PCL. It was concluded that the mechanical properties of the composites were enhanced due to the improvement of the compatibility between RC-NF and PCL by the surface modification with APS. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48599.     

Structural modification and evaluation of dielectric and ferromagnetic properties of Ce-modified BiFeO3–BaTiO3 ceramics

An investigation on Rare earth constituent Ce incorporated BiFeO₃–BaTiO₃ ceramics has been focused in the present study. The ceramic samples of (Bi₀.₇Ba_0.3)_1-xCe_x(Fe₀.₇Ti_0.3)O₃ (x = 0–0.12) were formulated adopting the cost-effective solid-state sintering method. The influence of aliovalent Ce ions on the structural, microstructural, dielectric, ferromagnetic, and optical properties of BiFeO₃–BaTiO₃ was evaluated in this paper. The coexistence of the Tetragonal and the Rhombohedral phases was established by the Rietveld refinement process. The refined crystallographic parameters showed maximum cell volume (V_cell) and the highest percentage of the Rhombohedral phase for x = 0.06; and consequently, the ceramic exhibited the topmost dielectric constant of 946 at x = 0.06. The scanning electron microscopy of the samples revealed the manifestation of polygonal grain morphology. Besides, remarkably improved ferromagnetic properties were evinced for Ce doped ceramics. The magnitude of saturation (M_s) and remnant (M_r) magnetizations were boosted from 0 emu/g and 0.0019 emu/g to 0.9186 emu/g and 0.3745 emu/g respectively with increasing x from 0 to 0.12. Additionally, the optical band gaps of all the samples were evaluated and found to be in the range of 2.941–3.077 eV.     

Preparation of quaternized poly(vinylidene fluoride) membranes by γ-ray irradiation induced graft polymerization and their antibacterial property

PVDF microfiltration membranes were modified by γ-ray irradiation induced grafting polymerization of 4-vinyl pyridine (4-VP) and then quaternization by n-butyl chloride. The effects of grafting method (simultaneous irradiation, pre-irradiation/UV), grafting conditions (absorbed dose, UV irradiation time, and 4-VP concentration) and quaternization conditions (temperature, time, and concentration of n-butyl chloride) were investigated. It was found that, the grafting degree initially increases with the absorbed dose and then reaches a plateau. The optimal concentration of 4-VP is around 15 wt.%. The ion exchange capacity increases with quaternization temperature, time, and concentration of n-butyl chloride. After modification, the pores size, permeation flux, as well as elongation at break, of quaternized membrane decreases, while the retention coefficient, tensile strength and Young’s modulus increase apparently. Upon contacting with the membranes, the Escherichia coli concentration decreases gradually. It is the adsorption mechanism for the pristine membranes whereas contact-killing mechanism for the quaternized membranes. Simultaneous irradiation is more effective than pre-irradiation/UV in the improvement of antibacterial property of membranes.     

Ferroelectric hysteresis behavior and dielectric properties of 1–3 lead zirconate titanate–cement composites

Lead zirconate titanate (PZT) ceramic was mixed with Portland cement (PC) to form 1–3 connectivity PZT–PC composite using a dice-and-fill technique. Ferroelectric hysteresis behavior and dielectric properties of these composites were investigated using PZT volume content of 60%, 70% and 80%. The results showed that the dielectric constant of the composite materials increased with PZT content and the dielectric constant (?_r) value is 781 for 80% PZT composite at 1 kHz. The dielectric loss tangent (tan δ) was found to decrease with increasing PZT content and the tan δ value of 80% PZT composite is 0.06. Parallel and series models were also compared to the dielectric measurement results. For the hysteresis measurements, the ferroelectric hysteresis loops can be seen for all composites. The “instantaneous” remnant polarization (P_ir) was found to increase with increasing PZT content from 3.20 to 4.28 μC/cm² at 90 Hz when PZT volume content used was 60% and 80% respectively.     

β-Polymorph enhancement in poly(vinylidene fluoride) by blending with polyamide 6 and barium titanate nanoparticles

Polyamide 6 (PA6) as a cost-effective polar polymer and barium titanate (BT) as piezoelectric ceramic nanofiller were melt compounded with poly(vinylidene fluoride) (PVDF) matrix to enhance the β electroactive phase. A series of samples with two blending ratios of PVDF/PA6 (90/10 and 70/30 [wt%/wt%]) each containing 1, 3, and 5 wt% of BT were prepared. The SEM results revealed that the addition of BT to the neat blends refined the biphasic morphology which is mainly due to selective localization of BTs in PA6 dispersed phase as confirmed by TEM observation and wetting parameter predictions. The EDX analysis demonstrated a uniform distribution of BT nanoparticles in the filled systems. FTIR and XRD results showed that β content increased as a result of blending while the α phase was suppressed. The BT nanoparticles inclusion to the blends showed a synergistic effect on the β-polymorph content. These results in combination with the data derived from DSC (indicating reduction of the total crystallinity) complement the idea of β enhancement by the addition of BT nanoparticles and PA6 into PVDF.     

Fabrication of dual-layer poly(styrene-b-4-vinyl pyridine)–poly(vinylidene fluoride) membranes with tailored pore sizes under 10 nm via surface quaternization

Block copolymer membranes can be applied to precise size-based separation because of their highly ordered surface morphology and adjustable pore sizes. However, there is a lower limit of the scale of block copolymer self-assembly; this makes it a challenge to tailor the pore size down to below 10 nm. In this study, poly(styrene-b-4-vinyl pyridine) membranes were modified to quaternized selective layers with pores smaller than 10 nm and were supported by poly(vinylidene fluoride) hollow fibers as the substrate to provide a high mechanical strength. Two reactants, methyl iodide and 2-chloroacetamide, were used in the quaternization. With this one-step chemical modification, the molecular weight cutoff was reduced from 190 to 8 kDa, and the surface pore sizes were narrowed down from 20–30 to 3 nm; this bridged the gap of tailored pore sizes down to below 10 nm. Such membranes are promising candidates for low-molecular-weight separation with high resolution. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47137.     

The effect of fibre concentration on the α to β-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites

Vapour growth carbon nanofibres/poly(vinilidene fluoride) - VGCNF/PVDF - composites prepared by solution casting were studied. The spherulitic crystallisation morphology of the pure polymer is maintained for the composites. Mechanical stretching of the composite films induces the α to β-phase transformation within the polymer matrix. This phase transition is accompanied by the destruction of the spherulitic microstructure in favour of a microfibrillar one. The incorporation of the VGCNF in the PVDF matrix increases the degree of crystallinity of the polymer composites for concentrations lower than ∼1%, remaining stable for higher VGCNF concentrations. With respect to the electrical properties, the stretching associated to the phase transformation induces a change in the conduction mechanism: the α-phase composite demonstrates a percolative behaviour on the measured conductivity whereas the β-phase demonstrates typical ionic conduction behaviour. Dielectric measurements in conjunction with the the two exponent percolation phenomenological equation demonstrates that in the β-phase an effective reduction in the ratio VGCNF length/domain length could induce the observed percolation behaviour.     

The processing and properties of (Zr,Hf)B2–SiC nanostructured composites

(Zr, Hf)B₂–SiC nanostructured composites were fabricated by high energy ball milling and reactive spark plasma sintering (RSPS) of HfB₂, ZrSi₂, B₄C and C. Highly dense composites with homogeneously intermixed ultra-fine (Zr, Hf)B₂ and SiC grains (100–300 nm) were obtained after RSPS at 1600 °C for 10 min. The densification was promoted by high energy ball milling and ZrSi₂ additive. The additives were almost completely transformed into ZrB₂ and SiC during densification. The improvement of flexural strength and fracture toughness (641 MPa and 5.36 MPa m^1/2, respectively) was achieved. The relationships between the ultra-fine microstructure and mechanical properties were discussed.     

Structure,relaxation behaviors and nonlinear dielectric properties of BaTiO3–Bi(Ti0.5Mg0.5)O3 ceramics

(1−x)BaTiO₃–xBi(Mg_1/2Ti_1/2)O₃ (BT–BMT, x=0–0.2, abbreviated as BT–BMT100x) ceramics were prepared by using a solid state reaction process. Their crystal structure, microstructure, conduction behavior, dielectric and tunability properties were investigated. It is found that the tetragonal phase and a pseudocubic phase coexist for x≤0.15 and transform to a pseudocubic phase at x=0.20. With the incorporation of BMT, BT–BMT becomes more insulating. The activation energies of the conduction are respectively 1.15(1) and 1.54(1) eV for grain and grain boundary of BT–BMT20. Furthermore, an abnormal nonlinear dielectric tunable behavior is observed. The dielectric permittivity first slightly increases until reaching the threshold electric field, and then suddenly decreases. This abnormal nonlinear dielectric behavior is attributed to the synergetic effects of the clamped oxygen vacancies and excessive aggregation of Bi at the grain boundaries.     

Effect of WO3 doping on dielectric and ferroelectric properties of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics

WO₃(0–6 mol%)-doped 0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃ lead-free ceramics were synthesized by conventional solid-state reaction. The effect of WO₃ addition on the structure and electrical properties were investigated. The result revealed that a small amount of WO₃ (≤1 mol%) can diffuse into the lattice and does not significantly affect the phase structure, however, more addition will result in distortion and enlargement of the unit cells. The maximum permittivity temperature (T_m) is suppressed dramatically as the dopant increasing, while the depolarization temperature (T_d) fall to the minimum with 1 mol% WO₃ additive. The remanent polarization (P_r) was enhanced and coercive field (E_c) was reduced by doping with WO₃. The strain shows the largest value for 1 mol% doped sample, which is due to a field-induced antiferroelectric–ferroelectric phase transition.