Ultra-fast firing: Effect of heating rate on sintering of 3YSZ,with and without an electric field

It has recently been reported that ceramics can be sintered in a few seconds with the aid of an electric field (“flash sintering”). This investigation tests the possibility that the accelerated sintering is a consequence of the rapid heating rate involved rather than a direct effect of the electric field on mass transport. The sintering of 3YSZ powder compacts at a temperature of ∼1300 °C was compared (i) in flash sintering, (ii) with rapid heating rates produced without the application of an electric field, and (iii) with conventional heating rates. The results show that rapid heating can accelerate sintering by over 2 orders of magnitude compared with heating to the same temperature at conventional rates, even without the application of an electric field. It is concluded that the rapid densification in flash sintering of 3YSZ is at least partly a consequence of the rapid heating involved. Possible explanations are discussed.     

The cold sintering process: A review on processing features,densification mechanisms and perspectives

Since its first introduction in 2016, cold sintering process (CSP) has gained worldwide interest from the scientific community as green and innovative fabrication route due to the dramatic reduction of processing time, energy, and costs. Cold sintering resembles the geological formation of rocks where a ceramic powder is densified with the aid of a liquid phase under an intense external pressure and limited heating conditions (below 350 °C). Up to date, tens of different materials, including composites, have been successfully processed through CSP and extraordinary results in terms of densification, microstructure and final properties have been achieved. In the present review, processing features and variables, possible densification mechanisms and issues also for the realization of ceramic-based composites are explored. Advantages with respect to existing techniques are analysed and current challenges are described to lay the ground for new processing opportunities to be faced in the near future.     

Flash sintering of lead zirconate titanate (PZT) ceramics: Influence of electrical field and current limit on densification and grain growth

Flash sintering of lead zirconate titanate ceramics were investigated under DC electric fields ranging from 300 to 600?V/cm. The onset temperature for flash sintering significantly decreased with the electrical field to a lower limit of furnace temperature of 538?°C at 600?V/cm. The retardation of grain growth was observed, and the grain size decreased with increasing the electrical field. The current limit had a great influence on the density and grain size of specimen. During the flash sintering process, power dissipation first rose abruptly to a maximum value, then declined sharply to a steady state. Meanwhile, optical glow of specimen was observed. Using black body radiation model, the actual specimen temperature was estimated, which was too low to obtain the full dense ceramics in 30?s. It was suggested that Joule heating, ultra-high heating rate and high concentrations of defects were responsible for flash sintering of PZT ceramics.     

A thermal perspective of flash sintering: The effect of AC current ramp rate on microstructure evolution

In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary.     

Spark plasma sintering of a lunar regolith simulant: effects of parameters on microstructure evolution,phase transformation,and mechanical properties

The spark plasma sintering (SPS) process is a potentially effective in-situ resource utilization (ISRU) technology for consolidating lunar regolith in order to produce structural components for future space exploration. This study examined the fundamental mechanisms of the effects of SPS conditions on microstructure evolution, phase transformation, and mechanical properties. For this purpose, a lunar regolith simulant (FJS-1) was selected and sintered for a total of 16 cases based on four primary SPS testing parameters: temperature, applied external pressure, dwell time, and heating rate. The Taguchi design method was used to examine the effects and sensitivity of each testing parameter. Laboratory tests were conducted in multiple length scales, including density, porosity, optical microscopy, scanning electron microscopy aided by energy-dispersive spectroscopy, transmission electron microscopy, nanoindentation, and strength testing (in both compressive and flexural). Taguchi analysis results of SPS parameters and sintering mechanism discussion indicated that the sintering temperature is the dominant factor changing microstructure heterogeneity and densification during the SPS process. The contribution of applied pressure to the surface and the grain boundary diffusion rate and the nucleation rate indicated that the applied pressure may have enhanced both phase transformation and homogeneity during the sintering process. Strength of the sintered samples were approximately 10 times greater than those of a typical plain concrete. The collective results indicate that the SPS technology, a potentially viable ISRU method, can be used to produce property-specific and application-targeted building components on the lunar surface.