Effect of 1 wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

Densification of nanocrystalline cubic yttria (nc-Y2O3) powder, with 18 nm crystal size and 1 wt% LiF as a sintering additive was investigated. Specimens were fabricated by spark plasma sintering at 100 MPa, within the temperature range of 700–1500 °C. Sintering at 700 °C for 5 and 20 min resulted in 95% and 99.7% dense specimens, with an average grain size of 84 and 130 nm, respectively. nc-Y2O3 without additive was only 65% dense at 700 °C for 5 min. The presence of LiF at low sintering temperatures facilitated rapid densification by particle sliding and jamming release. Sintering at high temperatures resulted in segregation of LiF to the grain boundaries and its entrapment as globular phase within the fast growing Y2O3 grains. The sintering enhancement advantage of LiF was lost at high SPS temperatures

Spark plasma sintered carbon electrodes for electrical double layer capacitor applications

The spark plasma sintering (SPS) is an emerging process for shaping any type of materials (metals, ceramic, polymers and their composites). The advantage of such a process is to prepare densified ceramic materials in a very short time, while keeping the materials internal porosity. In the present work, we have used the SPS technique to prepare activated carbon-based electrodes for Electrochemical Double Layer Capacitor applications (EDLC). Self-supported 600 and 300µm-thick electrodes were prepared and characterized using of Electrochemical Impedance Spectroscopy and galvanostatic cycling in a non-aqueous 1.5MNEt4BF4 in acetonitrile electrolyte. Electrochemical performance of these sintered electrodes were found to be in the same range – or even slightly better – than the conventional tape-casted activated carbon electrodes. Although organic liquid electrolyte was used to characterize the electrochemical performance of the sintered electrodes, these results demonstrate that the SPS technique could be worth of interest in the ultimate goal of designing solid-state supercapacitors

Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

Multiphase (MP) monoclinic and cubic Y2O3 nanoparticles, 40 nm in diameter, were densified by spark plasma sintering for 5–15 min and100 MPa at 1000 °C, 1100 °C, and 1500 °C. Densification started with pressure increase at room temperature. Densification stagnated during heating compared to the high shrinkage rate in cubic single-phase reference nanopowder. The limited densification of the MP nanopowder originated from the vermicular structure (skeleton) formed during the heating. Interface controlled monoclinic to cubic polymorphic transformation above 980 °C led to the formation of large spherical cubic grains within the vermicular matrix. This resulted in the loss of the nanocrystalline character and low final density

Hardness and friction behavior of bulk CoAl2O4 and Co–Al2O3 composite layers formed during Spark Plasma Sintering of CoAl2O4 powders

Materials made up of a Co–Al2O3 composite coating over a CoAl2O4 core are prepared during Spark Plasma Sintering of CoAl2O4 powders. The Co particles are precipitated because of a combination of high temperature and low O2 partial pressure. The precipitation and densification processes hamper each other and thus the way the uniaxial pressure is applied during the sintering cycle is an important parameter to control the microstructure of composite layer and its thickness (about 100 mm) and obtain a dense sample (about 4 g/cm3). The friction coefficient of the Co-Al2O3 composites against an Al2O3 ball is lower than that found for an Al2O3 specimen, which could reveal the lubricating role of submicrometer Co particles. However, increasing the load from 5 to 10 N load causes major changes in the friction contact, which are detrimental. Bulk CoAl2O4 was found to have a Vickers microhardness about 15.5 GPa and an average friction coefficient lower than that of an Al2O3 sample

SPS-prepared targets for sputtering deposition of phase change films.

Phase-change materials like thin films from the systems [Ge1-xPbx]Te and Ge[Te1-xSex] are of interest for data storage. For these compositions amorphous materials can not be obtained by melt quenching. However, Suitable films can be obtained using RF sputtering. Spark plasma sintering (SPS) was used to densify the powders to obtain large targets. Synthesis conditions and characterisations of the targets are reported. Amorphous nano films were obtained using the sintered targets and characterised

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

Mechanical and tribological properties of Fe/Cr-FeAl2O4-Al2O3 nano/micro hybrid composites prepared by Spark Plasma Sintering

Fe/Cr–Al2O3 nanocomposite powders are prepared by H2 selective reduction of oxide solid solutions. These powders, an alumina powder and a starting oxide powder, are sintered by spark plasma sintering. The microstructure of the resulting materials is studied. The composites show a lower microhardness and higher fracture strength than unreinforced alumina. The friction coefficient against an alumina ball is lower, revealing the role of the intergranular metal particles, whereas FeAl2O4 grains formed during SPS are beneficial for higher cycle numbers

Flash sintering of dielectric nanoparticles as a percolation phenomenon through a softened film

Recent work [Biesuz et al., J. Appl. Phys. 120, 145107 (2016)] showed analogies between the flash sintering and dielectric breakdown in α-aluminas pre-sintered to different densities. Here, we show that flash sintering of dielectric nanoparticles can be described as a universal behavior by the percolation model. The electrical system is composed of particles and their contact point resistances, the latter softened first due to preferred local Joule heating and thermal runaway during the flash. Local softening has a hierarchical and invasive nature and propagates between the electrodes. The flash event signals the percolation threshold by invasive nature of the softened layer at the particle surfaces. Rapid densification is associated with local particle rearrangements due to attractive capillary forces induced by the softened film at the particle contacts. Flash sintering is a critical phenomenon with a self-organizing character. The experimental electric conductivity results from flash sintering are in full agreement with those calculated from the percolation model

Influence of pulse current during Spark Plasma Sintering evidenced on reactive alumina–hematite powders

Spark Plasma Sintering (SPS) is increasingly used. The temperature and current are not independent parameters, making it difficult to separate the current intrinsic role from Joule heating. There is a debate on whether there are any specific SPS mechanisms. The influence of a key parameter, the (on:off) pulse pattern, is studied on the SPS of reactive α-Al2−2xFe2xO3 (x = 0.02; 0.05; 0.07; 0.10) powders. Changing it modifies the current crest intensity and has a great influence on the materials microstructure. Comparisons with runs where the current is blocked and hot-pressing reveal three competing phenomena: formation of FeAl2O4, dominant in the core and not peculiar to SPS, formation of Fe, producing Fe-Al2O3 composite surface layers, and most notably electrical-field induced diffusion of Fe3+ ions towards the cathode, which could have far-ranging implications for the consolidation of ionic materials and the in situ reactive shaping of composites and multimaterials

Spark plasma sintering of double-walled carbon nanotubes

Bulk samples of double-walled carbon nanotubes are prepared for the first time. The best spark plasma sintering conditions are (1100 °C, 100 MPa). Raman spectroscopy and scanning electron microscopy show that the nanotubes are undamaged. The density is equal to 1.29 g cm−3 and the pores are all below 6 nm in diameter. The electrical conductivity is equal to 1650 S cm−1. The transverse fracture strength is equal to 47 MPa