Jorgen Rufner

Electric Field Assisted Sintering (EFAS, also referred to as spark plasma sintering) is a powerful technology for the consolidation of powder materials. The high heating rate during the sintering process is critical for minimizing energy consumption, but it can also cause microstructure heterogeneities in sintered parts, such as spatially varied porosity. The examination of localized porosity usually requires the use of a scanning electron microscope with a carefully prepared surface. In this paper, photothermal radiometry is used to measure local thermal diffusivity and extract localized porosity of EFAS-sintered parts by using a percolation-threshold model. Applying this approach, we identified the radial position-dependent porosity variation in EFAS parts, which is likely formed due to the large temperature gradient during the sintering process. This approach has a unique advantage because it can measure samples with minimal or no surface preparation, enabling the possibility of in situ characterization in EFAS with proper system modification. Necessary modifications on the measurement approach for EFAS deployment and in situ characterization are also discussed.

Structural health monitoring is critical for components working in harsh environments to assessing their safety and integrity. This can be realized with advanced sensors, such as fiber optic sensors, embedded in these components. Recent studies have explored the use of ultrasonic additive manufacturing and laser-based additive manufacturing for fiber embedment. Fiber embedding using these techniques has been challenging for high-temperature materials commonly used in extreme environments, resulting in large gaps and voids around the embedded fiber. A good fiber-matrix bond is critical to guarantee free gas/liquid leaks as well as good temperature sensing and strain coupling. To this aim, this study proposed a novel fiber embedding technique using electric field-assisted sintering (EFAS). Sapphire fibers with a diameter of 125 μm and length of 55 mm were placed in SS316L powder, which was subsequently sintered using EFAS. To optimize the processing parameters for fiber embedment, a parametric study was scoped. The as-fabricated samples were first examined using optical transmission testing to inspect fiber integrity. After that, the samples were cross-sectioned for microscopy analysis to evaluate the quality of bond between the embedded fiber and the matrix. The density, microstructure, and hardness of the sintered SS316L were also studied to optimize EFAS parameters. The result shows that intact sapphire fibers can be embedded in high-density SS316L with good metallurgical bonds using the optimized EFAS parameters. Optical transmission inspection demonstrates the successful transmission of light through the embedded fibers.

Compact heat exchangers are of interest for a number of applications including advanced reactors. Alloy 617 is one of the top candidate materials for the gas-cooled reactor intermediate heat exchanger. Previous endeavors to diffusion weld Alloy 617 utilized hot pressing (HP). It was reported that grain boundary migration across the interface was hindered by extensive precipitation. Bonds of this nature have been observed to reduce the elevated-temperature mechanical properties compared to the wrought-product form. It was hypothesized that the electric current applied during electric-field-assisted sintering (EFAS) can overcome these challenges, resulting in improved diffusion welding (DW). This study investigated DW of Alloy 617 via EFAS. Stacks composed of three sheets that were 20 mm in diameter were welded using EFAS. Specimens were welded with an applied electric current, a pressure of 30 MPa, hold time of 30 min, and temperatures of 1050°C, 1100°C, and 1150°C. DW using HP as the zero-current analog of EFAS was also performed at the most promising EFAS conditions. Results revealed that both the applied electric current and temperature played a key role in precipitation and grain boundary migration in diffusion-welded Alloy 617. Precipitates were observed at the interface of the hot-pressed samples which limited grain boundary migration. Electric current was found to prevent precipitate formation along the interface at 1150°C. The electric current coupled with a temperature of 1150°C during EFAS resulted in significant grain boundary migration across the interface.

Magnesium aluminate spinel was sintered and annealed at 1300°C under an applied 1000 V/cm DC electric field. The experiment was designed such that current could be removed as a variable and just the effect of a noncontact electric field was studied. Enhanced grain growth was observed for both samples that were sintered or annealed after densification in the presence of an electric field. Grain‐boundary character distributions revealed that no microstructural changes were induced due to the field. However, the electric field was found to enhance the kinetic movement of cations within the lattice. Energy‐loss spectroscopy experiments revealed cation segregation resulting in regions of Mg‐rich and Al‐rich layers adjacent the grain‐boundary cores. The defects generated during segregation supported the generation of a space charge gradient radiating from the grain‐boundary core out into the bulk, which was significantly affected by the applied field. The interaction between the field and space charges effectively reduced the activation energy for cation movement across boundaries thereby enhanced grain‐boundary mobility and resultant grain growth.

This article reports a comparative characterization of ultrafine MgAl₂O₄ spinel nanoparticles synthesized by polymeric precursor (Pechini) and coprecipitation methods. The nanoparticles were evaluated in terms of purity and surface cleanliness, size distribution, state of agglomeration, and sintering behavior. Powders synthesized by the Pechini technique were highly agglomerated and revealed a bimodal particle size distribution centered around 12 and 27 nm. Thermal analysis and infrared spectroscopy measurements indicated that carbon species remained on the surface of the powders only to be released when temperatures exceeded 1000°C. Isothermal sintering of such nanopowders at 1300°C showed a maximum relative density of only 54%. MgAl₂O₄ synthesized via coprecipitation created small nanoparticles, around 5–6 nm after calcination at 800°C, with significantly less agglomeration. Compared with the precursor‐derived powders, excellent sinterability of the coprecipitated powders was obtained under the same sintering conditions. Relative densities above 90% were obtained after only 10 min, which further increased to greater than 95% after 20 min with no sintering aids or dopants. The results highlight the importance of purity and processing control to exploit the beneficial high sinterability of nanoparticles.

Using Field Assisted Sintering Technique/Spark Plasma Sintering the effect of heating rate on the sintering of zinc oxide at a temperature of 400°C has been investigated. For the highest heating rate of 100°C/min, relative density larger than 95% was achieved whereas at low heating rates only little shrinkage occurred. Hardness measurements, Transmission Electron Microscopy, and impedance spectroscopy revealed clear differences between heating rates. It was found that residual water is responsible for this behavior, enhancing particle rearrangement and diffusion kinetics.

Surface properties play an important role in understanding and controlling nanocrystalline materials. The accumulation of dopants on the surface, caused by surface segregation, can therefore significantly affect nanomaterials properties at low doping levels, offering a way to intentionally control nanoparticles features. In this work, we studied the distribution of chromium ions in SnO₂ nanoparticles prepared by a liquid precursor route at moderate temperatures (500°C). The powders were characterized by infrared spectroscopy, X‐ray diffraction, (scanning) transmission electron microscopy, Electron Energy Loss Spectroscopy, and Mössbauer spectroscopy. We showed that this synthesis method induces a limited solid solution of chromium into SnO₂ and a segregation of chromium to the surface. The s‐electron density and symmetry of Sn located on the surface were significantly affected by the doping, while Sn located in the bulk remained unchanged. Chromium ions located on the surface and in the bulk showed distinct oxidation states, giving rise to the intense violet color of the nanoparticles suitable for pigment application.