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High growth rate synthesis of zinc oxide and carbon based nanostructured materials using dense plasma focus device
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Type
Thesis
Author
Tan, Kin Seng
Supervisor
Rawat, Rajdeep Singh
Abstract
This thesis reports the high growth rate synthesis of zinc oxide (ZnO), carbon nanotubes (CNT), and graphene nanoflakes (GNF) in a DPF device based on the growth parameters used. Discussions regarding the possible growth mechanism and the relationship of plasma conditions with material grown are elucidated. Initial research was conducted on the growth of ZnO thin films using a solid Zn anode and oxygen gas environment as the source for the thin film. Substantial growth rate close to 100 nm/shot was achieved. This reported growth rate is supported by the well-developed polycrystalline structure of ZnO which exhibits strong c-axis orientation. Thickness of within 1.06 ± 0.14µm to about 2.82 ± 0.32 µm was achieved from 10 to 30 shots with respect to the substrate distance used. It is expected that a growth rate of up to 60 µm/min can be achieved with a 10 Hz repetitive DPF device.
Next, in the initial trial and error investigation, four significant key parameters were identified to influence the growth of CNT and GNF in a dense plasma focus device: (i) substrate temperature, (ii) plasma presence, (iii) catalyst thin film, and (iv) precursor gas. Growth of CNT was initiated at a minimum growth temperature of 600 °C for CNT whereas substrates at room temperature resulted in GNF growth. GNF growth was achievable at room temperature in DPF, with and without catalyst. A study on the growth of graphene nanoflakes was conducted based on the findings from the trial and error approach. A gas mixture of methane and nitrogen was utilized as working gas. The highly wrinkled and curved nature of the GNF films was observed and strong D, G, D’, and 2D peak in Raman spectroscopy suggests that highly defective graphene nanoflakes were formed due to the use of nitrogen gas. Pure methane gas synthesis yielded little growth at large substrate distances, which contrasts with 30% nitrogen addition, producing substantial growth at short substrate distances.
Growth of CNT was investigated on the effects of plasma conditions by varying parameters such as: (i) capacitor charging voltage, (ii) number of DPF shots, (iii) gas refresh rate, and (iv) the use of a plasma shield. The capacitor charging voltage influences the plasma characteristics in the DPF device which determines the amount of dissociation of gaseous species and dynamics of the plasma. The minimum voltage threshold of 12.0 kV was found to provide reliable growth of CNT at a substrate temperature of 650 °C. Multiple numbers of shots increases the CNT film thickness linearly but damaging effect is observed for more than 2 shots. High speed shockwave and ablated anode particles produced during pinch phase are responsible for the damage which can be suppressed by refreshing the gas for every two shots. Addition of a plasma shield (a mechanical shutter) reduces the damaging effect from the direct exposure of the hot and dense plasma. Various catalyst types were investigated and it was found that Ni, FePt and Fe were able to grow CNT but the growth density was affected. Fe provided the highest density while Ag did not show any CNT growth. The catalyst thin film below 10 nm thickness produced CNT with the highest structural anisotropy while a thicker or bulk catalyst provided CNTs with varying sizes. Furthermore, effects of different gas densities revealed significant effects on the CNT length and tube diameter. Pure methane environment at 20.0 and 25.0 mbar yielded CNT, while it was absent at a low pressure of 5.0 mbar. Upon dilution of methane gas with argon gas as much as 90%, the growth of CNT occurs between pressures of 8.0 mbar to 20.0 mbar as the additional inert gas provided a lower activation path in dissociation of neutral methane gas into active carbon species. Highly uniform and dense film of CNT with lengths of up to 2.28 µm and average diameter between 7.5 nm to 9.8 nm was possible with a single plasma exposure.
Using two high speed cameras (PI-Max Roper Scientific and Vision Research V211), the plasma dynamics was optically recorded and investigated for plasma sheath speed, observation of plasma pinch efficiency and occurrences, ionization wave bubble formation, and the life time of the dense and hot plasma. The life time of the plasma was determined to last as long as 840 µs for a gas mixture of 30% methane and 70% argon at gas pressure of 10.0 mbar while a pure methane gas pressure of 25.0 mbar only lasted as long as 250 µs. The characteristic dense plasma observed using the PI-Max high speed camera revealed unique plasma dynamics where the dense plasma hovered on the substrate for a period of 55.4 µs and 76.5 µs for an argon gas pressure of 2.5 mbar and 7.0 mbar, respectively.
To conclude, this report presents the parameters for synthesis of ZnO, GNF, and CNT using high energy density plasma focus device in a short time scale and the effects of additional inert gas. In addition, the results show high growth rates for layered polycrystalline defect free ZnO synthesis, catalyst free growth of GNF, and high growth rate single shot synthesis of vertically aligned CNT film using the DPF device.
Next, in the initial trial and error investigation, four significant key parameters were identified to influence the growth of CNT and GNF in a dense plasma focus device: (i) substrate temperature, (ii) plasma presence, (iii) catalyst thin film, and (iv) precursor gas. Growth of CNT was initiated at a minimum growth temperature of 600 °C for CNT whereas substrates at room temperature resulted in GNF growth. GNF growth was achievable at room temperature in DPF, with and without catalyst. A study on the growth of graphene nanoflakes was conducted based on the findings from the trial and error approach. A gas mixture of methane and nitrogen was utilized as working gas. The highly wrinkled and curved nature of the GNF films was observed and strong D, G, D’, and 2D peak in Raman spectroscopy suggests that highly defective graphene nanoflakes were formed due to the use of nitrogen gas. Pure methane gas synthesis yielded little growth at large substrate distances, which contrasts with 30% nitrogen addition, producing substantial growth at short substrate distances.
Growth of CNT was investigated on the effects of plasma conditions by varying parameters such as: (i) capacitor charging voltage, (ii) number of DPF shots, (iii) gas refresh rate, and (iv) the use of a plasma shield. The capacitor charging voltage influences the plasma characteristics in the DPF device which determines the amount of dissociation of gaseous species and dynamics of the plasma. The minimum voltage threshold of 12.0 kV was found to provide reliable growth of CNT at a substrate temperature of 650 °C. Multiple numbers of shots increases the CNT film thickness linearly but damaging effect is observed for more than 2 shots. High speed shockwave and ablated anode particles produced during pinch phase are responsible for the damage which can be suppressed by refreshing the gas for every two shots. Addition of a plasma shield (a mechanical shutter) reduces the damaging effect from the direct exposure of the hot and dense plasma. Various catalyst types were investigated and it was found that Ni, FePt and Fe were able to grow CNT but the growth density was affected. Fe provided the highest density while Ag did not show any CNT growth. The catalyst thin film below 10 nm thickness produced CNT with the highest structural anisotropy while a thicker or bulk catalyst provided CNTs with varying sizes. Furthermore, effects of different gas densities revealed significant effects on the CNT length and tube diameter. Pure methane environment at 20.0 and 25.0 mbar yielded CNT, while it was absent at a low pressure of 5.0 mbar. Upon dilution of methane gas with argon gas as much as 90%, the growth of CNT occurs between pressures of 8.0 mbar to 20.0 mbar as the additional inert gas provided a lower activation path in dissociation of neutral methane gas into active carbon species. Highly uniform and dense film of CNT with lengths of up to 2.28 µm and average diameter between 7.5 nm to 9.8 nm was possible with a single plasma exposure.
Using two high speed cameras (PI-Max Roper Scientific and Vision Research V211), the plasma dynamics was optically recorded and investigated for plasma sheath speed, observation of plasma pinch efficiency and occurrences, ionization wave bubble formation, and the life time of the dense and hot plasma. The life time of the plasma was determined to last as long as 840 µs for a gas mixture of 30% methane and 70% argon at gas pressure of 10.0 mbar while a pure methane gas pressure of 25.0 mbar only lasted as long as 250 µs. The characteristic dense plasma observed using the PI-Max high speed camera revealed unique plasma dynamics where the dense plasma hovered on the substrate for a period of 55.4 µs and 76.5 µs for an argon gas pressure of 2.5 mbar and 7.0 mbar, respectively.
To conclude, this report presents the parameters for synthesis of ZnO, GNF, and CNT using high energy density plasma focus device in a short time scale and the effects of additional inert gas. In addition, the results show high growth rates for layered polycrystalline defect free ZnO synthesis, catalyst free growth of GNF, and high growth rate single shot synthesis of vertically aligned CNT film using the DPF device.
Date Issued
2017
Call Number
TA418.9.N35 Tan
Date Submitted
2017