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Keywords: Aircraft instrumentation, Phase change material, Packaging
To better understand the thermal behaviour of critical components, it may be interesting to integrate advanced measuring systems (electronics systems) in harsh thermal environments. For example, by integrating an infrared camera in the engine compartment of an aircraft, a thermal mapping of various components during flight tests can be obtained. The operating temperature of these cameras does not exceed 80 � C while the thermal environment can vary transiently between 40 � C and 120 � C for the coldest engine compartments. [1 & 2] show that PCM-based design is an excellent candidate design for transient electronic cooling applications and IR camera. The purpose of this work is to ensure acceptable temperature for advanced but sensitive flight test instrumentation without disturbing the environment thermal behaviour and involving a too complex integration (such as dedicated cooling or blowers). Attention has been paid to the easy adaptation of the prototypes to various electronics systems. Several studies have looked specifically avionics and spacecraft applications with Phase Change Materials (PCM) to limit thermal peak [3, 4, 5, and 6]. Without protection the IR camera temperature is too high during several flight phases: such as ground or take-off phases. Integration of PCM must protect the camera primarily during these two phases. However, insulation of the camera from the external environment should not induce excessive internal temperature rises due to its dissipated power. A double encapsulation with PCM and thermal gel should help manage both functions. In this study, a new approach is proposed with an innovative passive packaging (double encapsulation) integrating PCM and thermal gel with a high thermal conductivity. This solution can reduce thermal peaks and can smooth the temperature of the camera in a harsh environment. This study presents a sizing approach with 3D simulations using COMSOL Multiphysics. The impact of different parameters was studied (PCM materials, PCM quantity, casing materials, design, dissipated power�) to investigate the thermal performance and assess the efficiency of each solution with respect to objectives. A review of several PCM materials can be found in several publications [7, 8, and 9]. Modeling shows that PCM with thermal gel could be an attractive option for passive thermal control in an aircraft environment (camera temperature below 80�C). Based on modeling, a prototype was manufactured and integrated into a representative test bench. The IR camera is integrated into a double encapsulation comprising a thermal gel with good conductivity and a PCM. The PCM is a paraffin RT64HC from Rubitherm. The prototype was instrumented with several thermocouples and was tested in a representative test bench where the flow and the temperature of the air can be varied A comparison was made between a component with and without double encapsulation. The results show the interest of this double encapsulation which makes it possible to smooth the temperature (interest for the measurements) and to limit the thermal peaks of the component. The packaging keeps the temperature of the camera stable at 65 � C while the thermal environment fluctuates between 40 and 120 � C during several hours. The ability of the solution to maintain its performance over time is very important for this application. In the literature, there is not a lot of data concerning lifetime and evolution of PCMs. Several publications have shown variations in thermal properties for different PCMs [10, 11 and 12]. Generally, paraffin wax has a good stability and is chemically stable. Cyclability tests were be performed to evaluate the stability of the PCM and its compatibility with the material of the container (Aluminum alloys). The purpose is to give first elements related to the durability of the solution. Material compatibility and aging tests were also carried out (520 cycles). These tests did not reveal any significant evolution of the thermal properties of the material chosen. A PCM which shows chemical stability and compatibility with other materials is a good candidate. Similar behaviour between experimental results and modelling results was obtained. These results showed that innovative packaging with a double encapsulation comprising PCM and thermal gel could be an attractive solution for thermal management of an electronic system in aircraft environment. This solution could also be proposed for other applications (automotive, aerospace...).
Sebastien QUENARD, R&D Engineer
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