Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Abstract

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.

Introduction

The increasing problem of CO₂ emissions and energy security concerns such as nuclear safety, radioactive waste management and resource dependence, have given interest in alternative sources of energy. Solar energy is unlimited and clean, and concentrated solar power (CSP) using optical concentration is a particularly good candidate for providing a clean and renewable source of energy [1,2]. In CSP systems, the solar radiation is converted into heat by a solar receiver, which is passed through a heat transfer fluid. Volumetric solar receivers (VSR) are probably the most efficient variety of solar receivers. These macroporous cellular ceramics can achieve particularly high energy efficiency thanks to the so-called volumetric effect, characterized by (i) a temperature of the solid that is higher at the exit than at the entrance and (ii) a temperature of the fluid that reaches that of the solid at the exit [[3], [4], [5]]. The volumetric effect can be attained only by finding the best compromise in terms of pore volume geometry (e. g. cell shape and size) and material properties (e. g. optical selectivity, mechanical strength/stiffness, thermal conductivity/expansion, oxidation resistance). The pore volume geometry of VSR is currently very limited, mostly due to fabrication constrains. Two major types can be found: extruded structures such as honeycombs [6,7] and open foams made by replication [[7], [8], [9], [10], [11]]. For both categories, the geometrical pattern, the cell size and the open porosity are not easily adjustable, so a satisfactory compromise is difficult to find. Another type of cellular ceramics is 3D lattice structures. These synthetic structures are often periodic (yet not necessarily: [12]), but non-extrudable. Provided they can be effectively fabricated, they could be designed specifically to demonstrate the volumetric effect [13]. In order to improve the performance of VSR, it would be then beneficial to turn to a manufacturing method that is able to generate any morphology and especially those likely to demonstrate the volumetric effect. The rapid growth of 3D printing in recent years has led us to consider this technology as a practical solution.

The basic principle of 3D printing –or additive manufacturing– is to generate a 3D computer-assisted design (CAD) model to directly manufacture a three-dimensional object layers by layers. Technologies such as selective laser sintering, fused deposition modeling and stereolithography were initially developed for polymer materials [14]. They were successfully adapted to 3D ceramic parts only a few years later [[15], [16], [17]]. Binder jetting (BJ) is another technique derived from inkjet printing that was soon applied to produce green ceramic parts before sintering [18]. Finally, robocasting, based on the extrusion of a filament from a paste, was also used for the fabrication of simple 3D ceramic structures after sintering [19]. All of these techniques and a few others were employed as at least one step in the manufacturing of complex shaped ceramics [14,20,21]. Thermal post-treatments are indeed often required to obtain dense ceramic parts. These final ceramization/densification stages depend on the nature of the ceramic itself.

The solid material constituting the porous structure of VSR must resist to oxidation at high operating temperatures in air (typically around 1000 °C, but up to 1200−1300 °C [4,6,8]) and supposedly for very long periods of time. It has also to absorb a maximum of solar radiation, emit a minimum of IR thermal radiation, diffuse sufficiently heat and, finally, resist to thermal shock, i.e. meet a subtle combination of high strength, low stiffness, low coefficient of thermal expansion (CTE) and high thermal conductivity [22]. These very strict –sometimes conflicting– requirements rule out metals and oxides and put forward silicon carbide (SiC), which is still considered as the reference material for an application as VSR [4,6,7,11,[23], [24], [25]].

3D SiC-based structures were prepared by using the different additive manufacturing techniques mentionned above (except direct selective laser sintering), but the processing routes followed are often multi-step and hybrid. For instance, Ortona et al. produced Si-SiC cellular ceramics by replication, with a ceramic slurry, of a polymer structure printed by strereolithography, firing and liquid silicon infiltration (LSI) [26]. Similarly, Wahl et al. densified by LSI relatively complex shapes –yet, with a rougher surface finish– made by robocasting [27]. Schlier et al. or Fleisher et al. also used LSI, but after BJ on a SiC powder bed of a water-based solution containing a carbon precursor [28,29], or after BJ and polymer impregnation and pyrolysis (PIP) with a phenolic resin [30]. Preceramic polymers can also be used as raw materials for printing as an alternative to SiC powders. Zocca et al. indeed synthesized Si-O-C ceramics by BJ of solid precursors and pyrolysis [31]. Liquid preceramic polymers were also modified to become UV curable and suitable for stereolithography, resulting in Si-C-O or SiC-based ceramics after pyrolysis [32,33]. Most of these routes leads to a poor SiC crystallinity and a high amount of impurities in the final material: e. g. free silicon after LSI, or free turbostratic carbon –sometimes even combined with an amorphous silicon oxycarbide phase– when starting from preceramic polymers. These microstructures could be sources of thermochemical instability, high susceptibility to oxidation, corrosion and creep, and finally a low level of thermal conductivity. It is indeed known to what extent the overall properties of SiC-based materials vary according to their purity, microstructure and structure [34].

Our approach is to take advantage of the ease of 3D printing for the formatting of complex shaped cellular materials, but not at the expense of the solid constituent purity. We therefore oriented our choice towards binder jetting 3D printing because it respects the purity of the starting SiC powder. The first original aspect of our work is the deliberate introduction of a high multiscale residual porosity into the printed and fired material. This is achieved by adding a pore forming agent to the SiC powder bed and by an intermediate PIP step to consolidate the SiC porous body. The second main feature of this method is the use of chemical vapor infiltration and deposition (CVI, CVD) to fill in the residual microporosity and cover up the solid with pure and crystalline SiC. This process has been studied and used for many years [[35], [36], [37]] and CVD-SiC is recognized as a high performance material whose properties are very well documented [34].

The first main objective of this work is to examine the feasibility and understand the various stages of the process along the synthesis of a model SiC-based 3D lattice structure. The composition and the microstructure of the solid and the porous network will be analyzed in details at different scales.

The second objective is to evaluate the various intrinsic properties of the constituting SiC-based material that are most relevant to the application as VSR, namely, mechanical properties, oxidation resistance, thermo-physical properties, thermal expansion, thermal micro-diffusivity and thermal shock resistance. These tests will be performed at the various stages of the process, mostly on model specimens.

The optimization of the VSR 3D structures by numerical simulation and the evaluation of their macroscopic properties, in conditions close to the application, is not to be addressed here but will appear in a forthcoming paper.