Dissociative Oxygen Adsorption and Incorporation in Co3O4-Dispersed BaZr0.9Sc0.1O2.95 for PCFC Cathode

The development of air electrodes with superior surface oxygen exchange properties at intermediate temperatures is crucial for improving the efficiency of protonic ceramic fuel cells. This study evaluated the surface exchange properties of Co3O4 dispersed protonic conductors, BaZr0.9Sc0.1O2.95. Although Co3O4 is widely acknowledged as superior dissociative adsorption catalysts, there is still ambiguity regarding the enhancement mechanisms of their surface exchange properties by Co3O4, as well as their optimal composition to achieve high catalytic activity. To overcome these difficulties, this study elucidated the effect of the chemical states and composition of composites on their surface exchange properties by evaluating their chemical states and surface exchange reaction rates with several compositions prepared at different temperature conditions using a vibrating-sample magnetometer and the pulse isotope exchange technique. For samples annealed at a high temperature, it became evident that the surface exchange activity became the most active by adding only 1 vol % Co3O4 and indicated an abrupt decline above this composition despite an increase in the volume of the catalysts. This was attributed to the combined effect of the high dissociative adsorption activity of the Co-containing solid solutions formed at a high temperature and a decrease in oxygen vacancies due to hole compensation. For samples annealed at intermediate temperature, their chemical states remained unchanged from those of the original milled powders, and their surface exchange properties monotonically improved with an increase in the volume of Co3O4. Based on the results, different chemical states of composites derived from different preparation conditions lead to completely different activation behavior of the surface exchange reaction.

Cold Laser Sintering of Medicines: Toward Carbon Neutral Pharmaceutical Printing

Selective laser sintering (SLS) is an emerging three-dimensional (3D) printing technology that uses a laser to fuse powder particles together, which allows the fabrication of personalized solid dosage forms. It possesses great potential for commercial use. However, a major drawback of SLS is the need to heat the powder bed while printing; this leads to high energy consumption (and hence a large carbon footprint), which may hinder its translation to industry. In this study, the concept of cold laser sintering (CLS) is introduced. In CLS, the aim is to sinter particles without heating the powder bed, where the energy from the laser, alone, is sufficient to fuse adjacent particles. The study demonstrated that a laser power above 1.8 W was sufficient to sinter both KollicoatIR and Eudragit L100-55-based formulations at room temperature. The cold sintering printing process was found to reduce carbon emissions by 99% compared to a commercial SLS printer. The CLS printed formulations possessed characteristics comparable to those made with conventional SLS printing, including a porous microstructure, fast disintegration time, and molecular dispersion of the drug. It was also possible to achieve higher drug loadings than was possible with conventional SLS printing. Increasing the laser power from 1.8 to 3.0 W increased the flexural strength of the printed formulations from 0.6 to 1.6 MPa, concomitantly increasing the disintegration time from 5 to over 300 s. CLS appears to offer a new route to laser-sintered pharmaceuticals that minimizes impact on the environment and is fit for purpose in Industry 5.0.

Upcycling plastic waste into fully recyclable composites through cold sintering

Plastics have substantial societal benefits, but their widespread use has led to a critical waste management challenge. While mechanical recycling dominates the reuse of post-consumer plastics, it is limited in efficacy, especially for composites. To address this, we propose a direct reprocessing approach that enables the creation of hybrid, long-lasting, and durable composites from difficult-to-recycle plastics. This approach utilizes cold sintering, a process that consolidates inorganic powders through fractional dissolution and precipitation at temperatures far below conventional sintering; these temperatures are compatible with plastic processing. We show that this process can create inorganic-matrix composites with significant enhancements in tensile strength and toughness over pure gypsum, which is commonly found in construction waste. These composites can be recycled multiple times through direct reprocessing with the addition of only water as a processing promoter. This approach to recycling leads to composites with orders of magnitude lower energy demand, global warming potential, and water demand, when compared against common construction products. Altogether, we demonstrate the potential for cold sintering to integrate waste into high-performance recyclable composites.

Cold-Sintered ZnO Ceramic Composites Co-Doped with Polytetrafluoroethylene and Oxides

Grain boundaries play a significant role in determining the performance of ceramic-based materials. The modulation of interfacial structures provides a promising approach to improve the physicochemical and electrical properties of ceramic materials. In this work, the grain boundary structures of ZnO-based ceramics were manipulated by incorporating polytetrafluoroethylene (PTFE) and metal oxides through the cold sintering process (CSP). It was found that the grain size of ZnO-based ceramics can be effectively reduced from 525.93 nm to 338.08 nm with an addition of PTFE and metal oxides of CoO and Mn2O3. Microstructural results show that most of the PTFE phase and metal oxides were distributed along the grain boundaries, which may lead to the increased grain boundary resistance from 1.59 × 106 ohm of pure ZnO to 6.21 × 1010 ohm of ZnO-based ceramics doped with PTFE and metal oxides, and enhanced Schottky barrier height from 0.32 eV to 0.59 eV. As a result, the breakdown field and nonlinear coefficient of the ZnO-based ceramics were improved to 3555.56 V/mm and 13.55, respectively. Therefore, this work indicates that CSP presents a feasible approach to design functional ceramic composites through the integration of polymer and metal oxides.

Aqueous Cold Sintering of Li-Based Compounds

Aqueous cold sintering of two lithium-based compounds, the electrolyte Li_6.25La₃Zr₂Al_0.25O₁₂ (LLZAO) and cathode material LiCoO₂ (LCO), is reported. For LLZAO, a relative density of ∼87% was achieved, whereas LCO was sintered to ∼95% with 20 wt % LLZAO as a flux/binder. As-cold sintered LLZAO exhibited a low total conductivity (10^-8 S/cm) attributed to an insulating grain boundary blocking layer of Li₂CO₃. The blocking layer was reduced with a post-annealing process or, more effectively, by replacing deionized water with 5 M LiCl during cold sintering to achieve a total conductivity of ∼3 × 10^-5 S/cm (similar to the bulk conductivity). For LCO-LLZAO composites, scanning electron microscopy and X-ray computer tomography indicated a continuous LCO matrix with the LLZAO phase evenly distributed but isolated throughout the ceramics. [001] texturing during cold sintering resulted in an order of magnitude difference in electronic conductivity between directions perpendicular and parallel to the c-axis at room temperature. The electronic conductivity (∼10^-2 S/cm) of cold sintered LCO-LLZAO ceramics at room temperature was comparable to that of single crystals and higher than those synthesized via either conventional sintering or hot pressing.

Cold Sintering of Li6.4La3Zr1.4Ta0.6O12/PEO Composite Solid Electrolytes

Ceramic/polymer composite solid electrolytes integrate the high ionic conductivity of in ceramics and the flexibility of organic polymers. In practice, ceramic/polymer composite solid electrolytes are generally made into thin films rather than sintered into bulk due to processing temperature limitations. In this work, Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO)/polyethylene-oxide (PEO) electrolyte containing bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium salt was successfully fabricated into bulk pellets via the cold sintering process (CSP). Using CSP, above 80% dense composite electrolyte pellets were obtained, and a high Li-ion conductivity of 2.4 × 10⁻⁴ S cm^–1 was achieved at room temperature. This work focuses on the conductivity contributions and microstructural development within the CSP process of composite solid electrolytes. Cold sintering provides an approach for bridging the gap in processing temperatures of ceramics and polymers, thereby enabling high-performance composites for electrochemical systems.

Cold Sintering of PZT 2-2 Composites for High Frequency Ultrasound Transducer Arrays

Medical ultrasound and other devices that require transducer arrays are difficult to manufacture, particularly for high frequency devices (>30 MHz). To enable focusing and beam steering, it is necessary to reduce the center-to-center element spacing to half of the acoustic wavelength. Conventional methodologies prevent co-sintering ceramic–polymer composites due to the low decomposition temperatures of the polymer. Moreover, for ultrasound transducer arrays exceeding 30 MHz, methods such as dice-and-fill cannot provide the dimensional tolerances required. Other techniques in which the ceramic is formed in the green state often fail to retain the required dimensions without distortion on firing the ceramic. This paper explores the use of the cold sintering process to produce dense lead zirconate titanate (PZT) ceramics for application in high frequency transducer arrays. PZT–polymer 2-2 composites were fabricated by cold sintering tape cast PZT with Pb nitrate as a sintering aid and ZnO as the sacrificial layer. PZT beams of 35 μm width with ~5.4 μm kerfs were produced by this technique. The ZnO sacrificial layer was also found to serve as a liquid phase sintering aid that led to grain growth in adjacent PZT. This composite produced resonance frequencies of >17 MHz.

Thermal crystallization of amorphous calcium phosphate combined with citrate and fluoride doping: a novel route to produce hydroxyapatite bioceramics

Amorphous calcium phosphate (ACP) is a material of high interest for dentistry, orthopedics, and other biomedical sectors. Being intrinsically metastable, the process of transformation of ACP into a crystalline phase upon heating is of high relevance for the development of innovative bioceramics. Here we have first studied the thermal behavior of a citrate-stabilized ACP (Cit-ACP) also doped with fluoride ions (Cit-FACP) prepared at three different nominal Cit/Ca ratios (i.e. 4, 2, 1) by differential thermal analysis. Next, the physico-chemical features of the crystalline products as well as the in vitro cell response to the materials were investigated. A citrate and fluoride free ACP sample was also tested as the blank. We have found that the activation energy of crystallization of Cit-(F)ACP samples is lower in comparison to the blank ACP and this is influenced by the nominal Cit/Ca molar ratio. Interestingly, we have discovered that the thermal treatment of Cit-(F)ACP at 800 °C yields hydroxyapatite (HA) or fluorapatite (FHA) as the main products differently from blank ACP that, like most of the ACPs reported in the literature, yields β-tricalcium phosphate. This was attributed to the Ca/P ratio of Cit-(F)ACP, which is similar to HA. A study of the crystalline products has revealed that all the (F)HA samples were non-cytotoxic, and retained carbonate ions in the crystal structure despite the heat treatment that should have induced decarbonation. The morphology of the products is influenced by the nominal Cit/Ca ratio and the presence of fluoride, ranging from spherical nanoparticles to micrometric hexagonal rods. Overall, our results prove that the thermal crystallization of Cit-(F)ACP is markedly different from classic ACP based materials and the thermal treatment of Cit-(F)ACP represents an attractive route for producing pure bioactive HA ceramics.

Precursor Design Strategies for the Low-Temperature Synthesis of Functional Oxides: It's All in the Chemistry

Solution-based (multi)metal oxide synthesis has been carried out employing a large diversity of precursor routes. The selection of an appropriate synthesis strategy is frequently dictated by the resulting material properties, although this choice should also be based on green chemistry principles, atom economy considerations and energy efficiency. In order to limit the required energy budget to convert the chemical precursor to the target oxide material, various approaches were recently reported. This Review summarizes some frequently encountered low-temperature routes, critically assessing their application window and advantages. More specifically, auto-combustion synthesis, UV-assisted decomposition routes, sol-gel network adjustments and precursor complex design concepts are discussed. It is expected that this toolbox of low-temperature strategies may assist further progress in the field, stimulating novel applications, such as flexible electronics or organic-oxide hybrid materials, which are very sensitive to the temperature requirements.

Cold sintering of YBa2Cu3O7-δ

Cold sintering is a sintering technique which enables ceramic powders to be densified at greatly reduced temperatures compared to traditional solid state techniques, which often require temperatures in excess of 1000 °C. These temperatures often preclude the exploitation of size or orientational effects in ceramics as these are lost during heating. One such effect is the orientation of the crystallographic c axis in YBa₂Cu₃O_7−δ (YBCO) which can be controlled through applied pressure. This effect is of interest for increasing critical current density which is highly dependent on the orientation of the a–b (CuO₂) planes within the ceramic. Using cold sintering, we demonstrate that dense YBCO can be created at 180 °C (vs. 1000 °C using solid state) and demonstrate that the likely sintering mechanism is mediated by the cracking which occurs in YBCO when exposed to water. In addition, the ceramics produced show and retain the orientational effect, representing a unique opportunity to study the effect on critical current density. We show that the intergranular critical current when the a–b planes are parallel to the applied field is around 15% higher than when perpendicular.

Cold sintered superconducting YBa₂Cu₃O_7−δ densified at 180 °C shows enhanced critical current densities by exploiting grain alignment created during pressing.