Energy efficient and cost effective method for generation of in-situ silver nanofluids: Formation, morphology and thermal properties

Abstract

In the present work, silver nanoparticles (AgNPs) are generated using the Micro-Electro Discharge Machining process (micro-EDM). Nanofluids are synthesised in dielectric fluids such as polar fluid like Deionized water (DI), Deionized water with 4 wt% of Poly-Vinyl Alcohol (DI + PVA), and non-polar fluids like Ethylene Glycol (EG) and Kerosene (KR). Low energy consumption, in-situ nanofluid synthesis, cleaner work environment, non-essential chemical post-processing during the synthesis using micro-EDM are the significant reasons creating broad scope for this process exploration. To understand the process, theoretical approach is explored to study the effect of dielectric fluids on particle formation mechanism, critical radius and nucleation rate of nanoparticles. In the experimental approach, silver nanoparticles are generated and characterized for the particle concentration, morphology and size distributions in all four dielectric fluids. High Resolution Scanning Electron Microscopy (HRSEM), Dynamic Light Scattering (DLS), and UV–Visible spectroscopy (UV–Vis) are used for the study. Nanofluid's decomposition temperatures and latent heat of vaporization are investigated using ThermoGravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), respectively. Particles generated in DI + PVA fluid found to be smaller mean size of (30.06 ± 1.12) nm followed KR fluid of (40.32 ± 1.29) nm, EG fluid of (47.85 ± 1.24) nm, and in DI fluid of (149.04 ± 1.93) nm. Also polar liquids yielded wider and non-polar liquids yielded narrower particle distribution. KR nanofluid is thermally stable followed by DI + PVA, DI, and EG nanofluid. With the spark energy of 1.15 mJ, the in-house developed micro-EDM process yielded highest nanoparticles concentration of 2.68 g/L in KR fluid followed by DI + PVA fluid of 2.13 g/L, DI fluid of 2.09 g/L and least by EG fluid of 1.04 g/L.

Introduction

Nanoscience and nanotechnology is the field of study of minuscule particles with the dimensions being less than 100 nm. These nanoparticles are extensively exploited in almost all fields of science and technology, such as optics, photonics, catalysis, bacterial inhibition, drug delivery, heat transfer, lubrication, etc. The properties of nanoparticles are very different from their bulk materials. Smaller the particle, lesser the crystallinity and change in atomic density differs the physical properties such as phase transition temperatures, optical absorption, electrical conductivity, magnetism, and chemical properties such as lubrication, adhesion, colloidal dispersions, catalysis etc. [1]. With this much intense importance, the generation and tuning of nanoparticle properties and their application suitability studies are the prime focus of research worldwide [2]. Based on the applications, there are a variety of nanoparticles, such as metallic, metallic oxides, alloyed, graphene, doped, hybrid nanoparticles are commercially available in the market. Among these particles, metallic nanoparticles found a lot of attention due to their distinctive possibilities to bind with ligands, drugs, antibodies, and their exceptional heat transfer, electrical conductivity, and magnetic properties.

Silver, in particular, has an ability to showcase antimicrobial/antifungal characteristics, efficient drug carriers, and exhibit high electric and thermal conductivity [3]. These properties enabled the use of silver nanoparticles mainly in textiles, food packaging, cosmetics, wound dressing, theranostics, etc. lead to increase requirements of silver nanoparticles and is likely to require around 800 tonnes of production by 2025 [4].

To meet the requirements, there is a need for efficient, eco-friendly, and simple techniques to produce the silver nanoparticles. Currently there are four major categories for producing the particles namely chemical synthesis by reducing the silver nitrate (AgNO₃) by using suitable reducing agents, physical synthesis by decomposition of silver bulk material using thermal, electric, ion beam energy, photochemical synthesis by reducing silver nitrate (AgNO₃) by using suitable reducing agents along with UV or light source, and biological synthesis using bio-control strains such as Bacillus sp, Peptide etc. [5]. Among all these processes, physical synthesis can produce large quantities of nanoparticles with comparatively high initial investment costs. Physical synthesis includes, wire explosion used for generating aluminium nanoparticles [6], ball milling for iron oxide [7], [8], electric discharge machining for nickel [9] and aluminium [10], laser spraying for silver [11], ion beam machining for silver [12], flame spraying techniques for titanium oxide nanoparticles [13].

Micro-EDM is one of the non-conventional mechanical micromachining processes which has the same working principle as of conventional EDM but differs in operating parameters in particular voltage, pulse-on time, inter-electrode gap, current, etc. due to scaling effects [14]. Researchers worldwide explored this process that requires low energy and has clean production environment to generate metallic nanoparticles. [15] used this process to generate copper nanoparticles, [16] generated gold nanoparticles and [10] aluminium nanoparticles are also generated. They have reported the effects of stabilizers on the agglomeration of nanoparticles during generation.

Various researchers have considered the spark energy (E = Voltage × Current × Pulse-on time) as one of the parameters to quantify energy consumption [17]. Spark energy involved in die-sink EDM reported as 3.6 mJ, hybrid-EDM is 3 mJ, and in micro-EDM, spark energy is 0.2 mJ [18] whereas in the present study it is 1.15 mJ. Among the physical synthesis processes, the production rate obtained is 2.88 g/hr for 48.3 mJ of spark energy [19], 1.14 g/hr for 192 mJ [20], 21 g/hr for 224.6 mJ [21] and in the present study maximum of 2.68 g/hr is achieved in kerosene nanofluid for 1.15 mJ of spark energy.

In this process, particle formation inside the dielectric fluid follows two different ways. The majority of particle formation takes place by condensation of metallic vapor, and the rest of the particles are by freezing of molten metal. Such generated particles are pure and monolithic [22], [23], [24]. The detailed mechanism is discussed in the subsequent Sections. Another significant advantage of this process is in-situ synthesis of nanofluids, which avoids two-step preparation [25] saves efforts and energy in handling nanopowders and post-processing for nanofluid stabilization.

Nanofluids are the homogeneous mixture of nanoparticles suspended in a basefluid. The base fluids may be bio-fluids, lubricants, organic/inorganic fluids, water etc. depending on the applications. The major difficulties in handling nanofluids are the stability of suspended particles, ever-changing optical, thermal properties, and insufficient information on the interaction between particles, surfactants with the liquid molecules [26], [27].

Based on extensive study on existing literature on the generation of nanoparticles, characterization of nanofluids, and understanding of particle interactions in the different types of fluids, the current paper has been prepared. It is evident that there is a scope for increasing the productivity of the micro-EDM process by studying the influence of dielectric fluids on particle formation mechanism and the characterization of in-situ generated nanofluids. Particle formation and growth mechanisms in micro-EDM setup are discussed based on established theories. Effect of dielectric fluid on critical radius formation, number of nucleation sites based on the surface tension, vapor pressure and latent heat of vaporization of nanofluids are studied. Change in surface tension, decomposition temperatures, and latent heat of vaporization are examined using thermogravimetric analysis and differential scanning calorimetry techniques. Concentration measurements, particle size distribution and purity of nanoparticles are discussed by using UV–Vis spectroscopy, dynamic light spectroscopy and scanning electron microscopy.