Firing and milling method for producing a manganese activated zinc silicate phosphor

BACKGROUND OF THE INVENTION
In the production of manganese activated zinc silicate phosphors, excess silicic acid with long blending times are used with fluxes and long firing times in order to achieve a good reaction. This phosphor requires a high manganese concentration that is necessary for high luminescence efficiency and it is difficult to achieve a good blend, especially with the zinc component. Sometimes this phosphor manufactured by these methods results in a brown or yellow discoloration and very poor fluorescent lamps. This is due possibly to minute traces of unincorporated manganese with the zinc component.
Manganese activated zinc silicate phosphors are disclosed in U.S. Pat. Nos. 2,109,984, 2,206,280, 2,210,087, 2,222,509, 2,241,030, 2,245,414, 2,247,142, 2,544,999 and 3,416,019.
Another problem with manganese activated zinc silicate phosphors is the relatively poor fluorescent lamp maintenance. That is, the light output or lumens per watt of the phosphor decreases to a greater extent during lamp life than is desirable. This invention is concerned with a manganese activated zinc silicate phosphor having improved maintenance.
SUMMARY OF THE INVENTION
The present invention is concerned with a method for producing a manganese activated zinc silicate phosphor, the individual particles of which are surrounded with a non-particulate, conformal aluminum oxide coating. The method of the invention comprises dry blending a mixture of components consisting essentially of zinc oxide, silicic acid, a source of manganese, ammonium chloride, ammonium fluoride, tungstic oxide, and silica, wherein the Zn+Mn/Si mole ratio is from about 1.95 to about 2.02, wherein the silica is colloidal and has a surface area of from about 50 to about 410 m.sup.2 per gram, and wherein the colloidal silica makes up from about 0.01% to about 1.0% by weight of the mixture; firing the resulting dry blend of components in a nitrogen atmosphere at a temperature of from about 1200.degree. C. to about 1300.degree. C. for a sufficient time to produce the phosphor; milling the resulting phosphor for a period of time of from about 60 minutes to about 120 minutes; firing the resulting milled phosphor in air at a temperature of from about 1175.degree. C. to about 1275.degree. C. for a sufficient time to diffuse the tungsten and manganese to the surfaces of the phosphor particles; applying a non-particulate, conformal aluminum oxide coating to the outer surface of individual particles of the phosphor while the phosphor particles are suspended in a fluid bed having a midpoint temperature between 400.degree. and 480.degree. C., the aluminum oxide coating being formed by a chemical vapor deposition reaction involving oxidation of trimethyl aluminum; and annealing the aluminum oxide coated phosphor for a period of time from 4-16 hours at a temperature between 750.degree. and 800.degree. C.
The parent hereof is directed to a manganese activated zinc silicate phosphor and method for producing same which comprises dry blending a mixture of components consisting essentially of zinc oxide, silicic acid, a source of manganese, ammonium chloride, ammonium fluoride, tungstic oxide and silica wherein the Zn+Mn/Si mole ratio is from about 1.95 to about 2.02, wherein the silica is colloidal and has a surface area of from about 50 to about 410 m.sup.2 /g, and wherein the colloidal silica makes up from about 0.01% to about 1.0% by weight, firing the blend in a nitrogen atmosphere at a temperature of from about 1200.degree. C. to about 1300.degree. C. for a sufficient time to produce the phosphor, and firing the phosphor in air at a temperature of from about 1175.degree. C. to about 1275.degree. C. for a sufficient time to diffuse the tungsten and manganese to the surfaces of the phosphor particles. Between the firing steps, the phosphor can be milled for from about 60 to 120 minutes to increase luminescence. The phosphor can be coated with a continuous coating of aluminum oxide to improve maintenance of the phosphor.
The parent hereof is also concerned with a fluorescent lamp incorporating the phosphor.

BRIEF DESCRIPTION OF THE FIGURE
In the drawings:
FIG. 1 is a schematic diagram of the steps of a preferred method of practicing the present invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above described drawing and description of some of the aspects of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The embodiment of the invention described herein is a method for producing a manganese activated zinc silicate phosphor, the individual particles of which are surrounded with a non-particulate, conformal aluminum oxide coating. The method of the invention comprises dry blending a mixture of components consisting essentially of zinc oxide, silicic acid, a source of manganese, ammonium chloride, ammonium fluoride, tungstic oxide and silica wherein the Zn+Mn/Si mole ratio is from about 1.95 to about 2.02, wherein the silica is colloidal and has a surface area of from about 50 to about 410 m.sup.2 /g, and wherein the colloidal silica makes up from about 0.01% to about 1.0% by weight, firing the blend in a nitrogen atmosphere at a temperature of from about 1200.degree. C. to about 1300.degree. C. for a sufficient time to produce the phosphor, and firing the phosphor in air at a temperature of from about 1175.degree. C. to about 1275.degree. C. for a sufficient time to diffuse the tungsten and manganese to the surfaces of the phosphor particles. Between the firing steps, the phosphor is milled for from about 60 to 120 minutes to increase luminescence. The method further comprises applying a non-particulate, conformal aluminum oxide coating to the outer surface of individual particles of the phosphor while said phosphor particles are suspended in a fluid bed having a midpoint temperature between 400.degree. and 480.degree. C., said aluminum oxide coating being formed by a chemical vapor deposition reaction involving oxidation of trimethyl aluminum; and annealing the aluminum oxide coated phosphor for a period of time from 4-16 hours at a temperature between 750.degree. and 800.degree. C.
In a preferred embodiment of the process, the midpoint temperature (also referred to herein as mid bed temperature) is from about 420.degree. to about 440.degree. C. and the annealing temperature for the coated phosphor is from about 750.degree. to about 800.degree. C., with an annealing time of about 4-16 hours. Most preferably, the midpoint temperature is about 430.degree. C. and the coated phosphor anneal temperature is about 760.degree. C. to about 770.degree. C. with an anneal time of about 4 hours.
The phosphor produced by the process of this invention is a manganese activated zinc silicate phosphor, also referred to as synthetic willemite phosphor.
The first step in making the phosphor is to dry blend a mixture of the starting components which will be subsequently fired to produce the phosphor. The starting components are zinc oxide, silicic acid, a source of manganese, preferably manganese carbonate, and fluxes such as ammonium chloride, ammonium fluoride and tungstic oxide. Tungstic oxide improves the maintenance of the phosphor as described in U.S. Pat. No. 4,728,450. A typical composition of this mixture is given in the example that follows. The mole ratio of Zn+Mn/Si is from about 1.95 to about 2.02. In addition to the above components, the mixture also includes a source of colloidal silica. The content of the colloidal silica in the mixture ranges from about 0.01% to about 1.0% by weight and preferably from about 0.03% to about 0.3% by weight. This range of silica content allows the resulting phosphor to have reproducible brightness when tested in fluorescent lamps. The colloidal silica is characterized by a high surface area. The surface area is from about 50 to about 410 m.sup.2 per gram, preferably from about 100 to about 300 m.sup.2 per gram and most preferably from about 175 to about 225 m.sup.2 per gram. A preferred colloidal silica for use in the invention is manufactured by Degussa, Inc., New York, N.Y., under the trade name of "Aerosil". According to a technical bulletin by Degussa, Aerosil.RTM. is a colloidal silica manufactured by a flame hydrolysis of silicon tetrachloride in the gas phase. Aerosil.RTM. is supplied in various surface areas as designated by the number after the name. Especially suited to the purposes of the present invention is Aerosil.RTM. which has a high surface area of about 200.+-.25 m.sup.2 /g with a primary particle size of about 12 millimicrons. This is a very pure silica which has unusual electrical and optical properties.
The resulting blended mixture is then fired in nitrogen at a temperature of from about 1200.degree. C. to about 1300.degree. C. and preferably at about 1250.degree. C. for a sufficient time, preferably about 2.5 hours, to produce the phosphor.
The resulting phosphor can be broken up or crushed into smaller pieces or particles at this point.
The resulting phosphor is then fired in air at a temperature of from about 1175.degree. C. to about 1275.degree. C. for a sufficient time to diffuse the tungsten and manganese to the surfaces of the phosphor particles. This step is referred to as the "refiring" step.
The phosphor is preferably subjected to light milling to clean the surfaces of the phosphor particles and to remove any surface manganese which is present. By light milling is meant that the duration of milling time is relatively short, that is, the duration is typically from about 20 to about 30 minutes. The milling is done preferably with citric acid in water. The milled material is washed with water, filtered, dried, and classified such as by sieving to obtain the desired particle size for use in the application. One or more of this series of steps of milling, washing, and classifying is called finishing the phosphor in preparation for its use in the application. The milling is done according to methods well known in the art.
The addition of colloidal silica to the formulation blend as described above achieves a more intimate blend and results in reproducible and high quality phosphor lots when evaluated in a fluorescent lamp. Table 1 exhibits lamp data on nine lots made with 0.1% by weight colloidal silica in the mixture. The data is obtained on material in which the blended mixture is fired followed by the light milling step. The second firing of the phosphor or refiring step is not done. Over about 5,000 lumens at zero hours are achieved in six of the eight logs (1-1, 1-4, 1-5, 1-6, 1-7, and 1-8) with the remaining three lots achieving over about 4900 lumens at zero hours (1-2, 1-3, and 1-9). Table 1 also demonstrates that this high luminosity is achieved at lower NH.sub.4 F concentrations (1/2, 1/4), by drum blending half the normal time (DB, 8 hr) or by V-blending (a preferred blending technique). These results are contrasted with previous lots as listed on Table 2 that are formulated without colloidal silica. Again, the data is obtained on material in which the blended mixture is fired, followed by light milling. The refiring step is not done. These lots are inferior when evaluated in a fluorescent lamp and occasionally result in a brown fired cake. V-blending does not result in high luminosity phosphors as exhibited in lots 2- 3 and 2-14, although this is normally a preferred blending technique.
TABLE 1__________________________________________________________________________Phosphor Formulated with 0.1% by Weight Colloidal Silica LampFormulation Blending Firing Powder DataLot NH.sub.4 F NH.sub.4 Cl Type Time Temp. Wt. Lumens__________________________________________________________________________1-1 Std Std DB 16 hr 7 min Std 7.7 50231-2 1/2* Std DB 8 hr 8 min Std 8.2 49321-3 1/2 Std DB 8 hr 8 min Std 6.4 49781-4 Std Std DB 8 hr 8 min Std 7.6 51151-5 Std Std DB 8 hr 8 min Std 8.1 51091-6 Std Std VB 1 hr 8 min Std 6.1 50701-7 1/4** 2.12x*** DB 8 hr 8 min Std 8.9 51111-8 1/2 Std VB 11/4 hr 8 min Std 6.4 50731-9 1/2 Std VB 11/4 hr 8 min Std 6.4 4906__________________________________________________________________________ Std Standard concentration DB Drum blending VB Vblending * 1/2 of Std ** 1/4 of Std *** 2.12 x Std
TABLE 2__________________________________________________________________________Phosphor Formulated Without Colloidal Silica LampFormulation Blending Firing Powder DataLot NH.sub.4 F NH.sub.4 Cl Type Time Temp. Wt. Lumens__________________________________________________________________________2-1 Std Std DB 16 hr 7 min Std -- 43262-2 Std Std DB 16 hr 7 min Std -- 46992-3 Std Std VB 1 hr 7 min Std -- 44192-4 Std Std DB 16 hr 7 min Std -- 46642-5 Std Std DB 16 hr 7 min Std 8.3 48202-6 Std Std DB 16 hr 7 min Std 9.7 47392-7 Std Std DB 16 hr 7 min Std 10.6 45122-8 Std Std DB 16 hr 7 min Std 10.6 45122-9 Std Std DB 16 hr 7 min Std 10.3 45652-10 Std Std DB 16 hr 7 min Std 11.5 47822-11 Std Std DB 16 hr 7 min Std 11.7 46662-12 Std Std DB 16 hr 7 min Std 12.0 48052-13 Std Std DB 16 hr 7 min Std 11.9 47192-14 Std Std VB 1 hr 7 min Std 13.3 48262-15 Std Std DB 16 hr 7 min Std 10.1 4836__________________________________________________________________________
To demonstrate that the addition of colloidal silica improves the intimacy of the blend, a test of reduced blending time is completed using a drum blender. The blending time is reduced from the typical 16 hours to about 4 hours when the colloidal silica is used according to the present invention.
Table 3 below shows the length of drum blending time with addition of Aerosil.RTM. on lamp performance. The data is obtained on material in which the dry blended mixture is fired, followed by the light milling. The refiring is not done.
TABLE 3______________________________________ 0 Hr. Blending Lamp Time Powder OutputSample (HR) Weight Lumens Comments______________________________________3-1 4 6.5 50713-2 8 7.1 50153-3 10 7.4 50423-4 12 7.0 50653-5 14 6.6 50433-6 16 7.2 4928 Normal Blending Time Control3-7 -- 4.9 4367 Commercial Phosphor - Lamp Control______________________________________
It can be seen that when Aerosil.RTM. is used, reproducible brightness of over about 5000 lumens is attained with less blending time (a decrease from about 16 hours to about 4 hours). As previously demonstrated in Table 2, without Aerosil.RTM. a 16 hour blending time does not reproducibly achieve a high lumen quality.
The incorporation of the colloidal silica into the initial mixture of components results in greater flexibility of the mole ratio of (Zn+Mn)/Si than when this silica is not used.
Prior to the present invention, the mole ratio of (Zn+Mn)/Si is limited to 1.97 to achieve a high degree of reactivity and good quality brightness in a lamp. With the addition of colloidal silica according to the present invention, greater latitude in the mole ratio is possible. For example, with colloidal silica, a mole ratio of (Zn+Mn)/Si of >2 such as 2.02, a brightness of >5000 lumens at zero hours is achieved when the phosphor is tested in a lamp. The brightness is high also at lower mole ratios of Zn+Mn/Si of, for example, 1.99, 1.98, and 1.95. Accordingly, a mole ratio of (Zn+Mn)/Si in the range of from about 1.95 to about 1.99 is now also useful.
It is believed that the advantage of the incorporation of the colloidal silica into the mixture is that the reaction of the zinc with the manganese and the silicon is more complete as opposed to incompleteness when the colloidal silica is not used. This incomplete reaction is indicated by a brownish color in the fired material or poor luminosity in fluorescent lamps. The brown material contains unreacted zinc oxide with manganese present.
Once the manganese activated zinc silicate phosphor is made as described above, that is, by blending the components, firing and refiring, with the optional light milling, water washing, filtering and drying steps, the phosphor particles can be coated with a continuous coating of aluminum oxide and annealed. This coated and annealed phosphor exhibits increased maintenance or longer life compared to phosphors produced which are not alumina coated and annealed.
The advantages associated with the phosphor of the present invention are illustrated by the lamp performance of the phosphor when particles are individually coated with a continuous protective coating. Individually and continuously coated phosphor particles are prepared by depositing a continuous, nonparticulate, conformal, aluminum oxide coating on the outer surfaces of the individual phosphor particles. Such coating can be applied by coating methods such as, for example, chemical vapor deposition or preferably chemical vapor deposition in a fluidized bed.
The coated phosphor particles are annealed in an air atmosphere at a temperature of from about 700.degree. C. to about 850.degree. C. for a period of from about 15 minutes to about 20 hours. Preferably, the coated phosphor particles are annealed in air at a temperature of from about 750.degree. C. to about 800.degree. C. for a period of from about 4 to about 16 hours; and most preferably at a temperature of from about 760.degree. C. to about 770.degree. C. for about 4 hours.
Between the firing and refiring steps, the phosphor can be milled usually at room temperature for a period of time of from about 60 minutes to about 120 minutes and preferably for about 90 minutes. This milling step is called long-milling to distinguish it over the previously described light milling step. Normally, the long and light milling procedures are the same except for the milling times. The milling is done by methods well known in the art. The milling is done in citric acid. Prior to the refiring step, the long-milled phosphor is normally washed, filtered, dried and classified.
The long-milled and refired phosphor can then be light milled, washed, filtered, dried, and classified (finished) as described previously. FIG. 1 is a schematic diagram of the preferred steps of the process of the present invention that includes the firing step, the long milling step, the refiring step, the finishing steps (light milling and washing) and finally, the coating and annealing steps.
The long-milled and refired phosphor can be coated or coated and annealed as described previously.
In accordance with one embodiment of this invention, there is provided a fluorescent lamp comprising a light transmitting sealed glass envelope of circular cross section containing electrodes at each end supported by lead-in wires which extend through glass presses in mount stems to the contacts in bases which are affixed to the ends of the lamp. The envelope is filled with an inert gas such as argon and neon at a lower pressure, for example, two torr, and a small quantity of mercury, at least enough to provide a low vapor pressure of about six microns during operation. The interior of the envelope is coated with at least one layer of a blend of phosphors. The blend comprises at least one phosphor of the present invention.
A phosphor coating suspension is prepared by dispersing the phosphor particles in a water base system employing polyethylene oxide as the binder with water as the solvent. The phosphor suspension is applied in the usual manner of causing the suspension to flow down the inner surface of the envelope and allowing the water to evaporate, leaving the binder and phosphor particles adhered to the envelope wall. The phosphor coated envelope is then heated in a lehr to volatilize the organic components, the phosphor layer remaining on the envelope wall.
The envelope is processed into a fluorescent lamp by conventional lamp manufacturing techniques.
To more fully illustrate this invention, the following non-limiting example is presented.
EXAMPLE 1
The following blend is made up:
______________________________________ Mole Ratio Amount (kg)______________________________________ZnO 2.00 282.0Silicic acid 1.077 127.152Aerosil .RTM. 200 0.0043 0.448MnCO.sub.3 0.1217 26.222 Total 435.822______________________________________
The above materials, except for the ZnO, are put into a 40 cubic foot V-blender with an intensifing bar and blended for about 20 minutes. The ZnO is added to this same V-blender and then blended for about 20 minutes followed by intensifying with blending for about 20 minutes. (This is blend #1). To an 8 quart V-blender with an intensifying bar is added about 2.5 kg of the above blend and the fluxes (blend #2) given below. The resulting new blend with the fluxes is blended with intensifying bar for about 30 minutes.
______________________________________ Mole Ratio Amount (kg)______________________________________NH.sub.4 Cl 0.0465 383.0NH.sub.4 F 0.0040 22.8WO.sub.3 0.00116 41.4______________________________________
About 36.237 kg of blend #1 is added to a 5 cubic foot V-blender with an intensifying bar and blended for about 10 minutes. To this blended mixture is added blend #2 and the resulting new blend is blended for about 15 minutes and blended and intensified for another 60 minutes. This results in a blend with a formulation as given below.
______________________________________ Mole Ratio______________________________________ZnO 2.00Silicic acid 1.077Aerosil .RTM. 200 0.0043MnCO.sub.3 0.1217NH.sub.4 Cl 0.0465NH.sub.4 F 0.0040WO.sub.3 0.00116______________________________________
This formulation is fired in a continuously stoked furnace at about 1250.degree. C., stoked at about 8 minutes per crucible, and purged with nitrogen. The crucibles are alumina with covers that each contain about 500 g of this formulation. The resulting phosphor cakes are skinned to remove any dull luminescence material. The material at this stage is called the fired cake (FC).
This fired cake is finished by the following procedure. A 12 gallon ball mill is filled with from about 8 to about 12 kg of the fired cake, about 14 liters of deionized water, and about 4 g of citric acid per kg of fired cake. This mill contains also 94 lb. of 3/8" alumina balls and is rotated at about 27 RPM for about 20 minutes to light-mill the phosphor. The light-milled phosphor is transferred to a 50 gallon tank and washed six times for 10 minutes with settling times of 3.0, 2.0, 1.5, 1.0, 0.5 hours, respectively. This material is filtered and oven dried at about 230.degree. C. for about 16 hours. Finally, the material is sieved through a 400 mesh screen.
EXAMPLE 2
In order to make refired material, the above fired cake (FC) is then fired again at about 1225.degree. C., stoked at from about 8 to about 12 minutes/crucible, open to air. Silica crucibles without covers are used in this firing with about 500 g of the FC material/crucible. The material is skimmed of dull luminescent material and is called the refired cake (RFC) at this stage. This material is finished as in Example 1.
EXAMPLE 3
In order to make milled and refired material, the above material from Example 1 is first long-milled. This is done is the same way as the finishing step except that the milling time is increased to about 90 minutes. The material is then washed, filtered and dried the same as in the finishing step, but is not sieved. The long-milled phosphor is then refired as previously described. This material is then finished as described previously.
The phosphors of Examples 1, 2, and 3 are coated and annealed as follows:
Approximately 1500 grams of phosphor with approximately 0.05-0.1% by weight of a fluidization aid such as Aluminum Oxide C, available from Degussa, Inc., is loaded into a fluid bed column comprising a 80 millimeter ID quartz tube having a quartz frit fused to the bottom, acting as a distributor plate. A 65 millimeter stainless steel agitator disc is positioned inside the quartz tube. The agitator disc is attached to a vibromixer agitator. Approximately 50 millimeters from the base of the agitator, a two micron stainless steel filter element is welded in line and functions as the diffuser of the oxygen mixture. The agitator disc itself is located approximately 25 millimeters above the quartz distributor. A series of approximately eleven copper coil windings of 1/4" tubing are located immediately around the frit located at the bottom of the quartz tube, such that one coil is below the distributor and the remaining ten coils are above the distributor plate. In addition, there is a copper foil of approximately 120 millimeters which is sandwiched between the cooling coil and the quartz tube to provide improved heat transfer, and there is insulation located above the copper coil windings to further reduce heat transfer between the heated and the unheated portions of the tube. This insulation comprises approximately 50 millimeters of 1" wide by 1/2" thick Fiberfax roll insulation. The edge of the Fiberfax insulation matches exactly the level between the unheated and heated part of the 3-Zone Lindberg furnace, that is, between the bottom and center zones of the furnace. The furnace zones are 6", 12", and 6" in length, and a spike thermocouple is located at the midpoint of each zone. The first two furnace zones are set for 500.degree. C. and the last zone is shut off. The amount of phosphor charged into the column is 1500 grams, and the height of the phosphor bed is approximately 500 mm. The operating conditions for the fluid bed CVD alumina coating of the phosphors are shown in Table 4. The fluid bed temperature is recorded by a thermocouple located within the fluid bed half way between the distributor plate and the top of the expanded bed. A second thermocouple is located between the agitator and distributor. The temperatures quoted in Table 4 are the minimum and maximum temperatures measured during the actual coating operation. The gasses that pass through the distributor plate at the bottom of the quartz column have flow rates as shown in Table 4. For the bubbler, 1500 cc/min. of an inert gas such as nitrogen, argon, helium, neon, or mixtures thereof, and for the carrier, 1800 cc/min. of the inert gas. A fluidized phosphor bed is formed by passing the inert gas upwardly through the phosphor particles in order to suspend the particles in the inert gas stream. In addition to supporting the phosphor particles in a fluidized bed, the inert gas functions as a carrier gas for the vaporized trimethyl aluminum. The inert gas is passed through the bubbler containing liquid trimethyl aluminum at approximately 30.degree. C. and the liquid trimethyl aluminum is vaporized into the inert gas before passing through the distributor plate into the fluidized phosphor bed. A continuous protective alumina coating is formed on the surface of the individual phosphor particles when the vaporized trimethyl aluminum is exposed to the oxygen at a temperature sufficient for a reaction between the oxygen and the vaporized trimethyl aluminum to occur. The oxygen, as an oxygen/inert gas mixture is introduced at 2500 cc/min. for O.sub.2 and 50 cc/min. for N.sub.2 into the fluidized bed through the two micron filter elements located on the shaft of the vibrating mixer above the vibrating disc. The coating times are shown in Table 4. Once the particles of the phosphor are coated with a continuous coating of alumina, the alumina coated phosphor powder is transferred into a quartz boat of dimensions 21/4" (height).times.4" (width).times.10" (length). Typical powder loading is 750 gms. The boat is inserted into a six-inch diameter MAXIBRUTE.RTM. tube furnace which is purged with 11/2 liters per minute of air. The furnace is then brought up to the annealing temperature from room temperature in about an hour. The annealing temperature, as measured by a calibrated thermocouple embedded in the powder, is 765.degree. C. The powder remains at temperature for 4 hours and then is allowed to cool overnight. After the annealing step, the phosphor is coated in 20W-T12 or 40W-T12 lamps using a conventional water base suspension system. The coated lamps are then processed into finished lamps and evaluated.
Tables 5, 6, and 7 show data on fluorescent lamps in which the phosphors are prepared according to the general procedures described in Examples 1, 2, and 3, compared to a control Type 2293 (Ce, Tb) MgAl.sub.11 O.sub.19 phosphor.
TABLE 4______________________________________Fluid Bed Coating ParametersTemperature** .degree.C. Alon C* Below AgitatorSample wt. % Agitator Mid Bed Coating Time______________________________________Example 1 0.1 401/420 410/434 4 hr.Example 2 0.1 392/450 415/446 3 hr. 45 min.Example 3 0.1 400/418 410/442 3 hr. 30 min.Lot #1Example 3 0.05 320/410 378/448 4 hr. 7 min.Lot #2Example 3 0.05 359/448 392/449 4 hr. 7 min.Lot #3______________________________________*Aluminum Oxide C**Temperatures quoted are the minimum and maximumtemperatures measured during the actual coatingoperation.Charge - 1500 gmsFlow Rates:Bubbler 1500 cc/min.Carrier for bubbler 1800 cc/min.Oxygen 2500 cc/min.Carrier for oxygen 50 cc/min.______________________________________
TABLE 5__________________________________________________________________________ % M % M Density Powder 0 Hr. 100 Hr. % M 500 Hr. 100- 1000 Hr. 100-Example # g/cc Weight Lumens Lumens 0-100 Hr. Lumens 500 Hr. Lumens 1000 Hr.__________________________________________________________________________Set I (40WT12)Control 78.5 6.37 4913 4773 97.2 4704 98.6 4689 98.21 76.3 8.53 4678 4490 96.0 4388 97.7 4332 96.5withoutrefiring2 RF 76.1 8.44 4806 4601 95.7 4480 97.4 4401 95.73 RF 76.9 8.17 4938 4698 95.1 4598 97.9 4615 98.2Set IIControl 78.5 2.97 2008 1919 95.6 1872 97.6 1854 96.63 MF 77.5 4.25 2049 1926 94.0 1841 95.6 1819 94.43 MF 77.3 4.05 2026 1929 95.2 1852 96.0 1835 95.1__________________________________________________________________________ Control (Ce, Tb)MgAl.sub.11 O.sub.19 RF Refired phosphor MF Longmilled and refired phosphor
Table 5 contrasts the three procedures of "without refiring, " (Example 1), "refiring," (Example 2) and "milling and refiring" (Example 3). This Table details fluorescent lamp data on the alumina coated and annealed Zn.sub.2 SiO.sub.4 :Mn phosphor after applying the coating. From this data it can be seen that the refired phosphor RF and the long-milled and refired phosphor MF processes exhibit superior luminosity at 0 hr. and 100 hr. and through to 1,000 hr. of lamp operation, compared to the material which is not refired. The processes of Examples 2 and 3 exhibit a lower powder weight and better lamp texture. The long-milled and refired phosphor results in the highest lumen output at the lowest powder weight. These processes appear to be reproducible as exhibited in the samples in Set II. It should be noted also that the high 100 hr. lumen and maintenance levels obtained with refired and long-milled and refired phosphors are comparable to the (Ce, Tb)MgAl.sub.11 O.sub.19 phosphor which serves as the control.
The phosphor characteristics before the coating is applied are shown in Table 6 for these same sample lots. Table 6 details powder data for particle size and chemical composition. From this data it can be seen that the refired and long-milled and refired phosphors of Examples 2 and 3, respectively, both exhibit good brightness. These processes appear to narrow the particle size distribution (lower QD as measured by Coulter Counter technique), especially be lowering the coarse fraction (higher NS). The particle size appears to be slightly reduced, as measured by Coulter Counter or BET (higher surface area) measurements. These processes result in reduction of the bulk tungsten level from about 610 to about 520 or 290 weight ppm, respectively. Finally, the 0 hr. output in a fluorescent lamp appears to be slightly improved, even though uncoated for Set I. The remilled and refired phosphor results in improvement in all these results, compared to the refired phosphor. These processes appear to be reproducible in all the powder and fluorescent lamp characteristics, as exhibited on the samples of Set II.
Tungsten is purposely added to the phosphors of Examples 1, 2, and 3 in the initial blending step, mainly to improve particle growth and to improve fluorescent lamp maintenance, as taught in U.S. Pat. No. 4,728,459. Too high a tungsten level results in an undesirable product after coating. This tungsten level is significantly reduced by the long-milling and refiring process and slightly by the refiring process. These results can be seen in Table 7. To further explore this point, Table 7 exhibits chemical analysis by bulk (atomic absorption) and surface (ESCA) of Zn.sub.2 SiO.sub.4 :Mn phosphors. Table 7 includes samples of materials during various stages of the processes and finished phosphors. Theoretical atomic percentages are also included, that are based on actual formulation for the particular lots. A brief description of the relationships of these samples to the processes is given below.
TABLE 6__________________________________________________________________________Set I Particle Size (.mu.m) Lamp DataBET Counter Counter Bulk W Lumens % MNumber SA 50% Sonic QD NS (ppm) 0 Hr. 100 Hr.__________________________________________________________________________Example 1 .21 11.0 0.33 2.5 610 4940 80.0withoutrefiringExample 2 .23 11.2 0.32 3.5 520 4943 80.7RFExample 3 .29 9.6 0.25 4.0 290 5009 74.9MFLot #1Set IIExample 3 .26 9.5 0.25 4.0 250 4699 70.6MFLot #2Example 3 .23 9.4 0.24 4.0 170 4952 80.1MFLot #3__________________________________________________________________________
7-1 no refiring or long-milling (as in Example 1) without finishing.
7-2 no refiring or long-milling (as in Example 1) with finishing.
7-3 refired (as in Example 2) without finishing.
7-4 refiring (as in Example 2) with finishing.
7-5 long-milled and finished, but without classifying.
7-6 long-milled and refired (as in Example 3) without finishing.
7-7 long-milled and refired (as in Example 3) with finishing.
TABLE 7______________________________________Atomic Percent Bulk Surface (Atomic Absorption) ESCANo. Mn W Mn W______________________________________Theoretical 1.625 0.0155 1.625 0.01557-1 1.81 0.0130 4.0 2.17-2 1.78 0.0105 1.3 0.147-3 1.75 0.0113 3.9 2.07-4 1.75 0.0088 1.3 07-5 1.75 0.0101 0.95 07-6 1.78 0.0103 4.7 2.57-7 1.67 0.0048 1.0 0.06______________________________________
From the data in Table 7, it appears that the surface tungsten and manganese level is increase on fired materials (7-1, 7-3, and 7-6) and that finishing reduces or eliminates these surface concentrations. This results in reduced bulk levels, especially in the tungsten as exhibited in Tables 6 and 7. Thus, the MF, and to a lesser extent, the refiring procedure, allows for the bulk tungsten to be reduced in the finished phosphor so that its detrimental effect on the overall product is not observed, and to utilize its particle growth and maintenance benefits.
Lamp Tests
Two lots of "MF" phosphor were prepared by the method similar to that described in Example 3, above. The two phosphor lots were blended to form Lot 51/52. Three series of lamp tests were conducted using samples of Lot 51/52: (1) to test the effect of the mid bed temperature during the coating process on the performance of the coated phosphor (Tables 8 and 9); (2) to test the effect of anneal time on the performance of the coated phosphor (Tables 8 and 9); and (3) to test the effect of coating thickness on the performance of the coated phosphor (Table 10).
Samples of Lot 51/52 were coated using the above-described coating apparatus.
The first two furnace zones were set for temperatures between approximately 400.degree. and 600.degree. C. and the last zone was shut off. Using approximately 2000 gms of phosphor for each coating run, the height of the phosphor bed was approximatelY 525 mm. The fluid bed temperature was controlled by a thermocouple located within the fluid bed halfway between the distributor plate and the top of the expanded bed. Adjustments were made to the furnace temperature to achieve and maintain an approximately constant mid bed temperature. The gasses that passed through the distributor plate at the bottom of the quartz column had flow rates of 1750 cc/min of nitrogen gas which had passed through a bubbler held at 30.degree. C. containing trimethyl aluminum liquid. An additional 1800 cc/min of nitrogen was used as a carrier gas. The oxygen as an oxygen/nitrogen gas mixture was introduced at 2500 cc/min with an additional 50 cc/min as a nitrogen carrier gas into the fluidized bed through the two-micron filter element located on the shaft of the vibrating mixer above the vibrating disc. The coating time was selected to achieve the desired coating thickness. The effective coating thickness is derived from the following equation: ##EQU1## The surface area of the base phosphor was determined to be 0.40 meters .sup.2 /gm from a Quantachrome single-point BET measurement. The thickness of the coating was achieved by varying the coating time, keeping all other coating parameters constant. Once the particles of the phosphor were coated with a continuous coating of aluminum oxide, the coated phosphor powder was transferred into a quartz boat of dimensions 21/4" (height).times.4" (width).times.10" (length). The powder loading was 750 gms. The boat was inserted into a six-inch diameter MAXIBRUTE.RTM. tube furnace which was purged with 11/2 liters per minutes of air. The furnace was then brought up to the annealing temperature from room temperature in about an hour. The annealing temperature, as measured by a calibrated thermocouple embedded in the powder, was approximately 768.degree. C. The powder remained at temperature for the appropriate annealing time and then was allowed to cool overnight. After the annealing step, the phosphor was incorporated into 40WT12 fluorescent lamps using known techniques and evaluated.
Tables 8-10 report the data for the three above-mentioned series of lamp tests. Each lamp test included 5 lamps. The Tables also report the optical density of the phosphor coating, as measured by an optical density test kit. Such parameter and measurement technique are well-known in the lamp manufacturing art. The Tables also report the total weight of the coated phosphor included in the lamp (P.Wt.) The data reported under the heading "0 Hour", "100 Hour", and "500 Hour" in each of Tables 8-10 are the lumen output measured after the stated number of hours of operation. The heading "%M" represents the maintenance of the lamp as a percentage. Maintenance is calculated by the following formula: ##EQU2## For the data reported in Tables 8-10, "x" is zero (0), and "y" has the value corresponding to the number of hours indicated in the heading for the column in which the data appears. The data reported for each lamp test represents the average value obtained for the lamps used in the particular test. Also shown in Tables 8 and 10 is the carbon content of the finished phosphor (coating and base phosphor) as determined by LECO carrier gas fusion. The virgin (uncoated) phosphor was evaluated at 20-30 ppm.+-.10 ppm. The lifetest data in Table 8 was generated by the coated phosphor included in 20WT12 fluorescent lamps and then converted to corresponding 40WT12 performance values using established conversion factors. This conversion permits the data to be compared with that obtained from 40 watt T12 fluorescent lamps. The data in Tables 8 and 9 show that maximum performance is achieved when the mid bed coating temperature is 430.degree. C. with an annealing time of 4 hours. Table 10 also reports the % of aluminum oxide in the coated phosphor included in the lamp. Data is also given for lamps employing cerium terbium magnesium aluminate phosphor (`CAT` phosphor (FHX164)), which lamps were simultaneously fabricated and lifetested. The CAT phosphor was not coated with aluminum oxide in any of the CAT lamp tests. Comparisons show that the annealed phosphor is superior to the CAT in both initial brightness and maintenance compared to CAT at 430.degree. C. coating temperature and 4 hour annealing schedule. At the lower coating temperature, the initial brightness and/or maintenance suffers. Extended annealing reduces initial brightness. At the higher coating temperature, the maintenance at 500 hours starts to suffer.
The effects of coating thickness have been examined over the range 75 to 300A. As the data in Table 10 demonstrates, there is no dependence of brightness or maintenance on coating thickness over the range of investigation. An aluminum oxide coated phosphor having from about 2.6 to 2.8% aluminum oxide is useful.
TABLE 8__________________________________________________________________________COATING TEMPERATURE SERIES, LOT 51/52 REMILLED & REFIRED PHOSPHOR,20WT12 (CORRECTED LUMENS = 2.42X) ANNEAL 768.degree. C.Lamp Mid bed (0-100)Test Temp(.degree.C.) Density P. Wt. 0 Hr 100 Hr % M 500 Hr % M Comments.sup.t Carbon__________________________________________________________________________1 320 78.7 2.38 2813 2611 92.8 2265 80.5 Coated 1,0002 320 77.8 3.77 4780 4442 92.9 3903 81.6 4 hr. anneal 8603 320 78.1 3.88 4496 4142 92.1 3678 81.8 8 hr anneal 5704 320 78.1 3.89 4499 4067 90.4 3625 80.6 16 hr anneal 4905 360 78.5 3.53 4082 3616 88.6 3298 80.8 Coated 4506 360 77.9 3.89 5041 4709 93.4 4453 88.3 4 hr anneal 4107 360 77.5 3.64 4963 4649 93.7 4242 85.5 8 hr anneal 3908 360 77.7 3.78 4830 4616 95.6 4262 88.2 16 hr anneal 360CAT 360 78.3 2.81 4940 4776 96.7 4596 93.59 400 77.5 3.81 4770 4427 92.8 4145 86.9 Coated 11010 400 77.7 3.86 4984 4809 96.5 4721 94.7 4 hr anneal 9511 400 77.5 3.68 5018 4795 95.6 4750 94.6 8 hr anneal 8712 430 77.4 3.75 4775 4436 92.9 4242 88.8 Coated 5613 430 76.9 3.38 5023 4865 96.9 4787 95.3 4 hr 4714 430 77.2 3.56 4936 4741 96.1 4716 95.6 8 hr 4515 475 77.2 3.55 4481 4249 94.8 4196 93.7 Coated 3816 475 77.2 3.52 4972 4726 95.0 4660 93.7 4 hr 3217 475 77.8 3.87 4913 4644 94.5 4547 92.5 8 hr 31CAT 475 78.3 2.81 4912 4789 97.5 4596 93.5__________________________________________________________________________ .sup.t `coated` means CVD coated only, not annealed; times refer to annealing time carried out on the CVD coated phosphor. The % by weight of Al.sub.2 O.sub. 3 is determined to be between 2.6 and 2.8 for these samples which corresponds to approximately 150A effective coating thickness.
TABLE 9__________________________________________________________________________COATING TEMPERATURE SERIES, LOT 51/52 REMILLED & REFIRED PHOSPHOR,40WT12 ANNEAL 768.degree. C.Lamp Mid bed 0-100 HrTest Temp(.degree.C.) Density P. Wt. 0 Hr 100 Hr % M 500 Hr % M Comments Carbon__________________________________________________________________________18 430 78.3 9.15 4203 4045 96.2 3673 87.4 Coated 5419 430 78.7 9.83 4884 4784 98.0 4687 96.0 4 hr Anneal 4720 460 78.3 8.77 4250 4128 97.1 3874 91.2 Coated 3921 460 78.5 9.33 4884 4712 96.5 4564 93.5 4 hr Anneal 3122 480 78.4 9.25 4559 4414 96.8 4156 91.2 Coated 3223 480 78.7 9.62 4836 4722 97.6 4536 93.8 4 hr Anneal 32CAT 480 79.2 5.98 4808 4655 96.8 4554 94.7__________________________________________________________________________
TABLE 10__________________________________________________________________________COATING THICKNESS SERIES, LOT 51/52 RMF - 40WT12 Coating Carbon Al.sub.2 O.sub.3Example Thickness Density P. Wt. 0 Hr 100 Hr % M 500 Hr % M (ppm) (by weight)__________________________________________________________________________24 75A 78.8 9.09 4904 4667 95.1 4541 92.6 35 1.325 100A 78.7 9.08 4932 4704 95.4 4551 92.3 40 1.726 125A 78.9 9.48 4901 4691 95.7 4529 92.4 38 2.127 150A 79.2 10.09 4952 4767 96.3 4696 94.8 47 2.628 175A 79.2 9.74 4855 4691 96.6 4537 93.4 47 2.729 200A 79.2 9.46 4931 4703 95.3 4594 93.2 50 3.030 300A 78.7 9.33 4928 4730 96.0 4604 93.4 57 4.4CAT 79.0 6.49 4921 4740 96.3 4654 94.6__________________________________________________________________________
While there has been shown and described what are at present considered the preferred embodiments of the invention, 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.

Number	Name	Date
2118091	Levenenz	May 1938
2210087	Leverenz	Aug 1940
3208950	Yocum et al.	Sep 1965
3416019	Kaduk	Dec 1968
4585673	Sigai	Apr 1986
4710674	Sigai	Dec 1987
4728459	Fanetal	Mar 1988
4797594	Sigai et al.	Jan 1989
4803400	Peters et al.	Feb 1989
4825124	Sigai	Apr 1989

Firing and milling method for producing a manganese activated zinc silicate phosphor

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