| 1. |
- Nzulu, Gabriel Kofi, 1974-
(författare)
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Characterization of Pathfinders and Indicators of Gold
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research on natural minerals and their chemical bonding to economically critical raw materials is a viable industrially relevant research area due to its increasing demand. Meeting demands requires fast, robust, and efficient techniques to explore new ore deposits and continuous operation of active mines as well as recycling. One of the most critical metals is gold which occurs in three main types of ore deposits: i) hydrothermal quartz veins and related deposits in metamorphic and igneous rocks; ii) volcanic-exhalative sulfide deposits, and iii) consolidated to unconsolidated placer deposits. Gold is commonly found as disseminated grains in quartz veins in pyrite and other sulfides or as rounded grains, flakes or nuggets in deposits in riverbanks, in contact with metamorphic or hypothermal deposits (e.g., skarns) or epithermal deposits such as volcanic fumaroles. Pathfinder elements and indicator minerals provide means to explore large areas for their potential mineral commodities such as gold, diamond, base metals, platinum group of elements, and rare earth elements by narrowing the search area to reduce exploration costs. The recent technological advancement in obtaining rapid geochemical results using field portable analytical devices as alternatives to the old approach where collected field samples are carried to the laboratory calls for further investigation to explore other techniques in mineral and metal exploration.In this Thesis, I investigate the properties of artisanal small-scale gold mining concentrate, outcrop, bulk Au, and drill hole samples from the Kubi Gold Project of the Asante Gold Corporation near Dunkwa-on-Offin in the Central Region of Ghana with a materials science perspective. X-ray diffraction (XRD) is used to identify SiO2 (quartz), Fe3O4 (magnetite), garnet, pyrite (FeS2), periclase (MgO), arsenopyrites, pyrrhotite, biotite, titanium oxide, and Fe2O3 (hematite) as the main indicator minerals in the mining site with less significant contributions from chalcopyrite, iridosmine, scheelite, tetradymite, gypsum, and a few other sulfates. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) indicate that Fe, Ag, Al, N, O, Si, Hg, C, Ba, P, Ca, Mg, Na, Mn, Cl, S, K, and Ti are important host elements that form alloys with Au or are inherent in the sediment at the concession site. The results also indicate that Si and Ag are in strong co-occurrence with Au due to their eutectic qualities, while N, C, and O occur due to their attraction to Si. Also, the XPS results indicate that the relationship between Au and pathfinder elements or indicator minerals depends on the d-orbital of Au and other elements that possess octahedral or tetrahedral geometry to split into two states, eg and t2g that can acquire either higher or lower energy depending on the geometry and are responsible for the covalent, metallic, and ionic states of Au with other ligands. From the air anneal furnace (AAF) and differential scanning calorimetry (DSC), I investigated the transformations in quartz and pyrite minerals that alter to hematite minerals. The quartz samples are observed to transform from α-quartz to β-quartz and finally to cristobalite while the pyrite transforms to magnetite and later to hematite. These findings suggest that during the hydrothermal flow regime impurity materials are trapped by voids and faults and can be altered at different depositional stages by oxidation and reduction processes. Results from the scanning electron microscopy (SEM) revealed the presence of carbonates in fracture zones in the quartz, pyrite, and almandine-type garnet mineral in gabbroic rocks.The findings indicate that, from the top of the oxide zone, grains within sediments are seen to be controlled by quartz, and hematite, the bedrock consists of pyrite and pyrrhotite, and the orebody contains garnet, arsenopyrite, periclase, and biotite as pathfinder minerals within the concession area. Therefore, the Au mineralogy of the alluvial environment that is mined by artisanal small-scale miners is traced from the chemical weathering reaction of garnet minerals from the orebody that produces fractions of other indicator minerals as by-products in the Kubi mining area. These findings also indicate that primary geochemical dispersion evolving from the crystallization of magma and hydrothermal liquids are the main attributes and constitute the identification of indicator minerals and pathfinding elements in this mineralogical study area.Furthermore, the findings suggest that XRD, XPS, TEM, and EDX could be combined in other mineralogical laboratories to aid in identifying indicator minerals of Au and the location of ore bodies, to increase the knowledge in this field, and reduce environmental and exploration costs.
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| 2. |
- Honnali, Sanath Kumar, 1996-
(författare)
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Energy-efficient physical vapor deposition of transition metal nitride thin films
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on providing insights into energy-efficient ways of growing protective thin films using physical vapor deposition (PVD) by magnetron sputtering, specifically high-power impulse magnetron sputtering (HiPIMS). This technique involves ionizing the material to be deposited to a high degree. The properties of the film for applications such as protective coatings could thus be controlled by modulating the energy and guiding the ions using electric and magnetic fields, respectively. The multiprincipal element TiZrNbTa nitride system is of interest for its corrosion-resistant coating applications. This material system consists of refractory metals that exhibit different ionic charge states with significant mass contrast. Thus, when sputtered with HiPIMS, the properties of the films strongly depend on the mass and energy of the bombarding metal ions. The transport of these ions to the substrate is influenced by the magnetic field distribution in the chamber. To demonstrate the influence of the magnetron arrangement, the deposition is performed without external heating. Two magnetron arrangements were chosen: a tilted closed-field design with four magnetrons and a single magnetron. The films exhibited different properties depending on the magnetron design used. The single magnetron design induces changes in the preferred orientation of the films from 111 to 200 along with film composition and density. A reduction in residual stress was observed with only a ~6 % degradation in the hardness compared to the closed-field design. I also demonstrate epitaxial growth of TiZrNbTaNx films without external heating. The films were grown with a single magnetron design on single crystal sapphire substrate. Applying a pulsed substrate bias with a long pulse width instead of a constant bias, resulted in low argon (~1 at. %) and oxygen (0.5 at. %) content in the films. In addition, the films exhibited a higher optical absorbance in the near-infrared region than the high-temperature grown films. The total energy consumption for film deposition was reduced by approximately 50 % compared to dc magnetron sputtering (DCMS) at 400°C growth temperature. This reduction is without considering the substrate heating and stabilization phase, which is shorter compared to the industry standard where the entire chamber is heated up to ~500-600°C. These findings are beneficial in designing film growth conditions for energy-efficient processes without compromising film quality. This could also address the challenges of growing high-quality films on temperature-sensitive substrates.
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| 3. |
- Xin, Binbin, 1989-
(författare)
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Flexible inorganic and hybrid thermoelectric thin films based on layered calcium cobaltate
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the development of wearable and miniaturized electronics, self-sustaining energy sources have drawn extensive attention. Flexible thermoelectric materials and devices is an approach to convert waste heat into electricity as continuous power supply for such applications.Traditional inorganic thermoelectric materials, such as Bi2Te3, PbTe, and SnTe, exhibit high thermoelectric properties, but their disadvantages of toxicity and oxidation when exposed to high temperature in air, as well as the extreme rarity of tellurium, restrict them from widespread use in applications. Compared to conventional thermoelectric materials, oxides, especially misfit-layered Ca3Co4O9, have advantages as thermoelectric materials, not only has the typical advantages of oxides including low cost and good chemical stability at high temperatures, but they are also based on abundant raw materials and have relatively high thermoelectric properties due to the complex structure which composed of CoO2 conductive layers and rock-salt type Ca2CoO3 insulating layers. Many strategies have been used to enhance the thermoelectric performance of Ca3Co4O9. However, inorganic materials are generally rigid, limiting their use in flexible devices. Fully inorganic flexible thermoelectrics can be obtained through novel fabrication technologies, miniaturization, and structural design. Otherwise, organic/inorganic hybrids materials simultaneously combine the respective features of the good flexibility and low thermal conductivity from conducting polymers, and the electrical transport properties from inorganic materials.In order to explore flexible thermoelectric thin films based on layered calcium cobaltate, I have investigated the Ca3Co4O9 and CaxCoO2 systems. Nanoporous Ca3Co4O9 thin films were synthesized using sequential reactive magnetron sputtering and post annealing. The key factors, Ca(OH)2 content, bilayer thickness, and Ca elemental ratio in multilayers film, for the formation of nanoporous Ca3Co4O9 have been studied and can further tailor the porosity and morphology. Nanoporous Ca3Co4O9 with different porosity and discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles, have been obtained. Based on the nanoporous Ca3Co4O9 thin film, flexible double-layer nanoporous Ca3Co4O9/PEDOT:PSS thin films were synthesized by spin-coating PEDOT:PSS into the nanopores.
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| 4. |
- Gangaprasad Rao, Smita, 1992-
(författare)
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Cantor-Alloy-Based Multicomponent Nitride Thin Films
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this Thesis, I have investigated multicomponent alloy based thin films synthesized by magnetron sputtering. The studies in the thesis are centered around the phase diagram of the CrFeCoNi nitrogen containing system. Theoretical and experimental methods were employed to understand the phase formation in this system which is related to the archetypical Cantor alloy (CrMnFeCoNi). CrFeCoNi thin films of approximately equimolar composition crystallize with fcc structure when grown at room temperature. This structure, however, is not retained when nitrogen (x) is added into the lattice. Density functional theory calculations together with the experimental investigation on the (CrFeCoNi)Nx system revealed the stabilization of the metallic fcc when x ≤ 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films. The importance of the phase diagram is realized when attempting to grow much more complex structures for application-oriented research such as irradiation resistance, corrosion resistance as well as epitaxial films for a fundamental understanding of the material system. The phase diagram of the CrFeCoNi system indicated that higher stability of the single-phase solid solution Cantor nitride lay in a limited temperature range of 200 to 300 °C. In order to compensate for the higher deposition temperature required to grow epitaxial films magnetic field assisted dc magnetron sputtering was used. This technique allows for the control of the flux of Ar ions bombarding the substrate during growth thereby providing the growing film with kinetic energy as opposed to thermal. The results from the study indicated that the quality of epitaxy can be improved by increasing low ion energy, high ion-flux bombardment. The thesis in whole, gives a fundamental understanding of the nitride cantor alloy material system in terms of crystal structure, mechanical and electrical properties.
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| 5. |
- Xin, Binbin, 1989-
(författare)
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Synthesis of nanoporous Ca3Co4O9 thin films for flexible thermoelectrics
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During energy generation, transportation and usage, large amounts of energy are lost as waste heat. With increasing energy consumption and environmental issues, exploiting this waste heat has drawn extensive attention. Thermoelectric energy conversion is an approach to take advantage of the ability of thermoelectric materials to convert waste heat into electricity.The thermoelectric effect was initially studied in the early 19th century with the discovery of the Seebeck effect. Thermoelectric materials and devices can directly convert thermal energy (temperature gradients) into electric energy (voltage) and vice versa. Thermoelectric devices have been used in space as energy generators and as coolers in small-scale instruments and devices. However, thermoelectrics remain limited in terms of applications. The traditional state-of-the-art thermoelectric materials, such as Bi2Te3, PbTe, and SnTe, exhibit high thermoelectric properties, but their disadvantages of toxicity, extreme rarity of tellurium, and oxidation when exposed to high temperature air restrict them from widespread use in applications. Compared to traditional thermoelectric materials, misfit-layered Ca3Co4O9 not only has the typical advantages of oxides including low cost, being environmentally friendly, and good chemical stability at high temperatures, but also has relatively high thermoelectric properties due to the complex structure which composed of CoO2 conductive layers and rock-salt type Ca2CoO3 insulating layers. Many strategies have been used to enhance the thermoelectric performance of Ca3Co4O9. Compared with bulk materials, thermoelectric thin films can exhibit improved thermoelectric properties and new application in flexible devices and miniaturization. Flexibility can be induced in Ca3Co4O9 by nanostructural tailoring to act as fully inorganic flexible thermoelectrics.In order to explore how to produce Ca3Co4O9 nanoporous thin film and control the porosity in the films, I have investigated the nanoporous Ca3Co4O9 system. Nanoporous Ca3Co4O9 thin films were synthesized using sequential reactive magnetron sputtering and post annealing to determine the key factors of nanoporous Ca3Co4O9 formation and tailoring of the porosity.
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| 6. |
- Alijan Farzad Lahiji, Faezeh
(författare)
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Epitaxy of oxide and nitride thin films grown by magnetron sputtering
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The need for electronic devices with new functionalities has caused research to move in a way to design and utilize materials with high-performance thermoelectricity, widely used in batteries, sensors, and electronic devices. Two-dimensional materials (2D) with unique structures and remarkable properties have been identified to fabricate oxide heteroepitaxy. The growth of heteroepitaxy has been focused on the growth of high-quality films on single crystalline substrates. The preferred orientation and the crystallization of the materials with thin or two-dimensional structures require an understanding of epitaxy. In epitaxial growth, using a specific, well- defined substrate with lattice constants close to that film is decisive in controlling the film orientation with high epitaxial quality. The electrical, optical, magnetic, and structural properties of the film are strongly determined by the texture and its epitaxial alignment. The majority of studies report epitaxial growth on Si and sapphire with different crystallographic orientations. The family of NaCl-structured materials covers a variety of nitrides and oxides broadly used in science and technology that have been epitaxially grown on monocrystalline Si and sapphire (Al2O3). In this thesis, the structure and optical properties of NiO are investigated as functions of oxygen content on Si(100) and c-Al2O3 using pulsed dc reactive magnetron sputtering. It is found that NiO with cubic structure is a single phase with predominant orientation along (111) on both substrates. It is fiber textured on Si(100), while twin domain epitaxy is achieved on c-Al2O3. The growth of two cases of metal oxide and nitride films (NiO and CrN) with rock-salt (NaCl) structure is also demonstrated on r-plane sapphire. It is revealed that the NaCl-structured materials NiO and CrN grow with a tilted orientation relative to the substrate. This characterization and analysis of the epitaxy, crystallography, and growth modes yield a single and identical epitaxial relationship of these two cubic materials on r-plane sapphire, in contrast to earlier studies on NaCl-structured materials on r-plane sapphire, indicating several different orientation relationships. The results advance the understanding of growth modes and unusual epitaxial relationships of two cases of metal oxide and nitride films with rock-salt (NaCl) structure broadly used in science and technology on r-plane sapphire.
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| 7. |
- Gangaprasad Rao, Smita, 1992-
(författare)
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Phase formation in multicomponent films based on 3d transition metals
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The need for materials that enhance life span, performance, and sustainability has propelled research in alloy design from binary alloys to more complex systems such as multicomponent alloys. The CoCrFeMnNi alloy, more commonly known as the Cantor alloy, is one of the most studied systems in bulk as well as thin film. The addition of light elements such as boron, carbon, nitrogen, and oxygen is a means to alter the properties of these materials. The challenge lies in understanding the process of phase formation and microstructure evolution on addition of these light elements. To address this challenge, I investigate multicomponent alloys based on a simplified version of the Cantor alloy.My thesis investigates the addition of nitrogen into a Cantor variant system as a step towards understanding the full Cantor alloy. Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤0.28 thin films were grown by reactive magnetron sputtering. The films showed a change in structure from fcc to mixed fcc+bcc and finally a bcc-dominant film with increasing nitrogen content. The change in phase and microstructure influenced the mechanical and electrical properties of the films. A maximum hardness of 11 ± 0.7 GPa and lowest electrical resistivity of 28 ± 5 μΩcm were recorded in the film with mixed phase (fcc+bcc) crystal structure.Copper was added as a fourth metallic alloying element into the film with the mixed fcc + bcc structure, resulting in stabilization of the bcc phase even though Cu has been reported to be a fcc stabilizer. The energy brought to the substrate increases on Cu addition which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness recorded by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.05. While it is generally believed that large amounts of Cu can be detrimental to thin film properties due to segregation, this study shows that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as for mechanical properties.This thesis thus provides insights into the phase formation of nitrogen-containing multicomponent alloys.
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| 8. |
- Shu, Rui, 1990-
(författare)
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Nonstoichiometric Multicomponent Nitride Thin Films
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High entropy ceramics have rapidly developed as a class of materials based on high entropy alloys; the latter being materials that contain five or more elements in near-equal proportions. Their unconventional compositions and chemical structures hold promise for achieving unprecedented combinations of mechanical, electrical and chemical properties. In this thesis, high entropy ceramic films, (TiNbZrTa)Nx were deposited using reactive magnetron sputtering with segmented targets. The stoichiometry x was tuned with two deposition parameters, i.e., substrate temperature and nitrogen flow ratio fN, their effect on microstructure and mechanical, electric, and electrochemical properties were investigated.Understoichiometric MeNx (Me = TiNbZrTa, 0.25 ≤ x ≤ 0.59) films were synthesized at a constant fN when substrate temperature was varied from room temperature (RT) to 700 °C. For low-temperature deposition, the coatings exhibited fcc solid-solution polycrystalline structures. A NaCl-type structure with (001) preferred orientation was observed in MeN0.46 coating deposited at 400 ºC, while an hcp structure was found for the coatings deposited above 500 ºC. The maximum hardness value of 26 GPa as well as the highest and values (0.12 and 0.34 GPa) were obtained for the MeN0.46 coating. These films exhibited low RT electrical resistivities. In 0.1 M H2SO4 aqueous solution, the most corrosion resistant film was MeN0.46 featured dense structure and low roughness.The MeNx films (x=0, 0.57 < x ≤ 0.83) were deposited with different fN. The maximum hardness was achieved at 22.1 GPa for MeN0.83 film. Their resistivities increased from 95 to 424 μΩcm with increasing nitrogen content. The corrosion resistance is related to the amount of nitrogen in the films. The corrosion current density was around 10-8 A/cm2, while the films with lower nitrogen contents (x < 0.60) exhibited a nearly stable current plateau up to 4.0 V, similar to the metallic films, while the films with a higher nitrogen content only featured a plateau up to 2.0 V, above which a higher nitrogen content resulted in higher currents. The reason was that the oxidation of these films at potentials above about 2.0 V vs. Ag/AgCl resulted in the formation of porous oxide layers as significant fraction of the generated N2 was lost to the electrolyte.Hence, these observed effects of deposition temperature and nitrogen content on the overall properties of nonstoichiometric MeNx films provide insights regarding protective multicomponent nitride films, e.g. as corrosion resistant coatings on metallic bipolar plates in fuel cells or batteries.
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| 9. |
- Shu, Rui, 1990-
(författare)
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Refractory High-entropy Alloy and Nitride Thin Films
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on understanding the process-structure-property relation-ships for several refractory-metal-based high-entropy alloys and nitrides synthesized by magnetron sputtering. The thesis begins with the growth of TiZrNbTaNx understoichiometric nitrides by controlling substrate temperature and nitrogen flow ratio fN. Their effects on microstructure and mechanical, electric, and electrochemical properties were investigated. TiZrNbTaN0.46 deposited at 400 ºC shows a NaCl-type structure with (001) preferred orientation and exhibits the highest corrosion resistance in 0.1 M H2SO4 aqueous solution. A stable passive region up to 3.0 V vs. Ag/AgCl could be achieved when x< 0.64. The densification effects were explored by ion energy for (TiZrTaMe)N1–x (Me = Hf, Nb, Mo, or Cr) films and by high-power impulse magnetron sputtering technique for TiNbCrAl films.The local chemical distortions in the TiZrTaNb-based system with different nitrogen content were investigated by X-ray absorption fine structure spectros-copy. The influence of crystallinity on superconducting transition behavior was studied in (TiZrHf)x(TaNb)1-x, (TiZrNbTa)1-xWx, and (TiZrNbTa)1-xVx systems. The highest superconducting transition temperature (Tc) reaches 8.05 K for the TiZrNbTa film (x=0). The superconducting transition temperature Tc of these films deposited at the fixed temperature decreases monotonically as a function of x, and Tc can be increased by elevating the deposition temperature. Furthermore, the structural stability and elemental segregation under Xe-ion irradiation of TiZrNbTaV-based HEA and HEN films, and high-entropy TiZrN-bTa/CrFeCoNi metallic and nitride multilayer coatings were investigated. The microstructure of TiZrNbTaVN film remain stable after irradiation at room temperature and 500 °C. The as-deposited TiZrNbTaV film exhibited an amorphous structure and became a bcc phase structure after irradiation at 500 °C. Thermal-induced and irradiation-induced grain growth resulted in a grain-size distribution. For the multilayer coatings, the microstructure of metallic multilayers was not stable and the interdiffusion or mixing of the constituent elements is prominent under ion irradiation and/or heat treatment. no diffusion and phase trans-formation were observed for the nitride multilayers after irradiation at 500 °C.
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| 10. |
- Ekström, Erik, 1989-, et al.
(författare)
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Epitaxial Growth of CaMnO3-y Films on LaAlO3 (112 over bar 0) by Pulsed Direct Current Reactive Magnetron Sputtering
- 2022
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Ingår i: Physica Status Solidi. Rapid Research Letters. - : Wiley-V C H Verlag GMBH. - 1862-6254 .- 1862-6270. ; 16:4
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Tidskriftsartikel (refereegranskat)abstract
- CaMnO3 is a perovskite with attractive magnetic and thermoelectric properties. CaMnO3 films are usually grown by pulsed laser deposition or radio frequency magnetron sputtering from ceramic targets. Herein, epitaxial growth of CaMnO3-y (002) films on a (112 over bar 0)-oriented LaAlO3 substrate using pulsed direct current reactive magnetron sputtering is demonstrated, which is more suitable for industrial scale depositions. The CaMnO3-y shows growth with a small in-plane tilt of <approximate to 0.2 degrees toward the (200) plane of CaMnO3-y and the (1 over bar 104) with respect to the LaAlO3 (112 over bar 0) substrate. X-ray photoelectron spectroscopy of the electronic core levels shows an oxygen deficiency described by CaMnO2.58 that yields a lower Seebeck coefficient and a higher electrical resistivity when compared to stoichiometric CaMnO3. The LaAlO3 (112 over bar 0) substrate promotes tensile-strained growth of single crystals. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal antiphase boundaries composed of Ca on Mn sites along and , forming stacking faults.
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