**15. Combustion synthesis**

**Self-propagating synthesis (SHS),** also known as combustion synthesis, is a process that involves a reaction, which is sufficiently exothermic to sustain itself. This has led to a low cost, energhetically efficient method for the development of advanced materials. **Solution combustion synthesis (SCS)** is a promising method to prepare high-purity, small-sized and spherical particle phosphors because the starting raw materials are homogeneously mixed in liquid phases, and the high temperature generated instantly by exothermic reaction can volatilize low boiling point impurities leading to purer products. In addition, SCS results in products with narrow particle distribution because of the decrease in reaction time (a few seconds during the combustion reaction) **[16].** The mechanism of nanoparticle forming in SCS is shown in **Fig.16.** When heated rapidly at 5500 C, the solution containing stoichiometric amount of redox mixture boils, dehydrate, followed by decomposition generating combustible gases. The volatile combustible gases ignite and burn with a flame. The large amount of escaping gases dissipates heat and thereby prevents the material from sintering and thus provides conditions for formation of nanocrystalline phase. Also, as the gases escape they leave voluminous, foaming and crystalline fine powder occupying the entire volume of the container and have no chance of forming agglomerations unlike in the other conventional processes. Therefore, in combustion synthesis, instantaneous and *in situ* very high temperature, combined with release of large volume of volatiles from liquid

(ZnxCd1-x)S phosphors are of considerable interest because of their use as window material in solar cells. While powder phosphors were prepared by heating at 9000C in argon atmosphere, thin films (15-25 µm) were obtained by painting uniformly the mixture of phosphor powder along with 30% of CdCl2 and 65% propylene glycol on SnO2 substrate, drying in air at 1200C for 2h and then sintering at 6250C in N2 atmosphere for 30 min. CdCl2 is used as a sintering aid in the preparation of thin films of (ZnxCd1-x)S. During sintering Zn is gradually replaced by Cd through the reaction ZnS + CdCl2 ZnCl2 + CdS. This is confirmed by the red shift in the optical absorption edge of thin film when compared to that of powder phosphor. In sintered films new centres are created which increases the luminescence efficiency and emission occurs in green, yellow-orange and red regions as against only yellow-orange emission in (Zn,Cd)S:Mn,Sm phosphors. While the 525 nm emission band was interpreted as due to free to bound transition from conduction band and acceptor band, the 625 nm emission band was attributed to the radiative recombination of electrons from the donor and acceptor levels. Excitation from Sm3+ ion can be transferred to donor-acceptor pair which leads to enhanced emission at 625 nm. In addition, new traps (donor levels) are created in ZnS type phosphors

as co-activator when Zn was gradually replaced by Cd. Moreover, during

While annealing Zr2O in air at 13000C was accompanied with pronounced phase changes, phase composition remained unchanged even with prolonged holding at 13000C in vacuum. Vacuum sintering of powders with complex morphology at a high temperature thus allowed obtaining a dense ceramic based on Zr2O3 and Y2O3 with no monoclinic phase,

**Self-propagating synthesis (SHS),** also known as combustion synthesis, is a process that involves a reaction, which is sufficiently exothermic to sustain itself. This has led to a low cost, energhetically efficient method for the development of advanced materials. **Solution combustion synthesis (SCS)** is a promising method to prepare high-purity, small-sized and spherical particle phosphors because the starting raw materials are homogeneously mixed in liquid phases, and the high temperature generated instantly by exothermic reaction can volatilize low boiling point impurities leading to purer products. In addition, SCS results in products with narrow particle distribution because of the decrease in reaction time (a few seconds during the combustion reaction) **[16].** The mechanism of nanoparticle forming in SCS is shown in **Fig.16.** When heated rapidly at 5500 C, the solution containing stoichiometric amount of redox mixture boils, dehydrate, followed by decomposition generating combustible gases. The volatile combustible gases ignite and burn with a flame. The large amount of escaping gases dissipates heat and thereby prevents the material from sintering and thus provides conditions for formation of nanocrystalline phase. Also, as the gases escape they leave voluminous, foaming and crystalline fine powder occupying the entire volume of the container and have no chance of forming agglomerations unlike in the other conventional processes. Therefore, in combustion synthesis, instantaneous and *in situ* very high temperature, combined with release of large volume of volatiles from liquid

**13. Sintering enhances thin film electroluminescence** 

sintering, Zn vacancies (VZn) are created which act as deep acceptors.

despite the strong recrystallization of grains of the tetragonal phase.

containing Cl-

**14. Sintering in vacuum** 

**15. Combustion synthesis** 

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+ by solution combustion synthesis under UV illumination.

(c) Propagation (d) Final Product

Fig. 16. Various steps in solution combustion process.

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

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

The Role of Sintering in the Synthesis of Luminescence Phosphors 343

homogenization; thirdly, Eu(Ac)3 and Tb(Ac)3 were added into the precursor in the amount of 10 mol% (Eu:Zn2SiO4). Finally, a proper amount of 0.1M HCl was applied as catalyst for the hydrolysis of TEOS. The obtained precursor was transparent and clear, and was stable for several months if sealed at room temperature. Transparent gel can be obtained by leaving the precursor in air for about 2 or 3 days. Dry gel was prepared by baking the gel at 1200C for 5 min. Powder phosphors were prepared by sintering the dry gel for 30 min at different temperatures from 650 to 8500C. XRD data revealed that good crystallization of the powders could be obtained at about 8500C, which is about 4500C lower than that of the conventional solid-state reaction method **(Fig.19).** Phase analysis indicates that the obtained product has a willemite structure (rhombohedral). At the same time, a little amount of triclinic Zn2SiO4 was also detected. The TEM observation of the powders indicates that the

particles are in the range 40–100 nm in diameter and have a good crystallinity.

Fig. 19. XRD patterns of sol–gel derived Zn2SiO4 powders [19].

**TG and DTA analysis in sol-gel route** - In order to study the thermal decomposition of undoped xerogels, thermal gravimetry analysis (TGA) combined with infrared (IR) spectroscopy were used. TG and DTA (differential thermal analysis) results show cinetics of emitted gases during TG analysis, determined through the intensity evolution of the infrared main characteristic bands of each species. The total weight loss remains between -35 and -40% whatever the Tb3+ concentration is and occurs in three steps as shown in **Fig.20**. The first one (25–200◦C) corresponds to the *departure of adsorbed moisture and chemically bonded alcohol molecules, i*n agreement with **Fig. 21.** This stage leads to a wide *endothermic peak*. The second and main weight loss (−19%) lying from 200 to 600◦C is associated with an *exothermic phenomenon.* This stage can be *assigned to the pyrolysis of the organic parts* of the alkoxide groups and likely to more strongly adsorbed alcohol molecules. It is in good agreement with IR measurements which show that most of the residual organic groups are removed before

patterns show that the YAG phase can be produced with this method by sintering at 1000 °C for 2 h. This temperature is much lower than that required to synthesize the YAG phase using the conventional solid-state reaction method. No intermediate phases such as yttrium aluminum perovskite (YAP; YAlO3) or yttrium aluminum monoclinic (YAM; Y4Al2O9) were observed as a result of the sintering process. The powders were found to absorb excitation energies in the range of 410–510 nm. Furthermore, the crystalline YAG:Ce powder phosphor produced broad emission peaks in the range of 480–600 nm with the maximum intensity at 528 nm **[18]. Fig. 18** shows the TEM picture of BAM:Eu nano particles prepared by solution combustion synthesis [15].
