# Structured Alumina Substrates for Environmental Catalysis Produced by Stereolithography
Measurements also showed that the polyhedral lattices have very similar light-off behavior in spite of the four times lower surface area. A comprehensive analysis of the heat transfer of such structures during transients as well as the comparison to benchmark honeycombs is presented by [PERSON] et al. [16]. It is clear that ceramic Additive Manufacturing plays nowadays a fundamental role in the realization of components such as the catalysts described above. The main benefits are that AM allows for the production of geometries designed and reproduced according to the final needs. In substitution of the conventional HC catalysts, to allow gases to follow tortuous paths, a network of solid struts (i.e., lattice) can be actually 3D printed. Since the introduction of the first AM technique in the late 1980s, several approaches were developed in recent years for the production of ceramic components. They can be subdivided according to the basic working principle: (i) laser-assisted sintering, where a high-energy source selectively fuses powdered particles: Selective Laser Sintering (SLS) and Laser Engineered Net Shaping (LENS); (ii) extrusion, where a material is continuously extruded through a nozzle: Fused Deposition of Ceramics (FDC) and Direct Ink writing (DIW); (iii)vat polymerization, where a liquid photopolymer in a vat is selectively cured by UV light: Stereolithography (SLA); and (iv) powder bed-based, where a liquid binding material is selectively deposited on a powder bed: Binder Jetting (BJ). In the mentioned techniques, the numerical model of the component, previously designed by a Computer Aided Design (CAD) software, has to be converted into a Standard Tessellation Language (STL) format. Then, the model is sliced into layers having a defined thickness to create a consecutive series of 2D images representing the cross sections of the component. The generated file is finally provided to the machine that builds the component layer by layer, by physically or chemically consolidating the feedstock material. Currently, vat polymerization technologies (i.e., Stereolithography, SLA) offer the best compromise between printing resolution (i.e., accuracy) and surface quality, allowing reproducing very thin details with geometrical tolerances in the order of a few tens of microns. SLA, if compared to other techniques, also allows building green components in a relatively short time, permitting to manufacture a layer in a few seconds. Only recently, regular structures were realized with SLA techniques in corderite by [PERSON] et al. [16], while [PERSON] et al. produced, with the same method, extremely thin and complex structures with reduced flaw size [17]. Further studies showed that the SLA technique is the most suitable for the production of ceramic substrates for catalysis. [18; 19; 20]. SLA employs an UV light beam that induces photopolymerization of a reactive suspension containing ceramic particles. These are dispersed in a suitable photopolymerizable resin in which a photoinitiator is dissolved. The three basic steps of the photopolymerization reaction are (i) initiation: the photoinitiator absorbs the photons and it cleaves to produce a free radical, (ii) propagation: the growing macro-radical becomes a cross linked gel at a relatively low degree of conversion, and (iii) termination: after time, termination starts to occur, mainly due to recombination and oxygen inhibition (see Figure 1). Once polymerized, the photopolymer constitutes a rigid matrix around the ceramic particles and confers the cohesion to the green body. The organic phase is subsequently removed by an appropriate thermal treatment at low temperature (i.e., debinding). Then, the sintering of the part at high temperature ensures the final properties of the ceramic piece [21; 22]. In the present work, 3D-printed alumina struts-based monoliths were produced via SLA.