Densification and microstructure evolution during sintering of silicon under controlled water vapor pressure

Sintering of fine silicon powder was studied under controlled water vapor pressures using the Temperature–Pressure–Sintering Diagram approach. The water vapor pressure surrounding the sample was deduced from thermogravimetric analysis and related to the water content of the incoming gas flux with a simple mass transfer model. The thickness of the silica layer covering silicon particles was then monitored by the water vapor pressure and the microstructure evolution and densification during sintering could be controlled. Stabilizing the silica layer indeed inhibits grain coarsening and allows better densification of the compacts under humidified atmosphere as compared to dry atmosphere.     

Influence of the PuO2 content on the sintering behaviour of UO2-PuO2 freeze-granulated powders under reducing conditions

The sintering behaviour of freeze-granulated UO₂-PuO₂ powders containing 33 and 15 mol% Pu/(U + Pu) was investigated under reducing conditions up to 1700 °C. For both compositions, the “grain size versus relative density” trajectory was constructed. All the experimental points form a single trajectory meaning that a relative density/grain size pair obtained after sintering seems independent of the thermal path (heating rate, soak time, soak temperature) and of the Pu content. Exploiting the “grain size versus relative density trajectory” enabled also to propose that densification was controlled by grain boundary diffusion and grain growth by the grain boundaries whatever the Pu content. An activation energy around 510 kJ/mol was obtained for densification, which was close to the value reported for the grain boundary diffusion of plutonium cations in U_1-xPu_xO₂ polycrystals. Whatever the Pu/(U + Pu) content, the sintered microstructure of 98 % dense samples possesses a homogeneous distribution of plutonium and uranium cations.     

Studies on the effect of sintering conditions on the magnetostriction characteristics of cobalt ferrite derived from nanocrystalline powders

The effect of sintering conditions on the magnetostriction characteristics of sintered cobalt ferrite derived from nanocrystalline powders is studied. It is observed that the magnetostriction coefficient as well as the slope of the magnetostriction as a function of field depends on the microstructure of the sintered product. There is a direct correlation between the magnetostriction characteristics and density as well as porosity of the sintered products. Intragranular pores are found to be strongly influencing the magnetostriction characteristics. Similarly, correlation is found between the magnetostriction parameters and the Curie temperature, indicating the role of cation distribution which affects the magnetic anisotropy. Sintering the compacts made from nanocrystalline powders at 1450 °C for a very short duration is found to give high values of magnetostriction coefficient. However, higher values of magnetostriction at low magnetic fields are obtained for samples sintered at lower temperatures for longer duration and having relatively lower density.     

Direct sintering of UO2+x oxides prepared under hydrothermal conditions

The sintering of uranium oxide powders prepared by hydrothermal conversion of oxalates was studied. A dilatometric study first showed that among the synthesis conditions, only the pH largely impacted the densification, notably through the powders morphology. Three samples (single crystals – pH = 1, microspheres – pH = 2 and nano‑powders – pH = 8) were further selected to study their sintering behaviour. Densitometric and granulometric data then allowed establishing sintering maps. For single crystals and microspheres, densities above 96 %TD were obtained at 1700 °C. In contrast, only 2–8 hours at 1500 °C were needed to yield comparable results for nano-powders. These observations confirm that the hydrothermal conversion of oxalates can be considered as a promising route for the synthesis of actinide oxides, which can subsequently be sintered directly.     

MOX fuel sintering under oxidizing conditions: A comprehensive study of the solarisation phenomenon

Sintering and thermogravimetric experiments, coupled to microstructure investigations and phase structure analyses, were carried out on a homogeneous U_0.89Pu_0.11O_2+x MOX fuel under an atmosphere made of Ar + 4 vpm O₂. A significant weight gain (+2.4 wt %, oxidation step) is observed at low temperatures (≤590 °C) without any swelling of the pellet. Raman spectroscopy characterizations indicated the presence of the M₃O₇ phase (M=U/Pu). At higher temperatures (between 1200 and 1540 °C), a weight loss (−2.2 wt %, reduction step) is observed. In the same time, swelling of the pellet is detected due mostly to the release of dioxygen from the sample over-oxidized areas, leading ultimately to the typical solarisation microstructure (core/shell structure, large grains in the core and small grains in the shell, residual pores located at multiple grain junctions in both cases). The temperature range 590–1200 °C is related mainly to densification and grain growth.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Microstructural evolution and phase transition mechanism of Ti(C,N)-based cermets during vacuum sintering process

Ti(C,N)-based cermets were fabricated using section vacuum sintering. The density and shrinkage ratio of sintered samples were recorded and analyzed. Phase transition, microstructure and interface behaviors of cermets were examined by transmission electron microscope, X-ray diffractometer and scanning electron microscope. As the sintering temperature rising, the number of pores in cermets was decreased and the density of samples was increased. Moreover, with the progress of sintering, atoms of various elements were constantly diffusing and gathering, affecting the formation of different microstructure, and the angle and intensity of each XRD peak were also constantly changing slightly, or disappearing at a certain temperature. These relevant results indicated that the proportion of heavy element to Ti in the hard phase would affect its lattice parameter. After the appearance of liquid phase, the densification mechanism changed from surface diffusion to grain boundary diffusion, thus accelerating the densification. With the proceeding of sintering process, various microstructures were formed such as core - rim/rimless structure. Finally, the process of interface change in core rimless phase transition was discussed. This rim phase, formed in solid state sintering process while failing to develop into core-rim structure after liquid sintering, was formed by some heavy elements short-range diffusion into core phase. The orientation different and the misfit between this core and rim were relatively large. While after liquid sintering, the semi-coherency or even coherency state at the interface of core-rimless and binder was formed.     

Impact of pressure in static and dynamic pressing of zirconia ultradisperse powders on compact density and compaction efficiency during sintering

This paper studies the impact of pressure in static and dynamic pressing on densification of stabilized zirconia ultradisperse powder compacts and on compaction kinetics during sintering. Ultradisperse powders of 97 ZrO₂ + 3 Y₂O₃ zirconia were synthesized using the plasma chemical method. Dry uniaxial static pressing and double-action magnetic pulse compaction were employed. It is shown that double-action magnetic pulse compaction provides the maximum density of the product in comparison to that obtained through static pressing. The dilatometric studies showed that the increased density of compacts from stabilized zirconia powders obtained in dynamic pressing does not make ceramics less compact during isothermal aging as it typically occurs during static pressing. This increases the density of ceramics and improves its mechanical characteristics.     

Impact of pressure in static and dynamic pressing of ultrafine plasmochemical ZrO2 (Y)-Al2O3 powders on compact density and compaction efficiency during sintering

The effect of different methods of pretreatment and compacting of ultrafine alumina-zirconia powders of composition (in mass%): 20 Al₂O₃-80 (ZrO₂-Y₂O₃) on densification processes during pressing and subsequent calcination has been studied. Ultrafine powders were prepared by plasma-chemical method. It was found that the initial nanocomposite is a mechanical mixture made up of zirconia nanoparticles and amorphous alumina in a thermodynamically nonequilibrium state. Grinding of powders did not affect their phase state. Powder compacts were produced by means of uniaxial static pressing and magnetic pulse compaction. The impact of mechanical processing of powders on ceramics density was studied. It was shown that dry grinding of powders in a planetary ball mill does not increase the ceramics density. The best and virtually identical results were obtained using preliminary static pressing of powders at increased pressure P?=?900?MPa and their subsequent grinding in a ball mill. Dilatometric studies showed that double-action magnetic pulse compaction provides the maximum shrinkage rate at lower temperatures in comparison to that observed under static pressing. The ceramic density achieved is higher than that obtained using other pressing methods.     

Removal of the herbicide Bentazon from contaminated water in the presence of synthesized nanocrystalline TiO2 powders under irradiation of UV-C light

A solution-based processing method has been used to synthesize nanocrystalline TiO₂ powders by controlling the hydrolysis of TiCl₄ in an aqueous solution in both anatase and rutile phases. The primary particle sizes of the powders were in the range of 5-15 nm. To determine the crystal phase composition and size of the prepared photocatalysts, X-ray diffraction (XRD) measurements were used. We also studied the photocatalytic removal of the herbicide, Bentazon, from contaminated water in the presence of synthesized nanocrystalline TiO₂ powders under UV light illumination (30 W). The removal efficiency of Bentazon was 16% when the photolysis was carried out in the absence of TiO₂ and it was negligible in the absence of UV light. We have studied the influence of the basic operational parameters such as the different kinds of TiO₂, amount of TiO₂, irradiation time and initial concentration of Bentazon on the photocatalytic removal efficiency of Bentazon. Our results indicated that 99% removal of the herbicide from the solution containing 15 ppm of Bentazon after selecting desired operational parameters could be achieved in a relatively short time, about 90 min. A kinetic model was successfully established for the prediction of removal of Bentazon by the UV/TiO₂ system with any concentration of the herbicide. In this work, we also compared the photocatalytic activity between the commercial TiO₂ and synthesized nanocrystalline TiO₂ powders. The photocatalytic activities of different photocatalysts were tested using the herbicide solution.     

Green synthesis of thiiranes from epoxides catalyzed by magnetically separable CuFe2O4/Mg(OH)2 nanocomposite in water under benign conditions

The magnetically separable CuFe₂O₄/Mg(OH)₂ nanocomposite was prepared and characterized by FT-IR, XRD, SEM, EDS, and VSM techniques. The synthesized nanoparticles were used as a new and efficient heterogeneous catalyst for the conversion of various epoxides to the corresponding thiiranes with thiourea in water solvent at room temperature. The reactions were completed within 1–3.7?h to give thiiranes in 70–99% yields. The applied CuFe₂O₄/Mg(OH)₂ nanocomposite was separated easily using an external magnet and reused for several times without any considerable loss of activity.