Title of Invention

A PROCESS FOR THE PREPARATION OF AFTERGLOW PHOSPHORS

Abstract

A process for the preparation of afterglow phosphors which comprises of substances selected from Group A to Group C in the proportion mentioned thereagainst- The above mentioned substances are dissolved in sol vents selected from Group D in the proportion indicated therein. Fuel selected from Group E is added thereto in the proportion indicated therein and the same is made into a s1urry, The said slurry is placed in a furnace preheated to a temperature between 100 deg.C and 300 deg.C to combust the slurry and result in a white fluffy solid which is removed from the furnaceand cooled to room temperature. The resulting mixture is heated between 300 deg.C and 1000 deg.C for 0.5 to 3 hours in air/N2/Ar/running vacuum and the heated mixture is cooled to room temperature;- mixing a substance selected from Group F in the proportion indicated therein and the resultant mixture is pelletised. The pellets are then heated between 1000 deg.C and 1800 deg.C in a furnace in a reducing atmosphere using H<SUB>2</SUB>-Ar mixture/ H<SUB>2</SUB> N<SUB>2</SUB> mixture/NR<SUB>3</SUB>/ CO<SUB>2</SUB>. mixture for 0.2 hours to 6 hours. The pellets are then quickly cooled to room temperature in a reducing atmosphere and are crushed thereafter to the desired particle size to obtain the said after-glow phosphor.

Full Text

This invention relates to a process for the preparation of afterglow phosphors. More particularly, this invention relates to a process for the preparation of alkaline earth aluminate phosphors exhibiting a range of colours depending on the choice of the starting materials. The intensity and duration of the glow are higher than those of phosphors known to the art. The process can be carried out economically. Basically, the process proposed herein has three steps: (i) the first step involves the homogenization of metal salts in a solution medium (ii) the second step involves the combustion or flame pyrolysis or spray pyrolysis of the aforesaid mixture (this step yields an atomic level homogeneous precursor) and (iii) the third step involves a reduction step yielding the final product. The final product is superior in glow quality at lower dopant levels and is also cost effective. Copper doped ZnS is a well known phosphor exhibiting green emission which peaks at 530 nm (nano metres) in the visible region. However, the phosphor is not bright enough for many applications and the glow does not last more than 1-2 hours. In the last few years a new rare earth doped aluminate (oxide) phosphor has been developed which emits at 520 nm (green). The brightness and afterglow of the new phosphor are more than 10 times the best known ZnS based phosphors. Such phosphor materials are synthesized by conventional ceramic route. This method involves the reaction of two or more solids in the required atmosphere at high temperatures (1300 - 1600 °C) for several hours. The reactions are very slow and require repeated grindings in order to obtain homogeneous end products. In addition, other major disadvantages are the secondary phase formation as well as inhomogeneous distribution of the dopant ions in the crystal lattice of the product. The above inadequacies can be overcome by employing the process proposed herein involving self-propagating high-temperature synthesis or "combustion synthesis". The combustion synthesis process when applied to the preparation of oxide materials typically involves mixing of fuel and solution of metal salts of reactants and combusting the mixture. Since the reactants are mixed in solution at molecular level, this process can yield oxide phosphorescent products with high homogeneity in a short period of time. Alternatively, the solution can be flame pyrolyzed or spray pyrolyzed at high temperatures to obtain the product. The aluminate compositions synthesized by the process proposed herein are stable. Powder X-ray diffraction analysis revealed the formation of single phase crystalline aluminates. Samples exhibit long after-glow lasting for 6-12 hours depending upon the colour and the property is uniform throughout the product. The luminescence intensity of the samples is substantially more than samples synthesized by conventional solid state reaction method. The process proposed herein has the advantage of shorter cycle time thus increasing throughput and reducing the cost. Due to the uniform distribution of the dopant the process proposed herein calls for lower level of otherwise costly dopant, that is , rare earths, for a comparable after-glow. The phosphor has the general formula Ax.aMy.bOzR aR b where A is the alkaline earth metal ion (Mg, Ca, Sr, Ba or a mixture of them), M is the metal ion chosen from the group consisting of Al, B, Y, Si, Ga or a mixture of them and R1 is the rare earth metal ion, Eu which acts as the activator and R is the rare earth metal ion chosen from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y or a mixture of them which acts as the co-activator. The starting materials for the preparation of the said afterglow phosphor are selected from the following groups: Group A: A group of alkaline earth metal ions consisting of Mg, Ca, Sr, Ba or a mixture of them constituting 2-45% of the weight of the final product. These metal ions are in the form of oxides, hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonates, a mixture of them or any other soluble salts which do not interfere with the phosphorescent property. Group B: A group of rare earth metal ions with Eu as activator which constitutes 0.001-10% of the weight of the final product and another metal ion chosen from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y or a mixture of them as co-activator, constituting 0.001-10% of the weight of the final product. These are in the form of oxides hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonates, a mixture of them or, any other soluble salts which do not interfere with the phosphorescent property. Group C: A group of metal ions consisting of Al, B, Y, Si, Ga or a mixture of them constituting 2-45% of the weight of the final product. These metal ions can be taken in the form of oxides, hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonates, a mixture of them or any other soluble salts which do not interfere with the phosphorescent property. Group D: The solvent can be water, mineral acid which includes nitric acid and perchloric acid or a base which includes NH3/NF4OH, glycine or a mixture of them. The weight ratio of the reaction mixture to solvent ranges from 1:0.2 to 1:50. Group E: The fuel used can be urea, oxalyldihydrazide, substituted urea, carbohydrazide, maleic hydrazide, maleno hydrazide, tetraformyl hydrazine, dimethyl hydrazones, diformyl hydrazine, glycine, substituted glycine, 3-methyl-5-pyrazolone, or a mixture of them and the fuel ratio is 20-500% by weight of the reaction mixture. Group F. A flux is selected from the group consisting of H3BO3, H3BO3 B2O3, HBO3,borax or A1F3 -The quantity of theflux added is in the range 0.2- 2(m by weight of the final product The process, according to this invention, for the preparation of afterglow phosphors compris the dissolution of substances, selected from Group A Group B and Group C in the solvents selected from Group D; adding thereto a fuel selected from Group E; and making the same into a slurry; placing the slurry in a furnace pre-heated to a temperature between 100 degree C and 800 degree C to combust the said shiny and result in a white fluffy solid; removing the said solid from the furnace and cooling the same to room temperature; heating the resulting mixture between 300 degree C and 1000 degree C for 0.5 hours to 3 hours in air/N2 7Ar/running vacuum and cooling the heated mixture to room temperature; mixing a substance selected from Group F with the resultant mixture and pelletising the same; heating the pellets betwwen 1000 degree C and 1800 degree C in a furnace in a reducing atmosphere using H2 - Ar mixture/Hb -N2 mixture/NHj/CO - CO2 mixture for 0,2 hours to 6 hours; cooling the pellets quickly to room temperature in reducing atmosphere and crushing the pellets thereafter to a predetermined particle size to obtain the said after-glow phosphor. EXAMPLE-I 11.074 % by weight of strontium carbonate, 0,269 % by weight of europium oxide, 0.286 % by weight of dysprosium oxide and 57.431 % by weight of aluminium nitrate are dissolved in 10 ml of 1:1 dilute nitric acid to form a solution. 30.940 % by weight of urea is then mixed with the solution to form a slurry The said slurry is then combusted in a furnace preheated to 5S0 degree C to yield a fluffy white solid resembling foam. This susbstance is then remove from the furnace and cooled to room temperture which is then heated at 800 degree C for 3 hours and then cooled to room temperature. Boric acid in the proportion of 0.1 moles per mole of the final product composition is then mixed with the cooled solid and pelletised. The pellets are then heated to 1350°C in hydrogen atmosphere for 3 hours. Thereafter the pellets are rapidly cooled to room temperature in hydrogen flow. The cooled pellets are then crushed to the desired particle size to obtain the said afterglow phosphor ready for use. The product exhibited yellowish green phosphorescence lasting for 12 hours. EXAMPLE-II 7.034% by weight of calcium carbonate, 0.279% by weight of europium oxide, 0.268% by weight of neodymium oxide and 59.573% by weight of aluminum nitrate are dissolved in 10 ml of 1:1 dilute nitric acid to form a solution. 32.093% by weight of urea is then mixed with the solution to form a slurry. The said slurry is then combusted in a furnace preheated to 540°C to yield a fluffy white solid resembling foam. This substance is then removed from the furnace and cooled to room temperature which is then heated at 800°C for 3 hours and then cooled to room temperature. Boric acid in the proportion of 0.1 moles per mole of the final product composition is then mixed with the cooled solid and pelletised. The pellets are then heated to 1370°C in hydrogen atmosphere for 3 hours. Thereafter the pellets are rapidly cooled to room temperature in hydrogen flow. The cooled pellets are then crushed to the desired particle size to obtain the said afterglow phosphor ready for use. The product exhibited intense purple phosphorescence lasting for 6 hours. EXAMPLE-III 7.034% by weight of strontium carbonate, 0.171% by weight of europium oxide, 0.182% by weight of dysprosium oxide and 72.960% by weight of aluminum nitrate are dissolved in 12 ml of 1:1 dilute nitric acid to form a solution. 19.653% by weight of urea is then mixed with the solution to form a slurry. The said slurry is then combusted in a furnace preheated to 550 C to yield a fluffy white solid resembling foam. This substance is then removed from the furnace and cooled to room temperature which is then heated at 800o C for 3 hours and then cooled to room temperature. Boric acid in the proportion of 0.1 moles per mole of the final product composition is then mixed with the cooled solid and pelletised. The pellets are then heated to 1350 C in hydrogen atmosphere for 3 hours. Thereafter the pellets are rapidly cooled to room temperature in hydrogen flow. The cooled pellets are then crushed to the desired particle size to obtain the said afterglow phosphor ready for use. The product exhibited intense aqua marine blue phosphorescence lasting for 8 hours. The choice of the starting material determines the colour of the end product. The terms and expressions in this specification are of description and not of limitation, since there is no intention of excluding any equivalents of the features i1lustrated and described, but it is understood that various other modes of carrying out the said process are possible without departing from the scope and ambit of this invention. We Claim LA process for the preparation of afterglow phosphors comprising the dissolution of substances, selected from Group A Group B and groupC in the solvents selected from group D; adding thereto a fuel selected from Group E; and making the same into a slurry; placing the slurry in a furnace pre-heated to a temperature between 100 degree C and 800 degree C to combust the said stony sod result in a white fluffy solid; removing the said solid from the furnace and cooling the same to room temperature; heating the resulting mixture between 300 degree C and 1000 degree C for 0.5 hours to 3 hours in air/N2 /Ar/ running vacuum and cooling the heated mixture to room temperature; mixing a substance selected from Group F with the resultant mixture and pelletising the same; heating the pellets between 1000 degree C and 1800 degree C in a furnace in a reducing atmosphere using H2 - Ar mixture/Ha -N2 mixture/NH3/CO - CO2 mixture for 0.2 hours to 6 hours; cooling the pellets quickly to room temperature in reducing atmosphere and crushing the pellets thereafter to a predetermined particle size to obtain the said after-glow phosphor. 2. A process as claimed in Claim 1 wherein the substances in Group A comprise alkaline earth metal ions consisting of Mg, Ca Sr, Ba or a mixture of them constituting 2-45 % of the weight of the final product, the said metal ions being in the form of oxides, hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonates, a mixture of them or any other soluble salts which (to not interfere with the phosphorescent property. 3.A process as claimed in any one of the preceding Claims wherein the substances in Group B comprise rare earth metal ions with Eu as activator which constitutes 0.001-10% of the weight of the final product and and other metal ion chosen from the group consisting of La,Ce,Pr,Nd,Sm,Gd,Tb, Dy,Ho,Er,Tm, Yb, Lu, Yor a mixture of them as co-activator constituting 0.001 -10 % of the weight of the final product, alt being in the form of oxides hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonates, a mixture of them, or other soluble salts which do not interfere with the phosphorescent property. 4.A process as claimed in any one of the preceding Claims wherein the substances in Group C comprise metal ions of Al, B, Y, Si, Ga or a mixture of them constituting 2-45% of the weight of the final product, the said metal ions being in the form of oxides, hydroxides, oxalates, citrates, nitrates, oxynitrates, tartarates, acetates, carbonates, bicarbonaies, a mixture of them or other salts which do not interfere with the phosphorescent property. 5. A process as claimed in any one of the preceding Claims wherein the solvent in Group D comprises water, mineral acid, including nitric acid and perchloric acid, a base including NHy/NH4OH, glycine or a mixture of them, the weight ratio of reaction mixture to solvent ranging from 1:0.2 to 1:30 6. A process as claimed in any one of the preceding Claims wherein the fuel in Group E comprises area, oxalyldihyctazide, substituted urea, carbohydrazide, maleic hydrazide, nmleno hydrazide, tetraformyl hydrazine, dimethyl hydrazones, diformyl hydrazine- glycine, substituted glycine, 3-methyl-5-pyrazolone or a mixture of them, the fuel ratio being 20-500% by Weight of the reaction mixture. 7.A process as claimed in any one of the preceding Claims wherein Group F comprises flux H2BO3, H2BO3, B2O3, HBO3, borax or AIF3, the flux being in the range 0.2 - 20% 8 A process for the preparation of after-glow phosphors substantially as herein described with reference to, and as illustrated by, the Examples.

Documents:

408-mas-2001 abstract granted.pdf

408-mas-2001 claims granted.pdf

408-mas-2001 description (complete) granted.pdf

408-mas-2001-abstract.pdf

408-mas-2001-claims.pdf

408-mas-2001-correspondence others.pdf

408-mas-2001-correspondence po.pdf

408-mas-2001-description complete.pdf

408-mas-2001-form 1.pdf

408-mas-2001-form 19.pdf

408-mas-2001-form 26.pdf