Showing papers by "Christian M. Julien published in 2008"

Thermal stabilization of tin- and cobalt-doped manganese dioxide

Abstract: We study the thermal stability, local structure, and electrical properties of the α-MnO2 phase doped with Sn and Co. It is found that doping prevents the transformation from α-MnO2 to α-Mn2O3 that occurred in the temperature range of 500–600 °C. Samples have been synthesized in an acidic medium using the reduction of potassium permanganate by fumaric acid. X-ray diffraction patterns (XRD) of pure and doped α-MnO2 prepared at 450 °C do not show new peaks related to dopant species. Thermogravimetric analysis (TGA) of the Sn and Co doped MnO2 reveals that transformation from MnO2 to α-Mn2O3 starts above 700 °C. The increase in the thermal stability is attributed to the presence of Sn or Co ions incorporated inside the large 2 × 2 tunnels as revealed by Fourier transform infrared (FTIR) spectra measurements. An increase in the electrical conductivity with the presence of Sn or Co ions is observed. Electrochemical features of the doped MnO2 samples in alkaline cells are reported and compared with that of the pristine α-MnO2 phase.

Oxidation and metal-insertion in molybdenite surfaces: evaluation of charge-transfer mechanisms and dynamics

Abstract: Molybdenum disulfide (MoS2), a layered transition-metal dichalcogenide, has been of special importance to the research community of geochemistry, materials and environmental chemistry, and geotechnical engineering. Understanding the oxidation behavior and charge-transfer mechanisms in MoS2 is important to gain better insight into the degradation of this mineral in the environment. In addition, understanding the insertion of metals into molybdenite and evaluation of charge-transfer mechanism and dynamics is important to utilize these minerals in technological applications. Furthermore, a detailed investigation of thermal oxidation behavior and metal-insertion will provide a basis to further explore and model the mechanism of adsorption of metal ions onto geomedia. The present work was performed to understand thermal oxidation and metal-insertion processes of molybdenite surfaces. The analysis was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA). Structural studies using SEM and TEM indicate the local-disordering of the structure as a result of charge-transfer process between the inserted lithium and the molybdenite layer. Selected area electron diffraction measurements indicate the large variations in the diffusivity of lithium confirming that the charge-transfer is different along and perpendicular to the layers in molybdenite. Thermal heating of molybenite surface in air at 400°C induces surface oxidation, which is slow during the first hour of heating and then increases significantly. The SEM results indicate that the crystals formed on the molybdenite surface as a result of thermal oxidation exhibit regular thin-elongated shape. The average size and density of the crystals on the surface is dependent on the time of annealing; smaller size and high density during the first one-hour and significant increase in size associated with a decrease in density with further annealing.

Novel Lithium Iron Pyrophosphate (LiFe1.5P2O7) as a Positive Electrode for Li-Ion Batteries.

Abstract: Lithium iron pyrophosphate (LiFe1.5P2O7) has been synthesized by a wet-chemical method for application in lithium-ion batteries. Structural analysis using X-ray diffraction and high-resolution electron microscopy indicate that LiFe1.5P2O7 synthesized at 500 °C, using metal acetates, is crystallized in the monoclinic structure with lattice parameters a = 0.698 76 nm, b = 0.812 36 nm, c = 0.964 22 nm, and β=111.83° (P21/c space group). Electrochemical characterization of LiFe1.5P2O7 in lithium cells indicates a capacity of 95 mA·h/g obtained in the voltage range 2.5−4.2 V. The resulting incremental capacity indicates a stable structure for the first cycle with the redox peaks at 3.33 and 3.22 V versus Li0/Li+. The capacity of LiFe1.5P2O7 decreased with cycling number at the rate of 0.22% per cycle while the capacity falls off rapidly in the case of material prepared at higher temperatures.