Thermal conductivity: how materials transfer heat

Thermal conductivity measures a material’s ability to conduct heat. This article explains the concept, mechanisms, typical materials, measurement, applications, and practical examples such as metal spoons and insulation.

Thermal conductivity is a physical property that quantifies how readily heat energy moves through a material. In practical terms it describes the rate at which thermal energy flows in response to a temperature difference. The symbol commonly used for thermal conductivity is the Greek letter kappa, κ, and its SI units are watts per metre kelvin (W·m⁻¹·K⁻¹). At a basic level, a higher thermal conductivity means a material will transmit heat faster; a low value identifies it as a thermal insulator.

$\kappa$

Mechanisms and material dependence

Heat conduction in solids occurs primarily by two mechanisms: the transport of energy by mobile charge carriers (electrons) and by lattice vibrations (phonons). In metals the free electrons carry most of the thermal energy, which is why many metals are good conductors. In ceramics, polymers and other non‑metallic solids, phonons dominate and their interactions with defects, grain boundaries, and impurities strongly influence conductivity. Thermal conductivity generally depends on temperature, crystal structure, porosity and the presence of defects or composite phases.

Characteristic values and examples

Materials used for heat transfer or thermal insulation span a very wide range of conductivity values. Typical good conductors include silver and copper among common metals, and diamond among solids known for extremely high thermal conductivity. Common insulators include air, vacuum (which eliminates conduction by removing matter), foams and aerogels. Engineers choose materials based on these differences: metals and diamond are selected where rapid heat spread is desired, while foams and fibrous materials are chosen to resist heat flow.

Examples of conductors: silver, copper, aluminum, diamond.
Examples of insulators: vacuum, aerogel, polyurethane foam, mineral wool.

Measurement and formal description

The macroscopic behaviour of heat conduction is commonly described by Fourier’s law, which relates heat flux to the temperature gradient and thermal conductivity. Values of thermal conductivity are obtained using steady‑state or transient measurement techniques tailored to solids, liquids and gases. Steady‑state methods measure a constant heat flow across a specimen, while transient methods track temperature response after a brief heat pulse. Because microstructure and surface conditions affect results, standardised procedures are used in laboratory characterisation.

Applications and practical demonstrations

Understanding thermal conductivity is central to many technologies: heat sinks and thermal interface materials in electronics, cookery and cookware design, building insulation, clothing and cold‑chain systems for refrigeration. Simple demonstrations help illustrate the property: for example, a spoon made from a highly conductive metal in hot water will feel hotter farther from the heat source than a spoon made from a less conductive alloy; likewise, placing small metal objects on ice shows that better conductors transfer heat to the ice more rapidly, making the contact region melt faster.

Distinctions, limits and notable points

Thermal conductivity is distinct from thermal diffusivity and thermal resistance, though the concepts are related. Thermal resistivity is the reciprocal of conductivity and is often used when designing insulation systems. Materials can be anisotropic (conductivity varies with direction) or temperature dependent, and interfaces between different media introduce thermal boundary resistance that can dominate heat transfer in thin films and layered structures. Advances in composite materials and nanoscale engineering have expanded the practical range of thermal conductivities available to designers.