Design and Characterization of ZrW2O8/Al Composites Based on Zero Thermal Expansion

Author:Zhou Chang

Supervisor:wu gao hui


Degree Year:2017





In this paper,inertial navigation for the application of background,to study a new type of zero expansion material-negative thermal expansion particles reinforced Al matrix composites.Fully dense 50~73vol.%ZrW2O8/Al-Si composites were designed and successfully fabricated by squeeze-casting method.The scanning electron microscopy(SEM),transmission electron microscopy(TEM),thermal expansion tester,thermal conductivity tester,In-situ Raman,computer simulation and three-point bending tests were used to systematically study microstructure and mechanism of the composites.The thermophysical properties,mechanical properties and thermal cycling properties of the composites were tested,and the relevant influencing factors were analyzed.Based on the interfacial reaction formula,a method for predicting the thermal expansion coefficient of ZrW2O8/Al-Si composites were established here.In this paper,the goal is to design the ultra-low thermal expansion(zero thermal expansion) materials.According to the Kerner model,the Turner model and the Rule of mixed,the volume fraction of the reinforcement of the ultra-low expansion ZrW2O8/Al-Si composite material should be 73 vol.%.Based on the Grüneisen parameter and the Raman peak shifts,the calculated equation of the residual stress as function of temperature is obtained.The XRD and density results show that there is a large amount of metastable high-pressure phase,γ-ZrW2O8 in the cast composite system,reducing the negative thermal expansion effect.The high temperature Raman,DSC,high temperature XRD,theoretical calculation,in situ TEM methods were used to determine the 200℃ as the appropriate heat treatment temperature,thereby eliminate the high-pressure phaseγ-ZrW2O8 content,to achieve the low thermal expansion behavior.Based on the principle of interfacial reaction kinetics,the design of the interface is controlled by reducing the preparation temperature and shortening the holding time,and the ultra-low expansion performance is obtained.In this paper,the average coefficient of thermal expansion(0℃~100℃) of 73vol.%ZrW2O8/Al-Si composites are 0.79×10-6K-1,which is consistent with the application of zero expansion.The composition and interface microstructure of the composites were also studied.SEM analysis showed that the aluminum matrix was continuously distributed in the composites.TEM analysis showed that a layer of nanoscale interface was formed between the matrix and the negative expansion particles.The interface layer is combined well with the two phases.With the increase of preparation time,the interface layer thicken.Under the condition of low temperature preparation,the main phase structure of nano-interface layer is ZrW2O8.The thickness of the interfacial layer is increased to submicron at high temperature and the brittle interface products ZrO2 and WO3 are formed.The mechanical properties of the composites were reduced from 110 MPa to 80 MPa when the fabricated temperature changed from 680℃ to 770℃.The formation mechanism of the nanostructures of zero-expansion composites is explained from the atomic-scale structure by high-resolution TEM and First-principles Calculations.It is pointed out that the polyhedral gap in the ZrW2O8 lattice can hold the aluminum atoms in the matrix,which is proved to be a spontaneous behavior.The interstitial solid solutions eventually transform into a high-intensity,stable nano-interface.The bending strength,density and elastic modulus of 73vol.%ZrW2O8/Al-Si composites were consistent after 200 cycles of hot and cold cycles,and the service performance was excellent.Microstructure observation also showed that Al elements diffused into the interface and showed a monotonically decreasing trend.Oxygen elements diffuse into the Al matrix and form a small amount of Al,O compounds.It is proved that the reaction of ZrW2O8/Al interface is a diffusion behavior and the corresponding diffusion rate and diffusion energy are calculated.Based on the diffusion formul a,the theoretical calculation model of temperature and time parameters and interface reaction was established.The relationship between the extention of reaction,the thickness of interfacial layer and the macroscopic thermal expansion was analyzed.