Ni-Mn-Ga alloys can exhibit a thermoelastic phase transformation near room temperature, which is associated with the shape-memory effect (i.e., temperature-induced strain recovery after twinning) or superelasticity (i.e., strain recovery after a stress-induced phase transformation). This work uses a synchrotron X-ray transmission technique to investigate texture induced by applying magnetic fields during the phase transformation in polycrystalline Ni2MnGa. Synchrotron X-ray radiation is ideally suited for such investigations since the measurements are representative of twinning in the bulk, in contrast with measurements from conventional X-ray sources that represent surface measurements affected by surface relaxation. Magnetic texturing of polycrystalline Ni2MnGa, by cooling through the phase-transformation in the presence of a magnetic field, has potential to lead to polycrystalline materials with more compatible field- induced strains and hence increased twin boundary mobility upon application of a magnetic and/or stress field.

The structural properties of metallic nanowires prepared within the pores of nanoporous alumina membranes are investigated using EXAFS, XANES and High Energy X-Ray Diffraction (HED). Gold, silver, copper and iron nanowires show the same structure and atom-atom distances as the corresponding bulk metals. The structural properties of these metals do not change if the wire diameter is varied. Cobalt nanowires, however, show a size dependent mixture of fcc and hcp phases. The amount of fcc-Co, which in the bulk is only known as a high temperature modification, increases with decreasing wire diameter. This leads to interesting changes in the magnetic properties of Co nanowires.

The electronic structures of five polycrystalline YNi2−xCoxB2C (x=0, 0.05, 0.1, 0.15, and 0.2) borocarbide superconductors were studied by photoemission and photoabsorption spectroscopies and theoretical calculations. The valence-band (VB) photoemission spectrum is compared with the theoretical total and partial density-of-states (DOS) curves. The VB satellite is peaked at a binding energy (EB) of 6 eV. The Ni K-edge x-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. Extended x-ray absorption fine structure (EXAFS) spectra at the Ni and Co K edges are analyzed to yield the structural parameters. The decrease of the superconducting transition temperatures (Tc) with addition of Co dopant in these compounds is due to a decrease of the total DOS at the Fermi level (EF).

Single crystals of the in situ composite TaSi2-Si are candidates for wide bandpass monochromators and analyzers for x-ray experiments. Two large crystals (32 × 39 × 7 mm3 and 32 x 39 x 30 mm3) were studied in detail using 115 keV synchrotron radiation at the BESSRC beamline at the APS. For transmission geometry, the Si(111) reflection and 39 mm crystal thickness, rocking curve widths of 95 arc sec with reflectivities of 50% were measured over the entire crystal. Turning the crystal to an angle of 45° to the incident beam and translating it through the beam allowed investigation of the influence of crystal thickness on diffracted intensity. For sample thicknesses ranging between 5 mm and 12.5 mm, the full width at half maximum (FWHM) of the rocking curve increased linearly from 29 arc sec to 53 arc sec. The greatest gain in integrated intensity was obtained for 8-9 mm thickness where FWHM = 40 arc sec. If this crystal were used as a monochromator for synchrotron radiation, it would provide 40 times more intensity than a perfect silicon crystal and an energy bandwidth of 1 keV at 115 keV photon energy. With the increase of the rocking curve FWHM, a plateau across the peak maximum developed; its width increased from 5 arc sec to 25 arc sec when the crystal thickness increased from 5 mm to 12.5 mm, respectively. This plateau allows these crystals to be used as analyzer crystals in diffraction experiments, i.e., as very narrow slits to suppress background. A plateau width of about 5-20 arc sec for the analyzer crystal is needed to insure that the entire intensity scattered from a sample is delivered to the detector. The simple expedient of changing the crystal thickness alters the plateau width of the analyzer crystal and tunes its acceptance to the needs of the sample under study.

We have used synchrotron X-ray diffraction to study the thermal expansion of the Ag(111) surface. Throughout the temperature interval between 300 and 1100K, we observed that the separation between the first and the second atomic layers at the surface, 12 d, differs from its bulk counterpart, d, by less than 1%, indicating that the thermal expansion at the surface is similar to the one in the bulk. This result represents the first experimental confirmation of the predictions from molecular dynamics simulations, which indicate a small enhancement of the anharmonic effects at this surface.

An instrument equipped with total electron yield detectors was designed and constructed for in situ X-ray absorption spectroscopy (XAS) investigations in the soft X-ray range (100 eV ≤ hν ≤ 1000 eV) at elevated pressures (mbar range) and sample temperatures (T ≤ 1000 K) [1]. This allows, for the first time, XAS studies in a surface-sensitive mode of the light elements (Z = 3-15). Furthermore, the gas phase XAS and the surface-related XAS of the solid state phase can be collected simultaneously in order to correlate the gas/solid reaction rate with the surface electronic structure under working conditions in a flow-through mode.

The novel experimental tool represents a contribution to the experimental overcoming of the “pressure gap” in material science. In this work examples are presented belonging to the field of heterogeneous catalysis [2-4] and to the reactivity of diamond surfaces [5]. Additionally, prospects for in situ studies in material science will be given.

We demonstrate the feasibility of element-specific magnetization reversal studies applying x-ray magneto-optical Kerr effect (XMOKE) to lanthanide systems. In particular, we will first describe XMOKE spectra at 4d-4f excitation thresholds of Gd and Tb in some detail, before showing element specificity at the example of a Gd/Y/Tb-trilayer metal film. On the basis of experimental Tb M4,5 absorption spectra, we may anticipate substantial MO signals in specular reectivity at M4;5 edges of all lanthanide elements, provided they carry a non-vanishing net 4f-magnetization.

Grazing incidence small-angle x-ray scattering (GISAXS) allows to investigate precisely the microstructure of nanocomposite thin films containing metal nanocrystals produced using different synthesis techniques. We present results on the size, size distribution, shape, and correlation length of metallic nanoparticles embedded in different matrices fabricated by sequential pulsed laser deposition, magnetron sputtering, and ion-beam sputtering co-deposition. The morphology of the nanoparticles is discussed in terms of the different growth process that takes place in each case.

A series of martensitic, near-equiatomic NiTi shape-memory alloy wires was deformed to strains ranging from 1 to 40% up to stresses of 920 MPa. After deformation, the wires were exposed to a monochromatic, parallel beam of high energy x-rays oriented perpendicular to the wire axis. The transmitted low index diffraction rings show that martensitic texture is increasing with prestrain up to ε=15% after twinning is complete. Further prestraining in the plastic range lowers the texture again indicating that twinning- and plasticity-textures cancel partially each other. Also, deformed NiTi-wires were heated and cooled from 20°C to 200°C under a small constant stress of 6 MPa. The strain change due to the Two-Way Shape-Memory Effect was measured and correlated to the diffraction results.

The structural changes induced in single crystal silicon implanted with Krypton above the amorphisation threshold were studied by X-ray reflectivity together with Grazing Incidence Small Angle X-ray Scattering technique. Silicon samples were implanted with Krypton with two different ion energies. A well-defined layer, 220 nm thick of amorphous silicon, rich in Krypton, was formed below the top, undisturbed layer. A series of samples consist of as-implanted, relaxed, and a number of samples with increased level of defects induced by additional Kr implantation. Additional implantation caused changes in the films composition and thickness, which was well evidenced in reflectivity curve, while only minor changes of surface roughness and critical angle were detected in GISAXS spectra.

We report x-ray scattering studies of polarons and tilt ordering in the high-temperature, paramagnetic insulating phase of La0.7Ca0.3MnO3. The temperature dependence of scattering due to individual and correlated polarons was measured, and the latter was found to disappear at ∼400 K. The scattering due to tilt ordering, which was observed by tuning the incident photon energy near the La L1 edge, was also measured as a function of temperature. The destruction of tilt ordering at ∼690 K is believed to be associated with the orthorhombic-rhombohedral structural transition.

High-energy synchrotron x-rays were used to measure internal reinforcement strains in bulk metallic glass composites containing tungsten or tantalum particles during in-situ uniaxial tensile loading. Load transfer from the matrix to the stiffer but weaker particles was characterized in both their elastic and plastic regimes and compared to theoretical predictions.

The residual-stress-gradient distribution just below the surface of a material is an important factor to consider during the engineering and design of a component. With the availability of an intense energy-tunable synchrotron x-ray source, it becomes easier to analyze the stress gradient below the surface, using a multi-energy x-ray diffraction method. A program was developed to efficiently determine possible experimental parameters using a sample with a known stress gradient distribution. In addition, this program can also calculate the stress gradient distribution below the surface taking into account experimental results. It also includes a subroutine for calculating the x-ray absorption coefficients of all of the elements, generalizing it for use with any material. As an example, in the present study, the relationship between x-ray energy and the residual stress gradient is discussed according to the calculated result for a silicon nitride composition.

X-ray reflectivity using high-energy X-rays allows one to characterize interfaces between thick materials at nanometer scale. The technique combines the high penetration of X-rays allowing the crossing of the radiation through large thicknesses of material with the interface sensitivity of grazing angle techniques. In the case of a buried interface between two thick materials, the beam enters the sample through the side of one material and contributions of external surfaces are suppressed. Then, the technique is sensitive to the interface structure only. Examples are given using wafer bonding interfaces, both in the hydrophilic case (as used e.g. in Silicon-On-Insulator substrate fabrication) and in the hydrophobic case (Silicon/Silicon bonding).

In order to measure the residual strain (stress) in the silicon nitride based composites, synchrotron based x-ray diffraction was employed on the BESSRC beamline at the APS. The residual stress for both the baseline Si3N4 and the Si3N4-TiN composites were determined from the (441) and (531) reflections, obtained by applying the three-dimensional 2θ-sin2ψ method. In both the baseline Si3N4 and the Si3N4-TiN composites, after thermal shocking, compressive residual stresses were developed in both directions parallel and perpendicular to the specimen's surface. The average residual stresses in the direction parallel to the sample surface were much higher than in the perpendicular direction.

The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the strength and toughness of each composite. The results suggested that there should be a maximum thermal shock temperature, within the range of 1000 °C to 1100 °C, for improving the fracture toughness for both the baseline Si3N4 and the Si3N4-TiN composites. Also, the addition of the TiN appeared to improve both the strength and toughness of the baseline composition.

X-ray microtomography of the three-dimensional spatial distribution of crack opening as a function of applied load has helped clarify important processes of roughness-induced fatigue crack closure. Fracture mechanics dictates sample geometries which limit the crack opening sensitivity that can be obtained with microtomography, and there is a real need to be able to quantify smaller openings. Synchrotron phase contrast radiographs of an AA 2090 T8E41 (central 2 mm of a 12.7 mm thick plate) compact tension sample show the fatigue crack geometry clearly while absorption contrast radiographs of the same sample have difficulty even detecting the crack. The phase contrast expected in the projection images is determined from the microtomography-derived, three-dimensional crack geometry and compared to that observed experimentally. Good agreement is obtained between the phase contrast radiographs and the expected pattern of contrast.

Thallium based high temperature superconducting films were formed on LaAlO3 (LAO) substrates by thalliation of sputtered amorphous precursor films by reaction with Tl2O. High resolution strain measurements and diffraction topographic imaging studies were performed using the synchrotron at Stanford Synchrotron Radiation Laboratory (SSRL). From these results, we have reached preliminary conclusions regarding the effect of film strain and substrate twinning on the superconducting properties of Tl-2212 films on LAO substrates. High- resolution strain measurements showed a correlation between film strain and microwave properties. The relative strain in samples in which Q>105 at 5.55 GHz (at 50 K) was approximately 30% lower than that in samples in which Q<104 when tested under the same conditions. Examination of samples where Q<104 at 5.55 GHz (at 50 K) by SEM also indicated microstructure defects that are typical in films with a high level of residual stress. White beam x-ray diffraction topographic analysis was used to quantitatively measure the degree of twinning in the substrate. The results will discuss the relationship between the measured Q values of the microwave cavity and the strain state, microstructure and defect morphology in the superconducting film/substrate couple.

In many systems for which there are several atoms in the unit cell, the displacements of the atoms may be locally correlated even though there is no long range coherence. Such displacements can play an important role in determining various macroscopic properties. We consider several examples to demonstrate this phenomena - the local distortions in the colossal magneto-resistive (CMR) and charge-ordered manganites, magnetic field induced distortions in CMR materials that are connected to macroscopic magnetostriction, and correlated displacements of tetrahedral units within the negative thermal expansion material ZrW2O8. The distortions in the CMR materials observed using XAFS change rapidly just below Tc and are attributed to the formation of polarons as the temperature is increased through Tc. In the ferromagnetic state, the lattice is more ordered for CMR systems; consequently applying a magnetic field for T ∼ Tc should decrease the local distortions. Such an effect has been observed and is a much larger effect than the measured macroscopic magnetostriction. Finally, in ZrW2O8 the tetrahedral and octahedral units are found to be very rigid as expected. More surprising is that the width of the pair-distance distribution for the W-Zr atom-pair is also quite small, indicating that the W-Zr linkage is stiff. In contrast, for the W-W pair, the distribution width grows rapidly with T, indicating correlated displacements of two WO4 tetrahedral units.

Monomolecular films at the air-water interface can be investigated on the subnanometer scale with grazing incidence X-ray diffraction (GIXD) using synchrotron radiation. This surface semsitive technique utilizes the property of total external reflection of X-rays from a water surface : an evanescent wave generated within the film diffracts in the surface plane giving an image of the film reciprocal lattice. Three applications of GIXD are presented ranging from poorly to highly crystalline thin films. (i) Cholesteryl-L-glutamate forms a crystalline monolayer at the air-water interface within which the glutamate moieties are not closely packed. This system specifically incorporates hydrophobic amino acids from the subphase. (ii) Long-chain cholesteryl esters deposited on the water surface spontaneously self assemble, forming crystalline interdigitated bilayers. The molecular structure, solved at the atomic resolution, was found to be similar to the 3D counterpart. (iii) According to 2-D diffraction theory, the shape of Bragg peaks is related to the mechanical constants of the film. Rigidity of the film can be deduced from a detailed peak analysis for secondary short chain alcohols showing a softening of the monolayer close to melting.

EXAFS and XANES analysis have been applied in a study of ball-milled nano-crystalline iron-nickel alloys prepared with overall compositions spanning the two-phase region. EXAFS analysis was used to determine bond distances for like and unlike atom pairs in single phase face-centered cubic Fe-Ni alloys, demonstrating the presence of bond dilation for unlike neighbors. Using XANES analysis, the compositions of the two coexisting solid solutions were determined to characterize the ball-milled two-phase region. The compositions of the coexisting face-centered and body-centered cubic phases were found to be consistent with the overall composition of the mixture. The results obtained for the negative heat-of-mixing Fe-Ni system were used for comparison to the positive heat-of-mixing Fe-Cu system.