Figure 1 shows the measured Seebeck coefficient for the VPS and hot-press samples. Seebeck coefficient of Ca3Co4O9 films at room temperature was found to be comparable to those of the single-crystal samples, which shows that the Seebeck coefficient is relatively insensitive to microstructure. However, the Seebeck coefficient is very sensitive to carrier concentration and is inversely proportional to electrical conductivity because of the opposite dependence of carrier concentration.
FIG. 1. Seebeck of Mg2Si by VPS and hot press, VPS with 21 kW (V3) has highest Seebeck coefficient and hot press with Mg rich (H2) has the lowest.
Figure 2 validates that the Seebeck coefficient decreases with increased carrier concentration. Generally as temperature increases, the Seebeck coefficient will increase in magnitude because the ratio Nc=n in Eq. (1) is proportional to expðkTÞ. The fact that the Seebeck coefficient is negative indicates that un-doped Mg2Si is an n-type semiconductor. The curve in the above figure is of the form S ¼ A ln n B, with A ¼ 81:143 and B ¼ 610:55. The Seebeck coefficient for hot press sample H2 is the lowest because the carrier concentration is highest among all the samples. Also, VPS sample V2 with Mg-rich powder has a lower Seebeck coefficient than VPS samples with nominal ratios of Mg because more Mg in the sample increase carrier concentration and thus decreases Seebeck coefficient.
FIG. 2. Seebeck coefficient variations due to carrier concentration change at room temperature.
For the hot-press samples, the Mg-rich sample H2 had a lower Seebeck coefficient because of its larger carrier concentration. The hot press samples had carrier concentrations larger than all of VPS samples V1, V2, and V3, resulting in a lower Seebeck coefficient, compared to the VPS samples.
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