Investigation on hybrid nanofluids based on carbon nanotubes filled with metal nanoparticles: Stability, thermal conductivity, and viscosity

Field Emission Properties of Cu-Filled Vertically Aligned Carbon Nanotubes Grown Directly on Thin Cu Foils

Copper-filled vertically aligned carbon nanotubes (Cu@VACNTs) were grown directly on Cu foil substrates of 0.1 mm thicknesses at different temperatures via plasma-enhanced chemical vapor deposition (PECVD). By circumventing the need for additional catalyst layers or intensive substrate treatments, our in-situ technique offers a simplified and potentially scalable route for fabricating Cu@VACNTs with enhanced electrical and thermal properties on thin Cu foils. Comprehensive analysis using field emission scanning microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) mappings, and X-ray diffraction (XRD) revealed uniform Cu filling within the VACNTs across a range of synthesis temperatures (650 °C, 700 °C, and 760 °C). Field emission (FE) measurements of the sample synthesized at 700 °C (S700) showed low turn-on and threshold fields of 2.33 V/μm and 3.29 V/μm, respectively. The findings demonstrate the viability of thin Cu substrates in creating dense and highly conductive Cu-filled VACNT arrays for advanced electronic and nanoelectronics applications.

Optimizing MWCNT-Based Nanofluids for Photovoltaic/Thermal Cooling through Preparation Parameters

Multiwalled carbon nanotubes (MWCNTs) were employed as added particles for nanofluids in this practical investigation. To identify the most appropriate nanofluid for cooling PVT systems that are functional in the extreme summer environment of Baghdad, the parameters of base fluid, surfactant, and sonication time used for mixing were examined. Water was chosen as the base fluid instead of other potential candidates such as ethylene glycol (EG), propylene glycol (PG), and heat transfer oil (HTO). Thermal conductivity and stability were important thermophysical qualities that were impacted by the chosen parameters. The nanofluid tested in Baghdad city (consisting of 0.5% MWCNTs, water, and CTAB with a sonication period of three and a quarter hours) resulted in a 119.5, 308, and 210% enhancement of thermal conductivity (TC) for water compared with EG, PG, and oil, respectively. In addition, the nanofluid-cooled PVT system had an electrical efficiency that was 88.85% higher than standalone PV technology and 44% higher than water-cooled PVT systems. Moreover, the thermal efficiency of the nanofluid-cooled PVT system was 20% higher than the water-cooled PVT system. Finally, the nanofluid-cooled PVT system displayed the least decrease in electrical efficiency and a greater thermal efficiency even when the PV panel was at its hottest at noon.

Investigation of the Three-Dimensional Hybrid Casson Nanofluid Flow: A Cattaneo-Christov Theory

We consider the Casson hybrid nanofluid (HN) (ZnO + Ag/Casson fluid) that flows steadily along a two-directional stretchable sheet under the influence of an applied changing magnetic flux and is electrically conducting. The basic Casson and Cattaneo-Christov double diffusion (CCDD) formulations are used for the simulation of the problem. This is the first study on the analysis of the Casson hybrid nanofluid by using the CCDD model. The use of these models generalize basic Fick's and Fourier's laws. The current produced due to the magnetic parameter is taken into consideration by using the generalized Oham law. The problem is formulated and then transformed to a coupled set of ordinary differential equations. The simplified set of equations is solved using the homotopy analysis method. The obtained results are presented through tables and graphs for various state variables. A comparative survey in all the graphs is presented for the nanofluid (ZnO/Casson fluid) with the HN (ZnO + Ag/Casson fluid). These graphs depict the effect of various pertinent parameters, like Pr, M, Sc, γ, Nt, m, Nb, δ₁, and δ₂, varying values over the flow. The Hall current parameter m and stretching ratio parameter γ show increasing trends for the velocity gradient, while the magnetic parameter and the flux of mass depict opposite trends for the same profile. The increasing values of the relaxation coefficients show an opposite trend. Furthermore, the ZnO + Ag/Casson fluid shows a good performance in the transfer of heat and thus can be used for cooling purposes to increase the efficiency of the system.

Understanding the intermediates and carbon dioxide adsorption of potassium chloride-incorporated graphitic carbon nitride with tailoring melamine and urea as precursors

In this study, we demonstrated the synthesis of potassium chloride (KCl)-incorporated graphitic carbon nitride, (g-C₃N₄, CN) with varying amounts of N-vacancies and pyridinic-N as well as enhanced Lewis basicity, via a single-step thermal polymerization by tailoring the precursors of melamine and urea for carbon oxide (CO₂) capture. Melamine, as a precursor, undergoes a phase transformation into melam and triazine-rich g-C₃N₄, whereas the addition of urea polymerizes the mixture to form melem and heptazine-rich g-C₃N₄ (CN11). Owing to the abundance of pyridinic-N and the high surface area, CN11 adsorbed higher amounts of CO₂ (44.52 μmol m^-2 at 25 °C and 1 bar of CO₂) than those reported for other template-free carbon materials. Spectroscopic analysis revealed that the enhanced CO₂ adsorption is due to the presence of pyridinic-N and Lewis basic sites on the surface. The intermediates of CO₂adsorption, including carbonate and bicarbonate species, attached to the CN samples were identified using in-situ Fourier-transform infrared spectroscopy (FTIR). This work provides insights into the mechanism of CO₂ adsorption by comparing the structural features of the synthesized KCl-incorporated g-C₃N₄ samples_. CN11, with an excellent CO₂ uptake capacity, is viewed as a promising candidate for CO₂ capture and storage.

Numerical Analysis of Heat Transfer Enhancement Due to Nanoparticles under the Magnetic Field in a Solar-Driven Hydrothermal Pretreatment System

Solar-driven hydrothermal pretreatment is an efficient approach for the pretreatment of microalgae biomass for biofuel production. In order to enhance the heat transfer, the magnetic fields effects on flow and heat transfer of nanofluids were investigated in a three-dimensional circular pipe. The magnetic fields were applied in different directions and magnetic field intensities to the flow. In this paper, Finite Volume Method was used to simulate flow and heat transfer of nanofluids under a magnetic field, and the Discrete Phase Model was selected to calculate two-phase flow, which was water mixed with metal nanoparticles. The research was also carried out with the various physical properties of nanoparticles, including the volume share of nanoparticles, particle diameter, and particle types. When the magnetic fields were applied along the X, Y, and Z directions and the intensity of magnetic fields was 0.5 T, the heat transfer coefficients of Cu-H2O nanofluids flow were increased evenly by 9.17%, 10.28%, and 10.32%, respectively. When the magnetic field was applied, the heat transfer coefficients and the Nusselt numbers were both increased with the increment of intensities of the magnetic field.

Surface-Oxidised Carbon Nanofibre-Based Nanofluids: Structural, Morphological, Stability and Thermal Properties

The reputation of nanofluids as a convenient heat transfer media has grown in recent years. The synthesis of nanofluids is often challenging, particularly carbon-based nanofluids, due to the rapid agglomeration of the nanoparticles and the instability of the nanofluids. In this regard, surface modification and surfactant addition are potential approaches to improve the physical and thermal properties of carbon-based nanofluids that have been studied and the structural, morphological, and thermal characteristics of surface-oxidised carbon nanofibre (CNF)-based nanofluids has been characterised. Commercial CNF was first subjected to three different acid treatments to introduce surface oxygen functional groups on the CNF surface. Following the physical and thermal characterisation of the three surface-oxidised CNFs (CNF-MA, CNF-MB, and CNF-MC), including Raman spectroscopy, Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FESEM), the CNF-MB was selected as the best method to synthesise the surface-oxidised CNF-based nanofluid. A total of 40 mL of ultrapure water was used as a pure base fluid and mixed with the surface-oxidised CNF at a concentration range of 0.1-1.0 wt.%, with a fixed of 10 wt.% amount of polyvinylpyrrolidone (PVP). The thermal conductivity of CNF-based nanofluid was then characterised at different temperatures (6, 25, and 40 °C). Based on the results, surface oxidation via Method B significantly affected the extent of surface defects and effectively enhanced the group functionality on the CNF surface. Aside from the partially defective and rough surface of CNF-MB surfaces from the FESEM analysis, the presence of surface oxygen functional groups on the CNF wall was confirmed via the Raman analysis, TGA curve, and FTIR analysis. The visual sedimentation observation also showed that the surface-oxidised CNF particles remained dispersed in the nanofluid due to the weakened van der Waals interaction. The dispersion of CNF particles was improved by the presence of PVP, which further stabilised the CNF-based nanofluids. Ultimately, the thermal conductivity of the surface-oxidised CNF-based nanofluid with PVP was significantly improved with the highest enhancement percentage of 18.50, 16.84, and 19.83% at 6, 25, and 40 °C, respectively, at an optimum CNF concentration of 0.7 wt.%.

Interfacial interaction-driven rheological properties of quartz nanofluids from molecular dynamics simulations and density functional theory calculations

Correlations of the shear viscosity of quartz nanofluids with particle concentration, particle size, and temperature were investigated with molecular dynamics simulations and density functional theory (DFT) calculations. A new understanding to the experimentally concluded correlations was addressed in terms of microscopic particle-water interfacial interaction in three aspects. First, the viscosity of quartz nanofluids at different particle concentrations, particle sizes, and temperatures were simulated using the equilibrium molecular dynamics simulations method to reproduce the experimental observations. At the same particle size, the nanofluid viscosity decreases significantly with temperature and increases with nanoparticle volume concentration, and at the same volume concentration, the nanofluid viscosity increases with the decrease of particle size. Second, DFT calculations confirm a stronger particle-water interaction than that among water molecules. The important role of particle-water interaction in the viscosity determination of nanofluids was revealed. Finally, a correlation was proposed to fit the simulated results and compared with earlier two-parameter correlations. One parameter in the correlation is indeed a constant, while the other is a function of SiO₂-water interaction energy. Our study proposes a physical basis for the experimentally concluded correlations on the viscosity of nanofluids.

The Effect of Ag-Decoration on rGO/Water Nanofluid Thermal Conductivity and Viscosity

Carbon-based nanomaterials have a high thermal conductivity, which can be exploited to prepare nanofluids. Graphene is a hydrophobic substance, and consequently, graphene-based nanofluid stability is improved by adding surfactants. An attractive alternative is the decoration of reduced graphene oxide (rGO) with metallic materials to improve the thermal conductivity without affecting the stability of nanofluids. This study focuses on the synthesis and characterization of rGO/Ag (0.1 wt.%) aqueous nanofluids. Moreover, the effects of the Ag concentration (0.01−1 M) on the thermal conductivity and viscosity during the synthesis of rGO/Ag composite are analyzed. The nanofluid thermal conductivity showed increases in relation to the base fluid, the most promising being 28.43 and 26.25% for 0.1 and 1 M of Ag, respectively. Furthermore, the nanofluids were Newtonian in the analyzed range of shear rates and presented a moderate increase (<11%) in viscosity. Aqueous nanofluids based on rGO/Ag nanocomposites are a potential alternative for applications as heat transfer fluids.

Graphene Oxide and Nitrogen-Doped Graphene Coated Copper Nanoparticles in Water-Based Nanofluids for Thermal Management in Electronics

Graphene oxide (GO) and nitrogen-doped graphene (NG) coated copper nanoparticles (NPs) have been developed in this work and investigated as nanofiller for water as Heat Transfer Fluids (HTFs). The morphology and composition of the coating were characterized to confirm the presence of functional groups and the nitrogendoping of the graphene coating. Different fractions of the two types of coated nanoparticles NPs between 0.1 and 10 wt.% were dispersed in water. The thermal conductivity of the dispersions was evaluated at temperatures between 20 and 50 °C. A positive correlation between the thermal conductivity of the HTFs and the fraction and temperature are observed as a result of the increase of the solid phase contribution into the heat transfer. At a concentration of 0.5 wt.%, the thermal conductivity of the NG-CuNPs nanofluid reached its maximum increase of 78%, compared to a 13% increase in the case of GO-CuNPs. However, due to the poor stability of the NG-CuNPs, further increase of the solid phase did not result in any additional improvement. In contrast, the thermal conductivity of the GO-based dispersion resulted in a 103% enhancement at 10 wt.% at a temperature of 50 °C.

Stable Near-Infrared Photoluminescence of Single-Walled Carbon Nanotubes Dispersed Using a Coconut-Based Natural Detergent

We prepared single-walled carbon nanotube (SWNT) suspensions in phosphate buffer solutions containing 1% of a coconut-based natural detergent (COCO) or 1% of sodium dodecyl sulfate (SDS). The suspensions exhibited strong photoluminescence (PL) in the near-infrared region, suggesting that the SWNTs, such as those with (9, 4) and (7, 6) chiralities, were monodispersed. Upon diluting the suspensions with a detergent-free phosphate buffer solution, the PL intensity of the SDS-containing SWNT suspension was significantly lower than that of the COCO-containing SWNT suspension. The COCO-containing SWNT suspension was more stable than the SDS-containing SWNT suspension. The SWNT concentration of the suspensions prepared via bath-type sonication was lower than that of the suspensions prepared via probe-type sonication. However, near-infrared (NIR) PL intensity of the SWNT suspensions prepared via bath-type sonication was much higher than that of the SWNT suspensions prepared via probe-type sonication regardless of the detergent. This suggested that the fraction of monodispersed SWNTs of the suspensions prepared via bath-type sonication was larger than that of the suspensions prepared via probe-type sonication, although the SWNT concentration was low. Our results indicated that COCO favored the fabrication of SWNT suspensions with stable and strong NIR PL, which are useful for various biological applications.