Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to a method for making tungsten-thorium
alloys and in particular a method for making thin film tungsten-thorium
alloys.
, . .
` 2. Description of the Prior Art
It is well known in the prior art to use tungsten-thorium alloy as
wire filaments for incandescent lamps, displays and the like because it
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can be made in a single crystal state. A thermal compression extruding
; method is used in the prior art for making these devices. I~ requires
1 the providing of a powdery mixture oP the tungsten and thorium ingredi-
.. . .
,) ents, concurrently subjecting the mixture to high temperature, circa
2000C., and high pressure conditions, and extruding the mixture under
these conditions through a wire forming die. As such, the extruded wire
is in a polycrystal state.
EN9-72-045
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1 In order to make it a single crystal, the wire is subsequently annealed at
an elevated
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~ l 1 temperature, e.g. 1800C., for approximately two hours. ~-
i ~ 2 As is well known to those skilled in the art, a single
~ 3 crystal tungsten-thorium alloy filament has an extended
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life expectancy as compared to a polycrystal one.
The aforedescribed prior art extrusion
' 6 method is not conducive to making thin film W-Th alloys
7 such as thin film plates and sheets, or layers, and
8 particularly thin films of ten microns or less. Thus,
9 for example, it is not conducive to making a thin
. , - .:
;~ 10 film W-Th alloy in the form of a protective layer or
11 coating used for anti-thermal, anti-corrosive, and/or ;;`
~;1 12 wear resistance purposes. Furthermore, this prior
i 13 art method canno~ be employed for making W-Th alloy
1 14 components used in printed and/or integrated circuits.
It is known in the prior art to make
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il 16 planar, i.e. printed circuit type, pure W filaments by
17 chemical vapor deposition. Because W has a high boil-
t 18 lng point, i.e. 5660C., chemical vapor deposition is
19 not particularly suitable for depositing pure W in ~^-
these~type applications. Accordingly, a W compound,
21 such as WF6, is used in combination with a suitable
22 reducing agent, e.g. H, to provide the pure tungsten ~ I
~¦ 23 ~deposit at temperatures of 600C. approximately. In
j 24 addition, the pure W filament when formed is in a poly-
crystal state. As is well known to those amiliar with
I 26 the art, pure W cannot be produced in single crystal
; 27 state form. Hence, the polycrystal pure W filament
28 suffers from the defect of short life expectancy.
, 29 It has been suggested to make printed
:
circuit type filaments of a W-Th filament by the method
1 31 of chemical vapor deposition. However, because the
-,` EN972045 -2-
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1 boiling point of Th is also relatively high, i.e. 4790C., this method
is not suitable for these applications. Moreover, the system would
require that the W and Th each be compounded with other elements and
a suitable and compatible reducing agent be utilized. Thus, the system
complexity is increased and thereby the system's reliability is
decreased. Hence, the use of chemical vapor deposition for making
thin film W-Th alloy bodies is not deemed practical. ;
SUMMARY OF THE INVENTION
It is an obiect of this invention to provide a method of making
single crystal or polycrystal thin films of tungsten-thorium alloys.
Another object of this invention is to provide a method for making
~ thin film of W-Th alloys as incandescent filaments.
', Still another object of this inve~ntion ~s to prov~de a method o~
making thin film W-Th alloys as components for printed c~rcuits and/or
. integrated circuits~
;. Still another object of this invention is to provide a method for
, making thin film of W-Th alloy bodies and in particular bodies which i
... . .
are substantially planar such as plates or sheets and the like.
Another object of this invention is to provide a method for making
thin film of W-Th alloy bodies as a coating or layer for an object and
. . . .
in particular protecti~ve coatings used for anti-thermal, anti-corrosive
and/or wear resistance purposes.
The method~for making thih films of tungsten-thorium alloy according
to the invention ~
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1 includes providing a plating bath comprised of a first
2 aqueous solution of WO3, NaPO4 and H2O at a predeter-
3 mined temperature. The bath has immersed in it prede- -
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4 termined cathode and anode electrode means. The cathode ~
and anode electrode means are energized to provide a `
6 predetermined current density for a predetermined time
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period. During the energizing, another aqueous solution
~;i 8 of a predetermined concentration of Th(SO4)2 is added
' 1
g to the bath at a predetermined rate to produce plating --
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of the thLn film tungsten-thorium alloy on an appro-
11 priate substrate which may be part of the cathode elec-
,;
12 trode means.
13 The foregoing and other objects, ea-
14 tures, and advankages oE khe invention will be appar-
ent from the following more particular description of `
16 the preferred embodiments of the invention, as illus-
17 trated in the accompanying drawing.
18 DESCRIPTION OF THE DRAWING `~
19 FIG. 1 is a perspective view of a member
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used to apply thereto a thin film W-Th alloy according
21 to the method of the present invention;
~; 22 ~ FIG. 2 is a perspective view of the W-Th
~ 23 alloy layer or coating applied to the member of FIG. l;
li ~ 24 FIG. 3 is a perspective view of a thin
¦ 25 film W-Th alloy body after the member of FIG. 1 has
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26 been removed from the structure of FIG. 2;
27 FIG. 4 is an enlarged partial per~qpcc-
' 28 tive view of another member useful in applying thereto
;~, 29 a W-Th alloy ac~ording to the method of the present
¦ 30 invention:
31 FIG. 5 is an enlarged partial front view
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~ EN972045 -4- ~!
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1 of the member of FIG. 4 at a different stage of its formation; -
FIG. 6 is an enlarged partial cross-sectional view of the member
of FIG. 5 taken along the line 6-6 thereof; -.
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~ FIG. 7 is an enlarged partial cross-sectional view of a thin f;lm
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of W-Th alloy formed on the member of FIGS. 5 - 6 according to the method
~i of the present invention; and
FIG. 8 is an enlarged front view, shown partially in schematic,
of a W-Th alloy filament display of FIG. 7. -
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; In the figures, like elements are designated with similar reference `
numerals. ;
.i'' DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method for making a thin film tungsten-thor~um alloy according
to the present invention broadly contemplates electrodeposit~ng, i.e. ,~
'; electroplating, a W-Th alloy by pass~ng an electric current through a
,, heated bath consisting essentially of an aqueous solution of W03 and
Na3P04 and H20 is in the ratio of the order of about 35:100:200 parts ~;
by weight, respectively. A 10% concentration of Th(S04)2 ls used as a
second aqueous~solution. The bath is heated to a temperature of about
i 80C.
The W-Th a~lloy is deposited on a substrate which is immersed in
the bath. The substrate~is conductive and may be temporary or partially
~ temporary dependlng on the particular
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1 application. For example, in the application where the W-Th alloy is
to be used as a protective type coating such as a thermal, anti-
corrosive and/or anti-abrasive type for an object, the object would
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be immersed in the bath. As such, the object acts as the substrate ~-
- and is of the permanent type. In an application, for example, where ;`
an W-Th alloy body is required such as W-Th sheet it can be deposited
on a planar substrate which substrate is subsequently removed by, for
example, a suitable etchant or other means. Since the substrate is
'1 conductive, it also acts as the cathode electrode of the electroplating
system.
;, Accordingly, referring to FIG. 1, member 1 is a planar conduc-
tive body of, for example, Cu. Assume, for example, it is desired
to coat surface la of member I w~th a thin film of W-Th alloy. An
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approprlate anti-plating masking process is used to mask the opposite
.1 surface and edges of member 1. In addit~on, a suitable lead-~n con-
ductor is attached to the member I ZZn non-obstructing relationship
with surface la so that member 1 will also serve as the cathode
electrode. Thé anode electrode, not shown, and cathode electrode are
immersed in the~aforedescribed heated bath and connected to a suitable
i 20 electrlcal supply that passes the plating current through the bath at
the same time,~ i.e.~concurrently, the Th(S04)2 aqueous solution is
belng added. As a~result, the W-Th alloy coating 2, cf. FIG. 2, is -
deposited on the surface la of member 1.
In certain~applications the member 1 is retained after the elctro-
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~ depositing such as, for example, the aforementioned applications where
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-' 1 is a protective type coating. ;~`
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;, 2 However, if it is desired in certain ap-
;, 3 plications to provide a body, such as flat sheet of the
~' 4 W-Th alloy, the member 1 is removed by any suitable
means such as etching with a compatible etchant that ;
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-1 6 removes the member l without adverse efects to the
l 7 W-Th alloy. For the yiven copper substrate example, the
:~ ! 8 etchant may, for example, be FeC13. As a result, the
~ g W-Th alloy member 2, cf. FIG. 3, becomes an independent
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~;1 10 body which is substantiaIly rigid and of high strength
I ll even at thicknesses of 10 microns or less.
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,~ 12 ¦ In aertain applications the W-Th alloy
13 is annealed after the electrodepositing process. On~
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~1 14 such application is for use o~ the W-Th alloy as an
incandescent filament where annealing changes the W-Th
`¦l 16 alloy to a single crystal state. The annealing may be
i ¦ 17 done with or without the member 1 being attached to the
18 W-Th alloy film 2. Preferably, the annealing is done ~¦
19 below the melting point of the member 1 when it is at~
. 3 `, ~
tached to the film 2. Thus, for the given Cu example :-
; 21 which has a melting point of 1083C., the annealing
22 temperature~may~be 900C., for example. If the member
23 ~ 1 is not attached, then the film 2 may be annealed at
24 higher temperatures, e.g. 1800C.
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;` ` 26 ment of the present invention for making thin ~ilm
¦ 27 tungsten filaments for an alpha-numeric character dis-
28 play in which a part of the substrate is temporarily
~¦ 29 provided during the electrodepositing process and sub-
sequently removed. Referring to FIG. 4, the substrate
! 31 3 includes a ceramic planar member 4. A preferred and
~! EN972045 -7-
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1 suitable ceramic for this purpose is A1203. Imbedded in the ceramic
member are an array of conductive posts 5 of a suitable material.
Preferably, posts 5 are high temperature materials such as Au.
. Alternatively, posts 5 may be of W. The posts 5 may be formed, for
, . .
example, by drilling holes in the ceramic member 4 at spatial
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locations corresponding to the desired post array. Next, the insides
~; and edges of the holes so formed are electroplated with gold using
well known sensitizing and electroplating techni!ques.
Substrate 3 also includes a conductive layer 6 which is different
from the materlal of the posts 5. Preferably the layer 6 is copper.
It is formed on the face of the ceramic member 4 by printed circuit
deposition techniques. The exposed surface of the layer 6 is co-planar
with the end surfaces of the posts 5 which protrude above the front
surface 4a, as viewed ln FIG. 4. This can be done, for example, by
~; controlling the thickness of the layer 6 during its deposition and/or
by subsequent lapping.
Referring now to FIGS. 5 and 6, in the preferred embodiment, the
copper layer 6 is next processed using well known printed circuit
techniques to form a composite conductive pattern 6' made up of
individual conductive segments formed between mutually-exclusive pairs
of posts 5. One such technlque which is preferred is to form the pat-
tern using photorçslst methods. The composite pattern corresponds to
the pattern desired for the W-Th alloy filament to be formed on the
substrate 3 and more~particularly on the members S, 6' thereof. The ~`
pattern 6' comprises a rectangular
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1 array of individual segments each of which is s~uare
2 wave-shaped in form and onLy one segment of which is
3 shown in FIGS. 5 and 6 for sake of clarity. By way `
4 of example, a typical array includes 35 such segments - ;
arranged in a 5x7 matrix as will be apparent herein- ~;
6 after. Preferably, the ends of post 5 protruding out- `
7 wardly from the rear surface 4b of member 4 are coated
8 with a suitable masking material to prevent depositing
9 of the W-Th alloy thereat in the subsequent electro-
plating, i.e. electrodepositing, process. ~,
11 In accordance with the principles of the ;!~ '
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~ 12 present invention, an aqueous solution o WO3, Na3PO~, ;;
'1 13 and l12O is provided as part of the plating bath. The
~l 14 bath is heated to an elevated temperature o~ approxi-
;l 15 mately 80C. Immersed in the bath is the substrate 3
16 of FIG. 5. The copper segments 6' and gold posts 5
1 17 act as the cathode of the electroplating system. A
i 18 platinum anode is also immersed into the solution and ;; :
`', ~ 19 a plating current density of approximately 160 milli- .
amperes per square inch is conducted between the elec- ;~
21 trodes and through the bath. An appropriate electrical
22 ~ supply is~provided for this purpose. A plating time ;~!'' , '',
23 ~of between~;l5 to 30 minutes is utilized. During the
24 plating process~an aqueous solution having a concentra-
tion of about 10% of Th(SO4)2 is added to and is part
~;l 26 of the plating bath at the rate oE approximately 20
27 drops per minute. As shown in phantom outline, the
28 tungsten-thorium alloy film 7 is deposited at the com-
29 pletion of the plating process on the exposed copper
pattern 6' and exposed gold end surfaces of -th~ posts
1 31 5 of the substrate 3 and thus, has the desired display :~
~ EN972045 -9-
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After the layer 7 has been deposited and the composite structure
of elements 4, 5, 6' and 7 is removed from the bath, an etching process
takes place with an etchant that attacks only the conductive segments
.. : .
6' but does not affect the ceramic member 4, W-Th alloy film 7, or ~ ~
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posts 5. For the preferred choice of materials, to wit: Cu, A1203
and Au, an etchant such as the aforementioned FeC13 is preferred. As
a result, the conductive segments 6' are removed and the W-Th filament
pattern 7 is affixedly supported to the posts 5 as shown in FIG. 7.
Subsequently, the film 7 is annealed to place the film 7 in a
; single crystal state.
In one way of annealing, a temperature is utilized which is below
the temperatureassociated with the material of the members 4, 5 and 7
having the lowest melting polnt. For the preferred materials of
, A1203, Au and W-Th, a suitable annealing temperature is 900C., for
; example.
, A preferred way of annealing the W-Th filament pattern 7 is to pass ;
:'!
~` an electrical current through it while it is located in a vacuum or
inert atmosphere. The relative high resistance W-Th generates heats
~ .
and incandesces. The W-Th is thus at a very`high temperature which is
!
controlled by the amount of current being passed. Preferably, the `~
~ temperature is about 1800C. Ev,en though the material of posts 5 may
,~; have a melting point below this temperature, it will not be adversely
i~ affected if it is judiciously selected to have a low resistance, i.e.
be a good conductor such
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1 as gold, so that the heat generated therein ky the
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2 current passing through the posts 5 is at a temperature
3 well below their melting point~ This operation may be
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; 4 performed prior to mounting the W-Th filament pattern
7 in its associated display housing, or alternatively .:~
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: 6 may be performed after it is mounted in the housing -
. 7 and the housing is evacuated~
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8 After annealing, a suitable printed : :
. 9 circuit conductive pattern may be provided on the rear .~
,. j~ . .
' 10 surface 4b of the ceramic member 4 to provide lead-in
... .
11 electrode connections to the individual filament seg-
.: 12 ments located on the front face 4a, c~. FIG. 8. When
. .
13 incorporated in a suitabIe evacuated display housing,
14 the ~ilament segments via their respective electrode j. .
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l 15 connections are selectively energized to provide the .`
j 16 desired chaxacter to be read out. In FIG. 8, the
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i. 17 re~erence numbers S' indicate the W-Th alloy plated end ` .
18 surfaces of the posts 5 for sake of clarity.
19 An example of the plating bath suitable
20~ for use in:the present method, is as follows: -
21 ~ WO3~ ~ 35 grams -~ :
22 ~ 3po4 100 grams
.., ~
; 2~3 ~ H2O~ 200 grams
24 In one such thin film W-Th alloy built :; :
:! 25 in accordanae with the principles of the present inven- . .
~ 26 tion, the ~ilm was approximately 4 microns thick.
;.~ 27 It should be understood that alterna- .
:~ 28 tively a co-extensive W-Th alloy thin film can be de-
i 29 posited directly on the exposed sur~ace 6a of layer 6
`1 30 and the exposed co-planar end surfaces of posts 5 of
~`l 31 substrate 3 shown in FIG. 4 in accordance with the :
1:~ EN972045 -11- ; .
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l principles of the inventive method. After the W-Th
2 film is deposited, the pattern 7 is formed using printed
3 circuit techniques. Preferably, a conventional photo-
4 resist is applied to the exposed surface of the deposited
W-Th film. Photoresist is exposed through a mask con- -
6 taining the desired conductive display pattern and `
7 developed. Thereafter, subsequent etching processes are
; 8 employed to remove the unwanted portions of the W-Th
9 alloy film and to remove the temporary copper layer 6.
A mixture o~ K Fe(CN) and NaOH is a suitable etchant
3 6
ll for removing the W-Th alloy and FeC13 is a suitable
12 etchant ~or removing the copper layer 6. Thereaeter, ~'
13 the remaining W-Th ~ilament pattern i~ annealed to
14 place the W-Th alloy in a single crystal state. As a
result, a W-Th alloy ~ilament display identical to the
16 one shown in FIGS. 7 - 8 is provided.
i~, 17 Thus, it has been demonstrated from the
18 foregoing description that the W-Th alloy in some ap-
~, 19 plications remains on the object on which it is deposited, ;
~''''.'! 1 20 cf. FIG. 2. In other applications, it is completely re-
21 moved from the ob~ect, cf. FIG. 3. Still in other ap-
22 plications, it is only partially removed, cf. FIGS. 7 - 8.
23 However, while the invention has been
~ 24 particularly shown and described with reference to pre-
;~ 25 ferred embodiments thereof, it will be understood by ;
26 those skilled in the art that various changes in form
27 and details may be made therein without departing from
~l 28 the spirit and scope of the invention.
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EN972045 -12-
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