Porous metal materials, characterized by their porous structures, are innovative engineering materials that offer impressive strength while being light. These materials are used across different industries, including aerospace, metallurgy, mechanics, petrochemicals, energy, pharmaceuticals, architecture, and transportation. Their unique properties make them suitable for specialized applications, such as in life support systems, energy storage, hydrogen generation, and filtration systems.
Porous metal materials can be categorized into three types:
- Metal Foams. Metal foams are lightweight cellular structures composed of a solid base metal with gas-filled pores, inspired by natural materials like wood, bones, and sea sponges. This design gives metal foams high strength-to-weight ratios and excellent energy absorption properties, making them ideal for use in diverse industries such as aerospace and automotive applications.
- Sintered Metal Powder. Sintered metal powder is a porous material produced by sintering, a process wherein the metallic powder is compressed and then heated at temperatures below its melting point. Sintering causes the particles to bond into a solid piece with small pores. Typically, sintered metal powders have a high solid volume fraction, ranging from 0.35 to 0.65; thus, this type of porous material is commonly used in applications where good mechanical strength is required.
- Sintered Fiber Felts. Advancements in fiber-pullout techniques have led to the development of sintered metal fiber felt, a non-woven, porous material made of long metallic fibers typically over 1.5 µm in diameter. These fiber felts are used in structural applications, such as the core of sandwich panels, as well as in functional applications like anodic gas diffusion layers, catalyst supports, and filtration nets.
Manufacturing Sintered Metal Powders vs. Sintered Metal Fiber Felts
Unlike metal foams, which are typically created through a foaming process that introduces gas into metallic melts, sintered metal powders and fiber felts are formed by sintering compacted powders or laminated fibers, respectively.
Sintering is the key step in the production of both sintered metal powders and sintered metal fibers. It is a manufacturing process where metal raw materials are heated to high temperatures, just below their melting point. This causes the particles to fuse together, forming a solid mass. During sintering, several changes occur in the metal powder particles, improving various properties like strength, ductility, corrosion resistance, conductivity, and magnetic permeability.
These changes are vital for different applications, as they determine the material’s porosity, strength, and overall performance. Therefore, sintering plays a major role in achieving the properties needed for specific uses.
Sintering Mechanism in Metal Powders
The different sintering stages show how loose metal powders transform into a solid object:
At the initial stage of sintering, particles start sticking together due to weak forces, like van der Waals’ forces. At higher temperatures, they rearrange and pack together, sometimes rotating and twisting to achieve lower energy states in terms of their arrangement.
In the next stage, a sinter bond begins to form between the contact points established earlier, forming necks between particles. This occurs because atoms move from the surface of the particles to the contact points, reducing the surface area and forming interparticle bonds. This process, driven by surface transport mechanisms, strengthens the connections between particles but does not reduce the distance between them. Although this marks the start of sintering, there is little to no significant densification yet.
This stage is followed by the intermediate stage, where neck growth occurs, leading to densification. During this phase, the necks lose their distinct identities, and the pores become rounded but remain interconnected. At this point, the centers of the two spheres start to get closer, resulting in shrinkage. Bulk transport mechanisms predominate, facilitating neck growth and the elimination of pores.
In the final stage of sintering, interconnected open pores close and turn into isolated closed pores. As this happens, grain growth occurs, which slows down the surface and bulk diffusion processes. Consequently, this stage becomes the slowest, as densification increases from 95% to 99%.
(SD: Surface Diffusion, VD: Vacancy Diffusion, GB: Grain Boundary Diffusion)
Sintering Mechanism in Metal Fiber Felts
Throughout the sintering process of fiber felts, several transformations occur, such as the development of necks between fibers, the enlargement of grains within fibers, and changes in porosity. Similar to powder sintering, sintering of fiber metal felts involves six modes of material transport. Three of these modes lead to sintering without densification: vapor transport, surface diffusion, and lattice diffusion from the surface.
Conversely, the other three modes lead to densification: boundary diffusion, lattice diffusion from the grain boundary, and lattice diffusion from dislocation. To predict the sintering conditions necessary to achieve desired properties, sintering diagrams have been developed for different powders and wires. Originally, these diagrams were based on simple models, like the two-sphere model, which worked well for powders and wires. However, fiber felts, with their complex geometry, require a different approach.
Unlike in powders, where sintering occurs between particles bonded by van der Waals forces, sintering in fiber felts takes place in the joints between adjacent fibers at random angles. During the pressing or shaping of fibers, sintering joints primarily develop at points where fibers make contact. Under pressure, fibers interlock, forming many contact areas. These contact regions can be categorized as either fiber-to-fiber contact joints or fiber-to-fiber mechanical meshing.
During sintering, material migrates in fiber-to-fiber contact joints or mechanical meshing to reduce surface energy. Initially, sintering begins on microstructures’ surfaces, forming contact points between fibers, which then strengthen. This process continues across the fiber network, forming a mesh-like structure. In comparison to sintering powders, sintering metal fiber felts undergo less densification. This is because surface processes, grain growth, and neck growth mechanisms dominate over densification processes like grain boundary diffusion.
Difference between Sintered Metal Powder and Sintered Metal Fiber Felt
(Left) Powder and (Right) Fiber Felt [3]
Compared to sintered metal powder, sintered metal fiber felts are less dense, resulting in higher porosity and permeability. Sintered metal powders typically have porosities lower than 50%, whereas sintered metal fibers can achieve porosities higher than 50%.
For instance, sintered metal fiber felts can have porosities as high as 98%, with pore sizes smaller than 10 µm. In the case of LINQCELL sintered titanium fiber felts, our products boast a tailored porosity level ranging from 50% to 80%. Additionally, sintered metal fiber felts exhibit a three-dimensional reticulated structure. This structure not only provides well-defined conductive paths but also offers controlled electrical conductivity-temperature characteristics. The high porosity and decreased electrical resistivity due to the rupture of joint fiber contacts after sintering make sintered metal fiber felt an excellent material for applications such as water electrolyzers and fuel cells.
| Product | Porosity (%) | Thickness (um)* | Basis Weight (g) | Actual weight (g)** |
| LINQCELL TFP250 | 50–60 | 250 | 45.5 | 18.3–22.6 |
| LINQCELL TFP250S | 60–70 | 250 | 45.5 | 13.7–18.1 |
| LINQCELL TFP400 | 50–60 | 400 | 72.8 | 29–36.3 |
| LINQCELL TFP400S | 60–70 | 400 | 72.8 | 22–29 |
| LINQCELL TFP600 | 50–60 | 600 | 109.3 | 44–54.5 |
| LINQCELL TFP600S | 60–70 | 600 | 109.3 | 32.8–43.6 |
| LINQCELL TFP800 | 50–60 | 800 | 145.7 | 58.3–72.9 |
| LINQCELL TFP800S | 60–70 | 800 | 145.7 | 43.7–58.2 |
Conclusion
Having said that… should we prefer sintered metal fiber felts over sintered metal powders? The answer lies within your target application. For example, water electrolyzer manufacturing typically can work with porous transport layers with porosities as low as 40% (something achievable using sintered metal powders). In contrast, fuel cells require gas diffusion layers with high porosities, and so sintered metal fibers will be more appropriate in that case.
At CAPLINQ, we offer a wide range of sintered metal fibers including nickel, stainless steel, and titanium, available in various dimensions and porosities. Additionally, we provide Pt coating to ensure optimal performance in specific water electrolysis and fuel cell configurations. If your application demands sintered metal powder, we can supply that as well. For more information or to discuss your specific needs, contact us.
