Structure and properties of nanocrystalline Cu70Fe18Co12 obtained by mechanical alloying

Cu₇₀Fe₁₈Co₁₂ alloy is prepared by mechanical alloying of pure Cu, Fe, Co powder using a high energy ball mill, with increasing milling time ranging from 4 to 8, 24, 36 and 54 h. The variation of the morphology and the elemental distribution were measured at these different stages on various grains of the alloy using a scanning electron microscope with a dispersive energy analyzer. Atomic clusters of iron were observed on some grains after 8 h of milling, confirming the non-homogenisation of the powder at this stage. Beyond 12 h, the homogenisation is ensured over a volume of one cube micron. Microstructural changes during the mechanical alloying have been studied by X-ray diffractometry (XRD) and Mössbauer spectroscopy. X-ray diffraction measurements confirm the dissolution of iron and cobalt phases in the FCC matrix of copper after 24 h of milling with increase of the structural parameter. This same dissolution was also measured by Mössbauer spectroscopy, confirming that after 4 h of milling the CuFe phase begins to form and iron dissolution is incomplete with partial amount of alpha Fe phase surviving after 36 h of milling.     

Amorphous and nanocrystalline silicon made by varying deposition pressure in PECVD process

Silicon thin films are deposited using plasma enhanced chemical vapor deposition (PECVD) of silane, argon, hydrogen mixture at various pressures in the range of 2–8 Torr. Raman scattering shows these to be amorphous in the pressure range 6–8 Torr, and nanocrystalline in the range 2–4 Torr. The volume fraction of nanocrystals is estimated by fitting the Raman data to three peaks and is found to be ～75% for the films deposited at low pressure, density of states of these films was measured. It is observed that the electrical conduction in these films depends on the crystalline volume fraction (ρ), estimated from the laser Raman Spectroscopy. Temperature dependence electrical conductivity shows that at lower temperatures thermionic emission dominates for the films with lower ρ, whereas, hopping is the main conduction mechanism for the films having high ρ. The density of states is estimated from the space charge limited currents (SCLC) observed at high fields. Photoconductivity at room temperature is also measured. The amorphous films are found to be more photosensitive than the nanocrystalline one. In the context of these findings, changes in the properties of silicon from amorphous to nanocrystalline are described.     

Electrical and optical properties of CdS nanocrystalline semiconductors

II‐VI semiconductor nanoparticles have recently attracted a lot of attention due to the possibility of their application in various devices. In the present study, chemical method has been used in synthesis of CdS nanoparticles and thiophenol was used as capping agent. X‐ray diffraction studies of both samples were done. The dc conductivity of CdS increases at a lower rate or is approximately constant upto 500K and thereafter the conductivity increases at a rapid rate. Beyond T_c it is seen that the portion of the σ_dc versus 1000/T is almost a straight line showing an Arrehenious behaviour. The dielectric constant of nanoparticles of CdS is found to be larger than the corresponding values of CdS crystals. It is clearly observed that at lower wavelengths nanocrystalline samples show a blue‐shift. The three peaks of sample (S2), A, Band C can be ascribed to the transition from Cd‐O complex donor formed by adsorbed oxygen to the valance band, Cd ‐ excess acceptor and the surface states, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of nanocrystalline zinc blende ZnTe by mechanical alloying

Nanocrystalline ZnTe was prepared by mechanical alloying from equiatomic powder mixture. X-ray diffraction and differential scanning calorimetry techniques were used to study the structural and thermal properties of the milled powder. An annealing of the as-milled sample at 600 °C for 6 h was performed to clarify thermal reactions showed in its calorimetry measurement. Minority crystalline phases (Te and ZnO) and residual amorphous ones were observed for both annealed and as-milled samples. The structural parameters, phase fractions, mean crystallite sizes and strains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns using the program package GSAS.     

Preparation and reactivities of composite nanocrystalline solids prepared by mechanical alloying

The formation of Fe---SiC composite nanocrystalline solids through ball milling is reported. The effects of mechanical deformation, crystallite size and gaseous elements on the solid state reaction between Fe and SiC are also investigated. The formation of Fe₃C after a long milling time is attributed to the kinetics of the reaction between nanocrystalline Fe and SiC.     

Syntheses and magnetic properties of nanocrystalline CuCr2Se4

Nanocrystalline powder of ferromagnetic spinel CuCr₂Se₄ is synthesized with an average particle size of about 20–50 nm. The nano-powder is characterized using transmission electron microscopy, X-ray diffraction and magnetic measurements. The Curie temperature of the nanocrystals exceeds the value obtained for usual bulk polycrystalline spinel sample. Exposure of the samples by laser radiation at wavelength of 1.5 μm under an external field leads to an increase in magnetization, that is caused by photo and thermo stimulated remagnetization of monodomain nanocrystals.     

Thermoelectric properties of nanocrystalline solid solutions of bismuth and antimony chalcogenides

The samples structure of solid solutions of p- and n-type bismuth and antimony chalcogenides obtained in the form of rods 1.6 mm in diameter by hot extrusion have been studied. These rods are found to consists of nanocrystallites 8?C60 nm in size. In p-type samples, nanocrystallites aggregated into spherical crystallites 50?C100 nm in diameter, whereas nanocrystallites in n-type samples formed disoriented fibers with a transverse cross section of several ten nanometers to several micrometers and a length of a few micrometers to several dozen micrometers. The presence of nanocrystallites of this size in the sample structure reduces the thermal conductivity of bismuth and antimony chalcogenides by 10% but barely changes their resistivity and thermoelectric power. The thermoelectric efficiencies of the p- and n-type chalcogenides are 3.27 × 10^?3 and 2.78 × 10^?3 K^?1, respectively.     

Surface and bulk magnetic properties of amorphous and nanocrystalline Ni-substituted FINEMET samples

The effect of partial substitution of Fe by Ni on the bulk and surface magnetic properties in as-quenched and nanocrystalline Fe_73.5?xNi_xCu₁Nb₃Si_13.5 (x = 0, 8, 10 and 20) alloys has been investigated. A study of the variation of the surface magnetic properties with oxidation in these alloys is also presented. The surface hysteresis loops were obtained by the transverse magneto-optical Kerr effect. The experimental results show that the as-quenched and annealed samples with lower surface coercive field values are those with lower Fe/Si ratio in the sample surfaces. No significant changes between the values of surface H_c for the oxidized and non-oxidized annealed samples with the same Ni content have been observed.     

Micromechanical properties of amorphous,nanocrystalline and transition phase hot-wire thin-silicon MEMS

Thin film silicon MEMS-based electrostatic bilayer microresonators are fabricated on glass substrates by hot-wire chemical vapor deposition with the silicon structural layer spanning the amorphous to nanocrystalline transition. Five sets of bridge and cantilever microresonators are fabricated with hydrogen dilutions of 0%, 60%, 85%, 90% and 95%. The silicon structural layers for 0% and 60% dilution are amorphous, for 90% and 95%, nanocrystalline, and for 85% dilution, an intermediate structure. All processing steps were carried out at temperatures ⩽110 °C. Microresonators are electrostatically actuated and the resulting deflection is optically monitored. The crystallinity of the structural layer does not have an observable effect in the rigidity of the resonators. The quality factor shows a maximum at 85% H₂ dilution, corresponding to a material with a structure intermediate between amorphous and nanocrystalline. A sharp decrease in quality factors is observed for higher dilutions.     

Structural relaxation and optical properties in transparent nanocrystalline β-quartz glass-ceramic

We studied structural relaxation in a nanocrystalline β-quartz solid solution (s.s.) glass-ceramic through density measurement, and the effect of structural relaxation on its optical properties. The density of the glass-ceramic changed as structural relaxation proceeded in the glass phase. It was found that the glass-ceramic whose glass phase has a high fictive temperature shows a high optical transmittance because of less optical scattering. We also demonstrated that the refractive index difference between the crystal and glass phases can be calculated from the scattering coefficients of the specimens with different fictive temperatures. The calculated refractive index of the crystalline phase was in fair agreement with the indices of a high-quartz crystal.     

Engineering of the magnetic properties of Finemet based nanocrystalline glass-coated microwires

The magnetic properties of Finemet nanocrystallized ribbons have been extensively studied and well understood. However, results obtained on microwires of such compositions are ambiguous considering the form of the hysteresis loop after the crystallization thermal treatment. In this paper, we show results on the static characterization of annealed Finemet based microwires (Fe_73.2Si_15.8B_6.9Cu_1.2Nb_2.9) of different geometries and illustrate how the magnetic anisotropy of Finemet based nanocrystalline microwires depends on the annealing temperature and the geometric characteristics of the microwires.     

Development and properties of a glass made from MSWI bottom ash

A uniform shiny black-coloured glass was obtained using bottom ash produced by a Portuguese municipal solid waste incinerator (MSWI). The bottom ash was the single batch material used in the formation of the glass, which was obtained by vitrification of the solid waste at 1400 °C for 2 h. Under these conditions, a homogeneous melt with an appropriate viscosity to be shaped was obtained, indicating the suitability of this waste material to be employed in the development of vitreous products. The characterization of the resulting glass was performed in order to assess its structural, physical, mechanical, thermal and chemical features. The glass had a density of 2.69 g cm⁻³, a hardness of 5.5 GPa, a fracture strength of 75 MPa, a thermal expansion coefficient of 9.5 × 10⁻⁶ °C⁻¹ and it exhibited a very good chemical stability. In summary, the MSWI bottom ash glass has good mechanical and chemical properties and may, therefore, be used in several applications, particularly as a construction material.     

Structure of nanocrystalline ammonothermal GaN in dependence of temperature and type of acidic mineralizer

This study investigated ammonothermal synthesis of nanocrystalline gallium nitride (GaN) in supercritical ammonia with acidic mineralizers NH₄X (X=Cl, Br, I) at 400–500 °C. Results showed that three types of acidic mineralizers could effectively accelerate the formation of GaN. The mixed hexagonal/cubic phase fractions and lattice parameters of nanocrystalline GaN were calculated by the Rietveld refinement method. SEM showed an agglomerate of nanocrystalline GaN. A considerable amount of GaN was synthesized using NH₄Cl as the mineralizer, however, there was no yield using NH₄Br or NH₄I at 400 °C. For acidic mineralizers, both hexagonal structures (wurtzite) and cubic structures (zincblende) were obtained in ammonothermal synthesis by XRD and Raman measurement. GaN synthesized with NH₄Br and NH₄I showed mixed phases of hexagonal-GaN (h-GaN) and cubic-GaN (c-GaN) at 450–500 °C. In the case of NH₄Cl mineralizer, GaN only exhibited mixed phases of h-GaN and c-GaN at 500 °C, but pure h-GaN at 400–450 °C. Based on the results, NH₄Cl favored pure h-GaN, and NH₄Br and NH₄I favored c-GaN at 400–450 °C.     

Dynamic mechanical properties of metallic glasses

Dynamic mechanical properties of two metallic glasses, Fe₄₀Ni₄₀P₁₄B₆ and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆, have been studied at frequencies ranging from 0.1 to 3 kHz and at temperatures between ? 160 and 390°C. Each of the samples exhibits an internal friction maximum at about ?20°C with activation energies of 25 and 34 kcal/mol. A possible mechanism for the low-temperature internal friction peak is suggested.     

Enhancing mechanical properties of silica aerogels

Low-density monolithic silica and hexylene-bridged polysilsesquioxane aerogels were chemical vapor deposition (CVD) treated with hexamethyldisilazane or hexachlorodisilane silylating agents producing TMS (trimethylsilane) or Si layers on the aerogel. Reinforcing the weak aerogels by controlled deposition process improved their compressive strength and preserved their properties characteristic of low-density aerogels. When the silica and hexylene-bridged polysilsesquioxane aerogels were CVD treated with hexamethyldisilazane, the compressive modulus more than doubled in some cases. However, when treating hexylene-bridged aerogels with hexachlorodisilane the compressive modulus increased six fold. Not only did CVD treatment of the aerogels improve the compressive modulus, but the low densities, high surface areas, high porosities, and the transparency of the aerogels were not significantly affected.     

Quasicrystals: Structure and properties

The structure and properties of quasicrystals are discussed. The short-and long-range atomic orders and the effect of these factors on the physical characteristics are considered. It is noted that investigations of the physical properties of quasicrystals at temperatures above room temperature should be performed. Promising applications are briefly outlined.     

Structure and properties of ANOF Layers

Oxide layers produced by ANOF technique show other structures and properties than such ones anodized as usual. Some characteristic mechanical, electrical and chemical properties are described. Their dependence on structure is discussed.     

Structure and properties of N-phenylmaleimide derivatives

The crystal structures of 2-chloro-N-phenyl-triphenylphosphoranaminomaleimide 1, N-phenyl-di(triphenylphosphoranamino)maleimide 2a, a nitromethane solvate 2b, N-phenyl-2,3-dithiomaleimide 3, and N-phenyl-2,3-di(thiophenyl)maleimide have been determined. 1 crystallizes in space group P-1 with cell dimensions a = 10.432(6), b = 14.661(5), c = 9.376(4) Å, = 93.13(4), = 92.09(5), and = 79.08(4)°, 2a crystallizes in space group P2₁/c with cell dimensions a = 11.272(2), b = 28.910(7), c = 12.702(2) Å, and = 115.31(2)°, 2b crystallizes in space group P-1 with cell dimensions a = 13.140(2), b = 13.796(3), c = 11.755(3) Å, = 99.62(2), = 100.23(2), and = 102.74(2)°, 3 crystallizes in space group C2/c with cell dimensions a = 42.926(5), b = 5.757(6), c = 8.259(3) Å, and = 99.71(3)°, and 4 crystallizes in space group C2/c with cell dimensions a = 20.055(4), b = 10.370(6), c = 18.690(7) Å, and = 100.32(2)°. Vicinal diazides of five-membered rings undergo the normal Staudinger reaction to form triphenylphosphoranamino derivatives. Compounds 1 and 2 fluoresce strongly, but nitromethane quenches the fluorescence of 2.     

Doping-induced formation of bulk nanocrystalline alloy from metallic glass with controllable microstructure and properties

We report the formation of bulk nanocrystalline alloys from a Pr-based bulk metallic glass-forming alloy by doping iron. The microstructure of the alloys can be tuned progressively from full glassy state to composite with nanocrystalline particles in the glassy matrix, and finally into nanostructured state accompanying with the gradual magnetic and mechanical changes. The role of the addition in the control of microstructure and magnetic property, the mechanism for the nanocrystalline formation induced by addition, and the relation between the microstructure and properties are discussed.