The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

Spectral Reflectivity of Ceramics of Cubic Zirconium Oxide on the Wavelength of 0.63 μm under Intense Surface Heating above the Melting Point and Subsequent Cooling

The experimental data on the normally hemispherical reflectivity of zirconium oxide ceramics are obtained for the wavelength of 0.63 m in the course of the first cycle of heating by concentrated laser radiation up to temperatures above the melting point and free cooling in ambient air. The obtained data are compared with analogous results of the second and subsequent cycles of heating and cooling. Analysis is made of the effect of the temperature field in the heated layer on the reflectivity and on the effective temperature measured by a pyrometer.     

A comparison of exact first order and spring boundary conditions for scattering by thin layers

In scattering problems for time-harmonic elastic waves, thin elastic layers are often of interest, e.g., in laminates. Various ways of substituting such layers by some effective boundary conditions have been proposed, and these are briefly reviewed. A rational way of obtaining boundary conditions that are exact to first order in the layer thickness is then described. For a thin spherical layer numerical comparisons are performed between these exact first order boundary conditions, the commonly used spring boundary conditions and the exact solution, and it is shown that the exact boundary conditions are far superior to the spring boundary conditions in most situations. A drawback with the exact boundary conditions is that they are quite complicated.     

Underpinning of a simple blunt flaw fracture initiation relation

With the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture as a basis, the author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. In earlier work, a Mode I fracture initiation relation has been derived, subject to the restriction that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius . The relation gives the critical elastic flaw-tip peak stress _pcr, and has been derived using a two-extremes procedure, whereby the separate, and indeed exact, solutions for small and large s/ values are blended together to give an all-embracing relation that is valid for all s/ values. _pcr is expressed in terms of the process zone material parameters and geometrical parameters but, for a wide range of flaw geometry parameters, _pcr essentially depends on only one geometrical parameter . This paper provides underpinning for the general thrust of the two-extremes procedure by appealing to exact results for the complete spectrum of s/ values from analyses of appropriate Mode III models. Results obtained by applying the two-extremes procedure are shown to be in very good agreement with the exact results.     

Study of the Temperature Fields in Oil and Gas Strata with Water Injection Based on the Perturbation Method

The problem of the thermal action on oil and gas strata by injection of a heating medium using expansion in a small parameter is reduced to an infinite sequence of boundaryvalue problems that are solved by the method of integral transforms. It is shown that, with an appropriate selection of the small parameter, the zerothorder approximation corresponds to a spaceaveraged (across the stratum thickness) solution of the main problem and leads to a concentratedcapacity scheme that is constructed assuming that the stratum temperature is independent of the vertical coordinate. The first approximation permitted marked refinement of calculations according to the concentratedcapacity scheme and an evaluation of its error. Spacetime temperature distributions are presented that have been calculated using the analytical solutions obtained.     

“Reservoir Model” for Shallow Modulation-Doped Digital Magnetic Quantum Wells

Digital magnetic heterostructures (DMH) are semiconductor structures with magnetic monolayers. Here we study electronic and magnetotransport properties of shallow modulation-doped (ZnSe/ZnCdSe) DMHs with spin-5/2 Mn impurities. We compare the reservoir model, possibly relevant to shallow geometries, to the usual constant-density model. Our results are obtained by solving the Kohn-Sham equations within the local spin density approximation (LSDA). In the presence of a magnetic field, we show that both models exhibit characteristic behaviors for the electronic structure, two-dimensional carrier density, Fermi level and transport properties. Our results illustrate the relevance of exchange and correlation effects in the study of shallow heterostructures of the group II-VI.     

Yield and transient effects during the plastic deformation of solid polymers

Tensile tests were performed on seven commercial polymers at 22° C and at constant true strain rates of 10^–4 to 10^–1 sec^–1. The constant strain rates were imposed on the minimum section of each sample with the aid of a diametral transducer, an exponential function generator and a closed-loop hydraulic testing machine. The polymers investigated were: high and low density polyethylene, polytetrafluoroethylene, polypropylene. polyvinylchloride and polyamide 6 and 66. True yield drops were observed in the rigid glassy polymers, whereas yielding was more gradual in the semi-crystalline or plasticized polymers. Strain rate change tests were also performed, during which one order of magnitude increases and decreases were imposed on the specimens. Normal transients were observed at small strains in the samples containing a rubbery phase, while the transients were of an inverse nature in the samples containing a glassy phase. With an increase in the strain at which the change was initiated, the normal transients changed in character to inverse. Transient tests were also performed in which straining was interrupted to permit a period of stress relaxation or of holding in the unloaded condition prior to the resumption of straining. A quantitative model is proposed, based on the dynamics of plastic waves which accounts for the transition from normal to inverse transient behaviour with increasing strain, and also explains the opposite effects of stress relaxation and of specimen unloading on the restraining transients.     

Clustering research fields for macro-strategic analysis: A comparative specialization approach

The goal of this article is to show that it is possible to construct an index to measure a country's relative specialization in different scientific fields in a way which is both reliable and relevant for macro-strategic analysis. We will call this index a Revealed Scientific Advantages Index. The technical problem to be discussed is one of aggregation: how can we be sure that an index calculated for a small number of relatively large fields does not mask significant policy needs synthetic measures which are easy to interpret. We will show that the Revealed Scientific Advantages approach offers the possibility of building them. The study itself is based on figures obtained through an exploitation of the INIST/CNRS PASCAL database classification of science. 107 sub-fields of this classification were initially used to determine the areas of specialization for 11 countries (revealed national advantages). Clustering techniques were then used to aggregate this data and 13 specific fields were identified. The science policy information produced during the study concerned these 13 fields. It proved to be both easily understandable and relevant for macro-strategic analysis.     

A rank-frequency model for scientific productivity

If f(r) is the number of contributions of an author or rank r, then it is shown that f(r) is proportional to r^–, where >0. The model is dependent on the definitions of a contribution and rank of an author. Three estimation procedures are illustrated and four other scientific productivity studies and two data sets on Canadian Mathematicians are shown to adequately fit this rank-frequency relationship.Research partially supported by National Research Council Grant No. A9229.     

Effect of Intensification of Selenium‐Protection Properties in Laboratory Animals Drinking “Flint” Water Due to the Increase in the Coefficient of Diffusion of Selenium in Organs and Tissues

The influence of flint water on the development of biochemical processes which cause the accumulation of selenium in the body of laboratory animals has been studied experimentally. A decrease in the bond energy of the molecule of flint water with the anion SeO₃ ^2– in relation to the bond energy of the molecule of ordinary water and an increase in the coefficient of diffusion of this anion in bone and muscular tissues in filtration of flint water have been substantiated.     

Spin hydrodynamic equations with external disturbances and suitable Green's functions for superfluid3He-B. New onsager relations

The spin hydrodynamic equations for superfluid³He-B have been obtained for the case of external, time-dependent fields. On the basis of a microscopic approach, expressions are found for additional terms in equations containing these fields. Considering the linear response of the system to the switching on of external fields, formulas are found for suitable Green's functions (magnetization-magnetization, rotation-rotation, magnetization-rotation, rotation-magnetization). The rotation-rotation Green's function has the 1/q ² singularity characteristic of superfluid systems. Connections between Green's functions lead to relations among kinetic coefficients v, ₁, and ₂. It is also shown that there is a conserved quantityQ ^(B) = div v _s ^(B) that describes sources or magnetic type charges (monopoles) of the superfluid velocity v _s ^(B) . Comparison with the phenomenological approach suggests thatQ ^(B) is proportional to a pseudoscalar giving the projection of the spin density onto the vector describing the axis of rotation.     

A new acoustic spectroscopy: Resonance spectroscopy by the MIIR

Recent and remarkable advances in the experimental study of acoustic scattering from targets immersed in water are leading to a new spectroscopy: resonance acoustic spectroscopy. The discovery and improvement of an intriguing method, the Method of Isolation and Identification of Resonances (MIIR), has made possible experimental determination of the eigenfrequency spectra of aluminum-elastic cylinders and cylindrical shells. This method gives a quasilinear resonance spectra. In addition, it shows the importance of circumferential waves which generate standing waves. They allow us to explain the reradiation of targets after the end of insonification. The MIIR has numerous applications, especially in underwater acoustics and nondestructive testing.     

Lamellar thickening growth of an extended chain single crystal of polyethylene (II) : ΔT dependence of lamellar thickening growth rate and comparison with lamellar thickening

The degree of supercooling (T ⁰) dependence of lamellar thickening growth rate (U) of an isolated extended chain single crystal (ECSC) of polyethylene is studied. The experimental formula,U = C exp(–D/T ⁰), where C = 130 nm/s and D = 20.0 K is obtained for the first time. The formula is the same as that of lateral growth rate (V). The reason why U and V obey the same formula is well explained by a model named sliding diffusion model of the lamellar thickening growth. The model proposed that the lamellar thickening growth is controlled by both chain sliding diffusion within the ECSC and the nucleation on the side surface. The observed fact that the U increases with increase of T ⁰ is opposite to the well known fact that lamellar thickening rate W decreases with increase of T ⁰. This siginificant difference was well explained by the difference between the primary crystallization and the secondary crystallization, which is a kind of Ostwald's ripening process. The origin of the tapered shape is well explained by coupling of lamellar thickening and lateral growths.     

Evaluation of the economic effects of Brite-Euram programmes on the European industry

This article deals with an evaluation performed by BETA group about the economic effects of EU R & D programmes (Brite, Euram and Brite-Euram I) on the European industry. The approach used is based on an original methodology designed by BETA, which aims at evaluating those effects at a micro level (i.e. the participants to the programmes) by means of direct interviews of 176 partners involved in 50 projects. The definition of these economic effects is firstly described, as well as the different steps of the evaluation work. Then the overall results of the study are presented, showing the importance of both direct and indirect observed effects in monetary terms. Finally, some more detailed results highlight the positive impact of some aspects of the organization structure set up for the analyzed R & D projects on the amount of observed effects: i) the participation of a university lab; ii) the participation of at least one partner involved in a fundamental research work; iii) the diversity of research tasks over a scale ranging from fundamental research to industrialization work; iv) the combination of user-type and producer-type of activity in one given organisation (integration effect) or in one given project (consortia effect), etc...     

Where Are We in HTCS?

Address given at the closing of the conference on Major Trends in Superconductivity in the New Millennium, MTSC 2000 and the opening of the Symposium on Itinerant and Localized States in HTCS SILS in HTCS.     

Analysis of the complex impedance data for β-PbF2

Previously published complex impedance plots for the ionic conductor -PbF₂ show the familiar depressed circular arcs with an angle 0.7 /2 and an activation energy for the volume conductivity W _v=0.45 eV. A detailed analysis of this behaviour in terms of the recently developed non-Debye model shows the real part of the relative dielectric permittivity to have a frequency dependence ^0.7–1, with a high frequency limit of 50 Hz and with only weak dependence on temperature. The low-frequency spurs on the impedance plots are shown to indicate an interfacial barrier at each electrode having a similar non-Debye frequency characteristic to the bulk but showing a strong temperature dependence with an activation energy equal to W _v/2. This suggests the presence of low Debye-screened barriers of about k T height, resulting from depletion and accumulation of ionic carriers at incompletely transmitting electrodes. There is no visible effect of inter-grain boundaries on the flow of direct current.     

Shear viscosity of liquid4He and3He-4He mixtures,especially near the superfluid transition

High-resolution measurements of are reported for liquid⁴He and³He-⁴He mixtures at saturated vapor pressures between 1.2 and 4.2 K with particular emphasis on the superfluid transition. Here is the mass density, the shear viscosity, and in the superfluid phase both and are the contributions from the normal component of the fluid ( _n and _n ). The experiments were performed with a torsional oscillator operating at 151 Hz. The mole fraction X of³He in the mixtures ranged from 0.03 to 0.65. New data for the total density and data for _n by various authors led to the calculation of . For⁴He, the results for are compared with published ones, both in the normal and superfluid phases, and also with predictions in the normal phase both over a broad range and close to T. The behavior of and of in mixtures if presented. The sloped/dT near T and its change at the superfluid transition are found to decrease with increasing³He concentration. Measurements at one temperature of versus pressure indicate a decreasing dependence of on molar volume asX(³He) increases. Comparison of at T, the minimum of _n in the superfluid phase and the temperature of this minimum is made with previous measurements. Thermal conductivity measurements in the mixtures, carried out simultaneously with those of , revealed no difference in the recorded superfluid transition, contrary to earlier work. In the appendices, we present data from new measurements of the total density for the same mixtures used in viscosity experiments. Furthermore, we discuss the data for _n determined for⁴He and for³He-⁴He mixtures, and which are used in the analysis of the data.     

A novel X-ray technique for inspection of steel pipes

The corrosion-induced material loss in crude oil carrying steel pipelines was originally studied by making use of the backscatter X-ray technique. The steel thickness can be determined by studying the density profile of the backscatter intensity vs. the depth location of a voxel. There are, however, some practical limitations to the above method, and a new method for evaluating steel thickness, namely, the transcatter technique is described. This technique uses the intensity of the beam which is transmitted by the pipe wall and subsequently scattered by the hydrocarbon inside the pipeline. The thickness is evaluated using three techniques, namely, the sequential technique, the dual angle technique, and the reference technique. Of the three techniques, the sequential technique has been studied in detail. The mathematical equations and experimental results related to the transcatter technique show that the thickness can be measured with an accuracy of better than 10% for a nominal steel thickness of 8 mm with a measurement time of several minutes.     

Fracture initiation and crack propagation of acrylonitrile-butadiene-styrene (ABS) in organic solvents

The effects of organic liquid environments on the fracture behaviour of acrylonitrile-butadiene-styrene (ABS) have been investigated. Fracture initiation experiments showed thatK _i ² , (K _i being the stress intensity factor at crack/craze initiation), could be meaningfully correlated with the solvent solubility parameter ( _s) of the different liquid environments and had a minimum value at _s= _p, where _p was the solubility parameter of ABS. For the range of organic liquids used, hydrogen bonding did not have any significant effects on the correlations. It was demonstrated that theK _i ² – _s correlations could also be usefully extended to other materials such as plain and glass-filled polystyrenes. At a common crack speed (å), the fracture toughness (R) values in crazing liquids (i.e. alcohols) were greater than those in cracking solvents (i.e. acetone, benzene, toluene, etc.) which usually caused a dissolution effect on the plastic. From crack propagation experiments, and using fracture mechanics analyses, definiteR(å) andK _c(å) relationships for ABS immersed in toluene, carbon tetrachloride and methanol were determined. These experimental results showed that crack propagation was relaxation controlled and agreed well with a recent theoretical analysis due to Williams and Marshall for environmental crack and craze growth in polymers. Finally, SEM pictures were presented to show the remarkable differences in the fracture morphologies of ABS in both crazing and cracking liquid environments.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.