Showing papers by "James E. Martin published in 2010"

Isothermal Magnetic Advection: Creating functional fluid flows for heat and mass transfer

Abstract: Natural convection has been of interest for over a century due to its rich nonlinear dynamics and applications to heat transfer. However, convection occurs only when both gravity and a destabilizing thermal gradient exist. We have discovered a unique class of vigorous, emergent fluid flows that have the full functionality of natural convection but can be stimulated regardless of gravity or thermal gradients, simply by subjecting a platelet suspension to certain time-dependent biaxial magnetic fields of modest strength. This enigmatic phenomenon may facilitate cooling in microgravity environments and in other circumstances where convection fails.

Field‐Structured Chemiresistors

Abstract: A significantly improved material is developed for application to chemiresistors, which are resistance-based sensors for volatile organic compounds. This material is a polymer composite containing Au-coated magnetic particles organized into electrically conducting pathways by magnetic fields. This improved material overcomes the various problems inherent to conventional carbon-black chemiresistors, while achieving an unprecedented response magnitude. When exposed to chemical vapors, the polymer swells only slightly, yet this is amplified into large, reversible resistance changes, as much as (1 × 1011)% at a swelling of only 1.5%. These conductor–insulator transitions occur over such a narrow range of analyte vapor concentration that these devices can be described as chemical switches. The sensitivity and response range of these sensors can be tailored over a wide range by controlling the stress within the composite, including through the application of a magnetic field. Such tailorable sensors can be used to create sensor arrays that can accurately determine analyte concentration over a broad concentration range, or can be used to create logic circuits that signal a particular chemical environment.

Increasing the Luminescent Quantum Yield of CdS Nanoparticles Having Broadband Emission

Abstract: Studies of the quantum yield (QY) of CdS quantum dots (QDs) with broadband luminescence are presented. These QDs are synthesized using the classic inverse micelle method employing the surfactant dioctyl sulfosuccinate sodium salt in heptane. Previous studies have shown that the QY of these materials is in the range of 10-13%. We have found that dehydration and UV photolysis roughly double the QY to typically ∼20%, while only slightly redshifting the emission. Dehydration is accomplished by cooling the samples to -30°C to remove the water from the inverse micelles. The addition of trialkylamines to dehydrated, photolyzed solutions results in a further improvement in QY to 37%. This effect is independent of amine chain length over the range studied.

Strain-Tunable Chemiresistor

Abstract: We have developed a resistance-based chemical sensor (chemiresistor) for volatile organic compounds whose sensitivity can be reversibly increased over a range of nearly 2 decades by the application of tensile strain. This polymer-based sensor is comprised of Au-plated magnetic particles structured into conducting chains by the application of a magnetic field during the curing of the prepolymer resin. The resistance of this field-structured composite increases when an analyte vapor swells the polymer and reduces the contact pressure between particles. We have found that applying a tensile strain increases both the sensor resistance and sensitivity, as defined by its relative resistance change. This increase in sensitivity is a smooth, continuous function of the applied strain, and the effect is fully reversible. Sensitivity tuning enables the response curve of the sensor to be dynamically optimized for sensing analytes over a wide concentration range.

Vortex magnetic field mixing with anisometric particles

Abstract: Recently, we reported a vigorous, scale-adaptive mixing technique suitable for microfluidic applications, wherein a suspension of spherical magnetic particles is subjected to a vortex magnetic field, which induces the formation of dynamic particle chains that efficiently stir the solution. Here we explore the dependence of the mixing torque on particle shape, and show that increasing degrees of shape anisometry (i.e., spheres, platelets, rods) give increased mixing torque at the same particle volume fraction. Moreover, all particles, regardless of shape, exhibit similar dependencies of the mixing torque on the vortex field parameters: the torque is maximized in a balanced vortex magnetic field, is proportional to the square of the field strength, and is independent of the field frequency. However, the torque advantage of anisometric particles is somewhat offset by their increased packing volume, which can make the removal of trapped fluid difficult.

Analyte discrimination from chemiresistor response kinetics.

Abstract: Chemiresistors are polymer-based sensors that transduce the sorption of a volatile organic compound into a resistance change. Like other polymer-based gas sensors that function through sorption, chemiresistors can be selective for analytes on the basis of the affinity of the analyte for the polymer. However, a single sensor cannot, in and of itself, discriminate between analytes, since a small concentration of an analyte that has a high affinity for the polymer might give the same response as a high concentration of another analyte with a low affinity. In this paper we use a field-structured chemiresistor to demonstrate that its response kinetics can be used to discriminate between analytes, even between those that have identical chemical affinities for the polymer phase of the sensor. The response kinetics is shown to be independent of the analyte concentration, and thus the magnitude of the sensor response, but is found to vary inversely with the analyte's saturation vapor pressure. Saturation vapor pressures often vary greatly from analyte to analyte, so analysis of the response kinetics offers a powerful method for obtaining analyte discrimination from a single sensor.

Master Transduction Curve for Field-Structured Chemiresistor Calibration

Abstract: Chemiresistors are gas sensors for volatile organic compounds that are composed of conducting particle networks in a polymer matrix. In the presence of an analyte that is compatible with the polymer phase, the sensor conductance decreases as the analyte is absorbed, eventually reaching a steady-state value that is a measure of the analyte’s concentration. The response curve, which is the relationship between steady-state conductance and analyte activity (normalized concentration), is strongly dependent on both the chemical affinity of the analyte for the polymer and the stress field within the chemiresistor composite. Calibration of an individual sensor would seem to necessitate mapping out the response curve for each analyte of interest, a tedious and expensive proposition. In a recent paper, we have shown that the transduction curve of any particular sensor is a function of polymer swelling alone, regardless of the chemical nature of the analyte. This master transduction curve implies that sensor calibr...

Hydrothermal method of synthesis of rare-earth tantalates and niobates

Abstract: A hydrothermal method of synthesis of a family of rare-earth Group 5 oxides, where the Group 5 oxide is a niobate or tantalate. The rare-earth Group 5 oxides can be doped with suitable emitter ions to form nanophosphors.

Magnetic method for stimulating transport in fluids

Abstract: A method for producing mass and heat transport in fluids, wherein the method does not rely on conventional convection, that is, it does not require gravity, a thermal gradient, or a magnetic field gradient. This method gives rise to a unique class of vigorous, field-controllable flow patterns termed advection lattices. The advection lattices can be used to transport heat and/or mass in any desired direction using only magnetic fields.

Chemical strategies for die/wafer submicron alignment and bonding.

Abstract: This late-start LDRD explores chemical strategies that will enable sub-micron alignment accuracy of dies and wafers by exploiting the interfacial energies of chemical ligands. We have micropatterned commensurate features, such as 2-d arrays of micron-sized gold lines on the die to be bonded. Each gold line is functionalized with alkanethiol ligands before the die are brought into contact. The ligand interfacial energy is minimized when the lines on the die are brought into registration, due to favorable interactions between the complementary ligand tails. After registration is achieved, standard bonding techniques are used to create precision permanent bonds. We have computed the alignment forces and torque between two surfaces patterned with arrays of lines or square pads to illustrate how best to maximize the tendency to align. We also discuss complex, aperiodic patterns such as rectilinear pad assemblies, concentric circles, and spirals that point the way towards extremely precise alignment.