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Interest in ultrafast sintering has been expanding from different directions that include not only electric and electromagnetic field driven accelerations, but also the influence of heating rates. Among them, the electrical method of flash sintering is most quantitative (and, therefore, most amenable to manufacturing). The electric field, the current density and temperature can be explicitly controlled and measured as a function of time, and related to the sintering behavior and microstructure evolution. In this article, we attempt to explain the greater generality of flash sintering, not just unto itself, but also to heating rate experiments carried out without electrical fields. The rise in electrical conductivity and electroluminescence are the principal features of flash; they are likely to be of general significance in accelerated sintering. Their occurrence in high heating rate experiments, highlighted in this article, is particularly salient, raising new fundamental issues in the science of flash sintering.

We develop an algorithm that relates the incubation time for flash initiation to the workpiece velocity as it is pulled through stationary electrodes in a continuous flash sintering experiment. Experiments with a whiteware green body that is sintered in this way are compared with the model. A processing map when the workpiece is drawn at a constant velocity is developed. The parameter space for the map is given by the speed and the current flowing through the workpiece. It distinguishes between the regimes for uniform and inhomogeneous sintering. All experiments were carried out at a furnace temperature of 900°C. The study shows the viability of continuous flash sintering of ceramics at speeds of up to 3 mm s−1 with electrodes that form a line contact with the workpiece. Remarkably, higher velocities and higher currents yield better sintering.

We report results from in-situ measurements of lattice expansion during flash sintering of 3 mol% yttria stabilized tetragonal zirconia taken at the Advanced Photon Source, Argonne National Laboratory. The expansion is anisotropic, with the relative expansion of the a-lattice constant exceeding that of the c-lattice constant. The anisotropic expansion cannot be explained by thermal expansion and is consistent with predictions from ab-initio calculations based upon the generation of vacancy-interstitial pairs of zirconium and oxygen.

We show that BiFeO, that is electrically homogeneous, is a good insulator, and has a low dielectric constant (the properties desired in its applications), can be produced by flash sintering, which is nominally difficult to achieve by conventional and spark plasma sintering processes. The flash-sintered specimens had a uniform microstructure with a nanometric grain size of ~20 nm.

We show that phase transformation in the alumina–titania system, which produces aluminum-titanate, follows an unusual trajectory during flash sintering. The experiments begin with mixed powders of alumina–titania and end in dense microstructures that are transformed into aluminum-titanate. The sintering and the phase transformation are separated in time, with the sintering occurs during Stage II, and phase transformation during Stage III of the flash sintering experiment. Stage III is the steady-state condition of flash activated state that is established under current control, while Stage II is the period of transition from voltage to current control. The extent of phase transformation increases with the current density and the hold time in Stage III.

Diffraction peaks of nanoscale particles of 3 mol% yttria-stabilized zirconia become sharper as the powder sinters. The reduction in the peak width is correlated with the increase in density. The sharpening of the peak agrees reasonably well with the remaining free surface area as the sample sinters. Therefore, high curvature of the free surface of the pores is assumed to lead to peak broadening (the grain boundaries that grow at the expense of the free surfaces of the pores do not have this curvature). The change in the grain size during sintering does not make a significant contribution to peak width.

We show that (1 1 0) and (1 1 1) peaks weaken while the (2 1 1) X-ray diffraction peak gains in intensity in titanium dioxide under the influence of the electric field, during in-situ experiments of flash sintering, carried out at the Advanced Photon Source (APS). The effect is reversible, appearing and vanishing when the field is turned on and off. The changes in the peak intensities are immediate, but they fluctuate with time even though the electric field remains constant. Similar experiments with 3 mol% yttria stabilized zirconia have shown time dependent emergence of new peaks signaling the formation of a different phase, apparently by a diffusion controlled process. In contrast, in titania the existing peaks change intensity under the electric field. We attribute the events in titania to short range movement of atoms and defects in the lattice.

The optical glow of ceramics that becomes established during the constant state of flash, known as Stage III in flash sintering experiments, is investigated. The specimen temperature in this state is obtained from in situ experiments at the Pohang Light Source II. The measurements of the specimen temperature agree very well with the predictions from the black body radiation model. The optical emission spectrum from the specimen is measured from the visible into the deep infrared, and compared with black body radiation that would have been expected from Joule heating. It is concluded that the specimens radiate by electroluminescence, which is ascribed to electron–hole recombination of excitons. The phenomenon is likely the same as discovered by Nernst at the turn of the twentieth century.

Flash sintering is a nonlinear phenomenon characterized by a sharp increase in the conductivity of the sample and concomitant rapid densification under an electric field in low temperatures in a matter of seconds. Since it is a transient phenomenon, the power dissipation on the sample is not uniform during the process. Thus, a transient analysis is needed to estimate the temperature of the sample during flash sintering due to Joule heating. In this work, the Finite Element Method on a coupled electrothermal nonlinear analysis was used in order to obtain the specimen temperature of 8YSZ after 5 s of flashing. The results agree with the experimental data obtained by the flashing of dense samples and with previous literature.

The flash phenomenon occurs when oxide ceramics are heated above a threshold temperature under an applied electric field. It is defined as an abrupt increase in the conductivity of the specimen. The specimen then can be held in this state of high conductivity by switching the power supply from voltage to current control. Here, we report on the emergence of new X-ray diffraction peaks in 3 mol% yttria-stabilized zirconia (3YSZ) when the specimen is held in this current controlled state. These peaks are indexed as a pseudocubic phase of zirconia. The peaks extinguish and reappear when the field is turned off and on. The specimen temperature in the flash state is measured from the thermal expansion of platinum, which is placed as a thin film on a small portion of the specimen surface. Experiments without the electric field, at even higher temperatures than those measured with the platinum standard, do not show any change of phase, thus ruling out Joule heating as the cause of this phenomenon. The time dependency of the growth and dissolution of the pseudo cubic phase is reported. These in situ experiments were carried out at the Advanced Photon Source Synchrotron at the Argonne National Laboratory.

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.

Conventional sintering of undoped Y 2 O 3 requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y 2 O 3 was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.

International audience; The aim of this work was to revisit the two-sphere model of solid-state sintering kinetics to make it consistent with the morphological evolution of the particle contacts during most of the sintering process. Mass transport equations for solid-state sintering are written in a closed-form. The usual simplifications of tangent or secant spheres are avoided and the true evolution of the neck geometry is taken into account, depending on the relative weight of densifying / nondensifying mechanisms. Finite difference methods are used to compute the evolution of geometrical parameters and of the shrinkage rate as a function of time and temperature. The model combines the usual mechanisms of volume, surface, and grain-boundary diffusion as well as vapor transport. The ratio of concave to convex surfaces is deduced from neck growth and from the packing coordination number. A smooth transition from the classical first stage to the intermediate stage of sintering, where curvature gradients have vanished at the particle surface, is obtained. The same model can then be applied to describe sintering kinetics until pore closure. This model providing a realistic evolution of the contact morphology makes it possible to analyze the competition between densifying and nondensifying mechanisms throughout the early and intermediate stages of sintering. The competition between lattice diffusion and vapor transport or surface diffusion is thus analyzed.

International audience; The development of functional devices compatible with standard microelectronic processes is central to the More‐than‐Moore and Beyond‐CMOS (complementary metal oxide semiconductor) electronic fields. Devices based on nanowires (NWs) are very promising, but their integration remains complex and submitted to variability limiting the potential scalability. The field of flexible electronics is another one in which the standard microelectronic industry struggles to propose a solution. Despite tremendous progress, organic materials remain highly sensitive to oxygen and humidity and deteriorate under UV irradiation, thus limiting their long‐term operation. Here, it is shown that Si NW networks, also called Si nanonets, provide an easy‐to‐process single answer to develop flexible electronics and NW‐based devices. As a major contribution to the state of the art, it is demonstrated that stable Si NW–NW junctions, insensitive to oxidation, can be formed with low variability, which opens up a new route to form reproducible and reliable devices, with long‐term performances, presumably over several years, for NW‐based or flexible devices using Si as active element.

International audience; Sintering of fine silicon powder covered with a native oxide, SiO2(s), has been studied under controlled atmosphere in a dilatometric apparatus. The densification kinetics is marked by the presence of two peaks on the shrinkage rate curve which are respectively attributed to the first and second stages of sintering. A complete description of sintering kinetics involving lattice diffusion and vapor transport is proposed to explain this unusual behavior. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; Sintering of fine silicon powder was studied under controlled water vapor pressures using the Temperature Pressure Sintering Diagram approach. The water vapor pressure surrounding the sample was deduced from thermogravimetric analysis and related to the water content of the incoming gas flux with a simple mass transfer model. The thickness of the silica layer covering silicon particles was then monitored by the water vapor pressure and the microstructure evolution and densification during sintering could be controlled. Stabilizing the silica layer indeed inhibits grain coarsening and allows better densification of the compacts under humidified atmosphere as compared to dry atmosphere. (C) 2013 Elsevier Ltd. All rights reserved.

International audience; Sintering of fine silicon powder was studied using thermogravimetric experiments associated with dilatometry. The reduction of the silica layer at the particle surface is consistent with a kinetic model taking into account the powder oxygen content, the partial pressure of oxidizing species, and the pore morphology of the sintered material. The presence of silica at the particle surface inhibits coarsening and promotes densification of the powder compact, which is confirmed by kinetic calculations

International audience; In recent years, silicon nanostructures have been investigated extensively for their potential use in photonic and photovoltaic applications. So far, for silicon quantum dots embedded in SiO2, control over inter-dot distance and size has only been observed in multiple bilayer stacks of silicon-rich oxides and silicon dioxide. In this work, for the first time the fabrication of spatially well-ordered Si quantum dots (QDs) in SiO2 is demonstrated, without using the multilayer approach. This ordered formation, confirmed with TEM micrographs, depends on the thickness of the initially deposited sub-stoichiometric silicon oxide film. Grazing incidence x-ray diffraction confirms the crystallinity of the 5 nm QDs while photoluminescence shows augmented bandgap values. Low-temperature current–voltage measurements demonstrate film thickness and order-dependent conduction mechanisms, showing the transition from temperature-dependent conduction in randomly placed dots to temperature-independent tunnelling for geometrically ordered nanocrystals. Contrary to expectations from dielectric materials, significant conduction and photocarrier generation have been observed in our Si QDs embedded in SiO2 demonstrating the possibility of forming initial film-thickness-controlled conductive films. This conduction via the silicon quantum dots in thick single layers is a promising result for integration into photovoltaic devices.

International audience; Temperature-(water vapor) Pressure-Sintering (TPS) diagrams were established to control the microstructure evolution and densification during sintering of silicon. Experiments were performed under controlled humidified atmospheres and corroborated diagram predictions.

International audience; Silicon powders, initially covered with a native oxide, SiO(2(s)), have been studied during high-temperature processing under an He-H(2) atmosphere. Thermogravimetric experiments allowed identification of the mechanisms involved in the reduction of the oxide layer. The experimental results were compared to previous thermochemical analyses and experiments from the literature. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

International audience

International audience; The development of functional devices compatible with standard microelectronic processes is central to the More-than-Moore and Beyond-CMOS (complementary metal oxide semiconductor) electronic fields. Devices based on nanowires (NWs) are very promising, but their integration remains complex and submitted to variability limiting the potential scalability. The field of flexible electronics is another one in which the standard microelectronic industry struggles to propose a solution. Despite tremendous progress, organic materials remain highly sensitive to oxygen and humidity and deteriorate under UV irradiation, thus limiting their long-term operation. Here, it is shown that Si NW networks, also called Si nanonets, provide an easy-to-process single answer to develop flexible electronics and NW-based devices. As a major contribution to the state of the art, it is demonstrated that stable Si NW-NW junctions, insensitive to oxidation, can be formed with low variability, which opens up a new route to form reproducible and reliable devices, with long-term performances, presumably over several years, for NW-based or flexible devices using Si as active element.

Partial trisomy of the region 12q24.1 q24.2 is rare and usually associated with other rearrangements. We report on the clinical and cytogenetic findings in a girl with a pure de novo direct duplication dup(12)(q24.1 q24.2). She had developmental and growth retardation, facial dysmorphism with upslanting palpebral fissures, wide downturned mouth, short neck, and Marcus Gunn phenomenon. She also had single transverse creases, hypoplasia of the corpus callosum, and cardiac malformations consisting of a bicuspid aortic valve, multiple ventricular septal defects, and kinking of the aorta. The size of the duplication was characterized by molecular cytogenetics and comparative genomic hybridization (CGH) to be 11.5 Mb in size and extended from the BAC probe RP11-256L11 loci (108.2 Mb) ± 1 Mb to the BAC probe RP11-665J20 loci (119.7 Mb) ± 1 Mb. No such pure 12q24 duplication was detected out of the 23 patients reported in the literature with duplications in 12q region. Comparison with these reported 12q trisomies suggests the duplication dup(12)(q24.1 q24.2) is associated with a recognizable phenotype consisting of characteristic facial dysmorphism, growth retardation, and cardiac malformation. © 2006 Wiley-Liss, Inc.