Tony Spassov

Hydrogenation of amorphous and nanocrystalline Mg-based alloys

Abstract Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline/amorphous Mg 75 Ni 20 Mm 5 (Mm=Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of α-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 160°C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd ( a o =2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300°C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 .

Nanocrystallization and hydrogen storage in rapidly solidified Mg–Ni–RE alloys

Hydrogen storage in rapidly solidified magnesium based Mg-Ni-RE (RE 5Y or Mm) alloys was studied. The as-quenched alloys were amorphous or nano- / amorphous, consisting of nanocrystals (with average size of 3 nm) embedded in large amounts of amorphous phase. The hydrogenation properties (hydrogen storage capacity and hydrogenation kinetics) of Mg Ni Y and Mg Ni Y alloys were 76 19 5 78 18 4 compared mainly with those of Mg Ni Mm (Mm5Ce, La-rich mischmetal), reported recently. Alloys with the above mentioned 75 20 5 compositions were found to attain the best hydriding characteristics among the Mg-Ni-RE alloys. A more beneficial effect on the hydrogen storage capacity of Mm (La,Ce-rich mischmetal) than Y was detected. Crystallization of these alloys leads to a nanocrystalline material through a metastable fcc Mg Ni phase, as for the Mg Ni Y alloy the nanostructure and the intermediate phase are thermally 67 8 18 4 more stable. The kinetics of crystallization was studied at non-isothermal conditions and the kinetic parameters for different magnesium alloys were compared. The results from previous and the present study show that stable nanocrystalline microstructures appropriate for hydrogen storage can be produced by controlled crystallization of amorphous or nano- / amorphous magnesium based Mg-Ni-RE precursors.

Grain-growth in nanocrystalline zirconium-based alloys

Crystallization and subsequent grain growth in nanocrystalline Fe33Zr67 and (Fe, Co)33Zr67 alloys were studied by TEM, differential scanning calorimetry and X-ray diffraction. The grain-growth data for both alloys obtained over a wide temperature range (about 200 °C) were fitted to different kinetic equations (with different grain-growth exponent, n). The model with n=3 (Equations 4 and 5) was found to predict in the best way the isothermal experimental data. This result gives strong evidence that crystallization (in our case by a polymorphic reaction) is indeed observed of a glass into a nanocrystalline material prior to the coarsening, rather than grain growth in an extremely fine-grained material which was never glassy at all. The activation energies for grain-growth, — 260±25 kJ mol−1, were found to be practically the same for both systems. Additional information about the crystal growth kinetics of the nanocrystals in the amorphous matrix was obtained for (Fe, Co)33Zr67 glass.

Primary crystallization kinetics in rapidly quenched Mg-based Mg–Ni–Y alloys

Abstract The primary crystallization kinetics of two magnesium based Mg–Ni–Y amorphous alloys (Mg76Ni19Y5, Mg78Ni18Y4) with attractive hydrogen storage characteristics were studied. Although the compositional difference of the alloys is very small they show essentially different devitrification behaviour. In Mg76Ni19Y5 the crystallization starts with formation of the equilibrium hexagonal Mg2Ni phase, obeying a continuous nucleation and parabolic growth kinetics law during the whole transformation. Mg78Ni18Y4 alloy forms a nanocrystalline metastable phase during primary crystallization, which was previously identified by us as fcc Mg6Ni and was found to be responsible for the improved hydrogenation/dehydrogenation kinetics [1] , [2] . The kinetics of the nanocrystal formation in the last alloy were found to deviate from the JMKA model, including nucleation and diffusion controlled growth, due to diffusion field impingements in the advanced stage of the primary crystallization reaction. On the basis of the experimental data on the transformed fraction and grain size as a function of time, important information about the mechanism of these transformations was obtained.

Effects of non-steady state nucleation in the kinetics of crystallization of the amorphous alloy Fe80B20

The existence of non-steady state nucleation in the isothermal crystallization of the amorphous alloy Fe80B20 is shown. The incubation time τ0 of isothermal volume crystallization of the alloy is investigated in a wide range as a function of temperature. From these data an equation for the temperature dependence of the viscosity η = η(T) is derived: η = 6.62 exp (2526.97T) × exp [836.52/(T − 530)].[/p]

Oxidation of rapidly solidified Mg87Ni12Y1 alloy

Abstract Oxidation of a rapidly solidified Mg–Ni–Y alloy was studied in a wide temperature range in air by means of TG, DSC, XRD, TEM, SEM/EDAX. The alloy was in different microstructural states and with a different phase composition depending on the temperature of oxidation. The oxidation kinetics have been found to obey different laws (linear, logarithmic, parabolic) at the different temperatures. At low temperatures of oxidation (200–350°C) the alloy oxidizes slowly with linear kinetic law. In the temperature range of 350–430°C generally a logarithmic model describes best the experimental kinetic data, as the initial stage of oxidation follows also a linear kinetic equation. At temperatures above 430°C a parabolic law is valid. During further increase of the temperature, again a linear kinetic law was followed due to partial cracking of the oxide film. The rate constants, activation energies and pre-exponential factors were also derived. The products of oxidation, the morphology and microstructure of the oxide film, alloy matrix and oxide/alloy matrix interface were investigated. The phase composition and microstructure of the metal matrix do not differ very much from those resulting after annealing in a protective argon atmosphere. Grain growth proceeds with a low rate even after long term annealing at temperatures of about 400–450°C. It was found that the oxide film consists of MgO and MgNiO2 and traces of yttrium oxides. The outer layer of the scale is nanocrystalline. At the oxide/alloy matrix interface dense coarse crystals with most probable composition Mg24Y5 were observed. The microscopic observations as well as the kinetic results indicate that the oxide film in Mg87Ni12Y1 works successfully as a protective layer at temperatures up to 500–520°C, retarding the oxygen transport in the scale and to the oxide/metal interface.

Crystallization behaviour of Fe-(Nb,Cu)-Si-B metallic glasses

The crystallization behaviour and Curie temperatures of Fe−(Nb,Cu)−Si−B metallic glasses were studied by means of differential scanning calorimetry (DSC), thermomagnetic gravimetry (TMG) and X-ray diffraction. The agreement between the DSC and TMG results was complete. For all Fe−Si−B amorphous alloys, two-peak crystallization was observed with the primary crystallization of α-Fe(Si) followed by eutectic crystallisation. The effects of Cu and Nb additions on the crystallization behaviour and on the activation energies for each stage of the crystallization process of Fe−Si−B glass were investigated.ZusammenfassungMittels Differential-Scanningkalorimetrie (DSC), thermomagnetischer Gravimetrie (TMG) und Röntgendiffraktion wurde das Kristallisationsverhalten und die Curie-Punkte der metallischen Gläser Fe−(Nb,Cu)−Si−B untersucht. Zwischen DSC- und TMG-Ergebnissen besteht volle Übereinstimmung. Bei allen amorphen Fe−Si−B-Legierungen wurden zwei Peaks Kristallisation beobachtet, wobei die primäre Kristallisation von α-Fe(Si) von einer eutektischen Kristallisation gefolgt wird. Der Einfluß des Zusatzes von Cu und Nb auf das Kristallisationsverhalten und auf die Aktivierungsenergien für jede einzelne Stufe des Kristallisationsprozesses von Fe−Si−B-Gläsern wird untersucht.

Microstructural investigations of carbon foams derived from modified coal-tar pitch

This work reports the microstructural evaluation of carbon foams derived from coal-tar pitch precursors treated with H2SO4 and HNO3 and finally annealed at 1000°C and 2000°C. Our experimental investigations combine scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) imaging, X-ray photoelectron spectroscopy (XPS) and micro-spot near-edge X-ray absorption fine structure (μ-NEXAFS) spectroscopy. This set of complementary techniques provides detailed structural and chemical information of the surface and the bulk of the carbon foams. The high-resolution microscopy data indicate the formation of carbonaceous amorphous microspheres (average diameters of 0.28±0.01μm) embedded in the partially graphitized carbon foam matrix at 1000°C. The microspheres are enriched with sp-bonded species and their microstructural characteristics depend on the reagent (nitric vs. sulfuric acid) used for pitch treatment. A complete chemical transformation of the microspheres at temperatures >1000°C occurs and at 2000°C they are spectroscopically identical with the bulk material (sp(2)- and sp(3)-hybridised forms of carbon). The microstructure-property relationship is exemplified by the compressive strength measurements. These results allow a better description of coal-tar pitch-derived carbon foams at the atomic level, and may account for a better understanding of the processes during graphitization step.

Influence of milling conditions on the hydriding properties of Mg-C nanocomposites

Mg75 at.%, CB25 at.% (CB: carbon black) composites were synthesized at different ball milling conditions (milling energy, milling duration, and environment) and their hydriding properties were characterized by high-pressure DSC. The SEM observations revealed that the samples consist of 5-15 µm Mg particles, surrounded and in some cases coated by carbon particles. X-ray diffraction analysis showed that the Mg phase of all as-obtained composite powders is nanocrystalline with average crystallite size in the range 20-30 nm, depending on the milling conditions. The best hydriding properties, expressed in low-temperature hydriding (below 150°C) and improved cycle life, showed the composites milled at dry conditions. This is obviously due mainly to the successful Mg surface protection by the carbon. Additional decrease of the hydriding temperature (<100°C) was achieved applying higher-energy milling, but at the same time the cycling stability deteriorated, due to the extremely fine particles and microstructure achieved under these conditions. The composites milled in the presence of heptane showed rapid capacity decline during cycling as well. The observed difference in the hydriding behavior of the Mg-CB composites is attributed to the different coating efficiency of the carbon milled under different conditions with Mg, which is supposed to protect magnesium from oxidation and plays a catalytic role for the hydriding reaction.

Liquid crystal nanocomposites produced by mixtures of hydrogen bonded achiral liquid crystals and functionalized carbon nanotubes

The liquid crystalline (LC) nature of alkyloxybenzoic acids is preserved after adding of any mesogenic or non-mesogenic compound through hydrogen bonding. However, this noncovalent interaction provokes a sizable effect on the physical properties as, e. g. melting point and mesomorphic states. In the present work we investigate nanocomposites, prepared by mixture of the eighth homologue of p-n-alkyloxybenzoic acids (8OBA) with single-walled carbon nanotubes (SWCNT) with the purpose to modify the optical properties of the liquid crystal. We exercise optical control on the LC system by inserting SWCNT specially functionalized by carboxylic groups. Since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level, molecular modification can lead to different macroscopical nanocomposite symmetry. The thermal properties of the functionalized nanocomposite are confirmed by DSC analyses. The mechanism of the interaction between surface-treated nanoparticles (functionalized nanotubes) and the liquid crystal 8OBA bent- dimer molecules is briefly discussed.