Understanding and utilization of thermal gradients in spark plasma sintering for graded microstructure and mechanical properties

2022 Summer.Includes bibliographical references.Spark plasma sintering (SPS), also commonly known as electric field assisted sintering, utilizes high density electric currents and pressure to achieve rapid heating and significantly shorter sintering times for consolidating metal and ceramic powders, which could otherwise be difficult, time consuming, and energy intensive. SPS has attracted extensive research interests since the early 1990's, with the promise of efficient manufacturing of refractory materials, ultrahigh temperature ceramics, nanostructured materials, functionally graded materials, and non-equilibrium materials. Thermal gradients occur in SPS tooling and the samples during sintering, which can be a drawback if homogeneous properties are desirable, as the temperature inhomogeneity can lead to large gradients in microstructure such as porosity, grain size, and phase distribution. Many researchers have looked to mitigate or control these gradients by design and use of specialized tooling. However, the effect of the starting powder is relatively less investigated or overlooked. Feedstock powders can come in various shapes, particle size distributions, and surface chemistry. Effects of these powder characteristics on the SPS process and the consequent microstructure of the sintered parts remain as a gap in the fundamental knowledge of SPS. To fill in this gap, my research investigated the role of thermal gradients during SPS, and how the thermal gradients subsequently affect the location-specific pore distribution, and the consequent mechanical properties of the materials. From a practical point of view, design and fabrication of a bulk sample with a fully dense surface and an engineered pore architecture in the sample interior via one-step SPS will enable mechanical properties unattainable via conventional processing of fully dense bulk materials, such as alike combination of lightweight, high surface hardness, and wear resistance, and high toughness. Therefore, the overarching goal of my research was to provide fundamental insights into the material processing - microstructure - properties correlation so that the field assisted sintering technology can be advanced to control location-specific microstructure. To fulfill this goal, two metallic materials were selected in my study, austenitic stainless steel and commercially pure titanium, representing inherently heavy but widely used alloys, and a pure metal that is inherently lightweight, these materials were used to investigate the effects of powder morphology on the sintering behavior. The pure Ti was selected specifically to gain fundamental insight into the effect of powder shape on sintering, while mitigating the concern of alloying/precipitation events and integrating FEM with my experimental work. This work identified a relationship between decreasing pore size and increasing yield strength in stainless steel, which was attributed to fine precipitate formation surrounding submicron pores inducing local stiffening. Whereas larger pores where precipitates were not found are concluded to not have the necessary driving force for the precipitation event to occur. Ball milled stainless steel powders with higher aspect ratios were also shown to have smaller porosity gradients in comparison to their spherical gas atomized counterparts. A thermal electric finite element model is also proposed which incorporates the master sintering curve to simulate densification as an alternative to the more computationally costly and difficult to parametrize fully coupled thermal-electric-mechanical finite element model. Results from the combined model indicate strong agreement with experimental results within 2% accuracy of measured densification. Additionally, the model predicts higher porosity gradients for gas atomized powders in comparison to ball milled powders which is experimentally verified

Geometrical Modeling of Material Aging

Material aging is understood as changes of material properties with time. The aging is usually observed as an improvement of some properties and a deterioration of others. For example an increase of rigidity and strength and reduction in toughness with time are commonly observed in engineering materials. In an attempt to model aging phenomena on a continuum (macroscopical) level one faces three major tasks. The first is to identify an adequate age parameter that represents, on a macroscopic scale, the micro and sub microscopical features, underlying the aging phenomena such as nucleation, growth and coalescence of microdefects, physico-chemical transformations etc. The age parameter should be considered as a parameter of state, in addition to the conventional parameters such as stress tensor and temperature. The second task consists of formulation of a constitutive equation of aging, i.e., equations of age parameter evolution expressed in terms of controlling factors, e.g., load and temperature. It is expected that at common circumstances a small variation of controlling factors results in a small variation of age parameter. However, at certain conditions, a sudden large variation of age parameter may result from a small perturbation of controlling factors. Experimental examination, classification and analysis of the condition that lead to such a catastrophic behavior, constitute the third task of the modeling. Formulation of local failure criteria within the scope of continuum mechanics is an example of this task. In many engineering materials the aging is manifested in variations of mass density as well as in the spectrum of relaxation time. Thus in a macroscopic test the aging can be detected in variations of intrinsic (material) length and time scales. Following this notion, in the present paper we employ the material metric tensor G as an age parameter. An evolution of Gin 4D -material space-time determines in our approach an inelastic behavior and time dependent material properties recorded by an external observer. The objective of the present work is to derive the constitutive equations of aging based on Extremal Action Principle. The variational approach seems to be most promising in view of complexity of the problem and lack of experimental data. It provides with a guide line for the experimental examination of the basic assumptions and modifications, if necessary

A Bayesian approach to the analysis of time symmetry in light curves: Reconsidering Scorpius X-1 occultations

We present a new approach to the analysis of time symmetry in light curves, such as those in the x-ray at the center of the Scorpius X-1 occultation debate. Our method uses a new parameterization for such events (the bilogistic event profile) and provides a clear, physically relevant characterization of each event's key features. We also demonstrate a Markov Chain Monte Carlo algorithm to carry out this analysis, including a novel independence chain configuration for the estimation of each event's location in the light curve. These tools are applied to the Scorpius X-1 light curves presented in Chang et al. (2007), providing additional evidence based on the time series that the events detected thus far are most likely not occultations by TNOs.Comment: 24 pages, 18 figures. Preprint typeset using LaTeX style emulateapj v. 04/20/0

Investigation Of Hydrologic Change In The South Fork Edisto Basin 1981-2016

Increased demands on water resources coupled with an increased awareness of the need to preserve environmental flows have led to a need to understand how anthropogenic activities may have impacted water resources in South Carolina. The South Fork of the Edisto River has been facing increasing demands for both surface water and groundwater. This research investigated if and why hydrologic change, as indicated by a change in the rainfall/ stream discharge relationship, occurred in the South Fork Edisto Basin, in South Carolina, between the years 1980-2016. A double-mass curve analysis, which is corroborated by runoff coefficients, indicates that a fundamental change in the precipitation-runoff relationship in the South Fork Edisto Basin occurred in the early 2000s. Documented water diversions from the basin were examined as a possible driver of the changing relationship. A Soil Water Balance Model was developed to isolate water budget components and to examine how changes in climatic variables and land use and land cover acted as potential drivers of the change documented by the double mass curve. The results indicate that all three variables have had an impact on the basin. The results have implications to water availability, environmental flows, and water resources management within the basin

Competition among Sellers in Securities Auctions

We study simultaneous security-bid second-price auctions with competition among sellers for potential bidders. The sellers compete by designing ordered sets of securities that the bidders can offer as payment for the assets. Upon observing auction designs, potential bidders decide which auctions to enter. We characterize all symmetric equilibria and show that there always exist equilibria in which auctions are in standard securities or their combinations. In large markets the unique equilibrium is auctions in pure cash. We extend the model for competition in reserve prices and show that binding reserve prices never constitute equilibrium as long as equilibrium security designs are not call options. (JEL D44, D82, G10)

A national survey of community rehabilitation service provision for people with long Covid in Scotland

Background: Over 50 million cases of COVID-19 have been confirmed globally as of November 2020. Evidence is rapidly emerging on the epidemiology of COVID-19, and its impact on individuals and potential burden on health services and society. Between 10–35% of people with COVID-19 may experience post-acute long Covid. This currently equates to between 8,129 and 28,453 people in Scotland. Some of these people will require rehabilitation to support their recovery. Currently, we do not know how to optimally configure community rehabilitation services for people with long Covid. Methods: This national survey aimed to provide a detailed description of current community rehabilitation provision for people with long Covid in Scotland. We developed, piloted, and conducted a national electronic survey of current community rehabilitation service provision for people presenting with long Covid symptomatology. Our sample were the Allied Health Professions Directors of all 14 territorial NHS Health Boards in Scotland. Fixed response and narrative data were analysed descriptively. Results: Responses were received from all respondents (14/14), enabling a national picture to be gained. Almost all Health Boards (13/14) currently deliver rehabilitation for people with long Covid within pre-existing services. Fatigue (11/14) and respiratory conditions (9/14) were the two most common presenting problems of patients. Most long Covid community rehabilitation services are delivered through a combination of face-to-face and digital contact (13/14). Conclusions: Community rehabilitation for people with long Covid is an emerging reality. This survey provides a national picture of current community rehabilitation for people with long Covid. We do not know how community rehabilitation can be optimally delivered for this population. This is vital as community rehabilitation services were already under pressure prior to the emergence of COVID-19. Further research is urgently required to investigate the implementation, outcomes and cost-effectiveness of differing models of community rehabilitation for this patient population.[Version 2; peer review: 2 approved

Designing hollow nano gold golf balls.

Hollow/porous nanoparticles, including nanocarriers, nanoshells, and mesoporous materials have applications in catalysis, photonics, biosensing, and delivery of theranostic agents. Using a hierarchical template synthesis scheme, we have synthesized a nanocarrier mimicking a golf ball, consisting of (i) solid silica core with a pitted gold surface and (ii) a hollow/porous gold shell without silica. The template consisted of 100 nm polystyrene beads attached to a larger silica core. Selective gold plating of the core followed by removal of the polystyrene beads produced a golf ball-like nanostructure with 100 nm pits. Dissolution of the silica core produced a hollow/porous golf ball-like nanostructure