Luminescence Study of Ion-Implanted Gallium Nitride

Luminescence and absorption measurements were used to demonstrate the efficacy of ion implantation for introducing various classes of dopants into GaN. A wide range of implantation and annealing studies were performed with several dopant species (Ar, Zn, C, O, Si, Be, Mg, Nd, Er). Room temperature ion implantation was performed on MOCVD- and MBE-grown GaN samples at energies between 100 and 1150 keV with doses ranging from 1 x 1013 to 1 x 1015/cm-2. Conventional furnace annealing in flowing NH3 or N2 gas resulted in good implantation damage recovery at an annealing temperature of 1000°C for 90 min. Annealing temperature was found to be the determining factor in implantation damage recovery. It was discovered that surface degradation occurred for annealing in an NH3 environment at temperatures above 1000°C. An optimal annealing temperature of 1000°C and an optimal annealing gas environment of NH3 were found for the optical activation of Zn, Mg, Er, and Nd. Several new luminescence features were observed for the various dopants. Zn-implanted GaN was found to have a strong luminescence peak in the blue at 2.86 eV. The energetic location and width of this luminescence peak was insensitive to temperature changes and excitation intensity changes. These properties suggested that an internal Zn center transition was responsible

Measurements of rotor stalling in a matched and a mismatched multistage compressor

April 1995Includes bibliographical references (page 21)This paper presents the results from a set of experiments on stall inception in multistage axial flow compressors. The experiments were tailored to investigate phenomena having a wide range of time and length scales. This range of scales was motivated by two previously observed paths to stall. Parametric changes such as tip clearance, inlet distortion and mismatch were carried out to demonstrate the importance of component coupling in the stall inception process. Evidence is presented for the importance of the local compressor characteristic in determining where and when the initiation of the stall inception process will occur. Although the stall inception process may begin as a localized event, its growth into rotating stall is governed by the environment established by the coupling of the various compression system components. Finally, the tip flow field, specifically the rotor tip leakage jet, is shown to be a key feature in the stall inception process.Supported by the Air Force Office of Scientific Research, General Electric Aircraft Engine Co. and the Air Force Research in Aero Propulsion Technology (AFRAPT) Progra

Aerodynamic design of moveable inlet guide vanes for active control of rotating stall

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1990.Includes bibliographical references (leaves 57-59).by Peter D. Silkowski.M.S

An assessment on the unsteady flow distortion generated by an S-duct intake

Closer integration between the fuselage and the propulsion system is expected for futureaircraft toreducefuel consumption, emissions, weight and drag. The use of embedded or partially embedded propulsion systems may require the use of complex intakes. However, thiscanresult in unsteady flow distortion which can adversely affect the propulsion system efficiency and stability. This works assesses the characteristics of the unsteady flow with a view to the potential flow distortion presented to the compression system.Particle image velocimetry is used to measure the flow distortion generated by an S-shaped intake.The time-resolved tracking of the idealized relative incidence angle revealed that most frequent distortion events exhibited90°exposure sector and upto±5°meanrelativeincidence. The imposition of a thicker boundary at the S-duct inlet increased the probability of distortion events that are characterized by a longer exposure sector and higher relative incidence angles.Because of these characteristics, thedistortion caused by the S-duct intake could induce instabilities that are detrimental for the propulsion system performances and stability. Overall, this work proposes a new method to assess thepossible relativeincidence angle on the compressor rotor taking into account the intake flow unsteadiness

Assessing sedimentation equilibrium profiles in analytical ultracentrifugation experiments on macromolecules: from simple average molecular weight analysis to molecular weight distribution and interaction analysis

Molecular weights (molar masses), molecular weight distributions, dissociation constants and other interaction parameters are fundamental characteristics of proteins, nucleic acids, polysaccharides and glycoconjugates in solution. Sedimentation equilibrium in the analytical ultracentrifugation provides a powerful method with no supplementary immobilization, columns or membranes required. It is particularly powerful when used in conjunction with its sister technique, namely sedimentation velocity analysis. We describe key approaches now available and their application to the characterisation of antibodies polysaccharides and glycoconjugates. We indicate how major complications such as thermodynamic non-ideality can now be routinely dealt with, thanks to a great extent to the extensive contribution of Professor DonWinzor over several decades of research

Purification of bacterial membrane sensor kinases and biophysical methods for determination of their ligand and inhibitor interactions

This article reviews current methods for the reliable heterologous overexpression in Escherichia coli and purification of milligram quantities of bacterial membrane sensor kinase (MSK) proteins belonging to the two-component signal transduction family of integral membrane proteins. Many of these methods were developedatLeedsalongsideProfessor SteveBaldwintowhomthisreviewisdedicated.Italsoreviewstwo biophysical methods that we have adapted successfully for studies of purified MSKs and other membrane proteins – synchrotron radiation circular dichroism (SRCD) spectroscopy and analytical ultracentrifugation (AUC), both of which are non-immobilization and matrix-free methods that require no labelling strategies. Other techniques such as isothermal titration calorimetry (ITC) also share these features but generally require high concentrations of material. In common with many other biophysical techniques, both of these biophysical methods provide information regarding membrane protein conformation, oligomerization state and ligand binding, but they possess the additional advantage of providing direct assessments of whether ligand binding interactions are accompanied by conformational changes. Therefore, both methods provide a powerful means by which to identify and characterize inhibitor binding and any associated protein conformational changes, thereby contributing valuable information for future drug intervention strategies directed towards bacterial MSKs

First beta-decay spectroscopy of In-135 and new beta-decay branches of In-134

The beta decay of the neutron-rich In-134 and In-135 was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number Z = 50 above the N = 82 shell. The beta-delayed gamma-ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three beta-decay branches of In-134 were established, two of which were observed for the first time. Population of neutron-unbound states decaying via. rays was identified in the two daughter nuclei of In-134, Sn-134 and Sn-133, at excitation energies exceeding the neutron separation energy by 1 MeV. The beta-delayed one- and two-neutron emission branching ratios of In-134 were determined and compared with theoretical calculations. The beta-delayed one-neutron decay was observed to be dominant beta-decay branch of In-134 even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of Sn-134. Transitions following the beta decay of In-135 are reported for the first time, including. rays tentatively attributed to Sn-135. In total, six new levels were identified in Sn-134 on the basis of the beta.. coincidences observed in the In-134 and In-135 beta decays. A transition that might be a candidate for deexciting the missing neutron single-particle 13/2(+) state in Sn-133 was observed in both beta decays and its assignment is discussed. Experimental level schemes of Sn-134 and Sn-135 are compared with shell-model predictions. Using the fast timing technique, half-lives of the 2(+), 4(+), and 6(+) levels in Sn-134 were determined. From the lifetime of the 4(+) state measured for the first time, an unexpectedly large B(E2; 4(+)-> 2(+)) transition strength was deduced, which is not reproduced by the shell-model calculations.Peer reviewe