THE EFFECTS OF SINTERING ATMOSPHERE ON THE PROPERTIES OF STRONTIUM FERRITE PERMANENT MAGNETS

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1 5, le rit id l el 13 il tt CANADA DEPARTMENT OF ENERGY, MINES A RESOURCES MINES BRANCH OTTAWA, THE EFFECTS OF SINTERING ATMOSPHERE ON THE PROPERTIES OF STRONTIUM FERRITE PERMANENT MAGNETS SUTARNO. W. S. BOWMAN A G. E. ALEXAER MINERAL SCIENCES DIVISION Reprinted frm the Jurnal f the Canadian Ceramic Sciety, Vl. 40, pp. 914 (1971) Reprint Series RS 108 Price 25 cents

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3 The effects f sintering atmsphere n the prperties f strntium ferrite permanent magnets Sutarn*, W. S. Bwman** and G. E. Alexander*** ABSTRACT. The effects f variatins in the partial pressure f xygen in the sintering atmsphere n the ceramic and magnetic prperties f strntium ferrite ceramic magnets have been investigated. It was fund that, at given sintering parameters (i.e. temperature, time, heating rate, etc.), the increase f xygen partial pressure reduced the degree f sintering, and cnsequently decreased the remanent magnetizatin and increased the cercive frce. Intrductin The prcess used t manufacture a ferrite permanent magnet is basically similar t thse used t manufacture ther ceramic pieces. It cnsists mainly f mixing the raw materials in prescribed prprtins, with each raw material being intrduced int the mixture at a given stage f the prcess, reacting them t frm a hexaferrite pwder, cmminuting this pwder t a suitable particle size fr further fabricatin stages, fabricating the pieces, sintering them and finishing the prduct t the desired gemetry. Even assuming that ne is able t btain surces f suitable raw materials having acceptable reprducibility, the cmplexity f the prcess can still prduce a wide variatin in the quality f the finished prducts. In rder t be able t apply adequate prcess cntrl t yield prducts with an acceptable qualitytlerance required fr a particular applicatin, the sensitivity t each f the prcess variables shuld be knwn. Sme f the effects f these prcess variables, mainly the calcinatin temperature and time, the milling time, the frming pressure, the sintering time and temperature, have been previusly studied and reprted.'. 2 The purpse f the present wrk is t investigate the effect f Research Scientist; Technical Officer; Electrnics Technlgist, Mineral Sciences Divisin, Mines Branch, Department f Energy, Mines and Resurces, Ottawa, Canada. sintering atmsphere n the ceramic and magnetic prperties f strntium ferrite permanent magnets. Strntium ferrite was chsen because f its superir prpertiis, by cmparisn with thse f either its barium r lead cunterparts. 3 Experimental prcedures In rder t avid the prblem f nnreprducibility f the raw materials, the ferrite pwders used in this wrk were frm the same batches as thse used in previus wrk.' The raw materials were reagentgrade strntium carbnate and irn xide (Fe 203 ) pwders. The spectrgraphic analyses f these pwders are listed in Table I. The experimental prcedure is illustrated schl matically in Figure 1. Fr each f three batches, five punds f irn xide pwder were mixed with the apprpriate amunt f strntium carbnate t yield the nminal cmpsitin Sr0.5.5Fe The mixing was dne in a rd mill, using an alchl medium in place f water, t guard against pssible strntium lsses by slutin. After batch calcinatins at 900 and 1 C, the masses were ballmilled in an 8 in.diameter steel mill using 3/8 in.diameter steel balls. The mass rati f ferrite:alchl:balls was 1:4:35. Discs, 1.5 in. in diameter, were pressed under a magnetic field f apprximately 13,000 ersted frm VOLUME 40,

4 Table I. &miquantitative spectrgraphie analysis f raw materials and f the nminal Sr0.5.5&203 %)* Spectr graphie Analyses Strntium ferrite (nminal) Batch Batch Batch Elements SrCO, Fe203 N. 1 N. 2 N. 3 Ba B Mn Sb Mg As M W Pb Sn Cr Si 0.03 Nb Ta Fe Ge Bi Al In Zr Cu Ag Na Zn Ti Ni C Sr PC Ca PC PC 0.01 PC * Nndetectable; PC Principal cnstituent. I\JD PC PC PC PC the ferrite slurries. 4 The frming pressure, which was read n a calibrated hydrstatic pressure gauge, was 5000 psi. The cmpacts were demagnetized by heating them t 500 C in rder t facilitate handling and measurement f "green" density. The discs were sintered in an electricallyheated tube furnace at 1160 C, at 1180 C r at 1200 C. The heating rate was C per hur and the saking time was 120 minutes. The sintering atmspheres were nitrgen (cntaining abut 0.01% xygen), a mixture f 4% (by vlume) xygen and 96% nitrgen, air (which cntains 20.95% xygen), and pure xygen. The gas flwrate was kept cnstant and the specimens were arranged in the furnace in the same way fr each run, s that the effective gas flwrate with respect t the specimens remained cnstant. A staticair atmsphere was als emplyed. The specimens were cled in the same atmsphere at the natural cling rate f the furnace. The discs were lapped t cylinders with their faces parallel t within in. using silicn carbide grinding cmpund, grade 600. The sintered densities f the lapped discs were determined frm their weights and dmensins. Values Mix in Rd Mill lb hr Dry and Calcine 9 00ru: Mix inmrtar Calcine 1 C hr Mix in Rd Mill 16 hr Calcine 1*C 2 hr Ball Mill FIGURE 1. Schematic diagram f the experimental prcedure. Press under N1agnetic Field Demagnetize 500"C; Measure Green Density (D g l Sinter; Measure Diamet rai Shrinkage (8d) Lap; Measure 5intered Density (D.) Measure Magnetic Prperties Reprt f the thickness shrinkage (S1) were calculated frm the "green" density (D,), the sintered density (D.) and the diametral shrinkage (Se), since slight warping made direct measurement f this prperty difficult. The magnetic prperties f the specimens were measured by the "plecil" methd. 5 (BH) vs H curves were pltted autmatically and the intrinsic cercive frce (JI), cercive frce (.11 5), remanent magnetizatin (B,) and maximum energy prduct [(BH)...] were measured graphically. Each disc was measured in five places in rder t average ut lcal variatins within the specimen. Results and discussin The spectrgraphic analyses f the raw materials and f the ferrite pwders are given in Table I. The analyzed cmpsitins f these ferrite pwders were Sr0.5 73Fe 20 3, Sr0.5.77Fe 203 and Sr0.5.76Fe 203 fr Batch Ns. 1, 2 and 3, respectively, as cmpared with the nminal cmpsitin f Sr0.5.5Fe These differences frm batch t batch are nt cnsidered t be chemically significant. The ceramic and magnetic prperties f the discs are listed in Table II; sme f them are shwn graphically in Figures 2 t 7. The regressin equatins f B Ji, and as functins f the mle fractin f xygen, were cmputed by the stepwise methd. 6 The 10 JOURNAL OF THE CANADIAN CERANI1C scirry

5 Table H. Ceramic and magnetic rperties f SrO. 5 5Fe,O3, sintered at varius temperatures and in varius atmspheres Remanent Green Oxygen Sintered magnetizatin dcnsity Sintering partial Diametral Thickness Shrinkage density ( N,) (gauss) Batch (D,) temp. pressure shrinkage shrinkage rati ( D,) Std. N. (g/cm,) ( C) (Atm) (Sd)(%) (S,)(%) B/A (g/cmi) Mean Dev. Intrinsic cercive Maximum frce Cercive frce energy prduct (;1ij (ersted) (y11,) (ersted) ((B!l),,,,,j Std. Std. (104 gausscrstcd) Mean Dev. Mean Dev. Mean Std. Dev S S * * , C *Static air atmsphere. fllwing plynmial was cnsidered as the starting mdel: Lg Y, = a + al lg Ox, + a2 (lg Ox, ) z+ E, where Y, = the bserved value f ne f the dependent variables B,, X. r (13H)...; Ox, = mle fractin f xygen in Run,; a, al and a2 are cnstants; and E, = errr in Run. The results f these regressin analyses are summarized in Table III. The results f the runs in the staticair atmsphere were nt included in the regressin analyses. It is shwn in Figures 2 t 7 that the presence f xygen in the sintering atmsphere, in general, inhibits the prgress f sintering. It causes a decrease in the remanent magnetizatin and in the sintered density, and an increase in the intrinsic cercive frce. The intrinsic cercive frce increases with increasing xygen partial pressure at a different rate frm bth the remanent; magnetizatin and the sintered density. As a result, at a given sintering temperature, there is an ptimum partial pressure f xygen in the sintering atmsphere that will prduce a maximum (BH). value. The relative magnitude f the effect f xygen partial pressure is, f curse, dependent n the sintering temperature. VOLUME 40,

6 ' Table III. Summary f the regressin analysis f the magnetic prperties f Sr Fe20 3 as functins f the mle fractin f xygen in the sintering atmspheres Sintered at 1160 C Sintered at 1180 C Batch N. 1 Batch N. 2 Batch N. 3 Prperty Ceff. S.E. Ceff. S.E. Ceff. S.E. Batch N. 1 Ceff. S.E. Batch N. 2 Batch N. 3 Ceff. S.E. Ceff. S.E. B, a a l a S. Y C.V. <I% <1% <I% <1% R 2 98% 99% 99% 96% H, ( al dg S. Y C.V. <1% <1% <1% <1% R 2 99% 99% 99% 99% (BII), a a l a S. Y C.V. I% I% 2% 1% RI 99% 99% 99% 99% <1% 96% <1% 98% % 92% <1% 94% <1% 99% % S.E. Standard errr in the cefficients. a, al, and a2. S.V. Standard errr in Y. C.V. Cefficient f variatin; i.e., the rati f S.Y. t the average value f dependent variables. R 1 Multiple crrelatin ce fficient. It is shwn als in Table II that there is a signe cant difference between discs sintered in a flwingair atmsphere and thse sintered in a staticair atmsphere. The remanent magnetizatin and the sintered density are generally higher and the intrinsic cercive frce lwer fr thse sintered in flwing air. The staticair atmsphere seems t behave as thugh it has a higher effective xygen partial pressure than the flwingair atmsphere. The abvementined phenmena suggest that there is an xygen transfer frm the sample t the atmsphere during sintering, with the ratecntrlling factr being the xygen partial pressure in the layer immediately in cntact with the surface f the sample. Thus, by increasing the flw f gas, this layer is mre rapidly swept away and, cnsequently, the xygen partial pressure in this layer decreases and, hence, the rate f xygen transfer t the atmsphere increases. This g 0 CI CS I t; 2, 6 Q. le 5 > `^" D 8 0 Ix 0 g IN TRINSIC COE RCI V E c. 8 0 j X 0 t.; z u 1 9 ) VOLUME % OF OXYGEN FIGURE 2. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prperties f Sr0.5 5Fe203, Batch # 1, sintered at 1160C fr 2 hrs I 10 VOLUME % Of OXYGEN FIGURE 3. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prperties f Sr0.5. 5Fe103, Batch #2, sintered at 1160 C fr 2 hrs. 12 JOURNAL OF THE CANADIAN CERAMIC SOCIETY

7 î BATCH NO C DENS ITY, D s (g /c m 3) m :?! O t. w e E Z Î d ; 00 _z e isa c3 D a Li 200 ± VOLUME X OF OXYGE N VOLUME % OF OXYGEN FIGURE 4. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prperties f Sr0.5.5Fe203, Batch #3, sintered at 1160*C fr 2 hrs. FIGURE 5. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prpe rties f Sr0.5.5Fe203, Batch #1, sintered at 1180'C fr 2 hrs. xygeri lss creates anin vacancies and, presumably, increases the rate f sintering. The chemical analysis f a sample frm Batch N. 3, sintered at 1160 C, shwed that there was n difference in the ferrus cntent between discs sintered in nitrgen, in nitrgen with 4% xygen, in air (i.e., apprximately 20% xygen) and in a pure xygen atmsphere. (These discs had been stred in air during the evaluatin f their ceramic and magnetic prperties.) Hwever, when a small piece f a disc (weighing abut 0.5 g) was heated t 1200 C fr tw hurs in a nitrgen atmsphere, cled vernight in nitrgen, and then quickly analyzed chemically, it was shwn that it cntained 0.16% Fe 2 *, while anther similar piece, treated exactly the same except that an xygen atmsphere was used, was shwn t cntain less than 0.04% Fe 2 *. This fact suggested BATCH NO C BATCH NO C E 7_ 0 D ô et 4,_ z 4 9» 3, H Ds _ t! a g 2000 al ' a, 5 g w ± ks u g,y, u. z & i 1, 0 gf ct DE N SI TY, Ds fg 1c m 3 ) i _ Ds if, 5.k (BH/ max La, 1./ ce u 14.1 Z 200 & VOLUME '4 OF OX YGE N FIGURE 6. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prperties f Sr0.5 5Fe103, Batch #2, sintered at 1180*C fr 2 hrs VOLUME X OF OXYGEN FIGURE 7. Effect f vlume % f xygen in the sintering atmsphere n the magnetic prpe rties f Sr0.5.5Fe203, Batch #3, sintered at 1180*C fr 2 hrs. VOLUME 40,

8 that snie xygen lss during sintering des ccur and is enugh t prmte the sintering prcess; the sample is presumably quickly rexidized after expsure in the air atmsphere. Anther prperty that is als imprtant is the shrinkage. The value f thickness shrinkage, Si, (shrinkage alng the crystallgraphic caxis) ranges frm 17.3% t 33.8%, while the value f diametral shrinkage alng the crystallgraphic caxis) ranges frm 4.0% t 10.9%. The thickness shrinkage decreases with the increase f xygen partial pressure. The behaviur f the diametral shrinkage with respect t the xygen partial pressure did nt exhibit a clear trend. Hwever, the shrinkage rati B/A, where B is defined as (S,) and A as ( ), increases with increasing xygen partial préssure. Althugh Batch Ns. 1, 2 and 3 shw n apparent chemical differences, there are significant differences in the ceramic and magnetic prperties f discs prepared frm them. These discs shw different levels f respnse t the change in xygen partial pressure. These differences can be clearly seen in the regressin cefficients listed in Table III. Cnclusins Frm the freging discussin, it is cncluded that the partial pressure f xygen in the sintering atmsphere plays an imprtant rle in the sintering prcess f strntium ferrite ceramic magnets. The lwer the xygen pressure, the higher will be the rate f xygen lss frm the sample and, hence, the greater will be the increase in the rate f sintering. The intrin sic cercive frce is mre sensitive t variatins in the xygen partial pressure in the sintering atmsphere than are the remanent magnetizatin and the sintered density. Since the abve effects depend apprximately n the lgarithm f the xygen partial pressure, they are mre nticeable at the lwer xygen partial pressures than at the higher values. Acknwledgments This investigatin was cnducted under the general directin f Dr. N. F. H. Bright, Head, Physical Chemistry Sectin. The authrs wish t thank G. A. C. Wills, Physical Chemistry Sectin, fr his experimental assistance. The abvementined persns are members f the staff f the Mineral Sciences Divisin. References 1 Sutarn, W. S. Bwman, G. E. Alexander and J. D. Childs. The Effect f Sme Operatinal Variables n the Prperties f Strntium Hexaferrite, J. Can. Ceram. Sc., 38: 913, Sutarn, W. S. Bwman and G. E. Alexander. The Effect f Frming Pressure and f Sintering Time and Temperature n the Magnetic and Ceramic Prperties f Strntium Ferrite, J. Can. Ceram. Sc., 39: 4550, ' Cchardt, A. Recent Ferrite Magnet Develpments, J. Appl. Phys., 37(3): , ' Sutarn, W. S. Bwman, J. F. Tippins and G. E. Alexander. Ferrites: Part 111. Cnstructin and Operatin f a Magnetic Orienting Press fr the Fabricatin f Anistrpic Ferrite Magnets, Mines Branch Investigatin Reprt JR 6746, Steingrever, E. Ein Magnetstahlprüfer mit Richtkraftkmpensiertem Flussmesser, Arc/i. Elektrtech., 40: , 'Draper, N. R. and H. Smith. Applied Regressin Analysis, Wiley, New Yrk, (1966), pp JOURNAL OF THE CANADIAN CERAMIC SOCIETY