Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate

The microstructural evolution and the adhesion of an Ag nanopaste screen-printed on a silicon substrate were investigated as a function of sintering temperature. Through the two thermal analysis methods, such as differential scanning calorimeter and thermo-gravimetric analysis, the sintering conditions were defined where the temperature was raised from 150°C to 300°C, all with a fixed sintering time of 30 min. The microstructure and the volume of the printed Ag nanopaste were observed using a field emission scanning electron microscope and a 3-D surface profiler, respectively. The apparent density of the printed Ag nanopaste was calculated depending on the sintering conditions, and the adhesion was evaluated by a scratch test. As the sintering temperature increased from 150°C to 300°C, the apparent density and the adhesion increased by 22.7% and 43%, respectively. It is confirmed that the printed Ag nanopaste sintered at higher temperatures showed higher apparent density in the microstructural evolution and void aggregation, resulting in the lower electrical resistivity and various scratched fractures

MONOLITHICALLY INTEGRATED, PRINTED SOLID-STATE RECHARGEABLE BATTERIES WITH AESTHETIC VERSATILITY

Department of Energy Engineering (Battery Science and Technology)With the advent of flexible/wearable electronics and Internet of Things (IoT) which are expected to drastically change our daily lives, printed electronics has drawn much attention as a low cost, efficient, and scalable platform technology. The printed electronics requires so-called “printed batteries” as a monolithically integrated power source that can be prepared by the same printing processes. The printing technology is a facile and reproducible process in which slurries or inks are deposited to make pre-defined patterns. The slurries/inks should be designed to fulfill requirements (such as rheology and particle dispersion) of the printing process. Development of printed batteries involves the design and fabrication of battery component slurries/inks. Most studies of the printed batteries have been devoted to the development of printed electrodes. However, in order to reach an ultimate goal of so-called “all-printed-batteries”, printed separator membranes and printed electrolytes should be also developed along with the printed electrodes. The objective of the research presented in this dissertation is to develop materials and printing-based strategies to fabricate a new class of monolithically integrated, printed solid-state rechargeable batteries with aesthetic versatility to address the aforementioned formidable challenges, with particular attention to comprehensive understanding of colloidal microstructure and rheological/electrochemical properties of printable battery component slurries/inks. Colloidal microstructure of the battery component slurries/inks is expected to play a viable role in realizing the monolithically integrated printed batteries, as it can significantly affect fluidic characteristics of the slurries/inks and also electrochemical properties. In particular, our interest is devoted to concentrated colloidal gels that exhibit thixotropic fluid behavior (i.e., they readily flow upon being subjected to external stress and quickly return to a quiescent state). Driven by such unique viscoelastic response, the slurries/inks show good dimensional stability and shape diversity on various objects. In addition to the viscoelasticity control of the slurries/inks, the interaction between colloidal conductive particles should be carefully tuned in order to secure facile ion and electron transport pathways. When the attractive interaction is dominant, the colloidal particles tend to be aggregated in disordered and dynamically arrested forms, yielding the highly reticulated three-dimensional networks. In an electrochemical system, these interconnected conductive particle networks act as electron conduction channels while the interstitial voids formed between the particle networks allows ion transport. In this dissertation, as a proof-of-concept, lithium-ion batteries (LIBs), electric double layer capacitors (EDLCs), and Zn-air batteries are chosen to explore the feasibility of this approach. The resultant solid-state printed batteries are fabricated through various printing processes such as stencil printing, inkjet printing, and pen-based writing. Notably, the printed batteries can be seamlessly integrated with objects or electronic devices, thus offering unprecedented opportunities in battery design and form factors that lie far beyond those achievable with conventional battery technologies.ope

The Development and Biocompatibility of Low Temperature Co-Fired Ceramic (LTCC) for Microfluidic and Biosensor Applications

Low temperature co-fired ceramic (LTCC) electronic packaging materials are applied for their electrical and mechanical properties, high reliability, chemical stability and ease of fabrication. Three dimensional features can also be prepared allowing integration of microfluidic channels and cavities inside LTCC modules. Mechanical, optical, electrical, microfluidic functions have been realized in single LTCC modules. For these reasons LTCC is attractive for biomedical microfluidics and Lab-on-a-Chip systems. However, commercial LTCC systems, optimized for microelectrics applications, have unknown cytocompatibility, and are not compatible with common surface functionalization chemistries. The first goal of this work is to develop biocompatible LTCC materials for biomedical applications. In the current work, two different biocompatible LTCC substrate materials are conceived, formulated and evaluated. Both materials are based from well-known and widely utilized biocompatible materials. The biocompatibilities of the developed LTCC materials for in-vitro applications are studied by cytotoxicity assays, including culturing endothelial cells (EC) both in LTCC leachate and directly on the LTCC substrates. The results demonstrate the developed LTCC materials are biocompatible for in-vitro biological applications involving EC. The second goal of this work is to develop functional capabilities in LTCC microfluidic systems suitable for in-vitro and biomedical applications. One proposed application is the evaluation of oxygen tension and oxidative stress in perfusion cell culture and bioreactors. A Clark-type oxygen sensor is successfully integrated with LTCC technique in this work. In the current work, a solid state proton conductive electrolyte is used to integrate an oxygen sensor into the LTCC. The measurement of oxygen concentration in Clark-type oxygen sensor is based on the electrochemical reaction between working electrode and counter electrode. Cyclic voltammetry and chronoamperometry are measured to determine the electrochemical properties of oxygen reduction in the LTCC based oxygen sensor. The reduction current showed a linear relationship with oxygen concentration. In addition, LTCC sensor exhibits rapid response and sensitivity in the physiological range 1─9 mg/L. The fabricated devices have the capabilities to regulate oxygen supply and determination of local dissolved oxygen concentration in the proposed applications including perfusion cell culture and biological assays

Laser-based three-dimensional manufacturing technologies for rechargeable batteries.

Laser three-dimensional (3D) manufacturing technologies have gained substantial attention to fabricate 3D structured electrochemical rechargeable batteries. Laser 3D manufacturing techniques offer excellent 3D microstructure controllability, good design flexibility, process simplicity, and high energy and cost efficiencies, which are beneficial for rechargeable battery cell manufacturing. In this review, notable progress in development of the rechargeable battery cells via laser 3D manufacturing techniques is introduced and discussed. The basic concepts and remarkable achievements of four representative laser 3D manufacturing techniques such as selective laser sintering (or melting) techniques, direct laser writing for graphene-based electrodes, laser-induced forward transfer technique and laser ablation subtractive manufacturing are highlighted. Finally, major challenges and prospects of the laser 3D manufacturing technologies for battery cell manufacturing will be provided

3D printed neuromorphic sensing systems

Thanks to the high energy efficiency, neuromorphic devices are spotlighted recently by mimicking the calculation principle of the human brain through the parallel computation and the memory function. Various bio-inspired \u27in-memory computing\u27 (IMC) devices were developed during the past decades, such as synaptic transistors for artificial synapses. By integrating with specific sensors, neuromorphic sensing systems are achievable with the bio-inspired signal perception function. A signal perception process is possible by a combination of stimuli sensing, signal conversion/transmission, and signal processing. However, most neuromorphic sensing systems were demonstrated without signal conversion/transmission functions. Therefore, those cannot fully mimic the function provides by the sensory neuron in the biological system. This thesis aims to design a neuromorphic sensing system with a complete function as biological sensory neurons. To reach such a target, 3D printed sensors, electrical oscillators, and synaptic transistors were developed as functions of artificial receptors, artificial neurons, and artificial synapses, respectively. Moreover, since the 3D printing technology has demonstrated a facile process due to fast prototyping, the proposed 3D neuromorphic sensing system was designed as a 3D integrated structure and fabricated by 3D printing technologies. A novel multi-axis robot 3D printing system was also utilized to increase the fabrication efficiency with the capability of printing on vertical and tilted surfaces seamlessly. Furthermore, the developed 3D neuromorphic system was easily adapted to the application of tactile sensing. A portable neuromorphic system was integrated with a tactile sensing system for the intelligent tactile sensing application of the humanoid robot. Finally, the bio-inspired reflex arc for the unconscious response was also demonstrated by training the neuromorphic tactile sensing system

Promotional Effect of Molten Carbonates on Proton Conductivity and Oxygen Reduction Reaction – An Experimental and Computational Study

Recent research of Solid oxide fuel cells (SOFCs) is aimed to lower the operating temperature to an intermediate temperature (IT) range of 500 to 700oC, while maintaining a proper performance. This Ph.D. research project investigates the promotional effects of alkaline carbonate eutectics on the proton conductivity of proton conducting electrolytes and cathodic ORR reactivity in SOFCs by both experimental and computational methods. The ionic conductivity of the MC-BZY composite above 500oC increases with the higher loading of MC. The sample exhibited nearly a factor of two higher conductivity in H2-containing atmosphere than in air. First-principles DFT modeling further investigated proton transfer at the interface of BaZrO3 and molten carbonate. With the presence of carbonate ion, the energy barrier for proton migration becomes as low as 0.332 eV. The modeling indicates the reduction of energy barrier is resulted from the change of ratedetermining step from proton transfer between oxygen atoms to proton rotation around oxygen atom. Infiltration of MC into porous cathode can reduce the polarization of resistance (Rp), i.e., enhance the oxygen reduction reaction (ORR) activity. The EIS analysis shows that MC has a beneficial effect on reducing Rp for different cathodes including Au,La0.8Sr0.2MnO3-δ(LSM), La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) and La2NiO4+δ (LNO). Specifically, the study on MC loading effect was carried out on LSCF cathode. It shows that a higher loading makes a greater reduction on Rp and the degree of reduction is the same from 500 to 600oC. As the loading increases to 1.4 wt%, the degree of Rpreduction tends to reach a limit. First-principles DFT modeling was further used to investigate the incorporation of oxygen into MC. The formation of CO52- in molten carbonate was considered as a chemisorption of gas oxygen on the surface of MC infiltrated cathodes. After the formation of CO52-, it reacts with another CO32- to form two CO42-, which is a rate-limiting step on potential energy surface. After dissociation, oxygen atoms migrate in molten carbonate, which is energetically favor by intermolecular pathways. An O-O-O linkage is formed between carbonate ions, which facilitates the oxygen migration between carbonate ions

Layered perovskites as electrocatalysts for energy conversion and storage systems

Department of Energy EngineeringWith growing concerns over energy and environmental issues, sustainable and environmentally-friendly energy conversion and storage devices have received significant attention from both the academic and industrial communities. Solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), which are collectively referred to as solid oxide cells (SOCs), are attractive energy conversion and storage systems with high energy conversion efficiency and environmental benefits. SOFCs, which directly convert chemical energy (such as H2, CH4, C3H8, etc) into electricity, represent an efficient alternative combustion system for the production of electricity. Similarly, SOECs, which convert water to hydrogen, are a clean and efficient hydrogen production system. Ideally, SOFCs and SOECs should meet several criteria, such as high performance, long term stability, and relatively inexpensive cost. However, the operation of SOFCs and SOECs still accompanies several problems, especially in relation to the electrode materials. Conventional electrode materials suffer from insufficient performance, performance degradation, redox instability, coarsening, electrode delamination, and the formation of a secondary phase. In this regard, the development of electrode materials with both high conductivity and high and stable electrocatalytic activity is a vital step for the commercialization of SOFCs and SOECs. This dissertation focuses on layered perovskite based electrode materials for SOFCs and SOECs with an aim to overcome the problems noted above. These materials show outstanding performance and stability with fast electrochemical reaction kinetics. I first discuss basic principles and present a theoretical overview of solid oxide fuel cells and solid oxide electrolysis cells in chapter 1 and then describe the experimental techniques for the fabrication and characterization of electrode materials for SOFCs and SOECs in chapter 2. Finally, my research papers on the properties of electrode materials for SOFCs and SOECs are presented as outlined below, 1. Thermodynamic and electrical properties of Ba0.5Sr0.5Co0.8Fe0.2O3-?? and La0.6Sr0.4Co0.2Fe0.8O3-?? for intermediate-temperature solid oxide fuel cells. 2. Optimization of Sr content in layered SmBa1-xSrxCo2O5+?? perovskite cathodes for intermediate-temperature solid oxide fuel cells. 3. High redox and performance stability of layered SmBa0.5Sr0.5Co1.5Cu0.5O5+?? perovskite cathodes for intermediate-temperature solid oxide fuel cells. 4. Electrochemical properties of B-site Ni-doped layered perovskite cathodes for IT-SOFCs. 5. Correlation between fast oxygen kinetics and enhanced performance in Fe doped layered perovskite cathode for solid oxide fuel cells. 6. Achieving high efficiency and eliminating degradation in solid oxide electrochemical cells by using high oxygen capacity perovskite. 7. Novel hydrogen production system: Dual Solid Oxide Electrolyzer.ope