Materials Science & Engineering Theses and Dissertations

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The Determination of Preferred Orientation in Rolled Electrical Steels Using Single Diffraction of Neutrons
(1963) Eugenio, Manuel; Duffey, Dick; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
Preferred orientation in rolled electrical steels has been determined using single diffraction of neutrons from the University of Maryland pool-type nuclear reactor (DMR) operating at 10 KW thermal . X-rays are used extensively to determine preferred orientations in metallic wires and rolled sheets, but X-rays suffer the disadvantage of high absorption and cannot be used effectively on thick samples without chemical or mechanical treatment which ultimately results in the destruction of the samples. The use of reactor neutrons for this purpose is believed to offer particular advantages such as the use of thicker samples and wider beams. To this end, neutrons from the UMR were scattered directly from metallic sheet samples to obtain diffraction patterns from which preferred orientations of the crystallographic axes could be deduced. The neutron diffraction data were obtained in the form of : 1) Maxwellian curves; and 2) rocking curves. To obtain the first type of curve, the sample and neutron detector were rotated at a 1-to-2 angular ratio, respectively, and the diffraction pattern was essentially the Maxwellian neutron energy distribution. From the maximum of the Maxwellian curve, the crystallographic plane mainly responsible for the reflection was calculated; from this, the main orientation was deduced. For the second type of curve, the sample was rocked back and forth, with the neutron detector fixed, and the resulting pattern was used to infer the variation of a given crystallographic direction about its main orientation. The results of this study, particularly on grain-oriented and cube-textured silicon-iron (Si-Fe) alloy sheets demonstrate that single diffraction techniques can be used to determine preferred orientation in highly oriented materials. The results on Si-Fe sheets described as non-oriented indicate the possibility that these techniques may be applicable to ordinary rolled metallic sheets, which are not highly oriented.
Magnetic Anisotropy of Fe1-xGax Alloys
(2003-12-22) Rafique, Sadia; Wuttig, Manfred; Salamanca-Riba, Lourdes; Cullen, James R; Material Science and Engineering
Cubic magnetocrystalline anisotropy constants, K1 and K2, for Fe1-xGax alloys were measured using magnetization curves with x = 0.05, 0.125, 0.14, 0.18 and 0.20. Thin circular {110} disks with all <100>, <110> and <111> in the plane of the disk were used to measure K1 and K2. K1 was also measured with {100} circular disks. K1 for 5 at% Ga content has been found to be larger than that of pure Fe. K1 and K2 both drops gradually till 18 % Ga substitution. Then there is a sharp drop in the magnitude of both the constants. <110> and <111> directions were magnetically equivalent for all the compositions considered for this study resulting in K2 to be equal to 9K1/4. A calculation of anisotropy energy density verifies this result. Magnitude of K1 measured from both {110} and {100} disks were reasonably consistent.
Transmission Electron Microscopy Studies of FeGa Alloys
(2004-01-27) Koda, Nobuko; Wuttig, Manfred; Salamanca-Riba, Lourdes; Cullen, James R; Material Science and Engineering
Microstructural analysis of rapid-solidified (melt-spun) FeGa alloys with 17.3at.%Ga and bulk alloys with 12-20at.%Ga using transmission electron microscope was carried out. Dark field and high resolution image analysis show all the samples are inhomogeneous with fine texture of 5-10nm. Although the expected crystal structure is bcc for all the samples, bulk samples with 12-20at.%-Ga contain additional phases other than bcc phase. Tetragonal modulation of DO3 and superlattice with diagonal arrangement of two B2(CsCl-type) cells in bcc gave the best identification for observed diffraction patterns. Tetragonality calculated from mismatch between observed diffraction spots and the calculation for the assumed tetragonal structure was approximately 1.1.
Fabrication and Packaging Optimization for Polymer-Based Microfluidics
(2004-04-30) Valentine, Theresa Michelle; Rubloff, Gary W; Material Science and Engineering
Packaging microelectromechanical systems (MEMS) often accounts for 80 percent of both the cost and the failures of the devices. For biological MEMS with microfluidic channels, packaging requires reliable fluid and electrical connections. This work describes various strategies for optimizing the fabrication of microfluidic circuits and the design of leak-tight, re-usable, multi-functional packaging systems. Various materials are surveyed to determine the appropriate microfluidic chip substrate for an all-polymer device. Three unique test site designs allow combinatorial experiments and improve the functionality of three proven leak-tight packaging fixtures. Finally, the successful deposition of chitosan, a polysaccharide biopolymer that can act as the interface layer between inorganic electrodes and biological components such as proteins and nucleic acids, is shown in a packaged microfluidic environment for the first time. This study lays the groundwork for future applications in miniaturized bio-reactors and chemical and biological sensors.
Combinatorial Exploration of Artificial Multiferroic Thin Films
(2004-05-12) Lin, Chuan-Lan; Takeuchi, Ichiro; Material Science and Engineering
Combinatorial synthesis consists of high throughput fabrication and rapid characterization of compositionally varying samples to speed up the process of materials development. In this thesis, synthesis of composition spreads of artificial multiferroic thin film heterostructures consisting of alternating layers of (PTO) - (CFO) has been demonstrated using our combinatorial pulsed laser deposition (PLD) system. In the spread samples, the average composition changes continuously from pure PTO to pure CFO so that we can observe the changes in physical properties as a function of average composition. The coexistence of ferromagnetic and ferroelectric properties has been observed in a large extended region between pure CFO and PTO. An unexpected peak in the dielectric property has been observed in the middle of the spread, and it was identified as the composition where the ferroelectric phase transition takes place in PTO doped with CFO. The of ferroelectricity is found to be tunable from 500 ℃ to room temperature by controlling the average volume ratio of CFO and PTO. We have also found that the magnetic anisotropy in the materials changes by introducing PTO to CFO.
Evolution of Patterned Gallium Arsenide (001) surface subjected to Molecular Beam Epitaxy Growth
(2004-05-14) Shah, Sonam Shantilal; Phaneuf, Raymond J; Material Science and Engineering
We describe here a study of lateral length scale dependence of the transient evolution of surface corrugation during MBE growth of alternating layers of AlAs and GaAs with individual layer thickness of 50 nm (approximate configuration as used in the distributed Bragg reflectors of a VCSEL) or GaAs layers onto the patterned GaAs (001) substrates. By patterning the surface with arrays of cylindrical pits of varying diameter and spacing, we were able to study selectively the changes which occur as a function of lateral period over a range of corrugation amplitudes. The evolution in the surface morphology after various stages of growth was characterized in air with AFM. We show that there exists a critical length scale which separates regimes of amplification and decay of corrugation amplitude with further growth. We compare our observations with the predictions of existing continuum models.
Small Angle Neutron Scattering of Arborescent Graft Polymer Solutions
(2004-06-01) Lai, Kai-Chi; Briber, Robert M.; Material Science and Engineering
Small Angle Neutron Scattering (SANS) was used to characterize the size and shape of generation 3 and generation 4 polystyrene (PS) based arborescent graft polymers where the final generation of the polymer molecule composed of deuterated PS. Contrast variation techniques were used to match the solvent to the either the PS core or the deuterated PS shell. A core-shell model was used to fit the SANS data with good success. Density profiles of generation 4 and generation 3 arborescent graft polymers in different solvents are calculated, and the size of the molecule is found to be dominated by the solvent quality. The radius of gyration is found to be determined by the scattering length density of core and shell, core radius and total hydrodynamic radius. The radius of gyration from Guinier plot is discussed and found unreliable due to non-uniform density distribution of the molecule.
NEUTRONIC EFFECTS ON TUNGSTEN-186 DOUBLE NEUTRON CAPTURE
(2004-07-02) Garland, Marc; Al-Sheikhly, Mohamad I; Mirzadeh, Saed; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Rhenium-188, a daughter product of tungsten-188, is an isotope of great interest in therapeutic nuclear medicine, being used in dozens of laboratory and clinical investigations worldwide. Applications include various cancer therapy strategies, treatment of rheumatoid arthritis, prevention of restenosis following coronary artery angioplasty, and palliation of bone pain associated with cancer metastases. With its half-life of 17 hours, 2.12 MeV (maximum) beta-particle emission, chemical similarity to technetium-99m (the most widely used diagnostic radioisotope), and its availability in a convenient tungsten-188/rhenium-188 generator system, rhenium-188 is a superb candidate for a broad range of applications. Production of 188W is typically via double neutron capture by 186W in a high flux nuclear reactor, predominantly the High Flux Isotope Reactor at the Oak Ridge National Laboratory in Tennessee. Experience at HFIR has shown that production yields (measured in Ci of 188W produced per g of 186W target) decrease considerably as target size increases. While the phenomenon of neutron resonance self-shielding would be expected to produce such an effect, temperature effects on neutron flux distribution and neutron capture rates may also be involved. Experimental investigations of these phenomena have not been previously performed. The work presented in this thesis evaluates the factors that contribute to the decrease in 188W yield from both theoretical and experimental standpoints. Neutron self-shielding and temperature effects were characterized to develop a strategy for target design that would optimize production yield, an important factor in minimizing health care costs. It was determined that decrease in yield due to neutron self-shielding can be attributed to depletion of epithermal neutrons at resonant energies, most significantly within the initial 0.4 mm depth of the target. The results from these studies further show that 188W yield in the interior of the target (beyond 0.4 mm depth) does not decrease as would be expected due to neutron attenuation. This observation was explained by the fact elevated temperatures in the interior of the target result in an increase in the 188W yield through Doppler broadening of cross sections, compensating for reduced yield due to neutron attenuation. Finally, this work supports earlier analyses that questioned the accuracy of the 187W thermal cross section and resonance integral.
Investigation of Low Temperature Creep Deformation Behavior of a Metastable Beta Titanium-14.8Wt%Vanadium Alloy
(2004-07-13) Hudson, Candi Monica; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation presents the results of investigated low temperature creep behavior of a metastable beta phase Ti-14.8Weight%V alloy (Ti-14.8V). It is the first such study which relates the activation energy and microstructure with low temperature creep deformation mechanisms in the temperature range of 298K to 800K. A Ti-14.8V alloy with a grain size of 350 m was tensile and creep tested in the temperature range of 298 - 458 K; creep tests were conducted at 95% of the 0.2% yield stress. Activation energies were determined by utilizing strain rate models and resulting least squared Arrhenius plots, which were found to be in the range of 36.6-112.42 kJ/mole for the measured temperature range of 298 - 458K. The resulting activation energies plotted as a function of strain was found to be linear dependent. The determined activation energy values of 36.6 57.55 kJ/mole at the low end of the strain are within the range of activation energy values for dislocation motion. The higher activation energy value of 112.42 kJ/mole is within range of for activation energy value for diffusion of oxygen in beta titanium alloy. These activation energy values are consistent with SEM and TEM observations of deformation mechanisms as dislocations, slip, and stress induced plates (SIP) in the form of twinning were the dominant creep deformation mechanisms for this alloy. The deformation mechanisms changed from predominantly slip to SIP in the form of twins at the higher test temperatures. Further, these findings are consistent with observations, characterization by TEM analysis identified slip dislocations of the 1/2<111> type and twins of the {332}<113> type, which are consistent with time dependent twinning deformation. The results strongly support the mechanism of oxygen controlled time dependent twinning deformation as proposed earlier.
Real-Time In-Situ Chemical Sensing in AlGaN/GaN Metal-Organic Chemical Vapor Deposition Processes for Advanced Process Control
(2004-08-04) Cho, Soon; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Gallium nitride and its alloys promise to be key materials for future semiconductor devices aimed at high frequency, high power electronic applications. However, manufacturing for such high performance products is challenged by reproducibility and material quality constraints that are notably more stringent than those required for optoelectronic applications. To meet this challenge, in-situ mass spectrometry was implemented as a real-time process- and wafer-state metrology tool in AlGaN/GaN/AlN metal-organic chemical vapor deposition processes on semi-insulating SiC substrate wafers. Dynamic chemical sensing through the process cycle, carried out downstream from the wafer, revealed generation of methane and ethane reaction byproducts, as well as other residual gas species. Real-time metrics were derived based on the chemical signals to predict/control material quality and thickness of critical layers within the heterostructure in real time during growth, and corresponding metrologies were used for real-time advanced process control. Using the methane/ethane ratio, GaN epilayer crystal quality was predicted in real time to 2 5% precision, which was verified by post-process x-ray diffraction. Moreover, the same real-time metric predicted material quality as indicated by post-process photoluminescence band-edge intensities to ~5% precision. The methane/ethane ratio has a fundamental significance in terms of the intrinsic chemistry in that the two byproducts are believed to reflect two parallel reaction pathways leading to GaN-based material growth, namely the gas phase adduct formation route and the surface route for direct precursor decomposition, respectively. The fact that lower methane/ethane ratios consistently yield better material quality suggests that the surface pathway is preferred for high quality GaN growth. In addition, a metric based on methane and ethane signals integrated through the AlGaN growth period (~1 min or less) enabled prediction of the cap layer thickness (~20 nm) to within ~1% precision, which was verified by post-process x-ray reflectance. These types of real-time advanced process control activities in terms of fault detection and management, course correction, and pre-growth contamination control have made significant contributions to the GaN-based semiconductor development and manufacturing at Northrop Grumman Electronics Systems in terms of improved material quality, yield, and consequent cost reduction, and they are now in routine use.
Dynamics of Near-Alpha Titanium Welding
(2004-10-12) Neuberger, Brett William; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Typically, when gas tungsten arc welding (GTAW) is employed to join near-alpha titanium alloys, the resulting weld fusion zone (FZ) is much harder than that of the base metal (BM), thereby leading to lost ductility. The aim of this investigation was to improve FZ ductility of Ti-5Al-1Sn-1V-1Zr-0.8Mo by modifying filler metal chemistry. In this regard, metallic yttrium was added to the filler metal and aluminum concentration reduced. It was believed that additions of yttrium would lead to formation of yttria in the weld melt, thereby promoting heterogeneous nucleation. Since oxygen and aluminum both act as alpha-stabilizers, expected pickup of oxygen during the welding process will be offset by the aluminum reduction. Tensile testing indicated that modified filler metal welds showed a dramatic increase in ductility of the FZ. Fracture toughness testing showed that while JIC values decreased in all welds, the tearing modulus, T, in modified filler metal welds was significantly higher than that of matching filler metal welds. Microhardness mapping of the weld zones illustrated that modified filler metal welds were significantly softer than matching filler metal welds. Microstructural examinations were completed through the use of optical, SEM and TEM studies, indicating that there was a presence of nano-particles in the weld FZ. XPS analysis identified these particles as yttrium oxysulfate. WDS analysis across the welds' heat affected zones demonstrated that there is an internal diffusion of oxygen from the BM into the FZ. Research results indicate yttrium oxysulfide particles form in the weld pool, act as a drag force on the solidification front and limit growth of prior-beta grain boundaries. The reduced prior-beta grain size and removal of interstitial oxygen from the matrix in modified filler metal welds, further enhanced by oxidation of yttrium oxysulfide to yttrium oxysulfate, leads to increased ductility in the weld's FZ. Addition of yttrium to the weld also acts to modify the surface tension of the melt, leading to an increased weld depth penetration. Results of this work indicate that the goals of this project and a significant advancement in the understanding of yttrium effects on titanium grain refinement have been achieved.
Elasticity in Ferromagnetic Shape Memory Alloys
(2004-11-23) Dai, Liyang; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Ferromagnetic shape memory alloys (FSMAs) are a new class of active materials, which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials are technologically interesting due to the possibility of inducing large shape changes with an external applied magnetic field; either inducing the austenite/ martensite transformation or rearranging the martensitic variant structure with an applied field will induce a reversible shape change. The dependence of a solid's elastic properties on temperature in the vicinity of a structural transformation provides insight into the nature of the transition. Therefore, the elasticity of Ni2MnGa and Fe3Pd were studied. The temperature dependence of the elastic constants of the austenitic Ni0.50Mn0.284Ga0.216 and Ni0.49Mn0.234Ga0.276 were studied by an ultrasonic continuous wave method. Anomalous behavior in austenite was observed, which indicates a premartensitic transition. The temperature dependence of the elastic constants in martensitic Ni0.50Mn0.284Ga0.216 indicates a structural phase change from the tetragonal to a second phase at lower temperature. Modeling this phase change as a reentrance transition reproduces the major aspects of the temperature dependence of the shear elastic constant, (C11-C12)/2. The elasticity as a function of temperature and magnetic field of Fe3Pd was studied as well. An abrupt change of the elastic constants at around 45OC indicates a possible premartensitic transformation. The magnetic field dependence of elastic constants also indicates a probably magnetic field induced transition.
Growth and Characterization of Multiferroic BaTiO3-CoFe2O4 Thin Film Nanostructures
(2004-12-08) Zheng, Haimei; Salamanca-Riba, Lourdes; Ramesh, Ramamoorthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Multiferroic materials which display simultaneous ferroelectricity and magnetism have been stimulating significant interest both from the basic science and application point of view. It was proposed that composites with one piezoelectric phase and one magnetostrictive phase can be magnetoelectrically coupled via a stress mediation. The coexistence of magnetic and electric subsystems as well as the magnetoelectric effect of the material allows an additional degree of freedom in the design of actuators, transducers, and storage devices. Previous work on such materials has been focused on bulk ceramics. In the present work, we created vertically aligned multiferroic BaTiO3-CoFe2O4 thin film nanostructures using pulsed laser deposition. Spinel CoFe2O4 and perovskite BaTiO3 spontaneously separated during the film growth. CoFe2O4 forms nano-pillar arrays embedded in a BaTiO3 matrix, which show three-dimensional heteroepitaxy. CoFe2O4 pillars have uniform size and spacing. As the growth temperature increases the lateral size of the pillars also increases. The size of the CoFe2O4 pillars as a function of growth temperature at a constant growth rate follows an Arrhenius behaviour. The formation of the BaTiO3-CoFe2O4 nanostructures is a process directed by both thermodynamic equilibrium and kinetic diffusion. Lattice mismatch strain, interface energy, elastic moduli and molar ratio of the two phases, etc., are considered to play important roles in the growth dynamics leading to the nanoscale pattern formation of BaTiO3-CoFe2O4 nanostructures. Magnetic measurements exhibit that all the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It was calculated that stress anisotropy is the main contribution to the anisotropy field. We measured the ferroelectric and piezoelectric properties of the films, which correspond to the present of BaTiO3 phase. The system shows a strong coupling of the two order parameters of polarization and magnetization through the coupled lattices. This approach to the formation of self-assembled ferroelectric/ferro(ferri-)magnetic nanostructures is generic. We have created similar nanostructures from other spinel-perovskite systems such as BiFeO3-CoFe2O4, BaTiO3-NiFe2O4, etc., thus making it of great interest and value to a broad materials community.
Deposition and Characterization of Multiferroic BiFeO3 Thin Films
(2005-01-20) Wang, Junling; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Multiferroics, defined as materials with coexistence of at least two of the electric, elastic, and magnetic orders, have attracted enormous research activities recently. A subsystem of multiferroics is the ferroelectromagnet, which possesses both electric and magnetic orders. One of the natural ferroelectromagnets is BiFeO3, which has ferroelectric (TC~1100K) and antiferromagnetic (TN~640K) orders at room temperature. Even though bulk samples have been synthesized back in 1950s, characterizations of its intrinsic properties have been difficult due to poor sample quality. This work is the first study on epitaxial BiFeO3 thin films. Highly resistive films have been prepared using Pulsed Laser Deposition. (001), (110) and (111) cut SrTiO3 substrates were used to control the film orientation. Film structures were characterized using both X-ray diffraction and transmission electron microscope. It was found that epitaxial stress changes the film structure. Monoclinic domain splitting was observed from both (101) and (001) oriented films, while (111) films remain rhombohedral similliar to single crystals. Much larger polarizations were observed for all three orientations (~55 C/cm2 for (001) films, ~80 C/cm2 for (101) films, and ~100 C/cm2 for (111) films). Calculation using the effective charges and reported ion displacements is performed; indicating that the large observed polarization is likely the intrinsic property of BiFeO3. Magnetic measurements reveal that these resistive BiFeO3 thin films show hysteresis behavior at room temperature, which was not observed in bulk single crystal under the same field range. Thickness dependence of the magnetic property was studied. It is proposed that epitaxial stress destroys the cycloidal spin structure of BiFeO3, releasing the weak ferromagnetic property due to spin canting. In addition, integration of BiFeO3 with Si using SrTiO3 template layer was also studied. Large dielectric constant and piezoelectric coefficients were observed, showing promise for applications in MEMs and actuators.
EFFECTS OF ELECTRIC FIELD ON PIEZOELECTRIC RESPONSES OF FERROELECTRIC THIN FILMS
(2005-01-24) Chen, Lang; Roytburd, Alexander L; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The results of theoretical and experimental studies of the piezoeffect in thin ferroelectric films constrained by substrates are presented. The elastic interactions between the film and the substrate has been calculated and proven to be a key factor in determining total piezoresponse of thin film. Thermodynamic theory is employed to explain the electric field dependence of piezoelectric properties of a single domain PbZrxTi1-xO3 (PZT) material. The strong nonlinearity of converse piezoelectric coefficient under large external electric field is proven to be intrinsic both in bulk crystal and in epitaxial thin films of tetragonal PZT. The tunability of piezoelectric responses by external electric field and its dependence on film/substrate misfit and elastic compliance of thin films are characterized quantitatively. The theoretical predictions are in good agreement with the experimental results of piezoresponse force microscopy. It is shown that under an applied electric field, 180° ferroelectric domains act as elastic domains due to the converse piezoelectric effect. The effective dielectric and piezoelectric responses of thin films are determined by interdomain elastic interactions in addition to the substrate clamping effects. Characteristics of the piezoelectric loops of thin ferroelectric films have been explained from this point of view. Experimental proof of the existence of strong interdomain elastic interactions has been obtained via piezoresponse force microscopy. It has been demonstrated that new 90° elastic domains are observed in epitaxial PZT 20/80 (x=0.2) thin films in vicinity of 180° domains, which are formed under strong local electric field created by the AFM tip. The area of internal stress arises under a conductive tip due to opposite signs of the converse piezoelectric strains in the switched 180° domain and the unswitched film. The formation of 90° domains leads to the relaxation of the internal stress. The necessary conditions for realization of this relaxation mechanism are presented. While most of studies on piezoresponse of thin films are based on the assumption that the substrate is rigid, we have found that elasticity and thickness of the substrate play a significant role in the converse piezoresponse of the films and therefore cannot be neglected. Through the theoretical analysis based on different boundary conditions, it has been shown that the elastic deformation of the substrate contributes positively to the total piezodisplacement, while the film/substrate bending can give much larger negative deflections.
EFFECT OF MICROSTRUCTURE ON THE ROOM TEMPERATURE TENSILE AND CREEP DEFORMATION MECHANISMS OF ALPHA-BETA TITANIUM ALLOYS
(2005-04-13) Jaworski, Allan Wayne; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Two-phase alpha-beta titanium alloys are used in many applications because of their high specific strength, corrosion resistance, processability, and biocompatibility. The room temperature tensile and creep deformation mechanisms of alpha-beta alloys must be understood in order to design alloys with desired properties and improved creep resistance. There is a lack of understanding in this regard. The aim of this investigation is to systematically study the effects of microstructure, stability of the beta phase, and alloying elements on the deformation mechanisms of alpha-beta titanium alloys using Ti-6.0wt%Mn and Ti-8.1wt%V as the model systems. The tensile and creep deformation mechanisms and microstructure were studied using SEM, TEM, HREM, and optical microscopy. In addition, theoretical modeling was performed in terms of crystallographic principles and stress analysis. It was found for the first time in an alpha-beta titanium alloy (Ti- 8.1wt%V) that the alpha phase deforms by twinning and the beta phase deforms by stress induced martensite, different mechanisms than the single-phase alpha and beta alloys with similar grain size. Single-phase alpha deforms predominantly by slip, and single-phase beta deforms predominately by twinning. This is also the first time that stress induced martensite has been observed in a creep deformed alpha-beta titanium alloy. However in the case of Ti-6.0wt%Mn, where the beta phase stability is higher, stress induced martensite was not observed. The deformation mechanisms are modeled in terms of the beta phase stability and interactions between phases, including elastic interaction stresses, alpha phase templating, interactions of deformation products, and alpha-omega interactions. A model is also proposed which explains anisotropic interface sliding based on locking of growth ledges. These results are extremely valuable when designing new alloys with improved resistance to creep and other failure modes. The observed deformation mechanisms can directly affect the mechanical reliability of systems. For instance, increased creep strain can alter the dimensional tolerances of components and the observed stress induced products can act as nucleation sites for fracture initiation and stress corrosion cracking. This work was supported by the National Science Foundation under grant number DMR-0102320.
INVESTIGATION OF AMORPHOUS HYDROGENATED Si AS A RESIST FOR VACUUM-COMPATIBLE LITHOGRAPHY OF HgCdTe/CdTe FILMS
(2005-04-13) Jacobs, Randolph N; Salamanca-Riba, Lourdes; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The vision of achieving a completely in-vacuum process for fabricating HgCdTe Infrared detector arrays is contingent on the availability of a vacuum-compatible lithography technology. One such technology for vacuum-lithography involves the use of amorphous hydrogenated Si (a-Si:H) as a dry photoresist. The basic concept has recently been demonstrated whereby a-Si:H resists were deposited via plasma enhanced chemical vapor deposition (PECVD), and then patterned using an excimer laser. The patterns were then hydrogen plasma developed to remove unirradiated areas. Finally, an Ar/H2 electron cyclotron resonance (ECR) plasma was used to transfer patterns to underlying Hg1-xCdxTe film layers. This thesis presents a continued investigation of a-Si:H as a resist material wherein the resists are deposited using an Ar-diluted silane precursor. To determine the best conditions for the technique, the effects of different laser fluences, and exposure environments were studied. Analysis via transmission electron microscopy (TEM) reveals that the excimer-exposed surfaces are polycrystalline in nature, indicating that the mechanism for pattern generation in this study is based on melting and crystallization of the exposed areas. To reduce undesirable surface roughness induced by laser irradiation, a step-wise crystallization/dehydrogenation technique is demonstrated. Fundamental aspects of pattern transfer (via ECR plasma etching) to CdTe and HgCdTe films are also demonstrated, where etch selectivities of 8:1 and 16:1 (respectively) are observed. These values represent a significant improvement to etch selectivities obtained using commercially available organic resists. To address concerns regarding possible damage to HgCdTe caused by the a-Si:H dry lithography process, preliminary studies were carried out using double-crystal rocking curve X-Ray diffraction and high-resolution TEM. The results indicate no evidence of microstructural damage to the HgCdTe film. Other characterization techniques used throughout this thesis include Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and stylus profilometry. The implementation of the a-Si:H dry lithography process represents a crucial step toward achieving totally integrated fabrication of HgCdTe IR detector arrays. In addition this lithography technique is both low temperature and contamination-free, so that other semiconductor microfabrication processes could potentially benefit from its use.
A Process Model to Characterize Airborne Radionuclide Emissions and Transport using Radiological and Meteorological Measurements
(2005-04-27) Williams, Dwight; Pertmer, Gary A; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The radionuclide analysis model developed and validated in this study is the first one ever to integrate human judgment throughout the analytical process. Therefore, besides relating the generation, transport, and measurement of anomalous anthropogenic radionuclides, this model enables many associated tasks to be achieved that could not be performed using existing models. These tasks include thoroughly characterizing radionuclide detection sites, effectively processing qualitative data, and correcting data during processing. The study outlines the model as a highly detailed itemized procedure and validates the model through four case studies. Each case study is able to demonstrate a specific novelty of the model, although multiple novel and useful qualities of the model can be found in all of the case studies. Case Study 1 shows the model's ability to perform site characterizations by determining the presence of 50 radionuclides at a site where only seven had been identified previously. In Case Study 2, the model is shown to be able to isolate a specific emission location through the effective incorporation of qualitative data. Case Study 3 demonstrates the model's ability to perform complicated radionuclide analysis completely independent of computational models. Through Case Study 4, the model is shown to be capable of processing errant data that could not be analyzed computationally. Besides the usefulness of each of the novelties, the model offers many practical values, including its ability to normalize analysis amongst radionuclide analysts with varied levels of experience -- effectively enabling junior level analysts to perform senior level analysis.
Developement of High Throughput Polarization Maintaining NSOM Probes
(2005-05-02) Adiga, Vivekananda Parampalli; Phaneuf, Raymond J; Drew, Howard D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High throughput, polarization maintaining probes for Near-field Scanning Optical Microscopy (NSOM) were fabricated by chemical etching. An aperture was formed with Focused Ion Beam after coating the tips with 300 nm of aluminum. Tips showed a typical far field polarization extinction ratio of 100 to 1. The throughput of the tips depends on the polarization of the incident light because of the boundary conditions imposed by elliptical aperture. We show NSOM images obtained with these probes on gold dots on a GaAs substrate. NSOM images show a contrast inversion on the gold dots compared to the far-field imaging. We were not able to determine the spatial resolution of the probes since the topographic effects dominate the optical response. Interference effects and variation of the light intensity in close proximity to the sample are observed and studied.
ORIENTATION DEPENDENCE OF THE PIEZOELECTRIC PROPERTIES OF EPITAXIAL FERROELECTRIC THIN FILMS
(2005-07-08) Ouyang, Jun; Roytburd, Alexander L.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
There are both intrinsic piezoelectric response and extrinsic piezoelectric response in ferroelectric materials. The intrinsic piezoelectric response is due to the lattice deformation of a single-domain crystal, which can be characterized by tensors of piezoelectric constants. The extrinsic piezoelectric response depends on extrinsic sources of displacement under the electric field, which can be the movement of domain walls, phase boundaries, or even defects like grain boundaries or dislocations. Due to the elastic interaction between an epitaxial ferroelectric thin film and a substrate, the piezoelectric properties of an epitaxial ferroelectric film are different from those of bulk ferroelectric materials. This work is the first study on the general orientation dependence of the piezoelectric properties of epitaxial ferroelectric thin films, which includes both theoretical and experiment work on intrinsic and extrinsic piezoelectric properties of epitaxial ferroelectric films. A complete theoretical analysis of intrinsic piezoelectric responses in a single domain ferroelectric film, which are characterized by effective longitudinal,transverse and shear piezoelectric coefficients, is presented in this dissertation. On the part of extrinsic piezoelectric response, our recent work on the piezoelectric properties of epitaxial thick lead titanate zirconate (Pb(ZrxTi1-x)O3 with x=0.52) films with tetragonal distorted structures will be presented as an example. It is shown that(011) oriented epitaxial films had much enhanced piezoelectric responses as compared with those of (001) and (111) oriented films. Detailed structure analysis showed that instead of an interconnected 3-domain (3-D) architecture that is usually found in a (001) oriented thick film, the (011) films consisted of a dominant 2-domain (2-D) architecture, by which the pinning between neighboring domain walls is much reduced. This study demonstrate the possibility of achieving high extrinsic piezoelectric responses by optimizing the epitaxial relationship between the film and substrate with respect to the domain mobility, and should also be instructive to the design of ferromagnetic and ferroelastic thin film devices used for transducer applications.

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