Oxidation Kinetics of Amorphous,Polycrystalline, and Nanocrystalline Co33Zr67 Alloys

The oxidation kinetics of amorphous, polycrystalline, and nanocrystalline CoZr₂ alloys have been studied under isothermal conditions in air atmosphere with a microbalance. DSC and X-ray diffraction were used to study the influence of oxidation on crystallization. The three alloys under investigation which have the same composition but differ in the microstructure showed noticeable differences in their oxidation behaviour: the oxidation of the amorphous alloy is characterized by a very fast initial oxygen absorption step, completed in less than 1–2 min, followed by a slow oxide growth obeying a logarithmic time law at 400 °C. The polycrystalline thermodynamically stable tetragonal alloy absorbs small amounts of oxygen and obeys a parabolic rate law in the temperature range 500–800 °C. The activation energy of oxygen diffusion in tetragonal CoZr₂ was determined from the temperature dependence of the parabolic rate constant. The intermediate nano-crystalline phase with a local structure more closely packed and ordered than that of the precursor amorphous phase, shows an oxidation behaviour similar to that of the tetragonal phase, but with a different time exponent.     

Stabilization Effect of Zr and Ti Additions on the Ageing Characteristics of Al-1 wt% Si Alloy Through a Creep Study

Al-1 wt% Si and Al-1 wt% Si-0.1 wt% Zr–0.1 wt% Ti alloys were used to trace the effect of Zr and Ti additions on the behaviour of the steady state creep. After solid solution treatment specimens of both alloys were aged at 623, 673, 723 and 773 K and creep tests were performed at room temperature by applying stresses of 60.0, 62.4, 64.7 and 67.1 MPa. The results showed a sound stabilization effect of Zr and Ti on the ageing characterstics of binary Al-1 wt% Si alloy. Values of the applied stress sensitivity parameter, m, obtained were in the range of (20–34) for Al Si alloy and (14–19) for Al Si Zr Ti alloy. Time to rupture was found to be strongly increased by Zr and Ti additions. The activation energies of the precipitation process involved were found to be 81.9 kJ/mole and 33.7 kJ/mole of the Al Si and Al Si Zr Ti alloys respectively.     

Synthesis of nano‐crystalline zeolite β: Effects of crystallization parameters

The effects of variation in Si/Al ratio (25 and 100) and crystallization temperature (80 °C to 180 °C, at an interval of 20 K) on crystal size of zeolite β were studied. Products obtained at different synthesis parameters were characterized by powder X‐ray diffraction, IR spectroscopy, thermal analysis, scanning electron microscopy and nitrogen adsorption. Increase in crystal size with crystallization temperature and Si/Al molar ratio was observed. Crystal morphology at 140 °C was spherical whereas at 180 °C it was of irregular shape. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the kinetics of the decomposition of ternary Al-Zn(6at.%)?Mg(X) and Al?Mg(X)?Si(Y) alloys

Investigations on Al-Zn(3)-Mg(X) alloys gave the rather surprising result that the migration energies depend on the physical quantities (resistivity, Young's modulus) and the ageing time used. Therefore it is the aim of this paper to test, if similar effects occur using other ternary alloys and additional measuring methods. The ageing kinetics of 7 Al-Zn(6)-Mg(X) alloys are examined by isothermal measurements of resistivity, Young's modulus, change of length, and microhardness over a relatively long time interval of about 3 h within the temperature range from 20°C to 90°C.

• 1 The activation energies got for the Al-ZN(6)-Mg(X) alloys depend on the physical quantity as well as on the ageing time used within the temperature range from 20°C to 40°C as known from the Al-Zn(3)-Mg(X) alloys.
• 2 The activation energies obtained for the Al-Zn(6)-Mg(1.3) alloy in the temperature interval from 50°C to 90°C decrease monotonously with increasing time for the measuring methods considered.

These results different with respect to those two temperatur ranges are probably caused by a change of the decomposition mechanism. Below 40°C presumably several kinds of clusters are formed in the beginning of the decomposition, but above 50°C probably only one sort of clusters should grow. The late stage of ageing should be caused mainly by the diffusion of VZn₂ complexes (E^M = 0.21 eV) in the temperature range investigated, since 0.2 eV holds for that stage. – Finally the results of the investigations of the kinetics of some Al-Mg(X)-Si(Y) alloys are discussed by means of the jump model.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Investigation of the formation of nanowires from silicon whiskers

Nanowires have been prepared by the high-temperature oxidation of Si whiskers. The dependences of the nanowire formation on the oxidation parameters have been investigated. The oxidation rate is shown to depend on the whisker diameter. Oxidation in dry oxygen at temperatures no higher than 950°C results in self-stopping; i.e., the nanowire diameter is stabilized. Stabilization is not observed at oxidation temperatures above 950°C or at oxidation in wet oxygen. Oxidation at higher temperatures made it possible to obtain nanowires ≤5 nm in diameter in relatively thick (up to 200 nm in diameter) whiskers.     

A positron annihilation study of vacancies and their clusters in diluted aluminium alloys quenched or neutron-irradiated

Pure Al and Al alloyed with Mg, Zn, Ge, and Si was quenched from 600°C in water at room temperature or irradiated with fast neutrons. With the help of positron lifetime and angular correlation peak height measurements vacancies, their clustering and their annealing behaviour were studied.     

Effect of Cyclic Stress Reduction on the Creep Behaviour of Al‐Ag and Al‐Ag‐Zr Alloys Containing γ′ and γ Precipitates

Creep tests under the effect of cyclic stress reduction were conducted to Al‐16 wt%Ag and Al‐16 wt%Ag‐0.1 wt%Zr alloys aged at 548 and 673 K. The steady state creep rate was found to be higher under cyclic stress reduction condition than that under static creep condition for the same maximum stress. An expression for the temperature dependence of the threshold stress for cyclic creep acceleration is offered. Cyclic creep acceleration is interpreted using the dislocation cross‐slip model. TEM investigations confirmed that the addition of Zr to Al‐Ag alloy accelerates the formation and coarsening of γ′ and γ‐phases. The mean value of activation energy for both alloys aged at 548 K was found to be 35±0.3 kJ/mol, and 58±0.4 kJ/mol for alloys aged at 673 K.     

The recovery of Quenched-in Vacancies in Fe–Al (6.3 to 28.3 at.%) Alloys studied by Positron Annihilation

Measurements of the peak height of the 2γ-angular correlation curve were carried out during constant rate heating of Fe–Al alloys with an Al-content between 6.3 and 28.3 at.%. The samples had been quenched form 1000°C into water at room temperature. Two recovery stages are observed. The first stage at 100 to 200°C shows a small decrease of the peak height. The main stage at 300 to 400°C is attributed to the migration and disappearance of monovacancies. From isothermal measurements the migration enthalpy of vacancies is estimated to (1.15 ± 0.1) eV in Fe–Al (19.9 at.%) and to (1.31 ± 0.1) eV in Fe–Al (24.4 at.%). – In samples with an Al-content between 13.8 and 28.3 at.% a minimum in the peak height at 400°C is found. Obviously a particular stage is formed in these samples during migration of quenched-in vacancies. Possible causes of the effect are discussed.     

Aluminum induced formation of SiGe alloy in Ge/Si/Al thin film structure

In this work, a study of aluminum induced crystallization (AIC) of thin film germanium/silicon/aluminum (Ge/Si/Al) structure on oxidized silicon is presented. The Ge/Si/Al trilayer structure was prepared in three consecutive thin film deposition processes. The AIC was performed in nitrogen at 500 °C within time duration between 1 and 9 h. The progress of crystallization was monitored by optical microscopy, Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS). It was found that in Ge/Si/Al structure the AIC can lead to formation of SiGe alloy at temperature of 500 °C. This presents an alternative low-temperature formation method of SiGe which is suitable for integration with the conventional Si technology in electronic device fabrication.     

Mechanical Behaviour of LiF Single Crystals in Compression and Tension at Temperatures from −196°C to 800°C

The work hardening characteristics of LiF single crystals in compression and tension has been investigated as a function of temperature. It was shown that in temperature range from 200 °C to 600 °C four stages of hardening were observed and the main part of plastic flow proceeds in stage IV. The parabolic hardening was not observed. The flow stresses and hardening rates in all stages in compression are higher than those in tension. As a temperature increase this difference decreases. The hardening rates, stresses which correspond to the beginning of the linear hardening stages and the duration of those stages decrease as a temperature increase. At temperatures above 600 °C the yield drop occured. Taking into account a good agreement between the temperature dependences of the critical resolved shear stress and τIII the assumption was made that τIII corresponds to resolved shear stress in a secondary glide system.     

Formation and thermal properties of glassy NiFe based alloys

The formation and thermal properties of glassy NiFe based alloys were investigated. The NiFePBSiAl alloys were found to exhibit a wide range of glass-forming compositions and could be quenched to the glassy state with relative ease and reproducibility. The variation of the glass transition temperature with a change in the ratio of Ni to Fe and with variations in the P, B, C, Si and Al content are reported.     

Solid phase epitaxial growth of Si through Al film

Uniform epitaxial growth has been obtained by dissolution and transport of an evaporated Si film through an evaporated Al film at temperatures below 500°C. By analyzing the samples made in different ways we show that the presence of a cap on the metal layer, which inhibits the diffusion of the metal through the evaporated Si, plays a fundamental role. The cap consists of a thin oxide layer grown on top of the metal. The cap is made by leaving the sample in vacuum for two days or by heating the sample in vacuum before the Si deposition. The study of the initial growth rate on 〈100〉 and 〈111〉Si substrates reveals that the growth starts as islands which grow until they coalesce to form a continuous layer. Different growth rates have been obtained by using 〈100〉 and 〈111〉Si substrates. Typical growth rates are 50 Å/min at 330°C on 〈100〉 and 100 Å/min at 392°C on 〈111〉. The activation energy of the process is 1.2 eV.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

On the position of the upper limit temperature of homogeneous nucleation of precipitates in low concentrated Al?Zn alloys

The upper limit temperature T_hn of the onset of homogeneous nucleation of Guinier-Preston (GP) zones was determined for three Al Zn alloys with xZn = 4.5, 6.0 and 8.0 at.% by SAXS investigations (cooling from the range of homogeneity to various temperatures T_a and ageing at T_a). The results are T_hn = (95 ± 3) °C for xZn = 4.5 at.%, (118 ± 3) °C (6.0 at.%) and (154 ± 3) °C (8.0 at.%), respectively. The obtained results fit well the data known for the alloys with higher contents of Zn. It is stressed that one has to distinguish between T_hn, determined by isothermal measurements after a direct quench or cooling to the respective T_a, and the upper limit temperature T_rhn of the onset of the rapid homogeneous nucleation of GP zones (continuous cooling).     

Investigations on the HCl gas-phase etching of differently doped and oriented gaas crystals

The influence of the orientation and dopant on the dissolution of GaAs surfaces in H₂–HCl gas mixtures has been investigated. For this purpose the etch rates, temperature regions of different etching behaviour and the corresponding activation energies were determined for temperatures ranging from 540 °C to 945 °C. The etch rate anisotropy is correlated with the three existing temperature regions of different dissolution mechanisms. The orientation-dependent transition temperatures are influenced by the dopant and can be related to the thermodynamic data of the dopant.     

Thermal analysis of geopolymer pastes synthesised from five fly ashes of variable composition

This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥ 5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000 °C, whereas geopolymers synthesised from fly ashes with low Si:Al (< 2) in the glass exhibited strength losses and reduced dimensional stability upon high temperature exposure.     

Morphology and crystal structure of A1-doped TiO2 nanoparticles synthesized by vapor phase oxidation of titanium tetrachloride

Al-doped titanium dioxide nanoparticles with precisely controlled characteristics were synthesized in an aerosol reactor between 900 °C and 1500 °C by vapor-phase oxidation of titanium tetrachloride. The effect of process variables (reactor temperature, initial TiCl₄ concentration, residence time and feeding temperature of oxygen) on particle morphology and phase characteristics was investigated using TEM, XRD, EDS, ICP and XPS, etc. The average particle size increased with decreasing oxygen feeding temperature and increasing reaction temperature, residence time and TiCl₄ concentration. The presence of aluminum during gas phase reaction increased the rate of phase transformation from anatase to rutile and altered the particle morphology from polyhedral to irregular crystals. TiO₂ and Al₂O₃ co-precipitated during particle formation which lead to the aluminum solid solution in titania. α-Al₂O₃ and Al₂TiO₅ were observed at AlCl₃/TiCl₄ ratios higher than 1.1 and reactor temperatures in excess of 1400 °C. The rutile content, which increased with increasing Al/Ti ratio and residence time, was at a maximum at about 1200 °C and decreased at both lower and higher reactor temperatures.     

Nucleation in quiet supersaturated ADP solutions

The nucleation study has been made with supersaturated ADP solution. Rate of nucleation as indicated by the induction period was measured for non-agitated system over the temperature range 20°C–45°C. The nucleation rate increases with increase in temperature but supersaturation has the dominant effect as predicted by classical nucleation theory. However, attempts to analyse the results in accordance with classical theory were not entirely successful, but it is shown how the assumption of (a) a variation of crystal surface energy with temperature and (b) the influence of heterogeneous nucleation can account for the discrepancies. Activation energies, surface free energies and sizes of crystal nuclei were determined for nuclei of ammonium dihydrogen orthophosphate over the range of temperature 20°C–45°C.