Browsing by Subject "Nanowires"

The majority of consumer electronic devices, electric vehicles, and aerospace electronics are powered by lithium ion batteries because of their high energy and power densities. Commercially available lithium ion batteries consist of electrodes, separators and current collectors fabricated in multilayer rolls that are packaged in cylindrical or rectangular cases. The size and shape of the package as well as the composition of the electrode has a significant impact on the battery life and design of the products they power. For example, the battery life and shape of portable electronics such as cell phones or laptops, is governed by the volume that is dedicated to the battery. In the case of electric vehicles, decreasing the size and weight of the battery while increasing capacity is an engineering challenge that affects vehicle range and cost. Therefore, the of my dissertation consists of the development of a novel 3D printable lithium ion battery nanocomposites and the integration of conductive metal nanomaterials into conventional lithium ion anodes. Here, we report the development of PLA-anode, cathode, and separator materials that enable 3D printing of complete lithium ion batteries with a low-cost FFF printer for the first time. The most common 3D printing polymer polylactic acid (PLA) is an insulator. However, our work demonstrates that 3D printed PLA can be infused with a mixture of ethyl methyl carbonate, propylene carbonate, and LiClO4 provides an ionic conductivity of 2.3 x 10−4 S cm−1 which is comparable to that of polymer and hybrid electrolytes (10−3 to 10−4 S cm−1). It was found that up to 12-30 volume % of solids, depending on the filler morphology, could be mixed into PLA without causing it to clog during 3D printing. It was also found that not only is electrical conductivity crucial to the performance of a 3D printed lithium ion battery, but efficient electrical contact to the active materials is as well. To that effect, we investigated the effect of aspect ratio of silver-copper core-shell nanowires on the performance enhancement of a commercially fabricated graphite lithium ion anodes. Currently, carbon is the most common conductive filler used in commercial lithium ion battery anodes. We hypothesize that a more conductive, high aspect ratio would improve the performance of a lithium ion battery. We examined the effect of exchanging carbon with CuAg nanowires as the conductive filler in graphite lithium ion batteries. We tested 4 different aspect ratios and found that not only does aspect ratio matter, diameter and length have profound effect on capacity and energy of the anode at the same volume percent as carbon conductive filler.

In the field of electronic materials, metal nanowires have been extensively studied for both their syntheses and their properties in electronic composites and devices. This dissertation addresses challenges in the field of electronic materials development with the use of copper nanowires synthesized in gram-scale syntheses, as well as provides analysis of devices and composites that could only be feasibly manufactured thanks to the large-scale syntheses.

In the field of printed electronics, there has been research into the development of fully printed memories. One of the challenges has been developing a memory that has switching characteristics that are on par with existing commercial memories, such as Flash memory. This can be achieved with a composite of Cu-SiO2 nanowires dispersed in ethylcellulose, which acts as a resistive switch when between printed Cu and Au electrodes. A 16-cell crossbar array of these memristors was printed with an aerosol jet. The memristors exhibited moderate operating voltages (~3 V), no degradation over 104 switching cycles, write speeds of 3 µs, and extrapolated retention times of 10 years. The low operating voltage enabled the programming of a fully printed 4-bit memristor array with an Arduino. The excellent performance of these fully printed memristors could help enable the creation of fully printed RFID tags and sensors with integrated data storage. Thanks to the large-scale synthesis of copper nanowires, this can allow for the expanded production of high-quality, fully printed memories.

Materials that retain a high conductivity under strain are essential for wearable electronics. I describe a new conductive, stretchable composite consisting of a Cu-Ag core-shell nanowire felt infiltrated with a silicone elastomer. This composite exhibits a retention of conductivity under strain that is superior to any composite with a conductivity greater than 1000 S cm-1. This work also shows how the mechanical properties, conductivity, and deformation mechanisms of the composite changes as a function of the stiffness of the silicone matrix. The retention of conductivity under strain was found to decrease as the Young’s modulus of the matrix increased. This was attributed to void formation as a result of debonding between the nanowire felt and the elastomer. The nanowire composite was also patterned to create serpentine circuits with a stretchability of 300%. Composites of this scale and density could only be feasibly manufactured thanks to large-scale syntheses of copper nanowires and the silver coating of copper nanowires. With the advances made in the quality of stretchable conductive composites, alternate methods were employed as to manufacture new composites and structures, such as the cofiltration of nanowires and waterborne rubber to accelerate production, or the manufacturing of Cu-Ag nanowire aerogels with density tunable via the aspect ratio of the nanowires.

Research and development bismides have proven bismides to be a promising field for material science with important applications in optoelectronics. However, the development of a complete description of the electrical and material properties of bismide ternaries is not comprehensive or straightforward. One of the main benefits of this ternary system is the opportunity for bandgap tuning, which opens doors to new applications. Tuning the bandgap is achieved by means of varying the composition; this allows access to a wider energy spectrum with particular applications in long wavelength emitters and detectors. In addition to bandgap tuning, Bi provides an opportunity to decrease lasing threshold currents, the temperature sensitivity and a major loss mechanism of today’s telecom lasers.

We propose to characterize the electronic and chemical structure of GaAsBi grown by molecular beam epitaxy. We probe the binding structure using x-ray photoelectron spectroscopy. This provides insights into the antisite incorporation of Bi and the reactivity of the surface. Furthermore, we use XPS to track the energy variation in the valence band with dilute Bi incorporation into GaAs. These insights provide valuable perspective into improving the predictability of bandgaps and of heterostructure band offsets for the realization of bismides in future electronics.

The stringent growth conditions required by GaAsBi and the surfactant properties of Bi provide a unique opportunity to study nanostructure formation and epitaxial growth control mechanisms. The GaAsBi epitaxial films under Ga-rich growth conditions self-catalyze Ga droplet seeds for Vapor-Liquid-Solid growth of embedded nanowires. We demonstrate a means to direct the nanowires unidirectionally along preferential crystallographic directions utilizing the step-flow growth mode. We mediated the step-flow growth by employing vicinal surfaces and Bi’s surfactant-like properties to enhance the properties of the step-flow growth mode. Semiconductor nanostructures are becoming a cornerstone of future optoelectronics and the work presented herein exploits the power of a bottom-up architecture to self-assemble aligned unidirectional planar nanowires.

The demand for printable electronics has sharply increased in recent years and is projected to continue to rise. Unfortunately, electronic materials which are suitable for desired applications while being compatible with available printing techniques are still often lacking. This thesis addresses two such challenging areas.

In the realm of two-dimensional ink-based printing of electronics, a major barrier to the realization of printable computers that can run programs is the lack of a solution-coatable non-volatile memory with performance metrics comparable to silicon-based devices. To address this deficiency, I developed a nonvolatile memory based on Cu-SiO2 core-shell nanowires that can be printed from solution and exhibits on-off ratios of 106, switching speeds of 50 ns, a low operating voltage of 2 V, and operates for at least 104 cycles without failure. Each of these metrics is similar to or better than Flash memory (the write speed is 20 times faster than Flash). Memory architectures based on the individual memory cells demonstrated here could enable the printing of the more complex, embedded computing devices that are expected to make up an internet of things.

Recently, the exploration of three-dimensional printing techniques to fabricate electronic materials began. A suitable general-purpose conductive thermoplastic filament was not available, however. In this work I examine the current state of conductive thermoplastic filaments, including a newly-released highly conductive filament that my lab has produced which we call Electrifi. I focus on the use of dual-material fused filament fabrication (FFF) to 3D print electronic components (conductive traces, resistors, capacitors, inductors) and circuits (a fully-printed high-pass filter). The resistivity of traces printed from conductive thermoplastic filaments made with carbon-black, graphene, and copper as conductive fillers was found to be 12, 0.78, and 0.014 ohm cm, respectively, enabling the creation of resistors with resistances spanning 3 orders of magnitude. The carbon black and graphene filaments were brittle and fractured easily, but the copper-based filament could be bent at least 500 times with little change in its resistance. Impedance measurements made on the thermoplastic filaments demonstrate that the copper-based filament had an impedance similar to a conductive PCB trace at 1 MHz. Dual material 3D printing was used to fabricate a variety of inductors and capacitors with properties that could be predictably tuned by modifying either the geometry of the components, or the materials used to fabricate the components. These resistors, capacitors, and inductors were combined to create a fully 3D printed high-pass filter with properties comparable to its conventional counterparts. The relatively low impedance of the copper-based filament enable its use to 3D print a receiver coil for wireless power transfer. We also demonstrate the ability to embed and connect surface mounted components in 3D printed objects with a low-cost ($1,000 in parts), open source dual-material 3D printer. This work thus demonstrates the potential for FFF 3D printing to create complex, three-dimensional circuits composed of either embedded or fully-printed electronic components.