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The aim of this article was an explanation of the gold nanoparticle (AuNP) formation mechanisms which take place in a redesigned Ultrasonic Spray Pyrolysis (USP). Depending on the synthesis parameters of gold concentration in the precursor solution, gas flow and reaction temperature, we have previously obtained a combination of spherical, irregular and cylindrical AuNPs. Two parameters (gold concentration and gas flow) were determined to be the most influential on the shapes and sizes of the produced AuNPs. The effects of these two influential synthesis parameters were evaluated on controlling the formation mechanisms: Gold concentration (2.5 and 0.5 g/l Au) in the precursor solution of HAuCl 4 dissolved in water and gas flows of (i) aerosol carrier gas N 2 (1.5, 3.0 and 4.5 l/min) and (ii) reduction gas H 2 (1.0, 1.5 and 2.0 l/min). Depending on the parameter conditions, the AuNPs are formed from a combination of the liquid/solid phase (Droplet-to-Particle mechanism, DTP), the gas phase (Gas-to-Particle mechanism, GTP) and from intermediate secondary droplets, formed from primary droplet explosions. Increasing the gas flow affected the evaporation of the solvent (water) and diffusion of the solute ([AuCl 4 ] − ) in the aerosol droplets, which resulted in the formation of more uniformly shaped AuNPs and with narrower size distributions than before. With favorable parameter conditions increased control over AuNP synthesis with USP has been achieved.

The present investigation reports the first-time successful synthesis of AuNPs using a new precursor salt of Au(III) acetate through USP. An aqueous solution of this salt was prepared with very limited solubility with H2O. HCl and HNO3 were then added separately to increase the solubility, resulting in a clear, yellowish solution. This enabled the successful formation of AuNPs with USP. In order to improve AuNPs synthesis, NaOH and Na2CO3 were added into the precursor to increase its pH (6–7). With such approach, it was possible to perform USP synthesis using varying concentrations of [Au] in the precursor. Evaporation and reaction temperatures (100 and 300 °C) of USP were chosen based on detected decomposition temperatures of Au(III) acetate with TGA-DT. TEM confirmed the presence of circular shaped, unagglomerated AuNPs having an Fm-3m space group with diameter range of 15–30 and circularity value range of 0.89–0.92. The UV–Vis spectroscopy showed absorbance peaks at 528 and 532 nm. ICP-MS indicated the highest concentration of AuNPs, 79 ppm, by the precursor with the lower initial concentration of [Au]. This could be due to the smallest sedimentation and turbulent losses of larger AuNPs in transport tubes and reaction USP zones.

To prevent the evaporation of alkali elements during the calcination of the lead-free piezoelectric K 0.50 Na 0.50 NbO 3 (KNN), lower temperatures for the solid-state synthesis of the KNN from a mixture of Na 2 CO 3 , K 2 CO 3 and Nb 2 O 5 were investigated with a variety of powder-characterization techniques. The collected data suggested 550 °C as a possible low calcination temperature, as well as being sufficiently high for a completed synthesis of the KNN. The XRD and TEM analyses of double-calcined KNN powder at 550 °C revealed nanocrystalline, cube-shaped particles with a perovskite structure, but without noticeable unreacted precursors or secondary phases. Our investigations revealed that the morphology of the calcined powders underwent a dramatic change from small, differently shaped particles at 600 °C/6 h, to fully rounded, large, spherical agglomerates produced at 1000 °C/6 h. To explain this behavior, a mechanism involving the formation of a liquid phase during the reaction of the alkali elements with the moisture and CO 2 in the atmosphere is proposed.

The manufacturing and properties of the AE44 magnesium alloy reinforced with SiC-Al2O3-SiO2 ceramic foam were studied. The interpenetrating phase composite was manufactured by gravity casting at different preheating temperatures of the ceramic foam. The samples were investigated using optical and electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and compression testing. The interfacial reaction products (AlSiRE and AlMgSiRE) between the metal phase and ceramics were influenced by the preheating temperatures of ceramic foam and reduced the compression strength significantly. V delu je obravnavan kovinsko-keramični material z infiltrirano magnezijevo zlitino (kompozit IPC; ang. interpenetrating phase composite). Preiskovani kompozit IPC je bil izdelan s postopkom gravitacijskega kokilnega litja. Metalografsko smo ga preiskali s svetlobno mikroskopijo, vrstično in presevno elektronsko mikroskopijo, energijsko disperzijsko spektroskopijo in rentgensko difrakcijo, prav tako pa smo opredelili njegove mehanske lastnosti.

Cr2N0.61-7Ag nanocomposite coatings were deposited on substrates made of Cr-V ledeburitic tool steel Vanadis 6 using reactive magnetron sputtering at a deposition temperature of 500 °C. Investigations of as-deposited films and annealing experiments in closed-air atmosphere at temperatures of 300, 400 and 500 °C and the durations up to 24 h, followed by quantitative scanning electron microscopy, transmission electron microscopy, Auger electron spectroscopy and X-ray diffraction revealed that the films were composed of Cr2N0.61 matrix and individual silver agglomerates located along columnar crystals of the matrix. The maximal size of Ag-agglomerates was 80 nm. The surface population density of silver agglomerates increased with prolonging the annealing time up to 2 h and then decreased. The increase was more pronounced at lower annealing temperatures. This behaviour was referred to the competition between three phenomena, namely the transport of detached Ag atoms to the free surface, formation of oxide layer on the surface and sublimation of silver from the surface. At lower temperatures and/or shorter annealing times, the Ag-transport to the free surface was determined to be prevalent, thus, an increase in population density of silver agglomerates was determined. On the other hand, for higher temperatures and/or longer annealing times the population density of Ag-agglomerates rather decreased due to retarding effect of thicker oxide layer and sublimation of silver.

This article reports about a new synthesis approach by the Ultrasonic Spray Pyrolysis of Au nanoparticles which consists of a separate heating zone for evaporation and subsequent connection of reduction gas by entering directly into the reaction zone. This redesign was made in order to control each step of the USP to find those influential parameters which dictate final gold nanoparticles’ morphology and size. For the precursor a starting solution of HAuCl4 and water was used with various concentrations of Au (0.625 g/l, 1.25 g/l and 2.5 g/l). Other variable parameters were evaporation temperatures (75 � C, 80 � C and 85 � C) and time of synthesis (3 h, 6 h, 9 h), while the temperature in the heating zone was constant (350 � C) during performing all the experiments. Characterization of synthesized Au nanoparticles was carried out by Transmission Electron Microscopy (TEM). It was found that Au nanoparticles have bimodal size distribution. An investigation of the Au nanoparticles’ electron diffractions enabled us to find the possible growth of Au nanocrystal types which was finally the base for setting up the synthesis mechanisms of Au nanoparticles. The cytocompatibility investigations of the Au nanoparticles suggested that they were not cytotoxic for L929 cells in vitro, but they can exhibit anti-proliferative properties, depending on their size distribution.

Nanostructured zinc oxide (ZnO) particles were synthesized by the one step Ultrasonic Spray Pyrolysis (USP) process from nitrate salt solution (Zn(NO3)2&middot, 6H2O). Various influential parameters, from Zn(NO3)2&middot, 6H2O concentrations (0.01875&ndash, 0.0375 M) in the initial solution, carrier gas (N2) flow rates (0.5&ndash, 0.75 L/min) to reaction temperature (400&ndash, 800 &deg, C), were tested to investigate their role on the final ZnO particles&rsquo, morphology. For this purpose, Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and (Selected Area Electron Diffraction) SAED techniques were used to gain insight into how the ZnO morphology is dependent on the USP process. It was revealed that, by certain parameter selection, different ZnO morphology could be achieved, from spherical to sphere-like structures assembled by interwoven nanoplate and nanoplate ZnO particles. Further, a more detailed crystallographic investigation was performed by XRD and Williamson-Hall (W-H) analysis on the ZnO with unique and non-typical planar morphology that was not reported before by USP synthesis. Moreover, for the first time, 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.

Microstructure, phase constitution and hardness of Cr–V ledeburitic steel Vanadis 6 subjected to sub-zero treatment with a soaking time of 4 h and at a temperature of −196 °C have been examined with reference to the same material after conventional heat treatment. Microstructures have been characterized using light microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Hardness has been evaluated by Vickers method. As-annealed material consists of the ferritic matrix and of two carbide phases, namely M7C3 and MC. After the heat treatment, the matrix is martensitic with certain amount of retained austenite. Sub-zero treatment results in more complete martensitic transformation. The martensite contains areas with extremely enhanced carbon content. Sub-zero treatment refines the size of martensitic domains and increases the dislocation density inside the domains. Microstructure of sub-zero treated steel contains enhanced amount of small globular carbides, as compared to conventionally heat treated material. These particles have a size of around 100 nm in most cases. Small globular carbides are of the cementitic nature for both the no sub-zero treated steels and the sub-zero treated material. Bulk hardness manifests a moderate increase due to the sub-zero treatment. The increase in hardness is more significant when higher austenitizing temperature is used for the treatment, which is associated with more pronounced reduction of the retained austenite content.

This article explains the mechanism of the metal/oxide core-shell Ag/TiO2 and Au/TiO2 nanoparticle formation via one-step ultrasonic spray pyrolysis (USP) by establishing a new model. The general knowledge on the standard “droplet-to-particle” (DTP) mechanism, nucleation, and growth processes of noble metals, as well as physical and chemical properties of core and shell materials and experimental knowledge, were utilized with the purpose of the construction of this new model. This hypothesis was assessed on silver (Ag)/titanium oxide (TiO2) and gold (Au) TiO2 binary complex nanoparticles’ experimental findings revealed by scanning electron microscopy (SEM), focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM), and simulation of crystal lattices. It was seen that two mechanisms run as proposed in the new model. However, there were some variations in size, morphology, and distribution of Ag and Au through the TiO2 core particle and these variations could be explained by the inherent physical and chemical property differences of Ag and Au.

The Cr-V ledeburitic steel Vanadis 6 was vacuum austenitized at different temperatures, nitrogen gas quenched and double tempered at 530 °C for 2 h. For the selected samples, a sub-zero period was inserted between quenching and tempering. The obtained results infer that: I) as-quenched microstructure consisted of martensite, retained austenite and undissolved carbides, II) sub-zero processing reduced the amount of the retained austenite and increased the tetragonality of the martensitic lattice, III) as-quenched hardness of the steel was higher by 2 – 3 HRC due to sub-zero processing, IV) as-tempered hardness increased with increasing austenitizing temperature but it decreased slightly with the sub-zero period, V) no negative impact of sub-zero processing on toughness was recorded, VI) wear resistance increased with sub-zero period when 100Cr6 steel has been used as a counterpart.

The Cr–V ledeburitic steel Vanadis 6 was vacuum austenitised, nitrogen gas quenched and double tempered under various combinations of heat treatment regimes. For selected samples, a subzero period was inserted between quenching and tempering. The results obtained infer that: (1) the as quenched microstructure consisted of martensite, retained austenite and undissolved carbides; (2) the subzero processing reduced the amount of the retained austenite and increased the tetragonality of the martensitic lattice; (3) the as quenched hardness of the steel was higher by 2–3 HRC due to subzero processing; (4) the as tempered hardness increased with increasing austenitising temperature, but it decreased slightly during the subzero period; (5) no negative impact of subzero processing on toughness was recorded; (6) the wear resistance increased with subzero period when a 100Cr6 steel was used as a counterpart.

Metals : open access journal 8(4), 278 (2018). doi:10.3390/met8040278 special issue: "Special Issue "Powder Synthesis and Processing" / Guest Editor Prof. Dr. Claude Estournès", Published by MDPI, Basel

The aim of the study was to assess the influence of adding Al2O3 nano-particles of 0.5 wt. % with the mean particle size of 500 nm on the mechanical properties and wear behaviour of the austenitic stainless steel matrix reinforced with nano-particles produced by conventional ingot casting. The focus was on the methods and possibilities of homogeneous and uniform distribution of the particles within the steel matrix using conventional casting routes. The main drawback of the casting method used is the agglomeration of the particles and poor interface between the particles and the metal matrix. The results show that through a proper insertion method, nano-particles can be successfully introduced into the metal matrix. The Al2O3 nano-particles were successfully incorporated into the steel matrix with no signs of clustering and intermetallic reactions taking place between the nano-particles and the steel matrix. This led to improved mechanical properties as well as the wear behaviour of the stainless steel, achieved by using conventional casting routes.

This paper presents the crystal structure and the phase transitions of K x Na 1− x NbO 3 (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100–800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic–tetragonal and tetragonal–cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.

The sequence of reactions during the synthesis of sodium niobate, potassium niobate, and sodium potassium niobate from alkaline carbonates and niobium oxide has been studied by diffusion couples in the temperature range between 500° and 700°C for up to 48 h. The reactions proceed by coupled diffusion of alkaline and oxygen ions into niobium oxide. The first phase to form at the interface Na 2 CO 3 /Nb 2 O 5 is Na 2 Nb 4 O 11 at 500°C. The perovskite phase forms only after heating at 700°C at the boundary between Na 2 Nb 4 O 11 and Na 2 CO 3 . In the K 2 CO 3 /Nb 2 O 5 diffusion couple, the sequence of phases after heating at 600°C is Nb 2 O 5 /K 6 ,Nb 10.88 O 30 /K 4 Nb 6 O 17 /KNb03/ K 2 CO 3 . In the (K 2 CO 3 +Na 2 CO 3 )/Nb 2 O 5 diffusion couple the (K,Na)NbO 3 solid solution forms via the intermediate phase (K,Na) 2 Nb 4 O 11 at 600°C. The order of magnitude of the parabolic reaction rate constant for the diffusion-controlled reaction at 600°C is about 10 -15 m 2 /s for the (K 2 CO 3 +Na 2 CO 3 )/ Nb 2 O 5 and the K 2 CO 3 /Nb 2 O 5 systems, which is about one order of magnitude less than that for Na 2 CO 3 /Nb 2 O 5 (10 -14 m 2 /s). The reaction rate in the ternary system is determined by the diffusion of the slower species, i.e., the potassium ions.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030°C for 8 h and 1100°C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100°C, the larger grains (up to 30 μm across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.

In this contribution the effect of alkaline-earth (AE) dopants Mg2+, Ca2+, Sr2+ and Ba2+ on sinterability and functional response of KNN is studied. Ceramic powders with the general formula (K0.5Na0.5)1−2yAEyNbO3, y = 0.005 were prepared by classical ceramic processing. The values of 94–95% of theoretical density (TD) are obtained for stoichiometric KNN after sintering at 1115 °C for 2 h. Calcium and strontium doping promotes densification while magnesium doping inhibits it. In the case of Ba-doping a secondary phase forms. The dielectric constant and piezo d33 coefficient determined by Berlincourt piezo d33-meter of Ca and Sr-doped KNN ceramics are 500 and 95 pC/N, respectively, while the values for undoped KNN are slightly lower.

This paper presents the crystal structure and the phase transitions of K x Na 1− x NbO 3 (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100–800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic–tetragonal and tetragonal–cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.