Jon Almer

In situ small-angle x-ray scattering study of nanostructure evolution during decomposition of arc evaporated TiAlN coatings

Small-angle x-ray scattering was used to study in situ decomposition of an arc evaporated TiAlN coating into cubic-TiN and cubic-AlN particles at elevated temperature. At the early stages of decomp ...

Post-yield nanomechanics of human cortical bone in compression using synchrotron X-ray scattering techniques

The ultrastructural response to applied loads governs the post-yield deformation and failure behavior of bone, and is correlated with bone fragility fractures. Combining a novel progressive loading protocol and synchrotron X-ray scattering techniques, this study investigated the correlation of the local deformation (i.e., internal strains of the mineral and collagen phases) with the bulk mechanical behavior of bone. The results indicated that the internal strains of the longitudinally oriented collagen fibrils and mineral crystals increased almost linearly with respect to the macroscopic strain prior to yielding, but markedly decreased first and then gradually leveled off after yielding. Similar changes were also observed in the applied stress before and after yielding of bone. However, the collagen to mineral strain ratio remained nearly constant throughout the loading process. In addition, the internal strains of longitudinal mineral and collagen phases did not exhibit a linear relationship with either the modulus loss or the plastic deformation of bulk bone tissue. Finally, the time-dependent response of local deformation in the mineral phase was observed after yielding. Based on the results, we speculate that the mineral crystals and collagen fibrils aligned with the loading axis only partially explain the post-yield deformation, suggesting that shear deformation involving obliquely oriented crystals and fibrils (off axis) is dominant mechanism of yielding for human cortical bone in compression.

High-energy x-ray scattering quantification of in-situ-loading-related strain gradients spanning the dentinoenamel junction (DEJ) in bovine tooth specimens.

High energy X-ray scattering (80.7keV photons) at station 1-ID of the Advanced Photon Source quantified internal strains as a function of applied stress in mature bovine tooth. These strains were mapped from dentin through the dentinoenamel junction (DEJ) into enamel as a function of applied compressive stress in two small parallelepiped specimens. One specimen was loaded perpendicular to the DEJ and the second parallel to the DEJ. Internal strains in enamel and dentin increased and, as expected from the relative values of the Youngs modulus, the observed strains were much higher in dentin than in enamel. Large strain gradients were observed across the DEJ, and the data suggest that the mantle dentin-DEJ-aprismatic enamel structure may shield the near-surface volume of the enamel from large strains. In the enamel, drops in internal strain for applied stresses above 40MPa also suggest that this structure had cracked.

Synchrotron applications in archaeometallurgy: analysis of high zinc brass astrolabes.

Synchrotron X-rays were used to perform non-destructive transmission diffraction and fluorescence experiments on a group of 24 European and Islamic astrolabes dated between 13501720 A.D. in order to determine their compositions. A group of six astrolabes produced in Lahore between 1601-1662 A.D. were found to contain a mixed α + β brass microstructure, proving that the brass was produced by a co-melting technique rather than the traditional cementation process. The results also show evidence of dezincification, attributed to heavy annealing of the brass during astrolabe manufacture. This effect was so severe that accurate analysis of the bulk Zn composition could not be determined from the fluorescence results alone; transmission X-ray diffraction gives a more accurate measurement of the bulk Zn composition.

Internal strain gradients quantified in bone under load using high-energy X-ray scattering

High-energy synchrotron X-ray scattering (>60 keV) allows noninvasive quantification of internal strains within bone. In this proof-of-principle study, wide angle X-ray scattering maps internal strain vs position in cortical bone (murine tibia, bovine femur) under compression, specifically using the response of the mineral phase of carbonated hydroxyapatite. The technique relies on the response of the carbonated hydroxyapatite unit cells and their Debye cones (from nanocrystals correctly oriented for diffraction) to applied stress. Unstressed, the Debye cones produce circular rings on the two-dimensional X-ray detector while applied stress deforms the rings to ellipses centered on the transmitted beam. Ring ellipticity is then converted to strain via standard methods. Strain is measured repeatedly, at each specimen location for each applied stress. Experimental strains from wide angle X-ray scattering and an attached strain gage show bending of the rat tibia and agree qualitatively with results of a simplified finite element model. At their greatest, the apatite-derived strains approach 2500 με on one side of the tibia and are near zero on the other. Strains maps around a hole in the femoral bone block demonstrate the effect of the stress concentrator as loading increased and agree qualitatively with the finite element model. Experimentally, residual strains of approximately 2000 με are present initially, and strain rises to approximately 4500 με at 95 MPa applied stress (about 1000 με above the strain in the surrounding material). The experimental data suggest uneven loading which is reproduced qualitatively with finite element modeling.

Emerging Opportunities in High-Energy X-ray Science: The Diffraction-Limited Storage Ring Frontier

The worldwide march to electron storage rings with diffraction-limited photon properties in the X-ray regime is well underway. First out of the gate is MAX-IV in Sweden, scheduled for operation in 2016, followed by SIRIUS in Brazil in 2018; both are greenfield rings operating at 3 GeV with ~520 m circumference and emittances of ~250 pm-rad. They will be followed by the upgrade of ESRF, operating at 6 GeV with a target emittance of 150 pm-rad and operational date of 2020. The upgrade of the APS at Argonne National Laboratory (6 GeV, 60 pm-rad) is anticipated to follow shortly thereafter.

Strain and Phase Mapping of Ceramic Matrix Composites and Protective Coatings

Coatings are frequently required to provide oxidation protection for high temperature materials. Silicon carbide (SiC) coatings have been used to protect carbon-carbon composites on leading edges and zirconia coatings are used as thermal barriers on gas turbine aerofoils. The effectiveness and durability of these coatings is dependent on the residual strains created in these coatings during their formation or deposition and also during service. Tensile strains in the plane of the coating can lead to through thickness cracks that expose the substrate, while compressive strains can cause the coating to delaminate. This paper presents strain measurements of these high temperature material systems obtained with high energy X-ray diffraction. The diffraction also provided useful information on phase, crystallite size and texture as a function of depth. Tensile strains were found in the SiC coatings, and compressive strains were found in the zirconia coatings. Both these strains were parallel to their coatings’ surfaces. The differences in thermal expansion coefficients between the coatings and their substrates can account for both the compressive strain in the zirconia and the tensile strain in the SiC.Copyright

Creep Mechanisms in Bone and Dentin Via High-Energy X-ray Diffraction

Bone and dentin are highly complex, hierarchical composite materials with exceptional properties due to their unique composition and structure. They are essentially the same material with varied structural organization. They are three phase composites made up of a ceramic component, hydroxyapatite (HAP), a polymeric or proteinaceous component, collagen, and fluid filled porosity. A number of macroscopic studies have shown that both dentin [1-6] and bone [7-9] undergo visco-elastic, creep deformation and stress-relaxation behaviors. The problem with these bulk experiments is that they do not give information about which phase is contributing to the macroscopic creep or how. Some of these inquiries have suggested that the collagen is not responsible [9] and that creep in hard biological materials is primarily due to dislocations in the HAP mineral. On the other hand, others have said that collagen is completely responsible for the creep [8, 10]. These uncertainties make it essential to use techniques that allow for the study of the behavior of these very different components simultaneously during loading, determining their participation in creep. One such technique is synchrotron diffraction.

Shear strain of mineral crystals calculated using wide-angle X-Ray scattering (WAXS) techniques

The post-yield properties of bone mainly provide the estimation of its toughness and are directly related to the composition and structure of the tissue at basic building unit. Combining the recently developed synchrotron X-ray scattering techniques with a novel progressive loading scheme [1], a recent study has evinced that the internal strain of mineral and collagen phases in bone varies significantly before and after yielding of the tissue [2]. The minerals as well as collagen fibrils in bone have random and preferred orientation distribution according to the location within a body. Such orientation distribution will have significant influence on the deformation mechanisms of bone. This study hence aims to develop a methodology to determine local strain tensors in the mineral phase aligned in different orientations. To do so, the deformation of different lattice planes within individual mineral crystals is taken in account using the full WAXS spectrum.Copyright

Interface structure in solid oxide fuel cells by anomalous/high-energy SAXS/WAXS

Control of solid oxide fuel cell (SOFC) microstructure and chemistry is needed to optimize SOFC performance and cost. Within the composite electrodes, the variation in the triple-phase-boundary (TPB) morphology (where the gas-, electronand ion-conducting phases all meet), as a function of distance from the electrolyte layer, is particularly significant in defining electrochemical performance. Thus, characterization of the void and phase microstructures within the anode and cathode, at sufficient resolution to infer a quantitative characterization of the TPB interface, is highly desirable. This has become possible by utilizing the high brilliance and high x-ray energies available at a 3rd generation hard x-ray synchrotron source. Anomalous ultrasmall-angle x-ray scattering (USAXS), close to the Zr absorption edge, has been combined with high-energy smallangle and wide-angle scattering (HE-SAXS and HE-WAXS). For the first time this has provided correlated variations in void size distribution, interface surface area, and solid phase, to below 10 micrometers spatial resolution, and has enabled the ion-conducting YSZ phase to be distinguished from the voids and electron-conducting phase (LSM or Ni). From thses data, TPB properties can be inferred.