Molecular dynamics simulations of shock waves in oriented nitromethane single crystals: plane-specific effects

Molecular dynamics simulations of supported shock waves (shock pressure P(s) ～ 15 GPa) propagating along the [110], [011], [101], and [111] directions in crystalline nitromethane initially at T = 200 K were performed using the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. These simulations, combined with those from a preceding study of shocks propagating along [100], [010], and [001] directions in nitromethane for similar conditions of temperature and shock pressure [L. He, T. D. Sewell, and D. L. Thompson, J. Chem. Phys. 134, 124506 (2011)], have been used to study the post-shock relaxation phenomena. Shocks along [010] and [101] lead to a crystal-crystal structure transformation. Shocks propagating along [011], [110], [111], [100], and [001] exhibit plane-specific disordering, which was characterized by calculating as functions of time the 1D mean square displacement (MSD), 2D radial distribution function (RDF), and 2D orientation order parameter P(2)(θ) in orthogonal planes mutually perpendicular to the shock plane; and by calculating as functions of distance behind the shock front the Cartesian components of intermolecular, intramolecular, and total kinetic energies. The 2D RDF results show that the structural disordering for shocks along [100], [110], and [111] is strongly plane-specific; whereas for shocks along [001] and [011], the loss of crystal structural order is almost equivalent in the orthogonal planes perpendicular to the shock plane. Based on the entire set of simulations, there is a trend for the most extensive disordering to occur in the (010) and (110) planes, less extensive disordering to occur in the (100) plane, and essentially no disordering to occur in the (001) plane. The 2D P(2)(θ) and 1D MSD profiles show, respectively, that the orientational and translational disordering is plane-specific, which results in the plane-specific structural disordering observed in the 2D RDF. By contrast, the kinetic energy partitioning and redistribution do not exhibit plane specificity, as shown by the similarity of spatial profiles of the Cartesian components of the intermolecular, intramolecular, and total kinetic energies in orthogonal planes perpendicular to the shock plane.     

The Anisotropic Chemical Reaction Mechanism of 1,3,3-trinitroazetidine (TNAZ) under Different Shock Wave Directions by ReaxFF Reactive Molecular Dynamics Simulations

1,3,3-Trinitroazetidine (TNAZ) has good thermal stability and low shock sensitivity, among other properties, and it has broad prospects in insensitive ammunition applications. In this study, a molecular dynamics calculation based on the ReaxFF-lg force field and multiscale shock technique (MSST) was used to simulate the shock-induced chemical reaction of TNAZ with different shock wave directions. The results showed that the shock sensitivity of TNAZ was in the order of [100] > [010] > [001]. There were significant differences in molecular arrangements in different shock directions, which affected the reaction rate and reaction path in different directions. The molecular arrangement in the [010] and [001] directions formed a “buffer” effect. The formation and cleavage of bonds, formation of small molecules and growth of clusters were analyzed to show the effect of the “buffer”. The polymerization reactions in the [010] and [001] directions appeared later than that in the [100] direction, and the cluster growth in the [010] and [001] directions was slower than that in the [100] direction. In different shock loading directions, the formation and cleavage mechanisms of the N-O bonds of the TNAZ molecules were different, which resulted in differences in the initial reaction path and reaction rate in the three directions     

冲击载荷下TATB晶体滑移和各向异性的分子动力学研究

采用ReaxFF反应力场和分子动力学方法,研究了1,3,5-三氨基-2,4,6-三硝基苯(TATB)炸药晶体在沿不同方向冲击载荷下的滑移和各向异性。冲击方向分别垂直于(101)、(111)、(011)、(110)、(010)、(100)和(001)晶面,冲击强度为10 GPa。研究结果表明,各冲击方向下可能被激发的滑移系均在{001}面,而其它滑移系均因很大的剪切阻力不容易被激发,这与TATB晶体沿c轴的层状结构和平面分子结构相符。预测了七个冲击方向下最容易被激发的滑移系,分别为(101)/{001}100、(111)/{001}010、(011)/{001}010、(110)/{001}010、(010)/{001}110、(100)/{001}120和(001)/{001}010。TATB晶体的冲击响应具有各向异性,动力学过程中体系的应力、能量、温度和化学反应都依赖于冲击方向。对垂直于(100)和(001)晶面的冲击,体系在滑移过程中遭遇的剪切阻力较高、持续时间较长,使得能量和温度较快升高,化学反应较容易发生;对垂直于(101)和(111)晶面的冲击,体系在滑移过程中遭遇的阻力较小且出现次数少,使得能量和温度缓慢升高,化学反应不易发生;对其余冲击方向,体系的响应居中。据此评价了7个冲击方向的相对敏感程度:(101)、(111)(011)、(110)、(010)(100)、(001)。本研究有助于在微观层次深入认识动载荷下TATB的响应机制、结构与性能的关系,为高能低感炸药的设计和研制提供理论参考。     

Anisotropy of the elastic properties of crystalline cellulose Iβ from first principles density functional theory with Van der Waals interactions

In spite of the significant potential of cellulose nanocrystals as functional nanoparticles for numerous applications, a fundamental understanding of the mechanical properties of defect-free, crystalline cellulose is still lacking. In this paper, the elasticity matrix for cellulose I_β with hydrogen bonding network A was calculated using ab initio density functional theory with a semi-empirical correction for van der Waals interactions. The computed Young’s modulus is found to be 206 GPa along [001] (c-axis), 98 GPa along [010] (b-axis), and 19 GPa along [100] (a-axis). Full compliance matrices are reported for 1.0, 1.5 and 2.0 % applied strains Color contour surfaces that show variations of the Young’s modulus and average Poisson’s ratio with crystallographic direction revealed the extreme anisotropies of these important mechanical properties. The sensitivity of the elastic parameters to misalignments in the crystal were examined with 2D polar plots within selected planes containing specific bonding characteristics; these are used to explain the substantial variability in the reported experimental Young’s moduli values. Results for the lattice directions [001], [010] and [100] are within the range of reported experimental and other numerical values.     

Molecular dynamics study of the crystallization of nitromethane from the melt

The crystallization of nitromethane, CH(3)NO(2), from the melt on the (100), (010), (001), and (110) crystal surfaces at 170, 180, 190, 200, 210, and 220 K has been investigated using constant-volume and -temperature (NVT) molecular dynamics simulations with a realistic, fully flexible force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The crystallization process and the nature of the solid-liquid interface have been investigated by computing the molecular orientations, density, and radial distribution functions as functions of time and location in the simulation cell. During crystallization the translational motion of the molecules ceases first, after which molecular rotation ceases as the molecules assume proper orientations in the crystal lattice. The methyl groups are hindered rotors in the liquid; hindrance to rotation is reduced upon crystallization. The width of the solid-liquid interface varies between 6 and 13 ? (about two to five molecular layers) depending on which crystal surface is exposed to the melt and which order parameter is used to define the interface. The maximum rate of crystallization varies from 0.08 molecules ns(-1) ?(-2) for the (010) surface at 190 K to 0.41 molecules ns(-1) ?(-2) for the (001) surface at 220 K.     

Compression Behavior and Vibrational Properties of New Energetic Material LLM-105 Analyzed Using the Dispersion-Corrected Density Functional Theory

The 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a newly energetic material with an excellent performance and low sensitivity and has attracted considerable attention. On the basis of the dispersion-corrected density functional theory (DFT-D), the high-pressure responses of vibrational properties, in conjunction with structural properties, are used to understand its intermolecular interactions and anisotropic properties under hydrostatic and uniaxial compressions. At ambient and pressure conditions, the DFT-D scheme could reasonably describe the structural parameters of LLM-105. The hydrogen bond network, resembling a parallelogram shape, links two adjacent molecules and contributes to the structure stability under hydrostatic compression. The anisotropy of LLM-105 is pronounced, especially for Raman spectra under uniaxial compression. Specifically, the red-shifts of modes are obtained for [100] and [010] compressions, which are caused by the pressure-induced enhance of the strength of the hydrogen bonds. Importantly, coupling modes and discontinuous Raman shifts are observed along [010] and [001] compressions, which are related to the intramolecular vibrational redistribution and possible structural transformations under uniaxial compressions. Overall, the detailed knowledge of the high-pressure responses of LLM-105 is established from the atomistic level. Uniaxial compression responses provide useful insights for realistic shock conditions.     

Structural change of layered perovskite La2Ti2O7 at high pressures

The perovskite-related layered structure of La₂Ti₂O₇ has been studied at pressures up to 30 GPa using synchrotron radiation powder X-ray diffraction (XRD) and Raman scattering. The XRD results indicate a pronounced anisotropy for the compressibility of the monoclinic unit cell. The ratio of the relative compressibilities along the [100], [010] and [001] directions is ∼1:3:5. The greatest compressibility is along the [001] direction, perpendicular to the interlayer. A pressure-induced phase transition occurs at 16.7 GPa. Both Raman and XRD measurements reveal that the pressure-induced phase transition is reversible. The high-pressure phase has a close structural relation to the low-pressure monoclinic phase and the phase transition may be due to the tilting of TiO₆ octahedra at high pressures.     

Communication: determination of the bond dissociation energy (D0) of the water dimer, (H2O)2, by velocity map imaging

The bond dissociation energy (D(0)) of the water dimer is determined by using state-to-state vibrational predissociation measurements following excitation of the bound OH stretch fundamental of the donor unit of the dimer. Velocity map imaging and resonance-enhanced multiphoton ionization (REMPI) are used to determine pair-correlated product velocity and translational energy distributions. H(2)O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via the C? (1)B(1) (000) ← X? (1)A(1) (000 and 010) transitions. The fragments' velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H(2)O. An accurate value for D(0) is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, D(0) = 1105 ± 10 cm(-1) (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of D(0) = 1103 ± 4 cm(-1) (13.2 ± 0.05 kJ∕mol) suggested as a benchmark by Shank et al. [J. Chem. Phys. 130, 144314 (2009)].     

Texture development in polyethylene II. Unidirectional rolling

The effect of degree of strain on texture development in high-density polyethylene has been studied by pole figure analysis for unidirectional rolling. The crystallite orientation distribution in rolling textures has been quantified with an efficient technique which fits three-parameter, two-dimensional Gaussian-type distributions to pole figure intensity data around ideal single crystal orientations. During flat rolling of polyethylene a texture consisting of a strong (100) [001] component and a weak (110) [001] component develops continuously from the lowest true strain of 0.24 (21% reduction) up to the highest true strain of 1.36 (74% reduction). The peak intensity of the Gaussian distributions of both (100) [001] and (110) [001] components increase continuously to the highest strain. The maximum angular breadth of both component distributions, which are roughly perpendicular to the strain direction, remains constant with increasing strain. The minimum angular breadth of both component distributions, which are roughly parallel to the strain direction, decreases continuously owing to gradual alignment of the covalently bonded chain backbone parallel to the strain direction. The development of the (100) [001] component is explained by slip on (100) planes while the weak (110) [001] component is explained by slip on (110) planes. Although the latter component was previously attributed to (110) or (310) relaxation twinning, this seems unlikely because of the lateral constraint during plane strain deformation conditions used in this study.     

Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films

Na_xCoO₂ [x = 0.51, 0.54, and 0.59] thin films have been grown on SrTiO₃ (100)-oriented single crystals with a 5° vicinal cut towards [010] by pulsed laser deposition. We analysed the films by X-ray diffractometry, atomic force microscopy (AFM), and dc-transport measurements. X-ray diffraction patterns of the films show single phase and c-axis textured growth with the film plane closely aligned to the [001]-direction of 5° miscut SrTiO₃ (001) substrates. In addition to the structural analysis of these films we performed transport measurements along and perpendicular to the substrate tilt direction and determined the resistivity anisotropy as a function of temperature. The results enable the development of a strategy for the fabrication of Na_xCoO₂ based thermoelectric thin film devices.     

低压长脉冲载荷下β-HMX单晶滑移系的微观物理化学响应

采用基于ReaxFF反应力场的分子动力学方法,从炸药弹塑性微观机制出发,研究了在低压长脉冲载荷下β-1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(β-HMX)炸药单晶中最有可能的七组滑移系的微观物理化学响应.模拟结果表明沿着垂直于(001)、(101)、(100)、(011)、(111)、(110)、(010)晶面的长脉冲作用方向,这七组滑移系呈现不同的物理化学响应.体系的剪切应力、能量、温度以及化学反应与长脉冲作用方向存在明显的依赖性:对(010)晶面,体系的剪切应力位垒高,能量和温度升高得快,化学反应很快发生,反应敏感度最高;对(001)晶面,体系的剪切应力位垒低,能量和温度变化缓慢,化学反应很难发生,因此反应敏感度低.滑移系的反应敏感度与滑移面两侧的分子间接触程度(即空间位阻)以及接触原子或基团间的反应活性紧密相关.对空间位阻大且相互接触的原子或基团容易发生反应的方向,滑移系的反应敏感度就高;对空间位阻小或相互接触的原子或基团不容易发生反应的方向,滑移系的反应敏感度就低.具有较高化学反应敏感度的滑移系被认为与单晶炸药中的"热点"起源有关.本研究为进一步发展更加合理和可靠的感度评价方法提供了理论支撑.     

低压长脉冲载荷下β-HMX单晶滑移系的微观物理化学响应

采用基于ReaxFF反应力场的分子动力学方法, 从炸药弹塑性微观机制出发, 研究了在低压长脉冲载荷下β-1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(β-HMX)炸药单晶中最有可能的七组滑移系的微观物理化学响应.模拟结果表明沿着垂直于(001)、(101)、(100)、(011)、(111)、(110)、(010)晶面的长脉冲作用方向, 这七组滑移系呈现不同的物理化学响应. 体系的剪切应力、能量、温度以及化学反应与长脉冲作用方向存在明显的依赖性: 对(010)晶面, 体系的剪切应力位垒高, 能量和温度升高得快, 化学反应很快发生, 反应敏感度最高; 对(001)晶面,体系的剪切应力位垒低, 能量和温度变化缓慢, 化学反应很难发生, 因此反应敏感度低. 滑移系的反应敏感度与滑移面两侧的分子间接触程度(即空间位阻)以及接触原子或基团间的反应活性紧密相关. 对空间位阻大且相互接触的原子或基团容易发生反应的方向, 滑移系的反应敏感度就高; 对空间位阻小或相互接触的原子或基团不容易发生反应的方向, 滑移系的反应敏感度就低. 具有较高化学反应敏感度的滑移系被认为与单晶炸药中的“热点”起源有关. 本研究为进一步发展更加合理和可靠的感度评价方法提供了理论支撑.     

Mechanism of the formation of micropores in the thermal decomposition of goethite to hematite

The mechanisms of the formation of micropores in the thermal decomposition of goethite to hematite have been studied through the measurements of X‐ray diffraction, Brunauer–Emmett–Teller adsorption and scanning electronic microscopy, as well as the estimation of crystal cell parameters in this work. The results have shown that the thermal decomposition led to the crystal cells of goethite to be contracted along the [010] and [001] directions and to be elongated along the [100] direction, forming nano‐scale and rod‐like hematite. It was the contractions along the two directions that led to the formation of the micropores in the hematite particles. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular dynamics simulations of melting and the glass transition of nitromethane

Molecular dynamics simulations have been used to investigate the thermodynamic melting point of the crystalline nitromethane, the melting mechanism of superheated crystalline nitromethane, and the physical properties of crystalline and glassy nitromethane. The maximum superheating and glass transition temperatures of nitromethane are calculated to be 316 and 160 K, respectively, for heating and cooling rates of 8.9 x 10(9) Ks. Using the hysteresis method [Luo et al., J. Chem. Phys. 120, 11640 (2004)] and by taking the glass transition temperature as the supercooling temperature, we calculate a value of 251.1 K for the thermodynamic melting point, which is in excellent agreement with the two-phase result [Agrawal et al., J. Chem. Phys. 119, 9617 (2003)] of 255.5 K and measured value of 244.73 K. In the melting process, the nitromethane molecules begin to rotate about their lattice positions in the crystal, followed by translational freedom of the molecules. A nucleation mechanism for the melting is illustrated by the distribution of the local translational order parameter. The critical values of the Lindemann index for the C and N atoms immediately prior to melting (the Lindemann criterion) are found to be around 0.155 at 1 atm. The intramolecular motions and molecular structure of nitromethane undergo no abrupt changes upon melting, indicating that the intramolecular degrees of freedom have little effect on the melting. The thermal expansion coefficient and bulk modulus are predicted to be about two or three times larger in crystalline nitromethane than in glassy nitromethane. The vibrational density of states is almost identical in both phases.     

Three-dimensional crystallographic determination of the body-centered-cubic morphologies of shear-aligned block copolymer systems

The texture of ordered phases of block copolymer melts and gels is highly sensitive to shear. In the body-centered-cubic phase of a block copolymer system [polystyrene–poly(ethylene butylene)–polystyrene] mixed with oil, we show how a given textures can be controlled with the application of a specific shear rate and amplitude. The low-amplitude shear texture is dominated by {001} planes perpendicular to the shear gradient and by the [110] axis parallel to the flow direction, that is, the {001}/[110] slip system. Detailed crystallographic studies show that both intermediate-amplitude oscillatory shear and large-amplitude oscillatory shear lead to twin structures with {112} planes sharing neighboring twins and [111] axes parallel to the shear flow. At an intermediate shear amplitude, the ve shear plane, defined by the shear flow direction (v) and shear vorticity direction (e), is parallel to the {112} twin planes. At a high shear amplitude, the orientation is rotated 90°, and this makes the ve shear plane parallel to the {110} crystallographic planes. The crystalline slip system is accordingly ({112 }/[111] + {11 2}/[111]) under intermediate-amplitude shear and ({11 0}/[111] + {1 10}/[111]) under large-amplitude shear. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3095–3101, 2004     

Rainbow scattering of CO and N2 from LiF(001)

The angular intensity distributions of CO and N(2) molecules scattered from a LiF(001) surface have been measured as functions of surface temperature, incident translational energy, and incident azimuthal direction affecting surface corrugation at a high resolution. Although both molecules have the same molecular mass and linear structure, only the CO molecule shows a rainbow feature in its scattering pattern, while the N(2) molecule shows a single peak distribution. From the comparisons of the obtained results with the calculated predictions based on the newly developed classical theory of the ellipsoid-washboard model, the differences in scattering distribution are attributed to the effects of molecular anisotropy and center-of-mass position. With an increase in the extent of the molecular anisotropy such as that of N(2) and CO as compared with rare-gas atoms, the summation of several scattering distributions depending on molecular orientation results in smearing the rainbow scattering on the corrugated surface. This smearing effect, however, attenuates when center-of-mass position deviates from the molecular center, as that for CO.     

Interaction of Pt clusters with the anatase TiO(2)(101) surface: a first principles study

The adsorption of Pt(n)() (n = 1-3) clusters on the defect-free anatase TiO(2)(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO(2)(101) surface has a stepped structure with a step width of two O-Ti bond distances in the (100) plane along the [10] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt-O bonds formed upon Pt adsorption are covalent. Among six stable Pt(2) adsorption configurations examined, Pt(2) was found to energetically favor the O-O bridge sites on the step edge along [010] with the Pt-Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O-O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt(3) structures were determined as minima for Pt(3) adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt(3) occupies the edge O-O site with the Pt(3) plane being upright and almost perpendicular to the [001] terrace. The preference of Pt(n)() to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.     

Studies of Adjacent Re‐Entry Folds of Chains of Syndiotactic l,2‐Poly(1,3‐butadiene) by Molecular Mechanics Calculations

The mode of packing and the adjacent re‐entry folds of chains of syndiotactic 1,2‐poly(1,3‐butadiene) have been studied by molecular mechanics calculations with the use of various sets of potential functions. The results of the packing analysis indicate that the crystal grows preferentially along the [100] and [110] directions. Models of fold have been built up on an infinite ab surface completely covered by adjacent re‐entry folds in the (100) and (110) planes. The results of energy minimizations show that several almost isoenergetic folds, constituted by four monomeric units, can be realized in the (100) planes, while the fold in the (110) planes has higher energy. The calculated value of the work of fold is in satisfactory agreement with that derived by crystallization kinetics reported in literature.

     

Probing crystallization of calcium oxalate monohydrate and the role of macromolecule additives with in situ atomic force microscopy

Kidney stones are crystal aggregates, most commonly containing calcium oxalate monohydrate (COM) microcrystals as the primary constituent. Macromolecules, specifically proteins rich with anionic side chains, are thought to play an important role in the regulation of COM growth, aggregation, and attachment to cells, all key processes in kidney stone formation. The microscopic events associated with crystal growth on the [010], [121], and [100] faces have been examined with in situ atomic force microscopy (AFM). Lattice images of each face reveal two-dimensional unit cells consistent with the COM crystal structure. Each face exhibits hillocks with step sites that can be assigned to specific crystal planes, enabling direct determination of growth rates along specific crystallographic directions. The rates of growth are found to depend on the degree of supersaturation of calcium oxalate in the growth medium, and the growth rates are very sensitive to the manner in which the growth solutions are prepared and introduced to the AFM cell. The addition of macromolecules with anionic side chains, specifically poly(acrylic acid), poly(aspartic acid), and poly(glutamic acid), results in inhibition of growth on the hillock step planes. The magnitude of this effect depends on the macromolecule structure, macromolecule concentration, and the identity of the step site. Poly(acrylic acid) was the most effective inhibitor of growth. Whereas poly(aspartic acid) inhibited growth on the (021) step planes of the (100) hillocks more than poly(glutamic acid), the opposite was found for the same step planes on the (010) hillocks. This suggests that growth inhibition is due to macromolecule binding to both planes of the step site or pinning of the steps due to binding to the (100) and (010) faces alone. The different profiles observed for these three macromolecules argue that local binding of anionic side chains to crystal surface sites governs growth inhibition rather than any secondary polymer structure. Growth inhibition by cationic macromolecules is negligible, further supporting an important role for proteins rich in anionic side chains in the regulation of kidney stone formation.