Oxidative protection characteristics

Other antioxidants may be substituted, but will impart different cured physical properties and oxidative protection characteristics that must be evaluated on a case-... 

Protection of Carbon-Carbon. A common aerospace material used in aircraft brakes, rockets, and reentry systems is carbon-carbon which comprises a carbon-fiber reinforcement and a carbon matrix. It has the highest specific strength of any material above 1000°C.I l But like all carbon materials, it is susceptible to oxidation and oxidizes rapidly above 500 C. Oxidation protection is provided by silicon carbide which is normally applied by pack cementation or CVD, usually in combination with other materials such as boron to promote self-healing characteristics. 

Due to the potency of the redox system of vitamin C s two forms, several further interactions with readily oxidizable nutrients exist. The redox system is able to protect sulfhydryl groups of proteins as well as polyunsaturated fatty acids from oxidation. The close relationship of the nutritive antioxidants vitamin C, vitamin E, and P-carotene can be seen not only in cellular antioxidative metabolism but also during absorption. In the presence of ascorbic acid the absorption of these antioxidants is improved due to the protecting characteristics of vitamin C. 

A number of studies have compared the performance of Ce to La, arrd found that Ce performs better than La on several aluminium alloys. However, the work of Yasakau et demonstrated that when aluminium alloys are exposed to lanthanide solutions (Fig. 7.4(a)) then in the case of cerium chloride solutions, cerimn deposits, such as Ce(OH)3 at cathodic intermetallic particles (Fig. 7.4(b)), form a porous oxide with poorer protection characteristics than either La(OH)3 (Fig. 7.4) or Ce(OH) Ce(OH), which form denser oxides that provide better protection than Ce(OH)3, although an alternative view has been reported. This distinction may explain some of the discrepancies reported between Ce and La performance. Aramaki suggests that the better performance of Ce(OH) over... 

The temperature dependence of the conductivity has been investigated by several authors. The conductivity increases over one order of magnitude from helium to room temperatiue, but it doesn t follow a single activation mechanism. The last important matter is the electrode for n-type wafers. Ti, in combination with A1 or Au as oxidation protective layers, has been proved to provide a good Ohmic contact for n-type -Ga203 samples as shown in Fig. 5 (right). Later, the Ohmic characteristic of the Ti electrode can be improved a prior Si ion-implantation. The latest reported specffic contact resistance and resistivity are as low as 4.6 x 10 (1cm and 1.4 X 10 flcm, respectively. ... 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

Plasticity Retention Index. The oxidation behavior of natural mbber may affect both the processing characteristics and final vulcanizate performance, and the plasticity retention index (PRI) test can be used to give an indication of both. Natural antioxidants present in natural mbber give some protection and a measure of the efficacy of protection is given by PRI. PRI% = P q j Pq x 100, where Pq is the initial Wallace plasticity and P q is the... 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

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