A new type of crystal could enable refrigerators and air conditioners to keep us cool without warming the planet.

Refrigerators and air conditioners get their cooling power by circulating a liquid through the device, which absorbs heat and causes chilling through a cycle of evaporation and condensation. But many such liquids contribute to the greenhouse effect, causing further warming when they leak. Now, at Deakin University in Australia and her colleagues have made a climate-friendly alternative to these liquids using “plastic crystals” – crystals with molecules that can move just enough to make them pliable.

Under enough pressure, these plastic crystals can transform. Their molecules go from being randomly oriented to aligning themselves into a neat grid. Then, when pressure is removed, they become disordered again. As part of this disordering process, the crystals absorb heat, effectively cooling their surroundings.

Such pressure-based cooling has been investigated before, but most materials capable of this transition could only do so at balmy temperatures, limiting their cooling power, says Pringle. In contrast, the heat-sucking ability of her team’s crystals kicks in at temperatures from -37°C (-34.6°F) to 10°C (50°F), a suitable range for household refrigerators and freezers.

However, the new crystals are not ready to leave the lab yet. That’s because the pressures needed to make them work are very high – hundreds of times greater than atmospheric pressure and equivalent to being thousands of metres underwater, says Pringle.

at the University of Glasgow, UK, says materials like those in the new study have “the potential to almost completely decarbonise this huge [cooling] industry”, but he shares the concern about the high pressures required.

There may be another practical issue with this approach, says at the Chinese Academy of Sciences. With every repeated use, each crystal may absorb less heat, as the grid the molecules form becomes more strained. Still, Li is optimistic and says he is confident the technology could be applied in the “near future”.

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Annalee’s week

An abundant mineral called olivine can help make carbon-negative cement. This process could help tackle cement’s large carbon footprint – the material contributes about 8 per cent of global CO2 emissions.

Olivine is one of the main components of Earth’s mantle and reserves sit on every continent. “It’s one of the few minerals that is available at the gigatonne scale,” says at Seratech, a UK-based company that has patented a process to turn olivine into cement.

Dozens of start-ups like Seratech are developing low-carbon methods to produce cement, such as supplementing with steel by-products or recycling the CO2 released in cement production. Most emissions occur when heating limestone to produce clinker, a binder in cement, along with burning fossil fuels to generate the heat.

Draper and his colleagues looked to the more abundant olivine to find a replacement for some of the usual clinker. Olivine contains silica, which makes cement stronger and more durable. Magnesium sulphate can also be extracted from it, and this salt reacts with CO2 to form minerals that sequester the gas.

The researchers extracted these compounds by dissolving powdered olivine in sulphuric acid. After separating the silica and magnesium sulphate, they bubbled CO2 through the magnesium slurry to form a mineral called nesquehonite. To scale up the process, Draper says a cement plant would use CO2 captured from an emissions source or from the air, rendering the entire process carbon negative. The leftover nesquehonite could be recycled into new construction materials like bricks.

Replacing 35 per cent of the regular cement in a concrete mix with silica from this process would produce a carbon-neutral cement, the researchers estimated, while subbing 40 per cent or more would make it carbon negative. Draper says current building standards allow this type of material to replace up to 55 per cent of cement, although he says they haven’t yet made enough of it for robust testing.

The process utilises well-known reactions, says at the University of Guelph in Canada, but offers a novel and “reasonable” way to combine them. However, some of the chemicals involved may prove tricky to recycle, he says.

Glare-free windows could be made from a material studded with billions of tiny metal patches. This makes a one-way window that is matte – it looks opaque, reflecting barely 1 per cent of light from the outside – but still transparent, which could allow cars and offices to have privacy without blurring the view or significantly lowering the amount of light they let in.

Yun Lai at Nanjing University in China and his colleagues created a glass-like material that has a similar matte finish to frosted glass called a transparent matter surface (TMS). Unlike existing anti-glare coatings and films, you can look at or photograph an object through TMS without it looking blurry.

Lai says that being matte and transparent are typically mutually exclusive properties – to achieve both simultaneously, he and his team covered the material in nano-sized patches that scatter and reflect light in just the right way. They made these components from a reflective metal, such as gold, and materials like silicon that conduct electricity poorly, which reduces how much light they reflect. Then they arranged billions of them on a glass wafer.

They tested it by shining light on it and found that it can reflect as little as 1.3 per cent of light. But it transmitted enough light that a camera with the material overlaid on its lens could still take clear photographs. The researchers also tested the TMS with a camera connected to an AI that labels objects and asked it to identify a tennis ball on the other side. The AI was successful when viewing the ball through the TMS, but labelled it as “unidentified” when viewed through the blur created by a conventional anti-glare film.

“What excited us most in our experiments was the fascinating moment that we first saw the freshly prepared 4-inch sample with our own eyes, without any special equipment. It was difficult to imagine how a material could be clear and undistorted when light passes through it but also have a hazy appearance when reflecting light,” says Lai.

at the University of Exeter in the UK says this material is well suited to produce “quite large windows” and could be useful for reducing light pollution. Reflection is a big contributor to light pollution, and this material could minimise it as it reflects soft, diffuse light instead of a more bothersome bright glare.