Atomic scale mechanisms for the amorphisation of irradiated graphite

Molecular dynamics simulations using empirical potentials reveal HOPG graphite’s response to irradiations. Two different methodologies: displacement cascades and Frenkel pair accumulations, probe the primary damage and dose effect, respectively. This work reveals that in HOPG graphite primary knock-on atoms with initial energies less than 40 keV do not induce amorphisation by direct impact. Rather, defects stabilise and persist after a single irradiation event. However, amorphisation occurs via the accumulation of defects mimicking multiple events. Before amorphisation the graphite structure undergoes three stages of evolution characterised by (i) an increase in point defects; (ii) a wrinkling of graphene layers pinned by small amorphous pockets; and (iii) a full amorphisation of the structure via percolation of the small amorphous pockets. This structural evolution gives way to an irradiation induced volume change of the HOPG graphite. In the first stage, interstitials contribute, as expected, to c-axis swelling, while vacancies contribute to basal plane shrinkage. Subsequently, rippling of the graphene layers induces the overall volume to change. A power law relation illustrates the relation between the c-axis swelling and the basal-plane shrinkage as a function of the irradiation dose.     

Glass-formation and hardness of Cu–Y alloys

Metallic glasses exhibit particularly attractive mechanical properties, like high stresses to fracture and large elastic strain (up to 2%), but they show generally low plasticity. Aim of this work is to investigate the glass forming range in the Cu–Y system, in order to form the ductile CuY phase (CsCl structure) upon crystallization. Cu₅₈Y₄₂, Cu₅₀Y₅₀ and Cu₃₃Y₆₇ alloys have been prepared by rapid solidification and copper mould casting, obtaining ribbons and cylindrical shaped ingots, with diameter of 2 mm. Fully amorphous, partially amorphous and fully crystalline samples have been obtained for different compositions and quenching conditions. In some cases, the X-ray diffraction results, analysed using the Rietveld method, showed CuY nanocrystals embedded in an amorphous matrix. The microstructure was studied by transmission electron microscopy (TEM) and the presence of nanocrystals of the ductile phase CuY has been confirmed. Microhardness results showed a softening of the amorphous phase due to the presence of CuY nanocrystals and a hardening due to the Cu₂Y phase.     

Phase transformations in materials studied by micro-Raman spectroscopy of indentations

 Phase transformations occurring in materials under high pressures are important for a wide range of problems in materials science and solid-state physics. Most of the results in this area have been obtained using various sophisticated high-pressure cells. We studied solid-state phase transformations and amorphisation under high non-hydrostatic pressures in very simple experiments using a combination of hardness indentation tests with micro-Raman spectroscopy. Amorphisation of diamond, that did not occur under hydrostatic loading, has been observed. Shearing and distortion of cubic diamond structure above 100 GPa resulted not only in its amorphisation, but also in the formation of threefold coordinated carbon. A carbon film that was squeezed between a SiC substrate and diamond indenter lost its graphitic structure and produced a Raman band typical of diamond-like carbon (DLC). Even for such a well-studied material as Si, principally new data have been obtained. High spatial resolution of the method allowed us to show that the Raman spectrum that was previously ascribed to a metastable Si-III phase originates from two different high-pressure phases of Si. Up to five different phases of Si were found within a single impression. Studies of reversible transformations that occur upon unloading or heating of samples by the laser beam have also been carried out. Amorphisation and/or phase transformations have been observed for some other materials, such as SiC, quartz, Ge, GaAs and other. The combination of indentation tests with micro-Raman spectroscopy provides a powerful and fast tool for in-situ and ex-situ monitoring of pressure-induced phase transformations in materials. Received: 2 January 1997 / Accepted: March 1997     

Temperature dependence of damage formation in Ag ion irradiated 4H-SiC

Rutherford backscattering spectrometry (RBS) in channelling mode was used to study the defect formation in silver (Ag) ion irradiated silicon carbide (SiC). The 4H-SiC samples were irradiated with 360 keV Ag ions at different temperatures (15, 295, 375, 475, 625 and 875 K) over a wide range of fluences (1×10¹¹ to , depending on the irradiation temperature). The results can be divided into two groups: (i) for irradiation temperatures between 15 and 475 K amorphisation of the implanted layers is reached for ion fluences between 7×10¹³ and . The over-all cross-section of defect production at very low ion fluences which comprises the formation of point defects and of amorphous clusters, is almost identical for all data sets measured in this temperature range. Differences in the damage evolution which occur at higher ion fluences, suggest that the relative contribution of amorphous clusters within single ion impacts in crystalline material decreases with rising temperature. (ii) For irradiations performed at 625 and 875 K no amorphisation is found for ion fluences as high as . With increasing ion fluence the defect concentration exhibits a distinctive plateau due to the balance between formation and recombination of point defects before increasing up to a saturation level well below amorphisation. For this final stage our results indicate a mixture of point defect clusters and extended defects most probably dislocations. A comparison with data from the literature suggests that the damage evolution for implantation at 625 and 875 K is strongly influenced by the mobility of vacancies starting at around 600 K.     

Solid state amorphisation in binary systems prepared by mechanical alloying

In the present work a detailed study of amorphisation in different systems prepared by mechanical alloying under the same experimental conditions was carried out, milling up to 50 and 100 h in some cases. The systems studied were: AlTi, AlNi, AlFe, FeNi, FeCo, NiMo, NiW, NiCo, MoW, CoMo. These systems were chosen to study the effect of Al-transition metal, transition metal–transition metal and also systems with large and small negative heat of mixing, different and similar crystal structures, atomic sizes and diffusion coefficients. Calculations based on the Miedema model for alloy formation and amorphisation on all the alloys studied were performed. The experimental results from X-ray diffraction and transmission electron microscopy showed that the systems based on Fe (FeNi, FeCo and FeAl) did not amorphised, even after milling for 100 h, and formed a stable solid solution with a nanometric grain size of 7 nm. The systems NiMo, NiW, MoW and CoMo (systems with small negative heat of mixing), showed amorphisation after 50 h of milling. NiAl and TiAl form an intermediate amorphous phase after around 20 h of milling and with further milling they recrystallize into a fcc solid solution. Agreement between the theoretical calculations based on the Miedema model and the experimental results was found in most of the systems.     

Structural investigations of Y2O3 dispersoids during mechanical milling and high-temperature annealing of Fe-15Y2O3-xTi (x = 0–15) model ODS alloys

A model Oxide Dispersion Strengthened (ODS) alloy powder of composition Fe – 15 wt. % Y₂O₃ – x wt. % Ti (x = 0, 2, 5, 10 and 15) were synthesized by high energy mechanical milling in Ar atmosphere for a prolonged duration of 60 h. Synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations suggested the amorphisation of Y₂O₃ nano-crystallites, irrespective of Ti content, which is further studied by Raman spectroscopy. The Raman spectroscopy analysis confirms the presence of YO bonding in the milled powder and ruled out the possibility of elemental dissociation of Y₂O₃ and dissolution into the Fe matrix. Annealing of the milled powders containing different amounts of Ti led to formation of different types of oxide complexes which were also studied using synchrotron XRD and TEM studies. The role of Ti in refining the dispersoids through formation of Y_1.6Ti_1.8Fe_0.4O_6.6 is established through these studies.     

Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite

Mechanical activation of boehmite (γ-AlOOH), synthesized by thermal decomposition of gibbsite, has been carried out in a planetary mill up to 240 min. After an initial decrease in particle size up to 15 min, the particle size shows an increase with further milling; the median size (d₅₀) has increased from 1.8 to 5 μm during 15 to 240 min of milling. Quite unexpectedly, the BET specific surface area of the sample (N₂ adsorption method) decreases continuously from 264 m²/g to 67 m²/g with milling. A detailed analysis of N₂ adsorption/desorption isotherms has indicated that the decrease in surface area is associated with: (a) change in narrow slit like pores with microporosity to slit shaped pores originating from loose aggregate of platelet type particles; and (b) shift of maxima in pore size distribution plot at ~ 2 nm and ~ 4 nm to dominantly ~ 23 nm size pores. Scanning electron microscopy (SEM) studies have revealed that during milling, initial breakage is followed by agglomeration/fusion of particles with consequent loss in porosity. Amorphisation, decrease in microcrystallite dimension (MCD) and increase in microstrain (ε) are indicated from a detailed analysis of X-ray powder diffraction patterns and Fourier Transform Infrared (FTIR) spectra. Reactivity of samples, expressed in terms of increase in dissolution in alkali (in 8 M NaOH at 90 °C) and decrease in boehmite to γ-Al₂O₃ transformation temperature, increases with milling time. The nature of correlations between reactivity and physico-chemical changes during milling has been analyzed and discussed.     

Alloying evolution and stability of Al65Cu20Ti15 during process of amorphisation by high energy ball milling

Abstract

Mechanical alloying of Al₆₅Cu₂₀Ti₁₅ powder blend has been carried out by high energy vibrating ball mill. The process of amorphisation in the mechanically alloyed Al₆₅Cu₂₀Ti₁₅ powder and the stability of the amorphous phase during ball milling were investigated. Almost completely amorphous powder was achieved after 25 h ball milling. Examination of the microstructural constituents using X-ray diffraction and transmission electron microscopy shows that the amorphisation process was controlled by the transformation of both Al based solid solution and intermetallic compounds (Al₂Cu, Cu₉Al₄ and AlCu₂Ti). However, that prolonging the ball milling time to 30 h led to the appearance of Cu₉Al₄, the Al₆₅Cu₂₀Ti₁₅ composite comprising nanocrystalline and amorphous phases could be stable after 50 h ball milling.     

Characteristics of aluminum-implanted 6H-SiC samples after different thermal treatments

Multiple energy aluminum (Al) implantations were performed at room temperature in n-type epitaxial 6H-SiC layers, aiming at amorphizing the material from the surface up to a depth inferior to 0.5 μm. Annealings were then carried out in an induction furnace. The goal of this paper is to optimize the furnace geometrical configuration, in order to reduce the surface degradation and improve the crystal reordering. This optimization was established for one-side amorphized wafers, which need restricting annealing parameters, and is therefore supposed to be valid for less crystal damaging implantations. Two types of geometrical parameters were essentially studied: the internal configuration, which tends to increase the silicon partial pressure inside the reactor, and the position of the sample, which has a direct influence on the recrystallization and on the dopant electrical activation. The annealings are compared for the same thermal parameters: the plateau temperature (1700 °C), the annealing duration (30 min), and the heating rate (60 °C s⁻¹). The surface roughness was evaluated by using atomic force microscopy. Two final configurations were retained, leading to satisfactory results with respect to the as-implanted material: (i) Rutherford backscattering spectrometry in channeling geometry revealed a very good recrystallization in both cases, giving a signal level similar to the virgin crystal one; (ii) secondary ion mass spectrometry showed two distinct results depending on the sample position: one position led to some material etching, especially the SiC part which was amorphized by the implantation, and the second position gave rise to the deposition of a crudely monocrystalline SiC layer on the surface of the sample implanted side. This coating was found to prevent from any dopant loss by exodiffusion or material etching. Electrical measurements (four-point probe at 300 K) proved an Al substitutional ratio of 97 and 78% depending on the configuration, giving room temperature sheet resistances of about 2×10⁴ and 4×10⁴ Ω sq.⁻¹, respectively, for 4×10¹⁹ cm⁻³ Al implanted samples.     

Influence of Fermi energy equalization on crystal nucleation in glass melts

The energy saving produced by the equalization of Fermi energies of a crystal and a melt has been associated to the crystal formation in undercooled melts to determine the new homogeneous-nucleation critical-temperature T₂ and the new nucleation critical barrier as a function of the temperature T. Small unmelted clusters act as nuclei by reducing the critical energy barrier and the nucleation times; the glass transition temperature T_g occurs near T = T₂. The temperature dependence of the specific heat difference of the undercooled melt with the glass, the viscosity weakening below T_g and the nose of the temperature–time–transformation diagrams are successfully predicted.