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The compression-induced deformation mechanisms of boron carbide (B4C) have not been well understood due to its complex crystal structure. Here, we investigate the mechanical behaviors and deformation mechanisms of B4C under various compression conditions, using molecular dynamics simulations with a machine-learning force field. Two structural transitions in B4C under bulk compression are identified: the formation of new chains–icosahedra bonds, and icosahedral deconstruction. The former triggers the "pop-in" event observed in the nanoindentation experiments due to chain bending, while the latter facilitates icosahedral sliding and amorphization. Furthermore, the results reveal a mechanism involving an intermediate structure characterized by the development of new chains–icosahedra bonds across the entire model, followed by the amorphization process. This mechanism induced by increased stress in icosahedra due to the new chains–icosahedra bond formation plays an important role in significantly improving the strength and ductility of B4C. In contrast, B4C with free surfaces or nanopores exhibits a direct transformation from chain bending to icosahedral deconstruction without the intermediate structure formation, consequently reducing strength and ductility. However, this intermediate configuration is not stable and maintained only under stress, because structural recovery within non-amorphized regions occurs after amorphization under quasi-static compression with a relaxed stress state. This study provides a thorough understanding of the deformation behaviors and mechanisms of B4C. [ABSTRACT FROM AUTHOR]

The Hall-Petch and inverse Hall-Petch behaviors in nanocrystalline ceramics are not well understood because dislocation activities, which are important in metals, are usually limited in these materials. In this study, we use molecular dynamics simulations with a machine-learning force field to investigate the shear deformation behaviors of nanocrystalline Al-doped boron carbide (n -B 12 -CAlC) as the grain size varies. We find a transition from the Hall-Petch to inverse Hall-Petch behavior in n -B 12 -CAlC when the grain size reaches its critical value of 6.09 nm, with a maximum shear strength of 14.93 GPa. Interestingly, mobile dislocation nucleated from grain boundaries (GBs) is activated in n -B 12 -CAlC due to the breakage of weakened C-Al chain bonds, which plays a significant role in its Hall-Petch behaviors. As the grain size decreases, the increasing GB regions homogenize the shear stress, which suppresses dislocation nucleation from GBs and strengthens the material, as observed in the conventional Hall-Petch relationship. However, with a further decrease in the grain size (< ∼ 6 nm), the increasing GB regions provide more favorable sites and a higher possibility for dislocation nucleation, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. These findings shed light on deformation behaviors and mechanisms of nanocrystalline ceramics and provide an explanation for the transition from Hall-Petch to inverse Hall-Petch behaviors. [ABSTRACT FROM AUTHOR]

Semiconductors are promising in photoelectric and thermoelectric devices, for which the thermal transport properties are of particular interest. However, they have not been fully understood, especially when crystalline imperfections are present. Here, using cadmium telluride (CdTe) as an example, we illustrate how grain boundaries (GBs) affect the thermal transport properties of semiconductors. We develop a machine-learning force field from density functional theory calculations for predicting the lattice thermal conductivity (LTC) via equilibrium molecular dynamics simulations. The LTC of crystalline CdTe decreases with the relationship of κL∼1/T in the simulation temperature range of 300 – 900 K, in which the isotropic LTC decreases from 3.34 to 0.23 W/(m⋅K) due to the enhanced anharmonicity. More important, after introducing GBs, the LTC is suppressed in all directions, especially in the direction normal to the GB planes. More severe LTC suppression occurs in CdTe with Σ9 GB than that with Σ3 GB at 300 K, decreasing by 92.8% and 61.4% along the direction normal to the GB planes compared to the isotropic LTC of the crystalline CdTe, respectively. The decreased LTC is consistent with the weaker bonding near GB planes and lower shear modulus of the defective material. The analyses of the phonon dispersion curves, vibrational density of states, and phonon participation ratio indicate that the decreased LTC mainly arises from phonon scattering at GBs. Overall, our work highlights that GBs can greatly influence the LTC of semiconductors, thus providing a promising approach for thermal property design.

The isomerisation energies and thermal stabilities of the entire set of C40 fullerene isomers are investigated using molecular dynamics simulations with the recently developed machine-learning-based Gaussian Approximation Potential (GAP-20) force field. Our simulations predict high statistical correlation between the relative isomerisation energy distribution of all C40 isomers and their thermal fragmentation temperatures. The most energetically and thermally stable isomer is the symmetric C40-D2 isomer and its fragmentation temperature is 3875 ± 25 K. In contrast, the least stable isomer is the D5d nanorod-shaped isomer which is 146.8 kcal mol−1 higher in energy and decomposes at 2975 ± 25 K, i.e., around 900 K below the most stable isomer. In addition, we show that most isomers lie in the range between 20 and 60 kcal mol−1 above the most stable isomer, and nearly 60% of isomers decompose in the temperature range of 3425–3675 K. These computational results provide valuable insights into the synthesis of C40 fullerenes at high-temperatures.

The adsorption, reaction and thermal stability of bromine on Rh(111)-supported hexagonal boron nitride ( h -BN) and graphene were investigated. Synchrotron radiation-based high-resolution x-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS allowed us to follow the adsorption process and the thermal evolution in situ on the molecular scale. On h -BN/Rh(111), bromine adsorbs exclusively in the pores of the nanomesh while we observe no such selectivity for graphene/Rh(111). Upon heating, bromine undergoes an on-surface reaction on h -BN to form polybromides (170-240 K), which subsequently decompose to bromide (240-640 K). The high thermal stability of Br/ h -BN/Rh(111) suggests strong/covalent bonding. Bromine on graphene/Rh(111), on the other hand, reveals no distinct reactivity except for intercalation of small amounts of bromine underneath the 2D layer at high temperatures. In both cases, adsorption is reversible upon heating. Our experiments are supported by a comprehensive theoretical study. DFT calculations were used to describe the nature of the h -BN nanomesh and the graphene moiré in detail and to study the adsorption energetics and substrate interaction of bromine. In addition, the adsorption of bromine on h -BN/Rh(111) was simulated by molecular dynamics using a machine-learning force field., (Creative Commons Attribution license.)

• Dislocation core reconstructions are not favorable in CdTe, a typical II-VI semiconductor, due to the predominant ionic bonding characteristics. • The screw dislocation in cdte prefers to be in the shuffle set and starts to glide at relatively low temperatures due to the low peierls stress. • The 60° perfect dislocation in cdte is more stable in the shuffle set. As it moves to the glide set, the 60° perfect dislocation tends to dissociate into a pair of 30° and 90° partial dislocations connected by a stacking fault. • The undissociated screw and 60° shuffle dislocations, as well as dissociated 30° and 90° glide partial dislocations play an essential role in the mechanical deformation of cdte at ambient temperature. Dislocation plays a crucial role in many material properties of semiconductors, ranging from plastic deformation to electronic transport. But the dislocation structures and reactions remain controversial in many semiconductors. Here, we systematically examine the dislocation properties of cadmium telluride (CdTe), a prototype of II-VI compound semiconductors, using molecular statics and molecular dynamics simulations with a machine-learning force field. We find that dislocation cores in CdTe are not reconstructed along the dislocation line due to the significant ionic bonding characteristics. Moreover, the undissociated screw dislocation in the shuffle set is more stable than that in the glide set, and the glide dislocation tends to move to the shuffle set, followed by random movement rather than dissociation, arising from its low Peierls stress. For 60° perfect dislocation, it also prefers to locate in the shuffle set, but the 60° glide dislocation is dissociated into a pair of 30° and 90° partial dislocations connected by a stacking fault with a width of ∼9.03 nm at low temperatures through reconfiguring atomic bonds in the core. This work provides an atomic-level understanding of dislocation core properties in CdTe, laying a basis for interpreting the mechanical and electronic performance of CdTe and other II-VI semiconductors. [ABSTRACT FROM AUTHOR]

• A unique pseudoplastic deformation behavior is identified in nanotwinned boron carbide. • Introducing high-density nanoscale twins into boron carbide can enhance its low-temperature ductility. • Amorphization preferentially nucleates near twin boundaries due to planar-defect-induced stress concentrations, indicating the twin-boundary-related softening mechanism in boron carbide. Nanotwinned metals exhibit enhanced strength due to the twin-boundary-dislocation interaction, but nanotwins may play a different role in strong, yet brittle ceramics due to the absence of mobile dislocations under low temperatures. Here, we carry out molecular dynamics (MD) simulations using a machine-learning force field to illustrate how nanoscale twins influence the mechanical properties and deformation mechanisms of superhard boron carbide (B 4 C). Surprisingly, in contrast to brittle failure in single-crystal B 4 C, we identify an abnormal pseudoplastic deformation behavior in nanotwinned B 4 C (nt-B 4 C) which arises from successive breakage of stretched inter-icosahedral B-C bonds. This pseudoplastic behavior shows a strong dependence on temperature and twin density, and it becomes more prominent as temperature decreases and twin density increases. This abnormal anti-correlation between pseudoplasticity and temperature mainly arises from enhanced atomic shear strain induced by thermodynamic vibrations at high temperatures. Moreover, a nanotwinning-induced softening mechanism is observed in nt-B 4 C, since amorphization preferentially nucleates near twin boundaries due to stress concentration. This work demonstrates that introducing high-density nanotwins into B 4 C is effective to enhance its low-temperature ductility. [Display omitted] [ABSTRACT FROM AUTHOR]