Tailoring And Mechanism Of Thermal Expansion With A₂P₂O₇（A=Ni,Co） Based Compounds

In optics,precision instruments,aerospace and other fields,during the change of ambient temperature the thermal expansion of the material leads to the huge size change and the functional failure of the component,which greatly reduces the precision and service life of devices.The advent of negative thermal expansion materials presents an opportunity for thermal expansion regulation.However,the variety and quantity discovered are still very few,especially few materials with the advantages of simple preparation,large coefficient of thermal expansion and low price,which seriously limit their application.Therefore,this work takes A₂P₂O₇series materials as the research object,designs and prepares the continuous regulation of the coefficient thermal expansion from huge negative to positive,and elucidates the mechanism of negative thermal expansion by using variable temperature XRD and temperature dependent Raman method.Based on a novel phase variant negative thermal expansion material Ni₂P₂O₇(α_l=-35.8×10^-6K^-1,820～860 K).By replacing Ni atom with Mn atom,not only the phase transition temperature of Ni₂P₂O₇base material is effectively reduced,but also the temperature range of negative thermal expansion is greatly widened,and the continuous control of Ni₂P₂O₇base material from negative to nearly zero to positive thermal expansion is realized.The ratio ofαphase toβphase in Ni_2-xMn_xP₂O₇（x=0.4,0.6）samples was measured by variable temperature XRD.It is found that the ratio ofαphase decreases and that ofβphase increases with the increase of temperature,which is consistent with the variation law of the ratio of two phases extracted by temperature dependent Raman.Therefore,we determined that the thermal expansion regulation mechanism of Ni₂P₂O₇based materials is the change of phase proportion.Although both phases（αandβphases）of Ni₂P₂O₇exhibit positive thermal expansion behaviour,we can modulate the thermal expansion behaviour by controlling the phase transition rate,which provides a new idea for the thermal expansion modulation of other phase changing materials.In order to further reveal the thermal expansion regulation mechanism of A₂P₂O₇series,we form a new solid solution Co_2-xCu_xP₂O₇（0≤x≤2）of two negative thermal expansion materials,Co₂P₂O₇with phase variation and Cu₂P₂O₇with transverse vibration of bridging atoms.Co₂P₂O₇exhibits a large NTE effect(α_l=-96×10^-6K^-1,540～577 K).The substitution of Cu atom for Co atom not only reduces the phase transition temperature of Co₂P₂O₇,but also expands the temperature range of negative thermal expansion.Co_1.85Cu_0.15P₂O₇exhibits a low negative thermal expansion(α_l=-3.27×10^-6K^-1,300～460 K).The mechanism of negative thermal expansion of Co_1.85Cu_0.15P₂O₇was investigated by temperature dependent Raman test.It was found that the regulation mechanism of negative thermal expansion was still phase proportion change.Through vickers hardness and SEM tests,it is found that the chemical substitution of Cu atom for Co atom not only improves the thermal expansion property of Co₂P₂O₇base compound,but also enhances the density and hardness,which improves the accuracy of linear expansion coefficient.