Sang Joon John Lee

Layer position accuracy in powder‐based rapid prototyping

Emerging technologies commonly known as “rapid prototyping” fabricate solid objects directly from computer models by building parts in thin layers. Three‐dimensional printing is one such process that creates engineering prototypes and tooling by joining powder particles selectively on a layer‐by‐layer basis. The powder‐based approach offers tremendous flexibility in geometry and materials, but it makes layer position accuracy a fundamental concern for dimensional control in the vertical direction. Ideally, each powder layer is generated at a vertical position that remains fixed, at a prescribed distance with respect to a machine reference. However, compressive loads imparted to a stack of layers (by the weight of subsequent layers, for example) may cause the layers to displace downward. Develops a model for layer displacement using experimental data for compressibility and applied load. Compares predictions made from the model to measured displacements, and the predictions successfully captured the relative magnitudes of actual errors at various positions within layered powder beds. Position changes were most severe in the middle regions of the powder beds, with diminishing magnitude towards the top and bottom. Uses aluminium oxide powder in two different sizes (approximately of 10‐micron and 30‐micron diameter) and two different shapes (platelet and spherical) in the studies. The average measured displacement in a 76.2mm deep bed ranged from 23 microns for a 30‐micron platelet‐shaped powder to over 260 microns for a 9‐micron platelet‐shaped sample.

Geometric Scale Effect of Flow Channels on Performance of Fuel Cells

Department of Mechanical and Aerospace Engineering, San Jose State University,San Jose, California 95192-0087, USAThis paper studies the effect of flow channel scaling on fuel cell performance. In particular, the impact of dimensional scale on theorder of 100 micrometers and below has been investigated. A model based on three-dimensional computational flow dynamics hasbeen developed which predicts that very small channels result in significantly higher peak power densities compared to their largercounterparts. For experimental verification, microchannel flow structures fabricated with varying sizes in SU-8 photoepoxy havebeen tested with polymer electrolyte membrane electrode assemblies. The experimental results confirm the predicted outcome atrelatively large scales. At especially small scales ~,100 mm!, the model ~which does not consider two-phase flow! disagrees withthe measured data. Liquid water flooding at the small channel scale is hypothesized as a primary cause for this discrepancy.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1799471# All rights reserved.Manuscript submitted October 14, 2003; revised manuscript received April 6, 2004. Available electronically October 8, 2004.

Lateral Ionic Conduction in Planar Array Fuel Cells

A performance degradation phenomenon is observed in planar array fuel cells. This effect occurs when multiple cells sharing the same electrolyte membrane are connected in series to build voltage. The open circuit voltage (OCV) and low current behavior of such a series connected planar stack is lower than should be expected. The flow of ionic cross currents between cells in the array, dubbed membrane cross-conduction, is proposed as the likely cause for this loss phenomenon. This hypothesis is confirmed by experimental observations. An equivalent circuit model for a planar double cell is developed which takes into account membrane cross conduction. This model is shown to predict the observed current-voltage behavior of an experimental planar double cell while a simple series model does not. The validated model is used to investigate the impact of various fuel cell parameters on the membrane cross-conduction effect. Design rules are extracted to minimize membrane cross-conduction losses for a linear fuel cell array. It is concluded that the membrane cross-conduction phenomenon primarily affects the OCV and low current density behavior of planar fuel cell arrays. Losses due to membrane cross conduction are minimal for conservative cell spacing, but can be significant for densely packed fuel cell arrays.

Characterization of laterally deformable elastomer membranes for microfluidics

This work presents experimental characterization and numerical modeling of laterally deflecting polydimethylsiloxane (PDMS) membranes under pneumatic actuation. The device for this study is a membrane valve seat that partially closes a perpendicular fluid microchannel, fabricated using single-layer soft lithography. Membranes with thickness between 8 and 14 µm have been experimentally tested up to 207 kPa, and maximum lateral displacement beyond 20 µm has been demonstrated. Investigation of geometric parameters by factorial design shows that the height of the membrane is more dominant than the width and thickness, and this is attributed to the zero-displacement boundary condition at the foot of the membrane where it is bonded to a flat substrate. A numerical model that incorporates hyperelastic material testing data shows close agreement with the deflection behavior of experimental samples, accurately predicting that a membrane of 10 µm thick, 100 µm wide and 45 µm tall deflects approximately 13 µm at 207 kPa. Simulation further shows that sidewall effects from bulk compression of the elastomer material in the actuation cavity have a significant effect, reducing maximum displacement by as much as 15% over predictions based on deformation that is limited strictly to the membrane only. Experimental yield, SEM imaging and stress simulations emphasize that the membrane foot region requires the greatest attention in terms of process development.

Investigation of Transport Phenomena in Micro Flow Channels for Miniature Fuel Cells

This paper presents a study on the transport phenomena related to gas flow through fuel cell micro-channels, specifically the impact of dimensional scale on the order of 100 microns and below. Especially critical is the ability to experimentally verify model predictions, and this is made efficiently possible by the use of structural photopolymer (SU-8) to directly fabricate functional fuel cell micro-channels. The design and analysis components of this investigation apply 3-D multi-physics modeling to predict cell performance under micro-channel conditions. Interestingly, the model predicts that very small channels (specifically 100 microns and below) result in a significantly higher peak power density than larger counterparts. SU-8 micro-channels with different feature sizes have been integrated into fuel cell prototypes and tested for comparison against model predictions. The results not only demonstrate that the SU-8 channels with metal current collector show quite appreciable performance, but also provide experimental verification of the merits of channel miniaturization. As predicted, the performance in terms of peak power density increases as the feature size of the channel decreases, even though the pressure drop is higher in the more narrow channels. So it has been observed both theoretically and experimentally that cell performance shows an improving trend with micro-channels, and design optimization for miniature fuel cell provides a powerful method for increasing power density.Copyright

Polymethylhydrosiloxane (PMHS) as a functional material for microfluidic chips

Polymethylhydrosiloxane (PMHS) has been investigated as a candidate material for microfluidic chips. The ability to modify the surface of PMHS by hydrosilation is particularly advantageous for separation processes. The chemical modification of PMHS is verified by diffuse reflectance infrared Fourier transform (DRIFT) analysis, and the modified PMHS is shown to be stable when exposed to extreme pH conditions between 2 and 9. Spectrophotometer measurements show that PMHS exhibits over 40% transmittance for ultraviolet (UV) wavelength as low as 220 nm, indicating viability for sensor applications based on UV absorption. The UV transmittance is furthermore observed to be insensitive to thickness for specimens tested between 1.6 mm and 6.4 mm thick. Full curing of PMHS liquid resin occurs between 48 and 72 h at 110 °C with no secondary additives. Casting of microscale features is achieved by using soft lithography methods similar to established techniques for fabrication based on polydimethylsiloxane (PDMS). Microchannels approximately 100 µm wide and 50 µm deep are also demonstrated by carbon dioxide laser ablation, with uniform channels produced using an energy dose of 0.2 mJ mm−1 with respect to line length. Other basic functional requirements for microfluidic chips are discussed, including the ability to bond PMHS substrates by plasma treatment.

Experimental Studies on the Effects of Geometric Parameters in a Planar Pneumatic Microvalve

The lateral deflection of a microscale elastomer membrane patterned in polydimethylsiloxane (PDMS) has been experimentally characterized in terms of how geometric parameters affect the compliance of the membrane, and consequently the magnitude of deflection for a given pressure. The concept of a laterally-deformable membrane had been previously demonstrated as viable for applications including sealing, mixing, and sorting. The present work specifically applies fractional factorial experiments to examine the effects of thickness, width, and height of the membrane. It was observed that deflection magnitudes in excess of 10 microns could routinely be achieved with actuation pressure less than 200 kPa. In some cases deflection magnitude as high as 20 microns was also demonstrated. Functional membrane fabrication with dimensions as aggressive as 8 microns thick, 200 microns wide, and 45 microns tall has been successfully demonstrated. Thinner membranes and wider span exhibited similar magnitude of effects in terms of improving compliance. Height of the membrane exhibited a notably larger magnitude of effect, attributed to boundary conditions where the foot of the membrane is bonded to the base substrate. Seal reliability at the foot of the membrane was a significant limitation to sample yield, and represents the primary need for future development.

A “Bottom-Up” Approach to Engineering Education in Nanotechnology

This paper presents a pilot project for a “bottom-up” approach to reform of undergraduate engineering education in nanotechnology, supported by a planning grant from the National Science Foundation (Engineering Education and Centers, Award #0431970). A core principle is to have individuals from different disciplines be the ones not only to build concise modules, but also to share them with content developers in other disciplines under a common organized framework. This bottom-up approach is an efficient way of introducing new content in existing curricula, and is especially helpful in university environments that may have no comprehensive “experts” in nanotechnology per se. Having individuals work together to develop bottom-up pieces from their own specialized fields provides a mechanism not only for curriculum enhancement but also for faculty professional development. In this work, pilot modules are developed as new content infused into existing courses in mechanical engineering, electrical engineering, and materials engineering at San Jose State University. Topics span different aspects of nanoscale materials, phenomena, devices, and manufacturing, and the content is structured in a framework such that components may be packaged as modular entities. We present pilot work accomplished in this work-in-progress, with emphasis on how lessons learned can be applied to expandability and sustainability of this bottom-up approach.

A monolithic micro four-bar mechanism with flexure hinges

A monolithic micro four-bar mechanism was fabricated in silicon to examine motion amplification as well as the effect of non-ideal geometric profiles in its flexure hinges. Through-wafer deep reactive ion etching (DRIE) was used to produce high-aspect-ratio flexure joints that allow compliant motion within the plane of a silicon wafer. The flexures were approximately 20 microns wide and 530 microns deep, micromachined through the entire wafer thickness. A taper angle of approximately 0.5 degree narrowing toward the bottom of the wafer was measured in the flexure cross section. A finite element model was developed to predict the output rotation of one link in response to the displacement applied at the drive link. For a 1-micron linear input, the model predicted a 0.39-degree angular displacement for the output link. This showed close agreement with experimental data that measured 0.41 degree. An enhanced finite element model that accounted for the tapered cross-section, however, predicted a slightly smaller input/output relation of 0.37 degree per micron.Copyright

Fabrication Process Development and Characterization of Polymethylhydrosiloxane (PMHS) for Surface-Modified Microfluidic Chips

Microfluidic chips made of polymer materials such as polydimethylsiloxane (PDMS), polyimide, and cyclic olefin co-polymer have cost and manufacturing advantages over materials such as fused silica and borosilicate glass. While these materials have been extensively investigated, polymethylhydrosiloxane (PMHS) is an alternative that has a unique combination of properties in terms of UV transparency and potential for chemical surface modification. The present study investigates process development and characterization of PMHS as a new candidate material for microfluidic chip applications, in particular separation processes that would benefit from the ability to custom-engineer its surface conditions. This paper compares different approaches for fabricating microchannel features as well as options for enhancing the surface area of the channel walls. The fabrication methods include replication by casting over patterned molds, soft lithography casting, and material removal by laser ablation. Casting into solid form is achieved in 48-hours at 110 °C. Laser ablation is studied with energy dose varying from 2 mJ to 160 mJ per millimeter scanned, with channels approximately 100 microns wide occurring at 0.2 mJ/mm. Mechanical characterization is applied to quantify the hardness of cast PMHS, because fine-resolution features are limited by mold removal. PMHS samples have been measured to have a Shore A hardness of 46.2, similar to PDMS that is well-established in polymer microfluidic devices. Surface enhancement techniques including laser and plasma treatment are investigated for the prospective benefit of separation processes that require high surface-to-volume ratio. Spectrophotometry shows that PMHS exhibits transmittance even below 250 nm, which is favorable for sample analysis by UV absorption methods.Copyright