Reliable metal alloy contact for Mg3+δBi1.5Sb0.5 thermoelectric devices

Abstract

Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg3+δBi2-xSbx exhibits very good TE performance near room temperature, which makes Mg3+δBi2-xSbx-based compounds highly promising candidates to replace the Bi2Te3-ySey alloys, but ideal contacts that can match their TE performance have not yet been well studied. Here we investigate different metal (Ni and Fe) and metal alloy (NiFe, NiCr, NiCrFe, and stainless steel) contacts on n-type Mg3+δBi1.5Sb0.5. It is first shown that the low Schottky barrier and narrow depletion region resulting from the band degeneracy and high carrier concentration of a heavily doped TE material are beneficial for the formation of a low-resistivity ohmic contact with a metal or a metal alloy. Most fully optimized TE materials can take advantage of this. Second, it is found that the NiFe/Mg3+δBi1.5Sb0.5 contact exhibits excellent thermal stability and the lowest ohmic contact resistivity among those studied after aging for over 2100 h, which is attributed to the formation of metallic NiMgBi between the NiFe and Mg3+δBi1.5Sb0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg3+δBi1.5Sb0.5/Bi0.4Sb1.6Te3 unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K.

Mg3Sb2-based Thermoelectrics: Materials, Interfaces, and Devices

“…optimizing TEcMs 10,38 , designing TEiMs 29,[34][35][36]58 , and applying protective coating 28,58 . Typically, He et al successfully fabricated reliable TE devices by higher temperature sintering and Mg heavy overcompensation, and the robust modules are proved via thermal shock testing over 30,000 times when T h varied between 50 and 227 °C 38 .…”

“…assembling 24,25 . As a result, there is still a yawning gap between theoretical and experimental conversion efficiency (η), and output power density (ω) for the reported Mg 3 Sb 2 -based TE devices [18][19][20][26][27][28][29][30][31][32][33][34][35][36][37][38] .…”

“…It is difficult to balance the high σ s and low ρ c for a single-element TEiM; thus, the synergetic achievement in low ρ c and high σ s is a great challenge for the whole TE community because of the mutual exclusivity. 10,18,20,29,30,[34][35][36][37]62 are included for comparison.…”

“…Compared with quick and significant progress in the Mg 3 Sb 2 -based TE materials, the Mg 3 Sb 2 -based devices progress hardly due to the challenges in electrode interface design and device assembling [24][25] . As a result, there is still a yawning gap between theoretical and experimental conversion efficiency (η), and output power density (ω) for the reported Mg 3 Sb 2 -based TE devices [18][19][20][26][27][28][29][30][31][32][33][34][35][36][37][38] .…”

Interface and Surface Engineering Realized High Efficiency of 13% and Improved Thermal Stability in Mg3Sb1.5Bi0.5‐Based Thermoelectric Generation Devices

“…Some alloying TEiM layers for n‐type Mg 3 Sb 1‐ x Bi x TEcM legs, including 304 stainless steel (304SS), [ 36 ] Mg 2 Cu, [ 10 ] Fe 90 Sb 10 , [ 37 ] and Mg 3.4 Sb 3 Ni [ 38 ] were subsequently reported to show lower ρ c values. Very recently, Song et al reported that the NiFe/Mg 3 Bi 1.5 Sb 0.5 contact exhibits a highly stable ρ c of 13 µΩ cm 2 after aging for over 2100 h. [ 39 ] The authors have suggested an alloying approach to design the TEiM rather than using available alloys, which has already been verified as an effective technique route with some necessary characteristics, including high bonding propensity, coefficient of thermal expansion (CTE) matching, diffusion passivation, and dopant inactivation. [ 25 ] For example, in our previous work, we selected Fe from 15 elements as the matrix element, considering the criteria of high σ s and low ρ c , and further designed two ternary alloys (Fe 7 Mg 2 Cr and Fe 7 Mg 2 Ti) with σ s > 40 MPa and ρ c < 5 µΩ cm 2 according to the above principles.…”