Mechanochemical Synthesis And Permanent Magnetic Properties Of Nd₂Fe₁₄C And Sm₂Fe₁₇C_x Materials

In this paper,a novel synthesis method,namely mechanochemical method for Nd₂Fe₁₄C and Sm₂Fe₁₇C_x permanent magnetic materials is proposed.In our previous investigation,it is found that being ball-milled in heptane(C₇H₁₆),SmCo₅ alloy disproportionates into SmH_2+δ and α-Co,similar to the disproportionation reaction happened in the HDDR（hydrogenation,disproportionation,desorption and recombination）process of Nd-Fe-B materials.Annelaed in vacuum,H₂ is released and SmCo₇ compound with better permanent magnetic properties and small amount of SmCoC₂ phase are formed.H and C atom come from heptane.This disproportionation reaction induced by ball milling in heptane avoids the difficulty in the disproportionation of Sm-Co alloy when apply HDDR technology to Sm-Co system.In addition,Nd₂Fe₁₄C and Sm₂Fe₁₇C_x compounds have excellent intrinsic magnetic properties while the difficulty in synthesis limits their development.In this paper,the above-mentioned mechanochemical synthesis technology,namely ball milling in heptane followed by annealing in vacuum,applied to Sm-Co system is investigated further on one hand,and the mechanochemical systhesis of Nd₂Fe₁₄C and Sm₂Fe₁₇C_x compounds by means of the C element obtained from the disproportionation reaction is explored on the other hand.The major research works are as follows:1.The effect of Sm to Co ratio of the Sm-Co alloy on the disproportionation reaction and the constituent phases of the final procuct undergoing ball milling and annealing under vacuum is investigated and it is proved that single SmCo₅ phase,which owns the highest magnetocrystalline anisotropic field（HA）,can not be obtained by this mechanochemical method.All the alloys with nominal composition of Sm_0.7Co₅,SmCo₅ and Sm_1.3Co₅,ball milled in heptane for 8 hours,disproportionate into SmH_2+δ and α-Co.The sample with high Sm content contains more SmH_2+δ phase after ball milling and there is more H₂ released during the annealing process.After the annealing to 800℃,SmCo₇ and soft magnetic α-Co phases were formed in Sm_0.7Co₅ alloy.Only SmCo₇ phase is formed in SmCo₅ alloy.Sm₂Co₇,SmCo₅ and SmCo₇ phases are formed in Sm1.3Co5 alloy.With the increase of Sm content,the coercivity increases and the magnetization decrease.In addition,non magnetic SmCoC₂ has been observed in all samples.2.Nd₂Fe₁₄C compound is synthesized by this mechanochemical method,with the Nd-Fe alloy(containing Nd₂Fe₁₇ and Nd)as the precursor.Ball milled in heptane,the Nd2Fe7 alloy disproportionates into NdH_2+δ and α-Fe.During the subsequent annealing process,H₂ is released and the Nd₂Fe₁₄C compound is formed successfully.After annealing,there is a little residual soft magnetic Nd₂Fe₁₇ phase in the sample with ball-milling time t = 6h,due to the insufficiency disproportionation reaction of Nd-Fe alloy.There appears α-Fe phase in the sample with ball-milling time t = 10 h.This is because long ball milling results in too much C which consumes more Nd by means of the formation of Nd-C phase.In contrast,the sample ball milled for 8 hours contains single permanent magnetic Nd₂Fe₁₄C phase and exihibits relatively good permanent magnetic properties: coercivity iHc = 13.9 kOe,remanent magnetization Mr = 6 kGs,maximum magnetic energy product（BH）max = 7.84 MGOe.A Nd-C phase compound was formed in all the samples after annealing.3.By adding small amount of B element in Nd-Fe alloy（Nd2Fe11Bx,x=0-0.15）,the Nd-C phase is suppressed and the Nd₂Fe₁₄C content in the final product and also the permanent magnteic properties are increased.Besides,the ball milling is shortened and the Sm to Co ratio can decrease to 2:11.The minimum value of Nd-C phase was obtained in Nd₂Fe₁₁B_0.06 with ball milling time t=3.5h.The best magnetic properties,coercivity iHc = 15 kOe,remanent magnetization Mr = 7.2 kGs,maximum magnetic energy product（BH）max = 11.3 MGOe,are achieved in this sample.4.Permaenent magnetic Sm₂Fe₁₇C_x interstitial compounds were synthesized by this mechanochemical method,with SmFe₃ alloy as precursor.Ball milled in heptane,the SmFe₃ alloy disproportionates into SmH_2+δ,α-Fe,Fe₃C and C phases.H₂ is released during the annealing process.After annealing to 850℃,the main phase in samples is soft magnetic Sm₂Fe₁₄ C.After second ball milling for 1.5h in Ar and second annealing to 600 ℃ for 15 minutes,partial Sm₂Fe₁₄ C phase turns into the permanent magnetic Sm₂Fe₁₇C_x phase.Due to the intergrain exchange coupling between permanent magnetic Sm₂Fe₁₇C_x and soft magnetic Sm₂Fe₁₇C_x and α-Fe,the nanocomposite magnetic material exhibits a behavior similar to the magnet with single permanent magnetic phase,and the highest coercivity of 4kOe is achieved.In addition,Sm-C compounds were formed all samples.In summary,Nd₂Fe₁₄C and Sm₂Fe₁₇C_x permanent magnetic materials with good permanent magnetic properties were successfully synthesized by mechanochemical method.Mechanochemical method does not need high-pressure hydrocarbon gas and long time annealing,C atom is directly from heptane(C₇H₁₆).Hence,it is simpler,safer and more efficient.Nd₂Fe₁₄C and Sm₂Fe₁₇C_x permanent magnetic materials synthesized by mechanochemical method have great potential to develop into commercial permanent magnetic materials.