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.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Atmosphere effects on micron-sized ZnO ridges during sintering

During the fabrication of micron-sized ceramic features, sintering plays a critical role in shape retention and microstructure evolution. Because of the evaporation tendency of zinc oxide (ZnO) during sintering, atmosphere has a significant influence on the sintering behavior and feature fidelity. In this study, micron-sized ZnO ridge features were sintered under air and argon atmospheres. Ridge size, line edge roughness, and shrinkage were characterized. Sintering mechanisms and evaporation mass loss were analyzed in order to obtain fundamental understanding of the micron-sized ZnO feature sintering. Oxygen partial pressure is the deciding factor in the ridge feature evolution. The argon atmosphere leads to faster grain growth and densification but compromises the ridge size and ridge grain size due to increased mass loss.     

Particle transport mode during flash sintering of sodium bismuth titanate ceramic

In this work, the NBT ceramics are successfully flash sintered at a direct current 30 mA/mm² for 30 s under different initial electric field. Subsequently, the actual temperature of samples is estimated by blackbody radiation theory under different conditions. The calculation results show that the sample temperature is close to the densification temperature of NBT ceramics used in conventional method. The rapid densification mechanism is discussed by wetting of local contact particles due to the asymmetrical Joule heating. In particular, we analyzed the particle transport mode during flash sintering in terms of particle diffusion. It is believed that the transform in the particles transport mode from solid diffusion to flow mass transfer accelerated the particles diffusion rate, which ensure the particle rearrangement and achieve the local shrinkage of particles in a short period of time.     

The cold sintering process: A review on processing features,densification mechanisms and perspectives

Since its first introduction in 2016, cold sintering process (CSP) has gained worldwide interest from the scientific community as green and innovative fabrication route due to the dramatic reduction of processing time, energy, and costs. Cold sintering resembles the geological formation of rocks where a ceramic powder is densified with the aid of a liquid phase under an intense external pressure and limited heating conditions (below 350 °C). Up to date, tens of different materials, including composites, have been successfully processed through CSP and extraordinary results in terms of densification, microstructure and final properties have been achieved. In the present review, processing features and variables, possible densification mechanisms and issues also for the realization of ceramic-based composites are explored. Advantages with respect to existing techniques are analysed and current challenges are described to lay the ground for new processing opportunities to be faced in the near future.     

Microstructure evolution during spark plasma sintering of FJS-1 lunar soil simulant

A spark plasma sintering (SPS) process has been explored to densify FJS-lunar soil simulants for structural applications in space explorations. The effect of SPS conditions, such as temperature and pressure, on the densification behavior, phase transformation, microstructural evolution, and mechanical properties of FJS-1 have been examined by conducting the X-ray diffraction analysis, electron microscopy imaging, and nano/micro indentation testing. Test analysis results were also compared to results from the FJS-1 powder and sintered samples without pressure. The FJS-1 powder was composed of sodian anorthite, augite, pigeonite, and iron titanium oxide. When FJS-lunar soil simulants were sintered without pressure, the main phase evolved from sodian anorthite to the intermediate sodian anorthite, jadeite and glass, and iron titanium oxide at 1000°C, which were further transformed into filiform and feather-shaped augite and schorlomite at 1100°C. Most densification processes in pressureless sintering occurred at 1050°C-1100°C. During the SPS process, the main phases were sodian anorthite, pigeonite, and iron titanium oxide at 900°C. These phases were transformed to sodian anorthite, glass, and feather-shaped augite at 1000°C and 1050°C, with the nucleation of dendritic schorlomite at 1050°C. Significant densification by SPS can be observed as low as 900°C, which indicates that the application of pressure can substantially lower the sintering temperature. The SPSed samples showed higher Vickers microhardness than the pressureless sintered samples. The mechanical properties of the local phases were represented by the contour maps of elastic modulus and nanohardness. Multiscale mechanical test results along with the microstructural characteristics further imply that the SPS can be considered a promising in-situ resource utilization (ISRU) method to densify lunar soils.     

Modeling and in-situ evaluation of thermal gradients during sintering of large ceramic balls

During the sintering process of ceramic pieces, temperature gradients may arise inside the piece if the thermal cycle is too fast with regards to the size of the ceramic body. Too large temperature gradients may lead to density gradients, thus to strain gradients which may result in defects or in the fracture of the pieces. This article presents both an experimental setup and the first steps towards a numerical method to evaluate these thermal gradients and their evolutions during sintering, in the case of large zirconia, alumina and zirconia-toughened alumina balls. Finally it discusses the advantages of Rate Controlled Sintering thermal cycles in terms of diminishing the temperature gradients during densification.     

不同烧结法对3Y-TZP陶瓷力学性能的影响

本文研究了低温烧结含3mol%氧化钇的四方多晶氧化锆(3Y-TZP)的烧结性能和力学性能,以及进行热等静压(HIP)后其力学性能的变化。成形后的3Y-TZP在常压、1300～1450℃温度下进行烧结。由于该粉料有很高的烧结活性,在1300℃低温烧成下就可获得相对密度大于94%的烧结体;在1350℃烧成温度下3Y-TZP获得了最佳的力学性能。其断裂韧性(KIC)和维氏硬度(HV)分别达到18.7MPa.m1/2和13.7GPa,其中应力诱导相变是其主要的增韧机理。对低温烧成的3Y-TZP陶瓷进行热等静压烧结后发现,HIP增大3Y-TZP陶瓷HV的作用显著,可使其增至14.3GPa。     

Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic

The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 °C and 1350 °C for 15 min on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13–14.6 GPa) and fracture toughness (~3 MPam^1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 °C and reduced significantly at 800 °C thus resulting in the enhancement of electrical conduction.     

Effects of sintering temperature on microstructure,properties, and crushing behavior of ceramic proppants

Ceramic proppants with excellent performance were successfully prepared by second grade bauxite. The phase structure and the microstructure were investigated via X-Ray diffraction and scanning electron microscopy. The results suggested that bulk density and apparent density constantly increased, while the breakage ratio gradually decreased with the increasing in sintering temperature. When the sintering temperature reached to 1500°C, the breakage ratio obtained lowest value of 3.6% under 52 MPa closed pressure, which satisfied the requirement of Chinese Petroleum and Gas Industry standard. Furthermore, the mode of fracture for ceramic proppants was transformed from intergranular fracture into transgranular fracture and open pores had larger influence than closed pores for breakage ratio of ceramic proppants.     

Microstructural evolution of ZnO via hybrid cold sintering/spark plasma sintering

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions.     

Graphene nanosheets toughened TiB2-based ceramic tool material by spark plasma sintering

One kind of TiB₂/TiC composite ceramic tool material toughened by graphene nanosheets was fabricated by spark plasma sintering. Effects of graphene nanosheets on microstructure, mechanical properties and toughening mechanisms were investigated. The results indicated that TiB₂/TiC with 0.1?wt% graphene nanosheets sintered at 1800?°C with the holding time of 5?min obtained full densification and optimal mechanical properties. Its fracture toughness and Vickers hardness were 7.9?±?1.2?MPa?m^1/2 and 20.0?±?0.7?GPa, respectively. Excess graphene nanosheets had no effects to toughness improvement. Fracture toughness was increased by 31.7% in comparison with the TiB₂/TiC without graphene nanosheets. Toughness enhancement mainly benefited from crack bridging, also slip-stick effect of graphene made it hard to detach and effectively restrained crack extension.     

Model for the cold sintering of lead zirconate titanate ceramic composites

A model was developed to describe the cold sintering process (CSP) of lead zirconate titanate (PZT) using moistened lead nitrate as a sintering aid. The densities of PZT powder with different volume fractions of lead nitrate were evaluated after cold sintering at 300°C and 500 MPa for 3 hours. The densities were categorized into three zones. In zone I, the relative density following cold sintering increases from 66% to 80%, as the lead nitrate contents rise from 0 to 14 vol%. In this case, the lead nitrate acts to fill some of the pore volume between PZT grains. Zone II serves as a transition region, where there is both pore filling and dilution of the PZT grains associated with lead nitrate contents from 14 to 34 vol%. In zone III, the relative density drops due to dilution at lead nitrate contents exceeding 34 vol%. To slow the process down so that the kinetics could be studied more readily, samples were cold sintered at room-temperature and 500 MPa. It was found that during the first few seconds of compaction, 85PZT/15Pb(NO₃)₂ rapidly densified from 51% to 61% relative density due to particle re-arrangement. For longer times at pressure, the CSP improved the packing relative to PZT compacted without the lead nitrate, yielding a higher relative density. The late stages of the PZT/Pb(NO₃)₂ CSP could be well described using a viscous sintering model for pressures from 50 MPa to 1000 MPa and temperatures from 25°C to 300°C.     

Si-rich pressureless sintering of the gelcasted Ti3SiC2 bulk ceramic

The Si-rich pressureless sintering was used to fabricate the Ti₃SiC₂ bulk ceramic. The results show that the optimized Ti₃SiC₂ suspension could be prepared at the absolute value of zeta potential, pH level, PAA-NH₄ dosage, and solid loading of 62.1 mV, 11, 2.0 wt%, and 50 vol%, respectively. The channels existing in the Si-free sintered body facilitated the reactants and products to diffuse to the interior and out of the Ti₃SiC₂ matrix, thereby forming the porous reaction layer of TiC-Ti₃SiC₂. The co-effects of the channels and the reaction layer of TiC-Ti₃SiC₂ severely lowered mechanical properties of the Si-free sintered Ti₃SiC₂ ceramic. On the contrary, the Si-rich sintering method isolated the volatile carbon and established a closed Si-rich atmosphere to sinter the green Ti₃SiC₂ cylinder. The porosity, density, fracture toughness, hardness, and flexural strength of the Si-rich sintered Ti₃SiC₂ ceramic reached 0.74 vol%, 4.36 g/cm³, 5.49 MPa·m^1/2, 4.03 GPa, and 383 MPa, respectively.     

担载膜受限烧结孔结构模型的改进及验证

针对担载膜受限条件下,复杂的堆积方式及垂直膜面方向出现的加速收缩将影响孔径的问题,对前期建立的担载膜孔结构预测模型进行了改进,从两个方面对模型的关键参数进行定量修正。考虑制备过程中颗粒可能形成的软团聚体对初始堆积方式的影响,对模型重要参数——初始孔隙率进行调整;并结合受限烧结应力模型计算担载膜在垂直膜面方向上的收缩速率和收缩率,研究该方向加速收缩对孔径的影响。在此基础上,采用改进的模型对ZrO2担载膜在800～1200℃烧结后,孔径、孔隙率、膜厚等微结构进行了预测。结果表明:与原模型相比,改进模型具有更好的适应性以及更高的计算精度,能更准确地预测不同烧结温度制备的担载膜的孔径和膜厚,为陶瓷膜孔径的预测与定量控制提供了有效的工具。进一步将模型计算值代入Hagen-Poiseuille方程,可以预测膜的纯水渗透通量。     

Low-temperature sintering of porous silicon carbide ceramic support with SDBS as sintering aid

The sintering temperature of porous silicon carbide ceramic support (PSCS) is typically higher than 1500 °C. In this paper, sodium dodecyl benzene sulfonate (SDBS) was used as a sintering additive to fabricate PSCS with high gas permeance and high bending strength at a sintering temperature less than 1200 °C. The PSCS was prepared by the dry pressing method followed by in-situ reaction. The effects of SDBS loading on the porosity, bending strength, gas permeation performance, and microstructure of the PSCS were investigated. The results showed that without SDBS, the required sintering temperature was as high as 1550 °C and resulted in a bending strength of 6.5 MPa but the sintering temperature decreased to 1150 °C with 8% SDBS and the bending strength increased to 16 MPa. The main reason was that SDBS decomposed into Na₂O which reacted with SiO₂ and ZrO₂ to form strong bonding connections. The prepared PSCS with SDBS also showed good gas permeance of 900 m³/(m² h kPa), higher than the 750 m³/(m²·h·kPa) without SDBS. This work describes the effective use of SDBS as a ceramic additive to reduce sintering temperature, while achieving high gas permeation and bending strength. The use of the low cost and commercially available SDBS produces an excellent ceramic filter with much lower energy consumption, and could also be implemented in other ceramic systems.     

Numerical simulation and experimental verification of dry pressed MgTiO3 ceramic body during pressureless sintering

To clarify the densification law of dry pressed MgTiO₃ ceramic body during pressureless sintering, SOVS model (Skorohod-Olevsky Viscous Sintering model) modified with creep characteristics was embedded into finite element software Abaqus. The selected model can effectively express the grain boundary characteristics and densification mechanism. The change law of relative density, shrinkage rate, sintering stress, and grain size of MgTiO₃ cylindrical specimens was investigated by the above numerical simulation method. It showed that the average relative density of ceramic body rose from 60% to 97%, and the shrinkage rate respectively reached 17.28% and 11.99% in axial and radial direction. The average grain size increased from 1 to 6 μm. In order to verify the accuracy of the simulation results, corresponding sintering experiments on cylindrical specimens were carried out to obtain actual sintering densities and shrinkage rates. It showed that the errors of relative density and shrinkage were below 5% and 2%. Grain growth trend was also basically consistent with the simulation results. After that, the above numerical simulation method was applied into the prediction of fabricating MgTiO₃ filter with complex structure. Therefore, the present work provided a reliable numerical simulation method to predict the densification behavior of MgTiO₃ ceramics during the pressureless sintering process, which was helpful to design and fabricate microwave dielectric products.     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Effect of oscillatory pressure on the sintering behavior of ZrO2 ceramic

Zirconia ceramics were prepared by oscillatory pressure sintering (OPS) and hot pressing (HP). The result revealed that OPS could enhance densification compared to HP when sintering temperature was higher than a critical value. The onset temperature for rapid grain growth was found to be same for both techniques. However, rate of grain growth in OPS was lower than that in HP. Furthermore, the result also showed that samples prepared by OPS exhibited higher hardness than those prepared by HP when sintering temperature was higher than the critical value. The improved hardness was solely due to the higher density of the samples prepared by OPS.     

Influence of yttrium oxide addition and sintering temperature on properties of alumina-based ceramic cores

Al₂O₃-based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al₂O₃ powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al₂O₃ and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al₂O₃ particles, exerting a good role in connecting Al₂O₃ particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al₂O₃-based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C.