Co-B Alloys Research Articles

Simultaneously enhancing mechanical and tribological performance in undercooled Co-18.5 at. % B alloys

The wear behavior and its mechanism greatly depend on the microstructure of alloys. Herein, the Co-18.5 at. % B eutectic alloys with three different morphologies and phase constitutions were obtained by undercooling solidification under different magnetic field intensity (0T, 10T, 20T), and the relative mechanical properties and tribological performance were investigated systematically. Results show that the grain size of Co–B alloy decreases first and then increases with the increase of magnetic field strength, the average grain size of samples under 0 T and 20 T was about 3–5 μm, while the grain size of sample with 10 T refined to 20–40 nm. The sample treated under 10 T exhibits the lowest wear rate (1.53 × 10−5 mm3N−1m−1) and highest micro-hardness (868.27 HV). The fine grain strengthening effect is the main reason for the increase in strength and hardness of Co–B alloy. The fine crystalline nano-heterostructures (20–40 nm) and the precipitation enhancement of secondary precipitated Co nanoparticles also has a beneficial effect on mechanical properties. The oxide friction layer consisting mainly of layered H3BO3 lubricant, detected on the worn surface of Co–B alloy (10T) has anti-wear properties. By contrast, Co–B alloys with coarse Co3B grains (0T) and inhomogeneous FCC-Co/Co2B lamellar structure (20T) present higher wear rates and lower hardness. This work can provide guidance for the initial control of mechanical properties from the design of the microstructure.

Temperature-induced structure evolution in CoxBy liquids studied by ab initio molecular dynamics simulation

The relationship between the local structures and the thermodynamic properties of metallic liquids has been a fundamental issue, which significantly impacts the comprehensive performances of metallic materials. The temperature-induced structure evolution in CoxBy alloy liquids, involving CoB, Co2B, Co3B, and Co23B6, was investigated using ab initio molecular dynamics simulations. The results show that the clusters of icosahedral-like polyhedrons are found predominantly in all alloys at low temperature, while the heating induces the decrease in ideal icosahedral and the increase in defected icosahedral. At high temperature, the polyhedron clusters decrease significantly and the ideal icosahedra transforms into smaller short-range polyhedrons, signifying the temperature-induced chemical structure change from long-range to short-range ordering. The aggregation of B atoms at high temperature is confirmed by atomic configuration, charge density, and B-centered Voronoi polyhedrons, and the α-Co single phase is observed below 1600 K. This study is helpful for understanding the local structure variability in eutectic Co-B alloys and guiding solidification path in theory.

The Activation Energy of Viscous Flow and Liquid–Liquid Structure Transition in Co-B Alloys

The temperature dependences of the kinematic viscosity during heating and cooling have been investigated in Co-B melts with a boron content of up to 30.8 at. A liquid–liquid structural transition was found, which is accompanied by an increase in the activation energy and cluster size, as well as a significant decrease in the density of the melt. The liquid–liquid structural transition was associated with the formation of clusters with a short-range order of Co23B6 in the intermediate temperature region. At low and high temperatures, clusters of the order of an atomic size are active participants in the viscous flow. It was shown that with an increase in the cluster size, the activation energy increases and the viscosity of melts decreases. The formation of large Co23B6 clusters during the cooling of melt with low boron content leads to undercooling and the appearance of the transition temperature region with high activation energy, although this region does not exist during the heating stage.

Zn-MOF-74-derived graphene nanosheets supporting CoB alloys for promoting hydrolytic dehydrogenation of sodium borohydride

The development of an efficient tactic for enhancing the catalytic activity toward hydrolytic dehydrogenation of sodium borohydride (NaBH4) is of vital significance but remains a long-standing challenge. In this work, CoB alloys were successfully anchored on MOF-74-derived graphene nanosheets (GNS) via the chemical reduction process. As a catalyst support, GNS can provide a high dispersion of CoB alloys to expose more active sites. Moreover, the existence of a synergistic effect between CoB alloys and GNS is verified by a series of well-designed control experiments. Therefore, a low activation energy (Ea = 38.8 kJ·mol–1) and a high hydrogen generation rate (HGR = 7937 mlH2·min−1·g−1) are simultaneously achieved for the optimized Co1B/GNS catalyst. In addition, Co1B/GNS has decent recyclability, i.e., 58 % of the initial HGR remaining after ten successive cycles of NaBH4 hydrolysis. This work provides a feasible approach for fabricating high-performance non-noble metal catalysts for hydrolytic dehydrogenation of NaBH4.

Liquid state dependent solidification of a Co-B eutectic alloy under a high magnetic field

• Solidification behaviors in Co-B alloy dependent on the liquid state are studied. • The undercooling and recalescence extent are enlarged after experiencing LLST. • The wear resistance are significantly enhanced after experiencing LLST. • The concept of magentic-induced LLST is put forward firstly in Co-B alloys. The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement. A crossover was observed on the 1/ M - T curve during the overheating process, indicating that a liquid-liquid structure transition (LLST) took place in the melt. Based on this information, the effects of LLST on the solidification behavior, microstructure and tribology property were investigated experimentally. The sample solidified with the LLST exhibits significantly different solidification behaviors, i.e., the nucleation undercooling and the recalescence extent are conspicuously enlarged, and the solidification time is shortened. As a result, the microstructure is effectively refined and homogenized, and the hardness and wear resistance are significantly enhanced. The present work might be helpful for not only theoretically understanding the influence of LLST on the solidification behavior but also providing an alternative approach to tailor the microstructure and properties.

Effects of an ultra-high magnetic field up to 25 T on the phase transformations of undercooled Co-B eutectic alloy

While there have been multiple recent reports in the literature focusing on the effects of magnetic field on the phase transformation behaviors, the research conducted with an ultra-high magnetic field greater than 20 T is still preliminary. In the current study, the structure evolution of Co-B alloys are experimentally studied with undercooling. The effects of a 25 T magnetic field on the solidification behavior and the subsequent solid-state phase transformation behavior have been investigated. The 25 T magnetic field is confirmed to have little effect on the homogeneous nucleation, but have some influence on the heterogeneous nucleation of Co3B and Co23B6 phases by modifying the wetting angle θ. The decomposition of Co23B6 phase in the subsequent cooling process can be effectively suppressed by applying the 25 T magnetic field. The present work might be helpful for not only theoretically understanding the influence of ultra-high magnetic field on the phase transformation behaviors but a potential technology of field-manipulation of magnetic materials.

(Invited) Film Properties of Various Electroless Plated Co Alloy Films Formed on SiO2/Si Substrate and Its Interdiffusion Properties Against Cu

There is a serious problem of poor side wall coverage for the sputtered barrier metals in high aspect ratio TSVs. We have proposed electroless plated Co-alloy barrier metals for TSV. For this purpose, we prepared various CoB and CoWB alloys with different atomic compositions and crystalline structures. Furthermore, we evaluated interdiffusion properties of Cu/Co-alloy/Si stacked structure by SIMS analysis. It turned out that the electroless CoWB film containing larger amount of W has excellent diffusion barrier property than other CoWB films with smaller W content. We formed electroless barrier films of CoB and CoWB using Pd nanoparticle catalyst on SiO2/Si substrate [1]. For evaluation of Cu interdiffusion properties by SIMS, we deposited Cu and TiN thin films successively by sputtering on the barrier film. The ratio of the atomic composition was changed by adjusting the ratio of cobalt sulfate, tungstic acid, and some other contents of the plating bath. SIMS depth profiles of CoB (Fig.1-a) and CoWB films after annealing at 400 ℃ (Fig.1-b) are shown in Fig.1. There was a large interdiffusion between Cu and CoB after 400 ℃ annealing (Fig.1-a). Co-Silicide formation significantly occurred and Si atoms diffused into CoB layer. However, interdiffusion was significantly suppressed in the case of Co-20%W-B film (Fig.1-b). These results suggest that W atoms existed in CoWB have a strong effect of suppressing Cu diffusion in Co film, and CoWB with 20 atom % W showed a good diffusion barrier property. The present study suggests that the electroless plated CoWB having a larger W content has a high potential as a diffusion barrier film for the via last TSV process.[1] F. Inoue, T.Shimizu, F.Miyake, R.Arima, T.Ito, H.Seki, Y.Shinozaki, T.Yamamto, and S.Shingubara, Microelectronic Engineering, 106 (2013) 164–167.[2] T. Iseri, S.Shindo, Y.Miyachi, A.Hirate, S.Tanaka, T.Shimizu, T.Ito, S.Shingubara, Jpn. J. Appl. Phys. 57 (2018), 07MB02-1~6. Figure 1

Efficient Hydrogen Generation from the NaBH4 Hydrolysis by Cobalt-Based Catalysts: Positive Roles of Sulfur-Containing Salts.

Development of a simple and efficient strategy for improving the catalytic activity of cobalt-based catalysts toward hydrogen evolution from sodium borohydride (NaBH4) is paramount but remains challenging. Here, we reported a facile and efficient approach to tune the catalytic performance for NaBH4 hydrolysis with Co-based catalysts prepared by using cobalt sulfate as a precursor or a mixture of sulfur-containing sodium salts/cobalt salts as a raw material. With the use of cobalt sulfate as the precursor, the CoSO4-doped Co3O4 sample was formed and it exhibited excellent activity with the generation of ∼500 mL of hydrogen gas from NaBH4 hydrolysis under mild conditions. In terms of sulfur-free cobalt salts (e.g., cobalt chloride, cobalt nitrate, and cobalt acetate) as precursors, the obtained Co-based samples were found to be entirely ineffective for hydrogen production. Interestingly, during the cobalt-based catalyst preparation, the introduction of sodium sulfate or sodium sulfide can considerably accelerate hydrogen production. On the contrary, adding sulfur-bearing salts did not inspire any activity improvement only during the hydrogen generation reaction. Control experiments indicate that during catalyst preparation, the presence of Na2SO4 and Na2S is beneficial for the in situ transformation of Co3O4 into catalytically active Co-B alloys, accompanying a positive change in surface morphology during the NaBH4 hydrolysis, thereby inducing an excellent hydrogen generation rate of up to 4425 mL·min-1·gcat-1.

Transition from hypereutectic to hypoeutectic for rapid solidification in an undercooled Co-B alloy

Phase selection, a common and important phenomenon in rapid solidification, determines the solidification sequence and the mechanical/physical properties. In this work, a Co-20 at%B hypereutectic alloy was undercooled by the melt fluxing technique and the microstructure was characterized by the back-scattering diffraction technique. A transition from hypereutectic to hypoeutectic was found at a critical undercooling of ΔT = 119 K. When ΔT < 119 K, a primary directional dendritic β-Co3B phase surrounded by the regular α-Co + β-Co3B lamellar eutectics was found. When ΔT > 119 K, the above hypereutectic microstructure changes into hypoeutectic structure with the α-Co phase as the primary phase. According to dendrite growth model, the transition from hypereutectic to hypoeutectic can be ascribed to the higher growth velocity of the α-Co phase than the β-Co3B phase, i.e., the growth-controlled mechanism. The current work shows also that there is a coupled zone skewed to the β-Co3B phase in the Co-B alloys system.

Effect of synthesizing temperature on the electrochemical properties of nitrogen-doped Co-B alloys

Using urea as nitrogen source, nitrogen-doped Co-B alloys were prepared by chemical reduction method at different reaction temperatures (25, 50, and 75 °C). The structure and surface morphology were characterized by XRD and SEM. Then they were used as negative electrode materials for alkaline secondary batteries (ASBs). The electrochemical properties were measured by galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), and anodic polarization (AP). The experimental results show that the lower the synthesizing temperature is, the better the electrochemical performance of nitrogen-doped Co-B alloy is. For example, nitrogen-doped CoB alloy prepared at 25 °C has an initial discharge capacity of 825.8 mAh/g. After 100 charge/discharge cycles, the discharge specific capacity has remained to be 369.5 mAh/g with capacity retention of 44.7%.

The Thermodynamic Functions of Monoborides XB (X=Ti, Mn, Fе, Co)

In the paper the physical properties and thermodynamic functions of monoborides ХВ (Х=Ti, Mn, Fe, Co) are studied with accounting for fluctuation processes. The research was performed for alloys with boron content of 9,0-15,0 % (wt.), the rest is metal Х (Х=Ti, Mn, Fe, Co). We use the microstructure analysis, the X-ray structural and the durometric analyses to determine the physical properties of alloys. In the paper it is determined the phase composition of Ti-B, Mn-B, Fe-B and Co-B alloys and physical properties of monoborides. In this paper for the first time it is determined the thermodynamic functions of monoborides using the Hillert and Staffansson model with accounting for the first degree approximation of high-temperature expansion for the free energy potential of binary alloys. We obtain the temperature dependences for such thermodynamic functions as Gibbs free energy, entropy, enthalpy and heat capacity Ср along with their values at the formation temperature for ХВ monoborides (Х=Ti, Mn, Fe, Co). The approach under consideration enables to give more thorough from the thermodynamic point of view description of monoborides formed from the liquid. The outcomes of the thermodynamic function calculation for TiB, MnB, CoB та FeB monoborides are in good agreement with experimental data and results of other authors.

Preparation of Co-B Alloy with the Assistance of the Sonication and its Electrochemical Properties

In order to enhance the electrochemical properties of Co-B alloys used as negative electrode materials of alkaline rechargeable batteries, Co-B alloy was successfully prepared by a chemical reduction method with the assistance of the sonication. The phase structure and the surface morphology of the as-prepared Co-B alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen physisorption. Moreover, the electrochemical performance was characterized by galvonostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS) and anodic polarization (AP). Co-B alloy prepared with the assistance of the sonication consists of small particles with a uniform distribution. The electrochemical measurements showed that at a discharge current density of 100 mA/g, the initial discharge capacity was 858.1 mAh/g and the discharge capacity was 322.6 mA/g even at the 100th cycle with the capacity retention of 37.6%.

Magnetic Properties of Co-B Nanostructures Prepared via Electroless Deposition

We have investigated the magnetic properties of nanostructured Co-B alloys, that were prepared via electroless deposition. The deposition process results in the formation of a nanostructure consisting of nanotubes connected to thin films at both ends. Depending on the deposition time end-open or end-closed nanotubes can be formed. The overall nanostructure of Co-B deposit has a specific magnetization of 65.6 ± 8 JT-1Kg-1 (0.75 ± 0.09 μB per Co atom). We also investigated the anisotropy of the nanostructure by carrying out magnetic measurements with and without the top and base films. We only observed magnetic anisotropy in nanostructures with thin films, which had minimum coercivities of 557 A/m (7 Oe) and 4536 A/m (57 Oe) measured parallel and perpendicular to the nanotube axis. The nanotubes do not show any significant anisotropy with coercivities of 8753 A/m (110 Oe) and 7161 A/m (90 Oe) parallel and perpendicular to the nanotube axis.

Electroplated cobalt-boron alloys: Formation and structure features

Co-B alloys containing 3–22 at % boron were produced by electroplating from a cobalting bath containing sodium decahydro-closo-decaborate as a source of boron. Alloys containing 3–8 at % boron are found to be mixed substitutional-interstitial solid solutions with an α-Co crystal lattice. At a boron content of 15 at % or higher, amorphous coatings are formed. The microstructure of amorphous alloys is characterized by the existence of regions with different substance density. With an increase in the boron content from 16 to 20 at %, the regions of the increased and decreased substance density become larger and more pronounced. The existence of chemical bonds between cobalt and boron atoms was discovered with the use of X-ray photoelectron spectroscopy.

Effects of the Pre-Freezing Time on the Structures and Electrochemical Performances of Co-B Alloys

A series of Co-B alloys with uniform nanoparticles were prepared via a vacuum freeze-drying method, which the precursors of Co-B alloys were prepared by the reduction of bivalent cobalt chloride with aqueous sodium borohydride solution. The alloys were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and brunauer emmett teller (BET) surface area measurement. The electrochemical activities of Co-B samples were examined by charge-discharge test. This work studied the influence of pre-freezing time on the microstructure and electrochemical performance. The sample pre-freezing for 3 hours exhibits the best cycling performance than others, the discharge capacity increase to its maximum 438mAh/g, and it still remained 232 mAh/g after 60 cycles at a current of 50 mAh/g; while those of samples pre-freezing for 1 hour and 5 hours are 399 mAh/g and 413mAh/g, respectively.

A New Preparation Method for Amorphous Co-B Alloys Nanoparticles

The Co–B alloy can be prepared facilely by a solid-solid reaction of CoCl2•6H2O and KBH4 powders at room temperature. Various characterizations, such as the chemical analysis, inductively coupled plasma-atomic emission spectroscopy (ICP), powder X-ray diffraction, electron diffraction and TEM have been performed. The results indicate that the Co-B alloy obtained by the solid-solid reaction is amorphous nanoparticles. The composition of the alloy is Co1.36B. The average diameter of the Co–B alloy nanoparticles is 30nm–50nm. The room temperature solid-solid reaction is mainly a surface reaction. The direct solid solid reaction between the borohydride and some metal-salts is thermodynamically possible. This simple preparation method may also be used for the large-scale production of the amorphous nanoparticles of some metal-boron alloys.

Facile preparation and good electrochemical hydrogen storage properties of chain-like and rod-like Co-B nanomaterials

Chain-like and rod-like Co-B nanomaterials are prepared by chemical reduction method in cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP) aqueous solution, respectively. XRD patterns demonstrate that the two materials both have amorphous structures. SEM and TEM images show that the chain-like Co-B constructs of one-by-one tactic ball-like particles with nanoflakes on the surface, whereas the rod-like Co-B alloy possesses a porous nanostructure. The results of electrochemical measurements indicate that, as negative electrode materials of Ni-MH batteries, their electrochemical properties are both better than those of regular Co-B alloy. At the discharge current density 25 mA g −1, the discharge capacities of the chain-like and the rod-like Co-B alloys are 314 mAh g −1 and 292 mAh g −1 after 50 cycles, respectively, which are both higher than that of regular Co-B alloy. XRD patterns of the electrodes on different charge–discharge states illustrate that the discharge capacity is attributed to hydrogenation of Co-B alloy.

PREPARATION AND ELECTROCHEMICAL BEHAVIORS OF Co-B ALLOY POWDERS

Alkaline rechargeable batteries,such as Ni/Cd batteries and Ni/MH batteries have been widely used as power sources,however,their further applications are limited due to contamination of Cd in Ni/Cd batteries and lower discharge capacities of Ni/MH batteries.Some metal borides, such as Co-B,Ni-B,Fe-B,V-B and Ti-B,have been known to own very high discharge capacity in alkaline aqueous solution,in which Co-B alloy exhibits the highest reversible discharge capacity and the best cyclic stability.However,the reversible capacity of Co-B alloy prepared by arc melting process is usually less than 250 mA·h/g,which is only one fourth of the theoretical capacity of the alloy electrode(908 mA·h/g).In the present study,chemical reduction method was used to prepare Co-B alloy to further enhance the electrochemical capacity of the alloy.A series of ultrafine powders of amorphous Co-B alloys,Co_(0.68)B_(0.32),Co_(0.55)B_(0.45) and Co_(0.50)B_(0.50),were prepared by reducing CoSO_4 with solution of NaBH_4.Electrochemical measurements indicate that the prepared alloys exhibit excellent electrochemical properties.At a high current density of 300 mA/g,the initial discharge capacities of these alloys are 510.6,666.4 and 667.2 mA·h/g,respectively,their discharge capacities still keep 331.6,379.5 and 390.5 mA·h/g after 60 cyc,respectively.Even at a discharge current density as high as 1200 mA/g,the three alloys still deliver reversible capacities of 336.2,373.4 and 390.1 mA·h/g,respectively.In the Co-B alloy electrodes,the boron atoms have two functions.First, boron can be oxidized to BO_3~(3-),thereby partly contributes to the discharge capacity.Second,most importantly,the gradual dissolution of boron into the electrolyte(in the form of BO_3~(3-)) during the charging/discharging creates a new surface in the electrodes,which can effectively increase the surface area of the active material in contact with the electrolyte.The alloy with higher boron content thereby can produce a larger reaction surface area by boron dissolution than the alloy prepared with lower B content.So the higher B content in the Co-B alloys can be helpful for improving their electrochemical properties.

Amorphous Co-B/Al nanocomposites prepared by electroless coating technique and their excellent catalytic activity

To improve the catalytic activity of amorphous Co-B alloys, Co-B coated aluminum (Co-B/Al) nanocomposites were prepared by electroless coating technique and evaluated as additives for the catalytic performance of ammonium perchlorate (AP) and AP-based solid state propellants. X-ray diffractometry (XRD), scanning electron microscopy (SEM), inductive coupled plasma emission spectrometry (ICP), differential scanning calorimetry (DSC) as well as strand burner method were employed to characterize the crystal phase, morphologies, chemical composition, and catalytic activity of the as-synthesized material. The results show that a continuous layer of about 100 nm amorphous Co72.6B27.4 covers the surfaces of Al particles. Addition of the as-synthesized Co-B/Al nanocomposites as catalysts promotes AP decomposition, enhances the burning rate, and lowers the pressure exponent of the AP-based propellants considerably.

Electroless Deposition of Thin-Film CoB alloys: The Influence of Some Bath Parameters on its Magnetic Behavior

The influence of citrate and glycine at different concentrations on the morphology and magnetic properties of thin film CoB alloys is studied. With both additives, coercivity (Hc) is inversely proportional to the complexant concentration. In deposits obtained from glycine containing baths higher magnetization saturation (Ms) values were obtained (1.3 T). Possibly the higher Ms in deposits grown from glycine baths is due to the low boron content (less than 1%). In deposits obtained in citrate containing baths, the boron content increased until a maximum of c.a. 15 % as the citrate concentration was increased. There are evidences that not only boron but the additive influences microstructure and therefore the magnetic properties of such films.