Effect of Silicon Nitride (Si3N4) on Mechanical and Dielectric Properties of Fused Silica Ceramic Composites

Fused silica ceramics composites were widely using for the fabrication of radomes which is the vital component of a missile. The required properties were attained by the addition of small amounts of Silicon Nitride (Si3N4) to the fused silica for enhancing flexural strength and without disturbing the dielectric properties. In the present work, porous fused silica ceramic composites were fabricated using gelcasting process, a near net shaping technique. The experiments were conducted using Response Surface Methodology (RSM) central composite with face centered design with six centre points approach. The process parameters of gelcasting process such as solid loading, monomer ratio, monomer content and additive (Si3N4) were considered as input parameters and flexural strength, porosity and dielectric constant as response parameters. Single parameter and interaction parameter effect on responses were studied. The effectiveness of derived models was compared with Analysis of Variance (ANOVA) at 95% confidence level. Statistical analysis proven that input parameters have critical effect on responses. The derived RSM mathematical models have a higher R2 values (Flexural strength 97.62%, Porosity 95.12% and dielectric constant 95.93%) which shows the critical relation between actual and predicted values. The optimal responses obtained were flexural strength 86.173 MPa, porosity 39.767% and dielectric constant 6.949 corresponding parameters process parameters solid loading 47.95%, additive (Si3N4) 10.43 wt%, monomer content 15 wt% and monomer ratio 3.

Microfabrication of High-Aspect Ratio KNN Lead-Free Piezoceramic Pillar Arrays by Aqueous Gelcasting

The present paper reported a novel approach for the fabrication of a high-aspect ratio (K, Na)NbO3 (KNN) piezoelectric micropillar array via epoxy gelcasting, which involves the in situ consolidation of aqueous KNN suspensions with added hydantoin epoxy resin on a polydimethylsiloxane (PDMS) soft micromold. KNN suspensions with solid loadings of up to 45.0 vol.% have rheological behavior, which was suitable for the gelcasting process. The uniform green KNN bodies derived from the optimized suspension of 42.0 vol.% solid loading and 15.0 wt.% resin had exceptionally high mechanical strength (9.14 MPa), which was responsible for the integrity of the piezoceramic micropattern structure. The square-shaped KNN piezoelectric pillar array with lateral dimensions of up to 5 μm and an aspect ratio of up to five was successfully fabricated.

Desenvolvimento de uma metodologia de processamento para obtenção de materiais com variação contínua de funcionalidade

Functionally Graded Materials (FGM) are those that have variation of properties along one or more directions. They are an alternative to conventional composites in applications that have stress gradients (mechanical, thermal, electrical etc.) or when you want to reduce the negative effects caused by the incompatibility of materials applied directly on each other, in the case of metals coated with ceramic components . There are several techniques to produce FGMs, however, most of them result in a discrete gradation in bulky parts which, for many applications, can be a negative factor since there is an abrupt change of properties between the layers. The objective of this research study was, therefore, the development of a methodology to obtain bulky materials with continuous gradation of functionality from the mixture of particulate systems and consolidation by the gelcasting process. The systems used were suspensions of stainless steel and alumina, the latter being used with two particle size distributions to evaluate the effect of this variable on the processing and properties of the formed material. Such suspensions were optimized for the ideal dispersant concentration and introduced into a static mixer in a variable proportion through a device composed of syringes, hoses, stepper motors and spindles controlled by a microcontroller board, so that the suspension at the mixer outlet was transferred to a mold with a continuous variation of composition. Some gradation profiles have been defined - along the length of all parts produced or in specific regions of them. The consolidation of the suspension inside the mold was done through the gelcasting process, reason why they were prepared with high levels of solid, but with viscosity compatible with the variable flow system. In order to harmonize the sintering temperatures between stainless steel and alumina, an additive composed of titanium dioxide (TiO2) and copper oxide (CuO) was applied in a proportion of 4: 1 by weight. Results of sample dilatometry before mixing showed the densification of alumina after sintering at 1400 ºC. The gradation formed was macroscopically visible in green body. The analysis of the sintered specimens was performed by obtaining some properties along its length, such as shrinkage after sintering, density, apparent porosity and ferromagnetic phase content. The gradation was verified by observing the variation of these properties along the total length of the samples or in specific regions planned for gradation. The methodology proved to be viable to produce bulky pieces with functionality varying continuously along the length, however future studies should be carried out with to improve the densification of the regions rich in alumina, that were far from expected

Fabrication and characterization of aluminum nitride sponges using a mixture of two porous formation methods

ABSTRACTA method for the fabrication of interconnected ceramic sponges was used in the present work, designed by using a combination of two different, aqueous gel casting and sacrificial template, using aluminum nitride powder (99.97%) with a mean size of 2.4 micrometers. Two types of sponges were made by using two different monomers, acrylamide and methacrylamide, the resultants sponges have 60% of porosity after being sintered and pyrolyzed at temperature of 1673 K using an inert atmosphere of argon for 1 h. The hydrolysis evolution of this ceramic powder during the gelcasting process was studied by measuring the pH during the stirring time, the microstructure changes during the time of exposure were observed in a SEM. XRD were made to study the present phases after the gel was eliminated by thermal treatment at 873 K using an oxidizing atmosphere, observing a formation of up to 4 %wt. of cubic alumina phase which was made after the hydrolysis products. Infrared spectroscopy was used to study the changes in the ceramic powder.