Medios de vida:
2. HUMANITARISMO: ENTRE LA VISIÓN CLÁSICA Y SU MAXIMIZACIÓN HUMANITARIA
2.2 El humanitarismo: entre su carácter apolítico y su objetivo de mitigar los efectos de las decisiones políticas
To investigate the influence of the heating rate on the structural properties, a FeSb3 film with a Sb content of (79 ± 2) at.% was codeposited at room temperature on a Ohara ”‘S-TIH23”’ special glass substrate ([153], see chapter 9) and broken into several pieces. Based on the results of chapter 7.2.2, an annealing temperature of 280◦C was chosen, since decomposition processes can not be excluded for higher temperatures. This annealing temperature was reached by using three different heat-ing rates (94 K/min, 10 K/min, and 3 K/min) and kept constant afterwards for 1 h.
The medium rate of 10 K/min is the standard heating rate usually used for annealing in this work.
The XRD investigation shows for all annealed samples the formation of the skut-terudite phase. Minor phases were not detected, but the background caused by the glass substrate decreases the signal-to-noise ratio and thus the sensitivity for small amounts of impurity phases is also decreased.
The AFM images are similar for the different heating rates (fig. 7.14 top row). No cracks occur since the thermal expansion coefficients of film and substrate are in the same range (see chapter 9). The structures observed on the surface are also found on the blank substrates and caused by polishing. The different heating rates have therefore no visible influence on the film morphology.
To get information about the grain structure and grain sizes, EBSD was performed.
The band contrast maps are presented in the bottom row of figure 7.14. Band con-trast maps reveal a bright concon-trast for rastered points, where a crystalline structure is clearly observed, and a dark contrast for points, where no crystalline structure is detected (like amorphous regions or grain boundaries). The images are comparable to the EBSD images achieved for CoSb3. No amorphous regions can be found and the grain boundaries exhibit a thickness smaller than 50 nm.
7.2 Structural properties of FeSb3 thin films
Figure 7.14: top row) AFM images of FeSb3-films with a Sb content of 79 at.% obtained after annealing at 280◦C by using different heating rates [138]. No differences can be found. bottom row) EBSD band contrast maps of these films. A bright color corresponds to a good identification of a crystal structure, hence grain boundaries occur black. It can be seen, that the grain size is decreasing for faster annealing [138].
A clear dependence on the heating rate is observed by analyzing the grain size. With increasing heating rate the grain size decreases (see fig. 7.14) [138]. Because of posi-tion drifts in vertical direcposi-tion during the EBSD measurement, which was proven by recording a SEM picture before and after the EBSD measurement, it was found, that only the horizontal grain size can be analyzed accurately. The average horizontal grain size was extracted by measuring the size of several grains in the image. For a heating rate of 94 K/min, 10 K/min, and 3 K/min a grain size of 0.9µm, 1.1 µm, and 1.6µm was obtained, respectively. It is therefore possible to adjust the grain size of skutterudite thin films by the heating rate of the annealing process. If the heat-ing rate is large, the crystallization temperature is passed very fast and the crystal growth starts from many different nucleation sides simultaneously. The growth is then limited by collisions of the growing grains. If the heating rate is lower, only a few grains start to grow. The grains can grow further and reach larger size, before they collide with others. Since the annealed FeSb3 films behave in most cases exactly like the annealed CoSb3 films, there is some confidence that different heating rates should yield also for CoSb3 different grain sizes.
Figure 7.15:Thermoelectric properties in dependence of grain size (or heating rate). For increasing grain size the resistivity ρ is decreased, the Seebeck coefficient S increased and therefore the power factor S2/ρ enhanced. Hall measurements reveal a constant mobility and an increased charge carrier density.
Additionally, these films were thermoelectrically pre-characterized at room tempera-ture. The data is presented in figure 7.15. The resistivity decreases with increasing grain size by 50 %. Since a larger grain size yield also a lower fraction of grain boundaries, the lower resistivity could be caused by decreased scattering leading to an increase in mobility. Other possibilities are a lower number of trapping states or barrier effects for the charge carriers occurring at the grain boundaries, which would be indicated by a higher number of charge carriers. Therefore Hall measurements were performed and reveal a constant mobility and an increase of the charge carrier density (see fig. 7.15), which supports the second explanation. A more detailed dis-cussion about the influence of the grain boundaries on the electronic properties are for instance given by Suzuki et al. [23].
By changing the heating rate the Seebeck coefficient is only slightly affected and shows an increase of 6.3 % for larger grain sizes. The Seebeck coefficient exhibits only a weak dependence on grain size, grain boundaries, doping or charge carrier density for these films and its magnitude is dominated by the fact, if the skutterudite phase is formed or not (see chapter 8).
7.2 Structural properties of FeSb3 thin films
To summarize, it is possible to enhance the power factor (fig. 7.15) by a controlled adjustment of the heating rates and thus the grain size of the film. A larger grain size is connected with a lower resistivity and a larger Seebeck coefficient. How the thermal conductivity κ and therefore ZT is influenced by the different grain sizes has to be investigated by future measurements, but it is expected that κ is increased with larger grain size and a balance has to be found between large power factor and low κ for achieving high ZT values.