Flash‐sintering of magnesium aluminate spinel (MgAl2O4) ceramics

The sintering behavior of commercially available MgAl₂O₄ spinel was investigated under DC electric field in a range of 0 and 1000 V/cm. Flash‐sintering results in densification close to theoretical density at 1410°C under the DC field of 1000 V/cm, in comparison to the higher sintering temperature of 1650°C in case of conventional sintering. It was observed that the fields less than 750 V/cm had no significant effect on the densification behavior. An abrupt increase in power dissipation was observed corresponding to the occurrence of the flash event. A significant enhancement in grain size was observed in case of flash‐sintered dense spinel samples. The gradual increase in the specimen conductivity observed in the electric field‐assisted sintering (FAST) regime led to Joule heating within the specimen. The increased specimen temperature triggered further increment of current and Joule heating, resulting in the immediate densification.     

Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.     

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO-doped UO2

In this work, the sintering kinetics of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by a high-temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO₂, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO₂ was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO₂ powder. The microstructure evolution of the sintered sphere and UO₂ matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO₂ matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO₂ matrix.     

Densification of Y2O3 by flash sintering under an AC electric field

Frequency dependence of the densification behavior of undoped Y₂O₃ sintered by the AC-flash sintering was systematically investigated at 500 V·cm^?1 over a frequency range from 0.05 Hz to 1 kHz. The Y₂O₃ bodies sintered under an AC field showed a uniform microstructure, without an asymmetric grain size distribution between the electrodes. Almost fully-densified Y₂O₃ body was consolidated at 1 kHz exhibited a relative density greater than 99 % and an average grain size of 1.6 μm. The almost full densification probably resulted from the high input power at the relatively high onset temperature of 1300 °C at this frequency. The temperature dependence of the power dissipation during the AC-flash sintering experiments can be ascribed to the periodic fluctuations of the specimen temperature at low frequencies and to the phase shift between the applied field and the specimen current at high frequencies.     

Progress in pressureless sintering of boron carbide ceramics – a review

ABSTRACT

Boron carbide (B₄C) ceramics has many outstanding performance, such as extremely high hardness, low density, high melting point, high elastic modulus, high thermoelectromotive force, high chemical resistance, high neutron absorption cross section, high impact and excellent wear resistance. Therefore, B₄C ceramics can be used in various industrial applications, such as lightweight ceramic armour, high temperature thermocouples, neutron absorber, reactor control rods in nuclear power engineering, polishing media for hard materials, abrasive media for lapping and grinding, and wear resistant components (blasting nozzles, die tips and grinding wheels). Pressureless sintering is the method with industrialised application value for B₄C ceramics, however, it is impossible to sinter pure B₄C ceramics to high densities without additives by pressureless sintering. So sintering additives must be used to promote the densification of B₄C ceramics. The different sintering additives used to promote the densification of boron carbide will be described in this review, including carbon additives, metallic additives, oxide additives, non-oxide additives, combined additives and rare earth oxide additives. Finally, the recent research trends for sintering methods and sintering additives of B₄C ceramics will also be proposed.     

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.     

Effect of Ni additives on pressureless sintering of SHS ZrB2

Abstract

Pressureless sintering of ultrafine zirconium diboride ZrB₂ produced by self-propagating high temperature synthesis (SHS) was carried out over a temperature range of 1573 to 1873 K using a nickel additive. The additive improved densification behaviour and a maximum densification of 88% was achieved at 1873 K. The XRD pattern showed formation of Ni₃Zr phase during pressureless sintering. The microhardness of sintered ZrB₂ was found to increase with Ni content and a maximum hardness of 1150 kg mm^?2 was found at 40 wt-%Ni addition. The coefficient of thermal expansion of different sintered samples was also investigated.     

Low temperature sintered MnZn ferrites for power applications at the frequency of 1 MHz

The low power loss Mn-Zn ferrites with fine grains were developed by the low-temperature-sintering ceramic process for power applications at a high frequency of 1 MHz. The LiBO₂ sintering aid was added to promote the low temperature sintering and densification. The effects of LiBO₂ on micromorphology and magnetic properties of the sintered Mn-Zn ferrites were investigated. With the aid of LiBO₂, sintering temperature could be reduced as low as 990 °C. The optimum sample was obtained by the addition of 500 ppm LiBO₂ sintered at 1020 °C. The average grain size of this sample is 2.78 μm, the density reaches 4.82 g/cm³, and the minimum power loss is 310 kW/m³ at 1 MHz/30 m T and 25 °C. This sample shows good wide-temperature stability of power loss. The mechanism of power loss affected by the LiBO₂ addition was also discussed. The ceramic sintering process combining the low temperature sintering and the sintering aid offers a new way to develop high-frequency Mn-Zn ferrites.     

Kinetic field approach to study liquid phase sintering of ZnO based ceramics

Liquid phase sintering kinetics in the system ZnO–Bi₂O₃–Sb₂O₃ was studied using closed crucibles and an optical dilatometer. A modified kinetic field technique was applied for the first time to investigate the densification rates. The values obtained were assessed with existing liquid phase sintering models. Grain growth data were derived from the kinetic field diagram and compared to those obtained from microstructure analysis of quenched samples. Good agreement was obtained between both techniques. Values for both the activation energies (activation energies for grain growth and densification) were also reported for the ZnO–Bi₂O₃–Sb₂O₃ system for the first time. In the initial sintering stage mechanisms were identified which retard densification and are essentially unaffected by temperature. It was shown how the position and slope of the iso-strain lines in the modified kinetic field diagram can be used for a qualitative understanding of the interaction of coarsening, liquid redistribution and densification during sintering.     

Effects of CaCO3 content on the densification of aluminum nitride

The effect of adding up to 13.4 wt.% CaCO₃ on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C. The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves. Two microstructural aspects determined the densification of aluminum nitride with CaCO₃: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO₃ caused the formation of higher melting point calcium aluminates (mainly CA₂) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO₃ amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C₁₂A₇ related eutectic temperature. Irrespective of the initial CaCO₃ content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions. The effect of this composition changing on densification is discussed. Large pores were formed in the sites originally occupied by large particles of CaCO₃ and retarded the bulk densification in samples with high additive contents.     

Particle atomic layer deposition of alumina for sintering yttria-stabilized cubic zirconia

The addition of aluminum oxide (Al₂O₃) as a sintering aid to yttria-stabilized zirconia (YSZ) reduces the required densification temperature. Sintering aids are incorporated using a number of processes which can lead to ambiguity when determining the effect of the sintering aid on the densification mechanism. In this study, a novel method for sintering aid addition, Particle Atomic Layer Deposition (ALD), was used to deposit an amorphous Al₂O₃ thin film on YSZ particles. Transmission electron microscopy confirmed the deposition of conformal Al₂O₃ thin films on the surface of the YSZ particles. The addition of Al₂O₃ to YSZ reduced the temperature at which densification began by ~75°C, and 2.2 wt% Al₂O₃ addition resulted in a minimum activation energy for the intermediate stage of densification. This concentration is well in excess of the solubility limit of Al₂O₃ in YSZ, showing that Al₂O₃ does not enhance the densification of YSZ solely by dissolving into the YSZ lattice and activating volume diffusion. The addition of 0.7 wt% Al₂O₃ with one Particle ALD cycle enhanced the ionic conductivity of YSZ by 23% after sintering at 1350°C for 2 hours, demonstrating that dense parts with high oxygen ion conductivities can be produced after sintering at reduced temperatures. One Particle ALD cycle is a fast, easily scaled-up process that eliminates the use of solvents and has substantial cost/performance advantages over conventional processing.     

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.     

Flash sintering feasibility study and localized densification in boron carbide

The feasibility of flash sintering boron carbide (B₄C) was investigated using a direct current (DC) electric field across different electrodes, field strengths, and thermal profiles. Flash behavior was observed at furnace temperatures as low as 386°C with field strengths of 68-278 V/cm, but only a small channel of the specimen was densified due to hot spot effects. Application of a 2.2 V/cm·s voltage ramp at a constant temperature of 550°C caused uniform heating, but at temperatures too low for sintering. Scalable densification of B₄C at low furnace temperatures with flash sintering is theorized to be possible by applying a higher current density through power supply or specimen modifications.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Densification of fine-grained alumina ceramics doped by magnesia,yttria and zirconia evaluated by two different sintering models

The influence of various dopants (500 ppm MgO and Y₂O_3; 250 ppm ZrO₂) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y₂O₃ and ZrO₂ significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.     

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.     

Sintering and mass transport features of (Sn,Ti)O2 polycrystalline ceramics

Different (Sn,Ti)O₂ compositions were sintered at 1450 °C for 2 h with the purpose of investigating their sintering and mass transport properties. Highly dense ceramics were obtained and their structural properties studied by X-ray diffraction and scanning electron microscopy. The changes in lattice parameters were analyzed by the Rietveld method and two mass transport mechanisms were observed during sintering in different temperature ranges, evidenced by the linear shrinkage rate as a function of temperature. The effect of the concentration of TiO₂ on mass transport and densification during sintering was analyzed by considering the intrinsic defects. System densification was attributed to a mass transport mechanism in the SnO₂ matrix, caused by the presence of TiO₂, which formed a solid solution phase. The change in the mass transport mechanism was attributed to chemical bonding between SnO₂ and TiO₂, which improves ionic diffusion as the concentration of TiO₂ increased in (Sn,Ti)O₂ compositions.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Dielectric properties of Ba1−xSrxTiO3 ceramics prepared by microwave sintering

A comparative study on the dielectric properties of Ba_1?xSr_xTiO₃ (x=0.1–0.6) ceramics prepared by microwave sintering (MS) and conventional sintering (CS) has been done. It was found that MS samples need lower temperature and much shorter time than CS samples to obtain the same degree of densification. Compared with CS samples, MS samples possessed smaller grain size, better densification and more uniform grain growth. The dielectric properties of the samples were measured as a function of temperature. It was observed that the dielectric constant was higher for MS samples compared with that of CS samples especially in the ferroelectric phase.