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Colorado School of Mines
1.
Crouse, Dustin Ray.
Controlled spalling in (100)-oriented germanium by electroplating.
Degree: MS(M.S.), Metallurgical and Materials Engineering, 2017, Colorado School of Mines
URL: http://hdl.handle.net/11124/171015 
► This work investigates controlled spalling as a method to exfoliate thin films of various thickness from rigid, crystalline germanium (Ge) substrates and to enable substrate…
(more)
▼ This work investigates controlled spalling as a method to exfoliate thin films of various thickness from rigid, crystalline germanium (Ge) substrates and to enable substrate reuse for III-V single junction photovoltaic devices. Technological limitations impeding wide-spread adoption of flexible electronics and high-material-cost photovoltaic devices have motivated significant interest in a method to remove devices from their substrates. DC magnetron sputtering has been previously utilized to remove semiconductor devices of various thicknesses from Ge substrates, but this method is expensive and time-consuming. Controlled spalling via high-speed electrodeposition is a fast, inexpensive exfoliation method that utilizes a tensile-stressed metal layer deposited on a (100)-oriented Ge substrate and an external force to mechanically propagate a crack parallel to the surface at a desired depth in the substrate material. Suo and Hutchinson’s quantitative models describe critical combinations of film thickness and strain mismatch between a film and substrate at which a stressed bilayer system spontaneously spalls; however, fine control over a wide steady-state spall depth range has been limited by the ability to experimentally tailor strain mismatch caused by residual stress within deposited stressor layers. This work investigates the effect of tuning electroplating current density and electrolyte chemistry on the residual stress in a nickel stressor film and their impact on the achievable spall depth range. Steady-state spall depth is found to increase with increasing stressor layer thickness and decrease with increasing residual stress. By tailoring residual stress through adjusting plating conditions and the electrolyte’s phosphorous concentration, wide control over spall depth within Ge substrates from sub-micron to ~76µm-thicknesses were achieved. To assess the viability of utilizing controlled spalling for substrate reuse, this dissertation demonstrates the first III-V solar cells (GaInAsP, Eg~1.7 eV) grown directly on a spalled-Ge substrate without any additional surface preparation. Widespread adoption of high-efficiency III-V solar cells has been limited by expensive deposition processes and high material cost of substrates. Substrate reuse offers a promising route towards enabling III-V devices to become cost-competitive for one-sun terrestrial applications. In this study, the quality of spalled Ge surfaces is characterized to assess lattice matching capability between the device layer materials and the substrate. GaAs films grown on spalled Ge substrates by hydride vapor phase epitaxy were single-crystal in nature. III-V solar cells grown on spalled and pristine Ge substrates show nearly equivalent efficiency of ~8%, despite the roughness of the spalled-Ge substrate. Principles of fractography were used to deduce that surface roughness originated from non-uniform crack propagation and mixed-mode loading during the spalling process.
Advisors/Committee Members: Packard, Corinne E. (advisor), Brennecka, Geoffrey (committee member), Reimanis, Ivar E. (Ivar Edmund) (committee member).
Subjects/Keywords: Fracture; Solar cells; Substrate reuse; III-V; Exfoliation; Spalling
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APA (6th Edition):
Crouse, D. R. (2017). Controlled spalling in (100)-oriented germanium by electroplating. (Masters Thesis). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/171015
Chicago Manual of Style (16th Edition):
Crouse, Dustin Ray. “Controlled spalling in (100)-oriented germanium by electroplating.” 2017. Masters Thesis, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/171015.
MLA Handbook (7th Edition):
Crouse, Dustin Ray. “Controlled spalling in (100)-oriented germanium by electroplating.” 2017. Web. 17 Feb 2019.
Vancouver:
Crouse DR. Controlled spalling in (100)-oriented germanium by electroplating. [Internet] [Masters thesis]. Colorado School of Mines; 2017. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/171015.
Council of Science Editors:
Crouse DR. Controlled spalling in (100)-oriented germanium by electroplating. [Masters Thesis]. Colorado School of Mines; 2017. Available from: http://hdl.handle.net/11124/171015
Colorado School of Mines
2.
Musselman, Matthew Anthony.
In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄.
Degree: MS(M.S.), Metallurgical and Materials Engineering, 2017, Colorado School of Mines
URL: http://hdl.handle.net/11124/170999
► Xenotime DyPO4 and GdxDy(1-x)PO4 (x = 0.4, 0.5, 0.6) (tetragonal I41/amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond…
(more)
▼ Xenotime DyPO4 and GdxDy(1-x)PO4 (x = 0.4, 0.5, 0.6) (tetragonal I41/amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The Raman active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicate a phase transformation to a lower symmetry structure, likely monoclinic. The onset phase transformation pressure of DyPO4 was 15.3 GPa. The transformation pressures of the solid solutions (GdxDy(1-x)PO4) were 12.2 GPa (40/60, x=0.4), 10.1 GPa (50/50, x=0.5), and 11.9 GPa (60/40, x=0.6). The addition of Gd resulted in a decrease in transformation pressure from pure DyPO4. A line crossing as a result of Raman mode softening is observed at approximately 7-8 GPa for each material, preceding the phase transformation. DyPO4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The solid solutions yielded partially reversible phase transformations and retained peaks from the high-pressure phase while reforming xenotime peaks upon decompression.
Advisors/Committee Members: Packard, Corinne E. (advisor), Reimanis, Ivar E. (Ivar Edmund) (committee member), Brennecka, Geoffrey (committee member).
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APA (6th Edition):
Musselman, M. A. (2017). In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄. (Masters Thesis). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/170999
Chicago Manual of Style (16th Edition):
Musselman, Matthew Anthony. “In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄.” 2017. Masters Thesis, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/170999.
MLA Handbook (7th Edition):
Musselman, Matthew Anthony. “In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄.” 2017. Web. 17 Feb 2019.
Vancouver:
Musselman MA. In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄. [Internet] [Masters thesis]. Colorado School of Mines; 2017. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/170999.
Council of Science Editors:
Musselman MA. In situ Raman spectroscopy of pressure-induced phase transformations in DyPO₄ and GdxDy(1-x)PO₄. [Masters Thesis]. Colorado School of Mines; 2017. Available from: http://hdl.handle.net/11124/170999
Colorado School of Mines
3.
Cox, Brandon M.
Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications.
Degree: MS(M.S.), Metallurgical and Materials Engineering, 2018, Colorado School of Mines
URL: http://hdl.handle.net/11124/172346
► Dielectric resonators (DRs) are ceramic components used primarily in wireless communication devices for their high relative permittivities and low loss properties, with much room for…
(more)
▼ Dielectric resonators (DRs) are ceramic components used primarily in wireless communication devices for their high relative permittivities and low loss properties, with much room for improvement aimed at maximizing their electrical properties. Additive manufacturing (AM) of ceramic materials opens the door for new methods of fabrication of these DRs to produce functionally graded permittivities. An extrusion-based ceramic AM technique has been employed to print structures with graded permittivity and custom geometries. Through a series of rheological studies, shear thinning ceramic suspensions of titanium dioxide and barium titanate have been developed to demonstrate this technique and improve the ability to print such structures. Co-sintering studies were performed to ensure that these materials are compatible during densification and sintering, with validation through microstructural characterization, to verify that the essential material properties meet the requirements of these DR devices. Graded permittivity parts have the ability to enable new electrical properties of DRs, which can be used as a host of new opportunities in the field of microwave dielectrics, such as DR oscillators and DR antennas.
Advisors/Committee Members: Brennecka, Geoffrey (advisor), Gorman, Brian P. (committee member), Diercks, David R. (committee member).
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APA (6th Edition):
Cox, B. M. (2018). Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications. (Masters Thesis). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/172346
Chicago Manual of Style (16th Edition):
Cox, Brandon M. “Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications.” 2018. Masters Thesis, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/172346.
MLA Handbook (7th Edition):
Cox, Brandon M. “Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications.” 2018. Web. 17 Feb 2019.
Vancouver:
Cox BM. Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications. [Internet] [Masters thesis]. Colorado School of Mines; 2018. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/172346.
Council of Science Editors:
Cox BM. Critical evaluation of additively manufactured electrical ceramics for dielectric resonator applications. [Masters Thesis]. Colorado School of Mines; 2018. Available from: http://hdl.handle.net/11124/172346
Colorado School of Mines
4.
Manna, Sukriti.
Design and discovery of new piezoelectric materials using density functional theory.
Degree: PhD, Mechanical Engineering, 2018, Colorado School of Mines
URL: http://hdl.handle.net/11124/172532
► Piezoelectric materials find applications in microelectromechanical systems (MEMS), such as surface acoustic wave (SAW) resonators, radio frequency (RF) filters, resonators, and energy harvesters. Using density…
(more)
▼ Piezoelectric materials find applications in microelectromechanical systems (MEMS), such as surface acoustic wave (SAW) resonators, radio frequency (RF) filters, resonators, and energy harvesters. Using density functional theory calculations, the present study illustrates the influence of alloying and co-alloying with different nitrides on piezoelectric and mechanical properties of an existing piezoelectric material such as aluminum nitride (AlN). Besides improving the performance of existing piezoelectric material, a high-throughput screening method is used to discover new piezoelectric materials. AlN has several beneficial properties such as high temperature stability, low dielectric permittivity, high hardness, large stiffness constant, high sound velocity, and complementary metal-oxide-semiconductor (CMOS) compatibility. This makes it widely accepted material in RF and resonant devices. However, it remains a challenge to enhance the piezoelectric modulus of AlN. The first part of this thesis establishes that the piezoelectric modulus of AlN could be improved by alloying with rocksalt transition metal nitrides such as scandium nitride (ScN), yttrium nitride (YN), and chromium nitride (CrN). As the content of the rocksalt end member in the alloy increases, the accompanying structural frustration enables a greater piezoelectric response. This structural frustration is also accompanied by thermodynamic driving forces for phase separation which, with increased alloy concentration, lead to the destruction of the piezoelectric response upon transition to the (centrosymmetric, cubic) rocksalt structure. Thus, it becomes necessary to identify suitable alloying elements that may yield highest piezoelectric response with minimal alloying additions. The study reveals that the alloying with CrN would lead to the lowest transition composition (occurring at approximately 25% CrN concentration) between the wurtzite and rocksalt structures, thereby allowing piezoelectric enhancements at alloying levels that are easier to stabilize during the synthesis. The present study indicates that, for 25% CrN alloying, the piezoelectric modulus is about 4 times larger than that of pure AlN. Thus, it is proposed to use Cr
xAl
1-xN as a suitable replacement for AlN. One of the adverse effects of transition metal nitride alloying for improvement of the piezoelectric response is accompanied by the softening of AlN lattice. This would make the material less desirable for resonant applications. Our subsequent research establishes that co-alloying with YN and BN would enable the most superior combination of piezoelectric and mechanical properties of AlN. The rest of the thesis describes our efforts for identifying new piezoelectric materials. This requires high-throughput screening of inorganic materials for their piezoelectric properties. Our quest to search for new piezoelectric compounds is motivated by the notion that soft materials have the opportunity to exhibit large piezoelectric response (piezoelectric modulus bf{d}), and…
Advisors/Committee Members: Ciobanu, Cristian V. (advisor), Stevanovic, Vladan (advisor), Brennecka, Geoffrey (committee member), Stebner, Aaron P. (committee member), Berger, John R. (committee member).
Subjects/Keywords: Piezoelectrics; AlN; Thin film
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APA (6th Edition):
Manna, S. (2018). Design and discovery of new piezoelectric materials using density functional theory. (Doctoral Dissertation). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/172532
Chicago Manual of Style (16th Edition):
Manna, Sukriti. “Design and discovery of new piezoelectric materials using density functional theory.” 2018. Doctoral Dissertation, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/172532.
MLA Handbook (7th Edition):
Manna, Sukriti. “Design and discovery of new piezoelectric materials using density functional theory.” 2018. Web. 17 Feb 2019.
Vancouver:
Manna S. Design and discovery of new piezoelectric materials using density functional theory. [Internet] [Doctoral dissertation]. Colorado School of Mines; 2018. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/172532.
Council of Science Editors:
Manna S. Design and discovery of new piezoelectric materials using density functional theory. [Doctoral Dissertation]. Colorado School of Mines; 2018. Available from: http://hdl.handle.net/11124/172532
Colorado School of Mines
5.
Barcellos de Oliveira, Debora R.
Design of materials for solar thermochemical hydrogen production.
Degree: PhD, Metallurgical and Materials Engineering, 2018, Colorado School of Mines
URL: http://hdl.handle.net/11124/172540
► Hydrogen is one of the most widely produced and used commodity “chemicals” in the world. Besides its current widespread use in the petrochemicals and ammonia…
(more)
▼ Hydrogen is one of the most widely produced and used commodity “chemicals” in the world. Besides its current widespread use in the petrochemicals and ammonia production industries, it also has the potential to be used as an energy carrier for future power, transportation and storage applications. However, the environmental sustainability of hydrogen for such applications can only be assured if it can be obtained from renewable sources. Solar thermochemical hydrogen production (STCH) is a carbon-free technique that uses solar thermal heat to split water and produce hydrogen. The predicted high process efficiencies and scalability make this technique preferred for large scale hydrogen production over electrolysis or thermolysis. The current state-of-the-art material for STCH is ceria (CeO2), which produces hydrogen via a relatively straightforward two-step water splitting (WS) cycle. In the first step, carried out at a high temperature, the oxide reduces by creating oxygen vacancies and releases oxygen. In the second step, carried out at a lower temperature, the oxygen deficient oxide splits the water molecule and reoxidizes, thereby releasing hydrogen. The great challenge of using ceria is the high temperature required for the first reduction step, which is over 1600 °C. In this work I propose new materials for water splitting BaCe0.25Mn0.75O3 and three materials of composition CeaSr2-aMnO4 (a = 0.1, 0.2 and 0.3) that meet or exceed ceria’s water splitting performance, but at significantly lower temperatures (~1350-1400 °C). BaCe0.25Mn0.75O3 (BCM) is shown to produce about 3X more H2 than ceria at a lower reduction temperature (1350 °C) and is demonstrated to have a higher steam-to-hydrogen conversion than other promising perovskite candidates. This steam-to-hydrogen conversion concept was shown to be of extreme importance for the application of the STCH process in a realistic reactor. The thermodynamic properties of BCM were investigated to further understand the underlying reasons for its outstanding performance. This study led to the discovery of a beneficial polytype phase change that may take place during STCH cycling in this materials system and that likely contributes to its performance. Motivated by my discovery of the BCM system, we subsequently identified a Ruddlesden-Popper phase, CeaSr2-aMnO4 (CSM) which also showed excellent potential for STCH WS. Importantly, this phase had never before been reported in the literature. After a structure characterization study, this new materials system was demonstrated to produce 2-3X more hydrogen than ceria at the reduction temperature of 1400 °C. Both the BCM and CSM systems open up new directions for the deisgn and optimization of redox-active STCH materials that can provide higher performance at lower temperatures than CeO2, thereby underscoring remaining oportunities to further discover new materials for this important renewable energy application.
Advisors/Committee Members: O'Hayre, Ryan P. (advisor), Tong, Jianhua (advisor), Sanders, Michael (committee member), McDaniel, Anthony H. (committee member), Pylypenko, Svitlana (committee member), Brennecka, Geoffrey (committee member).
Subjects/Keywords: Perovskites; Water splitting; Solar thermochemical; Hydrogen production
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APA (6th Edition):
Barcellos de Oliveira, D. R. (2018). Design of materials for solar thermochemical hydrogen production. (Doctoral Dissertation). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/172540
Chicago Manual of Style (16th Edition):
Barcellos de Oliveira, Debora R. “Design of materials for solar thermochemical hydrogen production.” 2018. Doctoral Dissertation, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/172540.
MLA Handbook (7th Edition):
Barcellos de Oliveira, Debora R. “Design of materials for solar thermochemical hydrogen production.” 2018. Web. 17 Feb 2019.
Vancouver:
Barcellos de Oliveira DR. Design of materials for solar thermochemical hydrogen production. [Internet] [Doctoral dissertation]. Colorado School of Mines; 2018. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/172540.
Council of Science Editors:
Barcellos de Oliveira DR. Design of materials for solar thermochemical hydrogen production. [Doctoral Dissertation]. Colorado School of Mines; 2018. Available from: http://hdl.handle.net/11124/172540
6.
Beuerlein, Michaela.
Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics.
Degree: MS(M.S.), Metallurgical and Materials Engineering, 2017, Colorado School of Mines
URL: http://hdl.handle.net/11124/171594
► Technological advancements in power electronics devices require the development of capacitors that are capable of operating at elevated temperature and under large applied electric fields.…
(more)
▼ Technological advancements in power electronics devices require the development of capacitors that are capable of operating at elevated temperature and under large applied electric fields. The BaTiO3 – Bi(M)O3 family of dielectrics (where M represents a net-trivalent cation or cation pair) has proven to be a promising alternative to currently available X7R- and C0G-type capacitors. However, much is yet to be known about this class of materials in order to enable design optimization and proper integration into industrial applications. This work studies the effects of atomic structure, phase formation, processing conditions, and microstructure development of (1-x)BaTiO3 – xBi(M)O3 on its measured dielectric properties, with a focus on (1-x)BaTiO3-xBi(Zn1/2Ti1/2)O3. Studies included in this work are in situ hot-stage x-ray diffraction, which elucidated the reaction pathways of precursors as they reacted to form 0.8BaTiO3-0.2Bi(Zn1/2Ti1/2)O3; Rietveld refinements of high resolution x-ray diffraction scans of 0.8BaTiO3-0.2Bi(Zn1/2Ti1/2)O3, which provided valuable information about the structure(s) at room temperature; studies about overcoming the thermodynamic challenge of cofiring copper electrodes with bismuth oxide-based dielectrics, accomplished through x-ray diffraction and thermal analysis; and a general review of the current understanding of structure-processing-property relationships in BaTiO3-Bi(M)O3 dielectrics, accompanied by a case study of the microstructure development and dielectric properties of a subset of BaTiO3-Bi(M)O3 compositions.
Advisors/Committee Members: Brennecka, Geoffrey (advisor), Sanders, Michael (committee member), Gorman, Brian P. (committee member).
Subjects/Keywords: BT-BZT; Dielectric; Barium titanate; Microstructure; Ceramic
…of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO
80401
2…
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APA (6th Edition):
Beuerlein, M. (2017). Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics. (Masters Thesis). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/171594
Chicago Manual of Style (16th Edition):
Beuerlein, Michaela. “Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics.” 2017. Masters Thesis, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/171594.
MLA Handbook (7th Edition):
Beuerlein, Michaela. “Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics.” 2017. Web. 17 Feb 2019.
Vancouver:
Beuerlein M. Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics. [Internet] [Masters thesis]. Colorado School of Mines; 2017. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/171594.
Council of Science Editors:
Beuerlein M. Phase formation, microstructure development, and copper co-firing of barium titanate - bismuth zinc titanate and related dielectrics. [Masters Thesis]. Colorado School of Mines; 2017. Available from: http://hdl.handle.net/11124/171594
7.
Fioretti, Angela N.
Development of zinc tin nitride for application as an earth abundant photovoltaic absorber.
Degree: PhD, Physics, 2018, Colorado School of Mines
URL: http://hdl.handle.net/11124/172032
► In recent years, many new potential absorber materials based on earth-abundant and non-toxic elements have been predicted. These materials, often made in thin film form…
(more)
▼ In recent years, many new potential absorber materials based on earth-abundant and non-toxic elements have been predicted. These materials, often made in thin film form and known to absorb light 10-1000 times more e ciently than crystalline silicon, could lower module cost and enable broader solar deployment. One such material is zinc tin nitride (ZnSnN2), a II-IV-nitride analog of the III-nitride materials, which was identified as a suitable solar absorber due to its direct bandgap, large absorption coefficient, and disorder-driven bandgap tunability. Despite these desirable properties, initial attempts at synthesis resulted in degenerate n-type carrier density. Computational work on the point defect formation energies for this material revealed three donor defects were likely the cause; specifically Sn_Zn antisites, V_N sites, and O_N substitutions. Given this framework, a defect-driven hypothesis was proposed as a starting point for the present work: if each donor defect could be addressed by tuning deposition parameters, n-type degeneracy may be defeated. By using combinatorial co- sputtering to grow compositionally-graded thin film samples, n-type carrier density was reduced by two orders of magnitude compared to state-of-the-art. This reduction in carrier density was observed for zinc-rich samples, which supported the defect-driven hypothesis initially proposed. These results and their implications are the topic of Chapter 2. Further carrier density control in zinc-rich ZTN was achieved via hydrogen incorporation and post-growth annealing. This strategy was hypothesized to operate by passivating acceptor defects to avoid self-compensation, which were then activated by hydrogen drive- out upon annealing. Carrier density was reduced another order of magnitude using this technique, which is presented in Chapter 3. After defeating n-type degeneracy, a deeper understanding of the electronic structure was pursued. Photoluminescence (PL) was used to study electronic structure and recombination pathways in zinc-rich ZTN, and excitonic emission was observed despite its many crystallographic defects. PL results are presented in Chapter 4. Ultimately, this work has advanced the field of ZTN research both technologically and scientifically, by providing strategies for self-doping control and identifying critical defect interactions giving rise to n-type degeneracy and carrier density reduction.
Advisors/Committee Members: Toberer, Eric (advisor), Tamboli, Adele C. (advisor), Packard, Corinne E. (committee member), Brennecka, Geoffrey (committee member), Zimmerman, Jeramy D. (committee member), Zakutayev, Andriy (committee member).
Subjects/Keywords: Defect physics; Nitrides; Semiconductors; Material development; Combinatorial; Photovoltaics
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APA (6th Edition):
Fioretti, A. N. (2018). Development of zinc tin nitride for application as an earth abundant photovoltaic absorber. (Doctoral Dissertation). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/172032
Chicago Manual of Style (16th Edition):
Fioretti, Angela N. “Development of zinc tin nitride for application as an earth abundant photovoltaic absorber.” 2018. Doctoral Dissertation, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/172032.
MLA Handbook (7th Edition):
Fioretti, Angela N. “Development of zinc tin nitride for application as an earth abundant photovoltaic absorber.” 2018. Web. 17 Feb 2019.
Vancouver:
Fioretti AN. Development of zinc tin nitride for application as an earth abundant photovoltaic absorber. [Internet] [Doctoral dissertation]. Colorado School of Mines; 2018. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/172032.
Council of Science Editors:
Fioretti AN. Development of zinc tin nitride for application as an earth abundant photovoltaic absorber. [Doctoral Dissertation]. Colorado School of Mines; 2018. Available from: http://hdl.handle.net/11124/172032
8.
Miller, J. Aaron.
Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties.
Degree: PhD, Metallurgical and Materials Engineering, 2017, Colorado School of Mines
URL: http://hdl.handle.net/11124/170972
► Magnesium aluminate spinel is a material of interest for transparent armor applications. Owing to its unique combination of transparency to large portions of the electromagnetic…
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▼ Magnesium aluminate spinel is a material of interest for transparent armor applications. Owing to its unique combination of transparency to large portions of the electromagnetic spectrum and mechanical robustness, spinel is among the front runners for applications including transparent armor windows for military vehicles and space craft windows, missile radomes, and infrared windows. However, failure in such applications may lead to severe outcomes, creating motivation to further improve the mechanical reliability of the materials used. In this thesis, potential toughening mechanisms that utilize unique control over the evolution of second phase particles are explored. Al-rich spinel (MgO•nAl2O3) with a composition of n = 2 is investigated. First, it is demonstrated that precipitation of second phase Al2O3 from single phase spinel can be achieved by modifying the densification routines typically used to produce transparent spinel. Subsequent heat treatments in air and in vacuum result in varying amounts of precipitation, demonstrating that the single phase is stabilized by the creation of oxygen vacancies during densification, and a modified defect reaction for precipitation is proposed. The location of precipitation can be varied by controlling the reintroduction of oxygen, which is beneficial for toughening specific locations of material with complex shapes, such as toughening the surface of a curved missile radome. The fracture toughness ranges from 0.88 – 2.47 MPa√m depending on the local microstructure. Improved toughness within precipitated regions is due to increased crack tortuosity at phase boundaries. However, precipitation from the spinel matrix causes local volume contraction, creating porosity and residual tensile stresses in regions immediately adjacent to precipitated regions. The light scatter caused by porosity is detrimental to the transmission properties of the material, especially for precipitation layers greater than 60 m. The dissolution of second phase Al2O3 particles into a stoichiometric spinel matrix is also investigated. Complete dissolution of all Al2O3 demonstrates the capability to control the size of the second phase particles, limiting light scatter at phase boundaries. Furthermore, dissolution results in compressive, rather than tensile, stresses within the composite material. A maximum toughness of 4.34 MPa√m was measured in the two-phase composite compared to 2.26 MPa√m once complete dissolution had occurred. However, the toughness of the dissolved specimen is still an improvement from 1.72 MPa√m measured for single-phase, Al-rich spinel of the same overall composition as densified by traditional methods. The observed enhancement in toughness is attributed to a combination of residual stresses that arise from the coefficient of thermal expansion mismatch between particle and matrix, crack deflection caused by second phase particles, and the volume expansion as Al2O3 dissolves into the spinel matrix.
Advisors/Committee Members: Reimanis, Ivar E. (Ivar Edmund) (advisor), Berger, John R. (committee member), Brennecka, Geoffrey (committee member), Packard, Corinne E. (committee member), Gorman, Brian P. (committee member).
Subjects/Keywords: Armor; Spinel; Transparent; Hardness; Alumina; Toughness
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APA (6th Edition):
Miller, J. A. (2017). Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties. (Doctoral Dissertation). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/170972
Chicago Manual of Style (16th Edition):
Miller, J Aaron. “Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties.” 2017. Doctoral Dissertation, Colorado School of Mines. Accessed February 17, 2019.
http://hdl.handle.net/11124/170972.
MLA Handbook (7th Edition):
Miller, J Aaron. “Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties.” 2017. Web. 17 Feb 2019.
Vancouver:
Miller JA. Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties. [Internet] [Doctoral dissertation]. Colorado School of Mines; 2017. [cited 2019 Feb 17].
Available from: http://hdl.handle.net/11124/170972.
Council of Science Editors:
Miller JA. Non-stoichiometric magnesium aluminate spinel : microstructure evolution and its effect on properties. [Doctoral Dissertation]. Colorado School of Mines; 2017. Available from: http://hdl.handle.net/11124/170972
. 
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