Liquid coolant is one of the key factors in liquid cooling technology.In the application of immersion (contact) liquid cooling technology, in addition to hardware equipment requirements, liquid coolant is also one of the most critical factors.For a suitable contact liquid cooling coolant it requires:1) Good thermophysical properties. High thermal conductivity and specific heat capacity, low viscosity, and high latent heat of vaporization are required for phase change.2) Low freezing point and expansion coefficient.3) Single-phase liquid cooling requires a high boiling point.4）Two-phase liquid cooling requires a suitable boiling point and a narrow boiling range.5) Good chemical and thermal stability to electronic devices.6) High flash point and auto-ignition temperature.7) Non-corrosive to system materials (metals, non-metals and other organic substances.8) No or only minimal regulatory restrictions are required (environmentally friendly, non-environmentally toxic, biodegradable, etc.)9) Economy.。
Coolants can be mainly divided into fluorinated chemicals (or fluorocarbons) and hydrocarbons (such as mineral oil, synthetic oil and natural oil)。A liquid with a higher boiling point (above the maximum temperature of the system) is required to ensure that the liquid remains in the liquid phase.The following factors need to be considered when deciding between different fluorochemicals and hydrocarbons: heat transfer performance (stability and reliability, etc.), ease of IT hardware maintenance, fluid hygiene and replacement needs, material compatibility, electrical properties, flammability or combustibility, environmental impact, safety related issues and total fluid cost over the life of the tank or data center. Hydrocarbons are used as coolants, which have defects such as incompatibility, flammability, relatively viscous, and easy evaporation. Hydrocarbons (such as mineral, synthetic and natural oils) consist primarily of hydrogen and carbon, but they may also contain nitrogen and/or oxygen.These materials readily dissolve hydrocarbon-based polymers, so they are less likely to be compatible with adhesives, elastomers, and thermal interface materials.Additionally, most hydrocarbons are combustible and/or flammableTherefore, for many applications, especially in two-phase immersion cooling, hydrocarbons may pose an unacceptable risk to safety and infrastructure.Hydrocarbon fluids with sufficiently high boiling and flash points can be used in some single-phase applications, but they have the disadvantage of being relatively viscous (especially at low temperatures) and not easily evaporating from the hardware.
At present, aromatic substances, silicates (25R), aliphatic compounds, organosilicon and fluorocarbons have all been tried for direct contact cooling. Fluorocarbons are one of the most common and popular liquid coolants for electronic equipment due to their suitable dielectric constant, specific heat capacity, stability and safety.
Fluorinated liquid has good thermal conductivity, electrical insulation, and chemical inertness, and is suitable for immersion liquid cooling systems. Fluorinated fluid is a thermally stable perfluorinated fluid. Due to its chemical inertness, it can be used as a single-phase or two-phase cooling fluid, used in supercomputer systems and sensitive electronic components for military use.Fluorocarbons consist primarily of the elements fluorine and carbon, and may also contain hydrogen, nitrogen and/or oxygen. In organic chemistry, the bond between carbon and fluorine is known as the strongest single bond, which is why fluorocarbons exhibit high chemical and thermal stability.Krypton refrigerant has chemical properties and will not corrode electronic components when in contact.No special cleaning procedures are required after use. At the same time, it is also used as a stable coolant due to its good thermal conductivity.Fluorinated coolants can be widely compatible with substances, have good dielectric constant and strength, can achieve electrical insulation, have complete toxicity data, and complete occupational exposure guidance, and can be used in immersion liquid cooling systems to cool IT equipment；Suitable for new construction and renovation of data centers, it does not contain restricted substances such as nPB, HAP, trichlorethylene, perchlorethylene and 26 kinds of harmful substances common in electronic equipment, and its ozone depletion potential (ODP) is zero.
Perfluorocarbons are most suitable for use in data center coolants, and the market demand is expected to increase significantly.According to the composition and structure of fluorocarbons, they can be further divided into chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), hydrofluoroethers ( HFE) and other types.At present, CFC types have been phased out globally; HFCs were developed in the 1990s to replace hydrochlorofluorocarbons (HCFCs) and other ozone-depleting substances, and some HFCs (such as HFC-365mfc) can be used in solvent cleaning applications, Although it does not destroy the ozone layer, it has a high global warming potential (GWP). Perfluorocarbons (PFC) include perfluoroalkane, perfluoroamine, perfluoropolyether (PFPE) and other types. The characteristics of boiling point and dielectric constant are more suitable for semiconductor equipment cooling scenarios, but they also have the effect of greenhouse effect;Hydrofluoroether (HFE) has little influence on the greenhouse effect and does not damage the ozone layer, but usually has a high dielectric constant, and has a greater impact on signal transmission when it is in direct contact with the microstrip line or connector of the printed circuit board. On the whole, perfluorocarbons are currently more suitable coolants for data center liquid cooling systems. With the accelerated construction of data centers, the market demand for fluorocarbon coolants is expected to increase significantly.
Electronic fluorinated fluid is mainly monopolized by foreign countries.In the semiconductor manufacturing process, in order to obtain accurate processing capabilities under smaller process dimensions, it is necessary to use coolants to precisely control the temperature in some links of chip production。Since the semiconductor production line usually runs continuously, it is often cooled by electronic grade fluorinated liquid to ensure stable operation.Electronic fluorinated liquid is a key supply chain raw material for controlling the temperature of the wafer surface in the semiconductor etching process. Its production technology is difficult and its quality is demanding.At present, electronic fluorinated fluid is mainly monopolized by overseas companies, and domestic companies are in a state of accelerating catch-up. Only a few companies in the world, such as 3M and Solvay in the United States, can provide electronic fluorinated fluid cooling solutions.
3M coolants fall into two main categories: 1) Perfluorocarbons (PFCs), named after 3M™ Fluorinert™ Electronic Fluids; 2) Hydrofluoroethers (HFEs), named after 3M™ Novec™ Engineered Fluids. Of these, some models are suitable for immersion cooling, including: 1) PFCs: 3M™ Fluorinert™ Electronic Fluids in models FC-3283, FC-40, FC-43, FC-3284, FC-72, and FC-702 ) HFEs: 3M™ Novec™ Engineered Fluids Models 7000, 7100, 7200, 7300, 7500, 7700.
