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Biomedical subjects

Dave H A Blank

Publications and source records attributed to Dave H A Blank.

13 recordsLinked to original sources
PubMed2006

Chemistry of 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) modification of zirconium and hafnium propoxide precursors.

The modification of different zirconium propoxide and hafnium propoxide precursors with 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) was investigated by characterization of the isolated modified species. The complexes [Zr(OnPr)3(thd)](2), [Zr(OnPr)(OiPr)2(thd)]2, Zr(OiPr)(thd)3, [Hf(OnPr)3(thd)]2, and Hf(OiPr)(thd)3 were isolated and characterized. The structure of the n-propoxide analogue of Zr(OiPr)(thd)3 could not be refined, but its existence was clearly demonstrated by XRD and 1H NMR. The modification of the propoxide precursors involves mono- and trisubstituted intermediate compounds and does not involve a disubstituted compound; thus, the commercial product that is claimed to be "Zr(OiPr)2(thd)2" and is most commonly used for the MOCVD preparation of ZrO2 does not exist. No evidence was found for the presence of such a compound in either zirconium- or hafnium-based systems. Formation of the dimeric hydroxo-di-thd-substituted complex, [Hf(OH)(OiPr)(thd)2]2, which could be isolated only for hafnium-based systems, occurs on microhydrolysis. All heteroleptic intermediates are eventually transformed to the thermodynamically stable Zr(thd)4 or Hf(thd)4) The compounds obtained from isopropoxide precursors showed a higher stability than those with n-propoxide ligands or a combination of both types. In addition, it is important to note that residual alcohol facilitates the transformation and strongly enhances its rate. The unusually low solubility and volatility of MIV(thd)4 has been shown to be due to close packing and strong van der Waals interactions in the crystal structures of these compounds.

Journal Article↗
PubMed2006

Electronically coupled complementary interfaces between perovskite band insulators.

Perovskite oxides exhibit a plethora of exceptional properties, providing the basis for novel concepts of oxide-electronic devices. The interest in these materials is even extended by the remarkable characteristics of their interfaces. Studies on single epitaxial connections between the wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either high-mobility electron conductors or insulating, depending on the atomic stacking sequences. For device applications, as well as for a basic understanding of the interface conduction mechanism, it is important to investigate the electronic coupling of closely spaced complementary interfaces. Here we report the successful realization of such coupled interfaces in SrTiO3-LaAlO3 thin-film multilayer structures. We found a critical separation distance of six perovskite unit cell layers, corresponding to approximately 23 A, below which a decrease of the interface conductivity and carrier density occurs. Interestingly, the high carrier mobilities characterizing the separate conducting interfaces are found to be maintained in coupled structures down to subnanometre interface spacing.

Journal Article↗
PubMed2006

Recovery of homogeneous polyoxometallate catalysts from aqueous and organic media by a mesoporous ceramic membrane without loss of catalytic activity.

The recovery of homogeneous polyoxometallate (POM) oxidation catalysts from aqueous and non-aqueous media by a nanofiltration process using mesoporous gamma-alumina membranes is reported. The recovery of Q(12)[WZn(3)(ZnW(9)O(34))(2)] (Q=[MeN(n-C(8)H(17))(3)](+)) from toluene-based media was quantitative within experimental error, while up to 97 % of Na(12)[WZn(3)(ZnW(9)O(34))(2)] could be recovered from water. The toluene-soluble POM catalyst was used repeatedly in the conversion of cyclooctene to cyclooctene oxide and separated from the product mixture after each reaction. The catalytic activity increased steadily with the number of times that the catalyst had been recycled, which was attributed to partial removal of the excess QCl that is known to have a negative influence on the catalytic activity. Differences in the permeability of the membrane for different liquid media can be attributed to viscosity differences and/or capillary condensation effects. The influence of membrane pore radius on permeability and recovery is discussed.

Catalysis↗
PubMed2005

Factors influencing the transport of short-chain alcohols through mesoporous gamma-alumina membranes.

The pressure-driven transport of water, ethanol, and 1-propanol through supported gamma-alumina membranes with different pore diameters is reported. Water and alcohols had similar permeabilities when they were transported through gamma-alumina membranes with average pore diameters of 4.4 and 6.0 nm, and the permeability coefficient was found to be proportional to the square of pore size, in accordance with a viscous flow mechanism. For transport through membranes with an average diameter of 3.2 nm, the behavior of water was in accordance with the viscous flow mechanism, but the permeability of the membrane for ethanol and 1-propanol was much smaller than expected and could not be explained in terms of viscous flow. Although the low permeability of the membrane with 3.2 nm pores for ethanol and 1-propanol was partly due to the presence of small amounts of water in the alcohols, the permeability coefficients were still substantially smaller when water was absent. This intrinsic difference between water and alcohol may be due to differences in molecular size, chemisorption of alcohols on the oxide pore wall, which would lead to a reduction of the effective pore size, and/or a certain degree of translational ordering of the alcohol molecules inside the membrane pores, which leads to an effectively higher viscosity and, therefore, to a higher transport resistance.

Journal Article↗
PubMed2005

Selective pervaporation of water through a nonselective microporous titania membrane by a dynamically induced molecular sieving mechanism.

Pervaporation experiments were performed on microporous titania membranes using several binary liquids containing 2-20 wt % water. The membrane was nonselective in the separation of water from alcohols and p-dioxane but showed a remarkably high selectivity in the separation of water from ethylene glycol/water mixtures with < or =15 mol % water. The absence of selectivity under most conditions is explained by the large pore size (0.9 nm) of microporous titania. The high selectivity for water in the separation from ethylene glycol can be explained by the formation of a hydrogen-bonded network of ethylene glycol in the micropores, which blocks transport of ethylene glycol, while water can still permeate through. These networks are disrupted by water at higher concentrations, leading to full loss of membrane selectivity.

Letter↗
PubMed2005

Si-supported mesoporous and microporous oxide interconnects as electrophoretic gates for application in microfluidic devices.

Microfluidic analysis systems are becoming an important technology in the field of analytical chemistry. An expanding area is concerned with the control of fluids and species in microchannels by means of an electric field. This paper discusses a new class of Si-compatible porous oxide interconnects for gateable transport of ions. The integration of such thin oxide films in microfluidics devices has been hampered in the past by the compatibility of oxides with silicon technology. A general fabrication method is given for the manufacture of silicon microsieve support structures by micromachining, on which a thin oxide layer is deposited by the spin-coating method. The deposition method was used for constructing gamma-alumina, MCM-48 silica, and amorphous titania films on the support structures, from both water-based and solvent-based oxide sols. The final structures can be applied as microporous and mesoporous interconnecting walls between two microchannels. It is demonstrated that the oxide interconnects can be operated as ion-selective electrophoretic gates. The interconnects suppress Fick diffusion of both charged and uncharged species, so that they can be utilized as ionic gates with complete external control over the transport rates of anionic and cationic species, thus realizing the possibility for implementation of these Si-compatible oxide interconnects in microchip analyses for use as dosing valves or sensors.

Diffusion↗
PubMed2005

Noble metal nanoparticles deposited on self-assembled monolayers by pulsed laser deposition show coulomb blockade at room temperature.

Nanometer-sized noble-metal clusters are fabricated on top of alkylthiolate self-assembled monolayers (SAMs) on annealed gold by pulsed laser deposition at elevated pressures. The size distribution of the clusters depends on the metal and on the pressure during the deposition. Scanning tunneling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM) showed that the metal clusters are insulated from the substrate on top of the SAM. Coulomb blockades could be measured at room temperature by STM for palladium clusters on decanethiol SAMs.

Electric Conductivity↗
PubMed2004

Effect of trace amounts of water on organic solvent transport through gamma-alumina membranes with varying pore sizes.

The transport behavior of toluene and n-hexane in gamma-alumina membranes with different pore diameters was studied. It was shown that the permeability of water-lean hexane and toluene is in agreement with Darcy's law down to membrane pore diameters of 3.5 nm. The presence of molar water fractions of 5-8 x 10(-4) in these solvents led to a permeability decrease of the gamma-alumina layer by a factor of 2-4 depending on pore size. In general, a lower permeability was found for hexane than for toluene. Moreover, in the presence of water a minimum applied pressure of 0.5-1.5 bar was required to induce net liquid flow through the membrane. These phenomena were interpreted in terms of capillary condensation of water in membrane pores with a size below a certain critical diameter. This is thought to lead to substantial blocking of these pores for transport, so that the effective tortuosity of the membrane for transport of hydrophobic solvents increases.

Journal Article↗
PubMed2004

Controlling the transport of cations through permselective mesoporous alumina layers by manipulation of electric field and ionic strength.

The electric field-driven transport of ions through supported mesoporous gamma-alumina membranes was investigated. The influence of ion concentration, ion valency, pH, ionic strength, and electrolyte composition on transport behavior was determined. The permselectivity of the membrane was found to be highly dependent on the ionic strength. When the ionic strength was sufficiently low for electrical double-layer overlap to occur inside the pores, the membrane was found to be cation-permselective and the transport rate of cations could be tuned by variation of the potential difference over the membrane. The cation permselectivity is thought to be due to the adsorption of anions onto the pore walls, causing a net negative immobile surface charge density, and consequently, a positively charged mobile double layer. The transport mechanism of cations was interpreted in terms of a combination of Fick diffusion and ion migration. By variation of the potential difference over the membrane the transport of double-charged cations, Cu2+, could be controlled accurately, effectively resulting in on/off tunable transport. In the absence of double-layer overlap at high ionic strength, the membrane was found to be nonselective.

Journal Article↗
PubMed2003

Ordering and manipulation of the magnetic moments in large-scale superconducting pi-loop arrays.

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2pi when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating pi-Josephson junctions, a phase shift of pi can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Phi(0)(Phi(0) = h/2e = 2.07 x 10(-15) Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled pi-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These pi-loop arrays are of interest as model systems for studying magnetic phenomena--including frustration effects--in Ising antiferromagnets. Furthermore, studies of coupled pi-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

Journal Article↗