Bragg-edge Imaging Research Articles

Operando lateral state-of-charge inhomogeneity mapping via wavelength-resolved neutron imaging

The continuous advancement in battery technology requires the development of robust and more precise diagnostic tools for the characterization of commercial-scale battery cells. In this work, operando Bragg edge imaging (BEI), based on a time-of-flight approach, provides spatially resolved state-of-charge (SoC) mapping of a commercial-size pouch cell. The changes in the position and intensity of Bragg edges are used to track the Li intercalation into the graphite, thereby identifying the spatial distribution of various lithiation stages and the corresponding SoC. The operando wavelength-resolved BEI assessment revealed an inhomogeneous Li+ intercalation, dependent on the distance to the electrical tabs, effectively identifying features that contribute to the overall loss of performance and the accelerated degradation of battery materials. The presented results demonstrate the potential of advanced wavelength-resolved neutron imaging to be used as a diagnostic tool for commercial battery cells in a configuration without special casings or isotopes for enhancement of imaging contrast. It thus highlights its applicability for operando characterization of future batteries for portable and automotive applications.

Exploring crucial technological advancements in Anger-Camera neutron detectors for single crystal neutron diffraction

In accordance with the second phase construction blueprint for the China Spallation Neutron Source (CSNS-II), the proposal includes the establishment of a single crystal neutron diffractometer. This apparatus necessitates a neutron detector array spanning over 1 m2 and capable of covering neutron diffraction angles from 20° to 160°. Additionally, the detector's position resolution must better than 1 mm × 1 mm. To meet such requirements, a scintillator neutron detector based on the Anger-Camera principle was designed. The detector is composed of a 6LiF/ZnS (Ag) scintillation screen, optical guide, silicon photomultipliers (SiPMs) array and self-design composite compression readout electronic systems. A detector prototype, featuring a detection area of 50 mm × 50 mm, was fabricated and subjected to testing at the BL20 neutron test line in the China Spallation Neutron Source (CSNS). The detector system achieved an optimal position resolution of 0.6 mm, while the maximum count rate was approximately 20 kHz. Moreover, the detector's Bragg edge imaging capability was preliminarily investigated using stainless steel samples, demonstrating its future potential for Bragg edge transmission imaging. The proposed detector demonstrates promising potential as a candidate for the single crystal diffractometer in CSNS-II.

The use of time-of-flight neutron Bragg edge imaging to measure the residual strains in W/Cu dissimilar joints for fusion reactors

The European DEMOnstration power plant (EU DEMO) project is currently leading the research endeavours within the fusion field with the goal of developing a next generation fusion reactor. Given the challenging nature of the application, it is essential to establish methodologies that can provide convenient and reliable characterisation of the chosen materials within DEMO. In this paper, the recently developed Time-of-Flight Neutron Bragg Edge Imaging (TOF-NBEI) was used on Tungsten (W)/ Copper (Cu) dissimilar joints sample mock-ups of the cooling system design used in the critical divertor component with the goal of mapping the residual stresses across the sample. Residual strain mapping was performed on the W phase with considerable tensile residual strains identified close to the W-Cu interface. The large-grain microstructure of the Cu phase was analysed using the energy-resolved neutron radiographs. The results will be used as a basis for future TOF-NBEI experiments of tungsten monoblocks related to DEMO.

Time-of-flight polarization contrast neutron imaging for enhanced characterization of ferritic phase fractions in Fe-Mn-Si shape memory alloys

The determination of the amount and distribution of different phase fractions in additively manufactured shape memory alloys processed with laser powder bed fusion is crucial for understanding the correlation between processing parameters, microstructure, and mechanical properties. Neutron imaging techniques, such as Bragg edge imaging and polarization contrast neutron imaging (PNI), have been introduced to complement and overcome the limitations of traditional characterization methods, which are often destructive and limited to surface analyses and small-sized specimens. Bragg edge imaging can distinguish and quantify crystallographic phase fractions with spatial resolutions of a few tens of micrometers, while PNI is highly sensitive to crystallographic phases and is particularly suited for sub-percent phase fractions and in-situ, time-resolved, and tomographic analyses. In this work, we present a time-of-flight PNI method that enables simultaneous measurements of phase fractions.

The VENUS imaging beamline at the Spallation Neutron Source: layout, expected performance and status of the construction project

Located at the Spallation Neutron Source, VENUS is an instrument optimized for wavelength-dependent neutron imaging techniques, namely Bragg edge and resonance imaging, across a broad range of neutron energies (from meV to hundreds of eV). The VENUS construction project started in Fall 2018, establishing a conceptual design of the major components and timeline for purchasing, testing, and installation. Completion of the VENUS instrument is anticipated late 2024. This instrument comprises components such as a set of boron carbide apertures designed for both thermal/cold and epithermal neutrons, a suite of choppers (T0 and single disks), beam collimators, and a cadmium filter. The time-of-flight detector at VENUS will be the micro-channel-plate Timepix detector and will be positioned 25 m away from the source. This manuscript describes the instrument general layout, explains the choice of the moderator, and specifies the VENUS key capabilities. It also provides the status of the construction project with installed components. Discussions about software development are also presented.

A new method to obtain the crystallographic texture of polycrystalline materials in Bragg edge imaging experiments

In polycrystalline materials, the shape and height of Bragg edges observed in wavelength resolved neutron transmission experiments depend on the crystallographic texture. Previous works have proved the capacity of actual models to predict the transmission spectra once the texture of the material is known. In this work, we summarize the recent advances done trying to obtain information of the texture from the experimental transmission data. The basis of this method resides in a compact expression of the coherent elastic cross section in terms of the Fourier coefficients of the orientation distribution function, which describes the crystallographic texture. The validity of this method is restricted to the applicability of the kinematic approximation of diffraction (polycrystalline materials with grain sizes below 10μm) and to materials with spatially uniform texture.

Bragg edge imaging characterization of multi-material laser powder-bed fusion specimens

Multi-material laser powder-bed fusion (M2LPBF) is a novel additive manufacturing approach that makes it possible to print different materials along the built direction and within a single layer of a component. At the interface between the different materials, the deposited powders melt, mix and solidify very rapidly, than can produce a range of desired and undesired phases, residual stresses and defects. Here we applied Bragg edge imaging to characterize M2LPBF specimens of stainless steel and CuCrZr with vertical and horizontal interfaces. A diffuse interface is observed in the samples with both vertical and horizontal interfaces. The analysis of the (111) and (200) Bragg edges height across the samples demonstrated a clear difference between the crystallographic texture of both alloys, with a strong alignment of the (002) planes along one of the transversal directions in the steel and a random texture within the copper alloy.

Neutron Bragg edge tomography characterisation of residual strain in a laser-welded Eurofer97 joint

Nuclear fusion is a potential source of electricity which can address the environmental problems posed by fossil fuels. Eurofer97 steel is a primary structural material for breeding blanket and divertor components in fusion Tokamaks. Assembling and maintaining the structural integrity of these in-vessel components requires remote joint techniques, such as laser welding, although it induces immersive residual stress. The interaction of the residual strain and the heterogeneous microstructure degrades the mechanical performance of fusion components. However, an inspection of bulk residual strain distribution is still challenging. This study presents the residual strain distribution in the bulk of the weldment using volumetric tomographic reconstruction. A neutron Bragg edge imaging technique is used to obtain 2D angular projections. The 3D volumetric strain map is reconstructed from 2D residual strain projections using the filtered back projection technique. It is found that the laser welding technique generates a uniform residual strain field in the through-thickness direction. The results also demonstrate the potential of reconstructing volumetric residual strain distribution in bulk materials using fewer projections to reduce data redundancy and acquisition time for the neutron Bragg edge imaging technique.

Microstructure and texture analysis of 304 austenitic stainless steel using Bragg edge transmission imaging

Bragg edge imaging using pulsed neutrons is a non-destructive technique for studying microstructure and texture of materials. It provides two-dimensional visualization of crystallographic information using a pixelated gas electron multiplier detector and a time-of-flight method. In this work, the properties of type 304 austenitic stainless steel samples were studied via Bragg edge imaging. The samples included hot-rolled, cold-rolled and heat-treated specimens, which were characterized to investigate texture, phase fraction and grain growth. The results showed that the crystallite size increased with increasing annealing temperature. The cold-rolled and annealed samples exhibited strong textures, while the hot-rolled sample showed no preferred orientation. The phase volume fraction of induced martensite in the cold-rolled sample was also obtained. Two-dimensional maps of microstructures and textures were obtained without destructive processes. The results were validated by electron backscatter diffraction and found to be consistent. This work provides valuable information for non-destructive characterization of bulk materials by performing Bragg edge imaging using the Hokkaido University compact accelerator neutron source.

Residual stress relaxation by bending fatigue in induction-hardened gear studied by neutron Bragg edge transmission imaging and X-ray diffraction

The compressive residual stress on the gear tooth’s surface, a vital parameter to control mechanical properties, such as strength, has a beneficial effect on the component’s fatigue life. A novel procedure, double induction quenching (DIQ), effective for improving the fatigue strength of gear products, has been used for producing gears with steep gradients of compressive residual stress generated in the tooth surface. We performed a Bragg edge imaging experiment at a pulsed neutron source to determine the spatial distribution of the {110} lattice spacing (d110) and the broadening of the {110} Bragg edge (w110) on the DIQ gear product after tooth-bending fatigue tests to which different loading cycles were applied. No significant difference occurred in the d110 and the w110 at Hofer’s critical section (tensile side) of the teeth with different loading conditions within the accuracy of data analysis. However, we detected a decrease in the w110 and changes in the residual lattice strain distribution in the axial direction (through the thickness) along the tooth root directions at the opposite side of Hofer’s critical section for both teeth after 3 × 105 and 8 × 105 cycles, relieving the compressive residual stresses during the fatigue process. The residual stress close to the gear tooth surface determined by X-ray diffraction using sequential polishing showed a slight relaxation and redistribution from the tensile side in the hoop direction, complementary to the neutron Bragg edge imaging information in the axial direction.

The Complex, Unique, and Powerful Imaging Instrument for Dynamics (CUPI2D) at the Spallation Neutron Source (invited).

The Oak Ridge National Laboratory is planning to build the Second Target Station (STS) at the Spallation Neutron Source (SNS). STS will host a suite of novel instruments that complement the First Target Station's beamline capabilities by offering an increased flux for cold neutrons and a broader wavelength bandwidth. A novel neutron imaging beamline, named the Complex, Unique, and Powerful Imaging Instrument for Dynamics (CUPI2D), is among the first eight instruments that will be commissioned at STS as part of the construction project. CUPI2D is designed for a broad range of neutron imaging scientific applications, such as energy storage and conversion (batteries and fuel cells), materials science and engineering (additive manufacturing, superalloys, and archaeometry), nuclear materials (novel cladding materials, nuclear fuel, and moderators), cementitious materials, biology/medical/dental applications (regenerative medicine and cancer), and life sciences (plant-soil interactions and nutrient dynamics). The innovation of this instrument lies in the utilization of a high flux of wavelength-separated cold neutrons to perform real time in situ neutron grating interferometry and Bragg edge imaging-with a wavelength resolution of δλ/λ ≈ 0.3%-simultaneously when required, across a broad range of length and time scales. This manuscript briefly describes the science enabled at CUPI2D based on its unique capabilities. The preliminary beamline performance, a design concept, and future development requirements are also presented.

Monte Carlo Study of an Electron-Based Neutron Source for Bragg Edge Imaging

Neutron imaging is a powerful and nondestructive tool for testing materials in industrial and research applications. Compact accelerator neutron sources are gaining interest in neutron application techniques, such as Bragg edge transmission imaging. Delivering a high neutron flux with a narrow pulse width and suppressed photons at the sample position are fundamental factors for designing a neutron source for Bragg edge imaging. In this study, Monte Carlo calculations were performed to simulate a 40-MeV electron beam impinging on a cylindrical tungsten target. Different target moderator and reflector (TMR) geometries were investigated to produce cold neutrons, and their results were compared. Polyethylene (PE) and graphite were used as the moderator and the reflector, respectively. The structures and dimensions of the moderator and reflector were optimized using a Monte Carlo simulation with the PHITS-3.28 code. The effect of the PE moderator temperature on the cold neutron flux was investigated. The results showed that the optimum size of the PE at 77 K inside the reflector was 3 × 15 × 15 cm3 to achieve the wavelength resolution of 1.05% and the neutron flux of 1.16 × 104 n/cm2/s at 1000 cm from the target station by assuming the electron beam current of 275 µA. In addition, the FLUKA 4-2.1 code was used to calculate the neutron spectrum from the designed neutron production target at room temperature, and the results were consistent with the PHITS calculations. The neutron spectrum together with its pulse width from the designed TMR were used to simulate the Bragg edges of an α -Fe sample, and it was concluded that the TMR is suitable for performing Bragg edge imaging.

Neutron performance and future prospect of the compact electron accelerator-driven neutron facility AISTANS

The neutron wavelength resolution, Bragg-edge measurement ability and future prospects regarding the neutron count rate were investigated at AISTANS. The wavelength resolution evaluated by Bragg-edge analysis using an iron powder sample is in reasonably good agreement with calculations at neutron wavelengths of 0.2–0.3 nm. Bragg-edge imaging was also performed on a steel plate sample with distinct regions of BCC and FCC crystallinity, and the different crystal structures were successfully discriminated. In addition, an aluminum sample containing a friction stir spot welding region was also measured and the 200 and 111 Bragg-edges at the two joining areas were observed. However, it was hard to characterize the difference in texture between the two areas. The neutron counting rate for Bragg-edge imaging is expected to increase by approximately a factor 50 in the near future thanks to the planned improvements of the electron beam power and the detection efficiency of the neutron detector.

Neutron Bragg edge imaging for strain characterization in powder bed additive manufacturing environments

Spatially resolved studies of crystalline structures, e.g. lattice spacings, are enabled by recording the transmitted spectra in neutron Bragg edge imaging. The recorded signals are, however, a result of through-thickness averaging of the probed specimen in the beam direction. Therefore, it is challenging to extract the strain distribution when the strain varies across the thickness, which applies for studies on different materials or material states along the beam. Here we introduce the approach to disentangle contributions to the recorded signals, i.e. separating the transmission spectra of two different material states. This is particularly applicable to powder bed additive manufacturing environments where operando strain characterization of the printed specimen using neutrons is intended. In this work, Laser Powder Bed Fusion (PBF-LB/M)-built 316L and IN718 samples embedded in their corresponding powders are used, extracting the desired spectra of the printed specimen. The disentanglement is proven to be satisfactory by obtaining coinciding strain maps of identical specimens embedded in powder layers of different thicknesses. Furthermore, the obtained residual strain distributions of 316L samples were verified by conventional neutron diffraction with lower spatial resolution due to the gauge volume averaging.

Mechanical surface treatment studies by Bragg edge neutron imaging

Mechanical surface treatment technologies such as laser peening and cavitation peening require detailed characterization, including residual stress analysis, to optimize their processing parameters. Recent developments at neutron facilities allow non-destructive 2-dimensional residual strain mapping through Bragg edge imaging, which provides specific advantages over more established methods. The present work highlights the application of Bragg edge neutron imaging for the study of mechanical surface treatments, through determination of lattice spacing distributions by energy-resolved radiography. Through three different examples, the unique capabilities of the method are demonstrated, particularly for providing near surface residual strain maps within samples with complex geometries with relatively high spatial resolution. By providing a comparison with X-ray diffraction and neutron diffraction results, the present work emphasizes the potential of Bragg edge neutron imaging as a tool for surface treatment research.

Spatial distribution and preferred orientation of crystalline microstructure of lead-bismuth eutectic

To develop a lead-bismuth eutectic (LBE) cooled nuclear reactor, phase transition phenomena of LBE are very important. In the solidification of LBE, the crystalline structure is varied with the cooling process. The volumetric expansion of LBE must be clarified for the safety of an LBE cooled nuclear reactor. The time dependence of the volumetric expansion depends on the crystalline microstructure. In this study, the crystalline microstructure of the LBE samples solidified with the different cooling processes was investigated by the neutron Bragg edge imaging technique. Spatially integrated and local microstructure characteristics of LBE samples were analyzed. Characteristics of preferred orientation of LBE microstructure were clarified.

Revealing the residual stress distribution in laser welded Eurofer97 steel by neutron diffraction and Bragg edge imaging

• Through-thickness residual stress distribution in three dimensions is evaluated. • High-resolution residual strain is mapped by neutron Bragg edge imaging. • Location-dependant reference lattice spacing is first applied on neutron imaging. • Correlation between microstructure, residual stress and micro-hardness is studied. Eurofer97 steel is a primary structural material for applications in fusion reactors. Laser welding is a promising technique to join Eurofer97 plasma-facing components and overcome remote handling and maintenance challenges. The interaction of the induced residual stress and the heterogeneous microstructure degrades the mechanical performance of such fusion components. The present study investigates the distribution of residual stress in as-welded and post-heat treated Eurofer97 joints. The mechanistic connections between microstructure, material properties, and residual stress are also studied. Neutron diffraction is used to study the through-thickness residual stress distribution in three directions, and neutron Bragg edge imaging (NBEI) is applied to study the residual strain in high spatial resolution. The microstructures and micro-hardness are characterised by electron backscatter diffraction and nanoindentation, respectively. The M-shaped residual stress distribution through the thickness of the as-welded weldment is observed by neutron diffraction line scans over a region of 1.41 × 10 mm 2 . These profiles are cross-validated over a larger area (∼56 × 40 mm 2 ) with the higher spatial resolution by NBEI. The micro-hardness value in the fusion zone of the as-welded sample almost doubles from 2.75 ± 0.09 GPa to 5.06 ± 0.29 GPa due to a combination of residual stress and cooling-induced martensite. Conventional post weld heat treatment (PWHT) is shown to release ∼ 90% of the residual stress but not fully restore the microstructure. By comparing its hardness with that of stress-free samples, it is found that the microstructure is the primary contribution to the hardening. This study provides insight into the prediction of structural integrity for critical structural components of fusion reactors.

Strain distribution visualization of punched electrical steel sheets using neutron Bragg-edge transmission imaging

Residual strains in a punched electrical steel sheet increase the iron loss in the steel sheet. To accurately estimate the effect of residual strain on iron loss, the residual strain distribution in a punched electrical steel sheet should be evaluated. In this study, we demonstrated the two-dimensional imaging of the residual strain distribution in a punched electrical steel sheet using the neutron Bragg-edge transmission imaging method. To improve the accuracy of strain measurement with minimal deterioration of spatial resolution, we applied a process of superposing many specimen images. The tensile strain near the punched edge and the compressive strain inside the core were experimentally confirmed using this method. Finally, the neutron Bragg-edge imaging results and those obtained from kernel average misorientation map using electron backscattered diffraction were compared to verify the validity of the proposed method.

Nondestructive quantitative imaging for spatially nonuniform degradation in a commercial lithium-ion battery using a pulsed neutron beam

A quantitative nondestructive Bragg-edge imaging technique for power-storage degradation was developed using commercial prismatic-shaped lithium-ion battery cells. The C or Li atoms/ions column density [cm−2] images for five phases of the negative electrode material (graphite, Li0.04C6, Li0.2C6, Li0.5C6, and Li1.0C6) with a 1 mm spatial resolution were obtained by considering the crystalline orientation for both fresh and fatigue cells. In the fresh cell, most of the graphite was uniformly converted to Li1.0C6 by charging, and in the fatigue cell, the graphite changed to various phases (especially, Li1.0C6 and Li0.5C6) by charging, and their spatial distributions were nonuniform.

Intact, Commercial Lithium-Polymer Batteries: Spatially Resolved Grating-Based Interferometry Imaging, Bragg Edge Imaging, and Neutron Diffraction

We survey several neutron imaging and diffraction methods for non-destructive testing and evaluation of intact, commercial lithium-ion batteries. Specifically, far-field interferometry was explored as an option to probe a wide range of autocorrelation lengths within the batteries via neutron imaging. The dark-field interferometry images change remarkably from fresh to worn batteries, and from charged to discharged batteries. When attempting to search for visual evidence of battery degradation, neutron Talbot-Lau grating interferometry exposed battery layering and particle scattering through dark-field imaging. Bragg edge imaging also reveals battery wear and state of charge. Neutron diffraction observed chemical changes between fresh and worn, charged and discharged batteries. However, the utility of these methods, for commercial batteries, is dependent upon battery size and shape, with 19 to 43 mAh prismatic batteries proving most convenient for these experimental methods. This study reports some of the first spatially resolved, small angle scattering (dark-field) images showing battery degradation.