Karel Maca

Fabrication of graded porous ceramics using alumina–carbon powder mixtures

Green bodies consisting of mixtures of submicrometric alumina powder and carbon coal were prepared by cold isostatic pressing. Sintering in air atmosphere resulted in alumina ceramics with controlled porous structure and a highly homogenous distribution of pores. Sintering in vacuum (at 1 to 5 Pa) followed by sintering in air for 2 h at 1530°C led to a graded porous structure. These samples exhibited a highly porous core and a dense surface layer. The conditions of thermodynamic equilibrium for the chemical reactions occurring in the studied system during sintering were calculated. These were used to explain the formation of the graded porosity.

Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics

Infrared reflectivity and time-domain terahertz transmission spectra of EuTiO3 ceramics revealed a polar optic phonon at 6–300 K whose softening is fully responsible for the recently observed quantum paraelectric behaviour. Even if our EuTiO3 ceramics show lower permittivity than the single crystal due to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7 phase, we confirmed a magnetic field dependence of the permittivity, also slightly smaller than in single crystal. An attempt to reveal the soft phonon dependence at 1.8 K on the magnetic field up to 13 T remained below the accuracy of our infrared reflectivity experiment.

Antiferrodistortive phase transition in EuTiO3

X-ray diffraction, dynamical mechanical analysis and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics. Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in Glazer notation). The phase transition is analogous to SrTiO3. However, some ceramics as well as single crystals of EuTiO3 show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples electron diffraction revealed an incommensurate tetragonal structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO3.

Sintering densification curve: A practical approach for its construction from dilatometric shrinkage data

This article summarizes the usage of high-temperature dilatometry in ceramic processing and powder technology with special attention on the description of the sintering process. A practical method for transformation of dilatometric shrinkage data into densification curves (the dependence of the sample density on sintering temperature or time) is described in detail. A new automatic procedure to recalculate sintering shrinkage data allowing the plot of the densification curve has been developed, which is presented here.

TWO-STEP SINTERING AND SPARK PLASMA SINTERING OF Al2O3, ZrO2 AND SrTiO3 CERAMICS

ABSTRACT The influence of Two Step Sintering and Spark Plasma Sintering processes on the microstructure of three kinds of oxide ceramics was studied. Al2O3 (with initial particle size of 240 nm), ZrO2 (60 nm, stabilized with 3 mol% Y2O3) and SrTiO3 (50 nm) powders were cold isostatically pressed and pressureless sintered with different heating regimes. It was found that the method of TSS does not bring about significant reduction of final grain size in Al2O3 and ZrO2 ceramics, but does reduce the grain size in perovskite SrTiO3 ceramics. Pressureless sintering of SrTiO3 leads only to 95%TD dense ceramics with the minimum grain size of 400 nm. In comparison, by SPS, with an applied uniaxial pressure of 200 MPa, SrTiO3 ceramic of 98%TD dense with grain size of 80 nm was prepared.

Master Sintering Curve - A Practical Approach to its Construction

The concept of a Master Sintering Curve (MSC) is a strong tool for optimizing the sintering process. However, constructing the MSC from sintering data involves complicated and time- consuming calculations. A practical method for the construction of a MSC is presented in the paper. With the help of a few dilatometric sintering experiments the newly developed software calculates the MSC and finds the optimal activation energy of a given material. The software, which also enables sintering prediction, was verified by sintering tetragonal and cubic zirconia, and alumina of two different particle sizes.

Grain Boundary and Size Effect on the Dielectric, Infrared and Raman Response of SrTiO3 Nanograin Ceramics

The recently revealed giant grain size effect on dielectric properties in undoped SrTiO 3 ceramics (J. Petzelt et al., J. Phys.: Condens. Matter 19 , 196222 (2007) and references therein), was extended to smaller grains of 80 nm mean grain size. Like for previously studied ceramics with larger grain size, in addition to dielectric permittivity also the infrared and Raman responses were studied and discussed. It was shown that the reduced effective permittivity is fully accounted for by the infrared soft mode behaviour and, similar to single crystals and other ceramics studied, no dielectric dispersion appears below the THz frequency range. The rather universal (independent of the grain size and sintering process) double dead layer structure was proposed to be responsible for the observed changes in the infrared and Raman spectra, allowing the grain core to keep the single crystal dielectric function. The outer dead layer shell (obviously charged due to an oxygen deficit) is very thin (∼ 1 nm) having frequency and temperature independent low permittivity (∼ 10) and is responsible for the static permittivity suppression. The inner layer of only slightly distorted perovskite structure is polar with local polarization normal to grain boundaries gradually decreasing towards the grain centre. This polarization and/or the thickness of the polar layers, which compensate the charged grain boundaries, appear to increase on decreasing temperature, particularly below the structural phase transition. Its nature is still not fully understood. In agreement with our previous suggestions, from the Raman data it can be also concluded that in the low-temperature tetragonal phase of all SrTiO 3 ceramics, the local tetragonal axes tend perpendicular to the grain boundaries and the tetragonality is strongly reduced compared to single crystals.

Advanced Ceramic Processes

The chapter is focused on the processing of bulk advanced ceramics. A general overview of ceramic processes is presented with the focus on processes relevant to advanced ceramics in dentistry. The processing of ceramics is divided into four parts that describe the basic steps: powder treatment, shaping of ceramic green bodies, drying and binder removal, and sintering. The first part discusses the reasons for powder treatment. The causes of powder agglomeration are explained and possible dispersion techniques are given. The principles of the most important methods of dry, wet, and plastic shaping, and consolidation of green bodies are explained. The mechanisms of solvent drying and binder removal from consolidated green bodies are discussed and the potential problems of this processing step are highlighted. The densification of green bodies via sintering is explained and possible sintering techniques are described. Advantages and disadvantages of particular sintering methods are discussed.

Master Sintering Surface – A Practical Approach to Its Construction and Utilization for Spark Plasma Sintering Prediction

The sintering is a complex thermally activated process, thus any prediction of sintering behaviour is very welcome not only for industrial purposes. Presented paper shows the possibility of densification prediction based on concept of Master Sintering Surface (MSS) for pressure assisted Spark Plasma Sintering (SPS). User friendly software for evaluation of the MSS is presented. The concept was used for densification prediction of alumina ceramics sintered by SPS.

Alumina and Zirconia Based Composites: Part 1 Preparation

Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The similar charge and electrophoretic mobility of Al2O3 and ZrO2 in the suspensions was found. Adjusting to properly controlled kinetics of deposition deposits were prepared of pre-defined thicknesses. In view of the negative charge of Al2O3 and ZrO2 particles in the isopropanol suspensions used, the prepared layers were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure.