Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (12 results)

  • Article
LBL Publications (2003)

Alumina has been bonded via copper/niobium/copper interlayers, and correlations have been made between various processing conditions (applied load, processing temperature, copper film thickness, surface roughness, etc.) and strength. Four-point bend strengths and micrographs of fracture surfaces have been used to determine the relationship between processing, microstructure, and properties. Transparent sapphire substrates bonded with copper/niobium/copper interlayers were used in model experiments to track the microstructural development of these ceramic/metalinterfaces and to identify the important mechanisms that contribute. High interfacial strengths were generally associated with small unbonded regions, extensive breakup of the copper film into isolated particles, ceramic pullout, and regions of niobium/alumina contact where the grain boundary grooves of the alumina are visible on both sides of the fracture surface. Experiments with sapphire substrates showed that asperities in the niobium and grain boundary grooves in the niobium play an important role in the initiation and growth of sapphire/niobium contact. The presence of a liquid film can enhance the kinetics of sapphire/niobium contact and growth by providing a low-temperature high-diffusivity path. The breakup of the copper film was described using two models that were in fairly close agreement. The breakup of the copper film depended on the asperity density in the niobium, niobium grain boundary density, liquid film redistribution, and the breakup of liquid patches via Rayleigh instabilities. The redistribution of the liquid was affected by defect geometry, local film thickness, and local interfacial crystallography. Thermal grooving effects of liquid copper on alumina and niobium were studied using conventional sessile drop experiments. The thermal grooving of one particular grain boundary in alumina when in contact with copper and niobium was studied using a fabricated bicrystal. Both diffusion mechanisms and the dissolution-precipitation reaction of alumina in niobium limited the kinetics of thermal grooving.

    Cover page: Mechanisms of microstructure development at metallic-interlayer/ceramic interfaces during liquid-film-assisted bonding
    • Article
    LBL Publications (2002)
    Alumina has been joined at 1400 degrees C using niobium-based interlayers. Two different joining approaches were compared: solid-state diffusion bonding using a niobium foil as an interlayer, and liquid-film assisted bonding using a multilayer copper/niobium/copper interlayer. In both cases, a 127-(mu)m thick niobium foil was used; =1.4-(mu)m or =3-(mu)m thick copper films flanked the niobium. Room-temperature four-point bend tests showed that the introduction of a copper film had a significant beneficial effect on the average strength and the strength distribution. Experiments using sapphire substrates indicated that during bonding the initially continuous copper film evolved into isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. Film breakup appeared to initiate at either niobium grain boundary ridges, or at asperities or irregularities on the niobium surface that caused localized contact with the sapphire.
      Cover page: Liquid-film assisted formation of alumina/niobium interfaces
      • Article
      • Peer Reviewed
      LBL Publications (2005)
        Cover page: Effects of Impurities on Alumina-Niobium Interfacial Microstructures
        • Article
        • Peer Reviewed
        LBL Publications (2005)
          Cover page: Processing of alumina-niobium interfaces via liquid-film-assisted joining
          • Article
          LBL Publications (2005)
            Cover page: Transient-Liquid-Phase and Liquid-Film-Assisted Joining of Ceramics
            • Article
            • Peer Reviewed
            LBL Publications (2018)
            Incorporation of ions into battery electrodes can lead to phase transformations. When multiparticle phase-transforming electrodes charge or discharge, two processes must occur in each particle: the new phase must nucleate, and then grow until the particle is fully charged or discharged. A fundamental question is which of these two processes is rate limiting. Here we construct a simple stochastic model that shows how the relative rate of nucleation compared with growth determines the particle state-of-charge distributions in the electrode. We find that the number of particles that are partially charged at any time increases as the relative nucleation rate increases. The maximum number of particles that are actively charging occurs just before the time when the first particles are becoming completely charged. By comparing measured state-of-charge distributions with the model, we determine the relative rate of nucleation. We apply this procedure to measurements of the evolution of particles in LiFePO4 cathodes and show we can account for the particle state-of-charge distribution as a function of the electrode state of charge.
              Cover page: Simple Stochastic Model of Multiparticle Battery Electrodes Undergoing Phase Transformations
              • Article
              • Peer Reviewed
              UC Santa Cruz Previously Published Works (2020)
              The lower limit of metal hydride nanoconfinement is demonstrated through the coordination of a molecular hydride species to binding sites inside the pores of a metal-organic framework (MOF). Magnesium borohydride, which has a high hydrogen capacity, is incorporated into the pores of UiO-67bpy (Zr6O4(OH)4(bpydc)6 with bpydc2- = 2,2'-bipyridine-5,5'-dicarboxylate) by solvent impregnation. The MOF retained its long-range order, and transmission electron microscopy and elemental mapping confirmed the retention of the crystal morphology and revealed a homogeneous distribution of the hydride within the MOF host. Notably, the B-, N-, and Mg-edge XAS data confirm the coordination of Mg(II) to the N atoms of the chelating bipyridine groups. In situ 11B MAS NMR studies helped elucidate the reaction mechanism and revealed that complete hydrogen release from Mg(BH4)2 occurs as low as 200 °C. Sieverts and thermogravimetric measurements indicate an increase in the rate of hydrogen release, with the onset of hydrogen desorption as low as 120 °C, which is approximately 150 °C lower than that of the bulk material. Furthermore, density functional theory calculations support the improved dehydrogenation properties and confirm the drastically lower activation energy for B-H bond dissociation.
              • 1 supplemental ZIP
              Cover page: Nanoconfinement of Molecular Magnesium Borohydride Captured in a Bipyridine-Functionalized Metal–Organic Framework
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2021)
              Liquid organic hydrogen carriers such as alcohols and polyols are a high-capacity means of transporting and reversibly storing hydrogen that demands effective catalysts to drive the (de)hydrogenation reactions under mild conditions. We employed a combined theory/experiment approach to develop MOF-74 catalysts for alcohol dehydrogenation and examine the performance of the open metal sites (OMS), which have properties analogous to the active sites in high-performance single-site catalysts and homogeneous catalysts. Methanol dehydrogenation was used as a model reaction system for assessing the performance of five monometallic M-MOF-74 variants (M = Co, Cu, Mg, Mn, Ni). Co-MOF-74 and Ni-MOF-74 give the highest H2 productivity. However, Ni-MOF-74 is unstable under reaction conditions and forms metallic nickel particles. To improve catalyst activity and stability, bimetallic (NixMg1-x)-MOF-74 catalysts were developed that stabilize the Ni OMS and promote the dehydrogenation reaction. An optimal composition exists at (Ni0.32Mg0.68)-MOF-74 that gives the greatest H2 productivity, up to 203 mL gcat-1 min-1 at 300 °C, and maintains 100% selectivity to CO and H2 between 225-275 °C. The optimized catalyst is also active for the dehydrogenation of other alcohols. DFT calculations reveal that synergistic interactions between the open metal site and the organic linker lead to lower reaction barriers in the MOF catalysts compared to the open metal site alone. This work expands the suite of hydrogen-related reactions catalyzed by MOF-74 which includes recent work on hydroformulation and our earlier reports of aryl-ether hydrogenolysis. Moreover, it highlights the use of bimetallic frameworks as an effective strategy for stabilizing a high density of catalytically active open metal sites. This journal is
                Cover page: Stabilized open metal sites in bimetallic metal–organic framework catalysts for hydrogen production from alcohols
                • Article
                • Peer Reviewed
                UC Santa Cruz Previously Published Works (2023)
                Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3-4 nm ultrathin MgB2 nanosheets (multilayers) in high yield. High-pressure hydrogenation of these multilayers at 70 MPa and 330 °C followed by dehydrogenation at 390 °C reveals a hydrogen capacity of 5.1 wt%, which is ≈50 times larger than the capacity of bulk MgB2 under the same conditions. This enhancement is attributed to the creation of defective sites by ball-milling and incomplete Mg surface coverage in MgB2 multilayers, which disrupts the stable boron-boron ring structure. The density functional theory calculations indicate that the balance of Mg on the MgB2 nanosheet surface changes as the material hydrogenates, as it is energetically favorable to trade a small number of Mg vacancies in Mg(BH4 )2 for greater Mg coverage on the MgB2 surface. The exfoliation and creation of ultrathin layers is a promising new direction for 2D metal boride/borohydride research with the potential to achieve high-capacity reversible hydrogen storage at more moderate pressures and temperatures.
                  Cover page: Hydrogen Storage in Partially Exfoliated Magnesium Diboride MultilayersCreative Commons 'BY-NC' version 4.0 license
                  • Article
                  • Peer Reviewed
                  LBL Publications (2021)
                  Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here we report the energy landscape for surface atomic configurations of MgB2 by combining first-principles calculations, global optimization, material synthesis and characterization. We demonstrate that contrary to previous assumptions, multiple disordered reconstructions are thermodynamically preferred and kinetically accessible within exposed B surfaces in MgB2 and other layered metal diborides at low boron chemical potentials. Such a dynamic environment and intrinsic disordering of the B surface atoms present new opportunities to realize a diverse set of 2D boron structures. We validated the predicted surface disorder by characterizing exfoliated boron-terminated MgB2 nanosheets. We further discuss application-relevant implications, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance.
                    Cover page: Spontaneous dynamical disordering of borophenes in MgB2 and related metal borides
                    Top