The thermal expansion coefficient (TEC) measures how a material’s dimensions change with temperature. For tungsten disulfide (WS₂), a layered hexagonal material, the TEC varies depending on the crystallographic direction due to its anisotropic structure. Along the a-axis (in-plane, within layers), the TEC is approximately 6-8 × 10⁻⁶ K⁻¹, while along the c-axis (out-of-plane, between layers), it is higher, around 12-15 × 10⁻⁶ K⁻¹, reflecting weaker van der Waals bonding between layers compared to strong covalent W-S bonds within them.
This anisotropy arises from WS₂’s 2H crystal structure, where layers of tungsten atoms are sandwiched between sulfur atoms, stacked via weak interlayer forces. When heated, the layers expand more readily along the c-axis as van der Waals gaps widen, while in-plane expansion is constrained by the robust W-S lattice. These values are derived from X-ray diffraction (XRD) studies tracking lattice parameter changes with temperature, typically up to 500°C in air or higher in inert conditions before oxidation or decomposition occurs.
Compared to metals like steel (TEC ~12 × 10⁻⁶ K⁻¹) or ceramics like alumina (8 × 10⁻⁶ K⁻¹), WS₂’s in-plane TEC is moderate, while its c-axis value is closer to that of layered materials like graphite (c-axis ~25 × 10⁻⁶ K⁻¹). The lower in-plane TEC reflects WS₂’s stability under thermal stress, beneficial for applications like thin films or coatings where dimensional consistency is key. However, the higher c-axis TEC can lead to delamination or cracking in thick multilayered structures if thermal gradients are significant.
Temperature affects the TEC slightly, with values increasing marginally as bonds weaken near decomposition (1250°C in inert conditions) or oxidation (510°C in air). However, within typical operating ranges (e.g., -50°C to 400°C), the TEC remains relatively stable, supporting WS₂’s use in high-temperature lubricants or composites. In nanoscale forms (e.g., nanosheets), the effective TEC may differ due to defects or edge effects, but bulk values provide a reliable baseline.
The TEC is crucial for integrating WS₂ into systems with dissimilar materials, as thermal mismatch can cause stress or failure. For example, in WS₂-coated bearings, a low in-plane TEC minimizes distortion, enhancing durability. Accurate measurement requires techniques like dilatometry or XRD, adjusted for WS₂’s anisotropy, ensuring design precision in thermal environments.
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