Research progress on metal oxides for transparent thermal insulation coatings

In recent years, with the rise of high-rise buildings, large glass windows and glass curtain walls are more and more widely used, how to reduce the energy loss caused by glass use, curb the energy crisis has become one

At present, there are a variety of thermal insulation and energy-saving products in the market, such as coated glass, foil glass, vacuum glass, Low-E glass and other architectural glass, but they all have a low visible light transmittance, too high a cost and the need to replace the existing glass, which limits the application of thermal energy-saving materials in the online coatingol.

Nano-inorganic fillers for transparent thermal insulation coatings can absorb a large amount of infrared light while the transmittance of visible light exceeds 80%, which has strong prospects for

Although nano-metal oxides have excellent optical properties, there are also improvements, such as indium tin oxide (ITO) is expensive, and intestivity is toxic, antimony tin oxide (ATO) and Al doped ZnO (AZO) in the 800 ~ 1200 nm band of infrared blocking effect is not good, CsxWO3 stability is poor, unstable under ultraviolet light, these problems need to be solved

1 Reflective nano-metal oxides

Reflective nano-metal oxides can impart the ability

1.

Tin dioxide is an N-type wide-gap semiconductor with a regular tetrahedral rutile structure with a band gap width of 3.

1.

ITO nano powders have excellent optical properties
with high light transmission in the visible light region and high reflection in the near-infrared light region.
In addition, ITO has excellent chemical properties, abrasion resistance, corrosion resistance and electrical conductivity
.
Katagiri et al.
prepared a nano-silica infrared shielding coating with a uniformly dispersed ITO with a visible light transmittance of more than 80%, completely blocking infrared light
with a wavelength of >1400 nm, using all hydrogen polysilica nitrogen as a precursor.
The pencil hardness of the coating at 750g load is 9H, which solves the problem that it was too hard to commercialize ITO films in the
past.

Tao et al.
prepared ITO/epoxy nanocomposites by thermal decomposition of indium acetate and tin acetate under argon using oleic acid and oleamine as sealants under argon, and then grafted polyglyceryl methacrylate (PGMA) and then dispersed into epoxy
resins.
The modified ITO particles are evenly dispersed in epoxy resin, and the visible light transmittance > 90%, while the ultraviolet and infrared light transmittance is lower
.
Grafting polymer macromolecular chains on the surface of inorganic nanoparticles is considered one
of the most promising technologies for overcoming inter-nanoparticle attraction and stable dispersion at high concentrations.

Guo et al.
adopted the method of combining ultrasonic spray and microwave-assisted pyrolysis to obtain ITO ultrafine powder with uniform particle size distribution, denseness and smooth surface morphology under the condition of microwave pyrolysis temperature of 600 °C and the amount concentration of precursor solution substances
of 0.
05mol/L.
This method is simple, fast and requires no post-heat treatment
.

Chen et al.
named In (NO3) 3 · H2O, SnC2O4, dilute nitric acid and NH4NO3 as raw materials, urea/glucose as composite fuel, using a new spray combustion method, prepared nano-ITO powder with a particle size of about 40 nm, which provided a way
for large-scale preparation of ITO powder.

1.
1.
2 ATO

The addition of Sb ions in SnO2 yields the ATO nanopowder
.
Compared with ITO, ATO has similar optical properties, better mechanical properties and thermal stability performance, is widely used, and greatly reduces costs
.

Li et al.
by oxidation co-precipitation-hydrothermal method, C2H5OH, NH3 · H2O and H2O2 were used as solvents, precipitants and oxidants, respectively, to explore the effects
on ATO synthesis.
The study found that Sb3+ was oxidized to Sb5+ by H2O2, and then entered the lattice of SnO2, replacing Sn4+, C2H5OH can dissolve SnCl4·5H2O very well, tin ions and antimony ions are NH3 · H2O precipitates, reaching the atomic level of the mixture
.
When the molar ratio of Sb/Sn is 6.
25%, the ATO has excellent NIR shielding performance
.

ATO hollow microsphere coating is a promising energy-efficient material
.
Wang et al.
used carbon balls as a template to synthesize ATO hollow microspheres by hydrothermal method, which were added to silicon propylene coatings in
different proportions.
Compared with solid microspheres, ATO hollow microspheres have higher absorption rate, reflectivity, specific heat capacity and infrared emissivity, and lower thermal conductivity
.
ATO hollow microspheres reduce the near-red outer transmittance of the silicon propylene coating, slowing heat transfer and enhancing radiative cooling
.

Wu et al.
prepared PUA/ATO thermal insulation coatings
by sol gel method.
The carbon-carbon double bond is pre-attached to the ATO surface and polymerized onto the
PU chain by ultraviolet light initiation.
Ultraviolet radiation for 40s, the conversion rate of double bonds reaches 93%.

When the ATO mass fraction is 3%, the composite film drops the temperature of the design room by 10 °C, but only blocks about 16% of the visible light
.

1.
1.
3 FTO

ITO is widely used in various fields due to its excellent optical and electrical properties, but the high cost and toxicity of raw material indium require the replacement of indium with other elements, such as fluorine-doped tin oxide (FTO
).
In addition to excellent optical properties, FTO also has good thermal and chemical stability
.

Malek et al.
synthesized nano-FTO powder by gel combustion method using SnCl4·5H2O, ammonium fluoride as raw material, and citric acid as fuel, with an average particle size of 20 nm
.

Han et al.
used acetylene black as fuel and also synthesized FTO nano powders by gel combustion method, with a particle size of 16 to 38 nm
.
Asl et al.
used SnCl2·2H2O and ammonium fluoride as raw materials to deposit FTO on
soda-lime glass through self-made spray pyrolysis equipment.
It was found that HCl can destroy the strong ionic bonds in SnCl4 and promote the growth
of the tin oxide layer.
Instability is formed between HCl and SnCl2 HSnCl3
.
When the HCl concentration is 0.
2mol/L, the conversion of SnCl2·2H2O to HSnCl3 is the largest, and the light transmittance of FTO film at wavelengths of 400~900 nm is 78.
60%.

1.
2 nano zinc oxide based filler

ZnO is a wideband (3.
4eV) N-type semiconductor with extremely widespread applications in optics and electricity, such as: solar conversion, photocatalytic and UV-resistant coatings
.
There are defects in the unopoted ZnO crystal structure, such as oxygen vacancies, zinc gaps, and hydrogen gaps
in the ZnO lattice.
The conductivity of ZnO is thermally unstable, and replacing Zn2+ with Al and Ga doped Zn2+ yields Al-doped ZnO (AZO) and Ga-doped ZnO (GZO), which can generate additional electrons, increase carrier concentration, and improve the optical and electrical properties
of ZnO.

1.
2.
1 AZO

Unoppoped ZnO is a thermally unstable.

Replacing Zn2+ ions with IIIA ions (B3+, Al3+, Ga3+, and In3+) generates additional electrons and improves the optical, electrical, thermal, and magnetic properties
of ZnO.
Al3+ has a small radius of ions and low material costs, making it the most commonly used doped element
.

Chen et al.
prepared Al-doped ZnO particles (AZO) with dual functions of photocatalytic and thermal insulation by Zn (CH3COO)2 and AlCl3 as raw materials and citric acid-induced sol-hydrothermal method, which has broad application prospects
in the field of energy conservation and environmental protection.

Li et al.
prepared AZO nanoparticles with different Al doped amounts by pyrolysis with Al(NO3)3·9H2O and Zn(CH3COO)2·2H2O as raw materials, with an average particle size of about 50 nm
.
As the amount of Al doping increases, the AZO resistivity decreases
significantly.
AZO nanoparticles with the best electrical conductivity, with the best
infrared light shielding performance.
AZO was introduced in a transparent epoxy resin and an insulating coating
was prepared.
Coatings containing 0.
5% (mass fraction) AZO have a visible light transmittance of more than 50% and a shading coefficient of 0.
45
.

Qu et al.
use DC magnetron sputtering method, add 10% hydrogen to argon, and prepare AZO films
at room temperature.
The results show that the increase in carrier concentration improves the infrared light reflectivity of the film due to the introduction of hydrogen and oxygen vacancies
in the film.
The average transmittance of light in the film at 400~900 nm is close to 86%, and the infrared light reflectance at 2500 nm reaches 75%.

1.
2.
2 GZO

Since the atomic radius of Ga3+ (0.
062 nm) is similar to that of Zn2+ (0.
074 nm), Ga is also one
of the best chemical elements doped with ZnO.

Chen et al.
used the sol-gel dip coating method to prepare Ga-doped ZnO (GZO) films
.
Coating solutions
were prepared using zinc acetate dihydrate, gallium acetylacetone, isopropanol, and monoethanolamine.
The coating was prepared by dip coating method, annealed for 1h in a reducing atmosphere (95% N2 and 5% H2) at 500 °C, cooled, and the GZO film was
obtained.
The study found that the carrier concentration has a greater
effect on the optical properties of the coating than the carrier mobility.

Li et al.
prepared GZO nanoparticles
by polymer pyrolysis by polymer pyrolysis, using Ga(NO3)3·9H2O and Zn(CH3COO)2·2H2O as raw materials, acrylic aqueous solution as solvent, and (NH4) 2S2O8 as initiators.
At a Ga doped amount of 4% (molar fraction), the average particle size of GZO is 26 nm
.
This method is simple to operate and is suitable for various metals and N-type semiconductors
doped with metal ions.

Cheng Hui used rfmosarcomattage method to prepare GZO film on the glass substrate, the highest visible light transmittance >80%, the near-infrared light region showed a low transmittance, and the infrared light after the 1500nm band was completely blocked, indicating that the film had selective transmittance to visible light and was suitable for the preparation of transparent thermal insulation coatings
.

1.
3 nano vanadium oxide based filler

In recent years, VO2 has been widely used as a phase change material in smart windows, and the phase change temperature (Tc) is 68 °C, which is much higher than the room temperature, so its operating temperature needs to be considered
.
Typically, doping high-valent cations, such as W6+, Nb6+, and Ta5+, or doping monovalent atom H+, increases carrier concentrations, decreases Tc, and alters the optical properties of
VO2.

Zomaya et al.
studied the reflux reaction of ammonium metavanadate with aspartic acid in water, and prepared tungsten-doped VO2
.
This method can be repeatedly prepared for large batches of tungsten-doped VO2, and the Tc is 53 °C
.
After being doped with tungsten, VO2 is oxidized in air, improving stability
.
The optimal light transmittance (Tlum) is 68.
30% at 22°C, 47.
6% at 80°C, and the optimal sunlight control capacity (ΔTsol) is 20.
37%.

Shen et al.
used solid-phase reaction method to synthesize tungsten-doped VO2
with good crystallization, low Tc, high solar thermal shielding ability and strong oxidation resistance.
After doping 2.
0% tungsten, Tc dropped from 67.
3 °C to 10.
8 °C
.
Tungsten-doped VO2 particles are exposed to air at 300 °C for more than 5h
.
Tungsten-doped 2.
0% VO2 composite film showed excellent light transmittance and solar thermal shielding performance, Tlum was 49.
9%, and the transmission ratio Tsol= 44.
8%.

2 Absorbent nano metal oxides

2.
1 nano tungsten oxide based filler

In recent years, tungsten oxide (WO3) has attracted widespread attention
with its unique electrochromic, photochemical and photocatalytic properties.
In WO3, elements such as Li, Na, K, Rb and Cs are mixed to form tungsten bronze, which has excellent optical properties
.
The dispersion of cesium tungsten bronze (CsxWO3) exhibits significant near-infrared light shielding while maintaining a high transmittance of visible light, and CsxWO3 will have great potential applications
in the field of architectural and automotive window glass.

2.
1.
1 CsxWO3

Transparent oxides such as ITO, ATO and AZO films can only shield near-infrared light
with a wavelength of > 1500 nm.
The hexagonal tungsten bronze phase of Cs0.
33WO3 has been widely studied
for its excellent shielding performance of near-red external radiation.

Eyassu et al.
are known as CsOH · H2O and WCl6 as raw materials, benzyl alcohol as the main solvent, oleic acid as the capping agent, the use of solvothermal method at lower temperature and shorter reaction time, the average particle size of 80 nm hexWO3 nanorods were prepared, visible light transmittance of 80% ~ 90%, near-infrared light shielding rate of 80% ~ 90%.

CsxWO3 is an attractive future filler
for transparent insulation coatings for building and automotive windows.

Yao et al.
proposed a two-step method for preparing hexagonal CsxWO3 nanoparticles
.
Prepare precursors by drying a mixture of WO3 sol and CsCl to obtain CsxWO3 nanoparticles
in a short time.
Using oleic acid as a solvent, wrapped on the surface of CsxWO3 nanoparticles, can reduce the particle size and reduce agglomeration
.
Cs0.
32WO3 nanoparticles were prepared into thin films, covered on ordinary glass sheets, and about 99% of the near-infrared light radiation in the wavelength range of 780~2600 nm was blocked, and the visible light transmittance at 465 nm was 78.
9%.

Wu et al.
with H2WO4, CsOH · H2O was used as raw material, triethylamine (C6H15N) and deionized water as solvents, and Hexagon Cs0.
32WO3 powder
was synthesized by solution method.
Cs0.
32WO3 powder is used as the target, and a thin film
is deposited on the quartz glass by electron beam evaporation method.
The study found that Cs0.
32WO3 films annealed at 500 °C in Ar/H2 atmosphere had a maximum visible light transmittance of 80% and a near-infrared light transmittance of 42%.

2.
1.
2 Other WO3-based oxides

Guo et al.
use acetic acid and ethanol as solvents, under vigorous stirring, dissolve a certain amount of WCl6 in absolute ethanol, add RbOH, and add acetic acid
after mixing well.
The solution was transferred to an autoclave, and after the reaction at 235 °C for 20 h, the dark blue product was centrifuged, washed and dried to obtain The RbxWO3 nanoparticles
.
A rod-shaped ammonium tungstate bronze
with a diameter of about 120 nm was obtained by the same method.

Luo et al.
used sodium tungstate as raw material and citric acid as the reducing agent, and prepared NaxWO3 powder
by hydrothermal method.
Among them, Na0.
1WO3 is rod-shaped, about 20 μm long and 5 μm
in diameter.

Song et al.
synthesized KmCsnWO3 powder
using KNO3, CsNO3 and WO3 as raw materials and using the high-temperature solid phase method.
It was evenly dispersed in polyvinyl alcohol solution and KmCsnWO3 thin film glass
was prepared by roll coating.
When n(K)∶n(Cs) = 0.
2:0.
8, the maximum visible light transmittance is 66.
89%, and the near-infrared shielding rate is 98.
25%.

The performance of various metal oxides for transparent thermal insulation coatings was compared, as shown in
Table 1.

Table 1 Comparison of metal oxide properties for transparent thermal insulation coatings

The advantages and disadvantages of various metal oxides are shown in
Table 2.

Table 2 Advantages and disadvantages of various types of metal oxides

3 Factors affecting the properties of metal oxides

3.
1 Effect of preparation method on nanometallic oxides

Different nano metal oxide preparation methods have advantages and disadvantages, but for micro-doped metal oxides, it is more appropriate to use the sol gel method, the dispersion of nanoparticles is more uniform, and the sol can be directly added to the composite coating for use, and the transparent thermal insulation effect is better
.
The advantages and disadvantages of different doped metal oxide preparation methods are compared as shown in
Table 3.

Table 3 Advantages and disadvantages of the method for preparing doped metal oxides

3.
2 Agglomeration of nano-powders

Due to the small particle size of the nano-powder, the specific surface area and surface energy are large, extremely unstable and easy to agglomerate, which limits the characteristics of
the nano-powder.
Among them, reunification has become a key issue
.
During the preparation process, agglomeration
can be controlled by the following methods.

(1) Organic solvent cleaning method
.
During the washing process of nanopowders, washed with deionized water first, and then washed with organic solvents such as ethanol, can eliminate the hydroxyl junction between the nanopowders and reduce hard agglomeration
.

(2) Azeotropic distillation method
.
Numerous studies have shown that the water molecules contained in the sintering precursor are the main cause of particle agglomeration, and the water molecule content
must be reduced.
Azeotropic distillation uses organic solvents to form an azeotrope with water, and the water in the colloid is exuded in the form of azeotrope to prevent calcination from forming hard agglomeration
.
Commonly used azeotropes are n-butanol and polyethylene glycol
.

(3) Precursor calcination
.
While ensuring that the precursor decomposes or converts into the desired phase, the calcination temperature and time are reduced as much as possible, which helps to reduce hard agglomeration
.

(4) Wash and dry
.
It is recommended to wash the nanoparticles using centrifugation, which can remove Na+ and Cl-
better than suction filtration.
The particle size arrangement obtained by using different instruments in the drying process is the blast drying box> the vacuum drying box> freeze drying > azeotropic distillation and then drying
.

(5) Others
.
The addition of surfactants, dispersants and pH regulation has an impact on the particle size and dispersion of the nanoparticles
.

3.
3 Effects of Na+ and Cl-

When preparing nano-powders, the presence of chlorine in the raw material is prone to hard agglomeration, which reduces the purity of the particles
.
Removing chloride ions wastes a lot of solvents, increases costs, and requires exploring the non-use of chlorides
.
For the first time, Wang et al.
synthesized chlorine-free indium tin oxide nanoparticles without the use of expensive metal alkoxides
.
Zhang et al.
used granular tin and Sb2O3 as raw materials for the first time, and successfully synthesized antimony-doped tin
oxide by wet chemical co-precipitation method.

3.
4 Effect of filler particle size on the performance of transparent thermal insulation coatings

Usually, the filler particle size is too small, easy to agglomerate, and unevenly dispersed in the coating; The particle size is too large, the dispersion performance is poor, and the unique optical properties
of the nano powder are lost.
Trenque et al.
explored the scattering intensity of visible light in suspensions of GZO with different particle sizes, and the transmittance of 25 nm GZO in visible light was comparable to that of GZO at 10 nm, which was much better than that of GZO
at 100 nm.
Under the premise of ensuring optical properties and coating dispersion performance, appropriately increase the particle size of the filler and improve the barrier rate
of infrared light.

3.
5 Dispersion of fillers in coatings

The uniform and stable dispersion of nano powder in the coating is the key to the preparation of transparent thermal insulation coating, and the main influencing factors are as follows
.

(1) Physical dispersion method
.
The ball mill dispersion of the nano powder and then the ultrasonic dispersion of the slurry can significantly improve the uniform dispersion stability
of the nano powder in the coating.

(2) The use of
dispersants.
For different resins and fillers, the choice of dispersant is particularly important
for the uniform and stable dispersion of nano powders.

(3) The consistency of the slurry
.
Add a certain amount of thickener to the slurry, increase the viscosity in an appropriate amount, and the powder can be well suspended, improving the stability
of the nano powder.

4 Conclusion

This paper introduces the advantages and disadvantages of various metal oxides and preparation methods for transparent thermal insulation coatings, compared with other metal oxides, CsxWO3 has more excellent visible light transmission and near-infrared light shielding performance, and is relatively environmentally friendly, the future application prospects are very extensive, but its stability under ultraviolet light needs to be improved
.
In the future, controlling the agglomeration of nano powders, solving their compatibility in coatings, and reducing raw material costs are still urgent problems that need to be solved in the development of transparent thermal insulation coatings
.

Source of this article: Paint Industry, No.
8, 2022

Page range: 81~88

Authors: Wang Song, Wang Jihu, Xie Chen
.
Li Shuaibiao, Mei Dajiang, Wen Shaoguo, Yuan Chunping