Tan, Augustine Tuck Lee

The NX2 device, a low energy plasma focus, at the Nanyang Technological University in Singapore, was used as a soft X-ray (SXR) source for micromachining. The gas used was neon which produced SXRs in a narrow spectral range of 0.9 - 1.6 keV. The SXR yield from repetitive operation of the NX2 device was monitored and measured using a cost effective multi-channel SXR spectrometric system. The system consists of filtered BPX65 PIN diodes, with the associated electronics --- an integrator, sample and peak holder, analogue switch, an A/D converter and a microcontroller. The system enables easy shot-to-shot statistical analysis under repetitive operation at adjustable preset trigger frequencies. A total of 4000 shots were fired at 0.5 Hz, using the same gas filling. The SXR production was at an average yield of 60 J/shot and a maximum single-shot yield of more than 100 J. The SXRs emitted by the NX2 device was used for contact micromachining, producing structures with an excellent aspect ratio of up to 20:1 on 25 μm SU-8 resist.

A 3 kJ plasma focus was operated with a 3He-D2 gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1:1 for 3He and D atoms. The fusion reactions D(3He,p)4He and D(d,p)3H were measured simultaneously using CR-39 polymer nuclear track detectors placed inside a pinhole camera positioned on the forward plasma focus axis. A sandwich arrangement of two 1000 μm thick CR-39 detectors enabled the simultaneous registration of two groups of protons with approximate energies of 16 MeV and 3 MeV arising from the D(3He,p)4He and D(d,p)3H reactions, respectively. Radial track density distributions were obtained from each CR-39 detector and per-shot average distributions were calculated for the two groups of protons. It is found that the D(3He,p)4He and D(d,p)3H proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(3He,p)4He fusion is concentrated close to the plasma focus pinch column, while the D(d,p)3H fusion occurs relatively far from the pinch. The relative absence of D(d,p)3H fusion in the pinch is one significant reason for concluding that the D(3He,p)4He fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(3He,p)4He fusion is due to energetic 3He2+ ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(3He,p)4He and D(d,p)3H fusion in the plasma focus are discussed.

A total of 11 fundamental and 3 overtone bands of the formaldoxime isotopologue 13CD2NOH were identified using its Fourier transform infrared (FTIR) spectra which were recorded with a low resolution (0.50 cm−1) in the 500–4000 cm−1 region, and high resolution (0.00096 cm−1) in the 280–500 cm−1 region. Their relative infrared (IR) band intensities were also measured. Furthermore, a rovibrational analysis of the IR transitions of the band of 13CD2NOH was carried out using its high-resolution FTIR spectrum which was recorded at the Australian Synchrotron. A total of 1077 IR transitions of the C-type band were assigned and fitted using the Watson's A-reduced Hamiltonian in the Ir representation to derive its band center and the = 1 state rovibrational constants up to all 5 quartic centrifugal distortion terms for the first time, with a root-mean-square (rms) deviation of 0.00044 cm−1. The band center of the band of 13CD2NOH were found to be 391.054446(36) cm−1. The ground state rovibrational constants up to all 5 quartic terms were determined for the first time by fitting 407 ground state combination differences (GSCDs) derived from the assigned IR transitions of the band of 13CD2NOH of this work. The rms deviation of the GSCD fit was 0.00040 cm−1. Additionally, all 3 rotational constants and 5 quartic centrifugal distortion terms of the ground state and 3 rotational constants of the = 1 state of 13CD2NOH were computed from theoretical anharmonic calculations at two different levels of theory, B3LYP and MP2 with the cc-pVTZ basis set, for comparison with the experimental results. Close agreement was found for the calculated and experimental rovibrational constants of 13CD2NOH for both ground and = 1 states. The vibrational anharmonic frequencies of the 12 fundamental bands of 13CD2NOH in the 280–4000 cm−1 region, and their IR band intensities were also calculated using B3LYP and MP2 with the cc-pVTZ basis set, and they were compared with the respective experimental data. Finally, the ground state rotational constants and the band center of the band of the cis conformer of 13CD2NOH were calculated and compared with those of the trans conformer of this work.

The ability to produce scientific explanations is an important learning goal in our local physics curriculum. Yeo and Gilbert (2014) showed that its production entails attending to its function, form (structural organisation of meanings) and level (of precision [model used], abstractness and complexity [use of representations]). These multi-dimensions might explain why students find producing an “acceptable” scientific explanation difficult. There are also few studies that examines the kinds of explanations students need to construct, the process of constructing one and the difficulties encountered. There is also an absence of a comprehensive framework for teachers to assess students’ explanations. The goals of this study are thus to (a) develop a framework of scientific explanations that is comprehensive and parsimonious, and (b) produce a characterisation of the process that students undergo in producing scientific explanations.

This study reports the enhanced ferromagnetic ordering in ZnO:Mn nanoparticle thin films, grown at different substrate temperatures using pulsed laser deposition. The optimum growth conditions were deduced from X-ray, photoemission and magnetic measurements. The X-ray measurements reveal that there was an optimum substrate temperature where the thin films showed relatively stronger texture, better crystallinity and lower strain. Substrate temperature tuned the deep level recombination centers in ZnO:Mn, which changed the optical quality by altering the electronic structure. The M-H curves, in the present study, revealed superior ferromagnetic response of 20-nm sized particles in ZnO:Mn thin film grown at a substrate temperature of 450 °C. Ferromagnetic ordering becomes weaker at higher/lower substrate temperatures due to the activation of native defects in ZnO host matrix.

The Fourier transform infrared (FTIR) spectrum of formaldoxime^-13C (¹³CH₂NOH) was recorded in the 600–3800 cm⁻¹ region with a resolution of 0.50 cm⁻¹ to identify its fundamental and overtone bands and to measure their relative infrared band intensities. Furthermore, the high-resolution (0.00096 cm⁻¹) FTIR spectrum of the ν₁₂ band of ¹³CH₂NOH was recorded at the Australian Synchrotron in the 250–600 cm⁻¹ region for a rovibrational analysis. A total of 1506 infrared (IR) transitions of the C-type ν₁₂ band were fitted using the Watson's A-reduced and S-reduced Hamiltonians in the I^r representation with a root-mean-square (rms) deviation of 0.000364 cm⁻¹ for both fits. From the rovibrational analysis, the ν₁₂= 1 state rovibrational constants up to one sextic centrifugal distortion term were derived for the first time. The band center of the ν₁₂ band of ¹³CH₂NOH were found to be 397.365846(32) cm⁻¹ in the A-reduced Hamiltonian. The ground state rovibrational constants up to one sextic term were improved with higher accuracy by fitting 1374 ground state combination differences (GSCDs) derived from the IR transitions of the ν₈ and ν₁₂ bands of ¹³CH₂NOH, together with two previously reported microwave frequencies. Furthermore, a comparison of the experimental vibrational frequencies, IR band intensities, rovibrational constants (ν₁₂ = 1 and ground states) of ¹³CH₂NOH and those derived from theoretical calculations at two levels of theory: B3LYP and MP2 with cc-pVTZ basis set were made. Close agreement was found for the calculated and experimental rovibrational constants of ¹³CH₂NOH for both ground and ν₁₂ = 1 states.

A time resolved imaging study of pulsed laser ablated Fe and Al plasma plumes with specific interest in the splitting of plumes into the slow and fast moving components as they expand through the background argon gas at different pressures is reported. The material ablation was achieved using a Q-switched Nd:YAG yttrium aluminum garnet laser operating at 532 nm with a pulse duration of 8 ns full width at half maximum and a fluence of 30 Jcm−2 at the target surface. Typical time resolved images with low magnification show that the splitting occurs at moderate background gas pressures 0.5 and 1.0 mbar for Fe, and 0.2 mbar for Al plasma plumes. The plume splitting did not occur for higher background gas pressures.

The effect of varied concentrations of deuterium-krypton (D 2 –Kr) admixture on the neutron emission of a fast miniature plasma focusdevice was investigated. It was found that a judicious concentration of Kr in D 2 can significantly enhance the neutron yield. The maximum average neutron yield of (1±0.27)×10 4  n/shot for pure D 2 filling at 3 mbars was enhanced to (3.14±0.4)×10 5  n/shot with D 2 +2% Kr admixture operation, which represents a >30 -fold increase. More than an order of magnitude enhancement in the average neutron yield was observed over the broader operating range of 1–4 mbars for D 2 +2% Kr and D 2 +5% Kr admixtures.