L. C. Damonte

Nanocrystalline HfN produced by mechanical milling: Kinetic aspects

Nanocrystalline HfN powders were obtained by mechanically assisted gas-solid reaction. An analysis of the reaction kinetics under different milling frequencies and gas pressure is presented. The results shown the formation of an interstitial solid solution a-Hf, prior to its transformation to cubic HfN. Rietveld refinements of the X-ray diffraction data were done in order to analyze the progress of the structural change. The short range order was characterized by perturbed angular correlations, which indicated a random distri- bution of N around Hf. The reaction kinetics were obtained from the measured value of absorbed nitrogen per Hf (n). Two main features were evident: the reaction advance dependence on the dose of mechanical energy and a slowing down in the reaction kinetics for n0.5 associated with the saturation of surface sites. These findings are discussed taking into account the structural change and the role of surface adsorption and volume diffusion.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Effect of preparation conditions on the short-range order in Zr-based bulk glass-forming alloys

Abstract The short-range order in as-prepared and annealed Zr–Cu–Al–Ni glassy alloys, obtained by quenching from the melt at different cooling rates as well as by solid state reaction via mechanical alloying, was investigated by perturbed angular correlations technique (PAC). All the samples were also characterised by X-ray diffraction and differential scanning calorimetry. For all the as-prepared amorphous samples, the PAC spectra are well described by a Czjzek distribution revealing that the short-range order corresponds to that of a dense random packing of ions. The observed electric field gradient seems to be independent of the preparation method and, when compared with previous measurements, of the Zr content. These trends are discussed in comparison with the similar behaviour observed in binary amorphous alloys. Samples annealed above the glass transition temperature but below the crystallisation step showed two broad distributions of frequencies, which were assigned, to a metastable Ti 2 Ni-type phase.

Short-range order in bulk Zr- and Hf-based amorphous alloys

Abstract The local order in mechanically alloyed Hf 65 Al 7.5 Cu 17.5 Ni 10 and Zr 65 Al 7.5 Cu 17.5 Ni 10 amorphous alloys was studied by means of perturbed angular correlation measurements. The amorphous state of the starting material was confirmed by X-ray diffraction. Differential scanning calorimetry analysis showed the existence of an extended supercooled liquid region. Two exothermic effects, indicating a two-step crystallization process, were also observed. Perturbed angular correlation measurements indicated similar short-range order in the supercooled liquid state and in the starting amorphous state. The evolution of short-range order with annealing temperature was also studied ,being the intermediate and final phases characterized by this hyperfine technique.

ZnO Nanoestructured Layers Processing with Morphology Control by Pulsed Electrodeposition

The fabrication of nanostructured ZnO thin films is a critic process for a lot of applications of this semiconductor material. The final properties of this film depend fundamentally of the morphology of the sintered layer. In this paper a process is presented for the fabrication of ZnO nanostructured layers with morphology control by pulsed electrodeposition over ITO. Process optimization is achieved by pulsed electrodeposition and results are assessed after a careful characterization of both morphology and electrical properties. SEM is used for nucleation analysis on pulsed deposited samples. Optical properties like transmission spectra and Indirect Optical Band Gap are used to evaluate the quality of the obtained ZnO structures. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3593004] All rights reserved.

Theoretical treatment of a self-sustained, ball milling induced, mechanochemical reaction in the Fe2O3–Al system

Abstract A theoretical approach to self-sustained reactions (SSR) is established based on self-propagating high temperature synthesis. The corresponding heat conduction equation has been solved numerically and conclusions about the occurrence of the reaction through the whole sample have been obtained from the relationship between the dissipation mechanisms. These results strongly suggest that radiation heat flux plays a dominant role in the propagation of the reaction in reactive ball milling processes. The influence of different parameters on the occurrence of the SSR or its development were studied. Data for the formation of iron–alumina composites by reducing hematite with aluminum were used, in order to obtain realistic values. However the results are sufficiently generic to extrapolate the conclusions to other high enthalpy reactions.

Mechanochemical Reactions in Fe2O3–M (M: Al, Ti)

The production of metal–ceramic nanodispersion by mechanical milling of powders through the displacement reaction Fe2O3+M→Fe+M-oxide (with M: Al, Ti) was studied. The reaction progress with milling time was followed by recording the temperature and pressure during the process. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy at the intermediate and final stages. In both cases self-sustained reactions were observed with different activation times. The results confirm that mechanical work at room temperature yields the reduction of hematite by Ti and Al. The final oxides were identified as Ti2O3 and Al2O3, respectively. The dependence of the intermediate and final stages on the milling conditions and the starting composition will be discussed.

Nucleant layer effect on nanocolumnar ZnO films grown by electrodeposition.

Different ZnO nanostructured films were electrochemically grown, using an aqueous solution based on ZnCl2, on three types of transparent conductive oxides grow on commercial ITO (In2O3:Sn)-covered glass substrates: (1) ZnO prepared by spin coating, (2) ZnO prepared by direct current magnetron sputtering, and (3) commercial ITO-covered glass substrates. Although thin, these primary oxide layers play an important role on the properties of the nanostructured films grown on top of them. Additionally, these primary oxide layers prevent direct hole combination when used in optoelectronic devices. Structural and optical characterizations were carried out by scanning electron microscopy, atomic force microscopy, and optical transmission spectroscopy. We show that the properties of the ZnO nanostructured films depend strongly on the type of primary oxide-covered substrate used. Previous studies on different electrodeposition methods for nucleation and growth are considered in the final discussion.

Phase Transformations in Fe–Mn Alloys Induced by Ball Milling

We have studied the dependence on the milling time of the amounts of metastable fcc and hcp phases produced by ball milling on Fe-13.7 wt% Mn alloys by Mössbauer spectroscopy and X-ray diffraction. The relative proportions of γ-Fe(Mn) fcc and ε-Fe(Mn) hcp increase with milling times up to 9 h and decline afterwards. A random solid solution of Mn in the α-Fe matrix is found for milling times up to 15 h. Mn segregation is observed after 25 h. The evolution of the relative percentages suggests that ball milling brings about processes that on the one hand induce the fcc/hcp martensitic transformation, and on the other favour transformations towards the phases dictated by the thermodynamic phase diagram.

Nuclear techniques characterization of short-range order in Zr-TM-Cu-Al-Ni (TM=Hf, Ti, Fe) bulk metallic glasses

Abstract In this paper an overview of recent and new perturbed angular correlation (PAC), Mossbauer (MS) and positron annihilation (PAS) experiments on Zr- and Hf-based bulk metallic glasses is presented. PAC results showed that all samples in their as-prepared amorphous state, irrespective of the preparation method used, are well described by a dense random packing of ions (DRP). Hf-based samples showed a greater electric field gradient (EFG) value than Zr-based ones for same composition. A general observation is that the relaxed amorphous and supercooled liquid has the same local arrangements than the initial amorphous state. The crystallization behaviour is not preparation-method dependant but composition and impurities have a major influence on it. Preliminary positron annihilation lifetime measurements on amorphous samples yielded one characteristic lifetime, with a value intermediate between those for the well-annealed and defected metallic constituents of the alloys. Mossbauer spectra of as-prepared amorphous alloys consist of a broad asymmetric doublet, originating from a distribution of electric field gradients characteristic for amorphous samples.

Tdpac studies in the semiconductors SnO2 and Cu2O

We present here preliminary TDPAC results on the semiconductors SnO2 and Cu2O in the temperature range 17–1200K, using111Cd. We discuss the presence of a time-dependent interaction associated to EC aftereffects processes in terms of the specific character (acceptor/donnor) of the impurity.