Keke Chang

Modeling of metastable phase formation diagrams for sputtered thin films

Abstract A method to model the metastable phase formation in the Cu–W system based on the critical surface diffusion distance has been developed. The driver for the formation of a second phase is the critical diffusion distance which is dependent on the solubility of W in Cu and on the solubility of Cu in W. Based on comparative theoretical and experimental data, we can describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation. Metastable phase formation diagrams for Cu–W and Cu–V thin films are predicted and validated by combinatorial magnetron sputtering experiments. The correlative experimental and theoretical research strategy adopted here enables us to efficiently describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation during magnetron sputtering.

Experimental investigation and thermodynamic modeling of the Cu―Mn―Zn system

Abstract The phase equilibria of the Cu–Mn–Zn system at 550 °C were investigated using 4 diffusion couples and 25 equilibrated alloys. The preliminary phase relations resulting from diffusion couples were used to select the compositions of the equilibrated alloys. The samples were examined by means of X-ray diffraction, optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry and electron probe microanalysis. The experimental results show no existence of any ternary compound at 550 °C. The phase separation of the β(CuZn)-phase (bcc_A2 structure based on approximate Cu49Zn51, in wt.%) into β and β2 was observed in the diffusion couple of Cu73Mn27 (in wt.%)/Zn and in the ternary alloy of Cu42.4Mn8.8Zn48.5 (in wt.%). There is a debate on such a phase separation in the literature. An order/disorder (A2/B2) transition was inferred to exist according to the present experimental result. All the experimental phase diagram data available from the literature were critically reviewed. A set of self-consistent thermodynamic parameters for the Gibbs energies of individual phases in the Cu–Mn–Zn system was then obtained by using the CALPHAD approach taking into account the reliable data from the literature and from the present work. Comprehensive comparisons between the calculated and measured phase diagrams showed that most of the experimental information is satisfactorily accounted for by the present thermodynamic modeling.

Dependence of the constitution, microstructure and electrochemical behaviour of magnetron sputtered Li-Ni-Mn-Co-O thin film cathodes for lithium-ion batteries on the working gas pressure and annealing conditions

Abstract Li(Ni1/3Mn1/3Co1/3)O2 as a cathode material for lithium ion batteries shows good thermal stability, high reversible capacity (290 mAh g−1), good rate capability and better results in terms of environmental friendliness. In this paper thin film cathodes in the material system Li–Ni–Mn–Co–O were deposited onto silicon and stainless steel substrates, by non-reactive r.f. magnetron sputtering from a ceramic Li1.18(Ni0.39Mn0.19Co0.35)O1.97 target at various argon working gas pressures between 0.2 Pa and 20 Pa. A comprehensive study on the composition and microstructure was carried out. The results showed that the elemental composition varies depending on argon working gas pressure. The elemental composition was determined by inductively coupled plasma optical emission spectroscopy in combination with carrier gas hot extraction. The films showed different grain orientations depending argon working gas pressures. The degree of cation order in the lattice structure of the films deposited at 0.5 Pa and 7 Pa argon working gas pressure, was increased by annealing in an argon/oxygen atmosphere at different pressures for one hour. The microstructure of the films varies with annealing gas pressure and is characterized using X-ray diffraction and unpolarized micro-Raman spectroscopy at room temperature. Electrochemical characterization of as-deposited and annealed films was carried out by galvanostatic cycling in Li–Ni–Mn–Co–O half-cells against metallic lithium. Correlations between process parameters, constitution, microstructure and electrochemical behaviour are discussed in detail.

Enthalpies of formation of layered LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) compounds as lithium ion battery cathode materials

Abstract Layer-structured mixed transition metal oxides with the formula LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) are considered as important cathode materials for lithium-ion batteries. In an effort to evaluate the relative thermodynamic stabilities of individual compositions in this series, the enthalpies of formation of selected stoichiometries are determined by high temperature oxide melt drop solution calorimetry and verified by ab-initio calculations. The measured and calculated data are in good agreement with each other, and the results show that LiCoO2–LiNi0.5Mn0.5O2 solid solution approaches ideal behavior. By increasing x, i. e. by equimolar substitution of Mn4+ and Ni2+ for Co3+, the enthalpy of formation of LiNixMnxCo1–2xO2 from the elements becomes more exothermic, implying increased energetic stability. This conclusion is in agreement with the literature results showing improved structural stability and cycling performance of Ni/Mn-rich LiNixMnxCo1–2xO2 compounds cycled to higher cut-off voltages.