Cold sintering process (CSP) radica

Cold sintering process (CSP) radically changes our concepts of sintering temperature ranges[6,7]. Sintering is generally regarded as a thermally-driven process where atomic diffusion lead to a decrease in the excess of surface energy in a particulate ensemble [8]. This is generally accomplished by a densification and grain growth process. The typical sintering temperatures are considered in relation to the melting temperature of the material. Mechanistically, Ashby et al. have most comprehensively outlined the important behavior enabling and controlling the diffusion mass transport processes under applied stress and temperature [9]. The ratio of sintering temperatures, Ts, and melting point, Tm, in those sintering diagrams ranges between 0.5 to 0.95 in terms of the Ts/Tm ratio. With the introduction of CSP, which is a process that utilizes a transient chemical phase, with intermediate pressures and low temperatures ≈ 300 °C, it has now been demonstrated for many materials, over 80 different materials with a wide variation of compositions, crystal structures, and chemical bonding [10–12]. Cold sintering uses an open system that allows the evaporation and loss of the transient phases. At interfaces between particles undergoing sintering, it aids rearrangement, dissolution-precipitation creep, and grain growth. Transport is permitted to have increased kinetics through a pressure solution creep mechanism that drives chemical dissolution from highly constrained areas between particles into the liquid solvents, and then rapidly diffuses from the contact points along the grain boundaries, to then precipitate at less constrained pore surfaces [13,14]. In addition to demonstrations of the densification process with temperature ratios Ts/Tm < 0.1 with cold sintering, there are also demonstrations of co-sintering different materials, allowing the ability to fabricate novel composite materials. This has been with nanomaterials at grain boundaries, such as 2D materials [15], thermosetting [16], and thermoplastic polymers [17–19], all targeting grain boundary properties. Another important demonstration of the power of CSP has been with modifying conventional manufacturing thick film processes to form films on metal substrates and multilayer devices; then, after low temperature debinding of forming organics, enabling prototyping of devices such as thermoelectric generators [20], CoG capacitors [21], and microstrip patch antennas [22], with different electrochemical active electrode materials.