Hydrogen sensors based on Pt-AlGaN/GaN back-to-back Schottky diode

In this paper, platinum (Pt) with a thickness of 45 nm was sputtered on the surface of AlGaN/GaN heterostructure to form the Schottky contact and the back-to-back Schottky diodes were characterized for H-2 sensing at room temperature. Both the forward and reverse current of the devices increased with exposure to H-2 gas, which was attributed to Schottky barrier height reduction caused by hydrogen absorption in the catalytic metals. A shift of 0.7 V at 297 K was obtained at a fixed forward current of 0.1 mA after switching from N-2 to 40% H-2 in N-2. The sensor's responses under different concentrations from 2500 ppm H-2 to 40% H-2 in N-2 at 297 K were investigated. Time response of the sensor at a fixed bias of 1 V was given. Finally, the decrease of the Schottky barrier height and the sensitivity of the sensor were calculated. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Investigation of the Mechanical Properties of Thin Films by Nanoindentation, Considering the Effects of Thickness and Different Coating-Substrate Combinations

In order to further investigate nanoindentation data of film-substrate systems and to learn more about the mechanical properties of nanometer film-substrate systems, two kinds of films on different substrate systems have been tested with a systematic variation in film thickness and substrate characteristics. The two kinds of films are aluminum and tungsten, which have been sputtered on to glass and silicon substrates, respectively. Indentation experiments were performed with a Nano Indent XP II with indenter displacements typically about two times the nominal film thicknesses. The resulting data are analyzed in terms of load-displacement curves and various comparative parameters, such as hardness, Young's modulus, unloading stiffness and elastic recovery. Hardness and Young's modulus are investigated when the substrate effects are considered. The results show how the composite hardness and Young's modulus are different for different substrates, different films and different film thicknesses. An assumption of constant Young's modulus is used for the film-substrate system, in which the film and substrate have similar Young's moduli. Composite hardness obtained by the Joslin and Oliver method is compared with the directly measured hardness obtained by the Oliver and Pharr method.

Influence of the thickness ratio of air layer to liquid layer on the onset of Benard-Marangoni convection

An experimental investigation of the onset of Benard-Marangoni convection has been performed in a liquid layer of rectangular configuration. The critical temperature difference was measured via the detections of both temperature field pattern (IR-imaging) on the free surface and fluid convection (PIV) in the liquid layer. The critical temperature difference or the critical Marangoni number was given. The experiments were performed for a fixed depth of air layer and a changeable depth of the liquid layer, and then the influence of the thickness ratio of the air layer to liquid layer on the Marangoni instability was studied.

The relationship of SIF between plate and plane fracture problems and the effect of the plate thickness on SIF

In the present paper, it is shown that the zero series eigenfunctions of Reissner plate cracks/notches fracture problems are analogous to the eigenfunctions of anti-plane and in-plane. The singularity in the double series expression of plate problems only arises in zero series parts. In view of the relationship with eigen-values of anti-plane and in-plane problem, the solution of eigen-values for Reissner plates consists of two parts: anti-plane problem and in-plane problem. As a result the corresponding eigen-values or the corresponding eigen-value solving programs with respect to the anti-plane and in-plane problems can be employed and many aggressive SIF computed methods of plane problems can be employed in the plate. Based on those, the approximate relationship of SIFs between the plate and the plane fracture problems is figured out, and the effect relationship of the plate thickness on SIF is given.

Two bifurcation transitions of the floating half zone convection in a fat liquid bridge of larger Pr

The transient process of the thermocapillary convection was obtained for the large Pu floating half zone by using the method of three-dimensional and unsteady numerical simulation. The convection transits directly from steady and axisymmetric state to oscillatory flow for slender liquid bridge, and transits first from steady and axisymmetric convection to the steady and non-axisymmetric convection, then, secondly to the oscillatory convection for the fatter liquid bridge. This result implies that the volume of liquid bridge is not only a sensitive critical parameter for the onset of oscillation, but also relates to the new mechanism for the onset of instability in the floating half zone convection even in case of large Prandtl number fluid.

Instability from steady and axisymmetric to steady and asymmetric floating half zone convection in a fat liquid bridge of larger Prandtl number

The linear instability analysis of the present paper shows that the thermocapillary convection in a half floating zone of larger Prandtl number has a steady instability mode w(i) = 0 and m = 1 for a fat liquid bridge V = 1.2 with small geometrical aspect ratio A = 0.6. This conclusion is different from the usual idea of hydrothermal instability, and implies that the instability of the system may excite a steady and axial asymmetric state before the onset of oscillation in the ease of large Prandtl number.

Refractive index and thickness analysis of natural silicon dioxide film growing on silicon with variable-angle spectroscopic ellipsometry

The refractive index and thickness of SiO2 thin films naturally grown on Si substrates were determined simultaneously within the wavelength range of 220-1100 nm with variable-angle spectroscopic ellipsometry. Different angles of incidence and wavelength ranges were chosen to enhance the analysis sensitivity for more accurate results. Several optical models describing the practical SiO2-Si system were investigated, and best results were obtained with the optical model, including an interface layer between SiO2 and Si, which proved the existence of the interface layer in this work as described in other publications.

An Approximate Method for Evaluating the Shear Band Thickness in Saturated Soils

formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations.

Thickness And Component Distributions Of Yttrium-Titanium Alloy Films In Electron-Beam Physical Vapor Deposition

Thickness and component distributions of large-area thin films are an issue of international concern in the field of material processing. The present work employs experiments and direct simulation Monte Carlo (DSMC) method to investigate three-dimensional low-density, non-equilibrium jets of yttrium and titanium vapor atoms in an electron-beams physical vapor deposition (EBPVD) system furnished with two or three electron-beams, and obtains their deposition thickness and component distributions onto 4-inch and 6-inch mono-crystal silicon wafers. The DSMC results are found in excellent agreement with our measurements, such as evaporation rates of yttrium and titanium measured in-situ by quartz crystal resonators, deposited film thickness distribution measured by Rutherford backscattering spectrometer (RBS) and surface profilometer and deposited film molar ratio distribution measured by RBS and inductively coupled plasma atomic emission spectrometer (ICP-AES). This can be taken as an indication that a combination of DSMC method with elaborate measurements may be satisfactory for predicting and designing accurately the transport process of EBPVD at the atomic level.

Finite element analysis on stresses field of normalized layer thickness within ceramic coating on aluminized steel

Multilayer ceramic coatings were fabricated on steel substrate using a combined technique of hot dipping aluminum(HDA) and plasma electrolytic oxidation(PEO). A triangle of normalized layer thickness was created for describing thickness ratios of HDA/PEO coatings. Then, the effect of thickness ratio on stresses field of HDA/PEO coatings subjected to uniform normal contact load was investigated by finite element method. Results show that the surface tensile stress is mainly affected by the thickness ratio of Al layer when the total thickness of coating is unchanged. With the increase of A] layer thickness, the surface tensile stress rises quickly. When Al2O3 layer thickness increases, surface tensile stress is diminished. 'Meanwhile, the maximum shear stress moves rapidly towards internal part of HDA/PEO coatings. Shear stress at the Al2O3/Al interface is minimal when Al2O3 layer and Al layer have the same thickness.

The Effect of the Thickness of Fe2 Al5 Phase Layer at Fe/Al Interface on the Mechanics Behavior of Hot Dip Aluminizing Coating

Hot Dip Aluminized Coatings with different thickness were prepared on Q235 steel in aluminum solutions with different temperature for certain time. Through tensile tests and in-situ SEM observations, the effect of the coating's microstructure on the tensile strength of the samples was studied. It was disclosed at certain aluminum solution temperature,transaction layers mainly composed of Fe2 Al5 phase got thicker with time prolonging, and this changed initial crack's extending direction from parallel with to vertical with stretching direction. The change in crack direction decreased tensile strength of samples, thus made the coating easy to break. It was concluded that the existence of thick Fe2 Al5 phase layer was the basic reason for the lowering of tensile strength of the coating.

Prediction Of Shear-Band Thickness In Metallic Glasses

We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effects of geometric shape on the hydrodynamics of a self-propelled flapping foil

The hydrodynamics of a free flapping foil is studied numerically. The foil undergoes a forced vertical oscillation and is free to move horizontally. The effect of chord-thickness ratio is investigated by varying this parameter while fixing other ones such as the Reynolds number, the density ratio, and the flapping amplitude. Three different flow regimes have been identified when we increase the chord-thickness ratio, i.e., left-right symmetry, back-and-forth chaotic motion, and unidirectional motion with staggered vortex street. It is observed that the chord-thickness ratio can affect the symmetry-breaking bifurcation, the arrangement of vortices in the wake, and the terminal velocity of the foil. The similarity in the symmetry-breaking bifurcation of the present problem to that of a flapping body under constraint is discussed. A comparison between the dynamic behaviors of an elliptic foil and a rectangular foil at various chord-thickness ratios is also presented.

Elastic deformation of soft membrane with finite thickness induced by a sessile liquid droplet

In this paper, the role of vertical component of Surface tension of a droplet on the elastic deformation of a finite-thickness flexible membrane was theoretically analyzed using Hankel transformation. The vertical displacement at the Surface was derived and can be reduced to Lester's or Rusanov's solutions when the thickness is infinite. Moreover, some Simulations of the effect of a liquid droplet on a membrane with a finite thickness were made. The numerical results showed that there exists a saturated membrane thickness of the order of millimeter, when the thickness of a membrane is larger than such a value, the membrane can be regarded as a half-infinite body. Further numerical calculations for soft membrane whose thickness is far below the saturated thickness were made. By comparison between the maximum vertical displacement of an ultrathin soft membrane and a half-infinite body, we found that Lester's or Rusanov's solutions for a half-infinite body cannot correctly describe Such cases. In other words, the thickness of a soft membrane has great effect on the surface deformation of the ultrathin membrane induced by a liquid droplet. (C) 2009 Elsevier Inc. All rights reserved.