Ablators metal-base

Metals Metal-base ablators are a fourth major class of thermal protection materials. The most common type of ablator is a porous refractory skeleton containing a lower melting point metallic infiltrant. 

In production technologies, pulsed laser and electron beams are mostly applied for the ablation of material, generally for the production of holes. Figure 5 shows the process principle of laser ablation. A pulsed laser beam is focused onto a workpiece and absorbed on its surface. Typical penetration depths are less than 100 nm on metal surfaces. The material melts up and finally evaporates. Ablation is based on two mechanisms evaporation and melt expulsion. Depending on the duration of the laser pulse, the one or other mechanism is dominant. 

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Laser ablation ICP-AES and LA-ICP-MS were also proposed for rapid, direct analysis of tablets containing metallic species.194 Compendial heavy metal tests based on wet chemistry are among the most labor-intensive tasks in pharmaceutical laboratories. Both ICP-MS and ICP-OES have been proposed as alternatives for compendial methods and have the advantages of smaller sample size, element-specific information, quantitation, rapid sample throughput, and significantly improved accuracy.195-198... 

Excimer Laser Micromachining [132, 133] is a technique based on laser ablation. Currently, this process can routinely ablate vias as small as 6 pm in diameter in polymers, glass, ceramics and metals. The minimum size of the features that this method can produce is limited by diffraction and by heat/mass transport. Commercial instruments and services are available from a number of companies (for example, Resonetics, Itek). 

In conclusion, the developed techniques (based on laser ablation and electrical discharges in liquids) have been shown to be efficient for fabrication of metal and semiconductor nanoparticles of different composition and structure. 

The tetrameric Zr complexes [Zr(0H)2(H20)2L]4X8 (L = bipyridyl, phenanthroline, various Schiff-bases X = Cl, NCS) were the presumed products from the reaction of ZrOCl2 with various heterocyclic bidentate ligands.336 The interaction of H20 with Zr0(C104)2 or Zr0(N03)2 was probed by NMR spectroscopy.337,338 In the latter case, the cation [Zr4(0H)8(H20)i6]8+ (80) was proposed as the product.338 The related trimer [Zr02Ci2H8(/u2-0H)]3(//3-0)Li5(THF)g(H20)5 (81) was isolated from the hydrolysis product of an organometallic precursor. X-ray structural data confirmed the planarity of the six-membered Zr3(/r2-OH)3 core.339 In related work, protometric studies of Zr hydroxide complexes have probed thermodynamic stability,340 while FT IR and theoretical investigations have addressed the details of laser-ablated group IV metal atoms that... 

A comparative study of spark-ablation ICP-MS and GD-MS in the case of steel has been reported by Jakubowski and coworkers [536, 613], The RSFs for a number of trace elements and the measurement precision are very similar in both cases. Steel analysis by GD-MS benefits from the addition of 1% of H2 to the Ar discharge gas [614], the explanation for which is certainly complex. For certified reference steels, including superalloys, reliable analysis results can be obtained. The determination of Mo, Nb and Zr in steels by GD-MS was found to be affected by the formation of multiply-charged cluster ions (metal argides) [615]. A correction based on the assumption that the rate of formation of the singly-charged argide is the same for all analytes and coincident with that of FeAr+ was used. The capabilities of low resolution GD-MS were shown by the example of steel analysis [616], where detection limits were down to 1 ng/g and up to 30 elements could be determined. 

Kong et al (2000) have compared the sensitivity to NO2 and NH3 of (a) a CNTFET based on individual semiconducting carbon nanotubes with the sensitivity of (b) a CNTFET-based SWCNT network. The SWCNTs which composed this second device were synthesised by laser ablation. The resistance of device (a) increases and the resistance of device (b) decreases when exposed, respectively, to NH3 and N02.The responses of a CNTFET based on an SWCNT network to 1% of NH3 and 200 ppm of NO2 were approximately 50%. Compared with a CNTFET based on individual SWCNTs, the sensitivity is dramatically reduced. In the case of SWCNT networks, the response of semiconducting SWCNTs is averaged by the small response of metallic carbon nanotubes. Furthermore, due to the van der Waals... 

Ceramics Ceramic-base ablators constitute another class of heat shielding materials. They generally have high thermal efficiency, but this capability is difficult to realize because of their susceptibility to thermal stress failure. During thermal shock, the material may crack extensively and fail catastrophically. Placing the ceramic in a metal honeycomb tends to alleviate this problem by restricting any cracks to the outer walls of the cell structure. 

---