Cold Sintering Process And Densification Mechanism Of The ZrW₂O₈/ZrO₂ Composites With Low-near Zero Thermal Expansion

ZrW₂O₈ exhibits an excellent isotropic negative thermal expansion property over a wide temperature range,which can be composited by ZrO₂ to fabricate the ZrW₂O₈/ZrO₂ composites with low-near zero thermal expansion,since these composites have the advantages of fine designability,good wetting of biphase and wear and corrosion resistance.Therefore,the thermal deformation and function failure issues of the components,such as the Bragg grating,substrate of fiber optic measuring stick,microelectronic circuit board,etc.,resulted by the thermal expansion effect of materials under the large temperature changes are expected to be solved by ZrW₂O₈/ZrO₂ composites with low-near zero thermal expansion,thus,these composites have great application prospects in the fields including precision manufacturing and measurement,optical instrument,optical fibre communication,electronic precision system,etc.of the the ground environment.However,the decomposition issue of ZrW₂O₈ phaseT_d ∈777 ℃,1105 ℃)have seriously hindered the engineering application of these composites when using the traditional solid phase sintering method to fabricate ZrW₂O₈/ZrO₂ composites at high temperature.Cold sintering process（CSP）is a novel technique to realize the fabrication of ceramic materials with an ultra low sintering temperature（Ts ≤ 200 ℃）assisted by the suited external pressure and aqueous liquid phase,which is expected to solve the decomposition issue of ZrW₂O₈/ZrO₂ composites.Therefore,the dense and undecomposed ZrW₂O₈ ceramics are firstly fabricated by CSP and on this basis,a mosaic structure with the ZrW₂O₈ matrix particles embedded by ZrO₂ nano particles is constructed by the hydrothermal assembly in order to modify the interface combination mode of biphase.Then the undecomposed ZrW₂O₈/ZrO₂ composites with low-near zero thermal expansion are fabricated by CSP at ultra low sintering temperatures（T = 190 ℃ & TA = 550 ℃）.As a consequence,the decomposition issue is effectively solved.Compared with the traditional solid-state sintering at high temperatures,the research of this paper has realized the fine low-near zero thermal expansion of ZrW₂O₈/ZrO₂ composites with greatly reducing the sintering temperatures（the rate of reduction reaches to 54%）,and this research has provided the theoretical basis and technical support for the engineering application of these composites.In order to solve the problem relating the steady supply of raw ZrW₂O₈ powders with high quality,the ZrW₂O₈ powders with high purity（≥ 99.9%）,small particle size（average particle size: 103 nm）and high activity are firstly prepared by hydrothermal synthesis.Since ZrW₂O₈ is the core functional phase in ZrW₂O₈/ZrO₂ composites,so the single-phase ZrW₂O₈ ceramic with negative thermal expansion fabricated by CSP is investigated in priority.The optimized parameters relating CSP densification for fabricating the ZrW₂O₈ ceramic are obtained through the decoupling research of CSP parameters（including the sintering temperature T,external pressure P,sintering time t,liquid phase type and variable liquid phase content c,etc.）.And a dense（bulk density ρZW = 4.79 g/cm3）and undecomposed ZrW₂O₈ ceramic with negative thermal expansion(the sample after annealing at TA = 660 ℃ with t A = 360 min shows the coefficients of thermal expansion（CTE）:α_0-1.55℃=-8.9 10 / ℃×-6155-500 ℃α=-3.9 10 / ℃×)is obtained with the parameters of T = 190 ℃,P = 350 MPa,t = 180 min,c = 30 wt%,in which the hydrothermal precursor suspension HS（ZrOCl2·8H2O（aq）+ Na2WO4·2H2O（aq）+ HCl（aq））is chosen as the liquid phase.The microstructure analysis proves that the ZrW₂O₈ has been densely sintered.Combined with the powder sintering theory,the CSP implementation mechanism of ZrW₂O₈ ceramics is illustrated,in which the ZrW₂O₈ ceramics are densely sintered from the particles by the flow mass transport,dissolution and precipitation with the assistance of liquid phase HS.Then the microstructure is optimized through the annealing treatment controlled by diffusion mass transport and the ZrW₂O₈ ceramics with fine negative thermal expansion are obtained finally.Based on the above investigation,the fabrication of ZrW₂O₈/ZrO₂ composites utilizing CSP is investigated.Firstly,the ZrW₂O₈/ZrO₂ composite powders are obtained by the mechanical mix,ZrW₂O₈/ZrO₂ composites are fabricated by CSP with the same parameters used in the fabrication of ZrW₂O₈ ceramic,in which HS is adopted as the liquid phase.The results reveal that an undecomposed ZrW₂O₈/ZrO₂ composite can be obtained when the volume fraction of ZrW₂O₈ VZW = 81.4%.But the sintering properties of the cold-sintered samples are poor with a terrace-like layered microstructure,indicating that the composite does not been densely sintered.The direct reasons of the poor sintering properties are the biphase distribution and the interface combination of the directly physical contact caused by the mechanical mixing.And the essential reason is attributed to the insolubility of ZrO₂ in HS liquid phase.In order to improve the sintering properties of the ZrW₂O₈/ZrO₂ composites fabricated by CSP,ZrW₂O₈/ZrO₂ composite powders with a mosaic structure,in which the difficult sintered ZrO₂ particles embedded into the easy sintered ZrW₂O₈ matrix particles,are hydrothermally assembled to modify the interface combination.And the fundamental mechanism of this hydrothermal assembly is discussed in further.The hydrothermally assembled ZrW₂O₈/ZrO₂ composite powders are used to investigate the CSP densification evolution,as well as the effect of the hydrothermal liquid phase content c on the microstructure and properties of the hydrothermally assembled ZrW₂O₈/ZrO₂ composites fabricated by CSP.The relative dense（ρ30% = 4.53 g/cm3）and undecomposed ZrW₂O₈/ZrO₂ composite（hydrothermal assembly）with near-zero thermal expansion（the sample after annealing at TA = 550 ℃ with t A = 600 min shows the CTE:-630%α= 0.23×10 /℃,[ ]T ∈50℃,220℃）,which can be used in the terrestrial environment,is fabricated by CSP with the parameters of T = 190 ℃,P = 350 MPa,t = 180 min,c = 30 wt%.In addition,a denser（ρ10% = 4.86 g/cm3）and undecomposed ZrW₂O₈/ZrO₂ composite（hydrothermal assembly）with low thermal expansion（CTE:-610%α= 2.49 ×10 /℃,[ ]T ∈50℃,220℃）can be obtained by adjusting the liquid phase content c（c = 10 wt%）.The interface combination of biphase modified by hydrothermal assembly has significantly improved the sintering properties of the cold-sintered ZrW₂O₈/ZrO₂ composites（hydrothermal assembly）.According to the analysis of the CSP densification evolution and microstructure evolution,the densification of ZrW₂O₈ phase in the composite is realized by the rodlike ZrW₂O₈ matrix particles embedded by ZrO₂ particles through the flow mass transport,dissolution and precipitation with the assistance of liquid phase HS,and ZrO₂ phase is driven to realize the densified fabrication of ZrW₂O₈/ZrO₂ composites by CSP.Then the excellent low-near zero thermal expansion property of the composites is obtained though the optimization of microstructure during the subsequent annealing treatment dominated by the diffusion mass transport.