The performance of silicone-based pastes is of course also heavily dependent on their fillers, which determine their thermal, physical and chemical properties. I now divide the whole thing into three groups again, although there can of course also be special hybrid cases.
Metal-based fillers
Advantages:
Metals such as silver, copper and aluminum offer excellent thermal conductivity, which leads to more effective heat dissipation. Metal-based fillers retain their thermal properties even at high operating temperatures. A special case is the so-called liquid metal (LM)
Disadvantages:
Most metals are electrically conductive, which poses a risk of short circuits if the paste gets onto electrical contacts. The effort required for application is much greater than usual. Some metals can also corrode, especially in humid environments, which can degrade the performance of the paste over time. I’ll write a little more about liquid metal paste in a moment, as we need to go into a little more detail here.
Ceramic-based fillers
Advantages:
Ceramic fillers such as boron nitride, aluminum oxide and zinc oxide are electrically insulating, making them safe for applications where electrical insulation is required. They are also chemically inert and do not react with other materials or environmental conditions. Ergo, they offer high chemical stability.
Disadvantages:
Although they offer good thermal conductivity, they do not achieve the values of metal-based fillers. High-quality ceramic fillers can be more expensive than some metal-based alternatives, but they don’t have to be.
Corundum (aluminum oxide, Al2O3) and zinc oxide (ZnO) are both ceramic materials that are used as fillers in thermal conductive pastes to improve thermal conductivity. Both have their specific properties and advantages, but their thermal conductivities differ significantly. The thermal conductivity of corundum is typically in the range of 20-30 W/mK, although it can vary depending on purity and processing. Corundum is a very hard material with good thermal conductivity and excellent chemical stability. It is also electrically insulating. Due to its higher thermal conductivity, corundum is often used in high-performance thermal conductive pastes, especially where efficient heat dissipation is required.
Zinc oxide has a lower thermal conductivity compared to corundum, typically in the range of 2-5 W/mK. ZnO is also an electrically insulating material and is often chosen because of its availability and lower cost. Zinc oxide is therefore very often used in standard thermal conductive pastes where moderate thermal conductivity is sufficient and cost is an important factor. In practice, the choice between corundum and zinc oxide in a thermal compound is made on the basis of specific requirements for thermal conductivity, electrical properties, cost and other factors. In many cases, blends of both materials can also be used to achieve an optimal balance between performance and cost.
Carbon-based fillers
Advantages:
Carbon materials such as graphene and carbon nanotubes offer very high thermal conductivity and electrical insulation at the same time. These materials can be produced in different structures to meet specific requirements. This makes them extremely flexible to use.
Disadvantages:
The production of high-purity carbon materials can be expensive. Dispersion in the matrix is also a sticking point. The uniform distribution of carbon materials in the carrier matrix can be very challenging, which can affect the consistency of the thermal conductivity.
By combining different fillers, manufacturers can take advantage of each material to achieve an optimized balance between thermal conductivity, electrical insulation and cost. The development of such a paste containing different fillers requires extensive research and testing to ensure compatibility and performance. The so-called graphite pads, which have been promoted by marketing for many years, occupy a special position here.
Croy-compatible fillers
The choice of fillers plays a decisive role in the functionality of thermally conductive pastes at ultra-low temperatures. Fillers such as silver, aluminum nitride or boron nitride improve the thermal conductivity of silicone pastes. For use at ultra-low temperatures, however, it is important that these fillers do not undergo significant contraction or expansion, which could lead to microcracks or delamination. Fillers with similar thermal expansion coefficients as the silicone matrix material are preferable in order to minimize mechanical stresses during extreme temperature fluctuations. There are also various additives to improve cryogenic resistance.
- 1 - The three big P's - introduction to pastes, pads and putty
- 2 - The purpose of thermal pastes
- 3 - The big debate between cheap and expensive
- 4 - The matrix as the basis for all pastes and pads
- 5 - Silicone-based pastes: optimization, durability, decomposition
- 6 - Thermally conductive fillers are important
- 7 - How the degree of grinding influences performance
- 8 - Silicone modification for low temperatures and LN2 overclocking
- 9 - The paste production process and possible hurdles
- 10 - Special case liquid metal (LM)
- 11 - Special case of graphite pads and phase changers
- 12 - Temperature window, expansion behavior, application
- 13 - Ageing and decomposition of pastes and pads
- 14 - Manufacturer vs. bottler, misleading marketing and conclusion
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