The large bank of data for ceramics from experiments in flash sintering reveal a surprising characteristic: that the transition to a highly nonlinear rise in electrical conductivity—a signature event ...for the onset of the flash—occurs within a narrow range of power density. This condition holds for ceramics that are semiconductors, ionic conductors, electronic conductors, and insulators.They flash at temperatures that range from 300°C to 1300°C, and at electric fields from 10 V/cm to over 1000 V/cm. Yet, the power expenditure at the transition for all of them still falls within this narrow range. This, rather uniform value of power dissipation suggests that Joule heating is a key factor in instigating the flash. A general formulation is developed to test if indeed Joule heating alone can lead to the progression of such nonlinear behavior. It is concluded that Joule heating is a necessary but not a sufficient condition for flash sintering.
Recent work on reactive flash sintering of powders of two oxides, bismuth and of iron oxide, into pure single phase bismuth ferrite, which was accomplished in a few seconds at low furnace ...temperatures, is expanded to four constituents, alumina, lithia, zirconia, and lanthana, to produce reasonably dense polycrystals of a predominantly single phase, cubic LLZO(Al). Transformation and sintering occur concurrently at a furnace temperature near 700°C, in ambient atmosphere, in just a few seconds. The process may simplify the preparation of complex ceramics with new chemistries and dopants, which are predicted from ab intio calculations to have special attributes, not only because the powders sinter quickly at low temperatures, but also because the need for stoichiometric powders as starting materials is obviated.
AMP kinase (AMPK) is an evolutionarily conserved enzyme required for adaptive responses to various physiological and pathological conditions. AMPK executes numerous cellular functions, some of which ...are often perceived at odds with each other. While AMPK is essential for embryonic growth and development, its full impact in adult tissues is revealed under stressful situations that organisms face in the real world. Conflicting reports about its cellular functions, particularly in cancer, are intriguing and a growing number of AMPK activators are being developed to treat human diseases such as cancer and diabetes. Whether these drugs will have only context-specific benefits or detrimental effects in the treatment of human cancer will be a subject of intense research. Here we review the current state of AMPK research with an emphasis on cancer and discuss the yet unresolved context-dependent functions of AMPK in human cancer.
We report in-situ measurements of impedance spectroscopy during flash experiments with single crystals of cubic-yttria stabilized zirconia, conducted in ambient air at 300 °C to 700 °C under a direct ...current electric field. Platinum electrodes transition from blocking to non-blocking behavior before the onset of flash, indicating a transition to partial electronic conductivity during the incubation period. Even though the conductivity continued to increase with temperature the electron transport remained at a plateau of approximately 0.25. This result has ramifications not only in the fundamental understanding but also in technological significance of the flash phenomenon.
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We show that flash experiments with three phase mixed‐powders of yttria‐stabilized zirconia (8YSZ), MgO, and α‐Al2O3 not only produce polycrystals of high density, but also the transformation of ...magnesia and alumina into single‐phase spinel. The presence of zirconia facilitates the onset of the flash. The sintering experiments in the laboratory were extended to live experiments at the National Synchrotron Light Source II at Brookhaven National Laboratory in order to measure the time‐dependent evolution of single‐phase spinel. The phase transformation occurred in <3 seconds during Stage II. Later, during Stage III the cubic zirconia transformed partly into the monoclinic phase, which reverted back to the cubic phase when the flash was extinguished by turning off the current to the specimen. The results underpin a recent report on the synthesis of single‐phase bismuth ferrite from constituent oxides in reactive flash experiments, raising the specter of flash as a method for synthesis as well as sintering of complex oxide ceramics. The role of zirconia in catalyzing the flash in the present study is discussed.
The oxidation behavior of SiC fibers coated with (a) undoped polysilazane and (b) precursors containing a mixture of polysilazane and hafnium butoxide in equal weight fractions, is reported. The ...coatings were prepared by repetitive cycles of nanolayer depositions, as reported in recent publications. The oxidation experiments were carried out at 1400°C in ambient air (Boulder, CO) for up to 100 hours. The extent of degradation of SiC was measured by the recession in the diameter of the fibers as a function of time. The fibers with undoped polymer precursor recessed significantly, whereas the fibers coated with HfSiCNO remained essentially unchanged. These results are in agreement with earlier work from our laboratory where the resilience of hafnon and zircon, as well as hafnia and zirconia, against high‐temperature corrosion in streaming humid environments had been highlighted.
Flash sintering has evolved into touch‐free sintering, where free‐standing workpieces can be sintered without attaching electrodes. Instead, the flash is transmitted from the surface of a reactor ...into the workpiece with superimposition of a magnetic field. Thus, sintering now depends on two independent parameters: the current used to sustain the flash in the reactor and the current flowing through the induction coil. We present a first report on the influence of these two parameters on the quality of the sintered workpiece. The specimens were made from whiteware, consisting of aggregates of ceramic particles interspersed with particles of a glass phase. The results are presented in a map with the reactor current and the induction current as the control variables. Three regimes are identified: insufficient sintering, good sintering, and the formation of defects. The reactor current emerges as an important variable: densification is poor if it is too low, and defects form if it is too high, with high density achieved in the intermediate regime. High induction currents are needed to achieve good sintering. Touch‐free flash sintering has also been shown to sinter and at the same time transform powders of elemental oxides into a single‐phase multicomponent ceramic.
We measure the sintering behavior of thin and thick films of ceramics made by stacking several layers of tapes. The stack thickness was varied from 0.12 to 0.50 mm. A powder‐pressed dog‐bone sample, ...~1 mm thick served as the baseline. While all specimens exhibit flash sintering, the following differences are noted. (a) The onset of flash, which has been shown to occur within a narrow band of volumetric power density for powder‐pressed dog‐bone shaped specimens, moves to a higher value for the thinner specimens. (b) The power transition collapses, again, to within a narrow band when the power density is normalized with respect to the surface area rather than the volume. (c) Sintering of thinner specimens to full density required a higher current‐density limit. These results point to a deeper significance of black body radiation than its use in estimating the specimen temperature in a steady state of flash that is calculated from the energy balance between electrical and radiated energy. The experiments were carried out on 3 mol% yttria stabilized zirconia at constant heating rate
Isothermal field-assisted sintering of two-phase, 50vol% 3YSZ-alumina composites exhibit an incubation time for the onset of the flash event. Weaker applied fields and lower temperatures lengthen the ...incubation period. The effect is highly non-linear. For example at 1300°C and 150Vcm−1 the flash occurs nearly instantaneously (in 10s), but extends to 2h at 1275°C and 65Vcm−1. This behavior is reminiscent of nucleation and growth phenomena in chemically driven experiments involving phase transformations in the solid state. Here, a model for nucleation under electrical driving forces, based upon the growth of embryos of colossal permittivity is presented. Nucleation is the precursor to the onset of a flash. Therefore it occurs at the furnace temperature. Joule heating is a consequence of nucleation not the cause of it.
The effect of DC electric field on sintering, and on the electrical conductivity of undoped rutile, TiO2 (99.99%), has been investigated at fields ranging from 0 V to 1000 V/cm. The experiments were ...carried out at a constant heating rate of 10°C/min with the furnace temperatures reaching up to 1150°C. The sintering behavior falls into two regimes: at lower fields, up to 150 V/cm, sintering is enhanced, but densification occurs gradually with time (Type A or FAST sintering). At higher fields sintering occurs abruptly, and is accompanied by a highly nonlinear increase in conductivity, which has been called flash sintering (Type B or FLASH sintering). Arrhenius plots of conductivity yield an activation energy of 1.6 eV in Type A and 0.6 eV in Type B behavior; the first is explained as ionic and the second as electronic conductivity. The evolution of grain size under both types of sintering behavior are reported. These results highlight that the dominant mechanism of field‐assisted sintering can change with the field strength and temperature. We are in the very early stages of identifying these mechanisms and mapping them in the field, frequency, and temperature space.
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