Design and Synthesis of Oriented Guest-Host Nanostructures for Enhanced Membrane Performances Page: 3 of 5
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interface. We have aimed at a proving the concept of a nanostructured material designed to maximize the
grain boundary and grain-substrate interfacial effects.
Results and Discussion
Proof-of-principle studies have focused on development of membrane synthesis by infiltrating and
growing YSZ (typically ZrO2:1 6%Y) nanophases inside oriented nanoporous host layers of anodized
alumina discs, with pore-channel diameters in the range of 10-100nm.
Synthesis of host-guest nanocomposite membranes. Initial effort to prepare ordered nanopore host
materials was by an "engineered molecular assembly templated synthesis" (EMATS) [Hu et al., 2003],
which allows the formation of large-area domains of hexagonally ordered cylinder channels (<10 nm),
perpendicular to the layer surface. However, at this stage, oriented mesoporous silica in the form of
membrane discs is difficult to make. Alternatively, we have been cooperating with Synkera Technologies
and have successfully obtained anodized aluminum oxide (AAO) discs of various desirable pore channel
diameters (13 2, 18+3, 35+3, 55+6, and 73+7 nm). Various methods have been attempted to infiltrate
guest YSZ nanophases into host matrix. A vacuum-assisted polymer or nanoparticle precursor infiltration
method has been developed to infiltrate or impregnate YSZ precursor into the host nanopore channels,
followed by thermally induced crystallization and grain growth. The nanostructures of the resulting
composite materials (e.g., pore channels and confined nanograins) were examined by scanning
transmission electron microscope (STEM) and X-ray diffraction (XRD), showing - Snm nanocrystalline
grains present at typical 600 C, 4h calcination. Back-scatter SEM images have shown long YSZ
nanowires across the channels, in some cases segmented wires.
Conductivity enhancement in oriented guest-host nanocomposite membranes. The conductivity of the
nanostructured samples were measured by impedance spectroscopy (IS), which allows the separation of
resistances related to grains, grain boundaries, and electrode effects. The diffusivity of ions in
nanocrystalline materials corresponds to the results of conductivity measurement by impedance
spectroscopy. The cross-membrane conductivity at room temperature of the materials synthesized in this
study was the highest ever reported for YSZ materials; in fact, this is the first time values have been
obtainable at room temperature. As shown in Figure lb, the oriented host-guest interfaces further enhance
the conductivity beyond what grain boundaries in nanocrystalline phase can provide. It is noted that
conductivity of the materials from this study is lower than other nanocrystalline YSZ at high
temperatures; low guest density and nanowire discontinuity and interface segmentation at high
temperatures might have contributed to this reduced performance at high temperature.
Stability and maintenance of high-conductivity nanostructures. Thermal stability of nanocrystalline
phases, which is related to grain growth phenomena under relatively high temperature processing or
operation conditions, is known to be a technology bottleneck for maintaining nanophase-enhanced
conductivity in conventionally supported nanocrystalline films [Dong and Hu et al., 2002]. Our STEM
examinations have visually proven the nanopore-confinement effect that has restricted the growth of grain
size within the dimension of nanopore channels of the AAO host. In an extreme case, a single crystal
nanowire may form, but still provide the desirable nanograin dimension and the oriented guest-host
interfaces. Therefore, this guest-host design resolves the thermal stability problem by surviving the
desirable nanograin size in the nanocomposite membrane.
Future work. The demonstration of the enhanced conductivity and improved thermal stability for our
nanocomposite membrane samples is expected to attract follow-on funding for further development of the
membrane fabrication method for fuel cells and other applications. Future development needs to address
various issues, including systematic studies of nanostructure and composition; improvement of YSZ
nanophase density, homogeneity, and the contact interface between electrode and the membrane; the
effect of channel diameter, aspect ratio, channel length, and host and guest composition effect. The size of
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Hu, M.Z. Design and Synthesis of Oriented Guest-Host Nanostructures for Enhanced Membrane Performances, report, November 15, 2005; [Tennessee]. (digital.library.unt.edu/ark:/67531/metadc892710/m1/3/: accessed July 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.