David D. Allred received Alumni Professorship Award

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Abstract: An intact thermal barrier coating (TBC) specimen consisting of electron-beam physical vapor deposited (EB-PVD) ZrO2-7 wt-%Y2O3 (YSZ) topcoat, thermally grown oxide (TGO), grit-blasted (Ni,Pt)Al bond coat and CMSX-4 single crystalline superalloy, was characterized after 645 thermal cycles. Each thermal cycle was carried out in air, and consisted of 10-min heat-up to 1038 degreesC, 10-h hold at 1038 degreesC, and 10-min forced air-quench to ambient temperature., Characteristics of TGO scale were initially examined by photostimulated luminescence spectroscopy (PSLS) and scanning electron microscopy equipped with energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) and scanning TEM (STEM) with nano-spot energy dispersive spectroscopy, high-angle annular dark field (HAADF) imaging, selected area diffraction (SAD), convergent beam electron diffraction (CBED), and electron energy loss spectroscopy was carried out for detailed microstructural characterization. A site-specific preparation of TEM specimen for the thermally cycled TBC was successfully carried out using focused ion beam in-situ lift out technique. Photostimulated luminescence corresponding to negligible residual stress in TGO scale was observed for this intact TBC. Microstructural analysis showed undulation of interface between TGO and bond coat, and the corresponding damage near YSZ/TGO interface. Extensive decohesion within TGO (nearly 9 mum thick), near the YSZ/TGO interface, was observed by TEM/STEM, and related to the observation made by PSLS. Microstructure and phase constituents of mixed-oxide zone containing Zr, Y, Ta, Ni, Co, Cr, Ti in Al2O3 matrix and continuous-columnar Al2O3 TGO are presented based on TEM/STEM and related analytical techniques. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The depth profiles of high dosage Cr-52(+) and V-51(+) ions implanted in (100) crystalline silicon after thermal anneal at temperatures between 300 degreesC and 1000 degreesC are studied by secondary ion mass spectrometry and cross-sectional transmission electron microscopy. At dosages of 1x10(15) ions/cm(2) and above, the surface layer of silicon substrate is amorphorized. During the subsequent thermal annealing, the depth profiles of the implanted ions are strongly coupled with the solid phase epitaxial growth of amorphous silicon. Silicide precipitate formation is important to understand the differences between Cr and V diffusion. After anneal of the 1x10(15) ions/cm(2) implanted samples at 900 degreesC and 1000 degreesC, most of the Cr has left the silicon, but only 10% of the V has escaped. The 1x10(14) ions/cm(2) Cr-implanted sample shows Cr ions exist only near the surface after 1000 degreesC anneal. The V-implanted sample, on the other hand, only shows a narrowing of the V profile after 1000 degreesC anneal. (C) 2004 American Institute of Physics.

Abstract: We report the preparation and investigation of ferromagnetic La0.67Sr0.33MnO3 (LSMO) thin films with orthogonally (along (110) and (1 (1) over bar0)) distributed nanometer-scale cracks. When the density of cracks reaches similar to10(4) cm(-1), the magnetoresistance MR = (R-H - R-0)/R-0 > - 30% was observed in the magnetic fields less than or equal to200 Oe and temperature range from 5 to > 300 K. The discussion about such unique feature will be done based on the spin-polarized electron tunneling and spin scattering, (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: Amorphous silica nanowires (a-SiONWs) were produced by direct solid state transformation from silica films. The silica nanowires grow on TiN/Ni/SiO2/Si substrates during the annealing in H-2 or a H-2:CH4 mixture at 1050 degreesC. Titanium nitride (TiN) films were used to induce a solid state reaction with silica (SiO2) films on silicon wafers to provide silicon atoms into growing nanowires. The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built inside the films. The nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched gamma-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [10 (1) over bar0]. Stacking faults in the basal plane are seen when viewing the specimen in the [1 (2) over bar 10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3x10(8) cm(-2) when viewing the [1 (2) over bar 10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as "seeds" for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations. (C) 2003 American Institute of Physics.