Helium bubbles

MOCVD Reactions. A great deal of interest has been generated by the availability of two metallo-organic titanium compounds, tetrakis-diethylamino titanium (TDEAT) andtetrakis-dimethylamino titanium (TDMAT). These precursors make possible the deposition of TiN at lower temperature.[ " k l These compounds are liquid at room temperature. A flow of helium bubbling through the warm precursor entrains the vapor into the deposition chamber. Deposition temperature is approximately 320°C. The following reactions occur ... 

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

One final test was conducted on a heat treated sample to elucidate the effect of heat treatment and sensitization on the fracture behavior of this steel. A CF sample was heated for 24 hours at 650°C to see if the fracture mode would continue to change with an even larger amount of carbide precipitation. Note in Figure 15-a this heavily sensitized steel has a fracture appearance that is completely dominated by small microvoids associated with carbides. The bimodal distribution of microvoids like those in Figure 7 has been eliminated. In fact, the fracture appearance is remarkably similar to that of the tritium-exposed-and-aged steels albeit at a different magnification (Figure 15-b). It appears that carbides in the microstructure affect the fracture mode in a similar manner as the decay helium bubbles but on a different scale. 

M. H. Tosten and M. J. Morgan, The Effects of Helium Bubble Microstructure on Ductility in Annealed and HERF 21Cr-6Ni-9Mn Stainless Steel , WSRC-TR-92-551, February, 1998. 

Anodic fluorination of thiols was reported [28] providing a route to perfluo-roalkyl derivatives of sulfur hexafluoride though the yields of products are not high. The process, achieved in a system of HF/NaF at a Ni anode with helium bubbling through the solution, leads to different perfluorinated products (Scheme 8). 

Systematic diffusion experiments were also conducted with self-supported zeolite wafers (7-14 mg cm ) which were activated at 10 Pa and 675 K for 1.5 h. Prior to contact with the sorbate, the IR cell was filled with dried helium as a carrier gas. Subsequently, one or two components (benzene or ethylbenzene), carried by helium bubbling through thermostatted saturators, could be admitted. A system of mass-flow controllers allowed for an independent change of the partial pressures while the total pressure could be kept constant [22]. The time required to pass the sorbate from the inlet valve to the place of the zeolite wafer was about 4 s. IR spectra were obtained in intervals as short as 0.37 s. 

Figure 2, Logarithms of absorbance (480 nm) are plotted vs. time for basic solution of an osmarin reacting with air. Key /, no pretreatment II, helium bubbled through prior to beginning reaction by adding strong base and III, O, gas pre ...

Airflow/helium bubble analog flow visualization. 

Af John Zink Company s COOLflozv Physical Modeling Facilify, a differenf flow visualization fech-nique has been successfully applied to scale model sfud-ies. In fhis technique, small quanfifies of minufe helium bubbles are suspended in fhe airflow and used as fracer spheres. The helium bubbles are neufrally buoyanf wifh a nominal densify close to fhaf of fhe air af ambienf femperafures. Figure 10.4 shows fhe helium bubble flow visualization technique applied to a scale model. 

The microcrystalline properties of metals are particularly influ ced by irradiation. Although low-alloy steel in modem reactor tanks are rather radiation resistant (provided they are free of Cu, P and S in urities), stainless steel (e.g. of the 18% Cr, 8% Ni type) has been found to become brittle upon irradiation due to the formation of microscopic helium bubbles, probably due to n,a reactions in Fe and inq)urities of light elem ts (N, B, etc.). This behavior is accentuated for metallic uranium in reactors because of the formation of fission products, some of which are gases. As a result of this radiation effect it is not possible to use uranium metal in modem power reactors, where high radiation doses are accumulated in a very short time. The fuel elemrats for power reactors are therefore made of nonmetallic uranium compounds. 

A. Hasegawa, M. Saito, S. Nogami, K. Abe, R.H. Jones, and H. Takahashi, Helium Bubble Formation Behavior of SiCf/SiC Composites After Helium Implantation, J. Nucl. Mater., 264, 355-58(1999). 

Figure 6. An Imacon camera sequence showing the initiation of explosion in a silver azide crystal by the collapse of a helium bubble. Frame interval, 0.1 nsec [28].

Under certain conditions, embrittlement can be enhanced by the presence of the helium bubbles (helium embrittlement). The accepted view is that this embrittlement is the result of stress-induced growth of helium gas bubbles at the grain boundaries. The bubbles eventually link up and cause intergranular failure. 

FIGURE 24.20 (a) Helium bubble density as a function of depth from the surface in Cu/V 50 nm and CuA 2.5 nm nanolaminates, (b) Hardness change of Cu/V nanolaminates as a function of the individual layer thickness—h. (From J. Nucl. Mater., 385(3), Fu, E., Carter, J., Swadener, J. et al.. Size dependence enhancement of helium ion irradiation tolerance in sputtered Cu/V nanolaminates, 629-632, Copyright 2009, with permission from Elsevier.)... 

The multilayered films of Cu/V, Cu/Nb, and Fe/W with the thickness of some layers varying in the range of 2.5-200 nm were exposed to irradiation with helium ions and the formation of gas-filled pores has been analyzed [81-84]. Whatever the combination of metals is, helium bubbles accumulate along the boundaries between layers. Moreover, as the thickness of layers decreases the pore size reduces (Figure 24.20a). At a thickness of 2.5 nm, the resolution of electron microscope failed to show any helium formations. In addition, it was noticed that the hardening of multilayered structures degrades as the thickness of individual films decreases (Figure 24.20b). 

Helium bubble formation (HBF) uses electron microscopy to measure the distribution of bubble sizes that form when a solid inqtlanted with He ions is annealed [73Aitl]. For He in Nb, bubble formation occurs by migration and coalescence, not by He diffusion. Thus, the size distribution can be related to the surface diffusion coefficient, which governs bubble migratioa Spatial resolution is about 10 nm. 

Helium Refrigeration (1) 41 Helium Production Process (1) 171 Hydrogen-Helium Liquefier (2) 8 Gas Chromatography as Applied to the Industrial Separation of Neon from Nitrogen and Helium (2) 197 Helium Heat Rectifiers and a Simple Magnetic Refrigerator (2) 220 A Liquid Helium Bubble Chamber (2) 330 Economic and Other Aspects of the Distribution of Navy Helium in Liquid Form (3) 114... 

The aqueous solution of EtOH was deoxygenated by helium bubbling in the solution for 10 min before use. Methyl vinyl ketone (from Aldrich) was distilled under 20 torr and collected in a round-bottom flask cooled in a C02-acetone bath, immediately before use. Purifications of copper iodide (from Merck, Darmstadt, Germany) and Zn dust (60 fim... 

FiGURE 9.25 Exposed Screen Height as a Function of Tank Liquid Conditions and Mass Fiow Rate through the Standard 325 x 2300 Channei at (a) First Helium Bubble Ingestion and (b) Totai Breakdown. 

The explanation appears to lie in the accumulation of lattice vacancies at the nucleation sites of the helium bubbles. The average energy of a neutron in the fast reactor is above 100 keV, greatly in excess of the 25 eV or so which is required to displace an atom from a lattice site. Neutron collisions, followed by multiple cascade processes, therefore lead to a high density of vacancies and interstitials. The interstitials have a higher mobility than the vacancies and tend to be more rapidly absorbed at grain boundaries and dislocations, where they lose their identity. The surplus vacancies are then available for the formation of voids at the nucleation centers of the helium produced by the (n, a) reactions. 

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