Download A primary x-ray investigation of the turning of ferroelectric

A primary x-ray investigation of the turning of ferroelectric microspheres
contained in electrorheological fluids under a direct current electric
field
Weijia Wena)
Department of Physics, Hong Kong University of Science and Technology, Hong Kong and Institute
of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Kunquan Lu
Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
~Received 17 October 1995; accepted for publication 15 December 1995!
An x-ray diffraction method is introduced to probe the turning of ferroelectric singlecrystal
microspheres suspended in electrorheological ~ER! fluids. The microspheres used in the experiment
have their permanent dipoles polarized in the same direction. Our initial results show that the x-ray
diffraction amplitude in some crystal planes will increase when a fixed dc electric field is applied to
the ER fluids. We argue that the enhancement of the diffraction is due to the reorientation of the
crystal planes which is caused by the turning of the permanent dipoles in the ferroelectric
microspheres under an external electric field. The particle turning has not attracted much attention
in the past. We propose that further work should be performed. © 1996 American Institute of
Physics. @S0003-6951~96!04208-3#
Electrorheological ~ER! fluids, which consist of microscopic particles suspended in an insulating liquid, can be
transformed into a solidlike state when an electric field is
applied. It is thought that this phenomenon is caused by the
structural change of ER fluids. A widely accepted model is
the polarization theory which assumes that the interaction
between the electric dipoles induced on the particles by the
external electric field will lead to the formation of chain and
then column structures which cause an increase in the shear
stress of ER fluids. Many papers have proved this view.1– 8
However, certain other parameters appear to have been overlooked because there is as yet no clear explanation for the
quantitative difference between the experimental results and
theoretical predictions.
Another significant phenomenon is that ER fluids composed of waterless ferroelectric material display a totally different frequency response and the shear stress of this kind of
ER fluid increases when the frequency of an ac electric field
is increased.9 In contrast, ER fluids composed of nonferroelectric materials show the opposite frequency response.10
The reason for this is probably due to some difference in
their structure. The permanent dipole of a ferroelectric particle turns continuously when an ac electric field is applied.
The turning of the dipole causes the particle’s vibration to
deviate from the direction of the applied electric field and at
the same time leads to a change in the force branch of the
shear direction of ER fluid. However, the effect of vibrating
on the shear stress only appears in a low frequency range
from a few Hz to about 3 kHz. In a previous experiment, we
observed under a microscope that the microspheres contained in ER fluids turned when an electric field was applied.
However, quantitative results have not been obtained.
In this letter, we introduce an x-ray diffraction method to
investigate the turning of the particles suspended in an insulating oil. In order to get quantitative results, special ferro-
electric spheres with a single-crystal structure and a unified
polarization direction were fabricated. Our experiments revealed that the diffraction spectra in some crystal planes of
the particles were enhanced when a fixed dc electric field
was applied. The changes in the x-ray diffraction spectra are
thought to be caused by the reorientation of the dipoles in the
crystal microsphere when a fixed dc electric field is applied.
The cell ~Fig. 1! used in our experiment is made of a
piece of glass with a 10 mm320 mm31.5 mm window on
which two pieces of Al foil are mounted to serve as electrodes. The ER sample consists of silicon oil containing
BaTiO3 single-crystal spheres with a radius of 20 mm. These
ferroelectric microspheres were specially fabricated for the
experiment so that their permanent dipoles are all polarized
in the same direction.11 The volume fraction of the spheres is
31.5%. Before taking any measurements, the ER fluid was
stirred in order to keep it uniform. An x-ray measurement
was carried out with a Cu target. The voltage and current
were fixed at 40 kV and 150 mA, respectively. An optical
microscope was used to observe the structure of the ER fluid
and found that the ferroelectric microspheres can easily form
a chain structure even when exposed to a very low electric
field.
The measured results of the x-ray diffraction spectra at
the electric strengths E50 V/mm and E52500 V/mm are
shown in Figs. 2~a! and 2~b!, respectively. We can clearly see
the differences between the two spectra. The reason can be
explained from Fig. 3. At E50 @Fig. 3~a!#, the distribution of
the microspheres suspended in the oil was random, and the
a!
FIG. 1. Schematic of the cell employed in the x-ray diffraction experiment.
Electronic mail: [email protected]
1046
Appl. Phys. Lett. 68 (8), 19 February 1996
0003-6951/96/68(8)/1046/2/$10.00
© 1996 American Institute of Physics
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FIG. 4. The relative variation of the diffraction spectra.
FIG. 2. X-ray diffraction spectra of the BaTiO3 crystal microspheres with a
volume fraction of 31.5%, where the field strengths of ~a! and ~b! are 0 and
2500 V/mm, respectively.
orientation of their dipoles and that of their fixed crystal
planes were random as well. Therefore, the diffraction spectra in Fig. 2~a! are the average results of all random orientations of the crystal plane, which give weak diffraction peaks.
We can see from Fig. 3~b! that when the electric field was
applied to the ER fluids, the dipoles as well as the fixed
crystal planes of the suspended microspheres would turn toward the direction of the external field. The reorientation of
the crystal planes gives rise to the observed change in the
diffraction spectra. This fact can be seen in Fig. 2~b!, where
the spectra amplitudes in some crystal planes increase dramatically.
The relative difference between E50 and E52500
V/mm is plotted in Fig. 4, where I j and I n represent the x-ray
diffraction amplitudes with and without the electric field, respectively. From the plot we can see that the diffraction from
the ~102! and ~104,110! planes increases dramatically, while
diffraction in the high-angles range remains unchanged.
In conclusion, an x-ray diffraction method to probe the
turning of the ferroelectric microspheres suspended in ER
fluids is presented in this letter. Our initial results show that
the particles first turn and then remain in a certain position
when a fixed dc electric field is applied. We argue that this
phenomenon is due to the turning of the dipoles in the ferroelectric microsphere under a dc electric field. The turning of
the particles was actually observed under an optical microscope. This phenomenon involving turning of the ferroelectric particles in ER liquids under the electric field especially
in the ac electric field has not attracted much attention up to
now. We propose that further theoretical and experimental
work be performed in the future.
We would like to thank Dr. Guanfar Yi and Dr. Hongru
Ma for their helpful discussion, and Dr. R. A. Dragan for his
editorial suggestions.
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1
2
FIG. 3. Schematic representation of the microsphere arrangements. The arrow and line in each sphere represent the permanent dipoles and crystal
planes, respectively.
Appl. Phys. Lett., Vol. 68, No. 8, 19 February 1996
W. Wen and K. Lu
1047
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