Nelson S. Bell

Electrospun Teflon AF fibers for superhydrophobic membranes

Superhydrophobic membranes have the potential to protect devices from incidental exposure to water. This paper reports on the processing of Teflon AF fluoropolymers through electrospinning. Teflon AF is difficult to electrospin due to its low dielectric constant and the low dielectric constants of the liquids in which it is soluble. The two approaches that have been utilized to produce fibers are direct electrospinning in Novec engineering liquids and core-shell electrospinning. Both methods produced superhydrophobic membranes. Fibers with an average diameter of 290 nm and average water contact angle of 151° were obtained by core-shell electrospinning. One suggested application for electrospun superhydrophobic membranes is the lithium-air battery.

Electrical conduction and photoluminescence properties of solution-grown ZnO nanowires

We report on the optical and electrical properties of zinc oxide nanorods synthesized in solution using Oswald ripening of ZnO nanodots with the addition of ethylenediamene growth directing agent. This method results in high quality, single crystalline ZnO nanorods that extend up to 3μm in length and have an average diameter of 25±7nm, compared to ∼75nm diameter for similarly prepared nanorods but without the addition of the growth directing agent. Furthermore, we find that the higher aspect ratio nanorods exhibit strong size-dependent electrical characteristics, with a critical diameter of about 27nm delimiting nonconductive and conductive behaviors. Theoretical calculations indicate that the origin of this size-dependent conductivity is the presence of surface states that deplete the carriers in the smaller diameter nanorods, and an estimate of the density of these states is provided.

Microcombustor array and micro-flame ionization detector for hydrocarbon detection

This paper describes results from using a microcombustor to create two hydrocarbon gas sensors: one utilizing calorimetry and the other a flame ionization detector (FID) mechanism. The microcombustor consists of a catalytic film deposited on the surface of a microhotplate. This micromachined design has low heat capacity and thermal conductivity, making it ideal for heating catalysts placed on its surface. The catalytic materials provide a natural surface-based method for flame ignition and stabilization and are deposited using a micropen system, which allows precise and repeatable placement of the materials. The catalytic nature of the microcombustor design expands the limits of flammability (LoF) as compared with conventional diffusion flames; an unoptimized LoF of 1-32% for natural gas in air was demonstrated with the microcombustor, whereas conventionally 4-16% is observed. The LoF for hydrogen, methane, propane and ethane are likewise expanded. Expanded LoF permit the use of this technology in applications needing reduced temperatures, lean fuel/air mixes, or low gas flows. By coupling electrodes and an electrometer circuit with the microcombustor, the first ever demonstration of a microFID utilizing premixed fuel and a catalytically-stabilized flame has been performed; the detection of 1.2-2.9 % of ethane in a hydrogen/air mix is shown.

Colloidal processing of chemically prepared zinc oxide varistors. Part I: Milling and dispersion of powder

Chemically prepared zinc oxide powders are fabricated for the production of high aspect ratio varistor components. Colloidal processing was performed to reduce agglomerates to primary particles, form a high solids loadingslurry, and prevent dopant migration. The milled and dispersed powder exhibited a viscoelastic to elastic behavioral transition at a volume loading of 43-46%. The origin of this transition was studied using acoustic spectroscopy, zeta potential measurements, and oscillatory rheology. The phenomenon occurs due to a volume fraction solids dependent reduction in the zeta potential of the solid phase. It is postulated to result from divalent ion binding within the polyelectrolyte dispersant chain and was mitigated using a polyethylene glycol plasticizing additive. This allowed for increased solids loading in the slurry and a green body fabrication study to be presented in our companion paper.

Electrospinning of nanofiber Chevrel phase materials

A modified sol–gel synthesis for non-oxide sulfide ceramics is presented. Sols are electrospun into continuous nanofiber precursors and then heat treated to obtain Chevrel-phase sulfide materials. In particular, the Mg-Chevrel fibers formed have average diameters of 230 ± 57 nm with grain sizes of 10 ± 3 nm after heat-treatment.

Synthesis and self-assembly of zinc oxide nanoparticles with septahedral morphology

The formation of 10-nm ZnO nanopyramids using a simple synthetic route has been isolated from the reaction of Zn(OAc) 2 ·2H 2 O in 1,4-butanediol followed by ripening at 90 °C. This was accomplished by establishing control over the Ostwald ripening process through the use of a carboxylic acid specific adsorbate. Using a variety of analytical methods, it is proposed that the carboxylate groups in the acetate precursor stabilize the {101} habit planes, creating septahedral shapes or nanopyramids. Particle assembly into crystallographically oriented dimers was observed with high specificity, and the association mechanism is suggested to relate to the crystal polarity and the variation in specific adsorption of the carboxylic acid to the surface facets. These materials are a candidate for biological labeling applications in living cells.

Colloidal processing of chemically prepared zinc oxide varistors. Part II: Near-net-shape forming and fired electrical properties

Chemically prepared zinc oxide powders were processed for the production of high aspect ratio varistor components (length/diameter >5). Near-net-shape casting methods including slip casting and agarose gelcasting were evaluated for effectiveness in achieving a uniform green microstructure that densifies to near theoretical values during sintering. The structure of the green parts was examined by mercury porisimetry. Agarose gelcasting produced green parts having low solids loading values and did not achieve high fired density. Isopressing the agarose cast parts after drying raised the fired density to greater than 95%, but the parts exhibited catastrophic shorting during electrical testing. Slip casting produced high green density parts, which exhibit high fired density values. The electrical characteristics of slip-cast parts are comparable with dry-pressed powder compacts.

Micropen Printing of Electronic Components

This chapter explains a direct-write approach for fabricating highly integrated, multilayer components using a Micropen to deposit slurries in precise patterns. Micropen deposition enables printing of multilayer material structures on nonplanar substrates, enabling high-density circuitry with integrated passive components. Furthermore, this chapter presents the technologies instrumental to material integration by the Micropen, including equipment operation, slurry preparation, deposition constraints, and cofiring of multimaterial integrated passives. The four example device elements include: high precision resistors, high-capacitance density dielectrics, integrated inductor coils, and chemical sensors. The direct-write approach provides the ability to fabricate multifunctional, multimaterial integrated ceramic components (MMICCs) in an agile way with rapid turnaround. This technique is used to fabricate devices such as integrated RC filters, multilayer voltage transformers, and other passive components. Thus with advent of new levels of pen tip sophistication, vision control systems, agile materials delivery, and ultralow temperature materials, pen-dispense direct write appears to be most promising route towards high-speed, multimaterial electronics fabrication.

Calculation of Hamaker constants in non-aqueous fluid media

Calculations of the Hamaker constants representing the van der Waals interactions between conductor, resistor and dielectric materials are performed using Lifshitz theory. The calculation of the parameters for the Ninham-Parsegian relationship for several non-aqueous liquids has been derived based on literature dielectric data. Discussion of the role of van der Waals forces in the dispersion of particles is given for understanding paste formulation. Experimental measurements of viscosity are presented to show the role of dispersant truncation of attractive van der Waals forces.

In situ characterization of silver nanoparticle synthesis in maltodextrin supramolecular structures.

The use of maltodextrin supramolecular structures (MD SMS) as a reducing agent and colloidal stabilizing agent for the synthesis of Ag nanoparticles (Ag NPs) identified three key points. First, the maltodextrin (MD) solutions are effective in the formation of well-dispersed Ag NPs utilizing alkaline solution conditions, with the resulting Ag NPs ranging in size from 5 to 50 nm diameter. Second, in situ characterization by Raman spectroscopy and small angle X-ray scattering (SAXS) are consistent with initial nucleation of Ag NPs within the MD SMS up to a critical size of ca. 1 nm, followed by a transition to more rapid growth by aggregation and fusion between MD SMS, similar to micelle aggregation reactions. Third, the stabilization of larger Ag NPs by adsorbed MD SMS is similar to hemi-micelle stabilization, and monomodal size distributions are proposed to relate to integer surface coverage of the Ag NPs. Conditions were identified for preparing Ag NPs with monomodal distributions centered at 30-35 nm Ag NPs.