Due to the limitations of macro- and microparticles, researchers had to prepare suspensions of nano-particles and found that copper nanofluids were three times more conductive than water was (S. K. Das et al., 2006). The reason behind these characteristics arises because the surface area of the same usual volume greatly increases; for example, 1 kg of 1 mm3 particles has the same surface area as 1 mg of 1 nm3 particles (King et al., 2024). Nanofluids aim to maximize thermal properties while using minimal concentrations (ideally less than 1% by volume). This is achieved by uniformly dispersing and stably suspending nanoparticles (preferably smaller than 10 nanometers) within the host fluids (Choi, 2008).

Different sol-gel synthesis methods include bioactive metal nanoparticles (BGNs), base-catalyzed methods (the Stöber method), acid/base catalyzed methods, post modification of sol-gel derived nanoparticles, microemulsion-assisted sol-gel methods, aerosol-assisted sol-gel methods and many other sol-gel-based methods (Zheng & Boccaccini, 2017). The synthesis of bioactive metal nanoparticles (BGNs) has numerous advantages, and options, for example, Ag-silica nanocomposites, were synthesized by applying acetonitrile bifunctionally as a solvent and metal ion stabilizer with high concentrations of Ag nanoparticles without the use of silicon alkoxide to settle metal ions. This process has also been used for “block copolymer-directed self-assembly of meso-porous material, spin coating of film and electro-spinning of nanofibers”. In addition, many organic and inorganic hybrid nanoparticles can be synthesized by UV radiation or hydrolytic sol-gel methods (Chibac et al., 2012). Silver nanoparticles are synthesized through a number of processes and methods that are repeatedly used by researchers for different purposes. Physical methods could include elector deposition or an explosion wire process. The nanoparticles were 16 nm in size, which is smaller than most other methods described in other papers (Solanki & Murthy, 2011). Chemical methods are among the most used methods for synthesizing nanoparticles. Silver nanoparticles synthesized in two solvents with special microscopic irradiation wavelengths of 7 and 12 nm had the same efficiency as synthesized nanoparticles synthesized via other methods, and the synthesized Ag colloids were stable for 6 months. It was possible to produce a monolayer of AgNPs of 48 nm with modified substrates of macrocations (Michna et al., 2019; Noroozi et al., 2014; Islam et al., 2020). Green synthesis has recently become the most effective method for producing nanoparticles because of its natural availability and environmental friendliness. Crystalized nanoparticles of various sizes can be produced from the biomass of Spirulina platensis, the leaf extract of Enicostemma axillare (Lam.), inside the human gut, the algae Botryo-coccus braunii, Poulownia tomentosa tree, silkworm cocoon fibroins and Penicillium cyclopium were found (Arya et al., 2019; Bian et al., 2019; Cui et al., 2018; Escobar-Hernández & Escobar-Remolina, 2019; Mahdieh et al., 2012; Pontaza-Licona et al., 2019; Raj et al., 2018; Wanarska & Maliszewska, 2019).

Graphene nanofluids increase the heat transfer rate by 29% at 450°C with different power inputs, temperatures and angles of inclination (S. Das et al., 2019). Compared with those of the base fluid, heat transfer via silver nanoparticles of 0.1% and 0.3% at 40°C enhanced heat transfer by approximately 18% and 5%, respectively (Rodrigues et al., 2014). In an experiment with two sizes of silver nanoparticles and oil as the mixing fluid, the heat transfer increment was approximately 30% in laminar flow and increased further with increasing Reynolds number (Hosseini & Ghorbani, 2018). The heat transfer enhancement of hybrid nanofluids works in both laminar and turbulent flows (Huminic & Huminic, 2018). Several researchers have synthesized 9 kinds of nanoparticles by chemical solution methods with controllable microstructures, and the addition of a low thermal conductivity liquid can also strongly increase heat transfer (Wang & Wei, 2009). Silicon dioxide–water nanofluids with a diameter of 20 nm were used to measure the heat transfer coefficient and pressure drop compared to those of water alone. They concluded that using nanofluids results in a pressure drop higher than that of only the base fluid, which may limit the use of these nanofluids (Anoop et al., 2013). Several studies of magnetic impact and different temperatures utilizing water as the base fluid and 0.112 vol.% Cu/CuO nanoparticles have shown that the same experiment can be reproduced using the maximum magnetic field influence (Roszko et al., n.d.).

Most silver nanoparticle synthesis processes are green because of their antibacterial properties. The majority of metal chemical processes in nanoparticle synthesis begin with the reduction of metal alkoxides, which is a common beginning step for sol-gel method. In this process, the initial molecular precursor (typically a metal alkoxide) dissolves in water or alcohol. Then, through heating and stirring, it transforms into a gel via hydrolysis or alcoholysis (Bokov et al., 2021). Silver alkoxides are not as abundant as other metal alkoxides; rather, they can be classified as rare (Edworthy et al., 2005). 

Therefore, different other methods are utilized to synthesize silver nanoparticles in the sol-gel process. Several chemical reduction methods are also available, from which a synthesis technique performed at room temperature was chosen. The reducer concentration and silver nitrate concentration were varied to determine the effect of these concentrations on the particle size. Later, these nanoparticles were mixed with water as the base fluid to produce nanofluids. The heat transfer coefficient of the nanofluid sample was measured using a shell and tube heat exchanger.

Silver nanoparticle synthesis

The nanoparticles were prepared by mixing silver nitrate (AgNO3), citric acid (C6H8O7), sodium hydroxide (NaOH), and ammonia (NH3), which was used to adjust the pH to neutral, as per the flow diagram shown in Fig. 1.

Silver ion reduction was confirmed when the solution turned black and the solution was evaporated in the presence of hydrazine hydrate, which acted as a reducing agent to separate the nanoparticles (Shahjahan et al., 2017).

The prepared nanoparticles are shown in Fig. 2. The weight fraction of the nanofluid was 0.72%; that is, 1.8 gm of nanoparticles was used to make 250 ml of nanofluid.

Chemical Compositions

A total of three samples were made where the silver nanoparticle or AgNO3 concentration was increased, as was the concentration of the reducing agent gradually, as shown in Table 1. Silver nitrate, silver chloride, silver carbonate and silver sulfate can also be used with different approaches for the reduction of ions. However, in wet chemistry for reducing silver, silver nitrate and silver perchlorate are usually used, which, when reduced using a reduction agent, can form colloids.

XRD results and analysis

The following graphs show the XRD results for the different samples, and the curves were analyzed using the software “fityk”. There was no noticeable difference in the pattern determined from the fitted curve, as shown in Fig. 5, compared with the unfitted curve shown in Fig. 6. The Ag nanoparticle size for all the samples was calculated similarly to the calculation shown in Table 1 for sample 1.

Three samples of silver nanoparticles 15 nm, 20 nm and 21 nm in size were synthesized for this study, and it was found that the silver nanoparticle size increased with increasing concentration and addition of silver nitride. Adding nanoparticles to a base fluid also increases the heat transfer coefficient of that base fluid. The increases in the heat transfer coefficient for the 1st, 2nd and 3rd samples are 68%, 75% and 58%, respectively, greater than that of the base fluid. However, the extent of heat transfer is not satisfactory because there are other metal nanoparticles, such as copper nanoparticles, which also have similar heat transfer coefficients (Luna et al., 2015).

M.A.S. Data collection, Analysis and interpretation of results, and Manuscript preparation; M.A.I. Data collection, Manuscript writing and review; S.S. Data collection, Manuscript writing and review; M.A.S. Study conception and design, Manuscript review; M.A.I. and S.S. Study conception and design, Analysis and interpretation of results, Manuscript writing and review.

The Authors would like to thank Dr. M. A. Gaffur, Principle Scientific Officer, Pilot Plant & Process Development Center, Bangladesh Council of Scien-tific and Industrial Research (BCSIR) for helping and conducting XRD for the project work.

The authors declare they have no financial interests.