Experimental studies on the generation and evolution of mineral porosity during fluid-mediated mineral replacement reactions

The porosity in minerals contributes to enhanced permeability for fluid flow in natural systems and engineering processes. Porosity can be created by fluid-mediated mineral replacement reactions. Such reaction-induced porosity can evolve with time, yet the mechanisms and kinetics of porosity creation and evolution remain poorly understood. This thesis presents experimental investigations on the creation and evolution of mineral porosity in two model replacement reactions, i.e., the replacement of calcite by gypsum and anhydrite with a positive volume change and the replacement of pentlandite by violarite and millerite with a negative volume change. These replacement reactions were conducted under mildly acidic hydrothermal conditions for up to 18 months, and the mineralogy, microstructure and porosity of the reaction products were quantitatively analysed by powder X-ray diffraction, (ultra) small-angle neutron scattering, high resolution scanning electron microscopy, focused-ion beam scanning electron microscopy, and X-ray micro-tomography. The results showed that porosity creation and evolution are highly dependent on mineral systems and reaction conditions. In the calcite-gypsum-anhydrite mineral system, the experiments at 25-60 °C produced intragranular nanopores in gypsum replacing calcite. Because of the positive volume change, gypsum overgrowth also occurred on the grain surface, and the gypsum in the overgrowth region contained intergranular micropores. Porosity coarsening was rapid (a few weeks) in the replacement region, leading to the formation of micro-voids in the core of gypsum grains. The replacement reaction was sensitive to temperature. When the experiments were conducted at a higher temperature of 220 °C, anhydrite was formed instead of gypsum. Porosity evolution in anhydrite was different when compared to gypsum at lower temperatures. In the pentlandite-violarite-millerite mineral system, only replacement occurred, likely because the negative volume change does not require overgrowth for additional space. The replacement was sensitive to temperature and solution pH. The experiments conducted at 125 °C and pH 4 produced permeable nanopores leading to the complete replacement of pentlandite; these nanopores coarsened slowly during the 17 months of experiment and occurred preferentially near the grain surface. However, in experiments conducted at 125 °C and pH 5, violarite became impermeable in partially replaced grains due to hematite precipitation in the pore space, blocking the fluid flow. At a higher temperature of 220 °C and pH 4, the formation of millerite in addition to violarite resulted in faster porosity coarsening and formed micropores within 4 weeks. Fundamentally, these complex porosity creation and evolution phenomena observed in the two model mineral replacement reactions are controlled by the interplay between dissolution, precipitation, epitaxial nucleation, and Ostwald ripening processes which are all sensitive to reaction conditions. This understanding should generally be applicable to other mineral replacement reactions. Finally, a case study of the application of porosity control was presented. The leaching of chalcopyrite is often kinetically inhibited by surface passivation layers, which are formed by the replacement of chalcopyrite during leaching. Common passivation layers are elemental sulphur and jarosite. Our leaching experimental results showed that surface sulphur could be removed by adding sulphur-dissolving solvent tetrachloroethylene (TCE) into the sulfuric acid leaching solution. The removal of surface sulphur significantly improved the leaching rate by almost 6 times compared with TCE-free leaching. At the later stage of leaching, chalcopyrite was replaced by potassium jarosite. The jarosite shell did not passivate TCE-free leaching due to its porous structure. However, the jarosite shell became nearly impermeable in TCE-assisted leaching because elemental sulphur filled the pores in the jarosite. This case study suggests that chalcopyrite leaching can be significantly enhanced by either removing the surface passivating layer or by controlling the porosity and permeability of the surface layers formed on the chalcopyrite surface

Elastic foundation effects on arch dams

Earthquake response of an arch dam should be calculated under ground motion effects. This study presents three-dimensional linear earthquake response of an arch dam. Thereby, we considered different ground motion effects and also foundation conditions in the finite element analyses. For this purpose, the Type 3 double curvature arch dam was selected for application. All numerical analyses are carried out by SAP2000 program for empty reservoir cases. In the scope of this study, linear modal time-history analyses are performed using three dimensional finite element model of the arch dam and arch dam-foundation interaction systems. According to numerical analyses, maximum horizontal displacements and maximum normal stresses are presented by dam height in the largest section. These results are evaluated for rigid and various elastic foundation conditions. Furthermore, near-fault and far-field ground motion effects on the selected arch dam are taken into account by different accelerograms obtained from the Loma Prieta earthquake at various distances

PREPARATION OF TIN@CARBON FIBER COMPOSITES BY ELECTROLESS DEPOSITION

The synthesis of tin-coated carbon fibers (CFs) by electroless deposition method in aqueous solution was studied. Sodium hypophosphite was used as a reducing reagent. The composites were characterized by Xray diffraction, energy-dispersive X-ray spectroscopy and scanning electron microscopy techniques. The experiment results showed that the car fiber surfaces can be fully deposited by tin layers and a continuous tin coating on carbon fiber was obtained by electroless deposition. These tin@carbon fiber composites can be used in some possible applications such as electrode materials, fiber-matrix composites, and so on

Pulse Electrodeposition of Copper-Zinc Coatings from an Alkaline Bath

A copper-zinc bath containing EDTA was used for deposition of multi-functional copper-zinc coatings. Copper substrates were used for pulse electrodeposition of copper-zinc coating. Microhardness and wear resistance of copper-zinc coatings has been studied. The films were characterized by scanning electron microscopy and X-ray diffraction. EDS and EDS-dot mapping were also performed to analyse the amount and the distribution of Cu-Zn atoms

Electrical Conductivity, Viscosity and Thermal Properties of TEGDME-Based Composite Electrolytes for Lithium-Air Batteries

Some important properties of the electrolytes used in Li-air batteries were investigated. Electrolyte composed of a solution of 1 M LiPF6 in tetra ethylene glycol dimethyl ether (TEGDME) was reinforced with SiO2, Al2O3, poly(ethylene) oxide (PEO) and tris (pentafluorophenyl) borane (TPFPB) additives. The effects of these reinforcements on conductivity, viscosity and thermal stability were investigated. Electrical conductivity tests were carried out using a multiparameter meter. Viscosity tests were performed in a viscometer using tuning-fork vibration method. Thermal stability of the electrolytes was tested by both TG and DSC

Production of pulse electrodeposited Ni-TiC nanocomposite coatings

In the present work, the Ni-TiC nanocomposites were electrodeposited from Watts' bath with suspended TiC nano-particles by pulse current (PC) deposition method. Effects of the peak current density on the TiC phase precipitation, surface morphology, crystalline size and the amount of co-deposited TiC nano-particles were investigated. TiC nano-powders were co-deposited with nickel matrix on the copper substrates. The characterization of the coatings was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS) and Vicker's hardness test methods. (C) 2017 Elsevier Ltd. All rights reserved

Tris(pentafluorophenyl) borane as an electrolyte additive for Li-O-2 batteries

TPFPB was used as an additive in a rechargeable Li-O-2. cell utilizing an electrolyte composed of a solution 1 M LiPF6 in tetraethylene glycol dimethyl ether (TEGDME) to investigate the effects on capacity and cycling performances. Capacity limitation was also applied to the cells and results were compared with full charging and discharging condition. Galvanostatic charge/discharge (GC) measurements were performed in the assembled Li-O-2 cells using assembled cells, designed with different electrolytes. Cell discharge capacities were cyclically tested by a battery tester at a constant current in voltage range between 2.15 V-4.25 V. Discharge products of the electrolytes characterized by SEM and XRD techniques. The electrochemical results showed increasing discharge capacity and cycleability of the assembled cells produced with TPFPB as compared to TPFPB free electrolytes. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Graphene supported heterogeneous catalysts for Li-O-2 batteries

In this study production and characterization of free-standing and flexible (i) graphene, (ii) alpha-MnO2/graphene, (iii) Pt/graphene (iv) alpha-MnO2/Pt/graphene composite cathodes for Li-air batteries were reported. Graphene supported heterogeneous catalysts were produced by a facile method. In order to prevent aggregation of graphene sheets and increase not only interlayer distance but also surface area, a trace amount multi-wall carbon nano tube (MWCNT) was introduced to the composite structure. The obtained composite catalysts were characterized by SEM, X-ray diffraction, N-2 adsorption-desorption analyze and Raman spectroscopy. The electrochemical characterization tests including galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) measurement of catalyst were carried out by using an ECC-Air test cell. These highly active graphene supported heterogeneous composite catalysts provide competitive properties relative to other catalyst materials for Li-air batteries. (C) 2016 Elsevier B.V. All rights reserved