What does Thermal Conductivity Mean?

Thermal conductivity can be a primary factor in selecting one material over another, especially when deciding between metal and non-metal engineering materials. But what is thermal conductivity, and why is it important to choose a metal for your application?

What is Thermal Conductivity?

Thermal Conductivity is a physical property of a material, and refers to the materials ability to conduct or transfer heat. The SI unit is W/mK or watts per meter kelvin.

It is a property that can be easily visualized; consider how hot a steel frying pan handle gets when compared to a wooden or plastic-covered one. This is solely due to the metal handle having a much higher thermal conductivity than the wood or plastic, transferring the heat from the stove easily.

The difference between different metals is not as large as metal to plastic, but it does still vary, read on for a table comparing common engineering materials.

Why is Thermal Conductivity Important?

The thermal conductivity of a material can have an effect on both the manufacturing process and the performance of the completed part. Any part that encounters changes in temperature may need to have the ability to transmit or restrict that temperature change to adjoining parts.

One key example of this is a heat sink, these are used on all kinds of electrical equipment where heat is generated at specific points, such as a processor on a computer motherboard. As heat sinks are designed to remove and dissipate heat, materials with high conductivity such as copper and aluminium are commonly used.

Conversely, a material having poor thermal conductivity can be an asset, such as in heat shields on engines or other high-temperature assemblies. Stainless steel is often used in this application, as its low thermal conductivity means heat transfers through the material more slowly.

Thermal Conductivity and Manufacturing

As well as considering how a component will perform in service, it’s also important to consider the effect thermal conductivity can have on manufacturing, especially as so many processes involve high temperatures.

One issue commonly caused by low thermal conductivity in particular is warping when welding. Stainless steel is infamous for this issue, owing to its low conductivity but relatively high thermal expansion rate.

What Affects a Metal’s Thermal Conductivity?

There are seemingly few trends to the thermal conductivity of metals, with hardness and strength being mostly irrelevant, one property that does tend to increase alongside thermal conductivity is density. The more dense a metal is, the closer-packed the atoms are, enabling an easier path for thermal energy to pass through.

That said, some metals, such as aluminium, do not follow this trend, being less dense but more thermally conductive than many other metals including steel, stainless steel and zinc.

This shows there is more at play than just density, one other factor is the atomic structure of the metal, which is also a critical factor in strength and other mechanical properties. This is down to the grain boundaries in the material, they inhibit heat transfer, therefore materials with larger grain sizes typically have better heat transfer.

Typical Thermal Conductivity in Metals

The thermal conductivity within metals ranges wildly and is not necessarily linked to other properties such as hardness, corrosion resistance, or strength. Below are some common engineering metals:

How is Thermal Conductivity Measured?

To ascertain the above figures, there are a few different testing methods, the most commonly used process for metals is the Guarded Hot Plate method.

The guarded hot plate test involves sandwiching a sample piece of the material between an electric heating element and a cooling plate. The sample is heated via the hot side, and an internal probe measures the rate of the heat transfer.

This can then be used to determine the differential between the hot and cold side, this combined with knowing the sample thickness and surface area, allows thermal conductivity to be calculated using Fourier’s Law.

What Are the Best Metal Conductors?

If heat and electric conductivity are important to your application, it’s important to select the correct material for the task, here are some materials you might consider, depending on your need:

Thermal Conductivity

Copper and Aluminium are two of the most thermally conductive, commonly used engineering materials. Silver and Gold surpass them, but are extremely expensive in comparison.

For the peak of thermal conductivity, Diamond is the most thermally conductive material known to man, but that is prohibitively difficult to manufacture and purchase!

Electrical Conductivity

In this case, the best materials for thermal conductivity are also the best for electrical, at least within a typical engineering budget, Copper is the most commonly used metal for electrical conductors, followed by aluminium.

Again, Gold and Silver can offer even higher conductive properties, but start at around 50x the price of copper.

What are the Worst Metal Conductors?

In certain applications, poor conductivity is the goal, components such as heat shields and enclosures are examples of this.

Thermal Conductivity

One of the worst metals for conducting heat is Stainless Steel, it is also extremely easy to work, allowing all traditional welding and forming processes to be used with ease. This makes it ideal for heat shields.

For higher strength, or high-temperature resistant material, Titanium offers nearly as low thermal conductivity as stainless steel, it is more expensive and harder to work with, though.

Electrical Conductivity

Whilst it’s true that some metals conduct electricity better than others, all metals do conduct electricity in some form. Bismuth and Tungsten are among those with the lowest conductivity, if you need electrical isolation, though, insulating plastics are the best bet.

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