【作者】 李顺；

【导师】 肖加余； 白书欣；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2008， 博士

【摘要】 纳米复合永磁材料是一种新型的永磁材料,它是由高饱和磁化强度的软磁相和高各向异性场的硬磁相在纳米范围内复合组成,通过纳米尺度下软、硬磁性相之间的交换耦合相互作用来获得磁性能。纳米复合Nd-Fe-B系永磁材料和传统的Nd-Fe-B系永磁材料相比,稀土含量较低,化学稳定性较好,而且具有高剩磁、高磁能积的特点,是一种新型的、有广泛应用前景的稀土永磁材料。本文利用熔体快淬法结合晶化退火工艺制备了一种具有新型成分范围的Nd₂Fe₁₄B/α-Fe（Fe₃B）型纳米复合永磁合金,通过X射线衍射（XRD）、振动样品磁强计（VSM）、示差扫描量热分析（DSC）、透射电镜（TEM）、热磁分析（TMA）等分析仪器和手段,重点研究了高熔点金属Ti和C添加对其结构和磁性能的影响规律。并在上述基础上,研究了合金中稀土Nd含量、B含量、Pr部分取代Nd以及与Ti性质相似的高熔点金属元素Nb、Zr、Cr和C的联合添加对合金结构和磁性能的影响规律。文章最后对Ti和C联合添加纳米复合磁体的磁化和反磁化过程、纳米复合永磁粉及其粘结磁体的特性进行了研究。在研究Ti和C添加对合金结构和磁性能的影响规律中发现,一定量Ti添加能够抑制Nd₂Fe₂₃B₃和Fe₃B相的形成,并且可以在Nd_9.4Fe_79.6B₁₁合金中形成细小且分布均匀的高熔点TiB₂质点相,细化晶粒,增强晶粒之间的交换耦合相互作用,提高合金的磁性能。综合磁性能较佳的Nd_9.4Fe_75.6Ti₄B₁₁合金薄带最佳晶化条件下的剩磁Br为0.87T,矫顽力iHc达到931.1kA/m,磁能积（BH）max达115.4kJ/m³。在Nd_9.4Fe_75.6Ti₄B₁₁合金中添加一定量的C,能够抑制合金中TiB₂沉淀相的生成,使TiC优先从体系中析出,将减少由于TiB₂析出而从体系中夺取的B元素量,从而在一定程度上增加体系中硬磁性Nd₂Fe₁₄B相的含量。适量Ti和C的联合添加改变了合金的晶化方式,使软磁性α-Fe相和硬磁性Nd₂Fe₁₄B相同时从非晶基体中析出,这种晶化方式避免了先析出相晶粒的长大,利于获得细小均匀的纳米晶结构。综合磁性能较佳的Nd_9.4Fe_75.6Ti₄B_10.5C_0.5合金薄带晶化后平均晶粒尺寸在15nm左右,其剩磁Br=0.91T,矫顽力iHc=975.6kA/m,磁能积（BH）max=135.4kJ/m³。文章从动力学理论的观点揭示了合金显微结构与合金晶化动力学特性之间的关系,指出Ti和C添加改变了Nd_9.4Fe_79.6B₁₁合金晶相析出时的动力学参量值,使合金晶化时晶相的析出方式由难形核易长大型转变为易形核难长大型,这种易形核而难长大的晶化动力学特征,是Ti和C添加合金获得细小均匀的纳米晶结构的根本原因。研究了Nd和B含量及Pr部分取代Nd对Nd-Fe-Ti-B-C合金结构和磁性能的影响规律,结果表明Nd和B含量可以改变合金薄带晶化后的相组成、相分布和晶粒尺寸,因而改变了软硬磁性相之间的交换耦合相互作用,从而使不同Nd和B含量的合金呈现出不同的永磁特性。成分为Nd₉Fe₇₆Ti₄B_10.5C_0.5的合金晶化后具有最佳的永磁性能。Pr取代Nd没有改变Nd-Fe-Ti-B-C永磁合金晶化后相的组成,但Pr使合金薄带晶化后晶粒变得粗大,不利于合金矫顽力的提高。Pr对合金薄带磁性能影响不大,直接利用稀土矿的次级分离产物Di来制备高性能低稀土含量纳米复合Di-Fe-Ti-B-C永磁合金完全可行,制备出的合金剩磁Br在0.86T与0.90T之间,内禀矫顽力iHc在1000kA/m左右,最大磁能积（BH）max介于130kJ/m³与136kJ/m³之间。在此基础上指出当稀土含量为8⁹.5at.%,Ti含量约为4at.%,C含量约为0.5at.%,B含量为8.5¹2.0at.%,其余为Fe时,Re-Fe-B-Ti-C永磁合金在“过快淬+最佳退火”后可以得到平均磁能积约为130kJ/m³的纳米复合磁体。当稀土含量达到9at.%时,B含量约10.5at.%时,纳米复合永磁合金的内禀矫顽力可以达到1000kA/m。在Nd_9.4Fe_79.6B₁₁合金中添加高熔点金属元素Ti、Nb、Zr和Cr,均能形成质点相,从而抑制合金中Nd₂Fe₂₃B₃和Fe₃B相的生成,细化晶粒,提高矫顽力。但Ti、Nb和C的联合添加能够在不降低合金薄带剩磁的情况下获得高矫顽力,实现剩磁和矫顽力的有效平衡,而相同含量Zr、Cr和C的添加降低了合金薄带的剩磁,不能实现矫顽力和剩磁的有效平衡。通过对合金薄带起始磁化曲线、矫顽力与磁化场的关系、回复曲线、退磁曲线可逆与不可逆行为的研究,发现未添加Ti和C的Nd_9.4Fe_79.6B₁₁合金的矫顽力同时具有形核和钉扎两种机制,形核场与钉扎场中较大的场决定了合金的矫顽力;而添加Ti的Nd_9.4Fe_75.6Ti₄B₁₁合金及Ti和C联合添加Nd_9.4Fe_75.6Ti₄B_10.5C_0.5合金的矫顽力由畴壁位移的钉扎机制控制。元素添加后在合金中晶粒与晶粒之间形成的薄层晶界相是Ti和C添加合金畴壁运动时的钉扎中心。温度稳定性研究结果表明,含Ti和C的纳米复合磁粉在25¹00℃之间具有较好的温度稳定性,其温度系数与MQ公司生产的具有较低温度系数的MQP-C和MQP-D磁粉相当,优于MQ公司生产的纳米复合磁粉MQP-15-7和MQP-16-7。Ti和C的纳米复合磁粉温度稳定性的提高源于元素添加后合金显微结构的改变。抗氧化性研究结果表明,含Ti和C的纳米复合磁粉具有比MQP系列磁粉优异的抗氧化性能,更适宜于在复杂环境中的应用。文章最后用含Ti和C纳米复合磁粉制备了粘结磁体,在相同制备工艺条件下,纳米复合Nd₉Fe₇₆B_10.5Ti₄C_0.5粘结磁体与用MQ-D磁粉制备的粘结磁体剩磁和最大磁能积基本一致,但矫顽力更高,稀土含量低,仅为9at.%,且不含战略元素Co,具有性价比高的特点。更多还原

【Abstract】 Nanocomposite permanent magnetic materials are composed of hard magnetic phases with high anisotropic field and soft magnetic phases with large saturation magnetization. In compared with the traditional sintered Nd-Fe-B magnets, Nd-Fe-B based nanocomposite magnets have received much attention for their potential applications because of their enhanced remanence and maximal energy product, low rare earth content, low cost and high corrosion resistance.In this dissertation, nanocomposite Nd₂Fe₁₄B/α-Fe（Fe₃B） type hard magnetic alloys have been prepared by melt-spinning technique and subsequent crystallization annealing. With the help of X-ray diffraction （XRD）, vibrating sample magnetometer （VSM）, differential scanning calorimeter （DSC）, transmission electron microscope （TEM）, and thermal magnetic analyzer （TMA）, the effects of Ti and C additions on the microstructure and magnetic properties of the alloys have been investigated, and the influences of Nd content, B content, partial substitution of Nd by Pr, and the addition of refractory elements such as Nb, Zr, and Cr together with C on the microstructure and magnetic properties of the alloys have also been researched. At the end of this dissertation, the magnetization and demagnetization behavior, the magnetic properties of magnetic powders and their bonded magnets, of Ti and C doped nanocomposite alloys have been studied.The results show that the formation of unfavorable soft Nd₂Fe₂₃B₃ and Fe₃B phases can be suppressed effectively by the addition of Ti. When Ti content reaches a certain amount, Ti may precipitate as TiB₂ from the Nd_9.4Fe_79.6B₁₁ alloys, which can refine the structure, thus the exchange coupling between the hard and soft phases is enhanced. As a result, amazing magnetic properties of Br=0.87T, iHc=931.1kA/m, and （BH）_max=115.4kJ/m³ are achieved for Nd_9.4Fe_75.6Ti₄B₁₁ alloy ribbons. Additional C addition in Nd_9.4Fe_75.6Ti₄B₁₁ alloys can suppress the formation of TiB₂ by the preprecipitation of TiC. This can release abundant boron from TiB₂ to insure the formation of hard Nd₂Fe₁₄B phases. The results also show that Ti and C addition changes the crystallization behavior of Nd_9.4Fe_79.6B₁₁ alloys, leading to the simultaneous precipitation ofα-Fe and Nd₂Fe₁₄B phases from amorphous Nd_9.4Fe_75.6Ti₄B_10.5C_0.5 alloys. This behavior can avoid the growth of the first precipitating phases and let the soft and hard phases grow together, which is of avail for attaining a fine and uniform microstructure. Excellent magnetic properties of Br=0.91T, iHc=975.6kA/m, and （BH）max=135.4kJ/m³ , together with a fine microstructure with a average grain size of about 15nm have been attained in Nd_9.4Fe_75.6Ti₄B_10.5C_0.5 ribbons. The relationship between the microstructure and the crystallization behavior of the alloys has been discussed by using the kinetic theories. The results point out that the values of kinetic parameters are altered with the addition of Ti and C, which change the crystallization type from difficult nucleation and easy growth pattern to easy nucleation and difficult growth pattern, and this difficult nucleation and easy growth pattern may be the main reason why fine and even microstructure is gained in Ti and C doped alloys.The influences of Nd content, B content, partial substitution of Nd by Pr on the microstructure and magnetic properties have also been researched. The results reveal that the distribution, content, and grain size of hard and soft phases are remodeled with the change of Nd and B contents, thus alter the exchange coupling between hard and soft phases, resulting in ribbons with different magnetic properties. The ribbons with nominal composition of Nd₉Fe₇₆Ti₄B_10.5C_0.5 have the best magnetic properties. Partial substitutions of Nd by Pr do not change the constitution of alloys after crystallization. But microstructures with an inhomogeneous and coarse grain size are gained with the substitution of Pr, which is of disadvantage for the increase of coercivity. Pr has little effect on the magnetic properties of (Nd_1-xPr_x)_9.4Fe_75.6Ti₄B_10.5C_0.5 ribbons, ribbons with Br between 0.86T and 0.90T, iHc about 1000kA/m, and （BH）max between 130kJ/m³ and 136kJ/m³ are all achieved with different ratio of Nd and Pr following their optimal crystallization annealing, which implies that it is possible to prepare high performance, low rare earth content nanocomposite Di-Fe-Ti-B-C permanent alloys with didymium directly. For nanocomposite Re-Fe-B-Ti-C alloys with 8⁹.5at.% rare earth metal, 4at.% titanium, 0.5at.% carbon, and 8.5¹2.0at.% boron, an average energy product of about 130kJ/m³ would be attained, following over melt spinning and subsequent crystallization annealing. Especially when the rare earth metal content is 9at.%, and boron content is 10.5at.%, a high coercive above 1000kA/m could be achieved.Stable particles can appear in all nanocomposite Nd_9.4Fe_79.6B₁₁ alloys with Ti, Nb, Zr, and Cr addition, which can avoid the formation of Nd₂Fe₂₃B₃ and Fe₃B phase, refine the structure, and thus increase the exchange coupling interaction between the soft and hard phases. The addition of Ti, Nb and C are found to be particularly effective in increasing the coercivity without sacrificing much remanence, but the effects of Zr, Cr and C additions are negative.Through the investigation of the initial magnetization curves, the relationship between coercivity and the magnetized field, the recoil curves, the reversible and irreversible portions of demagnetization curves of the ribbons, it founds that the coercivity mechanism of Nd_9.4Fe_79.6B₁₁ alloys is dominated by both nucleation and domain wall pinning, and the larger domain wall pinning field determines the coercivity. But in Ti or Ti and C doped alloys, the coercivity mechanism is only controlled by domain wall pinning. The thin interface phases between the crystal grains may be responsible for the magnetic hardness for alloys with Ti and C additions.Excellent thermal stability has been attained in magnetic powders containing Ti and C, whose temperature coefficient of remanence and coercivity between 25¹00℃are about the same with that of MQP-C and MQP-D magnetic powders, but better than that of MQP-15-7 and MQP-16-7 nanocomposite magnetic powders. The excellent thermal stability may due to the change of microstructure by Ti and C additions. The results also reveal that nanocomposite magnetic powder containing Ti and C has more outstanding antioxidation properties than that of MQP magnetic powders, and is more suitable for use in high temperature environment.At last, bonded magnets are prepared with magnetic powders containing Ti and C and MQ-D. The remanence and maximum energy product of bonded Nd₉Fe₇₆B_10.5Ti₄C_0.5 and MQ-D magnets are about the same, but the former has a larger coercivity. In contrast with MQ-D, Ti and C doped magntic powders has only 9at.% rare earth, and does not contain cobalt, which presents a property of high performance with low cost.更多还原