Field of the Invention
This invention relates to a method for forming an image on a magnetic
composite medium and to apparatus particularly suited for such image formation.
Background of the Invention
In the January 24, 1992 issue of Science (Vol. 255, p. 446), applicants
Jin and Tiefel describe a class of composite materials which are optically transparent
and, at the same time, electrically conductive. These composite materials comprise
sheets of polymer containing columns of magnetic conducting spheres.
Such composite materials have a variety of uses due to their anisotropic
electrical conductivity. They conduct through the thickness of the material but
not laterally. United States Patent No. 4,644,101 issued to Sungho Jin et al. on
February 17, 1987 discloses the use of such materials in a pressure-responsive
position sensor. The operative principle is that applied pressure forces the spheres
through any intervening polymer into contact with one another and through the
polymer to the surface. United States Patent No. 5,049,249 shows the use of such
material as a means for providing electrical contact between protruding electrical
contact regions. The protruding contacts press on the conductive columns to enhance
The present invention is concerned with the magnetic properties of
a composite medium rather than its electrical properties, and it is specifically
concerned with the use of a composite medium as a material upon which erasable
magnetic images can be written and developed.
Summary of the Invention
An image is formed by applying a local magnetic field to selected
regions of a magnetic composite medium comprising columns of magnetic particles
distributed in a matrix medium. The particles are "hard" or "semi-hard" magnetic
materials in order to retain the latent image as residual magnetism, and the image
is developed by exposure to magnetic fluid or powders. The image can be erased
by exposure to an AC demagnetizing field or a DC sweep magnet. Preferred apparatus
for making such images comprises a sheet of such composite material having a pair
of major surfaces with columns of magnetic particles oriented between the surfaces.
A local magnetic field, such as a magnetic pen, can be used to write a latent magnetic
image on one of the major surfaces. The magnetic columns present the latent image
for development at either major surface. In preferred apparatus, one major surface
is adapted for magnetic image writing and the other major surface is positioned
in sealed relationship with a chamber for exposing the image to magnetic development
material. In this arrangement the columns provide a high resolution image on the
second surface despite the thickness of the medium between the write and development
Brief Description of the Drawings
In the drawings:
- FIG. 1 is a schematic cross section illustrating a method and a preferred apparatus
for forming an image in accordance with the invention; and
- FIGS. 2-4 are schematic cross sections of preferred magnetic media for image
Referring to the drawings, FIG. 1 is a schematic cross section illustrating
a method and a preferred apparatus for forming an image. In essence, the method
of image formation comprises the steps of providing a magnetic composite medium
10 comprising columns 11 of magnetic particles distributed in a nonmagnetic medium
12, forming a latent image 13 by applying a local magnetic field, as from a magnetic
pen 14, to a selected portion of the medium. The latent image is developed by applying
magnetic fluid or powder 15 and allowing the applied material to accumulate on
In the preferred apparatus for forming such an image, the composite
medium 10 is in the form of a layer having two major surfaces 16 and 17. One major
surface, e.g. 16, which can be called a write surface, is adapted to permit the
writing of a magnetic image without loss of magnetic particles. For example, a
wear resistant polymer such as polyurethane is coated on the surface in sufficient
thickness that the columnar particles are not extracted by the write pen. The second
major surface 17, which can be called the development surface, can be positioned
in sealed relation with a development chamber 18 containing the development fluid
15. The presence of magnetic columns 11 extending substantially between the two
major surfaces enables a magnetic image written on surface 16 to be developed as
a high resolution image on surface 17 despite the intervening distance between
the two surfaces. Alternatively, the latent image can be developed on the same
surface on which it is written.
The preferred magnetic composite medium 10 is shown in greater detail
in FIG. 2. The composite medium 10 is similar to those described in the aforementioned
Jin et al article and patents except that the composite medium is made of higher
coercivity Hc magnetic materials with permanent remanent induction.
The earlier composites use soft magnetic particles such as nickel, with typical
coercive force (Hc) of less than 10 Oe. See R. M. Bozorth,
Ferromagnetism, D. Van Nostrand Co., Inc, New York, 1951, p. 275. Such soft
magnetic materials do not retain much magnet strength, and they exhibit small or
negligible remanent induction after the applied field is removed. See
Metals Handbook, 8th ed., Vol. 1. American Society for Metals, 1961,
p. 779, and B. D. Cullity, Introduction to Magnetic Materials,
Addison-Wesley, Menlo Park (A, 1972, p. 491 ). They are easily demagnetized especially
if the magnetized material has an aspect ratio of less than about 100.
The medium for the present application is made so that the particles
will not escape the write surface. The medium comprises columns 11 of high coercivity
magnetic particles 20 distributed in a matrix medium 12. Preferably, a protective
layer 21 is disposed on the write surface of the medium to prevent the particles
20 from breaking through to the surface where they could be removed by the magnetic
wring pen 14. If the matrix material is an adhesive or rigid material such as epoxy
or glass, then the protective layer is not needed.
The particles 20 are magnetic particles made of permanent or semi-hard
magnet materials having Hc > 100 Oe. For example, they
can be magnetic alloys such as Nd&sub2;Fe&sub1;&sub4;B, Alnico, Fe - Cr - Co, or
rare-earth cobalt magnets SmCo&sub5; or Sm&sub2;Co&sub1;&sub7;. Alternatively,
they can be non-conductive or weedy conductive ferrite magnets such as BaO.6Fe&sub2;O&sub3;
or SrO.6Fe&sub2;O&sub3;. For permanent image storage, materials having Hc
> 200 Oe and preferably Hc > 1000 Oe are
desirable. Advantageously, the particles are coated with a corrosion resistant
material such as gold or silver for corrosion resistance and to reduce light absorption.
Typical particle diameters are in the range 0.1 to 2000 micrometers with a preferred
range of 10-500 micrometers.
The matrix material 12 can be a polymeric material such as an elastomer
or adhesive or it can be a glass. For typical magnetic image applications the material
can be compliant or rigid. It is important for the fabrication of medium 10 that
the matrix be a material that goes through a viscous state before curing or setting.
Useful materials include silicone elastomers, epoxies, polyurethane resins and
glasses. While transparent media are preferred for a number of applications, the
material can be lightly colored for decoration. Typical thicknesses are on the
order 2-5000 micrometers and preferably 10-500 micrometers.
Medium 10 can be fabricated starting with matrix material 12 in a
viscous state. Magnetic particles 20 are demagnetized and mixed with the viscous
material in a volume fraction of 0.1-20% but preferably 0.5-5%%. After mixing,
the material is formed into a layer, as by doctor blading, and, while initially
in the viscous state, is subjected to a magnetic field of 50-5000 Oe,
and preferably 200-1000 Oe during hardening or cure. The effect of the
magnetic field is to cause the magnetic particles to move in the viscous material
into a configuration of columns 11 extending substantially through the medium at
random locations distributed with substantially uniform density in the medium.
The method of cure or hardening depends on the nature of the matrix
material. Polymerizing and thermosetting materials can be heated in an oven. Light
sensitive resins can be cured by exposure to radiation of appropriate frequency,
and glasses, thermoplastic materials or inorganic compounds can be solidified by
cooling. After hardening a protective layer 21, such as polyurethane, can be formed
on the write surface of the medium to keep the particles 20 from being extracted
during the write operation.
The advantages of this medium and apparatus for magnetic image formation
are manifold. Resolution is enhanced because it is easier to magnetize particles
in a column and obtain stronger flux from their ends due to the improved aspect
ratio when the particles are in a column configuration. Moreover the columnar configuration
extending substantially through medium 10 permits writing on one surface, e.g.
the top surface, and development of a sharply defined image on the other surface,
e.g. the bottom. This establishes magnetic flux lines close to the display medium
while permitting enclosure of the development medium away from the user. This feature
can be used to prevent leakage of magnetic powders and ferrofluids. Moreover, the
use of a column configuration -- as distinguished from a random distribution of
magnetic particles -- permits better transparency for medium 10 than would be present
for the same content of randomly distributed particles.
Writing of an image can be accomplished by using either a permanent
magnet pen or an electromagnet pen. The pen can be hand-held or machine-controlled,
such as the stylus on an Z-Y recorder.
Erasure of a written image can be effected in a variety of ways. One
approach is to use a permanent magnet or electromagnet to uniformly magnetize the
write surface. Another approach is to use a permanent magnet or electromagnet to
demagnetize the surface. Yet another approach is to use an erase pen of opposite
polarity to erase the image locally.
FIG. 3 is a schematic cross section of an alternative form of the
medium 10 where the magnetic particles 30 are in the form of magnetic rods having
a length approximately equal to the medium thickness.
FIG. 4 is a schematic cross section of yet another embodiment where
the magnetic particles 40 are spheres having diameters approximately equal to the
medium thickness. Fabrication of such a medium is described in greater detail in
applicants United States Patent No. 4,737,112 issued April 12, 1988 and entitled
"Anisotropically Conductive Composite Medium".
The fabrication and structure can be understood in greater detail
by consideration of the following specific example. 3.5% by volume of Sm&sub2;Co&sub1;&sub7;
magnet particles having diameters in the range 200-250 micrometers were mixed in
General Electric RTV#615 elastomer. The mixture was then sheeted out as a 600
micrometer sheet onto a glass substrate and exposed to a vertical magnetic field
(across the thickness) of 300 Oe while curing the elastomer at 130°C
for 20 min. The resulting medium comprised columns of magnetic particles extending
substantially through the 600 micrometer thickness and distributed with a substantially
uniform average distribution spacing. The medium exhibit a transmittance of about
75% in the visible light range.
An image of the letter "A" was then written on the medium by a Nd-Fe-B
magnetic pen having a 1/16'' radius tip (field estimated to be 1600 Oe).
The image was developed by placing a sheet of white paper over the same and sprinkling
Fe powder (25-100 micrometer diameters) onto the sheet and gently tapping. The
result was a visible image of the written "A".
An eraser pen with opposite polarity field of 600 Oe, was
moved over the written "A" on the composite medium, and it was erased. In other
experiments the image was erased by uniform magnetizing effected by sweeping a
vertical field of 3400 Oe, across the surface. Alternatively, a similar
image was erased using demagnetization by applying an opposite polarity field of
1100 Oe across an air gap.