Field–assisted and flash sintering of nanocrystalline yttria: Densification and microstructural evolution

Y2O3 ceramics have special chemical and physical properties such as high resistance to halogen-plasma corrosion and thermal stability. At the same time they are difficult to sinter. Conventional sintering requires very high temperatures typically \u3e1400°C, and a vacuum or hydrogen atmosphere. We show that high-purity, undoped Y2O3 can be sintered nearly instantaneously to almost full density by flash-sintering, where densification occurs in a few seconds under a threshold condition of temperature and applied field [1]. The Y2O3 shows flash-sintering at the fields above 300 V/cm. For instance, full densification is achieved at 1133°C under a field of 500 V/cm. The flash event in Y2O3 is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behavior differs from earlier work on Y2O3-stabilized ZrO2 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 Y2O3 was confirmed by high-resolution transmission electron microscopy (HRTEM). 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

Micropillar compression of anisotropic Al2O3-based eutectic composite

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Orientation-dependent plastic deformability in micropillar compression of oxide ceramics

Micromechanical testing, such as nanoindentation and micropillar compression, can be the promising tool for characterizing the plastic deformability of ceramics even at temperatures below macroscopic ductile-to-brittle transitions [1,2]. Such plasticity is possibly achieved by the fracture strength increase at small scales. The fracture strength, C , is conventionally determined as a function of a crack dimension: Please click Download on the upper right corner to see the full abstract

Low temperature and high strain rate superplastic flow in structural oxide ceramics induced by flash event

High-strength oxide ceramics are known to be brittle, with limited plastic deformability, even at high temperatures above 1000°C. However, structural ceramics with grain sizes of 1 μm or less can exhibit superplasticity, i.e., tensile elongation exceeding 100% via grain boundary sliding (GBS). Because deformation by GBS requires diffusion of atoms (ions) as a stress-relaxation process, a short accommodation length, i.e. a small grain size is critical for achieving superplasticity in ceramics. Superplastic forming is currently employed as a manufacturing technology for metals and alloys, typically at operation temperatures \u3c1000°C and strain rates \u3e10−3 s−1. In contrast, in typical superplastic ceramics such as Y2O3-stabilized tetragonal ZrO2 polycrystals (TZP), superplastic deformation only occurs at higher temperatures (\u3e1400°C) and lower strain rates (\u3c10−4 s−1), though an average grain size of superplastic TZP is usually in a range of 0.3-0.5 μm. Some of nano-grained ceramic composites have been shown to exhibit superplasticity at higher strain rates, but only at higher temperatures beyond 1600°C. In order to reduce the superplastic temperature and to increase the deformation speed, an approach other than grain-size refinement is required to activate GBS in structural ceramics. The application of a strong electric field during sintering is known to enhance densification in ceramics by accelerating diffusion mass transport. The use of flash sintering, in which a material is directly exposed to heat and a strong electrical field beyond threshold values, enables rapid sintering to achieve a fully dense sample at significantly lower furnace temperatures than conventional sintering. Field-assisted sintering and flash sintering have been used for fast manufacturing of TZP and other ceramic materials. The rapid densification during flash sintering is a result of accelerated self-diffusion and is accompanied by a non-linear increase in the electric conductivity of the material. Electron energy-loss spectrometry revealed the existence of extrinsic oxygen anion vacancies in flash-sintered Y2O3 and TZP, suggesting that flash sintering proceeds via the generation of atomic defects under the strong electric field. In addition, it has been pointed out that grain growth in Y2O3-stabilized ZrO2 is accelerated by an electric current as well as by reduction in N2+5%H2 atmosphere. Thus, one can expect that the application of a strong field could also facilitate high-temperature mass-transport phenomena, such as GBS. We demonstrate in this paper that by employing flash event under a strong DC field higher than 50 V·cm−1, conventional TZP ceramics can exhibit superplastic deformation with an elongation to failure of \u3e150%, at a lower furnace temperature of 800°C and a higher strain rate of 2 × 10−3 s−1 compared to previous methods. The flash event can also enhance bending deformation as well as tensile deformation. The flow stress-strain rate relationship indicated that the enhancement in the plastic flow of TZP resulted not only from increased specimen temperature due to Joule heating but also from accelerated diffusion by electric field and/or current. The field/current effect was equivalent to increase in temperature of about 200ºC

Local mechanical response in the vicinity of single grain boundary in YSZ measured by nanoindentation

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Grain growth behavior during spark plasma sintering of ceramics

During sintering, most of densification process proceeds in the intermediate stage where channel-like open pores and large isolated pores shrink by the movement of particles or grains towards the pores. The grain rearrangement without significant shape change, one of the characteristics of sintering, is a result of the grain-boundary sliding which is the most important mechanism for high-temperature deformation, such as superplastic deformation. The grain-boundary sliding is an essential process during densification. During spark plasma sintering of alumina, the effects of heating rate, pressure and loading schedule on the grain size were examined. Usually, high heating rates results in small grain sizes because of short heating time. However, when alumina was densified at low temperatures, high heating rates accelerated grain growth, though the total heating time was reduced. The grain growth rate after full densification was also accelerated for high heating rates. The accelerated grain growth might result from the generation of defects during densification. The densification in the intermediate stage of sintering includes the deformation of powder particles, and the deformation occurs mainly by grain-boundary sliding or grain re-arrangement. The defects generated during grain-boundary sliding may enhance the grain-boundary mobility and accelerate the grain growth rate, that is the dynamic grain growth. It is considered, therefore, that the high deformation rate at high heating rates accelerated grain growth during sintering. The accelerated grain growth also appeared for high-pressure sintering. The grain size after sintering increased with the applied pressure. High pressures lowered the deformation temperature and increased the deformation rate. As a result, the high deformation rate during heating may generate defects and enhance the grain-boundary mobility. Lastly, the loading schedule during heating also affected the deformation and the grain growth. Applying pressure at low temperatures or at high rates may generate more defects and resultantly accelerate the grain growth. These unusual grain growth behaviors during spark plasma sintering are explained by using a concept of dynamic grain growth [1]. Hence, one of our conclusions is that the deformation of grain-boundary sliding plays an important role in both densification and grain growth during sintering. [1] BN Kim et al., Scripta Mater., 80 (2014) 29

Charge Transport through Polyene Self-Assembled Monolayers from Multiscale Computer Simulations

We combine first-principles density-functional theory with matrix Green’s function calculations to predict the structures and charge transport characteristics of self-assembled monolayers (SAMs) of four classes of systems in contact with Au(111) electrodes: conjugated polyene chains (n = 4, 8, 12, 16, and 30) thiolated at one or both ends and saturated alkane chains (n = 4, 8, 12, and 16) thiolated at one or both ends. For the polyene SAMs, we find no decay in the current as a function of chain length and conclude that these 1−3 nm long polyene SAMs act as metallic wires. We also find that the polyene-monothiolate leads to a contact resistance only 2.8 times higher than that for the polyene-dithiolate chains, indicating that the device conductance is dominated by the properties of the molecular connector with less importance in having a second molecule−electrode contact. For the alkane SAMs, we observe the normal exponential decay in the current as a function of the chain length with a decay constant of βn = 0.82 for the alkane-monothiolate and 0.88 for the alkane-dithiolate. We find that the contact resistance for the alkane-monothiolate is 12.5 times higher than that for the alkane-dithiolate chains, reflecting the extra resistance due to the weak contact on the nonthiolated end. These contrasting charge transport characteristics of alkane and polyene SAMs and their contact dependence are explained in terms of the atomic projected density of states

Athermally Enhanced High Temperature Plastic Flow in Zirconia Ceramics under Flash Event

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High temperature tensile behavior of zirconia ceramics under DC current

These Flash sintering phenomena, which occurs by applying DC current directly to ceramic powder compacts, has been the subject of many paper of ceramic sintering. This is because the flash event can succeed to lower the sintering temperature/time of several ceramic powders. On the other hand, Conrad and his colleagues examined the effect of electric fields on the high temperature tensile properties of 3Y-TZP and confirmed that the fields can lower the tensile flow stresses of 3Y-TZP enough to attain superplasticity. The enhanced deformation was explained by suppressed grain growth due to the electric bias effect. However, the mechanism/phenomena of the flash event are still unclear. In order to clarify the effect of electric current on high temperature deformation, therefore, the present study was carried out to examine the tensile behavior of polycrystalline zirconia ceramics under the several temperature and electric field/current conditions. By applying the DC electric power higher than a critical value Ec, the flash event similar to that of powder sintering occurs even in dense zirconia ceramics. At around 1000 °C, for example, the Ec value is about 100 - 200 mW/mm3, which is slightly larger than those reported in the powder compacts. For lower than Ec, the applied electric current increases sample temperature depending on the applied value, but does not enhance the rate of deformation. For higher than Ec, on the other hand, the electric current enhances the rate of the deformation to about several times as compared with that of without current conditions. The enhanced deformation cannot be interpreted only by the increment of sample temperatures and is likely to occur by the flash event. After the deformation under the electric current conditions, the tested sample shows slight gray color even under air condition. This suggests that the enhanced deformation would be related to oxygen vacancy formation. In the presentation, we will discuss the detailed current effect obtained at wide range testing conditions