Abstract The preparation of pure Ca3Co4O9 materials in the form of dense bodies made of nano-sized grains was investigated by combining a sol-gel route, different thermal cycles, a ball-milling post-treatment and final densification with HP-FAST (High Pressure-Field Assisted Sintering). We found that an effective way for obtaining nano-sized compact bodies with only a marginal increase of the particle sizes of the original powder was the operation of HP-FAST at extremely high pressures (up to 430 MPa) and comparatively low temperatures. A ball-milling treatment before HP-FAST compaction was then required to hinder the large plastic deformation occurring when using these pressures. In contrast, un-milled powders could be densified by HP-FAST only at lower pressure and therefore required higher temperatures, thus losing their nanostructure. For all powders, X-ray absorption spectroscopy assessed a mean Co oxidation state slightly higher than III, coupled to absence of localized Co(II) and presence of localized Co(IV). Ball milling induced a decrease of the mean oxidation state of Co coupled to an increased disorder. The latter effect was confirmed by X-ray Absorption Fine Structure. Nanostructure had a complex effect on the different properties of compact bodies. According to the different conditions of HP-FAST, densities of 75–98% were achieved; the decrease in thermal conductivity was of a factor 2–3, while the electronic transport properties – in particular electrical conductivity – of nano-sized compact bodies were reduced to a larger extent with respect to their micro-sized counterparts. This demonstrates that, for bulk Ca3Co4O9 materials, nanostructure was not an effective approach toward a performing thermoelectric material.

中文翻译：

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纳米结构的钙钴氧化物 Ca3Co4O9 作为热电材料。纳米结构对局部配位、钴电荷态和热电性能的影响

摘要 通过溶胶-凝胶途径、不同的热循环、球磨后处理和最终致密化与 HP-FAST（高压）相结合，研究了制备纳米尺寸致密体形式的纯 Ca3Co4O9 材料。 -现场辅助烧结）。我们发现，在极高压（高达 430 MPa）和相对较低的温度下运行 HP-FAST 是获得纳米级致密体的有效方法，而原始粉末的粒径仅略有增加。然后需要在 HP-FAST 压实之前进行球磨处理，以阻止使用这些压力时发生的大塑性变形。相比之下，未研磨的粉末只能在较低的压力下通过 HP-FAST 进行致密化，因此需要较高的温度，从而失去其纳米结构。对于所有粉末，X 射线吸收光谱评估的平均 Co 氧化态略高于 III，加上局部 Co(II) 不存在和局部 Co(IV) 存在。球磨导致 Co 的平均氧化态降低，同时无序度增加。X射线吸收精细结构证实了后一种效果。纳米结构对致密体的不同性质有复杂的影响。根据 HP-FAST 的不同条件，密度达到 75-98%；热导率的降低是 2-3 倍，而纳米尺寸的致密体的电子传输特性——尤其是导电性——相对于它们的微米尺寸的对应物而言降低了更大的程度。这表明，对于块状 Ca3Co4O9 材料，