Note: Descriptions are shown in the official language in which they were submitted.
~Q~66~3 PHN 82~0
The invention relates to a luminescent screen
provided with a luminescent a:Luminate of trivalent cerium
and of magnesium, which aluminate is activated by trivalent
terbium. Furthermore the invention relates to a low-
pressure mercury vapour discharge lamp provided with such
a luminescent screen and to the luminescent aluminate
itself.
Our Canadian Patent 1,028,844 which issued on
April 4, 1978 discloses a lar~e number of luminescent
aluminates and gallates, the host lattices of which are
ternary compounds which contain besides aluminium and/or
gallium a large positive ion and a small bivalent ion,
particularly magnesium. Of the luminescent materials
described, the terbium-activated aluminates of cerium and
magnesium constitutes a particularly important group.
These aluminates, which appear to have a crystal structure
corresponding to that of magnetoplumbite (PbFel2019), emit
namely when excited by, for example ultraviolet radiation,
in a very efficient manner the characteristic Tb3
20` emmission~ They are therefore used with much advantage
in low pressure mercury vapour discharge lamps ~see also
our Canadian Patent 1,033,158 which issued on June 20,
` 1978).
The molar ratios of the elements present in
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PHN 8240
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said ternary compounds may generally be within wide ranges
of ~alues. It has appeared, howevert ~hat in the case of
the terbium-activated aluminates of cerium and magnesium,
optimum luminescent properties, especially as regards the
lumlnous flux, are achieved with materials which satisfy
the formula (Ce, Tb) MgAll1019. When preparing these
aluminates a deviation is often made, as described in the
above-men-tioned Canadian Patent 1,028,844, from the
stoichiometry of this optimally luminescent phase and a
small excess of A1203 is added in order to obtain a
reaction process which proceeds better. When using, for
example, 10 mole ~ A1203 in excess the composition of the
reaction product may be represented as ~(Ce,Tb)203.
- MgO.6.05 A1203.
- 15 Terbium-activated aluminates of cerium and
magnesium are also disclosed in German Patent 2,501,783,
which issued to Thorn Electrical Industries on July 24,
1975, in which wide ranges are also indicated for the
molar ratios of the composite elements. The compounds
described in this place have the crystal structure of
-alumina (a structure which i~ strongly related to that
of magnetoplumbite~. As optimally luminescent aluminates
of cerium and magnesium are mentioned (Ce, Tb)Mgl 25A122035 75
' ) g2A122036.5; however, no values of the
luminous flu~ of these luminescent phases are given.
It is an object of the invention to provide
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6~3 PHN 8240
improved terbium-actlvated lu~inescent aluminates on the
basis of the known (Ce,Tb)MgA:LllOlg-phase with magneto-
plumbite structure.
A luminescent screen according to the invention
is provided with a luminescent aluminate of trivalent
cerium and of magnesium which is activated by trivalent
terbium and is characterized in that the aluminate has
the composition (~-x-y)ce2o3~xLa2o3 yTb2o3~Mgo pAl2o3 where
0 ~ x ~ 0.20
0.05 ~ y ~ 0.25
6.2 ~ p C 25,
and where the Mg may be replaced partly or wholly by
Zn and/or Be, and the Al to a maximum of 25 mole % by Ga
and/or Sc.
A luminescent screen according to the invention
contains a cerium magnesium aluminate in which in known
manner (see Canadian Patents l,028,844 and l,033,158
already mentioned above) part of the Ce may be replaced
by La and part (maximum 25 mole %) of the Al ma~ be
replaced by Ga and/or Sc. Furthermore, it is possible
to replace the Mg wholly or partly by Zn and/or Be. Said
substitutions affect the luminescent properties of the
aluminate only sllghtly and yield no additional advantages.
The luminescent aluminates according to the
invention contain a comparativeIy large to a very
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PHN. 82~0.
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large excess of A12O3 with respect to the stoichiome-
trical quantity (p = 5.5). They are obtained by heat-
ing to a high temperature, for example a 1200C or
higher, a starting mixture of the composite oxides in
the specified quantities or of compounds which form
these oxides on heating x-ray diffraction analyses
of the luminescent aluminates according to the inven-
tion display the characteristic pattern of magneto~
plumbite together with the diffraction lines of A1203. -
It is consequently assumed that the luminescent phase
proper in a luminescent aluminate according to the
invention is the stoichiometric ~Ce, Tb)MgA111019
phase. This is also confirmed by the spectral en~rgy
distribution of the omitted radiation which, for the
aluminates according to the invention, is identical
to that of the stoichiometric phase. Actually, the
results of x-ray diffraction analyses performed on
the aluminates according to the invention are sub-
stantially equal to those of similar analyses on
mechanical mixtures of the stoichiometric phase with
corresponding large quantities of A1203.
It has surprisingly appeared that with the
aluminates according to the invention luminous fluxes
are o~tained which can be compared with or are even
slightly higher than those obtained with the stoi-
chiometric phase. Only with very high values of p
. ..
~ in the above-mentioned range (6.2 ~ p ~25~ a slight
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PHN. 8~40.
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decrease is observed in the luminous ~lux. This was
to~ally unexpected, as 90 ~ar diluting a luminescent
phase with an inert material gave rise to considerable
losses in the luminous ~lux.
A great advanl:age of the aluminates according
to the invention is that they are considerably cheaper
than the pure stoichiometric phase. With a value o~ p
in the above-mentioned composition equal to 15, for
example, a material is obtained which consists for
less than 50 weight % o~ the stoichiometric phase and
whose luminous flux is Iess than 2% lower than that
of the stoichiometric phase. This yields a saving in
cost of approximately 50% when a same pow~er weight
is used in the luminescent screen. In the aluminates
according to the invention the value of p is not chosen
to be lower than 6.2 because then the savings obtained
would be too small. Values of p which exceed 25 are
not used because then too large a loss in luminous flux
would occur.
The aluminates according to the invention
have the additional advantage that they are obtained
in the form of a powder with an average grain diameter,
which is smaller than that of the stoichiometric com-
pound. This offers a possibility to use lower powder
weights in the luminescent screen whilst maintaining
the luminous flux. This also means a further economic
advantage.
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PHN. ~240.
~,@~66~
The alumina-tes according to the inven-tion
appear, when used in the luminescent screen of low-
pressure mercury vapour discha~ge lamps to have during
the operating life of the lamps a smaller decline of the
luminous flux than the stoichiometric compound, which o~
course is very advantageous. An aluminate according to
the invention which initially yields, in a lamp, a
luminous flux which is a few per cents smaller may
already emit a larger luminous flux after 100 operating
hours than the stoichiometric compound.
Preference is given to aluminates according to
the invention where x = O, 0.15 ~ Y 0~20 and 10 ~ p
15. Namely with these materials the most favourable
combinations of cost reduction and high luminous fluxes
are obtained.
The invention will now be further explained
with reference to a number of embodiments and measure-
ments and a drawing.
Starting from the basic materials A1203, MgO,
CeO2 and Tb407 materials of the composition
/ (Cel.33Tbo~67) 03 MgO-pA1103 with respectively
p = 7.0; 10.4; 14.2 and 21.1 are made by carefully
mixing these basic materials in the required quantities
and by heating the mixture obtained twice, in air, each
25 time for 1 hour at 1550C. Each heating operation is
followed by cooling in nitrogen. The product thus ob-
tained is pulverized and sieved and thereafter heated
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for 2 hours at 1200C in a nitrogen current which contains
some vol. ~, for example 2 vo:L. %, of hydrogen and which
has been passed through water of approximately 15C.
After cooling and pulverizing the product is ready.
In a manner which is known in itself the lumine-
scent aluminates thus obtainecl are applied to the luminescent
screen of a low-pressure mercury vapour discharge lamp.
Measurements of the luminous flux (in Lm/W) of these lamps
at different times during burning of the lamps are
summarized in the table which follows herebelow. For
comparison the table shows under example (a~ the measure-
ments of a known aluminate with a very slight excess of
A1203 (p = 6.05 ; not according to the invention; see our
Canadian Patent 1,028,844). Also for comparison, under
(b) the measuring results are shown which are obtained
with lamps provided with a mechanical mixture of the
material specified under (a) with 15.05 mol A1203. Prior
to mixing the A1203 was submitted to a heat treatment
analogous to the heat treatment as used in the preparation
described above of the aluminates according to the
invention. The material mentioned under (b) contains a
total of 21.1 mole A1203 (compare with example 4).
A material of a composition equal to example
2 of the tab:Le (p = 10.4) is obtained by suspending
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I'l]N 8i'~
3 1 o . ~
tlle :requ:i.red q~lalltl-t:~ oI` :E`illely clis~e.rs(?cl l~:L203 ;:rl a
so] u ti on o:E` Ce-, Mg-, ~.ri~ ancl, op ti ona:l :l y, I~l.~ni tra te,
ereaf`-ter b~r add i.nrr Nl-l~101-l the me l;al llyclroxicles arc
percipitated onto -the 1\1203. 'Nle coprec:ipitate ob~
tained is -tllerea:~ter heated in tl~e same 3nan~e:r as
descri~ed abov-e. Measurements made on lamps prov:i.c1ed
.~ith the 1ulllirlescellt alulllinate thus obtailled are :L:ist-
ed under examp1e 5 in the tab~ e.
.. ~
lum:inoll.s :f` 1 ux
... ~ . .__~
E~am- .0 hour 100 1000
ple P . hours hoL1rs
.~
~m/l~ 1n % ~ r t .Lm/W ~m/W
~ ........ _ ~ ~_ ~ :
. .. (a) 6,o5109,5 103,5105,3 9717
. 1 7, 109~6 100,2-109,~ 105.1
2 10,11109,2 101~1108,0 102
3 lL~2 107~ 100.7107,0 101,1
Ll 21 ~ 1102~ ~l 101 ,5 . . 100,9 97.0
~1~) 6,o5 ~91,S 107,585~1~ 77~1
1~ 0 mo1e ~
. , ~ ~ lO~./I108,6 -10!~,7_ L
-- ~ ~ ~ ~._. _ .~..... .
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lo. 1 1.7G
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T]-le dra~ ng shows a low~pressure mercllry vapollr
dischalge la~np accordin~: to t:l~e :i:nveIltion. The lalnp com-
prises all envelope 1 wi-th electrodes 2 and 3 place(l one
at each encZ thereo:r. The inte:rnal sur~ace o:~ the en-
velope 1, whic}l is, for exalnp:Le, made o~ glass :i.s coat~
ed Wit}l a luminescent layer 4 whicll contains a ,Lum;.-
nescent aluinina-te accorcdin~ tol:the invention. Tlle lu-
minescellt al.umiliate, optionally m:ixed wi-tll o-ther ].umi-
nescent materia:Ls ma~r be applied to the enve].ope 1 by
applying a suspension o~ th.e luminescent mate:rial and
nitrocel:Lulose in butyl-acetate to the internal sur-
~ace o~ the eIlvelope so -that a thin coating of this
suspension is ~ormed. lIerein nitrocellulose serves as
a temporary binder. Thereafter the envelope is sub-
mit-ted to a tempera-ture treatment which expels -the
binder and which results in a proper adhesion o~ -the
l~inescont coat.
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