热电氧化物钛酸锶电热输运的理论研究
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摘要
热电效应(又称温差电效应)是可以将热能与电能相互转换的一种效应,即热能可以发电(温差发电),而电能可以制冷(热电制冷)。而实现以上功能,只需要全固体的热电材料器件,故其具有可靠性高,无污染等优点。热电发电与传统的机械能发电模式相比,温差发电具有可靠性高等优点,并已成功应用于空间探测器的动力装置,如1977年发射的旅行者号飞船(Voyager)上采用的温差发电器件在运行2.5亿装置时(device hour)后无一故障；制冷方面,与传统的氟利昂压缩机制冷模式相比,具有无噪声、无污染等优点,并已广泛应用到卧室与宾馆中使用的无噪音冰箱中。近年来,由于能源与环境成为可持续发展的两大主题,热电材料作为清洁、绿色的能源材料,有望在汽车尾气废热发电,家用制冷,便携式冰箱等方面得到广泛的应用
为构造转换效率较高的热电器件,寻找高性能的热电材料成为研究者的主要目标。热电材料的性能一般由无量纲优值系数ZT值表示,ZT值越高,表明材料转换能量的效率越高。ZT=S2σ／K,其中S,σ与K分别为Seebeck系数,电导率与热导率,分子部分称为功率因子(PF值)。高性能热电材料需要具有高的PF值和低的热导率。从这点出发,寻找高性能热电材料主要分为两个方向：一是将传统的热电材料(如Bi2Te3,PbTe,SiGe等)低维化以提高其热电性能,二是寻找新型的体相热电材料。低维化可以加强界面散射以有效降低材料热导率,并且可以引入“量子限域”“能量过滤”等效应增大PF值。而新型体相热电材料一般含有特殊的微结构,如三维空洞(CoSb3,笼状结构),二维层间弱耦合(NaxCoO2,Ca3Co4O9)等。本文即从这两个方面探索提高材料热电性能的可能途径。
本论文以环保型热电氧化物材料作为研究方向,选取钙钛矿结构氧化物SrTiO3作为研究对象。SrTiO3由原料丰富、价格便宜的轻元素构成,具有无毒、环保、热稳定性好、价格低廉等优点；且其有效质量大,迁移率高,具有较好的热电性能。本论文中,结合密度泛函理论计算和分子动力学模拟,采用基于玻尔兹曼方程的多带模型和声子传输唯象理论对热电氧化物SrTiO3的电热输运性质进行了理论研究,分析了纳米化方法和掺杂方法提高SrTiO3热电性能的可能性,并且对SrTiO3陶瓷中的固有缺陷氧空位和晶界进行了理论研究。本论文的主要创新之处有：
(1)发展了与密度泛函理论相结合的、基于波尔兹曼方程的计算重掺杂体系Seebeck系数的方法。与传统的以电子群速度作为输入参数的方法不同,该方法利用密度泛函理论计算的能态密度(DOS)作为输入参数,可以求得Seebeck系数随温度和载流子浓度变化的关系。将该方法应用于重掺杂La：SrTiO3体系,计算结果与实验值取得较好的符合。对Seebeck系数的分析表明,Seebeck系数的大小取决于Fermi能级上下DOS的不对称度,不对称度越大Seebeck系数越大。Seebeck系数随温度升高、掺杂浓度降低而升高的趋势也可以由不对称度的变化很好的解释。
(2)采用纳米化的方法提高SrTi03陶瓷的热电性质。首先采用材料设计的理念,设计了一种SrTi03纳米陶瓷作为新型的氧化物热电材料。这种陶瓷具有纳米尺度的晶粒,晶界作为功能界而散射声子以降低晶格热导率。同时,在晶界处束缚电子形成二维电子气,则连通的晶界将构成二维电子气网络,对沿晶界传输的电子引入“量子限域”效应；晶界旁边的空间电荷区形成势垒,对垂直于晶界传输的电子引入“能量过滤”效应。这两个效应均可提高功率因子。晶格热导率的降低和功率因子的提高均会提升材料的热电性能。然后,利用密度泛函理论和玻尔兹曼方程计算了含有晶界二维电子气的SrTi03纳米陶瓷的热电性质。计算发现,当a)晶界厚度小b)晶界势垒高度合适c)晶粒内载流子浓度高时,该纳米陶瓷材料具有最高的ZT值。一是由于量子限域效应,材料ZT值随晶界厚度的增大而减小；二是由于能量过滤效应,材料ZT值随晶界势垒高度的增大呈现先增大后减小的趋势,而最大ZT值对应的势垒高度即为最佳晶界势垒高度,其值大约比Fermi能级高0.06eV。对其高温热电性质的计算表明,该种纳米陶瓷的ZT值随温度升高而降低,说明该材料适于应用在室温下。我们的计算结果对于设计新型环保的氧化物热电材料会起到帮助作用。
(3)探讨了固溶、掺杂提高SrTi03单晶热电性能的可能性,利用密度泛函理论计算和玻尔兹曼方程计算了Ca、Ba元素固溶及V,Ta,W,Sb元素掺杂对SrTi03单晶热电性质的影响。计算发现：a)Ba固溶可以有效提高功率因子。结合紧束缚模型分析能带色散关系和DOS得知,Ba固溶提高功率因子的原因是增大晶格常数。b)通过B位掺杂过渡元素原子,在能带结构中形成局域态,引入DOS峰并将Fermi能级调节到DOS峰附近,增大Fermi能级上下DOS的不对称度,进而增大Seebeck系数。通过对过渡元素掺杂的能带结构计算表明,V掺杂可以引入DOS峰,但由于载流子浓度较低,Fermi能级并未移动到合适位置。Sb掺杂由于Fermi能级处于DOS“阶梯”附近,具有较高的Seebeck系数。
(4)分析了双元素掺杂在SrTi03禁带中形成级联杂质能级的可能性。计算发现Bi和Cu共同掺杂可以实现这一目标,Bi掺杂在导带底附近引入深杂质能级,Cu掺杂在价带顶附近引入深杂质能级,二者形成的级联能级有效提高了电子从价带顶到导带底的受激跃迁几率。因为与直接跃迁到导带底相比,价带顶电子在通过杂质能级进行级联跃迁时所需要的能量低,可以提供的能量的声子较多。
(5)对SrTi03的氧空位进行了理论研究,包括氧空位对晶格常数、声子振动模式以及晶格热容的影响,并研究了氧空位与掺杂原子的相互作用。计算发现：a)随氧空位浓度的增加,SrTi03的晶格常数先增大,在氧空位浓度5%左右达到最大值,然后减小。b)对含氧空位体系的声子计算和原子间力常数矩阵计算得到,氧空位对声子振动模式的影响主要体现在Ti-O振动模式的变化。对含氧空位体系的晶格热容的计算表明,氧空位的形成可以有效降低晶格热容,说明氧空位不仅可以散射声子降低声子平均自由程,而且可以降低热容,从两方面降低热导。c)对氧空位与掺杂原子的结合能计算表明,当掺杂原子价态越低,半径越小,越易于与氧空位形成缺陷对。这是因为价态低消耗的电子弛豫能较低,而半径小消耗的晶格弛豫能较低。
(6)自编程序探索了在周期性边界条件下SrTi03晶界的可能构型,通过对晶界电子结构的计算,发现(111)[-110]晶界的SrO构型可以在导带底形成势阱,从而支持了创新点(2)中的晶界势阱模型。首先采用自编程序找到超晶胞小于300个原子的5种晶界构型,然后进行结构优化并计算其电子结构。通过分析原子的投影态密度发现,晶界层的厚度为3个原子层,而晶界距离大于11.3A时其相互作用即可以忽略。分析晶界和晶粒的分态密度可知,在(111)[-110]晶界中,SrO构型的晶界可以在导带底形成势阱,而TiO2构型的晶界在价带顶形成势阱。势阱的形成是Ti原子和O原子在晶界处的成键与在晶粒内部不同所致。
论文最后部分对本论文进行了总结,但由于时间篇幅等原因,在以下几个方面未能进行深入研究：(1)理论计算结果的实验验证。现在只是在实验上证实了SrTiO3的晶格热导率可以通过纳米化在室温下降低到2Wm／K左右,而对其电学性质的预测并未通过实验结果证实。(2)体材料中实现纳米化结构的其它方法的探索。本文只提出了在陶瓷的晶界处形成纳米结构。实际上在体材料中引入纳米线或量子点等也是形成纳米结构的有效方式,而且还有很多体材料本身含有独特的纳米微结构。这些都是值得探索的提高热电性能的方法。(3)其它氧化物的热电体系的系统研究。本论文仅对SrTi03的热电性能进行探索。但氧化物种类繁多,其中存在有潜力的性能优异的热电材料。由于时间所限并未对氧化物热电材料进行系统的探索并寻找其中潜在的规律。
Thermoelectric effect refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. The realization of the above features just need all solid-state thermoelectric devices, so it has the merits of high reliability, non-polluting and so on. For power generation, it has the merit of high reliability compared to traditional mechanical energy generation mode, and has been successfully applied to space probes, such as the thermoelectric power generation devices in the Voyager spacecraft launched in 1977. As for refrigeration, it has the merits of no noise, no pollution, etc., compared with the traditional freon compressor cooling mode, and has been widely applied to the bedroom and the guest house use of noise-free refrigerator. In recent years, due to the two major themes of energy and environment for sustainable development, thermoelectric materials, as a clean, green energy materials, are expected to be widely used asautomobile exhaust heat power generation, home cooling, portable refrigerators, etc.
The performance of thermoelectric materials depends on ZT value, higher ZT value gives higher efficiency of energy conversion. ZT= S2σ/k, where S,σandκ, are Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. High-performance thermoelectric material needs to have a high PF value and a low thermal conductivity. From this point of view, looking for high-performance thermoelectric materials can be divided into two directions:First, using low-dimensional technology to improve their thermoelectric properties the traditional thermoelectric materials (such as Bi2Te3, PbTe, SiGe, etc.); second, looking for a new type of bulk thermoelectric materials.
In this thesis, environmental friendly materials:thermoelectric oxides are chosen as research directions, and the perovskite oxides SrTiO3 is selected as an object. Combined with density functional theory calculations and molecular dynamics simulation, Boltzmann equation-based multi-band model and the phenomenological phonon transport theory are used to calculate thermal and electrical transport in SrTiO3. Nanocrystallization and doping are both analyzed to find the e possibility of enhancing the thermoelectric performance of SrTiO3. The mainly defects in SrTiO3 ceramics, including oxygen vacancies and grain boundaries are theoretically studied. Main contents of each chapter are as follows:
In chapter I, the history and current status of thermoelectric field are introduced, as well as the current status of research about SrTiO3. Starting with the discover of thermoelectric phenomenon in 1823, the development of thermoelectric field for nearly 200 years is introduced. Then from point of view of application, the structure and working principle of thermoelectric device are introduced. Then the most important components:thermoelectric materials are discussed. From two aspects, the low-dimensional thermoelectric materials and new bulk thermoelectric materials, the current research status is described in detail, and two parts are highlighted:the oxide thermoelectric materials and theoretical studies of thermoelectric materials. Finally, the research status of SrTiO3 is introduced.
In chapter II, a novel approach was developed to calculate temperature dependent Seebeck coefficient of heavily doped systems. The electronic density of states (DOS) and Fermi energy were determined and then, using these two parameters, the Seebeck coefficient was calculated by using Boltzmann transport theory. This approach is applied to heavily La doped SrTiO3. The calculated Seebeck coefficient agrees well with the experimental data. By analyzing the results, it was shown that Seebeck coefficient is greatly affected by the asymmetry of DOS with respect to Fermi energy.
In chapter III, (1) firstly, density functional theory calculations and Boltzmann transport theory are employed to simulate the thermoelectric properties of SrTiO3 ceramics with two-dimensional electron gas (2DEG) grain boundaries (GBs). This material can achieve a large thermoelectric figure of merit (ZT>1 at room temperature) by utilizing quantum confinement and energy filtering at GBs. The latter causes ZT value to reach a maximum before decreasing with increasing GB barrier height. The optimum barrier height was approximately 0.06 eV higher than the Fermi energy of the grain interior. Our results may aid the design of materials with environmentally benign thermoelectric oxides. (2) Secondly, thermoelectric performance of nanocrystalline SrTiO3 ceramics with 2DEG GBs was theoretically investigated. The GBs of STO ceramics consist of 20% Nb-doped STO with thickness of 1,2,4 unit cells, respectively, and the GBs are separated by 10% La-doped STO grain interior (GI) with thickness of 16 unit cells. The calculated transport coefficients indicated that phonon confinement in GI and GBs greatly reduces the lattice thermal conductivity, and quantum confinement 2DEG at GBs greatly enhances the power factor. The energy filtering effect on electrons and boundary scattering of phonons at GBs further enhances the ZT value, which reaches 0.8 at 300K due to the co-existence of these effects. In addition, The ZT value reaches 1.2 at 300K if the minimum lattice thermal conductivity can be achieved by this nanocrystalline ceramics.
In chapter IV, (1) firstly, Boltzmann transport theory and density functional theory calculations were employed to calculate transport coefficients of CaTiO3, SrTiO3 and BaTiO3. The transport coefficients of these materials were compared and analyzed by using'Tight Binding Model'. The band narrowing, caused by different lattice constants of these materials, was the mainly reason for their different transport properties. The calculated electronic conductivity and thermal conductivity in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 1.45 for these wide band gap semiconductors. Finally ZT values were estimated and BaTiO3 had the largest ZT value among the three materials, mainly due to its largest lattice constant. (2) V doping can modify the shape of DOS, and effectively increase the Seebeck coefficient. (3) Thirdly, the possibility of forming cascading energy levels in SrTiO3 is analyzed by using the Density Functional Theory based first principles calculations of the electronic structure by doping Bi and Cu as example. The results show that Bi doping and Cu doping introduce defect level in the forbidden band, respectively, and Co-doping of Bi and Cu can introduced two defect levels in forbidden band. The electrons at the top of valence band can transit to the bottom of conduction band through a'cascade transition'process. With No Radical Transition Model, the analysis points out that the probability of electronic transition from the valence band to the conduction band through a cascade transition is much higher than that of direct transition from the valence band to the conduction band. The cascade transitions can effectively increase the carrier concentration in the conduction band.
In chapterⅤ, the main defects in SrTiO3 ceramic, including oxygen vacancies and grain boundary, are theoretically studied. (1) It was found that in SrTiO3 with increasing oxygen vacancy concentration, the lattice constant first increases before reaching the maximum at 5% concentration, then decreases. This is due to the lattice distortion caused by oxygen vacancies. By analyzing the atomic force constant matrix, we can see oxygen vacancies mainly change the Ti-O optical branch. The calculations of the lattice heat capacity show that oxygen vacancies can effectively reduce the lattice heat capacity, indicating that oxygen vacancies not only scatter phonons, but also can reduce the heat capacity, so they can reduce the thermal conductivity in two ways. (2) For grain boundaries, theoretical studies focus on its electronic structure. the possibility of grain boundary supercells are constructed by using a self-compiled small program. By analyzing the atomic projection density of states, it is found that the grain boundary interactions can be neglected when their interval is greater than 11.3A; and the grain boundary consists of three atomic layers. By analysising of the partial density of states of grain boundary and grain, it can be seen SrO grain boundary configuration can form a potential well in the conduction band edge, while the TiO2 configuration can form a potential well at the top of valence band. These are due to of the Ti atom and O atoms at the grain boundaries have different bonding character with those in the grain.
ChapterⅥsummarizes this thesis, and points out that the formation of nano-structure in bulk materials is an effective way to improve materials' thermoelectric performance, based on the main conclusions in this thesis. To further illustrate this point, the intercalated structure (SnS)1.2TiS2 is taken as an example: pure TiS2 single crystal is a layered N-type thermoelectric materials, PF value is high, but the thermal conductivity is also high. By intercalating SnS layers between TiS2 layers, the lattice thermal conductivity can be effectively reduced, which its electrical properties maintained. This point is demonstrated by calculating partial density of states of the intercalated TiS2 layered structure.
This thesis proposes, for the first time, the concept of "forming two-dimensional electron gas at grain boundaries in SrTiO3 ceramics" and "forming cascade impurity levels in the band gap of SrTiO3 a by double-doping" in order to improve the materials'thermoelectric performance, and tests these concepts by doing calculation under Boltzmann equation and density functional theory. It is found that two-dimensional electron gas grain boundary can effectively increase the power factorof the nano-ceramics, and the formation of cascade impurity levels in the band gap can effectively increase the probablity of electrons transition from the valence band maximum to the conduction band minimum.
为构造转换效率较高的热电器件,寻找高性能的热电材料成为研究者的主要目标。热电材料的性能一般由无量纲优值系数ZT值表示,ZT值越高,表明材料转换能量的效率越高。ZT=S2σ／K,其中S,σ与K分别为Seebeck系数,电导率与热导率,分子部分称为功率因子(PF值)。高性能热电材料需要具有高的PF值和低的热导率。从这点出发,寻找高性能热电材料主要分为两个方向：一是将传统的热电材料(如Bi2Te3,PbTe,SiGe等)低维化以提高其热电性能,二是寻找新型的体相热电材料。低维化可以加强界面散射以有效降低材料热导率,并且可以引入“量子限域”“能量过滤”等效应增大PF值。而新型体相热电材料一般含有特殊的微结构,如三维空洞(CoSb3,笼状结构),二维层间弱耦合(NaxCoO2,Ca3Co4O9)等。本文即从这两个方面探索提高材料热电性能的可能途径。
本论文以环保型热电氧化物材料作为研究方向,选取钙钛矿结构氧化物SrTiO3作为研究对象。SrTiO3由原料丰富、价格便宜的轻元素构成,具有无毒、环保、热稳定性好、价格低廉等优点；且其有效质量大,迁移率高,具有较好的热电性能。本论文中,结合密度泛函理论计算和分子动力学模拟,采用基于玻尔兹曼方程的多带模型和声子传输唯象理论对热电氧化物SrTiO3的电热输运性质进行了理论研究,分析了纳米化方法和掺杂方法提高SrTiO3热电性能的可能性,并且对SrTiO3陶瓷中的固有缺陷氧空位和晶界进行了理论研究。本论文的主要创新之处有：
(1)发展了与密度泛函理论相结合的、基于波尔兹曼方程的计算重掺杂体系Seebeck系数的方法。与传统的以电子群速度作为输入参数的方法不同,该方法利用密度泛函理论计算的能态密度(DOS)作为输入参数,可以求得Seebeck系数随温度和载流子浓度变化的关系。将该方法应用于重掺杂La：SrTiO3体系,计算结果与实验值取得较好的符合。对Seebeck系数的分析表明,Seebeck系数的大小取决于Fermi能级上下DOS的不对称度,不对称度越大Seebeck系数越大。Seebeck系数随温度升高、掺杂浓度降低而升高的趋势也可以由不对称度的变化很好的解释。
(2)采用纳米化的方法提高SrTi03陶瓷的热电性质。首先采用材料设计的理念,设计了一种SrTi03纳米陶瓷作为新型的氧化物热电材料。这种陶瓷具有纳米尺度的晶粒,晶界作为功能界而散射声子以降低晶格热导率。同时,在晶界处束缚电子形成二维电子气,则连通的晶界将构成二维电子气网络,对沿晶界传输的电子引入“量子限域”效应；晶界旁边的空间电荷区形成势垒,对垂直于晶界传输的电子引入“能量过滤”效应。这两个效应均可提高功率因子。晶格热导率的降低和功率因子的提高均会提升材料的热电性能。然后,利用密度泛函理论和玻尔兹曼方程计算了含有晶界二维电子气的SrTi03纳米陶瓷的热电性质。计算发现,当a)晶界厚度小b)晶界势垒高度合适c)晶粒内载流子浓度高时,该纳米陶瓷材料具有最高的ZT值。一是由于量子限域效应,材料ZT值随晶界厚度的增大而减小；二是由于能量过滤效应,材料ZT值随晶界势垒高度的增大呈现先增大后减小的趋势,而最大ZT值对应的势垒高度即为最佳晶界势垒高度,其值大约比Fermi能级高0.06eV。对其高温热电性质的计算表明,该种纳米陶瓷的ZT值随温度升高而降低,说明该材料适于应用在室温下。我们的计算结果对于设计新型环保的氧化物热电材料会起到帮助作用。
(3)探讨了固溶、掺杂提高SrTi03单晶热电性能的可能性,利用密度泛函理论计算和玻尔兹曼方程计算了Ca、Ba元素固溶及V,Ta,W,Sb元素掺杂对SrTi03单晶热电性质的影响。计算发现：a)Ba固溶可以有效提高功率因子。结合紧束缚模型分析能带色散关系和DOS得知,Ba固溶提高功率因子的原因是增大晶格常数。b)通过B位掺杂过渡元素原子,在能带结构中形成局域态,引入DOS峰并将Fermi能级调节到DOS峰附近,增大Fermi能级上下DOS的不对称度,进而增大Seebeck系数。通过对过渡元素掺杂的能带结构计算表明,V掺杂可以引入DOS峰,但由于载流子浓度较低,Fermi能级并未移动到合适位置。Sb掺杂由于Fermi能级处于DOS“阶梯”附近,具有较高的Seebeck系数。
(4)分析了双元素掺杂在SrTi03禁带中形成级联杂质能级的可能性。计算发现Bi和Cu共同掺杂可以实现这一目标,Bi掺杂在导带底附近引入深杂质能级,Cu掺杂在价带顶附近引入深杂质能级,二者形成的级联能级有效提高了电子从价带顶到导带底的受激跃迁几率。因为与直接跃迁到导带底相比,价带顶电子在通过杂质能级进行级联跃迁时所需要的能量低,可以提供的能量的声子较多。
(5)对SrTi03的氧空位进行了理论研究,包括氧空位对晶格常数、声子振动模式以及晶格热容的影响,并研究了氧空位与掺杂原子的相互作用。计算发现：a)随氧空位浓度的增加,SrTi03的晶格常数先增大,在氧空位浓度5%左右达到最大值,然后减小。b)对含氧空位体系的声子计算和原子间力常数矩阵计算得到,氧空位对声子振动模式的影响主要体现在Ti-O振动模式的变化。对含氧空位体系的晶格热容的计算表明,氧空位的形成可以有效降低晶格热容,说明氧空位不仅可以散射声子降低声子平均自由程,而且可以降低热容,从两方面降低热导。c)对氧空位与掺杂原子的结合能计算表明,当掺杂原子价态越低,半径越小,越易于与氧空位形成缺陷对。这是因为价态低消耗的电子弛豫能较低,而半径小消耗的晶格弛豫能较低。
(6)自编程序探索了在周期性边界条件下SrTi03晶界的可能构型,通过对晶界电子结构的计算,发现(111)[-110]晶界的SrO构型可以在导带底形成势阱,从而支持了创新点(2)中的晶界势阱模型。首先采用自编程序找到超晶胞小于300个原子的5种晶界构型,然后进行结构优化并计算其电子结构。通过分析原子的投影态密度发现,晶界层的厚度为3个原子层,而晶界距离大于11.3A时其相互作用即可以忽略。分析晶界和晶粒的分态密度可知,在(111)[-110]晶界中,SrO构型的晶界可以在导带底形成势阱,而TiO2构型的晶界在价带顶形成势阱。势阱的形成是Ti原子和O原子在晶界处的成键与在晶粒内部不同所致。
论文最后部分对本论文进行了总结,但由于时间篇幅等原因,在以下几个方面未能进行深入研究：(1)理论计算结果的实验验证。现在只是在实验上证实了SrTiO3的晶格热导率可以通过纳米化在室温下降低到2Wm／K左右,而对其电学性质的预测并未通过实验结果证实。(2)体材料中实现纳米化结构的其它方法的探索。本文只提出了在陶瓷的晶界处形成纳米结构。实际上在体材料中引入纳米线或量子点等也是形成纳米结构的有效方式,而且还有很多体材料本身含有独特的纳米微结构。这些都是值得探索的提高热电性能的方法。(3)其它氧化物的热电体系的系统研究。本论文仅对SrTi03的热电性能进行探索。但氧化物种类繁多,其中存在有潜力的性能优异的热电材料。由于时间所限并未对氧化物热电材料进行系统的探索并寻找其中潜在的规律。
Thermoelectric effect refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. The realization of the above features just need all solid-state thermoelectric devices, so it has the merits of high reliability, non-polluting and so on. For power generation, it has the merit of high reliability compared to traditional mechanical energy generation mode, and has been successfully applied to space probes, such as the thermoelectric power generation devices in the Voyager spacecraft launched in 1977. As for refrigeration, it has the merits of no noise, no pollution, etc., compared with the traditional freon compressor cooling mode, and has been widely applied to the bedroom and the guest house use of noise-free refrigerator. In recent years, due to the two major themes of energy and environment for sustainable development, thermoelectric materials, as a clean, green energy materials, are expected to be widely used asautomobile exhaust heat power generation, home cooling, portable refrigerators, etc.
The performance of thermoelectric materials depends on ZT value, higher ZT value gives higher efficiency of energy conversion. ZT= S2σ/k, where S,σandκ, are Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. High-performance thermoelectric material needs to have a high PF value and a low thermal conductivity. From this point of view, looking for high-performance thermoelectric materials can be divided into two directions:First, using low-dimensional technology to improve their thermoelectric properties the traditional thermoelectric materials (such as Bi2Te3, PbTe, SiGe, etc.); second, looking for a new type of bulk thermoelectric materials.
In this thesis, environmental friendly materials:thermoelectric oxides are chosen as research directions, and the perovskite oxides SrTiO3 is selected as an object. Combined with density functional theory calculations and molecular dynamics simulation, Boltzmann equation-based multi-band model and the phenomenological phonon transport theory are used to calculate thermal and electrical transport in SrTiO3. Nanocrystallization and doping are both analyzed to find the e possibility of enhancing the thermoelectric performance of SrTiO3. The mainly defects in SrTiO3 ceramics, including oxygen vacancies and grain boundaries are theoretically studied. Main contents of each chapter are as follows:
In chapter I, the history and current status of thermoelectric field are introduced, as well as the current status of research about SrTiO3. Starting with the discover of thermoelectric phenomenon in 1823, the development of thermoelectric field for nearly 200 years is introduced. Then from point of view of application, the structure and working principle of thermoelectric device are introduced. Then the most important components:thermoelectric materials are discussed. From two aspects, the low-dimensional thermoelectric materials and new bulk thermoelectric materials, the current research status is described in detail, and two parts are highlighted:the oxide thermoelectric materials and theoretical studies of thermoelectric materials. Finally, the research status of SrTiO3 is introduced.
In chapter II, a novel approach was developed to calculate temperature dependent Seebeck coefficient of heavily doped systems. The electronic density of states (DOS) and Fermi energy were determined and then, using these two parameters, the Seebeck coefficient was calculated by using Boltzmann transport theory. This approach is applied to heavily La doped SrTiO3. The calculated Seebeck coefficient agrees well with the experimental data. By analyzing the results, it was shown that Seebeck coefficient is greatly affected by the asymmetry of DOS with respect to Fermi energy.
In chapter III, (1) firstly, density functional theory calculations and Boltzmann transport theory are employed to simulate the thermoelectric properties of SrTiO3 ceramics with two-dimensional electron gas (2DEG) grain boundaries (GBs). This material can achieve a large thermoelectric figure of merit (ZT>1 at room temperature) by utilizing quantum confinement and energy filtering at GBs. The latter causes ZT value to reach a maximum before decreasing with increasing GB barrier height. The optimum barrier height was approximately 0.06 eV higher than the Fermi energy of the grain interior. Our results may aid the design of materials with environmentally benign thermoelectric oxides. (2) Secondly, thermoelectric performance of nanocrystalline SrTiO3 ceramics with 2DEG GBs was theoretically investigated. The GBs of STO ceramics consist of 20% Nb-doped STO with thickness of 1,2,4 unit cells, respectively, and the GBs are separated by 10% La-doped STO grain interior (GI) with thickness of 16 unit cells. The calculated transport coefficients indicated that phonon confinement in GI and GBs greatly reduces the lattice thermal conductivity, and quantum confinement 2DEG at GBs greatly enhances the power factor. The energy filtering effect on electrons and boundary scattering of phonons at GBs further enhances the ZT value, which reaches 0.8 at 300K due to the co-existence of these effects. In addition, The ZT value reaches 1.2 at 300K if the minimum lattice thermal conductivity can be achieved by this nanocrystalline ceramics.
In chapter IV, (1) firstly, Boltzmann transport theory and density functional theory calculations were employed to calculate transport coefficients of CaTiO3, SrTiO3 and BaTiO3. The transport coefficients of these materials were compared and analyzed by using'Tight Binding Model'. The band narrowing, caused by different lattice constants of these materials, was the mainly reason for their different transport properties. The calculated electronic conductivity and thermal conductivity in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 1.45 for these wide band gap semiconductors. Finally ZT values were estimated and BaTiO3 had the largest ZT value among the three materials, mainly due to its largest lattice constant. (2) V doping can modify the shape of DOS, and effectively increase the Seebeck coefficient. (3) Thirdly, the possibility of forming cascading energy levels in SrTiO3 is analyzed by using the Density Functional Theory based first principles calculations of the electronic structure by doping Bi and Cu as example. The results show that Bi doping and Cu doping introduce defect level in the forbidden band, respectively, and Co-doping of Bi and Cu can introduced two defect levels in forbidden band. The electrons at the top of valence band can transit to the bottom of conduction band through a'cascade transition'process. With No Radical Transition Model, the analysis points out that the probability of electronic transition from the valence band to the conduction band through a cascade transition is much higher than that of direct transition from the valence band to the conduction band. The cascade transitions can effectively increase the carrier concentration in the conduction band.
In chapterⅤ, the main defects in SrTiO3 ceramic, including oxygen vacancies and grain boundary, are theoretically studied. (1) It was found that in SrTiO3 with increasing oxygen vacancy concentration, the lattice constant first increases before reaching the maximum at 5% concentration, then decreases. This is due to the lattice distortion caused by oxygen vacancies. By analyzing the atomic force constant matrix, we can see oxygen vacancies mainly change the Ti-O optical branch. The calculations of the lattice heat capacity show that oxygen vacancies can effectively reduce the lattice heat capacity, indicating that oxygen vacancies not only scatter phonons, but also can reduce the heat capacity, so they can reduce the thermal conductivity in two ways. (2) For grain boundaries, theoretical studies focus on its electronic structure. the possibility of grain boundary supercells are constructed by using a self-compiled small program. By analyzing the atomic projection density of states, it is found that the grain boundary interactions can be neglected when their interval is greater than 11.3A; and the grain boundary consists of three atomic layers. By analysising of the partial density of states of grain boundary and grain, it can be seen SrO grain boundary configuration can form a potential well in the conduction band edge, while the TiO2 configuration can form a potential well at the top of valence band. These are due to of the Ti atom and O atoms at the grain boundaries have different bonding character with those in the grain.
ChapterⅥsummarizes this thesis, and points out that the formation of nano-structure in bulk materials is an effective way to improve materials' thermoelectric performance, based on the main conclusions in this thesis. To further illustrate this point, the intercalated structure (SnS)1.2TiS2 is taken as an example: pure TiS2 single crystal is a layered N-type thermoelectric materials, PF value is high, but the thermal conductivity is also high. By intercalating SnS layers between TiS2 layers, the lattice thermal conductivity can be effectively reduced, which its electrical properties maintained. This point is demonstrated by calculating partial density of states of the intercalated TiS2 layered structure.
This thesis proposes, for the first time, the concept of "forming two-dimensional electron gas at grain boundaries in SrTiO3 ceramics" and "forming cascade impurity levels in the band gap of SrTiO3 a by double-doping" in order to improve the materials'thermoelectric performance, and tests these concepts by doing calculation under Boltzmann equation and density functional theory. It is found that two-dimensional electron gas grain boundary can effectively increase the power factorof the nano-ceramics, and the formation of cascade impurity levels in the band gap can effectively increase the probablity of electrons transition from the valence band maximum to the conduction band minimum.
引文
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