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Conductive Coating for Matrix Display Magnet

IP.com Disclosure Number: IPCOM000122802D
Original Publication Date: 1998-Jan-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 92K

Publishing Venue

IBM

Related People

Beeteson, J: AUTHOR [+2]

Abstract

A Magnetic Matrix Display (MMD) uses a large area permanent magnet between 0.4 and 2mm thick, perforated with a large number of small holes. The material of the magnet is typically ferrite or glass bonded ferrite, which itself is nonconducting. The hole dimensions are typically 0.2mm diameter on 0.3mm centres.

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Conductive Coating for Matrix Display Magnet

      A Magnetic Matrix Display (MMD) uses a large area permanent
magnet between 0.4 and 2mm thick, perforated with a large number of
small holes.  The material of the magnet is typically ferrite or
glass bonded ferrite, which itself is nonconducting.  The hole
dimensions are  typically 0.2mm diameter on 0.3mm centres.

      In use, electron beams are directed into the holes and
collimated by the presence of the strong magnetic field present,
before passing onto a final phosphor screen.

      Inevitably, a few stray electrons will impinge on the sides of
the hole and the outer surface of the magnet, and since the ferrite
is nonconducting, they have no discharge path.  Thus, electrons will
gradually collect and build up a negative voltage in the hole
aperture.  Eventually, the charge accumulation and hence, voltage,
will build up to a level which will prevent proper operation of the
display.

      Because it is necessary that an electric field be present
across the magnet, to accelerate electrons to the final screen, it is
not possible to allow a high degree of electrical conductivity in the
ferrite to allow electrons to discharge.

      In the improved magnet assembly disclosed here, the holes are
coated with a thin conductive film which has a resistivity of the
order of 10e8ohm-cm or greater.  The process could be extended to
coat spacers attached to the magnet, and so solve two problems at
once.  A coating with a resistivity as high as this would not prevent
the establishment of an electric field, but would allow electron
discharge.

Processes

      Coating substrates with a high resistivity tin oxide film is
well known.  For example, the NESA process used to coat CRT
faceplates with an antistatic coating, where Stannic Chloride
dissolved in alcohol  was atomized and allowed to settle at
atmospheric pressure onto the glass screen heated to between 200 -
400ºC.  The compound reacts with the glass to form Tin Oxide
compounds and the alcohol evaporates in a reaction, such as:
                  SnCl4 + 2H2O  4HCl + Sn02

      The problem in the case of the magnet is that existing
processes will not give proper penetration of the vapor through the
small holes.  It is, therefore, proposed that the magnet be supported
at the edges or corners (where there are no holes) and that a
differential pressure be created across the heated magnet plate to
force vapor through the holes.  This can be done i...