(90d) Nanofluid Generated Via a Multi-Pulse Laser Ablation Technique

The application of microreactor technology for fuel processing gains a lot of attentions recently due to their size and portability as well as microchemical systems. However, major challenges and key issues still exist. One of the critical issues is the heat management within the microreactor [Shah et al., 2005]. Nanofluids are stable colloidal suspensions of nanoparticles in common base fluids such as water, oil, or polymer solutions. It has been reported that the presence of nanoparticles could greatly enhance the thermal conductivity of the base fluids.

There are two techniques that have been commonly used to prepare nanofluids: the single-step technique and the two-step method. In most studies, sample sizes generated by these techniques have been limited to less than a few hundred milliliters of nanofluids. The single-step simultaneously makes and disperses the nanoparticles directly into the base fluids. The two-step technique makes nanoparticles and then disperses them into the base fluids. Nanoparticles can be produced from several processes such as gas condensation, mechanical attrition or chemical precipitation. Chemical techniques always involve with reduction reactions or ion exchange, the base fluids always contain other ions and reaction products, which are difficult to separate from the fluids. In addition, nanoparticles produced by chemical processes have a tendency to agglomerate which limits the potential of the high surface areas of the nanoparticles. Thus, particle dispersion must be carried out. Several approaches have been used to disperse the particles [Xuan and Li, 2000] : (1) changing the pH value of suspension; (2) using surface activator and dispersants; and (3) using ultrasonic vibration. These dispersion techniques can also alter the surface properties of the particles. The presence of dispersants and chemical reagents could influence the physical, thermal, and chemical properties of the nanofluids. This may be the critical issue that results in large inconsistencies among the existing experimental data on thermal conductivity, and on convective and boiling heat transfer in nanofluids.

Laser ablation in liquid medium occurs when a high-power laser is focused at the submerged surface for an appropriate time. The laser ablation in liquid is simple and, if no additives are involved, the generated nanofluids are basically free of extraneous ions or other chemicals.

In this study, multi-beam laser ablation in liquid was used to prepare nanofluids of silver, aluminum, magnesium, calcite and dolomite in deionized water. In our studies, two single-mode, Q-switched Nd-Yag lasers operating at 1064 nm, with various pulse duration from 5.5 ns to 10 Hz rep rate were used. The results have demonstrated that both laser intensities and multi-beam ablation can increase the ablation rate and promote reduction of the particle sizes and particle size distribution. For the case of silver, various samples of nanofluids with silver particles of sizes mainly in the range of 20 to 30 nm were generated. These samples were stable for several months without the need of using dispersants or surfactants. However, for the case of aluminum, the generated fluids were not stable and significant amount of solid precipitates were observed in the stock solutions. The size distribution was from 5 nm to 400 nm indicating that the particle agglomerated and large distribution in sizes. TEM analysis indicated that, for silver-water nanofluids, the fluids contain only Ag nanoparticles are fcc single crystals. For the case of aluminum, the fluid contents different shape and size particles. These species are triangular (bayerite), rectangular (gibbsite) and fibrous (boehmite). For the case of dolomite and calcite samples in water, a significant increase in concentration of Ca and Mg in solution with calcite and dolomite was observed, respectively.