Prediction And Synthesis Of Novel Sc₂SC MAX Phase

The MAX phase is a general term for a series of layered materials.Due to the special bonding type between M?A and X atoms,this kind of materials have excellent properties of both ceramics and metals.The unique properties of this kind of materials make them a wide application prospect.Though more and more MAX phases have been successfully prepared,researchers have stayed in the exploration of new MAX phase materials.Therefore,the prediction and synthesis of new MAX phase materials have attracted more and more interests of researchers.In this thesis,through the combination of theoretical calculations and experimental researches,we predict and synthesis new MAX phasees.By using the high-throughout calculations based on first-principles,stable MAX phases are predicted.Firstly,the crystal structures of 108 M₂AC phases were constructed,the low-energy structures of each component were studied,the stabilities of thermodynamics,mechanics and lattice dynamics of each low-energy structure were studied,and the stabilities of each low-energy structures in the phase diagram were explored.54 M₂AC phases which are stable in thermodynamics,mechanics and dynamics were successfully predicted.Among them,37 phases have been synthesized,and 17 phases have not yet been synthesizedIn this thesis,Sc₂SC was selected as the target phase for the experimental synthesis from 17 M₂AC phases that have not been experimentally synthesized.Sc powder,S powder and C powder were used as raw materials.By adjusting the experimental conditions such as the ratio of raw materials,temperature,and holding time,an attempt was made to synthesize the Sc₂SC phase at high temperature under vacuum and argon environments.X-ray diffractometer(XRD)and scanning electron microscope(SEM)were used to characterize and analyze the samples prepared under different conditions to determine the optimal synthesis conditions for the Sc₂SC phase.Under a flowing argon atmosphere,the synthesized sample contains a lot of impurity substances,and the Sc₂SC phase content is small.In a vacuum environment,the optimal synthesis conditions for the Sc₂SC phase are obtained by adjusting the holding time,ratio of raw materials,temperature and other conditions:the molar ratio of the raw materials is determined to be Sc:S:C=2:0.2:1,the temperature is 1300°C,time is for 3 hours.Under this condition,the Sc₂SC phase is the main phase in the synthesized samples,but a certain amount of Sc₃C₄and Sc₂OC impurity phases often appear in the samples.By adjusting the experimental conditions such as the type of acid,reaction temperature,reaction time,and concentration of acid,the successful selection of a nitric acid solution can successfully remove the Sc₃C₄ phase.The optimal conditions are to be:the temperature for 40?;and the reaction time for 3-4 hours.The XRD results of the Sc₂SC sample after removing the Sc₃C₄ were subjected to Retiveld refinement.It is found that the lattice parameters of the Sc₂SC phase are:a=3.455?,b=3.453?,and c=17.727?.The content of the target product Sc₂SC in the sample was determined to be 92.4wt%,and the content of the impurity Sc₂OC was only 7.6wt%.SEM results show that the Sc₂SC phase in the sample is a layered structure with typical MAX phase morphology.STEM clearly shows the atomic arrangement of the Sc₂SC phase,which is consistent with the theoretical calculations.The thermal stability of Sc₂SC phase was studied.It was found that Sc₂O₃ is stable under the temperature of 500?in the air environment,and almost no oxidation occurred.When the temperature was above 500?,Sc₂O₃ began to appear in the sample.When the temperature is over 700?,all Sc₂SC is oxidized to Sc₂O₃.Under the condition of argon,the Sc₂SC phase can keep stable to 1100?.