1,043 research outputs found

    Optimal design of multi-channel microreactor for uniform residence time distribution

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    Multi-channel microreactors can be used for various applications that require chemical or electrochemical reactions in either liquid, gaseous or multi phase. For an optimal control of the chemical reactions, one key parameter for the design of such microreactors is the residence time distribution of the fluid, which should be as uniform as possible in the series of microchannels that make up the core of the reactor. Based on simplifying assumptions, an analytical model is proposed for optimizing the design of the collecting and distributing channels which supply the series of rectangular microchannels of the reactor, in the case of liquid flows. The accuracy of this analytical approach is discussed after comparison with CFD simulations and hybrid analytical-CFD calculations that allow an improved refinement of the meshing in the most complex zones of the flow. The analytical model is then extended to the case of microchannels with other cross-sections (trapezoidal or circular segment) and to gaseous flows, in the continuum and slip flow regimes. In the latter case, the model is based on second-order slip flow boundary conditions, and takes into account the compressibility as well as the rarefaction of the gas flow

    Fabrication and Enhancement of Aluminum-Based Microchannel Devices

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    Microscale molding replication and transient liquid phase (TLP) bonding were used to fabricate Al-based microchannel heat exchangers (MHEs) and micro gas chromatograph (mGC) columns. Metal-based microchannel heat exchangers often experience corrosion as a result of their operating conditions. To address this problem, an internal anodization method was developed in Al microtubes by pulsing the flow of electrolyte through a microtube when the current dropped below a set value. The anodic aluminum oxide (AAO) films were characterized by scanning electron microscopy (SEM), focused ion beam (FIB) cross sections, and X-ray energy dispersive spectroscopy (EDS) to determine their growth rate and morphology. The AAO was sealed by immersing in near-boiling water, and then subjected to a linear sweep voltammetry corrosion test in NaCl, which showed an order of magnitude decrease in the corrosion current between un-anodized and anodized/sealed microtubes. The anodization process was extended to MHEs, and subsequent corrosion testing showed superior resistance to ion diffusion within the AAO film. Al-based mGC columns were fabricated and tested for carrier gas flow rate and single-compound efficiency, then they were subsequently anodized using the same process as the MHEs at 30V or 50V for 3 hours or 8 hours. They were re-tested for single compound efficiency then used to separate an n-C4 through n-C9 hydrocarbon standard. Comparison with a commercial column of the same length showed that the mGC columns had a lower resolution because of lower retention times for all compounds. Additionally, the n-C8 and n-C9 peaks had significant tailing. On the other hand, the mGC columns had higher efficiency than the commercial column for n-C5, n-C6, and n-C7. Characterization via SEM and EDS showed that inconsistent AAO morphology was one likely cause of the lower resolution and tailing. A series of recommendations for manufacturing improvements were provided, including changes to the anodization process and surface treatment of the AAO to lower the distribution of active site energies

    Sacrificial Adhesive Bonding: A Powerful Method For Fabrication Of Glass Microchips

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    Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A new protocol for fabrication of glass microchips is addressed in this research paper. Initially, the method involves the use of an uncured SU-8 intermediate to seal two glass slides irreversibly as in conventional adhesive bonding-based approaches. Subsequently, an additional step removes the adhesive layer from the channels. This step relies on a selective development to remove the SU-8 only inside the microchannel, generating glass-like surface properties as demonstrated by specific tests. Named sacrificial adhesive layer (SAB), the protocol meets the requirements of an ideal microfabrication technique such as throughput, relatively low cost, feasibility for ultra largescale integration (ULSI), and high adhesion strength, supporting pressures on the order of 5 MPa. Furthermore, SAB eliminates the use of high temperature, pressure, or potential, enabling the deposition of thin films for electrical or electrochemical experiments. Finally, the SAB protocol is an improvement on SU-8-based bondings described in the literature. Aspects such as substrate/resist adherence, formation of bubbles, and thermal stress were effectively solved by using simple and inexpensive alternatives.5Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2010/08559-9]CNPq [482696/2011-7

    Sacrificial Adhesive Bonding: A Powerful Method For Fabrication Of Glass Microchips.

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    A new protocol for fabrication of glass microchips is addressed in this research paper. Initially, the method involves the use of an uncured SU-8 intermediate to seal two glass slides irreversibly as in conventional adhesive bonding-based approaches. Subsequently, an additional step removes the adhesive layer from the channels. This step relies on a selective development to remove the SU-8 only inside the microchannel, generating glass-like surface properties as demonstrated by specific tests. Named sacrificial adhesive layer (SAB), the protocol meets the requirements of an ideal microfabrication technique such as throughput, relatively low cost, feasibility for ultra large-scale integration (ULSI), and high adhesion strength, supporting pressures on the order of 5 MPa. Furthermore, SAB eliminates the use of high temperature, pressure, or potential, enabling the deposition of thin films for electrical or electrochemical experiments. Finally, the SAB protocol is an improvement on SU-8-based bondings described in the literature. Aspects such as substrate/resist adherence, formation of bubbles, and thermal stress were effectively solved by using simple and inexpensive alternatives.51327

    Fundamental Studies of Electrochemical Reactions and Microfluidics in Proton Exchange Membrane Electrolyzer Cells

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    In electrochemical energy devices, including fuel cells, electrolyzers and batteries, the electrochemical reactions occur only on triple phase boundaries (TPBs). The boundaries provide the conductors for electros and protons, the catalysts for electrochemical reactions and the effective pathways for transport of reactants and products. The interfaces have a critical impact on the overall performance and cost of the devices in which they are incorporated, and therefore could be a key feature to optimize in order to turn a prototype into a commercially viable product. For electrolysis of water, proton exchange membrane electrolyzer cells (PEMECs) have several advantages compared to other electrolysis processes, including greater energy efficiency, higher product purity, and a more compact design. In addition, the integration of renewable energy sources with water electrolysis is very attractive because it can be accomplished with high efficiency, flexibility, and sustainability. However, there is a lack in fundamental understanding of rapid and microscale electrochemical reactions and microfluidics in PEMECs. This research investigates the multiscale behaviors of electrochemical reactions and microfluidics in a PEMEC by coupling an innovative design of the PEMEC with a high-speed and microscale visualization system (HMVS). The results of the investigation are used to aid in revealing the electrochemical reaction mechanisms and the microfluidics behavior including bubble generation, growth and detachment, which all together play a very important role in the optimization of the design of PEMECs. The effects of operating parameters such as current density, temperature and pressure on the electrochemical reactions and the microfluidics are determined and analyzed by mathematical models of PEMECs, which also match the experimental results. Improved understanding of the electrochemical reactions and microfluidics in PEMECs can not only help to optimize their designs, but can also help advance many other applications in energy, environment and defense research fields

    Fabrication, bonding, assembly, and testing of metal-based microchannel devices

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    Microsystem technologies are believed to be an important part of the contemporary technological foundation and are becoming a commercially significant specialty area in manufacturing. The design and fabrication of microscale engineering structures has the potential of generating revolutionary changes in many products over a wide range of industrial sectors. Metal-based microchannel heat exchangers (MHEs) promise high heat transfer coefficients together with mechanical robustness, and are of interest for a wide range of applications. Fabrication technologies capable of creating high-aspect-ratio microscale structures (HARMSs) in metals such as Cu at low cost and high throughput are of particular interest. Likewise, simple and reliable bonding and assembly techniques are critical for building functional metal-based microfluidic devices. This dissertation focuses on various aspects of fabrication, bonding, assembly, and testing of metal-based microdevices. In chapter 1, existing techniques for fabricating metal-based HARMSs are reviewed briefly and compared with each other. A new technique for fabricating metal-based HARMSs, high temperature compression molding, is introduced. Two related issues, bonding and assembly of metal-based HARMSs and testing of assembled metal-based microdevices are discussed respectively. In chapters 2-6, Cu- and Al- based HARMSs were successfully bonded using Al or Sn thin foil intermediate layers and co-deposited Al-Ge thin film intermediate layers, respectively. Quantitative evaluation of bond strengths was carried out as a function of various bonding parameters. Tensile bond strengths are shown to be ~30MPa for bonded Cu pieces and to exceed 75MPa, reaching as high as 165MPa, for boned Al pieces. Detailed characterizations of the micro-/nano- scale structure of buried bonding interfaces were conducted to rationalize results of mechanical testing. Chapters 7&8 talk about systematic experimentation of fabrication, bonding, and testing of Cu- and Al- based MHEs, and detailed results and discussion on flow and heat transfer performance of these MHEs under two different testing configurations, constant heat flux and constant wall temperature. The results show the increase of surface roughness in the replicated microchannels can cause significant improvements to microchannel heat exchanger performance. Finally, chapter 9 summarizes this dissertation research with main results and achievements. Future work is also discussed in this chapter

    Single Compartment Micro Direct Glucose Fuel Cell

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    Micro fuel cells have received considerable attention over the past decade due to their high efficiency, large energy density, rapid refuelling capability and their inherent non-polluting aspect. An air breathing abiotically catalyzed single compartment micro direct glucose fuel cell (SC-µDGFC) has been developed using microfabrication technologies. The single compartment of the fuel cell was shared by the anode and cathode that had an interdigitating comb electrodes configuration. The SC-µDGFC compartment was formed of polydimethylsiloxane (PDMS) which exhibits high permeability to oxygen and served as the membrane through which oxygen from ambient environment was able to permeate to the cathode. To minimize the losses associated with fuel crossover, two features were incorporated in the fuel cell: (i) silver was used as the catalyst to selectively reduce oxygen in the presence of glucose and (ii) cathodes were made 25-45µm higher than the anode to reduce access of oxygen to the anode with nickel or platinum catalyst. For 1M glucose/2M KOH solution, an initial OCV of 120-160mV was recorded, which gradually decreased with time and stabilized at 60-75mV. For a fuel cell tested without PDMS membrane, maximum OCV of 135mV and power density of 0.38µW/cm2 was obtained
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