*3.1.2 Effect of capping agent in liquid-state conditions*

The capping agents can be employed to overcome the drawbacks of the synthesis procedures and to produce particles with homogenous and novel morphologies. The selected surfactant can be cationic, anionic, or non-ionic. In the use of capping agent, the type of capping agent is not able to highlight the relationship between the agent and the core particle. Some type of capping agents use could decrease the particle size; however, the crystallinity of samples could be affected adversely [15]. This situation might require a more detailed examination of the relationship between the capping agent and the magnesium borate particle.

In the modified liquid-state synthesis of magnesium borates, the examples of the preferred capping agent are polyvinyl pyrrolidone (PVP), sodium dodecyl sulfate (SDS), nickel nitrate (Ni(NO3)2), cetyltrimethylammonium bromide (CTAB) and triton (T) [15, 37, 44, 45]. The examples of the effects of different capping agents on the synthesized magnesium borates were presented in **Figure 7**. Kumari et al. [15] reported the characteristic effects of surfactant addition to the liquid-state synthesis of magnesium borate particles and indicated the notable changes in morphology to obtain nano-whiskers [15]. In the synthesis of inorganic ceramic compounds, PVP is utilized to decrease particle size and/or to sustain homogeneous morphology. However, hierarchic porous structures were obtained in the PVP-based synthesis of magnesium borates [44].

## **3.2 Solid-state (thermal) synthesis**

Solid-state synthesis of magnesium borates fundamentally involves the mixture of the powders of raw materials without any liquid component and the reaction of solid powders occurs with the impulsive effect of temperature increase in high-temperature furnaces. The common magnesium sources preferred in this synthesis method are

### **Figure 6.**

*The comparison of the Admontite morphologies synthesized (a) by ultrasonic liquid-state method [42], and (b) by traditional liquid-state method [6].*

**Figure 7.** *The effects of different capping agents on the synthesized magnesium borates (a) CTAB, (b) SDS, (c) T at low magnification, and (d) T at high magnification [15].*

MgO and magnesium hydroxide (Mg(OH)2). Mostly, the raw materials are reacted in air atmosphere. The prepared sample is commonly in micron-scale at heterogeneous morphology. Mg2B2O7, MgB4O7, and Mg3B4O6 are notable combinations of dehydrated magnesium borates [45, 46].

In the phase diagram of Liu et al. [40] for the solid-state synthesis of magnesium borates, the reaction commonly occurs at higher reaction temperatures than 800°C [40]. In this synthesis procedure, the main drawback of solid-state is the requirement of grinding and sieving processes after the solid-state reaction. Chen et al. [47] used capping agent addition of Ni(NO3)2 to eliminate these extra processes of the solidstate synthesis procedure [47].
