of functional complexity [7].
The hierarchical complexity involves the fabrication of features with shape complexity across multiple size scales. This multiscale approach can cover up to five orders of magnitude for some of the available 3D printing techniques. For instance, SLA printing scale ranges from tens of micrometres to almost one meter, which allows including small details in high aspect ratio printed parts. This feature is extremely relevant for many applications like energy where the performance of devices is typically proportional to the active area such as in chemical reactors or electrochemical cells.
A high level of material complexity can also be introduced in printed pieces by processing in a different way at different points of the part, for example, introducing different levels of porosity all along the object by changing the laser parameters in SLA or the gray level in inkjet printing. This material complexity can also refer to a more advanced multi‐material printing. This has been traditionally employed in inkjet implementing a matrix of printheads for colorful printing but it is still under development for other types of technologies. This approach allows the deposition of different materials or, eventually, materials of different nature in a single process. The multi‐material capability enables the fabrication of graded compositions or, ultimately, complete devices. Independently on the printing deposition technique, one of the unsolved critical issues in processing of multi‐material parts is the quality of the interfaces between dissimilar materials [8, 9]. This represents one of the most interesting fields of current research on 3D printing.
Finally, 3D printing is able to lend a high level of functional complexity to the parts by design or by direct implementation of functional materials. By design, one can easily imagine the fabrication of specific shapes with certain functionality such as plasmonic patterns or cantilevers for energy harvesting as well as the deposition of thin layers acting as antireflective or water‐proof coatings for photovoltaics. By using functional materials, the conceptual design of 3D printed parts with high level of functional complexity is even more straightforward since the printed material has specific functionalities by itself. Despite the enormous potential of this approach, its implementation is very much limited by the short list of available advanced materials for printing uses [8]. Just to give an example of the low availability of printable functional materials, Table I.1 shows a comprehensive compilation of most of the advanced oxides reported so far (beyond structural materials or bioceramics).
Table I.1 Functional ceramics processed in the past with additive manufacturing technologies
Source: Based on Mueller et al. [8]. © 2017 John Wiley & Sons.
| AM technique | Inkjet | SLS | SLA | FDM | LOM |
|---|---|---|---|---|---|
| Functional ceramic | BaTiO3 PZT TiO2 LSMO/YBCO YSZ/CGO SDC/SSC BaTiO3 [10] BaTiO3/P(VDF‐TrFE) [11] ZnO ink [12] | PZTBaTiO3 SiCN YSZ | PZTFe2O3/Fe(C2O4) SiCN BaTiO3/photo‐resin [2] PZT@Ag/photo‐resin [13] ZnO/photo‐resin [14] YSZ [15, 16] | BaTiO3 PZT, PMN LiFePO4/Li4Ti5O12 BaZrO3, SrTiO3 BaMn2Al10O19−x ITO, ZnO La(Mg0.5, Ti0.5)O3 Zr0.8Sn0.2TiO4 TiO2 BaTiO3/PVA paste [15] | SiO2‐Al2O3‐RO‐glass LZSA‐Glass PZT |
This migration from structural to functional materials will result in the fabrication of advanced devices with high value added, which will extend the markets where 3D printing is applied from prototyping to manufacturing. Up to now, most of the 3D printing techniques have been developed and commercialized for the fabrication of polymeric and metallic structural parts but recently the focus has moved to the production of functional‐quality components made of advanced materials including, for instance, composites, ceramics, and nanomaterials.
In the case of composites, enhanced properties are expected from the fabrication of complex shapes using inorganic‐polymer matrix based materials [8]. The most evident application is probably the printing of fibre‐reinforced polymeric composites for improving mechanical properties of structural parts [17] but also other applications in which the inorganic loading has functional properties are envisaged, for example, 3D‐printed dielectric/plastic composites [18]. Beyond polymer‐based materials, it is of great interest the printing of metal–ceramic or ceramic–ceramic composites since they can have a strong impact in strategic fields such as electronics or energy [19].
The relevance of the recent progress on 3D printing of ceramics lies in the broadest spectrum of functional properties of this type of materials compared to all other classes, such as metals or polymers. The unique functional properties of advanced ceramics (electrical, optical, or magnetic) make them of critical importance to face upcoming technological challenges especially in the fields of electronics, information and communication technologies (ICT), and energy and environment. In this regard, recent advances in printing piezoelectric materials [20], dielectrics [21] or ionic conductors [22, 23] represent the beginning of a big revolution.
The use of nanomaterials for 3D printing will also bring interesting advantages, especially if they are functional nanomaterials, because they will enable the increase of complexity in shape (fine surface finishing, high vertical resolution, or improved layer assembly), hierarchy (multi‐length‐scale structures or graded porosity), materials (low sintering temperature in ceramics and metals or multi‐material deposition in suspension), and functionality (use of nanocomposites or nanomaterials with high surface area or core‐shell structures).
I.3 3D Printing for Energy
The possibility of printing functional materials with applications in energy has been attracting a growing attention since 3D printing technologies represent a new paradigm for the manufacture of energy conversion and storage technologies [13]. Among other advantages, additive manufacturing offers unique capabilities for increasing the specific performance per unit mass and volume of energy devices by implementing high levels of hierarchical and shape complexity. While the implementation of multiscale features can be of great interest for chemical reactors or batteries, a high level of shape flexibility becomes crucial in harvesting applications where the efficiency of the generators strongly depends on their capability to properly couple to variable scenarios, for example, for a good adaptability of thermoelectric generators to the source of waste heat. Complementary, the opportunity to implement fully controlled graded compositions or tuneable porosity, that is, a certain level of materials complexity, also represents a big advantage for those energy devices with multiple interfaces or porous electrodes, such are fuel cells, gas separation membranes, or electrochemical capacitors. Despite this potential, the fabrication of highly complex devices for the energy sector by using 3D printing is just an emerging field [17]. This is probably due to the complexity of the devices usually employed. However, since the complexity adds cost to traditional processes, the more complex the final part, the more likely that AM will be of benefit for the sector [16].
This emerging application of 3D printing already end up with remarkable examples of the fabrication of components for solid oxide fuel cells [14, 24, 25], batteries [15, 26], or photovoltaic systems [27, 28]. Despite most of the existing examples in energy applications correspond to low‐aspect ratio deposition of functional layers (mainly by inkjet), increasing activity is devoted to proper 3D printing of complex shape multi‐material parts and devices. In this direction, the most inspiring examples were reported by Kim et al. [21] and Pesce et al. [29] who were able to fabricate high‐aspect ratio interdigitated Li‐ion microbatteries by inkjet printing of concentrated viscoelastic inks of