Migration and correlation in highly defective systems: Fast-diffusion in lithium oxide

The high-temperature superionic phase of lithium oxide is characterized by a high concentration of Frenkel defects and a diffusion mechanism involving several types of atomic jumps. We have calculated the tracer-correlation factor and analyzed the migration paths of the Li ions obtained by molecular dynamics (MD). A kinetic Monte Carlo code, simulating the lithium vacancy diffusion, has been developed and used to predict the correlation factor as a function of the atomic fraction of defects. There is a good agreement with the result directly obtained by MD. The analysis of the jump paths shows that the direct exchange between a vacancy and a migrating atom is the main part of the diffusion mechanism. The other atomic jumps, although complex, mostly imply vacancies. The Li⁺ fast-diffusion proceeds by a vacancy mechanism involving several jump types.     

Sulfidation of iron at high temperatures and diffusion kinetics in ferrous sulfide

The kinetics and mechanism of iron sulfidation have been studied as a function of temperature (950–1200 K) and sulfur pressure (10^–3-0.065 atm). It has been stated that a compact Fe_1–yS scale on iron grows according to the parabolic rate law as a result of outward lattice diffusion of metal ions through cation vacancies. The activation energy of sulfidation increases with sulfur pressure and the 1/n exponent increases with temperature. This nontypical dependence of iron sulfidation kinetics on temperature and pressure results from the analogous effect of both these parameters on defect concentration in ferrous sulfide. The chemical diffusion coefficients,D_FeS, and diffusion coefficients of defects, D_d, in ferrous sulfide have been calculated on the basis of parabolic rate contacts of iron sulfidation and deviations from stoichiometry in ferrous sulfide. It has been shown thatD_FeS is practically independent of cation vacancy concentration whereas the diffusion coefficient of defects depends strongly on that parameter. A comparison of self-diffusion coefficients of iron in Fe_1–yS calculated from the kinetics of iron sulfidation to those obtained from radioisotopic studies indicates that within the range studied of temperatures and sulfur vapor pressures the outward diffusion of iron across the scale occurs preferentially along the c axis of columnar ferrous sulfide crystals.     

Niobium and titanium diffusion in the high niobium-containing Ti–54Al–10Nb alloy

Titanium and niobium diffusion in the ternary Ti–54Al–10Nb alloy (the composition are in at.%) is investigated in an extended temperature interval by the radiotracer technique. The mechanical sectioning was performed by grinding or sputtering in order to measure the tracer diffusivities at higher and lower temperatures, respectively. Almost linear Arrhenius dependencies are established for Nb (Ti) diffusion with the pre-exponential factor and the activation enthalpy of 1.9×10⁻⁵ (4.0×10⁻⁴) m² s⁻¹ and 280 (304) kJ mol⁻¹, respectively. Niobium is a slower diffuser in comparison to Ti in the ternary as well as in the binary (α₂-Ti₃Al and γ-TiAl) titanium aluminides. The heavy alloying of γ-TiAl with 10 at.% niobium, however, enhances significantly both the Ti and the Nb diffusivities. This effect is attributed to an elastic distortion of the L1₀ structure of TiAl induced by the oversized Nb atoms, which may decrease the activation barriers for atomic diffusion.     

Grain boundary diffusion and recrystallization in ultrafine grain copper produced by equal channel angular pressing

The diffusion of ⁶³Ni radiotracer in ultrafine grain (UFG) Cu produced by equal channel angular pressing (ECAP) was studied using the serial-sectioning method. The diffusion annealings were performed in the temperature range of 424–553 K for annealing times at which volume diffusion is negligible and only short-circuit diffusion occurs. Complete or partial recrystallization occurred during all heat treatments, and the explicit expression describing the kinetics of recrystallization was obtained from observations of the microstructure after annealing treatments. The measured radiotracer penetration profiles exhibited two distinct slopes, indicating the co-existence of “slow” and “fast” short-circuit diffusion paths in the system. Based on the results of previous studies, the former were associated with the general high-angle grain boundaries in the non-recrystallized, UFG matrix. A model that considers diffusion in UFG polycrystal undergoing recrystallization was developed. Application of this model enabled us deriving the diffusion coefficients along the grain boundaries in UFG matrix from the experimentally measured radiotracer penetration profiles.     

Effect of Mg24Y5 intermetallic particles on grain refinement of Mg-9Li alloy

The microstructures of the as-cast and as-extruded Mg-9Li-xY alloys (x = 0, 0.3; wt%) were observed to investigate the effect of Y on the Mg-9Li alloy, and the crystallographic calculations between Mg₂₄Y₅ and the matrix were examined on the basis of the edge-to-edge matching model. The results indicated that with the addition of 0.3 wt% Y, the average grain size of α-Mg phases in the as-cast Mg-9Li alloy and β-Li phases in the as-extruded Mg-9Li alloy were reduced remarkably, which was caused by the formation of Mg₂₄Y₅ intermetallic compound. Furthermore, crystallographic calculations confirmed that Mg₂₄Y₅ particles were effective grain refiners for both α-Mg and β-Li phases in Mg-9Li alloy.     

Point defects and diffusion in ordered alloys: An ab initio study of the effect of vibrations

We present a detailed investigation of the influence of atomic vibrations on the point defect and diffusion properties of ordered metallic alloys, by means of ab initio calculations with density-functional theory. Considering the case of Ni₂Al₃ which provides a rich panel of defect-related properties, our study reveals that the behaviour of this compound is largely monitored by self-interstitials, whereas such defects are usually ignored in metallic compounds. The vibration free energies are obtained for the full set of point defects of Ni₂Al₃, showing that these quantities are strongly defect-dependent, and significantly modify the free energy of the compound in an intricate composition-dependent manner. The second key-issue is the first ab initio full analysis of attempt frequencies, via the coupling of vibration analysis and saddle-point search for significant atomic jumps. This analysis indicates that attempt frequencies range over several orders of magnitude and exponentially increase with migration energies. We show the importance of these factors in reaching realistic composition-dependent diffusion coefficients.     

Nano- and micro-scale free volume in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing

Nano- and micro-scale free volumes in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing (ECAP) were examined by a combination of radiotracer diffusion measurements, transmission electron microscopy, depth milling by focused ion beam, and positron annihilation spectroscopy. The positron lifetime spectroscopy revealed the existence of vacancy-type defects, presumably related to grain boundary states, and vacancy clusters. The positron annihilation sites probed by coincident Doppler broadening spectroscopy are surrounded by both Cu and Pb atoms, implying preferred nucleation of nanovoids near small Pb inclusions. Larger free volume defects, such as nano- and submicron-sized pores and submicrometer cracks, were directly observed by the focused ion beam technique. Ultra-fast penetration of liquid tracer solution in the sample bulk was established in the radiotracer diffusion experiments. The results provide an unambiguous proof of the existence of open porosity in ultrafine grained Cu–Pb alloy deformed by ECAP. Under specific deformation conditions, a connected network of open channels, which promotes long-range penetration of a liquid radiotracer solution, was shown to form during the severe deformation. The relative volume fraction of the percolating porosity was estimated at about 2 × 10⁻⁶.     

Electrodeposition of Li and electrochemical formation of Mg–Li alloys from the eutectic LiCl–KCl

The electrochemical behavior of Li⁺ was studied at the inert and active electrodes in the molten LiCl–KCl eutectic. Transient electrochemical techniques, such as cyclic voltammetry, chronoamperometry and chronopotentiometry were used in order to explore the deposition mechanism of Li. The reduction process of Li⁺ is irreversible and the diffusion coefficient of Li⁺ at 723 K was determined as 6.68(±0.07) × 10⁻⁶ cm² s⁻¹. During the electrodeposition process of Li, the electrocrystallization played an important role. The chronoamperometric studies indicated that instantaneous nucleation existed during the electrodeposition process of Li metal. At a Mg electrode, the electroreduction of Li⁺ takes place at a less cathodic potential values than that at Mo electrode which indicated the formation of Mg–Li alloy. The Mg–Li alloy films with different crystal phase were obtained by potentiostatic electrolysis, and the samples were characterized by X-ray diffraction and scanning electron microscopy.     

Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system

Tracer diffusion coefficients are calculated in different phases in the Mo–Si system from diffusion couple experiments using the data available on thermodynamic parameters. Following, possible atomic diffusion mechanism of the species is discussed based on the crystal structure. Unusual diffusion behaviour is found in the Mo₅Si₃ and Mo₃Si phases, which indicate the nature of defects present on different sublattices. Further the growth mechanism of the phases is discussed and morphological evolution during interdiffusion is explained.     

Corrosion characterization of microarc oxidation coatings formed on Mg–7Li alloy

Ceramic coatings with thickness of 27 µm were fabricated on Mg–7Li alloy in Na₂SiO₃–C₆H₁₈O₂₄P₆ solution by microarc oxidation (MAO). The morphology and phase composition of MAO coatings were characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The corrosion behavior of the bare and MAO coated Mg–7Li alloy was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the MAO coatings were composed of MgO, Li₂O, and Mg₂SiO₄, and there existed some micropores on the coating surface with a diameter of 3–20 µm. The corrosion potential (E_corr) and corrosion current density (I_corr) of the MAO coated alloy were about ?1.4761 V and 7.204 × 10^?7 A/cm², respectively. The E_corr of the MAO coated alloy increased by 109.6 mV and its I_corr decreased by three orders compared with that of the bare Mg–7Li alloy. The EIS plots indicated that the impedance of the MAO coated alloy was 15 times higher than that of the bare alloy. The fitting parameters showed that the resistance of the MAO coatings was far greater than that of the bare alloy. The dense intermediate layer and the transition layer of the MAO coatings acted as a barrier to hinder the proceeding of solution permeation, remarkably improving the corrosion resistance of the Mg–7Li alloy.     

Influence of Ca on mierostructure and corrosion resistance of Mg-14Li alloy

In order to improve the corrosion resistance of magnesium lithium alloy, Mg-14Li alloy with different content of Ca (0, 3, 5, 10wt.%) was prepared with a induction melting furnace. Electrochemical test and corrosion test were carried out in NaCI solution with different Cl^- concentrations. The results indicate that the microstructure of the Mg-14Li alloy with Ca consists of dendritic β phase and eutectic structure (β+CaMg₂). With the increase of Ca addition from 0, to 3, 5, 10wt.%, the corrosion resistance of the Mg-14Li alloy initially increases first and then decreases, and that of alloy with 3% Ca is the best. Therefore, the corrosion resistance of Mg-14Li alloy in NaCI solution can be effectively improved by adding proper amount of Ca. In addition, the concentration of Cl^- was one of important factors affecting the corrosion resistance of the Mg-14Li alloy, and the influence of Ca was slighter than that of Cl^-.

     

Synthesis of Li excess LiFePO4/C using iron chloride extracted from steel scrap pickling

Olivine-type Li rich Li_1+xFePO₄/C composites are synthesized by a solid state reaction process using premilled Li₂CO₃ and pre-synthesized amorphous FePO₄·xH₂O powders. The amorphous FePO₄·xH₂O powders are prepared from an industrial waste liquid (by-product), a FeCl₃ (38%) solution, via a precipitation process. In addition, lithium carbonate is pre-milled using a high energy nano mill to control particle sizes and shape differences for enhancing the reaction activity in the starting materials. The main purpose of this study is to investigate the effect of excess Li on the electrochemical properties of LiFePO₄ cathode materials. The pre-synthesized FePO₄ powders are mixed with pre-milled lithium carbonate and glucose (8 wt%) using a ball-mill process. The structural characteristics of the Li_1+xFePO₄/C composites are examined by XRD and SEM. To investigate the effect of excess Li content on the electrochemical properties in Li_1+xFePO₄/C composites, a Li[LiPF₆ (Ethylene carbonate + Dimethyl carbonate)] Li_1+xFePO₄/C model cell is used. It is demonstrated that the 1% Li rich Li/[Li_1.01FePO₄/C] cell exhibits the best electrochemical performance and delivers an initial discharge capacity of 161 mAhg^?1, which is 25 mAhg^?1 higher than that of the Li/[LiFePO₄/C] cell.     

Influence of Y^3＋ doping on structure and electrochemical performance of layered Li1.05V3O8

LiOH.H2O,V2O5 and Y(NO3)3 were used as raw materials to synthesize the precursors containing Li,V and Y by liquid-state reaction,then the cathode materials Li1.05YxV3-xO8(x=0,0.002 5,0.005,0.01,0.02,0.1,0.2)for lithium-ion battery were obtained by calcining the precursors.The influence of Y3 doping on structure,conductivity and electrochemical performance of Li1.05V3O8 were investigated by using XRD,cyclic voltammograms,AC impedance,etc.The results show that Li1.05YxV3-xO8 with different doping amounts have well-developed crystal structure of layered Li1.05V3O8 and lengthened interlayer distance of(100) crystal plane.Y3 can insert into crystal lattice completely when the doping amount is small and the impurity phase of YVO4 is found when x≥0.1.There is no change in the process of Li insertion-deinsertion with Y 3 doping.The conductivity is clearly improved due to small amount of Y 3 doping and it tends to increase first and then decrease with increasing doping amount.The initial discharge capacity and plateau potential are both enhanced with proper amount of Y3 doping.When x is 0.005,the first specific discharge capacity reaches 288.9 mA.h/g,4.60%larger than that of undoped sample(276.2 mA.h/g).When x≤0.1,the average discharge plateau potentials are enhanced by about 0.15 V,which makes for higher energy density.     

Interstitial and substitutional diffusion of metallic solutes in Ti3Al

Solute diffusion of Ni, Fe and Nb in the intermetallic compound Ti₃Al was studied. The diffusion measurements of Ni and Fe were performed with radiotracers applying the standard precision grinding technique for sectioning, while the concentration profiles of Nb in Ti₃Al were analysed through in-depth profiling by secondary ion mass spectrometry (SIMS). Ni and Fe exhibit fast diffusion behaviour of about 4 and 2 orders of magnitude, respectively, larger than titanium self-diffusion. The temperature dependencies of the diffusion coefficients yield the Arrhenius parameters D₀^Ni=1.76 × 10⁻⁵ m²/s, Q^Ni=195 kJ/mol and D₀^Fe=1.96 × 10⁻³ m²/s, Q^Fe=277.2 kJ/mol, respectively. The high mobility of these solutes with comparatively small atomic radii suggests in the D0₁₉-structure of Ti₃Al some type of interstitial diffusion that is discussed in the context of results of analogous diffusion investigations in α-Ti. Nb-diffusion coefficients in Ti₃Al were found to be lower by about one order of magnitude than self-diffusion. The Arrhenius relation is characterized by D₀^Nb=3.15 × 10⁻⁴ m²/s and Q^Nb=338.7 kJ/mol. It is concluded that Nb-diffusion occurs substitutionally via thermal vacancies in the Ti-sublattice.     

机械化学法强化废弃锂离子电池中钴和锂的浸出

以SiO2为助磨剂,采用机械化学法提高废弃锂离子电池中Co和Li的浸出率.实验结果表明,在SiO2与LiCoO2质量比为1:1、研磨转速为500 r/min及研磨时间为30 min的最优条件下,Co和Li在柠檬酸中的浸出率分别达到94.91％和97.22％.表征结果表明,通过机械化学研磨,LiCoO2形成新的活性表面,...     

Wetting behavior and interfacial characteristics of In–Sn alloy on CuZr-based bulk metallic glass

The wettabilities of In–Sn alloy on Cu₄₀Zr₄₄Al₈Ag₈ BMG substrate were investigated using the sessile-drop method at different temperature. The result shows that the equilibrium contact angle decreased with increasing temperature. The interfacial reaction of the active Sn atoms in molten In–Sn alloy and the active Zr atoms in Cu₄₀Zr₄₄Al₈Ag₈ BMG caused crystallization reaction. Ion beam sputtering profiling in combination with AES technique was employed to investigate the Sn diffusion in Cu₄₀Zr₄₄Al₈Ag₈ BMG. Between 473 K and 673 K, the diffusion coefficients vary from 0.7 × 10⁻¹⁶ m²/s to 12.9 × 10⁻¹⁶ m²/s. It is concluded that the interfacial reaction is favorable to the crystallization and then the crystallization also promotes Sn diffusion.     

Defect structure of NiO and rates and mechanisms of formation from atomic oxygen and nickel

The oxidation of nickel by atomic oxygen at pressures from 6×10^–3 to 0.33 Torr between 1050 and 1250 K has been investigated. In these ranges, the oxidation was found to follow the parabolic rate law, viz.k _p = 1.14×10^–5 exp(–13410/T)g² cm^–4sec^–1 for films of greater than 1 m thickness and was pressure-independent. The activation enthalpy for the oxidation reaction was 27±3 kcal mole^–1. Of a number of possible mechanisms and defect structures considered, it was shown that, based on reaction activation enthalpies, impurity effects, pressure independence, and magnitudes of the rates, the most likely was a saturated surface defect model for atomic oxidation. A possible model judged somewhat less likely was one having equilibrium concentrations of doubly ionized cationic defects rate-controlling in both atomic and molecular oxygen. From comparisons of the appropriate processes, the following enthalpy values were derived: H* (Ni diffusion in NiO) = 26.5 ± 8 kcal mole^–1 and H _f ⁰ (doubly ionized cation vacancies in NiO from atomic oxygen) = – 2.1 ± 6.0 kcal mole^–1. The recombination coefficient of atomic oxygen on oxidized nickel was determined to be 0.14 ± 0.06 in the temperature range 985 to 1100 K.     

Electrochemical behavior of Li+, Mg2+, Na+ and K+ in LiFePO4/ FePO4 structures

Besides Li⁺ and Mg²⁺, the electrochemical behavior of Na⁺ and K⁺ in LiFePO₄/ FePO₄ structures was studied since they naturally coexist with Li⁺ and Mg²⁺ in brine. The cyclic voltammogram (CV) results indicated that Na⁺ exhibits some reversibility in LiFePO₄/FePO₄ structures. Its reduction peak appears at ?0.511 V, more negative than that of Li⁺ (?0.197 V), meaning that a relatively positive potential is beneficial for decreasing Na⁺ insertion. The reduction peak of K⁺ could not be found clearly, indicating that K⁺ is difficult to insert into the FePO₄ structure. Furthermore, technical experiments using real brine with a super high Mg/Li ratio (493) at a cell voltage of 0.7V showed that the final extracted capacity of Li⁺, Mg²⁺ and Na⁺ that can be attained in 1 g LiFePO₄ is 24.1 mg, 7.32 mg and 4.61 mg, respectively. The Mg/Li ratio can be reduced to 0.30 from 493, and the Na/Li ratio to 0.19 from 16.7, which proves that, even in super high Mg/Li ratio brine, if a cell voltage is appropriately controlled, it is possible to separate Li⁺ and other impurities effectively.     

Effect of Li on electrochemical properties of Ti1.4V0.6Ni quasicrystal alloy produced by rapid quenching

The effect of Li on the electrochemical performance of electrodes consisting of Ti_1.4V_0.6Ni quasicrystal was investigated at room temperature in three-electrode cell set-up. The quasicrystal sample was initially synthesized by arc melting, followed by melt-spinning, and then infiltrated with Li atoms by electroosmosis. According to X-ray diffraction, all ribbon samples were determined to be icosahedral quasicrystal phase (I-phase), V-based solid solution phase with BCC structure and face centered cubic (FCC) phase with Ti₂Ni-type structure. After infiltrating some Li atoms into the Ti_1.4V_0.6Ni quasicrystal lattice under the condition of an electroosmosis current of 0.6 A, we could observe the appearance of Li diffraction peaks. Importantly, the slight shift to the left observed in the diffraction peaks indicated that lattice expansion was caused by the infiltration of Li. The discharge capacity of Ti_1.4V_0.6Ni–Li material was higher than that of Ti_1.4V_0.6Ni.The maximum discharge capacity of 307.1 mAh/g was recorded for Ti_1.4V_0.6Ni–Li at a current density of 30 mAh/g. Both high-rate dischargeability and cycling stability were enhanced as a result of infiltrating Li. The lithium could get into the lattice, which resulted in the formation of microspores on surface of alloy, thus improving electrochemical activity of the alloy electrode. At the same time, the electrochemical reaction kinetics of alloy electrodes was also researched.     

Crystal structure and thermoelectric properties of β-MoSi2

A powder sample of a metastable phase β-MoSi₂ having the C40-type crystal structure was prepared by heating Mo sheets with a Na–Si melt at 858 K for 12 h to determine details of the crystal structure. The lattice constants (a = 4.6016(3) Å and c = 6.5700(3) Å) and Si atom coordinate (y = 0.1658(2)) of β-MoSi₂ were determined by Rietveld analysis of the X-ray powder diffraction. The thermoelectric properties were refined for a bulk sample prepared by sintering the β-MoSi₂ powder at 773 K and 600 MPa. The electrical resistivity of the sintered β-MoSi₂ sample with a relative density of 65% of the theoretical one was 2.5 mΩ cm at 300 K, and slightly increased with increasing temperature from 300 to 725 K. The Seebeck coefficients changed from +60 to +89 μV/K in the temperature range from 330 to 725 K. The maximum thermoelectric power factor was 2.2 × 10^?6 W cm^?1 K^?2 at 725 K.