Will phosphorescent materials one day light up our cities?


Enlarge / Daan Roosegaarde is the artist behind Van Gogh’s phosphorescent fietspad (cycle path).

Around 1603, the Italian shoemaker and amateur alchemist Vincenzo Casciarolo attempted to melt a particularly dense stone he had found on the slopes of Mount Paderno, near Bologna. No gold, silver or other precious metal turned out as he had hoped. But after the stone cooled, Casciarolo discovered something interesting: if he exposed the material to the sun and then took it to a dark room, the stone would glow.

This “Bologna stone” was the first persistent luminescent substance artificially prepared. Many more would follow and today persistent luminescent materials are used for decorations, emergency lighting, ground markings and medical imaging.

Someday they could give us radiant cities that stay cooler and use less electricity.

A new generation of luminescent materials has the potential to cool cities by re-emitting light that would otherwise be converted into heat. They could also reduce energy consumption, as luminescent sidewalks, illuminated road markers or even illuminated buildings could replace some public lighting. Already, some cities in Europe have installed glowing cycle paths and some researchers have studied the use of glowing paint for road markings.

The Van Gogh cycle path in Eindhoven is inspired by the artist's painting <em>The starry Night</em>.  Glow-in-the-dark paths and roads could potentially save energy for lighting while cooling cities.  (Enlarge to animate this GIF.)
Enlarge / The Van Gogh cycle path in Eindhoven is inspired by the artist’s painting The starry Night. Glow-in-the-dark paths and roads could potentially save energy for lighting while cooling cities. (Enlarge to animate this GIF.)

“It’s better for the environment,” says Paul Berdahl, an environmental physicist now retired from Lawrence Berkeley National Laboratory in Berkeley, California. “If the technology can be improved, we can use less energy … It’s an interesting thing to do.”

Bologna stone, a form of mineral barite, fascinated natural philosophers at the time,]but was never particularly useful. But in the 1990s, chemists developed new types of persistent photoluminescent materials, such as strontium aluminate, that maintained a strong glow for hours after exposure to light. Most of these newer materials give off a blue or green glow, although a few are yellow, red, or orange.

Such photoluminescent materials work by “trapping” the energy of a photon and then re-emitting that energy as lower wavelength light. Sometimes the light is emitted immediately, such as in a fluorescent bulb. Other materials, called persistent luminescent, store energy longer and emit it more slowly.

Over 250 types of luminescent materials have been identified.  Above, they are grouped by a) traces of materials that serve as a luminescent center;  b) the host compound;  and c) the color emitted by the material.
Enlarge / Over 250 types of luminescent materials have been identified. Above, they are grouped by a) traces of materials that serve as a luminescent center; b) the host compound; and c) the color emitted by the material.

These materials that shine brightly for hours open up possibilities, such as glow-in-the-dark cities lit by sidewalks and luminescent buildings. Given that 19% of all global energy consumption is for lighting, and in Europe around 1.6% specifically for street lighting, the potential energy savings are significant, writes the building engineer Anna Laura Pisello and colleagues in the Annual Review of Materials Research 2021.

One problem with the approach is that most luminescent materials don’t glow overnight. Better materials could help solve this problem, says Pisello, of the University of Perugia, which studies energy-efficient building materials. In the meantime, existing materials could be paired with electric lighting that would turn on long enough to recharge the ground marking before turning off again.

Luminescent paint could also provide exterior lighting. Pisello’s lab developed such a phosphorescent paint and, in a 2019 report, simulated what would happen if he painted a public road near a train station with it. By shining through the night, the paint would reduce the energy required for lighting by about 27% in the immediate area, the scientists found.

If this raises concerns for entire cities glowing all night long and adding to harmful light pollution, Pisello says it’s unlikely. Luminescent materials would likely only replace existing lighting, not enrich it. The color of the incandescent materials could be chosen to avoid blue frequencies which have been shown to be particularly harmful to wildlife.

Luminescent materials could also help combat what’s known as the urban heat island effect. Roofs and sidewalks absorb the sun’s energy and emit it as heat, raising the city’s summer temperatures by an average of 7.7 degrees Celsius compared to the surrounding countryside. High temperatures are a potential health hazard and also result in greater energy consumption to cool buildings.

An increasingly common solution is to use “cool” materials that reflect light, such as white paint and light-colored asphalt. It turns out that adding luminescent materials can help even more.

Anna Laura Pisello and her colleagues at the University of Perugia try to create practical sidewalks that glow in the dark.  They experiment with different luminescent substances and test how to add them to the pavement material to achieve the best performance and durability.  Above, samples of luminescent materials and a paving stone in which they have been embedded.

Anna Laura Pisello and her colleagues at the University of Perugia try to create practical sidewalks that glow in the dark. They experiment with different luminescent substances and test how to add them to the pavement material to achieve the best performance and durability. Above, samples of luminescent materials and a paving stone in which they have been embedded.

Anna Laura Pisello

At the Lawrence Berkeley Lab, Berdahl and his team experimented with synthetic ruby, a material that luminescent in sunlight, to create colorful coatings that stay cool. In a first experiment, they reported that a ruby ​​pigmented surface stayed cooler in the sun than a similar colored material without a special pigment.


Pisello’s lab went further and added several persistent luminescent materials to concrete, which stored light energy and slowly released it. Compared to non-luminescent surfaces of the same color, the best of them lowered the temperature of the surrounding air on sunny days by up to 3.3 ° C.

“You can do [a surface] as reflective as possible. But can you go beyond that? The idea is that you can maybe take it a step further by using persistent luminescence as another way to transfer energy … It’s interesting, ”says Patrick E. Phelan, mechanical engineer at the Arizona State University who co-authored an article. on the urban heat island effect in the Annual Review of the Environment and Resources.

There are 250 known luminescent materials, many of which have not yet been investigated for practical applications. Pisello says there is potential for glossy paints and coatings that last longer and shine more in more colors.

“In the short term, the best and easiest solution is to improve what we already have,” she says. This includes fine-tuning the materials so that they diffuse light longer, harder, or in different colors, and ensuring that they continue to function in real environments.

In the longer term, she adds, new classes of engineering materials might work even better. For example, one could look to “quantum dots” – tiny semiconductor particles that can be made to glow and which are already used in biological imaging – or perovskites, materials used in solar cells which are also being studied. for their luminescent properties.

Kurt Kleiner is a Toronto-based freelance science journalist.

This story originally appeared on Knowable Magazine.


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