Ceramic powders porosity

Tape can be cast on a stainless steel table or belt, glass plate, or a Mylar, Teflon, or ceUulose acetate film carrier. The tape should adhere to the carrier sufficiently to prevent curling, but should be easily removable. In a continuous casting process, the tape is dried by air flowing 1—2 m/min counter to the casting direction. A typical dry green tape contains approximately 35 vol % organics, 50% ceramic powder, and 15% porosity. 

The process has the advantage that the green tape is free from porosity since there is no solvent loss and the ceramic powder is very well dispersed with the elastomer uniformly distributed. The tape can be further processed for substrates or multilayer capacitors in the same way as band-cast material. 

Porosity can be an advantage or a disadvantage in ceramic powders, depending upon the processing and final application. Tailoring of the pore size distribution is very important for catalytic substrates, because access to the catalytic sites depends on these diflusional pathways. 

The major physical properties of ceramic powders constitute size distribution of primary particles and agglomerates, specific surface area, density, porosity, and morphology (e.g., shape, texture, and angularity). 

Teijin Solufill Limited produces Solufill a high porosity PE engineering film formerly made by a joint venture with DSM whose interests were bought out by Teijin. Solufill is filled with a special ceramic powder. Solufill was formerly used in the manufacture of bulk multilayer ceramic capacitors but in 2005 Teijin decided to withdraw from this application due to weakening electronic component prices and a worsening market environment. However, Solupor, a sister product is being supplied for use as a battery separator. 

Qrmposite samples of PMMA/BaTIOi were prepared by polymerization of MMA in which fine partides of BaTiO were dispersed [931-33]. Samples with 0 to about 60 vol% BaTiOt were successfully obtained without any considerable gradient of ceramic powder concentration. Further enrichment of the composite with barium titanate led to a sharp rise in porosity of the material. This is a known property of mixtures of solid dielectrics (109]. 

Since the process is basically independent on the ceramic powder composition while driven by the ice growth, samples with homogeneous isotropic rounded porosity could be obtained pouring the slurry into moulds cooled at the liquid nitrogen temperature of -196 °C, as found for other materials (Landi et al 2008). Similarly, samples with graded porosity, from globular to elongated shapes, could be obtained within the same sample (Landi et al., 2008). 

The shaping of these fine, submicrometer powders into complex components and their subsequent consoHdation into dense ceramic parts of ideally zero porosity is a major technological challenge. The parts formed need to be consoHdated to near-net shape because Si N machining requires expensive diamond grinding. Additionally, Si N dissociates at or near the typical densiftcation temperatures used in the fabrication of stmctural ceramics and, therefore, special measures have to be taken to preserve the compositional integrity of the material. 

---