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Conduction with Large Transport Entropy in Electrically Conducting Oxides and Its Applications to Thermoelectric Materials

导电氧化物中大输运熵的传导及其在热电材料中的应用

基本信息

批准号：
13650740
负责人：
OHTAKI Michitaka
金额：
$ 2.11万
依托单位：
Kyushu University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2003
项目状态：
已结题

项目摘要

We have investigated large transport entropy in metal oxides with particular emphasis on their thermoelectric properties. The summary is as follows:1.Selective substitution for Na and Co in NaCo_2O_4 by Ni revealed that the substitution for Na decreased the electrical conductivity, whereas that for Co decreased the Seebeck coefficient. These results suggest that Ni at the Co sites disturbs the spin-spin exchange between the Co atoms, which has been considered to be responsible for unusually large thermopower of the oxide.2.Layered vanadate LiV_2O_4 and LiVO_2 with the layered crystal structure similar to that of NaCo_2O_4 showed large positive thermopower and rather low electrical conductivity. Attempted p-type doping failed to increase the conductivity, probably due to the large electron specific heat of the oxide comparable to those observed for"heavy-fermion" materials, suggesting large transport entropy in these oxides.3.Large enhancement of thermopower was observed for oxide materials with nanoviod structure. The nanovoid structure was induced by mixing polymer beads of 150 nm in diameter as a void forming agent (VFA) with the starting oxide powder mixture, followed by firing in an inert atmosphere. The electrical conductivity of the resulting sintered samples was almost the same as that of the pristine sample without VFA, yet the thermopower showed intense negative maxima at around 5 00-600 ーC. This enhancement of the thermopower can be attributed to the porous structure in oxides. Moreover, the thermal conductivity of the nanovoid samples showed a marked decrease by 30-35% from room temperature up to 760℃ These results strongly suggest that the nanovoid structure serves as selective scattering centers for phonons.

我们研究了金属氧化物中的大输运熵，特别强调了它们的热电性质。结果表明：（1）Ni对NaCo_2O_4中Na和Co的选择性取代使电导率降低，而Co的取代使Seebeck系数降低。这些结果表明，Co位上的Ni干扰了Co原子间的自旋-自旋交换，这被认为是该氧化物具有异常大的热电势的原因。2.层状钒酸盐LiV_2O_4和LiVO_2具有类似于NaCo_2O_4的层状晶体结构，具有较大的正热电势和较低的电导率。尝试p型掺杂未能提高电导率，这可能是由于氧化物的电子比热与“重费米子”材料相当，表明这些氧化物具有较大的输运熵。通过将直径为150 nm的聚合物珠粒作为空隙形成剂（VFA）与起始氧化物粉末混合物混合，随后在惰性气氛中烧制来诱导纳米空隙结构。所得烧结样品的电导率几乎与不含VFA的原始样品相同，但热电势在约500 -600 ℃时显示出强烈的负最大值。这种热电势的增强可以归因于氧化物中的多孔结构。从室温到760℃，纳米孔洞样品的热导率显著下降30-35%，这表明纳米孔洞结构是声子的选择性散射中心。