cold sintering
Sintering usually means fire — a furnace at a thousand degrees or more. Cold sintering asks a startling question: can you densify a ceramic near room temperature, with no furnace at all? The recipe is almost homely. Wet the ceramic powder with a few drops of a transient liquid, usually water or a simple solution, then squeeze it hard in a die warmed to just a few hundred degrees (often 120 to 300 degrees C, cooler than an oven roasts a chicken). Under that pressure the powder consolidates into a dense solid, much the way sedimentary rock forms, and the liquid is then driven off as steam, leaving a fired-looking part behind.
The mechanism is pressure-driven dissolution and precipitation. Where two particles press together, the contact points are under high stress, which raises the chemical potential of the material there and makes it dissolve into the thin film of transient liquid. That dissolved material is carried a short distance and re-precipitates in the neighbouring pore spaces, where the stress is low. Particle contacts flatten, pores fill, density climbs — and only then is the liquid removed. It is essentially the geologist's pressure-solution creep, sped up and put to work. It succeeds best for materials whose surfaces are soluble in or reactive with the chosen liquid: zinc oxide, many lithium and sodium salts, some perovskites. Pressures run to hundreds of MPa.
Cold sintering matters most for things you simply cannot do in a hot furnace. Because it stays cool, you can co-sinter a ceramic together with a polymer or a metal into an intimate composite that would burn or melt at 1400 degrees C, and you save enormous amounts of energy in the process — attractive for integrated electronics and batteries. Be honest about its narrowness. It needs a transient-liquid chemistry that actually dissolves the powder, so it does not work for insoluble covalent ceramics like silicon carbide, silicon nitride or alumina without tricks. Residual liquid or a second phase left along the grain boundaries can degrade properties, and the process is young, materials-specific, and still being understood.
Zinc oxide powder dampened with a little water can be pressed to over 95 percent of full density at just 90 to 250 degrees C — cool enough to co-sinter it in the same body as a plastic, making a dense ceramic-polymer composite no ordinary kiln could ever produce.
A transient liquid does the work heat usually does — dissolve, transport, re-precipitate — but near room temperature.
Cold sintering is not universal: it relies on a transient liquid that can dissolve the particle surfaces, so covalent ceramics like silicon carbide and alumina, which are essentially insoluble, are poor candidates.