Diffusion Of Cobalt Research Articles

Enhancing performance and thermal stability in GeTe thermoelectric joints with cobalt diffusion barrier

This study investigates the influence on thermoelectric properties of p-type GeTe after depositing cobalt (Co) diffusion barrier, and its ability on inhibiting the interfacial reactions that degrades thermoelectric performance of the joint at medium temperatures. Establishing stable interconnections within the joint is essential for ensuring the reliability of the device. By examining the improved thermoelectric properties of GeTe, the research highlights a discovering that the addition of a Co diffusion barrier significantly elevates figure of merit (zT) values of GeTe. Electron probe microanalysis (EPMA) reveals the diffusion of Co into GeTe and the formation of Co–Te intermetallic compounds. As the device ages, there is a substantial increase in contact resistivity at the GeTe joint. After depositing a Co layer and subjecting it to long-term aging, electrical conductivity improves, and thermal conductivity decreases. This improvement occurs because the Co atoms, which have diffused into the GeTe lattice, occupy Ge vacancy sites. Instead of degrading the zT value, Co deposition leads to a 56 % increase in the zT value for the p-type GeTe joint. This study emphasizes the improvement of the zT value rather than pursuing a high peak zT value for GeTe. These results affirm the role of Co as an effective diffusion barrier, contributing to the stability of the joint interface and the enhancement of the thermoelectric properties of GeTe material, offering a viable route for creating more efficient and durable thermoelectric energy conversion devices.

Intermixing of iron and cobalt with oxygen-rich magnesium oxide in CoFeB/MgO/CoFeB magnetic tunneling junctions

Atomic-scale spectroscopic imaging of sputtered magnetic tunnel junction structures with a thick oxygen-rich MgO barrier reveals the diffusion of iron and cobalt into the MgO barrier from CoFeB electrodes. First principles calculations are performed to (1) confirm that Fe diffusion through Mg vacancies is energetically favorable, (2) quantify the reduction of interfacial perpendicular magnetic anisotropy due to Fe diffusion into MgO, and (3) predict that the presence of Fe impurities in MgO causes an increased leakage and a tunneling magnetoresistance decrease. Through the chemical shift of the Fe L3 edge and the peak ratio Fe L3/Fe L2 measured by electron energy loss spectroscopy, we suggest that, within MgO, iron with mixed oxidation state Fe2+ and Fe3+ or higher is found in the as-grown structure, which is reduced by annealing to Fe2+. These results indicate that the stoichiometry of as-deposited MgO barrier layers plays an important role in controlling the microstructure and optimizing the performance of magnetic tunnel junctions.

An investigation into low-temperature HFCVD and derivative techniques for the enhancement of diamond-coated cemented carbide tool performance

The performance of diamond-coated cemented carbide tools manufactured via the conventional hot filament chemical vapor deposition (HFCVD) technique exhibits certain limitations, including issues with poor toughness and reduced adhesive strength of film-substrate systems, primarily attributed to the cobalt diffusion at elevated temperatures (> 800 °C). In this study, a pioneering exploration of a low-temperature diamond film deposition technique on cemented carbide is developed, and a comprehensive evaluation of the film properties is conducted. The results indicate that owing to constraints related to growth rate and diamond film purity, it may not serve as an effective approach for enhancing the performance of conventional coated tools. Nonetheless, the inherent capability of this technique in inhibiting cobalt diffusion is strategically harnessed to pioneer the development of a derivative technique referred to as the “low-temperature to high-temperature HFCVD (LT-HT-HFCVD)”. This innovative method leverages a low-temperature deposited diamond layer as an intermediate layer to impede cobalt diffusion while serving as a densely nucleated layer for subsequent diamond growth. Subsequently, the conventional technique is employed to facilitate an effective growth rate and maintain high growth purity. The diamond-coated tools generated by this technique are subject to a comprehensive comparative analysis against those generated by low-temperature HFCVD (LT-HFCVD) and conventional HFCVD (CT-HFCVD) techniques. The results reveal that tools fabricated using the LT-HT-HFCVD combine the advantages inherent to both the LT-HFCVD and CT-HFCVD techniques, thereby manifesting superior toughness, enhanced film-substrate adhesive strength, and outstanding cutting performance.

Grain boundary diffusion and grain boundary phase transition in tungsten in the temperature range of activated sintering

Abstract Grain boundary self- and solute (cobalt) diffusion in tungsten was found [Lee et al., Scr. Metall, 1988; Lee et al., Col. de Physique, 1990] to exhibit discontinuities in the Arrhenius behavior at the homologous temperatures of 0.36 < T/T m < 0.4 that surprisingly match the activation sintering temperature of W (T m is the melting point). In the present work, this unusual grain boundary diffusion phenomenon is discussed in terms of a fundamental grain boundary phase transition in W. The experimental data are analysed with respect to predicted segregation-induced grain boundary phase transformation. Competing co-segregation of impurity elements (carbon and phosphor) might induce a discontinuous grain boundary segregation and invoke a grain boundary phase transition which modifies the grain boundary mobilities of substitutional atoms. The improved understanding of grain boundary phase transitions is expected to provide a breakthrough in interpreting the exact mechanism of W-activated sintering.

<em>In situ</em> study of mechanical properties and structural transformations during heating of WC–TaC–Co cemented carbides in a transmission electron microscope column

This study investigated the hardness of lamella with varying thickness, obtained from a massive, fine-grained cemented carbide comprising WC–6 %Co–0.2 %TaC, characterized by an average grain size of approximately 5 μm. The picoindentation method was employed for this analysis. Picoindentation was carried out using a Berkovich diamond indenter with a radius of curvature around 50 nm, and the experimental data were analyzed using the Oliver–Pharr model. The results revealed a significant correlation between hardness and lamella thickness. The hardness of the electron transparent section (thickness less than 100 nm) of the lamella measured 11.3±2.8 GPa, while the electron nontransparent section (thickness more than 200 nm) exhibited a hardness of 20.8±1.2 GPa. The lower hardness in electron transparent objects (thickness ~100 nm) is likely attributed to a combination of factors, including the potential bending of thin cobalt layers, the presence of edge effect, and closely spaced structural defect dislocations on the lamella surface. In situ TEM studies were conducted to examine structural transformations during the heating of WC–6 %Co–0.2 %TaC lamella, including in the presence of oxide phases (WOx ). Oxide phases on the lamella’s surface were generated by oxidizing the lamella at 200 °C in an air atmosphere. The results indicated that heating up to 500 °C did not bring about significant changes in the structure. However, at 600 °C, there was a notable thinning of cobalt layers due to intense surface diffusion of cobalt. Simultaneously, the formation of nanosized particles of the Co3W3C phase, ranging in size from 5 to 20 nm, was observed in the binder. These particles resulted from a shift in the equilibrium phase composition of the carbide, changing from a two phase region (WC + γ) to a three phase region (WC + γ + Co3W3C) as a consequence of the lamella’s oxidation.

Cobalt diffusion during the initial stage of CVD diamond growth on cemented carbide – A molecular dynamics and experimental study

Systematical researches on cobalt diffusion during the initial stage of diamond growth on cemented carbide (WC) are conducted by combining molecular dynamics (MD) and experimental investigations. Simulation results and characterizations indicate that temperature, WC grain size, initial cobalt distribution (including cobalt content and Co-removal depth after acid treatment), diamond crystal orientation, and the grain composition of polycrystalline diamond significantly affect cobalt diffusion. It is proven that cobalt tends to diffuse along grain boundaries in both cemented carbide and diamond, and the intricate nature of grain boundaries in smaller grain sizes results in higher activation energy for cobalt diffusion. However, due to the strong interaction between cobalt and parallel double six-membered ring structures at grain boundaries, cobalt diffusion to the diamond surface is exceedingly challenging. The accumulation of cobalt at the interface between the diamond film and substrate leads to graphitization, which ultimately deteriorates the adhesion of the diamond film during prolonged deposition. In conclusion, the reduction of cobalt diffusion in chemical vapor deposition (CVD) processes can be achieved by employing cemented carbide with smaller grain sizes and lower initial cobalt content. Additionally, decreasing the growth temperature or increasing the Co-removal depth significantly inhibits cobalt diffusion to the diamond film.

Optimized conditions for cobalt diffusion in Sri Lankan colorless topaz and coloration mechanism elucidation through spectro-chemical investigation

Most of natural topaz is colorless; thus, methods of color enhancement are widely used for coloring this mineral. Currently, blue color is obtained by cobalt diffusion due to drawbacks in existing coloration methods. In this study, optimum conditions suitable for Cobalt diffusion in Sri Lankan colorless topaz were investigated and coloration mechanism was elucidated. The diffusion agent was prepared by mixing CoCO3 with Na2CO3, CaCO3 and carbon powder and diffusion was carried-out by varying the temperature and soaking time. Chemical analysis, UV-Vis absorption spectrum, infrared absorption spectra, and Raman peaks of diffused and non-diffused topaz were tested. The results clearly indicated that the optimum condition for Co diffusion in Sri Lankan topaz is 950℃ for 11 h. The EPMA analysis showed that the Co concentration in the diffused sample varied from 0.001 wt% to 0.027 wt% while colorless topaz showed <0.001 wt%. The UV-Vis spectrum of Co diffused blue topaz gave three absorption peaks at 556, 588, and 627 nm corresponding to three spin-allowed electronic transitions of Co2+ ion in teterahedaral coordination. In case of Co diffused topaz, one additional new broader IR absorption peak was noticed around 6640 cm-1 presumably arising by optical transitions of 4A2 → 4T1 in Co2+ (4F). Our results lead to the conclusion that, blue color of the Co diffused topaz is arising by spin-allowed electronic transitions of Co2+ ions in tetrahedral site of topaz matrix through substitution of Si4+ ions.

Degradation of multi-layer CVD-coated cemented carbide in finish milling compacted graphite iron

Stricter regulations on emissions and higher demands on engine performance drive automotive industries to replace conventional gray cast iron with compacted graphite iron (CGI). CGI has a higher wear resistance, strength, elastic modulus, and almost double fatigue strength as to gray cast iron, yet the same positive properties make CGI more difficult to machine. This study focuses on finish milling CGI EN-GJV-450 with current industrial standard of tooling: cemented carbide with multi-layer CVD-coating of Ti(C,N)–Al2O3. At cutting speed of 150 m/min a total of 3126 cm3 of material after 190 passes was removed, while 1645 cm3 was removed after 100 passes in case of 250 m/min. Studying the wear evolution at different engagement times demonstrated an accelerated chemical wear of the Al2O3 top coating due to its reaction with MgO-containing inclusions that creates a softer reaction product of (Mg,Fe,Mn)Al2O4 spinel which is abraded and removed with the chip flow. Ti(C,N) coating layer experiences diffusional and mechanical (abrasion, debonding, micro-fracture) wear mechanisms. Exposure of cemented carbide substrate resulted in its rapid wear in the form of cratering and massive material loss on the flank where the latter eventually causes tool failure. The adherence of CGI to WC-Co facilitates diffusional loss of carbon and tungsten from WC grains. Carbon reacts with iron at the interface forming iron carbide, while residual tungsten alloys it thus forming (Fex,W1-x)3C. This phase can reduce the wear rate as it acts as a diffusion barrier, but its high brittleness enables its periodic removal by adhesive wear. Outward cobalt diffusion was also an active wear mechanism that facilitates further diffusion of carbon and tungsten but also weakens the WC-grain bonding, further facilitating adhesive wear by CGI flow. Deposition of oxide inclusions to exposed WC-Co work as an anti-stick for CGI adhesion and thus reduces the wear rate.

Experimental and clinical study of the concentrations of chemical elements in oral fluid from prostheses and implants

BACKGROUND: Electrochemical reactions can occur in the mouth of patients with prostheses made of metal alloys. The phenomena of galvanism reflect the hidden process of corrosion of structural alloys in oral fluid, which makes it necessary to analyze the chemical contents of prostheses under clinical conditions in patients with implants and experimental conditions.
 AIM: Comparative analysis of the trace element content in the oral fluid from prostheses in implants in the clinic and an experiment.
 MATERIALS AND METHODS: National Research Technological University MISIS analyzed artificial saliva after prostheses were in contact with grade 4 and grade 5 implants for 3 months. Titanium, cobalt, chromium, aluminum, vanadium, molybdenum, tungsten, manganese, nickel, and iron were determined by spectrometry. The same chemical elements were analyzed using an identical spectrometric method in 32 patients with fixed dentures on implants.
 RESULTS: Under the experimental conditions, nine trace elements were found in the container around one grade 4 implant with a crown, including titanium (0.4 g/l), cobalt (3.8 g/l), chromium (0.5 g/l), aluminum (2.6 g/l), vanadium (0.6 g/l), molybdenum (1.5 g/l), tungsten (3.0 g/l), manganese (0.9 g/l), nickel (0.5 g/l), and iron (0.6 g/l). The grade 5 implant was characterized by a greater degree of diffusion of aluminum, nickel, and manganese into artificial saliva. Mass spectrometry of the oral fluid in patients with dental implants revealed more trace elements than experimental data. The difference in the concentration of microelements from the prostheses under clinical conditions and in the experiment ranged from 0 to 128 times the minimum.
 CONCLUSION: Fixing experimental prostheses on implants for 3 months in artificial saliva was accompanied by the diffusion of microelements from the prostheses and implants into the environment, particularly cobalt, aluminum, and vanadium. Prostheses on implants with a lower titanium content are characterized by a greater degree of diffusion of aluminum, cobalt, nickel, and manganese. More significant diffusion of microelements occurred from the prostheses into the saliva under clinical conditions, according to the spectrometric data.

Enhancement of opto-electrical properties in Co doped CdS–TiO2 nanocomposite thin film as photoanode for Semiconductor Sensitized Solar Cells (SSSCs)

Cobalt doped CdS–TiO 2 nanocomposite thin films were coated on glass substrates by bottom up approaching methods. ZnS nanolayer over cobalt doped CdS–TiO 2 films act as a protective layer of CdS nanocrystal which enhances the photo stability. The structural, optical, electrical and surface morphological properties of ZnS-cobalt CdS–TiO 2 composite thin film were systematically evaluated. Structural analysis indicates that Co 2+ ions have been successfully incorporated in CdS site. The doping of cobalt in ZnS–CdS–TiO 2 film gives rise to an increase in the lattice parameter while the band gap decreased with increasing cobalt concentration. The bandgap of doped film decreases indicating the easy way of electron transfer from valence band to conduction band of CdS and TiO 2 . Surface morphological studies shows the diffusion of cobalt doped CdS nanoparticles into TiO 2 nanoporous structure of the composite film . The emission peaks in photoluminescence spectrum of ZnS-cobalt CdS–TiO 2 film proved the clear interaction of cobalt dopant with CdS nanocrystals. An enhancement in conductivity was observed due to incorporation of cobalt in CdS site and the resistance value (∼47%) was significantly decreased for cobalt doped CdS under illumination condition. Electrical analysis proved that the doped film is preferred as conductive electrode in solar cells than undoped film. • Cobalt doped CdS–TiO 2 composite thin films were synthesized by simple wet chemical method. • Interlocked CdS nanocrystals observed in SEM analysis, facilitate easy penetration into TiO 2 nanoporous structure of the composite film. • The doped film exhibit shifting of absorption edge towards longer wavelength (red shift), indicates decreased optical band gap. • The doped film shows superior electrical performance due to decreased resistance, preferred as conductive electrode in solar cells.

The Interdiffusion Behavior of NiCoCrAlYHf Coating Deposited by Arc Ion Plating on Carburized Ni-Based Single Crystal Superalloy.

In the present study, arc ion plating (AIP) was used to prepare a NiCoCrAlYHf coating (HY5 coating) on a carburized third-generation single-crystal superalloy DD10. The interdiffusion behavior of the carburized superalloy with an HY5 coating was investigated for a 1000 h oxidation time at 1100 °C. Carburization enhanced the interfacial bonding force and improved the microstructure of the NiCoCrAlYHf coating. An interdiffusion zone (IDZ) formed after a 300 h oxidation time, and the formation of a carburized layer effectively suppressed an inward diffusion of cobalt, aluminium, and chromium to the DD10 superalloy as well as an outward diffusion of nickel and refractory elements for instance rhenium and tungsten to the HY5 coating that occurred in static air at 1100 °C. The roles of the carburized layer in affecting thermal cyclic oxidation and element interdiffusion were studied. Subsequently, a modified form of the Boltzmann–Matano analysis was used to present the interdiffusion coefficients of aluminium.

(Invited) In Situ Study on Nucleation and Growth of Single-Walled Carbon Nanotubes on Catalysts

Understanding the growth mechanism is prerequisite to design the methodology for controlled growth of single-walled carbon nanotubes (SWNTs). By using an environmental aberration-corrected transmission electron microscope in combination with in situ synchrotron X-ray absorption spectroscopy, we directly present the structural and composition evolution of cobalt and cobalt-tungsten intermetallic catalysts during the nucleation and growth of SWCNTs under carbon feeding. The co-existence of cobalt and cobalt carbide was generally observed among the pure cobalt catalysts which are active in nucleating tubes. The nucleation and growth behaviors of tubes on solid state cobalt nanoparticles and molten ones have been compared. It was found that the tubes grown from solid state catalysts always present smaller diameter than the catalysts. No structure-correlation between catalyst and nanotube was found. Distinct different from cobalt, no carbides or carbon diffusion induced structural change was observed from the in situ characterization. The atomic structure of Co7W6 was stable when CNTs nucleating. These observations reveal that cobalt-tungsten intermetallic catalysts act as the stable structure template for the chirality controlled growth of SWNTs.

Radiation-enhanced high-temperature cobalt diffusion at grain boundaries of nanostructured hardmetal

In situ annealing a FIB lamella obtained from nanostructured WC-Co hardmetal in a transmission electron microscope at 950 °C leads to the dissolution of some Co interlayers and small Co pools. The surface of the WC grains adjacent to the dissolved Co pools contains significant amounts of cobalt diffused to the surface of these WC grains from the Co pools. The observed new phenomenon of the abnormally fast Co diffusion at WC/Co grain boundaries is presumably caused by the combined effect of the high temperature and electron irradiation by an electron beam in the transmission electron microscope.

Promoting the formation of Co (III) electrocatalyst with diamond anodes

In this work, the catalytic effect of electrode substrate on the electrochemical oxidation of cobalt (II) using boron doped diamond (BDD) anodes was studied. A significant increase of the Co (III) formation was observed in using BDD coatings on tantalum substrate (91.43% at around 90 min of galvanostatic electrolysis), with respect to the Si-BDD anode, which only attained a poorer Co (III) conversion (8.33%) after applying the same electric charge. Electrochemical impedance spectroscopy showed that the Ta/BDD electrode has a lower charge-transfer resistance for the Co (II)/Co(III) metallic pair and the process is controlled by diffusion of cobalt towards the electrode surface, due to its rapid adsorption at the interface. This was further confirmed by cyclic voltammetry (CV) where the electrodeposition of Co (II) on the electrode surface is obviously significant if we compare it with that of Si/BDD anode. This difference in the electrode behavior, caused by the substrate on which the diamond coating is deposited, opens a line of research to improve the efficiency of mediated electrolytic treatments based on the use of Co, with important industrial applications in the treatment of many pollutants including volatile organic compounds (VOCs).

First principle study on lithium-ion diffusion, electronic and electrochemical properties of cobalt doped lithium metal borates

• XRD analysis reveals the Co doping improves structural stability. • First-principle calculations reveal that the Co doping tunes the band gap. • CDD, NDD analyses reveal the 1D wave like pathway of Li-ion along [001]-direction. • Electrochemical calculations predict that the Co-doping shifts the redox potential. • The average voltage of LMCB, LFCB, LNCB are 4.9, 4.9, 5.2 V.

Effect of Ta diffusion layer on the adhesion properties of diamond coating on WC-Co substrate

To enhance diamond nucleation and adhesion on cemented carbide substrates, the Ta diffusion layers with gradient structure were deposited by the combination with double glow plasma surface alloying technique (DGPSA) and the reverse-sputtering process. The effect of reverse-sputtering times on phase composition and morphology of the Ta surface layers was studied. It was found that both the thickness of the Ta deposition layer and the amount of Ta elements decreased during the removal of the Ta deposition layer. With the reverse-sputtering time increases from 0 to 60 min, the thickness of the Ta deposition layers decreased from 1.6 μm to 0 μm. When the reverse-sputtering time reached 30 min, the as-prepared sample surface exhibited the maximum micro-hardness (3496.3 HV1.96N). Rockwell C indentation tests were used to investigate the effect of the Ta diffusion layers on the adhesion between the WC-Co substrate and the diamond coating. The results show that diamond coating exhibits good adhesion performance on WC-Co substrate with the Ta diffusion layer after reverse-sputtering for 60 min, indicating that the barrier effect of cobalt diffusion is enhanced by introducing the Ta diffusion layer.

Microstructural analysis of failure progression for coated carbide tools during high-speed milling of Ti-6Al-4V

In this study, the microstructural failure progression of coated carbide tools was analyzed during milling Ti-6Al-4V under dry conditions. The microstructure and compositional characterization of worn tool inserts were examined using SEM and EDS analysis. It was found that the coated carbide cutting tools undergo several stages during interrupted cutting Ti-6Al-4V alloy: coating delamination, cobalt diffusion, attrition wear, cracking and flaking. Firstly, coating delamination firstly occurred around the cutting edge and then extended to the rake face and flank face as cutting process proceeds. The diffusion of the binder phase cobalt (Co) was observed after coating delamination and cutting speed significantly promoted the process of cobalt diffusion. Then, attrition wear occurred around the cutting edge as the WC particles were pulled out and removed from the tool substrate. In addition, Microcracks initiated and propagated from the voids introduced by cobalt diffusion due to cyclic thermomechanical loadings. Brittle fractures were found to be the main failure modes of coated carbide tools at high cutting speeds.

(Invited) In Situ Study on Cleation and Growth of Single-Walled Carbon Nanotubes on Catalysts

Understanding the growth mechanism is prerequisite to design the methodology for controlled growth of single-walled carbon nanotubes (SWNTs). By using an environmental aberration-corrected transmission electron microscope in combination with in situ synchrotron X-ray absorption spectroscopy, we directly present the structural and composition evolution of cobalt and cobalt-tungsten intermetallic catalysts during the nucleation and growth of SWCNTs under carbon feeding. The co-existence of cobalt and cobalt carbide was generally observed among the pure cobalt catalysts which are active in nucleating tubes. The nucleation and growth behaviors of tubes on solid state cobalt nanoparticles and molten ones have been compared. It was found that the tubes grown from solid state catalysts always present smaller diameter than the catalysts. No structure-correlation between catalyst and nanotube was found. Distinct different from cobalt, no carbides or carbon diffusion induced structural change was observed from the in situ characterization. The atomic structure of Co7W6 was stable when CNTs nucleating. These observations reveal that cobalt-tungsten intermetallic catalysts act as the stable structure template for the chirality controlled growth of SWNTs.

Grain Boundary Diffusion of 57Co in Nickel

Grain boundary diffusion of cobalt in Ni of purity level 99.98 wt% was measured over a wide temperature interval using the radiotracer technique and applying the 57Co radioisotope. The diffusion experiments were performed in Harrison’s type B and C kinetic regimes. Temperature dependences of triple product and grain boundary diffusivity of Co in Ni have been determined. The value of grain-boundary segregation factor of cobalt in nickel was determined to be about unity in the temperature interval under investigation. It is demonstrated that the purer the material, the higher is the grain-boundary diffusivity and the lower the corresponding activation enthalpy of diffusion.

Gradient-Modified HfC-SiC Mixed Bi-Interlayers Synthesized under Different TMS Flow Rate Increment for Depositing Diamond Coating onto WC-Co Substrate.

To deposit well-adhered diamond coating, gradient-modified hafnium carbide-silicon carbide (HfC-SiC) mixed bi-interlayers were prepared on cemented carbides (WC-Co) by plasma surface metallurgy technique under the different tetramethylsiline (TMS) flow rate increment. The effects of the TMS flow rate increment on the composition, microstructure, adhesion, and hardness of the bi-interlayers were investigated. Then, the well-adhered bi-interlayer was chosen for the deposition of the diamond coating. It was found that the HfC-SiC mixed bi-interlayers consisted of a diffusion-modified HfC-riched inner layer and a SiC-riched outer layer. The TMS flow rate increment played a key role in tailoring the surface morphology, thickness, and interface character of the bi-interlayer. The dense nanocrystalline diamond coating was formed on the optimized bi-interlayer at the increment of 0.20 sccm/2 min. The diamond coating showed excellent adhesion, which was benefited from the cobalt (Co) diffusion inhibition, gradient composition distribution, and mechanical interlocking.