Crystalline to amorphous phase transformation during initial lithiation in (100) Si wafers is studied in an electrochemical cell with Li metal as the counter and reference electrode. During initial lithiation, a moving phase boundary advances into the wafer starting from the surface facing the lithium electrode, transforming crystalline Si into amorphous Li(x)Si. The resulting biaxial compressive stress in the amorphous layer is measured in situ, and it was observed to be ca. 0.5 GPa. High-resolution TEM images reveal a very sharp crystalline-amorphous phase boundary, with a thickness of ∼1  nm. Upon delithiation, the stress rapidly reverses and becomes tensile, and the amorphous layer begins to deform plastically at around 0.5 GPa. With continued delithiation, the yield stress increases in magnitude, culminating in a sudden fracture of the amorphous layer into microfragments, and the cracks extend into the underlying crystalline Si.

A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia. © 2000 American Institute of Physics. ͓S0003-6951͑00͒05034-8͔The efficiency of gas turbine engines is dictated by the maximum sustained operating temperature of their typically Ni-or Co-based alloy turbine rotors. The development of new, higher temperature, high-strength, lightweight alloys is desirable. 1 However, recent studies have concluded that significant near-term progress in increasing turbine engine operating temperatures is more likely to come from the development of improved thermal barrier coatings ͑TBCs͒, typically yttria-stabilized zirconia ͑YSZ͒, than from the design of new alloys. 2 New processing techniques that result in TBC microstructures with lower thermal conductivity could lead either to higher operating temperatures of turbine engines, resulting in greater efficiency, or thinner coatings for the same operating temperature, which would reduce overall weight. Nanocrystalline YSZ coatings are of interest because they offer the possibility of lowering thermal conductivity, and may also provide additional benefits for TBC applications because of the possibility of improved toughness and ductility compared to that of coarser-grained ceramics. 3,4 The low thermal conductivity of YSZ ͑ϳ2.3 W/mK for high-density, polycrystalline material with a yttria-content of 10 mol. % at 20°C 5 ͒ is due primarily to phonon scattering by vacancies on the material's highly defective oxygen sublattice. 6 The potential for reduced thermal conductivity in nanocrystalline coatings arises from the predicted enhanced phonon scattering due to the presence of numerous closely spaced grain boundaries. For example, Klemens and Gell 6 have theoretically predicted that the room temperature thermal conductivity of 10 nm grain-sized YSZ containing 7 wt. % Y 2 O 3 will be decreased more than 50% compared to 1 m grain-sized YSZ of the same composition. The goal of the present study was to experimentally determine the effect of grain size on the room-temperature thermal conductivity of YSZ, thus contributing to the fundamental understanding of grain-size-dependent phonon scattering processes.Nanocrystalline YSZ films were grown by metal-organic chemical vapor deposition ͑MOCVD͒ using a low-pressure, horizontal, cold-walled deposition system. Yttrium b-diketonate ͓Y͑thd͒ 3 ͔ and zirconium t-butoxide ͓ZrOC͑CH 3 ͒ 4 ͔ 7 were chosen as precursor materials. Highpurity nitrogen was used as the precursor carrier gas. The precursors were mixed with high-purity ...

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The diffusion of Cs+, Rb+, and K+ ions was measured in three grades of vitreous SiOl by the radiotracer-sectioning technique or Rutherford ,backscattering spectroscopy. The values of the diffusion coefficient, D , at 1000°C decrease strongly with increasing ionic radius, changing by about two and one-half orders of magnitude per row of the periodic table. The difference between Dcs and DRb is largely in the preexponential factor Do in the Arrhenius expression rather than in the activation enthalpy Q . The values of D are much smaller than the tracer D values for the alkali metal ion in homogeneous Rb or Cs silicate glasses. Residual metallic impurities in the Si02 decrease both Q and Do for the diffusion of Rb. The results are analyzed in terms of the interstitial structure of vitreous SO2.

Polycarbonate (LexanTM) (PC) was implanted with 2 MeV B+ and O+ ions separately to fluences of 5 × 1017, 1 × 1018, and 5 × 1018 ions/m2, and characterized for changes in surface hardness and tribological properties. Results of tests showed that hardness values of all implanted specimens increased over those of the unirradiated material, and the O+ implantation was more effective in improving hardness for a given fluence than the B+ implantation. Reciprocating sliding wear tests using a nylon ball counterface yielded significant improvements for all implanted specimens except for the 5 × 1017 ions/m2 B+-implanted PC. Wear tests conducted with a 52100 steel ball yielded significant improvements for the highest fluence of 5 × 1018 ions/m2 for both ions, but not for the two lower fluences. The improvements in properties were related to Linear Energy Transfer (LET) mechanisms, where it was shown that the O+ implantation caused greater ionization, thereby greater cross-linking at the surface corresponding to much better improvements in properties. The results were also compared with a previous study on PC using 200 keV B+ ions. The present study indicates that high energy ion irradiation produces thicker, more cross-linked, harder, and more wear-resistant surfaces on polymers and thereby improves properties to a greater extent and more efficiently than lower energy ion implantation.

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A modified approach to silicon-on-insulator (SOI) by bond-and-etch-back technology was studied where a high-energy (MeV) boron implant was utilized as an etch stop to eliminate the need for an epitaxial layer growth in forming a device film. Also a second (retro) MeV implant, applied after the first stage of the etch-back process, was investigated as an improved method for achieving uniform thinning of a thick (3 μm) SOI film. Significantly improved thickness uniformities (σ<10 nm across a 3×3 in. area) were obtained by this method for a 490-nm-thick silicon device film.