Phase transitions in wetting films at the surface of Ga–Pb alloys

Surface phase transitions at Ga-rich liquid surfaces have been investigated in Ga–Pb alloys with low lead content. In the region of the liquid–liquid miscibility gap, the Pb-rich liquid phase completely wets the surface of the Ga-rich phase at coexistence. Observations have been made of demixing and solidification of the Pb-rich liquid film. Ga-rich alloys, which are single-phase below the monotectic temperature, can be undercooled below the liquidus, as far as the metastable binodal line where the Pb-rich wetting liquid film forms and solidifies into thin {111} Pb crystals. These films completely redissolve upon reheating to the liquidus temperature. Freezing occurs at surfaces because of complete wetting of the liquid rich in the high melting point component and the hysteretic character of the solidification transformation. Such “surface” experiments allow assessment of the stable and metastable liquidus lines of the Ga–Pb phase diagram in the vicinity of the monotectic temperature.     

Equilibrium segregation of Ti to Au–sapphire interfaces

Equilibrium segregation of Ti to Au–sapphire interfaces was measured from dewetted Au(Ti) films on the (0001) surface of sapphire. Quantitative energy dispersive spectroscopy was used to determine a Ti excess at the Au–sapphire interface of 2.2 Ti atoms/nm², which together with an excess of 4.6 Ti atoms/nm² at the (0001) sapphire surface, is associated with a decrease in the solid–solid Au–sapphire interface energy. Quantitative high resolution transmission electron microscopy showed that the segregated Ti is distributed within a 1.54-nm thick intergranular film at the Au–sapphire interface, which is not a bulk phase but rather an equilibrium interface state. As a result, Ti segregation without the formation of a bulk reaction at the interface is associated with a decreased interface energy, improved wetting, and may be an important part of the total complex mechanism responsible for improved wetting and spreading in “reactive” braze systems.     

Solid-state wetting transitions at grain boundaries

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point (T_PWC), and higher order at temperatures above T_PWC. Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.     

Wetting transition of grain boundaries in the Sn-rich part of the Sn–Bi phase diagram

The microstructural evolution of tin-rich Sn–Bi alloys after the grain boundary wetting phase transition in the (liquid + β-Sn) two-phase region of the Sn–Bi phase diagram was investigated. Three Sn–Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 °C for 24 h. The micrographs of Sn–Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures.     

A criterion for optimum adhesion applied to fibre reinforced composites

The effects of physical adhesion on the mechanical properties of a composite structure are examined in this work. A criterion for optimum adhesion between matrix and reinforcing fibres is proposed based on maximizing the wetting tension. It is shown that the maximum wetting tension criterion best fulfils two important requirements for a strong interface:(i) the physical interactions at the molecular level between the resin and the fibres must be maximized, and (ii) the liquid resin must spontaneously wet the fibre surface in order to minimize the flow density at the interface. The conditions on the surface energy of the various phases leading to maximum wetting tension are analysed considering three mixing rules: two based on dispersive–polar interactions, and a third one based on acid–base interactions. The optimum adherend for a given adhesive, and the optimum adhesive for a given adherend, are examined. The analysis shows that maximum wetting tension is obtained when the substrate and adhesive surface energies are very high and equal, so that their polar and dispersive components are equal when the polar–dispersive mixing rule is used, and e.g. their Lifshitz–van der Waals’ components are equal and the acid component of one phase is equal to the basic component of the other phase when the acid–base approach is considered. It is shown using data from the literature that interfacial strength correlates with the wetting tension for fibre reinforced composites. Additional observations show that under poor wetting conditions the voids tend to concentrate at the fibre–resin interface, whereas under favourable wetting conditions they tend to coalesce in regions away from the fibre surface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Novel organic monotectic alloy and its thermal,physicochemical, and microstructural studies

The phase diagram study of an organic analogue of a nonmetal–nonmetal system involving 4-bromochlorobenzene and resorcinol shows the formation of a eutectic and a monotectic. The phase equilibrium shows the large miscibility gap region with the consolute temperature 143.0 °C. Growth kinetics of the eutectic, the monotectic and the pure components studied at different undercooling, suggests the applicability of Hillig–Turnbull’s equation. For binary materials and parent compounds, the heat of mixing, entropy of fusion, roughness parameter, interfacial energy and excess thermodynamic functions were calculated from the enthalpy of fusion values determined by the DSC method. The solid–liquid interfacial energy shows the applicability of Cahn’s wetting condition. The effects of solid–liquid interfacial energy on solidification behaviour of monotectic alloy have also been discussed. The microstructures of monotectic and eutectic show the uniform array of droplets and broken lamella, respectively.     

Effect of Nb?+?Ti coating on the wetting behavior, interfacial microstructure, and mechanical properties of Al/Al2O3 joints

The subject of the work was to study the effect of Nb + Ti thin film deposited by PVD method on alumina substrates on the wetting behavior, bond strength properties, and structure of interface in the Al/Al₂O₃ joints. Applying the sessile drop method, the wetting behavior of molten Al (99,999%) on coated alumina substrates was studied in the temperature range between 953 and 1373 K under a vacuum of 0.2 mPa for 30 min of contact. The sessile drop samples were used to examine the interface structure, shear strength, and interfacial fracture toughness under the concentrated load. The introduction of the thin Nb + Ti film layer of 900 nm thickness: (1) greatly improves the wettability of alumina by molten Al at above 1223 K and the shear strength of Al/Al₂O₃ joints produced at 1223 K, (2) has positive effect on structure transformation in the interface and leads to fabrication of reliable metal–ceramic joints. Microstructural investigations of the interface indicated that the precipitates of Nb and Ti-rich intermetallic phases were formed at the Al/Al₂O₃ interface, which influenced strengthening of these joints. Hence a conclusion can be drawn that the interface structure influences the durability increase in Al/Al₂O₃ joints.     

The effect of thermodynamic properties of Me–Ti (Me = In,Sn, Ga,Au, and Ge) melts on the wetting of the CaF<Subscript>2</Subscript> substrate

The effect of Ti additions on the wetting behavior of CaF₂ by non-reactive liquid metals (In, Sn, Ga, Au, Ge) was investigated. Pure metals do not wet CaF₂ while minor additions of Ti improve wetting. Small changes of the contact angle were observed in the CaF₂/Au–Ti and CaF₂/Ge–Ti systems, which are characterized by strong Me–Ti interaction in the melt, while considerable decrease of contact angle was obtained in the CaF₂/In–Ti, CaF₂/Sn–Ti and CaF₂/Ga–Ti systems, which display a relatively weak Me–Ti interaction. According to a thermodynamic analysis and experiential observations, Ti does not react with the substrate to form condensed phases at the metal/CaF₂ interface. Therefore, it was assumed that the mechanism of the wetting improvement is attributed to the Ti segregation at the interface. The results of the XPS analysis confirm a Ti enrichment of the region close to the interface, moreover, according to the high resolution XPS spectrum, obtained from this region, the position of the In4d peak has a chemical shift, which is typical for In–Ti intermetallic compounds. The XPS analysis does not provide sufficient evidence for the formation of the intermetallic interfacial layer at elevated temperature. Thus, further investigations have to be designed and conducted in order to clarify this issue.     

Effects of composition and transesterification catalysts on the physico-chemical and dynamic properties of PC/PET blends rich in PC

Melt blending of polycarbonate (PC)/poly(ethylene terephthalate) (PET) rich in PC at absence/present of different type of tranesterification catalysts was carried out by using reactive extrusion method. The thermal, dynamic, and morphological properties were studied. It was found that all blends are formed by a PC matrix and a semicrystalline (12–20% of crystallinity) of PET dispersed phase. The addition of a catalyst in the mixing process promotes a refined and homogeneous dispersion of PET, as well as it enhances the dynamicmechanical behavior of PC/PET blends compared with PC. These effects are attributed to the emulsifying effect of the PC–PET copolymer generated by transesterification. Additionally, this copolymer contributes to the miscibility between phases as demonstrated by the glass transition (T _g) shift of PC phase and PET phase.     

Properties and microstructure of Sn–0.7Cu–0.05Ni solder bearing rare earth element Pr

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)₆Sn₅ is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu₃Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)₆Sn₅ IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.     

Y2O3/(Cu–Me) systems (Me=Al, Ti): interface reactions and wetting

Wetting behavior and interface interaction between Y₂O₃ and Cu–alloys were investigated at 1,423 K. Pure copper does not wet yttria substrate but the wettability is significantly improved by additions of Al and Ti. Different interface structures were observed in the Y₂O₃/(Cu–Al) and Y₂O₃/(Cu–Ti) systems. Relatively deep crater was detected at the interface in the first system, while Cu alloying by Ti led to formation of a flat interface with a thin reaction layer. The results of the wetting experiments and the interface features were well accounted of by thermodynamic analysis of the Y₂O₃/(Cu–Me) systems.     

Effect of microstructural evolution on magnetic properties of Ni thin films

The magnetic properties of Ni thin films, in the range 20–500 nm, at the crystalline-nanocrystalline interface are reported. The effect of thickness, substrate and substrate temperature has been studied. For the films deposited at ambient temperatures on borosilicate glass substrates, the crystallite size, coercive field and magnetization energy density first increase and achieve a maximum at a critical value of thickness and decrease thereafter. At a thickness of 50 nm, the films deposited at ambient temperature onto borosilicate glass, MgO and silicon do not exhibit long-range order but are magnetic as is evident from the non-zero coercive field and magnetization energy. Phase contrast microscopy revealed that the grain sizes increase from a value of 30–50 nm at ambient temperature to 120–150 nm at 503 K and remain approximately constant in this range up to 593 K. The existence of grain boundary walls of width 30–50 nm is demonstrated using phase contrast images. The grain boundary area also stagnates at higher substrate temperature. There is pronounced shape anisotropy as evidenced by the increased aspect ratio of the grains as a function of substrate temperature. Nickel thin films of 50 nm show the absence of long-range crystalline order at ambient temperature growth conditions and a preferred [111] orientation at higher substrate temperatures. Thin films are found to be thermally relaxed at elevated deposition temperature and having large compressive strain at ambient temperature. This transition from nanocrystalline to crystalline order causes a peak in the coercive field in the region of transition as a function of thickness and substrate temperature. The saturation magnetization on the other hand increases with increase in substrate temperature.     

On the metastable miscibility gap in liquid Cu–Cr alloys

Electromagnetically levitated Cu–Cr alloy melts containing 5–70 at.% Cr were splat-quenched onto a chill substrate. The microstructure of the solidified alloys was investigated by scanning electron microscopy. The alloys containing 5–60 at.% Cr showed a droplet-shaped microstructure consisting of Cr-rich spheroids or and dendrites in a Cu-rich matrix, whereas those containing 65 and 70 at.% Cr showed a banded microstructure consisting of alternative Cu-rich and Cr-rich bands. Both types of microstructure presented evidence for metastable phase separation in Cu–Cr alloy compositions, thus verifying the existence of a broad miscibility gap in the undercooled liquid. However, the results suggested that the miscibility gap has a Cr-rich critical composition and a skewed geometry.     

Dimensional effects observed for the electrical, dielectrical and optical properties of TiO2 DC magnetron thin films

In this paper, we present the dimensional effects observed for TiO₂ films deposited by DC circular magnetron sputtering. TiO₂ thin films are deposited in an Al–TiO₂–Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al–TiO₂ interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO₂ thin films is also investigated, and is observed to vary with TiO₂ film thickness. Further, TiO₂ band gap is observed to vary with film thickness.     

Prediction of enthalpy of formation and Gibbs energy change in pseudo-binary (Ti–Zr)(Fe–Cr)<Subscript>2</Subscript> and pseudo-ternary (Ti–Zr)(Fe–Cr)<Subscript>2</Subscript>-H system using extended Miedema model

The thermodynamic model proposed by Miedema is capable of predicting the enthalpy of formation (ΔH) and relative stability of phases in binary but not in ternary or multi-component systems. While developing nanocrystalline binary/ternary metal hydrides for compressor-driven reversible heating–cooling applications, it is necessary to identify appropriate alloy compositions with suitable hydrogen storage capacity and reversible hydrogen absorption–desorption capability. Accordingly, a suitable modification of the Miedema model is proposed in the present study for calculating ΔH of AB₂ type of pseudo-binary (Ti–Zr)(Fe–Cr)₂ and pseudo-ternary (Ti–Zr)(Fe–Cr)₂-H alloys. Subsequently, Gibbs energy (ΔG) of the possible phases is estimated to predict relative phase stability/equilibrium in a given system. It is shown that grain size or interfacial energy contribution exerts a significant influence on ΔG and relative stability of the phases beyond a critical value/limit. Finally, the predicted phase equilibrium from this model-based calculation is validated by suitable comparison with relevant experimental data reported in the literature.     

Thermal study on structural changes and phase transition in high-energy electron-irradiated blends of P(VDF–TrFE) copolymers

Thermal study of high-energy electron-irradiation binary blends of ferroelectric P(VDF–TrFE) copolymers has been investigated by X-ray diffraction, differential scanning calorimetry (DSC) and thermally stimulated depolarization current (TSDC). X-ray diffraction shows some degree of mixing inside a crystal lattice due to significant changes in the ferroelectric-to-paraelectric phase transition behavior from all-trans to trans-gauche conformation in nanometer range after irradiation. In DSC thermograms, it is found that both the Curie temperature and melting temperature decrease with the dose and that the F–P transition temperatures and enthalpies of two individual copolymers and the blend merge into one with the increase of dose, indicating that there exists a strong lattice coupling between the two copolymers. The peaks observed in TSDC spectra of the blends exhibit the phase transitional characters of the parent copolymers which demonstrates that the miscibility in the crystalline region for their large compositional discrepancy. The distribution of temperature peaks in TSDC also show that the existence of two types of crystallite in the blend which become more clear after irradiation, confirming the X-ray and DSC results.     

Interfacial reactions between Sn-3.5 Ag solder and Ni–W alloy films

Nickel based alloys are currently being investigated in an effort to develop stable barrier films between lead free solder and copper substrate. In this study, interfacial reactions between Ni–W alloy films and Sn-3.5 Ag solder have been investigated. Ni–W alloys films with tungsten content in the range of 5.0–18.0 at.% were prepared on copper substrate by electrodeposition in ammonia citrate bath. Solder joints were prepared on the Ni–W coated substrate at a reflow temperature of 250 °C. The solder joint interface was investigated by Cross-sectional scanning electron microscopy, energy dispersive X-ray spectroscopy and electron back scatter diffraction. It has been observed that a Ni₃Sn₄ layer with faceted morphology formed on the Ni–W alloy film after reflow. The thickness of the bright layer was found to decrease with the increase of tungsten content in the Ni–W film. An additional layer with a bright appearance was also found to form below the Ni₃Sn₄ layer. The bright layer was identified to be a ternary phase containing Sn, W and Ni. The bright layer is found to be amorphous and is suggested to have formed through solid state amorphization caused by anomalously fast diffusion of Sn into Ni–W film.     

Interface interaction in the (B4C + TiB2)/Cu system

Interface phenomena in the TiB₂/(Cu–B) and (B₄C + TiB₂)/(Cu–B) systems were investigated in order to determine conditions for cermet preparation by free infiltration. The wetting behavior of the two-phase ceramic substrate may be accounted for as a superposition of behavior patterns characteristic of the two ceramic phases. The relatively low wetting angles of the liquid Cu on the titanium diboride substrate is attributed to a minor departure of the ceramic phase composition from stoichiometry. Copper alloyed with above 8 at.% B yields contact angles less than 20° sufficient for fabrication of cermets based on the two-phase ceramic by free infiltration. The enhanced wetting and the absence of a new phase formation were confirmed by SEM and TEM analysis of the infiltrated cermets.     

Electrical and optical properties of InSbSe<Subscript>3</Subscript> amorphous thin films

Electrical conductivity, I–V characteristics and optical properties are investigated for InSbSe₃ amorphous thin films of different thicknesses prepared by thermal evaporation at room temperature. The composition of both the synthesized material and thin films were checked by energy dispersive X-ray spectroscopy (EDX). X-ray analysis indicated that all samples under investigation have amorphous structure. The dc electrical conductivity was measured in the temperature range (303–393 K) and thickness range (149–691 nm). The activation energy ΔE _σ was found to be independent of film thickness in the investigated range. The obtained I–V characteristic curves for the investigated samples are typical for memory switches. The switching voltage increases linearly with film thickness in the range (113–750 nm), while it decreases exponentially with temperature in the range (303–393 K). The switching process can be explained according to an electrothermal process initiated by Joule-heating of the current channel. Measurements of transmittance and reflectance in the spectral range (400–2,500 nm) are used to calculate optical constants (refractive index n and absorption index k). Both n and k are practically independent of film thickness in the investigated range (149–691 nm). By analysis of the refractive index n the high frequency dielectric constant ε_∞ was determined via two procedures and was found to have the values of 9.3 and 9.15. Beyond the absorption edge, the absorption is due to allowed indirect transitions with energy gap of 1.46 eV independent on film thickness in the investigated range.     

Coupled morphological and thin film instabilities generated by inclusions in crystal growth

The interaction of a foreign particle that is suspended in the melt with a planar solidifying interface may induce the onset of morphological instabilities provided that its distance from the interface falls below a critical value. This distance, which is of the order of the particle’s radius, depends on the governing processing and physical parameters. When the particle is in nearcontact with the solid-liquid interface, the disjoining pressure in the melt film that separates the particle from the interface influences the interaction. We derive an expression for the film thickness at which rupture occurs. The critical film thickness, which depicts the competition between the stabilizing influence of surface tension and thermal gradients and the destabilizing influence of the intermolecular forces, varies as (Sh)^1/4, where Sh is the Scheludko number that is modifed by the imposed thermal gradients. We note the existence of a critical value for the particle’s radius below which the stabilizing effects are primarily due to surface tension and above which they are due to the thermal gradients.