IEEE TRANSACTIONS ON MAGNETICS, VOL. 28. NO. 5, SEPTEMBER 1992

Dispersion Quality of Metal Particle Inks Measured By a Magnetic Probing Technique and Its Relation to Tape Quality

C. Jung and S. Raghavan, Department of Materials Science and Engineering, The University of Arizona, Tucson, Arizona 85721

M. C. A. Mathur, IBM Corporation, ADSTAR Line of Business, Tucson, Arizona 85744

&smct-The dispersion quality of magnetic inks made from iron based metal particles has been investigated using a magnetic probing technique known as DIMAG. A comparison of the DCON values measured at different binder levels to the tape characteristics shows that disper- sions with large negative DCON signals result in good quality tapes.

I. INTRODUCTION

Many methods based on optical, flow and magnetic techniques have been described in literature for the charac- terization of dispersion quality of inks'4. A magnetic probing method known as DIMAG (DIspersion by MAG- netic measurement) has been claimed to be successful in identifying the presence of single particles, doublets and agglomerates in y-Fe20B based magnetic inks'". The instrument used in this method is similar to a M-H loop tester and is shown in Fig. 1. A sample of magnetic ink placed in the pick-up coil is initially subjected to an alter- nating magnetic field (c33 Oe) in a vertical direction. After a predetermined time, the sample is exposed to a DC magnetic field of much higher intensity (up to 600 Oe) acting in a direction perpendicular to the AC field for a short duration (- 10 sec). The sinusoidal signal generated by the ink in the pick-up coil is sampled at two points per each cycle for the calculation of amplitude and phase values

sample AC coil / pick-up coil

c(

DC coil

, ! I ,

Fig. 1. A schematic sketch of the DIMAG coil system.

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with respect to the input signal. The average of eight successive values are stored to generate plots of amplitude and phase as a function of time. It is claimed that the percent change in the amplitude that OCCUTS during the application of the DC magnetic field @CON) can be related to the dispersion quality of the magnetic ink.

The response (amplitude) of a single particle and a doublet to the applied fields (alternating in one direction and constant in an orthogonal direction) is shown in Fig. 2. Case A depicts the waveform of the constant magnetic field applied in the form of a DC pulse, HE, which is larger than HAC. Case B is for a situation where H, is less than the coercivity of the particles (He) while case C depicts the response when H, > H,. The induced signal (DCON) for single and doublet particles are given in case D as a function of time.

An ink containing individual particles should yield a negative DCON signal6. This is primarily due to the reduction in the oscillation of the particle and its magnetic moment in response to the AC field.

In the case of a "hard doublet, the weak alternating magnetic field does not cause the magnetic moment vectors to oscillate. When H, c H, the induced signal DCON should not change from the zero value when the DC field is applied and removed. On the other hand, when H, > H, a repulsive force would arise between particles and cause them to separate. This separation makes the magnet- ic moment vectors to oscillate in response to the AC field and hence the induced DCON signal increases.

In soft doublets, the application of a DC field can break apart the doublets into single particles. This will happen by the rotation of the particles themselves in the DC field. Thus at H, < H, the induced signal should increase on the application of the DC field and should show a further increase when DC field is turned off. At higher DC fields, the particles and the magnetic moment vectors are pinned in the direction of the DC field and respond less to the oscillating AC field. This would result in a signal which is smaller than that for the case of H, c H,.

This paper reports the results of the work carried out to characterize the dispersion quality of metal particle inks by the DIMAG technique and its relation to the magnetic characteristics of the tapes.

001 8-9464/92$03.00 0 1992 IEEE

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* 0 rtf of hs (Slurry) 0 1.5 WU of YE

ot--s-*-t-c* I .-.I .......................................... -.*

I I 5 "tf Of hs a 18.2 w t z WE

.......=......* ..... ..... .*...

- -6

Fig. 2. The response of single particles, hard and soft doublets to a DC field.

MATERIALS AND METHODS

Samples of passivated metal particles (H, = 1500 Oe, S, = 59.9 m'/gm) were used in this research. These particles averaged 0.15 pm in length with an aspect ratio of 1O: l and contained 79.2% of Fe, 2.4% of Al, 3.3% of Ni, 1.6% of Zn and 0.64% of Si. HPLC grade THF and cyclohexanone were used as solvents. A commercially available wetting binder, a vinyl chloride/vinyl acetate copolymer containing hydroxyl and carboxyl groups (w = 68,OOO), was used as received.

A DIMAG instrument built at IBM (GMTC), Sindelfing- en, was slightly modified to apply DC fields of up to 840 Oe. The following conditions were used during the measurement: AC field of 33 Oe at a frequency of 80 Hz, DC field of 60 - 840 Oe, a 10 sec DC pulse and a relaxation time of 17 sec after the removal of the DC pulse. The DIMAG measurements were carried out with attritor milled magnetic inks consisting of passivated iron particles (- 22% by weight of ink), binder, and mixed solvents (75% THF, 25% cyclohexanone). Ten ml samples of ink were periodi- cally withdrawn from the attritor and characterized using the DIMAG instrument.

Tapes were hand drawn from the milled inks and the magnetic properties of tapes such as switching field distribu- tion (SFD) and squareness ratio (SR) were measured using a B-H meter.

RESULTS AND DISCUSSION

A. DIMAG Measurements

The results of DIMAG measurements will be presented in two parts: (1) effect of milling time at different binder concentrations and (2) magnetization curves (DCON vs. DC field) at different binder concentrations.

1) Effect of Milling 7ime: The effect of milling time on the degree of dispersion was investigated at six different binder concentrations. Samples were taken every ten minutes after starting the milling process and a DC field of 4.5 A (- 270 Oe) was applied to measure the DCON signal. The induced signals (DCON) for six magnetic inks have been plotted as a function of milling time in Fig. 3. The samples containing 0 and 1.5% of binder resulted in positive DCON values while the 5,16,25 and 33% samples generated negative DCON values. Based on the theory presented earlier, it can be concluded that in inks with binder levels in excess of 5% by weight of particles the magnetic particles behave like single particles. The ma@- tude of the negative DCON signal increased with binder concentration up to 16% but interestingly became less negative at higher binder levels. Increasing the milling time did not significantly improve the DCON signals for these six ink samples. Small positive DCON values for 0 and 1.5% samples indicate the existence of aggregates in these inks.

2) Magnetization Curves: Information on the cohesive strength of agglomerates can be obtained by measuring DCON values at different strengths of DC field'. Based on the information in Fig. 2, the theoretical magnetization curves for inks containing hard agglomerates, soft agglomer- ates and single particles can be easily shown to follow the profiles shown in Fig. 4(a).

The magnetization curves of four different magnetic inks measured after 2 hours milling time are plotted in Fig. 4@). These samples were subjected to a DC-field strength in the range 84 - 840 Oe. The DCON signals for slurry (no binder) and an ink sample at a binder level of 1.1% did not change significantly (0 - 1.5%), even at a DC field of 840 Oe, indicating the presence of hard agglomerates. The magnetization curves measured for the ink samples with binder levels of 16% and 33% showed a behavior typical of ferrofluids, indicating that the particles were well dispersed and act as single particles or chains of single particles.

0 33.8 w t z Of w .. ~ -lol 0 25 *U of WE

Fig. 3. Effect of binder level and milling time on DCON values.

2373 D"

mlbdomars t n ~hrd

,-, I "

8 '

OL

. 33 W U O f w 1-10 7 . . .

Fig. 4. (a) Theoretical field dependence of DCON signal (from ref. 6). (b) Magnetization curves of prepared inks.

B. Magnetic Properties of Tapes

Switching field distribution (SFD) and squareness ratio (SR) of the hand-drawn tapes are plotted in Figs. 5 and 6, respectively, as a function of milling time. In the drawing direction, the SFD was improved by the milling time as well as the binder level, The squareness ratio of 5% sample did not change significantly with increasing milling time. Except for a milling time of 40 minutes, squareness ratio of the tapes was found to increase steadily with milling time for binder levels of 16% and 33%.

A comparison of the DCON values measured at different binder levels to the tape characteristics would indicate that dispersions with large negative DCON signals result in good quality tapes. Interestingly, the squareness ratio of the tapes was found to increase steadily with milling time while the DCON signals did not change significantly with milling time. In spite of this apparent lack of correlation, it appears that the DIMAG technique can be successfully used to identify the proper wetting binder concentration required for the production of good quality metal particle tapes.

CONCLUSIONS

The results of this investigation clearly show that the DIMAG technique is useful in a qualitative characterization of the dispersion quality of metal particle inks prepared at different wetting binder concentrations. The sensitivity of the DIMAG signal to the binder level and the correlation between the DIMAG signal and tape quality imply that the

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Fig. 5.

9i

1 o...... ...... ............................................

Switching field distribution in hand drawn tapes.

- I OI-tim

a m a im la0 0.4

W I I ! 1~ T l n (mlm.)

Fig. 6. Squareness ratio of hand drawn tapes.

DIMAG technique has the potential of providing informa- tion on optimum ink formulations.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support provided by IBM Corporation through a SUR grant.

REFERENCES

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[4] K. Sumiya, N. Hirayama, F. Hayama and T. Matsumoto, "Deter- mination of dispersibility and stability of magnetic paint by mtation- vibration method," E E E Zkm. Mugn., vol. 20, p. 745 (1984).

[5] A. Brunsch, W. Steiner and G. Trippet, "Methods of determining dispersion, particle density or viscoSity or resin/sohrcnt mixture containing magnetic particles," US. Patent No. 4,651,092 (1987).

[6] A. Brunsch, W. Steiner and G. Trippel, "Method and apparatus for characterizing magnetic coating compositions as well as improving magnetic particle dispersions," US. Patent No. 4,785,239 (1988).

(1991).

105% (1982).