Cold sintering and microwave dielectric properties of dense HBO2-II ceramics

HBO₂-II ceramics were prepared by cold sintering with 10wt% dehydrated ethanol as the transient liquid phase. When the processing temperature is 30°C, the relative density of the mechanically robust HBO₂-II ceramics increases from 77.5% to 84.5% with increasing the uniaxial pressure from 200 to 500 MPa. It changes less than 0.2% for higher pressure up to 700 MPa. Under a constant uniaxial pressure of 500 MPa, the relative density further increases to 94.7% for the processing temperature of 120°C. HBO₂-I is observed as the secondary phase when the processing temperature is 150°C. In comparison, the compacts prepared in the absence of ethanol are fragile, and the relative densities are 78.5%-84.5% for the processing temperatures of 30-120°C and uniaxial pressure of 500 MPa. It is indicated that ethanol promotes the densification significantly through the dissolution-precipitation mechanism. The permittivity increases with increasing the processing temperature, while the Qf value decreases. The optimal properties with the relative density of 94.7%, ε_r = 4.21, Qf = 47 500 GHz, and τ_f = −70.0 ppm/°C were obtained in the single-phase HBO₂-II ceramics cold sintered at 120°C under 500 MPa for 10 minutes. The relative density and Qf value are significantly higher than those of the HBO₂-II ceramic prepared by sintering the H₃BO₃ compact at 180°C for 2 hours (70.3% and 32 700 GHz, respectively). The results indicate that the nonaqueous solvent can also be used as the transient liquid phase for cold sintering, so that more materials that are unstable or insoluble in water can be densified by this method.     

Lightweight bricks made of diatomaceous earth,lime and gypsum

Diatomaceous earth from Lampang Province in the north of Thailand composes of diatom, kaolinite, montmorillonite and illite, and has porous cellular structure. In this work, the diatomite, hydrated lime and gypsum are the main ingredients in making autoclaved lightweight bricks. Water content, pre-curing period, lime content, gypsum content and calcined temperature are the factors investigated. Mechanical and thermal properties are used to indicate their quality. The nature of hydration products and morphological characteristics of the lightweight bricks are also investigated.     

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.     

Cold sintering assisted processing of Mn-Zn ferrites

Typically, commercial Mn-Zn ferrites are sintered at high temperatures with prolong times. In this work, commercial Fe₂O₃-rich ferrite powders with the composition of 0.21Mn_0.8Zn_0.2Fe₂O₄-0.79Fe₂O₃ (wt%) are densified by cold sintering at 300 °C with the assistance of organic salts, including MnC₂O₄·2H₂O, FeC₂O₄·2H₂O, and Zn(C₂H₃O₂)₂·2H₂O. Excessive Fe₂O₃ enters into spinel structure forming a solid solution through annealing in low pO₂ at 1350 °C. The sintering behaviors, microstructures, magnetic properties and impedances are investigated. The dehydration of organic salts provides mediate liquid phase to trigger the dissolution-precipitation process, which assists the densification of ceramics. The grains grow from 0.15 µm to 0.52 µm and 7.67 µm after cold sintering at 300 °C and annealing at 1350 °C, respectively. The initial permeability of cold sintered sample is improved to 11000 with a Curie temperature of 125 °C. This work provides a feasible route for cold sintering assisted processing of commercial soft magnetic ferrites.     

Cold sintering process: A new era for ceramic packaging and microwave device development

Cold sintering process (CSP) is an extremely low‐temperature sintering process (room temperature to ~200°C) that uses aqueous‐based solutions as transient solvents to aid densification by a nonequilibrium dissolution‐precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic‐polymer composites. Some dielectric materials, namely Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and (LiBi)_0.5MoO₄ ceramics, and also (1?x)Li₂MoO₄?xPTFE and (1?x)(LiBi)_0.5MoO₄?xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%‐95%) and good microwave dielectric properties (permittivity, 5.6‐37.1; Q × f, 1700‐30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co‐fired ceramic technology, namely CSCC. Li₂MoO₄?Ag multilayer co‐fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy‐saving strategy for the ceramic packaging and microwave device development.     

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.     

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.     

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.     

Cold sintering of bioglass and bioglass/polymer composites

Bioactive glasses are widely utilized to regenerate bone tissue and aid bonding of orthopedic implants. Forming composites of bioglass with bioactive polymers allow the mechanical properties and biological response to be tailored. Although several methods for creating bioglass–polymer composites exist, they require dissolution of the polymer, controlled phase separation, and appear to have an upper limit of ∼30 vol.% bioglass. Cold sintering is a novel technique for the densification of ceramics and glasses which utilizes a liquid phase and pressure to allow the production of components at reduced temperatures. We demonstrate that cold sintering (100°C) of Bioglass 45S5 powder produced via flame spray pyrolysis and the fabrication of Bioglass 45S5–polymer composites. Assessment of the in vitro response revealed that composites were not cytotoxic. Solid-state ³¹P and ²⁹Si MAS NMR studies of the silicon and phosphorus speciation in the glass powder, as-received, wetted, and sintered samples show similarities to reactions expected when bioglass is implanted in the body which along with Raman spectroscopy data gave insight into the cold sintering densification mechanism.     

Current understanding and applications of the cold sintering process

In traditional ceramic processing techniques, high sintering temperature is necessary to achieve fully dense microstructures. But it can cause various problems including warpage, overfiring, element evaporation, and polymorphic transformation. To overcome these drawbacks, a novel processing technique called “cold sintering process (CSP)” has been explored by Randall et al. CSP enables densification of ceramics at ultra-low temperature (≤300°C) with the assistance of transient aqueous solution and applied pressure. In CSP, the processing conditions including aqueous solution, pressure, temperature, and sintering duration play critical roles in the densification and properties of ceramics, which will be reviewed. The review will also include the applications of CSP in solid-state rechargeable batteries. Finally, the perspectives about CSP is proposed.     

Cold sintering assisted CaF2 microwave dielectric ceramics for C-band antenna applications

A cold sintering process is adopted to pre-densify CaF₂ ceramics from 85.7% at 300 MPa to 91.7% at 750 MPa. Subsequent post-annealings at 1000–1150 °C lead to further improvements in densification, where great enhancements of grain size and crystallinity are also observed from the scanning and transmission electron micrographs. Significant advances in Qf values are achieved in the post-annealed CaF₂ ceramics. The optimum Qf value (80,522 GHz) is achieved after cold sintering at 750 MPa and post-annealing at 1000 °C, which is three times higher than the conventional sintered one at 1000 °C (26,448 GHz). Moreover, the obtained low-ε_r (5.9–6.5) of CaF₂ ceramics suggests broad application prospects in the high-band microwave communications. A microstrip patch antenna is fabricated using the CaF₂ ceramics as the substrate, which operates at 7.89 GHz in the C-band, with an S₁₁ of ?13.4 dB, simulated high gain and efficiency of 6.41 dBi and ?0.56 dB, respectively.     

Cold sintering of the Mg–C–O–H system

The cold sintering process reproduces the formation of sedimentary rocks in the Earth's crust by molding raw powder and a small amount of solvent at temperatures of about 300 °C or less under uniaxial pressure of several hundred megapascals. Generally, carbonates and hydroxides cannot be hardened by conventional sintering process due to thermal decomposition. In contrast, when this cold sintering process is selected, since the densification of the starting powder can be achieved by a dissolution-precipitation reaction with water as a solvent, carbonates and hydroxides can be hardened at temperatures below their decomposition temperatures. In the Mg–C–O–H system, magnesium hydroxide and a basic magnesium carbonate were selected as starting compounds, and the mechanism of their densification by a cold sintering process was investigated. The average compressive strength of the obtained magnesium hydroxide solidified products after cold sintering at 250 °C and 270 MPa for 60 min, and basic magnesium carbonate solidified products after cold sintering at 150 °C and 270 MPa for 60 min, were 121 MPa (84% relative density) and 275 MPa (88% relative density), respectively. The added water was found to play an important role in promoting a solution-precipitation process inside the magnesium hydroxide or basic magnesium carbonate powder compact, resulting in low-temperature sintering to form hardened bodies in the Mg–C–O–H system.     

Single-step densification of nanocrystalline CeO2 by the cold sintering process

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.     

Flash sintering of ceramics

Flash sintering is a novel densification technology for ceramics, which allows a dramatic reduction of processing time and temperature. It represents a promising sintering route to reduce economic, energetic and environmental costs associated to firing. Moreover, it allows to develop peculiar and out-of-equilibrium microstructures.The flash process is complex and unusual, including different simultaneous physical and chemical phenomena and their understanding, explanation and implementation require an interdisciplinary approach from physics, to chemistry and engineering. In spite of the intensive work of several researchers, there is still a wide debate as for the predominant mechanisms responsible for flash sintering process.In the present review, the most significant and appealing mechanisms proposed for explaining the “flash” event are analyzed and discussed, with the aim to point out the level of knowledge reached so far and identify, at least, possible shared theories useful to propose future scientific activities and potential technological implementations.     

Role of different rare earth oxides on the reaction sintering of magnesium aluminate spinel

Role of three rare earth oxides, viz., La₂O₃, CeO₂ and Yb₂O₃ on reaction sintering of magnesium aluminate spinel having molar ratio of MgO:Al₂O₃?=?1:2 from its solid oxide precursors was investigated in static and dynamic heating conditions. Effect of these additives (3?wt%) on densification behavior, phase assemblage and microstructure development were studied in the temperatures of 1500–1700?°C. Yb₂O₃ enhanced the sintering of spinel, while La₂O₃ and CeO₂ negatively impacted the sintering of magnesium aluminate spinel which can be discerned from the shrinkage curve of TMA as well as from static firing regime. This is ascribed to the formation of secondary phases in La₂O₃ and CeO₂ containing samples which have different crystalline structures to that of spinel. This anisotropy due to different crystallinity hindered the pore shrinkage and pore removal and thereby retarded the densification. Whereas, the cubic structure of the secondary phase formed in Yb₂O₃ containing sample which is isotropic with the crystalline orientation of the parental spinel phase assisted the densification.     

Thermosetting polymers in cold sintering: The fabrication of ZnO-polydimethylsiloxane composites

This study reports the fabrication of the first ceramics-thermosetting polymer composites by the cold sintering process, for high ceramic volume fractions (v/v >95%). The (1 − x) ZnO − x polydimethylsiloxane (PDMS) composites, with 0.00 ≤ x ≤ 0.05, were cold sintered at 250°C, 320 MPa for 60 minutes. In situ densification studies conducted with the help of a semi-automated press revealed that the mechanisms driving the densification of the material changes with the polymer content. Relative densities of the (1 − x) ZnO − x PDMS composites (0.00 ≤ x ≤ 0.05) were above 90%. Impedance spectroscopy of the composites yields insight into the ceramic-polymer interfaces within the sintered ZnO bulk and suggests long-range conduction governed by ZnO-PDMS interface properties for x = 0.03 and x = 0.05. The study also emphasizes on the complexities and opportunities of such ceramic-dominated ceramic-polymer composites.     

On the confluence of ultrafast high-temperature sintering and flash sintering phenomena

Ultrafast high-temperature sintering (UHS) and flash sintering are novel methods for rapid sintering of ceramics, often completed in just a few seconds. Here, we show that both also share two additional features: an abrupt rise in electrical conductivity, which is electronic, and electroluminescence. More fundamentally, both are related to phonon physics where MD calculations have shown that proliferation of phonons at the edge of the Brillouin zone can induce Frenkel pairs without the application of electrical fields. Here, we show that, indeed, heating without the application of electric field, can also induce flash: Rapid heating processes of thin films of an oxide-salt deposited on silk fibers, with a propane torch, are shown to induce electronic conductivity, electroluminescence, and rapid sintering of the oxide. The discussion in this article harkens back to two inventions, more than a century ago, which can now be related to flash and UHS: (i) the Nernst glow lamp circa 1900, made from zirconia, and (ii) the Welsbach mantle, constituted from ceria doped thorium oxide, in the late nineteenth century. Thus, the confluence between high heating rate and electric field induced flash phenomena links the past to the new. The emerging question is how injection of phonons that has been shown to create Frenkels can further induce high electronic conductivity and electroluminescence in oxides. Both electronic conductivity and luminescence are likely related to the generation of electron–hole pairs.     

Pressureless low temperature sintering of nanocrystalline zirconia ceramics via dry powder processing

Pressureless sintering approaches provide a simple avenue to manufacture dense ceramic parts with minimal processing equipment, but current pressureless sintering techniques have yet to demonstrate capabilities of producing dense ceramics while maintaining sub-50 nm grain sizes. Nanocrystalline yttria stablized zirconia ceramics were process from 4 mol% yttria stablized zirconia (4YSZ) nanopowders with a crystallite size of 7.5 nm using dry cold isostatic pressing (CIP) where powders are dried immediately prior to green compact formation and CIP vacuum bagging. It is shown that CIP pressures >75 000 psi (517 MPa) effectively remove pores larger than 100 nm and that pressureless sintering occurs at reduced temperatures for green densities ≥50%. Though the sintering kinetics are shown to be similar to other zirconia nanopowder sintering studies, the small initial crystallize size and reduced sintering temperature allowed densities as high as 97.2%, while retaining a ceramic grain size at or below 40 nm. Produced nanocrystalline 4YSZ ceramics with a grain size of 30.3 nm and a density of 96.3% had Vicker's hardnesses as high as 14.2 GPa and Vicker's indentation fracture resistance of 3.43 MPa·, demonstrating that simple processing approaches can be refined to fabricate nanocrystalline ceramics while maintaining high hardness and indentation fracture resistance.     

A study of the sintering of diatomaceous earth to produce porous ceramic monoliths with bimodal porosity and high strength

Diatomite powder, a naturally occurring porous raw material, was used to fabricate ceramic materials with bimodal porosity and high strength. The effect of the sintering temperature on the density and porosity of dry pressed diatomite green bodies was evaluated using mercury porosimetry and water immersion measurements. It was found that the intrinsic porosity of the diatomite particles with a pore size around 0.2 µm was lost at sintering temperatures above 1200 °C. Maintaining the sintering temperature at around 1000 °C resulted in highly porous materials that also displayed a high compressive strength. Microstructural studies by scanning electron microscopy and energy-dispersive X-ray analysis suggested that the pore collapse was facilitated by the presence of low melting impurities like Na₂O and K₂O.