Electric current as a driving force for interphase growth in spark plasma sintered dielectric composites

All-oxide composites are increasingly investigated in the field of microelectronics and telecommunications because of their multi-functionalities. Dielectric properties can be tuned in ferroelectric-based composites performed by Spark plasma sintering (SPS) through a control of composition, grain size, architecture and charged defects at interfaces. Using SPS on ceramic bilayers made of ferroelectric Ba0.65Sr0.35TiO3 (BST65/35) and TiO2 dielectric phases, we show that the reactivity at their interfaces is dependent on the direction of the current and/or on the sintering temperature. The electric current promotes an interphase growth whose thickness depends on the bilayer stacking and on the current direction. The influences of the current density and direction on the rate of growth of interphases were mainly studied for intermetallic multilayers. We show here that this can be extended to dielectric oxides and that properties can be tuned by adjusting the nature of the composite (BST/TiO2, BST/ZrO2) and the architecture (bilayer, multilayer or random 3D mixing). The specific SPS conditions enable the control of both the interface between the two components and the reduction level leading to interface-driven dielectric properties. We suggest that such approach can be extended to other interphases between oxides of different reduction ability

Linking hopping conductivity to giant dielectric permittivity in oxides

With the promise of electronics breakthrough, giant dielectric permittivity materials are under deep investigations. In most of the oxides where such behavior was observed, charged defects at interfaces are quoted for such giant behavior to occur but the underlying conduction and localization mechanisms are not well known. Comparing macroscopic dielectric relaxation to microscopic dynamics of charged defects resulting from electron paramagnetic resonance investigations we identify the actual charged defects in the case of BaTiO3 ceramics and composites. This link between the thermal activation at these two complementary scales may be extended to the numerous oxides were giant dielectric behavior was found

Stoichiometry and Grain Boundaries Control by Spark Plasma Sintering in Ba0.6Sr0.4TiO3:Mn/MgO Composites

LaNbO4/La3NbO7 and LaNbO4/LaNb3O9 cer-cer composites were prepared by impregnating Ca-doped LaNbO4 powder, synthesized by spray pyrolysis, with La- or Nb-precursor solutions. The sintering of the calcined powders was investigated by dilatometry, and dense composites were prepared by conventional sintering, hot pressing, and spark plasma sintering. The particle size of the starting powders was about 50 nm, and the average grain size of the dense materials ranged from 100 nm and upwards, depending on the sintering temperature, sintering procedure, and the phase composition. The unit cell parameters of LaNbO4 showed a finite size effect and approached the cell parameters of tetragonal LaNbO4 with decreasing crystallite size, both for the single-phase material and the composites. The minority phase (La3NbO7 or LaNb3O9) were observed as isolated grains and accumulated at triple points and not along the grain boundaries, pointing to a large dihedral angle between the phases. The calcium-solubility in the minority phases was larger than in LaNbO4, which corresponds well with previous reports. The electrical conductivity of the heterodoped materials was similar to, or lower than, that for Ca-doped LaNbO4

Low-losses, highly tunable Ba0. 6Sr0. 4TiO3/MgO composite

Spark plasma sintering SPS is an efficient tool to obtain highly densified ferroelectric-dielectric ceramic composites with clean interfaces and tunable properties. Dielectric MgO and ferroelectric Ba0.6Sr0.4TiO3 BST were combined in two-dimensional multilayer and three-dimensional random powders design. Their unmodified BST Curie temperature proves the suppression of interdiffusion while dielectric losses are below 0.5% and the tunability is 40% at room temperature. The composites and pure BST with similar densities 95% were obtained, owing reliable comparison of their dielectric properties. Such SPS ceramics can be used as experimental input for simulation and are potential candidates for high frequency applications

Some strategies to (co)-sinter refractory functional oxides at low temperature by spark plasma sintering

The sintering at high temperatures (1000-1400°C) of refractory oxides widely used in electronic devices, raises several issues related to defects, chemistry, microstructure and interface control. Reducing sintering temperatures below 900°C is a major challenge in particular when high relative densities, optimal microstructure and the control of reactivity at interfaces (grain boundaries, multi-materials) are mandatory. In this context, we propose to highlight some strategies focused on interfaces and phases control through two different illustrations of our recent works. The first one is focused on Micro-Electromechanical System (MEMS) energy harvesters (EH) using piezoelectric materials[1]. We will show the potentiality of SPS to co-sinter in one step and below 900°C complex devices such as screen-printed PbZrTiO3 in sandwich between two gold electrodes and supported on a stainless steel substrate. Here, the sintering aids in the pastes should be removed if possible or adapted for good adhesion, and delamination and bending of the multilayer EH must be avoided. We also intent to get rid of the annealing process after the SPS sintering. The second illustration reports on the ambitious goal to sinter zirconia ceramics at temperatures below 400°C. Recently, the exploration of non-equilibrium sintering, through transient liquid phase, hydrated precursors, or by using solvent assisted sintering, Flash sintering and Spark Plasma Sintering has been investigated to sinter ZnO and thermodynamically unstable materials at very low temperature [2-5]. Here, our approach is based on the use of specific precursors and deals with the control of transient non-equilibrium phases to find the driving force to establish the most favorable pathway for enhanced densification. [1]. M. I. Rua-Taborda, O. Santawitee, A. Phongphut, B. Chayasombat, C. Thanachayanont, S. Prichanont, C. Elissalde, J. Bernard, H. Debéda, Printed PZT Thick Films Implemented for Functionalized Gas Sensors , Key Engineering Materials, 777,158, 2018 [2].B. Dargatz, J. Gonzalez Julian, M. Bram, P. Jakes, A. Besmehn, L. Schade, R. Röder, C. Ronning and O. Guillon, “FAST/SPS sintering of nanocrystalline zinc oxide—Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water, J. Eur. Ceram Soc. 36, 1207, 2016 [3]. S. Funahashi, J. Guo, H. Guo, K. Wang, A. L. Baker, K. Shiratsuyu, and C. A. Randall, Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics, Journal of the American Ceramic Society, 100, 546, 2017. [4]. Luo J., “The scientific questions and technological opportunities of Flash sintering: from a case study of ZnO to others ceramics”, Scripta Mater., 146, 260, 2018 [5]. T. Herisson de Beauvoir, A Sangregorio, I. Cornu, C. Elissalde and M. Josse, “Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials” J. Mater. Chem. C, 6, 2229, 201

Size effect on properties of varistors made from zinc oxide nanoparticles through low temperature spark plasma sintering

Conditions for the elaboration of nanostructured varistors by spark plasma sintering (SPS) are investigated, using 8-nm zinc oxide nanoparticles synthesized following an organometallic approach. A binary system constituted of zinc oxide and bismuth oxide nanoparticles is used for this purpose. It is synthesized at roomtemperature in an organic solution through the hydrolysis of dicyclohexylzinc and bismuth acetate precursors. Sintering of this material is performed by SPS at various temperatures and dwell times. The determination of the microstructure and the chemical composition of the as-prepared ceramics are based on scanning electron microscopy and X-ray diffraction analysis. The nonlinear electrical characteristics are evidenced by current–voltage measurements. The breakdown voltage of these nanostructured varistors strongly depends on grain sizes. The results show that nanostructured varistors are obtained by SPS at sintering temperatures ranging from 550 to 600 8C

Shaping of nanostructured materials or coatings through Spark Plasma Sintering

In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al2O3 and Y2O3) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO2 or Al2O3) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored FeœFeAl2O4œAl2O3 nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy

Shaping of nanostructured materials or coatings through Spark Plasma Sintering

In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al2O3 and Y2O3) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO2 or Al2O3) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored FeœFeAl2O4œAl2O3 nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy