The temperature dependence of luminescence from a long-lasting phosphor exposed to ionizing radiation

The temperature dependence of luminescence from a long-lasting phosphor (LLP), SrAl₂O₄ : Eu²⁺,Dy³⁺, exposed to ionizing radiation has been measured to understand the LLP luminescence mechanism. Evaluation of the decay constants of the LLP exposed to -, β- or γ-rays at temperatures from 200 to 390 K showed that the decay constant is divided into four components ranging from 10⁻⁴ to 10⁻¹ s⁻¹ with activation energies of 0.02–0.35 eV.

Total luminous intensity from the LLP with changing irradiation temperature has its maximum value around the room temperature. Irradiation at elevated temperature (390 K) has the total luminescence pattern with monotonous decrease as temperature rises. As a result of evaluating the temperature dependence of luminescence, the luminescence mechanism is considered as follows:

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On asymptotic behavior of oscillatory solutions of operator differential equations perturbed by a fast Markov process

We study the asymptotic behavior of the distributions of the solution of the differential equation of the form

in a separable Hilbert space H where y(t) is an ergodic homogenous Markov process in a measurable space (Y, C) satisfying some mixing conditions and A(y), y Y is a family of commuting closed linear operators with the same dense domain. Using the spectral representation of the solution we construct an H-valued process û_ε(t) which is expressed in terms of the solution of the averaged equation

where = ƒ A(y)(dy) and is the ergodic distribution of Y(t), and some Gaussian random fields with independent increments. We show that the distributions of u_ε(t/ε) and û_ε(t) asymptotically coincide.     

Near-tip fields of growing cracks and resistance curves

This paper gives the formulation and solution of near-tip fields of mode-I cracks growing quasi-statically in compressible elastic perfectly plastic materials. As Poisson's ratio v tends to , the solution approaches the solution of crack growth in incompressible elastic perfectly plastic materials. The rate of crack opening is determined as

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, where β = 5.454 for V = o.3. The evaluation of fracturing based on the criterion of the near-tip opening is discussed.     

Determining crystallization kinetic parameters of Li2O–Al2O3–SiO2 glass from derivative differential thermal analysis curves

The kinetic parameters such as crystallization activation energy, E, and the frequency factor, ν, of Li₂O–Al₂O₃–SiO₂ glass were determined by a new non-isothermal method. The method is described by the equation

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, where β is the heating rate and T_f is the inflection-point temperature of differential thermal analysis (DTA). The value of T_f is determined as the maximum peak temperature on derivative differential thermal analysis (DDTA) curves. Values of E and ν of Li₂O–Al₂O₃–SiO₂ glass were also determined by two existing non-isothermal methods, namely the Kissinger plot and the Ozawa plot, and compared with those determined by isothermal method. Values of E and ν determined by the proposed equation were 332 kJ/mol and 1.4×10¹³ s⁻¹, respectively. They are excellent agreement with the isothermal analysis results, 336 kJ/mol and 1.8×10¹³ s⁻¹, respectively. In contrast, both the Kissinger equation and the Ozawa equation give much higher values of E and ν.     

Orientation and Habit Plane Relationships for Carbide and Nitride Precipitates Molybdenum

Precipitation of group IVa carbides and nitrides (TiC, TiN, ZrC, ZrN, HfC, and HfN) in molybdenum has been studied with electron microscopy. The precipitates display a characteristic platelike morphology, and contrast effects suggesting coherent nucleation have been observed. The zirconium and hafnium carbides and nitrides exhibit a cubic {100} habit plane and an orientation relationship with the matrix characterized as follows:

Platelike precipitates of titanium carbide and titanium nitride, however, exhibit an apparent {310}_Mo habit plane.

An analysis of the mismatch parameters and an approximate calculation of the relative strain energies associated with coherency for various possible habit planes and matrix-precipitate orientation relationships confirm the {100}_Mo habit for the zirconium and hafnium carbide and nitride precipitates. The theory predicts the observed {310}_Mo habit plane for the titanium carbide and nitride phases with the following orientation relationship:

     

Methodology for determining the radial fracture toughness of Zr---Nb pressure tube material

With regard to the determination of the radial fracture toughness of Zr-Nb pressure tube material, using a curved bend specimen machined from a pressure tube, a theoretical analysis shows that one can use the same relation to give the initiation J value as is used for pure bending of a small remaining ligament, namely

, whereB is the specimen thickness, b is the ligament width, P is the load and Δ is the load-line displacement; the integration is performed up to the onset of crack extension. With the curved specimen, this relation should give an initiation J value that is never more than 4% greater than the true J value for where t is the tube thickness; it should in fact underestimate the initiation J value with a very ductile material.     

Initial stages of growth of GaN over (0001) Al2O3 substrate using MBE: a crystallographic analysis of the defects

An AlN buffer layer grown on (0001) sapphire substrate by molecular beam epitaxy has been studied. It is found to be made of small grains having a common [0001] axis parallel to that of the substrate. Some grains are rotated around this axis and the angle rotation can reach 20° leading to a new epitaxial relationship (0001)_sap//(0001)_AlN and [11

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0]_sap//[21

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0]_AlN. A model for the atomic structure of one of these grain boundaries is proposed using high resolution electron microscopy and extensive image simulation.     

An electron metallographic study of the commercial zinc-based pressure-die-cast alloy 3

The solidification structure of pressure-die-cast commercial alloy 3 was examined using scanning electron microscopy and thin foil transmission electron microscopy techniques. It was found that the rapidly cooled alloy commenced solidification by the formation of small rounded primary zinc (η) particles followed by pseudo-primary β particles and then a fine eutectic of β + η. The high temperature β phase subsequently decomposed into -aluminum and η, the phases stable at ambient temperature, but an intermediate transitional phase was found in alloys examined shortly after casting. Chemical analysis by energy-dispersive spectroscopy showed that this phase contained zinc with 11.8 wt.% Al, suggesting that it was a transitional phase since none of the stable phases in the Al---Zn system has that composition. This transitional phase had almost entirely disappeared after aging for a period of 5 years.

Precipitates had formed during or immediately after casting within the central regions of the zinc-rich primary dendrites. These were identified as the aluminium-rich solid solution with f.c.c. structure. The orientation relationship between the phases was determined as

[0001][111]

Each grain contained two families of precipitates with a common (0001)_η habit plane, each adopting one of the two non-equivalent variants of the orientation relationship.

Since this work had shown that aluminium precipitation from η was completed rapidly, the long-term dimensional changes found in zinc alloy castings on aging are considered to be due to the gradual replacement of the zinc-rich metastable phase by equilibrium and η.     

Characterisation of dislocations, nanopipes and inversion domains in GaN by transmission electron microscopy

Transmission electron microscopy is used to analyse a range of defects observed in hexagonal GaN films grown on sapphire and GaN substrates by metalorganic chemical vapour deposition. Large angle convergent beam electron diffraction is used to analyse the Burgers vectors of dislocations and to show that hollow tubes, or nanopipes, are associated with screw dislocations having Burgers vectors±c. Weak-beam electron microscopy shows that dislocations are dissociated into partials in the (0001) basal plane, but that threading segments are generally undissociated. The presence of high densities of inversion domains in GaN/sapphire films is confirmed using convergent beam electron diffraction and the atomic structure of the {

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} inversion domain boundary is determined by an analysis of displacement fringes seen in inclined domains.     

Driver age, car mass and accident exposure— A synthesis of available data

The tendency of younger drivers to be more likely than older drivers to drive smaller cars has been an important consideration in a number of prior investigations of the relation between car size and traffic safety. The purpose of the present study is to quantify this effect on a firmer basis than hitherto by fitting data from seven independent sources to a unified general model. More specifically, when the exposure measures “per unit distance of travel” or “per registered car” are used in studies of car mass effects on traffic safety, the exposure information often does not contain the variable driver age. This work develops a general procedure for disaggregating such exposure data into three driver (or owner) age categories; A₁: 16–24; A₂: 25–34; and A₃: 35 years. Data from the seven sources are fined to the equation

f(i,m) = H_i[1 + G _i(m/900 − 1)]

where m is the ear mass in kg, and f(i,m) is the fraction of cars of mass m which are driven (owned) by persons in the A_i, (i = 1, 2, 3) age category. The form of this equation permits easy comparison of 900 and 1800 kg cars. Those particular masses that have been chosen for illustrative comparisons in earlier work. The seven sets of data are used to derive overall average values of the parameters H₁ and G₁. The data from all seven sources show consistent effects which are summarized in one analytical expression which is well suited for use in future studies of car size effects because it reflects a synthesis of much prior data and it permits sensitivity analyses to be performed conveniently.     

Classification of 2 × 2 non-strictly hyperbolic systems for plane waves in isotropic elastic solids

The governing equations for plane waves in generally nonlinear isotropic elastic solids are a system of 6 × 6 hyperbolic conversation laws. For the half-space Riemann problem in which the initial conditions at t = 0, x > 0 and the boundary conditions at x = 0, t > 0 are constant, the system is equivalent to 3 × 3 system in the full-space Riemann problem. It is further reduced to a 2 × 2 system due to the fact that one of the characteristic wave speeds is linearly degenerate. For hyperelastic materials for which there exists a potential W whose gradients provide the strains, the wave curves near an isolated umbilic point are represented by the potential of the form

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which contains two parameters k and m. The classification of the geometry of wave curves depends on the values of k and m and can be classified into five cases. The potential function considered here is equivalent to considered by Schaeffer and Shearer where a and b are parameters. The classification presented here seems to provide simpler algebraic expressions. It also renders more refined classification as shown in the paper.     

Use of tensorial polynomial strength function for strength prediction in laminated polymeric composites

It has been shown recently that a strength function, expressed by a tensorial polynomial for an anisotropic material, can be only of the form

F₄(σ) + F₂(σ) + F₁(σ) ≤ 1

where F_i(σ) are the respective polynomials of order i of the σ tensor components. Some further work in this field is now presented. It will be seen that a notable simplification has been achieved, bringing down the initial number of components of an 8th rank tensor from 3⁸ = 6561 to a more managable number of 42 for orthotropic materials and 11 for the planar case. It is also shown that the three F_i(σ) polynomials cannot exist simulataneously, leading either to the known Tsai and Wu proposal or a new tensorial form. Some considerations to strength theory for isotropic materials are given as well. Finally, applicability of strength theories for composite laminates is discussed, with some suggestions for practical use.     

Deformation behavior of semi-solid A356 Al–Si alloy at low shear rates: Effect of fraction solid

Conventional casting was employed to produce semi-solid metal, SSM, billets of A356 Al–Si alloy. The rheological behavior of the as-cast SSM billets was studied using the parallel plate compression viscometry. The compression tests were carried out at different applied pressures, billet temperatures and fractions solid. The rheological studies were conducted assuming the SSM slurry behaving as Newtonian or non-Newtonian fluid. The calculated viscosity numbers confirmed the pseudoplastic behavior of the SSM billets and were correlated to fraction solid and shear rate, through an empirical equation, to highlight the effect of metallurgical and process parameters on the viscosity of the SSM A356 alloy.

This equation is valid at very low shear rates, less than 0.01 s⁻¹.     

Thermodynamics and phase equilibria in the Ga---Sb system

The optimum values of the solution parameters of a multiparameter integral free-energy function have been determined using experimental data from the Ga---Sb system. The equation is represented as

ΔG^XS=x(1-x)[(a₁+a₂T+a₃T ln T)+x(a₄+a₅T+a₆T ln T)+x(1-x)(a₇+a₈T+a₉xT)]

The integral and the corresponding partial form of the free energy function have been found to be of use when interpreting the high temperature thermodynamic data, atomic interactions and phase equilibria in the Ga---Sb system.     

The surface of carbon fibres continuously treated by cold plasma

The surface of carbon fibres prepared from polyacrylonitrile (PAN) precursor have been continuously treated by means of a cold plasma. The interlaminar shear strength of carbon fibre reinforced epoxy composites containing fibres so treated was increased from about 60 MPa to 100 MPa. There are four possible mechanisms for this increase:

1. (1) Higher reactivity between fibre surface and matrix as a result of an increase of

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groups on the fibre surface.

2. (2) The surface constitution was changed by the plasma treatment so as to improve the wetting properties of fibre surface. The contact angle θ between water and the carbon fibre was decreased from 75° to 61°.

3. (3) Electron micrographs of the surface of carbon fibre show that the surface striations and surface roughness were changed increasingly on fibre surfaces after plasma etching. This increases the interfacial adhesion and the effect of mechanical interlocking.

4. (4) The strength of the carbon fibre was decreased very little (about 1·6%) by this surface treatment method.

Analysis of fracture morphology by scanning electron microscopy (SEM) indicates that debonding and fibre pull-out between fibre and matrix do not occur in the treated carbon fibre reinforced epoxy composite. These results all show that adhesion between fibre and matrix is very strong. In addition, the plasma treatment technology is very simple and the cost is low. This treatment process produces no environmental pollution and has promising future for engineering applications.     

Comparison of luminescence and physical morphologies of GaN epilayers

In this paper we examine a series of four GaN epilayers grown by MOVPE on sapphire substrates with different AlN buffer layer thicknesses. We examine the effect of the buffer layer thickness on the physical and optical properties of the samples via optical microscopy, cathodoluminescence imaging, photoluminescence, and cathodoluminescence spectroscopy. While the morphological and optical properties of all the films (excepting that with the thinnest buffer layer of 30 nm) are good, i.e. the films are smooth and the luminescence is dominated by excitonic luminescence, a number of circular island like features are observed in all the films whose density decrease with increasing buffer layer thickness. A large circular island present on the sample with the thinnest buffer layer and surrounded by cracks in the 11

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0 directions, displays some interesting acceptor related luminescence.     

GaN thin films deposited by pulsed laser ablation in nitrogen and ammonia reactive atmospheres

Well-oriented, crystalline GaN films were grown on (11

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0) sapphire substrates in reactive atmospheres of N₂ and NH₃ by pulsed laser deposition. GaN targets were ablated at 2.8 J cm⁻² and the substrate temperature was varied from 500 to 700°C. The background gas pressure was varied from 0.04 to 0.3 mbar. All the films had a wurtzite structure. The crystal quality and preferential orientation depended on the substrate temperature, laser fluence and the presence of the nitriding atmosphere. For both N₂ and NH₃, the most resistive films were preferentially orientated in the [000l] direction. For 700°C the film resistivity was found to increase from 10⁻³ Ω cm when deposited in NH₃ to 10² Ω cm when deposited in N₂. The band-gap, obtained from optical transmission measurements shifted from 3.1 to 3.4 eV. Violet photoluminescence was found in all samples and was centered at 3.2 eV with a full width at half maximum of 0.2 eV. A broad peak in the yellow, centered at 2.1 eV, was detected for films grown in vacuum and ammonia.     

Thermally induced logarithmic stress singularities in a composite wedge and other anomalies

Wedge paradoxes, which were first studied by Sternberg and Koiter (Sternberg E, Koiter WT. The wedge under a concentrated couple: a paradox in the two-dimensional theory of elasticity. ASME Journal of Applied Mechanics 1958;4:575–81), occur due to multiple roots in the Williams (Williams ML. Stress singularities resulting from various boundary conditions in angular corners of plates in extension. ASME Journal of Applied Mechanics 1952;19:526–28) eigenfunction expansion. The consequence of such a paradox is a change in behavior of the stresses from , to the ‘non-separable’ form,

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. The focus of this study is the problem of thermally induced logarithmic stress singularities in a composite wedge associated with ω=0. Both double and triple root examples are presented which lead to and behavior in the stresses, respectively. This behavior is primarily associated with incompressible materials for the clamped–clamped single material case, and for the full range of Poisson’s ratio for the clamped-free case. The study also includes non-separable eigenfunctions that occur when complex conjugate roots transition to double real roots. Perhaps the most interesting result is that for the clamped–clamped wedge with Poisson’s ratio equal to 1/2, the hydrostatic stress has a logarithmic singularity proportional to the thermal strain for all wedge angles. This result can be extended to conclude that for a confined, incompressible or nearly incompressible material with a relatively sharp corner, and subject to some expansion or contraction phenomena, high hydrostatic stresses can result.     

Neutrino properties studied with a triton source and a large spherical TPC

The purpose of the present paper is to study the neutrino properties as they may appear in the low-energy neutrinos emitted in triton decay:

with maximum neutrino energy of 18.6 keV. The technical challenges to this end can be summarized as building a very large Time Projection Counters (TPC) capable of detecting low-energy recoils, down to 100 eV, within the required low background constraints. More specifically we propose the development of a spherical gaseous TPC of about 10-m in radius and a 200 Mcurie triton source in the center of curvature. One can list a number of exciting studies, concerning fundamental physics issues, that could be made using a large-volume TPC and low-energy antineutrinos: (1) The oscillation length involving the small angle δ=sin θ₁₃, directly measured in our ν_e disappearance experiment, is fully contained inside the detector. Measuring the counting rate of neutrino–electron elastic scattering as function of the distance of the source will give a precise and unambiguous measurement of the oscillation parameters free of systematic errors. In fact first estimations show that even with a year's of data taking a sensitivity of a few percent for the measurement of the above angle will be achieved. (2) The low-energy detection threshold offers a unique sensitivity for the neutrino magnetic moment which is about two orders of magnitude beyond the current experimental limit of 10⁻¹⁰μ_B. (3) Scattering at such low neutrino energies has never been studied and any departure from the expected behavior may be an indication of new physics beyond the standard model. We present a summary of various theoretical studies and possible measurements, including a precise measurement of the Weinberg angle at very low momentum transfer.     

Spectroscopic studies of Nd-doped alkali fluoroborophosphate glasses

A new family of three optical glasses of the following chemical composition with 1 mol% of Nd³⁺ were prepared to examine the effects of alkali fluorides in unmixed form:

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, where RF=LiF, NaF and KF. On the basis of the measured values of densities and refractive indices, the dielectric constant, reflection losses, molar refraction, Nd³⁺ ion concentration in glasses and several other physical properties were determined. Absorption spectra of these glasses were recorded and Judd–Ofelt intensity parameters (Ω_λ) have been calculated. Radiative lifetimes (τ_R) and branching ratios (β_R) for the fluorescent levels have been determined. To understand the laser efficiency of these materials, the values of the spectroscopic quality factor (Ω₄/Ω₆) has been evaluated and it is found that glass C could be suggested as a suitable lasing material.