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Thermionic Cathode Magnetic Matrix Display

IP.com Disclosure Number: IPCOM000123094D
Original Publication Date: 1998-May-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 4 page(s) / 228K

Publishing Venue

IBM

Related People

Beeteson, JS: AUTHOR [+3]

Abstract

A Magnetic Matrix Display (MMD) is essentially a flat, thin CRT. If a thermionic area cathode is used as an electron source then good thermal design is essential to remove the heat generated by the hot filament wires. In addition the filaments and grids used must be precisely positioned relative to the magnet, and this positioning must be accurate over the whole area of the display. The improved display described here embodies techniques to achieve these objectives.

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Thermionic Cathode Magnetic Matrix Display

   A Magnetic Matrix Display (MMD) is essentially a flat, thin
CRT.  If a thermionic area cathode is used as an electron source
then good thermal design is essential to remove the heat generated
by the hot filament wires.  In addition the filaments and grids used
must be precisely positioned relative to the magnet, and this
positioning must be accurate over the whole area of the display.  The
improved display described here embodies techniques to achieve these
objectives.

   Figure 1 is a cross section through the display.  A is the
front glass plate coated with phosphors and black matrix.  B is the
glass side plates frit sealed to glass plate A and the rear
substrate.  C is the rear substrate, made of metal for the rear
surface, sealed to glass sides.  The metal rear surface provides a
good thermal conductivity path for the cathode construction
described.  D is a flat, thin magnet carrying a grid structure front
and back.  The grids on the back surface operate at a few volts
negative.  A particular feature of this magnet is that residual
stresses in manufacture may cause a bowing of the plate.  To allow
the magnet to be maintained flat it is manufactured in such a way
that the bow can be predicted and will lie in a direction away from
the front glass plate A.  E is part of the cathode structure and is a
mesh grid with about 95% open apertures with typically 10 mum wires
and 380 mum openings.  E operates at a few tens of volts.  F are
oxide coated tungsten filament wires, typically 20 mum diameter, and
preferably run vertically the full height of the display (they may
alternatively run horizontally, but this may increase their tendency
to sag under their own weight).  Because the filaments operate at a
temperature of 993degK they are mounted on springs to keep them taut
as they heat up after switch on.  G is the rear plate of the cathode
structure which is typically held at a low negative voltage.  Current
flat thin CRTs typically use the rear substrate C as the cathode rear
plate while the improved device described here uses a separate plate
to allow proper alignment of the structures, as will be described
later.  H is a plane of low velocity, high density electrons,
produced during cathode operation.  This electron plane must be
precisely positioned close to the magnet, which requires close
dimensional control of the grid E, filaments F and rear plate G.

   Because the total power required to heat the filament wires
may be 30W or more, and since these are contained within the
internal vacuum of the display, it is necessary to have a very good
thermal design to avoid excessive temperatures of the internal
structures.  In particular the ferrite magnet material must not get
so hot as to unduly reduce its magnet field, must not have a high
differential temperature across it and, to minimize purity losses,
must not have a high differential temperature between it and the
front p...