Search results for " pressing"

Consolidation and densification mechanisms of copper powder during Spark plasma sintering (SPS)

2015

Spark plasma sintering is a manufacturing process that leads to dense materials with fine microstructures. SPS combines heating and uniaxial load as well as the Hot Pressing (HP) process but the material is heated using a pulsed current. The phenomena occurring during SPS are not fully understood and are still an open point: -Which densification and consolidation mechanisms are involved during SPS? -Why is sintering by SPS more efficient than sintering by traditional ways such as HP? –Does electrical current modify the sintering mechanisms? The aim of this work is to answer these questions in the case of spherical copper powder (from 10 to 50 µm). Comparisons between SPS and HP were perform…

Modeling of yttrium, oxygen atoms and vacancies in γ-iron lattice

2011

Abstract Development of the oxide dispersion strengthened (ODS) steels for fission and fusion reactors requires a deep understanding of the mechanism and kinetics of Y 2 O 3 nanoparticle precipitation in the steel matrix. Therefore, it is necessary to perform a large-scale theoretical modeling of the Y 2 O 3 formation. In the current study, a series of first-principles calculations have been performed on different elementary clusters consisting of pair and triple solute atoms and containing: (i) the Y–Fe-vacancy pairs, (ii) the two Y atoms substituted for Fe lattice atoms and (iii) the O impurity atoms dissolved in the steel matrix. The latter is represented by a face-centered cubic γ-Fe si…

Uniaxial stress dependence of the dielectric and ferroelectric properties of the Na0.5K0.5NbO3and Na0.5K0.5NbO3 + 0.5 mol%MnO2ceramics

2013

Na0.5K0.5NbO3 and Na0.5K0.5NbO3 + 0.5 mol%MnO2 lead-free ceramics have been produced by solid phase hot pressing sintering process at 1120–1145°C. The results of the X-ray diffraction measurements showed that the obtained samples possessed a perovskite structure. The micrograph of the fractured surface showed a dense ceramic structure, in compliance with 93% relative density determined by the Archimedes method. The dependence of dielectric and ferroelectric properties on the uniaxial compressive stress (0–3000 bar) of the Na0.5K0.5NbO3 and Na0.5K0.5NbO3 + 0.5 mol%MnO2 ceramics was investigated. Significant influence of the external stress on these properties was revealed. It includes a shif…

Influence of Compressive Stress on Dielectric and Ferroelectric Properties of the Na0. 5K0.5NbO3, Na0.5K0.5(Nb0.94Sb0.06)O3and Na0.5K0.5(Nb0.94Sb0.06…

2013

Good quality lead-free ceramics of Na0.5K0.5NbO3, Na0.5K0.5(Nb0.94Sb0.06)O3 and Na0.5K0.5(Nb0.94Sb0.06)O3 +0.5mol%MnO2 have been produced by a solid phase hot pressing sintering process at 1120–1145°C. The dependence of dielectric and ferroelectric properties on the uniaxial pressure (0–3000 bar) of the these samples was investigated, revealing a significant influence of the external stress on their properties. This includes a shift of the phase transition, a diffuseness of the permittivity characteristics, a change of the dielectric dispersion and a decrease of the polarization. We discuss our results based on hardening of the soft-mode and domain switching processes under applying of pres…

Influence of the suspension of continued treatment with flurazepam and amobarbital on two discrimination learning schedules.

1977

The authors have studied the effect of the suspension of chronic treatment with flurazepam and amobarbital on the operant behavior of rats that for the first time were in the presence of two fixed-interval discrimination schedules. With the sound discrimination schedule, the responses emitted by the treated animals had characteristics similar to those of control animals. With the temporal discrimination schedule, though it is not possible to distinguish between learning rates, modifications in the intensity of the effect (increases in lever pressing) indicate that, considering the doses, the action of flurazepam is slight and that of amobarbital clear and statistically significant.

DIELECTRIC PROPERTIES OF LEAD-FREE [(1-x)(Na0.5Bi0.5)-xBa]Zr1−yTiyO3CERAMICS (x = 0.01, 0.06, 0.085, 0.09, 0.1 AND y = 0.96, 0.97)

2008

ABSTRACT Lead-free ceramics based on (Na0.5Bi0.5TiO3, NBT)-(Ba(Ti, Zr)O3, BTZ) were prepared by solid phase synthesis and hot pressing sintering process and their dielectric properties have been studied. The obtained results were compared with these for pure NBT. The obtained samples show pure perovskite structure. Low frequency (100 Hz-100 kHz) investigations reveal the diffuse phase transitions and relaxorlike behaviour. The obtained results are discussed in terms of inhomogeneity of ions distribution, their ferro- and antiferroactivity and internal stresses. The NBT-BTZ system is expected to be a new promising candidate for lead-free electronic ceramics.

Production and Properties of (1-x)Pb(Sc<sub>1/2</sub>Nb<sub>1/2</sub>)O<sub>3</sub> – xPb(Yb<sub>1/2</su…

2004

Dielectric Properties of Na0.5K0.5(Nb1-xSbx)O3+MnO2 Ceramics (x = 0.04, 0.05 and 0.06)

2011

Lead-free ceramics Na0.5K0.5(Nb1-xSbx)O3+0.5mol%MnO2 (x = 0.04, 0.05 and 0.06) were prepared by conventional sintering and hot pressing process; their microstructure and dielectric properties have been studied. It is found that increase of sintering temperature promotes the obtaining highly dense microstructure and increase in grain size while doping with Sb contributes to more dense and homogeneous microstructure and smaller grain size. Low frequency (100 Hz-200 kHz) investigations revealed the diffuse phase transitions. It can be concluded from experimental results that Sb doping improves dielectric properties. The obtained materials are expected to be promising candidates for lead-free e…

Structural, thermal, dielectric and ferroelectric properties of Na0.5K0.5NbO3and Na0.5K0.5NbO3+0.5mol%MnO2ceramics

2014

Lead-free Na0.5K0.5NbO3 and Na0.5K0.5NbO3+0.5mol%MnO2 ceramics were fabricated by solid state hot pressing sintering process. The phase structure and thermal, dielectric and ferroelectric properties of these ceramics were investigated. X-ray measurements showed that all samples have a single perovskite structure and possess orthorhombic–tetragonal and tetragonal–cubic phase transitions. Sequence of phase transitions was confirmed by differential scanning calorimetry (DSC) and dielectric measurements. Obtained results showed that optimal temperature sintering of Na0.5K0.5 NbO3+0.5mol%MnO2 ceramics is 1100 °C. The results show that investigated ceramics are one of the promising materials as e…

Structure, nanohardness and photoluminescence of ZnO ceramics based on nanopowders

2015

ZnO ceramics obtained from grained powders with different grain size by hot pressing and ceramics from tetrapods nanopowders obtained by press-less sintering have been investigated under identical conditions. Ceramics obtained by hot pressing were optically transparent but were composed of large inhomogeneous grains (d = 8–35 μm) exhibiting a substructure. Decreased values of elastic modulus within a grain and a wide defect-associated ('green') photoluminescence (PL) band at 2.2–2.8 eV in conjunction with a weak excitonic band indicate a high concentration of residual point defects in hot pressed ZnO ceramics. Utilization of more small-grained powders contributes to the formation of more un…