John J. Vajo

Aqueous room temperature synthesis of cobalt and zinc sodalite zeolitic imidizolate frameworks

Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) were synthesized at room temperature under aqueous conditions in 10 min. A trialkylamine deprotonated the 2-methylimidazole ligand and nucleated the frameworks. Furthermore, the ligand acted as a structure directing agent in place of an organic solvent.

A comparative study of beam ion implantation, plasma ion implantation and nitriding of AISI 304 stainless steel

Abstract This paper presents the results of a comparative study using beam ion implantation (BII), plasma ion implantation (PII), ion nitriding and gas nitriding of AISI 304 stainless steel. We have demonstrated that under controlled conditions (the same treatment times of 30 and 60 min, and the same treatment temperature of 400 °C), the microstructures produced by all four techniques are similar, being mainly the formation of nitrogen in solid solution (γ N phase). However, the concentrations of nitrogen and the detectable depths of the nitrogen-enriched layers are significantly different, depending on the process. Both BII and PII produce thick nitrogen-enriched layers (greater than 1 μm) at high concentrations (20–30 at.%) compared with either ion nitriding or gas nitriding (layers less than 1 μm thick with low nitrogen concentrations). As a result, the load-bearing capacity after either BII or PII is much greater than after either ion or gas nitriding. It has also been found that high current density implantation is crucial for the formation of the thick N-enriched layers.

The synthesis and hydrogen storage properties of a MgH2 incorporated carbon aerogel scaffold

A new approach to the incorporation of MgH2 in the nanometer-sized pores of a carbon aerogel scaffold was developed, by infiltrating the aerogel with a solution of dibutylmagnesium (MgBu2) precursor, and then hydrogenating the incorporated MgBu2 to MgH2. The resulting impregnated material showed broad x-ray diffraction peaks of MgH2. The incorporated MgH2 was not visible using a transmission electron microscope, which indicated that the incorporated hydride was nanosized and confined in the nanoporous structure of the aerogel. The loading of MgH2 was determined as 15-17 wt%, of which 75% is reversible over ten cycles. Incorporated MgH2 had >5 times faster dehydrogenation kinetics than ball-milled activated MgH2, which may be attributed to the particle size of the former being smaller than that of the latter. Cycling tests of the incorporated MgH(2) showed that the dehydrogenation kinetics are unchanged over four cycles. Our results demonstrate that confinement of metal hydride materials in a nanoporous scaffold is an efficient way to avoid aggregation and improve cycling kinetics for hydrogen storage materials.

Influence of O+2 energy, flux, and fluence on the formation and growth of sputtering‐induced ripple topography on silicon

The formation of ripples on Si(100) by O+2 sputtering at an angle of incidence of 40° and energies from 1 to 9 keV has been studied using secondary ion mass spectrometry and scanning electron microscopy. At 1 keV no ripples are observed. Between 1.5 and 9 keV ripples are observed oriented perpendicular to the ion direction with average wavelengths that increase, from ∼100 to 400 nm, approximately linearly with O+2 energy. Two‐dimensional fast Fourier transforms of secondary electron images are used to investigate the frequency distribution of the ripples. For the conditions studied, the distributions of frequencies appear approximately Gaussian. At 1.5 keV, the wavelength and growth rate with sputtered depth are independent of flux for fluxes from 15 to 150 μA/cm2. Accompanying ripple formation are changes in secondary ion yields. The changes occur abruptly at depths that increase, from ∼0.2 to 5.6 μm, with O+2 energy. In contrast, sputtering with Ar+ at 1.5 and 7 keV to depths 5–10 times those that produ...

The kinetic enhancement of hydrogen cycling in NaAlH4 by melt infusion into nanoporous carbon aerogel

Enhanced kinetic performance and reversibility have been achieved with uncatalyzed NaAlH4 by incorporation into nanoporous carbon aerogel. Aerogel with a pore size distribution peaked at 13 nm and a pore volume of 0.8 cm(3) g(-1) was filled with NaAlH4 to 94% capacity by melt infusion at 189 degrees C under 183 bar H(2) gas overpressure. Dehydrogenation to NaH + Al with reasonable kinetics was accomplished at 150 degrees C, well below the NaAlH4 melting temperature (183 degrees C), compared to hydrogen release above 230 degrees C for bulk uncatalyzed NaAlH4. Uncatalyzed bulk samples did not rehydrogenate under laboratory conditions, whereas NaAlH4 in a carbon aerogel host was readily rehydrogenated at approximately 160 degrees C and 100 bar H(2) to approximately 85% of its initial capacity. Ball-milled NaAlH4 catalyzed with 4 mol% TiCl3 showed somewhat better kinetics compared to the infused aerogel; nevertheless, the large kinetic enhancement obtained by incorporation into carbon aerogel, even in the absence of a catalyst, demonstrates the substantial benefit of confining the NaAlH4 to nanoscale dimensions.

Relative roles of ion energy, ion flux, and sample temperature in low-energy nitrogen ion implantation of FeCrNi stainless steel

Abstract A matrix of nitrogen ion energies (0.4, 0.7 and 1.0 keV) and ion fluxes (1, 2, and 3 mA/cm2) has been selected to investigate the systematics of ion-beam processing of an austenitic FeCrNi stainless steel (AISI 304) at 400°C for 60 min. In addition, the role of temperature was examined over the range from 280°C to 475°C at fixed processing conditions of 0.7 keV, 2 mA/cm2, and 60 min. Characterization of the composition, structure, magnetic nature, thickness and strength of the nitrogen-containing layers was made by a combination of Auger electron spectroscopy, X-ray diffraction, backscatter Mossbauer spectroscopy, and microhardness measurements. A high N content layer, which is a magnetic, fcc phase, is predominant for all conditions and its thickness increases linearly with either ion energy at fixed ion flux or with ion flux at fixed ion energy. This behavior is consistent with layer growth that is controlled primarily by the N supply rate into the first few atomic layers rather than by thermal diffusion.

Fabrication and hydrogen sorption behaviour of nanoparticulate MgH2 incorporated in a porous carbon host

Nanoparticles of MgH2 incorporated in a mesoporous carbon aerogel demonstrated accelerated hydrogen exchange kinetics but no thermodynamic change in the equilibrium hydrogen pressure. Aerogels contained pores from <2 to approximately 30 nm in diameter with a peak at 13 nm in the pore size distribution. Nanoscale MgH2 was fabricated by depositing wetting layers of nickel or copper on the aerogel surface, melting Mg into the aerogel, and hydrogenating the Mg to MgH2. Aerogels with metal wetting layers incorporated 9-16 wt% MgH2, while a metal free aerogel incorporated only 3.6 wt% MgH2. The improved hydrogen sorption kinetics are due to both the aerogel limiting the maximum MgH(2) particle diameter and a catalytic effect from the Ni and Cu wetting layers. At 250 degrees C, MgH2 filled Ni decorated and Cu decorated carbon aerogels released H(2) at 25 wt% h(-1) and 5.5 wt% h(-1), respectively, while a MgH(2) filled aerogel without catalyst desorbed only 2.2 wt% h(-1) (all wt% h(-1) values are with respect to MgH2 mass). At the same temperature, MgH2 ball milled with synthetic graphite desorbed only 0.12 wt% h(-1), which demonstrated the advantage of incorporating nanoparticles in a porous host.

Zeolite templated carbon materials for high pressure hydrogen storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 m(2) g(-1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g(-1) (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g(-1) (0.46 wt %) uptake per 1000 m(2) g(-1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5-6.6 kJ mol(-1) in the Henrys law limit.

Ion‐induced topography, depth resolution, and ion yield during secondary ion mass spectrometry depth profiling of a GaAs/AlGaAs superlattice: Effects of sample rotation

Effects of sample rotation and sputtering conditions on the depth resolution and ion yield during secondary ion mass spectrometry (SIMS) sputter depth profiles have been studied on bulk GaAs and a GaAs(5 nm)/Al0.3Ga0.7As (5 nm) superlattice. Profiles without sample rotation with 1.0–7.0 keV O+2 show a rapid degradation of the depth resolution with increasing sputter depth. Profiles with Ar+ show only slight degradation. Scanning electron microscope (SEM) studies indicate that degradation is associated with development of periodic surface ripples. The wavelength of the ripples is energy dependent and increases with increasing ion impact energy. With sample rotation, no degradation of the depth resolution is observed and SEM micrographs indicate that surfaces sputtered with rotation are smooth. In addition, with 3.0 keV O+2 significant changes in the secondary ion yield of AsO+ from bulk GaAs are observed at a depth of ∼200 nm. No changes are observed with sample rotation. Our results demonstrate that sampl...

Ternary metal fluorides as high-energy cathodes with low cycling hysteresis

Transition metal fluorides are an appealing alternative to conventional intercalation compounds for use as cathodes in next-generation lithium batteries due to their extremely high capacity (3–4 times greater than the current state-of-the-art). However, issues related to reversibility, energy efficiency and kinetics prevent their practical application. Here we report on the synthesis, structural and electrochemical properties of ternary metal fluorides (M1yM21-yFx: M1, M2=Fe, Cu), which may overcome these issues. By substituting Cu into the Fe lattice, forming the solid–solution CuyFe1-yF2, reversible Cu and Fe redox reactions are achieved with surprisingly small hysteresis (<150 mV). This finding indicates that cation substitution may provide a new avenue for tailoring key electrochemical properties of conversion electrodes. Although the reversible capacity of Cu conversion fades rapidly, likely due to Cu+ dissolution, the low hysteresis and high energy suggest that a Cu-based fluoride cathode remains an intriguing candidate for rechargeable lithium batteries.