夹层对MxIn4Se3-δ化合物热电性能的影响与机理研究

The In4Se3 semiconductor crystal forms a layered structure of (In3)5+ clusters covalently bonded to Se ions in the b-c planes held together by van der waals interactions along the a axis. The In4Se3 compounds show high negative thermopower (Seebeck coefficient, S) (～-500μVK-1) and very low thermal conductivity (κ) (～1Wm-1K-1) at room-temperature owing to the inherent quasi-two-dimensional structure and lattice distortion (charge density wave and Peierls distortion). The thermoelectric properties of polycrystalline In4Se3 samples have been investigated by several groups and reported to have a maximum ZT (=σS2/κ) about 1 at about 700K. It is significantly that In4Se3 compounds are promising applications in "environmentally friendly" power generation. However, the electrical resistivities (ρ) of In4Se3 are too high (～10-3Ωcm) for the materials to be good thermoelectric materials with very high ZT due to the low carrier concentration (～1017cm-3), resulting in the low power factor (PF). So there is much room for improvement in ZT for In4Se3 if a key strategy could be found to significantly reduce the electrical resistivity without too much affecting the thermopower. According to the phonon-glass and electron-crystal (PGEC) concept and the experimental results of the thermoelectric properties for intercalation compounds NxTiS2 (N=Bi, Sb, Nd), the elements like Bi, Pb, Gd, Nd could intercalate into the van der Waals gap of In4Se3, forming MxIn4Se3 (M=Bi, Pb, Gd, Nd…) intercalation compounds. The valence electrons of intercalation atoms (M) transfer to the host of In4Se3, leading to increase of electrical conductivities (σ) of MxIn4Se3, which may originate from the increase of carrier concentration. It is speculated that the power factor could be improved, though the thermopower of MxIn4Se3 samples decrease with increasing carrier concentration. In addition, the intercalation atoms may reduce thermal conductivity of MxIn4Se3, which may be presumably attributed to the enhancement of the phonon scattering by atomic "rattling" in the van der Waals gap of In4Se3. As a result, the ZT values would be improved obviously. Furthermore, it was reported that the control of Se deficiency δ could enhance the thermoelectric properties of In4Se3-δ caused by self doping. Thus, it is possible that the ZT values of MxIn4Se3-δ could be enhanced further by control of intercalation atoms and Se deficiency with appropriately optimized routes. Therefore, in this research, the mechanism and effect of intercalation elements and Se deficiency on the thermoelectric properties, charge density wave (CDW) and Peierls distortion for MxIn4Se3-δ prepared by the ball milling and hot-pressing approach will be investigated carefully by adjusting the intercalation atoms (M), the content of intercalation atoms (x), and the Se deficiency (δ), which could provide reliable experimental basis and valuable guidance for the improvement of thermoelectric properties of In4Se3 compounds.

In4Se3-δ具有内禀准二维结构，室温下热电势S较高(～-500μVK-1)，热导率κ较低(～1Wm-1K-1), 700K时ZT值约为1，具有广阔的应用前景。但其载流子浓度n较低(～1017cm-3)，电阻率ρ较大(～10-3Ωcm)，使得功率因子PF较低。如何有效降低ρ并维持较大S成为提高ZT值的关键。对此问题，我们将Bi、Pb、Gd、Nd等元素引入范德瓦尔斯层，形成夹层化合物MxIn4Se3-δ，M的电荷转移可增加n，降低ρ，提高PF；而M的"晃动"可增强声子散射使κ减小，从而提高ZT值。此外，利用夹层与δ的调节组合来提高ZT值。本课题采用球磨结合热压技术制备MxIn4Se3-δ多晶材料。通过调节M、x、δ，研究其对MxIn4Se3-δ热电性能的影响规律和机理，探讨其对In4Se3-δ的电荷密度波和派尔斯畸变的影响规律及机制，为提高In4Se3-δ的热电性能奠定可靠的实验基础。

采用溶胶-凝胶方法结合高温烧结制备了钴基氧化物YCoO3和Ca3Co4O9体系样品，并系统地研究了掺杂对该体系电输运和热电性能的影响。. 随着Sr掺杂含量的增加，Y1-xSrxCoO3 的电阻率和热电势逐渐减小，这主要是Sr替代后空穴浓度增加所致；随着Sr替代量的增加，Y1-xSrxCoO3的功率因子在650K达到极值，比YCoO3的功率因子提高了大约5倍，这表明Sr替代可以有效提高YCoO3体系的高温热电性能。对于Ni掺杂YCo1-xNixO3体系，随着Ni的替代含量的增加，YCo1-xNixO3的电阻率和热电势逐渐减小，这主要是Ni掺杂后空穴浓度增加所致。尽管样品的热电势有所下降，但在300-740K温区内，YCo1-xNixO3的热电功率因子明显增加。例如YCo0.98Ni0.02O3 的功率因子是YCoO3 的6倍。因此，Ni替代Co可以有效地提高YCoO3的高温热电性能。对于Mn掺杂YCo1-xMnxO3体系，随着Mn的掺杂量的增加, YCo1-xMnxO3低温电阻率先增加(x≤0.01)然后逐渐地减小，这主要是Mn掺杂引入电子所致。Mn替代后，样品YCo1-xMnxO3 (x≠0)热电势正值变为负值，这种变化可以由霍尔系数的变化来解释。当Mn掺杂量为1%时，YCo1-xMnxO3体系的热电势就发生了正负值变化，这种变化可以用双载流子模型解释。Mn替代后YCo1-xMnxO3体系功率因子增加，Mn的替代可以有效地提高YCoO3的高温热电性能。. 随着Fe (Al)掺杂含量的增加，Ca3(Co1-xMx)4O9 (M=Fe，Al)的电阻率和热电势逐渐增加，这主要是Fe(Al)替代后空穴浓度减小所致；随着Fe(Al)替代量的增加，Ca3(Co1-xMx)4O9的热导率显著减小，这主要有掺杂后声子散射增强所致。由于电阻率的增加，M掺杂不能有效改善Ca3Co4O9的热电性能。.