Ex-situ Transmission Electron Microscopy Research Articles

Reversion of sub-boundaries into dense dislocations in aluminum by electric pulsing treatment

Rapid dissolution of sub-boundaries into dense dislocation arrays in electric-pulse-treated aluminum was observed using ex-situ transmission electron microscopy. This unusual increase of interior dislocations, which has rarely been observed in materials treated by conventional thermal treatments, suggests the rapid and possibly simultaneous operations of three mechanisms for sub-boundary dissolution, i.e., emission, extraction and collective migration of boundary dislocations. This phenomenon could be attributed to the enhanced dislocation mobility and the reduced obstacle to dislocation extraction by electric pulsing. This finding provides a new pathway to tailor the microstructure of metallic materials by concurrently coarsening subgrains and increasing dislocation density.

Interactive contraction nanotwins-stacking faults strengthening mechanism of Mg alloys

Light-weight Mg alloys with higher strength are especially desirable for the applications in transportation, aerospace, electronic components and implants owing to their high stiffness, abundant raw materials and environmental friendliness. Nevertheless, the majority of traditional strengthening approaches involving grain refining and precipitation strengthening could effectively prohibit dislocation movement as well as compromise ductility invariably. Here we report a novel strategy for simultaneously achieving a high specific yield strength (182 ± 8 kNmKg−1) and a good elongation (21 ± 2%) in the Mg-13 wt% Li at room temperature, based on the formation of a hierarchical contraction nanotwins-stacking faults (CTWSFs) structure by cryorolling followed by ultrahigh pressure. Both of them are the highest values reported so far, even compared to commercial Al/Ti alloys and steel. The formation process and strengthening mechanism have been clarified by ex-situ transmission electron microscopy observation and molecule dynamics simulations. It demonstrates that this unique nanoscale interactive coherent interface structure is effective to prohibit dislocation motion in Mg crystal, analogous to twin boundaries. Those new results provide insights towards designing alternative and more innovative HCP-type structural materials with higher mechanical properties.

Hierarchical Zn1.67 Mn1.33 O4 /graphene nanoaggregates as new anode material for lithium-ion batteries

Cubic spinel type Zn1.67Mn1.33O4 porous sub-micro spheres were synthesized by the calcination of solvothermally prepared ZnxMn1 − xCO3 precursor powders and evaluated as new anode materials for Li-ion batteries for the first time. Each sphere exhibited aggregated morphology, constructed entirely from nanoparticles with a primary particle size of 11 nm. Electrochemical investigations and ex-situ transmission electron microscopy analyses revealed that the reaction mechanism of obtained Zn1.67Mn1.33O4 nanoaggregates is the combined conversion and alloying reaction, similar to that of ZnMn2O4 systems. In favor of the uniform porous sphere structure, these resulting Zn1.67Mn1.33O4 nanoaggregates enabled the mitigation of volume change upon cycling. In addition, graphene composites with Zn1.67Mn1.33O4 nanoaggregates were fabricated to improve electrical conductivity, simply by adding graphenes during solvothermal reaction for the formation of ZnxMn1 − xCO3 precursors. Zn1.67Mn1.33O4/graphene composites showed a capacity of 670 mA h g−1 higher than that of pure Zn1.67Mn1.33O4 (518 mA h g−1) after 200 cycle at a current density of 100 mA g−1.

La2(MoO4)3@C as novel anode for lithium ion battery: Structural and chemical evolutions upon electrochemical cycling

La2(MoO4)3 is a kind of transition metal molybdate. It has been proved to be a possible anode material for lithium ion batteries due to the high theoretical capacity. In this work, molybdenum-based La2(MoO4)3 material is successfully synthesized through a traditional solid-state reaction method. Carbon-coated La2(MoO4)3 is designed by using glucose as carbon source. By comparing the carbon-coated La2(MoO4)3 with the carbon-free La2(MoO4)3, carbon-coated La2(MoO4)3 delivers better cyclic stability and rate performance. Carbon-coated La2(MoO4)3 obtains a reversible charge capacity of 525.9 mA h g−1 at 100 mA g−1 after 100 cycles, corresponding to capacity retention of 75.2%. However, the capacity retention of carbon-free La2(MoO4)3 is only 33.4%. The results indicate that carbon layer can evidently alleviate the irreversibility of structure and improve capacity retention of La2(MoO4)3. It demonstrates that the carbon-coated La2(MoO4)3 composite can be a possible anode candidate for high capacity lithium ion batteries. In addition, in-situ X-ray diffraction investigation (XRD), ex-situ X-ray photoelectron spectroscopy (XPS), and ex-situ transmission electron microscopy (TEM) are employed to analyze the electrochemical behavior during process of cycling. These experimental results indicate that the conversion behavior of La2(MoO4)3 upon lithiation/delithiation is a process of structural pulverization. Based on that, our work provides guidance for the research of molybdenum-based electrode materials.

Revisiting the conversion reaction in ultrafine SnO2 nanoparticles for exceptionally high-capacity Li-ion battery anodes: The synergetic effect of graphene and copper

Generally, in SnO2-based anode materials, the reversible alloying/dealloying reaction is the main Li-ion storage mechanism. Interestingly, these materials can show an exceptionally high capacity that is beyond the theoretical value (i.e., 783 mA h g−1 based on Sn + 4.4Li+ + 4.4e− ⇌ Li4.4Sn reaction), owing to the reversibility of the reaction between Sn and Li2O to form SnOx (x = 1, 2), so-called conversion reaction. Herein, we prepare Cu-reduced graphene oxide (rGO)-SnO2 nanocomposites as a model system in order to demonstrate an effective strategy to improve the reversibility of the conversion reaction in SnO2. The incorporation of rGO can prevent the aggregation of SnO2 nanoparticles. Furthermore, the Cu-rGO-SnO2 nanocomposite exhibits the most improved conversion reaction reversibility, resulting in improved cycling performance and high capacity. Ex-situ transmission electron microscopy analysis confirms the high reversibility of the conversion as well as the alloying/dealloying reactions. Also, Cu nanoparticles promote the decomposition of amorphous Li2O, leading to enhancement of the conversion reaction between Sn and Li2O. Therefore, these results demonstrate a strategy for significantly improving the electrochemical performances of SnO2-based anodes for Li-ion batteries.

Morphological transformations during Fischer-Tropsch synthesis on a titania-supported cobalt catalyst

Deactivation mechanisms for a rhenium-promoted titania-supported cobalt catalyst are investigated during Fischer-Tropsch (FT) synthesis. Bench-scale reactor tests, chemisorption studies, thermogravimetric analyses (TG), and transmission electron microscopy (TEM) are used to probe environmental effects on catalytic activity.Bench-scale reactor studies show a steady decrease in activity with time. While a fraction of this loss can be recovered with a low temperature reduction (rejuvenation) inside the reactor, multiple data indicate this phenomenon is primarily attributable to water-induced oxidation of small cobalt particles.Data from long-term FT runs indicate the presence of three non-rejuvenable deactivation mechanisms: metal agglomeration, strong metal-support interaction (SMSI), and mixed metal oxide formation. High conversion studies implicate byproduct water in the agglomeration process, and ex-situ TEM data conclusively reveals that growth occurs via a coalescence mechanism. Combined kinetic and chemisorption studies reveal that SMSI results from the gradual buildup of titania decoration on the surface of the active cobalt and is exacerbated with multiple rejuvenation cycles. TGA data indicate that mixed metal oxide formation occurs in long-duration, pilot plant runs. In all cases, the aggregate of reactor kinetic, chemisorption, TG, and TEM results point to chemically-assisted deactivation phenomena attributable to the byproduct water.

BaNb3.6O10 nanowires with superior electrochemical performance towards ultrafast and highly stable lithium storage

High-performance anode materials for development of long cycling life and environmental benignity lithium-ion batteries (LIBs) are greatly desired. Here, the nanostructure, electrochemical performance and structural evolution of barium niobates in LIBs are reported of the first time. One-dimensional barium niobium oxide (BaNb3.6O10) nanowires are prepared by electrospinning technique, displaying the “nanoparticle-in-nanowire” morphology. The BaNb3.6O10 nanowires show high structural stability, high reversible specific capacity (263.8 mAh g−1 at 100 mA g−1), superior rate performance and long-term cycling capability (keeping 60% of the initial reversible capacity at a high current density of 1000 mA g−1 after 5000 cycles). In addition, the mechanism for lithium storage in BaNb3.6O10 is also studied by using ex-situ transmission electron microscopy (TEM), in-situ X-ray diffraction (XRD), in-situ TEM and theory calculations. The results show that BaNb3.6O10 nanowires have reversibility and structural stability for the lithiation and delithiation process. The fabricated BaNb3.6O10/LiMn2O4 full cell demonstrates a practical energy density of 270 Wh Kg−1. This work perfectly confirms that the one-dimensional BaNb3.6O10 nanowires are an ideal material for LIBs.

Multiscale analysis of nanoindentation-induced defect structures in gum metal

Using ex-situ transmission electron microscopy and the recently developed nanoprobe diffraction (NPD) technique, we characterize a nanoindented solution treated gum metal. Lattice rotations are resolved at a 1.2 nm length-scale and shown to be continuous within the nanoindentation pit; further, it is shown that these can be accommodated by a reasonable number of geometrically necessary dislocations at a density of ∼1015/m2. We additionally provide direct evidence that dislocations within the nanoindent, rather than secondary phase nanoparticles, can serve as potent barriers to dislocation motion. We also demonstrate that plasticity in these alloys under nanoindentation can be accommodated solely by dislocation nucleation and propagation, with no competing deformation mechanisms present. Conventional transmission electron microscopy and “g•b” analysis reveal the presence of dislocations on 〈1¯11〉{110} slip systems and highly localized plastic deformation in the form of shear bands on <111>{1¯1¯2} slip systems, similar to previously observed “giant faults”.

High capacity Mg batteries based on surface-controlled electrochemical reactions

Mg batteries are one of several new battery technologies expected to partially substitute lithium-based batteries in the future due to the lower cost and higher safety. However, the development of Mg batteries has been greatly hindered by the sluggish Mg migration kinetics in the solid state. Here, we exploit a high performance cathode for Mg battery based on a tailored nanocomposite, synthesized by in-situ growth of nanocrystalline Mn3O4 on graphene substrates, which provides high reversible capacities (~ 220 mA h g−1 at 15.4 mA g−1 and ~ 80 mA h g−1 at 1.54 A g−1), good rate performance (high reversibility at various current rates), and excellent cycling stability (no capacity decay after 700 hundred cycles). The magnesiation mechanism in our cell system has been identified as a combination of capacitive processes and diffusion-controlled reactions involving electrolyte solvents. Characterization is performed by ex-situ transmission electron microscopy (TEM)/scanning TEM (STEM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) in addition to quantitative kinetics analysis. Exploiting the high-performance capacitive-type electrodes, where the specific capacity is limited by the kinetics of surface processes and not by bulk Mg ion diffusion governing the properties of conventional intercalation-type electrodes, could reveal a new approach to developing commercially viable Mg batteries.

Enhanced photoelectrochemical performance of defect-rich ReS2 nanosheets in visible-light assisted hydrogen generation

Defect introduction is one of the most important motivations to produce highly efficient electrocatalyst in the field of water splitting. Here we report the visible-light enhanced photoelectrochemical performance derived from defect-rich ReS2 nanosheets with high activity towards hydrogen generation from water. After continuous pre-electrolysis under visible-light irradiation for about 2 h, the activated ReS2 nanosheets electrocatalyst performs highly enhanced catalytic activity, with an onset potential decreasing from 170 to 88 mV and a long stability over 10 h. It is worth noting that only a small overpotential of 116 mV is required to afford the current density of 10 mA cm−2, which is the highest activity to date for ReS2 electrocatalysts and much better than most transition-metal dichalcogenide materials electrocatalysts. Ex-situ transmission electron microscopy (TEM) and inductively coupled plasma emission spectrometer (ICP) examinations demonstrate that the performance improvement is attributed to the defect introduction during the visible-light assisted pre-electrolysis process involving the rhenium atoms vacancies on the basal planes and the lattice fringes of ReS2. Theoretical calculations further confirm that the abundant Re atoms vacancies with lower Gibbs free energy for H adsorption activate the inert basal plane as highly active sites for hydrogen evolution.

Deformation induced martensitic transformation in 304 austenitic stainless steel: In-situ vs. ex-situ transmission electron microscopy characterization

304 stainless steel is known to be metastable as the austenite phase can transform into martensite under deformation. In this work, both ex-situ and in-situ transmission electron microscopy (TEM) characterization were used to investigate the mechanisms of the deformation-induced transformation at room temperature. The ex-situ tensile tests were conducted at a strain rate of 10−3s−1 until rupture, followed by TEM and X-Ray Diffraction (XRD). Samples were also interrupted at strains of 7%, 18%, and 30% with the goal of investigating the intermediate microstructure. In addition, tensile tests were conducted in-situ in a TEM at 25°C using a special straining-stage with the goal of capturing the nucleation and growth of the martensitic phase as it develops under deformation. The formation of stacking faults and the subsequent formation of ε-martensite (hcp) through their overlapping/bundling was captured in-situ, confirming the role played by Stacking Faults (SFs) as intermediate step during the transformation from γ-austenite to ε-martensite. Direct transformation of γ-austenite (fcc) to α’-martensite (bcc) was also captured upon straining and characterized. Such unique in-situ observations showcase how in-situ straining in a TEM, as a small scale tensile technique, is a powerful technique to visualize and investigate the mechanisms of deformation induced phase transformations.

Regulating kinetics of deformation-induced phase transformation in amorphous alloy composite via tuning nano-scale compositional heterogeneity in crystalline phase

Nano-scale compositional heterogeneity of crystalline phase in Ti-based amorphous alloy composites (AACs) has been successfully tuned by tuning cooling rate in solidification process. And the effect of compositional heterogeneity on the kinetics of the deformation-induced phase transformation was investigated by in-situ synchrotron-based high-energy X-ray diffraction (HE-XRD) and ex-situ transmission electron microscopy. In-situ HEXRD experiments provide obvious evidence that with the decrease of the cooling rate during solidification, the critical stress of the deformation-induced phase transformation becomes lower, and the phase transformation rate becomes higher. Further high angle annular dark field-scanning electron microscopy investigation shows that the occurrence of the nano-scale Zr-lean compositional heterogeneity, which can favor the nucleation of the martensite, is the reason for the variation of the phase transformation kinetics.

In-situ kinetics of modifications induced by swift heavy ions in Al2O3: Colour centre formation, structural modification and amorphization

This paper details in-situ studies of modifications induced by swift heavy ion irradiation in α-Al2O3. This complex behaviour is intermediary between the behaviour of amorphizable and non-amorphizable materials, respectively. A unique combination of irradiation experiments was performed at the IRRSUD beam line of the GANIL facility, with three different characterisation techniques: in-situ UV–Vis absorption, in-situ grazing incidence X-Ray diffraction and ex-situ transmission electron microscopy. This allows a complete study of point defects, and by depth profile of structural and microstructural modifications created on the trajectory of the incident ion. The α-Al2O3 crystals have been irradiated by 92 MeV Xenon and 74 MeV Krypton ions, the irradiation conditions have been chosen rather similar with an energy range where the ratio between electronic and nuclear stopping power changes dramatically as function of depth penetration. The main contribution of electronic excitation, above the threshold for track formation, is present beneath the surface to finally get almost only elastic collisions at the end of the projected range. Amorphization kinetics by the overlapping of multiple ion tracks is observed. In the crystalline matrix, long range strains, unit-cell swelling, local microstrain, domain size decrease, disordering of oxygen sublattice as well as colour centre formation are found. This study highlights the relationship between ion energy losses into a material and its response. While amorphization requires electronic stopping values above a certain threshold, point defects are predominantly induced by elastic collisions, while some structural modifications of the crystalline matrix, such as unit-cell swelling, are due to contribution of both electronic and nuclear processes.

Mechanisms for phase transformation induced slip in shape memory alloy micro-crystals

Compression of a single crystal, superelastic NiTi shape memory alloy (SMA) micro-pillar and the stress-field around an ellipsoidal twinned martensite (M) plate embedded in an austenite (A) matrix were simulated using a coupled phase transformation and crystal plasticity model. Post-mortem transmission electron microscopy (TEM) analysis of the dislocation structures in a foil extracted from a compressed NiTi micro-pillar was also performed. Based on these modeling and experimental data, we propose mechanisms for phase-transformation-induced defect generation in superelastically stressed NiTi SMA. The geometry of the simulated slip bands shows that dislocations nucleate and grow in the austenite phase adjacent to a growing or receding martensite plate to accommodate local strain gradients. The simulated resolved shear stress on individual slip systems, and Burgers vector analysis of dislocations in the TEM data show that the slip system and amount of slip activity depend on the magnitude of the strain gradients, which are controlled by the martensite crystallography, the dynamics of martensite plate growth, and scale of the twinned structure and A-M interface. In addition to a[0 1 0] (1 0 1¯) and a [0 0 1] (1¯1 0) slip systems observed in prior experiments, we report the activation of a third slip system: a[0 0 1](1 1 0). We show that the three slip systems are likely to be active at different locations around a martensite plate. The modeling component in this work complements ex-situ TEM characterization by furnishing the resolved shear stress and slip activity on austenite slip systems throughout the cyclic loading.

Monometallic Palladium for Oxygen Reduction in PEM Fuel Cells: Particle-Size Effect, Reaction Mechanism, and Voltage Cycling Stability

In this experimental study, we determine the size dependent activity as well as accelerated voltage cycling stability of various carbon supported palladium electrocatalysts for the oxygen reduction in acidic medium. Furthermore, ex-situ transmission electron microscopy studies before and after accelerated voltage cycling provide a deeper understanding regarding particle stability during voltage cycling. Regarding oxygen reduction, a particle size effect on the specific activity is observed, with bulky Pd-black ( ≈ 4 m2 g− 1Pd) exhibiting ≈x6 times higher activity than Pd supported on Vulcan carbon ( ≈ 190 m2 g− 1Pd). Mass activities, however, exhibit a strong correlation with catalyst surface area at small electrochemically active surface area (ECSA) values, but are observed to be nearly constant between ≈ 50 − 200 m2 g− 1Pd. As stability tests during voltage cycling reveal a benefit for smaller surface areas, i.e. bigger particles, a limited gain in stability can be achieved by increasing catalyst particle size at a negligible cost of electrocatalytic mass-based activity. Moreover, we provide a deeper insight regarding the oxygen reduction reaction mechanism and show significant hints that a sequential two-plus-two electron reduction mechanism via intermediate hydrogen peroxide is likely to occur on carbon supported Pd catalysts.

Phosphorus-Rich Copper Phosphide Nanowires for Field-Effect Transistors and Lithium-Ion Batteries.

Phosphorus-rich transition metal phosphide CuP2 nanowires were synthesized with high quality and high yield (∼60%) via the supercritical fluid-liquid-solid (SFLS) growth at 410 °C and 10.2 MPa. The obtained CuP2 nanowires have a high aspect ratio and exhibit a single crystal structure of monoclinic CuP2 without any impurity phase. CuP2 nanowires have progressive improvement for semiconductors and energy storages compared with bulk CuP2. Being utilized for back-gate field effect transistor (FET) measurement, CuP2 nanowires possess a p-type behavior intrinsically with an on/off ratio larger than 10(4) and its single nanowire electrical transport property exhibits a hole mobility of 147 cm(2) V(-1) s(-1), representing the example of a CuP2 transistor. In addition, CuP2 nanowires can serve as an appealing anode material for a lithium-ion battery electrode. The discharge capacity remained at 945 mA h g(-1) after 100 cycles, showing a good capacity retention of 88% based on the first discharge capacity. Even at a high rate of 6 C, the electrode still exhibited an outstanding result with a capacity of ∼600 mA h g(-1). Ex-situ transmission electron microscopy and CV tests demonstrate that the stability of capacity retention and remarkable rate capability of the CuP2 nanowires electrode are attributed to the role of the metal phosphide conversion-type lithium storage mechanism. Finally, CuP2 nanowire anodes and LiFePO4 cathodes were assembled into pouch-type lithium batteries offering a capacity over 60 mA h. The full cell shows high capacity and stable capacity retention and can be used as an energy supply to operate electronic devices such as mobile phones and mini 4WD cars.

Ex-Situ TEM Investigation on Additional Capacity of SnO2 Lithium-Ion Battery Anode Material

Having high energy densities, lithium ion rechargeable batteries are widely used for energy storage and still are the subject of intensive investigation. As anode materials, tin-based materials have long been considered as a promising candidate due to their twice larger theoretical specific capacity compared to commercially used graphite carbon material. Another anode candidates with extremely high electrochemical performances have been developed steadily resulting from intensive researches, but the interests on tin-based anode materials recently have been increased again since they also show great performance as an anode for next-generation sodium-ion batteries. Most of the various tin-based materials researched for high energy battery anodes can be subdivided into two groups, pure tin (Sn) metal based and tin oxide (SnOx) based compounds. Both anode materials are based on reversible alloying reaction mechanism forming LixSn alloys in lithium-ion batteries, but tin oxide undergoes additional reaction, i.e. conversion reaction, before alloying reaction occurs in the first charging process. Even though the conversion reaction, which indicates the reaction forming reduced tin metal and lithium oxide from tin oxide, is known as almost irreversible and it consumes plenty of lithium ions, it is normally considered as an advantage because the resulting lithium oxide is believed to act as mechanical buffer layers preventing from drastic volume expansion/contraction and therefore contributes capacity retention. However, there is still a lack of understanding on the reaction mechanism of tin oxide. In particular, the SnO2 anode usually shows additional specific capacity that greatly exceeds the theoretical value. On the basis of our experimental data, the extra capacity reaches 130% of its theoretical value on the first charging process. The extra capacity commonly found in transition metal oxide (TMO) anodes has also been a subject of great interest, and Hu et al. explained that reaction of lithium hydroxide from electrolyte decomposition mainly contributes to additional capacity in RuO2 anode (Nature Materials, 2013.12.). But it still does not give an explanation of SnO2 anode because they have different fundamental electrochemical reaction mechanisms that enable lithium-ion battery to be rechargeable; TMO anodes such as the RuO2 anode are operated by the reversible conversion reaction, whereas the SnO2 anode is operated by the reversible alloying reaction. Furthermore, the partial reversibility of conversion reaction has long been in debates on SnO2 anode materials. The partial reverse conversion reaction of the SnO2 anode has been commonly accepted and considered to contribute the extra capacity, but previous reports are mostly based on indirect experimental evidences. Here, we elucidated the origin of additional capacities and studied the possibility of partial reversibility of conversion reaction through ex situ transmission electron microscopy (TEM) research. We improved and optimized an ex situ TEM analysis technique to investigate electrochemical reactions inside the actual battery-operating system. Direct (dis)charging of SnO2 particles on TEM grid in a coin cell enabled us to track the same locations of nano-areas throughout the full reaction cycle and, therefore, all of the phase evolutions including solid electrolyte interphase (SEI) layers were identified. We additionally performed same experiments with pure Sn particles to draw an exact conclusion. Eliminating the conversion reaction step, pure Sn particles give us better understanding on the reaction mechanism of SnO2. Combining all of the experimental results as well as the aid of computational works, we found out that reactions of the Li2O phase contribute to the extra capacity and the reverse conversion reaction of SnO2 hardly occurs in the real battery system. We expect that this study will provide fundamental knowledge on behaviors of various tin-based materials in lithium-ion batteries.

Why Is Li2MnSiO4 Unstable in Li-Ion Battery Cell? Structural Studies at Different Stages of Electrochemical Reaction

Application of orthosilicates Li2MSiO4 (M=Fe, Mn, Co, Ni) compounds as insertion materials for LIBs was firstly proposed by Prof. Goodenough [1]. The materials mainly crystallize in orthorhombic system of Pmn21 space group in olivine structure [2-4]. The Li2MSiO4 olivine structure may be described as wavy layers of [SiMO4]∞ on ac axis plane and connected along b axis by LiO4 tetrahedra [2]. Within the layers every SiO4 tetrahedron shares its corner with four next MO4 tetrahedra. Lithium ions occupy the tetrahedral sites (LiO4) between two layers and share 3 and 1 oxygen atoms with the layers. In fact, diffusion of lithium ions in this structure is possible only through the canals formed by LiO4 tetrahedra. Li2MSiO4 silicates reveal a possibility of reversible insertion of two lithium ions per molecule, which leads to exchange of two electrons by transition metal. As a results the silicates reveal high theoretical capacity, up to 330 mA/g for Li2MnSiO4 (LMS). LMS, which works at potentials 4.2 and ~4.5 V for first and second redox reactions respectively, seems to be the most useful from all of dilithiumorthosilicates family compounds. The main disadvantage of LMS is its extremely low electrical conductivity [2,5] and structural instability [7-11]. While the conductivity issues can be fixed by the preparation of nanograined material and its modification by carbon coating, similarly to the procedures applied in case of LiFePO4, the reasons of structural instability of LMS are not entirely known. There is a number of reports showing that Li2MnSiO4 cathode material undergoes amorphization in the first few charging/discharging cycles but there is no comprehensive explanation of this phenomenon. The degradation of crystalline structure can be caused by Jahn–Taller distortion associated with changes in lattice parameters during Mn3+→Mn4+ transition [8]. Another explanation may be the occurrence of secondary reactions between electrolyte and delithated forms of lithium manganese silicate (LiMnSiO4 and MnSiO4). The aforementioned structural changes of the material entail variations in electrochemical properties of Li2MnSiO4[9]. The goal of this studies is to examine structural instability of Li2MnSiO4which occurs in first electrochemical charging/discharging cycle via microscopic analysis of individual grains at different stages of electrochemical reaction. The Li2MnSiO4material was prepared using Pechini's sol–gel method which was previously reported by our group [9-11]. The structural changes in the material was studied using in-situ X-Ray diffraction as well as ex-situ transmission electron microscopy. To determine degradation mechanism of Li2MnSiO4 and C/Li2MnSiO4 in working Li-ion cell two different aspects of LMS performance were analyzed. Firstly corrosive properties of liquid electrolytes were taken under consideration. Solubility of manganese from LMS and C/LMS samples (occurring during electrochemical reaction) in LiPF6solution in EC:DMC (1:1) was studied. On the other hand using in-situ XRD and ex-situ TEM analysis in different SOC (in initial cycle in 1.5-4.8V potential range) the process of amorphization of crystalline LMS was examined. ACKNOWLEDGMENT This work is supported by National Science Centre, Poland under research grant no. 2014/13/B/ST5/04531. REFERENCES Padhi A.K., Nanjundaswamy K.S., Goodenough J.B. J. Electrochem. Society 144 (1997) 1188-1194.M.E. Arroyo-de Dompablo, M. Armand, J.M. Tarascon, U. Amator, Electrochem. Commun. 8 (2006) 1292–1298.A. Nyten, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun. 7 (2005) 156–160.Z.L. Gong, Y.X. Li, Y. Yang, J. Power Sources 174 (2007) 524–527.A. Kokalj, R. Dominko, G. Mali, A. Meden, M. Gaberscek, J. Jamnik, Chem. Mater. 19 (2007) 3633–3640.R. Dominko, M. Bele, M. Gaberscek, A. Meden, M. Remskar, J. Jamnik, Electrochem. Commun. 8 (2006) 217–222.R. Dominko, M. Bele, A. Kokalj, M. Gaberscek, J. Jamnik, J. Power Sources 174 (2007) 457–461.Y.-X. Li, Z.-L. Gong, Y. Yang, J. Power Sources 174 (2007) 528–532.M. Świętosławski, M. Molenda, M. Grabowska, A.Wach, P. Kuśtrowski, R. Dziembaj, Solid State Ionics 263 (2014) 99–102M. Molenda, M. Świętosławski, A. Rafalska-Łasocha, R. Dziembaj, Funct. Mater. Lett. 4 (2011) 135–138.M. Świętosławski, M.Molenda, K. Furczoń, R. Dziembaj, J. Power Sources 244 (2013) 510–514.

The role of stress relaxation and creep during high temperature deformation in Ni-base single crystal superalloys – Implications to strain build-up during directional solidification

The study of high temperature deformation of Ni-base superalloys finds a number of applications. In this study we consider how stress/strain resulting from directional solidification can be alleviated through phenomena such as relaxation and how this can be measured. Based on a modelling study, a range of stresses between 550 and 650 MPa were chosen for tensile testing at 900 °C in the Ni-base superalloy, CMSX4. In-situ neutron diffractometry together with ex-situ transmission electron microscopy have been used to study relaxation occurring during high-temperature deformation. The behaviour of (100) and (200) lattice strains with macroscopic stress has been quantitatively analysed for single crystals with axial orientations within 5° from [100]. The (200) γ+γ/ fundamental peak has been used to relate the decay in applied macroscopic stress with lattice strain. At high stresses (above 600 MPa), relaxation was particularly pronounced with an immediate appreciable decay in lattice strain (and stress) within 20 min. At lower stresses, significant relaxation is only observed after prolonged hold. Relaxation occurs in both γ and γ/, as confirmed by presence of dislocations within both phases and it was also with minimal lattice rotation (<3°). Also, the decrease in lattice strain from relaxation was at least two/threefold lower than the creep strain, obtained from sample elongation. It is shown that in modelling of strain during solidification, it is important to consider the relaxation of lattice strain, rather than just creep. Implications of this study to the critical plastic strain for re-crystallisation are addressed.

Activity, Stability and Degradation of Carbon Supported Palladium (Pd/C) Fuel Cell Electrocatalysts for the Oxygen Reduction

In this experimental study, we determine the size dependent activity as well as accelerated voltage cycling stability of various carbon supported palladium electrocatalysts for the oxygen reduction in acidic medium. Furthermore, ex-situ transmission electron microscopy studies before and after accelerated voltage cycles provide a deeper understanding regarding particle stability during operating conditions. Regarding oxygen reduction, a particle size effect on the specific activity is observed, with bulky Pd-black (≈4 m2/gPd) exhibiting ≈x6 times higher activity than Pd supported on Vulcan carbon (≈190 m2/gPd). Mass activities, however exhibit a strong correlation with catalyst surface area at small values, but are observed to be weakly dependent only between ≈50-200 m2/gPd. As stability tests during voltage cycling reveal a benefit for smaller surface areas, i.e. bigger particles, a limited gain in stability can be achieved by increasing catalyst particle size at a negligible cost of electrocatalytic mass-based activity.