Instituto Tecnológico y de Estudios Superiores de Monterrey
Low-Emissivity Nanocomposite Film
Sergio Alfonso Aranda Márquez //A00824120// Industrial EngineerProfessor: Dr. Jaime Bonilla
May 12th , 2018
The present work proposes the development of a low-emissivity film composite of PVC/AgNPs/TiO2 NPs. The synthesis of nanoparticles shall be bottom-Up techniques such as Sol-Gel and seed growth for the generation of TiNO2NPs and Ag nanodisks respectively. Since the shape, size and aspect ratio of the nanoparticles are crucial aspects for the final application, the characterization will be carried out by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS). These techniques will allow us to determine the morphology of the particles, and not only the size of a nanoparticle sample but also the distribution. When it comes to optical properties, such as transmittance, absorbance and reflectance characterization of nanoparticles and the nanocomposite itself will be by spectroscopy techniques. infrared spectrometer.
The emissivity (?) is a measure of a material’s radiative properties, i.e. the emission of infrared radiation. 1 The higher emissivity, the higher emission. Highly reflective materials of infrared radiation have low emissivity values, e.g. polished surfaces of gold, silver, aluminum or copper.
The ? value will be a number between 0 and 1. Oxidation of metallic surfaces will increase the emissivity substantially, e.g. polished aluminum with ? = 0.05 (reflectance 0.95) and oxidized aluminum with ? = 0.30 (reflectance 0.70).
Determination of the emissivity is required in order to further determine the solar factor (SF) and the thermal transmittance (Uvalue). 1
Low emissivity (low-e) materials reduce the thermal energy used in both the visible (transparent) and the opaque areas of the construction building. The objective and goal of this low-e materials are to minimize the heat transfer through thermal radiation. These materials will influence on the daylight and solar energy that goes through the windows. This research will focus on finding a new way through nanotechnology that reduces the heat transfer that goes through the window without reducing the daylight throughput in the window. A large part of the energy usage is directly related to heating and cooling demands for buildings. Transparent surfaces, e.g. windows, contribute to a substantial part of this energy loss.
Low-e materials have been developed to minimize the amount of Ultraviolet light (UV) light and infrared (IR) light that can go through the glass without reducing the daylight or visible light passing through it. When heat or light energy is absorbed by the surface, it is shifted away by air or radiated by the same glass surface. Emissivity is the ability to radiate this energy. All materials radiate heat in a long-wave form, IR energy depends on the emissivity and temperature of the surface.
Ultraviolet light, visible light and infrared light all occupy different parts of the solar spectrum – the differences between the three are determined by their wavelengths. 2 Ultraviolet light, that causes interior materials such as fabrics and wall coverings to fade, has wavelengths of 310-380 nanometers when reporting glass performance. Visible light occupies the part of the spectrum between wavelengths from about 380-780 nanometers. Infrared light (or heat energy) is transmitted as heat into a building, and begins at wavelengths of 780 nanometers Fig.1.
Fig. 2 Solar Energy Distribution. 2
Low-e coatings have a microscopically thin, transparent coating, that reflects long-wave infrared energy. 3 Thin silver layers sandwiched with dielectric layers have been used in order to enable de-reflection of the silver surface attributing to optical interference of multilayer films.
Nanostructured silver particles exhibit unique optical characteristics. In contrast to their corresponding bulk counterparts, metallic nanoparticles can absorb electromagnetic radiation, 4 resulting in surface plasmon polaritons at the metal dielectric interface.
Ag nanoparticle films showed a typical surface emissivity of about 0.793, compared to about 0.837 of the plain glass substrate. After a mild heat treatment at 200 °C, the annealed Ag nanoparticle films showed a substantially reduced surface emissivity value as low as 0.015. 5 forming an interconnected, porous network of Ag nanoparticles is essential for achieving the low-e effect for this material.
Sun screen lotions containing organic UV absorbers, which typically are compounds containing phenolic groups with intramolecular hydrogen bonds, mixed with TiO2 or ZnO particles, which contribute to protection with both strong UV light scattering and absorption, are widely spread. 3
As mentioned before there are various demands depending on the region’s climate or the season, e.g.in winter when the heat energy tries to “escape” to the outside to the colder climate, the low-e coating reflects the heat back to the inside preventing the loss of heat in the building. The same thing happens in summer but the other way around.
There are basically two types of low-e coatings: passive and solar control. Passive low-e coatings objective is to maximize the heat energy obtained from the sun and transfer it to the inside of the building. Solar control low-e coatings objective is to limit the amount of heat energy that goes through the windows and keeping the buildings colder. This designs are used in order to reduce the energy consumption of the heater or the air conditioner respectively.
To design a structure that achieved desired properties, we turn our attention to metamaterials based on the principle of localized plasmon resonance (LPR). LPR is the collective motion of free electrons inside metals in resonance with the vibration of the electric field of light. Light is absorbed and scattered intensely near the resonant frequency.
There are two primary production methods – pyrolytic (fig. 3), and Magnetron Sputter Vacuum Deposition (MSVD) shown in figure 4. In the pyrolytic process the coating is applied to the glass ribbon while it is being produced on the float line. The coating then “fuses” to the hot glass surface, creating a strong bond that is very durable for glass processing during fabrication. Finally, the glass is cut into stock sheets of various sizes for shipment to fabricators. In the MSVD process the coating is applied off-line to pre-cut glass in a vacuum chambers at room temperature. More information about low-e coating processes and low-e material properties can be found on Vitro page in the following link;
16630652199640Fig. 4 Magnetron Sputter Vacuum Deposition
00Fig. 4 Magnetron Sputter Vacuum Deposition
186309010795Fig. 3 Pyrolytic Process
00Fig. 3 Pyrolytic Process
In order to characterize the silver nanodisks and TiO2 nanoparticles in the sputtered film The scanning electron microscope (SEM) will be used. SEM uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample.
The transmission electron microscope (TEM) is a very powerful tool for material science. A high energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe features such as the crystal structure and features in the structure like dislocations and grain boundaries. Chemical analysis can also be performed. TEM can be used to study the growth of layers, their composition and defects in semiconductors. High resolution can be used to analyze the quality, shape, size and density of quantum wells, wires and dots.
Glass represents an ideal material for the use in solar energy applications thanks to its high solar transmittance, long-term stability and low cost. To combine these advantageous properties with an additional heat protection function, spectrally selective coatings reflecting radiation in the infrared wavelength range, can be applied to its surface, thus significantly reducing thermal losses. 5
Thin polymer films with low-e had also been applied to the glass surface. Thin films present a mayor advantage compared to the traditional coating, which is that the glass is not needed at hand in order to produce this films. 6 They can be manufactured separately and at the same time. The possibilities of improving the film technology and make it achieve the same level of performance as factory applied coatings may make for a process where low-e glazing can be manufactured faster and at a lower cost.
Transparent materials should be to achieve the same transparency as clear glass. As of today, the reduction in visible transmittance when applying low-e coatings is considered an undesirable characteristic, as it may increase the need for artificial lighting, i.e. increased energy usage.
The objective of the proposal is to implement through nanotechnology a new way to have a thin polymer film with low emissivity characteristics applying nanoparticles. This in order to improve the performance of films and find a cheaper way for glass surfaces to achieve low-e without the coating process. The nanoparticles will help the film repel both the ultraviolet light and the infrared light without the glass loosing transparency and allowing the daylight transmitted through the window.
Via simulation Kiyoto et. all 7 investigated the ideal structure and discovered that if silver nanodisks can be arranged like a stone pavement as shown in figure 3. the intended properties could be realized. Applying photographic film technology, it was proved that this distribution can be obtained.
Figure 3. Formation and arrangement of silver nanodisks
Why thin films?
The current manufacturing method of low-e coating is a reason for relative high cost as well as, the price on silver. One option that has been tested out is to add anti-reflection treatment to low-e coatings. Hammarberg and Roos 8 showed that it was possible to increase the visible transmittance by up to 9.8% by depositing a thin film of silicon dioxide on both sides of a commercial glazing. The film will be a nanocomposite of silver (Ag) and Titanium dioxide (TiO2).
Polyvinyl or poly(vinyl chloride) (PVC) will be used as the polymer in the film nanocomposite. PVC will be used since it is very popular commercially due to its versatility, is a cost-efficient thermoplastic and has excellent weathering properties. It is also a clear material depending on the composition. The PVC film will be bought form Alibaba.
As mentioned before silver nanoparticles have very good low-e properties. Silver nanodisks will be produced, according to N. Kiyoto et al. 6 the shape of nanodisks is an important parameter to control resonance wavelengths. They found that round silver nanoparticles are resonant with visible light. However, if the nanodisks are shaped into flat disks, by adjusting the aspect ratio (division of the equivalent circle diameter by thickness), silver nanoparticles can be resonant with a wider range of wavelengths from visible to infrared light.
TiO2will be used as a dielectric layer which protects the silver nanoparticles and act as an anti-reflective layer for visible light. The synthesis of TiO2 will be through a Sol-Gel process.
Materials used for the silver nanoplates are the following vial (liquid scintillation vial, with a white cap and polyethylene liner, Research Products International, Chicago, IL), ethylene glycol (EG, J. T. Baker, 9300-01), AgNO3 (Aldrich, 209139-25G) and PAM (Aldrich, 43494-9, M.W.=10,000, 50 wt.% solution in water).
For the TiO2 nanoparticles synthesis the following materials will be bought Analytical grade of Titanium isopropoxide Ti(OCH(C3H8 )2)4, anhydrous 2-propanol
(C3H8O) were procured from SIGMA-Aldrich, whereas acrylic binder (VISYCRYL-8350) from N. R. Chemicals.
The silver nanoplates is divided in two main steps that are divided in triangular nanoplates synthesis and then in a hexagonal nanodisks.
To accurately predict properties such as light scattering decided by the shape and arrangement of nanoparticles, Kiyoto et al. 7 employed a computational electromagnetics technique, the finite-difference time- domain (FDTD) method. They came to the conclusion that, if silver nanodisks can be arranged like a stone pavement the intended properties could be realized.
By adjusting the aspect ratio of silver nanodisks it is possible for silver nanoparticles to be resonant with a wider range of wavelengths form visible to infrared light. Fig. 4 7 shows examples of the extinction cross section calculated with different aspect ratios of silver nanodisks. The results show that nanodisks with an aspect ratio of about 10 or larger can be resonant in the infrared region.
15201908890Fig 4. Resonance wavelength variation according to aspect ratio of silver nanodisks 7
00Fig 4. Resonance wavelength variation according to aspect ratio of silver nanodisks 7
Preparation of Ag triangular nanoplates
Y. Xiong 9 used relatively easy method to generate triangular nanoplates that consists in; a small amount of ethylene glycol hosted in a 20- mL vial, and heated in air under magnetic stirring at 135 °C for 1 h. At the same time, 0.024 g of AgNO3 was dissolved in 0.50 mL of EG and 0.17 mL of PAM was mixed with 0.33 mL of EG at room temperature. These two solutions were then added simultaneously into the vial using glass pipettes. After the vial had been capped, the reaction was allowed to proceed at 135 °C for 3 h. The product was collected by centrifugation and washed with water three times to remove EG and excess PAM.
Preparation of Ag hexagonal nanodisks
Y. Chao 10 reported the following steps to achieve the optimal aspect ratio of the silver nanodisks; 10.0 mL of PVP (0.001 mol L-1), 0.1 mL of AgSCN (0.5 mol L-1) and 0.6 mL of N2H4_H2O (85 wt%) were added into 250.0 mL of the seed solution and reacted for 10 min at 300C under stirring.Then 100.0 mL of AgNO3 (0.2 mol L-1) solution was added at a rate of 400.0 µL min_1 by a peristaltic pump. In this process, two five-way flow regulating valves were used. The reaction was retained for 30 min before the addition of AgNO3 solution finished. The products were collected by centrifugation at 8000 rpm for 5 min and washed with water for several times.
Fig. 5 Triangular to Hexagonal silver nanodisks
Preparation of TiO2 nanoparticles
Sol-gel technique has been used to synthesize TiO2 nanoparticles at room temperature 11. In a typical experiment, titania isopropoxide was dissolved in 100 ml of anhydrous 2-propanol (0.4 M). A measured quantity of distilled water was mixed with 100 ml of anhydrous 2-propanol to prepare a second solution. Both the solutions were covered and stirred for 45 minutes. With the help of a burette, the water solution was added drop-wise to the first solution under constant magnetic stirring for another 6 hours. As a result of hydrolysis of titania isopropoxide, the color of solution changed from transparent to white indicating the formation of precipitates. Different solutions were prepared by varying the L (ratio of molar concentration of water to that of alkoxide precursor) within the range of 20 – 50. The reaction of hydrolysis of the titania isopropoxide proceeds as follows:
Ti(C3H7O)3 + 2( +n) H2O ?TiO2 .nH2O+4C3H7OHThe precipitates were filtered and dried in an oven at 60°C for 8 hours; after crushing in pestle and mortar, powder of titania was obtained. The dried powder was then calcined at 350°C, 550°C, 750°C and 900°C for 15 minutes to remove the solvent completely as well as to crystallize the amorphous titania powder.
Measurement of diffuse reflectance
The diffuse reflectance measurement of the nanoparticles was performed by UV-Visible spectrophotometer, attached with integrating sphere to spatially integrate the radiant flux. For the measurement, the nanoparticles were pressed into thick pellet, and placed at the entrance port of the integrating sphere. The same set up was used to measure diffuse reflectance of the developed reflectors. Calibration of the reflectance scale was done by standard reference material (WS-1-SL, Spectralon) as used by Kumbar et al. 11
The obtained TiO2 nanoparticles can be deposited on the first PVC film through magnetron sputtering. The most common magnetron sputter cathode/target shapes are circular and rectangular. Circular sputtering magnetrons are more commonly found in smaller scale “Confocal” batch systems or single wafer stations. The second film will be placed on top after the nanoparticle deposition, again through magnetron sputtering the nanodisks will be deposed to the surface of the second PVC film followed by the deposition of TiO2 having two thin nano-layers. The TiO2 coating is in order to act as a dielectric layer and also to protect the silver nanodisks form degradation. Finally the third and final PVC film will be placed over the sputtered surface in order to have completed the low-emissivity film composite of PVC/Ag NPs/ TiO2 NPs. Graphic representation shown on figure 6.
Figure 6. Visual representation of Nanocomposite thin film
Silver nanodisks are distributed widely throughout the layer thickness while in the latter are aligned on the same plane (figure 7). Kiyoto et al. 6 reported that the properties gained an infrared radiation of 900 nm set as the central value for plasmon resonance. The results revealed that in the resonant state, energy is lost mainly by absorption after being scattered but, in the resonant state of the latter, reflection is dominant over the energy loss.
That could occur because the dense arrangement of silver nanodisks on the same plane allows plasmon resonance to grow into an electromagnetic field vibration over a large region across multiple nanoparticles, which facilitates the release of electromagnetic waves of light to the outside.
168211511430Fig. 7 SEM images of silver nanodisks
00Fig. 7 SEM images of silver nanodisks
TEM images were captured (fig 8) using a Phillips 420 transmission electron microscope operated at 120 kV. High-resolution TEM images were taken on a JEOL 2100F field-emission microscope or a JEOL 2010 LaB6 high-resolution microscope operated at 200 kV.
center150495Fig. 8 TEM images of silver nanodisks
00Fig. 8 TEM images of silver nanodisks
Measurement and calculation method
The Ultraviolet Solar Transmittance
The Ultraviolet Solar Transmittance (Tuv) is given by the following expression 1:
where S? = relative spectral distribution of ultraviolet solar radiation 1, T(?)¼spectral transmittance of the glass, ? = wavelength, ?? = wavelength interval. The Tuv value will thus be a number between 0 and 1, calculated in the ultraviolet part of the solar spectrum, i.e. 300-380 nm. A low number indicates a low transmission of ultraviolet solar radiation, whereas a high number represents a high ultraviolet solar radiation transmission. In common usage the Tuv values may often be chosen in percentage, i.e. between 0 and 100%
Visible solar transmittance
The Visible Solar Transmittance (Tvis), often denoted Light Transmittance, is given by the following expression 1:
where D? = relative spectral distribution of illuminant D65 1, V(?) = spectral luminous efficiency for photopic vision defining the standard observer for photometry, T(?) = spectral transmittance of the glass, ? = wavelength, ?? = wavelength interval, The Tvis value will thus be a number between 0 and 1, calculated in the visible part of the solar spectrum, i.e. 380- 780 nm. A low number indicates a low transmission of visible light, whereas a high number represents a high visible light transmission.
Solar heat gain and glare are among the few downsides of daylighting, negatively impacting occupant comfort levels and driving up cooling-related energy consumption. In addition, there is also the benefits from using low-e materials for windows. In recent years, it has become more common to design buildings with extensive glass facades. Hence, the focus on more energy-efficient glazing solutions has increased to meet the challenge of reducing heating and cooling loads. With this nanocomposite film UV light and IR light will be reflected at a much more efficient way and it is also cheaper than the current glass coating. The processes can also be faster since they can be worked on separately. This film will not affect the visibility light that goes through the glass because of the TiO2 properties.
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