Browsing by Author "Carl C. Koch, Committee Member"

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Growth and Characterization of ZnO and ZnO-Based Alloys MgxZn1-xO and MnxZn1-xO
(2004-11-19) Jin, Chunming; Robert M. Kolbas, Committee Member; Carl C. Koch, Committee Member; Jagdish Narayan, Committee Chair; Ronald O. Scattergood, Committee Member
The goals of this work were to synthesize ZnO and ZnO based alloy thin films by using PLD and to study the structural, stoichiometric, optical and electrical properties of these films. Epitaxial hexagonal MgZnO thin films have been grown on sapphire (0001) with domain-matching epitaxy by using PLD. The films show the high single-crystalline quality and bright excitonic luminescence. The maximum Mg concentration was found to be 34 at. %, which is almost ten times of the value allowed by the phase diagram. The bandgap of MgZnO alloy film can be tuned from 3.40 eV to 4.19 eV. Epitaxial ZnMgO thin films with cubic (NaCl) structure were also synthesized on MgO (001) sapphire (0001) and TiN/Si(001) by using PLD. The maximum Zn concentration in these cubic alloy films was 18 at. %. The epitaxial growth of cubic ZnMgO on Si(001) substrate is of significant importance for integrating ZnO-based alloys to the Si-based electronics. The phase stability of MgZnO/ZnO/MgZnO superlattice structures was studied using XRD and HRTEM methods. The diffusion of Mg from the MgZnO barrier to the ZnO well was observed by using the HRTEM and optical measurements. The cubic nanoinclusions were also observed with HRTEM. Epitaxial MnZnO thin films were synthesized on sapphire (0001) substrates. The maximum Mn concentration was 35 at. %. The bandgap of these films shifts to the higher energy side with increasing Mn content. Magnetic investigations indicate that these films are paramagnetic. Epitaxial ZnO films have been grown on Si (111) substrates by using PLD with two different heterostructures, ZnO/AlN/Si(111) and ZnO/MgO/TiN/Si(111). These thin films show the excellent single crystalline quality and extremely bright excitonic emission. C-axis orientated ZnO thin films have been grown on the amorphous silica substrates. The PL characteristics of these films are comparable to that of the films grown on the sapphire substrates. An ultraviolet illumination-enhanced luminescence effect was observed. This new phenomenon is attributed to the oxygen desorption on the surface. A phenomenological model was proposed to explain this new effect.
Machining of Bulk Metallic Glass
(2004-06-10) Bakkal, Mustafa; Eric C Klang, Committee Member; Albert J Shih, Committee Chair; Jeffrey W Eischen, Committee Member; Carl C. Koch, Committee Member
The turning and drilling of Zr52.5Ti5Cu17.9Ni14.6Al10 metallic glass (BMG) are evaluated in this study. The mechanics of machining and chip formation and characterization are investigated. In the lathe turning of BMG, above a threshold cutting speed, the low thermal conductivity of BMG leads to chip temperatures high enough to cause the chip oxidation and associated light emission. The high temperature produced by this exothermic chemical reaction causes crystallization within the chips. Oxide layer, amorphous region, fully crystalline region, and crystalline-amorphous transition region are observed in the cross-section of the chips. The x-ray diffraction peaks match the pattern for monoclinic ZrO2. Turning chips morphology suggests that increasing amounts of viscous flow control the chip-removal process. Moreover, viscous flow and crystallization can occur during the machining of the bulk metallic glass, even under the high temperature gradient and strain rate. For the BMG chip without light emission, the serrated chip with adiabatic shear band and void formation was observed. High cutting speed significantly reduced the forces for BMG machining due to thermal softening. Roughness of machined BMG surfaces is generally better than that of Al6061-T6 and SS304. Tool wear is a problem for BMG turning. Chipping and thermal softening on the lathe tool cutting edges can be observed. Drilling of BMG shows that holes with precision geometry and good surface roughness can be efficiently produced in BMG using the high speed steel and WC-Co drills at spindle speed that does not exceed the limit for chip light emission. Morphology of BMG drilling chip are classified and analyzed. The thermal conductivity of tool material and cutting speed are concluded as two critical factors that triggered the chip exothermic oxidation and light emission. The chip light emission has profound impact on the drill wear, as shown by the experimentally measured thrust force and torque. This study concludes the precision machining of BMG is possible with the selection of feasible tools and process parameters.
Molecular Dynamics Simulations of Plastic Deformation in Nanocrystalline Metal and Alloy
(2007-10-24) Jang, Seonhee; Ronald O. Scattergood, Committee Chair; Donald W. Brenner, Committee Member; Carl C. Koch, Committee Member; Korukonda L. Murty, Committee Member
Nanocrystalline metals have different mechanical properties from conventional grain sized metals. Hardness and yield strength have been found to increase with decreasing grain size in the nanocrystalline regime down to at least 15 nm on the basis of Hall-Petch mechanisms. Below grain sizes of ˜10 nm, the strength decreases with further grain refinement, leading to the inverse Hall-Petch effect. Although the experimental evidence has found these deformation responses in nanocrystalline materials, the underlying mechanisms are not well identified. Molecular dynamics simulations were carried out for uniaxial tensile straining of two-dimensional columnar microstructures of aluminum (Al) and aluminum-lead (Al-Pb) alloys. Pure Al has a critical grain size at dc ≈ 15 to 20 nm, the crossover from "normal" to "inverse" Hall-Petch effect, accompanied with intra-grain mechanisms by partial dislocations and twins as grain sizes increases. With increasing grain size there exists a transition in plastic deformation mechanism from inter-grain processes to one that consists of both inter-grain and intra-grain processes. For Al-Pb alloys with a 10 nm grain size, Pb segregates completely to the grain boundaries and the grain boundaries become wider and more disorganized as the Pb content increases. A softening effect was observed in agreement with, but less than that found experimentally. As the Pb content increases, partial dislocation nucleation at grain boundaries is completely suppressed and the plastic strain is accommodated by mechanisms other than dislocation slip. As the grain sizes increase up to 15 or 20 nm, dislocation generation at grain boundaries is also suppressed. However, dislocation generation is not entirely suppressed at 3 equivalent at% Pb, compared to the 10 nm grain size showing complete suppression.
Process and Properties of Nitride-based Thin Film Heterostructures
(2003-11-05) Wang, Haiyan; Jagdish Narayan, Committee Chair; J. Michael Rigsbee, Committee Member; Carl C. Koch, Committee Member; John Muth, Committee Member
The goals of this work were to synthesize nitride-based thin film heterostructures by Pulsed Laser Deposition, study the structural, mechanical, electrical and optical properties of these heterostructures and establish structure-property relations for these materials in order to further improve their properties and design new structures. Domain matching epitaxy was explored in most of these heterostructures and studied in detail for each case. Mechanical and electrical properties of TiN as a function of microstructure varying from nanocrystalline to single crystal TiN films deposited on (100) silicon substrates were investigated. By varying the substrate temperature from 25°C to 700°C during PLD, the microstructure of TiN films changed from nanocrystalline (having uniform grain size of 8 nm) to a single crystal epitaxial film on the silicon (100) substrate. The hardness of TiN films decreased with decreasing grain size. The dependence of resistivity of TiN as a function of the substrate temperature is discussed and correlated with hardness results. High-quality epitaxial B1 NaCl-structured TaN films were deposited on Si(100) and Si(111) substrates with TiN as buffer layer, using pulsed laser deposition. Our method exploits the concept of lattice-matching epitaxy between TiN and TaN and domain-matching epitaxy between TiN and silicon. XRD, TEM, and STEM experiments confirmed the single-crystalline nature of the films with cube-on-cube epitaxy. The stoichiometry of TaN films was determined to be nitrogen deficient (TaN[subscript 0.95]) by RBS. Resistivity of the TaN films was found to be 220μΩ-cm at room temperature with temperature coefficient of resistivity of -0.005K⁻¹. Diffusivity of copper in single-crystal (NaCl-structured) and polycrystalline TaN thin films grown by PLD was investigated. The polycrystalline TaN films were grown directly on Si(100), while single-crystal films were grown with TiN buffer layers. The diffusion distances in epitaxial TaN are found to be about 5nm at 650°C for 30 min annealing. Cu diffusion in polycrystalline TaN thin films is found to be nonuniform with enhanced diffusivities along the grain boundary. By PLD, TiN and TaN targets were arranged in a special configuration that they can be ablated in a sequential manner to obtain TiN-TaN alloy or TiN/TaN superlattice structure. The 60% TaN resulted in superlattice of TaN(3nm) /TiN(2nm), while 30% and 70% TaN generated uniform TaXTi1-XN alloys. TiN buffer layers were deposited first to achieve those epitaxial binary components. XRD and TEM analysis showed the epitaxial nature of these films. Microstructure and uniformity of the superlattice and alloy structures were studied by TEM and STEM. Nanoindentation results suggested high hardness and future hard coating applications for these TiN-TaN composites. Four point probe electrical resistivity measurements and Cu diffusion characteristics study prove that TiN-TaN binary components provide a superior diffusion barrier for copper. Uniform AlxTi1-xN alloys (x up to 70%) and highly aligned TiN/AlN superlattices were deposited by PLD. Microstructure and uniformity for the superlattice structures and alloys were studied by TEM and STEM. Nanoindentation results suggested high hardness for these new structures and four point probe electrical resistivity measurements showed overall insulating behavior for both alloys and superlattices. The eptaxial wurtzite AlN thin films were grown on (0001) &alpha-Al2O3 substrates by PLD. XRD and SAD in TEM revealed the epitaxial growth of AlN on (0001) α -Al2O3 substrate. These AlN films were post-deposition annealed at 1300°C for 30mins. Bright field and dark field TEM and transmittance spectra for the samples before and after annealing prove the annealing can effectively improve the quality of the film. Post-deposition annealing for AlN on α-Al2O3 substrates could be a very promising procedure for high quality optical device fabrications. The eptaxial wurtzite AlN thin films were grown on (111) Si substrates by PLD and Laser-MBE. XRD and SAD in TEM revealed the epitaxial growth of AlN on Si(111) substrate. The interface structure and growth mechanism were studied by high-resolution TEM. Fourier filtered image of cross-sectional AlN/Si(111) samples from both Si (112) zone axes revealed the domain matching epitaxy of 4:5 ratio between a[subscript Si(110)] and a[subscript AlN(2110)].
Self-assembled Magnetic Nanostructures: Epitaxial Ni on TiN (001) Surface
(2006-01-10) Zhou, Honghui; Gerd Duscher, Committee Member; Carl C. Koch, Committee Member; Robert M. Kolbas, Committee Member; Jagdish Narayan, Committee Chair
Systems that contain single domain magnetic particles have been receiving intensive attentions over recent years since they are possible candidates for applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this research is self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer-Weber mode. The growth was conducted by pulsed laser deposition (PLD) technique using epitaxial titanium nitride film as the template, which was in turn grown on silicon (100) substrate. The structural characterization includes X-ray diffraction and both cross-sectional and plan-view transmission electron microscopy. The results showed that the nickel islands formed exhibit a self-assembled nature, i.e., a certain degree of uniformity in orientation, shape, and size. The orientation relationship observed is Ni [100] // TiN [100] // Si [100], the so-called 'cube-on-cube' relationship. The islands are faceted, forming truncated pyramids with walls of (111) planes and a flat top of (100) plane. The base of islands is rectangular with the two principal edges parallel to two orthogonal 011 directions. The size distribution is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski-Krastanow (S-K) mode. A certain degree of self-organization was also found in the island lateral distribution: island chains were observed along the directions close to 011, which are also the edge directions. The island faceting could be explained by surface energy minimization. The interaction of island edge induced strain field between neighboring islands is believed to be responsible for the size uniformity and the lateral ordering. Magnetic measurements were also conducted on these crystallographically aligned nickel islands using superconducting quantum interference device (SQUID) magnetometer, and the results were compared with that obtained from the ensemble of randomly oriented nickel islands, which were grown on polycrystalline/amorphous Al2O3 matrix layer. It is found that both blocking temperature and coercivity of aligned nickel islands are significantly higher than that of the randomly oriented nickel islands. The enhancement in ferromagnetism is attributed to the increased collective effects resulting from the particle interactions in the ensemble of aligned islands, which are self-assembled and self-organized to some degree.
Synthesis and Monte Carlo Simulation of Metallic Nanoparticles and Thermophysical Property Studies of Nanofluids
(2007-01-11) Wu, Chunwei; Kevin M. Lyons, Committee Co-Chair; William L. Roberts, Committee Member; Carl C. Koch, Committee Member; Taofang Zeng, Committee Chair
Nanostructured materials, including versatile nano-objects such as nanoparticles, nanotubes, nanowires, quantum dots and other nano-units as the building blocks for new bottom-up approaches to device and system assembly, are at the leading edge of the rapid developing field of nanoscience and nanotechnology. Metallic nanoparticles have captivated scientists' enduring attention and passion for their novel physical and chemical properties and promising application in numerous areas. In this work, we present for the first time, a whole new metallic nanoparticles synthetic strategy based on a heterogeneous metal displacement reduction mechanism. In association with this underlying principle, we developed hydrodynamically and mechanically-assisted, and ultrasonication-assisted displacement reduction methods to successfully prepare a series of silver, copper, iron oxide, gold and platinum nanoparticles. By controlling reactant concentration and particle average residence time, we achieve size selectivity and size distribution control, which provides the possibility for exploitable scalability in commercial production. Based on our experimental system, we established a kinetic model using a Monte Carlo stochastic algorithm and FORTRAN programming to explain the formation of dispersions of various sizes and size distributions. The model was tested with parameters of our real system of silver nanoparticles formation with a variety of mean size and size distribution. The simulated average size, size distribution and the time scale of the process agree reasonably well with the experimental values. Thus the established theoretical model was proven to simulate and predict the practical system adequately and effectively. Thermophysical property of copper nanofluids we produced was studied, and the effective thermal conductivity of nanofluids at room temperature enhances with increased nanoparticles volume fraction; a 7.4 % of enhancement was obtained with 1% volume fraction.