S. Dregia - Academia.edu (original) (raw)

Papers by S. Dregia

Journal of the American Ceramic Society, 1993

Molecular statics calculations of mechanical properties have been carried out for NaCI using the ... more Molecular statics calculations of mechanical properties have been carried out for NaCI using the Born–Mayer–Huggins pair potential and periodic boundary conditions. Three truncation techniques are utilized. For each, calculations have been performed as a function of the truncation distance. The stress–strain curve, the fracture strength, and the fracture strain are found to be much more sensitive to the truncation technique and to variations in the truncation distance than other properties such as the elastic constants, the cohesive energy, or the lattice parameter.

Journal of Materials Research, 2006

Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays of single-crystal... more Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays of single-crystal titania nanofibers by reaction with a noncombustible, hydrogen-bearing gas mixture at only 680–780 °C. Such nanofiber formation resulted from anisotropic etching (“nanocarving”) of the titania grains. The fibers possessed diameters of 20–50 nm and lengths of up to several microns, with the long fiber axes oriented parallel to the [001] crystallographic direction of rutile. Mass spectroscopy and inductively coupled plasma spectroscopy indicated that oxygen, but not titanium, was removed from the specimen during the reaction with hydrogen. The removal of substantial oxygen and solid volume from the reacting surfaces, without an appreciable change in the Ti:O ratio at such surfaces, was consistent with the solid-state diffusion of titanium cations from the surface into the bulk of the specimen. The reaction-induced weight loss followed a parabolic rate law, which was also consistent with a...

IEEE Transactions on Applied Superconductivity, 2007

Ceramics International, 2013

Acta Metallurgica et Materialia, 1991

Journal of Materials Research, 1994

The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and g... more The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and growth of LiNb3O8 precipitates which exhibit an epitaxial orientation relationship. The same orientation relationship is observed on x-, y-, and z-cut LN substrates. The epitaxy arises from similarities between the two crystal structures and provides for an essentially continuous oxygen-ion framework from parent to product. On y-cut LN, the precipitates have a well-defined habit plane, and the interfacial misfit between the two structures is accommodated by a rectangular grid of misfit dislocations. The density, geometry, and imaging behavior of the misfit dislocations suggest that their Burgers vectors serve to accommodate the disregistry in the oxygen-ion framework. Based on these observations, it is concluded that the mechanism of formation of LiNb3O8 consists of preferential nucleation on the surface and subsequent growth, possibly by counterdiffusion of cations in a closed system whe...

Differential Scanning Calorimetry (DSC) was used to study the formation of from amorphous boron p... more Differential Scanning Calorimetry (DSC) was used to study the formation of from amorphous boron powder and powder. The B and powder mixture demon- strated that forms near 600 with an estimated activation energy of 239 kJ/mole. The B and Mg powder mixture has an ad- ditional low temperature reaction near 425 that is initiated by decomposition. Similar effects can be duplicated with the hydride powder via moisture contamination. Index Terms—Magnesium diboride, superconductor, thermal analysis.

Proceedings, annual meeting, Electron Microscopy Society of America

Most previous studies of epitaxy have shown that growth of fee thin films on fee substrates leads... more Most previous studies of epitaxy have shown that growth of fee thin films on fee substrates leads to parallel, cube-on-cube orientation relationships. However, recent studies of Ag/Ni interfaces indicated that non-parallel orientation relationships may be preferred in fcc/fcc systems of large misfit. One objective of this study is to investigate the orientation relationships produced by epitaxy in Au/Cu bilayers. The Au/Cu system is especially interesting because it has complete mutual solid solubility despite the large lattice parameter misfit (∼ 12%). Hence, the second objective of this study is to investigate low-temperature interfacial diffusion phenomena in the special microstructures produced by multivariant epitaxy.

Proceedings, annual meeting, Electron Microscopy Society of America

Misfit dislocations are known to accommodate disregistry at interfaces which separate crystals of... more Misfit dislocations are known to accommodate disregistry at interfaces which separate crystals of different orientation and/or lattice parameter. Most studies of misfit dislocations have addressed epitaxial bicrystals in ‘parallel’ orientation relationships (OR), e.g, cube-on-cube in fee systems. Recent work, though, has demonstrated that non-parallel epitaxial ORs are energetically favored in some systems, e.g, (111)/(001) in fee systems of large misfit (∼12%). In these systems coexistence of equivalent variants of the same OR gives rise to ‘multivariant epitaxy’ wherein a film grows in a special polycrystalline form on a monocrystalline substrate. In this study we examine the accommodation of misfit in the special structures produced by multivariant epitaxy of (111)Ag on (001)Cu.

Applied Surface Science, 2017

Abstract Ordered surface step arrays were successfully prepared on two different types of miscut ... more Abstract Ordered surface step arrays were successfully prepared on two different types of miscut yttria-stabilized zirconia (YSZ) substrates via one-step pre-annealing. These steps were subsequently used as templates for the self-assembly of the nanoislands, which results in significantly improved alignment of the islands along the step direction. Additionally, a unique step faceting phenomenon at island growth front was observed only on the (001)–[100] miscut YSZ substrates. Preliminary investigation indicates that this faceting is induced by doping via surface diffusion, while the elastic interaction between the faceted steps also plays an important role.

MRS Proceedings, 1985

Computer simulations of the Ni/Ag-(001) interphase boundary have been performed with the Embedded... more Computer simulations of the Ni/Ag-(001) interphase boundary have been performed with the Embedded Atom Method. The relaxed atomic configurations, the periodic interaction potential, the shear modulus of the interface, and the interaction of an Au impurity with the interface have been evaluated. In addition, experimental studies have been conducted on the same system; and the structure and composition of the interface have been simultaneously examined.

Analyse, avant et apres recuit a 600 C pendant 9 heures, par microscopie electronique en transmis... more Analyse, avant et apres recuit a 600 C pendant 9 heures, par microscopie electronique en transmission des joints d'interphase dans le systeme MgO/Ni. Le nickel est une couche monocristalline de surface (100) dans laquelle sont deposees des particules cubiques de MgO (0,02 a 0,2 μm) de surfaces {100}. Les joints correspondent a des joints d'interphase de coincidence

MRS Proceedings, 1991

Relative orientations which correspond to minimum energy can be found by particle rotation method... more Relative orientations which correspond to minimum energy can be found by particle rotation methods, both for particles on free surface and particles inside a solid. For common fcc metals (Ni,Ag,Cu,Ag) the minimum energy orientations predicted by Embedded Atom Method calculations correspond well with experimental observations. Epitaxial studies of growth on (001)Cu and (111)Cu show the observed orientation relationships of vapor deposited Ag and Au are consistent with EAM calculations and the limited particle rotation experiments available.

Thermochimica Acta, 2014

ABSTRACT We have studied thermodynamic phase stability in the Mg–B system through use of a high-p... more ABSTRACT We have studied thermodynamic phase stability in the Mg–B system through use of a high-pressure, high-temperature apparatus consisting of a large pressure vessel and an RF induction heater. The incongruent melting temperature for MgB2 was found to be ∼1450 °C at 10 MPa using thermal analysis data applied to both MgB2 powder samples and Mg/B mixtures. The experimental temperature is ∼300 °C lower than temperatures shown in calculated phase diagrams of the Mg–B system at the same pressure and the discrepancy demonstrates the need for further experimental investigations of phase stability in binary Mg–B and ternary Mg–B–X systems. In this study C (as an impurity in boron) was found to have a large effect on the peritectic temperature, with a relatively small (0.7 wt% C) impurity concentration resulting in a ∼40 °C elevation of the peritectic temperature. Along with morphological characterization, XRD and EPMA analyses were used to identify phases and confirm the peritectic transformation in the Mg–B phase diagram.

Journal of the American Ceramic Society, 1993

Molecular statics calculations of mechanical properties have been carried out for NaCI using the ... more Molecular statics calculations of mechanical properties have been carried out for NaCI using the Born–Mayer–Huggins pair potential and periodic boundary conditions. Three truncation techniques are utilized. For each, calculations have been performed as a function of the truncation distance. The stress–strain curve, the fracture strength, and the fracture strain are found to be much more sensitive to the truncation technique and to variations in the truncation distance than other properties such as the elastic constants, the cohesive energy, or the lattice parameter.

Journal of Materials Research, 2006

Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays of single-crystal... more Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays of single-crystal titania nanofibers by reaction with a noncombustible, hydrogen-bearing gas mixture at only 680–780 °C. Such nanofiber formation resulted from anisotropic etching (“nanocarving”) of the titania grains. The fibers possessed diameters of 20–50 nm and lengths of up to several microns, with the long fiber axes oriented parallel to the [001] crystallographic direction of rutile. Mass spectroscopy and inductively coupled plasma spectroscopy indicated that oxygen, but not titanium, was removed from the specimen during the reaction with hydrogen. The removal of substantial oxygen and solid volume from the reacting surfaces, without an appreciable change in the Ti:O ratio at such surfaces, was consistent with the solid-state diffusion of titanium cations from the surface into the bulk of the specimen. The reaction-induced weight loss followed a parabolic rate law, which was also consistent with a...

IEEE Transactions on Applied Superconductivity, 2007

Ceramics International, 2013

Acta Metallurgica et Materialia, 1991

Journal of Materials Research, 1994

The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and g... more The decomposition of congruent lithium niobate (LN) crystals proceeds by surface nucleation and growth of LiNb3O8 precipitates which exhibit an epitaxial orientation relationship. The same orientation relationship is observed on x-, y-, and z-cut LN substrates. The epitaxy arises from similarities between the two crystal structures and provides for an essentially continuous oxygen-ion framework from parent to product. On y-cut LN, the precipitates have a well-defined habit plane, and the interfacial misfit between the two structures is accommodated by a rectangular grid of misfit dislocations. The density, geometry, and imaging behavior of the misfit dislocations suggest that their Burgers vectors serve to accommodate the disregistry in the oxygen-ion framework. Based on these observations, it is concluded that the mechanism of formation of LiNb3O8 consists of preferential nucleation on the surface and subsequent growth, possibly by counterdiffusion of cations in a closed system whe...

Differential Scanning Calorimetry (DSC) was used to study the formation of from amorphous boron p... more Differential Scanning Calorimetry (DSC) was used to study the formation of from amorphous boron powder and powder. The B and powder mixture demon- strated that forms near 600 with an estimated activation energy of 239 kJ/mole. The B and Mg powder mixture has an ad- ditional low temperature reaction near 425 that is initiated by decomposition. Similar effects can be duplicated with the hydride powder via moisture contamination. Index Terms—Magnesium diboride, superconductor, thermal analysis.

Proceedings, annual meeting, Electron Microscopy Society of America

Most previous studies of epitaxy have shown that growth of fee thin films on fee substrates leads... more Most previous studies of epitaxy have shown that growth of fee thin films on fee substrates leads to parallel, cube-on-cube orientation relationships. However, recent studies of Ag/Ni interfaces indicated that non-parallel orientation relationships may be preferred in fcc/fcc systems of large misfit. One objective of this study is to investigate the orientation relationships produced by epitaxy in Au/Cu bilayers. The Au/Cu system is especially interesting because it has complete mutual solid solubility despite the large lattice parameter misfit (∼ 12%). Hence, the second objective of this study is to investigate low-temperature interfacial diffusion phenomena in the special microstructures produced by multivariant epitaxy.

Proceedings, annual meeting, Electron Microscopy Society of America

Misfit dislocations are known to accommodate disregistry at interfaces which separate crystals of... more Misfit dislocations are known to accommodate disregistry at interfaces which separate crystals of different orientation and/or lattice parameter. Most studies of misfit dislocations have addressed epitaxial bicrystals in ‘parallel’ orientation relationships (OR), e.g, cube-on-cube in fee systems. Recent work, though, has demonstrated that non-parallel epitaxial ORs are energetically favored in some systems, e.g, (111)/(001) in fee systems of large misfit (∼12%). In these systems coexistence of equivalent variants of the same OR gives rise to ‘multivariant epitaxy’ wherein a film grows in a special polycrystalline form on a monocrystalline substrate. In this study we examine the accommodation of misfit in the special structures produced by multivariant epitaxy of (111)Ag on (001)Cu.

Applied Surface Science, 2017

Abstract Ordered surface step arrays were successfully prepared on two different types of miscut ... more Abstract Ordered surface step arrays were successfully prepared on two different types of miscut yttria-stabilized zirconia (YSZ) substrates via one-step pre-annealing. These steps were subsequently used as templates for the self-assembly of the nanoislands, which results in significantly improved alignment of the islands along the step direction. Additionally, a unique step faceting phenomenon at island growth front was observed only on the (001)–[100] miscut YSZ substrates. Preliminary investigation indicates that this faceting is induced by doping via surface diffusion, while the elastic interaction between the faceted steps also plays an important role.

MRS Proceedings, 1985

Computer simulations of the Ni/Ag-(001) interphase boundary have been performed with the Embedded... more Computer simulations of the Ni/Ag-(001) interphase boundary have been performed with the Embedded Atom Method. The relaxed atomic configurations, the periodic interaction potential, the shear modulus of the interface, and the interaction of an Au impurity with the interface have been evaluated. In addition, experimental studies have been conducted on the same system; and the structure and composition of the interface have been simultaneously examined.

Analyse, avant et apres recuit a 600 C pendant 9 heures, par microscopie electronique en transmis... more Analyse, avant et apres recuit a 600 C pendant 9 heures, par microscopie electronique en transmission des joints d'interphase dans le systeme MgO/Ni. Le nickel est une couche monocristalline de surface (100) dans laquelle sont deposees des particules cubiques de MgO (0,02 a 0,2 μm) de surfaces {100}. Les joints correspondent a des joints d'interphase de coincidence

MRS Proceedings, 1991

Relative orientations which correspond to minimum energy can be found by particle rotation method... more Relative orientations which correspond to minimum energy can be found by particle rotation methods, both for particles on free surface and particles inside a solid. For common fcc metals (Ni,Ag,Cu,Ag) the minimum energy orientations predicted by Embedded Atom Method calculations correspond well with experimental observations. Epitaxial studies of growth on (001)Cu and (111)Cu show the observed orientation relationships of vapor deposited Ag and Au are consistent with EAM calculations and the limited particle rotation experiments available.

Thermochimica Acta, 2014

ABSTRACT We have studied thermodynamic phase stability in the Mg–B system through use of a high-p... more ABSTRACT We have studied thermodynamic phase stability in the Mg–B system through use of a high-pressure, high-temperature apparatus consisting of a large pressure vessel and an RF induction heater. The incongruent melting temperature for MgB2 was found to be ∼1450 °C at 10 MPa using thermal analysis data applied to both MgB2 powder samples and Mg/B mixtures. The experimental temperature is ∼300 °C lower than temperatures shown in calculated phase diagrams of the Mg–B system at the same pressure and the discrepancy demonstrates the need for further experimental investigations of phase stability in binary Mg–B and ternary Mg–B–X systems. In this study C (as an impurity in boron) was found to have a large effect on the peritectic temperature, with a relatively small (0.7 wt% C) impurity concentration resulting in a ∼40 °C elevation of the peritectic temperature. Along with morphological characterization, XRD and EPMA analyses were used to identify phases and confirm the peritectic transformation in the Mg–B phase diagram.