Indian Journal of ChemistryVol 35A, May 1996, pp. 353-365

Advances in Contemporary Research

Recent developments in processing of Mn -- Zn ferrites-An overview t

C E Deshpande & S K Date·

Physical Chemistry Division, National Chemical Laboratory, Pune 411 001'

Received 21 August 1995; revised 11 January 1996

DrS K Date or C E Deshpande

Sadgopal KashilUllh Dale (born 30th Nov. 1942) receivedB.Sc. (1962), M.Sc. (1964) and Ph.D. (1969) degree fromBombay University. He joined the Chemical Physics Group atTlFR, Bombay in 1969 and wor.ked on structure-propertycorrelations in a variety of solid state and inorganic materials.Since 1981, he is' with the National Chemical Laboratory,Pune. At present, his. major interests are magnetic oxides, elec-tronic ceramics, characterization of materials, Mossbauer spec-troscopy and hyperfine interaction in solids. He is a memberof American Physical Society, Materials Research Society(USA & India), life member of Indian Physics Association,Catalysis Society of India, National Academy of Sciences andFeilow of Indian Thermal Analysis Society and MaharashtraAcademy of Sciences. He is also presently visiting Professor atDept. of Physics and Chemistry, University of Poona.

Chandrakant Eknath Deshpande (born 21 August 1939) re-ceived; B.Sc. (1959), M.Sc. (1962) and Ph.D. (1982) degreesfrom the University of POO.l1lJ.He joined NCL. Pune in 1965arid has been working in the Judd of solid state chemistry andelectronic ceramics. His current interests are the processing. ofmagnetic oxides and [errites and their structure-property cor-relations. He is a member of Materials Research Society of In-dia. .

INCL Communication No. 6324

IntroductionMn ~ Zn ferrites belong to a category of soft

ferrites which are ferrimagnetic oxides with spinelstructure having low coercivities. They are techni-cally important materials because. of their highmagnetic permeability and low core losses. Theseferrites have been used for several decades asmagnetic cores in high frequency coils and trans-formers. There has been a continuous increase inmagnetic permeability and decrease in magneticlosses in ferrite materials leading to improvedperformance over years! (Fig. 1). The soft ferritesare widely' used in telecommunication devices andentertainment electronics applications? as sum-.marized in Table 1. During the last decade,Mn - Zn ferrites were used in the frequency rangeof about 16-25 kHz. However, with the emerg-ence of switched-mode power supply very rapiddevelopment of high frequency ferrites (power.ferrites) was seen in electronics industries. Fre-quency operation increased rapidly and power-supplies operating at 1 MHz and above werecommerically available by 1990. Demand. forcompact power supplies increased due to thecomputers, microprocessors and VCR systemswhich enhanced the market for power ferrites '.Ruthner has estimated total world soft ferrite pro-duction" in metric tonnes per year (MTPY) alongwith the consumption of iron oxide for their pro-duction (Table 2). As per the estimates, total fer-rite production in India was around 9000 MTPYin 1989 which included 3000 MTPY of soft fer-rite.Keeping in view the scientific and technical im-

portance of Mn - Zn ferrites", we have attemptedto review some of the recent developments onprocessing of this important electronic ceramicmaterial along with its basic background.

354 INDIAN J CHEM, SEe. A, MAY 1996

Table I-Summary of Mn - Zn ferrite applications!

S.No. Device Frequencies Desired ferrite props.

I. Inductor "I MHz High Jl, high JlQ, high stability of Jl with temp. and time.

2. Transformer Up to 500 MHz High Jl, low hysteresis losses(Pulse and wide band)

3. Loading coil Audio High Jl, high B" high stability of Jl with temp. and time and de bias.

4. Flyback transformer < 100kHz High Jl. high B, , low hysteresis losses.

5. Deflection yoke < 100kHz High Jl, high B, .

6. Suppression bead Upto250MHz Mod. high u,high B, , high hyst. losses.

7. Choke coil Upto250MHz -do-

ll. Recording head Up to 10 MHz High Jl, high density, high JlQ, high wear resistance.

9. Power transformer <60kHz High B" low hyst. losses.

200 Table 2-Estimated world ferrite production"

(11)1)0-2000 AD)

Year 1990 1995 2000

I. Toted ferrite production 516,000 665,500 974,000(MTPY)

2. Soft ferrite production 150,000 185,500 221,500(MTPY)

3. Fe203 consumption for 105,000 130,000 155,000soft ferrite (MTPY)

oL-~~~--~--~--~~--~~20 30 40 so 60 70 80 90 100

TEMPERATURE ("CI

Fig. l=-The reduction in core losses of power ferrites during1970-19901•

AB B8 AA

Fig. 2-(a) Two subcells of a unit cell of the spinel structure.(b) Interionic distances and angles in the spinel struc-ture for the different types of lattice site interactions".

2 Theoretical BackgroundCrystal structure

Mn - Zn ferrite has a cubic spinel structure(Fig. 2a). It consists of a typical face-centred cubicstructure of oxygen ions with two types of lattice/crystallographic sites-tetrahedral (A) and octahe-dral (B)-available for meal ions. Fig. 2b showsthe configurations of the ion pairs in the spinellattice for which the distance and angles are mostfavourable for different types of lattice site inter-actions", The unit cell of the spinel is representedby MMFel~1I032'Thus, there are eight formula un-its of MilO' Fe~1I03 (MFe204) in a unit cell (Milrepresenting the divalent ions such as Mn2+. andZn2 + in case of Mn - Zn ferrite). The site occu-pancy. of an ion depends on its site preference en-ergy 7 which in turn is temperature dependent. Inaddition, the occupancy of a particular lattice sitedepends on thermal history of the spinel, i.e., thehigh temperature structure is frozen duringquenching while in slow cooling, sufficient time isavailable for the readjustment of ions to the equi-librium configuration.

DESHPANDE et al: RECENT DEVELOPMENTS IN PROCESSING OF Mn-Zn FERRITES 355

+

J~•i! ot--.....,.~-----20....L...----•I D .'-'Ie ••••••11_.: 7·~CIi:-'."-_-:t;"."" ••~s••~••••~t

Fig. 3-Exchange integral as a function of the ratio of inter-nuclear separation to the radius. of the d orbit (D/d)8.

Magnetic interactionsMagnetic interaction between the two atoms of

spins Si and Sjis given by

E= -2 Jesi·Sj ... (1)where Je is the exchange integral and is a measureof the extent of overlap of the electronic chargeof the two atoms. This direct exchange interactionmay be either positive or negative depending up-on the ratio of atomic sepanltion(D) to the diame-ter of tffirbit (d), i.e., Old. For Old < 1.5, the ex-change interaction isnegative, attains maximumvalue at Old = 1.8 and subsequently diminishes toa very small but positive value of Did" 3 (Fig. 3).In ferrimagnetic spinels Old" 2.5, indicatingweak positive interaction by direct exchanges.However, experimental evidence favours a strongnegative A-B interaction in spinel ferrite which ac-counts for the experimental magnetic moment va-lue which is drastically different from the valuecalculated on the basis of positive ferrimagneticA-B interaction. The mechanism by which the ne-gative A-B interaction is obtained via the inter-vening oxygen ions known as 'superexchange' hasbeen suggested by Anderson", This mechanismresults in a negative interaction favouring antifer-romagnetism when the d orbitals of the metal ionsare half-full or more than half-full, while a posi-tive interaction accompanied by ferromagnetismresults when the d orbital is less than half-full.The mechanism may be explained in brief as fol-lows. The electrons in the outermost shell of 02-

are paired and occupy JrU and p-n orbitals. The02 - is diamagnetic but an excited state is possiblein which an electron from JrU orbital of 02- istransferred to one of the neighbouring Fe3+ ionswhich momentarily becomes Fe2+ ion. The tran-sient oxygen ion becomes paramagnetic becauseof its unpaired electrons. Its moment can have di-rect exchange with the moment of other Fe3+ions.

The superexchange may also involve p-n orbi-tals which have positive overlaps with the t2g orbi-

BeS)

H

(.)

BIS)

-ii•••iiii••• -dII••••ce-r.-IHoe)

( .)

Fig. 4-(a). Characteristic hysteresis loop for a magnetic ma-terial.(b) 'TYpical magnetization curve of a virgin speci-menl2a.

tals of the metal ions, while p-o have strongeroverlaps with eg orbitals of the meal ions. p-inwith eg or p-o with t2g give negative overlaps.Therefore, in fer rites the result of the indirect ex-change through oxygen is a net antiferromagneticinteraction between any two transition meta'ions 10. This mechanism predicts a strong antifer-romagnetic interaction when the metal-oxygen-metal (M - 0 - M) angle is 1800 and also wherethe interatomic distance is the shortest. A weakerinteraction exists when this angle is in the neigh-bourhood of 900 and the interatomic distancesare too large. In a spinel structure, the AOB angleis in the .neighbourhood of two right angles whilethe AOA and BOB angles are closer to one rightangle. One should expect, therefore, a very strongAOB antiferromagnetic interaction which domi-nates and virtually suppresses the weaker AOAand BOB antiferromagnetic interactions. Oneshould thus expect the metal ions on A sites to bealigned parallel to one another as also those r-n B·sites, while the net moments at the A and B sitesshould align antiparallel to one another. A modelbased on ferrimagnetism was proposed II to ac-count for the observed magnetic moments of fer-rites by Nee!. It is the basis of our present under-standing of the various types of magnetic interac-tion occurring in spinel lattice.

356 INDIAN J CHEM, SEe. A, MAY 1996

Magnetic properties and performance parametersThe magnetic properties/performance parame-

ters of soft ferrite magnet are best understoodwith the help of its hysteresis loop [Fig. 4(a)]. Thearea of the hysteresis loop is a measure of themagnetic losses that take place during the magne-tization processes. The value of magnetic induc-tion (B) at applied magnetic field (H) = 0 on thedemagnetization curve is called the remanent in-duction or remanence (Br) and the value of thereverse field applied to reduce the magnetic in-duction (B) to zero is called the coercive force orcoereivity (HJ. They are expressed in units ofGauss and Oersteds respectively. Both these par-amcters are very important from application pointof view of ferrites.

initial permeability (Jtj)- The magnetic permea-bility (Jt) is the ratio of magnetic induction B tothe magnetizing field H. This parameter can bemeasured under different sets of conditions, e.g.,if the magnetizing field is very low, approachingzero, the ratio of B/H is called initial permeabilityJti. u, = limit (B/H) as +I -+ O. With increasing H,permeability (Jt) increases to a maximum valueJtma• and then decreases again as B gets saturatedand attains the value Bs. The parameter Jti is oneof the most important parameters for telecommu-nication applications of soft ferrites 12.

Crystal anisotropy and permeability-An un-magnetized ferrite material consists of a largenumber of small domains with parallel spins inin-dividual domains called Weiss domains. These do-mains are oriented in all possible directions andthe resulting magnetic moment is zero. A trans-ition zone exists between two adjacent domains in,which the orientation of domains is graduallychanged. This zone is called a Bloch wall. Duringthe magnetization with a large magnetic field, alldomains are oriented in the direction of appliedmagnetic field. The orientation can take place-either by rotation of domains or by wall displace-·ments as shown in Fig. 4b (ref. 12a). It shows theprocesses taking place in the three different. re-gions of the curve for magnetization of thevirginspecimen. The first is the lower section of thecurve characterized by reversible domain wall dis-placements or domain rotations. It means that af-ter changing the magnetization slightly (by in-creasing the applied field) the original magnetiza-tion condition can be returned if the field is re-duced to the original value. The second stage ofthe magnetization curve is one with increasingslope in which irreversible domain wall displace-ment takes place. The third section of the curve isone of irreversible domain rotations with a flat

slope indicating the large amount of energy th~t·isrequired to rotate the domain magnetization inthe direction of H. In ferrites, magnetization takesplace preferably by domain rotations since theBloch walls arc more difficult to move due tovoids present in the material. However, it is clearfrom Fig. 4 (a&b) that initial permeability u, isdue to the domain wall displacement and not dueto domain rotation. Thus, large u, is primarily as-sociated with low crystal anisotropy in Mn - Znferrites and attains maximum value as the 'crystalanisotropy K approaches zero. This is achievedby introducing a small calculated amount of Fc~+

ions which contributes a positive anisotropy.Mn - Zn ferrite has a negative crystal anisotropywhich makes the resultant K ""zero.

Magnetostriction and permeability-Anotherimportant factor that affects the permeability ismagnetostriction (A),which is a change ·in mechan-ical shape of a magnetic material on magnctiza-tion. Conversely, a magnetization is caused bychanges in shape by means of internal stresses,This preferred direction of magnetization in thedirection of stress lowers the permeability. Mag-nctostriction can be minimized by making mixedcrystals of cubic ferrucs having negative and posi-tive rnagnctostrictions. It is reported I~h that thecompositional dependence of the permeahility isinterpreted from those of crystal anisotropy (K),magnetostriction (A).and Curie temperature (TJ.High permeability (Jti) and minimum loss factor(tan NIi;) were found for the composition rangecorresponding to K = 0 with minimum magnetos-triction (A). Fig. 9 shows the phase diagram whichclearly indicates the region of ferrite compositionswith liima• and the regions were desirable magne-tic properties can be optimized.

Shape anisotropy and permeability-Shape anis-otropy is created in the ferrite by the presence ofnonmagnetic second phase present a.s an impurityor even due to voids filled with air. These impu-rities result in incomplete magnetic circuit in thematerial causing a demagnetizing field. Permeabil-ity is lowered due to the preferred orientation inthe direction of demagnetizing field., j

Magnetic lossesWhen a magnetic material is employed ~ a

core material for a high frequency transformersome energy gets dissipated as heat-energy whichis termed as core losses. For soft magnetic mated-als used in low power transformer cores lossesare described in terms of complex permeability atlow field strengths. Permeability Jt separates intou' and Jt-, which are related as Jt '= Jt' - iu",

DESHPANDE et al.: RECENT DEVELOPMENTS IN PROCESSING OF Mn-Zn FERRITES 357

,,u,K

Fig. 5-CrystaJ anisotropy (K) and permeability (11) as a func-tion of temperature (T), of an extremely homogeneous

MnZnFe" ferrite 1.1.

where u' represents the component of B in phasewith H (real permeability), -while u" refers to thepart of B which is 90° out, of phase with H (imagi-nary permeability). The loss tangent is given bytan 0= Il will' where a is the loss angle of the corematerial and the inverse of tan a is quality factor'Q'. Normalised loss tangent per unit of permea-bility i.e. tan 01u, represents the 'Loss factor'(LF).

The losses in magnetic material are classifiedinto three grops: (1) eddy current losses, (2) hys-terisis losses and (3)· residual losses. The .eddycurrent loss factor is proportional to dZf21 p,where d is the smallest dimension of the material,f the frequency and p the resisitivity. In metalmagnets, .this is the prominent loss factor contri-buting to the core losses. However, in ferrites theeddy current is normally negligible because ofhigh p, though in Mn - Zn ferrite resisitivity islow due to its Fe2 + ion content resulting in higherlosses at high frequencies. The hysteresis loss percycle is proportional to the area of the B - H loopof the ferrite material [Fig. 4(a)]. At high magneticfields (H) and high magnetic induction (B) values,these losses become very prominent and are mea-sured in watts/em.'. For high field applications, theheat developed in the core becomes more import-ant than LF alone since it contributes to the totallosses of the material. The residual losses areusually negligible for metallic core materials, butIn ferrite cores these losses constitute the majorpart of tan 01Il at low fields.

Temperature factor ( TF).of permeabilityPermeability of a ferrite is the result of intrinsic

and extrinsic effects acting simultaneously on it.These are governed by the chemistry and struc-ture-dependent properties of ferrites, namely,anisotropy and magnetostriction. Therefore, the

permeability variation with temperature dependson many parameters that are temperature-de-pendent in themselves. We see a wide variation inthe shape of the permeability versus temperaturecurve (Fig. 5) for the high permeability Mn - Znferrite!". The permeability rises and attains a max-imum at a temperature-little before the curie tem-perature (Tc) since the exchange energy, the anis-otropy and internal stress all decrease due toheating. This maximum (peak) drops to zero atthe curie point since the ferrimagnctism is lost.For Mn - Zn ferrite, one more peak called thesecondary maximum of permeability (SMP) oc-curs close to the temperature at which the crystalanisotropy (K) goes through zero. This peak canbe moved to the temperature at which the ferriteis to be used by varying its chemistry and chang-ing the shape of Il-T curves!". SMP is normallyadjusted to be in the temperature region of oper-ations, i.e., 60 to 100°C for optimum use of theferrite material. The temperature factor (TF) ofpermeability is calculated as 1/1l2-dllldTwhich isa material constant" useful to predict the varia-tion in magnetic properties of a magnetic compo-nent; TF can be brought down by introducing anair gap in the magnetic circuit 10.

Disaccommodation factor (DF)The disaccommodation of Mn - Zn ferrite is an

ageing effect indicating the fall in permeabilitywith time. This factor has. to be minimum for theapplication of" ferrite in inductors which requirehigh stability of Il with temperature and time. InMn-Zn-ferrite Mn3 +, Fez + pairs are formed dueto the electron exchange (Fe] + + Mn Z + r! Fez +Mn3+) between Fe3+ and Mrr' + which constitutean anisotropic configuration and diffuse graduallywith time. The relaxation time of the inducedanisotropy due to diffusion depends on cation va-cancies and the inhomogeneity of the oxygen bal-ance. Disaccommodation is therefore liable to oc-cur more in iron-excess ferrite than for an iron-deficient one. However, it was also noticed loa thatfor Mn - Zn ferrites with iron-excess composi-tions fired in pure (oxygen-free) nitrogen gas, dis-accommodation is not observed in the absence ofeation vacancies. This implies that in addition toFe2 + ions, cation vacancies have a direct effect ondisaccommodation. According to Yanase 16", theinduced anisotropy which is associated with disac-commodation arises from cation vacancies andthe presence of Fe2 + ions is necessary to allowthe vacancies to migrate through the lattice. Inother words, 'Fe2+ -vacancy' pairs Cause the disac-commodation in Mn - Zn ferrites since the vacan-

358 INDIAN J CHEM, SEe. A, MAY 1996

(0) Ceramic method (bl-Co ••••tlllit.II!MI ••• 'hM

Fig. 6-Schematic flow chart for (he synthesis of Mn - Znferrite by (a) ceramic method and (b) coprecipitation method.

cies permit diffusion of Fe2 + ions to their energet-ically favourable sites. The effect can be mini-mised in stoichiometric ferrites and by a properhomogenization of the oxygen balance. D.F. is cal-culated as,

... (1.)

where )1. ( and )1.2 are the permeabilities measuredat I( and {~minutes after demagnetization.

Synthesis of Mn-Zn FerriteCeramic method

Mn - Zn ferrite powders 'are traditionally pre-pared by the ceramic method 17. The method issuccessful for large scale preparation of bulk fer-rite powders because of its low cost and easy ada-patability. However, the process has several limit-ations due to some inherent drawbacks such aslong heating cycles at high temperatures, ball-mill-ing of the powders for a few days to get the re-quired particle size etc. High temperature resultsin sintering of powders and increasing its particlesize than the optimum for a given ferrite. Ball-

milling leads to a broad distribution of shape andsize of particles with a poor control on morphol-ogy and microscopic homogeneity. In addition,the stoichiometry of the ferrite powder is dis-turbed due to the grinding of the material in steelball-mills as it introduces variable amounts of ironas impurity.

A flow chart depicts the various steps of ferriteprocessing by ceramic process (Fig. 6). High pur-ity starting materials namely oxides, carbonates,oxalates of the metal ions (iron, manganese &zinc in case of Mn - Zn ferrite) are weighed andmixed in the required proportions by wet-millingusing a suitable medium like water or alcohol in astainless steel ball-mill. After milling, the mixtureis dried and calcined in inert atmosphere of ni-trogen (N 2) at temperatures ranging from 900 to1300°C when the formation of Mn - Zn ferritetakes place. The next step is the grinding of cal-cined ferrite material by ball-milling to achievebetter homogenization of the product with re-duced particle size to achieve the desired powdermorphology and sinterability. It is one of the im-portant processing steps of the ceramic methodwhich controls particle size and size distributionof the resulting fine powders. Somasundaram (II

has reviewed concepts in grinding and discussedthe grinding of ceramics by different methodsused at present in the industrial units.

Compaction- The micron size powders aregranulated by adding binders such as PVA. algi-nates, waxes like gum arabic, halowax ere. to getthe free-flowing nature of ferrite powders re-quired to compact into the desired shapes. This isdone either by conventional compaction in a die-puch assembly by hydrostatic or isostatic compac-tion of the powder. A lubricant such as stearic oroleic acid is used for the die' walls to avoid fric-tion between the ferrite powder particles and withdie walls also. The die-punch compacting methodof pressing is commonly used in a laboratory aswell as in ferrite manufacturing units with auto-matic presses; Johnson and Ghate'? have de-scribed compaction of spray-dried ferrite powdersusing various binders and their effect on magneticproperties of the resulting compacts.

Sintering-Sintering of Mn - Zn ferrites is animportant and elaborate step in its processing forthe following reasons:

1. Zinc loss poses a problem when high sintereddensity is required.

2. Oxidation of Mn2+ to higher valency states(and consequent reduction of Fe3+ to lowervalence state) during sintering.

DESHPANDE et al: RECENT DEVEWPMENTS IN PROCESSING OF Mn-Zn FERRITES 359

•~i I

f ~/II~/rAir

Fig. 7- Typical firing programme of high-performanceMn - Zn ferrite-",

z.oFig. 8- Initial permeabilities of Mn - Zn ferrites represented

in miscibility diagram".

3. Stabilization of Fe2 + content consistent withthe composition is necessary.

It is therefore carried out under a typical firingprogrammes" (Fig. 7). Some critical requirementsessential for sintering of Mn - Zn ferrites?' aregiven below:1. Very high sintering temperatures are avoided

to minimize zinc loss.2. Firing temperatures are adjusted and main-

tained very closely during sintering.3. For maintaining the stoichiometry and to esta-

blish the correct Fe2 + content, carefully con-trolled heating and cooling rates are effectedmaintaining the inert gas atmosphere with var-ying oxygen contents to adjust equilibrium oxy-gen partial pressure at the corresponding tem-peratures.

4. An inert gas atmosphere of pure N2 gas (oxy-gen-free) is absolutely necessary to avoid thereoxidation of Mn2+ and Fe2+ during coolingcycle from llooae down to RT. This avoidsmicrocracks in the-sintered compacts and alsomaintains the ratio ofFe?" IFe3+ in the ferrite.

Mn - Zn ferrites, sintered with these precau-tions, result in the desired composition and mic-rostructure leading to high performance materialfor a particular application. Nomura/" has pre-pared Mn - Zn ferrites with low loss characteris-

Table 3-Performance parameters of Mn - Zn ferrites forpower supply applications'"

Material A BFrequency for use 100kHz 1 MHz

#i 2480 ~473BIII(mT~ 514 448iJ,(mT) 61 ~4Hc(Nm) ~.2 10.9

Power loss at 80·C (mW/cm') 232 2264(tOO kHz) (1 MHz)

tics by doping with Ti02 and Sn02 for powersupply applications at 100 kHz and 1 MHz fre-quencies (Table 3).

Isostatic pressing-It is based on the principlethat pressure and heat causes densification of cer-amics at much lower temperatures, Ferrite pow-ders are isostatically pressed to get compacts withuniform density due to the uniform pressure ap-plied in all directions on surfaces of thecompact". In cold isostatic pressing the hydraulicpressure of the order of 10,000 to 30,000 psi istransmitted through a liquid medium such as gly-cerine, water or hydraulic oil against a rubber orplastic bag containing the ferrite powder. Thetechnique has the following advantages. It givesmore uniform and high density compacts. It elimi-nates differential shrinkage, avoids pressurecracks and is useful for pressing complex shapes.

Hot isostating pressing (HIP) is used to producedense ceramics for critical applications such as re-cording heads or high power microwave ferrites.Hot pressing is the only means of achieving adense ferrite compact at a pressure of 15,000 psiand low temperatures of 200-300ae withoutgrain-growth. This makes it more appropriate forferrites with a volatile component. High perform-ance Mn - Zn ferrites with u; > 3000 and porosity< 0.1% have been prepared using uniaxial HIp22.

Wet chemical methodsIn the last two decades, there has been a grea-

ter interest in the synthesis of fine powders ofceramics by using newer merhods=":" of ultras-tructure processing. These methods have resultedin tailor-making of materials having desired mor-phology, microscopic homogeneity, texture, chem-ical purity etc. Some of the important wet chemi-cal routes are: (i) spray roasting, (ii) freeze-drying,(iii) hydrothermal oxidation, (iv) sol-gel/sol-precipitation.Tv) combustion method and (vi) co-

360 INDIAN J CHEM, SEC. A, MAY 1996

Table 4(a)-Comparison of the performance parameters ofMn - Zn ferrites prepared by citrate gel and ceramic

method"

Process Citrate gel Ceramic

I. Density (g/cm'] 4:!~4 4.93::!. ,II :245.11 30()(1

3. tan bill ( x 10 ') 1.1 4.2-I, B",(G) 4R50 5100

Table 4(b)- The variation of the permeability with the firing-temperature and frequency for Ni7~Zn",2~Fe20. prepared by

combustion method"

SiJ1l, Density Grain Permeahility u' attemp. \·}~>Theor.) sizeI"C) (Il) 1 kHz 10 kHz 1 MHz 10 MHz

<JO() 9XA <I 23.72 4R.R5 49.31 49.771100 99.5 ::! 50.R9 70.43 69.07 13.16

II 00 99.5 4 138.84 140.67 129.66 121.71

precipitation. Without going into the experimentaldetails of each method, we shall review only theapplication of these routes for the synthesis offine powders of Mn - Zn ferrites with high perm-eability and low core looses. Miscibility diagram"of Mn - Zn ferrite showing the lines of constantinitial permeability (J..tj) at room temperature isgiven in Fig. 8. .

Spray roasting-The method is versatile for fer-ric oxide and ferrite synthesis. An aqueous solu-tion mixture of the metal chlorides is sprayed intoa large preheated reaction vessel where the metalsalts are hydrolyzed followed by oxidation tot herespective oxides. The hydrochloric acid is re-covered and the mixed oxide is collected, at thehottom of the roaster. Ruthner" used it for thepreparation of ferrite grade Fe203 using wastepickle liquor (FeCI, solution) from steel mills. Itresulted in very pure, spherically shaped, highlyuniform submicron sized particles of Fez03. Ochi-ai30 adopted this method for the preparation ofMn - Zn ferrite powders using FeCI3, MrrCl; andZnClz 'solutions on commercial scales claimingthe following advantages over other methods: (1)increased homogeneity, (2) elimination of calcin-ing step, (3) good magnetic properties and (4)commercial viability. Wagner!' has described 'spayfiring' technique for Mn - Zn ferrite preparationwhere a mixed slurry is sprayed into a verticalhigh temperature furnace held at about 1200°CThe reaction time reported is only one minute re-sulting in very fine, reactive ferrite powder with

better sinterability. Flame spraying'? also pro-duced fine ferrite powders.

Hydrothermal oxidation methbd+ Takada andKiyama " pioneered this technique for the prepar-ation of Mn - Zn ferrite using aqueous solutionsof sulphates of manganese, zinc an diron mixed inthe required proportion. Complex hydroxides ofthese metal ions were precipitated by addingNaOH solution to the solution of sulphates untilthe pH value of the resultant suspension wasmore than 10. The suspension containing complexprecipitate was heated to a temperature between60 and 90°C and air was then bubbled through itat an optimized. rate for stirring and simultaneousoxidation of complex hydroxides to Mn - Zn fer-rite. Thus, ferrite particles of 0.05 to 1.0 J..tmcould be obtained by choosing suitable reactionconditions. Particles of about 0.1 J..tm were ob-tained if pH of the suspension was kept less than9.

Hydrothermal preparation of Mno.sZIlo.sFe20.jultrafine particles (10-20 nm) has been reportedby Komarneni" at 135°C by evaporative decom-position of solutions (EDS) technique givingspherical particles of ferrites. The synthesis ofMn - Zn ferrite particles with narrow particle sizedistribution (the average size being in the range of0.3 to 8 pm) has been reported by Hasegawa"using hydrothermal oxidation process.

Sol-gel/sol-precipitation method-This is a veryrecent technique used for the preparation of mag-netic ceramics. The sol-gel technique is useful forpreparing dense microspheres at low sinteringtemperatures. The technique provides very reac-tive agglomerates of very fine particles. The pro-cess consists of the preparation of a sol (an aque-ous colloidal suspension of the desired oxides inhydrated form) from the nitrates or other salts bycontrolled precipitation. The sol is converted to agel by partial dehydration. Sometimes the dehy-dration is achieved by evaporation or by disper-sion of the sol as droplets in a column containinga long chain alcohol and a surfactant. The gel iscompletely dried and fired to a dense cerarnic-"-".Recently; ultrafine Mn - Zn ferrites have beenprepared by amorphous citrate gel process withlow loss and high permeability compositions byOchiai and Kimura3R (Table 4a). The table com-pares the performance parameters of Mn - Znferrites prepared by citrate gel process and usualceramic process as reproted by the authors. It isvery clear that loss factor is drastically reduced bygel processing. Saimanthip and Amarakoon "made use of sol-gel technique to achieve a un-iform coating of CaO - Si02 film on Mn - Zn fer-

DESHPANDE el aL: RECENT DEVELOPMENTS IN PROCESSING OF Mn-Zn FERRITES 361

Table 5-Mlignetic performance parameters of Mn - Zn ferrite prepared using 'Stabilized MnO'S6

Sr. Composition IIi LFx 1()6 D r, DFx 1()6 TFx 106No. 4kHz 4kHz gem"? 'C

FezOJ Zno MnO (100~)

1. 51.75 20.25 28.0 2640 1.15 4.72 135 0.87 1.06(2620)

2. 5i.71 23.13 ~5.26 2800 1.72 4.72 125 0.45 1.74(2760)

3. 52.00 19.25 2'8.75 2720 1.53 4.77 150 1.09 1.42(2700)

4. 53.06 17.80 29.14 2730 2.70 4.82 160 1.08 0.13(2570)

5. 3H. Philipss1 2300 ± 20% •••1 4.7-4.9__130 :10;4.3 0.5-1.5

Table 6-Comparative evaluation of conventional. copreeipi-tation and spray roasting processes"

Criteria Conventional Coprecipita- Spray roastingprocess tion process process

Process Breakdown Buildup Build up

Processing Standard More Lesssteps

Calcination Necessary Necessary Unnecessary

Comminution Longtime Short time Short timeCost perfor- GoOC Not good ?mance

Powder properties

Primary 0.8-3.0.urn 0.05-5.um 0.4-1.0.urnpanicle size

Size distribu- Wide Narrow Narrowtion

Homogeneity Not good Good GoodPurity Low High High

Core properties Good Excellent Excellent

rite particles. The coated ferrite (MIlo.52ZnO.44Fe204)showed better microstructure than that preparedfrom uncoated powder. The difference in the mic-rostructure was revealed by electron micrograph,indicating many intragranular pores in uncoatedsample which were absent in CaO - Si02 coatedferrite samples. The absence of intragranularpores helps in increasing magnetic permeability ofMn - Zn ferrites.

Freeze-drying method-This is a cryochemicalmeth(>d of preparation for ceramic oxides intro-duced by Schnettler et aL40-42 It consists of atomi-zation of droplets' of salt solution/mixed salt solu-tions of the metal ions of interest through a hy-draulic nozzle at a pressure of a few psi. The

droplets are sprayed into a bath of immiscible li-quid such as hexane. chilled by dryice-acetone ordirectly into liquid N 2 where they are subjected tothe rapid freezing. The rapid freezing minimizesice-salt segregation. The hexane has a better ther-mal contact with the droplets as compared togaseous layer of nitrogen around the droplets inliquid N 2' which impede the heat transfer. Thefrozen product is collected from the top of therefrigerant and is dried in vacuum at about-70°C by sublimation of ice. The freeze-dryingtechnique gives spherical aggregated crystalliteshaving a very low bulk density. The crystallite sizecan be varied by changing the calcination temper-ature or the concentration of the solutions to startwith.

Combustion methods-(a) Solution combustionmethods include those preparation techniques inwhich solutions are actually burned to form solidparticles. A self propagating combustion methodfor the synthesis of spinel ferrites using metal nitr-ate (oxidiser) and oxalic acid dihydrazide (ODH)as fuel is reported", Ferrite is formed at •••350°Calmost instantaneously in less than 3 minutes. Themechanism of combustion method can be ex-plained in terms of solution pyrolysis utilizingoxygen from metal nitrates. The exothermic reac-tion proceeds with the evolution of large amountsof CO2, N2 and water vapour. The reaction, onceinitiated, is catalyzed by the ferrite formed till thereaction is completed. Preparation of differentcompositions of NixZn,_xf'e204 (x=O.2 to 0.8)having large surface area (85-95 m//g) and nar-row particle size distribution has been reported.Maximum saturation magnetization of 73.4 emu/gis obtained for x =0.5 composition. However,other magnetic performance parameters such aspermeability, loss factor, T.F., DF, etc., have notbeen measured for these materials. The method is

362 INDIAN J CHEM, SEe. A, MAY ]996

Table 7:"Magnetic property values for high-frequency manganese zinc ferntes'"

Supplier Material Permeability Power loss Conditions(mW/cm3)

Kristinel Corp. KB4 2000 130 400 kHz/500 GKB4 2000 730 I MHz/500GKB5 1000 130 400 kHz/500 GKB5 1000 520 1 MHz/500G

KB5 1000 300 2MHz/200G'Fuji Electrochemical Co. 120 500 kHz/500 G*Niederlandse Philis 2000 200 500 kHzl200 mTBedrijven

500 1 MHzl200mT

1000 2MHz/200mTTDK H74 700 500 MHz/100 mT*

2000 1 MHz/100 mT*

*At 100·C

used for other spinels" and hexagonal" ferritesalso.

(b) Combustion of solid solution precursors in-clude combustion of precursors of the type(N2Hs)3 Nil_AFe2(N2H3COOk3H20 (O~ x ~1).Hydrazinium metal hydrazine carboxylate,' at250°C in oxygen atmosphere has been used forthe preparation of Ni - Zn ferrite powders". Thespeciality of the technique is that the complex isdecomposed exothermally in a single step be-tween 120 and 200°C resulting in fine particles ofthe ferrite. High densities (99% of the theoretical)have been achieved with these fine materials whenheated at llOO°C for 24 h as also the increasedsaturation magnetization of 70 emu/g. The varia-tion of permeability with the firing temperatureand frequency for a typical Ni - Zn ferrite com-position is given in Table 4(b).

Coprecipitation method-The chemical copreci-pitation of a suitable precursor such as hydroxide,carbonate or oxalate of the metal ions of interestis carried out using solutions of sulphates, chlo-rides or -nitrates and a suitable precipitating agentsuch as sodium hydroxide, ammonium hydroxide,sodium carbonate or ammonium oxalate underoptimized processing conditions. The processingconditions include:(i) ratio of cations; (ii) concentration of solutions;(iii) alkali proportion; (iv) rate of precipitation; (v)temperature of precipitation; (vi) pH of slurry;(vii) method of washing ppt; (viii) pH of wash; (ix)drying temperature of the ppt; and (x) calcinationtemperature/time.

The precursor on ignition/decomposition yieldsthe ferrite powder of different morphologies de-pending on the nature of the precursor and theprocessing parameters used for a given ferrite.The flow sheet in Fig. 6 explains the method inbrief. Bo and Leyi'" used this method to preparelow loss high stability Mn - Zn ferrites by prepar-ing complex oxalates of manganese, zinc and iron.Precipitation of the complex oxalates was carriedout at 60- 70°C using I-2M solution of sulphatesand 1.2-1.6M ammonium oxalate solution as aprecipitating agent. Deionized water was used forwashing the precipitate free from sulphate ions,followed by drying at "" 1800e and calciningaround 8000e to get the ferrite. The performanceparameters reported are given below:

1. Ilj= 1800

tan d -02. -.-= 1 x 10 (at 100 KHz)

III

3. DF= 5 x 10-6

4. TF= 1 X 10-6 ( -10 to + 55°C)

Other prominent contributions are fromAkashi'" who reported synthesis of low loss highstability Mn - Zn ferrite using coprecipitationroute and also Goldman et al.49•so who obtainedmonodispersed particles of this ferrite by controll-ing the agglomeration of fine particles. Recently,colloidally precipitated Mn - Zn ferrites are re-ported by Petrovich et al.51

DESHPANDE et 01.: RECENT DEVELOPMENTS IN PROCESSING OF Mn-Zn FERRITES 363

Tc·2S00CJ.--4~IPo--+~-7'I-+~f--k--?\••••0•0L-~~L-~~ __L-~~~~-L~5

45 10 II 10 15 1'0Tc· 3OO-c FlzOI

Fig. 9-Composition diagram for Mn - Zn ferrite system?".

In our laboratory, we have carried out copreci-pitation of Mn-oxalate with small quantities of Znoxalate to prepare solid solution of oxides ofmanganese and zinc, Mn, _xZnxO (x = 0.001-0.1 ).This is. a stable variety of MnO which does notabsorb oxygen from ambient even on heating to atemperature of 200°C in air and therefore re-ferred to as 'stabilized MnO'52. It was studied indetail for its optical, electrical, thermal and mag-netic properties and also used for the preparationof high permeability Mn - Zn ferrite53-50 with lowloss factor in .the frequency range of 4 to 100kHz. The magnetic performance parameters ob-tained are given in Table 5. They were reproduci-ble for batch to batch preparations of the ferritecompositions and were also comparable with thereported data on professional grade high permea-bility, low loss Mn - Zn ferrites'", The above dis-cussion clearly brings out the correlation betweenprocessing and the performance of Mn - Zn fer-rites. Each method described above is importantin its own right though one has to adopt a suit-able method keeping in view the ferrite applica-tion (Table 6)5R.

High frequency Mn- ZnferritesIn processing of high frequency Mn - Zn fer-

rites, minimization of the eddy current losses is avery important factor. Eddy current loss isbrought about by high electrical resistivities of theferrites. Amongst the soft ferrites, Mn - Zn fer-rites have. the desirable magnetic properties whichmake them suitable for applications in MHz range.of frequencies. These properties are high satura-tion magnetization, high permeability and lowlosses upto a frequency of about 2 MHz (Table7)59. These characteristics properties can be

SOOO~------------------------,

5 '0 '1 20DI •• "I' CAI"

Fig. 10- Permeability of Mn - Zn ferrite as a function ofgrain sizefttr•

achieved by selecting some suitable compositionswith the help of the composition diagram (Fig .9)59. While choosing a composition care should

. be taken to ensure that crystalline anisotropy andmagnetostriction are minimum. The phase dia-gram also depicts the regions where the desirablemagnetic properties can be obtained. The stoichi-ometric composition of Mn - Zn ferrite is repre-sented by Mn2+Zn2+ Fe2+Fe3+0' and the opt-x I-x-y y 2 4,imum composition range for power ferrite as giv-en by Ochiai is 52.5-54.5 mol % Fe203 and 9.0 to14:0 mol% ZnO, the balance being MnO. Thisrange of MnO and ZnO concentrations has beenexpanded to include a little higher MnO and ZnOconcentrations in recent developments resulting inthe change in position of secondary maximum ofpermeability (SMP) to the desired temperaturerange of application (such as transformer operat-ing temperature range).

Another important factor in achieving the re-quired characteristics of power ferrites is the mic-rostructure. The desired microstructure for a highfrequency Mn - Zn ferrite core material is a fine,uniform rnicl'ostructure with grain size > 5 pmhaving high grain boundary resistance. The grainsize is important since the permeability of the fer-rite is drastically decreased below the critical sizeas shown in Fig. 10 (ref. 60). Guilland related theinflection of the curve at 5 pm to a change fromdomain rotation to wall-movement above 5 p.m.However, many other researchers+'<' have con-firmed the linear dependence of grain size uptopermeabilities of 40,000 and grain size to 40 p.m.An important and necessary precaution was to li-mit included porosity since the permeabilitiesdrop because of intragranular porosity. It is evi-denced from the fact that if pores are suppressedor located at the grain boundaries, the permeabi-lities increased with grain size. Apparently, forhigh frequency ferrites a compromise has to bemade between the large grain, dense ferrites forhigh. permeability at low frequencies and the smallgrain, porous ferrites for low losses at high fre-

364 INDIAN J CHEM, SEC. A. MAY 1996

quencies with prominent role of grain boundaries,as quoted by Goldman'" while describing highfrequency materials.

ConclusionTo summarize, we arrive at the following con-

clusions regarding the processing of this profes-sionally important ferrite:

Conventional ceramic methods have been im-proved through the use of active raw materialsand dopants. The new methods of processingdeveloped over the last two decades have. turnedout to be fruitful in this regard.

Processing of Mn - Zn ferrites by wet-chemicalmethods has proved to be advantageous with re-spect to the quality of ferrite powders as com-pared to the conventional method of processing.These new methods produce powders of highpurity, better homogeneity and fine particle sizewith .narrow size distribution and better sinterabi-lity.

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