Siqin Meng

Controlled synthesis of anatase TiO2 nanotube and nanowire arrays via AAO template-based hydrolysis

A novel and facile method was successfully developed to fabricate nanotube (NT) and nanowire (NW) arrays through an anodic aluminum oxide (AAO) template-based Ti(OC4H9)4 hydrolysis process. The wall thickness of the TiO2 NTs can be controlled by adjusting the concentration of Ti(OC4H9)4 solutions. A continuous series from thin-walled NTs to completely filled NWs were synthesized using Ti(OC4H9)4 solutions of different concentrations from 0.1 to 2.0 M. The TiO2 NTs and NWs in the AAO membrane transformed from amorphous to nanocrystalline anatase completely after annealing at 650 °C, which is higher than the phase transformation temperature of TiO2 powder. Optical spectra of the synthesized TiO2 NT and NW arrays/AAO composite nanostructures were characterized and showed a blue shift of the optical absorption edge with the decrease of NT wall thickness, due to the nanoscale effect and shape dependent transformation.

Magnetic properties of in-plane oriented barium hexaferrite thin films prepared by direct current magnetron sputtering

In-plane c-axis oriented Ba-hexaferrite (BaM) thin films were prepared on a-plane (112¯0) sapphire (Al2O3) substrates by DC magnetron sputtering followed by ex-situ annealing. The DC magnetron sputtering was demonstrated to have obvious advantages over the traditionally used RF magnetron sputtering in sputtering rate and operation simplicity. The sputtering power had a remarkable influence on the Ba/Fe ratio, the hematite secondary phase, and the grain morphology of the as-prepared BaM films. Under 80 W of sputtering power, in-plane c-axis highly oriented BaM films were obtained. These films had strong magnetic anisotropy with high hysteresis loop squareness (Mr/Ms of 0.96) along the in-plane easy axis and low Mr/Ms of 0.03 along the in-plane hard axis. X-ray diffraction patterns and pole figures revealed that the oriented BaM films grew via an epitaxy-like growth process with the crystallographic relationship BaM (101¯0)//α-Fe2O3 (112¯0)//Al2O3 (112¯0).

Dual band microwave ferromagnetic resonance absorption in annealed cobalt nanowire arrays

In this letter, we present a dual band microwave absorption phenomenon in 60 nm diameter Co ferromagnetic nanowire arrays annealed at 500 °C in nitrogen atmosphere. The frequency dependent microwave response of the wires is obtained for applied field parallel to the nanowire axis, below 4 kOe and above saturation magnetization up to 8 kOe, in steps of 1 kOe. For applied magnetic fields above saturation magnetization, two sets of absorption peaks are observed, while below saturation, only one single peak is obtained. Combining structural characterization and ferromagnetic resonance measurements, it is proven that the two ferromagnetic resonance peaks are associated with a coexistence of two kinds of nanopillars with different crystal structure in annealed nanowires, one for nanopillars with hexagonal close packed structure at higher ferromagnetic resonance frequency, and the other with face center cubic structure.

Phase Characterization and Dielectric Properties of Zn2SiO4 Ceramics Derived from a Sol-Gel Process

The Zn 2 SiO 4 based ceramics with the compositions of Zn 2 SiO 4 , Zn 1.8 SiO 3.8 , Zn 2.2 SiO 4.2 , and Zn 2 SiO 4 -11wt%TiO 2 were prepared by a sol-gel process. The formation process of the crystal phases in the precursor powders was characterized by XRD techniques. The results showed that the willemite phase was formed from the reaction between ZnO and SiO 2 , and the presence of TiO 2 precursor could reduce the temperature of phase formation. The excellent microwave properties of ϵ r ∼ 6.55, Q× f ∼ 198,400 GHz, τ f ∼ -41.6ppm/°C and ϵ r ∼ 9.13, Q× f∼ 150,800GHz, τ f ∼ 1.0ppm/°C were achieved for the pure and TiO 2 -doped Zn 2 SiO 4 ceramics, respectively. The phase characterization revealed that the pure phases formed from sol-gel process might be responsible for the excellent properties.

Structural Transitions and Microwave Dielectric Properties of (Ba, Sr)2 LnSbO6 (Ln = La, Pr, Nd, Sm, Gd, Dy) Double Perovskites

Structures and microwave dielectric properties of (Ba, Sr)2 LnSbO6 (Ln = La, Pr, Nd, Sm, Gd, Dy) ceramics were studied. Ba2 LnSbO6 undergoes a cubic-rhombohedral structural transition, and Sr2 LnSbO6 adopts the monoclinic P21/n structure. The microwave dielectric properties, especially the ϵr and τf values, change with composition obviously. The Ba2 LnSbO6 ceramics have ϵr in the range of 19.2 ∼ 20.7, Q×f = 6 700 ∼ 86 500 GHz, and τf = −37 ∼ 38 ppm/°C; the Sr2 LnSbO6 ceramics have ϵr in the range of 15.9 ∼ 16.8, Q×f = 5 700 ∼ 93 000 GHz, and τf = −46 ∼ −55 ppm/°C.

Epitaxial Spinel Cobalt Ferrite Films Prepared by Two-Step Spin-Coating Method

Epitaxial spinel cobalt ferrite thin films have been prepared by spin-coating method. <111>-axis in-plane and <111>-axis out-of-plane oriented films are prepared on a-plane and c-plane sapphire substrates, respectively. The key process is to fabricate a layer of α-Fe2O3 precursor film first. The final cobalt ferrite film is formed by coating the precursor film with Cobalt nitrate solution and subsequently activating the topotactic reactive diffusion by thermal annealing. In-plane magnetic anisotropy has been confirmed in the <111> in-plane oriented film. This process enables the fabrication of epitaxial cobalt ferrite film with desired orientations without utilizing expensive physical vapor deposition systems.

Epitaxially grown BaM hexaferrite films having uniaxial axis in the film plane for self-biased devices

Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.

Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3-BaWO4 Composite Ceramics

Microwave dielectric composite ceramics with compositions of (1-x)Ba(Zn 1/3 Nb 2/3 )O 3 -BaWO 4 (x = 0.1∼ 0.4) were prepared. The increasing BaWO 4 content can effectively lower the sintering temperature of the composite ceramics from 1400°C to 1250°C. XRD indicated that Ba(Zn 1/3 Nb 2/3 )O 3 and BaWO 4 coexisted. SEM observation revealed that the grain size increased with the sintering temperature rising up. The microwave dielectric properties of the sintered ceramics varied with the BaWO 4 content. The samples with x = 0.4 sintered at 1225°C exhibited the best microwave dielectric property: a near zero temperature coefficient of resonant frequency of τ f = −2.1 ppm/°C, ϵ r = 24.0, Q×f = 65300 GHz was obtained.