Dielectric,magnetoelectric and magnetodielectric properties in CMFO-SBN composites

The paper reports synthesis of Sr_0.5Ba_0.5Nb₂O₆ (SBN) and Co_1.2−xMn_xFe_1.8O₄ (CMFO) via ceramic and hydroxide co-precipitation routes respectively. The nanopowders of SBN-CMFO0.1 (MSBN0.1) and SBN-CMFO0.3 (MSBN0.3) are compacted to form the desired magnetoelectric (ME)/magnetodielectric (MD) composites. The Bi₂O₃ is used as a sintering aid. The Bi₂O₃ at three weight percent is observed to cause agglomeration of SBN and CMFO particles and improve the magneto-mechanical coupling. The composites are investigated for their ferroelectric, ferromagnetic, dielectric, magnetoelectric (ME) and magnetodielectric (MD) properties. The results on the magnetocapacitance (M_c) are observed interesting and could be correctly understood in terms of the stress induced variation in the dielectric constant. The MC is observed to remain fairly constant between 10 to 500 kHz and possess a useful magnitude of M_c nearly 4%.     

Dielectric and mechanical properties of rubber ferrite composites

Abstract

Rubber ferrite composites (RFCs) containing powdered nickel zinc ferrite (Ni_{1 - x}Zn_xFe₂O₄ ) in a natural rubber matrix have been prepared and their mechanical and dielectric properties have been evaluated. Variations in the relative permittivity of both the ferrite ceramics and RFCs have been studied over a range of frequencies, ceramic compositions, ceramic filler loadings, and temperatures, and the results have been correlated. Appropriate mixture equations have been formulated to calculate the dielectric permittivity of the composite from the dielectric permittivity of its constituents. Values calculated using these equations have been compared with experimental data on relative permittivity, and the two have been found to be in good agreement. In the present investigation it was also observed that for x = 0 4 and for the maximum ferrite loading, the composite sample exhibits maximum magnetisation and optimum flexibility.     

Dielectric relaxation studies in some Polymer-PZT composites

Composites of PZT with different polymeric compositions viz. Vinyl Acetate-Vinyl Chloride (VA-VC) co-polymer, Amide cured Epoxy (ACE), Nitrile Rubber-Poly Vnyl Chloride (NR-PVC) blend and piezoelectric poly Vinyledene Fluoride (PVDF) were prepared and the dielectric properties were studied. New dielectric relaxation peaks were observed in the VA-VC and PVDF containing composites owing to the interaction of the molecular motion of the polymer chains with PZT.     

Magnetic and magnetoelectric properties of nickel ferrite–lead iron niobate relaxor composites

Magnetoelectric effect in bulk ceramic and multilayer (laminated) structures consisting of 6 nickel ferrite and 7 lead iron niobate relaxor (PFN) layers was investigated. This paper describes the synthesis and tape-casting process for ferrimagnetic Ni_0.3Zn_0.62Cu_0.08Fe₂O₄ ferrite and multiferroic relaxor Pb(Fe_0.5Nb_0.5)O₃. X-ray analysis and studies of the electrical and magnetic properties were performed for bulk and layered composites. Complex impedance and dielectric permittivity of bulk and layered composites were studied in a temperature range from ?40 to 300 °C and a frequency range of 10 Hz to 2 MHz. Magnetic hysteresis, ZFC–FC curves and dependencies of magnetization versus temperature for nickel ferrite, PFN relaxor and magnetoelectric composites were measured with a vibrating sample magnetometer (VSM) in an applied magnetic field up to 85 kOe at ?269 °C. Magnetoelectric effect at room temperature was investigated as a function of static magnetic field (0.3–6.5 kOe).     

Dielectric and magnetic properties of random and segregated ferrite polystyrene composites

The dielectric and magnetic properties of polystyrene composites containing barium or nickel-zinc ferrites were studied as function of the ferrite concentration and field frequency. The composites were prepared by methods yielding a random distribution of the ferrite particles or segregated structures. Barium ferrite-poly-styrene composites exhibited a typical insulator behavior, and only above 60% ferrite were high values of the dielectric properties noted at the lower frequencies, decreasing gradually with frequency to the low values typical of the higher frequencies. The mode of barium ferrite particle distribution did not affect the dielectric properties. The nickel-zinc ferrite systems demonstrated a conductor type behavior. An apparent insulator-conductor transition was observed, having lower values for segregated than for random distributions. The magnetic permability of barium ferrite-polystyrene composites above 10% ferrite increases with the ferrite concentration, whereas the magnetic dissipation factor steeply increases with concentration above 40% ferrite.     

Dielectric relaxation studies of low thermal expansion polymer composites

The results of temperature‐dependent dielectric and rheological measurements are reported on polymer‐ceramic composite films, poly(methyl methacrylate) (PMMA) : lead titanate (PbTiO₃). Analyses of relaxational processes of the PMMA host matrix have been investigated using temperature‐dependent dielectric and rheological measurements. It is found that the α‐relaxation is more significantly affected by the addition of filler in comparison to β‐relaxation. The composite films are found to have much lower dielectric constants in comparison to the pure ceramic material. Suitable models have been used to explain the observed dielectric constant of the composite films. Using rheological measurements, occurrence of reinforcement in these composite films due to the addition of ceramic filler has also been observed and the results are discussed in the article. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Dielectric relaxation and ac conductivity in magnetoelectric YCrO3 ceramics: A temperature dependent impedance spectroscopy analysis

Here, we report on the temperature and frequency dependent electrical conduction and dielectric behaviour of YCrO₃ ceramics. Dielectric studies reveal a peak in the dielectric constant ～230?K, suggesting presence of spin-charge coupling. Also, an additional broad peak found at ～450?K is reminiscent of a relaxor like behaviour for YCrO₃, attributed to a diffused phase transition. The nature of dc conductivity is of Arrhenius type and shows an abrupt change in the activation energy at ～230?K and ～450?K. The activation energy suggests that the polaronic hopping mechanism stabilizes at low temperature while, at higher temperatures, the process is associated with the diffusion of double ionized oxygen vacancies. However, ac conductivity suggests that the overlapping large polaron tunnelling conduction mechanism drives the ac conduction below 300?K and above 300?K, the conduction behaviour is consistent with the correlated barrier hopping conduction mechanism.     

Dielectric relaxation dynamics in liquid crystal – dye composites

Dielectric relaxation spectroscopic studies were undertaken to investigate the role of the resistivity of a host liquid crystal in the relaxation dynamics of dyed systems. Two different types of liquid crystal mixture were used, namely, one that was characterized by high ion contamination and low resistivity and the other which was of high resistivity and virtual ion-free; both types were doped using an azo or anthraquinone dye. Dielectric spectra of both the pristine liquid crystals and their composites at different external fields were measured in homogenously-aligned cells. The use of both dyes on an ion-enriched liquid crystal not only enhanced ion concentration but also slowed down the relaxation process. In contrast, the relaxation process for composites with a high-resistivity liquid crystal host was faster than that of the pristine liquid crystal.     

Dielectric relaxation induced internal electric field and its effect on magnetoelectric state in Co4Nb2O9

Pyrocurrent and dielectric properties of magnetoelectric antiferromagnet Co₄Nb₂O₉ have been investigated. The sample shows 2 thermally activated dielectric relaxations in the temperature range far above Néel temperature with the low‐ and high‐temperature ones being a dipolar and Maxwell‐Wagner relaxations, respectively. Two types of dipoles: relaxing dipoles and magnetic‐field‐induced dipoles were found in the sample. An internal electric field is formed due to the ordering of relaxing dipoles. Three current peaks were observed. The 2 higher temperature current peaks correspond to the dielectric relaxations. The lowest temperature current peak contains a positive tip and negative dip. The tip and dip are related to the depolarization of ferroelectricity induced by external and internal fields, respectively.     

Dielectric and dynamic mechanical relaxation behavior of exfoliated nano graphite reinforced flouroelastomer composites

Dynamic mechanical analysis and dielectric relaxation spectra of exfoliated nano graphite reinforced flouroelastomer composites were used to study their relaxation behavior as a function of temperature (−80°C to +40°C) and frequency (0.01 to 10⁵ Hz). The effect of filler loadings on glass transition temperature was marginal for all the composites and T_g value was in the narrow range of 7.8–8.4°C, which has been explained on the basis of relaxation dynamics of polymer chains in the vicinity of fillers. Strain‐dependent dynamical parameters were evaluated at dynamic strain amplitudes of 0.01–10%. The nonlinearity in storage modulus has been explained on the concept of filler‐polymer interaction and filler aggregation of the nano graphite platelets. The variation in real and complex part of impedance with frequency has been studied as a function of filler. The percolation of the nano graphite as studied by conductivity measurements is also reported. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers.     

Dielectric relaxation study of poly(styrene-co-4-vinylbenzoic acid)/poly(ethyl methacrylate-co-4-vinylpyridine) blends

Summary The α-relaxations, located in the neighborhood of glass-transition region (Tg), of a styrene-co-4-vinylbenzoic acid (PSVBA) random copolymer blended in several weight proportions with an ethyl methacrylate-co-4-vinylpyridine (PEMAVP) random copolymer have been investigated by dielectric spectroscopy in the temperature and frequency ranging from 25 to 180 °C and 0.1 to 100 kHz, respectively. The analysis of the obtained results shows that the dielectric spectra of the miscible blends, essentially dominated by the response of PEMAVP, are composition dependent becoming broader with its content. These observations are correlated with segmental dynamic heterogeneities and blend concentration fluctuations. Received: 21 December 1998/Revised version: 21 March 1999/Accepted: 20 April 1999     

A sintering strategy for lithium zinc ferrite with a high saturation magnetization and low ferromagnetic resonance line-width

Based on the activation energy and diffusion kinetics theory of grain growth, Li_0.42Zn_0.28Ti_0.1Fe_2.2O₄ ceramics with a low ferromagnetic resonance line-width (ΔH) and high saturation magnetization (Ms) were synthesized by adopting LTCC (low-temperature cofired ceramics) technology. The critical sintering temperature of ferrite synthesis was reduced to 925 °C due to activation energy reduction and the liquid phase sintering mechanism of Bi₂O₃. The sintering agent B₂O₃ further improved the grain size, homogeneity, density and properties. EDS and XRD refinement showed that Bi³⁺ and B³⁺ ions did not enter lattices, but Ti⁴⁺ ions entered lattices and replaced part of the Fe³⁺ ions, leading to the lattice expansion. Finally, homogeneous and compact Li_0.42Zn_0.28Ti_0.1Fe_2.2O₄ ferrite with Ms up to 354.6 kA/m and ΔH as low as 184.2 Oe was synthesized at temperatures as low as 925 °C by adding an appropriate content of Bi₂O₃ and B₂O₃. In the present study, the exploration and practice of reducing the sintering temperature and improving the material properties based on sintering agents is a beneficial supplement and improvement to the wider application of the LTCC technique.     

Dielectric relaxation in poly(1,4-phenylene terephthalamide)

The dielectric behavior of poly(1,4-phenylene terephthalamide) is reported in this paper as a function of frequency (400 Hz–100 kHz), temperature (30–290°C), and the degree of polymerization (134–228). The general behavior of all the samples of different degrees of polymerization is the same except for the occurrence of the peaks at different positions on the frequency and temperature scales. This difference in behavior could be because of the combined effect of the difference in molecular weights and packing of molecules. The data are completely analyzed for the sample with degree of polymerization n = 210. Two distinct processes are observed, one being observed in the entire temperature range, another orginating in the vicinity of glass transition temperature. The former is analyzed in terms of Cole–Cole distribution and the latter is conformed with a nonexponential decay function. Activation energies are calculated from plots of I/T vs. In τ.     

Dielectric relaxation in epoxy-polyglycol elastomers

Dielectric relaxation data is reported on a series of elastomers formed between bisphenol A and either polyethylene glycol or polypropylene glycol. The observed dispersion may be ascribed to relaxation of the “soft” block and is found to vary with both change in the glycol block length and with the ratio of bisphenol A to polyglycol. Elastomers prepared with a poly-glycol component of mixed composition allowed the relaxation frequency at a particular temperature to be shifted over a frequency range of almost three decades. Activation parameters obtained from the temperature dependence of the mean relaxation frequency are used in evaluation of the factors influencing the molecular motion of the “soft” block.     

Controlling magnetoelectric coupling by nanoscale phase transformation in strain engineered bismuth ferrite

The magnetoelectric coupling in multiferroic materials is promising for a wide range of applications, yet manipulating magnetic ordering by electric field proves elusive to obtain and difficult to control. In this paper, we explore the prospect of controlling magnetic ordering in misfit strained bismuth ferrite (BiFeO(3), BFO) films, combining theoretical analysis, numerical simulations, and experimental characterizations. Electric field induced transformation from a tetragonal phase to a distorted rhombohedral one in strain engineered BFO films has been identified by thermodynamic analysis, and realized by scanning probe microscopy (SPM) experiment. By breaking the rotational symmetry of a tip-induced electric field as suggested by phase field simulation, the morphology of distorted rhombohedral variants has been delicately controlled and regulated. Such capabilities enable nanoscale control of magnetoelectric coupling in strain engineered BFO films that is difficult to achieve otherwise, as demonstrated by phase field simulations.     

Dielectric relaxation spectroscopy of poly(ethylene terephthalate)

Contour maps of dielectric loss tangent within the ranges 0.1 Hz to 3 MHz and ?175 °C to +190 °C are presented for a commercial poly(ethylene terephthalate) (PET) in two initial states of crystallinity. Individual absorption regions resemble those for poly(butylene terephthalate) and are attributed to carbonyl‐driven α‐ and β‐relaxation processes and to Maxwell–Wagner–Sillars polarizations. Possible causes are considered for the asymmetry and structure apparent in the α‐peak of partially crystalline PET. © 2001 Society of Chemical Industry     

Molecular orientation and relaxation in poly(butylene terephthalate)/polycarbonate blends

Rheo-optical Fourier-transform infrared (FTIR) spectroscopy is based on the simultaneous acquisition of stress-strain data and FTIR spectra on-line to the mechanical treatment of polymers and is frequently applied for the characterization of transient structural changes during deformation and stress-relaxation. In the present communication, this technique has been employed in order to investigate the distribution of molecular orientation and its relaxation in uniaxially drawn solution-cast films of semicrystalline partial miscible blends of poly(butylene terephthalate) (PBT) with polycarbonate (PC) containing 10, 30 and 50 wt% PC. The uniaxial deformation of these blend films having a PBT-crystallinity degree ranging from 31 to 12%, in unstretched blends, leads to a appreciable high segmental orientation for the crystalline PBT due to a structural transformation from lamellae to microfibrils. The formation of this fibrillar structure is attributed to non-reversibility of an extended phase with all-trans conformational sequence of the aliphatic segments of PBT, occurring during elongation. The rate of relaxation of this conformational transition, however, increases with increasing amorphous content in the blends. Also it is observed that even with increasing amorphous content in the PBT/PC blends the crystalline PBT shows significant orientation. In such cases, apart from the few lamellae which transform to microfibrils, it is suggested that a stress induced transformation of PBT chains in amorphous PBT-component to irreversible all-trans extended crystalline form also contributes to PBT crystalline orientation. In contrast with this high crystalline orientation, the amorphous PBT located in the interlamellar regions inside the PBT-spherulites show a lower orientation in blends as compared in pure PBT.On the other hand, an overall segmental orientation of PC chains in blends is of lower order which is attributed mainly to low stretching temperature compared to T_g of pure PC. The results are discussed in terms of the resulting spherulitic morphology and the temporary network formed by the elongated PBT and PC chains inside the interlamellar regions, in blends.