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Functionally graded materials

 

Functionally graded materials (FGMs) are established as an attractive class of materials in which it is possible to create a gradient of properties that cannot be attained in any spatially homogeneous materials. FGMs’ properties vary as a function of position; continuous changes in the characteristics such as: chemical composition, grain size, porosity, etc., result in the gradient of electrical and/or magnetic features, also enhancing structural performances, such as mechanical and thermal expansion. FGMs have been used for the fabrication of various technological components, such as electrical devices (piezoelectric ceramics, thermoelectric semiconductors, etc.), electrochemical ones (e.g. solid oxide fuel cells - SOFC, or high-efficiency hybrid direct energy conversion systems - HYDECS), as well as biomaterials.

At the Institute of Technical Sciences of SASA we work on design and characterization of barium-titanate stannate (BaTi1-xSnxO3, BTS) functionally graded materials with an uniaxial gradient of dielectric and ferroelectric properties.

For more than a decade, BTS FGMs have been used in semiconductors industry for the production of electronic components such as capacitors, thermistors, computer memories and bending actuators; in all the cases, the main scientists’ task was the investigation of bending behavior and mechanical stress in those components.

In our research group, the stress is put on the study of electrical characterization of BTS FGM as a function of the concentration gradient, as well as on attempts to determine the influence of the microstructure on FGMs’ electrical characteristics. Until now, by varying the Ti/Sn concentration gradient we successfully created FGMs with a maximum of dielectric permittivity in desirable temperature intervals, furthermore we managed to design a range of temperature intervals in which dielectric permittivity reaches the maximum.

An important processing goal for FGMs is to obtain a high-quality microstructure with the desired grain size and density. The fabrication of FGMs using powder technology can be accompanied with significant problems of components’ shape distortion. Precisely, during the thermal treatment, the graded layers in FGM show different shrinkage rates and extents of shrinkage during sintering, as well as different final density. This phenomenon can lead to excessive shape distortion, warping, delamination, development of cracks and micro-structural damage in the sintered FGMs. Therefore, it is desirable to predict the sintering process for every graded layer in FGM and design sintering strategies to achieve high-quality FGM free from any form of deformation. In our research group, for the modeling of FGMs’ sintering processes, we used the concepts of master sintering curve (MSC).

At the moment, a new challenge for us is the creation of hydroxyapatite FGMs with a gradient of porosity. Such bioceramic materials have a potential application in the reparation of bone defects.

 

SEM micrograph of BTS2.5/BTS5/BTS7/BTS10 FGM. Cone shape is a consequence of different shrinkage of graded layers; there are no defects in the structure
In situ sintering of BTS2.5/BTS5/BTS7/BTS10 FGM

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