Key Takeaways & Executive Findings
- •• • Cr foil barrier layer pre-densification suppresses Sn phase penetration, eliminating Ag diffusion pathways and maintaining a clean interface after aging at 973 K for 30 days, with contact resistivity as low as 0.27 μΩ cm2—critical for long-term device reliability. • • The Hf-free Half-Heusler module achieves a conversion efficiency of 10.4% at a hot-side temperature of 976 K, demonstrating that cost reduction via Hf elimination does not compromise performance when interfacial design is optimized. • • Anomalous Ag diffusion through Cr powder barrier layers is identified as a key failure mechanism, driven by Sn phase penetration—highlighting the necessity of barrier layer morphology control to prevent interfacial degradation. • • The study provides a practical pathway to overcome the trade-off between material cost and device stability, enabling commercial deployment of Half-Heusler modules in mid-to-high temperature waste heat recovery.
Abstract
High-temperature interfacial diffusion in Half-Heusler (HH) thermoelectric devices poses significant challenges for practical applications, particularly the diffusion of Ag from conventional solders, which degrades material performance and device stability. This study reveals anomalous Ag diffusion through a Cr powder barrier layer into Ti0.5Zr0.5NiSn0.98Sb0.02, driven by Sn phase penetration. In contrast, employing a Cr foil barrier layer pre-densified the material, effectively preventing Sn phase penetration and eliminating Ag diffusion pathways, thereby preserving junction integrity. After aging at 973 K for 30 days, the Cr foil junction maintained a clean interface with a low contact resistivity of 0.27 μΩ cm2. Benefiting from this interfacial design, a Hf-free HH module achieved a high conversion efficiency of 10.4% at a hot-side temperature of 976 K, alongside long-term stability. This work addresses critical bottlenecks in developing high-performance, low-cost HH modules, facilitating their commercial application in waste heat recovery.
1. Introduction
Half-Heusler (HH) thermoelectric devices promise efficient waste heat recovery at mid-to-high temperatures, yet their commercialization is hindered by interfacial degradation at electrode-material junctions. Conventional barrier layers, such as Cr powder, fail to block the diffusion of Ag from solders, which penetrates into the TE material and degrades its thermoelectric properties. This diffusion is exacerbated by the penetration of Sn-rich phases, which create fast diffusion pathways. The resulting increase in contact resistivity and loss of mechanical integrity severely limits device lifetime and efficiency.
This study addresses this bottleneck by employing a Cr foil barrier layer that pre-densifies the TE material, effectively closing grain boundaries and preventing Sn phase penetration. This approach eliminates Ag diffusion pathways, preserving a clean interface and low contact resistivity (0.27 μΩ cm2) even after prolonged aging at 973 K. The resulting Hf-free HH module achieves a high efficiency of 10.4% at 976 K, demonstrating that cost-effective, Hf-free materials can be reliably integrated into high-performance thermoelectric generators when interfacial design is optimized.
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Fushan Li, Shuyao Li, Hao Wu, Yu-Ke Zhu, Wenjing Shi, Xin Bao, Yifan Jin, Lankun Wang, Xingyan Dong, Fengkai Guo, Wei Cai, Jianbo Zhu, Zihang Liu, Jiehe Sui (2026). Phase Penetration: Key Drivers in Barrier Layer Failure of Hf-free Half-Heusler Thermoelectric Modules. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3633-5
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Frequently Asked Questions
What is the root cause of Ag diffusion through Cr powder barrier layers, and how does Cr foil mitigate it?
Ag diffusion is driven by Sn phase penetration through the porous Cr powder layer, which creates fast diffusion channels. Cr foil, being dense, pre-densifies the TE material and blocks Sn penetration, thereby eliminating Ag pathways and maintaining interface integrity.
How does the contact resistivity of the Cr foil junction compare to industry benchmarks after high-temperature aging?
After aging at 973 K for 30 days, the Cr foil junction exhibits a contact resistivity of 0.27 μΩ cm2, which is exceptionally low and meets the stringent requirements for high-efficiency thermoelectric modules, ensuring minimal energy loss.
What is the maximum conversion efficiency achieved, and at what operating temperature?
The Hf-free Half-Heusler module achieves a conversion efficiency of 10.4% at a hot-side temperature of 976 K, demonstrating that high performance is attainable without Hf, provided interfacial stability is ensured.
What are the long-term stability implications of using Cr foil barrier layers in commercial devices?
The Cr foil barrier layer maintains a clean interface and low contact resistivity after 30 days of aging at 973 K, indicating excellent long-term stability. This is critical for commercial applications where devices must operate reliably for years.
How does the cost of Hf-free modules with Cr foil barriers compare to traditional Hf-containing modules?
Eliminating Hf significantly reduces raw material costs, and the Cr foil barrier layer is a cost-effective solution compared to complex multi-layer barriers. This combination enables low-cost manufacturing without sacrificing performance or stability.
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