Shock wave-induced phase transition in RDX single crystals

The real-time, molecular-level response of oriented single crystals of hexahydro-1,3,5-trinitro-s-triazine (RDX) to shock compression was examined using Raman spectroscopy. Single crystals of [111], [210], or [100] orientation were shocked under stepwise loading to peak stresses from 3.0 to 5.5 GPa. Two types of measurements were performed: (i) high-resolution Raman spectroscopy to probe the material at peak stress and (ii) time-resolved Raman spectroscopy to monitor the evolution of molecular changes as the shock wave reverberated through the material. The frequency shift of the CH stretching modes under shock loading appeared to be similar for all three crystal orientations below 3.5 GPa. Significant spectral changes were observed in crystals shocked above 4.5 GPa. These changes were similar to those observed in static pressure measurements, indicating the occurrence of the alpha-gamma phase transition in shocked RDX crystals. No apparent orientation dependence in the molecular response of RDX to shock compression up to 5.5 GPa was observed. The phase transition had an incubation time of approximately 100 ns when RDX was shocked to 5.5 GPa peak stress. The observation of the alpha-gamma phase transition under shock wave loading is briefly discussed in connection with the onset of chemical decomposition in shocked RDX.     

Shock wave induced decomposition of RDX: quantum chemistry calculations

Quantum chemical calculations on single molecules were performed to provide insight into the decomposition mechanism of shocked RDX. These calculations complement time-resolved spectroscopy measurements on shock wave compressed RDX crystals (previous paper, this issue). It is proposed that unimolecular decomposition is the primary pathway for RDX decomposition in its early stages and at stresses lower than approximately 10 GPa. This decomposition leads to the generation of broadband emission from 350 to 850 nm. Chemiluminescence from (2)B1 and (2)B2 excited states of NO2 radicals is associated with a major portion of the experimentally observed emission spectrum (>400 nm). The remaining portion (<400 nm) of the emission spectrum primarily results from excited HONO intermediates. It is proposed that for stresses higher than 10 GPa, bimolecular reactions between radical decomposition products and unreacted RDX molecules become the dominant pathway. This radical assisted homolysis pathway is cyclic and leads to the acceleration of decomposition, with increased production of low energy NO2 radicals. These radicals produce emission that is stronger in the long wavelength portion of the spectrum. Finally, a comprehensive chemical decomposition mechanism is put forward that is consistent with the experimental observations of shock-induced emission in RDX crystals.     

Spectroscopic study of shock-induced decomposition in ammonium perchlorate single crystals

Time-resolved Raman scattering measurements were performed on ammonium perchlorate (AP) single crystals under stepwise shock loading. For particular temperature and pressure conditions, the intensity of the Raman spectra in shocked AP decayed exponentially with time. This decay is attributed to shock-induced chemical decomposition in AP. A series of shock experiments, reaching peak stresses from 10-18 GPa, demonstrated that higher stresses inhibit decomposition while higher temperatures promote it. No orientation dependence was found when AP crystals were shocked normal to the (210) and (001) crystallographic planes. VISAR (velocity interferometer system for any reflector) particle velocity measurements and time-resolved optical extinction measurements carried out to verify these observations are consistent with the Raman data. The combined kinetic and spectroscopic results are consistent with a proton-transfer reaction as the first decomposition step in shocked AP.     

Shock wave induced decomposition of RDX: time-resolved spectroscopy

Time-resolved optical spectroscopy was used to examine chemical decomposition of RDX crystals shocked along the [111] orientation to peak stresses between 7 and 20 GPa. Shock-induced emission, produced by decomposition intermediates, was observed over a broad spectral range from 350 to 850 nm. A threshold in the emission response of RDX was found at about 10 GPa peak stress. Below this threshold, the emission spectrum remained unchanged during shock compression. Above 10 GPa, the emission spectrum changed with a long wavelength component dominating the spectrum. The long wavelength emission is attributed to the formation of NO2 radicals. Above the 10 GPa threshold, the spectrally integrated intensity increased significantly, suggesting the acceleration of chemical decomposition. This acceleration is attributed to bimolecular reactions between unreacted RDX and free radicals. These results provide a significant experimental foundation for further development of a decomposition mechanism for shocked RDX (following paper in this issue).     

Decomposition of γ-Cyclotrimethylene Trinitramine (γ-RDX): Relevance for Shock Wave Initiation

To elucidate the reactive behavior of RDX crystals at pressures and temperatures relevant to shock wave initiation, Raman spectroscopy and optical imaging were used to determine the pressure-temperature (P-T) stability and the decomposition of γ-RDX, the high pressure phase of RDX. Experiments were performed on single crystals in a diamond anvil cell at pressures from 6 to 12 GPa and at temperatures up to 600 K. Evidence for the direct decomposition of γ-RDX above 6 GPa, without the involvement of other phases, is provided. The upper limit of the P-T locus for the γ-RDX thermal decomposition was determined. A refined P-T phase diagram of RDX is presented that includes the current findings for γ-RDX. The static compression results are used to gain key insight into the shock initiation of RDX, including a determination of the RDX phase at decomposition and understanding the role of pressure and temperature in accelerating shock induced decomposition. This study has established the important role that γ-RDX plays in decomposition of RDX under static and shock compression conditions; thus theoretical modeling of RDX decomposition at high pressures and temperatures needs to incorporate the γ-phase response.     

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 in photocatalytic oxidization activity of NaNbO(3) photocatalyst

NaNbO(3) single-crystal films with (100), (110) and (111) oriented crystal planes were grown on LaAlO(3) (100), (110) and (111) substrates by pulsed laser deposition. The NaNbO(3) films exhibit anisotropy in the photocatalytic oxidization activity for Rhodamine B (RhB) degradation. The increasing order of RhB degradation in Ar atmosphere under full arc irradiation of a Xe lamp is (100) < (110) < (111), which is consistent with that of ˙OH generation. The good linear relationship between the activity of RhB degradation and the coercive electric field indicates that the photocatalytic activity is closely related to the ferroelectric property.     

Shock-induced chemical reactions in nickel-aluminum powder mixtures: Radiation pyrometer measurements

Time-resolved emission of visible and near-infrared thermal radiation has been measured from powders of pure nickel and mixed nickel-aluminum shocked to peak pressures of 14 GPa. Temperatures determined from the measurements indicate that the Ni-Al mixture has a source of heat in addition to that supplied by shock compression. The extra heat, produced on a time scale of 100 ns, is inferred to come from an exothermic reaction between the two metals forming a binary alloy. If the alloy is Ni₃Al, the measured temperatures are consistent with prompt shock-induced reaction of at least 45% of the reactants.     

Island shapes and aggregation steered by the geometry of the substrate lattice

We find that island shapes and aggregation in diindenoperylene deposited on Au(100), Au(110), and Au(111) single crystals are steered by the anisotropy due to the lattice geometry of the substrate. This phenomenon may be exploited as a tool for molecular patterning of surfaces.     

Characterization of Nano-Scale Parallel Lamellar Defects in RDX and HMX Single Crystals by Two-Dimension Small Angle X-ray Scattering

Nano-scale crystal defects extremely affect the security and reliability of explosive charges of weapons. In this work, the nano-scale crystal defects of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) single crystals were characterized by two-dimension SAXS. Deducing from the changes of SAXS pattern with sample stage rotating, we firstly found the parallel lamellar nano-scale defects in both RDX and HMX single crystals. Further analysis shows that the average diameter and thickness of nano-scale lamellar defects for RDX single crystal are 66.4 nm and 19.3 nm, respectively. The results of X-ray diffraction (XRD) indicate that the lamellar nano-scale defects distribute along the (001) in RDX and the (011) in HMX, which are verified to be the crystal planes with the lowest binding energy by the theoretical calculation.     

1,3,5-trinitro-1,3,5-triazine decomposition and chemisorption on Al(111) surface: first-principles molecular dynamics study

We have investigated the decomposition and chemisorption of a 1,3,5-trinitro-1,3,5-triazine (RDX) molecule on Al(111) surface using molecular dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). The real-space DFT calculations are based on higher-order finite difference and norm-conserving pseudopotential methods. Strong attractive forces between oxygen and aluminum atoms break N-O and N-N bonds in the RDX and, subsequently, the dissociated oxygen atoms and NO molecules oxidize the Al surface. In addition to these Al surface-assisted decompositions, ring cleavage of the RDX molecule is also observed. These reactions occur spontaneously without potential barriers and result in the attachment of the rest of the RDX molecule to the surface. This opens up the possibility of coating Al nanoparticles with RDX molecules to avoid the detrimental effect of oxidation in high energy density material applications.     

Adsorbed intermediates of formaldehyde oxidation and their role in the reaction mechanism

Formaldehyde oxidation was studied on the basal planes of platinum single crystals. Electrochemical and IR spectroscopy data give new information on the mechanism of oxidation. Formaldehyde oxidation at platinum electrodes is a surface-sensitive reaction. From the three basal planes of Pt(hkl), Pt(111) is the most active one. The less active surfaces Pt(100) and Pt(110) are blocked by adsorbed carbon monoxide at the initial stages of the reaction as the formaldehyde is admitted in the solution with the electrode polarized at 0.05 V. Besides CO(ad), other adsorbed species are formed. From these, methylene glycolate, H2COO(ad), is the intermediate of the fast oxidation pathways forming CO2 and HCOOH as soluble products. According to IR data the yields of soluble products at Pt(111) were calculated at 0.6 V, giving 63% for HCOOH and 37% for CO2. At 0.05 V the Pt(111) surface becomes slowly blocked by CO(ad), as observed when the electrode was left in contact with the formaldehyde solution over a period of several minutes. The same blockage occurs during a cyclic voltammogram, which causes a lowering of activity during the second potential scan. A general scheme of the reaction is proposed.     

A comparative study of hydroxide adsorption on the (111), (110), and (100) faces of silver with cyclic voltammetry, ex situ electron diffraction, and in situ second harmonic generation

Hydroxide adsorption on the (111), (110), and (100) faces of silver electrodes from mixed NaOH/NaF solution is studied using cyclic voltammetry and in situ second harmonic generation (SHG). Cyclic voltammograms for the three low index silver planes in alkaline electrolytes are for the first time compared. They show two pairs of anodic and cathodic peaks in the potential interval below the equilibrium Ag/Ag(2)O potential. These are attributed to the specific adsorption of hydroxide ions followed by submonolayer oxide formation. The differences in the cyclic voltammograms for the (111), (110), and (100) planes are attributed to different (i) work functions, (ii) surface atomic densities, and (iii) corrugation potentials for these surfaces. Ex situ low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) show that disordered adlayers are formed on Ag(111) and Ag(100), in contrast to Ag(110), where ordered structures are produced in the region of the first pair of current peaks. In the region of the second pair of peaks, LEED indicates disordered oxide phases on each crystal plane and RHEED shows the presence of small islands of c(2 x 2) structure at some potentials on (110) and (100). SHG measurements were performed (i) in the potential scan mode at constant rotational angle and (ii) at constant potential as a function of the rotational angle. The isotropic (for the (111), (110), and (100) planes) and anisotropic (for the (110) and (111) planes) contributions to the SHG intensity were calculated by fitting the experimental data and are discussed in terms of their dependence on the charge density at the interface, on hydroxide adsorption, and on submonolayer oxide formation.     

Thermal and mechanical cracking in bis(triisopropylsilylethnyl) pentacene thin films

Bis(triisopropylsilylethnyl) pentacene (TIPS pentacene) was synthesized to increase its solubility in common liquid solvents and, at the same time, enhance the π–π stacking between neighboring acenes in the crystallized state in comparison with unmodified pentacene. Hot-stage microscopy experiments revealed that during heating voids develop along the long axis of the TIPS pentacene films {along the [210] direction/parallel to the (120 ) planes} and crystals overlap along the short axis {along the [120 ] direction/parallel to the (210) planes}. From molecular mechanics simulations, the predominant twin boundaries of (120 ) and commonly observed cracking planes of (120), (120 ), and (210) had relatively low surface energies in comparison with planes with similar Miller indices. Organic thin-film transistors with TIPS pentacene as the active layer were fabricated, and the mobility values decreased from 0.4–1.0 cm²/V s before cracking to ∼0.2 cm²/V s after cracking. To maintain the high charge carrier mobility of TIPS pentacene devices, these cracks should be avoided. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3631–3641, 2006     

Epitaxial electrodeposition of Fe(3)O(4) thin films on the low-index planes of gold

Half-metallic ferrimagnetic materials such as Fe(3)O(4) are of interest for use in spintronic devices. These devices exploit both the spin and charge of an electron in spin-dependent charge transport. Epitaxial thin films of Fe(3)O(4) have been grown on the three low-index planes of gold by electrodeposition. On Au(110), a [110] Fe(3)O(4) orientation that is aligned with the underlying Au(110) substrate is observed. Thin films on Au(100) grow with three different orientations: [100], [111], and [511]. On Au(111), both [111] and [511] orientations of Fe(3)O(4) are observed. The [511] orientations are the result of twinning on [111] planes. A polarization value of approximately -40% at the Fermi level was measured by spin-polarized photoemission at room temperature for a thin film on Au(111).     

Elucidating the effect of additives on the growth and stability of Cu2O surfaces via shape transformation of pre-grown crystals

A new strategy of using pre-grown crystals to study preferential adsorption of various additives is demonstrated for the electrocrystallization of Cu2O. In this method, micron-size Cu2O crystals with well-defined cubic and octahedral shapes were first electrochemically grown, and their crystallization was resumed in a medium containing the additive to be investigated (e.g., Na+, NH4+, SO42-, Cl-, dodecyl sulfate). This method makes it possible to systematically study the interaction of additives with specific planes (e.g., {100} of a cube and {111} of an octahedron) already present. By observing shape transformation over time, the relative stabilities of {100}, {111}, and {110} planes of Cu2O in various growth media could be determined. During this study, a general scheme of forming new crystal shapes containing crystallographic planes that cannot be directly stabilized by preferential adsorption alone was also established (i.e., rhombicuboctahedral shape of Cu2O containing {110} planes). This method can be extended to other crystal systems, which will enable us to classify common features of additives (e.g., charges, type of atoms) and crystallographic planes (e.g., atomic arrangement, surface termination, surface charge) required to allow for strong preferential adsorption.     

Epitaxial growth and electrochemical properties of Li4Ti5O12 thin-film lithium battery anodes

Epitaxial Li(4)Ti(5)O(12) thin-films were successfully synthesized on SrTiO(3) single-crystal substrates with (111), (110), and (100) lattice plane orientations using pulsed laser deposition (PLD). Thin-film X-ray diffraction (XRD) revealed that the Li(4)Ti(5)O(12) films had the same orientation as the SrTiO(3) substrates: Li(4)Ti(5)O(12) (111) on SrTiO(3) (111), Li(4)Ti(5)O(12) (110) on SrTiO(3) (110), and Li(4)Ti(5)O(12) (100) on SrTiO(3) (100). These epitaxial films contained island structures, and the morphology of the (111), (110), and (100) films, observed by field emission scanning electron microscopy (FE-SEM), exhibited angular, needle-like, and circular shapes, respectively. The electrochemical properties of 20 nm thick Li(4)Ti(5)O(12) (111) and (110) films were investigated by cyclic voltammetry. Reversible intercalation proceeded through both lattice planes due to the three-dimensional diffusion pathway of lithium in the spinel framework. Reduction peaks in the first cathodic scan appeared at different positions from those in subsequent scans, suggesting a surface reconstruction at the Li(4)Ti(5)O(12) surface due to interfacial reactions.     

Density Functional Theory Calculations Revealing Metal‐like Band Structures and Work Function Variation for Ultrathin Gallium Arsenide (111) Surface Layers

Density functional theory (DFT) calculations have been performed on tunable numbers of gallium arsenide (100), (110), and (111) planes for their electron density of states (DOS) plots and the corresponding band diagrams. The GaAs (100) and (110) planes show the same semiconducting band structure with tunable plane layers and a band gap of 1.35 eV around the Fermi level. In contrast, metal‐like band structures are obtained with a continuous band structure around the Fermi level for 1, 2, 4, 5, 7, and 8 layers of GaAs (111) planes. For 3, 6, and 9 GaAs (111) planes, the same semiconducting band structure as seen in the (100) and (110) planes returns. The results suggest the GaAs {111} face should be more electrically conductive than its {100} and {110} faces, due to the merged conduction band and valence band. GaAs (100) and (110) planes give a fixed work function, but the (111) planes have variable work function values that are smaller than that obtained for the (100) and (110) planes. Furthermore, bond length, bond geometry, and frontier orbital electron number and energy distribution show notable differences between the metal‐like and semiconducting plane cases, so the emergence of plane‐dependent electronic properties have quantum mechanical origin at the orbital level. GaAs should possess similar facet‐dependent electronic properties to those of Si and Ge.     

The turnover rate for the catalytic combustion of methane over palladium is not sensitive to the structure of the catalyst

The kinetics for the catalytic combustion of methane on Pd in its metal and oxide forms was measured on a foil and on large single crystals exposing initially the (111), (100), and (110) planes. The rates, reaction orders, and activation energies were not dependent on the structure of the active phase. Turnover rates were strictly proportional to the total surface area. The kinetic data were in agreement with the ones for supported catalysts. The values reported can be used as benchmark kinetic values for this reaction since purity and effect of support could be controlled.     

Thermophysical, mechanical and dielectric studies on piperidinium p-hydroxybenzoate

Single crystals of recently identified nonlinear optical material, piperidinium p-hydroxybenzoate (PDPHB), were grown by solution cooling method. It crystallizes in a monoclinic system with a noncentrosymmetric space group Cc. Thermal stability and decomposition behavior of PDPHB are illustrated through thermogravimetric, differential thermal and differential calorimetric analysis. The thermal diffusivity, thermal conductivity, and specific heat capacity of the grown crystals, which are the three most important thermo-physical parameters in heat transfer calculations, are calculated by an improved photopyroelectric technique. The room temperature hardness test has been performed on crystallographic planes (200), (020), and (002) using Vickers microhardness tester and the results are analyzed through the classical Meyer’s law. The dielectric constant, dielectric loss, and ac conductivity are studied as a function of frequency (100 Hz to 1 MHz) and temperature (313–363 K). All these studies are performed for the first time and aimed to explore the useful and safe region of thermal, mechanical, and electrical properties to enhance effectiveness of PDPHB crystals for device fabrications.