We show that flash-sintering in MgO-doped alumina is accompanied by a sharp increase in electrical conductivity. Experiments that measure conductivity in fully dense specimens, prepared by ...conventional sintering, prove that this is not a cause-and-effect relationship, but instead that the concomitant increase in the sintering rate and the conductivity share a common mechanism. The underlying mechanism, however, is mystifying since electrical conductivity is controlled by the transport of the fastest moving charged species, while sintering, which requires molecular transport or chemical diffusion, is limited by the slow moving charged species. Joule heating of the specimen during flash sintering cannot account for the anomalously high sintering rates. The sintering behavior of MgO-doped alumina is compared to that of nominally pure-alumina: the differences provide insight into the underlying mechanism for flash-sintering. We show that the pre-exponential in the Arrhenius equation for conductivity is enhanced in the non-linear regime, while the activation energy remains unchanged. The nucleation of Frenkel pairs is proposed as a mechanism to explain the coupling between flash-sintering and the non-linear increase in the conductivity.
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.
We show that cubic 8 mol% yttria (8YSZ) can be sintered at 750°C with the application of DC electrical fields; in comparison the lowest sintering temperature for 3YSZ was 850°C. Furthermore, cubic ...zirconia exhibits the onset of flash sintering at 30 V/cm, whereas 3YSZ begins flash sintering at 60 V/cm. However, the volume specific power dissipation for the onset of flash sintering remains similar at ∼10 mW/mm3. The easier sintering of 8YSZ is ascribed to its higher ionic conductivity.
Flash sintering of strontium titanate (SrTiO3) is studied at different applied fields to understand its effect on density and grain growth. In particular, the defect structure is investigated by ...optical and structural analysis. SrTiO3 exhibited a trend in densification opposite that of ionically or electronically conductive ceramics: as the applied voltage decreased, the density increased. Abnormal grain growth in conventionally sintered SrTiO3 is arrested by flash sintering. Interestingly, undoped SrTiO3 behaved differently than undoped Al2O3, which did not exhibit any signs of flash sintering. Previous attempts at flash sintering could only be achieved in MgO‐doped Al2O3. We believe that non‐stoichiometric Ruddlesden‐Popper phases in SrTiO3, as indicated by ultrafast optical spectroscopy, X‐ray diffraction, conductivity measurements, and transmission electron microscopy, assist flash sintering by increasing local conductivity through enhanced defect content.
Nuclear energy is presently the single major low-carbon electricity source in Europe and is overall expected to maintain (perhaps eventually even increase) its current installed power from now to ...2045. Long-term operation (LTO) is a reality in essentially all nuclear European countries, even when planning to phase out. New builds are planned. Moreover, several European countries, including non-nuclear or phasing out ones, have interests in next generation nuclear systems. In this framework, materials and material science play a crucial role towards safer, more efficient, more economical and overall more sustainable nuclear energy. This paper proposes a research agenda that combines modern digital technologies with materials science practices to pursue a change of paradigm that promotes innovation, equally serving the different nuclear energy interests and positions throughout Europe. This paper chooses to overview structural and fuel materials used in current generation reactors, as well as their wider spectrum for next generation reactors, summarising the relevant issues. Next, it describes the materials science approaches that are common to any nuclear materials (including classes that are not addressed here, such as concrete, polymers and functional materials), identifying for each of them a research agenda goal. It is concluded that among these goals are the development of structured materials qualification test-beds and materials acceleration platforms (MAPs) for materials that operate under harsh conditions. Another goal is the development of multi-parameter-based approaches for materials health monitoring based on different non-destructive examination and testing (NDE&T) techniques. Hybrid models that suitably combine physics-based and data-driven approaches for materials behaviour prediction can valuably support these developments, together with the creation and population of a centralised, “smart” database for nuclear materials.
Simulated corium samples were prepared using a sol-gel process to yield U-Zr-oxide materials representative of a molten core covering the whole range of compositions in the U-Zr series. Discs of ...U-Zr-oxide were compacted by Spark Plasma Sintering (SPS). The materials were characterised by XRD and optical/electron microscopy techniques as well as SEM-EDX. The thermal diffusivity of all samples has been measured between 500 and 1600 K by the laser-flash technique and thermal conductivity was calculated. For comparison, a sample extracted from the fully melted core of the Three Mile Island reactor Unit 2 (TMI-2) was also investigated.
The results for the simulated and real corium were analysed and compared to literature data. A substantial decrease of the thermal diffusivity occurred as the fraction of ZrO2 increased up to 18 mol% in the simulated corium. In the range 18–74 mol% ZrO2 only a weak composition dependence was observed. In this range the thermal conductivity at 500 K is between 2.5 and 3 W m−1 K−1, in agreement with other experimental data.
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•Simulated corium samples produced by sol-gel process and spark plasma sintering.•Thermal diffusivity measured for the simulants and a real corium sample.•Already low Zr-content simulants show substantial decrease of the thermal conductivity.•No further Zr-composition dependence of thermal conductivity is observed.•A lower thermal conductivity value for mixed (U,Zr)O2 corium is suggested.
Dense tantalum-based carbonitride materials featuring >95 % relative density, TaC, Ta2C1.5N0.5, Ta2CN, Ta2C0.5N1.5 and TaN, were prepared by Spark Plasma Sintering (SPS) at 1873 K with a dwell time ...of 10 min and a pressure of 50 MPa. Despite the presence of an oxide phase (i.e. 5 vol%) and some W inclusions (i.e. 1 vol%), the mechanical properties of such ultra-high temperature ceramics (UHTC) show promising values. Indeed, the Young's moduli measured by nano-indentation were approx. 600–700 GPa, which is higher than literature values reported for similar UHTC. The hardness values increased from 16.4 ± 0.8 GPa for TaC to 27.9 ± 1.3 GPa for TaN, with an approximately linear trend for the carbonitride samples while the ratio of plastic deformation work over the total indentation work followed the opposite trend.
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A novel method is proposed for the determination of the uniaxial viscosity of porous ceramic layers upon sintering. This approach is based on the application of a continuous but very low tensile ...stress to the densifying powder compact whose deformation is continuously monitored by an optical system. The viscosity of the system can be determined as a function of temperature and density from the sintering rate differences measured between loaded and unloaded samples. The uniaxial viscosity of porous Y2O3 doped ZrO2 (YSZ) and NiO–YSZ composites was measured using the proposed approach. The results were used to predict the curvature evolution of bilayers used in solid oxide fuel cell applications, obtaining a fairly good agreement between the model and the data recorded experimentally.
We show that yttrium‐stabilized zirconia can be sintered in a few seconds at ∼850°C to full density, starting from a green density of 0.5, by the application of a dc electrical field (nominally, ...several hours at 1450°C are needed to complete the sintering process). This finding is explained by the local Joule heating at grain boundaries, which, on the one hand, promotes grain‐boundary diffusion (a kinetic effect), while at the same time restricts grain growth (a thermodynamic effect). The smaller grain size and the higher temperature at grain boundaries can then act synergistically to enhance the rate of sintering. These results have a bearing in explaining the widespread spark plasma and microwave‐assisted techniques for enhanced sintering.
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