MgO-Al2O3 Ceramics Research Articles

Microstructures and strengths of microporous MgO–Al2O3 ceramics from Al(OH)3 and calcined magnesite

AbstractSeven microporous MgO–Al2O3 ceramics with an Al2O3 content of 15–90 wt% were prepared using Al(OH)3 and calcined magnesite as raw materials. A wet mixing process was employed during sample preparation to transform the calcined magnesite with a larger particle size to smaller Mg(OH)2 particles. The in situ decomposition synthesis method and the Kirkendall effect were utilized to produce and control the pore structure of the microporous MgO–Al2O3 ceramics. There were two kinds of pores in the microporous MgO–Al2O3 ceramics. The first one resulted from the in situ decomposition of Al(OH)3 and Mg(OH)2 particles, which were small and equally distributed. Another one originated from the position of the Mg(OH)2 particles due to the Kirkendall effect caused by MgO diffusion. They were similar in size to the Mg(OH)2 pseudomorph particles. Simultaneously, the Al2O3 content affected the packing behavior and the spinel formation, which changed the characteristics of the pores and necks among the particles. These mechanisms also affected the strengths of the microporous MgO–Al2O3 ceramics. Thus, when the Al2O3 content was 45–90 wt%, the microporous MgO–Al2O3 ceramics had a high compressive strength (10.0–18.3 MPa) and apparent porosity (52.2%–58.4%).

Multicomponent positron–electron annihilation kinetics in the MgO–Al2O3 ceramics in the frequency domain

Multicomponent positron–electron annihilation kinetics in initially dried (after natural water adsorption and before water treated process) and with following immersion into water MgO–Al2O3 ceramics sintered at different temperatures (1100 ÷ 1400 °C) has been calculated and analyzed in the frequency domain. The spectra of real (in-phase) $$\chi_{1} \left( \omega \right)$$ and imaginary (quadrature) $$\chi_{2} \left( \omega \right)$$ components of modulated positron–electron annihilation response have been obtained numerically from temporal kinetic characteristics using integral Fourier transform. To calculate Fourier transform combined with spline cubic interpolation for short time range (below 5.5 ns) and piecewise linear interpolation for highly noisy longer times range, domain kinetics have been used. A special method has been proposed to obtain clear frequency response of low-intensive long time components of PEA kinetics, which are almost invisible on spectra because of dominating high-frequency Debye-type component. Strong high-frequency dispersion apparently consists of two Debye-type components with close characteristic frequencies, that leads to widening of large maximum. Low-frequency decay component with characteristic time of about 70 ns shows stronger dependence on sintering temperature of the ceramics and is characterized by significant deviation from Debye form of relaxation. This indicates the formation of some changing with time structures around positrons localized on long time traps.

Effect of water adsorption on positron–electron annihilation kinetics in the MgO–Al2O3 ceramics in the frequency domain

Positron–electron annihilation kinetics in the MgO–Al2O3 ceramics sintered at different temperatures (1100, 1200 and 1400 °C) with the following water adsorbtion procedure has been calculated and analyzed in a frequency domain. The spectra of real (in-phase) $$\chi_{ 1} \left( \omega \right)$$ and imaginary (quadrature) $$\chi_{2} \left( \omega \right)$$ components of modulated positron–electron annihilation response have been obtained numerically from temporal kinetic characteristics using integral Fourier transform. The obtained complex spectra of positron-electron annihilation in MgO–Al2O3 ceramics in the frequency domain obey a sum of two Debye law components denying a correlation between elementary positron annihilation processes. Strong increasing of amplitude of lower frequency Debye component caused by water adsorbtion on the frequency spectra has been observed. Characteristic frequencies of Debye-type components of water-immersed samples show weaker dependencies on sintering temperature than in just-sintered ones. It is shown that position of large maxima on the frequency dependencies of imaginary part corresponds to the fastest average relaxation lifetime representing the most intensive interaction process of positrons with small cavity traps in solids.

Nanoporous characterization of modified humidity-sensitive MgO-Al2O3 ceramics by positron annihilation lifetime spectroscopy method

Investigation of nanopores in the humidity-sensitive MgO-Al2O3 ceramics modified at 1100°C for 2 hours was performed using positron annihilation lifetime spectroscopy method. Lifetimes of the third and fourth components of positron annihilation lifetime spectra obtained by four-component fitting procedure were used for calculation of nanopores radii. Transformation in free volumes was studied in ceramics after drying and exposure to water vapor. It is established that the lifetime of the third component decreases and the intensity of this component increases in ceramics exposed to water vapor reflecting ortho-positronium decay in nanopores simultaneously with annihilation in water bubbles. The characteristics of the fourth component (lifetime and intensity) decrease in ceramics exposed to water vapor reflecting ortho-positronium trapping in free-water volume of nanopores. Final drying of the MgO-Al2O3 ceramics results in returning of ortho-positronium lifetimes and intensities to the initial values confirming good desorption of water from nanopores of ceramics.

Water-Sorption Effects near Grain Boundaries in Modified MgO-Al2O3 Ceramics Tested with Positron-Positronium Trapping Algorithm

Water-Sorption Effects near Grain Boundaries in Modified MgO-Al2O3 Ceramics Tested with Positron-Positronium Trapping Algorithm

Frequency domain kinetic of positron–electron annihilation in the MgO–Al2O3 spinel-type ceramics

In this work, the kinetic of positron–electron annihilation in the MgO–Al2O3 spinel-type ceramics sintered at different temperatures (1100, 1200 and 1400 °C) has been calculated and analyzed in a frequency domain. The spectra of real (in-phase) and imaginary (quadrature) components of positron–electron annihilation kinetic have been obtained numerically from usual temporal characteristics using integral Fourier transform. The numerical calculations were carried out using cubic spline interpolation of the pulse characteristics of MgO–Al2O3 ceramics in time domain with following analytical calculations of integrals. The obtained spectra as real so imaginary part of MgO–Al2O3 ceramics in frequency domain almost good obey a Debye law denying correlation between elementary positron annihilation processes. Complex diagrams of frequency domain responses of as-prepared samples have a shape of semicircles with close characteristic frequencies. Some deviation on low-frequency side of the semicircles is observed confirming an availability of longer time kinetic processes. Sintering temperature dependencies of the relaxation times and characteristic frequencies of positron–electron annihilation processes have been obtained. It is shown that position of large maxima on the frequency dependencies of imaginary part corresponds to fast average relaxation lifetime representing the most intensive interaction process of positrons with small cavity traps in solids.

Water-Vapor Sorption Processes in Nanoporous MgO-Al2O3 Ceramics: the PAL Spectroscopy Study.

The water-vapor sorption processes in nanoporous MgO-Al2O3 ceramics are studied with positron annihilation lifetime (PAL) spectroscopy employing positron trapping and positronium (Ps)-decaying modes. It is demonstrated that the longest-lived components in the four-term reconstructed PAL spectra with characteristic lifetimes near 2 and 60–70 ns can be, respectively, attributed to ortho-positronium (o-Ps) traps in nanopores with 0.3- and 1.5–1.8-nm radii. The first o-Ps decaying process includes “pick-off” annihilation in the “bubbles” of liquid water, while the second is based on o-Ps interaction with physisorbed water molecules at the walls of the pores. In addition, the water vapor modifies structural defects located at the grain boundaries in a vicinity of pores, this process being accompanied by void fragmentation during water adsorption and agglomeration during water desorption after drying.

Nanostructural Free-Volume Effects in Humidity-Sensitive MgO-Al2O3 Ceramics for Sensor Applications

Technologically modified spinel MgO-Al2O3 ceramics were prepared from Al2O3 and 4MgCO3·Mg(OH)2·5H2O powders at sintering temperatures of 1200, 1300, and 1400 °C. Free-volume structural effects in MgO-Al2O3 ceramics and their electrophysical properties were studied using combined x-ray diffraction, scanning electron microscopy, Hg-porosimetry, and positron annihilation lifetime spectroscopy. It is shown that increasing of sintering temperature from 1200 to 1400 °C results in the transformation of pore size distribution in ceramics from tri- to bi-modal including open macro- and meso(micro)pores with sizes from ten to hundreds nm and nanopores with sizes up to a few nm. Microstructure of these ceramics is improved with the increase of sintering temperature, which results in decreased amount of additional phases located near grain boundaries. These phase extractions serve as specific trapping centers for positrons penetrating the ceramics. The positron trapping and ortho-positronium decaying components are considered in the mathematical treatment of the measured spectra. Classic Tao-Eldrup model is used to draw the correlation between the ortho-positronium lifetime and the size of nanopores, which is complementary to porosimetry data. The studied ceramics with optimal nanoporous structure are highly sensitive to humidity changes in the region of 31-96% with minimal hysteresis in adsorption-desorption cycles.

Influence of Sintering Temperature on Pore Structure and Electrical properties of Technologically Modified MgO-Al2O3 Ceramics

Technologically modified spinel ceramics are prepared from Al2O3 and 4MgCO3×Mg(OH)2×5H2O powders at 1200, 1300 and 1400 oC. The influence of sintering temperature on porous structure and exploitation properties of obtained humidity-sensitive MgO-Al2O3 ceramics are studied. It is shown that increasing of preparing temperature from 1200 to 1400 oC result in transformation of pore size distribution in ceramics from tri- to bi-modal including the open macro- and mesopores with sizes from tem to hundreds nm and nanopores until to a few nm. The studied ceramic elements with electrical resistances ~ 10-2-102 MОhm are high humidity sensitive in the region of 30-95 % with minimal hysteresis in adsorption-desorption cycles. It is established that increasing of humidity sensitivity in ceramics are related to achievement near to optimum pore size distribution and quantity of pores in the all regions. Prolonged degradation transformation in ceramics at higher temperature and relative humidity result in lose sensitivity up to 40-50 %.DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5189

Dephosphorization of complexly alloyed nickel melts under vacuum induction melting conditions: II. Experimental results and their interpretation

The activities of the components of BaO-BaF2-BaCl2-NiO and CaO-CaF2-CaCl2-NiO slags systems, which were considered as a phase having a collective electron system, are calculated, and it is shown that the barium, calcium, and oxygen activities at 1600°C are maximal in the BaO- and CaO-based systems depending on the main oxide content. The dephosphorization of three types of nickel-based melts by slags of 14 compositions in crucibles made of Al2O3, MgO, and MgO-Al2O3 (80-20 wt %) is experimentally studied, and the degree of dephosphorization is shown to depend on the phosphorus content in a metal, the slag composition, and the crucible material (degree of dephosphorization is maximal in Al2O3 crucibles). The forming slag is assimilated by Al2O3 and MgO-Al2O3 ceramics with a porosity of about 30%. If 4–10 wt % NiO are present in a slag, the wettability of the Al2O3 ceramic is significantly higher than that of the MgO-based ceramic.

Preparation and Properties of ZrO<sub>2</sub> Dispersed MgO-Al<sub>2</sub>O<sub>3</sub> Ceramics (Part 2)

ZrO2-dispersed MgO-Al2O3 ceramics with various MgO to Al2O3 compositional ratios were prepared from Mg(OH)2 powder, α-Al2O3 powder and Zr-n-butoxide by calcining at 1000°C for 2h and sintering at 1600°C for 1h. The influence of the MgO-Al2O3 compositional ratio on the sinterability and mechanical properties was studied. Addition of 10vol% ZrO2 improved the relative density of the sintered specimens containing 50 and 62.5mol% Al2O3. However, the addition was not effective at other compositional ratios. The bending strength was improved at composition containing more than 50mol% Al2O3, where the dispersed ZrO2 particles were in the form of tetragonal phase. The bending strength increased with increase in Al2O3 content in the ZrO2 dispersed specimens. The fracture toughness was improved for all specimens by ZrO2 dispersion, especially, for the specimen in which MgO-Al2O3 component was in a single crystalline phase.

Preparation and Properties of ZrO<sub>2</sub>-Dispersed MgO-Al<sub>2</sub>O<sub>3</sub> Ceramics

Preparation and Properties of ZrO<sub>2</sub>-Dispersed MgO-Al<sub>2</sub>O<sub>3</sub> Ceramics