Pranjal Nautiyal

Morphological and chemical transformations in boron nitride nanotubes under high temperature atmospheric conditions is probed in this study. We report atmospheric oxygen induced cleavage of boron nitride nanotubes at temperatures exceeding 750 °C for the first time. Unzipping is then followed by coalescence of these densely clustered multiple uncurled ribbons to form stacks of 2D sheets. FTIR and EDS analysis suggest these 2D platelets to be Boron Nitride Oxide platelets, with analogous structure to Graphene Oxide, and therefore we term them as " White Graphene Oxide " (WGO). However, not all BNNTs deteriorate even at temperatures as high as 1000 °C. This leads to the formation of a hybrid nanomaterial system comprising of 1D BN nanotubes and 2D BN oxide platelets, potentially having advanced high temperature sensing, radiation shielding, mechanical strengthening, electron emission and thermal management applications due to synergistic improvement of multi-plane transport and mechanical properties. This is the first report on transformation of BNNT bundles to a continuous array of White Graphene Oxide nanoplatelet stacks. The brilliant mechanical, thermal and electrical properties of Carbon Nanotubes (CNT) propelled the scientific community towards exploration of this lightweight material for diverse applications over the past two decades 1. One major shortcoming of CNTs is their poor high temperature stability; they start oxidizing around 400 °C 2,3 , limiting their applications to lower temperatures. Boron Nitride Nanotube (BNNT), a structural analogue of CNT, displays a unique combination of exceptional strength and flexibility with low density, thermal conductivity coupled with electrical insulation, piezoelectric behavior, radiation shielding effect and high temperature stability 2–5. BNNTs are resistant to oxidation at temperatures as high as 800–900 °C (twice the temperature for CNT) 2,3. This makes BNNT greatly desirable for numerous applications, viz. light weight aircraft and space vehicle bodies which are usually subjected to tremendous aerodynamic heating, thermal management of high density power electronics systems, high temperature piezoelectric sensors and high temperature field emitters 4–13. Despite the fanfare around superior oxidation resistance, there is very little understanding of BNNT behavior at elevated temperatures. Chen and co-workers 2 found that the oxidation performance of BNNT depends on their nanostructure. Coarse nanotubes, such as bamboo-like and cone-shaped BNNTs exhibited broken walls, whereas thin nanotubes retained their multiwalled structure on exposure to high temperatures. In addition, thin nanotubes were found to withstand higher temperatures (upto 900 °C) without oxidizing. Golberg et al. 3 fabricated BNNT ropes comprising of multiwalled nanotubes and investigated high temperature behavior of these ropes. First signs of oxide formation were noticed only after 800 °C. While these studies report that BNNT reacts to form Boron Trioxide (B 2 O 3) around 750–900 °C 2,3 , the oxidative transformation mechanism of BNNTs remains elusive. Being a 1-D nanomaterial, BNNT is anisotropic with strong directionality in its properties. This makes the understanding of morphological and phase transformations associated with oxidation critical for high temperature applications.

Graphene foam based hierarchical polyimide composites with nano-engineered interface are fabricated in this study. Damping behavior of graphene foam is probed for the first time. Multi-scale mechanisms contribute to highly impressive damping in graphene foam. Rippling, spring-like interlayer van der Waals interactions and flexing of graphene foam branches are believed to be responsible for damping at the intrinsic, inter-layer and anatomical scales, respectively. Merely 1.5 wt.% graphene foam addition to the polyimide matrix leads to as high as ~300% improvement in loss tangent. Graphene nanoplatelets are employed to improve polymer-foam interfacial adhesion by arresting polymer shrinkage during imidization and π-π interactions between nanoplatelets and foam walls. As a result, damping behavior is further improved due to effective stress transfer from the polymer matrix to the foam. Thermo-oxidative stability of these nanocomposites is investigated by exposing the specimens to glass transition temperature of the polyimide (~400ºC). The composites were found to retain their damping characteristics even after being subjected to such extreme temperature, attesting their suitability in high temperature structural applications. Their unique hierarchical nanostructure provides colossal opportunity to engineer and program material properties.

Interactions between long boron nitride nanotube (BNNT) fibrils and molten aluminum (Al) pool are probed in this study to assess the feasibility of fabricating composite materials by solidification route. BNNTs were found to survive high temperature and reactive conditions present in molten aluminum. Very limited interfacial reaction was observed, resulting in the formation of AlN, AlB 2 and AlB 10 in trace amounts. AlN was the principal reaction product, resulting in improved interfacial wetting. Calculations based on surface energies revealed improved work of interfacial adhesion due to AlN formation. BNNTs were found to be well integrated in the aluminum matrix, signifying AlN induced excellent wetting. We also report capillarity-induced high temperature filling of BNNT by molten Al. The filling was promoted by AlN formation. In addition, formation of B-rich AlB 10 phase inside the nanotube was observed. Nanotube filling by metal and subsequent reaction to form nano-ceramic phases is expected to alter mechanical properties of the cast Aluminum-BNNT composites. This study establishes the suitability of solidification route for developing high strength Al-BNNT composites in future.

In this study, nanoindentation induced creep in Mg metal system is investigated theoretically as well as experimentally. Analytical equations relating indentation creep rate to penetration depth for different creep deformation mechanisms were derived. The theoretical model was fitted with experimental results obtained for pure magnesium and AZ61 alloy. Dislocation glide was found to be the predominant indentation induced creep mechanism for both the materials at room temperature, for the entire range of load (from 50 to 150 mN). To gain insight into kinetics of deformation mechanism, activation energy and 0 K flow stress were determined for the two materials by fitting the experimental curves with the derived equations. Notable enhancement was found in the value of 0 K flow stress due to alloying, signifying highly effective solid solution strengthening on addition of 6 wt% Al and 1 wt% Zn. Stress exponents exhibited size effect, showing an increasing trend with increase in the value of indentation load or penetration depth; however, transition in the values of stress exponent did not correspond to transition in major creep mechanism (the theoretical results indicate prominence of glide throughout the investigated range). This leads to an important conclusion: unlike uniaxial tests, in nanoindentation creep tests, deformation mechanisms cannot be deduced based on stress exponent values.

Indentation scratch-induced wear in Mg-8 wt% Al-0.5 wt% Zn (AZ80) alloy is investigated in this study for solution-treated and peak-aged conditions. Aged alloy exhibited higher wear resistance, with 12–14 % reduction in wear volume. This is attributed to enhanced microstructural resistance to scratch wear due to precipitate phase. Ploughing was found to be the dominating material removal mechanism for both solution-treated and aged specimens. Microcutting was also active in solution-treated alloy, but not in aged alloy due to the presence of brittle Mg 17 Al 12 intermetallic particles in the microstructure. The localized stresses induced by indenter tip were found to exceed theoretical shear strength for magnesium crystal, resulting in intergranular as well as transgranular fracture phenomena. Cracks were more prominent in aged alloy, which is ascribed to the suppression of twinning activity in aged alloy, resulting in non-fulfilment of von Mises criterion of plastic deformation. Scratch-induced plastic flow was more pronounced in solution-treated alloy, with twinning as the major plasticity mechanism.

This study reports the effect of loading path and precipitates on indentation induced creep behavior of AZ61 magnesium alloy. Indentation creep tests were performed on solution-treated and peak-aged extruded AZ61 magnesium alloy, and Atomic Force Microscopy (AFM) investigations were carried out to study deformation mechanisms. Twinning is the dominant creep mechanism for indentation along the extrusion direction (ED) in solution-treated alloy. A combination of slip and twinning appears to be the prominent mechanisms for indentation creep perpendicular to ED. Creep flow is arrested for indentation perpendicular to ED, due to slip–twin interactions. Influence of precipitates on creep deformation was also studied. Aged specimen exhibited higher creep resistance than solution-treated specimen. Unlike solution-treated specimens, twinning was not observed in aged alloy. Creep in aged specimen was attributed to slip.