Metal penetration

The magnitude of the phase shift relates to the total depth of the metal penetrated and hence is a sensitive measure of the wall thiekness and loss of thiekness. 

Elsewhere, large block carbons are utilized as wall material, generally with thicknesses in the range of 1.5—2.5 m. However, the single-thickness blocks have a tendency to crack and spall because of high mechanical and thermal stress and lack of expansion provisions. To combat this problem, various exotic carbons have been developed to resist hot metal penetration and increase thermal conductivities, but it should be noted that these measures do not solve the cause of the cracking, which is a lack of provisions to accommodate differential expansion. 

Figure 4-426. Sketch of deepest pit with relation to average metal penetration and the pitting factor. (From Ref. [186].)...

Material Rate of metal penetration at various exposure depths (mm/y) Form of... 

Detailed studies about metal deposition from the gas phase onto SAMs have been published [108-110], The central question for the system substrate/SAM/deposit there (as well as in electrochemistry) is the exact location of the deposited metal On top of the SAM or underneath Three clearly different situations are easily foreseen (Fig. 31). (1) Metal on top of the SAM. Depending on a strong or weak chemical interaction between metal and SAM (e.g., functional end group of the SAM), the deposit will spread out on top of the SAM or it will cluster on the SAM. (2) Metal penetrating the SAM (e.g., at defects in the SAM) and connecting to the metal substrate underneath the SAM. This configuration is often pictured as a mushroom, with a thin connective neck and a large, bulky head. (3) Deposited metal is inserted be-... 

Reactive metals are of interest for two primary reasons (1) reaction with the uppermost part of the SAM which can drive uniform nucleation with no penetration and (2) for electropositive metals, injection of electrons into the SAM to create a favorable dipole at the metal/SAM interface for device operation. With respect to the first, as opposed to the results with non-reactive metal deposition, some reports of reactive metal deposition appear to show prevention of metal penetration with the avoidance of short-circuits across the M junction. In general, serious concerns remain that some of metal atoms react destructively with the SAM backbone to produce inorganic species, e.g., carbides and oxides in the case of aggressive metals such as titanium. 

Eor inert SAMs such as n-aUcanethiolates/Au, alkaline earth and alkali metal deposition on inert SAMs tends to exhibit low sticking coefficients of the nascent metal atoms due to quite weak interactions with the -CH3 terminus sometimes <10 of the impinging metal atoms stick to the surface while the rest scatter off the smface [23, 58]. Bammel and co-workers observed quite slow penetration of Na through this inert SAM [59]. In the case of Mg and Ca depositions on n-aUcanethiolate SAMs it was observed that while Mg does not react it does undergo continuous penetration thorough the SAM. In contrast, Ca does react to some extent resulting in calcium carbide species formation [56, 57]. In the case of K on an n-aUcanethiolate SAM the results are more complicated. For example, at 10 K atoms per SAM molecule, it has been reported that half of the deposited metal penetrates to the SAM/Au interface while the remainder is claimed to remain embedded within the SAM matrix [60], though such space is not available theoretically in a dense SAM. 

Whiskers can be incorporated into the metallic matrix using a number of compositeprocessing techniques. Melt infiltration is a common technique used for the production of SiC whisker-aluminum matrix MMCs. In one version of the infiltration technique, the whiskers are blended with binders to form a thick slurry, which is poured into a cavity and vacuum-molded to form a pre-impregnation body, or pre-preg, of the desired shape. The cured slurry is then fired at elevated temperature to remove moisture and binders. After firing, the preform consists of a partially bonded collection of interlocked whiskers that have a very open structure that is ideal for molten metal penetration. The whisker preform is heated to promote easy metal flow, or infiltration, which is usually performed at low pressures. The infiltration process can be done in air, but is usually performed in vacuum. 

ERM methods give a direct measure of metal penetration but can be of low accuracy, especially in slow-moving cooling systems, thus requiring long periods of measurement. As a result, for most systems the probes are permanently installed and the multichannel monitor transmits data directly to a control room computer, although hand-held equipment has become available in recent years. 

In this section, two examples are presented for the application of a technique of low-melting-point alloy (LMPA) impregnation that provides for a visualization of the invasion of a nonwetting fluid into the pore spaces in a typical porous article. The visualization can be linked to the modeling of mercury porosimeter curves using 3-D stochastic pore networks. This makes the quantification of pore structure more direct. Quantified structures can be visually examined against sample particle sections. The visual comparison can be made more precise by image analysis of the accessible porosity made visible by metal penetration over a series of pressures. 

Silica Catalysts. - Little problem is to be expected from depositing metals by simple cation exchange onto the surface of silica gel with appropriate control of pH to control the depth of metal penetration. Many high-surface-... 

From the microprobe analysis, the metal penetration parameter Qv can be derived. This parameter is defined as follows ... 

FIGURE 6.14. Flowchart of the fabrication process of tungsten heavy metal penetrators [6.26]. 

FIGURE 7.24. Schematic drawing of a heavy metal tank ammunition cartridge (1) windshield, (2) tungsten heavy-metal penetrator (subprojectile), (3) three-section sabot, (4) stabilizing fin, (5) propellant. By courtesy of TAAS— Israel Industries Ltd., Israel. 

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