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You searched for +publisher:"University of Virginia" +contributor:("Poon, Joseph"). Showing records 1 – 3 of 3 total matches.

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University of Virginia

1. Petersen, Alexander. Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis.

Degree: MS, 2015, University of Virginia

The focus of this study was on further understanding bulk metallic glasses through the twofold path of theoretical modeling to predict elastic properties and experimental creation of large bulk metallic glass systems. In order to accurately predict the elastic properties of bulk metallic glass systems a reliable mathematical model had to be created. Existing elastic models for crystalline systems were explored with a focus given to the Voigt and Reuss models. The method selected for further investigation was the Coherent Potential Approximation (CPA) which proved to more accurately predict the elastic moduli. Beyond this, the short and medium range order was modeled and predicted based on the efficient packing of atomic clusters, as discussed by Miracle and others. In addition to geometric cluster modeling bonding priority and atomic arrangement were considered to predict the probable locations of specific elements during the metallic glass synthesis process. Through this adjustment the CPA model was allowed to reach a coefficient of determination in excess of 0.89 on a collection of 117 select experimental bulk metallic glass systems. The experimental creation was done on a variety of Nickel and Tungsten based systems with the goal of creating high density, highly compact, high hardness, and fairly ductile amorphous systems. This was done through optimization of amorphous system creation via synthesis of amorphous powder precursors and compaction. The sample compaction was conducted through the use of a spark plasma sintering system to ensure rapid temperature transitions, high pressure, and high compaction. The optimal path combined amorphous powders synthesized through a combination of methods including arc melting, melt spinning, annealing, and ball milling. The optimal systems created were Ni-based alloys constructed through a combination of multiple methods that retained their amorphous structure while demonstrating a hardness of 12.6 GPa, a density in excess of 11 g/cc, and ductility demonstrated through shear banding. Advisors/Committee Members: Poon, Joseph (advisor).

Subjects/Keywords: Metallic Glass

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Petersen, A. (2015). Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis. (Doctoral Dissertation). University of Virginia. Retrieved from http://libra.virginia.edu/catalog/libra-oa:9488

Chicago Manual of Style (16th Edition):

Petersen, Alexander. “Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis.” 2015. Doctoral Dissertation, University of Virginia. Accessed February 19, 2019. http://libra.virginia.edu/catalog/libra-oa:9488.

MLA Handbook (7th Edition):

Petersen, Alexander. “Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis.” 2015. Web. 19 Feb 2019.

Vancouver:

Petersen A. Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis. [Internet] [Doctoral dissertation]. University of Virginia; 2015. [cited 2019 Feb 19]. Available from: http://libra.virginia.edu/catalog/libra-oa:9488.

Council of Science Editors:

Petersen A. Bulk Metallic Glasses: Atomic Modeling and Spark Plasma Sintering Synthesis. [Doctoral Dissertation]. University of Virginia; 2015. Available from: http://libra.virginia.edu/catalog/libra-oa:9488


University of Virginia

2. Ding, Manli. Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition.

Degree: PhD, 2014, University of Virginia

Spin-Torque-Transfer Magnetic Random Access Memory (STT-MRAM) has the potential to become a universal memory due to its superior properties compared with conventional memory technologies. However, the main challenge for implementing STT writing mode in high-density and high-speed memory is the substantial reduction of the intrinsic current density Jc0 required to switch the magnetization of the free layer (FL) while maintaining high thermal stability required for long-term data retention. The intrinsic switching current reduction can be pursued by using materials with low saturation magnetization (Ms), low damping constant (α), high spin polarization (η) and high magnetic anisotropy energy density (Ku) at low dimension. Previous experiments showed that CoFeB incorporated in MgO-based MTJ is one of the most efficient materials for this purpose. In this work chromium was substituted into CoFeB alloys to further reduce their magnetization attributed to the antiferromagnetism of Cr. Films in composition of Cr at 30% showed a reduced magnetization of 300 emu/cm3 and an enhanced coercive field of 500 Oe, making this material suitable for device application. Moreover, the switching current density Jc0 is predicted to be reduced significantly by using materials with perpendicular magnetic anisotropy (PMA), compared with the normal in-plane ones. Perpendicular Magnetic anisotropy was found in crystalline D022 Mn2.5Ga films with optimal magnetic anisotropy energy of 0.5 ×107 erg/cm3, which leads to high thermal stability. A low saturation moment (Ms = 260 emu/cc) was investigated, attributed to the strong antiferromagntic coupling between Mn atoms at different sites. With a reported high spin polarization of 88%, D022- Mn2.5Ga film has the potential to be used in spintronic devices. Amorphous ferrimagnetic GdxFe93-xCo7 films were also found to exhibit low magnetization and perpendicular magnetic anisotropy, which is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal™ ions and ferromagnetic interaction between the TM ions. Furthermore, low magnetization was achieved at room temperature in previously unreported high-Gd region of x=52-59, accompanied by a large PMA with coercivity reaching ~6.6 kOe. Our findings have broadened the composition range of transition metal-rare earth alloys for designing PMA films, making it attractive for tunable magnetic anisotropy in nanoscale devices. In addition to Ms and η, one can also reduce FL intrinsic damping constant for lowering Jc0, but practically PMA films often have higher damping constant α due to strong spin-orbit coupling. With reported low damping constant of 0.0025, CoFeGe film is promising spintronic material. Perpendicular magnetic anisotropy was achieved in the MgO/CoFeGe/MgO structure with an optimized magnetic anisotropy energy density of 2 106 erg/cm3, which is contributed by the interfacial anisotropy between CoFeGe and MgO. The B2-ordered CoFeGe is not half metallic but has a perfectly spin-polarized ∆1 band along… Advisors/Committee Members: Poon, Joseph (advisor).

Subjects/Keywords: magnetic thin films

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Ding, M. (2014). Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition. (Doctoral Dissertation). University of Virginia. Retrieved from http://libra.virginia.edu/catalog/libra-oa:6421

Chicago Manual of Style (16th Edition):

Ding, Manli. “Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition.” 2014. Doctoral Dissertation, University of Virginia. Accessed February 19, 2019. http://libra.virginia.edu/catalog/libra-oa:6421.

MLA Handbook (7th Edition):

Ding, Manli. “Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition.” 2014. Web. 19 Feb 2019.

Vancouver:

Ding M. Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition. [Internet] [Doctoral dissertation]. University of Virginia; 2014. [cited 2019 Feb 19]. Available from: http://libra.virginia.edu/catalog/libra-oa:6421.

Council of Science Editors:

Ding M. Growth and Properties of Magnetic Thin Films using Magnetron Sputtering Deposition. [Doctoral Dissertation]. University of Virginia; 2014. Available from: http://libra.virginia.edu/catalog/libra-oa:6421

3. Wu, Di. "Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach".

Degree: PhD, 2013, University of Virginia

Nanostructured Half Heusler alloys MNiSn, MCoSb (M=Hf, Zr and Ti) and SiGe nanocomposites were synthesized with high energy vibrational ball mill, followed by spark plasma sintering (SPS) consolidation. The thermoelectric properties were measured from room temperature to 1100K, the dependence of thermoelectric properties on consolidation parameters and microstructures were investigated. A differential effective medium (DEM) model was also developed to evaluate the lattice thermal conductivity in heterogeneous two-phase systems. Advisors/Committee Members: Poon, Joseph (advisor).

Subjects/Keywords: thermoelectric; Half Heusler; Silicon Germanium; nanostructure; Differential Effective Medium

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Wu, D. (2013). "Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach". (Doctoral Dissertation). University of Virginia. Retrieved from http://libra.virginia.edu/catalog/libra-oa:3565

Chicago Manual of Style (16th Edition):

Wu, Di. “"Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach".” 2013. Doctoral Dissertation, University of Virginia. Accessed February 19, 2019. http://libra.virginia.edu/catalog/libra-oa:3565.

MLA Handbook (7th Edition):

Wu, Di. “"Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach".” 2013. Web. 19 Feb 2019.

Vancouver:

Wu D. "Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach". [Internet] [Doctoral dissertation]. University of Virginia; 2013. [cited 2019 Feb 19]. Available from: http://libra.virginia.edu/catalog/libra-oa:3565.

Council of Science Editors:

Wu D. "Enhanced thermoelectric performance of Half Heulser and Silicon Germanium, and calculation on lattice thermal conductivity with Effective Medium Approach". [Doctoral Dissertation]. University of Virginia; 2013. Available from: http://libra.virginia.edu/catalog/libra-oa:3565

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