Li Chen

Colossal negative thermal expansion with an extended temperature interval covering room temperature in fine-powdered Mn0.98CoGe

MnM′X (M′u2009=u2009Co, Ni; Xu2009=u2009Ge, Si, etc.) alloys usually present a large volumetric change during the Martensitic (MA) transformation. This offers a great opportunity for exploring new negative thermal expansion (NTE) materials if the temperature interval of NTE can be extended. Here, we report colossal NTE in fine-powdered Mn0.98CoGe prepared by repeated thermal cycling (TC) through the MA transition or ball milling. Both treatments can expand the MA transformation, and thus broaden the NTE temperature window (ΔT). For the powders that have gone through TC for ten times, ΔT reaches 90u2009K (309u2009K–399u2009K), and the linear expansion coefficient (αL) is about −141u2009ppm/K, which rank among the largest values of colossal NTE materials. The difference between two kinds of treatments and the possible mechanisms of the extended MA transformation window are discussed based on the introduced strain.

Giant negative thermal expansion covering room temperature in nanocrystalline GaNxMn3

Nanocrystalline antiperovskite GaNxMn3 powders were prepared by mechanically milling. The micrograin GaNxMn3 exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50u2009K as the average grain size (⟨D⟩) is reduced to ∼30u2009nm. The corresponding coefficient of linear thermal expansion (α) reaches ∼ −70u2009ppm/K, which are comparable to those of giant NTE materials. Further reducing ⟨D⟩ to ∼10u2009nm, ΔT exceeds 100u2009K and α remains as large as −30u2009ppm/K (−21u2009ppm/K) for xu2009=u20091.0 (xu2009=u20090.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling ⟨D⟩, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions.

BiFeO3 thin films prepared on metallic Ni tapes by chemical solution deposition: effects of annealing temperature and a La0.5Sr0.5TiO3 buffer layer on the dielectric, ferroelectric and leakage properties

In this work, BiFeO3 (BFO) thin films were prepared on metallic Ni (200) tapes with and without a La0.5Sr0.5TiO3 (LSTO) buffer layer at different temperatures by chemical solution deposition. The effects of the annealing temperature as well as the LSTO buffer layer on the dielectric, leakage and ferroelectric properties have been studied in detail. The crystallite size, dielectric constant and leakage current density increase, while the coercive field decreases with increasing annealing temperature. The BFO thin films deposited directly on the Ni tapes are prone to wrinkling, while the wrinkles are smoothed by introducing a thin LSTO buffer layer. Decreased compressive microstrain as well as improved ferroelectric and leakage properties are observed in the BFO thin films deposited on the LSTO buffered Ni tapes. The results will provide an instructive route to optimize BiFeO3-based thin films on metallic tapes by chemical solution deposition methods.

Evolution of the resistive switching in chemical solution deposited-derived BiFeO3 thin films with dwell time and annealing temperature

Different resistive switching has been observed in the Au/BiFeO3/Pt structure composed with the BiFeO3 (BFO) thin films annealed at different temperature for different dwell time. Resistive switching decreases and disappearances with increasing annealing temperature, while reappear with prolonging dwell time. The evolution in the resistive switching is understand by the polarization modulated interfacial barriers for the low temperature annealing BFO films, oxygen vacancies rectifying interfacial depletion layer barriers for the high temperature and long time, and p-n junction for the films annealed for the longest time at high temperature.

Forming-free unipolar resistive switching behavior with conical conducting filaments in LaVO4 thin films

LaVO4 thin films have been deposited on Pt / Ti /SiO2 /Si substrates by pulsed laser deposition in order to explore the resistive switching (RS) behavior of the Au/LaVO4/Pt devices. It is found that the as-prepared Au/LaVO4/Pt devices stay in the low resistance state (LRS) and no electroforming process (forming-free) is needed to trigger fresh devices for the subsequent RS. Furthermore, the devices show excellent switching parameters such as a reproducible switching effect, a high resistance ratio of ~104 between the LRS and high resistance state (HRS), good endurance and retention characteristics, and non-overlapping switching voltages. The dominant conduction mechanisms are Ohmic conduction in the LRS and the lower voltage region of the HRS, and Poole–Frenkel emission in the higher voltage region of the HRS. The combination of temperature dependence of resistance, x-ray photoelectron spectroscopy, and model analysis suggests that the forming-free unipolar RS behavior can be explained by the formation and rupture of conical conducting filaments formed out of oxygen vacancies. These results may be important for practical application in nonvolatile resistive switching memory.

Enantioselectivity of D-amino acid oxidase in the presence of ionic liquids

In this paper, enantioselectivities of D-amino acid oxidase (DAAO) in ten ionic liquids were investigated in detail. Among the ionic liquids studied, 3-methylimidazolium formate ([MIM][COO]) was able to trigger enzyme activation with L-Ala as substrate. The enzyme activity of DAAO reached a maximum of 4.26 U mg−1 min−1 in the presence of 40% [MIM][COO], while no activity toward L-Ala was observed when DAAO was incubated with other ILs. Moreover, DAAO activities towards other L-amino acids such as L-Pro and L-Arg were also improved in [MIM][COO], which indicated that the enzymatic enantioselectivity of amino acids was lowered. This was further confirmed by capillary electrophoresis, circular dichrosim and fluorescence analysis. Molecular optimization supported that the protein–ionic liquid interactions modulated the structure of the enzyme, especially in [MIM][COO], leading to relaxation in the enantioselectivity of DAAO. This observation and exploration might open a new window for modulation enantioselectivity resolution in ionic liquids.