Tantalum layer

Schwoerer et al. used tantalum for both accelerator and radiator but in two separate targets and the Jena 15 TW laser system at an intensity of 102°Wcm-2. A 5() pm first foil employed for the accelerator and a second slab of 1mm that acted as radiator [89]. As a result, 104 fission events per Joule of laser energy of 232Th and 238U, placed behind the second tantalum layer, were obtained with a reaction rate of the order of 1 event per laser shot. 

Tantalum—Layer of 40% HF on cone. H2SO4 0.5-30 volts dc. Hydrogen embrittlement is produced by ae. 

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

Mass transport may influence material growth in ionic liquids. 1-Butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, for example, is, at room temperature, about 60 times more viscous than water. At temperatures above 150°C its viscosity is similar to most molecular solvents at ambient conditions. Indeed, temperatures between 150 and 200 °C were best to deposit tantalum from TaFs in the presence of LiF. One has to keep in mind that the deposition of tantalum from TaFs or an anionic complex delivers one Ta atom and 5-7 fluorides. If the deposition is too fast F may not diffuse rapidly enough from the surface to the bulk of the solution and may be trapped in the deposit. This might explain why we only got crystalline tantalum layers at low current densities. 

As was shown in Chapter 4, elemental tantalum can be electrodeposited in the water- and air-stable ionic liquid [Pyi,4] TFSA at 200°C using TaFs as a source of tantalum [ 15,16]. The quality of the deposit was found to be improved upon addition of LiF to the deposition bath. At room temperature only ultrathin tantalum layers can be deposited as the element. The electrodeposition of tantalum was investigated by in situ STM to gain insight into the electrodeposition process. 

The tantalum layer were prepared by PVD on previously polished substrates. The substrates are degassed under argon atmosphere (450°C, 0.2 Pa for 60 min) and then ionically cleaned (18 min, V = 200 V) prior to the deposition stage at 300°C (0.2 Pa, 4 kW, -50 V polarization). The layer obtained is dense but has a number of defects as nodules and craters dispersed over it s surface. 

The ratio between strain at separation debonding to strain on cracking (e t aack) is smaller than the same ratio for the SiC/steel reference system for thick tantalum layers (SiC/ Taj 2 niuch higher for Ta/steel and... 

The methods applied to assess mechanical stability give proof of the beneficial role of the interposition of a PVD tantalum layer between the PECVD deposited ceramic layer and the substrate a factor of 2 gain in strain and in critical load with respect to the SiC/steel pair. However, it is nonetheless clear that the optimum thickness of the interlayer could not be established with certainty. At the present time, there does not appear to be any experimental explanation for this. The only factor that might be stressed is... 

FIGURE 41.7 GD-MS spectra of a tantalum layer containing fluorine. Reproduced with permission from Springer, Reference [58]. 

The glow discharge optical spectroscopy (GDOS) of the A 301L sample indicated the coating composition (Figs.7,8). The results of the depth profile analysis proved carbon enrichment in the interface of the steel substrate and the tantalum layer. 

Experiments to test the residual stress of the borided tantalum layer on steel have been started with an X-ray diffractive goniometer. Thick layers of TaB2 showed tensile stress crack formation. In the boron depleated TaB films compressive stress has been detected (Fig. 10). 

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