Production and characterization of FeNiCoCu high entropy alloys via ultrasonic spray pyrolysis method

High-Entropy Alloys (HEAs) are considerably new field in metallurgical and materials engineering. By the time, this new research area taking more attentions with their unique properties and their wide range of functional and structural applications. As a basic definition, high entropy alloys (HEAs) are solid solution alloys which contain multi principal elements with equatomic or near equatomic percent (at.%) (commonly 5% to 35%.). Up to present, the development of conventional alloy systems has been based on one or two principal element and other additive elements with substantial amounts to improve properties. However, HEAs depend to have multi component principal elements and suprisingly they tend to form with simple solid solution phases instead of intermetallic phases. This forming phenomenonas and special properties of high entropy alloys are depended to four core effects which are related with the high amount of the different alloy elements. These four core effects are, high entropy effect, lattice distortion effect, sluggish diffusion effect and cocktail effect. The name of high entropy alloys derived from that dominant high entropy effect. There are different prosessing routes to obtain high entropy alloys, the by far most common method is melting and casting the alloy. Another processing routes to synthesize high entropy alloys are powder metallurgy methods and deposition techniques. On the other hand, when mixing the many principal elements, it is not the say all multi-component alloys on the center of phase diagrams with equatomic ratio will form solid solution phases. Indeed, only accurately chosen compositions and elements will form as solid solutions. For instance, Hume-Rottery Rules is very useful to obtain solid solution for two component systems. Similarly, there is some criterias for high entropy alloys to form solid solutions. These criterias include these parameters, atomic size difference (δ), entalpy of mixing (ΔHmix), entropy of mixing (ΔSmix) and bond parameter (Ω). Obtaining high entropy alloys depend chosing these parameters carefully. In addition, these high entropy alloys have some application areas. So far, magnetic materials, nuclear industry, heat–wear resistant coatings, refractory industry, electronic studies, biomedical area, aero engine materials and battery materials have been reported as potential usage areas for HEAs. Up to now, many studies have reported that HEAs are nanostructured. Since nanostructured materials have been synthesized for the first time, they have attracted attentions with their superior properties. The nanostructured metals usually exhibit very interesting and desired chemical, mechanical, magnetic, electronic and optical properties. The properties of bulk materials change when formed from nanocrystalline particles thanks to they have a greater surface area per weight than larger particles. Therefore, nanoparticles have great scientific and technological interest areas. Development of different methods to synthesize nanostructured metal particles have attracted significant attention because of the future applications. There are two main approaches to synthesis of nanoparticles, which are top down and bottom up methods. Ultrasonic Spray Pyrolysis (USP) is a bottom-up nanoparticle synthesis method. For the synthesis of nanoparticles it is important if a process have controlling on the particle characteristics including particle morphology, mean particle size, size distribution, and composition. USP method is based on the precursor atomization and enables the obtain high purity and size controlled particles with narrow particle size distrubition and desired particle morphologies. High purity metal salt solutions or leached secondary raw material solutions can be used to production of metallic nanoparticles via USP method. That method begins with the atomization of the precursor solution of these salts or raw materials by using ultrasonic atomizer. After atomization the obtained aerosol is carried into the furnace by carrier gases. If hydrogen gas is used here, it serves both as a reducing agent and as a carrier gas and the method called as Ultrasonic Spray Pyrolysis-Hydrogen Reduction (USP-HR) method. Inside the furnace, assumed that the following steps occur: evaporation of solvent, diffusion of solutes, precipitation, decomposition and densification. Then the obtained particles are can be deposit on the layers as thin coatings or can be collected in a collecting agent which is usually ethanol. The USP-HR method is a simple and inexpensive technique and enabling one-step process. The aim of this study, synthesis of FeNiCoCu High Entropy Alloy particles via USP-HR method and determine the effects of the synthesis parameters on the obtained particles. The affect of the temperature and precursor solution molarity were examined in the production process. Characterization of FeNiCoCu HEAs were carried on through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS). Crystal structure, morphology, crystallite size, chemical composition and morphology of particles were studied. The crystallite sizes were calculated with Scherrer equation using XRD datas. In the design of experiment part, the phase formation of FeNiCoCu HEAs were investigated according to available HEA solid solution formation rules. Parameters like Hume-Rothery rules for HEAs were listed in literature review and used to calculate for FeNiCoCu equatomic composition. Moreover, the estimation of the lattice structure of the alloy was made with the help of these parameters. The calculations showed that the FeNiCoCu alloy is suitable for forming HEA solid solution and predicted that the alloy would form in the FCC lattice structure. Experimental studies were carried out under conditions of precursor solution concentration of 0.1, 0.2 and 0.4 M solutions with 800 oC furnace temperature and 700, 800 and 900 C furnace temperatures with 0.1 M precursor molarity. High purity Fe(NO3)3.6H2O, Ni(NO3)2.6H2O, Co(NO3)2.6H2O and Cu(NO3)2.6H2O nitrate salts were prepared with equamolar compositions. For atomization of the precursor solutions, ultrasonic atomizer with 1.3 MHz frequency was used. Water cooled thermostat was used to control the temperature of the solution heated by the atomization. During the heating of the furnace to the experiment temperature, high purity nitrogen gas was supplied to the system to provide an inert atmosphere. The aerosol, formed during the experiment was transported to the furnace environment using high purity hydrogen. The flow of these gases to the system is measured and controlled by flow meter devices. The aerosol droplets of iron-nickel-cobalt-copper nitrates with hydrogen gas which acting as both carrier and reducing gas, pass through different temperature zones in the furnace and they are reduced to high entropy alloy nanoparticles through evaporation, drying, precipitation and thermal decomposition / reduction processes, respectively. Then collected in ethanol, passing through gas wash bottles in which the analytical grade ethanol was included inside and the particles were collected. In the characterization studies, the results of the EDS analysis show that the constituent elements are in near equimolar proportions and SEM images have shown that the particles are uniform and spherical at all reduction temperatures and all different precursor molarity values. It has been observed that with increasing reduction temperature, average particle sizes tend to increase. In the same way, coarser particles were obtained with increasing solution concentrations. In all SEM images, the submicron-sized particles seen which were originated from smaller sizes of nanoparticles of agglomerated. Smaller nanostructured particles forming larger particles. This phonemena was better displayed in higher magnification SEM images. XPS characterization results were showed, a thin layer of Cu element was found on the surface of high entropy alloy nanoparticles produced by USP-HR method and it was evaluated that FeNiCoCu HEA particles also could define as hybrid nanoparticles.