SAXS study on crystallization of an amorphous Pd76Au6Si18 alloy

The crystallization behaviour of an amorphous Pd₇₆Au₆Si₁₈ alloy has been investigated mainly by means of small angle X-ray scattering measurements. The amorphous alloy crystallized to form MS-I phase with composition Pd₇₈Au₂₂ in the amorphous matrix, which later crystallized as MS-II. The crystallization kinetics of MS-I phase were analysed in the framework of conventional nucleation theory. It was suggested that the considerable number of nuclei of MS-I phase had been already formed in the as-quenched specimen and the remaining nucleation attained within the short period of isothermal ageing. The remarkable increase of the scattering intensity corresponded to the growth of MS-I phase, the kinetics of which were found to be controlled by the diffusion mechanism. The interdiffusion constant was obtained fromD=6.0×10¹⁵ exp (–420 kJ mol^–1/RT) m² sec^–1, which agreed fairly with the reported values.     

Crystallization of amorphous Ni60Nb40

The crystallization of Ni₆₀Nb₄₀ metallic glass during continuous heating and isothermal annealing at temperatures from 845 to 904 K, was studied by differential scanning calorimetry (DSC), electrical resistance measurements, X-ray diffraction and transmission electron microscopy. Crystallization occurred in four clearly defined stages. In the initial stage a metastable phase, with structure similar to the M-phase in the Ni-Nb-Al ternary system, forms in the amorphous matrix. In the two subsequent stages the remaining glass crystallizes to the Ni₃Nb- and-phases found in the equilibrium Ni-Nb phase diagram. The M-phase transforms into the equilibrium Ni₃Nb- and-phases only at high temperature. The crystallization of the M-phase could be described by Johnson-Mehl-Avrami kinetics with the time exponentn=1.3 and activation energyE ₁=628 kJ mol^–1. M-phase crystal growth was apparently diffusion controlled and the diffusion coefficient was estimated to be 4.2×10^–20 m² sec^–1. Activation energies for the second and third stages of crystallization were found to beE ₂=446 kJ mol^–1 andE ₃ = 430 kJ mol^–1.     

Crystallization characteristics of an amorphous Nb81Si19 alloy under high pressure and formation of the A15 phase

Amorphous Nb-19 at% Si alloy, prepared by rapid quenching from the molten state, was annealed while being subjected to a pressure of 10 GPA. X-ray diffraction investigations on the alloy specimens quenched to ambient conditions have shown that pressure greatly alters the crystallization characteristics and the cubic A15 (Nb₃Si)-phase forms in preference to the tetragonal Nb₃Si-phase at temperatures in the range from 710° C to 800° C. Up to 680° C, the component atoms do not show any tendency towards ordering upon crystallization and the body-centred tetragonal solid solution forms; while, at 830° C, niobium atoms diffuse to form the body-centred cubic Nb precipitates. Superconducting properties have been measured for the single-phase A15 structure with the lattice parametera=0.5155 nm with the results that the transition temperature,T _C, is 3.4 K and the temperature coefficient of the upper critical field,H _C2, is 1.2 MA m^–1 K^–1 (15 kOe K^–1).     

Structural,electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2–xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu_2–xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10^–5 -m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of 10⁸ m^–1. The band gap for direct transition, E_g.dir varies in the range of 2.0–2.3 eV and that for indirect transition E_g.indir is in the range of 1.25–1.5 eV.     

Non-isothermal crystallization and isothermal transformation kinetics of the Ni68.5Cr14.5P17 metallic glass

Non-isothermal crystallization of the Ni_68.5Cr_14.5P₁₇ alloy is characterized by differential scanning calorimetry (DSC) and X-ray techniques. Transformation occurs in two stages at peak temperatures 622±1 and 692±1 K (at 20 K min^–1), with H ₁=1.50+0.1 kJ mol^–1 and H ₂=3.0±0.1 kJ mol^–1. Precipitation of a nickel phase occurs in the first stage and a (NiCr)₃ P phase is formed during the second stage. An approach to the isothermal kinetics of the two crystallization events is derived within the frame work of the Johnson-Mehl-Avrami theory.     

Film-thickness effects on the optical and electrical properties of Cu-GeO2 thin cermet films

The effect of film thickness on the optical and electrical properties of Cu-30 wt % GeO₂-70 wt % thin cermet films prepared by electron-beam deposition at about 10^–3 Pa and at a substrate temperature of 300 K is reported. The ultraviolet, visible and direct current (d.c.) conductivity results are analysed with the aim of determining the optical band gap,E _opt, the width of the band tails,E _e, and the d.c. thermal activation energy,E _a. It was found that the optical energy gap increases with increasing thickness and that the absorption was due to indirect transitions ink-space. The general feature of the absorption edge remains similar for both unannealed and annealed films, but annealing has the effect of decreasingE _opt. The d.c. conductivity results show thatE _a decreases with increasing thickness. From a knowledge ofE _opt andE _a, a probable model of the electronic band structure in Cu-GeO₂ thin films has been suggested.     

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part II Surface and internal crystallization processes investigated by differential thermal analysis (DTA)

Controlled crystallization of (TiO₂-ZrO₂)-rich calcium aluminosilicate glasses led to zirconolite in the bulk, and titanite and anorthite on the surface. Such glass-ceramics can be envisaged for minor actinides immobilization. In this study, the crystallization of three glass compositions with increasing TiO₂, ZrO₂ and CaO amounts was followed by differential thermal analysis (DTA). The effect of glass particle size and of heating rate on DTA curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E _c for the different crystalline phases. Exothermic effects associated with the crystallization of a phase having a defect-fluorite structure in the bulk and its consecutive transformation into zirconolite were only detected for the highly TiO₂, ZrO₂ and CaO enriched glasses due to their higher crystallization rate. Using an Avrami constant n = 3 and a dimensionality of crystal growth m = 3, the activation energy of defect-fluorite crystal growth was found to be E _c = 440 kJ · mol^–1 (modified Kissinger method). Titanite and anorthite grow only from glass surface with activation energies of respectively 493 and 405 kJ · mol^–1 (n = m = 1, Kissinger method). DTA study of melt crystallization during cooling showed that baddeleyite (ZrO₂) crystals firstly crystallize but become unstable versus zirconolite for higher undercooling.     

Crystallization kinetics of Ni-P glass — activation energies for nucleation and growth of nuclei

A pre-anneal treatment was introduced to a melt-spun Ni-P alloy. By crystallizing the asquenched and pre-annealed samples in a differential scanning calorimeter at different heating rates and isothermal process, the apparent activation energies for the total crystallization of the Ni-P samples were obtained by using the Kissinger technique and the Arrhenius relation. It was found that the apparent activation energy, E _c, decreased and tended to a certain value of 180 kJ mol^–1, with increase of pre-anneal time. According to the classical phase transformation theory, the activation energies for nucleation and growth of nuclei in the Ni-P glasses were deduced to be 324 and 180 kJ mol^–1, respectively, which are very close to the values previously reported by our laboratory. The weight parameters of nucleation, a, and growth, b, decrease monotonically and increase and tend to certain values with an increase of pre-anneal time, respectively.     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

Electrical resistivity,nucleation and crystal growth in amorphous Se100−x -Sb x ,

The process of nucleation and crystal growth has been studied, using the change of the electrical resistance with time, for the system Se_100–x -Sb _x , wherex = 5, 10 and 15. The X-ray analysis of different records was also used to investigate the same processes. The results show that the apparent activation energies of nucleation were 17.89, 19.88 and 24.25 kcal mol^–1 (74.90, 83.23 and 101.53 kJ mol^–1) for x = 5, 10 and 15, respectively. The values of the apparent activation energy of crystal growth were 39.78, 45.67 and 52.70 kcal mol^–1 (166.55, 191.21 and 220.64 kJ mol^–1 ) for the same respective samples. The activation energy of conduction and the electrical resistance were found to decrease as the antimony content increases in both amorphous and crystalline states.     

Formation and characteristics of lead lanthanum zirconium titanate thin films formed by using the r.f. sputtering method

PLZT (lead lanthanum zirconium titanate) thin films were prepared by using the r.f. magnetron sputtering method and post-annealing for crystallization at 650 C. The films which were annealed at 650 C for 10 min consisted of a metastable phase and a stable phase. However, another film which was annealed at 650 C for 20 min had only stable perfect perovskite phase. The stability of the post-annealed thin film and substrate interfaces was observed by using scanning electron microscopy. The longer the annealing time, the more unstable were the interfaces. By analysing the EDX data, the composition difference between the sputtering target and thin films, and the composition variation between as-deposited and post-annealed PLZT were studied. The films annealed at 650 C for 20 min showed good ferroelectric and electrical properties with a remanent polarization (P _r) of 11.5 C cm^–2, and a coercive field (E _c) of 164 kV cm^–1.     

Preparation of fatigue-free SrBi2Ta2O9 thin films by r.f. magnetron sputtering and their ferroelectric properties

Fatigue-free bismuth-layered SrBi₂Ta₂O₉ (SBT) films were deposited on Pt/Ti/SiO₂/Si substrates by r.f. magnetron sputtering at room temperature. The variation of structure and electrical properties were studied as a function of annealing temperatures from 750–850 °C. The films annealed at 800 °C had a composition ratio of Sr:Br:Ta = 0.7:2.0:2.0. X-ray photoelectron spectroscopy signals of bismuth show an oxygen-deficient state within the SBT films. The films annealed at 800 °C have a thickness of 200 nm and a relatively dense microstructure. The remanent polarization (2P _r), and the coercive field (2E _c), obtained for the SIBT films, were 9.1 C cm^–2 and 85 kV cm^–1 at an applied voltage of 3 V, respectively. The films showed fatigue-free characteristics up to 10¹⁰ cycles under 5 V bipolar square pulses. The leakage current density was about 7 × 10^–7 A cm^–2 at 150 kV cm^–1. The SBT films prepared by r.f. magnetron sputtering were attractive for application to non-volatile memories.     

Investigation of Microinhomogeneities in a-SiC r :H Films Using the Chemical Induction Model

Amorphous hydrogenated silicon–carbon (a-SiC _r :H) films grown by decomposing silane–methane mixtures in a low-frequency (55 kHz) glow discharge at different methane concentrations are studied by IR spectroscopy. The absorption band in the range 1850–2300 cm^–1 is decomposed into four Gaussian components, and the results are compared with calculations in the chemical induction model. It is found that the carbon atoms are nonuniformly distributed in the nearest neighbor environment of the SiH groups in the form of HSi–Si_{3 – n} C _n (n= 0–3) structures. The random bonding model is used to evaluate the probability of formation of each HSi–Si_{3 – n} C _n structure as a function of the C/Si ratio. Comparison with experimental data points to an inhomogeneous microstructure of the films.     

Synthesis of melt-quenched Bi1.7V0.3Sr2Ca2Cu3O10+y superconducting glass-ceramics

A glass in the BSCCO system with Bi₁₇V_0.3Sr₂Ca₂Cu₃O_10+y nominal composition was prepared by the melt-quenching (glass) method. The suitability of the glass ceramic method has been assessed in terms of physical and electrical properties. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization has been obtained. The result obtained indicated that substitution of vanadium for bismuth increased the activation energy compared to the unsubstituted BSCCO system but did not enhance the superconducting phase formation. The activation energy for crystallization of glass has been found, E _a=355 kJ mol^–1. The crystal structure was found to differ from that in the unsubstituted BSCCO system. Most importantly, the HT _c phase was formed by reaction between the constituent phases at lower temperatures and not directly from the glass material. The best electrical properties were obtained at T ₀=75 K and J _c=12 × 10³A cm^–2 at 4.2 K.     

Optical and electrical properties of carbon nitride films deposited by cathode electrodeposition

Carbon nitride (CN _x ) films were prepared on silicon (100) wafers and ITO conductive glasses by cathode electrodeposition, using dicyandiamide (C₂H₄N₄) in acetone as precursors. The composition ratios (N/C) were approximated to or larger than 1 from XPS. The optical properties and electrical resistivities of the films were investigated. Intense PL with two bands in the range 2.5–3.5 eV was observed on the CN _x films. The band gaps (E _opt) deduced from measurements of the optical absorption coefficients in the UV-VIS spectra were found to be in the range of 1.1–1.6 eV. From the PL and UV-VIS spectra, the nitrogen content has a large effect on the PL band gap and E _opt. The electrical resistivities of the films on Si wafers are in the 10⁹–10¹⁰ · cm range.     

Characterization of crystallization in Metglas 2826 MB alloy

The crystallization behaviour of the Metglas 2826 MB alloy (Fe₄₀Ni₃₈Mo₄B₁₈) has been studied using resistance measurements and X-ray diffraction techniques. Three annealing sequences were used to follow the process. Samples were annealed isothermally (a) at 780° C in a vacuum of 2×10^–5 torr for times in the range 1 sec to 4 h, (b) for 2 h in an argon atmosphere at temperatures where the resistance curve indicated phase changes to occur, and (c) for 300 h in 100 torr of helium at 400, 600, 700 and 850° C. From these annealing sequences it was found that the alloy did not crystallize below 410° C and followed a crystallization process of: amorphous Fe₄₀Ni₃₈Mo₄B₁₈ Fe _x Ni_23–x B₆ (cubic)+glassy matrix Fe _x Ni_23–x B₆+(Fe, Ni) (FCC) (Fe, Ni)₃B(bct). This series of transformations was followed for Sequences (a) and (c) above, but was slightly different for Sequence (b). An orthorhombic (Fe, Ni)₃ B phase was found in the samples annealed in a vacuum of 2×10^–5 torr.Trademark of Allied Chemical Co.     

Preparation and photovoltaic properties of anodically grown Ag2O films

The semiconducting and photovoltaic properties of p-type Ag₂O films grown anodically on silver electrodes were studied, in view of possible applications in solar energy conversion. Films were grown in different alkaline solutions; the best results were obtained for 0.02M Ag₂SO₄ + 0.17M NH₄OH + 5.7 × 10^–3M Ba(OH)₂ saturated with Ag₂O powder, stirred mechanically at room temperature. Film thicknesses of up to 10m were thus obtained for the first time in anodically grown Ag₂O. Photovoltaic spectra taken at 300 K give a bandgap ofEg = 1.42 ± 0.04 eV. Evaporated gold on Ag₂O appears to be ohmic while aluminium and platinum are rectifying. The barrier height of Ag/Ag₂O is 0.90 ± 0.02 eV, that of Al/Ag₂O is 0.93 ± 0.02 eV, and that of platinum 0.94 ± 0.02 eV. The best cells give an open-circuit voltage,V _oc, of over 150 mV, and a short circuit current,I _sc = 100A cm^–2 under 50 mW cm^–2 illumination.     

A study of the effects of phosphorus on the kinetics of recrystallization and mechanical properties of Ni2Cr alloy

The recrystallization process of 50% cold-rolled Ni₂Cr alloy, with phosphorus (P) contents of 0.002, 0.0017 and 0.078 wt% was investigated, in specimens annealed at temperatures from 600 to 100° C, for times ranging from 10 to 60 min, as functions of surface hardness and microstructure change. It was found that annealing at temperatures below 700° C for times less than 30 min resulted in an increase in hardness from about 430 to 50 KH, depending on the phosphorus content. This increase was thought to be due to interactions between dislocations and the strain fields of impurity atoms such as phosphorus. It was also found that phosphorus increases the activation energy of the recovery and recrystallization process, from about 23 to 27 kcal mol^–1 for low and medium phosphorus material to 43 kcal mol^–1 for high phosphorus material. The effects of thermomechanical treatment and order transformation on the mechanical properties of Ni₂Cr alloy with 0.002 and 0.078 wt% P were also studied. It was found that 50% cold-working increases the yield stress of 0.002 and 0.078 wt% P materials by 190% and 145%, respectively, compared to the solution-annealed materials. Similarly, an increase in yield stress of 106% and 85%, respectively, was observed for 0.002 and 0.078 wt% material, which was transformed to the long-range ordered condition.On leave of absence from Suez Canal University, Faculty of Petroleum and Mining Engineers, Suez, Egypt.     

Dissolution kinetics and diffusivity of silver in glassy layers for hybrid microelectronics

Silver and palladium/silver compositions are widely used in hybrid microelectronics, as electrodes for dielectric layers and multilayers, terminations of thick film resistors and interconnections. Interactions between Ag and the adjacent films are known to affect the microcircuit performances. The present study is aimed at collecting data on the behavior of Ag-based films in contact with glassy layers. Most experiments were performed with a glass with composition 68.2 PbO : 30.5 SiO₂ : 1.3 Al₂O₃ wt %. Two different systems were analyzed. The first system consists of thick films prepared from a paste containing glass and either 3 or 15 wt % silver particles; both fine (spherical grains, 0.5–1 m diameter) and coarse (flakes, 2–5 m, <1 m thick) Ag powders were used for these pastes. The distribution of Ag in the film was studied with X-ray diffraction, scanning electron microscopy and fluorescence analysis. The results show that Ag floats on the glassy layer. Diffraction of X-rays generated by a synchrotron radiation source allowed us to study the kinetics of silver dissolution in the glass; this phenomenon is consistent with the Avrami theory, with an apparent activation energy E _dis=0.69±0.04 eV. The second system analyzed, Ag-based terminations of glass layers fired at various peak temperatures, enabled us to obtain quantitative values for both Ag solid solubility (about 2.5 wt %) and Ag diffusion coefficients D _Ag(T ). Typical values of D _Ag(850 °C) are 30.3±11.9 10^–8 cm²/s; an apparent activation energy of the diffusion process is E _a=0.6±0.1 eV.     

Effects of temperature,thickness and atmosphere on mixing in Au-Ti bilayer thin films

The interdiffusion and intermetallic compound formation of Au-Ti bilayer thin films annealed at 125 to 350 °C have been investigated. The bilayer thin films were prepared through electron beam deposition at comparatively low temperature. The interdiffusion of annealed specimens was examined by measuring electrical resistance and the depth-composition profile, and by observation using a transmission electron microscope. Interdiffusion between the thin films was detected at temperature above 175°C in a vacuum of 10^–4 Pa. The starting temperature at which interdiffusion occurred decreases with lowering annealing vacuum. The intermetallic compounds AuTi, Au₄Ti, Au₂Ti and Ti₃Au form during annealing at over 250 °C. The activation energies of Au in Ti and Ti in Au obtained by the penetration depth are approximately 0.45 and 0.41 eV, respectively. These measurements indicate that the diffusion is controlled by a short-circuit mechanism. The diffusion of Ti species in Au depends on the annealing vacuum and Au thickness.