Dear,

The concept of microstructure can be defined independently of the materials considered (metals, ceramics or plastics). Schatt and Worch define it as follows: "The concept of microstructure designates the overall conformation of a particle of matter, the orientation of which is, to a first approximation, homogeneous, with regard to its composition and the arrangement of its components. The microstructure is characterized by the shape, size, proportions and texture of the phases1.

The zones of the microstructure, called crystallites (grains, fillers or amorphous zones) are generally of microscopic size and can be characterized, both qualitatively and quantitatively, under an optical microscope.

The main reason for change in optical properties at nanoscale level is that nanoparticles are so small that electrons in them are not as much free to move as in case of bulk material. Due to this restricted movement of electrons, nanoparticles react differently with light as compared to bulk material.

Dear Saeed Khazaee, thanks for your question, or to be more precise your two questions because you have included two different printing technologies in your query: SLAS and SLS, and while they are related, there are relevant differences between them. SLA (Stereolithography) generally uses a liquid resin. This resin can be a polymer or a prepolymer that undergoes a photopolymerization process -a photochemical process- by the interaction of a light source, commonly a laser that describes a path in a plane of the liquid, normally at its free surface (interface polymer-air) or at the bottom of the container which holds the liquid resin. The 3D object is buit by displacing the plane where the light is focused (either moving the laser or the polimer container or the growing structure).

SLS means Selective Laser Sintering. Here the material we deal with to build a 3D object by aplication of a laser source is a powder. The process involved, unlike in SLA is a physical process, where the heat delivered by the laser on the powder particles starts to melt them, and because the laser contact with the powder is quite short the powder grains get bonded, fused together as they get cold, forming a continous body. This process only happens on the areas where the laser passed by, the rest of particles remain as powder. SLS machines normally have a powder bed and a level arm which removes the excess of powder on the surface of the bed to leave a flat surface, where the laser beam describe a path following a cut or slice of the 3D model. The process is repeated layer by layer to build the final 3D object. Finally the solid object is removed from the bed and the remaining powder can be reused for a new piece.

Let me go now with your question, -why we cannot go nanoscale with SLS?-

There are several reasons, the first one can be worked out from the SLS definition: it uses powders, and powders are normally not nanometric. So if you are binding particles in the range of micrometers you cannot get structures in the nm range. The second problem is related to the sintering method, in this case a laser, to get nanoscale structures the laser beam should be sharp enough to just write patterns with a nm size over a material small enough to register that nm resolution. You also should keep in mind that this is a physical process, where the laser is used to melt the powder, so you also have to consider the heat diffusion on your material, I mean, you could have a tightly focused laser, write your structure with the wished nano-resolution and when you check it under a SEM or TEM it would be thicker than you wanted because the heat spreaded and enlarged the area sintered.

Let me go with the laser focusing. Well, traditionally, optical resolution- the well you can focus a light beam- was limited by diffraction, so even with the more perfect lenses you cannot go down a certain resolution - the size of your laser spot- (it is roughly speaking around half the wavelength of your laser source, so imagine you are using a green laser at 532 nm, at best you could get a resolution around 266 nm. Remember, that would be just a theoretical value, because if your powder particles have a size of say 10 or 20 micrometers their size will be what define resolution in your 3D object.

The foscusing problem would be the same for SLA, but here you have a liquid as precursor, so the resolution is not dependent on particle size. Here the laser light is the trigger for a chemical reaction, directly over the resin or over a photocatalist. Once more you must have into account the effects of diffusion. If light is creating free radicals they can migrate beyond the laser path and so your structure would be thicker than your laser beam.

There are several ways to overcome these problems, for example:

SLS: you can use a tiny source of heat for writing your layers on a nanopowder. Usually that tiny sourfce of heat is an AFM head electrically heated.

SLA: you can use the same idea but with light, so you can use a hollow AFM probe with a nanometric aperture at its tip from where you send the light. It is called a Scanning Near-field Optical Microscope (SNOM). Another beautiful and amazing idea consist in using some super-resolution microscopy technique to focus your light below the diffraction limit.

Note that these kind of systems are already available as commercial devices.

Hope it helps.

My best wishes and greetings for the New Year.

In addition to the good points of the previous experts, another important limitation of using nanopowders for SLS applications is the poor flow properties. SLS is not only about the printing process, but deposition of a dense powder layer is essential for the quality of the final built part.

Powder flow is, among other parameters, highly influenced by the particle size and size distribution. A high surface area in the powder leads to a higher concentration of interactions among particles, which plays in detriment of the flow and leads to powder "caking". Thus, deposited nanopowder layers are normally of poor quality. Some additives can be used to limit the interactions between particles and enhance the powder flow (e.g. fumed silica or magnesium stearate), but those are effective only if they can be deposited onto the surface of the particles, which would not be possible with nanosized powders due to the similarities in size.