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Pointe-Claire, Canada

Hendaoui A.,INRS - Institute National de la Recherche Scientifique | Emond N.,INRS - Institute National de la Recherche Scientifique | Dorval S.,INRS - Institute National de la Recherche Scientifique | Chaker M.,INRS - Institute National de la Recherche Scientifique | Haddad E.,MPB Communications Inc.
Current Applied Physics | Year: 2013

This work involves the study of the positive emittance-switching (i.e. emittance that increases with increasing the temperature) of thermochromic VO2 films deposited using reactive pulsed laser deposition (RPLD) on Al substrates. The temperature dependence of the emittance of a 260 nm-thick VO2 film on Al substrate revealed a maximum of the emittance of 0.29 around 68°C. It is attributed to an increase in the infrared radiation absorption by the VO2 film due to the coexistence of both insulating and metallic phases in the vicinity of the transition temperature of VO 2. The emittance tunability between 25°C and 68°C is 0.21. Since practical SRD application requires both high emittance at high temperature and large tunability, we demonstrate, by both simulation and fabrication, that these goals can be accomplished to some extent by a top dielectric a-Si:H/SiO2 λ/4 stack layer. In fact, the addition of a-Si:H/SiO2 λ/4 overlayer results in an increase of the maximum value of the emittance by 114% (from 0.29 to 0.62) as well as an increase of the tunability by 81% (from 0.21 to 0.38). This work reports an important improvement of the positive emittance-switching efficiency of the VO2-based structures and holds promise for a new generation of smart radiator devices (SRDs) for a passive thermal control of spacecrafts. © 2013 Elsevier B.V. All rights reserved.


Hendaoui A.,University of Quebec | Emond N.,University of Quebec | Chaker M.,University of Quebec | Haddad E.,MPB Communications Inc.
Applied Physics Letters | Year: 2013

This paper describes a VO2-based smart structure with an emittance that increases with the temperature. A large tunability of the spectral emittance, which can be as high as 0.90, was achieved. The transition of the total emittance with the temperature was fully reversible according to a hysteresis cycle, with a transition temperature of 66.5 °C. The total emittance of the device was found to be 0.22 and 0.71 at 25 °C and 100 °C, respectively. This emittance performance and the structure simplicity are promising for the next generation of energy-efficient cost-effective passive thermal control systems of spacecrafts. © 2013 American Institute of Physics.


Gong S.,York University | Zhu Z.H.,York University | Haddad E.I.,MPB Communications Inc.
Journal of Applied Physics | Year: 2013

This paper investigates the effect of carbon nanotube (CNT) deformation on the electrical conductivity of CNT polymer composites at crossed nanotube junctions using a revised 3-dimensional CNT percolating network model. Two aspects of the work are considered. The first is concerned with the effect of CNT deformation on its intrinsic and contact resistances at CNT-CNT junctions. An analytical model based on electron ballistic tunneling theory and Landauer-Büttiker formula is proposed to describe the variation of CNT-CNT contact resistance at the CNT-CNT junction in terms of local deformation of CNT walls and CNT-CNT distance. In addition, a model exclusively based on experimental data to describe the change of CNT intrinsic resistance in terms of its cross-section deformation is adopted. The second is concerned with the relationship among the CNT-CNT distance, the angle between two adjacent CNTs, and the dimensions of local deformation of CNT walls and its impact on the corresponding intrinsic and contact resistance of CNTs near and at a CNT-CNT junction. Finally, Monte Carlo simulations are conducted to evaluate these effects on the electrical conductivity of nanocomposites for different CNT weight fractions. Our results reveal that the local deformation of CNT walls plays a significant role in the evaluation of electrical conductivity of CNT polymer composites. The intrinsic resistance in the deformed part of CNTs near a CNT-CNT junction increases much faster than the decrease of CNT-CNT contact resistance at the same junction when two CNTs are getting closer, resulting in a net increase of resistance at the junction. Numerical results show that the current model agrees with existing experimental data better than existing models without considering the effect of CNT deformation, which tends to overestimate the electrical conductivity of CNT polymer composites containing homogeneously dispersed percolating CNT network. © 2013 AIP Publishing LLC.


Hendaoui A.,INRS - Institute National de la Recherche Scientifique | Emond N.,INRS - Institute National de la Recherche Scientifique | Dorval S.,INRS - Institute National de la Recherche Scientifique | Chaker M.,INRS - Institute National de la Recherche Scientifique | Haddad E.,MPB Communications Inc.
Solar Energy Materials and Solar Cells | Year: 2013

One of the major challenges facing the use of thermochromic coatings as efficient Smart Radiator Devices (SRDs) for a passive energy-efficient thermal control of spacecrafts is the limited high temperature emittance that can be achieved with present devices. Through an elegant choice of the thickness of an SiO2 layer being part of a VO2-based SRD multilayer structure, we obtained an emittance as high as 0.80 at high temperature, while maintaining a large emittance tunability. More interestingly, by doping VO 2 with tungsten (W), the transition temperature of pure VO 2 occurring originally at about 68 C could be decreased down to about 19.5 C with 2.9 at% of W. The average decrease rate of the transition temperature was found to be ~16.5 C per 1 at% of W. This reduction was accompanied by a displacement of the emittance transition region that can be tailored over a wide range of temperatures, and by a shrinking of the hysteresis width. This work constitutes an important breakthrough toward practical applications of thermochromic VO2-based smart coatings for passive energy-efficient thermal control of spacecrafts close to room temperature. © 2013 Elsevier B.V.


Patent
MPB Communications Inc. | Date: 2010-06-08

The emittance value is a measure of an amount of energy expelled from a given surface area relative to a black-body reference. Depending on the specific coating a change in the emittance value is actively or passively effected. There are known active variable emittance thermal control coatings. However, such coatings are actually panels housing a mixture of both high and low emissivity materials that are electrically manipulated to control the emittance value of the panel. These coatings are classified as either electrochromic or electrophorectic. Both electrochromic and electrophorectic coatings require an applied voltage to cause a change in the emittance value of the coating. By contrast, aspects of the present invention do not include active variable emittance thermal control coatings. Aspects of the present invention do include passive variable emittance thermal control coatings and materials. In accordance with aspects of the present invention passive means that the variable emittance value changes in response to changes in the environment without active control (e.g. neither a voltage nor a current is applied). More specifically, in accordance with one aspect of the invention a passive variable emittance thermochromic material is provided that has a relatively low emittance value at low temperatures and a relatively high emittance value at high temperatures.

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