Synthetic willemite phosphors and fluorescent lamp containing the same

Synthetic willemite phosphor for use in fluorescent lamps in the form of particles coated with from 0.001% to 0.5% of an oxide of scandium, yttrium, lanthanum or a rare earth element. Fluorescent lamps comprising the phosphor which have improved aging qualities are also provided.

The present invention relates to improved synthetic willemite phosphors for 
use in fluorescent lamps and to a process for their production. It also 
relates to fluorescent lamps incorporating the improved phosphors. 
Throughout the present specification, by the term `synthetic willemite 
phosphors`, we mean manganese activated zinc silicate phosphors of the 
type which are well known in the lighting art as fluorescent materials and 
which emit green light when exposed to ultra-violet irradiation. An 
example of such a material is zinc orthosilicate, Zn.sub.2 SiO.sub.4 
activated by approximately 1% by weight of manganese. 
Synthetic willemite phosphors are particularly useful when the ultraviolet 
irradiation is of a wavelength in the region of 250 nm (e.g. 253.7 nm as 
in the low pressure mercury vapour discharge tubes widely used in 
commercially available fluorescent lamps) since at such a wavelength the 
visible green emission is particularly bright. Unfortunately the efficacy 
of the fluorescent emission from existing synthetic willemite phosphors 
decreases with use at a rate which is greater than the comparable rate of 
decline of other well known and commonly used phosphors such as the 
alkaline earth halophosphates or europuim activated yttrium oxide. In 
practice, in order to achieve light of other colours or substantially 
white light, the synthetic willemite phosphor is normally used in 
conjunction with one or more of these or other phosphors and its 
relatively more rapid deterioration means that the colour of the light 
emitted from a fluorescent lamp incorporating such a material will tend to 
change during operation. 
It is an object of the present invention to provide an improved synthetic 
willemite phosphor which ages less rapidly than existing phosphors of this 
type. Accordingly we provide a synthetic willemite phosphor as 
hereinbefore defined in the form of particles suitable for fluorescent 
lamp production in which said particles are coated with from 0.001% to 
0.5% by weight of the total phosphor of one or more oxides of formula 
R.sub.2 O.sub.3 where R is an element selected from the group consisting 
of scandium, yttrium, lanthanum and the lanthanon or rare earth elements 
of atomic number 58 to 71. Particularly useful oxides are those wherein R 
is yttrium, gadolinium or lanthanum. Most conveniently the amount of oxide 
coating is within the range of from 0.03% to 0.3% by weight and a 
particularly suitable improved phosphor of the invention is coated with 
about 0.1% by weight of oxide. 
Particles suitable for fluorescent lamp production are normally in the size 
range of from 3 to 30 microns. 
Our invention also provides a process for the production of the above 
phosphor which comprises the steps of suspending particles of a synthetic 
willemite phosphor as hereinbefore defined in a solution of a salt of the 
element R, as hereinbefore defined, and whilst maintaining the suspension 
causing a compound containing the element R to be precipitated onto said 
particles. If the precipitated compound is not the oxide, further 
treatment to convert it to the oxide must be carried out. The particular 
salt used will of course depend on solubility in readily available 
solvents but we have found that, in the case of yttrium, lanthanum or 
gadolinium, an aqueous solution of the nitrate may be used. In this latter 
case precipitation of the hydroxide is easily accomplished by the addition 
of excess ammonia. The improved phosphor of the invention is then 
recovered by filtration and subsequent drying. The drying may optionally 
be followed by further heat treatment, for example at a temperature of 
from 200.degree. C. to 800.degree. C. for 10 to 60 minutes. 
Our invention also provides fluorescent lamps comprising the above phosphor 
having improved ageing qualities. In particular we find that such a lamp 
after 500 hours of operation retains at least 90% and preferably from 92 
to 96% of its original light output (in terms of lumens/watt) and this 
compares with figures of from 84 to 89% for the uncoated previously known 
material.

The invention is illustrated but in no way limited by the following 
examples wherein all parts and percentages are by weight. 
EXAMPLE 1 
A synthetic willemite phosphor (zinc orthosilicate containing approximately 
1% by weight of manganese) (200 g.) of particle size distribution from 3 
to 30 microns and median particle size (as determined by a Coulter 
counter) of 7.6. microns, was mixed into a slurry with demineralised water 
(400 ml.) at a temperature of 80.degree. C. and 2 or 3 drops of 
phenolphthalein indicator added. The slurry was stirred and maintained at 
a temperature of about 80.degree. C. and aqueous yttrium nitrate solution 
(20 ml.) containing the equivalent of 0.2 grams Y.sub.2 O.sub.3 was 
stirred in. Ammonia solution was added drop-wise until the phenolphthalein 
indicator just changed from colourless to pink and the stirring of the 
slurry was maintained for a further 15 minutes. The willemite was then 
filtered off, washed several times on the filter with demineralized water 
and then dried at 120.degree. C. and sieved through a 175 mesh sieve. 
Fluorescent lamps (40 watt) were then made up in which this treated 
phosphor was used for the fluorescent coating; at the same time an equal 
number of fluorescent lamps were made up in which the phosphor coating was 
synthetic willemite which had not been treated with the yttrium nitrate 
solution. Measurements of the light outputs in terms of lumens/watt of 
these fluorescent lamps after various times of operation gave the 
following results: 
__________________________________________________________________________ 
TIME (Hours) 
0 100 500 1000 2000 5000 
__________________________________________________________________________ 
Average of Control 
Lamps (no yttrium 
104.6 
98.9 89.6 86.1 77.6 67.7 
treatment) 
(100%) 
(94.6%) 
(85.7%) 
(82.4%) 
(74.2%) 
(64.7%) 
Average of Test 
Lamps (Yttrium 
101.2 
97.9 93.8 89.3 82.6 73.3 
treated willemite) 
(100%) 
(96.7%) 
(92.7%) 
(88.2%) 
(81.6%) 
(72.4%) 
__________________________________________________________________________ 
It will be seen that although the initial output from the lamps using the 
treated willemite was slightly reduced compared with the untreated 
willemite, the fall in brightness during operation of the lamps was 
appreciably reduced. 
EXAMPLE 2 
The method of Example 1 was repeated, but in this case part of the yttrium 
treated willemite was heated to 600.degree. C. for 30 minutes in air 
before being used to make fluorescent lamps. The lamp light output 
(lumens/watt) measurements were as follows: 
______________________________________ 
0 hours 100 hours 
500 hours 
______________________________________ 
Average of Control 
lamps (no yttrium 
104.5 99.8 90.0 
treatment) (100%) (95.5%) (86.1%) 
Average of Test Lamps 
(yttrium treated 
willemite heated to 
102.2 98.6 95.0 
120.degree. C.) 
(100%) (96.5%) (93.0%) 
Average of Test Lamps 
(yttrium treated 
willemite heated to 
101.3 98.9 94.4 
120.degree. C. then 600.degree. C.) 
(100%) (97.6%) (93.2%) 
______________________________________ 
Again, the yttrium treated willemite lamps show a greater improvement in 
maintenance of light output during operation than those using untreated 
willemite. 
EXAMPLE 3 
The method of Example 2 was repeated but using an aqueous solution of 
lanthanum nitrate (20 ml) containing the equivalent of 0.2 grams of 
La.sub.2 O.sub.3 in place of the yttrium nitrate. 
EXAMPLE 4 
The method of Example 2 was repeated but using an aqueous solution of 
gadolinium nitrate (20 ml) containing the equivalent of 0.2 grams of 
Gd.sub.2 O.sub.3 in place of the yttrium nitrate. 
The materials produced by Examples 3 and 4 were used to make fluorescent 
lamps which gave the following light output (lumers/watt) results 
__________________________________________________________________________ 
TIME (Hours) 
0 100 500 1000 2000 5000 
__________________________________________________________________________ 
Average of Control 
102.7 
97.7 92.3 85.7 78.1 66.5 
Lamps (no treatment) 
(100%) 
(95.3%) 
(89.9%) 
(83.4%) 
(76.0%) 
(64.8%) 
Average of lamps 
made using the 
phosphor of 
99.7 
96.6 95.3 89.4 84.2 73.9 
Example 3 (100%) 
(96.9%) 
(95.6%) 
(89.7%) 
(84.5%) 
(74.1%) 
Average of lamps 
made using the 
phosphor of 
99.6 
98.9 95.9 89.5 84.1 72.5 
Example 4 (100%) 
(99.3%) 
(96.3%) 
(89.9%) 
(84.4%) 
(72.8%) 
__________________________________________________________________________