Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

Yb3+ doped Lu2O3 transparent ceramics by spark plasma sintering

Transparent ceramics of 10% Yb doped Lu₂O₃ was fabricated by spark plasma sintering. The operating vital parameters in yielding transparency and mutual effects of sintering, pressure, dwell time, heating rate and annealing temperature on microstructure have been investigated. Fully compacted specimens were obtained at 1250 °C and the average grain size increased from few nm up to 5 μm until 1700 °C, above which abnormal grain growth was witnessed. The post-annealing of sintered ceramics at 1200 °C removes discoloration and improves transparency. The ceramics prepared at 1700 °C with dwell time of 5 min and heating rate at 50 °C/min shows the maximum transmittance with a thickness of 2 mm of 55% at a wavelength of 2 μm.     

Co-precipitation synthesis route to yttrium aluminum garnet (YAG) transparent ceramics

Yttrium aluminum garnet (YAG) precursor was synthesized via a coprecipitation method with aluminum nitrate and yttrium nitrate as raw materials, using ammonium hydrogen carbonate (AHC) as the precipitant. Fine and low-agglomerated YAG powder was obtained by calcining the precursor at 1200 °C. The primary crystallites were measured to be ～120 nm in size and weakly agglomerated to a particle size of ～500 nm, indicating a high degree of sinterability. With 0.5 wt% tetraethyl orthosilicate (TEOS) and 0.1 wt% magnesia as sintering aids, transparent YAG ceramics were fabricated by vacuum sintering at 1730–1790 °C for various hours. The influences of sintering temperature and holding time on the microstructure and transmittance of YAG ceramics were discussed.     

Fabrication,microstructure and mechanical properties of novel bulk binderless (Ti0.8Zr0.2)C carbides prepared by mechanical alloying and spark plasma sintering

Nanocrystalline (Ti_0.8Zr_0.2)C powder consisting in grains of about 200 nm in diameter obtained by mechanical alloying was sintered by a spark plasma sintering (SPS) process without the addition of any binder phase. The microstructure, Vickers micro hardness and density in relation to the sintering time and temperature are carefully described. The most suitable sintering condition under pressure of 50 MPa is 1650 °C for 5 min. In this sintering condition, the hardness can reach 2760 Hv and the relative density can reach 98%.     

Transmittance improvement of Dy–α-SiAlON in infrared range by post hot-isostatic-pressing

At present stage, the transmittance improvement is still a thorny problem in SiAlON ceramics due to their complex composition and processing. In the present research, Dy–α-SiAlON ceramics were selected to be translucent in the medium infrared range. The samples had a higher densification value by using hot-pressing (HP) sintering method at 1650–1700 °C with or without LiF additive. The as-sintered specimens experienced the post-hot-isostatic-pressing (PHIP) treatment at 1650–1700 °C for 30–90 min in either N₂ or Ar environment to increase the optical transmittance. A significant optical transparency improvement has been found in Dy–α-SiAlON, with or without LiF co-doping, undergone a PHIP at 1700 °C for 30 min under an Argon gas pressure of 180 MPa. The improved transmittance attributes to a fully developed α-Sialon crystalline phase, a uniform grain size, a denser grain arrangement, and a clean grain boundary, based on the X-ray diffraction analysis, SEM and TEM microstructure observation, and optical transmittance measurement.     

Effect of spark plasma sintering parameters on microstructure and room-temperature hardness and toughness of fine-grained boron carbide (B4C)

Spark plasma sintering (SPS) parameters are reported to have a remarkable effect on microstructure and fracture toughness of boron carbide. Fully dense fine-grained boron carbide samples have been sintered by SPS at 1700 °C for 3 min as optimal conditions. Both temperature and the current density applied during sintering seem to be important parameters for fully dense output and can induce significant changes on the final microstructure. The initial grain size of the powder is a crucial factor to perform fine-grained fully dense specimens. The improvement on room-temperature hardness and toughness is discussed.     

Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering

The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.Fully dense materials were obtained at 1400 °C with a heating rate of 100 °C/min, 10 min soaking time and 100 MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400 °C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.     

Effect of pressure and temperature on densification,microstructure and mechanical properties of spark plasma sintered silicon carbide processed with β-silicon carbide nanopowder and sintering additives

The effects of applied pressure and temperature during spark plasma sintering (SPS) of additive-containing nanocrystalline silicon carbide on its densification, microstructure, and mechanical properties have been investigated. Both relative density and grain size are found to increase with temperature. Furthermore, with increase in pressure at constant temperature, the relative density improves significantly, whereas the grain size decreases. Reasonably high relative density (~96%) is achieved on carrying out SPS at 1300 °C under applied pressure of 75 MPa for 5 min, with a maximum of ~97.7% at 1500 °C under 50 MPa for 5 min. TEM studies have shown the presence of an amorphous phase at grain boundaries and triple points, which confirms the formation of liquid phase during sintering and its significant contribution to densification of SiC at relatively lower temperatures (≤1400 °C). The relative density decreases on raising the SPS temperature beyond 1500 °C, probably due to pores caused by vaporization of the liquid phase. Whereas β-SiC is observed in the microstructures for SPS carried out at temperatures ≤1500 °C, α-SiC evolves and its volume fraction increases with further increase in SPS temperatures. Both hardness and Young׳s modulus increase with increase in relative density, whereas indentation fracture toughness appears to be higher in case of two-phase microstructure containing α and β-SiC.     

Transparent yttrium aluminum garnet (YAG) ceramics by spark plasma sintering

Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 °C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 °C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.     

Effect of pre-oxidation on the microstructure,mechanical and dielectric properties of highly porous silicon nitride ceramics

Highly porous Si₃N₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si₃N₄ ceramics were investigated. As the pre-oxidation temperature increased from 1200 °C to 1400 °C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 °C, and then decreased to 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si₃N₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

On the effects of LiF on the synthesis and reactive sintering of gahnite (ZnAl2O4)

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl₂O₄) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al₂O₃ mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.     

Crack healing and strength recovery in SiC/spinel nanocomposite

The crack healing behavior and the strength recovery of the newly introduced SiC/spinel nanocomposite were investigated. SiC/spinel nanocomposite containing 27.26 wt% SiC was prepared by the ball milling of talc, graphite and aluminum powders with subsequent annealing at 1200 °C for 1 h in a vacuum. The SEM results showed that the surface cracks produced by Vickers indenter on the prepared SiC/spinel pellets can be completely healed after sintering at 1545 °C for 1 min in air atmosphere. Furthermore, an almost complete strength recovery of the specimens can be obtained in those samples heat treated at 1550 °C for 1 min, as evaluated by diametral tensile strength (DTS) test. The formations of mullite and aluminosilicate glassy phases are the major factors which are responsible for the crack healing and strength recovery in the structure. It was found that the healing efficiency of those specimens healed at 1550 °C for 1 min is 99%.     

Effect of consolidation parameters on structural,microstructural and electrical properties of magnesium titanate ceramics

The aim of this study is to explore the influence of the processing route on the structural and physical properties of bulk MgTiO₃ ceramics. Commercially available MgO and TiO₂ powders were mechanically activated in a planetary ball mill. Green bodies were formed by an isostatic pressure of 300 MPa. The sintering of these samples was done either by the Two-Step Sintering (TSS) approach or by conventional pressureless sintering followed by Hot Isostatic Pressing (post-HIPing). The first set of compacts was sintered by TSS in air at 1300 °C for 30 min and the next step was performed at 1200 °C for 20 h. The density of the obtained samples after the two-step sintering reached almost 90% of the theoretical density (%TD). The second set of compacts was sintered at 1400 °C for 30 min in air. The samples without open porosity were post-sintered by the HIP at 1200 °C for 2 h in an argon atmosphere at a pressure of 200 MPa. The density significantly increased up to 96%TD. The differences between samples prepared by these two techniques were also analyzed by XRD and SEM. The lattice vibration spectra were obtained using Raman spectroscopy and they indicate a high degree of lattice disorder, as well as high values of the oxygen vacancy concentration. Electrical characteristic were established in the frequency range from 10 kHz to 10 GHz. The choice of the processing route had considerable influence on structural and physical properties of MgTiO₃ ceramics.     

Optimization of spark plasma sintering parameters of Si3N4-SiC composite using response surface methodology (RSM)

The Si₃N₄-SiC micro-nano composites were fabricated via the spark plasma sintering method using MgSiN₂ as an additive. Response surface methodology and central composite design were applied to optimize the spark plasma sintering process for the fabrication of Si₃N₄-SiC/MgSiN₂ with improved density. The relation between the three parameters of sintering including temperature, pressure, and holding time was modeled and the optimized parameters were obtained. The best sintering results obtained for the sintering temperature, holding time, and pressure are 1700 °C, 487 s, and 49 MPa, respectively. The addition of MgSiN₂ as an additive and SiC as a secondary phase were also investigated in the present work. The Si₃N₄−5 vol% SiC composite exhibited high hardness (19 GPa) and fracture toughness values (6.5 MPa m^1/2) at room temperature.     

Characterization of IT-SOFC non-symmetrical anode sintered through conventional furnace and microwave

This work investigated the mechanical and electrical properties of NiO–SDC/SDC anode sintered by two different methods: in a microwave at about 1200 °C for 1 h and in a conventional furnace at 1200 °C with a holding time of 1 h (total sintering time of 21 h). Nano-powders Sm_0.2Ce_0.8O_1.9 (SDC) and NiO were mixed using a high-energy ball mill, followed by the co-pressing technique at a compaction pressure of 400 MPa. No binder was used between the layers. The electrical behaviors of all sintered samples were studied using electrochemical impedance spectra in the frequency range of 0.01–10⁵ Hz under 97% H₂–3% H₂O, an amplitude of 10 mV, and at high temperature range of 600–800 °C. Results indicate that the non-symmetrical NiO–SDC/SDC anode achieved through microwave sintering has finer grain size and higher electrochemical performance. However, hardness and Young׳s modulus increased in the samples sintered through a conventional furnace.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

Microwave sintering and grain growth behavior of nano-grained BaTiO3 materials

In this paper, we investigated the effect of microwave sintering parameters on the development of the microstructure of nano-grained BaTiO₃ materials co-doped with Y and Mg species. It is observed that the materials can not only be sintered densely at a lower temperature (1150 °C) and a shorter soaking time (20 min), but also the grain growth can be suppressed by 2.45 GHz microwave heating process. However, the grain growth exhibits a unique tendency in some processing conditions such as microwave sintering for longer intervals (≧60 min) or at higher temperatures (1200 °C). The grain growth behavior after densification was investigated in terms of the phenomenological kinetics, and the activation energy for grain growth using microwave sintering (59.4 kJ/mol) is considerably less than that of the conventionally sintered ones (96.0 kJ/mol), which indicates that microwave sintering process can accelerate the densification rate of the BaTiO₃ materials comparing with the conventional sintering process.