Facile Method for Fabricating Flexible Substrates with Embedded, Printed Silver Lines

Insertion, curing and delamination is presented as a simple and scalable method for creating flexible substrates with embedded, printed silver lines. In a sequential process, aerosol-jet printed silver lines are transferred from a donor substrate to a thin reactive polymer that is directly adhered to a flexible substrate. Due to the unique ability of the aerosol jet to print continuous lines on a low energy surface, a 100% transfer of the printed electrodes is obtained, as confirmed by electrical measurements. Moreover, the root-mean-square roughness of the embedded electrodes is less than 10 nm, which is much lower than that for their as-printed form. The embedded electrodes are robust and do not show a significant degradation in electrical performance after thousands of bending cycles

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

CryoSEM Investigation of Latex Coatings Dried in Walled Substrates

Nonuniformities, such as heavy edges or “coffee rings”, frequently develop as particulate coatings dry. One idea for avoiding these nonuniformities is to engineer the substrate edges. In this work, monodisperse latex coatings were deposited on substrates with photoresist walls around their edges. Cryogenic scanning electron microscopy (cryoSEM) results show particle accumulation near the walls and at the free surface. The contact line, pinned at the wall, generates lateral transport of water and particles, leading to a nonuniform coating thickness. Still, coatings on substrates with walls were shown to have a higher degree of thickness uniformity after drying than those without walls

Deformation Processes in Block Copolymer Toughened Epoxies

Real-time deformation events in epoxies with different cross-link density, modified with rubbery and glassy core block copolymer micelles, were captured by collecting small-angle X-ray scattering patterns while simultaneously straining the sample. Analysis and interpretation of the scattering patterns provide quantitative information about deformation processes leading to toughness in these materials. These experiments yielded direct evidence of cavitation in 30 nm rubber particles as anticipated by theory. We found that the extent of void growth after cavitation is strongly affected by the cross-link density of the matrix and is directly correlated to the toughness enhancement of the material. Our findings imply that the combination of micelle and matrix properties strongly affects the processes leading to toughness in block copolymer modified epoxies

CryoSEM Investigation of Latex Coatings Dried in Walled Substrates

Nonuniformities, such as heavy edges or “coffee rings”, frequently develop as particulate coatings dry. One idea for avoiding these nonuniformities is to engineer the substrate edges. In this work, monodisperse latex coatings were deposited on substrates with photoresist walls around their edges. Cryogenic scanning electron microscopy (cryoSEM) results show particle accumulation near the walls and at the free surface. The contact line, pinned at the wall, generates lateral transport of water and particles, leading to a nonuniform coating thickness. Still, coatings on substrates with walls were shown to have a higher degree of thickness uniformity after drying than those without walls

Leveraging Process Fundamentals to Improve Semiconductor Thickness Control and Uniformity in Inkjet-Printed Schottky Diodes

Liquid-applied coating and printing methods are attractive options for the production of large-area, low-cost flexible electronics. However, controlling the deposited functional layer thickness and uniformity, particularly at submicrometer thicknesses, is challenging. This study focuses on thickness uniformity and control in Schottky diodes made by self-aligned capillarity-assisted lithography for electronics (SCALE). SCALE combines UV imprinting to structure a substrate surface and inkjet printing of functional inks to make flexible electronic devices. In the diode described here, the key functional layer is the poly(3-hexylthiophene-2,5-diyl) (P3HT) semiconductor, which was deposited from a 1,2-dichlorobenzene solution. Thin, uniform P3HT layers with no shorts are required for optimal diode performance. Thickness nonuniformities in the P3HT layer, including the coffee-ring effect and lack of planarization over adjacent electrode channels, occurred during drying. These nonuniformities were most severe when drying was carried out at elevated temperatures (≥50 °C). By drying P3HT layers at 23 °C, the film uniformity and planarization improved significantly, and the device yield was nearly 8× higher. P3HT layers less than 300 nm thick were demonstrated. The improvements in uniformity and planarization are discussed in terms of the competition between solvent evaporation and P3HT diffusion. Self-aligned, printed Schottky diodes demonstrated up to 4.0 × 104 rectification ratio at ±1 V, minimal hysteresis, and ∼0.3 V turn-on voltage

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Magnetic Microrheology of Block Copolymer Solutions

The viscosity of poly­(styrene)-<i>b</i>-poly­(lactide) [PS-<i>b</i>-PLA] solutions in a neutral solvent was characterized by magnetic microrheology. The effect of polymer concentration on the viscosity of the block polymer solutions was compared with that of the PS and PLA homopolymers in the same solvent. The viscosity of PS-<i>b</i>-PLA solution, unlike the homopolymer solutions, showed a steep increase over a narrow concentration range. The steep rise was concomitant with microphase separation into an ordered cylindrical microstructure as determined by small-angle X-ray scattering. Hence microrheology proved effective as a means of characterizing the order–disorder transition concentration. During an in situ drying experiment, changes in local viscosity through the depth of a block copolymer solution were characterized as a function of drying time. Early in the drying process, the viscosity rose steadily and was uniform through the depth, a result consistent with steadily increasing and uniform polymer concentration. However, later in the drying process as the overall polymer concentration approached that required for microphase separation, the viscosity of the polymer solution near the free surface became an order of magnitude higher than that near the bottom of the container. The zone of high viscosity moved downward as drying proceeded, consistent with a microphase separation front

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10^–3–10 s) to the formation of a dry thin film (a “capillary coating”; 10²–10³ s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings

Dynamics of Capillary-Driven Flow in 3D Printed Open Microchannels

Microchannels have applications in microfluidic devices, patterns for micromolding, and even flexible electronic devices. Three-dimensional (3D) printing presents a promising alternative manufacturing route for these microchannels due to the technology’s relative speed and the design freedom it affords its users. However, the roughness of 3D printed surfaces can significantly influence flow dynamics inside of a microchannel. In this work, open microchannels are fabricated using four different 3D printing techniques: fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering, and multi jet modeling. Microchannels printed with each technology are evaluated with respect to their surface roughness, morphology, and how conducive they are to spontaneous capillary filling. Based on this initial assessment, microchannels printed with FDM and SLA are chosen as models to study spontaneous, capillary-driven flow dynamics in 3D printed microchannels. Flow dynamics are investigated over short (∼10^–3 s), intermediate (∼1 s), and long (∼10² s) time scales. Surface roughness causes a start–stop motion down the channel due to contact line pinning, while the cross-sectional shape imparted onto the channels during the printing process is shown to reduce the expected filling velocity. A significant delay in the onset of Lucas-Washburn dynamics (a long-time equilibrium state where meniscus position advances proportionally to the square root of time) is also observed. Flow dynamics are assessed as a function of printing technology, print orientation, channel dimensions, and liquid properties. This study provides the first in-depth investigation of the effect of 3D printing on microchannel flow dynamics as well as a set of rules on how to account for these effects in practice. The extension of these effects to closed microchannels and microchannels fabricated with other 3D printing technologies is also discussed