Nonequilibrium Dynamics in Amorphous Si₃B₃N₇

Abstract

We present extensive numerical investigations of the structural relaxation dynamics of a realistic model of the amorphous high-temperature ceramic a-Si3B3N7, probing the mean-square displacement of the atoms, the bond survival probability, the average energy, the specific heat, and the two-point energy average. Combining the information from these different sources, we identify a transition temperature Tc approximately 2000 K below which the system is no longer ergodic and physical quantities observed over a time t(obs) show a systematic parametric dependence on the waiting time t(w), or age, elapsed after the quench. The aging dynamics "stiffens" as the system becomes older, which is similar to the behavior of highly idealized models such as Ising spin glasses and Lennard-Jones glasses.

Thermodynamic stability of solid and fluid phases in the Si3B3N7 system

“…Thus, within the range of our simulations, the mean-field picture is only marginally appropriate for a quantitative description. This agrees with our earlier work [26,20] studying the glass transition: Judging from the specific heat curves, there appears to occur a considerable release of configurational entropy at temperatures far above the actual glass transition at T G ≈ 2000 − 2500 K that was computed based on the peak in the specific heat and the decrease of the diffusion constants.…”

“…We have investigated the stability of these void-containing structures [19] and have found that they should be kinetically stable up to ca. 1750 K. Above this temperature, a very slow transformation into a denser amorphous phase without larger voids should take place -the same phase one would also obtain if one could synthesize a-Si 3 B 3 N 7 via a glass transition from the melt [15] with a glass transition temperature somewhere in the range of 2000 -2250 K according to our simulations [20]. Up to now, however, the melt phase of Si 3 B 3 N 7 has not yet been accessible experimentally, most likely due to the evolution of N 2 from the melt.…”

“…Note that in the non-equilibrium regime, equation 3 only gives an estimate for the diffusion coefficient, since in that regime the linear relationship between t obs and the MSD(t obs ; V, T ) does not hold in general [20]. But for the present purpose this estimate suffices.…”

“…For each temperature and volume we performed runs of length 10 6 MCC, resulting in about 600 data points for each of the two procedures described below. Since we had found that the system exhibits aging effects [26,20], we have only used the last 2 · 10 5 MCC (corresponding to a time window of about 100 ps) after a waiting time of t w = 8 · 10 5 MCC, for analysis purposes in order to allow the system to reach at least quasi-equilibrium. Furthermore the runs were repeated 8 times at each temperature in order to improve the statistics of our simulations.…”

The Route to the Structure Determination of Amorphous Solids: A Case Study of the Ceramic Si3B3N7

“…Angewandte Chemie time dependence is found for the slow improvement in energy during the aging process, as observed in long-time simulations for Si 3 B 3 N 7 models at temperatures below the melting point. [85] The effect of high pressures on the nanoporous material could have great ramifications for potential applications. [84] Constant-pressure (1-50 GPa) simulations at 300 K show that the system can be compacted significantly, and that much of the densification is irreversible.…”