Thermal Conductivity of Materials

Material selection for thermal management starts with thermal conductivity, the property that sets how fast heat moves through a solid. From heat sinks to insulation, the right material depends on whether you want to conduct heat or block it.

What Is Thermal Conductivity?

Thermal conductivity, given the symbol k (sometimes written λ), measures how readily a material carries heat by conduction. It comes from Fourier's law, which says heat flows through a material in proportion to the temperature difference across it and the area it covers, and in inverse proportion to its thickness. A high value means heat passes easily, while a low value means the material resists heat flow and works as an insulator.

In metals, free electrons carry most of the heat, so strong electrical conductors like copper and silver also conduct heat well. Non-metals rely on lattice vibrations instead, so their values run far lower.

Units and How It Is Measured

The SI unit is the watt per metre-kelvin, written W/m·K. Because a kelvin and a degree Celsius span the same size of interval, W/m·K and W/m·°C are numerically the same. In the United States, building and insulation products are often rated in BTU per hour per foot per degree Fahrenheit instead.

Laboratories measure the value in two broad ways. Steady-state methods, such as the guarded hot plate, hold a fixed temperature difference across a sample and read the heat that passes through. Transient methods, such as laser flash analysis and the hot-wire technique, apply a brief pulse of heat and track how fast the temperature responds.

Converting Between Common Units

A few factors cover most everyday conversions:

• 1 W/m·K = 0.5778 BTU/(hr·ft·°F)
• 1 W/m·K = 6.933 BTU·in/(hr·ft²·°F)
• 1 cal/(s·cm·°C) = 418.4 W/m·K

For a worked example, take copper at 385 W/m·K. Multiply by 0.5778 and you get about 222 BTU/(hr·ft·°F). The BTU-inch form is useful for insulation, where thickness is quoted in inches rather than feet.

Materials by Application

Where Thermal Conductivity Matters

The right choice depends on the task. A processor heat spreader or a cookware base needs a high value so heat moves fast and evenly, which is why copper and aluminum lead there. A refrigerator wall or a winter coat needs the opposite, a low value that slows heat loss. Windows, engine parts, and circuit boards sit in between, trading heat transfer against weight, cost, and strength.

Conclusion

Material thermal conductivity spans five orders of magnitude, from diamond above 1000 W/m·K down to aerogel near 0.01. Engineers balance conductivity against cost, weight, and other properties. Copper and silver excel at moving heat, foams and aerogels excel at stopping it, and ceramics bridge both worlds for specialized jobs.

Related Articles

• Understanding Thermal Conductivity: How Heat Moves Through Materials
• Insulation R-Values: Understanding Thermal Resistance