The Study On Thermoelectric Properties And Mechanisms Of CoSb₃ And RE_xCo₄Sb₁₂ Prepared By MA-SPS

Thermoelectric materials are the functional materials that can convert heatenergy to electric energy or vise versa directly using thermoelectric effects and haveimmense prospects with many applications in thermoelectric power generation andrefrigeration. Although the thermoelectric materials with skutterudite structure (AB3type) have large Seebeck coefficients and higher melting points, they need doping orfilling with other elements to improve their properties due to that are still low nowand the dependence of properties with the material structures and preparingtechnologies are needed to be investigated. CoSb3 and RExCo4Sb12 (RE-rare earth elements) are chosen as study objectsand their preparing technologies, structures, thermoelectric properties andmechanism are investigated by advanced ways of high energy mill, spark plasmasintering (SPS), X-ray diffraction, scanning electronic microscope, transmittingelectronic microscope (TEM) , electric constants instrument, Laser thermalconductivity instrument etc. At first the effects of milling technologies on phase changes of Co-Sb binarypowder in milling were investigated. Experimental results show that CoSb3 powdercan be synthesized by mechanical alloying (MA) after milled for 5h under theconditions of ball-powder ratio 20:1 and mill working voltage 110V. The averagecrystal grain size of above powder is 20～35nm. High density bulk CoSb3 withaverage crystal grain size less than 100nm were synthesized successfully byMA-SPS for the first time. Results show that the mechanism of preparingnanocrystalline bulk CoSb3 is that MA makes the powder grain size to be fine tonanometer and the fast, short time and low temperature technology and specialsintering mechanism of SPS restrain the crystal grain from growing up in sintering.Thermoelectric properties of bulk CoSb3 prepared by different technologies weresystemically investigated. Results show that different preparing technologies have noevident effects on thermoelectric properties of bulk CoSb3 samples which show thecharacteristics of typical semiconductor electricity that the resistivity decreases with III北京工业大学工学博士学位论文the temperature rising and have dimensionless thermoelectric figure of merit ZT lessthan 0.0571. CexCo4Sb12 thermoelectric materials were prepared by MA-SPS for the firsttime, the dependence of their thermoelectric properties with temperature and Cecontent were investigated systemically and the effects of La, Ce and Nd onthermoelectric properties of RExCo4Sb12 samples were compared: (1) Doped Cemakes the resistivity decrease to a lower magnitude and increases electric conductingproperty greatly; The resistivities of samples decrease with the Ce content increasingwhen x=0.7～2.0, the increasing Ce content does not evidently decrease theresistivity when x>1.25; (2) The samples with positive α are p type when x=0.1,0.3 and 0.5 and the higher Ce content samples with negative α are n type whenx=0.7～2.0; (3) The thermoelectric power factors of CexCo4Sb12 get the maximum of15.30～23.70×10-4 wK-2m-1 at 100～500℃ when x=1.25; (4) Ce0.5Co4Sb12 has thelowest thermal conductivity of 1.922～1.609 Wm-1K-1 at 100～500℃ lower thanthat of CoSb3 in the same group. The thermal conductivity of Ce2.0Co4Sb12 is severaltimes than those of the lower Ce content samples and more Ce doped makes thethermal conductivity increase; (5) The highest ZT figure of p type is 0.1739 at 500℃when x=0.5; The ZT figures of n type CexCo4Sb12 (x=0.7～2.0) increase first andthen decrease with the Ce content increasing, When x=1.0 it gets the highest figuresof 0.1675～0.374 at 100～500℃ in which the maximum is 0.374 at 400℃ that isthe highest ZT figure in n type samples and also in the experiments of this paper. (6)Phases are similar in RExCo4Sb12 with the same RE content (La, Ce, or Nd)respectively and Ce1.0Co4Sb12 has the highest ZT. It is shown that Ce is the optimumd...