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Enhancing the densification of ceria ceramic at low temperature via the cold sintering assisted two-step sintering process
Abstract
The cold sintering process (CSP) in the transient aqueous environment of malonic acid is applied in this study to enhance the densification behavior of ceria ceramics at a significantly reduced sintering temperature. Dense ceria ceramics (> 98% relative density) could be achieved after CSP at 180 °C, followed by the sintering at 1100 °C/4 h. As expected, the CSP can improve the compaction of the particles before sintering, resulting in ceria ceramics with a fine microstructure of submicrometer scale of grain sizes. All samples show the single phase of cubic fluorite structure with no significant change of lattice parameter. Finally, electrical conductivity of CSP and conventionally sintered ceramics is investigated. This communication emphasizes the advantage of the CSP to reduce the sintering temperature without deterioration of the structure and properties of the final ceramics.
... In the case of yttria-stabilized zirconia (YSZ) and Gd-doped CeO 2 , annealing was fundamental not only for improving densification but also to stabilize the microstructure, thus avoiding undesired phase transformation [52,46]. CSP as preliminary densification step was reported to be beneficial both in reducing the time and the temperature of the overall process [53,54]. As an example, Charoonsuk et al. [53] achieved the final stage of sintering of ceria (CeO 2 ) already at 1100 • C, when by conventional sintering a temperature of at least 1500 • C is necessary. ...
... CSP as preliminary densification step was reported to be beneficial both in reducing the time and the temperature of the overall process [53,54]. As an example, Charoonsuk et al. [53] achieved the final stage of sintering of ceria (CeO 2 ) already at 1100 • C, when by conventional sintering a temperature of at least 1500 • C is necessary. Despite annealing is typically performed above 600 • C, in the majority of literature works a fine microstructure was still preserved after the thermal treatment [45,46,49,51,52,12,50,40,44]. ...
... A similar outcome was reported by Charoonsuk et al. [53] for cerium oxide processed at 120 • C, 150 • C and 180 • C. Furthermore, Bang et al. [56] found that a slower heating rate was beneficial in improving densification. Slower heating rate means longer time to reach the boiling point of the solvent, thus dissolution is enhanced because the solvent is in intimate contact with the powder for longer time. ...
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.
... Due to their importance in the construction of solid oxide fuel cells, attempts have been made to densify ceria-based electrolyte ceramics via the cold sintering process. Chanroonsuk et al. [32] studied the cold sintering of CeO 2 at 180 C for 60 h in malonic acid solution media. Zaengle et al. [31] also investigated cold sintering of CeO 2 by utilizing NaOH and KOH transient solvents from 250 to 400 C and reached 91% of its theoretical density. ...
... However, CeO 2 ceramics are known to be practically insoluble in water at room temperature. Even the solubility of ceria in the presence of malonic acid was reported to be only 243.1 ppb [32]. In the present work, CSP was conducted at room temperature and water was used as a transient liquid without any chemical additives. ...
Cold sintering is a novel technique that promotes the densification of ceramics at a much reduced temperature as compared to conventionally sintered ones. With the aid of this technique, gas-tight GDC electrolytes were sintered up to 96% of their theoretical densities at 1100 °C, a remarkable 300 °C below the conventional sintering temperature. It is one of the rare cases in which the cold sintering process was carried out at room temperature. The activation energy for electrical conduction and OCV values at 600 and 800 °C were found to be 0.69 eV and 0.97 and 0.88 V, respectively. The achieved OCV values were slightly higher than the ones produced by the conventional sintering densification process.
... Several strategies can be adopted to apply CSP to such materials. For instance, fluorite-structured ceramics (like stabilized zirconia, gadolinia-doped ceria etc.) cannot be completely densified by CSP, this being substantially used as a shaping technology to develop high-quality green bodies [148][149][150][151]. The improved green density has indeed beneficial effects on the densification process since (i) a smaller deformation is required to achieve full density and (ii) the number of the inter-particles contact points increases, thus reducing the diffusion distance needed for sintering. ...
... This is a fingerprint of the initial stage of LPS [176]. Although it does not lead to real necking, it allows to improve the initial density of the green and this is widely used when CSP is followed by high temperature treatment [148,149,152]. Water reduces the viscosity of silicate glasses [177] (it acts as a network modifier) and can form hydrated phases with lower yield stress on the particles surface [178]. ...
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Despite sintering has a history even longer than human civilization (its discovery dates back at least to 25,000 years ago), in the past decade, new exciting challenges have emerged in the field: reduction of environmental impact, densification of metastable phases, complete consolidation of ultra-refractory compounds, precise microstructural design to control properties of functional ceramics and integration between inorganic-organic compounds. In order to meet such challenges, new sintering routes employing electric fields/currents, water/solvents and external loads have been developed. The research also opened new questions about unexpected (and still not completely understood) interactions between electricity, presence of water/liquid, heating and diffusion processes.
In this manuscript, we have rationalized the last-ten-years research in the field of sintering for the consolidation of ceramics. The processes are collected into three main groups: flash-like (sintering under relatively large electric fields and the material is internally heated by the Joule effect), spark plasma sintering-like (combination of pressure and limited electric field) and hydro-consolidation (sintering at temperature below ≈ 350 °C in the presence of a liquid under an applied pressure). This paper aims to point out common features and differences among different techniques. Finally, future research trends and new paradigm in material processing are anticipated.
... Several strategies can be adopted to apply CSP to such materials. For instance, fluorite-structured ceramics (like stabilized zirconia, gadolinia-doped ceria etc.) cannot be completely densified by CSP, this being substantially used as a shaping technology to develop high-quality green bodies [147][148][149][150]. The improved green density has indeed beneficial effects on the densification process since (i) a smaller deformation is required to achieve full density and (ii) the number of the inter-particles contact points increases, thus reducing the diffusion distance needed for sintering. ...
... This is a fingerprint of the initial stage of LPS [175]. Although it does not lead to real necking, it allows to improve the initial density of the green and this is widely used when CSP is followed by high temperature treatment [147,148,151]. Water reduces the viscosity of silicate glasses [176] (it acts as a network modifier) and can form hydrated phases with lower yield stress on the particles surface [177]. ...
The broad and far-reaching (beyond sintering) implications of flash processes are critically assessed. Different mass transport-based flash-like technologies are currently emerging: synthesis, pyrolysis, joining, plastic and viscous shaping. These could be used to develop new high entropy materials, to change wetting properties in ceramic-metals joints, to shape ceramics by plastic deformation. Moreover, the flash can change the chemistry of oxides thus introducing far-from-equilibrium crystallographic and/or electronic features. The development of predictive models to determine the field-current-material interactions and the produced defects could represent a new tool to design and engineer ceramics in the next future.
... The CSP method is simple, requiring a steel die, press, and heat controller [28]. Despite the tremendous technological relevance of the CSP, there is not much literature data available for ceria based compound, and only a few reports were published in recent times [29][30][31]. ...
... Typical geometry normalized (a) Nyquist plots (ρ' vs ρ") characterized by frequencies, recorded at 300°C in air for the GDC pellets. (b) Arrhenius plot for the estimation of total electrical conductivities of the GDC samples, compared with the sample fabricated via CSP and two-step post thermal treatment respectively byMaruyama et al. (1300°C-800°C for 5 h) [30] and Charoonsuk et al. (1400°C-1100°C for 4 h)[31]. ...
In this report, the effect of the cold sintering process (CSP) on the electro-chemo-mechanical properties of 10 mol% Gd-doped ceria (GDC) is investigated. High purity nanoscale GDC powder is sintered via a cold sintering process (CSP) in pure water followed by post-annealing at 1000 °C. The resultant ceramics exhibits high relative density (∼90%) with a fine grain size of ∼100 nm. This sample illustrates comparable electrochemical properties at intermediate-high temperatures and electromechanical properties at room temperature to the material prepared via conventional firing, i.e., sintering in the air at 1450 °C. Moreover, a large creep constant, as well as a low elastic modulus and hardness, are also observed in the CSP sample.
... Charoonsuk et al. reported the increased density of cerium oxide due to an increase in temperature up to 180°C. 75 Berbano et al. reported a 60 to 80% increase in the relative density of the Lithium aluminum phosphate sample. The processing temperature was increased from room temperature to 160°C. ...
The functional ABO3 structured materials can be processed by high-temperature sintering methods like conventional solid state, pressure-assisted and liquid phase sintering. It is necessary to adapt energy efficient sintering methods for the improvement of densification and crystalline properties of electroceramics. Among these, a low-temperature pressure-assisted cold sintering method is one of the wise approaches for the fabrication of perovskite-based, ABO3 type oxide materials. This article aims to explore an in-depth analysis of cold sintering approaches among several ABO3 structured materials. The cold sintering techniques of various recently emerging perovskites are summarized. The parameters influencing the cold sintering process, low-temperature densification mechanisms, and crystal structure designs were detailed. A comprehensive discussion on the dielectric consolidation principles of few functional perovskites was performed. The advantages & disadvantages of conventional and cold sintering methods were deliberated. It is speculated that energy efficient cold sintering ABO3 materials will play a vital role in future electronic industries.
... Compared to other sintering techniques, CSP is relatively simple to implement although optimizing the process can be challenging, due to various variables affecting the outcome [20,21]. In particular, the selection of the appropriate LP is crucial, determining whether the partial dissolution of the solid would be congruent or incongruent and different liquid phases, such as deionized water, and citric or malonic acids, have been used in the literature [19,[22][23][24][25][26][27]. Some works have also explored the significance of LP amount (wt%) [28][29][30][31][32], and concentration [18,27], on achieving highly densified ceramics. ...
This study evaluates the impact of incorporating varying contents (10–40 wt%) and molar concentrations (0.001–1 M) of citric acid solutions, as transient liquid phases in the Cold Sintering Assisted Sintering (CSAS) process of dysprosium zirconate (Dy2Zr2O7). CSAS processed samples achieved relative densities up to 98% of the theoretical maximum and significantly increased Vickers microhardness by over 2.5 times, compared to the traditional ‘press and fired’ sintering method. The Dy2Zr2O7 crystal structure remained consistent with the fluorite-type, with no secondary phases detected. Our findings underscore the benefits of using CSAS to enhance the mechanical strength of Dy2Zr2O7, while reducing the lengthy processing times at very high temperatures typically required for sintering refractory materials such as lanthanide zirconates.
... For compounds with low solubility, ongoing research indicates that effective densification can be achieved through subsequent post-sintering, a process known as the cold sintering-assisted two-step sintering route (abbreviated as CSAS). For instance, Charoonsuk et al. demonstrated that dense ceria ceramics (> 98% relative density) could be attained by cold sintering at 180 °C, followed by sintering at 1100 °C/4 h [25]. Guo et al. reported a reduction in the sintering temperature of ZrO 2 ceramics from 1400 to 1200 °C through the CSAS process [26]. ...
The effects of cold sintering assisted two-step sintering on the structural evolution and microwave dielectric properties of Al2O3 ceramics are systematically investigated in this work. XRD analysis reveals that the crystal structure remains stable throughout the cold sintering and following high-temperature sintering processes. Rietveld refinement confirms the trigonal Al2O3 configuration and the absence of impurities. Increasing cold sintering pressure results in improved relative density, reaching over 94% after post-sintering at 1575 °C. Additionally, the sintering temperature of Al2O3 ceramics via cold-sintering assisted two-step sintering route is 100 °C lower than that via direct conventional sintering. The microwave dielectric properties, including εr and Qf, exhibit enhancements with increasing cold sintering pressure and post-sintering temperature. The measured εr closely aligns with the theoretical value, indicating a successful densification. Qf achieves its peak of 100,220 GHz at 1575 °C under 375 MPa, nearly 2 times higher than the value obtained at 125 MPa (53,280 GHz). The optimal microwave dielectric properties (εr = 9.67, Qf = 100,220 GHz, and τf = − 54.5 ppm/°C) are achieved at a cold sintering pressure of 375 MPa and post sintering temperature of 1575 °C.
... Even small variations in the moisture content resulted in a remarkable increase in the theoretical density values. CeO 2 ceramics are known to be nearly insoluble in water, unlike Na 2 CO 3 which has a very high solubility limit (∼34 gr/100 mL at 25 • C). 45,56 Therefore, it was believed that the carbonate phase, which dissolved in water easily in huge quantities and enabled rapid mass transport with the aid of pressure and temperature, was responsible for the increases in density. Due to its high solubility limit in water, some of the dissolved carbonates during processing leaked with the water from the mold, therefore, water contents more than 4 wt.% could not be studied. ...
Sm0.2Ce0.8O1.9‐ 30% Na2CO3 (Sm doped ceria (SDC)‐30N) nano‐composite electrolytes were densified in a single step via cold sintering process (CSP). At 200°C and 450 MPa of uniaxial pressure, samples up to 97% of their theoretical density could be obtained. The effect of processing parameters, such as temperature, uniaxial pressure, processing duration, and moisture content, on the densification of the nano‐composite electrolytes was investigated. The thermal, microstructural, and electrical properties of nano‐composites were investigated by differential scanning calorimetry, X‐ray diffractometer, scanning electron microscope, and EIS analysis. SDC crystallite sizes were found to be around 25 nm, barely coarsened after CSP by which the true nano nature of the nano‐composite could be preserved. Because, by conventional processing high density values could not be attained and high processing temperatures in excess of 600°C had to be used, promoting particle coarsening. The highest total electrical conductivity was found to be 2.2 × 10⁻² S cm⁻¹ at 600°C, with an activation energy of 0.83 eV for SDC‐30N nano‐composites. The present investigation revealed that the implementation of cold sintering technique resulted in significant enhancements in the densification of nano‐composite electrolytes, thereby rendering them suitable for efficient utilization in SOFC applications, as compared to the conventional production methods.
... Ji et al. [22] and Wang et al. [23] have reported two prototype MPAs using cold-sintered CaSnSiO 5 -K 2 MoO 4 and CaTiO 3 -K 2 MoO 4 ceramic composites for application in 5G. Moreover, the cold-sintering process (CSP) has captivated a huge research intrigue in areas mostly confined to ionic conductors, semiconductors, Li-ion batteries, piezoelectric ceramics, and nanocomposites, low-dielectric-loss ceramics and thermo electrics [24][25][26][27][28][29][30]. These works have opened up new avenues for designing cold-sintered smart materials to manufacture affordable, directly unified devices ranging from single dielectric resonator antennas (DRAs) to array antennas. ...
Preparation of (1 – x)NaCa2Mg2V3O12–xLi2MoO4 (NCMVO-LMO; x = 0.4, 0.5, 0.6, 0.7) composites via cold sintering technique under 200 °C using uniaxial pressure of 450 MPa for 50 min is reported in this article. Microstructures of these composites suggest their enhanced densification with increased LMO concentration. Relative permittivity (εr) of (1 – x) NCMVO–xLMO ceramics varies from 7 to 8, and the quality factor (Qu × f) changes from 7000 to 13,500 GHz. A cylindrical dielectric resonator antenna (CDRA) and a microstrip patch antenna (MPA) were outlined, fabricated, and tried using the 0.4NCMVO–0.6LMO composite. CDRA and MPA exhibit excellent performance with centre frequencies of 13.24 and 5.48 GHz, maximum return losses of –36 and –31 dB, and maximum impedance bandwidths of 0.77 and 1.4 GHz, respectively. These values suggest that the fabricated antennas hold the potential to be a key component in future microwave communication systems, especially for 5G and Ku-band applications.
... Wang et al. (2018bWang et al. ( , 2019a reported a novel dielectric GRIN lens based on the cold sintering of NBMO-xLMO and BLVMO-xNMO systems. Moreover CSP has attracted a lot of research interest in a wide range of areas especially in the field of semiconductors, ionic conductors, piezoelectric ceramics, Li ion batteries, nano composites, thermo electrics and low dielectric loss ceramics (Berbano et al., 2017;Funahashi et al., 2017a;Seo et al., 2017;Charoonsuk et al., 2018;Gonzalez-Julian et al., 2018;Induja and Sebastian, 2018;Leng et al., 2018;Ndayishimiye et al., 2018a,b;Wang et al., 2018a;Sibi et al., 2020). Induja and Sebastian (2018) reported that Al 2 SiO 5 ceramics can be easily densified using CSP with the aid of NaCl as water soluble additive. ...
Cold sintering process (CSP) was successfully employed to fabricate (1 − x ) NaCa 2 Mg 2 V 3 O 12 -xNaCl [abbreviated as (1 − x ) NCMVO-xNaCl] microwave dielectric ceramics. (1 − x )NCMVO-xNaCl ceramics prepared at 200°C and at a pressure of 450 MPa had a high relative density of 80–94%. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy showed that both NCMVO and NaCl phases co-exist in all composite ceramics without forming any secondary phase. Further, dependence of microstructure and dielectric properties on cold sintering temperature and duration were investigated in detail and their optimized values to obtain maximum density of ceramic composites were 200°C and 50 min, respectively. (1 − x )NCMVO-xNaCl ( x = 0.4–0.7) composites have relative permittivity (ε r ) in the range of 6.9–7.4, and a reasonably high microwave quality factor (Q × f) of 5,000 to 13,830 GHz.
... [11][12][13][14][15] The first materials produced by CSP were mainly salts and soluble compounds, 3,16 although several activities have been then carried out on ferroelectrics, [16][17][18] piezoelectrics, 19 magnetic ceramics, 20 semiconductors, 21-23 transparent ceramics, 24 bioceramics, 14 and ionic conductors. [25][26][27] More recent works regard water-assisted cold processing of silicate ceramics including cold sintering of quartz, 28 soda-lime silicate glass, 15 and quartz-PTFE composite 29 or cold joining of silica 30 and cold isotactic pressing of glass monoliths. 31 Although CSP is inspired by phenomena taking place in nature (like the formation of sedimentary rocks 32 ), no research activities regarding the application of CSP on nonsynthetic industrial minerals have been reported to the best of the authors' knowledge. ...
Diatomite, a natural silicate‐based sedimentary rock, was densified by cold sintering at room temperature and 150°C under various pressures (100, 200, and 300 MPa) and using different NaOH water solutions (0–3 M). The relative density of cold sintered diatomite can be as high as 90%, a condition that can be achieved by conventional firing only at 1200–1300°C. The cold sintered materials maintain the same mineralogical composition of the starting powder (quartz, glass, and illite) and are constituted by well‐deformed and flattened grains oriented orthogonally to the applied pressure. Conversely, an evident phase evolution takes place upon conventional firing with the formation of cristobalite and mullite. The bending strength of cold sintered artifacts can exceed 40 MPa and increases to ≈80 MPa after post‐annealing at 800°C, such mechanical strength is much larger than that of conventionally pressed samples sintered at 800°C, which is only ≈1 MPa.
... On the other hand, the cold sintering process (CSP) is based on a transient liquid-phase sintering and it was first proposed by Guo et al. [18] where the synergic role of a solvent and pressure (≈300-500 MPa) enables consolidation at temperatures as low as 300° C. The CSP includes several mechanisms: facilitated particle rearrangement, hydroplasticity (plasticity enhanced by presence of a liquid), dissolutionprecipitation and evaporation of the liquid. CSP is a low temperature and low energy-consuming technique fairly and it has been applied to several inorganic and hybrid materials including conductors [19,20], semiconductors [21][22][23], nanostructured ceramics [24], functional ceramics (i.e. electroceramics, thermoelectrics, ferroelectrics, solid-state-battery, piezoelectrics [25]). ...
We report on a novel consolidation technique based on the combination of a liquid (cold sintering) and electricity (flash sintering). The evolution of hydrogen from electrolysis of the solvent triggered the Flash Cold Sintering (FCS) event without need for external heating. Electrochemical effects became pronounced in presence of acetic acid solution as the electrolyte had an increased conductivity compared to pure water. By adjusting the acetic acid molar concentration to 0.5 M and by inverting the polarity during FCS a 5 mm diameter 0.8 mm thick Nb2O5 sample was evenly sintered up to 99% in 100 s at a temperature as low as 350 °C. Conventional sintered counterparts achieved a relative 95% density when dwelled for 240 min at 1400°C. The inherent complexity of the proposed FCS approach resulted in enhanced densification and weakly translucent samples.
... In this method, which is called cold sintering; it is used at temperatures below 300 ° C, with a pressure of 100 to 500 MPa. In the first instance, a liquid phase as a solvent and very little is added to the interface between particles [49][50][51]. A portion of the particle surface is dissolved in the liquid phase. ...
In this study, hydroxyapatite-reduced graphene oxide (HA-rGO) powders were first synthesized in situ by a hydrothermal method. These powders were then consolidated using a cold sintering method. The solvent used in this method was water+ dimethylformamide+ Brushite which was added to the powders at different ratios. The sintered samples were then evaluated using X-ray diffraction, Fourier transforms infrared spectroscopy, Raman spectroscopy, High-resolution transmission electron microscopy, and Vickers microindentation techniques. The results of this study showed that the best conditions for the sintering of rGO-HA nanopowders were the temperature of 200 °C , the holding time of >30 min, and the pressure of 500 MPa. The highest mechanical properties were obtained when the solvent content was considered to be 20 wt%. The crack deflection and graphene bridging were among the mechanisms that increase the fracture toughness of these nanocomposites. By adding 1.5% rGO, the fracture toughness of this nanocomposite (by the Cold Sintering method) was approximately equivalent to the fracture toughness of the spark plasma sintered HA.
... A first attempt was made by Charoonsuk et al using water as a transient solvent for cold sintering at 180°C, 500 MPa and up to 60 hours, followed by an annealing step at 1100°C to obtain 96% dense CeO 2 samples. 22 In this study, a flux composed of sodium hydroxide (NaOH) and potassium hydroxide (KOH) was used as a "high temperature solution" to aid in the cold sintering of CeO 2 . The use of flux solvents in the cold sintering process was shown to be effective by Tsuji et al for the single step densification of BaTiO 3 at 300°C. ...
This study reports the successful single‐step cold sintering of nanocrystalline cerium dioxide (CeO2) at temperatures ranging from 250°C to 400°C under 500 MPa, using molten hydroxides flux solvents. CeO2 ceramics obtained were 82 to 91% of the theoretical density. Structural and microstructural investigations of the as‐cold sintered CeO2 ceramics were conducted to further understand this new approach to cold sinter ceramics. Electrical conductivity measured by two‐point AC impedance demonstrated an activation energy for grain conductivity of 0.49 eV, with impedance spectra characteristic of nanoscale CeO2.
... The possibility to integrate ceramics with organic materials [4,5] allows the fabrication of a new class of hybrid (organic/inorganic) components. Since its first report in 2016 by Guo et al. [1], CSP has attracted a lot of research interest [3] especially in the field of semiconductors [6][7][8][9][10], structural materials [11], ionic conductors [12,13], piezoelectric ceramics [14][15][16][17], ferroelectrics [18], Li-ion battery materials [19][20][21], thermoelectrics [5], nanocomposites [22] and low dielectric loss ceramics [23][24][25]. ...
Cold sintering is an innovative low-temperature processing technique which allows consolidation of several ceramics. Despite recent research activities on the cold sintering of functional and structural ceramics, an analytical study accounting for consolidation and grain growth phenomena is still missing in the literature. In this work, we provide a theoretical analysis of the mechanisms active during cold sintering. The analysis considers two cold sintering approaches, characterised by the application either of isostatic or uniaxial pressure. Physical phenomena and microstructural features are discussed in view of the applied cold sintering approach. The developed pressure-assisted densification models indicate that the processes governing densification during uniaxial cold sintering are more complex than those of conventional liquid phase sintering. A key role is played by the water/material interaction which promotes several effects such as formation of surface defects and secondary phases, dynamic recrystallization and other phenomena still partially unknown.
Publisher page: https://www.tandfonline.com/doi/abs/10.1080/17436753.2019.1692173?journalCode=yaac20
... To overcome this major drawback, many strategies have been proposed. These strategies include the use of more reactive ceria powders synthesized by co-precipitation [9][10][11], the sol-gel method [12], and hydrothermal treatments [13,14], or the improvement of the sintering cycle by using sintering aids [15], or using innovative densification techniques [16,17]. Among these methods, flash sintering (FS) has recently been shown to be a very promising consolidation route suitable for densifying ceramics at a reduced temperature via the application of an external electric field [18][19][20]. ...
In this work, ceria-based ceramics with the composition Gd0.14Pr0.06Ce0.8O2-δ and Sm0.14Pr0.06Ce0.8O2-δ, were synthesized by a simple co-precipitation process using either ammonium carbonate or ammonia solution as a precipitating agent. After the calcination, all of the produced samples were constituted by fluorite-structured ceria only, thus showing that both dopant and co-dopant cations were dissolved in the fluorite lattice. The ceria-based nanopowders were uniaxially compacted and consequently flash-sintered using different electrical cycles (including current-ramps). Different results were obtained as a function of both the adopted precipitating agent and the applied electrical cycle. In particular, highly densified products were obtained using current-ramps instead of “traditional” flash treatments (with the power source switching from voltage to current control at the flash event). Moreover, the powders that were synthesized using ammonia solution exhibited a low tendency to hotspot formation, whereas the materials obtained using carbonates as the precipitating agent were highly inhomogeneous. This points out for the first time the unexpected relevance of the precipitating agent (and of the powder shape/degree of agglomeration) for the flash sintering behavior.
Proton-conducting ceramic materials have emerged as effective candidates for improving the performance of solid oxide cells (SOCs) and electrolyzers (SOEs) at intermediate temperatures. BaCeO3 and BaZrO3 perovskites doped with rare-earth elements such as Y2O3 (BCZY) are well known for their high proton conductivity, low operating temperature, and chemical stability, which lead to SOCs’ improved performance. However, the high sintering temperature and extended processing time needed to obtain dense BCZY-type electrolytes (typically > 1350 °C) to be used as SOC electrolytes can cause severe barium evaporation, altering the stoichiometry of the system and consequently reducing the performance of the final device. The cold sintering process (CSP) is a novel sintering technique that allows a drastic reduction in the sintering temperature needed to obtain dense ceramics. Using the CSP, materials can be sintered in a short time using an appropriate amount of a liquid phase at temperatures < 300 °C under a few hundred MPa of uniaxial pressure. For these reasons, cold sintering is considered one of the most promising ways to obtain ceramic proton conductors in mild conditions. This review aims to collect novel insights into the application of the CSP with a focus on BCZY-type materials, highlighting the opportunities and challenges and giving a vision of future trends and perspectives.
The cold sintering process (CSP) has emerged as a revolutionary technique for low-temperature processing of ceramics and composites, enabling high-density fabrication at low temperatures. In this study, we demonstrated the implementation of CSP in fabricating the γ-glycine (γ-G)-bacterial cellulose (BC) composite and evaluated the effect of sintering temperature and holding time on the microstructure and electrical properties. Our findings revealed that an increase in sintering temperature and holding time leads to grain growth, as the transient solvent (water) facilitates the closely-packed microstructure. Moreover, the addition of BC as a filler into the γ-G matrix leads to a composite with a 10% increase in hardness when BC was uniformly distributed in γ-G. The composite with a relative density of 97% was successfully obtained at 120 °C/24 h, preserving the γ polymorph of glycine without the unwanted transformation commonly observed with traditional sintering. We also reported the dielectric and ferroelectric properties of the γ-G-BC composite, exhibiting a remanent polarization of 0.004 μC/cm2 and a coercive field of 1.201 kV/cm. Our findings suggest that CSP is a promising approach for low-temperature processing and fabrication of ceramics, especially when incorporating structurally sensitive filler such as organic ferroelectric, to achieve high-performance composites.
A generalized cold sintering densification strategy based on a hydroxide precursor transformation route is proposed for oxides. The densification of MgO, CuO, ZnO and WO3 was achieved via cold sintering by using their corresponding hydroxides at temperatures not exceeding 450 °C. Nano-oxides formed by the decomposition of the hydroxides exhibited good low-temperature sinterability. The densification mechanisms mainly involved particle rearrangement promoted by in situ released water and intergranular diffusion accelerated by surface defects of the oxide particles generated from hydroxide decomposition. During the cold sintering process, the oxides with relatively higher solubility in a water vapor environment are more likely to form surface defects, which promoted water-aided densification. Owing to the possibility of obtaining the corresponding hydroxides for almost all oxides, this strategy renders cold sintering feasible for a wide range of materials.
Due to its strong ionic conductivity, doped ceria has been employed as an electrolyte in medium-temperature solid oxide fuel cell applications. For the first time, we demonstrated the feasibility of cold sintering high-density Ce0.8Sm0.2O1.9 (SDC)@Na2CO3 composite electrolytes with Na2CO3 as a shell at temperatures less than 200 °C. The effects of Na2CO3 and water content on cold-sintered samples’ relative density, microstructure, and ionic conductivity were examined. The core-shell structure is significant in densification, but water addition has an opposite effect. The bulk and grain-boundary conductivities of cold-sintered [email protected]2CO3 electrolytes were studied at 350−700 °C and analyzed in detail based on carbonate and water effects. This work presents a significant and scalable method for synthesizing dense solid electrolytes by forming a core-shell structure by cold sintering.
High-density (∼99.34%), fine-grained (∼685 nm) ZnO ceramic was successfully fabricated by the high-pressure cold sintering. The influences of solvent (liquid phase) fraction and the applied pressure on densification in this process have been carefully studied. The excess solvent can result in grain refinement, but impedes the densification of ceramic. Significantly, the role of pressure is unlike that in the traditional pressure-assisted sintering, which can improve the densification and inhibit grain growth. There is a critical pressure (∼1 GPa) that can be associated with the transition of recrystallization from heterogeneous nucleation to homogeneous nucleation. When the pressure is lower than this critical value, the applied pressure can improve densification, but cannot inhibit grain growth. Above the critical value, the densification is dramatically weakened and many small crystals in the interstitial space between grains can be found.
The refractory nature of oxide-ion conducting Sm-doped CeO2 (SDC) has hindered the development of SDC devices. Lithium has been found to be an excellent sintering aid that enables sintering at temperatures as low as 900°C and also increases grain boundary conductivity. Even though Li–Si oxides melt only above the sintering temperature in reality, it is generally believed that these benefits are given by virtue of a formation of liquid phases with silicon contaminants. In this study, the key element for low-temperature sintering was revealed to be aluminum, which is spontaneously introduced into SDC by a covert but intense chemical reaction between the mixed lithium and the alumina crucible. The results show that 1 wt% lithium addition is responsible for the unintended introduction of 32 mol% aluminum. Computational thermodynamic calculations for a Li–Si–Al oxide system clarified that aluminum introduction is essential to reduce the melting point below the sintering temperature. A nano-scale analysis clearly indicates the segregation of aluminum at SDC grain-boundary triple junctions, like LiAlO2, while silicon appears to migrate towards the SDC-SDC grain boundaries as amorphous SiO2. This suggests that the decomposition of the Li–Si–Al liquid oxide filling the grain boundaries is associated with the reflow of silicon and the loss of lithium as sintering proceeds. It was shown that zirconium is also introduced into SDC due to ball-mill processing, and that it potentially forms Li–Si–Zr oxides which remain in their solid state during sintering. The results of this study highlight the importance of taking the impact of unintended aluminum introduction into consideration for a variety of ceramics mixed with even minute quantities of lithium.
Traditionally ceramic artifacts are processed at high temperatures (>1000 °C) by classical sintering techniques such as solid state, liquid phase and pressure-assisted sintering. Recently, inspired from the geology, novel sintering approaches that allow the densification of ceramic components at relatively low temperatures ≤400 °C have been proposed. While initial efforts for such low temperature densification concept were developed in the mid-70s, the topic has become increasingly prominent in the last decade. Currently, these low temperature methods can be classified into four main groups: (i) hydrothermal reaction sintering (HRS), (ii) hydrothermal hot pressing (HHP), (iii) pressure-assisted densification techniques: room-temperature densification (RTD), cold sintering (CS), warm press (WP), and finally no-pressure assisted method called (iv) reactive hydrothermal liquid phase densification (rHLPD). Above named techniques are commonly assisted by an aqueous solution used as either reactant or transient liquid phase to assist densification. Starting from the background in traditional sintering processes, this review aims to explore in depth the existing literature about low temperature densification approaches along with their advantages & disadvantages, and probable application areas.
Ceria-based ceramics can be considered among the most promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC). In the present work, variously doped nanocrystalline ceria powders were flash-sintered, and the role of doping (Gd and Sm, 5–20 mol%) and sintering aid (Li and Co) on the final microstructure and the electrical behavior was investigated. Gd- or Sm-doped nanocrystalline ceria powders were synthesized by co-precipitation method using ammonia solution as precipitating agent. The synthesized nanopowders were characterized by DSC-TG, XRD, and nitrogen physisorption analysis. The nanopowders were isostatically pressed and flash-sintered. The relative density was measured by hydrostatic balance, and the corresponding microstructure was observed by SEM. The electrical behavior was studied by EIS. Flash-sintered powder pellets showed different behaviors depending on the dopant and sintering aid. The electrical conductivity of the samples increased by increasing the relative density. Fully dense Gd-doped ceria samples, synthesized by co-precipitation, were obtained by flash sintering in very short times at 700 °C. The total conductivity was comparable to that measured on samples sintered with conventional route at much higher temperatures such as 1500 °C.
https://link.springer.com/article/10.1007/s10853-017-0980-2
In general, many attempts have been focused
on enhancement of oxy-ion conductivity at intermediate
temperature range in order to have a feasible operating
temperature of solid oxide fuel cell (SOFC). Every effort is directed towards the creation of defect-rich structure so as to get either ion or vacancy movement,
which, in turn, offers more ionic conductivity. Doping
is one of the strategies where isovalent, aliovalent,
codopant, and multidopant are the sources to create defects. Doped ceria electrolytes have shown potential to
reduce the operating temperature of SOFCs at an intermediate temperature (350–550 °C) range due to high
ionic conductivity at comparatively low temperature. In
the present work, a local structure of nano-sized
aliovalent Ce0.85(M)0.15O2–8 and codoped ceria systems
Ce0.85(Sm)0.075(M)0.075O2 − δ (where M = Sm, Sr, Gd,
Nd, Ca, and Dy) prepared by a hydrothermal synthesis
route followed by characterization with extended X-ray
absorption fine structure (EXAFS) and X-ray absorption
near-edge structure (XANES) is reviewed.
We report the effect of the charge compensation on the electronic transport and optical properties of CeO2 co-doped with donor, Nb, and acceptor, Y, ions. As expected, the concentration of Ce³⁺ decreases with an increase in the Y content in Ce0.992−xNb0.008YxO2, where 0 ≤ x ≤ 0.008. More importantly, random electric fields generated by the Y ions bring additional disorder into the system. As a result, the high-temperature activation energy of conductivity increases significantly from 189 to 430 meV. A similar energy shift in the optical absorption peak centered at 1.3–1.5 eV is attributed to an increase in the energy gap separating the localized f-electrons from the empty Ce 4f band. The results underline the paramount importance of the disorder-induced Anderson localization of the f-electrons in ceria.
We report magnetic susceptibility, electrical conductivity and optical absorption of Ce1−xMxO2 where M = Nb,Ta and 0≤x≤0.03. The dc conductivity follows a simple thermally activated Arrhenius-type behavior in the T=70–700 K range with a change in slope at T*≈155 K. The high-temperature activation energy shows gradual increase from ≈170 to 220 meV as the dopant concentration increases. The activation energy of the low-temperature conductivity shows a broad minimum of ≈77 meV at x≈0.01. Electron transport and localization mechanisms are analyzed in the framework of the Holstein small polaron, Anderson localization, and Jahn-Teller distortion models. The fit to the small polaron mobility is dramatically improved when, instead of the longitudinal phonons, the transverse optical phonons are considered in the phonon-assisted electron transport. This serves as an indirect evidence of a strong 4f1 orbital interaction with the oxygen ligands, similar to the case of PrO2. Based on comparison of the experimental data to the models, it is proposed that the defect-induced random electric fields make the dominant contribution to the electron localization in donor-doped ceria.
Research on sintering of dense ceramic materials has been very active in the past decades and still keeps gaining in popularity. Although a number of new techniques have been developed, the sintering process is still performed at high temperatures. Very recently we established a novel protocol, the "Cold Sintering Process (CSP)", to achieve dense ceramic solids at extraordinarily low temperatures of <300°C. A wide variety of chemistries and composites were successfully densified using this technique. In this article, a comprehensive CSP tutorial will be delivered by employing three classic ferroelectric materials (KH2PO4, NaNO2, and BaTiO3) as examples. Together with detailed experimental demonstrations, fundamental mechanisms, as well as the underlying physics from a thermodynamics perspective, are collaboratively outlined. Such an impactful technique opens up a new way for cost-effective and energy-saving ceramic processing. We hope that this article will provide a promising route to guide future studies on ultralow temperature ceramic sintering or ceramic materials related integration.
It has been reported that some transition metal oxides are effective aids both for the densification and the grain boundary behavior of ceria-based electrolytes. In the present work, NiO which is the most popular component of the anode of solid oxide fuel cells was added directly into the electrolyte ceramic, Ce0.8Gd0.2O1.9, to investigate the effects of the presence of NiO on the properties of GDC electrolyte. All of the samples possess a single phase with cubic fluorite structure. The grain size is increased by the addition of NiO when the sintering temperature is 1400 °C. This modification in chemical composition also results in a decrease in activation energy and thus a tendency to enhance grain boundary mobility. The maximum power density of the composite electrolyte single cell is higher than that of a GDC single cell. Therefore, NiO can be used as an effective aid for ceria-based electrolytes.
Gd-doped ceria (CGO) nanopowders were mixed with up to 1 mol% of Zn (as nitrate) to exploit the role of this dopant on sintering and electrical performance. All samples were prepared by two-step sintering schedules to try to combine the advantages of both effects on low-temperature densification. Structural and microstructural analyses were complemented with impedance spectroscopy (wide range of temperatures and oxygen partial pressures) and ion-blocking measurements. The combination of all data suggests that Zn is preferentially located in the grain boundary region, playing a significant role on sintering. Zn additions (I mol%) allowed for a 100 degrees C lower sintering temperature with respect to pure CGO. A marginal effect on the oxide-ion conductivity and a small enhancement (less than 20%) in n-type electronic conductivity was found between Zn-free and 1 mol% Zn-doped CGO. The removal of Zn from CGO after sintering is discussed.
Tetragonal ZrO2 polycrystalline (TZP) composites with 2 wt.% Al2O3 and co-stabilised with 1 mol% Y2O3 and (4, 6 or 8) mol% CeO2 were sintered at 1450 °C for 20 min in a single mode 2.45 GHz microwave furnace. For comparison, conventional sintering was performed in air at 1450 °C for 20 min. The starting powder mixture was obtained by a suspension coating technique using yttrium nitrate, cerium nitrate and pure m-ZrO2 nanopowder. Fully dense material grades were obtained by both sintering methods. The influence of the composition and the sintering methods on the final phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscopy. Finer and more uniform microstructures were observed in the microwave sintered ceramics when compared to the conventionally sintered samples. The fracture toughness increases with decreasing stabiliser content, whereas a reverse relation was found for the Vickers hardness. Comparable toughness and hardness values were obtained for the microwave and conventionally sintered samples.
The influence of dysprosia addition on the sintering and resulting microstructure of nano-grained CeO2 ceramics was investigated as functions of the spark plasma sintering (SPS) parameters. The addition of Dy2O3 (forming a solid solution) resulted in an increase in relative density and a decrease in grain size. The relative density of samples with Dy2O3 content of 6 and 10 mol% was over 95% when sintered at 1050 C under 500 MPa for holding times as short as 5 minutes. The application of high pressure facilitated the consolidation to relatively high densities with minimal grain growth. Heating rate and holding time, however, had insignificant effect on density but a measurable effect on grain size.
The solubility of 17 commonly available metal oxides in the elemental mass series Ti through Zn have been determined in three ionic liquids based on choline chloride. The hydrogen bond donors used were urea, malonic acid, and ethylene glycol. The results obtained are compared with aqueous solutions of HCl and NaCl. Some correlation is observed between the solubility in the deep eutectic solvents and that in aqueous solutions but some significant exceptions offer an opportunity for novel solvato-metallurgical processes.