Natalya Froumin

Wetting of TiC by non-reactive liquid metals

Abstract This paper is concerned with the wetting of TiC substrates by the non-reactive metals ( Me=Cu, Ag, Au and Sn ). The contact angle in the TiC/Sn and TiC/Cu systems reaches an equilibrium value slightly below 90° owing to the partial dissolution of titanium carbide in the molten metals. The intrinsic non-wetting behavior in the TiC/Ag system was confirmed. In the TiC/Au system, the non-wetting behavior is attributed to the strong metal–ceramic interaction that takes place at the interface and leads to the formation of a graphite layer. The addition of Fe or Ni increases the solubility of carbon in molten Au and decreases considerably the contact angle by preventing the formation of a graphite layer at the interface. Alloying non-reactive metals with Ti was done either by direct addition to the melt or by transfer from a sub-stoichiometric titanium carbide substrate. In both instances, the presence of Ti in the liquid metal, irrespective of its origin, improves wetting dramatically. The specific wetting behavior of the different Me–Ti alloys are well accounted for by the basic thermodynamic properties of the respective Me–Ti systems and of the titanium carbide phase.

Wetting phenomena in the TiC/(Cu-Al) system

The wetting behavior in the TiC/(Cu–Al) system was studied by the sessile drop method over the entire Cu–Al alloy concentration range. The experimental results show that the wetting evolution in the TiC/Cu system is controlled by partial dissolution of the titanium carbide phase. The presence of oxygen on the TiC surface strongly inhibits the interaction between the ceramic and molten Cu, thus leading to non-wetting conditions. The improved wetting of the oxygen-free TiC substrate by Cu–Al alloys is due to the enhanced transfer of titanium from the carbide phase into the melt, and results from its increased solubility in the Al-containing molten alloy. The wetting of TiC substrates covered with an oxygen-containing layer by a molten Cu–Al alloy is affected by the reduction of this layer and transfer of the released Ti into the molten metal. Enhanced wetting may also result from in situ deoxidization of the surface of the Cu–Al liquid drop and of the TiC substrate due to the evaporation of the Al2O sub-oxide at elevated temperature. A thermodynamic analysis of the systems under consideration is in good agreement with the experimental observations.

Ceramic–metal interaction and wetting phenomena in the B4C/Cu system

Abstract The experimental study of the wetting phenomena in the boron carbide–copper system, using the sessile drop method at 1150xa0°C shows that molten Cu attacks boron carbide substrates forming a crater below the contact area. Boron additions prevent crater formation, improve wetting and, for a 10 at.% B alloy, reduce the equilibrium wetting angle to 40°. Thermodynamic analysis of the Cu–B–C system in the Cu-rich corner confirms the experimental results. These results and similar ones obtained in the TiC–Cu system suggest that the presence of a carbide phase over a concentration range is a key feature that governs wetting phenomena and chemical interaction at these metal–carbide interfaces.

Sealing technique for wafer-level integrated cavity using In-Ag multilayers

The performance of MOEMS (Micro-Optical-Electronic-Micro-Systems) may be significantly improved and their service life-time extended by packaging under vacuum. For numerous applications the bonding process temperature has to be below 200°C and sealed joints must withstand a reflow temperature (about 250 - 280°C) without debonding and unsealing. The Ag-In system has been selected for developing such a bonding process. In this study, the kinetics and sequence of intermetallic phase growth in a Ti/Ag/In/Ag multilayer structure with various ratios of thicknesses, deposited on a silicon substrate, was investigated by DSC (Differential Scanning Callorimetry), SEM and XRD analysis. The joints were examined by DSC to determine the re-melting temperatures and the quality of a cavity sealing was evaluated using a helium leak detection system. It was established that annealing a Ag/In multilayer with a total AG to In layer thickness ratio equal to 3 and an overall thickness of about 10 μm under vacuum of 10-7 torr at 190°C for 40 min allows achieving a void-free joint consisting of two intermetallic phases Ag2In (γ) and AgIn2 (φ). A further appropriate thermal treatment at a relatively low temperature in air leads to the formation of the joint with a re-melting temperature higher than 300°C. The helium leak rate of the integrated cavity was experimentally estimated in the 5 - 10 x 10-9 mbar 1 s-1 range.

Conductive and SERS-active colloidal gold films spontaneously formed at a liquid/liquid interface

We show that water-soluble gold thiocyanate salt [KAu(SCN)4] spontaneously formed electrically-conductive and surface enhanced Raman scattering (SERS)-active films at the interface between water and pentane. Microscopy and spectroscopic analyses reveal that the films comprised of condensed Au nanoparticles, assembled via crystallization of KAu(SCN)4 and self-reduction of the Au3+ ions by the thiocyanate ligands. Importantly, the pentane phase was crucial in promoting the crystallization/reduction reactions. The Au films exhibited useful applications, including excellent conductivity and SERS sensing. We also show that the new film self-assembly process could be readily harnessed for producing macro-scale Au patterns.

Nanostructure Synthesis at the Solid–Water Interface: Spontaneous Assembly and Chemical Transformations of Tellurium Nanorods

Bottom-up synthesis offers novel routes to obtain nanostructures for nanotechnology applications. Most self-assembly processes are carried out in three dimensions (i.e. solutions); however, the large majority of nanostructure-based devices function in two dimensions (i.e. on surfaces). Accordingly, an essential and often cumbersome step in bottom-up applications involves harvesting and transferring the synthesized nanostructures from the solution onto target surfaces. We demonstrate a simple strategy for the synthesis and chemical transformation of tellurium nanorods, which is carried out directly at the solid-solution interface. The technique involves binding the nanorod precursors onto amine-functionalized surfaces, followed by in situ crystallization/oxidation. We show that the surface-anchored tellurium nanorods can be further transformed in situ into Ag2Te, Cu2Te, and SERS-active Au-Te nanorods. This new approach offers a way to construct functional nanostructures directly on surfaces.

The redox chemistry of copper tetraphenylporphyrin revisited

Mixing Cu(II)Cl2 with free base tetraphenylporphyrin ((H2(TPP)) in dry deaerated 1,2-dichloroethane surprisingly yields a mixture of Cu(II)(TPP) and Cu(III)(TPP). The later is long-lived in the absence of water and slowly decomposes within days to yield Cu(II)(TPP). Furthermore, in the presence of air, a slightly different CuIII porphyrin species forms, presumably, Cu(III)(TPP)•+, in which the Q-band is subject to a 10 nm red shift. The new Cu(III)(TPP) and Cu(III)(TPP)•+ differ in terms of absorption, fluorescence, electron paramagnetic resonance (EPR), and X-ray photoemission spectroscopy (XPS) when compared to the well-characterized Cu(II)(TPP), Cu(II)(TPP)•+, and Cu(II)(TPP)2+ species.

Interfacial interaction between quasi-binary oxides (MgAl2O4 and Y3Al5O12) and liquid aluminum

The lack of compatibility between reactive melts, such as aluminum, and crucible materials is a recurrent problem in metallurgical practice. Thermodynamically stable oxides are commonly used as crucible materials. In the present work, the interaction of the quasi-binary oxides (MgAl2O4 and YAG) with liquid Al was studied by the sessile drop method. In both systems, the contact angle decreases from 120 to 40° in the 1273–1523xa0K temperature range. No presence of Mg in Al was detected after the interaction between MgAl2O4 and liquid Al, whereas a detectable amount of Y was dissolved in the melt following the interaction between YAG and molten Al. The differences in the interaction at the interface in the studied systems are attributed to the thermodynamic properties of the ternary Al–Mg–O and Al–Y–O systems.

Wetting of calcium fluoride by liquid metals

The results of wetting experiments for the CaF2/Me and CaF2/Me–Ti systems (Mexa0=xa0Cu, Ge, Al, In, Ga, Sn, and Au) are presented and discussed. It was found that pure metals do not wet the CaF2 substrate, while a small quantity of Ti added to the melt improves the wetting. The effect of Ti depends on its thermodynamic activity in the melts. According to the thermodynamic analysis and experimental observations, Ti dissolved in the metals does not react with the substrate to form any new condensed phase at the interface and its effect cannot be attributed to the “reactive wetting” phenomenon. Density functional theory (DFT) was applied to focus on the nature of chemical bonding between the atoms in the melt and the surface of the substrate in these systems. It was demonstrated that partly filled d-states of Ti stimulate its adsorption onto F ions. Ab initio calculations show that Ti may segregate to the interface, decreasing the energy of CaF2/Me–Ti system. Based on the results of thermodynamic and DFT analyses, it is proposed that Ti segregation at the interface may be considered as the source of the improved wetting.

Fluorescent single-digit detonation nanodiamond for biomedical applications

Detonation nanodiamonds (DNDs) have emerged as promising candidates for a variety of biomedical applications, thanks to different physicochemical and biological properties, such as small size and reactive surfaces. In this study, we propose carbon dot decorated single digit (4-5 nm diameter) primary particles of detonation nanodiamond as promising fluorescent probes. Due to their intrinsic fluorescence originating from tiny (1-2 atomic layer thickness) carbonaceous structures on their surfaces, they exhibit brightness suitable for in vitro imaging. Moreover, this material offers a unique, cost effective alternative to sub-10 nm nanodiamonds containing fluorescent nitrogen-vacancy color centers, which have not yet been produced at large scale. In this paper, carbon dot decorated nanodiamonds are characterized by several analytical techniques. In addition, the efficient cellular uptake and fluorescence of these particles are observed in vitro on MDA-MD-231 breast cancer cells by means of confocal imaging. Finally, the in vivo biocompatibility of carbon dot decorated nanodiamonds is demonstrated in zebrafish during the development. Our results indicate the potential of single-digit detonation nanodiamonds as biocompatible fluorescent probes. This unique material will find application in correlative light and electron microscopy, where small sized NDs can be attached to antibodies to act as a suitable dual marker for intracellular correlative microscopy of biomolecules.