A Summary of Methods for Fire Tests of Roof Coverings

AbstractThe testing method about the fire performance of roof covering and materials has not been put into operation in China. This article focuses on two main international testing about fire performance of roof covering and materials, comparing the difference between the two test methods

Atomistic Investigation of the Solubility of 3‑Alkylthiophene Polymers in Tetrahydrofuran Solvent

We study the solubility properties of regioregular oligo­(3-alkylthiophene)­s in tetrahydrofuran solvent as a function of their alkyl chains length by an atomistic investigation based on model potential molecular dynamics. We make use of the Flory–Huggins theory that is typically used to study the miscibility of macromolecules and that is here applied for the first time to study the solubility of conjugated conducting polymers in a typical organic solvent. The properties of the isolated solvent and polymer are correctly reproduced, and the calculated solubilities of the oligo­(3-alkylthiophene)­s in tetrahydrofuran as a function of their side chains lengths are in agreement with available experimental data. Present investigation shows that the atomistic approach based on molecular dynamics is a powerful tool to study the solubility of alkylthiophenes in molecular solvents

Atomistic Simulations of P(NDI2OD-T2) Morphologies: From Single Chain to Condensed Phases

We investigate theoretically the structure, crystallinity, and solubility of a high-mobility n-type semiconducting copolymer, P­(NDI2OD-T2), and we propose a set of new force field parameters. The force field is reparametrized against density functional theory (DFT) calculations, with the aim to reproduce the correct torsional angles that govern the polymer chain flexibility and morphology. We simulate P­(NDI2OD-T2) oligomers in different environments, namely, in vacuo, in the bulk phase, and in liquid toluene and chloronaphthalene solution. The choice of these solvents is motivated by the fact that they induce different kinds of molecular preaggregates during the casting procedures, resulting in variable device performances. Our results are in good agreement with the available experimental data; the polymer bulk structure, in which the chains are quite planar, is correcly reproduced, yet the isolated chains are flexible enough to fold in vacuo. We also calculate the solubility of P­(NDI2OD-T2) in toluene and chloronaphthalene, predicting a much better solubility of the polymer in the latter, also in accordance to experimental observations. Different morphologies and dynamics of the oligomers in the two solvents have been observed. The proposed parameters make it possible to obtain the description of P­(NDI2OD-T2) in different environments and can serve as a basis for extensive studies of this polymer semiconductor, such as, for example, the dynamics of aggregation in solvent

Development of a Classical Interatomic Potential for MAPbBr₃

We develop a classical interatomic potential for MAPbBr₃. The model belongs to the class of MYP force-fields for hybrid perovskites based on two-body Buckhingam-Coulomb and dispersive terms to describe organic–inorganic interactions and already successfully applied to MAPbI₃. The model calibration is based on a simplified procedure able to extend one existing parametrization to a different halide by suitable scaling of selected subgroups of parameters. The main static and dynamical properties of MAPbBr₃ are well reproduced by the developed model: the lattice constant, cohesive energy curve, bulk modulus, energy barriers for cation rotations (both static and dynamic), the phase transition temperatures, and structural parameters evolution with temperature. The model also provides a valid relationship between MAPbBr₃ and MAPbI₃: MAPbBr₃ has shorter lattice constant, higher cohesive energy, lower phase transition temperatures, and larger anisotropy in orthorhombic phase. The good comparison also extends to the vibrational properties at finite temperatures that have been benchmarked on experimental and DFT results. The developed MAPbBr₃ model is further used to calculate the MA dynamics in MAPbBr₃ at room temperature finding a reorientation time of ∼3 ps in good agreement with experimental data. Present work represents an important step toward the large-scale atomistic modeling of MAPbBr₃ and the development of a general class of force fields for hybrid perovskites

Pinpointing the Cause of Platinum Tipping on CdS Nanorods

We computationally identify the precise mechanism by which metallic platinum aggregates at the tips of cadmium sulfide (CdS) nanostructures. Large-scale atomistic simulations of physically realistic nanorods are used to quantify the chemical, dispersive, and electrostatic contributions to platinum interaction with CdS. Crystallographic anisotropy as well as facet, edge, and tip effects are accounted for to show that Pt aggregation, known as “tipping”, is not due to the dynamics of adhesion and diffusion. Instead, efficient tipping is found to be due to long-range electrostatic interactions of metallic ions with polar tips set up by CdS surface stoichiometry. The results are used to stipulate the physical conditions by which metallic decoration of ionic nanostructures can be optimized. This is expected to be useful in the realization of nanoscale metal–semiconductor devices

Electronic Properties of Hybrid Zinc Oxide–Oligothiophene Nanostructures

Using density functional theory in combination with model potential molecular dynamics, we study hybrid systems consisting of oligothiophene molecules with increasing chain length (two, four, and six rings) adsorbed onto a ZnO nanoparticle model. We investigate the energetics of adhesion and the morphological features at the curved interface. We compute the energy-level alignment taking many body effects into account within the ΔSCF approach. Our results show that, as a consequence of the local curvature of the interface, the electronic coupling between the organic and inorganic component affects the energy-level alignment in all systems, making it less favorable for charge separation. In particular, the energy-level alignment for sexithiophene on the ZnO curved nanoparticle does not lead to a type-II junction with staggered band gaps, contrary to what was recently found for sexithiophene on a flat (101̅0) ZnO surface. Although the limited size (and hence the large curvature) of the nanoparticle does not allow us to make a general statement, this indicates a trend that is valid for systems in which quantum confinement effects are important. As a side result of our study, we propose a simple practical model to predict the energy-level alignment in hybrid systems, which gives consistent results compared to ΔSCF

Collective Molecular Mechanisms in the CH₃NH₃PbI₃ Dissolution by Liquid Water

The origin of the dissolution of methylammonium lead trihalide (MAPI) crystals in liquid water is clarified by finite-temperature molecular dynamics by developing a MYP-based force field (MYP1) for water–MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI₂ films and solvation of MA and I ions. We rationalize the effect of water on MAPI by identifying a transition from a reversible absorption and diffusion in the presence of vapor to the irreversible destruction of the crystal lattice in liquid due to a cooperative action of water molecules. A strong water–MAPI interaction is found with a binding energy of 0.41 eV/H₂O and wetting energy of 0.23 N/m. The water vapor absorption is energetically favored (0.29 eV/H₂O), and the infiltrated molecules can migrate within the crystal with a diffusion coefficient <i>D</i> = 1.7 × 10^–8 cm²/s and activation energy of 0.28 eV

Bottom-Up Mechanical Nanometrology of Granular Ag Nanoparticles Thin Films

Ultrathin metal nanoparticles coatings, synthesized by gas-phase deposition, are emerging as go-to materials in a variety of fields ranging from pathogens control and sensing to energy storage. Predicting their morphology and mechanical properties beyond a trial-and-error approach is a crucial issue limiting their exploitation in real-life applications. The morphology and mechanical properties of Ag nanoparticle ultrathin films, synthesized by supersonic cluster beam deposition, are here assessed adopting a bottom-up, multitechnique approach. A virtual film model is proposed merging high resolution scanning transmission electron microscopy, supersonic cluster beam dynamics, and molecular dynamics simulations. The model is validated against mechanical nanometrology measurements and is readily extendable to metals other than Ag. The virtual film is shown to be a flexible and reliable predictive tool to access morphology-dependent properties such as mesoscale gas-dynamics and elasticity of ultrathin films synthesized by gas-phase deposition

Bottom-Up Mechanical Nanometrology of Granular Ag Nanoparticles Thin Films

Ultrathin metal nanoparticles coatings, synthesized by gas-phase deposition, are emerging as go-to materials in a variety of fields ranging from pathogens control and sensing to energy storage. Predicting their morphology and mechanical properties beyond a trial-and-error approach is a crucial issue limiting their exploitation in real-life applications. The morphology and mechanical properties of Ag nanoparticle ultrathin films, synthesized by supersonic cluster beam deposition, are here assessed adopting a bottom-up, multitechnique approach. A virtual film model is proposed merging high resolution scanning transmission electron microscopy, supersonic cluster beam dynamics, and molecular dynamics simulations. The model is validated against mechanical nanometrology measurements and is readily extendable to metals other than Ag. The virtual film is shown to be a flexible and reliable predictive tool to access morphology-dependent properties such as mesoscale gas-dynamics and elasticity of ultrathin films synthesized by gas-phase deposition