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.     

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.     

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.     

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.     

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.     

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.     

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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Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K_1d and J_1d, arrest toughness K_1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from to . At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived:

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. It can describe the fracture process at different temperatures and loading rates. At a loading rate of , the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×10⁶T^−2.8K_1d(K_1a), which correlates K_1d and K_1a with CVN in one equation. This means that we can calculate K_1d and K_1a when we get CVN.     

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.     

Simple formulas for the energy loss of ultrarelativistic muons by direct pair production

Simple formulas for direct pair production are derived from the general equation for deeply inelastic lepton scattering. Applications to energy loss by ultrarelativistic muons are discussed. For muons above the critical energy, E_c^μ 200 GeV, where the radiative effects of direct pair production and bremsstrahlung are dominant, the expressions for energy losses are considerably simplified when quoted in terms of the fractional energy loss per radiation length. The differential probability for direct pair production in a thickness x of material of radiation length X₀ for an incident lepton of energy E, mass M, can be expressed as

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where ν is the energy of the produced pair, υ is the fractional energy loss, υ = ν/E, m_e is the electron mass and the variable z is defined by

For indicent muons, this simple expression agrees very well with the exact calculation to within 30% over the entire range of υ, for E 1 TeV. At higher energies complete screening occurs, and the agreement is further improved, to better than 15% (except for the range 0.005 ≤ υ ≤ 0.01 where it is 25%). The integral of this expression gives the energy loss due to direct pair production by muons (complete screening) which is accurate to 10%:

.     

An exploration of the perceptions of the average driver''s speed compared to perceived driver safety and driving skill

This study investigates the self-enhancement bias in driver attitudes, the finding that drivers rate themselves better than the average driver on safety and skill perceptions (Svenson, 1978 and Svenson, 1981; McCormick et al., 1986). A sample of 86 New Zealand drivers were asked their perceptions of their own and others' speeds in two conditions, 50 km/h and 100 km/h. The results established the self-enhancement bias for speed and safety, but not skill. Between 85% and 90% of drivers claimed to drive slower than the ‘average driver.' A new methodological technique derived from Harré and Gillett (1994) was used to investigate the direction of the self-enhancement bias. The results support the Downward Comparison Theory (Wills, 1981) because drivers consider other drivers negatively, rather than exaggerating their self-perceptions.     

Magnetoelectroelastic multi-inclusion and inhomogeneity problems and their applications in composite materials

We have studied the average magnetoelectroelastic field in a multi-inclusion or inhomogeneity embedded in an infinite matrix. The magnetoelectroelastic inclusion and inhomogeneity problems are discussed [1], and a numerical algorithm to evaluate the magnetoelectroelastic Eshelby’s tensors for the general material symmetry and ellipsoidal inclusion shape is developed. The solutions for the magnetoelectroelastic inclusion and inhomogeneity problems are applied to study the multi-inclusion and inhomogeneity problems. It is shown that the average field in an annulus surrounding an inclusion embedded in an infinite magnetoelectroelastic medium only depends on the shapes and orientations of two ellipsoids, which generalizes Tanaka and Mori's observation in elasticity [2]. The average field in a multi-inclusion is then determined exactly, from which the average field in a multi-inhomogeneity is obtained, using the equivalent-inclusion concept [3]. The solutions of multi-inclusion and inhomogeneity problems serve as basis for an averaging scheme to model the effective magnetoelectroelastic moduli of heterogeneous materials, which generalizes Nemat-Nasser and Hori's multi-inclusion model in elasticity [4]. The model is further extended to predict the effective thermal moduli of the heterogeneous magnetoelectroelastic solids, generalizing the recent work of Li on the thermal expansion coefficients of elastic composites [5]. The proposed model recovers Mori–Tanaka and self-consistent approaches as special cases. Finally, some numerical results are given to demonstrate the applicability of the model. The potential techniques to enhance the magnetoelectric effect in practical composites are also discussed.     

On the concept of e-maintenance: Review and current research

The importance of the maintenance¹ function has increased because of its role in keeping and improving system availability and safety, as well as product quality. To support this role, the development of the communication and information technologies has allowed the emergence of the concept of e-maintenance. Within the era of e-manufacturing and e-business, e-maintenance provides the opportunity for a new maintenance generation. As we will discuss later in this paper, e-maintenance integrates existing telemaintenance principles, with Web services and modern e-collaboration principles. Collaboration allows to share and exchange not only information but also knowledge and (e)-intelligence. By means of a collaborative environment, pertinent knowledge and intelligence become available and usable at the right place and time, in order to facilitate reaching the best maintenance decisions.

This paper outlines the basic ideas within the e-maintenance concept and then provides an overview² of the current research and challenges in this emerging field. An underlying objective is to identify the industrial/academic actors involved in the technological, organizational or management issues related to the development of e-maintenance. Today, this heterogeneous community has to be federated in order to bring up e-maintenance as a new scientific discipline.     

Target system neutronics study for NXGENS

The Materials Test Station (MTS) [E. Pitcher, G. Muhrer, H. Trellue, Neutronics Assessment of the LANSCE Materials Test Station as an Irradiation Facility for the JIMO Space Reactor, LA-CP-04-0903.[1]], a spallation target station, planned for construction at the Los Alamos Neutron Science Center (LANSCE), will provide the opportunity to test the prototype of a long-pulse spallation source neutron scattering instrument (NXGENS). In this paper, we present the target-moderator neutronics optimization study that was performed in support of NXGENS.     

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.     

Punching behavior of biaxial hollow slabs

The invention of a new kind of hollow slab with plastic balls as hollow bodies entailed the necessity to investigate its structural behavior. Because of its main field of application as a flat slab, the punching shear capacity is one of the most interesting properties of this slab. To investigate the influence of the cavities on the punching behavior, tests were carried out at the Institute for Concrete Structures in Darmstadt. In addition to these tests nonlinear computations using the Finite Element Method were performed. The computations allowed parametric studies to get a better understanding of the structural behavior without doing further expensive tests. Finally, necessary modifications of existing design recommendations according to the German design code DIN 1045 [1] were developed.     

An integrated database and expert system for failure mechanism identification: Part II—the system and performance testing

An integrated database and expert system has been developed for assisting the human analyst in identifying the failure mechanism of mechanical components. The system comprises six major modules: database and management system; case maintenance; knowledge acquisition and editing; expert system; explanation and test-recommendation facilities, and user interface. The knowledge acquisition and editing module has been detailed in Part I of a two-part paper [6]. The overall system and the details of other modules are described here. The system has been implemented using the M4 expert system shell, Microsoft Access database software, and Visual Basic. Illustrative examples are used to show the capability of the system. Also reported are the results obtained from four different tests to determine the system performance.     

Vertical MOS-gated pin-diodes: MOS-gated tunneling transistors in Si(100) and Si(111)

Tunneling devices are an interesting alternative to conventional MOS-devices due to their high speed switching capabilities. Recently, it was shown that tunneling transistors based on vertical MOS-gated pin-diodes can be fabricated. The pin-diodes themselves were grown by means of UHV-MBE on highly n⁺-doped Si(100)-substrates with a 100 nm thick intrinsic channel region. The top contact was formed by the deposition of a highly-doped B δ-layer with a peak doping amount of approximately 10²¹ cm⁻³ for the necessary abrupt pn-junction and 300-nm p⁺-contact region. At a low supply voltage of −0.2 V, a current gain of three orders of magnitude with saturation behavior is achieved [1]. In the present contribution, we have shown the influence of the amount of B in the δ-layer and of the abruptness of the drain-channel-junction on the transistor behavior. For that, we have discussed the characteristics of MOS-gated pin-diodes on Si(111) with ultra-sharp B δ's with a peak doping amount between 10²⁰ and 10²¹ cm⁻³ and a peak width <3 nm, in comparison with MOS-gated pin-diodes on Si(100) presented in Hansch et al. [1]. In order to obtain these highly doped ultra-sharp B δ-layers, a phase-transition from an electrically inactive Si(111)- -R30° B surface phase into an electrically active one was induced by rapid thermal annealing.     

An integrated database and expert system for failure mechanism identification: Part I—automated knowledge acquisition

An integrated database and expert system has been developed for identifying the failure mechanism of mechanical components. The system comprises six major modules: database and management system, case maintenance; knowledge acquisition and editing; expert system; explanation and test-recommendation facilities; and user interface. Part I of a two-part paper details the knowledge acquisition and editing module, as presented here. Part II describes the remaining modules and also gives test results [9]. The method used for automated knowledge acquisition is an inductive learning algorithm, which was modified from PRISM [2] to handle noisy and missing data. Using the algorithm, a total of 48 rules were induced from 477 training examples gathered for the identification of 15 different failure mechanisms such as brittle fracture, fatigue, and stress corrosion cracking. Fifty-nine attributes were used to distinguish one failure mechanism from the others. They include pitted, beach marks, microvoids, etc. The knowledge editing function is provided to allow the verification of induced rules by the human expert.