2. Dissolution method

processing to achieve high-purity substances. Furthermore, natural nano-ceramic powders are hardly found. As a result, various approaches have been proposed to synthesize such materials. Two general ways were usually used, that is, bottom-up and top-down methods. The former requires precursors of the desired cation(s) and

By definition, a nanometric powder means it has crystallite or grain or particle size less than 100 nm, although some researchers claimed that sub-nanometric size

Several examples of synthesis of oxide nano-ceramic powders are solvothermal, sol-gel, and (co-)precipitation methods. The solvothermal involves the use of (usually) nonaqueous precursors and an autoclave to produce nanoparticles with unique microstructures. This method, for example, has been used to produce nanorods [1, 2], nanoclusters [3], and hollow spheres [4]. It is, however, a complex procedure. Meanwhile, the sol-gel method includes the use of complex precursors as the raw materials. For instance, synthesis of nanoparticles of magnesium and titanium oxides [5–8] has been reported recently. Despite its potential in controlling the size and shape of the products, the sol-gel process usually is time-consuming and costly. Finally, the precipitation or coprecipitation method has been reported by several researchers as an effective method to produce magnesia, titania (anatase), and zircon nano-powders [9–11]. The use of a precursor, washing with a certain liquid (usually distilled water), drying in air, and calcination are basic attributes in coprecipitation synthesis. The crystallite size of the synthesized powders for each method depends on many factors, particularly the type of precursors, as well as media, time, and temperature for processing. Some examples of nano-ceramics synthesized by these methods are presented in Table 1. Examples of bi-cationic

Recently, an approach to processing oxide nano-ceramic powders has been developed. The approach, here designated as the dissolution method, comprises dissolution of a raw powder into a strong acid, followed by drying and washing, and finally calcination in air. There were several pure metals and minerals under our

Precursor Method Calcination

MgO Mg(NO3)2.6H2O Sol-gel 500 30 [5]

TiO2 Tetrabutyl titanate Sol-gel 160 6 [14]

temperature (°C)

Sol-gel 500 30 [15]

Coprecipitation 700 30–40 [16]

Sol-gel 400 12 [17]

Sol-gel 900 33 [18]

MgCl2 Sol-gel 800 100 [12] Mg(CH3COO)2.4H2O Solid 800 53 [13]

TiOSO4/CO(NH2)2 Hydrothermal 1000 9 [10]

Crystallite size (nm)

Refs.

usually uses heating in air or oxygen-controlled environment to develop the ceramics. The latter is basically a "breakdown" approach of a larger ceramic grains

Ceramic Materials ‐ Synthesis, Characterization, Applications and Recycling

or particles by milling.

of <200 nm was still acceptable.

ceramic nano-powders are also given.

Titanium tetraisopropoxide

(NO3)2.6H2O

Mg (NO3)26H2O + Ti [OCH(CH3)2]4

and C8H20O4Si

Methods to produce mono- and bi-cationic oxide nano-ceramics from references.

MgTiO3 Ti(OH)4 + Mg

Mg2SiO4 (CH3COO)2Mg.4H2O

Nanomaterial

Table 1.

44

In principle, dissolution of solids into a liquid or other solvents is a process by which the original states become dissolved components (solutes), hence forming a solution of the solid in the original solvent (see the schematic diagram in Figure 1). When a dissolution occurs, the dissolved component separates into ions or molecules, and each of them is surrounded by the molecules of the solvent. Using the dissolution process, one can generate a precursor of a cation from a metal or a mineral if it is soluble in a selected (strong) acid. For example, magnesium reacts with hydrochloric acid according to Mg(s) + 2HCl(aq) ! MgCl2 (aq) + H2(g) where the hydrogen gas is released [19]. On the other hand, titanium is a rather unreactive metal, making it difficult to dissolve unless more external energy such as heat is provided. Dissolving titanium powder in hydrochloric acid is possible as long as the process is run at approximately 60–70°C where the product is a purple solution of titanium trichloride.

#### Figure 1.

Schematic diagram for dissolution process, an example for separate dissolution of A and B powders in HCl to synthesize AB oxide nano-powder.

Dissolution of metal oxide MO is also possible [20, 21], where M denotes a metal. Several factors may affect the dissolution kinetics including physical form and constitution of the oxides, as well as pH (acid or base), redox potential, chelating strength, concentration, and temperature of the solution. Examples of metal oxide dissolution in strong acids are for lanthanum oxide [22], iron oxides [23], and zinc ferrite [24]. In this work, the dissolution method was further used for selectively extracting the cations from natural mineral to produce high-purity ceramic nanopowders.

### 3. Synthesis and phase analyses of the powders

Tables 2 and 3 present a series of metal oxide powders which were produced by metal and mineral dissolutions, respectively. For the metal-dissolved powders, hydrochloric acid was used as the solvent. The oxide precursors were obtained by


#### Table 2.

Dissolution of metal powders in acid to produce nanocrystalline oxide powders.

drying the precipitates after subsequent washing with water. The solvent for processing zircon sand, as shown in Table 3, depends on the purposed powder. Zircon sand can be processed to obtain nano-zircon, silica, or nano-zirconia powders. The calcination temperatures were selected after inspecting the thermal data from the precursors.

The structure, composition, and nanocrystallinity of the powders were examined using CuKα X-ray diffraction (XRD) data to which the Rietveld-based Rietica [25] and Material Analysis Using Diffraction (MAUD) [26] softwares were employed. The former was applied for phase composition calculation according to the "ZMV" method, while the latter for average crystallite size estimation by implementing the Thomson-Cox-Hastings [14] model in a pseudo-Voigt peak-shape function after instrumental correction using an annealed Y2O3 as suggested previously.

The relative weight fraction of phase i at each depth was determined by:

$$\begin{aligned} \boldsymbol{W}\_{l} &= \frac{s\_{l}(\boldsymbol{Z}\boldsymbol{M}\boldsymbol{V})\_{l}}{\sum\_{j=1}^{n} s\_{j}(\boldsymbol{Z}\boldsymbol{M}\boldsymbol{V})\_{j}} \end{aligned} \tag{1}$$

High-resolution transmission electron microscope (TEM) was used to investigate the crystallite and particle sizes as well as the morphology of the synthesized

Synthesis of High-Purity Ceramic Nano-Powders Using Dissolution Method

The dried powder of Mg-dissolved solution was run for DTA/TG characterization where the associated plot is given in Figure 2. A significant drop of the mass of the powdered sample between RT and approximately 600°C is related to some thermodynamic phenomena recorded as endothermic peaks. The peaks between RT and 300°C are attributed to release water molecules with a mass loss of approximately 50%, while the peak at �500°C is due to the liberation of chloride ions and molecules. The last release is also indicated by some mass drop up to 600°C. The DTA/TG observations led the synthesis of MgO. A calcination temperature range of 400–800°C was then selected. The success of the syntheses was examined mainly

The XRD patterns of the calcined Mg-dissolved powders clearly show the phase evolution with calcination temperature. At 400°C, only bischofite (MgCl2�6H2O) was detected. Increasing the temperature to 500°C resulted in the formation of MgO, and further calcination at 700°C gave pure MgO. This result indicates that between 400 and 700°C, there was a reaction of MgCl2�6H2O (s) ! MgO (s) + H2O (g) + Cl2 (g). As implied by the DTA/TG data, there is a drop of mass and exothermic phenomena which can be ascribed to this reaction. Further analysis shows that the composition and crystallite size of the products change subsequently (Table 4). Therefore, the Mg metal-dissolved method can produce nanocrystalline, pure MgO powder after calcination at 700°C. Calcination at 800°C does not signif-

DTA/TG plot of the dried Ti-dissolved solution is given in Figure 4. The XRD patterns of the calcined powders are presented in Figure 5. Similar behavior in the phase formation of MgO was found here for the Ti-dissolved powders. Significant mass release occurs and followed by crystallite formation. Interestingly, the dried Ti-dissolved powder has already exhibited rutile with a relatively low degree of crystallinity as indicated by low diffraction intensities. The crystallinity becomes clearer at higher calcination temperatures. Pure rutile is achieved at 400°C, but anatase forms above 600°C. In most cases, the formation of anatase occurs at a

powders.

3.1 Magnesium oxide (MgO)

DOI: http://dx.doi.org/10.5772/intechopen.81983

using XRD measurements (Figure 3).

3.2 Titanium dioxide (TiO2)

Figure 2.

47

DTA/TG plot of the Mg-dissolved dried powder.

icantly alter the phase characteristics of the powder.

where si denotes the Rietveld scale factors of sample i and n is the number of the phases. Zi is the number of formula unit of phase i (calculated from the refined lattice parameters) with mass Mi in the unit cell volume Vi.

