Onset Of Flash Research Articles

Silver catalytic effect on flash sintering of ceramics

We investigated the use of metallic silver as a catalyst for flash sintering on hydroxyapatite/zirconia (HZ) composites. Adding 3 wt% of silver to the ceramic powder reduced the flash onset temperature (from 1084 °C to 925 °C) and also the electric field (from 300 V cm−1 to 150 V cm−1) used in the process. We attribute this behaviour to the formation of a silver percolation path that acts as a heat source, accelerating the thermal runaway. As a result, both the flash temperature and the required electric field were reduced. Furthermore, the results suggest that the temperatures along the percolation paths are high enough to reach the boiling point of silver, forming a gaseous phase that leaves the sample.

Flash Suppression Reveals an Additional Nonvisual Extrastriate Contribution for Amblyopic Suppression.

A growing body of evidence suggests that anomalous binocular interactions underlie the deficits in amblyopia, but their nature and neural basis are still not fully understood. We examined the behavioral and neural correlates of interocular suppression in 13 adult amblyopes and 13 matched controls using a flash suppression paradigm while recording steady-state visual evoked potentials. The strength of suppression was manipulated by changing the contrast (10%, 20%, 30%, or 100%) of the flash stimulus, or the suppressor, presented either in the dominant (fellow) or nondominant (amblyopic) eye. At the behavioral level, interocular suppression in normal observers was found, regardless of the eye origin of the flash onset. However, the pattern of suppression in the amblyopes was not symmetric, meaning that the suppression from the dominant eye was stronger, supporting a putative chronic suppression of the amblyopic eye. Interestingly, the amblyopic eye was able to suppress the dominant eye but only at the highest contrast level. At the electrophysiology level, suppression of the steady-state visual evoked potential responses in both groups in all conditions was similar over the occipital region, but differed over the frontal region. Our findings suggest that, although suppression in amblyopia involves an imbalanced interaction between the inputs to the two eyes in the visual cortex, there is also involvement of nonvisual extrastriate areas.

Utility of Light-Adapted Full-Field Electroretinogram ON and OFF Responses for Detecting Glaucomatous Functional Damage.

To compare parameters of electroretinogram (ERG) responses for their ability to detect functional loss in early stages of nonhuman primate (NHP) experimental glaucoma (EG), including photopic negative responses (PhNR) to a standard brief red flash on a blue background (R/B) and 200-ms-long R/B and white-on-white (W/W) flashes, to W/W flicker stimuli (5-50Hz), and to a dark-adapted intensity series. Light-adapted ERGs were recorded in 12 anesthetized monkeys with unilateral EG. Amplitudes and implicit times of the a-wave, b-wave, and d-wave were measured, as well as amplitudes of PhNRs and oscillatory potentials for flash onset and offset. Flicker ERGs were measured using peak-trough and fundamental frequency analyses. Dark-adapted ERG parameters were modeled by Naka-Rushton relationships. Only PhNR amplitudes were significantly reduced in EG eyes compared to fellow control (FC) eyes. The d-wave implicit time was delayed in EG versus FC eyes only for the W/W long flash, but in all eyes it was 10 to 20 ms slower for R/B versus the W/W condition. Flicker ERGs were <0.5 ms delayed in EG versus FC overall, but amplitudes were affected only at 5Hz. The brief R/B PhNR amplitude had the highest sensitivity to detect EG and strongest correlation to parameters of structural damage. The PhNR to the standard brief R/B stimulus was best for detecting and following early-stage functional loss in NHP EG. These results suggest that there would be no benefit in using longer duration flashes to separate onset and offset responses for clinical management of glaucoma.

Probabilistic analysis on the influences of heatwaves during the onset of flash droughts over China

Abstract The increasing concurrences of heatwaves and droughts in the context of global warming have attracted much attention from the scientific community given their devastating social and environmental impacts. In this study, the effects of heatwaves in each adjacent week of flash drought onset on the intensification rate of soil moisture were quantified through a meta-Gaussian-based conditional probability model. Results showed that both heatwaves and flash droughts have become more frequent since the middle of the 1990s. For the seasonal distributions, except for the southwestern region where flash droughts lagged behind heatwaves, there was a good synchronization between the two climate extremes. Strong correlations between heatwaves and flash droughts were found in the northeastern, northern, and southwestern regions. Heatwaves with varied timing of emergence behave differently on the formation of flash droughts, along with significant regional differences. Short-term impending hot conditions were crucial for the breakout of flash droughts, especially for the week when flash droughts were initiated, the emergence of heatwaves was likely to increase the intensification rate of soil moisture by 20% compared to those with no heatwaves in their development stage.

A novel two-step field-assisted sintering method to prepare (MgCoNiCuZn)O high entropy oxide

A two-step sintering experiment of (MgCoNiCuZn)O high entropy oxide was performed to lower the flash onset temperature and shorten the process time. The flash onset temperature was efficiently minimized by regulating the first step heating process due to the variation of the conductivity and density of the compacts. The lowest onset temperature was only 307 °C at a 300 °C first step sintering. Subsequently, the as-sintered material with mainly rock-salt structure can be obtained in just 20 s soaking time after the field-assisted flash sintering procedure. The unusual densification was mainly caused by the higher configuration entropy and significant temperature gradients resulted from the preferential Joule heating of grain boundaries. This novel two-step sintering method would promote the rapid synthesize of high entropy oxides.

Characterization of scotopic and mesopic rod signaling pathways in dogs using the On–Off electroretinogram

BackgroundThe On–Off, or long flash, full field electroretinogram (ERG) separates retinal responses to flash onset and offset. Depending on degree of dark-adaptation and stimulus strength the On and Off ERG can be shaped by rod and cone photoreceptors and postreceptoral cells, including ON and OFF bipolar cells. Interspecies differences have been shown, with predominantly positive Off-response in humans and other primates and a negative Off-response in rodents and dogs. However, the rod signaling pathways that contribute to these differential responses have not been characterized. In this study, we designed a long flash protocol in the dog that varied in background luminance and stimulus strength allowing for some rod components to be present to better characterize how rod pathways vary from scotopic to mesopic conditions.ResultsWith low background light the rod a-wave remains while the b-wave is significantly reduced resulting in a predominantly negative waveform in mesopic conditions. Through modeling and subtraction of the rod-driven response, we show that rod bipolar cells saturate with dimmer backgrounds than rod photoreceptors, resulting in rod hyperpolarization contributing to a large underlying negativity with mesopic backgrounds.ConclusionsReduction in rod bipolar cell responses in mesopic conditions prior to suppression of rod photoreceptor responses may reflect the changes in signaling pathway of rod-driven responses needed to extend the range of lighting conditions over which the retina functions.

Room temperature flash of single crystal titania: Electronic and optical properties

AbstractA single‐crystal specimen of rutile (titania) was flashed repetitively, while increasing the electric field after each cycle. As expected, the flash onset temperature continued to drop modestly at higher fields. However, when the field was increased from 400 to 450 V cm–1, the flashed onset fell dramatically down to room temperature. We have investigated the electrical and optical properties of this room temperature flashed specimen (called SZ). The specimen was electronically conducting. Optical absorption spectroscopy revealed a narrow band of new energy levels that were generated just below the conduction band. The gap between the conduction band and this flash‐induced energy level agreed with the peak in the electroluminescence spectrum. Optical second harmonic generation (SHG) is reported. The flash‐on condition significantly lowered the SHG, which rebounded when the flash was turned off. This result suggests that the structure becomes more centrosymmetric in the state of flash, which may represent a disordered state of defects. The possibility of studying flash behavior at room temperature, without a furnace (as in SZ type specimens), opens a considerable simplification for in‐situ characterization of flash behavior. For example, a possible relationship between memristor physics and the flash phenomenon can be studied.

On the current percolation-induced hot-spot issue during flash sintering: The case of 8 mol% yttria-stabilized zirconia and Gd2Zr2O7 composites

Hot-spot creation during flash sintering is considered a key limitation for its industrialization. Establishing a method to predict this phenomenon can address this limitation. Here, a three-phase Gd2O3-m-ZrO2-8YSZ composite was designed to introduce a percolative current, and the induced hot-spot issue was systematically investigated. We show that the percolative current can be induced when the 8YSZ fraction reaches a certain value, but it will be suppressed as Gd2Zr2O7 generated at flash onset. On these bases, the effect of percolative current on sintering behavior was investigated by performing isothermal flash experiments. We demonstrated that hot spots tend to form in the presence of percolative current, and their size increases with increasing voltage. This was further distinguished with respect to onset power density, from which an abnormal rise was observed in the hot-spot samples. This finding was explained in terms of thermal runaway and could be considered an indicator of local heating.

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ∼90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.

On the catalytic effect of zirconia on flash sintering of alumina

AbstractFlash 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.

Accessing the role of Joule heating on densification during flash sintering of YSZ

Flash sintering is a novel electric field and current assisted sintering technique which densify ceramics at moderate furnace temperature and in short time (few seconds) as compared to the conventional sintering techniques. The onset of flash encompasses three unique characteristics, (i) non-linear rise in conductivity, (ii) luminescence and (iii) rapid densification. the mechanism behind the flash phenomena is not fully understood yet. The present work experimentally demonstrated that the densification in flash sintering is primarily dominated by Joule heating. Herein, we distinguished the onset of flash and the degree of densification of cubic zirconia in sintering atmospheres with different oxygen partial pressures. In this study, we demonstrate that the flash onset temperature and densification are independently influenced by the sintering atmosphere during flash sintering. Furthermore, the extent of densification is not directly influenced by the current density rather, by the power dissipated in the sample.

Initiation of lightning flashes simultaneously observed from space and the ground: Narrow bipolar events

We investigate the initiation of four lightning flashes detected from ground by means of the Colombia Lightning Mapping Array (Colombia-LMA) and simultaneously observed from space by the optical sensors of the Atmosphere-Space Interactions Monitor (ASIM) on board the International Space Station (ISS), the Geostationary Lightning Mapper (GLM), and the Lightning Imaging Sensor on the ISS. The initiations of the flashes are characterized by isolated and predominant optical blue pulses (337.0 nm). In three of the flashes, red emissions (777.4 nm), a dominant line of hot lightning, were not detected during their initiation. In these cases, the initiations were also accompanied by bipolar VLF/LF waveform with a narrow short duration (<40 μs) and VHF emissions with high radio frequency power (<269 kW). The detection of the blue emissions without any red luminosity supports that the fast breakdown processes at the flash initiation can be exclusively of streamer nature. The onset of the fourth flash was associated with both blue and red radiation, and with weak narrow bipolar waveform in VLF/LF and low VHF power. The flashes initiated between the midlevel negative and upper positive charge regions. This paper presents and discusses the first fast breakdown processes observed simultaneously from ground by means a Lightning Mapping Array (LMA) and from space during the onset of lightning flashes.

Effect of Processing Conditions on the Flash Onset Temperature in Hydroxyapatite.

When heat and electric field are applied to the sample, sintering takes place within a short time of a few seconds by the flash phenomenon that occurs. In what condition flash does occur is a main issue for the flash sintering technique. In this study, the effect of processing conditions such as sintering atmosphere, sample size, density and grain size on the flash onset of hydroxyapatite was investigated. In a vacuum atmosphere, a flash occurred at a lower temperature by 50–100 °C than in air. The smaller the thickness of the sample, the higher the flash onset temperature due to the larger specific surface area. Flash was also observed in samples which were presintered, having a density of 86–100% and a grain size of 0.2–0.9 μm. When the density and grain size of the sample were higher and larger, the flash onset temperature was higher. It was because the diffusion and conduction path through the grain boundary and the inner surface of the pores with high defect concentration are blocked with an increase of density or grain size. When an electric field was applied during flash sintering, a color change of the sample was observed and the reason was discussed.

Effect of thermal insulation on microstructural homogeneity and onset temperature of flash sintered materials

The present work aimed to reduce the microstructure heterogeneity inherent to flash sintering by using alumina blankets as a thermal insulator around ZnO cylindrical samples during the sintering process, under different electric field conditions. Thermal insulation significantly reduced the flash onset temperature and the grain size heterogeneity. For higher electric fields, a temperature reduction as high as 480 °C was observed, which also led to lower densification. These findings were discussed in terms of changes in the heat loss dynamics coupled with the adsorbed water retention, both promoted by the applied thermal insulation. A model to estimate the temperature at stage III of flash sintering was proposed. The final temperature reached with thermal insulation did not differ significantly from the ones without it. Thus, thermal insulation could represent an alternative route to flash sinter materials with lower furnace temperatures with energy savings up to 78 % and a more homogeneous microstructure.

On the power density at the onset for flash sintering in ceramic composites

Flash sintering is usually said to be activated in a narrow range of power density regardless of the onset flash temperature and material. However, the typical logarithmic scale employed in the flash power plots could hide some finer tendencies in the flash onset conditions. Herein, we investigated the flash onset in YSZ/Al2O3 composites taking into account the power density normalized to the total composite (won-comp) and the conductive phase volume (won-YSZ). The results show that at the flash onset (i) won-YSZ strongly increases with the Al2O3 amount in the composite, (ii) won-comp weakly depends on the composite composition, and (iii) both won-comp and won-YSZ increase with the flash onset temperature (which is a function of the electric field). The results offer new light on the mechanisms of flash ignition and a new insight on the power required to activate the phenomenon, which appears to be consistent with a thermally-driven process.

Flash Sintering of YSZ/Al2O3 Composites: Effect of Processing and Testing Conditions.

The flash sintering behavior of yttria-stabilized zirconia/alumina composites was investigated to understand the role of the fundamental processing and testing parameters (electric field intensity, electric current limit, thermal insulation, homogeneity and dispersion of the two phases) on densification. A strong relation between the composite compositions and the electric parameters needed to promote flash sintering is revealed. Interestingly, the composite preparation method, which affects the two-phases dispersion homogeneity, was shown to have a relevant effect on the flash onset conditions, where the more homogeneous material is more difficult to be flashed. Moreover, the use of a simple thermal insulation system around the green body allowed to improve the final density of the composites under constant electric current.

Flash sintering of yttria‐stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

AbstractThe addition of small quantities of aluminum oxide (Al2O3) to 8 mol% yttria‐stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al2O3 are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD‐coated powders were flash sintered using voltage‐to‐current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al2O3 films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al2O3 can enhance grain growth and sintering of 8YSZ. This suggests that Al2O3 dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.

Effects of powder dispersion on reactive flash sintering of 8 mol% yttria-stabilized zirconia and MgAl2O4 composites

A three-phase system consisting of alumina (Al2O3), magnesia (MgO) and 8 mol% yttria‐stabilized zirconia (8YSZ) was reactive flash sintered to produce MgAl2O4-8YSZ composites. We show that a critical volume fraction of the ionic conductor 8YSZ is required to promote the flash phenomenon, indicating a typical percolation behavior. The easy path for conduction could be reached with a smaller critical volume fraction when using agglomerated powders for producing the green bodies. Furthermore, microstructure and X-ray diffraction analyses demonstrate that both sintering and phase transformation largely depend on the mixing level of the precursors. Although the presence of agglomerates resulted in a decrease of the flash onset temperature, poor sintering and incomplete phase transformation were verified. This behavior is attributed to inhomogeneous heat distribution within those samples during the flash state, which could be explained by the electrical current percolation.

Elastic Softening in Zirconia During Stage III of Flash Experiments

We report in-situ measurements of the elastic modulus of dense polycrystalline zirconia held in a constant state of flash, i.e. during Stage III of flash experiments. The modulus, measured with and without flash, is lowered by 10 - 35 %, in the temperature range of 1100-1400 oC. The specimen temperature, measured with a pyrometer, was consistent with the estimate from a black body radiation model. These measurements echo recent publications on the role of phonons in the onset of flash, namely the Debye temperature as a lower bound for flash, and the formation of Frenkel pairs in molecular dynamics calculations by the proliferation of phonons above the Debye temperature. However, several questions remain, for example, the rise in electrical conductivity, and electroluminescence that are nearly universal signatures of the flash phenomenon.

Electric forces effect on field-assisted sintering

Electric forces acting on crystallographic defects generated at the flash onset or due to the surface polarization of the dielectric surrounding pores have been claimed to interact with the sintering kinetics upon flash processes. Herein, we estimated the magnitude of the external field-induced forces pointing out that the surface polarization does not play significant role on sintering. Moreover, the electric forces acting on charged defects are negligible when compared with the sintering stresses in the majority of flash experiments, only becoming comparable with the capillarity pressures when the external field is in the order of 103 V cm−1. Therefore, they can be of some relevance during flash sintering of resistive compounds like alumina or silicate glasses, they remaining normally not relevant otherwise.