Electric Field-assisted Sintering Research Articles

Synthesis and electric field-assisted sintering behavior of Al2O3–ZrO2 composite nanopowders by polyacrylamide gel method

Al2O3–ZrO2 composite ceramic nanopowders were synthesized using a polyacrylamide gel method. The mean particle size of the Al2O3–ZrO2 nanopowders was found to decrease with increasing mole ratio of monomer to the precursor salt. The Al2O3–ZrO2 nanopowders with mean particle size of 10 nm can be densified in 1 h at 965 °C, by application of a dc electrical field. Under a constant dc electrical field, the current density through the sample of Al2O3–ZrO2 rose rapidly when the temperature increased to a certain value. In the sintering process, the current density was restricted when a sharp increase occurred. It was found that current density had a great effect on the sintering process in electric field-assisted sintering. Joule heating was proposed to be the main mechanism of the electric field-assisted sintering process. The Al2O3–ZrO2 composite ceramic nanopowders were synthesized using a polyacrylamide gel method. The Al2O3–ZrO2 nanopowders can be densified in 1 h at low temperature, by the application of a dc electrical field. Under a constant dc electrical field, the current density through the sample of Al2O3–ZrO2 rose rapidly when the temperature increased to a certain value.

Electric field-assisted flash sintering of CaCu3Ti4O12: Microstructure characteristics and dielectric properties

The electric field-assisted flash sintering method recently introduced in the literature was here considered for producing high-quality CaCu3Ti4O12 electroceramics, from a powder originally synthesized by a Pechini-derived method and calcined at 800 °C for 2 h. The study included analyzing the dynamics of material shrinkage and densification. With increasing the electric field (E), three distinct regimes were observed: a conventional-like sintering behavior for E < 15 V/cm, followed by a region of accelerated (fast-dominated) sintering for 15 ≲ E < 30 V/cm, and then the flash-dominated regime, for E ≳ 30 V/cm, where sintering is not only accelerated but occurs suddenly. In consequence, sintering of the material under field input takes place at furnace temperatures sensibly lower, reaching a value as low as 750 °C for E = 60 V/cm versus 1050 °C in conventional processing. The probable physical mechanism behind each regime, including the role of the rise in sample temperature induced by the applied field (Joule heating), is also discussed. Finally, a correlation was found between the microstructure characteristics achieved during sintering and the dielectric response measured.

Joining of carbon fiber reinforced carbon composites with Ti3SiC2 tape film by electric field assisted sintering technique

Carbon fiber reinforced carbon composites (Cf/C) were first joined by combining electric field-assisted sintering technology and using a Ti3SiC2 (TSC) tape film as interlayer. A joint with shear strength of 26.3 ± 1.7 MPa was obtained within 12 min at a joining temperature of 1200 °C. The joint morphology, interface reaction, shear fracture behavior, and joining mechanism were investigated in detail. To achieve reliable joining of a Cf/C composite to itself via a TSC interlayer, the current work show the crucial controlling factor of interface reaction on the shear strength and the shear fracture behavior. Optimized bonding could be achieved without sacrificing the high strength of the carbon fibers and also taking advantage of the pseudo-plasticity feature of the TSC interlayer. A possible high-toughness joining structure was also proposed based on these results. The current-aid joining technique shortened significantly the bonding process of the Cf/C composites at moderate temperatures, and simplifies the manufacture of components with complex shapes.

Fast joining SiC ceramics with Ti3SiC2 tape film by electric field-assisted sintering technology

Commercial SiC ceramics were joined by electric field-assisted sintering technology using a Ti3SiC2 (TSC) tape film. A SiC/TSC/SiC joining sample with bend strength of 80.4 MPa was obtained at a low joining temperature of 1300 °C within a total time of 15 min. Three simple failure mechanism models were established, and the failure mechanisms of the joints joined at different temperatures were then revealed. The element diffusion and phase transition behaviour in the joining interface were investigated.

Consolidation of Partially Stabilized ZrO_{2} in the Presence of a Noncontacting Electric Field.

Electric field-assisted sintering techniques demonstrate accelerated densification at lower temperatures than the conventional sintering methods. However, it is still debated whether the applied field and/or resulting currents are responsible for the densification enhancement. To distinguish the effects of an applied field from current flow, in situ scanning transmission electron microscopy experiments with soft agglomerates of partially stabilized yttria-doped zirconia particles are carried out. A new microelectromechanical system-based sample support is used to heat particle agglomerates while simultaneously exposing them to an externally applied noncontacting electric field. Under isothermal condition at 900 °C, an electric field strength of 500 V/cm shows a sudden threefold enhancement in the shrinkage of the agglomerates. The applied electrostatic potential lowers the activation energy for point defect formation within the space charge zone and therefore promotes consolidation. Obtaining similar magnitudes of shrinkage in the absence of any electric field requires a higher temperature and longer time.

Light emission during electric field-assisted sintering of electroceramics

Electric field-assisted sintering has been shown to occur in ion, electron conducting as well as in insulating polycrystalline ceramics. Considerable linear shrinkage with microstructure control at temperatures lower than the conventionally utilized has been attained mainly in oxide ion conducting zirconia-based solid electrolytes. Some mechanisms, based on localized Joule heating and enhancement of defects diffusion, have been set forth to explain the sudden volume reduction in those ceramics. The results here reported contribute to the understanding of what happens during electric field-assisted sintering, by evidencing the occurrence of light emission simultaneously to the quasi instantaneous specimen shrinkage. An experimental setup with an optical fiber inserted in a dilatometer furnace allowed for light emission detection simultaneously to the electric current pulse in zirconia-yttria and also in tin dioxide specimens upon electric field-assisted sintering.

Electric field-assisted sintering of tin dioxide with manganese dioxide addition

Abstract Green compacts of pure SnO 2 and with addition of 0.25, 0.5 and 1.0 wt.% MnO 2 were sintered by applying 100 V cm −1 at 1 kHz and limiting the current to 5 A during 5 min at 1100 and 1200 °C. The shrinkage was monitored precisely during the electric current pulses. The role played by the additive was clearly seen in the shrinkage data: the higher is the additive content the lower is the onset of the shrinkage and the higher is the attained final shrinkage level. Sintering experiments on cylindrical samples with 0.5 wt.% MnO 2 with different thickness-to-diameter ratio show that the lower is that ratio, the higher is the shrinkage level, showing that the imparted Joule heating play a key role in the mechanisms responsible for sintering. The total electrical resistivity, evaluated by impedance spectroscopy, depends on the maximum attained shrinkage level, due to pore elimination upon sintering.

Shrinkage control of yttria-stabilized zirconia during ac electric field-assisted sintering

Abstract Yttria-stabilized zirconia pellets were easily and accurately sintered to a predetermined sintering level, including near full density, in an experimental arrangement consisting of a vertical dilatometer and an ac adjustable power supply. Conventional and electric field-assisted sintering steps can be combined, starting from temperatures above 800 °C and applying 1000 Hz alternating electric fields in the range 80–160 V cm −1 . A systematic comparison of the microstructures and impedance diagrams of samples conventionally and electric field-assisted sintered to the same density levels shows that the non-conventional sintering method gives significantly small grains in agreement with previous observations. The results show that this sintering method can be applied to produce materials partially sintered at any desired shrinkage level.

Phase formation of Si-based ceramic composites from polyureasilazane prepared by hot pressing and EFAS

Si-based ceramic composites were fabricated using two sintering techniques: hot pressing and electric-field-assisted sintering (EFAS). Liquid polyureasilazane was cross-linked and pyrolysed, then subjected to planetary ball milling for 24h, followed by sintering at 1500–1700°C in a hot press for 1h and EFAS for 10min. The phase formation behavior was different in the hot pressing than in the EFAS. The decomposition of Si2N2O began at 1600 and 1500°C in the hot pressing and EFAS, respectively.

Effect of hexagonal-BN on phase transformation of additive-free Si3N4/SiC nanocomposites prepared from amorphous precursor

Si3N4/SiC nanocomposites are well known and attractive for advanced ceramic applications due to excellent mechanical and thermal properties, which make them suitable for use in turbine engines, heat exchangers, and other sophisticated applications. However, without the presence of additives, the fabrication of Si3N4/SiC composites is difficult. The additives form a liquid phase during sintering and facilitate the densification of the composite. However, the additives present a drawback at high temperatures since they decrease the mechanical properties of the composites. Recently, Si3N4/SiC composites were fabricated via the polymer precursor route without any additives, using electric field assisted sintering (EFAS). In this study, fully densified Si3N4/SiC nanocomposites incorporating hexagonal-BN were successfully fabricated by hot pressing without any additives at 1700 °C for 2 h under vacuum at a pressure of 50 MPa (via the amorphous precursor route). Moreover, the incorporation of additives and h-BN is found to decrease the content of SiC. The phase transformation, densification, microstructure, and mechanical properties were discussed and presented.

An experimental setup for shrinkage evaluation during electric field-assisted flash sintering: Application to yttria-stabilized zirconia

Abstract Analyses of electrochemical impedance spectroscopy plots and dilatometric curves of ZrO 2 :Y 2 O 3 (YSZ) green compacts upon sintering show the possibility of choosing the shrinkage level of the green pellet by passing a current flash through the interparticle regions. The experiments consisted on inserting a YSZ green compact in a dilatometer sample holder, connecting either to a power supply or to an impedance analyzer, and monitoring the shrinkage upon sintering with and without applying an ac voltage in the 800–1000 °C temperature range. This procedure allows taking the sample from the first to the second sintering stage in few seconds without the occurrence of significant grain growth.

In situ transmission electron microscopy study of dielectric breakdown of surface oxides during electric field-assisted sintering of nickel nanoparticles

The removal of ultra-thin oxide surface layers on nanometric nickel particles is investigated in the framework of electric field-induced dielectric breakdown. In situ transmission electron microscopy was used to directly apply electrical biasing to agglomerates of nanoparticles during simultaneous imaging of the contact area between two adjacent particles. The applied electrical field initiated dielectric breakdown of the surface layers through percolation of oxygen vacancies and the migration of oxygen away from the particle contact, which leads to the formation of metallic necks and their subsequent growth. The experimental results represent direct evidence for surface cleaning effects during electric field-assisted sintering.

A comparative study of Spark Plasma Sintering (SPS), Hot Isostatic Pressing (HIP) and microwaves sintering techniques on p-type Bi2Te3 thermoelectric properties

The sintering of a synthesized p-type Bi2Te3 nano-powder has been investigated by three different techniques. Sintering techniques such as Hot Isostatic Pressing (HIP), microwave sintering and Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters i.e. temperature, pressure, and power, microstructure and thermoelectric properties of the prepared ceramics. This study demonstrates that the highest figure of merit ZT has been obtained using microwaves or SPS techniques. Ceramics observations reveal differences in microstructure as well as the presence of intra-granular precipitates in the pellets sintered by the three techniques. We finally conclude about the relationship between properties and microstructure to get optimum thermoelectric materials.

Electric Field-Assisted Sintering and Hot Pressing of Semiconductive Zinc Oxide: A Comparative Study

This study aims at comparing hot pressing (HP) and the electric field-assisted sintering (FAST, also known as spark plasma sintering, SPS) of zinc oxide. Two semiconductive nano- and submicrometer powders were investigated, with a particle size of 20–30 and 90–200 nm, respectively. Processing parameters were kept as identical as possible for both sintering methods: sample geometry, heating schedule (however affected by the temperature measurement method), applied pressure, and atmosphere. FAST and HP samples densify by grain-boundary diffusion. Zinc oxide is nevertheless very sensitive to transient overheating occurring in FAST with pyrometer temperature control, leading to different densification curves. The sintering trajectory reveals that grain size also depends on the temperature history. The electrical conductivity of ZnO dramatically increases at the sintering temperature, but not enough to affect significantly the densification behavior. Ex situ conductivity measurements do not reveal any difference between FAST and HP samples.

A comparative study of ZnS powders sintering by Hot Uniaxial Pressing (HUP) and Spark Plasma Sintering (SPS)

The sintering of two different polycrystalline zinc sulphide powders has been investigated by two different techniques. Conventional sintering technique (Hot Uniaxial Pressing, HUP) and the Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters (temperature, pressure) and optical properties of the prepared ZnS ceramics. This study demonstrates the potentiality to sinter ZnS by SPS in very short times, at lower sintering temperatures, compared with HUP, with good densification (close to the theoretical density) and good optical transmission in the infrared range 8–12 μm.

Electric Field-Assisted Sintering in Comparison with the Hot Pressing of Yttria-Stabilized Zirconia

This study aims at comparing hot pressing (HP) and the electric field-assisted sintering technique (FAST) of 8 mol% yttria-stabilized zirconia (8YSZ). Three different ionic conductive nano- and submicron powders were investigated, with a median particle size of 20–30, 51–65, and 150 nm, respectively. Processing parameters were kept identical for both sintering methods: sample geometry, heating schedule, applied pressure, and atmosphere. Investigations of densification and microstructural characterization reveal that under the same conditions FAST and HP sintered samples behave similarly. The analysis of the sintering curves shows that the densification mechanism is the same for both sintering methods (grain-boundary diffusion). An increase of the heating rate up to 150 K/min does not modify the densification mechanism. The sintering trajectory reveals that grain size depends only on density and not on the sintering method and grains have no preferential orientation. Secondary ion mass spectroscopy shows that both types of specimens are contaminated with carbon only superficially. Measurements of the electrical conductivity at high temperature show no significant difference between HP and FAST samples.

On the processing of hetero-nanostructured metals for improved strength/ductility balance by ECAE and SPS techniques

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by equal channel angular extrusion) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) techniques for producing multi-scale microstructures when a conducting milled powder is used is demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of spark plasma sintering), altering the nature of the milled powder – by Y 2O 3 addition during the milling stage – is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

Direct comparison between hot pressing and electric field-assisted sintering of submicron alumina

This study compares hot pressing (HP) and the electric field-assisted sintering technique (FAST) of two different electrically insulating Al 2O 3 submicron powders with median particle sizes of 150 and 500 nm. Sample geometry, heating schedule, applied pressure and atmosphere were identical for both sintering methods. The densification behavior and characterization of the microstructure revealed that FAST sintered samples reached a higher density compared with HP, in particular for the finer powder. It was found that an increase in dwell time was required to reach the same final density by HP. However, analysis of the sintering curves showed that the densification mechanism for both sintering methods was grain boundary diffusion. Increasing the heating rate up to 150 K min −1 did not modify the densification mechanism. The sintering trajectory showed that the grain size was only dependent on density and was insensitive to the sintering method, in addition to showing a lack of preferential grain orientation.

Analysis of magnetization relaxation in MgB 2 bulk samples obtained by electric-field assisted sintering

The relaxation of the irreversible magnetization of MgB 2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H ∝ T −2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low- T limit.

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric‐Field‐Assisted Sintering

Starting with Si‐C‐N(‐O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm3) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si3N4 increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano‐nanocomposites of silicon nitride and silicon carbide.