NI-based FTM shadow masks having a nickel phosphide black layer

The power handling capacity of nickel-iron-based flat tensioned foil shadow masks for flat faceplate cathode ray tubes (CRTs) is enhanced by providing the flat tensioned mask (FTM) with a thin surface layer which is enriched in nickel, which is converted to a nickel phosphide compound. The nickel phosphide surface layer is formed on the mask by immersing the mask in a first bath of a strong reducing acid followed by immersing the mask in a second strong reducing acid having an effective amount of hypophosphite ion. The thin surface layer of a nickel phosphide compound increases the FTM emissivity and reduces FTM color impurity at high electron beam energies. The nickel phosphide compound may be stabilized in the frit-lehr cycle during CRT assembly or in a separate heating operation.

BACKGROUND OF THE INVENTION 
This invention relates generally to flat faceplate cathode ray tubes, and 
more particularly to tubes of this type which have a tensioned foil shadow 
mask made from an alloy comprising nickel and iron. The invention also 
relates to a process for the manufacture of such masks, including 
contacting the mask with a strong reducing acid which dissolves iron 
faster than nickel to provide a nickel rich surface layer followed by 
contacting the shadow mask with a mixture of a strong reducing acid and 
hypophosphite salt to provide improved shadow mask emissivity for 
accommodating high electron beam energies. Also disclosed is a cathode ray 
tube assembly containing such a mask. 
Cathode ray tubes having flat faceplates and flat tensioned foil shadow 
masks are known to provide many advantages over conventional cathode ray 
tubes having a curved faceplate and a curved shadow mask. A chief 
advantage of a flat faceplate cathode ray tube with a tensioned mask is a 
greater electron beam power-handling capability, a capability which can 
provide greater picture brightness. The power-handling capability of tubes 
having the conventional curved mask is limited due to the thickness of the 
mask (5 to 7 mils), and the fact that it is not mounted under tension. As 
a result, the mask tends to expand or "dome" in picture areas of high 
brightness where the intensity of electron beam bombardment, and 
consequently the heat, is greatest. Color impurities result when the mask 
expands toward the faceplate and the beam-passing apertures in the mask 
move out of registration with their associated phosphor dots or lines on 
the faceplate. 
A tensioned mask when heated acts in a manner quite different from a 
curved, untensioned mask. For example, if the entire mask is heated 
uniformly, the mask expands and relaxes the tension. The mask remains 
planar and there is no doming and no distortion until the mask has 
expanded to the point that tension is completely lost. Just before all 
tension is lost, wrinkling may occur in the corners. When small areas of a 
tensioned foil mask are differentially heated, the heated areas expand and 
the unheated areas correspondingly contract, resulting in only small 
displacements within the plane of the mask. However, the mask remains 
planar and properly spaced from the faceplate and, consequently, any color 
impurities are unnoticeable. 
The mask must be supported in tension in order to maintain the mask in a 
planar state during operation of the cathode ray tube. The amount of 
tension required will depend upon how much the mask material expands upon 
heating during operation of the cathode ray tube. Materials with very low 
thermal coefficients of expansion need only a low tension. Generally, 
however, the tension should be as high as possible because the higher the 
tension, the greater the heat incurred, and the greater the electron beam 
current that can be handled. There is a limit to mask tension, however, as 
too great a tension can cause the mask to tear. 
The mask may be tensioned in accordance with known practices. A convenient 
method is to thermally expand the mask by means of heated platens applied 
to both sides of the mask. The expanded mask is then clamped in a fixture 
and, upon cooling, remains under tension. The mask may also be expanded by 
exposure to infrared radiation, by electrical resistance heating, or by 
stretching through the application of mechanical forces to its edges. 
PRIOR ART 
It is well known in the manufacture of standard color cathode ray tubes of 
the curved-mask, curved-screen type to heat-treat the masks prior to their 
being formed into a domed shape. Conventional (non-tensioned) masks are 
typically delivered to cathode ray tube manufacturers in a work-hardened 
state due to the multiple rolling operations which are performed on the 
steel to reduce it to the specified thickness, typically about 6 mils. In 
order that the masks may be stamped into a domed shape, they must be 
softened by use of an annealing heat treatment--typically to temperatures 
on the order of 700.degree.-800.degree. C. Annealing also enhances the 
magnetic coercivity of the masks, a desirable property from the standpoint 
of magnetic shielding of the electron beams. After stamping, and the 
consequent moderate work hardening of the mask which may result from the 
stamping operation, it is known in the prior art to again anneal the masks 
while in their domed shape to further enhance their magnetic shielding 
properties. 
Foils intended for use as tensioned masks are also delivered in a hardened 
state--in fact, much harder than standard masks in order to provide the 
very high tensile strength needed to sustain the necessary high tension 
levels; for example, 30,000 psi, or greater. The prior art annealing 
process, with its relatively high annealing temperatures, would be 
absolutely unacceptable if applied to flat tension masks, as any extensive 
softening or reduction of tensile strength of the mask resulting from the 
process would make the material unsuited for use as a tension mask. 
The disclosure of U.S. Pat. No. 4,210,843 to Avedani, of common ownership 
herewith, sets forth an improved method of making a conventional color 
cathode ray tube shadow mask; that is, a curved shadow mask having a 
thickness of about 6 mils, and designed for use with a correlatively 
curved faceplate. The method comprises providing a plurality of mask 
blanks composed of an interstitial-free steel, each with a pattern of 
apertures photo-etched therein, which blanks have been cut from a foil of 
steel, precision cold-rolled to a full hard condition, and with a 
thickness of from 6 to 8 mils. A stack of blanks is subjected to a limited 
annealing operation carried out at a relatively low maximum temperature, 
and for a relatively brief period sufficient only to achieve 
recrystallization of the material without causing significant grain 
growth. Each blank is clamped and drawn to form a dished mask without the 
imposition of vibration or roller leveling operations, and thus avoids 
undesired creasing, roller marking, denting, tearing or work-hardening of 
the blank normally associated with these operations. The end-product mask, 
due to the use of the interstitial-free steel material, has an aperture 
pattern of improved definition as a result of more uniform stretching of 
the mask blank. The annealing operation has little effect on the magnetic 
properties of this type of steel, and the coercivity of the material, 
after forming, is about 2.0 oersteds. 
A foil shadow mask is maintained under high tension within the cathode ray 
tube, and the mask is subjected to predetermined relatively high 
temperatures during tube manufacture. A process for pre-treating a metal 
foil shadow mask is disclosed in referenced co-pending application Ser. 
No. 948,212, now U.S. Pat. No. 4,656,702; of common ownership herewith. 
The process comprises preheating the shadow mask in a predetermined cycle 
of temperature and time effective to minimize subsequent permanent 
dimensional changes in the mask that occur when it is subjected to 
predetermined relatively high temperatures, but ineffective to 
significantly reduce the tensile strength of the mask by annealing. 
Earlier foil mask materials have limitations in terms of the desired 
combination of mechanical and magnetic properties described herein. One 
material used for mask applications in flat faceplate cathode ray tubes 
has been aluminum-killed (AK), AISI 1005 cold-rolled capped steel, 
generally referred to as "AK steel." AK steel has a composition of 0.04 
percent silicon, 0.16 percent manganese, 0.028 percent carbon, 0.020 
percent phosphorus, 0.018 percent sulfur, and 0.04 percent aluminum, with 
the balance iron and incidental impurities. Throughout the specification 
and claims, all percentages and parts are considered weight-percentages 
and parts by weight, unless otherwise indicated.) Invar, which has a 
nominal composition of 36 percent nickel, balance iron, has also been 
suggested as a possible material for tensioned foil shadow masks. Invar 
however has a thermal coefficient of expansion far lower than that of the 
glass commonly used in cathode ray tube faceplates and so is considered 
generally unacceptable. 
The material of the masks treated according to the Ser. No. 948,212 
disclosure is the aforedescribed AK steel. AK steel, while it can be 
formed into a fairly acceptable foil shadow mask, is deficient in certain 
important properties. For example, the yield strength of AK steel foil one 
mil thick is typically in the range of 75-80 ksi. This makes it only 
marginally acceptable from a strength standpoint. More importantly, AK 
steel has a permeability that is much lower than desired, for example, 
5,000 in a 1 mil foil. Since the ability of a material to carry magnetic 
flux decreases with decreasing cross-section, cathode ray tubes having 
masks made of AK steel thinner than about 1 mil may require both internal 
and external magnetic shielding. With internal shielding only, the beam 
landing misregistration due to the earth's magnetic field, i.e., the 
change in beam landing position upon reversal of the axial field 
component, is typically 1.5 mils, which is much greater than the maximum 
of about 1 mil that is generally considered tolerable. 
In addition, AK steel is metallurgically dirty, having inclusions, defects 
and dislocations which interfere with both the foil rolling process and 
the photo resist etching of the apertures in the foil resulting in higher 
scrap rates and consequently lower yields. 
Another significant disadvantage of an AK steel tensioned foil shadow mask 
is the fact that as the tension applied is increased, the permeability 
decreases and the coercivity increases. Translated into picture 
performance, this means that as the tension of the AK foil shadow mask is 
increased in order to permit increased beam current and, therefore, 
greater picture brightness, its ability to shield the electron beams from 
the earth's magnetic field deteriorates, resulting in increased beam 
misregistration. 
U.S. Pat. No. 3,867,207 to Decker, et al. describes a method for blackening 
steel components of a cathode ray tube, such as the aperture mask, by 
immersing the component in an electroless nickel or cobalt plating bath to 
provide a surface layer of nickel or cobalt on the component and, after 
subsequent rinses, immersing the component in a strong oxidizing acid and 
firing the component in air at about 450.degree. C. to form a black, 
complex nickel or cobalt phosphide compound on the surface of the 
component. 
The present invention overcomes the aforementioned limitations of the prior 
art by providing a tensioned foil shadow mask having a thin surface layer 
of a blackened nickel compound which substantially increases the 
emissivity of the shadow mask and retards its rate of temperature increase 
thus reducing color purity loss at high electron beam energies and which 
is provided by a simple process. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved flat tensioned foil shadow mask for use in a color cathode ray 
tube having a flat faceplate. 
Another object of the present invention is to provide an improved process 
for fabricating a cathode ray tube incorporating a flat tensioned foil 
shadow mask. 
A further object of the present invention is to provide a flat tensioned 
foil shadow mask having improved mechanical and emissivity properties. 
Yet another object of the present invention is to provide for the treatment 
of prior art flat tensioned foil shadow masks so as to substantially 
increase their thermal radiation characteristics and current handling 
capabilities.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To facilitate understanding of the process and material according to the 
invention and their relation to the manufacture of a color cathode ray 
tube having a tensioned foil shadow mask, a brief description of a tube of 
this type and its major components is offered in the following paragraphs. 
A color cathode ray tube 20 having a tensioned foil shadow mask is depicted 
in FIG. 1 The faceplate assembly 22 essentially comprises a flat faceplate 
and a tensioned flat foil shadow mask mounted adjacent thereto. Faceplate 
24, indicated as being rectangular, is shown as having on its inner 
surface 26 a centrally located phosphor screen 28 depicted 
diagrammatically as having a pattern of phosphors thereon. A film of 
aluminum 30 is indicated as covering the pattern of phosphors. A funnel 4 
is represented as being attached to faceplate assembly 22 at their 
interfaces 35; the funnel sealing surface 36 of faceplate 24 is indicated 
as being peripheral to screen 28. A frame-like shadow mask support 
structure 48 is indicated as being located on opposed sides of the screen 
between funnel sealing surface 36 and screen 28, and mounted adjacent to 
faceplate 24. Support structure 48 provides a surface for receiving and 
mounting in tension a metal foil shadow mask 50 a Q-distance away from the 
screen 28. The pattern of phosphors corresponds to the pattern of 
apertures in mask 50. The apertures depicted are greatly exaggerated for 
purposes of illustration; in a high-resolution color tube for example, the 
mask has as many as 750,000 such apertures, with aperture diameter being 
on the average about 5 mils. As is well-known in the art, the foil shadow 
mask acts as a color-selection electrode, or "parallax barrier" which 
ensures that each of the beamlets formed by the three beams lands only on 
its assigned phosphor deposits on the screen. 
The anterior-posterior axis of tube 20 is indicated by reference number 56. 
A magnetic shield 58 is shown as being enclosed within funnel 34. High 
voltage for tube operation is indicated as being applied to a conductive 
coating 60 o the inner surface of funnel 34 by way of an anode button 62 
connected in turn to a high-voltage conductor 64. 
The neck 66 of tube 20 is represented as enclosing an in-line electron gun 
68 depicted as providing three discrete in-line electron beams 70, 72 and 
74 for exciting respective red-light-emitting, green-light-emitting, and 
blue-light-emitting phosphor elements deposited on screen 28. Yoke 76 
receives scanning signals and provides for the scanning of beams 70, 72 
and 74 across screen 28. An electrical conductor 78 is located in an 
opening in shield 58 and is in contact with conductive coating 60 to 
provide a high-voltage connection between the coating 60, the screen 28, 
and shadow mask 50. This means of electrical conduction is described and 
claimed in referent co-pending application Ser. No. 060,142 of common 
ownership herewith. 
Two of the major components, designated as being "in-process," are depicted 
and described as follows. One is a shadow mask indicated diagrammatically 
in FIG. 2. In-process shadow mask 86 includes a central area 104 of 
apertures corresponding to the pattern of phosphors that is photodeposited 
on the screen of the faceplate by using the mask as an optical stencil. 
Center field 104 is indicated as being surrounded by an unperforated 
section 106, the periphery of which is engaged by a tensing frame during 
the mask tensing and clamping process, and which is removed in a later 
procedure. 
An in-process faceplate 108 is depicted diagrammatically in FIG. 3 as 
having on its inner surface 110 a centrally located screening area 112 for 
receiving a predetermined phosphor pattern in an ensuing operation. A 
funnel sealing surface 113 as indicated as being peripheral to screen 112. 
A frame-like shadow mask support structure 114 is depicted as being 
secured on opposed sides of screen 112; the structure provides a surface 
115 for receiving and mounting a foil shadow mask under tension a 
Q-distance from the screen. 
A process according to the invention essentially comprises providing an 
apertured foil shadow mask 86 comprised of a nickel-iron alloy, and 
securing the mask 86 to the mask support structure 114 of the faceplate 
108 while under tension, and in registration with the phosphor screen. The 
process is further characterized by first subjecting the mask 86 to 
contact with a strong reducing acid which dissolves iron faster than 
nickel to provide a nickel-rich surface layer followed by blackening the 
surface layer by contacting the mask with a mixture of a strong reducing 
acid and a hypophosphite salt to provide a blackened surface layer of 
nickel and molybdenum phosphides. 
A class of nickel-iron alloys, desirably containing minor additions of 
certain alloying agents, when heat-treated and cooled under controlled 
conditions, yield a material which, when fabricated into a thin foil, has 
mechanical and magnetic properties not found in known alloys that makes 
them uniquely suited for use as tensioned foil shadow masks. 
With regard to the alloy composition, a nickel-iron alloy is provided 
comprising between about 30 and 85 weight-percent of nickel, between about 
0 and 5 weight-percent of molybdenum, between 0 and 2 weight-percent of 
one or more of vanadium, titanium, hafnium, and niobium, with the balance 
iron and incidental impurities; e.g., carbon, chromium, silicon, sulfur, 
copper and manganese. Typically, the incidental impurities combined do not 
exceed 1.0 percent. Preferably and also according to the invention, the 
alloy may comprise between about 75 and 85 weight-percent of nickel, 
between about 3 and 5 weight-percent of molybdenum, with the balance iron 
and incidental impurities. Most preferably, the alloy may comprise about 
80 weight-percent nickel, about 4 weight-percent molybdenum, with the 
balance iron and incidental impurities. These examples of foil mask 
materials are generally referred to as molypermalloys. 
MASK HEAT TREATMENT DURING FRIT CYCLE 
The heat treatment of the masks described in the following paragraphs 
closely approximates the processing steps in frit sealing cathode ray 
tube, and the sealing of the funnel and faceplate in the manufacturing 
process. 
As indicated in FIG. 3 a shadow mask support structure 114 is secured on 
the inner surface 110 of faceplate 108 between the peripheral sealing 
area, noted as being the funnel sealing surface 113, and the screening 
area 112. The mask support structure 114 provides a surface 115 for 
receiving and supporting a foil shadow mask in tension. The mask support 
structure 114 may comprise, by way of example, a stainless steel metal 
alloy according to the disclosure of referent co-pending application Ser. 
No. 832,556, now U.S. Pat. No. 4,695,761, or alternately, a ceramic 
structure according to the disclosure of referent co-pending application 
Ser. No. 866,030now U.S. Pat. No. 4,737,681. Attachment of the support 
structure is preferably by means of a devitrifying frit. 
The alloy according to the invention is formed into a foil having a 
thickness of about 0.001 inch or less. A central area 112 of the foil is 
apertured to form a foil mask 108 consonant in dimensions with the 
screening area 112 for color selection. Aperturing of the mask can be 
accomplished by a photo-etching process in which a light-sensitive resist 
is applied to the foil. The resist is hardened by exposure to light except 
in those areas where apertures are defined. The exposed metal defining 
The foil mask is then tensed in a tensing frame to a tension of at least 
about 25 Newton/centimeters. A tensing frame suitable for use in tensing a 
mask foil, and the process for tensing, is fully described and claimed in 
referent co-pending application Ser. No. 051,896, now U.S. Pat. No. 
4,790,786 of common ownership herewith In essence, the foil may be 
expanded by enclosing it between two platens heated to 360.degree. C. for 
one minute, clamped in the tensing frame, and air cooling it to provide a 
tensioned foil having a greater length and width than the faceplate to 
which it will be secured. A pattern of red-light-emitting, 
green-light-emitting, and blue-light-emitting phosphor deposits are 
sequentially photoscreened on screening area 112. The photoscreening 
process includes repetitively registering the foil to the phosphor 
screening area by registering the tensing frame with the faceplate. The 
means of registration is fully set forth in the referent '896 application. 
The foil comprising the mask 86 is secured to the mask support structure 
114, with the apertures of the mask in registration with the pattern of 
phosphor deposits on screening area 112. The means for securing the mask 
to the mask support structure may be by welding with a laser beam, with 
the excess mask material removed by the same beam, as fully described and 
claimed in referent co-pending application Ser. No. 058,095, now U.S. Pat. 
No. 4,828,523 of common ownership herewith. Inasmuch as the faceplate 108 
and tensioned foil shadow mask 86 are rigidly interconnected by their 
mutual attachment to the mask support structure, the thermal coefficient 
of expansion of the alloy foil must approximate that of the faceplate, 
which is typically a glass having a coefficient of expansion of between 
about 12.times.10.sup.-6 in/in/.degree. C. This is necessary due to the 
relatively high temperatures to which the faceplate and mask are subjected 
during the cathode ray tube manufacturing process. A coefficient of 
expansion somewhat greater than that of the faceplate can be tolerated, 
but a coefficient of expansion substantially less than that of the 
faceplate is to be avoided as this may lead to mask failure during the 
manufacturing process. 
FIGS. 4 and 5 depict the use of a funnel referencing and fritting fixture 
186 for mating of a faceplate 108 with a funnel 188 to form a 
faceplate-funnel assembly. Faceplate 108 is indicated as being installed 
face down on the surface 190 of fixture 186. Funnel 188 is depicted as 
being positioned thereon and in contact with funnel sealing surface 113, 
noted as being peripheral to screening area 112 on which is deposited a 
pattern of phosphors 187 as a result of the preceding screening operation. 
With reference to FIG. 4, three posts 192, 193 and 194 are indicated as 
providing for alignment of the funnel and faceplate. FIG. 5 depicts 
details of the interface between post 194, the faceplate 108, and funnel 
188. Flat 117c on faceplate 108 is shown as being in alignment with 
reference area "c" on funnel 188. Shadow mask 86, noted as being in 
tension, is depicted as being mounted on shadow mask support structure 
114; this configuration of a shadow mask support structure is the subject 
of U.S. Pat. No. 4,686,416 of common ownership herewith. 
Post 194 is shown as having two reference points 196 and 198 for locating 
the funnel 188 with reference to the faceplate 108. The reference points 
preferably comprise buttons of carbon as they must be immune to the 
effects of the elevated oven temperature incurred during the frit cycle. 
A devitrifiable frit in paste form is applied to the peripheral sealing 
area of the faceplate 108, noted as being funnel sealing area 113, for 
receiving funnel 188. The faceplate 108 is then mated with the funnel 188 
to form a faceplate-funnel assembly. The frit, which is indicated by 
reference No. 200 in FIG. 5, may for example, comprise frit No. CV-130, 
manufactured by Owens-Illinois, Inc. of Toledo, Ohio. 
The faceplate-funnel assembly is then heated to a temperature effective to 
devitrify the frit and permanently attach the funnel to the faceplate, 
after which the assembly is cooled. The process of fusing of the funnel to 
the faceplate is generally carried out under conditions referred to as the 
frit cycle. In a typical frit cycle, the faceplate, to which the tensioned 
foil mask is adhered, and funnel are slowly heated to 435.degree. C., then 
cooled to room temperature or slightly thereabove over a period of 3-31/2 
hours. The foil must be cooled to the temperature at which the alloy is 
substantially recrystallized at a cooling rate of less than about 
5.degree. C. per minute, preferably less than about 3.degree. C. per 
minute, and most desirably at a rate of between about 2.degree. C. and 
about 3.degree. C.. per minute. The heating of the assembly and the foil 
is effective to blacken, or oxidize, a thin surface layer of a nickel 
compound deposited on the foil mask in accordance with the present 
invention as described in detail below. 
Referring to FIG. 6, there is shown a simplified flow chart for a procedure 
for treating a foil mask in accordance with the principles of the present 
invention. The first step at block 210 in the process involves degreasing 
of the foil tension mask (FTM). The FTM may be degreased by dipping it 
into a hot alkaline solution for on the order of 10 minutes. The next step 
at block 212 is the ultrasonic cleaning of the degreased FTM. The 
degreasing and ultrasonic cleaning procedures remove contaminants from the 
surface of the FTM which decrease the effectiveness of the subsequent 
steps. 
At step 214, the FTM is immersed in a strong reducing acid bath. Since iron 
has an electrochemical potential of -440 mV as compared to the 
electrochemical potential of -250 mV for nickel, the iron can be 
selectively removed from the surface of the FTM to provide a nickel 
enriched surface layer. The preferred strong reducing acid is preferably 
concentrated hydrochloric acid having from about 38% to about 50% HCL. The 
FTM preferably remains in contact with the strong reducing acid for a 
period of from about 1 to about 10 minutes while the acid is maintained at 
a temperature of from about 25.degree. to about 75.degree. C. After 
treatment with the strong reducing acid a nickel enriched surface layer 
from about 0.01 to about 0.1 mil. thick is formed which has an average 
nickel content of from about 75% to about 96% with the balance being 
primarily molybdenum. After being treated with a strong reducing acid, the 
FTM is cleaned with water at step 216. 
At step 218, the FTM is immersed into a strong reducing acid having a 
substantial level of hypophosphite ions. A suitable reducing acid is 
concentrated hydrochloric acid having from about 38% to about 50% HCL. The 
reducing acid is mixed with an effective amount of a soluble hypophosphite 
salt, such as sodium hypophosphite or potassium hypophosphite. Any 
hypophosphite salt which has a solubility of at least about 75 grams in 
25.degree. C. water can be used. Preferably, the hypophosphite salt is 
added to the strong reducing acid at a level from about 50 grams to about 
250 grams per liter. The FTM preferably remains in contact with the 
mixture of acid and hypophosphite salt for a period of from about 5 to 
about 30 minutes while the acid is maintained at a temperature of from 
about 25.degree. to about 85.degree. C. After treatment with the mixture 
of acid and hypophosphite salt, the nickel in the nickel enriched layer 
(and any molybdenum which may be present) are converted to complex nickel 
or molybdenum phosphide compounds, such as Ni.sub.3 P.sub.2, which are 
black in color. After acid treatment, the FTM is then cleaned in tap water 
at step 220 to remove any excess acid solution, followed by air blow 
drying of the iron coated FTM at step 222. The FTM may then be stored 
until required for use as a shadow mask in the manufacture of flat 
faceplate cathode ray tubes. 
It should be understood that discrete steps 214 and 218 are not required. 
That is, the formation of the nickel enriched surface layer, as depicted 
in step 214, and the conversion of the nickel (and any molybdenum which 
may be present) to complex phosphide compounds, as depicted in step 218, 
can be accomplished in a single step by immersing the FTM into a strong 
reducing acid having a substantial level of hypophosphite ions. This 
alternate method for forming the blackened surface layer of nickel and 
molybdenum phosphide compounds is shown by the dashed line route in FIG. 
6. 
After treatment in accordance with the invention to provide a blackened 
surface layer of complex nickel phosphide compounds, the complex nickel 
phosphide compound surface layer of the FTM may be stabilized at step 224 
by heat treatment prior to use or such stabilization may take place during 
the first cycle used in the manufacture of the cathode ray tube. In this 
connection, the complex nickel phosphide compound formed on the surface of 
the FTM is easily abraded and care must be exercised in handling the FTM 
prior to the stabilization heat treatment. 
In one example of the present invention, the foil mask is heated to a 
temperature of 435.degree. C. for 55 minutes to effect stabilization. The 
stabilized and blackened Ni.sub.3 P.sub.2 surface layer substantially 
increases the heat dissipating capability of the foil mask and retards the 
rate of temperature increase of the mask upon bombardment by electron 
beams by efficiently and effectively radiating away heat buildup so as to 
minimize its thermal distortion. Heating of the foil mask may be 
accomplished either before or after the foil mask is secured to the 
faceplate of a cathode ray tube. In the latter case, foil mask heating may 
be accomplished during a conventional frit-lehr cycle as described above. 
In heating the foil mask during the frit-lehr cycle, the assembled 
faceplate and funnel together with the foil mask was positioned on a belt 
moving at a speed of 9 inches per minute and was passed through an open 
furnace and exposed to a peak temperature of 435.degree. C. for 55 
minutes. Subjecting the foil mask to temperatures in the range 400.degree. 
C. to 600.degree. C. for a period ranging from 1/2 hour to 1 hour has also 
resulted in stabilizing of the foil mask and a substantial increase in its 
emissivity. 
Referring to Table I, there is shown the results of emissivity measurements 
of foil masks. The emittance data was taken at 40.degree. C. using a 
typical IR spectrometer. The upper row of data is for a foil mask treated 
in accordance with the invention to provide a surface layer of Ni.sub.3 
P.sub.2. Measurements were made in the infrared spectrum at various 
wavelengths as indicated in the table. 
TABLE I 
______________________________________ 
Emissivity of Various Metal Masks 
After Lehr Cycle 
Emissivity 
Material 5 .mu.M 8 .mu.M 14 .mu.M 
______________________________________ 
Moly-Permalloy* 
0.878 0.782 0.306 
Ni.sub.3 P.sub.2 Surface Layer 
AK Steel-Blackened 
0.757 0.645 0.528 
______________________________________ 
*Moly-Permalloy is the tradename for nickel alloy having 80% nickel, 4% 
molybdenum, 20% iron, balance impurities. 
The lower row of data represents measured thermal emissivity for uncoated 
AK steel shadow masks after blackening by oxidizing heat treatment. From 
the measured data it can be seen that the emissivity of the molypermalloy 
masks treated in accordance with the invention closely approximates the 
thermal emissivity of prior art AK steel masks. 
There has thus been shown a nickel-iron-based flat tensioned foil shadow 
mask for use in a color cathode ray tube having a blackened, or oxidized, 
thin surface layer of a complex nickel phosphite compound which 
substantially increases the emissivity of the shadow mask and, by 
retarding its rate of temperature increase and reducing shadow mask 
doming, permits the shadow mask to operate at high electron beam energies 
and is economically and simply manufactured. More energetic electrons 
allow for increased brightness of the video image visible on the faceplate 
of the cathode ray tube. The thin surface layer of a complex nickel 
phosphide compound is formed on the flat tensioned foil shadow mask by 
subjecting the foil to successive baths of a strong reducing acid and a 
strong reducing acid having an effective level of hypophosphite in using a 
procedure readily adapted for large scale, commercial fabrication of 
cathode ray tubes with flat tensioned foil shadow masks. The thin surface 
layer of a complex nickel phosphide compound is then stabilized, either 
during frit sealing of the cathode ray tube or by subjecting the shadow 
mask to high temperature in a separate step. 
While particular embodiments of the present invention have been shown and 
described, it will be obvious to those skilled in the art that changes and 
modifications may be made without departing from the invention in its 
broader aspects. Therefore, the aim in the appended claims is to cover all 
such changes and modifications as fall within the true spirit and scope of 
the invention. The matter set forth in the foregoing description and 
accompanying drawings is offered by way of illustration only and not as a 
limitation. The actual scope of the invention is intended to be defined in 
the following claims when viewed in their proper perspective based on the 
prior art.