Strontium Ferrite Permanent Magnet-An Overview

Indian Journal of Engineering & Materials Sciences Vol. 7, October-December 2000, pp. 364-369

Strontium ferrite permanent magnet-An overview

a b Amitabh Verma , 0 P Pandeyb & Puneet Sharma b Thapar Institute of Engineering and Technology, Patiala 147 001, India a Materials Science Di vision, Thapar Centre for Industrial Research and Development, Patiala 147001 , India Received 25 February 2000; accepted 23 Novell/ber 2000

Among the permanent magnets hard ferrites, particularly M-type hexagonal ferrites (Sr-Ferrite and Sa-Ferrite) have special place by virtue of their low cost and reasonable magnetic properties. This paper gives a brief introduction of hard ferrites with special reference to strontium ferrite permanent magnet. Processing of, sintered and plastic - bonded magnet

has been di scussed. The effect of composition of strontium ferrite powders and that of various additives, such as CaO, Si02, AI 20 ), etc. on the magnetic properties have also been outlined.

Magnetic materials can be classified into two groups, per unit of available magnetic energy is low . i.e. magnetically soft (easy to magnetize and Comparative properties of M-type ferrite with that of demagnetize) and magnetically hard materials other magnets are given below in Table 11. (difficult to magnetize and demagnetize). Some Comparison shows that rare earth-based magnets have important hard magnetic materials are ferrites, very high magnetic properties than ferrites, but at Alnicos, rare earth - transition metal compounds. In present they cost more than ferrites. Presently, their the class of magnets, ferrites enjoy a unique position use is limited to these applications which call for due to their commercial importance. Ferrites can be miniaturization. described as a class of magnetic oxide which contain iron oxide as the principal component. Hard fen"ites Crystal Chemistry and Physical Properties have a hexagonal structure and can be classified in Barium and strontium ferrite have the same crystal M-, X-, Y- , W-, Z- type fen"ites and given by the structure as magnetoplumbite. The hexagonal unit cell formulae: of Sr-Ferrite is shown in Fig. 11 and schematic representation of Sr.Ferrite structure in Fig. 22 M-type - R Fel 201 9 R = Ba, Sr, Pb. Sr Ferrite has the Chemical Formula Sr" Fe"" 0 ',,, 2 2 2 W-type - R Me2 Fe1 60 27 Me =Fe +, Ni +, Mn + etc. The hexagonal unit cell of Strontium Ferrite contains 2 molecules or total 2x32 = 64 atoms. It is very lon g X - type - R Me Fe28 0 46 in c-direction (c=2.32nm and a=0.588 nm) . Sr" and 0 ' Y - type -R2 Me2 Fel 2 0 22 are both large and closely packed. The smaller Fe" Z - type -R3 Me2 Fe24 0 41 ions are located in the interstices. The structural unit is bui lt up of smaller units: a W- , X- , Y- , Z- type ferrites are not interesting cubic block having spinel structure and a hexagonal economically because of their relatively difficult block containing Sr" -ion. The unit cell is built up by processing, but the M-type ferrites, isostructural with stacking of layers of these atoms one on top of magnetoplumbite, are by far the most important another. Fig. 2 shows 10 layers of large ions (Sr",O') hexagonal fen"ites. M-type Ferrites are mainly used as permanent magnet materials, that have strong Table I- Magneti c properti es of Common permanent magnets resistance to demagnetizing field once they get S. Materi als ;Hc (B H)ma, Bf magnetized and have a domin ant pos ItI on in No. permanent magnet market. They are preferred over (mT) kAhn ( I-; J/m') Alnicos due to lo wer material and processing cost and I. Alnico 650 40-45 42-45 superi or coercivity. Sr-Ferrite and Ba-Ferrite are the 2. Ferrites 350-400 240-320 27 .5-31.5 two main material s in th e M-type ferrite fam il y. These 3. Sm Co 1050- 11 50 880- 1360 145- 160 ferrites have moderate mag netic properties and pri ce 4. Nd-Fe-B 1200- 1300 800- 1200 2 1 0-~50 VERMA el at: STRONTIUM FERRITE PERMANENT MAGNET 365

8 Cubic C A C ® H(!xogonol A C Cubic B A B ~ A

Fi g. 2-Schematic layer wise representation of Sr-Ferrite struc­ ture

Table 2-Primary and secondary properti es of Sr M I2 Primary Properti es Saturation magneti zati on, mT 475 Ani sotropic constant, kJ/m 3 360 Curie temperature, K 750 Secondary pro perti es Specific wall energy, J/m2 54.2xlO·-l Anisotropy fi eld , kNm 1506 Max. coercivit y, (Hc)max kA/m 1240

+C axis of the hexagonal cell. Of the 24Fe" ions per unit cell, 4 are in tetrahedral sites, 18 in octahedral and 2 in hexahedral. The intrinsic magnetic properties may be subdivided into primary and secondary. Saturati on magnetization (M,) and magnetocrystalline anisotropy constant (K.) are directly related to the magnetic structure. The secondary properties, sLlch as the anisotropy field strength (H.) and the specific domain

2 wall energy (Ym) are derived from the primary Fig. I- Hexagonal unit cell of Sr-Ferrite where, 0 represents 0 - ; 3 • represents and Sr2+ and . 00 are for Fe + ions at different crys­ properties. Primary and secondary magnetic tallographic positi ons properties of Sr M are given in the Table 21.2. with 4 ions per layer, eight of these are wholly Processing oxygen, while two contain one strontium ion each. In Sintered Magnets each hexagonal block, a Sr-ion substitutes for an Powder processing technique is adopted for making oxygen ion in the centre of the three layer and the sintered isotropic and anisotropic ferrite magnets. layers are stacked in the hexagonal sequence. Isotropic ferrites have uniform magnetic properties in The magnetism of SrFe"O" comes from the Fe" ion all directions, whereas anisotropic magnets have each carrying a magnetic moment of 5f.1l1' These are higher flux density in the orientation direction. The located in three crystallographically different kinds of properties of the magnets are largely dependent on the sites-tetrahedral, octahedral and hexahedral. The Fe" process parameters which, in turn, affect the grain ions have their moments normal to the plane of the size, shape, volume fraction of phases and their oxygen layers and thus parallel or anti parallel to the alignment. The schematic process flow diagram for 366 INDIAN J ENG. MATER. SCI., OCTOBER-DECEMBER 2000

Table 3--Nominal specification of -Fe203 powder (for Sr-M magnets) Compound wt. % u-Fe20 3 99.0 (min.) SiOz 0.30 - DAD Mixing CaO 0.05 (max.) MgO 0.05 (max.) MnO 0.05 (max. ) Pelletizing Alz0 3 0.20 (max.) Cl. 0.10 (max.) LO! 0.3-0.5 Calcination (1373- 1533 K) Moisture 0.1-0.3

Table 4--(:hemical composition of SrCOJ . Wl:I-Grilldillg Com~u~ wt. % SrCO) 98-99 BaCO) 1.0-2.5 I'rl:ssing without magncti c Na zO 0.01-0.03 field (isotropic) Pressing under magnotic CaO 0.01 -0.25 lI eld (Anisotropic) AI 20 3 0.01 -0.06 Fez0 3 0.005-0.0 1 LO! 0.25-0.60 Moisture 0.03-0.5

achievable in the sintered magnets. It is recommended that the average particle size of Fe,O, shall be kept small and size distribution within a narro w range. An average particle size in the range of 0.7-0.9 ~m and 4 standard daviation within 0.1 4-0.16 is recommended . Fi g. 3--Process fl ow diagram of sintered SrM (Strontium Ferrite) The powder shall be fin er then - 400 mesh. magnets Similarly, the chemical composition of SrCO, is specified in table 4. sintered magnets is given in Fi g. 3. The individual Composition process steps are further elaborated. The mole ratio of Fe,QJSrO is critical in achieving Raw Materials high remanence and coercivity. As per the chemical formula (SrFe"O,.,), this ratio shall be 6.0. However, it u -Fe,Q, (Rhombohedral) and SrCQ, are the major is observed that good magnets are never obtained raw material s used for the manufacture of SrM. Other when the Fe,O/SrO ratio is 6.0. This is due to th e fact oxides such as CaO, ALO ... SiO" Cr,O" etc are added th at Fe,Q, and SrCO, used are never 100% pure. The in small quantity to obtain or enhance certain specific common impurities, therefore, shall be accounted for properti es. u -Fe,O, is a byproduct of steel mills. while fixing the mole ratio of Fe,O, and SrO. The Pi ck ling of billets is done to remove the oxide scale in following condition shall be met for obtaining hi gh Br the steel mills. This leads to th e formation of FeCI,. and He values. 5 Fen-ic chloride is roasted in the oven to obtain u­ 5.57-0.25xlog (CI)-0.95xSi02sintering, and coercive force is reduced due to cavity has been found to be adequate for magnetic abnormal grain growth. In order to check the grain alignment. The remanence is strongly affected by the growth during calcination, a small percentage of SiO, magnetic pressing as it is given by the following (0.1-1.0 wt.%) is added to the starting mixture. formula' : Normally, the SiO, percentage is kept below 0.5 wt%. Br =j(d/dx).S.1 . Presence of SiO, allows calcination at higher temperature without grain growth. where j is the degree of alignment; d/dx the relative density and S the fraction of pure ferrite in the solid. Calcination The value of j is 0.5 and 0.9 for isotropic and Calcination facilitates solid state diffusion of SrO anisotropic magnets, respectively. The relative density and Fe,O,. If the calcination temperature is low then is generally 0.9 and fraction of pure ferrite 0.98. grains of uniform SrM are not formed. Similarly, if Compaction pressure in the range of 5-15 MPa has the calcination temperature is too high, excessive been found to be sufficient in wet pressing. However, grai n growth occurs and coarse grain SrM is formed. higher compaction pressure is required for dry powder An optimum temperature for calcination is decided and it is in the range of 40-80 MPa. based on sil ica content in Fe,O, powder. Silltering Wet Grinding Sintering is carried ou t to bring about bondin g Wet grinding is carried out in ball mills to obtain between the powder particles and effect densification. powder of narrow particle size distribution. Average Sintering is carried out in the temperature ran ge of particle size in the range of 0.7 - 0.9 f..Im is 1373 - 1573 K. However, care is taken to avo id recommended for achieving good magneti c excessive grain coarsening during sintering. Typical properties. Moreover, the standard deviation shall not sintered magnets have - grain size of I f..Im, which is be very large. The standard deviation in the range of somewhat bigger than the critical domain size, but 0.14-0.16 has been found to be optimum. Particle size sufficiently small to avoid domains down to below 0.7 f..Im poses problem in compaction and gives considerable reverse fields . Magnetization reversal poor green density. Particles above 0.9 f..Im adversely proceeds by nucleation and growth of domains. On a affect the coercivity. Needless to say , the critical macroscale the reversal process is nonuniform, being domain size of SrM has been calculated to be 0.5 f..Im . governed by the initiation and growth of multicrystal A broad particle size distribution may lead to reversed regions. On this basis the coercivity abnormal grain growth. Abnormal grain growth expression is proposed as follows: affects the magnet properties as the properties change from grain to grain. Hh= aH, - bis/p" COli/pact ion where the first term represents the average internal The powder is compacted wet or dry in the field needed to nucleate a reverse domain. The presence or absence of magnetic field. Pressing under coefficient 'a' ranges from 0 to 1 and depends mainl y the magnetic field is done to obtain an isotropic on grain size. The second term represents the effect of magnet by aligni ng powder in the direction of internal demagnetization field . The coefficient 'b' magnetic field. Invariably, powder in the form of a ranges from 0 to 2 and depends on the remanence and sluny is compacted to obtain high performance the crystal demagnetizing factor N, determined by the anisotropic magnets. Wet sltlITY assists in better crystal shape. The value of N is 0 if the particles are orientation as well as density. Dry compaction of the needle shaped and I for thin plates. For platelet shape powders is also possible. Polyvinyl alcohol and particles, which is generally obtained in the case of 6 camphor are used as binder . These binders are SrM, the value of N li es in the range of 0.6 - 0.9. removed during sintering. It is possible to The crystal shape and size can be controlled with manufacture both isotropic and anisotropic magnets the help of additives and sintering temperature. SiO" by dry pressing. The direction of the applied magnetic CaO, Cr,O" AI ,O" Sb,O, and SrO are added to field during pressing may be parallel or perpendicular calcined powder to control grain growth. However, to the pressing direction. The magnetic properties the concentration of SiO" CaO, Sb,O" etc have to be obtained in perpendicular pressing has been found to controlled in a narrow range if magnets of high Br 368 INDIAN J ENG. MATER. SCI.. OCTOBER-DECEMBER 2000 and H values are to be obtained. The range in which A number of processing techniques, such as 7 the additions are used is given in Table 5 ,8 . compression moulding, injection moulding, rolling Ah03 and Cr203 can be used alternatively as both etc. may be employed for making polymer bonded have grain pinning effect. Si02 and CaO are generally magnets. It is possible to make anisotropic magnets used in combination as together they form a liquid by carrying out the said processes under a magnetic phase which helps in densification. The mole ratio of field which is suitably oriented. By alignment of CaO/Si02 is kept between 0.6-0.8. It has been powders it is possible to achieve energy product as observed that coercivity is low when Si02 is less than high as 12 kJ/m' in bonded ferrite magnets. The Br 0.3 wt %. If Si02 percentage is more than 0.6 wt.% the value of these magnets is largely dependent on the Br value is reduced. Similarly, the coercivity was volume fraction of the strontium ferrite powder. found to be higher when CaO percentage was less Typical magnetic properties achieved in various than 0.3 wt. %. However, the Br value was found to be sintered and bonded SrM magnets are given in less as the sintering was not proper below this value. Table 610. Above 0.6 wt. % of CaO, the sintering was satisfactory, but coercivity reduced. Both Ah03 and Applications Cr20 3 improve coercivity but reduce remanence as the Ferrite magnets are used for a number of percentage of ferrite is reduced. Sb20 3 in small applications ranging from magnetic holding tools to percentage alongwith CaO and Si02 has been found to motors and generators. It is not possible to describe give high Br as well as He. each of these applications in a limited space. However, Invariably, the sintering temperature is 80-120k some of the common applications in gadgets used in lower than the calcination temperature in order to everyday life are highlighted in Table i I. check the grain growth. A general expression for Needless to add, application of magnets in calculating optimum sintering temperature when magnetotherapy, purification, magnetic bearing, CaO/SiO, ratio is in the range of 0.6-0.8 and powder automatic camera are a few more applications which contains small percentage of ALO, is given below.7 are increasingly becoming important. Table 6-Typical Magnetic Properties of SrM magnets. TSint(OC) =(-30A-13) ...JTeal + 1100 (A) + 1670 Magnet Type B~(T) He (kAlm) (B·H.) max where T", is the calcination temperature in °C and A is kJ/m 3 the wt. % of ALO, Sintered isotropic 0.20 - 0.23 136 - 152 6.4 - 8.4 Sintered Anisotropic 0.39 - 0.43 192 - 200 28.8 - 34.4 Final Magnetization (High B,) Final magnetization is carried out at a field which Sintered Anisotropic 0.35 - 0.40 260 - 292 22.4 - 30.4 is 2.5-3 times higher then the intrinsic coercivity. (High He) Thus, the final magnetization is done at 1.2-1.5T. Bonded Flexible (Iso- 0.1 - 0.17 76 - 128 2.4 - 5.6 tropic) Bonded Magnets Bonded Flexible (Aniso- 0.20 - 0.25 140 - 176 8.0 - 12.0 Useful magnets can be made by bonding strontium tropic) ferrite powder in various resins, plastics or natural Bonded Rigid 0.13 - 0.14 72 - 84 2.8 - 3.2 rubber. If no special steps are taken, the material is Bonded Rigid isotropic and energy product is unlikely to exceed 5.5 (Anisotropic) 0.20 - 0.30 120 - 185 7.3 - 16.0 kJ/m'. If good flexibility is required the proportion of Table 7-Common Application of SrM magnets ferrite powder must not be too large and (B.H.) ~. Gadget Parts becomes considerably lower. Casette recorder Speaker, synchronous motor, mike. etc. Video cassette Recorder Main wheel motor 7 8 Table 5-Percentage range of additives in SrM • TV sets Speaker. colour adjusting magnet Additives wt. % Air conditioner Fan motor CaO 0.3-0.6 Refrigerator Fan motor. compressor motor. rubber SiOl 0.3-0.6 linning

AI 20 3 0.1-0.4 Car Starter motor. window motor. viper Cr20 3 0.1-0.4 motor Sb20 3 0.05-0.15 Computers Di sk drive. fan motor. speaker. etc. srO 0.01-1.0 VCD and DVD Main wheel motor VERMA et al : STRONTIUM FERRITE PERMANENT MAGNET 369

Table ~utput of Hard ferrites world wide has saturation magnetization about 10% hi gher than COllntries Output (tons) %Annual M-type, but control method of Fe" contents is 1985 1990 1995 1997 Growth difficult. (B.H.) ~ , value reported for these magnets is Rate 42 kJ/m'. However, H of 198.5 kA/m is too small to Japan 72000 84000 64000 50000 -3 .0 be useful for normal application. Moreover, the USA 31000 4)000 50000 49000 3.9 process cost is high. China 11000 25000 8000 100000 20.2 It is expected M-type high performance magnets South East-Asia 8000 35000 55000 70000 19.8 with (B.H.) ~ , value greater then 38 kJ/m' will be de­ l4 Asia and 20000 32000 32000 31000 3.7 veloped in the near future . Magnets of this energy Europe product are expected to have H greater than 238.5 Others 22000 40000 50000 53000 7.6 kA/m. These will be useful for many applications, especially for automobile starter motor. They are ex­ Table 9--Ferrite consumption (i n tons) in major countries. pected to replace some of the bonded rare earth mag­ Year Countries Total nets as the properties will be comparable with lower Japan USA Europe China cost. These properties can be achieved by the addi­ 1997 223000 132000 86000 63000 504000 tives substitution of lanthanum, light-rare earth ele­ 1998 234000 142000 92000 70000 538000 ments, Co, Ni, Zn and/or Mn elements. 1999 246000 153000 98000 77000 574000 2000 259000 164000 103000 86000 612000 References I Krasehwitz Jacqueline I, Howe-grant Mary; Encylopedia of Market Information chemical technology, 4th ed., (John Wiley & Sons), Vol. 10, The global permanent magnet market is estimated to P.381-4 11. be $5.0 billion which is growing at the rate of 12.5%. 2 Cullity B 0 ; Introduction to magnetic lIlaterials, (Addison­ Ferrites constitute 56% of the total market. Japan, USA Wesley Publishing Company Inc. Philippines), 1972. 3 Karoiwa Kyoji , Ohya Seiroku & Abe Mikio; Kawasaki Steel and China are the major producers of the ferrite Te chnical Report No. 19, November 1988, 131. magnets. While the annual growth rate in USA and 4 Nakamura, Okmori, Inaba; Japanese Patent No. 31534. (Ka­ Japan is not appreciable, the growth in China and other wasaki Iron Industry Ltd.,) 1985. South East Asian countries is substantial. The output of 5 Aragawa Ume Nari, Shimadzu Takahide Ji suga Nao, Zo; Japa­ tt nese Patent No. 260609 (Shine Nihonlron Company) 1990. hard ferrites in last fifteen years is given in Table 8 . 6 James S. Reed; Introduction to the Principles of Ceramic Japan and USA are still the largest market for Processing, (John Wiley & Sons, New York), 1995. ferrite. Japan and USA consume about 42% and 26% 7 Terada Nabno, Murata Yasuji, Ko bayeshi; Japanese Patent respectively of the world production. The ferrite No. 147809 (Nihon Metal Company Ltd.) 1989. consumption and its forecast in the major consumer 8 Nakamura, Okumori, Inaba; Japanese Patent No. 152167 (Kawasaki Iron Industry Ltd.) 1989. states is given in Table 9. 9 Wang Jun, Zhao Jian, Zhang kin, Heng, Zhang Tu Guo; Chi­ The price of ferrite magnet is falling continuosly nese Patent No. 1050641 A (Magnetic Materials Factory) due to large production capacity of China. The ferrite 1991. magnet price has fallen to $2.2-2.5/kg in the recent 10 Malcolm Me Caig and Alan G. clegg; "Permanent-Magnets years. in Th eory and Practice, Pen tech Press (London) 1987. II Proceedings of "China Magnets 98" held at Beijing, October 18-21 , 1998. Future Trends 12 Toyta, Y. J.; Powder and Powder Metallurgy, 44 No. I, (1997) 17. Recently two grades of high performance hard 13 Kubota, Y. J.; Powder and Powder Metallurgy, Proceedings ferrite magnets were reported. One of them is W-type 1998 spring 2-52A. (1998), 207. I2 13 crystal structure and the other is M_type . • W-type 14 Taguehi, H.; Japan Magnetic Society, 21 , No.5, (1997) 901.