Thermomechanical properties of amorphous hydrogenated carbon–germanium alloys

Thermomechanical properties of amorphous hydrogenated carbon-germanium alloys prepared by the rf sputtering technique were determined for films in the 0 at. % to 100 at. % carbon content range. The stress, thermal expansion coefficient, and elastic modulus were obtained using the thermally induced bending technique. The stress was related to the concentration of hydrogen and argon, to the difference in the Ge-Ge and Ge-C bond length, and to the carbon hybridization. The thermal expansion coefficients of pure amorphous germanium and amorphous carbon are higher than that of their corresponding crystalline counterparts, which was attributed to the compressive stress of the films. The biaxial modulus, on the other hand, are always smaller than that of their crystalline counterparts, but increases as the concentration of carbon increases due to the substitution of Ge-Ge bonds by energetically stronger Ge-C and C-C bonds. Received: 9 May 2000 / Accepted: 10 May 2000 / Published online: 13 July 2000     

In-situ monitoring and control of hydrogenated amorphous silicon–germanium band-gap profiling during plasma deposition process

In-situ germanium content monitoring and its characteristics in SiH₄/GeH₄/H₂ plasmas was studied during hydrogenated amorphous silicon–germanium (a-SiGe:H) film depositions. Since an appropriate band-gap profiling in a-SiGe:H deposition is very important to achieve high efficiency solar cell, the accurate monitoring and control of Ge contents are required. In this work, we found the spectral intensity ratio of silicon atom (288.2 nm) and germanium atom (303.9 nm) emission has strong relation with Ge content in plasmas. In typical, band-gap energy of films was decreased with the increasing of gas flow ratio GeH₄/SiH₄. However, at different total flow rate of GeH₄, the band-gap was different for same gas flow ratio cases because the Ge content in plasmas was changed due to the changes of electron temperature by hydrogen dilution. On the other hand, the emission intensity ratio Ge/Si detected the band-gap variation. Using this method, therefore, we measured and control Ge/Si to make a U-shape band-gap profile which was proved by an ellipsometer and Auger electron spectroscopy depth profile analysis.     

Thermal evolution of short-range order in Cu–Hf-based amorphous alloys

A Perturbed Angular Correlation study on melt-spun Cu₆₀Hf₂₀Ti₂₀ and Cu₆₀Hf₄₀ is presented. The influence of Ti addition on thermal stability and crystallization mechanism is followed by differential scanning calorimetry. The evolution of quadrupole parameters with measurement temperature is analyzed in both alloys in order to get insight into the crystallization process. Although an intricate crystallization mechanism is observed for the Ti containing alloy, the final stage is similar, irrespective of minority atom.     

On the formation of a phase with tenfold symmetry in amorphous Ni–Zr alloys

The magnetic anisotropy energy (MAE) of 3d transition-metal wires, stripes, and films is calculated self-consistently as a function of stripe width and film thickness. The magnetization-reorientation transitions in stripes are determined along the crossover from the mono-atomic one-dimensional chain to the two-dimensional monolayer. It is shown that the MAE oscillates as a function of stripe width and depends strongly on the considered transition metal. The reorientation transitions in Co films deposited on a highly polarizable substrate such as Pd are discussed. A local analysis of the layer-resolved MAEs provides new insights into the off-plane magnetization observed in Pd-capped Co films on Pd(111). The interfaces responsible for the stability of the off-plane easy axis are characterized microscopically. An unexpected internal magnetic structure of the Co–Pd interfaces is revealed in which the magnetic moments and spin–orbit interactions at Pd atoms play a crucial role. The nature of the reorientation transition from perpendicular to in-plane magnetization with increasing film thickness is studied by means of full-vectorial calculations. The existence of a spin-canted phase at intermediate film thickness is demonstrated.     

Antiferromagnetic exchange coupling in amorphous Fe–Sc alloys

Results of ⁵⁷Fe Mössbauer, AC and DC susceptibility, grazing incidence X-ray diffraction, resistivity and Rutherford-backscattering measurements on the amorphous alloys ${\text{Fe}}_{100 - x} {\text{Sc}}_x (8 \leqslant x \leqslant 70)$ give for the first time convincing evidence for the antiferromagnetic exchange coupling between Fe-moments. The antiferromagnetic coupling between Fe-moments is for $(8 \leqslant x \leqslant 70)$ limited to certain regions (magnetic clusters) of the sample. The nature of the coupling is of the Heisenberg type. For $8 \leqslant x < 20$ , the magnetic coupling between Fe-moments is across nonmagnetic Sc-atoms. The conduction electrons mediate an indirect magnetic interaction between the Fe-moments. The magnetic exchange coupling between Fe-moments across Sc-atoms is negative. The antiferromagnetic coupling between Fe-moments can be explained by the Ruderman-Kittel-Kasuya-Yosida, RKKY, interaction by taking into account the damped oscillatory behavior of the RKKY interactions.     

Role of amorphous silicon domains of Er^3＋ emission in the Er—doped hydrogenated amorphous silicon suboxide film

An investigation on the correlation between amorphous Si (a-Si) domains and Er^{3+} emission in the Er-doped hydrogenated amorphous silicon suboxide (a-Si:O:H) film is presented. On one hand, a-Si domains provide sufficient carriers for Er^{3+} carrier-mediated excitation which has been proved to be the highest excitation path for Er^{3+} ion; on the other hand, hydrogen diffusion from a-Si domains to amorphous silicon oxide (a-SiO_x) matrix during annealing has been found and this possibly decreases the number of nonradiative centres around Er^{3+} ions. This study provides a better understanding of the role of a-Si domains on Er^{3+} emission in a-Si:O:H films.     

Determination of activation energy for crystallizations in Ni–Cu–P amorphous alloys

The effect of plastic deformation on the crystallization kinetics of the ternary Ni–Cu–P amorphous alloy coatings prepared by electroless plating was investigated using differential scanning calorimetry. It was shown that the effective crystallization activation energy of the amorphous coatings is pronouncedly affected by the plastic deformation, indicating a decreasing tendency with deformation, the effective activation energy decreases from 199.02 to 163.71?kJ?mol^?1 as the plastic deformation from 0% to 40%. And, accordingly, this leads to the decrease of crystallization temperature. Analyses were presented to discuss the possible mechanism for the notable effects of plastic deformation on the crystallization kinetics of the amorphous coatings.     

Mössbauer study of isothermally annealed amorphous Fe-Nb-Cu-Si-B alloys

Amorphous ribbons of Fe_73.5Nb₃Cu₁Si_13.5B₉ have been annealed above the crystallization temperature. Annealed samples consisted of crystalline and amorphous phases in a wide temperature range. Two samples of different thicknesses of 33 µm and 27 µm were isothermally annealed at a temperature of 545°C from 0.5 to 5 h in a vacuum furnace. The amount of crystalline phase increases rapidly in the ticker sample. The crystalline part of the Mössbauer spectrum consists of four sharp sextets which can be assigned to a DO₃-structure FeSi alloy. After 700°C annealing the amorphous phase was not observed and the crystalline phase consisted of the DO₃-structure FeSi alloy, paramagnetic FeNbB and presumably Fe₂₃B₆ and Fe₃SiB₂.     

Metal–insulator transition in boron-doped amorphous carbon films

Boron-doped amorphous graphite-like carbon (GLC) films have been prepared with different boron concentrations. Electrical transport measurements in the temperature range 1.3–300?K on the films shows a doping-induced metal–insulator (MI) transition. On the metallic side of the transition, the experimental data are interpreted in terms of weak localization and the effect of electron–electron interactions. Data on the insulator side of transition are analyzed in terms of hopping conduction. Critical behaviour is observed near the transition, with the resistivity obeying a power-law temperature dependence.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Auger parameters of ternary Pd–Ni–P amorphous alloys

A series of bulk ternary amorphous alloys of Pd_{100?3 x} Ni_{2 x} P _x were prepared and their amorphicity checked by X-ray diffraction (XRD) and neutron scattering. Auger parameters of these alloys have been derived from X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectra (AES). Experiments of XPS and AES on each elemental metal and Ni₂P were also performed for comparison. The results show a clear compositional dependence for the 3d binding energies and the MNN Auger energies of Pd on alloying, while Ni 2p core levels and the Ni LMM Auger lines show a weaker variation. Analysis of the data using “excited atom” models indicates electron transfer from Pd, with P carrying negative charge, but the differences in behaviour of the two metallic species suggests that covalent bonding changes may be significant.     

Study on stability of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon （a-Si：H） films with high and same order of magnitude photosensitivity （-10^5） but different stability were prepared by using microwave electron cyclotron resonance chemical vapour deposition system under the different deposition conditions. It was proposed that there was no direct correlation between the photosensitivity and the hydrogen content （CH） as well as H-Si bonding configurations, but for the stability, they were the critical factors. The experimental results indicated that higher substrate temperature, hydrogen dilution ratio and lower deposition rate played an important role in improving the microstructure of a-Si：H films. We used hydrogen elimination model to explain our experimental results.     

Quantum chemical studies on a series of transition metal carbon dioxide complexes: Metal–carbon bonding and electronic structures

The bonding features and electronic structures of a series of transition metal carbon dioxide complexes have been studied by density functional theory (DFT) calculations combined with natural bond orbital (NBO) analysis and energy-decomposition analysis (EDA). NBO analysis shows that the interaction between the metal center and the carbon atom of the carbon dioxide ligand (M–C) is stronger than the other interaction between the metal center and the carbon dioxide ligand. Natural hybrid orbital (NHO) analysis gives the detailed bonding features of the M–C bond for each complex. The NBO charge distribution on the carbon dioxide unit in all studied complexes is negative, which indicates charge transfer from the metal center to the carbon dioxide ligand for all studied complexes. The hyperconjugation effect of the metal center and the two C–O bonds of the carbon dioxide ligand has been estimated using the NBO second-order perturbation stabilization energy. It was found that the NBO second-order stabilization energy of C–O?→?nM* is sensitive to the coordinated sphere and the metal center. Frontier molecular orbital (FMO) analysis shows that complexes 1 and 4 may be good nucleophilic reagents for activation of the carbon dioxide molecule. However, the EDAs show that the M–CO₂ bond interaction energy of complex 4 is about two times as large as that of complex 1. The high M–CO₂ bond interaction energy of complex 4 may limit its practical application.     

Positron annihilation study of the vacancy clusters in ODS Fe–14Cr alloys

Abstract

Oxide dispersion strengthened Fe14Cr and Fe14CrWTi alloys produced by mechanical alloying and hot isostatic pressing were subjected to isochronal annealing up to 1400 °C, and the evolution and thermal stability of the vacancy-type defects were investigated by positron annihilation spectroscopy (PAS). The results were compared to those from a non-oxide dispersion strengthened Fe14Cr alloy produced by following the same powder metallurgy route. The long lifetime component of the PAS revealed the existence of tridimensional vacancy clusters, or nanovoids, in all these alloys. Two recovery stages are found in the oxide dispersion strengthened alloys irrespective of the starting conditions of the samples. The first one starting at T > 750 °C is attributed to thermal shrinkage of large vacancy clusters, or voids. A strong increase in the intensity of the long lifetime after annealing at temperatures in the 800–1050 °C range indicates the development of new vacancy clusters. These defects appear to be unstable above 1050 °C, but some of them remain at temperatures as high as 1400 °C, at least for 90 min.     

Mössbauer study of rapidly solidified Al-Fe based amorphous alloys

Mössbauer spetroscopy and X-ray diffraction were used to study the structure of rapidly quenched Al₈₈Fe₅Y₇, Al₈₈Fe₄Y₇Sb₁ and Al₉₃Fe₅Sb₂ alloys. Sb addition diminishes the glass forming ability and results in precipitation of the stable AlSb compound. The X-ray amorphous Al₈₈Fe₅Y₇ alloy shows a short range order resembling to the Al₆Fe compound as it can be judged from the Mössbauer data.     

What we have learned from studies on chemical properties of amorphous alloys?

Amorphous alloys have many attractive characteristics including extremely high corrosion resistance if the sufficient amounts of corrosion-resistant elements are added. The superiority of amorphous alloys is based on the homogeneous single phase nature without any chemical and physical heterogeneities. Although there are processing limitations to avoid the formation of heterogeneous crystalline structure in addition to no welding technology without crystallization, the application of corrosion-resistant amorphous alloys is expected particularly to the very aggressive environments, where any conventional crystalline metallic materials cannot be used. Some amorphous bulk alloys showed zero corrosion mass loss due to spontaneous passivation even in 12 M HCl. Production of amorphous bulk alloys became possible for selected compositions. The homogeneous single phase nature is also effective to form useful catalysts with unique composition and structure. An example of catalysts is for carbon dioxide methanation useful for supply of renewable energy in the form of methane.     

First principles study of interactions of oxygen–carbon–vacancy in bcc Fe

Behaviors of C or O in bcc Fe and interactions of C–O and oxygen–carbon–vacancy(O–C–□) are investigated by first principles calculations. Octahedral interstitial site is the most stable position for an O atom in bcc Fe. The migration energy of an O atom in bcc Fe is 0.46 eV. The strength of O–Fe(1 nn) bond(0.32) is slightly greater than that of Fe–Fe metallic bond(0.26). Repulsive interactions of C–C, O–O, and C–O exist in bcc Fe. When the concentration of FIA(FIA refers to C or O) is relatively high, a vacancy can attract four FIAs and form stable FIAs–□ complex.