Jeffrey P. Hayes

Novel proton exchange membrane thin-film fuel cell for microscale energy conversion

Thin-film, proton exchange membrane fuel cells are developed using photolithographic patterning, physical vapor deposition, and spin-cast deposition techniques. In this study, micrometer-thick layers of nickel (Ni) and platinum (Pt) electrodes, as well as the proton conducting electrolyte layer of perfluoronated sulfonic acid, are synthesized. The anode layer is conductive to pass the electric current and provides mechanical support to the electrolyte and cathode layer that enables combination of the reactive gases. The morphology desired for both the anode and cathode layers facilitates generation of maximum current density from the fuel cell. For these purposes, the parameters of the deposition process and post-deposition patterning are optimized for continuous porosity across both electrode layers. The power output generated through current–voltage measurement is characterized at various temperatures in the range of 60–90 °C using dilute (4%) hydrogen fuel.

Sputter deposition of yttrium-oxides

The yttrium–oxide phase diagram contains only one equilibrium compound, a Y2O3 cubic structure. The process of reactive sputter deposition is used to synthesize previously unreported, yttrium‐rich oxide compounds. A planar magnetron is operated in the direct–current mode using a working gas mixture of argon–20% oxygen. For low sputter gas pressure and flow conditions, the yttrium content of the coating is directly controlled as a function of the applied target power, i.e., the deposition rate. The composition and structure of the coatings are characterized using Auger depth profiling and x‐ray diffraction. A continuous series of compounds are formed with an orthorhombic crystal structure for concentrations ranging from 40 to 75 at. % yttrium.

Reactive sputter deposition of yttria-stabilized zirconia

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.

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

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.

Sputter deposition of yttria‐stabilized zirconia onto a porous Au substrate

Process issues key to thin‐film/solid–oxide fuel cells include the deposition of defect‐free electrolyte layers on porous electrodes, gas transport through the porous conducting electrodes, and sufficient structural integrity for stack assembly and temperature cycling. This study addresses the method of electrolyte layer deposition. Our initial approach uses a porous metal substrate to permit measurement of the electrolyte performance as well as provide a pore size similar to conventional cermet electrodes. The sputter deposition of Au under controlled process parameters provides the porous substrate. An optimum choice for the electrolyte material is yttria‐stabilized zirconia (YSZ). Our focus is to evaluate the process parameters of rf sputtering a YSZ target to densely coat the porous (Au) substrate. A high sputter gas pressure of Argon facilitates filling surface voids of the porous substrate leading to the formation of a defect‐free layer of cubic YSZ as examined with electron microscopy techniques.

Chemical bonding in hard boronhexagonal boron nitride multilayers

Abstract 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 through the use of a layered structure. Alternate stiff layers of boron (B) and compliant layers of hexagonal boron nitride (hBN) are formed by modulating the sputter gas composition during deposition from a pure B target. The B/hBN thin films are characterized with transmission electron microscopy to evaluate the microstructure, nanoindentation to measure hardness and X-ray absorption spectroscopy to determine chemical bonding. The effects of layer pair spacing on chemical bonding and hardness are evaluated for the B/hBN films.

Superlattice effects on solid-state amorphization

Solid-state amorphization (SSA) is observed in Ni/Ti multilayers. Microscopy and diffraction analyses have shown that the sputter deposited structures are epitaxial with crystalline interfaces. The amorphization reaction nucleates at the multilayer interfaces, i.e. the lattice sites of disorder. The interfacial disorder that arises from lattice distortions is dependent on the layer pair spacing. The onset temperature of SSA as measured using differential thermal analysis is shown to depend on the layer pair spacing.

Novel thin film solid-oxide fuel cell for microscale energy conversion

A novel approach for the fabrication and assembly of a solid oxide fuel cell system is described which enables effective scaling of the fuel delivery, manifold, and fuel cell stack components for applications in miniature and microscale energy conversion. Scaling towards miniaturization is accomplished by utilizing thin film deposition combined with novel micromachining approaches which allow manifold channels and fuel delivery system to be formed within the substrate which the thin film fuel cell stack is fabricated on, thereby circumventing the need for bulky manifold components which are not directly scalable. Results demonstrating the generation of electrical current in the temperature range of 200 - 400 degrees Celsius for a thin film solid oxide fuel cell stack fabricated on a silicon wafer will be presented.

Elastic Property Dependence on Layer Periodicity in Cu/Ni Superlattices

Cu/Ni superlattices are prepared by magnetron sputter deposition and structurally characterized with X-ray diffraction measurement. A 1.2–4.5 nm range of layer pair spacings is produced in a series of 1–2-µm-thick films which have a [111] textured growth. Uniaxial tensile testing is used to produce load-displacement curves from free-standing Cu/Ni films and calibration standards of Cu and Ni films. Direct measurement of the yield and ultimate stresses as well as Youngs modulus is performed for the Cu/Ni films. The measured Youngs modulus shows a bimodal variation with layer pair spacing. A maximum increase of 50% in Youngs modulus, above the rule-of-mixtures value, is measured for a 2-nm-thick layer pair sample. The yield stress behavior follows the modulus trend with layer pair spacing, whereas the ultimate stress inversely follows the trend, that is, the stiffest samples are the most brittle. Isothermal annealing of the 2-nm-thick Cu/Ni sample progressively homogenizes the layered structure and diminishes the modulus enhancement to the rule-of-mixtures value.

Porous thin-film anode materials for solid-oxide fuel cells

Thin film, solid-oxide fuel cells (TFSOFCs) synthesized from an electrolyte and conductive material are developed using photolithographic patterning and physical vapor deposition. The anode layer must enable combination of the reactive gases, be conductive to pass the electric current, and provide mechanical support to the electrolyte and cathode layers. The microstructure and morphology desired for the anode layer should facilitate generation of maximum current density from the fuel cell. For these purposes, the parameters of the deposition process and post-deposition patterning are developed to optimize a continuous porosity across the anode layer. The anode microstructure is characterized using scanning electron microscopy and the power output generated through current-voltage measurement.