2.1. Drip Casting

In this study, the forming method used to produce Li₄SiO₄ ceramic pebbles is based on drip casting, which is a new process in the field of nuclear fusion material. Therefore, drip casting is a process for producing pebbles by dripping a ceramic suspension through a nozzle plate to form droplets, before hardening the droplets in a saline solution. Moreover, through the modification of the suspension (microstructure) characteristics and the ceramic production process parameters, it is possible to directly modify the properties of pebbles, including mechanical properties, such as compressive resistance, fracture toughness, hardness and abrasion resistance. This process allows the production of stable and well-sized ceramic pebbles, which at the same time also guarantees the sphericity and surface smoothness, the theoretical density and the other properties, such as the resistance, the hardness and fracture toughness, etc., which are traditionally desirable.

The innovative character of the new methodology under development at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa jointly with Industrie Bitossi is based on the drip casting method (main phases synthetized in Figure 2), which operates at room temperature.

Flow diagram with the main phases of the proposed fabrication process [15].

Li₄SiO₄ in powder form purchased from Albemarle (Frankfurt, Germany) was used as the starting material. First, the process involved the preparation of a Li₄SiO₄ aqueous suspension with sodium alginate ((C₆H₇NaO₆)n purchased from Sigma-Aldrich, Darmstadt, Germany). After this, the suspension was forced to pass through a nozzle plate to form droplets. Finally, the droplets were dropped into an aqueous solution of calcium chloride (CaCl₂ purchased from Sigma-Aldrich, Darmstadt, Germany) where they coagulated in solid spheres (Figure 3). After these steps, the obtained spheres were dried and sintered to form Li₄SiO₄ pebbles.

Drip casting process: cross-linking involves the cooperative bonding of divalent metal ions.

For the slurry preparation, a sodium alginate solution was added to the Li₄SiO₄ aqueous suspension. First, 20 wt % of lithium orthosilicate was grinded and dispersed in deionized water for 2 h using a rapid mill equipped with a porcelain alumina jar and alumina balls with 5-mm diameter. This step is necessary to reduce the particle dimensions of Li₄SiO₄ powders to a size (medium value of the particle size distribution 500 nm) that ensures a colloidal suspension without the precipitation phenomena, which allows us to obtain a homogeneous dispersion of particles. Second, 5 wt % of sodium alginate was dissolved in deionized water at 80 °C for 2 h using a magnetic stirrer. This temperature helps the dissolution of sodium alginate in water, reducing the viscosity of the solution. After this, the alginate solution was added to Li₄SiO₄ suspension in order to obtain a slurry with a global concentration of sodium alginate of 3 wt % (total water basis). The slurry was mixed with a magnetic stirrer for 30 min. Finally, 5 wt % calcium chloride solution was prepared by mixing CaCl₂ in deionized water for 30 min.