Growing larger: Scaling up during spark plasma sintering of high– temperature ceramics

Recently, extensive efforts were made towards investigation and development of spark plasma sintering (SPS) as a promising technique for rapid densification of ceramics at relatively low temperatures. In this respect, SPS was used for various processes and materials including fabrication of bulk metals and ceramics and their composites. Furthermore, this ‘low temperature’ processing allowed consolidation of ceramics with strong covalent bonding to the full density. However, significant gap exists between the technological and fabrication achievements to the fundamental understanding of the SPS mechanisms. This gap is due to the complexity of the thermal, electrical and mechanical processes that may be involved during the SPS, in addition to their dependence on the SPS parameters, as well as a reasonable question of possibility of scaling up. The majority of reports provided in peer-reviewed journals usually focuses on the ceramic specimens not larger than 40 mm, which is a reasonable size even for more “conventional” consolidation method that of hot-pressing. Hence, the present work focuses on the natural scaling up processes for fabrication of a large size specimens approaching 100 mm in diameter and 20 mm in height, and provides an analysis for the densification process, structure evolution (including homogeneity), and, crucially, concentrates on the change in the mechanical properties that arises during scaling up process for high-temperature ceramics consolidated at temperatures higher than 1800 °C

Engineering of lightweight ceramic composites by spark plasma sintering

It is well recognised that value of porosity and grain size influences mechanical properties such as fracture toughness, elastic modulus and strength dramatically. Boron carbide (B4C), as well as other lightweight ceramics like TiB2, B6O requires high consolidation temperatures owing to its poor sinterability. Therefore such decrease of ceramics properties becomes a materials processing issue since during consolidation process on the final stage of sintering (either pressure-assisted or pressureless one) grain growth starts. To overcome this problem, various metallic and non-metallic binders are used to obtain dense borides. However, the presence of a metallic binder is not desirable for high-temperature structural applications. We propose that reaction-driven consolidation by means of spark plasma sintering (SPS) at temperatures exceeding 1800 °C as an alternative method for fabrication of high-temperature lightweight ceramic composites. This work summarizes recent activity on processing of lightweight ceramics based on boron carbide, boron suboxide and titanium diboride in the respect to mechanical properties: such as hardness, fracture toughness and room and high-temperature strength. Application of various techniques for powders preparation and consolidation by SPS is thoroughly discussed in respect to obtained lightweight ceramic composites properties. A thorough discussion of high-temperature properties for these ceramic composites is also provided

Consolidation of B4C-TaB2 eutectic composites by spark plasma sintering

The in situ synthesis/consolidation of B4C-TaB2 eutectic composites by spark plasma sintering (SPS) is reported. The microstructure–property relations were determined for composites with the B4C-TaB2 eutectic composition as functions of TaB2 content, and TaB2-TaB2 interlamellar spacing. A clear maximum in fracture toughness was identified (∼4.5 MPa m1/2) for eutectic composites with interlamellar spacing between 0.9 and 1.1 μm. The composites with the hypereutectic composition of 40 mol.% TaB2 obtained by SPS exhibited lower Vickers hardness (25–26 GPa) but higher indentation fracture toughness (up to 4.9 MPa m1/2) than eutectic composites with 30–35 mol.% of TaB2.Published versio

Consolidation of B4C–VB2 eutectic ceramics by spark plasma sintering

The in situ synthesis and consolidation of B4C–VB2 eutectic composites by spark plasma sintering (SPS) is reported. The microstructure properties were determined for composites with the B4C–VB2 eutectic composition as functions of the VB2 content and the VB2–VB2 interlamellar spacing. A change of VB2 concentration from 45 to 48 mol.% resulted in the formation of the rod-like eutectic microstructure. Indentation fracture toughness values higher than 4 MPa·m1/2 were identified for eutectic composites with interlamellar spacings between 0.9 and 1.2 µm. The composites obtained by SPS with a composition of 45 mol.% VB2 exhibited a lower Vickers hardness (23–24 GPa) and higher indentation fracture toughness (up to 4.5 MPa·m1/2) than the eutectic composites with 48 mol.% of VB2.Published versio

Metal-ceramic/ceramic nanostructured layered composites for solid oxide fuel cells by spark plasma sintering

In this work, bi-layered Fe–Ni–Co–YSZ/YSZ nanostructured composites for solid oxide fuel cells were obtained using the spark plasma sintering (SPS) technique. The microstructures of the anode and electrolyte were controlled by optimization of SPS consolidation parameters. The resulting bilayers have a full dense YSZ electrolyte and porous Fe–Ni–Co/YSZ anode as well as crack-free and well-bonded anode/electrolyte interface. On the other hand, SPS under non-optimized processing parameters cannot yield the desired results. The high resistance to thermal stresses of the fabricated half-cells was achieved with Fe–Ni–Co/YSZ anode. The developed anode showed higher thermal compatibility with YSZ electrolyte than usual Ni/YSZ cermet. Thus, with the successful combination of SPS parameters and anode material, we have obtained bi-layers for SOFCs with required microstructure and thermal compatibility

Spark plasma sintered Ni-YSZ/YSZ bi-layers for solid oxide fuel cell

Ni-YSZ/YSZ bi-layers for SOFCs were fabricated by spark plasma sintering (SPS). Optimization of SPS parameters of YSZ and NiO/YSZ powders was performed in order to fabricate anode and electrolyte with desired microstructures. The effect of sintering conditions on microstructure and electrical properties of YSZ was studied. The influence of processing parameters and amount of pore-forming agent on the microstructures of Ni/YSZ cermets was also investigated. It was shown that the amount of pore-former and in situ reduction of nickel oxide had a substantial effect on microstructure of the cermets. The in situ reduced anode demonstrates sufficiently homogeneous distribution of Ni and YSZ making a conduction path for electrons and ions. Ni-YSZ/YSZ bi-layers with crack-free and well-bonded anode/electrolyte interface were obtained. Furthermore, warping was not observed for the produced bi-layers

Grain boundary diffusion driven spark plasma sintering of nanocrystalline zirconia

A methodology is proposed to investigate in detail shrinkage kinetics under isothermal spark plasma sintering (SPS) conditions applied to ceramic nano powders such as Y2O3 stabilized ZrO2. To do so, mild SPS conditions were used (low temperatures and pressure, long dwell times). The extracted experimental activation energy has the value of 246 ± 37 kJ mol−1 and the slope of the curves on the intense densification stage is around 0.33. Results are in agreement with densification by a grain-boundary diffusion mechanism as for conventional sintering and the contribution from the specific pressure-assisted mechanisms as for hot pressing is insignificant. This result suggests that exploration of mild SPS might prove rewarding in separation and control of the sintering mechanisms leading to production of specific ceramic with new or improved functionality

Densification kinetics of nanocrystalline zirconia powder using microwave and spark plasma sintering—a comparative study

Two-stage densification process of nanosized 3 mol% yttria-stabilized zirconia (3Y-SZ) polycrystalline compacts during consolidation via microwave and spark-plasma sintering have been observed. The values of activation energies obtained for microwave and spark-plasma sintering 260–275 kJ·mol−1 are quite similar to that of conventional sintering of zirconia, suggesting that densification during initial stage is controlled by the grain-boundary diffusion mechanism. The sintering behavior during microwave sintering was significantly affected by preliminary pressing conditions, as the surface diffusion mechanism (230 kJ·mol−1) is active in case of cold-isostatic pressing procedure was applied