37 Matching Results

Search Results

Advanced search parameters have been applied.

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

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 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

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

Layer formation in CuNi/C x-ray optics

Description: A series of Cu/C and CuNi/C multilayer films are prepared for potential use as normal incidence x-ray reflectors. The use of a metal alloy layer is intended to enhance layer formation without metastable metallic carbide formation. The films are characterized both in composition and structure. Addition of Ni to the Cu-rich layer is seen to markedly improve the formation of continuous metal layers. 9 refs., 4 figs.
Date: November 1, 1990
Creator: Jankowski, A.F. & Stearns, D.G.
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

Activation Energy for Grain Growth in Aluminum Coatings

Description: To produce a specific grain size in metallic coatings requires precise control of the time at temperature during the deposition process. Aluminum coatings are deposited using electron-beam evaporation onto heated substrate surfaces. The grain size of the coating is determined upon examination of the microstructure in plan view and cross-section. Ideal grain growth is observed over the entire experimental range of temperature examined from 413 to 843 K. A transition in the activation energy for grain growth from 0.7 to 3.8 eV {center_dot} atom{sup -1} is observed as the temperature increases from <526 K to >588 K. The transition is indicative of the dominant mechanism for grain growth shifting with increasing temperature from grain boundary to lattice diffusion.
Date: October 14, 2004
Creator: Jankowski, A F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

The Thermal Stability of Nanocrystalline Au-Cu Alloys

Description: Grain refinement to the nanocrystalline scale is known to enhance physical properties as strength and surface hardness. For the case of Au-Cu alloys, development of the pulsed electroplating has led to the functional control of nanocrystalline grain size in the as-deposited condition. The thermal aging of Au-Cu electrodeposits is now investigated to assess the stability of the nanocrystalline grain structure and the difference between two diffusion mechanisms. The mobility of grain boundaries, dominant at low temperatures, leads to coarsening of grain size whereas at high temperature the process of bulk diffusion dominates. Although the kinetics of bulk diffusion are slow below 500 K at 10{sup -20} cm{sup 2} {center_dot} sec, the kinetics of grain boundary diffusion are faster at 10{sup -16} cm{sup 2} {center_dot} sec. The diffusivity values indicate that the grain boundaries of the as-deposited nanocrystalline Au-Cu are mobile and sensitive to low-temperature anneal treatments affecting the grain size, hence the strength of the material.
Date: February 15, 2006
Creator: Jankowski, A F; Saw, C K & Hayes, J P
Partner: UNT Libraries Government Documents Department

THERMAL STABILITY AND MECHANICAL BEHAVIOR OF ULTRA-FINE BCC TA AND V COATINGS

Description: Ultra-refined microstructures of both tantalum (Ta) and vanadium (V) are produced using electron-beam evaporation and magnetron sputtering deposition. The thermal stability of the micron-to-submicron grain size foils is examined to quantify the kinetics and activation energy of diffusion, as well as identify the temperature transition in dominant mechanism from grain boundary to lattice diffusion. The activation energies for boundary diffusion in Ta and V determined from grain growth are 0.3 and 0.2 eV{center_dot}atom{sup -1}, respectively, versus lattice diffusion values of 4.3 and 3.2 eV{center_dot}atom{sup -1}, respectively. The mechanical behavior, as characterized by strength and hardness, is found to inversely scale with square-root grain size according to the Hall-Petch relationship. The strength of Ta and V increases two-fold from 400 MPa, as the grain size decreases from 2 to 0.75 {micro}m.
Date: November 3, 2006
Creator: Jankowski, A F; Go, J & Hayes, J P
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

Application of MXCD to magnetic thin-film sensors

Description: While Magnetic X-ray Circular Dichroism (MXCD) has been applied extensively to the extraction of elemental magnetic moments in various magnetic multilayers, the configuration of actual devices imposes certain constraints on the application of MXCD to devices. Using a set of real, thin-film spin valve devices with varying Cu spacer layer thicknesses, we demonstrate the correlation between MXCD and R-H measurements on those devices as well as the restrictions on the interpretation of MXCD data imposed by both the device topology and the formulation of realistic error estimates.
Date: May 17, 1996
Creator: Bedrossian, P.J.; Tobin, J.G. & Jankowski, A.F.
Partner: UNT Libraries Government Documents Department

Eutectic bonding of a Ti sputter coated, carbon aerogel wafer to a Ni foil

Description: The formation of high energy density, storage devices is achievable using composite material systems. Alternate layering of carbon aerogel wafers and Ni foils with rnicroporous separators is a prospective composite for capacitor applications. An inherent problem exists to form a physical bond between Ni and the porous carbon wafer. The bonding process must be limited to temperatures less than 1000{degrees}C, at which point the aerogel begins to degrade. The advantage of a low temperature eutectic in the Ni-Ti alloy system solves this problem. Ti, a carbide former, is readily adherent as a sputter deposited thin film onto the carbon wafer. A vacuum bonding process is then used to join the Ni foil and Ti coating through eutectic phase formation. The parameters required for successfld bonding are described along with a structural characterization of the Ni foil-carbon aerogel wafer interface.
Date: June 1, 1994
Creator: Jankowski, A.F.; Hayes, J.P. & Kanna, R.L.
Partner: UNT Libraries Government Documents Department

Formation of cubic boron-nitride by the reactive sputter deposition of boron

Description: Boron-nitride films are synthesized by RF magnetron sputtering boron targets where the deposition parameters of gas pressure, flow and composition are varied along with substrate temperature and applied bias. The films are analyzed using Auger electron spectroscopy, transmission electron microscopy, nanoindentation, Raman spectroscopy and x-ray absorption spectroscopy. These techniques provide characterization of film composition, crystalline structure, hardness and chemical bonding, respectively. Reactive, rf-sputtering process parameters are established which lead to the growth of crystalline BN phases. The deposition of stable and adherent boron nitride coatings consisting of the cubic phase requires 400 `C substrate heating and the application of a 300 V negative bias.
Date: March 1, 1997
Creator: Jankowski, A.F.; Hayes, J.P.; Makowiecki, D.W. & McKeman, M.A.
Partner: UNT Libraries Government Documents Department

Characterizing and modeling the apparent anomalous behavior of resistivity in Cr-Si-O thin films

Description: The Cr-Si-O material system is of interest for use as a thin film resistor. The films are sputter deposited onto conducting substrates from metal oxide compacts using a reactive gas mixture. the cermet films composition range from 50 to 100 vol.% SiO{sub 2} as determined from elemental measurements of the Cr, Si and O content. In a wide range of resistivities from 10{sup 1} to 10 {sup 14} {omega}-cm measured through the film thickness, an apparent anomalous behavior is found with the Cr, Si and O composition. The anomaly can be deducted to a discontinuous variation of resistivity with film composition near 80 vol.% SiO{sub 2}. The film microstructure is characterized as a distribution of conducting metal-silicide particles within an insulating matrix. The effective medium theory is used to predict the variation of conductivity and successfully models the anomalous resistivity behavior.
Date: March 1, 1998
Creator: Jankowski, A.F.
Partner: UNT Libraries Government Documents Department

Near-edge x-ray absorption fine structure examination of chemical bonding in sputter deposited boron and boron-nitride films

Description: Near-edge x-ray absorption fine structure (NEXAFS) is used to examine the chemical bonding in boron and boron-nitride films sputter deposited from a fully-dense, pure boron target. Reactive sputtering is used to prepare the boron-nitride and multilayered films. Although the process of sputter deposition often produces films that lack long range order, NEXAFS reveals the distinguishing features of sp{sup 2} and sp{sup 3} hybridization that are associated with different crystalline structures. The sensitivity of NEXAFS to local order further provides details in bonding modifications that exist in these films.
Date: May 1, 1996
Creator: Jankowski, A.F.; Hayes, J.P. & Suthreland, D.G.J.
Partner: UNT Libraries Government Documents Department