Focused Ion Beam Cross-sectioning Research Articles

Basic properties of solidified organic liquids at a cryogenic temperature for electron microscopic visualization and sample preparation of dispersion systems.

It is challenging to image structures in liquids for electron microscopy (EM); thus, low-temperature imaging has been developed, initially for aqueous systems. Organic liquids (OLs) are widely used as dispersants, although their cryogenic EM (cryo-EM) imaging is less common than that of aqueous systems. This is because the basic properties (e.g., vapor pressure, density, and amorphousness) of OL in the solid state have not been extensively investigated, preventing the determination of whether or not the observed structure is free from artifacts. Herein, I summarized physical data related to the phase change, and the solid density at 77 K and sublimation speed for some OLs were measured independently, to discuss the applicability of OLs for cryo-EM. Among various OL properties, the sublimation temperature, pressure, and rate, and crystallinity are important for cryo-EM. The sublimation-related properties are used to judge whether the OL is stable during storage, observation, and sample preparation such as etching. These properties were calculated, and the calculated sublimation speed matched with that measured by cryo-SEM movie imaging. Crystallinity was estimated using the difference between the extrapolated temperature-dependent liquid density and the solid density of frozen OLs measured in liquid nitrogen. Artifacts observed upon freezing were exemplified by focused ion beam cross-sections of OL-in-water emulsions, and cracks, voids, and wrinkles are found in the OL phase at a large shrinkage ratio. The study findings show that the applicability of OLs largely differs for structural isomers and that appropriate OLs are required for the cryo-EM imaging of nonaqueous systems.

Copper Metallization of Nano-through Silicon Vias for Backside Power Delivery Networks

I. Introduction Backside power delivery networks (BS-PDNs) represents an exciting technological concept that has been steadily advancing over the past few years, particularly in the integration of leading-edge nodes. With BS-PDN approaches, the entire power network is migrated to the backside of the wafer, eliminating interference, and allowing more area for signal routing on the frontside. In addition, the route for power delivery to transistor is shorter and more direct, resulting in reduction by 30% in IR drop1.Nano-Through Silicon Vias (n-TSVs) is one of the key enabling processes for BS-PDN integration1-4. The application of n-TSVs enhances the interconnect packing density, potentially increase the efficiency of power delivery up to 7 times compared to frontside power delivery, based on the simulation studies conducted by Arm and Imec5. The scaling down of TSV diameter to 200 nm brings challenges in fabrication process steps such as wafer thinning and metallization. To maintain the aspect ratio between 3 and 5 for reliable n-TSVs etch and filling processes, Si substrate must be thinned down to below 1µm6. Cu is one of the options for the filling of n-TSVs. However, significant challenges exist in Cu metallization of high aspect ratio n-TSVs (diameter <200 nm, aspect ratio 3-5). These include poor Cu seed sidewall coverage and overhang at via entrance with conventional physical vapor deposition (PVD) setups, as well as fill voids during electroplating (ECP) in structures with aspect ratio >3 due to limited additive diffusion into the nano features. In this paper, we will co-optimize PVD and ECP processes to achieve robust void-free Cu metallization of n-TSVs. II. Experimental 250x760 nm n-TSVs with pattern density approximately 1% were fabricated on 300mm silicon wafers. Immersion lithography was employed for pattern definition, followed by plasma dry etching on 760nm oxide/silicon/oxide stack, to mimic typical n-TSVs film structure. Subsequently, Al2O3 passivating dielectric and Ta barrier/Cu seed deposition by atomic layer deposition (ALD) and PVD, respectively. To enhance the Cu seed coverage, bias conditions during PVD process were investigated.Copper electroplating was conducted in AMAT-Nokota platform using a commercially available acidic copper electrolyte. To gain insights into copper filling mechanism of n-TSVs, ECP process was conducted at different conditions with varying current densities, agitation, and pulse current waveforms. The n-TSVs copper filling was characterized using cross-sectional focused ion beam transmission electron microscopy (FIB-TEM). III. Results Fig. 1 shows a cross-sectional TEM image illustrating n-TSVs filling under baseline condition. There are systematic voids at the center, throughout the top-half of the n-TSVs. These seam voids might be due to the poor diffusion of additives to nano scale features, leading to sub-conformal plating configuration, resulting in pinch off voids near the top of n-TSVs. To facilitate the diffusion and adsorption of appropriate additives towards n-TSVs, periodic pulse current waveforms are applied. Besides the enhancement of additive diffusion during pulse off-time, high peak current pulsing results in higher overpotential deposition which promotes copper nucleation. In addition, the average current density is reduced by 20% to allow sufficient replenishment of copper ions inside the n-TSVs.Apart from the center voids at the top region of the vias, there are voids along the sidewalls around the Si layer due to poor Cu seed coverage. To improve the Cu seed coverage, the wafer bias during PVD process is modulated. The increase in bias condition is beneficial not only for the continuous Cu seed at sidewalls, but also to reduce overhang at the top of the vias. With the approach of high bias PVD, via opening CD is larger, allowing better additive and copper ion diffusion during electroplating.Fig. 2 shows a cross-sectional TEM image representing n-TSVs filling under optimized condition. Void-free filling is achieved with the co-optimization of PVD and ECP processes. The details will be discussed in the full paper. IV. Novelty This work demonstrates a robust void-free Cu metallization for n-TSVs which is important for the integration and fabrication of backside power delivery networks. V. References Hafez et al., 2023 IEEE Symposium on VLSI Technology and Circuits, p. 1-2 (2023).Veloso et al., 2021 Symposium on VLSI Technology, p. 1-2 (2021).Veloso et al., IEEE Transactions on Electron Devices, 69, p. 7173–7179, (2022).Anne Jourdain et al., IEEE Electronic Components and Technology Conf., p. 1531-1538 (2022).Cline, D. Presaderic, E. Beyne, O. Zografos, “Next-Gen Chips Will Be Powered from Below,” IEEE Spectrum, (2021).Anne Jourdain et al., Proc. IEEE Electronic Components and Technology Conf., p. 42-48 (2020). Figure 1

Post-mortem analysis of the deposit layers on the lower divertor after the 2023 high particle fluence campaign of WEST

We analysed deposits collected on the lower divertor of WEST after the first high fluence campaign performed in 2023. Deposits were collected on the high field side (thick deposition area) of 2 ITER-grade plasma facing units (PFUs), located on different toroidal positions. Using focused ion beam cross sectioning, the deposits were found to be very thick (12–55 µm). A significant difference was observed in the toroidal direction with thicker deposits for the PFU located at the maximal heat load in the inner side, showing a deposition pattern due to the toroidal magnetic field modulation. Deposits present a complex layer-by-layer structure with dense layers, some melted parts and porosities within the layers. The deposition is mainly composed of tungsten with oxygen, boron, carbon and traces of nitrogen whose composition vary along the radial direction. We identified W-rich deposits in the high plasma flux area near the inner strike point and deposits rich in O, B, C and N in the low plasma flux area further away from this strike point. W dense layers of about 5 up to 40 µm thick in the thick deposit area near the strike point were attributed to the high fluence campaign. Pure boron layers resulting from wall conditioning and processed by plasma wall interactions were also observed.

Duplex surface engineering of cold spray Ti coatings and physical vapor-deposited TiN and AlTiN thin films

The feasibility of a duplex coating based on cold spray technology and magnetron sputtering was evaluated for repair applications requiring a ‘thin-on-thick’ layered structure. Commercially pure angular-shaped Ti grade 4 particles are fed to a cold spray gun and accelerated toward a Ti alloy substrate to deposit thick coatings (∼4.5 mm). TiN and AlTiN thin films are deposited on polished cold spray coatings using a four-source closed-field unbalanced magnetron sputtering (CFUBMS) system. Microstructure was characterized using focused ion beam (FIB) lift-out, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI). The nanoindentation technique was used to evaluate the mechanical properties of coatings. The H/E ratios and H3/E2 ratios for TiN films were found to be 0.098 and 0.26 GPa, respectively, while those for AlTiN films were measured at 0.066 and 0.052 GPa, respectively, suggesting higher capacity of TiN films to withstand both elastic and plastic deformation. Using scratch testing, the adhesion of TiN and AlTiN thin films to cold spray Ti was investigated, with TiN-Ti duplex coatings exhibiting better performance compared to AlTiN-Ti coatings. Tribological testing was performed on duplex coatings using a reciprocating tribometer equipped with an alumina ball counterface. The wear rate for AlTiN-Ti coatings after 2000 sliding cycles was found to be (1.0 × 10−3 ± 0.1 × 10−3 mm3/Nm), three orders of magnitudes higher than that for TiN-Ti (8 × 10−6 ± 2 × 10−6 mm3/Nm. SEM was used to reveal worn surface morphologies and cross-sectional analysis of the wear track. Subsurface microstructural changes due to wear were examined using focused ion beam cross-sectioning, revealing bending cracks and tribofilm formation.

New insights on bonding mechanism of FCC and BCC high entropy alloy microparticles upon supersonic impact using micromechanical adhesion test

Coating buildup in cold spraying (CS) relies on particle bonding upon impact, thus making it a significant area of study. High entropy alloys (HEAs) are a class of materials with superior work-hardening ability and resistance to softening, making their particle impact and bonding particularly important to understand for their cold sprayability. In this study, the splat adhesion strength is measured with a scratch test for FCC CoCrFeMnNi and BCC AlCrFeMnNi HEA microparticles deposited on mirror-polished SS304 substrates. Nanoindentation tests were conducted to estimate the tensile and shear strengths of the HEA powders. The results showed that particle deformation and jetting were dominant in the FCC HEA/SS304 system, while substrate deformation was dominant in the BCC HEA/SS304 system. Focused ion beam cross sections of particles confirmed that the FCC HEA had good bonding and underwent significant deformation and grain refinement at the substrate/particle interface, while the BCC HEA had limited bonding only in the sheared zone of the particles.In spite of the very limited deformation of BCC splats, their average adhesion strength was measured to be 451 ± 141 MPa, which is significantly higher than that of FCC splats, with an average value of 308.7 ± 69.3 MPa. Fracture surface analysis revealed that FCC splats consistently exhibit ductile fracture, regardless of whether it occurs at the particle, substrate, or interface. In contrast, for BCC splats, fracture was ductile when occurring at the interface and substrate sides, while it was cleavage when occurring on the particle side. These findings suggest that the toughness and shear strength of the bonding developed at the interface were higher than those of the particle. It was found that the flattening ratio (FR) of the particles decreases for both FCC and BCC HEA splats with an increase in particle size, resulting in a decrease in adhesion strength. Metallurgical bonding has been identified as the primary factor contributing to the adhesion strength of both FCC and BCC HEA splats. Particle penetration, indicative of mechanical interlocking, appears to have little or no effect on adhesion strength. However, it does positively influence the increase in adhesion energy of the splats.

On fatigue crack origin with a fine granular area in matrix without defect

Fatigue crack initiation in metallic materials during very high cycle fatigue has been characterized by a subsurface crack origin with defect such as an inclusion. In this paper, fatigue damage behavior in an austenitic stainless steel has been studied using a novel progressive stepwise load increasing test method with each cycle step higher than 108 cycles. Subsurface crack origin with a fine granular area has formed in the matrix without defect. This is a new phenomenon. The mechanism has been investigated using Focused Ion Beam cross-sectioning and electron channeling contrast imaging techniques. Strain localization, grain fragmentation and local plasticity exhaustion are the main factors that cause fatigue damage and crack initiation in the matrix. This study provides a fundamental understanding how material damage and crack initiation occur in material matrix during very high cycle fatigue.

Whisker-free lithium electrodeposition by tuning electrode microstructure.

Growth of lithium whiskers or dendrites is the major obstacle towards safe and stable utilization of lithium metal anodes in rechargeable batteries. In this study, we look deeper into the mechanism of lithium electrodeposition. We find that before lithium whisker or dendrite nucleation occurs, lithium is deposited into the grain boundaries of the metal electrode, which we directly observed in the focused ion beam cross-sections of the lithium electrode. The structure of grain boundaries near the surface predetermines the capacity for smooth deposition, which is proportional to the surface area of such boundaries. We then demonstrate that by reducing the metal grain average size we can extend whisker- and dendrite-free deposition of lithium. With a eutectic Ga-Li alloy containing only 2.5 atomic percent of gallium we observed morphologically stable lithium deposition for 10 hours. The results demonstrate deep interconnections between the metal microstructure and the whisker growth.

Effect of Mechanical Pressure on Lifetime, Expansion, and Porosity of Silicon Dominant Anodes in Laboratory Lithium-Ion Cells

Silicon (Si) is a promising next-generation anode material for lithium-ion batteries due to its nearly ten times higher specific capacity of 3579 mAh/gSi 1. The high and economic availability of Si due to existing industrial infrastructure is another non-technical advantage.2 The main drawback of Si is its large volume expansion of nearly 300% during full lithiation causing SEI (re-) formation, electrochemical milling, and electrode instability leading to thickness changes resulting in the disruption of the electronic pathways and decoupling of particles.2 These problems still exist in partially lithiated µm-sized Si particles with lower volume expansion.2 In this work, we study the influence of compressive force on electrode stability in full cells composed of NCA3 cathodes and silicon-dominant anodes based on µm-sized particles.2 The different mechanical pressures were applied using spring-compressed 3.34 mAh Swagelok™ T-cells (A = 0.94 cm²) with metallic lithium reference electrodes. The investigated mechanical pressure ranges from 0.02 MPa as low pressure (LP) to 2.00 MPa, as high high pressure (HHP) to determine the optimal pressure for cyclic lifetime. Firstly, we introduce an experimental methodology to precisely adjust the common three-electrode SwagelokTM cell setup. Therefore, we take the compression from the single components into account, which were measured in a universal testing machine4 for the pressure range mentioned above. The cells were built with different mechanical pressures and cycled with C/2 until a SoH of 80% of the initial capacity was reached which was measured periodically in CheckUp tests. In the post-mortem analysis, Si anodes cycled at different mechanical pressures are prepared for scanning electron microscopy using a focused ion beam cross-section polisher. The images are then analyzed regarding electrochemical milling, thickness increase, and porosity decrease at 80% SoH by comparing them to the pristine state.The HHP cells with 2 MPa show a detrimental effect on lifetime during cell cycling as depicted in Figure 1 and with only 100 cycles nearly a halved lifetime compared to a normal pressure (NP) of 0.20 MPa. Moreover, the specific discharge capacity is significantly reduced for HHP or LP compressed cells compared to NP cells (see Figure 1b). The post-mortem analysis showed a dependency of the porosity and thickness of the aged Si anodes on the applied pressure. On the particle level, no electrochemical milling was observed due to the partial lithiation of the Si anodes. Vein-like microstructures were observed for the lithiated probes which might originate from different lithiation pathways of polycrystalline Si.5 Based on these experimental results, multilayer pouch cells with the same electrode setup and balancing are produced and cycled, and the expansion of the cells is measured operando in a compression test bench. Acknowledgments: S. F. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany), under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank the research battery production team of iwb for the manufacturing of the electrodes

FIB line marking as a tool for local erosion/deposition/fuzz formation measurements in ASDEX Upgrade during the He campaign

Tungsten (W) is the prime candidate for the plasma facing material in present and future fusion devices. When exposed to a plasma containing He ions, W can exhibit creation of sub-surface nano-bubbles leading to formation of nano-tendrils called fuzz. Formation of W fuzz was confirmed by many laboratory experiments including He loading in linear plasma devices. There is, however, limited experience related to the parameter space for fuzz formation and re-erosion in a tokamak environment. For this reason, a dedicated experiment was carried out in ASDEX Upgrade (AUG) during its 2022 He campaign to study the evolution of fuzz-like W structures.Twelve tungsten samples were mechanically polished to a mirror-like surface finish. Six of them were subsequently exposed to a He plasma in the PSI-2 linear device in order to establish a fuzz layer with a thickness of 600 – 800 nm. Before exposure in AUG, all samples were pre-characterized by Focused Ion Beam (FIB) cross-sectioning. In total 48 FIB cross-sections with line markings for quantification of local erosion, deposition and fuzz formation were prepared – 4 on each sample. The samples were placed in two parallel poloidal rows spanning a range of 20 cm around the outer strike line position (OSP). They were subsequently exposed in AUG to a series of 14 consecutive discharges, 8 in H-mode and 6 in L-mode.Detailed analysis by means of electron microscopy revealed on the samples regions of erosion, deposition and fuzz formation. Below the H-mode OSP, homogeneous co-deposits containing W and O, with a thickness up to 400 nm, were found. In the close vicinity of the H-mode OSP, significant erosion of pre-exposure PSI-2 fuzz was observed. The erosion reached up to 100 – 250 nm, depending on the location. In addition, the initially polished samples did not show any newly formed fuzz in that zone. Above the H-mode OSP, new fuzz was formed with a thickness of up to 1 μm. There was no Mo found in the newly formed fuzz. Pre-exposure PSI-2 fuzz was either removed or modified.

Abrasion resistance of a Nb3Sn magnetron-sputtered coating on copper substrates for radio frequency superconducting cavities

The paper presents the first study of the abrasion resistance and degradation processes of magnetron-sputtered Nb3Sn coatings on electropolished (EP) or chemical polished (CP) copper substrates. The coating-substrate system is of interest for application in radio frequency superconducting cavities as an improvement on bulk niobium cavities. The microstructural and chemical properties of the substrates and coating were examined by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), electron backscatter diffraction, X-ray diffraction, and focused ion beam cross-sectioning. The abrasion resistance was investigated by scratch testing under progressive loads up to 200 N, after which the scratched surfaces were characterised by SEM/EDS to determine the failure mechanisms of the coatings. Several stages of coating damage were identified, including nodule compression, cracking, pitting, fragmentation, comminution and detachment. Nevertheless, substantial amounts of fragmented and embedded coating remained attached to the copper up to the highest load. The findings indicated similar abrasion resistance behaviour for the EP and CP conditions.

Gas-assisted femtosecond pulsed laser machining: A high-throughput alternative to focused ion beam for creating large, high-resolution cross sections.

Cross sectioning is a critical sample preparation technique used in a wide range of applications, that enables investigation of buried layers and subsurface features or defects. State-of-the-art cross-sectioning methods, each have their own pros and cons, but generally suffer from a tradeoff between throughput and accuracy. Mechanical methods are fast but lack accuracy. On the other hand, ion-based methods, such as focused ion beam (FIB), offer high resolutions but are slow. Lasers, which can potentially improve this tradeoff, face multiple challenges that include creation of heat affected zones (HAZs), undesirably large spot size as well as material redeposition. In this work, we utilized, for the first time, a femtosecond pulsed laser, which has been shown to cause minimal to zero HAZ, for rapid creation of large cross sections that are comparable with FIB cross sections in quality. The laser was integrated with a targeted CO2 gas delivery system for redeposition control and beam tail curtailing, and a hard mask for top surface protection and further shrinkage of the effective spot size. The performance of the proposed system is showcased through real world examples that compare the throughput and quality resulted from the laser and FIB cross sectioning techniques.

Surface modification of ITER-like mirrors after one hundred cleaning cycles using radio-frequency plasma

In ITER, the metallic first mirrors (FMs) will undergo erosion due to their proximity to the fusion plasma and deposition of materials originated from the first walls (mainly beryllium). In-situ plasma cleaning is a promising technique to conserve the FMs optical properties by means of ion sputtering. In this work, the evolution of the optical properties of single-crystal (Sc) and nanocrystalline (Nc) molybdenum (Mo) and rhodium (Rh) mirrors were investigated up to 100 cycles of consecutive contamination and cleaning. Aluminum oxide (Al2O3) was used as contaminant to replace the toxic beryllium. The plasma cleaning was carried out using a capacitively coupled argon (Ar) plasma excited by a 60 MHz radio-frequency generator resulting in the formation of a self-bias applied on the mirrors of -280 V. The plasma potential being around 30 V, the Ar ion energy was about 310 eV. The optical properties of the mirrors were assessed using ex-situ reflectivity measurements. Moreover, the surface topography was characterized by means of scanning electron microscopy (SEM), focused ion beam (FIB) and roughness measurements using atomic force microscopy (AFM). ScMo and ScRh mirrors formerly exposed to 80 successful cleaning cycles using aluminum/tungsten (Al/W) deposits and air storage exhibit drastic changes in their optical properties after being subject to cleaning cycles using Al2O3 as contaminant. Additionally, freshly polished ScRh were exposed to identical cleaning cycles. All Sc mirrors exhibited pits induced by the polishing procedure using diamond paste in addition of mounds/wavy patterns. The carbon incorporated during the polishing process was demonstrated to be responsible for the pitting of the surface. The Nc mirrors preserved their initial reflectivities after up to 100 cycles. The surface topography was systematically characterized and an average erosion rate for NcRh mirrors of about 59 nm per cycle has been estimated from FIB cross-sections. The optical properties of the Nc mirrors showed a superiority in the present study in comparison to the Sc materials due to the influence of their polishing procedures.

Efficient Utilization of Macropores as Artificial Solid–Electrolyte Interphase Channels for High-Capacity Silicon/Graphite Anode Materials

Porous architectures for silicon/graphite (Si/Gr) composites can buffer the massive volume expansion of Si particles during electrochemical cycling. However, the large surface area derived from the high porosity leads to unavoidable side reactions at the electrode–electrolyte interface, leading to the formation of thick resistive natural solid–electrolyte interphase (SEI). Herein, a simple and scalable route is developed for coating a polymeric artificial SEI (A-SEI) inside the porous architecture for the Si/Gr composite via a facile incipient wetness impregnation (IWI) method. Cross-sectional focused ion beam microscopic results infer that the polymer coating is successful. Polymer coating for the porous matrix as A-SEI induces sufficient porosity as well as prevents excessive electrolyte penetration into the highly porous matrix. Furthermore, it prevents the direct contact of active materials with electrolytes, minimizing the parasitic reactions that form natural SEIs. Consequently, the polymer coating obtained by IWI enables remarkable enhancement in the long-cycle stability of the porous Si/Gr electrode, in contrast to the nonimpregnated electrode displaying capacity roll-over due to excessive SEI formation. Moreover, it is demonstrated that the coating effectively prevents the formation of dendritic lithium plating on the surface of the Si/Gr electrode, thereby enhancing the safety of the battery.

Carbon erosion/deposition on the divertor of W7-X during the operational period OP 1.2b

Carbon net erosion and deposition at the test divertor unit (TDU) of Wendelstein 7-X (W7-X) were measured for the discharge period OP 1.2b in the year 2018 using 18 specially prepared target elements in all 10 TDUs. These had lengths between 30 and 60 cm and were coated with marker layers for erosion/deposition investigations of 5–10 μm carbon on top of about 300 nm molybdenum. The marker layer thicknesses were measured by elastic backscattering spectrometry (EBS) before and after plasma exposure using 2.5 MeV protons; the surface morphology was investigated using scanning electron microscopy (SEM) and focused ion beam cross-sectioning (FIB), the surface roughness was determined using a two-dimensional optical profiler. Plasma-exposed surfaces were considerably smoother than unexposed surfaces with decreased mean roughness and a shift of the inclination angle distribution towards lower values. The erosion on the 10 TDUs was unequal within a factor of about two. During the discharge period in total 20.4 ± 5.7 g carbon was eroded from the 10 TDUs. Adjacent to the strike line some deposition of carbon was observed. Compared to the discharge period OP 1.2a in the year 2017, the net carbon erosion rate dropped by a factor of 5–6 due to regular boronizations, which reduced the oxygen (and subsequently also the carbon) content in the plasma by 1–2 orders of magnitude. The significance of erosion/deposition processes for long-pulse discharges is discussed.

Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

Micron-sized polystyrene spherical projectiles made from three different molecular weight polymers spanning from below to well above the entanglement molecular weight were launched against a rigid substrate, causing ultrahigh strain rate deformation. Scanning electron microscopy, focused ion beam cross sections, and atomic force microscopy were used to elucidate the deformation mechanisms from the observed morphologies of the deformed specimens and to evaluate the important role of entanglements in the various deformation processes that dissipate the kinetic energy. Due to adiabatic heating from shock compression, the temperature at the bottom region of the polymer projectile is elevated above the glass transition temperature, enabling viscoplastic flow. The cooler portions of the unentangled sample undergo strain localization via micro-shear banding and brittle fracture, while the two entangled samples resist fracture and exhibit extensive viscoplastic flow and, in the upper, cooler regions, extensive shear banding followed by crazing. The interaction of molecular entanglements, temperature, shear rate, and total shear strain on melt viscosity influences the amount of additional plastic work and adiabatic heating occurring in the sample and hence the extent of lateral spreading of the projectile over the substrate.

Time-Resolved Characterization of Electrochemically-Induced Solid Oxide Cell Microstructure Evolution

Solid oxide cells (SOC) are exciting both for their capability of fuel-flexible electricity generation and their promise in the production of green H2; however, their degradation and long-term performance remain important questions. Operating individual button cells to characterize these changes is prone to cell-to-cell variations, whereas large-area cell/stack life testing requires considerable resources. In both cases, full cell characterization usually must wait until the completion of the life test, which may delay getting results for months or years. While progress has been made in observing microstructural changes over time (termed here time-resolved characterization) using techniques like non-destructive transmission x-ray microscope tomography, such experiments are difficult to implement, especially the effect of current is unexplored due to geometrical and gas environmental limitations of the system.Presented here is a method to rapidly obtain time-resolved microstructural information of the same cell in tandem with electrochemical measurements. While SOC degradation arises due to a range of processes, this work focuses on the microstructural evolution of Ni-YSZ fuel electrodes that occurs during electrolysis operation, which is known to be a key issue. Two key experimental features allowed time-resolved characterization of multiple cells simultaneously: (1) the use of Ni-YSZ symmetric cells allows multiple cells of different size in a single test by avoiding the need for gas seals, and (2) the use of laser milling to allow controlled removal of a portion of each cell during the test.Here we illustrate the method for the case of low steam content and high current density, designed to exacerbate and accelerate electrolysis degradation due to the highly reducing conditions achieved. Tape-cast planar electrode-supported Ni-YSZ symmetric cells were laser cut into precise geometries with well-defined projected areas and easily removable sections. Four cells with projected areas of 1, 1, 0.75, and 0.5 cm2 were connected in series within the same furnace at 800 C and a gas environment of 97% H2 and 3% H2O. A current of 0.75 A was run through three of the cells, resulting in current densities of 0.75, 1.0, and 1.5 A/cm2, while one was maintained with no current. The life test lasted for 500 h, and the microstructure was observed at 0, 200, and 500 hours.Figure 1 shows the microstructures observed at the highest current density by polished cross-sectional SEM and FIB-polished sections under conditions that provided Ni/YSZ contrast. Microstructural degradation had occurred by 200 h and became immense at 500 h, due to high current density in these reducing conditions. By 200 hours, grain boundaries become enriched with Ni, likely due to the formation of Ni-Zr intermetallics in the ultra-low pO2 induced by local overpotential as reported by Chen[1] and Szasz[2]. Additionally, the electrode-electrolyte interface becomes nanoporous and a Ni enriched fracture is present ~2 μm into the electrolyte. By 500 hours, the Ni rich nanoporosity has progressed more than 10 μm into the electrolyte and created islands of large grains surrounded by Ni-rich deposits.Electrochemical impedance spectroscopy measurements show a clear initial decrease in polarization resistance followed by an overall impedance increase by 200 hours. These correspond to the initial electrochemical boost from the production of a nanoporous structure, followed by the deactivation of areas of the cell due to the large fractures through the electrolyte.[1] M. Chen et al., “Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current,” J. Electrochem. Soc., vol. 160, no. 8, pp. F883–F891, 2013, doi: 10.1149/2.098308jes.[2] J. Szász et al., “High-Resolution Studies on Nanoscaled Ni/YSZ Anodes,” Chem. Mater., vol. 29, no. 12, pp. 5113–5123, 2017, doi: 10.1021/acs.chemmater.7b00360.Figure 1: Microstructural and electrochemical changes of a symmetric Ni-YSZ SOC undergoing 1.5 A/cm2 in reducing conditions at 0, 200, and 500 hours. Cathode is on the left in all images; anode is on the right. a, b, c) Backscatter electron imaging reveals destruction of the dense YSZ electrolyte over the lifetime of the experiment. Lines of brighter phases on the anode side of fractures are Ni deposits. d, e, f) Focused Ion Beam cross sections reveal nanoporosity forming in the first 10 μm from the cathode-electrolyte interface at 200 and 500 hours. Charging is evident as a result of curtaining from the nanoporous features in the electrolyte. g) Electrochemical impedance spectroscopy shows the initial decrease in resistance from 0 to 48 hours followed by increasing resistance due to microstructure evolution. Figure 1

Elucidation of Discharge Mechanism in CFx As a High Energy Density Cathode Material for Lithium Primary Battery

Fluorinated graphite (CF x ) is a class of cathode materials with the high theoretical capacity (865 mAh g-1 in case of x=1) for primary (non-rechargeable) batteries. When using Li metal as the anode material, the system possesses a very low self-discharge rate (< 0.5% per year at 25 °C) compared to other current alternative chemistries. This system, with the proposed general governing reaction of CF x +Li→LiF+C, is one of the best candidates for a wide range of applications, such as implantable medical devices and equipment for extreme environment.Although the lithium fluorinated graphite system has been under investigation for a few decades, there is still a lack of fundamental understanding of the reaction mechanism in this system. Here, a multiscale investigation on the CFx discharge mechanism was performed using a novel cathode structure to minimize the carbon and fluorine additives for precise cathode characterizations. Titration gas chromatography (TGC), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), cross-sectional focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) were utilized to investigate this system.The results demonstrated that: (i) There is no lithium deposition or intercalation through the entire discharge. (ii) The CFx structure transforms to a hard-carbon like structure with less sp2 content by increasing depth of discharge. (iii) The crystalline LiF particles uniformly covered the layers of CFx structure with a size range of < 10 nm. A three-step discharge reaction mechanism is proposed in agreement with our electrochemical performances. This work deepens the understanding of CFx as a high energy density cathode material and highlights the need for future investigations on primary battery materials to advance performance. Figure 1

Influence of porosity and blistering on the thermal fatigue behavior of tungsten

Tungsten is the leading plasma-facing material (PFM) for nuclear fusion applications. It faces severe operating conditions, including intense hydrogen plasma exposure and high-cycle transient heat loading, which create various defects in tungsten. Additionally, defects have often already been introduced during manufacturing. Little is understood regarding the synergistic effect of such defects on the lifetime of tungsten so far. Here, we investigate the influence of porosity and blistering on the thermal fatigue behavior of tungsten. The pores resulted from powder metallurgy whereas the blistering was induced by hydrogen plasma exposure. Both conditions were subjected to transient heat loading by a high-power pulsed laser. The exposure was performed in the linear plasma generator Magnum-PSI, which closely mimics the expected particle and heat flux in the world’s largest fusion experiment, ITER. Both porosity and blistering degraded the fatigue resistance of tungsten. Pores tended to aggregate at high-angle grain boundaries (HAGBs) and assisted crack initiation therein, as revealed by focused ion beam cross-sectioning and electron backscatter diffraction (EBSD) analysis. The blisters were characteristic of subsurface cavities, which were located at a depth close to the surface roughness induced by transient heat loading. The stress concentration at the tip of the cavities is considered to promote crack initiation. The results highlight the necessity of a ‘life cycle assessment’ of the tungsten PFM for nuclear fusion reactors.

Coating delamination analysis of diamond/Ti and diamond/Ti-6Al-4V systems using cohesive damage and extended finite element modeling

Diamond coatings are known to improve the wear resistance properties of substrate materials. This paper aims at analyzing damage behavior of brittle diamond coating deposited on two different ductile substrates, namely pure-Ti and Ti-6Al-4V alloy, subjected to uniaxial tensile loading. In-situ tensile tests were conducted on diamond/Ti and diamond/Ti-6Al-4V coated samples to identify the stress-strain response and the damage behavior of the coating. Surface imaging was performed using scanning electron microscope (SEM). The cross sections were made with the focused ion beam (FIB) gun installed in SEM to observe the coating failure modes. Energy dispersive x-ray and x-ray diffraction analyses were carried out to determine composition. The SEM analysis revealed micro-crystalline morphology of the coating and FIB cross-sections showed diamond coating top-layer, TiC interlayer and substrate at the bottom. The XRD and EDX findings confirmed the regions observed in FIB cross-section via chemical analysis. The in situ tensile tests revealed that the coating had undergone failure through crack initiation, fragmentation and delamination. The bucking of the coating under transverse compressive strain of the substrate was also observed. The Ti-6Al-4V alloy caused early cracking and delamination of the coating owing to its higher yield strength and higher rate of stress transfer to the coating. Two-dimensional finite element models were developed to simulate various damage modes, such as cracking, buckling and delamination. Extended finite element method and cohesive damage modeling were exploited to observe cracking and buckling, respectively. Numerical results revealed that the coating was more prone to failure when Ti-6Al-4V, a high yielding less ductile, substrate was used instead of a low yielding highly ductile Ti substrate. Moreover, buckling was found to be the major reason of delamination than bulk cracking. The coating is, therefore, likely to delaminate more severely under the compressive (buckling) than the tensile (bulk cracking) mode of deformation.

TiN/NbN Nanoscale Multilayer Coatings Deposited by High Power Impulse Magnetron Sputtering to Protect Medical-Grade CoCrMo Alloys

This study describes the performance of nanoscale multilayer TiN/NbN coatings deposited on CoCrMo medical-grade alloys by utilising novel mixed high power impulse magnetron sputtering (HIPIMS) and direct current unbalanced magnetron sputtering (UBM) technique in an industrial size vacuum coater. Scanning electron microscopy analysis showed that these coatings were extremely dense without any intercolumnar voids. The coating exhibited high hardness of 28 GPa, as well as low friction and wear coefficient of 0.7 and 1.4 × 10−14 m3·N−1·m−1, respectively, as compared to the bare material. Scratch tests revealed superior coating to substrate adhesion due to the HIPIMS etching prior to coating deposition. Energy-dispersive X-ray analysis of the wear debris generated during the impact test together with focused ion beam cross-section analysis in different locations of the impact crater revealed the coating failure mechanism and further confirmed the excellent coating to substrate bonding strength. Potentiodynamic polarisation tests in NaCl and Hank’s solutions revealed the clear passivation behaviour, several orders of magnitude lower corrosion currents, and high pitting potentials of the coating, which guarantee excellent protection to the base alloy in such aggressive environments. Inductively coupled plasma mass spectrometry analysis of Hank’s solution containing corrosion debris of the coated sample revealed that the leaching of harmful metal ions from the base material was reduced to below the detection limit of the technique, thus demonstrating the high barrier properties of the coating.