The need to reduce the energy consumed and the carbon footprint generated by firing ceramics has stimulated research to develop consolidation techniques operating at lower temperatures, ideally not exceeding 300°C. This has been realised in Ultra Low Energy Sintering (ULES) using high pressure (hundreds of MPa) in the presence of a transient liquid phase, which accelerates plasticity, grain boundary/surface diffusion and mass transport. Several ULES techniques have been developed in the past 50 years, and a common feature of all of them is low temperature consolidation, through mechanisms not yet fully understood, enabling multi-material integration (e.g. organics and inorganics). This research could transform the traditional firing of functional and structural materials. Early stage work on ULES, started in the 1960s, clearly demonstrated cohesion between the compacted particles exceeding what was possible if simply produced by Van der Waals bonding, suggesting the formation of primary inter-particle bonds. Surprisingly, metals Cold Sintered (CS) in dry conditions at room temperature can be even stronger than their counterparts sintered at high temperatures (typically ≈ 2/3 T m). Hydrothermal Hot Pressing (HHP) was originally conceived in the context of sustainability and environmental preservation, with some examples being the concept of 'synthetic rock' for immobilisation of toxic/radioactive waste and the consolidation of high surface area porous ceramics for filtration. Follow up work on HHP considered the possibility of recreating in the lab bio-mineralisation using hydroxyapatite and bioglass (including hybrids) as proof of concept. Recent work on the Cold Sintering Process has demonstrated the potential to bridge the processing gap of multimaterial devices (sensors, batteries, 5G antennas, electronic components and biomaterials), enabling integration of polymers, ceramics and metals without degradation of the individual components both at the bulk and interface level. The absence of heating unlocks grain boundary design to an unprecedented level, offering further degrees of freedom in tuning functional properties. This review provides a wide perspective on room temperature consolidation, and covers the related but fragmented work published (≈ 450 papers) during the past 50 years, encompassing the relevant work developed in different disciplines including chemistry, physics, biology and geoscience. Liquid-assisted or liquid-mediated phenomena involving diffusion, plasticity, rheology, and grain growth are still largely unexplored in material science. The purpose of bringing together this literature is to build a general and multidisciplinary knowledge to guide future research directions. Both the reduction of energy consumption and carbon footprint are driving the growing interest in ULES, which could reinvent the concept of sintering, 'rendering kilns obsolete'. Also, ULES has the potential to produce new classes of materials that cannot be fabricated using conventional routes.

Cold sintering is an innovative low-temperature processing technique which allows consolidation of several ceramics. Despite recent research activities on the cold sintering of functional and structural ceramics, an analytical study accounting for consolidation and grain growth phenomena is still missing in the literature. In this work, we provide a theoretical analysis of the mechanisms active during cold sintering. The analysis considers two cold sintering approaches, characterised by the application either of isostatic or uniaxial pressure. Physical phenomena and microstructural features are discussed in view of the applied cold sintering approach. The developed pressure-assisted densification models indicate that the processes governing densification during uniaxial cold sintering are more complex than those of conventional liquid phase sintering. A key role is played by the water/material interaction which promotes several effects such as formation of surface defects and secondary phases, dynamic recrystallization and other phenomena still partially unknown.

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