Densification Of MgO And NaNO₃/MgO By Cold Sintering

The cold sintering process,as a new method for densifying ceramics and ceramic-based composites at low temperatures,has attracted widespread attention once it came out in 2016.Through liquid phase and uniaxial pressure,the cold sintering process provides a new path for the densification of ceramics and has a broad application prospect.The“dissolution-precipitation”during cold sintering is generally considered to be the main driving force of densification.Therefore,this technology requires raw materials to be soluble in liquid media,which greatly limits the choice of raw materials.Many important industrial ceramics that are poorly soluble or insoluble in the liquid phase cannot be densified via cold sintering.Breaking through the limitations on the solubility of raw materials is crucial for the future development of cold sintering.This work takes MgO,an important industrial ceramic that is almost insoluble in water,as the research object,and explores the feasibility of achieving its densification through cold sintering with the aid of the hydration and dehydration reactions of MgO.The mixture of MgO and water(the mass ratio of MgO to water was preferably 1:1.6)should be aged properly before heating(preferably at 25? for 30 min)to help the liquid phase and powder to mix uniformly,which was beneficial to the subsequent cold sintering.In the initial heating stage(?290?),MgO would undergo a hydration reaction to form MgO-Mg(OH)₂ particles with a“core-shell”structure.At this stage,the amount of Mg(OH)₂ produced was positively correlated with the initial liquid content in the mixture.With the temperature increases(290-450?),Mg(OH)₂ gradually decomposed.When heated to 440?,a uniaxial pressure of 500 MPa was applied and sintered at 450? for 30 min to make the pellet dense.Mg(OH)₂contributes to the densification of MgO,and it was finally completely decomposed.A pure MgO pellet with a relative density of 93%was obtained.This method of combining cold sintering with hydration and dehydration reactions breaks through the limitations of cold sintering on the solubility of materials and greatly expands the low-temperature sintering path of insoluble or even insoluble ceramics.In addition,we applied the cold sintering of MgO to the preparation of NaNO₃/MgO composite phase change heat storage materials.The effects of uniaxial pressure,sintering time,liquid content,and sintering temperature on the densification of the composites were discussed.Besides,Na⁺might enter the crystal lattice of MgO during cold sintering,and the resulting lattice defects significantly reduced the sintering temperature of MgO,so that the NaNO₃/MgO composite could be quickly prepared at 120?.The phase change material was effectively encapsulated in a MgO skeleton,and the composite exhibited good mechanical property,phase change performance,and thermal cycle stability.Cold sintering provides a new method for the economical and efficient preparation of inorganic salt/ceramic composite phase change heat storage materials.