Dielectric Properties Of Porous Ceramics Research Articles

Dominant role of ceramic connectivity in microwave dielectric properties of porous ceramics

The connectivity of constituting phases plays an important role in the physical properties of composites and porous materials, while it is still overlooked in many cases. In the present work, CaTiO3 is used as the model material to clarify the effect of ceramic connectivity on the microwave dielectric properties of porous ceramics. CaTiO3-A and CaTiO3-B porous ceramics are prepared by partial sintering at 700–1200 °C and sintering at the optimum temperature of 1350 °C with the aid of porogen, respectively. With increasing the sintering temperature from 700 to 1000 °C, the relative density of CaTiO3-A ceramics maintains around 58%, while their εr, Qf, and τf increase dramatically (20.4–48.2, 968–4,237 GHz, and 443.9–724.2 ppm/ °C). The three parameters are even higher in CaTiO3-B ceramic with a similar relative density of 54.1% (69.1, 5,658 GHz, and 782.9 ppm/ °C). The huge variation in microwave dielectric properties is attributed to the significantly improved ceramic connectivity with sintering temperature observed from microstructure analysis. Finite element analysis further reveals that the ceramic connectivity plays the dominant role in the microwave dielectric properties by deciding the electric field distribution in CaTiO3 phase and pores and hence their contribution to dielectric response. The exponent α in exponential dielectric mixing rule is employed to describe the ceramic connectivity, and the trinary relationship among the microwave dielectric properties, constitution, and microstructure of porous ceramics is well constructed. This relationship can be extended to ceramic-polymer and ceramic-ceramic composites, which brings new perspectives for designing such microwave dielectric materials.

Effect of closed pores on dielectric properties of 0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3 porous ceramics

Porous 0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3 ceramics are fabricated via the pore-forming agent method with polymethyl methacrylate (PMMA) and stearic acid (SA) as pore forming agents, and microstructure observations demonstrate that the porosity, pore shape, and pore sizes can be controlled by the synthesis technology. The dielectric properties of porous ceramics are found not only correlated to the pore-matrix composite model, but also have a significant grain-size effect. Based on the Zener Theory, pining forces exerted by pores on the grain boundary are calculated, to explain the shape effect of pores on grain boundary migration. A phase-field simulation is carried out to investigate pore shape effect on the grain size regulation in porous polycrystalline, and simulation results are in good agreements with experiential results as well as theoretical calculations. Thus, a modified equation is proposed to predict the effective permittivity of the porous piezoelectric ceramics by considering effects of porosity, pore shape and grain size.

Investigation of low field dielectric properties of anisotropic porous Pb(Zr,Ti)O3 ceramics: Experiment and modeling

Anisotropic porous Pb(Zr,Ti)O3 ceramics with various porosity degrees have been studied in order to determine the role of the pore shape and orientation on the low-field dielectric properties. Ceramic samples with formula Pb(Zr0.52Ti0.48)0.976Nb0.024O3 with different porosity degrees (dense, 10%, 20%, 40% vol.) have been prepared by solid state reaction. Taking into consideration the shape and orientation of the pore inclusions, the dielectric properties of porous ceramics have been described by using adapted mixing rules models. Rigorous bounds, derived on the basis on Variational Principle, were used to frame dielectric properties of porous composites. The finite element method (FEM) was additionally used to simulate the dielectric response of the porous composites under various applied fields. Among the few effective medium approximation models adapted for anisotropic oriented inclusions, the best results were obtained in case of needle-like shape inclusions (which do not correspond to the real shape of microstructure inclusions). The general case of Wiener bounds limited well the dielectric properties of anisotropic porous composites in case of parallel orientation. Among the theoretical approaches, FEM technique allowed to simulate the distribution of potential and electric field inside composites and provided a very good agreement between the computed permittivity values and experimental ones.

Effects of solid content on the phase assemblages, mechanical and dielectric properties of porous α-SiAlON ceramics fabricated by freeze casting

Porous Y-α-SiAlON ceramics were prepared by freezing camphene-based suspensions at 0°C and subsequently sintering at 1900°C for 1h. The effects of solid loading content in the suspensions on porosities and formation of α-SiAlON as well as mechanical and dielectric properties of the porous ceramics were investigated. An XRD analysis performed on sintered samples indicated that the α-SiAlON did not fully form in the sample with initial solid loading content of 10vol%, due to the high porosity of 90vol% and interconnected pore of the green body. With the increase of initial solid loading content from 10vol% to 30vol%, the porosity decreased from 62.3% to 23.1% and the average pore size decreased from 19μm to 8μm. As a result, the flexural strength increased significantly from 72.4MPa to 190.2MPa, fracture toughness increased from 1.20MPam1/2 to 3.48MPam1/2, as well as the dielectric constant increased from 3.3 to 6.3. The dielectric loss (tanδ) of obtained material varied between 1.4×10−2 and 2.8×10−2, which did not depend on the porosity of samples.

Highly coupled dielectric behavior of porous ceramics embedding a polymer

We present a theoretical approach to study the dielectric properties of porous ceramics and composite materials and explain the experimentally observed notable influence of the coupling between the components of a saturated porous ceramic on the final behavior of the sample. This model is based on the assumption of a dielectric coupling between the components that modifies the expected averaged properties of the material.