In situ gold adsorption experiment at an acidic hot spring using a blue-green algal sheet.

Gold (Au), as one of the most precious metal resources that is used for both industrial products and private ornaments, is a global investment target, and mining companies are making huge investments to discover new Au deposits. Here, we report in situ Au adsorption in an acidic hot spring by a unique adsorption sheet made from blue-green algae with a high preferential adsorption ability for Au. The results of in situ Au adsorption experiments conducted for various reaction times ranging from 0.2 h to 7 months showed that a maximum Au concentration of 30 ppm was adsorbed onto the blue-green algal sheet after a reaction time of 7 months. The Au concentration in the hot spring water was below the detection limit (< 1 ppt); therefore, Au was enriched by preferential adsorption onto the blue-green algal sheet by a factor of more than ~ 3 × 107. Thus, our gold recovery method has a high potential to recover Au even from an Au-poor solution such as hot spring water or mine wastewater with a low impact on the environment.

Kinetic study on heterogeneous nucleation and incubation period during chemical vapor deposition.

The kinetics of heterogeneous nucleation during chemical vapor deposition (CVD) is still unclear despite its importance. Nucleation delay is often observed in many CVD processes, which is known as the incubation period (τi). In this study, the effects of concentration (C) and sticking probability (η) of film-forming species on τi were formulated based on our kinetic model. To discuss the kinetics, τi -1 with the rate dimension was used and formulated using C and η. Because η onto heterogeneous surfaces (ηhetero) is difficult to evaluate, the study was initiated with η onto homogeneous surfaces (ηhomo), followed by a discussion on its reasonability. The formulation was validated using the experimental dataset for SiC-CVD from CH3SiCl3/H2 onto BN underlayers because CVD involves multiple film-forming species with different ηhomo ranging from 10-6 to 10-2 and thus is a suitable system for studying the effect of ηhomo. High-aspect-ratio (1000:1) parallel-plate microchannels consisting of τi-involving BN and a τi-free Si surface were utilized to separate these film-forming species along the microchannel depth. τi was exceptionally long, up to several hours, depending on the CVD conditions. τi -1 was found to be proportional to Cn, where n is the reaction order. n was quantified as ≈1.6, suggesting the initial nucleation was triggered by the impingement of two adspecies in the second order and lowered possibly by the discrepancy between C in the gas-phase and that actually producing adspecies on the surface. τi -1 was also found to be proportional to ηhomo. The exceptionally long τi was likely originated from the significantly lower ηhetero than ηhomo and the higher activation energy for ηhetero than that for ηhomo.

Conformal and Stoichiometric Chemical Vapor Deposition of Silicon Carbide onto Ultradeep Heterogeneous Micropores by Controlling the Initial Nucleation Stage.

Conformal chemical vapor deposition (CVD) of silicon carbide (SiC) from methyltrichlorosilane (MTS) and hydrogen (H2) onto high-aspect-ratio (HAR; typically >100:1) three-dimensional features has been a challenge in the fabrication of ceramic matrix composites. In this study, the impact of heterogeneous underlayers on the initial nucleation of SiC-CVD was studied using HAR (1000:1) microchannels with a tailored wetting underlayer of Si(100) and dewetting underlayers of thermally formed amorphous silicon dioxide (a-SiO2) and turbostratic boron nitride (t-BN). Incubation periods were distributed in the microchannels on a-SiO2 and t-BN underlayers, with the longest period of 70 min found at the feature-bottom due to a decreased concentration (C) of film-forming species. The longer incubation periods with more dewetting underlayers arose due to demoted initial nucleation. Prolonged incubation at the feature bottom led to poor conformality because thick films had already formed at the inlet when film formation began at the feature bottom. The incubation periods were eliminated by increasing the supply of MTS/H2, in accordance with classical heterogeneous nucleation theory. In the meantime, carbon-rich SiC films formed in the vicinity of dewetting a-SiO2 and t-BN underlayers at the feature bottoms, with greater carbon segregation on more dewetting underlayers. This was probably due to the deposition of pyrocarbons (CH4, C2H2, and/or C2H4) generated from MTS/H2 in the gas phase. Decreasing the temperature (T) from 1000 to 900 °C prevented carbon-rich film formation, and the expected deposition rate of pyrocarbon decreased to 0.6% for the case of CH4. A higher C of MTS/H2 combined with a lower T enabled conformal and stoichiometric film formation on the heterogeneous HAR features.

Porous Membranes as Sacrificial Layers Enabling Conformal Chemical Vapor Deposition Involving Multiple Film-Forming Species.

We propose a new, concise method for conformal chemical vapor deposition (CVD) using sacrificial layers (SLs) to fill three-dimensional features with microscopic pores. SLs are porous membranes (e.g., ceramic felts) that filter film-forming species having high sticking-probability (η). CVD processes with multiple film-forming species generally suffer from poor conformality due to preferential film deposition at the inlets of features by the high-η species, such as reactive intermediates. An SL traps such high-η species before they reach the target features and selectively supplies film-forming species with lower η (e.g., source precursors or stable intermediates) that enables conformal film deposition. Here the trapping efficiency of an SL was predicted and a procedure for designing an optimal SL was established. The procedure was demonstrated by CVD of silicon carbide (SiC) with multiple film-forming species of high-η species (η = 8.0 × 10-3) and lower-η species (η = 5.9 × 10-5 and 2.2 × 10-7). The trapping of 99.2% of incident high-η species was achieved with an optimized SL, wherein the deposition rate (m/s) contribution by high-η species declined from 0.546 at the SL inlet to 0.014 at its outlet. Finally, using these optimized SLs, SiC-CVD filling of micron-scale trenches was demonstrated with an aspect-ratio of 16:1.

Multiscale analysis of silicon carbide-chemical vapor deposition process.

A kinetic study, which was performed by using multi-scale (a macro and a micro-scale) analysis, is presented in order to determine the reaction mechanism of the chemical vapor deposition (CVD) of silicon carbide (SiC) from CH3SiCl3 (MTS)/H2 gaseous mixture. The multi-scale analysis provides two well-defined reaction fields, corresponding to the flat substrates placed in a hot wall reactor and micro trenches on the substrate surface, with centimeter and submicron characteristic length scales, respectively. The microcavity method is a micro-scale analysis used to study the relative contributions of gas-phase and surface reactions to the SiC growth, and to determine the sticking probability of growth species in CVD reaction systems. From the macro-scale analysis, activation energy of the growth rate was estimated to be 43.0 kcal/mol at the up-stream part and the sticking probability was estimated to be 9.5 x 10(-7) at 1273 K and 6.8 x 10(-6) at 1373 K. On the other hand, we examined a sticking probability (eta) and the reaction mechanism by using the microcavity method. From the micro-scale analysis, we found that at least two growth species, a stable intermediate 1 (eta 1, = 1.3 x 10(-3) at 1273 K and 4.5 x 10(-3) at 1373 K) and a highly active intermediate 2 (eta 2 = 2.0 x 10(-1) at 1273 K and 5.4 x 10(-1) at 1373 K), are formed as byproducts of the gas-phase reaction. Activation energy of the sticking probability was 43.9 kcal/mol in the case of the intermediate 1 and 34.5 kcal/mol in the case of the intermediate 2. We could also confirm that the source precursor, MTS, was not the film growth species. Another analytical model based on Monte Carlo simulations correlates the film profile in the microcavity to the sticking probability of the deposition species. The combination of these two analysis techniques presents an overall picture of the reaction scheme.