Browsing by Author "Zhang, R"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    Chemomechanical influences during replacement of limestones by siderite
    (Goldschmidt, 2022-07-15) Weber, J; Starchenko, V; Zhang, R; Ilavsky, J; Debeer-Schmidt, L; Mata, JP; Littrell, K; He, L; Chen, WR; Allard, LF; Stack, AG; Anovitz, L
    A fundamental and predictive understanding of mineral-fluid interactions is important for a wide range of energy topics including carbon sequestration, nuclear waste management and legacy contamination clean up. The properties of aqueous solution are altered by confinement, which can be present within natural geomaterials, e.g., in grain boundaries and nanopores. Mineral replacement reactions have been reported to proceed via grain boundaries possibly due to higher diffusion rates than in solids. In addition to confinement effects, chemomechanical effects such as crystallization pressure induced fracturing can also alter mineral-fluid interactions. To test the effects of porosity and grain boundaries on replacement in single component and impurity-containing systems we experimentally investigated the model system of limestone replacement by siderite by batch reactor experiments at 200°C from 2 to 120 days with FeCl2. Variation in initial microstructure and solid impurities were used to identify reaction controls. Changes in porosity were spatially resolved analyzed using inverse scattering techniques ((ultra) small angle neutron/X-ray scattering), and these were combined with imaging by scanning (SEM) and transmission electron microscopy (TEM). In high-porosity limestones replacement is rapid (complete replacement within 2 days), and transport controlled, whereas in low-porosity limestones elevated porosity throughout the whole rock volume was observed that was independent of the reaction rim. Image analysis showed widening of selected grain boundaries with increasing reaction time. This led to increased grain boundary width distributions that were observed as higher porosity by scattering methods. SEM imaging showed that nucleation of siderite crystals either at dolomite impurities within the limestone or other defects lead to exertion of crystallization pressure, widening grain boundaries, which led to formation of preferential transport pathways that limited replacement of solid impurity-containing limestone rocks. This highlights how chemomechanical effects can alter reaction pathways.
  • No Thumbnail Available
    Item
    Dopant distribution in co-free high-energy layered cathode materials
    (American Chemical Society, 2019-11-21) Mu, L; Zhang, R; Kan, WH; Zhang, Y; Li, LX; Kuai, C; Zydlewski, B; Rahman, MM; Sun, CJ; Sainio, S; Avdeev, M; Nordlund, D; Xin, HL; Lin, F
    The practical implementation of Co-free, LiNiO2-derived cathodes has been prohibited by their poor cycle life and thermal stability, resulting from the structural instability, phase transformations, reactive surfaces, and chemomechanical breakdown. With the hierarchical distribution of Mg/Ti dual dopants in LiNiO2, we report a Co-free layered oxide that exhibits enhanced bulk and surface stability. Ti shows a gradient distribution and is enriched at the surface, whereas Mg distributes homogeneously throughout the primary particles. The resulting Mg/Ti codoped LiNiO2 delivers a material-level specific energy of ∼780 W h/kg at C/10 with 96% retention after 50 cycles. The specific energy reaches ∼680 W h/kg at 1C with 77% retention after 300 cycles. Furthermore, the Mg/Ti dual dopants improve the rate capability, thermal stability, and self-discharge resistance of LiNiO2. Our synchrotron X-ray, electron, and electrochemical diagnostics reveal that the Mg/Ti dual dopants mitigate phase transformations, reduce nickel dissolution, and stabilize the cathode–electrolyte interface, thus leading to the favorable battery performance in lithium metal and graphite cells. The present study suggests that engineering the dopant distribution in cathodes may provide an effective path toward lower cost, safer, and higher energy density Co-free lithium batteries. © 2019 American Chemical Society
  • No Thumbnail Available
    Item
    Electron doping evolution of the magnetic excitations in BaFe(2-x)NixAs2
    (American Physical Society., 2013-10-25) Luo, HQ; Lu, XY; Zhang, R; Wang, M; Goremychkin, EA; Adroja, DT; Danilkin, SA; Deng, GC; Yamani, Z; Dai, PC
    We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brillouin zone in electron-doped iron pnictide superconductors BaFe2-xNixAs2 with x = 0.096,0.15,0.18. While the x = 0.096 sample is near optimal superconductivity with T-c = 20 K and has coexisting static incommensurate magnetic order, the x = 0.15,0.18 samples are electron overdoped with reduced T-c of 14 and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x = 0) and electron optimally doped (x = 0.1) samples, the effect of electron doping was found to modify spin waves in the parent compound BaFe2As2 below similar to 100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x = 0.096 sample confirm the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q width below T-c similar to that of the optimally hole-doped iron pnictides Ba0.67K0.33Fe2As2. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron doping to overdoped iron pnictides with x = 0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above similar to 100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x = 0.096,0.15,0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent. © 2013, American Physical Society.