# Nanofluids for the Next Generation Thermal Management of Electronics: A Review

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Preparation Methods

## 3. Parameters That Affect Thermal Conductivity

_{2}O

_{3}nanoparticles and a reduction with increasing sizes of the nanoparticles was observed. Spherical, cylindrical, rod, rectangular, platelet, and blade shapes of nanoparticles were studied with nanofluids. With instrumental analysis techniques like the Rayleigh scattering diagrams, it can be stated that characteristics such as the spherical/cylindrical shapes of nanoparticles, appear to be clearly symmetric/asymmetric [26]. The results indicated that the particles with larger aspect ratios contribute more to the enhancement of the thermal conductivity, since thermal penetration increases and the interfacial thermal resistance on heat transfer is reduced. Murshed et al. [27] observed a greater increase in the thermal conductivity values ofwater-based TiO

_{2}nanofluids when using rod-shape nanoparticles than when using spherical ones. In addition, the nanofluids with platelet shape particles were the lower ones among the testing group.

_{2}O

_{3}and CuO, measured the thermal conductivity of nanofluids with concentrations lower than 0.05 vol. % and found that the thermal conductivity ratios increased almost linearly with volume fraction, but with different rates of increase for each group of nanoparticles tested. However, some reports noted a reduction of thermal conductivity for high concentration values. The concentration of the nanoparticles can be correlated with the thermal conductivity of the nanofluids by the Hamilton and Crosser model [32] equation:

_{nf}, K

_{p}and K

_{bf}are the thermal conductivity of the nanofluid, nanoparticles and base fluid, respectively, Φ is the volume fraction of the nanoparticles and n is the shape factor connected to the particles’ sphericity (n = 3/Φ). This model is valid when the thermal conductivity of the particles is at least 100 times higher than the thermal conductivity of the base fluid. Similarly, Xue et al. [33] introduced a model which derives from the Maxwell model and includes the effect of the axial ratio and space distribution of the particles and can be expressed by the following equation:

_{bf}/K

_{p}<< 1 and sphericity of the nanoparticles, can also correlate the thermal conductivity of the nanofluid with the concentration of the nanoparticles:

_{K}is the constant determined by matching the Equation (4) with the experimental results. C

_{K}= 3 (for a dilute system with spherical particles) can be used as a medium theory criterion for a specific nanofluid. If there is an increase in the K

_{nf}by a factor of 3, the heat transfer coefficient of the same working fluid will double. Accordingly, it should be noted that the concentration of the nanoparticles can also be directly correlated with all the viscosity terms present in the system by the following equation under the same assumptions:

_{µ}is equal to 2.5 in the pioneering model of Einstein [35] for rigid spherical particles under low particle volume fractions. However, it is usual to determine c

_{µ}by also matching the Equation (5) with the experimental results. In this sense, a first order estimation of C

_{µ}= 10 can be used.

_{2}nanofluids. The enhancement of the thermal conductivity with an increasing temperature is thought to be caused by the improvement of the Brownian motion and the reduction of the surface energy of the particles. On the contrary, a reduction of the thermal conductivity with increasing temperature was reported for nanofluids with non-spherical particles, which indicated that the aspect ratio of the particle is of influence. Moreover, the pH of the nanofluids affects the aggregation degree of the nanoparticles and, hence, the thermal conductivity. Wang et al. [38] reported an increasing enhancement of the thermal conductivity with increasing pH until the isoelectric point, which is then followed by a decrease. This was attributed to the increase in electrical charge on the surface of the nanoparticles, leading to greater electrostatic repulsion. An experimental work [39] has also shown that the pH influences the zeta potential, particle size distribution, rheology, viscosity andstability, which are factors that affect the thermal conductivity of the nanofluids containing ZrO

_{2}and TiO

_{2}nanoparticles.

_{2}O

_{3}and CuO nanoparticles dispersed in water, ethylene glycol, and water with SDBS.

## 4. Cooling of Electronics

#### 4.1. Challenges and Novel Coolants

^{2}at a temperature gradient of 50 K, it requires a heat transfer coefficient of around 20,000 W/m

^{2}·K, which is not possible through conventional free and forced convections [45]. Thus, there is the pressing need to find cooling fluids with superior heat transfer performance and, consequently, there are fluids like the nanofluids, which can be used as novel advanced coolants. With improved thermal properties, the nanofluids offer benefits in a wide range of applications, including the cooling of electronic components and devices [46]. Despite the progress made on electronic cooling systems, the required high heat flux removal remains very challenging. The existing cooling modes can be classified into the following categories [47]:

- Natural convection;
- Forced convection air cooling;
- Forced convection liquid cooling;
- Liquid evaporation.

**Conventional coolants:**The liquid coolants for electronics should present most of the following [48]:

- Non-flammability and non-toxicity;
- Cost-effectiveness;
- High thermal conductivity, specific heat and heat transfer coefficient;
- High boiling point and low viscosity;

**Dielectric coolants:**

- Aromatic-based liquids: Due to lower cost and better performance, alkylated aromatics such astoluene, benzene, and xylene are the most often used coolants;
- Aliphatic-based liquids: Aliphatic hydrocarbons of paraffinic and iso-paraffinic types (including mineral oils) and aliphatic polyolefins are used in the cooling of electronics;
- Silicone-based liquids: The main advantage of those coolants, commonly known as silicone oils, is their properties such as viscosity and freezing point;
- Fluorocarbon-based fluids: These fluids are inert, stable, non-flammable and non-reactive. The commercially available FC-72 and FC-77 are the most commonly used for electronic cooling.

**Non-Dielectric coolants:**The non-dielectric liquid coolants are often used in cooling kits and systems of electronic equipment due to their optimal thermal properties when compared to the ones of the dielectric coolants. The non-dielectric coolants are usually aqueous solutions with enhanced thermal conductivity and heat capacity and low viscosity. Therefore, water, ethylene glycol, and a mixture of both are widely used as coolants for electronic devices. Other often-used, non-dielectric coolants include propylene glycol, water/methanol, water/ethanol, NaCl solution and potassium formate solution [48].

**Innovative coolants:**The application of nanofluids in conventional and emerging techniques such as microchannels and heat pipes will constitute the next generation cooling systems of electronic components and devices.

#### 4.2. Cooling Methods of Electronics

#### 4.2.1. Thermosyphons

#### 4.2.2. Heat Pipes

_{2}O

_{3}/water nanofluid, the thermal resistance decreased by 22%. This fact stipulates that this nanofluid can perform better when compared to water in cooling a CPU. Figure 4 schematically illustrates the operating principle of a generic heat pipe.

#### 4.2.3. Microchannel-Based Forced Convection

^{2}. After the single-phase convection cooling technique emerged the two-phase (boiling) liquid cooling in devices with microchannels as a cooling technique with great potential for high-heat-generating electronic equipment. The impact of the flow rate of nanofluids with SiO

_{2}, Al

_{2}O

_{3}, and TiO

_{2}nanoparticles suspended in various volume fractions of a mixture of water and ethylene glycol in the cooling of chips was investigated by Rafati et al. [67]. The enhanced cooling of the microchip was observed due to nanofluids. The heat transfer performance of aqueous and EG-based Al

_{2}O

_{3}and carbon nanotubes nanofluids in a CPU cooling system was evaluated by Nazari et al. [68]. The results were compared with the cooling performances of the base fluids and the CPU temperature was found to decrease from about 20% to 22% when using Al

_{2}O

_{3}and carbon nanotubes nanofluids, respectively. Khaleduzzaman et al. [69] investigated the stability impact of alumina water-based nanofluids through a heat sink with minichannels. The authors studied nanofluids with several volume fractions varying from 0.1% to 0.25 vol. % and reported that the volume fraction of 0.1 vol. % showed the best performance for cooling the electronic system by analyzing this nanofluid in terms of microstructure, cluster size, sedimentation, and zeta potential. Mohsen et al. [70] simulated and compared the numerical results with the experimental ones through the usage of copper oxide nanofluids in a heat sink of a CPU cooling kit. The researchers concluded that the thermal resistance of the heat sink decreased approximately 5.4% when compared with water under the same flow rate. Similarly, the conductance value of the heat sink increased by 7.7%. Sarafraz et al. [71] studied the performance of copper oxide and gallium in water in the Intel core i5 4760 processor cooling system. The experiments were carried out for the standby, normal, and overload operating conditions of the processor, and it was observed that the temperature of the CPU was found to be 67 °C, 62 °C, and 53.1 °C for water, copper oxide, and gallium nanofluids. Seyed et al. [72] analyzed the performance of alumina nanofluids in a circular heat sink for chips cooling. The authors observed an increment in the heat transfer coefficient for higher concentrations and a decrease in the thermal resistance of the nanofluids, together with a modest setback coming from the need to increase the pumping power with the incorporation of a higher volume fraction of nanoparticles. Figure 5 presents a horizontal heat sink with and without embedded heat pipes used in CPU cooling. Mehdi et al. [73] performed an experimental work on different models of conventional CPU coolers. The authors used graphene with silver nanoparticles as working fluid and concluded that this hybrid nanofluid exhibited superior thermal performance when compared with water. While considering the geometries, serpentine liquid block offers better cooling at a constant Reynolds number but possesses the disadvantage of high-pumping power requirements. Bin Sun et al. [74] studied the thermal performance of copper and aluminum oxide nanoparticles in water in the liquid-cooled CPU heat radiator.

#### 4.2.4. Thermoelectric Cooling

#### 4.2.5. Free Cooling

#### 4.2.6. Phase Change Materials Based Cooling

#### 4.2.7. Jet Cooling

_{2}O

_{3}based nanofluid using the free jet impingement technique, and reported that the heat transfer coefficient increased, with an increase in the weight fraction of the nanoparticles until 0.0597 wt. %. Thereafter, the heat transfer coefficient decreased to 0.0757 wt. % upon loading more nanoparticles. Naphon and Wongwises [113] experimentally investigated the heat transfer characteristics of the jet impingement of a mini-rectangular fin heat sink for the cooling of a CPU, using a TiO

_{2}nanofluid. The authors made a comparison between the jet nanofluids impingement cooling system, jet liquid cooling system, and the conventional cooling system. Out of these three cooling systems, the Nusselt number for the jet nanofluids impingement cooling was higher than the other cooling techniques, due to the alteration in the fluid transport properties and flow characteristics of the working fluid because of the suspension of the nanoparticles. In contrast, the thermal resistance of the jet nanofluid thermal management technique was lower when compared to the one of other cooling techniques because of the thermal dispersion of jet technique. In addition, the operating temperature of the CPU was 3% as compared with the jet technique without nanofluid impingement cooling. Several investigations revealed that, when the fluid commences to flow outward, the liquid film starts to thicken, producing thin thermal boundary layers derived from jet deceleration and pressure. Consequently, very high heat transfer coefficients are obtained in the stagnation zone, although with a fast decrease on the radial flow region. The non-uniformity of jet cooling has prompted the use of array jets for cooling applications, where the nozzle array arrangement is of vital importance [114].

#### 4.2.8. Absorption Refrigeration Systems

_{2}ammonia-water nanofluid proved to be suitable as working fluid for an AARS.

#### 4.2.9. Spray Cooling

^{2}) and high heat flux (≥115 W/cm

^{2}) a heat transfer augment for the Ag nanofluid occurred with 0.0075% of volume fraction. The results also demonstrated that the heat transfer coefficient increases by adding Ag nanoparticles or multi-walled carbon nanotubes. The critical heat flux was found to be 48.5 W/cm

^{2}and 274.3 W/cm

^{2}for 0.0075% of volume fraction of carbon nanotubes and Ag nanoparticles, respectively. Figure 8 illustrates the heat transfer mechanisms closely related with the spray cooling technique.

## 5. Conclusions

- The thermal conductivity and heat transfer ability of nanofluids is greater than that of the base fluids;
- Long-term stability is mandatory for miniaturized fluidic systems;
- The production costs for systems using nanofluids are still high;
- The nanofluids enhance the critical heat flux, which can be attributed to the enhanced surface wettability, verified in the deposition of the nanoparticles. The findings concerning the influence of the nanofluids on nucleate boiling are, nevertheless, not consistent with each other, which is due to the involvement of a large number of not yet completely understood factors, such as the type of working fluid, surface roughness, heat flux, nanoparticle type, size, and concentration, and preparation procedures, which can strongly affect the thermal properties of the nanofluids;
- Comparing Equations (4) and (5) that correlate the thermal conductivity and viscosity of the nanofluids with the volume fraction of nanoparticles and, particularly, the empirical constants, i.e., C
_{µ}= 10 and C_{K}= 3, it can be stated that the enhancement in viscosity is higher than the increment in thermal conductivity. However, there is still a real benefit derived from the use of nanofluids, given that the addition of surfactants to the system does indeed mitigate the dynamic viscosity effect. Moreover, a thermal conductivity increase can correspond to a double heat transfer coefficient, which in many applications, such as in the cooling of electronic equipment, there is the need for removing high-density heat loads in the shortest time possible, and the heat transfer characteristics are critical to accomplish that purpose. This fact usually comes first in terms of importance despite the setback of an overall greater pumping power in the cooling devices working with fluids with increased viscosity. Furthermore, this work mentioned experimental studies carried out with only slightly extra pumping power of the cooling devices operating with nanofluids; - Despite the progress during the past decades, the electronic industry is still facing technical limitations related to the thermal management of their electronic products. This fact is closely related to the traditional cooling techniques and coolants that are gradually falling short in fulfilling the evermore compelling cooling requirements of the high heat-generating electronic equipment. Consequently, high performance electronic components and devices (chips, CPU, etc.) require novel methods and coolants, having improved heat transfer properties for their better performance and longevity.
- Many research works reported that heat pipes and devices with microchannels filled with forced convection cooling flows are very propitious techniques for the cooling of electronic components and devices. Moreover, findings from the investigations on the application of nanofluids demonstrated that they showed an improved performance when compared to traditional coolants. However, it is still of paramount relevance to use the nanofluids in a wider range of cooling methods and systems in order to address their thermal behavior as coolants of electronic equipment. The nascent cooling approaches with the simultaneous utilization of these innovative coolants can improve their heat dissipation performance and can fulfill the cooling demands of the high heat generating electronic components and devices.

- Long-term stable nanofluids are difficult to prepare because they require continuous maintenance. The influence of the preparation should be further studied, namely the sonication time, volume fraction, and type of the nanofluids, in order to avoid the sedimentation and agglomeration, and achieve optimal performance;
- Although adding dispersants to promote the dispersion of nanoparticles in the base fluid can maintain a long-term improved suspension performance, this addition may cause the decrease in the overall thermal conductivity of nanofluids, due to the lower thermal conductivity associated to the dispersants;
- Lack of multi-scale common protocols for the preparation of the nanofluids. Moreover, a considerable gap still remains between laboratory experimental studies and engineering applications. In this regard, the industrial application of nanofluids will be realized only when their long-term stability is guaranteed;
- Methods to scale-up production for commercialization are still in development. Despite the relative maturity of nanofluid-related studies, issues such as long-term stability, clogging, erosion, corrosion, maintenance and cleaning procedures have remained as obstacles to mass commercialization;
- No comprehensive theory has been introduced to justify the critical heat flux amelioration for a wide spectrum of nanoparticle sizes and chemical compositions. A lack of such a framework has limited the nanofluids commercialization in various fields, including nuclear reactor, fossil fuel boiler, and spray cooling.

- The use of nanofluids having particle nanosheets with a larger surface area that may enable the achievement of the same levels of thermal conductivity with lower volume fractions;
- The use of nanofluids as a response for the growing demand of green manufacturing based on environmentally benevolent processes;
- The development of methods to determine the specific enthalpy of the nanofluids;
- The development of nanofluids with self-assembling nanotubes;
- The determination of theoretical and empirical mathematical expressions to predict the thermal conductivities of hybrid nanoparticles (e.g., metal oxide and carbon);
- The resuming of experimental works conducting an accurate evaluation of the Brownian motion and thermophoresis’ (thermodiffusion or Soret effect) impact on the thermal properties of the nanofluids;
- The determination of the potential adverse impact of the nanoparticles on industrial equipment (it was concluded so far that these ones are harmful to stainless steel but may provoke corrosion in equipment made of copper or aluminum);
- An underway research project concerning the use of nanofluids in the cooling modules of power electronics in electric and hybrid vehicles;
- An underway research project regarding the cooling of a data center with the use of silicon carbide and alumina nanofluids in demonstrator units such as a server blade cabinet.

- The stability of the nanofluids should be improved by optimizing the concentration of the nanoparticles and the base fluid characteristics. The stability of nanofluids should be predicted by experimental studies of the surface tension of the nanofluid overtime, when operating in microfluidic devices;
- The average size of the nanoparticles should become smaller. In this sense, cost-effective techniques of preparation of reduced-dimensioned nanoparticles should be developed. Moreover, the impact of the size effect of the nanoparticles in the base fluid needs further investigation in order to prevent clustering and sedimentation;
- The development of a database of the thermo-physical properties, together with details about nanoparticle shape, size and suspension stability over time, with and without the addition of surfactants and dispersants, and in which database the most prominent nanofluids should be prioritized, is recommended;
- The novel types of nanofluids such as the shell-core, microfluidic, and hybrid are also to be exploited, owing to their phase-change characteristics and optimal thermal properties, which result in improved heat transfer performance, as well as in greater stability;
- Additives for decreasing the nanofluids viscosity while maintaining the same level of thermal conductivity are of paramount importance to achieve a better performance of the nanofluids;
- The systems using nanofluids as working fluids should become more cost-effective, without the need for extra pumping power and expensive maintenance. One path to be followed is the optimization of the design of, for instance, heat sinks, with the adjustment of the microchannels configuration parameters such as the total number of channels and/or the inlet/outlet positioning.

- The development of physical models that can be used to better understand the thermal performance of suspended nanoparticles in a base fluid, especially regarding the thermal management of electronic equipment in the final application;
- The development of boiling heat-transfer experimental works and numerical simulations involving the use of nanofluids as preferential coolants. In this sense, these are necessary research projects concerning the composing materials of biphilic surfaces in pool boiling scenarios, as well as the optimal geometric arrangements, numbers, dimensions, and pitches of the superhydrophobic spots of these kind of surfaces;
- The contribution to the development of small-scale pilot lines for the preparation of nanofluids and also to the project of future large-scale pilot lines, where the knowledge of the industrial equipment characteristics and turbulent flow regime conditions are essential.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Nomenclature

m | Flow Rate [kg/s] |

ρ | Fluid Density [kg·m^{−3}] |

h | Heat Transfer Coefficient [Wm^{−2}K^{−1}] |

TS | Interface Temperature [K] |

Cv | Latent Heat of Vaporization [J·kg^{−1}] |

Cp | Specific Heat [J·Kg^{−1}K^{−1}] |

C | Thermal Conductance [W·m^{−2}·K^{−1}] |

K | Thermal Conductivity [Wm^{−1}K^{−1}] |

R | Thermal Resistance [K·W^{−1}] |

µ | Dynamic Viscosity of Suspensions [Pa·s] |

vol. % | Volume Concentration of Nanoparticles |

wt. % | Weight Concentration of Nanoparticles |

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Single-Step | Methodology | Aim/Results | Advantages | Limitations |

Pulsed Wire Evaporation | Applying high-voltage to a metal wire that evaporates and condensates into nanoparticles | Uniform dispersion of nanoparticles in the base fluid | Avoids contamination. Improves particle size control and minimizes agglomeration | Need of high temperatures to evaporate materials hinders the production of certain nanofluids |

Pulsed Laser Ablation | Irradiate with laser the surface of a material submerged in the base fluid to disperse the nanoparticles | Uniform dispersion of the vaporized nanoparticles in the base fluid | Reliable and cost-effective technique. Relatively fast method | In some cases requires stirring and addition of surfactants to the base fluid |

Submerged Arc | Apply an electric pulse to evaporate the material in the Base fluid | Uniform dispersion of the vaporized nanoparticles in The base fluid | Stable nanofluids with uniform distribution of nanoparticles | In some cases requires stirring and the addition of surfactants to the Base fluid |

Two-Step | Methodology | Aim/Results | Advantages | Limitations |

Sol-Gel | Dissolution of nanoparticles, sonication, hydrolysis and drying of the gel | Crystalline nanopowder dispersion | Nanoparticles with high surface area; Low-cost method | Agglomeration of nanoparticles requires additives or surfactants |

Hydrothermal Synthesis | Synthesizes ingle-crystal under high temperature And pressure | Synthesize single-crystal from aqueous solutions | Low energy consumption and environmental benevolence | Cost of the autoclave and in some cases needs acid catalysts |

Sonication | Applying frequency higher than 20 kHz to the nanofluid | Reducing cluster formation by breaking molecular interactions | Avoids agglomeration/sedimentation of nanoparticles | Adjustment of sonication time that influences the size of the nanoparticles |

Magnetic Agitation | Spinning of a magnetic stir bar under a rotating magnetic field | Uniform dispersion of particles in the base fluid | Avoids agglomeration/sedimentation of nanoparticles | In some cases requires a surfactant; Time-consuming |

pH Adjustment | Addition of HCl or NaOH to adjust the Ph of the suspension | Raise the zeta potential and stability | More electrically stable suspension | It requires pH measuring equipment |

Combined Usage | Combination of sonication, magnetic agitation and pH adjustment | Uniform dispersion and stability | Avoids clusters and agglomeration | In some cases requires surfactants and solvents |

**Table 2.**Experimental works and numeric simulations of various geometries of heat sinks using nanofluids. MF stands for magnetic field.

Geometry | Nanoparticles in Water (conc.) | Nusselt Number Enhancement (%) | Thermal Resistance Reduction (%) | Heat Transfer Coefficient Enhancement (%) | Method | Ref. |
---|---|---|---|---|---|---|

Rectangular | Al_{2}O_{3} (2.0%) | n.a. | 25.0 | 70.0 | Exp | [79] |

Commercial | Al_{2}O_{3} (1.0%) | n.a. | n.a. | 18.0 | Exp | [80] |

Trapezoidal | CuO (4.0%) | 17.6 | n.a. | n.a. | NS | [81] |

Triangular | Al_{2}O_{3} (2.0%) | n.a. | n.a. | n.a. | NS | [82] |

Circular | SiO_{2} (5.0%) | n.a. | n.a. | 15.0 | Exp | [83] |

Circular | Fe_{3}O_{4} | n.a. | n.a. | n.a. | Exp | [84] |

Complex | Al_{2}O_{3} (1.0%) | 40.0 | 22.5 | 38.0 (MF) | Exp | [85] |

Complex | Al_{2}O_{3} (0.3%) | n.a. | 15.2 | n.a. | Exp | [86] |

Wide | Al_{2}O_{3} (0.2%) | 20.0 | n.a. | n.a. | Exp | [87] |

Wide | CuO (2.0%) | 100.0 | n.a. | n.a. | NS | [88] |

Cylindrical | Cu (0.3%) | 23.0 | n.a. | n.a. | Exp | [89] |

Cylindrical | Cu (0.3%) | 43.0 | 21.0 | 80.0 | Exp | [90] |

Dedicated | Al_{2}O_{3} (0.2%) | 23.9 | n.a. | n.a. | Exp | [91] |

Enclosure | CuO (1.0%) | 110.0 | n.a. | n.a. | NS | [92] |

Pin Finned | TiO_{2} (3.9%) | 37.8 | n.a. | n.a. | Exp | [93] |

Pin Finned | Al_{2}O_{3} (2.0%) | n.a. | 13.5 | 16.0 | Exp | [94] |

Micro Pin Finned | SiO_{2} (0.6%) | 14.0 | n.a. | n.a. | Exp | [95] |

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Moita, A.; Moreira, A.; Pereira, J.
Nanofluids for the Next Generation Thermal Management of Electronics: A Review. *Symmetry* **2021**, *13*, 1362.
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Moita A, Moreira A, Pereira J.
Nanofluids for the Next Generation Thermal Management of Electronics: A Review. *Symmetry*. 2021; 13(8):1362.
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