Void Cavities Research Articles

Bridging Methoxy Groups in NaY, NaX and NaLSX Zeolites

Distribution of chemisorbed methyl groups and sodium cations in the structure of NaY, NaX and NaLSX zeolites was estimated by neutron diffraction. Chemisorbed methyl groups were prepared in the structure by reaction of methyl iodide with reactive sodium cations available in SII and SIII positions of faujasites. Methyl cations CH3+, formed in the reaction, react immediately with the lattice oxygen forming surface bonded methyl groups in bridging configuration. 13C NMR signals of chemisorbed surface species and their linear dependence on the intermediate electronegativity of the zeolite lie in the interval from 53 ppm for most basic CsLSX to 58 ppm TMS for stabilized and acid leached sample of H,NaY-St. Changes in the distribution of structural sodium cations in the lattice after chemisorption of methyl cations have been detected. C-O distances in surface methoxy groups in void cavities were longer than in ordinary crystalline organometallic compounds with bridging methoxy groups. The location of chemisorbed methyl groups at the O1 lattice oxygen type was most probable for NaY. Nuclear densities of chemisorbed methyl groups were detected in NaX at O1 and at O4 lattice oxygens. The origin of the split signal at 58 ppm on NaX and NaLSX samples has been discussed.

Precracking Structures in a Creep-Ruptured Low-Carbon Cr-Mo Steel: Their Nature and Detection by Light Microscopy and Scanning Electron Microscopy

The precracking structures developed in a creep-ruptured Cr-Mo ferritic steel have been investigated metallographically using surface preparation methods that can be expected to produce truly representative surfaces. It is concluded that the structures are grain boundary carbides that have decohered from the matrix ferrite during creep straining. Traditional void cavities were not present. It is established, however, that several commonly used metallographic preparation procedures develop artifacts at the sites of decohered carbides, which could be mistaken for void cavities. The development of these artifacts greatly enhances the detectability of decohered carbides but gives a false impression of their size and destroys the evidence on which the nature of the precracking structures could be elucidated. Methods of reliably investigating the structure of decohered carbides, which is somewhat difficult, or of merely detecting their presence, which is comparatively simple, are discussed in the light of this new information, as is also the consequence of their presence instead of voids on the methods currently used in estimating the residual life of affected components. The implications to theoretical considerations of creep failure are also discussed, a number of questions being raised that require further investigation. Foremost among them is the frequency with which, and the reason why, carbide decohesion occurs without subsequent growth of significant voids. The morphology of the boundary carbides that grow during the earlier stages of creep appears to be central to these matters, and this, in turn, is in need of investigation. It apparently has to be accepted until these questions are settled that two mecanisms of creep failure are possible in these steels.

Heat transfer characteristics in low-temperature latent heat storage systems using salt-hydrates at heat recovery stage

The heat transfer characteristics of a low temperature latent heat storage system have been determined for circular finned and unfinned tubes using sodium acetate trihydrate as a phase change material (PCM). In the heat recovery stage, supercooling of PCM in the finned-tube system is larger than that in the unfinned-tube system. The heat-transfer coefficient between the PCM and the heat-transfer tube surface can be predicted from the steady-state heat conduction equation except the beginning of freezing with some degree of supercooling. The heat transfer is significantly reduced by the void cavities upon shrinkage of PCM in the finned-tube system. The enhancement of heat transfer by thin finned-tube over the unfinned-tube is found to be negligible. The heat-transfer coefficient in the thick finned-tube system is approximately two times higher than that in the unfinned-tube system. The heat transfer coefficients for the unfinned-tube and thick finned-tube systems are found to be 45 ∼ 150 W/m 2-K and 90 ∼ 250 W/m 2-K, respectively. The thermal performance for three different tube systems is found to be strongly affected by the inlet temperature but not by the flow rate of the heat transfer fluid. The amount of heat recovered has been correlated in terms of the Fourier, Stefan, and Reynolds numbers.

Precracking structures in a creep-ruptured low-carbon CrMo steel: Their nature and detection by light microscopy and scanning electron microscopy

The precracking structures developed in a creep-ruptured CrMo ferritic steel have been investigated metallographically using surface preparation methods that can be expected to produce truly representative surfaces. It is concluded that the structures are grain boundary carbides that have decohered from the matrix ferrite during creep straining. Traditional void cavities were not present. It is established, however, that several commonly used metallographic preparation procedures develop artifacts at the sites of decohered carbides, which could be mistaken for void cavities. The development of these artifacts greatly enhances the detectability of decohered carbides but gives a false impression of their size and destroys the evidence on which the nature of the precracking structures could be elucidated. Methods of reliably investigating the structure of decohered carbides, which is somewhat difficult, or of merely detecting their presence, which is comparatively simple, are discussed in the light of this new information, as is also the consequence of their presence instead of voids on the methods currently used in estimating the residual life of affected components. The implications to theoretical considerations of creep failure are also discussed, a number of questions being raised that require further investigation. Foremost among them is the frequency with which, and the reason why, carbide decohesion occurs without subsequent growth of significant voids. The morphology of the boundary carbides that grow during the earlier stages of creep appears to be central to these matters, and this, in turn, is in need of investigation. It apparently has to be accepted until these questions are settled that two mecanisms of creep failure are possible in these steels.

Recent observations and theoretical approaches for ordering of microdefects and clusters

Formation of ordered arrays of defects has been observed during irradiation for different irradiation conditions and in a wide class of materials. These range from ordering of void cavities, gas bubbles, solid inert gas bubbles, precipitates and formation of periodic walls of defect clusters. This paper gives a brief summary of these experimental observations and outlines the main features of the theoretical approach based on non-equilibrium stability of the irradiation process.

Coordination chemistry of rhodium and iridium in constrained zeolite cavities: methanol carbonylation

Abstract

Low-cost video distribution amplifier gives increased reliability : G. A. Stevens, Electron. Engng, October (1970), p. 47

In this work, the interaction, crystallization and morphology of membranes derived from polyamides/poly (vinylalcohol) (PA/PVA, PA: PA66, PA69, PA610 and PA612) blend materials are studied at various weight fractions and various crystallization temperatures. The experimental work includes differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), polarized optical microscopy (POM) and scanning clectron microscopy (SEM). The equilibrium melting temperatures of polyamides in the blends are obtained using Hoffman-Weeks plots, and the interaction parameters are calculated using the Nishi-Wang equation, which is based on the Flory-Huggins theory. The values of the Flory-Huggins interaction parameters χ12 are negative. Significant upward shifts of νNH are observed with increasing volume fraction of PVA. The morphology of membranes is impressively complex, diverse, and irregular. All compositions that show nodular morphology are in the μm-scale and the size of the nodules increases with increasing PA6 content. Large void cavities are observed in the substructure for the lower polymer concentrations.