This study investigates the shelving stability of dispersed aluminium nanoparticles in water mixtures fabricated by the conventional and the controlled bath temperature two-step methods. The nanofluids were prepared with water of pH 9 and nanoparticles of 0.1–1.0 vol.%. A bath type ultrasonicator was employed for dispersing the nanoparticles into the base fluid. The sonication process, for all as-prepared samples, lasted for 4 hours and was either device bath temperature uncontrolled or controlled in the range of 10–60°C. Furthermore, the stability of the as-produced nanosuspensions was evaluated using the sedimentation photograph capturing method by capturing images at equal intervals of time for 12 hours then analysing the data based on the sample sedimentation height ratios. It was found that the sedimentation behaviour of the nanofluids fabricated via the controlled temperatures of less than 30°C was of dispersed sedimentation type, while those produced by the conventional method and the fixed temperatures of 30°C and higher were of flocculated sedimentation type. In addition, increasing the controlled sonication temperature has shown to increase the settling process of the sediments. Moreover, the rise in nanoparticle concentration was seen to reduce the variation in sedimentation height ratio between the fixed temperature samples. A comparison between the two fabrication methods has shown that the 30°C nanofluids had better short- and long-term stability than the conventionally produced suspensions.
Aluminium (Al) is one of the most abundant crystal metals found on earth, which due to its capability of being fully recyclable, it is considered as a very sustainable material. The element itself and its alloys possess valuable electrical, mechanical, and thermal properties, which make their usages in various fields, such as construction and building, electrical engineering, and packaging favourable to the industry. Because of its relatively low density of 2700 kg/m3, Al is known to be the lightest among most, if not all, commonly used metals [
Often this is hardly even possible, as one of the main challenges that is associated with nanofluids is their poor stability, whereby the NPs tend to attract each other into forming different sizes of clusters of particles or agglomerations. The reason behind such attraction behaviour was previously found to be due to the imbalance between the electrostatic repulsion force caused by the electrical double layers on the particle large surface area and the strong van der Waals force of attraction among the NPs [
Furthermore, the gravitational force tends to separate the agglomerated particles from the base fluid causing the sediments to settle at the bottom of the hosting fluid, and hence, the kinetic stability of the nanofluid gets negatively impacted. There are three types of sedimentation behaviours that can be observed in any unstable nanofluid [
Forms of sedimentation mechanism in unstable nanofluids, where
Several methods were developed to evaluate the stability of nanofluids, such as (1) zeta potential analysis, (2) centrifugation method, (3) spectral analysis approach, (4) 3
Our review of the available literature [
Therefore, in this study, an evaluation of the stability of dispersed Al NPs in water fabricated via the conventional two-step method and the controlled sonication bath temperature approaches was performed. The sedimentation photograph capturing method was employed to determine the nanofluid stability variation with time. The examined nanofluids were prepared at equal sonication time using different concentrations of NPs, in the range of 0.1–1.0 vol.%. Furthermore, for the controlled temperature method, the ultrasonicator bath temperature was fixed at a set of temperatures of 10°C to 60°C, while the conventional fabrication route was initiated at room temperature conditions. Stability monitoring for all as-prepared samples lasted for the same duration of time. The outcome of this research is expected to widen the understanding of both researchers and manufactures of the significant and importance role of the production process on the stability of nanofluids.
A purity of 99.9% Al nanoparticles, of spherical particle shape and size between 40 and 60 nm, were purchased from SkySpring Nanomaterials Incorporated. A set of 60 mL clear glass vials, of 27.5 mm outer diameter and 140 mm height, with screwed top were provided by Sigma-Aldrich. Polypropylene holed caps, which include a polytetrafluoroethylene (PTFE)/silicone septa for each, were obtained from Sigma-Aldrich to seal the aforementioned vials. Deionised water, produced by an Elga PR030BPM1-US Purelab Prima 30 water purification system, was used as the base fluid for the nanofluid preparation after adjusting its pH value to 9, at an in-lab temperature of 25°C. The reason behind selecting the liquid pH value to be 9 is because other authors have reported high alumina nanofluid stability when using water of
Elemental test was performed for the Al NPs through a 9 kW Rigaku SmartLab, Japan, and X-ray diffraction (XRD) analyser and its software, SmartLab Guidance, using a CuK
Each nanofluid sample was prepared by placing the NPs first inside the vial then injecting 20 mL of as-prepared water, using a disposable syringe, on top of the nanopowder after which the vial was tightly sealed using the provided caps. The concentrations of NPs used were 0.1, 0.5, and 1.0 vol.%, for each experimental setup, which was calculated by using the mixing theory (Equations (
Ultrasonicator bath temperature changes with operation time.
Schematic procedure for the two-step nanofluid preparation.
To determine the natural settling behaviour of the nanosuspensions, the as-sonicated nanofluids were placed individually on a measuring stand to allow the separation mechanism to take place under gravitational force after which their sedimentation heights were measured with respect to time by capturing their photographical images, using a Canon EOS 700D professional camera that is equipped with a Sigma 105 mm F2.8 EX DG micro lens and a Phottix Company TR-90 remote switch with digital timer, at the start then for every 30 seconds for a total duration of 12 hours. The configuration used for the stability measurements is shown in Figure
Setup for nanofluid stability measurement.
The accuracy of the two previously mentioned heights (i.e.,
The diffraction pattern of the as-received Al NPs is shown in Figure
X-ray diffraction patterns of as-received aluminium nanoparticles.
The SEM analysis of the as-received nanopowder has shown that the morphology of the examined NPs is of spherical shape and that some agglomerations between the particles do exist, as illustrated by the SEM patterns (Figures
SEM and EDS analysis of the as-received Al nanopowder, where (a, b) are the SEM images of the sample at low and high magnifications, respectively, and (c) is the EDS X-ray spectrum of the elements within the characterised specimen.
EDS elemental percentage of the as-received Al nanopowder.
Element | Mass (%) | Atom (%) | Sigma | Net | |
---|---|---|---|---|---|
Aluminium | 62.35 | 49.55 | 0.09 | 1582595 | 0.3617546 |
Oxygen | 37.65 | 50.45 | 0.07 | 210593 | 0.1499887 |
Total | 100 | 100 | — | — | — |
Settling characterisation of the as-fabricated nanofluids has shown two types of sedimentation behaviours, which are the dispersed sedimentation and the flocculated sedimentation. The dispersed sedimentation was observed by the nanofluids prepared with 0.1 and 1.0 vol.% at controlled temperatures of 10°C and 20°C, while the other samples have illustrated a flocculated sedimentation settling mechanism. Such variation in settling behaviour is believed to be caused by the timing of NPs oxidation, where the reaction rate of the particles starts to prominently increase, within the aforementioned samples of dispersed sedimentation, after the sonication phase, in contrast to the nanofluids of flocculated sedimentation behaviour which most of its particles oxidise during the preparation stage. This is clearly seen by the notable hydrogen generation in the nanofluids that had experienced a dispersed sedimentation mechanism in comparison to the other as-fabricated suspensions. The hydrogen production is due to the following two possible reactions between the Al NPs and the hosting solution.
Equations (
An example of the two previous settling behaviours is demonstrated in Figure
Settling behaviour of the 0.5 vol.% nanofluids fabricated by a controlled ultrasonicator bath temperature of 20°C (top) and 40°C (bottom).
Furthermore, the changes in the
Sediment height ratio variation with settling time for the nanofluids fabricated with (a) 0.1 vol.%, (b) 0.5 vol.%, and (c) 1.0 vol.%.
In general, the nanofluids that were fabricated at 30°C have demonstrated better short- and long-term stability than the ones produced by the conventional two-step approach, as illustrated by the data in Figure
Photographical images of the nanofluid settling behaviour with time using the uncontrolled and 30°C controlled sonication temperature approaches, where the NP concentrations used were (a) 0.1 vol.%, (b) 0.5 vol.%, and (c) 1.0 vol.%.
Water-based colloid containing dispersed Al nanoparticles has been characterised via the sedimentation photograph capturing method to emphasize the role of the fabrication approach on the stability of the mixture. Two procedures were undertaken for the production of the as-prepared nanofluids, which are the conventional two-step approach and the two-step controlled sonication bath temperature method. The parameters studied include the nanoparticle concentration, nanofluid fabrication temperature, and sediment height ratio in the fluid.
Sodium hydroxide was used to adjust the pH value of the base fluid to 9, as lower pH values were reported in the literature to highly stabilise similar types of nanofluids. Mixing of the colloid was performed using an ultrasonic bath type device to induce the dispersion of the particles. It was found that the conventional two-step approach caused the bath temperature to increase with time, thus confirming other researchers’ findings. Moreover, the experiments have revealed that nanofluids produced at controlled temperatures lower than 30°C follow a dispersed sedimentation behaviour, whereas those fabricated at 30°C and above obeyed a flocculated sedimentation settling mechanism.
Evaluation of the nanofluids prepared by the controlled temperature method has generally shown a decrease in their stability with the increase in fabrication temperature. In addition, the increase in nanoparticle concentration has shown to reduce the variation in sedimentation height ratio between the samples that were produced at different fixed temperatures. Furthermore, when comparing the nanofluids fabricated by the two aforementioned preparation methods, it was seen that the stability of the 30°C colloid has exceeded all other controlled temperature samples, which obeyed the same sedimentation mechanism, beyond the rapid settling region. The 30°C nanosuspensions have also demonstrated better short- and long-term stability behaviour than the conventionally fabricated nanofluids. Thus, confirming that the controlled temperature two-step nanofluid fabrication approach is much promising in terms of the colloidal shelving stability than the conventional method.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflict of interest.
We acknowledge the help provided by Prof. M. Sherif El-Eskandarany, the program manager of Nanotechnology and Advanced Materials Program at KISR, for his help and support throughout the conducted work. This work was financially supported by the Kuwait Institute for Scientific Research (KISR) and Cranfield University.