| By converting waste heat into electricity,thermoelectrics are attracting more and more attention recently due to the exasperated energy crisis,however,a large-scale utilization of such materials are still hampered by their low efficiency.It is important to improve the Figure of Merit of thermoelectrics.Zintl phases Ca1-xCexAg1-ySb(0?x?1;0?y?1)have exhibited high-temperature thermoelectric potentials in previous studies.With the trivalent rare-earth cations doped into the system,an interesting structure transformation was observed from the TiNiSi-type CaAgSb to the LiGaGe-type Ca1-xRExAg1-ySb.Such a structure change also corresponded to a significant enhancement on the thermoelectric performance,which was explained by the converged band structures in Ca1-xRExA1-ySb.In this work,defect chemistry and thermoelectric properties related to the Ca1-xCexAg1-ySb series were carefully investigated,summerized as following.First,in order to facilitate the material preparation,the syntheses were carried out through a home-made micro induction furnace incorporated in the glovebox.With the aid of such a instrument,a reaction temperature as high as 1873 K can be approached only in 30 seconds,which offer a very convenient way to obtain such materials with high purity in a short time.Second,in this work,defect chemistry and thermoelectric properties related to the Ca1-xCexAg1-ySb series were carefully investigated.The results revealed that the Ce contents in Ca1-xCexAg1-ySb were very critical in governing the formation of various Ag-defect structures.With the Ce concentrations increased,several phase transition processes were observed,corresponding to closely related but obviously different[Ag1-ySb]anionic super lattices.Such a defect model makes these compounds very flexible in the optimization of thermoelectric properties.For a material with the nominal composition of Ca0.83Ce0.17Ag0.85Sb,high figure of merits(ZT?0.6)can be maintained over a wide temperature range from 800 to 1025 K,which is a favorable character for practical applications.Third,in order to better understand the mechanism related to defect formation and control in this system,we conducted a series of experiments aiming at controlling the point defects in Ca0.85Ce0.15Ag0.85Sb.This strategy was realized by intentional Nb doping,which resulted in the discovery of a series of low defect density materials Ca0.725+xNb0.1-xCe0.15AgSb(x?0.05).This work demonstrated an interesting defect controlling strategy by Zintl chemistry as well as the high flexibility of Ca1-xRExAg1-xSb in optimization of the thermoelectric properties.The results indicated strong electronic effects originating from the cations in affecting the concentration of the Ag defects in the anionic structure.Such a character makes the modulation of Ag defects in Ca1-xRExAg1-ySb very flexible,which as well seems tunable over a very broad range.In addition,replacement of the alkaline-earth by transition metals with localized d-states may cause potential resonant level effect,which is also interesting in study of related properties.As indicated in Ca0.725+xNb0.1-xCe0.15AgSb(x?0.05)series,wide platforms with zT?0.4 were observed over a broad high temperature range(above 573K),which is a favorable character for the practical applications of thermoelectrics.Forth,by utilization of the induction melting technique,a series of As-,Se-,Ge-doped materials of Ca1-xRExAg1-xSb were successfully prepared,and such a synthetic method was also proved to be very effective in the preparation of other Zintl phases containg high melting point rare-earth elements. |
PDF Full Text Request