Theo Saunders

Review of flash sintering: materials, mechanisms and modelling

ABSTRACT Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60 s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theories applied to electric current assisted/activated sintering techniques. The possibility of applying FS to a wider range of materials and its implementation in industrial scale processes are discussed. GRAPHICAL ABSTRACT Abbreviations: ECAS: Electric Current Assisted/Assisted Sintering; FS: flash sintering; SPS: spark plasma sintering; FSPS: flash spark plasma sintering; HIP: hot isostatic press; HP: hot press; TF: furnace temperature; TOnset: onset temperature; E: electrical field; TS: sample temperature; TSoft: softening temperature for glasses; Tm: melting temperature; PTC or NTC: positive or negative temperature coefficient of electrical resistance; IS: impedance spectroscopy; OES: optical emission spectroscopy; AES: atomic emission spectroscopy; DBS: Dog-bone shape, L: length, W: width, T: thickness, D: diameter, H: height, R: rectangular, CS: cross-section; P: particle size; C: crystallite size; 3YSZ: 3 mol-% yttria-stabilised zirconia; I: ion, H: hole, V: vacancy, E: electron, P: proton

Ultrafast-Contactless Flash Sintering using Plasma Electrodes.

This paper presents a novel derivative of flash sintering, in which contactless flash sintering (CFS) is achieved using plasma electrodes. In this setup, electrical contact with the sample to be sintered is made by two arc plasma electrodes, one on either side, allowing current to pass through the sample. This opens up the possibility of continuous throughput flash sintering. Preheating, a usual precondition for flash sintering, is provided by the arc electrodes which heat the sample to 1400 °C. The best results were produced with pre-compacted samples (bars 1.8 mm thick) of pure B4C (discharge time 2s, current 4A) and SiC:B4C 50 wt% (3s at 6A), which were fully consolidated under a heating rate approaching 20000 °C/min. For the composite a cylindrical volume of 14 mm3 was sintered to full density with limited grain growth.

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.

Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb18SbTe20

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (κlatt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed-tube melt processing, including (i) slow cooling of the melt, (ii) quenching and annealing, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κtot) conductivities of the CuPb18SbTe20 samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κlatt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κlatt without degrading the carrier mobility. In spite of their comparable σ and κtot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag) when Cu replaces Ag.

Effect of Heat Treatment on the Properties of Wood-Derived Biocarbon Structures

Wood-derived porous graphitic biocarbons with hierarchical structures were obtained by high-temperature (2200–2400 °C) non-catalytic graphitization, and their mechanical, electrical and thermal properties are reported for the first time. Compared to amorphous biocarbon produced at 1000 °C, the graphitized biocarbon-2200 °C and biocarbon-2400 °C exhibited increased compressive strength by ~38% (~36 MPa), increased electrical conductivity by ~8 fold (~29 S/cm), and increased thermal conductivity by ~5 fold (~9.5 W/(m·K) at 25 °C). The increase of duration time at 2200 °C contributed to increased thermal conductivity by ~12%, while the increase of temperature from 2200 to 2400 °C did not change their thermal conductivity, indicating that 2200 °C is sufficient for non-catalytic graphitization of wood-derived biocarbon.