Search for: All records

To fulfill the demands of more bandwidth in 5G and 6G communication technology, new dielectric substrates that can be co‐fired into packages and devices that have low dielectric loss and improved thermal conductivity are desired. The motivation for this study is to design composites with low dielectric loss (tan δ) and high thermal conductivity (κ), while still limiting the electrical conductivity, for microwave applications involving high power and high frequency. This work describes the fabrication of high‐density electroceramic composites with a model dielectric material for cold sintering, namely sodium molybdate (Na₂Mo₂O₇), and fillers with higher thermal conductivity such as hexagonal boron nitride. The physical properties of the composites were characterized as a function of filler vol.%, temperature, and frequency. Understanding the variation in measured properties is achieved through analyzing the respective transport mechanisms.

 

Cold sintering densification and coarsening mechanisms are considered from the perspective of the nonequilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolutionprecipitation creep). This is an important mechanism of densification and deformation in many geological rock formations in the Earth’s upper crust, and although very slow in nature, it is of direct relevance to the cold sintering process. In cold sintering, we select particulate materials and identify experimental processing parameters to significantly accelerate the kinetics of dissolution-precipitation phenomena, with appropriate consideration of chemistry, applied stress, particle size and temperatures. In the theory of pressure solution, pressure-driven densification is considered to involve the consecutive stages of dissolution at grain contact points, then diffusive transport along the grain boundaries towards open pore surfaces, and then precipitation, all driven by chemical potential gradients. In this study, it is shown that cold sintering of BaTiO3, ZnO and KH2PO4 (KDP) ceramic materials proceeds by the same type of serial process, with the pressure solution creep rate being controlled by the slowest kinetic step. This is demonstrated by the values of activation energy (Ea) for densification, which are in good agreement with the existing literature on dissolution, precipitation, or coarsening. The influence of pressure on the morphology of ZnO grains also supports the pressure solution mechanism. Other characteristics that can be understood qualitatively in terms of pressure solution are observed in the in systems such as BaTiO3 and KDP. We further consider activation energies for grain growth with respect to the precipitation process, as well as evidence for coalescence and Ostwald ripening during cold sintering. For completeness we also consider materials that show significant plastic deformation under compression. Our findings point the way for new advances in densification, microstructural control, and reductions in cold sintering pressure, via the use of appropriate transient solvents in metals and hybrid organic-inorganic systems, such as the Methylammonium lead bromide (MAPBr) perovskite. 

Cold sintering is an emerging non-equilibrium process methodology that densifies ceramic powders at significantly reduced temperatures. This study proposes a fundamental framework to investigate its densification kinetics. By controlling four densification process variables including the transient chemistry, sintering temperature, uniaxial pressure and dwell time, the anisothermal sintering kinetics of highly densified ZnO is identified and phenomenologically modeled for its relative activation energetics.