Some physicochemical studies on organic eutectics

The phase diagrams of phenothiazine with each of m-nitrobenzoic acid (m-NBA) and m-dinitrobenzene (m-DNB) have been studied by thaw-melt method. These materials have been characterized by X-ray diffraction. Growth behavior of the parent components, eutectic and charge transfer complex (CTC) studied by measuring the rate of movement of the growth front in a capillary suggests the applicability of Hillig–Turnbull equation for the system. Microstructure and electrical conductivities of congruent melting complexes and eutectics have been determined. The low electrical conductivities of these materials have been due to weak interaction and mixed stacking of donor and acceptor. Excess thermodynamics functions of the charge-transfer (CT) materials and eutectics have been determined.     

The solidification behavior and phase equilibrium of o-Nitroaniline-β-Naphthol binary eutectic system

Phase diagram studies of o-nitroaniline-β-naphthol system have been made by the thaw melt method. Simple eutectic type phase diagram was formed. Formation of eutectic mixture occurred at 0.7222 mole fractions of o-nitroaniline and melted at 326.85 K. The heat of fusion of components and eutectic mixtures were determined with the help of DSC technique and excess thermodynamic functions were calculated. Experiments were performed to study the undercooling, microstructure and linear velocities of crystallization. Flexural strength measurements were also made. Eutectic mixture showed higher flexural strength as compared to the components.     

Effect of a premelted film on the dynamics of particle–solidification front interactions

Numerical simulations are performed to study the interaction of a solidification front with an embedded particle. A sharp-interface method is used to track both the phase boundary and the particle. The solidification front dynamics is fully coupled with particle motion. The main objective of the paper is to distinguish the role played by the premelted layer between the solidification front and the particle in determining conditions for particle engulfment. Results are obtained by assuming a premelted layer exists in the gap between the particle and the solidification front and compared to those assuming no premelted layer. In the absence of a premelted layer, arbitrary cut-off values for particle-front gap thickness need to be invoked in order to define the critical velocity for which the pushing–engulfment transition occurs. When a premelted layer is assumed to exist, the prediction of the critical velocity is determined solely from the dynamics of the coupled front–particle interaction. In addition, model predictions for the critical velocity based on a steady-state heat transfer analysis are shown to differ from that when the full dynamics of the phase boundary are taken into account.     

Eutectic structures in the Ni–Co–Cr–Al system obtained by plasma spraying and by Bridgman growth

Formation of a regular fibrous two-phase microstructure was found in low-pressure plasma spray (LPPS) deposited Ni–Co–Cr–Al–Y coatings by transmission electron microscopy and analyzed by energy dispersive spectroscopy. The structure is compared to aligned lamellar three-phase structures of an Ni–Co–Cr–Al alloy obtained on Bridgman growth under slow unidirectional solidification (UDS) conditions. The composition of the Ni–Co–Cr–Al alloy for UDS experiments has been identified by DTA. The conditions for the formation of both the LPPS and the UDS structures are discussed.     

Porous crystal structures obtained from directionally solidified eutectic precursors

Interconnecting cage-like porous structures of several halide compounds were prepared by the selective leaching of one eutectic phase method. The binary eutectic precursors were prepared by directional solidification using the Bridgman crystal growth technique. Porous NaMgF₃ (40% pore volume), CaF₂ (57% pore volume) and BaF₂ (43% pore volume) crystals were obtained after water leaching the NaF component of the directionally solidified NaF/NaMgF₃, NaF/CaF₂ and NaF/BaF₂ eutectics with the appropriate entangled microstructure. The growth conditions for eutectic-coupled growth and the morphology of the eutectics have been determined. In the coupled growth regime, the size of the eutectic phases “λ” is fairly uniform and varies with the eutectic growth rate “v” as λ²v=constant, which allows us to control the pore size within the 0.5–10 μm range. The simplicity and versatility of the eutectic growth also allows us to fabricate highly aligned porous structures at relatively high production rates.     

Impurity induced periodic mesostructures in Sb-doped SnO2 nanowires

Impurity doping on semiconductor nanowires formed via vapor–liquid–solid (VLS) mechanism has been investigated with the intention being to control the transport properties. Here we demonstrate that an addition of excess impurity dopants induces a mesostructure of long range periodic arched-shape in Sb-doped SnO₂ nanowires. The microstructural and composition analysis demonstrated the importance of the presence of impurities at the growth interface during VLS growth rather than the dopant incorporation into nanowires, indicating kinetically induced mechanisms.     

Growth of CuS platelet single crystals by the high-temperature solution growth technique

Millimeter size CuS single crystals with a dark indigo blue color and a plate hexagonal shape have been successfully grown by the high-temperature solution growth technique using the KCl–LiCl eutectic as solvent. Surface microtopographic studies of the crystals indicated that the growth is made by the lateral spreading of the layers. Electrical resistivity measurements clearly show an anomaly at T55 K, related with the low-temperature structural transition, a high residual resistivity ratio of 400 and a sharp superconducting transition at T≈1.7 K confirming the very good quality of the crystals.     

Synthesis, growth of organic nonlinear optical crystal: Semicarbazone of 2-amino-5-chloro-benzophenone (S2A5CB) and its characterisation

A new organic crystal of semicarbazone of 2–amino–5–chloro–benzophenone has been grown as a single crystal by slow evaporation solution growth technique for the first time in the literature. The grown crystal has been characterised by proton nuclear magnetic resonance spectral analysis and single crystal and powder X-ray diffraction studies. Functional groups of the crystallised molecules were confirmed by FT-IR and FT-Raman analyses. Mechanical strength of the crystals was studied by microhardness test. Optical transparency of the grown crystals has been studied by UV-Visible spectra. The second harmonic generation property of the compound was analysed.     

An experimental investigation of the columnar-to-equiaxed grain transition in aluminum–copper hypoeutectic and eutectic alloys

Providing benchmark data of the thermal and metallographic parameters during the columnar-to-equiaxed transition (CET) in a wide range of alloy concentrations is of fundamental importance for understanding this phenomenon as well as for metallurgical and modeling purposes. The aim of this study was to investigate the columnar-to-equiaxed transition (CET) in aluminum–copper alloys of different compositions covering a wide range from 2 to 33.2 wt%Cu (eutectic composition), which were directionally solidified from a chill face. The thermal parameters studied included recalescence, cooling rates, temperature gradients and interphase velocities. We found that the temperature gradient and velocity of the liquidus interphase reached critical values at the CET; these critical values were between −0.44 and 0.09 K/mm and between 0.67 and 2.16 mm/s, respectively. The metallographic parameters analyzed were grain size, primary and secondary dendritic arm spacing and also eutectic spacing. The results obtained were compared with previous experimental studies, published predictions and models of the CET for similar alloys.     

Synthesis and characterization of PbSe and PbSe/PbS core–shell colloidal nanocrystals

A new colloidal procedure, for the synthesis of PbSe and PbSe/PbS core–shell semiconductor nanocrystals (NCs), is reported. The synthesis includes the reaction between tributyl-phosphine selenium and lead 2-ethyl-hexanoate, under inert gas at room temperature. High-resolution transmission electron microscopy (HRTEM), X-ray energy dispersive spectroscopy (EDS), absorption and photoluminescence (PL) spectroscopy were used to characterize the samples. The EDS and HRTEM confirmed the existence of rock-salt cubic structures of the PbSe. Furthermore, the HRTEM showed the formation of PbSe/PbS core–shell structures, with epitaxial shell layer with thickness varying between 1 and 4 ML. The absorption spectra of these materials exhibited distinct transitions, related to the e1–h1, e2–h2 and e2–h3 and e2–h1 transitions. These bands are blue shifted with decrease in the NCs diameter. However, the e1–h1 is slightly red shifted with increase in the PbS shell thickness. The last effect can be due to an electronic mixing of the PbSe and PbS conduction states. The PL measurements showed a substantial increase of the exciton emission in the core–shell structures, arising by an efficient chemical passivation of the PbSe core.     

Low-energy ion-assisted control of interfacial structures in metallic multilayers

A molecular dynamics method has been used to simulate the argon ion-assisted deposition of Cu/Co/Cu multilayers and to explore ion beam assistance strategies that can be used during or after the growth of each layer to control interfacial structures. A low-argon ion energy of 5–10 eV was found to minimize a combination of interfacial roughness and interlayer mixing (alloying) during the ion-assisted deposition of multilayers. However, complete flattening with simultaneous ion assistance could not be achieved without some mixing between the layers when a constant ion energy approach was used. It was found that multilayers with lower interfacial roughness and intermixing could be grown either by modulating the ion energy during the growth of each metal layer or by utilizing ion assistance only after the completion of each layers deposition. In these latter approaches, relatively high-energy ions could be used since the interface is buried and less susceptible to intermixing. The interlayer mixing dependence upon the thickness of the over layer has been determined as a function of ion energy.     

Fabrication of nano-crystalline silicon thin film at low temperature by using a neutral beam deposition method

Low temperature (<80 °C) neutral beam deposition (LTNBD) was investigated as a new approach to the fabrication and development of nano-crystalline silicon (nc-Si), which has better properties than that of amorphous silicon (α-Si). The difference between LTNBD and conventional PECVD is that the film formation energy of the nc-Si in LTNBD is supplied by controlled neutral beam energies at a low temperature rather than by heating. Especially, in this study, the characteristics of the nc-Si thin film were investigated by adding 10% of an inert gas such as Ne, Ar or Xe to SiH₄/H₂. Increasing the beam energy resulted in an increase in the deposition rate, but the crystallinity was decreased, due to the increased damage to the substrate. However, the addition of a higher mass inert gas to the gas mixture at a fixed beam energy resulted not only in a higher deposition rate but also in a higher crystallization volume fraction. The high resolution transmission electron microscopy image showed that the grown film is composed of about 10 nm-size grains.     

Optimization of the mineral content in polymeric gels: The effect of calcium to phosphate molar ratio

The influence of calcium to phosphate (Ca/P) molar ratio on the extent of mineralization in a model (poly)acrylamide gel was investigated under simulated physiological conditions. We hypothesized that the optimal growth of hydroxyapatite crystals will take place at the stoichiometric Ca/P molar ratio of 1.67. Phosphate ions were incorporated during the polymerization of the gel and mineralization was initiated by submersion of the gel in calcium acetate solution. Ca/P molar ratios were varied in the range of 0.5–5.0. The mineralized gel was characterized by Raman spectroscopy, scanning electron microscopy (SEM) and mineral weight fraction analysis via ashing. Raman spectra captured across the bulk of the gels indicated the presence of mineral at the core section. The phosphate symmetric stretching peak was observed in the range of 955–960 cm⁻¹ which is characteristic of hydroxyapatite. SEM images showed that crystals formed at Ca/P=2.0 were denser and larger in size than at other molar ratios. In agreement with SEM images, the dry weight fraction of mineral reached the maximum at the molar ratio of 2.0 and the extent of mineralization rapidly declined as the molar ratio diverged from 2.0. Also, the crystallinity of the mineral was optimum at the molar ratio of 2.0. Thus it appears that for effective mineralization, the molar ratio of the two ions needs to be in excess of the stoichiometric requirement, suggesting that ions are expended in processes other than the formation and growth of hydroxyapatite crystals. Therefore, the optimal level of mineralization in biomimetic-based growth of calcium phosphate crystals in sol–gel environment requires consideration of a range of molar ratios as opposed to using the molar ratios corresponding to that of the crystal species intended to grow.     

Incorporation of group III,IV and V elements in 3C–SiC(1 1 1) layers grown by the vapour–liquid–solid mechanism

We report on a comparative investigation of the incorporation of group III, IV and V impurities in 3C–SiC heteroepitaxial layers grown by the vapour–liquid–solid (VLS) mechanism on on-axis α-SiC substrates. To this end, various Si-based melts have been used with addition of Al, Ga, Ge and Sn species. Homoepitaxial α-SiC layers grown using Al-based melts were used for comparison purposed for Al incorporation. Nitrogen incorporation depth profile systematically displays an overshoot at the substrate/epilayer interface for all the layers. Ga and Al incorporations follow the same distribution shape as N whereas this is not the case for the isoelectronic impurities Ge and Sn. This suggests some interaction between Ga/Al and N coming from the high bonding energy between the group III and V elements, which does not exist with Ge and Sn. This is why both incorporate as a cluster. A model of incorporation is proposed taking into account metal-N and metal-C bonding energies together with the solid solubility of the corresponding nitrides.     

Microstructure selection map for rapidly solidified Al-rich Al–Ce alloys

The effect of rapid solidification on the microstructure of Al–8 Ce, Al–20 Ce and Al–36.6 Ce (wt%) alloys has been investigated using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A microstructure selection map has been established for the rapidly solidified Al-rich Al–Ce alloys. Rapid solidification has no effect on the phase constitution but has a marked effect on the microstructure of the Al–8 Ce and Al–20 Ce alloys. For the Al–36.6 Ce alloy, however, rapid solidification has a significant effect on both the microstructure and the phase constitution.     

Role of pore structure in salt crystallisation in unsaturated porous stone

Laboratory driven crystallisation of sodium sulphate and sodium chloride from concentrated solution in unsaturated porous stones has been performed. This contributes to a better understanding of the mechanisms by which salts crystallise and as a consequence limit the durability of porous materials which has an impact on buildings, civil constructions, and historical monuments. The identification of minerals in porous materials has been performed by scanning electron microscopy (SEM), Environmental Scanning Electron Microscopy (ESEM) and sequential profiles of X-ray diffraction (XRD) under temperature control of sample. The study of porous stones has been combined with experiments in capillary tubes. Data from SEM show that halite tends to precipitate on the surface of the stone with a similar distribution in all samples. However, the mirabilite–thenardite precipitation takes place preferably inside the stone and its depth from the surface and its relative concentration depends on the pore size distribution. In addition, mirabilite (Na₂SO₄·10H₂O) crystallises homogeneously, whereas thenardite (Na₂SO₄) and halite (NaCl) tend to nucleate heterogeneously. To explain the precipitation sequence from concentrated solutions in unsaturated porous materials, a detailed analysis of the thermodynamic equations has been carried out by establishing a simple model. The proposed model shows the influence of the pore structure both on the water activity and saturation degree of involved salts.     

Structural characteristics and connection mechanism of gold-catalyzed bridging silicon nanowires

Lateral, single-crystalline silicon nanowires were synthesized using chemical vapor deposition catalyzed by gold nanoparticles deposited on one of the vertical {1 1 1} sidewalls of trenches etched in Si(0 1 1) substrates. Upon encountering the opposing sidewalls of the trenches, the lateral nanowires formed a mechanically strong connection. The bridging connection at the opposing sidewall was observed using high-resolution transmission electron microscopy (TEM) to be epitaxial and unstrained silicon-to-silicon. Using energy-dispersive X-ray spectroscopy in TEM, gold could not be detected at the interface region where the nanowires formed a connection with the opposing sidewall silicon deposit but was detected on the surface adjacent to the impingement region. We postulate that a silicon-to-silicon connection is formed as the gold–silicon liquid eutectic is forced out of the region between the growing nanowire and the opposing sidewall.     

Mechanochemical metathesis synthesis and characterization of nano-structured MnV2O6·xH2O (x=2, 4)

A solid-state metathesis approach for the synthesis of hydrated MnV₂O₆·xH₂O (x=2, 4) materials driven by mechanochemical activation energy has been demonstrated. The metathesis pathway of forming the desired product is confirmed by the presence of high lattice energy by-product such as NaCl. The structural, optical, and chemical properties of the synthesized materials are examined by powder X-ray diffraction, X-ray photoelectron spectroscopy, thermo gravimetric analysis, scanning electron microscopy, transmission electron microscopy, and diffused reflectance measurements in the UV–vis range. The valence state of Mn and V was determined to be +2 and +5, respectively, for the title compounds and the bandgap values determined showed these materials are likely to be semiconductors.     

Growth and characterization of 2-amino-5-chlorobenzophenone (2A-5CB) single crystals

Organic nonlinear optical single crystals of 2-amino-5-chlorobenzophenone (2A-5CB) were grown in ethanol by slow solvent evaporation technique. The grown crystals were characterized by single-crystal XRD, FTIR, FT-Raman and UV–vis–NIR techniques. The UV–vis–NIR spectrum ascertains the cut-off wavelength of the sample as 390 nm. The powder second harmonic generation (SHG) technique reveals that 2A-5CB crystal has its SHG efficiency nearly three times that of KDP. The dielectric response of the sample was studied in the frequency region of 50 Hz–1 MHz at varying temperatures. The photoconductivity studies indicate that the 2A-5CB crystal exhibits negative photoconductivity. TGA–DTA studies confirm the melting point of the sample as 101.5 °C.