The heat transfer characteristics of solid material are measured by a property called the **thermal conductivity**, k (or λ), measured in **W/m.K**. It measures a substance’s ability to transfer heat through a material by conduction. Note that **Fourier’s law** applies to all matter, regardless of its state (solid, liquid, or gas). Therefore, it is also defined for liquids and gases.

The **thermal conductivity** of most liquids and solids varies with temperature, and for vapors, it also depends upon pressure.

**What is Thermal Conductivity – Definition**

Most materials are very nearly homogeneous. Therefore, we can usually write ** k = k (T)**. Similar definitions are associated with thermal conductivities in the y- and z-directions (k

_{y}, k

_{z}), but for an isotropic material, the thermal conductivity is independent of the direction of transfer, k

_{x}= k

_{y}= k

_{z}= k.

The previous equation follows that the conduction heat flux increases with increasing thermal conductivity and increasing temperature differences. In general, the thermal conductivity of a solid is larger than that of a liquid, which is larger than that of a gas. This trend is due largely to differences in **intermolecular spacing** for the two states of matter. In particular, diamond has the highest hardness and thermal conductivity of any bulk material.