Effect Of Antiferromagnetic Nd₆Fe₁₃M Compound On The Microstructure And Permanent Magnetic Properties Of NdFeB Magnet

With the development of society and the growing demand within the industrial field,higher requirements are placed on the coercivity and remanence as well as the temperature coefficient of NdFeB permanent magnets.The increasing price of rare earth resources has made the development of high-performance NdFeB permanent magnets without heavy rare earth a mainstream development trend.Based on the relationship between microstructure and magnet coercivity,and this characteristic of the antiferromagnetic phase Nd₆Fe₁₃M（M=Al,Cu,Ge）pegging the main ferromagnetic phase,this thesis obtained high-performance NdFeB magnets with the antiferromagnetic phase as the intergranular phase by three preparation methods of grain boundary diffusion,elemental doping,and double alloying method,and regulated the suitable heat treatment process,and elucidated in detail.The mechanism of the effect of different antiferromagnetism on the magnetic properties of NdFeB in the thin strip was elucidated in detail.The main research results obtained are as follows:（1）The grain boundary diffusion of commercial NdFeB magnets was carried out by using free-heavy rare-earth diffusion source Nd₂₈Fe₆₇M₅（M=Cu,Al,Ge）.The effects of different diffusion sources on the magnetic properties of magnets under different heat treatment processes were studied.Under the heat treatment system of900°C-10h-550-1h40min,the coercivity and remanence of the magnet of the diffusion source Nd₂₈Fe₆₇Ge₅ are improved.Under the reasonable heat treatment system of other diffusion sources,the coercivity of the magnet increases obviously,and the remanence decreases.According to the microstructure of the magnet after diffusion,the number and distribution of grain boundary phases in the magnet have been significantly improved,which is the key to improve the magnetic properties of the magnet.In addition,the antiferromagnetic Nd₆Fe₁₃Cu phase was found in the intergranular phase of some magnets,and the demagnetization curves of the corresponding magnets have good squareness and high magnetic properties,indicating that Nd₆Fe₁₃Cu phase has an important influence on the magnetic properties of magnets.（2）The original composition of NdFeB magnet was designed.Three different elements of Cu,Al and Ge were doped at the Fe site.NdFeB ribbons with antiferromagnetic Nd₆Fe₁₃M（M=Cu,Al,Ge）intergranular phase were obtained by melt spinning technology.The effect of the antiferromagnetic phase on the microstructure and magnetic properties of NdFeB ribbons was studied.Firstly,the crystal structure and phase composition of the ribbons were determined by XRD.It was found that Cu,Al and Ge elements did not enter the main phase,but Ge element formed Nd₅Ge₃ phase in the intercrystalline.The microstructure and micro-area composition of the obtained ribbons were characterized by SEM and EDS techniques.It was found that the ribbons had continuous and uniform intercrystalline phase,and the number of bulk Nd-rich phases was small.Nd₆Fe₁₃M（M=Cu,Al,Ge）phase also existed in the intercrystalline phase.Through the characterization of the magnetic properties of the ribbons,it is found that the exchange bias effect exists in the hysteresis loops of all the ribbons at room temperature and low field,and the exchange bias effect is weakened at high field.However,by deriving the initial magnetization curve,it can be determined that the coercivity mechanism of the prepared ribbons is a pinning field dominated mixing mechanism.In addition,the ribbons with Nd₆Fe₁₃M（M=Cu,Al,Ge）phase have low temperature coefficient and can maintain good magnetic properties at high temperature.（3）The microstructure and magnetic properties of Nd₆Fe₁₃Cu on NdFeB recycled magnets under different heat treatment systems were studied and analyzed by adding different mass ratios of Nd₆Fe₁₃Cu to waste NdFeB quenched magnetic powder.Through SEM and EDS techniques,it is determined that the addition of Nd₆Fe₁₃Cu can change the sintering mode of the rapidly quenched magnetic powder from solid-phase sintering to solid-liquid sintering,which helps to improve the density of the magnet.With the increase of the addition amount,the proportion of intergranular phase gradually increases,and the magnetic properties are further enhanced.After the completion of sintering,the magnet was tempered at low temperature.The low temperature tempering made the grain grow up,and the grain size was too large,which led to a significant decrease in the magnetic properties of the magnet.The maximum decrease of coercivity and remanence could reach 81.7%and 53.0%.