Synchrotron X-ray Characterization Research Articles

Multimodal and Operando Synchrotron X-ray Characterization for Advanced Energy Materials

Multimodal and Operando Synchrotron X-ray Characterization for Advanced Energy Materials

Epitaxial oxide ionotronics: Interfaces and oxygen vacancies

Epitaxial oxide ionotronics: Interfaces and oxygen vacancies

Integrating in-situ multi-modal characterizations with signatures to investigate localized deformation

Multi-modal approaches are often essential for exploring the intricate facets of complex materials phenomena. Each characterization technique contributes a unique perspective that, when linked with other techniques, enables a more holistic and comprehensive study. Sometimes direct interpretations of material response between techniques become infeasible, necessitating the use of indirect interpretations or signatures to build a complete understanding. In this work, we present a case study that combines signature discovery with electron and synchrotron X-ray characterization techniques to identify regions of intense slip localization - a complex material phenomenon - via a signature within an Inconel-718 alloy undergoing mechanical deformation. This signature relies on the higher order moments of single grain orientation distributions, which prove sensitive to localized deformation. We report the intergranular stress states within a grain neighborhood identified through this signature.

Relating Near-Surface Ti Electrode Structure with Electrochemical Nitrate Reduction Performance Via Synchrotron X-Ray Characterization

The global nitrogen cycle has been severely skewed since the widespread adoption of the Haber-Bosch process to produce ammonia (NH3). Currently, removal of reactive nitrogen (inorganic forms besides N2) from the environment lags behind its production and emission to the environment. Nitrate is one of the most prevalent waterborne nitrogen pollutants and, in its excess, threatens the health of ecosystems. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. Therefore, selective electrochemical nitrate reduction (NO3RR) to NH3 can couple water purification and NH3 production, helping offset the energy- and carbon-intensive Haber Bosch process. NH3 recovery from emissions could contribute over 20 million tons N per year by 2050 (~ 10% of projected N demand).Noble and transition metal catalysts including single metals (e.g., Pt, Rh, Ru, Ir, Pd, Cu, Ag, Au)[1,2] and alloys (e.g., CuNi)[3] have been studied for NO3RR. Notably, under acidic conditions, polycrystalline titanium has been demonstrated to display robust and efficient NO3RR.[4] Ti is corrosion-resistant, a poor hydrogen evolution catalyst, and a readily available, abundant metal. However, under acidic and reducing conditions, Ti forms a water-stable hydride (TiHx, 0<x≤2).[5] It remains unclear how the degree of surface hydride formation – which alters the physical and electronic properties of the electrode surface – impacts NO3RR performance. Thus, rationally implementing Ti-catalyzed NO3RR requires improved understanding of how near-surface Ti-hydride forms and influences NO3RR activity and selectivity.In this work, we show that near-surface Ti-hydride formation is a function of the duration and magnitude of applied NO3RR potential. A combination of ex situ grazing-incidence X-ray diffraction (GIXRD – probes long-range, crystalline order) and X-ray absorption spectroscopy (XAS – probes short-range, atom-specific probe into local coordination environments) enabled quantitative near-surface characterization of Ti-hydride electrodes. Through electrochemical testing and density functional theory calculations, we investigated the role near-surface Ti-hydride may play in electrochemical nitrate conversion. Our results preliminary suggest that near-surface hydride content plays a relatively minor role in steering NO3RR performance compared to applied potential and electrolyte effects. In addition, we are performing ongoing in situ GIXRD and XAS measurements to interrogate the transient nature and interplay of near-surface hydride formation and NO3RR. Put in context, our results help prioritize how Ti-catalyzed NO3RR processes can be optimized for electrified ammonia production and wastewater remediation.[1] G. E. Dima, A. C. A. de Vooys, M. T. M. Koper, Journal of Electroanalytical Chemistry 2003, 554–555, 15–23.[2] G. E. Dima, G. L. Beltramo, M. T. M. Koper, Electrochimica Acta 2005, 50, 4318–4326.[3] Y. Wang, A. Xu, Z. Wang, L. Huang, J. Li, F. Li, J. Wicks, M. Luo, D.-H. Nam, C.-S. Tan, Y. Ding, J. Wu, Y. Lum, C.-T. Dinh, D. Sinton, G. Zheng, E. H. Sargent, J. Am. Chem. Soc. 2020, 142, 5702–5708.[4] J. M. McEnaney, S. J. Blair, A. C. Nielander, J. A. Schwalbe, D. M. Koshy, M. Cargnello, T. F. Jaramillo, ACS Sustainable Chem. Eng. 2020, 8, 2672–2681.[5] Y. Liu, Z. H. Ren, J. Liu, R. F. Schaller, E. Asselin, J. Electrochem. Soc. 2019, 166, C3096–C3105.

Elucidating a dissolution–deposition reaction mechanism by multimodal synchrotron X-ray characterization in aqueous Zn/MnO2 batteries

Aqueous Zn/MnO2 batteries with their environmental sustainability and competitive cost, are becoming a promising, safe alternative for grid-scale electrochemical energy storage.

Correction: Elucidating a dissolution–deposition reaction mechanism by multimodal synchrotron X-ray characterization in aqueous Zn/MnO2 batteries

Correction for ‘Elucidating a dissolution–deposition reaction mechanism by multimodal synchrotron X-ray characterization in aqueous Zn/MnO2 batteries’ by Varun R. Kankanallu et al., Energy Environ. Sci., 2023, https://doi.org/10.1039/D2EE03731A.

(Invited) Synchrotron X-Ray Nano-Tomography and Multimodal Studies of Li-Ion Batteries

As batteries revolutionize all technological areas – from miniaturized electronic devices to electric vehicles and to large-scale energy storage, understanding the complex morphological, chemical and structural evolution and their interplays has been at the forefront of the research. Synchrotron X-ray characterization techniques provide insights into the electrochemical reactions and multiscale, multiphysics environments to address the fundamental mechanisms in these systems. The presentation will highlight the application of synchrotron X-ray analysis in two Li-ion battery systems, including both aqueous and non-aqueous systems. X-ray nano-tomography via transmission X-ray microscopy and spectroscopic imaging, complemented by other diffraction, spectroscopy and microscopy techniques, will be discussed. We will present how synchrotron X-ray nano-tomography and quantitative 3D morphological analysis were instrumental in revealing the dimensionality effect of conductive carbon fillers in LiNi 1/3Mn1/3Co1/3O2 (NMC111) cathode [1]. Additionally, we will discuss how a multimodal characterization approach offered insights when probing kinetics of water-in-salt aqueous batteries with thick, porous LiV3O8-LiMn2O4 electrodes [2, 3]. Through the morphological and chemical analyses, the work aims to facilitate the design of future advanced energy storage materials, as well as provide a novel characterization framework for studying a wider range of electrochemical systems.

(Industrial Electrochemistry and Electrochemical Engineering Division Student Achievement Award) Reactive Separations to Remediate and Valorize Nitrogen-Polluted Wastewaters

Anthropogenic perturbations to the global nitrogen cycle due to industrial Haber-Bosch fertilizer production threaten large-scale food production, energy inputs for chemical manufacturing, and protection of water quality. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. In this talk, I introduce Electrodialysis and Nitrate Reduction (EDNR), a novel electrochemical process that couples water purification with electrified ammonia production from nitrogen-polluted wastewaters.The EDNR reactor consists of three chambers and operates in two stages (Figure 1), with the influent entering the middle chamber and products recovered from the left and right chambers. In Stage 1, influent nitrate () and ammonium () are separated via electrodialysis (ED) and ammonia is recovered in the right chamber. In Stage 2, ammonia is synthesized from the electrochemical nitrate reduction (NR) in the left chamber. EDNR enables rational design of electrochemical environments in each chamber (e.g., electrolyte pH; cationic and anionic constituents; species concentrations) through tunable operating parameters such as applied potential, electrolyte flow rate, and duration of the ED and NR stages. This modular, tunable design facilitates robust water remediation and ammonia production from wastewaters of transient composition.We have demonstrated proof-of-concept EDNR reactors using titanium foil (left chamber, NR electrode), Ti/IrO2-Ta2O5 mesh (left and middle chambers, ED electrode), and platinum foil (right chamber, ED electrode). With recirculating batches of simulated wastewater (100 ppm + 500 ppm ), 75% influent NH4 + was recovered into the right chamber and 25% influent was converted to in the left chamber after three EDNR cycles. As the rate-limiting stage, NR on titanium merited further fundamental investigation. In particular, the reasons for titanium’s electrocatalytic NR performance remain largely unclear to date, especially regarding the role of the role of titanium hydride (TiHx, 0<x<2), which forms during NR. Rationally implementing Ti-catalyzed NR requires improved understanding of how near-surface Ti-hydride forms and influences NR activity and selectivity. Through systematic synchrotron x-ray characterization of Ti-hydride electrodes, electrochemical testing, and density functional theory calculations, we found that near-surface hydride content plays a relatively minor role in steering NR performance compared to applied potential and electrolyte effects. Put in context, our results help prioritize how EDNR operation can be optimized for ammonia production. As a validated platform with ongoing work to improve performance, EDNR shows great potential in realizing sustainable and distributed water remediation and ammonia production.

In-situ synchrotron X-ray analysis of metal Additive Manufacturing: Current state, opportunities and challenges

Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy Deposition (DED) have attracted significant interest from both the scientific community and the industry since these technologies offer great manufacturing opportunities for niche applications and complex geometries. Understanding the physics behind the complex and dynamic phenomena occurring during these processes is essential for overcoming the barriers that constrain the metal AM development. In-situ synchrotron X-ray characterization is suitable for investigating the microstructure evolution during processing and provides new profound insights. Here, we provide an overview of the research on metal PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled. We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included.

Structural and Electrochemical Properties of the Type VIII Ba8Ga16–δ Sn30+δ (δ ≈ 1)clathrate during Lithiation

Clathrates of the Tetrel (Tt = Si, Ge, Sn) elements are host-guest structures that can undergo Li alloying reactions with high capacities. However, little is known about how the cage structure affects the phase transformations that take place during lithiation. To further this understanding, the structural changes of the type VIII clathrate Ba8Ga16–δSn30+δ (δ ≈ 1) during lithiation are investigated and compared to those in β-Sn with ex situ X-ray total scattering measurements and pair distribution function (PDF) analysis.The results show that the type VIII clathrate undergoes an alloying reaction to form Li-rich amorphous phases (Li x Ba0.17Ga0.33Sn0.67, x = 2 – 3) with local structures similar to those in the crystalline binary Li-Sn phases that form during the lithiation of β-Sn. As a result of the amorphous phase transition, the type VIII clathrate reacts at a lower voltage (0.25 V vs. Li/Li+) compared to β-Sn (0.45 V) and goes through a solid-solution reaction after the initial conversion of the crystalline clathrate phase. Cycling experiments suggest that the amorphous phase persists after the first lithiation and results in considerably better cycling than β-Sn. Density functional theory (DFT) calculations suggest that topotactic Li insertion into the clathrate lattice is not favorable due to the high energy of the Li sites, which is consistent with the experimentally observed amorphous phase transformation. The clathrate structure is hypothesized to kinetically circumvent the formation of Li-Sn or Li-Ga crystalline phases, which results in better cycling and a lower reaction voltage.The electrochemical lithiation pathway of Ba8Ga16–δSn30+δ (δ ≈ 1) is similar to that of the previously investigated type I Ba8Ge43 1 suggesting that the amorphous lithiation could be a common mechanism for guest-filled Tetrel clathrates. As amorphous phase transformations are a beneficial property for alloying anodes, Tetrel clathrates could act as precursors for obtaining amorphous phases with tunable electrochemical properties. With this assumption of a common mechanism for guest-filled clathrates, we will then discuss design rules for clathrate based alloying anodes in terms of optimizing the gravimetric capacity and reaction voltage. References (1) Dopilka, A.; Childs, A.; Bobev, S.; Chan, C. K. Understanding the Amorphous Lithiation Pathway of the Type I Ba8Ge43 Clathrate with Synchrotron X-Ray Characterization. Chem. Mater. 2020, 32, 9444–9457.

Composition-Driven Structural Modulation and Guest-Induced Nanotemplate Effects of the Host–Guest Complexes Made by a Unimolecular Q-Clip

The composition of a self-assembly system has long been an essential factor that governs the resulting supramolecular structures. Nevertheless, due to the rigorous demands of structural compatibility in host–guest chemistry, the molar ratio of host–guest mixtures has not been applied as a significant factor in modulating the supramolecular structures of host–guest complexes. In this study, via using a novel unimolecular quaternary clip (Q-clip) as the host molecule and pyrene as the guest molecule and a nanotemplating reagent, we discovered that the changes in the molar ratios of the Q-clip:pyrene mixtures resulted in the structural modulation of the host–guest complexes. Supramolecular lamellae made by the 1:4 Q-clip:pyrene mixture and a hexagonal columnar phase (Colh) made by the 1:8 Q-clip:pyrene mixture were identified by the 1D powder and 2D fiber WAXS characterizations. Time-resolved synchrotron X-ray characterization and thermal analysis further showed that removing the easily sublimed guest molecules (pyrene) via thermal annealing simply hollowed out the supramolecular structures without damaging the supramolecular scaffolds made by the Q-clips. Pyrene is thus a nanotemplating reagent that can be used to create the hollowed-out 2D lamellar and 1D cylindrical scaffolds of Q-clips. The study thus gave a novel example of a host–guest mixture that is able to carry out composition-driven structural modulation and guest-induced nanotemplates. The finding broadens the horizon of host–guest chemistry and may inspire more nanoinnovation.

The Role of Cobalt and Manganese for the Safety of Ni-Rich NMC Cathode

The intrinsic poor thermal stability of layered LiNi x Co y Mn1–x–y O2 (NCM) cathodes and the exothermic side reactions triggered by the associated oxygen release are the main safety threats for their large-scale implantation. In the NCM family, it is widely accepted that Ni is the stability troublemaker, while Mn has long been considered as a structure stabilizer, whereas the role of Co remains elusive. Here, via Co/Mn exchange in a Ni-rich LiNi0.83Co0.11Mn0.06O2 cathode, we demonstrate that the chemical and structural stability of the deep delithiated NCM cathodes are significantly dominated by Co rather than the widely reported Mn. Operando synchrotron X-ray characterization coupling with in situ mass spectrometry reveal that the Co4+ reduces prior to the reduction of Ni4+ and could thus prolong the Ni migration by occupying the tetrahedra sites and, hence, postpone the oxygen release and thermal failure. In contrast, the Mn itself is stable, but barely stabilizes the Ni4+. Our results highlight the importance of evaluating the intrinsic role of compositional tuning on the Ni-rich/Co-free layered oxide cathode materials to guarantee the safe operation of high-energy Li-ion batteries.

Understanding the Lithiation Pathways of Tetrel Clathrates with Synchrotron X-Ray Characterization

Tetrel clathrates are materials composed of cage-like frameworks of covalently bonded Group IV (Si, Ge, Sn) elements containing guest atoms in the center of each cage. Our group has been investigating the electrochemical properties of Si and Ge clathrates for potential applications as Li-ion battery anodes. Our studies have found that in some cases the Li-alloying pathways are distinctly different in clathrates compared to those seen in the diamond cubic analogs. For example, the Ba8Al16Si30 Type I clathrate does not undergo a Li-alloying reaction [1] while the Ba8Ge43 clathrate does [2]. In the case of Ba8Ge43, the reaction pathway appears to follow an amorphous phase transformation, in contrast to diamond cubic Ge, which undergoes several phase transformations to form crystalline Li-Ge phases [3].In this work, we employ synchrotron X-ray pair-distribution function (PDF) analysis, X-ray diffraction (XRD), and in situ heating experiments to elucidate the electrochemically induced structural changes in Ge and Sn clathrates and how they differ from those in the elemental analogs. The Type I Ba8Ge43 and Type VIII Ba8Ga16Sn30 clathrates are chosen and compared to diamond cubic Ge and beta-Sn. The PDF method allows us to probe the local structure of the amorphous phases that form after lithiation, while in situ heating experiments enable us to observe structural changes to the amorphous phases upon moderate heating. The insights learned from the structural characterizations are then used to explain the differences in the electrochemistry between the Tetrel clathrates and their elemental analogs.The results suggest that the amorphous phases that are part of the final lithiated state of Ba8Ge43 have local structures similar to the Li-Ge crystalline phases. We conclude that the Ba atoms prevent the bulk nucleation of Li-Ge crystalline phases and restrict the Li-Ge ordering to the scale of 10-15 Å, resulting in a solid solution mechanism and a lower reaction voltage compared to cubic-Ge. Similar analysis of the Type VIII Ga-Sn clathrate (Ba8Ga16Sn30) results in a similar lithiation path showing a solely amorphous phase transformation. These results give insight to the fundamentals of how the initial structure of group IV materials affect their Li-alloying properties and structures. Figure 1 (a) Unit cell of Ba8Ge43, purple atoms are Ge and green atoms are Ba. (b) PDFs of the lithiation of Ba8Ge43 at different states of lithiation. (c) PDFs of cubic-Ge at different states of lithiation.[1] R. Zhao. ACS Appl. Mater. Interfaces. 9 (2017) 41246–41257.[2] A. Dopilka. ACS Appl. Mater. Interfaces. 10 (2018) 37981–37993.[3] H. Jung. Chem. Mater. 27 (2015) 1031–1041. Figure 1

Probing the Thermal-Driven Structural and Chemical Degradation of Ni-Rich Layered Cathodes by Co/Mn Exchange.

The intrinsic poor thermal stability of layered LiNixCoyMn1-x-yO2 (NCM) cathodes and the exothermic side reactions triggered by the associated oxygen release are the main safety threats for their large-scale implantation. In the NCM family, it is widely accepted that Ni is the stability troublemaker, while Mn has long been considered as a structure stabilizer, whereas the role of Co remains elusive. Here, via Co/Mn exchange in a Ni-rich LiNi0.83Co0.11Mn0.06O2 cathode, we demonstrate that the chemical and structural stability of the deep delithiated NCM cathodes are significantly dominated by Co rather than the widely reported Mn. Operando synchrotron X-ray characterization coupling with in situ mass spectrometry reveal that the Co4+ reduces prior to the reduction of Ni4+ and could thus prolong the Ni migration by occupying the tetrahedra sites and, hence, postpone the oxygen release and thermal failure. In contrast, the Mn itself is stable, but barely stabilizes the Ni4+. Our results highlight the importance of evaluating the intrinsic role of compositional tuning on the Ni-rich/Co-free layered oxide cathode materials to guarantee the safe operation of high-energy Li-ion batteries.

Understanding the Amorphous Lithiation Pathway of the Type I Ba8Ge43 Clathrate with Synchrotron X-ray Characterization

Tetrel (Tt = Si, Ge, and Sn) clathrates have highly tunable host–guest structures and have been investigated as novel electrode materials for Li-ion batteries. However, there is little understanding of how the clathrate structure affects the lithiation processes and phase evolution. Herein, the electrochemical lithiation pathway of type I clathrate Ba8Ge43 is investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to the lithiation of germanium with a diamond cubic structure (α-Ge). The results confirm previous laboratory XRD studies showing that Ba8Ge43 goes through a solely amorphous phase transformation, which contrasts with the crystalline phase transformations that take place during lithiation of micrometer-sized α-Ge particles. The local structure of framework-substituted clathrate Ba8Al16Ge30 after lithiation is found to proceed through an amorphous phase transformation similar to that in Ba8Ge43. In situ PDF and XRD during heating show that the amorphous phases derived from lithiation of Ba8Ge43 are structurally related to various Li–Ge phases and crystallize at low temperatures (350–420 K). We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li–Ge clusters and kinetically prevent the nucleation and growth of bulk crystalline phases. The amorphous phase evolution of the clathrate structure during lithiation results in electrochemical properties distinct from those in α-Ge, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as anodes in Li-ion batteries.

Quantifying microscale drivers for fatigue failure via coupled synchrotron X-ray characterization and simulations

During cyclic loading, localization of intragranular deformation due to crystallographic slip acts as a precursor for crack initiation, often at coherent twin boundaries. A suite of high-resolution synchrotron X-ray characterizations, coupled with a crystal plasticity simulation, was conducted on a polycrystalline nickel-based superalloy microstructure near a parent-twin boundary in order to understand the deformation localization behavior of this critical, 3D microstructural configuration. Dark-field X-ray microscopy was spatially linked to high energy X-ray diffraction microscopy and X-ray diffraction contrast tomography in order to quantify, with cutting-edge resolution, an intragranular misorientation and high elastic strain gradients near a twin boundary. These observations quantify the extreme sub-grain scale stress gradients present in polycrystalline microstructures, which often lead to fatigue failure.

Synchrotron X-ray characterization and the development of a biomaterials pipeline at DuPont

Synchrotron X-ray characterization and the development of a biomaterials pipeline at DuPont

Structure of Confined Ionic Liquids in Graphitic Nanopores from Ab-Initio Molecular Dynamics Simulations

Room temperature ionic liquids (ILs) have recently emerged as highly promising electrolytes for a wide range of emerging energy technologies, including next-generation supercapacitors and ion-batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. In this presentation, we combine first-principles simulations and synchrotron X-ray characterization experiments to unravel the structural properties of several imidazolium-based ILs in the bulk and at the graphitic interface, which play an important role in determining the performance of carbon-based supercapacitors. In particular, we utilize extensive ab initio molecular dynamics simulations to probe the local density distribution and medium-range order of bulk ILs, which can be directly compared and validated by X-ray scattering measurements. Having established this cross-validation, we then compare and contrast via simulation the structural, chemical and electronic properties of the ILs in the bulk and under confinement in sub-2-nm carbon slit nanopores constructed of graphene layers, which serve as model systems for understanding confinement effects in porous graphitic carbon. Our integrated theoretical and experimental approach relates these structural and chemical signatures with the intrinsic cation-anion interactions, by considering ILs with anions having significant differences in the molecular geometry, chemical composition, and charge distribution. This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Synthesis and synchrotron X-ray characterization of two 2D Hoffman related compounds [Ni(p-Xylylenediamine)nNi(CN)4] and [Ni(p-tetrafluoroxylylenediamine)nNi(CN)4

Synchrotron X-ray single crystal structure determination of two 2D Hofmann-related compounds, [Ni(p-Xylyenediamine)n-tetracyanonickelate] (abbreviated as Ni-pXdam) and [Ni(tetrafluoro-p-Xylyenediamine)n-tetracyanonickelate] (abbreviated as Ni-pXdamF4), have been conducted. Both the pXdam and pXdamF4 ligands contain two short chains of -CH2NH2 at the para-positions of a phenyl ring. These flexible chains link the 6-fold coordinated Ni2 sites throughout the network. In Ni-pXdam, the closed-2D network of [Ni-(CN-Ni1/4-)4]∞ is broken into 1D chains, leaving the C≡N groups at the trans-positions of the Ni(CN)4 moiety unbridged. The resulting 1D chains [(trans-)-NC-Ni(CN)2-CN-Ni-]∞ runs along the [010] direction of the unit cell. The pXdam ligands bridge in pair between the Ni atoms of the adjacent chains. The catenation structure of [Ni{(pXdam)}]∞ could be referred to as double -1D. In Ni-pXdamF4, the -CH2NH2 ligands connect the neighboring chains via the 6-fold Ni2 site. Surrounding the 4-fold Ni1 site, the two trans terminal C≡N groups were replaced by the Lewis base NH3 during the synthesis process, therefore preventing the propagation of the 2D net to form a 3D network. Computed pore volume of both compounds indicated that there is not sufficient space in the structure to accommodate gas molecules. In both compounds, hydrogen bonds were found, and solvent of crystallization was absent due to the limited free space in the structure.

Synchrotron X-ray characterization of crack strain fields in polygranular graphite

The strain field of a crack in polygranular isotropic nuclear graphite, a quasi-brittle material, has been studied during stable fracture propagation. Synchrotron X-ray computed tomography and strain mapping by diffraction were combined with digital volume correlation and phase congruency image analysis to extract the full field displacements and elastic crystal strains. The measured displacement fields have been analysed using a Finite Element method to extract the elastic strain energy release rate as a J-integral. Non-linear properties described the effect of microcracking on the elastic modulus in the fracture process zone. The analysis was verified by the good agreement of the predicted and measured elastic strain fields when using the non-linear model. The intrinsic critical elastic strain energy release rate for mode I crack propagation is approximately 200 J m−2.