Brief Study of Nano-transparent Thermal Insulation Coating

In Australia and New Zealand, the penetration rate of glass nano-transparent insulation coating is above 75%. From the Pentagon, the White House, the Sydney Opera House, and the FBI headquarters to many civilian buildings, the use of glass nano-transparent thermal insulation coating is everywhere.

In the European region, the penetration rate of nano-transparent thermal insulation coatings for architectural glass has also reached over 80%. Among them, the penetration rate of transparent thermal insulation coatings for architectural glass in the Netherlands, Denmark, Switzerland and other countries are more than 85%. In Asia, except for Hong Kong, Taiwan, Japan and Korea, the average penetration rate of transparent thermal insulation coatings for architectural glass in other countries is less than 20%.

Using thermal insulating transparent coatings on building glass is one of the important ways to save energy in buildings. Tin-based coatings such as indium tin oxide (ITO) and antimony tin oxide (ATO) have high transparency and excellent thermal insulation and have been the focus of research in the field of thermal insulating transparent coatings.

In recent years, the use of large windows and glass curtain walls has become common in Chinese buildings. While improving the interior view, they allow sunlight to shine into the interior. The indoor temperature rises significantly in summer, and in winter, it causes the energy from the interior to radiate to the exterior. This reduces the indoor temperature and increases energy consumption for cooling in summer and heating in winter.

Building energy consumption accounts for 30% to 40% of total energy consumption. About half of them are caused by building cooling or heating. The heat-insulating transparent coating is a safe and energy-saving material, and it is widely used in construction in Europe, America, Southeast Asia and Japan, Korea, and other countries.

The use of heat-insulating and transparent energy-saving coatings on building glass can reduce building energy consumption. This is of great significance to alleviate the energy crisis and maintain the sustainable development of our economy.

Nano metal oxide powders have ideal selectivity for the solar spectrum. It has high transmittance in the visible region and good shielding for infrared light. And tin-based transparent semiconductor materials such as nano-ITO, ATO and gallium tin oxide (GTO) have even higher transparency and conductivity. In recent years, researchers have actively explored the application of nano-tin-based materials in thermal insulation transparent coatings and developed some new products.

Nano-SnO₂ is an important chemical material with a wide band gap (3.6eV) and high exciton binding energy (130meV). It is a tetragonal rutile structure with good electrical conductivity, flame retardancy, and absorption of infrared radiation, light shielding, and stable chemical properties.

In recent years, scholars have devoted themselves to studying the effects of dopants on the properties of SnO₂. Theoretically, SnO₂ has a lattice defect and is an n-type semiconductor whose conductivity is between that of conventional semiconductors and metals.

A light wave is a kind of electromagnetic wave. The electric field of light waves causes the electrons inside the metal to move, thus absorbing the energy of light. Therefore, the higher the conductivity of metal the more energy it absorbs from light.

New tin-based materials are prepared by doping indium, antimony, ytterbium, gallium and other elements in a certain proportion into SnO₂. Such as ITO, ATO, nano-tin ytterbium oxide (YTO), etc. Can increase the carrier density of SnO₂. to make it appear cavity conductive. Thus, the conductivity, thermal stability and mechanical properties of SnO₂ can be improved.

Heat insulation principle of nano-tin-based heat insulation transparent coating. The visible light electromagnetic waves in the solar spectrum with wavelengths from 400 to 800 nm can pass through the nano-tin-based material. The electrons or lattice vibrators inside the nano-tin-based material are excited to jump to the high-energy state in the near-infrared light electromagnetic wave of wavelength 800~2500nm, and the energy absorption is needed to jump from the low-energy state to the high-energy state.

And sunlight happens to provide energy, and the leap process absorbs the energy of sunlight to form light absorption. Or the nano-tin-based material forms a plasma in the violent motion of the electric and magnetic field environment of electromagnetic waves. Thus, the sunlight energy is converted into thermal energy. The thermal energy distributed in the infrared band, which accounts for 43% of the sun’s energy, acts as an absorption barrier, thus achieving thermal insulation.

China is rich in tin reserves. Nanotin-based materials have excellent electrical and optical properties. Nano-ATO has a good thermal insulation effect and low cost. It is the most suitable semiconductor oxide for the preparation of thermal insulating transparent coatings. Nano ATO has a wide band gap, high transparency and sufficiently high free carrier concentration in the visible region. Therefore, it has excellent electrical conductivity and high infrared reflectivity.

In addition, nano-ATO can absorb visible and near-infrared radiation due to its free charge carriers and lattice. Therefore, when used in transparent thermal insulation coatings, nano-ATO is an excellent absorber of the near-infrared spectrum.

The degree of absorption depends on the content of nano-ATO in the coating. The higher the nano-ATO content, the stronger the absorption of the NIR spectrum. The reflection or absorption of the NIR spectrum by nano ATO depends on the electrical conductivity of the coating.

At present, the infrared blocking rate of nano-insulated transparent coatings is around 60%, which is not good for thermal insulation. If the thermal insulation performance can be improved, it will substantially increase the application value of nano thermal insulation transparent coatings.