Wen-Zhu Shao

Aqueous Solution Synthesis of CaF2 Hollow Microspheres via the Ostwald Ripening Process at Room Temperature

Nearly monodispersive CaF2 hollow microspheres were synthesized by a facile aqueous solution route from the mixed aqueous solutions of CaCl2, Na2WO4, and NaF at room temperature. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and N2 adsorption-desorption techniques. The CaF2 hollow microspheres have an average diameter of about 1.5 microm and a hollow interior of 0.5 microm. The shell is composed of numerous single-crystalline nanoparticles with diameter of about 20 nm. The morphologies and diameters of the CaF2 products are strongly dependent on the experimental parameters, such as the concentration of the aqueous NaF solution and the reaction temperature. The synthetic experiments indicate that the growth process of CaF2 hollow microspheres involves first the formation of CaWO4 solid microspheres and then the formation of CaF2 solid microspheres through the reaction between CaWO4 and F(-) ions controlled by the difference of the solubility product for CaWO4 and CaF2. Phenomenological elucidation based on TEM observations and XRD patterns of intermediate products at different precipitation stages indicates that the formation mechanism for the CaF2 hollow microspheres is related to the Ostwald ripening mechanism. N2 adsorption-desorption measurement shows that the CaF2 hollow microspheres possess a high Brunauer-Emmett-Teller surface area and porosity properties. The synthetic procedure is straightforward and represents a new example of the Ostwald ripening mechanism for the formation of inorganic hollow structures in an aqueous solution at room temperature.

Effect of electroactive phase transformation on electron structure and dielectric properties of uniaxial stretching poly(vinylidene fluoride) films

The effect of stretching conditions, such as stretching ratios, temperatures and rates of extension, on the relative fraction of β-phase and electron structure as well as dielectric properties of poly(vinylidene fluoride) (PVDF) films was investigated. The fraction of β-phase in PVDF film increases greatly and reaches the peak of 93% after uniaxial stretching. Meanwhile, stretched spherulites and micro-strips paralleling with the stretching direction were observed in the matrix by atomic force microscopy (AFM). The results of fine structure of fluorine and carbon elements in PVDF, identified by X-ray absorption near edge structure (XANES), indicate that the coordination between F and H atoms of the adjacent chains is produced and thus enhances the dielectric response of the stretched film. As a result, the dielectric constant of the stretched film increases by 50% and achieves up to 12.1, as well as the dielectric loss being as low as 0.02.

Formation of CdMoO4 porous hollow nanospheres via a self-assembly accompanied with Ostwald ripening process and their photocatalytic performance

A simple direct precipitation process in an aqueous solution at room temperature has been developed for the synthesis of monodispersed CdMoO4 porous hollow nanospheres (PHNs). The obtained CdMoO4 PHNs have an average diameter of ∼500 nm and the shell is constructed from nanoparticles with an average diameter of ∼40 nm. The effects of the concentrations of the reaction reagents, pH value and reaction duration have been systematically studied. The formation mechanism of the CdMoO4 PHNs is related to a self-assembly process accompanied with an Ostwald ripening process, during which the CdMoO4 solid nanospheres are first formed through the self-assembly of CdMoO4 nanoparticles and then gradually developed into porous nanospheres with hollow interiors by an Ostwald ripening process. An optical adsorption investigation shows that the CdMoO4 PHNs exhibit a strong absorption peak in the wavelength of 230–365 nm, centered at 256 nm. The photocatalytic activities of the CdMoO4 samples for the degradation of Rhodamine B (RhB) under UV light were studied. The CdMoO4 PHNs calcined at 300 °C for 2 h exhibit excellent photocatalytic efficiency for the degradation of RhB under UV light, which is up to 97% within 40 minutes. The high photocatalytic performance of the CdMoO4 PHNs is associated with the well-crystallized structure and plenty of nanopores which exist in the spheres, serving as transport paths for small molecules.

Synthesis of Fe–ferrite composite nanotubes with excellent microwave absorption performance

Fe–ferrite composite nanotubes were successfully prepared by thermal hydrogen reduction of α-FeOOH nanowires. The nanotubes have diameters of about 100 nm and lengths of tens of micrometres. The formation mechanism of Fe–ferrite composite nanotubes is discussed, and the non-equilibrium diffusion between hydrogen and oxygen was found to be responsible for the formation of the hollow interior structure. Because of the high shape anisotropy of the 1-D shape, the coercivity of composite nanotubes was higher than that of reported granular Fe–ferrite composite nanoparticles. Since the eddy current is effectively suppressed by the thin wall characteristic of nanotubes, the composite nanotubes exhibit higher permeability than that of the reported ferromagnetic metal nanowires. Due to the better impedance matching and higher dissipation efficiency, a superior microwave absorption performance was obtained in Fe–ferrite composite nanotubes, in which the maximum reflection loss is −18 dB and the effective absorption band (<−10 dB) covers the entire frequency band of 12.5–17.5 GHz.

Formation of FeMoO4 hollow microspheres via a chemical conversion-induced Ostwald ripening process

FeMoO4 hollow microspheres were prepared via a template-free hydrothermal method using FeCl2 and Na2MoO4 as the starting reaction reagents and distilled water as the solvent. The crystal structure, morphology and UV-vis reflection property of the as-synthesized products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The hollow microspheres have diameters ranging from 3 to 5 μm, hollow interiors of ∼2.5 μm and the shell is constructed by numerous nanoparticles with sizes of 100–200 nm. The effects of reaction temperature, concentration of reaction reagents and reaction times have been studied. The formation mechanism of the FeMoO4 hollow microspheres is related to the chemical conversion-induced Ostwald ripening process.

Synthesis and formation process of SrSO4 sisal-like hierarchical structures at room temperature

Strontium sulfate (SrSO4) sisal-like hierarchical structures (SLHS) were successfully synthesized by using SrCrO4 nanowires in mother solution as precursor to react with Na2SO4 aqueous solution at room temperature. The SrSO4 SLHS have a uniform diameter of about 6 μm, which are composed of single-crystalline SrSO4 nanopyramids oriented radially to their centre. The nanopyramids, which grow along [010] direction, are about 300–500 nm and 20–50 nm for the roots and tips in diameter and 1–3 μm in length. The controlled experimental results indicate that the formation mechanism of the SrSO4 SLHS is related to the “two-step” growth process.

Colloidal synthesis and formation mechanism of calcium molybdate notched microspheres

A special structure, calcium molybdate (CaMoO4) notched microspheres, were prepared by a solution-phase rapid-injection-based route using only CaCl2, (NH4)6Mo7O24, and sodium dodecyl sulfate as reagents and ethylene glycol as a solvent. X-ray diffraction, scanning electron microscopy, transmission electronic microscopy and an X-ray energy dispersive spectrometer were used to characterize the obtained samples. The notched microspheres were uniform in size and shape, with each sphere containing one notch on its surface. The results show that the formation process of the CaMoO4 notched microspheres is due to a “two-step” growth process, in which CaSO4 nanorods are formed first and then CaMoO4 microspheres are grown on the surfaces of the CaSO4 nanorods. The reaction intermediate, the CaSO4 nanorods, acts as a self-template for the formation of “CaMoO4 sphere gripped CaSO4 nanorod” structures, and finally produce very special structures – CaMoO4 notched microspheres. The shape and size of the notched CaMoO4 samples can be tuned from ellipses to spheres with diameters in the range of 0.1–1 μm by controlling the reaction conditions.

Effect of electron irradiation on electroactive phase and dielectric properties of PVDF films

The effect of electron irradiation on the crystal structure and dielectric properties of poly(vinylidene fluoride) (PVDF) films was investigated. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were employed to examine the degree of participation in PVDF and the relative fraction of electroactive phase as well as the thermal property of PVDF films. The results showed that both the exothermal peak temperature and the crystallinity of the irradiated film decreased with increasing doses. Huge damage on the morphology of spherulite was observed in irradiated films by atomic force microscopy (AFM). The results of fine structures in PVDF films characterized by X-ray absorption near edge structure (XANES) implied that the dominance of the oxidation reaction occurred during irradiation, and further induced destruction of the regular packed structure. The results indicate that the β-phase exhibits good anti-irradiation ability during the electron irradiation on PVDF film. Because a large portion of the electroactive phase is maintained and intermediate phases are produced in the irradiated PVDF sample, the irradiated film exhibits a relatively high dielectric constant (around 7.8 under 2200 kGy) with low loss.

Relationship between boundary misorientation angle and true strain during high temperature deformation of 7050 aluminum alloy

Abstract Tensile tests of solid solution treated 7050 aluminum alloy were conducted to different strain degrees (0.1, 0.4, 0.6 and failure) at 460 °C with the strain rate of 1.0 × 10 −4 –1.0 × 10 −1 s −1 . The boundary misorientation angle evolution during hot deformation of the 7050 aluminum alloy was studied by EBSD technique and the fracture surfaces were observed using SEM. A linear relationship between the increase in the average boundary misorientation angle and the true strain at different strain rates is assumed when aluminum alloy is deformed at 460 °C. The increasing rate of average boundary misorientation angle is 15.1°, 15.7° and −0.75° corresponding to the strain rate of 1.0 × 10 −4 , 1.0 × 10 −2 and 0.1 s −1 , respectively. The main softening mechanism is continuous dynamic recrystallization when the strain rates are 1.0 × 10 −4 and 1.0 × 10 −2 s −1 , and it is dynamic recovery when strain rate is 0.1 s −1 .

Dielectric and electrocaloric responses of Ba(Zr 0.2 Ti 0.8 )O 3 bulk ceramics and thick films with sintering aids

Large electrocaloric effect (ECE) has been reported in Ba(Zr0.2Ti0.8)O3 (BZT) bulk ceramics near room temperature. The finding opens up opportunities for developing high performance EC ceramic based refrigeration. To further reduce the operating voltage and enhance the reliability of such EC material, we investigate synthesis of BZT thick films can be fabricated into multilayer EC modules using the commercial multilayer ceramic capacitor (MLCC) technology. However, the high (1450 °C) sintering temperature of BZT EC ceramics poses challenge for the MLCC with low-cost electrodes. This paper investigate sintering aids that can significantly reduce sintering temperature of BZT ceramics. The bulk and thick-film BZT with 1 wt% PbO and B2O3 were sintered at 1200 °C and exhibited high dielectric constant and low loss around room temperature. Dielectric and electrocaloric responses of thus fabricated BZT thick films are studied. The low temperature sintered BZT thick films with proper sintering aids exhibit high breakdown field and larger ECE than the bulk BZT, thus paving the way for the future transition to EC MLCC suitable for EC cooling systems.