Lorraine F. Francis

Dispersible exfoliated zeolite nanosheets and their application as a selective membrane

Thin zeolite films prepared through a polymer exfoliation method were used as selective membranes. Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.

High‐Resolution Patterning of Graphene by Screen Printing with a Silicon Stencil for Highly Flexible Printed Electronics

High-resolution screen printing of pristine graphene is introduced for the rapid fabrication of conductive lines on flexible substrates. Well-defined silicon stencils and viscosity-controlled inks facilitate the preparation of high-quality graphene patterns as narrow as 40 μm. This strategy provides an efficient method to produce highly flexible graphene electrodes for printed electronics.

Gravure Printing of Graphene for Large‐area Flexible Electronics

Gravure printing of graphene is demonstrated for the rapid production of conductive patterns on flexible substrates. Development of suitable inks and printing parameters enables the fabrication of patterns with a resolution down to 30 μm. A mild annealing step yields conductive lines with high reliability and uniformity, providing an efficient method for the integration of graphene into large-area printed and flexible electronics.

Lowering the percolation threshold of conductive composites using particulate polymer microstructure

The percolation thresholds of carbon black–polymer composites have been successfully lowered using particulate polymer starting materials (i.e., latex and water-dispersible powder). Composites prepared using carbon black (CB) and commercial poly(vinyl acetate) (PVAc) latex exhibit a percolation threshold near 2.5 vol % CB. This threshold value is significantly lower than that of a comparable reference composite made from poly(N-vinylpyrrolidone) (PNVP) solution and the same CB, which exhibits a sharp rise in electrical conductivity near 15 vol % CB. This dramatic difference in critical CB concentration results from the segregated microstructure induced by the latex during composite film formation. Carbon black particles are forced into conductive pathways at low concentration because of their inability to occupy volume already claimed by the much larger latex particles. There appears to be good qualitative agreement between experimental findings and current models dealing with conductive behavior of composites with segregated microstructures. Lack of quantitative agreement with the models is attributed to the polydispersity of the polymer particles in the latex.

Addition of alumina to cellulose acetate membranes

Abstract Asymmetric ceramic-polymer membranes were prepared by dry and wet phase inversion processes using dispersions containing cellulose acetate, acetone, water and colloidal alumina particles. When the alumina content is low, the microstructure contains macrovoids (pore diameter ∼ 15 μm) below a dense polymer rich skin. The addition of greater amounts of alumina results in a microstructural transition from macrovoids to a uniform small pore network (pore diameter ∼ 2μm). However, the alumina particles have a negligible effect on the porosity of the dense skin that forms at the air/membrane or water immersion bath/ membrane interface. In all membranes, the alumina particles are unagglomerated and uniformly dispersed throughout the cellulose acetate network. Changes in the microstructure are attributed to delayed phase separation of cellulose acetate in the presence of ceramic particles.

Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines

Aerosol jet printing requires control of a number of process parameters, including the flow rate of the carrier gas that transports the aerosol mist to the substrate, the flow rate of the sheath gas that collimates the aerosol into a narrow beam, and the speed of the stage that transports the substrate beneath the beam. In this paper, the influence of process parameters on the geometry of aerosol-jet-printed silver lines is studied with the aim of creating high-resolution conductive lines of high current carrying capacity. A systematic study of process conditions revealed a key parameter: the ratio of the sheath gas flow rate to the carrier gas flow rate, defined here as the focusing ratio. Line width decreases with increasing the focusing ratio and stage speed. Simultaneously, the thickness increases with increasing the focusing ratio but decreases with increasing stage speed. Geometry control also influences the resistance per unit length and single pass printing of low-resistance silver lines is demonstrated. The results are used to develop an operability window and locate the regime for printing tall and narrow silver lines in a single pass. Under optimum conditions, lines as narrow as 20 μm with aspect ratios (thickness/width) greater than 0.1 are obtained.

Sol-gel processing and properties of lead magnesium niobate powders and thin layers

Lead magnesium niobate powders and thin layers were formed from an alkoxide-based solution by sol-gel methods. The solution was synthesized by reacting a magnesium-niobium alkoxide solution with a lead acetate-based precursor solution. The effects of gelation conditions on the properties of the dried gel, and on the organic decomposition behaviour and crystalline phase development in gel-derived powders are reported. Gels prepared with greater molar ratios of water to alkoxide (3∶1) had the largest surface areas (130 m22 g−1) and required the lowest temperature (320 ° C) for organic removal. The perovskite phase first appeared at temperatures near 700 ° C, and developed at a faster rate in gels prepared with higher water contents. Approximately 95% developed after 1 h at 700 ° C, or 5 min at 775 ° C. Dielectric thin layers were prepared on platinum-coated silicon substrates by a multilayered spin-casting method. The perovskite phase formed most readily in the thin layers by a fast-firing treatment at 800 ° C. Microstructures and electrical properties are reported for the integrated thin layer dielectrics. Ferroelectric hysteresis loops were observed.

Phase development in Si modified sol-gel-derived lead titanate

Sol-gel methods were used to prepare chemically modified lead titanate (PT) powders. A PT alkoxide solution was synthesized and doped with Si (2–12 mole%) or with equimolar amounts (2–12 mole%) of Si and Pb through the addition of Si and Pb–Si alkoxide solutions, respectively. PT alkoxide solutions were also prepared with excess Pb and Ti (7 and 10 mole%). Gels were prepared through controlled additions of water. Crystalline phase development of gel-derived powders with heat treatment (400–700 °C) was studied using x-ray diffraction (XRD) and differential thermal analysis (DTA). While PT powders without added Si crystallized directly into a perovskite phase, Si modified materials crystallized first into a pyrochlore phase and at a higher temperature transformed into perovskite. The pyrochlore lattice parameter and the temperature for the transformation to perovskite increased with Si content. In all cases, the crystal structure of the final perovskite phase was not affected by the Si addition. The effect of Si on phase development and mechanisms of transformation is discussed.

All-Printed, Foldable Organic Thin-Film Transistors on Glassine Paper

All-printed, foldable organic thin-film transistors are demonstrated on glassine paper with a combination of advanced materials and processing techniques. Glassine paper provides a suitable surface for high-performance printing methods, while graphene electrodes and an ion-gel gate dielectric enable robust stability over 100 folding cycles. Altogether, this study features a practical platform for low-cost, large-area, and foldable electronics.

Oriented MFI membranes by gel-less secondary growth of sub-100 nm MFI-nanosheet seed layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.