Mechanochemical Processes in Energetic Materials: A Computational and Experimental Investigation PDF
Preview Mechanochemical Processes in Energetic Materials: A Computational and Experimental Investigation
Springer Theses Recognizing Outstanding Ph.D. Research Adam A. L. Michalchuk Mechanochemical Processes in Energetic Materials A Computational and Experimental Investigation Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Adam A. L. Michalchuk Mechanochemical Processes in Energetic Materials A Computational and Experimental Investigation Doctoral Thesis accepted by University of Edinburgh, Edinburgh, UK 123 Author Supervisor Dr. AdamA.L. Michalchuk Prof. Colin R.Pulham Schoolof Chemistry Schoolof Chemistry University of Edinburgh University of Edinburgh Edinburgh,UK Edinburgh,UK ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-56965-5 ISBN978-3-030-56966-2 (eBook) https://doi.org/10.1007/978-3-030-56966-2 ©SpringerNatureSwitzerlandAG2020 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained hereinorforanyerrorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregard tojurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland ’ Supervisor s Foreword This thesis describes a truly innovative approach to rationalize and predict for the first time the relative impact and shock sensitivities to initiation of a range of energeticmaterials.Energeticmaterials(exemplifiedbyexplosives,propellants,gas generators, and pyrotechnics) release heat and/or gaseous products at a high rate upon stimulus by heat, impact, shock, spark, etc. They have widespread military andcivilianusesthatincludemunitions,mining,quarrying,demolition,emergency signaling, automotive safety, and space exploration. One of their most important properties is sensitivity to accidental initiation during manufacture, transport, storage,andoperation;Consequently, thishasimportantimplicationsfortheirsafe andreliableuse.Thepredictionofthesensitivityofenergeticmaterialstoinitiation byshockandimpactiswidelyrecognizedasbeingverychallengingandrepresents a significant barrier for the design, discovery, and preparation of safer materials. The research described in this thesis highlights how experimental and compu- tational methods have been used to develop various models with increasing levels ofcomplexityandsophistication,whichcanbeusedrationalize theexperimentally observed sensitivities of a range of energetic materials to initiation by impact and shock. A key aspect of the research has been to correlate successfully the experi- mentally measured sensitivities with knowledge of crystal structures, vibrational properties, and energy-transfer mechanisms. In particular, the importance of the various mechanisms of vibrational up-pumping of energy within the crystalline solidshasbeenhighlighted.Thesecorrelationshavebeendemonstratedforseveral classes of compounds including azides and nitramines, thereby highlighting the wide applicability of this approach. This work therefore opens the door to a new, fully ab initio approach for the design of safer energetic materials based solely on knowledge of their solid-state structures. Theintroductorychapterofthisthesisprovidesaveryaccessibleintroductionto energeticmaterialsandthevariousmethodsthathavebeenattemptedpreviouslyin order to rationalize and predict sensitivities to initiation. Chapter 2 describes the experimental and computational approaches that have been employed during this research. The subsequent chapters detail the results of these studies on a series of compounds, demonstrating excellent clarity of expression and a highly developed v vi Supervisor’sForeword critical analysis by the author. The final chapter points clearly to future opportu- nitiesforextendingthesestudiesnotonlytootherclassesofenergeticmaterials,but also as to how this predictive capability could be combined with ab initio crystal structure prediction methods. Such a combination offers a new paradigm for research into energetic materials whereby new energetic molecules could be designed in silico, with subsequent ab initio prediction of crystal structures, and then application of vibrational up-pumping models in order to determine the sen- sitivities to initiation of these chemical compounds. Edinburgh, UK Colin R. Pulham July 2019 Abstract Energetic materials (explosives, propellants and pyrotechnics; EMs) encompass a broad range of materials. These materials are used across a wide spectrum of applications, including civil and defence. For example, HMX, RDX and TNT are well-known EMs with defence applications. Silver fulminate is instead used in household Christmas crackers, and ammonium nitrate is used for numerous industrial applications. Common to all EMs is their propensity to rapidly release energy upon external perturbation. The amount and type of energy that is required to initiate an EM can vary across orders of magnitude. Some materials (e.g., tri- acetonetriperoxide,TATP)initiatewith<1 Jofimpactenergy,whileothers(e.g., triaminotrinitrobenzene, TATB) cannot be initiated without > 100 J of impact energy.Understandingwhichmaterialscanbehandledsafelyisthereforeofcritical importance for maintaining the safe use of EMs across all sectors. Current trends in EM research include a drive to develop new materials with decreased sensitivities. While it is relatively straightforward to selectively modify some properties (e.g., environmental impact), very little is understood about what constitutes a sensitive material. At present, a new EM must be synthesized and its sensitivity tested. However, with no a priori knowledge of the potential sensitivity ofanovelEM,synthesisisaccompaniedbysubstantialhazard,aswellastimeand financial costs. It is therefore pressing to develop a fundamental understanding of whatdictatesasensitivematerial,andhencedevelopamechanismtopredictthese properties. A particularly promising model to explore impact sensitivity of EMs is basedonvibrationalup-pumping,i.e.,theup-conversionofvibrationalenergy.This thesisexplorestheapplicationofthismodeltoasetofazide,organicmolecularand polymorphic materials. (cid:1) Azide-basedEMssharethecommon N explosophore. Theelectronicstructure 3 of this anion was followed as a function of its normal modes of vibration. It was foundthatexcitationofthebendingmodeissufficienttoinduceathermalelectronic excitationofthemoleculeandspontaneousdecomposition.Thisisvalidbothinthe gas and solid states. It is therefore suggested that this vibrational mode is largely responsible for decomposition of the azide materials. Based on calculations of the vii viii Abstract complete phonon dispersion curves, the various pathways to vibrational energy up-pumping were explored, namely via overtone and combination pathways. In (cid:1) particular, the relative rates of up-pumping into the N bending mode were 3 investigated. Remarkable agreement is found between these up-pumping rates and the relative ordering of the impact sensitivity of these azides. ThecalculatedvibrationalstructuresoforganicmolecularEMswerefirstcompared with experimental inelastic neutron scattering spectra and found to provide accurate representationofthelow-temperaturevibrationalstructureofthesecomplexcrystals. The decomposition pathways for organic EMs are not known, and hence, no target frequency could be unambiguously identified. Instead, the up-pumping model was developedforthesematerialsbyinvestigatingthetotalrateofenergyconversionintothe internalvibrationalmanifold.Anumberofqualitativetrendswereidentified,whichmay provide a mechanism for the rapid classification of EMs from limited vibrational information. The overtone pathways were found to offer a good agreement with experimental impact sensitivities of these compounds. However, the increased com- plexityofthevibrationalstructureoftheorganicEMsascomparedtotheazidesrequired amorethoroughtreatmentoftheup-pumpingmechanismtocorrectlyreflectexperi- mentalsensitivities.Theeffectsoftemperatureonup-pumpingwerealsoexplored. The sensitivity of organic EMs is known to differ across polymorphic forms. MostnotablearetheHMXpolymorphs.Thecalculatedvibrationalstructureoftwo HMXpolymorphswasconfirmedbyinelasticneutronscatteringspectroscopy.The up-pumpingmodeldevelopedformolecularorganicEMswasthereforeextendedto a comparison of these two HMX polymorphs. The polymorphic forms of FOX-7 were also investigated under the premise of the up-pumping model. Upon heating, FOX-7 undergoes two polymorphic transformations, which increases the layering of the materials. It therefore offered an opportunity to explore the widely held hypothesis that layered materials are less sensitive than non-layered materials. The metastable c-form was successfully recovered, and its experimental impact sensi- tivity investigated by the BAM drop-hammer method. However, upon impact, the c-polymorph appeared to convert to the a-form and initiate at the same input energy. Hence, a considerable deficiency of experimental methods is identified when studying polymorphic materials. FOX-7 was therefore explored within the frameworkoftheup-pumpingmodel.Theinelasticneutronscatteringspectrumwas collected forc-FOX-7,whichconfirmedthecalculated vibrationalstructure.Itwas shown that within the up-pumping model, the layered c-polymorph is predicted to be less sensitive than the a-form and results from a decrease in the maximum phonon-bath frequency. Hence, a new mechanism is proposed to describe the insensitivity of layered compounds. The work presented in this thesis explores the applications of vibrational up-pumpingtorationalizeandpredicttherelativeimpactsensitivitiesofarangeof EMs.Despitetheapproximationsemployedinconstructionofthemodel,itleadsto excellent correlation with experimental results in all cases. This work therefore opensthedoortoanewfullyabinitioapproachtodesigningnewEMsbasedsolely on knowledge of the solid-state structure. Parts of this thesis have been published in the following journal articles: Adam A.L. Michalchuk, Peter T. Fincham, Peter Portius, Colin R. Pulham, and Carole A. Morrison, A Pathway to the Athermal Impact Initiation of Energetic Azides, J. Phys. Chem. C, 2018, 122(34), 19395–19408 AdamA.L.Michalchuk,SvemirRudić,ColinR.Pulham,andCaroleA.Morrison, Vibrationally induced metallisation of the energetic azide a-NaN , 3 PhysChemChemPhys, 2018, 20(46), 29061–29069 Adam A.L. Michalchuk, Morris Trestman, Svemir Rudić, Peter Portius, Peter T. Fincham, Colin R. Pulham, and Carole A. Morrison, Predicting the reactivity of energetic materials: an ab initio multi-phonon approach, J. Mater. Chem. A, 2019, 7, 19539–19553 ix
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