Engeering Horizon

THE GROWING PULL OF PERMANENT MAGNETS

THE GROWING PULL OF PERMANENT MAGNETS

August 08
10:02 2014

THE GROWING PULL OF PERMANENT MAGNETS

By: Dr. Sara Qaisar, Zubair Ahmad*, Mi Yan* IICS, P.O. Box 1398, Rawalpindi, Pakistan *State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China

Permanent magnet provides magnetic flux into the air gap of a magnetic circuit without continuous supply of energy. The flux density may be uniform or non-uniform, steady or time varying. A good permanent magnet must have a high coercivity (Hc), which permits to avoid easy demagnetization, high remanence (Br) induces high magnetism that allows to design smaller and efficient components and a high energy product ((BH)max), allows to work for high performance devices with small magnet size. Magnetic properties of permanent magnets are divided into two general categories: those that are structure-sensitive and those that are structure-insensitive. Structure insensitive refers to the properties, which are not affected either by materials processing (thermal treatment or mechanical deformation) or by changes in alloy composition, including small amounts of certain impurities. Structure-insensitive properties include the saturation magnetization and resistivity. These properties are largely dependent on the composition of the particular alloy and are not changed substantially in the process of manufacturing of the component from the alloy. Structure-sensitive properties are those that are drastically affected by impurities. Small amounts of elements such as carbon, oxygen, nitrogen, and sulfur are commonly found in small quantities in magnetic materials. These elements tend to occupy interstitial sites in the crystalline lattice and consequently the lattice can be severely strained. As a result, small concentrations of these elements can have large effects on some of the magnetic properties of the materials. Permeability, coercivity, hysteresis losses, remanence, and magnetic stability are all considered to be structure-sensitive. The structure sensitive-properties are controlled through processing of the material.

Permanent magnets, which are currently of topical interest, can be classified into three important categories namely conventional magnets, rare earth magnets and nano-crystalline magnets.

Conventional permanent magnets include a vast array of alloys, intermetallic and ceramic magnetic materials. The most common magnetic materials are carbon or tungsten-steel, Vicalloy, Alnicos, iron-chromium-cobalt, cobalt-platinum and ferrites (SrO-Fe2O3 or BaO-6Fe2O3). The energy product up to 14.3 MGOe can be achieved by process optimization and microstructure refinement with Alnicos or iron-chromium-cobalt-molybdenum magnetic alloys.

Conventional magnets are widely used in electromechanical devices such as high speed rotors, generators, relays and electronic applications e.g. loudspeakers, traveling wave tubes, telephone ringers and holding devices of various applications.

Motors Need Permanent Magnets to Rotate Axle

Rare earth based magnets like SmCo5, Sm2Co17, Sm2Fe17Nx and Nd2Fe14B are produced by powder metallurgy, mechanical alloying and melt spinning techniques. Magnetic properties in these magnets are sensitive to the alloy composition and processing conditions. These magnets need a special protective atmosphere during each processing stage. The non-optimal micro-chemistry, inhomogeneity and non-ideal distribution of the grain boundary phase in the Rare Earth magnet microstructure may deteriorate the magnetic properties. Rare Earth magnets are superior to conventional magnets, where energy of magnetic field per unit volume and significant reduction in size and weight of devices are required. The energy product of SmCo5, Sm2Co17 and Nd2Fe14B permanent magnets have been almost reached to their theoretical limit. The world record energy product of 57 MGOe is for NdFeB magnet. Therefore, it is stringent to search for new permanent magnets for electro-mechanical device and electrical vehicles.

Nano-crystalline or nano-composite permanent magnets (NCM) are the new generation permanent magnets. They are known as exchange coupled magnets. These magnets have attracted world-wide interest and have opened new opportunities for scientific as well as commercial applications. According to models, the predicted energy product (BH) max of permanent nano-crystalline magnets can be exceeds 100 MGOe, which is higher than existing permanent magnets. Many achievements have been made with Nd2Fe14B (Pr2Fe14B)/Fe3B, Nd2Fe14B (Pr2Fe14B)/α-Fe, Sm2Fe17Nx/Fe and Sm2 (Fe,Co)17/(Fe,Co) nano-composite magnet system. These magnets are developed by mechanical alloying, mechanical milling, deposition technique (i.e., pulse laser deposition, magnetron sputtering or atomic-beam lithography) and rapid solidification process. These magnets are suitable for smart electro-mechanical, especially for thin magnet applications such as micro-gear for micro-motors, vibrator for cellular phone, relays for electro-mechanical devices and magnetic sensors micro-gears.

Permanent magnets play a role of increasing importance in modern society, serving as an essential component in anything from earphones, quartz analogue watches to microwave ovens and the space shuttles. They have wide range of industrial, domestic, automotive, biosurgical and aerospace applications. A modern permanent magnet may be a tiny ring at the heart of a compact disc player or a huge block providing the levitation force for the train of the future. For example, a modern family car contains almost 70 permanent magnets used in electronic ignition to automatic brake control and are helping us in adjusting seats, windows and sun roofs of the car. The current market value of permanent magnets is in excess of US$ 5.0 billion and is expected to rise to US$ 6.5 billion by 2013. Ferrites have shared around 41% of total magnet production in the world due

to economical raw materials. Nd-Fe-B magnets attained 28%, Sm-Co 8% and others 23% share values. Magnets have good prospects for innovative applications, especially if the properties of cost-effective magnet grades can be tailored to new requirements such as thermal stability or high-temperature operation. Scientific programs have been launched for the development of RE-free permanent magnets in Japan, China and USA with the aiming to eliminate dependence of crucial RE-metals due to supply chain problem as well as cutting cost for the magnetic products.

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