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Advanced Strained-Superlattice Photocathodes for Polarized Electron Sources

Description: Polarized electrons have been essential for high-energy parity-violating experiments and measurements of the nucleon spin structure. The availability of a polarized electron beam was crucial to the success of the Stanford Linear Collider (SLC) in achieving a precise measurement of the electroweak mixing angle, and polarized electron beams will be required for all future linear colliders. Polarized electrons are readily produced by GaAs photocathode sources. When a circularly polarized laser beam tuned to the bandgap minimum is directed to the negative-electron-affinity (NEA) surface of a GaAs crystal, longitudinally polarized electrons are emitted into vacuum. The electron polarization is easily reversed by reversing the laser polarization. The important properties of these photocathodes for accelerator applications are: degree of polarization of the extracted beam; ability to extract sufficient charge to meet accelerator pulse-structure requirements; efficiency and stability of operation; and absence of any asymmetries in the beam properties (charge, position, energy, etc.) upon polarization reversal. The performance of GaAs photocathodes has improved significantly since they were first introduced in 1978 [1]. The theoretical maximum polarization of 50% for natural GaAs was first exceeded in 1991 using the lattice mismatch of a thin InGaAs layer epitaxially grown over a GaAs substrate to generate a strain in the former that broke the natural degeneracy between the heavy- and light-hole valence bands [2]. Polarizations as high as 78% were produced for the SLC from photocathodes based on a thin GaAs epilayer grown on GaAsP [3,4]. After 10 years of experience with many cathode samples at several laboratories [5], the maximum polarization using the GaAs/GaAsP single strained-layer cathode remained limited to 80%, while the quantum efficiency (QE) for a 100-nm epilayer is only 0.3% or less. Two factors were known to limit the polarization of these cathodes: (1) the limited band splitting; and (2) a relaxation ...
Date: January 31, 2005
Creator: Moy, Dr. Aaron
Partner: UNT Libraries Government Documents Department

Amorphization threshold in Si-implanted strained SiGe alloy layers

Description: The authors have examined the damage produced by Si-ion implantation into strained Si{sub 1{minus}x}Ge{sub x} epilayers. Damage accumulation in the implanted layers was monitored in situ by time-resolved reflectivity and measured by ion channeling techniques to determine the amorphization threshold in strained Si{sub 1{minus}x}Ge{sub x} (x = 0.16 and 0.29) over the temperature range 30--110 C. The results are compared with previously reported measurements on unstrained Si{sub 1{minus}x}Ge{sub x}, and with the simple model used to describe those results. They report here data which lend support to this model and which indicate that pre-existing strain does not enhance damage accumulation in the alloy layer.
Date: December 1, 1994
Creator: Simpson, T.W.; Love, D.; Endisch, E.; Goldberg, R.D.; Mitchell, I.V.; Haynes, T.E. et al.
Partner: UNT Libraries Government Documents Department

Anisotropy of the sublattice magnetization and magnetoresistance in Co/Re superlattices on Al{sub 2}O(1120).

Description: [Co(20 {angstrom})/Re(6{angstrom})]{sub 20} superlattices were grown on a (11{bar 2}0) surface of a Al{sub 2}O{sub 3} single crystal, with the [0001] direction of their hcp structure in the plane of the film. The Co layers were found to be antiferromagnetically coupled (AF), with a saturating field of 6 kOe. Polarized neutron reflectivityy was used to determine the direction of the sublattice magnetization. In zero applied field, the AF moments are aligned along the Co [0001] axis. In a magnetic field H perpendicular to the Co [0001] axis, the sublattices moments evolve to a canted arrangement, with the AF component always perpendicular to the field. With H along the Co[0001] axis, the AF moments flop in a direction perpendicular to Co[0001] axis. The spin flop transition is not abrupt, but can be described as a gradual rotation that is completed at 2 kOe. The anisotropy of the sublattice magnetization is related to the anisotropy of the magnetoresistance. This has the conventional dumbbell behavior with the field applied perpendicular to the Co[0001] axis, but exhibits an extended plateau near H = 0, and a slight increase up to H {approximately} 2 kOe, when H is parallel to Co[0001] axis.
Date: September 29, 1998
Creator: Charloton, T.
Partner: UNT Libraries Government Documents Department

Artificially structured magnetic materials

Description: This document reports the progress made during the first six months of the current three-year DOE grant on Artificially Structured Magnetic Materials.'' However, because some of the results of our previous three-year DOE grant on Artificially Structured Superconductors'' continue to emerge, both topics are addressed in this Progress Report. This report describes progress with DOE funding during the current calendar year; description of the research to be conducted during the remaining six months of the current grant year; a description of the status of the graduate students working on this research; lists of the invited talks, seminars and colloquia, of other recognition of our research, and of the publications crediting DOE sponsorship; and a summary of current and pending federal support. Since the research proposed to be conducted during the next 2 1/2 years is described in detail in our DOE proposal, it is only briefly reviewed here.
Date: September 28, 1990
Creator: Falco, C.M.
Partner: UNT Libraries Government Documents Department

Auger rates in mid-IR InAsSb laser structures

Description: Auger rates are calculated for three InAsSb mid-infrared laser structures as a function of temperature. Compressive strain in the quantum wells reduces the mass of the holes; it is shown that this leads to a reduction in the Auger rate compared with an unstrained quantum well. The Auger rates for these structures are similar primarily due to their similar bandgap energies.
Date: February 1, 1995
Creator: Hjalmarson, H.P. & Kurtz, S.R.
Partner: UNT Libraries Government Documents Department

Boron-enhanced-diffusion of boron: The limiting factor for ultra-shallow junctions

Description: Reducing implant energy is an effective way to eliminate transient enhanced diffusion (TED) due to excess interstitials from the implant. It is shown that TED from a fixed Si dose implanted at energies from 0.5 to 20 keV into boron doping-superlattices decreases linearly with decreasing Si ion range, virtually disappearing at sub-keV energies. However, for sub-keV B implants diffusion remains enhanced and x{sub j} is limited to {ge} 100 nm at 1,050 C. The authors term this enhancement, which arises in the presence of B atomic concentrations at the surface of {approx} 6%, Boron-Enhanced-Diffusion (BED).
Date: December 1, 1997
Creator: Agarwal, A.; Eaglesham, D.J.; Gossmann, H.J.; Pelaz, L.; Herner, S.B.; Jacobson, D.C. et al.
Partner: UNT Libraries Government Documents Department

Characterization of an Irradiated RERTR-7 Fuel Plate Using Transmission Electron Microscopy

Description: Transmission electron microscopy (TEM) has been used to characterize an irradiated fuel plate with Al-2Si matrix from the RERTR-7 experiment that was irradiated under moderate reactor conditions. The results of this work showed the presence of a bubble superlattice within the U-7Mo grains that accommodated fission gases (e.g., Xe). The presence of this structure helps the U-7Mo exhibit a stable swelling behaviour during irradiation. Furthermore, TEM analysis showed that the Si-rich interaction layers that develop around the fuel particles at the U-7Mo/matrix interface during fuel plate fabrication and irradiation become amorphous during irradiation, and in regions of the interaction layer that have relatively high Si concentrations the fission gas bubbles remain small and contained within the layer but in areas with lower Si concentrations the bubbles grow in size. An important question that remains to be answered about the irradiation behaviour of U-Mo dispersion fuels, is how do more aggressive irradiation conditions affect the behaviour of fission gases within the U-7Mo fuel particles and in the amorphous interaction layers on the microstructural scale that can be characterized using TEM? This paper discusses the results of TEM analysis that was performed on a sample taken from an irradiated RERTR-7 fuel plate with Al-2Si matrix. This plate was exposed to more aggressive irradiation conditions than was the sample taken from the RERTR-6 plate. The microstructural features present within the U-7Mo and the amorphous interaction layers will be discussed. The results of this analysis will be compared to what was observed in the earlier RERTR-6 fuel plate characterization.
Date: March 1, 2010
Creator: Gan, J.; D. D. Keiser, Jr.; Miller, B. D.; Robinson, A. B. & Medvedev, P.
Partner: UNT Libraries Government Documents Department

Characterizing Composition Modulations in InAs/AIAs Short-Period Superlattices

Description: The formation of quantum wires has much interest due to their novel electronic properties which may lead to enhanced optoelectronic device performance and greater photovoltaic efficiencies. One method of forming these structures is through spontaneous lateral modulation found during the epitaxial growth of III/V alloys. In this paper, we report and summarize our investigations on the formation of lateral moduation in the MBE grown InAlAs/InP(001) system. This system was grown as a short-period superlattice where n-monolayers of InAs are deposited followed by m-Monolayers of AlAs (with n and m~2) and this sequence is repeated to grown a low strain InAlAs ternary alloy on InP(001) that exhibits lateral modulation. Films were grown under a variety of condition (growth temperature, effective alloy composition, superlattice period, and growth rate). These films have been extensively analyzed using X-ray diffraction, atomic force microscopy, and transmission electron microscopy (TEM) and microcharacterization, in addition to photon-based spectroscopes. Here we present results of several microstructural characterizations using a wide range of TEM-based techniques, and compare them to results from the other methods to obtain a unified understanding of composition modulation. Two strong points consistently emerge: 1) The lateral modulation wavelength is insensitive to growth temperature and effective alloy composition, but the strength of the lateral modulation is greatest near an effective alloy composition of In(0.46)Al(0.54)As, which corresponds to a slightly tensile global strain with respect to InP. 2) The composition variation for the strongly modulated films is as much as 0.38 InAs mole fraction. In addition, for these strongly modulated films, the modulation wave is asymmetric showing strongly peaked, narrower InAs-rich regions separated by flat AlAs-rich regions. We discuss these results and their possible implications in addition to detailing the techniques used to obtain them.
Date: April 26, 1999
Creator: Ahrenkiel, S.P.; Follstaedt, D.M.; Jones, E.D.; Lee, S.R.; Mascarenhas, A.; Millunchick, J. Mirecki et al.
Partner: UNT Libraries Government Documents Department

Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure

Description: This is a milestone document covering the activities to validate theoretical calculations with experimental data for the effect of the decay of 90Sr to 90Zr on materials properties. This was done for a surragate waste form strontium titanate.
Date: April 1, 2011
Creator: Van Ginhoven, Renee M.; Jaffe, John E.; Jiang, Weilin & Strachan, Denis M.
Partner: UNT Libraries Government Documents Department

Coherent Control of Optically Generated and Detected Picosecond Surface Acoustic Phonons

Description: Coherent control of elementary optical excitations is a key issue in ultrafast materials science. Manipulation of electronic and vibronic excitations in solids as well as chemical and biological systems on ultrafast time scales has attracted a great deal of attention recently. In semiconductors, coherent control of vibronic excitations has been demonstrated for bulk acoustic and optical phonons generated in superlattice structures. The bandwidth of these approaches is typically fully utilized by employing a 1-D geometry where the laser spot size is much larger than the superlattice repeat length. In this presentation we demonstrate coherent control of optically generated picosecond surface acoustic waves using sub-optical wavelength absorption gratings. The generation and detection characteristics of two material systems are investigated (aluminum absorption gratings on Si and GaAs substrates).
Date: November 1, 2006
Creator: Hurley, David H.
Partner: UNT Libraries Government Documents Department

Coherent coupling in ferroelectric superlattices

Description: The phase transition and dielectric behavior of ferroelectric multilayers have been discussed. The coherent interaction between ultra-thin layers can be significantly strong, resulting in a broad diffuse phase transition. The thicknesses of layers and their spatial distributions hold the keys of enhancing dielectric properties in a broad temperature range.
Date: July 1, 1996
Creator: Li, S.; Eastman, J.A.; Vetrone, J.; Newnham, R.E. & Cross, L.E.
Partner: UNT Libraries Government Documents Department

Collaborative project: research on strongly coupled plasmas. Final technical report for period July 15, 1998--July 14, 2002

Description: The main research accomplishments/findings of the project were the following: (1) Publication of an in-depth review article in Physics of Plasmas on the quasilocalized charge approximation (QLCA) in strongly coupled plasma physics and its application to a variety of Coulomb systems: the model one-component plasma in three and two dimensions, binary ionic mixtures, charged particle bilayers, and laboratory dusty plasmas. (2) In the strongly coupled Coulomb liquid phase, the physical basis of the QLCA, namely, the caging of particles trapped in slowly fluctuating local potential minima, is supported by molecular dynamics simulation of the classical three-dimensional one-component plasma. (3) The QLCA theory, when applied to the analysis of the collective modes in strongly coupled charged particle bilayers, predicts the existence of a remarkable long-wavelength energy gap in the out-of-phase excitation spectrum. More recent theoretical calculations based on the three principal frequency-moment sum rules reveal that the gap persists for arbitrary coupling strengths and over the entire classical to quantum domain all the way down to zero temperature. The existence of the energy gap has now been confirmed in a molecular dynamics simulation of the charged particle bilayer. (4) New compressibility and third-frequency-moment sum rules for multilayer plasmas were formulated and applied to the analysis of the dynamical structure function of charged particle bilayers and superlattices. (5) An equivalent of the Debye-Huckel weak coupling equilibrium theory for classical charged particle bilayer and superlattice plasmas was formulated. (6) The quadratic fluctuation-dissipation theorem (QFDT) for layered classical plasmas was formulated. (7) The QFDT was applied to a powerful kinetic theory-based description of the density-density response function and long-wavelength plasma mode behavior in strongly coupled two-dimensional Coulomb fluids in the weakly degenerate quantum domain.
Date: September 16, 2002
Creator: Golden, Kenneth I.
Partner: UNT Libraries Government Documents Department

Comparison of ALINGAAS/GAAS Superlattice Photocathodes Having Low Conduction Band Offset

Description: The main advantage of superlattice (SL) structures as spin polarized electron emitters is the ability to provide a large splitting between the heavy hole (HH) and light hole (LH) valence bands (VB) over a large active thickness compared to single strained layers. Two important depolarization mechanisms in these structures are the scattering effects during the transit of the electrons in the active region and the depolarization that takes place in the band bending region (BBR) near the surface. In this paper, we systematically study the effects of the electron mobility and transit time by using an InAlGaAs/GaAs SL with a flat conduction band (CB). Initial results by the SPTU-SLAC collaboration using such structures grown by the Ioffe Institute showed polarization and quantum yield (QE) of 92% and 0.2% respectively. We report measurements using similar structures grown by SVT Associates. The results (polarization up to 90%) are also compared with simulations.
Date: March 31, 2006
Creator: Ioakeimidi, K,; Maruyama, T.; Clendenin, J.E.; Brachmann, A.; Garwin, E.L.; Kirby, R.E. et al.
Partner: UNT Libraries Government Documents Department

Computer simulation of the anomalous elastic behavior of thin films and superlattices

Description: Atomistic simulations are reviewed that elucidate the causes of the anomalous elastic behavior of thin films and superlattices (the so-called supermodulus effect). The investigation of free-standing thin films and of superlattices of grain boundaries shows that the supermodulus effect is not an electronic but a structural interface effect intricately connected with the local atomic disorder at the interfaces. The consequent predictions that (1) coherent strained-layer superlattices should show the smallest elastic anomalies and (2) the introduction of incoherency at the interfaces should enhance all anomalies are validated by simulations of dissimilar-material superlattices. 38 refs, 10 figs.
Date: October 1, 1992
Creator: Wolf, D.
Partner: UNT Libraries Government Documents Department

Computer simulation of the anomalous elastic behavior of thin films and superlattices

Description: Atomistic simulations are reviewed that elucidate the causes of the anomalous elastic behavior of thin films and superlattices (the so-called supermodulus effect). The investigation of free-standing thin films and of superlattices of grain boundaries shows that the supermodulus effect is not an electronic but a structural interface effect intricately connected with the local atomic disorder at the interfaces. The consequent predictions that (1) coherent strained-layer superlattices should show the smallest elastic anomalies and (2) the introduction of incoherency at the interfaces should enhance all anomalies are validated by simulations of dissimilar-material superlattices. 38 refs, 10 figs.
Date: October 1, 1992
Creator: Wolf, D.
Partner: UNT Libraries Government Documents Department

Control of the interparticle spacing in gold nanoparticle superlattices

Description: The authors have investigated the formation of 2-D and 3-D superlattices of Au nanoclusters synthesized in nonionic inverse micelles, and capped with alkyl thiol ligands, with alkane chains ranging from C{sub 6} to C1{sub 18}. The thiols are found to play a significant role in the ripening of these nanoclusters, and in the formation of superlattices. Image processing techniques were developed to reliably extract from transmission electron micrographs (TEMs) the particle size distribution, and information about the superlattice domains and their boundaries. The latter permits one to compute the intradomain vector pair correlation function, from which one can accurately determine the lattice spacing and the coherent domain size. From these data the gap between the particles in the coherent domains can be determined as a function of the thiol chain length. It is found that as the thiol chain length increases, the nanoclusters become more polydisperse and larger, and the gaps between particles within superlattice domains increases. Annealing studies at elevated temperatures confirm nanocluster ripening. Finally, the effect of the particle gaps on physical properties is illustrated by computing the effective dielectric constant, and it is shown that the gap size now accessible in superlattices is rather large for dielectric applications.
Date: April 6, 2000
Creator: MARTIN,JAMES E.; WILCOXON,JESS P.; ODINEK,JUDY G. & PROVENCIO,PAULA P.
Partner: UNT Libraries Government Documents Department

The Controller Synthesis of Metastable Oxides Utilizing Epitaxy and Epitaxial Stabilization

Description: Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer. These advances were made through the use of epitaxy, epitaxial stabilization, and a combination of composition-control techniques including adsorption-controlled growth and RHEED-based composition control that we have developed, understood, and utilized for the growth of oxides. Also key was extensive characterization (utilizing RHEED, four-circle x-ray diffraction, AFM, TEM, and electrical characterization techniques) in order to study growth modes, optimize growth conditions, and probe the structural, dielectric, and ferroelectric properties of the materials grown. The materials that we have successfully engineered include titanates (PbTiO3, Bi4Ti3O12), tantalates (SrBi2Ta2O9), and niobates (SrBi2Nb2O9); layered combinations of these perovskite-related materials (Bi4Ti3O12-SrTiO3 and Bi4Ti3O12-PbTiO3 Aurivillius phases and metastable PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices), and new metastable phases (Srn+1TinO3n+1 Ruddlesden-Popper phases). The films were grown by reactive MBE and pulsed laser deposition (PLD). Many of these materials are either new or have been synthesized with the highest perfection ever reported. The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the superconducting, ferroelectric, and dielectric properties of these materials. These properties are important for energy technologies.
Date: December 2, 2003
Creator: Schlom, Darrell
Partner: UNT Libraries Government Documents Department

Cross-Sectional Scanning Tunneling Microscopy of InAsSb/InAsP Superlattices

Description: Cross-sectional scanning tunneling microscopy has been used to characterize compositional structure in InAs{sub 0.87}Sb{sub 0.13}/InAs{sub 0.73}P{sub 0.27} and InAs{sub 0.83}Sb{sub 0.17}/InAs{sub 0.60}P{sub 0.40} strained-layer superlattice structures grown by metal-organic chemical vapor deposition. High-resolution STM images of the (110) cross section reveal compositional features within both the InAs{sub x}Sb{sub 1{minus}x} and InAs{sub y}P{sub 1{minus}y} alloy layers oriented along the [{bar 1}12] and [1{bar 1}2] directions--the same as those in which features would be observed for CuPt-B type ordered alloys. Typically one variant dominates in a given area, although occasionally the coexistence of both variants is observed. Furthermore, such features in the alloy layers appear to be correlated across heterojunction interfaces in a manner that provides support for III-V alloy ordering models which suggest that compositional order can arise from strain-induced order near the surface of an epitaxially growing crystal. Finally, atomically resolved (1{bar 1}0) images obtained from the InAs{sub 0.87}Sb{sub 0.13}/InAs{sub 0.73}P{sub 0.27} sample reveal compositional features in the [112] and [{bar 1}{bar 1}2] directions, i.e., those in which features would be observed for CuPt-A type ordering.
Date: February 10, 1999
Creator: Allerman, A.A.; Biefeld, R.M.; Yu, E.T. & Zuo, S.L.
Partner: UNT Libraries Government Documents Department

Cu/sub 3/Pd observed by high-voltage electron microscopy

Description: Cu-Pd samples of compositions varying from 16 to 26 at.% Pd were irradiated in situ in a 1.5-MeV electron microscope at various temperatures. Low-temperature (90/sup 0/K) irradiation produced completely disordered solid solutions. Irradiation at room temperature up to as high as about 500/sup 0/K produced steady state short range order (SRO) which, for specimens of 18% or more Pd, is characterized by diffuse intensity at (1 +-q,0) and equivalent positions in reciprocal space (modulated SRO). In general, q is a function of composition, temperature and irradiation dose. High temperature irradiation tended to produce the expected equilibrium long range order - either L1/sub 2/ or a long period superstructure depending on composition and temperature. The 18 to 20% samples irradiated at room temperature exhibited steady state modulated SRO even though the expected equilibrium structure is one of unmodulated order (L1/sub 2/). It is suggested that spinodal ordering is responsible for this latter effect. An f.c.c. based Cu-Pd phase diagram is proposed incorporating ordering stability loci and a metastable Lifshitz point. 37 refs., 12 figs.
Date: July 1, 1987
Creator: Kulik, J.; Takeda, S. & de Fontaine, D.
Partner: UNT Libraries Government Documents Department

Defect and impurity effects on the initial growth of Ag on Si(111)

Description: Step and impurity effects on the initial growth of a thin film have been demonstrated in the ({radical}3 {times} {radical}3)R30{degree} domain growth of Ag on Si(111) using high angular resolution LEED. Anisotropy in the {radical}3 domain shape and growth during deposition are found on the stepped Si(111) with the preferential growth along the step edge direction. The {radical}3 superlattice grows with coverage principally by domain coalescence at the temperature T {approximately} 450{degree}C and is self-similar at different coverages (scaling) as observed on a flat Si(111). The size distribution is shown to follow a Gamma distribution by a simple model calculation. A dramatic change in the growth mechanism is observed when oxygen impurities ({le}0.02 ML) appear. The {radical}3 domains in the presence of impurities grow with coverage more randomly and isotropically in contrast with the step edge effects and coalescence is inhibited. As a result, the {radical}3 superlattice stays in a microdomain morphology without long range order. 17 refs., 6 figs.
Date: October 1, 1990
Creator: Zuo, J.K. & Wendelken, J.F.
Partner: UNT Libraries Government Documents Department

Deposition and properties of novel nitride superlattice coatings

Description: We have demonstrated that polycrystalline TiN/NbN superlattice thin films on tool steel substrates can have hardnesses H as high as 5200 kg mm{sup {minus}2} HV{sub 0.05} (52 Gpa). This is more than twice the hardness of polycrystalline sputtered TiN and NbN deposited using the same conditions, and matches the highest H observed for single-crystal TiN/NbN superlattices. Initial parametric studies showed that H increased with decreasing superlattice period down to 4 nm, similar to the dependence for single-crystal TiN/NbN. These results are the first indication that polycrystalline superlattices with high hardness can be deposited on practical substrates.
Date: December 1, 1991
Creator: Barnett, S.A. & Sproul, W.D.
Partner: UNT Libraries Government Documents Department

Deposition and properties of novel nitride superlattice coatings

Description: We have carried out detailed studies of the processing, structure, and properties of certain polycrystalline superlattice coatings, including TiN/NbN, TiN/VN, TiN/Ni, and TiN/NiCr, which were produced using an opposed, dual-cathode, high-rate, reactive, unbalanced-magnetron sputtering system. The coatings exhibited hardness values as high as 5200 kgf/mm[sup 2] for TiN/NbN, 5100 for TiN/VN, 3500 for TiN/Ni, and 3200 for TiN/NiCr. These hardness values are all twice higher than their corresponding rule-of-mixture hardness values. The structure and properties of the coatings are a strong function of superlattice period, partial pressure of N[sub 2], and energy and flux of ion bombardment during deposition. Possible mechanisms for hardness enhancement in the polycrystalline superlattice appear to be a result of dislocation blocking due to coherency strains, difference in dislocation line energies between layers, and small grain sizes. In response to high interest shown by industry, BIRL formed a 2-year Industrial Group Program, currently with 12 members, to transfer the superlattice coating technology.
Date: April 1, 1993
Creator: Barnett, S.A.; Sproul, W.D. & Wong, M.S.
Partner: UNT Libraries Government Documents Department

Deposition and properties of novel nitride superlattice coatings. Final report, May 1990--January 1996

Description: The authors have fulfilled the primary objectives of this research program which were to establish the feasibility of depositing polycrystalline nitride superlattices with high hardnesses on steel substrates, to understand the relationship between deposition process parameters, film microstructure, and resulting film properties, and to investigate the theories of superlattice strengthening/hardening. An opposed-cathode unbalanced magnetron sputtering system was modified for the deposition of the superlattices. Thin films of several microns thick, composed of many alternating thin layers of two (or more) different materials with periods between 3 and 150 nm, were deposited at low temperature onto various engineering substrate mate5rials such as steel. They have developed several superlattices with superior properties unattainable from either single-layer or thicker multilayer coatings. The nitride superlattices that they have explored so far include TiN/NbN, TiN/VN, NbN/VN, TiN/CrN, AlN/TiN, TiN/Ni, and TiN/NiCr. The effects of deposition parameters on film structure and properties were systematically studied, and optimized parameters for both monolithic films and superlattices were developed. The results of this research show that polycrystalline nitride superlattices have great promise for replacing commonly used nitride-based wear protective coatings. The existence of metastable crystalline phases has been observed in some of the superlattices explored, which they have named nanolayer-stabilized materials. The theoretical knowledge and experimental methods developed in this work have already been used to design and produce superlattice coatings for industrial work. A parallel industrial group program has been established to transfer the superlattice coating technology to industry. The detailed results achieved in this project have been discussed in over a dozen publications. The most important results from the work are summarized.
Date: March 1, 1996
Creator: Barnett, S.A.; Sproul, W.D. & Wong, M.S.
Partner: UNT Libraries Government Documents Department

Deposition and properties of novel nitride superlattice coatings. Progress report, 28 September 1990--October 1994

Description: The major effort during this report period was to elucidate the hardening mechanisms in polycrystalline superlattices. Since the last progress report, the authors have been working on theoretical modeling of hardening mechanisms in superlattices and have carried out detailed studies of two new polycrystalline superlattice coatings, NbN/VN and CrN/TiN, which were produced using an opposed, dual-cathode, high-rate, reactive, unbalanced-magnetron sputtering system. Significant hardness enhancement the above rule-of-mixture hardness value was found for the CrN/TiN, but not for the NbN/VN. Correlating these results with prior results for TiN/NbN and TiN/VN shows that the major hardening mechanism in polycrystalline superlattices is the difference in layer elastic moduli, which is minimal in the NbN/VN case. The modulus difference provides a barrier to dislocation flow across the layers. In the absence of modulus-difference hardening in NbN/VN, it was found that other hardening mechanisms such as coherency strains and the small grain sizes observed in the superlattices play a minor role. The results achieved in this report period are described in this paper along with publications and presentations.
Date: October 1, 1994
Creator: Barnett, S. A.; Sproul, W. D. & Wong, M. S.
Partner: UNT Libraries Government Documents Department