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Analyzation of radiation resistance of carbon-based materials for accelerator components

Bolz, Philipp (2023)
Analyzation of radiation resistance of carbon-based materials for accelerator components.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00023113
Ph.D. Thesis, Primary publication, Publisher's Version

Abstract

Functional materials in high-dose environments have to withstand extreme radiation conditions but factors that limit their radiation hardness are poorly understood. An example are materials for components in particle accelerators such as beam dumps, targets and collimators. With increasing energy and pulse intensities of new accelerator facilities, these beam intercepting devices are exposed to loads with high strain rates. To secure the safe operation of future facilities, the suitability of materials exposed to extreme pulsed beam conditions need to be tested. This work mainly concentrates on graphitic materials including isotropic graphite, carbon fibre reinforced carbon, highly oriented pyrolytic graphite and flexible graphite. The mechanical properties and material changes with increasing ion fluence are investigated using nano- and microindentation and nanoimpact measurements. Samples are irradiated at the universal linear accelerator UNILAC of the GSI Helmholtz Centre for Heavy Ion Research. The experiments are performed with various types of ions of MeV to GeV energies achieving fluences up to 2e14 ions/cm². In isotropic graphite and carbon fibre reinforced carbon, large changes in Young’s modulus of up to 300 % and in hardness by more than 1000 % compared to the pristine values are observed. These pronounced material modifications occur if the energy loss of the ions surpasses approximately 18 keV/nm. By nanoimpact measurements hardening is revealed, leading to embrittlement at fluences above 3e13 ions/cm². Raman spectroscopy indicates that these severe changes of mechanical properties are related to beam-induced allotropic transformation of the graphite structure into a disordered structure similar to glassy carbon. To obtain further information about the dynamic response of the materials to ion impacts, in-situ measurements during the irradiation are required. Disc-shaped samples are exposed to short pulses of uranium ions corresponding to a deposited power density of ~3 MW/cm³. The resulting thermal stress produces pressure waves in the samples. The velocity of the respective motion of the target surface is monitored by laser Doppler vibrometry. The velocity signal recorded as a function of time reveals bending modes as the dominant components. With accumulated radiation damage, the bending mode frequency shifts toward higher values. Based on this shift, the Young’s modulus of irradiated isotropic graphite and carbon fibre reinforced carbon are determined by comparison with FEM simulations. Young’s modulus values deduced from microindentation measurements are similar confirming the validity of the method. Beam-induced stress waves remain in the elastic regime and no large-scale damage effects are observed in graphite. Tungsten and copper show no beam-induced changes while glassy carbon and hexagonal boron nitride have lower radiation resistance evident by chipping and cracks risking material failure when applied in high dose environment.

Item Type: Ph.D. Thesis
Erschienen: 2023
Creators: Bolz, Philipp
Type of entry: Primary publication
Title: Analyzation of radiation resistance of carbon-based materials for accelerator components
Language: English
Referees: Trautmann, Prof. Dr. Christina ; Wilde, Prof. Dr. Gerhard
Date: 2023
Place of Publication: Darmstadt
Collation: x, 138 Seiten
Refereed: 19 July 2022
DOI: 10.26083/tuprints-00023113
URL / URN: https://tuprints.ulb.tu-darmstadt.de/23113
Abstract:

Functional materials in high-dose environments have to withstand extreme radiation conditions but factors that limit their radiation hardness are poorly understood. An example are materials for components in particle accelerators such as beam dumps, targets and collimators. With increasing energy and pulse intensities of new accelerator facilities, these beam intercepting devices are exposed to loads with high strain rates. To secure the safe operation of future facilities, the suitability of materials exposed to extreme pulsed beam conditions need to be tested. This work mainly concentrates on graphitic materials including isotropic graphite, carbon fibre reinforced carbon, highly oriented pyrolytic graphite and flexible graphite. The mechanical properties and material changes with increasing ion fluence are investigated using nano- and microindentation and nanoimpact measurements. Samples are irradiated at the universal linear accelerator UNILAC of the GSI Helmholtz Centre for Heavy Ion Research. The experiments are performed with various types of ions of MeV to GeV energies achieving fluences up to 2e14 ions/cm². In isotropic graphite and carbon fibre reinforced carbon, large changes in Young’s modulus of up to 300 % and in hardness by more than 1000 % compared to the pristine values are observed. These pronounced material modifications occur if the energy loss of the ions surpasses approximately 18 keV/nm. By nanoimpact measurements hardening is revealed, leading to embrittlement at fluences above 3e13 ions/cm². Raman spectroscopy indicates that these severe changes of mechanical properties are related to beam-induced allotropic transformation of the graphite structure into a disordered structure similar to glassy carbon. To obtain further information about the dynamic response of the materials to ion impacts, in-situ measurements during the irradiation are required. Disc-shaped samples are exposed to short pulses of uranium ions corresponding to a deposited power density of ~3 MW/cm³. The resulting thermal stress produces pressure waves in the samples. The velocity of the respective motion of the target surface is monitored by laser Doppler vibrometry. The velocity signal recorded as a function of time reveals bending modes as the dominant components. With accumulated radiation damage, the bending mode frequency shifts toward higher values. Based on this shift, the Young’s modulus of irradiated isotropic graphite and carbon fibre reinforced carbon are determined by comparison with FEM simulations. Young’s modulus values deduced from microindentation measurements are similar confirming the validity of the method. Beam-induced stress waves remain in the elastic regime and no large-scale damage effects are observed in graphite. Tungsten and copper show no beam-induced changes while glassy carbon and hexagonal boron nitride have lower radiation resistance evident by chipping and cracks risking material failure when applied in high dose environment.

Alternative Abstract:
Alternative abstract Language

Funktionale Materialien, die hohen Strahlungsdosen ausgesetzt werden, müssen gegen extreme Bedingungen resistent sein. Faktoren, die ihre Strahlungshärte limitieren, sind allerdings wenig erforscht. Beispiele sind Materialien, die in Teilchenbeschleunigern als Strahlblocker, Target oder Kollimator eingesetzt werden. Durch die steigende Energie und Pulsintensität zukünftiger Beschleunigeranlagen werden diese Bauteile Belastungen mit sehr hohen Dehnungsraten ausgesetzt. Um die sichere Operation von zukünftigen Anlagen zu gewährleisten, muss die Eignung von Materialien in extremen gepulsten Bedingungen getestet werden. In dieser Arbeit werden hauptsächlich die graphitischen Materialien isotroper Graphit, kohlenstofffaserverstärkter Kohlenstoff, hochorientierter pyrolytischer Graphit und flexibler Graphit untersucht. Die mechanischen Eigenschaften und Materialveränderungen mit steigender Ionenfluenz wurden mit Nano- und Mikroindentation sowie Nanoimpaktmessungen untersucht. Die Proben wurden am UNILAC (Universal Linear Accelerator) des GSI Helmholtzzentrum für Schwerionenforschung bestrahlt. Verschiedene Arten von Ionen mit MeV bis GeV Energien wurden verwendet und Fluenzen bis zu 2e14 Ionen/cm² erreicht. Isotroper Graphit und kohlenstofffaserverstärktem Kohlenstoff weisen große Veränderungen des Elastizitätsmoduls um 300 % und der Härte um 1000 % im Vergleich zur unbestrahlten Probe auf. Diese ausgeprägten Materialmodifikationen treten bei Bestrahlung mit Ionen mit Energieverlusten über 18 keV/nm auf. Nanoimpaktmessungen zeigen eine Erhärtung des Materials, die zu Versprödung bei Fluenzen über 3e13 Ionen/cm² führt. Ramanspektroskopie zeigt, dass diese starken Veränderungen der mechanischen Eigenschaften durch eine allotropische Transformation der Graphitstruktur in eine fehlgeordnete Struktur, die glasartigem Kohlenstoff ähnelt, verursacht wird. Um weitere Informationen über die dynamische Materialreaktion, verursacht durch Ionenpulse, zu bekommen, werden Messungen während der Bestrahlung benötigt. Dafür wurden scheibenförmige Proben kurzen Uranionenpulsen mit einer Leistungsdichte von ~3 MW/cm³ ausgesetzt. Die resultierende thermische Spannung produziert Druckwellen in den Proben. Die resultierende Geschwindigkeit dieser Bewegung kann an der Probenoberfläche mittels eines Laser-Doppler-Vibrometers gemessen werden. Das Geschwindigkeitssignal offenbart die Biegungsmoden der Proben als dominante Komponenten. Mit steigendem Strahlungsschaden verschiebt sich die Frequenz der Biegungsmoden zu höheren Werten. Durch diese Verschiebung und mittels Vergleiches mit ANSYS Simulationen kann das Elastizitätsmodul von isotropem Graphit und kohlenstofffaserverstärktem Kohlenstoff bestimmt werden. Der Vergleich mit den Elastizitätsmodulen, die durch Mikroindentation erhalten wurden, zeigt eine gute Übereinstimmung, was die Aussagekraft der Methode bestätigt. Stresswellen verursacht durch die Bestrahlung bleiben im elastischen Bereich und keine großräumigen Defekte werden in Graphit beobachtet. Wolfram und Kupfer zeigen keine strahlungsbedingten Veränderungen. Glasartiger Kohlenstoff und hexagonales Bornitrid dagegen haben eine geringe Strahlenresistenz, was durch Abplatzen und Rissbildung verdeutlicht wird. Materialversagen wird beim Einsatz dieser Materialien in Hochdosisumgebungen riskiert.

German
Status: Publisher's Version
URN: urn:nbn:de:tuda-tuprints-231136
Classification DDC: 500 Science and mathematics > 500 Science
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Ion-Beam-Modified Materials
Date Deposited: 25 Jan 2023 13:01
Last Modified: 26 Jan 2023 06:28
PPN:
Referees: Trautmann, Prof. Dr. Christina ; Wilde, Prof. Dr. Gerhard
Refereed / Verteidigung / mdl. Prüfung: 19 July 2022
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