Preparation And Characterization Of Low-dimensional Thermoelectric Materials

Thermoelectric materials attract much attention because of its potential application for refrigerators or power generators. If the thermal electric figure of merit ZT could be improved by a factor of about 3, quiet and rugged solid-state devices could eventually replace conventional compress or based cooling systems. Recently, thermoelectric low dimensional structures such as quantum wells, quantum wires and quantum dots have been proven to have much high thermoelectric coefficients according to both theoretical and experimental investigations. In this thesis, thermoelectric theories and recent progresses were reviewed. (Bi, Sb)2(Se, Te)₃ ( the best thermoelectric property known at room temperatures) and Filled-Skutterudite NaFe₄P₁2 ( most potential thermoelectric material) nanostructures were synthesized at different temperatures for different time by a hydrothermal co-reduction method, a sonchemical method and a solvothermal synthesis method respectively. Composition, microstructures and properties of powder product synthesized were investigated. Formation mechanism and growth models of the nanostructures were proposed. Some original results were obtained either in preparing low dimensional thermoelectric material, nanocrystal growth or in thermoelectric properties of synthesized materials. The main points are as follows:A low-cost, timesaving hydrothermal methods have been Used to synthesize some nanostructures. In the present work, Sb₂Se₃ nanostyloids and nanobelts have been synthesized by using a hydrothermal-co-reduction method. Sb₂Se₃ nanobelts of 40-60 nm in width and nanostyloids of 100-250 nm in diameter were synthesized by using a hydrothermal-co-reduction method at 105-180℃ for 24 hours. X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), and high resolution transmission electron microscopy (HRTEM) were used to characterize the products. The experimental results indicated that the Sb₂Se₃ nanobelts and nanostyloids were well crystallized nanocrystals, and grew along the (001) direction. With increasing heating temperature the content of nanostyloids in the product increases and that the size of the nanostructures varies from the nanometer to the micron range.Therefore the morphology and size of Sb₂Se₃ nanocrystals can be controlled by changingthe reaction temperatures. A model of formation and growth of Sb₂Se₃ nanobelts and nanostyloids is proposed.Ultrasound irradiation offers an attractive method for the preparation oflow-dimensional materials and has shown rapid growth in its application to nanomaterials science due to its unique reaction effects. Sb2Se3 nanostructures were synthesized at 25, 30, 40°C by using a sonochemical method. The products were characterized by using XRD, TEM, ED, FESM (field emission scanning microscopy) and HRTEM. The results indicate that the Sb2Se3 nanostructures synthesized at 25°C are octahedrals, [SbaSee]3" semi-cubes or cubes form octahedrals. And when reaction temperature raises to 30°C, except for octahedrals, there are some spherical particles and tubes in the product. At higher reaction temperature, some small particles are easy to reunite and form some spherical particles because of their high surface energy. In the same time, [Sb3Se6]3" units link together to form sheets, but some undersaturated danling bonds of atom Sb, Se existing at the edge of sheets would induce the interaction of between them, which leads to the rolling-up and the formation of tube structure. Rod clusters exist in the product synthesized at 40°C. Chain units combine with each other through dangling bonds to form clusters, and then form nanorods.Bi2Se3 nanostructures were prepared by a hydrothermal co-reduction method at 150, 180, 200, and 210°C, respectively. Bi2Se3 nanosheets, nanobelts and nanotubes were obtained. The Bi2Se3 nanoflakes are 50-500nm in width and 2-5nm in thickness. The Bi2Se3 nanotubes are 5-10nm in diameter, 80-120nm in length, and 1.3nm in wall thickness. XRD, ED and HRTEM were performed to characterize the products. Experimental results showed that the nanosheets and the nanotubes were hexagonal structure. The nanosheets are well crystallized and grow along the (0001) face. Nanotubes grow along [1100] direction. A possible formation and crystal growth mechanism of Bi2Se3 nanostructures were discussed. The thermoelectric properties of Bi2Se3 nanostructures were measured. The Bi2Se3 hot-pressed sample exhibits as a N-type semi-conductor. The Seebeck coeffcient of the sample is negative, which is stable, and holds in the range of 150-160UV.K'1. Electrical conductivity and power factor of the sample increase with the increase of the tmperature.Bi2Se3 nanonbelts with a typical width of 20-80nm, thickness of 8-10nm and length of several micrometers have been successfully synthesized by sonchemical method. The product was characterized by using XRD, TEM, ED and HRTEM. The results indicate the Bi2Se3 nanonbelts are well-crystallized nanocrystals and grow along the (001) direction. The morphology and size of nanostructures vary with synthesis temperature. A possibleformation mechanism of Bi2Se3 nanonbelts is proposed. Ultrasonic irradiation plays an important role during the formation of Bi2Se3 nanobelts.Bi2Te3 nanostructures were prepared by a solvothermal method at 110, 180 °C, respectively. TEM results indicate that the product synthesized at 110 °C are mainly nanosheets, but there are many long nanotubes, 100-300nm in diameter in the powder prepared at 180 °C. Bismuth telluride nanotubes have a uniform width along their entire length. Physical formation model of bismuth telluride nanotubes is proposed. The sample exhibits P-type semi-conduction. Seebeck coeffcient of the sample decreases with the increase of temperature, and reaches a very high Seebeck coeffcient 400nV.K at the room temperature. Electrical conductivity increase with the increase of temperature. Power factor decrease after increase with the increases of temperature, and reaches the highest value of S^xlO^Wm-'K'2 at 473K.NaFe4pi2 nanosprings were synthesized using a hydrothermal method at 170 °C for 24 h. XRD, TEM, ED, HRTEM and FESM results indicate that the nanosprings have the skutterudite crystal structure with cell parameter a= 7.782 A. The nanospringsare 1000-5000 nm in length and 200-1000 nm in diameter and made of coiled nanobelts of 80-J50 nm in width and 20-50 nm in thickness. With an increase annealing time, the nanospring grows as a macrorod with a multilayered helical tubular structure. It is proved that the nanobelts grow along the (111) plane and that their growth direction is not parallel to the [110] direction in the growth plane. A possible growth process for NaFe4Pi2 nanosprings is suggested.Some innovate results were obtained either in preparation of low dimensional sulfide and Filled-Skutterudite NaFe4Pi2 thermoelectric material, and investigation on growth of nanocrystal or investigation on properties of the products.