The rutile TiO2(110) (1 × 1) surface is considered the prototypical 'well-defined' system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by electron spectroscopies, can be prepared reproducibly by sputtering and annealing in oxygen. We present results that show that this commonly applied preparation procedure may result in a surface structure that is by far more complex than generally anticipated. Flat, (1 × 1)-terminated surfaces are obtained by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxygen at moderate temperatures (470-660 K), irregular networks of partially connected, pseudohexagonal rosettes (6.5 × 6 Å wide), one-unit cell wide strands, and small (≈tens of Å) (1 × 1) islands appear. This new surface phase is formed through reaction of oxygen gas with interstitial Ti from the reduced bulk. Because it consists of an incomplete, kinetically limited (1 × 1) layer, this phenomenon has been termed 'restructuring'. We report a combined experimental and theoretical study that systematically explores this restructuring process. The influence of several parameters (annealing time, temperature, pressure, sample history, gas) on the surface morphology is investigated using STM. The surface coverage of the added phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in 18O-enriched gas. Atomic models of the essential structural elements are presented and are shown to be stable with first-principles density functional calculations. The effect of oxygen-induced restructuring on surface chemistry and its importance for TiO2 and other bulk-reduced oxide materials is briefly discussed.
A computational procedure for extracting substructure-by-substructure flexibility properties from global modal parameters is presented. The present procedure consists of two key features: an element-based direct flexibility method, which uniquely determines the global flexibility without resorting to case-dependent redundancy selections, and the projection of kinematically inadmissible modes that are contained in the iterated substructural matrices. The direct flexibility method is used as the basis of an inverse problem, whose goal is to determine substructural flexibilities given the global flexibility, geometrically determined substructural rigid-body modes, and the local-to-global assembly operators. The resulting procedure, given accurate global flexibility, extracts the exact element-by-element substructural flexibilities for determinate structures. For indeterminate structures, the accuracy depends on the iteration tolerance limits. The procedure is illustrated using both simple and complex numerical examples and appears to be effective for structural applications such as damage localization and finite element model reconciliation.
Evidence for capillary waves at a liquid-vapor interface are presented from extensive molecular dynamics simulations of a system containing up to 1.24 million Lennard-Jones particles. Careful measurements show that the total interfacial width depends logarithmically on L∥, the length of the simulation cell parallel to the interface, as predicted theoretically. The strength of the divergence of the interfacial width on L∥ depends inversely on the surface tension γ. This allows us to measure γ two ways since γ can also be obtained from the difference in the pressure parallel and perpendicular to the interface. These two independent measures of γ agree provided that the interfacial order parameter profile is fit to an error function and not a hyperbolic tangent, as often assumed. We explore why these two common fitting functions give different results for γ.
Previous photopumping studies of coupled vertical cavity laser structures have demonstrated three mode coupling (two photonic and one excitonic), dual wavelength emission, and short pulse generation. This paper reports on electrically injected coupled resonator vertical-cavity laser diodes, including two novel modulation approaches.
Diamond films were grown by chemical deposition of hydrocarbon species in a vapor composed predominantly of hydrogen. The rate constants of the surface reactions and the concentrations of the gas-phase species were used as input to a variable time step Monte Carlo algorithm, which simulates the evolution of the diamond growth surface by tracking the occupancies of surface sites. The results of the combined tight binding and density functional theory quantum mechanical calculations are presented, suggesting that the etching of isolated, monomolecular moieties occurred at an appreciable rate, while etching from larger carbon islands was nor favorable.
We describe a very-low power consumption circuit for processing the pulses from a semiconductor radiation detector. The circuit was designed for use with a cadmium zinc telluride (CZT) detector for unattended monitoring of stored nuclear materials. The device is intended to be battery powered and operate at low duty-cycles over a long period of time. This system will provide adequate performance for medium resolution gamma-ray pulse-height spectroscopy applications. The circuit incorporates the functions of a charge sensitive preamplifier, shaping amplifier, and peak sample and hold circuit. An application specific integrated circuit (ASIC) version of the design has been designed, built and tested. With the exception of the input field effect transistor (FET), the circuit is constructed using bipolar components. In this paper the design philosophy and measured performance characteristics of the circuit are described.
We report the stability of TlBa2CaCu2O7 and Tl2Ba2CaCu2O8 on LaAlO3(100) epitaxial thin films, under a variety of conditions. All films are stable in acetone and methanol and with repeated thermal cycling to cryogenic temperatures. Moisture, especially vapor, degrades film quality rapidly. These materials are stable to high temperatures in either N2 or O2 ambients. While total degradation, resulting from Tl depletion, occurs at the same temperatures for both phases, 600 °C in N2 and 700 °C in O2, the onset of degradation occurs at somewhat lower temperatures for TlBa2CaCu2O7 than for Tl2Ba2CaCu2O8.
Tailoring of porous materials involves not only chemical synthetic techniques for tailoring microscopic properties such as pore size, pore shape, pore connectivity, and pore surface reactivity, but also materials processing techniques for tailoring the meso- and the macroscopic properties of bulk materials in the form of fibers, thin films and monoliths. These issues are addressed in the context of five specific classes of porous materials: oxide molecular sieves, porous coordination solids, porous carbons, sol-gel derived oxides, and porous heteropolyanion salts. Reviews of these specific areas are preceded by a presentation of background material and review of current theoretical approaches to adsorption phenomena. A concluding section outlines current research needs and opportunities.
Project 'Rolling Thunder' is a dish/Stirling demonstration project at Ft. Huachuca in southeastern Arizona. As part of the project, Sandia decided to retrofit a SOLO 161 Stirling engine on the CPG-460 at Ft. Huachuca. Although the SOLO 161 PCU has operated nearly flawlessly and the CPG-460 has been, for the most part, a solid and reliable component, integration of the SOLO PCU with the CPG-460 into a functional system required significant attention.
Materials Research Society Symposium - Proceedings
Newcomer, P.P.; Venturini, E.L.; Doyle, B.L.; Brice, D.K.; Schoene, H.
Lattice defects are introduced into the structure to suppress the motion of magnetic vortices and enhance the critical current density in high temperature superconductors. Point defects are not very effective pinning sites for the cuprate superconductors; however, extended defects, such as linear tracks, have been shown to be strong pinning sites. We study the superconducting cuprate Tl-2212 (the numbers designate Tl-Ba-Ca-Cu stoichiometry). Large enhancements of vortex pinning potential were observed in Tl-2212 after high-intermediate energy heavy-ion irradiations where non-continuous extended defects were induced at dE/dx of 9 to 15.2 keV/nm (60 MeV Au, 60 MeV Cu, and 30 MeV Au) and continuous linear defects were induced at 19.5 keV/nm (88 MeV Au). Our research addresses the question of pinning in highly anisotropic materials like Tl-2212 where the vortices are `pancakes' rather than `rods' and suitable defect structures may be discontinuous extended damage domains. The defect microstructure and the effectiveness of the pinning potential in Tl-2212 after irradiation by intermediate energy Au at lower dE/dx of 5-15 keV/nm, where recoils are more significant, is studied using high resolution transmission electron microscopy digital imaging and a SQUID magnetometer. The nature of the ion irradiation damage at these intermediate dE/dx will be correlated to the average vortex pinning potential and the TRIMRC calculations for recoils.
Sol-gel processing of materials is plagued by shrinkage during polymerization of the alkoxide monomers and processing (aging and drying) of the resulting gels. We have developed a new class of hybrid organic-inorganic materials based on the solventless ring-opening polymerization (ROP) of monomers bearing the 2,2,5,5-tetramethyl-2,5-disilaoxacyclopentyl group, which permits us to drastically reduce shrinkage in sol-gel processed materials. Because the monomers are polymerized through a chain growth mechanism catalyzed by base rather than the step growth mechanism normally used in sol-gel systems, hydrolysis and condensation products are entirely eliminated. Furthermore, since water is not required for hydrolysis, an alcohol solvent is not necessary. Monomers with two disilaoxacyclopentyl groups, separated by a rigid phenylene group or a more flexible alkylene group, were prepared through disilylation of the corresponding diacetylenes, followed by ring closure and hydrogenation. Anionic polymerization of these materials, either neat or with 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane as a copolymer, affords thermally stable transparent gels with no visible shrinkage. These materials provide an easy route to the introduction of sol-gel type materials in encapsulation of microelectronics, which we have successfully demonstrated.
Thermally stimulated current (TSC) techniques provide information about oxide-trap charge densities and energy distributions in MOS (metal-oxide-semiconductor) capacitors exposed to ionizing radiation or high-field stress that is difficult or impossible to obtain via standard capacitance-voltage or current-voltage techniques. The precision and reproducibility of measurements through repeated irradiation/TSC cycles on a single capacitor is demonstrated with a radiation-hardened oxide, and small sample-to-sample variations are observed. A small increase in E′δ center density may occur in some non-radiation-hardened oxides during repeated irradiation/TSC measurement cycles. The importance of choosing an appropriate bias to obtain accurate measurements of trapped charge densities and energy distributions is emphasized. A 10 nm deposited oxide with no subsequent annealing above 400 °C shows a different trapped-hole energy distribution than thermally grown oxides, but a similar distribution to thermal oxides is found for deposited oxides annealed at higher temperatures. Charge neutralization during switched-bias irradiation is found to occur both because of hole-electron annihilation and increased electron trapping in the near-interfacial SiO2. Limitations in applying TSC to oxides thinner than approximately 5 nm are discussed.
The grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AlGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AlN buffer layers results in a reduced and possibly controllable strain.
A comparison of KOH, NaOH and AZ400K solutions for UV photo-assisted etching of undoped and n+ GaN is discussed. The etching is diffusion-limited (Ea < 6kCal·mol-1) under all conditions and is significantly faster with bias applied to the sample during light exposure. No etching of InN was observed, due to the very high n-type background doping (> 1020cm-3) in the material.
A critical issue in the long-term reliability of solder connections used in electronic packages is joint failure during thermal cycling. At present, solder is assumed to be a homogeneous single-phase metal in most finite element analyses to predict solder joint fatigue failures. However, in the last decade, several metallurgical studies have shown that solder microstructure may have a role in early solder joint failures. Investigators have observed that solder microstructure coarsens in local bands during aging and during thermal cycle fatigue. In a failed solder joint, the fatigue cracks are found in these bands of coarse grains. It is speculated that the grain coarsening increases local strains within the microstructure, thereby increasing the likelihood for a crack to initiate. The objective of this study is to model and simulate the effect of grain coarsening on local stresses and strains. During solidification of eutectic Pb/Sn solder, two types of microstructures form: lamellar and equiaxed. In this study, the author has developed a computer code to generate both types of microstructures of varying grain coarseness. This code is incorporated into the finite element code that analyzes the local stresses and strains within the computer-generated microstructure. The FE code, specifically developed for this study, uses an algorithm involving the sparse matrix and iterative solver. This code on a typical single-processor machine will allow the analyst to use over 1 million degrees of freedom.
The minimum expended energy for fracture is the free energy required to form two new surfaces. For intergranular fracture, the minimum surface formation energy is complicated by the rough fracture surface, with area greater than the specimen cross-section. We utilize network optimization algorithms (max-flow/min-cut) to determine the minimum surface formation energies and surfaces for intergranular fracture in 3D polycrystals. For equiaxed grains and uniform boundary strength, the minimum energy fracture area is independent of grain size and is 45% larger than the specimen cross-section, and intergranular fracture will occur when surface energy is less than 1.6 times the grain boundary energy. The 3D fracture area is larger than projected from 2D systems. In systems with microcracked boundaries, the fracture surface deviates to preferentially include microcracked boundaries, creating interlocking grain configurations. Two-dimensional percolation of microcracks occurs at about 80% microcracked boundaries.
