Design And Performance Of Sn-Bi-Pb-Zn Based High Temperature Heat Transfer And Heat Storage Materials

In solar thermal power generation systems,high temperature heat transfer and heat storage media have always been a key factor affecting the efficiency of thermal power generation.Molten salts and heat transfer oils are currently used more.The maximum temperature of the heat transfer oil is 400°C,and the upper limit of the molten salt is only 600°C.Low melting point alloy has high thermal conductivity,low solidus temperature,wide temperature range,and the highest temperature can be higher than 900°C.It is a potential high temperature heat transfer medium.It has broad application prospects in the fields of nuclear energy engineering and solar thermal utilization.A series of Sn–Bi–Pb–Zn-based low melting point alloy materials were designed and prepared for the performance requirements of high temperature heat transfer and heat storage fluids.The microstructure,thermal properties,oxidation resistance and container compatibility were systematically studied.The relationship between the microstructure and thermal properties of the alloy is revealed,and the mechanism of specific heat of the nano-particle reinforced alloy is also discussed.The thermophysical properties of Sn–Bi–Pb–Zn alloys with different compositions were investigated by differential scanning calorimetry(DSC)and X-ray diffraction analysis(XRD).The results show that Sn–50Bi–2Zn has the best thermal performance.The change in the density and thermal properties of the alloy from solid to liquid makes the alloy have a higher thermal conductivity in the liquid state.The phase of the alloy affects the crystalline form of the alloy,resulting in a microscopic morphology of the alloy in the form of sheets or dendrites.The microstructure,phase and thermophysical properties of(Sn–50Bi–2Zn)–x In and(Sn–50Bi–2Zn)–x Ga alloys were investigated.A new structure and phase are formed in the alloy matrix to which the Ga,In element is added,so that the microstructure of the matrix alloy is more uniform and small.The Ga,In element can lower the melting point of the Sn–50Bi–2Zn alloy and increase the phase transition enthalpy,specific heat capacity and thermal conductivity.The thermal properties of the alloy are consistent with the trend of the matrix alloy as a function of temperature and physical state.The oxidation weight gain curves of(Sn–50Bi–2Zn)–x In and(Sn–50Bi–2Zn)–x Ga alloys were determined by thermogravimetric analysis(TGA)and the activation energy of the oxidation reaction was calculated.The Ga,In element can improve the activation energy of the oxidation reaction of the base alloy and improve the microstructure and oxidation resistance of the oxide film at 400°C.The Ga element segregates to the surface to form a dense Ga₂O₃ oxide film,which reduces the thickness of the oxide layer.The Gibbs free energy of the oxidation of Ga element is the smallest,and the mixed oxide film of Zn O and Ga₂O₃ remains stable once formed.The container compatibility of the Sn–50Bi–2Zn,(Sn–50Bi–2Zn)–7In and(Sn–50Bi–2Zn)–7Ga alloys were studied at 700°C with 316 stainless steel,304stainless steel,20 carbon steel and ceramic sheets by constant temperature static full immersion method.316 stainless steel has the best corrosion resistance in liquid alloys.The oxidation resistance mechanism similar to Ga element and Cr element enhances the corrosion resistance of 20 carbon steel and has little effect on the corrosion resistance of stainless steel.After the corrosion,Fe,Cr,and Ni elements are dissolved into the inside of the alloy liquid,and Ni and Zn elements are concentrated in a small range inside the alloy.The melting point and phase transition enthalpy of the(Sn–50Bi–2Zn)–7In and(Sn–50Bi–2Zn)–7Ga alloys after corrosion have been improved to some extent.During the corrosion process,the thermal conductivity of the alloy matrix is slightly increased due to the dissolution of elements in the steel sheet and the diffusion of the matrix alloy elements.Aiming at the low specific heat capacity of alloy materials,the effects of three kinds of nanoparticles on the microstructure and thermal properties of alloy matrix were studied.Due to the change of the microstructure of the nanoparticles as the second phase particles,the specific heat capacity and thermal conductivity of the alloy matrix are obviously improved.The analysis of the specific heat enhancement mechanism shows that the interface thermal resistance and the reduction of the alloy matrix spacing are the main reasons for the increase of specific heat.After the addition of nano-Ni particles,the proportion of phonons as heat transfer carriers in(Sn–50Bi–2Zn)–0.5Ni alloys increased,which is consistent with the results of specific heat mechanism analysis.