William Barvosa-Carter

Modelling a growth instability in a stressed solid

The growth of crystalline silicon from the amorphous phase in the presence of an applied stress is modelled using advanced numerical methods. The crystal region is modelled as a linear elastic solid and the amorphous as a viscous fluid with a time-dependent viscosity to reflect structural relaxation. Appropriate coupling conditions across the boundary are defined, and both problems are solved using a symmetric-Galerkin boundary integral method. The interface is advanced in time using the level set technique. The results match well with experiments and support the proposed kinetic mechanism for the observed interface growth instability.

Growth oscillation decay rates for control of III–V molecular beam epitaxy near stoichiometry

We have investigated the decay of reflection high-energy electron diffraction oscillations during growth of InAs (001), as a function of growth parameters, such as the V/III ratio. We have shown that the decay constants are sensitive to changes in growth morphology, as measured by scanning tunneling microscopy. Our experiments show that the values of these decay constants decrease at high V/III ratios, in agreement with previous work. Additionally, we have found that the values of the decay constants diverge as the transition between the (2×4) and (4×2) reconstructions is approached. We propose that the decay constants of the growth oscillations may be used as inputs for control of interface morphology.

Reversible work by electrochemical intercalation of graphitic materials

Graphite intercalation compounds are a class of materials systems formed as ions diffuse into a host graphite structure. The volume expansion associated with this process has been shown to be capable of performing work up to 3.8 MJ/m3. To evaluate GICs for solid state actuation, this study explores some factors affecting the rate at which the volume expansion occurs. Given that diffusion length has an exponential effect on rate, we tested a graphite paper comprised of 7-micron diameter PAN fibers. We found that the paper had ultimate strain and loading properties comparable to HOPG. The paper was cycled under various loads and temperatures to examine the strain rate and repeatability of the material. Testing showed a strong correlation between rate and temperature, while pressure had relatively little effect.

Graphite Intercalation Compounds as Actuation Materials

The intrinsic electrochemical behavior of Graphite Intercalated Compounds (GICs) during formation offers the potential for high-force, high-strain solid-state actuation applications. To explore this behavior we submitted a “model” system, highly-oriented pyrolytic graphite (HOPG)/sulfuric acid (H2 SO4 ), to axial compressive loads from 0 to 8 MPa, and measured the intercalation response in terms of voltage and displacement. We observed strains greater than 30% between 2 and 6 MPa, confirming the potential of GIC formation as a viable actuating mechanism. Further studies are planned in order to perform more precise analysis and examine alternate GICs.Copyright

Solid-state actuation based on reversible Li electroplating

Reversible electrochemical compound formation has considerable potential to form the basis of a high-strain high-force multifunctional actuator technology. We present preliminary experimental demonstrations of the reversible work capability of solid-state electroplating. Our experimental test case is the volume expansion incurred during the reversible electrochemical formation of thin-film Li metal from a ceramic lithium ion storage medium, LiCoO2 as part of the standard operation of a state-of-the-art Li-ion battery. Reversible work is accomplished through the plating or stripping of the pure Li film against an external load. With the active portion of the structure as a basis, we observe ~10% strain against loads up to 2 MPa, with the load being limited by battery failure. No change in actuation characteristics is observed up to failure.

Sb-terminated InAs(001)-(2×4) and (2×8) studied using scanning tunneling microscopy and ab initio density functional theory

Abstract : We have studied the structure of MBE-grown InAs(001)-(2 x 4) surfaces exposed to low Sb2 fluxes by scanning tunneling microscopy (STM) and ab initio density functional theory (DFT). Experimentally, we observe an Sb-terminated alpha 2(2 x 4) phase over a wide range of temperatures (400 - 510 deg C) for low Sb2 flux (< 0.1 ML/s), whereas temperature and As2 flux must be carefully controlled to achieve the same As-terminated surface structure. At lower temperatures, we observe indications of an Sb-terminated (2 x 8) symmetry surface phase, and we report briefly on its proposed structure and stability, as well as its possible role in subsequent formation of the Sb-terminated (1 x 3) phase found at typical Sb2 fluxes used during heterostructure growth.

Towards High Temperature Actuation Using Graphite Intercalation Compounds

Electrochemically formed graphite intercalation compounds (GICs) have many intrinsic properties well-suited for compact actuation in applications at high temperatures. GICs using ionic liquids are of interest because of their good thermal stability at elevated temperatures, high ionic conductivity, and low volatility. In this study we observed the potential and strain behavior of highly oriented pyrolytic graphite and 1-ethyl-3-methylimidazolium hexafluorophosphate subjected to a light compressive load and constant current. In situ measurements of the anode during intercalation showed a reversible strain of 2.5% to 4.5% from 100°C up to 250°C.Copyright

Atomistic Modeling of III-V Semiconductors: Thermodynamic Equilibrium and Growth Kinetics

Growth kinetics and thermodynamic equilibrium can both be determining factors at different stages of III-V semiconductor heteroepitaxy. We study their interplay, employing kinetic Monte Carlo simulations for the InAs(001) surface. The simulation contains atomistic details of both species, including the stability of different reconstructions and their kinetics. The behavior of the surface in thermodynamic equilibrium, including different reconstructions, is determined exclusively by extensive total energy calculations employing ab initio density functional theory. The continuous phase transition between the a2(2x4) and j32(2x4), predicted by theory, is confirmed by experiment. At full layer coverage, a recovery of the stable reconstruction is observed. The different time scales associated with As 2 and In are discussed with respect to equilibrium and