Stratified media theory interpretation of measurements of the spectral polarized directional emissivity of some oxidized metals

The emissivities of metals are strongly affected by the growth of oxide films. For small film thicknesses, perturbations are limited to the visible range. As thicknesses increase, they spread to the infrared range. Various samples of optically polished metals (Fe, Ni, Cr) and stainless steels (ELI.T 1803 MoT and AISI 304 or 316) have been oxidized at temperatures ranging from 400 to 800C. Their spectral polarized directional emissivities, and , have been measured with two experimental techniques. The spectral range studied extends from 0.4 to 14 m; the measurement directions vary between 0 and 80 from the normal to the sample. After measurement, each sample was analyzed by glowdischarge optical spectroscopy (GDOS). From the results of the analysis and from the survey of bibliographical data, we characterized the structure of the oxide films, i.e., their approximate thicknesses and compositions. If the complex optical indices of the metals and oxides are known, the stratified media theory enables the computation of the emissivities and . Computed and measured values have been compared. It appears that the theory accounts well for experimental data when the thicknesses of the oxides are small as in stainless steels. But for thicker oxide films, discrepancies are ascribed to several reasons.     

The Possibility of Reproducing the Units of Complex Permittivity at High Frequencies

The results of theoretical investigations of the reproduction of the units of the components of complex permittivity and by a method based on the use of the wave properties of a coaxial line are presented. The errors in reproducing and are analyzed.     

Deformation of single crystals of gallium arsenide

Deformation of single crystals of gallium arsenide is reported in bend and tension up to 1000° C whilst maintaining stoichiometry in an arsenic atmosphere. Surface defects and impurity segregation are shown to be dislocation sources. The dislocation density is low enough, however, to show large yield drops which are analysed in detail. Strains of 39% are possible. The activation energy for dislocation movement is increased by heat-treatment owing to an increase in point defect population.Electron microscopy shows that the predominant slip systems are {111} 110 and the majority of dislocations have b=a/2 110, the axes lying along 110 and 112 directions. Sub-cell formation occurs with sub-boundaries lying along 110 directions.     

Corrosion of Iron in a Carbonate-Bicarbonate Solution. Part 1. Crystallographic Analysis of Passive Films

Visible passive films are formed on the surface of 1010 steel at 325K in a solution of 1N NaHCO₃ + 1N Na₂CO₃ at anodic potentials up to –320mV_NHE inclusively. For potentials below –400mV, they are dull and contain FeCO₃. For –(390–320)mV, the films are thin blackish bright. Most probably, the diffraction peaks are caused by Fe₃O₄–Fe₂O₃. For –(430–400)mV, the films are dull blackish. Probably, this color is caused by a composite of magnetite and siderite.     

Mechanical modelling of the universal superplastic curve

The mechanical response of various combinations of non-linear viscous elements (dashpots) is analysed in this paper. It is assumed that the properties of i-th element can be described by the relation _i = K _{i i} ^mi , where _i is the stress, _i is the strain rate, K_i and m_i are constants 0 m_i 1). Parallel, series and combined combinations are considered. The main object is to find out the potential for various combinations to describe the sigmoidal variation of the flow stress, , with the strain rate, , which is a typical feature of superplastic materials. It is found that elements are connected in parallel as well as elements are connected in series are characterised by non-sigmoidal dependency of the net stress on the net strain rate . At the same time a mixed combination with two elements in parallel connected with a third one in series is shown to describe the sigmoidal curve with reasonable accuracy.     

Thermodynamic properties of superconducting niobium

The thermodynamic properties of superconducting Nb are calculated numerically from the solution of the Eliashberg equations on the imaginary axis for several possible electron-phonon spectral densities ²()F(). Comparison with experiment is made in order to see which spectrum gives the best agreement, and functional derivatives with respect to ²()F() are used to estimate how this agreement might be improved by small changes in ²()F(). Possible gap anisotropy effects are also considered with the help of a simple model anisotropy for the interaction.Research supported by Natural Science and Engineering Research Council of Canada.     

Subgap peak and Tomash-McMillan-Anderson oscillations in the density of states of SNS Bridges

The quasiparticle spectra and the densities of states of superconducting-normal-superconducting junctions are computed from the WKBJ transformed Bogoliubov-de Gennes Equations (BdGE), which are solved by Picard iteration and numerical integration. It is shown that the influence of the proximity effect on the bound states can be modeled by a rectangular pair potential well of effective normal layer thickness 2a*= _–L ^L [1–(z)/]dz, where (z) is the pair potential of the junction, is its asymptotic constant value, and 2L is the total length of the sample. The density of states exhibits a subgap peak at energies less than besides the BCS peak atE=; forE> there are geometrical resonances which are due to electron-hole interferences in finiteS layers of thicknessL-a*.     

Observation of hydrogen distribution in high-strength steel

The diffusible hydrogen in Cr-Mo steels are observed with autoradiography technique. Specimens with the diffusible hydrogen are prepared by an electrochemical cathodic charging method and those without the diffusible hydrogen by annealing at 373 K after charging hydrogen. TEM autoradiographs suggests, by the developed silver grains, that the hydrogen trapping sites are the grain boundary and internal interface of ferrite-cementite and ferrite-lath structure. After keeping the sample at 373 K, the silver grains disappeared. Most of hydrogen trapping sites release almost all the hydrogen at 373 K. It is clear that these sites of high-strength steels supplies the diffusible hydrogen. Hydrogen absorption characteristics of quench hardening tempering Cr-Mo steels have been evaluated by thermal desorption spectrometry (TDS). From tritium electron microscopic autoradiography and TDS analysis, the lower temperature (360 K–370 K) peaks show the diffusing hydrogen which is released a few days. The diffusible hydrogen from trapping sites such as the internal interface of ferrite-cementite or ferrite-lath structure are distinguished to the diffusing hydrogen.     

Theoretical estimation of the effect of interfacial energy on the mechanical strength of spinodally decomposed alloys

New interfaces are produced on the slip plane when a crystal with continuous composition fluctuation arising from spinodal decomposition is deformed by slip. In this work, the energy of such interfaces is evaluated for both modulated and mottled structures, and their effects on slip behaviour are discussed. It is concluded that the contribution of this interfacial energy is large enough to account for the age-hardening concomitant with spinodal decomposition.     

Interfacial spring model for ultrasonic interactions with imperfect interfaces: Theory of oblique incidence and application to diffusion-bonded butt joints

The quasi-static distributed spring model is used to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence. The results are then incorporated in a model for the corner reflection from a diffusion-bonded joint between two abutting plates, the corner being defined by the bond plane and the common lower surface plane of the plates. An immersion-inspection geometry is assumed, and seven categories of corner reflections are identified and examined in detail. These fall into two classes: those having parallel incident and exiting rays in water (=), and those having nonparallel water rays ( ). The = categories are suitable for single probe (pulse-echo) inspections of the joint. Based on the amplitude of the outgoing corner-reflected signal, two = geometries appear promising. These employ, respectively, a corner reflection involving only longitudinal waves with the interface illuminated at near-grazing incidence (LLL), and a corner reflection involving only transverse waves with the interface illuminated at near 45° incidence (TTT). In addition, two practical geometries are indicated; these both involve mode conversion upon reflection from the interface, with the incident or outgoing longitudinal wave traveling nearly parallel to the interface. Model predictions for LLL and TTT reflections are compared to measurements on diffusion-bonded Inconel specimens, and techniques for applying the model results to more complicated bond geometries are discussed.     

Modern Problems of Intensification of Heat Exchange in Channels

The current achievements in the field of intensification of heat exchange in tube heatexchange apparatus are considered. Requirements for highefficiency heattransfer surfaces have been formulated. Problems of intensification of heat exchange in bundles of finned tubes and in heatexchange apparatus under conditions of condensation and boiling of heattransfer agents and under scaling conditions have been investigated. Highefficiency designs of heatexchange apparatus are presented.     

Experimental status of the ac Josephson effect in liquid helium

The presence of an ac sound field causes several distinct effects in a system of two baths of superfluid liquid helium which are coupled by a small orifice. The primary effects of the sound field can be classified as a Z-T structure, steps, and anomalies. Each of these three effects has been subjected to various criteria which are necessary for an interpretation in terms of the ac Josephson effect. These three effects failed the criteria which were applied, and therefore they are not due to the Josephson effect. However, it is shown that the properties of these three effects correlate quite well with the expected properties of first-sound resonances within one of the two liquid baths.Financial support was provided in part by the National Science Foundation.     

Ultrasonic study of the temperature and pressure dependences of the elastic properties of a single-crystal Heusler structure Cu41Mn20Al39 alloy

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic-stiffness tensor components Cu and the adiabatic bulk modulus, BS, of a ferromagnetic Heusler structure Cu41Mn20Al39 at % alloy single crystal as functions of temperature in the range 14–300 K and hydrostatic pressure up to 0.2 GPa at room temperature. At 295 K the elastic stiffnesses are: C11=133 GPa, C44=92 GPa, C (=(C11–C12)/2)=17 GPa, C12=99 GPa, CL(=C11+C44–C)=205 GPa, and BS (=C11–4C/3)=106 GPa. Cu41Mn20Al39 is a comparatively soft material elastically because its elastic properties are influenced strongly by magnetoelastic effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to obtain the hydrostatic-pressure derivatives of the elastic – stiffness tensor components. At 295 K (C11/P)P=0, (C44/P)P=0, (C/P)P=0, (C12/P)P=0, (CL/P)P=0, and (BS/P)P=0 are 5.0±0.1, 3.0±0.1, 1.0±0.2, 3.0±0.3, 7.7±0.4 and 3.7±0.4, respectively. Application of hydrostatic pressure does not induce acoustic-mode softening: the pressure derivatives (CIJ/P)P=0 and (BS/P)P=0 and the acoustic-mode Grüneisen parameters are positive. An interesting feature of the non-linear acoustic behaviour of this alloy is that the value obtained for (C/P)P=0, associated with the softer shear mode propagated along the [1 1 0] direction and polarized along the [1 1 0] direction, is small in comparison with those of the other shear and longitudinal modes. The Grüneisen parameter of this mode, and hence its vibrational anharmonicity, is much larger than those of the other long-wavelength acoustic phonon modes. © 1998 Kluwer Academic Publishers     

Kinetics of nucleation and halt-in-growth processes in a thin layer

The methods of determining the kinetic exponents in the equation, dX/dVex = (1 – X)2–, used for nucleation and halt-in-growth processes where X is the transformed fraction, Vex the KJMA extended volume fraction which is related to time t, and is the overlap factor which accounts for the overlap between a crystallite and a phantom crystallite, are presented. The applications of the Kolmogorov–Johnson–Mehl–Avrami plot ( = 1) and the Austin–Rickett plot ( = 0) to this process are inappropriate, because the overlap factor is 0 < < 1. The impingement exponent 2- and the time exponent are determined from the linear relation of In {[(1 – X) – 1 – 1]/(1 – )} versus In t. From the value of , the crystal shape and growth dimension can be estimated by referring to the mathematical value of . The methods of evaluating the activation energy, Q, are presented using the Arrhenius relation. The value of Q is not directly related to the overlap factor however, appears as a constant term in the expression for Q.     

Fatigue analysis of brittle materials using indentation flaws

The results of an experimental dynamic fatigue study on glass-ceramic specimens containing indentation flaws are analysed in terms of the theory developed in Part 1. A Vickers indenter is used to introduce the flaws, and a conventional four-point bend apparatus to break the specimens. Base-line data for testing the essential theoretical predictions and for evaluating key material/environment parameters are obtained from polished surfaces, i.e. surfaces prepared to a sufficient finish to ensure removal of any pre-existing spurious stresses. The fatigue tests are carried out in water. Inert strength tests in dry nitrogen are used to calibrate appropriate equilibrium fracture parameters, with dummy indentations on selected control specimens providing a convenient measure of the critical crack dimensions at failure. Regression analysis of the dynamic fatigue data yields values for apparent kinetic parameters, which are converted to true kinetic parameters via the transformation equations of Part I. Regeneration of the fatigue function from the theory using the parameters thus determined gives a curve which passes closely through the experimental data points, thereby providing a self-consistent check of the formalism. The implications of the results in relation to the use of macroscopic fracture parameters in the prediction of strength properties for materials with small-scale flaws is an important adjunct to this work. Finally, a recommended procedure for the general testing of dynamic fatigue properties of ceramics using indentation flaws is described.     

Devitrification of sodium disilicate glass: a NMR study

Several crystallographically non-equivalent structures of sodium disilicate are known to exist. Successive heat treatments of the sodium disilicate glasses show -, -, - and -Na2Si2O5 as the devitrification products, depending upon the nature of heat treatment. At higher temperatures and for longer treatment times, only -Na2Si2O5 is observed but for intermediate temperature and time, and forms of the polymorphs are produced. For shorter times, all the polymorphs are formed. The results are verified using magic angle spinning nuclear magnetic resonance as well as X-ray diffraction. A way of estimating the unknown Si–O–Si mean bond angle of the polymorphs, with the help of the known values of the angles, is also presented.     

Crystal structure and thermal expansion of quartz-type aluminosilicates

The thermal expansion of quartz-type aluminosilicates, which is very important for their usefulness in ceramics, can be negative or positive, depending on composition. The structures of these phases are generally described as being -quartz-like, however, an analysis of T-O bond lengths and thermal parameters of the oxygen atoms indicates that in several of these aluminosilicates the local atomic arrangement is -quartz like and that the structures derived from diffraction data are average structures only. Computer modelling of the structures strongly supports this view. In the -quartz-like phases there are static atomic displacements from the -quartz positions that, as in -quartz itself, become smaller with increasing temperature. This mechanism makes a positive contribution to thermal expansion. In phases where static displacements are absent, only thermal vibrations contribute to thermal expansion. Among these, the vibrations of the oxygen atoms normal to the T-O-T bonding plane are the most important ones and make a negative contribution to thermal expansion which is dominant in -quartz as well as in some of the aluminosilicate phases like -eucryptite.     

Torsional Oscillations of a Rotating Column of 3He-B

No Heading We have analysed the axisymmetric and non-axisymmetric modes of a continuum of vortices in a rotating superfluid. We have investigated how changing the temperature affects the growth rate of the disturbances. We find that, in the long axial wavelength limit the condition q = /(1 – ) = 1, where and are temperature-dependent mutual friction parameters, is the crossover between damped and propagating Kelvin waves. Thus at temperatures for which q > 1, perturbations on the vortices are unlikely to cause vortex reconnections and turbulence. These results are in agreement with the recent discovery of Finne et al¹ of an intrinsic condition for the onset of quantum turbulence in ³He-B.PACS numbers: 67.40. Vs, 67.57.–z     

Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks

Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks are considered. The work extends the earlier analysis of 2-D crack-microcrack interactions to the 3-D configurations.After analysing simple elementary interaction events (involving only one microcrack) we solve the interaction problem for a number of sample arrays (containing up to 50 microcracks)-realizations of certain microcrack statistics.Statistical aspects of the problem are examined. The interaction effects are found to fluctuate, even qualitatively (from shielding to amplification) along the crack front: the intervals of reduced stress intensity factors (SIFs) alternate with local peaks of SIFs that enhance local front advances. Thus, no statistically stable effect of stress shielding is found (at least, for the microcrack statistics considered): the toughening by microcracking, if it exists, may be due to a statistics of the microcrack centers which is biased towards shielding configurations or to expenditure of energy on nucleation of new microcracks, rather than elastic interactions with them. Similarly to the 2-D case, stochastic asymmetries in the microcrack field produce noticeable secondary modes on the main crack (i.e., modes II and III under mode I loading); this may be partially responsible for crack kinking and an irregular crack path.The short range interactions (several microcracks closest to the main crack tip) play a dominant role. Their impact on the main crack is quite sensitive to the individual microcrack locations and cannot be adequately reproduced by modelling the short range microcracking zone by an effective elastic material of reduced stiffness.The interaction effects in 3-D are found to be weaker than in 2-D.