Fabrication of a vitreous porcelain by conventional sintering and spark plasma sintering

A vitreous porcelain body was fabricated by a conventional sintering and spark plasma sintering (SPS). The effects of sintering parameters such as heating rate, dwell time, temperature, and pressure were investigated. The porcelain sintered using the SPS techniques attained higher apparent bulk density of 2.52 g/cm3 compared with a conventionally sintered porcelain, which is 2.39 g/cm3, however, both of them exhibited similar water absorption of 0.01%. The SPS sintered porcelain presented a dense microstructure, which cannot be seen in conventionally sintered porcelain products. Mineralogically, the SPS sintered porcelain contained residual albite, quartz, mullite and glass while the conventionally sintered porcelain contained mullite, quartz, and glass. From the apparent bulk density, water absorption, microstructure and X-ray diffraction results, it can be deduced that the SPS sample required only a small amount of glass phase to achieve the fully dense microstructure compared with conventionally sintered porcelain. However, a comparative study of phase evolution of porcelains via conventional sintering and SPS such as glass and mullite formation in spark plasma sintering are being verified and will be discussed

Phase evolution and properties of a porcelain body fabricated using different rapid firing techniques

A porcelain powder was sintered using different rapid sintering techniques including direct sintering (DS), spark plasma sintering (SPS), microwave sintering (MWS), and flash sintering (FS). Densification, phase evolution, and physicomechanical properties of the fully sintered porcelain samples were investigated to improve understanding of the role of particular process parameters. Densification of the porcelains was controlled by liquid phase formation. Study of the DS revealed that formation of liquid glass is rapid even at 780 C/min heating rate, and the porcelain was fully densified within 15 min dwelling at 1175 °C. SPS enhanced densification rate about 10 times greater over that of conventional sintering, starting at 920 C. The dwelling step was negligible due to the rapid flow of the liquid glass filling the pores, assisted by the applied pressure. On using microwave energy, the sintering temperature of the porcelain was reduced by ~75 °C and dwell time from 15 min to 5 min compared to conventional sintering. Formation of the liquid phase was observed at 900-1000 °C and it was the key for the rapid densification because it promoted microwave absorbability. Densification in FS was difficult to resolve, but it could be deduced that by applying an electric potential of 1.5 kV/cm for 30 seconds, large amounts of glassy phase were produced. Various microstructures were observed as a result of the different processing routes. DS and MWS produced a typical porcelain microstructure which is dense but has numbers of residual pores embedded in the glassy phase. SPS produced a highly dense microstructure with a few residual pores assisted by applied pressure. FS, in contrast, produced a nonuniform microstructure containing under-sintered, well sintered, and over-sintered regions due to localised melting. Etched microstructures revealed special mullite morphologies. The applied pressure in SPS caused mullite orients perpendicular to the direction of the applied load. SPS also induced mullite dissolution at 1200 °C. Microwave produced fibre-like mullite via a vapour-liquid-solid mechanism having Fe(l) as a catalyst. Flash sintered samples contained dendritic mullite and some mullite needles were formed via a screw dislocation mechanism. However, mullite crystallite sizes calculated by the Scherrer equation revealed that in fully dense samples from each processing route, mullite crystallite sizes were in a narrow range of 25-40 nm. Physicomechanical of the porcelains produced using the different processing routes were similar; for example, apparent bulk density ranged 2.35-2.46 g/cm3 (MWSDS>SPS), hardness varied 5-7 GPa (MWS<SPS<DS) while fracture toughness varied from 3-6 MPa· m1/2 (MWS<DS<SPS). A comparison of energy consumption during the various sintering routes for this porcelain composition showed that DS consumed ~11 MJ/kg, MWS ~18 MJ/kg, FS ~3060 MJ/kg and SPS ~1612 MJ/Kg. While, the energy consumption listed here must be viewed with caution, the values are useful for comparison of the amounts of energy required for the different rapid sintering processes.Open Acces

Impact of microwave processing on porcelain microstructure

[EN] Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by similar to 75 degrees C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 degrees C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (similar to 32 +/- 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having similar to 63 mol% alumina content compared to similar to 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al+3 diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (similar to 900-1000 degrees C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.W. Lerdprom acknowledges Imperial College London funding no. MMRE_PG54200. A. Borrell acknowledges the Spanish Ministry of Economy and Competitiveness for her Juan de la Cierva-Incorporacion contract (IJCI-2014-49839).Lerdprom, W.; Zapata-Solvas, E.; Jayaseelan, DD.; Borrell Tomás, MA.; Salvador Moya, MD.; Lee, WE. (2017). Impact of microwave processing on porcelain microstructure. Ceramics International. 43(16):13765-13771. https://doi.org/10.1016/j.ceramint.2017.07.090S1376513771431