Utilizing the Cold Sintering Process for Flexible–Printable Electroceramic Device Fabrication

Conventional thermal sintering of ceramics is generally accomplished at high temperatures in kilns or furnaces. We have recently developed a procedure where the sintering of a ceramic can take place at temperatures below 200°C, using aqueous solutions as transient solvents to control dissolution and precipitation and enable densification (i.e., sintering). We have named this approach as the “Cold Sintering Process” because of the drastic reduction in sintering temperature and time relative to the conventional thermal process. In this study, we fabricate basic monolithic capacitor array structures using a ceramic paste that is printed on nickel foils and polymer sheets, with silver electrodes. The sintered capacitors, using a dielectric Lithium Molybdenum Oxide ceramic, were then cold sintered and tested for capacitance, loss, and microstructural development. Simple structures demonstrate that this approach could provide a cost‐effective strategy to print and densify different materials such as ceramics, polymers, and metals on the same substrate to obtain functional circuitry.     

Influence of platinum foil impurities on sintering of functional ceramics

Impurities in ceramic powders or in the sintering atmosphere can have a strong influence on the densification and microstructure development during sintering. Our sintering studies have shown that the presence of bulk platinum and adsorbed minor amounts of adsorbed impurities during the sintering process can affect the microstructural and property development of materials via the gas phase. Four different oxide powders were shaped into green bodies and sintered in presence and absence of bulk platinum. Analyses of the materials after sintering show clear differences in the microstructure and the electronic properties between samples sintered in a furnace environment and those sintered in contact or in the close vicinity of platinum foil. When Pt foil had been present in the sintering set-up, trace chemical analysis detected accumulations of the platinum metal and other trace impurities at crucible surfaces which had not been in direct physical contact with Pt foil.     

放电等离子烧结技术在陶瓷材料制备中的应用

放电等离子烧结(SPS)作为一种先进的材料制备技术,与传统烧结方法相比,在材料制备效率及所制备的材料的性能方面都有明显的提高,因而引起了世界范围的关注与深入研究。基于相关文献报道,本文针对SPS技术在陶瓷材料中应用中的若干基础问题进行了评述,分别就陶瓷SPS过程中的等离子放电现象、温度与温度场分布、电流电场的促进作用、升温制度选择与设置、压力的作用与使用、模具的设计与开发等控制参数对材料烧结行为、微观结构与宏观性能的影响进行了讨论,并简要介绍了纳米陶瓷SPS过程中微观结构自发均匀化现象。     

Thick-film NTC thermistors and LTCC materials: The dependence of the electrical and microstructural characteristics on the firing temperature

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film NTC thermistors (4993, EMCA Remex) fired either on LTCC (low-temperature cofired ceramic) substrates or buried within LTCC structures were evaluated. The thermistors were fired at different temperatures to study the influence of firing temperature on the electrical characteristics. The results were compared with the characteristics obtained on alumina substrates. The sheet resistivities were higher than the resistivities of thick-film thermistors on alumina substrates. The increase of the sheet resistivities was attributed to the diffusion of the glass phase from the rather glassy LTCC substrates into the NTC thermistors. This was confirmed by EDS analyses. However, the increase in the resistivity was linked to an increase of the beta factors. Therefore, the results show that the evaluated NTC thermistors on LTCC substrates can be used for temperature sensors in MCM-Cs as well as in MEMS LTCC structures. When the thermistors were buried in the LTCC substrates, the LTCC structures delaminated during firing, leading to high sheet resistivities and high noise indices. This delamination is attributed to the different sintering rates of the NTC and LTCC materials.     

Pre-eutectic densification in MgF2---CaF2

Increased densification rates were found as much as 200°C below the eutectic temperature (980°C) for MgF₂ containing small amounts of CaF₂. Constant heating rate and constant temperature sintering data, as well as microstructural developments indicated that solid state grain boundary transport rates had been enhanced by the eutectic forming additive. The effect saturated at about 1 wt% CaF₂. The results suggest that densification of ceramic powders could be favorably affected without a substantial increase in the grain growth rate, by the addition of small amounts of eutectic forming additives, and sintering below the eutectic temperature.     

One-dimensional electrospun ceramic nanomaterials and their sensing applications

One-dimensional sensing materials that are prepared via electrospinning and controlled annealing exhibit intrinsic properties, such as electron transmissivity, magnetic susceptibility, specific heat capacity, as well as optical and mechanical characteristics. Particularly, the electronic transmission characteristics of the ceramic fiber materials, such as the electrical conductivity, photocurrent, magnetoresistance, nanocontact resistance, and dielectric properties, exhibited great potential for applications in the next generation of electronic sensing devices. First, electrospun ceramic materials with different structural and functional characteristics were reviewed here, after which the strategies for improving their properties, as well as the method for assembling the flexible devices, are summarized. Moreover, the electrospun ceramic nanofibers were detailedly discussed regarding applications in device construction and wearable electronics, such as photosensors, gas sensors, mechanical sensors, and other energy storage devices. Finally, the future development direction of the electrospinning technology for multifunctional and wearable electronics skin was proposed.     

A review of two-step sintering for ceramics

The development of high-density ceramic materials with fine-grained microstructures has been studied to considerably improve their properties for high-performance applications. Many alternatives have been searched to refine their microstructure by changing their composition and/or processing. Among such alternatives, the densification of ceramic materials by sintering curve control is an effective, simple and economical microstructure refinement method. Thus, different thermal treatment techniques such as spark plasma sintering and microstructural forms of control such as the control of sintering conditions have been used to obtain nanostructured materials. One of the techniques widely used in recent years is two-step sintering. Two-step sintering (TSS) is a promising method used to obtain high-density bodies and smaller grain sizes. Two TSS methodologies are known: sintering with thermal pretreatment at a low sintering temperature, followed by a second stage at elevated temperature, and the more recent approach presented by Chen and Wang, which has been the most widely used. In addition to the sintering conditions (temperature, heating rate and sintering holding times) that must be suitable for each composition type, the starting materials, particle size and processing method may influence the obtained microstructure, especially the reduced grain size and increased densification. The current review of two-step sintering presents the effect of this technique on the grain density and sizes of different ceramic materials. The influence of the addition of doping agents and its effect on the mechanical properties in different systems is also presented in the current study.     

Effects of Current Confinement on the Spark Plasma Sintering of Silicon Carbide Ceramics

Despite numerous works have recently reported the densification of silicon carbide (SiC) ceramics using the Spark Plasma Sintering (SPS) technique, the effect of the localized electric current flow near the specimen over the liquid phase sintering process of SiC ceramics and on their microstructural features has not been completely addressed. In the present work, two different SPS setups affecting current flow are selected, one based on the ordinary die/punch setup configuration, and the other employing a BN electrically insulating coating on the inner wall of the die to force the electric current to locally flow through the inner graphite foil in contact with the ceramic compact. The effective electrical resistance and the energy consumed during the SPS runs for both setups, as well as the sintering behavior, microstructure, and mechanical properties of the SPSed materials are analyzed. The BN die coating considerably increases the effective resistance of the system, decreases the power consumption, and accelerates the SiC densification. Besides, ceramic specimens experience significantly higher real temperatures than the set values and, accordingly, coarser microstructures and tougher materials than those for the ordinary setting are produced. The thermoelectrical properties of SiC materials are proposed as fundamental in their SPS process, especially when electrical current is forced through the inner part of the SPS setting around the specimen.     

Anatase-to-Rutile Transformation in Titania Powders

Titania (TiO₂) is an important electronic ceramic material for use in diverse applications such as gas sensors, catalysts, dielectrics, and ceramic membranes. TiO₂ exists as several polymorphic phases, most commonly as rutile or anatase. This paper investigates the microstructural evolution of anatase-based commercial TiO₂ powders, with an average size of 100 nm, at high temperatures. These powders transform to the rutile structure at 1000°C. The characteristics of the anatase-to-rutile transformation have been studied using transmission electron microscopy analysis, and new information regarding the nature and mechanisms of this polymorphic reaction has been revealed.     

Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite

The present study investigates the effect of Ti addition on the microstructure development and phase evolution during spark plasma sintering of ZrB₂–SiC ceramic composite. A ZrB₂–20?vol% SiC sample with 15?wt% Ti was prepared by high-energy milling and spark plasma sintering at 2000?°C for 7?min under 50?MPa. The X-ray diffraction test, microstructural studies and thermodynamic assessments indicated the in-situ formation of several compounds due to the chemical reactions of Ti with ZrB₂ and SiC. The Ti additive was completely consumed during the sintering process and converted to the ceramic compounds of TiC, TiB and TiSi₂. In addition, another refractory phase of ZrC was also formed as a result of sidelong reaction of ZrB₂ and SiC with the Ti additive.     

Modelling of electrical properties of Ni-YSZ composites

Ni-YSZ cermets with tailored microstructural characteristics are of interest in many solid-state electrochemical applications such as fuel cell electrode processes or oxygen sensors. In order to elucidate the role of microstructural parameters on overall Ni-YSZ electrical characteristics, different cermets were synthesized by the citrate–nitrate combustion route and their electrical characteristics were tested. The combustion derived samples exhibited fairly high electronic conductivity, even at a relatively low metal volume fraction of 24 vol.% of Ni. To determine the electrical conductivity behaviour of the composites in a broad range of metal content and its relation to material porosity, the general effective media approach and the sine-wave approximation were used, respectively.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

3D analysis of ceramic powder sintering by synchrotron X-ray nano-tomography

In-depth investigation of the sintering phenomena in ceramic powders remains challenging, with the typical size of the individual particles being around 1 µm or less, i.e., at the resolution limit of X-ray micro-tomography (μCT). This has been dealt with, thanks to the state-of-the-art hard X-ray nano-analysis beamline at the upgraded European Synchrotron Radiation Facility (ESRF). Complete 3D images were obtained for representative ceramic powder systems with a voxel size as low as 25 nm, so as to depict particles and pores with adequate details and follow the entire sintering process. Subsequent quantitative image analyses were used to explore microstructural changes, including the evolution of relevant sintering parameters with respect to the grains and the pores. Notably, a study adopted in this research on the advancement of pore curvatures can be linked to tracking the stages of sintering.     

A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics

Nowadays, ceramic exhaust gas sensors are installed in quantities of millions in automotive exhaust gas systems. Almost any automobile being powered by a gasoline combustion engine is equipped with at least one zirconia exhaust gas oxygen sensor (λ probe) for detection of the air-to-fuel ratio λ. The first part of this short overview focuses on potentiometric as well as on amperometric zirconia exhaust gas oxygen sensors. It is remarkable that in the past years a leap in manufacturing technology has occurred from classical ceramic technology to tape and thick-film technology. The advent of novel combustion concepts like lean-burn operating gasoline direct injection required novel exhaust gas aftertreatment concepts. It pushed the development of the NO _x sensor, which is manufactured in the same technology. It is also shown how development of exhaust gas sensors has always to be considered in interaction with exhaust gas aftertreatment systems. This elucidates why novel sensors have gained in importance just recently when stricter emission regulations were announced, meaning that the time is ripe for novel exhaust gas aftertreatment concepts. Appropriate sensors—ammonia sensors, hydrocarbon sensors, and particulate matter sensors—are still in the R&D stage. Several possible sensor principles are discussed. The materials that are used for sensors in the automotive exhaust are electroceramics. Besides ion-conducting zirconia and zirconia cermets, electrically insulating alumina is used for substrate purposes. Novel functional materials in the R&D state are strontium–iron titanate for temperature-independent resistive oxygen sensing and zeolites for selective detection of specific gases like hydrocarbons or ammonia.     

Effect of Microstructure on the PTCR Effect in Semiconducting Barium Titanate Ceramics

The microstructural influence on the PTCR effect in semiconducting barium titanate ceramics was studied and a method for preparing the ceramic bodies exhibiting a PTCR effect of more than seven orders of magnitude was established. Commercial barium titanyl oxalate was used as a starting material and Sb₂O₃ was added as a doping substance. The average grain sizes of the ceramic bodies prepared were 2 to 5 μm over a sintering range of 60 to 92%, to examine in detail the microstructural influence on the PTCR effect. No extra element, such as Mn or Cr, was added to develop the PTCR effect in the present PTCR materials.     

Ceramic paste for space stereolithography 3D printing technology in microgravity environment

Additive manufacturing techniques have demonstrated great potential in space manufacturing for long-duration human spaceflight and colonization on alien planets. Herein, we present the design and manufacturing of previously inaccessible complex ceramic components with high precision in a microgravity environment. The proposed approach is based on controlling the rheological properties of ceramic slurry in reduced-gravity. Briefly, HE-cellulose and carbomer 940 are added as thickening agents to transform the ceramic slurry into a ceramic paste(soft matter), which exhibits Bingham pseudoplastic behavior and is not sensitive to gravity variations during parabolic flight. Additionally, X-ray computed tomography (XCT) results reveal that the as-prepared alumina samples, 3D printed in a microgravity environment, render a higher density of 99.3% after sintering. The proposed route can be applied to fabricate complex-shaped ceramic components in space, enabling various possibilities of manufacturing structural and functional materials, including gradient materials, solid fuel cells, active insulation ceramic structures and piezoelectric sensors.     

Spark Plasma Sintering and Densification Mechanisms of Conductive Ceramics under Coupled Thermal/Electric Fields

In spark plasma sintering (SPS), thermal and electric fields are applied simultaneously as a material is densified under pressure. The interactions between these two types of physical fields influence the densification behavior during SPS. Moreover, the uniformity and spatial distribution of these fields are also influenced by sample size. In the current investigation, the densification behavior of electrically conductive aluminum‐doped zinc oxide (AZO) ceramics is studied to provide insight into the role played by the thermal and electric fields on densification mechanisms, as a function of sample size. Our results demonstrate that field uniformity and densification behavior depend on sample size, and that ultimately, this behavior can be rationalized in terms of the electrical conductivity characteristics. Our results show that in small samples with a diameter of 20 mm, both thermal and electric fields are spatially uniform, which result in homogeneous microstructure. In large samples with a diameter of 80 mm, however, spatial variations in both thermal and electric fields lead to microstructural inhomogeneities, such as incomplete particle–particle bonding. Furthermore, as the density of the AZO sample increases, the effective electrical conductivity increases due to a decrease in void/pore volume, which changes the densification mechanisms, especially for the larger sample. Thus, for effective sintering of larger samples, a two‐stage sintering sequence is proposed, which relies on the thermal field that evolves once the effective electrical conductivity increases in the sample. We provide experimental confirmation to this suggestion on the basis of results which demonstrate that by extending the hold time from 3 to 30 min, high‐density (99.4%), homogeneous AZO ceramic samples with a diameter of 80 mm can be achieved after sintering at 1200°C.     

钛酸钡基电子陶瓷材料的改性进展

钛酸钡基电子陶瓷材料广泛应用于电容器、集成电路、传感器及热敏电阻等领域。高容量、小型化、抗击穿及低损耗等工业需求对钛酸钡基电子陶瓷材料的性能提出更高的要求,改性则是提高陶瓷材料性能的主要手段。综述了近年来钛酸钡基电子陶瓷材料在掺杂改性、复合改性及物理改性方面的研究进展。分析了钛酸钡基电子陶瓷材料在改性中存在的问题,比如：常规元素掺杂制备参数优化不足、稀土元素掺杂种类偏少、包覆效果待提升、聚合物陶瓷复合体综合性能欠佳、烧结工艺尚待优化等。提出了解决方法,比如：探索多种元素掺杂、优化工艺参数、改进包覆与聚合方式等。指出了钛酸钡基电子陶瓷材料的未来发展方向,即：强化烧结过程中晶粒尺寸、晶体形状、组分调控的机理研究,选取更多稀土元素进行改性,探索包覆掺杂改性、聚合物复合改性等新工艺。     

Electrically conductive ceramic composites prepared with printer toner as the conductive phase

Conductive ceramic composite was prepared by sintering the mixture of clay and printer toner at 1050°C and in the N₂ atmosphere. The microstructure and mineral phases of the ceramic composite were characterised by SEM, EDX, TG and XRD, and its electrical conductivity and mechanical properties were also investigated. The results show that, in the sintering process, a series of physical and chemical reactions take place, and mineral phases with excellent electrical conductivity, such as metal iron, carbon and Fe–Al solid solution material, are formed. The electrical conductivity mechanism can be explained by the percolation theory. The threshold value for electrical conductive percolation is between 3.5 and 7.0?wt-%. At the content of printer toner 10?wt-%, the volume electrical resistance of the ceramic composite is as low as 8.5?Ω?cm, and the composite exhibits excellent flexural strength higher than 14?MPa.     

SiC-based ceramics with remarkable electrical conductivity prepared by ultrafast high-temperature sintering

SiC-based ceramics with high electrical conductivity are applied widely as electrode materials and semiconductor materials. In this study, a SiC-based ceramic with relative density of 96% was prepared by ultrafast high-temperature sintering (UHS) at 2000 ℃ (with a heating rate of 1000 ℃/min) for 40 s. The resistivity of as UHS-ed SiC-based ceramic was 1/15 of that prepared by the pressureless sintering. We found that the components of as-sintered body (SiC, Si and Y₃Si₅) by UHS were different from those (SiC and YAG) prepared by the pressureless sintering. The reason for the remarkable increase of the electrical conductivity of UHS-ed body was that the Si with higher electrical conductivity than SiC had emerged. Besides, the reaction mechanism was proposed and the unusual composition of the SiC-based ceramic sintered by UHS may also provide new reference for the application of SiC in specific fields.