• Electromagnetic wave absorption properties of ternary poly (vinylidene fluoride)/magnetite nanocomposites with carbon nanotubes and graphene

      Tsonos, Christos; Soin, Navneet; Tomara, Georgia N.; Yang, Bin; Psarras, Georgios C.; Kanapitsas, Athanasios; Siores, Elias; University of Chester (Royal Society of Chemistry, 2015-12-21)
      Ternary nanocomposite systems of poly(vinylidene fluoride)/magnetite/carbon nanotube (PVDF/Fe3O4/CNT) and poly(vinylidene fluoride)/magnetite/graphene (PVDF/Fe3O4/GN), were prepared using high shear twin screw compounding followed by compression moulding. The electromagnetic (EM) microwave absorption properties of the nanocomposites were investigated in the frequency range of 3–10 GHz. PVDF/Fe3O4/CNT samples with the thickness d = 0.7 mm present a minimum reflection loss (RL) of −28.8 dB at 5.6 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB. PVDF/Fe3O4/GN with a thickness of 0.9 mm, presents a minimum RL of −22.6 dB at 5.4 GHz, while all the RL values in the measurement frequency range 3–10 GHz are lower than −10 dB as well. The excellent microwave absorption properties of both nanocomposites, in terms of minimum RL value and broad absorption bandwidth, are mainly due to the enhanced magnetic losses. The results indicate that the ternary nanocomposites studied here, can be used as an attractive candidate for EM absorption materials in diverse fields of various technological applications, not only in the frequency range 3–10 GHz, but also at frequencies <3 GHz for PVDF/Fe3O4/CNT and >10 GHz for PVDF/Fe3O4/GN with a realistic thickness of close to 1 mm.
    • Multiscale Understanding of Electric Polarization in Poly(vinylidene fluoride)-Based Ferroelectric Polymers

      Yang, Bin; Meng, Nan; Xintong, Ren; Zhu, Xiaojing; Wu, Jiyue; Gao, Feng; Zhang, Han; Liao, Yaozu; Bilotti, Emiliano; Reece, Michael; et al.
      Poly(vinylidene fluoride) (PVDF) and PVDF-based copolymers with trifluoroethylene (PVDF-TrFE) have attracted considerable academic and industrial interest due to their ferroelectric properties, which are only presented in very few polymers. However, the underlying fundamentals of molecular ordering and induced polarizations are complex and not fully understood. Herein, PVDF, PVDF-TrFE and their blends, prepared using melt extrusion and hot pressing, have been selected to obtain controlled case studies with well-defined chain ordering and microstructures. Impedance analysis and terahertz time-domain spectroscopy are exploited to investigate electric polarization in PVDF-based polymers at different length scales. The extruded ferroelectric films show in-plane chain orientation and higher domain wall density compared to hot pressed films with randomly-distributed polymer chains, which favors the polarization at low frequencies (Hz to MHz), as concluded from the higher dielectric constants and more prominent high electric field polarization switching features. However, the domain walls cannot respond at high frequencies, which leads to lower dielectric constants in the extruded films at THz frequencies.
    • Probing NaCl hydrate formation from aqueous solutions by Terahertz Time-Domain Spectroscopy

      Yang, Bin; University of Chester
      The cooling-induced formation of hydrate in aqueous NaCl solutions was probed using terahertz time-domain spectroscopy (THz-TDS). It was found that the NaCl hydrate formation is accompanied with emergence of four new absorption peaks at 1.60, 2.43, 3.34 and 3.78 THz. Combining the X-ray diffraction measurement with the solid-state based density functional theory (DFT) calculations, we assign the observed terahertz absorption peaks to the vibrational modes of the formed NaCl⋅2H2O hydrate during cooling. This work dedicates THz-TDS based analysis great potential in studying ionic hydrate and the newly revealed collective vibrational modes could be the sensitive indicators to achieve quantitative analysis in phase transitions and lattice dynamics.
    • Structure and dielectric properties of double A-site doped bismuth sodium titanate relaxor ferroelectrics for high power energy storage applications

      Yang, Bin; Zhang, Hangfeng; Fortes, Dominic; Yan, Haixue; Abrahams, Isaac; University of Chester; Queen Mary University of London; Rutherford Appleton Laboratory
      The structural and dielectric properties of barium/strontium substituted Bi0.5Na0.5TiO3 were examined in compositions of general formula (Ba0.4Sr0.6TiO3)x(Bi0.5Na0.5TiO3)1-x. An average classic cubic perovskite structure is maintained from x = 0.5 to 1.0. The temperature dependence of dielectric properties of studied compositions shows relaxor-ferroelectric behaviour attributed to the existence of polar nano-regions. Ferroelectric measurements under variable temperature demonstrated two transitions from normal ferroelectric to relaxor-ferroelectric and relaxor-ferroelectric to paraelectric, at the dipole freezing temperature, Tf, and temperature of maximum permittivity, Tm, respectively. The obtained value of Tf coincides with the onset of linear thermal expansion of the cubic unit cell parameter obtained from high resolution powder neutron diffraction data. Careful analysis of the neutron diffraction data revealed no significant change in the average cubic structure from -263 to 150 C. However, changes in the Gaussian variance component of the neutron peak shape, reveal three distinct regions with transitions at about -100 and 100 C corresponding to the beginning and end of the dielectric dispersion seen in the permittivity and loss spectra. This variation in the Gaussian variance parameter is attributed to the activity of the polar nano-regions. The composition (Ba0.4Sr0.6)0.5(Bi0.5Na0.5)0.5TiO3 was found to exhibit the maximum recoverable energy storage density, with a value of 1.618 J cm-3 and 76.9% storage efficiency at a field of 17 kV mm-1.