Metal-barium hexaferrite composites as permanent magnets

CERAMIJRGIA

INTERNATIONAL.

Vol.

3. n. 2.

1977

70

n&tabBarium Hexaferrite Composites as Permanent Magnets G. ASTI *, P. CAVALLOTTI Laboratorio MASPEC. di Chimica-Fisica! Eiettrochimica ??

* CNPM,

??

istituto

Ikrium ferrite Powders have been coated with a thin layer of a cobalt phosphorus or nickel phosphorus alloy by chemical reduction from aqueous solutions with hypophosphite, after the usual sensitization-activation pretreatments. The operation parameters of the deposition baths have been vsried to modify the phosphorus content, the morphology and crystallographic structure of the coatings, pointing out their influence on the magnetic and mechanical characte ristics of the composites. the utilization of slurry electrodes has shown the possibility of producing composites of barium ferrite with various metals, such as cobalt. nickel, iron, cop Per and zinc by electrochemical treatments. The different Powders produced with these new methods have been compacted and sintered at several temperatures: the influence of the thermal treatment on the characteristics of the produced magnets Is pointed wt. Encouraging results have been obtained, showing the possibility of producing magnets with improved magnetic and/or mechanical characteristics.

I . INTRODUCTION Barium ferrite ceramics are the most widely used materials for making permanent magnets. Their main advantages are: very high coercivity, high stability and specific electrical resistance. However they present some disadvantages, such as a relatively low remanence and poor mechanical properties, being brittle and difficult to machine. The finished products are obtained by pressing barium hexaferrite powders into shape ‘and sintering them at 1200°C + 1400°C. A magnetic field is often imposed onto the powder dispersed in a slurry during the pressing operation in order to obtain anisotropic ferrites of increased properties: e.g. the remanence is almost doubled. Shrinkage is very great during sintering and its control is difficult. Rubber magnets are made directly from powders using elastic or plastic binders. These flexible products are easy to work and shape, but have a decreased remanence bacause of the decreased weigth per volume of barium hexaferrite. Furthermore, it is difficult to obtain a strong orientation of the particles. The use of barium hexaferrite ceramics might be further extended and also new fields of application might be found by improving some of their magnetic properties, such as the energy product (BH), or their mechanical properties. We have undertaken research work with the aim of producing metal-ferrite composites. With this method it would be possible to increase the mechanical strength of the. magnet, its machinability

and to improve the control of shrinkage during sintering. The combination of two magnetic materials with different properties and their direct interaction seemed worth atudylng. We have adopted deposition methods from aqueous solutions to obtain a metallic plating of the ferrite powders. By this method it is possible to obtain a cioae contact between barium hexaferrite and

coating ‘. An increase of the anisotropy of the compact aan be expected when metal plated powders are oriented in a magnetic field. A lower tendency of metal plated powders can be expected towards agglo-

* * and R. ROBERTI ** C.N.R. Parma, Italy e Metaiiurgia dei Politecnico

di Milano,

Italy

meration because of a _decreased e!actrostatic and magnetostatic interaction. This effect is known to be a negative factor in obtaining highly oriented compacts. Cobalt alloys films, chemically reduced with hypephosphite, are usually employed for information storage, because of their good magnetic ::qooerties, particuforce. Coercivitlarly for their relatively high coerr,ive ies up to 1.2 and 2.0 K Oe and satuc-ation magnetization of 120 + 150 and 90 + i i3 ekTu/‘g have been reported for Co-P and Co-Ni-P films resp~.:ctively’. The study of the coupling of these 7 i.7. .:z i:3rium hexaferrite has been considered a ~di’.!~:!~-3c+ject of research.

2 - EXPERIMENTAL Cobalt alloys have been chemically deposited by reduction with hypophosphite on barium hexaferrite powders or single crystals, and on Cu or Ni single crystals for comparison. We used barium and strontium ferrite powders, synthesized by Montedison Novara ‘.‘, with the characteristics given in Table I. Fig. 1 shows a typical picture of the barium hexaferrite powders. Barium ferrite single crystals were obtained by a flux method, as described by Fano ‘. Copper arid nickel single crystals were obtained by a modified Bridgman technique frc;m “‘: pure materials. Their synthesis and orisntation ha?ie oeen described elsewhere ‘. The metal single crv.;L?ls were mechanically and chemically polished. For barium hexaferrite plating a sensitization-activation tres:mcnt of the following type was adopted: a. immersion in a 5.10-‘M SnCI. solution, aged for at least a week ‘; b. immersion in a 70 g/i SnCL 40 cc/l HCI cont. solution; c. immersion in a 0.5 g,/l PdCh, 5 cc/i HCI cont. solution I. Every immersion was done at 30” + 40°C for two minutes, followed by a prolonged water rinse. In some cases, before the sensitization-activation treatment, a grinding operation of the barium hexaferrite powders was performed in a Retsch-mill for about 1 h. In other cases, wet grinding was carried out simuitaneously with the activation treatment, using the solution a or a solution obtained by addition of 10 cc/i of the solutions b and c to the solution a. The composition and operating conditions of the chemical plating baths used for depositing Co-P or Co-Ni-P are given in Table Ii. A non-magnetic coating was often deposited after the magnetic one. These were Cu or Ni-P coatings obtained from baths whose composition is given in Table Ill. The deposit thickness on metallic substrates was measured by amperostatic anodic dissolution of a selected area in a cobalt solution of 100% anodic current efficiency. Ceil voltage variation gives the end point with a very good sensitivity. Electrochemical treatments of ferrite powders were made for comparison and carried out in the cell Of Fig. 2. Powders were placed onto Pt or platinized-Tl electrodes. A paper diaphragm was placed over the slurry electrode. Compaction of the powders and sintering in nitrogen

ynA~_@ARIuM

HBL~~+R’TECOMPOSITES

AS PERMANENT MAGNETS

71

_e f _ Cheract@ristiCe rr@s ,,euferrite powdere.

of the etartlW

Synthesis temperature Fe/Ba or Fe/Sr by X-ray fluorescence Analysis by X-ray diffraction

of the chemical

plating

1150°C 11.21

1000°C 12.96 a-Fen, 4 - 6% SrSO, 1 - 2% 0.236 0.445 3.97 65.1 5359

as in gasout__ 11

1

3 - RESULTS 3.1 - Characteristics

SrFeuOl,

BaSO, 1% 0.676 1.63 66.4 4340

Medium diameter by TEM (pm] Weighted medium diameter (pm] Surface area by BET (m’/g) cmKa (emu/g1 ,Hc (0 erstedl

were done by the usual P/M techniques. The morphoio By and crystallographic structure of the specimens were determined by several techniques: optical microscopy. scanning electron microscopy (SEMI, transmission electron microscopy [TEM) by the replica technique, reflection high energy electron diffraction (RHEED) at 50 KV, X-ray diffraction. The combination of the different techniques has proved to be particularly useful. The magnetic properties of the processed powders were obtained with a PAR vibrating sample magnetometer, at room temperature, and by use of an external field of intensity up to 20 KOe. The saturation magna tization was reproducible to * 3 emu/g, the coercitivity to f 10 Oe. Hysteresis curves of sintered speci: mens were recorded by means of a Permagraph Hyste resigraph.

BaFellOlc

4

anode

diaphragm 4

baths

- pwder

The three baths reported in Table II were chosen because they give deposits of specific preferred orientations. Bath A gives well-textured cl1 .O> deposits, bath 0 ~10.0, ones, and bath C

‘Pt or Ti,, FIGURE

2 - Electrolysis

cell

with a slurry electrode.

FIGURE S - Rae of deposition of Co-Ni-P agelnst nlckal addition to the solution. [Cd+] + [Nl”] = 03 melee/l.

A and B have deposition rates of 12 and 13 mg/cm’/h at 80°C and an activation energy of 10.3 and 11.8 Kcal/mole respectively. The general behaviour of these baths is different. it is difficult to observe inhibition in bath A and it has the greatest hydrogen evolution; when the pH is increased it tends to decompose. Bath B is easily inhibited, and bath C even more so. When the pH is increased the last two baths give .dark depo-

G. ASTI. P. CAVALLOTTI snd ri. ROBERTI L___----

72 v&E

;l-i

Composition and operating condition of chemical plating

&SO,

+ NiSO,

CaH,O,[NHJt t4&Xt+COOH pH iat ‘25°C with T”C

B

A

Bath name

NaOH)

0.3 0.3

M M

0.3 1.2

M

0.3 0.3

C M M

0.3 0.3

M M

0.6 M 0.3 10.5

0.4 0.3 115

M

10.5 80.0

80.0

80.0

sits, because of the precipitation of hydroxides or basic salts. When nickel is par$ially substituted to cobalt, the deposition rate decreases, as shown in Fig. 3. The nickel/cobalt ratio in the deposit is smaller than the ratio in the bath in the range investigated. Bath A gives deposits containing less nickel than the corresponding deposits from bath B. When further increasing the nickel content of the bath, the deposition trend is reversed and the nickel/cobalt ratio In the deposit becomes greater than its ratio in the bath.

b FIGURE 4 bath are:

. SEM micrograph of Co+ cftemfcaf deposits on (00.1) Aa-Odpm,bathA: h-1 v,bathA; h-4 pm.bMhA;

4”m*1~m,

fat:f)s of BaFe,&

single

Ab.g.Bpm,batftB: Bb-1 pm.bathB; Cb-4 pm,bathB.

! crystals.

Deposit thickness and deposition

MnA~_SARI,,,,d

HEXAFERRITE

Ccbtd~Sl~

AS

PERMANENT

-I---

MAGNETS

__----

FIGURE

6 . SEM micrograph of Co-? deporlts

on Ill11

h&s

of

bath are: a - Nf, 1 32

73 -

-

- Chemical

pm, bath A;

deposition

b-NI,O.Bm.bathI;

of Co-P on single

and NI singlo crystals. Substrate, .. c-C&l

p,bathA:

deposit thickness and deposition d - Nl. 4

p,

bath B.

crysta!s

The morphology of Co-P deposits obtained by chemical deposition with the baths of Table II has been s:iIdiiti using single crystals of Ba FellOlv and, for LXXX::-YS.. ;:. of Cu and Ni as substrates. Fig. 4 shows r.ome typlcal features of the deposits onto (00.1) faces of barium; hexaferrite single crystals. Bath A gives well resoi\& Structures showing basal planes of cob&It and ;i hsePacked direction developing perpendicular to pi;, SGJatrate with increasing thickness. The dimension of tl;-~ crystallites incrctises with the deposition time. F.k~al Pianos growing outwards are already present at G ;h.zknoss of the layer less than 1000 A. At low thickness, some epitaxial retationshlp with the depcsi; : -.I, bt: observed. The structure of the coating ic. :-:I .c. ‘-P tYPe for deposition frorii’%th A and B at icv.; il.,... ). _;;. However, deposits from bath B give ctysi&llrcVI smaffer size In this range (see Fig. 4 Aa and AL). For we have deposlted Co-P onto 1111) faces of comP?riSnn fCc sw$e crystals, which have a nearest ncighbsur distance very similar to that of cobalt, nameiy mz!!s! and copper. We have observed that Co-P grows pzrallet to the substrate, up to 0.5 urn and 0.2 urn for deposition from bath A and B respectively. At this thickness a twinning transformation occurs with composition planes {IO.1 1 I”. The new structure has been interpreted according to the orientation (10.1) [01.21c,// (111) [112l,k --;d 1‘4. morphology is shown in fig. 5A and 58. Its RHEED patterns, together with the points obtained by calculation, are given in Fig. 6.

FIGURE S - RHEED patterns of 0.8 pm Co-P deposits on (1111 fades of Cu and Ni single crystals. Beam along: A - [llO]; B - [ll’i]. a - substrate Cu. bath A; b - substrate Ni, bath B: c - interpretation.

A similar structure is obtained at lower thickness by deposition onto the (00.1) face of a barium hexaferrite single crystal. As the thickness increases the basal planes become oriented perpendicular to the substrate, taking the orientation imposed by the bath. Furthermore

G. ASTI. P. CAVALLOTTI end R. flOBERTI -~-

_

TABLE IV - Magnetic characteristics of 8aFe,O,, powders plated with Co-P alloys followed by a plating with Cu or NI.

Bath

Deposition time IminI

Deposition time 0nlnI

Bath

66.4 65.2 Zf :,: 67.3 63.9 59.3 59.8 53.1

FIGURE 7 - SEM micrograph of a Co-Ni.P chemical daporlt on the (111) face of a Cu single crystal. Bath A, [@+l/[Ni’+] = 2.

they also develop along other directions, as is shown in Fig. 5c and D. When nickel is added to the plating bath, the deposits become smooth with a typical rounded structure, see Fig. 7. The structure obtained by (IO.1 1 twinning occurs at very low thickness. Amorphous deposits are obtained when the nickel content is increased.

3.3 - Chemical plating of barium and strontium hexeferrite powders. We have deposited Co-P from baths A, B and C onto barium hexaferrite powders with the characteristics given in Table I. The Co-P deposition was usually foil@ wed by the deposition of a non-magnetic metal, such as Cu or Ni-P. from the baths given in Table Ill. This treatment can slightly increase the coerclvity of the powders as is shown in Table IV. Deposition of Co-P

T+BlE III - Chemical plating batf~ for

from bath A can also increase, to a small extent, the saturation magnetization. Deposition from bath B and bath C always decreases the saturation magnetization, whilst the coerclvity Is nearly the same. Table V shows some results obtained by deposition with Co-Ni-P alloys. In this case the coercivity is slightly increased. When grinding the powder a coercivity decrease is usually observed for this type of powders, whilst the saturation magnetization values are still high. A measured increase of the G/V. ratio from -47 to .5 value indicates thei the grinding produces particles small enough tc h3,!: single domain behaviour. Strontium hexaferrite powders show a similar __..” ” I -viour. We have been able to increase the coercivity by about 5% by plating with Ni-P or Co-Ni-P + :<,-F alloys; however. in this case the saturation magnetization decreased. Fig. 8 shows TEM and SEM micrographs of the treated powders. On increasing the deposition time the powder is more easily separated into separate particles.

3d - Electrochemlcel powdere.

CuSO, 5 Hz0 HCHO K Na tartrate Thioglycolic acid Glycerophosphoric NaOH Room temperature

of barium hexaferrite

We have produced composites of barium hexaferrite with various metals, such as cobalt, nickel, Iron, copper

Ni bath NiSO, ::aH.901 Lactic acid hialeic acid pti (adjusted T

14 g/t 48 gg/l 70 g/j acid

O*oo: % 20 g/I

with

0.1 mole,/1 0.3 moles/l 60 ccjl 2 g/l NaOH) 4.5 80°C

- ..---

..a

TABLE V - Magnetic characteristics of WeoGs powden plated with ~XU-P + + Cu or Ni.

Bath TYpe

kr. --.. 2. A ??. .

B

8 Ground for 1 h In a SnCI, solution. ?? * Ground for 1 h In 8 colloidal palladium rmtutlon. ?? ** Ground for 10 h in a SnCl, solution.

treatment

Cu bath

obtaining Cu or Nl deposits.

4340 4470 4370 4250 4270 4280 4535 4300 4315 4120 4270

Co/Ni

ratio 2 -. i 2 2 5 3

Deposition time Imin)

Bath

1

Ni -

3: 1 1 :

-

8 : 3 2

1 : 1

Ni Ni

“111 nvr ii.-fiiitigj

ia,:i

_ 4uuu 47W 385G

-

n 00.1 66.5 64.6 59.4 $2, _ . .-c 01.3 63.1

.5 .5 .5

612 61.8 61.9

_
.5 -

-

??

! B

C<~;inr;i:inn time Imin)

-

65

&AL-BARIUM

WAFERRITE

COMPOSITES

AB

~--._.._-.

PERMANENT MAGNETS -.~_~~.. .__

.__

__._.~~

3

Pf~t~ft~

3 . Micrographs

of barium

haxaferritc

powders

treated

by

dtemiul plating of Co alloys. i - TEM; Co-P, bath A. 1’;

and zinc by electrochemical treatments with slurry electrodes ‘I. We have been able to increase the saturation magnetization by cobalt plating onto Ba Fe120Eo powders. up to a cobalt content of about lo%, from 64.8 emu/g to 66.5 emu/g. In this case the coercitivy decrease from 4050 Oe to 3325 Oe. The t~,/u. value was very low: .4. 3.5. - Compaction and sintering ferrite powders Preliminary

results

on

FIGURE 9 - SEM micrographs dtr

end s’nt*W

at different

the

of treated

behaviour

of fracture temperatum.

of

surfaces ti A,

barium hexa-

isotrcpicaily

of barium hexaferrite [~o**]/[~fa+] = 2.

b c d

,\--

-

:\!

_

_~

., 1’ + xi i:,

_,

SEM; TEM; SEM:

i’;_

d -

Cu Xi

Co-N&P. bath A, [CO’~]/[NI’+] Co-NLP. bath A, [Coz+]/[Ni*+] Co-Ni-P. bath A. [Co?*]/[Ni”]

P,

bat?]

A,

x

= 2. 3’: = 2. I’: Ni 5’; = 2. 30’.

compacted barium he~?iz~ :i:s powders chemically plated with Co-Ni-P have S!IC:.XI that already at 1100°C a certain degree of sin&ring occurs. At 1200°C the mechanical strength of the compact increases greatly and shrinkage of the compact is observed (about 2.5%], At 1250°C there is grain growth of the barium hexaferrite powder. Fig. 9 shows SEM micrographs which illustrate the effect of the temperature on the sintering of the powder plated with a ,Co-Ni-P alloy. Demagnetization curves are presented in Fig. 10. The coercivity H, decreases and the remanence 0, increases with the sintering temperature. At 1250°C there is a large decrease

magnets,

treated

by chemical

deposition

of Co-Ni-P

on

the pow-

G.

76

FIGURE 10 - Demagnetisation curves of Co-Ni-P plated Ba Fe,20,p Powders, compacted and sintered at different temperatures. of the magnetic characteristics, in accordance with the observed grain growth in the material. Grain growth is favoured by a greater content of the deposited cobalt alloy in the material. 4 - DISCUSSION By a suitable choice of the composition of the chemical plating baths used it is possible to obtain different preferred orientations of the Co-P deposits. These may be distinguished according to the position of a Close packed direction: normal to the surface in deposits from bath A. tilted from bath B. parallel from bath C. The occurrence of these preferred orientations may be related to the general character of the process. A theory proposed in this laboratory “e” has stressed the importance of hydrolysis phenomena in chemical plating. in the case of cobalt, a two-step reduction of a cobalt hydrolized species is proposed: Co (OH)? + HZPOI- + Co (OH).d, + HIPO-3 + H Co (OH).d, + HIPOS- + Co + HIPO,- + H

ASTI.

P.

CAVALLOTTI

and

R.

ROBERTI

-

_

(al (b)

where Co (OH), is assumed to represent any hydrolized reacting species present in the solution, and Co (OH),d, any adsorbed specie& of partially reduced cobalt ions. According to which reaction is the rate determining step, the different preferred orientations are obtained. Hydrolysis reaction becomes, according to us. rate determining when the {ll.OI preferred orientation is obtained, reaction (a) for (10.0) preferred orientation, reaction (b) for (00.1) preferred orientation. A different activation energy is observed in correspondence to the given preferred orientations, and inhibition Phenomena are easily observed in the case of baths giving the (00.1) preferred Orientation. The influence of the operation parameters is in accordance with the Proposed scheme. When nickel is added to the solution, several phenomena must be taken into account:

a - the preferential

adsorption of cobalt on the surface with respect to nickel; b - the greater stabilization. of the hydrolized species by nickel ions and the greater reactivity towards the tetrahedral hypophosphite anion, negatively the cobalt-nickel baths is strongly ineffects. onto single crystals we have been h three different phases: _ yer range: in this range the structure of the sit Is mainly influenced by the substrate. Hcp 1% together with some fee Co, obtained in a . mflensitic relationship with fee substrates;

b - medium thick deposit; in this range the internal stesses provoked by .the difference in lattice parameter between deposit and substrate give a network of dislocations from which a new structure developes in a twinning relationship with the substrate. The observed (IO.1 1 twinning has been related ” to the occurrence of stacking faults in the deposits. This range begins very early when nickel is added to the solutions or when the lattice parameters of substrate and deposit are very different; c - thick deposits: in this range the orienting power of the bath prevails, a& :L; ;.L:L, LA orientation of the bath is obtained by tilting or rotation of planes. It has already been observed “ that the coercivity has a maximum as a function of the film thickness. The occurrence of this maximum may be related to the above mentioned structural change in the deposit. We have observed a very clear influence of the morphology of the deposit on the magnetic properties also in the case of the chemically plated barium hexaferrite powders. Powder plated in bath A has a greater saturation magnetization and remanence than those deposited in bath B or C. and it was also possible to obtain an increased value of croto.with respect to the calculated value, assuming that the barium hexaferrite crystallites are pure and randomly distributed. When nickel was present in the deposit, either by codeposition with Co and P, or subsequent deposition, or by direct deposition onto the powder a slight increase of the coercivity could be observed. The increase of magnetic characteristics’ observed in treated hexaferrite powder can be, in general, related to typical phenomena due to interaction and coupling between substances with different magnetic properties. In fact, the contact between a permanent magnet and an isotropic substance of constant permability decrease:5 the maenergy. This can occur here ?::ause the gnetostatic location of the easy magnetization oxis .:i :!x plated cobalt promotes a dispersion of the- ;:.:---; :harges. _I ..:eposits This is particularly the case for bath .I, of more definite structure. In this C:‘YS 2 .,otiid obtain a higher coercive field. Nickel 2“ : .t4:)per deposition, as is well known, separates :;:c.: .~~iyf:ctic particles promoting a more effective dori-j~:in ?vall pinning at the grain boundaries, which is kno.;n !o be a leading process in determining the coerci\ie icr<;t I?f a permanent magnet material. A careful e:xa*??j~:jo:. of the data given in Table IV and V shows :!I;;: :ht~ e.:perinlental results are in accordance with this simp:r? explanation. On the other hand, the saturation 1;+;?
M~~AL.SAA,“MHEXAFERRITE COMPOSITES AS PERMANENT MAGNETS ~___ ~__. -.. ___B___ --,-_-. --son with chemical plating. It has several advantages, Such as: the possibility of an easy Variation of the operation condition. with an easy deposition of different metals or elioys. and also the low cost of the electrochemical deposition. But it suffers from one main disadvantage, 1.e. the difficulty of obtaining powders with a uniform treatment. In the use of a fluidized bed or circulating electrodes the above difficulty can be avoided. On the other hand, chemical plating Is easily carried out. also on a laboratory scale as no special equipment is needed. It gives compact and uniform deposits of predetermined preferred orientation. The possible Increase of the saturation magnetization in the case of cobalt electroplating has been shown. The greater increase of u may be related to the purity of electrodeposited cobalt. The u,/Q. ratio is, in this case, greatly decreased. This may be understood by taking into account that pure cobalt, which covers the hexaferrite powders, is a soft magnetic substance,, and its magnetization continuosly’ changes with the change of the external field so that the local magnetic field remains nearly zero. Only preliminary experiments have been performed on sintered products. The main advantages which can be expected from these treatments are: a - an increase of the mechanical properties, particularly of the workability: b - an increase of the orienting power of the dispersed powder in a magnetic field, when these particles are covered with a non-magnetic metal, and consequently a better anisotropy of the compact: c - an optimization of the magnetic properties, by increasing the remanence of the ferrite with a metal having high saturation magnetization, andjor of the coercivity of the compact. The production of these metal-ceramic composites gives one more variance degree to the possibility of the fabrication of permanent magnets. Hot pressing techniques may permit the manufacture of better composite magnetic materials. In fact, in this case grain growth phenomena, whose Importance Is clear from Fig. 9, may be minimized nevertheless obtaining good densification of the materials.

ACKNOWLEDGEMENTS The authors are grateful to dr. L. Giarda and A. Corradi, 1st. Donegani, Monte&son, Novara for supplying the powders and useful discussions. starting hexaferrite The authors also wish to thank dr Greppi and Buzztti, Centro Magneti Permanenti, Caronno Pertusella, Mi/ano, for some magnetic measurements on sintered products.

REFERENCES

I

A. PASSERONE. R. B!A.ClNl arid V. LORENZELLI. Ceramurgia iz ternational 1 (1975) 23. 2. W.H. SAFRANEK. The Properties of Electrodeposited Metal: and Alloys. Elsevier. N.Y. (1974). Ch. 21. 3. G FAGHERAZZI. C F.JAGGI and G. SIAONI. Ceramurgia 2 (1972) 181. 4. L. GIARDA. Ceramurgia 6 11976) 33. 5. V. FANO. Ceramurgia 6 (1976) 21. 6. F. PEARLSTEIN, Met. Fin., Aug. [1965] 59. 7. N. FELDSTEIN and J.A. WEINER, J. Electrochem. Sot. 120 (1973) 475. 8. R. PIONTELLI. Electrochim. Met. 1 (1966) 5. 9. L. CADORNA. P. CAVALLOTTI and G. SALVAGO, Electroch\m 1 (19661 177: P. CAVALLOTTI and G. SALVAGO .! Met. Electrochem. Sot. it6 (1969) 819. 10. P. CAVALLOTTI and S NOER, J. Mat. Sci. 11 119761 645: P. CAVALOTTi, S I.,liER and G. CAIRONI, J. Mat, Sci. 1: I19761 1419. 11. P. CAVALLOTT! 1 ,_!i4Tl and R. ROBERTI. Ceramurgla E (1976) 17. 12. P. CAVALLOTTI ;:?d G SALVAGO, Electrochim Met 3 119681 329. 13. G. SALVAGO anc P CAVALLOTTI, Plating 60 (1972) 665. 14. J.S. JUDGE, J.R. LTORRISON. P.E. SPELtOTiS and G. BATE. J. Electrochem. Sot. 112 (19651 681.

Received

May

26.

1976;

revised

copy

received

October

28.

19%.