Effect of the Processing Parameters on the Fabrication of MgAl2O4 Foams

Stable MgAl2O4 foams (7–21 vol.%) were manufactured using a natural extract from the pericarp of the soap-nut fruit, saponin being the main component, as the foaming agent. The soap-nut extract is soluble in water, biodegradable, non-toxic, and has similar properties to commercial tensoactives. The stability and characteristics of the porous structure of the ceramic foams were evaluated in terms of the amount of foaming agent, content of MgAl2O4 particles, time and speed of stirring of the slurry, type of agitator, and drying temperature. It was found that the foaming capacity decreased with the percent of foaming agent and ceramic, whereas the time and speed of stirring enhanced the foamability. Foaming trials showed that stirring aqueous slurries with 3 wt.% of soap-nut extract for 2 min at 1070 or 2120 rpm, depending on the type of agitator, produced stable MgAl2O4 foams when drying at 60 °C. The mechanism of foaming is discussed. The foams were sintered at 1400 °C for 1 h under an Ar atmosphere. Observation of the sintered foam structures in the scanning electron microscope revealed nearly spherical cells with very good interconnectivity and strength to be manipulated, making them suitable as preforms for manufacturing Al-based composites by pressureless infiltration.

Fabrication of the Fe/PZT Functionally Graded Material and its Application on the Thermal Match for Transducers

To solve the debonding and fracture problems caused by the different thermal expansion coefficients of the transducer element and the measured object, a new method combined with the powder lamination, pressureless infiltration and hot pressing sintering is proposed to prepare Fe/PZT functionally graded material as the matching layer of transducers. The microstructure and element distribution of the functionally graded layer are analyzed by the SEM and EDS. The results show that the volume fraction and elements of the functionally graded layer after the pressureless infiltration at 1100 °C and the hot pressing at 800 °C are linearly and continuously distributed along the thickness direction. The thermal cycle contrast test shows the well matching performance of the functionally graded disk as the transducer matching layer from room temperature to 180 °C. Finally, Lamb wave propagation experiment at the 180 °C shows that the functionally graded matching layer can be used effectively.