Study On Fabrication And Phase Transformation Of Porous Cobalt

The Co40Al60 alloy was melted in vacuum arc melting furnace and then annealed. Optical microscopy examination and X-ray diffraction proved that Co40Al60 alloy consists of Al9Co2 and Al13Co4 phase.After the dealloying of bulk Co40Al60 alloy in 6 M NaOH solution, we did not observe porous structure except a large number of cracks on the surface of alloy. This morphology is called"brick wall"structure. It is because the reaction of Co40Al60 alloy and NaOH is violent, extensive cracking is from shrinkage due to removal of aluminum. The analysis showed that the"brick wall"structure is a layer of oxide. In order to accelerate the leaching reaction, bulk alloy was crushed into powder. And the dealloying was conducted in 6 M NaOH which was heated to different temperatures (17℃,40℃,60℃,80℃,90℃) with constant stirring for 2h. Also in 6M NaOH which was heated to 80℃with constant stirring for different times (20 min,1 h,2 h). By analyzing the morphology of dealloyed powders,the XRD results,the EDS results and so on. We obtained the best dealloying condition was that Co40Al60 alloy powder was dealloyed in 6M NaOH heated to 80℃with constant stirring for 2 h. we obtained porous cobalt under above condition. The porous cobalt we obtained was consisted of fcc-Co,a little hcp-Co and CoO. The BET surface area of porous cobalt is 42.65 m2/g. The pore structure is mesoporous structure. And average pore size of porous cobalt is 9 nm.The porous cobalt prepared was annealed at different temperatures under vacuum. Annealing process: in the high vacuum condition 10-4 Pa, 360℃,500℃,700℃,1000℃annealing for 2 h, 1200℃and 1300℃annealing for 4 h. The relationship between crystal structure and annealing temperature (grain size) was studied by analyzing the XRD patterns of porous cobalt after annealing. The results showed that the grain size of fcc-Co is increasing with annealing temperature.After 360℃,500℃,700℃and 1000℃annealing for 2h, the porous cobalt was mainly consisted of fcc-Co and CoO. After 1200℃annealing for 4h there appeared hcp-Co in porous cobalt. This implied that the porous cobalt had a phase transformation. The grain size of fcc-Co is 42.9 nm and that of after 1000℃annealing for 2 h is 20.9 nm. The result showed that when the grain size of fcc-Co is bigger than 42.9 nm, the transformation of cobalt will happen, the critical size of nanocrystalline cobalt having phase transformation is 20.9 nm~42.9 nm.