The basic physical properties of Co metal
Cobalt powders have been used extensively in cemented carbides, high-temperature alloys, PCD and PCBN, and magnetic materials, etc., due to its excellent physical, chemical, and mechanical properties. The fabrication and final properties of the above materials are strongly affected by the quality (purity, phase, size, shape, dispersity, fluidity, etc.) of Co powders. In order to fabricate a homogeneous and densified microstructure without pores, Co-pool, Co-poor and Co-free zones, etc., ultra-fine spherical Co powders have been desired with the development of ultra-fine and even nano grain materials. However, it is very difficult to industrial fabricate ultra-fine spherical Co powders with good quality by the conventional decomposition and hydrogen-reduction technology. In this paper, a new low energy-consumption industrial production technology-a continuously dynamic-controlled combustion synthesis (CDCCS) technology has been proposed about investigating how to obtain ultra-fine spherical Co powders (the average particle size is smaller than 0.8μm, and the length-diameter ratio is smaller than 2) with a lower impurity content.
2. Cobalt: Properties, minerals, extraction and applications
2.1.1. Physical properties
Cobalt does not occur naturally as a pure metal, but is a component of more than near a hundred naturally occurring minerals, including various sulfides, arsenides, sulfoarsenides, hydrates, and oxides. Pure cobalt can be produced by reductive smelting, and was firstly prepared by G. Brandt in 1735. Cobalt was confirmed as an element by T. Bergman in 1780. Cobalt is a metallic transition element, and its position in the Periodic Table is characterized:
Name, symbol, number: Cobalt, Co, 27
Element category: Transition metal
Group, period, block: 9(VIIIA), 4, d
Standard atomic weight: 58.933195
Electron configuration: 4s3d7
Electrons per shell: 2, 8, 15, 2
Co is a brittle, hard metal, resembling iron and nickel in appearance. Pure cobalt produced by reductive smelting is a hard, lustrous, silver-gray metal. Co has a relative permeability two thirds that of iron . Its Curie temperature and magnetic moment are 1115C  and 1.6~1.7 Bohr magnetons/atom , respectively. The basic physical properties are listed in Tab.1, and vapor pressure vs. temperature is shown in Fig.1. The transformation is sluggish and accounts in part for the wide variation in reported data on its physical properties. Metallic Co occurs as two crystallographic structures: a hexagonal closed-packed crystal structure (hcp) and a face-centered cubic crystal structure (fcc). During cooling, Co usually undergoes a polymorphous transformation from fcc to hcp. The temperature corresponding to the equilibrium between the high-temperature β (fcc) and low-temperature α (hcp) phases in Co is 417C , but in fact, the energy difference is so small that random intergrowth of the two is common .
|Liquid density at melting point temperature||7.75g/cm-3|
|Melting point||1768K, 1495C, 2723F|
|Boiling point||3200K, 2927C, 5301F|
|Heat of fusion||16.06kJ/mol|
|Heat of vaporization||377kJ/mol|
|Molar heat capacity||24.81J/(mol·K)|
2.1.2. Chemical properties
Pure Co does not dissolve in water and soil, and can stay for years at room temperature (RT). Co can stay in the air for a few days, and can be oxidized to CoO at about 300C, and can be burnt to Co3O4 at high temperature. Fine Co powders prepared by the hydrogen-reduction technology are easily oxidized to cobalt oxide and even generate self-ignite in the air. So the fine Co powders must be carefully preserved in a vacuum or an inert gas container.
Co is a weakly reducing metal, and is easily attacked by halogens and sulfur, and is also rapidly dissolved by HCl, H2SO4 and HNO3 acid solution, and is slowly eroded by HF, NH4OH, NaOH solution.
Co can solid-dissolve many metal and nonmetal atoms to form many intermetallic compounds. Co can well infiltrate many ceramics (WC, TaC, TiC, ZrC, TiN, Al2O3, cBN, diamond, etc.), and the almost all wetting angles are lower than 50 , so it is often used as a binder in the above ceramic-metal composites.
2.1.3. Mechanical properties
Metallic Co occurs as two crystallographic structures: hcp-α and fcc-β. Strength and hardness of hcp-α should be higher than those of fcc-β, but inverse for their plasticity because there are more slipping systems in fcc-β than hcp-α. Some mechanical properties of Co are listed in Tab.2.
|Mechanical properties||Crystal structures|
|Electrical resistivity (Ω·m)||6.24×10-8 at 20C||-|
|Thermal conductivity (W/(m·K))||100||-|
|Thermal expansion (/K)||13.36×10-6 at 25C||-|
|Young’s modulus (GPa)||209||<hcp-Co|
|Shear modulus (GPa)||75||<hcp-Co|
|Bulk modulus (GPa)||180||<hcp-Co|
|Vickers hardness (MPa)||1043||<hcp-Co|
2.2. Co minerals and extraction
Co is not a typically rare metal since it ranks 33 in abundance. Content of Co in the earth’s crust is about 0.035wt.%, and about 2.3 billion ton in the sea. Nearly, all Co is always found associated with metallic-lustered ores of other metals (for example, Cu, Ni, Fe, Pb, Zn, etc.), and Co minerals without other metals is very less except cobaltite in Morocco. So generally it is produced as a by-product of other metals mining. Near a hundred cobalt minerals have been already found in the nature, but about only 20 cobalt minerals are valuable and available, listed in Tab.3.
Based on the complexity of the Co minerals, the extraction processing is very complicated and efficiency of recovery is also very low. As usual, Co in the minerals is firstly concentrated or is transformed into the soluble states by the pyro-refining, and then Co in the calcine of pyro-refining is further enriched and extracted by the hydrometallurgy, the finally the cobalt compounds or pure cobalt are obtained. An extraction processing is shown in Fig.2 . Section (the production of Co powders) in the dotted line scope in Fig.2 will be emphasized in the chapter.
|Cobalt minerals||Chemical formula||Theory cobalt contents (wt.%)||Actual cobalt contents (wt.%)||Distribution|
|Arsenides||CoAs2||23.2||15~21||Canada, Morocco, USA|
|(Co,Fe)As2||28.2||9~23||Morocco, Canada, Russian|
|(Co,Ni)As3||20.8||16~20||Morocco, Canada, Russian|
|Sulfides||CoAsS||35.5||29~35.3||China, Canada, Morocco, USA, Australia|
|Oxides||<32||<30||Zaire, China, New Caledonia|
Due to the excellent physical, chemical, and mechanical properties, it is widely used in fabricating various alloys (which are used as high-temperature and wear resistant components, dies, saws, cutting tools, etc.) by powder-metallurgy technology, such as super alloys, high speed steels, cemented carbides, PCD (polycrystalline diamond) and PCBN (polycrystalline cubic boron nitride), etc. Co in the above alloys is usually used as an additive, an alloying element or a binder.
An important use for Co is in the field of high-temperature alloys. Required in gas turbines, jet engines, and similar applications, such alloys retain their strength above 650C; these alloys contain 5~65wt.% Co. Even higher operating temperatures in turbines have resulted in an increased use of cobalt-containing and cobalt-based alloys known generally as super alloys. These can withstand severe operating conditions and temperatures up to 1150C. For example, Nimonic 90 is a nickel-based alloy containing 18wt.% Co, a similar amount of Cr, and some Ti, and Waspaloy is another alloy of this type.
Cemented carbides, PCD and PCBN are used as cutting tools, wear-resistant components, dies, saws. In the production of a so-called cemented carbide, i.e. tungsten carbide composites, a briquetted mixture of tungsten carbide and soft cobalt powders is compacted and sintered at a temperature above the melting point of cobalt. The latter melts and binds the hard carbides, giving them the toughness and shock resistance needed to make carbides of practical value for cutting tools, drill bits, dies, and saws, etc. Co is the most satisfactory matrix metal for this purpose and may be adjusted in amounts from 3 to 25 percent by weight. A briquetted mixture of diamond or cBN and Co powders are sintered at high temperature and high pressure, and PCD and PCBN materials are obtained.
Very strong magnets are created when Co is alloyed with other metals. So Co is used in fabricating many advanced magnetic materials.
Cobalt’s use in rechargeable batteries is the fastest growing use. Notably in 2007, the percentage of cobalt use for rechargeable batteries rose to 25% of total cobalt demand from 22% in 2006.
3. Expectation qualities of cobalt powders in powder metallurgy industry
Powder metallurgy materials (PMM) have been usually fabricated by mixing, pressing, and sintering processing. The quality (purity, phase, size, shape, dispersity, fluidity, etc.) of raw powders acts an important role in determining the fabrication and final properties of PMM. Especially with the rapid development of ultra-fine grain cutting tool materials (such as, WC/Co alloys, PCBN, PCD, etc.), ultra-fine spherical Co powders (the size is 0.8μm and the length-diameter ratio is smaller than 2) have been desired in order to fabricate a homogeneous and densified microstructure without pores, Co-pool, Co-poor and Co-free zones, etc.
Though hcp-α Co is a room temperature stable phase, fcc-β Co can also be steadily retained at RT by some especial techniques (for example, rapidly cooling). For metal alloys, the strength and plasticity are generally contrary, so hcp-α and fcc-β structures must be alternative in applications. For cemented carbides (WC/Co alloys), hcp-α Co powders are desired because the cold welding among Co particles due to its higher brittleness can be decreased during ball-milling. The cold welding (seen as “hard” agglomerations, as shown in Fig.4) among Co particles can easily result in “Co-pool” which result in a rapidly decrease of mechanical properties of WC/Co alloys. However, fcc-β cobalt in the sintered WC/Co alloys is desired by rapidly cooling during sintering because its good plasticity can improve the toughness of WC/Co alloys.
4. Decomposition-hydrogen-reduction to fabricate cobalt powders
Co powders in powder metallurgy industry are mainly fabricated by a decomposition- hydrogen-reduction technique using cobalt compounds of cobalt oxalate (CoC2O4∙2H2O), cobalt carbonate (CoCO3∙
In order to well understand the decomposition mechanisms of CoC2O4∙2H2O, CoCO3∙
4.1. Cobalt oxalate
The chemical formula of the commercial cobalt oxalate is CoC2O4 2H2O. Fig.6 shows TG and DTA curves of CoC2O4 2H2O in an inert gas (such as N2, Ar) and air, respectively. DTA curve in the inert gas exhibits two endothermic peaks, which are accompanied by the weight loss of ~19.67% and ~48.09% in the TG curve, respectively. The ranges of reactive temperatures at the peaks are in about 170~225C and 350~440C, respectively, which will change a few with different testing conditions (such as changing heating rate). Such weight loss agrees with the value calculated for the following two transformations:
DTA curve in the air exhibits an endothermic peak and an exothermal peak, which are accompanied by the weight loss of ~19.67% and ~36.43% in the TG curve, respectively. The ranges of reactive temperatures at the peaks are in about 170~225C and 270~320C, respectively, which will also change a few with different testing conditions (such as changing heating rate). Such weight loss agrees with the value calculated for the following two transformations:
In actual industrial production of Co powders, the decomposition-hydrogen-reduction processing of CoC2O4 2H2O occurs in a H2 atmosphere in the device of Fig.5. Tikkanen
Fig.7 shows the effect of decomposition temperature and time of CoC2O4 2H2O on the specific surface area of Co powders . There are two activation energies for the growth of Co powders, which implies two different growth mechanisms: the aggregating growth among Co crystalline nuclei at lower temperature and the second aggregating growth among Co particles. It is obvious that the second growth must be inhibited during decomposition. The temperature occurring the second growth is about ~450C, which is similar to the results of DTA. Therefore, the temperature in the decomposition stage can not usually be higher than 450C in order to obtain ultra-fine Co powders. In fact, it is very difficulty to completely inhibit the second growth due to the local overheating in the boat. The following methods are usually used to improve the temperature homogeneity in the boat: increasing the surface of the boat, decreasing the layer thickness of CoC2O4 2H2O powders, and increasing the flow rate of H2.
Morphology of the commercial CoC2O4 2H2O powders is a short-fibrous structure with an average diameter of about 0.5~1μm and an average length of about 4~10μm, or a bunch (3~4μm) of fibrous particles is formed when several short-fibrous cobalt oxalate particles are adhered or bonded together during crystallization, as shown in Fig.8(a). Because morphology of Co powders can easily inherit that of cobalt oxalate, it is very difficulty to obtain spherical Co powders by decomposing cobalt oxalate; usually replacing a short-string or dendritic structure, shown in Fig.8(b). Furthermore, the production practice shows that there is always 2~4% “hard” agglomeration (a size of 10~30μm) in Co powders. The decomposition processing of cobalt oxalate is schematically shown in Fig.9. The short-fibrous CoC2O4 2H2O particles stacked disorderly in the boat are decomposed and reduced
In order to obtain ultra-fine spherical cobalt powders, there are two methods: to obtain spherical cobalt oxalate powders and to impede or break the adhesion of cobalt nuclei during the decomposing processing of cobalt oxalate. Spherical cobalt oxalate powders can be crystallized by adding a few spheroidizer [13,14] into the cobalt salt leachate, but the spheroidizer can deteriorate the properties of the final sintered alloys. Du
4.2. Cobalt carbonate
The chemical formula of the commercial cobalt carbonate is CoCO3
DTA curve in the air also exhibits two obvious endothermic peaks. The ranges of reactive temperatures at the peaks are in about 130~220C and 250~320C, respectively. According to XRD results in Fig.11, there are the following two reactions:
In addition, there seems a weak exothermic peak in about 220~270C, in which the weight loss on the TG curve is a few slower than that in 270~320C. According to XRD result at 220C in the air, there also are the following two possible reactions:
A similar result was reported in Ref. , namely the decomposed products of Co5(OH)6(CO3)2
Being different from the short-fibrous CoC2O4 2H2O powders, the production practice [19,20] indicates that the spherical Co powders can be easily obtained by decomposing a nearly spherical CoCO3
Another disadvantage is: the content of some impurities (such as S and Ca) in CoCO3
4.3. Cobalt oxide
There are some advantages using Co3O4 powders to fabricate Co powders by hydrogen-reduction technology: (1) For CoC2O4 2H2O and CoCO3
|Cobalt contents (wt.%)||32.24||49.58||78.67||71.08||73.44|
5. The continuous and controllable combustion synthesis
Basing on the above analysis, it is very difficulty to obtain the satisfied ultra-fine spherical Co powders by the conventional technique while using CoC2O4 2H2O and CoCO3
5.1. Basic principles
5.1.1. Selecting the raw powders
As mentioned above, although spherical Co powders can be obtained by decomposing spherical CoCO3
5.1.2. Utilizing the exothermal reaction of CoC2O4 2H2O — Low energy-consumption
Selecting CoC2O4 2H2O powders as the raw material to fabricate ultra-fine Co3O4 powders, a main advantage is that the decomposition processing of CoC2O4 2H2O in the air is an exothermal processing according to DTA. Seham
5.1.3. Utilizing airflow dispersion to in situ break the adhesion among Co3O4 particles
Morphology of Co3O4 powders can still inherit the short-fibrous structure of CoC2O4 2H2O powders when the decomposition processing is carried out in a fixed bed, namely Co3O4 particles can easily grow together to form a short-fibrous structure (see Fig.14(a)) due to the exothermal reaction. But a short-fiber is composed of ultra-fine spherical Co3O4 particles with a size of about 0.1~0.2μm, see Fig.14(b). The Co3O4 particles arrange a short-string along to the fiber, and there is a weak adhesion between particles. The results further explain why the short-string or dendritic Co powders is easily obtained by decomposing and reducing CoC2O4 2H2O powders in a fixed bed. Ultra-fine Co3O4 powders with the size of 0.1~0.2μm can obtained if the weak adhesion between them before forming a strong bond can be broken
5.2. Production equipments
According to the above desires, a continuously dynamic-controlled combustion synthesis (CDCCS) process is proposed and the bottlenecks of continuous production and process controls of combustion synthesis (CS) have also been solved satisfactorily. The preparation of ultra-fine Co3O4 powders via CDCCS is carried out in a gas-solid fluidized bed unit (CS unit), as shown in Fig.15. The unit is patented equipment consisting of fluidized bed roaster, feeding systems, receiving systems, dust collection systems and air supply system [24,25].
There are reaction and cooling boiling (11 and 21) that formed by gas-solid fluidization on the upper and lower gas distribution plates (61 and 62), respectively. The raw material on the upper gas distribution plate (61) is fleetly penetrated by the gas from the upper gas pipeline (32). The boiling bed with the flowing property like liquid is formed when the superficial gas velocity reaches a critical value of 0.03m/s. In our experiments, the superficial gas velocity is 0.09~0.18m/s. The height of boiling beds, locating between the gas distribution plates to the top of overflow gates, is about 500mm. And several thermocouples are installed on the CS unit’ wall and the location is about at 2/3 height of boiling beds.
When the boiling bed (11) containing CoC2O4 2H2O powders which are fed into by the screw feeder (51) is preheated to 380C by the hot carrier gas, CoC2O4 2H2O powders will react with O2 in the air. Co3O4 powders pour from the upper overflow pipe (35) under gravity and enter the cooling boiling bed (21), followed by collecting from the lower overflow pipe (34). The temperature of CS in reaction region can be perfectly controlled though the linkage of the upper thermocouple (71) and the upper gas pipeline (32).
5.3. Process analysis of CDCCS to fabricate Co3O4 powders
When the reactant CoC2O4 2H2O powders are fed by the screw feeder (51) into the boiling bed, no self continuous reaction occurs. Only when CoC2O4 2H2O powders in the combustion zone are heated by hot carrier gas, and the continuous reaction is ignited. This means that once the combustion reaction between O2 and CoC2O4 2H2O powders is ignited, the released heat by the combustion reaction can be used to ignite the following exothermic reaction and no external heat is needed, and then the hot carrier can be transformed into the cool carrier gas.
Fig.16 shows the effect of the feeding rate of CoC2O4 2H2O powders and the superficial gas velocities in the boiling bed on the temperature of combustion wave . Temperature of combustion wave decreases with increasing gas velocity at a given feeding rate. Therefore, the temperature of combustion wave can be adjusted and controlled dynamically by changing the feeding rate of CoC2O4 2H2O powders and/or the superficial gas velocity. According to the thermal analysis of CoC2O4 2H2O in the air, the CoC2O4 can be completely converted to Co3O4 at a temperature higher than 320C. Therefore, in order to ensure the dehydration reaction of CoC2O4 2H2O and the oxidation reaction of CoC2O4 can be carried out rapidly and completely, and the combustion temperature is set at 380~400C.
Being different from the conventional CS, the combustion wave here is almost full of the combustion zone in the middle of the boiling bed and the temperatures in whole space are homogeneous. The combustion wave is relatively suspended in the fluidized bed, and the temperature in the combustion zone can be dynamically controlled by adjusting the flow of carrier gas, as mentioned above. During the preparation, CoC2O4 2H2O powders are continuously fed by the screw feeder into the upper area of reaction boiling bed and contacted with the combustion wave. The Co3O4 particles are dispersed by the carrier gas flow. After the CS, the dispersed Co3O4 powders fall down and leave the combustion wave in time through flowing out off the overflow gate on the wall of the unit continuously.
The CS unit in the paper is different from the conventional one reported in Ref. [27,28]. In the conventional unit, the reactants and products are nearly stationary while the combustion wave propagates from reactants to products. The advantages of the continuous fluidization technologies and the CS are well utilized and combined in our unit. The combustion wave is relatively stationary, but the reactants and products are continuous and mobile at a given speed.
5.4. Quality of products
Fig.17(a) shows the morphology of Co3O4 powders fabricated by the CDCCS. The adhesion among Co3O4 powders shown in Fig.14(b) is broken by the carrier gas flow. The particles exhibit a spherical or quasi-spherical shape, and the size of Co3O4 particles is about 0.1~0.3μm. In the gas-solid fluidized bed unit, the temperature of the whole boiling bed is nearly homogeneous, and almost each solid particle in the boiling bed has a similar surrounding, namely all CoC2O4 2H2O particles can be completely decomposed to form Co3O4. The suspended solid particles collide and grind each other without aggregation by the action of airflow, which accelerates the formation of the spherical Co3O4 powders. The uneven distributions of particle size or hard aggregation appear scarcely due to the favorable diffusion condition and rapid transfer of heat and mass. The Co3O4 powders with a narrow particle size distribution, good dispersity and excellent fluidity are fabricated by the CDCCS.
As mentioned above, using the ultra-fine spherical Co3O4 powders fabricated by the CDCCS as raw materials, the ultra-fine spherical or quasi-spherical Co powders is easily obtained by hydrogen-reduction technology, as shown in Fig.17(b). The Co powders with an average size of 0.6μm possess a narrow particle size distribution, good dispersity, and excellent fluidity.
5.5. Advantages of the CDCCS
The preparation of ultra-fine spherical Co3O4 powders (the size is smaller than 0.8μm and the length-diameter radio is smaller than 2) by the CDCCS has been successfully applied in the industrialization production in Xiamen Golden Egret Special Alloy Co., Ltd [24,25].The novel method has the following several advantages:
Comparison with the fixed bed, ultra-fine spherical Co3O4 powders are continuously produced in the fluidized bed unit and the production efficiency is improved.
No external heat is needed after the reaction is once ignited, and then the energy consumption is reduced.
The fabrication process is in a closed unit so that a clean operation environment is realized and a high pure product is fabricated.
The device operations are automated, except the transport of reactants and products, thus the labor intensity is largely decreased.
The most important advantage is that the properties of the products fabricated by CDCCS are more excellent than those fabricated by the conventional method.
Ultra-fine spherical Co3O4 powders are firstly fabricated by CDCCS, and then ultra-fine spherical Co powders can be fabricated by hydrogen-reduction technology. Can ultra-fine spherical Co powders be directly fabricated by CDCCS if a hydrogen flow is blown into the CS unit? However, the hydrogen is a flammable and explosive gas. Therefore, our future work is how to directly fabricate ultra-fine spherical Co powders in the CS unit by solving the key technology difficulty of the hydrogen safety. Furthmore, the fabrication of many other metal powders (for example, W, Mo powders, etc.) can be applied in the CS unit.