**3. Synthesis procedures of magnesium borates**

The characteristic features of the samples associated with the composition, crystalline phases, and morphology are dependent on synthesis procedures. The

*Magnesium Borates: The Relationship between the Characteristics, Properties, and Novel… DOI: http://dx.doi.org/10.5772/intechopen.104487*

magnesium borates can be fabricated in various morphologies of the rod, sphere, tube, whisker, belt, wire, porous, or multi-angular at both nanoscale and microscale [15, 40]. Examples of the different morphologies of magnesium borates were presented in **Figure 4**. Kumari et al. [15], synthesized the nano-scale whiskers of magnesium borates in hydrothermal conditions without a capping agent [15]. Liu et al. [40], prepared submicron rods of a dehydrated form of magnesium borates by calcination at higher temperatures than 600°C [40]. Guo et al. [38], designed a hybrid method to fabricate the 3D hierarchical flower-like particles of magnesium borates [38].

The experimental design should be both low-cost and eliminate the risk of byproduct formation. The design can be shaped according to the required features of particles. Therefore, synthesis procedures can be classified as liquid-state, solid-state, and hybrid synthesis with the effect of development in production technologies.

## **3.1 Liquid-state (hydrothermal) synthesis**

Liquid-state synthesis of magnesium borates principally includes the dissolution of raw materials in a suitable solvent medium and the reaction occurs with the impulsive effect of temperature increase. Commonly, the type of magnesium salts such as magnesium oxide (MgO), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), and magnesium nitrate (Mg(NO3)2) is reacted with boric acid (H3BO3) or tincal (Na2B4O7�10H2O). At the end of the reaction, the solution is filtrated and dried. The growth mechanism could be explained by dissolution, nucleation, and recrystallization.

The growth mechanism according to the study of Ma and Liu is explained in Eqs. (1) and (2) [22]:

$$\text{B}\_{5}\text{O}\_{6}(\text{OH})\_{4}^{-}+\text{H}\_{2}\text{O}\rightarrow 2\text{B}\_{2}\text{O}\_{4}(\text{OH})\_{2}^{4-}+\text{H}\_{3}\text{BO}\_{3}(\text{aq})+7\text{H}^{+}\tag{1}$$

$$\text{g }\text{2Mg}^{2+} + \text{B}\_2\text{O}\_4(\text{OH})\_2^{\cdot 4-} \rightarrow \text{Mg}\_2\text{[B}\_2\text{O}\_4(\text{OH})\_2] \tag{2}$$

### **Figure 4.**

*Examples of the different morphologies of magnesium borates (a) nano-scale whisker by Kumari et al. [15], (b) sub-micron rod by Liu et al. [40], and (c) flower-like particle by Guo et al. [38].*

The particle shape and sizes can be controlled by optimizing the liquid-state reaction conditions. Derun et al. [6], fabricated the multi-angular particles of magnesium borate hydrates between the reaction temperatures of 80 and 100°C by using a traditional liquid-state method.

With the developing technology, liquid-state synthesis techniques can also be modified by the use of sonochemistry and capping agents.
