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Electrostriction is the basis of electromechanical coupling in all insulators. The quadratic electrostrictive strain x ij associated with induced polarization components Pk and P l is given by x ij = Q ijkl P k P l . Two converse electrostrictive effects may also be defined. In this paper, some trends in structure−property relationships that govern electrostriction are identified, along with the problems that limit our understanding of this fundamental electromechanical property. Electrostrictive coefficients range from the ∼10-3 m4/C2 in relaxor ferroelectrics to ∼103 m4/C2 in some polymers. High-sensitivity techniques, such as interferometry or compressometry, are necessary to accurately measure electrostrictive effects in most insulators. But even in low-K dielectrics, electrostrictive stresses may initiate breakdown in high-field environments such as microelectronic components with small dimensions, high-voltage insulators, or in high-power lasers. In polymeric materials, charge injection mechanisms may produce local electric field concentrations that can cause large electrostrictive strains. The electromechanical properties in polymers have also been observed to vary with the thickness of the specimen. A brief description of the anharmonic nature of electrostriction and its frequency dependence is included.

We report here an all-polymer high-dielectric (dielectric constant K>1000 at 1 kHz) percolative composite material, fabricated by a combination of conductive polyaniline particles (K>105) within a poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer matrix (K>50). These high-K polymer hybrid materials also exhibit high electromechanical responses. For example, 1.5% strain, which is proportional to the square of the field applied, can be induced by a field of 9.5 MV/m, an eightfold reduction in field applied compared with that in a fluoroterpolymer matrix.

Scientists have predicted that carbon's immediate neighbors on the periodic chart, boron and nitrogen, may also form perfect nanotubes, since the advent of carbon nanotubes (CNTs) in 1991. First proposed then synthesized by researchers at UC Berkeley in the mid 1990's, the boron nitride nanotube (BNNT) has proven very difficult to make until now. Herein we provide an update on a catalyst-free method for synthesizing highly crystalline, small diameter BNNTs with a high aspect ratio using a high power laser under a high pressure and high temperature environment first discovered jointly by NASA/NIA/JSA. Progress in purification methods, dispersion studies, BNNT mat and composite formation, and modeling and diagnostics will also be presented. The white BNNTs offer extraordinary properties including neutron radiation shielding, piezoelectricity, thermal oxidative stability (> 800˚C in air), mechanical strength, and toughness. The characteristics of the novel BNNTs and BNNT polymer composites and their potential applications are discussed.

The electromechanical response of a polyurethane elastomer was investigated at room temperature and in the temperature range near its glass transition. It was found that the Maxwell stress contribution to the strain response can be significant at temperatures higher than the glass transition temperature. In addition, the material exhibits a very high electrostrictive coefficient Q, about two orders of magnitude higher than that of polyvinylidene fluoride. It was also found that in a polymeric material, the chain segment motions can be divided into those related to the polarization response and those related to the mechanical response and the overlap between the two yields the electromechanical response of the material. In general, the activation energies for different types of motion can be different, resulting in different relaxation times in the dielectric, the elastic compliance, and the electrostrictive data, as observed in the polyurethane elastomer investigated. The experimental results indicate that at the temperatures investigated, the activation energy for the mechanical related segment motions is higher than that of nonmechanical related segment motions.

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Although Ni-rich layered oxides are considered a candidate of next-generation cathode materials, their inherent properties, such as surface lithium residues and structural destruction, cause detrimental electrochemical performance, especially at elevated temperatures. Here, a facile ball-milling method is proposed to remove the lithium residues and enhance the electrochemical performance of LiNi0.6Co0.2Mn0.2O2. After NH4VO3 treatment, a lithium ion-conductive Li3VO4 coating layer is found on the LiNi0.6Co0.2Mn0.2O2 surface at heat-treatment temperatures of 300 and 450 °C, with a small part of vanadium ions diffusing into the surface lattice. When the temperature surpasses 600 °C, almost all vanadium ions dope into the bulk structure. The complex relationships between the post-sintering temperature and surface structure and their impact on electrochemical properties are discussed in detail. Electrochemical tests show that 0.5 wt% NH4VO3 treated LiNi0.6Co0.2Mn0.2O2 at 450 °C exhibits much improved cycling stability (96.1% cycling retention at 0.5C after 100 cycles and 97.2% after 50 cycles at 55 °C), rate capability (117.0 mA h g-1 at 5C), and storage property (4683 ppm lithium residue amount after storing in air for 7 days). Such superior performance is ascribed to the Li3VO4 coating layer that inhibits the electrolyte decomposition and helps create a stable and thinner cathode-electrolyte interface, resulting in decreased interfacial resistance. In addition, this coating layer suppresses internal micro-stress and phase transformation from a layered to spinel and rock-salt structure, which increases the structural integrity of LiNi0.6Co0.2Mn0.2O2 during repeated charge-discharge cycling.

graphene, however, is problematic for many optoelectronic applications. Semiconducting layered metal dichalcogenides (LMDs) have generated an immense amount of interest, owing to their specific bandgaps in the ≈1-2.4 eV range. [2] Most LMDs undergo a transition from an indirect bandgap material in the multilayer form to a direct bandgap material in the monolayer form and exhibit strong lightmatter interaction at the 2D limit. [3,4] All of these features make 2D LMDs promising materials for several applications, such as transistors [5] and optoelectronic [6] and valleytronic [7] devices. Nonetheless, the finite bandgap of such reported LMDs largely restricts their applications in IR and ultraviolet (UV) photodetection. [8] UV photodetection, particularly in the solar blind range (200 to 280 nm), has great potential in the missile detection, flame alarms, and so forth. However, the lack of wide bandgap LMD materials makes realizing 2D UV photodetectors a challenge.As a new important member of the LMD family, germanium diselenide (GeSe 2 ), with a direct wide bandgap, has many potential applications in UV detection and IR waveguides. [9] Moreover, GeSe 2 exhibits a monoclinic crystal structure, [10] within two crystallographically different types of GeSe 4/2 tetrahedra centered on the Ge1 and Ge2 atoms, as shown in As an important 2D layered metal dichalcogenide, germanium diselenide (GeSe 2 ) with a direct wide bandgap is attracting increasing attention for its potential applications in ultraviolet (UV) detection. However, only few-layer GeSe 2 has been reported to date. Here, a joint theoretical-experimental study on the optical and electronic properties of monolayer GeSe 2 is presented, and monolayer GeSe 2 is shown to have a direct wide bandgap of 2.96 eV. Consequently, monolayer GeSe 2 does not respond to a major fraction of the visible spectrum. Notably, the photofield effect transistors based on the GeSe 2 monolayer show p-type behavior, high responsivity, superior detectivity, and a fast response time, competitive with state-of-the-art UV detectors. In addition to the excellent photoresponse properties, 2D GeSe 2 crystals also exhibit perpendicular optical reversal of the linear dichroism and polarized photodetection under wavelength modulation. Theoretical calculations of the band structure are used to shed light on these experimental results. The findings suggest that 2D GeSe 2 is a promising candidate for highly selective polarization-sensitive UV detection.