One-step cold sintering process towards translucent BaF2 ceramics

BaF₂ ceramics were prepared using a one-step cold sintering process with an ultra-low sintering temperature of 150 °C and uniaxial pressures ranging from 450 to 900 MPa. The relative density and microstructure improved steadily with the increasing pressure, and a fully densified microstructure with a relative density of 97.2% was achieved at 900 MPa. For BaF₂ ceramics with a thickness of 1 mm, the optimum in-line transmittance in the visible light region (58.5%) was achieved at a wavelength of 720 nm, and the maximum value (65.3%) was obtained at 1864 nm. The permittivity of the ceramics increased gradually from 6.18 to 7.09 with increasing pressure, and the dielectric loss was optimized from 0.01 to 0.003. Additionally, the mechanical properties improved continuously with the increasing pressure, and the optimal compressive strength (257 MPa), hardness (2.01 GPa), and Young's modulus (54.8 GPa) were achieved when cold sintered at 900 MPa.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.     

Dense gypsum ceramics prepared by room-temperature cold sintering with greatly improved mechanical properties

Dense gypsum (CaSO₄·2H₂O) ceramics were successfully fabricated by a simple room-temperature cold sintering process with 5 wt% water. The relative density of gypsum ceramics increased from 89.6% to 96.8% with increasing the applied uniaxial pressure from 100 to 400 MPa during cold sintering. The relative density changed slightly for higher pressure, and microcracks were observed as well as abnormal grain growth. Both the compressive and flexural strengths reached the peaks at 98.5 MPa and 26.5 MPa for the uniaxial pressure of 400 MPa, which were improved by 2.6 and 2.0 times, respectively comparing with the bulk gypsum prepared by traditional method from α-plaster. Furthermore, the dry-pressed gypsum compacts were very fragile, and had relative densities 5–12 % lower than the cold-sintered ceramics, indicating that the slight solubility of gypsum in water (0.2 g/100 g) played a critical role in the densification, microstructural evolution and greatly improved mechanical properties of cold-sintered gypsum ceramics.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Preparation of Na0.5Bi0.5TiO3 ceramics by hydrothermal-assisted cold sintering

A recently proposed novel technique, termed “cold sintering process” (CSP), can provide dense ceramic solids at remarkably low temperatures around 180?°C. In a recent work, we successfully obtained dense Na_0.5Bi_0.5TiO₃ ceramics by this method. Bismuth titanate sodium nanoparticles were prepared as the raw material powder by the hydrothermal synthesis route. A hydrothermal precursor solution was used as the transient solvent for cold sintering. Under the combined action of pressure and temperature, the Na_0.5Bi_0.5TiO₃ green body was densified by dissolution-precipitation, and a preliminary densified ceramic sheet was obtained. The amorphous phase in the ceramic sheet was then transformed into a crystalline phase by annealing. Finally, densified Na_0.5Bi_0.5TiO₃ ceramic sheets were obtained, with density of up to 99%, relative permittivity of 681, and dielectric loss of 0.08 at 10?kHz and room temperature. The piezoelectric coefficient d₃₃ of the sample was 52.5?pC/N. The properties of the prepared ceramics were comparable to those of the conventional sintered ceramics.     

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.     

Grain size effect on microwave dielectric properties of Na2WO4 ceramics prepared by cold sintering process

In this work, cold sintering was adopted to prepare Na₂WO₄ ceramics with different grain sizes ranging from 0.632 μm to 17.825 μm. Their microstructures, complex impedance, and microwave dielectric properties were studied in-depth. It was found that samples with relative densities higher than 92% can be successfully synthesized by cold sintering process at a low temperature of 240 °C. However, their electrical properties have strong dependence on the grain size. Specifically, the resistance of grain boundaries decreases dramatically with the increase of grain sizes, while the quality factor has a positive correlation with the grain sizes of Na₂WO₄ ceramics. Excellent microwave dielectric properties, including permittivity = 5.80, Q × f = 22,000 GHz, and TCF = −70 ppm/°C, are obtained for Na₂WO₄ ceramics with a grain size of 4.477 μm prepared by cold sintering process.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Synthesis of transparent YIG ceramics by pressureless sintering

Transparent YIG (Y₃Fe₅O₁₂) ceramics are successfully synthesized by reactive sintering at normal pressure using γ-Fe₂O₃ and Y₂O₃ as starting materials. The grain size of the sintered YIG ceramics is ca. 10–15 µm. Residual pores are not observed on the surface of sample, but numerous residual pores are observed by infrared transmission microscopy. In-line transmittance of a commercially available high-quality YIG single crystal (thickness 1 mm) fabricated by the floating zone method is 75 % in the near to mid-infrared region, whereas the sample produced in this study shows an in-line transmittance of 71 % in the wavelength range above 1.5 µm.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Scaling up the cold sintering process of ceramics

The cold sintering process (CSP) densifies ceramics below 300 °C by utilizing a transient phase and applied pressure. Although CSP has been employed for densifying a variety of functional systems, their structural integrity does not always reach that of conventionally sintered parts. On the example of ZnO, this study aims to eliminate processing-induced defects that compromise the strength of cold sintered materials. Ultrasonic evaluation was employed for nondestructive detection of flaws prior to mechanical testing. Load transfer misalignments and fast heating rates were found as major sources of defects, impairing the mechanical strength. Based on these findings, multiple disc-shaped samples (13 mm diameter and ∼1.3 mm thickness) were cold sintered simultaneously using precisely aligned punches and slow heating rates. The obtained homogeneous densification, high relative density (>97%) and relatively high strength (∼120 MPa), i.e. two times superior to previously reported values, demonstrates the feasibility of scaling up the CSP towards industrial implementation.     

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.     

Nano-to-macroporous TiO2 (anatase) by cold sintering process

Cold Sintering Process (CSP) was applied on commercial nanopowders to produce nanostructured TiO₂ anatase with nano-to-macro porosity. Nanoporous TiO₂ based materials were obtained by applying CSP at 150 °C and pressures up to 500 MPa on three TiO₂ nanopowders with different specific surface area (s.s.a. = 50, 90 and 370 m²/g), using water as transient aqueous environment. Although TiO₂ is insoluble in water, a density of 68% and s.s.a. = 117 m²/g were achieved from the powder with the highest specific surface area. A post annealing process at 500 °C increased the density up to 73% with a s.s.a. = 59 m²/g, and the crystallites dimensions passed from 110 Å in the powder to 130 Å in CSP material and 172 Å after post annealing. Finally, macroporosity was produced by using thermoplastic polymer beads as sacrificial templates within TiO₂ nanopowder during CSP, followed by a debonding at 500 °C.     

Highly dense 0.3CaCeNbWO8-0.7LaMnO3 composite ceramics fabricated by cold sintering process

The cold sintering process (CSP) has been used for fabricating functional ceramics at a low sintering temperature. In this study, highly dense 0.3CaCeNbWO₈-0.7LaMnO₃ composite ceramics have been successfully fabricated by CSP. The phase structure, microstructure, and electrical properties of composite ceramics have been investigated. The composite ceramic is mainly composed of a tetragonal CaCeNbWO₈ phase with scheelite structure and an orthorhombic LaMnO₃ phase with perovskite structure. The relative density of composite ceramic is 94.5%, and is higher than that of single phase ceramic. The resistivity of composite ceramic exhibits negative temperature coefficient characteristics, with an aging coefficient less than 2%. Such a sintering methodology is of great significance, since it provides a feasible idea for preparing composite ceramics.     

The effect of yttrium nitrate addition on the densification behaviour of Y2O3 ceramics during the cold sintering process

The densification behaviour and phase development of Y₂O₃ ceramics were investigated as a function of yttrium nitrate (Y(NO₃)₃·6H₂O) solution addition during the cold sintering process at 200 °C. Second phases such as Y₄O(OH)₉NO₃ and Y(OH)₃ were observed after the cold sintering process. The amount of Y₄O(OH)₉NO₃ increased with increasing amount of yttrium nitrate, while the amount of Y(OH)₃ decreased. The second phases were transformed to fine sized Y₂O₃ (∼30 nm) particles smaller than those of the raw powder (<400 μm) by sintering at 600 °C. The fine sized Y₂O₃ particles were located in the voids between the larger Y₂O₃ particles, thus increasing the packing density and enhancing the densification of the Y₂O₃ ceramics after the final sintering process.     

Mechanism of densification of calcium carbonate by cold sintering process

In general, carbonates cannot be easily hardened by the conventional ceramic sintering process due to their thermal decomposition during heating. However, when the cold sintering process (CSP) is selected, carbonates can be hardened at lower temperatures. It has been demonstrated that calcium carbonate can be hardened by CSP, but the detailed densification mechanisms of cold sintering at various temperatures have not been fully clarified. In this study, the vaterite phase of calcium carbonate was selected as the starting material. As the cold sintering temperature for calcium carbonate powder increased, the bulk density of the hardened calcium carbonate body increased. The compressive strength was maximized when cold sintered at 80 °C due to the balance between the solubility of calcium carbonate and the reactivity of cold sintering. Almost no crystal phase transformation from vaterite to calcite occurred during cold sintering, and reprecipitation of the vaterite phase though dissolution-precipitation densified the body.     

Characterisation of transparent hydroxyapatite nanoceramics prepared by spark plasma sintering

Abstract

Hydroxyapatites (HA) have good biocompatibility and are used as bioceramics for artificial bones. The application areas can be extended further if transparent and dense HA ceramics can be prepared. The preparation of dense and transparent HA ceramics were attempted using a spark plasma sintering technique at relatively low temperatures (900–1000°C) under a pressure of 80 MPa for a short time of 10 min. The sintered body was almost fully dense (>99%) and transparent with a transmittance >70%. The microstructure was examined by SEM, TEM, STEM and EDX. The HA ceramics exhibited a microstructure with grains, approximately 100 nm size. A number of intragranular voids, 5–10 nm in size, with flat boundaries were also observed. The voids were believed to have been generated by evaporation during spark plasma sintering and were stabilised during cooling. The grain boundaries were clean without a glassy phase.     

Influence of process parameters in gel casting of a pure yttria nanopowder to fabricate transparent ceramics

The goal of this research is to fabricate pure transparent yttria ceramics through gel casting and vacuum sintering. A specific processing method has been used and optimized for this purpose. A pure yttria nanopowder was synthesized as the starting material to produce pure transparent ceramics through a low-temperature sintering process. It was attempted to minimize the undesirable nanopowder hydration by using the as-synthesized yttria nanopowder and a rapid deagglomeration and slurry preparation process. The synthesized nanopowders were deagglomerated to enhance the efficiency of both powder shaping and sintering stages. Carrageenan was used as the gelling agent because it is a low-cost and abundant material, and because the temperature is the only catalyst needed for its gelation; therefore, it is possible to control its gelation to obtain high-density and pure optical ceramics. The effect of the deagglomeration method and the processing parameters, including the amounts of dispersant, gelling agent, solid loading, pH, and deagglomeration time, on the rheology of slurry, density, and microstructure of the obtained green yttria ceramics was examined and optimized in order to obtain high solid loading nanoyttria suspensions of 38 vol%, which is more than those obtained in many of the previous investigations. The precise gelling temperature and time were measured, and green gel cast ceramics with a density of 63 % of the theoretical density were produced. A rapid deagglomeration and slurry preparation method was used instead of using a conventional planetary ball-milling approach to minimize the risk of the hydrolysis of yttria nanopowder. No sintering aid was necessary, and transparent yttria ceramics with 99 % of the theoretical density were produced after vacuum-furnace sintering at 10^-2 mbar and 1715 °C.     

Zeolite ceramics with ordered microporous structure and high crystallinity prepared by cold sintering process

4A [Na₉₆(Al₉₆Si₉₆O₃₈₄)⋅216H₂O] and 13X [Na₂(Al₂Si_3.3O_10.6)⋅7H₂O] zeolite ceramics were prepared by cold sintering processes, and the physical properties were investigated together with the structure characterization. There were four primary parameters controlling the preparation process: content of NaOH, pressure, temperature, and holding time. Zeolite ceramics with the idea ordered microporous structure and high crystallinity were obtained at 393 K under 450~550 MPa with a holding time of 5 min from the solution with 20 wt% NaOH. A high compressive strength up to 60 MPa and low Young's modulus down to 76 MPa were achieved in 13X zeolite ceramics, and the present ceramics might provide potential applications in damping materials and thermal insulating materials.