Barium titanate (BaTiO 3 ) powders were synthesized by a solid state reaction between BaCO 3 and TiO 2 at high temperatures. Based on the empirical model for BaTiO 3 formation proposed by Beauger et al. [6], a simple mathematical model to represent the experimental results was derived. The assumptions of the model within a physical framework are discussed. It is believed that the proposed mathematical model can contribute to the understanding of the mechanisms that control BaTiO 3 formation at high temperatures.
In this research paper the synthesis of aluminum-copper/silicon carbide–based (Al-Cu/SiC) functionally gradient material using the innovative technique of gradient slurry disintegration and deposition is presented and discussed. Using the gradient slurry disintegration and deposition technique a gradient of SiC was established using an initial weight percentage of 15%. Results of the synthesis were confirmed by characterizing the microstructure and recording microhardness measurements. Test results revealed an increase in the weight percentage of SiC particulates along the direction of deposition to result in an increase in porosity, an elevation in microhardness, and an enhanced degree of clustering with the nature of SiC-Al alloy interfacial integrity remaining essentially unaltered. An attempt is made to highlight the interrelationships between processing technique, microstructural development, and microhardness of the material.
Self-propagating high-temperature synthesis was performed on an equimolar mixture of titanium and graphite embedded in a copper tube of variable thickness. Such an experimental setup allows us to study the interaction between the reaction front and copper. Special attention was focused on copper melting, copper penetration within the sample, and quenching of the sample: it is shown that a minimum of 1 mm for the copper thickness is necessary to ensure the physical integrity of the surrounding metal. Nevertheless, reaction is not complete under these conditions. As a consequence, copper is not the most appropriate metal for field assisted combustion synthesis electrodes. As for thermal explosion and high isostatic pressing combustion synthesis, some valuable insights are provided, and a metal with slightly lower thermal diffusivity, lower electrical conductivity, and/or higher melting temperature would likely be a good candidate for such applications.
Microwave-assisted aging Zn-Al-layered double hydroxide with nitrate as the interlayer anion (ZANOL) and its nanohybrid with an organic moiety, α-naphthaleneacetate (ZANAN), was done and the resulting properties of the materials compared. The results showed that intercalation of the α-naphthaleneacetate (NAA) anion into the Zn-Al layered double hydroxide lamella is readily accomplished, resulting in the expansion of the interlayer spacing from 9.0 Å in the layered double hydroxide to 20.0 Å in the nanohybrid. This expansion accommodates the NAA anion of larger size than nitrate, as indicated by its molecular structure. For both methods, the resulting materials afforded a well-ordered organic–inorganic nanolayered structure. The Zn to Al ratio of the resulting nanocomposite is lower than the ratio present in the mother liquor at the beginning of the reaction, which implied less incorporation of aluminum ions from the mother liquor into the inorganic metal hydroxide layers. By using the microwave-assisted method, however, slightly more Al3+ ions were incorporated into the inorganic metal double hydroxide layers. In general, there is not much difference in the physicochemical properties of ZANANs aged by either the microwave or the conventional oil bath method. For both methods, longer aging time slightly enriched the organic content of the resulting nanohybrid and the inorganic Zn to Al ratio remained the same, independent of the aging time.
Urea or biuret was added to the thermal synthetic system of Rhabdophane-type neodymium and cerium phosphates. The mixture of a rare earth compound, a phosphorus compound, and an additive [CO(NH2)2 or NH(CONH2)2] was heated at 150°C or 300°C for 20 hr, and the thermal products were analyzed by the XRD, FT-IR, and BET methods. H3PO4 and (NH4)2HPO4 were used for phosphorus compounds, and for rare earth compounds, Nd2O3, Nd(NO3)3 · 6H2O, NdCl3 · 6H2O, Nd2(CO3)3 · 8H2O, CeO2, Ce(NO3)3 · 6H2O, CeCl3 · 7H2O, and Ce2(CO3)3 · 8H2O were used. Urea and biuret worked not only as a dispersing agent but also as a reactant. By the addition of biuret, the thermal products changed from cerium oxide to Rhabdophane-type cerium phosphate in the system using CeCl3 · 7H2O and (NH4)2HPO4. Addition of urea or biuret influenced the specific surface area of Rhabdophane-type neodymium and cerium phosphates. Furthermore, to increase the reactivity of the raw solid materials, mechanical treatment was performed. The mixture of diammonium hydrogenphosphate and a rare earth compound was ground with water and then heated. The influence of the addition of urea or biuret was also studied in these systems.
The (Ba,Sr)TiO3 amorphous gel was prepared by sol-gel process and calcined in the 2.45-GHz multimode microwave furnace to synthesize (Ba,Sr)TiO3 nanopowder. The calcination temperature of the (Ba,Sr)TiO3 ceramic powders that convert the material into prevoskite phase can be reduced from 1100°C to 900°C, the nanopowder displays the highest sinterability. Using a new kind of insulator materials made of MgAl2O4–LaCrO3, the crack-free and dense (Ba0.80Sr0.20)TiO3 ceramics with fine grain size (<1 µm) were prepared by microwave sintering at 1310°C for 15 min. The fine (Ba,Sr)TiO3 ceramics sintered by microwave sintering technique display lower dielectric loss than that of conventional samples, indicating a reduction of the influence of defects with the microwave process.
Magnetic Fe 3 O 4 nanoparticles with size below 10 nm have been prepared by the aqueous phase coprecipitation method. The Fe 3 O 4 nanoparticles show typical superparamagnetism. Comparison is made between the dispersed sample and the powder sample, and the results are discussed.
Alumina (Al2O3) powders were synthesized by extracting Al2O3 from kaolin via kaolin-H2SO4 reactions with and without ultrasounds. The amount of Al2O3 extracted from kaolin via the ultrasonic extraction process was investigated by comparing reaction time and reaction temperature with the same factors under the conventional extraction process. The time to obtaining a given amount of Al2O3 by the ultrasonic process was shorter than that by the conventional process, indicating that the application of ultrasound in the synthesis of Al2O3 powders is an efficient way to reduce synthesizing time.
The motion of a fine inclusion in a viscous medium is subjected to theoretical examination. This motion can be qualified as an elementary act of the motion of fine phase particles in mixtures: suspensions, emulsions, airsols, and foamed liquid materials. The erroneous character of the theory of thermocapillary motion of inclusions, which is universally recognized now, is revealed. It is proposed to take into account the “energy of location” of the inclusion in a nonuniform temperature field, to formulate more correctly the Stokes equation describing the velocity field inside the fluid inclusion. The solution of the set of the Stokes equations referring to a liquid particle and a surrounding medium has allowed obtaining the formula for the motion velocity of a particle under joint influence of electromagnetism and thermocapillarity. This formula is suitable for theoretical calculations of transfer processes in suspensions, emulsions, and gaseous media containing various inclusions. The calculation on inclusions in molten metals (e.g., in Cu, Al, Fe, Ag) shows that the exertions caused by electromagnetism and thermocapillarity are comparable.
The dispersion and rheological behavior of alumina, zirconia, and alumina/zirconia mixed slurries were investigated using various solvent ratios of methyl isobutyl ketone (MIBK)/ethanol (EtOH), by measuring sedimentation bulk density, particle size distribution, and viscosity. Well-dispersed suspensions were obtained in MIBK-rich solvents with additional dispersant and in EtOH-rich without dispersant. The shear viscosity of the slurries was dependent on both the Al2O3/ZrO2 ratio and MIBK/EtOH ratio. At a constant solvent ratio, however, similar rheological behavior was shown regardless of the relative amounts of the two solids. At low shear rate, a Newtonian plateau was absent in the Al2O3/ZrO2 slurries. With increasing shear rate (>600 s−1), Al2O3 slurries exhibited a Newtonian plateau while ZrO2 demonstrated continuous shear thinning.
The interaction of chromium vapors with powderlike silicon carbide (SiC) was investigated by X-ray phase analysis, X-ray microanalysis, the EPR method, electron microscopy, and the BET method. It has been established that in the temperature range of 147 K to 2023 K under a pressure of 1.3 Pa, the main interaction product is chromium silicide (Cr 5 Si 3 ), which forms a surface layer on SiC particles in zones with a mean temperature ( T mean ) of 1773 K to 1473 K. Moreover, Cr 5 Si 3 vapors passing through the disperse SiC system condense in cold-temperature zones on SiC particles and aggregates.
La0.7Ca0.3MnO3 powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La0.7Ca0.3MnO3 was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La0.7Ca0.3MnO3 powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La0.7Ca0.3MnO3 powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m2/g. The resistance as a function of temperature and the magnetoresistance ratio in La0.7Ca0.3MnO3 thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.
Incommensurability of interacting sublattices in inorganic suprastructures can lead to formation of defects, including fragmentation of one of the sublattices. A theoretical approach is proposed, which is based on the analytical and numerical studies of the static one-dimensional Frenkel–Kontorova model for chains of finite length. This approach provides the possibility of relating geometric and energy parameters of the interacting sublattices to the values of bond deformations that arise. These deformations, in turn, may result in the fragmentation of one of the sublattices and formation of imperfect structures.
The experimental results of the influence of drying control chemical additives (DCCAs)—such as formamide (FA), N-methyl formamide (NMF), N-N′-dimethyl formamide (DMF), acetamide (AA), glycerol (GLY), and oxalic acid (OXA)—on the physical and optical properties of nanocrystalline cadmium sulfide (CdS)–doped silica xerogels were reported in this paper. Tetraethylorthosilicate [TEOS, Si(OC2H5)4] was used as a precursor for the three-dimensional silica network in which the CdS nanocrystallites were trapped. Silica alcosol was prepared by taking the mixture of ethanolic (EtOH) TEOS, water (H2O), hydrochloric acid (HCI), and ammonium hydroxide (NH4OH). Ethanolic cadmium acetate [Cd(CH3COO)2 ⋅ 2H2O] and thiourea [CS(NH2)2] were added to the alcosol for the formation of CdS crystallites. To study the effect of DCCAs on the physical and optical properties of nanocrystalline CdS-doped silica xerogels, the molar ratio of TEOS:EtOH:H2O:HCl:NH4OH:Cd(CH3COO2)2 2H2O:CS(NH2)2 was kept constant at 1:5:7:0.01:0.0027:0.001:0.002, respectively, and the molar ratio of DCCA/TEOS was varied from 0.001 to 1. The addition of DCCAs (e.g., formamide) resulted in decreased gelation time (tg) from 240 to <20 hrs. Transparent CdS-doped alcogels and xerogels were obtained for the DCCA/TEOS molar ratios of <0.5, whereas turbid alcogels and translucent nanocrystalline CdS-doped silica xerogels were obtained for higher DCCA/TEOS molar ratios (>0.5). It has been found that the percentage of volume shrinkage of the samples, when heated at 300°C for 3 hr, was more (>15%) for OXA, GLY, and DMF and the shrinkage was less (<15%) for AA, NMF, and FA. The density of the CdS-doped silica xerogels with DCCAs was less than that without DCCAs. Red shift was observed in the optical absorption spectra of the samples with addition of the DCCAs. An increase in CdS crystallite size in the silica matrix with the incorporation of DCCAs was observed in the X-ray diffraction patterns in the order: OXA < GLY < DMF < AA < NMF < FA. From XRD studies, the structure of the CdS crystallites in the silica matrix was found to be hexagonal wurtzite.
At electrodischarge sintering of powder mixtures by electric direct current, some essential displacements of molten metal and solid particles were revealed. Such displacements at conventional sintering were not known to us. The objects were powders (mixtures): Sn–Cu (dispersivity 100–200 µm); Sn–high-temperature alloy on nickel base or electrocorundum; copper alloy-electrocorundum (dispersivity 63–500 µm); Ni (dispersivity 10 µm). The electric direct current was passed in vertical direction parallel with the axis of cylindrical mould or through the series circuit steel-powder nickel layer-hard alloy. If the cylindrical container (mould) did not have the upper punch (a load on top was absent), the sample after solidification had the form of a “bullet.” This sample became convex on top and concave on bottom. At the application of prepressing (start pressing) on top, the movement of suspension (molten matrix and solid particles) occurred in a similar way. Under the action of electromagnetic forces, a redistribution of solid particles in volume of suspension occurs. Particles with more high conductivity than liquid phase begin to cluster at the center. For the case of lower conductivity, the motion of the particles to the outer side surface of mould prevails. As the current passed through the series of circuit steel-nickel powder layer-hard alloy, three named parts of the circuit were sintered into a single block. After destruction at testing, the interlayer had the form of a ring. This shows the nonuniformity of the passing of electric current through the interlayer. The cause of displacements of solid (nonmolten) particles may be the simultaneous influence of thermic convection and squeezing out by the electromagnetic Lorentz-forces, which are directed radially. The phenomena described may be applied in the production of objects with a gradient (variable in volume) concentration of particles added specially. This refers to products of their interaction with molten matrix as well.
The effect of polyacrylic acid (PAA) molecular weight on the rheological properties of TIF aqueous alumina suspensions that have gelation behavior at elevated temperature is studied. The volume fraction gelation threshold of such alumina suspensions is estimated based on the viscoelastic properties. The volume fraction gelation thresholds are 0.20 and 0.35 for the alumina suspensions containing 0.04 wt% PAA of Mw 10,000 and Mw 5,000, respectively. Both values are higher than that of the suspensions with 0.04 wt% PAA of Mw 50,000, which is about 0.15. These results can give us guidance to tailor both the composition and gelation character of the TIF alumina suspensions for direct casting.
The products formed during thermolysis of poly(germasilethyne) have been studied within temperature range 20–1600°C by means of Raman spectroscopy, XRD and EDS analysis, and SEM analysis. The evolution of solid products derived from poly(germasilethyne) as a function of heat-treatment temperature passes through several stages. At earlier stages the cross-linking of polymer molecules and the formation of rigid three-dimensional network occur. The particularity of the first stage of thermal transformation of poly(germasilethyne) in comparison with silicon analogue is not only the formation of the skeleton in the solid phase but also the release of disordered carbon phase. The stage of intensive thermal decomposition of germanium-containing polymer includes the escape of gaseous products, the formation of strongly disordered carbon, amorphous covalent silicon–germanium-carbon network and germanium as inorganic phase (most likely Si 1−x−y Ge x C y ). High-temperature treatment results in the evaporation of germanium, the formation and ordering of carbon and α -SiC phases. The presence of germanium in the products of thermolysis of polymer leads to a faster growth of the oxides during exposition of thermolyzed polymer to air at high temperatures.
In situ synthesis of calcium carbonate particles has been carried out in poly(vinylalcohol) solution and physical gels under free drift conditions. A multitude of morphologies, ranging from spherulites to helical structures of calcite, are produced by systematically varying the polymer concentration both in solution as well as in gels.
In order to obtain glass-ceramics reinforced by β-wollastonite, pressed specimens prepared by powdered waste fluorescent glass and refuse shell dumped in the seashore were heat-treated at 800°C, 900°C, and 1000°C. Chemical stability of the heat-treated specimens was analyzed by weight changes and by observation of surface morphology and composition, by field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Chemical durability of the samples, especially at the glass-matrix area, was decreased with an increase in the heat-treatment temperature from 800°C to 1000°C.
The grain growth kinetics of commercially pure Titanium with different contents of oxygen have been studied. The grain size parameters have been measured by means of the Image Analysis Technique for different heat treatment temperatures and times. The growth exponents and activation energies have been calculated. Finally, tensile tests on the same materials have been carried out. When the interstitial oxygen content in titanium increases the grain growth kinetics and the ductility decrease and the α→β transition temperature, activation energy for grain growth and mechanical strength increase.
