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Hot-Pressing at very high Temperatures
Published in Ian L. Spain, Jac Paauwe, High Pressure Technology, 2017
Synthetic graphite with a very high preferred-orientation of crystallite c-axes is produced by the hot-pressing (compression-annealing) of pyrolytic carbon [14, 15]. Pyrolytic carbon is a dense (2.2 Mg/m3) carbon made up of small graphite crystallites which have a high degree of preferred orientation of the c-axes, defined as the full width at half maximum intensity of the (002) orientation distribution function. This parameter, also called mosaic spread, is typically 45° for as-deposited pyrolytic carbons. Annealing pyrolytic carbon at temperatures of 3000 – 3500°C increases the crystallite size and greatly improves the preferred orientation, reducing the mosaic spread to as low as 3°. Compression-annealing, however, at temperatures of above 2800°C and pressures of 30 – 50 MPa (~4,350 – 7250 psi) can reduce the mosaic spread to values an order of magnitude lower than this [14, 15]. The compression-annealed pyrolytic graphite is not only useful in fundamental research on graphite, but has found widespread use as monochromators for both x-rays and thermal neutrons [33] because of the high efficiency of graphite in these applications and because of the high preferred-orientation and near-ideal mosaic structure of the graphite. A large flat graphite monochromator for thermal neutrons is shown in Figure 4 and a doubly-bent focusing monochromator for x-rays is illustrated in Figure 5. Highly oriented pyrolytic graphite has also been used successfully in polarizing beams of high energy (10 – 16 Gev) photons by an attenuation method.
Chapter 4 Biocompatibility and Tissue Damage
Published in B H Brown, R H Smallwood, D C Barber, P V Lawford, D R Hose, Medical Physics and Biomedical Engineering, 2017
Carbon-based materials are strong and are isotropic. They also have the advantage of low thrombogenicity. Specific types can be manufactured which have values of elastic modulus and density similar to those of bone. They have become quite widely used. For example, carbon-fibre-reinforced PTFE is used as artificial articular cartilage. Polysulphone carbon fibre is used for the manufacture of bone plates. Unlike metal bone plates, these have a similar stiffness to the natural bone. Pyrolytic carbon is used in the manufacture of heart valve prostheses (see Chapter 22).
Soft Tissue Replacements
Published in Joyce Y. Wong, Joseph D. Bronzino, Biomaterials, 2007
K.B. Chandran, K.J.L. Burg, S.W. Shalaby
In the last four decades, we have observed significant advances in the development of biocompatible materials to be used in blood interfacing implants. In the case of mechanical heart valve prostheses, pyrolytic carbon has become the material of choice for the occluder and the housing. The pyrolytic carbon is chemically inert and exhibits very little wear even after more than 20 years of use. However, thrombo-embolic complications still remain significant with mechanical valve implantation. The complex dynamics of valve function and the resulting mechanical stresses on the formed elements of blood appear to be the main cause for initiation of thrombus. More recent reports of structural failure with implanted mechanical valves and pitting and erosion observed on the pyrolytic carbon surfaces have resulted in investigations on cavitation bubble formation during valve closure. Along with further improvements in biomaterials for heart valves, detailed analysis of the closing dynamics and design improvements to minimize the adverse effects of mechanical stresses may be the key to reducing thrombus deposition. Improvements on mechanical heart valves or further developments in durable synthetic leaflet valves may also be vital for the development of TAHs for long-term implantation without neurologic complications.
Dynamic Modeling and Performance Analysis of a Two-Fluid Molten-Salt Breeder Reactor System
Published in Nuclear Technology, 2018
Vikram Singh, Matthew R. Lish, Alexander M. Wheeler, Ondřej Chvála, Belle R. Upadhyaya
Helium is used as the cover gas in the pump bowl and all other salt surfaces in the system. It also serves as the medium for sparging gaseous fission products from the salt. Small bubbles of helium (~0.5% salt volume) are injected into the salt in the suction line to the pump to encourage nucleation of 135Xe, 83Kr, and small quantities of other gases formed due to both fission and fission product decay. These gases are only slightly soluble in the high-temperature salt mixture and readily nucleate with the helium bubbles. The helium is then removed with its burden of xenon and krypton in a centrifugal separator in the line from the outlet of the heat exchanger to the reactor vessel.8 This mechanism is illustrated in Fig. 1 at the core fuel-salt outlet. With a 33% removal rate per fuel transit cycle (~15 s for MSBR), the time to process the complete fuel inventory would be ~45 s. It was shown that a poison fraction of 0.5% or less can be achieved for the MSBR design with the above removal rate.9 Additionally, the graphite assemblies are coated with pyrolytic carbon to reduce their permeability to gaseous fission products. Since only a small fraction of the fissions take place in the blanket, there is no need for a gaseous fission product removal system for the blanket stream.