Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology

We have developed an efficient order-N real-space Kubo approach for the calculation of the phonon conductivity which outperforms state-of-the-art alternative implementations based on the Green's function formalism. The method treats efficiently the time-dependent propagation of phonon wave packets in real space, and this dynamics is related to the calculation of the thermal conductance. Without loss of generality, we validate the accuracy of the method by comparing the calculated phonon mean free paths in disordered carbon nanotubes (isotope impurities) with other approaches, and further illustrate its upscalability by exploring the thermal conductance features in large width edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach of more conventional techniques. We show that edge-disorder is the most important scattering mechanism for phonons in graphene nanoribbons with realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication

The influence of relative fluid depth on initial bedform dynamics in closed, horizontal pipe flow

Measurements of time-dependent bedforms produced by the deposition of solid plastic particles in two-phase liquid-solid flows were performed using a novel ultrasonic echo method and via video image analysis in a 100-liter, closed-pipe slurry flow loop. Results are presented for the settled bed thicknesses over a range of nominal flow rates and initial bed depths and are combined into several phase diagrams based on various combinations of parameters, with the bedforms categorized into five types. The novel observation is made that the type of bedform that arises depends on both the flow rate and the initial relative bed or fluid depth, with both ripples and dunes being observed in the same system and in a single experiment. In addition, the critical Shields number at incipient particle motion is measured to be θsc = 0.094 ± 0.043, hysteretic behavior is observed, and the evolution and scaling of each time-dependent type of bedform is analyzed in detail and compared against several expressions for initial and equilibrium dimensions from the literature. A number of universal scalings for bedforms in any type of conduit are proposed with a view ultimately to unifying the observations of bedforms in pipes with those in channels and natural flows

The effect of cationic surfactants on improving natural clinoptilolite for the flotation of cesium

Flotation using cationic surfactants has been investigated as a rapid separation technique to dewater clinoptilolite ion exchange resins, for the decontamination of radioactive cesium ions (Cs+) from nuclear waste effluent. Initial kinetic and equilibrium adsorption studies of cesium, suggested the large surface area to volume ratio of the fine zeolite contributed to fast adsorption kinetics and high capacities (qc = 158.3 mg/g). Adsorption of ethylhexadecyldimethylammonium bromide (EHDa-Br) and cetylpyridinium chloride (CPC) surfactant collectors onto both clean and 5 ppm Cs+ contaminated clinoptilolite was then measured, where distribution coefficients (Kd) as high as 10,000 mL/g were evident with moderate concentrations CPC. Measurements of particle sizes confirmed that adsorption of surfactant monolayers did not lead to significant aggregation of the clinoptilolite, while 4, highlighting the great viability of flotation to separate and concentrate the contaminated powder in the froth phase

The effect of pre-activation and milling on improving natural clinoptilolite for ion exchange of cesium and strontium

Natural clinoptilolite, of relatively low-grade, was investigated for its capability to remove cesium and strontium ions from water and simulated seawater. To improve its capacity, the material was pre-activated with concentrated NaCl and HCl solutions. Additionally, it was milled to a number of < 300 μm size fractions, to expose exchange sites. Electron microscopy was used to characterise the naturally occurring impurities, where regions of high iron and potassium content was shown to correlate to lower levels of cesium adsorption. Adsorption kinetics for natural and activated resins with 5, 300 and 1500 ppm salt solutions were fitted with the Pseudo-Second Order (PSO) rate model. Activation led to clear increases in initial adsorption rate for both Cs+ and Sr2+, but only enhanced the overall rate constant for Cs+, due to the weaker interaction of the Sr2+. Equilibrium isotherms were compared with Langmuir and Freundlich monolayer models, where the adsorption capacity (Qc) for Cs+ was 67 mg/g which increased by over 100% with NaCl activation to 140 mg/g. Values for Sr2+ were significantly lower at 35 mg/g, with a considerably smaller enhancement with activation to 52 mg/g. Milling of the natural clinoptilolite was found to increase Cs+ uptake to similar levels as activation, in a linear correlation with specific surface area; although, improvements for Sr2+ were again lower, due to its weaker interaction with surface sites. In simulated seawater solutions, all materials gave considerably reduced performance due to K+ ion competition, with Sr2+ uptake decreased more extensively compared to Cs+. Overall, this work highlights that pre-activation and milling of clinoptilolite can be used to significantly enhance the grade of the ore for nuclear effluent treatment in low-salinity conditions

In situ characterization of mixing and sedimentation dynamics in an impinging jet ballast tank via acoustic backscatter

Impinging jets are utilized in numerous applications, including nuclear waste treatment, for both the erosion of sediment beds and maintaining particulates in suspension. Pulse-echo ultrasonic methods offer great potential for the in situ monitoring of critical mixing and settling dynamics, in concentrated dispersions. A non-active scaled version of a Highly Active Storage Tank at Sellafield, UK, was profiled with an acoustic backscatter system under various jet firing conditions. An advanced analysis technique enabled the direct quantification of dispersion concentration changes from the converted backscatter attenuation. Hence, the erosion and mixing capability of the jets, and settling kinetics were characterized. It was found that jet operation alone provided inadequate localized mixing of eroded sediment. An additional air-lift process operation was required to hinder the rapid re-settling of dispersed particulates

Synthesis and Physical Property Characterisation of Spheroidal and Cuboidal Nuclear Waste Simulant Dispersions

This study investigated dispersions analogous to highly active nuclear waste, formed from the reprocessing of Spent Nuclear Fuel (SNF). Non-radioactive simulants of spheroidal caesium phosphomolybdate (CPM) and cuboidal zirconium molybdate (ZM-a) were successfully synthesised; confirmed via Scanning Electron Microscopy (SEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectroscopy. In addition, a supplied ZM (ZM-b) with a rod-like/wheatsheaf morphology was also analysed along with titanium dioxide (TiO2). The simulants underwent thermal gravimetric analysis (TGA) and size analysis, where CPM was found to have a D50 value of 300 nm and a chemical formula of Cs3PMo12O40·13H2O, ZM-a a D50 value of 10 μm and a chemical formula of ZrMo2O7(OH)2·3H2O and ZM-b to have a D50 value of 14 μm and a chemical formula of ZrMo2O7(OH)2·4H2O. The synthesis of CPM was tracked via Ultraviolet-visible (UV-Vis) spectroscopy at both 25 °C and 50 °C, where the reaction was found to be first order with the rate constant highly temperature dependent. The morphology change from spheroidal CPM to cuboidal ZM-a was tracked via SEM, reporting to take 10 days. For the onward processing and immobilisation of these waste dispersions, centrifugal analysis was utilised to understand their settling behaviours, in both aqueous and 2 M nitric acid environments (mimicking current storage conditions). Spheroidal CPM was present in both conditions as agglomerated clusters, with relatively high settling rates. Conversely, the ZM were found to be stable in water, where their settling rate exponents were related to the morphology. In acid, the high effective electrolyte resulted in agglomeration and faster sedimentation