**11. Effect of calcination temperature**

Rare-earth ions can easily replace yttrium ions because their properties are similar. All peaks in XRD pattern of YInGe2O7 doped with 5 mol.% Eu3+ and calcined at various temperatures from 11000C to 14000C in air for 10 h could be attributed to the monoclinic YInGe2O7 phase. Trivalent europium ions (94.7 pm) were introduced to substitute trivalent yttrium ions (90 pm) in the (Y, Eu) InGe2O7 system. The variations are almost the same for Eu3+ and Y3+ ion radii, both easily forming a solid solution. Additionally, there were no charge compensation issues when Eu3+ ions substituted Y3+ ions in the YInGe2O7 lattice because both have the same valence. The full-width at half-maximum (FWHM) of these peaks seemed to decrease and the crystallinity of YInGe2O7:Eu3+ became better with an increase of the calcination temperature to 12000C. From Fig.14 it is observed that the emission intensity increases with calcination temperature, with a maximum value at12000C.

This was caused by the reduction of nonradiative recombination effects, i.e., quenching sites and surface defects trapped by increased crystallinity and decreased defects in the crystal and is in agreement with the results of the XRD analysis, as optimum crystallinity was presented at 1200oC. Higher calcination temperature enhances atomic mobility and causes grain growth, resulting in better crystallinity. When calcination temperature increased further, the emission intensity decreased significantly as shown in **Fig.14.** When the calcination temperature was higher than 12000C, the second phase of In2O3 (JCPD no. 06- 0416) was observed in the XRD pattern. The amount of the second phase increased with calcination temperature. The second phase might be produced because the increased calcination temperature leads to a nonstoichiometric system because the melting point of In2O3 (8500C) is lower than that of Y2O3 (24100C) and GeO2 (10860 C).

For the preparation of ZnWO4 phosphor, ZnO powder used as the source material was mixed with WO3 (99.9%) at different concentrations (10–60 mol%). The mixed powders were blended with deionized water then milled for 24 h. Subsequently, the solution was dried in an oven and sintered at 700–1,200°C for 1–8 h. Finally, the synthesized powders were ground, and the ZnWO4 phosphor was prepared. The crystallization of the ZnWO4 phosphor was improved by increasing the sintering temperature from 800°C to 1,200°C as seen in **Fig.15.** Meanwhile, the decreased FWHMs in the XRD patterns show that the grain size of the phosphor increased with the sintering temperature. Improvement in crystallization is normally accompanied by an increase in phosphor particle size. The particle sizes of ZnWO4 phosphor were approximately 16.8, 17.9, 20.2, 19.6, 19.6, and 26.5 nm for sintering temperatures increasing in steps of 100°C from 700°C to 1,200°C, respectively. SEM result is consistent with the results of XRD analysis. Significant changes in

**Yttria-stabilized zirconia** (YSZ) is a zirconium-oxide based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (ZrO2) and "yttria" (Y2O3), hence the name. The addition of yttria to pure zirconia replaces some of the Zr4+ ions in the zirconia lattice with Y3+ ions. This produces oxygen vacancies, as three O2- ions replace four O2- ions. It also permits yttrium stabilized zirconia to conduct O2- ions, provided there is sufficient vacancy site mobility, a property that increases with temperature. This ability to conduct O2- ions makes yttria-stabilized zirconia well suited to use in solid oxide fuel cells,

Rare-earth ions can easily replace yttrium ions because their properties are similar. All peaks in XRD pattern of YInGe2O7 doped with 5 mol.% Eu3+ and calcined at various temperatures from 11000C to 14000C in air for 10 h could be attributed to the monoclinic YInGe2O7 phase. Trivalent europium ions (94.7 pm) were introduced to substitute trivalent yttrium ions (90 pm) in the (Y, Eu) InGe2O7 system. The variations are almost the same for Eu3+ and Y3+ ion radii, both easily forming a solid solution. Additionally, there were no charge compensation issues when Eu3+ ions substituted Y3+ ions in the YInGe2O7 lattice because both have the same valence. The full-width at half-maximum (FWHM) of these peaks seemed to decrease and the crystallinity of YInGe2O7:Eu3+ became better with an increase of the calcination temperature to 12000C. From Fig.14 it is observed that the emission intensity increases with

This was caused by the reduction of nonradiative recombination effects, i.e., quenching sites and surface defects trapped by increased crystallinity and decreased defects in the crystal and is in agreement with the results of the XRD analysis, as optimum crystallinity was presented at 1200oC. Higher calcination temperature enhances atomic mobility and causes grain growth, resulting in better crystallinity. When calcination temperature increased further, the emission intensity decreased significantly as shown in **Fig.14.** When the calcination temperature was higher than 12000C, the second phase of In2O3 (JCPD no. 06- 0416) was observed in the XRD pattern. The amount of the second phase increased with calcination temperature. The second phase might be produced because the increased calcination temperature leads to a nonstoichiometric system because the melting point of

For the preparation of ZnWO4 phosphor, ZnO powder used as the source material was mixed with WO3 (99.9%) at different concentrations (10–60 mol%). The mixed powders were blended with deionized water then milled for 24 h. Subsequently, the solution was dried in an oven and sintered at 700–1,200°C for 1–8 h. Finally, the synthesized powders were ground, and the ZnWO4 phosphor was prepared. The crystallization of the ZnWO4 phosphor was improved by increasing the sintering temperature from 800°C to 1,200°C as seen in **Fig.15.** Meanwhile, the decreased FWHMs in the XRD patterns show that the grain size of the phosphor increased with the sintering temperature. Improvement in crystallization is normally accompanied by an increase in phosphor particle size. The particle sizes of ZnWO4 phosphor were approximately 16.8, 17.9, 20.2, 19.6, 19.6, and 26.5 nm for sintering temperatures increasing in steps of 100°C from 700°C to 1,200°C, respectively. SEM result is consistent with the results of XRD analysis. Significant changes in

although it requires that they operate at high enough temperatures.

calcination temperature, with a maximum value at12000C.

In2O3 (8500C) is lower than that of Y2O3 (24100C) and GeO2 (10860 C).

**11. Effect of calcination temperature** 

grain shape and size were observed with an increase in the sintering time. Optimal crystallization was realized in the case of the ZnWO4 phosphor synthesized using 50 mol% WO3 at 1,100°C for 3 h. The maximum emission intensity was achieved when the phosphor exhibited optimal crystallization.

Fig. 14. The relative emission intensity versus the calcined temperature of YInGe2O7:5 mole% Eu3+ under an excitation of 393 nm. The signals were detected at 611 nm **[10].**

#### **12. Phase change during sintering**

Luminescence properties (both PL and TL) of Tricalcium phosphate (TCP) are very sensitive to its crystal phase (α/β). TCP crystals, in both α and β phases, were synthesized through two different routes, viz. wet precipitation and high temperature solid state reaction **[12].** The doping was done during the synthesis using suitable compounds of Dy and Eu. In the wet precipitation method used, the wet reaction is carried out using calcium nitrate and diammonium hydrogen phosphate in an ammoniated solution. The precipitation of tricalcium phosphate occurs through the chemical reaction 3Ca(NO3)2 + 2(NH4)2HPO4 + 2NH4OH → Ca3(PO4)2+ 6NH4NO3 + 2H2O. the supernatant liquid was decanted to collect the precipitate. It was then centrifuged thrice using distilled water and finely filtered. The filtrate was dried at 100°C in a hot air oven overnight and then calcined at 300°C in a muffle furnace for 3 h to remove any traces of other compounds. The calcined material was ground to form fine powder and graded using standard sieves. It was then sintered at high temperatures for 2 h in a programmable furnace to obtain the required phase (900°C for β-TCP and 1300°C for α-TCP). Various samples were prepared using dysprosium and europium as dopant. The doping was done by adding oxides of the dopant elements (dysprosium and europium) dissolved in minimum quantity of dilute nitric acid. The solid state synthesis of tricalcium phosphate was done through a high temperature firing of the

The Role of Sintering in the Synthesis of Luminescence Phosphors 339

and graded using standard sieves. The samples prepared by wet precipitation annealed at 900°C) and by solid state sintering (annealed at 1100°C) techniques gave the spectra which match perfectly with that of β-TCP (Whitlockite mineral phase, JCPDS File No. 09-0169). The XRD pattern of sample prepared by wet precipitation method and annealed at 1300°C matched with that of α-TCP (JCPDS file number 09-0348). Though the TL efficiency of α-TCP is more compared to β-TCP, the former is less suited for TL applications as its peak temperature is on the lower side, which indicates high fading rate. An interesting outcome of the TL studies on doped tricalcium phosphate is that the phase transition from β to α during annealing could easily be identified through emission parameters. Dy is found to be an efficient dopant in β- TCP matrix compared to europium for maximum TL efficiency.

Aluminum oxide, commonly referred to as alumina, possesses strong ionic interatomic bonding giving rise to its desirable material characteristics. It can exist in several crystalline phases which all revert to the most stable hexagonal alpha phase at elevated temperatures. This is the phase of particular interest for structural applications. Alpha phase alumina is the strongest and stiffest of the oxide ceramics. Its high hardness, excellent dielectric properties, refractoriness and good thermal properties make it the material of choice for a wide range of applications. In a recent study, the α-alumina samples were prepared using a commercial ultra-pure γ-alumina powder obtained by thermal decomposition of ammonium alum (NH4Al(SO4)2, 12H2O). The study of Zr, Th and Ca doping is performed through the impregnation of the γ-alumina powder by an alcoholic solution of zirconium or calcium chloride or of thorium nitrate **[13].** A further thermal treatment allows decomposition of the solvent and the salts (drying at 100°C for 24 h) and diffusion of the doping species inside the host material (600°C for 24 h). The doped γ-alumina is then transformed into doped αalumina by a calcination at 1450°C for 2 h under a pure gas flow (O2 or Ar+2% H2). Th4+ and Zr4+ give rise to the same peaks on the glow curves, but the intensity is higher for Th4+. For both dopants, the doping promotes shrinkage in the case of very low concentrations, in fact,

Carbon doped α-Al2O3 is a well known optically stimulated luminescence material used in radiation dosimetry **[14].** α-Al2O3 doped either with Tb3+ or Tm3+ has been prepared by combustion synthesis techniques for TL ionizing radiation dosimetry applications. In this method, the reactants (aluminum nitrate, urea and terbium or thulium nitrate) are ignited in a muffle furnace at temperatures as low as 500 °C. This synthesis route is an alternative technique to the conventional fabrication methods of materials based on α-Al2O3 (Czochralsky, Vernuil), where high melting temperatures and reducing atmospheres are required. After combustion, the samples were annealed at temperatures ranging from 1000 to 1400 °C for 4 h in order to obtain the pure α-phase structure and were then irradiated with a Co-60 gamma radiation source. The annealed samples present a well defined TL glow peak with a maximum at approximately 200 °C and linear TL response in the dose range 0.5–5 Gy. It was observed that a 0.1 mol% concentration of Tb3+ or Tm3+ and annealing at 1400 °C optimize the TL sensitivity. The highest sensitivity was found for Tm3+ doped samples which were approximately 25 times more sensitive than Tb3+ doped samples. These results strongly suggest that combustion synthesis is a suitable technique to prepare doped aluminum oxide material and that Tm3+ doped α-Al2O3 is a potential material for TL

until the cation integrates into the host material.

radiation dosimetry **[15].**

Fig. 15. Scanning electron microscope images of the ZnWO4 phosphor prepared under different synthesis conditions; WO3 concentrations at (a) 10 and (b) 50 mol% and sintered at 700°C for 3 h; WO3 concentration at 50 wt% and sintered at (c) 1,100°C and (d) 1,200°C for 3 h; WO3 concentration at 50 wt% and sintered at 1,100°C for (e) 1 and (f) 8 h **[11].** 

powder mixture of calcium oxide (CaO) and dicalcium phosphate (CaHPO4). The reaction governing the process is CaO + 2CaHPO4 → Ca3(PO4)2 + H2O. The dopant, in powder form, in appropriate amount was added to this, again mixed thoroughly for 1 h, and then transferred to a porcelain crucible. The powder was heated at about 300°C, then mixed for 1 h, and annealed at 1100°C for 2 h. The resulting compound was crushed to powder from

Fig. 15. Scanning electron microscope images of the ZnWO4 phosphor prepared under different synthesis conditions; WO3 concentrations at (a) 10 and (b) 50 mol% and sintered at 700°C for 3 h; WO3 concentration at 50 wt% and sintered at (c) 1,100°C and (d) 1,200°C for 3

powder mixture of calcium oxide (CaO) and dicalcium phosphate (CaHPO4). The reaction governing the process is CaO + 2CaHPO4 → Ca3(PO4)2 + H2O. The dopant, in powder form, in appropriate amount was added to this, again mixed thoroughly for 1 h, and then transferred to a porcelain crucible. The powder was heated at about 300°C, then mixed for 1 h, and annealed at 1100°C for 2 h. The resulting compound was crushed to powder from

h; WO3 concentration at 50 wt% and sintered at 1,100°C for (e) 1 and (f) 8 h **[11].** 

and graded using standard sieves. The samples prepared by wet precipitation annealed at 900°C) and by solid state sintering (annealed at 1100°C) techniques gave the spectra which match perfectly with that of β-TCP (Whitlockite mineral phase, JCPDS File No. 09-0169). The XRD pattern of sample prepared by wet precipitation method and annealed at 1300°C matched with that of α-TCP (JCPDS file number 09-0348). Though the TL efficiency of α-TCP is more compared to β-TCP, the former is less suited for TL applications as its peak temperature is on the lower side, which indicates high fading rate. An interesting outcome of the TL studies on doped tricalcium phosphate is that the phase transition from β to α during annealing could easily be identified through emission parameters. Dy is found to be an efficient dopant in β- TCP matrix compared to europium for maximum TL efficiency.

Aluminum oxide, commonly referred to as alumina, possesses strong ionic interatomic bonding giving rise to its desirable material characteristics. It can exist in several crystalline phases which all revert to the most stable hexagonal alpha phase at elevated temperatures. This is the phase of particular interest for structural applications. Alpha phase alumina is the strongest and stiffest of the oxide ceramics. Its high hardness, excellent dielectric properties, refractoriness and good thermal properties make it the material of choice for a wide range of applications. In a recent study, the α-alumina samples were prepared using a commercial ultra-pure γ-alumina powder obtained by thermal decomposition of ammonium alum (NH4Al(SO4)2, 12H2O). The study of Zr, Th and Ca doping is performed through the impregnation of the γ-alumina powder by an alcoholic solution of zirconium or calcium chloride or of thorium nitrate **[13].** A further thermal treatment allows decomposition of the solvent and the salts (drying at 100°C for 24 h) and diffusion of the doping species inside the host material (600°C for 24 h). The doped γ-alumina is then transformed into doped αalumina by a calcination at 1450°C for 2 h under a pure gas flow (O2 or Ar+2% H2). Th4+ and Zr4+ give rise to the same peaks on the glow curves, but the intensity is higher for Th4+. For both dopants, the doping promotes shrinkage in the case of very low concentrations, in fact, until the cation integrates into the host material.

Carbon doped α-Al2O3 is a well known optically stimulated luminescence material used in radiation dosimetry **[14].** α-Al2O3 doped either with Tb3+ or Tm3+ has been prepared by combustion synthesis techniques for TL ionizing radiation dosimetry applications. In this method, the reactants (aluminum nitrate, urea and terbium or thulium nitrate) are ignited in a muffle furnace at temperatures as low as 500 °C. This synthesis route is an alternative technique to the conventional fabrication methods of materials based on α-Al2O3 (Czochralsky, Vernuil), where high melting temperatures and reducing atmospheres are required. After combustion, the samples were annealed at temperatures ranging from 1000 to 1400 °C for 4 h in order to obtain the pure α-phase structure and were then irradiated with a Co-60 gamma radiation source. The annealed samples present a well defined TL glow peak with a maximum at approximately 200 °C and linear TL response in the dose range 0.5–5 Gy. It was observed that a 0.1 mol% concentration of Tb3+ or Tm3+ and annealing at 1400 °C optimize the TL sensitivity. The highest sensitivity was found for Tm3+ doped samples which were approximately 25 times more sensitive than Tb3+ doped samples. These results strongly suggest that combustion synthesis is a suitable technique to prepare doped aluminum oxide material and that Tm3+ doped α-Al2O3 is a potential material for TL radiation dosimetry **[15].**

The Role of Sintering in the Synthesis of Luminescence Phosphors 341

mixture results in production of nanoparticles. The photographs in **Figs.17a and b** show the blue luminescence emission of as prepared (a) BaMgAl11O17:Eu2+ and (b) BaMgAl11O17:Mn2+

(a) Boiling (b) Ignition

(c) Propagation (d) Final Product

Fig. 17. Photographs of as prepared (a) BaMgAl11O17:Eu2+ and (b) BaMgAl11O17:Mn2+ by solutiuon combustion synthesis show their blue luminescence emission under UV

Several luminescence phosphors have been successfully prepared by SCS technique. For instance, Ce-doped yttrium aluminum garnet (YAG, Y3Al5O12) phosphor powders were synthesized using the combustion method. The luminescence, formation process, and structure of the phosphor powders were investigated by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The XRD

by solution combustion synthesis under UV illumination.

Fig. 16. Various steps in solution combustion process.

(a) (b)

illumination **[17].**
