Cold Sintering Preparation And Properties Of CaSO₄·2H₂O And SrMoO₄ Ceramics

Cold sintering process?CSP?is a novel method to prepare ceramics at ultra-low temperatures or even room temperature,which has attracted wide attention since its emerging in 2016.The most important densification mechanism in cold sintering is dissolution-precipitation mechanism,so a suitable solubility is required for the materials that are expected to be densified by this method.However,the lower limit of required solubility for CSP is unclear till now.In this thesis,the slightly soluble materials gypsum?CaSO₄·2H₂O?and SrMoO₄ ceramics were studied,and the prepration possibility by cold sintering and related properties were investigated.Dense CaSO₄·2H₂O ceramics were prepared by CSP at room temperature.For samples cold-sintered under 400 MPa,the relative density climbed from 90.8%to 96.8%with increasing the water content during cold sintering from 0 wt%to 5 wt%,and the microstructure became homogeneous and dense.The further increase of water content led to the slightly decreasing relative density,inhomogeneous microstructure and the appearance of water agglomertions.Therefore,5 wt%water was used in the following work.For samples cold-sintered with 5 wt%water,the relative density rose from 89.6%to 96.8%when the preparation pressure increased from 100 MPa to 400 MPa,and the microstructure became homogeneous together with the appearance of large aggreagations.With the pressure further increasing,the relative density decreased slightly,and abnormal grain growth was observed as well as large pores and microcracks.Correspondingly,with increasing the preparation pressure,both the compressive strength and flexural strength first increased and reached the maximum values when the preparation pressure was 400 MPa,and then dropped sharply.The highest compressive strength and flexural strength are 98.5 MPa and 26.5 MPa respectively,which were impreoved by 2.6 and 2.0 times,repectively comparing with the bulk gypsum prepared by traditional method.Furthermore,the dry-pressed compacts were very fragile,and had relative densities 5-12%lower than cold-sintered ceramics,indicating that the solubility of gypsum?0.2 g?is high enough to induce the dissolution-precipitation mechanism,and plays a critical role in the densification,microstructural evolution and greatly improved mechanical properties of cold-sintered gypsum ceramics.Dense SrMoO₄ ceramics were prepared room-temperature cold sintering and subsequent high-temperature sintering.Under the same preparation pressure,the relative densities of green compacts pressed with PVA solution were 1-7%lower than those cold-sintered with water,indicating that water promotes the mass transport and densification for slightly soluble SrMoO₄,and water is a better binder than PVA solution for obtaining high relative density of green compacts.The optimal CSP condition was 5 wt%water,500 MPa and 10 min,and the highest relative density of green compact was 78.0%.Although dense SrMoO₄ ceramics were not obtained in one step by CSP,the sintering temperature could be greatly reduced from 1050 ^oC to 850-950 ^oC due to the high relative densities of green bodies.With the increase of sintering temperature,the microstructures became more homogeneous and denser,together with the rapid grain growth.The dielectric constant first increased and then remained unchanged while the Qf value first increased,then decreased,and finally remained steady.The following optimal combination of microwave dielectric properties was achieved for the sintering temperature of 850 ^oC:?_r=9.3,Qf=13600 GHz,?_f=-64.4ppm/^oC,and the corresponding relative density was 94.6%.