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2,409,574
Patented Oct. 15', 1946
UNITED’ STATES PATENT OFFICE
2,409,574
LUMINESCENT MATERIAL AND 'METHOD OF
MANUFACTURE
Humboldt W. Leverenz, South Orange, N. .L, as
signor to Radio Corporation of America, a cor
poration of Delaware
‘
No Drawing. Application June 11, 1942,
Serial No. 446,626
9 Claims. (Cl. 252-30113)
.
1
.
,
dependent upon the puri?cation of the constitu
ents of my phosphor. These requirements for
exceedingly pure constituents apparently are due
in part to the characteristics of my preferred
constituents.
The above-mentioned objects are attained in
part by controlling the constituent content of
My invention relates to luminescent materials
and their preparation and particularly to the
preparation of luminescent zinc compounds.
It has been customary in the art to synthesize
luminescent materials or phosphors of zinc and
cadmium sulphides by various methods although
the majority of these methods result in a product
which has only moderate phosphorescent prop
erties. While I have disclosed in my copending
certain impurities. For example, the initial con
stituents from which I synthesize my phosphor
application, Serial No. 412,687, ?led September 10 material and particularly the zinc salt solution
27, 1941, a sulphide phosphor having relatively
high phosphorescence of relatively long duration
from which I form a sulphide should have less
than the following percentage of impurities:
following excitation to luminescence, even great
er intensities and longer periods of phosphores
cence following excitation are desirable in cer
Per cent
Mn
16
tain applications in which such phosphors are
utilized. For example, in aircraft position and
distance indicating equipment, it is quite desir
F'e
________________________________ __
0.0008
_____________________. ____________ __
0.0005
As ___________________________________
0.0008
Ni _________________________________ __ 0.00001
I prefer to synthesize my phosphor material
able to provide a cathode ray tube having a long
persistence phosphor, such persistence rendering 20 starting with zinc halide or, more preferably, zinc
sulphate and I remove a large proportion of in
herently contained iron and certain other metals
the tube suitable for portraying the trajectory of
an approaching aircraft. Sulphide phosphors
by an oxidation process to form insoluble com
are usually characterized by their low resistance
pounds which are precipitated and removed.
to heat. For example, the baking to which they
are usually subjected during cathode ray tube 25 More particularly I make a solution of zinc sul
phate ZnSO4-7H2O) in the proportion of ap
manufacture reduces their ei?ciency and for
proximately 50 grams to 33 milliliters of pure
certain applications they are wholly unsuitable
water and add thereto a solution of hydrogen
peroxide in an amount approximating 5 milli
of zinc compounds possessing high luminescent 30 liters of 30 per cent saturated solution. I then
add ammonium hydroxide until a slight White
properties and particularly high phosphorescence
turbidity is produced. Following the addition of
without undue sacrifice of fluorescence. An
ammonium hydroxide I bring the solution to a
other object is to provide a material which is
boil as rapidly as possible to remove certain vol
phosphorescent over a relatively ‘long period of
time following excitation to luminescence. A 35 atile products, such as ammonia gas, and con
tinue to boil until ammonia fumes are unnotice
further object is to provide phosphors which are
able. I then allow the solution to stand for at
insensitive to heat following synthesis so that
least 24 hours followed by decanting or by ?lter
they can be used in applications where high tem
ing to remove the precipitate. In addition to the
perature processing, such as encountered in tube
manufacture, has little or no effect on the phos 40 removal of iron, these steps likewise remove alu
minum, nickel and copper. Centrifuging may be
phor efficiency. These and other objects, fea
employed to reduce the time necessary to bring
tures and advantages of my invention are ob
down the precipitate.
tained in accordance therewith by combining a
Following the removal of the above impurities,
zinc sulphide and a ?uoride of zinc to form a
mixed sulphide-fluoride mass which is co-crys 45 the pH of the zinc sulphate solution should be
for use because of their low resistance to heat.
My invention has as an object, the production
between 4-7, any adjustment, if necessary, being
made by adding an acid, such as sulphuric acid,
following which I heat the zinc sulphate solution
to approximately 90° C‘. and allow it to trickle
tallized to form mixed sulphide-?uoride crystals.
Further in accordance with my invention I pro
vide a material in which the sulphide of zinc
either may be replaced in part by zinc oxide or
the zinc may be replaced in part by cadmium 50 over pure zinc for the purpose of removing all
with or Without zinc oxide prior to the formation
elements below zinc in the electromotive force
of the mixed sulphide-?uoride crystals. Fur
series of the elements. The treatment with zinc
metal may be accomplished at the very outset,
such as by reacting an acid such as hydrochloric
thermore, in accordance with my invention I
control the purity of the constituents within very
narrow limits to provide a material having high
?uorescent and phosphorescent e?iciencies. I
have found that such properties are exceedingly
55
or sulphuric acid with zinc metal or zinc oxide
in the presence of excess zinc metal. Following
such a treatment the purity of the zinc sulphate _
2,409,574.
3
4
solution will be approximately as indicated above,
preferably performed in platinum receptacles to
although to further assure removal of any ex
prevent formation of undesired ?uorides although
this may be obviated by forming the zinc ?uoride
from the ?ltered precipitated Zinc sulphide or
adding it thereto following precipitation and ?l
tering. I then thoroughly mix the sulphide-?uo
ceedingly small amount of manganese, lead and
copper, I prefer to electrolyze the solution be
tween platinum gauze electrodes at' 2-2.2 VOltS'
for a period ranging between 50-400 hours. To
still further assure purity I then fractionally pre
ride mixture with a platinum rod or spatula. If
an unactivated phosphor is desired, the mixture is
cipitate a small portion of the zinc as a zinc sul
phide by passing a small quantity of well-puri?ed
dried in an oven or over a water bath, broken up
hydrogen sulphide into the zinc sulphate solu 10 and fired at a temperature of from ‘750° C. to 1600°
tion following which the precipitated zinc sul
C. for aperiod of from 10 minutes to 10 hours
phide is removed by ?ltration and discarded.
depending upon the particle size desired and the
This fractional precipitation step may be repeat
temperature of ?ring. However, if an activated
ed to further lower the concentrations of un
material is desired, the activator metal is added
desirable impurities.
’
15 to the semi-?uid paste as a soluble salt in a water
In accordance with my invention I precipitate
zinc sulphide and zinc ?uoride (with or without
an activator, such as copper, silver or gold) from
solution and then the material is dried and ?red
as above. As explained below in greater detail
certain ?uxing agents may be added to the acti
the previously prepared zinc sulphate solution or
vated or unactivated material to assist the
alternatively I precipitate only a pure zinc sul 20 thermo- synthesis of the phosphor.
phide and then add to this sulphide a fluoride in a
Asa speci?c example of one teaching of my
form which will convert a portion of the zinc
invention, I add to 100 grams of the pure precipi
sulphide to zinc ?uoride, or still alternatively add
tated zinc sulphide and contained in a platinum
zinc ?uoride to the prepared zinc sulphide prior
crucible a quantity of zinc ?uoride between 0.5%
to calcining or heating of the mixture to produce 25 and 5% based on the zinc sulphide by weight such
the sulphide-?uoride phosphor. The amount of
as 2 grams of zinc ?uoride. Alternatively I add
zinc ?uoride either coprecipitated with the zinc
a sufficient quantity of a substance, such as hy
sulphide or added thereto as a ?uoride is relatively
drogen ?uoride or ammonium ?uoride, which will
critical to obtain the objects of my invention. For
react with the zinc sulphide to yield the desired
example, the ?uoride range is preferably between
quantity of zinc ?uoride. The specific quantity
0.1% and 15% by weight based on the zinc sul
of zinc ?uoride cited above, that is, 2% based on
phide content. I have found that in addition
the zinc sulphide content, is preferred inasmuch
to the zinc ?uoride a limited quantity of zinc oxide
as sulphides of this composition exhibit optimum
in the resultant phosphor i's bene?cial in enhanc
phosphorescence under 3650 A. radiation Without
ing the phosphorescent properties thereof; and 35 undue aggregation. One or more activator metals
furthermore, that while a single activator, such as
selected from the group consisting of copper, sil
copper, is preferably incorporated in my phosphor
ver and gold may be used and I prefer to add this
in known and controlled amounts, it is particu
activator as a soluble salt of the metal either pre
larly with the teaching of my invention to utilize
ceeding or following the precipitation of the zinc
a plurality of activators selected from the group 40 sulphide or following the preparation of the zinc
consisting of copper, silver and gold. As indicated
sulphide-zinc ?uoride mixture. Since the sul
above, my phosphor consists of a mixed crystal
phides of copper, silver and gold are extremely in
composition and it should be understood that the
soluble, it is immaterial whether their soluble salts
zinc ?uoride and zinc oxide which may be present
be added before or after the precipitation of the
are not necessarily present as strict chemical com
45
Zinc sulphide. Thus, for example, to the purified
binations in the final product. These materials
zinc sulphate solution or to the wet mixture of
merely symbolize the presence of ?uorine and
zinc sulphide and zinc fluoride I add an aqueous
oxygen or their ions in the sulphide phosphor.
solution containing the activator metal or metals
The resultant phosphor may be symbolized as
as soluble salts. I have found for the above prep
ZnS(ZnF2).e(ZnO)1/:Cu, wherein the subscripts a: 50 arations, the amount of copper may vary over
and y are representative of the amount of ?uoride
‘ wide limits; for example, the amount may vary
and oxide in the combined phosphor crystal com
from 0.0000l% to 0.1% molal with respect to the
position by weight based on the zinc sulphide
zinc sulphide content, although I prefer in the
content. More particularly the ?uoride and oxide
above example to add 0.01 gram of copper as the
content should be within limits such that x plus y
?uoride. I have found that the use of silver or
is equal to 0.1% to 15% based on the zinc sul
gold as an activator with the copper increases the
phide content of which the value 3/ is preferably
phosphorescence of the resultant material and the
equal to or greater than zero but not greater
amount of copper, silver and gold may vary from
than 5 %.
'
0.00001% to 0.1% molal, each with respect to the
As one speci?c example of preparing the ‘sul 60 zinc sulphide content. In addition I have found
phide-?uoride mixture prior to calcination, I pre
that in a multi-activated phosphor the quantity
cipitate zinc sulphide from the acidi?ed zinc sul~
of the individual activators may decrease in the
phate solution by bubbling puri?ed and thor
order of silver, copper and gold, the gold being
oughly washed hydrogen sulphide through the so
less than the copper and the copper being less
lution to form a precipitate of zinc sulphide. 85 than‘ the silver. The relative proportion of these
Simultaneously with or preferably following the
three activators may be one part gold, two parts
formation of the zinc sulphide precipitate, I pre
copper and four. parts silver. However, for vary
cipitate zinc ?uoride by adding gaseous or liquid
' ing the phosphorescent characteristic these rela
hydrogen fluoride or an aqueous solution of hy
tive proportions may vary above and below the
drogen fluoride to the zinc sulphate solution. I' 70 optimum proportion. I have found that the pe
obtain somewhat greater purity by forming the
riod of useful phosphorescence in a silver-gold
zinc ?uoride by precipitation subsequent to the
activated material increases in accordance with
precipitation of the zinc sulphide especially when.
the amount of gold with respect to silver and sim
I decant the supernatant liquid following the for-_
ilarl‘y. with a copper-silver or copper-gold acti
mation of the Zinc sulphide; The above steps are‘ 75 vator, the period of useful phosphorescence in
2,409,574
creases with the quantity of copper and gold re
spectively. The use of 0.1% of silver and 005%
copper by weight, to weight of zinc sulphide, gives
the group of metals consisting of zinc and cad
mium ’wherein the ?uoride is between 0.1% and
15% by weight based on the weight of said
optimum phosphorescence under cathode ray ex-‘ 7 sulphide.
2. A luminescent phosphor essentially consist
citation.
ing of a thermally crystallized zinc sulphide-zinc
To aid in the thermo-synthesis or calcination of
?uoride combination wherein the zinc ?uoride is
my phosphor I have found that speci?c preferred
between 0.1% and 15% by weight based on the
?uxing agents or catalysts may be mixed with
zinc sulphide content.
‘
the sulphide-?uoride mixture prior to the cal
3. A crystalline luminescent phosphor composi
cining or ?ring steps. I speci?cally avoid such 10
tion represented by the general formula
commonly used fluxing agents or materials as
boric acid, magnesium-?uoridevand calcium car~
bonate, which do not completely evaporate or
form readily removable compounds, by using a
wherein M is a metal selected from the group of
sodium or potassium chloride or bromide ?uxing 15 metals consisting of zinc and cadmium, F is
agent which controls the crystal growth and
' ?uorine, S is sulphur, O is oxygen, and :r and y
represent the weight of ?uoride and oxide re
phosphor formation upon calcination to far bet
spectively in the composition and each being
ter advantage than other ?uxes in the prepara
tion of the conventional sulphide phosphors. The
‘ greater than zero and wherein the sum of .r and
sodium or potassium chlorides (or bromides) re 20 y lies between ‘0.1% and 15% of the weight of MS.
act with the zinc sulphide to form zinc chloride
> 4. A crystalline phosphoras'claimed in claim 3
(or bromide) which sublimes at ‘732° C. leaving
wherein the value of y is greater than zero and
easily soluble sodium or potassium sulphides. I,
is not greater than 5%.
therefore, add to the wet zinc sulphide-zinc ?u
5. A crystalline luminescent phosphor repre
oride an aqueous solution containing 2 grams of
sented by the general formula ZnS(ZI1F2) :c(Zl'lO‘) 11
sodium or potassium chloride (or bromide) fol
wherein m and y represent the weight of zinc
lowing which I evaporate to dryness, such as in
?uoride and zinc oxide respectively in the com
an oven or under an infra-red lamp or over a
bined composition and wherein the sum of at and
water bath. I then place the material in the pow
3/ lies between 0.1% to 15% of the weight of zinc
i
dered state in a clean quartz crucible although a 30 sulphide content. .
platinum crucible may be used where the life of
6. The method of manufacturing a lumines
cent phosphor composition comprising the steps
the crucible is an economic factor in commercial
preparation. I then place the crucible and its 1 of adding a fluoride to a sulphur compound of a
contents in a clean electric furnace and raise the
metal selected from the group of metals consist
temperature of the furnace to between I750" C. 35 ing of zinc and cadmium to form a mixture there~
of, adding tosaid mixture a ?ux to aid crystal~
and 1600° C. and maintain this temperature for a
lization, and crystallizing said mixture by ?ring at
time interval of from 10 minutes to 10 hours,
a temperature between 750° C. and 1600° C.
the total heating time and temperature depend
, 7. The method of manufacturing a luminescent
ing upon the phosphorescence intensity and par
ticle size desired in the resulting phosphor mate 40 phosphor‘ composition comprising the steps of
rial, the particle size and phosphorescence inten
sity increasing with the time of calcination. Low
group of metals consisting’ of zinc and cadmium
er temperatures, such as 750° C.-1000° C. yield
to a sulphide of said metal to ‘form a mixture of
smaller crystals, but higher temperatures, such
as 1100° C‘.-1300° C. give higher phosphorescent
said ?uoride and sulphide, adding to said mixture
a flux to aid crystallization, and crystallizing said
intensity. Following crystallization the material
mixture as a combined ?uoride and sulphide by
adding a ?uoride of a metal selected from the
?ring said mixture and said ?ux at a temper
is removed from the crucible, those portions
ature between 750° C. and 1600° C.
which reacted therewith being detected under
8. The method of manufacturing a sulphide
ultra-violet excitation and usually discarded
whereupon the crystallized material in accord 50 phosphor comprising preparing a solution of a
sulphide of a metal in ‘the group consisting of
ance with my invention is then washed ‘with
zinc and cadmium having a pH ratio between 4
vigorous‘ stirring in pure distilled water satu
and 7, the said solution having a maximum im
rated with hydrogen sulphide. I have found that
the period of Washing should be short and that ' purity by weight with respect to the solution of:
preferably the material may be washed two to -
three times in pure water saturated with hydrogen
sulphide. The material should not be subjected
to the washing water for a period in excess of
Per cent
Mn
_________________________________ __ 0.0008
Fe
_____
_
As __________________________________ __
0.0005
0.0008
10-100 minutes (shorter time for higher ?uoride
content) following which the material is im 60
mediately dried at a temperature from. 100° C.
precipitating a sulphide and a ?uoride of said
to 250° C. whereupon it is ready for use. I have
metal fromsaid solution, adding a ?ux to the
found that my product not only has exception
precipitate, drying said precipitate and ?ux, and
ally high phosphorescent properties and excel
?ring the precipitate and flux at a temperature
lent stability to heat but that such phosphor 65 sufficient to crystallize the dried precipitate as
a mixed sulphide-?uoride phosphor.
escence is not obtained ata sacri?ce of ?uores
cence and further that my method of production
,9. The method of synthesizing a luminescent
may provide a smooth ?owing, substantially non
sulphide compound of a metal selected from the
group consisting of zinc and cadmium compris
aggregated, non-?occulent material which may
be easily applied during the subsequent manufac 70 ing mixing with a compound of said metal a quan
Ni
ture of luminescent coatings.
I claim:
‘
‘
1. A luminescent phosphor essentially consist
ing of a thermally crystallized combination of
a sulphide and ?uoride of a metal selected from 75
__ _ _ _ _ _ _
_ _ _ _ _ _ _
_ _ _ __
0.00001
tity of a ?uoride of an element selected from the
group consisting of said metal and hydrogen and
?ring said mixture to crystallize said compound
in the presence of said ?uoride.
HUMBOLDT W. LEVERENZ.
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