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根据统计,目前人类对化石能源的消耗已经超过了现有预测储量的一半以上,很快地球上的化石能源将会枯竭耗尽[1]。而且,化石能源的使用过程也对环境造成了极大的破坏,不利于环境的可持续性发展。因此,发展新型的清洁可持续的绿色能源技术已迫在眉睫。
事实上,除了风能、潮汐能和太阳能等清洁能源外,自然界和人类活动中还蕴藏着大量的热能未被开发利用,例如地热能以及现代工业生产和生活中排放的各种废热能等。利用塞贝克(Seebeck)效应,热电材料可以直接将地热能和废热能转换成电能,从而提高能源的利用率并减少环境污染,其研究为发展新型清洁能源技术打开了新的视角[2-5]。
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影响热电系统的能量转换效率有很多因素,如:温差电元件的类型及性能、热量的损失、设备的整体精确度等,其中,最关键的因素是热电材料的性能。用无量纲的热电优值zT值来衡量热电材料的性能[6-7],表达式为:
$$ {\rm{zT}} = \frac{{{\alpha ^2}T\sigma }}{\kappa } $$ 式中,T为绝对温度;α、
$ \sigma$ 和κ分别为材料的塞贝克系数、电导率和热导率。图1列出了近年来热电材料的发展现状[8],总体而言,其发展可以分为3个时期[9]:1) zT值较低的第一个时期,此时热电材料的zT值不到1,热电器件的能量转换效率只有4%~5%;2) zT值突飞猛进的第二个时期,此时热电材料的zT值达到1.7,热电器件的能量转换效率提升到11%~15%[10-13];3) zT值突破2的第三个时期,此时热电器件的能量转换效率可以达到15%~20%[14-17]。
图 1 N型和P型热电材料的zT值随温度的变化趋势[8]
高zT值意味着材料需要具有高的塞贝克系数、高的电导率以及低的热导率。然而,塞贝克系数、电导率、热导率这3个参数是相互影响彼此制约的,改变其中任何一个都会对另两个带来不利影响,很难同时优化。基于此,1995年,美国科学家Slack率先提出了“声子玻璃−电子晶体(PGEC)”的热电材料设计理念[18-19],认为好的热电材料应该同时具备像玻璃一样的声子传导特性以及像晶体一样的电子传导特性,从而同时具有低热导率和高电导率。方钴矿、笼化物材料(clathrates)和β-Zn4Sb3化合物等笼状化合物热电材料由于具有“声子玻璃−电子晶体(PGEC)”的特征而获得人们的广泛关注[20-25]。以方钴矿材料为例,由于其晶体结构中存在大量的本征晶格空洞,在这些晶格空洞中可以填充其他原子,通过填充原子在晶格空洞中的“rattling”振动极大地散射声子,从而显著降低材料的晶格热导率,进而优化材料的热电性能。近年来,随着热电理论体系的迅速发展,一些新型热电材料体系引起了人们的注意,2006年欧盟颁发的“WEEE”和“RoHS”条令,使得方钴矿作为中温段性能最好的热电材料之一,依然是热电领域研究的热点。
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方钴矿的英文名为Skutterudite,因最初是在挪威小镇Skutterud被发现而命名。方钴矿的化学通式为MX3,其中M表示铁(Fe)、钌(Ru)、锇(Os)、钴(Co)等过渡金属元素,X表示磷(P)、砷(As)、锑(Sb)等V族元素[26],其晶体结构如图2所示,具有复杂的体心立方结构,空间群为Im3。方钴矿的每个晶胞中共包含32个原子,为8个MX3单元,其中M原子占据8c位置,形成8个小立方体结构;X原子占据24g位置,每4个X原子组成一个四方环,形成6个四方环结构。在这8个小立方体结构中,有6个小立方体的中心被X原子构成的四方环占据,剩余2个没有被四方环占据的小立方体就形成了两个本征晶格空洞。而方钴矿材料的特点就在于这两个晶格空洞的存在,每个晶格空洞的半径可达1.89Å[27-28],可以在晶格空洞中填充稀土原子、碱土金属原子或镧系原子等其他原子,形成填充方钴矿。填充方钴矿的化学通式为RM4X12,其中R原子为填充原子,每个晶胞中有2个R原子。填充原子在空洞中的位移参数远远大于M原子和X原子,相当于这些原子在一个很大的由原子组成的笼子中“rattling”振动,因而可以极大地降低热导率。同时,方钴矿的电子传输主要由X原子构成的电子轨道决定,与形成“rattling”振动的原子在空间上是分离的,因而电导率受到的影响较小,从而表现出良好的“电子晶体−声子玻璃”特性。
从方钴矿的晶体结构中可知,在CoSb3基方钴矿材料中,Sb原子与Sb原子之间存在较强的共价键,起到了稳定结构的作用,而Sb原子与Co原子之间存在较明显的共价杂化和弱离子相互作用,使材料显示出窄带隙特性[29]。文献[29]系统地研究了CoSb3基方钴矿的电子结构,发现CoSb3的导带底存在三重简并现象,主要由Sb原子轨道、Co原子轨道以及Sb-Co原子间的相互共价耦合组成,而价带顶主要由Sb原子轨道、Sb-Sb原子间的相互共价耦合以及少量的Co原子轨道组成,如图3所示,在CoSb3的能带结构中,导带底和价带顶均位于Γ点,是一种直接带隙半导体,室温环境下其费米能级位于带隙的中间位置,表现出本征半导体材料的特性[29]。
表1为CoSb3基方钴矿材料在室温下的各个物理参数,包括晶格常数、热膨胀系数、电阻率、塞贝克系数、晶格热导率等[30]。
表 1 CoSb3基方钴矿室温下的物理性能参数
性能参数 值 晶格常数/A 9.0345 德拜温度/K 307 热膨胀系数/10−6K 6.36 格林艾森常数 0.952 电阻率/mΩ·cm 1.894 霍尔迁移率/cm2·V−1·s−1 2835 霍尔载流子浓度/1019cm−3 0.116 塞贝克系数/μV·K−1 220 晶格热导率/W·m−1·K−1 10 能带宽度/eV 0.55 从表中可以看到,未填充的CoSb3基方钴矿具有十分优异的电输运性能,其电阻率仅为1.9 mΩ·cm左右,但是晶格热导率很大,达到了10 W·m−1·K−1,过高的晶格热导率导致其热电性能较差,当温度为610 K时,未填充的CoSb3基方钴矿材料的最大zT值仅为0.17。由此可见,降低CoSb3基热电材料的晶格热导率是优化其热电性能的关键。
CoSb3 Based Skutterudites Thermoelectric Materials
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摘要: 热电材料是一种能够实现电能和热能相互转化的新型清洁能源材料。方钴矿热电材料利用其独特的笼状结构能大幅降低热导率,是中温段性能最好的热电材料之一。该文回顾了方钴矿化合物的热电性能,对方钴矿化合物热电性能的主要改善途径做了一些归纳总结,包括替位方钴矿、填充方钴矿、纳米结构方钴矿等。下一步的研究方向是扩展到热电器件的商业应用中。Abstract: Thermoelectric material as a new clean energy material can realize transfer between electricity and heat. Skutterudite compound is one of the best thermoelectric materials in the middle temperature range due to its unique cage-like structure. In this paper, the thermoelectric properties of the CoSb3 based skutterudites compounds are reviewed, and the main ways to improve the thermoelectric properties of the CoSb3 based skutterudites compounds are summarized, including doped skutterudites, filled skutterudites and nanostructured skutterudites. Hope to extend to the application of thermoelectric skutterudites.
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图 1 N型和P型热电材料的zT值随温度的变化趋势[8]
表 1 CoSb3基方钴矿室温下的物理性能参数
性能参数 值 晶格常数/A 9.0345 德拜温度/K 307 热膨胀系数/10−6K 6.36 格林艾森常数 0.952 电阻率/mΩ·cm 1.894 霍尔迁移率/cm2·V−1·s−1 2835 霍尔载流子浓度/1019cm−3 0.116 塞贝克系数/μV·K−1 220 晶格热导率/W·m−1·K−1 10 能带宽度/eV 0.55 -
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