CA1056151A - Lead-manganese activated caf2-casio3 phosphor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An orange-red emitting, UV energizable phosphor comprising basically lead-manganese activated calcium meta-silicate has incorporated therein CaF2. The composition com-pares in color rendition and brightness to phosphors not in-cluding the CaF2; however, it can be synthesized at much lower temperatures. Processes for manufacturing the phosphor are also disclosed.

Description

3ArKGROUND OF THE Ii1VE~JTIO~I
This invention relates to luminescent materials of the type genexally classified as phosphors; i.e~, materials which, upon being impinged by a given frequency of radiation, will emit radiation of a different frequency. In particular, the inventio`n relates tG a novel orange-red emitting, UV
excitable phosphor of the general composition lead-manganese activated calcium metasilicate having incorporated therein a given amount of CaF20 Lead-manganese activated calcium meta-silicate is a known orange-red emitting, W excitable phosphor.
A particular type of this phosphor is known as Sylvania No. 290, and is available from GTE Sylvania Incorporated, Towanda, Pennsylvania. The phosphor is excitable by 254 nm radiation (ultra violet) and is used in fluorescent lamps. The phosphor has a relatively high synthesis temperature; i.e., in the neighborhood of 1975F to 2050F, it being known that synthesis temperatures lower than the above, e.g., 1860F, will produce an under-reacted and nonluminescent material. Since the higher the firing or synthesizing temperature the greater will be the cost of the phosphor, it would be an advance in the art if a lower ~iring temperature could be achieved while maintaining the emission and excitation characteristics of the known material.
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OBJECTS AND SVMMARY OF_THE_INVENTION
It is, therefore, an object of this invention to obviate the disadvantages of the prior art. ~ ;
It is another object of the invention to provide a ``~
new phosphor material that is less costly to produce than similar materials.
Yet another object of the invention is the provision of a method for making such a phosphor.
These objects are accomplished in one aspect of the ~
invention by a luminescent composition of matter which consists ~ ~-essentially of lead-manganese activated calcium metasilicate containing a small amount of calcium difluoride. Emission and `~
excitation spectra of the new material are equal to that of the older phosphor without the CaF2, and it can be synthesized at `~
a much lower temperature, thereby reducing the cost of prepara-tion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS `~
For a better understanding of the present invention, together with other and further objects, advantages and cap- ;
abilities thereof, reference is made to the following disclosure `~

and appended claims~
~:~
In one aspect of the invention there is provided a luminescent composition of matter consisting essentially of a host matrix of calcium fluoride silicate containing an activator consisting essentially of lead and manganese and wherein the ~ `-said composition has the approximate general formula lCaF2 5Ca- ~ ;
MnSiO3:Pb and the composition range is 0.05-0.5 mole CaF2 per ;
mole of Ca-MnSiO3:Pb. ~
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In the preparation of lead-manganese activated calciurn ;-metasilicate of type No. 290, the synthesis Piring is carried out in the temperature range of 1975-2050F. With firing of the above composition at lower temperatures, say 1860F, the reaction products are under-reacted and nonluminescent.
It has been discovered that the inclusion of calcium difluoride in the above composition lowers the synthesis firing to about 1860F, at which temperature a phosphor material is now ~-~
produced which, as compared to the No. 290, has equivalent color ;~
emission and brightness. The calcium difluoride modified pro- ;
duct has the crystal structure of mixed wollastonite and fluorite minerals.
In the ternary system CaO+activator-CaF2-SiO2 with`lead ;
plus manganese the activator at a composition formulated to provide a ratio of 1 mole or slightly more of silica per l mole of calcium plus manganese, any CaF2 over the very small amount ~ ;
required to form Ca-MnSiO3:PbF2 remains as uncombined CaF2:Mn (fluorite) in the fired product. The fired product in this case, with x equaling an indefinite quantity, can be expressed as:
Ga-MnSiO3 : x PbF2 x Ca-MnF2.
~ Manganese activated fluorite, such as appears in the above-described product, emits ~reen with some lag under spark discharge tube excitation but is inert under ultraviolet excitation. It, therefore, contributes nothing`to emission ~ `
brightness or color under UV excitation and, since it has ~;
very little absorption of the emission of its companion Ca-MnSiO3:Pb, its presence does not appreciably depreciate the total emission intensity.
In Table I, the data show the effect of varying CaF2 `;
doped into a constant amount of lead-manganese activated calcium metasilicate mix prior to firing on emission color and bright-ness when fired at 1860F. Note that sample A fired without ,, ~, . . .

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calcium fluoride catalyst was under-reacted and nonluminescent.
The X-ray diffraction patterns of samples B to F inclusive shows mixture ofL3 CaSiO3 and CaF2 with increasing CaF2 in-tensity from sample s to sample F.
TABLE I ;~

Moles MolesRelative Emission Sample Ca-Mn SiO3: Pb CaF2 Intensity -~
, _ ~
No. 290 (Ca-MnSiO3:Pb Phosphor)100.0 A 1.00 None Inert B 1.00 0.10 100.0 : ~ , C 1.00 0.20 101.0 D 1.00 0.30 100.5 E 1.00 0.40 100.0 F 1.00 0.50 99.0 Calcium carbonate, manganous carbonate, silicic acid, lead fluoride, and calcium fluoride are weighed out in amounts `~
corresponding to a mole ratio of 1.0 calcium carbonate plus ~`
manganous carbonate to about 1.125 silicic acid and about 0.005 to 0.007 lead fluoride and about 0.10 to 0.50 calcium fluoride.
Any mixture of starting materials that when fired form luminescent lead-manganese activated wollastonite-fluorite mixed crystal structure material, may be used without departing from the scope of this disclosure. For example, Sylvania Type No. 290 lead-manganese activated calcium metasilicate may be easily converted to ~ CaSiO3-CaF2 crystal structure by simply -~;
doping with calcium fluoride and firing at abaut 1~60F.
The prefired mixture is rendered homogeneous by wet milling or other suitable means. Water is removed from wet `~

milled mixtures usually by filtering and drying prior to the ;
firing step. The mix is then placed in boats or crucibles and fired in an atmosphere of air or nitrogen. The optimum firing ,' ~ ',~

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temperature depends on -the amount of ca]cium fluoride used as a catalyst and the firing time on the firing container~charge size.
Particular examples of the preparation of these phosphors are described below:

Weigh the following materials in finely-powdered form into a 4 gallon pebble mill containing a standard 28 pound charge of flint pebbles:

Mole Batch ~
Ratio Grams ~ `
CaCO3 0.940 1600 g MnCO3 (44% Mn) 0.060 128. ~ -~
SiO2 x H2O (90% Sio2~ 1.125 1277.
PbF2 0.006 26. ;~
CaF2 0.200 265.
Add 6 liters deionized water to the mill and then roll it for ~ 12 hours at 47 rpm. Then filter as much water as possible from - the slurry before drying it to near bone dry in an oven at about 120C, then break up the oven dried cake by passing it through a Mikropulverizer or by any other suitable means.
Then fire a portion of the pulverized material in a nitrogen ;
atmosphere for 1 hour at 1860F in a silica boat 6 inches long by 3 inches wide by 2-l/2 inches deep filled level full and then cool it in nitrogen to room temperature. Break up the material with mortar and pestle or okher suitable means and then refire it in a nitrogen atmosphere in a 6 inch by 3 inch by 2-l/2 inch deep silica boat for 2 hours at 1860F and then cool it in nitrogen to room temperature. When broken up by mortaring or other means the material will be a soft white body colored product having equivalent emission color and brightness under about 254 nm radiation to Sylvania Type No. 290 lead-manganese ; ' . : .

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activated calcium metasilicate phosphox. -~
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EXAMPLE 2 -~
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Fire a portion of wet milled, dried, pulverized raw mix as described in Example 1 in air for 4 hours at 1860E in a crucible of 2 liter volume about 8 inches tall filled full and then cool it in air to room temperature. Break up the fired, cooled cake and then re~ire it in air in a 2 liter crucible for 4 hours at 1860F and then cool it in air to room temperature. When broken up by hammermilling or other suitable means the material will be soft white body colored having equivalent emission color and brightness under about 254 nm radiation to Sylvania Type No. 290 phosphor. ~- -Weigh out and combine materials at the mole ratio composition described in Example l. Render the mix homogeneous -~
by dry blending in a twin shell blender and then by passing dry ;
through a Mikropulverizer. Fire and refire in 2 liter volume crucibles the same procedure described in Example 2. -~
While there have been shown and described what are at present considered the preferred embodiments of the in~
` vention, it will be obvious to those skilled in the art that -~
various changes and modifications may be made therein without departing from the scope of the invention as defined by the ;`
appended claims. ~
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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A luminescent composition of matter consisting essentially of a host matrix of calcium fluoride silicate containing an activator consisting essentially of lead and manganese and wherein the said composition has the approximate general formula 1CaF2?5Ca-MnSiO3:Pb and the composition range is 0.05-0.5 mole CaF2 per mole of Ca-MnSiO3:Pb.

2. The method of making a luminescent composition of matter having the general formula 1CaF2?5Ca-MnSiO3:Pb which comprises the steps of: homogeneously mixing starting materials which when fired will yield the properly combined reaction products of said general formula; firing said starting materials in an atmosphere selected from-the group consisting of air and nitrogen at a temperature of about 1860°F for about 1 to 4 hours; cooling the first fired product to room temperature in said selected atmosphere;
breaking up the cake formed by said first firing; second firing the resulting product in said selected atmosphere at about 1860°F for about 2 hours; and cooling the resultant product to room temperature in said selected atmosphere and breaking up the resultant product.

3. The method of claim 2 wherein said starting materials consist essentially of: CaCO3; MnCO3; Sio2xH2O;
PbF2; and CaF2.

4. The method of claim 3 wherein said starting materials have the following molar ratios: 0.940 CaCO3;
0.060 MnCO3 (44%MN); 1.125 SiO2xH2O (90% SiO2); 0.006 PbF2;
and 0.200 CaF2.