Category Archives: Nano-technology

Nanofluids in Electronics Cooling Applications

By Dr. Reza Azizian
Research Scientist
Advanced Thermal Solutions, Inc.

1. Introduction

Power requirements for electronic devices have risen steadily in recent years, with the rate of increase sloping upwards, and that has necessitated enhanced thermal management solutions to preserve performance and maintain the mean time between failures (MTBF) of these devices. There are a variety of solutions that can be implemented for cooling high power electronic devices from air to liquid cooling. While air cooling is the default, liquid cooling is necessary when high-power electronic devices dissipate more than of 300-520 W/cm2. [1,2]

The addition of nanoparticles to a coolant (i.e. Nanofluid) is another alternative that can provide further improvement within a liquid cooled system.

Nanofluids

Engineered suspensions of nanoparticles in liquids have become known recently as nanofluids. The nanoparticles dispersed in a base fluid are typically metal or metal oxide particles with a size range of 1-100 nanometers. When suspended in the base fluid, nanoparticles create a colloidal solution that has been shown to eliminate the issues of erosion, sedimentation and clogging that plagued earlier solid-liquid mixtures that used larger particles.

Dispersion of nanoparticles in a base fluid alters the fluid’s overall thermo-physical properties (such as enhancing the thermal conductivity). Researchers were able to demonstrate as much as 20% enhancement in heat transfer performance of the single phase, liquid cooled system when nanoparticles were introduced. [3]

This white paper tries to offer a basic overview of nanoparticles and nanofluids for engineers unfamiliar with this topic, including a basic introduction of what nanofluids are, terminology and selecting criteria for engineers, price points, market trends, basic applications, challenges and conclusions. The end of this paper contains both a definition of terms for this topic and a list of further references.

2. Nanofluid Composition

Nanofluids are made up of two basic components: nanoparticles and a base fluid. Surfactant is also used in some cases to increase colloidal stability.

The nanoparticles used in nanofluids are typically metals, metal oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil. [4] The mixture of nanoparticles into a base fluid creates a colloid, as shown in Fig. 1.

Nanofluids in Electronics Cooling

Figure 1. Colloidal dispersion of nanoparticles in a base fluid (Nanofluids). [5]

The addition of nanoparticles to a base fluid enhances thermal conductivity and viscosity of the resultant nanofluids. The degree of enhancement depends on various factors such as volume fraction of the particles being added, particle shape, etc. The reasons for the enhancement are attributed to several microscopic phenomena including particle dynamic effect, liquid layering on the surface of the nanoparticles, and particle clustering. [6]

3. Markets and Applications

Several key markets and applications for nanofluids include:

  • Heat Transfer Enhancements
    • Power Systems (nuclear and conventional); heat exchangers (chemical industry); heat pipes; thermal management of power electronics and other semiconductors; air conditioning; and refrigeration systems
  • Mass Transfer Enhancements
    • Fermentations; protein/cell separation; drug delivery; and catalysts
  • Rheology
    • Lubricants (deep hole drilling)
  • Magnetic Nanofluids (Ferrofluids)
    • Rotary seals; cancer therapy (hyperthermia); NMR imaging; and magnetic cell sorting

4. Selecting the right nanofluid from the thermal standpoint

In the case of single-phase heat transfer, it has been shown that the heat transfer coefficient and the amount of pressure loss can be predicted by means of traditional correlations and models (Eq. 1 and 2 for heat transfer coefficient of laminar and turbulent flow respectively), as long as the measured temperature and loading dependent nanofluid properties are used. [7, 8]

Where hnf is the convective heat transfer coefficient of nanofluids; Knf, µnf, ρnf, cp,nf, are the thermal conductivity, viscosity, density and specific heat of nanofluid respectively; V is the fluid velocity, x is the distance from the entrance of the tube and D is the tube diameter.

It is clear that if accurate values of the thermo-physical properties for a specific nanofluid are available, the thermal behavior can be predicted. Apart from the specific heat and density, which can be identified by mixture rules, thermal conductivity and viscosity are the two most important parameters for overall thermal evaluation of a specific nanofluid. It is expected that by dispersing nanoparticles with higher thermal conductivity in comparison to the base fluid overall thermal conductivity of the resultant nanofluid will be higher than that of the original base fluid.

This is beneficial because having a working fluid with higher thermal conductivity would result in a reduction of pumping power while achieving the same amount of heat removal. It can also be shown that if one can increase the thermal conductivity of the working fluid by a factor of three (enhancing overall thermal conductivity is basically what nanofluid does), the heat transfer in the same apparatus doubles. [3] To give readers an idea, to increase the heat transfer of conventional fluid by a factor of two, pumping power must usually be increased by a factor of eight. Therefore, using nanofluid with high value of thermal conductivity can provide large pumping power reduction in the system.

Effective medium theory (i.e. Eq. 3) with Ck = 3 (for a dilute system of spherical particles) can be used as a rule of thumb criteria of the thermal conductivity estimation for the specific nanofluid. Obviously, measuring thermal conductivity and identifying Ck provides much more accurate value.

where knf and kbf are the thermal conductivity of nanofluid and base fluid respectively; Ck is the constant that should be found by matching Eq. 3 to the experimental data; and is the volume fraction of the particles.

However, this is just half of the story, in addition to thermal conductivity, it was also evident that viscosity has a significant impact on the overall performance of a heat transfer fluid. Adding nanoparticles to the base fluid results in a higher viscosity of the resultant nanofluid. Clearly, pumping a fluid with increased viscosity through a heat exchanger requires an increase in pumping energy, thereby reducing the overall benefit of a fluid with higher thermal conductivity. [9] Unfortunately, most of the equations for predicting viscosity of nanofluids under-predicts the experimental data by a huge margin. It was shown that for the first order estimation value of Cµ = 10 can be used in Eq. 4.

where µnf and µbf are the viscosity of nanofluid and base fluid respectively and Cµ is the constant that should be found by matching Eq. 4 with the experimental data.

This shows that generally the enhancement in viscosity is higher than enhancement in thermal conductivity (Cµ = 10 compared to Ck = 3). Then the key question to be answered is: “Is there any benefit of using nanofluids as a heat transfer fluid?” [10]

Criteria for the overall effectiveness of nanofluids as heat transfer fluids have been proposed, [9,11] which suggest that the increase in the viscosity has to be more than four times larger than the comparable increase in thermal conductivity for a nanofluid to be not beneficial at all.

Therefore, the overall quick guide line for selecting a nanofluid for a specific application would be as follows:

A. Use Eq. 3 and 4 to estimate the thermal conductivity and viscosity of the nanofluid (measured values of the thermal conductivity and viscosity are obviously preferred).

B. If the measured thermal conductivity and viscosity of the nanofluid of interest meets the requirements of Cµ <4 Ck move to the next step. If not, the choice of nanofluid should be changed.

C. Calculated/measured thermal conductivity and viscosity values should be implemented in Eq. 1 or Eq. 2 (depending on the flow) for the first order estimation of heat transfer coefficient enhancement.

In applications that involve boiling, it was shown that the boiling of nanofluids result in a deposition of nanoparticles on the heated surface (Fig. 2). [12,13] Deposition of nanoparticles on the surface provides a porous, hydrophilic layer that alters the wettability of the surface. Due to the nano-scale porous nature of this layer, capillary wicking also plays an important role. Lateral flow of the liquid due to the capillary wicking supplies a fresh liquid to the dry region beneath the bubble and delays the irreversible growth of the hot spot, which results in critical heat flux (CHF). [13] The exact mechanism behind the observed enhancement in the CHF value is still subject of debate in heat transfer community. [14]

Figure 2. Scanning electron microscopy of the heated surface after boiling with (a) DI-water and (b) nanofluid. [12]

5. Cost estimation of Nanofluids

The overall cost of a nanofluid is determined by the price of the chosen base fluid and the price of the nanoparticles themselves. Prices are higher when the nanoparticles are more difficult to synthesize or the process for creating the stable colloidal suspension is more challenging. Table 1 presents an estimation of the price for some common nanoparticles with water as the base fluid. The thermal conductivity of the resultant nanofluid is a function of the nanoparticle’s thermal conductivity. Clearly, nanoparticles with higher value of thermal conductivity provide higher enhancement.

6. Geographic Considerations for Nanofluid R & D

Nanotechnology is a field that has worldwide investment in R&D both from governments and private enterprises. The Industrial Research Institute’s “2016 Global R&D Funding Forecast” [15] notes that the most important technologies from a world-wide Industrial R&D investment standpoint include nanotechnology for both biological and industrial applications. Indeed, nanotechnology research for industrial applications (including thermal management of electronics) is forecast to have a 30% increase in R&D investment over the next two years. Only Information Technologies is forecast to have a larger increase at 31%.

The United States nanotechnology industry is led by the National Nanotechnology Initiative, which (in 2017 alone) has budgeted $1.4 billion for R&D. The budget supports nanoscale science, engineering, and technology R&D at 11 agencies, including the National Science Foundation, National Institute of Health, Department of Energy and the Department of Defense. [16] In addition, the United States has published 6,926 scientific papers on nanotechnology as of 2015. [17]

Other countries that are also heavily investing in nanotechnology include:

– China is second to the United States in investment for nanotechnology research having invested $1.3 billion in R&D and having published 2,898 scientific papers.
– Japan has $850 million dollars invested in nanotechnology research and has published 2,626 papers.
– Germany also has $850 million dollars invested with 1,767 published scientific papers.
– South Korea has $400 million dollars invested and 1,461 published scientific papers.

There has been a major effort in the European Union (EU) to make nanotechnology available for commercial purposes. In the European Commission’s “Horizon 2020” framework for research and innovation, nanofluids were labeled a “Key Enabling Technology,” although it also noted that many nanofluids are at Technical Readiness Level (TLR) 1-3, making them several years away from full commercial viability. [18] The European Commission is devoting €80 billion to spearhead research into various technologies, in addition to private investments.

While the amount of investment shows the overall commitment of a country to this technology, another metric is the concentration of nanotechnology startup companies by geography. For example, in the United States, the highest concentrations of nanotechnology startup companies are in California, Massachusetts, New York and Texas. The densest locations (with 30 or more companies) are Boston, Mass.; San Francisco, Calif.; San Jose, Calif.; Raleigh, N.C.; Middlesex-Essex, Mass.; and Oakland, Calif. [19]

7. Regulatory Considerations

Many nanoscale materials are regarded as “chemical substances” under the U.S. Environmental Protection Agency (EPA) Toxic Substance Control Act (TSCA). [20] TSCA requires manufacturers of new chemical substances to provide specific information to the Agency for review prior to manufacturing chemicals or introducing them into commerce. The EPA can take action to ensure that chemicals that may pose an unreasonable risk to human health or the environment are effectively controlled. The EPA’s bulletin, “Control of Nanoscale Materials under the Toxic Substances Control Act” outlines the regulatory framework for nanoscale materials. Most engineers implementing nanofluids for thermal management would be acquiring these materials from another manufacturer.

8. Key Market Drivers

Market drivers are forces that compel products and services to change in order to meet consumer or business demand. A useful construct for those drivers is a STEEPLE analysis, which considers seven broad factors of market drivers: Socio-Cultural, Technological, Economic, Environmental, Political, Legal and Ethical. This section deals with two to three key technological drivers for the most relevant heat transfer enhancement applications such as: power plants, power electronics and other semiconductors, and heat pipes.

8.1. Power Systems (nuclear and conventional)

Virtually all developed countries rely on a national power grid for their nations to grow and prosper. Nanofluids can play an important role in this space by improving the outcome and cost-efficiency of existing power plants. The use of nanofluids to cool a power plant can significantly reduce the resources consumed and the cost in electricity to run it. These cost savings and the greater efficiency that nanofluids provide can also eliminate the need for building additional power plants, which is a massive financial boost for a country, state, or municipality.

In power systems, the key technological drivers to consider for the implementation of nanofluids include:

o Nuclear [21]
– Improving the energy efficiency of a nuclear power plant by improving the functionality of existing power systems.
– Cost and regulatory avoidance by not having to build additional infrastructure since the efficiency of existing infrastructure can be improved to the degree that more building is unnecessary.
– Safety of nuclear power plants by improving the cooling of nuclear rods (in a pressurized water reactor, Fig. 3 [22]) by pushing the point at which a rod might overheat (CHF) to a higher value.

Figure 3. Pressurized water reactor (PWR). [22]

o Solar [23]
– Improvement to heat transfer by increasing the heat transfer performance of the fluid in the solar collectors for hot water supply (Fig. 4).

Figure 4. Solar hot water system. [24]

o Conventional [24]
– Improvement in power plant efficiency through various methods including higher heat capacity of the heat transfer fluid.
– Reduction in the size of heat exchangers.
– Reducing vapor pressure to reduce plant risk.
– Improvement in the stability of heat transfer fluids with a goal of keeping the consistency of energy conversion stable.

8.2. Power electronics and semiconductors [25, 26, 27]

With the advent of cloud computing, electric and hybrid vehicles, and the Internet of Things (IoT), electronics have become increasingly integrated into our lives. This integration of and the growth in applications drives an increasing need for more performance but at lower power and in smaller packages.

In power electronics and semiconductors the key technology drivers are:

• Increasing speeds of semiconductors with simultaneous increase in component densities.
• Increasing the performance of fluids for thermal management of electronics and migration from air to liquid cooling systems.
• Improvement in energy efficiency of electronic systems.
• Improving rack density for computing systems through reducing systems to sub-1U platforms.
• Improvement in power module life (MTBF).
• Thermal management of ever-increasing power densities.

8.3. Heat Pipes

Heat pipes are increasingly used as a key component in thermal management solutions. Their ability to transfer large amounts of heat from one section of a system to another makes them invaluable for keeping semiconductor junction temperatures within their proper range.

Key technology drivers for heat pipes and nanofluids are:

• Improving the heat transfer performance and temperature uniformity in heat pipes.
• Reduction of the thermal resistance (because of higher thermal conductivity of nanofluids).
• Increasing maximum allowable heat load of heat pipes (Qmax) by improving the wettability and providing larger surface area for thin-film evaporation in an evaporator (because of particle deposition as a result of evaporation).

9. Applications and Specifications

Nanofluids are a broad category of engineered fluids. The composition and selection of a particular colloid is generally specific to a given application category. The following are some examples of applications that could use nanofluids and some general specifications as to why an engineer may want to choose them.

9.1. Computer servers for cloud computing

Cloud computing, with its large scale of servers, continues to grow and with this trend so too does the amount of energy data centers consume. Moving from air cooling to liquid cooling can create a more energy- and computationally-efficient data center. When using water as the base fluid with nanoscale particles of zinc oxide, copper oxide or carbon nanotubes, for example, the colloid’s capability to conduct heat is improved 10-40% over water alone. The lower range of improvement is with a conventional heat exchanger, while the higher range requires a specially designed heat exchanger. [29]

9.2. Power Electronics

Insulated-gate bipolar transistor (IGBT) modules, power brick and power semiconductors have ever-increasing heat dissipation requirements reaching 300 W/cm2 and higher. At those heat dissipation levels the constraints of cabinet size, system density, noise levels and fan size make using liquid cooling with nanofluids a good thermal management strategy. As in other applications, an improvement of 10-40% may be possible by using nanofluids rather than water alone.

The implementations of liquid cooling can vary, but commonly involve heat pipes, vapor chambers or cold plates (either single or two-phase) with a pump. Because of the unique thermal capabilities of nanofluids, all three implementations can be smaller, lighter, quieter and thinner than when deployed without nanofluids. [30]

10. Trends

The idea of using liquid cooling for power electronics applications is no longer confined to theoretical observations or laboratory experiments. There is widespread use of heat pipes, for example, and personal computers frequently incorporate elaborate liquid cooling systems. Nanofluids represent an enhancement to the technologies that are increasingly being used for cooling electronics.

Much of the ongoing research is focused on immersion cooling and boiling. In tests, boiling with nanofluids has been shown to improve the value of critical heat flux (CHF) by as much as 200% as a result of nanoparticle deposition on the surface of component. This has led to other research on, potentially, engineering the surface of high-powered electronic devices with nanoparticles to improve heat dissipation without the need for an engineered liquid.

Researchers are also continually testing new materials to disperse in different fluids. For instance, aluminum oxide, iron oxide, zinc oxide, cerium oxide, and bismuth oxide are just some of the options that Nanophase Technologies Corporation offers to customers. In addition, there is work being done with carbon nanotubes that have shown promising results in heat transfer. [31] New materials, such as graphene, are also being developed and tested to determine their potential for application in thermal management.

Other trends in the technology include the use of different materials such as copper, aluminum, and newly developed polymers for the loop of a liquid cooling system or different wicking materials in the production of heat pipes and vapor chambers. In addition, improvements are being made in heat exchangers, pumps, and other components of liquid cooling systems.

11. Challenges

While nanofluids have gained momentum and notoriety in the past two decades, there are still several challenges to their widespread commercialization. [32] One of those challenges is that the concept of nanofluids is still relatively young. The term was only coined in the early 1990s and, while the number of studies have increased in recent years, there are no long-term assessments of how the addition of nanoparticles could affect a cooling system over time. Nanoparticles could collect and cause degradation in a pump or heat exchanger, for example. There is a widespread belief in the stability and reliability of nanofluid solutions, but it has not been possible to document that over the course of a long period of time.

Also, there are questions that have been raised about the environmental impacts of disposing nanofluids from a system, whether as vapor in the case of a nuclear power reactor or as liquid when replacing the fluid in a data center system. While it is possible that nanoparticles in high quantities could be harmful, there are few studies that have focused on the environmental impacts [33] or the impact on humans. [34]

12. Conclusions

This white paper summarized the potential applications of nanofluid in electronic cooling. It was shown that the thermal performance of the liquid cooled system can be further improved by dispersing nanoparticles in a base fluid (i.e. nanofluid). The guide for selection of the right nanofluid for specific application is provided. It was shown that the heat transfer coefficient enhancement in nanofluid can be predicted simply by implementing thermo-physical properties of the nanofluid of interest in the traditional models/correlation for single phase convective heat transfer. It was also shown that the large enhancement in CHF value can be achieved by boiling nanofluid on a heated surface. Based on these basic principles employing nanofluid in different applications, their opportunities and challenges were discussed.

Acknowledgment

The author would like to thank Ms. Rebecca O’Day and Mr. Joshua Perry for their help in preparing this manuscript.

Definition of Terms
———————————————-
There are several terms that are used in discussion nanoparticles and nanofluids in electronics cooling applications. These terms are listed below and defined for reference.

Base Fluid – Fluids generally used to contain nano particles to create a colloidal suspension. Base fluids typically but not exclusively used include water, ethylene glycol, engine oil, Tolouene, refrigerants.

Colloid – Nanoscale or microscale particles suspended in another medium.

Nanoparticle – Particles with size on the order of 1-100 nm. [35]

Nanoparticle Types: Nanoparticles can be made of materials with good thermal conductivity such as metallic oxides (Al2O3, CuO, SiO2, Fe3O4), nitride ceramics (AlN, SiN), Carbide ceramics (SiC, TiC), metals (Cu, Ag, Au), semiconductors (TiO2, SiC), carbon nanotubes (SWCNT, DWCNT, MWCNT).

Nanofluid – Fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloids of nanoparticles in a base fluid.


References
1. J. Valenzuela, T. Jasinski, Z. Sheikh, “Liquid Cooling for High-Power Electronics” Power Electronics Technology 2005 www.powerelectronics.com
2. R. Saidur, K.Y. Leong, H.A. Mohammad, “A review on applications and challenges of nanofluids”, Renewable and Sustainable Energy Review 15 (2011) 1646-1668.
3. K.V. Wong, O.D. Leon, “Applications of Nanofluids: Current and Future,” Advances In Mechanical Engineering 2 (2010) 519659.
4. S.K. Das, S.U.S. Choi, H. E. Patel, “Heat Transfer in Nanofluids—A Review,” Heat Transfer Engineering 27 (2006) 3-19.
5. W.S. Williams, “Experimental and Theoretical Investigation of Transport Phenomenon in Nanoparticle Colloids (nanofluids)”, Ph.D. Thesis, Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, December 2006.
6. S. Akilu, K.V. Sharma, A.T. Baheta, R. Mamat, “A Review of Thermophysical Properties of Water Based Composite Nanofluids” 66 (2016) 654-678.
7. U. Rea, T. McKrell, L-W. Hu, J. Buongiorno, “Laminar convective heat transfer and viscous pressure loss of alumina–water and zirconia–water nanofluids”, International Journal of Heat and Mass Transfer 52 (2009) 2042–2048.
8. W. Williams, J. Buongiorno, L-W. Hu, “Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes”, ASME Journal of Heat Transfer 130 (2008) 042412-7.
9. D.C. Venerus, J. Buongiorno, R. Christianson, et al., “Viscosity Measurements on Colloidal Dispersions (Nanofluids) for Heat Transfer Applications”, Applied Rheology 20 (2010) 44582-7.
10. R. Prasher, D. Song, J. Wang, “Measurements of nanofluid viscosity and its implications for thermal applications”, Applied Physics Letters 89 (2006) 133108-3.
11. J. Garg, B. Poudel, M. Chiesa, J.B. Gordon, J.J. Ma, J.B. Wang, Z.F. Ren, W.T. Kang, H. Ohtani, J. Nanda, G.H. McKinley, G. Chen, “Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid,” Journal of Applied Physics 103 (2008) 074301.
12. S.J. Kim, I.C. Bang, J. Buongiorno, L-W. Hu, “Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux”, International Journal of Heat and Mass Transfer 50 (2007) 4105–4116.
13. H.D. Kim, M.H. Kim, “Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids”, Applied Physics Letters 91 (2007) 014104-3.
14. M. Tetreault-Friend, R. Azizian, M. Bucci, T. McKrell, J. Buongiorno, M. Rubner, R. Cohen, “Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers”, Applied Physics Letters 108 (2016) 243102-4.
15. Industrial Research Institute’s “2016 Global R&D Funding Forecast” https://www.iriweb.org/sites/default/files/2016GlobalR%26DFundingForecast_2.pdf
16. “National Nanotechnology Initiative (NNI) 2017 Budget” https://www.nano.gov/about-nni/what/funding
17. T.y. Haqqi, “10 Best Countries in Nanotechnology”, Insider Monkey, http://www.insidermonkey.com/blog/10-best-countries-in-nanotechnology-370714/?singlepage=1, September 2015
18. “HORIZON 2020: The EU Framework Programme for Research and Innovation,” European Commission, accessed on Nov. 23, 2016, https://ec.europa.eu/programmes/horizon2020.
19. “US NanoMetro Map” http://www.nanotechproject.org/inventories/map/
20. “Control of Nanoscale Materials Under the Toxic Substances Control Act”, https://www.epa.gov/reviewing-new-chemicals-under-toxic-substances-control-act-tsca/control-nanoscale-materials-under
21. “Nanotech + nuclear = more electricity”, The MIT Energy Research Council, http://web.mit.edu/erc/spotlights/nano_nuclear.html
22. http://scientificgamer.com/pwr-overwhelmin/
23. H. Tyagi, P. Phelan, R. Prasher, “Predicted Efficiency of a low-temperature nanofluid – based on direct absorption solar collector,” Journal of Solar Energy Engineering 131 (2009) 041004.
24. M.E. Mondejar, M. Thern, “Non-conventional working fluids for thermal power generation: A review”, Journal of Postdoctoral Research 2 (2014) 1-14.
25. R. Azizian, “PSMA Webinar – Nanofluids for Electronic Cooling”, 2016 https://vimeo.com/189223573
26. “Liquid Cooling Moves onto the Chip for Denser Electronics”, Georgia Tech News Center, October 5, 2015 http://www.news.gatech.edu/2015/10/05/liquid-cooling-moves-chip-denser-electronics
27. “Water Cooling of Power Modules”, The Power Guru, http://www.powerguru.org/water-cooling-of-power-modules/, 2013
28. “Nanofluids in Heat Pipes” Qpedia Thermal eMagazine, Issue 96, http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues-Content/127.aspx
29. “Nano technology could cool the heat from server farms”, CNN, September 2010, http://www.cnn.com/2010/TECH/innovation/09/07/eco.nano.web/
30. D.L. Saums, “Liquid Cooling Systems for Electronics Thermal Management”, IMAPS Chesapeake Chapter Symposium, 2012 http://www.calce.umd.edu/whats_new/2012/Presentations/David%20Saums%20PPt.pdf
31. S. Halefadl, T. Maré, P. Estellé, “Efficiency of carbon nanotubes water based nanofluids as coolants,” Experimental Thermal and Fluid Science 53 (2014) 104-110.
32. M.M. Kostic, “Critical Issues in Nanofluids Research and Application Potentials,” Nanofluids (2013): 1-54.
33. “What are potential harmful effects of nanoparticles,” European Commission, accessed on Nov. 23, 2016,
http://ec.europa.eu/health/scientific_committees/opinions_layman/en/nanotechnologies/l-2/6-health-effects-nanoparticles.htm.
34. “The appropriateness of existing methodologies to assess the potential risks associated with engineered and adventitious products of nanotechnologies,” Scientific Committee on Emerging and Newly Identified Heath Risks (paper presented to European Commission Health & Consumer Protection Directorate-General, March 10, 2006).
35. http://www.trynano.org/resources/glossary

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

Industry Developments: Heat Exchangers for Electronics Cooling

By Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

(This article will be featured in an upcoming issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine. To read other stories from Norman Quesnel, visit https://www.qats.com/cms/?s=norman+quesnel.)

Heat exchangers are thermal management tools that are widely used across a variety of industries. Their basic function is to remove heat from designated locations by transferring it into a fluid. Inside the heat exchanger, the heat from this fluid passes to a second fluid without the fluids mixing or coming into direct contact. The original fluid, now cooled, returns to the assigned area to begin the heat transfer process again.

The fluids referred to above can be gases (e.g. air), or liquids (e.g. water or dielectric fluids), and they don’t have to be symmetrical. Therefore, heat exchangers can be air-to-air, liquid-to-air, or liquid-to-liquid. Typically, fans and/or pumps are used to keep these heat transfer medium in motion and heat pipes may be added to increase heat transfer capabilities.

Figure 1 shows a basic heat exchanger schematic. A hot fluid (red) flows through a container filled with a cold fluid (blue) but the two fluids are not in direct contact.

Heat Exchanger

Figure 1. In a Simple Heat Exchanger Heat Transfers from the Hot (Red) Fluid to the Cold (Blue) Fluid, and the Cooler After Fluid Re-Circulates to Retrieve More Heat. [1]

One example of a common heat exchanger is the internal combustion engine under the hood of most cars. A fluid (in this case, liquid coolant) circulates through radiator coils while another fluid (air) flows past these coils. The air flow lowers the liquid coolant’s temperature and heats the incoming air.

Applied to electronics enclosures, heat exchangers draw heated air from a cabinet, cool it, and then return the cooled air to the cabinet. These heat exchangers should be designed to provide adequate cooling for expected worst case conditions. Typically, those conditions occur when the ambient is the highest and when electrical loads through the enclosure are very high. Under typical conditions, heat exchangers can cool cabinet interiors to within 5°F above the ambient air temperature outside the enclosure.

Air-to-Air

Air-to-air heat exchangers have no loops, liquids or pumps. Their heat dissipation capabilities are moderate. Common applications are in indoor or outdoor telecommunications cabinetry or in manufacturing facilities that don’t have a lot of dust or debris.

Air-to-air heat exchangers provide moderate to good cooling performance. They don’t allow outside air to enter or mix with the air inside the enclosure. This protects the enclosure’s contents from possible contamination by dirt or dust, which could damage sensitive electronics and electrical devices and cause malfunctions.

An example of higher performance, air-to-air heat exchangers is the Aavid Thermacore HX series. These heat exchangers feature rows of heat pipes that add effective, two-phase heat absorbing properties when moving hot air away from a cooling area. The liquid inside the heat pipes turns to vapor. This transition occurs inside a hot cabinet. (See Figure 2)

The vapor travels to the other end of the heat pipe, which is outside the cabinet. Here it is cooled by a fan, transitions back to liquid form, and cycles back inside the cabinet environment.

Heat Exchangers

Figure 2. An Air-to-Air Heat Exchanger with Heat Pipes Extending Inside (top) and Outside (bottom) a Cabinet. Internal Heat is Transferred Outside the Enclosure. (Aavid Themacore) [1]

Other air-to-air heat exchangers feature impingement cooling functionality that can provide better performance than using heat pipes. Aavid Thermacore’s HXi Impingement core technology uses a folded fin core that separates the enclosure inside and outside. A set of inside fans draws in the hotter, inside air and blows it toward the fin core. This inside impingement efficiently transfers the heat to the fin core. Similarly, a set of outside fans draws in the cooler, ambient air and blows it toward the outer side of the fin core removing the waste heat. See Figure 3 below.

Heat Exchangers

Figure 3. Air-to-Air Heat Exchangers with Double-Sided Impingement Cooling Technology Can Move Twice the Heat Load of Conventional Exchangers. (Aavid Themacore) [3]

Liquid-to-Air

In some electronic cabinets, high power components can’t be cooled by circulating air alone or the external ambient air temperature is not cool enough to allow an air-to-air heat exchanger to solve the problem unaided. In these applications, liquid-to-air heat exchangers provide additional cooling to maintain proper cabinet temperatures.

For example, in a situation where heat is collected through a liquid-cooled cold plate attached directly to high power components. Even with the cold plate, the ambient air external to the cabinet is not cool enough to maintain the internal cabinet temperature at an acceptable or required level. Here, a liquid coolant in an active liquid-to-air heat exchanger can be used to cool the enclosure.

Heat Exchangers

Figure 4. Tube-to-Fin, Liquid-to-Air Heat Exchangers Provide High-Performance Thermal Transfer. [4] (Advanced Thermal Solutions, Inc.)

Advanced Thermal Solutions, Inc. (ATS) tube-to-fin, liquid-to-air heat exchangers have the industry’s highest density of fins. This maximizes heat transfer from liquid to air, allowing the liquid to be cooled to lower temperatures than other exchangers can achieve. All tubes and fins are made of copper and stainless steel to accommodate a wide choice of fluids.

Available with or without fans, ATS heat exchangers are available in a range of sizes and heat transfer capacities up to 250W per 1°C difference between inlet liquid and inlet air temperatures. They can be used in a wide variety of automotive, industrial, HVAC, electronics and medical applications. [4]

Heat Exchanger

Figure 5. Small, Light-Weight Liquid-to-Liquid Heat Exchanger Provides Efficient Cooling Performance. [5]

Lytron’s liquid-to-liquid heat exchangers are only 10-20% the size and weight of conventional shell-and-tube designs. Their internal counter-flow design features stainless steel sheets stamped with a herringbone pattern of grooves, stacked in alternating directions to form separate flow channels for the two liquid streams. This efficient design allows 90% of the material to be used for heat transfer. Copper-brazed and nickel-brazed versions provide compatibility with a wide range of fluids. [5]

Nanofluids

The development of nanomaterials has made it possible to structure a new type of heat transfer fluid formed by suspending nanoparticles (particles with a diameter lower than 100nm). A mixture of nanoparticles suspended in a base liquid is called a nanofluid. The choice of base fluid depends on the heat transfer properties required of the nanofluid. Water is widely used as the base fluid. Experimental data indicates that particle size, volume fraction and properties of the nanoparticles influence the heat transfer characteristics of nanofluids. [5]

When compared to conventional liquids, nanofluids have many advantages such as higher thermal conductivity, better flow, and the pressure drop induced is very small. They can also prevent sedimentation and provide higher surface area. From various research, it has been found that adding even very small amounts of nanoparticles to the base fluid can significantly enhance thermal conductivity.

Heat Exchangers

Figure 6. 3D Design of Curved Tube Heat Exchanger. Increased Turbulence and Velocity Increases Heat Transfer Rate. [6]

A recent paper by Fredric et al. proposes a theoretical heat exchanger with curved tubes and with nanofluids as the coolant. Nanofluids in place of regular water provide improved thermal conductivity due to the increased surface area. The heat transfer rate is further improved using curved tubes in place of straight tubes because the used of curved tubes increases the turbulence and fluid velocity, which helps increase the heat transfer rate. [6]

References
1. Advanced Thermal Solutions, Inc., https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
2. Aavid Thermacore, http://www.thermacore.com/documents/system-level-cooling-products.pdf.
3. Aavid Thermacore, http://www.thermacore.com/products/air-to-air-heat-exchangers.aspx.
4. Advanced Thermal Solutions, https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
5. Kannan, S., Vekatamuni, T. and Vijayasarathi, P., “Enhancement of Heat Transfer Rate in Heat Exchanger Using Nanofluids,” Intl Journal of Research, September 2014.
6. Fredric, F., Afzal, M. and Sikkandar, M., “A Review on Shell & Tube Heat Exchanger Using Nanofluids for Enhancement of Thermal Conductivity,” Intl. Journal of Innovative Research in Science, Engineering and Technology, March 2017.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Dr. Reza Azizian Giving Nanofluid Presentation for PSMA Webinar

Reza Azizian

Dr. Reza Azizian, a research scientist at ATS and an expert on nanofluids, will speak about nanofluid technology as part of a PSMA webinar on Thursday, Oct. 6. (Josh Perry/Advanced Thermal Solutions, Inc.)


On Thursday, Oct. 6, Dr. Reza Azizian, a research scientist at Advanced Thermal Solutions, Inc. (ATS), a leading-edge engineering and manufacturing company focused on the thermal management of electronics, and an expert on nanofluid technology, nano-engineered surfaces, fluid dynamics, heat transfer and two-phase flow, will present “Nanofluids for Electric Cooling” as part of a webinar sponsored by the Power Sources Manufacturers Association (PSMA).

Dr. Azizian will join a panel of experts to discuss the enhanced heat transfer properties of nanofluids and their potential for the thermal management of compact, liquid-cooled electronics. Dr. Azizian will present an overview of the current stage of nanofluids technology, state-of-the-art research into nanofluid thermos-physical properties, convective heat transfer, and boiling heat transfer.

Prior to the webinar, Dr. Azizian sat down with the Josh Perry, Marketing Communications Specialist at ATS, to speak about his career, his interest in nanofluids technology, and the upcoming webinar.

JP: Thank you for sitting down with us. We want to highlight the work that the engineers are doing here at ATS, so I appreciate you taking a few minutes out of your schedule for this Q&A. I saw on your bio that you got your doctorate in Australia, is that where you’re from?
RA: Thanks for having me! No, originally I am from Iran and I did my undergraduate there; then I moved to Turkey and did my Master’s in Turkey. After that I moved to Australia and I did my Ph.D. in Australia. And then I ended up in Boston and did my post-doc at MIT.

JP: How did you end up at MIT?
RA: There is a very famous professor at MIT who was working on heat transfer in nanofluids back then. I invited him to Australia. He came and visited our facility in Australia and gave a talk and then he became interested in my research. Then he invited me over and during my Ph.D. I came to MIT as a visiting student and I was here for a year and then I went back to finish my Ph.D. and came back as a post-doc.

JP: How did you end up joining the team at ATS?
RA: It was four years ago as a visiting student. I have a very good friend in Australia and I was always interested in high technology, heat transfer, electronic cooling, and then he sent me the link to the ATS website and said, ‘Hey Reza, while you’re in Boston, you might want to visit this company.’ I thought, wow this is cool. I went through the website to see what ATS does and saw some fascinating projects done by ATS. So, I emailed Dr. Kaveh Azar and he responded to one of my emails and then that’s how we got in touch and then I visited the ATS facility, and coincidentally when I went back to MIT and I was talking to my supervisor and I said, ‘Oh, I went and visited this company and they’re doing a great job.’ He said, ‘Oh, the name is very familiar.’ We realized that when he graduated, something like 16 years ago, he applied here for a job and got a job offer but he got a position at MIT so now he’s a professor there. I kept my contact with Kaveh and then I went back to Australia and finished my Ph.D. After I came back to the U.S. as a post-doc, I invited them to MIT to come and visit our laboratory. So, we stayed in touch.

That’s how I came to know ATS and I realized that they are doing a great job in electric cooling and I was always interested because in electronic cooling there is no limit basically. Electronic equipment is becoming smaller and smaller every day and the only limit is thermal, at least at the moment. The only barrier is thermal issue for the advancement of electronic cooling and that’s why basically all of the funding from the Department of Defense, NASA, etc., it’s all on cooling. Because again, at this stage with all of these nanotechnologies and manufacturing capabilities, they don’t have any barrier to make things smaller except thermal. It’s a very interesting area of research and, you know, when you’re at the university you do cutting-edge research, which is cool, but it’s always nicer to do the research and then build something and use your knowledge in a more applicable way.

JP: Many of the people who read this will probably know, but what are nanofluids?
RA: Nanofluid is the term that you use when you disperse metal or metal oxide nanoparticles, which with the dimensions of 109 m, which is like .000000001 meter…very tiny, and you disperse these in your base fluid, whatever it is – could be water, oil, anything – and because they are tiny they are going to stay dispersed and at the same time because they are metal or metal oxide their thermal conductivity is going to be much higher than your base fluid. In simple language, thermal conductivity means the ability of the material to transfer heat. So, for example, for water the thermal conductivity is .6 W/mK, but for copper it’s like 400 W/mK, so you can assume that by mixing these two, again because the particles are tiny you will still have your liquid, which can easily flow, but at the same time it has higher thermal conductivity compared to the base fluid that you have.

The nanofluid term comes into play because of the heat transfer limitations that you normally have. In very general terms, there are two ways that you can increase the rate of heat transfer. One of them is increasing the surface area and the other is to increase the flow rate. Increasing the surface area, you are normally limited by the space that you have, right, and also increasing the flow rate you should use a bigger pump for example, to have a higher flow rate, which these are all costly. The only option left is if you can play with your working fluid and see how you can improve that and one of the ways you can improve that is by dispersing these nanoparticles to increase the overall thermal conductivity of your working fluid.

JP: How did you get interested in nanofluids? How did that become the focus of your studies and work?
RA: I’ve been working on nanofluids for the past 10 years. I came to know nanofluids during my Master’s and it was for my final-year project. I was looking for something cool and, even back then, nanotechnology was everywhere and then I was looking for something in the area of nanotechnology and heat transfer. I remember, my supervisor didn’t know much and he was like, ‘If you’re going to do this then you’re going to be on your own. I can’t help you much.’ It was funny, I went to the Internet to look up nanofluids and the first thing that came up was the name of this professor at MIT that I was working with during my post-doc. Back then, I remember I was sitting in my office and his name came up and I was telling my office mate, ‘This guy is cool. I’m going to go and work with him one day.’ And he laughed at me like, ‘Oh from here you’re going to go and work with him at MIT? Such a dream.’ And I’m here now.

JP: Obviously there is quite a bit more known about them now, how much has the subject matter changed in the 10 years that you’ve been studying nanofluids?
RA: The good thing is that now there are companies that are actually making nanofluids with very good stability – the particles don’t settle, they stay stable for a long time – and they commercialized a couple of nanofluids that are available now. They even use them in car engines, in the radiators, to increase the rate of cooling. They use it for CPU cooling. Next month, I’m going to go to Europe, there’s an event for the European Union, and they’re trying to basically commercialize nanofluids by 2020. They’re trying to see what are the barriers. The field’s improved a lot. The whole term of nanofluid was invented in 1999, so it’s only 17-18 years. So, it’s a fairly new area of research and seeing this technology commercialized now…the progress was quite fast.

JP: What will you be talking about in the PSMA webinar taking place on Thursday, Oct. 6?
RA: I’m going to be talking about nanofluids in general. What are nanofluids, basically, and what are the applications of nanofluids, in particular, in electronic cooling and high-powered electronics, which is the interest to PSMA. Then I’m going to give a brief explanation about the thermo-physical properties of the nanofluids followed by how they behave under laminar and turbulent flow conditions or even boiling for immersed cooling of electronics. And then I will conclude my talk by [explaining] what is the state-of-the-art and what are the future directions we expect nanofluids are heading to.

JP: Why do you think this is an important topic? Why do you think nanofluids are important as we go forward in the world of electronics cooling?
RA: These tiny particles, you add them to your working fluid and you don’t add much to the pumping power that you’re going to use because they are tiny, but at the same time you see 15-20 percent enhancement (depending on the nanoparticles and working fluid combination) in the heat transfer coefficient without changing any hardware. So, it has a very good potential and, again, this is only for single-phase heat transfer. In the case of immerse cooling of high-powered electronics, which boiling is the main heat transfer mechanism, we were able to see 200-250 percent enhancement in the value of critical heat flux by just changing the working fluid to nanofluid. It’s a very convenient way of doing it.

JP: Do you see nanofluids as the future of the industry? Do you see this is where electronics cooling is heading?
RA: I have to highlight that there are still problems with using nanofluids. This is why there is still research going on in this area. Stability is a big issue. You can use definitely some form of surfactant, which is a polymer that covers these particles’ surfaces and that keeps them dispersed. But in general if you don’t have that these particles, because they are tiny, they are under constant Brownian motion and when they become close to each other they stick to each other and then they agglomerate and they settle. So, there are still some issues that different research groups are trying to address but definitely it’s an area that I think is very useful for electronic cooling.

JP: Is research still going on here at ATS? Are you still really involved in the research and trying to find more applications for it?
RA: Yeah, yeah…we are always trying to push more towards using nanofluids. And hopefully we’ll see more in the future.

If you are interested in the PSMA webinar on Oct. 6, contact power@psma.com no later than Oct. 4. For more information about Advanced Thermal Solutions, Inc., its thermal management products, testing equipment, and consulting services, visit www.qats.com.

Cutting edge semiconductor cooling news: Nano-coating cools chips four times faster

Many of us at ATS Thermal Labs are on the continued quest to continue to extend air only solutions for the thermal management of electronics.  We’ve invented some remarkable solutions to keep using air mainly because customer’s want it. No one wants to use liquid cooling in their electronic systems if they don’t have to. Such innovations as maxiFLOWTM heat sinks and superGRIPTM heat sink clips that allows phase change thermal interface material to work optimally, were driven by our customer’s demands to keep cooling with air. Soon there may be another tool for the thermal engineer to use: nanoscale coating.

As reported in EE Times:

Nanoscale coatings could boost the efficiency with which heat can be removed from semiconductors and other devices, according to an Army Research Laboratory funded study by researchers at the Pacific Northwest National Laboratory (PNNL) and Oregon State University (OSU).

The industry has known about nano-cooling for some time of course.  Including Intel’s invention of integrating circuits with carbon nano-tubes to enhance thermal management. But nanoscale coating might provide up to a 10x improvement in heat transfer coefficient. Pretty remarkable cooling and one our team is happy to see develop. You can read all about it at EE Times at their article, “Nano-coating cools chips four times faster“.

Intel’s nano-technology breakthrough for heat sinks has R&D promise but real-world applications are in the future; An ATS thermal management technology analysis

Just last week, Intel Corporation (Santa Clara, CA) earned U.S. Patent 7,704,791 for packaging of integrated circuits with carbon nanotube arrays to enhance heat dissipation through a thermal interface.

On the surface of it, this sounds like an exciting development in thermal management and heat sink invention, as noted in the article written by Alton Parrish on the news site “Before It’s News“:

According to inventors Valery M. Dubin and Thomas S. Dory a layer of metal is formed on a backside of a semiconductor wafer. Then, a porous layer is formed on the metal layer. A barrier layer of the porous layer at the bottom of the pores is thinned down. Then, a catalyst is deposited at the bottom of the pores. Carbon nanotubes are then grown in the pores. Another layer of metal is then formed over the porous layer and the carbon nanotubes. The semiconductor wafer is then separated into microelectronic dies. The dies are bonded to a semiconductor substrate, a heat spreader is placed on top of the die, and a semiconductor package resulting from such assembly is sealed. A thermal interface is formed on the top of the heat spreader. Then a heat sink is placed on top of the thermal interface.

Nano-carbon tube based materials have promised a revolution in thermal interface material technology but have never really delivered. In fact, many of us in thermal engineering have been anxiously awaiting a breakthrough development using nano-technology.
Nano-carbon tube technology has a lot of promise to solve the age old problem of contact resistance. The promise of nano-technology includes a few approaches from growing it on the semiconductor chip and eliminating TIM1 to growing it on the backside of the heat sink and minimizing or eliminating TIM2.

Unfortunately, many of these venture funded companies are no longer in business as the technologies maturation and commercialization simply hasn’t been there. From our vantage point as thermal scientists and engineers at ATS, nano-materials are an attractive proposition in certain university or corporate laboratories; with funding you can experiment and see if these technologies lead someplace. However, the real world issues of production in volume, sustaining production quality, cost, application and the biggest of all – the environmental hazard (nano-carbon tube has similar characteristics as asbestos) continue to create barriers to the real world utility of nano-carbon tube materials. While being a fascinating material to work with, the widespread, real world applications are limited at this juncture of technology life cycle and perhaps for the foreseeable future.

Perhaps most useful to the thermal management industry and to real world thermal engineering problems would be a comparison between nano-material based heat sinks and Aluminum or Copper; especially on a cost performance basis. ATS did a comparison between three geometrically identical heat sinks made of Copper, Aluminum and High-performance Graphite (though not nano-tube) and we did not see any difference. Surprisingly, the only advantage that high-performance Graphite offered was its light-weight, but, thermally and mechanically it was the worst (readers may download a copy of our study at ECNMag.com at this link: “Comparing the Impact of Different Heat Sink Materials on Cooling Performance“).

If we extrapolate our concerns and findings in our study to a heat sink made of nano-carbon tube material, (laying aside environmental factors and poor performance as nano-carbon tubes are isotropic – heat goes only in one direction), cost is a concern. Today’s electronics’ market is highly cost sensitive to their thermal development budget on a given project. Also, RoHS compliance is mandatory and studies are continuing on the health impact of nano-technology materials. Such work may not yield products for another five to 10 years. For the latest in the U.S. National Initiative in nano-scale technology, please visit the NNI’s 2011 Budget Supplement and Annual Report.

ATS applauds Intel for thinking out of the box and trying to create new ways of approaching the issue of thermal management. Semiconductor companies such as Intel who see themselves as part of the solution to thermal management are welcomed as fellow travellers to cooler and more reliable electronics.