*3.2.1 Advantages of microwaves in solid-state conditions*

Microwave energy can be defined as non-ionizing electromagnetic radiation with frequencies between 300 MHz and 300 GHz [48]. Similar to the effects of sonochemistry in liquid-state conditions, microwaves can be beneficial to solid-state synthesis procedures to increase the interaction between the powders of raw materials. Unlike traditional calcination techniques, the heating direction is from the inside to the outside of the heated sample in microwave conditions. This situation could assist both to increase reaction yield in solid-state conditions and to decrease the byproduct formation. The temperature increase is supplied with the microwave effects. The reaction costs can be reduced effectively with the optimization of reaction

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

conditions. In the reaction procedure, microwave power level and microwave treatment time are notable operating parameters. However, the relationship between the microwave parameters and supplied temperature increase has not been comprehensively studied. In that case, more detailed experimental setups should be designed for the determination of the reaction mechanisms.

Very few studies indicated the possible use of microwave energy in magnesium borate synthesis. Kipcak et al. [49, 50], proved the beneficial effects of microwave to decrease the reaction time in magnesium borate synthesis in comparison with traditional calcination techniques [49, 50].

## **3.3 Hybrid synthesis**

Hybrid synthesis procedure can be defined as the combination of liquid and solidstate conditions. The experimental procedure generally begins in hydrothermal conditions and continues with a solid-state step. Pechini, combustion, and sol-gel synthesis techniques can be assumed as examples of hybrid synthesis [51]. The steps of the hybrid procedure could be more complex than the traditional techniques; however, high purity and homogeneous morphology can be obtained. To strengthen the properties of magnesium borates, hybrid methods should be supported with novel technologies.

Gonzalez et al. [27], synthesized the Tm and Ag-doped MgB4O7 with the reaction of Mg(NO3)2 and H3BO3 in urea medium, at the calcination temperature range of 750– 950°C by using the combustion method [27]. Zhang et al. [37]; preferred the capping agent of N, N, dimethylformamide nitrate to fabricate the hierarchic porous particles of magnesium borates. The liquid-state reaction occurred at 150°C for 12 hours whereas the solid-state reaction continued at 600°C for 12 hours [37]. Chen et al. [44], fabricated the mesoporous structure of Mg2B2O7 in microsphere morphology by adding the SDS to the reaction medium of Mg(NO3)2 and borax. The two-step process began with the 80°C for 2 hours and continued with 500°C for 4 hours [44]. Wang et al. [52], prepared the fibers of magnesium borates with a two-step reaction. The mixture of MgCl2 and borax is reacted at 80°C for 12 hours and then sintered at 800°C for 6 hours [52]. In the hybrid synthesis of Zhu et al., the high purity of Mg2B2O5 was synthesized with the reaction of MgCl2, H3BO3, and NaOH [53].

The comparison of the obtained morphological features of hybrid and traditional procedures can be seen in **Figure 8**. The uniform particle formation was obtained in

### **Figure 8.**

*The comparison of the obtained particle morphologies (a) hybrid [44], and (b) traditional [6] synthesis.*


### **Table 3.**

*Comparison of traditional and advanced synthesis techniques.*

the hybrid synthesis of Chen et al. [44] whereas the heterogeneous morphology can be seen in the traditional hydrothermal synthesis of Derun et al. [6].
