Instantaneous growth approximation describing the nanocrystallization process of amorphous alloys: A cellular automata model

A cellular automata simulation based on an instantaneous growth approximation is developed to model the nanocrystallization kinetics. In this frame, the time required for a nucleus to grow up to its maximum size is neglected in comparison with the time required for the crystallization process. This approach allows a simple interpretation of the very low values of Avrami exponent found for nanocrystallization processes, ?1, in the theoretical frame of Johnson-Mehl-Avrami-Kolmogorov theory. Kinetics and microstructure predictions from the simulations are compared with experimental data for FeCoNbB alloys and a good qualitative agreement is found.     

Phenomenon of the maximum inhomogeneity in solidification of alloys from the peritectic-eutectic phase systems

An application of the Scheil equation with the purpose to: 1 describe the maximum dendritic microsegregation phenomenon, i.e. amount of the solid solution in function of the initial composition of alloys from the Ag Sb phase system, 2 determine the nature of the maximum microsegregation in the Ag–30 Sb and Ag–10 Sb alloys is presented. A new relation, supplementary to the Scheil equation is proposed.     

On the mechanism of lamellar spacing adjustment in eutectic alloys

Well controlled undirectional solidification experiments with continous increased rate were performed for eutectic alloys in the systems Ag-Cu, Al-Zn, Al-Cu and Al-Ni. In the Ag-Cu and Al-Zn alloys it was observed that discontinous changes in the lamellar spacing occured at low solidification rates. At high solidification rates such discontinuities could not be observed. Anyway the lamellar spacing always larger than observed at constant rate experiments. In Al-Cu alloys discontinous changes in the lamellar spacing with increasing solidification rate were also observed. However, after each change the lamellar spacing increased gradually up to a lamellar distance of 20% lower than before the abrupt decrease. All-Ni eutectic alloys gives a rod like eutectic structure. In this alloy no discontinous changes could be observed. A theoretical analyses of the observation based on well known theoretical models are discussed.     

On the compositional dependence of the optical gap in amorphous semiconducting alloys

It is found that the optical gap E_AB for amorphous A-B alloys can be determined by the energy gap E_A for element A and E_AB for the element B in the equation E_AB(Y) = YE_A + (1?Y) E_B where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for SiGe, SbSe, and AsTe films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the AsSe, AsS, GeTe and Sb₂Se₃As₂Se₃ systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the TeSe, As₂Te₃As₂Se₃ and GeTe₂GeSe₂ systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.     

On the thermodynamic stability of disperse systems

Investigation has been carried out on the effect of the surface activity of capillary or surface active substances (impurities), called surfactants (i.e. the effect of the lowering (Δσ) of the specific surface energy after impurity adsorption at the solid/gas, solid/liquid, liquid/gas or liquid/liquid interface), on the thermodynamic equilibrium in a polydisperse system. Attention has been paid to the fact that the presence of a surfactant with an activity within the range 0 < Δσ < σ₀ (where σ₀ is the specific surface energy of an adsorption-free interface) reduces the thermodynamic driving force for the appearance of the Gibbs-Thomson effect and for the recondensation (recrystallization) processes in the heterogeneous polydisperse system. It is shown that with an impurity activity Δσ = σ₀, the supersaturation becomes zero and the condensed phase formations should be, irrespective of their size and shape, in equilibrium with one another because their vapour pressure becomes equal to that of an infinitely large phase. In this case the Gibbs-Thomson effect completely disappears, the system become thermodynamically stable and, regardless of its polydispersity, it can exist infinitely long. The behaviour of such a heterogeneous system, which is far from its critical point (the system temperature is T ≪ T_critical), is similar to the behaviour of a system in the region of its critical point: the surface tension becomes nearly equal to zero, and the fluctuations sharply increase. The case when the surfactant activity is so high (Δσ > σ₀) that σ_a = σ₀ − Δσ < 0 (σ_a, specific surface energy of the interface in the presence of a surfactant), is of special interest. It represents a paradox in the phase formation theory: as a result of the adsorption of an impurity with such a high activity, the condensed phase surface would be destabilized and the phase would be spontaneously dispersed.     

An improved prediction model of morphological modifications of organic crystals induced by additives

Additives have a great effect on the crystal morphology of organic compounds. The molecular modeling techniques were applied to predict the morphological modifications induced by additives. Besides molecular modeling software additionally some simulation approaches are needed. The build‐in and the surface docking approaches were applied to some host‐additive‐systems. It will be presented here that the suitability of these two approaches depends on the degree of anisotropy of the intermolecular bonding of the host substance. A procedure on how to choose an appropriate prediction approach for the individual crystal systems on investigation is suggested.     

On the backscattering factor in quantitative Auger electron spectroscopy of binary alloys

The matrix dependent relative sensitivity factor S_rel(c) is considered for use in quantitative Auger analysis. It is affected by matrix dependent factors like the backscattering factor, the escape depth of Auger electrons and the number of atoms per unit volume. The backscattering factor, especially, is evaluated and possibilities of its determination are shown. Measurements of Auger intensities and of the energy distribution of backscattered electrons were carried out for this purpose on a series of CuPd and CuSb alloys, respectively. The backscattering factor thus obtained is compared with values resulting from a formula of JABLONSKI and by experimental data of SMITH and GALLON .     

以尿素为氮源合成AlN粉体过程中非晶碳的石墨化

针对有机合成过程中碳及碳化物的残余,传统方法中普遍使用除碳的工艺,而很少有文章针对非晶碳的结构和形貌进行表征。为此,本文采用高尿素含量的前驱盐体系,通过在氮气保护气氛中煅烧获得AlN粉体。采用X射线衍射分析、红外和拉曼光谱分析、扫描电子显微镜和透射电子显微镜对850~1 500 ℃温度范围内煅烧获得产物的结构和形貌进行表征,对AlN合成过程中含碳产物结构形貌的变化,以及AlN和含碳产物之间相互的依存生长关系进行分析。结果表明,AlN生长的过程中伴随着无定形碳的石墨化转变,AlN颗粒的形貌也受含碳残余产物形貌的影响而出现有规律的变化。     

Controlling the group II composition in CdZnTe alloys grown by organometallic vapor phase epitaxy: a kinetic model

The organometallic vapor phase epitaxy (OMVPE) of CdTe and ZnTe has been examined in a hot-wall, laminar-flow reactor. It was found that cadmium and zinc are produced in excess on the film surface during OMVPE. The excess group II elements sublime off the surface and are deposited downstream on the cold reactor walls. Based on these and other results, a kinetic model has been derived for CdZnTe OMVPE. The elementary reactions included in this model are the adsorption of the organometallic precursors, the desorption of the alkyl ligands, film growth, and the desorption of Zn and Cd metal. The predictions of the model have been compared to the Zn segregation data reported in the literature. This analysis reveals that the distribution of the group II elements between phases is relatively insensitive to the process conditions, i.e. temperature and VI/II ratio. However, it is strongly influenced by the intrinsic kinetic parameters, i.e. the difference in the Zn and Cd sublimation energies and the relative sticking probabilities of the organometallic precursors.     

On the electronic energy spectrum of disordered systems

The present paper deals with the investigation of the electronic energy spectrum of a one-dimensional model of monatomic disordered systems with rectangular-well type atomic potentials. First, an equation describing the connection between the wavefunctions at three consecutive atomic sites is derived, the method followed for this purpose being based on the use of well-known “transfer matrix” together with an approach reported earlier by the author. This connection equation is further analysed with the help of continued fraction methods as well as a perturbational approach (first used by Phariseau), and explicit equations, giving allowed electronic energies for various one-dimensional models with small deviations from periodicity, are obtained thereby. It is found in general that, for the types of disorder considered here, a band structure of electronic energies exists; in addition, there appear extra localised energy levels. The treatments involving continued fractions show that such extra energies lie unambiguously within the forbidden regions of the band structure, while the extra energies obtained with the help of perturbational approach, are seen to have the possibility of lying both in the allowed and forbidden regions. With regard to localisation of electronic states, we have found that, for all tratments, the electronic states other than those giving the band structure are localised. As for the band states Phariseau's approach predicts them to be localised, while the continued fraction methods fail to predict whether the band states are localised or not.     

On the enhancement of methanol and CO electro-oxidation by amorphous (NiNb)PtSnRu alloys versus bifunctional PtRu and PtSn alloys

It has been observed in this work and previous works [J. Barranco, A.R. Pierna, J. Non-Cryst. Solids 35 (2007) 851; J. Barranco, A.R. Pierna, J. Power Sources 169 (2007) 71 [1], [2]] that (NiNb)₉₉(PtSn)₁ alloys surfaces are very effective catalyst for CO electro-oxidation, but not for methanol electro-oxidation. On the contrary, (NiNb)₉₉(PtRu)₁ alloys seem to be a very effective catalyst for methanol, but not for CO, electro-oxidation. Since CO_ads is postulated to be a by-product in methanol electro-oxidation on Pt alloy surfaces, the lack of inactivity of (NiNb)₉₉(PtSn)₁ amorphous alloys appears paradoxical. In this work we present the behaviour of a tricatalytic amorphous alloy, in methanol and CO_ads electro-oxidation in an attend to enhance the methanol and CO oxidation mechanisms of the Sn and Ru containing alloys, respectively. It has been observed that the specific activity for (NiNb)₉₉(PtSnRu)₁ for methanol electro-oxidation, is enhanced around 50–80% with respect the bicatalytic PtSn alloys, at low methanol concentration, whereas PtRu catalysts seems to be more effective. At high methanol concentrations, (NiNb)₉₉(PtSnRu)₁ catalysts seems to be more effective. The enhanced in the CO_ads, is observed by the lower rate of poisoning of such alloy compare with the bicatalytic ones. The onset of CO_ads oxidation, appear to shift 0.25 V towards negative values of potentials, with respect (NiNb)₉₉(PtRu)₁ electrodes.     

On the decomposition behaviour of supersaturated Al-Zn-Mg alloys in dependence on heat treatment

The influence of the various heat treatments applied on the decomposition processes taking place in some Al-Zn-Mg alloys (varying in Zn as well as Mg content) are discussed based on the main assumptions that 1) the upper limit of the temperatur of homogeneous nucleation, T_hn, plays an important role on the kind of nucleation taking place at the aging temperature, T_a, and 2) vacancy-rich clusters, VRC, as well as overcritical GP zones can act as nucleation sites for η′;-particles at T_a > T_hn. The influence of the VRC is decreasing with rise of the Zn-content.     

On the Method of Constructing the Radial Distribution Function in the Structure of Amorphous Materials on the Basis of Diffraction Data

Crystallography Reports - The method of constructing the radial distribution function, proposed by the authors previously, has been developed. The method includes calculation of the normalization...     

Amorphous FeCrMo13P7C alloys unsusceptible to hydrogen embrittlement in deaerated solutions of different pH

Bending tests were performed to clarify the effect of chromium and molybdenum contents of amorphous FeCrMo13P7C alloys on the susceptibility to hydrogen embrittlement in deaerated solutions of different pH at 40°C. Some alloys suffered hydrogen embrittlement in deaerated 1 M HCl, 1 M NaCl and deionized water. The addition of sufficient sound amounts of both the elements improved the corrosion resistance and resulted in immunity to the embrittlement due to spontaneous passivation. The increase in pH also decreased the susceptibility to hydrogen embrittlement.     

On the electronic transport properties of amorphous (Si,Ge) alloys: charged scattering centers and compositional disorder

The electronic transport properties of a-(Si,Ge):H alloys over the entire range of Ge content (0-100%) grown using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping have been investigated in detail by employing the microwave photomixing technique. Strong evidence for the existence of long-range potential fluctuations in a-(Si,Ge):H alloys has been found from the measurements of electric field dependence of the drift mobility. It was observed for the first time that the film transport properties do not degrade monotonically with increasing Ge content; there exists a valley point near the middle of the composition range, where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again, but still much worse than that for the Si end. The effect of potential fluctuations can be enhanced by adding Ge or Si to the alloy system resulting in deterioration of the transport properties of a-(Si,Ge):H alloys. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Ge or Si to the alloys play an important role in the potential fluctuations.     

Development of CaZn based glassy alloys as potential biodegradable bone graft substitute

We report the development of a CaZn based bulk glassy alloy (BGA) with combining properties of low Young's modulus, high fracture strength, good corrosion resistance and cytocompatibility, which make it be potential as biodegradable bone graft substitute. Compared to commonly investigated biodegradable metals, the Young's modulus of the BGA is closer to that of cortical bone, and its fracture strength reaches 600 MPa, which is 2 times higher than that of pure magnesium. The magnetic susceptibility of the CaZn based BGA is as low as 22.3 × 10^? 6, which is compatible with magnetic resonance imaging (MRI) diagnosis. The BGAs also show slow degradation rate and good cytocompatibility. The new class of biodegradable CaZn based BGAs enriches the family of biomaterials, and the work has implications for developing new biodegradable alloys with active elements. The formation and the origins of excellent properties of the BGA are discussed.     

On the characteristic structural clustering in metal-metalloid amorphous systems

A Voronoi analysis of selected Fe---B, Fe---P, Ni---B and Ni---P amorphous alloys is made. The alloys were modeled by molecular dynamics, using pseudo- and Morse-pair potentials. Characteristic properties of the cluster formation are discussed.