Plastic deformation and effects of water in room‐temperature cold sintering of NaCl microwave dielectric ceramics

Abstract

NaCl ceramics were prepared by room‐temperature cold sintering using moistened NaCl powder with 4 wt% water and dry pressing using dehydrated powder. When the applied uniaxial pressure is low, the relative density of dry‐pressed NaCl ceramic is significantly lower than that of cold‐sintered ceramic, while the former is 98.5%‐99.3% and much higher than the latter (94.3%‐94.6%) for high applied pressure of 200‐300 MPa. The uniaxial pressure‐induced plastic deformation dominates the densification of dry‐pressed NaCl ceramic, and also plays a role during cold sintering as well as the dissolution‐precipitation process. The lower density of cold‐sintered NaCl ceramic under high applied pressure is attributed to the trapped water in ceramic body during cold sintering. Besides, the presence of water always promotes the microstructural homogeneity, which is responsible for the much higher Qf value of cold‐sintered NaCl ceramic. The optimal microwave dielectric properties with εr = 5.55, Qf = 49 600 GHz, and τf = −173 ppm/°C are obtained in cold‐sintered NaCl ceramic under the applied pressure of 300 MPa, indicating that it is a promising candidate as a microwave dielectric material.

Cold sintering and microwave dielectric properties of dense HBO2‐II ceramics

“…On the other hand, a new sintering method known as “cold sintering” has attracted extensive attention after its appearance in 2014, since many ceramics can be densified by this method at extremely low temperatures from room temperature to 300°C . Cold sintering is also very attractive for the low‐temperature preparation of ceramic‐polymer and ceramic‐ceramic composites .…”

“…For a regular cold sintering process, the ceramic powder is mixed with a small amount of transient liquid phase (usually water) in which the ceramic is soluble, and a uniaxial pressure is applied on the mixture. The transient liquid phase activates the dissolution‐precipitation mechanism and accelerates the mass transport greatly, so that the densification of the ceramics can be realized easily near the evaporation temperature of the transient solvent . Although HBO 2 is soluble in water, the cold sintering of HBO 2 ceramics with water as the transient liquid phase is not feasible because of the reaction between HBO 2 and H 2 O into H 3 BO 3.…”

“…On the other hand, a new sintering method known as "cold sintering" has attracted extensive attention after its appearance in 2014, 17 since many ceramics can be densified by this method at extremely low temperatures from room temperature to 300°C. [18][19][20] Cold sintering is also very attractive for the low-temperature preparation of ceramic-polymer and ceramic-ceramic composites. 21,22 For a regular cold sintering process, the ceramic powder is mixed with a small amount of transient liquid phase (usually water) in which the ceramic is soluble, and a uniaxial pressure is applied on the mixture.…”

“…The transient liquid phase activates the dissolution-precipitation mechanism and accelerates the mass transport greatly, so that the densification of the ceramics can be realized easily near the evaporation temperature of the transient solvent. 19,20,23,24 Although HBO 2 is soluble in water, the cold sintering of HBO 2 ceramics with water as the transient liquid phase is not feasible because of the reaction between HBO 2 and H 2 O into H 3 BO 3. 12 Noting that HBO 2 is also soluble in ethanol and no chemical reaction occurs between them, ethanol is probably a suitable transient liquid phase for the cold sintering of HBO 2 ceramics.…”

Preparation and microwave dielectric properties of B2O3 bulk

“…1 The sintering process requires the bulk diffusion of atoms from the particle interiors to the inter-particle necks, so high temperature (usually >1000°C) is needed to provide the energy for diffusion. [1][2][3][4][5][6][7][8] Among these methods, the liquid phase sintering is of significant importance, since it is compatible with the standard solid-state sintering method, and no extra device or process is needed. Many methods have been developed to lower the sintering temperature of ceramics, such as using fine ceramic powder, liquid phase sintering, spark plasma sintering, field assisted sintering, flash sintering, cold sintering and etc.…”