Thermal instability of a rotating curved plate subjected to an applied magnetic field

The effect of a magnetic field on thermal instability in mixed convection flow on a heated rotating convex surface is studied in this paper. The onset position characterized by the Goertler number G _δ depends on the Grashof number, the rotational number, the Prandtl number, the magnetic field parameter, and the wave number. The buoyancy force, the centrifugal force, the Lorentz force, and the Coriolis force are found to significantly affect the flow structure and heat transfer of the flow. Negative rotation (clockwise) destabilizes the boundary layer flow on a convex surface. However, the Lorentz force stabilizes the flow. Numerical data in this study show the same order of magnitude like experimental data.     

Effect of rotation on surface tension driven flow during protein crystallization

The effect of rotation on surface tension gradient driven flow, also known as Marangoni convective flow, during protein crystallization is modeled and studied computationally under microgravity conditions, where the surface tension gradient force is the main significant driving force. The main parameters are the solutal Marangoni number M_c, representing the surface tension gradient force and the Taylor number Ta representing the rotational effect. The numerical computations for various values of the parameters and low gravity levels indicated nontrivial competing effects, due to surface tension gradient, centrifugal and Coriolis forces on the flow adjacent to the protein crystal interface and the associated solute flux. In particular, for given values of M_c, certain values of Ta were detected where the Sherwood number (Sh), representing the convective solute flux, and the convective flow effects are noticeably reduced. These results can provide conditions under which convective flow transport during the protein crystallization approaches the diffusion limited transport, which is desirable for the production of higher quality protein crystals.     

Nonlinear thermal instability in a horizontal porous layer with an internal heat source and mass flow

Linear and nonlinear stability analyses of Hadley–Prats flow in a horizontal fluid-saturated porous medium with a heat source are performed. The results indicate that, in the linear case, an increase in the horizontal thermal Rayleigh number is stabilizing for both positive and negative values of mass flow. In the nonlinear case, a destabilizing effect is identified at higher mass flow rates. An increase in the heat source has a destabilizing effect. Qualitative changes appear in R_z as the mass flow moves from negative to positive for different internal heat sources.     

Electrical characteristics of Ti-O/Ta2O5 thin film sputtered on Ta/Ti/Al2O3 substrate

The electrical characteristics of Ti-O/Ta₂O₅ films sputtered on Ta/Ti/Al₂O₃ substrate were investigated. Ta (tantalum) was used for the bottom and upper electrodes for the purpose of simplifying the fabrication process and Al₂O₃ substrates were used, which are needed in integral passive devices. Ta/Ti-O/Ta₂O₅/Ta/Ti/Al₂O₃ capacitors were annealed at 700 °C for 60 s in vacuum. The X-ray diffraction pattern (XRD) results showed that as-deposited Ta had a highly preferred orientation, but Ta₂O₅ film had amorphous structure, which was transformed to crystallization structure by rapid thermal heat treatment. We examined the log J-E and C-V characteristics of the dielectric thin films deposited on the Ta bottom electrode. From these results, we concluded that the leakage current could be reduced by introducing a Ti-O buffer layer. The conduction mechanisms of Ta/Ti-O/Ta₂O₅/Ta/Ti/Al₂O₃ capacitors could be interpreted appropriately by hopping conduction in lower field (E<1×10⁵ V/cm) and space-charge-limited current in higher fields (1×10⁵ V/cm<E).     

Phases Resulting from the Reduction of Manganese Metaniobate by Niobium and Tantalum

The sequence of reactions in the reduction of manganese metaniobate with Nb, NbO, and Ta is studied. The reduction of MnNb₂O₆ by Nb or NbO differs in some respects from that by Ta. No Mn(Nb_{2 – x} Ta _x )O_4.33 or Mn(Nb_{2 – x} Ta _x )O_3.67 solid solutions were detected in the reduction products. Tantalum converts directly into manganese metatantalate, without oxidation according to the scheme Ta–TaO–TaO₂–Ta₂O₅. A reaction scheme is suggested for the reduction of MnNb₂O₆ by niobium and tantalum. The unit-cell parameters of the orthorhombic phase MnNb₂O_4.33 resulting from the reduction of MnNb₂O₆ with Nb or Ta are determined.     

Effects of the Ta content on the microstructure and electrical property of reactively sputtered TaxZr1−xN thin films

The Ta_xZr_1−xN films were prepared by reactive magnetron sputtering and the concentration of zirconium and tantalum was regulated by controlling the power to the sputtering guns. The effects of the Ta content on the microstructure, composition and electrical properties of Ta_xZr_1−xN films were investigated by x-ray diffraction, field-emission electron probe micro-analyzer, atomic force microscopy, x-ray photoelectron spectroscopy, four point probe and Hall-effect measurements. Results indicated that the Ta_xZr_1−xN films with different Ta contents were crystallized in NaCl-type structure. However, the lattice constant of Ta_xZr_1−xN films decreased with the increase of the Ta content due to the smaller ionic radius of Ta⁵⁺ comparing with that of Zr⁴⁺. The decreasing lattice constant of Ta_xZr_1−xN films with the Ta content evidenced the successful substitution of Zr with Ta. The electrical resistivity of Ta_xZr_1−xN films showed a minimum value of 78 μΩ cm at Ta content of 3.5 at.% and then increased with the increase of Ta content. Hall measurements indicated that the electrical conduction of films was essentially due to electrons (n-type). And the increase of carrier density and mobility at a Ta content of 3.5 at.%, caused by the extra d valence electron of Ta and the less electron scattering of grain boundaries, was responsible for the further decreasing of resistivity from pure ZrN_x films.     

Microstructure and electrical properties of (Ta, Co, Pr) doped TiO2 based electroceramics

The addition of different dopants affects the densification, mean grain size and electrical properties of TiO₂-based varistor ceramics. This paper discusses the microstructural and electrical properties of (Ta, Co, Pr) doped TiO₂ systems, demonstrating that some of these systems display electrical properties that allow for their use as low voltage varistor. Dopants such as Ta₂O₅ play a special role in the formation of barriers at the grain boundary and in the nonlinear behavior in TiO₂-based systems. The higher values of nonlinear coefficient and breakdown electric field were obtained in the system just doped with Ta₂O₅ and CoO.     

Role of iodate ions in chemical mechanical and electrochemical mechanical planarization of Ta investigated using time-resolved impedance spectroscopy

Chemical mechanical planarization (CMP) is currently used in the processing of Cu/Ta interconnect structures. Electrochemical mechanical planarization (ECMP) is an emerging extension of CMP that can potentially allow low down-pressure planarization of newer interconnect structures containing easily breakable porous dielectrics. In both CMP and ECMP of Ta, it is necessary to chemically (or electrochemically) form a “soft” surface film that can be easily removed by minimum mechanical abrasion. Alkaline KIO₃ solutions appear to serve this purpose in CMP of Ta and, as we show in this work, may also be utilized in ECMP of Ta. Using time-resolved impedance spectroscopy we study here the relevant surface reactions of IO₃⁻ that lead to a structurally weak surface film of soluble hextantalate [(Ta₆O₁₉)⁸⁻] embedded in (native or electro-generated) Ta₂O₅ on Ta. Catalytic reduction of IO₃⁻ on Ta₂O₅ enhances the local pH at the oxidized surface and promotes the conversion of Ta₂O₅ to (Ta₆O₁₉)⁸⁻. The chemical role of iodate ions in material removal through these reactions is similar to that of hydrogen peroxide in CMP of Ta in alkaline media.     

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

The Rutherford backscattering technique utilizing 2 MeV He⁺ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta₂O₅ layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta₂O₅ films on tantalum, there is no sharp boundary between Ta and Ta₂O₅. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm^-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.     

Improving the hard magnetic properties by intragrain pinning for Ta doped nanocrystalline Ce-Fe-B alloys

To develop Ce based permanent magnets with high performance/cost ratio, Ta doping is was employed to enhance the magnetic performance of Ce-Fe-B alloys. For melt spun Ce₁₇Fe_78-xTa_xB₆ (x = 0–1) alloys, the coercivity H_c increases from 439 to 553 kA/m with increasing x value from 0 to 0.75. Microstructure characterizations indicate that Ta doping is helpful for grain refinement. A second phase of TaB₂ is observed in Ce₁₇Fe_77.25Ta_0.75B₆ alloy, which acts as the pinning center of the magnetic domains, resulting in the change of coercivity mechanism from nucleation type to nucleation + pinning type. The micromagnetic simulation confirms that non-magnetic particles within hard magnetic phase can increase the demagnetization field around them and it is crucial for preventing the further magnetization reverse by pinning effect. Take the advantage of Ta doping for enhancing the coercivity, Ce content of Ce-Fe-B alloy can be further cut down to increase the remanence J_r due to the reduced volume fraction of CeFe₂ phase and increased Fe/Ce ratio. As a result, a good combination of magnetic properties with H_c = 514 kA/m, J_r = 0.49 T, and the maximum energy product (BH)_max = 36 kJ/m³ have been obtained in Ce₁₅Fe_79.25Ta_0.75B₆ alloy. It is expected that the present work can serve as a useful reference for designing new permanent magnetic materials with low-cost.     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

The use of Ta/Ta2O5 electrode as an indicator electrode in potentiometric acid-base titrations in fused KNO3

The Ta/Ta₂O₅ electrode was prepared by the electrolytic oxidation of molten KNO₃ on tantalum. This electrode can be used as an indicator electrode in potentiometric acid-base titrations. It behaves reversibily and responds theoretically to the oxide-ion concentration in molten KNO₃. Its potential varies linearly with the logarithm of [O^2?] and the slope of the line amounts to 65 mV at 350°C. The standard potential of Ta/Ta₂O₅,O^2? electrode, i.e. the potential at [O^2?] = 1, is computed as -985 mV relative to the Ag/Ag(I) reference electrode.     

Structural evolution of a Ta-filament during hot-wire chemical vapour deposition of Silicon investigated by electron backscatter diffraction

We report on the application of electron backscatter diffraction to investigate the structural changes of a tantalum filament operated at typical hot-wire chemical vapour deposition conditions for the synthesis of hydrogenated nanocrystalline silicon. Various tantalum-silicides, identified by electron backscatter diffraction, form preferentially along the length of the filament. The filament has a recrystallized Ta inner core and a TaSi₂ layer encapsulated with a Si layer at the cooler ends. The αTa₅Si₃, metastable Ta₅Si₃ and Ta₂Si phases formed in addition to recrystallized Ta and TaSi₂ at the centre regions. Cracks and porosity were prevalent throughout the length of the filament. The microstructural evolution of the aged tantalum filament can be ascribed to the thermal gradient along the filament length, recrystallization of Ta and the variation of silicon content within the filament.     

Electrical and structural properties of Ta-N thin film and Ta/Ta-N multilayer for embedded resistor

Ta/Ta-N multilayer has been developed to control temperature coefficient of resistance (TCR) in a thin-film embedded resistor with the incorporation of Ta layer (+ TCR) inserted into Ta-N layers (− TCR). Electrical and structural properties of sputtered Ta, Ta-N and the multilayer films were investigated. The stable resistivity value of 0.0065 Ω·cm in β-Ta film was obtained, and phase change from fcc-TaN to orthorhombic Ta₃N₅ in Ta-N films was observed at nitrogen partial pressure of 22%. The multilayer of Si/Ta(60 nm)/Ta₃N₅(104 nm)/Ta(60 nm)/Ta₃N₅(104 nm) showed TCR value of − 284 ppm/K, where TCR of Ta was − 183 ppm/K and that of Ta₃N₅ was − 3193 ppm/K.     

Spontaneous oxide reduction in metal stacks

Aluminum and copper interconnects are widely used for microelectronic applications. A problem can arise when interfacial oxides are present. Such oxides can significantly degrade device performance by increasing electrical resistance. This paper describes analyses of interfacial oxide layers found in Al/Ta and Ta/Cu metal stacks. The analyses were performed through transmission electron microscopy (TEM). The data indicated that the interfacial oxides resulted from spontaneous reductions; that is, Al spontaneously reduced Ta₂O₅ to form Al₂O₃, while Ta spontaneously reduced Cu oxide to form Ta₂O₅.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

Fabrication of perovskite-type Ba(Sn1−x Ta x )O3 ceramics and their power factors

Perovskite-type Ba(Sn_1?x Ta _x )O₃ (0.01 ≤ x ≤ 0.06) ceramics with high relative densities (92.7–94.4 %) were fabricated using the hot isostatic pressing (HIP) method at 1273 K and 196 MPa for 4 h in an atmosphere of argon gas. The lattice parameter decreased slightly with increasing x. From the XPS measurement, the Ta⁵⁺ ion was stable in Ba(Sn_1?x Ta _x )O₃ ceramics and the broad peak of the Ta4f level was the overlap between the Ta⁵⁺4f_5/2 and Ta⁵⁺4f_7/2 levels. Ba(Sn_1?x Ta _x )O₃ ceramics were n-type semiconductors, and their electrical resistivities increased with increasing x. The increase in the electrical resistivity was explained by impurity scattering due to the presence of the Ta ions. The absolute value of the Seebeck coefficient (S) increased with increasing temperature and x. The power factor (S ² σ), which was calculated from electrical conductivity (σ) and the Seebeck coefficient, was ca. 1.0 × 10^?5 W m^?1 K^?2 at x = 0.01.     

Stability of Thermocapillary Convection in Rotating Shallow Annular Pool of Silicon Melt

In order to understand the effect of pool rotation on stability of thermocapillary convection, the critical conditions for the incipience of oscillatory flow in rotating shallow annular pools of silicon melt (Pr = 0.011) are investigated by means of linear stability analysis (LSA) under different pool rotation rates ranging from Ta = 0 to 1,513.9 and under microgravity condition. The results indicate that with increase of Ta numbers, the critical Marangoni for the incipience of hydrothermal wave (HTW) increases, i.e., pool rotation stabilizes the steady axisymmetric thermocapillary convection of silicon melt. The critical azimuthal wave number m _c decreases linearly with increase of Ta number when Ta ≤ 378.5. However, when Ta > 378.5, m _c almost keeps constant with m _c = 34. When Ta < 550 the HTW propagates in the same direction as the pool rotation direction, while it is reversed when Ta > 550.     

Energetic electronic processes and negative resistance in amorphous TaTa2O5Au and AlAl2O3Au diodes

Voltage-controlled negative resistance (VCNR) can be established in metal-insulator-metal structures by applying a potential to the diode, which is usually in vacuum. Light emission due to electroluminescence (E.L.), and electron emission into vacuum, accompany the formation of conductivity; these energetic electronic phenomena are closely related to the “forming” of VCNR and to the resultant current-voltage characteristics. For Al₂O₃ diodes with impure oxides, VCNR forms at constant voltage, independent of oxide thickness; for clean oxides, forming depends on field and on the metal counterelectrode. The intensity and energy distribution of light emitted from TaTa₂O₅Au diodes have been measured, and are compared to E.L. from AlAl₂O₃Au diodes. The voltage threshold for the appearance of E.L. in TaTa₂O₅Au diodes is 1.2 V, for AlAl₂O₃Au it is 1.4 V. The respective voltages for maximum current, V_m, are 1.9 V and 2.8 V. In Al₂O₃, the E.L. spectrum covers the visible range with peaks at 1.8 eV, 2.3 eV, and 4.0 eV. For Ta₂O₅, the E.L. intensity is constant over most of the visible, but has a maximum between 1.6 eV and 1.8 eV which is also the energy of E.L. of TaTa₂O₅Au diodes before the establishment of VCNR. For both Al₂O₃ and Ta₂O₅ diodes, electron emission into vacuum is anomalous since emission is detectable at diode voltages of ∼2 V. Electron emission in the two insulators, though qualitatively similar, shows differences that depend on differences in trapping levels and band gaps of the insulators.     

Nonlinear transient dynamic response of a blade subject to a pulsating load in a decaying centrifugal force field

This study is concerned with an analytical solution for the nonlinear transient dynamic response of a rotating blade in a gas turbine engine experiencing dynamic unbalance due to a blade-off scenario leading to rotor unbalance. As a consequence of rotor unbalance, the blade experiences a pulsating load at its tip due to contact with the casing and a decaying centrifugal force field due to the rotor deceleration. The governing equations of motion consider the blade as an elastic Timoshenko beam of varying thickness subject to changing rotational speeds. The analysis takes into account the variation in the thickness of the beam, the coupled axial and lateral displacements of the beam as a result of Coriolis component of acceleration, shear deformation, rotary inertia, and friction resulting from the contact between the blade and the casing. Our findings indicate that the thickness profile of the beam plays a significant role in the transient response. It further reveals that the decaying centrifugal force field leads to a dramatic change in the dynamic response and the resulting forces on the blade.