The effect of surface defects on the optical and electrical properties of ZnO nanorods

University of Technology Sydney. Faculty of Science.This thesis reports on the effect of native point defects on both the NH₃ gas sensing properties and the surface electronic structure of ZnO nanorods. Low temperature hydrothermal growth at 90°C was utilised to synthesize ZnO nanorods approximately 55 ± 5 nm in diameter. The type and density of intrinsic surface defects on the ZnO nanorod was controlled using post growth annealing in O₂ gas and Zn vapor environments. Low voltage cathodoluminescence (CL) spectroscopy confirmed that the heat treatment process produced different surface defect structures. The as-grown, O₂, and Zn annealed nanorods exhibited broad CL peaks centered at 1.90 eV (YL), 1.70 eV (RL), and 2.44 eV (GL), which were attributed to O interstitials or LiZn deep acceptors, acceptor-like VZn complexes, and donor-like VO related centers, respectively. The first part of this thesis focuses on the influence of deep level surface defects on NH₃ gas response of ZnO nanorods. Electrical and gas sensing measurements revealed that the NH₃ gas sensitivity was 4.1 for the as-grown (YL), 22.6 for O₂ anneal (RL), and 1.4 for Zn vapor anneal (GL) samples. Hydrogen plasma treatment quenched the RL and inverted the ammonia electrical response due to the incorporation of shallow hydrogen donors. Changes to the gas sensing response were attributed to a shift in the ZnO Fermi level position relative to the ammonia gas chemical potential due to the formation near surface donor or acceptor centers. In the second part of this thesis the effect of native point defects on the surface electronic structure of all ZnO nanorod samples was studies utilizing X-ray photoemission spectroscopy (XPS), X-ray Absorption Near-Edge Structure (XANES), electron spin resonance (ESR), low voltage CL spectroscopy, sub band gap photoluminescence (PL) spectroscopy and electrical I-V measurements. Correlative characterization measurements established that the degree of surface band bending and surface conductivity is controlled by the amount of adsorbed ambient gas species (O₂, H₂O and OH), which is mediated by type and concentration of intrinsic surface point defects. The last part of this thesis concentrates on the chemical origin of RL peak and its optical properties. A wide range of analysis techniques, including PL and CL spectroscopy, XANES and ESR was used to comprehensively characterize the RL emission. From these data, the RL has been assigned to highly lattice coupled VZn-related acceptor-like center. No correlation was found between the RL and nitrogen impurities

Store and Forward CubeSat using LoRa Technology and Private LoRaWAN-Server

In the THAIIOT project, we built a 3U CubeSat with LoRa module and private LoRaWAN-server payload. The main goal of this project was to develop the payload for CubeSat in the low-earth orbit to receive uplink data via LoRa technology between LoRa module and private LoRaWAN-server on CubeSat to store data and then use S-band transceiver to downlink the data to ground station. In addition, we improved network security between ground LoRa nodes and the private LoRaWAN-server by using Advanced Encryption Standard (AES). In this paper, we present the efficiency of THAIIOT payload, focusing on store and forward data via LoRa technology. From the calculation results, the calculated link budget showed that the LoRa technology can transmit and receive data at a distance up to 2000 km. By assuming the transmitted data size of 35 bytes, the possible maximum data rate was 292 bps, which required 1.81 s of Time on Air (ToA). Moreover, the experimental results verify the capability of the THAIIOT payload to successfully transmit the data up to 2,200 km

Polarity in ZnO nanowires: A critical issue for piezotronic and piezoelectric devices

The polar and piezoelectric nature of the wurtzite structure of ZnO nanowires with a high aspect ratio at nanoscale dimensions is of high interest for piezotronic and piezoelectric devices, but a number of issues related to polarity are still open and deserve a particular attention. In this context, chemical bath deposition offers a unique opportunity to select the O- or Zn-polarity of the resultant nanowires and is further compatible with the fabrication processes of flexible devices. The control and use of the polarity in ZnO nanowires grown by chemical bath deposition opens a new way to greatly enhance the performance of the related piezotronic and piezoelectric devices. However, polarity as an additional tunable parameter should be considered with care because it has a strong influence on many processes and properties. The present review is intended to report the most important consequences related to the polarity in ZnO nanowires for piezotronic and piezoelectric devices. After introducing the basic principles involving crystal polarity in ZnO, a special emphasis is placed on the effects of polarity on the nucleation and growth mechanisms of ZnO nanowires using chemical bath deposition, defect incorporation and doping, electrical contacts and device properties

Fractional Order PID Control of Rotor Suspension by Active Magnetic Bearings

One of the key issues in control design for Active Magnetic Bearing (AMB) systems is the tradeoff between the simplicity of the controller structure and the performance of the closed-loop system. To achieve this tradeoff, this paper proposes the design of a fractional order Proportional-Integral-Derivative (FOPID) controller. The FOPID controller consists of only two additional parameters in comparison with a conventional PID controller. The feasibility of FOPID for AMB systems is investigated for rotor suspension in both the radial and axial directions. Tuning methods are developed based on the evolutionary algorithms for searching the optimal values of the controller parameters. The resulting FOPID controllers are then tested and compared with a conventional PID controller, as well as with some advanced controllers such as Linear Quadratic Gausian (LQG) and H ∞ controllers. The comparison is made in terms of various stability and robustness specifications, as well as the dimensions of the controllers as implemented. Lastly, to validate the proposed method, experimental testing is carried out on a single-stage centrifugal compressor test rig equipped with magnetic bearings. The results show that, with a proper selection of gains and fractional orders, the performance of the resulting FOPID is similar to those of the advanced controllers

FABRICATION OF SHAPE MEMORY ALLOYS USING AFFORDABLE ADDITIVE MANUFACTURING ROUTES

The primary objective of this research was to develop paste formulations to enable the fabrication of shape memory alloys (SMA) using low-cost additive manufacturing routes. We focused on determining which precursor particulates, binding agents and post-processing conditions could sinter the 3D printed parts. Nickel-titanium nano/micron-sized particulates along paraffin/alcoholic gel binders were tested as raw materials to produce pastes. Scanning electron and optical microscopy, energy dispersive spectroscopy and X-ray diffractometry were employed to determine the microstructural features in the raw material and final products. Thermal treatments to sinter the parts were performed in a tube furnace or a hot isostatic press. It was found that it is possible to 3D print metal pastes using micrometric particles, gel binders and a modified low-cost polymeric printer. Nanoparticle formulations were found to behave as shear thinning fluids and are not recommended for this application. The post-processing conditions that were successful creating sintered structures used 1000℃ and pressures above 138MPa. The Ni-Ti phases generated deviated from the ideal NiTi stoichiometry, however, were close to those found in commercially available SMA wires. SMAs could be employed to produce actuators and deployment mechanisms in space applications. This research introduces a novel way to fabricate SMAs that could reduce costs and widen the environments where their production could take place.http://archive.org/details/fabricationofsha1094561375Lieutenant, United States NavyApproved for public release; distribution is unlimited

Fractional Order PID Control of Rotor Suspension by Active Magnetic Bearings

One of the key issues in control design for Active Magnetic Bearing (AMB) systems is the tradeoff between the simplicity of the controller structure and the performance of the closed-loop system. To achieve this tradeoff, this paper proposes the design of a fractional order Proportional-Integral-Derivative (FOPID) controller. The FOPID controller consists of only two additional parameters in comparison with a conventional PID controller. The feasibility of FOPID for AMB systems is investigated for rotor suspension in both the radial and axial directions. Tuning methods are developed based on the evolutionary algorithms for searching the optimal values of the controller parameters. The resulting FOPID controllers are then tested and compared with a conventional PID controller, as well as with some advanced controllers such as Linear Quadratic Gausian (LQG) and H ∞ controllers. The comparison is made in terms of various stability and robustness specifications, as well as the dimensions of the controllers as implemented. Lastly, to validate the proposed method, experimental testing is carried out on a single-stage centrifugal compressor test rig equipped with magnetic bearings. The results show that, with a proper selection of gains and fractional orders, the performance of the resulting FOPID is similar to those of the advanced controllers

Development Of Nanoparticle Based Alternative for Metal Additive Manufacturing

NPS NRP Executive SummaryProject Summary: The research conducted had the objective of developing an additive manufacturing (AM) strategy to produce metal or alloy parts through the use of layer-by-layer extrusion of small particle paste formulations along post-processing steps. That is, we aimed to hybridize additive manufacturing approaches with known powder metallurgy (PM) processes to produce alloys of naval relevance. The work that we are reporting herein generated metal parts using, instead of expensive direct sintering equipment (metal tri dimensional (3D) printer), a conventional 3D printer (as the ones used for polymeric filaments). The later was equipped with an extruder capable of deliver paste composed of metal nano or micron particles and binding media that evaporated after the printing operation was completed. The 3D parts produced using those paste formulations had similar characteristics that the ones observed in green specimens generated by PM. Using the same type of post-treatments that AM and PM routes employ, such as annealing and hot isostatic pressing (HIP), we were able to generate 3D specimens of NiTi and NiCu alloys. We demonstrated that the new process could successfully generate solid specimens, which, after HIP operations, showed mechanical robustness. This new approach could be easier to adapt than laser or e-beam sintering routes and has potential to be used for metal/alloy parts that do not require stringent load bearing specifications.N4 Materials Readiness and LogisticsNPS-18-N337-AApproved for public release; distribution is unlimited

Tailoring deep level surface defects in ZnO nanorods for high sensitivity ammonia gas sensing

© 2014 American Chemical Society. The influence of deep level surface defects on electrical and gas sensing properties of ZnO nanorods NH3(g) sensors was studied. ZnO nanorods 50-60 nm in diameter were synthesized via low-temperature hydrothermal growth at 90°C on sapphire substrates. The as-grown nanorods exhibited a cathodoluminescence (CL) peak centered at 1.90 eV (YL), attributed to LiZn deep acceptors or O interstitials. Subsequent annealing in O2 at 1 atm and Zn vapor at 650°C produced broad CL peaks centered at 1.70 eV (RL) and 2.44 eV (GL), respectively. The RL and GL have been ascribed to acceptor-like VZn and donor-like VO related centers, respectively. Electrical and gas sensing measurements established that the NH3 gas response sensitivity was 22.6 for O2 anneal (RL), 1.4 for Zn vapor anneal (GL), and 4.1 for the as-grown (YL) samples. Additionally, treatment in H-plasma quenched the RL and inverted the NH3 electrical response due to the incorporation of H donors. Changes in the gas sensing response are explained by a shift in the position of the ZnO Fermi level relative to the chemical potential of NH3 gas due to the creation of near surface donor or acceptors. These data confirm that ZnO nanorods arrays can be tailored to detect specific gas species. (Chemical Equation Presented)