Christoph Genzel

Rietveld refinement of energy-dispersive synchrotron measurements

Abstract In the past two decades the energy-dispersive diffraction (EDD) method has become a powerful tool in many fields of materials research such as residual stress, texture, and crystal structure analysis, because of its favorable ratio of a comparatively low experimental effort in form of a simple and fixed instrumental setup to a high information content included in the measured diffraction patterns. However, mainly due to the rather poor instrumental resolution only little work has been done so far to apply the well-established methods of diffraction line profile analysis to the EDD data. In the paper, a Rietveld program is introduced that allows for particle size and strain broadening analysis by refining the whole EDD äpattern. With the examples of synchrotron measurements performed on the materials science beamline EDDI at BESSY II on instrumental standard as well as samples exhibiting size and/or strain broadened diffraction lines, it is demonstrated that the generalized Thompson, Cox & Hastings approach (TCH) using pseudo-Voigt functions for describing the diffraction line profiles yields sound and reliable results on the materials microstructure. For a first proof of the theoretical assumptions this Rietveld program is based on, the Pawley approach was used to extract the peak intensities obtained from powder samples affected by microstructural broadening. An excellent agreement with the results of the size-strain round robin was obtained. Future enhancements of the program code which aim at its application to full residual stress and microstructure analysis in the near surface zone and also in the material voläume of polycrystalline materials are discussed.

The materials science beamline EDDI for energy-dispersive analysis of subsurface residual stress gradients

In April 2005 the materials science beamline EDDI (Energy Dispersive DIffraction), which the HMI operates at the Berlin synchrotron storage ring BESSY, started user service. The high energy white synchrotron beam up to about 150 keV used for the diffraction experiments is provided by a superconducting 7 Tesla multipole wiggler. Starting with some basic information on the technical parameters of the beamline, its set-up and measuring facilities, the paper focuses on the application of white beam diffraction to the analysis of residual stress fields in the near surface zone of polycrystalline materials. The concept of a program system is introduced, which we offer to our users for preparing and evaluating their measurements performed at the EDDI beamline.

Determination of Real Space Residual Stress Distributions σij(z) of Surface Treated Materials with Diffraction Methods Part II: Energy Dispersive Approach

The detection of near surface residual stress gradients in real space requires high depth resolution for any orientation of the diffraction vector with respect to the sample co-ordinate system. In order to meet this demand, the slits are no longer being fixed in the laboratory co-ordinate system as in strain scanning experiments but directly coupled with the sample. Hence, the gauge volume orientation within the sample remains constant and allows performing depth-resolved sin2ψ measurements in real space. The method’s accuracy is determined by the gauge volume definition, which is investigated in detail. Apart from the evaluation of the σ(τ) versus σ(z) relation, which is of fundamental interest in X-ray residual stress gradient analysis, the method will be shown to have a unique applicability in rather delicate sample geometries such as multilayer systems.

An Assessment of Subsurface Residual Stress Analysis in SLM Ti-6Al-4V

Ti-6Al-4V bridges were additively fabricated by selective laser melting (SLM) under different scanning speed conditions, to compare the effect of process energy density on the residual stress state. Subsurface lattice strain characterization was conducted by means of synchrotron diffraction in energy dispersive mode. High tensile strain gradients were found at the frontal surface for samples in an as-built condition. The geometry of the samples promotes increasing strains towards the pillar of the bridges. We observed that the higher the laser energy density during fabrication, the lower the lattice strains. A relief of lattice strains takes place after heat treatment.

Improvements in Energy Dispersive Diffraction in respect of Residual Stress Analysis

In residual stress analysis (RSA) using energy dispersive (ED) diffraction care has to be taken of the detector energy stability. For a given detector system it is demonstrated that the energy position decreases significantly with dead time. Correction of the RSA data both in reflection and scanning experiments allows a significant improvement in the reliability of RSA under different conditions. Owing the small diffraction angles in ED experiments, the effect of adjustment errors in reflection geometry is investigated revealing the need of a wide incoming beam combined with high collimation of the secondary beam. The differences in the used absorber materials are shown in respect of sample heating and beam widening due to diffuse scattering.

X-Ray Diffraction Analysis of Nonuniform Residual Stress Fields σii(τ) under Difficult Conditions

Thin hard coatings for wear protection usually do not consist of single layers but of stacks of alternating sublayers which have to meet different demands. With respect to X-ray residual stress analysis (XSA) such multilayer systems pose a series of challenges. In addition to those problems which generally arise in thin film diffraction like small layer thickness or strong texture, neighbouring sublayers with similar chemical composition may superimpose each other, or sublayers of identical structure and composition, which contribute to the same diffraction line, are separated by other sublayers. Starting from a formalism that yields the X-ray penetration depth τ in multilayer systems of arbitrary sublayer sequences, we show how a combination of measurements using ‘conventional’ photon sources available at any X-ray lab and synchrotron radiation allowing for wavelength tuning near the TiKβ absorption edge, can be used to evaluate the residual stress state in the top sublayer stack of a hard coating multilayer system deposited by chemical vapour deposition.

Influence of the Grain Size on the Properties of CH3NH3PbI3 Thin Films

Hybrid perovskites have already shown a huge success as an absorber in solar cells, resulting in the skyrocketing rise in the power conversion efficiency to more than η = 22%. Recently, it has been established that the crystal quality is one of the most important parameters to obtain devices with high efficiencies. However, the influence of the crystal quality on the material properties is not fully understood. Here, the influence of the morphology on electronic properties of CH3NH3PbI3 thin films is investigated. Postannealing was used to vary the average grain size continuously from ≈150 to ≈1000 nm. Secondary grain growth is thermally activated with an activation energy of Ea = 0.16 eV. The increase in the grain size leads to an enhancement of the photoluminescence, indicating an improvement in the material quality. According to surface photovoltage measurements, the charge-carrier transport length exhibits a linear increase with increasing grain size. The charge-carrier diffusion length is limited by grain boundaries. Moreover, an improved morphology leads to a drastic increase in power conversion efficiency of the devices.

Internal stresses and textures of nanostructured alumina scales growing on polycrystalline Fe3Al alloy

The evolution of internal stresses in oxide scales growing on polycrystalline Fe3Al alloy in atmospheric air at 700 °C was determined using in situ energy-dispersive synchrotron X-ray diffraction. Ex situ texture analyses were performed after 5 h of oxidation at 700 °C. Under these conditions, the oxide-scale thickness, as determined by X-ray photoelectron spectroscopy, lies between 80 and 100 nm. The main phase present in the oxide scales is -Al2O3, with minor quantities of metastable -Al2O3 detected in the first minutes of oxidation, as well as -Fe2O3. -Al2O3 grows with a weak 0001 fiber texture in the normal direction. During the initial stages of oxidation the scale develops, increasing levels of compressive stresses which later evolve to a steady state condition situated around 300 MPa.

Pressure Dependent Rapid Thermal Processing of CuInS 2 Thin Films Investigated by In-Situ Energy Dispersive X-ray Diffraction

The rapid thermal processing (RTP) of Cu-rich Cu/In precursors for the synthesis of CuInS 2 thin films is possible within a broad processing window regarding leading parameters like top temperature, heating rate, and Cu excess. The key reaction pathway for the CuInS 2 phase formation has already been investigated by in-situ energy dispersive X-ray diffraction (EDXRD) for various precursor stoichiometries, heating rates and top temperatures at sulphur partial pressure conditions which are typical for physical vapour deposition processes. According to the phase diagrams of the binary sulphide phases, the sulfur partial pressure strongly determines the occuring crystalline phases. However, a controlled variation of the maximum sulphur partial in a typical RTP experiment has not been carried out yet. In order to study the influence of this parameter a special RTP reaction chamber was designed suitable for in-situ EDXRD experiments at the EDDI beamline at BESSY, Berlin. In a typical in-situ RTP/EDXRD experiment sulphur and a Cu/In/Mo/glass precursor are placed in an evacuated graphite reactor. The amount of sulphur determines the maximum pressure available at the top temperature of the experiment. As the RTP process proceeds a complete EDXRD spectrum is acquired every 10 seconds and thus the various stages of the reaction path and the crystalline phases can be monitored. The first experiments show already a significant change in the reaction pathway and the secondary Cu-S phases which segregate on top of the CuInS 2 thin film during the reaction.