Freeze Casting: From Low‐Dimensional Building Blocks to Aligned Porous Structures—A Review of Novel Materials, Methods, and Applications

Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well‐controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze‐casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze‐casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro‐ and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze‐cast geometries—beads, fibers, films, complex macrostructures, and nacre‐mimetic composites—are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze‐casting techniques and low‐dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.TU Berlin, Open-Access-Mittel - 202

Geopolymer composites and their applications in stress wave mitigation

In recent years, environmentally friendly materials have been the focus as alternatives to ordinary Portland cement (OPC), which generates about 6% of the total carbon dioxide emission in the world today and contributes to global warming. An alternative material to cement is aluminosilicate inorganic polymer, also known as “geopolymer” which emits about 80% less CO2 than OPC. Therefore, the chemical, mechanical, thermal and electrical properties of geopolymers have been of interest for the past two decades. The processes and microstructures of potassium-based geopolymer (K2O ∙ Al2O3 ∙ 4SiO2 ∙ 11H2O) have been studied by many researchers with various techniques. However, the brittleness and relatively lower strength limited the use of geopolymer in certain applications. Therefore, short carbon fibers (60 and 100 μm) have been introduced to reinforce the mechanical properties of potassium based geopolymer. The proper mixing and drying conditions of carbon fiber reinforced potassium geopolymer (Cf KGP) were determined since the mechanical properties varied in wide range depending on the drying conditions. Various static mechanical tests (flexure, uniaxial compression, hardness, toughness and biaxial tensile tests) and statistical analyses of brittle fractures (Weibull distribution) have been performed to investigate the optimal mechanical strengths of Cf KGP composites. In addition to the static measurements, the Young’s and shear moduli of Cf KGP have been measured by dynamic methods such as impulse excitation (IE) and resonant ultrasound spectroscopy (RUS) and compared with various theoretical models. Graphene nanoplatelets have high mechanical, electrical and thermal properties that can significantly improve the desired properties of composites at even low contents. 1, 2 and 3 wt% of graphene nanoplatelet-reinforced, potassium geopolymers (GNP KGP) were prepared and their microstructures were investigated by SEM, XRD and Raman spectroscopy. The mechanical properties such as flexure strength, Weibull modulus, Vickers hardness and Young’s modulus were measured by four-point flexure, microindentation and impulse excitation testing. In addition to mechanical properties, the electrical and thermal properties of GNP KGP were investigated by measuring electrical resistances and thermal conductivities. Moreover, silicon functionalized graphene nanoplatelets (sGNP) were prepared in order to enhance interfacial bonding between GNP and the geopolymer matrix. The various mechanical properties of sGNP KGP were measured and compared with GNP KGP, in order to investigate the effect of silicon functionalization. Due to their similarity with concrete, geopolymers have been mainly used as structural materials. With documented chemical and mechanical properties of geopolymers, their use could be expanded to applications in many disciplines. In this work, geopolymers were used as granular media and displayed interesting dynamic behavior for stress wave mitigation and as acoustic metamaterials. The fabrication of geopolymer beads was achieved by simple injection into a Polydimethylsiloxane (PDMS) polymer mold. Based on the mechanical properties of geopolymers, the dynamic responses of homogeneous and dimer chains were theoretically predicted and experimentally investigated under impulse excitation. By substituting metal beads for geopolymer beads which are 6 times lighter, a linear array of lightweight granular media was created to mitigate the effects of a stress wave

Pt nanowire growth induced by Pt nanoparticles in application of the cathodes for Polymer Electrolyte Membrane Fuel Cells (PEMFCs)

Improving cathode performance at a lower Pt loading is critical in commercial PEMFC applications. A novel Pt nanowire (Pt-NW) cathode was developed by in-situ growth of Pt nanowires in carbon matrix consisting Pt nanoparticles (Pt-NPs). Characterization of TEM and XRD shows that the pre-existing Pt-NPs from Pt/C affect Pt-NW morphology and crystallinity and Pt profile crossing the matrix thickness. The cathode with Pt-NP loading of 0.005 mgPt-NP cm−2 and total cathode Pt loading of 0.205 mgPt cm−2 has the specific current density of 89.56 A gPt−1 at 0.9 V, which is about 110% higher than that of 42.58 A gPt−1 of the commercial gas diffusion layer (GDE) with Pt loading of 0.40 mg cm−2. When cell voltage is below 0.48 V, the Pt-NW cathode has better performance than the commercial GDE. It is believed that the excellent performance of the Pt-NW cathode is attributed to Pt-NP induction, therefore producing unique Pt-NW structure and efficient Pt utilization. A Pt-NW growth mechanism was proposed that Pt precursor diffuses into the matrix consisting of pre-existent Pt-NPs by concentration driving, and Pt-NPs provide priority sites for platinum depositing at early stage and facilitate Pt-NW growth

Interdisciplinary research on the nature and properties of ceramic materials

The advancement of material performance and design methodology as related to brittle materials was investigated. The processing and properties of ceramic materials as related to design requirements was also studied

Multiscale modelling on material properties and mechanical behaviours of graphene reinforced polymer nanocomposites

Graphene possesses many superior properties, such as ultrahigh mechanical stiffness and strength, exceptional thermal and electrical conductivities as well as excellent optical properties. In many of the envisioned applications, graphene or its derivatives are incorporated into the polymer matrix to form graphene based nanocomposite systems in which the polymer matrices can work synergically with graphene fillers as functional components providing supports and protections to the embedded graphene. Two types of additive manufacturing (AM) techniques have been developed for the graphene reinforced polymer nanocomposites. One is the layer-by-layer (LbL) assembly technique which is a versatile process and capable of manipulating material composition and architectures at the nanoscale. The other AM technique is conventionally known as the extrusion-based 3D printing. This research focuses on the computational method and numerical modelling of material properties and mechanical behaviours of graphene-based polymeric nanocomposites. A hierarchical multiscale analysis approach is adopted and tailored specifically for the graphene-based polymeric nanocomposites fabricated using the AM techniques. Some of the important material characteristics at nano- and meso-scales such as molecular interactions and microstructure morphologies are simulated and discussed in details. The nonlinear mechanical behaviours e.g., bending, post-buckling and vibration of functionally graded graphene reinforced nanocomposite (FG-GRC) beams fabricated by LbL technique are subsequently carried out. Numerical analysis with various macroscaled parameters such as functionally graded patterns, temperature rises as well as foundation stiffnesses are presented and discussed. This study is crucial for engineering applications to evaluate mechanical behaviours of such nanocomposite materials with optimal arrangements and manufactured by using these two above-mentioned methods

Graphene Quantum Dot-Based Electrochemical Immunosensors for Biomedical Applications

In the area of biomedicine, research for designing electrochemical sensors has evolved over the past decade, since it is crucial to selectively quantify biomarkers or pathogens in clinical samples for the efficacious diagnosis and/or treatment of various diseases. To fulfil the demand of rapid, specific, economic, and easy detection of such biomolecules in ultralow amounts, numerous nanomaterials have been explored to effectively enhance the sensitivity, selectivity, and reproducibility of immunosensors. Graphene quantum dots (GQDs) have garnered tremendous attention in immunosensor development, owing to their special attributes such as large surface area, excellent biocompatibility, quantum confinement, edge effects, and abundant sites for chemical modification. Besides these distinct features, GQDs acquire peroxidase (POD)-mimicking electro-catalytic activity, and hence, they can replace horseradish peroxidase (HRP)-based systems to conduct facile, quick, and inexpensive label-free immunoassays. The chief motive of this review article is to summarize and focus on the recent advances in GQD-based electrochemical immunosensors for the early and rapid detection of cancer, cardiovascular disorders, and pathogenic diseases. Moreover, the underlying principles of electrochemical immunosensing techniques are also highlighted. These GQD immunosensors are ubiquitous in biomedical diagnosis and conducive for miniaturization, encouraging low-cost disease diagnostics in developing nations using point-of-care testing (POCT) and similar allusive techniques.TU Berlin, Open-Access-Mittel - 201

REMOTE CONTROLLED HYDROGEL NANOCOMPOSITES: SYNTHESIS, CHARACTERIZATION, AND APPLICATIONS

There is significant interest in the development of hydrogels and hydrogel nanocomposites for a variety of biomedical applications including drug delivery, sensors and actuators, and hyperthermia cancer treatment. The incorporation of nanoparticulates into a hydrogel matrix can result in unique material characteristics such as enhanced mechanical properties, swelling response, and capability of remote controlled (RC) actuation. In this dissertation, the development of hydrogel nanocomposites containing magnetic nanoparticles/carbon nanotubes, actuation with remote stimulus, and some of their applications are highlighted. The primary hydrogel nanocomposite systems were synthesized by incorporation of magnetic nanoparticles into temperature responsive N-isopropylacrylamide (NIPAAm) matrices. Various nanocomposite properties were characterized such as temperature responsive swelling, RC heating with a 300 kHz alternating magnetic field (AMF), and resultant collapse. The nanoparticle loadings and hydrogel composition were tailored to obtain a nanocomposite system that exhibited significant change in its volume when exposed to AMF. The nanocomposites were loaded with model drugs of varying molecular weights, and RC pulsatile release was demonstrated. A microfluidic device was fabricated using the low temperature co-fired ceramic (LTCC) processing technique. A magnetic nanocomposite of PNIPAAm was placed as a valve in one of the channels. The remote controlled liquid flow with AMF was observed for multiple on-off cycles, and the kinetics of the RC valve were quantified by pressure measurements. The addition of multi-walled carbon nanotubes (MWCNTs) in NIPAAm matrices was also explored for the possibility of enhancement in mechanical properties and achieving remote heating capabilities. The application of a radiofrequency (RF) field of 13.56 MHz resulted in the remote heating of the nanocomposites. The intensity of the resultant heating was dependent on the MWCNT loadings. In order to further understand the RC actuation phenomenon, a semi-empirical heat transfer model was developed for heating of a nanocomposite disc in air. The model successfully predicted the temperature rise as well as equilibrium temperatures for different hydrogel dimensions, swelling properties, nanoparticles loadings, and AMF amplitude. COMSOL was used to simulate temperature rise of the hydrogel nanocomposite and the surrounding tissue for hyperthermia cancer treatment application