Field‐assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure‐assisted sintering, and synthesis technique, which has been widely ...applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non‐equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.
The processing of novel inorganic materials at the lab scale or the rapid manufacturing of industrial products with higher output and reduced energy costs can be achieved by using versatile electric field‐assisted technologies. This review addresses their historical, technical, and scientific development, highlighting successes but also future research needs.
When one thinks of the Gram+ cell wall, the peptidoglycan (PG) scaffold in particular comes to mind. However, the cell wall also consists of many other components, for example those that are ...covalently linked to the PG: the wall teichoic acid and the cell wall proteins tethered by the sortase. In addition, there are completely different molecules that are anchored in the cytoplasmic membrane and span the cell wall. These are lipoteichoic acids and bacterial lipoproteins (Lpp). The latter are in the focus of this review. Lpp are present in almost all bacteria. They fulfill a wealth of different tasks. They represent the window to the outside world by recognizing nutrients and incorporating them into the bacterial cell via special transport systems. Furthermore, they perform very diverse and special tasks such as acting as chaperonin, as cyclomodulin, contributing to invasion of host cells or uptake of plasmids via conjugation. All these functions are taken over by the protein part. Nevertheless, the lipid part of the Lpp plays an as important role as the protein part. It is the released lipoproteins and derived lipopeptides that massively modulate our immune system and ultimately play an important role in immune tolerance or non-tolerance. All these varied activities of the Lpp are considered in this review article.
Dense (Hf, Ta, Nb, Ti, V)C- and (Ta, Nb, Ti, V, W)C-based high-entropy carbides (HEC) were produced by three different sintering techniques: gas pressure sintering/sinter–HIP at 1900 °C and 100 bar ...Ar, vacuum sintering at 2250 °C and 0.001 bar as well as SPS/FAST at 2000 °C and 60 MPa pressure. The relative density varied from 97.9 to 100%, with SPS producing 100% dense samples with both compositions. Grain size measurements showed that the substitution of Hf with W leads to an increase in the mean grain size of 5–10 times the size of the (Hf, Ta, Nb, Ti, V,)C samples. Vacuum-sintered samples showed uniform grain size distribution regardless of composition. EDS mapping revealed the formation of a solid solution with no intermetallic phases or element clustering. X-ray diffraction analysis showed the structure of mostly single-phase cubic high-entropy carbides. Hardness measurements revealed that (Hf, Ta, Nb, Ti, V)C samples possess higher hardness values than (Ta, Nb, Ti, V, W)C samples.
The wear behavior of SiC bonded diamond materials produced by liquid silicon infiltration in diamond preforms was investigated. The wear behavior in sand blasting tests (SiC abrasives, 5 bar ...pressure) was correlated with the microstructure. All SiC bonded diamond materials showed a wear, which was approximately 10 times less than the wear behavior of the reference SiC material.
Systematic changed microstructures were created by increasing the infiltration temperature. As the infiltration temperature increases, a graphite layer is formed at the diamond-SiC interface. At the highest infiltration temperature (1670 °C), the layer thickness reaches approx. 580 nm. The results show that wear resistance is not negatively affected by the graphite layer. On the contrary, for materials with a graphite layer thickness of up to 70 nm, the wear resistance increases by up to 30 %. The wear increases again only at the highest infiltration temperature. However, this is probably caused more by the internal damage to the diamonds and not by the graphite layer at the interface.
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•First time detailed analysis of the wear of SiC-bonded diamond materials under sandblasting conditions was carried out.•The wear resistance is appr. 10 times higher than that of sintered SiC Ceramic.•Thin graphitic interlayers at the diamond-silicon carbide interface do not reduce the wear resistance. On the contrary, they can even increase the wear resistance.
Staphylococcus aureus is a major human pathogen that causes a range of infections from acute invasive to chronic and difficult-to-treat. Infection strategies associated with persisting S. aureus ...infections are bacterial host cell invasion and the bacterial ability to dynamically change phenotypes from the aggressive wild-type to small colony variants (SCVs), which are adapted for intracellular long-term persistence. The underlying mechanisms of the bacterial switching and adaptation mechanisms appear to be very dynamic, but are largely unknown. Here, we analyzed the role and the crosstalk of the global S. aureus regulators agr, sarA and SigB by generating single, double and triple mutants, and testing them with proteome analysis and in different in vitro and in vivo infection models. We were able to demonstrate that SigB is the crucial factor for adaptation in chronic infections. During acute infection, the bacteria require the simultaneous action of the agr and sarA loci to defend against invading immune cells by causing inflammation and cytotoxicity and to escape from phagosomes in their host cells that enable them to settle an infection at high bacterial density. To persist intracellularly the bacteria subsequently need to silence agr and sarA. Indeed agr and sarA deletion mutants expressed a much lower number of virulence factors and could persist at high numbers intracellularly. SigB plays a crucial function to promote bacterial intracellular persistence. In fact, ΔsigB-mutants did not generate SCVs and were completely cleared by the host cells within a few days. In this study we identified SigB as an essential factor that enables the bacteria to switch from the highly aggressive phenotype that settles an acute infection to a silent SCV-phenotype that allows for long-term intracellular persistence. Consequently, the SigB-operon represents a possible target to develop preventive and therapeutic strategies against chronic and therapy-refractory infections.
Advancement of Tooling for Spark Plasma Sintering Giuntini, Diletta; Raethel, Jan; Herrmann, Mathias ...
Journal of the American Ceramic Society,
November 2015, Volume:
98, Issue:
11
Journal Article
Peer reviewed
Open access
A combined experimental and numerical study is conducted to investigate temperature nonhomogeneities within a Spark Plasma Sintering tooling setup. Radial thermal gradients through a powder compact ...are encountered, a cause of microstructural nonuniformities in sintered specimens, which tend to become more significant when increasing the setup's characteristic size. In the insulating silicon nitride powder compact employed for the experimental procedures, a double pyrometer arrangement detects a strong temperature disparity between the overheated die and the area adjacent to the tooling's axis. A previous finite‐element simulations campaign had individuated a possible solution in a novel punch design, consisting in the drilling of three concentric ring‐shaped holes according to a specific geometrical pattern, whose efficacy is here experimentally verified. Further punch optimization strategies are drawn, involving a refinement of the three‐rings geometry by linearly varying the drilled holes characteristic dimensions along the radial direction, or the selective coating and consequent insulation of the punch cross section with a thin layer of hexagonal boron nitride. Ideal configurations are identified, consisting in a concentration of the graphite punch's mass at its center by means of a tailored holes pattern, or in the coating of a portion of the conventionally shaped punch with boron nitride.
Materials with high thermal conductivity are required in a wide range of thermal management applications. Silicon carbide (SiC) bonded diamond materials, which can be produced without pressure, are a ...possible candidate for such applications. They exhibit thermal conductivities >650 W/(m·K) depending on the diamond content, diamond grain size and residual silicon content. It is difficult to experimentally determine the influence of these factors on the thermal conductivity separately, as changing one of these parameters affects the other parameters. Therefore, a method for generating digital microstructures of SiC- bonded diamond materials was developed and the thermal conductivity was calculated as a function of the grain size These data were compared with real structures.
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•A method for generating the 3D microstructure of SiC bonded diamond materials was proposed first time.•The thermal conductivity of SiC bonded diamond materials was modeled first time based on a 3 D model.•The model is in sufficient agreement with experiment and allows virtual materials design in the future.
During Field Assisted Sintering Technology (FAST) the temperature differences at two different positions were investigated using two pyrometers, an internal and an external one. Two substances, an ...electrically conductive (tungsten carbide) and a non-conductive material (96
wt.% silicon nitride with 2
wt.% alumina and yttria) were used to monitor the temperature differences between both pyrometers during heating, sintering shrinkage and dwell time by varying die geometry and heating rate. It was shown that the temperature distribution is strongly influenced by the electrical conductivity of the material as well as by tool design and setup. The alpha–beta transformation of silicon nitride was analyzed to predict the radial temperature distribution within the sample. For comparison and for visualization a dynamical FE model including piston movement for simulating sintering shrinkage was introduced. With this, a complete time dependent FAST run could be simulated. The modeled differences in temperature distribution are in good agreement with real temperature measurements as well as phase analyses.
A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in ...focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review.
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