Browsing by Author "Vrcelj, Ranko"
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Item Open Access Additively manufactured (3DP) thermite structures vs conventionally manufactured equivalents(IPSUSA Seminars, 2022-07-14) McGee, Christine; Vrcelj, RankoResearch into additive manufacturing (AM) has been steadily expanding over the past five decades. Where once only polymeric materials could be reliably printed, AM has been adapted to print with a range of materials such as biological, metallic, ceramic and even foodstuffs. The advantages of manufacturing in an additive manner include; a) a layer-by-layer approach allows the creation of architecturally complex structures, b) a reduction in weight, c) lessening of waste and d) the ability to create parts that that are otherwise difficult or too costly to produce. Pyrotechnic materials, including thermites, are used in a wide range of commercial and defence applications. However, hazards present during manufacturing and storage have resulted in major accidents around the world, with subsequent loss of life and in some cases loss of public infrastructure. AM, using a dry powder printing technique means that parts can be manufactured on demand, reducing the need for storage of large volumes of fully formed products or mixes, thus increasing the safety over lifetime of a product. The performance of pyrotechnics materials is dependent on a number of properties, including chemical composition, thermodynamic properties and physical form. In combination with composition, architecture could be utilised to understand and control these properties. A bespoke printer capable of additively manufacturing pyrotechnic materials has been constructed with the aim to explore this research area. In this presentation, we compare the burn rates of AM thermites and compare them to conventionally fabricated compositions and discuss the effects of the print parameters and confinement. We conclude with the results from the burning of AM thermite structures and compare their performance with conventionally prepared equivalent thermite examples.Item Open Access The growth and perfection of β-cyclotetramethylene-tetranitramine (HMX) studied by laboratory and synchrotron X-ray topography(Elsevier, 2017-05-31) Gallagher, H G; Sherwood, J N; Vrcelj, RankoAn examination has been made of the defect structure of crystals of the energetic material β-cyclotetramethylene-tetranitramine (HMX) using both Laboratory (Lang method) and Synchrotron (Bragg Reflection and Laue method) techniques. The results of the three methods are compared with particular attention to the influence of potential radiation damage caused to the samples by the latter, more energetic, technique. The comparison shows that both techniques can be confidently used to evaluate the defect structures yielding closely similar results. The results show that, even under the relatively casual preparative methods used (slow evaporation of unstirred solutions at constant temperature), HMX crystals of high perfection can be produced. The crystals show well defined bulk defect structures characteristic of organic materials in general: growth dislocations, twins, growth sector boundaries, growth banding and solvent inclusions. The distribution of the defects in specific samples is correlated with the morphological variation of the grown crystals. The results show promise for the further evaluation and characterisation of the structure and properties of dislocations and other defects and their involvement in mechanical and energetic processes in this materialItem Open Access John N. Sherwood: studies of energetic materials(American Chemical Society, 2023-07-03) Vrcelj, Ranko; Gallagher, Hugh G.; Halfpenny, Peter J.The studies on the physico-mechanical properties of commonly used energetic materials (EMs) that were pursued by the group led by Professor John Sherwood are reviewed in this paper. The studies ranged from the growth of high quality single crystals and the characterisation of their defect and dislocation structures, mechanical testing, through to the study of polymorphism of EM crystals and fundamental aspects of crystallization processes generally. The work performed lead to the definition of good growth conditions for all the EMs studied and to the full characterization of the defect structure, slip systems and hardness properties of cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN). Partial characterization of the defect structures and hardness properties of cyclotetramethylene tetranitramine (HMX) and 2-4-6 trinitrotoluene (TNT) were also achieved. Additionally, fundamental crystal growth and polymorph information was defined, allowing a deeper understanding of the crystallization and crystal structure of TNT. In addition to the general review, some thoughts as to possible future routes for further study that could suitably utilize the complementary nature of established and modern techniques.Item Open Access Laser ignitibility of energetic crystals doped with gold nanoparticles(Elsevier, 2019-01-04) Churchyard, Sam; Fang, Xiao; Vrcelj, RankoLaser 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.Item Open Access Material Chemistry Control for the Additive Manufacture of Composite Propellants(2022-06) Brash, J P A; Vrcelj, Ranko; Moniruzzaman, MThis thesis has sought to aid the additive manufacture of propellants using a novel dry powder printing system developed at Cranfield. The energetic performance and hazard safety of crystalline energetic materials is intrinsically linked to crystal properties such as size, morphology, and crystalline phase. By optimisation of cooling, antisolvent, sonocrystallisation, spray drying and microencapsulation, the properties of cyclotrimethylenetrinitramine (RDX) and ammonium perchlorate (AP) have been engineered towards better performance within our printing system. Crystallisation of AP from solution in water has been assessed as a means of producing particles with a controllable particle size and morphology. Slow cooling processes (−7.5 °C hr-1 ) failed to produce material suited for use in propellant formulations. However, by significantly increasing the nucleation rate using rapid cooling crystallisation processes (~ −5 °C min-1 ) the size of generated crystals was greatly reduced, with a d50 range of 79.1 - 152.3 µm, compared to ~ 500 – 2000 µm. The application of ultrasonic radiation via a horn to the rapid cooling crystallisation gave promising results – leading to particle size reduction (d50 range: 33.5 – 43.4 µm) and a reduced frequency of secondary nucleation. Moreover, the average particle size distribution width was reduced from 245 µm to 75 µm by the application of sonication. Flow character, as assessed by angle of repose measurements, was good for these sonicated materials (31.0° to 34.1°). Spray drying and micro encapsulation was assessed as a means of RDX particle size reduction. Initial studies using paracetamol as an inert simulant demonstrated that modifications to spray drying process parameters (flow, atomisation pressure, nozzle diameter and feed concentration) produced measurable changes in particle size and size distribution. However, attempts to rationalise these effects using a multifactorial design of experiment were inhibited with the significant errors retrieved from the model. Attempts to understand how particle properties impact the flow character of a powder led to the observation that increased particle size gave decreased angle of repose. However, the magnitude of change was negligible when compared to the effect of reformulation in the presence of known glidant nanomaterials. Microencapsulation of RDX with cellulose acetate butyrate (CAB) was conducted at a range of operating temperatures between 55 and 100 °C. Both particle morphology and impact Figure of insensitiveness were demonstrably affected by drying temperature, and both were minimised by the use of lower drying temperatures (d50 = 2.60 µm, FoI = 102.0). FoI values for RDX/CAB microparticles correlated negatively with drying temperature, suggesting that the strain imparted by this rapid crystallisation process may be retained in the material thereby acting to influence its hazardous nature. Crystallisation of RDX by antisolvent precipitation and spray drying was assessed with the inclusion of five different tailor-made additives (TMAs). Of the assessed TMAs, 2,4-dimethyl-1nitrobenzene and 1,2-diemthyl-3- nitrobenzene were noteworthy for causing significant particle size reduction of antisolvent precipitated RDX. Crystal size enlargement and aspect ratio elongation was most pronounced when 1,3,5-triazine-2,4-diamine impurity was present. A novel application of the Scherrer equation was employed, to study the effect of TMA inclusion on the constituent crystallites within spray dried microparticles. The investigation revealed reduced coherence length of the (002) plane and extension of the (210) plane when RDX was spray dried in the presence of TMAs.Item Open Access Microhardness indentation studies of 2-4-6 trinitrotoluene(Wiley, 2021-10-20) Gallagher, Hugh G.; Sherwood, John N.; Vrcelj, RankoThe 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].Item Open Access Nonlinear ptychographic coherent diffractive imaging(Optical Society of America, 2016-08-24) Odstrcil, M.; Baksh, P.; Gawith, C.; Vrcelj, Ranko; Frey, J. G.; Brocklesby, W. S.Ptychographic Coherent diffractive imaging (PCDI) is a significant advance in imaging allowing the measurement of the full electric field at a sample without use of any imaging optics. So far it has been confined solely to imaging of linear optical responses. In this paper we show that because of the coherence-preserving nature of nonlinear optical interactions, PCDI can be generalised to nonlinear optical imaging. We demonstrate second harmonic generation PCDI, directly revealing phase information about the nonlinear coefficients, and showing the general applicability of PCDI to nonlinear interactions.Item Open Access Seeded crystal growth of the acentric organic non-linear optical material Methyl-p-Hydroxybenzoate (MHB) from the vapour phase(American Chemical Society, 2023-06-06) Hou, Wenbo B.; Ristic, Radoljub I.; Sherwood, John N.; Vrcelj, RankoUsing in-situ differential interference contrast microscopy (DICM), growth morphology, structure, and step velocities of the vicinal hillocks on {110} and {111̅} faces of MHB crystal seeds growing from the vapour phase have been investigated over a supersaturation (σ) range of (0.2 < σ < 0.6). Under these conditions of supersaturation, a dislocation induced growth mechanism was identified. Ex-situ atomic force microscopy (AFM) shows that some dislocation induced hillocks exhibit hollow cores. The general observations of the {110} and {111̅} surfaces reveal that these faces follow a classical mode of layer growth, continuous generation of new layers by dislocation outcrops, which subsequently bunch and spread to cover the entire facets. A tangential step velocity of the slow and fast sides of {110} and {111̅} growth hillocks show a linear dependence with supersaturation in the region of (0.2 < σ < 0.4). Analysis of this dependence leads to the respective growth parameters for the identified growth mechanism: the activation energies for the slow and fast step motion of a growth hillock (EaS and EaF) and the corresponding kinetic coefficients (βaS and βaF), for both faces. The growth from physical vapour transport (PVT) shows that for the title material, as with a number of other polar materials, solvent poisoning is not the cause of the highly differential growth rates and is an intrinsic feature of the crystal. The results suggest that in terms of the production of large single crystals of high perfection by PVT, the supersaturation range for dislocation growth should be between 0.2 and 0.4. These findings provide a foundation for the rational design of large MHB crystals that may find applications utilizing their high optoelectronic potential.Item Unknown Towards understanding the detonation properties of additively manufactured RDX: Dry powder printed(Royal Society of Chemistry, 2022-06-22) McGee, Christine; Stennett, Christopher; Clements, Jim; Vrcelj, RankoResearch into additive manufacturing (AM) has been steadily expanding over the past five decades. Where once only polymeric materials could be reliably printed, AM has been adapted to print with a range of materials such as biological, metallic, ceramic and even foodstuffs. The advantages of manufacturing in an additive manner include; a) a layer-by-layer approach allows the creation of architecturally complex structures, b) a reduction in weight, c) lessening of waste and d) the ability to create parts that that are otherwise difficult or too costly to produce. 1,3,5-Trinitro-1,3,5-triazinane (RDX) is regularly used in explosive systems. Its detonation properties when conventionally manufactured are widely researched and broadly understood. However, recent advances in additive manufacturing technologies have led to greater interest in utilising RDX in this manner. There is growing evidence that emerging formulations and printing methods are changing the detonation properties of RDX composites, the critical diameter among them.1 This study reports on beginning to understand the detonation properties of additively manufactured RDX via a dry powder printing method.Item Unknown Ultrasonic studies of solid azobenzene decorated polymer thin films(American Chemical Society, 2018-12-19) Moniruzzaman, Mohammed; Christogianni, Paraskevi; Vrcelj, Ranko; Gill, Philip P.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.