David T. Marx

Phase separation in Nd2_xCexCuO4 evidence for superconductivity at a single composition

Abstract A series of samples with bulk compositions Nd 2-χ Ce χ CuO 4 , 0≤χ≤0.25, have been examined by pulsed neutron and high-resolution synchrotron X-ray diffraction. The diffraction data show that single phase material is formed only for the undoped compound, Nd 2 CuO 4 , and (possibly) for the optimum superconducting composition, χ≈0.165. Other samples appear to be a mixture of this superconducting composition and non-superconducting material of other compositions. The phase separation cannot be explained in the context of an equilibrium phase diagram and, thus, must result from non-equilibrium behavior during synthesis.

Variable-range hopping conduction in Ba1−xKxBiO3−y system

Abstract Normal state resistivity of the Ba1−xKxBiO3−y system is reported for various K concentrations. In the concentration range where the material is superconducting, phonon-assisted variable-range hopping [exp(T0/T) 1 4 ] is observed between 160 and 300 K.

The upper critical field of Ba1−xKxBiO3

Abstract Resistive measurements in magnetic fields up to 8 T and at temperatures between 0.5 and 150 K for Ba 1− x K x BiO 3 polycrystalline samples with various values of x are presented. Low-field magnetic measurements indicate bulk superconductivity for all concentrations measured with T c ranging from 25.6 K to 28 K for x ranging from 0.35 to 0.45. The upper critical field as a function of concentration and temperature is determined from the resistive measurements. A lower limit of 40 A is derived for the superconductive coherence length. There is a negative magnetoresistance in the normal state at low temperatures. The resistive transition curves depend strongly on the measuring current. We propose a granular model of superconductivity to explain this and other unsual features of the data.

Mechanical properties of carbon—carbon composite components determined using nanoindentation

Abstract The near-surface mechanical properties of the individual components (fiber, resin char, and CVD carbon) within several carbon—carbon composites are reported from the analysis of nanoindentation data. These near-surface properties (elastic modulus, stiffness, and hardness) are important parameters in thermal and mechanical models of friction processes occurring in carbon—carbon composite brakes.

Oxygen content and the synthesis of Ba1−xKxBiO3−y

Abstract In situ thermogravimetric analysis during the synthesis of Ba 1− x K x BiO 3− y shows the various processes occurring as a function of temperature and the gas composition of the ambient atmosphere. This allows the synthesis procedure to be optimized. Single-phase material can be produced with the metal-ion stoichiometry of the starting oxide mixture.

Measurement of interfacial temperatures during testing of a subscale aircraft brake

Interfacial temperatures have been measured using fibre optic inserts in the stationary brake ring during ring-on-ring, subscale dynamometer aircraft brake testing. The brake materials are carbon fibre-reinforced, carbon-matrix (carbon-carbon) composites. The temperature distribution varies with dynamometer test conditions and demonstrates the non-uniformity of contact pressure within the interface. A two-dimensional, axis-symmetric finite element model (FEM) is presented that is used to estimate temperature profiles during braking using either a constant pressure or constant energy flux assumption. The model incorporates the measured temperature-dependence of the thermal diffusivity and specific heat capacity for the composite materials. The surface temperatures obtained from the FEM are compared with the measured surface temperatures. Substantial differences between the two results are observed and discussed.

Multiphase behavior and the superconducting composition in Nd2−xCexCuO4

High resolution neutron and x-ray powder diffraction data show that samples of Nd 2−x Ce x CuO 4 are multiphase except at x=O and the optimum superconducting composition x≈0.165. Cerium doped samples with compositions other than x≈0.165 appear to be mixtures of the superconducting phase, which has a fixed composition, and non-superconducting phases with different compositions. The phase separation could result from non-equilibrium behavior during synthesis.

Application of fractals to the contact of carbon-carbon surfaces

Fractal contact mechanics has been applied to the study of carbon-carbon aircraft brake materials. Prepared aluminum surfaces were also studied. The approach originally taken by Yan and Komvopoulos [J. Appl. Phys. 84, 3617 (1998)] has been corrected and modified to determine the real contact area as a function of applied load for any surfaces in contact. The methods are then applied to determine the real area of contact for the carbon brake materials in their application. A key finding of this approach is that the measured friction coefficient is shown to strongly depend on the contact area.

Magnetoresistance studies of spin-slip transitions in single crystal holmium

Abstract Previous studies of single crystal holmium using a variety of techniques including thermal expansion, magnetization, and X-ray and neutron scattering have confirmed the presence of “spin-slips” below the antiferromagnetic ordering temperature T N = 132 K. Magnetization versus temperature studies taken below T N showed several anomalies which occur above the ferromagnetic ordering temperature T c = 13 K. These anomalies correspond to re-alignments of the moments within the magnetic unit cell as described in the spin-slip model developed by Bohr et al . Since magnetoresistance is sensitive to subtle changes in the magnetic structure, in this report we have studied the H-T phase diagram for applied fields along the b -axis of holmium up to 5.5 T and temperatures between 5 and 125 K using magnetoresistance (MR). Anomalies in the MR data correspond well with published transitions. Transitions are observed as either step increases or inflection points in magnetoresistance as the field is increased at a given temperature. Our data confirm previously published phase diagrams determined by magnetization and neutron diffraction.

Microscopic Modeling of Phonon Modes in Semiconductor Nanocrystals

Phonons of nanocrystals are quite different from those of the bulk. In this chapter, phonon modes of nanocrystals are calculated theoretically by a valence force field model. Group theory is employed and symmetry of each phonon modes is obtained. Raman intensity of phonon modes is calculated by bond polarizability approximation. Microscopic phonon modes are compared with macroscopic Lamb modes in spherical nanocrystals. Calculated Raman spectra are compared with experiments. Size effects of nanocrystals and their Raman spectra are discussed.