An Energizing Lead

A project group headed by Angermaier is looking very closely into which silicone materials will be required for this new form of mobility. Around two to three kilograms of silicone can be found in a midsize car’s combustion engine today. For example, silicone rubbers are used in turbocharger hoses, protective jackets for spark plug connectors and ignition cables, vibration dampers and various gaskets and sealing mats. In addition, the electronics used in the vehicle control units are protected by dispensable silicone sealing adhesives or encapsulants. Even the functional safety of an airbag significantly profits from a silicone coating, since it makes this technical textile permanently flexible and resistant to temperature, aging and wear.

Some of these silicone applications will not be required in a fully electrically powered vehicle, but other new applications will be. “Electric cars are another vehicle category where silicone materials are essential,” emphasized Angermaier. “Our objective is to have the necessary silicone products available in time for the start of industrial-scale production of electric vehicles.” The drive train of a fully electric vehicle has three key components: the electric motor, traction battery and power electronics.

The combustion engine and transmission are omitted or can be of a simpler design. The compact, but powerful electric motor provides the necessary torque for the drive in electric vehicles.

WACKER is offering new silicone-based gap fillers with their SEMICOSIL^® 96x TC series, ideal for the thermal bonding of the battery modules with the temperature-control system. These pastes rapidly cure at room temperature with a platinum-catalyzed addition reaction. They achieve the thermal conductivity levels required in practice with values of 2 to 4 Watt per meter and Kelvin. The starting point of this development was based on products that have been used successfully for years for heat management in power electronic modules.

Like conventional products, the SEMICOSIL^® 96x TC series grades form elastically deformable silicone pads in the gaps. Closely following the contours of both surfaces, these pads perfectly fill even the slightest roughness and unevenness in the millimeter range. As silicone products, such gap fillers have major advantages over products based on organic polymers. Because they do not age, they permanently ensure good heat transfer and thereby contribute significantly to the required service life and stability of charge-discharge cycles in expensive traction batteries. As they are virtually non-flammable, they also contribute to battery safety.

The processing properties are just as important for the automotive industry. “Thanks to a new formulation concept, we managed to match the flow properties of the gap fillers to the manufacturing processes of the automobile manufacturers,” explained Dr. Müller. These paste-like materials are a solid when at rest, but can be applied rapidly and accurately to large surfaces, enabling automated assembly. A single dispenser can be used to apply up to six kilograms of paste per minute. Such high application rates were previously unthinkable for gap fillers, but will be necessary for the planned large-scale production of batteries required in the automotive industry.

The new thermally conductive materials also exhibit considerably higher sedimentation stability than conventional products and can be supplied in 200-L containers – another prerequisite for use in industrial-scale production. The solid filler usually sediments and cakes together rapidly in conventional gap fillers. Once a filler has formed a sediment, it can no longer be remixed or only with great difficulty. Many conventional silicone gap fillers can therefore only be supplied in special small containers such as cartridges with stirring options. This problem has been solved by the newly formulated gap fillers from WACKER SILICONES.

Another heat source in electric vehicles comes from the power electronics installed in the power conversion unit: their active components, known as insulated-gate bipolar transistors (IGBTs), can become very hot during operation. Depending on the specific grade, IGBT operating temperatures can often reach well over 100 °C.

Overheating can damage the sensitive semiconductor structures of the IGBT and so lead to aging and eventually to component failure. Such failures can be prevented by actively cooling the printed circuit board (pcb) and IGBT assembly. It is necessary here to thermally couple the pcb to the cooling plate. If operating temperatures lie above 150 °C, a silicone-based thermally conductive material is the preferred choice – thermally conductive materials based on organic polymers cannot withstand this temperature load.

According to Dr. Markus Jandke, who, like his colleague Philipp Müller, heads an applications laboratory at WACKER SILICONES’ Industrial Solutions team: “The gap fillers in the SEMICOSIL^® 96x TC series have proven to be optimal in numerous cases.” He emphasized that “Even non-curing silicone pastes such as SEMICOSIL^® Paste 40 TC, which retains its consistency within the gap, or thermally conductive silicone adhesives such as SEMICOSIL^® 9712 TC or 9754 TC can be used.”

The choice of material depends on how the pcb is connected to the cooling plate. This means that gap fillers and silicone pastes are predestined for such structures where heat sinks and heat sources, when in use, are screwed together or are otherwise securely connected by mechanical means and squeezed together. Thermally conductive silicone adhesives, in contrast, bond the two adherenss through their adhesive strength so that additional mechanical fastening is not required in many cases.

Even an electric motor generates heat during operation – despite a high degree of efficiency, which is generally over 90 percent. In comparison, combustion engines reach an efficiency of around 30 to 45 percent. Most of the heat is released in the stator, the stationary component of the electric motor. Various automobile manufacturers have decided to produce stator coil windings for future motor generations by using hairpin technology as this unconventional technology permits faster large-scale production.

Electric motor manufacturers are currently investigating various approaches to heat dissipation. “It is still unclear which one will prevail. But it is clear that thermally conductive silicone products can play an important role here,” said Dr. Ochs.

One such approach proposes transferring the waste heat from the stator via the laminated core (armature) – an important component in the stator – to the motor housing with the aid of a thermally conductive silicone resin. WACKER has developed the SILRES^® H 68 TC silicone resin precisely for this purpose. The resin is so viscous at 60 °C that it can be easily trickled into the small gap between the copper hairpin and the stator sheets. Another approach to heat transfer uses the route along the copper hairpin toward the winding heads lying on the front of the stator. WACKER has an appropriate product for this application, too, as explained by Dr. Ochs: “In this case, a thermally conductive elastic silicone encapsulant such as ELASTOSIL^® RT 744 TC can effectively support heat transfer.”

Other discussions involve whether the laminated core of the stator should feature small channels that coolant medium can pass through. The medium in this case could be a heat-resistant silicone fluid, such as POWERSIL^® Fluid TR 50, which has been used for many years now to cool transformers. In comparison to water-glycol mixtures used for cooling in modern combustion engines, silicone fluids have the advantages of not being electrically conductive and of remaining free-flowing right down to -50 °C. Optimal cooling would therefore be ensured even during a cold start in the depths of Nordic winters.

An electric motor maintains a high torque across the entire speed range. The maximum torque is already available at speed zero in contrast to conventionally operated vehicles with transmission where torque increases up to a specific speed. Even with a comparably moderate motor performance, the electric vehicle achieves enormous acceleration from a stationary position, while the driver of a diesel or gasoline vehicle has to tediously shift gears – electric vehicles mean more driving fun, but also more frustration at gas stations. Even with a Tesla DC charger, also known as a supercharger, drivers need to wait 30 to 40 minutes until the battery is 80 percent charged.

Automotive manufacturers and their suppliers are hoping that electric vehicles will gain in appeal once the principle of inductive charging becomes more widespread. Charging is far more convenient with this wireless technology: drivers simply position their vehicles over a charging panel set into the ground. Although charging takes longer in this case, the process can be implemented with standard AC voltage systems. As soon as the vehicle is correctly positioned with the help of an electronic driver assistant, charging starts automatically. Such charging panels can be installed not only in garages, but also in customer parking lots. What’s more, visionary concepts envisage these installations at traffic lights, railroad crossing barriers or along traffic routes in general so that even fairly short stopping times could be used for charging.

This charging technology uses the principle of magnetic induction: high-frequency alternating current flows through a coil in the charging panel. The current is generated from a stationary AC voltage network by means of power electronic components. A second coil is located under the vehicle floor. Once both coils lie on top of one another, an alternating voltage is generated in the second coil. This is rectified by the vehicle’s on-board power electronics to charge the traction battery. The base plate and vehicle also contain electronic components that are networked via WLAN and which control the entire charging procedure. The efficiency of this contactless charging is today almost as good as charging with a cable.

The charging electronics are encapsulated to protect them against weather conditions and mechanical stress. WACKER offers a comprehensive range of encapsulants, sealants and adhesives for this purpose. These either rapidly cure at room temperature or at moderate temperatures or form bonds with the substrates involved – extremely interesting and important for innovative industrial-scale production. “Short production times are possible when our silicone products are used to manufacture electronic components. And the electronics are perfectly protected for many years under such rough operating conditions,” underlined Klaus Angermaier. Automotive companies are currently working intensively on the enhancement of inductive charging systems.