Ultra-thin coatings: a little goes a long way

The ability of glass to provide passive climate control has advanced rapidly in recent years, and this is largely thanks to developments in coating technology.

Complex multi-layer coatings that incorporate many microscopically thin layers of different materials at varying thicknesses change the properties of the glass in terms of the amounts and types of light and heat that can pass through.

This is an impressive achievement, especially given the whole ‘stack’ of materials building-up the coating is around 1000 times thinner than a human hair – just 100 to 200 nanometres. This means a tiny amount of material can be used to tweak the properties of a huge area of glass.

For example, in one recent project, just 8kg of silver – a piece about the size of a laptop – was enough to coat the 50,000 square metres of glass covering a 230-metre-tall tower. That’s an area equivalent to 192 tennis courts.

So how do ultra-thin coatings work?

Performance at the surface

When it comes to the way light and heat energy behaves when it encounters a pane of glass, the nature of its surface plays a huge role.

The science is extremely complex, and the mechanisms that determine the properties of glass is an area of ongoing academic study. However, extensive experimentation and development over the years have resulted in a wide choice of coatings being available to specifiers offering different performance benefits.

Coatings vary in their composition from a single layer of material to more than 10 in some cases.

Commonly applied materials include zinc, tin, aluminium and silver, as well as compounds of these materials with other elements including oxygen or nitrogen.

The choice and order of materials – as well as the exact thickness of each layer – is a closely guarded secret for glass manufacturers, as this make-up determines the properties the coating lends to the glass.

Hi-tech manufacturing

The process used to apply these extremely thin and precise coatings to glass is called magnetron sputtering, and it works by bombarding a piece of the coating material with ions -  positively charged atoms – at high speed, releasing very small particles of the material, which then stick to the glass.

While this is a relatively simple process in principle, performing it at scale in practice requires highly specialised equipment, as the glass needs to pass at a closely controlled speed through a series of vacuum chambers in which the sputtering takes place.

The glass must also be very carefully washed so it is perfectly clean when it enters the coater, and high-resolution quality control processes are also needed to ensure the material has been applied evenly across the whole surface.

Climate control

In the UK, the most popular application is low emissivity (low-e) glass. This reduces the amount of thermal infrared energy that can pass outwards through the glass, helping to keep heat on the side where it originated, while still allowing the wavelengths of light visible to the human eye to pass.

This means that less heat escapes through windows in a home, for example, and the spaces inside become easier and cheaper to heat while allowing occupants to enjoy clear views and daylight-flooded rooms.

Another application that is already widely used in warmer climates, but which is also gaining ground here as homes have become more airtight, is solar-control coated glass.

These coatings reduce the amount of thermal energy in sunlight that enters a building, lessening the likelihood of overheating when direct sunlight passes through the glass.

Using these two technologies together – with low-e glazing throughout and solar control glazing on south-facing windows that receive direct sunlight – can make a big difference in maintaining a stable climate all year round without over-reliance on heating and air conditioning.

So, applying just a very small amount of some other materials can have a drastic impact on the way glass performs and, therefore, how whole buildings perform.