S.N. Zhang

Improved thermoelectric performance in the Zintl phase compounds YbZn2−xMnxSb2 via isoelectronic substitution in the anionic framework

It has been shown previously that the thermoelectric properties of the Zintl phase compound, YbZn2Sb2 can be finely tuned via substitution at the cationic Yb-site. Here we report the results of the investigation of isoelectronic substitution of Zn by Mn in the anionic (Zn2Sb2)2− framework. The p-type YbZn2−xMnxSb2 (0.0⩽x⩽0.4) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grounding, and hot-pressing densification processes. In samples with x=0.0, 0.05, 0.10, 0.15, 0.2, 0.3, and 0.4, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been performed as a function of temperature from 300to750K. It is found that the Mn substitution effectively lowers the thermal conductivity for all samples, while it significantly increases the power factor for x⩽0.15. As a result, a dimensionless figure of merit ZT of ∼0.61–0.65 has been attained at 726K for x=0.05–0.15 as compared to the ZT of ∼0.48 in the unsubstituted YbZn2Sb2.

Preparation and thermoelectric properties of bulk in situ nanocomposites with amorphous/nanocrystal hybrid structure

The amorphous partial crystallization method has been applied to fabricate Ge–Te based bulk in situ thermoelectric amorphous/nanocrystal composites. High-resolution transmission electron microscopy showed that nanocrystals of only 4–8 nm were formed by nanoscale phase separation from the semiconductor amorphous matrix during annealing between the glass transition and the crystallization temperatures. The electrical conductivity and the thermoelectric power factor of the amorphous/nanocrystal composite annealed at 443 K for 2 h were about three orders of magnitude larger than those of the amorphous precursor. The amorphous nanocomposite exhibited a lower thermal conductivity and a higher figure of merit, compared with the crystalline GeTe alloy. This work provides a new approach to develop new bulk nanocomposites for thermoelectric applications.

Microstructure and electrical properties of quenched AgPb18Sb1−xTe20 thermoelectric materials

Bulk AgPbmSbTe2+m compounds exhibit outstanding thermoelectric properties. It is believed that their preparation by a higher cooling rate could improve the homogeneity of these materials, and thereby one expects a certain impact on their thermoelectric properties. We have prepared AgPb18Sb1−xTe20 compounds where x = 0.0, 0.1, 0.3 and 0.5 by quenching the melts in liquid nitrogen. High-resolution transmission electron microscopy has confirmed that the homogeneity of the samples is improved with respect to the samples prepared by furnace cooling. The decrease in the electric conductivity and the increase in the absolute value of the Seebeck coefficient, we ascribe to the improved homogeneity of the samples. The same effects on the conductivity and on the Seebeck coefficient are obtained with increasing Sb content. The influence of the Sb concentration are interpreted as follows: the number of Ag–Sb pairs and thereby the widening of the energy band increase with increasing Sb content.

Determination of bending stress of Si wafer using concentrated load

The technique of concentrated load with a simple O‐ring supporter is used to measure the deflection of Si wafers. The load varies so that the ratio of the deflection to the wafer thickness changes from 0 to 1. For some samples, this ratio goes up to 1.4 at which the samples are fractured. It is observed in the experiment that the stress of the wafer can be described by the linear deflection theory for small deflection. However, when the deflection is larger than 1/5 of the wafer thickness, nonlinear deflection theory should be used and the stress in the middle plane cannot be ignored anymore. The total stress for large deflection is approximately equal to the sum of the stress of a linear elastic plate and the stress in the middle plane. The experiment also shows that the bending strength of the Si wafer strongly depends on the surface conditions. Mirror finishing surface gives high mechanical strength.