| 1. |
- Nzulu, Gabriel Kofi, 1974-
(author)
-
Characterization of Pathfinders and Indicators of Gold
- 2023
-
Doctoral thesis (other academic/artistic)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-
(author)
-
Energy-efficient physical vapor deposition of transition metal nitride thin films
- 2024
-
Licentiate thesis (other academic/artistic)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-
(author)
-
Flexible inorganic and hybrid thermoelectric thin films based on layered calcium cobaltate
- 2022
-
Doctoral thesis (other academic/artistic)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-
(author)
-
Cantor-Alloy-Based Multicomponent Nitride Thin Films
- 2023
-
Doctoral thesis (other academic/artistic)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-
(author)
-
Synthesis of nanoporous Ca3Co4O9 thin films for flexible thermoelectrics
- 2020
-
Licentiate thesis (other academic/artistic)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
(author)
-
Epitaxy of oxide and nitride thin films grown by magnetron sputtering
- 2023
-
Licentiate thesis (other academic/artistic)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-
(author)
-
Phase formation in multicomponent films based on 3d transition metals
- 2021
-
Licentiate thesis (other academic/artistic)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-
(author)
-
Nonstoichiometric Multicomponent Nitride Thin Films
- 2020
-
Licentiate thesis (other academic/artistic)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-
(author)
-
Refractory High-entropy Alloy and Nitride Thin Films
- 2022
-
Doctoral thesis (other academic/artistic)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.
(author)
-
Epitaxial Growth of CaMnO3-y Films on LaAlO3 (112 over bar 0) by Pulsed Direct Current Reactive Magnetron Sputtering
- 2022
-
In: Physica Status Solidi. Rapid Research Letters. - : Wiley-V C H Verlag GMBH. - 1862-6254 .- 1862-6270. ; 16:4
-
Journal article (peer-reviewed)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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| 11. |
- Klarbring, Johan, 1989-
(author)
-
A First-Principles Study of Highly Anharmonic and Dynamically Disordered Solids
- 2020
-
Doctoral thesis (other academic/artistic)abstract
- This thesis is a first-principles theoretical investigation of solid materials with high degrees of anharmonicity. These are systems where the dynamics of the constituent atoms is too complex to be well-described by a set of uncoupled harmonic oscillators. While theoretical studies of such systems pose a significant challenge, they are under increasing demand due to the prevalence of these sytems in next-generation technological applications. Indeed, very anharmonic systems are ubiquitous in envisioned materials for future solid-state batteries and fuel-cells, thermoelectrics and optoelectronics. In some of these cases, the anharmonicity is a “side-effect” that simply has to be dealt with in order to accurately model certain properties, while in other cases the anharmonicity is the origin of the high-performance of the material.There are two main parts to the thesis: The first is on materials with perovskite-related structures. This is a very large class of materials, members of which are typically highly anharmonic, not least in relation to a series of complex phase transformations between different structural modifications. In the thesis, I have studied a specific class of such phase-transformations that relate to tilting of the framework of octahedra that make up the structure. The oxide CaMnO3 and a set of inorganic halide perovskites were taken as model systems. The results shed some light on the experimentally observed differences between the local and average atomic structure in these systems. I have further studied Cs2AgBiBr6, a member of the so-called lead-free halide double perovskites. I rationalized its temperature induced phase transformation and found high degrees of anharmonicity and ultra-low thermal conductivity. Finally, I studied the influence of nuclear quantum effects, which are often ignored in computational modelling, on the structure and bonding in the hybrid organic-inorganic lead-halide perovskite, CH3NH3PbI3.The second part of the thesis deals with theoretical studies of the phase stability of dynamically disordered solids. These are solids which have some sort of time-averaged long-range order, characteristic of a crystalline solid, but where the anharmonicity is so strong that the basic concept of an equilibrium atomic position cannot be statically assigned to all atoms. Examples include certain solids with very fast ionic conduction, so called superionics, and solids with rotating molecular units. This absence of equilibrium atomic positions makes many standard computational techniques to evaluate phase-stability inapplicable. I outline a method to deal with this issue, which is based on a stress-strain thermodynamic integration on a deformation path from an ordered variant to the dynamically disordered phase itself. I apply the method to study the phase stability of the high-temperature δ-phase of Bi2O3, which is the fastest know solid oxide ion conductor, and to Li2C2, whose high temperature cubic phase contains rotating C2 dimers.The thesis exemplifies the need to go beyond many of the standard approximations used in first-principles computational materials science if accurate theoretical predictions are to be made. This is true, in particular, for many emerging material classes in the field of energy materials.
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| 12. |
- Magnusson, Roger, 1977-, et al.
(author)
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Preparation and tunable optical properties of amorphous AlSiO thin films
- 2021
-
In: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 187
-
Journal article (peer-reviewed)abstract
- Thin films in the aluminosilicate (AlSiO) system containing up to 31 at. % Al and 23 at. % Si were prepared by reactive RF magnetron co-sputtering in order to investigate the dependence of film formation and optical properties on substrate temperature and Si and Al contents. The obtained films were amorphous with smooth microstructure. The growth rate at different substrate temperatures ranged from 1.2 to 3.3 nm/min and increase with increasing the Si target power. The roughness decreases and thickness increases with increasing Si content. The thickness of the films grown at a deposition temperature of 100 °C is found to be higher than the films deposited at 300 and 500 °C. The AlSiO-coated glasses have a higher transmission in the visible region than the uncoated glass. The spectroscopic ellipsometry analysis reveals that the refractive index value decreased with decreasing the Al content, having extinction coefficient values of zero in the measured spectral region and band gap values ≥ 3.4 eV. The obtained thin films have over 90% transmittance in the visible range and no systematic variation of transmittance was observed with substrate temperature. The results suggest that glass substrate coated with AlSiO thin films have improved optical properties.
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| 13. |
- Nzulu, Gabriel Kofi, 1974-, et al.
(author)
-
Chemical Reactivity and Alteration of Pyrite Mineral in the Kubi Gold Concession in Ghana
- 2024
-
In: Mining, Metallurgy & Exploration. - : Springer. - 2524-3462 .- 2524-3470.
-
Journal article (peer-reviewed)abstract
- Pyrite is the most common among the group of sulfide minerals in the Earth and abundant in most geological settings. This gangue mineral in association with garnet, hematite, magnetite, and other sulfide minerals acts as an indicator mineral in the Kubi concession of the Asante Gold corporation in Ghana. X-ray diffraction (XRD), air annealing in a furnace, energy-dispersive x-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) were applied to investigate the crystal structure, identify individual elements, permanence, transformation, and chemical/electronic properties of such pyrite. The study aims to identify individual elements and to gain an understanding of the surface reaction mechanisms, as well as the properties of precipitated pyrite particles observed during the hydrothermal formation of the ore deposit. XRD shows that pristine and annealed samples contain some hematite and quartz besides pyrite. Results from air annealing indicate that the relationship between pyrite and hematite-magnetite is controlled by temperature. EDX reveals that the sample has O and C as contaminants, while XPS in addition reveals Ba, Au, P, Al, and N. These elements are attributed to pyrite that bonds metallically or covalently to neighboring ligands/impurity minerals such as oxides, chalcogenide sulfides, as well as the gangue alteration minerals of magnetite and hematite in the pyrite sample.These findings suggest that during the hydrothermal flow regime, pyrite, pathfinder elements, and impurity minerals/metals were in contact with quartz minerals before undergoing hematite transformation, which thus becomes an indicator mineral in the Kubi gold concession.
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| 14. |
- Shu, Rui, et al.
(author)
-
Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg3Sb2-Based Thermoelectric Materials
- 2022
-
In: Advanced Science. - : Wiley. - 2198-3844. ; 9:25
-
Journal article (peer-reviewed)abstract
- Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg3Sb1.5Bi0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.
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| 15. |
- Burcea, Razvan, et al.
(author)
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Influence of Generated Defects by Ar Implantation on the Thermoelectric Properties of ScN
- 2022
-
In: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:9, s. 11025-11033
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Journal article (peer-reviewed)abstract
- Nowadays, making thermoelectric materials more efficient in energy conversion is still a challenge. In this work, to reduce the thermal conductivity and thus improve the overall thermoelectric performances, point and extended defects were generated in epitaxial 111-ScN thin films by implantation using argon ions. The films were investigated by structural, optical, electrical, and thermoelectric characterization methods. The results demonstrated that argon implantation leads to the formation of stable defects (up to 750 K operating temperature). These were identified as interstitial-type defect dusters and argon vacancy complexes. The insertion of these specific defects induces acceptor-type deep levels in the band gap, yielding a reduction in the free-carrier mobility. With a reduced electrical conductivity, the irradiated sample exhibited a higher Seebeck coefficient while maintaining the power factor of the film. The thermal conductivity is strongly reduced from 12 to 3 W.m(-1). K-1 at 300 K, showing the influence of defects in increasing phonon scattering. Subsequent high-temperature annealing at 1573 K leads to the progressive evolution of these defects: the initial dusters of interstitials evolved to the benefit of smaller dusters and the formation of bubbles. Thus, the number of free carriers, the resistivity, and the Seebeck coefficient are almost restored but the mobility of the carriers remains low and a 30% drop in thermal conductivity is still effective (k(total) similar to 8.5 Wm(-1).K-1). This study shows that control defect engineering with defects introduced by irradiation using noble gases in a thermoelectric coating can be an attractive method to enhance the figure of merit of thermoelectric materials.
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| 16. |
- Chowdhury, Susmita, et al.
(author)
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Thermoelectric properties and electronic structure of Cr(Mo,V)Nx thin films studied by synchrotron and lab-based x-ray spectroscopy
- 2023
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 108:20
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Journal article (peer-reviewed)abstract
- Chromium-based nitrides are used in hard, resilient coatings and show promise for thermoelectric applications due to their combination of structural, thermal, and electronic properties. Here, we investigate the electronic structures and chemical bonding correlated to the thermoelectric properties of epitaxially grown chromium-based multicomponent nitride Cr(Mo,V)Nx thin films. The small amount of N vacancies causes Cr 3d and N 2p states to appear at the Fermi level and reduces the band gap in Cr0.51N0.49. Incorporating holes by alloying of V in N-deficient CrN results in an enhanced thermoelectric power factor with marginal change in the charge transfer of Cr to N compared with Cr0.51N0.49. Further alloying of Mo, isoelectronic to Cr, increases the density of states at the Fermi level due to hybridization of the (Cr, V) 3d and Mo 4d-N 2p states in Cr(Mo,V)Nx. This hybridization and N off-stoichiometry result in more metal-like electrical resistivity and reduction in Seebeck coefficient. The N deficiency in Cr(Mo,V)Nx also depicts a critical role in reduction of the charge transfer from metal to N site compared with Cr0.51N0.49 and Cr0.50V0.03N0.47. In this paper, we envisage ways for enhancing thermoelectric properties through electronic band engineering by alloying and competing effects of N vacancies.
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| 17. |
- Gangaprasad Rao, Smita, 1992-, et al.
(author)
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Plasma diagnostics and film growth of multicomponent nitride thin films with magnetic-field-assisted-dc magnetron sputtering
- 2022
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In: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 204
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Journal article (peer-reviewed)abstract
- During direct current magnetron sputtering (dcMS) of thin films, the ion energy and flux are complex parameters that influence thin film growth and can be exploited to tailor their properties. The ion energy is generally controlled by the bias voltage applied at the substrate. The ion flux density however is controlled by more complex mechanisms. In this study, we look into magnetic-field-assisted dcMs, where a magnetic field applied in the deposition chamber by use of a solenoid coil at the substrate position, influences the energetic bombardment by Ar ions during deposition. Using this technique, CrFeCoNi multicomponent nitride thin films were grown on Si(100) substrates by varying the bias voltage and magnetic field systematically. Plasma diagnostics were performed by a Langmuir wire probe and a flat probe. On interpreting the data from the current-voltage curves it was confirmed that the ion flux at the substrate increased with increasing coil magnetic field with ion energies corresponding to the applied bias. The increased ion flux assisted by the magnetic field produced by the solenoid coil aids in the stabilization of NaCl B1 crystal structure without introducing Ar ion implantation.
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| 18. |
- Gharavi, Mohammad Amin
(author)
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Nitride Thin Films for Thermoelectric Applications : Synthesis, Characterization and Theoretical Predictions
- 2017
-
Licentiate thesis (other academic/artistic)abstract
- Thermoelectrics is the reversible process which transforms a temperature gradient across a material into an external voltage through a phenomenon known as the Seebeck effect. This has resulted in niche applications such as solid-state cooling for electronic and optoelectronic devices which exclude the need for a coolant or any moving parts and long-lasting, maintenance-free radioisotope thermoelectric generators used for deep-space exploration. However, the high price and low efficiency of thermoelectric generators have prompted scientists to search for new materials and/or methods to improve the efficiency of the already existing ones. Thermoelectric efficiency is governed by the dimensionless figure of merit ??, which depends on the electrical conductivity, thermal conductivity and Seebeck coefficient value of the material and has rarely surpassed unity.In order to address these issues, research conducted on early transition metal nitrides spearheaded by cubic scandium nitride (ScN) thin films showed promising results with high power factors close to 3000 μWm−1K−2 at 500 °C. In this thesis, rock-salt cubic chromium nitride (CrN) deposited in the form of thin films by reactive magnetron sputtering was chosen for its large Seebeck coefficient of approximately -200 μV/K and low thermal conductivity between 2 and 4 Wm−1K−1. The results show that CrN in single crystal form has a low electrical resistivity below 1 mΩcm, a Seebeck coefficient value of -230 μV/K and a power factor close to 5000 μWm−1K−2 at room temperature. These promising results could lead to CrN based thermoelectric modules which are cheaper and more stable compared to traditional thermoelectric material such as bismuth telluride (Bi2Te3) and lead telluride (PbTe).In addition, the project resulting this thesis was prompted to investigate prospective ternary nitrides equivalent to ScN with (hopefully) better thermoelectric properties. Scandium nitride has a relatively high thermal conductivity value (close to 10 Wm−1K−1), resulting in a low ??. A hypothetical ternary equivalent to ScN may have a similar electronic band structure and large power factor, but with a lower thermal conductivity value leading to better thermoelectric properties. Thus the elements magnesium, titanium, zirconium and hafnium were chosen for this purpose. DFT calculations were used to simulate TiMgN2, ZrMgN2 and HfMgN2. The results show the MeMgN2 stoichiometry to be stable, with two rivaling crystal structures: trigonal NaCrS2 and monoclinic LiUN2.
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| 19. |
- Linder, Clara, et al.
(author)
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Corrosion Resistance and Catalytic Activity toward the Oxygen Reduction Reaction of CoCrFexNi (0 ≤ x ≤ 0.7) Thin Films
- 2022
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In: ACS Applied Energy Materials. - : American Chemical Society. - 2574-0962. ; 5:9, s. 10838-10848
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Journal article (peer-reviewed)abstract
- Corrosion resistance and catalytic activity toward the oxygen reduction reaction (ORR) in an alkaline environment are two key properties for water recombination applications. In this work, CoCrFexNi (0 ≤ x ≤ 0.7) thin films were deposited by magnetron sputtering on polished steel substrates. The native passive layer was 2-4 nm thick and coherent to the columnar grains determined by transmission electron microscopy. The effect of Fe on the corrosion properties in 0.1 M NaCl and 1 M KOH and the catalytic activity of the films toward ORR were investigated. Electrochemical impedance spectroscopy and potentiodynamic polarization measurements indicate that CoCrFe0.7Ni and CoCrFe0.3Ni have the highest corrosion resistance of the studied films in NaCl and KOH, respectively. The high corrosion resistance of the CoCrFe0.7Ni film in NaCl was attributed to the smaller overall grain size, which leads to a more homogeneous film with a stronger passive layer. For CoCrFe0.3Ni in KOH, it was attributed to a lower Fe dissolution into the electrolyte and the build-up of a thick and protective hydroxide layer. Scanning Kelvin probe force microscopy showed no potential differences globally in any of the films, but locally, a potential gradient between the top of the columns and grain boundaries was observed. Corrosion of the films was likely initiated at the top of the columns where the potential was lowest. It was concluded that Fe is essential for the electrochemical activation of the surfaces and the catalytic activity toward ORR in an alkaline medium. The highest catalytic activity was recorded for high Fe-content films (x ≥ 0.5) and was attributed to the formation of platelet-like oxide particles on the film surface upon anodization. The study showed that the combination of corrosion resistance and catalytic activity toward ORR is possible for CoCrFexNi, making this material system a suitable candidate for water recombination in an alkaline environment.
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| 20. |
- Naumovska, Elena, 1995, et al.
(author)
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Local structure of hydrated nanocrystalline films of the proton conductor BaZr1-xScxO3-x/2 studied by infrared spectroscopy
- 2024
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In: Vibrational Spectroscopy. - : Elsevier. - 0924-2031 .- 1873-3697. ; 130
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Journal article (peer-reviewed)abstract
- We report results from a study of the local structure of hydrated nanocrystalline 2 m films of the well known proton conductor BaZr1-xScxO3-x/2 with x = 0.45, 0.54 and 0.64, using infrared (IR) spectroscopy. The films were prepared by magnetron sputtering. Analysis of the IR spectra focused on the O–H stretching region (2000—3700 cm-1), which reveals the presence of several distinct O–H stretching bands for which the intensity and frequency of each band vary in an unsystematic manner with Sc concentration. The spectra for the two higher Sc concentrations, x = 0.54 and 0.64, exhibit a distinct, highly intense O–H stretching band centered at around 3400–3500 cm-1, which is assigned to relatively symmetric, weakly hydrogen-bonding, proton configurations. The spectrum for the lower Sc concentration, x = 0.45, does not feature such a band but a broader, weaker, O–H stretching band between approximately 2500 and 3700 cm-1, suggesting that the protons are more homogeneously distributed over a range of different local proton coordinations in this relatively weakly doped material. A comparison to the IR spectra of powder samples of similar compositions suggests that for x = 0.45, the spectra and proton coordination of films and powder samples are similar, whereas for x = 0.54 and 0.64, a larger fraction of protons seems to be located in weakly hydrogen-bonding proton configurations in the films compared to the respective powder samples.
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| 21. |
- Nayak, Sanjay Kumar, et al.
(author)
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First-principles study on the superconductivity of doped zirconium diborides
- 2022
-
In: Physical Review Materials. - College Park, MD, United States : American Physical Society. - 2475-9953. ; 6:4
-
Journal article (peer-reviewed)abstract
- Recent experiments [Barbero et al. Phys. Rev. B 95, 094505 (2017)] have established that bulk superconductivity (Tc ∼ 8.3-8.7 K) can be induced in AlB2-type ZrB2 and HfB2, highly covalent refractory ceramics, by vanadium (V) doping. These AlB2-structured phases provide an alternative to earlier diamon-like or diamond-based superconducting and superhard materials. However, the underlying mechanism for doping-induced superconductivity in these materials is yet to be addressed. In this paper, we have used first-principles calculations to probe electronic structure, lattice dynamics, and electron-phonon coupling (EPC) in V-doped ZrB2 and consequently examine the origin of the superconductivity. We find that, while doping-induced stress weakens the EPC, the concurrently induced charges strengthen it. The calculated critical transition temperature (Tc) in electron (and V)-doped ZrB2 is at least one order of magnitude lower than experiments, despite considering the weakest possible Coulomb repulsion between electrons in the Cooper pair, hinting a complex origin of superconductivity in it.
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| 22. |
- Nzulu, Gabriel Kofi, 1974-, et al.
(author)
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Growth and thermal stability of Sc-doped BaZrO 3 thin films deposited on single crystal substrates
- 2023
-
In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 772
-
Journal article (peer-reviewed)abstract
- Thin films of BaZr1-xScxO3-x/2, (0 ≤ x ≤ 0.64), well known as proton conducting solid electrolytes for intermediate temperature solid oxide fuel cell, were deposited by magnetron sputtering. X-ray diffraction analysis of the as deposited films reveals the presence of single-phase perovskite structure. The films were deposited on four different substrates (c-Al2O3, LaAlO3〈100〉, LaAlO3〈110〉, LaAlO3〈111〉) yielding random, (110)- or (100)-oriented films. The stability of the as-deposited films was assessed by annealing in air at 600 °C for 2 h. The annealing treatment revealed instabilities of the perovskite structure for the (110) and randomly oriented films, but not for (100) oriented film. The instability of the coating under heat treatment was attributed to the low oxygen content in the film (understoichiometry) prior annealing combined with the surface energy and atomic layers stacking along the growth direction. An understoichiometric (100) oriented perovskite films showed higher stability of the structure under an annealing in air at 600 °C.
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| 23. |
- Nzulu, Gabriel Kofi, 1974-, et al.
(author)
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Growth and thermal stability of Sc-doped BaZrO3 thin films deposited on single crystal substrates
- 2023
-
In: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 772
-
Journal article (peer-reviewed)abstract
- Thin films of BaZr1-xScxO3-x/2, (0 ≤ x ≤ 0.64), well known as proton conducting solid electrolytes for intermediatetemperature solid oxide fuel cell, were deposited by magnetron sputtering. X-ray diffraction analysis of theas deposited films reveals the presence of single-phase perovskite structure. The films were deposited on fourdifferent substrates (c-Al2O3, LaAlO3〈100〉, LaAlO3〈110〉, LaAlO3〈111〉) yielding random, (110)- or (100)-orientedfilms. The stability of the as-deposited films was assessed by annealing in air at 600 ◦C for 2 h. Theannealing treatment revealed instabilities of the perovskite structure for the (110) and randomly oriented films,but not for (100) oriented film. The instability of the coating under heat treatment was attributed to the lowoxygen content in the film (understoichiometry) prior annealing combined with the surface energy and atomiclayers stacking along the growth direction. An understoichiometric (100) oriented perovskite films showedhigher stability of the structure under an annealing in air at 600 ◦C.
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| 24. |
- Shu, Rui, 1990-, et al.
(author)
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Stoichiometry Effects on the Chemical Ordering and Superconducting Properties in TiZrTaNbNx Refractory High Entropy Nitrides
- 2023
-
In: Annalen der Physik. - : Wiley-VCH Verlagsgesellschaft. - 0003-3804 .- 1521-3889.
-
Journal article (peer-reviewed)abstract
- High-entropy materials, an exciting new class of structural materials involvingfive or more elements, are emerging as unexplored ground forsuperconductors. Here, the effects of nitrogen stoichiometry are investigatedon local chemical structure of TiZrNbTa-based thin films by variousX-ray-based techniques. Lattice distortion and short-range order of a set ofTiZrNbTaNxsamples, including bond lengths of different atomic pairs andcoordination numbers of substituting atoms are quantitatively studied. Themaximum superconducting transition temperature Tcis found at 10 K for anear-stoichiometric (TiZrNbTa)N1.08film, which is>8 K measured for ametallic TiZrNbTa film. The underlying electronic structure and chemicalbonding in these high entropy nitrides thus influence the superconductingmacroscopic properties.
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| 25. |
- Xin, Binbin, 1989-, et al.
(author)
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Synthesis of textured discontinuous-nanoisland Ca3Co4O9 thin films
- 2022
-
In: Nanoscale Advances. - : Royal Society of Chemistry. - 2516-0230. ; 4, s. 3318-3322
-
Journal article (peer-reviewed)abstract
- Controllable engineering of the nanoporosity in layered Ca3Co4O9 remains a challenge. Here, we show the synthesis of discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles with controlled porosity and morphology. These discontinuously dispersed textured Ca3Co4O9 nanoparticles may be a candidate for hybrid thermoelectrics.
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