Investigations on high enthalpy shock wave exposed graphitic carbon nanoparticles
Introduction
After the discovery of fullerene carbon structures (C-60) [1], a variety of nanostructured carbonic materials including particles (NPs), rods (NRs), tubes (NTs), wires (NWs), balloons (NBs) etc. have emerged. These structures also received enormous attention from the world wide scientists due to their unique structural dependent chemical, mechanical, optical and electrical properties. For example, carbon NTs show high mechanical strength, high thermal stability and exotic electronic properties. These peculiar properties are quite different than that of its bulk counter parts and thus, the carbon nanostructures are under consideration for a wide range of practical applications [2], [3], [4], [5], [6], [7], [8], [9], [10] including hydrogen storage, lithium batteries and aerospace engineering.
At present, carbon has been well established in aerospace engineering as an ablative coating or thermal protection layer/system (TPS) material for space shuttles (SS) particularly at nose cones and wing leading edges. This is due to its high adsorbability of heat fluxes and emissivity (~ 0.8-0.98) of byproducts as steam into space along with its low density (1568 kg m− 3), high specific heat (~ 0.71 J gK− 1), high thermal conductivity (~ 90 W mK− 1) and high melting point (~ 3800 K) [11]. In real-situation, when SS enters into planetary atmosphere it encounters aerodynamic heating due to compression and surface friction. As a result, the carbon TPS undergoes partial dissociation and releases several gas molecules including oxides and nitrides due to pyrolysis [12], [13]. Thus, it is very crucial to understand the surface catalytic behavior and thereby radiated byproducts from carbon TPS into space in order to realize the possible environmental hazards. Moreover, the structural stability and phase transformation of the carbon at elevated conditions must be fully understood in order to adopt them not only in space-technology as TPS but also for various high temperature device applications.
On the other hand, the most important requirement for other applications of carbon is a control over three typical factors such as crystal structure, surface morphology and native defects [14], [15]. The first factor is important since carbon shows a phase conversion from graphite to diamond at very high pressures, whereas at higher temperatures the diamond phase becomes graphite. For example, ribbon-like nano graphite structures have been realized by annealing of diamond NPs (DNPs) at higher temperatures [16], [17]. The second factor is important because the hierarchical graphitic structures have been exploited as anode material in the fabrication of lithium ion batteries, which is due to its very flat potential and structural stability upon cycling along with high capacities beyond its theoretical limitation (372 mAh g− 1) [18], [19]. Zheng et al. have noticed a high capacitance of 550 mAh g− 1 with good cycling performance in carbon structures inserted lithium ion batteries [20]. The last factor is also very crucial since the crystalline quality of carbon nanostructures determines the performance of the carbon based device [21]. For example, the mechanical strength is observed as higher for less defective carbon nanostructures [22]. However, more defective nanostructures are also useful for the development of field emission devices [23]. In this view, reliable experimental evaluations on these structures are highly required in order to realize its large number of applications.
Free piston driven hypersonic shock tube (FPST) is a unique and novel technique invented for hypersonic aerodynamics. Recently, it has been utilized for the synthesis of new materials and/or structures, surface modifications, and also for various chemical and biological applications [24], [25], [26]. The temperature and pressure in this FPST can be significantly controlled by controlling the driver and test gases [27]. Moreover, by using this FPST system high flow enthalpies corresponding to temperatures ~ 16,000 K and also pressures of ~ 50 MPa can be produced within a short time (~ 2 ms). By keeping above two typical tasks in mind, the experiments were carried out in an inert atmosphere i.e. argon ambient as an initial step. It allows us to realize the impact of high enthalpy shock exclusively on the structural stability (crystallinity and phase) and surface morphology of carbon nanostructures. The obtained results including crystal structure, morphology, chemical composition and phase purity of the shock wave treated carbon nanostructures are presented by comparing with as-deposited ones and also discussed in view of space as well as chemical engineering applications.
Section snippets
Experimental procedure
Thick films (~ 50 μm) of graphitic carbon nanoparticles have been deposited on 3 mm thick aluminum substrates using a simple spray method (see Supporting Information SI_1a for schematic diagram of experimental setup) by spraying the nanopowder dispersed solution. In a typical process, the films were deposited at a substrate temperature of 250 °C by using high pure nitrogen gas as a carrier gas with a pressure of ~ 150 kPa. More details about the solution preparation and deposition procedure can be
Results and discussion
The GCNP films deposited by spray technique are well covered the substrate and however, their adhesion with substrate is slightly poor. Basic information about the shock wave treated films, typical parameters maintained at different shock wave treatments and evaluated technical data are given at SI-1d. Upon shock wave treatment a considerable loss in weight of GCNPs material has noticed. The loss in the deposited material was calculated by weighing the material (GCNPs + substrate) before and
Conclusions
GCNP films deposited on metal substrates have been exposed to high enthalpy shock waves at different Ar-pressures and analyzed with advanced analytical techniques in order to understand their crystal-structure, morphological, chemical and optical properties. The observed results from these investigations are summarized below.
The impact of shock wave on GCNPs film is exclusively limited to the surface atoms and the weight loss of deposited material per unit area is strongly dependent on its
Prime novelty statement
The work reported in this paper is novel and original, which is neither communicated nor published elsewhere.
Key issues:
Carbon–carbon nanocomposites expected candidates as thermal protection systems in space engineering applications.
These structures were studied by exposing them to a high enthalpy shock wave.
The observed results are interesting and novel, which are reported here.
Acknowledgements
We wish to thank Mr. Chinto, Mr. Harinath Reddy and Mr. Kiran for their timely help while executing the FPST experiments. Further, N. K. Reddy, wishes to acknowledge the CSIR for the sanction of SRA fellowship under the scheme of Scientist's pool (No: A-8525). All the authors are highly acknowledging the support of MNCF staff, IISc, Bangalore and Ye-Bin Kwon, Researcher, KBSI, Gwangju, South Korea in the characterization of these samples.
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