Browsing by Author "Zhou, Yu"

Now showing 1 - 7 of 7
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    Finite-basis many-electron approximation to the Anderson model
    Mancini, Jay D.; Bowen, Samuel P.; Zhou, Yu (American Physical Society, 1990-03)
    A relatively simple many-electron basis is used to construct a matrix for the Anderson impurity Hamiltonian. The basis states are each valid in the thermodynamic limit. The approximate ground-state energy compares well with Bethe-ansatz results for large Coulomb energies. The ground-state wave-function properties are not as well approximated. This method may be well suited to studies of more realistic Hamiltonians and their ground-state energy and its derivatives.
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    In Vitro Proliferation of Porcine Pancreatic Islet Cells for β-Cell Therapy Applications
    Niu, Guoguang; McQuilling, John P.; Zhou, Yu; Opara, Emmanuel C.; Orlando, Giuseppe; Soker, Shay (Hindawi, 2016-12-06)
    β-Cell replacement through transplantation is the only curative treatment to establish a long-term stable euglycemia in diabetic patients. Owing to the shortage of donor tissue, attempts are being made to develop alternative sources of insulin-secreting cells. Stem cells differentiation and reprograming as well as isolating pancreatic progenitors from different sources are some examples; however, no approach has yet yielded a clinically relevant solution. Dissociated islet cells that are cultured in cell numbers by in vitro proliferation provide a promising platform for redifferentiation towards β-cells phenotype. In this study, we cultured islet-derived cells in vitro and examined the expression of β-cell genes during the proliferation. Islets were isolated from porcine pancreases and enzymatically digested to dissociate the component cells. The cells proliferated well in tissue culture plates and were subcultured for no more than 5 passages. Only 10% of insulin expression, as measured by PCR, was preserved in each passage. High glucose media enhanced insulin expression by about 4–18 fold, suggesting a glucose-dependent effect in the proliferated islet-derived cells. The islet-derived cells also expressed other pancreatic genes such as Pdx1, NeuroD, glucagon, and somatostatin. Taken together, these results indicate that pancreatic islet-derived cells, proliferated in vitro, retained the expression capacity for key pancreatic genes, thus suggesting that the cells may be redifferentiated into insulin-secreting β-like cells.
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    Investigation of Research Commercialization at a University: A Case Study
    Zhou, Yu (Virginia Tech, 2015-05-06)
    With the increase of awareness and focus on university research commercialization, much research had been conducted to investigate this subject. It was revealed that because universities were not traditionally built to serve the purpose of commercialization, many obstacles existed in the path of university research commercialization. Historically, research had largely focused on identifying critical factors that impacted the performance of commercialization. However, it was not clear how those findings could be systematically incorporated into the commercialization improvement plan of individual cases. This research intended to fill this gap and provide a framework that could be used by most universities to access and improve their research commercialization process. A case study of a U.S. land-grant university was conducted and a narrative approach was mainly used as the method of data analysis. Under the scope of a single-case study, four sub-studies were conducted to address the goals of this research. First, a framework was developed that incorporated theories of existing research and the value stream map of lean management. Interviews with the intellectual property office and faculty were conducted to determine if the theoretical framework was applicable. It was found that the framework fitted well with the current process of university research commercialization. After that, a survey that covered a sample size of 1110 researchers at the targeted university was conducted to investigate the importance of different resources at different stages of the process. Resources that were under investigation were grouped into four categories: technical, human, social, and financial resources. This research identified the most important resources for research commercialization were industrial connections (social resource) and assistance from the intellectual property (IP) office (human resource), with industrial connections playing a more importance role at the beginning of the process and the IP office from the stage of patent application. To assess organizational characteristics of the targeted university, interviews were conducted with 22 faculty, three representatives from the administration, one representative from the intellectual property office, and one representative from an external organization. Six criteria derived from previous research were used to guide the assessment: (1) expenditures on research and development (RandD), (2) intellectual property policy, (3) research field, (4) key individuals, (5) commitment to innovation, and (6) networking with external relations. It was found that the targeted university had strong evidence of the advantages of expenditures on RandD and research field, however, it was relatively weak in the other four characteristics. The last part of the research involved interviews with two companies for the purpose of developing a best practice for research commercialization with the examples from the industry. Recommendations to improve targeted university's research commercialization were developed based on findings of the research.
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    Nanoscale surface modification of wood veneers for adhesion
    Zhou, Yu (Virginia Tech, 2008-09-01)
    Surface chemistry of wood is based on the exposed cut surface that is the combination of intact (lumen wall) and cut cell wall material. It is inherently complex and changes with history of processing. Modification of wood surface through noncovalent attachment of amine containing water soluble polyelectrolytes provides a path to create functional surfaces in a controlled manner. Furthermore, modification of the surface can be performed using layer-by-layer (LbL) assembly, where the adsorption of polyelectrolytes or nanoparticles in sequential steps yields a multilayer film with a defined layer sequence on a given substrate. The objective of this study was to quantify adsorption of polyelectrolytes onto wood surface and use these polyelectrolytes as adhesives. In this study, optimal pH conditions for modifying wood surfaces, by anchoring adsorbing polyelectrolytes, were detected using zeta- ( )-potential measurements. Positively charged wood surfaces were also detected by the same technique after a layer of poly(diallyldimethylammonium chloride) (PDDA) or poly (ethylenimine) (PEI) was adsorbed. Both X-ray photoelectron spectroscopy (XPS) and Carbon-Nitrogen-Sulfur analyzer (CNS) were used to quantify the amount of charged polymer on wood surfaces to elucidate optimal pH and ionic strength for polyelectrolyte adsorption. Confocal laser scanning microscopy (CLSM) and Environmental Scanning Electron Microscope (ESEM) were used to characterize adsorbed LbL multilayers of poly(acrylic) acid (PAA) and poly(allylamine hydrochloride) (PAH). Cross-linking between PAA and PAH at various temperatures was studied by Fourier Transform Infrared Spectroscopy (FTIR) and the evaluation of multilayer as bonding agents was carried out by compression shear test following ASTM D905 standard.
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    Occupational Skills and Gender Wage Gap
    Zhou, Yu (Virginia Tech, 2018-06-18)
    This dissertation consists of three essays studying the occupational wages, skills, and gender wage gap in U.S. and other OECD countries. The analysis especially focuses on how the gender differences in skill levels and skill returns could explain the gender wage gaps and changes. The first chapter outlines the dissertation by briefly discussing the motivations, methods, and main findings in each of the following chapters. Chapter 2 focuses on the well-documented wage and employment polarizations in the U.S.. The occupations moving into the lower tail ("in" occupations) have more immigrant workers, more part-time workers, and less female workers. In addition, the wage gaps between domestic/immigrant, full-time/part-time, and male/female workers are also larger in "in" occupations. The opposite facts hold true in the occupations moving out of the lower tail ("out" occupations). Utilizing the regional differences, we also find stronger spillover effect from high-wage occupations to the "out" occupations than the effect to the "in" occupations. Chapter 3 investigates how gender differences in skills beyond education and experience can account for the observed gender wage gap and its changes between 1980 and 2015 by using data from the Dictionary of Occupational Titles (DOT) and the Occupational Information Network (O*NET). The main empirical finding is that female workers possess much higher level of caring skills, and the returns to caring skills are significantly negative but have increased over time, accounting for a major part of the persistent gender wage gap and the narrowing gender wage gap from 1980 to 2015. Another significant portion of the narrowed gender wage gap can be attributed to the faster growth in female workers' average directness skills and the fact that the returns to directness skills are significantly positive and stable over time. In the last chapter, we document significant cross-country variation in gender wage gaps among OECD countries by using the data from Survey of Adult Skills (PIAAC). We find significant cross-country variation in the gender differences in returns. The gender differences in returns to basic labor and experience are the most important factors in explaining the gender wage gap. In addition, gender differences in returns to cognitive and directness skills are playing milder but substantial roles in explaining the wage gap. We also find the social institutions and attitudes indicators are related to the cross-country variation in gender differences.
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    Simulation of High-velocity Penetration for Rigid Projectile into Plain Concrete Target using Discrete Element Method
    Zhou, Yu (Virginia Tech, 2009-02-04)
    Penetration of high velocity is of concern for both civilian and military research for decades, and computerized simulation is the scholar's focus in recent years. This study presents a study on the Discrete Element Method (DEM) simulation of plain concrete target's behavior under high-velocity penetration of rigid projectile. In this thesis, different types of research works including empirical, analytical and numerical methods in penetration by the previous scholars were carefully reviewed. A DEM-based concrete model was established by using software PFC3D. The major micro-variables of the simulation program were calibrated according to the required macro-mechanical parameters. Meanwhile, their correlations within the concrete range were studied, with the sensitivity analysis and the corresponding regression equations. With the established digital concrete model, penetration simulation tests were carried out. The results of penetration depth versus impact velocity were compared with the experimental and empirical calculated results from Forrestal's work in 1994. A good agreement was obtained. Some other simulation studies, like projectile mass, geometry, penetrating acceleration, concrete response stress, strain, and strain-rate were also conducted to study the constitutive properties in this thesis.
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    The study of many-electron systems
    Zhou, Yu (Virginia Tech, 1991)
    Various methods and approximation schemes are used to study many-electron interacting systems. Two important many-particle models, the Anderson model and the Hubbard model, and their electromagnetic properties have been investigated in many parameter regimes, and applied to physical systems. An Anderson single-impurity model Hamiltonian based calculation of the magnetic susceptibility is performed for YbN in the presence of crystal fields using an alteration of the Non-Crossing Approximation proposed by Zwicknagl et.al., incorporating parameters obtained from ab initio band structure calculations. It yields good agreement with experimental data. For the Anderson lattice model, a variational scheme which uses specific many-electron wavefunctions as basis is applied to both one- and two-dimensional systems represented by symmetric Anderson lattice Hamiltonians. Without much computational effort, the ground state energy is well approximated, especially in strong-coupling limit. Some electronic properties are examined using the variational ground state wavefunction. The one-dimensional Hubbard model has been solved exactly for small-size clusters by diagonalizing the Hamiltonian in the basis of many-electron Bloch states. The results for the energy spectrum and eigenfunctions of the ground state and low-lying excited states are presented. Also, mean field calculations of the two-dimensional single-band Hubbard model and Cu-O lattice model (three-band Hubbard model) are carried out for various physical quantities including the energy, occupation probability, staggered magnetization, momentum distribution Fermi surface and density of states, by using a projection operator formalism. To develop a systematic approach to solving many-electron problems, the many-particle partition function for the free electron gas system is explored using a cumulant expansion scheme. Starting from the ground state, the partition function can be approximated to any order in terms of excitation energy. Its application to interacting systems such as the Anderson model and the Hubbard model is briefly discussed.