Crystal structure and centromere binding of the plasmid segregation protein ParB from pCXC100

Plasmid pCXC100 from the Gram-positive bacterium Leifsonia xyli subsp. cynodontis uses a type Ib partition system that includes a centromere region, a Walker-type ATPase ParA and a centromere-binding protein ParB for stable segregation. However, ParB shows no detectable sequence homology to any DNA-binding motif. Here, we study the ParB centromere interaction by structural and biochemical approaches. The crystal structure of the C-terminal DNA-binding domain of ParB at 1.4 Å resolution reveals a dimeric ribbon–helix–helix (RHH) motif, supporting the prevalence of RHH motif in centromere binding. Using hydroxyl radical footprinting and quantitative binding assays, we show that the centromere core comprises nine uninterrupted 9-nt direct repeats that can be successively bound by ParB dimers in a cooperative manner. However, the interaction of ParB with a single subsite requires 18 base pairs covering one immediate repeat as well as two halves of flanking repeats. Through mutagenesis, sequence specificity was determined for each position of an 18-bp subsite. These data suggest an unique centromere recognition mechanism by which the repeat sequence is jointly specified by adjacent ParB dimers bound to an overlapped region

Modeling and validation of chemical vapor deposition for tungsten fiber reinforced tungsten

In nuclear fusion reactors there are extreme conditions for plasma facing components. Currently, pure tungsten (W) is used to withstand the enormous heat and particle fluxes, neutron irradiation and plasma erosion. However, a significant drawback is that W is inherent brittle and thus can fail without warning. Therefore, W fiber-reinforced composites (W$_{f}$/W) are currently being developed. These can be produced by coating W fabrics via chemical vapor deposition (CVD). The aim of this work is to provide a profound and quantitative understanding of this process so that the material properties can be further improved. Models for the W-CVD process have been developed utilizing the commercial software COMSOL Multiphysics, and validated against experimental results. As a highlight a new description of the reaction kinetics was proposed solving controversies in literature with respect to the reaction order of the precursor WF$_{6}$ [1]. An increased W$_{f}$/W strength can be reached by higher relative density and finer W grains. Experimental CVD parameter studies and infiltration simulations showed that both can be achieved by operating at low WF$_{6}$ gas flow rates, high H$_{2}$ gas flow rates, medium total pressures and low temperatures. As lower temperatures increase the needed deposition time exponentially, 723–773 K are recommended. In addition, it is important to avoid WF$_{6}$ depletion within fiber inter-spaces, as this will lead to rapidly decreasing deposition rates, remaining pores, and thus to a reduced relative density. The minimum necessary WF$_{6}$ gas flow rate can be calculated with the developed model. The theoretically optimized CVD process parameters were applied experimentally to produce new bulk 15-layer W$_{f}$/W. In addition to finer grains and a higher relative density, further successes were a larger fiber volume fraction, a reduced WF$_{6}$ demand and a more uniform macro-scaled coating thickness. On the microscale, the new parameters resulted in such an uniform deposition that the optimization of the W fiber positions within the fabric (CVD substrate) was simplified based on geometric equations. However, in practice, pores can still remain at certain locations due to fiber positional deviations. Concepts for reducing these deviations and also for improving a continuous W$_{f}$/W production are presented in the Outlook chapter

Improving the W Coating Uniformity by a COMSOL Model-Based CVD Parameter Study for Denser Wf/W Composites

Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall and armor material for future fusion devices. However, its intrinsic brittleness and its embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (Wf/W) composites exhibiting extrinsic toughening are being developed. A possible Wf/W production route is chemical vapor deposition (CVD) by reducing WF6 with H2 on heated W fabrics. The challenge here is that the growing CVD-W can seal gaseous domains leading to strength reducing pores. In previous work, CVD models for Wf/W synthesis were developed with COMSOL Multiphysics and validated experimentally. In the present article, these models were applied to conduct a parameter study to optimize the coating uniformity, the relative density, the WF6 demand, and the process time. A low temperature and a low total pressure increase the process time, but in return lead to very uniform W layers at the micro and macro scales and thus to an optimized relative density of the Wf/W composite. High H2 and low WF6 gas flow rates lead to a slightly shorter process time and an improved coating uniformity as long as WF6 is not depleted, which can be avoided by applying the presented reactor model

Large-Scale Tungsten Fibre-Reinforced Tungsten and Its Mechanical Properties

Tungsten-fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidates for the first wall and divertor armour material in a future fusion power plant. As the development of Wf/W continues, the fracture toughness of the composite is one of the main design drivers. In this contribution, the efforts on size upscaling of Wf/W based on Chemical Vapour Deposition (CVD) are shown together with fracture mechanical tests of two different size samples of Wf/W produced by CVD. Three-point bending tests according to American Society for Testing and Materials (ASTM) Norm E399 for brittle materials were used to obtain a first estimation of the toughness. A provisional fracture toughness value of up to 346MPam1/2 was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied. A maximum value of the J-integral of 41kJ/m2 (134.8MPam1/2) was determined for the largest samples. Post mortem investigations were employed to detail the active mechanisms and crack propagation

Fiber Volume Fraction Influence on Randomly Distributed Short Fiber Tungsten Fiber Reinforced Tungsten Composites

For future fusion reactors, tungsten (W) is currently the main candidate for the application as plasma‐facing material due to its several advanced properties. To overcome the brittleness of W, randomly distributed short W fiber‐reinforced W (Wf/W) composites have been developed using field‐assisted sintering technology (FAST). Herein, Wf/W materials with different fiber volume fraction (20–60%) are manufactured by FAST process to study the fiber volume fraction influence on the composite properties. Wf/W with ductile fibers and brittle fibers is produced using different tool setups during the production. Three‐point bending tests on prenotched samples, 4‐point bending tests, and tensile tests have been performed to determine the fracture behavior and flexural/tensile strength of the material. Wf/W materials with 30–40% fiber volume fraction exhibit a promising pseudoductile behavior, similar to fiber‐reinforced ceramic composites. However, Wf/W with 20% and >50% fiber volume fraction shows only a limited extrinsic toughening effect. In terms of flexural strength, with increasing fiber volume fraction, the tensile/flexural strength does not show a clear increasing tendency, or even lightly decreases

Fiber Volume Fraction Influence on Randomly Distributed Short Fiber Tungsten Fiber Reinforced Tungsten Composites

For future fusion reactors, tungsten (W) is currently the main candidate for the application as plasma‐facing material due to its several advanced properties. To overcome the brittleness of W, randomly distributed short W fiber‐reinforced W (Wf/W) composites have been developed using field‐assisted sintering technology (FAST). Herein, Wf/W materials with different fiber volume fraction (20–60%) are manufactured by FAST process to study the fiber volume fraction influence on the composite properties. Wf/W with ductile fibers and brittle fibers is produced using different tool setups during the production. Three‐point bending tests on prenotched samples, 4‐point bending tests, and tensile tests have been performed to determine the fracture behavior and flexural/tensile strength of the material. Wf/W materials with 30–40% fiber volume fraction exhibit a promising pseudoductile behavior, similar to fiber‐reinforced ceramic composites. However, Wf/W with 20% and >50% fiber volume fraction shows only a limited extrinsic toughening effect. In terms of flexural strength, with increasing fiber volume fraction, the tensile/flexural strength does not show a clear increasing tendency, or even lightly decreases