Cold sintering process: A new era for ceramic packaging and microwave device development

Cold sintering process (CSP) is an extremely low-temperature sintering process (room temperature to ~200°C) that uses aqueous-based solutions as transient solvents to aid densification by a nonequilibrium dissolution-precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic-polymer composites. Some dielectric materials, namely Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and (LiBi)_0.5MoO₄ ceramics, and also (1−x)Li₂MoO₄−xPTFE and (1−x)(LiBi)_0.5MoO₄−xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%-95%) and good microwave dielectric properties (permittivity, 5.6-37.1; Q × f, 1700-30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co-fired ceramic technology, namely CSCC. Li₂MoO₄−Ag multilayer co-fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy-saving strategy for the ceramic packaging and microwave device development.