Browsing by Subject "Carbon nanotube"
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Item Carbon Nanotube Addition to Cement-Sand Based Piezoelectric Composites(2016) Kadlec, Alec; Wang, Shifa; Zhao, PingCarbon Nanotubes (CNTs) were added to a cement-sand based piezoelectric composite with consideration of Structural Health Monitoring (SHM) to improve conductivity and poling efficiency, increasing piezoelectric effects. The addition of CNTs to the composite structure formed continuous electric networks between the Lead Zirconate Titanate (PZT) particles, allowing more effective poling. Samples of 50 volume percent PZT were fabricated with a mixture of PZT powder, white Portland cement, graded silica sand, CNTs and a superplasticizer, and cured at room temperature. The properties of the composite, including piezoelectric coefficient and sensing effects were characterized for a range of CNT inclusion from 0 to 0.9 vol %. Results showed that CNT inclusion allowed for effective room temperature poling, improving piezoelectric properties of the composite. The modified composite was optimal at 0.6 vol % CNTs.Item Carbon nanotube thin films for active noise cancellation, solar energy harvesting, and energy storage in building windows(2014-07) Hu, ShanThis research explores the application of carbon nanotube (CNT) films for active noise cancellation, solar energy harvesting and energy storage in building windows. The CNT-based components developed herein can be integrated into a solar-powered active noise control system for a building window. First, the use of a transparent acoustic transducer as both an invisible speaker for auxiliary audio playback and for active noise cancellation is accomplished in this work. Several challenges related to active noise cancellation in the window are addressed. These include secondary path estimation and directional cancellation of noise so as to preserve auxiliary audio and internal sounds while preventing transmission of external noise into the building. Solar energy can be harvested at a low rate of power over long durations while acoustic sound cancellation requires short durations of high power. A supercapacitor based energy storage system is therefore considered for the window. Using CNTs as electrode materials, two generations of flexible, thin, and fully solid-state supercapacitors are developed that can be integrated into the window frame. Both generations consist of carbon nanotube films coated on supporting substrates as electrodes and a solid-state polymer gel layer for the electrolyte. The first generation is a single-cell parallel-plate supercapacitor with a working voltage of 3 Volts. Its energy density is competitive with commercially available supercapacitors (which use liquid electrolyte). For many applications that will require higher working voltage, the second-generation multi-cell supercapacitor is developed. A six-cell device with a working voltage as high as 12 Volts is demonstrated here. Unlike the first generation's 3D structure, the second generation has a novel planar (2D) architecture, which makes it easy to integrate multiple cells into a thin and flexible supercapacitor. The multi-cell planar supercapacitor has energy density exceeding that of other planar supercapacitors in literature by more than one order of magnitude. All-solution fabrication processes were developed for both generations to achieve economical and scalable production. In addition to carbon nanotubes, nickel/nickel oxide core-shell nanowires were also studied as electrode materials for supercapacitors, for which high specific capacitance but low working voltage were obtained. Semi-transparent solar cells with carbon nanotube counter electrodes are developed to power the active noise cancellation system. They can be directly mounted on the glass panes and become part of the home window. The 2.67% efficiency achieved is higher than the 1.8% efficiency required for harvesting adequate energy to cancel noise of 70dB Day-Night-Level, which impacts on a north-facing window. In summary, this project develops several fundamental technologies that together can contribute to a solar-powered active noise cancellation system for a building window. At the same time, since the component technologies being developed are fundamental, it is also likely that they will have wider applications in other domains beyond building windows.Item Electromechanical Switches Fabricated by Electrophoretic Deposition of Single Wall Carbon Nanotube Films(2015-08) Lim, Jun YoungPower dissipation is a critical problem of CMOS devices especially for mobile applications. Many efforts have been made to solve the problem, but there are still major issues associated with scaling the device size. Micro electromechanical (MEMS) and nano electromechanical (NEMS) devices are one candidate to solve the problems because of their excellent standby leakage. However, the switches have a tradeoff between low operating power and high device speed. Suspended beams with low mass density and good mechanical properties provide a way to optimize the device. Carbon nanotubes (CNTs) have the low mass density and excellent mechanical properties to enable high performance MEMS/NEMS devices. However, the high temperature required for the direct synthesis for CNTs makes it difficult for them to be compatible with a substrate containing transistors. Therefore, continuous film deposition techniques are investigated with low temperature (< 300 C). Electrophoretic deposition (EPD) is a simple and versatile processing method to deposit carbon nanotubes on the substrate at room temperature. The movement of the charged CNTs in suspension occurs by an applied electric field. The deposited CNT film thickness can be controlled through the applied voltage and process time. We demonstrate the use of an EPD process to deposit various thicknesses of CNT films. Film thicknesses are studied as a function of, deposition time, electric field strength, and suspension concentration. The deposition mechanism of the EPD process for carbon nanotube layers was explained with experimental data. We determined the film mass density and electrical/optical properties of SWCNT films. Rutherford backscattering spectroscopy was used to determine the film mass density. Films created in this manner had a mass density that varies with thickness from 0.12 to 0.54 g/cm3 and a resistivity of 2.1410-3 Ω∙cm. For the mechanical property measurements, we describe a technique to fabricate free-standing thin films using modified Langmuir-Blodgett method. Then we extracted the Young’s modulus of the film from the load-displacement data from nanoindentation using the appropriate modeling. The Young’s modulus had a range of 4.72 to 5.67 GPa, independent of deposited thickness. We fabricated two-terminal fixed beam switches with SWCNT thin films using the EPD process. Device pull-in voltages under 1V were achieved by decreasing the air-gap. The pull-in voltages were compared with the calculated results using the device geometry and extracted Young’s modulus from nanoindentation. Generally good agreement was observed. Also, we found a range of 2.4 to 3.5 MHz resonant frequency. However, we encountered several problems with the device including a gradual turn-on, hysteresis between pull-in and pull-out voltage, changes in the pull-in voltages with repeated on-off cycling, and early failure due to moisture absorption during testing in the air. Mechanisms for these observations are postulated. Further work is needed to improve device performance and reliability.Item Intelligent Pavement for Traffic Flow Detection – Phase I(Intelligent Transportation Systems Institute, Center for Transportation Studies, University of Minnesota, 2012-09) Yu, XunThis project explored a new approach in detecting vehicles on a roadway by making a roadway section itself a traffic flow detector. Sections of a given roadway are paved with carbon-nanotube (CNT)/cement composites; the piezoresitive property of carbon nanotubes enables the composite to detect the traffic flow. Meanwhile, CNTs can also work as the reinforcement elements to improve the strength and toughness of the concrete pavement. In contrast to current traffic flow detection technologies that require separate devices to be installed either in the pavement or over the road, the proposed sensing approach enables the pavement itself to detect traffic flow parameters. Therefore, the proposed sensor is expected to have a long service life with little maintenance and wide-area detection capability.Item Intelligent Pavement for Traffic Flow Detection – Phase II(Intelligent Transportation Systems Institute, Center for Transportation Studies, University of Minnesota, 2012-09) Yu, XunThis project is the extension of a Northland Advanced Transportation System Research Laboratory (NATSRL) FY09 project, titled as “Intelligent Pavement for Traffic Flow Detection”, which aims to explore a new approach in detecting vehicles on a roadway by making a roadway section as a traffic flow detector. Sections of a given roadway are paved with carbon-nanotube (CNT) enhanced pavement; the piezoresitive property of carbon nanotubes enables the pavement to detect the traffic flow. Meanwhile, CNTs can also work as reinforcement elements to improve the strength and toughness of the concrete pavement. The proposed sensor is expected to have a long service life with little maintenance and wide-area detection capability. In the FY09 project, lab tests demonstrated that CNT based cement composite can detect the mechanical stress levels for both static and dynamic loads. In the FY10 project, the research was extended to cement mortar, which has much higher mechanical strength and more useful in real applications. The effects of water level and CNT doping levels on the piezoresistivity of the composites were also studied. Preliminary road tests were performed for the evaluation of this new traffic sensor.