## 3 Micro- and Nanoparticles Generating Procedure in Sub- and Supercritical Water  
The usage of micro- and nanoparticles in chemical, pharmaceutical, or material industries is clearly booming, due to their mechanical, chemical, physical, electrical, optical, and thermal properties, provided by their high surface volume ratio and its peculiar shapes and structures in space [21]. The exclusive properties of micro- and nanoparticles make them widely usable for different technologies associated to ceramic, catalysis, data storage, biomedicine, pharmacy, energy, etc. Physical Processes  
* Thermal evaporation [23]
* Gas clusters preparation [24]
* Ion implantation [25]
* Chemical vapor deposition [26]
* Grinding or mechanical-chemical activation [27]. * Chemical Processes
* Radiolytic and photochemical reduction [28]
* Microwave irradiation [29]
* Solvothermal synthesis [30]
* Sol-gel process [31]. When compared to other conventional methods, the process to generate micro- and nanoparticles is favored when the precipitation is generated by means of near or supercritical water. This is due to the above-mentioned water properties, which provide a greater control of the nucleation, a greater reaction rate and an improved control of the particles shapes. Furthermore, they can be carried out by means of relatively simple and low-volume equipment [21]. The reactions that take place in hydrothermal synthesis are still to be thoroughly determined and studied. This is mainly due to lack of data on intermediate compounds. It is true that a large number of particle types implies some difficulties to develop the appropriate software application that may generate reliable predictive models, since each particle type may have its own intermediate compounds and crystallization rate. Failing to count on such predictive models, the influence of temperature, pressure, precursor, and time on the crystallization kinetics of the different compounds should be empirically determined. By getting to know how these variables work, we should learn how to control particle formation, their purity, formation time, and operating conditions [16]. The best approach in recent years to this problem was based on four simple steps [16]:  
1. Calculating the thermodynamic equilibrium as a function of the process variables. 2. Generating the equilibrium diagrams to map the phases of interest of the process variables. 3. Designing hydrothermal experiments to test and validate the diagrams previously calculated. 4. Using the process variables to control the reactions and the crystallization kinetics. In this way, a bidimensional phase diagram would be obtained by varying the media pH while the precursors concentration, temperature and pressure would remain constant. This would allow to determine the type of particle that would be generated and its purity depending on the pH value [32]. The diagrams can also be three-dimensional, that is, relating three of the variables mentioned, such as pH, temperature, and purity. Going deeper in this subject, Professor [PERSON] et al. [16] already completed some studies on the line of reactions. They hold that two types of reactions take place during the simple oxides forming process. The former is hydrolysis (Equation (1)) and the latter is dehydration (Equation (2)). First, themetal salt is hydrolyzed into metal hydroxide. Then it is immediately precipitated as metal oxide by dehydration due to its low solubility in water under hydrothermal conditions. \[\text{M}(\text{NO}_{3})_{2}\ (s)+\text{xH}_{2}\text{O}\ (l)\qquad\rightarrow\qquad \text{M}(\text{OH})_{\text{x}}\ (s)+\text{xHNO}_{3}\ (l) \tag{1}\]  
\[\text{M}(\text{OH})_{\text{x}}\ (s)\qquad\rightarrow\qquad\text{MO}_{\text{x}/2} \ (s)+\frac{\text{x}}{2}\text{H}_{2}\text{O}\ (l) \tag{2}\]  
Metal oxides' low solubility is due to water's density drop and dielectric constant variations of water when near the critical point (see Table 1).