Kaolinite-mullite reaction series: a TEM study

The kaolinite-mullite reaction series in single crystal kaolinite has been characterized by transmission electron microscopy. The exotherm observed at 980° C is attributed to the formation of a spinel phase. Mullite crystallites have also been observed with the spinel phase and both phases have a composition near that of 32 mullite. Subsequent heat treatment leads to the growth of mullite crystallites on the original kaolinite plates with the c-axis perpendicular to the plate.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO₂-Al₂O₃-SiO₂-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at 1425° C and ~ 1450° C respectively.     

Time-Temperature-Transformation Curves for Kaolinite-α-Alumina

The C-shaped time-temperature-transformation curves (T-T-T curves) of cristobalite formation and the L-shaped T-T-T curves of α-alumina reaction were established for a high-purity kaolinite-α-alumina mixture during heating. The results revealed that cristobalite formation in kaolinite was retarded by the presence of α-alumina between 1250° and 1350°C and was totally prohibited above 1380°C due to the reaction of kaolinite with α-alumina to form secondary mullite. The reaction of α-alumina with kaolinite was initiated at about 1250°C. It became quite extensive above 1380°C and was extremely fast at 1600°C and above, indicating the strong effect of the eutectic liquid formation at ∼1587°C in silica-alumina. The effectiveness of the established T-T-T curves was demonstrated and discussed.     

Aluminum-27 and Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite-Mullite Transformation

The ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of Al in tetra- and pentacoordination in dehydroxylated kaolinite. The ²⁹Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the ²⁷Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the modification of the coordination of Al, which changes from the tetra- or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C for 36 h produces the same transformation in the coordination of Al ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystallinity are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200°C, the 3:2 mullite NMR spectrum.     

Compatibility Relations of A1203 in the System Zr02-Al203-Si02-CaO

Compatibility relations of Al₂0₃ in the quaternary system Zr02-Al₂0₃-Si0₂-CaO were studied by firing and quenching followed by microscopy and energy-dispersive X-ray examination. A projection of the boundary surface of the primary crystallization volume of Al₂0₃ was constructed in terms of the CaO, Si0₂, and Zr0₂ contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, are defined, and the positions of the isotherms were established.     

Preparation and characterization of novel bioactive α-Tricalcium Phosphate doped with Dicalcium Silicate ceramics

This work describes the evaluation of pure α-Tricalcium Phosphate (TCP) and silica containing TCP ceramics in the system Dicalcium Silicate–Tricalcium Phosphate (C₂S–TCP) as a potential substrate for bone tissue engineering. The ceramics were soaked in dynamic simulated body fluid (SBF) for 2 weeks and characterized by SEM-WDS, XRD, and TEM analysis, and the results indicated that a carbonated hydroxyapatite (CHA) was formed on the surface of the ceramics. In addition, cell attachment assay showed that the ceramics supported the adult mesenchymal stem cells of human origin (hMSCs-A) adhesion and spreading, and the cells established close contacts with the ceramics after 24 h of culture. Also, cellular assays have shown a greater ability of hMSCs-A to express markers of the osteoblast phenotype (ALP, Col I, OC) in the C₂S doped α-TCP ceramics, indicating osteoblastic differentiation as a result of the increased concentration of silicon in solid solution in α-TCP. These findings indicate that the C₂S doped α-TCP ceramics possess good bioactivity, and biocompatibility, and might be promising for bone implant material.     

Detection of Defects Using Fault Model Oriented Test Sequences

This paper analyzes the possibilities and limitations of defect detection using fault model oriented test sequences. The analysis is conducted through the example of a short defect considering the static voltage test technique. Firstly, the problem of defect excitation and effect propagation is studied. It is shown that the effect can be either a defective effect or a defect-free effect depending on the value of unpredictable parameters. The concept of Analog Detectability Interval (ADI) is used to represent the range of the unpredictable parameters creating a defective effect. It is demonstrated that the ADIs are pattern dependent. New concepts (Global ADI, Covered ADI) are then proposed to optimize the defect detection taking into account the unpredictable parameters. Finally, the ability of a fault oriented test sequence to detect defect is discussed. In particular, it is shown that the test sequence generated to target the stuck-at faults can reasonably guarantee short defect detection till a limit given by the Analog Detectability Intervals.     

Temporal Talbot effect in fiber gratings and its applications

We show that a temporal effect equivalent to the spatial Talbot effect (self-imaging) applies to the reflection of periodic pulse trains from linearly chirped fiber gratings (LCFG's). For specific input repetition periods the reflected signal is an exact replica of the input signal. Input repetition period values that give rise to this effect depend on the dispersion coefficient of the grating. We propose to use this effect as an alternative for dispersion measurement in LCFG's. Furthermore, by using the properties of the temporal Talbot effect, we can design linear passive devices (LCFG's) for use as frequency multipliers, able to multiply the repetition rate of a given pulse train.     

Indirect cytotoxicity evaluation of pseudowollastonite

This study aimed to evaluate the cytotoxicity of substances leached by pseudowollastonite (CaSiO₃). It has been previously shown that calcium (Ca²⁺) and silicate (SiO₃ ^–) ions are released from pseudowollastonite into biological solutions. Both of these ions are known to influence the biological metabolism of osteoblastic cells essential in the mineralization process and bone-bonding mechanism. The indirect toxicity evaluation was performed by extraction method, according to International Standard Organization (ISO). Pseudowollastonite pellets obtained by solid-state reaction were incubated, in culture medium, during 24, 48, 72 or 168 h at different concentrations (5, 10, 15, 50, 100, 200 mg/ml). The cytotoxicity of each extract in presence of human osteoblastic cell line (SaOS-2) was quantitatively assessed by measuring the viability (succinate dehydrogenase activity, MTT), the membrane integrity (the uptake of the neutral red by viable cells, NR) as well as the cell necrosis by measuring the lactate dehydrogenase (LDH) released in the culture medium. No significant alteration of membrane integrity or cell suffering was detectable. However, increased cell metabolism was observed for cells exposed to pseudowollastonite extract with longest extraction time (168 h). In conclusion, mineral elements leached by pseudowollastonite do not significantly affect the metabolism of osteoblastic cells.