5 Werkstofftechnik
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Im Rahmen der Erstellung einer fraktographischen online-Datenbank an der BAM wurden systematisch Schwingversuche an einem Sinterstahl vorgenommen, dessen Sinterdichte variiert wurde. Die erzeugten Bruchflächen wurden analysiert, mit der Literatur verglichen und Datensätze in die Datenbank eingefügt.
Sintered bioactive glass scaffolds of defined shape and porosity, e.g. made via additive manufacturing, must provide sufficient bioactivity and sinterability. As higher bioactivity is often linked to high corrosion and crystallization tendency, a certain compromise between sintering ability and bioactivity is therefore required. Groh et al. developed a fluoride-containing bioactive glass (F3), which allows fiber drawing and shows a bioactivity well comparable to that of Bioglass®45S5.
To study whether and to what extent the sinterability of F3 glass powder is controlled by particle size, coarse and fine F3 glass powders (300-310µm and 0-32µm) were prepared by crushing, sieving and milling. Sintering, degassing and phase transformation during heating were studied with heating microscopy, vacuum hot extraction (VHE), DTA, XRD, and SEM.
For the coarse glass powder, sintering proceeds slowly and is limited by surface crystallization of primary Na2CaSi2O6 crystals. Although the crystallization onset of Na2CaSi2O6 is shifted to lower temperature, full densification is attained for the fine powder. This finding indicate that certain porosity might be tuned via particle size variation. Above 900°C, intensive foaming is evident for the fine powder. VHE studies revealed that carbon species are the main foaming source.
A novel method to generalize kinetic data of viscous crack healing in glasses is proposed. The method assumes that crack healing progress is proportional to the healing time, t, and indirect proportional to viscosity, n. This way, crack length and crack width data, normalized to the initial crack length and plotted versus t/n, allow to compare crack healing progress for different cracks and healing temperatures in a master curve. Crack healing experiments conducted in this study demonstrate the applicability of this method for a commercial microscope slide glass.
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Pressure-assisted sintering (PAS), as known from large-scale production of low temperature co-fired ceramics, was used to sinter multilayers of Ca3Co4O9 green tape at 900 °C with different pressures and dwell times. In-situ shrinkage measurements, microstructural investigations and electric measurements were performed. Pressure-less sintered multilayers have a 2.5 times higher electrical conductivity at room temperature than dry pressed test bars with randomly oriented particles. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. Relative density increases from 57 % after free sintering for 24 h to 94 % after 2 h of PAS with 10 MPa axial load. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (at 25°C) improves by a factor of 15 compared to test bars with randomly oriented particles. The high temperature thermoelectric properties show the same dependencies. The smaller the applied axial load, the lower the relative densities, and the lower the electrical conductivity. Longer dwell times may increase the density and the electrical conductivity significantly if the microstructure is less densified as in the case of a small axial load like 2 MPa. At higher applied pressures the dwell time has no significant influence on the thermoelectric properties. This study shows that PAS is a proper technique to produce dense Ca3Co4O9 panels with good thermoelectric properties similar to hot-pressed tablets, even in large-scale production.
Thermoelektrische Materialien können durch die Nutzung des Seebeckeffektes einen Temperaturunterschied direkt in eine Spannung umwandeln. Calciumcobaltit (p-typ) und Calciummanagant (n-typ) sind 2 der vielversprechendsten oxidischen thermoelektrischen Materialien. Für die Entwicklung von kostengünstigen Multilayergeneratoren ist das Co-sintern dieser beiden Materialien notwendig und deshalb eine Anpassung der Sintertemperatur nötig. Calciummangant wird herkömmlicherweise zwischen 1200°C und 1350°C gesintert. Calciumcobaltit erfährt einen ungewünschte Phasenumwandlung bei 926°C, es kann allerding bei 900°C unter 7.5MPa zu 95% dicht gesintert werden. Demzufolge, ist eine Co-sintertemperatur von 900°C anzustreben. Aus diesem Grund wurden mehrere Strategien zur Absenkung der Sintertemperatur von Calciummanaganat untersucht. Zum einen die Zugabe niedrigschmelzender Additive, zum anderen die Zugabe von Additiven, die eine eutektische Schmelze bilden. Es konnte gezeigt werden, dass für Calciummanganat die Verwendung von eutektischen Schmelzen besser geeignet ist als die Verwendung von niedrigschmelzenden Additiven um die Sintertemperatur zu senken.“
Lowering the sintering temperature of calcium manganate CaMnO3 for thermoelectric applications
(2018)
Thermoelectric materials can convert waste heat directly into electrical power by utilizing the Seebeck effect. Calcium cobaltite (p-type) and calcium manganate (n-type) are two of the most promising oxide thermoelectric materials. The development of cost-effective multilayer thermoelectric generators requires the co-firing of these materials and therefore the adjustment of sintering temperatures. Calcium manganate is conventionally sintered between 1200 °C and 1350 °C. Calcium cobaltite exhibits an undesired phase transition at 926 °C but can be sintered to high relative density of 95 % at 900 °C under axial pressure of 7.5 MPa. Hence, co-firing at 900 °C would be favourable. Therefore, strategies for lowering the sintering temperature of calcium manganate have been investigated. Basically, two approaches are common: i) addition of low melting additives like Bi2O3-ZnO-B2O3-SiO2 (BBSZ) glass or Bi2O3, and ii) addition of additives that form low-melting eutectics with the base material, for example CuO. In this study, several low melting additives including BBSZ glass and Bi2O3, as well as CuO were tested regarding their effect on calcium manganate densification. Bi2O3 did not improve the densification, whereas BBSZ glass led to 10 % higher relative density at 1200 °C. An addition of 4 wt% CuO decreases the temperature of maximum sinter rate from above 1200 °C to 1040 °C. By reducing the particle size of the raw materials from 2 μm to 0.7 μm the maximum sinter rate could be further shifted 20 K towards lower temperatures and the sinter begin decreased from 920 °C to 740 °C. It is shown that eutectic phase formation is more effective in lowering sintering temperature and accelerating densification than low-melting additives.
Calciumcobaltit und Calciummanganat gehören zu den vielversprechendsten thermoelektrischen Oxiden im Temperaturbereich zwischen 600 °C und 800 °C an Luft. Mittels thermoelektrischer Generatoren kann ein Temperaturgradient direkt in elektrische Leistung umgewandelt werden. Für die kostengünstige Pulverherstellung von Funktionsmaterialien wird im industriellen Maßstab meist die Festphasenreaktion (bzw. Kalzinierung) verwendet. Da es sich dabei um einen Hochtemperaturprozess handelt, ist diese Kalzinierung sehr energieintensiv. In der Literatur werden sehr unterschiedliche Prozessbedingungen zur Pulversynthese thermoelektrischer Oxide genutzt. Soweit dem Autor bekannt, ist keine systematische Untersuchung des Einflusses der Pulversynthesebedingungen auf die thermoelektrischen Eigenschaften publiziert.
Deshalb wurde eine systematische Untersuchung des Einflusses der Pulversynthesebedingungen (Temperatur, Haltezeit, Partikelgröße, Wiederholungen) auf die thermoelektrischen Eigenschaften von Calciumcobaltit und Calciummanganat durchgeführt.
Es konnte gezeigt werden, dass sich ein höherer Energieeintrag während der Kalzinierung negativ auf die thermoelektrischen Eigenschaften auswirkt.
A wide range of solid-state synthesis routes for calcium manganate is reported in the literature, but there is no systematic study about the influence of the solid-state synthesis conditions on thermoelectric properties. Therefore, this study examined the influence of calcination temperature and calcination cycles on the Seebeck coefficient, electrical conductivity, and thermal conductivity. Higher calcination temperatures and repeated calcination cycles minimized the driving force for sintering of the synthesized powder, leading to smaller shrinkage and lower densities of the sintered specimens. As the electrical conductivity increased monotonously with increasing density, a higher energy input during calcination caused deterioration of electrical conductivity. Phase composition and Seebeck coefficient of sintered calcium manganate were not influenced by the calcination procedure. The highest thermoelectric properties with the highest power factors and figures of merit were obtained by means of reaction-sintering of uncalcined powder.
Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2018)
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
A numerical pre-study has shown that cracks in a flat sample featuring a drilled hole can be classified into one of three crack shape classes based on the combined evaluation of various types of test data.
Es wird eine neu entwickelte Methode zur Thermographiebasierten Rissmessung vorgestellt. Darüber hinaus wird eine numerische Vorarbeit präsentiert, die zeigt, dass anhand der gemeisamen Auswertung der Versuchsdaten aus unterschiedlicher Sensorik die Möglichkeit besteht, die unter Ermüdungsbelastung in Bohrlochproben auftretenden Risse in Geometriekategorien zu unterteilen.
Es konnte anhand einer numerischen Voruntersuchung gezeigt werden, dass anhand der kombinierten Auswertung der im Versuch verwendeten Sensorik eine Einteilung der unter Ermüdung in Bohrlochproben auftretenden Rissformen in verschiedene Hauptkategorien (Eckriss, Oberflächenriss, Durchgangsriss) möglich ist.
Due to the increasing global demand for pure silver, native wire silver aggregates in very high purities are gaining more industrial attention. Up to the present, no substantial metallurgical Investigation of natural wire silver exists in the accessible literature. To convey urgently needed cross-disciplinary fundamental knowledge for geoscientists and metallurgical engineers, twenty natural wire silver specimens from eight different ore deposits have been investigated in detail for the first time by EBSD (Electron Back Scattering Diffraction), supported by light microscopy and micro-probe analyses. The improved understanding of the natural silver wire microstructure provides additional Information regarding the growth of natural silver aggregates in comparison to undesired artificial growth on electronic devices. Clear evidence is provided that natural silver curls and hairs exhibit a polycrystalline face-centered cubic microstructure associated with significant twinning. Although the investigated natural wire silver samples have relatively high purity (Ag > 99.7 wt.-%), they contain a variety of trace elements such as, S, Cu, Mn, Ni, Zn, Co and Bi, As and Sb. Additionally, Vickers micro-hardness measurements are provided for the first time which revealed that natural silver wires and curls are softer than it might be expected from conversion of the general Mohs hardness of 2.7.
A shape evolution approach based on the thermally activated self-organization of 3D printed parts into minimal surface area structures is presented. With this strategy, the present communication opposes currently established additive manufacturing strategies aiming to stipulate each individual volumetric element (voxel) of a part. Instead, a 3D structure is roughly defined in a 3D printing process, with all its advantages, and an externally triggered self-organization allows the formation of structural elements with a definition greatly exceeding the volumetric resolution of the printing process. For enabling the self-organization of printed objects by viscous flow of material, functionally graded structures are printed as rigid frame and melting filler. This approach uniquely combines the freedom in design, provided by 3D printing, with the mathematical formulation of minimal surface structures and the knowledge of the physical potentials governing self-organization, to overcome the paradigm which strictly orrelates the geometrical definition of 3D printed parts to the volumetric resolution of the printing process. Moreover, a transient liquid phase allows local programming of functionalities, such as the alignment of functional particles, by means of electric or magnetic fields.
A constitutive model for the mechanical behaviour of single crystal Ni-base superalloys under high temperature conditions has been developed in the framework of a Cooretec project in cooperation with Siemens AG, MTU Aero Engines AG and University Bayreuth. In addition to the conventional material properties e.g. elastic constants, the model requires the parameters of the initial microstructure as an input. Thus, the γ’-precipitate size and the channel width of the γ-matrix were obtained from SEM micrographs. The model uses the slip system theory and describes the movement, multiplication and annihilation of dislocations in the channels. Furthermore, the cutting of precipitates is another mechanism contributing to the plastic flow. The evolution of the morphology due to rafting and its effects on the deformation have been implemented according to. The kinematic hardening is introduced as a stress tensor to realistically represent the strain hardening of arbitrary oriented single crystals.
The mechanical behaviour of single crystal specimens has been experimentally investigated in tension tests at different strain rates and in creep tests under various loads. The constitutive model has been calibrated based on the experimental data for temperatures of 950°C and 850°C and the [001] and [111] crystallographic orientations.
Finally, a micromechanical model was created to simulate the creep response of additive manufactured polycrystalline structures. An EBSD image is taken to obtain the grain geometry and their respective orientation. The grain boundaries are discretised using cohesive elements, whereas the single crystal model was applied to each grain in the representative volume. The polycrystal model is generated using Dream3D, NetGen and other software previously developed at the BAM.
We invented the first non-metallic, self-adhesive and dry biosignalling electrode. The PEDOT polymer electrode changes its aggregate state and conductivity by a light curing procedure. The electrode can be applied as a gel underneath hair without shaving. With the aid of blue light, the electrode can be hardened within a few seconds at the desired location on the scalp. The cured polymer electrode is highly conductive and can be applied on a very small location. Unlike other EEG electrodes, our electrode does not lose conductivity upon drying. Furthermore, our electrode strongly bonds to Skin and does not require any additional adhesive. Short circuits due to an outflow of gel are prevented with this technique. Therefore, the PEDOT polymer electrode is extremely well suited for applications that, up to now, have been challenging, such as non-invasive EEG recordings from awake and freely moving animals, EEG recordings from preterm babies in the neonatal intensive care unit or long-term recordings in the case of sleep monitoring or epilepsy diagnostics. We addressed two technical questions in this work. First, is the EEG recorded with polymer electrodes comparable to a standard EEG? Second, is it possible to record full-band EEGs with our electrodes?