Small penetrants

Table 11 gives the properties for various commercial parylenes (pi ra-xylylene). These materials have excellent moisture and chemical resistance as well as good electrical and mechanical properties. They are noted for their abiHty to penetrate small spaces by vapor deposition (see Xylylenepolymers). 

Since supercritical fluids were chosen for their ability to penetrate small cracks and crevices, additional tests were performed to evaluate this characteristic. A test cube modeled after a similar fixture fiibricated by Ferranti Aerospace, was developed and manufactured to md in this study. The cube had a number of blind holes, tapped holes, dtannels and crevices to simulate actual hardware. Beryllium, 300 Series stainless steel and aluminum cubes were constructed to simulate the conunon metals found in the instrument. In addition, the sides of the cube were removable to facilitate deposition of the contaminants into these blind holes and crevices and later analysis of cleaning effectiveness. The base of the cube was equipped vrith a scanning electron microscope (SEM) mount so that the cube could be examined directly in the SEM. Figure 3 is a photograph of a test cube. Extensive evaluations with these test cubes indicated that supercritical fluids were indeed effective at removing contaminants from cracks and crewces. 

DMSO, ethylene glycol, and aqueous methanol. Such gels are very well suited for the fractionation of water-soluble natural products such as carbohydrates and small peptides. Even though the major mode of separation is that of gel filtration, (i.e., molecules are separated based on their relative abilities to penetrate small bead pores), additional adsorption mechanisms exist, such as hydrogen bonding, giving rise to good fractionations. 

The best adhesive for improved fracture toughness (1) does not develop shrinkage stresses during cure, (2) has a modulus close to that of wood perpendicular to the grain, (3) has a modulus that changes in parallel with the wood modulus as moisture content changes, (4) penetrates small-lumen, thick-walled cells but does not overpenetrate large-lumen thin-walled cells, and (5) can infiltrate the cell wall to reinforce the weak interphase between cell-wall layers. 

Experiments show the non wetting behavior (Fig. 3) of the fluoride salts of interest, that molten salts will not penetrate small cracks in the graphite and that the molten salt will not contact the fuel matrix (Briggs 1963 ORNL 1964 Fontana 1970). In a classical molten salt reactor where the uranium and fission products are dissolved in the fuel salt, the fuel salt is dumped to storage tanks during shutdown. 

Post-irradiation examination from the MSRE showed no interactions (erosion or corrosion) between the salt and the graphite. The original machining marks were still clearly visible. Out of reactor tests were conducted to 1400°C with no interactions between the salt and graphite. Experiments show the non-wetting behaviour of the fluoride salts of interest and demonstrate that liquid fluoride salts will not penetrate small cracks in the graphite. For the AHTR, this has the practical implication that the clean coolant will not contact the fuel micro spheres that are embedded in a carbon matrix. 

The disk sintered up to 1800 C was then subjected to an impregnation process to decrease porosity as illustrated in Scheme n. The disk was first soaked in a 50 wt% solution of the MAB condensate in chlorobenzene. The whole system was evacuated so that the solution could penetrate small pores inside the disk. The disk was then taken out, thoroughly dried in vacuo, and pyrolyzed up to 1000 C in an ammonia flow. Tins cycle was repeated several times, and the disk was finally sintered up to 1800 C in a nitrogen flow. Figure 4 shows the increase in bulk density with the number of impregnation cycles. It suggests that the increase in density leveled off beyond the fourth impregnation cycle where the bulk density reached 1.74 g/cm. ... 

The sidewall sampling tool (Sl/VS) can be used to obtain small plugs (2 cm diameter, 5 cm length, often less) directly from the borehole wall. The tool is run on wireline after the hole has been drilled. Some 20 to 30 individual bullets are fired from each gun (Fig. 5.35) at different depths. The hollow bullet will penetrate the formation and a rock sample will be trapped inside the steel cylinder. By pulling the tool upwards, wires connected to the gun pull the bullet and sample from the borehole wall. 

The tests implemented in this prograirune are mainly divided in two parts lab tests and shop tests. For the lab tests, artificial defects were integrated in small vessels, artificial defects are representative of real defects as lack of penetration, blowholes and inclusions. 

The case considered above corresponds to R < H. The calculation using formula (1) gives the next results. For example, consider the thickness of dry developer layer h = 20 pm. In the absence of sedimentation process our product family (penetrant and developer indicated above) could not detect the cracks with the depth lo < 1,33 mm of any widths. Nevertheless due to the sedimentation one can get the decrease of developer s thickness from h = 20 pm till h s 5 pm. As a result, our product family can ensure the detection of the cracks with H > 2,3 pm even with very small length lo = 0,4 mm. At the same time if lo = 1 mm, then the cracks with extremely small width H > 0,25 will be revealed. 

To clear up a role of two-side filling with liquids of dead-end capillaries in the practice of PT, we ve carried out some special experiments. It was established some years ago that it s almost impossible to reveal small defects applying dry powder developer in the case when defect s hollows are completely filled with a penetrant. But just such a situation one... 

The most widely used experimental method for determining surface excess quantities at the liquid-vapor interface makes use of radioactive tracers. The solute to be studied is labeled with a radioisotope that emits weak beta radiation, such as H, C, or One places a detector close to the surface of the solution and measures the intensity of beta radiation. Since the penetration range of such beta emitters is small (a ut 30 mg/cm for C, with most of the adsorption occurring in the first two-tenths of the range), the measured radioactivity corresponds to the surface region plus only a thin layer of solution (about 0.06 mm for C and even less for H). 

The diffraction pattern consists of a small number of spots whose symmetry of arrangement is that of the surface grid of atoms (see Fig. IV-10). The pattern is due primarily to the first layer of atoms because of the small penetrating power of the low-energy electrons (or, in HEED, because of the grazing angle of incidence used) there may, however, be weak indications of scattering from a second or third layer. 

None of the above methods is sufiBcient for neutrons, however. Neutrons penetrate mader so easily that the only effective approach is to use materials with a very high surface-to-vohuue ratio. This can be accomplished with small particles and exfoliated graphite, for instance, but the teclmique has essentially been abandoned in surface sdidies [7, 8]. 

Small molecules can penetrate and penneate tlirough polymers. Because of this property, polymers have found widespread use in separation teclmology, protection coating, and controlled delivery [53]. The key issue in these applications is the selective penneability of the polymer, which is detennined by the diffusivity and the solubility of a given set of low-molecular-weight compounds. The diffusion of a small penetrant occurs as a series of jumps... 

---