Flash sintering was discovered in 2010, where a dog‐bone‐shaped zirconia sample was sintered at a furnace temperature of 850°C in <5 s by applying electric fields of ~100 V cm−1 directly to the ...specimen. Since its discovery, it has been successfully applied to several if not all oxides and even ceramics of complex compositions. Among several processing parameters in flash sintering, the electrical parameters, i.e., electric field and electric current, were found to influence the onset temperature for flash and the degree of densification respectively. In this work, we have systematically investigated the influence of the electrical parameters on the onset temperature, densification behavior, and microstructure of the flash sintered samples. In particular, we focus on the development of a processing map that delineates the safe and fail regions for flash sintering over a wide range of applied current densities and electric fields. As a proof of concept, gadolinium‐doped ceria (GDC) is shown as an example for developing of such a processing map for flash sintering, which can also be transferred to different materials systems. Localization of current coupled with hot spot formation and crack formation is identified as two distinct failure modes in flash sintering. The grain size distribution across the current localized and nominal regions of the specimen was analyzed. The specimens show exaggerated grain growth near the positive electrode (anode). The region adjacent to the negative electrodes (cathode) showed retarded densification with large concentration of isolated pores. The electrical conductivity of the flash sintered and conventional sintered samples shows identical electrical conductivity. This quantitative analysis indicates that similar sintering quality of the GDC can be achieved by flash sintering at temperature as low as 680°C.
Microwaves and spark plasma sintering (SPS) enhance sinterability. Simple electrical fields, applied by means of a pair of electrodes to bare specimens, have been shown to accelerate the rate of ...superplastic deformation, reduce the time and temperature for sintering, and to retard the rate of grain growth. By inference, the influence of electrical and electromagnetic fields on grain boundary energetics and kinetics is unmistakable. Often, in ceramics, grain boundaries are themselves endowed with space charge that can couple with externally applied fields. The frequency dependence of this coupling ranging from zero frequency to microwave frequencies is discussed. The classical approach for modeling grain growth, creep, and sintering, considers chemical diffusion (self‐diffusion) under a thermodynamic driving force, underpinned by a physical mechanism that visualizes the flow of mass transport in a way that reproduces the phenomenological observations. In all instances, the final analytical result can be separated into a product of three functions: one of the grain size, the second related to the thermodynamic driving force, and the third to the kinetics of mass transport. The influence of an electrical field on each of these functions is addressed.The fundamental mechanisms of these electrical interactions are discussed in the following ways: (i) dielectric loss and Joule heating in the crystal and at the grain boundary, (ii) the coupling between mechanical stress and the electrochemical potential of charged species, (iii) the interaction between applied electrical fields and the intrinsic fields that exist within the space charge layers, (iv) and the possibility of nucleating defect avalanches under electrical fields. We limit ourselves to ceramics that have at least some degree of ionic character. In these experiments the electrical fields range from several volts to several hundred volts per centimeter, and the power dissipation from Joule heating is of the order of several watts per cubic centimeter of the specimen. Metals, where very high current densities are obtained at relatively low applied electric fields, leading to phenomenon such as electromigration, are not considered.
The ionic conductivity of flash-sintered, polycrystalline 8 mol% yttria stabilized zirconia (8YSZ) was enhanced compared with that of conventionally-sintered specimens. Flash sintering was carried ...out at a furnace temperature of 850 °C with an electric field of 100 V cm–1 to initiate flash. The current density limit was varied between 60 and 100 mA mm–2. Post-flash impedance measurements over the range 215–900 °C showed that both bulk and grain boundary conductivities had increased with the increased current density limit which was set prior to flash. The conductivity increases post-flash were ionic, not electronic, although electronic conductivity probably occurred, in addition to ionic conductivity, during flash. The conductivity increases were not attributable to sample densification or microstructural changes. The higher ionic conductivities are attributed to a change in YSZ defect structure that led to an increased concentration of mobile charge carriers; possible explanations for this are discussed.
Silicon‐based polymers evolve into refractory ceramics when heated gradually up to ~1000°C. The conversion is accompanied by the loss of gaseous species, and by a two‐fold increase in density. The ...shrinkage can produce microcracks if the heating rate is too high, or if the specimen is too thick. This communication builds on earlier work whereby the measurement of gas evolution, and its relationship with viscous flow, are related to the onset of fracture in disk‐shaped green (polymer) samples. The onset is determined as a function of the thickness of the disks, and of the heating rate. The results are presented in the form of a processing map. The overlay with gas evolution, and strain‐rate measurements, suggest that fracture initiates with the release of hydrogen and methane, starting at temperatures near 750°C.
Flash sintering of alumina is more difficult than of yttria‐stabilized zirconia (YSZ). Whereas (MgO doped) alumina requires fields greater than 1 kVcm–1 and temperatures often significantly higher ...than 1000°C, YSZ can be flashed sintered at ∼100 Vcm–1 at temperatures below 800°C. Mixed powders of such bi‐phasic ceramics, on the other hand, can be flashed under conditions below those for alumina. This effect is usually subscribed to the influence of YSZ on the overall electrical conductivity of the composite. However, such rationalization leaves open the mechanism by which YSZ catalyzes the flash event in alumina. Here, we present results for the onset of flash in a layered structure of YSZ and alumina where both constituents extend from one electrode to the other. We find that the flash initiates, at first, exclusively in the YSZ layer, under conditions identical to those in usual voltage‐to‐current experiments in single phase YSZ, and then, after a brief incubation period, spreads transversely through the thickness of the alumina layer at a speed of ∼3.3 mm s–1, while the power supply is held at constant current. This observation opens a new question as to how flash once initiated in an “easy” phase can migrate normal to itself into a second ceramic, which is nominally more‐difficult‐to flash. (In the present experiments, the alumina layer sintered to full density with all the shrinkage being accommodated in the thickness direction, consistent with an earlier study that articulated that flash obviates constrained sintering.) It is noteworthy that the catalytic effect depends not only on the volume fraction of YSZ, but also on the architecture of the green state (for example a two‐phase powder mixture vs. layered structure), which may affect the initiation of the flash in YSZ but, likely, not its migration behavior into the second phase.
A highly nonlinear increase in electrical conductivity and concomitant photoemission occurs not only during “flash sintering” but also in presintered, dense specimens. We report results from ...experiments with dense specimens that show a strong correspondence between the intensity of photoemission and electrical conductivity of specimens under a variety of conditions of the electrical parameters. It is proposed that both properties are related to the concentration of electron–hole pairs generated in such experiments.
Oxidation of SiC at 1400°C–1700°C produces carbon monoxide (CO), which can form bubbles at the interface between the substrate and the silica (SiO2) overgrowth. The bubbles add complexity to the ...mechanistic understanding of the oxidation process. We analyze the kinetics of bubble formation, where the driving force for their nucleation depends on the partial pressure of CO while their growth is determined by the viscosity of silica. The mechanism bears analogy to oxygen migrating from the atmosphere into superalloys, and reacting with carbide precipitates at the grain boundaries to nucleate bubbles of carbon monoxide, or hydrogen migrating into carbon‐steel to form methane. The unique feature of the present case is the ability of silica to release the gas pressure within the bubbles by viscous flow, and then, to heal the crater that is left behind. As the bubbles grow out from the interface and escape into the atmosphere, they can expose bare surface to the atmosphere rendering silicon carbide vulnerable to active oxidation. The relative significance of the mechanisms of oxidation of SiC is presented in the form of an oxidation map, where atmospheric pO2 and oxidation temperature are the experimental variables. We discover that experiments often lie in the regime of bubble formation.
We show, for the first time, that a dc electric field of 20 V/cm shifts the densification curve to a lower temperature in constant heating rate experiments with yttria‐stabilized tetragonal zirconia ...powder (3Y‐TZP). The enhanced sintering rate is ascribed, at least in part, to the reduced rate of grain growth under the applied field, consistent with earlier experiments on the influence of such fields on grain size in superplastic deformation and isothermal grain growth in zirconia polycrystals.
Understanding of the molecular mechanisms of prostate cancer has led to development of therapeutic strategies targeting androgen receptor (AR). These androgen-receptor signaling inhibitors (ARSI) ...include androgen synthesis inhibitor-abiraterone and androgen receptor antagonists-enzalutamide, apalutamide, and darolutamide. Although these medications provide significant improvement in survival among men with prostate cancer, drug resistance develops in nearly all patients with time. This could be through androgen-dependent or androgen-independent mechanisms. Even weaker signals and non-canonical steroid ligands can activate AR in the presence of truncated AR-splice variants, AR overexpression, or activating mutations in AR. AR splice variant, AR-V7 is the most studied among these and is not targeted by available ARSIs. Non-androgen receptor dependent resistance mechanisms are mediated by activation of an alternative signaling pathway when AR is inhibited. DNA repair pathway, PI3K/AKT/mTOR pathway, BRAF-MAPK and Wnt signaling pathway and activation by glucocorticoid receptors can restore downstream signaling in prostate cancer by alternative proteins. Multiple clinical trials are underway exploring therapeutic strategies to overcome these resistance mechanisms.
Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite ...significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid–liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.
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