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Research Papers

J. Micro Nano-Manuf. 2014;2(1):011001-011001-6. doi:10.1115/1.4026546.

This paper is one of three papers exploring and confirming a novel high rate nanomanufacturing method using laser to induce and accelerate chemical synthesis and deposition of nanotubes. We have shown elsewhere that the growth rate of SnO2 nanotubes by this method is a few orders faster than that by the state of the art electrochemical deposition method, the growth rate of the nanotubes is favorably affected by increasing the laser power under a constant number of scanning passes, and the process can grow nanotubes coalesced from ultrasmall particle size as small as 2 nm (Liu and Liu, 2013, "Laser Induced Chemical Solution Deposition of Nanomaterials: A Novel Process Demonstrated by Manufacturing SnO2 Nanotubes," Manuf. Lett., 1(1), pp. 42–35). In the second paper, we have shown that this novel method is generic, demonstrated by synthesizing various metal oxide and sulfide nanotubes (Liu and Liu, "Laser-Induced Solution Synthesis and Deposition: A Generic Method to Make Metal Chalcogenide Nanotubes at High Rate With High Consistency," J. Nanoeng. Nanosyst. (accepted)). Since the performance and properties of nanomaterials are highly dependent on its structure, we explore here how the basic processing variables affect the growth rate and crystal size. Our initial finding is that (1) the growth rate can be increased by increasing the pH value of the solution, resulting in little change on the crystal size and (2) the crystal size of the manufactured ferrihydrite nanotube arrays can be controlled by changing laser scanning passes. We found the increase of the pH value from 1.33 to 2.16 almost tripled the growth rate of ferrihydrite nanotubes, while the crystal size remained little changed as revealed by the transmission electron microscopy studies. However, increasing the number of laser scanning passes at a given power could coarsen the ferrihydrite nanocrystals. The crystal structure of the nanotubes could be converted to haematite by dry furnace annealing. These initial findings demonstrated the capability and controllability of the novel process.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(1):011002-011002-11. doi:10.1115/1.4026238.

Microtransfer printing is rapidly emerging as an effective method for heterogeneous materials integration. Laser microtransfer printing (LMTP) is a noncontact variant of the process that uses laser heating to drive the release of the microstructure from the stamp. This makes the process independent of the properties or preparation of the receiving substrate. In this paper, an extensive study is conducted to investigate the capability of the LMTP process. Furthermore, a thermomechanical finite element model (FEM) is developed, using the experimentally observed delamination times and absorbed powers, to estimate the delamination temperatures at the interface, as well as the strain, displacement, and thermal gradient fields.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(1):011003-011003-7. doi:10.1115/1.4026325.

Carbon nanoscrolls (CNS) of various forms are observed when highly ordered pyrolytic graphite (HOPG) is mechanically exfoliated using a wedge. We present two hypothesis of how such scrolls form. The first hypothesis is based on microscopy evidence of pre-existing folds in layer edges of the HOPG. The second hypothesis is based on the literature evidence that graphene sheets when subject to deformation can result in defects on the torn edges. The sample preparation process can induce such defects in the HOPG layers. We show using molecular simulations that the interaction of the moving wedge with certain fold geometries can trigger scroll formation, confirming the first hypothesis. To test the second hypothesis, we show using molecular simulations, that layers with edge defects, upon interacting with the moving wedge, can also form scrolls. In reality, both these factors could simultaneously cause scrolls to form. Opportunities exist in fine-tuning this wedge-based mechanical exfoliation process to synthesize CNS for use in potential applications.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(1):011004-011004-9. doi:10.1115/1.4026272.

A laboratory-scale inkjet printing system was designed for printing polymeric inks with the focus on PEDOT:PSS, a transparent, electrically conductive polymer. PEDOT:PSS inks with 0 and 1 wt. % Surfynol were tested rheologically in elongational and shear flows. A process model is presented and validated for the prediction of flow boundary after the ink exits the nozzle, including drop formation. Process optimization involved establishing a process window related to the voltage waveform, substrate temperature, speed and printed line-overlap, aiming at avoiding satellite drops, “coffee cup” rings, the Rayleigh instability, “stacked printed lines,” and discontinuities in the printed lines or films.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(1):011005-011005-12. doi:10.1115/1.4026518.

Friction effects during a progressive microforming process for production of micropins of various diameters were experimentally investigated and were analytically modeled, using a hybrid friction model. The response surface method and ANOVA analysis were used to generalize the findings for various pin diameters. Besides, it was shown that to get an accurate result in simulation, the friction model must be considered locally instead of a global friction model for the whole process. The effect of friction factor on the final micropart dimensions (the effect on the instantaneous location of the neutral plane) and the forming pressure were investigated. The results showed a reduction in the friction factor as die diameter increased. Following that, the optimum frictional condition to obtain the highest micropart aspect ratio was defined as the maximum friction on the interface between the die upper surface and the punch surface, together with a minimum friction inside the die orifice.

Topics: Friction , Simulation
Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(1):011006-011006-8. doi:10.1115/1.4026606.

When molding macroscale polymer parts with a high density of microfeatures (>1 × 106/cm2), a concern that presents itself is the ability to achieve uniform replication across the entire domain. In the given study, micro-injection molding was used to manufacture microfeatured polymer substrates containing over 10 × 106 microfeatures per cm2. Polystyrene (PS) plates containing microtopography were molded using different processing parameters to study the effect of flow rate and mold temperature on replication quality and uniformity. Flow rate was found to significantly affect replication at mold temperatures above the glass transition temperature (Tg) of PS while having no significant effect on filling at mold temperatures below Tg. Moreover, replication was dependent on distance from the main cavity entrance, with increased flow rate facilitating higher replication differentials and higher replication near the gate. Simulation of the molding process was used to corroborate experimental trials. A deeper understanding of polymer fluid behavior associated with micro-injection molding is vital to reliably manufacture parts containing consistent microtopography (Note: Values are expressed in average ± standard error).

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;2(1):011007-011007-9. doi:10.1115/1.4025978.

The method of roll molding is proposed as an alternative to compression molding for low-cost, high-throughput manufacturing of metal-based microchannel structures. Elemental aluminum- and copper- based microchannel arrays with depths of ∼600 μm and depth:width ratios ≥2:1 were successfully fabricated by roll molding at room temperature. Morphologies of roll molded Al and Cu microchannels were examined in detail. Response of roll molding was characterized by measuring depths of roll molded microchannels as a function of the normal loading force per width. This response of roll molding was further shown to scale with the flow stress of roll molded material. Roll molding offers the potential of fabricating microchannel structures with large footprints in a continuous manner.

Commentary by Dr. Valentin Fuster

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