Hot‑Press Fabrication of Bi‑Doped n‑Type Polycrystalline SnSe: Anisotropic Transport and Bipolar Transition

Abstract

We report the successful synthesis of Bi‑doped n‑type polycrystalline SnSe via a hot‑pressing route. The samples exhibit pronounced anisotropy in electrical and thermal transport, attributed to a preferential (h00) grain orientation along the pressing axis. At 773 K, the 8 % Bi‑doped specimen shows an electrical conductivity of 12.85 S cm⁻¹ perpendicular to the pressing direction and 6.46 S cm⁻¹ parallel to it; thermal conductivity values are 0.81 W m⁻¹ K⁻¹ and 0.60 W m⁻¹ K⁻¹, respectively. A bipolar conduction mechanism leads to an n‑to‑p transition whose temperature rises with increasing Bi content. Our findings demonstrate that hot‑pressing can effectively dope polycrystalline SnSe, paving the way for module‑scale thermoelectric devices.

Highlights

1. Bi‑doped n‑type polycrystalline SnSe was fabricated successfully using hot‑pressing.

2. Anisotropic transport properties arise from the (h00) preferred grain orientation along the pressing direction.

3. Bipolar conduction causes a temperature‑driven n‑to‑p transition in the samples.

Background

Thermoelectric materials convert waste heat into electricity or provide solid‑state cooling without moving parts, noise, or pollutants. Their efficiency is quantified by the dimensionless figure of merit, ZT = S²σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the total thermal conductivity. Optimizing ZT requires a high Seebeck coefficient and electrical conductivity while suppressing thermal conductivity—an inherently challenging balance because these properties are interdependent.

Traditional lead‑based thermoelectrics such as Bi₂Te₃ and PbTe have delivered ZT values up to 2.2, but their scarcity and toxicity limit scalability. In contrast, the IV–VI semiconductor SnSe has emerged as a promising lead‑free alternative, achieving ZT ≈ 2.6 at 923 K in p‑type single crystals and ZT ≈ 2.0 at 773 K in intentionally hole‑doped variants. The exceptional performance stems from SnSe’s ultralow lattice thermal conductivity, driven by resonant bonding and strong anharmonic phonon scattering.

While single‑crystal SnSe shows outstanding thermoelectric properties, its brittleness and the difficulty of large‑scale crystal growth hinder practical applications. Polycrystalline SnSe, fabricated by hot‑pressing or spark‑plasma sintering, offers a more manufacturable route, though reported ZT values for doped polycrystalline forms remain below 1.2. This study focuses on Bi‑doped n‑type polycrystalline SnSe, probing its anisotropic transport behavior and bipolar conduction to evaluate its suitability for module integration.

Methods/Experimental

The goal was to synthesize n‑type Bi‑doped SnSe with 0–8 % Bi and assess its thermoelectric performance. All procedures were carried out under an inert Ar atmosphere to prevent oxidation.

Fabrication of SnSe Compound by Temperature Gradient Technique

High‑purity (99.999 %) Sn and Se powders (1:1 atomic ratio) were sealed in an evacuated quartz ampoule (<10⁻⁴ Torr) and then encased in a secondary ampoule for safety. The mixture was heated to 600 °C for 30 h, then to 950 °C for 35 h, maintained for 16 h, and finally cooled slowly to room temperature. The resulting ingot measured 13 mm × 25 mm.

Fabrication of n‑Type Bi‑Doped SnSe Polycrystalline Samples by Hot‑Press Technique

Ingot powder was ground and blended with Bi (0–8 %) for 1 h, loaded into a 13 mm mold, and hot‑pressed at 800 °C under 30 MPa Ar pressure for 30 min, producing dense pellets (13 mm × 15 mm).

Characterizations

X‑ray diffraction (XRD) measured both parallel and perpendicular to the pressing direction. FE‑SEM examined fractured surfaces. Samples were cut into 2 × 1.5 × 8 mm bars for transport measurements and 13 × 13 × 1.5 mm disks for thermal diffusivity. Electrical conductivity and Seebeck coefficient were recorded from 300 K to 773 K using a four‑probe setup under Ar. Thermal diffusivity was obtained by laser flash (model LFA‑457, NETZSCH). Mass density was derived from dimensional measurements; specific heat was taken from literature for polycrystalline SnSe. Thermal conductivity was calculated as κ = DCₚρ.

Results and Discussion

XRD patterns of the 4 % Bi sample in both orientations reveal a pure orthorhombic SnSe phase (Pnma) with minor rhombohedral Bi impurities. The strong (400) peak in the parallel orientation confirms a (h00) grain alignment along the pressing axis.

FE‑SEM images show a layered microstructure with grain sizes increasing from ~3 µm (4 % Bi) to ~10 µm (6 % Bi), indicating that Bi acts as a flux and promotes grain growth.

Transport measurements exhibit clear anisotropy: electrical conductivity is higher perpendicular to the pressing direction. For the 8 % Bi sample, σ⊥ = 12.85 S cm⁻¹ and σ∥ = 6.46 S cm⁻¹ at 773 K. Seebeck coefficients remain negative for Bi‑doped samples, confirming n‑type behavior, but display a temperature‑driven sign change (n‑to‑p transition). Transition temperatures rise with Bi content (e.g., 492 K for 2 % Bi, 730 K for 4 % Bi, 762 K for 6 % Bi). The anisotropy in transition temperature reflects differing carrier mobilities along the two directions.

Power factors are modest, peaking at 0.19 µW cm⁻¹ K⁻² for the 6 % Bi sample along the parallel direction. Thermal conductivity values are 0.81 W m⁻¹ K⁻¹ (⊥) and 0.60 W m⁻¹ K⁻¹ (∥) at 773 K for the 8 % Bi specimen, slightly higher than single‑crystal counterparts, likely due to higher diffusivity and specific heat in the polycrystalline state.

The resulting figure of merit is low (ZT = 0.025 at 723 K for the 6 % Bi sample along ∥), dominated by the small Seebeck coefficient and electrical conductivity. Nevertheless, the observed anisotropy and bipolar conduction provide insight for further optimization of polycrystalline SnSe.

Conclusions

Bi‑doped n‑type polycrystalline SnSe has been successfully produced by hot‑pressing across a range of Bi concentrations. The material displays a pronounced (h00) grain orientation, leading to anisotropic electrical and thermal transport. At 773 K, the 8 % Bi sample shows σ⊥ = 12.85 S cm⁻¹ versus σ∥ = 6.46 S cm⁻¹, and κ⊥ = 0.81 W m⁻¹ K⁻¹ versus κ∥ = 0.60 W m⁻¹ K⁻¹. A temperature‑driven n‑to‑p transition is observed, with transition temperature increasing with Bi content. The optimum doping level is 6 %, yielding a maximum ZT of 0.025 at 723 K. While the current ZT is modest, the demonstrated hot‑pressing approach offers a scalable pathway toward module‑grade thermoelectric materials.