This thesis describes an investigation of flow phenomena of liquid helium II associated with temperature and pressure gradients. The experiments were done primarily in the range of heat flow critical with respect to mutual friction and with respect to turbulence in the normal fluid. The results are analysed on a phenomenological basis due to Gorter and Mellink,(12) and are interpreted by making use of the model put forward by Hall and Vinen. (14) This model assumes that the superfluid can develop vortex lines which interact with the thermal excitations of the normal fluid. A conclusion is reached, that a considerable amount of energy is stored in non-thermal motion in this vorticity. Friction in the normal fluid is reviewed in the light of the work of Staas, et al. (8) An experiment to measure the attenuation of secound sound at very low frequencies is described. A few suggestions for further investigation are tendered. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The conductance of a small electric probe has been determined for radial distances (2 - 15 cm) from a recovering spark gap as a function of time after discharge initiation. The times investigated were from 0.2 to 15 ms and the gas pressure was varied from 22 mmHg down to 0.1 mmHg. The voltage applied to the probes was a sawtooth pulse which rose to about 80 volts in 10 μs. It is shown theoretically that the probe conductance should be proportional to the three-halves power of the gas temperature. Prom a known value of the temperature, deduced from recovery measurements, and the known probe conductance the constant of proportionality was deduced. Hence it was possible to determine the temperature from the probe conductance. The probe conductance measurements show that at 200 mmHg pressure the spark channel is only 2 cm in radius and that there is no detectable ionization left 2 ms after the discharge. As the gas pressure decreases the spark channel increases in size and takes longer to deionize, until at 1 mmHg pressure the channel fills the whole spark chamber (spark channel radius is 15 cm) and requires almost 15 ms to deionize. At 1 mmHg gas pressure there is a radial temperature gradient, while at 0.1 mmHg pressure the gas everywhere in the channel recovers at the same rate. In this experiment it is theoretically predicted that volume recombination should be the dominant recovery method and this is experimentally verified. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The conductivities of the homologous series from tetra-methyl to tetra-n-amyl of the normal tetraalkylammonium iodides and bromides in acetonitrile at 25° C have been determined. The concentrations studied ranged from 0.006 to 0.00003 moles per litre. These salts are fairly strong electrolytes in acetonitrile. The iodides have higher conductivities and are more highly dissociated than the corresponding bromides. The conductivities decrease with increasing size of the alkyl group and the degrees of dissociation show a corresponding increase. There is a relatively large difference between the tetramethyl and tetraethyl ions and then smaller, fairly regular differences in going from tetraethyl to tetra-n-amyl. The limiting equivalent conductivities have been calculated by the methods of Shedlovsky and Fuoss. It is shown that the dissociation constants calculated by the method of Shedlovsky and those calculated by the method of Fuoss are not related in the manner predicted by these authors. Ionic conductances have been calculated by assuming that Walden's rule applies to the tetra-n-butylammonium ion. A comparison of ionic resistances in some organic solvents shows that the quaternary ammonium ions have little tendency to interact with solvent molecules. / Science, Faculty of / Chemistry, Department of / Graduate

Polymer nanocomposites are prepared by appropriately dispersing nanoscale fillers into polymer matrices. Graphene, a two-dimensional nano-carbon material with outstanding physical properties, has been widely studied as a conductive filler for nanocomposites. In this work, a gum Arabic aqueous solution was proposed as a new media to exfoliate graphite into few-layer graphene by liquid-phase sonication. Successful exfoliation was confirmed by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Four types of graphene nanoplatelets were used to study the effects of the filler's aspect ratio. The one with the largest aspect ratio showed the best performance, where the conductivity of neat epoxy was increased by five orders of magnitude at 10 wt.%. Using a hot sonication technique and adding a small amount of second fillers further improved the electrical conductivities. The highest conductivity obtained in this study was 0.025 S/cm, which met the requirements of electromagnetic shielding material.

The electrical conductivity of pure KI and CdI₂-doped KI has been studied in the temperature range 200 to 23°C. Two regions A and B (corresponding to different activation energy of conductivity) are identified. The region A can be given a conventional interpretation in terms of migration of cation vacancies in the bulk, their concentration being determined by impurities. U (the energy for migration of cation vacancies) amounts to 0.96 ± 0.02 eV. Observed activation energies higher than this value are accounted for by association and precipitation effects. Association energy of cation vacancies with impurities (0.48 eV. for Cd⁺₂) and heat of solution (0.25 eV. for CdI₂) obtained here are comparable with known values for other alkali halides. Region B found in this work represents unusual behaviour and has not previously been observed in any alkali halides. The activation energy of conductivity is considerably less than the energy needed for the migration of cation vacancies in the bulk. The activation energy E(formula omitted) (for region B) is about 0.57 eV. in a single crystal and 0.38 eV. in a pure KI pellet. Such low activation energies cannot be given a similar interpretation as for region A. It is suggested that the cation vacancies are in regions of unusually high mobility such as dislocations and grain boundaries. This effect may arise partly from a lower activation energy for motion of vacancies in these regions and partly from a vacancy concentration in these regions which increases with decreasing temperature, under the control of "space- charge" effects. The value 0.57 eV. appears to refer to isolated dislocations or low angle boundaries, while the value of 0.38 eV. refers to large angle intercrystalline boundaries in a pellet. A strong piece of evidence for this suggestion comes from the conductivity runs on single crystals. In an untreated single crystal, just as in pure pellets, two regions A and B are identified but region B disappears in crystals annealed overnight and reappears in a mechanically strained crystal. Moreover, region A remains almost undisturbed in each case. This means that the conduction process in region B is governed by dislocations and grain boundaries whereas region A is governed by motion of cation vacancies in the bulk. / Science, Faculty of / Chemistry, Department of / Graduate

In lightly doped semiconductors (LDSs), electrons can exist in localized states around impurities and dc electronic conduction can occur by electrons hopping between localized states. Such hopping is the dominant mechanism for conduction if the temperature is so low that the contribution from band electrons is negligible. According to theories of hopping conduction, at low enough temperature T, the conductivity σ will be o=σ₀e⁻(T₀/T)¼ where T₀ is a temperature which depends on the material. Experimental work on doped semiconductors which exhibits this form of σ is scarce. Recently, however, conductivities which were clearly of this form were reported for lightly doped n-GaAs and lightly doped n-InP. The experimental results were surprising in that the temperature ranges were well above, and the T₀ values well below, the limits set by the theories. To understand these experimental results, hopping in LDSs is modelled in this dissertation using a resistor network. This dissertation is unique in that the conductivity of the unabridged resistor network is examined in a temperature range (called "the high temperature regime") where kT is comparable to the spread ∆ε in the energies of localized electrons. A numerical simulation is performed and an analytic theory based on percolation methods is presented. In this dissertation, an analytic approach is developed for the first time for studying how, in the high temperature regime, the conductivity of the unabridged resistor network depends on the density of localized states. It is found that, in either two or three dimensions, if the density of states is flat, σ is of the activated form o=σ₀e ⁻εa/kt. The activation energies are found to be εa=0.28∆ε in two dimensions and εa =0.20∆ε in three dimensions. These values are considerable improvements over the estimates of previous workers, who used the low temperature asymptotic form of the resistance in the high temperature regime. It is also revealed that σ can be o=µσ₀e ⁻(T₀/T)¼ in the high temperature regime if the density of states decreases with |ε⁻µ₀| for energy e far enough away from the zero temperature chemical potential µ₀, These results are in accord with the experimental results described above. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Garnet structured cubic LLZO crystal (Li56La24Zr16O96) is one of the most promising solid electrolytes for next-generation solid-state lithium-ion batteries. Ab initio molecular dynamics simulations have been employed to study the impacts of lithium vacancy defect and doping concentration on the lithium ionic conductivity and stability of LLZO. The number of lithium atoms in a unit cell of LLZO has been reduced from 56 to 53, where 56 lithium atoms represent the structure of stoichiometric LLZO, i.e., Li7La3Zr2O12. Similarly, the effect of Al and Ga doping on the conductivity and stability of LLZO material was also investigated. Our computational results confirm that both the defects help in enhancing the conductivity of LLZO and the concentration of defect introduced controls the trade-off between the conductivity and stability. Overall, this study provides a valuable insight into the enhancement of conductivity of cubic LLZO garnet material along with structural stability.

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