Intergranular amorphous films formed by DC electric field in pure zirconia

It is difficult to obtain pure ZrO₂ sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO₂ can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO₂ flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.     

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.     

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.     

Athermal effect of flash event on high-temperature plastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal

This study experimentally demonstrates the athermal effect of a flash event under a DC electric field on high-temperature plastic deformation in a dense, fine-grained 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP). A high-temperature three-point flexural test in the conventionally sintered 3Y-TZP body with an average grain size of 0.36 µm was conducted under a flash event with a current density of 67 mA·mm^?2 at a furnace temperature between 900 °C and 1300 °C. The flash event significantly enhanced the high-temperature plastic flow in the 3Y-TZP body, even though subtracting the effect of specimen temperature rise by Joule heating, while grain growth during the flexural tests was effectively suppressed under the examined conditions. The athermal effect on the plastic flow was attributed to the reduction in the activation energy for the high-temperature plastic flow.     

Fabrication of Gd2O3‐MgO nanocomposite optical ceramics with varied crystallographic modifications of Gd2O3 constituent

The fabrication of Gd₂O₃‐MgO nanocomposite optical ceramics via hot‐pressing using sol‐gel derived cubic‐Gd₂O₃ and MgO nanopowders was investigated. The precursor powder calcined at 600°C had an average particle size of 12 nm. The effects of hot‐pressing temperature on constituent phases, microstructure, mid‐infrared transmittance, and microhardness were studied. The crystallographic modifications of Gd₂O₃ phase varied with the increase in sintering temperature from 1250 to 1350°C. The monoclinic‐Gd₂O₃ phase was retained for the composite sintered at 1350°C and the sample had an average grain size of 90 nm, excellent transmission (80.4%‐84.8%) over 3‐6 μm wavelength range, and enhanced hardness value of 14.1 GPa.     

Densification behavior of flash sintered boron suboxide

A study to quantify the flash sintering kinetics of boron suboxide (B₆O) under various electric field strengths and cut‐off amperages is presented. B₆O is conventionally sintered at a prolonged temperature above 1800°C, near its thermal decomposition temperature, with an overpressure >3 atm. By applying a direct current (DC) electric field across a green powder compact, B₆O can be sintered at 1000°C at atmospheric pressure. During the flash sintering process, an intensive radiation was emitted (electroluminescence), which is distinct from the thermal radiation (thermoluminescence) that is expected in conventional sintering. It was observed that the degree of sintering of the large B₆O specimen was heterogeneous due to apparent localization of electrically conducting paths. The material near the surface was sintered, but the core of the specimen was not. It was found that the flash event occurred at a critical temperature, which was obtained by combining external heating via ambient furnace conditions and internal Joule heating. The progressive densification behaviors of the B₆O are also presented.     

Effect of Ga2O3 on Sintering and Phase Stability of Sc2O3‐Doped Tetragonal Zirconia Prepared via Aqueous Solution Route

The effects of the presence of Ga₂O₃ on low‐temperature sintering and the phase stability of 4, 5, and 6 mol% Sc₂O₃‐doped tetragonal zirconia ceramics (4ScSZ, 5ScSZ, and 6ScSZ, respectively) were investigated. A series of zirconia sintered bodies with compositions (ZrO₂)_0.99?x(Sc₂O₃)_x(Ga₂O₃)_0.01, x = 0.04, 0.05, and 0.06 was fabricated by sintering at 1000°C to 1500°C for 1 h using fine powders that were prepared via the combination of homogeneous precipitation method and hydrolysis technique using monoclinic zirconia sols synthesized through the forced hydrolysis of an aqueous solution of zirconium oxychloride at 100°C for 168 h. The presence of 1 mol% Ga₂O₃ was effective in reducing sintering temperature necessary to fabricate dense bodies and enabled to obtain dense sintered bodies via sintering at 1100°C for 1 h. The phase stability, that is, low‐temperature degradation behavior of the resultant zirconia ceramics was determined under hydrothermal condition. The zirconia ceramics codoped with 1 mol% Ga₂O₃ and 6 mol% Sc₂O₃ (1Ga6ScZ) fabricated via sintering at 1300°C for 1 h showed high phase stability without the appearance of monoclinic zirconia phase, that is the tetragonal‐to‐monoclinic phase transformation was not observed in the 1Ga6ScZ after treatment under hydrothermal condition at 150°C for 30 h.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

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.     

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.     

The degradation behavior of high-voltage SnO2 based varistors sintered at different temperatures

In this research, for the first time, the stability of SnO₂ based varistor ceramics sintered in the range of 1250–1350 °C against DC-accelerated aging and impulse surge current tests, was systematically studied. Microstructural study of the sintered samples by XRD and FESEM indicated that the sintering temperature only affects densification and grain size, while phase composition remains intact. With the increase of sintering temperature from 1250 °C to 1350 °C, the mean grain size increased from 1.6 to 8 μm. The maximum nonlinear coefficient of 50 and the minimum leakage current density of 1.5 μA/cm² were obtained in the sample sintered at 1300 °C. The breakdown electric field decreased from 800 V/mm to 270 V/mm, when sintering temperature increased from 1250 °C to 1350 °C. The samples sintered at 1250 °C did not show stability against neither of DC-accelerated aging and impulse current tests. The varistors sintered at 1300 °C exhibited the excellent resistance to DC-accelerated aging degradation, while ceramics sintered at 1350 °C showed the best resistance to impulse current degradation.     

Influence of sintering temperature on the morphology of ceramic hollow fibers prepared from niobium pentoxide

Ceramic hollow fibers were prepared by the phase inversion and sintering method using niobium pentoxide (Nb₂O₅) as an innovative starting material. X-ray diffraction and Raman analyses revealed the same monoclinic crystalline phase for the ceramic material, H-Nb₂O₅, at all the evaluated sintering temperatures. According to SEM images, the starting material was composed of polydisperse particles of irregular size and shape with sizes ranging from 12.5 to 89.7 μm. The increase in the sintering temperature caused particles agglomeration. In the hollow fiber precursor (without sintering), Nb₂O₅ grains were surrounded by the coagulated polymer. The polymeric phase was eliminated when the fibers were sintered at temperatures above 600°C. When sintered at 1350°C, the outer surface of the fiber presented elongated crystals of well-defined shape, while agglomerated round shape grains were observed at the inner surface of the fiber. Formation of these elongated crystals was probable due to the material sintering at high temperatures (up to 1350°C) for more than 300 minutes. This study demonstrated the potential for general applicability of niobium pentoxide to fabricate ceramic hollow fiber membranes.     

Li2ZrO3 based Li-ion conductors doped with halide ions & sintered in oxygen-deficient atmosphere

All-solid-state batteries have recently attracted much attention for their high energy density and safety. Li₂ZrO₃-based Li-ion conductors with high electrochemical stability have potential applications for electrolytes in all-solid-state batteries. In this work, comparative investigations of Li₂ZrO₃ and halogen doped Li₂ZrO₃ ceramics were conducted by sintering at 700 °C in air or in oxygen-deficient atmosphere which was induced by a simple setup covering with corundum crucible. The analysis of phase composition reveals that the undoped Li₂ZrO₃ ceramic sintered in air contains pure monoclinic phase, while halogen-doped Li₂ZrO₃ sintered in air and all ceramics sintered in oxygen-deficient atmosphere are simultaneously composed of monoclinic and tetragonal phases. Li₂ZrO₃ ceramic with tetragonal phases has higher conductivity (0.28 mS cm⁻¹ for undoped Li₂ZrO₃) than the pure monoclinic Li₂ZrO₃ (0.07 mS cm⁻¹), and halogen doping can further enhance the conductivity of Li₂ZrO₃ ceramics higher than 0.5 mS cm⁻¹ at room temperature.     

Microstructure and electrical properties of ZnO–ZrO2–Bi2O3–M3O4 (MCo,Mn) varistors

Phase evolution, microstructure and the electrical properties of ZrO₂-added pyrochlore-free ZnO–Bi₂O₃–M₃O₄ (MCo, Mn) varistors have been studied as functions of ZrO₂ content up to 10 vol% and the sintering temperature between 900 and 1300 °C. Zirconia remained as intergranular second phase particles up to 1100 °C, which retarded densification and inhibited the grain growth of ZnO. At higher temperatures, on the contrary, ZrO₂ particles began to be entrapped in ZnO grains and irreversibly transform from monoclinic to stable cubic phase dissolving transition metal ions. The grain size of ZnO decreased with increasing ZrO₂ content, and increased with the increase of the sintering temperature. Accordingly breakdown voltage changed with both ZrO₂ content and the sintering temperature as was expected. Nonlinear coefficient (α) depended primarily on the sintering temperature: it increased to >40 up to 1000 °C, and significantly decreased to <30 at higher temperatures probably due to the volatilization of Bi₂O₃. While the specimens sintered at 1200 °C or above had relatively high leakage current (I_L) and large clamping ratio (C_R), those with ZrO₂ content of 0.5–5.0 vol% and sintered below 1200 °C revealed low I_L of ⩽20 μA/cm² and C_R well below 2.0. In spite that varistor characteristics of ZrO₂-added system could not match those of commercial ZnO varistors, its low temperature sinterability and ease of breakdown voltage control via ZrO₂ content without a serious loss of its figures of merit are worth noticing, particularly for multi-layered chip varistor (MLV) application.     

Densification behaviour and microstructural development in undoped yttria prepared by flash-sintering

Conventional sintering of undoped Y₂O₃ requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y₂O₃ was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

Detection of phases in Fe-doped YSZ by XRD and Raman spectroscopy

Samples of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) doped with Fe were prepared and sintered at 1350°C in air or argon atmosphere. The sintered samples were characterized by SEM, XRD, and Raman spectroscopy. Raman spectroscopy and XRD were found to complement each other in phase identification; all the sintered samples were shown to possess tetragonal phase (t”) whose axial ratio (c/a) is equal to 1 in the cubic matrix. The Raman peak at 616 cm⁻¹, in general, was seen to broaden with increase in Fe concentration for both sintering atmospheres.     

The influence of the processing parameters on the reactive flash sintering of ZrO2-CeO2

Reactive flash sintering (RFS) is a method that was recently developed to produce dense single-phase bulk ceramic parts through solid-state reactions in a single-step that only takes a few minutes. The influence of the RFS parameters on the phase purity of a simple mixed oxide, (Zr_0.8,Ce_0.2)O₂, was investigated. Parameters such as furnace temperature, furnace atmosphere, electric current density, and alternating current (AC) or direct current (DC) were examined. It was found that (Zr_0.8,Ce_0.2)O₂ pellets with high densities, above 90% of its theoretical density, can be produced by RFS in a few minutes when RFS occurs under oxidizing atmospheres, AC fields with current densities of 100 mA·mm⁻², and at a furnace temperature of 1200°C. Reducing conditions such as Ar-H₂ atmosphere and DC fields, low furnace temperatures, and low current densities resulted in phase impurities and poor reactions between the ZrO₂ and the CeO₂ powders. These results show that RFS is a useful method to produce mixed oxides, but it is very sensitive to the processing parameters. This is the first time that the influence of most of the RFS processing parameters has been studied systematically. Thus, the present work aims to provide guidelines on selecting the right processing parameters when exploring RFS.     

Low‐Temperature Sintering and Electric Properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 Ferroelectric Ceramics with CuO Additive

The low‐temperature sintering and electric properties of Pb_0.99(Zr_0.95Ti_0.05)_0.98Nb_0.02O₃ (PZTN 95/5) ferroelectric ceramics with CuO addition was investigated. The CuO addition significantly promoted the densification and reduced the sintering temperature of PZTN 95/5 ceramics by more than 200°C. The 0.2 wt% CuO‐added sample sintered at 1150°C exhibited the optimum relative density of 96.7% and excellent electric properties with values of P_r = 37.80 μC/cm², T_C = 223°C, ε_r = 329, and tan δ = 0.016, which were superior to that of PZTN 95/5 ceramics sintered at 1350°C.