[en] The short-time critical relaxation of an Ising model on a Sierpinski carpet is investigated using Monte Carlo simulation. We find that when the system is quenched from high temperature to the critical temperature, the evolution of the order parameter and its persistence probability, the susceptibility, and the autocorrelation function all show power-law scaling behavior at the short-time regime. The results suggest that the spatial heterogeneity and the fractal nature of the underlying structure do not influence the scaling behavior of the short-time critical dynamics. The critical temperature, dynamic exponent z, and other equilibrium critical exponents β and ν of the fractal spin system are determined accurately using conventional Monte Carlo simulation algorithms. The mechanism for short-time dynamic scaling is discussed

[en] Dynamic scaling for fracture or breakdown process in disordered systems is investigated in a two-dimensional random field Ising model (RFIM). We find two evolving stages in the avalanche process in the RFIM. At the short-time regime, a power-law growth of the avalanche size Δs is observed; and at late times, the conventional nucleation and growth process is found. At the critical point of the RFIM, the avalanche size is found to obey the dynamic scaling law Δs∼t^(d-β/ν)/z. From this dynamic scaling relation, the critical strength of the random field D_c and the critical exponents, β, ν, and z, are determined. The observed dynamics is explained by a simple nucleation theory of first-order phase transformations

[en] Ferroelectric dense cceramic compositions of (1–x)((Bi_1/2Na_1/2)_0.94Ba_0.06TiO₃)–xBa_1/2Sr_1/2TiO₃ (BNBT–BST), (0 ≤ x ≤ 0.1) were fabricated via sol–gel method. The phase, microstructural, dielectric, ferroelectric and temperature dependent properties were investigated in detail. The XRD revealed single phase perovskite structured Bi_1/2Na_1/2TiO_3. The temperature dependent dielectric properties revealed two phase transitions in the fabricated samples. The Energy storage properties were obtained indirectly from the Polarization versus Electric field (P–E) loops at various temperature and electric field. The maximum values of the energy storage density (W) and energy storage efficiency (η) of 0.57 J cm⁻³ and 43% were obtained at 150 °C and 70 KV cm⁻¹ for x = 0.08 respectively. The energy storage densities were observed to increase with increase in temperature and electric field. The optimum composition of BNBT–BST can be a promising candidate for energy storage applications. (paper)

[en] As fuel additives, ethyl levulinate (EL) can be used up to 5 wt% directly in the regular diesel engines, which can overcome the limited stock of fossil fuels and reduce the environment pollutions to some extent. In this work, the three-dimensional porous hybrids consisting of SiO₂ and graphene aerogel, which are denoted as SiO₂@GA, are facilely assembled and used as supports for H₃PW₁₂O₄₀ (HPW)-based solid acid catalysts. Structural analysis confirms that the resultant HPW/SiO₂@GA catalysts possess unique porous structure (S_BET ≥ 257 m² g⁻¹, V_p ≥ 0.450 cm³ g⁻¹) and exhibit excellent catalytic performance in the synthesis of EL by the esterification of levulinic acid (LA) with ethanol. The conversion of LA can be as high as 92.4% under the reaction conditions. Furthermore, various catalytic reaction parameters are also optimized over the 10 wt.% HPW/SiO₂@GA catalysts, which exhibit the highest turnover frequency (TOF = 83.91 mmol g⁻¹ h⁻¹) among the resultant catalysts. The results confirm the promising application of the HPW/SiO₂@GA heterogeneous catalysts in the synthesis of biofuel.

[en] As electrode materials for supercapacitors, biomass-derived activated carbons attract much attention owing to their natural abundance and low cost. In this work, porous activated carbons are facilely synthesized from cattail wool using Ni(NO₃)₂·6H₂O and KOH as co-etching agents. Compared with the carbons singly etched with KOH, these CWAC-x materials with hierarchical pores have much higher specific surface areas and exhibit much better capacitive performance. As for CWAC-10, the specific surface area and total pore volume are as high as 1581 m² g⁻¹ and 0.992 cm³ g⁻¹, respectively. For supercapacitor applications, CWAC-10 exhibits a high specific capacitance (314 F g⁻¹ at 1.0 A g⁻¹ in a three-electrode system), excellent cycling stability and high energy density (37.29 Wh kg⁻¹ at a power density of 159.98 W kg⁻¹ in a coin-type symmetric device). The enhanced electrochemical performance can be attributed to the unique structure and the existence of O and N elements.

[en] Phase transitions in sol–gel-derived (1 − x)(Bi_1_/_2Na_1_/_2TiO_3)-xBaTiO_3 (0  not ≦ x  not ≦ 0.06) solid solutions were investigated via dynamic mechanical, dielectric, and ferroelectric analyses. Structural phase transition was observed from both noncentrosymmetric ferroelectric to nonpolar (antiferroelectric) forms (FE-AFE) at temperatures ranging from ∼120 °C to ∼210 °C, and from antiferroelectric to paraelectric forms (AFE-PE) at temperatures ranging from ∼250 °C to ∼320 °C for 0 ≦ x ≦ 0.06. The former transition is accompanied by a significant and broad elastic softening or soft-mode process, and the latter is of typical diffused type. The dynamic mechanical scaling exponent and critical scaling exponent are used to characterize these two phase transitions, respectively. The temperature-dependent polarization showed an abnormal trend unlike other reported ferroelectrics, confirming the presence of two different phases: FE and AFE. The d_3_3 piezoelectric constant increases with increasing x up to the morphotropic phase boundary. It is suggested that the dynamic mechanical scaling exponents could be used to characterize the mobility of the polar domains and the elastic softening processes, which are closely related to abnormal pyroelectric properties and piezoelastic hardening behaviors. (paper)

[en] Due to the simultaneously broken inversion symmetries along multiple directions, $AB{P}_2{X}_6$ (A = Cu or Ag, B = In or Bi, and X = S or Se) monolayers exhibit the coexistence of sizable piezoelectric coefficients $d_{11},d_{21}$, and $d_{31}$, under unidirectional in-plane strains or stresses, as determined from first-principles calculations. Remarkably, the $AgBi{P}_2{S}_6$ and $AgBi{P}_{2}S{e}_6$ monolayers possess $d_{11}$ coefficients of 17.33 and 17.25 pm $V^{-<!--\; ???\; -->1}$, respectively; these are much higher than those of extensively studied 2D piezoelectric materials. Meanwhile, the out-of-plane $d_{31}$ coefficient of $AgBi{P}_{2}S{e}_6$ is as high as 0.36 pm $V^{-<!--\; ???\; -->1}$. The $AB{P}_2{X}_6$ monolayers possessing unique multidirectional piezoelectricity pave novel avenues for the application of piezoelectric devices with multiple functions. (© 2020 Wiley‐VCH GmbH)

[en] A three-dimensional hybrid consisting of nitrogen-doped graphene aerogel and the fusiform CuO nanoparticles is facilely assembled by using a gas-liquid interfacial reaction followed by freeze-drying. The results show that the CuO particles are dispersed evenly into the three-dimensional architecture of graphene aerogel to form a hierarchal porous structure. When used as anode materials for lithium-ion batteries, the resultant CuO@N-GA hybrid possesses a large first discharge specific capacity of 1442.2 mAh g⁻¹ at 100 mA g⁻¹ with a coulombic efficiency of 58.1%. Moreover, the hybrid exhibits superior cycle stability and excellent rate performance. Its discharge specific capacity maintains as high as 725.3 mAh g⁻¹ at 100 mA g⁻¹ after 100 cycles and 502.4 mAh g⁻¹ at 1000 mA g⁻¹. The enhanced electrochemical performance of CuO@N-GA is mainly originated from its unique interconnected porous structure, the incorporation of nitrogen, and the well dispersed CuO nanoparticles.

[en] Glucose-derived porous activated carbon materials (AGC-600-4 and AGC-180-x) are prepared using ZnCl₂ as the etching agent via impregnation treatment and hydrothermal method followed by the calcination process. The analytic results indicate that the obtained materials exhibit higher specific surface area and superior double-layer capacitive behavior than the corresponding pristine carbon (GC-600 and GC-180) when used as electrode materials for supercapacitors. Moreover, compared with the AGC-600-4 nanosheets, the optimal AGC-180-4 microspheres have a high specific surface area of 1713 m² g⁻¹ and a maximum specific capacitance of 235.9 F g⁻¹ at a current density of 1.0 A g⁻¹ in the three-electrode system. Meanwhile, AGC-180-4 also exhibits better capacitive properties than AGC-600-4 in the two-electrode system, showing an excellent cyclic stability with a high energy density of 24.63 Wh kg⁻¹ at the power density of 949.5 W kg⁻¹. It is thus demonstrated that AGC-180-4 could be ideal electrode materials for supercapacitor due to its unique etched spherical structure and excellent electrochemical properties.

[en] Bond breaking related with plastic deformation in a deformed metallic glass Zr₅₀Cu₅₀ is investigated by molecular dynamics simulations. The results show that the spatial distributions of broken bonds are closely correlated with local shear strains, and the clustering behaviors of atoms with broken bonds (flow defects) are characterized with different stages of plastic deformation. The average distance among those clusters of flow defects decreases as the strains increase, which follows the curvature quadrupole model for flow defects in ideal amorphous solids. For the first time, the features of bond breaking processes are quantitatively measured with the chemical composition, bond length and orientation, bond pairs, local five-fold symmetry and quasi-nearest atoms, whose threshold values are important factors that could characterize the flow defects in metallic glasses under plastic deformation. The shape, orientation and energetics of flow defects quantitatively characterized by the bond breaking analysis thus facilitate our understanding on the deformation mechanisms in metallic glasses.