Effect of grain boundary diffusing Dy-Al-Ga on the microstructure and magnetic properties of NdFeB magnets
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摘要: 通过晶界扩散Dy70Al10Ga20合金研究了烧结Nd-Fe-B磁体的磁性能和热稳定性能.用NIM-500C高温永磁测量仪和MLA650扫描电镜测出了磁体在扩散前后的磁性能和微观组织的变化.结果表明,在Dy70Al10Ga20合金扩散热处理后,磁体矫顽力从原始的1 080.968 kA/m显著提升到1 671.600 kA/m,提升幅度约为55 %,而剩磁下降很少. Dy、Al、Ga元素在晶界处扩散,很好地隔绝了磁交换作用,提升矫顽力. SEM图显示在扩散Dy70Al10Ga20合金后,可以很明显地看到晶粒外延层有一层灰色的壳层包覆着主相晶粒,很好地起到了隔离晶粒的磁交换作用. XRD显示主相的峰普遍往右偏移,这归因于重稀土元素Dy进入晶粒外延层形成(Nd, Dy)2Fe14B核壳结构. Dy的原子半径比Nd小,导致峰往右移.Abstract: The magnetic properties and thermal stability of sintered Nd-Fe-B magnets were studied by grain boundary diffusing Dy70Al10Ga20 alloy. The changes of magnetic properties and microstructure of the magnets were measured by NIM-500 C and SEM. The results show that after Dy70Al10Ga20 alloy receiving thermal diffusion treatment, its coercivity is enhanced significantly from 1 080.968 kA/m to 1 671.600 kA/m, a striking increment of 55 %. And there is only a slight reduction in remanence. Dy, Al and Ga elements diffuse at the grain boundary, which effectively insulates magnetic exchange so that the coercivity of the alloy is well improved. It is obvious that after Dy70Al10Ga20 alloy is diffused, in the epitaxial layer of grains is a gray shell covering the main phase grains via SEM, which plays an active role in magnetic exchange to isolate the grains. The peak of the main phase is generally shifted to the right via XRD observation, given the fact that the heavy rare earth element Dy enters the epitaxial layer of grains and forms the (Nd, Dy) core shell structure. The atomic radius of Dy is smaller than that of Nd, causing the peak to move to the right.
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Keywords:
- grain boundary diffusion /
- low-melting alloy /
- sintered NdFeB /
- coercivity
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在开采地下矿产资源的过程中,通常会形成形态各异、大小不一的空区,这些不规则复杂空区不仅直接威胁着井下设备和作业人员的安全,而且关系到矿山能否安全高效地回收地下矿产资源,因此对空区的空间形状、即时状态进行精密探测以及后期安全治理显得尤为重要,然而传统的空区探测方法,如高密度电阻率法、探地雷达法等探测深度和精度有限,过程繁琐,可视化程度低.
3D激光探测法是目前国际上广泛使用的一种新型空区探测方法,该方法是利用激光的高精确性对地下空区的位置、大小进行三维探测.C-ALS是一种用于探测地下空区的新型3 D激光探测技术,该探测系统可以迅速记录与目标物相关的三维数据信息,通过数据遥感勘测系统将所测得的数据输送到主控装置,同时利用计算机对数据进行获取和管理,再利用软件编辑和处理数据,构建空区三维模型,在此基础上,可以计算空区的体积、采场超欠挖量以及导入到数值模拟软件中进行稳定性分析,为空区有效治理提供了技术依据.
本文以龙桥铁矿的0~2线空区为工程背景,采用三维激光探测系统(C-ALS)进行了激光扫描,获得该空区的原始点云数据,同时借助Surpac矿业建模软件对原始数据进行处理以构建该空区的三维空间模型,准确地获取了其形状、空间位置、体积大小、暴露面积等信息,进而对相关数据进行了处理,并分析了顶板覆岩冒落高度及其原因.
1 三维激光探测系统(C-ALS)简介
三维激光探测系统(C-ALS)是英国Measurement Devices Limited公司生产的一种新型的采空区探测设备,该设备主要用于探测地下矿山隐伏空区,目前在国际上得到了广泛使用,由于其探头直径较小,仅为50 mm,故可以伸入地表直通地下空区的钻孔进行探测.
1.1 C-ALS基本构成
C-ALS主要由激光扫描探头、标准加长杆、钻孔摄像头、电源、电缆和计算机控制软件等构成,如图 1所示;其中计算机控制软件为C-ALS自带的软件CavityScan,它不仅可以处理空区探测后产生的原始数据,并且可以将其导入到CAD和其他建模软件中进行处理.
1.2 C-ALS工作原理
C-ALS探测系统采用激光测距的原理,扫描探头内置一个激光扫描仪,扫描仪发射脉冲激光,激光依次扫过空区后到达空区壁后被发射返回,返回的时间通过高精度的计时器记录下来,然后通过微处理器利用该时间自动计算出距离,设该距离值为s,每个激光脉冲横向扫描角度为α,纵向扫描角度θ,由此可得到每个三维激光测点坐标的计算公式[1-9],如图 2所示.
激光扫描探头伸入空区后可以上下180°、水平360°旋转,达到对空区完整扫描的目的.系统测量范围0.5~150 m,精确度为(±)5 cm,水平和垂直角精确度为0.1°,数据捕捉率为200点/s.C-ALS主要有3种扫描类型:单水平切面扫描、水平扫描和垂直扫描,其工作原理如图 3所示.
2 现场探测方法和内容
(1)设备架设.根据龙桥铁矿采空区现场情况,扫描探杆水平放置,扫描头朝向与0~2线联络巷平行,并设计了3根2 m长的连接杆,尽量将扫描探头伸入采空区,避免出现扫描盲区,空区探测位置和设备架设现场分别如图 4、图 5所示.
(2)空区探测.设备架设妥当之后,连接系统元件并接通激光发射装置的电源,打开计算机控制软件,根据扫描需要选择扫描类型、设置初始扫描角度和探头一次抬升的角度,准备就绪后就可以进行探测了,探测现场如图 6所示.
(3)探头定位.为了精确每个扫描点的坐标,需测定扫描探头的坐标及加长杆件的方位角,由于探测时没有探测扫描头坐标与扫描探杆的方位角,因此,本次探测时默认扫描头坐标为X=1000, Y=1000, Z=100.
3 采空区模型构建与三维信息获取
3.1 采空区三维模型构建
在采空区现场探测获得原始数据后,需在探测系统自带的软件CavityScan中将“.mdl”格式的原始数据转换为“.dxf”或“.str”格式的文件,然后将其导入到Surpac、Datemine、Dimine等建模软件中构建空区三维实体模型[10-13].本论文采用三维可视化程度高的Surpac软件来构建空区三维实体模型,采空区三维建模技术流程如图 7所示.
由于扫描现场条件制约,空区形状不规则,采场内矿堆太高,视野不够开阔,致使扫描空区有部分盲区,但不影响空区大致轮廓,将扫描数据导入Surpac软件构建采空区三维立体模型如图 8所示.
3.2 空区高度的计算
探测结果在Surpac中构建的三维实体模型,通过有效性验证后就可以求出采空区的体积,其计算结果如图 9所示.
由空区实体报告可知,空区体积为17607 m3,空区高度最高达45.451 m,结合该空区采矿活动历史沿革,首采层位于-320 m分层,采用浅孔爆破,分层高度大约为7 m,本次测试的水平标高为-355 m,依次推论此空区的高度理论值约为40 m以上,与空区实测值基本接近,这说明该区段空区顶板覆岩基本没有冒落,其主要原因是:该区段空区暴露的倾向跨度值较小,至今仍未满足空区顶板覆岩冒落的临界跨度.
4 结论
(1)三维激光探测系统(C-ALS)对不规则空区进行的探测,能够准确地扫描出空区的实际构造状况,根据扫描得到的三维点云数据可借助矿业建模软件进行处理,从而得到空区的三维表面模型,使得地下不明空区可视化,提高了空区探测数据的可靠性和实用性,为矿山的持续安全开采提供了详尽的信息.
(2)龙桥铁矿的0~2线空区进行无损探测的结果表明:实测空区的高度与理论推测值基本接近,该区段空区顶板覆岩基本没有冒落,其主要原因是,该区段空区暴露的倾向跨度值较小,至今仍未满足空区顶板覆岩冒落的临界跨度.
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表 1 N52磁体成分/%
Table 1 Composition of N52 magnet /%
表 2 原始磁体和扩散Dy70Al10Ga20合金后磁体的磁性能
Table 2 Magnetic properties of the original and diffused Dy70Al10Ga20 alloy magnet
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