Bulk thermoelectric materials are critical for energy conversion applications, leveraging the Seebeck effect for power generation and the Peltier effect for cooling. Among the most studied are bismuth telluride (Bi₂Te₃), lead telluride (PbTe), and silicon germanium (SiGe) alloys. These materials exhibit unique crystal structures and electronic properties that determine their thermoelectric efficiency, quantified by the dimensionless figure of merit (ZT). Their applications span waste heat recovery, refrigeration, and aerospace, though challenges like toxicity and high production costs persist.

Bismuth telluride is a layered van der Waals material with a rhombohedral crystal structure (space group R-3m). Its unit cell consists of quintuple layers stacked along the c-axis, with alternating Te(1)-Bi-Te(2)-Bi-Te(1) sequences. This anisotropic structure results in higher thermoelectric performance along the in-plane direction due to lower thermal conductivity. Bi₂Te₃ exhibits a narrow bandgap of approximately 0.15 eV, making it suitable for near-room-temperature applications. Doping with antimony (Sb) or selenium (Se) optimizes its carrier concentration, achieving ZT values around 1.0 for p-type and 0.9 for n-type at 300 K. Its high Seebeck coefficient (200–250 μV/K) and low thermal conductivity (1.5–2.0 W/m·K) contribute to its dominance in commercial Peltier coolers and portable refrigeration.

Lead telluride adopts a rock-salt cubic structure (space group Fm-3m), with Pb and Te atoms occupying alternating lattice sites. Its bandgap of 0.32 eV allows efficient operation at mid-temperature ranges (500–900 K). PbTe’s high ZT (1.5–2.0) stems from resonant states introduced by doping with sodium (Na) or thallium (Tl), which enhance the Seebeck coefficient without significantly reducing electrical conductivity. Its lattice thermal conductivity is intrinsically low (1.2–1.6 W/m·K) due to anharmonic bonding, further improved by nanostructuring or alloying with selenium. PbTe-based systems are widely used in automotive waste heat recovery and industrial power generation, where temperatures align with its optimal performance window.

Silicon germanium alloys are covalently bonded materials with a diamond cubic structure (space group Fd-3m). Their solid solutions (Si₁₋ₓGeₓ) exhibit tunable bandgaps (0.67–1.12 eV) and high thermal stability, making them ideal for high-temperature applications (900–1300 K). Heavy doping with phosphorus (n-type) or boron (p-type) ensures sufficient carrier concentration. Despite high thermal conductivity (5–10 W/m·K) for pure Si or Ge, alloy scattering reduces it to 3–5 W/m·K in SiGe, yielding ZT values of 0.7–1.0. These materials are indispensable in radioisotope thermoelectric generators (RTGs) for deep-space missions, where reliability at extreme temperatures is paramount.

Performance metrics for thermoelectric materials are governed by the interplay of electrical conductivity, Seebeck coefficient, and thermal conductivity. The figure of merit ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity, defines efficiency. Bi₂Te₃ excels near room temperature, PbTe in mid-range temperatures, and SiGe at high temperatures. Optimizing ZT requires balancing these parameters, often through doping, band engineering, or phonon scattering.

Industrial applications leverage these materials’ energy conversion capabilities. Waste heat recovery systems integrate PbTe modules into exhaust pipes of vehicles and factories, converting unused thermal energy into electricity. For instance, a 10% conversion efficiency in automotive systems can improve fuel economy by 3–5%. Bi₂Te₃-based Peltier coolers are ubiquitous in medical devices, electronic cooling, and beverage refrigerators, offering compact, vibration-free alternatives to compressor-based systems. SiGe alloys power RTGs in spacecraft like Voyager and Mars rovers, where solar energy is insufficient.

However, limitations hinder broader adoption. Bismuth telluride contains tellurium, a rare and expensive element (≈$50–100 per kilogram), and its toxicity raises environmental concerns. Lead telluride’s use of lead poses regulatory and health risks, particularly in consumer applications. Silicon germanium alloys, while non-toxic, require high-purity precursors and energy-intensive processing, increasing costs. Additionally, these materials exhibit performance degradation at temperature extremes, limiting their range.

Research continues to address these challenges. Advances in earth-abundant alternatives, such as magnesium silicides or skutterudites, aim to reduce reliance on toxic or costly elements. Engineering approaches, like graded doping or segmented legs, optimize temperature-dependent performance. Despite progress, bulk thermoelectric materials remain indispensable for their reliability and scalability in large-scale applications.

In summary, Bi₂Te₃, PbTe, and SiGe alloys represent the cornerstone of thermoelectric technology, each excelling in specific temperature regimes. Their crystal structures and electronic properties dictate their performance, enabling applications from refrigeration to space exploration. While toxicity and cost present hurdles, ongoing innovations seek to enhance their sustainability and efficiency, ensuring their role in future energy systems.