S. F. Xu

Investigating the Effects of Solid Surfaces on Ice Nucleation

Understanding the role played by solid surfaces in ice nucleation is a significant step toward designing anti-icing surfaces. However, the uncontrollable impurities in water and surface heterogeneities remain a great challenge for elucidating the effects of surfaces on ice nucleation. Via a designed process of evaporation, condensation, and subsequent ice formation in a closed cell, we investigate the ice nucleation of ensembles of condensed water microdroplets on flat, solid surfaces with completely different wettabilities. The water microdroplets formed on flat, solid surfaces by an evaporation and condensation process exclude the uncontrollable impurities in water, and the effects of surface heterogeneities can be minimized through studying the freezing of ensembles of separate and independent water microdroplets. It is found that the normalized surface ice nucleation rate on a hydrophilic surface is about 1 order of magnitude lower than that on a hydrophobic surface. This is ascribed to the difference in the viscosity of interfacial water and the surface roughness.

Large area, low microwave surface resistance thin films of YBa2Cu3O7 prepared by pulsed laser ablation

YBa2Cu3O7 (YBCO) superconducting thin films were deposited on (100)LaAlO3 substrates 35 mm in diameter by pulsed laser ablation. The substrates were heated resistively during deposition by a single-crystal Si heater and the laser beam was scanned on the rotating YBCO target. The films exhibited a thickness variation of +/- 3.7%, with zero resistance temperature T-c0 = 90.6 +/- 0.6 K, critical current density J(c) = 2.9 +/- 0.9 X 10(6) A/cm(2) at 77 K, and at 10 GHz a surface microwave resistance of < 250 mu Omega at 77 K. The above-mentioned properties showed that high-quality YBCO thin films over a large substrate area can be obtained with this laser ablation system.

Microstructure and properties of YBa2Cu3O7−δ thin films with BaO precipitates

BaO precipitates with sizes between 10–100 nm in laser ablated YBa2Cu3O7−δ (YBCO) thin films on Y‐stabilized Zirconia substrates have been identified by x‐ray diffraction and transmission electron microscopy. The precipitates exhibit equiaxed shapes and grow epitaxially inside and on the surface of the YBCO films, with (001)BaO plane parallel to the a,b plane of YBCO. Some of these smaller precipitates and BaO/YBCO boundaries probably provide potential pinning sites for magnetic flux lines, which might contribute to the observed increase of critical current density with magnetic field B under B≤500 G in the case of B perpendicular to the c axis of the film.

MICROSTRUCTURE OF OUTGROWTHS ON THE SURFACE OF LASER-ABLATED YBA2CU3O7 THIN-FILMS

Abstract The microstructure of the outgrowths on the surface of pulsed laser ablated YBa 2 Cu 3 O 7 (YBCO) thin films deposited on (100) Y stabilized zirconia substrates was studied by transmission electron microscopy. The outgrowths consist of misaligned superconducting c -axis YBCO grains and a -axis YBCO grains, non-superconducting CuO, Ba 2 CuO 3 , BaCuO 2 , CuYO 2 and Y 2 BaCuO 5 grains about 0.1–5 μm in size. The Y-123 type outgrowths are block-shaped misaligned with the YBCO matrix. The non-superconducting grains mostly are semi-spherical in shape and have a certain crystallographic relation with the YBCO matrix. The protruding heights of the outgrowths on the surface of the film are about 50–450 nm.

Viscosity of interfacial water regulates ice nucleation

Ice formation on solid surfaces is an important phenomenon in many fields, such as cloud formation and atmospheric icing, and a key factor for applications in preventing freezing. Here, we report temperature-dependent nucleation rates of ice for hydrophilic and hydrophobic surfaces. The results show that hydrophilic surface presents a lower ice nucleation rate. We develop a strategy to extract the thermodynamic parameters, J0 and Γ, in the context of classical nucleation theory. From the extracted J0 and Γ, we reveal the dominant role played by interfacial water. The results provide an insight into freezing mechanism on solid surfaces.

Correlation between distribution of outgrowths and microwave surface resistance for YBa2Cu3O7 thin films

A model was proposed to describe the effects of surface outgrowths on the microwave surface resistance of high Tc superconducting thin films. Calculated with experimental data the microwave surface resistance at 10 GHz for c‐axis oriented YBa2Cu3O7 thin films with no outgrowths could be as low as 70 and 4 μΩ at 77 and 4.2 K, respectively. The effect of the orientation of the surface outgrowths on the microwave loss has been discussed.

Effect of structure and morphology on resistive loss at 10 GHz of the large‐area laser‐deposited YBa2Cu3O7 thin films

Three large‐area YBa2Cu3O7(YBCO) superconducting thin films have been laser‐deposited under almost identical conditions. However, the microwave surface resistance Rs of the films deviated greatly, they are 280 μΩ, 3.78 mΩ, and 97 mΩ, respectively. It was found that the structure and morphology of the films greatly influence the resistive losses at 10 GHz and 77 K of the YBCO thin films. Different loss mechanisms were discussed. For high Rs, the large angle grain boundaries were the dominate defect in the thin films and increased the Rs of the film markedly. For low Rs, it was mainly due to the misaligned ‘‘123’’ grains in the thin films and the intrinsic loss. For Rs up to the mΩ range, besides the misaligned 123 grains, domain boundaries and nonsuperconducting outgrowths of different sizes appeared and this caused the rise of Rs value. All these results were given experimentally and discussed theoretically.

Triple-axis X-ray diffraction analysis of highly oriented YBa2Cu3O7 thin films on LaAlO3 substrates

High-quality YBa2Cu3O7 superconducting thin films have been deposited on LaAlO3 substrates by a pulsed laser deposition technique and have been studied by high-resolution triple-axis X-ray diffractometry. Reciprocal-space maps around both substrate and thin-film reciprocal-lattice points showed that the thin films are highly c oriented, while considerable strains exist in the substrate surface which originated from the mechanical polishing of the substrate. Twinning structures of the LaAlO3 substrates may have been copied into the films, which, in addition to the orientation disordering of the YBa2Cu3O7 crystal grains, causes a further broadening of the rocking curves of the film.

Microstructural features at the interface between laser ablated YBa2Cu3O7 films and LaAlO3 substrates

The microstructure at the interface between YBa2Cu3O7(YBCO) thin film and (100)LaAlO3 substrate has been investigated by using transmission electron microscopy. It has been observed that two distinct microstructural features existed in the interface: (1) A thin transitional layer of Ba3Al2O6 was frequently observed and the YBCO thin film grown on it showed stacking faults. (2) Sharp interface with no transitional layer was also occasionally observed and the YBCO film grown on it was single crystalline. In rare cases, a low symmetry phase was observed near the surface of the LaAlO3 substrate, however, the distortion caused by the lattice mismatch between this phase and the YBCO did not affect the quality of the YBCO thin film.

Magnetostriction and electrical resistivity of Mn doped Fe 81 Ga 19 alloys

The paper reports on the effect of substituting Mn for Fe on the magnetostrictive and related properties of Fe81−xMnxGa19 alloys. Samples of composition were prepared in polycrystalline form and were all single phase BCC. It is shown that the magnetostriction observed in Fe81Ga19 can be enhanced up to about 11% by Mn substitution with a maximum effect at x = 8. The change in the magnetostrictive coefficient of Fe81Ga19 with Mn substitution is accompanied by a monotonic increase in resistivity, being increased by almost a factor of 2 for x = 8. We also found that the lattice parameter of the alloys increased with the increase in Mn concentration and the Curie temperature decreased monotonically at the same substitution.