Preparation And Physical Properties Of As/P-based Max Phases

The MAX phase with A=As or P elements,also known as As/P-based MAX phases,has outstanding elastic and mechanical properties among the family of MAX phases.Besides the 211phases such as Nb₂As C and Nb₂PC discovered early,in 2019,a new series of MAX phases was discovered with a general formula of M₃A₂X fearturing in the trigonal-prism-coordinated MA bi-layer,including Nb₃As₂C,Nb₃P₂C,and so on.The traditional crystal structure of MAX phase can be viewed as an alternative stacking of single MA layer and n layers of MX-layer.The strong M-A bonding and the unique double-layer MA tri-prism structure in 321 phases,make As/P-based ones outstanding among MAX phases.Until now,only pure phase of Nb₃As₂C has been obtained,while the purity of other M₃A₂X phases is less than 60%.In addition,systematic studies on the preparation of As/P-based 211 phases are still lacking.The lacking of high-purity samples have hindered the study on the intrinsic properties of As/P-based MAX phases.In this dissertation,we used solid-state sintering（SSS）method and molten-salt sintering（MSS）method to prepare As/P-based MAX phases,and optimized the sintering temperature,heating time and other parameters to explore the pure-phase preparation process of As/P-based MAX phases.The pure phases of Nb₃As₂C and Nb₂As C can be obtained by SSS.Meanwhile,using MSS,pure Nb₃P₂C（>99%）and Nb₂PC with phase content of>75%was obtained,much higher than the one prepared by SSS（～30%）.Our results confirm that SSS and MSS are suitable to As-based and P-based MAX phases,respectively.The phase-formation mechanism of Nb₃P₂C was investigated by first-principle calculation and experimental analysis.Three chemical-reaction routes were considered.Route 1:2Nb P+Nb+C→Nb₃P₂C;Route 2:2Nb P+Nb C→Nb₃P₂C;Route 3:Nb₂PC+Nb P→Nb₃P₂C.First-principles calculations show that all three paths are thermodynamically favored,and Route 1 is the dominant one,which is supported by the experimental results.Comparing the results of MSS prepared and SSS prepared,we found MSS can effectively improve the phase purity of all possible reaction routes,especially Route 3,revealing the advantage of the MSS to synthesize high purity P-based MAX phases.Furthermore,with the help of first-principles calculations,the intrinsic physical properties and electronic structure of Nb₃P₂C and Nb₃As₂C were characterized by various physical characterizations.Well electrical and thermal conductivity was found in them.The dominant carriers are electrons as proved by the Hall effect and the Seebec effect results.The open-orbit Fermi surface topology was revealed by the first-principles claculations and the observed linear magnetoresistance.Meanwhile,weak thermoelectric effect,a coexistence of Currie-Weiss paramagnetism and diamagnetism,Relatively moderate hardness（6.21-7.77 GPa）were revealed.The intrinsic physical properties are essential for furture applications on 321 phases.