Spelling suggestions: "subject:"vertically aligned nanocomposites"" "subject:"ertically aligned nanocomposites""
1 |
Nanostructured thin films for solid oxide fuel cellsYoon, Jongsik 15 May 2009 (has links)
The goals of this work were to synthesize high performance perovskite based thin film
solid oxide fuel cell (TF-SOFC) cathodes by pulsed laser deposition (PLD), to study the
structural, electrical and electrochemical properties of these cathodes and to establish
structure-property relations for these cathodes in order to further improve their properties
and design new structures.
Nanostructured cathode thin films with vertically-aligned nanopores (VANP) were
processed using PLD. These VANP structures enhance the oxygen-gas phase diffusivity,
thus improve the overall TF-SOFC performance. La0.5Sr0.5CoO3 (LSCO) and
La0.4Sr0.6Co0.8Fe0.2O3 (LSCFO) were deposited on various substrates (YSZ, Si and
pressed Ce0.9Gd0.1O1.95 (CGO) disks). Microstructures and properties of the
nanostructured cathodes were characterized by transmission electron microscope (TEM),
high resolution TEM (HRTEM), scanning electron microscope (SEM) and
electrochemical impedance spectroscopy (EIS) measurements. A thin layer of vertically-aligned nanocomposite (VAN) structure was deposited in
between the CGO electrolyte and the thin film LSCO cathode layer for TF-SOFCs. The
VAN structure consists of the electrolyte and the cathode materials in the composition of
(CGO) 0.5 (LSCO) 0.5. The self-assembled VAN nanostructures contain highly ordered
alternating vertical columns formed through a one-step thin film deposition using a PLD
technique. These VAN structures significantly increase the interface area between the
electrolyte and the cathode as well as the area of active triple phase boundary (TPB),
thus improving the overall TF-SOFC performance at low temperatures, as low as 400oC,
demonstrated by EIS measurements. In addition, the binary VAN interlayer could act as
the transition layer that improves the adhesion and relieves the thermal stress and lattice
strain between the cathode and the electrolyte.
The microstructural properties and growth mechanisms of CGO thin film prepared by
PLD technique were investigated. Thin film CGO electrolytes with different grain sizes
and crystal structures were prepared on single crystal YSZ substrates under different
deposition conditions. The effect of the deposition conditions such as substrate
temperature and laser ablation energy on the microstructural properties of these films are
examined using XRD, TEM, SEM, and optical microscope. CGO thin film deposited
above 500 ºC starts to show epitaxial growth on YSZ substrates. The present study
suggests that substrate temperature significantly influences the microstructure of the
films especially film grain size.
|
2 |
Vertically Aligned Nanocomposite Thin FilmsBi, Zhenxing 2011 May 1900 (has links)
Vertically aligned nanocomposite (VAN) thin films have recently stimulated
significant research interest to achieve better material functionality or
multifunctionalities. In VAN thin films, both phases grow epitaxially in parallel on given
substrates and form a unique nano-checkerboard structure. Multiple strains, including
the vertical strain which along the vertical interface and the substrate induced strain
which along the film and substrate interface, exist in VAN thin films. The competition of
these strains gives a promise to tune the material lattice structure and future more the
nanocomposite film physical properties. Those two phases in the VAN thin films are
selected based on their growth kinetics, thermodynamic stability and epitaxial growth
ability on given substrates.
In the present work, we investigated unique epitaxial two-phase VAN
(BiFeO3)x:(Sm2O3)1-x and (La0.7Sr0.3MnO3)x:(Mn3O4)1-x thin film systems by pulsed laser
deposition. These VAN thin films exhibit a highly ordered vertical columnar structure
with good epitaxial quality. The strain of the two phases can be tuned by deposition
parameters, e.g. deposition frequency and film composition. Their strain tunability is found to be related directly to the systematic variation of the column widths and domain
structures. Their physical properties, such as dielectric loss and ferromagnetisms can be
tuned systematically by this variation.
The growth morphology, microstructure and material functionalities of VAN thin
films can be varied by modifying the phase ratio, substrate orientation or deposition
conditions. Systematic study has been done on growing (SrTiO3)0.5:(MgO)0.5 VAN thin
films on SrTiO3 and MgO substrates, respectively. The variation of column width
demonstrates the substrate induced strain plays another important role in the VAN thin
film growth.
The VAN thin films also hold promise in achieving porous thin films with ordered
nanopores by thermal treatment. We selected (BiFeO3)0.5:(Sm2O3)0.5 VAN thin films as a
template and get uniformly distributed bi-layered nanopores. Controllable porosity can
be achieved by adjusting the microstructure of VAN (BiFeO3):(Sm2O3) thin films and
the annealing parameters. In situ heating experiments within a transmission electron
microscope column provide direct observations into the phases transformation,
evaporation and structure reconstruction during the annealing.
Systematic study in this dissertation demonstrate that the vertically aligned
nanocomposite microstructure is a brand new architecture in thin films and an exciting
approach that promises tunable material functionalities as well as novel nanostructures.
|
3 |
Enhanced Flux-Pinning Properties in Superconducting YBa2Cu3O7-δ Thin Films with Nanoengineering MethodsTsai, Chen-Fong 03 October 2013 (has links)
Since the discovery of the high temperature superconductor YBa2Cu3O7-δ (YBCO), with transition temperature (Tc = 77 K), above liquid nitrogen point in 1987 many research projects have been dedicated to enhancing the high field performance of this material for practical applications. The 2nd generation YBCO-based coated conductors are believed to be the most promising approach for commercial applications including power transmission, motors, generators, and high field magnets. With the advances of nanotechnologies, different nanoengineering methods have been demonstrated to enhance the performance of YBCO thin films, include doping with 0-dimensional (0-D) self-assembled nanoparticles, 1-dimensional (1-D) nanorods, and 2-dimensional (2-D) nanolayers. Furthermore, dopants with ferromagnetic properties are also reported to provide enhanced pinning effects by Lorentz force, especially under high-applied magnetic fields. The principle of these methods is to generate high-density defects at the heterogeneous interfaces as artificial pinning centers in an effort to improve the flux-pinning properties. The morphology and dimensions of the nanoinclusions play an important role in pining enhancement. Optimized pinning structures are likely to be located at energetically favorable vortex cores, which form a triangular lattice with dimensions close to the YBCO coherence length ξ (ξab ~ 4 nm; ξc ~ 0.5 nm at 77 K.) However, it is challenging to achieve small dimensional nanodopants in the vapor deposited YBCO thin films. The purpose of this research is to utilize nanoengineering methods to produce optimized pinning structure in YBCO thin films.
In this thesis, we systematically study the effects of different nanoinclusions on the flux-pinning properties of YBCO thin films. The 0-D ferromagnetic Fe2O3 and CoFe2O4 nanoparticles, 2-D CeO2 multilayers, and tunable vertically aligned nanocomposites (VAN) of (Fe2O3)x:(CeO2)1-x and (CoFe2O4)x:(CeO2)1-x systems are introduced into the YBCO matrix as artificial pinning centers. Results suggest that all nanoinclusions showed significant enhancement in the superconducting properties of YBCO. The ferromagnetic pinning centers dominate at high field and low temperature regimes, however, the defect pinning centers dominate at low field and high temperature regimes. The uniquely arranged VAN structure of alternating magnetic and non-magnetic nanophases, which incorporates both high defect density and tunable distribution of magnetic dopants, is believed to be an ideal solution for flux-pinning enhancement.
|
4 |
Nanostructured Thin Film Electrolyte for Thin Film Solid Oxide Fuel CellsCho, Sungmee 2011 August 1900 (has links)
Solid oxide fuel cells (SOFCs) are very attractive as energy generation devices because they are clean, reliable, and almost entirely pollution-free. SOFCs have flexible fuel selections compared with other fuel cell technologies. The main disadvantage of SOFCs is their high operating temperature (~1000ºC for conventional SOFCs) which leads to cell cracking and formation of non-conducting compounds at electrolyte/electrode interfaces. Therefore, intermediate temperature SOFCs (ITSOFCs) in the range of 500-700 ºC has attracted extensive research interests. To achieve high cell performance at reduced temperatures, it requires high-catalytic activity, high ionic conductivity, and comparable thermal expansion coefficient (TEC) of the cell components. To address the above issues, the research focuses on two main approaches (i.e., the interlayer approach and the electrolyte approach) in order to improve the overall cell performance. First, the design of a thin layer of a vertically-aligned nanocomposite (VAN) structure as an interlayer between the electrolyte and cathode is demonstrated. The development of the VAN structures consisted of the cathode material as a perovskite or ordered double perovskite structure, La0.5Sr0.5CoO3 (LSCO) or PrBaCo2O5 delta (PBCO), and the electrolyte material as a fluorite structure, Ce0.9Gd0.1O1.95 (CGO or GDC), were achieved for thin film solid oxide fuel cell (TFSOFCs). The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode and also acts as a transition layer that improves adhesion and relieves both thermal stress and lattice strain. Second, microstructural and electrical properties of Gd-doped CeO2 (GDC, Ce0.9Gd0.1O1.95) thin films electrolyte are studied for intermediate temperature solid oxide fuel cells (SOFCs). The GDC thin film electrolytes with different grain sizes and grain morphologies were prepared by varying the deposition parameters such as substrate temperature, oxygen partial pressure, target repetition rate, and laser ablation energy. The electrical property of the GDC thin film is strongly affected by the grain size. Third, bilayer electrolytes composed of a gadolinium-doped CeO2 (GDC) layer (~6 micrometer thickness) and an yttria-stabilized ZrO2 (YSZ) layer with various thicknesses (~330 nm, ~440 nm, and ~1 micrometer) are achieved by a pulsed laser deposition (PLD) technique for thin film solid oxide fuel cells (TFSOFCs). One effective approach is to incorporate YSZ thin film as a blocking layer in between the GDC and anode for preventing chemical reduction of GDC and electrical current leakage. This bilayer approach effectively improves the GDC's chemical/ mechanical stability and reduces the OCV loss under reducing conditions. The results suggest that the YSZ thin film serves as a blocking layer for preventing electrical current leakage in the GDC layer and also provides chemical, mechanical, and structural integrity in the cell, which leads to the overall enhanced performance.
|
5 |
Metal-Oxide Nanocomposite for Tunable Physical PropertiesShikhar Misra (9132629) 05 August 2020 (has links)
<p>Understanding how light interacts
with the matter is essential for developing future opto-electronic devices.
Furthermore, tuning such light-matter interaction requires designing new
material platforms that is essential for developing devices which are functional
in different light wavelength regimes. Among these designs, particle-in-matrix,
multilayer or nanowire morphology, consisting of metal and dielectric materials,
have been demonstrated for achieving improved physical and optical properties,
such as ferroelectricity, ferromagnetism and negative refraction. For example,
Au-TiO<sub>2</sub> two phase nanocomposite has been explored in this
dissertation as a way of achieving enhanced photocatalysis. However, due to the
availability of a limited range of structures in terms of crystallinity and
morphology in the two-phase nanocomposites, a greater design flexibility and
structural complexity along with versatile growth techniques are needed for
developing next generation integrated photonic and electronic devices. This can
be achieved by incorporating a third phase through the three phase
nanocomposite designs by judicious selection of materials and functionalities. </p>
<p>In this dissertation, a new nanocomposite
design having three different phases has been introduced: Au, BaTiO<sub>3</sub>
and ZnO, which grow in a highly ordered ‘nanoman’-like structure. More
interestingly, the three phases in the novel ‘nanoman’-like structure combine
to give an emergent new property which are not found individually in the three
phases. The ordered ‘nanoman’-like structures enable a high degree of
tunability in their optical and electrical properties, including the hyperbolic
dispersion in the visible and near infrared regime, in addition to the
prominent ferroelectric/piezoelectric properties. Moreover, the growth kinetics
and the thermal stability (using in-situ Transmission Electron Microscopy) of
the ‘nanoman’ structures has also been studied. This study introduces a new
growth paradigm of fabricating three-phase nanocomposite that will surely generate
wide interests with potential applications to different systems. The ordered
three-phase ‘nanoman’ structures present enormous opportunities for novel
complex nanocomposite designs towards future optical, electrical and magnetic
property tuning.</p>
|
6 |
<b>Effect of Film Thickness on CeO</b><sub><strong>2</strong></sub><b>/Au Vertically Aligned Nanocomposite Morphology and Properties</b>Matteo T Moceri (18431868) 26 April 2024 (has links)
<p dir="ltr">The primary goal of this work is to gain a fundamental understanding on how growth conditions affect the morphology and crystallography orientation of CeO<sub>2</sub>/Au vertically aligned nanocomposite (VAN) thin films. Focus has been placed on how the changes in morphology and crystallography translate to tunable optical properties. The morphological effects have been observed and analyzed via two main approaches: the change in morphology was observed at multiple points along the film thickness, and the morphology at the film/substrate interface has been analyzed with respect to total film thickness. The changes in Au crystallography orientations have been observed by measuring peak shift in XRD patterns and determining the resulting in- and out-of-plane strain. To observe additional effects of this morphology change, optical measurements have been taken for films at the bottom, middle, and top of the thickness range. Strong trends in transmittance, plasmonic absorption peak shifts and hyperbolic permittivity behavior are correlated with the film thickness. This tunability of optical properties likely arises from changes in both Au pillar phase morphology and crystal orientation. These findings demonstrate that changing film thickness may be a desirable method to easily tune the morphology and optical properties of VAN thin films.</p>
|
Page generated in 0.4056 seconds