Cold sintering process for fabrication of a superhydrophobic ZnO-polytetrafluoroethylene(PTFE)ceramic composite

Composite coatings or films with polytetrafluoroethylene(PTFE)are typically utilized to offer superhydrophobic surfaces.However,the superhydrophobic surfaces usually have limited durability and require complicated fabrication methods.Herein,we report the successful integration of PTFE with ZnO ceramics to achieve superhydrophobicity via a one-step sintering method,cold sintering process(CSP),at 300℃.(1–x)ZnO–x PTFE ceramic composites with x ranging from 0 to 70 vol%are densified with relative density of over 97%.Micro/nano-scale PTFE polymer is dispersed among ZnO grains forming polymer grain boundary phases,which modulate surface morphology and surface energy of the ZnO–PTFE ceramic composites.For the 60 vol%ZnO–40 vol%PTFE ceramic composite,superhydrophobic properties are optimized with static water contact angles(WCAs)and sliding angles(SAs)of 162°and 7°,respectively.After abrading into various thicknesses(2.52,2.26,and 1.99 mm)and contaminating with graphite powders on the surface,WCA and SA are still maintained with a high level of 157°–160°and 7°–9.3°,respectively.This work indicates that CSP provides a promising pathway to integrate polymers with ceramics to realize stable superhydrophobicity.

Fabrication of form stable NaCl-Al2O3 composite for thermal energy storage by cold sintering process

A form stable NaCl-Al2O3(50-50 wt-%)composite material for high temperature thermal energy storage was fabricated by cold sintering process,a process recently applied to the densification of ceramics at low temperature 300℃ under uniaxial pressure in the presence of small amount o f transient liquid.The fabricated composite achieved as high as 98.65% of the theoretical density.The NaCl-Al2O3 composite also retained the chloride salt without leakage after 30 heating-cooling cycles between 750℃-850℃ together with a holding period o f 24h at 850℃.X-ray diffraction measurements indicated congruent solubility o f the alumina in chloride salt,excellent compatibility o f NaCl with Al2O3,and chemical stability at high temperature.Structural analysis by scanning electron microscope also showed limited grain growth,high density,uniform NaCl distribution and clear faceted composite structure without inter-diffusion.The latent heat storage density o f 252.5J/g was obtained from simultaneous thermal analysis.Fracture strength test showed high sintered strength around 5 GPa after 50 min.The composite was found to have fair mass losses due to volatilization.Overall,cold sintering process has the potential to be an efficient,safe and cost-effective strategy for the fabrication of high temperature thermal energy storage materials.

Combining cold sintering and Bi_(2)O_(3)-Activated liquid-phase sintering to fabricate high-conductivity Mg-doped NASICON at reduced temperatures

The cold sintering process(CSP)and Bi_(2)O_(3)-activated liquid-phase sintering(LPS)are combined to densify Mg-doped NASICON(Na_(3.256)Mg_(0.128)Zr_(1.872)Si_(2)PO_(12))to achieve high densities and conductivities at reduced temperatures.As an example,a cold-sintered specimen with the addition of 1.1wt%Bi_(2)O_(3)sintering additive achieved a high conductivity of 0.91 mS/cm(with~96%relative density)after annealing at 1000℃;this conductivity is>70%higher than that of a cold-sintered specimen without adding the Bi_(2)O_(3)sintering additive,and it is>700%of the conductivity of a dry-pressed counterpart with the same amount of Bi_(2)O_(3)added,all of which are subjected to the same heating profile.The highest conductivity achieved in this study via combining CSP and Bi_(2)O_(3)-activated LSP is>1.5 mS/cm.This study suggests an opportunity to combine the new CSP with the traditional LPS to sinter solid electrolytes to achieve high densities and conductivities at reduced temperatures.This combined CSP-LPS approach can be extended to a broad range of other materials to fabricate the“thermally fragile”solid electrolytes or solid-state battery systems,where reducing the processing temperature is often desirable.

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“tcold sintering process(CSP)”has been explored by Randall et al.CSP enables densification of ceramics at ultra-low temperature(<300℃)with the assistance o f 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.

Realizing translucency in aluminosilicate glass at ultralow temperature via cold sintering process

Glass with high visible-light transparency is widely considered as the most important optical material,which typically requires a processing temperature higher than 1000℃.Here,we report a translucent aluminosilicate glass that can be prepared by cold sintering process(CSP)at merely 300℃.After eliminating structural pores in hexagonal faujasite(EMT)-type zeolite by heat treatment,the obtained highly active nanoparticles are consolidated to have nearly full density by adding NaOH solution as liquid aids.However,direct densification of EMT powder cannot remove the structural pores of zeolite completely,leading to an opaque compact after the CSP.It is proved that the chemical reaction between the NaOH-and zeolite-derived powders is highly beneficial to dissolution–precipitation process during sintering,leading to the ultra-low activation energy of 27.13 kJ/mol.Although the addition of 5 M NaOH solution greatly promotes the densification via the reaction with aluminosilicate powder,lower or higher concentration of solvent can deteriorate the transmittance of glass.Additionally,the CSP-prepared glass exhibits a Vickers hardness of 4.3 GPa,reaching 60%of the reported value for spark plasma sintering(SPS)-prepared sample.

Cold Hydrostatic Sintering:From shaping to 3D printing

We developed a novel consolidation technique,Cold Hydrostatic Sintering(CHS),which allows near full densification of silica.The technique is inspired by biosilicification and geological formation of siliceous rocks.Unlike established cold sintering method which is based on uniaxial pressure,CHS employs an isostatic pressure to enable room temperature consolidation of bulks having a complex threedimensional shape.The resulting material is transparent(in line transmittance exceeding 70% in the visible range)and amorphous.After drying,the Vickers hardness was as high 1.4 GPa which half of materials consolidated at 1200℃ and it is the highest among all materials processed at room temperature.The CHS method,because of its simplicity,might be suitable for broad range of applications including 3D printing,mould forming and preparation of multi-layered devices.Because of the absence of the firing step,CHS could be directly integrated in the manufacturing of a wide range of hybrid(organic/inorganic)materials for functional and biological applications.

Room-temperature-densified H_(3)BO_(3) microwave dielectric ceramics with ultra-low permittivity and ultra-high Qf value

With the rapid development of mobile communication technology towards 5G and 6G,the microwave dielectric materials with ultra-low permittivity and ultra-high Qf value are urgently demanded.Here,the excellent microwave dielectric properties are reported in H3BO3 ceramics with the molecular crystal structure,whose permittivity(2.84)and density(1.46 g/cm^(3))are record-low among the low-loss ceramics.The ultra-high Qf value of 146,000 GHz(or the ultra-low dielectric loss of 1.03×10^(-4) at 15 GHz)is also distinguished.Besides,the H_(3)BO_(3) ceramics can be densified at room temperature by a simple cold sintering process in a short time of 10 min,and this brings many advantages for the integration with microwave circuits.The large molecule volume originating from the molecular crystal structure and the low dielectric polarizabilities of H^(+) and B^(3+) are responsible for the ultra-low permittivity of H_(3)BO_(3) ceramics,and more microwave dielectric materials with ultra-low permittivity and ultra-high Qf value are expected to be explored in the molecular crystals.