Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Flash sintering of alumina/reduced graphene oxide composites

The flash sintering behavior of Al₂O₃/reduced graphene oxide (rGO) composites was investigated. rGO was used as a composite component and a conductive additive. Under the electric fields of 250–400 V cm⁻¹, the flash event occurred at extremely low temperatures of 236–249 °C. The current density limit played a significant role in the degree of densification. A larger current density resulted in a higher density of the sample. However, current densities larger than 33.33 A cm⁻² resulted in broken samples because of the localization of high current density coupled with the formation of hot spots. Flash sintering at a furnace temperature of 800 °C, electric field of 300 V cm⁻¹ and current density limit of 33.33 A cm⁻² produced nearly completely dense Al₂O₃/rGO composites. In addition to the current limit, the furnace temperature is also a key parameter that controls the degree of densification to achieve “safe” flash sintering.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Rapid preparation of Bi0.5Na0.5TiO3 ceramics by reactive flash sintering of Bi2O3-NaCO3-TiO2 mixed powders

Lead-free Bi_0.5Na_0.5TiO₃ piezoelectric ceramics were successfully prepared by reactive flash sintering of Bi₂O₃-NaCO₃-TiO₂ mixed powders, where phase transformation and densification occurred simultaneously. The influence of electric field strength, current density and holding time at constant current state on the phase transformation and densification were investigated. The current density had a significant influence on the extent of phase transformation and densification. The holding time had no influence on the phase transformation, but had an important effect on crystallinity of sample. The sintered bulks exhibited the maximum polarization P_m of 16.8 μC/cm², remanent polarization P_r of 9.6 μC/cm², coercive field E_c of 29 kV/cmm, maximum electric-field-induced strain of 0.053 %, and piezoelectric coefficient d₃₃ of 85 pC/N. The reactive flash sintering can prepare the dense and single-phase ceramics from multiphase precursor powders in one step of flash, providing a new way for rapid production of ceramic materials.     

Sintering behavior,microstructural evolution,and mechanical properties of ultra-fine grained alumina synthesized via in-situ spark plasma sintering

Ultra-fine grained Al₂O₃ was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 °C. High relative density over 99% of in-situ sintered Al₂O₃ was obtained in the sintering condition of 1400 °C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al₂O₃ body was much finer (~400 nm) than that of Al₂O₃ sintered from the crystalline α-Al₂O₃ powders. For in-situ sintered Al₂O₃ from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al₂O₃ with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.     

Flash sintering of ultra-high pure alumina ceramics with fine microstructure

The nearly fully dense ultra-high pure (>99.99%) α-Al₂O₃ ceramics were prepared by flash sintering at the furnace temperature of 1300°C. Compared with the samples sintered at 1300 and 1650°C without electrical field, the flash-sintered samples exhibited remarkably improved density and finer grains. The flash-sintered samples also exhibited high hardness, which is even higher than that of the hot-pressed sample. Therefore, it is believed that flash sintering could be an effective technique for the preparation of ultra-high pure alumina ceramics.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

CuAlO2 thermoelectric compacts by SPS and thermoelectric performance improvement by orientation control

We fabricated CuO/Al₂O₃ green compacts from plate-like Al₂O₃ and granular CuO powders by multi-press forming and investigated the alumina orientation using Lotgering's method. The results showed that Al₂O₃ particles preferentially aligned perpendicular to the pressure direction and the orientation degree increased as the forming pressure was increased. We proposed a model describing the movement of the alumina particles to explain the pressure effect on their orientation. The orientation calculation was in good agreement with those by Lotgering's method. Furthermore, we prepared the CuAlO₂ compacts by regular or spark plasma sintering (SPS). However, the compacts sintered by SPS exhibited higher orientation degree and density than those produced by regular sintering. The electrical conductivity values of the orientation-controlled compacts sintered by SPS reached 770 S m⁻¹ at 928 K, which was close to that of CuAlO₂ single crystal. The power factor of the CuAlO₂ compacts with highest orientation degree is as high as 5.95 × 10⁻⁵ W m⁻¹ K⁻¹ at 928 K. Therefore, we can conclude that orientation control is an effective method to enhance the thermoelectric performance of compact polycrystalline CuAlO₂ bulks.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Influence of the electric field on flash-sintered (Zr + Ta) co-doped TiO2 colossal permittivity ceramics

In the preparation process for advanced ceramics, how to reduce the sintering temperature, shorten the processing time and refine grains is the key to obtaining high-performance ceramic materials. The flash sintering (FS) provides an effective method to solve this issue. Here, (Zr + Ta) co-doped TiO₂ colossal permittivity ceramics were successfully fabricated by conventional sintering (CS) and flash sintering under electric fields from 500 V/cm to 800 V/cm. The flash behavior, sintered crystal structure and microstructure, dielectric properties, and varistor characteristics were systematically investigated. The effects of the applied electric fields on the above behaviors were discussed. The results show that flash sintering can reduce the sintering temperature by 200 °C, decrease the processing time by 10 times and reduce grain sizes in TiO₂ ceramics. All sintered samples were single rutile structures. Flash sintering led to similar electrical properties to conventional sintering. In the flash-sintered samples, with increasing the electric field, the permittivity of co-doped TiO₂ ceramics increased at a frequency of 10³–10⁴ Hz. The flash-sintered sample under an electric field of 800 V/cm possessed the best comprehensive properties, a dielectric permittivity of >10⁵, a dielectric loss of ～0.77 at 10³ Hz, and a nonlinear coefficient of 5.2.     

A novel method for the fabrication of porous calcium hexaluminate (CA6) ceramics using pre-fired CaO/Al2O3 pellets as calcia source

Herein, porous calcium hexaluminate ceramics that contain pores exhibiting multiple morphologies were fabricated via in situ reaction sintering using α-Al₂O₃ powders and pre-fired CaO/Al₂O₃ pellets. The results indicated that the composition of the pre-fired CaO/Al₂O₃ pellets significantly affected the pore morphology, reaction-diffusion mechanisms, sintering behaviour and properties of the porous CA₆ ceramics. For the specimens containing low CaO/Al₂O₃-ratio (0.37) pellets, the main reaction occurred by solid state diffusion, i.e. ion diffusion through the solid reactant phase, which resulted in a slow process and low CA₆ formation rate at an elevated sintering temperature. With higher CaO/Al₂O₃-ratio (0.57) pellets, large-sized pores were observed because of transient liquid phase diffusion during the sintering process. The transient liquid phase diffusion effect increased the porosity of the porous ceramics and promoted the formation of a large number of plate-like CA₆ grains in the walls of the pores, enhancing their mechanical properties and high-temperature performance. The porous CA₆ ceramics containing high CaO/Al₂O₃-ratio (0.57) pellets sintered at 1700 °C exhibited high open porosity (55.88%), low thermal conductivity and excellent high-temperature performance.     

高纯超细氧化铝粉的常压烧结与高压烧结

以高纯超细α-Al₂O₃粉为原料,采用常压、高压烧结制备了高纯Al₂O₃陶瓷。研究表明,在乙醇溶剂中高速球磨可显著降低Al₂O₃粉料的平均颗粒尺寸,提高粉料的比表面积与烧结活性;在4.5 GPa、1230℃高压烧结30 min,制备了相对密度达98.71%的无烧结助剂掺杂的Al₂O₃陶瓷;与常压烧结相比,高压烧结可显著降低烧结温度,提高传质速率,大幅度缩短烧结时间,达到快速、低温烧结的效果;由于相对较低的烧结温度,掺杂微量MgO的Al₂O₃陶瓷在高压烧结中未出现液相,MgO对烧结致密化及Al₂O₃晶粒生长抑制几乎无影响。     

Flash sintering produces eutectic microstructures in Al2O3–LaPO4 versus conventional microstructures in 8YSZ–LaPO4

While monazite (LaPO₄) does not flash sinter even at high fields of 1130 V/cm and temperatures of 1450°C, composite systems of 8YSZ–LaPO₄ and Al₂O₃–LaPO₄ have been found to more readily flash sinter. 8YSZ added to LaPO₄ greatly lowered the furnace temperature for flash to 1100°C using a field of only 250 V/cm. In these experiments, -Al₂O₃ alone also did not flash sinter at 1450°C even with high fields of 1130 V/cm, but composites of Al₂O₃–LaPO₄ powders flash sintered at 900-1080 V/cm at 1450°C. Alumina–monazite (Al₂O₃–LaPO₄) composites with compositions ranging from 25 vol% to 75 vol% Al₂O₃ were flash sintered with current limits from 2 to 25 mA/mm². Microstructures were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A eutectic microstructure was observed to form in all flash sintered Al₂O₃–LaPO₄ composites. With higher power (higher current limits), eutectic structures with regular lamellar regions were found to coexist in the channeled region (where both the current and the temperature were the highest) with large hexagonal-shaped -Al₂O₃ grains (up to 75 m) and large irregular LaPO₄ grains. With lower power (lower current limits), an irregular eutectic microstructure was dominant, and there was minimal abnormal grain growth. These results indicate that Al₂O₃–LaPO₄ is a eutectic-forming system and the eutectic temperature was reached locally during flash sintering in regions. These eutectic microstructures with lamellar dimensions on the scale of 100 nm offer potential for improved mechanical properties.     

Rapid preparation of Gd2Zr2−xCexO7 waste forms by flash sintering and their chemical durability

A simple and low-energy-consuming approach for preparing ceramic nuclear waste forms is greatly preferred for disposal of ever-increasing amounts of radioactive nuclear wastes. Herein, simulated radionuclide Ce could be rapidly incorporated into Gd₂Zr₂O₇ ceramic via flash sintering technique. Under an electric field of 250 V/cm, Gd₂Zr_2−xCe_xO₇ (0.0 ≤x ≤ 1.2) waste forms with a single phase of defect-fluorite were flash sintered at relatively low temperatures of 889–997 °C in 60 s. The onset temperature of flash sintering was decreased with the enhancement of Ce content. Furthermore, the density and grain size of Gd₂Zr_2−xCe_xO₇ waste forms were increased with the increase of the current limit. The nearly full dense Gd₂Zr_2−xCe_xO₇ waste forms were flash sintered at a current limit of 200 mA. The normalized leaching rates of Gd, Ce, and Zr in Gd₂Zr_0.8Ce_1.2O₇ after 28 d were 6.5475 × 10⁻⁵, 1.1624 × 10⁻⁷, and 1.1613 × 10⁻⁷ g·m⁻²·d⁻¹, respectively, which exhibited a good chemical durability.     

High dielectric Al2O3-(20–30 wt%) HDPE composites processed via cold sintering

In the present work, the dielectric properties of cold sintered alumina (Al₂O₃) reinforced with 20–30 wt% HDPE composite was investigated. The Al₂O₃-HDPE composites were successfully processed via cold sintering at extremely low temperature in the range of 80–120 °C for 120 min with the application of uniaxial pressure of 500 MPa under vacuum. In fact, cold sintering is a promising method to consolidate ceramic-polymer composites with very large difference in melting points and other thermo-physical/chemical properties. In the present work, high dielectric constant (ε’) of 11.73 and low dielectric loss (tanδ) of 0.0076 measured for Cold Sintering Processed (CSPed) Al₂O₃-20HDPE and a little low ε’ of 9.13 and low tanδ of 0.0066 was evident for CSPed-Al₂O₃-30HDPE at 1 MHz. Such differences in the dielectric properties of the Al₂O₃-20,30 HDPE composites depend on the crystallite size, dangling bond density and microstrain of the materials. Increase in ε’ with temperature is noticed for CSPed-A20H. Moreover, for CSPed-A20H at 1 MHz the temperature variation of dielectric constant (TCC) of 186.94 ppm/°C (or 0.018694 %/°C) was estimated and it reflects a marginal variation of ε’ with temperature. The coefficient of thermal expansion (CTE) of 87.25 × 10⁻⁶ °C⁻¹ and 109.3 × 10⁻⁶ °C⁻¹ was estimated for CSPed-A20H and CSPed-A30H, respectively. Overall, the cold sintered Al₂O₃-20HDPE composites exhibited comparable or better dielectric properties than Al₂O₃ based materials (as reported in the literature) processed by conventional sintering or cold sintering processes.     

Colloidal Filtration of Silicon Nitride Aqueous Slips,Part II: Slip Casting and Pressure Casting Performance

In a previous part, the rheological behaviour of silicon nitride aqueous slips was optimized by dispersing with TMAH up to pH 11·5 using different mixing procedures and including different concentrations of sintering aids (Al₂O₃ and Y₂O₃). In this part, the obtention of pressureless sintered silicon nitride bodies by colloidal filtration techniques is studied. The kinetics of the different compositions is studied for both slip casting and pressure casting. The pressure casting kinetics is up to 20 times faster than that of slip casting, which allows the scale-up of the process for a low cost production, The obtained green density is slightly >58%th for slip casting and 57–55%th for pressure casting, depending on the applied pressure. This small difference does not influence sintered density. At 1750°C/2h, a final density around 96%th is obtained. The sintering conditions are studies considering the time, temperature, atmosphere and sintering bed. The best results are obtained when the sintering bed has the same composition to that of the sample to be sintered. The room temperature properties of the sintered materials show a K_IC value higher than 6 MPam^1/2, comparable to those found in the literature for pressure sintered materials.     

Mechanical and dielectric properties of reduced graphene oxide nanosheets/alumina composite ceramics

Reduced graphene oxide (rGO) nanosheets/alumina (Al₂O₃) composite ceramics were fabricated by hot-pressing sintering. The density, porosity, microhardness, flexural strength and complex permittivity were investigated to study their mechanical and dielectric properties. The results revealed that the rGO nanosheets were uniformly distributed in the Al₂O₃ matrix and that the composite ceramics were highly dense at 3.67–3.99 g/cm³. Due to low rGO hardness and elevated porosity, the microhardness exhibits a decreasing trend as the rGO content increases. The flexural strength first increased and then decreased with the escalation of rGO content, and the highest strength of 313.75 MPa was obtained at 3 wt%, increasing by 37.61% relative to that of the hot-pressing sintered Al₂O₃ ceramic. Owing to the enhanced interfacial polarization, dipole polarization, polarization relaxation loss and conductance loss, the real part and imaginary part of complex permittivity increase from 10.40 to 52.73 and from 0.08 to 28.86 as the rGO content rose from 0 wt% to 4 wt%, respectively.     

Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

Flash sintering of a three‐phase alumina,spinel, and yttria‐stabilized zirconia composite

Three‐phase ceramic composites constituted from equal volume fractions of α‐Al₂O₃, MgAl₂O₄ spinel, and cubic 8 mol% Y₂O₃‐stabilized ZrO₂ (8YSZ) were flash‐sintered under the influence of DC electric fields. The temperature for the onset of rapid densification (flash sintering) was measured using a constant heating rate at fields of 50‐500 V/cm. The experiments were carried out by heating the furnace at a constant rate. Flash sintering occurred at a furnace temperature of 1350°C at a field of 100 V/cm, which dropped to 1150°C at a field of 500 V/cm. The sintered densities ranged from 90% to 96%. Higher electric fields inhibited grain growth due to the lowering of the flash temperature and an accelerated sintering rate. During flash sintering, alumina reacted with the spinel phase to form a high‐alumina spinel solid solution, identified by electron dispersive spectroscopy and from a decrease in the spinel lattice parameter as measured by X‐ray diffraction. It is proposed that the solid solution reaction was promoted by a combination of electrical field and Joule heating.