Field assisted sintering of ceramic constituted by alumina and yttria stabilized zirconia

We show that a two-phase 50 vol% 3YSZ-alumina ceramic flash-sinters at a furnace temperature of 1060 °C under an electrical field of 150 V cm⁻¹. In contrast undoped, single-phase alumina remains immune to field assisted sintering at fields up to 1000 V cm⁻¹, although single-phase 3YSZ flash sinters at 750 °C (furnace temperature). The mechanisms of field assisted sintering are divided into two regimes. At low fields the sintering rate increases gradually (FAST), while at high fields sintering occurs abruptly (FLASH). Interestingly, alumina/zirconia composites show a hybrid behavior such that early sintering occurs in FAST mode, which is then followed by flash-sintering. The specimens held in the flashed state, after they had sintered to near full density, show much higher rate of grain growth than in conventional experiments. These results are in contrast to earlier work where the rate of grain growth had been shown to be slower under weak electrical fields.     

Abnormal grain growth in DC flash sintered 3-mol% yttria-stabilized zirconia ceramics

The origin of nonuniform microstructure and abnormal grain growth (AGG) was investigated in flash sintered 3 mol% yttria-stabilized zirconia (3YSZ) ceramics. The microstructural homogeneity decreased with increasing direct current (DC) density and with dwell time in a flash state, eventually resulting in AGG in the specimen core, the first observation of AGG in 3YSZ. Abnormal grains up to 100 μm in size emerged when the DC density was ≥160 mA/mm², and the specimen's density exceeded 99% of theoretical, starting from the cathode and propagating toward the anode. The results are discussed by comparison with established mechanisms and previous experimental evidence concerning AGG in oxides, focusing on the possible effects of the electrochemical reduction at the cathode end of the specimen.     

Effect of oxygen partial pressure on temperature for onset of flash sintering 3YSZ

In this paper, it is demonstrated that in an oxygen-enriched environment, the oxygen partial pressure affects the onset temperature of flash sintering of 3 mol% yttria-stabilized zirconia (3YSZ). Flash sintering experiments were performed with oxygen partial pressures in the range of 0.25–1 atm. The results indicate that the onset temperature increased by increasing oxygen partial pressure. According to the plots of the conductivity as a function of temperature, the oxygen partial pressure might affect the onset temperature, by changing the conductivity in the pre-flash stage. Combined to the analysis of the law of mass action, it was established that electron conduction might also represent a critical parameter during the pre-flash stage of flash sintering, excluding the oxygen vacancy conduction.     

A thermal perspective of flash sintering: The effect of AC current ramp rate on microstructure evolution

In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary.     

Fabrication of lead zirconate titanate ceramics by reaction flash sintering of PbO-ZrO2-TiO2 mixed oxides

Dense and pure PZT ceramics are successfully fabricated by reaction flash sintering of the PbO-ZrO₂-TiO₂ mixed oxides, which enables the chemical reaction and sintering to take place simultaneously, skipping the preparation process of PZT powders that is always required in the industrial processes. The reaction flash sintering occurs at 524, 533, 546 and 564 °C under the electric field of 600, 500, 400 and 300 V/cm, respectively. The average grain size of obtained sample is increased with electric field. Besides the Joule heating, generation of high concentrations of defects are proposed to explain the mechanisms of reaction flash sintering. This study presents a cost-effective way to fabricate dense and pure ceramic materials from their basic constituents in one step at low temperature in short time.     

Study of the densification and grain growth mechanisms occurring during spark plasma sintering of different submicronic yttria-stabilized zirconia powders

Densification and grain growth mechanisms of Yttria-Stabilized Zirconia sintered by Spark Plasma Sintering are investigated. Sintering trajectories of four commercial submicronic powders with different average particle sizes and yttria amounts have been established and sintering regimes determined. Densification mechanisms are determined in the regime where densification is occurring without grain growth using a model derived from hot-pressing. Grain growth mechanisms are determined using the conventional power law in the regime where ceramics are fully densified. Densification occurs by grain boundary sliding accommodated by an in-series interface-reaction/lattice diffusion of cations or by an overlapping of surface diffusion and grain boundary sliding mechanisms for tetragonal stabilized zirconia and by dislocation climbing for fully stabilized zirconia. A normal grain growth occurs for each ceramic, all composed of a single phase, contrary to the two-phased ceramics obtained in literature where grain growth occurs by segregation at grain boundaries.     

Suppression of nitridation of yttria-doped zirconia during flash sintering

The effect of direct current (DC) and alternating current (AC) on nitridation of 3 mol% Y₂O₃-doped ZrO₂ (3YSZ) after keeping in a flash state for 1 hour was investigated. The inside of the DC-flashed compact was confirmed to exhibit blacking. Scanning transmission electron microscopy, electron energy loss spectroscopy, and X-ray diffraction analysis revealed that zirconium nitrides formed in the blackened area. In contrast, a uniformly densified compact without blackening was obtained by AC fields. No zirconium nitrides formed in the compacts exposed to AC fields even when the flash state was maintained for 1 hour. Therefore, AC fields are effective to suppress nitridation of 3YSZ during flash sintering.     

Flash sintering of Mg-doped tricalcium phosphate (TCP) nanopowders

β-tricalcium phosphate is a bioceramic with unique osteoinductive and osteoconductive properties. Its processing is limited by the undesired β→α phase transition which occurs upon sintering at 1398 K. The reduction of sintering temperature or the stabilization of β phase (by doping) are therefore of particular interest.In this work, flash sintering was used to consolidate β-tricalcium phosphate nanopowders synthesized by wet chemical methods. Pure and Mg-doped powder was studied, pointing out a strong effect of the dopant on the flash behavior and on the phase transitions. The results point out that tricalcium phosphate can be consolidated in few seconds at relatively low furnace temperature. The sintered pellets contain only the desired β phase in the case of Mg-doped powder, whereas some retained α phase is present in the undoped ones. Starting from the electrical behavior of the material a first processing map to attain β-tricalcium phosphate by flash sintering is proposed.     

Phase transformation kinetics of 3 mol% yttria partially stabilized zirconia (3Y-PSZ) nanopowders prepared by a non-isothermal process

A crystalline nanopowder of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) has been synthesized using ZrOCl₂ and Y(NO₃)₃ as raw materials throughout a co-precipitation process in an alcohol-water solution. The phase transformation kinetics of the 3Y-PSZ freeze dried precursor powders have been investigated by nonisothermal methods. Differential thermal and thermogravimetric analyses (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been utilized to characterize the 3Y-PSZ nanocrystallites. When the 3Y-PSZ freeze dried powders are calcined in the range of 703-1073 K for 2 h, the crystal structure is composed of tetragonal and monoclinic ZrO₂. The BET specific surface area of the 3Y-PSZ freeze dried precursor powders calcined at 703 K for 2 h is 118.42 m²/g, which is equivalent to a crystallite size of 8.14 nm. The activation energy from tetragonal ZrO₂ converted to monoclinic ZrO₂ in the 3Y-PSZ freeze dried precursor powders was determined as 401.89 kJ/mol. The tetragonal (T) and monoclinic (M) ZrO₂ phases coexist with a spherical morphology, and based on TEM examination have a size distribution between 10 and 20 nm. When sintering green compacts of the 3Y-PSZ, a significant linear shrinkage of 8% is observed at about 1283 K. On sintering the densification cycle is complete at approximately 1623 K when a total shrinkage of 32% is observed and a final density above 99% of theoretical was achieved.     

Influence of flash sintering on the ionic conductivity of 8 mol% yttria stabilized zirconia

The ionic conductivity of flash-sintered, polycrystalline 8 mol% yttria stabilized zirconia (8YSZ) was enhanced compared with that of conventionally-sintered specimens. Flash sintering was carried out at a furnace temperature of 850 °C with an electric field of 100 V cm^–1 to initiate flash. The current density limit was varied between 60 and 100 mA mm^–2. Post-flash impedance measurements over the range 215–900 °C showed that both bulk and grain boundary conductivities had increased with the increased current density limit which was set prior to flash. The conductivity increases post-flash were ionic, not electronic, although electronic conductivity probably occurred, in addition to ionic conductivity, during flash. The conductivity increases were not attributable to sample densification or microstructural changes. The higher ionic conductivities are attributed to a change in YSZ defect structure that led to an increased concentration of mobile charge carriers; possible explanations for this are discussed.     

Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.     

Electromechanical properties of flash sintered BNT-based piezoelectric ceramic

Lead-free, (BiNa_0.88K_0.08Li_0.04)_0.5Ti_0.995 Mn_0.015 O₃ piezoceramic has been successfully densified by a novel electrical current applied technique known as flash sintering (FS) at 880 °C. The effect of alternating and direct current, current density limit and holding time on the densification, crystal structure, electromechanical and electrical properties have been investigated. The optimum flash condition was obtained with a 1 KHz alternating current, 100 V·cm^?1 initial electric field and preset maximum current limit of 1.5 A·cm^-2. The flash sintered specimen is characterized with finer grain size (10–15 μm), slightly higher electromechanical properties and higher symmetry butterfly shape strain hysteresis loop compared to conventional sintering. Under both sintering conditions uniform distribution of elements and pure rhombohedral structure were observed. Flash sintering also results in lower resistivity and more significant grain boundaries contributions in the conduction mechanism.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

Two-step sintering of Sm2O3 doped ceria stabilized zirconia

Effect of Sm₂O₃ addition and two-step sintering of Ceria Stabilized Zirconia (CSZ) on microstructure and mechanical properties were investigated in the present work. Samaria doped CSZ (SmCSZ) nanopowders were prepared by co-precipitation synthesis from their respective nitrate salts. Synthesized powders were calcined at 1000?°C for 2?h and then compacted to ?10?mm pellets using a uniaxial hydraulic press. Single step & two-step sintering methods were used to sinter the compacted pellets. Powders and sintered pellets were characterized for phase and microstructure using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) technique. Rietveld method was used for quantification of obtained phases. The hardness of the sintered samples was evaluated by Vicker's hardness tester, and toughness was estimated by indentation fracture toughness method. Samples sintered using two-step sintering method shown optimum hardness and toughness (up to 1288 HV10 and 5.37?MPa?m^1/2) values compared to conventionally sintered samples because of reduced grain size.     

Flash spark plasma sintering of 3YSZ

Flash SPS (FSPS) consolidation of 3YSZ cold pressed pellets was investigated. The results show that FSPS allows ultra-rapid consolidation of 3YSZ samples in 30–90 s under an applied electric field. The DC field induces electrochemical blackening. The partial reduction process starts from the cathode (-) and propagates toward the anode (+). This phenomenon, not previously discussed in FSPS literature, induces the development of internal temperature gradients resulting in a polarity dependent grain size/densification.     

The microstructural origin of rapid densification in 3YSZ during ultra-fast firing with or without an electric field

Recent research has shown that very rapid heating of 3YSZ powder compacts (ultra-fast firing), whether by passing an electric current through the sample (flash sintering) or by using external heat sources, causes a great acceleration of densification rate for a given relative density and temperature. Here, the microstructural evolution of 3YSZ is studied using four sintering methods with widely differing heating rates, produced with or without electric fields. The microstructural development depended greatly on thermal history. Most significantly, slow, conventional heating resulted in pores much larger than the grain size, whereas most pores were smaller than the grain size with the rapid heating methods, whether the heating involved an electric field or not. The smaller pore size clearly provides a major contribution to the acceleration of densification following rapid heating. In contrast, grain growth was not suppressed by rapid heating but was suppressed by an electric field.     

Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y₂O₃-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.     

Grain growth kinetics in microwave sintered graphene platelets reinforced ZrO2/Al2O3 composites

The grain growth kinetics and mechanical properties of graphene platelets(GPLs) reinforced ZrO₂/Al₂O₃(ZTA) composites prepared by microwave sintering were investigated. The calculated grain growth kinetics exponent n indicated that the GPLs could accelerate the process of the Al₂O₃ columnar crystal growth. And the grain growth activation energy of the Al₂O₃ columnar crystal indicated that the grain growth activation energy of the GPLs doped ZTA composites is much higher than those of pure Al₂O₃ and ZTA in microwave sintering. The optimal mechanical properties were achieved with 0.4?vol% GPLs, whose relative density, Vickers hardness and fracture toughness were 98.76%, 18.10?GPa and 8.86?MPa?m^1/2, respectively. The toughening mechanisms were crack deflection, bridging, branching and pull-out of GPLs. The results suggested that GPLs-doped are good for the Al₂O₃ columnar crystal growth in the ZTA ceramic and have a potentially improvement for the fracture toughness of the ceramics.