Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1091~91
PHN 8480
DUYS/BKS/MS
22-6-1977
The invention relates to a method of
manufacturing a dispenser cathode in which an impregnated
and sintered moulding is embedded in a holder.
The invention also relates to a dispenser
cathode manufactured according to said method.
Such dispenser cathodes are used in
electron guns for television display tubes, picture
pick-up tubes, travelling wave tubes, clystrons, and
the like.
Such a dispenser cathode and method
- are known from U.S. Patent 3,467,879 which issued to
~ U.S. Philips Corporation on September 16, 1969 in which
:
a sintered porous tungsten moulding which is impregnated
with varium aluminate is compressed in a holder which is
clamped to the cathode shank. The moulding is laid
~ in the holder in the form of a pill and is compressed
- by means of a cylindrical die. The holder has been
; obtained by deep drawing from a molybdenum sheet
having a thickness of 100/um. The impregnated and
sintered m~ulding is clamped in the holder by deformation
` of the edge of said holder so that a gap remains present
-2-
.
,,
,,,
:~.
., ,
~ B
9 ~ ~ 9 l PllN ~4~o
~2-f - 1977
bet;ween the molllding and the holcler.
The use of` semiconductors in television
cameras and telvision sets has resulted in the need
for rapidly warming-up cathodes. These cathodes are
cathodes which emit a sufficient amount of e]ectrons
already after less than 5 seconds from switching on
to be able to produce a picture on the display screen
of the display tube. It is known inter alia from
the Philips Product Note 67, Quick Vision CTV picture
tube A66~ll10 X, that the warming-up time (th) of a
cathode is proportional to the radio of the thermal
capacity (Cth) to the effective heating power (P )
of the heating body of the cathode.
~th
th ~
- 15 e
In order to be able to shorten the warming-up time
of the known cathode described above, it should be
composed of smaller components than the conventional
cathodes so as to reduce its thermal capacity. The
drawback of this is, however, that the life of such a
; cathode is considerably reduced. As a matter of fact,
the life is determined inter alia by the ratio between
the quantity of introduced impregnate which is proportional
to the volume of the moulding, and the quantity of
2 reaction products of the impregnate which leave the
moulding by evaporation from the surface of the mouldin~,
which quantity is proportional to the surface area of
'
.~
,,
~lO9i~9~l PHN 8480
22-6-1977
the moulding. It is thus obvious that reducing the size
of the moulding results in shortening of the life. It
has been found that reduction in life is in fact
even larger than follows from the above consideration.
This is because the exhaustion of the cathode is diffusion-
limited. It has therefore been found a cathode of
the known described construction and of the rapidly
warming-up type (less than 5 seconds) which is operated
at a voltage 11~ above the usual voltage has a
life of only 5000 hours. An advantage of a cathode of
the known construction is that the impregnated and
sintered moulding, prior to being placed in the holder,
can be tested for its composition, for example by
accurate weighing.
German published Patent Application 1,764,260,
which is an application of Telefunken and published on
July 1, 1971, discloses a method which does not have this
advantage. In this application, the material in powder form
; from which the moulding is manufactured is compressed
by means of a compression tool in the hollow end face
of a cathode shank to form the moulding and is then
sintered. This method has been found to be unsatisfactory
in manufacturing small cathodes of the rapidly warming-up
typei since the deformation of the end face necessary
to make it hollow of the very small cathode shank is
~- substantially impossible.
It is therefore an object of the invention
.
. . . .
1~9iZ9~ l'llN ~480
22-G-1977
to provide a me-thod of manufacturing a disl>enser cathode
having both ~ rapid w~rming-up time and/or ~ long life.
According to the invention, in a method
of manufacturing a dispenser cathode of the kind
described in the preamble is characterized in that
the holder is formed around t1~e moulding in such manner
that the width of any gap remaining between the moulding
and the side wall of the holder is less than 10/um wide,
the holder being manufactured from a metal foil which is
from 10/um to 100/um thick before forming.
The invention is based on the recognition
of the fact that the life is considerably extended
when a holder is used of metal foil which i~ from
10/um to 100/um thick which effectively seals the
surfaces of the ~oulding which are not to be used for
emission. The width of any gap remaining between the
holder and the moulding should for that purpose be
of the same order of magnitude as the pores in the
moulding, and in any case smaller than 10/um. Thus
substantially no migration of reaction products takes
place from the moulding via said gap. This is possible
by forming the holder which consists of foil around
the compressed and sintered moulding.
;i Since the mass of the holder is also an
important factor in obtaining a cathode of the rapidly
warming-up type, the holder suitably consists of a metal
foil which is from 10/um to 50/um thick. With a thickness
. .
,
:
:- .
.,
1091Zg~
PIIN ~ O
22-6-1977
sm.lller 1}Iall I0/um, the possibility ot pores in the metal
foil is too 1arge and with a thickness exceeding 100/um
the manufactllre of the holder around the moulding
becomes difficult. The meta] foil may consist preferably
of W, Mo, Ta~ Zr or alloys thereof.
Such a cathode is preferably obtained
according to the invention in that the moulding is
placed against a flat metal foil and is pressed
through an apertured die by means of a pressing tool,
the moulding acting as a plunger to draw the metal
foil over the moulding.
It is also possible to manufacture such
a cathode by compressing the impregnated and
sintered moulding into a partly pre-shaped holder
obtained by deep drawing, the dimensions of moulding
and holder being such that the holder is expanded.
Another possibility is to provide the
holder in the hot state around the cold moulding and
then to shrink the holder around the moulding by cooling.
The invention will now be described in
greater detail with reference to a drawing, in which
Fig. 1 is a diagrammatic partial sectional
view of a dispenser cathode according to the invention
Figs 2, 3, 4 and 5 show diagrammatically
a method of manufacturing a dispenser cathode according
to the invention,
.. .
:`
~091~91 ,~-6-1977
~ ig9. 6 and 7 show a few other possible
embodiments of the holders with moulds~ and
Fig. 8 S]lOWS graphically some lifetime
; measurements.
Fig. 1 is a diagrammatic partia] sectional
view of a dispenser cathode according to the invention.
The cathode shank 1 is manufactured from molybdenum foil,
40/um thick, and is blackened in its internal surface 2
so as to be able to readily absorb the thermal energy
irradiated by a heater filament 3. The length of the
cathode shank 1 is approximately 2.3 mm and its outer
diameter is 1.75 mm. The filament 3 consists of a metal
core 4 which is covered with a coating 5 at least
the surface of which is black and which readily radlates
', 15 the thermal energy. A 30/um thick holder 7 is secured
to the 100/um thick end face 6 of the cathode
shank by means of resistance welding. The holder 7
has been drawn around a moulding 8 impregnated with
barium aluminate. As a result of the small dimensions
of the mouIding 8 (diameter approximately 1.2 mm,
height 0.4 mm) and the thin holder 7 (30/um thick) in
i~:
~ combination with the thin blackened cathode shank 1
... .
b,
and the black filament 3, the cathode can reach its required
operating temperature in less than 5 seconds after being
" 25 swltched on. An annular gap 9 between the holder 7 and
the moulding 8 has a width of the same order of magnitllde
as the dimensions of the pores in the moulding, namely
~ -7-
, . .
109iZ9i 2~-6-1~77
le~ss than 1G/1~m. As a result of this, evaporation takes place
subst.antial~y on]y via ~h~ free end surface 10,
so that the life of such a cathode is 2 to 3 times
longer than that of comparable cathodes manufactured
by known methods.
In display tubes for displaying coloured
pictures, usually three electron beams are generated.
In such tubes it is very annoying if the life of one
of the cathodes is shorter than that of the other
cathodes, causing colour shifts.It is therefore
advantageous to mak~ the life of all the cathodes so
long that the electron beams generated by the three
cathodes maintain substantially the same intensity
d~ring the life of the display tube. This may be done
with cathodes according to the invention.
As a result of their relative long life~
such dispenser cathodes are also very suitable for use
in other beam tubes, such as television camera tubes,
e.g. plumbicons (Trade Mark) in travelling wave tubes,
clystrons and the like. It is not necessary for such
tubes to have cathodes of the rapidly warming~-lp
type.
Figs. 2, 3 and 4 show diagrammatically
: :;
a method of manufacturing a dispenser cathode according
to the invention in three steps. The previously manufactured
,~,.
and impregnated and sintered moulding 8 (Fig. 1 ) is placed
~-on a metal foil 11 of approximately 30/um thick,
~ , . I
-8-
,
~ !
~09lZ91 l'llN 8ll80
22~G- 1 977
the rO-ll be-;ng laid over an aperture 13 ln a die 12
(figure 2). 1`he apert;ure whicll is adapted to the desired
shape of the holder 7 and has a minimum diameter slightly
smaller than the diameter of the moulding ~ plus two
times the thickness of the foil 11 not only to give
the metal foil a deepdraw operation but also to
produce a reduction in wall thickness by 5/um to 15/Um
(so-called tapering) so that the resistance to`
deformation is ensured because the metal is work-hardened
and the gap 9 is small (l~ss than 10/um). By means of
the pressing tool 14 the moulding 8 is forced through
the aperture 13 (Fig. 3~, said moulding serving as a
plunger to draw the metal foil 11 into the form of
the holder 7. The supporting member 15 also ser~es
;. .
` 15 to eject the holder with the moulding (see figure 4).
Fig 5 shows another method of
~r,, manufacturing a dispenser cathode according to the
inventlon. According to this method, the moulding 8
is pressed into a partly preshaped holder 18 which
20 is placed on a supporting block 17. During the
pressing operation, the pressure members 16 are moved
- inwardly and pressed against the holder. As a
result of this, the holder 18 is expanded by the moulding
8, while the wall thickness is reduced.
The dimensions of the foil 11 may be
t~j chosen to be so that the moulding 8 proJects above the
~ holder 7 as is shown in Fig. 6, or that the edge 15
_g_
,
. - .
lO91Z9l
PHN 8480
22-6-1977
of the holder 7 projects just slightly above the
moulding 8, as shown in Fig. 7. It has been found
that as a result of the drawing process said edge 15
projects slightly inwardly. In the last case the
plunger 14 has to be of a smaller diameter than the
moulding 8.
Figure 8 shows three graphs I to III
of measured saturation emission current against time
during the life of three cathodes. Graph I relates to
a known cathode manufactured by the method as described
in the afore-mentioned U.S. Patent 3,467,879. The
diameter of the moulding was 1.8 mm and its height
was 0.6mm. The operating temperature was approximately
1317 K and the warming-up time to reach an emission of
O.lA/cm2 was 12 seconds. By making the diameter of the
moulding 1.2mm and reducing its height to 0.4mm, a
cathode was obtained having a warming-up time of 5 seconds
to reach the same current density. A life measurement
, (graph II) has demonstrated that the life decreases
considerably to less than 5000 hours by reducing the
moulding. By using the invention~ so by giving the
holder a thickness between 10/um and lOOlum and making
the slit between the holder and the moulding minimum
(smaller than 10/um), a cathode is obtained having a
short warming-up time (less than 5 seconds~ and a long
life (graph III~. All the life measurements were
performed with a heater voltage which
`: :
-10-
iV~ lZ91 PIIN 8lI80
22-6-1977
wa~11/~ hi~her tharI the usual heater voltage of 6.3 volts.
In cathodes in which the startl.ng material
is an impregnated and sintered moulding it is thus
favourable according to the invention to provide a
holder of thLn metaI foil around sai.d mou]dlng.
. ' ~
,, .
:
^.
,,'~j . . ,
,,
. . ._. __ ~ ,
,~, .
,........................... .
.:,, .
:
f~ ,
,, .
.
,~
.
,:~
... .
. ..
:.
... . .
:
.
