**21. Combustion and pyrolysis**

346 Sintering of Ceramics – New Emerging Techniques

Y3Al5O12 generally adopts a cubic garnet structure with lattice parameter of 12 Å (space group Ia¯3d). Its structure consists of a network where aluminum atoms reside both in octahedral and tetrahedral interstices whereas yttrium atoms occupy dodecahedral sites. Since the chemical shift of 27Al NMR is sensitive to the local coordination, this technique is largely applied for checking the different phases and coordination states of Al centers in aluminates. As a result, MAS NMR 27Al study has been undertaken on YAG samples in order to apprehend what kind of interatomic movements occur during sol-gel process. octahedral AlO6 sites resonate between 15 and 30 ppm, the much less common AlO5 sites between 40 and 25 ppm and tetrahedral AlO4 between 80 and 50 ppm. On this account, the 2.6 ppm major resonance in the spectrum of the uncalcined xerogel corresponds to sixcoordinate aluminum. Besides, two other distinct peaks located at about 35.4 and 61 ppm can be assigned to five and four-fold coordinated aluminum atoms. It can be noticed that the resonance at 35.4 ppm can also be consistent with a tetrahedral site distorted due to the presence of oxygen defects. 27Al NMR spectral features of the sintered YAG powder reveal three signals at 0.426, 23.6 and 62.7 ppm, which can be imputed to the three types of coordination. The octahedral band has shrinked into the sharp signal at 0.426 ppm. Furthermore, since it has been demonstrated that Al atoms can only occupy tetrahedral and octahedral sites in crystallized YAG, we can deduce that the 35.4 and 23.6 ppm signals respectively in xerogel and crystallized powder correspond to distorted tetrahedral sites and not to five-fold coordinated one. Besides, by integrating peak areas, the 27Al tetrahedral/octahedral ratio has been determined. Contrary to other studies, it remains the same after calcination, even if the resonance relative to "distorted tetrahedral" sites has

The author has developed a simplified co-precipitation technique for the synthesis of CaSO4:Dy TLD phosphor, which circumvent the cumbersome procedure, used so far, namely, evaporation of (highly corrosive) concentrated H2SO4 by recrysttalization. High TL sensitivity, uniform microcrystalline morphology, lower grain size (see SEM pictures shown below left) suitable for manufacturing dosimeters in solid form, better glow curve structure, lesser glow peak shift and better linearity and simplified preparation technique make the new phosphor a better alternative **(Figs.23 & 24).** The new recipe of CaSO4:Dy based on coprecipitation technique is not only economical but also compatible for large scale production. Sintering of the co-precipitated phosphor at 7000C in air increased its TL sensitivity by more than a factor of 2 (see TL glow curves shown below right) due to improved crystallinity and diffusion of Dy3+ ions from the surface to the whole volume of

In this experiment, initially, CaSO4.2H2O and Dy2O3 salts were dissolved in hot concentrated H2SO4 acid and during slow dilution – water was added drop wise into the above hot solution – CaSO4:Dy was found to precipitate. As per conventional solution chemistry, only CaSO4 should precipitate since CaSO4 does not dissolve in dilute H2SO4 acid. The Dy3+ ions should remain dissolved in the acid-water solution due to the high solubility of Dy2(SO4)3 in water and in H2SO4 acid. The formation CaSO4:Dy, as confirmed

**19. NMR** 

weakened in favour of tetrahedral one **[20].** 

**20. Co-precipitation and sintering** 

the grains as stated earlier.

A number of display phosphors has been recently synthesized in the author's laboratory using combustion and pyrolysis technique. In the pyrolysis technique, the constituent chemicals decompose and fuse by the action of heat at relatively low external temperatures (≤ 10000C) in air with adequate ventilation so that the gaseous products released during decomposition reactions escape and the fusion process is complete, Limitations of conventional solid state method are inhomogeneity of the product, formation of large particles with low surface area and hence mechanical particle size reduction is required, which introduces impurity and defects and presence of defects, which are harmful to luminescence. The problem of inhomogeneity could be mitigated by the use of nonconventional methods (wet-chemical) which include solution combustion. Combustion is an exothermic reaction and occurs with the evolution of heat and light. This method was accidentally discovered in 1988 in Prof. Patil's laboratory in India. The first synthesis of Eu3+ doped LnBO3 (Ln=La, Gd and Y) borate phosphors by combustion method was made by his group. The emission spectrum of LaBO3:Eu3+ consisted of two bands at 615 and 595 nm and these bands were attributed to 5D0→7F2 and 5D0→7F1 transition of 9-coordinated Eu3+ ions, respectively. But, there were three bands at 625, 610 and 595 nm observed for GdBO3:Eu3+ and YBO3:Eu3+ phosphors. The band at 595 nm was attributed to magnetic dipole 5D0→7F1 transition of Eu3+, whereas the bands at 625 nm and 610 nm were attributed to electric dipole 5D0→7F2 transition for 12 and 8 coordinated Eu3+ ions, respectively. Since electric dipole transition, 5D0→7F2 depends upon the structure, two bands were observed for differently coordinated Eu3+ in GdBO3:Eu3+ and YBO3:Eu3+.

(Y,Gd)BO3:Eu3+ phosphor used in PDP displays was prepared earlier by combustion method using amino acetic acid as the combustion agent and then sintered at 1000 °C for 30 min. However, details of the recipe used and the PL sensitivity comparisons with commercial phosphor were not reported. In author's lab, raw materials used for the synthesis of (Y,Gd)BO3:Eu3+ and YBO3:Eu3+ phosphors using combustion technique were Y2O3, Gd2O3, H3BO3 and Eu2O3. NH4NO3 and urea (CH4N2O) were used as oxidizer (O) and fuel (F) respectively. The O/F ratio was kept at unity. After wet mixing in a porcelain crucible, combustion was carried out at 600 °C for 15 min in a muffle furnace with adequate ventilation so that the gaseous products released during combustion escape and the combustion process is complete. Combustion of the borate materials with urea fuel resulted in smoldering without flame. In contrast, with Oxalyl dihydrazide (ODH — C2H6N4O2) fuel, the combustion was reported to be flaming and the flame temperature measured using optical pyrometer was about 1400±100 °C. ODH process showed very sharp powder XRD pattern whereas urea process exhibited very broad powder XRD peaks. Therefore, by changing the fuel one could control particulate properties. However, ODH is nearly 50 times more expensive than urea

The Role of Sintering in the Synthesis of Luminescence Phosphors 349

Fig. 24. Comparison of TL glow curves of CaSO4:Dy phosphor obtained by recrystallization (c) and co-precipitation (a and b) The TL sensitivity of sample b increases by more than a

The PL intensities of 592 nm emission peak on 393 nm excitation of (Y0.54Gd046)x (BO3)y :Eu3+ (5 mol%) were found to vary with y/x molar ratio. Optimal PL efficiency is obtained at the y/x molar ratio of 1.38. XRD data reveal that at this molar ratio, interfering phases such as (Y0.95,Eu0.05)2O3 and (Y,Eu)3BO6 are negligible and H3BO3 had been fully used to build (Y,Gd,Eu)BO3 crystal resulting in the maximized luminescence efficiency **(Fig.25).** A 12-fold increase in its luminescence efficiency was seen with an increase in Eu concentration from 0.5 to 8.4 mol%. No saturation or activation quenching of the PL efficiency was seen till the highest Eu concentration (8.4 mol%) level studied. PL efficiencies at still higher Eu concentrations are being studied. Interestingly, at 8.4 mol% Eu concentration, the PL sensitivity of (Y0.54Gd046)x (BO3)y :Eu3+ is 40% higher than that of the commercial (Y,Gd)BO3:Eu3+ PDP phosphor. It is concluded that (Y0.54Gd046)x (BO3)y :Eu3+, at the y/x molar ratio of 1.38, developed in this work is a potential candidate for application as a red

factor of 2 on sintering at 7000C for 1h **[21].**

phosphor for nUV LED.

and hence we used only urea as the fuel in the combustion process. The material obtained after combustion was agglomerated and since it was not a hard material, a mild grinding in an agate mortar and pestle was found sufficient to remove the agglomeration which was then transferred to an alumina crucible before sintering at 1000 °C for 2 h in air in the furnace. The material obtained was once again hand ground mildly to obtain grain sizes below 30 μm. The grain morphology and their size distribution are being studied. The increase in emission intensity of the combustion synthesized red phosphors on calcination at high temperatures has been attributed to improved crystallinity. For PL intensity comparisons, (Y,Gd)BO3:Eu3+ PDP phosphor obtained from LG chemicals (Korea) was used.

Fig. 23. SEM photographs of CaSO4:Dy TL phosphor grains obtained by (A) co-precipitation (in as-grown condition) and (B) conventional recrystallization (after grinding) **[21].** 

and hence we used only urea as the fuel in the combustion process. The material obtained after combustion was agglomerated and since it was not a hard material, a mild grinding in an agate mortar and pestle was found sufficient to remove the agglomeration which was then transferred to an alumina crucible before sintering at 1000 °C for 2 h in air in the furnace. The material obtained was once again hand ground mildly to obtain grain sizes below 30 μm. The grain morphology and their size distribution are being studied. The increase in emission intensity of the combustion synthesized red phosphors on calcination at high temperatures has been attributed to improved crystallinity. For PL intensity comparisons, (Y,Gd)BO3:Eu3+ PDP

phosphor obtained from LG chemicals (Korea) was used.

Fig. 23. SEM photographs of CaSO4:Dy TL phosphor grains obtained by

grinding) **[21].** 

(A) co-precipitation (in as-grown condition) and (B) conventional recrystallization (after

Fig. 24. Comparison of TL glow curves of CaSO4:Dy phosphor obtained by recrystallization (c) and co-precipitation (a and b) The TL sensitivity of sample b increases by more than a factor of 2 on sintering at 7000C for 1h **[21].**

The PL intensities of 592 nm emission peak on 393 nm excitation of (Y0.54Gd046)x (BO3)y :Eu3+ (5 mol%) were found to vary with y/x molar ratio. Optimal PL efficiency is obtained at the y/x molar ratio of 1.38. XRD data reveal that at this molar ratio, interfering phases such as (Y0.95,Eu0.05)2O3 and (Y,Eu)3BO6 are negligible and H3BO3 had been fully used to build (Y,Gd,Eu)BO3 crystal resulting in the maximized luminescence efficiency **(Fig.25).** A 12-fold increase in its luminescence efficiency was seen with an increase in Eu concentration from 0.5 to 8.4 mol%. No saturation or activation quenching of the PL efficiency was seen till the highest Eu concentration (8.4 mol%) level studied. PL efficiencies at still higher Eu concentrations are being studied. Interestingly, at 8.4 mol% Eu concentration, the PL sensitivity of (Y0.54Gd046)x (BO3)y :Eu3+ is 40% higher than that of the commercial (Y,Gd)BO3:Eu3+ PDP phosphor. It is concluded that (Y0.54Gd046)x (BO3)y :Eu3+, at the y/x molar ratio of 1.38, developed in this work is a potential candidate for application as a red phosphor for nUV LED.

The Role of Sintering in the Synthesis of Luminescence Phosphors 351

Fig. 26. Luminescence from some of the phosphors prepared by pyrolysis in author's lab are shown below under UV and 450 nm LED illumination. From left to right: Zn2SiO4:Mn,

YAG:Ce LED and Y2O3:Eu3+.

Fig. 25. XRD spectra of (Y0.54Gd046)x (BO3)y :Eu3+ (5 mol%) sintered at 10000C as a function of BO3 concentration. y/x ratio: 0.57, 1.02, 1.38 and 1.50. The standard XRD spectra of YBO3 (JCPDS 16-0277), Y3BO6 (JCPDS 34-0291) and Y2O3 (JCPDS 25-1200) are also shown **[22].**

Development of high sensitive (Y,Gd)BO3:Eu3+ phosphor also assumes significance since it is widely employed as red phosphor for plasma display panels (PDP) due to the BO3 mediated energy absorption in the 130–170 nm VUV region giving rise to 593 nm emission. However the color purity of all YBO3 based phosphors developed needs improvement since the 593 nm orange emission corresponding to the 5D0–7F1 magnetic dipole transition is greater than the red components from electric dipole transitions in the 610–680 nm region. Studies carried out in this direction by others have succeeded only partially so far.**Fig. 26** shows luminescence from some of the phosphors prepared by pyrolysis.

Fig. 25. XRD spectra of (Y0.54Gd046)x (BO3)y :Eu3+ (5 mol%) sintered at 10000C as a function of BO3 concentration. y/x ratio: 0.57, 1.02, 1.38 and 1.50. The standard XRD spectra of YBO3 (JCPDS 16-0277), Y3BO6 (JCPDS 34-0291) and Y2O3 (JCPDS 25-1200) are also shown **[22].**

Development of high sensitive (Y,Gd)BO3:Eu3+ phosphor also assumes significance since it is widely employed as red phosphor for plasma display panels (PDP) due to the BO3 mediated energy absorption in the 130–170 nm VUV region giving rise to 593 nm emission. However the color purity of all YBO3 based phosphors developed needs improvement since the 593 nm orange emission corresponding to the 5D0–7F1 magnetic dipole transition is greater than the red components from electric dipole transitions in the 610–680 nm region. Studies carried out in this direction by others have succeeded only partially so far.**Fig. 26** shows

luminescence from some of the phosphors prepared by pyrolysis.

Fig. 26. Luminescence from some of the phosphors prepared by pyrolysis in author's lab are shown below under UV and 450 nm LED illumination. From left to right: Zn2SiO4:Mn, YAG:Ce LED and Y2O3:Eu3+.

The Role of Sintering in the Synthesis of Luminescence Phosphors 353

Fig. 27. Schematic diagram displaying the whole preparation steps of CaMgSi2O6:Eu blue

The hydrothermal (HT) method, which uses autogenous pressure developed at temperatures above the boiling point of water, has been used especially in the synthesis of various ceramic oxide powders. The advantage of the hydrothermal method is that ceramic materials can be synthesized at relatively low temperatures (100-300C) without milling or calcinations [11]. The particle size and shape can also be controlled by various processing variables such as temperature, pH, and the addition of surfactants ormineralizers. The reaction is controlled by dissolution/precipitation of reactants in an aqueous medium. Therefore, the above processing variables are thought to have a significant influence on the dissolution or precipitation behavior, even though the exact roles or effects are not fully

For the hydrothermal synthesis, stoichiometric amounts of Gd2O3 and Eu2O3 were dissolved in 100 ml of distilled water acidified by the addition of nitric acid. After the complete dissolution of these oxides, NH4OH solution was added drop wise until the pH of the solution reached 9. White precipitates were instantaneously formed and so-obtained precipitates were washed several times with distilled water by centrifugation. The washed precipitates were mixed with 50 ml solution of H3BO3 and distilled water. The amount of H3BO3 in the solution was adjusted so as to have an appropriatemole ratio with respect to Gd and Eu to give GdBO3:Eu3+. NH4OH was used to adjust pH of the precursor solution to 7–10. After vigorous stirring, the precursor solution was placed in a Teflon-lined stainless steel autoclave with a volume of 100 ml. The solution was heated at 200-240C for 3–10 h" and cooled to room temperature. The resulting powders were filtered and washed several

phosphor particles by the spray pyrolysis **[24].** 

understood and differ in various systems.

**24. Hydrothermal synthesis** 
