Browsing by Subject "Supercapacitor"
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Item Nanoporous and nanostructured materials for energy storage and sensor applications(2013-03) Vu, Anh D.The major objective of this work is to design nanostructured and nanoporous materials targeting the special needs of the energy storage and sensing fields. Nanostructured and nanoporous materials are increasingly finding applications in many fields, including electrical energy storage and explosive sensing. The advancement of energy storage devices is important to the development of three fields that have strong effects on human society: renewable energy, transportation, and portable devices. More sensitive explosive sensors will help to prevent terrorism activities and boost national security. Hierarchically porous LiFePO4 (LFP)/C composites were prepared using a surfactant and colloidal crystals as dual templates. The surfactant serves as the template for mesopores and polymeric colloidal spheres serve as the template for macropores. The confinement of the surfactant-LFP-carbon precursor in the colloidal templates is crucial to suppress the fast crystallization of LFP and helps to maintain the ordered structure. The obtained composites with high surface areas and ordered porous structure showed excellent rate performance when used as cathode materials for LIBs, which will allow them to be used as a power source for EVs and HEVs. The synthesis of LiFePO4 in three dimensionally confined spaces within the colloidal template resulted in the formation of spherical particles. Densely packed LiFePO4 spheres in a carbon matrix were obtained by spin-casting the LFP-carbon precursor on a quartz substrate and then pyrolyzing it. The product showed high capacity and could be charged /discharged with very little capacity fading over many cycles. Three-dimensionally ordered mesoporous carbons were prepared from nano-sized silica sphere colloidal crystal templates. These materials with very high surface areas and ordered porous structure showed high capacitance and excellent rate capability when used as electrodes for supercapacitors. Mesoporous silica thin films of different morphologies, including disordered (wormlike), 2D-hexagonal, 3D-hexagonal, and cubic structure, were prepared. The films were then doped or bridged with fluorescence compounds and used as sensors for nitroaromatic compounds. The sensor performance depended on both the film structure and the mode of fluorophore attachment. The best films showed high quenching rates and were stable during long time storage. The films can potentially be incorporated in portable sensing devices. (351 words)Item Power Management Techniques for Supercapacitor Based IoT Applications(2018-01) Hua, XingyiThe emerging internet of things (IoT) technology will connect many untethered devices, e.g. sensors, RFIDs and wearable devices, to improve health lifestyle, automotive, smart buildings, etc. This thesis proposes one typical application of IoT: RFID for blood temperature monitoring. Once the blood is donated and sealed in a blood bag, it is required to be stored in a certain temperature range (+2~+6°C for red cell component) before distribution. The proposed RFID tag is intended to be attached to the blood bag and continuously monitor the environmental temperature during transportation and storage. When a reader approaches, the temperature data is read out and the tag is fully recharged wirelessly within 2 minutes. Once the blood is distributed, the tag can be reset and reused again. Such a biomedical application has a strong aversion to toxic chemicals, so a batteryless design is required for the RFID tag. A passive RFID tag, however, cannot meet the longevity requirement for the monitoring system (at least 1 week). The solution of this thesis is using a supercapacitor (supercap) instead of a battery as the power supply, which not only lacks toxic heavy metals, but also has quicker charge time (~1000x over batteries), larger operating temperature range (-40~+65°C), and nearly infinite shelf life. Although nearly perfect for this RFID application, a supercap has its own disadvantages: lower energy density (~30x smaller than batteries) and unstable output voltage. To solve the quick charging and long lasting requirements of the RFID system, and to overcome the intrinsic disadvantages of supercaps, an overall power management solution is proposed in this thesis. A reconfigurable switched-capacitor DC-DC converter is proposed to convert the unstable supercap's voltage (3.5V~0.5V) to a stable 1V output voltage efficiently to power the subsequent circuits. With the help of the 6 conversion ratios (3 step-ups, 3 step-downs), voltage protection techniques, and low power designs, the converter can extract 98% of the stored energy from the supercap, and increase initial energy by 96%. Another switched-inductor buck-boost converter is designed to harvest the ambient RF energy to charge the supercap quickly. Because of the variation of the reader distance and incident wave angle, the input power level also has large fluctuation (5uW~5mW). The harvester handles this large power range by a power estimator enhanced MPPT controller with an adaptive integration capacitor array. Also, the contradiction between low power and high tracking speed is improved by adaptive MPPT frequency.Item Supercapacitive Sensors for Force/Strain Measurements in Biomedical Applications(2019-08) Zhang, YeThis dissertation develops a new class of flexible force and strain sensors based on the principle of supercapacitive sensing. The sensing mechanism consists of a change in capacitance in a double-layer supercapacitor in response to an applied force or strain by inducing a change in the contact area between an electrolyte and a pair of electrodes. The new sensors can provide a measurement sensitivity several orders of magnitude higher than traditional capacitive sensors, and have other advantages such as flexibility, soft material construction, ability to operate in liquid environments, tremendous ease of fabrication and unprecedented configurability. As a key component of the new sensors, a paper-based solid-state electrolyte with high deformability is developed. The paper substrate can be easily cut and shaped into complex three-dimensional geometries on which ionic gel can be coated. Paper dissolves in the ionic gel after determining the shape of the electrolyte, leaving behind transparent electrolyte structures with micro-structured fissures responsible for their high deformability. Exploiting this simple paper-based fabrication process, this dissertation develops diverse sensors of different configurations and demonstrates their operation and their advantages. First, force sensors in multiple configurations involving electrolytes that are arch-shaped, corrugated and dome-shaped are fabricated. They have sensitivity that is 1000 times larger than similarly sized capacitive sensors and have negligible parasitic capacitance when used in immersive liquid environments. The use of such force sensors on a urethral catheter which can be used to diagnose the cause of urinary incontinence in a Urology application is demonstrated. A urethral catheter with five distributed force sensors is fabricated that can be used to measure distributed urethral closure pressure in a human subject. Experimental results with the catheter, including cuff tests and ex vivo tests are presented. Next, their high sensitivity allows the use of multiple supercapacitive sensors together in a quad structure to enable a sensor in which normal and shear forces can be simultaneously measured. Such a sensor can have multiple applications in robotics and in wearable monitoring systems which can benefit from measurement of multi-axis forces. The performance of the multi-axis normal-shear force sensor is evaluated using extensive experimental data with a wide range of force combinations. Due to manufacturing imperfections, the sensor does not have uniform axisymmetric sensitivity. Hence, a learning algorithm which utilizes a deep neural network to model the sensor response to multi-axis forces is developed and implemented. The learning algorithm allows the sensor system to provide highly accurate normal and shear force estimates, no matter what the alignment of the forces applied on the sensor. Finally, the use of the supercapacitive sensors for strain measurement is evaluated. The paper-based electrolyte is strengthened with silicate nanoparticles to allow it to withstand over 110% stretch without failure. The strengthened electrolyte is used in a unique strain sensor design. The strain sensor is shown to have ultra-high sensitivity and its performance in a wearable home-monitoring application to measure the size of the leg and thus monitor leg-swelling is demonstrated. The contributions of this dissertation include the development of a new soft deformable electrolyte, the development of a paper-based supercapacitive sensor system, and the development of novel sensor configurations such as a simultaneous normal-shear force sensor, a distributed urethral catheter with multiple pressure sensors and a highly stretchable strain sensor. The developed class of new sensors provides extremely high sensitivity and other advantages in spite of easy fabrication with no requirement for clean room facilities.