Mo–Si–B alloys for ultra-high-temperature space and ground applications: liquid-assisted fabrication under various temperature and time conditions

Journal of Materials Science - Boron-doped molybdenum silicides have been already recognized as attractive candidates for space and ground ultra-high-temperature applications far beyond limits of...     

Wetting Behavior and Reactivity of Molten Silicon with h-BN Substrate at Ultrahigh Temperatures up to 1750 °C

For a successful implementation of newly proposed silicon-based latent heat thermal energy storage systems, proper ceramic materials that could withstand a contact heating with molten silicon at temperatures much higher than its melting point need to be developed. In this regard, a non-wetting behavior and low reactivity are the main criteria determining the applicability of ceramic as a potential crucible material for long-term ultrahigh temperature contact with molten silicon. In this work, the wetting of hexagonal boron nitride (h-BN) by molten silicon was examined for the first time at temperatures up to 1750 °C. For this purpose, the sessile drop technique combined with contact heating procedure under static argon was used. The reactivity in Si/h-BN system under proposed conditions was evaluated by SEM/EDS examinations of the solidified couple. It was demonstrated that increase in temperature improves wetting, and consequently, non-wetting-to-wetting transition takes place at around 1650 °C. The contact angle of 90° ± 5° is maintained at temperatures up to 1750 °C. The results of structural characterization supported by a thermodynamic modeling indicate that the wetting behavior of the Si/h-BN couple during heating to and cooling from ultrahigh temperature of 1750 °C is mainly controlled by the substrate dissolution/reprecipitation mechanism.     

Silicon-Boron Alloys as New Ultra-High Temperature Phase-Change Materials: Solid/Liquid State Interaction with the h-BN Composite

Silicon-boron alloys have been recently pointed out as novel ultra-high temperature phase change materials for applications in Latent Heat Thermal Energy S     

Relationship Between Mechanical Properties of Lead-Free Solders and Their Heat Treatment Parameters

The research was undertaken to establish mechanical properties of as-cast and heat-treated Sn-Zn-based alloys of binary and ternary systems as candidates for lead (Pb)-free solder materials for high-temperature applications. The heat treatment of as-cast alloys was made under different combinations of processing parameters (168 h/50 °C, 42 h/80 °C, and 24 h/110 °C). The systematic study of structure-property relationships in Sn-Zn, Sn-Zn-Ag, and Sn-Zn-Cu alloys containing the same amount of Zn (4.5, 9, 13.5 wt.%) and 1 wt.% of either Ag or Cu was conducted to identify the effects of chemical composition and heat treatment processing parameters on the alloy microstructure and mechanical behavior. Structural characterization was made using optical microscopy and scanning electron microscopy techniques coupled with EDS analysis. Mechanical properties (initial Young’s modulus E, ultimate tensile strength UTS, elastic limit R _0.05, yield point R _0.2, elongation A ₅, and necking Z) were determined by means of static tensile tests. All the examined Sn-Zn-based alloys have attractive combination of mechanical characteristics, especially tensile strength, having values higher than that of common leaded solders and their substitutes of Pb-free SAC family. The results obtained demonstrate that the Sn-Zn-based alloys present competitive Pb-free solder candidates for high-temperature applications.     

Effects of PCB Substrate Surface Finish and Flux on Solderability of Lead-Free SAC305 Alloy

The solderability of the SAC305 alloy in contact with printed circuit boards (PCB) having different surface finishes was examined using the wetting balance method. The study was performed at a temperature of 260 °C on three types of PCBs covered with (1) hot air solder leveling (HASL LF), (2) electroless nickel immersion gold (ENIG), and (3) organic surface protectant (OSP), organic finish, all on Cu substrates and two types of fluxes (EF2202 and RF800). The results showed that the PCB substrate surface finish has a strong effect on the value of both the wetting time t ₀ and the contact angle θ. The shortest wetting time was noted for the OSP finish (t ₀ = 0.6 s with EF2202 flux and t ₀ = 0.98 s with RF800 flux), while the ENIG finish showed the longest wetting time (t ₀ = 1.36 s with EF2202 flux and t ₀ = 1.55 s with RF800 flux). The θ values calculated from the wetting balance tests were as follows: the lowest θ of 45° was formed on HASL LF (EF2202 flux), the highest θ of 63° was noted on the OSP finish, while on the ENIG finish, it was 58° (EF2202 flux). After the solderability tests, the interface characterization of cross-sectional samples was performed by means of scanning electron microscopy coupled with energy dispersive spectroscopy.     

Interaction between liquid aluminum and NiO single crystals

The wetting behavior of NiO single crystal by liquid aluminum has been studied by the sessile drop method under vacuum at 973–1273 K for 2 h. Optical microscopy, SEM, EDS and X-ray analysis were applied to characterize the structure and chemistry of solidified cross-sectioned sessile drop couples. Under tested conditions, molten Al wets and reacts with NiO to form Al₂O₃ and Ni. This leads to alloying of the initially pure Al drop with Ni to the hypereutectic composition and to the formation of a thick reaction product region inside the NiO substrate, whose structure presents interpenetrated networks of fine alumina precipitates and an Al–Ni matrix. After solidification the Al–Ni matrix and the drop have the same phase composition, which is in agreement with Al–Ni phase diagram, showing the formation of Al₃Ni at T < 1128 K and Al₃Ni₂ at 1128 K < T < 1406 K. The strong reactivity of Al/NiO couples, accompanied with the drop deformation, fragmentation of the reaction product region and development of a crater under the drop, contributes to the perturbation of the triple line and to the formation of apparent contact angles at 1073–1273 K. This leads to unusual changes of measured contact angles with temperature, decreasing from 84^° at 973 K to 36^° at 1073 K, and then increasing to 75^° at 1273 K, while structural analysis suggests complete wetting at 1073 K.