T. E. Mitchell

Single crystal elastic constants of NbSi2

Abstract Room-temperature elastic properties of hexagonal C40 NbSi2 single crystal have been determined using resonant ultrasound spectroscopy (RUS). All five independent elastic stiffness constants cij were obtained: c11 = 380·2, c33 = 468·0, c44 = 145·3, c12 = 75·9 and c13 = 88·3 GPa. The shear modulus c66 in the (0001) plane, 152·2 GPa, is low relative to those of MoSi2 and WSi2. The isotropic elastic constants for polycrystalline material were also calculated. The results show that the bulk modulus K. shear modulus G and Youngs modulus E are much higher than those of the constituent elements. The room temperature Poissons ratio of the compound is 0·18, which is much smaller than those of the constituent elements and most observed materials. The elastic properties of C40 NbSi2 and C11b MoSi2 and WSi2 are compared and the possible influence on the mechanical behaviour is discussed.

Total energy and electronic structure calculations of C15 laves-phase compounds MV2 (M=Zr, Hf or Ta) : elastic properties

Abstract The total energy and electronic structure of early-transition-metal cubic Laves-phase compounds MV2 (M─Zr, Hf or Ta) have been calculated using the linear muffin-tin orbital method within the local-density and atomic sphere approximations. The elastic properties of the compounds have been examined from these results. The bulk moduli of the cubic MV2 alloys (M─Z, Hf or Ta) obtained from the total-energy calculations are 162, 172 and 218 GPa respectively. It is found from the electronic structures that there is a double degeneracy of electron energy levels with a linear dispersion relationship in the neighbourhood of the X point of the Brillouin zone and that the Fermi surfaces of the alloys pass near X-point of the Brillouin zone with an energy gap Δ∊. The contribution c(e) 44 from these electrons to the shear modulus c44 of the alloys, is found to be anomalous at high temperatures (T>400K), such that c(e) 44 increases with increasing temperature.

103 slip in MoSi2

Abstract {103}⟨331⟩ slip is found to be activated in MoSi2 single crystals by deformation along [001] in uniaxial compression at 1000°C. The {103} slip plane has been determined by slip trace analysis. Observations by transmission electron microscopy reveal the presence of screw dislocations with a Burgers vector of ½⟨331⟩. However, ½⟨331⟩ dislocations of mixed character lying along the ⟨ 010 ⟩ direction are observed to have decomposed into two other perfect dislocations by the energetically favourable reaction ½⟨ 331 ⟩→½⟨111⟩ + ⟨110⟩. The occurrence of such dislocation decomposition explains the difficulty of observing ½⟨331⟩ dislocations in previous work. The ½⟨331⟩ screw dislocations have been examined by weak-beam dark-field microscopy but dissociation into three ½⟨331⟩ collinear partials could not be detected.

Thin films of cubic boron nitride on silicon

Abstract Thin films of cubic boron nitride were grown on Si(100) surfaces using an ion-beam-assisted physical vapour deposition process. We studied the film microstructure with transmission electron microscopy and selected-area electron diffraction. The films are polycrystalline with a grain size as large as 0.4 μm. Sometimes individual grains are found to extend through the film. Electron diffraction and large-angle tilting were used to identify clearly the structure as cubic boron nitride. Multiple twins on {111} planes are abundant in individual grains, as revealed by electron diffraction.

Phase stability and elasticity of C15 transition-metal intermetallic compounds

First-principle quantum mechanical calculations based on the local-density-functional theory have been performed to study the electronic, physical and metallurgical properties of C15 intermetallics MV{sub 2} (M = Zr, Hf, or Ta). The elastic constants of C15 HfV{sub 2} + Nb were measured by the resonant ultrasound spectroscopy technique. The phase stability of C15 HfV{sub 2} + Nb was studied by specific heat measurements and by transmission electron microscopy in a low temperature specimen holder. The total energies and their lattice volume dependence were used to obtain the equilibrium lattice constants and bulk modulus. The band structures at the X-point near the Fermi level were employed to understand the anomalous temperature dependence of shear modulus of the C15 intermetallics. It was found that the double degeneracy with a linear dispersion relation of electronic levels at the X-point near the Fermi surface is mainly responsible for the C15 anomalous elasticity. The density of states at the Fermi level, N(E{sub F}), and the Fermi surface geometry were obtained to understand the low temperature phase instability of C15 HfV{sub 2} and ZrV{sub 2} and the stability of C15 TaV{sub 2}. It was proposed that the large N(E{sub F}) and Fermi surface nesting are the physical reasons for the structural instability of the C15 HfV{sub 2} and ZrV{sub 2} at low temperatures. The relation between anomalous elasticity and structural instability of C15 HfV{sub 2} and ZrV{sub 2} is also discussed.

Study of low-resistivity oxides on Pt/MgO

Abstract Highly crystalline SrRuO3 (SRO) and La0.5Sr0.5CoO3 (LSCO) thin films were deposited on Pt/MgO(100) by pulsed laser deposition. The films were mainly (001) textured normal to the substrate surface with a high degree of in-plane orientation with respect to the substrates major axes. These films were characterized using X-ray diffraction, Rutherford backscattering, four-point probe resistivity measurement and transmission electron microscopy. The room-temperature resistivities for LSCO and SRO films on Pt/MgO were found to be 35 and 40μωcm respectively. Ion beam minimum channelling yields of 43 and 33% were obtained for LSCO and SRO films respectively. The interface between Pt and oxide was found to be smooth and free from any interfacial diffusion. This result showed that high-quality low-resistivity oxide thin films can be deposited on Pt.

Synthesis of low resistivity complex oxides on MgO using Pt as buffer layer

Highly crystalline SrRuO3 (SRO) and La0.5Sr0.5CoO3 (LSCO) thin films were deposited on (100) Pt/ MgO by pulsed laser deposition. The films were mainly (001) textured normal to the substrate surface with a high degree of in-plane orientation with respect to the substrate’s major axes. These films were characterized using x-ray diffraction, Rutherford backscattering, four-point probe resistivity measurement, and transmission electron microscopy. The room temperature resistivity for LSCO and SRO films on Pt/MgO was found to be ∼35 and ∼40 μΩ-cm, respectively. An ion beam minimum channeling yield of ∼43% and ∼33% was obtained for LSCO and SRO films, respectively. The interface between Pt and oxide was found to be smooth and free from any interfacial diffusion. This result showed that high-quality low resistivity oxide thin films can be deposited on Pt.

Surface diffusion and surface atomic roughness on Ir(001) surface and terraces

Abstract Diffusion of iridium and rhenium on Ir(001) surfaces have been studied by field ion microscopy. Self-adsorbed iridium adatoms have been found to diffuse favorably along 〈100〉 directions by an atomic exchange-replacement mechanism. Rhenium adatoms force out one of four nearest neighboring iridium substrate atoms to form ReIr dimer vacancy complexes at ∼ 240 K, which can change orientation by 90° in the temperature range from 210 to 280 K. The configuration of the ReIr dimer vacancy complex is considered to be the intermediate step of the atomic exchange-replacement diffusion mechanism. Unlike the diffusion of adatoms on the Ir(001) surface, ledge atoms on Ir(001) terraces diffuse only along 〈110〉 steps at low temperatures by an ordinary atomic hopping diffusion mechanism. The diffusing ledge atom is trapped at one end of the step. The barrier potential that a trapped ledge atom needs to overcome for diffusing away from the trap is found to be 0.73 eV, which is about 0.11 eV higher than the barrier for a diffusing ledge atom migrating along the step (0.62 eV). Surface step atoms start to dissociate from various sites randomly at ∼ 1 6 of the melting temperature of iridium so that one-dimensional step roughening occurs. By measuring the mean square deviation of the step heights at various temperatures, the kink energy for the Ir(001) steps is found to be 1.36 eV.

Atomistic study of energy and structure of surfaces in NiO

The structure and energy of surfaces in NiO have been studied by atomistic calculations employing short range Buckingham potentials fitted to properties of NiO. The polarizability of lattice anions is included by using the shell model. The results show that the surface energy depends strongly on surface orientation, and the 100 surface has the lowest energy. Surfaces with higher energy prefer to reconstruct into 100 facets to lower their energy and to stabilize their structure.

High-resolution electron microscopy of advanced materials

This final report chronicles a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The High-Resolution Electron Microscopy Facility has doubled in size and tripled in quality since the beginning of the three-year period. The facility now includes a field-emission scanning electron microscope, a 100 kV field-emission scanning transmission electron microscope (FE-STEM), a 300 kV field-emission high-resolution transmission electron microscope (FE-HRTEM), and a 300 kV analytical transmission electron microscope. A new orientation imaging microscope is being installed. X-ray energy dispersive spectrometers for chemical analysis are available on all four microscopes; parallel electron energy loss spectrometers are operational on the FE-STEM and FE-HRTEM. These systems enable evaluation of local atomic bonding, as well as chemical composition in nanometer-scale regions. The FE-HRTEM has a point-to-point resolution of 1.6 {angstrom}, but the resolution can be pushed to its information limit of 1 {angstrom} by computer reconstruction of a focal series of images. HRTEM has been used to image the atomic structure of defects such as dislocations, grain boundaries, and interfaces in a variety of materials from superconductors and ferroelectrics to structural ceramics and intermetallics.