Lang Zhou

General synthesis of rare-earth orthochromites with quasi-hollow nanostructures and their magnetic properties

Rare-earth orthochromites are extremely interesting because of their potential applications as multifunctional materials. However, it is still a great challenge for the general synthesis of nanostructured full rare-earth orthochromites series. Here, a facile and versatile solvothermal reduction strategy is successfully employed in the preparation of rare-earth chromites with quasi-hollow nanostructures. X-ray diffraction data show that all the products have the orthorhombic perovskite structure. The electron microscopy analysis reveals that the morphology of the product is seriously affected by the rare-earth ionic radius. Tube-like and vesicle-like structures can be formed for the larger and smaller rare-earth cationic radii, respectively. The experimental results suggest that the room-temperature precursors of potassium rare-earth chromates serve as a self-template for the in situ reduction and formation of rare-earth orthochromites hollow structures. The magnetization studies demonstrate that all the products, as it would be expected, undergo a magnetic transition from paramagnetic to antiferromagnetic phase at the Neel temperature (TN1) attributed to Cr3+–Cr3+ exchange and this critical temperature goes up linearly with an increase in the rare-earth ionic radius. Additionally, some samples exhibit a variety of fancy magnetic properties, including thermal hysteresis suggesting a first-order magnetic transition, magnetization reversal due to the antiparallel polarization of the R3+ paramagnetic moments by the Cr3+ canted antiferromagnetic ones, and magnetic exchange bias related to the spin reorientation transition of the Cr3+ magnetic moments.

Self-template formation and properties study of Cr2O3 nanoparticle tubes

As an important semiconductor, Cr2O3 exhibits many attractive properties and significant industrial applications. Nevertheless, it remains a challenge to develop a simple synthetic methodology for the fabrication of Cr2O3 1D nanostructures. In this paper, Cr2O3 sub-microtubes, consisting of nanoparticles, have been successfully prepared by the in situreduction of SrCrO4 nanorods in a microemulsion system. X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy were employed to study the crystal structure and morphology of the products. Experiments showed that the solvent, reaction temperature and reaction time were critical for the formation of the Cr2O3 nanoparticle tubes. Magnetic properties of the products were investigated by the electron spin resonance and superconducting quantum interference device magnetometer. These magnetic results revealed a weak ferromagnetic behavior induced by the uncompensated surface spin below TB. The UV-Vis absorption spectrum showed three broad absorption peaks at 360, 450 and 600 nm, respectively. The nitrogen adsorption-desorption measurement was used to determine the surface area and pore size distribution of the as-obtained polycrystalline tubes. A possible growth mechanism for the Cr2O3 nanoparticle tubes was also proposed.

A facile in situ reduction route for preparation of spinel CoCr2O4 polycrystalline nanosheets and their magnetic properties

Considerable efforts have been exerted on the controllable synthesis of nanomagnetic materials due to their size- and morphology-dependent properties. Herein, a facile ethylene glycol in situ reduction strategy has been successfully employed in the preparation of CoCr2O4 nanosheets. X-ray diffraction patterns showed that the products have the cubic spinel structure. The electron microscopy analysis revealed that the obtained CoCr2O4 nanosheets consisted of nanoparticles with the diameters of 20–30 nm. Experiments proved that the volume ratio of ethylene glycol to water was crucial for the final morphology. The magnetization studies demonstrated that besides the long-range ferrimagnetic order below the Curie temperature (TC = 86 K), the sample exhibited two low-symmetry ordered states including the spiral magnetic order at TS = 20 K and the magnetic lock-in transition at TL = 13 K. The crystallinity- and size-dependent magnetic properties were also investigated. The temperature dependence of the specific heat revealed both phase transition at TC = 90 K and TS = 20 K, in line with the magnetic results.

The ferromagnetic–antiferromagnetic properties of Ni–Cr2O3 composite hollow spheres prepared by an in situ reduction method

Considerable efforts have been exerted on the facile synthesis of magnetic composite materials because of their unique properties and potential applications. Especially for ferromagnetic–antiferromagnetic systems, the magnetic exchange bias effect is essential for the development of magneto-electronic switching devices and magnetic storage media. In this research, a facile ethylene glycol in situ reduction strategy has been successfully employed in the preparation of Ni–Cr2O3 composite hollow spheres. X-Ray powder diffraction was used to determine the phase composition. Scanning electron microscopy and transmission electron microscopy was employed to characterize the morphologies of the as-prepared samples. Experiments proved that the volume ratio of ethylene glycol to water played a determinative role in the final morphology of the products. The magnetization vs. temperature results revealed a spin-glass-like behavior with blocking temperature of about 150 K for the as-prepared Ni–Cr2O3 composites. Induced by the coupling between ferromagnetic Ni and antiferromagnetic Cr2O3, a small exchange bias effect could be observed in the magnetic hysteresis loops. At lower temperature, a larger exchange bias field and coercivity are obtained. A high surface area of 145.1 m2 g−1 was obtained for the prepared porous hollow spheres.

Molecular dynamics simulation of the solidification process of multicrystalline silicon from homogeneous nucleation to grain coarsening

Based on the Stillinger–Weber potential, molecular dynamics simulations of the solidification processes of multicrystalline silicon were carried out. Every stage of the whole solidification process, including homogeneous nucleation, grain growth, grain coarsening and defect characterization, was investigated. In the nucleation stage, it showed two typical nucleation models (spontaneous nucleation and sporadic nucleation) at high and low temperatures. Local heterogeneity was occasionally observed during homogeneous nucleation. The simulated nucleation rates at different temperatures were measured, whose trends were in agreement with the results of theoretical calculations, and both of them reached the maximum nucleation rate at a critical temperature of ∼0.65Tm. In the growth stage, the nuclei showed the maximum growth exponent at ∼0.65Tm. The evolution of the grain number at different temperatures exhibited three different patterns. Furthermore, the grains showed modest anisotropic growth before the growth was influenced by other grains. In the grain coarsening stage, the grain size distribution could be described suitably by the log-normal distribution. The grain coarsening exponent was approximately one order of magnitude lower than the nuclei growth exponent. The analyses of crystal defects showed that the dislocation density is about 105 m−2, twin percentages are above 40%, and the percentage of CSL grain boundaries is about 30.30% in mc-Si of the simulations. The statistics of crystal defects are similar to experimental results.