Abstract

A set of ceramic powders has been synthesized using a "bottom-up" approach which is denoted here as the dissolution method. The raw materials were metal powders or minerals. The dissolution media were strong acid or base solutions. In the case of metallic raw materials, magnesium and titanium powders were separately dissolved in hydrochloric acid to obtain their precursors. They were then dried, washed, and calcined in air at various temperatures to produce pure MgO and TiO2 nano-powders. Pure MgTiO3 nano-powders by mixing the precursors at the stoichiometric ratio and calcining the dried mixture at a temperature as low as 700°C have also been successfully synthesized. In the mineral case, local zircon sand was used as the raw material. A standard procedure to extract the "clean" and pure zircon powder was applied which included washing, magnetic separation, and reactions using hydrochloric acid and sodium hydroxide. A pure zircon nano-powder was obtained by applying mechanical ball-milling to the zircon powder. The zircon powder was also chemically dissociated to give amorphous silica (SiO2), cristobalite, amorphous zirconia (ZrO2), and nanometric tetragonal zirconia powders.

Keywords: dissolution method, strong acid and base, metal and mineral, ceramic nano-powders

## 1. Introduction

Many efforts have been paid to produce nano-powders since they exhibit different and usually outstanding physical and chemical properties as compared to their larger counterparts. Ceramic nano-powders are even more attractive since they are thermally and chemically more stable than non-ceramic ones. Several applications of such nano-powders are in drug delivery, corrosion inhibitor, catalyst, and microwave communication. Oxide ceramics such as MgO, TiO2, MgTiO3, ZrO2, and SiO2 are more abundant than non-oxide ceramics. Their use in technological and industrial applications is hence more substantial.

Oxide ceramic powders are not easily found in nature in their simple mono- and bi-cationic forms. Yet, their technological benefits are very valuable. Their existence in nature is usually in the form of complex compounds and needs further

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 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 or particles by milling.

study. In terms of synthesis of nano-materials, it is also classified as a "bottom-up"

and demagnetization to improve the "purity" of the raw material. Further approaches were used to obtain nanometric zircon, silica, and zirconia powders. The success of the syntheses was affirmed by analytical methods like thermogravimetric and differential thermal analyses, X-ray fluorescence spectroscopy, X-ray

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

diffractometry, and high-resolution transmission electron microscopy.

The report of the synthesis is arranged by the type of the raw materials, that is, metal and mineral. The metal powders were magnesium and titanium, which produced magnesia, titania, and magnesium titanate nano-powders. The second group materials were synthesized with a slightly different way, that is, there were cleaning

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

Dissolution of metal oxide MO is also possible [20, 21], where M denotes a metal.

Schematic diagram for dissolution process, an example for separate dissolution of A and B powders in HCl to

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

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 nano-

3. Synthesis and phase analyses of the powders

approach and a wet method.

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

2. Dissolution method

titanium trichloride.

synthesize AB oxide nano-powder.

powders.

45

Figure 1.

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 of <200 nm was still acceptable.

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 ceramic nano-powders are also given.

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


#### Table 1.

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

Synthesis of High-Purity Ceramic Nano-Powders Using Dissolution Method DOI: http://dx.doi.org/10.5772/intechopen.81983

study. In terms of synthesis of nano-materials, it is also classified as a "bottom-up" approach and a wet method.

The report of the synthesis is arranged by the type of the raw materials, that is, metal and mineral. The metal powders were magnesium and titanium, which produced magnesia, titania, and magnesium titanate nano-powders. The second group materials were synthesized with a slightly different way, that is, there were cleaning and demagnetization to improve the "purity" of the raw material. Further approaches were used to obtain nanometric zircon, silica, and zirconia powders. The success of the syntheses was affirmed by analytical methods like thermogravimetric and differential thermal analyses, X-ray fluorescence spectroscopy, X-ray diffractometry, and high-resolution transmission electron microscopy.
