Effect of Electric Current on Ceramic Processing

PhDThis work was on the effect of electric current on the processing of ceramics. The focus was on electromigration/electrochemistry and plasma effects. While there is no solid evidence that there is plasma in Spark Plasma Sintering, (SPS), newer techniques e.g. flash, use different conditions so there is an interest in understanding the conditions under which a plasma forms. The minimum arcing voltage was found from literature to be from 10-15V for materials of interest. This is above that found in SPS (10V). However, due to the many contact points in a powder compact much higher voltages (50V) were required in practical experiments. Optical spectroscopy was used to verify the formation of a plasma, and emission peaks from the powder compact material were visible implying they were vaporised and formed the plasma. Electromigration was exploited to alter the oxidation of zirconium diboride, by passing current through the oxide layer (120μm zirconia base grown at 1200°C) oxygen could be pumped either away or toward the diboride bulk. Small cubes (3mm) of diboride had platinum foil electrodes applied on both sides and oxidation was performed at 1400°C for 5hr. Without a field the oxide grew to 360μm, by applying 10V and 100mA the oxide grew to 150μm under the +ve electrode but 1400μm under the -ve electrode. Electrochemical reduction was believed to have occurred due to the electrical properties of the material changing during oxidation and visible blackening of the oxide. Combining the techniques from both earlier works, a contactless flash sintering setup was developed. This used two plasma arcs as electrodes to heat and pass current through the sample. Various materials, currents and times were used, but the best result was with SiC:B4C which was sintered in 3s with 6A, the microstructure showed sharp grains, no segregation and limited grain growth ( initially 0.7μm SiC and 0.5μm B4C, this grew to 1.1μm and 1.4μm). This was the first recorded case of contactless flash sintering and the technique has the potential to sinter ceramics in a continuous manner.The European framework 7 ADMACOM (Advanced manufacturing routes for metal/Composite components for aerospace) (EC FP7 2007-2013) EPSRC Materials for Extreme Environments (EP/K008749/1, XMat)

Field Assisted Material Engineering (FAME)

In order to further improve the energy saving of Spark Plasma Sintering we have developed a very rapid sintering technique called Flash SPS (FSPS) with heating rates in the order of 104-105 ˚C/minute[1]. Unlike the Flash Sintering based on high voltage (≈100V), FSPS is based on low voltage (≈10V) and it can be up-scaled to samples volumes of several tens of cubic centimetres. Flash SPS allows densification of metallic conductors like ZrB2 and HfB2, under a discharge time as short as 20-30 seconds. FSPS of semiconductors like silicon carbide and boron carbide was also demonstrated. Highly customized and versatile equipment with ultrafast responsive controls and programmable bipolar power supplies (up to 20 kHz, 1 MA, 500V) has been built. The developed methodology has been applied to produce FSPSed samples even larger than 6 cm in diameter of ultra refractory materials. Understanding the intrinsic electrical field role in the triangle properties-microstructure-processing remains one our primary scientific goal and the main open question. We tried to give some answers by approaching the problem at different length scales (see figure 1) by developing dedicated equipment/controls, simulations (FEM and ab-initio), thermo-kinetic analysis, in situ observations and accurate temperature measurements/calibrations. Please click Additional Files below to see the full abstract

Electrochemical, optical and thermal effects during flash sintering of 8YSZ

We report on the electrochemical effects occurring during the flash sintering of 8YSZ. In-situ observations for both polycrystalline and single crystal specimens confirm electrochemical blackening/darkening during the incubation period prior to flash sintering (Figure1), even though chromatic alterations are usually visible only after the samples are cooled down in a protective atmosphere rather than in air. The phenomenon is induced by cathodic partial reduction under a DC field. When using a low frequency AC (square 0.1 – 10 Hz) field, the blackening becomes reversible and it follows the imposed polarity switching. Thermal imaging combined with sample color changes (transparent single crystals) and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. Please click Additional Files below to see the full abstract

Temperature and composition insensitivity of thermoelectric properties of high-entropy half-heusler compounds

Composition modification by doping and solid solution is a well-studied strategy in thermoelectric (TE) materials to optimize their properties. Recently, the concept of entropy stabilization has offered the possibility of forming random solid solutions that have properties that go beyond the rule of mixture. In this study, we prepared a series of high-entropy half-Heusler solid solutions (HEHHs) with varying valence electron counts (VEC), (Ti0.33Zr0.33Hf0.33)1-x(V0.33Nb0.33Ta0.33)xCoSb (x = 0.5 to 0.75). Compared to their medium- and low-entropy counterparts, the TE properties of HEHHs are less sensitive to temperature and composition variation (charge carrier concentration efficiency of ∼10 %). An ultra-low lattice thermal conductivity for half-Heusler of 1.19 W·m−1·K−1 was achieved.Peer reviewe

Microwave characterization of two Ba0.6Sr0.4TiO3 dielectric thin films with out-of-plane and in-plane electrode structures

Ferroelectric (FE) thin films have recently attracted renewed interest in research due to their great potential for designing novel tunable electromagnetic devices such as large intelligent surfaces (LISs). However, the mechanism of how a polar structure in the FE thin films contributes to desired tunable performance, especially within the microwave frequency range, which is the most widely used frequency range of electromagnetics, has not been illustrated clearly. In this paper, we described several straightforward and cost-effective methods to fabricate and characterize Ba0.6 Sr0.4 TiO3 (BST) thin films at microwave frequencies. The prepared BST thin films here exhibit homogenous structures and great tunability (h) in a wide frequency and temperature range when the applied field is in the out-of-plane direction. The high tunability can be attributed to high concentration of polar nanoclusters. Their response to the applied direct current (DC) field was directly visualized using a novel non-destructive near-field scanning microwave microscopy (NSMM) technique. Our results have provided some intriguing insights into the application of the FE thin films for future programmable high-frequency devices and systems

Investigation of electrochemical, optical and thermal effects during flash sintering of 8YSZ

This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1–10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used

Hybrid Flash-SPS of TiNiCu0.05Sn with reduced thermal conductivity

TiNiCu0·05Sn was sintered using Spark Plasma Sintering (SPS) and a new derivative processing method, hybrid Flash-SPS (hFSPS). The high heating rate achieved (7700 °C/min) produced almost single-phase samples with high density. The sample sintered at 1040 °C showed a higher power factor and a lower thermal conductivity than the SPS sample, resulting in a higher ZT at 350 °C (0.44 vs 0.35)