中文摘要：
热电材料是一种能将热能直接转换成电能的新能源材料,其具有无噪音,无排放,无泄漏等优点使其拥有广泛的潜在研究应用前景。优异的热电材料在具备高Seebeck系数、高电导率的同时还需具有较低的热导率。PbSe由于具有较低的热导率和较优的电学性能在中温区被认为是一种极具潜力的热电材料。据文献报道,其同族PbTe材料的ZT值达到2.0左右,但Te在自然界的含量很低,与之相比,Se的储量远高于Te,而且PbSe与PbTe在晶体和电子结构方面非常类似,通过优化,PbSe也应具有优异的热电性能。此外,与PbTe相比,PbSe的熔点较高,能在更高温度的环境下工作,因此近年来PbSe作为热电材料被广泛研究。本文尝试采用熔剂法制备PbSe基热电材料,通过调节熔剂起始量来改善材料的热电性能,并结合第一性原理研究熔剂法制备的样品中Bi取代Pb对PbSe能带结构的影响。本论文获得如下主要结论:1.通过Pb助熔剂法制备出晶体质量较好的PbSe单晶热电材料,并通过调节Pb熔剂的配比实现利用非掺杂手段对材料的载流子浓度进行微调。所有样品的晶格常数均随Pb含量的增加而增大。FESEM和TEM结果表明,通过Pb助熔剂法制备的PbSe样品具有完整的晶体结构。热分析表明,样品在中温范围内具有较高的热稳定性。与球磨法制备的多晶样品相比,室温附近Pb熔剂法制备的单晶PbSe样品具有更高的电导率,Pb:Se=1.3:1的样品电导率值高达10.53×10~4S?m~(-1)。材料的ZT值明显提高,该样品在300 K时取得最大ZT为1.25,较球磨法制备的多晶PbSe样品提高了100%。2.通过Bi熔剂法制备出一系列具有良好热电性能的单晶PbSeBix(x=2.5,3,3.5,4)样品。热分析结果表明,样品在中温范围内具有较高的热稳定性。XRD结果表明,样品的特征峰与空间群为Fm3?m的NaCl晶体结构类型一致。EDS线扫描显示各元素在样品内部均匀分布,TEM和SAED证实了制备的样品具有典型的单晶结构。Bi起始含量x=3的样品在300 K处最高电导率为20.53×10~4 S·m~(-1)。实验结果与理论结果表明部分Bi取代Pb在晶体中的位置,从而产生了N型导电和共振态。x=3.5的样品在550 K时的功率因子最高,为3.6×10~(-3) W·m~(-1)·K~(-2),比多晶PbSe样品(1.8×10~(-3) W·m~(-1)·K~(-2))提高了约50%。3、采用NaCl熔剂法制备的PbSe(NaCl)x(x=3,3.5,4,4.5)热电材料表现出优良电传输性能。制备的PbSe样品尺寸约13 mm。SEM显示NaCl熔剂法制备的样品内部无宏观缺陷。XPS和EPMA结果均表明样品中存在Na和Cl元素。实验表明,通过调节NaCl熔剂的初始含量,PbSe样品的载流子浓度逐渐降低,迁移率逐渐升高,x=4.5的PbSe样品表现出最优的电性能,在690 K处取得最大PF值为2.8×10~-33 W·m~(-1)·K~(-2),与热压法制备的Na掺杂PbSe样品相比提高了一倍,说明通过调节NaCl的含量能有效地调节PbSe样品的载流子浓度,使PbSe体系的功率因子增加。
英文摘要：
Thermoelectric material is a material that can directly convert thermal energy to electrical energy.It has the advantages of zero noise,no emissions,and the ability to reuse waste heat,which makes it widely used in research and application.Good thermoelectric materials need to have high Seebeck coefficients,high electrical conductivity,and low thermal conductivity.PbSe is considered to be a promising thermoelectric material in the mid-temperature region due to its lower thermal conductivity and better electrical properties.According to literature reports,the thermoelectric figure of merit of thermoelectric alloys PbTe has reached about 2.0,but the content of Te in nature is very small.Compared with this,the content of Se are much higher than Te.PbSe and PbTe are very similar in terms of lattice and electronic structure.In addition,PbSe has a higher melting point and can adapt to higher operating temperatures.Therefore,recently PbSe has been widely studied as a thermoelectric material.This article attempts to use the flux method to prepare PbSe-based single crystal thermoelectric materials.The thermoelectric properties of the material are improved by adjusting the initial amount of the flux.The effect of Bi substitution of Pb on the PbSe band structure is studied first-principles calculation.The main conclusions are as follows:PbSe single crystal thermoelectric material with better crystal quality was prepared by Pb flux method,and the carrier concentration of the material was fine-tuned by adjusting the solvent ratio of Pb.The lattice parameters of all samples increased with the increase of Pb content.The FESEM and TEM results showed that the single crystal PbSe sample prepared by Pb flux method had a complete crystal structure.Thermal analysis showed that the sample had high thermal stability in the mid-temperature range.The ZT value of the n-type PbSe system was significantly improved,and the sample with Pb content x=1.3 obtained a maximum ZT value 1.25at 300 K,which increased by 100%compared with the polycrystalline PbSe sample prepared by ball milling.the maximum electrical conductivity value of 10.53×10~4S?m~(-1) was obtained for the sample with x=1.3.A series of single crystal PbSeBix(x=2.5,3,3.5,4)samples with good thermoelectric properties were prepared by the Bi-flux method.Thermal analysis showed that the sample had high thermal stability in the medium temperature range.XRD results show that the characteristic peaks of all samples are consistent with the NaCl structure with space group Fm3?m.No grain boundaries,cracks or micropores are found in the prepared samples,and the crystal structure has good single crystal structure.The EDS line scan revealed that the elements were uniformly distribute on the single crystal PbSe,and the TEM and SAED further confirmed that the prepared sample had a typical single crystal structure.For sample with x=3,the highest conductivity at 300K reached 20.53×10~4 S·m~(-1).Combining the experimental and theoretical results,Bi replacing Pb is the reasonable result and causing both n-type conduction and resonant states.The sample with x=3.5 achieves the highest power factor of 3.6×10~(-3) W·m~(-1)·K~(-2) at 550 K and it increased by about 50%compared to the polycrystalline sample(1.8×10~(-3) W·m~(-1)·K~(-2)).The PbSe(NaCl)x(x=3,3.5,4,4.5)thermoelectric materials prepared by the NaCl flux method showed excellent electrical transmission properties.The size of the prepared PbSe sample is about 13 mm.SEM images showed that there were no macro defects inside the samples prepared by the NaCl flux method.Both XPS and EPMA results indicated the presence of Na and Cl elements in the sample.The experiments show that by adjusting the initial content of NaCl flux,the carrier concentration of the PbSe sample gradually decreases,and the mobility gradually increases.The PbSe sample with x=4.5 shows the best performance,and the maximum PF value at 690 K is 2.8×10~-33 W·m~(-1)·K~(-2),which has doubled compared with Na-doped PbSe samples prepared by hot pressing,which indicates that the carrier concentration of PbSe samples can be effectively adjusted by adjusting the content of NaCl,and the Power factor of PbSe system can be increased.
