Efficient thermal management is needed for drive batteries, electric motors and power electronics in electric vehicles. Due to their wide range of consistencies and their robustness, silicone-based thermal interface materials prove indispensable in this field.
Most experts agree: tomorrow’s cars will be electric. By 2025, roughly 25 percent of world light vehicle production will have an electric engine with a battery, as found in hybrids, plugin hybrids, battery electric or fuel cell vehicles.
The automotive industry has recognized the signs of the times and is now working flat out on the development of electromobility. One challenge is how to effectively dissipate the heat generated in the various components while the battery is being charged and when the vehicle is on the road.
Such thermal management is especially critical for the battery serving as the power source. Lithium-ion batteries only deliver their best performance at temperatures between 20 and 35 °C. Consequently, to ensure acceptable performance and life span, they need to be prevented from overheating. Heat is also generated by the electric motor and the power electronics. Again, to avoid heat-related damage or failure, this thermal energy also needs to be dissipated quickly and effectively.
Thermal interface materials (TIMs) play a key role here. They fill the gap between the assembly which needs to be temperature-controlled and the heat exchanger or heat sink, and thus lower the thermal transfer resistance. With this, they enhance thermal coupling between the components. Thermal interface materials are therefore becoming increasingly attractive to car makers as they develop electric vehicles for mass production.
Thermal Conductivity
Dispensing a bead of silicone-based gap filler onto the heat sink of a power electronics module. (Photo: Wacker)
The choice of thermal interface material and its presentation form – whether paste, curable gap filler, adhesive, prefabricated pad – depends on the application and the prospective operating conditions. Thermal interface materials are commonly made from polymers which have a high filler content of thermally conductive inorganic substances. The base polymer may be an organic polymer or a silicone. The desired thermal conductivity is achieved with filler loads well in excess of 90 percent. The fillers are typically metal oxides, such as aluminum oxide. They ensure that the TIM remains electrically insulating, a property which is essential for use in close proximity to live components.
Silicone-based thermal interface materials, i.e. heat-conducting materials comprising a matrix of cured or uncured silicones, have a long successful track record in power electronics assemblies. Silicones are widely known for their aging resistance – even upon exposure to high or low temperatures. Unlike organic polymers, their physical and technical properties show very little change over the temperature range -45°C to 180 °C and above. They are also more flame-resistant than organic polymers.
A further characteristic of silicones is their low surface energy. Liquid silicones, for instance, will wet nearly all solid surfaces. This makes silicone-based TIMs easier to work with because they will fill even the tiniest irregularities in the substrate surfaces. Aging resistance and flame resistance are also the main arguments in favor of using thermally conductive silicone products in vehicles fitted with all-electric drives – even for assemblies operating at temperatures which do not necessarily require silicones.
Easy Application
Many silicone-based thermal interface materials are available in paste form. These TIMs are shear thinning compounds and so will not sag when at rest, but will flow when exposed to shear forces. They are dispensed onto the heat sink in the form of a bead. The assembly to be cooled is then mounted on top and pressed into place. Pourable types are also available in the form of resins and encapsulation compounds.
The use of thermally conductive silicone adhesives obviates the need for other means of attachment, as they not only provide thermal coupling between the parts, but also bond them together. Silicone pastes maintain their consistency after application. In practice, their applications are limited to small substrates and thin film thicknesses which should not exceed 100 to 150 µm.
Silicone-based gap fillers and silicone adhesives undergo a change of consistency as a result of a platinum-catalyzed addition-cure reaction. This yields a relatively soft, elastically deformable pad in the gap which fills out the contours of the surfaces exactly. Such gap fillers can even out surfaces with roughness values in the millimeter range which occur especially with large substrates. This distinguishes them from prefabricated pads, which have a specific thickness and are therefore unable to accommodate large tolerances.
Thermally conductive encapsulants are used for surfaces of complex shape. They transport the heat to the heat sink and at the same time protect the surfaces from environmental factors. Such products are applied by pouring. Resins are also applied by dipping.
Special Products for Electromobility
(Chart: WACKER)
WACKER has taken products with a successful track record in power electronics assemblies and has used them to develop numerous silicone-based TIMs for the electromobility sector. The company is continually optimizing these products and their ease of processing to meet the requirements of mass production.
The primary requirement here has been to adjust the rheological properties of the paste-like compounds so that they can cope with high-speed, highly automated car production. The relevant parameters are the molecular chain lengths and the type of liquid silicone polymers as well as the size and shape of the filler particles. The new highly loaded silicone systems are much more resistant to sedimentation than conventional products. They can be readily conveyed over long distances on suitable handling equipment, dispensed at high rates. Gap fillers, for example, can be dispensed at up to 30 to 50 mL/s, supporting rapid, automated assembly of parts at low, reproducible pressure.
The speed of crosslinking of the silicone systems is also important. Gap fillers and adhesives need to be formulated in such a way that curing – and the development of adhesion in the case of adhesives – proceeds rapidly at moderate temperatures. Unlike conventional products, Semicosil® 9754 TC, a rapid-cure, thermally conductive silicone adhesive, develops good adhesion even at room temperature, cures quickly below 80 °Celsius and thus allows rapid processing down the production line. Such adhesives achieve thermal conductivity values of between 2 and 4 W/(m K).
Applications in Electric Cars
Electric vehicles currently use lithium-ion batteries as energy storage. These are usually installed below the passenger compartment, where they occupy most of the floor space.
A thermally conductive gap filler is needed to provide thermal coupling between the battery modules and the heat-dissipation system. It must be aging-resistant to prevent premature battery failure and must lend itself to rapid application to large surfaces. Ease of application is therefore key for this filler. Precisely for such applications, WACKER has developed gap fillers from the SEMICOSIL® 96x TC series which can be dispensed rapidly and permit short cycle times, even where large substrates are mass produced.
(Chart: WACKER)
A further source of heat in electric vehicles are power electronics. Their task is to transform and regulate the electric current. Inverters, for example, convert direct current into alternating current and vice versa, while voltage converters change the level of the voltage. Power module components such as Integrated Gate Bipolar Transistors (IGBT) can reach temperatures greater than 100 °C in operation. The power losses can exceed 100 W/cm² – more than the power density emitted from the surface of a cooker hob on full power.
Overheating can damage the sensitive semiconductor structures and so lead to aging and eventually to component failure. Such failures can be prevented by actively cooling the printed board and IGBT assembly. At operating temperatures above 150 °C, silicone-based TIMs are the materials of choice for thermal coupling because organic polymers would be unable to withstand the heat. Depending on the design, effective component cooling can be achieved with thermally conductive gap fillers from the SEMICOSIL® 96x TC series, heat-sink pastes (e.g. SEMICOSIL® Paste 40 TC) or heat-sink adhesives (SEMICOSIL® 9754 TC).
Summary
There is still a great deal of development and testing being done in the field of thermal management in electric vehicles. However, it is already apparent that silicone-based thermal interface materials will play a key role in future thermal management. They can be readily adapted to a wide variety of application and manufacturing methods and are therefore the thermal interface materials of choice for mass production of electric vehicles. Silicones thus go a long way toward ensuring that key components of electromobility such as batteries and power electronics function reliably over the long term.
Author
Peter Walter, Senior Marketing Manager E-Mobility at
WACKER SILICONES
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| For further information, please contact: Wacker Chemie AG Media Relations & Information Florian Degenhart Phone +49 89 6279-1601 florian.degenhart@wacker.com www.wacker.com The company in brief: WACKER is a globally-active chemical company with some 14,700 employees and annual sales of around €4.93 billion (2019). WACKER has a global network of 24 production sites, 23 technical competence centers and 51 sales offices. WACKER SILICONES Silicone fluids, emulsions, rubber grades and resins; silanes; pyrogenic silicas; thermoplastic silicone elastomers WACKER POLYMERS Polyvinyl acetates and vinyl acetate copolymers and terpolymers in the form of dispersible polymer powders, dispersions, solid resins and solutions WACKER BIOSOLUTIONS Biotech products such as cyclodextrins, cysteine and biologics, as well as fine chemicals and PVAc solid resins WACKER POLYSILICON Polysilicon for the semiconductor and photovoltaic industries |
