1.7 Preparation methods for nano fluids

1.7.1 Two-Step Method

Two-step method is the most widely used method for preparing nano fluids. Nano particles, nano fibers, nano tubes, or other nano materials used in this method are first produced as dry powders by chemical or physical methods. 

Then, the nano sized powder will be dispersed into a fluid in the second processing step with the help of intensive magnetic force agitation, ultrasonic agitation, high-shear mixing, homogenizing, and ball milling. Two-step method is the most economic method to produce nano fluids in large scale, because nano powder synthesis techniques have already been scaled up to industrial production levels. 

Due to the high surface area and surface activity, nano particles have the tendency to aggregate. The important technique to enhance the stability of nano particles in fluids is the use of surfactants. However, the functionality of the surfactants under high temperature is also a big concern, especially for high-temperature applications.

Due to the difficulty in preparing stable nano fluids by two-step method, several advanced techniques are developed to produce nano fluids, including one-step method. In the following part, we will introduce one-step method in detail.

1.7.2  One-Step Method

To reduce the agglomeration of nano particles, Eastmanetal. developed a one-step physical vapour condensation method to prepare Cu/ethylene glycol nano fluids . The one-step process consists of simultaneously making and dispersing the particles in the fluid. In this method, the processes of drying, storage, transportation, and dispersion of nano particles are avoided, so the agglomeration of nano particles is minimized, and the stability of fluids is increased . 

The one-step processes can prepare uniformly dispersed nano particles, and the particles can be stably suspended in the base fluid. The vacuum-SANSS (submerged arc nano particle synthesis system) is another efficient method to prepare nano fluids using different dielectric liquids. The different morphologies are mainly influenced and determined by various thermal conductivity properties of the dielectric liquids. 

The nano particles prepared exhibit needle-like, polygonal, square, and circular morphological shapes. The method avoids the undesired particle aggregation fairly well.

One-step physical method cannot synthesize nano fluids in large scale, and the cost is also high, so the one-step chemical method is developing rapidly. Zhuetal. presented a novel one-step chemical method for preparing copper nano fluids by reducing C uSO4⋅ 5H2O with NaH2PO2⋅ H2O in ethylene glycol under microwave irradiation. 

Well-dispersed and stably suspended copper nano fluids were obtained. Mineral oil-based nano fluids containing silver nano particles with a narrow-size distribution were also prepared by this method. The particles could be stabilized by K orating, which coordinated to the silver particle surfaces via two oxygen atoms forming a dense layer around the particles. The silver nano particle suspensions were stable for about 1 month. Stable ethanol-based nano fluids containing silver nano particles could be prepared by microwave-assisted one-step method .

 In the method, polyvinyl pyrrolidone (PVP) was employed as the stabilizer of colloidal silver and reducing agent for silver in solution. The cationic surfactant octadecylamine (ODA) is also an efficient phase-transfer agent to synthesize silver colloids . The phase transfer of the silver nano particles arises due to coupling of the silver nano particles with the ODA molecules present in organic phase via either coordination bond formation or weak covalent interaction. Phase transfer method has been developed for preparing homogeneous and stable graphene oxide colloids.

However, there are some disadvantages for one-step method. The most important one is that the residual reactants are left in the nano fluids due to incomplete reaction or stabilization. It is difficult to elucidate the nano particle effect without eliminating this impurity effect.

by adjusting synthesis parameters such as temperature, acidity, ultrasonic and microwave irradiation, types and concentrations of reactants and additives, and the order in which the additives are added to the solution.

1.7.4 The stability of nano fluid

The agglomeration of nano particles results in not only the settlement and clogging of micro channels but also the decreasing of thermal conductivity of nano fluids. 

So, the investigation on stability is also a key issue that influences the properties of nano fluids for application, and it is necessary to study and analyze influencing factors to the dispersion stability of nano fluids. This section will contain (a) the stability evaluation methods for nano fluids, (b) the ways to enhance the stability of nano fluids, and (c) the stability mechanisms of nano fluids.

1.7.5 The Stability Evaluation Methods for Nano fluids

 Sedimentation and Centrifugation Methods

Many methods have been developed to evaluate the stability of nano fluids. The simplest method is sedimentation method. The sediment weight or the sediment volume of nano particles in a nano fluid under an external force field is an indication of the stability of the characterized nano fluid. 

The variation of concentration or particle size of supernatant particle with sediment time can be obtained by special apparatus. The nano fluids are considered to be stable when the concentration or particle size of supernatant particles keeps constant. Sedimentation photograph of nano fluids in test tubes taken by a camera is also a usual method for observing the stability of nano fluids . 

Zhuetal. used a sedimentation balance method to measure the stability of the graphite suspension. The tray of sedimentation balance immerged in the fresh graphite suspension. The weight of sediment nano particles during a certain period was measured. The suspension fraction of graphite nano particles at a certain time could be calculated. For the sedimentation method, long period for observation is the defect. Therefore, centrifugation method is developed to evaluate the stability of nano fluids. Singhetal. 

applied the centrifugation method to observe the stability of silver nano fluids prepared by the microwave synthesis in ethanol by reduction of Ag NO₃ with PVP as stabilizing agent. It has been found that the obtained nano fluids are stable for more than 1 month in the stationary state and more than 10 h under centrifugation at 3,000 rpm without sedimentation. Excellent stability of the obtained nano fluid is due to the protective role of PVP, as it retards the growth and agglomeration of nano particles by steric effect. Li prepared the aqueous  polyaniline  colloids and used the centrifugation method to evaluate the stability of the colloids . Electrostatic repulsive forces between nano fibers enabled the long-term stability of the colloids.