Electronic Transport in Alloys with Phase Separation (Composites)

Abstract

A measure for the efficiency of a thermoelectric material is the figure of merit defined by 2 ZT S T ρκ = , where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the "Giant Hall effect"? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of () 1 σ ρ = , κ , S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ , κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Electron Theory of Metals - Answers to Unsolved Problems/Questions

“…A comparison has shown that other previously derived thermopower and Hall coefficient formulas for composites are based on approximations that may produce erreneous results (Sonntag et al, 2019). Therefore, in the following the Equations ( 2) and (3) are applyed for a discussion of the previously mentioned unsolved questions/problems.…”

Composites to Produce a Material with Zero Absolute Thermopower S = 0 or a Thermopower Switch between S = 0 and S ≠ 0

“…The essential difference between these two methods of deposition is the fact, that in the co-sputtered films there is a common for all the x realized during the deposition run, whereas for the films sputtered from different Cr Si targets, is different for the different x . Between these two series of a -Cr Si films there are also considerable differences regarding their specific resistivity versus x dependences ([ 23 ], Figure 7 therein).…”

“…On the first look all these formulas look different. However, comparing their formulas in detail, all provide an identical concentration dependence ([ 23 ], Section 2.5.1. therein), versus x , and what is especially remarkable, all these formulas contain , and . Reason for this fundamental difference to the formula Equation ( 1 ) is the fact, that the authors start with an approximation of the thermal current density, , for instance Webman et al [ 33 ], and are the total thermal conductivities in the composite and the phase i , respectively.…”