【作者】 潘志军；

【导师】 吴建生； 张澜庭；

【作者基本信息】 上海交通大学 ， 材料学， 2007， 博士

【摘要】 热电材料的热电性能与其电子结构密切相关,研究热电材料的电子结构及掺杂对其电子结构的影响能为提高现有热电材料的热电性能及开发新的高性能热电材料提供思路。单晶能消除晶界等微观缺陷,且可以单晶为研究对象,研究热电材料的沿不同晶向的热电性能。CrSi₂是一种潜在的中、高温区使用的热电材料,研究Si侧Al置换掺杂与Cr侧V置换掺杂对其单晶热电传输性能及电子结构的影响能为提高CrSi₂的热电性能提供新的方法与思路。使用阻尼牛顿动力学法（Damped Newton Dynamics Schemes）对Si侧Al、P置换掺杂的CoSi_1-xY_x（Y=Al、P）的晶体结构进行驰豫,采用基于第一性原理（First-principles）及密度泛函理论（Density Functional）的线性缀加平面波法（Linearized Augmented Plane Wave Method）计算热电材料CoSi及Si侧Al置换掺杂的CoSi_1-xAl_x（驰豫后）及Si侧P置换掺杂的CoSi_1-xPx（驰豫后）的电子结构,基于计算得到的CoSi和CoSi_1-xAl_x的电子结构,结合已有的CoSi_1-xAl_x单晶的热电传输性能的实验数据,分析Si侧Al置换掺杂影响CoSi_1-xAl_x的热电传输性能的本质。计算得到的CoSi的能带结构显示,在其费米面附近,CoSi的价带和导带存在部分交叠,CoSi的电传输性能体现半金属性质,Si侧Al置换掺杂使其电子结构变得较复杂,随着掺杂浓度的增大,CoSi_1-xAl_x的费米面逐渐向低能带移动,而Si侧P置换掺杂则使CoSi_1-xPx的费米面逐渐向高能带移动。Si侧Al置换掺杂对CoSi_1-xAl_x的能带结构的影响能很好地从原理上解释CoSi_1-xAl_x单晶的电阻率及塞贝克系数的变化趋势。掺杂是提高β-FeSi₂的电性能及降低其热传导性能的有效方法,在β-FeSi₂中,Fe具有FeⅠ与FeⅡ两种非等同位置,Si具有SiⅠ与SiⅡ两种非等同位置,采取计算总能量的方法,分别确定β-FeSi₂中Si侧Al置换掺杂时Al原子的置换位置及Fe侧Co置换掺杂时Co原子的置换位置,使用阻尼牛顿动力学法对Si侧Al置换掺杂的Fe(Si_0.96875Al_0.03125)₂和Fe侧Co置换掺杂的Fe_0.9375Co_0.0625Si₂的晶体结构进行驰豫,采用基于第一性原理的线性缀加平面波法,计算β-FeSi₂及驰豫后的Fe(Si_0.96875Al_0.03125)₂和Fe_0.9375Co_0.0625Si₂的电子结构,基于计算得到的β-FeSi₂、Si侧Al置换掺杂的Fe(Si_0.96875Al_0.03125)₂及Fe侧Co置换掺杂的Fe_0.9375Co_0.0625Si₂的电子结构,从理论上分析Al及Co置换掺杂对β-FeSi₂的热电传输性能的影响。电子结构显示,β-FeSi₂在费米面附近存在一能隙,为典型的半导体化合物,在点H处直接能隙为0.74eV,在点H与Λ/3之间存在一值为0.71eV的间接能隙。Fe侧Co置换掺杂时Co的置换位置应为FeⅡ,Si侧Al置换掺杂时Al的置换位置应为SiⅠ。Si侧Al置换掺杂使Fe(Si_0.96875Al_0.03125)₂的费米面向其价带偏移,而Fe侧Co置换掺杂则使Fe_0.9375Co_0.0625Si₂的费米面向其导带偏移。掺杂会提高β-FeSi₂的电导率,对其塞贝克系数及热导率也有重要的影响。La原子填充是降低CoSb₃的热导率的重要方法,La原子的填充会导致CoSb₃的几何结构产生驰豫。选择La_0.5Co₄Sb₁₂做为研究对象,采用阻尼牛顿动力学法研究La填充对其几何结构的影响,并采用基于第一性原理及密度泛函理论的线性缀加平面波法计算CoSb₃及结构驰豫后的La_0.5Co₄Sb₁₂的电子结构及电子云密度,研究La原子填充对CoSb₃的电子结构及电子云密度的影响。驰豫结果显示,La原子填充对CoSb₃的几何结构影响比较大,La填充形成的La_0.5Co₄Sb₁₂在结构驰豫后的键长、键角及电子云密度都发生了改变。CoSb₃为窄能隙半导体,在其能带结构的Γ点附近靠近费米面的能带呈直线的形状,且与k矢量成线性的关系。La填充使La_0.5Co₄Sb₁₂的费米面向导带偏移。由于La填充在降低La_0.5Co₄Sb₁₂的热导率的同时会提高其电传输性能,因此La填充会提高La_0.5Co₄Sb₁₂的热电性能。CrSi₂为中、高温区使用的热电材料。使用基于第一性原理及密度泛函理论的线性缀加平面波法,计算CrSi₂、Si侧Al置换掺杂的Cr(Si_1-xAl_x)₂及Cr侧V置换掺杂的Cr_1-xV_xSi₂的电子结构并分析Al及V置换掺杂分别对CrSi₂的电子结构的影响。采用电子探针及能谱分析等实验方法分别研究CrSi₂中Al、V的最大固溶度范围。以X射线衍射谱为基础,研究Al、V置换掺杂分别对Cr(Si_1-xAl_x)₂和Cr_1-xV_xSi₂的晶格常数的影响。在最大固溶度范围内,采用光学悬浮区熔法制备CrSi₂及Al、V置换掺杂的Cr(Si_1-xAl_x)₂和Cr_1-xV_xSi₂单晶,并研究Al、V置换掺杂分别对Cr(Si_1-xAl_x)₂和Cr_1-xV_xSi₂单晶的热电传输性能的影响。以计算得到的电子结构及实验得到的数据为基础,分析Si侧Al置换掺杂对Cr(Si_1-xAl_x)₂单晶及Cr侧V置换掺杂对Cr_1-xV_xSi₂单晶的热电传输性能影响的本质。CrSi₂的电子结构显示,CrSi₂为典型的p型半导体,空穴载流子控制其热电传输性能,在其能带结构中存在一大小为0.35eV的间接能隙。Si侧Al置换掺杂及Cr侧V置换掺杂都使Cr(Si_1-xAl_x)₂及Cr_1-xV_xSi₂的费米面向价带偏移,从而影响其热电传输性能。Al在CrSi₂中的最大固溶度介于0.20与0.30之间。对于V置换掺杂的Cr_1-xV_xSi₂,当V的含量x达到0.40时,V仍然可以全部固溶于Cr_1-xV_xSi₂中,电子探针显微照片显示V置换掺杂的Cr_1-xV_xSi₂中无第二相析出。Al及V置换掺杂都会使CrSi₂的晶格常数发生改变,且符合维加德定律。Al或V置换掺杂对CrSi₂单晶的热电传输性能有重要的影响,在研究的温度范围内,Al或V置换掺杂会提高CrSi₂单晶的热电性能。更多还原

【Abstract】 Thermoelectric properties are related to the electronic structure of thermoelectric materials. Studying the electronic structure and the effects of doping on the electronic structure of thermoelectric materials can provide means to improve the thermoelectric properties of the current thermoelectric materials and ideas to develop the new and high-performanced thermoelectric materials. Doping is one of the significant means to improve the thermoelectric transport performances of thermoelectric materials. The crystal boundary and the melting defects such as impurity and hole are eliminated in single crystals. Thus, single crystal can be used to investigate the thermoelectric properties along the different crystallographic directions of thermoelectric materials. CrSi₂ is a candidate thermoelectric material used at intermediate or high temperature. Studying the effects of Si substitution by Al as well as Cr substitution by V on the thermoelectric transport performances and electronic structure of the CrSi₂ single crystal is an important means to improve the thermoelectric properties of CrSi₂.Damped Newton Dynamics Schemes were used to study the relaxation process of CoSi_1-xY_x （Y=Al、P） where Si was substituted by Al or P. Linearized Augmented Plane Wave （LAPW） based on the Density Functional Theory （DFT） of the first-principles was utilized to calculate the electronic structure of CoSi and CoSi_1-xY_x （Y=Al、P）. Based on the calculated electronic structures of CoSi and CoSi_1-xAl_x as well as the existing experimental data on thermoelectric transport performances of the CoSi_1-xAl_x single crystal, the essence of the effects of Al doping on the thermoelectric properties of the CoSi_1-xAl_x single crystal were analyzed. The electronic structure of CoSi showed that the overlap between valence band and conduction one existed at its Fermi level, which meant that CoSi was a semi-metal. The Si substitution by Al made the electronic structure of CoSi_1-xAl_x complex. With the increase of Al, the Fermi level of CoSi_1-xAl_x shifted toward the low energy band. In contrast, the Si substitution by P made the Fermi level of CoSi_1-xP_x shift upward （toward high energy band）. The changing trends of electrical resistivity and Seebeck coefficient of the CoSi_1-xAl_x single crystal were explained by the effects of the Si substitution by Al on the electronic structure.Doping is an effective method to increase the electrical performances and decrease the thermal conductivity. Two inequivalent sites of Fe as well as Si exist inβ-FeSi₂. The method of comparing total energy was used to determine the substitution sites of Al and Co, respectively. Damped Newton Dynamics Themes were used to relax the crystal structures of Fe(Si_0.96875Al_0.03125)₂ and Fe_0.9375Co_0.0625Si₂. LAPW was used to calculate the electronic structures ofβ-FeSi₂ and Fe(Si_0.96875Al_0.03125)₂ as well as Fe_0.9375Co_0.0625Si₂. The effects of doping by Co and Al on the thermoelectric transport performances ofβ-FeSi₂ were analyzed theoretically. The electronic structure ofβ-FeSi₂ showed that an energy gap existed at the Fermi level, which meant thatβ-FeSi₂ was a typical semiconductor. A direct energy gap of 0.74eV existed at H and an indirect energy gap of 0.71eV lies between H andΛ/3. The substitution sites of Co and Al were FeⅡand SiⅠ, respectively. The Si substitution by Al made the Fermi level of Fe(Si_0.96875Al_0.03125)₂ shift toward its valence band. In comparison with that, doping by Co made the Fermi level of Fe_0.9375Co_0.0625Si₂ shift toward its conduction band. Doping could increase the Seebeck coefficient as well as the thermal coductivity greatly.La filling is an important method to reduce the thermal conductivity for CoSb3. The filling of La atom will result in the relaxation of geometrical structure of CoSb3. In this thesis, the effects of La filling on the geometrical structure of La0.5Co4Sb12 were investigated following the Damped Newton Dynamics Themes. LAPW was used to calculate the electronic structures and electron densities of CoSb3 as well as the relaxed La0.5Co4Sb12. The relaxation results showed that La filling had significant effects on the geometrical structure of CoSb3. In addition to that, La filling made the bond length, bond angle and charge density change. CoSb3 was a narrow-gap semiconductor. The energy band in the vicinity ofΓat the Fermi level was linear with the k vector. Ascribing to the La filling, the Fermi level of La0.5Co4Sb12 shifted toward the conduction band. La filling could increase the thermoelectric properties, which was attributed to the decrease of thermal conductivity while increasing the electrical transport performances.CrSi₂ is a thermoelectric materials utilized at the intermediate or high temperature. The electronic structures of CrSi₂, Cr(Si_1-xAl_x)₂ as well as Cr_1-xV_xSi₂ were calculated by LAPW. The effects of doping by Al and V on the electronic structures were analyzed. The range of solubility limits of Al and V in CrSi₂ were investigated with EPMA （Electronic Probe Microanalysis） and EDS （Energy Dispersive Spectroscopy）, respectively. Based on the X-ray diffraction spectrum, the effects of doping by Al and V on the lattice parameters of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ were studied, respectively. Within the solubility limit, the single crystals of CrSi₂, Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ were prepared by optical-heating floating zone melting method. The effects of doping by Al and V on the thermoelectric transport performances of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ single crystals were studied, respectively. Based on the calculated electronic structures and observed experimental data, the essence of the effects of doping by Al and V on the thermoelectric properties was analyzed. The electronic structure showed that CrSi₂ was a p-type semiconductor and hole carriers dominated its thermoelectric transport performances. An indirect energy gap of 0.35eV existed in the band structure of CrSi₂. The Fermi levels of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ shifted toward their valence bands resulting from the doping by Al and V, which had great effects on their thermoelectric properties. The solubility limit of Al in CrSi₂ was in the range of 0.20≤x<0.30, while V could be dissolved in CrSi₂ completely when the solubility x of V in CrSi₂ is less than or equal to 0.4. The change of the lattice parameters of CrSi₂ resulting from the doping by Al and V was in accord with the Vegard’s law. The dopings by Al and V had significant effects on the thermoelectric transport performances and could increase the thermoelectric properties of the CrSi₂ single crystal greatly.更多还原