## What is Thermal Conductivity – Definition

The heat transfer characteristics of a solid material are measured by a property called the **thermal conductivity**, k (or λ), measured in **W/m.K**. It is a measure of a substance’s ability to transfer heat through a material by conduction. Note that **Fourier’s law** applies for 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. 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.

From the foregoing equation, it follows that the conduction heat flux increases with increasing thermal conductivity and increases with increasing temperature difference. 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.