Preparation And Properties Of Negative Thermal Expansion Materials And Reinforced Aluminum Matrix Composites

Most materials in nature show the phenomenon that the volume of the material expands with the increase of temperature and shrinks with the decrease of temperature.This characteristic provides a lot of convenience for human beings,but it brings a lot of troubles to modern life,such as thermal stress and precision of precision parts.Because of its anomalous thermal expansion properties,negative thermal expansion materials have attracted extensive attention in the world since they were discovered in the1950s.However,the research on negative thermal expansion materials is still in the experimental stage to date,and the materials with excellent negative thermal expansion properties are still few,let alone those with practical application value.Therefore,the preparation of materials with excellent negative thermal expansion properties is an important task of this paper.So what is excellent negative thermal expansion performance?There are the following requirements:(1)negative heat expansion temperature range in the vicinity of room temperature;(2)the width of the negative thermal expansion temperature range is larger;(3)the coefficient of negative thermal expansion is large;(4)isotropic and stable.In order to obtain the negative thermal expansion materials with excellent properties,this paper explores from two directions.On the one hand,we control the grain size of Mn_3.1Zn_0.5Sn_0.4N to realize the translation and stretching of its negative thermal expansion temperature range.On the other hand,the thermal expansion properties of Mn₃(Mn_xZn_ySn_z)N(x+y+z=1)series negative thermal expansion materials were studied by adjusting the ratio of A doped elements in Mn₃AN.This negative thermal expansion behavior makes the negative thermal expansion material used as the expansion property regulator to prepare composite materials.In this paper,the negative thermal expansion materials with excellent properties obtained in the early stage were compounded with pure aluminum matrix to prepare aluminum matrix composites with near-zero-thermal-expansion properties.In a certain temperature range,the thermal expansion coefficient of the composites was close to 0×10^-6K^-1.In order to improve the comprehensive properties of composite materials,this paper explores how to improve the mechanical properties of composite materials by adjusting the particle size ratio of negative thermal expansion materials.This method not only found the influence of particle size ratio on mechanical properties,but also found the rule of influence on thermal conductivity.(1)Mn_3.1Zn_0.5Sn_0.4N negative thermal expansion material of normal size was sintered in tube furnace with Mn powder,Zn powder,Sn powder and high purity nitrogen as raw materials.Then,the nanoscale powder of negative thermal expansion material was prepared by ball grinding at250r/min for 10?30H.Finally,Mn_3.1Zn_0.5Sn_0.4N negative thermal expansion material with nanometer grain was sintered by SPS under pressure of 50MPa at 800?.With the ball grinding time prolongs,the material is less likely to appear the negative thermal expansion phenomenon.The powder particle size of ball mill for 10H reaches 10nm,and the negative thermal expansion temperature range of sintered body moves to the low temperature zone,proving that the size effect will affect the thermal expansion performance of negative thermal expansion materials.(2)By adjusting the contents of x,y and z in Mn₃(Mn_xZn_ySn_z)N(x+y+z=1)series of negative thermal expansion materials,the influence of the content of doping elements on the thermal expansion properties of negative thermal expansion materials was explored.In this paper,Mn₃(Mn_xZn_0.5Sn_0.5-x)N(x=0.1,0.2,0.3),Mn₃(Mn_0.1Zn_0.9-xSn_x)N(x=0.3,0.4,0.5),Mn₃(Mn_0.2Zn_0.8-xSn_x)N(x=0.2,0.3,0.4),Mn₃(Mn_0.3Zn_0.7-xSn_x)N(x=0.1,0.2,0.3)series negative thermal expansion material were prepared successively,and their thermal expansion behavior was characterized.The results show that the negative thermal expansion materials with excellent properties can be prepared by adjusting the content of doped elements,so that the temperature range of negative thermal expansion is near room temperature,has a wide temperature zone and has a large negative thermal expansion coefficient.For Mn₃(Mn_xZn_ySn_z)N(x+y+z=1)series of negative thermal expansion materials,the content of Mn,Zn and Sn has a significant influence on the temperature range of negative thermal expansion:as the content of Mn increases,the temperature range of negative thermal expansion of materials moves to the low temperature zone.When the content of Mn is fixed,the negative thermal expansion temperature range of the material moves towards low temperature as the content of Sn decreases and the content of Zn increases.(3)Near-zero-thermal-expansion aluminum matrix composites were prepared by combining the excellent negative thermal expansion material with the pure aluminum matrix.After fully mixing the negative thermal expansion material with pure aluminum powder,the composite material with a thermal expansion coefficient close to 0×10^-6K^-1 was successfully prepared by pressurizing 400MPa at 450?in a vacuum hot pressing furnace.By adjusting the particle size ratio of the reinforcement,the influence of particle size ratio of the reinforcement on the mechanical and thermal properties was found:the composite material with a higher proportion of small-size particles had a higher tensile strength,while the composite material with a higher proportion of large-size particles had a better plasticity;the composite materials with larger size particles have better thermal conductivity and higher thermal diffusion coefficient.