Superconducting state in M3C60: Strong vs. weak coupling

The superconducting state in the doped fullerenes is due to strong coupling (e.g.,2.1 for Rb₃C₆₀) to low-frequency intramolecular modes _L 250 cm^–1 (^21/2). The analysis is based on an equation describingT _c for any strength of the coupling and on recent isotope effect and NMR data.     

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl₃ Ni₂Al₃ NiAl NiAl (Al-rich) + Ni₃Al NiAl.     

Soft chemical synthesis and characterization of lithium nickel oxide electrode materials

Soft chemistry was used to prepare ordered nanophase Li_0.6NiO₂ electrode materials by completing the oxidation of Ni²⁺ to Ni³⁺ and/or Ni⁴⁺ species with H₂O₂ oxidant during solution reactions at 50–60 °C and evaporation at 105–150 °C rather than during sintering. Both elemental analysis and electron spectroscopy for chemical analysis (ESCA) results indicate that oxidation was completed. The deconvoluted ESCA spectra of nickel ions exhibited a semi-quantitative ratio of Ni⁴⁺Ni³⁺=6040 which presented no significant change with increase of sintering time. After sintering for up to 11 h at 700°C, ordered Li_0.61Ni_0.96O_2.0 ceramics were formed (R3m, a ₀=0.2837 nm, c ₀=1.417 nm). Distribution of the crystallite size was in the range of 80–200 nm. As sintering times were increased, the crystallite shapes exhibited a more distinct morphology, and the ordering degree of the cations was enhanced, while the conductivity was sharply enhanced up to 2.0×10^{–1 –1} cm^–1 at 30 °C. Compared to conventional ceramic and solution methods, the ordered nanophase Li_0.61 Ni_0.96O_2.0 ceramics was obtained at 700 °C with shorter sintering times ( 11 h).     

Microstructure and its correlation to magnetic properties in 2:17 type (Sm,Gd)-Co-Fe-Cu-Zr alloys

Studies on the evolution of microstructure and phase transformation at various stages of thermal processing have been carried out in 2:17 type (Sm,Gd)-Co-Fe-Cu-Zr alloys. Microstructural studies reveal that during the solution heat treatment stage, a lamellar structure is formed and its volume fraction is found to be more at 1463 K for alloys without Gd, while at 1473 K for alloys with Gd = 25 wt% of Sm. It appears that the formation of the lamellar structure is closely associated with the phase transformation from Th₂Ni₁₇ (TbCu₇ + Th₂Zn₁₇) structure during the solution heat treatment. Magnetic measurements in isothermally aged (1123 K) samples showed that the replacement of 25% of Sm by Gd reduces the energy product from 180 to 136 kJ/m³. The Gd substitution is also found to reduce the temperature coefficient of magnetisation from 350 to 225 ppm/°C in the temperature range of 30–100°C.     

Microstructure and thermal stability of melt-spun Al-Nd and Al-Ce alloy ribbons

Alloys of compositions Al-6Nd, Al-9Ce, Al-16Nd and Al-22Ce (wt%) were prepared by chill-block melt-spinning at a surface speed of 29 m s^–1 into ribbons of thickness between 20 and 70 m. The melt-spun ribbons were heat treated for 2 h at 200, 300, 400 and 500 °C. The as-spun and heat-treated ribbons were tested for microhardness and investigated by optical and transmission electron microscopy and X-ray diffraction. Age hardening was not observed for any of the alloys, indicating only a limited amount of extended solid solubility as a result of the melt-spinning conditions used. Al-9Ce and Al-6Nd were found to be relatively stable at a hardness of 90 HK0.01 and 75–80 HK0.01, respectively, unaffected by heat treatment at up to 300 °C for 2 h, but heat treatment for 2 h at 400 or 500 °C caused softening. Al-6Nd and Al-9Ce as-solidified ribbons comprised a cellular Zone B structure with Al cells (of size 0.5 m) and intercellular orthorhombic Al₁₁(Nd, Ce)₃. The hardness of Al-16Nd as-spun was measured as 330±20 HK0.01 unchanged by heat treatment at 200 °C for 2 h, while heat treatment for 2 h at higher temperatures caused softening. As-spun Al-16Nd consisted of Zone A only comprising Al solid solution and orthorhombic or tetragonal Al₁₁Nd₃ partly coherent with the matrix. Al-22Ce had a hardness of 330±20 HK0.01 as-spun but softened on treatment for 2 h at 200 °C. As-spun Al-22Ce consisted partly of Zone A structure extending from the wheel side of the ribbon through half of the thickness. The remaining part of the ribbon consisted of cellular Zone B (cell size 0.2 m) with intercellular Al/Al₁₁Ce₃ lamellar eutectic together with areas that were 100% lamellar eutectic (interlamellar spacing 0.09±0.02 m). All the as-spun microstructures showed coarsening upon heat treatment at sufficiently high temperature. The resulting microstructure after treatment at 500 °C for 2 h consisted of a bimodal distribution of Al₁₁ (Nd, Ce)₃ which formed as large particles on the grain boundaries and smaller ones in the grain interiors. The corresponding microstructure was less coarse for Al-16Nd and Al-22Ce.     

Stability of tetragonal ZrO2 phase in Al2O3 prepared from Zr-Al organometallic compounds

Zr-Al organometallic compounds have been spray-dried and heated at temperatures 600 to 1400°C to prepare ZrO₂-Al₂O₃ composite powders. The powders consist of balloon-like particles 0.5 to 2 m in diameter with homogeneously dispersed tetragonal ZrO₂ grains 0.1 to 0.2 m in diameter. The tetragonal fraction of ZrO₂ in the composite powders is higher than that in the powders prepared from sols of Zr(OBuⁿ)₄ and Al[OCH(CH₃)₂]₃. The fraction is affected by the organofunctional group in the Zr-Al compounds.Zr(OBuⁿ)₄ = Zr(OC₄Hgⁿ)₄; Al[OCH(CH₃)₂]₃ = Al(OPrⁱ)₃.     

Structural features of certain Ni-Al-Ta and Ni-Al-Hf alloys containing the γ′- and β -phases

An investigation has been made of the structure of certain alloys of the Ni-Al-Ta and Ni-Al-Hf systems. The compositions chosen for investigation were: Ni-28.75 at% Al-2.5 at% Ta; Ni-26.75 at% Al-2.5 at% Ta; Ni-20.0 at% Al-7.5 at% Ta; Ni-20.0 at% Al-7.5 at% Hf; Ni-22.5 at% Al-5.0 at% Hf. The Ni-Al-Ta alloys consisted of +-phases (or-phase transformed during rapid cooling to a martensitic product). Two types of morphology were observed in the transformed-phase, one consisting of parallel sided plates and the other of acicular shaped plates; the plates contained fine twins on {1 0 1} b c t planes. The as-cast Ni-Al-Nf alloys consisted of the -phase, together with eutectic regions of-phase and (Ni, Al)₇Hf₂; both lamellar and discontinuous eutectic morphologies were observed. Extensive solid solution of tantalum and hafnium in the -phase occurred.     

Structural,transport and infrared studies of oxidic spinels Zn1−xNixFeCrO4

X-ray, electrical conductivity and IR studies for the system Zn_1–x Ni _x FeCrO₄ were carried out. All the compounds, 0 x 1 showed cubic symmetry. X-ray intensity calculations and IR studies indicate the presence of Zn²⁺ at tetrahedral sites, Ni²⁺ and Cr³⁺ at octahedral sites, and Fe³⁺ ions are equally distributed at both the sites. Activation energy and thermoelectric coefficient decreases with the increasing values ofX. All the compounds exhibit P-type semiconductivity which may be due to transfer of Fe³⁺ ions from B-site to A-site creating holes. The electrical resistivity temperature behaviour obeys the relation= ₀ exp (E/KT). The mobility of holes as calculated from IR and conductivity data is of the order of 10^–9 cm² V sec^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ³⁺ [Fe _1–x ³⁺ Ni _x ²⁺ Cr³⁺] O ₄ ^2– .     

New high-tin phase found in electrolytic Sn–Ni deposits

Using X-ray diffraction technique and transmission electron microscopy it has been demonstrated that multiphase Sn–Ni electrodeposits with 3–34 at.% Ni contain a new phase of composition close to NiSn₉. Its lattice parameters are specified as follows: a ₀ = 1.2621(7), b ₀ = 0.5231(4), c ₀ = 0.6213(4) (in nm), = 105.8(7)° of the C 2/m space group. This new phase decomposes at 100°C into tin and Ni₃Sn₄.     

Influence of additions on the solidification behaviour of Ni-B alloys — crystallography of Ni-Ni3B eutectic

The influence of small additions (1 to 2 at.%) of some elements (chromium, copper, silicon, aluminium, iron, titanium, vanadium) on the solidification behaviour and crystallography of Ni()-Ni₃B eutectic composition is examined by differential thermal analysis, X-ray and electron diffractions, scanning and transmission electron microscopy. With the exception of Fe-doped eutectic alloys, all the alloys whether doped or undoped exhibit a very large undercooling for the nucleation of Ni₃B. We give an interpretation of the controversial hypothesis of this undercooling. We propose a crystallographic orientation relationship between the cubic phase Ni() and the orthorhombic phase Ni₃B in the lamellar eutectic Ni-Ni₃B. We also examine a complex transformation which occurs in Ni₃B during slow cooling.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

Oxidation behavior of FeAl alloys with and without titanium

The influence of Ti addition on the high temperature oxidation behavior of FeAl intermetallic alloys in air at 1000°C and 1100°C has been investigated. The oxidation kinetics of FeAl alloys was examined by the weight gain method and oxide products were examined by XRD, SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5Al alloys could be described as different parabolas that followed the formula: (W/S)² = K _p t + C. The parabolic rate constant, K _p values are approximately 2.4 and 3.3 mg² cm^–4 h^–1 for Fe-36.5Al alloy and about 1.3 and 2.0 mg² cm^–4 h^–1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only -Al₂O₃ oxide for Fe-36.5Al alloy while there are -Al₂O₃ and TiO oxides for Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were addressed based on the microstructural evidence.     

Phase equilibria in the Ti-rich corner of the Ti-Si-Sn-Al system Part I Section 9Si-1Sn (at.%)

Phase equilibria in Ti-rich corner of the Ti-Si-Sn-Al system were studied using differential thermal analysis, X-ray diffraction, microscopy, and electron microprobe analysis. Projections of solidus and liquidus surfaces, an isothermal section at 1300°C and an isopleth at 9Si-1Sn (at.%) were constructed. It was shown that in the concentration interval studied at the solidus and 1300°C temperatures two two-phase Ti₅(Si,Sn,Al)₃ + <-Ti< and Ti₅(Si,Sn,Al)₃ + <-Ti< regions are present. The liquidus surface is characterised by the regions of and Z primary crystallisation, resulting in bivariant L + Ti₅(Si,Sn,Al)₃ eutectic. The character of horizontal and vertical sections is similar to that for the Ti-Si-Al system.     

Mechanical properties of precipitation-strengthened Ni-Al-Cr alloy based on an NiAl intermetallic compound

Mechanical properties of a ternary alloy Ni-30.3 at. % Al-6.6 at. % Cr have been studied in the temperature range 25 to 1100° C. The material was heat-treated to produce a stable dispersion of incoherent rod-shaped Ni₃Al precipitates, 1m in diameter and 20m long. The tensile properties were found to be temperature dependent. Below 750° C the material had high strength, low ductility and low strain-rate sensitivity, whilst above 750° C the strength fell, ductility increased and the material became strain-rate sensitive. The room temperature fracture toughness of the single-phase material was 6 MN m^–3/2 and increased to 50 MN m^–3/2 in the two-phase material. This can be attributed to the effect of Ni₃Al on crack nucleation and propagation.     

Effect of Nb addition on the glass forming ability and mechanical properties in the Ni-Zr-Al-Hf-Nb alloys

The effect of substituting Nb for Zr on the glass forming ability (GFA) and mechanical properties of Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys was investigated. The substitution of Zr with Nb of 0 -- 14 at% improved the GFA. When increasing the Nb content x from 0 -- 14, the glass transition temperature, T_g, and the crystallization temperature, , of melt-spun Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys increased from 833 and 863 K to 877 and 914 K, respectively. The Ni₆₀ Al₆Hf₇ (x = 8, 10, 12 and 14) alloys exhibited high (>0.615), (>0.4) values and a wide supercooled liquid region, (= – T_g) (>36 K) enabling the fabrication of bulk metallic glass (BMG) with a diameter above 1 mm. As such, Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ and Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloys with a maximum diameter of 2 mm could be fabricated by injection casting. These bulk amorphous alloys also exhibited good mechanical properties, where the true ultimate compressive strength, strain and Vickers hardness (H_v) were approximately 3.0 GPa, 2.08% and 713, respectively, for the amorphous Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ alloy, and 3.1 GPa, 2.22% and 687, respectively, for the amorphous Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloy.     

Invariant three- and four-phase equilibria in the magnesium-rich corner of the Mg-Cu-Sn ternary system

Invariant three- and four-phase equilibria in the magnesium-rich corner of the Mg-Cu-Sn ternary system have been studied by differential thermal analysis, optical microscopy, electron probe microanalysis and X-ray diffraction. The L(Mg) + Mg₂ Cu + Mg₂Sn ternary eutectic reaction was found to be at 467 °C and at Mg–13.5 at % Cu–4.4 at% Sn. The LMg₂Cu + Mg₂Sn pseudo-binary eutectic reaction is tentatively located at 522°C and at Mg–26.0 at % Cu–7.7 at % Sn.     

Steady-state creep of an alloy based on the intermetallic compound Ni3Al(γ′)

An investigation of the steady-state creep of a Ni₃Al.10 at% Fe alloy () has shown that two creep mechanisms were operative over the temperature range 530 to 930° C. The experimental data at low temperatures (below 680° C) were not consistent with any of the established creep theories. However, the experimental data were in good agreement with a proposed model for cross-slip from octahedral {111} planes on to cube {100} planes in Li₂ crystals. Above 680° C, the rate-controlling mechanism, which had an activation energy of 3.27eV atom^–1, is considered to be the removal/production of APBs during climb.     

Nickel aluminide (Ni3Al) fabricated by reactive infiltration

Nickel aluminide Ni₃Al in the single phase form, with grain size 10 m, porosity 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800 °C of liquid aluminium into a porous preform containing 78 vol % nickel and made by sintering 3–7 m size nickel particles. Without sintering, the preform contained 58 vol % nickel and reactive infiltration resulted in an aluminium-matrix NiAl₃ particle ( 50 m size) composite and extensive growth of Ni-Al needles from the preform to the excess liquid aluminium around the preform.     

Controlled crystallization and characterization of cordierite glass-ceramics for magnetic memory disk substrate

Glass-ceramics containing cordierite (2MgO-2Al₂O₃-5SiO₂) as a crystal phase based on the glasses in the system MgO-Al₂O₃-SiO₂ were investigated for the application to magnetic disk substrate for higher storage capacity. Parent glasses were prepared with CeO₂ addition as a flux and were crystallized by a controlled 2-step heat treatment The maximum nucleation and crystal growth rates were 2.4 × 10⁹/mm³ · hr at 800 °C and 0.3 m/hr at 915°C respectively. Only - cordierite was precipitated after heat-treatment. After nucleation at 800 °C for 5 hours prior to crystallization at 915 °C for 1 hour, the resulting crystal volume fraction and crystal size were 17.6% and 0.3 m, respectively. Heat-treated specimens through the above condition showed the optimum properties for magnetic memory disk substrates as follows.; Bending strength of 192 MPa, Vickers hardness of 642.1 kg/mm², thermal expansion coefficient of 39 × 10^–7/K and surface roughness of 27 Å.     

Synthesis,characterization and electrical conductivity properties of homo- and hetero-di and trimetallic complexes of mixed azo dyes

The synthesis of seven new complexes of mixed ligands and metals are reported. Their stoichiometries (metals:ligands) were found to be 13, 22 and 32 corresponding to FeL₁2L₂H, M₂L₁L₂ and MM₂L₁L₂, respectively (where M = Cu²⁺, M = Cu²⁺, Co²⁺ or Ni²⁺; HL₁ = Methyl Red; H₂L₂ = Alizarin Yellow R). The complexes were characterized by elemental analysis, infrared and Nujol mull electronic spectra and magnetic moment measurements. The results of the electrical conductivity studies for the prepared complexes in the solid state (using compressed tablets) indicated reasonable semiconducting properties. The magnitude of these values, log₂₉₁ lay between –5.46 and –8.67, resulting from the structure and nature of the substituent of the two mixed ligands as well as the high metallic content of their complexes.