Browsing by Author "Zhang, Xin"

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    Envelopes for efficient multivariate parameter estimation
    (2014-04) Zhang, Xin
    Multivariate linear regression of response Y on predictor X is a cornerstone of multivariate statistics. When the dimensions of responses and predictors are not small, it is widely recognized that reducing the dimensionalities of X and Y may often result in improved performance.Cook, Li and Chiaromonte (2010) proposed a new statistical concept{envelopes for increasing efficiency in estimation and prediction in multivariate linear regression. The idea is to envelope the information in the data that is material to the estimation of the parameters of interest, while excluding the information that is immaterial to estimation. This is achieved by estimating an envelope, which is essentially a targeted dimension reduction subspace for particular parameters of interest, to reduce the dimensionality of original problems. In this dissertation, we first propose a fast and stable 1D algorithm for envelope estimation in general. Because envelope estimation involves Grassmann manifold optimizations, our algorithm largely lessens the computational burdens of past and future envelope methods. We then naturally propose two new envelope methods for simultaneously educing X and Y, and for combining envelopes with reduced-rank regression. At the final chapter, we extend the idea of envelope beyond multivariate linear model to rather arbitrary multivariate estimation problems. We propose a constructive definition and a unied framework for incorporating envelopes with many future applications.
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    Phosphorus removal and recovery from animal manure by electrocoagulation and manure fertilizer efficiency improvement
    (2018-09) Zhang, Xin
    Certain type of livestock waste, for instance, dairy and swine manure, containing high level of phosphorus, is conventionally used as a fertilizer because it contains necessary nutrients for plant growth. When the application rate is formulated based on plant nitrogen requirement, phosphorus will accumulate in the soil over the years and may cause eutrophication in surrounding water bodies. Phosphorus recovery from animal manure not only addresses its environmental concerns, also provides a value-added product as a fertilizer for better managing phosphorus. An appreciable amount of phosphorus exists in the form of fine particles smaller than 45 µm and is therefore difficult to separate by natural sedimentation or other mechanical solid-liquid separation systems. Electrocoagulation is proved to be an efficient approach that can be applied in the municipal and industrial wastewater for phosphorus removal and recovery; however, its application in the livestock manure management is less researched, especially with different selection of the electrode materials and the application of the byproduct in agriculture. In this dissertation, electrocoagulation process was evaluated on phosphorus removal and recovery from liquid dairy and swine manure. Four commonly seen electrodes, including aluminum, stainless steel, low carbon steel and cast iron, were compared. The results showed low carbon steel achieved the most efficient phosphorus removal (96.7%). The average particle size of dairy manure solids increased from 32.2 to 126.9 μm, while it didn’t change a lot in swine manure, although the settling property of electrocoagulation treated swine manure improved and about 90% of particles was removed. Further analyses indicated that the particle density of the majority part of swine manure solids (particles with sizes between 25 µm and 45 µm) increased from 1.29 g/mL to 1.36 g/mL after electrocoagulation, and that the background viscosity of liquid swine manure decreased from 1.99 g/m*s to 1.64 g/m*s. All those manure property changes expedited particle and phosphorus settling in swine manure. The simulation experiment suggested that iron release and hydrolysis, which then worked as coagulants, is the major mechanism for phosphorus removal. Electrocoagulation by low carbon steel is an effective method for P separation from liquid phase of dairy manure to solid phase. Operation parameters affect the process of coagulation and the phosphorus removal efficiency. An 800-ml electrocoagulation process with low carbon steel electrodes was investigated on four operation parameters including current, agitation speed, initial pH, aeration time and anaerobic digestion. The results show that higher current and higher initial pH can improve the phosphorus removal efficiency. Agitation assist the formation of coagulants, but high agitation speed didn’t improve the phosphorus removal efficiency more. Pretreatment, i.e. aeration or anaerobic digestion before electrocoagulation are beneficiary for the phosphorus removal. The suggested operation parameters are under 1 A, agitation of 75 rpm, and higher initial pH. Pretreatment of 10 days anaerobic digestion or aeration of 60 min before electrocoagulation are recommended. The pilot scale study of the electrocoagulation process showed that the ratio of nitrogen to phosphate in swine manure decreased from 1.71-2.44 to 0.52-0.78 after electrocoagulation, and 65.9-88.9% of the phosphate was concentrated in the sludge portion. The swine manure sludge was reduced more than 90% after electrocoagulation and natural precipitation. However, this system didn’t work well for dairy manure. More studies need to be done to find out the reasons. The purpose of the final step of the experiments is to evaluate the fertilizer effects of electrocoagulation treated manure and whether the phosphorus solubilizing fungi (Fusarium acuminatum and Epicoccum nigrum) can assist the manure fertilizer fertility. A soybean growth experiment was conducted. The results show that the phosphorus in electrocoagulation treated manure was able to be utilized by the soybean plants and it increased the soybean biomass and the TP content in the biomass. The dry biomass of soybean plants applied with raw swine manure, electrocoagulation processed dairy and swine manure were similar as that of using KH2PO4. Fusarium acuminatum could assist SM fertilizer and brought the soybean dry biomass to the same level as applying KH2PO4 (without fungi inoculation). Fusarium acuminatum helped promote the total phosphorus content in the biomass of soybean with EC SM fertilizer from 1.56± mg-P to 2.77±0.56 mg-P. Epicoccum nigrum didn’t have much effect on the soybean growth but it significantly increased the total phosphorus content in the biomass of soybean with KH2PO4 fertilizer (increased by 98.7%). Therefore, Fusarium acuminatum and Epicoccum nigrum 7 had the potential of being P fertilizer assistants (EC SM assistant and KH2PO4 assistant, respectively). Overall, the electrocoagulation method with low carbon steel electrodes developed in this dissertation successfully achieved the goal of improving phosphorus recovery as well as solid-liquid separation from liquid animal manure and has a potential to achieve better separation and to consume less energy after process optimization. The electrocoagulation treated manure is ready to be used as P fertilizer and can promote soybean growth compared with the untreated manure. Consequently, electrocoagulation is an effective pre-treatment method to recover the phosphorus in liquid animal manure and produce economical organic P fertilizer.
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    Synthesis, Characterization and Electronic Transport Properties of Thin Film Iron Pyrite for Photovoltaic Applications
    (2015-08) Zhang, Xin
    The pyrite form of FeS2 has long been recognized as an earth-abundant and non-toxic material with exceptional properties as a solar absorber for inexpensive photovoltaic devices. However, a significant research effort from the mid 1980’s achieved power conversion efficiencies of only less than 3 %. The reasons for such low efficiencies have not been fully elucidated yet, primarily because the electronic transport and doping mechanisms of pyrite are poorly understood. One classic example is well-known puzzle remaining in pyrite, where bulk single crystals are almost exclusively n-type based on Hall effect measurements, whereas polycrystalline thin films are typically deduced to be p-type, mostly from thermopower measurements. The fundamental reason(s) for this are not understood, and identifying the unintentional dopants in FeS2 remains an outstanding challenge. In this work we address, using ex situ sulfidation synthesis, this long-standing problem of understanding conduction mechanisms and doping in FeS2 films. This is done by systematically exploring the effects of film synthesis conditions on microstructure, surface morphology, chemical stoichiometry, electronic transport mechanisms, charge carrier mobility and charge density. More than a hundred of FeS2 thin films and synthetic crystals were probed in this study. In addition to conventional diffusive transport, hopping transport was also frequently observed in FeS2 thin films. This hopping transport was discovered to be caused by nanoscale inhomogeneity (e.g. nanoscale Fe or FeS clusters), which has been overlooked by the pyrite community until now. This hopping transport may explain the poor performance of some FeS2-based solar cells, since the carrier mobility and lifetime are significantly reduced in hopping. More importantly, accompanying the crossover from diffusive to hopping transport, we find significant suppression, and sign inversion from electron-like to hole-like, of Hall and themopower signals in FeS2 thin films. The results indicate that thin films with diffusive transport show n-type conduction, just like single crystals, which implies that the major n-type dopants may be the same for both FeS2 thin films and single crystals. As the transport crosses over to hopping, both Hall and thermopower measurements indicate sign inversions, which are not caused by real p-type doping, but are rather an artifact of hopping conduction. These findings provide the first potential resolution for the “doping puzzle” in FeS2, and emphasize that understanding the electronic transport mechanisms is mandatory for interpreting the sign of Hall and thermopower coefficients in FeS2. In the last part of this work, some preliminary results for identifying the unintentional dopant(s) in FeS2 are presented. The results suggest the major n-type dopants in FeS2 are unlikely to be metal impurities or oxygen. S vacancies are a genuine possibility however, although further study is still required to settle this issue. These findings answer several critical questions for understanding the electronic transport and doping mechanisms in pyrite FeS2 thin films. They also have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, (b) caution in interpreting carrier types and densities, and (c) doping control in pyrite FeS2 films.