High Piezoelectric Performance in Undoped (K,Na)NbO3 Piezoceramics over a Wide Range of Compositions.

In a previous study, an optimized low-temperature (LT) sintering process for the preparation of high-performance undoped (K,Na)NbO3 (KNN) ceramics with high density, high reproducibility, and high chemical stability was established for the K = 50% composition. In the current work, this optimized process is applied to other stoichiometries ranging from K = 20% to 90%, aiming at gaining more insights into the stoichiometry dependence of piezoelectric properties. Alike the case of K = 50%, fast melt-quenching and preannealing of calcined raw materials lead to high-crystallinity single-phase powders without parasitic phases regardless of composition. Grain growth upon recrystallization after pulverization can be seen to depend on the composition and recrystallization annealing temperature, which is also reflected in the microstructure of ceramics showing smaller grain sizes and piezodomains in high K-rich stoichiometries. After LT spark-plasma sintering of powders, high-density ceramics with high and stable properties [d33 ∼ (140 to 150) pC/N; kp and kt ∼ (40 to 45)%] are obtained over a wide range of middle stoichiometries. Such piezoelectric results contrast with the general assumption of higher piezoelectric performance around K = 50%, where two or more phases (orthorhombic and monoclinic) are supposed to coexist, like in the case of standard Pb(Zr,Ti)O3 ceramics. Here, it is demonstrated that the best properties are found within the orthorhombic KNN phase for K ≥ 40%. Therefore, this work demonstrates that at present, the main factor for the achievement of high-performance undoped KNN ceramics is not the stoichiometry, but rather the preparation process.

Thermoelectric Performance of Cr Doped and Cr-Fe Double-Doped Higher Manganese Silicides with Adjusted Carrier Concentration and Significant Electron-Phonon Interaction.

Polycrystalline higher manganese silicides Mn1-xCrxSi1.74 (x = 0, 0.10, 0.20) with Cr single doping and Mn1-2yCryFeySi1.74 (y = 0.10, 0.20) with Cr-Fe double doping have been prepared by arc melting and spark plasma sintering. Hall effect results and thermoelectric transport properties measurements demonstrate that Cr doping effectively increases the carrier concentration, thereby giving rise to enhanced electrical conductivity and power factor. Coupled with an enlarged effective mass and a reduction in the lattice thermal conductivity, a maximum zT is realized in Mn0.90Cr0.10Si1.74. It is also proved that the carrier concentration and carrier scattering mechanism could be altered through further doping on the Mn site by Fe, which leads to a lower electrical conductivity and higher Seebeck coefficient. Factors related to the suppression of the lattice thermal conductivity, like mass and strain field fluctuation scattering and electron-phonon scattering, are also analyzed. This work reveals the effects of Cr single doping and Cr-Fe dual-element doping on the carrier concentration, carrier scattering mechanism, and lattice thermal conductivity of higher manganese silicides.

Self-passivated ultra-thin SnS layers via mechanical exfoliation and post-oxidation.

Remarkable optical/electrical features are expected in two-dimensional group-IV monochalcogenides (MXs; M = Sn/Ge and X = S/Se) with a uniquely distorted layered structure. The lone pair electrons in the group-IV atoms are the origin of this structural distortion, while they also cause a strong interlayer force and high chemical reactivity. The fabrication of chemically stable few-to-monolayer MX has been a significant challenge. We have observed that, once the SnS surface is oxidized, the SnOx top layer works as a passivation layer for the SnS layer underneath. In this work, the SnOx/SnS hetero-structure is studied structurally, optically, and electrically. When tape-exfoliated bulk SnS is oxygen-annealed under a reduced pressure at 10 Pa, surface oxidation and SnS sublimation proceed simultaneously, resulting in a monolayer-thick SnS layer with the SnOx passivation layer. The field-effect transistor of nine-layer SnS prepared via mechanical exfoliation exhibits a p-type characteristic because of intrinsic Sn vacancies, whereas ambipolar behavior is observed for the monolayer-thick SnS obtained via oxygen annealing probably owing to the additional n-type doping by S vacancies. This work on monolayer-thick SnS fabrication can be applied to other unstable lone pair analogues and can facilitate future research on MXs.

Multifunctional Superelastic Foam-Like Boron Nitride Nanotubular Cellular-Network Architectures.

Construction of cellular architectures has been expected to enhance materials' mechanical tolerance and to stimulate and broaden their efficient utilizations in many potential fields. However, hitherto, there have been rather scarce developments in boron nitride (BN)-type cellular architectures because of well-known difficulties in the syntheses of BN-based structures. Herein, cellular-network multifunctional foams made of interconnective nanotubular hexagonal BN (h-BN) architectures are developed using carbothermal reduction-assisted in situ chemical vapor deposition conversion from N-doped tubular graphitic cellular foams. These ultralight, chemically inert, thermally stable, and robust-integrity (supporting about 25,000 times of their own weight) three-dimensional-BN foams exhibit a 98.5% porosity, remarkable shape recovery (even after cycling compressions with 90% deformations), excellent resistance to water intrusion, thermal diffusion stability, and high strength and stiffness. They remarkably reduce the coefficient of thermal expansion and dielectric constant of polymeric poly(methyl methacrylate) composites, greatly contribute to their thermal conductivity improvement, and effectively limit polymeric composite softening at elevated temperatures. The foams also demonstrate high-capacity adsorption-separation and removal ability for a wide range of oils and organic chemicals in oil/water systems and reliable recovery under their cycling usage as organic adsorbers. These created multifunctional foams should be valuable in many high-end practical applications.

Ubiquitous element approach to plasmonic enhanced photocatalytic water splitting: the case of Ti@TiO2 core-shell nanostructure.

We demonstrate a new approach to plasmonic enhanced photocatalytic water splitting by developing a novel core-shell Ti@TiO2 brush nanostructure where an elongated Ti nanorod forms a plasmonic core that concentrates light inside of a nanotubular anodic TiO2 shell. Following the ubiquitous element approach aimed at providing an enhanced device functionality without the usage of noble or rare earth elements, we utilized only inexpensive Ti to create a complex Ti@TiO2 nanostructure with an enhanced UV and Vis photocatalytic activity that emerges from the interplay between the surface plasmon resonance in the Ti core, Vis light absorption in the Ti-rich oxide layer at the Ti/TiO2 interface and UV light absorption in the nanotubular TiO2 shell.

Nanostructured WO3 /BiVO4 photoanodes for efficient photoelectrochemical water splitting.

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.

UV-visible Faraday rotators based on rare-earth fluoride single crystals: LiREF4 (RE = Tb, Dy, Ho, Er and Yb), PrF3 and CeF3.

High optical quality LiREF(4) (RE = Tb(3+), Dy(3+), Ho(3+), Er(3+) and Yb(3+)), PrF(3) and CeF(3) single crystals have been grown by the Czochralski technique. Their magneto-optical properties have been measured and analyzed in detail in the ultraviolet-visible wavelength region, and their figures of merit as Faraday rotators have been determined. CeF(3) presents superior properties above 300 nm, showing a figure of merit higher than that of the reference material, terbium-gallium-garnet, which is nowadays used in the visible-near infrared. PrF(3) is the best rotator for the 220-300 nm range. Towards shorter wavelength and in the vacuum ultraviolet, it is shown that the LiREF(4) crystals are unique rotators. Overall, the rare-earth fluoride single crystals studied here exhibit better properties than other materials considered so far, and therefore they have potential to cover the increasing demand for new and improved Faraday rotators in the ultraviolet-visible wavelength region.

Gadolinium ytterbium trifluoride, Gd0.81Yb0.19F3.

A new gadolinium ytterbium trifluoride has been grown for the first time by the Czochralski technique. Although GdF(3) and YbF(3) both present a high-temperature phase transition, the mixed compound Gd(0.81)Yb(0.19)F(3) maintains its crystallographic structure upon cooling to room temperature. Taking into account that both Gd(3+) and Yb(3+) ions are distributed randomly on a single site (Wyckoff position 4c), this is attributed to a mean cationic radius coincident with that of the Tb(3+) ion, so that the stability of the crystal structure resembles that of TbF(3). The grown crystal melts noncongruently at ∼1413 K, it is transparent and colourless, and it has a high density.

Birefringent- and quasi phase-matching with BaMgF4 for vacuum-UV/UV and mid-IR all solid-state lasers.

BaMgF(4) is a ferroelectric fluoride which shows a very wide transparency range extending from 125 nm to 13 microm. The conjunction of these properties confers to BaMgF(4) a unique chance for optical applications in the UV and mid-IR wavelength regions, where other nonlinear materials cannot be used. In particular its application as frequency converter in all solid-state lasers is considered. The wavelength dispersion of the refractive indices along the three optical principal axes is measured in the transparent region, and the Sellmeier coefficients for the three refractive indices are determined. The conditions for nonlinear optical processes are calculated for birefringent-matching and quasi phase-matching, with special emphasis in the UV and IR wavelength regions. Quasi phase-matching can be achieved in the whole transparent wavelength region, in contrast to birefringent-matching, which can be obtained in a limited range 573-5634 nm. First demonstration of second harmonic generation by quasi phase-matching with a ferroelectric fluoride is shown by frequency-doubling the emissions of a 1064 nm Nd:YAG laser and a tunable Ti:sapphire laser. The shortest emission is obtained in the UV at 368 nm, indicating the potential of BaMgF(4) as nonlinear medium for the fabrication of all solid-state lasers in the vacuum-UV/UV and mid-IR wavelength regions.

High-energy, all-solid-state, ultraviolet laser power-amplifier module design and its output-energy scaling principle.

We demonstrate that a coaxially pumped, large-aperture ultraviolet power-amplifier module with solid-state tunable laser medium Ce(3+):LiCaAlF6 has 98-mJ, 290-nm, and 3-ns output pulses with a sufficient extraction efficiency of 25%. The detailed information of design parameters, including the gain-coefficient dependence on pump condition, is successfully accumulated for further energy scaling for a terawatt-class ultraviolet chirped pulse amplification laser system or a high-pulse-energy laser system.