Effect of electric field/current on liquid phase sintering

Sintering behavior of ionic and electric conductor ceramics is enhanced when an external electric field is applied during the sintering process by Joule heating, generation of Frenkel defects, mobility of point defects, electrochemical reactions and electromigration mechanisms. The applied electric field modifies the sintering behavior in two ways, depending mainly on the strength of the field (V/cm) and the electrical properties of the sample. Intermediate fields enhance the sintering kinetics apparently due to the retardation of grain growth, which is called field assisted sintering. In contrast, high fields generate massive densification just in few seconds, which is recently referenced as flash sintering. Nonetheless, the majority of these studies are performed for solid state sintering (SS), whereas only a few works have been published on liquid phase sintering (LPS). In this work, the effect of the electric field on liquid phase sintering is analyzed in detail, being the first time that flash sintering is observed in LPS. Alumina containing different amount of a glassy phase (Al2O3/CAS) was chosen to ascertain the role of current on liquid phase sintering using a sinter-forging device. In situ laser dilatometry, evaluation of specimen temperature, real-time measurement of electrical field and current density are complemented by microstructure analysis and sintering rates calculations, which give insights of involved mechanisms as function of the sintering conditions and applied field. In addition, viscosity measurements and wettability studies of the glass at high temperature were carried out to reveal the effect of current on the response of glass. Current flowed only through the liquid phase at high temperature and enhanced the densification process by two effects: Joule heating and athermal response of the viscous liquid under the electric field. Joule heating increased the temperature within the specimen, whereas the applied electric field reduced the viscosity of the liquid phase promoting a more effective matter transport

Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method

Electrical field assisted sintering of yttrium-doped ceria investigated by sinter-forging

The production of traditional and advanced ceramics is an energy-intensive activity, which requires high sintering temperatures and long holding times to activate diffusional processes necessary for densification. Electric field assisted processing has the potential to significantly reduce the sintering time and temperature which are not obtainable by other methods. The role of electric fields in the densification and coarsening of oxide ceramics is still under debate. By using a sinter-forging device equipped with a versatile power source and high-resolution laser scanners, it is possible to investigate in detail field assisted sintering process by quantifying uniaxial viscosity, viscous Poisson’s ratio and sintering stress of oxide ceramics. The macroscopic Joule heating effect was eliminated by using Finite-Element Simulations calibrated experimentally and by lowering the furnace temperature accordingly. In other words, the sample temperature was kept constant under the different testing conditions, enabling a correct estimation of a thermal electric field effects. The sintering parameters of the ceramic pellets were measured without / with alternating electrical field well below flash sintering conditions. Clear effect of the electrical field on both uniaxial viscosity and sintering stress were observed. Microstructures of the specimens were investigated by SEM and TEM, and correlated to the electrical properties of the samples measured by Electrochemical Impedance Spectroscopy in order to understand the interplay between grain boundaries and electric field

Fields Matter intiative in Germany

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Modelling the effect of the porous support on the flux through asymmetric oxygen gas separation membranes

Oxygen Transport Membranes (OTM) represent a new technology for energy-efficient oxygen generation which can be used in low-pollutant power plants and oxygen generators or membrane reactors in the chemical industry and health care. The two competing demands of low ionic resistance of the functional separation membrane and high mechanical stability lead to an asymmetric design comprising of a thin membrane layer and a thicker porous support. However, the overall membrane performance is strongly affected by the microstructure of this support layer which prevented the use of the full potential of such a design in the past. Please download the full abstract below

Columnar Thermal Barrier Coatings Manufactured by Novel Laser Cladding Process

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Superior performance of plasma sprayed YSZ thermal barrier coatings with oxide dispersion strengthened bond coats

Advanced thermal barrier coatings are essential to increase the efficiency of next-generation gas turbine engines. Different materials and process technologies give the possibility to extend the lifetime of TBCs. One limiting factor of the TBC lifetime is the growth of the TGO during thermal exposure resulting in a accelerated crack growth at the top coat- bond coat interface. The oxidation resistance and the temperature of the bond coat are key factors influencing the TGO growth rate. Oxide dispersion strengthened (ODS) bond coats have a slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In previous studies TBC systems with an additional thin ODS bond coat on top of a standard bond coat showed a higher thermal cycling performance. These studies used Inconel 738 and Amdry 386 as substrate and bond coat material, respectively. This study investigates in the thermal cycling performance of the ODS bond coat TBC systems combined with a different substrate ERBO 1 and bond coat material Amdry 995. TBC systems with the new material combination show high cycling lifetimes and superior performance in comparison to previous samples. Samples were tested by a cyclic burner rig facility. Surface was heated by a gas burner to 1400°C while the backside is cooled by pressurized air to 1050°C. One cycle consists of 5 min heating followed by 2 min cooling. Cross sections of the samples were analyzed by SEM and laser microscope. The lifetime of the samples was evaluated especially with respect to diffusion processes, material properties, and bond coat temperature. ODS powders with higher aluminum oxide additions were produced by high energy milling to fit the CTE of the ODS bond coat to the one of the top coat. This will reduce the initial crack formation on the top a wavy top coat - bond coat interface and increase lifetime. The advanced bond coats were applied by low pressure plasma spraying, the standard YSZ top coat by atmospheric plasma spraying. The performance was evaluated by a gas burner rig test

Field assisted sintering of larger scaled ceramic parts using adapted tool design and hybrid heating

Field Assisted Sintering/Spark Plasma Sintering (FAST/SPS) is a promising technology for the energy efficient sintering of ceramic, composite and metal powders. The combination of direct current heating and applied pressure enables high heating rates, rapid densification and offers the potential to decrease the sintering temperature significantly. FAST/SPS is of special interest for materials, which are difficult to densify by conventional methods like pressure less sintering. To establish this processing technology on industrial scale, fundamental studies are required to better understand the relationship between processing parameters, specific FAST/SPS boundary conditions and resulting material properties. A challenging task – especially for non-conductive oxide ceramics – is the decrease of thermal gradients during FAST/SPS cycles to a minimum and to suppress interface reactions with the tool material. In the present work, a systematic study was conducted in our FAST/SPS device aiming on to homogeneously densifying commercial yttria (Y2O3) powder to discs with diameter up to 100 mm. Specific attention was laid on the formation of thermal gradients during the cycle and to investigate their influence on the resulting microstructure. Therefore, different tool set ups were used. Amongst others, carbon fiber reinforced carbon (CFC) inlays were implemented to adjust thermal conductivity of the tool. Furthermore, the effect of hybrid heating was evaluated. For this experimental series, an additional induction coil was mounted in the FAST/SPS device. For evaluating the efficiency of hybrid heating, total energy consumption of the FAST/SPS device – operated with and without induction coil – was measured. The experimental studies were accompanied by finite element modelling to estimate the temperature distribution of non-conductive yttria sample during FAST/SPS processing. The modelling results will be correlated with the grain size distribution along the cross section of the 100 mm disc. Additionally, Vickers hardness measurements were done to investigate how thermal gradients tend to influence mechanical properties