Effect of substrate bias voltage on the structure, electric and dielectric properties of TiO2 thin films by DC magnetron sputtering

Titanium dioxide (TiO₂) films have been deposited on glass and p-silicon (1 0 0) substrates by DC magnetron sputtering technique to investigate their structural, electrical and optical properties. The surface composition of the TiO₂ films has been analyzed by X-ray photoelectron spectroscopy. The TiO₂ films formed on unbiased substrates were amorphous. Application of negative bias voltage to the substrate transformed the amorphous TiO₂ into polycrystalline as confirmed by Raman spectroscopic studies. Thin film capacitors with configuration of Al/TiO₂/p-Si have been fabricated. The leakage current density of unbiased films was 1 × 10⁻⁶ A/cm² at a gate bias voltage of 1.5 V and it was decreased to 1.41 × 10⁻⁷ A/cm² with the increase of substrate bias voltage to −150 V owing to the increase in thickness of interfacial layer of SiO₂. Dielectric properties and AC electrical conductivity of the films were studied at various frequencies for unbiased and biased at −150 V. The capacitance at 1 MHz for unbiased films was 2.42 × 10⁻¹⁰ F and it increased to 5.8 × 10⁻¹⁰ F in the films formed at substrate bias voltage of −150 V. Dielectric constant of TiO₂ films were calculated from capacitance–voltage measurements at 1 MHz frequency. The dielectric constant of unbiased films was 6.2 while those formed at −150 V it increased to 19. The optical band gap of the films decreased from 3.50 to 3.42 eV with the increase of substrate bias voltage from 0 to −150 V.     

Interaction of Shallow Water Waves

In this paper, we consider the Riemann problem and interaction of elementary waves for a nonlinear hyperbolic system of conservation laws that arises in shallow water theory. This class of equations includes as a special case the equations of classical shallow water equations. We study the bore and dilatation waves and their properties, and show the existence and uniqueness of the solution to the Riemann problem. Towards the end, we discuss numerical results for different initial data along with all possible interactions of elementary waves. It is noticed that in contrast to the p -system, the Riemann problem is solvable for arbitrary initial data, and its solution does not contain vacuum state.     

Efficient interval partitioning for constrained global optimization

A new efficient interval partitioning approach to solve constrained global optimization problems is proposed. This involves a new parallel subdivision direction selection method as well as an adaptive tree search. The latter explores nodes (intervals in variable domains) using a restricted hybrid depth-first and best-first branching strategy. This hybrid approach is also used for activating local search to identify feasible stationary points. The new tree search management technique results in improved performance across standard solution and computational indicators when compared to previously proposed techniques. On the other hand, the new parallel subdivision direction selection rule detects infeasible and suboptimal boxes earlier than existing rules, and this contributes to performance by enabling earlier reliable deletion of such subintervals from the search space.     

Continuum thermodynamic formulation of models for electromagnetic thermoinelastic solids with application in electromagnetic metal forming

The purpose of this work is the formulation and application of a continuum thermodynamic approach to the phenomenological modeling of a class of engineering materials which can be dynamically formed using strong magnetic fields. This is carried out in the framework of a thermodynamic, internal-variable-based formulation in which the deformation, temperature and magnetic fields are in general coupled. This coupling takes the form of the Lorentz force as an additional supply of momentum, and the electromotive power as an additional supply of energy, in the material. In the current approach, the basic thermomechanical field relations for mass, momentum and moment of momentum are obtained from the total energy balance via invariance, and completed by Maxwells field equations. The constitutive formulation is based on the exploitation of the Müller-Liu entropy principle, here for the case of isotropic thermoelastic, viscoplastic material behaviour. The resulting reduced constitutive and field relations and restrictions are then applied to the modeling and simulation of high-speed electromagnetic forming of metal tubes and sheet metal. In this context, scaling arguments show that, over the relevant length- and timescales of engineering interest, the evolution of the magnetic field is diffusive in nature, and thermal conduction is negligible. Comparison of the simulation and experimental results for the final sheet metal form shows very good agreement.Received: 16 March 2004, Accepted: 6 May 2004, Published online: 17 September 2004PACS: 46.05. + b, 46.25.Hf, 46.35 + z Correspondence to: B. Svendsen     

Effect of time-periodic vertical oscillations of the Rayleigh–Bénard system on nonlinear convection in viscoelastic liquids

A study of heat transport in Rayleigh–Bénard convection in viscoelastic liquids with/without gravity modulation is made using a most minimal representation of Fourier series and a representation with higher modes. The Oldroyd-B constitutive relation is considered. The resulting non-autonomous Lorenz model (generalized Khayat–Lorenz model of four modes and seven modes) is solved numerically using the adaptive-grid Runge–Kutta–Fehlberg45 method to quantify the heat transport. The effect of gravity modulation is shown to be stabilizing there by leading to a situation of reduced heat transfer. The Deborah number is shown to have an antagonistic influence on convection compared to the stabilizing effect of modulation amplitude and elastic ratio. The results in respect of Maxwell, Rivlin–Ericksen and Newtonian liquids are obtained as particular cases of the present study. A transformation of the momentum equations illustrates the equivalence of present approach and the one due to Khayat that uses normal stresses explicitly.     

Tuning CO2 Uptake and Reversible Iodine Adsorption in Two Isoreticular MOFs through Ligand Functionalization

The synthesis and characterization of two isoreticular metal–organic frameworks (MOFs), {[Cd(bdc)(4‐bpmh)]}_n?2 n(H₂O) ( 1 ) and {[Cd(2‐NH₂bdc)(4‐bpmh)]}_n?2 n(H₂O) ( 2 ) [bdc=benzene dicarboxylic acid; 2‐NH₂bdc=2‐amino benzene dicarboxylic acid; 4‐bpmh=N,N‐bis‐pyridin‐4‐ylmethylene‐hydrazine], are reported. Both compounds possess similar two‐fold interpenetrated 3D frameworks bridged by dicarboxylates and a 4‐bpmh linker. The 2D Cd‐dicarboxylate layers are extended along the a‐axis to form distorted square grids which are further pillared by 4‐bpmh linkers to result in a 3D pillared‐bilayer interpenetrated framework. Gas adsorption studies demonstrate that the amino‐functionalized MOF 2 shows high selectivity for CO₂ (8.4 wt % 273 K and 7.0 wt % 298 K) over CH₄, and the uptake amounts are almost double that of non‐functional MOF 1 . Iodine (I₂) adsorption studies reveal that amino‐functionalized MOF 2 exhibits a faster I₂ adsorption rate and controlled delivery of I₂ over the non‐functionalized homolog 1 .     

Two CdII/CoII‐Imidazolate Coordination Polymers: Syntheses,Crystal Structures,Stabilities, and Luminescent/Magnetic Properties

Cadmium(II) based 2D coordination polymer [Cd(L1)₂(DMF)₂] ( 1 ) (L1 = 4,5‐dicyano‐2‐methylimidazolate, DMF = N,N′‐dimethylformamide) and 2D cobalt(II)‐imidazolate framework [Co(L3)₄] ( 2 ) (L3 = 4,5‐diamide‐2‐ethoxyimidazolate) were synthesized under solvothermal reaction conditions. The materials were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X‐ray diffraction measurement (PXRD) and single‐crystal X‐ray diffraction. Compound 1 has hexacoordinate Cd^II ions and forms a zigzag chain‐like coordination polymer structure, whereas compound 2 exhibits a 2D square grid type structure. The thermal stability analysis reveals that 2 showed an exceptional thermal stability up to 360 °C. Also, 2 maintained its fully crystalline integrity in boiling water as confirmed by PXRD. The solid state luminescent property of 1 was not observed at room temperature. Compound 2 showed an independent high spin central Co^II atom.     

Exact solutions to magnetogasdynamic equations in Lagrangian coordinates

The Lie group of point transformations, which leave the equations for a simplified model of one dimensional ideal gas in magnetogasdynamics invariant, are used to obtain some exact solutions for the governing system of hyperbolic partial differential equations (PDEs). Similarity variables which reduces the governing system of PDEs into system of ordinary differential equations (ODEs) are determined through the transformations. The resulting ODEs are solved analytically to obtain some exact solutions that exhibits space-time dependence. Further, we study the propagation of weak discontinuity through a state characterized by one of the solutions.     

Proton‐Conducting Magnetic Coordination Polymers

Three isostructural lanthanide‐based two‐ dimensional coordination polymers (CPs) {[Ln₂(L)₃(H₂O)₂]_n ? 2n CH₃OH) ? 2n H₂O} (Ln=Gd³⁺ ( 1 ), Tb³⁺ ( 2 ), Dy³⁺ ( 3 ); H₂L=cyclobutane‐1,1‐dicarboxylic acid) were synthesized by using a low molecular weight dicarboxylate ligand and characterized. Single‐crystal structure analysis showed that in complexes 1 – 3 lanthanide centers are connected by μ₃‐bridging cyclobutanedicarboxylate ligands along the c axis to form a rod‐shaped infinite 1D coordination chain, which is further linked with nearby chains by μ₄‐connected cyclobutanedicarboxylate ligands to form 2D CPs in the bc plane. Viewing the packing of the complexes down the b axis reveals that the lattice methanol molecules are located in the interlayer space between the adjacent 2D layers and form H‐bonds with lattice and coordinated water molecules to form 1D chains. Magnetic properties of complexes 1 – 3 were thoroughly investigated. Complex 1 exhibits dominant ferromagnetic interaction between two nearby gadolinium centers and also acts as a cryogenic magnetic refrigerant having a significant magnetic entropy change of ?ΔS_m=32.8 J kg^?1 K^?1 for ΔH=7 T at 4 K (calculated from isothermal magnetization data). Complex 3 shows slow relaxation of magnetization below 10 K. Impedance analysis revealed that the complexes show humidity‐dependent proton conductivity (σ=1.5×10^?5 S cm^?1 for 1 , σ=2.07×10^?4 S cm^?1 for 2 , and σ=1.1×10^?3 S cm^?1 for 3 ) at elevated temperature (>75 °C). They retain the conductivity for up to 10 h at high temperature and high humidity. Furthermore, the proton conductivity results were correlated with the number of water molecules from the water‐vapor adsorption measurements. Water‐vapor adsorption studies showed hysteretic and two‐step water vapor adsorption (182000 μL g^?1 for 1 , 184000 μL g^?1 for 2 , and 1874000 μL g^?1 for 3 ) in the experimental pressure range. Simulation of water‐vapor adsorption by the Monte Carlo method (for 1 ) confirmed the high density of adsorbed water molecules, preferentially in the interlayer space between the 2D layers.