**2.2 Combustion method**

The combustion method is a synthetic route for ceramics that requires less calcination compared to the conventional solid–solid method. It is simple, economical, and fast. It is interesting from research on the mechanism of the physicochemical processes involved, the dynamics of product formation, their limit of stability and process control.

Indeed, the method of combustion or synthesis by fire is known under the name of self-propagating synthesis at high temperature. To generate fire, fuel and temperature are needed. All these three elements form a triangle of fire. Fire could be described as uncontrolled combustion, which produces heat, light, and ash. The process uses highly exothermic redox chemical reactions between an oxidant and a fuel [13].

For the reaction to propagate on its own, the heat released should be greater than the heat required to initiate combustion. A redox reaction involves simultaneous oxidation and reduction processes. The classic definition of oxidation is the addition of oxygen or any other electronegative (non-metallic) element, while reduction is the addition of hydrogen or any other electropositive element (metal). The general formula of the combustion reaction is as follows:

$$\text{Combustion} + \text{O}\_2 \rightarrow \text{CO}\_2\text{(g)} + \text{H}\_2\text{O(g)}\tag{1}$$

These reactions are highly exothermic or even explosive. This method is used for the synthesis of refractory materials: borides, nitrides, oxides. Reagents are generally used in the form of nitrates because the nitrate group acts as an oxidizing agent and it has high solubility and allows homogenization of greater product yields. They are mixed, then they are "ignited" by laser, electric arc, or heating resistance. Materials based on hydrazine, urea, glycine, or citric acid form the fuel for this reaction [14, 15].

The combustion method was unexpectedly discovered during the reaction between aluminum nitrate and urea [16, 17]. A mixture of Al(NO3)3.9H2O and a urea solution, when rapidly heated to about 500 °C in a muffle furnace, to burn with an incandescent flame producing a bulky white product, which has been identified as α-Al2O3.

### **2.3 Sol–gel method**

Sol–gel process is considered as a soft chemistry method used for the synthesis of ceramics [18]. The sol–gel process is a wet-chemical technique that uses either a chemical solution or colloidal particles (sol) to produce an integrated network (gel). This later is a semi-rigid solid where the solvent is held captive in the network of solid material, which could be colloidal (concentrated sol) or polymeric [19].

The elaboration of materials by the sol–gel process, takes place via reactions of inorganic polymerizations in solution from molecular precursors, generally metal alkoxides: M(OR)n where M is a metal of oxidation degree n (for example: Si, Ti, Zr, Al, Sn…) and OR an alkoxyl group. After a drying step, a heat treatment leads to the elimination of organic compounds to form the inorganic oxide material. These reactions lead to gelation and the formation of a gel made up of M-OM (or M-OH-M) chains, the viscosity of which increases over time. This gel still contains solvents and precursors, which have not reacted. The "gel" phase in the sol–gel process is defined and characterized by a solid 3D "skeleton" embedded in liquid phase. The solid phase is typically a condensed polymeric sol where the particles have become entangled to form a three-dimensional network. The reactions allowing this material to be obtained are carried out at room temperature (**Figure 1**) [20, 21].

#### **Figure 1.**

*General scheme of sol–gel process.*

The parameters influencing the reactions are the temperature, the pH, the nature of the precursor and the solvent and the concentrations of the reactants. However, the most significant are the pH and the [H2O] / [M] ratio. The pH influences the morphology of the gel formed. In fact, an acidic pH accelerates hydrolysis and slows down condensation unlike basic pH. A high rate of hydrolysis (acidic pH) therefore promotes network growth and leads to a polymeric solution. Under acid catalysis, which is the fastest synthesis route, the gel formed is called a "polymer gel": after gelling, an open structure is obtained. However, a low rate of hydrolysis (basic pH) rather favors nucleation and leads to the formation of a colloidal solution. In the case of basic catalysis, the size of the pores is controllable (unlike acid catalysis). The gel formed is called a "colloidal gel" and has a large pore structure (clusters) [22]. Once gelation takes place, the material undergoes drying due to capillary forces in the pores and this drying could result in volume shrinkage. The drying process for obtaining the sol–gel material requires that the alcohol or water could escape as the gel solidifies. The evaporation process occurs through existing holes and channels in the porous sol–gel material. The method of drying dictate whether an aerogel or xerogel is formed. In fact, aerogel is obtained when the liquid phase of a gel is replaced by a gas in such a way that its solid network is retained, with only a slight or no shrinkage in the gel. It was usually achieved under supercritical conditions of the solvent. It is characterized by low density and high porosity. When the drying of gel is occurred by simple evaporation, xerogel is obtained. It may retain its original shape, but often cracks due to the extreme shrinkage that is experienced while being dried. After a drying process, the liquid phase is removed from the gel. Then, a thermal treatment (calcination) may be performed in order to favor further polycondensation and enhance mechanical properties [23, 24].

Although this process requires the use of relatively expensive precursors, it has many advantages:

• The high purity and the homogeneity of the solutions linked to the fact that the different constituents are mixed at the molecular scale in solution


Several materials were synthesized by the sol–gel process. CeO2-TiO2 mixed oxides aerogels with high surface area and stable anatase phase are obtained via sol–gel process [25, 26]. Bismuth barium ferrite nanoparticles doped with sodium and potassium metal ions (Bi0.85 − xMXBa0.15FeO3, x = 0 and 0.1, M = Na and K) were prepared by the sol–gel method by Haruna et al. [27]. Ahmoum et al. [28] synthesized Zn1-xNixO by sol–gel method. La2Mo2O9 nanoparticles were synthesized by Zhang et al. [29] by the sol–gel process using lanthanum nitrate La(NO)3.6H2O, and ammonium heptamolybdate (NH4)6Mo7O24.4H2O as precursors.
