High-resolution X-ray Radiography Research Articles

An X-ray Radiography Study of the Effect of Thermal Cycling on Damage Evolution in Large-Area Sn-3.5Ag Solder Joints

There is a need for next-generation, high-performance power electronic packages and systems utilizing wide-band-gap devices to operate at high temperatures in automotive and electricity transmission applications. Sn-3.5Ag solder is a candidate for use in such packages with potential maximum operating temperatures of about 200°C. However, there is a need to understand the thermal cycling reliability of Sn-3.5Ag solders subject to such high-temperature operating conditions. The results of a study on the damage evolution occurring in large-area Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates with Au/Ni-P metallization subject to thermal cycling between 200°C and 5°C are presented in this paper. Interface structure evolution and damage accumulation were followed using high-resolution X-ray radiography, cross-sectional optical and scanning electron microscopies, and X-ray microanalysis in these joints for up to 3000 thermal cycles. Optical and scanning electron microscopy results showed that the stresses introduced by the thermal cycling result in cracking and delamination at the copper–intermetallic compound interface. X-ray microanalysis showed that stresses due to thermal cycling resulted in physical cracking and breakdown of the Ni-P barrier layer, facilitating Cu-Sn interdiffusion. This interdiffusion resulted in the formation of Cu-Sn intermetallic compounds underneath the Ni-P layer, subsequently leading to delamination between the Ni-rich layer and Cu-Sn intermetallic compounds.

In situ diagnostic of liquid water distribution in cathode catalyst layer in an operating PEMFC by high-resolution soft X-ray radiography

To investigate the water transport behavior in the cathode catalyst layer (CCL) of an operating proton exchange membrane fuel cell (PEMFC), we examined transversal liquid water distributions in the CCL by using high-resolution soft X-ray radiography. The liquid water concentration gradient across the CCL was observed at a spatial resolution of 1.5μm. More liquid water accumulated in the CCL at the gas diffusion layer side than at the polymer electrolyte membrane side. The effect of accumulated liquid water in the CCL on the charge-transfer resistance was confirmed by electrochemical impedance spectroscopy. Liquid water accumulation in the CCL corresponds to deterioration of charge-transfer in the electrode.

X-ray radiography and tomography for monitoring the penetration depth of consolidants in Opuka – the building stone of Prague monuments

Abstract Stone consolidation is one of the major restoration treatments used for historical monuments preservation. A natural stone is a complicated heterogeneous porous system making the process of consolidation dependent on many variables. In practical restoration aims, for a given stone type, the selection of a suitable consolidant and consolidation conditions therefore remains a complex issue. The impregnation depth is a key factor for the assessment of the treatment efficiency. The combination of state-of-the-art hybrid pixel semiconductor detectors with newly available micro-focus X-ray sources makes possible to apply X-ray radiography, an ideal non-destructive tool, for penetration depth monitoring. In this study, high-resolution X-ray radiography is used for monitoring the penetration depth of organosilicon consolidants in the Opuka stone. The penetration depth has been evaluated in relation to the time of consolidation, stone porosity and consolidation mixtures properties. The exact influence of the X-ray contrast agent on the consolidation depth has been investigated as well. The information obtained provides supplementary knowledge on the suitability of investigated products for the treatment of this type of stone. In our study, the capabilities of X-ray radiography have been demonstrated on X-ray radiography simple projections, high-resolution computed tomography (CT) as well as on the dynamic processes monitoring. The results thus can serve also as an instrumental and methodological example applicable for consolidation monitoring of other stone types.

Scintillation efficiency and X-ray imaging with the RE-Doped LuAG thin films grown by liquid phase epitaxy

Very thin scintillator imaging plates have recently become of great interest. In high resolution X-ray radiography, very thin scintillator layers of about 5–20 μm are used to achieve 2D-spatial resolutions below 1 μm. Thin screens can be prepared by mechanical polishing from single crystals or by epitaxial growth on single-crystal substrates using the Liquid Phase Epitaxy technique (LPE). Other types of screens (e.g. deposited powder) do no reach required spatial resolutions. This work compares LPE-grown YAG and LuAG scintillator films doped with different rare earth ions (Cerium, Terbium and Europium). Two different fluxes were used in the LPE growth procedure. These LPE films are compared to YAG:Ce and LuAG:Ce screens made from bulk single crystals. Relative light yield was detected by a highly sensitive CCD camera. Scintillator screens were excited by a micro-focus X-ray source and the generated light was gathered by the CCD camera’s optical system. Scintillator 2D-homogeneity is examined in an X-ray imaging setup also using the CCD camera. ► Novel thin film scintillation detectors for high resolution 2D X-ray imaging. ► Comparison of LPE-made and mechanically prepared detectors. ► Practical setup for 2D X-ray micro imaging and its resolution capability.

Equation of state studies of warm dense matter samples heated by laser produced proton beams

Heating of matter by proton beams produced by short pulse, laser-solid target interaction has been demonstrated over the last ten years by a number of workers. In the work described in this paper heating by a pulse of laser produced protons has been combined with high-resolution soft x-ray radiography to record the expansion of thin wire targets. Analysis of the radiographs yields material properties in the warm dense matter regime. These measurements imply initial temperatures in the experimental samples over a range from 14 eV up to 40 eV; the sample densities varied from solid to a tenth solid density. Assuming an adiabatic expansion after the initial proton heating phase isentropes of the aluminium sample material were inferred and compared to tabulated data from the SESAME equation of state library. The proton spectrum was also measured using calibrated magnetic spectrometers and radiochromic film. The accuracy of the technique used to infer material data is discussed along with possible future development.

Investigation of water accumulation and discharge behaviors with variation of current density in PEMFC by high-resolution soft X-ray radiography

The management of water is a challenging issue to achieving high power density, long-term operation, and increased robustness in PEMFCs. Development of in-situ diagnostic techniques to understand the dynamic behavior of liquid water is needed. In this study, visualization of liquid water across the membrane electrolyte assembly (MEA) of operating PEMFCs was performed by using high-resolution soft X-ray radiography, with which was possible to distinguish the catalyst layer (CL), polymer electrolyte membrane, and gas diffusion layer (GDL) of the MEA. Dynamic liquid water transport behavior in the cathode CL and GDL was observed at high spatial and temporal resolution (1 frame/s). At low current density, generation of liquid water was recognized under the rib, while at high current density, liquid water was observed both under the rib and the channel. Temporal behaviors of liquid water accumulation in GDL under the rib and discharge process of liquid water to under the channel were also visualized.

High-resolution multi-MeV x-ray radiography using relativistic laser-solid interaction

When high intensity (≥1019 W cm−2) laser light interacts with matter, multi-MeV electrons are produced. These electrons can be utilized to generate a MeV bremsstrahlung x-ray emission spectrum as they propagate into a high-Z solid target positioned behind the interaction area. The short duration (<10 ps) and the small diameter (<500 μm) of the x-ray pulse combined with the MeV x-ray spectrum offers an interesting alternative to conventional bremsstrahlung x-ray sources based on an electron accelerator used to radiograph dense, rapidly moving objects. In experiments at the Omega EP laser, a multi-MeV x-ray source is characterized consistently with number of independent diagnostics. An unfiltered x-ray dose of approximately 2 rad in air at 1 m and a source diameter of less than 350 μm are inferred. Radiography of a complex and high area density (up to 61 g/cm2) object is then performed with few hundred microns spatial resolution.

Investigation of the Performance of Scintillator-Based CMOS Flat Panel Detectors for X-Ray and Thermal Neutron Imaging

Recent advances in of silicon-based CMOS (complementary metal-oxide-semiconductor) flat panel detectors have resulted in an attractive use of cost-effective radiation imaging devices for X-ray and neutron radiography/tomography system. Indirect detection methods consisted of an X-ray converter (or a scintillator screen) and photodiode arrays are more widely used in high resolution micro-CT (computed tomography), dental and industrial NDT applications. In this study, The terbium-doped gadolinium oxysulfide (Gd2O2 S:Tb, Gadox) scintillator screens with different thickness (several 30-140 m thickness) were directly coupled with a fiber-optic plate (FOP) of a commercially available CMOS imaging device for high resolution X-ray and thermal neutron radiography. The RadEye1 CMOS APS (active pixel sensor) imager having a large active area of 25 × 50 mm2 and 48 m pixel pitch was selected for X-ray and thermal neutron imaging with high resolution. The scintillation properties and imaging performance such as relative light output, linearity and spatial resolution were measured and evaluated under X-ray and thermal neutron beam exposure. The good linearity and high spatial resolution characteristics in X-ray and thermal neutron imaging experiments were achieved by using compact, cost-effective imaging detector with exchangeable Gadox scintillator screens.

Metastable and unstable cellular solidification of colloidal suspensions

Colloidal particles are often seen as big atoms that can be directly observed in real space. They are therefore becoming increasingly important as model systems to study processes of interest in condensed-matter physics such as melting, freezing and glass transitions. The solidification of colloidal suspensions has long been a puzzling phenomenon with many unexplained features. Here, we demonstrate and rationalize the existence of instability and metastability domains in cellular solidification of colloidal suspensions, by direct in situ high-resolution X-ray radiography and tomography observations. We explain such interface instabilities by a partial Brownian diffusion of the particles leading to constitutional supercooling situations. Processing under unstable conditions leads to localized and global kinetic instabilities of the solid/liquid interface, affecting the crystal morphology and particle redistribution behaviour.

Multi-disciplinary characterisation of a sandstone surface crust

Sandstones are used in many contemporary and historical buildings. With time, these buildings become dark and a surface crust is formed. Generally, these crusts mainly consist of minerals of which the constituents were mobilised from the interior of the stone, and material from the surrounding environment like air pollutants and dust. Many traditional techniques, such as optical microscopy, Scanning Electron Microscopy and the Drilling Resistance Measurement System, have been used to study these crusts in the past. The advent of more advanced techniques allows a better characterisation of sandstone surface crusts, commonly present in urban stone decay. High-resolution X-ray radiography and tomography, micro X-ray absorption spectroscopy (µ-XANES) and 2D micro-XRF scanning at laboratory and synchrotron sources, combined with more traditional research methods were applied to the study of a sandstone surface crust. This shows that the combination of these highly advanced techniques, which were not designed with the purpose of answering geological or environmental questions, can generate complementary 2D and 3D imagery of geological materials, opening up new approaches in the study of element migration inside porous geomaterials. Capsule: I hereby state that this work is significant to the rest of the scientific community.

High-resolution in-plane investigation of the water evolution and transport in PEM fuel cells

High-resolution synchrotron X-ray radiography is used to study the evolution of primary water clusters and the transport of liquid water from the catalyst layer through the gas diffusion layer (GDL) to the gas channels of a low temperature polymer electrolyte membrane (PEM) fuel cell. The liquid water content is quantified separately in the respective components; in the hydrophobic microporous layer (MPL) almost no liquid water can be observed. In the adjacent GDL, depending on the current density i 0 water clusters are formed which lead to a diffusion barrier for the reactant gases. Water transport dynamics are explained and a recently proposed eruptive mechanism describing the transport from the GDL to the gas channels is imaged in a pseudo three-dimensional representation [A. Bazylak, D. Sinton, Z.-S. Liu, N. Djilali, J. Power Sources 163 (2007) 784–792; S. Litster, D. Sinton, N. Djilali, J. Power Sources 154 (2006) 95–105; I. Manke, Ch. Hartnig, M. Grünerbel, W. Lehnert, N. Kardjilov, A. Haibel, A. Hilger, H. Riesemeier, J. Banhart, Appl. Phys. Lett. 90 (2007) 174105]. Based on a high temporal resolution the dynamics of the liquid water transport are observed; transient conditions resembling dynamic operation of the fuel cell are studied and an estimation of the time required to reach equilibrium conditions is given. The obtained spatial resolution of 3 μm is far below commonly used techniques such as neutron radiography or 1H NMR. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multi-phase flow are addressed.

Development of high spatial resolution x-ray radiography system equipped with multiwalled carbon nanotube field emission cathode

An x-ray radiography system (XRS) equipped with a multiwalled carbon nanotube cathode was developed. This system has ordinary scanning electron microscopy (SEM) functions available for the precise adjustment of focusing conditions including astigmatism and alignment of an electron beam. The electron beam diameter on a target, which is one factor that limits spatial resolution of XRS, was estimated as 50nm from the resolution of SEM images obtained. X-ray images were taken as a demonstration of the XRS, and clear images with resolution higher than 700nm were successfully obtained.

Development of compact field emission scanning electron microscope equipped with multiwalled carbon nanotube bundle cathode

As the first stage of development of a nanofocused x-ray source for high-resolution x-ray radiography, a commercial compact scanning electron microscope (SEM) was modified by replacing the thermionic cathode and two magnetic condenser lenses with a multiwalled carbon nanotube (MWCNT) bundle field emission (FE) cathode and a special designed Butler electrostatic lens, respectively. This modification also brought about a downsizing to the electron optical system, only 192mm of distance between the cathode and the sample stage was achieved. The MWCNT bundle FE cathode continuously emitted electrons for over 9h under a base pressure of ca. 10−5Torr which was a severer pressure for steady FE. The performance of electron gun system consisted of the MWCNT bundle FE cathode and the special Butler electrostatic lens was demonstrated by SEM images taken under the poor vacuum condition at room temperature.

Relevance of 2D radiographic texture analysis for the assessment of 3D bone micro-architecture

Although the diagnosis of osteoporosis is mainly based on dual x-ray absorptiometry, it has been shown that trabecular bone micro-architecture is also an important factor in regard to fracture risk. In vivo, techniques based on high-resolution x-ray radiography associated to texture analysis have been proposed to investigate bone micro-architecture, but their relevance for giving pertinent 3D information is unclear. Thirty-three calcaneus and femoral neck bone samples including the cortical shells (diameter: 14 mm, height: 30-40 mm) were imaged using 3D-synchrotron x-ray micro-CT at the ESRF. The 3D reconstructed images with a cubic voxel size of 15 microm were further used for two purposes: (1) quantification of three-dimensional trabecular bone micro-architecture, (2) simulation of realistic x-ray radiographs under different acquisition conditions. The simulated x-ray radiographs were then analyzed using a large variety of texture analysis methods (co-occurrence, spectral density, fractal, morphology, etc.). The range of micro-architecture parameters was in agreement with previous studies and rather large, suggesting that the population was representative. More than 350 texture parameters were tested. A small number of them were selected based on their correlation to micro-architectural morphometric parameters. Using this subset of texture parameters, multiple regression allowed one to predict up to 93% of the variance of micro-architecture parameters using three texture features. 2D texture features predicting 3D micro-architecture parameters other than BV/TV were identified. The methodology proposed for evaluating the relationships between 3D micro-architecture and 2D texture parameters may also be used for optimizing the conditions for radiographic imaging. Further work will include the application of the method to physical radiographs. In the future, this approach could be used in combination with DXA to refine osteoporosis diagnosis.

Point x-ray source using graphite nanofibers and its application to x-ray radiography

A point x-ray emission was obtained from a diode configuration composed of a graphite-nanofiber cold cathode and a conical-shaped copper metal anode. When combined with a highly sensitive charge coupled device (CCD) camera with a scintillator, this x-ray source was then used to obtain x-ray transmission images of a tungsten mesh and an x-ray test chart. The spatial resolution of this x-ray radiography system was on the order of 10 μm and the acquisition time for these images was less than 10 s. This combination of the point x-ray source and the sensitive detection system offers a high-resolution x-ray radiography system.

Plasma and ion barrier for electron beam spot stability

High-current electron beams of small spot size are used for high-resolution x-ray radiography of dense objects. Intense energy deposition in the bremsstrahlung target causes generation of ions which can propagate upstream and disrupt the electron beam. We have investigated the use of a thin beryllium foil placed 1–2 cm in front of the target, which serves as a barrier for the ions but is essentially transparent to the incoming electron beam. Analysis and computer simulations confirm that this confinement method will halt ion propagation and preserve the spot size stability of the electron beam.

X-ray point-source projection photography with a laser-produced source

Laser-produced plasmas are shown to have characteristics useful for high-resolution subnanosecond x-ray radiography. Plasmas produced by 3-J subnanosecond 1.06-μm laser pulses focused onto an Al target emit x rays (≳ 1.5 keV) from a 50-μm-diam volume in the order of 1 nsec. Such sources may be useful to probe other plasmas.