Synthesis of Porous Silicon Nitride with Unidirectionally Aligned Channels Using Freeze‐Drying Process

Abstract

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze‐drying process. Freezing of a water‐based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze‐drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si3N4 had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si3N4 matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…Nevertheless, compared to the other hard templating methods, the freeze casting technique offers not only the possibility to tailor the direction of the pores, the pore size distribution and its gradient along the freezing direction, 169 but also e.g. the porosity 176 of the resulting solid by adjusting the process parameters such as temporal and spatial temperature profile and the solid concentration.…”

Biomaterials by freeze casting

“…Three decades later, Tong et al (1984) and Tong & Gryte (1985) applied the technique to a water-soluble polymer; they created, from agar gels, highly porous materials with a well-controlled cellular architecture and studied the effect of freezing velocity and diffusion conditions on pore geometry. More recently, the potential of freeze casting as a means to create ceramics with a controlled and complex cellular architecture has started to attract research attention (Fukasawa et al 2001(Fukasawa et al , 2002, and the first reports on freeze-cast biomedical materials prepared from collagen solutions have appeared (Schoof et al 1998(Schoof et al , 2000von Heimburg et al 2001;Kuberka et al 2002). Freeze casting of ceramic-based biomaterials followed (Chow et al 2001;Araki & Halloran 2005;Zhang et al 2005;Deville et al 2006a;Deville 2008;Fu et al 2008a,b;Macchetta et al 2009).…”

Fabrication and characterization of highly porous calcium phosphate (CaP) ceramics by freezing foamed aqueous CaP suspensions

“…No cracks were observed in any of the fabricated samples due to the use of PVA polymer as the binder. Table 2 lists the overall porosity, porosity of the macropores and porosity of the CaP walls produced with various CaP contents (15,20, and 25 vol %). The overall porosity calculated by measuring the mass and dimensions of the samples decreased from 83 to 73 vol % with increasing initial CaP content from 15 to 25 vol %.…”

“…First, 0.4 g PVA (Sigma Aldrich, St. Louis, MO, U.S.A.) and 0.4 g non-ionic surfactant (Hypermer KD-6, UniQema, Everburg, Belgium) were dissolved in 5 cc DI water using a magnetic stirrer for 1 h. Subsequently, predetermined amounts (15,20, and 25 vol %) of the CaP powders were added to the PVA solutions and stirred for 1 h. This simple magnetic stirring at a stirring speed of 1000 rpm resulted in the vigorous formation of air bubbles throughout the suspension.…”

“…In particular, the effect of the initial CaP content in the aqueous suspension (15,20, and 25 vol %) on the development of a porous structure was examined. Compressive strength tests were carried out to evaluate the structural integrity of the samples.…”