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Патент USA US2408622

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2,408,621
Patented Oct. 1, 1946
PATENT oFFics
UNITED > STATES
2,408,621
rnosrnons AND THEIR STABILIZATION
C. Froelich, Cleveland, Ohio, assignor to
Company, a corporation of
vGeneral Electric
Herman
New York
'
No Drawing.
Application May 14, v1942,
Serial No. 442,993
,
9 Claims.
,
(01. Ill-33.5)
1,
phor mixturev havefundergone prolonged atmos
pheric heating at considerably higher tempera
This invention relates to ?uorescent lamps. or
tures than this, it was thought that such deterio
ration of the- phosphor mixture must be due to
some deactivating‘reaction between these com
tubes and the’like, and. particularly to lumines
cent materials or phosphors. ‘The inventionv is
especially. concerned with deterioration of phos
ponents when heated together. However, phos
phors arising from manganese or the like which
phors consisting solely of zinc-beryllium silicate
is used for purposes of activation with matrix
(or other silicates) activated with manganese ex
materials such as metal silicates and the like,’ ex
hibit similar deterioration. vWhen heated as
empli?ed by cadmium silicate, zinc silicate, or
zinc-beryllium silicate. Other matrix materials 10 above mentioned, phosphors consisting in whole
or in part of such activated silicate(s) also
of‘this character are metal phosphates, ‘chloro
those of cad
change in color- as observed under white light:
their natural pure white takes on light shades of
gray or brown. The ?ner the phosphor, and the
mium. The invention aims not only to prevent
deterioration of phosphors by manganese of their
own, but also to obviate deteriorating influence 15 greater its exposed surface, the more pronounced
is the deterioration byheating; and it is also
of manganese in one phosphor on an associated
more pronounced when the phosphor is heated
phosphor that may not itself'contain any man
in pure oxygen instead of in air. Even at atem
ganese. An example of such a phosphor mixture
perature of only 500° C. in air, standardzinc
is one of manganese-activated zinc-beryllium sil
beryllium silicate phosphor‘ changes color- and
icate with magnesium tungstate without any
added activator, which gives a white light when 20 loses about 8—10 percent in brightness. .» » - -.
As there isevery reason tobelieve that silicate
excited by the 2537 A. resonance radiation ‘of the
phosphors consist of a matrix of compound which
mercury arc discharges
'
.
‘is commonly represented, for example, by the
I have explained my invention hereinafter with
particular reference to the above-mentioned
formula (Zn,Be)2Si04, together with manganous
tungstate phosphor also undergoes atmospheric
utes below, red'heat, as .in lamp tube baking or
heating at fairly high temperature during manu
facture, though its properties would scarcely be
bending; and yet this is indubitably the case.
Furthermore, it. has so far proved impossible to
white phosphor mixture and to zinc-beryllium 25 oxide,‘MnO, in solid‘ solution in this matrix, and
as such phosphors undergo several hours of heat
silicate phosphor activated .with ‘manganese.
ing in airv at relatively high temperatures (such
Such zinc beryllium silicate phosphor is a mate--‘
asl100° ‘C. to 1300° C.) during manufacture, it
rial of almost ceramic properties, that has been
heated in the air at temperatures of about 1l00° 30 is very surprising that such phosphors should not
bev perfectly, stable when heated for a few min
to 1300° C. during its preparation. Magnesium
prepare manganese activated silicate phosphors
termed ceramic.
In the manufacture of ?uorescent lamps or
235 of anything like normal ?uorescence that do not
tubes of the usual'positive column discharge type,
show the deterioration above described, under
?nely powdered phosphor is intimately mixed
with organic binder solution (as-of nitrocellulose)
heat that is otherwise desirable in lamp process.
by grinding in a ball-mill. After applicationv of
the resulting phosphor suspension to a lamp tube
and drying out of the binder liquid or solvent, the
coated tube undergoes baking for some minutes
ing. By re?ring such phosphor after grinding, its
stabilityas against loss of brightness by heating
is improved; but it still remains subject to ob
jectionable
deterioration.
I have found that the discoloration and loss of
wall temperature of some 400° C. to 600° C.,'more
?uorescent brightness which silicate phosphors
andfwhite phosphor mixtures undergo when
or less.
heated is due to manganese which is not, appar
in an oven to burn out the binder, at'an actual
In some cases, as in the bending of
?uorescent electric sign tubes, the coated lamp
envelopes may be heated as high as GOO-‘625° C.,
ently, activatingly combined, With the silicate
phosphor matrix, or included in the crystal lat
In the case of ?uorescent lamps coated with
tice of this matrix-An other words, is not even in
solid solution in the silicate. In the ‘case of white
has been found that exposure to atmospheric
phosphor is also somewhat affected by such
or even 650° C.
'
a
the above-mentioned white phosphor mixture, it 50 phosphor mixtures, the magnesium tungstate
“stray” manganese of the associated silicate
phosphor; ‘but the maJo'r effect is on the ‘silicate
?uorescent brightness amounting to some 15;30 55 phosphor itself. For both cases, I'have'found
per cent. As both the components of this phos
temperatures of some 600° C. or more (as in
bending coated sign tubes) results in a loss of
2,408,621
3
4
means of obviating such phosphor deterioration.
I have determined that “stray” manganese is
leterious compound into something that is inert
commonly present even in silicate phosphor that
or innocuous, as by reducing it to a lower form
has not undergone the heating incident to lamp
processing, and is manifested in a demonstrable
compound in other Ways.
oxidizing power associated with this phosphor,
A method of eliminating the deleterious man
ganese compound by conversionvwithout removal
position as (Zn,Be)2SiO4 with activating MnO in
is to reduce this compound to a lower one in
solid solution therein. A test which shows this
oxidizing power is to make a slurry of the phos 10 which the manganese is divalent merely. This
and not to be accounted for by its essential com
can be effected by heating the phosphor in vacu
phor in a weakly acid solution of starch and po_
um to about 500-609" C., or in a
tassium iodide, which turns blue in response to
the oxidizing action of the stray manganese com
pound present in the phosphor. Titration with
very dilute sodium thiosulphate or the like makes
the test quantitative. The explanation seems to
10 minutes before being
exhausted and charged with mercury and starting
gas ( e. g., argon at a pressure of 2-4 mm. of mer
cury) , and sealed off.
A method of actually removing the deleterious
manganese compound is to treat the phosphor
with‘ dilute acid in the presence of a suitable re
ducer, such as sulphur dioxide, hydrogen peroxide,
etc., thus converting higher manganese oxide into
a water-soluble manganous salt. All that is nec~
This oxidizing power and the dark discolora
tion which the phosphor undergoes when heated
to some 500-650" C. would indicate a manganese
compound in which manganese is more than di 30
valent (as in MnO). Evidence is inconclusive as
under heating rather points to some of the many 35
Quantitative deter
minations show an amount of the compound
ranging from about 0.001 to
the zinc-beryllium silicate
With an average of about 0.006 per cent. Such an 40
before heating.
It is to the minute yet
dark matter that I attribute the light gray or
of the phosphor is
rescent brightness.
I have discovered that while it seems imprac
ticable to produce phosphors free from deleterious
advantage that its
dissolving action on the phosphor matrix is
milder than that of mineral acids,
'
' 2,408,621?
mix them thoroughly for applioation‘to a lamp,
this phosphor shows somewhat less improvement
pairment of its fluorescence.
in ?uorescent‘brightness on the lamp wall than
.
6
the heating of the lamp in processing it; and‘ thus
prevents discoloration‘ of the phosphor and im;
would be expected from brightness tests made
just before and just after the reducer treatment.’
For the convenience of those‘desiring to prac-‘
.
:.;v
:i
'
‘
My‘ oopending divisional-application .Serial No.
666,776, ?led May 2, 1946, claims certain methods
relative to thereduction of the uncombined and
super?cial manganese compounds higher valent
ing it), the following illustrative particulars of
than two, wherein the reduction is produced un
der conditions conducive to such reduction, such
the preferred wet method are given, the work be
ing carried on with baths or solutions at ordinary‘
methods therein claimed constitute further in--
tice the invention (but not as limiting or de?n
as in an atmosphere conducive to'reduction. The
ventions over and above the subject matter of the
,
>.
instant application. The above-mentioned di
To a bath of 200 cc. of distilled‘ water contain
visional application is assigned to the assignee of
ing 0.03 per cent of acetic acid is added 10 cc.
of a 5 per cent aqueous solution of sulphur diox 15'
What I claim as new and desire to secure by
ide (S02). 100 g. of zinc-beryllium silicate phos
Letters Patent of the United States is:
phor are suspended in 200 cc. of distilled water.
room ' temperature :
this
making a thin slurry.
The solutions are then ,
poured together and stirred for 5-10 minutes, and
application.
'
‘
‘
‘
‘
--
I
l. A generator of radiation consisting essen
tially of a thermally synthesized luminescent
the whole is then promptly ?ltered through or
dinary ?lter paper on a suction ?lter. The phos
phor ?lter-cake is at once thoroughly washed
with distilled water drawn through it on the ?l
composition of metal silicate as matrix and man
ganese as an activator wherein the manganese is
present only in a divalent condition activatingly
ter, until the ?ltrate no longer shows any acid
combined with the silicate matrix.
-
2. A generator of radiation consisting essen
reaction. After drying, the phosphor is ready for
incorporation with the binder for application to
tially of a thermally synthesized luminescent
?uorescent lamp envelopes as usual.
and manganese as an activator wherein the man
_
A somewhat more convenient method of treat
composition of zinc-beryllium silicate as matrix
ganese is exclusively in divalent condition and
activatingly combined in solid solution with the
ing the phosphor with reducer and dilute acid is
to pass moist sulphur dioxide gas through the 30 silicate matrix.
3. A method of assuring maximum brightness
dry phosphor powder for some minutes in a suit
in a ?uorescent discharge lamp in which is pro
able closed vessel. and afterward to wash the
vided a coating of a thermally synthesized lumi
gassed powder with suitably acidulated water on
nescent composition of metal silicate as matrix
a ?lter, followed by thorough washing with pure
‘water. However. the wet method described above 35 and manganese in divalent condition activatingly
combined with said silicate matrix, which method
is at present preferred for phosphors used in or
dinary ?uorescent lamps.
'Manganese-activated silicate phosphor that is
comprises removing from the surface of the
composition uncombined and super?cial manga
treated with reducer such as S02 and dilute acid
nese compound whose manganese is not held fast
according to the preferred wet method herein 40 by the silicate matrix and which is normally
present on the particles of the composition after
before described is somewhat whiter than be
its synthesis. the removing step comprising treat
fore: shows a gain in ?uorescent brightness of as
ing the composition with a reducing agent and
much as 10 per centwhen tested directly after
ward. or even more. without any apparent shift
in color toward yellow or green: gives no reac
that with starch '
tion to an oxygen test such as
acid in the presence of water to form a soluble
manganese salt.
4. A method of assuring maximum brightness
and potassium iodide. which indicates absence of
free manganese oxide or the like higher than
MnO. both at the surfaces and in the interior of
in a ?uorescent discharge lamp in which is pro-_
vided a coating of a thermally synthesized lumi
nescent composition of metal silicate as matrix
the silicate matrix crystals: remains 100 per cent
stable when heated to 500-650’ 0.. either alone or
and manganese. in divalent condition activat
ingly combined with said silicate matrix. which
in admixture with magnesium tungstate; and
method comprises removing the uncombined and
shows satisfactory maintenance of output in
super?cial manganese compound by treating the
luminescent composition with a reducing agent
?uorescent lamps.
A method of virtually eliminating their stray 55 and acid in the presence of 'water, and washing
manganese from phosphors and thus stabilizing
away the resulting soluble manganese compound.
them is to inactivate the phosphor against dele
5. A method of assuring maximum brightness
terious reaction of the stray manganese devel
in a ?uorescent discharge lamp in which is pro
oped during heating, as by protectively coating
vided a coating of a thermally synthesized lumi
the phosphor particles. For example, the pow 60 nescent composition of metal silicate as matrix
dered phosphor may be suspended in a 5 per cent
solution of boric oxide (B203) in methyl alcohol,
which may then be ?ltered through ?lter paper
on a suction ?lter, and allowed to dry.
and manganese in divalent condition activat
In the
case of ?uorescent lamp phosphors, this general
method may be used more advantageously to
treat the phosphor after it has been coated on
the inside of the lamp envelope. For this pur
pose, the phosphor-coated lamp envelope may be
?ushed out with a l per cent solution of boric
oxide in methyl alcohol, allowed to drain, and
then dried, after which the lamp may be proc
essed and completed as usual. The thin‘ ?lm of
boric oxide with which the phosphorparticles
6
ingly combined with said silicate matrix, which
method comprises removing the uncombined and
super?cial manganese compound by treating the
luminescent composition in ?nely divided form
with sulphur dioxide and acid in the presence of
water, and washing away the resulting soluble
manganese compound.
6. A method of assuring maximum brightness
in a ?uorescent discharge lamp in which is pro
vided a coating of a thermally synthesized lumi
nescent composition of metal silicate as matrix
and manganese in divalent condition activat
ingly combined with said silicate matrix, which
are thus coated and ensealed prevents oxidizing 75 method comprises removing the uncombined and
reaction of stray manganese developed during
7
2,408,621
super?cial manganese compound Whose manga
and is not held fast by
the silicate matrix, and which normally forms on
the particles of the composition after its synthe
9. A method of assuring maximum brightness
in a ?uorescent discharge lamp in which is pro
vided a coating of a thermally synthesized lumi~
sis, the removing step comprising treating the
nescent composition of metal silicate as matrix
composition with a reducing agent and acid in
the presence of water.
7. A method according to clam 6 wherein the
super?cial compound whose manganese is more
and manganese in divalent condition activat
ingly combined with said silicatev matrix, which
method comprises removing the uncombined and
super?cial manganese compound by treating the
than divalent is converted into a soluble man
ganese salt prior to removal.
10 luminescent composition, after being applied as
a coating to an envelope, with hydrogen peroxide
and acid in the presence of water, and washing
8. A method, according to claim 6 wherein the
away the resulting soluble. manganese compound.
super?cial compound whose manganese is higher
valent is converted into compound wherein the
manganese is divalent and is uncombined with 15‘
HERMAN C. FROELICH.
the matrix.
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