**Preface to "Advances in Synthesis of Metallic, Oxidic and Composite Powders"**

The high demand for new materials, such as metals, oxides, and composites, raises the need for an advanced synthesis of different materials, which are crucial for technological applications. Different process synthesis routes, such as atomization, reduction in aqueous phase, crystallization, chemical precipitation, high pressure reaction in autoclave, and electrolysis, can be used to create controlled powder characteristics with specific properties for a particular application or industry. Advances in synthesis explore a range of materials and techniques used for powder metallurgy and the use of this technology across a variety of application areas, such as medicine, catalysis and automotive industry. This Special Issue, "Advances in the Synthesis of Metallic, Oxidic and Composite Powders", is dedicated to the latest scientific achievements in the efficient preparation of metals, oxides and composite materials. In this issue, we are focused on description of the synthesis of metal, oxide and composite particles from the water, metalorganic and colloid solutions using different synthesis methods. The main challenge of this issue is the controlled synthesis via process parameters (conditions and modes atomization, the concentration of solution, residence time of aerosol in a reactor, presence of additives, flow rate, decomposition and reduction temperature, different precursors with reducing agents, and surrounding atmosphere), in order to guide the process to obtain powders with such a morphology that satisfies more and more complex requirements for the properties of advanced engineering materials. The synthesis of powders has two different strategies: "Top-Down" and "Bottom-Up". The meaning of "Top-Down" is based on the mechanical grinding of initial materials to small dimensions. It is necessary to decrease the powder size in order to perform Hall-Petch strengthening and apply a severe plastic deformation to powder particles to perform work hardening. High energy milling has a potential for realizing the new ideas of materials designers. The meaning of "Bottom-Up" is related to the physico-chemical preparation methods in gas phase (ultrasonic spray pyrolysis, flame pyrolysis and chemical vapor deposition) and in liquid phase (sol gel, hydrothermal processes, precipitation, electrolytic synthesis, high pressure reactions in an autoclave and crystallization). The precipitation methods are usually used for the purification of spent solution. In this regard, new approaches in material and synthesis design, structural engineering and morphological characteristics are provided. The preparation of metal particles by spray pyrolysis of metal salts is especially challenging. Using aerosol synthesis, a single-step and multistep preparation process of different core-shall particles is possible, thus avoiding several steps like drying, shrinkage, solute precipitation, thermolysis, and sintering to form uniform spherical particles in a nanosized and submicron range. Technical limitations of this technique, as well as a comparison with other synthesis methods (difficulty in controlling morphology-porous or hollow particles, relatively low production rate and process of large volume of gas), will be partly considered in order to prevent or solve these problems. Especially, the newest results in the synthesis of nanosized core-shell particles by ultrasonic spray pyrolysis method will be published. The HDH process consists of the following sequence: surface conditioning of the turnings, hydrogenation, ball milling (for powder production), and dehydrogenation. This Special Issue contains 17 papers from Europe, Asia, Australia, South Africa and Balkan countries, which confirms that there is a high interest for this research subject worldwide. The advances in the synthesis of metallic, oxidic and composite powders were presented via the following methods: ultrasound-assisted leaching process¸ ultrasonic spray pyrolysis, hydrogenation, dehydrogenation, ball milling, molten salt electrolysis, galvanostatic electrolysis, hydrogen reduction, thermochemical decomposition, inductively coupled thermal plasma, precipitation and high pressure carbonation in an autoclave. The synthesis methods are focused on metals: Co, Cu; Re; oxides: ZnO, MgO, SiO2; V2O5; sulfides: MoS2, core shell material: Cu-Al2O3, Pt/TiO2; Ca0.75Ce0.25ZrTi2O7, and compounds: Mo5Si3, Ti6Al4V.

The environment friendly strategy was presented at the carbonation of olivine, nuclear waste immobilization via the stability of zirconolite and treatment of acid mine drainage water. The application of the flotation tailings as an alternative material for an acid mine drainage remediation was successfully applied for an extremely acidic Lake Robule in Serbia.

Ultrasonic spray pyrolysis mentioned in three papers was applied for the synthesis of ZnO [2], core shell particles Pt/TiO2 [11] and MoS2 [7]. In addition, the ultrasonic spray pyrolysis of ammonium meta-tungstate hydrate (AMT) was used for the production of WO3 particles at 650 °C in air. The synthesis of tungsten disulfide (WS2) powder was performed by the sulfurization of tungsten trioxide (WO3) particles in the presence of additive potassium carbonate (K2CO3) in nitrogen (N2) atmosphere, first at a lower temperature (200 °C) and followed by reduction at higher temperature (900 °C). Nanostructured zinc oxide (ZnO) particles were synthesized by the one-step ultrasonic spray pyrolysis (USP) process from nitrate salt solution (Zn(NO3)2·6H2O). A flexible USP formation model was proposed, ending up in various ZnO morphologies rather than only ideal spheres, which is highly promising to target a wide application area. USP-synthesized Pt/TiO2 composites were generated in the form of a solid mixture, morphologically organized in nesting huge hollow and small solid spheres, or TiO2 core/Pt shell regular spheroids by in situ or ex situ methods, respectively. This paper exclusively reports on characteristic mechanisms of the formation of novel two-component solid composites, which are intrinsic from the USP approach, and controlled precursor composition, as shown in Figure 1.

Figure 1: Experimental setup for ultrasonic spray pyrolysis method with SEM analysis of produced particles and electrochemical measurements [11].

The carbonation process under high pressure conditions in autoclave is mentioned in two papers and applied for the synthesis of magnesium oxide (2–5 μm) and nanosized silica [4,8]. The subject of both studies is the carbonation of an olivine (Mg2SiO4) and synthetic magnesia sample (>97 wt% MgO) under high pressure and temperature in an autoclave. Early experiments have studied the influence of some additives, such as sodium bicarbonate, oxalic acid and ascorbic acid, solid/liquid ratio, and particle size on the carbonation efficiency. The obtained results for carbonation of olivine have confirmed the formation of magnesium carbonate in the presence of additives and complete carbonation of the MgO sample in the absence of additives. Differently to the traditional methods of the synthesis of nanosilica such as sol gel, ultrasonic spray pyrolysis method and hydrothermal synthesis using some acids and alkaline solutions; this synthesis method takes place in water solution at 175 °C and above 100 bar. The obtained particles of magnesium carbonate and nanosilica were shown in Figure 2:

Figure 2: Reaction path of direct forsterite carbonation in aqueous solution and SEM analysis of the obtained MgCO3 and spherical nanosilica [4, 8].

Electrochemical synthesis was mentioned in two papers using two methods: galvanostatic electrolysis and molten salt electrolysis. Al-Ti alloys were electrodeposited from equimolar chloroaluminate molten salts containing up to 0.1 M of titanium ions, which were added to the electrolyte by the potentiostatic dissolution of metallic Ti. Titanium dissolution and titanium and aluminium deposition were investigated by linear sweep voltammetry and chronoamperometry at 200 and 300 °C [13]. The obtained deposits were characterized by SEM, energy-dispersive spectrometry and XRD. In the deposits on the glassy carbon electrode, the analysis identified an Al and AlTi3 alloy formed at 200 °C and an Al2Ti and Al3Ti alloy obtained at 300 °C. Three different forms of copper powder particles obtained by either galvanostatic electrolysis or a non-electrolytic method were analyzed by a scanning electron microscope (SEM), X-ray diffraction (XRD) and particle size distribution (PSD). Electrolytic procedures were performed under different hydrogen evolution conditions, leading to the formation of either 3D branched dendrites or disperse cauliflower-like particles. The third type of particles were compact agglomerates of the Cu grains, whose structural characteristics indicated that they were formed by a non-electrolytic method [6].

Ultrasound-assisted leaching process and hydrogen reduction were mentioned in three papers [14, 16, 17] describing the synthesis of metallic powders such as rhenium and cobalt. The preparation of rhenium powder by a hydrogen reduction of ammonium perrhenate is the only industrial production method. However, due to the uneven particle size distribution and large particle size of rhenium powder, it is difficult to prepare high-density rhenium ingot. Moreover, the existing process requires a secondary high-temperature reduction and the deoxidization process is complex and requires a high-temperature resistance of the equipment. The leaching of industrial polycrystalline diamond (PCD) blanks in aqua regia at atmospheric pressure between 60 °C and 80 °C was performed using an ultrasound to improve the rate of cobalt removal, in order to be able to reuse very expensive polycrystalline diamond [16,17]. A transition from a reaction-controlled to a diffusion-controlled shrinking core model was observed for PCD with a thickness greater than 2.8–3.4 mm. Intermittent ultrasound doubles the reaction rate constant, and the full use of ultrasound provides a 1.5-fold further increase. The obtained maximum activation energy between 60 °C and 80 °C is 20 kJ/mol, for a leaching of diamond blank with grain size of 5 μm. Some results are shown in Figure 3.

Figure 3: Plots of ln(k) over PCD blank size and ultrasound time fraction (D14) [17].

Leaching and precipitation were mentioned in four papers describing the synthesis of poly-alumino-ferric sulphate (AMD-PAFS) [10], vanadium oxide [5] and hydroxide based on Al, Mn and Co [12]. Tests conducted in Erlenmeyer flasks showed that after neutralization of the lake water in Serbia to pH 7, over 99% of aluminum (Al), iron (Fe), and copper (Cu) precipitated, as well as 92% of Zn and 98% of Pb. In order to remove residual Mn and Ag, the water was further treated with NaOH. Flotation tailings rich in carbonate minerals from the tailings deposit of the copper mine Majdanpek (Serbia) were applied for neutralization of the water taken from the extremely acidic Lake Robule (Bor, Serbia). The co-precipitation of iron and aluminium from acid mine drainage water (AMD) from South Africa is conducted at pH values of 5.0, 6.0 and 7.0, using sodium hydroxide in order to evaluate the recovery of iron and aluminium as hydroxide precipitates, while minimizing the co-precipitation of the other heavy metals. The precipitation at pH 5.0 yields iron and aluminium recovery of 99.9 and 94.7%, respectively. An increase in the pH from 5.0 to 7.0 increases the recovery of aluminium to 99.1%, while the recovery of iron remains the same. The production of the coagulant is carried out by dissolving the precipitate in 5.0% (w/w) sulphuric acid. Subsequently, the treatment of the brewery wastewater shows that the AMD-PAFS coagulant is as efficient as the conventional poly ferric sulphate (PFS) coagulant. In contrast, to use ammonium solution for precipitation, an eco-friendly technology was investigated to prepare vanadium oxides from a typical vanadium (IV) strip liquor, obtained after the hydrometallurgical treatment of a vanadiumbearing shale. Thermodynamic analysis demonstrated that VO(OH)2 could be prepared as a precursor over a suitable solution pH range. Experimental results showed that by adjusting the pH to around 5.6, at room temperature, 98.6% of the vanadium in the strip liquor was formed into hydroxide, in 5 min. After obtaining the VO(OH)2, it was washed with dilute acid to minimize the level of impurities. VO2 and V2O5 were then produced by reacting the VO(OH)2

with air or argon, in a tube furnace. Consequently, this process could promote the sustainable development of the vanadium chemical industry. Synthetic zirconolite samples with a target composition Ca0.75Ce0.25ZrTi2O7, prepared using two different methods, were used to study the stability of zirconolite for nuclear waste immobilization [9]. Particular focus was on plutonium, with cerium used as a substitute. The testing of destabilisation was conducted under conditions previously applied to other highly refractory uranium minerals that have been considered for safe storage of nuclear waste, brannerite and betafite. Acid (HCl, H2SO4) leaching for up to 5 h and alkaline (NaHCOƁ, Na2CO3) leaching for up to 24 h was done to enable comparison with brannerite leached under the same conditions. Ferric ion was added as an oxidant. Given the demonstrated durability of zirconolite, long term criticality risks in the disposal environment seem a remote possibility, which supports its selection, above brannerite or betafite, as the optimal waste form for the disposition of nuclear waste, including of surplus plutonium.

Milling and thermal decomposition and hydrogenation process were mentioned in three papers and used for the synthesis of Mo5Si3 [3], Cu-Al2O3 [15] and Ti6Al4V [1] powders. A method was developed to fabricate spherical Mo5Si3 powder by milling and spheroidizing using inductively coupled thermal plasma. A Mo5Si3 alloy ingot was fabricated by vacuum arc melting, after which it was easily pulverized into powder by milling due to its brittle nature. The milled powders had an irregular shape, but after being spheroidized by the thermal plasma treatment, they had a spherical shape. Sphericity was increased with increasing plasma power. After plasma treatment, the percentage of the Mo3Si phase had increased due to Si evaporation. The hydrogenation–dehydrogenation (HDH) process for synthesis of Ti6Al4V consists of the following sequence: surface conditioning of the turnings, hydrogenation, ball milling (for powder production), and dehydrogenation. Promising results were obtained regarding the potential of the recycled powders in additive manufacturing after making minor adjustments in the HDH process. Thermochemical synthesis of copper/alumina nanocomposites in a Cu-Al2O3 system with 1–2.5 wt.% of alumina and their characterization, which included: transmission electron microscopy: focused ion beam (FIB), analytical electron microscopy (AEM) and high resolution transmission electron microscopy (HRTEM), confirming high potential for using this process in nanotechnology. Thermodynamic analysis was used to study the formation mechanism of desirable products during drying, thermal decomposition and reduction processes. Upon the synthesis of powders, samples were cold pressed (2 GPa) in tools dimension 8 × 32 × 2 mm and sintered at temperatures within the range 800–1000 °C for 15 to 120 min in a hydrogen atmosphere.

Additionally, the HSC Chemistry® software package 9.0 and FactSage were used for the analysis of chemistry and thermodynamic parameters of the processes for powder synthesis [15].

Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), focused ion beam (FIB), analytical electron microscopy (AEM), inductively coupled plasma optical emission spectroscopy (ICP OES), thermal gravimetric analysis (TGA), X-ray analysis, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were used for the characterization of morphology, structure and chemical phase and composition.

We hope that this Special Issue will offer new information and shed light on advances in the synthesis of metallic, oxidic, and composite powders.

#### **Srecko Stopic, Bernd Friedrich**

*Guest Editors* 

#### **References**

