Next-generation dielectric heat transfer fluids for cooling military electronics
StoryJuly 09, 2009
Long the gold standard for inert dielectric heat transfer fluids in coldplate and spray-cooling applications in the military, Perfluorocarbon (PFC) liquids have faced scrutiny due to their high Global Warming Potentials (GWPs). The utility of Polyalphaolefin (PAO) alternatives is limited by flammability, high viscosity at low temperature, and by low vapor pressure that prevents their use in evaporative applications like spray cooling. New segregated Hydrofluoroether (HFE) and Fluoroketone (FK) fluids are nonflammable and have much lower GWPs than PFCs. They compare favorably with the PAO fluids and Silicate Ester (SE) fluids often used in military electronics, for single phase applications, and may be suitable for use in spray and other evaporative cooling applications.
Perfluorocarbon (PFC) heat transfer fluids have been used in military applications since the 1950s. They are specified working fluids for direct contact electronic tests like gross leak and thermal shock under MIL-STD-202, -750, -883, -8129, and so on. PFCs are pumped through conduction-cooling systems like the Airborne Warning And Control System (AWACS), the Aegis ballistic missile defense system, and the ALQ-184 airborne Electronics Countermeasure (ECM) system. They have been used more recently in spray-cooled chassis aboard unmanned aerial vehicles like Predator and Global Hawk. They are also used in Environmental Control and Life Support (ECLS) systems aboard crewed space vehicles.
These “fully fluorinated” fluids are preferred because they are among the most chemically inert fluids and are excellent, very stable dielectrics. Direct contact or immersion systems require these properties. Other applications require a PFC for protection against short circuits should the fluid accidentally leak onto sensitive electronics. These nonflammable fluids are colorless and odorless, evaporate cleanly, and possess excellent toxicological properties.
The same chemical inertness that makes PFC fluids so well suited for the aforementioned heat transfer applications also makes them very long-lived in the upper atmosphere. Atmospheric lifetimes for these fluids range from 400 to 2,000-plus years. This coupled with their IR absorption profiles gives them high Global Warming Potentials (GWPs). Though PFC use in legacy defense-related systems is generally considered defensible, use in new platforms where other alternatives are suitable is inconsistent with a growing global trend to use materials with the lowest possible environmental impact.
In the past 13 years, Hydrofluoroether (HFE)1 and Fluoroketone (FK)2 fluids have been commercialized that share many of the aforementioned performance properties of PFCs, but have much lower GWPs while comparing favorably with Polyalphaolefin (PAO) fluids. In the sections that follow, the performance properties of these fluids are discussed along with some application examples.
Hydrofluoroethers, fluoroketones, and DoD applications
Segregated HFEs have atmospheric lifetimes of a few years and GWPs that overlap with naturally occurring compounds (Figure 1). Segregated HFEs and solvent blends based on them are already widely used for precision cleaning of embedded electronic assemblies throughout all branches of the military, NASA, and various national labs. Other common DoD cleaning applications for HFEs include Liquid and Gaseous Oxygen (LOX/GOX) line and system, aircraft engines, airframe, and general maintenance.
Figure 1: GWP of commercially available HFE and FK fluids compared with commercially significant HFCs and naturally occurring compounds.
(Click graphic to zoom by 1.2x)
|
|
Segregated HFEs have been used for nearly a decade in semiconductor test applications. HFE-7100 is pumped through precision-machined cold plates and blind Quick Disconnects (QDs) borrowed from embedded military applications. These test systems make use of 30 or more cold plates that can be quite large, often dissipating more than 1 kW each. HFE fluids are also flowed through high-voltage electrodes to maintain temperatures in plasma vapor deposition and etch applications. One such HFE was studied for use in crewed space vehicles and shown to generate no harmful decomposition products under conditions representative of ECLS systems. It is also being studied to replace perfluoropentane in ECM systems.
FK fluids have among the lowest GWPs of manmade compounds (see again Figure 1) and have thermophysical and dielectric properties nearly indistinguishable from PFCs. One FK fluid is used in fire extinguishing applications to replace ozone-depleting halons in telecommunication switch rooms, computer and electronic control rooms, and critical military applications such as engine and crew bays, hazard aboard ships, and flight-line protection. It is being used as the working fluid in DoD-commissioned Organic Rankine Cycles (ORCs) to convert heat into electrical energy.
While the suitability of FKs and segregated HFEs for immersion applications is still being investigated, these globally available fluids are well suited for indirect heat transfer applications and many in which the fluid briefly contacts electronic components (thermal test, for example). As will be shown, they compare favorably with PAO and Silicate Ester (SE) heat transfer fluids commonly used in military applications.
Performance
Table 1 shows properties of representative PFC, PAO, FK, and segregated HFE fluids. Though segregated HFE and FK fluids have lower specific heat and thermal conductivity than PAO, they have lower viscosity, particularly at low temperatures. Figure 2 shows results of pipe flow calculations comparing the HFE C4F9OC2H5 with PAO at matched heat transfer capacity (that is, mass flow rate times specific heat) using common methods3. Shown are the ratios of the various HFE calculation results to those of PAO as a function of temperature. At moderate temperatures, the HFE fluid maintains higher heat transfer coefficients with lower pressure drop and similar pumping power. At temperatures common in cold climates, these advantages become more apparent as the PAO becomes too viscous to maintain turbulent flow and its pumping power rises dramatically.
Table 1: Properties of representative HFE and FK fluids compared to a PFC and typical PAO.
(Click graphic to zoom by 1.8x)
|
|
Figure 2: Results of pipe flow calculations comparing the HFE C4F9OC2H5 (b.p. 76 ºC) with a common PAO. Heat transfer capacity is matched. (1) Higher is better. (2) Lower is better.
(Click graphic to zoom by 1.7x)
|
|
Sidebar 1: Drawing of a dual side solderable IGBT and results of passive, two-phase immersion cooling experiments in an HFE. Shown is the junction-to-fluid thermal resistance, DTjf, as a function of the die heat flux, Q”.
(Click graphic to zoom)
|
|
Electrical properties
Dielectric fluids are most often used because they will not short circuit electronics in the event of a leak. HFE and FK fluids have very high dielectric strength. The electrical resistivity of FKs and HFEs, though lower than PFCs, is well above the best attainable deionized water, a fluid widely used in high-voltage RF applications. Unlike deionized water, the resistivity of an FK or HFE is not prone to change. FKs share the low dielectric constant of PFCs. The dielectric constant of an HFE is substantially higher, and this may preclude HFE use in some immersion applications.
Material compatibility
HFE and FKs will often function as drop-in replacements for PFCs and are compatible with all metals, hard plastics, and a variety of inexpensive elastomers like Ethylene Propylene (EP) and butyl. HFE fluids have some hydrocarbon solvency, so heavily plasticized elastomers should not be used.
Stability
SE fluids hydrolyze over time, creating flammable alcohols as well as SiO2 deposits4 that have plugged heat exchangers, causing failures in ECM pods. No such breakdown mechanisms exist for FK and HFE fluids. HFEs are recommended for continuous duty up to 150 °C. Above this temperature, thermal decomposition might be expected under some conditions. FKs, though reactive with liquid water (that is, a separate water phase) are remarkably stable in its absence to more than 300 °C, allowing them to be used as working fluids in ORC systems applied to high-temperature heat sources like exhaust manifolds on internal combustion engines.
Safety and handling
Unlike PAO, HFE and FK fluids have no flash point (Figure 3). They are nonirritating, have low acute toxicity, and high inhalation exposure guidelines as would be expected for materials used in aerosol cleaners and fire extinguishing applications. Unlike PAO, HFE and FK fluids evaporate cleanly and quickly if spilled and will not trap grime that must be cleaned from hardware components.
Figure 3: Photo of 9 cSt PAO being sprayed with nitrogen into a candle flame.
(Click graphic to zoom by 1.3x)
|
|
The final analysis
Segregated hydrofluoroether and fluoroketones are safe, sustainable alternatives to perfluorocarbon and polyalphapolefin heat transfer fluids commonly used in military embedded systems. These fluids perform better than polyalphaolefins, particularly at low temperatures and can be used in an evaporative mode. They evaporate cleanly if spilled and are already widely used in DoD firefighting and cleaning applications.
Phil Tuma is an Advanced Application Development Specialist in the Electronics Markets Materials division of 3M Company. He has worked for 14 years developing applications for fluorinated heat transfer fluids in various industries including military and aerospace electronics, computer, fuel cell, pharmaceutical, and semiconductor manufacturing. He holds a BA from the University of St. Thomas, a BSME from the University of Minnesota, and an MSME from Arizona State University.
David A. Hesselroth is a Product Development and Technical Service Specialist at 3M, with primary focus on Novec Engineered Fluids used in precision cleaning applications for the aerospace, electronics, military, industrial, and medical markets. He joined 3M in 1978 and has been working with fluorochemicals the past 24 years. He holds a BA in Chemistry from Augsburg University, Minneapolis, Minnesota.
Tom Brodbeck is a Key Account Manager in the Electronic Markets Materials Division of 3M. He has worked for 28 years in a variety of positions involving 3M Specialty Chemicals and Fluids, focusing on heat transfer applications within the semiconductor and military and aerospace arenas. Specific application development activities include NASA, the U.S. Air Force, U.S. Navy, and the U.S. Army. Tom has a Bachelor of Science degree from Regis University.
3M Company
800-810-8513
www.mmm.com/novec
References:
1. Tuma, P.E., “Segregated Hydrofluoroethers: Long Term Alternative Heat Transfer Liquids,” Proceedings of the 2000 Earth Technologies Forum, Oct. 30-Nov. 1, Washington D.C., pp. 266-275.
2. Tuma, P.E., “Fluoroketone C2F5C(O)CF(CF3)2 as a Heat Transfer Fluid for Passive and Pumped 2-Phase Applications,” 24th IEEE Semi-Therm Symposium, San Jose, CA, March 16-20, 2008, pp. 174-181.
3. Tuma, P.E., “Hydrofluoroethers as Low-Temperature Heat-Transfer Liquids in the Pharmaceutical Industry,” Pharmaceutical Technology, March 2000, pp. 104-116.
4. DoD SBIR AF06-083, “Coolanol 25R Replacement for Military Aircraft Radar Cooling Systems.”
