BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

Approaches, Derivatives and Applications Vasilios Georgakilas,† Michal Otyepka,‡ Athanasios B. Bourlinos,‡ Vimlesh Chandra, Namdong Kim, K. Christian Kemp, Pavel Hobza,‡,§,⊥ Radek Zboril,*,‡ and Kwang S. Kim* †Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi Attikis, 15310 Athens, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo naḿ. 2, 166 10 Prague 6, Czech Republic

Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications

Multiferroic magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single-phase compounds are rare, and their magnetoelectric responses are either relatively weak or occurs at temperatures too low for practical applications. In contrast, multiferroic composites, which incorporate both ferroelectric and ferri-/ferromagnetic phases, typically yield giant magnetoelectric coupling response above room temperature, which makes them ready for technological applications. This review of mostly recent activities begins with a brief summary of the historical perspective of the multiferroic magnetoelectric composites since its appearance in 1972. In such composites the magnetoelectric effect is generated as a product property of a magnetostrictive and a piezoelectric substance. A...

Multiferroic magnetoelectric composites: Historical perspective, status, and future directions

http://staff.ulsu.ru/moliver/ref/polymers/pott11.pdf

Graphene-based polymer nanocomposites

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) have great potential for applications in micro-electromechanical devices and high-charge storage capacitors. In order to realize these applications, it is highly desirable to substantially improve the dielectric constants of such ferroelectric polymers. Up to now, much work has focused on the preparation of 0–3-type composites based on polymers and ceramics of high dielectric constant, and the resultant composites usually possess a relatively high dielectric permittivity (about 100). Nevertheless, the high volume fraction (> 50 vol%) of ceramics, which was necessary to achieve the high dielectric constants, presents a number of limitations, in terms of high weight, low flexibility, and poor mechanical performance, as a result of the weak matrix-filler bonding and agglomeration of ceramic nanoparticles. Furthermore, most ceramics of high dielectric constant are lead-based, and potentially harmful to our health. To overcome the limitations of ferroelectric polymer/ceramic composites, very promising work has been carried out recently based on percolation theory, in which a small volume fraction of some conductive filler was added to the polymer matrix to achieve a high dielectric constant, thus preserving the mechanical flexibility of the polymer. For example, Dang et al. fabricated a new poly(vinylidene fluoride)/carbon-nanotube composite, with a percolation threshold of 8 vol%, possessing a dielectric constant of 600 (the dielectric loss value, tan d, is about 2) at 1000Hz. For a P(VDF-TrFE-CFE)/carbon-nanotube system, the dielectric constant increased from 57 to 102 (tand! 0.36) at 100Hz, by inclusion of only 2wt% (1.2 vol%) carbon nanotubes. A dielectric constant as high as 56 was observed in a PVDF/acetylene-black composite, when the acetylene-black concentration was in the neighborhood of the percolation threshold (about 1.3 vol%). Recently, Panda et al. reported that, for PVDF/Ni composites, a high effective dielectric constant of 2050 (tand1⁄4 10) at 100Hz was observed near the percolation threshold of 27 vol%. Along this line, in this communication, we propose a novel nanocomposite system consisting of poly(vinylidene fluoride) and exfoliated graphite nanoplates (PVDF/xGnPs). The xGnPs were selected as the conductive filler, because of their good electrical and thermal conductivity, high mechanical strength, and more importantly, large aspect ratio and unique layered structure with nanoscale thickness, which give advantages in the formation of a large number of parallel-board microcapacitors with low filler loading. Moreover, functional groups, such as C–O–C, C–OH, and C––O, existing on the surface of graphite nanoplates, can promote the interaction between the PVDF and the graphite nanoplates, leading to good dispersion of xGnPs in thematrix. It is well known that the homogenous dispersion of conductive fillers in a polymer matrix is a critical factor in achieving a high-performance nanocomposite. Therefore, it was expected that a much lower volume fraction of xGnP in the PVDF/xGnP nanocomposite could result in a greater increase in dielectric permittivity. xGnPs were obtained from subjecting natural graphite flakes to acidic intercalation, rapid thermal treatment, and ultrasonic powdering, in sequence (see S1 in Supporting Information). Natural graphite flakes (see S2 in Supporting Information), a naturally abundant and low-cost carbon-based material, are composed of parallel carbon layers. Carbon atoms within the graphite layers connect to each other to form six-member rings through strong covalent bonds, while the parallel carbon layers are joined together by weak van der Waals force. Such a structure makes it possible to intercalate some small molecules into the interlayer space of graphite. In the present study, natural graphite flakes were first converted to graphite intercalation compounds (GICs), through intercalation and chemical oxidation in the presence of concentrated H2SO4 andHNO3.When heated at high temperatures, because of the volatilization of the mixed acid, the GICs could be expanded up to a few hundred times along the direction perpendicular to the carbon-layer plane of the intercalated graphite, so that expanded graphite (EG), which is a worm-like material (see the scanning electron microscopy (SEM) image, shown in Fig. 1a), could be obtained. It is also important to note that some functional groups could be introduced to the graphite during the preparation of the GICs and EG. After ultrasonic treatment, the graphite worms were fragmented into exfoliated graphite nanoplates with diameters of 0.5–25mm and thicknesses of 20–60 nm (see S3 in Supporting Information), as shown by the SEM image in Figure 1b. C O M M U N IC A TI O N www.advmat.de

High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates

The attachment of Fe3O4 nanoparticles to graphene oxide by covalent bonding

Terfenol-D/epoxy pseudo 1-3 composites were fabricated by embedding and aligning Terfenol-D particles with a size distribution of 10–300μm in a passive epoxy matrix using six Terfenol-D volume fractions (υf) ranging from 0.22 to 0.72. The dependence of the dynamic relative permeability (μr33T), elastic modulus (E3H), and dynamic strain coefficient (d33) on υf was investigated as a function of magnetic bias field (HBias). The HBias response data showed that the built-in non-180° domain states related to residual compressive stresses in the composites result in a significant decrease in μr33T for HBias 0.5. The present study provides a useful guide to optimize the composite properties for transducer design.

/pdf/dynamic-magnetomechanical-properties-of-terfenol-d-epoxy-54djmyezkf.pdf

Dynamic magnetomechanical properties of Terfenol-D/epoxy pseudo 1-3 composites

Piezoelectric and dielectric properties of CeO2-added (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

Ultra-small multifunctional KGdF4:Tm3+,Yb3+ nanoparticles with near-infrared to near-infrared upconversion are synthesized. The average sizes of KGdF4:Tm3+ 2%, Yb3+ 20% core-only and KGdF4:Tm3+ 2%, Yb3+ 20%/KGdF4 core–shell nanoparticles are ∼3.7 nm and ∼7.4 nm, respectively, which fall within the reported optimal sizes (<10 nm) for bioimaging probes. The excitation and emission at 980 and 803 nm are favorable to deeper tissue penetration and reduced autofluorescence. The weak upconversion luminescence of the ultra-small core-only nanoparticles is overcome by the use of the core–shell approach. The magnetic mass susceptibility and magnetization of the ultra-small core-only and core–shell nanoparticles were determined, and are close to those of large nanoparticles (26 and 50 nm) used for magnetic resonance imaging and bio-separation. There is no variation in the magnetic properties with the nanoparticles’ sizes between the core-only (∼3.7 nm) and core–shell (∼7.4 nm) nanoparticles, which differs from the size-dependent luminescence. The oleate-capped core–shell nanoparticles were further encapsulated with a PEG-phospholipid shell to endow them with dispersibility in water, which is indispensable for future biological applications.

Helen Lai Wa Chan

Papers

High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates

The attachment of Fe3O4 nanoparticles to graphene oxide by covalent bonding

Dynamic magnetomechanical properties of Terfenol-D/epoxy pseudo 1-3 composites

Piezoelectric and dielectric properties of CeO2-added (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

Water dispersible ultra-small multifunctional KGdF4:Tm3+, Yb3+ nanoparticles with near-infrared to near-infrared upconversion