Disordered enthalpy-entropy descriptor for high-entropy ceramics discovery.

The need for improved functionalities in extreme environments is fuelling interest in high-entropy ceramics1-3. Except for the computational discovery of high-entropy carbides, performed with the entropy-forming-ability descriptor4, most innovation has been slowly driven by experimental means1-3. Hence, advancement in the field needs more theoretical contributions. Here we introduce disordered enthalpy-entropy descriptor (DEED), a descriptor that captures the balance between entropy gains and enthalpy costs, allowing the correct classification of functional synthesizability of multicomponent ceramics, regardless of chemistry and structure. To make our calculations possible, we have developed a convolutional algorithm that drastically reduces computational resources. Moreover, DEED guides the experimental discovery of new single-phase high-entropy carbonitrides and borides. This work, integrated into the AFLOW computational ecosystem, provides an array of potential new candidates, ripe for experimental discoveries.

Settling the matter of the role of vibrations in the stability of high-entropy carbides.

High-entropy ceramics are attracting significant interest due to their exceptional chemical stability and physical properties. While configurational entropy descriptors have been successfully implemented to predict their formation and even to discover new materials, the contribution of vibrations to their stability has been contentious. This work unravels the issue by computationally integrating disorder parameterization, phonon modeling, and thermodynamic characterization. Three recently synthesized carbides are used as a testbed: (HfNbTaTiV)C, (HfNbTaTiW)C, and (HfNbTaTiZr)C. It is found that vibrational contributions should not be neglected when precursors or decomposition products have different nearest-neighbor environments from the high-entropy carbide.

Dynamics of Neutral and Charged Nanodiamonds in Aqueous Media Confined between Gold Surfaces under Normal and Shear Loading.

The dynamics of cubo-octahedral nanodiamonds (NDs) with three different surface treatments and confined in aqueous environments between gold surfaces under shear and normal loading conditions have been characterized via molecular dynamics (MD) simulations. The treatments consisted of carboxyl (-COO-) or amino (-NH3 +) groups attached to the NDs, producing either negatively or positively charged NDs, respectively, and hydrogen-terminated surfaces producing neutral NDs. Simulations were performed in the presence and absence of induced image charges to explore the impact of electrostatic interactions on friction and surface deformation. Significant deformation of the gold surfaces was observed for negatively charged NDs placed between gold surfaces under external loads that were sufficient to displace water from the contact. Rolling and relatively high friction levels were also observed for the negatively charged NDs under the same conditions. In contrast, the neutral and positively charged NDs exhibited sliding behavior with only minor deformation of the gold surfaces. The results suggest that the size of the surface functional group plays a major role in determining whether NDs slide or roll on solid contacts. Higher friction levels were also observed in conjunction with induced image charges in the gold contacts. The results demonstrate how surface functionalization and surface-induced charges can work in combination to profoundly influence tribological performance.

High-entropy high-hardness metal carbides discovered by entropy descriptors.

High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor-entropy forming ability-for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides-promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.

Charge-Induced Disorder Controls the Thermal Conductivity of Entropy-Stabilized Oxides.

Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.

Interdependent Roles of Electrostatics and Surface Functionalization on the Adhesion Strengths of Nanodiamonds to Gold in Aqueous Environments Revealed by Molecular Dynamics Simulations.

Molecular dynamics simulations demonstrate that adhesion strengths as a function of charge for aqueous nanodiamonds (NDs) interacting with a gold substrate result from an interdependence of electrostatics and surface functionalization. The simulations reveal a water layer containing Na+ counterions between a negative ND with surface -COO- functional groups that is not present for a positively charged ND with -NH3+ functional groups. The closer proximity of the positive ND to the gold surface and the lack of cancelation of electrostatic interactions due to counterions and the water layer lead to an electrostatic adhesion force for the positive ND that is nearly three times larger than that of the negative ND. Prior interpretations of experimental tribological studies of ND-gold systems suggested that electrostatics or surface functionalization could be responsible for observed adhesion strength differences. The present work demonstrates how these two effects work together in determining adhesion for this system.

Theory and modelling of diamond fracture from an atomic perspective.

Discussed in this paper are several theoretical and computational approaches that have been used to better understand the fracture of both single-crystal and polycrystalline diamond at the atomic level. The studies, which include first principles calculations, analytic models and molecular simulations, have been chosen to illustrate the different ways in which this problem has been approached, the conclusions and their reliability that have been reached by these methods, and how these theory and modelling methods can be effectively used together.

Stimuli-Responsive Polymer Brushes for Flow Control through Nanopores.

Responsive polymers attached to the inside of nano/micro-pores have attracted great interest owing to the prospect of designing flow-control devices and signal responsive delivery systems. An intriguing possibility involves functionalizing nanoporous materials with smart polymers to modulate biomolecular transport in response to pH, temperature, ionic concentration, light or electric field. These efforts open up avenues to develop smart medical devices that respond to specific physiological conditions. In this work, an overview of nanoporous materials functionalized with responsive polymers is given. Various examples of pH, temperature and solvent responsive polymers are discussed. A theoretical treatment that accounts for polymer conformational change in response to a stimulus and the associated flow-control effect is presented.

Computational study of nanometer-scale self-propulsion enabled by asymmetric chemical catalysis.

We present a detailed analysis of the self-propulsion of a model nanometer-scale motor by reactive molecular dynamics simulations. The nanomotor is decorated with catalysts on only one side that promotes exothermic reactions of the surrounding fuel. Unidirectional drift of the nanomotor is observed that is superimposed on its Brownian motion. The motor response upon the application of external loads is also investigated and the thermodynamic efficiency is calculated. It is shown that the propulsion of our nanomotor can be understood by a momentum transfer model which is akin to rocket propulsion.

Molecular simulation of the influence of interface faceting on the shock sensitivity of a model plastic bonded explosive.

Molecular dynamics simulations are used to model the shock loading of an interface with various degrees of nanometer scale faceting between an inert binder and an energetic crystal. The facets create regions of local compression that induce exothermic reaction that leads to local hotspots and an increased shock sensitivity to detonation. Two mechanisms for compression and hotspot formation are identified that depend on the shock impedance mismatch between the binder and energetic crystal, namely shock focusing and local compression of the facets. These results provide a possible explanation for why spherical RDX crystals in cast polymer-bonded explosives appear less shock sensitive than RDX with more faceted morphologies.

Simulated thermal decomposition and detonation of nitrogen cubane by molecular dynamics.

We present simulations of a model molecular solid of nitrogen cubane subject to thermal agitation and mechanical shock. A new approach, a reactive state summation potential, has been used to model nitrogen cubane dissociation. At elevated temperatures, the system decomposes to N(2) mixed with a small amount of oligomeric nitrogen. When subject to shock loading the system detonates above some critical threshold after which a shock front is self-sustained by the energy release from chemical reactions at a constant intrinsic speed. This is the first example of a fully three-dimensional atomic simulation of a chemically-sustained detonation. The spatial confinement of the shock front results in longer chain intermediates than in the case of thermal decomposition, suggesting that shock intermediates can be structurally very different from the same material subject to comparable temperatures and pressures.

Multiscale analysis of liquid lubrication trends from industrial machines to micro-electrical-mechanical systems.

An analytic multiscale expression is derived that yields conditions for effective liquid lubrication of oscillating contacts via surface flow over multiple time and length scales. The expression is a logistics function that depends on two quantities, the fraction of lubricant removed at each contact and a scaling parameter given by the logarithm of the ratio of the contact area to the product of the lubricant diffusion coefficient and the cycle time. For industrial machines the expression confirms the need for an oil mist. For magnetic disk drives, the expression predicts that existing lubricants are sufficient for next-generation data storage. For micro-electrical-mechanical systems, the expression predicts that a bound + mobile lubricant composed of tricresyl phosphate on an octadecyltrichlorosilane self-assembled monolayer will be effective only for temperatures greater than approximately 200 K and up to approximately MHz oscillation frequencies.

Diffusion on a self-assembled monolayer: molecular modeling of a bound + mobile lubricant.

The diffusion of tricresyl phosphate molecules on an octadecyltrichlorosilane self-assembled monolayer (SAM) was characterized using molecular dynamics simulations. The simulations predict that when placed on the top of a close-packed SAM, the molecules remain mobile on the surface with an isotropic diffusion activation energy of approximately 9 kJ/mol. In contrast, an anisotropic barrier that results from chain tilt within the SAM is predicted for diffusion into a defect created by reducing the alkane chain length within a cylinderical region of the surface. Once in the defect, the molecules become trapped by embedding part of the molecule into the side of the SAM.

Thermal conductivity of diamond nanorods: Molecular simulation and scaling relations.

Thermal conductivities of diamond nanorods are estimated from molecular simulations as a function of radius, length, and degree of surface functionalization. While thermal conductivity is predicted to be lower than carbon nanotubes, their thermal properties are less influenced by surface functionalization, making them prime candidates for thermal management where heat transfer is facilitated by cross-links. A scaling relation based on phonon surface scattering is developed that reproduces the simulation results and experimental measurements on silicon nanowires.

Ab initio study of the role of entropy in the kinetics of acetylene production in filament-assisted diamond growth environments.

We present a new theoretical strategy, ab initio rate constants plus integration of rate equations, that is used to characterize the role of entropy in driving high-temperature/low-pressure hydrocarbon chemical kinetics typical of filament-assisted diamond growth environments. Twelve elementary processes were analyzed that produce a viable pathway for converting methane in a feed gas to acetylene. These calculations clearly relate the kinetics of this conversion to the properties of individual species, demonstrating that (1) loss of translational entropy restricts addition of hydrogen (and other radical species) to unsaturated carbon-carbon bonds, (2) rotational entropy determines the direction of the rate-limiting abstraction reactions, and (3) the overall pathway is enhanced by high beta-scission reaction rates driven by translational entropy. These results suggest that the proposed strategy is likely applicable to understand gas-phase chemistry occurring in the systems of combustion and other chemical vapor depositions.

Flow control through polymer-grafted smart nanofluidic channels: molecular dynamics simulations.

Presented are results of molecular dynamics simulations that demonstrate flow gating through a polymer-grafted nanopore as a function of effective solvent quality. Analysis of density and flow profiles from the simulations show that the difference in drag force exerted on the flowing solvent due to different polymer brush configurations produces the effective fluid gating. Shear-induced permeability changes through these nanopores has also been investigated. These results establish a critical starting point in nanofluidics from which continuum modeling can be developed to design this emerging class of smart nanoporous materials with tailor-made properties.

First principles prediction of the gas-phase precursors for AlN sublimation growth.

Using a new, parameter-free first principles strategy for modeling sublimation growth, we show that while Al and N2 dominate gas concentrations in AlN sublimation growth chambers under typical growth conditions, N2 is undersaturated with respect to the crystal and therefore cannot be a growth precursor. Instead, our calculations predict that the nitrogen-containing precursors are Al(n)N (n=2,3,4), in stark contrast to assumptions used in all previous modeling studies of this system.