Conduction and sintering mechanism of high electrical conductivity Magnéli phase Ti4O7

In this paper, the crystal structure and electronic structure of Ti₄O₇ were calculated based on density functional theory, and Magnéli phase Ti₄O₇ bulks were successfully prepared by spark plasma sintering (SPS). Results indicated that the contribution of Ti 3d to Fermi level increased due to the lack of oxygen atom in lattice, and the energy band gap of Ti₄O₇ was reduced compared with that of TiO₂. By calculating the relationship between the densification rate and effective stress in the process of SPS, it can be known that the densification mechanism of Ti₄O₇ powders was controlled by diffusion. Based on this, under the conditions of sintering temperature of 1000 °C, holding time of 10 min and sintering pressure of 30 MPa, Ti₄O₇ bulks with the optimal electrical conductivity (961.5 S cm⁻¹) could be obtained, which was more than 30% higher than the graphite material reported in literature. The results reveals that Ti₄O₇ will be one of the most promising electrode materials in the electrochemical field.     

Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Q_act, and the stress exponent, n, for SPS of nearly stoichiometric UO₂. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Q_act and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Q_act. The average values were Q_act =96 kJ/mol (CHR), Q_act = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250?450 kJ/mol), creep (350?550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol).This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.     

Study of densification mechanisms during Spark Plasma Sintering of co-precipitated Ho:Lu2O3 nanopowders: Application to transparent ceramics for lasers

This work was focused on the determination of densification mechanisms during Spark Plasma Sintering (SPS) of Ho:Lu₂O₃ nanopowders. Strong variation of the stress exponent n was evidenced during the sintering process. At low relative density (i.e. ρ < 66 %), n = 3 and powder particles rearrangement and coalescence take place because of high value of effective stress and low size of primary nanoparticles. Then, for ρ between 66 % and 85 %, the stress exponent decreases to n = 2 then n = 1. Such values were related to Rachinger then Lifshitz sliding mechanisms, the last one was associated with an average activation energy of 565 kJ.mol⁻¹. At the final densification stage (ρ > 85 %), the stress exponent suddenly increases to 4 in accordance with a power-law creep. From these investigations, an optimized thermomechanical cycle was proposed to obtain highly transparent Ho:Lu₂O₃ ceramics suitable for laser applications.     

Influence of conductive secondary phase on thermal gradients development during Spark Plasma Sintering (SPS) of ceramic composites

Spark Plasma Sintering (SPS) has attracted a lot of interest in recent years owing to its ability to enable the densification of a broad range of materials in a very short processing time. It is well documented in the literature that the very high heating rates that can be applied with this technology can lead to the apparition of large thermal gradients in the tool and thus affect the homogeneity of the compact.In the present study, the influence of the compact thermal and electrical properties on the thermal gradients was studied. Al₂O₃, AlN and TiC powders were used to produce series of Al₂O₃-TiC and AlN-TiC composites (0, 25, 50, 75, 100 vol%TiC) showing different electrical and thermal conductivities. Two pyrometers were used in order to observe and measure the thermal gradients and the percolation of the current during sintering at a high heating rate and without insulation.Electrical conductivity measurements were carried out on samples presenting different relative densities. This samples were obtained through interrupted sintering cycles at temperatures below and above the identified percolation threshold temperature.It was shown that high thermal gradients can appear during SPS depending on the processing parameters (dimensions of the die and heating rate) but also on the composition of the compact (proportion of conductive phase) and on its density.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

Microwave versus conventional sintering: Estimate of the apparent activation energy for densification of α-alumina and zinc oxide

A comparative study between the conventional and 2.45 GHz microwave multimode sintering behavior of insulator (α-Al₂O₃) and semi-conductive ceramic (ZnO) was systematically investigated. The apparent activation energy of nonisothermal sintering was determined by way of the Arrhenius plot of densification data at constant heating rates (CHR) and the concepts of Master Sintering Curves (MSCs), respectively. During microwave densification process, the apparent activation energy was about 90 kJ/mol less than the value for conventional sintering of Al₂O₃ applying these two estimation methods. However, an opposite result was obtained in the case of ZnO, although its densification process had been also accelerated by microwave as well as Al₂O₃. The significant differences in activation energy give a good proof of the difference in diffusion mechanism induced by the electromagnetic field underlying microwave sintering.     

Development and evaluation of TiB2–AlN ceramic composites sintered by spark plasma

Titanium diboride (TiB₂) is a ceramic material with high mechanical resistance, chemical stability, and hardness at high temperatures. Sintering this material requires high temperatures and long sintering times. Non-conventional sintering techniques such as spark plasma sintering (SPS) can densify materials considered difficult to sinter. In this study, TiB₂–AIN (aluminum nitride) composites were sintered by using the SPS technique at different sintering temperatures (1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C). x-ray diffraction was used to identify the phases in the composites. mechanical properties such as hardness and indentation fracture toughness was obtained using a vickers indenter. Different toughening mechanisms were identified, and good densification results were obtained using shorter times and lower temperatures than those previously reported.     

Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization

Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca₃MgSi₂O₈ – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T₁ = 1300 °C, T₂ = 1250 °C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T₂ = 1250 °C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m^1/2.     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Microstructure-property relationships in composites of 8YSZ ceramics and in situ graphitized nanocellulose

Ceramic matrix composites (CMC) of 8 mol.% yttria-stabilized zirconia (8YSZ) mixed with natural fiber nanocellulose (0.75, 1, 2 wt%) were prepared by spark plasma sintering (SPS). Nanocellulose markedly improved the densification of the 8YSZ ceramic matrix and induced significant grain size refinement. It was demonstrated that in situ graphitization of nanocellulose during the SPS processing resulted in 6 nm thin turbostratic graphite layers homogeneously covering the 8YSZ ceramic grains. The dielectric properties were analyzed by electrical impedance spectroscopy suggesting a low percolation threshold near or below ≈ 1.6 vol% graphite, above which mixed ionic-electronic conduction dominates. The CMCs are stable under reducing conditions (5%H₂/Ar atmosphere) at least until 800 °C with a high conductivity of σ_dc = 0.17 S?cm^?1 even at 900 °C (8YSZ-2%CNF). These features make the 8YSZ-nanocellulose CMCs promising candidates for application in medium- to high-temperature electrochemical devices.     

Densification and mechanical properties of TiC by SPS-effects of holding time,sintering temperature and pressure condition

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness (H_V) increase with sintering temperature and holding time, but fracture toughness (K_IC) was not significantly influenced by sintering parameters. The H_V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H_V values of ～24–30 GPa and K_IC of ～3.7–5 MPa m^1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.     

A critique of master sintering curve analysis

In this work, we explore factors affecting the accuracy of the master sintering curve (MSC) approach for analyzing the complete sintering profile of ceramic powders. We show that the instantaneous anisotropic shrinkage must be accounted for to develop an accurate MSC. The MSC diverges at >90% density because of basic assumptions that oversimplify the analysis of the densification process. We also show that powder chemistry and forming techniques can affect the fitting parameter Q. Q should not be interpreted as the sintering activation energy, or used to interpret mechanistic differences since it is comprised of several mechanisms that influence densification throughout the sintering cycle. Despite these limitations, the MSC is a useful and practical tool for predicting thermal load (i.e. time and temperature) effects on the densification of a ceramic part fabricated from a singular powder that is fabricated by a singular forming process.     

Effect of polysilane addition on spark plasma sintering and superconducting properties of MgB2 bulks

Polycrystalline MgB₂ bulk samples were produced by ex-situ spark plasma sintering (SPS) using the oxygen-free preceramic polymer additive poly(dimethylsilane) as a source for carbon doping and as a sintering aid. Two major effects were identified. One is the significant enhancement of the densification kinetics during sintering for all tested compositions. The second one is the improvement of the high field critical current density for a certain level of the poly(dimethylsilane) addition.     

Static and dynamic mechanical properties of boron carbide processed by spark plasma sintering

Spark plasma sintering (SPS) has become a popular technique for the densification of covalent ceramics. The present investigation is focused on the static mechanical properties and dynamic compressive behavior of SPS consolidated boron carbide powder without any sintering additives. Fully dense boron carbide bodies were obtained by a short high temperature SPS treatment. The mechanical properties of the SPS-processed material, namely hardness (32 GPa), Young modulus (470 GPa), fracture toughness K_C (3.9–4.9 MPa m^0.5), flexural strength (430 MPa) and Hugoniot elastic limit (17–19 GPa) are close or even better than those of hot-pressed boron carbide.     

The impact of flash sintering on densification and plasticity of strontium titanate: High heating rates,dislocation nucleation and plastic flow

Flash sintering involves very rapid densification of ceramic powder compacts during a thermal runaway induced by an applied voltage and current. The mechanisms of fast densification are still not well-understood. The present study investigates the impact of high heating rates during flash sintering on densification, dislocation density and plasticity of SrTiO₃. After flash sintering, a high dislocation density of almost 10¹⁴ m⁻² was observed by TEM. Uniaxial compression at 1150 °C revealed very high deformation rates. It is argued that for SrTiO₃, dislocations are generated and migrate during flash sintering. This becomes possible by the very high heating rates, which conserve high driving forces for sintering up to high temperatures. High driving forces of several 10 MPa are preserved up to high temperatures. Thus, the sintering stress can be above the flow stress of SrTiO₃ (5 MPa), and the nucleation of dislocations occurs, paving the path for plastic flow.     

Sintering of nano-sized titania particles and the effect of chlorine impurities

The consolidation behaviour of two titania nanopowders with comparable specific surface area is reported in this study. Surface impurities were identified as a key element influencing the sintering behaviour of titania nanopowders. Raw materials for the synthesis processes of the nanopowders were titanium tetrachloride (TiCl₄) and titanium tetraisopropoxide (TTIP), respectively. Particle properties, densification and sintering behaviour were investigated by several techniques (SEM, TEM, XRD, MIP and dilatometry). Phase transformation and densification of the nanopowder, prepared from TiCl₄, started at lower temperature compared to the powder synthesized from TTIP. However, a drastic grain growth occurred during the final stage of sintering of the TiCl₄ synthesized nanopowder. Chlorine impurities were identified by STA-MS. After purification of this nanopowder, the chlorine was effectively removed from particle surfaces and grain growth was reduced.     

Densification kinetics of boron carbide with medium entropy alloy as a sintering aid during spark plasma sintering

The high sintering temperature of pure B₄C considerably limits its widespread application, thus searching an effective sintering aid is critical. In this work, B₄C-based ceramic with 1 vol.% nonequiatomic Fe₅₀Mn₃₀Co₁₀Cr₁₀ medium entropy alloy as a sintering aid were fabricated at 1900-2000°C by spark plasma sintering (SPS) under applied pressure, and their mechanical properties were examined and compared with pure B₄C ceramic sintered at same condition. The maximal flexural strength of 255.59 MPa, microhardness of 2297.6 Hv_0.2 and fracture toughness of 3.62 MPa m^1/2 could be obtained at optimized SPS pressure of 50 MPa, which were all higher than those of pure B₄C ceramic. To better understand the densification kinetics mechanisms, the densification ratio as a function of SPS temperature and pressure was theoretically analyzed using steady creep model. It was found that densification controlled by grain-boundary sliding at lower pressure transferred to power law creep regime at higher pressure, which were proved by the dislocation net shown in transmission electron microscopy image.     

Magnesium fluoride (MgF2) – A novel sintering additive for the preparation of transparent YAG ceramics via SPS

The effect of MgF₂ as a sintering additive for the preparation of YAG ceramics via spark plasma sintering (SPS) is investigated with promising results, as nearly complete densification (0.58% porosity) is achieved at relatively low temperature and moderate pressure. Higher temperature and dwell time resulted in a translucent/transparent body. On the other side, significant grain growth was observed with MgF₂ addition.     

Second-phase evolution and densification behavior of AlN with CaO–Y2O3–C multicomponent additive system

AlN compacts with different CaO–Y₂O₃–C mixtures were sintered between 1100 and 1850 °C to understand the effects of the in situ formed reducing atmosphere on the densification behavior and evolution of the second-phases. AlN with Y₂O₃ densified at 1750 °C, but the addition of C changed the second-phases evolution towards Y-rich phases that delayed the densification. For AlN containing CaO, the second-phases were little influenced by the reducing atmosphere, but the addition of C increased the evaporation of the second-phase compounds during sintering, limiting the densification due to the reduction of the liquid-phase fraction and the gas trapping inside the pores. AlN with CaO–Y₂O₃ mixtures could be completely densified at 1650 °C, but the addition of C inhibited the densification below this sintering temperature because liquid-phase had poor wetting and spreading characteristics and the second-phase a high melting point (>1800 °C).     

Effect of sintering temperature and dopant concentration on microstructure and electrical conductivity of ultra-fine-grained ZrO2–Sc2O3 ceramics

Phase transformations in ZrO₂ + xSc₂O₃ solid solutions (6.5 < x < 11 mol%) at sintering of ceramics obtained from nanopowders produced by laser evaporation of the ceramic targets have been studied. The Sc₂O₃ concentration increasing from 6.5 to 11 mol% is accompanied by the sintering temperature decreasing and the average grain size growth from 130 nm to 760 nm. At concentration of about 7 mol% Sc₂O₃ an abrupt increase of the average grain size and electric conductivity is observed. The sinterability of the ZrO₂ − хSc₂O₃ ceramics is affected by the prehistory of nanopowders preparation. The characteristics of ceramics obtained from nanopowders evaporated from the targets based on (ZrO₂ + xmol% Sc₂O₃) mixture and on the (ZrO₂ − 11mol% Sc₂O₃) solid solution significantly differ, namely, in the latter the sintering temperature is markedly lower and the shrinkage rate is higher. Besides, its average grain size is substantially lower and the conductivity is higher.