Chemical and structural modifications of laser treated iron surfaces: investigation of laser processing parameters

This study focuses on the chemical, morphological and structural characterization of iron surfaces treated by laser in ambient air. Incorporation of nitrogen over a 1–2 μm thickness (10–30 at.% at the profile maximum) and superficial oxidation on 200–400 nm depth have been evidenced by nuclear reaction analyses. X-ray diffraction at grazing incidence has shown the formation of FeO and Fe₃O₄ oxide phases as well as γ-Fe(N), and ε-Fe_xN for a sufficiently high amount of nitrogen incorporated. Treatments performed with different laser beams indicate that the parameter playing the major role in surface modification processes is the wavelength. Nitrogen incorporation has been found to occur via the interaction of reactive N, present in the laser-induced plasma, and the iron molten bath. The nitriding process is promoted in the IR wavelength range. Oxidation takes place by chemical reaction during the cooling step, and is furthered in the case of UV treatment.     

The dissolution kinetics of free iron in Nd-Fe-B permanent magnet alloys

Three kinds of NdFeB alloys, near the Nd₂Fe₁₄B ( φ phase ) composition of the Fe-(Nd₂B) pseudobinary phase diagram, were used to study the kinetics of free iron dissolution in the matrix, which is extremely important for the attainment of high coercivity. The dissolution equations together with the time exponent, n( , ) and activation energies of rate constants, Q(34–40.9 kcal/mol) were derived. These indicate that the quantity of Nd-rich liquid phase plays an important role in the dissolution kinetics. For the ‘hypo-φ’ composition (Fe-rich), grain boundary diffusion dominates. For φ and ‘hyper-φ’ (Nd-rich) composition, interface reaction dominates. The low activation energies of this system are attributed to the activated effect of φ phase formation and liquid phase assisted grain boundary diffusion, it is found that there will be no free iron left after powder pulverizing and sintering of the commercial Nd₁₅Fe₇₇B₈ alloy.     

Determination of Si-SiO2 interface trap properties of p-MOS structures with very thin oxides by conductance measurement

The interface traps of as-oxidized p-MOS structures with very thin (88–434 Å) oxides are characterized by conductance technique. The interface trap density is found to increase inversely with the oxide thickness. The hole capture cross sections σ_p show no obvious dependence on the oxide thickness, but decrease exponentially with energy towards the midgap. The dispersion parameters σ_s are compared with the theoretical predictions based on the patchwork model. Both the magnitude and bias dependence of the measured σ_s are much larger than the theory predicts. With numerical simulation, it is shown that the width of the conductance peak can be strongly influenced by the behaviour of the capture cross section. By incorporating the energy dependence of σ_p into the patchwork model, good agreement between the experimental values and theoretical estimations of σ_s is obtained.     

One-dimensional dynamics of nano-scale oxidation

A continuum model is proposed to describe the process of scanned probe oxidation in the presence of a thin water layer on the surface of a substrate. The model describes the electric field and ion transport in both the liquid and the oxide layers and incorporates the reaction mechanism at the substrate/oxide interface. Further, the influence of the space charge due to ions trapped near the substrate/oxide interface is taken into account.Separation of time scales for the chemical reactions and ion transport as well as the asymptotic limit in terms of a small aspect ratio of the oxide layer are used to reduce the complex system of partial differential equations to a one-dimensional system of ordinary differential equations. The analytical solution of the reduced system results in the evolution equation for the oxide thickness. Numerical simulations of the evolution equation predict features of oxide growth that qualitatively agree with the experimental observations. A parametric study is conducted to determine the influence of the key operating and material parameters on the thickness of the oxide, the electric field, and ion concentration in the system.     

Tracking the formation of 2223 BiSrCaCu oxygen compounds by resistivity and susceptibility measurements reaching Tc(0)=115 K

Unsubstituted and lead-antimony substituted 2223 BiSrCaCu Oxygen compounds have been synthesized and compared by resistivity R(T) and susceptibility χ(T) measurements and chemical analysis. The R(T) and χ(T) measurements show that interface effects occur which are reflected in the R(T) characteristics. The chemical analysis of samples with a resistance zero T_c(0)=114 K and a diamagnetic onset at 115 K yields a severe loss of lead from nominally 40 at% down to about 3 at%, whereas the T_c(0)=110 K samples contain up to about 30 at% of lead. It is assumed that for the formation of the 115 K phase lead oxide acts as a catalyst not being incorporated to a considerable amount into the lattice.     

Structural changes in thin SiO2 on Si after RIE-like nitrogen plasma action

Reactive ion etching (RIE) damage effects on thin (13 nm) thermal SiO₂ on Si have been studied using X-ray photoelectron spectroscopy. It is found that 5 min exposure of the oxide to N₂ plasma operating in RIE-mode causes structural modifications which manifest only as a deterioration of the oxide quality but without actual nitridation of the oxide. The presence of a small (<10%) constant amount of SiO species through the oxide and a broadening of Si---SiO₂ interface transition region are detected as consequences from the RIE process.     

The Ba/Si(100)-2 × 1 interface I. XPS and XAES studies of silicide formation

In two complementary papers we present the results and analysis of an extended study of the Ba/Si(100)−2 × 1 interface. In this paper, we will discuss X-ray excited Auger electron spectroscopy- and X-ray photoelectron spectroscopy chemical shifts, as well as plasmon losses, which have been studied to answer the question whether silicide formation occurs at this interface. It is found that no silicide formation takes place at room temperature. Two Ba-Si phases are detected as reaction products upon annealing the Ba/Si(100) system at ˜ 550 K.     

Coherent and incoherent η-photoproduction from nuclei

Elastic and inelastic η-photoproduction from complex nuclei is studied in a distorted-wave impulse approximation (DWIA) framework. The elementary operator is obtained by using a dynamical model which employs the reactions πN→πN, πN→ππN and π⁻p→ηn to fix the hadronic vertex as well as the isobar propagators and the process γN→πN to constrain the electromagnetic vertex. The nuclear structure input for the inelastic transitions has been extracted from electron-scattering form factors. The η final-state interaction has been included via a simple optical potential using the ηN t-matrix as an input. We find that coherent η-production is dominated by the D₁₃(1520) isobar while spin-flip transitions to excited nuclear states are sensitive to the S₁₁(1535) resonance. Predictions are given for coherent production on ⁴He, ¹²C and ⁴⁰Ca, as well as incoherent production on ⁶Li, ¹⁰B and ¹²C.     

Electrical characterization of low temperature deposited TiO2 films on strained-SiGe layers

Thin films of titanium dioxide have been deposited on strained Si_0.82Ge_0.18 epitaxial layers using titanium tetrakis-isopropoxide [TTIP, Ti(O-i-C₃H₇)₄] and oxygen by microwave plasma enhanced chemical vapor deposition (PECVD). The films have been characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Dielectric constant, equivalent oxide thickness (EOT), interface state density (D_it), fixed oxide charge density (Q_f/q) and flat-band voltage (V_FB) of as-deposited films were found to be 13.2, 40.6 Å, 6×10¹¹ eV⁻¹ cm⁻², 3.1×10¹¹ cm⁻² and −1.4 V, respectively. The capacitance–voltage (C–V), current–voltage (I–V) characteristics and charge trapping behavior of the films under constant current stressing exhibit an excellent interface quality and high dielectric reliability making the films suitable for microelectronic applications.     

High and low thermal nitridation of SiO2 thin films

Thin thermal SiO₂ films on crystalline silicon substrate were nitrided at low ammonia pressures (10^-6P_NH310^-1 mbar) for times varying from 1 to 10 h, by means of two techniques. (i) Surface nitridation has been achieved by thermal activation at high temperature (HT), in the range 800–1100°C. (ii) A new process at low temperature (LT), was employed at T ≈ 30°C, under electron-beam irradiation; the nitridation-reaction rate depends on the electron energy (reaching a maximum within the energy range 1 to 2 keV), and on the electron flux. Conduction and electron trapping on the nitrided oxide films depends on the chemical compositions and on the amount of nitrogen incorporated into the bulk of the films and/or at the SiO₂-Si interface.     

Adsorption and spreading of polymers at plane interfaces; theory and molecular dynamics simulations

Nonequilibrium processes play a key role in the adsorption kinetics of macromolecules. It is expected that the competition between transport of polymer towards an interface and its subsequent spreading has a significant influence on the adsorbed amount. An increase of the transport rate can lead to an increase of the adsorbed amount, especially when the polymer has too little time to spread at the interface. In this study we present both molecular dynamics simulations and analytical calculations to describe some aspects of the adsorption kinetics. From MD simulations on a poly(ethylene oxide) chain in vacuum near a graphite surface, we conclude that the spreading process can, in first approximation, be described by either a simple exponential function or by first-order reaction kinetics. Combining these spreading models with the transport equations for two different geometries (stagnation-point flow and overflowing cylinder) we are able to derive analytical equations for the adsorption kinetics of polymers at solid-liquid and at liquid-fluid interfaces. Received: 18 July 1997 / Received in final form: 27 October 1997 / Accepted: 6 November 1997     

Matrix Riccati equation formulation for radiative transfer in a plane-parallel geometry

In this paper, we formulate the radiative transfer problem as an initial value problem via a pair of nonlinear matrix differential equations (matrix Riccati equations or MREs) which describe the reflection (R) and transmission (T) matrices of the specific intensities in a plane-parallel geometry. One first computes R and T matrices of some small but finite layer thickness (equivalent optical thickness τ∼0.01 and then repetitively applies the doubling method to the reflection and transmission matrices R(τ)and T(τ) until reaching the desired layer thickness. The initial matrices R(τ₀)and T(τ₀) can be computed quite accurately by either of the following methods: multiple-order, multiple-scattering approximation, iterative method or fourth-order Runge-Kutta techniques. In addition, the reflection coefficient matrix of a semi-infinite medium satisfies an algebraic matrix equation which can be solved repetitively by a matrix method. MREs offer an alternative way of solving plane-parallel radiative transport problems. This method requires only elementary matrix operations (addition, multiplication and inversion). For vector and/or beam-wave radiative transfer problems, large matrices are required to describe the physics adequately, and the MRE method provides a significant reduction in computer memory and computation time.     

Double pion photoproduction on the nucleon: study of the isospin channels

A model for the γp → π⁺π⁻p reaction developed earlier is extended to account for all isospin channels. The model includes N, Δ(1232), N^*(1440) and N^*(1520) as intermediate baryonic states and the meson as an intermediate 2π resonance. Although many terms contribute to the cross section, some channels exhibit particular sensitivity to certain mechanisms of resonance excitation or decay and the reactions provide novel information on such mechanisms. In particular the γN → N^*(1520) → Δπ process affects all the channels and is a key ingredient in the interpretation of the data. Comparison is made with all available data and the agreement is good in some channels. The remaining discrepancies in some other channels are discussed.     

Relativistic field-theoretical equations for the three-body multichannel πN and γN scattering reactions and propagator of the Δ-resonance

A new form of the relativistic three-body equations for the coupled πN and γN scattering reactions with three particle final states ππN and γπN is suggested. These equations are derived in the framework of the time-ordered three dimensional field theory. The solutions of the considered equations satisfy the unitarity condition and are exactly gauge invariant. The form of these three-body equations is does not depend on the choice of the model of Lagrangian and is also the same for the formulations with and without quark degrees of freedom.

The effective potentials of the suggested equations are defined by the vertex functions with two on-mass shell particles. It is emphasized that these input vertex functions can be constructed from experimental data. Special attention is given to the construction of the propagator of the Δ-resonance in the framework of the separable πN potential model. The strong dependence of the multichannel πN and γN cross sections on the form of the Δ-resonance propagator[2] is discussed.

The used formulation of the relativistic three-dimensional and three-body equations allows us to overcome a number of approximations which are usually used by practical calculations of the πN and γN scattering reactions.     

Dynamics of surface roughening in disordered media

In this paper the roughening of interfaces moving in inhomogeneous media is investigated by examining the corresponding stochastic differential equations using (i) numerical methods and (ii) dimensional analysis. We consider interface evolution equations where disorder is represented by quenched noise which can be both additive and multiplicative. Our main finding is that quenched noise leads to a new universality class as concerning the exponents δ and β describing respectively the spatial and temporal scaling of surface roughness. In particular, additive noise close to the pinning transition results in a behaviour with δ = 0.71 ± 0.08 and β = 0.61±0.06 up to a crossover time. These estimates are in very good agreement with the theoretical prediction and that we derive from a dimensional analysis of the equation. Furthermore, we argue that multiplicative noise is the appropriate choice to describe experiments where the interface between two flowing phases is considered. By numerically integrating the proposed equation we have obtained (i) surfaces remarkably similar to those observed in the experiments and (ii) a scaling of the surface width as a function of time with an exponent β = 0.65 being in an excellent agreement with the experimental value. In addition to the exponents we discuss other relevant features of the surfaces, including the scaling of the average velocity of the surface ν_a close to pinning and the non-trivial, power law distribution of waiting times and noise along the interface in the stationary regime.     

Oxidation of low-index FeAl surfaces

The oxidation of the (100), (110) and (111) surfaces of the intermetallic compound FeAl has been investigated using LEED and XPS. On all three surfaces, oxidation at room temperature leads to the formation of an amorphous oxide film on top of an Al-depleted interlayer. The film growth can be divided into two regions of differing kinetics, i.e. the initial formation of a closed oxide film and a subsequent thickening. In the first region, the oxygen-uptake rate varies significantly with surface orientation, while in the thickening regime the uptake is the same for all surfaces. The maximum thickness as well as the composition of the oxide films were found to depend on the initial oxidation rate. At higher oxidation temperatures, ordered oxide films of around 5–8 Å in thickness are formed, very similar to those observed on NiAl. Photoemission spectra from these ordered phases showed evidence for Al atoms in two different chemical environments, i.e. the well-known oxide species in the interior of the film and an additional species present at the oxide/alloy interface.     

Temporary phase segregation processes during the oxidation of (Fe0.7Mn0.3)0.99O in N2---H2O atmosphere

A two-step thermochemical cycle with the ternary metal oxide system (Fe_{1 − x}Mn_x)₃O₄/(Fe_{1 − x}Mn_x)_{1 − y}O is applied to convert solar energy to chemical energy. Experimental investigations on the water splitting reaction of (Fe_{1 − x}Mn_x)_{1 − y}O revealed temporary formation of a manganese rich rock salt phase and an iron rich spinel phase due to phase segregation processes.     

On the dynamics of parabolic equations describing diffusion, convection and a chemical reaction

A tubular nonisothermal-nonadiabatic chemical reactor with a consecutive reactions described by a set of three nonlinear parabolic equations, shows a sequence of period-doubling bifurcations of a “limit cycle”. A numerical examination of the model reveals that complex periodic and irregular oscillations are possible. With increasing value of the Peclet number regular and irregular oscillations are suppressed and disappear. From the computed results may be informed that for reacting systems nonlinear parabolic equations may feature similar qualitative properties as the ordinary differential equations. The results of this numerical study may be used to explain turbulization of laminar flames.     

The microstructure of the base oxide on 304L steel

304L stainless steel specimens were subjected to 168 h of oxidation in dry oxygen at 600°C. The composition and microstructure of the oxide as well as the steel near the steel–oxide interface were investigated using SEM and TEM in conjunction with EDS analysis. It was found that the first oxide layers, referred to as the base oxide, consisted of 50–100 nm-sized corundum-type grains. Each grain contained oxides of chromium, iron plus a small amount of manganese in a solid solution with the chemical formula (Cr,Fe,Mn)₂O₃. In the oxide regions closest to the steel–oxide interface, the grain center compositions varied laterally from grain to grain, with the cation balance ranging between very chromium-rich (Cr:Fe=4:1) and very iron-rich (Cr:Fe=1:8). Within each oxide grain, the chromium level was generally higher in the center than near the boundary. This was due to the much quicker diffusion of iron (compared to chromium) at and through the oxide grain boundaries than in the bulk of the oxide grains, creating iron enrichment near the oxide grain boundaries. Compositional variation with respect to the distance from the steel–oxide interface was also noted. Chromium was richer in the oxide regions closer to the steel–oxide interface than farther out in the oxide and depleted from the steel regions bordering the steel–oxide interface. Manganese was also depleted in those steel regions although hardly enriched in the oxide just outside the steel–oxide interface. Nickel was less reactive than chromium, iron or manganese and hence was virtually absent from the oxide.     

The reactions pp→pΛK and pp→pΣK near their thresholds

The reactions pp→pΛK⁺ and pp→pΣ⁰K⁺ are studied near their thresholds. The strangeness production process is described by the π- and K exchange mechanisms. Effects from the final state interaction in the hyperon-nucleon system are taken into account rigorously. The Λ production turns out to be dominated by K exchange whereas K- as well as π exchange play an important role for the Σ⁰ case. It is shown that the experimentally observed strong suppression of Σ⁰ production compared to Λ production at the same excess energy can be explained by a destructive interference between π and K exchange in the reaction pp→pΣ⁰K⁺. Implications of such an interference on the reaction pp→nΣ⁺K⁺ are pointed out.