Boron nitride carbon foams for various thermal applications

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.Doctor of Philosophy (EEE

Effect of annealing temperature on physical properties of nanostructured TiN/3DG composite

Recently, three-dimensional graphene (3DG) has attracted much attention in many research fields due to its unique structure and considerable properties. In order to expand the range of applications of 3DG, the suitable nanomaterials can be grown on its surface. In this study, titanium nitride (TiN) phase was deposited on 3DG porous structure by chemical method. This method contains two steps of immersing 3DG into a solution containing Ti ions and then annealing under ammonia atmosphere. The effect of annealing temperature on type of synthesized phases, their morphology, and stoichiometry was investigated. For this purpose, the samples were annealed at different temperatures (750–900 °C) and analyzed via various techniques. The results showed that increasing annealing temperature results in increased crystallite size and lattice constant, while decreased oxygen content in TiN structure. Annealing at 850 °C resulted in the most stoichiometric composition with titanium/nitrogen atomic ratio of 1.09, which had the lowest electrical resistivity of 0.41 Ω cm and lowest work function of 4.68 eV. After applying TiN, the water contact angle of 3DG (127°) was reduced to lower than 90°. Such TiN/3DG composite can be a promising candidate as an electrode in solar cells.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore

Effect of titanium nitride coating on physical properties of three-dimensional graphene

In this paper, titanium nitride (TiN) was applied on the surface and into the porous structure of three-dimensional graphene (3DG) by chemical method. This method consists of immersing 3DG into a solution containing Ti ions and annealing under ammonia atmosphere at 850 °C. The effects of TiN coating and high temperature annealing under NH3 on the physical properties of 3DG were investigated. For this purpose, the 3DG samples, with and without TiN coating, were characterized via XRD, SEM, XPS, and Raman spectroscopy. Then, the electrical resistivity, work function, and wettability of samples were determined by Van der Pauw method, contact angle meter, and UV photoelectron spectroscopy (UPS), respectively. The results showed that an almost pure and very crystalline TiN phase with titanium/nitrogen atomic ratio of 1.09 was formed on the 3DG network. Annealing of 3DG under NH3 resulted in locally doping of graphene with nitrogen and generation of defects in its structure. After TiN coating, the work function value of 3DG (5 eV) was reduced to 4.68 eV, while its initial water contact angle decreased from 127° to 83°.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore

Configurable three-dimensional boron nitride-carbon architecture and its tunable electronic behavior with stable thermal performances

Recent developments of 3D-graphene and 3D-boron-nitride have become of great interest owing to their potential for ultra-light flexible electronics. Here we demonstrate the first synthesis of novel 3D-BNC hybrids. By specifically controlling the compositions of C and BN, new fascinating properties are observed, such as highly tunable electrical conductivity, controllable EMI shielding properties, and stable thermal conductivity. This ultra-light hybrid opens up many new applications such as for electronic packaging and thermal interface materials (TIMs)

Flexible thermal rectifier based on macroscopic PDMS@graphite composite film with asymmetric cone-shape interfaces

Thermal rectifier is a device with the higher heat transport capacity in one direction than the backward one, being similar to the electrical diode working for the control of the electrical current. In this work, we report a new thermal rectifier based on the flexible macroscopic polydimethylsiloxane (PDMS) film with asymmetric cone-shape holes embedded with micrometer sized graphite powder (denoted as PDMS@graphite). The PDMS@graphite shows thermal rectification behavior with an extracted thermal rectification coefficient of 1.1326 ± 0.009 under 129.8 K temperature bias, and this value can be further modulated by changing the asymmetric ratio of the cone-shape interface in the PDMS@graphite film. Two underlying mechanisms are invited to explain the thermal rectification effect in the PDMS@graphite system. The one is the opposite temperature dependence of thermal conductivity for PDMS and graphite powder. The other one is the different temperature dependent thermal conductivity of the asymmetric cone-shape graphite in PDMS@graphite film in the forward and backward heating direction when applying the same temperature bias, which can be demonstrated in the COMOSL theoretical simulated temperature distributions for the PDMS@graphite. The as-fabricated flexible macroscopic PDMS@graphite composite film thermal rectifier may provide the potential applications in thermal control and management.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore

Three-Dimensional Graphene: A Biocompatible and Biodegradable Scaffold with Enhanced Oxygenation

International audienc

Direct growth of nanocrystalline hexagonal boron nitride films on dielectric substrates

Atomically thin hexagonal-boron nitride (h-BN) films are primarily synthesized through chemical vapor deposition (CVD) on various catalytic transition metal substrates. In this work, a single-step metal-catalyst-free approach to obtain few- to multi-layer nanocrystalline h-BN (NCBN) directly on amorphous SiO2/Si and quartz substrates is demonstrated. The as-grown thin films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited substrate as inspected using optical and atomic force microscopy. The starting layers of NCBN orient itself parallel to the substrate, initiating the growth of the textured thin film. Formation of NCBN is due to the random and uncontrolled nucleation of h-BN on the dielectric substrate surface with no epitaxial relation, unlike on metal surfaces. The crystallite size is ∼25 nm as determined by Raman spectroscopy. Transmission electron microscopy shows that the NCBN formed sheets of multi-stacked layers with controllable thickness from ∼2 to 25 nm. The absence of transfer process in this technique avoids any additional degradation, such as wrinkles, tears or folding and residues on the film which are detrimental to device performance. This work provides a wider perspective of CVD-grown h-BN and presents a viable route towards large-scale manufacturing of h-BN substrates and for coating applications.Published versio

Human rett-derived neuronal progenitor cells in 3D graphene scaffold as an in vitro platform to study the effect of electrical stimulation on neuronal differentiation

Studies of electrical stimulation therapies for the treatment of neurological disorders, such as deep brain stimulation, have almost exclusively been performed using animal-models. However, because animal-models can only approximate human brain disorders, these studies should be supplemented with an in vitro human cell-culture based model to substantiate the results of animal-based studies and further investigate therapeutic benefit in humans. This study presents a novel approach to analyze the effect of electrical stimulation on the neurogenesis of patient-induced pluripotent stem cell (iPSC) derived neural progenitor cell (NPC) lines, in vitro using a 3D graphene scaffold system. The iPSC-derived hNPCs used to demonstrate the system were collected from patients with Rett syndrome, a debilitating neurodevelopmental disorder. The graphene scaffold readily supported both the wild-type and Rett NPCs. Electrical stimulation parameters were optimized to accommodate both wild-type and Rett cells. Increased cell maturation and improvements in cell morphology of the Rett cells was observed after electrical stimulation. The results of the pilot study of electrical stimulation to enhance Rett NPCs neurogenesis were promising and support further investigation of the therapy. Overall, this system provides a valuable tool to study electrical stimulation as a potential therapy for neurological disorders using patient-specific cells.NRF (Natl Research Foundation, S’pore)Accepted versio

Novel timed and self-resistive heating shape memory polymer hybrid for large area and energy efficient application

Shape memory polymers (SMPs) are a polymeric smart material that can register two or more temporary shapes and transform to one another through an external stimulus. Despite their compactness and customizability, SMPs haven't been able to be adopted for mainstream applications. Since the majority of SMPs are triggered by heat, and SMPs have a very poor thermal conductivity, large thermal gradients within the polymer appear which cause slow response, inhomogeneous heat distribution and thus non-uniform transformation of shapes and cracks. Many have attempted to improve their thermal performance through the incorporation of filler-based nanomaterials. However, the outcome is ineffective as the spatial dispersion of fillers within the SMP is inhomogeneous and leads to performance loss. Contrastingly, the herein presented new class of nanocomposite-SMP, composed by 3D-foam fillers, showcase a much more efficient SMP adaptable to larger area with faster transformation speed and without any performance loss. Furthermore, the improved thermal properties lead to a decrease in required input energy, as well as render the SMP a self-heating capability which can be further designed into timed multi-step SMP behavior

Trimethylamine Borane: A New Single-Source Precursor for Monolayer h‑BN Single Crystals and h‑BCN Thin Films

Due to their exceptional chemical and thermal stabilities as well as electrically insulating property, atomically thin hexagonal boron nitride (h-BN) films have been identified as a promising class of dielectric substrate and encapsulation material for high-performance two-dimensional (2D) heterostructure devices. Herein, we report a facile chemical vapor deposition synthesis of large-area atomically thin h-BN including monolayer single crystals and C-doped h-BN (h-BCN) films utilizing a relatively low-cost, commercially available trimethylamine borane (TMAB) as a single-source precursor. Importantly, pristine 2D h-BN films with a wide band gap of ∼6.1 eV can be achieved by limiting the sublimation temperature of TMAB at 40 °C, while C dopants are introduced to the h-BN films when the sublimation temperature is further increased. The h-BCN thin films displayed band gap narrowing effects as identified by an additional shoulder at 205 nm observed in their absorbance spectra. Presence of N–C bonds in the h-BCN structures with a doping concentration of ∼2 to 5% is confirmed by X-ray photoelectron spectroscopy. The inclusion of low C doping in the h-BN films is expected to result in constructive enhancement to its mechanical properties without significant alteration to its electrically insulating nature. This study provides new insights into the design and fabrication of large-area atomically thin h-BN/h-BCN films toward practical applications and suggests that the range of precursors can be potentially extended to other anime borane complexes as well