The Peculiarities Of Copper Nanofluid

Introduction

Colloids are heterogenous biphasic system in which the colloidal particles are dispersed or suspended over another substance. The colloidal particles have the size ranging from 1 to 1000 nano meters, these colloidal particles are larger than solution but not large enough to settle out. Colloidal systems can be classified into 3 types namely lyophilic colloids, lyophobic colloids and association colloids.1 Lyophilic colloids are highly solvated and charge. They are known as hydrophilic colloids when water acts as their dispersing medium. Association colloids are molecules that have both the hydrophilic and hydrophobic region. As for lyophobic colloids, thy have no affinity towards the dispersing phase and are not solvated. They are termed as hydrophobic colloids if the water is the dispersing medium.2

A hydrophobic colloid can be defined as a colloid system that is composed of inorganic particles as their dispersed phase. Some examples of the inorganic particles dispersed in water are gold, silver, sulphate and copper.1 As they are not solvated, they do not interact with water contribute to the instability of hydrophobic colloid. There are 3 types of instability in hydrophobic colloid, namely flocculation, creaming and coalescence. Flocculation refers to the condition where the colloids comes out from the dispersing medium in the form of flakes meanwhile creaming is the migration of one of the substances to the top or bottom whereas coalescence is the bumping of small colloid droplet, forming a larger droplets.3 These instabilities have always be a problems in the application of hydrophobic colloid.

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The instabilities of hydrophobic colloid can be improved by adding emulsifiers or surfactant. The emulsifiers or surfactants are the agent used to stabilized the colloid. Common surfactants seen are detergents that can stabilize the interface between oil and water droplets in the suspension.3 Besides the addition of emulsifiers, the adjustment of other parameters such as pH or concentrations may also help in the stabilization of hydrophobic colloid.

Case

Copper nanofluid is one of the examples of hydrophobic colloid. It is a nanoscale colloidal suspension containing copper nanoparticles as their dispersed phase. Copper nanofluid has a wide range of application especially in the development of heat transfer fluid for cooling system of machines and instruments due to its thermal conductivity. Copper nanofluids containing metallic nanoparticles are believed to have better thermal conductivity than pure fluids that do not contain suspended solid particles. 4

The preparation of copper nanofluids is a step-by-step method involving the drying, storage and transportation of nanoparticles. After the nanoparticles are dried as powder form, it will then be dispersed in a fluid in the second processing step along with agitation to produce the nanofluids. This method is widely used as it is the most economic method for the production of large scale nanofluids. However, agglomeration may take place during the preparation which may result in quick settlement and clogging of the microchannels. The occurrence of agglomeration is due to the high surface area and surface activity of the nanoparticles. 5The agglomeration has not only result in settlement and clogging of microchannel but has also decreases the thermal conductivity of nanofluids. The fast settling of nanofluid is undesirable in the practical use of nanofluid 6 and thus, it is important and essential to solve this problem by improving the stability of copper nanofluids.

The use of surfactants is said to be an important technique in enhancing the stability of nanoparticles for the preparation of copper nanofluids. However, concerns arise regarding the functionality of surfactants under high temperature as surfactants may produce foams when heating and may contaminate the heat transfer media. 5The highly reactive copper nanoparticles that undergo oxidation rapidly also contributed to the instability and problem of copper nanofluid.6 Hence, different approaches have to be conducted to improve the stability of copper nanofluid as the stability of copper nanofluids is very essential as a heat transfer medium in the industrial application.

Approaches

There are various approaches and technique that can be used to improve the stability of copper nanofluid by synthesizing nanofluids in single steps, avoiding the agglomerations in the previous step-by-step synthesis. Single step method can avoid agglomeration as the preparation of nanoparticles and nanofluids are combined together, avoiding the drying, storage and transportation and dispersion process of copper nanoparticles. Therefore, single step method is believed to be able to prepare uniformly dispersed nanoparticles that can stably suspended in the base fluid.5

One of the novel methods to formulate stable, non-agglomerated copper nanofluids is by using copper sulphate pentahydrate as the source for copper nanoparticles that is reduced by sodium hypophosphite in the ethylene glycol by the means of conventional heating. In this approach, sodium hypophosphite is used as a reducing agent whereas the ethylene glycol acts as base power fluid. In this single step method, high yield of stable copper nanofluids can be obtained in a comparatively shorter time period. The stability of copper nanofluid is enhanced by the preparation of small size nanoparticles and the addition of surfactants.4

One of the essential approaches to avoid agglomeration is by minimizing the size of the nanoparticles as smaller nanoparticles have better dispersion. In the method mentioned above, the concentration of copper sulphate and dilution are effective in reducing the size of the copper nanoparticles. Higher concentration of copper sulphate allows the nucleation and quantum growth of copper nanoparticles occurring at the same time, result in a wide distribution range of copper nanoparticles within the nanofluid in which uniform and desirable size of nanoparticles may not be obtained. Therefore, 0.1M of copper sulphate should be used in the synthesis of copper nanofluid, to avoid the growth and nucleation taking place simultaneously, resulting in getting desired size of nanoparticles. This is because the nucleation and growth stages are separated when the concentration is lower, giving rise to a narrow distribution range of copper nanoparticles7 thus giving a more uniform and smaller nanoparticles size, improving the stability of the copper nanofluids. The particle size in the copper nanofluids can be further minimized if dilution is carried out. The dilution of 0.1M copper nanofluid by 100 ml of water is consider to be able to reducing the size of particle effectively and thus and prevent the happening of agglomeration.4

Reducing agent is needed in the preparation of copper nanofluid by reducing copper ions into copper. The time taken to prepare copper nanoparticles can be shorten when the sodium hypophosphite is used in an acidic condition. This is because an acidic condition with a low pH can accelerate the reduction of copper ions as the action of sodium hypophosphite is favouring acidic condition. 4 Although ethylene glycol can also be used as a reducing agent, sodium hypophosphite is introduced into the synthesis as it is a strong reducing agent which can accelerate the reaction. However, the agglomeration of nanoparticles is often serious with the use of sodium hypophosphite. Therefore, here comes the advantage of polyol. Ethylene glycol as a polyol also acts as a surface protective agent in preventing agglomeration and the abnormal growth of nanoparticles beside being as a solvent in the synthesis of copper nanofluids. In this way, both the advantages of the polyol process and the aqueous chemical reduction method are secured in which the reaction is accelerated and the agglomeration is avoided.7

In this novel method to formulate stable copper nanofluid, the addition of surfactant is a must to improve its stability. Upon the introduction of sodium lauryl sulphate into the preparation of copper nanoparticles, growth and re-agglomeration of copper particles are retarded by keeping the nanoparticles suspended. The cooper nanofluid synthesized by this novel method has being proved to be more stable in which the copper nanofluid obtained is stable for more than 3 weeks in its stationary state whereas its stability can last for more than 8 hours without sedimentation under centrifugation at 4000rpm.

Besides the first method, there are also other single step method than can be used to produce stable copper nanofluid. The second method available is similar to the first method in which all the chemicals used are the same. The copper nanofluids are prepared by reducing copper sulphate by sodium hypophosphite monohydrate in ethylene glycol. However, different from the conventional heating used by the first method, microwave irradiation is used in the second method to produce well-dispersed and stable copper nanofluids.5 The use of microwave irradiation is fast, simple yet energy-saving as compared to the conventional heating methods. The intense friction and collision of molecules created by microwave irradiation can accelerate the nucleation of copper while depressing the straightforward growth of newly born copper, result in a uniform nucleation. Therefore, the use of microwave irradiation result in a higher rate of reaction, smaller particle size of copper nanoparticles and can produce copper nanofluid with better stabilization.

In addition, there is another physical single step method that can be used in synthesizing stable copper nanofluids. By using this method, the copper nanofluid synthesized are not only having weak particle agglomeration but also obtain an enhancement in its thermal conductivity up to 40%. In this method, copper nanofluids are synthesized in an alkaline environment at low temperature with copper tetra-ammonia hydroxide as the sources for copper nanoparticles, hydrazine hydrate as the reducing agent and sodium dodecylbenzenesulfonate (SDBS) as surfactant. This alkaline environment has not only promoted the reaction but also prevent the precipitation of copper hydroxide before the reduction of copper ions. 8This method seems to share some similarities as the method used above but it can produce copper nanofluids that are clean in its composition as the final products produced in this chemical reaction are nitrogen gas, excessive ammonia and water besides the copper nanoparticles and SDBS. The use of copper tetra-ammonia hydroxide in this method can prevent the prevent the precipitation of copper hydroxide and copper oxide that may affect the purity and growth of copper particles. The nitrogen gas produced as one of the end products can also help to prevent the oxidation of particles during incubation period. The introduction of hydrazine in the synthesis can terminate the growth of copper nanoparticles and accelerate the nucleation of copper particles and thus smaller size nanoparticles can be obtained. The use of SDBS as surfactant can narrow the size distribution of nanoparticles in which aggregation may occur due to the absence of SDBS.

Furthermore, a continuous-flow microfluidic microreactor has been developed to synthesize copper nanofluids with the purpose of synthesizing copper nanofluids continuously. By using this method, the microstructure and properties of copper nanofluids can be controlled with an adjustment in parameters. This is essential as the structure and shape of nanoparticles can influence the nanofluid properties significantly. By this method, the copper hydroxide as the precursor can be transformed completely into copper oxide nanoparticle in water with the help of microwave irradiation result in the synthesis of copper oxide nanofluids with high solid volume. The growth and aggregation of nanoparticles is prevented with the use of ammonium citrate as its surfactant. By using this synthesis method, the copper nanofluid obtained is having higher thermal conductivity than nanofluids prepared by other methods.

Conclusion

As mentioned earlier, the stabilization of copper nanofluid is very important for its industrial application as heat transfer medium used in machines and instruments. The copper nanofluid obtained from the previous conventional step-by-step method is not favourable as the copper nanofluid could only lasted for one week in the stationary state at room temperature without any accelerated stress. Therefore, many approaches and techniques are introduced to improve the stability of copper nanofluid by avoiding agglomeration of the copper nanoparticles. To sum up, the size of the nanoparticles should be minimized as smaller size particles are having better dispersion. The size of the nanoparticles can be minimized with the help of adjusting the concentration of copper source, diluting the copper nanofluid with water and by using microwave irradiation. Besides minimizing the size of the nanoparticles, the addition of surfactants are also important to improve the stability of copper nanofluid. The surfactants that can be added include sodium lauryl sulphate (SLS) and sodium dodecylbenzenesulfonate (SDBS). The surfactants added enhance the stability of copper nanofluids by retarding the growth of nanoparticles and accelerate the nucleation which eventually narrow down the size distribution of the nanoparticles.

Reference

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