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1. Why use porous metal to make filter ?
Porous metal is a material with a unique structure that contains interconnected pores or voids within its solid framework. The pore size and distribution can be tailored to meet specific application requirements, making it a versatile material for a range of industries.
So as those special function, Porous metals are commonly used in the fabrication of filters due to several reasons:
1. Controlled Pore Size: Porous metals can be engineered to have very precise pore sizes. This allows for the creation of filters with specific filtering capabilities, such as removing particles of a certain size.
2. High Strength: Metal filters have high mechanical strength, which makes them robust and durable. They can withstand high pressure and temperature conditions that might damage other types of filters.
3. Chemical Resistance: Metals are often resistant to a variety of chemicals, making them suitable for use in environments where they might be exposed to corrosive substances.
4. Reusability: Metal filters can be cleaned and reused, which is both cost-effective and environmentally friendly.
5. Thermal Stability: Metallic filters can sustain in high-temperature applications, which might not be the case with filters made from polymer materials.
6. Permeability: Due to their porous nature, these materials permit a high degree of fluid flow while efficiently capturing and retaining particles.
7. Backwash Capability: Metal filters can be backwashed to remove trapped particles, which allows for the recovery of valuable materials and prolongs the lifespan of the filter.
Hence, depending upon the application and the type of fluid that needs to be filtered, porous metal filters can be an excellent choice.
2. How is porous metal made?
Porous metal is typically produced through a process called sintering, which involves heating metal powder to a temperature just below its melting point, allowing the particles to fuse together while maintaining the desired porosity.
The process of making porous metals involves creating voids or pores within the metal. There are several methods used to achieve this, including powder metallurgy, sintering, and additive manufacturing. Below is a simplified description of a common method, powder metallurgy:
1. Metal Powder Production: The first step in creating porous metal is to produce metal powder. This can be done in several ways, including atomization (spraying a stream of molten metal into a chamber where it solidifies into powder) or mechanical milling.
2. Mixing and Compacting: The metal powder is mixed with a binding agent or space holder material to help maintain the structure during processing. The mixture is then compacted under high pressure in a die to form a "green" compact. The shape of the die will determine the final shape of the porous metal part.
3. Sintering: The green compact is then heated in a furnace to a temperature below the melting point of the metal. This process, known as sintering, causes the metal particles to bond together. The high temperature also causes the binder or space holder material to burn off or evaporate, leaving behind pores.
4. Cooling and Finishing: After sintering, the metal part is allowed to cool, then it may undergo additional processes like finishing or coating to improve its surface characteristics.
An alternative approach is using additive manufacturing (commonly known as 3D printing), where a metal powder is selectively melted layer by layer based on a digital model. This can create complex shapes and internal pore structures that would be difficult or impossible to achieve with traditional methods.
The pore size, distribution, and connectivity can be controlled to a large extent during the manufacturing process, which makes porous metals suitable for a wide range of applications, including filtration.
3. What are the advantages of porous metals?
The advantages of porous metals include high surface area-to-volume ratio, mechanical strength, thermal and electrical conductivity, and the ability to tailor pore size and distribution. These properties make it useful in applications such as catalysis, filtration, and energy storage.
4. What are the limitations of porous metals?
Porous metals may have lower bulk strength compared to non-porous metals due to the presence of voids within the material. Additionally, the manufacturing process can be complex and costly.
5. What is the difference between open-cell and closed-cell porous metals?
Open-cell porous metals have interconnected pores that are accessible from the surface of the material, while closed-cell porous metals have sealed pores that are not accessible from the surface.
6. What types of metals can be used to make porous metals?
Porous metals can be made from a range of metals, including aluminum, titanium, nickel, copper, and stainless steel, among others.
7. What are the applications of porous metals?
Porous metals have applications in industries such as aerospace, biomedical engineering, chemical processing, and energy storage, among others.
8. What are the challenges associated with manufacturing porous metals?
The challenges associated with manufacturing porous metals include maintaining the desired porosity, ensuring good mechanical properties, and controlling the pore size and distribution.
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9. What is the porosity of porous metals?
The porosity of porous metals can range from a few percent up to 90% or higher, depending on the application requirements.
10. What is the significance of pore size and distribution in porous metals?
The pore size and distribution in porous metals are critical to determining the material's properties, such as permeability, mechanical strength, and surface area. This is because pore size affects how easily fluids can flow through the material and how much surface area is available for reactions to occur.
11. Can porous metals be customized for specific applications?
Yes, porous metals can be customized for specific applications by adjusting the pore size and distribution, as well as the type of metal used.
12. What is the lifespan of porous metals?
The lifespan of porous metals depends on the application and the specific material used. Generally, porous metals have a long lifespan due to their high durability and resistance to corrosion.
13. Can porous metals be recycled?
Yes, porous metals can be recycled by melting the material down and reusing it in new applications.
14. Are porous metals safe for use in biomedical applications?
Certain types of porous metals, such as titanium and tantalum, are biocompatible and can be used safely in biomedical applications. The porous structure can encourage bone growth and improve integration with surrounding tissue.
15. How can porous metals be tested for their properties?
Porous metals can be tested for properties such as porosity, permeability, and mechanical strength using techniques such as scanning electron microscopy (SEM), gas permeability tests, and compression tests.
Most commonly, implants are fixed in place with the help of bone cement or metal screws. However, the major drawback to bone cement is fragmentation which can result in foreign body response to the released debris; this mode of implant fixation usually leads to periprosthetic osteolysis, early aseptic loosening, and failure of the implant [48]. For quicker healing after implantation, osseointegration is critical. Osseointegration, meaning, bone tissue ingrowth into the implant surface. The correlation between the morphology and size of the porous surface and the strength of fixation with the surrounding tissue has also been determined [49]. Many studies have concluded that open porosity improves implant wettability and aids bodily fluids flow, thus improving osseointegration when >100 μm pore size is present [50]. The porosity is either homogeneously or non-homogeneously distributed based on the processing method. Discussed are several fabrication methods for producing open-cell porous implants or coatings.
Sintering is one of the oldest and most evolved techniques in powder metallurgy for producing density-controlled materials. The basic concept of the process is to prepare powder (metal/ceramics) by compacting and binding powdered raw material and providing thermal energy for the compacted powder for densification and grain growth. In this process, the powder particles bond at high temperatures with minor changes to the initial powder particle shape. Binders hold the powder particles together, providing enough area for mass transport during solid-state diffusion. Due to this technique’s highly evolved nature, many studies on porous orthopedic implant structures have employed sintering or modified versions of sintering for preparing porous metal structures.
The porosity of the sintered structures can be controlled by tailoring the shape and size of the metal powder, the compaction pressure, and the temperature and time of sintering. It was observed that the compaction pressure and the sintering temperature significantly impact the microstructural and mechanical properties of the porous sintered metals. In general, it was observed that sintered Ti compacts at 1173 K, 1373 K, and 1573 K at no applied pressure; as the temperature increased, the porosity decreased. However, the porosity remained more significant than 30% for each temperature, and almost 100% was observed to be open porosity. Moreover, the effect of different applied pressures at different sintering temperatures has also been studied, and it was observed that the porosity decreases considerably (∼19%). However, most of the porosity still was found to be open porosity. The scanning electron microscope (SEM) images of the microstructure of Ti compacts sintered at 1173 K, 1373 K, and 1573 K with an applied pressure of 10 MPa can be seen in figures (a)–(c), respectively [51].
Additional investigations produced porous functionally graded Ti using the sintering method by stacking layers of powders with varying particle sizes and the volume fraction of the additive (silicon), i.e. the low volume fraction of additive with finer powder (20% with 45 μm) to the high-volume fraction of additive with coarser powder (45% with 200 μm). The cross-sectional microstructure of a functionally graded porous Ti structure prepared in this manner is shown in figure (d) [45].
Another alternative form of porous surface characteristics to implant the structure’s dense core via sintering is metal fibers instead of metal powder. This method has been investigated for both stainless steel and Ti fibers, and the procedure used to make such porous coatings is similar to the powder metal sintering process [53–56]. The metal fibers are laid complying with the form of the implant structure, compacted, and then sintered at high temperatures. The solid-state diffusion process forms a fully interconnected porous coating at each point of contact of the fibers [57]. However, the main drawback of this process is that compacting the metal fibers to the form of the implant structure is challenging and time-consuming. Moreover, the interfacial bond between the dense implant core and the fiber mesh coating depends on the complexity of the contours of the implant structure [53]. Even though the sintering method of building porous structures is relatively mature, several limitations exist. A most relevant limitation is that particle oxidation could inhibit the proper bonding of the particles because it is a high-temperature operation. Further, solid-state diffusion bonding of particles usually results in the neck formation comprised of brittle phases and might result in lower mechanical toughness and fatigue resistance. Moreover, the pore size and morphology are usually irregular and largely dependent on the particle size and shape. However, these limitations could be substantially improved upon by using appropriate sintering techniques [13].
Many modified sintering techniques have been investigated to produce porous metallic structures with improved porosity and controlled pore morphology, including the space holder, the spark plasma sintering (SPS), and the replication methods. In the space holder method, several investigations have used carbamide particles as the space holder in preparing porous Ti and porous Ti6Al4V alloys. Carbamide has been chosen in the space holder method because of its ideal spherical particle geometry and chemical properties, such as ease of removal before sintering [58, 59]. A properly sieved and sized mixture of Ti or Ti6Al4V with carbamide was weighed and compacted under pressure. The compaction was then heat treated so that the carbamide particles dissociated at lower temperatures (∼193 °C), and the dissociated by-products were expelled by either using a vacuum furnace or by the continued flow of argon. Thereby, the heat treatment cycle was typically followed where the compact was first heated up to 100 °C, and then slower heating rates were used up to 500 °C to ensure enough time for most of the carbamide particles to dissociate, and the consequent by-products were expelled. Following this, much faster heating rates could be used up to the sintering temperature at which the compaction was held for a considerable time, depending on the size of the compaction being sintered [58]. In this method, the size, shape of the pores, and porosity could be controlled primarily by controlling the volume fraction and the shape of the space holder particles. Other parameters that determine the porosity and the pore morphology are the compaction pressure and the holding time during sintering. While the compaction pressure applied to prepare the pre-sintered compact varied between investigations, the dependence on the mean porosity and mechanical properties (such as yield strength) and the compaction pressure seemed similar, i.e. as the compaction pressure increased, the mean porosity decreased. Also, it was observed among several investigations that the porosities that could be produced by this method ranged between 55% and 75%. It was observed that most of the pores were considered to possess consecutive and open cell morphology, with the sizes of the majority of these pores (<700 μm) being less than the largest space holder particle size (∼700 μm). The pores with larger sizes were observed to be the consequence of pore coalescence. Furthermore, the pore walls’ thickness is believed to significantly impact the porous structure’s mechanical strength. Thereby, it has been observed that, while the pore wall thickness of the structure produced by this method is in the same range as any other powder metallurgy process (i.e. ∼100–200 μm; compared with Berger’s report), interconnected angular-shaped micropores were observed along the pore walls suggesting that the sintering process was incomplete because of low diffusivity. While these micropores were considered beneficial in improving the interconnectivity of the pores, they might significantly deteriorate the structure’s mechanical properties. Different space holders or compaction techniques investigated other variants of this process [60, 61].
Another variant of the sintering technique is the SPS method. SPS or field assisted sintering technique (FAST) is a process similar to hot pressing where the heat required to sinter the powder particles in a compact is provided by joule heating due to the current flowing within the compact [62]. The general working principle of SPS, as indicated by figure (a), is that the sintering powder is compacted within a graphite die and DC voltage pulses pass through the die, and the compact (in the case of conductive sintering powder) produces joule heating, with heating rates up to 1000 °C·min−1, cooling rates of up to 400 °C·min−1 and maximum temperature of 2400 °C, sintering conditions could be facilitated by appropriately controlling the pulse voltages and durations [63]. Furthermore, this process could be controlled by controlling the die’s measured temperature, power, or current. The rapid heating and cooling rates make this one of the fastest sintering processes, thereby ensuring limited grain growth in the sintered metal [64].
The powder mixture is produced similarly to the space holder technique to produce porous metal structures using the SPS technique. The metal or alloy powder is mixed with varying volume fractions of space holder constituents such as NaCl or NH4HCO3. Then, the powder mixture is cold compacted under pressure to produce green compacts, sintered using SPS in a specially designed graphite die, as shown in figure (b) [65]. In the case of the study, where NaCl was used as an additive, post-sintering dissolution of NaCl in deionized water produced porous Ti6Al4V structures [66]. And in the case of the study where NH4HCO3 was used as an additive, it dissociates into NH3, H2O, and CO2 and is expelled during the sintering process and thus produces porous Ti structures [65]. In both studies, the x-ray diffraction (XRD) analysis of the porous structure showed negligible impurity content indicating that the space holders used were eliminated during the process. Also, it was observed that the majority phase appears to be α-Ti in the post-sintered structure, and the grains developed in the sintered porous structure are mostly fine. This indicates that the SPS process offers extremely fast heating and cooling rates.
The porosity, pore size, and morphology of the porous structures produced by the SPS method showed similarity with the previously discussed space holder method. In general, two types of pores were observed: Macro pores caused due to the dissociation of the space holder, and the micropores, within the pore walls of the macro pores, caused because of the incomplete sintering between adjacent metal particles (as shown in figures (c) and (e) in both the studies). Both studies also observed that as the sintering temperature was increased or by heat treatment post-sintering, the pore walls became thicker, and the porosity reduced (as shown in figures (d) and (f) in both studies). Also, both studies produced 40%–70% porosities; most pores were well interconnected.
Finally, the replication method’s third variant of the sintering method is discussed here. This process is typically three-step and has traditionally been used to prepare porous ceramics [13]. However, some investigations have used this method to produce porous Ti and Ti alloys. This method immersed polyurethane foams in a slurry and then rapidly dried for the metal powder to positively maintain the polyurethane foam replica. The slurry comprises 70% by weight Ti6Al4V and 20% by weight H2O and ammonia solution, where the ammonia solution has been added to improve the flow properties of the slurry. After repeating this process multiple times till the polyurethane foam struts were coated entirely with the Ti6Al4V powder, the polyurethane foam and binder were removed thermally, and the remaining Ti6Al4V powder arrangement was sintered, forming an open-cell reticulated Ti6Al4V foam [67]. This three-step process’s schematic is shown in figure (a) [13].
As expected, in this process, the flowability of the slurry, controlled via particle size distribution, binder chemistry, the pH of the slurry, air bubble quantity, and the solid–liquid ratio, determines the quality of foam produced by this process [68]. By this process, Ti alloy foams of 88% primarily open porosity have been attained, and the pore morphology could be observed in the SEM image shown in figure (b) [67]. The pores formed are chiefly found to be of three types: the primary porosity, comprising of smaller pores on the strut surface; secondary porosity, medium-sized pores formed at the core of the hollow strut formed by previously occupied polyurethane foam; and the tertiary porosity, larger open pores between the struts. A subsequent study observed that a second coating of the powder slurry and second sintering on a previously sintered foam enhanced the density and mechanical properties of the Ti or Ti alloy foams [69].
Even though sintering is a very mature metallurgical process, up to 80% of porosities could be achieved with slight modification. However, the process has inherent limitations. Sintered porous metallic structures are susceptible to brittle fracture and low fatigue resistance. Since, in this process, the metal particles bond via a solid-state diffusion process, the neck formed between metal particles after sintering is usually quite brittle. Also, the sintering process usually produces non-homogeneous pore distribution and pore morphology.
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