Sabyasachi Ganguli

Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions

The establishment of covalent junctions between carbon nanotubes (CNTs) and the modification of their straight tubular morphology are two strategies needed to successfully synthesize nanotube-based three-dimensional (3D) frameworks exhibiting superior material properties. Engineering such 3D structures in scalable synthetic processes still remains a challenge. This work pioneers the bulk synthesis of 3D macroscale nanotube elastic solids directly via a boron-doping strategy during chemical vapour deposition, which influences the formation of atomic-scale “elbow” junctions and nanotube covalent interconnections. Detailed elemental analysis revealed that the “elbow” junctions are preferred sites for excess boron atoms, indicating the role of boron and curvature in the junction formation mechanism, in agreement with our first principle theoretical calculations. Exploiting this material’s ultra-light weight, super-hydrophobicity, high porosity, thermal stability, and mechanical flexibility, the strongly oleophilic sponge-like solids are demonstrated as unique reusable sorbent scaffolds able to efficiently remove oil from contaminated seawater even after repeated use.

Covalently Interconnected Three- Dimensional Graphene Oxide Solids

The creation of three-dimensionally engineered nanoporous architectures via covalently interconnected nanoscale building blocks remains one of the fundamental challenges in nanotechnology. Here we report the synthesis of ordered, stacked macroscopic three-dimensional (3D) solid scaffolds of graphene oxide (GO) fabricated via chemical cross-linking of two-dimensional GO building blocks. The resulting 3D GO network solids form highly porous interconnected structures, and the controlled reduction of these structures leads to formation of 3D conductive graphene scaffolds. These 3D architectures show promise for potential applications such as gas storage; CO2 gas adsorption measurements carried out under ambient conditions show high sorption capacity, demonstrating the possibility of creating new functional carbon solids starting with two-dimensional carbon layers.

Importance of interfaces in governing thermal transport in composite materials: modeling and experimental perspectives.

Thermal management in polymeric composite materials has become increasingly critical in the air-vehicle industry because of the increasing thermal load in small-scale composite devices extensively used in electronics and aerospace systems. The thermal transport phenomenon in these small-scale heterogeneous systems is essentially controlled by the interface thermal resistance because of the large surface-to-volume ratio. In this review article, several modeling strategies are discussed for different length scales, complemented by our experimental efforts to tailor the thermal transport properties of polymeric composite materials. Progress in the molecular modeling of thermal transport in thermosets is reviewed along with a discussion on the interface thermal resistance between functionalized carbon nanotube and epoxy resin systems. For the thermal transport in fiber-reinforced composites, various micromechanics-based analytical and numerical modeling schemes are reviewed in predicting the transverse thermal conductivity. Numerical schemes used to realize and scale the interface thermal resistance and the finite mean free path of the energy carrier in the mesoscale are discussed in the frame of the lattice Boltzmann-Peierls-Callaway equation. Finally, guided by modeling, complementary experimental efforts are discussed for exfoliated graphite and vertically aligned nanotubes based composites toward improving their effective thermal conductivity by tailoring interface thermal resistance.

Vertically-aligned carbon nanotubes infiltrated with temperature-responsive polymers: smart nanocomposite films for self-cleaning and controlled release

We have demonstrated that the infiltration of temperature-responsive polymers (e.g., PNIPAAm) into vertically-aligned carbon nanotube forests created synergetic effects, which provided the basis for the development of smart nanocomposite films with temperature-induced self-cleaning and/or controlled release capabilities.

Prediction of the transverse thermal conductivity of pitch-based carbon fibers

In this paper, we utilized a bottom-up method to predict the transverse thermal conductivity of pitched-based carbon fibers. We used molecular dynamics simulations with Green-Kubo formalism to calculate the in-plane thermal conductivity and out-of-plane thermal conductivity of the graphite sheets. The effects of waviness on the thermal conductivity of the graphite sheets were studied by MD simulations. The calculated in-plane thermal conductivity and out-of-plane thermal conductivity of graphite sheets from MD simulations were then used for the prediction of transverse thermal conductivity of the pitch fibers by finite element method. In the finite element simulations, the waviness in the graphite sheets was found to decrease the transverse thermal conductivity of pitch fibers, though not significantly. The defects observed in the pitch fibers were simulated by the damage elements in the finite element analysis. The simulation results showed that the proposed model, in which 12.5% of damage was included, predicted the effective transverse thermal conductivity well compared to the value measured from experiments.

Improved Flow Rate in Electro-Osmotic Micropumps for Combinations of Substrates and Different Liquids With and Without Nanoparticles

A new design for an electro-osmotic flow (EOF) driven micropump was fabricated. Considering thermal management applications, three different types of micropumps were tested using multiple liquids. The micropumps were fabricated from a combination of materials, which included: silicon-polydimethylsiloxane (Si-PDMS), Glass-PDMS, or PDMS-PDMS. The flow rates of the micropumps were experimentally and numerically assessed. Different combinations of materials and liquids resulted in variable values of zeta-potential. The ranges of zeta-potential for Si-PDMS, Glass-PDMS, and PDMS-PDMS were −42.5–−50.7 mV, −76.0–−88.2 mV, and −76.0–−103.0 mV, respectively. The flow rates of the micropumps were proportional to their zeta-potential values. In particular, flow rate values were found to be linearly proportional to the applied voltages below 500 V. A maximum flow rate of 75.9 μL/min was achieved for the Glass-PDMS micropump at 1 kV. At higher voltages nonlinearity and reduction in flow rate occurred due to Joule heating and the axial electro-osmotic current leakage through the silicon substrate. The fabricated micropumps could deliver flow rates, which were orders of magnitude higher compared to the previously reported values for similar size micropumps. It is expected that such an increase in flow rate, particularly in the case of the Si-PDMS micropump, would lead to enhanced heat transfer for microchip cooling applications as well as for applications involving micrototal analysis systems (μTAS).

ENHANCED ELECTRO-OSMOTIC FLOW PUMP FOR MICRO-SCALE HEAT EXCHANGERS

Non-uniform heat flux generated by microchips can create “hot spots” in localized areas on the microchip surface. This research presents an improved design of an active cooling electro-osmotic flow (EOF) based micro-pump for hot spots thermal management. The design of the micro-pump was simpler and more practical for the application compared to designs presented in literature.Most micro-channel heat sink devices presented in literature were silicon based. Though silicon has better thermal conductivity when compared to polymers used in micro-devices fabrication, processes of silicon fabrication are complicated and time consuming. Also, most micro-channel fabrication processes use silicon etching which leads to rough walls within the micro-channel. An improved design, which uses a combination of silicon and Polydimethylsiloxane (PDMS), is being developed and tested. The main idea of this design is to utilize the favorable thermal properties of silicon while achieving both smoother charged surfaces and ease of fabrication of PDMS material.The EOF micro-pump was tested for four cooling fluid namely, DI water, distilled water, borax buffer, and Al2O3 nano-particles suspended in water solution. A maximum flow rate of 31.2 μL/min was achieved using distilled water at 500 V of EOF voltage. Such micro-pump with this flow rate range can be implemented in a closed loop heat rejection system for hot spot thermal management. Moreover, it can be used in Lap-on-chip and uTAS application for sample transport.© 2012 ASME

Thermal conductivity measurement at micrometer length scales based on a temperature-balance method

We have developed a novel thermal conductivity measurement technique that is applicable to micro- and nanoscale structures. The methodology integrates a device consisting of two identical microheaters with thin-filmed platinum heating elements and integrated resistance temperature devices (RTDs) on trenched, thermally isolated silicon substrates. The platinum RTDs used for temperature measurement were calibrated by monitoring the melting points of three metallic microspheres placed on the device as its power was increased. We validated our measurement technique by measuring thermal conductivities of several micrometer-sized metallic wire standards with known values. A multiphysical analysis based on a 3D finite element method has been conducted to simulate both the electric and thermal behaviors of the microheaters. These simulations describing the heating and heat flow in the device yield similar temperature profiles near the microheaters obtained in the experimental system.

Hybrid Nanomaterials for Flexible Electronics Interconnects

In emerging flexible electronics survivable to high strain-rate deformation (high impact environment), interconnects materials are to exhibit high strain to failure characteristics while maintaining the desired electrical, and in high power applications thermal properties as well. In this work we present a novel nano materials possessing high strain to failure properties with desired electrical and thermal characteristics. A junctioned interconnected network of nano materials (carbon nanofibers, in our case) is embedded in highly flexible polymers. Atomistic scale simulation reveals that design of network junctions critically influence the electrical and thermal properties, whereas the flexibility in the network provides the strain resiliency. The network contact electrical conductance is influenced by the overlapping electronic orbitals of the adjacent (joining nano elements) at the junction, whereas, the junction thermal conductance depends on the matching of the atomic mass and atomic interaction potentials of the junction materials composition. To facilitate welding of the junctions of the nano elements, junctions with metallic nano particles (Ti, Cr, Au, Ag) have also been studied. On processing of such junctioned hybrid network of carbon nanofibers in flexible polymers, bio-inspired Peptide assisted Au nanoparticle dispersion on carbon nanofibers is being pursued to create metallic nano junctions. In addition, characteristics for direct printing (additive manufacturing) of the material is demonstrated. Both the computational and supporting experimental work will be presented to discuss the potential of this novel hybrid nano material concept for high flexibility and strain resiliency as a viable interconnect materials for flexible electronics.

Thermally Conductive Epoxy Nanocomposites

The quest for improvement of thermal conductivity in aerospace structures is gaining momentum. This is even more important as modern day aerospace structures are embedded with electronics which generate considerable amounts of heat energy. This generated heat if not dissipated might potentially affect the structural integrity of the composite structure. The use of polymer based composites in aerospace applications has also increased due to their obvious superior specific properties. But the thermal conductivity of the polymer matrix is very low and not suited for the design demands in aerospace applications. Several research studies have been conducted to improve the thermal conductivity of the polymeric composites. Different fillers have been used to improve the thermal conductivity of the polymeric matrix. Fillers may be in the form of fibers or in the form of particles uniformly distributed in the polymer matrix. The thermophysical properties of fiber filled composites are anisotropic, except for the very short, randomly distributed fibers, while the thermophysical properties of particle filled polymers are isotropic. Numerous studies have also been conducted in recent years where nanoparticles have been dispersed in the polymeric matrix to improve the thermal conductivity. Putman et al. [1] used the 3ω method to study the thermal conductivity of composites of nanoscale alumina particles in polymethylmethacrylate (PMMA) matrices in the temperature range 40 to 280 K. For 10% of 60 nm of alumina particle filler by weight (3.5% by volume) thermal conductivity of the composite slightly decreased at low temperatures. Whereas, above 100 K, thermal conductivity of the nanocomposite increased by 4% at room temperature. Kruger and Alam [2] studied the thermal conductivity of aligned, vapor grown carbon nanoscale fiber reinforced polypropylene composite. They measured thermal conductivity by laser flash instrument in the longitudinal and transverse directions for 9%, 17% and 23% fiber reinforcements by volume. The values of thermal conductivity as reported by them were 2.09, 2.75, 5.38 W/m.K for the longitudinal directions and 2.42, 2.47, 2.49 W/m-K for the transverse direction respectively, while the thermal conductivity of unfilled PP was 0.24 W/m-K. Exfoliated graphite platelets are another filler material of promise for improving the thermo-mechanical properties of the polymeric matrix. Aylsworth [3, 4] developed and proposed expanded graphite as reinforcement of polymers in 1910s. Lincoln and Claude [5] in 1980s proposed the dispersion of intercalated graphite in polymeric resins by conventional composite processing techniques. Since that time, research has been conducted on exfoliated graphite reinforced polymers using graphite particles of various dimensions and a wide range of polymers. Drzal et al. [6] have demonstrated the use of exfoliated graphite platelets to enhance the thermal and mechanical properties of polymeric resins. They concluded that composites made by in situ processing have better mechanical properties compared to composites made by melt-mixing or other ex situ fabrication methods due to better dispersion, prevention of agglomeration and stronger interactions between the reinforcement and the polymer. In the present study we use silver nano-filaments, nickel nano-filaments, alumina and exfoliated graphite platelets to enhance the thermal conductivity of an epoxy thermoset resin. The objective of this research is to identify the right filler to achieve the thermal conductivity as required by aerospace design engineers which is around 10 W/ m-K. An arbitrary filler loading of 8 wt% was chosen to compare the different fillers used in this study.© 2007 ASME