Whiter than ever: paint that reflects up to 98.1% of sunlight

The invention of the bicycle often describes the process of creating something that has already been created. In other words, meaningless work. However, in the scientific world there are many works that can be described with this phrase. However, multiple creation of the same bike by different people allows you to look at it from different angles, thereby improving it. A similar situation exists with materials capable of reflecting a large percentage of solar heat in order to obtain passive cooling without the need for air conditioning systems. This topic has already been touched upon by us earlier (https://habr.com/ru/company/ua-hosting/blog/510582/), but scientists from Purdue University (USA) decided to look at this problem in their own way, creating an ultra-white paint that can reflect up to 98.1% of the sun’s rays. What is the secret of the new paint and varnish material, how was it created, and will its use in practice be really beneficial and environmentally friendly? We will find answers to these questions in the report of scientists. Go.

Basis of research

Radiation (radiative) cooling is the leitmotif of research related to reducing the economic and environmental burden on cooling. This method consists in passive cooling by means of special devices, materials, coatings and other things. Most often, complex multilayer structures or reflective metal layers are used to implement radiation cooling. Of course, there is an effect from them, but this option is not particularly practical and profitable.

Attempts to implement radiation cooling with a single paint layer also often end in failure, because in this case this layer will be very thick, and the cooling effect is insignificant.

However, radiation cooling still has its advantages if implemented correctly. For example, unlike active cooling, which requires electricity, radiation cooling uses an atmospheric transparent window (“sky window”) to emit thermal radiation directly into the deep sky without consuming energy. If the thermal radiation through the sky window exceeds the absorption of sunlight, then a cold environment can remain on the surface even in direct sunlight.

Previously, there have been attempts to create a paint capable of realizing radiation cooling. There was a variant that used a thin layer of TiO2 on an aluminum backing. On a winter day, such a structure exhibited a temperature 2 ° C lower than the ambient temperature. However, according to the scientists, this was likely due to the substrate rather than the paint itself.

There were also options without any paints, based on multi-layer structures. In one such embodiment, a metal layer, polyethylene airgel and delignified wood were used. Obviously, such structures are extremely complex and expensive to implement, not to mention the large thickness of the resulting coating.

In other words, there are quite a few methods for implementing radiation cooling; each of them has a number of advantages and disadvantages. The authors of the work we are considering today decided to try their luck in this area and created another passive cooling method based on the combination of a film of BaSO nanoparticlesfour and paint containing the same nanoparticles.

Research results

BaSO’s choicefour as the protagonist of this work was no coincidence. BaSOfour has a wide band gap, which is good for low solar absorption, and a phonon resonance at 9 microns, which is good for high emissivity. Taking these features into account, it was possible to create a film of BaSO nanoparticlesfour with high sunlight reflectance (97.6%) and transparent window emissivity (0.96).

To increase the stability and reliability of the film, an acrylic paint containing BaSO nanoparticles was created.four (60% of the volume). The high concentration of nanoparticles and their wide size distribution make it possible to reduce the refractive index of BaSOfour, which results in a sunlight reflectance of 98.1% and an emissivity of 0.95. According to scientists, their BaSOfour-acrylic paint has a quality index of 0.77, which is one of the highest among similar structures for radiation cooling. At the same time, their version is reliable, easy to use, and also perfectly implemented in the industrial process of paint production.

Image No. 1

Commercial white paints (TiO2-acrylic) cannot achieve complete cooling due to high absorption in the UV range (due to the band gap of TiO2 at 3.2 eV) and near infrared (NIR) range (due to acrylic absorption).

In this work, a film was made from BaSO particlesfour 150 μm thick on a silicon wafer (1a) in combination with commercial white paint. SEM images (SEM from scanning electron microscope) films BaSOfour (1b) the formation of air voids is visible. Interfaces between BaSO nanoparticlesfour and the air cavity increase the scattering of photons in the film, thereby increasing the overall reflectance of sunlight.

To improve the reliability of the structure, it is necessary to ensure the stability of BaSOfour films to the environment. This is exactly what acrylic paint was used for. However, paint based on BaSOfour (1c) has a low refractive index, unlike TiO2… To fix this, the concentration of BaSO particlesfour in paint was increased to 60%, which is significantly higher than in industrial paints.

Image No. 2

As shown in the picture 2aTo achieve successful cooling below ambient temperature, a high degree of sunlight reflection and a high degree of emissivity are required. To achieve this, it was necessary to reduce the absorption in the UV range. This was achieved at the expense of BaSOfourwith a band gap of ~ 6 eV.

And due to phonon resonance at 9 microns, it is possible to design particles of a certain size so that only one layer is needed to achieve both reflectivity and emissivity. As a result, the optimal particle size of BaSOfour was 400 nm. As a result, the BaSOfour had a sunlight reflectance of 97.6% and an emissivity of 0.96 (2b). These performances are superior to those of commercially available heat reflective paints (sunlight reflectance ranging from 80% to 91%).

Scientists note that the silicon substrate used in their structure was only a foundation, and did not participate in any way in increasing the cooling performance. On the chart 2c shows a comparison of the reflectance of different structures: with a substrate (different material and thickness) and without it. As we can see, the use of a substrate does not affect the cooling capacity of the entire structure in any way.

For paint, the high BaSO versionfour showed the best results: sunlight reflectance – 98.1%; emissivity – 0.95. The physics behind the high reflectivity has been modeled by Monte Carlo method * (2d).

Monte Carlo method * – a method for studying random processes, when they are described by a mathematical model using a random variable generator. The model is repeatedly calculated, and on the basis of the data obtained, the probabilistic characteristics of the process under study are calculated.

The thickness of the paint layer has also been determined through simulations and practical experiments. At a thickness of 400 µm, the maximum values ​​of reflection and emission indicators were achieved, while at other thicknesses they were slightly less: at 200 µm – 95.8%; at 224 microns – 96.2%; at 280 μm – 96.8% (2e).

Image No. 3

Further, field tests were carried out in order to personally observe the work of the created structure. The experiments were carried out on March 14-16, 2018 in West Lafayette (Indiana) with a peak solar radiation of 907 W / m2 and humidity 42% (3a).

The sample temperature dropped 10.5 ° C below ambient overnight and remained 4.5 ° C below ambient even with peak solar radiation. In comparison, commercial paint options were heated 6.8 ° C above ambient temperature under the same test conditions.

Additional tests in Reno, Nevada on July 28, 2018 showed that the cooling capacity reached an average of 117 W / m2 for a daily period at 10% humidity (3b).

The power of thermal radiation increased with increasing surface temperature during the daytime, which compensates for the higher absorption of solar energy. Therefore, estimating cooling capacity without taking into account surface temperature may be an incorrect indicator of cooling efficiency.

Thermal emission power of BaSO filmfour at 15 ° C reached 106 W / m2… Additionally, field tests of BaSO were carried outfour paints (3c and 3d), which remained colder than the environment during the day with a peak solar radiation of 993 W / m2 and humidity of about 50% (measured at noon).

Since the established BaSOfour the paint is intended for external use, it was also necessary to check its reliability. For this, abrasion, outdoor weathering and viscosity tests were carried out.

Image No. 4

During abrasion tests (4a) a pair of abrasive wheels were placed on the sample with a load of 250 g per wheel. The wheels were renewed every 500 cycles, between which the weight loss of the sample was measured. The wear factor was defined as the weight loss (mg) per 1000 cycles. Resulting wear factor BaSOfour paints reached 150, which is comparable to commercial paints (104). The environmental test was carried out quite simply: the sample was placed in the open air for 3 weeks (4b). Throughout the entire time, the sunlight reflectance and emissivity remained virtually unchanged. Viscosity BaSOfour paint was also measured and showed values ​​similar to those for commercial versions (4c).

For a more detailed acquaintance with the nuances of the study, I recommend that you look at scientists report


In this work, scientists once again turned their attention to the issue of radiation cooling, which tempts with its environmental friendliness and economy in comparison with classical methods. Their idea is to use microscopic BaSO particlesfour and creating a two-layer structure. One layer is a film of these particles, the second is acrylic paint, which again contains BaSO particlesfour

As a result, the resulting film was able to show a sunlight reflectance of 97.6% and an emissivity of 0.96. But this is not the maximum that the developed structure can do. By combining the BaSO filmfour with paint, which also contains BaSOfour, it was possible to achieve a sunlight reflectance of 98.1% and an emissivity of 0.95.

Field tests have shown that the temperature of a surface coated with BaSOfour paint was 4.5 ° C lower than the ambient temperature, and the average cooling power was 117 W / m2

In terms of reliability and durability, the resulting paint is in no way inferior to its commercial counterparts. In addition, the implementation of this development in the industry does not require large costs or specific equipment. In other words, it will be quite simple and profitable to create and use such material.

Thanks for your attention, stay curious and have a good work week, guys. 🙂

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