Note: Descriptions are shown in the official language in which they were submitted.
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'~ethod of manufacturing an electric discharge tube and
electric discharge tu~e manufactured by said method".
The invention relates to a method o manu-
facturing an electric discharge tube in which at least
one of the components of the tube is subjected to an
operation ln a vessel sealed from the atmosphere,
Nhich vessel is composed of at least two parts to
be sealed hermetically against each other.
The invention furthermore relates to an
electric discharge tube manufactured by said method.
In the manufacture of electric discharge
tube3 it frequently occurs that certain componen~s
of the tube are to be subjected to operations which
have an undesired and frequently detrimental influence
on other components of the tube. For that reason it
ls desired that the components in question be subjected
to the required treatments in separate spaces before
they are assembled to form a tube. Moreover, the spe-
c~fic problem may occur that the components after
t~eir treatment ma~ no longer be exposed to atmos-
p~eric influences. A known example hereof is found
in the manufacture of electric discharge tubes hav-
ing a given radiation-sensitive layer, for example,
televlsion camera tubes and photomultiplier tubes.
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It is kno~n that the properties of the radiation-sensitive
material with respect to, for example, the sensikivity or
the inertia, can vary unfavourably when they are exposed
to the atmosphere.
United States Patent Specification 3,334,955
describes a method of manufacturing a vacuum tu~e in
~hich the components of the tube are assembled in an
evacuated space after having been subjected in separate
vessels to various treatments. Said vessels are
temporarily sealed b~ a metal foil and are opened
again in the evacuated space by cutting said foil from
an annular frame.
German Patent Specification l,915,710 de-
scribes a method of manufacturing a television camera
tube in which a photocathode is vapour-deposited on a
window in a closed vessel comprising said window.
Said vessel together with the electrode system of
the camera tube uhich is already mounted in an envelope
~fiich is open at one end, is then accommodated in a
space communicating with an evacuating device. After
evacuating the space, the system of electrodes is de-
gassed, after which the window having a radiation-
sensit1ve layer is removed from the said vessel by
means of a thermo-shock treatment and is then placed
on the open end of the envelope of the camera tube
with the inter-position of an indium sealing ring,
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The methods which are used in -the known manufacturing techniques
to obtain a temporary seal of the vessels are far from ideal. The seals
can be used only once and require expensive and precise pre treatment.
The invention starts from the above described methods and has
for its object to provide an improved method of manufacturing an electric
discharge -tube.
According to the invention, there is provided a method of
manufacturing an electric discharge tube wherein at least one of the
components of the tube is subjected to a treatment in a vessel sealed from
the atmosphere, the tube having an envelope being formed of at least two
envelope parts, the vessel being composed of at least two vessel parts
including at least one of said envelope parts, said vessel parts being
hermetically sealed against each other by bringing together smooth facing
surfaces thereof with the interposition of a sealant substantially con-
sisting of gallium or a low melting point allo~ of gallium in the liquid
state provided on at least one of these surfaces.
According to another aspect of the invention there is provided
an electric discharge tube comprising an envelope which consists of at
least two parts hermetically sealed against each other by means of a sealant
substantially consisting of gallium or a low melting point alloy of gallium.
Gallium has a melting temperature of approximately 29C. In the
liquid state, said metal can readily wet glass and other metals. Another
property which is important for the use according to the invention is that
gallium has a vapour pressure lower than 10-8 mm Hg even at a temperature
of 500C.
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It is noted that seals of gallium are known as such from the
United States Patent Specification 3,038,731, in particular with respect to
liquid seals for valves and parts which are arranged so as to be rotatable
relative to each other. As described in said Patent Specification, the seal-
ing efect of gallium and low-melting-point alloys of gallium is based on an
equilibrium of forces between on the one hand the surface forces of the seal-
ing material and on the other hand the force acting on said material as a
result of the pressure differential across the seal. This equilibrium may
~xist so long as with a given pressure differential the distance between the
surfaces of the parts to be connected does not exceed a given value. In the
practical case in which the pressure differential across the seal is approxi-
mately one atmosphere, this means that the maximum distance of the parts to
be connected may not be larger than approximately 10 ~m with a surface ten-
sion of ~he sealing material of approximately 500 dyn/cm.
Seals of gallium or a low-melting-point alloy of gallium can be
realized in a particularly simple manner. The facing surfaces of the parts
to be connected are first polished so as to ensure that said surfaces join
each other within a tolerance of 10 ~m. At least one of the surfaces is
then wetted with gallium or a low-melting-point alloy of gallium, after which
the parts are provided on each other. According to the invention, the parts
~hich are provided on each other are preferably rotated relative to each other
so that a pattern of closed rings is formed in the sealing material. In this
manner, the occurrence of radial leakage paths by island forma~ion of the
sealing material between the surfaces is prevented. The sealing as such is
then completed and the space thus closed can be evacuated. The seal can
easily ~ithstand large temperature fluctuations. Temperatures exceeding
200C hardly constitute a drawback, while a hermetic seal is also maintained
if the sealing material changes into the solid phase as a result of a de-
crease of the temperature.
Not only can the above-described seal be realized in a simple man-
ner, but parts o an envelope connected together in this manner can also be
separated easily without damage thereto. It is thus these properties which
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make the seal ex~remel~ suitable for use as a temporary seal in the manufac-
ture of electric discharge tubes in which at least one of the composing parts
is sub~ected to a treatment in a vessel which is sealed from the atmosphere.
According to the invention the envelope of said vessel preferably constitutes
already wholly or partly the final envelope of the tube to be manufactured.
According to the invention, the gallium or a low-melting-point alloy of gal-
lium is also used for the final sealing of the tube.
Examples of low~-melting ~point alloys which satisfy the object
underlying the invention are alloys of gallium and at least one of the metals
lndium and tin containing at least 50% by weight of Ga, and in particular
eutectic alloys of gallium, indium and tin the melting point of which is below
room temperature.
The invention preferably relates to the manufacture of an electric
device having a vapour-deposited layer of radiation-sensitive material sealed
from the atmosphere. The part of the tube envelope to be provided with the
radiation-sensitive material is for that purpose arranged on a vapour-
deposition device with the interposition of a layer of gallium-containing
material for the hermetic seal of a first vessel thus formed. The complemen-
tary part of the tube envelope comprising the electrode system of the tube is
hermetically sealed in an analogous manner by means of a sealing member and
thus constitutes a second vessel. The said first vessel and second vessel
are then evacuated, after which the radiation-sensitive material is vapour-
deposited in the first vessel and the electrode system is degassed in the
second vessel. After vapour-depositing the radiation-sensitive material, the
first vessel is filled with a gas which does not influence the radiation-
sensitive material. When the pressures inside and outside the vessel are
substantially equal to each other, the relevant part of the tube envelope may
be removed from the vapour deposition device. In the second vessel the de-
gassed electrode system is surrounded by an inert gas, after which the seal-
3Q ing member of the said complementary part of the tube envelope may be re-
moved. Finally the part of the tube enve~lope having a radiation-sensitive
layer ~s transferred to the complementary part of the ~ube envelope having
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the said electrode system. The gallium-containing material which is already
present on the surfaces to be connected ancl which in the first instance en-
sured a temporary seal of the vessels, now produces the final seal of the
tu~e.
During the transfer of the part of the tube envelope having a
radiation-sensitive layer to the complementary part of the tube envelope, the
radiation-sensitive material should be shielded from atmospheric influences.
Said shielding can be obtained in known manner by surrounding the relevant
part during the transport with an inert gas, or causing the transport to take
place in a vacuum space.
As is already known from the above-mentioned United States Patent
Specification 3,038,731, measures are furthermore taken preferably to hold
surfaces connected together hermetically with the use of a seal as described
above in position relative to each other.
The invention will be described in greater detail with reference
to the drawing, in which:
Figure 1 is a diagrammatic sectional view of a device for vapour-
depositing a layer of radiation-sensitive material on a part of the envelope
of a television camera tube to be manufactured according to the invention,
2~ Figure 2 is a perspective view of a centring holder f~r the part
of t~e said envelope shown in Figure 1,
Pigure 3 is a diagrammatic sectional view of the complementary part
of the part of the tube envelope shown in Figure 1 and having a system of
electrodes ready for degassing,
~igures ~ and 5 are sectional views of two embodiments of a final
seal of a television camera tube manufactured according to the invention, and
Figure 6 is a diagrammatic sectional view of a further embodiment
of a television camera tube manufactured according to the invention.
Pigures 1 and 3 show two stages of the manufacture of a television
camera tu~e. The envelope of the tube consists of two complementary parts,
namely~ a glass window 1 and a glass envelope 20 which is open at one end.
~n t~e stage sho~n in ~i~ure 1J a radiation-sensitive la~er 3 of lead monoxide
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is vapour-deposited on the ulndow 1.
Preparatory to said treatment, a layer of liquid gallium 2 is pro-
vided on a smoo~h polished edge 4 of a hollo~ cylinder 5. For that purpose,
the edge 4 to be wetted is moved in a rotating manner on a heated flat plate
on which a layer of liquid gallium of a few microns thickness is present, a
layer of gallium 2 remaining on said edge 4. At its non-wetted end the
c~linder 5 is t~en connected hermetically to a vacuum line 6 of an evacuating
device not shown. The window l on which a transparent conductive layer 7 of
tin oxide or indium oxide has already been vapour-deposited, is placed on the
edge 4 by means of a centring holder 8 and rotated a fe~ times so as to wet
the surface of the window destined for the sealing~ Moreover, due to the
rotary movement, closed concentric rings are formed in the gallium so that
possible radial leakage paths are closed. In order to have a good grip on
the windou during said rotary movement two apertures 9 are recessed in the
centring holder 8 a perspective vie~ of which is shown in Figure 2~ By the
arrangement of the window on the edge 4 with the interposition of a layer of
gallium 2, a space is obtained which is sealed hermetically from the at-
mosphere and which is evacuated via the vacuum line 6. An electric current
is now conveyed through the wires 10 and 11 so that a tray 12 filled with
lead monoxide 13 is heated. The lead monoxide i5 vapour-deposited on the
conductive layer 7 ~hile forming the radiation-sensitive layer 3.
W~en the layer 3 has reached a thickness of approximately 8 microns,
the vapour-deposition process is discontinued and the space bounded by the
~ndo~ l and the cylinder 5 is filled, via a closable supply line 14, with a
gas, for example helium or nitrogen, which does not influence the radiation-
sensitive material. ~eanwhile, a pouch 15 manufactured from a synthetic
material, for example, polythene, is slid on the cylinder S and is also filled
with an inert gas, for example, helium or nitrogen, via a supply line 16, the
air present in said pouch being driven out. After a pressure equal to or
3a slightly larger than the ambient pressure has built up in the space communi-
cating with the supply line 1~, the window 1 may be removed from the cylinder
5 and be transported uhile protected by the inert gas present in the pouch,
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the opening of the pouch remaining directed downwards if the gas present
therein is lighter than air.
Before placing the window 1 on the complementary part 20 of the
ellvelope of the tube, the elec~rode system 21 already mounted in said part is
degassed. This phase of the manufacturing process will be explained with
reference to Pigure 3.
The glass part 20 of the envelope has a smooth polished edge 22
which is wetted wi*h the gallium in the above described manner. Via an ex-
haust tube 23, the part 20 of the envelope is hermetically connected to a
vacuum line 25 with the interposition of a sealing ring 24. An auxiliary
~indow 28 is placed on the edge 22 in the above-described manner with the
interposition of the layer of gallium 26 and by means of a centring holder
27. The space in which the electrode sys~em 21 shown diagrammatically in the
drawing is mounted on a number of pins 29 in the cap of the tube is hermetical-
ly sealed from the atmosphere. After evacuating said space, the assembly is
heated in a furnace to a temperature of 250C and the electrode system is
then degassed at a temperature of 700C by means of a high-frequency coil 30.
After degassing, nitrogen is admitted to the said space through the duct 31
until the pressure in the space is equal to that of the surrounding atmos-
phere, after which the auxiliary window 28 may be removed from the part 20
of the envelope. The window 1 having a layer of lead monoxide is now placed
on the edge 22 of the part 20 of the envelope. The nitrogen supply through
the line 31 is discontinued and ~he gallium present on the surfaces of the
w~ndo~r and envelope to be connected now serves for the final seal of the tube.
The final envelope of the tube thus formed is finally evacuated via the line
25 after which the tube is completed.
In the case of large temperature Eluctuations as they occur upon
degassing the tube, no large temperature differences should arise between
the ~rindow and the envelope of the tube since differences in expansion result-
3a ing therefrom may stimulate the formation of radial leakage paths in the seal.
An improvement in this respect is obtained by replacing the centring holder
27 in this phase of t~e manufacturing process by a holder in ~rhich the aper-
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tures 9 are not provided. It ls also po~sible, as is shown in broken lines
in Pigure 3, to provide a temporary envelope 32 around the device to be de-
gassed during firing, which envelope is pumped to a rough vacuum via a line
33, that ls to say, a pressure of a few mm Hg. If desired, the firing tem-
perature may then be increased from 250C to 450C without any objection,
while in addition the leadthrough pins 29 are protected against possible
oxidation.
In Figures 4 and 5 two embodiments of a completed gallium seal are
shown in an enlarged scale. A window 41 is hermetically sealed against a
c~linder 43 while using a gallium seal 42. Window and cylinder both have
ground edges 44 and 45, respectively, which together constitute a V-shaped
groove. A layer 46 of lead monoxide ls vapour-deposlted on a transparent
signal electrode 47. The signal electrode 47 is led through to the exterior
via a strip-shaped extension 48. In order to prevent a radial movement of
the window 41, the groove formed by the ground edges 44 and 45 is filled with
a hardening synthetlc material 49.
The embodlment shown in Figure 5 dlffers from that shown in Figure
4 in that a metal ring is provlded which covers the gallium seal and makes an
electric contact with the extension 48 of the signal electrode 47. Since
said ring locks the window against a radlal movement, the filllng of the V-
s~aped groove with a cement or hardenlng synthetic material 49 may be omitted.
The invention is not restrlcted to the above-described examples in
which one of the complementary parts of the en~elope of the tube is formed
b~ a ~ndow. In the television camera tube shown diagrammatically in Figure
6 the window 60 forms one assembly with the cylindrical envelope 61 while
said cylindrical envelope is sealed hermetically on its open end against a
closing plate 62 while using a gallium seal 63. The method of manufacturing
said tube is analogous to that described with reference to Figures 1 and 3.
Tnstead of an auxiliary windo~ an auxiliary envelope is used during the de-
3Q gassing of the electrode system 64 shown diagrammatically, while during trans-
port the lead mon~ de layer 65 formed on the window is protected against
atmospherlc influence by an lnert gas, for example helium, admitted insida
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the envelope 61.
The closing plate 62 has a raisecl edged 66 to lock it against a
radial movement. If desired, the gallium seal 63 may be protected against
atmospheric influences and dust by means of a synthetic material 67 or by
means of a ring as described with reference to Figure 5. Conductive pins 68
via which the various electrodes of the electrode system 6~ can be brought
at the desired potential are' sealed in the closing plate. A transparent sig-
nal electrode 69 vapour-deposited on the window 60 is electrically led through
the wall of the envelope 61 br means of a current supply conductor 70.
Photoelectric devices manufactured according to the above-described
method have highly reproducible properties. The individual phases in the
manufacturing process of the device can be carried out in conditions optimum
for each phase. A further advantage is that no high temperatures are required
either for producing or for disrupting the seals. As a result of this, not
only the manufacturing process of the tube is simplified, but it is also pre-
vented that temperature-sensitive co~ponents, for example, a radiation-
sensitive la~er, are detrimentally influenced.
