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|Title:||Boron nitride carbon foams for various thermal applications||Authors:||Loeblein, Manuela||Keywords:||DRNTU::Engineering::Electrical and electronic engineering||Issue Date:||2018||Source:||Loeblein, M. (2018). Boron nitride carbon foams for various thermal applications. Doctoral thesis, Nanyang Technological University, Singapore.||Abstract:||Three-dimensional graphene and hexagonal-boron nitride structures (3D-C and 3DBN) have recently attracted attention due to their enhanced mechanical, handling and surface area properties, while maintaining the well-reported properties of their twodimensional constituents. In this Thesis, we developed a new three-dimensional hexagonal-boron nitride carbon (3D-BNC) structures, a hybridized of the two, using chemical vapor deposition (CVD) of CH4/Ethanol and ammonia borane to further widen their application areas. Through the ability to specifically control the compositions of C and BN (demonstrated through Raman, X-ray photoelectron and energy dispersive Xray spectroscopy), a highly tunable electrical conductivity (0 – 0.6 Scm-1, measured using the Van-der-Pauw technique), controllable EMI shielding properties (0 – ~50 dB, measured through microwave network analyzer), while maintaining a high and stable thermal conductivity (0.84 – 1.2 Wm-1K-1, from laser flash measurements) is obtained. One of the key advantages of such 3D materials is for thermal management in electronics. The stable thermal conductivity, compressibility and temperature stability of the 3D-foams are ideal conditions to achieve improved thermal management performance. Through compression of the foams, a high cross-plane thermal conductivity of 62 – 86 Wm-1K-1, as well as excellent surface conformity is demonstrated. These values are among the highest cross-plane conductivities of freestanding graphene or h-BN structures, and in the same range of eutectic metal foils. Evaluation of the thermal extraction efficiency on a state-of-the-art 2.5D electronic platform along with state-of-the-art thermal interface materials (TIMs) reveals 3Dfoam’s improved performance of cooling by 20 – 30%, which means a temperature decrease by ΔT of 44 – 24°C. This is colder than any of the commercially available TIMs tested on the same platform (i.e. Sn/Au) and among the highest temperature decrease of hot spots on actual chips reported so far (e.g. highest values for alternative heat spreaders currently under research range around ΔT ~ 13°C for CVD-graphene and ΔT ~ 20°C for exfoliated few-layer graphene). This is a significant decrease, since it is known that the decrease of hot spot temperature by 20°C extends the transistors lifetime by one order of magnitude. In addition, the interconnected structure of these 3D-foams is also ideal as filler material for insulative polymeric film to enhance its electrical and thermal behaviour as the 3D structure prevents inhomogeneous distribution and aggregation commonly faced when using typical nanofillers. To further unveil its potential, we investigate the enhancement in flexible electronics, space shielding materials and as well as thermomechanical actuation in shape memory polymer (SMP). In flexible electronic, we incorporated 3D-C and 3D-BN with the currently state-ofthe-art flexible platform for flexible electronics, polyimide (PI), an improved thermal dissipation by 25-times (5 – 6 Wm-1K-1 ) is obtained, while preserving full flexibility and toughness of the PI. It is shown that these hybrid films can be directly used as printable substrates and can dissipate heat more efficiently from hot spots, which in turn allows increasing the maximum power applicable by at least 50%. In addition, by incorporating 3D-C into PI, also the electrical conductivity is improved, which can be used for other applications, such as space shielding. In the current space shielding materials there is always a coating of conductive material to prevent the build-up of electrostatic charge, however, these are prone to cracks and failure. Since the hybrid of 3D-C with PI has an electrical sheet resistance of 3 Ω/□, which fulfills the antistatic-criterion to dissipate the build-up of electrostatic charge, it is further developed and space-qualified according to European Space Standards, which requires to test its outgassing properties, as well as Gamma Ray, atomic oxygen (AO) and thermal cycling resistance. Through monitoring electrical, weight and optical properties it is shown that it withstands and keeps a stable performance throughout various thermal cycles (from -100°C to +160°C), as well as the oxidative and aggressive environment of ground-based simulated space environments (Gamma ray doses equivalent to 15 years in geosynchronous equatorial orbit, and AO exposure equivalent to 8 months in low Earth orbit). Lastly, the utilization of 3D-fillers in SMP has also been demonstrated. SMPs, despite their easiness in shaping, ultra-light weight and customizability, are limited in their applicability for mechanical actuation, as they are prone to cracks (due to poor thermal conductivity resulting in large thermal gradients across the material leading to internal stress). To target this issue, 3D-BNC foams of varying concentrations are infused with SMPs. Thanks to the homogeneous distribution of the foam within the polymer, a uniform spread of heat is obtained, demonstrated in this work through thermal camera imaging, thus leading to an even transformation of shape. It is demonstrated that through this technique, bigger sample sizes are attainable (maximum sizes without 3D-foam infusion are 3 cm in length, while with the 3D-foam infusion up to 7 cm in length are demonstrated to transform without any cracking). It is shown that the 3D-foams speed up the transformation process by three times, reduce the required energy to initiate the transformation process by 20% and in addition, thanks to the tunability of electrical conductivity of 3D-BNC, a self-heating and timed actuation can be incorporated to the polymer.||URI:||http://hdl.handle.net/10356/73382||Fulltext Permission:||open||Fulltext Availability:||With Fulltext|
|Appears in Collections:||EEE Theses|
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