Flash Sintering Method Research Articles

Microstructure and hardness of two-step reverse-polarity flash sintered high-purity Al2O3 ceramics

In this work, high-purity alumina ceramics were prepared by using two-step reverse polarity flash sintering (PR-FS). The effects of the time distribution between the first flash and the second flash on microstructure and hardness were studied. The results show that compared to the conventional DC flash sintered alumina, the RP–FS–ed samples exhibited more uniform-microstructure, finer grains, higher relative density, and higher hardness. The underlying mechanism for such more homogeneous microstructure was related to uniform distribution of point defects under effect of reverse polarity. Two-step reverse polarity flash sintering method offers a great potential in obtaining highly densified alumina ceramics with homogeneous microstructure.

Flash sintering of silicon carbide at room temperature

Although flash sintering (FS) is an effective method for rapidly preparing ceramics, FS of non-oxide ceramics at room temperature (RT) has not yet been achieved. In this study, we developed a feasible method for FS of SiC at room temperature (RT). A dog-bone-shape SiC sample prepared without using any sintering aid achieved a relative density of 96.5 % after sintering in a low-pressure Ar atmosphere for 60 s. Arc heating, which was constrained by the height of an alumina plate fixed above the sample, increased the conductivity of the sample, which then reached the stable stage of FS. The sintered sample exhibited a typical 6H–SiC crystal structure, which was consistent with that of the raw powder. The proposed FS method, based on constraining the arc using an alumina plate under a low-pressure inert gas atmosphere, provides a reference for developing further advanced RT FS strategies for non-oxide ceramics with low conductivities at RT.

Ultrafast fabrication of W/Cu composites using flash sintering

When the proportion of reinforcing phases is high and the wettability between the secondary phase and the metal is poor, preparing fully dense particle-reinforced metal matrix composites becomes challenging. This study employs the flash sintering method to prepare fully dense WCu20 composite materials within a few tens of seconds. It is found that during the “thermal runaway' process of flash sintering, the Cu phase undergoes rapid melting and solidification, and the core mechanism of material densification is that pressure causes Cu liquid to quickly fill the gaps between tungsten particles. Pressure not only promotes the contraction of the framework formed by tungsten particles but also facilitates the complete filling of gaps within the tungsten frameworks by the Cu liquid and promotes the rapid expulsion of bubbles from the Cu liquid, thereby achieving rapid densification of WCu20 composite materials. Moreover, an increase in the temperature enhances the fluidity of the Cu liquid, which aids in filling the gaps between tungsten particles and promotes the discharge of bubbles from the Cu liquid. Flash sintering is a common technique for the sintering and densification of metal powders, suitable for the rapid preparation of various metal matrix composites.

Effect of alumina morphology on flash sintering and mechanical properties of 3YSZ/alumina composites: Whiskers vs powders

In this paper, 3YSZ/alumina composites containing different content of alumina with different forms (powders and whiskers, respectively) were fabricated by flash sintering. The flash sintering behavior and mechanical properties of the composites were characterized and were compared with those prepared by conventional sintering. The onset temperature of the 3YSZ/alumina whiskers composites was lower than that of the 3YSZ/alumina powders composites with identical alumina content. The reason can be attributed to electrical conductivity; the conductivity of the former was lower than that of the latter. For the 3YSZ/alumina powders composites which can be densified by conventional sintering method, their relative density, Vickers hardness and fracture toughness were comparable to those prepared by flash sintering. While for the 3YSZ/alumina whiskers composites where constrained sintering existed, the flash sintering method revealed obvious advantage over the conventional sintering. A relative density of 94.5 %, hardness of 13.77 GPa and fracture toughness of 5.36 MPa m1/2 were obtained for the flash sintered 3YSZ/30 vol% alumina whisker composites. In contrast, the relative density, hardness, and fracture toughness of the conventional sintered ones were 86.4 %, 11.60 GPa and 4.82 MPa m1/2, respectively.

Flash sintering of ZnO ceramics with combined power supplies at room temperature

A flash sintering method at room temperature based on combined power supplies is proposed in this paper. Flash sintering is performed in two steps. First, a high voltage is used to break down the sample and then disconnect the power supply. Second, a low-voltage power supply is used to provide a large current to densify the sample. The method does not require changing the properties of the material or the sintering atmosphere. In addition to reducing the capacity of the required power supply, there is no need for external auxiliary equipment. Different types of high-voltage breakdown are also studied. The temperature and grain size corresponding to different currents of low-voltage flash sintering are studied. The results show that the sintered ZnO ceramics reach 98.5% relative density. A possible mechanism of flash sintering using combined power supplies is analysed.

Flash sintering BaTiO3-0.01MgO-0.01Nb2O5-xBi2O3 ceramics with superior dielectric temperature stability via defect engineering

In this paper, Bi2O3 doped BaTiO3-0.01MgO-0.01Nb2O5 ceramics are designed to increase dielectric constant, reduce dielectric loss and improve temperature dielectric stability through inducing defects, and are prepared by flash sintering method to inhibit the volatilization of Bi element and ensure the element ratio. The microstructure, polarization mechanisms, dielectric properties and conduction mechanism of the ceramics are studied. The doping of Bi2O3 can enhance the relaxation behavior and dielectric properties. The defects evolution in BaTiO3-0.01MgO-0.01Nb2O-0.01Bi2O3 ceramics are studied. The results show that the BaTiO3 ceramics doped with different amounts of Bi2O3 can form defect dipoles {[BiBa∙‐MgTi″‐BiBa∙]×, [NbTi∙‐VBa″‐NbTi∙]×, [MgTi″‐VO∙∙]×} and oxygen vacancy, which enhance the short-range transition within the grain and inhibit the movement of free electrons at the grain boundary. The ceramics have excellent room temperature properties, high dielectric constant (εr = 2894) and low dielectric loss (tanδ < 0.03). The ceramics have superior temperature stability and satisfying the EIA X8R (Δεr/ε25 ≤ ±15 %, over the temperature range from −55 °C to 150 °C). It is found that flash sintering can effectively guarantee the stoichiometric concentration of bismuth element and obtain good properties of ceramics. This study demonstrates the potential of Bi2O3 doped BaTiO3-based ceramics synthetized by flash sintering as high-performance electronic device materials.

Reusing blended leach residue by flash sintering method

AbstractIn this study, the effects of different sintering methodology on grain formation, density, and hardness of blended leach residue (B‐LR) pellets is investigated. For the first time, the flash sintering (FS) method, an environmentally friendly process, is used to make multiphase ceramic composites using B‐LR under 100 V/mm at 675C. The outcomes of this study revealed that the FS process is faster and more energy‐efficient than the conventional sintering (CS) methods performed at 800°C for 4 h. The formation of abnormal grain growth is prevented by FS in B‐LR material composition after it is exposed to a max power absorption 61.4 mW/mm3 and whole FS is completed less than 100 s. Besides, FS method does not lead to any melting on grain boundaries due to excessive joule heating, compared with the CS sample. It is noted that FS also provides better density and hardness values for this material system along with compositional integrity in this multiphase system. The significant outcome of this study is reducing lead volatility and emission by decrement on the sintering temperature. Ultimately, this study has planned the practicality of reusage and recycle of this material with green, safe, and eco‐friendly methods.

Sol-hydrothermal synthesis of Na0.5Bi0.5TiO3 powders and corresponding ceramics prepared by traditional and flash sintering

In this paper, a new wet chemical route named sol-hydrothermal method was used to synthesize Na0.5Bi0.5TiO3 (NBT) nanopowders. The effects of hydrothermal reaction conditions such as temperature, time and NaOH concentration on the properties of the powder were studied in detail. The results showed that as-synthesized NBT powders with were perovskite structure with rhombohedral symmetry and had submicron size (200 nm ∼ 800 nm). The morphology of NBT powders was mainly spherical and they had good dispersibility. NBT powders with homogeneous size and pure phase were synthesized under the optimum condition of hydrothermal reaction at 200 °C for 24 h. It is worth noting that pure NBT ceramic could not be obtained by traditional sintering at 1100 °C for 2 h. However, the relatviely dense ceramic with a relative density of 95.8% and an apparent porosity of 1.8% was obtained by electric field assisted flash sintering for only 3 s at 700 °C. Therefore, the very short time flash sintering method was suitable for the preparation of volatile NBT ceramic. In particular, a mechanism named Electric Field Assisted Solid Crystal Growth (EFASCG) was proposed to explain sintering process.

Investigation of Reusing Copper Converter Slag Residue with the Flash Sintering Method

Investigation of Reusing Copper Converter Slag Residue with the Flash Sintering Method

Enhanced densification and ionic conductivity of LLZO by flash sintering

ABSTRACT Flash sintering arouses the interest since high-density ceramics can be obtained at shorter dwell times and lower temperatures than conventional sintering. In this study, the cubic garnet Li6.25Al0.25La3Zr2O12 (Al-LLZO) was successfully synthesised by the solid-state method. The powders were uniaxially pressed and were subjected to flash sintering at 850°C in a tube furnace under a DC bias using various current densities. It is evidenced that control of the flash electric current is a crucial factor for densification of Al-LLZO. The sample sintered in 50 V cm−1 and 200 mA mm−2 showed a cubic LLZO, 94 ± 0.4% relative density, 0.37 mS cm−1 total ionic conductivity and 0.32 eV activation energy. In addition, it was demonstrated that increasing the current density had a considerable impact on the relative density. This outstanding ionic conductivity might be due to the lower lithium loss and higher density as a result of flash sintering method applied.

Reactive flash synthesis and phase compositions of Lix(CoCrFeMnNi)Oy (x = 0–1) oxides

In this paper, the reactive flash sintering method was adopted to synthesize a series of Lix(CoCrFeMnNi)Oy oxides with x in the range of 0–1. The phase compositions and electrical conductivities of the oxides at different Li incorporation level were characterized. A single phase of spinel structure (Fd-3m) was obtained for the sample (CoCrFeMnNi)3O4. With incorporation of Li, the phase transformed into Rock-salt structure (Fm-3m) and then layered structure (R-3m). The ionic and electronic conductivities of the samples were characterized by impedance spectrometry. The ionic and electronic conductivities of the samples decreased with increasing Li incorporation level at first along with the spinel to Rock-salt structure transformation, and then increased with further increasing Li incorporation level along with the Rock-salt to layered structure transformation.

Ultra-fast synthesis of Bi2Te3−xSex based compounds with micro-nano composite structure at room temperature by reactive flash sintering

The synthesis of Bi2Te3−xSex (x = 0 and 0.3) compounds in a few seconds at room temperature was realized by reactive flash sintering method using Bi, Te and Se raw powders. The phase transition and microstructure evolution during flash sintering were systematically studied relating to the duration time, power density and sample temperature. The synthesized Bi2Te3−xSex compounds have micro-nano composite structure, which are composed of 5–10 μm lamellar crystal and 10–20 nm nanograin. It is suggested that the instantaneously generated local Joule heat throughout the sample and the resulted ultra-high self-heating rate approaching tens of thousands K/min are responsible for the synthesis of highly crystalline Bi2Te3−xSex. The synthesis mechanism is proposed, including Joule heating, melting, chemical reaction, nucleation and grain growth. This highly efficient and economic synthesis method of Bi2Te3−xSex compounds with micro-nano composite structure has great potential for application in the field of thermoelectric industry.

Microstructural and electrical properties of flash sintered ZnO–Bi2O3–Sb2O3 based varistors: Effect of current density with a controlled current ramp

The ZnO–Bi2O3–Sb2O3-based varistors were produced by the flash sintering method. The effects of current density passed through the sample during the sintering process on the microstructure, and electrical properties of Varistors were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), impedance spectroscopy, and electrical measurements. The average grain size increased from 1.46 μm to 4.85 μm by increasing the current density from 80 to 200 mA/mm2, respectively. The XRD analysis showed that the pyrochlore phase was decomposed in the samples sintered by the current density of 200 mA/mm2. Increasing the current density passed through the sample during the flash sintering process resulted in the modified nonlinear characteristics of varistors. The highest relative density (94% of theoretical density) and the optimum electrical properties were obtained in varistors sintered by 200 mA/mm2, i.e., the nonlinear coefficient, leakage current density, and Schottky barrier height of 39.1, 12 μA/cm2, and 0.43 eV, respectively.

The effects of electric field on physical properties of LLZO made by flash sintering method

Li7La3Zr2O12 (LLZO) is one of the promising electrolytes for all-solid-state lithium-ion batteries. Since in conventional sintering, lithium retention is hard to control in LLZO, flash sintering, characterized by shorter sintering time and lower sintering temperature, can be beneficial. In this study, Al-doped LLZO (LALZO)powder was successfully synthesized by the combustion sol-gel method. The product was characterized by thermal analysis (TG/DTA) to evaluate its thermal behavior. Afterward, the effect of an electrical field in an isothermal approach on the relative density and phase composition of LALZO has been investigated. The obtained LALZO powder was uniaxially pressed into a dog-bone shape at 100 MPa. The samples were held in a tube furnace at 850 °C with an applied DC electrical field ranging from 40 V.cm−1 to 100 V.cm-1, while the current density was limited to 200 mA.mm−2. The phase composition was analyzed with X-ray diffraction (XRD), and the ionic conductivity was investigated using an AC impedance spectroscopy. Dense LALZO was flash sintered successfully, and the sample sintered with an electric field of 60 V.cm−1 showed a pure cubic LALZO with 92.4% relative density and a total ionic conductivity of 0.31 mS and activation energy of 0.37 eV. Moreover, it was revealed that the change in the electric field does not significantly affect the relative density. Also, increasing the electric field causes the incubation time to become shorter. This behavior is thought to be due to electric field strength as a driving force for nucleation mechanisms.

Two-step flash sintering of (Mg1/3Ta2/3)0.01Ti0.99O2 giant dielectric ceramics

PurposeThe purpose of this paper on the one hand is to reduce the sintering temperature, shorten the sintering time and improve the electrical properties of the sample through the two-step flash sintering method and on the other hand is to study the effect of electric field on the phase structure, microstructure and electrical properties of the flash sintering sample.Design/methodology/approachIn this paper, (Mg1/3Ta2/3)0.01Ti0.99O2 giant dielectric ceramics were prepared by conventional sintering and two-step flash sintering, respectively. Further, the effect of electric field (600–750 V/cm) on the electrical properties of (Mg1/3Ta2/3)0.01Ti0.99O2 giant dielectric ceramics was studied.FindingsThe results show that compared with the conventional sintering, the sintering temperature of the two-step flash sintering can be reduced by 200°C and the sintering time can be shortened by 12 times. All sintered samples were single rutile TiO2 structure. Compared with conventional sintering, two-step flash sintering samples have finer grain size. The two-step flash sintered sample has similar dielectric properties to the conventional sintered sample. The dielectric constant of flash sintered samples decreases with the increase of electric field. When the electric field is 700 V/cm, the ceramic sample has the optimal dielectric properties, where the dielectric constant is approximately 5.5 × 103 and the dielectric loss is about 0.18 at 1 kHz. Impedance spectroscopy analysis shows that the excellent dielectric properties are attributed to the internal barrier layer capacitance model.Originality/valueThis paper not only provides a new method for the preparation of co-doped TiO2 giant dielectric ceramics but also has great potential in greatly improving efficiency and saving energy.

Flaş sinterleme yönteminin uçucu kül mikroyapısına etkisi

Bu çalışmada termik santral atığı olan cam seramik malzemelerden uçucu kül’ ün yoğunlaştırılması (sinterlenmesi) elektrik alan destekli flaş sinterleme yöntemiyle gerçekleştirilmiştir. Uçucu Kül numuneleri Tunçbilek Termik Santralinden (Kütahya, Türkiye) temin edilmiştir. Flaş sinterleme deneyinde 0,85 A akım üst limiti belirlenerek 450 V/mm elektrik alan uygulanmıştır. XRD analizine göre yapıda camsı faz, kuvars, hematit, manyetit, magnezyoferrit ve müllit yapıları bulunmaktadır. Kimyasal analizi (XRF) ile içeriğinde belirlenen yüksek orandaki SiO2, Al2O3 ve Fe2O3’ün tek başlarına ve/veya birlikte oluşturdukları cam-seramik malzemelerin sinterlenmesi 1500℃ ve üzeri sıcaklıklarda gerçekleştiği literatürde rapor edilmiştir. Elektrik alanlı flaş sinterleme işlemi uçucu külün ortalama sinter sıcaklığını 1500℃’lerden 1137℃’ye düşürürken sinter süresini de 3 saatten 70 saniyeye çekmiştir. Bu yöntem ile gerçekleştirilen sinterleme işlemi geleneksel sinterleme yöntemlerinden daha düşük sıcaklık ve daha kısa sürede gerçekleştirilmiştir. Literatürdeki uçucu kül sinterleme işlemlerinin SEM mikrografları ile karşılaştırıldığında daha yoğun bir yapı sinter işleminin başarılı bir şekilde gerçekleştiğini kanıtlanmıştır.

High‐performance ZnO varistor ceramics prepared by arc‐induced flash sintering with low energy consumption at room temperature

The development of ultra-high voltage transmission lines requires a ZnO arrester with excellent electrical response and high voltage gradient. Compared with conventional preparation methods, flash sintering allows fast production of novel high-performance ZnO varistor ceramics with low energy consumption and controlled grain growth. Herein, ZnO varistor ceramics were prepared using the flash sintering method at an air pressure of 21 kPa for 60 s at 25°C. The results showed that the flash-sintered samples had fine grains and reduced loss of volatile elements. Moreover, the voltage gradient, leakage current density, and non-linear coefficient of the flash-sintered varistor ceramics were significantly improved compared with those of conventionally sintered samples. Owing to the short preparation time and absence of a heating process, the energy consumption of the proposed flash sintering method was significantly reduced to 10% of that by conventional sintering. The proposed method is a promising approach for the preparation of ZnO arresters with excellent electrical properties and serves as an effective method that realises energy saving and emission reduction.

Improvement of photocatalytic hydrogen evolution of La5Ti2AgS5O7 by flash sintering method

La5Ti2AgS5O7 (LTA) is a long-wavelength-responsive oxysulfide photocatalyst that exhibits considerable photocatalytic H2 evolution activity with an intense visible light absorption up to 570 nm. However, suppression of oxide impurities and sulfur defects when preparing LTA remains a big challenge. In this study, LTA was prepared by using a flash sintering (FS) method that allows for the synthesis of LTA (LTA-FS) with a significantly short heating period (a few seconds). The approach allowed a formation of high degree crystallinity with relatively small particle sizes without adding excessive sulfur that was employed in conventional solid-state reactions during the preparation process. Moreover, well-defined sulfur defects on the surface of LTA-FS were favorable for constructing the self-organized homojunction between the bulk and surface phases with a local built-in electric field, which is beneficial for charge separation and transfer, thus improving H2 evolution activity. The corresponding structure-to-performance relationship was further investigated by combining with the density functional theory of the electronic band structures and charge densities. This work suggests that FS may be very useful for the preparation of oxysulfide photocatalysts.

Flash Sintering Effect on PMN-PT Ceramics

Densification of PMN-PT piezoelectric ceramic powder is carried out by electric field assisted/flash sintering method. The composition selected in PMN-PT materials system is 0.67[Pb(Mg1/3Nb2/3)O3]−0.33(PbTiO3). During flash sintering experiment, 200 V/mm electric field is applied on sample with 0.1 amp current cutoff value. Flash sintering is accomplished in less than 90 sec at ~700 °C. Relative density for sintered sample is measured approximately 94 % which is reasonable value comparing with literature. SEM microstructure illustration exhibits proper consolidation morphology which is agreement with the relative density measurement. According to XRD analysis, the said materials system shows consistency in rhombohedral perovskite crystal structure and preserve its chemical and physical composition during whole experimental cycle. By electric field assisted/flash sintering technique, the sintering temperature is decreased from 1200 °C to 700 °C along with a decrement in sintering time from 2 hours to 90 seconds. PMN-PT is properly produced by flash sintering method for the first time with respect to open literature. Hence, this densification method will gain importance to reduce toxic Pb emission aroused at high temperature processes.

The Effects of Contact Paste Type and Electric Field on Physical Properties of Zirconia Bodies Made by Flash Sintering Method: Modeling Via Response Surface Methodology

The Effects of Contact Paste Type and Electric Field on Physical Properties of Zirconia Bodies Made by Flash Sintering Method: Modeling Via Response Surface Methodology