Cold Sintering of Functional Ceramics and Glasses

Ceramic and glass materials have had been used for millennia and have become ubiquitous in applications including communication, electronic and biomedical devices. Traditionally, ceramics are sintered at very high temperatures approaching ~80% of melting temperature. The high temperatures required to produce dense ceramics not only makes a significant contribution to processing energy but also limits the potential for development of novel ceramic and glass components which integrate polymeric materials and non-noble metals. A novel method of densification, known as ‘cold sintering’, utilises a transient liquid phase, pressure, and temperatures up to 300 °C to achieve the consolidation and densification of ceramics. In addition to energy reduction, benefits include the ability to co-sintering a broad variety of materials, greater control of component dimension and the final microstructures. This work demonstrates the application of the cold sintering process to three materials – lithium molybdate, a borosilicate glass frit and Bioglass® 45S5. The properties of lithium molybdate after cold sintering, heat treatment and conventional sintering are compared with heat treated and conventionally sintered samples. No residual water or secondary phases were found in cold sintered samples although some evidence of an amorphous grain boundary phases is seen. Lithium molybdate - bismuth molybdate composites were also fabricated and shown to remain as distinct phases within the component. The room temperature permittivity of the composites is reported and discussed. Coatings of lithium molybdate on glass substrates are produced by cold sintering, demonstrating the capability of the process to create well-adhered coatings on glass at very low temperatures. A glass frit provided by Johnson Matthey is also densified via the cold sintering method and compared to conventionally sintered materials. The effect of sintering conditions (cold and conventional) on the impedance behaviour is discussed. The creation of coatings and adhesive layers on steel and PTFE are also demonstrated. Bioglass® 45S5 is well-known for its bioactive responses driven by the ability to dissolve in the body and stimulate bone regeneration. Its solubility makes it an ideal candidate for densification using the cold sintering method. Bioglass powders are produced via flame spray pyrolysis and densified by cold sintering at 100 °C. NMR studies of the materials as received, wetted and after cold sintering provide insight into the potential mechanisms of the cold sintering of Bioglass. Bioglass-polymer composites are produced with a broad range of compositions and are shown not to be cytotoxic. Coatings of Bioglass on titanium have also been achieved. These results are promising steps towards a simple method of creating biomedical components utilising Bioglass-polymer composites and coatings thereof. The outcome of this work is to gain further understanding of the difference between properties of cold and conventionally sintered materials and to demonstrate the potential applications of cold sintering in areas beyond electroceramics.