Substrate assembly for a luminescent display panel having fired liquid gold layers for segmented display electrodes

A substrate assembly for a fluorescent or phosphorescent display panel comprises a layer of fired liquid gold, preferably three microns or less thick, for each of segmented display electrodes. The layer is in direct contact with an insulator substrate and with a mass of a luminescent material and may be an integral part of a lead for the electrode or electrodes. Alternatively, the layer may be formed on a resistive layer comprising powder of ruthenium (IV) oxide and formed, in turn, on the substrate with a conductive layer interposed for providing an electric connection to the lead.

BACKGROUND OF THE INVENTION 
This invention relates to a substrate assembly for a fluorescent or 
phosphorescent display panel. 
A fluorescent or phosphorescent display panel, namely, a luminescent 
display panel as generally referred to herein, comprises a substrate 
assembly comprising, in turn, a substrate of an electrically insulating 
material, a plurality of segmented electrodes on the substrate, masses of 
a luminescent material on the segmented electrodes, respectively, and a 
plurality of electroconductive leads for the segmented electrodes also on 
the substrate. Each segmented electrode is generally formed in an 
indentation formed in the substrate in a shape corresponding with the 
luminescent mass disposed therein and on the segmented electrode so as not 
to protrude outwardly of the general surface of the substrate. The 
expression "on the substrate" should therefore be understood to mean that 
the electrodes and luminescent masses do not necessarily protrude from the 
general substrate surface. Each segmented electrode and the luminescent 
mass placed thereon form a display electrode. The substrate may comprise a 
plurality of substrate layers. 
It has been the practice to form the segmented electrodes and the 
electroconductive leads by firing prints on the substrate of silver or 
silver-palladium paste at about 600.degree. C, particularly when the 
substrate is made of glass. The segmented electrodes and conductive leads 
are consequently about ten microns thick. The segmented electrodes thus 
formed are incapable of making the luminescent masses luminesce to their 
fullest brightness. A layer of graphite is therefore interposed between 
each segmented electrode and the luminescent mass as will later be 
illustrated with reference to one of the figures of the accompanying 
drawing. The graphite layer, however, does not adhere tenaciously to the 
underlying segmented electrode. The adhesion is enhanced by addition to 
the graphite of a glassy adhesive although brightness is somewhat reduced 
thereby, especially when the adhesive comprises lead glass that is 
excellent insofar as adhesion is concerned. The graphite layer is further 
objectionable because it comes off together with the luminescent mass when 
it is necessary to remove a luminescent mass that is inadvertently wrongly 
formed on the graphite layer. In addition, the thickness of about ten 
microns is undersiredly thick when a layer of an insulating material 
should be deposited on at least portions of the conductive leads. 
The segmented electrodes and the conductive leads have alternatively been 
formed of tungsten or molybdenum by resorting to metallization techniques 
specifically when the substrate is made of ceramics. During metallization, 
the tungsten or molybdenum is inevitably activated in a hydrogen 
atmosphere to be readily contaminated either during storage of the 
substrates for subsequent use or during manufacture of the display panels. 
It is therefore necessary to plate the metallized electrodes and leads 
with gold as will also be described with reference to the above-mentioned 
one figure. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a substrate 
assembly for a luminescent display panel, which comprises no graphite 
layer and may not comprise a plated gold layer. 
It is another object of this invention to provide a substrate assembly of 
the type described, capable of making the display panel give a brightest 
possible display. 
In is still another object of this invention to provide a substrate 
assembly of the type described, wherein the thickness of segmented 
electrodes and electroconductive leads is appreciably thinner than the 
conventional one. 
It is yet another object of this invention to provide a substrate assembly 
of the type described, with which it is possible to reduce loss of the 
assemblies. 
A substrate assembly for a luminescent display panel to which this 
invention is applicable comprises an insulator substrate, segmented 
electrodes, masses of a luminescent material, and electroconductive leads 
as set forth at the beginning of the instant specification. In accordance 
with this invention, each of the segmented electrodes comprises an 
electrode layer of fired liquid gold. Most preferably, each electrode 
layer is in direct contact with a luminescent mass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a luminescent display panel comprises a substrate 
assembly 20 comprising, in turn, a substrate 21 of an electrically 
insulating material, such as glass, alumina, for sterite, or other 
ceramics, a plurality of display electrodes 22 on the substrate 21, and a 
plurality of electroconductive leads 23 also on the substrate 21. In the 
example being illustrated, the display electrodes 22 are arranged in a 
substantially figure-of-eight configuration and in a plurality of groups, 
each for a selected one of the numerals 0 to 9. The substrate assembly 20 
further comprises a pair of cathode supports 25 connected to two of the 
conductive leads 23, respectively, and a plurality of pairs of grid 
supports 26, each pair being for a group of the display electrodes 22 and 
connected to one of the conductive leads 23. A grid 28 is attached to each 
pair of the grid supports 26 to cover a relevant one of the display 
electrode groups. At least one hot cahtode 29 is extended above the grids 
28 and attached at both ends to the cathode supports 25. The substrate 
assembly 20 with the grids 28 and cathode 29 attached thereto is 
hermetically sealed to a glass cover plate 30 with the display electrodes 
22, the grids 28, and the cathode 29 disposed in a hermetically sealed 
space. 
As shown, the conductive leads 23 are extended outwardly of the sealed 
space to serve as external leads 31 for supplying a heater voltage and a 
cathode potential to the cathode 29 and for selectively supplying a grid 
voltage to the grids 28 and also the display electrodes 22 with a 
potential that is positive with respect to the cathode potential. As is 
known in the art, the substrate assembly 20 may be enclosed with a vacuum 
envelope (not shown) together with the cathode 29 and grids 28 supported 
by the envelope. 
Referring to FIG. 2, each display electrode 22 of a conventional substrate 
assembly 20 comprises a segmented electrode 35 provided by a part of the 
electroconductive lead 23 therefor, a graphite or a plated gold layer 36 
mentioned in the preamble of the instant specification, and a mass of a 
luminescent material 37 deposited on the graphite or plated gold layer 36. 
A sheet of glass 39 overlying the substrate 21 is the layer of an 
insulating material mentioned also in the preamble and may be formed by 
printing a glassy material and subsequently firing the print at about 
600.degree. C before formation of the graphite or plated gold layer 36. 
The above-mentioned substrate 21 and the overlying insulator sheet 39 may 
be deemed as a two-layer composite substrate although this is not 
necessarily the case. The conventional substrate assembly 20 has various 
defects pointed out hereinabove. Furthermore, the graphite layer 36 is not 
sufficiently adherent to the overlying sheet 39 unless either an 
objectionably large amount of the glassy adhesive is used to adversely 
affect the brightness-enhancing capability of the graphite layer 36 or a 
glassy adhesive comprising lead glass is used to harm the electron 
emissivity of the hot cathode 29 gradually during use of the display panel 
and to thereby again reduce the brightness of the display. 
Referring now to FIG. 3, a substrate assembly 20 according to a first 
embodiment of the present invention comprises a segmented electrode 35 
formed solely of an electrode layer of fired liquid gold in direct contact 
with the substrate 21. As shown, the fired liquid gold layer is formed 
partly on an electroconductive lead 23 for the segmented electrode 35. A 
mass of a luminescent material 37 is deposited directly on the fired 
liquid gold layer. The conductive leads 23 are formed in a conventional 
manner by firing prints of silver or silver-palladium paste at about 
600.degree. for both a glass and a ceramic substrate 21. 
As described by Kenneth Shaw in a book entitled "Ceramic Colours and 
Pottery Decoration" and published 1962 by MacLaren & Sons Ltd., London, 
the United Kingdom, pages 69 to 74, "liquid gold" per se is known in the 
art of pottery and comprises an organic compound of gold and a vehicle 
therefor. As detailed in the book cited, gold resinate for use as an 
organic gold compound is prepared by producing gold chloride at first by 
dissolving gold in aqua regia. In the meantime, sulphur balsam is produced 
by subjecting sulphur, turpentine oil, and turpentine to reaction. The 
gold chloride and sulphur balsam are dissolved in chloroform and 
neutralized with sodium carbonate. Afer filtration, the filtrate is 
condensed. Methanol is added to the condensed filtrate to produce 
precipitates of gold resinate, which is separated from the mother liquor 
by filtration, rinsed, and subsequently dried. The gold resinate is mixed 
with a vehicle therefor together with resinates of bismuth, chromium, and 
rhodium to produce liquid gold. the vehicle may comprise ethylcellulose or 
nitrocellulose. In place of gold resinate, use may be made of any other 
organic gold compound miscible with a vehicle, such as gold mercaptide 
prepared by subjecting gold chloride and thioborneol to reaction with 
methanol. Firing is carried out at about 600.degree. C. Bismuth is added 
in order to strengthen the adhesion of the fired liquid gold to the 
substrate 21. Chromium is added to avoid aggregation of gold particles in 
the fired liquid gold layer. Rhodium is added to prevent the gold particle 
aggregation and to provide a fine luster to the electrode layer. 
Referring to FIG. 4, a substrate assembly 20 according to a second 
embodiment of this invention comprises a segmented electrode 35 formed of 
an electrode layer of fired liquid gold alone as an integral part of the 
associated electroconductive lead 23. It will readily be understood that 
the segmented electrodes 35 and the conductive leads 23 are simultaneously 
formed directly on the substrate 21 by coating with liquid gold those 
surface areas of the substrate 21 which correspond in shape to the 
electrodes 35 and leads 23 and thereafter firing the liquid gold. 
As will be appreciated from the description of the first and second 
embodiments of this invention, it is possible in accordance with this 
invention to manufacture stable substrate assemblies 20 with simple 
processes by the use of liquid gold known and used per se in the art of 
pottery. It has additionally and quite unexpectedly confirmed that the 
fired liquid gold layers tenaciously adhere to both the substrate 21 and, 
if any, the overlying insulator sheet 39 to make it possible to remove 
only the masses of luminescent material 37 if desired, that the 
luminescent masses 37 provide a brightest possible display without the use 
of the conventionally interposed graphite layers 36 (FIG. 2), that the 
fired liquid gold layers are less expensive and formed in a cleaner state 
than the conventional plated gold layers 36 (FIG. 2), or that the fired 
liquid gold layers are only about three microns or less thick and can be 
made even thinner than about one micron. 
The substrate assemblies 20 according to the second emboiment are preferred 
to those according to the first embodiment in that migration of silver 
which is still unavoidable in a luminescent display panel including a 
substrate assembly 20 according to the first embodiment is avoided to 
prevent the external leads 31 from being shorted, even in extreme cases 
where a luminescent display panel is put into operation in an extremely 
hot and humid atmosphere. 
Referring now to FIG. 5, a substrate assembly 20 according to a third 
embodiment of this invention comprises a segmented electrode 35 
comprising, in turn, an electrode layer 41 of fired liquid gold, an 
electroconductive base layer 42 directly on the substrate 21, and a 
resistive layer 43 between the electrode and base layers 41 and 42. The 
electrode layer 41 is in direct contact with a mass of a luminescent 
material 37 (not shown for simplicity of illustration). The base layer 42 
is in electrical contact with the associated electroconductive lead 23 and 
may be an integral part of the latter. The resistive layer 43 comprises 
powder of ruthenium(IV) oxide. The base layer 4 may be formed of silver or 
silver-palladium paste in a conventional manner. Alternatively, the base 
layer 42 and the associated conductive lead 23 may be an integral layer of 
fired liquid gold, In the example being illustrated, an insulator sheet 39 
is deposited on the substrate 21 with holes formed therethrough at the 
positions of the segmented electrodes 35 by firing glass in a conventional 
manner. The resistive layers 43 are subsequently formed by filling each 
hole with a mixture of ruthenium (IV) oxide powder and a binder or flux, 
such as lead borosilicate glass, and firing the mixture at about 
600.degree. C. With a substrate assembly 20 according to the third 
embodiment, it is possible to raise the brightness of the display and to 
remove, when necessary, only the masses of luminescent material 37 without 
loss of the remaining parts of the substrate assembly 20. 
Brightness and luminous efficiency of masses 37 of a luminescent material 
consisting essentially of zinc sulfide were measured with a fluorescent 
display panel having segmented electrodes 35 comprising fired liquid gold 
layers in direct contact with the zinc sulfide masses 37 in accordance 
with this invention and with a conventional fluorescent display panel 
having graphite layers 36 in the segmented electrodes 35. The results are 
as follows: 
______________________________________ 
Fired 
liquid gold layer 
Graphite layer 
______________________________________ 
Heater voltage (V.sub.ac) 
3.4 3.4 
Display electrode 
current (mA) 1.1 1.0 
Brightness (cd . m.sup.-2) 
89.3 74.0 
Luminous efficiency 
(cd . m.sup.-2 . mA.sup.-1) 
80.9 74.0 
______________________________________ 
While a few preferred embodiments of this invention have thus far been 
described, it should be understood that a sheet of an insulating material 
39 is not an essential element of a substrate assembly 20. Gold powder may 
be added to the liquid gold to raise the gold content of the fired liquid 
gold electrode layers. Although unnecessary, each of the fired liquid gold 
layers may be plated with gold in order to provide plated gold layers 
between the masses of luminescent material 37 and the fired liquid gold 
layers as is the case with the plated gold layers 36 in a conventional 
substrate assembly 20. When the luminescent masses 37 are disposed in 
indents formed in the substrate 21, the fired liquid gold layers may be 
extended along the side walls of the indents. 
Finally referring to FIGS. 6 to 11, it should be understood at first that 
the fired liquid gold electrode layers comprise very fine particles of 
gold because they can be made thinner than about one micron. Although the 
sizes of the gold particles are not yet determined, gold resinate is shown 
in FIGS. 6 and 7 by electron micrographs with magnifications of 2,000 and 
10,000, respectively. Electron micrographs shown in FIGS. 8 and 9 with 
magnifications of 2,000 and 10,000, respectively, clearly proove that the 
electrode layers made from the gold resinate shown in FIGS. 6 and 7 
comprise very fine gold particles. Furthermore, gold powder and several 
frit particles for preparing conventional gold paste and a fired layer of 
the gold paste are shown in FIGS. 10 and 11, respectively, both with a 
magnification of 2,000, for reference. All micrographs were taken by a 
scanning electron microscope.