Co-rich Regions Research Articles

Probing degradation at solid-state battery interfaces using machine-learning interatomic potential

Solid-state batteries featuring fast ion-conducting solid electrolytes are promising next-generation energy storage technologies, yet challenges remain for practical deployment due to electro-chemo-mechanical instabilities at solid-solid interfaces. These interfaces, which include homogeneous/internal interfaces such as grain boundaries (GBs) and heterogeneous/external interfaces between solid-electrolyte and electrode materials, can impede Li-ion transport, deteriorate performance, and eventually lead to cell failure. Here we leverage large-scale molecular simulations, enabled by validated machine-learning interatomic potentials, to directly probe the onset of interfacial degradation at the garnet Li7La3Zr2O12 (LLZO) solid-electrolyte/LiCoO2 (LCO) cathode interface. By surveying different interfacial geometries and compositions, it is found that Li-deficient interfaces can lead to severe interfacial disordering with cation mixing and Co interdiffusion from LCO into LLZO. By contrast, Li-sufficient interfaces are less disordered, although elemental segregation with local ordering is observed. As a consequence of Co interdiffusion, Co-rich regions are formed at the GBs of LLZO due to cation segregation and trapping effects. This behavior is independent of the GB tilting axis, degree of disorder at the GBs, and Co concentration, which implies Co clustering at GBs is a general phenomenon in polycrystalline LLZO and can dictate its overall transport and mechanical properties. Our findings elucidate the underlying fundamental mechanisms that give rise to experimentally observed physicochemical properties and provide guidelines for interface design that can mitigate interfacial degradation and improve cycling performance.

Effect of local chemical order on monovacancy diffusion in CoNiCrFe high-entropy alloy

High-entropy alloys (HEAs) are promising materials for nuclear applications. Understanding the influence of local chemical order (LCO) on defect diffusion and evolution in these alloys is crucial for enhancing their resistance to radiation damage. In this study, we used molecular dynamics simulation to investigate the effect of LCO on monovacancy diffusion in CoNiCrFe HEA. Alongside the Warren-Cowley parameter, which quantifies the degree of local elemental ordering, we propose a new parameter to characterize the spatial scale of LCOs. Based on their degree, LCO structures have been classified as either chemical medium-range order (CMRO) or chemical short-range order (CSRO). Our study reveals a non-monotonic variation in vacancy diffusion coefficients, transitioning from random solid solution to CSRO and then to CMRO structures. Moreover, we observe a preference for vacancy diffusion in low-energy Fe, Co-rich regions, with their spatial distribution and spatial connectivity significantly influencing the vacancy diffusion. Due to the spatial scale of the inhomogeneity introduced by LCO, both global average diffusion parameters and single migration barriers cannot fully reflect the actual diffusion dynamics. Therefore, our study emphasizes the importance of understanding the preferred diffusion pathways and the associated energy landscapes to fully assess the defect diffusion dynamics in HEAs. This deeper investigation into localized diffusion behaviors influenced by LCO is crucial for evaluating and enhancing the radiation damage tolerance of HEAs.

Properties of molecular clumps and cores in colliding magnetized flows

ABSTRACT We simulate the formation of molecular clouds in colliding flows of warm neutral medium with the adaptive mesh refinement code flash in eight simulations with varying initial magnetic field strength, between 0.01–5 μG. We include a chemical network to treat heating and cooling and to follow the formation of molecular gas. The initial magnetic field strength influences the fragmentation of the forming cloud because it prohibits motions perpendicular to the field direction and hence impacts the formation of large-scale filamentary structures. Molecular clump and core formation occurs anyhow. We identify 3D clumps and 3D cores, which are defined as connected, CO-rich regions. Additionally, 3D cores are heavily shielded. While we do not claim those 3D objects to be directly comparable to observations, this enables us to analyse their full virial state. With increasing field strength, we find more fragments with a smaller average mass; yet the dynamics of the forming clumps and cores only weakly depends on the initial magnetic field strength. The molecular clumps are mostly unbound, probably transient objects, which are weakly confined by ram pressure or thermal pressure, indicating that they are swept up by the turbulent flow. They experience significant fluctuations in the mass flux through their surface, such that the Eulerian reference frame shows a dominant time-dependent term due to their indistinct nature. We define the cores to encompass highly shielded molecular gas. Most cores are in gravitational-kinetic equipartition and are well described by the common virial parameter $\alpha _\mathrm{vir}$, while some undergo minor dispersion by kinetic surface effects.

Material extrusion additive manufacturing of WC-9Co cemented carbide

Additive manufacturing (AM) cemented carbide is primarily challenged by critical issues of difficult-to-eliminate pores, cracks, deleterious phases, and non-uniform microstructure, which result in poor mechanical properties. This study utilizes material extrusion (MEX) AM technology to prepare a WC-9Co cemented carbide. The results show that green body printing and solvent debinding process significantly affect its stacked voids, interlayer bonding defects, and debound cracks. These green body defects are eliminated through liquid phase flow and WC particles flow-rearrangement during sintering, resulting in Co-rich regions or Co pools, abnormal growth of WC grains, and residual pores. A green body free of visible defects is prepared using a feedstock with a powder loading of 54 vol%, a printing temperature of 150 ℃, a microfilament overlap ratio of 30 %, and a printing-layer thickness of 0.1 mm. Followed by a two-step solvent debinding and continuous thermal debinding-vacuum-pressure sintering, a nearly fully dense WC-9Co cemented carbide is fabricated, with a uniform microstructure of two phases and free of pores, cracks, and Co pools. Its Vickers hardness, transverse rupture strength, and fracture toughness reach 1525±3 HV30, 3492±45 MPa, and 20.4±0.5 MPa·m1/2, respectively, which are significantly better than those of similar reports and could be comparable to similar cemented carbides prepared by powder metallurgy. This study could provide a new technological path for developing high-performance cemented carbide complex-structured components.

Influences of Cr contents on oxidation behavior of WC-Co-Ni-Cr cemented carbides at 900 °C

Modification of binder composition is an effective way to improve the oxidation resistance of WC-Co-Ni-Cr cemented carbide. Since the adjustment of Ni and Cr contents in the Co-Ni-Cr binder alters the oxidation behavior and oxidation products, WC-Co-Ni-Cr cemented carbide is expected to achieve excellent oxidation resistance in severe service conditions. In this paper, the influences of Cr contents in Co-Ni-Cr binder phase with a fixed total amount of Co and Ni on the oxidation behavior of WC-Co-Ni-Cr cemented carbides at 900 °C was investigated. The oxide morphology and phase distribution were focused. The oxidation mechanism of WC-Co-Ni-Cr cemented carbide was discussed. It has been shown that Co-rich regions are preferentially oxidized in the initial stage of oxidation. As oxidation proceeds (60 min), the dominant oxidation product on the alloy surface is NiO. The oxidation resistance of the WC-Co-Ni-Cr cemented carbide increases with increasing Cr content in the binder. Compared to the series of WC-2Co-11Ni-xCr, WC-6.5Co-6.5Ni-xCr presents a better overall oxidation resistance due to the formation of Cr2O5 in the middle of the oxide layer.

Functional group inhomogeneity in graphene oxide using correlative absorption spectroscopy

Graphene oxide is a promising 2D material for industrial applications. However, understanding its hybrid electrical properties that result from different functional groups remains a fundamentally open question: experimental characterization of the electronic configuration contains convolved information, and identifying functional groups by classical approaches cannot adequately describe these groups at the nanoscale. Here, we correlate infrared and ultraviolet–visible absorption spectroscopy using imaging techniques. The CH and COrich regions are identified through infrared imaging, and their electronic behavior is analyzed via the ultraviolet–visible absorption spectra from the same region. We confirmed through correlation spectroscopy that CH and CO rich regions absorb relatively more at 280 nm (4.43 eV) and 380 nm (3.26 eV), respectively. These correlations are confirmed by analyzing several graphene oxide flakes and validated using density functional theory calculation.

Structural and magnetic properties in the Heusler compounds Co3−x Fe x Al thin films

The structural and magnetic properties are important parameters for spintronic applications of Heusler compounds. The Co2FeAl and CoFe2Al alloys are considered as the representatives of the regular and inverse Heusler structure, respectively. Here, we present a systematic study of the structural and magnetic properties of Co3−x Fe x Al (x = 1 ∼ 2) Heusler thin films grown on MgO (001) substrates. The lattice parameters and magnetic properties, such as magnetic coercivity, saturated moment and four-fold magnetic anisotropy constant, display different change trends in the Co-rich and Fe-rich regions, which suggest that magnetic properties have relevance to the Heusler structures. Our findings give a better understanding of the Heusler structures and magnetic properties, which is helpful for developing spintronic applications.

On the mechanism of crater wear in a high strength metastable β titanium alloy

The primary wear mechanism of WC-Co tools when machining aerospace titanium alloy components is crater wear. The diffusion controlled dissolution and thus the crater wear rate is more apparent when machining the high strength, fatigue resistant metastable β titanium alloys: an expensive class of titanium alloy used in aerostructural components such as twin aisle aircraft landing gear parts. To characterise the mechanistic process of crater wear progression, outer diameter turning trials were conducted on the metastable β alloy, Ti-5553. Uncoated WC-Co inserts were used for time steps of 30, 60 and 900 s at a cutting speed of 70 m/min. The crater region was analysed using high-resolution SEM, X-EDS, WDS and STEM/TEM. Such targeted characterisation enabled the detailed mechanism of progressive crater wear in titanium machining to be further understood. In the crater wear process, rapid diffusion of Co into adhered Ti-5553 occurs within the crater zone, proceeded by decarburisation of the WC (hcp). Thermodynamic modelling and high-resolution imaging of the interface suggests Co facilitates the formation of Ti2Co, and possibly a liquid, within prior Co rich regions at WC grain boundaries. Subsequently, this creates regions that exhibit similar characteristics to a Kirkendall morphology due to accelerated C depletion and W–Ti dissolution. The formation of abrasive titanium carbides in the workpiece material increases as C is made available from the WC transforming to W (bcc) and through binder regions. Once the W (bcc) layer is formed the rapid decarburisation process is concentrated at prior Co-rich (heavily β stabilised) Ti regions, this degradation dictates the crater wear rate during the machining of Ti-5553. Now an understanding of the crater wear mechanism has been characterised for Ti-5553, tooling manufacturers can engineer the Co-binder volume fraction and distribution, to improve tool life and provide significant cost savings when machining metastable β titanium alloy components.

Performance and wear mechanisms of uncoated cemented carbide cutting tools in Ti6Al4V machining

The primary tool material when machining Ti6Al4V titanium alloy is uncoated straight cemented carbide. This study examines the performance of these materials during high-speed finish turning, and uses the advanced microscopy methods of SEM, (S)TEM, XEDS, and SAED to explore the fundamental tool wear mechanisms. The wear modes include a combination of flank wear and rake cratering. Four individual TEM lamellae were extracted from the crater and flank of one as-worn tool to investigate the wear mechanisms of cemented carbide exposed to different temperatures and contact conditions. The main wear mechanism identified is temperature-driven diffusion. Outward carbon diffusion occurs from surface WC grains into the adhered Ti alloy, which results in a layer of metallic tungsten. Dissolution of the W layer leads to doping of the α-Ti, thus causing its transformation into the β-Ti phase. At the same time, carbon-depleted WC grains interact with the Co binder, inducing formation of Co3W. Additional wear mechanisms include inward titanium diffusion, resulting in formation of TiC on both sides of the W layer. Simultaneously, TiCo2 is formed in Co-rich regions in the vicinity of the tool-chip interface. These reaction products retard direct dissolution of tool material in Ti6Al4V, thus acting as localized tool protection layers.

Morphology and Interconnected Microstructure-Driven High-Rate Capability of Li-Rich Layered Oxide Cathodes.

A Li-rich layered oxide (LLO) cathode with morphology-dependent electrochemical performance with the composition Li1.23Mn0.538Ni0.117Co0.114O2 in three different microstructural forms, namely, randomly shaped particles, platelets, and nanofibers, is synthesized through the solid-state reaction (SSR-LLO), hydrothermal method (HT-LLO), and electrospinning process (ES-LLO), respectively. Even though the cathodes possess different morphologies, structurally they are identical. The elemental dispersion studies using energy-dispersive X-ray spectroscopy mapping in scanning transmission electron microscopy show uniform distribution of elements. However, SSR-LLO and ES-LLO nanofibers show slight Co-rich regions. The electrochemical studies of LLO cathodes are evaluated in terms of charging/discharging, C-rate capability, and cyclic stability performances. A high reversible capacity of 275 mA h g-1 is achieved in the fibrous LLO cathode which also demonstrates good high-rate capability (80 mA h g-1 at 10 C-rate). These capacities and rate capabilities are superior to those of SSR-LLO [210.5 mA h g-1 (0.1 C-rate) and 4 mA h g-1 (3 C-rate)] and HT-LLO [242 mA h g-1 (0.1 C-rate) and 22 mA h g-1 (10 C-rate)] cathodes. The ES-LLO cathode exhibits 88% capacity retention after 100 cycles at 1 C-rate. A decrease in voltage on cycling is found to be common in all three cathodes; however, minimal voltage decay and capacity loss are observed in ES-LLO upon cycling. Well-connected small LLO particles constituting fibrous microstructural forms in ES-LLO provide an enhanced electrolyte/cathode interfacial area and reduced diffusion path length for Li+. This, in turn, facilitates superior electrochemical performance of the electrospun Co-low LLO cathode suitable for quick charge battery applications.

WITHDRAWN: Effect of heat treatment on microstructural evolution and properties of cemented carbides (WC-17Co) processed by selective laser sintering

Abstract Processing materials such as cemented carbides (WC-Co) using metal-based Additive Manufacturing (AM) presents many challenges due to the material and complex mechanisms that occur during processing. Powder composition, processing parameters and post-processing treatments dictate the microstructural integrity and mechanical properties of the processed parts. In this study, WC-17Co cemented carbides were processed using Selective Laser Sintering (SLS) and heat treated at 400 °C, 600 °C, 800 °C and 1000 °C for 3 h to understand the effect of processing and post-processing heat treatment on the structure and properties of the WC-17Co cemented carbide. Electron microscopy and X-ray diffraction (XRD) analysis revealed that the microstructure of the as-printed specimen was characterized by relatively large polygonal WC/W2C chips, WC-Co dendritic structures, WC-Co “foggy” and Co-rich regions. During heat treatment between 0 °C and 600 °C, the large polygonal chips disintegrated to smaller polygonal chips as a result of the conversion of the unstable W2C phase to the more stable WC phase. Heat treatment above 600 °C resulted in the coalescence and growth of relatively large WC phase chips. In addition, globular WC-Co dendritic structures evolved with increase in temperature coupled with a reduction in the volume fraction of the Co-rich regions. There was significant increase in hardness of the samples during heat treatment when compared with the as-printed sample, with the sample heat-treated at 600 °C being 36% harder than the as-printed sample due to the breakdown of polygonal WC chips and the increase in volume fraction and spatial distribution of the observed “foggy” regions. The increase in hardness at 600 °C was coupled with the highest fracture toughness, representing a 34% increase in fracture toughness, when compared with the as-printed sample. The high fracture toughness is attributed to the evolution of the ductile W6Co6C phase in the sample after heat treatment. Nevertheless, the as-printed sample had approximately 15% higher wear resistance than the sample heat-treated at 600 °C. It is concluded that post-processing heat treatment of SLS printed WC-17Co alloy at 600 °C can be used to improve the structure and mechanical properties of the alloy.

Structural and Magnetic Properties of Ba1−xRexCo2ZnxFe16−xO27 W-Type Hexaferrites Prepared by Sol–Gel Auto-Combustion

Ba1−xRexCo2ZnxFe16−xO27 (Re = La, Nd, Pr; x = 0.0, 0.1, 0.2) hexaferrites were prepared by a sol–gel auto-combustion method and sintering at 1250 °C for 2 h. The effects of rare-earth (RE) substitution on the structural and magnetic properties of the hexaferrites were investigated. XRD patterns of the samples revealed crystallization of pure W-type hexaferrite phase in all samples, except for Nd–Zn (x = 0.2) sample, which contained a minor M-type phase in addition to the major W-type phase. The saturation magnetization did not change appreciably with RE–Zn substitution, although the magnetic measurements revealed slight improvement with Pr–Zn substitution. However, the magnetocrystalline anisotropy field (Ha) increased significantly with RE–Zn substitution, recording a maximum increase up to 9.23 kOe compared with 6.30 kOe for the unsubstituted sample. The results indicated the possibility of tuning the anisotropy field, and consequently, the ferromagnetic resonance frequency, to frequency ranges appropriate for desired microwave applications. The thermomagnetic curves revealed spin reorientation transitions below 300 °C, and magnetic phase transition in the range at 461–481 °C, which was associated with the Curie temperature of the W phase. In addition, the weak magnetic phase transition in the temperature range of 505–516 °C was associated with Co-rich impurity magnetic phase, with enhanced superexchange interactions in Co-rich regions.

Microstructural evolution and resulting properties of differently sintered and heat-treated binder-jet 3D-printed Stellite 6

Stellite 6 components are manufactured from gas-atomized powder using binder-jet 3D-printing (BJ3DP) followed by curing and sintering steps for densification. Green parts are sintered at temperatures ranging from 1260 °C to 1310 °C for 1 h. Microstructural evolution and phase formation during sintering and aging are studied by optical and scanning electron microscopy, elemental analysis and X-ray diffraction. It was found that solid-state sintering was present at temperatures below 1280 °C with Cr-rich carbides present within grains; while supersolidus liquid phase sintering was the dominant sintering mechanism during sintering at 1290 °C and higher in which the Co-rich solid solution regions are surrounded by eutectic carbides. Sintering at 1300 °C resulted in the maximum density of ~99.8%, mean grain size of ~98 ± 6 μm with an average hardness of 307 ± 15 HV0.1 and 484 ± 30 HV0.1 within grain and at the boundaries, respectively. Aging was performed at 900 °C for 10 h leading to the martensitic transformation (fcc → hcp) as well as an increase in eutectic carbides at boundaries and nano-sized carbides within grains where the average hardness within grains and boundaries was enhanced to 322 ± 29 HV0.1 and 491 ± 58 HV0.1, respectively. Fibroblasts seeded on top of 3D-printed Stellite 6 discs displayed a cell viability of 98.8% ± 0.2% after 48 h, which confirmed that these materials are non-cytotoxic. Presented results demonstrate that binder jetting can produce mechanically sound complex-shaped structures as shown here on a denture metal framework and small-scale knee model.

Structural characteristics of 98-atom Co-Pt clusters

The well-mixed morphology of 98-atom Co-Pt clusters is investigated. The global minimal structures of ComPt98-m (m = 1–97) clusters are optimized by adaptive immune optimization algorithm with inner cores (AIOA-IC) method based on the many-body Gupta potential. In Co-poor regions, the clusters are found to adopt face-centered cubic (fcc) and stacking-fault fcc (sf-fcc) motifs while in Co-rich regions, they present icosahedral motifs. Leary tetrahedron (LT) appears to be the dominant motif among all compositions. A Td point group is found in Co80Pt18 cluster, and the construction method for the Td LT is proposed starting from a regular tetrahedron. Moreover, the Co-Pt clusters favor well-mixed pattern except for Co-rich and Co-poor regions and the highest degree of mixing is at Co55Pt43, which is one of the 14 amorphous motifs but not LT motif. The phenomenon of atomic rearrangement is also reported.

Curie temperature study of and systems using mean field theory and Monte Carlo method

Cubic Laves phases including , , , and are considered as promising candidates for application in hydrogen storage and magnetic refrigeration. While and are ferromagnets, alloying with Co decreases magnetic moments and Curie temperatures (TC) of pseudobinary and systems, leading to the paramagnetic states of and . The following study focuses on the investigation of Curie temperature of the and system from first principles. To do it, Monte Carlo (MC) simulations and the mean field theory (MFT) based on the disordered local moments (DLM) calculations are used. The DLM-MFT results agree qualitatively with the experimental data from the literature and preserve the characteristic features of dependencies for both and . However, we have encountered complications in the Co-rich regions due to failure of the local density approximation (LDA) in describing the Co magnetic moment in the DLM state. The analysis of Fe–Fe exchange couplings for and phases indicates that the nearest-neighbor interactions play the main role in the formation of .

Cross-sectioning spatio-temporal Co-In electrodeposits: Disclosing a magnetically-patterned nanolaminated structure

Micrometer-thick cobalt-indium (Co-In) films consisting of self-assembled layers parallel to the cathode plane, and with a periodicity of 175±25nm, were fabricated by electrodeposition at a constant current density. These films, which exhibit spatio-temporal patterns on the surface, grow following a layer-by-layer mode. Films cross-sections were characterized by electron microscopies and electron energy loss spectroscopy. Results indicate the spontaneous formation of nanolayers that span the whole deposit thickness. A columnar structure was revealed inside each individual nanolayer which, in turn, was composed of well-distinguished In- and Co-rich regions. Due to the dissimilar magnetic character of these regions, a periodic magnetic nanopatterning was observed in the cross-sectioned films, as shown by magnetic force microscopy studies.

Fabrication and Magnetic Properties of Mn x B45Co100−x Alloys

Mn55−x B45Co x bulk alloys with x ranging from 0 to 30 were fabricated by an arc melting technique. Structural properties were investigated by XRD and SEM analysis revealing that Co replaces Mn atoms as x amount increases in the alloy and samples contain two phases with Mn-rich or Co-rich regions. Magnetic properties were characterized by field dependence (M-H) and temperature dependence of magnetization (M-T) measurements using a vibrating sample magnetometer (VSM). Results showed that as x increases from 0 to 30, saturation magnetization decreases from 130 to 53 emu/g. In addition, an increase of the Co content leads a decrease of Curie temperature from 565 to 412 K, enabling the Curie temperature tuning by Co addition.

Wear of a high cBN content PCBN cutting tool during hard milling of powder metallurgy cold work tool steels

The wear characteristics of a high cBN content PCBN cutting tool during hard milling of two different hardened cold work tool steels have been evaluated. Post-cutting examination of the worn cutting inserts was performed using high resolution field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy, Auger electron spectroscopy and optical surface profilometry. Also, the machined work material surfaces and collected chips were characterized in order to evaluate the prevailing wear mechanisms.The results show that both flank and crater wear are controlled by continuous wear due to tribochemical reactions, adhesive wear and mild abrasive wear. Besides, the cutting inserts show a tendency to micro-chipping along the cutting edge especially at higher cutting speed. The latter mechanism was also found to be dependent on type of work material. High lateral resolution Auger electron spectroscopy of the crater region shows that the worn surface is covered by a thin SixOy rich tribofilm with a thickness of 50–500nm, the tribofilm being thicker on the binder phase regions. Also, the Co-rich regions of the binder phase seem to be more tribochemically affected by the prevailing contact conditions as compared with the W-rich regions of the binder phase and the cBN phase.

Microstructure, chemical states, and mechanical properties of V–C–Co coatings prepared by non-reactive magnetron sputtering

V–C–Co coatings have been prepared by non-reactive magnetron co-sputtering from VC and Co targets. The microstructure, chemical states, and mechanical properties are examined as a function of Co content in the coatings. The coatings are dense, with columnar growth structures. High resolution transmission electron microscopy (HRTEM) studies identify a nanocomposite microstructure for the 12.4at.% Co coating, in which ligament-like Co-rich regions partially separate the nanocrystalline VC grains. X-ray photoelectron spectroscopy studies reveal a noticeable charge transfer from Co 2p states to C 1s states. This charge transfer, in addition to the ligament-like Co-rich regions as revealed by HRTEM, points to the formation of a strong Co/VC interface. The nanoindentation hardness of the coatings drops steadily with the Co content, from 29GPa for pure VC to ~21GPa for the 12.4at.% Co coating. Meanwhile, the plasticity characteristic increased from 0.42 to 0.53.

Influence of the surface structure on the magnetic properties of Zn1−xCoxO

The surface of ferromagnetic Zn1−xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. We found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, we show that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.