Uniaxial anisotropy and enhanced magnetostriction of CoFe2_2O4_4 induced by reaction under uniaxial pressure with SPS

In this study, we have compared magnetic and magnetostrictive properties of polycrystalline CoFe$_2$O$_4$ pellets, produced by three different methods, focusing on the use of Spark Plasma Sintering (SPS). This technique allows a very short heat treatment stage while a uniaxial pressure is applied. SPS was utilized to sinter cobalt ferrite but also to make the reaction and the sintering (reactive sintering) of the same ceramic composition. Magnetic and magnetostrictive measurements show that the reactive sintering with SPS induces a uniaxial anisotropy, while it is not the case with a simple sintering process. The induced anisotropy is then expected to be a consequence of the reaction under uniaxial pressure. This anisotropy enhanced the magnetostrictive properties of the sample, where a maximum longitudinal magnetostriction of $-229$~ppm is obtained. This process can be a promising alternative to the magnetic-annealing because of the short processing time required (22 minutes)

Dielectric behaviour of Hf-doped CaCu3Ti4O12 ceramics obtained by conventional synthesis and reactive sintering

CaCu3(Ti4xHfx)O12 ceramics (JC = 0.04, 0.1 and 0.2) were prepared by conventional synthesis (CS) and through reactive sintering (RS), in which synthesis and sintering of the material take place in one single step. The microstructure and the dielectric properties of Hf-doped CCTO (CCTOHf) have been studied by XRD, FE-SEM, AFM, Raman and impedance spectroscopy (IS) in order to correlate the structure, microstructure and the electrical properties. Samples prepared by reactive sintering show slightly higher dielectric constant than those prepared by conventional synthesis in the same way than the pure CCTO. Dielectric constant and dielectric losses decrease slightly increasing Hf content. For CCTOHf ceramics with x> 0.04 for CS and x> 0.1 for RS, a secondary phase HfTi04 appears. As expected, the reactive sintering processing method allows a higher incorporation of Hf in the CCTO lattice than the conventional synthesis one

Spark plasma sintering as a reactive sintering tool for the preparation of surface-tailored Fe–FeAl2O4–Al2O3 nanocomposites

Al1.86Fe0.14O3 powders were partially or totally reduced in H2. The fully reduced Fe–Al2O3 nanocomposite powder was sintered by spark plasma sintering (SPS) without any reaction taking place. For the other powders, the SPS induced the formation of FeAl2O4 and sometimes Fe. The most severe reducing conditions were found at the surface of the materials, producing nanocomposites with a surface layer composition and microstructure different to those of the core. This in situ formed composite layer confers a higher hardness and fracture strength

Processing YAG/뱉 Al2O3 composites via reactive sintering Y2O3/Al2O3 NP mixtures. A superior alternative to bottom up processing using atomically mixed YAlOx NPs

This effort contrasts â bottomâ upâ processing of YAG/αâ Al2O3 composites where both elements (as 40â 50 nm APSs nanopowders) are present at close to atomic mixing with reactive sintering where ballâ milled mixtures of the individual nanopowders (40â 50 nm APSs) give uniform elemental mixing at length scales closer to 100â 800 nm with correspondingly much longer diffusion distances. In contrast to expectations, densification with control of final grain sizes is best effected using reactive sintering. Thus, reactive sintering to densities â ¥95% occurs at only 1500°C with final grain sizes of â 1000 nm for all samples. In contrast â bottom upâ processing to â ¥95% densities is only achieved at 1600°C, and with final grain sizes of 1700 nm. The reason for this unexpected behavior is that YAG phase forms early in the bottom up approach greatly inhibiting diffusion promoted densification. In contrast, in reactive sintering, YAG is prevented from forming because of the longer diffusion distances such that densification occurs prior to full conversion of the Y2O3 component to YAG. The found hardness values are statistically superior to literature values for composites near the known eutectic composition. In an accompanying paper, the addition of a third component reverses this behavior.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138233/1/jace14980_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138233/2/jace14980.pd

High Total Proton Conductivity in Large-Grained Yttrium-Doped Barium Zirconate

Barium zirconate has attracted particular attention among candidate proton conducting electrolyte materials for fuel cells and other electrochemical applications because of its chemical stability, mechanical robustness, and high bulk proton conductivity. Development of electrochemical devices based on this material, however, has been hampered by the high resistance of grain boundaries, and, due to limited grain growth during sintering, the high number density of such boundaries. Here, we demonstrate a fabrication protocol based on the sol−gel synthesis of nanocrystalline precursor materials and reactive sintering that results in large-grained, polycrystalline BaZr_(0.8)Y_(0.2O3−δ) of total high conductivity, 1 × 10^(−2) Scm^(−1) at 450 °C. The detrimental role of grain boundaries in these materials is confirmed via a comparison of the conductivities of polycrystalline samples with different grain sizes. Specifically, two samples with grain sizes differing by a factor of 2.3 display essentially identical grain interior conductivities, whereas the total grain boundary conductivities differ by a factor of 2.5−3.2, depending on the temperature (with the larger-grained material displaying higher conductivity)

Low-Temperature Rapid Synthesis and Superconductivity of Fe-Based Oxypnictide Superconductors

we were able to develop a novel method to synthesize Fe-based oxypnictide superconductors. By using LnAs and FeO as the starting materials and a ball-milling process prior to solid-state sintering, Tc as high as 50.7 K was obtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering at temperatures as low as 1173 K for times as short as 20 min.Comment: 2 pages,2 figures, 1 tabl

A two-stage approach of manufacturing FeAl40 iron aluminides by self-propagating synthesis and pressureless sintering

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Powder Metallurgy on 11 June 2018, available online at https://doi.org/10.1080/00325899.2018.1478778. Under embargo until 11 June 2019.A two-stage sintering process was successfully used to sinter FeAl to densification levels of just above 95% at a temperature of 1300 ºC. In the first stage, mixed iron and aluminium powders were synthesised at 750°C via Self-Propagating High-temperature Synthesis (SHS) to form brittle and porous Fe2Al5. Then the pellets were crushed and milled to various sizes and mixed with iron powders in the nominal composition of FeAl40 and pressurelessly sintered at a higher temperature to obtain a higher densification by taking advantage of the less violent exothermic reaction of Fe2Al5 and Fe. The intermediate and end products in SHS and sintering were characterised by SEM/EDX and XRD. The porosity level of the final FeAl40 product was controlled by the heating rate and powder size, which was also strongly influenced by the temperature, holding time and the ratio of the two powders.Peer reviewedFinal Accepted Versio

Kinetic and thermodynamic description of intermediary phases formation in Ti-Al system during reactive sintering

Reactive sintering is currently considered as a promising production route for titanium aluminides in many research works. However, the published descriptions of the reaction mechanism are contradictory or lacking, especially at the temperatures below the melting point of aluminium. This work aims to fill this gap, providing the description of the reactive sintering process at the temperatures between 400 and 900 degrees C. The phases' formation sequence and reaction kinetics were studied and explained using experimental model (Ti/Al diffusion couple) and real reactively sintered samples of equiatomic Ti-Al compressed powder blend. Moreover, phase formation was thermodynamically assessed. It was revealed that Ti2Al5 phase formed preferentially. This phase has not been reported previously as a starting phase in reactive sintering. According to results obtained by experimental model, its formation is controlled by diffusion at 700 degrees C. This phase reacted with aluminium forming pure TiAl3 phase or with titanium, resulting in TiAl phase. Subsequently, TiAl phase reacted with titanium, leading to the Ti3Al phase, or with already present Ti2Al5 phase yielding TiAl2 intermetallic compound. Titanium-rich Ti3Al phase could form only at the temperature of 600 degrees C or above