Development and Application of Integrated Silicon-in-Plastic Microfabrication in Polymer Microfluidic Systems

Polymer-based microfluidic devices can offer a number of advantages over conventional devices, and have found many applications in chemical and biological analysis. In order to fully develop a lab-on-chip (LOC) device, the functional components, such as sensors and actuators, tend to be assembled to complete a functional device. But the integration of silicon chips into polymer-based microfluidic systems remains a virtually unexplored area. In this work, a novel silicon-in-plastic microfabrication technology is developed, which involves seamlessly integrating individual microfabricated silicon chips into a larger polymer substrate, where the silicon components provide functionality, and the plastic substrate provides system-level fluid handling. This technology employs low-cost polymer substrates and simple polymer processing techniques which are amenable to mass production. The fabrication and testing of two polymer microfluidic systems using the silicon-in-plastic technology are presented in this dissertation. The first integrated microsystem is a water-based chemical monitoring system based on microhotplate gas sensor and polymer microfluidics. The chemical monitoring system is designed to sample a water source, extract solvent present within the aqueous sample into the vapor phase, and direct the solvent vapor past the integrated gas sensor for analysis. Design, fabrication, and characterization of a prototype system are described, and results from illustrative measurements performed using methanol, toluene, and 1,2-dichloroethane in water are presented. The second one is an integrated UV absorbance detection system that uses silicon-in-plastic technology to seamlessly integrate bare photodiode chips into a polymer microfluidic system. Detection platforms fabricated using this approach exhibit excellent detection limits down to 1.5 x 10 8 M for bovine serum albumin (BSA) as a model protein. In addition to providing high sensitivity, sub-nanoliter detection volumes are enabled by the use of direct photodetector integration. The fabrication methodology is detailed, and system performance metrics including minimum detection limit, detection volume, dynamic range, and linearity are reported

A 3D microfluidic device fabrication method using thermopress bonding with multiple layers of polystyrene film

In this article, we present a fabrication method that is capable of making (3D) microfluidic devices with multiple layers of homogeneous polystyrene (PS) film. PS film was chosen as the primary device material because of its advantageous features for microfluidics applications. Thermopress is used as a bonding method because it provides sufficient bonding strength while requiring no heterogeneous bonding materials. By aligning and sequentially stacking multiple layers (3 to 20) of patterned PS film that were achieved by a craft cutter, complicated 3D structured microfluidic devices can be fabricated by multiple steps of thermopress bonding. The smallest feature that can be achieved with this method is approximately 100 μm, which is limited by the resolution of the cutter (25 μm) as well as the thickness of the PS films. Bonding characteristics of PS films are provided in this article, including a PS film bonding strength test, bonding precision assessment, and PS surface wettability manipulation. To demonstrate the capability of this method, the design, fabrication, and testing results of a 3D interacting L-shaped passive mixer are presented

Study on coalescence dynamics of unequal-sized microbubbles captive on solid substrate

The dynamics of bubble coalescence are of importance for a number of industrial processes, in which the size inequality of the parent bubbles plays a significant role in mass transport, topological change and overall motion. In this study, coalescence of unequal-sized microbubbles captive on a solid substrate was observed from cross-section view using synchrotron high-speed imaging technique and a microfluidic gas generation device. The bridging neck growth and surface wave propagation at the early stage of coalescence were investigated by experimental and numerical methods. The results show that theoretical half-power-law of neck growth rate is still valid when viscous effect is neglected. However, the inertial-capillary time scale is associated with the initial radius of the smaller parent microbubble. The surface wave propagation rate on the larger parent microbubble is proportional to the inertial-capillary time scale

Simulation of Heat Generation in a Reconstructed LiCoO2 Cathode during Galvanostatic Discharge

A three dimensional numerical framework with finite volume method was employed to simulate heat generation of a semi lithium ion battery (LIB) cell during isothermal galvanostatic discharge processes. The microstructure of the LIB cathode electrode was experimentally determined using X-ray nano computed tomography technology. Heat generation in the semi LIB cell during galvanostatic discharge processes from different mechanisms, such as electronic resistive heat, ionic resistive heat, contact resistive heat, reaction heat, entropic heat and heat of mixing, was investigated. The spatial distribution of heat generation rates from different mechanisms was also studied. The simulation results demonstrate that the magnitude of heat generation rates spans a wide range in the electrode due to structural inhomogeneity. The simulation results of heat generation from the three dimensional model and the porous-electrode theory model were compared in this study. It is found that the typical Bruggeman coefficient, 1.5, underestimated ionic resistance in the electrolyte and overestimated electronic resistance in the cathode particles. In general, the three dimensional model predicted more heat generation than the porous-electrode theory model at large discharge rates due to the wider distribution of physical and electrochemical properties

Polarization Analysis Based on Realistic Lithium Ion Battery Electrode Microstructure Using Numerical Simulation

poster abstractThe performance of lithium ion battery (LIB) is limited by the inner polarization and it is important to understand the factors that affect the polarization. This study focuses on the polarization analysis based on realistic 3D electrode microstructures. A c++ software was developed to rebuild and mesh the microstructure of cathode and anode electrodes through Nano-CT and Micro-CT scanned images respectively. As a result, the LIB model was composed of electrolyte, cathode and anode active materials and current collectors. By employing 3D finite volume method (FVM), another c++ code was developed to simulate the discharge and charge processes by solving coupled model equations. The simulation revealed the distribution of physical and electrochemical variables such as concentration, voltage, current density, reaction rate, et al. In order to explore the correlation of local effects and electrode structural heterogeneity, the cathode electrode were divided equally into 8 sub-divisions, of which the porosity, tortuosity, specific surface area were calculated. We computed the polarizations in the sub-divisions due to different sub-processes, i.e., the activation of electrochemical reactions and charge transport of species. As shown in Fig. 1, the tortuosity is very irregular because of unevenly distributed cathode particle size and packing pattern with low porosity. There are no exact and direct relations among porosity, tortuosity and specific surface area. Fig. 2 shows that the polarizations are related to the porosity in sub-divisions. The knowledge from the study will help to figure out the mechanism of polarization and power loss in LIB, which could be useful to improve LIB design and manufacturing. Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850. Fig. 1 Porosity and tortuosity in sub-divisions of a cathode electrode Fig. 2 Intercalation reaction polarization and ionic conduction polarization of sub-divisions at 120 sec during a 5 C charging proces

Geometric and Electrochemical Characteristics of NMC Electrodes with Different Calendering Conditions

poster abstractThe energy and power capabilities of Li ion batteries (LIBs) have been considered critical factors to determine the commercial values of the LIB powered applications. Many efforts have been done to improve the energy density and rate capability of LIBs. In addition to intrinsic material properties of anode and cathode active materials, the structure of electrode at micro and nano scales also plays a critical role in determining the energy density and rate capability of a LIB [1-3]. Calendering is a process in battery manufacturing to lower the porosity of the electrode and increase electrical contact. Increased calendering can increase the packing density of active materials in LIB electrodes, thereby increasing the volumetric energy density. The specific energy density is also increased by calendering via decreasing the percentage of inactive materials, such as current collector and separator. However, higher fraction of active materials in LIB electrodes can change electrodes’ structural properties significantly, such as porosity, specific surface area, pore size distribution and tortuosity [4]. To this end, there are few reports on the geometric characteristics and their impact on the electrochemical performance of LIB electrodes with different calendering conditions due to the inhomogeneity, complexity, and three-dimensional (3D) nature of the electrode’s microstructure [5-6]. Recently, porous electrode microstructures have been reconstructed by advanced tomography techniques such as X-ray nano-computed tomography (nano-CT) and focused ion beam scanning electron microscope (FIB-SEM)[7-8]. The reconstructed microstructures have been employed to investigate the geometric characteristics and spatial inhomogeneity of porous electrodes. In this study, we investigated real 3D Li[Ni1/3Mn1/3Co1/3]O2 (NMC) electrode microstructures under different calendering conditions and the effect of calendering on the performance of LIBs[4]. To investigate geometric characteristics of porous microstructures, cathode electrodes were fabricated from a 94:3:3 (weight %) mixture of NMC, PVDF, and super-P carbon black. To change the calendering condition, initial thickness of the electrodes was set 50μm, 80um, 90um, 100um. Then all electrodes were pressed down to 50 μm by using a rolling press machine. A synchrotron X-ray nano-CT at the Advanced Photon Source of Argonne National Lab was employed to obtain morphological data of the electrodes, with voxel size of 58.2 × 58.2 × 58.2 nm3. The morphology data sets were quantitatively analyzed to characterize their geometric properties. The geometric analysis showed that high packing density can result in smaller pore size and more uniform pore size distribution. The specific surface area and tortuosity of different electrodes will be reported. The charge/discharge experiments were also conducted for these electrodes. The geometric properties and cell testing results will be analyzed and reported

Modeling and simulation of heat of mixing in lithium ion batteries

poster abstractHeat generation is a major safety concern in the design and development of lithium ion batteries (LIBs) for large scale applications, such as electric vehicles. The total heat generation in LIBs includes entropic heat, enthalpy, reaction heat, and heat of mixing (1-3). The heat of mixing will be released during relaxation of Li ion concentration gradient. For instance, after the drivers turn off their vehicles, the generation of entropy, enthalpy and reaction heat in LIBs will stop, but the heat of mixing is still being generated. Thomas and Newman derived methods to compute heat of mixing in LIB cells and investigated the heat of mixing on a Li|LiPF6 in ethylene carbonate:dimethyl carbonate|LiAl0.2Mn1.8O4-δF0.2 cell (4). The objective of this study is to investigate the influence of heat of mixing on the LIBs with different materials, porosities, particle sizes, and charge/discharge rate and to understand whether it is necessary to consider heat of mixing during the design and development of LIBs. In this study, a mathematical model was built to simulate heat generation of LIBs using COMSOL Multiphysics. The LIB model was based on Newman’s model. LiCoO2 was applied as the cathode materials, and LiC6 was applied as the anode material. The results of heat of mixing were compared with the other heat sources to investigate the weight of heat of mixing in the total heat generation. Table 1 shows the heat of mixing, irreversible heat, and reversible heat in anode and cathode electrodes at 5 min during a 2 C discharge process. As shown in Table 1, the heat of mixing in cathode is smaller than the heat of mixing in anode, mainly due to the lower Li ion diffusivity and larger particle size of LiC6. The heat of mixing is not as much as the irreversible heat and reversible heat, but it cannot be neglected for this operating condition. The heat of mixing in different LIB cells and under different operating conditions will be reported. The mathematical model: Mathematical model equations: = ( − ) + + Σ Δ + Σ Σ ( − ) = [ 1 2 ∙ ( − ,∞)] =

Geometric characteristics of 3D reconstructed anode electrodes of lithium ion batteries

The realistic 3D microstructure of lithium ion battery electrodes plays a key role in studying the effects of inhomogeneous microstructures on the performance of LIBs. However, the complexity of realistic microstructures implements significant computational cost on numerical simulation of large size samples. In this work, we used tomographic data obtained for a commercial lithium ion battery graphite electrode to evaluate the geometric characteristics of the reconstructed electrode microstructure. Based on the analysis of geometric properties, such as porosity, specific surface area, tortuosity, and pore size distribution, a representative volume element that retains the geometric characteristics of the electrode material was obtained for further numerical studies. In this work, X-ray micro-CT with 0.56 μm resolution was employed to capture the inhomogeneous porous microstructures of lithium ion battery anode electrodes. The Sigmoid transform function was employed to convert the initial raw tomographic images to binary images. Moreover, geometric characteristics of an anode electrode after 2400 1 C charge/discharge cycles were compared with those of a new anode electrode to investigate morphological change of the electrode. In general, the cycled electrode shows larger porosity, smaller tortuosity, and similar specific surface area compared to the new electrode

3D Simulation of diffusion induced stress in realistic LiCoO2 electrode particles of lithium ion battery generated by nano-CT

Diffusion induces stresses in the electrode during charge and discharge processes of lithium ion batteries, which can cause deformation and even fracture, further result in the fade of capacity and duration. The 3D model coupling diffusion and induced stress is applied to the reconstructed LiCoO2 electrode particles determined by X-ray nanocomputed tomography technology, of which the nonuniform electrochemical intercalation reaction takes place on the surface. A code is developed to simulate the fully coupled diffusion and induced stress in the LiCoO2 electrode particles at different discharge rates. The simulations demonstrate the variable distribution such as concentration, reaction rate, hydrostatic stress, Von-Mises stress, and so on. The influence of the geometric characteristics of LiCoO2 electrode particle and material properties on the variables is revealed. The investigation can help to improve lithium ion battery design and manufacture through understanding the relationship between electrode morphology and mechanical endurance

CHEMICAL AND PHASE TRANSFORMATION FROM VANADIUM SULFIDE TO OXIDE VIA A NEW CHEMICAL ROUTE FOR THE SYNTHESIS OF Βʹ-LIXV2O5 AS A HIGH PERFORMANCE CATHODE

The used of rechargeable lithium ion batteries are so widely nowadays on consumer electronics especially portable devices such as cellphones, laptops and etc. The advancement of technology has created batteries with providing high energy density without memory effect and minimum the self-discharge on standby mode. Even with these features, researchers are still trying to improve the batteries with more energy density, low cost, better safety and high durability. The energy density improves with high operation voltage and high capacity. All these features came from one source, material. The resources for current commercial cathode material are decreasing and so new alternative cathode with high performance is needed to replace the commercial cathode in the future. The high temperature vanadium pentoxide phase, βʹ-LixV2O5, was synthesized via a new chemical synthesis involving the evolution of vanadium oxides from the 600°C heat treatment of the pure LiVS2 in air. By employing this method of synthesis, well-crystalized, rod-shaped βʹ-LixV2O5 particles 20 – 30 μm in length and 3 – 6 μm in width were obtained. Moreover, the surface of βʹ-LixV2O5 particles was found to be coated by an amorphous vanadium oxysulfide film (~20 nm in thickness). In contrast to a low temperature vanadium pentoxide phase (LixV2O5), the electrochemical intercalation of lithium into the βʹ-LixV2O5 was fully reversible where 0.0 < x < 2.0, and it delivered a capacity of 310 mAh/g at a current rate of 0.07 C between 1.5 V and 4 V. Good capacity retention of more than 88% was also observed after 50 cycles even at a higher current rate of 2 C