Contralable Synthesis Of Non-stoichiometric Cu₅FeS₄ And Thermoelectric Properties

Thermoelectric materials are one kind of new energy materials which can directly convert waste heat into electrical power based on the Seebeck effect.Good thermoelectric materials should possess large Seebeck coefficient,high electrical conductivity,and low thermal conductivity.For the large-scale application of thermoelectric materials for waste heat recovery,they are also required to have low price,in addition to high performance.Recent years,the development of Cu-based mineral thermoelectric materials provides a new strategy to achieve such goal.Cu₅FeS₄is a widespread copper sulfide natural mineral named after bornite.The low-cost,non-toxic,earth-abundant features and the potential high thermoelectric performance of bornite make it attractive for the researchers in thermoelectrics.Although stoichiometric Cu₅FeS₄ sample exhibits an ultralow thermal conductivity,its electricity conductivity value is quite low,resulting from the low carrier concentration,which largely hampers its thermoelectric performance.It is known that the micro-morphology nanocrystal compound has great influence on its chemical composition and crystal structure,which is further related to the thermoelectric performance.Traditional solid state synthesis method can barely control the micro-morphology of Cu₅FeS₄ compound.Thus,the development of novel chemical method for the synteisis of the Cu₅FeS₄ compounds with controllable micro-morphology and favorable chemical composition is of great importance to the futher enhancement of their thermoelectric perforamce and the study in the associated physical transport proerpties.In this thesis,we employed two wet chemical methods including colloidal synthesis and microwave-assisted hydrothermal wet chemical method,to produce non-stoichiometric Cu₅FeS₄ nanoamaterials.Using the spark plasma sintering?SPS?technology,most features of nanomaterials can be well maintained in the consolidated bulk samples,which provides great opportunity for us to investigate the effect of synthesis parameters on the chemical composition,micro-morphology,grain size and thermoelectric properties of Cu₅FeS₄ compounds.The achievements of this work can be briefly summarized as the following:?1?About 5 g non-stoichiometric Cu₅FeS₄ nanocrystal powders was obtained after one pot synthsis through a large-scale colloidal synthesis at different temperatures?493K,533 K and 553 K?.The SPS process at 723 K,under the pressure of 45 MPa allowed for the densification of the as-made powders to the bulk compounds with nano-structure for thermoelectric property measurment.The variation of synthesis temperature was performed to control the stoichiometry of Cu₅FeS₄ and optimize its thermoelectric performance.In this study,the carrier density of these non-stoichiometric Cu₅FeS₄nanocrystal produced by the colloidal synthesis is four orders of magnitude higher than that of stoichiometric Cu₅FeS₄ synthesized by solid state reaction method.The higher carrier density in non-stoichiometric Cu₅FeS₄ enlarges the electrical conductivities and thus the power factors over a wide temperature range.As a result,the thermoelectric figure of merit?zT?of Cu₅FeS₄ compound made by the colloidal synthesis process reaches 0.56 at 533 K,which is about 47%higher than that of stoichiometric Cu₅FeS₄bulk counterparts.Moreover,these non-stoichiometric Cu₅FeS₄ samples show a better mechanical performance compared to that of stoichiometric Cu₅FeS₄ bulk samples.?2?A scalable and facile colloidal solution method was employed to synthesize non-stoichiometric orthorhombic@cubic core-shell Cu₅FeS₄ compounds.By altering the synthesis time?10 min,30 min and 60 min?,we were able to control the micro-morphology of Cu₅FeS₄.When the synthesis time was prolonged to 30 min,the non-stoichiometric orthorhombic@cubic core-shell Cu₅FeS₄ compounds with icosahedral profile were obtained.The Cu-rich shell is cubic Cu₅FeS₄,while the Fe-rich core is orthorhombic Cu₅FeS₄.The ratio of cubic Cu₅FeS₄ increases with the extension of synthesis time.The compounds with icosahedral profile contain ultrahigh-density twin boundaries in the form of five-fold twins in these nanoparticles.The ultrahigh-density twin boundaries were retained after SPS,which caused the enhanced phonon scattering as well as the decreased thermal electrical conductivity.However,the lattice thermal conductivity of cubic Cu₅FeS₄ is higher than that of orthorhombic Cu₅FeS₄ due to its relatively simple crystal structure that hinders the futher reduction in the lattice thermal conductivity of these compounds.The SPS temperature also has a substantial effect on the ratio of cubic phase to orthorhombic phase for these samples:when the SPS temperature is 773 K,the constituting elements of the sample will be redistributed and then decrease the ratio of cubic phase to orthorhombic phase;when the SPS temperature is 723 K,it has little effect on the ratio.The tuned fraction of the secondary phase?Fe-deficient cubic Cu₅FeS₄?,with the ultrahigh-density twin boundaries synergistically optimize the thermal and electrical transport properties,which results in an enhanced zT of⁰.62 at 710 K.?3?Cu₅FeS₄ compounds was rapidly synthesized through microwave-assisted hydrothermal method combined with the SPS.By changing the parameters in the synthesis process,such as synthesis time,sulfur content and the different quantities of NaCl,etc.,we were capable of optimizing the thermoelectric performance of these samples.The synthesis time was found to have important effect on the thermoelectric performance of sampels:the Seebeck coefficient of sample synthesized in 15 min is higher than that in 10 min,while the electrical conductivity is diminshed.The lower electrical conductivity leads to the slightly lower power factor for the sample synthesized in 15 min as compared to that in 10 min.Nevertheless,the thermal conductivity of the former is much lower.As a result,the zT of the Cu₅FeS₄ sample synthesized in 15 min is increased to 0.4 at 580 K.We also investigated the effect of the variation of another parameter,the ratio of Cu:S,on the thermoelectric transport properties.It is found that the decline of such ratio will cause the increasing electric conductivity and the decreasing Seebeck coefficient.As a consquence,the Cu₅FeS₄ has the largest power factor whereas the Cu₅FeS_4.8 has the smallest one among all the samples.When the mole ratio of Cu:S is less than 5:4,the lattice thermal conductivity of the samples decreases with the decreasing sulfur content.Due to the effect of different sulfur content on the electrical and thermal properties,the zT of Cu₅FeS_4.2samples reached 0.43 at 577 K.By adding different quantity of mineralizer NaCl during the synthesis process,we found that the thermoelectric property of these materials can be also tuned.The Seebeck coefficient of these samples increases with the increasing additional content of the NaCl in the precursors,however,on the contrary,the electrical conductivity drops.Finally,the power factor of all these samples is larger than that of Cu₅FeS₄ synthesized by solid state reaction method.In all,when the starting molar ratio of Cu:S is 5:4.8,facilitated by 1 g mineralizer NaCl in the precursor,the Cu₅FeS_4.8compound with a maxmum zT of 0.55 at 632 K is obtained through microwave-assisted hydrothermal synthesis method in 15 min.