Dual Focused Ion Beam Research Articles

Analysing Tortuosity for Solid Oxide Fuel Cell Anode Material: Experiments and Modeling

Solid oxide fuel cells (SOFCs) directly convert chemical energy to electricity with high electrical efficiency. It involves gas transport through the porous electrode to the three-phase boundaries (TPB). The tortuosity of gas transport relates the bulk diffusion of gas in free space to the effective diffusion coefficient of gas migrating through a porous material. Therefore, determining the tortuosity is of great importance. This paper tests button SOFCs with NiO-YSZ as anode material followed by dual beam-focused ion beam scanning electron microscopy (FIB-SEM) to obtain 2D serial slice images. Based on processed 2D images and reconstructed 3D microstructure, the tortuosity is calculated using three approaches i.e., porosity-tortuosity correlations, voxel-based, and path-length-based approaches. The test results show that a decrease in Ni content in the anode greatly decreases the cell performance due to a decreased percolated electronic phase. The sample with low performance has high tortuosity. Different approaches vary regarding the tortuosity value and computational time. The path-length-based approach can achieve reasonable accuracy in a relatively short time but is only valid for using the longest path length.

Correlative high-resolution imaging of hydrogen in Mg2Ni hydrogen storage thin films

Nanometer scale imaging of hydrogen in solid materials remains an important challenge for the characterization of advanced materials, such as semiconductors, high-strength metallic alloys, and hydrogen storage materials. Within this work, we demonstrate high-resolution imaging of hydrogen and deuterium within Mg2Ni/Mg2NiH4 hydrogen storage thin films using an in-house developed secondary ion mass spectrometer (SIMS) system attached to a commercially available dual-beam focused ion beam - scanning electron microscope (FIB-SEM) instrument. We further demonstrate a novel approach to measure the size, shape, and distribution of the hydride phase in partially transformed films using laser scanning confocal microscopy (LSCM) to measure surface topography changes from the hydride phase volume expansion. Combining these techniques provides new insights on hydride nucleation and growth within the Mg2NiHx system. Finally, we demonstrate the efficacy of tracking deuterium as a hydrogen analog to reduce the background for SIMS imaging of hydrogen in high-vacuum chambers (∼10−6 mbar).

Characterization of the Native Oxide Shell of Copper Metal Powder Spherical Particles.

The native oxide layer that forms on copper (Cu) metal spherical particle surfaces under ambient handling conditions has been shown to have a significant effect on sintering behavior during microwave heating in a previous study, where an abnormal expansion was observed and characterized during sintering of Cu compacts using reducing gases. Because microwave (MW) heating is selective and depends greatly on the dielectric properties of the materials, this thin oxide layer will absorb MW energy easily and can consequently be heated drastically starting from room temperature until the reduction process occurs. In the current study, this oxide ceramic layer was qualitatively and quantitatively characterized using the carrier gas hot extraction (CGHE) method, Auger electron spectroscopy (AES), and a dual-beam focused ion beam (FIB)/scanning electron microscope (SEM) system that combines both FIB and SEM in one single instrument. Two different commercial gas-atomized spherical Cu metal powders with different particle sizes were investigated, where the average oxygen content of the powders was found to be around 0.575 wt% using the CGHE technique. Furthermore, AES spectra along with depth profile measurements were used to qualitatively characterize this oxide layer, with only a rough quantitative thickness approximation due to method limitations and the electron beam reduction effect. For the dual-beam FIB-SEM system, a platinum (Pt) coating was first deposited on the Cu particle surfaces prior to any characterization in order to protect and to preserve the oxide layer from any possible beam-induced reduction. Subsequently, the Pt-coated Cu particles were then cross-sectioned in the middle in situ using an FIB beam, where SEM micrographs of the resulted fresh sections were characterized at a 36° angle stage tilt with four different detector modes. Quantitative thickness characterization of this native oxide layer was successfully achieved using the adapted dual-beam FIB-SEM setup with more accuracy. Overall, the native Cu oxide layer was found to be inhomogeneous over the particles, and its thickness was strongly dependent on particle size. The thickness ranged from around 22–67 nm for Cu powder with a 10 µm average particle size (APS) and around 850–1050 nm for one with less than 149 µm.

Application of a Cryo-FIB-SEM-μRaman Instrument to Probe the Depth of Vitreous Ice in a Frozen Sample

The development of instruments combining multiple characterization and imaging tools drove huge advances in material science, engineering, biology, and other related fields. Notably, the coupling of SEM with micro-Raman spectrometry (μRaman) provides the means for the correlation between structural and physicochemical properties at the surface, while dual focused ion beam (FIB)-scanning electron microscopes (SEMs) operating under cryogenic conditions (cryo-FIB-SEM) allow for the analysis of the ultrastructure of materials in situ and in their native environment. In cryo-FIB-SEM, rapid and efficient methods for assessing vitrification conditions in situ are required for the accurate investigation of the original structure of hydrated samples. This work reports for the first time the use of a cryo-FIB-SEM-μRaman instrument to efficiently assess the accuracy of cryo-fixation methods. Analyses were performed on plunge-freezed highly hydrated calcium phosphate cement (CPC) and a gelatin composite. By making a trench of a defined thickness with FIB, μRaman analyses were carried out at a specific depth within the frozen material. Results show that the μRaman signal is sensitive to the changes in the molecular structures of the aqueous phase and can be used to examine the depth of vitreous ice in frozen samples. The method presented in this work provides a reliable way to avoid imaging artifacts in cryo-FIB-SEM that are related to cryo-fixation and therefore constitutes great interest in the study of vitreous materials exhibiting high water content, regardless of the sample preparation method (i.e., by HPF, plunge freezing, and so on).

Nuclear shell model for the ignition process of boron particle with binary surface oxide layer

Abstract The combustion of a single boron particle contains three continuous macroscopic processes, including ignition delay, ignition and combustion. Studies show that among these processes, the intermediate ignition process has the most complicated kinetic mechanism. On the other hand, conventional models have serious shortcomings to simulate this process. The main challenges in this regard are indeterminate initial thickness of the surface oxide layer, incomplete diffusion mechanism in the surface oxide layer, and neglected effects of the pressure and particle size on the control mechanism. In order to overcome the drawbacks of existing ignition models, a comprehensive kinetic ignition model is developed in the present study. To this end, the dual-beam focused ion beam micro/nanofabrication system is applied to etch and cut the heat-treated boron particles into slices. Then, the scanning transmission electron microscope with collateral energy dispersive spectrometer is applied for analyzing the microstructure and elementary composition of the slice. It is found that the average thickness of the surface oxide layer is higher than earlier assumed values, and the bidirectional diffusion mechanism of boron and oxygen in the liquid boron oxide is confirmed. During the model development, seven heterogeneous global reactions and two vapor-phase reactions are used in total for analyzing the kinetic mechanisms. In the present study, different indicators, including the vaporization process, O2 reactions, and H2O reactions on the internal and external surfaces of the binary oxide layer are investigated. Moreover, the conservation laws of mass and energy are used for the establishment of governing equations. Then, the accuracy of the developed model is evaluated by comparing the obtained results for the ignition time with that of the experiment, which shows a reasonable consistency between them. Then, the proposed model is applied to investigate the ignition characteristics of boron particles. Effects of different parameters, including the environment temperature, O2 concentration, and H2O concentration are studied on the ignition time. It is concluded that the established model, which is named as the nuclear shell (N-S) model is a powerful scheme to better understand the ignition mechanisms and characteristics of boron particles and can be further applied in the field of computational fluid mechanics.

3D focused ion beam sectioning of zirconium oxides in Zircaloy-4 for the characterisation of cracking

Sequential 2D sectioning and imaging using a dual-beam focused ion beam and scanning electron microscope has been used to characterise the interconnectivity of cracks in oxide films on Zircaloy-4. Evidence of cracks perpendicular to the metal/oxide interface have been observed in multiple oxides of different thicknesses. Such vertical features are considered critical in the provision of a percolation path between the environment and innermost band of lateral cracks (closest to oxide/environment interface), in between the lateral bands of cracks and between the final band of lateral cracks and the metal/oxide interface. Given the limited occurrence of cracks perpendicular to the metal/oxide interface, localised tensile surface strains have been considered as a potential source of initiation of these cracks.

Locally Condensed Water as a Solution for In Situ Wet Corrosion Electron Microscopy.

In microstructural corrosion studies, knowledge on the initiation of corrosion on an nm-scale is lacking. In situ transmission electron microscope (TEM) studies can elucidate where/how the corrosion starts, provided that the proper corrosive conditions are present during the investigation. In wet corrosion studies with liquid cell nanoreactors (NRs), the liquid along the electron beam direction leads to strong scattering and therefore image blurring. Thus, a quick liquid removal or thickness control of the liquid layer is preferred. This can be done by the use of a Peltier element embedded in an NR. As a prelude to such in situ work, we demonstrate the local wetting of a TEM sample, by creating a temperature decrease of 10 ± 2°C on the membrane of an NR with planar Sb/BiSb thermoelectric materials for the Peltier element. TEM samples were prepared and loaded in an NR using a dual-beam focused ion beam scanning electron microscope. A mixture of water vapor and carrier gas was passed through a chamber, which holds the micro-electromechanical system Peltier device and resulted in quick formation of a water layer/droplets on the sample. The TEM analysis after repeated corrosion of the same sample (ex situ studies) shows the onset and progression of O2 and H2S corrosion of the AA2024-T3 alloy and cold-rolled HCT980X steel lamellae.

Radial microstructural evolution in low burnup fast reactor MOX fuel

Examination of irradiated fast reactor mixed-oxide (MOX) fuel was conducted to analyze microstructural behavior and defect evolution as a function of radial position. The fuel analyzed in this work was irradiated to 3.4% fissions per initial metal atom (FIMA) in the Fast Flux Test Facility. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to analyze microstructural features and fission product secondary phases. A dual-beam focused ion beam (FIB)/SEM was used to prepare several samples along the fuel radius for transmission electron microscopy (TEM) defect analysis. The microstructure near the center of the fuel pellet showed separated grain boundaries accompanied by fission gas bubbles. The fuel pellet did not reach the threshold linear heating rate (LHR) needed for restructuring, formation of the columnar or equiaxed regions was not observed. Metallic fission products aggregated in pores along grain boundaries near the pellet center, but no metallic precipitates were visible in the outer region. Moreover, no appreciable amount of insoluble perovskite phase was formed. The dislocation loop density is highest near the fuel central void but decreases past the mid-point of the fuel radius likely due to the self-irradiation effects. Dislocation lines follow an inverse trend, with the lower density observed near the hottest regions and increasing in the pellet outer radius. These results indicate that the thermal gradient in MOX fuels heavily influences the localized fission product morphology and defect behavior.

Generating 3D virtual microstructures and statistically equivalent RVEs for subgranular gamma-gamma’ microstructures of nickel-based superalloys

Nickel-based superalloys are characterized by γ phase (FCC) nickel solid solution and γ′ phase (L12) intermetallics in their subgrain microstructures. The γ′ precipitates act as obstacles to the motion of dislocations and affect the mechanical behavior. It is important to capture the statistics of these microstructures in effective micromechanical models of these materials at the sub-grain scales. This paper develops a comprehensive suite of computational methods for generating optimally designed representative volume elements from experimental data on the material microstructure. Starting from 3D input datasets generated by a dual-beam focused ion beam (FIB-SEM) system, computational algorithms are developed to generate statistically equivalent virtual microstructures (SEVMs) and microstructure-based statistically equivalent representative volume elements (M-SERVEs) of γ-γ′ microstructures of Ni-based superalloys. Image processing, microstructural characterization, feature parametrization and representation of statistical distributions are conducted, along with novel interpolation schemes for statistically accurate precipitate packing. Subsequently, methods are developed for identifying the M-SERVE as an optimal subset of the SEVM, meeting requirements of statistical equivalence with respect to specific microstructural descriptors. Parametric representation of geometric features enable the incorporation of microstructural descriptors in parametrically homogenized constitutive models.

Insight into atmospheric pitting corrosion of carbon steel via a dual-beam FIB/SEM system associated with high-resolution TEM

Carbon steel experiences serious pitting corrosion in the atmospheres containing chloride ions. The present research is focused on revealing a three-dimensional chemical and microstructural insight into pitting corrosion with high spatial resolution nano-tomography by addressing 1018 carbon steel after exposed in the marine atmosphere. A well-defined core-shell structure for pitting corrosion products was revealed by using a combination of dual-beam focused ion beam system with scanning electron microscope (FIB/SEM) and high-resolution transmission electron microscope (HRTEM). The core is made of ferric chloride and the shell is constituted with iron oxides. Pitting corrosion is a self-catalytic process and the corrosion products grow to be porous flower-type structures. The porosity in the core provides a diffusion path for the ions. Inclusions promote pitting corrosion growth.

Simultaneous transport coefficient measurements for an individual bismuth wire embedded in a quartz template applying nano-fabrication

An individual bismuth wire (1.90 μm in diameter and 1554 μm in length) embedded in a quartz template was used to demonstrate simultaneous transport coefficient measurements for resistivity, magneto-resistivity, Hall coefficient, and Seebeck coefficient at 300 K. To obtain these measurements, we performed nano-fabrication using a dual-beam focused ion beam apparatus and fabricated local electrodes at the six-side and two-edge positions of the wire. Furthermore, using the measured values, we estimated carrier density, mobility, and Fermi energy. The obtained physical properties were similar to those of bulk bismuth, as expected, because of its large diameter.

Investigation of modelling and stress distribution of a coating/substrate system after an indentation test

A monolayer TiAlSiN coating and multilayer TiAlSiN nano-coating were deposited on an ISO P30 cemented carbide substrate, and their mechanical properties and fracture mechanisms were investigated by indentation and scratch tests. Mechanical models of indentation were proposed and optimized according to the results of numerical simulations, and it was found that these models could contribute to the quantitative analysis of stresses. The tensile stresses at the middle of the two adjacent corners along the indentation edge of the multilayer TiAlSiN nano-coating were found to be lower than those of the monolayer TiAlSiN coating, which could decrease the possibility of fracturing the coating. The crack initiation and propagation in the cross-sections of the coatings were observed using a dual-beam focused ion beam (FIB) machine to slot-cut the indentations, and the mechanisms of the crack propagation were analyzed. These cracks propagated perpendicularly to the interface in the monolayer TiAlSiN coating so the fractures could be mainly caused by tensile stress. With the multilayer TiAlSiN nano-coating, the orientation of the cracks observed in the same area were inclined at an angle rather than perpendicular to the interface, possibly due to the combination of tensile stress and shear stress at the interface between layers.

(Invited) Long-Term Stimulation Stability of IrOx Tip Metallization for the Utah Electrode Array

Long-term neural stimulation and recording efficacy is critical for clinical translation of neural interface technologies for use in therapeutic and diagnostic applications. As part of the DARPA HAPTIX program, we will be performing neural stimulation and recording in peripheral nerves of human subjects for more than 1 year, driving the need to improve the electrode metallization. This motivated the development of aggressive in vitro test to evaluate and improve the electrode material and its electrochemical stability. Utah Electrode Arrays (UEAs) with thin film IrOx electrode sites were pulsed at 1,667 Hz for 109 pulses using cathodal-first square biphasic current with 200 µs cathodal and anodal phases, a 100 µs interphase period, and amplitudes ranging from 100 µA to 2,100 µA. The pulse frequency and current werewell outside the typical ranges of 50 to 200 Hz and 1 to 100 µA used for physiological stimulation in order to accelerate failure. The voltage waveforms were sampled, and used in combination with electrochemical impedance spectroscopy and cyclic voltammetry to monitor the electrochemical properties of the electrodes. In addition, physical characterization of the electrodes was performed using back-scattered scanning electron microscopy and dual-beam focused ion beam analysis to investigate changes in surface morphology and the layer structure of the electrodes. Electrodes were stimulated at amplitudes up to 1,600 µA without observable degradation of the electrochemical properties or changes morphology. This stimulation had cathodic excursions (Emc) up to -2 V, which is well outside the water window. Some electrodes stimulated at amplitudes of 2,100 µA experienced modest degradation, and rarely an electrode was damaged catastrophically, with the majority of the tip metal delaminated. The damaged electrodes were investigated with electron microscopy. The dendritic morphology of the IrOx films was fully preserved except during catastrophic failure where no metal remained to observe. Delamination of the films, ranging from either small regions for the majority of samples to nearly complete metal delamination in rare cases, was observed. This strongly suggests that the IrOx material is highly robust and electrochemically stable during aggressive stimulation testing in-vitro. In addition, with cross-sectional analysis of the tip metal using db-FIB, we observed delamination between the tip metal and silicon electrode. We are investigating if this delamination is occurring through a physical process, or is the result of electrochemical etching of the silicon substrate resulting in undercutting of the electrode. In addition, we have developed a highly sensitive method for electrochemically testing neural electrode dielectric encapsulation. By fully encapsulating devices such that the active electrode region is completely insulated by the dielectric film under investigation, we have been able to resolve the time course and evolution of shunt pathways through dielectric defects with much great sensitivity. The fully encapsulated electrodes are characterized using EIS and current leakage measurements as a function of time to monitor changes in the encapsulation. In addition, electrodes that experience failure as, indicated by leakage currents > 1 nA, were decorated by electrochemical Cu plating to localize failures. Device lifetimes and failure modes characterized by these techniques will be reported.

Degradation mechanisms of AlGaN/GaN HEMTs under 800 MeV Bi ions irradiation

Degradation characteristics and mechanisms of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated under swift heavy-ion irradiation. AlGaN/GaN HEMTs were exposed to 800MeV Bi ions with fluence up to 5.28×1010ions/cm2. Post-irradiation measurement shows that the saturation drain current reduced by 15%, the maximum transconductance decreased by more than 27%, and both the positive and negative gate characteristics degraded dramatically. By the off-state examination using the Photo Emission Microscopy (PEM), electroluminescence hot spots were found in the gate areas, which indicates new leakage passages produced by heavy-ion bombardment. Micro cross sections were prepared at hot spot areas by dual-beam focused ion beam (FIB) and examined using transmission electron microscope (TEM). The presence of latent tracks throughout the whole hetero-junction area was confirmed. Latent tracks, which related to the high density ionizing energy-loss, play a role of new leakage paths and account for the increment of gate leakage. Moreover, nonionizing energy-loss induced defects decrease the charge density in the two-dimensional electron gas (2DEG) and reduce the carrier mobility, leading to the degradation of device output performances.

Melting relations in the MgO–MgSiO3 system up to 70 GPa

Melting experiments in a binary system MgO–MgSiO3 were performed up to 70 GPa using a CO2 laser heated diamond anvil cell. The quenched samples were polished and analyzed by a dualbeam focused ion beam (FIB) and a field emission scanning electron microscope (FE-SEM), respectively. The liquidus phase and the eutectic composition were determined on the basis of textual and chemical analyses of sample cross sections. Our experimental results show that the eutectic composition is the Si/Mg molar ratio of ~0.76 at 35 GPa and it decreases with increasing pressure. Above 45 GPa, it becomes relatively constant at about 0.64–0.65 Si/Mg molar ratio. Using our experimental data collected at a wide pressure range up to 70 GPa together with previous experimental data, we have constructed a thermodynamic model of the eutectic composition of the MgO–MgSiO3 system. The eutectic composition extrapolated to the pressure and temperature conditions at the base of the mantle is about 0.64 Si/Mg molar ratio. The modeled eutectic composition is quite consistent with a previous prediction from ab initio calculations (de Koker et al. in Earth Planet Sci Lett 361:58–63, 2013), suggesting that the simple assumption of a non-ideal regular solution model can well describe the melting relation of the MgO–MgSiO3 system at high pressure. Our results show that the liquidus phase changes from MgO-periclase to MgSiO3-bridgmanite at 35 GPa for the simplified pyrolite composition (~0.7 Si/Mg molar ratio), while MgSiO3-bridgmanite is the liquidus phase at the entire lower mantle conditions for the chondritic composition (~0.84 Si/Mg molar ratio).

Compact nanostructured TiO2 deposited by aerosol spray pyrolysis for the hole-blocking layer in a CH3NH3PbI3 perovskite solar cell

Layers of crystalline TiO2 were prepared using aerosol spray pyrolysis at different deposition temperatures and times. The effects of the deposition parameters on the structural, morphological and optical properties were investigated. The extent of the crystallinity and surface morphology of TiO2 depended very much on the growth temperatures. At a gas flow rate of 5l/min, an inhomogeneous morphology with a cauliflower-like feature was observed for the films sprayed at 375°C whereas they became denser and homogenous with the quality of both crystallization and diameters becoming better when the sprayed temperature was being increased up to 450°C. The optical band gap of the TiO2 films was found to decrease from 3.66 to 3.61eV as the film thicknesses was increased from 80 to 450nm. The refractive index for electromagnetic waves having wavelengths between 450 and 800nm varied between 2.0 and 2.12 for the films with thickness of 250–360nm. There was a significant decrease to 1.93 when the thickness of the films was 450nm. A layer of the sprayed-on TiO2 was then used as the hole-blocking layer in an organic-inorganic CH3NH3PbI3 perovskite based solar cell. A state-of-the-art transmission electron microscopy (TEM) technique for sample preparation was carried out using a dual focused ion beam (FIB) to investigate vertical morphology of the devices. It was found that the devices having the optimized thickness of the sprayed-on TiO2 displayed the power conversion efficiency (PCE) of 6.24%.

Effect of iron doping on the structural and optical properties of CeO2 films

Undoped and Fe-doped CeO2 thin films were fabricated by spin coating on F-doped tin oxide glass substrates followed by annealing at 500 °C for 15 h. The concentration of the dopant was varied from 2 to 10 % iron by weight (metal basic). Glancing angle X-ray diffraction and laser Raman microspectroscopy indicated that the films consisted only of CeO2 without any impurity phases of FexOy. The thickness of the films was determined by dual-beam focused ion beam milling to be ~150 nm for undoped film, while Fe-doped CeO2 films have thickness of ~200 nm for all dopant samples. The transmission spectra from UV–visible spectrophotometry showed a redshift of the absorption edge of the doped films, and the optical indirect band gap of the films decreased from 3.48 to 3.20 eV with increasing dopant concentration. Furthermore, the results have been proposed in the diagram of electron–hole trapping.

A study of dislocation transmission through a grain boundary in hcp Ti–6Al using micro-cantilevers

Micro-cantilevers with an equilateral triangular cross-section and an apex at the bottom were machined across grain boundaries in a Ti–6Al alloy sample using a focused ion beam (FIB). A nano-indenter was used to deform the micro-cantilevers. An <a> prism slip system was selectively activated in the upper grain by controlling the crystal orientation along the micro-cantilever. Specimens for transmission electron microscopy (TEM) were prepared from the deformed micro-cantilevers using dual-beam FIB. Bright field scanning transmission electron microscopy was used to investigate the process of slip propagation through the grain boundary.

Novel implementations of relaxation methods for measuring residual stresses at the micron scale

The advent of dual-beam focused ion beam - field emission gun scanning electron microscope (FIB-SEM) imaging and milling systems in combination with digital image correlation (DIC) analysis has made it possible to make very fine excisions and to record the resulting displacements with nanometre precision. This has led to novel implementations of macroscale residual stress measurement methods to the microscale, for example, micro-slotting, micro-hole drilling and so on. This review article outlines the different methods for residual stress mapping at the micron scale highlighting their relative merits and limitations with the aim of guiding the user towards the most appropriate techniques for a given application. These open new possibilities for residual stress measurement not just at the continuum scale but even for mapping stresses within grains. In addition, recent advances in milling throughput promised by new dual-beam plasma focused ion beam systems promise to help bridge the gap between the conventional macroscale methods and their micron-scale counterparts.

Characterization of Second-Phase Inclusion in Silicon Carbide Powders

A specific transition metal is used as a dopant element in silicon carbide powders to create the compensation effect. According to ab-initio simulation, vanadium, chromium, and manganese-induced compensation decrease the lifetime of the acceptor carrier and cause higher resistance when boron is the main impurity. Since the silicon carbide lattice has low solubility, excess metal precipitates on the surface of powders, particularly on the grain boundaries. The compositions of matrix and precipitation in the powders reveal obvious differences between the two areas. The X-ray diffraction (XRD) pattern shows the structure of VSi2, which indicates the existence of a second phase. Dual-beam focused ion beam (DBFIB) is used to further analyze the geography inside the powders. A cross-section view by DBFIB shows a second phase in the grains with a composition similar to that in the grain boundary. Metal-doped silicon carbide powders are used as starting materials to conduct crystal growth with better dopant element distribution.