A Practical Approach for the Calculation of the Activation Energy of the Sintering

Newly developed software for calculation of activation energy (Qs in the following) of sintering using the Wang and Raj model is presented. To demonstrate the practical potential of the software and to evaluate the behaviour of the Qs during the sintering process, alumina and cubic zirconia ceramic compacts were prepared from nanometric powders. The results obtained with both materials are in agreement with previously published data calculated by different approaches. In the interval of interest (relative densities from 60 % to almost 100 % of theoretical density), both materials show similar behaviour. Three distinct regions can be seen: the initial constant values of Qs 868 kJ/mol and 762 kJ/mol for alumina and cubic zirconia, respectively; a region containing linear drop of Qs and the final region of constant Qs values 625 kJ/mol and 645 kJ/mol for alumina and cubic zirconia, respectively

Microstructure and Phase Composition Of Steatite Ceramics Sintered by Traditional and Spark Plasma Sintering

The influence of the sintering method on the mineral phase transformations and development of the crystalline microstructure of steatite ceramics was investigated. The steatite samples were fabricated from talc and bentonite as low-cost raw materials. Feldspar and barium carbonate, as fluxing agents, were altered in the steatite composition. Dilatometric analysis was applied in the monitoring of the dimensional changes and thereby densification of steatite during the traditional sintering (TS) procedure up to 1200 degrees C. Spark plasma sintering (SPS) method was used under the following sintering conditions: 100 degrees C/min heating rate, uniaxial pressure of 50 MPa; sintering temperature 800 degrees C/1 min or 1000 degrees C/2 min. Crystallinity changes and mineral phase transition during sintering were observed by X-ray diffraction technique. Microstructural visualization of the samples and the spatial arrangements of individual chemical elements were achieved via scanning electron microscopy equipped with the EDS mapping. It was found that SPS sintering facilitated all microstructural changes during high temperature treatment and shifted them to lower temperatures. SPS treatment conducted at 1000 degrees C resulted in maximum densification of the steatite powder compacts and the formation stabilized protoenstatite structure

Derivation of losses from impedance spectrum for contour modes of ceramic resonator

Method of loss coefficients calculation from mechanical quality factors of transversal vibration modes is presented. Theoretical formulae are derived for the contour mode of thin square plate and width extensional mode of a thin rectangular plate. Mechanical quality factors are expressed as a function of loss coefficients for ceramic with known Poisson's ratio. Measured impedance spectra are compared with calculations and used for the characterization of losses on lead-free BCZT and lead-based PZT piezoelectric ceramics. Full set of loss coefficients active for planar vibration modes is characterized on a single sample. Mechanical quality factor at resonance is the same for the contour extensional mode of a thin square plate and for the width extensional mode of a thin rectangular plate. Loss coefficients reach the biggest values for the piezoelectric losses, followed by the dielectric and elastic losses. Our results show that the BCZT ceramics have losses of the same magnitude as soft PZT ceramic

The impedance analysis of sintered MgTiO3 ceramics

In the present study the effect of preparation conditions on microstructure and electrical properties of MgTiO3 ceramics was investigated. Mixture of MgO and TiO2 powders was subjected to mechanical activation for different time periods. Two types of sintering (two-step and HIP sintering) were performed on green bodies prepared by cold isostatic pressing. AC impedance spectroscopy in the low-frequency range (from 20 Hz to 1 MHz), at room and elevated temperatures, were performed in order to separate contribution of grains and grain boundaries to capacitance, resistance and conductivity of magnesium titanate ceramics. The special attention was directed to explanation of the influence of mechanical activation on electrical and morphological changes in sintered samples. The significant decrease of partial oxygen pressure in HP sintering atmosphere caused the change from insulating behavior of MgTiO3 samples after two-step sintering to semiconducting one

Assessing electrical properties of ceramic samples

We present an overview of electromagnetic numerical techniques and measurement apparatus that have been used for assessing electrical properties of ceramic samples. The complex permittivity of the ceramics is typically the first electromagnetic property of interest. In the measurement approach that we use, ceramic samples, in the shape of pills with metalized faces, are placed in either a proprietary coaxial chamber or in an open fixture. The scattering parameter (reflection coefficient) is measured using a network analyzer. For the deembedding of the complex permittivity of a sample one needs precise electromagnetic simulations, including detailed models of the chamber or the open fixture. For lower and intermediate frequencies we use a numerical technique for electrostatic analysis, based on the method of moments with Galerkin testing. We create a database that is used afterwards for evaluation of the complex permittivity practically in real time. For higher frequencies, a full electrodynamic model of the chamber is developed with WIPL-D software. For all our deembedding techniques, we have also developed a detailed model of losses in the SMA coaxial connectors, which is essential for a precise evaluation of the imaginary part of the complex permittivity for low-loss dielectrics. We present results of several measurements in order to illustrate the used techniques

Microstructural and Electrical Properties of Cordierite-based Ceramics Obtained After Two-step Sintering Technique

Cordierite-based ceramic materials are attracting much interest for their various applications in industry, for manufacturing multilayer circuit boards, catalytic converters, filters, thermal insulation, kiln furniture, components of portable electronic devices, etc. In order to reduce production costs and modify cordierite-based materials, mechanical activation can be used. In this study, microstructural and electrical properties of mechanically activated MgO-Al2O3-SiO2 system have been analyzed. The mixtures of MgO-Al2O3-SiO2 powders were mechanically activated in a planetary ball mill for the time periods from 0 to 160 min. Morphological investigations have been performed on the obtained powders. The effects of activation and two-step sintering process on microstructure were investigated by scanning electron microscopy (SEM). Electrical measurements showed variations of the dielectric constant (epsilon(r)) and loss tangent (tan delta) as a function of time of mechanical treatment

Processing and properties of dense cordierite ceramics obtained through solid-state reaction and pressure-less sintering

Sintering of pure cordierite 2MgO:2Al(2)O(3):5SiO(2), and cordierite with the addition of 5 mass % TeO2 was studied. Green bodies were prepared from powder mixtures mechanically activated in a high-energy planetary mill, shaped by uniaxial (20 MPa) and cold isostatic pressing (1000 MPa). The pressure-less sintering of these specimens was performed at 1350 degrees C for 1 h. High relative density over 95% of the theoretical value was obtained through solid-state reaction and pressure-less sintering of powder activated for 40 min, and for the first time reported in the literature. Phase composition and microstructures of sintered samples were determined by XRD and SEM, coupled with EDS mapping. The real part of the complex relative permittivity of the samples was measured at 200 MHz. The loss tangent of all samples was below the resolution of the measurement setup. A strong correlation between the relative permittivity and the density agrees with previously published data

Properties of Magnesium Titanate Ceramic Obtained by Two Stage Sintering

Poster presented at the III Serbian Ceramic Society Conference, 29th September - 1st October, Belgrade, 201