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Metallic multilayers at the nanoscale

Description: The development of multilayer structures has been driven by a wide range of commercial applications requiring enhanced material behaviors. Innovations in physical vapor deposition technologies, in particular magnetron sputtering, have enabled the synthesis of metallic-based structures with nanoscaled layer dimensions as small as one-to-two monolayers. Parameters used in the deposition process are paramount to the Formation of these small layer dimensions and the stability of the structure. Therefore, optimization of the desired material properties must be related to assessment of the actual microstructure. Characterization techniques as x-ray diffraction and high resolution microscopy are useful to reveal the interface and layer structure-whether ordered or disordered crystalline, amorphous, compositionally abrupt or graded, and/or lattice strained Techniques for the synthesis of metallic multilayers with subnanometric layers will be reviewed with applications based on enhancing material behaviors as reflectivity and magnetic anisotropy but with emphasis on experimental studies of mechanical properties.
Date: November 1994
Creator: Jankowski, A. F.
Partner: UNT Libraries Government Documents Department

Thin-film solid-oxide fuel cells

Description: Fuel cells are energy conversion devices that would save billions of dollars in fuel costs alone each year in the United States if they could be implemented today for stationary and transportation applications (1-5). There are a wide variety of fuel cells available, e.g. molten carbonate, phosphoric acid, proton exchange membrane and solid-oxide. However, solid-oxide fuel cells (SOFCS) are potentially more efficient and less expensive per kilowatt of power in comparison to other fuel cells. For transportation applications, the energy efficiency of a conventional internal combustion engine would be increased two-fold as replaced with a zero-emission SOFC. The basic unit of a SOFC consists of an anode and cathode separated by an oxygen-ion conducting, electrolyte layer. Manifolded stacks of fuel cells, with electrical interconnects, enable the transport and combination of a fuel and oxidant at elevated temperature to generate electrical current. Fuel cell development has proceeded along different paths based on the configuration of the anode-electrolyte-cathode. Various configurations include the tubular, monolithic and planar geometries. A planar geometry for the anode-electrolyte-cathode accompanied by a reduction in layer thickness offers the potential for high power density. Maximum power densities will require yet additional innovations in the assembly of fuel cell stacks with all of the manifolding stipulations for gas flow and electrical interconnects.
Date: May 1, 1997
Creator: Jankowski, A.F.
Partner: UNT Libraries Government Documents Department

MODELING GROWTH OF AU-CU NANOCRYSTALLIINE COATINGS

Description: The electrodeposition process parameters of current density, pulse duration, and cell potential affect both the structure and composition of the foils. The mechanism for nucleation and growth as determined from current transients yield relationships for nucleus density and nucleation rate. To develop an understanding of the role of the process parameters on grain size--as a design structural parameter to control strength, for example, a formulation is presented to model the affects of the deposition energetics on grain size and morphology. An activation energy for the deposition process is modeled that reveals different growth mechanisms, wherein nucleation and diffusion effects are each dominant as dependent upon pulse duration. A diffusion coefficient common for each of the pulsed growth modes demarcates an observed transition in growth from smooth to rough surfaces. Empirical relationships are developed that relate the parameters of the deposition process to the morphology and grain size at the nanoscale. Regimes for nanocrystalline growth include a short and long pulse mode, each with distinct activation energies. The long pulse has the additional contribution of bulk-like diffusion whereas the short pulse is limited to surface diffusion and nucleation. For either pulse condition, a transition from a rough (or nodular) growth to a smooth surface results with an increase in the kinetics of diffusion.
Date: September 22, 2005
Creator: Jankowski, A F
Partner: UNT Libraries Government Documents Department

MODELING NANOCRYSTALLINE GRAIN GROWTH DURING THE PULSED ELECTRODEPOSITION OF GOLD-COPPER

Description: The process parameters of current density, pulse duration, and cell potential affect both the structure and composition of electrodeposits. The mechanism for nucleation and growth as determined from current transients yield relationships for nucleus density and nucleation rate. To develop an understanding of the role of the process parameters on grain size, as a design structural parameter to control strength for example, a formulation is presented to model the affects of the deposition energy on grain size and morphology. An activation energy for the deposition process is modeled that reveals different growth mechanisms, wherein nucleation and diffusion effects are each dominant as dependent upon pulse duration. A diffusion coefficient common for each of the pulsed growth modes demarcates an observed transition in growth from smooth to rough surfaces.
Date: October 27, 2005
Creator: Jankowski, A F
Partner: UNT Libraries Government Documents Department

Rate controlled synthesis of composition modulated, metal-oxide thin films

Description: The development of advanced deposition technologies is continuously evolving for the synthesis of oxide coatings used in optical applications. Recent progress is made in the use of magnetron sputtering to reactively deposit metal-oxide thin films. Sputter deposition parameters are chosen to vary the composition along the film growth direction. The key process parameter to control is the sputtering rate of the target. The shape of the composition profile directly corresponds to the preselected variation of deposition rate. By simply varying the sputtering rate using a working gas that consists of an inert-oxygen mixture, structures are synthesized with composition profiles which can be either abrupt or graded in the growth direction. Result is a compositionally modulated structure of the metal-oxide system. This procedure for composition modulated synthesis is demonstrated for metals which are highly reactive with oxygen as well as for those metals which are not. The development of this deposition methodology will facilitate the design of metal oxide films for optical applications, as in gradient-index filters for example. Results are presented for the reactive sputter deposition of metal oxide coatings in the Y-O, Mo-O, and Cu-O systems.
Date: July 1, 1994
Creator: Jankowski, A. F.
Partner: UNT Libraries Government Documents Department

Superhardness Effect in Au/Ni multilayers

Description: The presence of an enhanced hardness, that is a `Superhardness Effect, is found as a behavior parallel to the Supermodulus Effect in Au/Ni multilayer structures. The submicron thick Au/Ni coatings are prepared by magnetron sputter deposition. A microindenter is used to measure load as a function of indentation depth. An increase in hardness is measured as the Au/Ni layer pair spacing decreases. A local maximum in hardness occurs for samples with layer pairs consisting of 4-6 atomic planes of each metal component.
Date: March 1, 1993
Creator: Jankowski, A. F.
Partner: UNT Libraries Government Documents Department

Superlattice effects on the amorphization of ni-ti multilayers

Description: A phenomenological model is proposed to correlate the onset of solid-state amorphization with the loss of interfacial stability in Ni-Ti multilayers. Additionally, a temperature dependence to the onset of amorphization is attributed to the effect of interfacial coherency that varies with the Ni-Ti layer pair spacing.
Date: February 10, 2000
Creator: Jankowski, A F
Partner: UNT Libraries Government Documents Department

LOW-TEMPERATURE DIFFUSION IN CRYSTALLINE COMPOSITION MODULATED FILMS

Description: The diffusivity (D) in alloy systems at low temperatures is determined using composition-modulated structures. An artificial concentration wave is produced by alternating a deposition of the alloy elements. A quantification of the interdiffusivity coefficient is determined by analyzing the decay of the composition fluctuation, that is, the static concentration wave using Khachaturyan's microscopic theory of diffusion. As it's customary to assume that there is a linear relationship between ln D and T over a wide range of temperature (T), the bulk diffusion coefficient represents the long wavelength approximation of the interdiffusivity. The dependency of interdiffusivity on structure is found in general expressions that account for the specific periodicity and growth orientation of the multilayer structure. The kinetics are quantified by analysis of changes in the composition fluctuation with time at temperature through x-ray scattering measurements. In addition to the examination of single-phase crystalline systems as Cu-Ni and Cr-Ti, the theory is now developed to assess diffusion in two-phase layered systems. Specifically, as in Ni-(Cr,Mo) where a face-centered cubic/body centered cubic combination form a pseudo-epitaxial multilayer.
Date: December 16, 2004
Creator: Jankowski, A F
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of nanophase face-centered-cubic titanium

Description: Unalloyed Ti is sputter deposited in the formation of two trilayer films. Each layer within the combinations of Ni-Ti-Ni and Ti-Ni-Ti is less than 0.1 {mu}m thick. High resolution imaging and electron diffraction results are presented for the microstructural characterization of both trilayer films. Nanophase fcc Ti is initially found in crystalline layers grown on Ni whereas hcp Ti is found in layers grown without a Ni epilayer. Conditions are further examined under which the fcc to hcp transition occurs for Ti deposited on Ni.
Date: February 1, 1995
Creator: Jankowski, A.F. & Wall, M.A.
Partner: UNT Libraries Government Documents Department

Anodically enhanced diffusion in Cu/Ag thin film couples

Description: Thin, 100-nm films of first silver and then copper were deposited consecutively onto pure single-crystal (111) silicon substrates by magneton sputter deposition. Controlled anodic current density was applied at room temperature to dissolve the outer copper film to varying depths approaching the copper/silver interface. Profiles of copper and silver concentration vs. depth below the anodically dissolved surfaces were subsequently obtained by argon ion sputtering and simultaneous Auger Electron Spectroscopy. Despite some intrinsic mixing during the profile analysis, there is clear evidence that diffusion of copper and silver into one another is enhanced by anodic currents at the outer copper surface. Preliminary interpretation leads the authors to believe that the effect is caused by surface formation of vacancies during anodic dissolution. The anodically generated vacancies can migrate as divacancies to the copper/silver interface where they enhance diffusion by the usual vacancy exchange mechanism.
Date: April 1, 1995
Creator: Jones, D.A. & Jankowski, A.F.
Partner: UNT Libraries Government Documents Department

Sputter deposition of nanocones for field emission

Description: Deposition into micron-sized holes is known to produce cone shapes as supported on substrates. Potential uses for the cones include field-forming devices as field ionizers and field emission cathodes. The application of such devices include flat panel displays and flash x-ray tubes. Process iterations to closely space arrays of sharp cones have been extensively documented during the past two decades using the physical vapor deposition method of evaporation. Sputter deposition is well known as a method to fill holes and trenches but has only recently been demonstrated as an alternative method to produce field emission cathodes. In a further reduction in size, we have been successful in demonstrating the ability to deposit a cone shape into a cavity with a 300nm diameter hole. Through comparison to the results of electron-beam evaporative deposition, a sputter deposited nanocone appears to be suitable for use as a field emission cathode.
Date: July 1, 1997
Creator: Jankowski, A.F. & Hayes, F.P.
Partner: UNT Libraries Government Documents Department

Deposition of field emissions cathodes over large areas

Description: Field emission cathodes (FECs) with characteristics of cold emission, low voltage operation, high current density and microscopic size meet the requirements for an electron source for use in vacuum microelectronics. Deposition efforts have focused on evaporation techniques, as electron beam, to produce the size and shape of cathode required for efficient operation. After two decades of development, the convention for FEC synthesis involves coating with very high tolerances for thickness uniformity using a planetary substrate fixture and a long source-to-substrate distance. A further reduction in the operating voltage results by increasing the density of emitters through a reduction of cathode size and spacing. In addition, the objective of scaling the substrate size from small to large areas has compounded the manufacturing requirements to a point beyond that which is obtainable through modifications to the conventional FEC deposition process. We have been successful in a new alternative approach to design, assemble and operate a system that enables FEC synthesis over large areas through the control of deposition source divergence and step-and-repeat substrate handling.
Date: April 3, 1997
Creator: Jankowski, A.F. & Hayes, J.P.
Partner: UNT Libraries Government Documents Department

Thin film synthesis of novel electrode materials for solid-oxide fuel cells

Description: Electrode materials for solid-oxide fuel cells are developed using sputter deposition. A thin film anode is formed by co-deposition of nickel and yttria-stabilized zirconia. This approach is suitable for composition grading and the provision of a mixed-conducting interracial layer to the electrolyte layer. Similarly, synthesis of a thin film cathode proceeds by co-deposition of silver and yttria- stabilized zirconia. The sputter deposition of a thin film solid- oxide fuel cell is next demonstrated. The thin film fuel cell microstructure is examined using scanning electron microscopy whereas the cell performance is characterized through current-voltage measurement and corresponding impedance spectroscopy.
Date: December 1, 1997
Creator: Jankowski, A.F. & Morse, J.D.
Partner: UNT Libraries Government Documents Department

Chemical bonding in hard boron-nitride multilayers

Description: The oxides and nitrides of boron show great potential for use as hard, wear resistant materials. However, large intrinsic stresses and poor adhesion often accompany the hard coatings as found for the cubic boron-nitride phase. These effects may be moderated for use of a layered structure. Alternate stiff layers of boron and compliant layers of nitride are formed by modulating the sputter gas composition during deposition of boron target. The B/BN thin films are characterized with transmission electronic microscope to evaluate the microstructure, nanoindentation to measure hardness and ex-ray absorption spectroscopy to determine chemical bonding. The effects of layer pair spacing on chemical bonding and hardness are evaluated for the B/BN films.
Date: June 1, 1997
Creator: Jankowski, A.F. & Hayes, J.P.
Partner: UNT Libraries Government Documents Department

Reactive sputter deposition of yttria-stabilized zirconia

Description: Yttria-stabilized zirconia (YSZ) films are synthesized using reactive de magnetron sputter deposition. A homogeneous alloy of Zr-Y is synthesized and processed into a planar magnetron target which is reactively sputtered with an Argon-Oxygen gas mixture to form Zr-Y-0 films. The sputtering conditions of gas flow, gas pressure, deposition rate and substrate temperature are determined in order to produce the cubic phase of zirconia as verified with x-ray diffraction. A higher rate of deposition is achievable when the sputtering mode of the Zr-Y alloy target is metallic as opposed to oxide. The Zr-Y composition of the planar magnetron target is designed for optimium oxygen-ion conductivity in the YSZ films, at elevated temperature for potential use in solid-oxide fuel cells. The oxygen concentration of the as-deposited films is measured using Auger electron spectroscopy and found to principally vary as a function of the sputter deposition rate. A fuel cell is produced with the reactive deposition process using Pt electrodes from which the growth morphology of the YSZ layer is characterized using scanning electron microscopy.
Date: May 1, 1995
Creator: Jankowski, A.F. & Hayes, J.P.
Partner: UNT Libraries Government Documents Department

Fabrication and evaluation of transmissive multilayer optics for 8 keV x rays. [Zone plates]

Description: We have investigated an alternative technique for fabricating zone plates that operate in the 5 to 10 keV regime. Ultimately we plan to make zone plates by sputtering alternating layers of opaque and transparent materials onto a thin wire core, then slicing perpendicular to the core axis to produce many zone plates. This technique shows promise for making x-ray optical elements that can be used in industrial crystallography, microprobe and radiography equipment. In a previous publication we reported on the favorable comparison between the measured performance of an Al/Ta diffraction grating and our numerical simulation. In this report we concentrate on the fabrication techniques used to produce diffraction gratings and linear zone plates. 2 refs., 10 figs.
Date: December 1, 1987
Creator: Bionta, R.M.; Jankowski, A.F. & Makowiecki, D.M.
Partner: UNT Libraries Government Documents Department

Fabrication and evaluation of transmissive multilayer optics for 8 keV x rays

Description: We have made and tested several sliced multilayer structures which can function as transmissive x-ray optical elements (diffraction gratings, zone plates, and phase gratings) at 8 keV. Our automated multilayer sputtering system is optimized to sputter layers of arbitrary thickness for very large total deposits at high deposition rates. Diffraction patterns produced by the multilayer devices closely match theoretical predictions. Such transmissive optics have the potential for wide application in high resolution microscope and spectrometer systems. 13 refs., 10 figs.
Date: January 21, 1988
Creator: Bionta, R.M.; Jankowski, A.F. & Makowiecki, D.M.
Partner: UNT Libraries Government Documents Department

MODELING AND THE SPUTTER DEPOSITION OF COATINGS ONTO SPHERICAL CAPSULES

Description: The sputter deposition of coatings onto capsules of polymer and oxide shells as well as solid metal spheres is accomplished using a chambered substrate platform. Oxides and metal coatings are sputter deposited through a screen-aperture array onto a 0.3-1.2 mm diameter, solid spheres and hollow shells. Each shell is contained within its own individual chamber within a larger array. Ultrasonic vibration is the method used to produce a random bounce of each capsule within each chamber, in order to produce a coating with uniform thickness. Characterization of thin aluminum-oxide coated, platinum solid spheres and thicker copper-gold layer coated, hollow capsules (of both glass and polymer) show that uniform coatings can be produced using a screen-aperture chambered, substrate platform. Potential advantages of this approach compared to open-bounce pans include improved sample yield and reduced surface roughness from debris minimization. A process model for the coating growth on the capsules is developed to assess selection of the screen aperture based on the effects of sputter deposition parameters and the coating materials.
Date: September 19, 2006
Creator: Jankowski, A F & Hayes, J P
Partner: UNT Libraries Government Documents Department

Interdiffusion in Ni/CrMo Composition-Modulated Films

Description: The measurement of diffusivity at low temperature in the Ni-CrMo alloy system, relative to the melt point, is accomplished through the use of a composition-modulated structure. The composition-modulated structure consists of numerous pairs of alternating Ni and Cr-Mo layers that are each just a few nanometers thick. A direct assessment of alloy stability is made possible through measurement of the atomic diffusion between these layers that occurs during anneal treatments. X ray diffraction under the Bragg condition in the {theta}/2{theta} mode is the method used to quantify the changes that occur in the short-range order, i.e. the artificial composition fluctuation. The relative intensities of satellite reflections about the Bragg peaks are monitored as a function of the time at temperature. The decay rate of the artificial composition fluctuation of Ni with Cr-Mo is analyzed using the microscopic theory of diffusion to quantify a macroscopic diffusion coefficient D as 1.52 x 10{sup -19} cm{sup 2} {center_dot} sec{sup -1} for Ni{sub 2}(Cr,Mo) at 760 K.
Date: February 18, 2003
Creator: Jankowski, A F & Saw, C K
Partner: UNT Libraries Government Documents Department

Sputter Deposition of Metallic Sponges

Description: Metallic films are grown with a sponge-like morphology in the as-deposited condition using planar magnetron sputtering. The morphology of the deposit is characterized by metallic continuity in three dimensions with continuous porosity on the sub-micron scale. The stabilization of the metallic sponge is directly correlated with a limited range for the sputter deposition parameters of working gas pressure and substrate temperature. This sponge-like morphology augments the features as generally understood in the classic zone models of growth for physical vapor deposits. Nickel coatings are deposited with working gas pressures up to 4 Pa and for substrate temperatures up to 1100 K. The morphology of the deposits is examined in plan and in cross-section with scanning electron microscopy. The parametric range of gas pressure and substrate temperature (relative to absolute melt point) for the deposition processing under which the metallic sponges are produced appear universal for many metals, as for example, including gold, silver, and aluminum.
Date: January 18, 2002
Creator: Jankowski, A F & Hayes, J P
Partner: UNT Libraries Government Documents Department

Ti-Cr-Al-O Thin Film Resistors

Description: Thin films of Ti-Cr-Al-O are produced for use as an electrical resistor material. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O{sub 2}. Vertical resistivity values from 10{sup 4} to 10{sup 10} Ohm-cm are measured for Ti-Cr-Al-O films. The film resistivity can be design selected through control of the target composition and the deposition parameters. The Ti-Cr-Al-O thin film resistor is found to be thermally stable unlike other metal-oxide films.
Date: March 21, 2002
Creator: Jankowski, A F & Hayes, J P
Partner: UNT Libraries Government Documents Department

Evaporative Deposition of Aluminum Coatings and Shapes with Grain Size Control

Description: The direct deposition of coatings with variable cross-section profiles presents a challenge for the use of physical vapor deposition technology. Coatings with constant and variable cross-section profiles are of interest for advancing the evaluation of material behavior under extreme loading conditions, as for example under high strain rate. The synthesis of a variable cross-section profile by design in the as-deposited condition requires process innovation. It is demonstrated that a thickness gradient in cross-section can be produced when the substrate is exposed to a highly collimated evaporation source. The exposure is governed using a variable position shutter as driven by a computer-controlled stepper motor. An example is shown for aluminum deposition in which the coating thickness varies linearly from one plateau to another forming a wedge shape. To deposit a controlled grain size in coatings as these wedge shapes, first requires an understanding of the affect of time at temperature. An examination of aluminum coatings with constant cross-section reveals that ideal-grain growth behavior is observed from the micron-to-millimeter scale for depositions at temperatures in excess of half the melt point.
Date: February 19, 2003
Creator: Jankowski, A F & Hayes, J P
Partner: UNT Libraries Government Documents Department

Magnetron sputtered boron films and Ti/B multilayer structures

Description: A method is described for the production of thin boron and titanium/boron films by magnetron sputter deposition. The amorphous boron films contain no morphological growth features, unlike those found when thin films are prepared by various physical vapor 5 deposition processes. Magnetron sputter deposition method requires the use of a high density crystalline boron sputter target which is prepared by hot isostatic pressing. Thin boron films prepared by this method are useful for ultra-thin band pass filters as well as the low Z element in low Z/high Z mirrors which enhance reflectivity 10 from grazing to normal incidence.
Date: March 11, 1991
Creator: Makowiecki, D. M. & Jankowski, A. F.
Partner: UNT Libraries Government Documents Department

Reactive sputter deposition of boron nitride

Description: The preparation of fully dense, boron targets for use in planar magnetron sources has lead to the synthesis of Boron Nitride (BN) films by reactive rf sputtering. The deposition parameters of gas pressure, flow and composition are varied along with substrate temperature and applied bias. The films are characterized for composition using Auger electron spectroscopy, for chemical bonding using Raman spectroscopy and for crystalline structure using transmission electron microscopy. The deposition conditions are established which lead to the growth of crystalline BN phases. In particular, the growth of an adherent cubic BN coating requires 400--500 C substrate heating and an applied {minus}300 V dc bias.
Date: October 1, 1995
Creator: Jankowski, A.F.; Hayes, J.P.; McKernan, M.A. & Makowiecki, D.M.
Partner: UNT Libraries Government Documents Department