Gadolinium-Doped Ceria (GDC) is a prospective material for application in electrochemical devices. Free sintering in air of GDC powder usually requires temperatures in the range of 1400 to 1600 °C and dwell time of several hours. Recently, it was demonstrated that sintering temperature can be significantly decreased, when sintering was performed in reducing atmosphere. Following re-oxidation at elevated temperatures was found to be a helpful measure to avoid sample failure. Sintering temperature and dwell time can be also decreased by use of Spark Plasma Sintering, also known as Field-Assisted Sintering Technique (FAST/SPS). In the present work, we combined for the first time the advantages of FAST/SPS technology and re-oxidation for sintering of GDC parts. However, GDC samples sintered by FAST/SPS were highly sensitive to fragmentation. Therefore, we investigated the factors responsible for this effect. Based on understanding of these factors, a special tool was designed enabling pressureless FAST/SPS sintering in controlled atmosphere. For proof of concept, a commercial GDC powder was sintered in this tool in reducing atmosphere (Ar-2.9%H2), followed by re-oxidation. The fragmentation of GDC samples was avoided and the number of micro-cracks was reduced to a minimum. Prospects of GDC sintering by FAST/SPS were discussed.
The microstructure and mechanical properties of high‐speed steel AISI M50 (80MoCrV42‐16, Mat. Nr. 1.3551), produced by laser powder bed fusion (LPBF), are analyzed. The mechanical properties in hardened and tempered condition are characterized by hardness, fatigue strength, and toughness and compared with the properties of conventionally produced samples. Moreover, the effects of an additional posttreatment by hot isostatic pressing (HIP) on the microstructure and mechanical properties are investigated. Dilatometric testing is used to investigate the influence of the different initial microstructures on hardening. All heat‐treated samples expose a fine martensitic microstructure with high hardness. The conventionally produced samples show a band‐like orientation of carbides due to the production by vacuum induction melting and vacuum arc remelting followed by a hot working process. This carbide structure is bypassed by the rapid cooling in the LPBF process. The LPBF samples show a comparable hardness after hardening and tempering to the conventionally produced material. In heat‐treated state, the LPBF samples show a low fatigue strength. Posttreatment by HIP included in the heat‐treatment chain significantly increases the fatigue strength. Nevertheless, the fatigue strength is still lower compared with the reference material. Both LPBF grades show a low toughness compared with the reference material.
In the last years, a lot of ceramic materials were developed that, at higher temperatures, have a high electrical conductivity and a high conductivity of oxygen ions. Such mixed ionic/electronic conductors can be used to produce high‐purity oxygen. This work focuses on the realization of a pilot membrane module, with BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3‐δ) perovskite selected as the membrane material. An amount of 500 kg of powder was industrially fabricated, spray‐granulized and pressed into tubes. The best operation conditions concerning energy consumption were calculated, and a module reactor was designed operating at 850 °C, with an air pressure of 15–20 bar on the feed site and a low vacuum of about 0.8 bar on the permeate site. Special emphasis was placed on joining alternatives for ceramic tubes in metallic bottoms. A first laboratory module was tested with a membrane area of 1 m2 and then advanced to a pilot module with 570 tubes and a capability of more than 300 000 L of pure oxygen per day.
Reactive air brazing (RAB) is a promising and economic method for joining ceramic to metal as well as ceramic to ceramic. Hybrid joints gain importance in the field of challenging applications such as gas separation or solid oxide fuel cells. While the ceramic partner is often directly chosen due to its functionality, the metallic partner needs to be chosen carefully in order to avoid high residual stresses due to the differences in the thermal expansion coefficients. This leads to the restriction of the usage of a significant number of potential joining partners. Ceramic/metals components with unadapted thermal expansion behavior (Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-d /Crofer 22 H and 3YSZ/AISI 314) were joined by RAB. Cu-containing as well as Cu-free Ag-based filler metals were used. Analysis of the cross-section of brazed samples showed that the microstructure of the samples depends especially on the used filler metal. The melting process as well as the exothermic reactions leading to a distinct reaction zone could be observed in the differential scanning calorimetric measurements. While the microstructure of the joints is not significantly influenced by the base material, the strength of the BSCF/Crofer 22 H specimens is nearly four times lower than the strength of the specimens with the adapted thermal expansion coefficients (BSCF/AISI 314 and 3YSZ/Crofer 22 H). The reasons are residual stresses caused by the different thermal expansion coefficients. Finite element simulations assuming a viscoplastic material model show that an amount of stress can be reduced by relaxation within the silver based braze. But the volume of the high residual stresses is still larger in the BSCF/Crofer 22 H joints than that of the BSCF/AISI 314 joint resulting in a lower fracture strength.
The field of additive manufacturing (AM), and especially laser powder‐bed fusion (LPBF), is constantly growing. Process windows for a large variety of materials are already developed. Nevertheless, some materials are still difficult to manufacture with LPBF. One of these materials is the tungsten carbide/cobalt‐based hard metal (WC–Co), which is conventionally produced by powder metallurgy including liquid‐phase sintering. Most approaches to manufacture WC–Co with LPBF show a high porosity, undesirable phases in the microstructure, and inhomogeneous carbide distribution. However, the production of WC–Co cutting tools by LPBF will offer some benefits such as production of geometrically optimized inner cooling channels or optimized geometry of the cutting edges. Herein, WC with 17 wt% Co is processed by LPBF with a powder‐bed heating of 900 °C. Afterwards microstructure, density, and hardness are determined. In addition, X‐ray diffraction (XRD) analysis is performed to determine the phase composition. To investigate the edge‐holding properties of LPBF‐manufactured WC–Co cutting tools, stock removal tests are conducted on three different workpiece materials.
This study aims to understand the effect of the electrical field on microstructure evolution during field‐assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium‐doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain‐boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain‐boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain‐boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations.
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