**4. Molten salt sintering**

Combined co-precipitation with the molten salt method, a new technology for preparation of Y2O3:Eu3+ and YAG:Ce3+ phosphors was proposed with the controlled size and higher luminescent intensity. With rare earths oxide as raw materials, the molten salt method was compared with solid phase method. Some main principles for the selection of molten salt system were, i) the melting point should lower the temperature of phosphor preparation, ii) the difference of boiling point and melting point should be as wide as possible, and iii) the molten salt must not be hazardous to luminescent intensity. The best multiple molten salt system for Y2O3:Eu3+ and YAG:Ce3+ were NaCl+S+Na2CO3 and Na2SO4+BaF2, respectively **[2].** Molten salt sintering improved the crystal degree and configuration of phosphors, resulting in higher luminescent intensity. Using YCl3 and EuCl3 as raw material, the preparation of Y2O3:Eu3+ precursor was investigated concerning some factors, such as temperature, complexing agent, precipitation agent and the dripping mode. The size of precursor was the smallest at pH=7 and the complexing agent could control the release velocity of rare ion effectively. With citric acid as a complexing agent, the size of precursor and sintering sample was the smallest and the luminescent intensity of sintering sample was

The Role of Sintering in the Synthesis of Luminescence Phosphors 329

Fig. 5. Emission spectra of BaAl2Si2O8:Eu2+ (BAS:Eu2+) phosphor under VUV (147 nm)

Sieving *before* the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermoluminescence (TL) sensitivity to γ-rays. The reduction in TL sensitivity of higher sized grains observed following the procedure of sieving *after* sintering has also more or less vanished. Maximum TL sensitivity is seen after sintering around 700°C, whereas maximum PL sensitivity is seen after sintering around 325°C. While the observed increase in TL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy3+ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy3+ environment on the grain surface perhaps due to oxygen incorporation **(Fig.6)**. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO4:Dy slightly; however, it does not influence TL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature. TG-DTA (thermogravimetric and differential thermal analysis) data indicate that dysprosium sulphate dopant in CaSO4 which are hydrated at RT become anhydrous at 4000C and the water molecules released possibly damage the crystal lattice which get restored at higher sintering temperatures. As a result, CaSO4:Dy annealed at 4000C show a slightly reduced TL while those annealed at 3000C or at 7000C do not show any such reduction. The water molecules released at 4000C possibly displace Dy from its lattice site causing a reduction in TL. No such reduction in TLis observed on annealing at 3000C since the water molecules do not get dislodged from the lattice. Annealing at 7000C possibly restore the Dy in its original lattice sites and hence

excitation **[3]**.

restore the TL sensitivity.

**6. Effect of sintering on photo and** 

**thermo-luminescence in CaSO4 : Dy phosphor** 

the highest. Probably, the citric acid could complex effectively the earth ionic and buffer the pH during the precipitation process in the presence of ammonia and therefore enhanced the precursor density and activity. For the preparation of Y3-xCexAl5O12 (YAG:Ce3+) precursor by co-precipitation, the optimal process condition were: the concentration of salt was as low as about 0.05M, precipitation agent was NH4HCO3, pH=8, temperature=90℃, and adverse dripping mode was preferred. Because the precursor was a sol mixture of Y2(CO3),nH2O and NH4AlO(OH)HCO3, it was easy to agglomerate after drying. It was found that the agglomeration problem could be solved by adding active carbon before precipitation. For active carbon, its numerous capillary frameworks might disconnect the sol effectively and its incomplete combustion in sintering was helpful for the deoxidization from Ce4+ to Ce3+. Through adjusting components of multiple molten salt and mole ratio of the molten salt to precursor, the optimal sintering conditions for preparation of Y2O3:Eu3+ and YAG:Ce3+ were obtained. The samples were sphere-like particles whose average size were 1~3μm and 3~5μm and luminescent intensity were 11% and 8% better than commercial phosphor respectively. The results for different sintering temperature indicated that molten salt could reduce activation energy of phosphor like a kind of catalyst, leading to lower sintering temperature than solid phase method. The formation of sphere-like particles might be owing to the surface tension difference between liquid molten salt and phosphor, and the existence of double layer insured the dispersion of particles. The liquid molten salt provided the stable high temperature field and liquid environment and promoted the crystal degree, resulted in the increased luminescent intensity. In addition, the mole ratio of Y:Al:Ce was investigated for increasing luminescent intensity of YAG:Ce3+. Luminescent intensity of sample was enhanced evidently when the mole ratio of Y:Al:Ce was reduced from 2.94:5:0.06 to 2.90:5:0.06 in Y3-xCexAl5O12. A little lack of Y in crystal lattice might help to increase luminescent intensity, which coincided with the theory of radiation from crystal lattice defect. The structure of Y2O3:Eu3+ and YAG:Ce3+ was body-centered cubic structure and yttrium aluminum garnet structure respectively showing that the molten salt did not enter into the crystal lattice of phosphor. Compared with the traditional solid phase method, the new technology can obtain the controlled size and higher luminescent intensity phosphor through only one sintering process, avoiding comminuting process required in solid phase method. In addition, it is a new energysaving process with lower sintering temperature and has a potential application in preparation of phosphor with excellent performance.

#### **5. Thermal stability against sintering and crystal structure**

The thermal stability of BaAl2Si2O8:Eu2+ (BAS:Eu2+) phosphor used in PDP was found to depend on its polymorph property - hexagonal and monoclinic crystal structure. The monoclinic BAS:Eu2+ when baked at 500 °C in air for 30 min, showed the same PL intensity as the fresh one, whereas the baked hexagonal one lost its PL intensity significantly **(Fig.5)**. Electron spin resonance studies on Eu2+ and Rietveld refinement showed that the difference of thermal stability between hexagonal and monoclinic BAS:Eu2+ could be ascribed to both the crystal structure of host materials and the average inter-atomic distances between the Eu2+ ion and oxygen which plays the key role of shield for Eu2+ ions against an oxidation atmosphere.

the highest. Probably, the citric acid could complex effectively the earth ionic and buffer the pH during the precipitation process in the presence of ammonia and therefore enhanced the precursor density and activity. For the preparation of Y3-xCexAl5O12 (YAG:Ce3+) precursor by co-precipitation, the optimal process condition were: the concentration of salt was as low as about 0.05M, precipitation agent was NH4HCO3, pH=8, temperature=90℃, and adverse dripping mode was preferred. Because the precursor was a sol mixture of Y2(CO3),nH2O and NH4AlO(OH)HCO3, it was easy to agglomerate after drying. It was found that the agglomeration problem could be solved by adding active carbon before precipitation. For active carbon, its numerous capillary frameworks might disconnect the sol effectively and its incomplete combustion in sintering was helpful for the deoxidization from Ce4+ to Ce3+. Through adjusting components of multiple molten salt and mole ratio of the molten salt to precursor, the optimal sintering conditions for preparation of Y2O3:Eu3+ and YAG:Ce3+ were obtained. The samples were sphere-like particles whose average size were 1~3μm and 3~5μm and luminescent intensity were 11% and 8% better than commercial phosphor respectively. The results for different sintering temperature indicated that molten salt could reduce activation energy of phosphor like a kind of catalyst, leading to lower sintering temperature than solid phase method. The formation of sphere-like particles might be owing to the surface tension difference between liquid molten salt and phosphor, and the existence of double layer insured the dispersion of particles. The liquid molten salt provided the stable high temperature field and liquid environment and promoted the crystal degree, resulted in the increased luminescent intensity. In addition, the mole ratio of Y:Al:Ce was investigated for increasing luminescent intensity of YAG:Ce3+. Luminescent intensity of sample was enhanced evidently when the mole ratio of Y:Al:Ce was reduced from 2.94:5:0.06 to 2.90:5:0.06 in Y3-xCexAl5O12. A little lack of Y in crystal lattice might help to increase luminescent intensity, which coincided with the theory of radiation from crystal lattice defect. The structure of Y2O3:Eu3+ and YAG:Ce3+ was body-centered cubic structure and yttrium aluminum garnet structure respectively showing that the molten salt did not enter into the crystal lattice of phosphor. Compared with the traditional solid phase method, the new technology can obtain the controlled size and higher luminescent intensity phosphor through only one sintering process, avoiding comminuting process required in solid phase method. In addition, it is a new energysaving process with lower sintering temperature and has a potential application in

preparation of phosphor with excellent performance.

atmosphere.

**5. Thermal stability against sintering and crystal structure** 

The thermal stability of BaAl2Si2O8:Eu2+ (BAS:Eu2+) phosphor used in PDP was found to depend on its polymorph property - hexagonal and monoclinic crystal structure. The monoclinic BAS:Eu2+ when baked at 500 °C in air for 30 min, showed the same PL intensity as the fresh one, whereas the baked hexagonal one lost its PL intensity significantly **(Fig.5)**. Electron spin resonance studies on Eu2+ and Rietveld refinement showed that the difference of thermal stability between hexagonal and monoclinic BAS:Eu2+ could be ascribed to both the crystal structure of host materials and the average inter-atomic distances between the Eu2+ ion and oxygen which plays the key role of shield for Eu2+ ions against an oxidation

Fig. 5. Emission spectra of BaAl2Si2O8:Eu2+ (BAS:Eu2+) phosphor under VUV (147 nm) excitation **[3]**.

### **6. Effect of sintering on photo and thermo-luminescence in CaSO4 : Dy phosphor**

Sieving *before* the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermoluminescence (TL) sensitivity to γ-rays. The reduction in TL sensitivity of higher sized grains observed following the procedure of sieving *after* sintering has also more or less vanished. Maximum TL sensitivity is seen after sintering around 700°C, whereas maximum PL sensitivity is seen after sintering around 325°C. While the observed increase in TL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy3+ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy3+ environment on the grain surface perhaps due to oxygen incorporation **(Fig.6)**. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO4:Dy slightly; however, it does not influence TL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature. TG-DTA (thermogravimetric and differential thermal analysis) data indicate that dysprosium sulphate dopant in CaSO4 which are hydrated at RT become anhydrous at 4000C and the water molecules released possibly damage the crystal lattice which get restored at higher sintering temperatures. As a result, CaSO4:Dy annealed at 4000C show a slightly reduced TL while those annealed at 3000C or at 7000C do not show any such reduction. The water molecules released at 4000C possibly displace Dy from its lattice site causing a reduction in TL. No such reduction in TLis observed on annealing at 3000C since the water molecules do not get dislodged from the lattice. Annealing at 7000C possibly restore the Dy in its original lattice sites and hence restore the TL sensitivity.

The Role of Sintering in the Synthesis of Luminescence Phosphors 331

1250◦C, some peaks of new phases, SrAl12O19 and Sr4Al14O25, are identified from the XRD pattern, showing the complex of the phase transition in this temperature region. However, with the calcination temperature increasing to 1300◦C, almost all the diffraction peaks can be indexed to the orthorhombic Sr4Al14O25 phase when referring to PDF 74-1810, implying that SrAl2O4 and SrAl12O19 can be viewed as the intermediate phases during the process of Sr4Al14O25:Eu2+,Dy3+ preparation. Sr4Al14O25:Eu2+,Dy3+ phosphor exhibited better afterglow property than the SrAl2O4:Eu2+,Dy3+ phosphor due to a deeper trap level and a higher trap concentration formed in the host material. When compared to the powder obtained in conventional method, the nano sized powders revealed a blue shift in emission spectrum

Fig. 7. DSC and TG profiles of precursors, showing the dehydration/decomposition reaction process during calcinations. Note- **DTA** detects any change in all categories of materials;

Fig. 8. XRD patterns of the precursors calcined at different temperature: (a) raw powder, (b) 1200 ◦C, (c) 1250◦C and (d) 1300 ◦C, indicating the phase transformation during the

**DSC** determines the temperature and heat of transformation **[5].**

due to the decrease in grain size **(Figs.7-10)**.

calcinations **[5].** 

Fig. 6. Dependence of the Thermostimulated luminescence (TSL, 136–359◦C area) as well as 484 nm PL emission (*λ*exi = 350 nm) intensities of unwashed as prepared samples (*<*75*μ*m grain size and sieving carried out before sintering) of CaSO4:Dy on the sintering temperature **[4].** 

#### **7. Precipitation and sintering – TG/DTA studies**

Eu2+ activated long lasting Sr4Al14O25 nano sized phosphor synthesized by precipitation method is a revealing study. Al(NO3)3·9H2O, Sr(NO3)2, (NH4)2CO3, Eu(NO3)3 and Dy(NO3)3, all in analytical purity, were the starting materials. The (NH4)2CO3 solution was added in droplets to produce a white precursor. After drying at 120◦C for 24 h, the final luminescent powders were obtained by calcinating the dried precursor at different temperature from 1200 to 1300 ◦C in a reducing environment of 5%H2 + 95%N2. Thermogravimetric (TG) and differential thermal analysis (DTA) studies revealed that the endothermic peaks A and B in DSC (differential scanning calorimetry) curve can be assigned to the dehydration of Al(OH)3 and the other two endothermic peaks C and D can be assigned to the decomposition of SrCO3, since the dehydration usually occurs at a relatively low-temperature. The TG curve also shows a good accordance with the DSC result. It can be seen from the TG curve that the weight loss (WL) after the dehydration is 20.6%, very close to the theoretical value 22.5%. And the weight loss after the decomposition of the carbonate is 10.2%, which is also very close to the theoretical value 10.5%.It is obvious that the TG curve reveals no weight loss after 1100◦C, indicating that the two exothermic peaks E and F may result from the chemical reaction between the calcined oxides at high-temperature. XRD data reveal the formation of the orthorhombic aluminate could take several steps, and the low-temperature products after the dehydration and decomposition, say Al2O3 and SrO, will react spontaneously to formthe monoclinic SrAl2O4 around 1200◦C. When the calcination temperature increased to

Fig. 6. Dependence of the Thermostimulated luminescence (TSL, 136–359◦C area) as well as 484 nm PL emission (*λ*exi = 350 nm) intensities of unwashed as prepared samples (*<*75*μ*m

Eu2+ activated long lasting Sr4Al14O25 nano sized phosphor synthesized by precipitation method is a revealing study. Al(NO3)3·9H2O, Sr(NO3)2, (NH4)2CO3, Eu(NO3)3 and Dy(NO3)3, all in analytical purity, were the starting materials. The (NH4)2CO3 solution was added in droplets to produce a white precursor. After drying at 120◦C for 24 h, the final luminescent powders were obtained by calcinating the dried precursor at different temperature from 1200 to 1300 ◦C in a reducing environment of 5%H2 + 95%N2. Thermogravimetric (TG) and differential thermal analysis (DTA) studies revealed that the endothermic peaks A and B in DSC (differential scanning calorimetry) curve can be assigned to the dehydration of Al(OH)3 and the other two endothermic peaks C and D can be assigned to the decomposition of SrCO3, since the dehydration usually occurs at a relatively low-temperature. The TG curve also shows a good accordance with the DSC result. It can be seen from the TG curve that the weight loss (WL) after the dehydration is 20.6%, very close to the theoretical value 22.5%. And the weight loss after the decomposition of the carbonate is 10.2%, which is also very close to the theoretical value 10.5%.It is obvious that the TG curve reveals no weight loss after 1100◦C, indicating that the two exothermic peaks E and F may result from the chemical reaction between the calcined oxides at high-temperature. XRD data reveal the formation of the orthorhombic aluminate could take several steps, and the low-temperature products after the dehydration and decomposition, say Al2O3 and SrO, will react spontaneously to formthe monoclinic SrAl2O4 around 1200◦C. When the calcination temperature increased to

grain size and sieving carried out before sintering) of CaSO4:Dy on the sintering

**7. Precipitation and sintering – TG/DTA studies** 

temperature **[4].** 

1250◦C, some peaks of new phases, SrAl12O19 and Sr4Al14O25, are identified from the XRD pattern, showing the complex of the phase transition in this temperature region. However, with the calcination temperature increasing to 1300◦C, almost all the diffraction peaks can be indexed to the orthorhombic Sr4Al14O25 phase when referring to PDF 74-1810, implying that SrAl2O4 and SrAl12O19 can be viewed as the intermediate phases during the process of Sr4Al14O25:Eu2+,Dy3+ preparation. Sr4Al14O25:Eu2+,Dy3+ phosphor exhibited better afterglow property than the SrAl2O4:Eu2+,Dy3+ phosphor due to a deeper trap level and a higher trap concentration formed in the host material. When compared to the powder obtained in conventional method, the nano sized powders revealed a blue shift in emission spectrum due to the decrease in grain size **(Figs.7-10)**.

Fig. 7. DSC and TG profiles of precursors, showing the dehydration/decomposition reaction process during calcinations. Note- **DTA** detects any change in all categories of materials; **DSC** determines the temperature and heat of transformation **[5].**

Fig. 8. XRD patterns of the precursors calcined at different temperature: (a) raw powder, (b) 1200 ◦C, (c) 1250◦C and (d) 1300 ◦C, indicating the phase transformation during the calcinations **[5].** 

The Role of Sintering in the Synthesis of Luminescence Phosphors 333

During the preparation of sintered pellets for applications in radiation dosimetry, Mg-doped LiF phosphors compressed with a pressure of 100 atm (10.13MPa) exhibited a significant glow curve change and a TL sensitivity decrease by a factor of about 10–16. Phosphors finely crushed in a mortar did not exhibit such a behavior. These changes are reportedly caused due to lattice deformations generated by the static pressure. The deformations quench the original TL peak near 2000C but produced new traps giving rise to peaks between 230 and 4000C. Pressure applied to the irradiated phosphor empty the filled traps and create new traps. However, the original glow curve shape and TL sensitivity were restored when the LiF phosphor was heated to a temperature higher than 2000C even for a shorter period. The sintered pellets were found reusable after a 4000C, 1h+ 800C, 4 h annealing treatment. Subsequent investigations revealed that the pressure-induced defomation disappears fully only when the annealing temperatures are above 350◦C; annealing temperatures higher than 100◦C cause further complicated variations in the glow curve shape of LiF-based phosphors. Plastic deformation of LiF:Mg,Ti followed by irradiation decreased the TL intensities, with 2000C peak decreasing more drastically than 1000C peak. The latter reported that it is more likely that new TL traps are created by dislocation intersections which then compete with the previously existing traps. Plastic deformation after exposure to ionizing radiation increase the F band (250 nm) absorption in LiF:Mg,Ti, while decreasing the absorptions at 310 and 380 nm which are related to TL traps. The rate of change with deformation of the three bands indicates that F-centres are created when 310-and 380-nm absorption centres are destroyed. These results were interpreted in terms of the removal of F-centres from the

Studies by the author **[6]** showed pressure-induced changes in the luminescent properties of gypsum (CaSO4.2H2O), calcite (CaCO3), CaSO4:Dy and anhydrite (CaSO4). A systematic reduction in the TL as well as PL (in the case of CaSO4:Dy) sensitivity with the pressure applied is seen with the former three materials **(Fig.11).** However, an exactly reverse trend,

Fig. 11. Dependence of PL emission spectra (λexi = 350 nm) of unirradiated CaSO4:Dy on the

pressure applied (in MPa), 1:0, 2:0.7, 3:1.05, 4:1.23, 5:1.4**.[6].** 

defect complexes causing the 310- and 380-nm absorption bands.

Fig. 9. Afterglow property of the nano scaled phosphors **[5].** 

Fig. 10. TL curve for nano scaled SrAl2O4:Eu,Dy and Sr4Al14O25:Eu,Dy phosphors **[5].**

### **8. Pelletization and sintering**

The change in TL properties due to compression and subsequent heating are well studied in LiF TL dosimetry phosphor. Changes in TL at contacts and in the vicinity of ores in carbonate host rocks and near faults have been studied. In the history of formation of terrestrial planets, the collision of solid bodies is central at an early stage of their evolution. Analysis of shock processes in natural minerals play an important role in geology. Influence of thermal history, deformation and stresses associated with major tectonic dislocations, meteoritic craters and underground nuclear explosions on the TL of several natural minerals have been discussed earlier. Shock-induced TL in Oligoclase, Quartz and Calcite have been studied in this regard. An enhancement in TL intensity and shifting of the glow peaks to relatively higher temperatures in pelletized kyanite samples when compared to single crystals which were attributed to the particle nature of the phosphor and/or pressure-induced defects. A shift in the temperature of glow peaks of γ- and X-ray irradiated quartz with pressure was attributed to an electron trap that is getting shallower with increasing pressure.

Fig. 9. Afterglow property of the nano scaled phosphors **[5].** 

**8. Pelletization and sintering** 

Fig. 10. TL curve for nano scaled SrAl2O4:Eu,Dy and Sr4Al14O25:Eu,Dy phosphors **[5].**

to an electron trap that is getting shallower with increasing pressure.

The change in TL properties due to compression and subsequent heating are well studied in LiF TL dosimetry phosphor. Changes in TL at contacts and in the vicinity of ores in carbonate host rocks and near faults have been studied. In the history of formation of terrestrial planets, the collision of solid bodies is central at an early stage of their evolution. Analysis of shock processes in natural minerals play an important role in geology. Influence of thermal history, deformation and stresses associated with major tectonic dislocations, meteoritic craters and underground nuclear explosions on the TL of several natural minerals have been discussed earlier. Shock-induced TL in Oligoclase, Quartz and Calcite have been studied in this regard. An enhancement in TL intensity and shifting of the glow peaks to relatively higher temperatures in pelletized kyanite samples when compared to single crystals which were attributed to the particle nature of the phosphor and/or pressure-induced defects. A shift in the temperature of glow peaks of γ- and X-ray irradiated quartz with pressure was attributed During the preparation of sintered pellets for applications in radiation dosimetry, Mg-doped LiF phosphors compressed with a pressure of 100 atm (10.13MPa) exhibited a significant glow curve change and a TL sensitivity decrease by a factor of about 10–16. Phosphors finely crushed in a mortar did not exhibit such a behavior. These changes are reportedly caused due to lattice deformations generated by the static pressure. The deformations quench the original TL peak near 2000C but produced new traps giving rise to peaks between 230 and 4000C. Pressure applied to the irradiated phosphor empty the filled traps and create new traps. However, the original glow curve shape and TL sensitivity were restored when the LiF phosphor was heated to a temperature higher than 2000C even for a shorter period. The sintered pellets were found reusable after a 4000C, 1h+ 800C, 4 h annealing treatment. Subsequent investigations revealed that the pressure-induced defomation disappears fully only when the annealing temperatures are above 350◦C; annealing temperatures higher than 100◦C cause further complicated variations in the glow curve shape of LiF-based phosphors. Plastic deformation of LiF:Mg,Ti followed by irradiation decreased the TL intensities, with 2000C peak decreasing more drastically than 1000C peak. The latter reported that it is more likely that new TL traps are created by dislocation intersections which then compete with the previously existing traps. Plastic deformation after exposure to ionizing radiation increase the F band (250 nm) absorption in LiF:Mg,Ti, while decreasing the absorptions at 310 and 380 nm which are related to TL traps. The rate of change with deformation of the three bands indicates that F-centres are created when 310-and 380-nm absorption centres are destroyed. These results were interpreted in terms of the removal of F-centres from the defect complexes causing the 310- and 380-nm absorption bands.

Studies by the author **[6]** showed pressure-induced changes in the luminescent properties of gypsum (CaSO4.2H2O), calcite (CaCO3), CaSO4:Dy and anhydrite (CaSO4). A systematic reduction in the TL as well as PL (in the case of CaSO4:Dy) sensitivity with the pressure applied is seen with the former three materials **(Fig.11).** However, an exactly reverse trend,

Fig. 11. Dependence of PL emission spectra (λexi = 350 nm) of unirradiated CaSO4:Dy on the pressure applied (in MPa), 1:0, 2:0.7, 3:1.05, 4:1.23, 5:1.4**.[6].** 

The Role of Sintering in the Synthesis of Luminescence Phosphors 335

could easily diffuse in. Such a technique could offer an easy recipe for the preparation of doped samples. Unlike the case with Li co-dopant, Dy3+ or Tm3+ could not be doped by this (pelletization followed by sintering) technique. This indicates that incorporation of trivalent dopants require greater activation energy than monovalent co-dopants. This is understandable since trivalent dopants in a divalent host require activation energy for the

Phosphors tend to get oxidised during sintering in air. For example CaSO4 gets oxidized to CaO at temperatures above 8000C in air. As a result the luminescence efficiency of CaSO4:Dy gets reduced at high sintering temperatures. In addition there are certain activators such as Mn, Cu etc which tend to get oxidised during sintering in air. Certain other activators such Eu and Ce tend to exhibit dual valence state. Quite often during sintering, reducing atmosphere is essential to prevent Eu2+ and Ce3+ from getting oxidized to Eu3+ and Ce4+, respectively. While phosphors containing Eu2+ activator (eg., BAM:Eu2+) gives intense blue emission, those containing Eu3+ activator (eg., Y2O3:Eu3+) gives intense red emission. So depending on the phosphor and activator, sintering should be carried out either in a reducing atmosphere or in air. Reducing atmospheres are usually obtained with H2/N2 mixture. Alternately, the phosphors to be sintered are covered with carbon powder in closed

crucibles so as to create reducing CO atmosphere when burnt with limited oxygen.

CaS:Eu2+ red-emitting phosphors particles, were prepared by the precipitation method with calcium acetate and Na2S as starting materials, followed by sintering in the atmosphere over the mixture of sulfur powder, Na2CO3, and carbon-containing compounds such as tartaric acid, citric avid, glucose, and cane sugar. CaS:Eu2+ particles without additive show inhomogeneous, rough and aggregation with the size of 75–125 nm, but the spherical particles with mean size of about 110 nm were obtained by adding carbon-containing compounds **(Fig.13).** Compared with phosphor without additive, the addition of carbon-containing materials induced a remarkable increase of PL, in the order of cane sugar, glucose, citric acid, and tartaric acid. This enhancement is due to the improvement of crystallinity, particle morphology and size distribution of the samples by adding carbon-containing additive.

Fig. 13. Transmission electron (TEM) micrographs of CaS:Eu2+ obtained by the precipitation

method without additive (a) and with cane sugar additive (b) **[9].**

creation of cation vacancies **(Fig.12).**

**10. Effect of atmosphere during sintering** 

i.e., a systematic increase in the TL sensitivity with the pressure was witnessed in the case of anhydrite. While the application of pressure (7MPa) reduced the TL sensitivity of CaSO4:Dy to 43% of its initial value, a 700◦C, 1 h anneal could restore its value to only 51%. This shows that the pressure-induced changes are more or less permanent, unlike the case reported with LiF. No change in the XRD data was seen in all these samples on the application of pressure (7MPa) which shows that the changes observed in TL and PL should be attributed to the damage (in the cases of calcite, gypsum and CaSO4:Dy) / creation (in the case of anhydrite) of traps / luminescent centres rather than to the damage to their crystal structure. Pressure applied before or after the irradiation produced no difference in CaSO4:Dy. However, in γray exposed anhydrite and gypsum, pressure-induced changes in the TL sensitivity were quite complex though no change in its glow curve structure was seen. Results showed that in anhydrite, there is a competition between two opposing pressure-induced phenomena one is radiation damage to the filled traps and the other is the increased luminescence efficiency.

#### **9. Activator diffusion during sintering**

CaSO4 can be pelletized only with suitable additives. However, Dy3+ could be incorporated in CaSO4 lattice only by recrystallization in H2SO4 medium. A recent study **[7]** has shown that Li coactivator could be successfully introduced in CaSO4:Dy/Tm at a concentration of 0.06% during a subsequent step of cold pressing at RT and sintering of the pellets at 7000C. A number of alternative lithium compounds have been used in this role and they include Li2CO3, LiCl, LiF, Li2B4O7 and Li2SO4. Addition of lithium significantly shifted the major TL peak of CaSO4:Dy/Tm from 220°C down to 120°C. The fact that similar results were obtained earlier with Li coactivator added during initial crystal growth show that certain impurities can easily diffuse into host crystal during pelletization followed by sintering. In this case Dy3+ incorporation in Ca2+ sites creates cation vacancies into which Li+ co-dopant

Fig. 12. Pictorial representation of CaSO4:Dy and CaSO4:Dy,Na. Trivalent Dy could be incorporated into CaSO4 lattice only by recrystallization in H2SO4 medium. Monovalent co-dopants such as Na+ or Li+ could, however, be incorporated into CaSO4:Dy lattice by cold pressing followed by sintering [8].

i.e., a systematic increase in the TL sensitivity with the pressure was witnessed in the case of anhydrite. While the application of pressure (7MPa) reduced the TL sensitivity of CaSO4:Dy to 43% of its initial value, a 700◦C, 1 h anneal could restore its value to only 51%. This shows that the pressure-induced changes are more or less permanent, unlike the case reported with LiF. No change in the XRD data was seen in all these samples on the application of pressure (7MPa) which shows that the changes observed in TL and PL should be attributed to the damage (in the cases of calcite, gypsum and CaSO4:Dy) / creation (in the case of anhydrite) of traps / luminescent centres rather than to the damage to their crystal structure. Pressure applied before or after the irradiation produced no difference in CaSO4:Dy. However, in γray exposed anhydrite and gypsum, pressure-induced changes in the TL sensitivity were quite complex though no change in its glow curve structure was seen. Results showed that in anhydrite, there is a competition between two opposing pressure-induced phenomena one is radiation damage to the filled traps and the other is the increased luminescence

CaSO4 can be pelletized only with suitable additives. However, Dy3+ could be incorporated in CaSO4 lattice only by recrystallization in H2SO4 medium. A recent study **[7]** has shown that Li coactivator could be successfully introduced in CaSO4:Dy/Tm at a concentration of 0.06% during a subsequent step of cold pressing at RT and sintering of the pellets at 7000C. A number of alternative lithium compounds have been used in this role and they include Li2CO3, LiCl, LiF, Li2B4O7 and Li2SO4. Addition of lithium significantly shifted the major TL peak of CaSO4:Dy/Tm from 220°C down to 120°C. The fact that similar results were obtained earlier with Li coactivator added during initial crystal growth show that certain impurities can easily diffuse into host crystal during pelletization followed by sintering. In this case Dy3+ incorporation in Ca2+ sites creates cation vacancies into which Li+ co-dopant

Fig. 12. Pictorial representation of CaSO4:Dy and CaSO4:Dy,Na. Trivalent Dy could be incorporated into CaSO4 lattice only by recrystallization in H2SO4 medium. Monovalent co-dopants such as Na+ or Li+ could, however, be incorporated into CaSO4:Dy lattice by

efficiency.

**9. Activator diffusion during sintering** 

cold pressing followed by sintering [8].

could easily diffuse in. Such a technique could offer an easy recipe for the preparation of doped samples. Unlike the case with Li co-dopant, Dy3+ or Tm3+ could not be doped by this (pelletization followed by sintering) technique. This indicates that incorporation of trivalent dopants require greater activation energy than monovalent co-dopants. This is understandable since trivalent dopants in a divalent host require activation energy for the creation of cation vacancies **(Fig.12).**
