Effects of point defects and microstructure on the pseudo-elasticity of ThCr2Si2-type crystals, The

2018 Fall.Includes bibliographical references.Ternary intermetallic compounds with the ThCr2Si2-type structure, which are known for their high-temperature superconductivity, have recently garnered interest due to the discovery of a pseudo-elastic mechanical response to compression along the c-axis. However, the effects of point defects and doping on this response remain unknown. In this work, these effects are investigated with density functional theory (DFT) in conjunction with continuum-scale models. DFT simulations of hydrostatic and uniaxial compression of pure ThCr2Si2-type crystals were conducted. The magnetic phase transition of CaFe2As2 was reproduced, while LaRu2P2 exhibited a continuous transition into its collapsed tetragonal phase. The two-phase DFT data was used to build a continuum-scale, thermodynamically-driven composite model which predicts the pseudo-elastic response of a large sample under displacement control and load control scenarios. Strain along the c-axis was shown to be the critical parameter in predicting crystal collapse. Then, DFT simulations of defected or doped unit cells were conducted to investigate their energetics and mechanical responses to compression. In some cases, the addition of vacancies effectively suppressed the pseudo-elastic response of the crystals. Simulations of crystals doped with varying concentrations revealed alterations of the mechanical properties as well. Tunable variability of the phase change with respect to dopant concentration was predicted in disordered doped structures, while multiple phase changes were predicted in ordered doped structures. Composite models were then built with the DFT data to predict the response of a sample comprised of multiple microstructures. The models predict a wide range of variability in the mechanical behavior and provide insight into how impurities and defects can be used to tune the response of these materials

Wastewater Disinfection with Peracetic Acid: Development of an Artificial Neural Network-Based Metamodel

Peracetic acid (PAA) is well-known as an antimicrobial agent for disinfection, and has particular applicability for treatment of wastewater for reuse. The performance of a per- acetic acid disinfection system is a function of site, speci c water characteristics, intrinsic kinetics, and reactor hydraulics. Often on-site pilot testing is important to determine the site-specific kinetics. Prediction and optimization of disinfection performance is often difficult without modeling the full-scale disinfection system. Metamodels o er an elegant solution to this problem. In this thesis, we present the results of microorganism inacti- vation in a mobile serpentine pilot reactor using a three-dimensional computational fluid dynamics (CFD) model. A 3-D CFD model was used for the prediction of transport of momentum as well as the concentrations of the disinfectant PAA and its decay, and the microorganisms (dead and alive). The CFD model developed was calibrated with tracer data from the on-site pilot reactor and used to analyze the chemical and biological performance of such a system. Polynomial t and ANN-based metamodels were developed from the CFD model predictions of microorganism inactivation in lieu of experimental disinfection data. It was determined through cross-validation analysis that the 5-neuron ANN was the optimum metamodel for this application. The metamodel was applied to obtain algebraic relations of the disinfection rate as a function of the inlet conditions of the microbial concentration and PAA dose.M.S., Mechanical Engineering and Mechanics -- Drexel University, 201

A Nanoindentation Study of the Plastic Deformation and Fracture Mechanisms in Single-Crystalline CaFe2As2

The plastic deformation and fracture mechanisms in single-crystalline CaFe2As2 has been studied using nanoindentation and density functional theory simulations. CaFe2As2 single crystals were grown in a Sn-flux, resulting in homogeneous and nearly defect-free crystals. Nanoindentation along the [001] direction produces strain bursts, radial cracking, and lateral cracking. Ideal cleavage simulations along the [001] and [100] directions using density functional theory calculations revealed that cleavage along the [001] direction requires a much lower stress than cleavage along the [100] direction. This strong anisotropy of cleavage strength implies that CaFe2As2 has an atomic-scale layered structure, which typically exhibits lateral cracking during nanoindentation. This special layered structure results from weak atomic bonding between the (001) Ca and Fe2As2 layers

Modeling pseudo-elasticity in small-scale ThCr2Si2-type crystals

These data files contain the raw DFT data of the compression of CaFe2As2 and LaRu2P2 unit cells used for the construction of the composite model. The output of the model is also included in this data set.Crystals of the ThCr2Si2-type structure comprise a large class of known compounds, and observations of superconductivity in some of the compounds generated significant interest in these materials. Recently, nano-indentation experiments have shown that at room temperature, small-scale crystals of CaFe2As2 exhibit pseudo-elastic behavior with recoverable strains of over 10%. These experiments also demonstrate the potential for shape memory effects at cryogenic temperatures, behavior which has previously been related to its magnetic phase transitions. In this work, the phase transitions of CaFe2As2 are investigated using density functional theory (DFT) in conjunction with analytical models. The models demonstrate that both uniaxial and hydrostatic loading can give rise to pseudo-elastic behavior. These models are then applied to LaRu2P2, which does not exhibit a magnetic phase change, but is still found to have a similar pseudo-elastic response. A suite of parameters useful in quantifying the complex responses of these compounds is presented and it is demonstrated that c-axis strain is the critical loading parameter in predicting the pseudo-elastic behavior. These results provide a method of connecting local chemical tuning to macroscopic behavior

A Nanoindentation Study of the Plastic Deformation and Fracture Mechanisms in Single-Crystalline CaFe2As2

The plastic deformation and fracture mechanisms in single-crystalline CaFe2As2 has been studied using nanoindentation and density functional theory simulations. CaFe2As2 single crystals were grown in a Sn-flux, resulting in homogeneous and nearly defect-free crystals. Nanoindentation along the [001] direction produces strain bursts, radial cracking, and lateral cracking. Ideal cleavage simulations along the [001] and [100] directions using density functional theory calculations revealed that cleavage along the [001] direction requires a much lower stress than cleavage along the [100] direction. This strong anisotropy of cleavage strength implies that CaFe2As2 has an atomic-scale layered structure, which typically exhibits lateral cracking during nanoindentation. This special layered structure results from weak atomic bonding between the (001) Ca and Fe2As2 layers.</p

Uniaxial compression of [001]-oriented CaFe2As2 single crystals: the effects of microstructure and temperature on superelasticity Part I: Experimental observations

Micropillar compression experiments on [001]-oriented CaFe2As2 single crystals have recently revealed the existence of superelasticity with a remarkably high elastic limit of over 10%. The collapsed tetragonal phase transition, which is a uni-axial contraction process in which As-As bonds are formed across an intervening Ca-plane, is the main mechanism of superelasticity. Usually, superelasticity and the related structural transitions are affected strongly by both the microstructure and the temperature. In this study, therefore, we investigated how the microstructure and temperature affect the superelasticity of [001]-oriented CaFe2As2 micropillars cut from solution-grown single crystals, by performing a combination of in-situ cryogenic micromechanical testing and transmission electron microscopy studies. Our results show that the microstructure of CaFe2As2 is influenced strongly by the crystal growth conditions and by subsequent heat treatment. The presence of Ca and As vacancies and FeAs nanoprecipitates affect the mechanical behavior significantly. In addition, the onset stress for the collapsed tetragonal transition decreases gradually as the temperature decreases. These experimental results are discussed primarily in terms of the formation of As-As bonds, which is the essential feature of this mechanism for superelasticity. Our research outcomes provide a more fundamental understanding of the superelasticity exhibited by CaFe2As2 under uni-axial compression.</p