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[en] The functionalization of fine primary particles by atomic layer deposition (particle ALD) provides for nearly perfect nanothick films to be deposited conformally on both external and internal particle surfaces, including nanoparticle surfaces. Film thickness is easily controlled from several angstroms to nanometers by the number of self-limiting surface reactions that are carried out sequentially. Films can be continuous or semi-continuous. This review starts with a short early history of particle ALD. The discussion includes agitated reactor processing, both atomic and molecular layer deposition (MLD), coating of both inorganic and polymer particles, nanoparticles, and nanotubes. A number of applications are presented, and a path forward, including likely near-term commercial products, is given.
[en] Highlights: • Hybrid biomass-methane steam processing was studied in the range 1600–1800 K. • Syngas yields, CGE, and carbon conversion were determined for three bio-feedstocks. • Production of CO was shown to be kinetically limited relative to H_2 production. • Gasifier efficiency and yields favored allothermal over autothermal operation. - Abstract: Hybrid thermochemical processes show promise to increase plant performance with respect to fungible hydrocarbon production as a substitute to petroleum-based transportation fuels. Biomass, methane, and steam were reacted in a high temperature, indirectly heated reactor to determine the effects of biomass type (microalgae, rice hulls, cotton stalk), temperature (1600–1800 K), and reactant ratios (α = 0–2.0; β = 1.0–4.8) on carbon conversion, cold gas efficiency, and syngas composition. This hybrid co-feed system was shown to achieve high H_2-content syngas with CO selectivity >0.90 and carbon conversion of both biomass and methane >0.90. Temperature was the dominant factor on the yields of CO, CO_2, and CH_4, while reactant ratios could be used to fine-tune the syngas composition. H_2 yield was only slightly dependent on temperature and excess steam. CO formation was highly kinetically-limited for this temperature range. Biomass type slightly affected gasifier performance, most likely due to total char and soot yield from devolatilization. Allothermal reactor design results in comparable gasifier efficiencies depending on steam input and thermal efficiency; a solarthermal reactor would negate 1.3–1.6 kgCO_2/kgC processed and represents the recommended configuration for this type of process operation.
[en] A solar-thermal aerosol flow reactor process is being developed to dissociate natural gas (NG) to hy drogen (H2) and carbon black at high rates. Concentrated sunlight approaching 10 kW heats a 9.4 cm long x2.4 cm diameter graphite reaction tube to temperatures ∼2000 K using a 74% theoretically efficient secondary concentrator. Pure methane feed has been dissociated to 70% for residence times less than 0.1 s. The resulting carbon black is 20-40 nm in size, amorphous, and pure. A 5 million (M) kg/yr carbon black/1.67 M kg/yr H2 plant is considered for process scale-up. The total permanent investment (TPI) of this plant is $12.7 M. A 15% IRR after tax is achieved when the carbon black is sold for $0.66/kg and the H2 for $13.80/GJ. This plant could supply 0.06% of the world carbon black market. For this scenario, the solar-thermal process avoids 277 MJ fossil fuel and 13.9 kg-equivalent CO2/kg H2 produced as compared to conventional steam-methane reforming and furnace black processing
[en] Mass spectrometry is used to study the reaction mechanism of platinum (Pt) atomic layer deposition (ALD) on large quantities of high surface area silica gel particles in a fluidized bed reactor. (Methylcyclopentadienyl)trimethylplatinum [(MeCp)PtMe3] and oxygen are used as precursors. Studies are conducted at a substrate temperature of 320 °C. The self-limiting behavior of ALD appears to be disrupted with overexposure of Pt precursor. The amount of the deposited Pt and the size of the Pt nanoparticles increase with an increasing overdose time of Pt precursor. This can be explained by the thermal decomposition of Pt precursor at the reaction temperature of 320 °C and the in situ sintering of Pt nanoparticles forming larger particles. This finding is significant and its understanding is essential for better control of Pt deposition to achieve desirable morphological and structural properties for different application requirements.
[en] Pigment-grade anatase TiO2 particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100-160 oC. The growth rate varied from 0.5 nm/cycle at 100 oC to 0.35 nm/cycle at 160 oC. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO2 particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al2O3 film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO2 particles.
[en] A fundamental understanding is developed for the chemical reaction mechanism that underlies platinum atomic layer deposition (ALD) on a carbon support, XC72R, for use as a fuel cell catalyst. Specifically, trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe_3) was fed as the 1st reactant for ALD on high surface area particles using a well-instrumented fluidized bed reactor equipped with an in-line mass spectrometer. The precursor’s organic ligands were removed by reaction with the 2nd reactant, either oxygen or hydrogen. These experiments were performed on both unmodified and functionalized XC72R. Carbon modification involved reflux with nitric acid, which oxygenated the XC72R. Platinum weight loading, average particle size, and particle dispersion depended on carbon treatment and on the reactant used for ligand removal (oxygen or hydrogen). Deposited platinum particle sizes ranged from 2.6 to 6.7 nm. Transmission electron microscopy, chemisorption, and diffuse reflectance infrared Fourier transform spectroscopy were used to characterize the Pt deposition and carbon support functionalization. More discrete and non-agglomerated platinum nanoparticles were produced using hydrogen, rather than oxygen, as a reactant and when deposition was conducted on functionalized, rather than unmodified, XC72R carbon. The platinum nanoparticles are stabilized by the underlying oxygen added during substrate functionalization and the avoidance of carbon substrate combustion when using hydrogen, instead of oxygen, as the 2nd reactant to remove residual ligands
[en] The modulation of optoelectronic properties, such as the bandgap of a pure-component semiconductor material, is a useful ability that can be achieved by few techniques. Atomic layer deposition (ALD) was used here to experimentally demonstrate the ability to deposit films that exhibit quantum confinement on three-dimensional surfaces. Polycrystalline ZnO films ranging from ∼1.5 to 15 nm in thickness were deposited via ALD using diethylzinc and hydrogen peroxide at 100 deg. C. Conformal, pinhole-free films were deposited on Si wafers and on nanosized spherical SiO2 particles using an augmented central composite design strategy. Powder x-ray diffraction was used to measure the crystallite size of the films and monitor size evolution on the basis of the number of ALD cycles and thermal annealing post-treatments. The absorbance of the ZnO films on Si wafers and SiO2 particles was measured using spectroscopic ellipsometry and diffuse transmittance techniques, respectively. Post-deposition annealing steps increased the crystallite size of the films, independently of the coating thickness. The ZnO bandgap was increasingly blue-shifted for films of decreasing crystallite size, approaching +0.3 eV at dimensions of 2-3 nm. The nonlinear bandgap response correlated well with the Brus model. This work represents an experimental demonstration of quantum confinement using ALD on two- and three-dimensional substrates.
[en] For carbothermal reduction (CTR) to be an economic and clean process for magnesium metal production, operational challenges must be overcome. Strong and reactive precursor pellets are necessary to effectively and selectively produce Mg(g) from any feedstock. In this study, the effects of ore (magnesia and dolime), carbon (petroleum coke, charcoal, algal char, and carbon black), and binder (organic and inorganic) on pellet strength and reactivity, product yield and purity, and reduction selectivity were analyzed. Theoretically and experimentally, the CTR of dolime (MgO·CaO) favored MgO reduction over CaO reduction; however, with enough carbon and heat, both oxides could be reduced. CaO carbothermal reduction produced CaC2 and Ca(g). The selectivity to CaC2 remained constant (7 ± 4 pct) for all C/MgO·CaO ratios analyzed, while the selectivity to Ca(g) increased (5 pct → 40 pct) when C/MgO·CaO was increased from 0.5 to 2.0. As the overall metal yield decreased (77.6 pct → 59.7 pct) with increasing CaO reduction (38.2 pct → 78.1 pct), Ca(g) reverted faster than Mg(g). Heavy metal impurities primarily remained in the residue (< 30 pct volatilized) and, when volatilized, condensed at high temperatures (700 °C to 1450 °C), relative to light metal impurities (350 °C to 1000 °C, > 78 pct volatilized). Organic binders added reducing power to the pellets but produced frail pellets (radial crush strength = 9.1 ± 0.7 N) after pyrolysis, relative to pellets with inorganic binders (15.1 ± 3.2 N). Kinetic parameters were determined for extruded pellets to predict the reaction rate as a continuous function of pressure and temperature.
[en] Despite the significant recent increase in quantum-based optoelectronics device research, few deposition techniques can reliably create the required functional nanoscale systems. Atomic layer deposition (ALD) was used here to study the quantum effects attainable through the use of this aangstroem-level controlled growth process. Size-dependent quantum confinement has been demonstrated using TiO2 layers of nanoscale thickness applied to the surfaces of silicon wafers. TiO2 films were deposited at 100 deg. C using TiCl4 and H2O2 in a viscous flow ALD reactor, at a rate of 0.61 A/cycle. The low-temperature process was utilized to guarantee the amorphous deposition of TiO2 layers and post-deposition thermal annealing was employed to promote crystallite-size modification. Hydrogen peroxide significantly reduced the residual chlorine that remained from a typical TiCl4-H2O ALD process at this temperature, down to 1.6%. Spectroscopic ellipsometry was used to quantify the optical properties both below and above the bandgap energy. A central composite design was employed to map the surface response of the film thickness-dependent bandgap shift for the as-deposited case and up to a thermal annealing temperature of 550 deg. C. The Brus model was used to develop a correlation between the amorphous TiO2 film thickness and the quantum length to promote equivalent bandgap shifts.
[en] During the atomic layer deposition (ALD) of ZnO films on a bulk quantity of high surface area particles, the thermal decomposition of diethlyzinc (DEZ) is problematic at normal operating temperatures. A low-temperature pathway to ZnO ALD on bulk quantities of nanoparticles was studied using concentrated H2O2 as the oxidant in a fluidized bed reactor. Decreasing the operating temperature effectively mitigated DEZ decomposition, but caused liquid bridging of particles. The mechanisms behind the non-ideal behaviors associated with high temperature precursor decomposition and low-temperature liquid bridging are discussed. An optimal deposition temperature was observed at 100 deg. C to balance these effects