Sintering of solid inorganic partic

Sintering of solid inorganic particulates into a dense polycrystalline ensemble with assistance of thermal energy and/or pressure is an essential process that underpins bulk ceramic manufacturing.1,2 Properties of ceramics, such as dielectric permittivity, dielectric breakdown strength, mechanical hardness and strength, and electrical/thermal conductivity are substantially affected by density. Studies on sintering have received broad research attention worldwide and continue to be of interest today.8,9 Owing to the high melting temperatures for most ceramic materials, conventional sintering is commonly accomplished at quite high-temperature ranges, as a rule of thumb, ~50%–75% of their melting points. For many materials like general oxides, the sintering temperature is typically ~1000°C, and the time necessary to sinter even a simple pellet of dense material can be up to several hours to several days.1,2 This high-temperature process is energy-consuming and often requires a sophisticated experimental setup for relevant facilities;10 moreover, the chemical stoichiometry of the final product may vary in the cases involving volatile elements (e.g. volatile of Bi, Pb, Na, K in piezoceramics) or co-firing of different materials (e.g. electrode-ceramic co-fired multilayer ceramic capacitors), yielding to property and crystal structure deviation caused by the alteration of defects concentration or intergranular diffusion.11–16 Therefore, developing low-temperature sintering techniques has driven global-wide research in scientific communities and industrial corporations during the past several decades. There have been many attempts to lower sintering temperatures through the addition of liquid phase additives, such as partially soluble inorganic liquid phase additives or glass fluxes, however, these typically only lower the conventional sintering temperature ~10%–20%.1,2,17–19 There are also alternative techniques being developed to sinter ceramics in a more efficient way with the utility of electrical energy or controllable thermal steps or high pressures. Examples of these techniques include Microwave Sintering (MVS),20 Flash Sintering (FS),21 Spark Plasma Sintering (SPS),22–26 Two-Step Sintering,27 Rate-Controlled Sintering,28 High-Pressure Sintering (HPS),29 or a combination of some of them,30 all of which have made impressive efforts to lower the sintering temperatures, and yet these temperatures are still typically well above 400°C. Therefore, the target of obtaining dense ceramic at ultralow temperatures, or even around room temperature like the formation of pearls and the aggregation of table salt or sugar particles, is hardly achieved.