Mechanism And Application In The Preparation Of Nd₂Fe₁₄C Rare Earth Permanent Magnet Of Mechanical Alloying Involving Hydrocarbons

Rare earth permanent magnet materials have important applications in medical equipment,electronic information and computers,vehicles,generators,sensors,etc.Nd₂Fe₁₄C compound with excellent inherent magnetic properties,has not been applied widly due to the difficulty in the preparation.The main reason is that Nd₂Fe₁₄C is not a stable phase at the high temperature stage of alloy smelting and powder metallurgy sintering.The powder metallurgy technology applied to Nd₂Fe₁₄B permanent magnetic material cannot be applied to Nd₂Fe₁₄C.In this thesis,a mechanical alloying involving hydrocarbons is applied to prepare Nd₂Fe₁₄C.The mechanism of the relevant chemical reaction and influence of the technological conditions on the component and permanent magnetic properties are investigated thoroughly.The detailed works are as follow.1.The mechanism of reaction between hexane(C₆H₁₄)and Nd induced by high energy ball milling is investigated firstly.C₆H₁₄、C₆H₁₄+Fe、C₆H₁₄+Nd、C₆H₁₄+Fe+Nd are milled respectively and the gas in the ball milling vial are detected by gas chromatography to determined the existence of H₂.H₂ is hard to be detected in the vial in which only C₆H₁₄is milled,meaning C₆H₁₄seldom decompose.When C₆H₁₄and Fe are milled together,large amount of H₂ is observed,meaning massive C₆H₁₄decomposes under the action of Fe.Milling NdH_2+δ+Nd and C₆H₁₄+Fe+Nd,small quantity of H₂ bas been observed.While considering the existence of NdH_2+δ in the milling product,C₆H₁₄also be decomposed and Nd also promotes the decomposing reaction.Hence it is verified that C₆H₁₄decompose to produce H₂ under the action of ball milling and the H₂ react with Nd to produce NdH_2+δ.Decomposing of C₆H₁₄should produce matter containing C element,which has not been obversed maybe due to the small amount and small grains size.2.C₆H₁₄+ Nd+Fe（Nd:Fe=20:80）,C₆H₁₄+ Nd+Fe（22:78）,C₆H₁₄+ Nd+Fe（24:76）are milled for 3h,3.5h,4h,4.5h and 5h.Nd and C₆H₁₄react to form NdH_2+δ.Annealing from room temperatue to 800°C and maintaining for 10 min under vacuum,H₂ release between700-800℃.According to the XRD,NdH_2+δ diminished and Nd₂Fe₁₄C formed.Samples with short milling time contain more soft magnetical phase Nd₂Fe₁₇and that with long milling time contain more soft magnetically α-Fe.Nd-C has been obversed in all samples.More Nd₂Fe₁₇exists in sample with less Nd and more Nd-C exists in sample with more Nd.The optimum permanent magnetic properties of_iH_c = 5.3kOe,M_r= 4.6 kGs and（BH）_max= 3.3MGOe are obtained in Nd₂₂Fe₇₈milled for 3.5h.3.Nd₂₂Fe₇₈B_0.05,Nd₂₂Fe₇₈B_0.075and Nd₂₂Fe₇₈B_0.1are processed by above mentioned technique.The addition of B promotes the formation of Nd₂Fe₁₄C and reduce the soft magnetically Nd₂Fe₁₇,α-Fe and no magnetic Nd-C obviously.The best raw material ratio is Nd₂₂Fe₇₈B_0.075,and the best synthesis conditions are ball milling for 5h and heat treatment at 800 ℃ for 10 min.The permanent magnetic properties of the materials obtained are: coercive force iHc = 9.0kOe,residual magnetization Mr = 5.5kGs,maximum magnetic energy product（BH）max = 5.7MGOe.The study also found that when the doping amount of B is 10%,the magnetic properties of the obtained material are significantly reduced,which shows that excessive B content leads to a decrease in permanent magnetic properties.