Mankang Zhu

Hydrothermal synthesis of zinc oxide powders with controllable morphology

Crystalline zinc oxide powders with various morphologies have been prepared by hydrothermal treatment of zinc acetate in pure water, KOH or ammonia aqueous solution. It was found that the selected solvents play a different role in controlling the morphologies of the obtained powders. The variation of morphology of the obtained ZnO powder with solvents mainly depends on the different zinc species and their environment during the hydrothermal processing.

Self-assembly of hydroxyapatite nanostructures by microwave irradiation

Hydroxyapatite (HAp) nanorods, bowknot-like nanostructures and flower-like architectures have been directly synthesized and assembled under microwave irradiation without the help of any templates. The uniform nanorods present an average diameter of about 40 nm and a length of up to about 400 nm. The as-prepared bowknot-like nanostructures consist of sword-like HAp nanorods with a typical width of 150 nm and lengths up to 1–2 µm. The flower-like architectures are composed of leaf-like flakes with typical diameters of 150–200 nm and lengths up to 1–2 µm. The SAED and HRTEM experiments imply that the sword-like HAp nanorods and leaf-like flakes are single crystalline in nature and preferentially grow along the [001] direction. It is found that the pH value and the complex reagent EDTA play important roles in synthesis of the final HAp nanostructures. The possible mechanism is discussed for the formation of the HAp nanostructures in the presence of EDTA under microwave irradiation.

Effect of Li2CO3 addition on the dielectric and piezoelectric responses in the low-temperature sintered 0.5PZN–0.5PZT systems

Low-temperature sintering of 0.5Pb(Zn1∕3Nb2∕3)O3–0.5Pb(Zr0.47Ti0.53)O3 ceramics (0.5PZN–0.5PZT) was investigated using Li2CO3 as sintering aids. The addition of Li2CO3 significantly improved the sinterability of 0.5PZN–0.5PZT ceramics, resulting in a reduction of sintering temperature from 1100to950°C. Moreover, the effect of Li2CO3 addition on the dielectric and piezoelectric responses in 0.5PZN–0.5PZT systems was systematically studied in this work. The analysis of x-ray diffraction patterns and scanning electron microscopy indicated that the solubility limit of Li ions in perovskite structures was near 0.5wt% in Li2CO3 form. Below the solubility limit, Li+ ions entered the six-fold coordinated B sites of oxygenic octahedral center and enhanced the compositional fluctuation in nanoscale, resulting in the increase of the degree of diffuseness γ. While at high doping level above the solubility limit, γ decreased subsequently, which was attributed to the formation of pyrochlore phase. Raman analysis on the...

Improving Dielectric Properties of PVDF Composites by Employing Surface Modified Strong Polarized BaTiO3 Particles Derived by Molten Salt Method

BaTiO3/polyvinylidene fluoride (BT/PVDF) is the extensive reported composite material for application in modern electric devices. However, there still exists some obstacles prohibiting the further improvement of dielectric performance, such as poor interfacial compatibility and low dielectric constant. Therefore, in depth study of the size dependent polarization and surface modification of BT particle is of technological importance in developing high performance BT/PVDF composites. Here, a facile molten-salt synthetic method has been applied to prepare different grain sized BT particles through tailoring the calcination temperature. The size dependent spontaneous polarizationof BT particle was thoroughly investigated by theoretical calculation based on powder X-ray diffraction Rietveld refinement data. The results revealed that 600 nm sized BT particles possess the strong polarization, ascribing to the ferroelectric size effect. Furthermore, the surface of optimal BT particles has been modified by water-soluble polyvinylprrolidone (PVP) agent, and the coated particles exhibited fine core-shell structure and homogeneous dispersion in the PVDF matrix. The dielectric constant of the resulted composites increased significantly, especially, the prepared composite with 40 vol % BT loading exhibited the largest dielectric constant (65, 25 °C, 1 kHz) compared with the literature values of BT/PVDF at the same concentration of filler. Moreover, the energy storage density of the composites with tailored structure was largely enhanced at the low electric field, showing promising application as dielectric material in energy storage device. Our work suggested that introduction of strong polarized ferroelectric particles with optimal size and construction of core-shell structured coated fillers by PVP in the PVDF matrix are efficacious in improving dielectric performance of composites. The demonstrated approach can also be applied to the design and preparation of other polymers-based nanocomposites filled with ferroelectric particles to achieve desirable dielectric properties.

Effect of lattice occupation behavior of Li+ cations on microstructure and electrical properties of (Bi1/2Na1/2)TiO3-based lead-free piezoceramics

The multisite occupation of Li+ cations in perovskite structure of (Bi1/2Na1/2)TiO3-based solid solution is investigated, which is rarely reported in previous studies. The multisite occupation in relation to the Li2CO3 doping level has been demonstrated with the aid of the microstructure and temperature-dependent conductivity analysis: As the addition of Li2CO3 is below 0.75 mol. %, the introduced Li+ cations precede to enter the A sites of the perovskite lattice to compensate for the A-site deficiency stemming from the high-temperature sintering process. Once the addition exceeds 0.75 mol. %, the excess Li+ cations will occupy B sites to substitute for Ti4+ and give rise to the generation of oxygen vacancies. The proposed multisite occupation behavior of Li+ cations and its derivative effects, involving the clamping effect or grain size effect, have made the piezo-/ferroelectric performances of the 0.85BNT–0.10BKT–0.05BT ceramics optimized at the Li2CO3 addition of 1.0 mol.%:d33 = 163 pC/N, Pr = 40.9 μC/cm2.

Novel non-hydrazine solution processing of earth-abundant Cu2ZnSn(S,Se)4 absorbers for thin-film solar cells

A novel non-hydrazine precursor solution followed by dip-coating has been developed to produce Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin films. The precursor solution is based on an ethanol solution of metal–thioacetamide (TAA) complex with monoethanolamine (MEA) as the additive agent. By forming coordination complexes with TAA, metal cations are found to have good solubility in ethanol–MEA solvents, producing molecular-level blending in metal precursor solutions. All the materials are low-cost and environmentally friendly. The annealing treatments are conducted under vacuum and Se vapor to form CZTS and CZTSSe absorber films. A solar cell fabricated with the CZTSSe thin film exhibits power conversion efficiency of 5.36%, which is much higher than that (2.86%) of the cell using the CZTS thin film as absorber.

Hydrothermal synthesis and photocatalytic properties of layered La2Ti2O7 nanosheets

Layered La2Ti2O7 nanosheets were prepared through a one-step hydrothermal method at low temperature. The concentration of NaOH mineralizer plays an important role in the synthesis. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the thickness of every nanosheet is about 5–10 nm, while the planar dimension is more than 1 µm. The photo-catalytic activities of the nanosheets were characterized by the decolourization of methyl orange solution and the evolution rate of H2. The results demonstrated that the La2Ti2O7 nanosheets possess significantly improved photocatalytic properties in water purification and evolution rate of H2 from water–ethanol solution compared with those of samples prepared by conventional solid-state reaction.

Band bending mechanism for field emission in wide-band gap semiconductors

A theoretical model based on the band bending theory was developed for explaining the field-emission mechanism of wide-band gap semiconductors (WBGSs). It was shown that the maximum value of the band bending, which is nearly linearly proportional to the band gap of WBGSs, may amount to a few eV. Furthermore, the calculated field-emission energy distribution combined with the band bending analyzed on cubic boron nitride (c-BN) as typical one of WBGSs, indicated that the electron emission originates from the conduction band minimum resulting from the band bending. These results present a perspective to explain the field-emission mechanism, in which it is considered that the band bending, as well as the negative electron affinity, is of equal importance to the excellent field emission performances of WBGSs.

Relaxor behavior of (K0.5Bi0.5)TiO3 ceramics derived from molten salt synthesized single-crystalline nanowires

Single-crystalline K0.5Bi0.5TiO3 nanowires have been fabricated by a large scale and facile molten salt synthetic method in a KCl medium. Pristine nanowires have diameters of about 40nm and lengths exceeding 4μm, and they possess tetragonal perovskite structure. The K0.5Bi0.5TiO3 ceramics with a relative density above 98% can be fabricated from high quality nanowires. A broad dielectric peak with frequency dependent dielectric maximum temperature was observed, which can be well fitted by a modified Curie-Weiss law and a Vogel-Fulcher relationship. The suggested relaxor behavior in K0.5Bi0.5TiO3 composition is possibly due to the A-site compositional fluctuations at the nanolevel.

Facile synthesis and high d33 of single-crystalline KNbO3 nanocubes

Single-crystalline KNbO(3) nanocubes with orthorhombic phase were prepared in a large scale by a simple one-step molten salt route without using any surfactant as template; the nanostructures exhibited high piezoelectric properties such as d(33)=105 pC/N and k(p)=0.34 as piezoelectric materials.