S K Das

The effect of ZnO addition on the densification and properties of magnesium aluminate spinel

Abstract Stoichiometric magnesium aluminate spinel can be developed by solid oxide reactions of calcined magnesia and calcined alumina. The raw materials were mixed; attrition milled, compacted under a uniaxial pressure of 100 MPa and finally fired in the temperature range of 1500 to 1650°C. Up to 2 wt% ZnO was incorporated as an additive. In this investigation the effect of ZnO on the densification and properties of the magnesium aluminate spinel has been studied. It was found that 99% of theoretical density was achieved on firing at 1550°C with the addition of 0.5 wt% ZnO. The optimum properties in terms of bulk density, hot strength and thermal shock resistance was obtained with 1 wt% ZnO. All the ZnO containing samples retained their strength up to 6–8th cycle on thermal shock. ZnO containing samples are comparatively more resistant to thermal shock than ZnO free samples.

A new high temperature resistant glass-ceramic coating for gas turbine engine components

A new high temperature and abrasion resistant glass-ceramic coating system (based on MgO-Al2O3-TiO2 and ZnO-Al2O3-SiO2 based glass systems) for gas turbine engine components has been developed. Thermal shock resistance, adherence at 90°-bend test and static oxidation resistance at the required working temperature (1000°C) for continuous service and abrasion resistance are evaluated using suitable standard methods. The coating materials and the resultant coatings are characterized using differential thermal analysis, differential thermogravimetric analysis, X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The properties evaluated clearly showed the suitability of these coatings for protection of different hot zone components in different types of engines. XRD analysis of the coating materials and the resultant coatings showed presence of a number of microcrystalline phases. SEM micrographs indicate strong chemical bonding at the metal-ceramic interface. Optical micrographs showed smooth glossy impervious defect free surface finish.

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO2

Abstract A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO2 (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO2 on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO2 slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO2. ZrO2 containing samples always show better resistance to thermal shock than the ZrO2 free samples. Scanning electron micrography shows that ZrO2 occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.

The effect of CuO addition on the sintering of lime

Abstract The sintering of lime was carried out with 1–4 wt.% CuO in the temperature range 1500–1650°C. A double calcination process was adopted in the study. The result showed that without additive the densification was 88% and with additive it maximised to 93% of the theoretical value at 1550°C with 1 wt.% CuO. The density decreased due to the presence of large closed pores with a higher percentage of CuO. Hydration resistance was measured at 50 o C in 95% relative humidity through the weight gain after 3 h. Addition of CuO up to 2 wt.% improved the hydration resistance, but it was not significantly high in comparison to that of 1 wt.% CuO. The use of a higher level of CuO in lime did not show any further improvement in hydration resistance. The CaO forms a low melting compound (2CaO.CuO) with CuO which helps liquid phase sintering of lime. When the liquid content increased in the sinterred lime grain growth takes place simultaneously along with pore growth

Aluminium Oxide Coating by Microwave Processing

Microwave processing is an energy-efficient advanced technique to prepare materials with finer microstructure and better mechanical properties. In the present paper, microwave energy was utilized for the development of oxide coatings on blocks of commercial aluminium of various geometrical shapes. Experimental results revealed that the geometrical term, i.e. the volume to surface ratio had a dominant influence on the microwave induced oxidation behaviour of the samples. In all the cases, coating thickness and its density progressively increased with increasing microwave exposure time. The coating phase was found out to be α-Al2O3 by XRD analysis. The coating microstructure and elemental analysis as examined by SEM and EDAX respectively have been discussed in detail. Adhesion of coatings was studied by using the scratch test.

Densification of reactive lime from limestone

Sintering of lime from natural limestone was carried out by a single stage (from natural carbonate) and double stage (from limestone converted hydroxide) process in the temperature range 1500 to 1650°C. In double stage process hydroxides were activated by three different techniques through pre-calcination and hydration. Different techniques employed are air-quenching the powder after precalcination, furnace cooling and water quenching of powder. The air quenching process showed better densification. Incorporation of hydroxide into carbonate powder up to an extent of 25 wt.% showed maximum densification. Hydration resistance was related to densification and grain size of sintered lime.

Calcination effect on magnesium hydroxide and aluminium hydroxide for the development of magnesium aluminate spinel

Abstract Spinellisation and densification of magnesium aluminate spinel was studied. Magnesium hydroxide and aluminium hydroxide were calcined separately between 900 and 1600°C. Stoichiometric mixtures of the uncalcined and calcined materials were sintered between 1550°C to 1750°C. Spinellisation was found to be reduced with increasing calcination temperature but sintering at 1750°C produces similar spinellisation for different batches due to higher diffusivities of the diffusing species. Above 99% spinellisation was obtained for the uncalcined batch sintered at 1750°C. Densification was also found to be greatly dependent on calcination temperature. Control of reactivity by calcination and slower rate of sintering have reduced the negative effects of the single stage sintering and a maximum of 91% densification has been found for the batch with materials calcined at 1400°C. ©

Fabrication of tapered single mode fiber by chemical etching and used as a chemical sensor based on evanescent field absorption

Single mode tapered fiber (SMTF) has been fabricated with core diameter of 8 μm and reduced cladding diameter up to 11 μm by hydrofluoric acid (HF) etching technique. To obtain the required cladding diameter, the time of etching has been optimized by using different HF concentrations. The mechanism as well as kinetics path of etching reaction on standard optical fiber is discussed. This study is related to surface catalyzed dissociation of HF followed by direct reaction with adsorbate molecules and the surface silicon oxide molecules. The etched tapered fibers are then packaged on quartz substrate to use as sensor element. Finally, the etched fiber is used as an element within chemical sensor based on evanescent field absorption. In this experiment, a 419-ppm cobalt nitrate solution is used for sensing.

Effects of elevating temperature and high-temperature annealing upon state-of-the-art of yttia-alumino-silicate fibers doped with Bismuth

We report an experimental analysis of attenuation and fluorescence (at low-power 750-nm excitation) spectra’ transformations in yttria-alumino-silicate fiber doped with Bismuth (Bi), which occur at higher than room, but not exceeding 700°C, temperatures. As well, we address impact of elevating temperature upon the fiber’s basic characteristics, such as fluorescence/resonant-absorption saturation, fluorescence lifetime, and pump-light backscattering, given by the presence of Bi-Al related active centers (BACs). The experimental data reveals dramatic impact of heating and high-temperature annealing in excess of 500…550°C on the fiber’s state-of-the-art, expressed as significant rise of resonant absorption, enhancement of BACs NIR fluorescence, and reduction of scattering loss. In the meantime, such microscopic parameters of the fiber as BACs fluorescence lifetime and saturation power are found to be kept almost unchanged in its post-annealed state as compared to the pristine one. Possible mechanisms responsible for the phenomena and advantages of utilizing temperature-treated fiber of such type for lasing/amplifying purposes are discussed.

Crystallization of Glass Coating By Microwave Heating

Microwave and conventional heat treatment processes were utilized to achieve adequate crystallization in MgO-Al2O3-TiO2 based glass coatings with identical compositions. The processed coatings were evaluated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and image analysis. XRD analysis showed that the misfit strain in the lattice of conventionally heat treated coating was higher than that in the corresponding microwave processed coating. SEM investigation along with image analysis confirmed that microwave heating generated finer crystallites in the glass coating than those obtained in the coating processed by conventional heating.