In situ doping of BaTiO3 and visualization of pressure solution in flux-assisted cold sintering

Cold sintering process (CSP) has attracted great interest due to its extremely low processing temperatures, fast processing times, and simplicity to allow for the densification of ceramics and composites. Understanding the detailed mechanisms underlying low temperature densification is crucial to develop advanced materials and facilitate sustainable and cost-effective industrial implementation to come. Here, by taking BaTiO₃ powder and Sr(OH)₂·8H₂O transient chemical flux as a model system, chemical transformation at solid/flux interfaces driving the dissolution-precipitation creep mechanism were investigated. We demonstrate that Sr(OH)₂·8H₂O acts both as a sintering flux and a solid solution doping additive, resulting in the formation of BaTiO₃ - Ba_1-xSr_xTiO₃ with lower Curie temperatures. Using strontium (Sr) as a tracer chemistry, transmission electron microscopy chemical mapping with energy-dispersive X-ray analysis indicates that there is a precipitation of a Ba_1-xSr_xTiO₃ mainly at grain/grain interfaces, while grain cores remain undoped. In addition, the difference in the interfacial Sr concentration, which is influenced by the applied uniaxial pressure direction, was clearly observed. This successful visualization of compositional distribution after CSP underlines the significant role of the pressure solution creep in densification process.     

In situ characterization of ceramic cold sintering by small-angle scattering

The first in situ characterization of the pore morphology evolution during the cold sintering process (CSP) is presented using small-angle X-ray scattering methods. For practical reasons, measurements have been made on a model system, KH₂PO₄ (KDP). The scattering signal revealed a striking behavior that could be modeled with nanoscale structural features associated with the dissolution and reprecipitation of KDP close to the grain/pore interface during CSP. The prospects for future more quantitative experiments under a range of temperature and pressure conditions, as well as for studies of more technologically important materials such as ZnO are considered.     

Densification of oxides via cold sintering of hydrate precursors

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.     

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

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

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.     

Mechanism studies of hydrothermal cold sintering of zinc oxide at near room temperature

Zinc oxide densification mechanisms occurring during the cold sintering process (CSP) are examined by investigating specifically the effects of ion concentration in solution, temperature, pressure, and die sealing. The experiments suggest that mass transport through solution is a primary densification mechanism and that either a pre-loaded solution or grain dissolution can supply migrating ions. Additionally, results indicate cold sintering zinc oxide requires a critical pressure value, above which densification is relatively pressure independent under the majority of process conditions. This critical pressure is related to thermal expansion of the liquid and determines the uniaxial pressure threshold for densification. The data supports a three-stage interpretation of cold sintering, which includes quick compaction, grain rearrangement, and dissolution-reprecipitation events. Further, it is observed that under the lowest temperature conditions a net decrease in particle size can occur during the cold sintering process.     

Influence of incongruent dissolution-precipitation on 8YSZ ceramics during cold sintering process

The incongruent dissolution-precipitation behaviors of 8YSZ (8 mol% yttria-stabilized zirconia) ceramics during cold sintering process is studied in this paper by changing the pH of liquid media. The different solubility of Y³⁺ and Zr⁴⁺ at the same lattice position causes the disparate dissolution behaviors, and results in incongruent precipitation. Compared with acidic or alkaline solution, neutral solution is more conducive to the incongruent dissolution-precipitation process, and the concentration ratio of dissolved Y³⁺ and Zr⁴⁺ can reach ～6385. The incongruent dissolution-precipitation process facilitates the formation of neck structure and promotes the ionic migration and diffusion at the subsequent high-temperature sintering process, which improves the mechanical properties and electrochemical properties of 8YSZ ceramics. This work reveals the principle of incongruent dissolution-precipitation process of zirconia-based ceramics, and it is of significance for selecting suitable liquid media to control the incongruent dissolution-precipitation behaviors in cold sintering process to prepare high-performance zirconia-based ceramics.     

High permittivity BaTiO3 and BaTiO3-polymer nanocomposites enabled by cold sintering with a new transient chemistry: Ba(OH)2∙8H2O

Cold sintering process (CSP) offers a promising strategy for the fabrication of innovative and advanced high permittivity dielectric nanocomposite materials. Here, we introduce Ba(OH)₂?8H₂O hydrated flux as a new transient chemistry that enables the densification of BaTiO₃ in a single step at a temperature as low as 150 °C. This remarkably low temperature is near its Curie transition of 125 °C, associated with a displacive phase transition. The cold sintered BaTiO₃ shows a relative density of 95 % and a room temperature relative permittivity over 1000. This new hydrated flux permits the fabrication of a unique dense BaTiO₃-polymer nanocomposite with a high volume fraction of ceramics ((1-x) BaTiO₃ – x PTFE, with x = 0.05). The composite exhibits a relative permittivity of approximately 800, at least an order of magnitude higher than previous reports on polymer composites with BaTiO₃ nanoparticle fillers that are typically well below 100. Unique high permittivity dielectric nanocomposites with enhanced resistivities can now be designed using polymers to engineer grain boundaries and CSP as a processing method opening up new possibilities in dielectric materials design.     

Densification of thermodynamically unstable tin monoxide using cold sintering process

SnO is a thermodynamically unstable phase and undergoes thermal decomposition into SnO₂ and Sn at a relatively low temperature when heating under ambient conditions. With the cold sintering process (CSP), SnO can be densified up to 89% of theoretical density within 100 min by applying uniaxial pressure of 350 MPa and transient liquid phase. 15-fold BET specific surface area reduction is observed between the ball-milled powder and the cold-sintered pellet, indicating experimental evidence of sintering. The temperature profiles of 70–265 °C show densification while maintaining the phase purity. Water and 2 M acetic acid solution are studied as transient liquid phases which promotes dissolution-precipitation on the particle surface and induces crystalline texture. Electrical properties of the cold sintered bulk, notably electrical conductivity and Seebeck coefficient, are measured as a function of temperature.     

Dense and strong calcite ceramics prepared by room-temperature cold sintering based on high-pressure-enhanced solubility

Dense and strong calcite (CaCO₃) ceramics were prepared by room-temperature cold sintering with the aid of water and high pressure of up to 900 MPa. Under atmospheric pressure, calcite is barely soluble in water. However, the microstructure evolution and stress-strain analysis during cold sintering revealed that the dissolution-precipitation, plastic deformation, and pressure-solution-creep mechanisms played a crucial role in the densification and mechanical robustness of calcite ceramics, which was attributed to the significantly enhanced solubility of calcite in water under high pressure. The calcite ceramic cold sintered under 900 MPa from micron powder exhibited the highest relative density of 92.1% and best mechanical properties with compressive strength, flexural strength, hardness, and Young's modulus of 276.5 MPa, 52.5 MPa, 1.64 GPa, and 53.7 GPa, respectively. The as-prepared calcite ceramic was stronger and harder than most stones and cement, indicating its promising application as novel building and biomimetic materials. The present study also provides a new strategy for densifying ceramics with low solubility by cold sintering.     

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics. Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C. Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering. By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N=3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering. The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process.     

Sintering densification behavior of ZnO–SnO2 binary ceramic targets

In this work, high-performance ZnO–SnO₂ binary ceramic targets for magnetron sputtering of transparent conductive oxide (TCO) films were prepared by pressureless oxygen atmosphere sintering. The sintering behavior and densification mechanism of the ZnO–SnO₂ binary targets were analyzed by systematically studying the oxide powder state, formation process of the solid reaction phase, and evolution of the target microstructure. The data revealed that the ZnO–SnO₂ powder treatment improved the sintering activity and the powder dispersion; furthermore, it promoted a mutual reaction between the different components during sintering and the homogeneity of the target composition. The densification of the pure SnO₂ ceramic target was difficult to achieve, and the addition of ZnO led to a continuous interaction between the ZnO and SnO₂ components. The Zn₂SnO₄ phase started to form, and a temporary shrinkage of the target occurred above 800°C. After formation of the stable Zn₂SnO₄ and SnO₂ phases, the target shrunk rapidly with increasing temperature, densification occurred during growth, and the two phases started to interact. The sintering temperature provided the driving force for the target densification, with the densification activation energy of the ZnO–SnO₂ binary ceramic target estimated to be 580 kJ/mol based on the master sintering curve. A binary ceramic target with a high density (99.78% relative density), a fine grain size, and a homogeneous phase structure was achieved at a temperature of 1600°C. These findings are promising for the further improvement and performance enhancement of SnO₂-based materials.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Single-step,high pressure,and two-step spark plasma sintering of UO2 nanopowders

Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO_2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650?550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO₂ samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO_2.00.     

Sintering highly dense ultra-high temperature ceramics with suppressed grain growth

Grain coarsening normally occurs at the final stage of sintering, resulting in trapped pores within grains, which deteriorates the density and the performance of ceramics, especially for ultra-high temperature ceramics (UHTCs). Here, we propose to sinter this class of ceramics in a specific temperature range and coupled with a relatively high pressure. The retarded grain boundary migration and pressure-enhanced diffusion ensure the proceeding of densification even at final stage. A highly dense TaC ceramic (98.6 %) with the average grain size of 1.48 μm was prepared under 250 MPa via high pressure spark plasma sintering using a C_f/C die at 1850 °C. It was suggested that the final-stage densification is mainly attributed to grain boundary plastic deformation-involved mechanisms. Compared to the usual sintering route using a high temperature (>2000 °C) and normal pressure (<100 MPa), this work provides a useful strategy to acquire highly dense and fine-grained UHTCs.     

Influence of powder characteristics on cold sintering of nano-sized ZnO with density above 99 %

Cold sintering parameters such as, temperature, pressure, aqueous phase, heating rate and dwelling time has been widely discussed in the literature but the role of starting powder with respective microstructure development is mostly overlooked. There is a need for understanding the effect of powder agglomerates and the role of inter particle friction on the densification behavior during cold sintering process. Present study encompasses investigation and optimization of these parameters for ZnO which enabled > 99 % of relative density with grain sizes below 200 nm. Additionally, role of external atmosphere was also studied to investigate its impact on densification during the process. All cold sintering experiments were carried out in a FAST/SPS device for studying aqueous phase evaporation and ensuring the reproducibility of process parameters. Microstructure characterization (scanning and transmission electron microscopy) showed – without any post heat treatment– defect free grain boundary structure opposite to what documented by previous studies.     

Cold densification and sintering of nanovaterite by pressing with water

While dissolution-precipitation, plastic deformation and fracture have been proposed to explain the compaction of carbonates in geological formations, the role of these mechanisms on the densification process of calcium carbonate nanoparticles in synthetic systems remains poorly understood. Here, we systematically investigate the effect of pH of the aqueous phase (1 ≤ pH ≤ 7), temperature (10 ≤ T ≤ 90 °C), and pressure (10 ≤ P ≤ 800 MPa) on the cold compaction of nanovaterite powder with water to shed light on the mechanisms underlying this unique densification. Compaction experiments reveal that the applied pressure plays a major role on the densification of vaterite nanopowder with water. Our experimental data thus suggest that plastic deformation or subcritical crack growth might be important densification mechanisms for vaterite nanoparticles. These findings provide a new perspective into the cold compaction of nanopowders with water and may open promising routes for the manufacturing of CO₂-based structural materials at mild processing conditions.     

Sintering behaviour and properties of manganese-doped alumina

The effect of manganese (0.1, 0.5 and 1.0?wt%) on the sintering and mechanical properties of alumina was studied. Sintering was carried out by the conventional heating method in a box furnace and in a hybrid multimode microwave furnace. XRD analysis revealed the precipitation of a spinel second phase (MnAl₂O₄) in manganese-doped samples as a result of manganese limited solubility in the corundum lattice. The addition of 0.1?wt% manganese was most beneficial in enhancing the densification of alumina (97.5% relative density when compared to 94.2% for the undoped sample), hindered grain growth, and improved the hardness of the ceramic when sintered at 1500?°C. The study also revealed that microwave sintering was effective in suppressing grain growth of alumina. In addition, the hardness was dependent on the sintered bulk density and that grain coarsening ensued as the density of the sintered alumina exceeded 95% of theoretical.     

Properties of spark plasma sintered pseudocubic BiFeO3–BaTiO3 ceramics

Perovskite solid solution materials, namely, 0.67BiFeO₃-0.33BaTiO₃, were synthesized by spark plasma sintering method. The effects of the spark plasma sintering temperature on phase purity, microstructure, and electric properties of the as-prepared materials were investigated. The materials could be referred as pseudocubic phases based on the X-ray diffraction patterns. The bulk density first increased and then decreased. The 880 °C-sintered-ceramics had the maximal density and a compact microstructure with grain size of 0.77 ± 0.34 μm. The dielectric constant as a function of temperature exhibited a broad peak. At the optimal spark-plasma-sintering temperature, enhanced ferroelectric properties were observed with a value of P_r ～ 21 μC/cm². This investigation on the spark plasma sintering process confirms it as an efficient approach to prepare outstanding performance BiFeO₃–BaTiO₃ ceramics.     

Spark plasma sintering of nanocrystalline BaTiO3-powders: Consolidation behavior and dielectric characteristics

BaTiO₃ nanopowders prepared by two different wet chemical routes, one based on microemulsion-mediated synthesis (M-BT) and the other one on the alkoxide-hydroxide method (A-BT) were consolidated by spark plasma sintering (SPS). The densification process, the linear shrinkage rates and the relative densities achieved were strongly dependant on the synthetic route. The results show that fully densified BaTiO₃ ceramics with a grain size of about 200 nm can be obtained in both cases by controlling the sintering temperature during the SPS process. The study of dielectric properties revealed that M-BT derived ceramics show higher permittivity values compared to those obtained for A-BT. The influence of the barium/titanium ratio on the sintering behavior and the dielectric properties is discussed.