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.     

Study of the lattice strain relaxation in the Ga1-X Al X Sb/GaSb system by x-ray topography and high resolution diffraction

High resolution x-ray diffraction and topographic methods have been used to study lattice strain relaxation in the Ga_1-X Al _X Sb/GaSb system. Samples with layer thickness ranging between 0.1 and 6 m and with Al concentration x=0.402±0.005 have been grown by molecular beam epitaxy at 550 °C on (0 0 1) oriented undoped GaSb LEC substrates. A first critical thickness (t _C1), related to the misfit dislocation generation, has been found to be between 0.16 < t _C1 < 0.20 m. Due to the weak sensitivity of the rocking curve to the onset of relaxation, this result has been obtained by means of a double crystal topographic technique. A plateau region in the curve of the residual strain versus thickness has been observed for t ranging between 0.2 and 0.5 m. The residual strain _res shows a dependence close to t^–0.5 above a second critical thickness value t _C2 slightly larger than 0.5 m. Finally, in the last region above a layer thickness of 3 m, strong dislocation interaction effects seem to affect the relaxation. A comparison with theoretical models has been made.     

Substrate/film interactions in thick films of YBaCuO on alumina

The substrate-film interaction in thick films (>10m) of YBa₂Cu₃O_7–x on alumina processed under normal conditions is investigated using electron-probe microanalysis. The formation of a mixture of barium aluminate and alumina over a thickness of about 2m in the interfacial region is established quantitatively using compositional mapping and point-count analysis across the substrate-film interface. Diffusion of aluminium into the film over severalm beyond the reaction layer is also observed. The variation of oxygen composition across the interface has been mapped. Leaching of oxygen from the 1-2-3 phase in the bulk is suggested as the reason for the observed decrease inT _c(0) and increase in T _c in films of YBa₂Cu₃O_7–x on alumina.     

Photodoping and relaxation of persistent photoconductivity in YBa2Cu3Ox studied byin-situ transport measurements

We report on in-situ resistivity and Hall-effect studies of partially oxygen-depleted YBa₂Cu₃O_x thin films during white light illumination. The measurements were performed at various temperatures between the superconducting transition and room temperature. At all temperatures the resistivity and the Hall coefficient decreased as a function of the illumination time. The Hall number p_H and the Hall mobility _H, calculated within a simple one-band model, showed an increase of p_H during the illumination without a saturation. At low temperatures, _H behaved similarly to p_H. At 200 K, however, _H became constant after several hours of illumination, while at 260 K and 290 K, we observed a short increase of _H followed by a long-term decrease. From the different time dependences of _H and p_H, we conclude that two mechanisms contribute to the photodoping effect. Additionally, we observed that after termination of the illumination at 290 K the photodoped values of p_H and _H relaxed with substantially different time constants.     

Microporosity of Dense Anodic Al2O3 Films

Dense Al₂O₃ films 2.5 and 5 m thick produced by high-voltage and two-step anodization were studied by electron microscopy and small-angle x-ray scattering. The volume microporosity of the films and the mean radius of inertia of pores were evaluated. The results indicate that the densification of porous films in the second step of anodization is a result of two concurrent processes: dense oxide growth and dissolution of the primary porous Al₂O₃ layer.     

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.     

Measurement of the Thermal Diffusivity of Metallic Foils and Films by the Photoacoustic Method1

The thermal diffusivities of four kinds of metallic foils from 20 to 200m in thickness were measured by a photoacoustic method on the basis of the Rosencwaig and Gersho theory. The measured data for continuous foils of uniform microscopic structure almost agreed with the literature values. Measurements were also carried out on two kinds of metallic thin films with of 10m thickness produced by sputtering. The difference in thermal diffusivity between the foils and the sputtered films depended on the uniformity of the microscopic structure.     

Manufacturing and friction welding properties of particulate reinforced 7005 Al

This study has successfully incorporated Al₂O₃, SiC particulates into the 7005Al alloy matrix by using a drag-push method. The reinforced particulates are uniformly distributed in the matrix. This study also discusses the influence of aging treatment on the friction welding properties of 7005Al/10 wt%, 15 m and 6 m SiC_(p) composites and 7005Al/10 wt%, 15 m Al₂O_3(p) composites joint system. Experimental results show that after peak aging treatment was performed, if SiC particulates were used in the strengthening phase, the heat-affected zone (HAZ) had higher density of strengthening particulate, this resulted an increase in the hardness and stress concentration at the fully plasticized zone (Zpl) of the HAZ region, but a decrease in the width of the Zpl zone and the welding strength. And the welded fracture surface morphology had a low-ductile fracture.     

Cutting cheese with wire

Wire cutting involves fracture, plastic deformation and surface friction effects so that, in principle, it provides a method for determining parameters for all these properties. This paper describes an analysis in terms of the fracture toughness, G _c, the yield stress, _y, and the coefficient of friction, . By measuring the cutting force as a function of wire diameter, G _c and (1 + )_y can be found. These values are compared with direct measurements in notched bending for G _c, in simple compression for and _y, and in sliding tests for . A comparison of values obtained for a range of cheeses shows encouraging agreement.     

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al_xGa_1–xN (0.1 < x < 0.8) films, 0.4 m in thickness, were prepared on (0001) sapphire substrates at 500,C by alternate supply of Ga and Al alkyls and ammonia (NH₃). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 m in thickness, were also grown at 1000,C on the LT-Al_xGa_1–xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al_xGa_1–xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al_0.43Ga_0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al_xGa_1–xN buffer layer induced mosaic microstructure and bonding defects in the films.     

Structure and optical properties of vacuum-deposited scandium films on crystalline substrates

Thin films of scandium have been deposited in high vacuum on to amorphous and crystalline substrates at room temperature. These films were examined by X-ray diffraction and transmission electron microscopy to study the structure and orientation of films of various thicknesses. It was observed that thin scandium films evaporated on to air-cleaved mica and rocksalt substrates have a polycrystalline structure. The variation of the refractive indexn, the extinction coefficientk and optical conductivity ₁ () with wavelength were studied. Measurements were made in the spectral region from 2.5 to 40m.     

The Magnetic Properties of Sputtered Pd1−xMnx Films for Thermometry and Bolometry

The magnetic susceptibility of thin films and microstructures consisting of a Pd_1–xMn_x alloy have been measured as a function of temperature and magnetic field. Sputtering from a 0.68% manganese target produced films with a concentration of approximately 0.90%, as judged by comparison with results from bulk PdMn sensitivity measurements. The thinnest films (thickness 1.0m) show significant domain scale noise below the Curie Temperature, T _c , while thicker films (thickness 10m) show reliable non-hysteretic behavior throughout the temperature range of interest. The thin films show the effects of demagnetization with the field perpendicular to the surface, but a fine screen in this orientation shows no evidence of saturation and a predictable decrease in sensitivity due to demagnetization. These films will serve as the thermometric element in a new class of bolometers and thermometers for fundamental physics applications.     

Characterization of deformation processes in a Zn-22% Al alloy using atomic force microscopy

The Zn-22% Al eutectoid alloy was subjected to equal-channel angular pressing at a temperature of 473 K to give an as-pressed grain size of 1.3 m. Subsequent tensile testing of the as-pressed alloy at room temperature revealed a transition from deformation by a dislocation mechanism at the higher strain rates to superplastic flow at strain rates below 5 × 10^–3 s^–1: this corresponds to the transition from region III to region II in conventional superplasticity. Samples were pulled to relatively low total strains, of the order of 0.2–0.5, and the surface topography was then examined using an atomic force microscope (AFM). The AFM observations confirm the transition in deformation mechanisms with decreasing strain rate and they provide direct evidence for the occurrence of grain boundary sliding within the superplastic regime.     

The effect of sheet thickness on the magnetic properties of 77 wt% Ni permalloys

The effect of sheet thickness,t, on the magnetic properties of some Ni-Fe alloys of approximate composition 77% Ni-14% Fe-5% Cu-4 wt% Mo for various heat treatments in the temperature range 300 to 1250° C has been investigated. The range of sheet thickness used was 50 to 375m and was obtained by cold-rolling without interstage annealing. It was found that permeability increases with decreasing sheet thickness, attains a maximum att75m and starts to fall as the thickness is further reduced. The increase in permeability with decreasing sheet thickness (t>75m) is thought to be due to a decrease in eddy current losses, changes in texture formation and possibly the degree of short range order (SRO) developed in the material. The permeability obeys a 1/t ² relationship with sheet thickness in the range 75 to 375m. Below a sheet thickness of 75m permeability starts to decrease with decreasing sheet thickness. The reason for this fall in permeability is thought to be due to a thickness dependent magnetostatic energy contribution to the wall energy associated with free poles along the domain wall and also to the loss ratio,, which increases rapidly at small thicknesses. This rapid increase in is due to factors such as the ratio of domain wall spacing to sheet thickness, which varies with sheet thickness, the spin relaxation term, which has a 1/t ² dependence on sheet thickness, and the increase in the wall energy which increases the domain wall spacing. Finally a decrease in permeability could be caused by a different degree of SRO in the thinner specimens due to different cooling rates which could lead to an increase in first anisotropy constant,K ₁, and saturation magnetostriction, _s which then produces a decrease in _i att<75m when the degree of SRO in the specimens is not optimum.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Effect of Grain Size on the Modulus of Elasticity and Strength of Synthetic Graphites

The effect of the grain size of the filler on the mechanical properties (compressive, bending, and tensile strength and modulus of elasticity) of synthetic graphite is analyzed using data for commercial structural graphites. As the mean particle size of the filler (_av) decreases from 3000 to 1 m, the modulus of elasticity increases, on the average, from 10 to 15 GPa, and the compressive, bending, and tensile strength increases by about one order of magnitude. The Griffith equation is used to evaluate the size of defects that initiate fracture (c _c) in different types of graphites. It is shown that the factors determining the critical defect size depend on the particle size of the filler. For _av > 150 m, c _c is comparable to _av or _max. In the range 30 < _av < 150 m, c _c is equal to or greater than _max. In graphites with _av < 30 m, c _c far exceeds _max and, presumably, corresponds to the particle size of the molding powder.     

Synthesis of (Pb,La) (Zr,Ti)O3 films using a diol based sol–gel route

Morphotropic phase boundary compositions of lead zirconate titanate (PZT) modified with 2, 5 and 10 mol% lanthanum (PLZT) have been prepared using a diol based sol–gel route. Thin films of these PLZT compositions were fabricated on platinized silicon substrates by a spin coating technique. The effects of firing temperature and lanthanum modifications were investigated with regard to phase development, microstructure, and ferroelectric and dielectric characteristics. A strong 1 1 1 orientation developed as the amount of lanthanum doping increased. The results indicate that the values of remanent polarization, P _r, and dielectric constant, _r, decrease, relative to unmodified PZT, for films modified with 2 and 5 mol% lanthanum. The 5 mol% La films for example had a P _r of 14 C cm^–2 and an _r value of 700 compared to 31 C cm^–2 and 1480 for undoped PZT films. At these La concentrations there was also an improvement in the leakage current density by two orders of magnitude compared with unmodified PZT. The 10 mol% La sample did not exhibit any switchable polarization behaviour.     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Spontaneous nuclear ferromagnetic ordering of in nuclei in AuIn2 Part I. Nuclear specific heat and nuclear susceptibility

We have measured the nuclear specific heat C_n and nuclear susceptibility _n of In nuclei (I=9/2, =5.5 _n) in the cubic intermetallic compound AuIn₂ (Korringa constant =0.11 Ksec) in the normal conducting state at 30K10mK and 2mTB115 mT. Our data show a positive nuclear Weiss temperature =+ 43 K and that the In nuclei undergo a nuclear ferromagnetic transition at T_c=35 K. The In nuclei experience an internal field of about 10 mT (obtained from C_n at T>T_c ). The nuclear ordering temperature T_c and the internal field increase with applied magnetic field. From the data we deduce exchange constants for the investigated system. The critical entropy reduction S(T_c)/S_max=8.6% and critical enthalpy E=0.28 RT_c are in reasonable agreement with the measured ordering temperature T_c,applying the Heisenberg model for a simple cubic I=9/2 spin system. The nuclear spin relaxation time calculated from the real and imaginary parts of _n is 10 msec at T>50 K, but drops to <1msec at T_c.This is the first observation of a spontaneous nuclear magnetic ordering transition in a not-hyperfine-enhanced metal at thermal equilibrium, i.e. at T _nuclear =T _electron .     

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

The effect of particle size (0.78 4.4 m) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed.Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 m and 1.5 m powders and 0.33 for 0.78 m powder. For the 0.78 m powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 m and 129 kcal/mole for 0.78 m powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 m powder is 10¹ to 10² times larger than that of 4.4 m powder, and for 0.78 m powder the diffusion constant is estimated to be still higher.The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.