| Thermoelectric material is a new kind of functional material which can convert heat into electricity.It can recycle waste heat or realize the fluorine-free refrigeration,and has received wide attention.In recent years,the thermoelectric material is in a rapid development period.With the successful applying of the performance optimization strategies,the properties of many excellent materials have been enhanced tremendously.However,thermoelectric materials are still difficult to be commercially used on a large scale due to the low efficiency and high cost.So it is significant to further improve the performance of thermoelectric materials or develop new materials.CuFeX2?X=O,S,Se?materials,containing abundant elements,have the advantage of low cost in application.CuFeO2 is a kind of delafossite oxide with layered structure.CuFeS2 is a kind of alloy with chalcopyrite structure.Both CuFeO2 and CuFeS2 have been studied in thermoelectric performance.CuFeSe2 is a new thermoelectric and the thermoelectric transport properties are not revealed.In this thesis,the synthesis and phase structure of this material system are studied in detail,and the relationship between them is established.Basing on that,the thermoelectric performance is studied and the thermoelectric transport is analyzed.The main content is as follows:?1?Solid solutions of?1-x?CuFeO2+xCuAlO2 ceramics are fabricated by the solid state reaction.From the X-ray diffraction?XRD?pattern,a phase transition of CuFeO2-Cu?Fe,Al?2O4-CuAlO2 has been found in the samples with x increasing.The electrical resistivity of delafossite samples can be decreased obviously near the phase change areas,which is due to the increased carriers.At last,the power factor and zT values are enhanced due to the improved electrical resistivity.The highest zT is 0.04 for sample of x=0.2 at 907K,which is 2.5 times of the pristine CuFeO2.Besides,it is found that the impurity phases are easy to exist in the samples.The relationship between the synthesis temperature and the sample phase structure has been studied in detail.The result shows that the temperature should be in the range of 1000?-1010? to achieve the sample with less impure phases.The CuFeO2-CuMnO2 solid solutions have also been prepared in this thesis.The lattice parameters of a/b are found decreasing with Mn replacing Fe.This is due to the Jahn-Teller distortion induced by Mn.?2?To get the pure phase of CuFeO2,the microwave hydrothermal method has been employed to prepare the pure CuFeO2 nano-percursors rapidly.Samples with different micro-morphology have been got by changing reaction temperatures between 130-210?,and the grain growth mechanism has been analyzed by studying the grain size.From the XRD patterns,the peaks of CuFeO2 are found increasing with the increase of temperature,indicating the poor crystallinity at low temperatures.The average grain sizes are almost the same below 180? and increase rapidly after the temperature is over 180?.The grain size distributions are also studied for samples at different temperatures.It is found that samples synthesized at high temperature have more large grains and less small grains.These results are consistent with the Oswald ripening mechanism which describes the process that small grains dissolve and deposit on larger grains.With the temperature increasing,the ripening progress is accelerated.Besides,it is found the CuFeO2 nano-precursors will be oxidized or decomposed when they are used to fabricate the thermoelectric bulk material.This is due to the high chemical reactivity which is relevant with the high surface energy of nano-materials.?3?Samples of Cu1+xFeSe2?x=0-0.05?are fabricated by the quenching,annealing,and hot-pressing progress.The secondary phase of Cu2Se is detected when x>0.01 from the XRD patterns.The peaks of Cu2Se increase with the increase of x,indicating the increase of the secondary phase.As the excessive Cu reacting with Se to form Cu2Se,the Se in the CuFeSe2 lattice will be deficient.This Se vacancies is called the competitive Se vacancy here.Besides,the X-ray fluorescence?XRF?shows the decreased ratios of Cu and Se in samples which is due to the elements volatilizations during the high-temperature progress,especially the quenching procedure.This kind of Se vacancies are defined as the Se intrinsic vacancy.The microstructure is seen laminar and no holes are detected,indicating the good density of the bulks.The energy dispersive spectrum of x=0.05 show the enriched area of Cu2Se in samples.The Seebeck coefficients of all samples show p-n transformation with the temperature increasing.This transformation is first discovered in this material.This transformation is due to the Se vacancies which contribute more electrons to the conduction band.As the number of the electrons is positive correlation with the amount of Cu2Se,the electrical resistivity is decreased with the x increasing,which lead to the increase of power factor.Moreover,the excessive Cu strengthen the point defect scattering and induce secondary phase scattering which decreased the thermal conductivity obviously.At last,zT is improved tremendously by the excess Cu?4?CuFeS2 samples are synthesized by the melting method,and different cooling ways are used to get samples.From the XRD patterns,samples with natural cooling?NC?and quenching cooling?QC?have different phase structures.The phase of QC-CuFeS2 is a mixed phase of chalcopyrite and putoranite,while that of NC-CuFeS2 is only chalcopyrite.The electrical properties measurement shows that these two samples are all n-type conductive behavior.But the values of two samples are much different as well as the change trend depend on temperature.The QC-sample has low electrical resistivity and Seebeck coefficient,and finally the power factor is much low than NC-sample.Besides,it is found that the phase structures of Ca doped samples change from the mixed phase to single putoranite phase.The electrical resistivity and Seebeck coefficient show completely different change tendency due to the phase change.The Ca-doped samples have the larger Seebeck coefficients which lead to the higher power factor.Moreover,the Mg doped NC-CuFeS2 samples have also been fabricated to study the electrical properties.The electrical resistivity and Seebeck coefficient decrease with the increase of doping amount.This result proves the increase of carriers and is consistent with the n-type doping.However,the power factor is decreased due to the much decreased Seebeck coefficient.In this thesis,the synthesis and performance characteristics of CuFeX2?X=O,S,Se?series thermoelectric materials are systematically expounded through the study of the above contents.The preparation methods of CuFeO2 by solid phase and microwave hydrothermal synthesis have been systematically explored,and the problems and advantages of different methods have been clarified.These results would be a reference for this material.For CuFeSe2 materials synthesized by the melting method,our work is the first complete study for the thermoelectric performance optimization.In addition,the p-n conductivity transformation with temperature change in the sample is first discovered,and the mechanism of this transformation is analyzed,which will provide a reference for future research and application of this material.For CuFeS2,samples with different phase structures have been obtained through different cooling treatments,and the presence of mixed phase of quenching samples has been found. |
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