Nanoparticle-coated microcrystals

Coprecipitation provides a rapid high-yield method for self-assembly of nanoparticles on the surface of flat water-soluble crystalline surfaces and a simple immobilisation technique prior to storage or thermal and chemical modification

Ultrasonic studies of solid azobenzene decorated polymer thin films

This work investigates the effect of ultrasound on switching of cis azobenzene isomers to their trans counterparts in solid films of methyl methacrylate and methacryloyloxyazobenzene copolymers [P(MMA/MOAB)]. UV/Vis and 1H NMR spectroscopy demonstrates that 46% of the cis isomer converts to the trans form purely by ultrasonic agitation and 46% converts to the trans isomer by localised ultrasound induced heating effects. Comparative studies of isomerisation by ultrasound wave, heat and visible irradiation shows that ultrasound exposure requires a longer time to switch the cis to trans conformation. The estimated activation energy for the cis to trans conversion in the solid polymer films is shown to be comparable to previous values of azobenzene isomerization, indicating that incorporation of the chromophore in a polymeric system affects the kinetics of transition, but not the barriers to conformational change

Laser ignitibility of energetic crystals doped with gold nanoparticles

Laser ignition mechanism enhances the safety of explosive applications. However, optical sensitisation of the energetic materials is required for their optical absorption. In this research, explosive Cyclotrimethylene Trinitramine (RDX) was doped with gold nanoparticles during recrystallisation to provide a suitable optical sensitisation method. The relationship between the precipitation rate used during RDX recrystallisation and the subsequent laser ignition properties using an 808-nm continuous wave diode laser was studied. Faster initial precipitation was found to reduce the laser ignition delay times and smaller nanoparticle size reduced ignition thresholds. It was shown that recrystallized RDX particle size, which is determined by both nanoparticle induced nucleation and precipitation rate, affects the ignition delay time and quality of the crystals. The largest crystals in each batch were investigated using both high-speed photography and microscopic etching, revealing that in contrast to the bulk sample (which was generally a powder), a slower precipitation seems to achieve better doping. The largest crystals in each sample therefore do not represent the bulk sample, a useful indication as such large crystals are easier to handle and therefore tend to be selected for detailed characterisation work

Dehydration mechanism of a small molecular solid: 5-nitrouracil hydrate

Previous studies of the dehydration of 5-nitrouracil (5NU) have resulted in it being classified as a ‘‘channel hydrate’’ in which dehydration proceeds principally by the exit of the water molecules along channels in the structure. We have re-examined this proposal and found that in fact there are no continuous channels in the 5NU structure that would contribute to such a mechanism. Product water molecules would be immediately trapped in unlinked voids in the crystal structure and would require some additional mechanism to break loose from the crystal. Through a detailed structural analysis of the macro and micro structure of the 5NU as it dehydrates, we have developed a model for the dehydration process based on the observed development of structural defects in the 5NU crystal and the basic crystallography of the material. The model was tested against standard kinetic measurements and found to present a satisfactory account of kinetic observations, thus defining the mechanism. Overall, the study shows the necessity of complementing standard kinetic studies with a parallel macro and micro examination of the dehydrating material when evaluating the mechanisms of dehydration and decomposition processes

Crystal growth of the acentric organic nonlinear optical material methyl-p-hydroxybenzoate : morphological variations in crystals grown by physical vapor transport

Single crystals of the acentric compound methyl-p-hydroxybenzoate were grown by self-nucleation and seeded growth from the vapor phase by the physical vapor transport (PVT) process. In the temperature range of 80-95 °C (nucleation supersaturation 0.97 to 0.88), all crystals were of the polymorphic form as produced by room-temperature solution growth. Self-nucleated crystals varied in macromorphology from columnar to octahedral to skewed octahedral and finally to skewed columnar but retained the same crystal forms indicated by theoretical calculations. Micromorphological studies of growth faces indicated that these variations result from changes in growth mechanisms that influence both the defect structure and perfection of the growing crystal. X-ray topographic studies confirmed that growth under the most ideal conditions, when the dominant faces of the crystals were growing by a dislocation induced Burton, Cabrera, and Frank mechanism, yielded the structurally most perfect crystals. Preliminary studies of seeded growth were performed as a prelude to using PVT for the growth of larger crystals. The seeded growth followed a different pattern of supersaturation dependence. All crystals showed the same asymmetric growth along the polar axis that has come to be regarded as characteristic of these highly polar acentric materials when grown from solution

Growth and dislocation studies of β-HMX

Background: The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in “hot-spot” detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX). Results: The as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip. Conclusions: Crystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b = [010] and pure screw dislocations with b = [010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b = [010]

Mechanical properties of β-HMX

Background: For a full understanding of the mechanical properties of a material, it is essential to understand the defect structures and associated properties and microhardness indentation is a technique that can aid this understanding. Results: The Vickers hardness on (010), {011} and {110} faces lay in the range of 304-363 MPa. The Knoop Hardnesses on the same faces lay in the range 314-482 MPa. From etching of three indented surfaces, the preferred slip planes have been identified as (001) and (101). For a dislocation glide, the most likely configuration for dislocation movement on the (001) planes is (001) [100] (|b| = 0.65 nm) and for the (101) plane as (101) 101~(|b| = 1.084 nm) although (101) [010] (|b| = 1.105 nm) is possible. Tensile testing showed that at a stress value of 2.3 MPa primary twinning occurred and grew with increasing stress. When the stress was relaxed, the twins decreased in size, but did not disappear. The twinning shear strain was calculated to be 0.353 for the (101) twin plane. Conclusions: HMX is considered to be brittle, compared to other secondary explosives. Comparing HMX with a range of organic solids, the values for hardness numbers are similar to those of other brittle systems. Under the conditions developed beneath a pyramidal indenter, dislocation slip plays a major part in accommodating the local deformation stresses. © 2015 Gallagher et al.; licensee Springer

Microhardness indentation studies of 2-4-6 trinitrotoluene

The microhardness of the {001} faces of 2-4-6 trinitrotoluene crystals has been investigated using both Vickers and Knoop indentation methods. The Vickers hardness number was found to be 22.5 kg mm−2 independent of crystal orientation and perfection. At ambient temperatures (∼20 °C) the Knoop hardness number varied between 20.5 kg mm−2 and 24.0 kg mm−2 with crystal orientation. At higher temperature (50 °C) the Knoop hardness anisotropy curve retained its shape, although the overall hardness decreased by 10 %. We interpret this change as reflecting a simple temperature dependant loosening of the crystal lattice rather than any change in deformation mechanism. No variation of Knoop hardness was evident with changing load. The hard direction was [010] and the soft [100]. The dominant operative slip system was defined to be {001}[010]

The crystal growth and perfection of 2,4,6-trinitrotoluene

Large crystals of TNT were grown from ethyl acetate solution by both temperature lowering and solvent evaporation. The perfection of crystals grown from seeds under carefully controlled conditions was generally higher than those prepared by uncontrolled solvent evaporation. Examination by X-ray topography revealed the crystals to have a characteristic growth induced defect structure comprising growth sectors and boundaries, growth banding, solvent inclusions and dislocations. Twins and stacking faults (SF) were also observed. Many of the defects noted in the topographs can be attributed to impurities. The influence of the highly anisotropic crystal structure on the nature of growth defects is discussed. A structural model proposed to explain twinning and SF formation is partially supported by topographic evidence

Polymorphism in 2,4,6-trinitrotoluene crystallized from solution

An examination has been made of the role of solvent type in the definition of the polymorphic nature of 2,4,6-trinitrotoluene precipitated from solution. A combination of calorimetric and structural techniques including in situ crystallization studies using synchrotron radiation has shown that the variations in polymorphic form following precipitation from solution do not arise specifically from any stereospecific guidance that the nature of the solvent might impose on the structural form. Rather the differences are linked to the variations in solubility and hence supersaturation which might build up prior to nucleation and growth and to the isolation of the metastable orthorhombic phase from the solvent. The final conclusion is that the changes fit well with Ostwald’s Law of Stages with the orthorhombic form always precipitating initially followed by its conversion to the stable monoclinic form. The previously observed tendency for some solvents to yield one or the other form then becomes attributable to kinetics in solution rather than structural control. It can be associated with the solubility of the material in the solvent used and the role of this factor in a solvent-mediated phase transformation. On this basis rules can be formulated for the isolation of the metastable forms of this and similarly related polymorphic systems