Low-emissivity Coatings Research Articles

Effect of TiO2 lowermost layer on crystal orientation and electrical resistivity of glass/TiO2/ZnO/Ag structure in Low-E glass

In order to improve the heat insulating properties of low-emissivity coatings, it is necessary to reduce the electrical resistivity of their multilayer structure, which consists of an Ag layer and oxide layers. In this study, structures comprising glass/ZnO/Ag and glass/TiO2/ZnO/Ag were deposited by sputtering, and the effects of these layers on the electrical resistivity were investigated. For a ZnO layer deposited at low sputtering gas pressure, lower surface roughness reduced the electrical resistivity of the Ag layer. Also, it was found that the electrical resistivity could be further decreased by inserting a TiO2 lowermost layer between the substrate and the ZnO layer. The results of X-ray diffraction and X-ray reflectivity measurements showed that the decrease in resistivity was mainly due to the improved crystal orientation of the ZnO and Ag layers.

Coatings utilization to modify the effective properties of high temperature packed bed thermal energy storage

High-temperature thermal energy storage is becoming more and more important as a key component in concentrating solar power systems and as an economically viable large-scale energy storage solution. Ceramics and natural rocks based packed beds are one of the attracting solutions. For application temperatures above 600 °C, radiation heat transfer becomes the dominant heat transfer phenomenon and it greatly influences the performance of thermal storage systems. Coatings with different thermal properties (mainly thermal emissivity and thermal conductivity) could be exploited to modify the effective thermal properties of packed beds. In this work, we present a methodology to account for the thermal effect of a coating layer applied over the pebbles of a packed bed. The influences on the packed bed effective thermal conductivity of several characteristics of the coating material, packed bed arrangement, and filler material are investigated. The results show that low emissivity coatings could reduce the effective thermal conductivity of a rock based packed bed of about 58%, with respect to a similar uncoated solution, already at 800 °C. A low emissivity coating could also limit the increase in the thermal effective conductivity from the cold to the hot zone of the storage. Coatings would have a higher influence when applied in packed beds with large size particles, relatively high thermal conductivity of the substrate and void fraction. The application of different coatings, with various thermo-physical properties, in different parts of the storage could modify the effective thermal conductivity distribution and enable a partial control of the thermocline degradation, increasing the storage thermal efficiency.

Combining Low-Emissivity Thin Coating and 3D-Printed Original Designs for Superior Fire-Protective Performance.

Three-dimensional (3D) printing is a very flexible process to design various objects of original shapes. Previous works highlighted the preparation of new multimaterials composed of an original sandwich structure made of the ethylene vinyl acetate copolymer containing 30 wt % of aluminum trihydroxide in which a hydrogel phase made of agar and vermiculite was incorporated. This original material revealed an extremely low heat release rate (HRR) (with a reduction of 86 and 64% with regard to the peak of the HRR and total heat release, respectively, when compared to the same sample without hydrogel filling) during its heat exposure at 50 kW/m2 according to the mass loss cone calorimetry test. However, the time to ignition (TTI) of this material was not improved. This work consequently focuses on delaying the time to ignition of this hydrogel sandwich 3D-printed multimaterial. Solution consists in depositing by pulsed DC magnetron sputtering a low-emissivity thin coating on the exposed skin surface. This coating reflects most of the infrared rays responsible for heat absorption and thus delays the ignition of the underlying material. The thermal resistance performances of this coated sandwich 3D-printed multimaterial were evaluated, and a mechanism of action was proposed to explain the dramatic enhancement of the properties.

Factors influencing the performance parameters of vacuum glazed smart windows to net zero energy buildings

The progression of smart technologies such as vacuum glazed windows are considered a realistic achievement of the net energy zero buildings (NZEBs). From designers to researchers to builders, there has been an increasing concern about understanding the inter-dependencies between the parameters and influencing factors that determine the performance of vacuum glazed smart windows. This research reviews the performance parameters such as thermal transmittance (U value), thermal resistance (R value), solar transmittance (g value), visible light transmittance (tv value) and  thermal resistance of residual gas space (Rgas value). These are inter-dependent on factors such as edge seal, support pillar array, low emittance coatings, getters, and effective evacuation process. This research implicates that effective hermetic edge seal provides longevity such as fusion and solder glass edge sealed vacuum glazing could be cost-effective and energy efficient solution. Stainless steel support pillar array is an unavoidable compromise on U value. This review shows that an increase of the size of glass sheet increases support pillar array improving the overall U value. Also, an addition of low emittance coatings enhances U value whilst maintaining tv value. To improve the overall life span of the vacuum glazed smart window, an incorporation of combo-getter that absorb any gases released from the internal glass surfaces in to into the vacuum cavity from the glass surface which prevents degradation of vacuum pressure and provide long term vacuum pressure stability in the vacuum glazed smart window. A recent improvement in the understanding of evacuation process shows that hot-plate surface heat induction of 60?C improved the vacuum pressure and mitigates the pump-out hole sealing process whilst lessening the temperature induced stresses.

Thermal performance and condensation risk of single-pane glazing with low emissivity coatings - ADDENDUM

Thermal performance and condensation risk of single-pane glazing with low emissivity coatings - ADDENDUM

Thermal performance and condensation risk of single-pane glazing with low emissivity coatings

Thermal performance and condensation risk of single-pane glazing with low emissivity coatings

Structural and electronic properties of SnO2 doped with non-metal elements.

Crystal structure and electronic properties of SnO2 doped with non-metal elements (F, S, C, B, and N) were studied using first-principles calculations. The theoretical results show that doping of non-metal elements cannot change the structure of SnO2 but result in a slight expansion of the lattice volume. The most obvious finding from the analysis is that F-doped SnO2 has the lowest defect binding energy. The doping with B and S introduced additional defect energy levels within the forbidden bandgap, which improved the crystal conductivity. The Fermi level shifts up due to the doping with B, F, and S, while the Fermi level of SnO2 doped with C or N has crossed the impurity level. The Fermi level of F-doped SnO2 is inside the conduction band, and the doped crystal possesses metallicity. The optical properties of SnO2 crystals doped with non-metal elements were analyzed and calculated. The SnO2 crystal doped with F had the highest reflectivity in the infrared region, and the reflectance of the crystals doped with N, C, S, and B decreased sequentially. Based on this theoretical calculations, F-doped SnO2 is found to be the best photoelectric material for preparing low-emissivity coatings.

Heat transfer through a double-glazed window by radiation

A method of separately combined analysis of the process of radiant-convective heat transfer through the central part of the double-glazed windows is developed. The problem of heat transfer by radiation through a double-glazed window is formulated and a solution is obtained in an analytical form. The solution is applicable for all types of double-glazed windows with a given number, characteristics and location of low-emission coatings. The possibilities of reducing radiant heat losses through double-glazed windows due to an increase in the number of glasses and the use of glasses with low-emission heat-protective coatings are analyzed in detail.

Improved thermal expansion performance of Aluminum/polysiloxane/glass coatings with low infrared emissivity by zinc powder

Thermal expansion performance of Aluminum/polysiloxane/glass coatings with low infrared emissivity was improved by adding zinc powder. The influences of zinc powder on phase structure, composition, surface microstructure, thermal expansion, adhesion and infrared radiation of Aluminum/polysiloxane/glass coating was systematically studied. The thermal mechanical analysis indicated that thermal expansion coefficient can be adjusted by adding zinc powder in the coating. The tensile strength test results show that the matching of the thermal expansion could improve the adhesion strength of the coatings. As a result, compared with the coating without zinc powder, the adhesion strength was enhanced by 2.0 ± 0.4 MPa when the heat treatment temperature at 450 and 500 °C, and increased by 3.0 ± 0.4 MPa at 550 °C. Simultaneously, average infrared emissivity value of the coating with good thermal stability was about 0.20 in the 3–5 μm wavebands, and approximately 0.34 within 8–14 μm after heated at 550 °C. These results indicated that the good thermal expansion performance and low infrared emissivity coatings were obtained by adding zinc powder.

Assessment and parametric analysis of solar trigeneration system integrating photovoltaic thermal collectors with thermal energy storage under time-of-use electricity pricing

The solar trigeneration system based on coupling photovoltaic thermal (PVT) collectors with an absorption-subcooled compression hybrid cooling configuration has the potential for enhancing solar energy utilization in buildings. Considering time-of-use electricity pricing, the availability of solar cooling often does not coincide with on-peak periods during which electricity prices are high. To benefit from time-of-use pricing for greater economic profitability, sensible heat thermal storage is potential options. However, changeable storage tank temperature affects not only solar cooling capacity but also the electrical output of PVT collectors. Thus, in this work, three operation schemes (i.e., a conventional scheme, heat energy storage and cool energy storage schemes) are compared, and the effect of key design parameters on system performance is analyzed. Models for different system layouts and a conventional photovoltaic (PV-only) system are developed by programming with MATLAB. Annual (8760-hour) simulations are performed based on a case study for a high-rise hotel in subtropical cities. The results for Guangzhou show that the system layout, integrating cool energy storage and glazed PVT collectors with low-emissivity coatings, achieves the highest total electricity cost saving, which is 16% higher than that of the PV-only system. Its total solar energy utilization efficiency is 0.321, which is 2.4 times that of the PV-only system. The novelty of this work is that it provides an appropriate energy storage strategy and the design guidelines of key parameters for PVT-based solar trigeneration systems. This work is helpful for PVT-based solar trigeneration systems to improve operating cost savings under time-of-use electricity pricing.

The compatible performance of three-dimensional SiO2–ZnO amorphous photonic crystals in adjustable structural color and low infrared emissivity

In high temperature environments, most low-emissivity coatings show monotonous and dull colors which make the visible-infrared compatible stealth still challenging. To achieve a wide range of color selection, in this work, three-dimensional (3D) SiO2–ZnO amorphous photonic crystals (APCs) are manufactured by electrodepositing Zn2+ into 3D SiO2 APCs, investigating the compatibility of adjustable structural color and low emissivity. A variety of structural colors could be easily obtained by modulating the diameter of SiO2 nanospheres. In addition, the experimental parameters of spin-coating counts and electrodeposited time are studied to optimize the compatible performance of color and infrared emissivity. The results indicate that the doped ZnO effectively reduces the infrared emissivity in 3–5 μm waveband without changing the structural color of SiO2 APCs. However, excess electrodeposited time of ZnO would cause the structural color faded gradually because of the increased refractive index and destroyed periodical structure. The adequate spin-coating counts is helpful for the bright structural color, but excess spin-coating counts would induce more defects in periodic structure and result in more absorption in 3–5 μm waveband. When the spin-coating counts and electrodeposition content of Zn2+ in the SiO2 APCs are properly modulated in this study, the SiO2–ZnO APCs can exhibit a good compatibility in desired structural color and low infrared emissivity (~0.4), as well as excellent thermal stability. This study provides a new approach for the design of multi-spectrum compatible stealth materials.

Thermal Management by Engineering the Alignment of Nanocellulose.

One of the grand current research challenges is to improve the energy efficiency of residential and commercial buildings, which cumulatively consume more than 40% of the energy generated globally. In addition to improving the comfort of the inhabitants and mitigating the growing energy consumption problem, new building materials and technologies could provide a safe strategy for geoengineering to mitigate global climate change. Herein, recent progress in developing such advanced materials from nanocellulose, which is often derived from wood or even dirty feedstocks like waste, is reviewed. By using chemical and bacteria-enabled processing, nanocellulose can be used to fabricate broadband photonic reflectors, thermally super-insulating aerogels, solar gain regulators, and low-emissivity coatings, with potential applications in windows, roofs, walls, and other components of buildings envelopes. These material developments draw inspiration from advanced energy management found in nature, such as the nanoporous photonic structures that evolved in cuticles of beetles. Fabrication of such materials takes advantage of mesoscale liquid crystalline self-assembly, which allows for pre-designed control of cellulose nanoparticle orientations at the mesoscale. With the potential fully realized, such materials could one day transform the current energy-lossy buildings into energy plants on Earth and possibly even enable extraterrestrial habitats.

Nano-Scale Residual Stress Profiling in Thin Multilayer Films with Non-Equibiaxial Stress State.

Silver-based low-emissivity (low-E) coatings are applied on architectural glazing to cost-effectively reduce heat losses, as they generally consist of dielectric/Ag/dielectric multilayer stacks, where the thin Ag layer reflects long- wavelength infrared (IR), while the dielectric layers both protect the Ag and act as an anti-reflective barrier. The architecture of the multilayer stack influences its mechanical properties and it is strongly dependent on the residual stress distribution in the stack. Residual stress evaluation by combining focused ion beam (FIB) milling and digital image correlation (DIC), using the micro-ring core configuration (FIB-DIC), offers micron-scale lateral resolution and provides information about the residual stress variation with depth, i.e., it allows depth profiling for both equibiaxial and non-equibiaxial stress distributions and hence can be effectively used to characterize low-E coatings. In this work, we propose an innovative approach to improve the depth resolution and surface sensitivity for residual stress depth profiling in the case of ultra-thin as-deposited and post-deposition annealed Si3N4/Ag/ZnO low-E coatings, by considering different fractions of area for DIC strain analysis and accordingly developing a unique influence function to maintain the sensitivity of the technique at is maximum during the calculation. Residual stress measurements performed using this novel FIB-DIC approach revealed that the individual Si3N4/ZnO layers in the multilayer stack are under different amounts of compressive stresses. The magnitude and orientation of these stresses changes significantly after heat treatment and provides a clear explanation for the observed differences in terms of scratch critical load. The results show that the proposed FIB-DIC combined-areas approach is a unique method for accurately probing non-equibiaxial residual stresses with nano-scale resolution in thin films, including multilayers.

The Properties of an Aluminum/UV-Curable, Infrared, Low-Emissivity Coating Modified by Nano-Silica Slurry

To improve the performances of UV-curable coatings, the effects of nano-silica slurry, aluminum and UV-curing time on the glossiness and infrared emissivity of UV-curable coatings were investigated by orthogonal experiments. The results showed that UV-curing time is a key factor affecting the performance of the coating. When the UV-curing time was increased from 30 to 360 s, the glossiness of the UV-curable coating slowly decreased from 11.1% to 9.0%. The L’ value decreased from 78.6 to 75.0. The infrared emissivity of the coating with UV-curing time of 180 s was 0.106, which was the lowest. The coating hardness with different curing time was 6H. The coating roughness was high when the UV-curing time was 30–120 s. When the UV-curing time was greater than 300 s, the coating adhesion was 0 and the coating had the best impact resistance of 500 N/cm. The overall performance of the aluminum/UV-curable coating was optimal when the UV-curing time was 180 s. This research is able to promote the industrial development of UV-curable, infrared, low-emissivity coatings.

Optical and Chromaticity Properties of Metal-Dielectric Composite-Based Multilayer Thin-Film Structures Prepared by RF Magnetron Sputtering

Coated glass products, and especially the low-emissivity coatings, have become a common building material used in modern architectural projects. More recently, these material systems became common in specialized glazing systems featuring solar energy harvesting. Apart from achieving the stability of optical parameters in multilayer coatings, it is also important to have improved control over the design of visual color properties of the coated glass. We prepare metal-dielectric composite (MDC)-based multilayer thin-film structures using the radio frequency (RF)-magnetron sputtering deposition and report on their optical and chromaticity properties in comparison with these obtained using pure metal-based Dielectric/Metal/Dielectric (DMD) trilayer structures of similar compositions. Experimentally achieved Hunter L, a, b values of MDC-based multilayer building blocks of coatings provide a new outlook on the engineering of future-generation optical coatings with better color consistency and developing approaches to broaden the range of achievable color coordinates and better environmental stability.

Fabrication and characterization of a PDMS modified polyurethane/Al composite coating with super-hydrophobicity and low infrared emissivity

Polydimethylsiloxane (PDMS) modified polyurethane (PU)/Al composite coating with super-hydrophobicity and low infrared emissivity was prepared via glass rod scraping method using nano-SiO2, PDMS modified PU, and Al powders as micro-nano structural modifier, resin matrix, and functional pigments, respectively. The effects of curing temperature, Al powder addition, and the ratio of Al powder and nano-SiO2 on the coating performance has also been discussed. The results show that when the curing temperature of coating was 80 °C, the coating had lowest infrared emissivity and glossiness properties. The Al powder addition had an important effect on the coating performance. When the amount of Al powder was 50 wt%, the emissivity and glossiness of the coating were as low as 0.167 and 13.5, respectively, while, the water contact angle (WCA) of the coating reached 117°, which is significantly higher than that for traditional resin-based low emissivity coatings. The modification of coating with appropriate amount of nano-SiO2 led to clear mastoid micro-nano structures on the coating surface, so that the coating could achieve super-hydrophobic properties. When the ratio of Al powder and nano-SiO2 in the coating was 6:4, the WCA of the coating reached 151.5°, the sliding angle reached 9°, and the coating had outstanding self-cleaning performance. At the same time, the super-hydrophobic coating had relatively low emissivity (0.614) and extremely low glossiness (3.5) properties.

Primary Seal Deformation in Multipane Glazing Units

Quadruple glazing has become a high-end standard in the field of sealed insulating glass units. With more than three glass panes a set of internal heat-related technical problems emerges. Durability, being the most pressing problem, requires careful management of the primary sealant deformation, especially under summer environmental conditions. Namely, the role of the primary seal in insulating glass units is to protect their humidity-sensitive, low-emissivity coatings from moisture and the quality of such protection is a key factor in durability. In this paper, a new methodology is proposed for a feasibility assessment of a new multipane insulating glass unit design, where proper design enables avoidance of excessive strain on the primary seal. It focuses on the calculation of the primary sealant strains, which are nonhomogeneous and multiaxial according to different loading conditions. This approach leads to analytical expressions that enable convenient identification of the critical location on the primary sealant. Finally, feasibility is assessed with the proposed methodology for the newly developed highly insulative six-pane facade unit by means of a comparison of the calculated strain state with the anticipated allowable strains, based on technical practice.

Effect of Different Color Paste on Properties of Fluorine Resin/Aluminum Infrared Low Emissivity Coating

The effect of the four kinds of red, dark yellow, purple, and black pastes on the properties of fluorine resin/aluminum low emissivity coating was studied. The infrared emissivity coatings with red and black pastes were higher than the coatings with dark yellow and purple pastes. The hardness of the coatings with red, dark yellow, and purple color pastes was 6H, and that with black pastes was 6B. The adhesion and impact resistance of dark yellow coating was better, followed by red and purple, and the adhesion and impact resistance of black coating was the worst. Electrochemical polarization curves indicated that fluorine resin coatings with purple paste had better corrosion resistance. After the salt water resistance test, there was no obvious loss of light in the coatings with the four kinds of color pastes. The purple paste coating had no obvious loss of light and less bubble, suggesting that the fluorine resin/aluminum low emissivity coating with purple paste had better performance. The results of this study provide a new prospect for the application of infrared low emissivity coating in infrared stealth and compatibility with visible light.

Damage Identification of Low Emissivity Coating Based on Convolution Neural Network

At present, low emissivity coating has been widely used in various fields, but damage will greatly reduce the efficiency of low emissivity coating, so the damage detection of low emissivity coating becomes an important work. Based on convolution neural network, a model for automatic identification of coating damage with low emissivity is proposed. Firstly, the optical image data set of low emissivity coating is constructed and extended by means of data enhancement. After that, VGG-19 and ResNet-50 models are built based on tensorflow, and the cross entropy loss function is used in the models. Then, SGD, momentum, RMSprop and Adam are used to optimize the model. In the process of model optimization, the learning rate is adjusted to get the optimal model. The results show that when the learning rate is $5\times 10^{-5}$ and Adam method is used to optimize the model, the recognition accuracy of VGG-19 model is 90.64%, while that of ResNet-50 model is 94.14%. This paper is of great significance for the study of automatic damage identification of low emissivity coatings.

Finite element analysis of heat transfer performance of vacuum glazing with low-emittance coatings by using ANSYS

As a new transparent insulation material, vacuum glazing with Low-E (low emittance) coating has drawn more and more attentions thanks to its excellent performances: outstanding thermal and sound insulations, anti-frosting, resisting wind press capacity, and good durability. However, a comprehensive analysis and evaluation on the heat transfer performance of Low-E vacuum glazing are still missing in the current literatures. In this work, the influences of various aspects on the heat transfer performance of Low-E vacuum glazing were analyzed based on the Finite Element Analysis (FEA) method, including thickness of substrate glass, width of the sealing edge, materials and spacing of support pillar array, and insulation frame structure. The simulated results show the following, 1) with the increase of the thickness of the substrate glass, the heat transfer coefficient of the Low-E vacuum glazing rises slowly in a linear trend. The larger the size of the whole piece of the glazing, the more slowly it rises; 2) the Low-E vacuum glazing heat transfer coefficient declines linearly with the decrease of the width of the sealing edge; 3) as the support pillar array spacing increases and the thermal conductivity of the support pillar array decreases, the Low-E vacuum glazing heat transfer coefficient decreases; 4) the use of thermal insulation frame structure can improve the insulation efficiency of the vacuum glazing. Experiments were carried out on a Low-E vacuum glazing, and the heat transfer coefficient of this vacuum glazing was determined by the Building Heat Resistance-III (BHR-III), an equipment for insulation performance testing. The simulated result matches well with the experimental result, with a relative deviation of 2.8%, which confirms the reliability of the simulation result. This work is expected to provide guidance for optimization design of Low-E vacuum glazing and its insulation performance evaluation.