Low-temperature isotropic carbons

Electrode Assembly. This device consists of a specially machined Teflon electrode holder, two disc electrodes (only one is energized), and a clamp machined from acrylic plastic (Figure 3). The electrode discs are of low-temperature isotropic carbon alloyed with SiC (Carbo-metics, Austin, TX). They were originally developed for use in artificial heart valves (14), and are approximately 1.6 cm in diameter and 1.25 mm thick, and have the surface properties of glassy carbon. Treatment of the discs requires only polishing to a high lustre with diamond grinding compounds of 14,000 and 50,000 mesh. 

Pyrolytic (low-temperature isotropic) carbon, ultralow-tem-peratuie isotropic carbon, pyroliz polymers for carbon-carbon composites, pyroli fibers for fiber composites Composites... 

Cardiac valves with components fabricated from low temperature isotropic carbons (pyrolytic carbon) are successfully used clinically [23]. These materials are appropriate for such applications as mechanical valves which require long-term chemical inertness, smoothness, and wear-resistance. The reasons for the marked improvement in the performance (reduced thrombosis and thromboembolic stroke rates) of these newer vs. older style heart valves are not entirely understood, but are undoubtedly multifactorial and related to improved patient management and valve design, as well as to the nature of the carbon surface. The specific benefits conferred by pyrolytic carbons with respect to blood cell and protein interactions, resulting in a very low frequency of clinical complications, remain to be defined. The use of carbon coatings has been proposed for other devices, i.e., vascular grafts, although such devices have not yet been used clinically. 

Hepler, B. R., Weber, S. G., and Purdy, W. G., 1978, The behavior of an electrochemical detector used in liquid chromatography and continuous flow voltammetry. 2. Evaluation of low-temperature isotropic carbon for use as an electrode material, Anal. Chim. Acta 102 41-59. 

LTIC low temperature isotropic carbon MASIF surface forces adhesion method NMR nuclear magnetic resonance RNase ribonuclease... 

Samples of polystyrene were cut from Petri dishes (Falcon 1008, Falcon Plastics, Oxnard, California). Unalloyed low temperature isotropic (LTl) carbon sjwiples were obtained from the General Atomic Company (Pyrolite - registered trademark of General Atomic Company, now Carbo-Medics, Inc.). 

Recently, success was achieved in depositing pyrolitic carbon onto the surfaces of blood vessel implants made of polymers. This type of carbon is called ultra low temperature isotropic (ULTI) carbon instead of low temperature isotropic (LTI) carbon. The deposited carbon has excellent compatibility with blood and is thin enough not to interfere with the flexibility of the grafts [Park and Lakes, 1992]. 

Examples AI2O3, low-temperature isotropic (LTI) carbon ultra LTI carbon vitreous carbon Z1O2 Tissue attachment Mechanical Bioactive ceramics... 

Isotropic carbon, on the other hand, has no preferred crystal orientation and hence possesses isotropic material properties. There ate three types of isotropic carbon pyrolytic, vitreous, and vapor-deposited carbon. Pyrolytic carbons are formed by the deposition of carbon from a fluidized bed onto a substrate. The fluidized bed is formed from pyrolysis of hydrocarbon gas at between 1000 and 2500°C (Hench and Ethridge, 1982). Low-temperature isotropic (LTI) carbons are formed at temperatures below 1500°C. LTI pyrolytic carbon possesses good frictional and wear properties, and incorporation of silicon can further increase hardness and wear resistance (Bokros, 1978). 

A pyrolytic, low-temperature isotropic LTt) carbon is also used by depositing it on substrates such as metals. For instance, an artificial heart valve is composed of a metal coated with LTI carbon. It has not been found out so far why this carbon has a good blood compatibility... 

Low-cost carbon fibers are produced from an isotropic pitch with a low-softening point. The precursor is melt-spun, thermoset at relatively low temperature, and carbonized. The resulting fibers generally have low strength and modulus ( 35 - 70 GPa). They are suitable for insulation and filler applications. Their cost dropped to less than 20/kg in 1992. ... 

Further evidence for isotropic chemical shifts was found in variable temperature studies. The aromatic carbon ortho to the oxygen resolved into two peaks at low temperatures. The aromatic carbons meta to the oxygen have two peaks, but these peaks are not as well resolved. As the temperature was raised, the splittings in both coalesced into one peak, indicating that there is rapid sampling from two magnetic environments. This is indicative of the motion of the phenyl group with respect to the backbone. At lower temperatures, the methyl resonance broadens and eventually... 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

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