Showing papers by "Y. A. Chang published in 1995"

Diffusional behavior in B2 intermetallic compounds

Abstract: The Huntington-McCombie-Elcock (HME) and antistructure bridge (ASB) mechanisms proposed for B2 intermetallic compounds are reviewed. By using a thermodynamic argument, the HME mechanism is shown to be operative near the stoichiometric composition. On the other hand, the ASB mechanism is shown to be important at compositions away from stoichiometry. A combination of these two mechanisms is able to describe the compositional dependences of the experimentally determined self-diffusion coefficients for several B2 intermetallic compounds. By using conclusions drawn from these two mechanisms, it is further shown that the compositional dependences of the activation energies and pre-exponential factors are related to the partial molar enthalpies and entropies of the component elements. The frequently observed decreases in the activation energy and pre-exponential factor with deviations from stoichiometry are in accord with the corresponding decreases in the partial molar enthalpy and entropy. The extensive experimental data for NiAl are used to verify these relations.

Synthesis of in situ composites through solid-state reactions: thermodynamic, mass-balance and kinetic considerations

Abstract: The applications of thermodynamics, mass balance and kinetics to the synthesis of in situ composites through solid-state reactions are discussed. The proper choice of starting materials, the principles governing the diffusion paths and the formation of desirable microstructures are considered. The use of stability diagrams for rationalizing diffusion and reactions during composite synthesis is also demonstrated. Applying these principles to the synthesis of NbS1 2 -SiC composites, it is shown that the proper starting materials are Nbc 1-r and Si. The probable microstructure is an aggregate type, composed of NbSi 2 and SiC. Preliminary experimental results of the study of bulk NbC 1-x -Si diffusion couples annealed at 1300 °C for 60 h revealed that the microstructure is indeed a two-phase mixture of NbSi 2 and SiC. Discontinous SiC particles with an average size of 1 μm are homogeneously dispersed in the NbSi 2 matrix. This microstructure is considered favorable for high-temperature structural composite application. Further characterization of mechanical and other properties for this material are currently being performed.

Development of multicomponent solidification micromodels using a thermodynamic phase diagram data base

Abstract: Solidification micromodeling of industrial alloys requires the treatment of multicomponent alloys. For particular alloys, some progress has been achieved using an approximate binary phase diagram of the important solute or using a multicomponent approach with values of the liquidus slopes and partition coefficients obtained either from the binary phase diagrams or from experiments on a small number of samples of the alloy of interest. The authors have sought to employ a multicomponent thermodynamic calculation of the phase boundaries between liquid and the various solid phases that can be directly linked to micromodel codes. This approach quite naturally treats the temperature and/or concentration dependence of partition coefficients and liquidus slopes, and the formation of secondary solid phases. In addition, the same data base can be used for alloys of the same base metal but with different compositions. The authors report a phase diagram subroutine using a preliminary thermodynamic data base for Ni-base superalloys containing up to 10 elements. This subroutine is used for a Scheil model and for a solidification model that includes solid diffusion for IN718.

Pdal schottky contact to in0.52al0.48as grown by metalorganic chemical vapor deposition

Abstract: β‐PdAl was studied as a Schottky contact to metalorganic chemical vapor deposition grown In0.52Al0.48As. Intermetallic alloy β‐PdAl was chosen in order to utilize the Al–In exchange reaction which may occur between PdAl and In0.52Al0.48As, which would result in an enhanced Schottky barrier height. I–V, C–V, and deep level transient spectroscopy (DLTS) were used to determine the contact characteristics. The contact barrier height (φb) was measured by I–V and C–V methods after different annealing conditions, and good agreement between I–V and C–V results were obtained. The largest φb value is 0.67 eV from I–V measurement (0.69 eV from C–V) after the diode was annealed at 450 °C for 1 min. DLTS measurements were carried out to examine the effect of deep traps in the In0.52Al0.48As layer. Two deep levels were found, but the concentrations are lower than the intrinsic donor concentration obtained from the Hall method. The activation energies for these two deep levels obtained from an Arrhenius plot are 0.38 an...

Schottky barrier enhancement using reacted Ni2Al3/Ni/n-GaAs, Ni/Al/Ni/n-GaAs, and NiAl/Al/Ni/n-GaAs contacts

Abstract: A systematic study of the enhancement of Schottky barriers to n‐GaAs has been carried out using the following diodes, Ni2Al3/n‐GaAs, Ni2Al3/Ni/n‐GaAs, Ni/Al/Ni/n‐GaAs and NiAl/Al/Ni/ n‐GaAs, prepared by sputter deposition at a base pressure ∼2×10−7 Torr. A high Schottky barrier height ranging from 0.95 to 0.98 eV (deduced from current‐voltage measurements) was observed for all the annealed contacts except for Ni2Al3/n‐GaAs. Enhancement of the Schottky barrier height in all the contacts was attributed to the formation of a high Al content (Al,Ga)As layer at the metal/semiconductor interface. A regrowth mechanism was used to rationalize the Schottky barrier enhancement. In this mechanism, Ni reacts with GaAs initially at low temperatures, forming NixGaAs. The NixGaAs layer then reacts with the Ni‐Al layer on top to form the (Al, Ga)As layer under subsequent high temperature annealing. A (200) dark‐field XTEM image of the annealed contact was used to demonstrate the existence of this (Al,Ga)As phase.

Phase equilibria in the Ga- In- Ni system at 600 °C

Abstract: Phase equilibria are established in the Ga-In-Ni system at 600 °C using X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA). The system is characterized by a notable absence of ternary solubility among the constituent binary phases. The only exception is the phase Ni2ln3, which dissolves a maximum of 12.7 at% Ga. Two ternary phases exist in the compositional vicinity of γ TVi13Ga9 and Ni5Ga3, both of which contain approximately 10 at.% In. These phases are considered to be superlattice structures derived from that of the binary, high-temperature phase γNi3Ga2, which possesses a partially filled NiAs (B81) structure. The phase diagram isotherm determined in the present investigation was also compared with an isotherm calculated using experimental thermodynamic data for the constituent binary phases, which are available in the literature. The calculated and experimental isotherms agree well.