Xu Shuyan

Information technology is a critical element in teaching and learning and is central to our mission of preparing students for the 21St century. This work explores the use and benefit of computer aided techniques in undergraduate physics teaching. It is demonstrated that the computer aided teaching techniques indeed promote students' deep learning and critical thinking skills, especially for undergraduate theoretical and experimental physics courses. The results show that such a non-conventional approach provides a great opportunity to effectively motivate students in understanding some abstractive physical concepts and principles.

Precise readout values for propellant flow rates obtained from mass flow controllers are vital for derivation of accurate and reliable figures that are indicative of thruster performance, such as the specific impulse and thruster effi-ciency. This is exceptionally important in thrusters employed for micro-propulsion for orbit maintenance and drag compen-sation in small satellites, where a small uncertainty and devia-tion in propellant flow would lead to large errors in derived values. Therefore, it is crucial for researchers engaged in pro-pulsion-based research to ensure that their gas flow controllers are calibrated and verified experimentally to be accurate be-fore engaging in thorough experimental work. In this work, an experimental method for the practical calibration of propellant feed systems in typical Hall thruster setups is tested. Results yield reliable calibration factors that have been applied for performance correction of miniature Hall thrusters. The cor-rected propellant flows correlate well with obtained ionization oscillation waveforms, indicating that the presented method for calibration is favorable and dependable for application to ground test facilities for thrust performance qualification.

The age of space electric propulsion arrived and found the space exploration endeavors at a paradigm shift in the context of new space. Mega-constellations of small satellites on low-Earth orbit (LEO) are proposed by many emerging commercial actors. Naturally, the boom in the small satellite market drives the necessity of propulsion systems that are both power and fuel efficient and accommodate small form-factors. Most of the existing electric propulsion technologies have reached the maturity level and can be the prime choices to enable mission versatility for small satellite platforms in Earth orbit and beyond. At the Plasma Sources and Applications Centre/Space Propulsion Centre (PSAC/SPC) Singapore, a continuous effort was dedicated to the development of low-power electric propulsion systems that can meet the small satellites market requirements. This review presents the recent progress in the field of electric propulsion at PSAC/SPC Singapore, from Hall thrusters and thermionic cathodes research to more ambitious devices such as the rotamak-like plasma thruster. On top of that, a review of the existing vacuum facilities and plasma diagnostics used for electric propulsion testing and characterization is included in the present research.

Silicon based photovoltaic cells still remain a mainstay in the industries due to its relatively low cost for manufacturing and implementation. A good knowledge base of the material has also been built up over the years and there is no doubt that silicon based photovoltaic cells would continue to lay the basis for renewable energy for many years to come. However, it is widely known that conventional silicon photovoltaic cells have relatively lower power conversion efficiencies as compared to its next generation counterparts. This is partly due to the high optical losses on surfaces, resulting in poor harvesting of energy from incident light. In this work, an ICP process was developed to fabricate ultra-low reflective silicon surfaces for photovoltaic applications. An Ar + H2 feedstock was used to texture nanocones on the surface of silicon wafers, reducing the reflective losses and forming a high quality pn junction simultaneously. Reflectivity of the samples were characterised with a Zolix SCS10-X150-DSSC UV-Vis spectrometer with an attached integrating sphere, while the photovoltaic properties were measured with a PV characterization suite from Sinton instruments. The low reflectivity with promising electronic properties of the processed materials shows propitious potential for applications in the field of photovoltaics.

Thermionic cathodes are essential for the operation of various electrostatic propulsion devices. They strongly influence the performance and lifetime of the propulsion system. In this study, a 1 A-class LaB6 laboratory model hollow cathode has been tested with krypton in diode and triode configurations in order to assess for the cathode discharge mode transition behavior. Measurements have been performed over a range of krypton mass flow rates (0.1, 0.15 and 0.21 mg/s, or 1.6, 2.4 and 3.4 sccm), keeper (0.1, 0.15, 0.2 A) and anode currents (0.1 to 1 A) at a fixed cathode-to- anode distance. Seven criteria were used to distinguish between the spot and plume mode operations. The results show that the mode transition in low-current cathodes may be a non-linear phenomenon and only some of the existing mode transition criteria can be used to accurately predict the spot/plume discharge regions at low emission currents.

Colonization of Mars: As humans gradually overcome technological challenges of deep space missions, the possibility of exploration and colonization of extraterrestrial outposts is being seriously considered by space agencies and commercial entities alike. But should we do it just because we potentially can? Is such an undoubtedly risky adventure justified from the economical, legal, and ethical points of view? And even if it is, do we have a system of instruments to effectively and fairly manage these aspects of colonisation? In this essay, a rich diversity of current opinions on the pros and cons of Mars colonization voiced by space enthusiasts with backgrounds in space technology, economics, and materials science are examined.

Heterostructural engineering and noble metal coupling are effective strategies to optimize semiconductor photocatalytic materials. In this work, (Ag, Mn₂O₃)-codecorated ZnO nanoparticles with different Mn₂O₃ contents (0–10 mol%) were synthesized by integrating the two strategies by a facile two-step polymer network-gel process. The photocatalytic activity of Ag/ZnO (AZM0) was significantly enhanced with the optimum Mn₂O₃ molar ratio of 3 mol%. The degradation efficiency of AZM3 is ∼3 times and ∼4.8 times higher than that of AZM0 for the degradation of methylene blue (MB) upon exposure to simulated sunlight and visible light, respectively. Also, this ternary nanocomposite exhibits enhanced gas sensing performance towards NO₂ under ultraviolet/visible light irradiation at room temperature. The analysis of its microstructural, optical and photoelectrical characteristics suggests the synergistic coupling effects of Ag and Mn₂O₃, in which the significantly enhanced visible light response and hetero-interface charge carrier migration are the critical factors for the improvement of photocatalytic efficiency and gas sensing activity. Furthermore, the effects of recycling ability, the influence of the initial solution pH, the catalyst dosage and the main active species during the catalysis process on photocatalytic activity were explored. This study develops a feasible pathway to consciously construct multiheterostructures for enhancing the photocatalytic activity with great potential applications in toxic pollution abatement and noxious gas detection.

Surface texturing is a method widely adopted by industries to reduce the reflective losses in photovoltaic (PV) cells. In this work, a multi-chambered Integrated Plasma Photovoltaic Nanofabrication Cluster facility was used to produce nanocone surface textured polycrystalline (PX) PV cells. An inductively coupled plasma (ICP) discharge of O2 and SF6 was used to remove damage on PX p-type silicon wafers. Following that, a mixture of H2 and Ar plasma was used to texture an anti-reflecting array of silicon nanocones on the surface, while simultaneously forming a p-n junction. A plasma enhanced chemical vapour deposition (PECVD) process was utilized using SiH4, CO2, N2 and H2 precursors for front and back surface passivation for growth of SiNx:H and SiOxH thin films. Aluminium electrodes were sputtered on using an RF magnetron sputtering facility to provide the contacts for the PV cell. Scanning electron microscopy of textured sample surfaces revealed uniform, well defined, high aspect ratio nanocones. The absorption spectra of the resulting surface show dramatic reductions in the reflectance of the wafers, and external quantum efficiency measurements show improved spectral response for the 300 nm – 1100 nm region. The resulting cells showed promising photovoltaic responses, with short circuit-currents of 36 mA/cm2, open circuit voltages of 560 mV, fill factors reaching 80% and conversion efficiencies of up to 14.8%. The feedstock gases utilized in this entire process were mostly environmentally friendly, and the single plasma based processing cluster eliminated the need for excessive waste generated from chemicals that would be otherwise found in commercial production lines. This work shows exciting potential in the pursuit of fabricating low cost, environmentally friendly and highly efficient PV modules to address the problems posed by the global energy crisis.

Plasma instabilities in the plume of hollow cathodes have been extensively researched in particular for high-current operation. The rise of plume mode ionization-like instability leads to a degradation of cathode’s performance along with the emergence of highly energetic ions that can produce sputtering of various cathode’s surfaces. Numerical simulations using 2D fluid or hybrid codes brought forward an interesting correlation between the evolution of ion acoustic turbulence (IAT) and emergence of plume mode oscillations. Such numerical findings were proven to be true by experimental measurements of wave dispersion and plume mode-IAT correlation in the plume of cathodes emitting currents >10 A. This study brings forward evidence of the correlation between plume mode oscillations and IAT in the plume of low-current cathodes operating with Kr at sub-ampere current levels. It is shown that at <1 A the plume mode instability is highly correlated with the IAT and the anomalous electron collision drives the electron transport in the cathode plume. The fluctuations in IAT wave energy lead to large temperature oscillations which then drive fluctuations in the density via ionization.

Heated thermionic cathodes based on LaB6 emitters are known to consume a relatively large amount of power during the heating part of the ignition phase. Miniaturized electric propulsion systems, and in particular low-power Hall thrusters, rely on the operation of self-sustained cathodes at low-current emission, typically below 1 A. At PSAC/SPC, an open-end knife-edge LaB6 emitter hollow cathode was developed and tested, denoted herein as the PSAC-KE cathode. Here, a detailed description of the new model's design and comparison to the older version, the PSAC cathode, is included. Electrostatic simulations suggested that the open-end emitter geometry allowed for an enhanced electric field within the emitter region during cathode startup and steady-state operation as compared to the orificed emitter PSAC cathode. The PSAC-KE cathode's thermal management was improved based on iterative thermal simulations driven by different cathode geometries and various combinations of materials. The new cathode was tested with xenon in diode mode with an external anode and triode mode with the external anode and the keeper electrode. Furthermore, results from the coupling tests with a low-power Hall thruster are presented. Cathode performance was monitored over a range of anode current from 0.1 to 1 A, mass flow rates from 0.057 to 0.3 mg s−1 and keeper current of 0.05, 0.1, and 0.15 A. The cathode's thermal design was assessed with thermocouple measurements and compared with thermal simulations and results obtained for the PSAC cathode. The preliminary tests showed that the PSAC-KE cathode achieved ignition at low heating power below 35 W, self-sustained operation at 1 A when in standalone mode, and <1 A when against a low-power Hall thruster.