Nicolas Pineau

Molecular dynamics and kinetic study of carbon coagulation in the release wave of detonation products

We present a combined molecular dynamics and kinetic study of a carbon cluster aggregation process in thermodynamic conditions relevant for the detonation products of oxygen deficient explosives. Molecular dynamics simulations with the LCBOPII potential under gigapascal pressure and high temperatures indicate that (i) the cluster motion in the detonation gas is compatible with Brownian diffusion and (ii) the coalescence probability is 100% for two clusters entering the interaction cutoff distance. We used these results for a subsequent kinetic study with the Smoluchowski model, with realistic models applied for the physical parameters such as viscosity and cluster size. We found that purely aggregational kinetics yield too fast clustering, with moderate influence of the model parameters. In agreement with previous studies, the introduction of surface reactivity through a simple kinetic model is necessary to approach the clustering time scales suggested by experiments (1000 atoms after 100 ns, 10 000 atoms after 1 μs). However, these models fail to reach all experimental criteria simultaneously and more complex modelling of the surface process seems desirable to go beyond these current limitations.

Theoretical study of the nucleation/growth process of carbon clusters under pressure

We used molecular dynamics and the empirical potential for carbon LCBOPII to simulate the nucleation/growth process of carbon clusters both in vacuum and under pressure. In vacuum, our results show that the growth process is homogeneous and yields mainly sp(2) structures such as fullerenes. We used an argon gas and Lennard-Jones potentials to mimic the high pressures and temperatures reached during the detonation of carbon-rich explosives. We found that these extreme thermodynamic conditions do not affect substantially the topologies of the clusters formed in the process. However, our estimation of the growth rates under pressure are in much better agreement with the values estimated experimentally than our vacuum simulations. The formation of sp(3) carbon was negligible both in vacuum and under pressure which suggests that larger simulation times and cluster sizes are needed to allow the nucleation of nanodiamonds.

Shock compression of diamond: Molecular dynamics simulations using different interatomic potentials

Two recently developed interatomic potentials for carbon, the screened environment dependent - reactive empirical bond order (SED-REBO) and the long-range carbon bond-order (LCBOPII), were used in molecular dynamics (MD) simulations of shocked diamond. Static uniaxial compressions showed that both potentials offer an improved accuracy compared to the commonly used REBO potential. MD simulations were run in the [110] direction, and a split elastic-elastic shock wave regime was observed with one of the potentials. Isothermal compression allowed us to explain the origin of this regime, characterized by a solidsolid phase transition, leading to a non-monotonic stress-strain response below the Hugoniot elastic limit of the material.

Calculation of a solid/liquid surface tension: A methodological study

The surface tension of a model solid/liquid interface constituted of a graphene sheet surrounded by liquid methane has been computed using molecular dynamics in the Kirkwood-Buff formalism. We show that contrary to the fluid/fluid case, the solid/liquid case can lead to different structurations of the first fluid layer, leading to significantly different values of surface tension. Therefore we present a statistical approach that consists in running a series of molecular simulations of similar systems with different initial conditions, leading to a distribution of surface tensions from which an average value and uncertainty can be extracted. Our results suggest that these distributions converge as the system size increases. Besides we show that surface tension is not particularly sensitive to the choice of the potential energy cutoff and that long-range corrections can be neglected contrary to what we observed in the liquid/vapour interfaces. We have not observed the previously reported commensurability effect.

Impact of the granularity of a high-explosive material on its shock properties

S of the Problem: In order to understand the formation of strain-induced martensite (SIM) of austenitic stainless steels, phase textures were investigated both before and after static and cyclic loading, namely plastic deformation is made intentionally. Moreover, in-situ measurements of the strain-induced martensitic transformation that takes place during tensile loading at room temperature were performed. Even in the low plastic strain regime, with loading to yield stress, the SIM transformation occurred. However, the area fraction of the martensite formation did not increase significantly even when the sample was loaded to the ultimate tensile strength. On the other hand, by the cyclic loading, the area fraction of the martensite formation increases significantly when the maximum cyclic load is more than 80%UTS. In other word, the SIM formation is apparently absent when the samples are loaded with less than 70%UTS, although those samples are fractured completely. No clear frequency effect (1Hz vs. 30Hz) is detected. With the analysis, two different SIM characteristics were clarified following plastic deformation. The martensitic structures were obtained in the twin deformation and slip bands. The severity of martensite formation increased with increasing C content. It was found that martensite was formed mainly in austenitic stainless steel lacking Mo, whereas a high Mo content led to a strong martensite structure, i.e., a weak martensite. The formation of martensite occurred from austenite viamartensite, and was related to the slip deformation. The Mo element in austenitic stainless steel had high slip resistance (or stress-induced martensite transformation), due to the large size of the Mo atom. This resulted in the creation of weak martensite. The phase structures of the strained austenitic stainless steels were interpreted using a proposed, i.e., the martensitic transformations.We present various types of group III-nitride microand nano-structures for novel classical and quantum photonic applications. We demonstrate phosphor-less white-color light generation, unidirectional light propagation, ultrafast single photon generation, and room temperature exciton-polariton generation using these group III-nitride based photonic structures. First, multicolor and broadband visible light emitting diodes based on GaN hexagonal truncated pyramid and columnar structures were demonstrated [1, 2]. Second, by using GaN/InGaN core−shell QW semiconductors grown on tapered GaN rods, which have a large gradient in their bandgap energy along their growth direction, highly asymmetric photonic diode behavior was observed [3]. Third, we utilized a novel approach of the self-aligned deterministic coupling of single quantum dots (QDs) to nanofocused plasmonic modes, which enhances spontaneous emission rate of QDs as high as ~ 22 over a wide spectral range [4]. We also discuss about effective method for enhancing collection efficiency of the QDs formed in these photonic structures [5]. Finally, we developed a novel excitonpolariton system working at room temperature resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core−shell hexagonal wire with GaN/InGaN multiple quantum wells [6]. An overview and comparison of the characteristics of the above nanostructures will be given.