Search

The problem of hydroelastic wave diffraction by a surface-piercing vertical circular cylinder mounted on the bottom of an ice-covered channel is considered. The ice sheet is modelled as an elastic thin plate with homogeneous properties, while the linearized velocity potential theory is adopted to describe the motion of the fluid. The solution starts from the Green function satisfying all other boundary conditions apart from that on the body surface. This is obtained through applying a Fourier transform in the longitudinal direction of the channel and adopting an eigenfunction expansion in the vertical direction. The boundary conditions on the side walls and ice edges are imposed through an orthogonal product. Through the Green function, the velocity potential due to a surface-piercing structure with arbitrary shape can be expressed through a source distribution formula derived in this work, in which only integrals over the body surface and its interaction line with the ice sheet need to be retained. For a vertical circular cylinder, the unknown source distribution can be expanded further into a Fourier series in the circumferential direction, and then the analytical solution of the velocity potential can be obtained further. Extensive results and discussions are provided for the hydrodynamic forces and vertical shear forces on the cylinder, as well as the deflection and strain of the ice sheet. In particular, the behaviour of the solution near one of the natural frequencies of the channel is investigated in detail.

The problem of interaction of a uniform current with a submerged horizontal circular cylinder in an ice-covered channel is considered. The fluid flow is described by linearized velocity potential theory and the ice sheet is treated as a thin elastic plate. The potential due to a source or the Green function satisfying all boundary conditions apart from that on the body surface is first derived. This can be used to derive the boundary integral equation for a body of arbitrary shape. It can also be used to obtain the solution due to multipoles by differentiating the Green function with its position directly. For a transverse circular cylinder, through distributing multipoles along its centre line, the velocity potential can be written in an infinite series with unknown coefficients, which can be determined from the impermeable condition on a body surface. A major feature here is that different from the free surface problem, or a channel without the ice sheet cover, this problem is fully three-dimensional because of the constraints along the intersection of the ice sheet with the channel wall. It has been also confirmed that there is an infinite number of critical speeds. Whenever the current speed passes a critical value, the force on the body and wave pattern change rapidly, and two more wave components are generated at the far-field. Extensive results are provided for hydroelastic waves and hydrodynamic forces when the ice sheet is under different edge conditions, and the insight of their physical features is discussed.

The dense TiC-TiB2/Ni composite was successfully fabricated through the pressure-assisted thermal explosion reaction from Ti, B4C and Ni powder blends in air. The ignition temperature (625 degrees C) in air was 415 degrees C lower than that in vacuum. The decreased ignition temperature resulted from a chemical oven mechanism in which the oxidation and nitrification of Ti and the oxidation of B4C released the heat and promoted the occurrence of the thermal explosion reaction. The composites prepared in air and vacuum had similar phase constituents and microstructure. Moreover, the composite prepared in air possessed comparable hardness, flexural strength and fracture toughness to the composite prepared in vacuum.

We report that pure microwave (MW) radiation, without the assistance of MW susceptors, can rapidly heat titanium hydride (TiH2) powder to 1300 C for isothermal sintering. The release of hydrogen, however, induces low-temperature plasmas which reflect incident MWs, leading to cyclic temperature fluctuations. Commercially pure titanium fabricated by MW radiation of TiH2 powder exhibits much higher sintered densities, tensile strength and ductility together with finer residual pores, compared to that sintered from hydride-dehydride titanium powder.© 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A small addition (≤0.5 wt%) of cerium silicide (CeSi 2 ) to powder metallurgy (PM) commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) results in substantial microstructural refinement and noticeably improved ductility with marginally improved sintered density. CeSi 2 is unstable and decomposes between 1423 K and 1473 K. The Si goes into solid solution in β-Ti and is responsible for the improved sintered density while the Ce scavenges both oxygen (O) and chlorine (Cl) from the Ti powder and therefore improves tensile ductility. The resulting CeO 2 and CeCl x O y particles generally exist along or close to the prior-β grain boundaries. The substantial microstructural refinement in terms of both the prior-β grain size and the subsequent α-Ti lath size is attributed to the grain boundary pinning effect of the CeO 2 particles. The optimum concentration of CeSi 2 is approximately 0.5 wt%, beyond which both the sintered density and tensile elongation drop with increasing addition of CeSi 2 . CeSi 2 can be a practical form of Ce addition to PM Ti alloys for impurity scavenging, microstructural refinement and tensile ductility improvement.

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si lower the solidus of Ti–Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti–3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti5Si3 form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti5Si3 phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.

A comprehensive study has been made of the sintering and microstructural evolution of Ti–10V–2Fe–3Al compacted from titanium and master alloy powder blends. The densification of Ti–10V–2Fe–3Al at ≤1300 °C occurs by solid-state sintering with apparent activation energy of 163 ± 13 kJ/mol, which falls into the reported activation energy range for the self-diffusion of titanium. The sintered density depends primarily on the titanium powder size while the sintered microstructure and mechanical properties depend mainly on the master alloy type or diffusion of vanadium. The real challenge for the fabrication of Ti–10V–2Fe–3Al by sintering is not densification, which is trivial with relatively fine titanium powder. Rather, it is the realisation of a desired microstructure through master alloy selection or design of the sintering pathway, which determines the distribution of the principal alloying element V, a slow diffuser in β-Ti. This distinguishes it from the sintering of lean alloys, where the focus has predominantly been on densification. The use of 10V–2Fe–3Al master alloy produced more uniform microstructures and therefore much better ductility than the use of 85V–15Al at similar densities.

The ignition temperature of the thermal explosion reaction from the Ti-C system under air is much lower than that under Ar atmosphere. The ignition mechanism for the Ti-C system under air is determined to be a mechanism of chemical oven, and the reaction mechanism is dissolution, reaction, and precipitation. Namely, the heat generated from the oxidation and nitrification of Ti and C can promote the melting of Ti inside the compact; subsequently, the carbon atoms dissolve into the Ti melt and TiC precipitate.

The effect of the reactant particle size on the self-propagating high-temperature synthesis (SHS) reaction behaviors in the Ni-Ti-B4C system was investigated in the present study. The B4C particle size has a significant effect on the ignition behavior and combustion wave velocity, while Ti and Ni particle sizes have only a limited influence. Increasing the B4C particle size retards the reaction initiation and remarkably lowers the combustion velocity and combustion temperature. The reaction products are mainly dependent on the B4C particle size. Nevertheless, decreasing Ni and Ti particle sizes is in favor of perfect products consisting of only TiC, TiB2, and Ni phases.

The SHS reaction in the Ni–Ti–B4C system starts with the formation of Ni–Ti and Ni–B intermetallic compounds from the solid interacted reaction among the reactants and, subsequently, the formation of Ni–Ti and Ni–B liquid at the eutectic point. Meanwhile, some C atoms from the reaction between Ni and B4C can dissolve into Ni–Ti liquid to form TiC. The heat generated from these reactions can promote the mutual diffusion of Ni–Ti–C and Ni–B liquid and simultaneously accelerate the formation of Ni–Ti–C–B liquid. Finally the precipitation of TiC and TiB2 occur when the C and B atoms in the liquid become supersaturated. The addition of Ni not only promotes the occurrence of the self-propagating high temperature synthesis (SHS) reaction by forming Ni–Ti liquid, but also accelerates the SHS reaction by forming Ni–B liquid and dissociative C. The early appearance of dissociative C from the reaction between Ni and B4C causes the formation of TiC prior to that of TiB2.

The solidification behavior of the cast TiB 2 particulate reinforced AZ91 composite, including the cooling curve, microstructure and grain refinement effect, was investigated in the present study. Compared with the AZ91 alloy, the composite possessed lower nucleation temperature of primary α-Mg, almost the same eutectic reaction temperature and faster cooling rate during the total solidification time. The TiB 2 particulates were pushed into the last solidified region and were surrounded by the eutectic Mg 17 Al 12 . The grain refinement effect in the TiB 2 /AZ91 composite can be attributed to the growth restriction effect exerted by TiB 2 particulates and the faster cooling rate of the composite.

The effect of Al–P–Ti–TiC–Nd2O3 modifier on the microstructure and mechanical properties of hypereutectic Al–20 wt.%Si alloy has been investigated. It has been concluded that Nd2O3 in Al–P–Ti–TiC–Nd2O3 modifier can modify primary silicon in hypereutectic Al–Si alloys. Statistical analysis shows that primary silicon size (approximately average 38 μm) in modified Al–20 wt.%Si alloy with Al–P–Ti–TiC modifier is larger than that (approximately average 20 μm) in modified Al–20 wt.%Si alloy with Al–P–Ti–TiC–Nd2O3 modifier. Compared with primary silicon modified with Al–P–Ti–TiC modifier, Primary silicon modified with Al–P–Ti–TiC–Nd2O3 modifier has the smaller size at different holding times such as 10, 60 and 120 min. The internal defects in coarser primary silicon particles play an important role in determining the tensile strength and elongation of the materials. Coarser primary silicon particles in unmodified Al–Si alloys can degrade the loading bearing capacity of the materials. Modification can eliminate the internal defects to a great extent and improve the tensile strength and elongation of hypereutectic Al–Si alloys. Compared with unmodified and heat-treated Al–20 wt.%Si alloy, the tensile strength of the modified with Al–P–Ti–TiC–Nd2O3 modifier and heat-treated Al–20 wt.%Si alloy increases by 20%.

A method is proposed to estimate quantitatively the shifting distance of the eutectic point in a horizontal direction (under non-equilibrium solidification condition). The shifting distances of the eutectic point in Al–20 wt.%Si alloys under the non-equilibrium solidification condition are 0.4 and 0.9 wt.% in a horizontal direction towards higher silicon contents with the addition of 0 and 3 wt.% master alloys (Al–20 wt.%Y 2 O 3 ), respectively.

Exothermic reaction path of the Ni–Ti–B4C system in the differential thermal analysis apparatus proceeded in such a way that Ni initially reacted with B4C and Ti to form Ni–B compounds and Ti2Ni with heat release, respectively; subsequently, the formation of Ni–B eutectic liquid between Ni2B and Ni4B3, and the appearance of Ni–Ti eutectic liquid phase between Ti2Ni and Ti were followed, which enabled the diffusion of Ni, B, C, and Ti in solution to form Ni–Ti–B–C liquid more easily and quickly at a low temperature; finally, TiC and TiB2 were precipitated out of the liquid. The addition of Ni provided an easier route for reactant mass transfer and accelerated the occurrence of complete reaction.

A 40 μm thick melt-spun ribbon was manufactured by using a single roller melt-spinning technique, and the temperature distribution of the melt-spun ribbon was obtained. The solidification time and the cooling rate of the melt-spun ribbon were estimated to be 6.59 × 10 −5 s and 1.11 × 10 6 °C/s, respectively. The morphologies of primary silicon in the melt-spun ribbon were drastically changed to fine block-shape (maximum size of primary silicon is 5 μm, minimum size of primary silicon is 0.5 μm and average size of primary silicon is 2 μm), and the smaller eutectic silicon can be obtained due to the rapid solidification.

Accurate temperature measurement remains a challenge for microwave heating of powder materials. We propose a temperature calibration method based on exothermic reactions and the resultant thermal runaway that occurs during microwave heating. The approach was demonstrated on microwave heating of four titanium alloys. Differential scanning calorimetry was used to determine the threshold reaction temperature for each selected titanium alloy. This served as a standard for the microwave heating of these titanium alloys. Infrared pyrometric temperature measurements were then calibrated by comparing the starting temperature of each thermal runaway event with the threshold reaction temperature.

InGaP/GaAs heterojunction bipolar transistors (HBTs) with and without passivation ledge in the extrinsic base region were investigated. Gummel plot changes before and after reliability testing were compared. The experimental results demonstrated that the devices featuring the lower quality of the extrinsic base surface are more sensitive to a temperature–current stress. The HBTs with a passivation ledge have an activation energy of 1.41 eV and a mean time to failure (MTTF) of 106 h whereas the HBTs without passivation ledge have an activation energy of 1.24 eV and a MTTF of 105 h.

Zinc-doped GaInP/GaAs heterojunction bipolar transistors (HBTs) grown by MOCVD were fabricated. Characteristics of HBTs with an undoped spacer layer and an n-type GaAs set-back layer (heterostructure-emitter) between base and emitter were investigated. The results show that the base dopant out-diffusion was effectively prevented in heterostructure-emitter bipolar transistors (HEBTs). In addition, a current gain of 100 at the collector current density of 1*10-2 A cm-2 and an offset voltage of 57 mV were obtained for Zn-doped HEBTs. In comparison with carbon-doped HBTs, HEBTs Offer a higher current gain at a low current level, a smaller offset voltage and better uniformity in characteristics across the wafer.

A series of samples with nominal composition of La3CaχBa4−χCu7Oy (0≤χ≤4.00) was prepared by the conventional powder reaction method. Single-phase materials with triple-perovskite structure were obtained for χ≤1.25 samples of which the lattice parameters monotonically decreased with the increase of Ca substitution. Superconductivity was observed for χ≤3.50 samples with 37 K≤Tconset≤86 K. A Tc plateau above 80 K was found in the 1.00≤χ≤1.25 samples with a-axis length of 3.879≥a≥3.872 A. In other words, an optimal rCu-O distance of ca. 1.94 A in the Cu-O planes was the most appropriate value for superconductivity in the La3CaχBa4−χCu7Oy system.

The composition of as-grown BiSrCaCuO superconducting thin films has been investigated as a function of the RF magnetron sputtering variables, i.e. total sputtering pressure, substrate temperature, and oxygen partial pressure in an off-axis configuration. This study shows that use of off-axis geometry alone is insufficient to completely overcome the resputtering problem due to energetic particle bombardment on the substrate. The bismuth loss in the sputtered films at low sputtering pressure and copper loss at high pressure, both of which were aggravated at increasing temperatures, were observed. The mechanisms of thermally enhanced resputtering and gas scattering effects at high pressure are proposed to interpret the discrepancy between target and film composition. >

An analysis of the effects of a magnetic field applied perpendicular and parallel to the initial injection velocity of ballistically injected energetic electrons in a quasiballistic heterojunction bipolar transistor has resulted in an expected reduction in electron energy directed towards the collector barrier. A measure of the high energy effective mass in InGaAs resulted in a value of 0.08 times the free electron mass. In addition, the application of a magnetic field parallel to the ballistic velocity induces a broadening and shift to higher energies of the thermalized distribution. This result has been interpreted in terms of a reduction in the scattering rate of energetic electrons due to the imposition of quasi one-dimensional constraints by the magnetic field.

A two-dimensional quantum model based on the solution of the Schrodinger and Poisson equations is first presented for an In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As/InP HEMT. According to the model, the two-dimensional distributions of electron density and transverse electric field in the channel of the InAlAs/InGaAs HEMT are discussed.