Papers by Keyword: Nucleation

Abstract: The solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni are overviewed and compared. In joints on Cu substrates, both solders begin solidification with primary Cu₆Sn₅ growing in the bulk liquid prior to tin nucleation. In freestanding balls and joints, SAC305 generally solidifies with a single tin nucleation event and exhibits a mutually-twinned tin grain structure. In contrast, SN100C BGA balls and joints exhibit multiple independent tin grains that grow as a columnar array in joints.

Abstract: In this work, directional solidification was performed for multicrystalline silicon (mc-Si) ingot casting. The initial nucleation at the bottom of the silicon melt could be controlled by changing the cooling rate from 9 to 20μm/s. Metallographic microscope, X-Ray Diffraction (XRD), Microwave photoconductivity decay meter (μ-PCD) and four-point probe resistivity tester were used to investigate the microstructure, crystal orientation and electrical properties of the mc-Si ingots. The obtained results showed that cooling rate at 17μm/s is the optimum condition for the mc-Si ingots casting, under which the prepared ingot has lower dislocation density of 6×10^-3 cm^-2, better electrical properties, more uniformer resistivity distribution with an average value of 0.68 Ω×cm and higher minority carrier lifetime with a maximum value of 1.8 μs than that of in the other cooling rate conditions.

Abstract: The demand for tunable surface plasmon (SP) of embedded metal nanoparticles (NPs) in rare earth doped inorganic lasing glasses is ever-growing. Following melt quenching method Er³⁺ doped zinc-boro-tellurite glasses containing silver (Ag) NPs are prepared. Glasses are heat treated (thermally annealed) at varying temperatures and time duration to alter the NPs morphology which generates SP. The annealing assisted SP resonance mediated modification in spectral features is discerned. Samples heat treatment at 410 °C for 6 hrs duration ensures the reduction of Ag+ ions to Ag^o NPs. Thermally annealed glasses are characterized via XRD, UV–Vis-IR absorption, photoluminescence spectroscopy, and TEM imaging. XRD spectra confirm the amorphous nature of the glass and TEM image reveals the existence of homogeneously distributed spherically shaped silver NPs of average diameter ~4.5 nm. NPs are found to grow with the increase of both annealing time and temperature. The UV–Vis spectra exhibit seven absorption bands corresponding to 4f–4f transitions of Er³⁺ ions in the wavelength range of 500-650 nm. The localized SPR band is evidenced at 550 and 580 nm. Heat treatment causes a red shift of the plasmon peaks ascribed to the alteration in glass refractive index. Furthermore, the glass sample annealed for 6 hrs displays maximum enhancement in the emission intensity corresponding to the peaks centered at 536 (²H_11/2→⁴I_15/2), 550 (⁴S_3/2→⁴I_15/2) and 632 nm (⁴F_9/2→⁴I_15/2). This enhancement is primarily attributed to the local field effect of the silver NPs. Admirable features of the results suggest that our systematic method for heat treatment in tuning NPs size assisted SPR may contribute towards the development of functional glass.

Abstract: Colloidal syntheses of PbSe nanocrystals (NCs) have been widely investigated and the properties of nanocrystals have been shown to vary with reaction conditions, time, concentration and chemistry of reagents as well as the surfactants used. In this work the effects of reaction temperature, solvents, ligand purity, lead and selenium sources on the optical and structural properties of PbSe nanocrystals were investigated. PbSe NCs synthesized at 90 °C were observed to be spherical and had a narrower size distribution as compared to those synthesized at higher temperatures. 1-octadecene, trioctylphosphine and oleylamine were investigated as solvents for NC synthesis with the non-coordinating solvent octadecene showing the fastest growth rate with medium sized NCs. The coordinating solvents trioctylphosphine and oleylamine produced larger and smaller NCs respectively; this could be attributed to solvent interference during NC nucleation and growth phases. Oleate ligands were used during these syntheses and the ligand purity was not observed to have a significant effect on the NC optical and structural properties. The selenium precursor used affected the NC size and their optical properties while the lead source influenced both the NC shape and size. Lead acetate produced cubic NCs which were larger than the spherical NCs obtained when lead oxide was used.

Abstract: Phase Change Material (PCM) is a material that uses the phase transition properties as latent heat storage for certain application. Salt hydrate CaCl₂.6H₂O (melting temperature, T_m = 29°C, and melting entalphy, DH = 190 kJ/kg) is a common inorganic PCM that can be used for regulating the air temperature of the room, in order to reduce the electrical energy consumption for energy conservation. General problems of inorganic PCM, in particular during the crystallization or latent heat release, are the subcooling and phase separation effects. It is worth mentioning that understanding the crystallization of PCM is important to keep its performance for application. In this research we choose two types additive materials as nucleator to CaCl₂.6H₂O to overcome those two negative effects, namely Ba₂CO₃ and K₂CO₃. We also study the amount of additives needed for effective reduction of subcooling and its phase stability by performing the cycling process. The average temperature and maximum temperature of the subcooling will be determined from temperature vs time recording.

Abstract: Photoinduced nucleation in an external magnetic field is investigated. A possibility of a week magnetic field strength measurement is showed.

Abstract: In this paper, the effect of nickel (Ni) catalyst on the growth of carbon nanotubes (CNTs) was studied where the CNTs were vertically grown by plasma enhanced chemical vapor deposition (PECVD) method. The growth conditions were fixed at a temperature of 700°C with a pressure of 1000mTorr for 40 minutes with various thicknesses of sputtered Ni catalyst. Experimental results show that high density of CNTs was observed especially towards thicker catalyst layers where larger and taller nanotubes were formed. The growth rate increases by ~0.7 times with increasing catalyst thickness from 4nm to 10nm. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. From the Raman results, we found that only slight variation in the intensity ratio of G-band over D-band as increasing catalyst thicknesses. The minor difference in G/D ratio indicates that the catalyst thickness does not significantly influence the quality of CNTs grown.

Abstract: The austenitization of steels can occur in a wide variety of initial microstructures. In this study we addressed the transformation of banded pearlite steels. Banded pearlite initial structures similar to the real ones were created. In these structures the entire transformation process was simulated whose part processes are nucleation and grain growth. The nucleation is described by a free energy based model, and the Fick II. diffusion equation by using Finite Difference Method describes the grain growth. These models have been coupled in cellular automata simulations.

Abstract: Increasing concerns on global warming and current environmental issues have directed research attention to the problems of energy saving, alternative energy sources, as well as to the improvement of the efficiency of existing chemical current sources. A particular focus is currently concentrated on solar energy use. High-purity silicon plates are employed as details in solar cells. Over the past ten years researchers have sought to create silicon nanomaterials able to significantly improve the efficiency of lithium-ion electrochemical batteries and photovoltaic cells. It is a common knowledge that the principle industrial method for producing high-purity silicon is based on vapor deposition; for example Siemens process [1]. However, it has a number of drawbacks in terms of power consumption, costly reagents, and sophisticated equipment. The development of a low-cost production process for solar and nanocrystalline silicon seems therefore an interesting research task. An alternative approach to the production of silicon is electrodeposition from molten salts containing silicon ions [2-7]. This method can be applied to obtain both coherent covers and Si-nanostructures, such as nanopowders and nanofibres.

Abstract: Arcjet plasma enhanced CVD was used to grow diamond. Nucleation of diamond was studied at the early stage of growth cause. The micro-structural probes (transmission electron microscope (TEM), high resolution electronic microscope (HREM), selected area diffraction (SAD) and electron energy loss spectra (EELS) were used to characterize the nuclei. It was found that nuclei formed following the amorphous carbon formation. The critical nucleus size was deduced to be less than 20 nm. The growth of nucleus would form the network-like structure. The incubation period of nucleation was deduced as 6–8min under the condition of high concentration of CH₄ in H₂.