Kinetics of Formation, Magnetic Moment, and Stability of Colloidal

28 Kinetics of Formation, Magnetic Moment, and Stability of Colloidal Magnetite MARIA RONAY

Downloaded by PURDUE UNIVERSITY on June 17, 2013 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch028

IBM, Thomas J. Watson Research Center, Yorktown Heights, NY 10598

The transformation of the jointly precipitated mixture of ferrous and ferric hydroxides to colloidal magnetite of 50-100 Ådiameter was studied by the continuous measurement of the magnetic moment. At large excess of ammonia magnetite forms at a fast rate by grain boundary nucleation and the transformation is complete. At small excess ammonia transformation is slow, diffusion controlled and incomplete. Magnetite produced in a magnetic field gradient has a saturation moment, 96.5 emu/g, that is larger than when produced without a magnetic field, 92 emu/g. In the course of the spontaneous oxidation of magnetite to γ-Fe O the magnetic moment of the particles changes linearly with the Fe /Fe ratio between that of pure magnetite and pure γ-Fe O . Implications to magnetic inkjet printing are discussed. 2

3

2+

3+

2

3

Magnetite belongs to the family of ferrites with a spinel structure, described by the general formula M e F e 0 , where Me represents a divalent metal ion. In the case of magnetite the divalent ion is iron. Natural magnetite exhibits a wide variation in composition; ideally it contains F e and F e in the ratio 0.5. The smallest cell of the spinel lattice that has cubic symmetry contains eight "molecules" of MeFe 0 . The relatively large oxygen ions form an f.c.c. lattice. In this cubic close-packed structure two kinds of interstitial sites occur, the tetrahedral and the octahedral sites. Of the 64 tetrahedral and 32 octahedral sites only 8 and 16 respectively are occupied by metal ions (called A and B sites). In normal spinels the eight divalent ions occupy 2

2 +

2

4

3 +

4

0097-6156/82/0200-0553$06.00/0 © 1982 American Chemical Society In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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the eight available tetrahedral sites and the sixteen trivalent ions the sixteen octahedral sites. In magnetite, which is an inverse spinel, the bivalent iron ions together with half of the trivalent iron ions are distributed at random over the octahedral sites, the other half of the trivalent ions occupying the tetrahedral sites giving (Fe [Fe Fe +]0 ) (p. Because of the relative magnitudes of the exchange interactions one may expect the spins of the A and B ions in ferrites with spinel structure to be oppositely oriented, so that when T = 0, there will be two saturated and oppositely magnetized sublattices present. The resulting magnetization is thus the difference between the magnetization of the octahedral lattice (B) and that of the tetrahedral lattice (A). This was first postulated by N6el (2). For magnetite the resultant moment is that of the ferrous ion, which at T = 0 is predicted to be 4 Bohr magnetons. Synthetic magnetite was made by Lefort (3) by wet chemical reaction already in 1852. A solution containing ferrous and ferric sulphates in a proportion 1:2, poured into a boiling solution of N a O H , gives a near colloidal precipitate. Krause and Tulecki (4) in 1931 precipitated magnetite at 1 8 ° C by adding ammonia to a mixed solution of ferrous and ferric chlorides yielding colloidal magnetite. Magnetite can also be produced by the partial oxidation of ferrous hydroxide; this reaction however leads to magnetite crystals that are larger than colloidal (5) Owing to the brilliant black color of magnetite a process similar to the above one was patented in 1905 for producing printing ink (6), but it was not until the seventies with the invention of the magnetic inkjet printer that a printing process depended on the magnetic properties of the ink. A magnetic inkjet printer (7) uses a continuous high speed capillary jet of a magnetic fluid. By direct periodic magnetic excitation uniform drops are generated from the jet. To create characters, single drops are magnetically selected, deflected in one direction and recirculated. The remaining drops are magnetically deflected in a direction orthogonal to the first, while the paper moves perpendicular to the direction of drop deflection. The magnetic fluid to be used in inkjet printing should exhibit a fast magnetization in response to an applied magnetic field and a rapid decay of the resultant moment upon removal of the field, as well as a high saturation magnetization. Also, the magnetic particles ought to be very small and monodispersed in order to go through a nozzle of about 3 +

2 +

3


4

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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555

50 /im diameter. Beyond the magnetic requirements, an ink for inkjet printing must fulfill a number of other requirements such as, an aiming stability (i.e. the jet direction should not "wander") and the nozzle should not become clogged, both of which are related to agglomeration. Requirements for inks for an electrostatic inkjet printer are given in reference 8, most of which are relevant to magnetic inks as well. Pure bulk natural magnetite has a saturation moment of 92 emu/g and its color is brilliant black. It can be synthesized in colloidal size, which makes it the first candidate for the pigment of magnetic inkjet ink. The question arises: Are the magnetic properties of the synthetic colloid the same as those of natural bulk magnetite? In the author's laboratory, magnetic inks were produced by precipitating magnetite from a mixture of ferrous and ferric chlorides with ammonia and coating the particles immediately with ammonium oleate to prevent agglomeration, while colloidal stability was achieved by the adsorption of a cationic surfactant such as amines or quaternary ammonium compounds on oleate coated particles (9). The question arises whether the magnetic moment of the particles develops before it is coated with surfactant. If not, will it fully develop after it is coated with surfactant? To answer these questions the kinetics of the magnetite formation need to be known. Also the reliable operation of a magnetic inkjet printer puts great demands on the magnetic and chemical stability of the ink. The aim of the work to be reported here was the study of the kinetics of formation and of the magnetic moment and stability of colloidal magnetite. The main results of this study have been disclosed (10). Since magnetite precipitated at or below room temperature with ammonia from a solution of a mixture of ferrous and ferric chlorides has the smallest particle size, this is the reaction we studied. The reaction takes place in two steps:

NH OH 4

FeCl

2

+ 2 FeCl

Fe(OH)

2

+

Fe 0 2

3

• xH 0

3

Step I trigonal non magnetic

Step II

amorphous paramagnetic

inverse spinel ferrimagnetic


2

556


Step I is the joint precipitation of an intimate mixture of F e ( O H ) and F e 0 • x H 0 by chemical reaction which takes place instantaneously. This product in statu nascendi has practically no magnetic moment. (The paramagnetic susceptibility of F e 0 • x H 0 produced as given is X = 60 - 80.10" emu/g). The second step is the transformation of F e ( O H ) and F e 0 • x H 0 to magnetite. Because the transformation product is magnetic, the transformation can be followed by the continuous measurement of the magnetic moment. It will be shown that the time dependence of this transformation and the extent to which it takes place depends on the amount of ammonia added in Step I. 2

2

3

2

2

3

2

6

g


2

2

3

2

Experimental conditions The chemical reaction. Baker analyzed reagents were dissolved in distilled and demineralized water of 0.18 Mfl resistivity. The alkyl benzene sulphonate concentration indicative of the surfactant content was less than 0.001/10 . Since ferrous ions oxidize readily to ferric ions, to counteract this the quantity of ferric chloride to be used in the reaction was less than equivalent. Reimers and Khalafalla (J_p found that the saturation magnetization of magnetite prepared by this reaction was the highest when the initial ratio of F e / F e was 1.75 instead of the stoichiometric 2, and this was the ratio we used. Typically 4 g of F e C l • 4 H 0 and 9.45 g F e C l • 6 H 0 were dissolved in 125 ml water and cooled to + 1 0 ° C . Separately 18.66 g N H O H (58%) were dissolved in 75 ml water and cooled to + 1 0 ° C . This amount of ammonia is four times the equivalent amount, designated 4 E , required for the amounts of ferrous and ferric salts. Reactions were also made in which 3E, 2.75E, 2.4E and 2E ammonia was used. In some of the reactions the ammonia was added at a slow rate (15 ml/min), in others at a quick rate (3 ml/sec) under continuous manual stirring. In one reaction together with the ammonia 0.1E oleic acid was added, in another 0.1E Ethomeen C-25 was added to the ferrous and ferric solution. Ethomeen C-25 (Armour Industrial Chemical Co.) is a (15) polyoxyethylene-dodecyl (coconut oil) amine and is a cationic surfactant. After the desired amount of ammonia was added at the desired rate, a sample was taken from the suspension into a preweighted plastic vial for the measurement of the magnetic moment. During the measurement the vial was closed and its total weight determined after the measurement. 6

3+

2

2

3

2 +

2

4



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The measurement of the magnetic moment. A force type measurement was used to determine the magnetic moment following the design of Bozorth and Williams (\2) in which strain gauges are used to detect the displacement. In the instrument the sample is contained in a small balancing coil on the end of a phenolic sample rod. The sample holder is placed between the poles of an electromagnet. In this instrument the horizontal deflection of the sample bends a thin vane to which the sample rod is attached. Four strain gauges are mounted on the flexible vane in Wheatstone bridge arrangement, two on either side of the vane. The measurement procedure involves returning the sample holder to zero position by a current in the balancing coil; this nullifies the sample moment. The current in this coil is a direct measure of the magnetic moment. The unbalanced condition is detected by an amplifier. The magnetic moment a of a sample was calculated by the formula o = i 0.0349/w emu/g, where i is the balancing current in amperes and w is the weight of the sample in grams (total weight - vial weight). The constant 0.0349 is determined by calibrating the instrument with nickel. When the magnetic moment of the suspension was measured as a function of time to follow the transformation to magnetite, the measurement was made in a magnetic field of 13K Oe. Since the suspension contained magnetite, water, excess ammonia in varying amounts, and a side product of the reaction N H C 1 , the magnetic moment of the suspension was multiplied by a factor consisting of the ratio of the total weight of reagents over the theoretical F e 0 yield of the reaction, both in grams. The theoretical yield of the reaction is 4.246 g. When the magnetic moment of a magnetite powder or suspension was measured as a reaction product and the time dependence was not a concern, the magnetic moment was measured in the magnetic fields H = 2, 4, 6, 8, 10, 12 and 13K Oe and the measured values were plotted against 1 / H and extrapolated to infinite field ( 1 / H = 0) to get the value for the saturation magnetization denoted a . 4

3

4

s

Chemical reaction in a magnetic field. When ferrous and ferric chloride solutions are reacted with ammonia, the side product of this reaction is N H C 1 in water. It is difficult to separate this from the magnetite particles due to their colloidal size. In order to make the transformation to magnetite complete as well as to facilitate the removal of N H C 1 , the precipitation from the ferrous and ferric solution with ammonia, both the transformation to magnetite as well as the decantation, was made in a magnetic field gradient. The aim was to make magnetite with the highest possible magnetic moment. The magnetic 4

4



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field gradient was produced by putting two wedges made of steel between the poles of a large electromagnet. The opening of the wedges was 3 6 ° . The gradient of magnetic field inside the opening was 2K Oe/cm. A wedge shaped reaction vessel to fit the opening was made of quartz. The ratio and dilution of the ferrous and ferric salts was the same as given previously. In every reaction 4E ammonia was added also in the dilution given. The ferrous and ferric solutions were cooled to + 1 0 ° C and placed in the magnetic field. The ammonia solution was added at the slow rate applying careful manual stirring. When the stirring stopped, the black precipitate settled to the bottom of the wedge. After time was allowed for the transformation to take place, determined from the kinetic studies, the clear solution from the top was sucked off with a syringe. Washing took place by manual stirring with the wash fluid while the magnetic field was turned off. Subsequently the magnetic field was turned on, the precipitate settled, and the wash fluid sucked off with a syringe. Washing was repeated three times. The magnetic moment and the F e / F e ratio of the suspension were determined immediately. 2 +

3 +

Chemical analysis Chemical analyses on suspension or powder samples were performed by a volumetric method (0.05 normal potassium dichromate) following dissolution in hydrochloric acid. The ferrous ( F e ) content was determined directly. The ferric ( F e ) content was analyzed via the Zimmermann-Reinhardt (SnCl reduction) technique, which gives the total iron. Subtraction of the F e gives the desired Fe value. The F e / F e ratio was determined with an accuracy of ± 0 . 0 1 . The accuracy of the determination of the total iron was ± 0 . 5 % of the result. Chloride content was determined by Volhard titration with an accuracy of ± 5 % of the result. Nitrogen content was analyzed with the Nessler method with a relative accuracy of ± 1 5 % . 2 +

3 +

2

2 +

3 +

2 +

3 +

Structure analysis. The particles were suspended in alcohol and put on a carbon substrate. They were examined in a Philips 301 electron microscope. Bright field, dark field and interference image micrographs on a high resolution stage were taken as well as transmission electron diffraction patterns. The lattice parameter of powders was determined by X-ray diffraction using C u K radiation. a


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559


Results and interpretation Kinetics of magnetite formation. After the chemical reaction was performed with the amount of ammonia and at the rate indicated, the magnetic moment of the suspension was measured continuously in order to follow the time dependence of the transformation to magnetite. It was found that the rate of transformation and the extent of transformation as indicated by the final moment is greatly influenced by the amount of ammonia and the rate it was added in Step I. Examples of the transformations are shown in Figure 1. It is to be seen that the more ammonia is added in Step I, the faster the transformation takes place. Surfactants lower somewhat the rate of transformation. The final moments achieved under the various conditions are given in Table I. These numbers show the extent to which the transformation took place, since their calculation was based on the assumption that the transformation took place to completion. These are not precise, absolute values and, since they were measured at only one field strength, they do not give the (larger) value of saturation magnetization. Such values will be given in the next section. Table I. shows that the magnetic moment at the end of the transformation is about 90 emu/g when the ammonia was between 2.4-4 equivalent and was added slowly. The same amount of ammonia added quickly produced a final moment of around 82 emu/g. When the ammonia is 2E and is added slowly, the final moment is 67 emu/g. The same ammonia added quickly produces the lowest final moment of about 60 emu/g. Similar low moment is achieved if, with the same rate and same total amount of ammonia, the ferrous and ferric hydroxides are precipitated separately and mixed together subsequently. While the presence of 0.1E Ethomeen C-25 does not affect the final moment, 0.1E oleic acid added together with 3E ammonia at a quick rate results in a final moment of 76 emu/g, compared to 82 emu/g resulting from the same reaction without oleic acid. It has been reported that the moment of ~ 100A diameter N i F e 0 particles decreases when coated with oleic acid (1_3). Continued investigation has demonstrated that the apparent moment decrease is due to strong pinning of the spins of those ferrite cations that are bonded to the organic molecules. (14). 2

4

The experimental determination of the magnetic moment as a function of time enables us to find the f—t relations, where f = V ^ / V is the volume fraction transformed at time t. Avrami (J_5) proposed that for a three-dimensional nucleation and growth process we should use the general relation f

=

1-

n

exp(-kt )


(1)


560

REPROGRAPHIC

E

EQUIVALENT

O

A M M O N I A A D D E D A T S L O W R A T E (15mlmin)

X

A M M O N I A A D D E D A T Q U I C K R A T E (3ml

•

Fe(0H)

2

AMMONIA

+ 175 Fe(OH)

3

[2E. QUICK

sec)

RATE]

•

3 E A M M O N I A + 0.1E O L E I C A C I D A T Q U I C K

•

0.1E E T H O M E E N C

Q

o

•

TECHNOLOGY

25 P R E S E N T

3E, 4E o o 3E, 4E x x « x

•

•

•

•

O

RATE

o

m

• o

2E -ooooo

RATE

3 E AMMONIA A D D E D AT QUICK

o

S

2

< 2 | C

p°

J

D

I

I

40 60 80 TIME (min)

L

I00

4 24 TIME (hrs)

48

Figure 1. Transformation of the jointly precipitated ferrous and ferric hydroxides to magnetite for different amounts of ammonia added at different rates in Step I. Also shown is the effect of surfactants and the transformation of separately precapitated and subsequently mixed hydroxides.


RONAY

Colloidal Magnetite

Table I.


The magnetic moment of magnetite at the end of transformation. Sample

a

ammonia equivalent

rate added

emu/g

M25M M26M

4 4

slow quick

89.68 81.6

M23M M22M

3 3

slow quick

88.3 81.9

quick Ethomeen quick oleic

82.27

M29M M28M

75.9

M34M M33M

2.75 2.75

slow quick

90.2 82.4

M32M M35M

2.4 2.4

slow quick

88.96 81.2

M20M M21M

2 2

slow quick

67.16 60.52

M30M

2

quick hydroxides

62.5


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where k is the rate constant and 3 < n < 4. This should cover all cases in which I, the nucleation rate per unit volume, is some decreasing function of time up to the limit when I is constant (n=4). The exponential growth law summarized in E q . (1) is valid for linear growth under most circumstances and approximately valid for the early stages of diffusion controlled growth. Plotting log log [1/(1 - f)] against log t, the slope of the line gives n. The value of n is indicative of the kinetic process (16). The existence of a straight line relationship might be thought to imply random volume nucleation since the kinetic law of a transformation nucleated on grain boundary surfaces, grain edges or corners can not be expressed in the simple form of E q . (1). We will see that this is not so. We plotted the results of a few of the kinetic investigations in the form described above, f was taken as the ratio of the magnetic moment at time t over the final magnetic moment, and t was given in seconds. Figure 2 shows log log [1/(1 — f)] as a function of log t for the transformations where 2.75E, or 3E ammonia was added at a slow rate as well as for transformations where 3E and 4 E ammonia were added at a quick rate. In the two former cases the plot consists of two straight lines of slopes four and one, with an intermediate region over which the slope decreases. In the 3E Q case a disappearing slope 4 is indicated by a single point, and in the case of 4 E Q slope 4 disappears completely and only slope 1 is recorded. Figure 3 shows the transformation when 3E ammonia is added at a quick rate, but in the presence of 0.1E Ethomeen C-25, or added together with 0.1E oleic acid. Since surfactants lower the rate of transformation, here the plot clearly shows the two straight lines of slope four and one Consider that nucleation is on grain boundary surfaces (H>). When the kinetic parameter a = ( I u ) / t, where I is nucleation rate per unit area of grain boundary and u is the growth velocity, is very small, the kinetic law approaches the limiting form V


V

£ V

=

1 -

B

B

2

exp

(-77 I V

B

1

u

3

3

B

4

t /3)

(2)

B

where I is the grain boundary nucleation rate per unit volume. This expression is identical with E q . (1), so that f depends only on the nucleation rate per unit volume, irrespective of where the nuclei are formed. When a is very large, the kinetic law has another limiting form B

S = v

1 -

v

exp ( - 2 O

B

u t)

B

(3)

where O is the grain boundary area per unit volume. The log log [1/(1 — f)] versus log t plot thus consists of two straight lines of slopes



RONAY

563

Colloidal Magnetite

0.5 r

y

y /

I Q

-0.5Q

CO

CO

-1.0I

2.0

2.5

3.0

2.5

3.0

Log t Figure 2. Log Log [1/(1 — £)] as a function of log t for transformations where 2.75E and 3E ammonia was added at a slow rate (S), and where 3E and 4E ammonia was added at a quick rate (Q).



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Figure 3. Log Log [1/(1 — 0] as a function of log t when 3E ammonia is added in the presence of 0.1E Ethomeen C-25 (X), or added together with 0.1 E oleic acid (%), both at a quick rate.


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Colloidal Magnetite

four and one; such is the plot for all grain boundary nucleated reactions. The physical explanation for the change in the slope was termed by Cahn "site saturation" 06). We cannot expect that the change in slope can always be observed. It is probable that the whole observable range of reaction will correspond to one or other of the straight line regions even when nucleation is confined to the grain boundaries. Site saturation will be observed if it occurs when f ^ 0.5. It was shown (H>) that site saturation occurs at half reaction when 4


V

I

B

* 6xl0

3

B

u/(L )

(4)

B

V

B

where L is the mean grain diameter. For values of I smaller than some value near that given by this equation, saturation of nucleation sites will not occur until a late stage of the reaction, and the kinetics are equivalent to those of random volume nucleation. For larger values of I^, saturation occurs early in the reaction. Only for a small critical range, where the condition in E q . (4) holds almost exactly, should the change be discernible on a log log [1/(1 — f)] versus log t plot. Figure 4 shows this plot for the transformation when the ferrous and ferric hydroxides were precipitated separately by the quick addition of the sum of 2E ammonia and mixed together subsequently. The plot consists of two straight lines of slopes 1 and 2.5. When 2E ammonia was added to the mixture of ferrous and ferric solutions quickly the slopes were 1 followed by 1.85; when added slowly, the slopes were 1 and 1.82. Slope 1 indicates grain boundary nucleation after saturation. Slopes between 1.5 and 2.5 are indicative of diffusion controlled growth from small dimensions (16). Note that the curve for 2E Q seems to begin with a larger slope. When the reaction takes place with a large excess of ammonia, 2.75E or greater, and the slopes 4 followed by 1 indicate grain boundary nucleation, a possible mechanism for magnetite formation is that the boundary surface of the ferric hydroxide reacts with F e ions in solution. There are two arguments supporting this mechanism. It is known that F e ( O H ) is soluble in excess ammonia in the presence of ammonium salts, so there is a supply of F e in solution. The other argument is based on Figure 5 which shows the change in pH of the ferrous and ferric solution upon the addition of ammonia at 1 0 ° C . The isoelectric point of F e ( O H ) is at pH ^ 7.7(Reference 4). The p H at 2.75E ammonia is 9.2 and at 4E ammonia 9.6. As the charge on the F e ( O H ) particles becomes more negative with increasing p H , more positive F e ions may be adsorbed. V

2 +

2

2 +

3

3

2 +


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REPROGRAPHIC T E C H N O L O G Y Or

-0.5


-1.0

-J -1.5

-2.0

2.5

3.0

3.5

Log t Figure 4. Log Log [1/(1 — 0] as a junction of log t when ferrous and ferric hydroxides were precipitated separately and mixed together subsequent ly>also when 2E ammonia was added to the mixture of ferrous and ferric solutions quickly and slowly. Key: • , hydroxides; O, slow; and X, quick.

I0

O

r

8

i.e.p. Fe(OH)^

CO

o

6

in 4 I0±I°C 2

x o.

x

x—x^ ^ J

I 10

I

L 20

J

30

L

40

50

60

J

L

70

N H 0 H (ml) 4

Figure 5. Change in pH of the ferrous and ferric solution upon the addition of ammonia at 10°C. The isoelectric point (i.e.p.) of Fe(OH) is at pH ~ 7.7. 3


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When 2 E ammonia was used in the reaction or pre-precipitated (with the sum of 2 E ammonia) ferrous and ferric hydroxides were mixed, the slopes were 1 followed by 1 . 8 - 2 . 5 indicating grain boundary nucleation after saturation followed by diffusion controlled growth. In this case magnetite probably nucleates at the boundary between solid ferrous and ferric hydroxide particles and grows by the counterdiffusion of F e and F e cations similarly to its formation in sintering (Wagner mechanism 1_7). As shown in Figure 5 at 2 E ammonia, the pH is 7 . 5 , about the same as the isoelectric point of F e ( O H ) . Few F e ions would adsorb, but the electrostatic repulsion between the primary particles is small and the particles can interact. 2 +

3 +

2 +


3

Magnetic moment and stability of magnetite. Magnetite was made and washed in a magnetic field gradient as described in the experimental section. The current proportional to the saturation magnetization of the suspension was determined immediately, as well as the F e / F e ratio. After the measurement of the current the vial containing the suspension was placed with its cover open in a vacuum dessicator. After the suspension dried to a powder the current proportional to the powder's saturation magnetization was measured. This current was less than the previous one indicating that the magnetic moment decreased in the course of drying. The F e / F e ratio of the dry powder was determined immediately together with total iron, chloride ion and nitrogen content. From these data it was possible to estimate the magnetic moment of magnetite when it was freshly formed and in suspension form. In the formula for the calculation of the saturation moment the current applied for the fresh suspension was used. This, however, was divided by the corrected weight of the powder. The correction took place as follows: First the weight of the powder was reduced by the chloride content (which was on the order of 0 . 3 - 0 . 5 % ) and with 0 . 5 % of its weight to account for adsorbed humidity. The nitrogen content of the powder was negligible. In the course of drying, the F e / F e ratio changed dramatically. If for example this ratio was 0 . 5 0 in the suspension, it became 0 . 3 1 in the powder. The reason for this change was that part of the magnetite oxidized to maghemite, y — F e 0 . Upon such oxidation the molecular weight of F e 0 , M | = 2 3 1 . 5 5 , becomes larger by 1 / 2 O, which makes it M = 2 3 9 . 5 5 . Denoting the ratio F e / F e in the powder r and in the suspension r , 2 +

2 +

2 +

2

3

+

3 +

3 +

3

3

4

2

2 +

3 +

s


568

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TECHNOLOGY

the factor to correct the weight of the powder is given as f _

2r x231.55 + 2 ( l - r ) x 2 3 9 . 5 n

n

p

p

2 r x 231.55 + 2 ( 1 - r ) x 239.55 s

s

After the corrections for chloride and humidity were made, the weight of the powder was multiplied by this factor. When magnetite was produced in the magnetic field gradient and washed with water three times, the F e / F e ratio was 0.50 ± 0.01 in the suspension, and the saturation moment of magnetite in the suspension was 96.5 emu/g. When magnetite was produced without the magnetic field gradient, the F e / F e ratio was again 0.50 ± 0.01, but the saturation moment was 91.45 emu/g in accordance with the accepted value of 92.0 emu/g. Producing magnetite in a magnetic field gradient thus increases the magnetic moment. The value 96.5 emu/g is the highest ever reported for magnetite. The freshly made suspension was frozen and its saturation moment measured at 250K, 77.2K and 4.2K. The saturation moment was 11% higher at 4.2K and 9.3% higher at 77.2 K than it was at 250K. Neglecting the moment change between R.T. and 250K, the saturation moment of magnetite at 4.2K is 107.1 emu/g if made in a magnetic field gradient.


2 +

2 +

3 +

3 +

In the following we made magnetite in the magnetic field gradient, but varied the F e / F e ratio. This ratio could be increased to 0.52 or 0.55 by using various dilute solutions of F e C l • 4 H 0 for washing instead of pure water. Magnetite with a ratio < 0.50 was made from suspensions with an initial 0.50 ratio by simply letting them stand for various lengths of time in suspension form, or drying them to powder, or a combination of these. The results are given in Figure 6. It shows that the maximum saturation moment is at exactly 0.50 ferrous to ferric ratio corresponding to pure magnetite and its value is 96.5 emu/g. This experiment was repeated four times. The figure shows that in the course of oxidation the magnetic moment of the particles changes linearly with the ferrous to ferric ratio between that of pure magnetite, a = 96.5 emu/g, F e / F e = 0.50 and that of pure maghemite, a = 73.5 emu/g, F e / F e = 0. It is interesting to note that in seven months the particles did not oxidize completely to maghemite, and 17% of the magnetite ( F e / F e = 0.085) was still preserved. The ease of oxidation is explainable in view of the fact that maghemite has the same spinel lattice as magnetite, but has no divalent ions; two thirds of the octahedral B sites vacated by the divalent iron 2 +

3 +

2

2 +

2 +

3 +

3 +

2 +

3 +


2

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3+

ions are occupied by F e ions, the other third remains vacant. The oxidation is accompanied by a small volume decrease. Another contributing factor to the ease of oxidation is the large surface area these particles exhibit. The lattice parameters of two powders with Fe +/Fe ratios 0.14 and 0.31 were determined by X-ray diffraction and are marked with crosses in Figure 6. The lattice parameters marked with circles are the standard ones for the pure compounds. It seems that the lattice parameter also changes linearly with the ferrous to ferric ratio. Downloaded by PURDUE UNIVERSITY on June 17, 2013 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch028

2

3 +

Particle size. Particle size was determined using transmission electronmicroscopy on samples made in the magnetic field and precipitated with 4E ammonia at a slow rate. Figure 7a shows the particles in bright field and Figure 7b in dark field, both in a magnification of 98,000X. The particle size is in the range of 50-100 A Figure 8 shows an interference image micrograph of the same particles using a high resolution stage. The magnification of this picture is 500,000X. The picture shows that the particles are rather perfect. The lattice planes of the magnetite show little evidence for the presence of imperfections or inhomogeneous strain. The particle size after precipitation in the magnetic field with 3E ammonia is the same as when precipitated with 4E ammonia. Figure 9a shows particles precipitated with 3E ammonia at a quick rate, Figure 9b with 3E ammonia at a slow rate. Figure 9c shows particles precipitated with 2E ammonia at a quick rate and Figure 9d with 2E ammonia at a slow rate. The particles precipitated with 2E ammonia are somewhat larger than those precipitated with 3E and 4E ammonia. The magnetite particles produced by adding ammonia at a fast rate are smaller and more uniform in size than particles produced by adding ammonia at a slow rate. All the pictures shown refer to particles produced in the magnetic field gradient. When the particles are produced without a magnetic field, the size of particles is the same as when the reaction and transformation takes place within the magnetic field gradient. The diffraction pattern of the freshly made magnetite particles in which the Fe /Fe ratio was 0.50 was identical with F e 0 . 2 +

3 +

3

4

Discussion The saturation moment of magnetite which was made in a magnetic field gradient is approximately 107.1 emu/g at 4.2K. The relation



570

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100 # 4 POINTS

90 80 E

70| 8.378 A

8.3961 h- o

0.6

J

^ I 0.4

+ 8.364 A I I I 0.2

o 8.350A I 2+ .3+

(±0.01)

I F e

Figure 6.

3°4 100%

XFe 0 2

3

100%

The magnetic moment and lattice parameter as a function of Fe''/Fe*' ratio.



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Colloidal Magnetite

571

Figure 7. Magnetite particles made with 4E ammonia at a slow rate in the magnetic field gradient, bright field (left) and dark field (right) (68.600X)-

Figure 8. Interference image micrograph of magnetite particles made as in Figure 7 (350,000X).


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Figure 9. Magnetite particles made with 3E ammonia at a quick rate (a), with 3E ammonia at a slow rate (h), with 2E ammonia at a quick rate (c), and with 2E ammonia at a slow rate (d) (68,600x)>


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28.

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Colloidal Magnetite

between a at 0 K and the number of Bohr magnetons n F e 0 is given by 3

n

R

=

a

B

N./x

=

per molecule

^Ul

lQ1 A = 4 44

5585

b

where M is the molecular weight of F e 0 , N is Avogadro number and j u B is the magnetic moment of the electron called the Bohr magneton. Substituting the relevant quantities, the saturation magnetization of a F e 0 molecule at 0 K is 4.44 Bohr magnetons if it was made in a magnetic field gradient. The possible cause for deviation from the theoretical value of 4, which takes only the spin moment into account, is that when magnetite is not made in a magnetic field the orbital moment of the F e ion is "quenched" by the crystalline field. In the presence of a magnetic field, the crystal field is not able to remove the orbital degeneracy, and the orbital moment is of the same order of magnitude as the spin moment. Similarly to magnetic annealing, the effect of a magnetic field applied in statu nascendi is permanent. In the following we discuss the implications of our study to magnetic inks and magnetic inkjet printing. The study of the kinetics of magnetite formation showed that in the case of a large excess of ammonia, which is required to make the transformation complete, oleic acid indeed has to be added very quickly to the suspension if the particles are to be surrounded with a surfactant before the magnetic moment develops. The investigation also showed that the presence of 0.1E oleic acid or Ethomeen C-25 does not affect the mechanism of transformation. The oxidation of magnetite to maghemite may however have profound consequences on the stability and reliability of magnetic inks for inkjet printing. Welo and Baudisch (1_8) had already found in 1925 that synthetic F e 0 particles as well as their magnetic oxidation product y — F e 0 are very active catalysts of oxidation. Many substances were oxidized by H 0 only in the presence of these particles. This finding can be explained by the y — F e 0 being a p-type oxide and of great surface area. Other p-type oxides ( C u 0 , C o O , NiO) are known to be the most active catalysts of oxidation. Metal soaps, particularly oleates, are used as driers in the paint, printing ink and linoleum industries to accelerate the change of a liquid oil to an elastic solid by oxidation and polymerization reactions. It has been long established that the metal cation of the metallic soap is responsible for the catalytic activity, and iron soaps are among the most effective. Magnetic inks containing F e 0 + y — F e 0 particles 3

3


B

4

4

4

2 +

3

2

4

3

2

2

2

3

2

3

4

2

3


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partially coated with oleic acid as well as some iron oleate thus contain powerful catalysts of oxidation and polymerization. When a few drops of dilute H 0 solution is added to oleate-based magnetic inks, they respond with violent and long-lasting gas formation and dry subsequently to a gum-like tough substance. Even though this is a harsh test, the ink in the course of inkjet printing is kept under high pressure in the container and is intermittently thoroughly exposed, in the form of small drops, to air. The possibility of oxidation and polymerization during the repeated recirculation of the ink is there. Even if the degree of such reactions is small and takes place mainly at micelles which are preferred sites of oxidation and polymerization (19), even small and loose agglomerates can cause a wandering of a jet by their radial migration. They can also deposit inside the nozzle causing first a change in the direction of the jet and later the clogging of the nozzle. To avoid agglomeration, one possibility is to use a saturated fatty acid instead of oleic acid. Also, the use of such surface-active agents that are both oxidation inhibitors and form micelles only at concentrations larger than the ones used is recommended. Examples of such surfactants are the polyoxyethylene fatty amines. Since the critical concentration for micelle formation increases with decreasing alkyl- and increasing ethylene oxide chain length, short alkyl and long polyoxyethylene chains are favorable, such as Ethomeen C-25. An open question remains whether natural magnetite and a magnetic fluid made of finely ground natural magnetite is not more stable than synthetic colloidal magnetite. Welo and Baudisch (20) reported that the complete oxidation of natural magnetite (size not known) to y — F e 0 takes place at a much higher temperature (about 8 0 0 ° C ) than the oxidation of synthetic magnetite (about 2 2 0 ° C ) . 2


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2

2

3

Conclusion

The transformation of the jointly precipitated (Step I) mixture of ferrous and ferric hydroxides to magnetite (Step II) was studied by the continuous measurement of the magnetic moment. It was found that at large excess ammonia, added in Step I, magnetite forms at a fast rate and the transformation is complete. At small excess ammonia the transformation is slow and incomplete. The application of Avrami's theory to the data indicated grain boundary nucleation in the former case and grain boundary nucleation after saturation followed by diffusion controlled growth in the latter case. Mechanisms are proposed for both cases.



28.

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Colloidal Magnetite

It was found that magnetite produced in a magnetic field gradient has a saturation moment, 96.5 emu/g, that is larger than when produced without a magnetic field, 92 emu/g. The saturation moment of a magnetite molecule made in a magnetic field gradient is 4.44 Bohr magnetons at 0 K, instead of the theoretically predicted 4. A n explanation for the increase in moment is offered. o The magnetite particle size is in the range 50-100 A , and the particles are of great perfection. Particle size depends only weakly on the variables investigated in this paper. In the course of the spontaneous oxidation of magnetite to y — F 2 ^ 3 ' the magnetic moment of the particles changes linearly with the F e * / F e + ratio between that of pure magnetite and pure y — Fe C>3. Since magnetite, y — Fe C>3 * * °' " powerful catalysts of oxidation, agglomeration due to oxidation and polymerization may take place in magnetic inks based on oleate coated magnetite particles. This may cause aiming instability or clogging if such inks are used in magnetic inkjet printers. e

+

3

2

a n c

r o n

e a t e

a

r

e

a

2

Acknowledgments I am grateful to P. Chaudhari for drawing my attention to Avrami's theory, indebted to B. L . Gilbert for the chemical analyses, S. Herd for the electron-microscopy, J. W. Mitchell for the use of the magnetometer and H . R. Lilienthal for the low temperature measurements.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Smit, J and Wijn, H . P. J. "Ferrites"; John Wiley and Sons: New York, 1959, p 141. Néel, L . Ann. de Phys. 1948, 3, 137-198. Lefort, Compt. Rend. Acad. 1852, XXXIV, 488. Krause, A . ; Tulecki, J. Zeit. Anorg. Chem. 1931, 195, 228. Baudisch; Mayer, Biochem. Z. 1920, CVii, 1. Fireman, Peter, U.S. Patent 189,944, Dec. 13, 1905. Fan, G . ; Toupin, R. A . , U.S. Patent 3,805,272, April 16, 1974. Ashley, C . T.; Edds, K. E.; Elbert, D. L . , IBM J. Res. Develop. 1977, 21, 69. Kovac, Z . ; Sambucetti, C . , this meeting. Magnetic inks for magnetic inkjet printing.


576


10. 2759, 11.

Ronay, M . , IBM Technical Disclosure Bulletin, 1976, 19, 2753ibid. 2760-2763, ibid. 1976, 18, 3490-3491. Reimers, G . W.; Khalafalla, S. E., Bureau of Mines Innovative Processes in Extractive Metallurgy Program Technical Progress Report - 59. September 1972. U.S. Department of the Interior. Bozorth, R. M.; Williams, H . J., Phys. Rev. 1956, 103, 572. Berkowitz, A . E.; Lahut, J . A . , AIP Conf. Proc. 1973, K), 966. Berkowitz, A . E.; Lahut, J. A.; Jacobs, I. S.; Levinson, L . M . , Phys. Rev. Lett. 1975, 34, 594. Avrami, M . J. Chem. Phys. 1939, 7, 1103; ibid. 1940, 8, 212; ibid 1941, 9, 177. Christian, J. W. "The Theory of Transformations in Metals and Alloys," Second edition, Part I; Pergamon Press: Oxford, etc., 1975; p 525-542. Wagner, C . , Z. Phys. Chem. 1936, , B34, 309. Welo, L . A.; Baudisch, O., Journal of Biological Chemistry, 1925, L X V , 215. Harkins, W. D., J. Amer. Chem. Soc. 1947, 69, 1428. Welo, L . A . ; Baudisch, O., PhiL Mag. SJ). 1925, 50, 399.

12. 13. 14.


15. 16.

17. 18. 19. 20.

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Kinetics of Formation, Magnetic Moment, and Stability of Colloidal

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