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.     

Flash sintering of lead zirconate titanate (PZT) ceramics: Influence of electrical field and current limit on densification and grain growth

Flash sintering of lead zirconate titanate ceramics were investigated under DC electric fields ranging from 300 to 600?V/cm. The onset temperature for flash sintering significantly decreased with the electrical field to a lower limit of furnace temperature of 538?°C at 600?V/cm. The retardation of grain growth was observed, and the grain size decreased with increasing the electrical field. The current limit had a great influence on the density and grain size of specimen. During the flash sintering process, power dissipation first rose abruptly to a maximum value, then declined sharply to a steady state. Meanwhile, optical glow of specimen was observed. Using black body radiation model, the actual specimen temperature was estimated, which was too low to obtain the full dense ceramics in 30?s. It was suggested that Joule heating, ultra-high heating rate and high concentrations of defects were responsible for flash sintering of PZT ceramics.     

Field-assisted sintering of undoped BaTiO3: Microstructure evolution and dielectric permittivity

We report, for first time, how electric fields influence the sintering of undoped BaTiO₃, a ferroelectric material, and how this process affects the microstructure and the dielectric properties. Flash sintering is achieved at a furnace temperature of 688 °C under a field of 500 V cm⁻¹, producing specimens that are 94% dense. As a consequence, the grain size is much finer than in conventional sintering, which is shown to influence the Curie temperature and dielectric permittivity. Data obtained at different strengths of the electrical field, and current limits imposed on the specimen are presented in the form of a “processing map” that separates the safe region, where sintering is uniform, from the fail region, where the current flow in the sample becomes localized. The map illustrates that ceramics can respond by different mechanisms, with the dominant mechanism changing with the strength of the electrical parameters.     

Development of a processing map for safe flash sintering of gadolinium-doped ceria

Flash sintering was discovered in 2010, where a dog-bone-shaped zirconia sample was sintered at a furnace temperature of 850°C in <5 s by applying electric fields of ~100 V cm⁻¹ directly to the specimen. Since its discovery, it has been successfully applied to several if not all oxides and even ceramics of complex compositions. Among several processing parameters in flash sintering, the electrical parameters, i.e., electric field and electric current, were found to influence the onset temperature for flash and the degree of densification respectively. In this work, we have systematically investigated the influence of the electrical parameters on the onset temperature, densification behavior, and microstructure of the flash sintered samples. In particular, we focus on the development of a processing map that delineates the safe and fail regions for flash sintering over a wide range of applied current densities and electric fields. As a proof of concept, gadolinium-doped ceria (GDC) is shown as an example for developing of such a processing map for flash sintering, which can also be transferred to different materials systems. Localization of current coupled with hot spot formation and crack formation is identified as two distinct failure modes in flash sintering. The grain size distribution across the current localized and nominal regions of the specimen was analyzed. The specimens show exaggerated grain growth near the positive electrode (anode). The region adjacent to the negative electrodes (cathode) showed retarded densification with large concentration of isolated pores. The electrical conductivity of the flash sintered and conventional sintered samples shows identical electrical conductivity. This quantitative analysis indicates that similar sintering quality of the GDC can be achieved by flash sintering at temperature as low as 680°C.     

High-Resolution Transmission Electron Microscopy Study of the Microstructural Development of a Silicon Carbonitride Nanocomposite

Nanocrystalline ceramics are expected to possess enhanced superplasticity over their microcrystalline counterparts. In this effort of producing nanocomposites of silicon nitride and silicon carbide, amorphous Si-C-N derived from pyrolysis of a polysilazane precursor was sintered with yttria as an additive. High-pressure sintering at different temperatures resulted in sintered materials ranging from amorphous to nanocrystalline. High-resolution transmission electron microscopy was conducted to characterize the development and grain-boundary features of the nanocrystalline microstructure. The results provide a preliminary understanding of the process of the formation of the nanocrystalline structure from an amorphous matrix, under the condition of high pressure and relatively low temperature. The wide variation in the thickness of grain-boundary phases observed in this material suggests a nonequilibrium state of the grain boundary, which might be related to the processing conditions.     

高纯镁砂及氧化镁陶瓷研究进展

高纯镁砂是重要的耐高温材料,氧化镁陶瓷则广泛应用于透光材料领域,对两种材料的生产工艺开展研究具有重要理论和实际意义。本文系统地综述了利用菱镁矿、卤水生产高纯氧化镁及镁砂的各种技术,以及氧化镁陶瓷的烧结方法和烧结助剂对烧结过程的影响;介绍了菱镁矿制备高纯镁砂,卤水沉淀法、卤水直接热解法制备高纯氧化镁,以及电熔法制备高纯镁砂等技术。指出了每种生产技术的优缺点及今后的研究与发展方向。介绍了常压烧结、热压烧结、热等静压烧结、放电等离子烧结、微波烧结和真空烧结等氧化镁陶瓷烧结技术及其进展,总结了烧结助剂对烧结过程的影响及其机理,指出氧化镁陶瓷未来的研究关键主要在于对粉体合成技术、致密化烧结技术及烧结助剂的研究。     

Formation of 3D tubular cannel in fully densified zirconia ceramics by free-foaming method using superplasticity

Three-dimensional (3D) tubular cannels in fully densified zirconia ceramics have been fabricated by a newly developed solid-state free-foaming method using superplasticity. Three mole percent of yttria-stabilised zirconia (3YSZ) and α-SiC were used as foam matrix and foaming agent simultaneously. Dried pattern of 15 wt% (wt%) α-SiC slurry based on 10 wt% methyl cellulose buried into 3YSZ powder compacts and proper heat treatment at the sintering temperature causes successful fabrication of 3D cannel in zirconia ceramics.     

Al2O3陶瓷及其复合材料的超塑性研究

陶瓷材料的超塑性是细晶多晶陶瓷在高温下的固有属性.本文提出了陶瓷材料超塑性产生的条件、特征和变形机制.综述了国内外Al2O3陶瓷及其复合材料超塑性研究的最新进展,最后指出了今后Al2O3陶瓷材料超塑性研究发展方向.     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

La掺杂(Na_(0.5)Bi_(0.5))_(0.82)(K_(0.5)Bi_(0.5))_(0.18)TiO_3系统无铅压电陶瓷的制备工艺探讨

采用传统陶瓷工艺,研究了制备[(Na0.5Bi0.5)0.82(K0.5Bi0.5)0.18]1-xLaxTiO3(x=0.00,0.01,0.03,0.05,0.10)无铅压电陶瓷的工艺条件对陶瓷的物相组成、显微结构和压电性能的影响。利用XRD、SEM等技术分析结果表明,合成温度的提高有利于主晶相的形成,且此系统烧成温度范围较窄,故需控制在合适的烧成温度下才能得到高致密度的陶瓷。同时,研究了极化工艺条件对材料压电性能的影响,结果表明,提高极化电场强度、控制适当的极化温度有利于提高材料的压电性能。     

La掺杂(Na0.5Bi0.5)0.82(K0.5Bi0.5)0.18TiO3系统无铅压电陶瓷的制备工艺探讨

采用传统陶瓷工艺，研究了制备[（Na0.5Bi0.5）0.82（K0.5Bi0.5）0.18]1-xLaxTiO3（x=0．00，0．01，0．03，0．05，0．10）无铅压电陶瓷的工艺条件对陶瓷的物相组成、显微结构和压电性能的影响。利用XRD、SEM等技术分析结果表明，合成温度的提高有利于主晶相的形成，且此系统烧成温度范围较窄，故需控制在合适的烧成温度下才能得到高致密度的陶瓷。同时，研究了极化工艺条件对材料压电性能的影响，结果表明，提高极化电场强度、控制适当的极化温度有利于提高材料的压电性能。     

Effect of graphite and Mn3O4 on clay-bonded SiC ceramics for the production of electrically conductive heatable filter

Electrically conductive porous SiC ceramics are attracting substantial attention due to their application in heatable filters, vacuum chuck, and semiconductor processing parts, etc. The main problem is their high processing cost. Ideal candidates from an engineering ceramic perspective will be mechanically durable and have the required electrical properties with sufficiently low fabrication costs. To decrease the sintering temperature, kaolin has been added, but it tended to render the material an insulator. Graphite was used to effectively decrease the electrical resistivity. Additionally, manganese oxide was used to decrease the quantity of kaolin (the component that leads to an insulator material after sintering) and decrease the electrical resistivity while maintaining the mechanical properties. In our study, we found that SiC with 35% kaolin, 20% graphite and 10% manganese oxide can produce samples with 6.5 × 10^?1 Ω cm electrical resistivity and 43.5 MPa flexural strength at a low sintering temperature of 1200 °C.     

Structure,electrical and luminescent properties of the NBT:Er-0.3CT:Pr composite ceramics

A binary composite ceramic Na_0.5Bi_0.5TiO₃:0.002 Er-0.3CaTiO₃:0.003 Pr (NBT:Er-0.3CT:Pr) was fabricated by a conventional solid state reaction route. The effect of sintering temperature on the microstructure and phase structure of the ceramics has been investigated. And the basic electrical properties such as dielectric properties and ferroelectric performances of the ceramics were examined. Macro long-ranged ferroelectric order cannot be induced via electric field in the composite ceramics. And under local electric field using PFM, obvious ferroelectric switching is identified in the ceramics. Furthermore, the up-conversion luminescence, photochromic reaction, and luminescence modulation have been achieved in the ceramics. Additionally, photo-stimulated luminescence and thermal luminescence are obtained. The present study indicates that optical properties are tightly related to the sintering temperature. The prepared ceramic NBT:Er-0.3CT:Pr owns a piezoelectric performance and multi luminescence properties, which can widen the applications of ferroelectric ceramics.     

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO₃ ceramics. The results reveal that the most suitable way to control the sintering process is by using non-linear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO₃, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials.     

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.     

Temperature Dependence of the Electrical Properties and Seebeck Coefficient of AlN–SiC Ceramics

AlN–SiC ceramics composed of AlN–SiC solid solutions were fabricated by pressureless sintering without sintering additives. The microstructure and electrical properties of the AlN–SiC ceramics were investigated for compositions between 0 and 75 mol% AlN. The AlN–SiC ceramics had a porous structure, and a 2H polytype was found in all compositions. The electrical conductivities and Seebeck coefficients of the AlN–SiC ceramics increased with temperature. The electrical conductivity of 25 mol% AlN–75 mol% SiC ceramics was the highest in all compositions: 32.7 S/m at 300°C. In contrast, the electrical conductivity of 75 mol% AlN–25 mol% SiC ceramics was much lower than that of other samples: 10⁻² S/m at 300°C. The Seebeck coefficient of 50 mol% AlN–50 mol% SiC ceramics was the highest of all samples: 210 μV/K at 300°C. The electrical and thermoelectrical properties of SiC can be controlled by the formation of AlN–SiC solid solutions.     

Pressure treatment of superplastic ceramic materials

Superplasticity is defined as viscoplastic deformation of a material in a special ultra-fine-grained structural state. Modern fine ceramics with submicrometer structural components are polycrystalline materials that can exhibit superplasticity under certain temperature and rate conditions. The prospects for industrial use of the effect of superplasticity in pressure treatment of ceramics are analyzed on the basis of results of research of the last decade. It is shown that the property of superplasticity makes it possible to manufacture preforms by virtually any of the known methods of hot pressure treatment of bulk and sheet preforms.     

Processing and characterizations of flash sintered ZnO-Bi2O3-MnO2 varistor ceramics under different electric fields

The dense ZnO-Bi₂O₃-MnO₂ (ZBM) varistors were prepared by flash sintering under electric fields ranging from 200 V/cm to 400 V/cm at constant heating rate (CHR) and constant furnace temperature (CFT), respectively. The structure and electrical properties of the ZBM varistors were studied via the XRD, SEM and a DC parameter instrument. The onset temperature and incubation time of flash sintering decrease with increasing electric field. The effects of the maximum limiting current on the density of the samples were also investigated. The results showed that the density of samples increase with the increasing current values. The improved electrical characteristics were obtained during constant furnace temperature flash sintering. The ZBM varistor ceramics exhibited superior comprehensive electrical characteristics under a field of 250 V/cm, in which the nonlinear coefficient is 26.4, the threshold voltage and the leakage current is 466 V/mm and 12.32 μA, respectively.     

The orientation mechanism of (Ca,Sr)Bi4Ti4O15 ceramics prepared by slip casting in high magnetic field and subsequent sintering

A high magnetic field of 10 T was introduced into a processing of slip casting for fabricating (Ca,Sr)Bi₄Ti₄O₁₅ (abbreviated as CSBT) ceramics. Feeble magnetic CSBT particles in green compacts were partially aligned through rotating a gypsum mold containing the CSBT slurry in the magnetic field. The green compacts were sintered at 1200 °C for different time without magnetic field. With increasing of the sintering time, the preferable orientation degree of CSBT ceramics rapidly went up at the initial stage, and then slowly increased at the medium and final stages. The mechanism of the orientation degree increasing during the sintering can be attributed to a processing in which large oriented particles coarsen small randomly oriented particle.