Microstructure and mechanical properties of Al–20Si–5Fe–2X (X = Cu, Ni, Cr) alloys produced by melt-spinning

Al–20Si–5Fe–2X (X = Cu, Ni and Cr) ribbons were produced by melt-spinning and consolidated by hot pressing at 400 °C for 60 min. The microstructure of the ribbons and the consolidated alloys was investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD) method, and transmission electron microscopy (TEM). The hardness and compressive strength of the specimens at ambient and elevated temperatures were examined. The microstructure of the ribbons exhibited featureless and dendritic zones. Results of XRD and TEM showed formation of spherically shaped Si particles with an average diameter of 20 nm. Ultrafine Si (110–150 nm) and iron-containing intermetallic particles were noticed in the microstructure of the consolidated ribbons. An improved strength was achieved by alloying of Al–20Si–5Fe with Cu, Ni, and Cr. Nickel was found to be the most effective element in increasing the maximum stress, particularly at elevated temperatures.     

Production of Al-(12–25) wt% Si alloys by rapid solidification: melt spinning versus centrifugal atomization

Al-Si-x(x=Cu, Mg, Ni, Co or Sr) alloys with Si content in the range 12–25 wt% were quenched from the liquid state using two methods: melt spinning and centrifugal atomization. The powders obtained were degassed followed by hot extrusion. Effects of chemical composition, quenching conditions, hot extrusion and heat treatment on the variation in the microstructure were examined. The present results show the necessary conditions for supersaturated solid solution, and those required for mechanism of solute trapping by moving the solid/liquid interface. Also, the mechanical properties of the products obtained were evaluated. It is observed that melt spun ribbons with Si concentrations of more than 12% possess high yield strength with low ductility. These materials undergo softening on ageing at temperatures above 150 °C. The properties of extruded alloy powders are markedly improved as compared to those made by ingot metallurgy. This effect is mainly due to the silicon particle refinement brought about by rapid solidification with cooling rates higher than 10⁵ Ks^–1.     

Ferromagnetic behavior of copper oxide-nanowire-covered carbon fibre synthesized by thermal oxidation

Copper oxide nanowires were synthesized on carbon fibre surfaces by annealing of copper thin films at 400 °C for 4 h in air. The nanowires were characterized with field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The magnetic properties of CuO nanowires were measured with a vibrating sample magnetometer (VSM). Its diameters range covered 40–100 nm and lengths range covered 1–3 μm. It is interesting that the CuO nanowires on the carbon fibres showed ferromagnetism at room temperature.     

Magnetic properties, phase evolution, and microstructure of melt spun Hf-substituted Sm(Co0.97Hf0.03)(x)Cy (x = 5-9; y = 0-0.1) nanocomposites

Magnetic properties, phase evolution, and microstructure of melt spun Hf-substituted Sm(Co0.97Hf0.03)(x)Cy (x = 5-9; y = 0-0.1) ribbons quenched at the wheel speed of 40 m/s are investigated. X-ray diffraction analysis shows that the main phases existed in Sm(Co0.97Hf0.03)(x) ribbons are 1:5 phase for x = 5-5.5; 1:5 and 1:7 phases for x = 6; 1:7 phase for x = 6.5-7.5; 1:7 and 2:17 phases for x = 8; and only 2:17 phase for x = 8.5-9, respectively. For Sm(Co0.97Hf0.03)(x) (x = 5-9) ribbons, the optimum magnetic properties of B(r) = 5.6 kG, (i)H(c)= 15.6 kOe and (BH)(max) = 7.1 MGOe are obtained for Sm(Co0.97Hf0.03)6.5 ribbons. Furthermore, a slight amount of C addition in Sm(Co0.97Hf0.03)(x) ribbons slightly modify phase constitution and effectively refine the grain size from 200-700 nm for C free ribbons to 10-70 nm, strengthening the exchange coupling effect between magnetic grains of the ribbons. As a result, magnetic properties are further improved. The magnetic properties of B(r) = 6.9 kG, (i)H(c) = 9.2 kOe and (BH)(max) = 10.0 MGOe can be achieved for Sm(Co0.97Hf0.03)7.5C0.1 nanocomposites.     

Rapidly solidified Si-Fe alloys

Si-Fe ribbons produced by the chill-block melt-spinning method, with silicon contents of 5.59 and 6.06 wt%, were annealed at temperatures between 1000 and 1215° C, in vacuum, 5.33 to 13.13 Pa. During annealing for 1 h abnormal grain growth was observed to occur, with modification of the as-cast texture. Two main texture components were identified: a(100)[0kl] texture as the main texture component at all temperatures; and the (421)[–5 84] or a combination of (421) [–584] and (321) [–563] as the second component. The growth of the two main components is affected by the annealing temperature and the annealing time.d.c. coercivity and core loss are significantly reduced by annealing with minimum values being obtained for a 6.06 wt% Si ribbon annealed at 1085° C for 1 h.     

Structures and Properties of Nanocrystalline Nd4.5Fe75Co1Si1B18.5Composite Magnets

1. IntroductionImprovement of hard magnetic properties, simplification of processing, and reduction of cost havebeen the principal motivating factors for much ofthe research on Nd-Fe-B permanent magnet materials since they were discovered. According to traditional Stoner-Wohlfarth model, the biggest Jr/Jsof hard magnetic materials with coarser microstructures is 0.sit]. When the grain size of Nd-Fe--B magnets is small enough, exchange coupled interactionwould be produced between soft and h…     

Dielectric properties of epoxy/silica composites used for microlectronic packaging, and their dependence on post-curing

We studied the dielectric properties (dielectric constant and loss factor) of epoxy molding compounds used for electronic packaging, as a function of frequency (100 Hz–100 kHz) and temperature (25–100 °C). Studies were performed for samples with different formulations (various silica and carbon black contents). At room temperature a loss peak is found at 50 kHz, whose intensity is enhanced by carbon black addition. Additional loss is detected below 1 kHz when the temperature is increased up to 100 °C. We also studied the influence of post-mold curing time (0–12 h at 165 °C) on the dielectric properties. The dielectric constant monotonically decreases with post-mold cure to level off to a minimum value for long post-cure durations. The loss factor first increases for short post-curing times, and then decreases as post-cure is continued. The origin of loss is discussed with reference to common relaxation processes observed in epoxy polymers.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

Magnetic and dielectric properties on sol–gel combustion synthesis of Pb(Zr0.52,Ti0.43X0.05)O3 (X = Fe,Ni, and Co) nanoparticles

Pure and X-doped (X = Fe, Ni, and Co) PZT nanoparticles (PZT-NPs) were synthesized by a sol–gel combustion method. The structural characterization of the obtained pure and doped PZT-NPs was carried out by X-ray diffraction (XRD). The crystallite size of the prepared samples was then evaluated by the size strain plot (SSP) method using the XRD results. The average particle sizes of 30–40 nm were measured for pure and doped PZT-NPs by a particle size analyzer. For dielectric measurements, the synthesized powders were pressed into pellets and then sintered at 1250 °C for 2 h. The magnetic properties of the synthesized doped PZT-NPs were studied by a vibration sample magnetometer (VSM) and ferromagnetic behavior was observed.     

Microstructures and properties of biomedical TiNbZrFe β-titanium alloy under aging conditions

A metastable β-titanium alloy Ti–28Nb–13Zr–0.5Fe (TNZF alloy for short) was designed for implant biomedical application. The forged specimens were solute-treated at 850 °C followed by water quenching and then aged at 350 °C, 450 °C, and 550 °C for 2–6 h in order to evaluate the effect of phase transformation during ageing on the biomechanical compatibility of the alloy. The quenched microstructure consists of lath α″ martensite and β phase. A large quantities of shuttle-like ω phase precipitate at 350 °C, leading to the drastic increase of strength and elastic modulus and the decrease of plasticity. Ageing at 450 °C for 4 h, small amount of elliptic ω phase and dot α phase precipitate from β matrix. With increasing ageing time α precipitations begin to coarsen and precipitation free zones (PFZs) form around prior β grain boundaries. Needle-like α phase precipitates on grain boundaries and intra-grains when aged at 550 °C. Both PFZs and grain boundary α precipitates are prone to bring about the intergranular fracture and thus have adverse effects on the tensile strength and fracture plasticity. The quenched microstructure has good combination properties of high strength, high plasticity and low elastic modulus.     

Effect of rapid solidification on the microstructure and mechanical properties of hot-pressed Al–20Si–5Fe alloys

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al–20Si–5Fe–2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 10⁵ K/s and 5 × 10⁷ K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated. Results showed a profound effect of the cooling rate, consolidation stage, and transition metals on the microstructure and mechanical strength of Al–20Si–5Fe alloys. While microstructural refining was obtained at both cooling rates, the microstructure of the atomized powder exhibited the formation of fine primary silicon (~ 1 μm), eutectic Al–Si phase with eutectic spacing of ~ 300 nm, and δ-iron intermetallic. Supersaturated Al matrix containing 5–7 at.% silicon and nanometric Si precipitates (20–40 nm) were determined in the microstructure of the melt-spun ribbons. The hot consolidation resulted in coarsening of Si particles in the atomized particles, and precipitation of Si and Fe-containing intermetallics from the supersaturated Al matrix in the ribbons. The consolidated ribbons exhibited higher mechanical strength compared to the atomized powders, particularly at elevated temperatures. The positive influence of the transition metals on the thermal stability of the Al–20Si–5Fe alloy was noticed, particularly in the Ni-containing alloy.     

Sol–gel derived PZT films doped with vanadium pentoxide

The present research investigated the sol–gel preparation, dielectric and ferroelectric properties of PZT films doped with 5 mol% vanadium oxide. Stable PZTV sols can be readily formed, and homogeneous, micrometer thick and pinhole-free PZTV films were obtained by using spin coating followed with rapid annealing. The X-ray diffraction patterns revealed that no parasitic or secondary phases were formed in the sol–gel PZT films with the addition of vanadium oxide. The material doped with vanadium pentoxide showed enhanced dielectric constant and remanent polarization with reduced loss tangent and coercive field.     

Poly(3-nonylthiophene-co-methylthiophene)s: New soluble conductive copolymers

New conductive soluble copolymers of 3-nonylthiophene (3NT) and 3-methylthiophene (3MT) were chemically synthetized using FeCl₃ in chloroform solution as a catalyst at room temperature and a N₂ atmosphere. The structural properties of the undoped and iodine doped 3NT-co-3MT have been studied by UV-Vis, FTIR, 1H- and ¹³C-NMR, GPC, DSC, TGA, WAXD, magnetic susceptibility and charge transfer measurements. The results show that copolymers (3NT-co-3MT) have a random arrangement. These copolymers have good thermal stability dependent on the 3NT. 3MT content and low magnetic susceptibility (typical for compounds of this class) which decreases with increasing temperature. The conductivity of the iodine doped copolymer (3NT-co-3MT) (measured in the dark at room temperature) increases distinctly in comparison to the undoped samples (2–8×10^–9 Sm^–1).     

AI-12.5 wt % Si ribbons prepared by melt spinning

Aluminium-silicon alloy with composition 12.5 wt% Si was rapidly quenched from the melt at a cooling rate of about 10⁶ Ks^–1 using the melt-spinning technique. The resulting ribbons were investigated by resistivity measurements and X-ray diffraction. The resistivity of the 20 m thick ribbons was 26.6 cm. This is 4.4 times its bulk resistivity value. The activation energy calculated from isothermal ageing was 1.14 ± 0.1 eV. The limit of primary solid solubility is extended almost to the eutectic composition and the large supersaturation is relieved by thermal annealing.     

Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3 and H2S. Part I: Preparation and microstructural characterization of the tungsten-oxide thin films

Tungsten-oxide thin films are promising materials for use in highly effective gas-sensing devices for NO₂, ozone and H₂S detection in ambient air. In this work tungsten-oxide thin films were obtained by electron-beam deposition and annealed in the temperature range 350–800 °C for 1–3 h. The structure, morphology and phase composition of the as-deposited and annealed films were characterized by X-ray diffraction, SEM and AFM. The changes of phase composition and the microstructure in dependence of the annealing conditions are described in detail. The direction of the phase transformations for different annealing conditions is influenced by the very high macrostresses that appear as an additional, independent thermodynamic factor. During annealing at 350–400 °C for 1–3 h and at 800 °C for 1 h predominantly semiconductor phases are formed, whereas the thin films annealed at 500–600 °C for 1–3 h and 800 °C for 2 h consist mainly of phases with more pronounced metallic properties. The processes of realignment of crystal structures during solid-phase transformation lead not only to the growth of new crystallites with a preferential orientation but also to a change in the direction of preferred growth with increasing annealing temperature and time. The films can be divided into two main groups: compact (as-deposited and annealed at 350–500 °C for 1–3 h) and porous (annealed at 600–800 °C for 1–3 h) layers. The gas-sensing properties of these films and the correlation between microstructure and sensing properties will be described in the second part of this paper.     

Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites

Long, square cross-section samples of a unidirectional and a cross-ply [0/90]3s silicon carbide (Nicalon) fibre calcium aluminosilicate glass-ceramic matrix composite have been subjected to a range of thermal treatments. They were held at temperatures up to 800 °C above room temperature for 1, 6 or 24 h then slowly cooled or quenched into water. The thermal cycle was repeated up to six times for a small number of samples. The effects of these thermal regimes on Young's modulus, onset of matrix cracking (as assessed by onset of non-linearity in the load–displacement curve) and flexure strength have been monitored using three-point flexure testing. In very broad terms, where clear trends emerged, the intermediate temperature differentials, i.e. 400–650 °C, have been found to have the most detrimental effects on properties, and this has been linked to expected changes in the carboneous interphase and its subsequent replacement by silica. © 1998 Kluwer Academic Publishers     

Rapid solidification and magnetism characteristics of Co-11.8 at%Gd and Co-7.5 at%Gd dual-magnetic ribbons at different wheel speeds

The rapidly solidified ribbons of Co-7.5 and 11.8 at%Gd alloys were obtained by the melt spinning technique. The magnetic properties were measured and the effects of the wheel speed and the heat treatment on the coercivity were examined. SEM and EDS results show that the microstructures of melt-spun ribbons vary at different wheel speeds and after the heat treatment (1073 K, 30 min), and the soft magnetic properties of annealed ribbons are better than those of quenched ones. For Co-11.8 at%Gd ribbons, the maximum saturation magnetization reaches to 65.11 emu/g at the wheel speed of 20 m/s, while the minimum value of coercivity is 75.32 Oe. For Co-7.5 at%Gd ribbons, the minimum value of coercivity is 43 Oe in the annealed ribbons at 30 m/s and the maximum value of saturation magnetization is 106.93 emu/g from the annealed ribbons at 40 m/s. Theoretical analysis indicates that the exchange coupling length of (Co) and Co₁₇Gd₂ phases is 65.3 nm, and the exchange coupling coefficient of Co-7.5 and 11.8 at%Gd ribbons is in the range of 0.023–0.089. The exchange coupling effect in Co-11.8 at%Gd ribbons is stronger than that in Co-7.5 at%Gd ribbons.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

The magnetic properties of Fe-based amorphous ribbons coated with various oxides using the sol–gel process

The metal oxides MgO, BaO, Al2O3, SiO2, MgO–Al2O3, CaO–Al2O3, SrO–Al2O3, BaO–Al2O3 and MgO–SiO2 have been coated onto ribbons of the Fe-based amorphous alloy Metglas 2605S3A using a sol–gel process. The effects of the surface coating on the magnetic properties of the alloy are investigated. The d.c. hysteresis loop for ribbons coated with MgO and MgO–Al2O3 is more square shaped than that for the uncoated ribbon. For ribbons coated with BaO, SrO–Al2O3 and BaO–Al2O3 it is more inclined than for the uncoated ribbon. Significant differences in the frequency dependence of the effective permeability are observed that depend on the nature of the coated oxides. The core loss is also affected by the coating. These results may be explained in terms of a stress induced by the coating and the modification of the domain structure via elastic and/or magnetoelastic interactions. It is thought from the magnetoelastic interactions that the MgO and MgO–Al2O3 coatings induce tensile stresses whilst those of BaO, SrO–Al2O3 and BaO–Al2O3 induce compressive stresses.     

Photoacoustic properties of softmagnetic amorphous Fe81B13Si4C2 ribbon

Photoacoustic properties of amorphous Fe81B13Si4C2 ribbons were investigated. The amplitude and phase photoacoustic spectra were measured as a function of the modulation frequency of an He–Ne laser beam. Thermal diffusivity was determined by comparison of obtained experimental results and calculated theoretical photoacoustic spectra for a non-annealed Fe81B13Si4C2 sample and an Fe81B13Si4C2 sample annealed at 600 °C. © 1998 Chapman & Hall