IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-17, No. 6, NOVEMBER 1981

y- Fe 0 WITH CONDENSED PHOSPHATES AS ANTISINTERING AGENTS 2 3

E. Me110 Ceresa

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Abstract - Magnetic and rheological properties for pure and phosphates surface modified y-Fe-0, were de- - - ;ermined in order to evaluate different k d s of phos- phate ions, as antisintering agents, during the trans- formation of a-FeOOH into y-Fe 0 via Fe 0 The magnetic data show that hexametaphosphate (HIT) and tripolypho'sphate (TPP) ions,which adsmh onto the a-FeW precursor surface, 'behave like effective antisintering additives in the same way as sili'ca coating. On the contrary, no antisintering properties are shown by pyrophosphate (PP) ions. The wettability and the specific surface area of the HMP and TPP modified.y-Fe 0 are higher than those of the pure and PP treated $-$e 0 since the increase of surface roughness produces a much higher "real surface area".

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INTRODUCTION

Acicular y-Fe 0 (maghemite) is the most common material for magnetic recording and its production has become .extremely important due to the increased use of magnetic tapes. The usual preparation, which involves a-FeOOH deydra- tion and reduction to Fe 0 at 35OaC-40O0C followed by controlled re-oxidation at lower temperature (200°C- 3 4

300°C), can be represented as follows:

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The properties of y-Fe 0 are mainly dependent on the shape and size distribution of the precursor, as the particles maintain the elongated shape of the original a-FeOOH. O n the other hand, several pores and voids are developed into the structure during the process, producing local demagnetizing fields,which reduce the coercivity values of the materials tl'l. The pores closure, which is ruled by the reduction tem- perature, can be achieved by coating the precursor par- ticles. This allows to perform the reduction at higher temperatures without morphological collapse and stick- ing of the particles in order to obtain more compact bulk structures and higher coercivity values C21.

of phosphates as surface modifiers and to compare the magnetic 'and rheological behaviour of' the products so obtained with that of untreated y-Fe 0

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The aim of this work is to evaluate different kinds

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EXPERIMENTAL

Purea-FeOOH was supplied from pilot-plant produc- tion of Montedison. The average length and width of the particles, determined by electron microscopy using the

Robinson and Hockings method E31 , were estimated to be 0.90 and 0.21 pm. The reduction and oxidation steps for pure and surface modified a-FeOOH were carried out using a fluid bed reactor, according to the previously described proce- dure C21 . The surface treatment of a-FeOOH was made in water so- lution at pH of 5.5, 'by specifically adsorbing onto SUE

pended and well stirred particles a proper amount (-1.0 w/w on the dried product) of different kinds of phos- phates: namely, sodium hexametaphosphate (HMP) (NaPOJ6, sodium tripolyphosphate (TPP) Na P 0 and sodium pyro- phosphate (PP) Na P 0

sured wlth a vlbrating sample magnetometer. ci r Specific surface areas were determined by low-tempera- ture N adsorption using the BET technique. The wettability was evaluated by the oil absorption me- thod which measures the amount in grams of linseed-oil required by a specified amount of y-Fe 0 pigment to form a putty-like mass C41. The weight'o3 oil in grams, calculated for 100 g of pigment, necessary to reach this state is the O.A. Z (Oil Absorption) value. The phosphorus content in the ferric oxides, before and after the reduction/oxidation treatment, was determined colorimetrically by the well-known molybdate blue me- thod.

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H and j (remanence/saturation ratio) were mea- 4 2 7'

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RESULTS AND DISCUSSION

Condensed phosphate ions are usually used to sta- bilize colloidal systems, because of their specific a2 sorption onto the particles surface with relative in- crease of particles zeta-potential ( [ ) C51. It has been reported that phosphate i.ons are adsorbed to a greater extent than simpler ions like chloride, sulfate and carbonate C61, in fact the stability of ferric oxide suspensions containing tripolyphosphate is comparable with that operated by a typical dispers- ing agent. It is also known that y-Fe 0 powders treated with so- dium metaphosphate and tripo?yphosphate show an anoma-

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lous increase of coercivity, which disappears when the samples were washed with water C7, 83. Forthereasons given above some kinds of phosphates have been tried as protective agents against tempera- ture. The relationship between coercive force and reduction temperature fofuncoated and phosphate surface modified y-Fe 0 samples is reported in Fig. 1. It is apparent that HMP and TPP, added to a-FeOOH, be- have like effective antisintering additives allowing to reach reduction temperatures higher than thoseforpure and PP treated samples, so obtaining coercivity values about 15 Oe higher.

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Manuscript received April 6 , 1981. E. Mello Ceresa is with Istituto G.Donegani S.p.A.,

via G. Fauser 4, 28100 Novara (Italy).

0018-9464/81/1100-3023$00.75 0 1981 IEEE

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Tr P C )

Fig. 1. Coercivity vs. reduction temperature for un- coated (TQ) and phosphates treated samples.

The plot of j = M /M versus T is shown in Fig. 2, the shape of the curves of the h and TPP treated sa2 pies is still shifted towards higher temperatures. Such a behaviour is similar to that of silica coated y-Pe 0 C21: coating with silica improves the material resistence against temperature effects so increasing coercivity values and yielding particles that, though still maintaining the original shape, have a lower amount of pores.

r r s

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TO & ...... pp

, x ----- HMB

Fig. 2. Remanence/saturation ratio ( j ) as a funttion of reduction temperature for uncoatgd (TQ) and phos- phates treated samples.

Since no differences were found between a-FeOOH ,created powders and the respective y-Fe 0 final pro- ducts as regards to phosphorus concentratlon (see Table 11, it is possible to conclude that the strong adsorp- tion of phosphate ions onto thea-FeOOH particles pro- duces, during the temperature treatment, a direct bond- ing between phosphorus and iron via oxygen ion Fe-0-P E815

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No protective action is shown by PP, this can be probably due to a lower interaction with surface sites sf the oxide particles. Indeed we observed that, during

the surface treatment, the stabilities ofu-FeOOH par- ticles in solutions of hexametaphosphates and tripoly- phosphates are greater than those in solutionsof pyro- phosphates,

TABLE 1

Condensed phosphate ions content Z in the ferric oxides before and after reduction/oxidation treatment.

Before treatment 1.11 0.98 0.89

After treatment 1.06 0.95 0.76

Since the effectiveness of phosphates in stabilizing the suspensions may be attributed to their strong ad- sorption onto the surface of the particles C61, it is probable that pyrophosphate does not preferentially ad sorbs at the solid-liquid interface.

The oil absorption values for different y-Fe 0 samples are reported as a function of the reduction 2 3

temperature in Fig. 3. They are determined pletely by the surface area of the pigment wetting,

almost com- and by its

Fig, 3 . O.A. '% vs. reduction temperature for uncoated (TQ) and phosphates.treated samples.

Results confirm the behaviour of the magnetic parame- ters already reported. From curves reported in Fig. 3, it is possible to con- clude that PP surface modified y-Fe 0 behaves as the uncoated material, while the higher O.A. values of the HMP and TPP treated oxides can be attributed to the roughness due to the coating which produces a much high er "real surface area". As seen in Fig. 4 , specific surface area decreases with increasing temperature because of the pores clo- sure and the sintering among the particles. Pure and PP surface modified y-Fe 0 show lower speci- fic surfaces than those of HMF' an2 ?PP treated samples. It is therefore confirmed the hypothesis that adsorbed HMP and TPP ions give rise to a real coating, increas- ing the surface roughness.

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N $2j I E

2 I2

.

o-TQ

0 - . - TPP

350 375 400 425 450 475 500 T, P C 1

Fig. 4. Specific surface area vs. reduction temperat: re for uncoated (TQ) and phosphates treated samples.

REFERENCES

c 11 c21

C31

C41

C51

C7 1

E81

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