External Field Assisted Freeze Casting

External field-assisted techniques for polymer matrix composites with electromagnetic interference shielding

The rapid development of mobile devices has greatly improved the lives of people, but they have also caused problems with electromagnetic interference (EMI) and information security. Therefore, there is an urgent need to develop high performance EMI shielding materials to suppress electromagnetic radiation and prevent information leakage. Some reports point out that the self-orientation behavior of fillers under external forces contributes to the improvement of EMI shielding performance. So how to construct an effective filler orientation structure in the polymer matrix is becoming a hot topic in the research of EMI shielding materials. In view of the fact that there are few reports on the preparation of polymer matrix EMI shielding composites by external field induction, from this perspective, we first highly focus on strategies for the construction of conductive networks within composites based on external field induction. Subsequently, the research progress on the preparation of polymer matrix EMI shielding composites by inducing the orientation of inorganic fillers through external fields, including temperature, electrostatic, gravity, mechanical force and magnetic fields, is organized and sorted out in detail. Notably, the particular response relationship between the unique composite structures prepared by external field induction and the incident electromagnetic waves is further dissected. Finally, the key scientific problems that need to be solved in the preparation of polymer matrix EMI shielding composites assisted by external fields are proposed. The approach discussed and the strategies proposed are expected to provide some guidance for the innovative design of high-performance polymer matrix EMI shielding composites.

From Forces to Assemblies: van der Waals Forces-Driven Assemblies in Anisotropic Quasi-2D Graphene and Quasi-1D Nanocellulose Heterointerfaces towards Quasi-3D Nanoarchitecture

In the pursuit of advanced functional materials, the role of low-dimensional van der Waals (vdW) heterointerfaces has recently ignited noteworthy scientific interest, particularly in assemblies that incorporate quasi-2D graphene and quasi-1D nanocellulose derivatives. The growing interest predominantly stems from the potential to fabricate distinct genres of quasi-2D/1D nanoarchitecture governed by vdW forces. Despite the possibilities, the inherent properties of these nanoscale entities are limited by in-plane covalent bonding and the existence of dangling π-bonds, constraints that inhibit emergent behavior at heterointerfaces. An innovative response to these limitations proposes a mechanism that binds multilayered quasi-2D nanosheets with quasi-1D nanochains, capitalizing on out-of-plane non-covalent interactions. The approach facilitates the generation of dangling bond-free iso-surfaces and promotes the functionalization of multilayered materials with exceptional properties. However, a gap still persists in understanding transition and alignment mechanisms in disordered multilayered structures, despite the extensive exploration of monolayer and asymmetric bilayer arrangements. In this perspective, we comprehensively review the sophisticated aspects of multidimensional vdW heterointerfaces composed of quasi-2D/1D graphene and nanocellulose derivatives. Further, we discuss the profound impacts of anisotropy nature and geometric configurations, including in-plane and out-of-plane dynamics on multiscale vdW heterointerfaces. Ultimately, we shed light on the emerging prospects and challenges linked to constructing advanced functional materials in the burgeoning domain of quasi-3D nanoarchitecture.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Bioinspired Techniques in Freeze Casting: A Survey of Processes, Current Advances, and Future Directions

Freeze casting, popularly known as ice templating or freeze gelation, is a mechanical method to fabricate scaffolds of desirable properties and materials. Aerospace engineering, the healthcare sector, manufacturing department, and automotive industries are the different fields where freeze casting has been used. Bioinspiration refers to the translation of biological systems into new and innovative creations. Bioinspired materials are extensively used in freeze casting methods such as ceramide, spines of porcupine fish, and collagen. Due to the tunable properties and production of complex structures with ease, biomaterials have found numerous applications in the ice templating method. This review rigorously explains the freeze casting process and the effect of thermal conductivity, stress, and electrostatic repulsion on the porous materials. Also, we have discussed the different biomaterial polymers used in freeze casting along with different methods involved.

Tailoring the Microstructure of Lamellar Ti3C2Tx MXene Aerogel by Compressive Straining

Aerogels are attracting increasing interest due to their functional properties, such as lightweight and high porosity, which make them promising materials for energy storage and advanced composites. Compressive deformation allows the nano- and microstructure of lamellar freeze-cast aerogels to be tailored toward the aforementioned applications, where a 3D nanostructure of closely spaced, aligned sheets is desired. Quantitatively characterizing their microstructural evolution during compression is needed to allow optimization of manufacturing, understand in-service structural changes, and determine how aerogel structure relates to functional properties. Herein we have developed methods to quantitatively analyze lamellar aerogel domains, sheet spacing, and sheet orientation in 3D and to track their evolution as a function of increasing compression through synchrotron phase contrast X-ray microcomputed tomography (μCT). The as-cast domains are predominantly aligned with the freezing direction with random orientation in the orthogonal plane. Generally the sheets rotate toward flat and their spacing narrows progressively with increasing compression with negligible lateral strain (zero Poisson's ratio). This is with the exception of sheets close to parallel with the loading direction (Z), which maintain their orientation and sheet spacing until ∼60% compression, beyond which they exhibit buckling. These data suggest that a single-domain, fully aligned as-cast aerogel is not necessary to produce a post-compression aligned lamellar structure and indicate how the spacing can be tailored as a function of compressive strain. The analysis methods presented herein are applicable to optimizing freeze-casting process and quantifying lamellar microdomain structures generally.

Identifying Structure-Property Relationships of Micro-Architectured Porous Scaffolds through 3D Printing and Finite Element Analysis

This study integrates 3D printing and finite element analysis (FEA) to investigate the effect of micro-architectural characteristics on the mechanical properties of porous scaffolds. The studied characteristics include the thickness of the scaffold walls and the number of domains at the cross-section. We use 3D printing to fabricate scaffolds of deliberately designed microstructures to enable strict control of the structures. The longitudinal compressive properties of different scaffolds are first analyzed through experimental testing. Then, FEA is conducted to investigate the mechanical properties and the deformation mechanisms of the scaffolds. We find that decreasing wall thickness leads to failure mechanism transition from wall compression failure to buckling instability. For scaffolds with different wall thicknesses, the failure mechanisms and the critical loads are evaluated using the theory of thin plate buckling. For the characteristic of the number of domains, both experimental and FEA results indicate increasing effective stiffness with increasing domains. Interestingly, we find that with the material properties extracted from a single wall scaffold, the computational models tend to overestimate the effective compression modulus of scaffolds with larger numbers of walls or domains than the experimental data. This observation indicates possible size-dependent material properties in 3D printed structs. Our study demonstrates that integrating experiments and computational modeling can provide fundamental insights into the mechanical properties and deformation mechanisms of micro-architectured scaffolds and unveil unique small-scale material behaviors.

Multidimensional Mechanics of Three-Dimensional Printed and Micro-Architectured Scaffolds

Mechanical properties of porous materials depend on their micro-architectural characteristics. Freeze casting is an effective method to fabricate micro-architectured porous scaffolds. Three key characteristics generated during freeze casting are wall thickness, number of domains at the cross-section, and transverse bridges connecting adjacent walls. To specifically study the effect of these structural characteristics on the mechanics and anisotropic compressive properties of scaffolds, we utilize additive manufacturing, i.e., 3D printing, to fabricate strictly designed cubic scaffolds with varying one characteristic at a time. We then compare strength, toughness, resilience, stiffness, and strain to failure in three orthogonal directions of the scaffolds, including longitudinal and transverse directions. To compare these multidimensional mechanics in a single diagram, we use a previously developed radar chart method to evaluate different scaffolds and unravel the effect of the structural characteristics. We find that the multidimensional mechanics can be effectively tuned by the micro-architectural characteristics. Notably, the buckling resistance of the scaffolds depends on all three structural characteristics. Our results show that an increased number of domains leads to enhanced toughness in all three directions. Increasing wall thickness leads to enhanced mechanical properties but comes at the price of losing small-sized pores, which is not favored for certain applications. In addition, adding transverse bridges increase not only the transverse strength of the scaffolds but also the longitudinal strength as they also enhance the buckling resistance. Our study provides important insights into the structure-property relationships of 3D-printed micro-architectured porous scaffolds.

Bioinspired Multifunctional Cellular Plastics with a Negative Poisson's Ratio for High Energy Dissipation

Cellular plastics have been widely used in transportation, aerospace, and personal safety applications owing to their excellent mechanical, thermal, and acoustic properties. It is highly desirable to impart them with a complex porous structure and composition distribution to obtain specific functionality for various engineering applications, which is challenging with conventional foaming technologies. Herein, it is demonstrated that this can be achieved through the controlled freezing process of a monomer/water emulsion, followed by cryopolymerization and room temperature thawing. As ice is used as a template, this method is environmentally friendly and capable of producing cellular plastics with various microstructures by harnessing the numerous morphologies of ice crystals. In particular, a cellular plastic with a radially aligned structure shows a negative Poisson's ratio under compression. The rigid plastic shows a much higher energy dissipation capability compared to other materials with similar negative Poisson's ratios. Additionally, the simplicity and scalability of this approach provides new possibilities for fabricating high-performance cellular plastics with well-defined porous structures and composition distributions.

Bone-like structure by modified freeze casting

Freeze casting has emerged as one of the most promising manufacturing methods to fabricate porous scaffolds in recent years. This is due to various reasons which include a wide range of materials which can be used in this process, easiness of the process, etc. One of the major objectives of this work was to fabricate bone-like structure by using a modified freeze casting process. In this work, Hydroxyapatite and Tricalcium phosphate scaffolds with bone-like structure were fabricated by understanding and utilizing the basic physics of freeze casting. Thermal conductivity of the base plate is a crucial factor for obtaining controlled pore and porosity distribution in a porous scaffold. It was found that designing the base plate with variable thermal conductivity has led to the formation of bone-like structure. Porous scaffolds were quantitatively analyzed for pore size and porosity distribution at center and circumference. Porosity at circumference was observed to be approximately dropped by 55%, a similar trend was seen for pore size. Therefore, it was significant evidence that modified freeze casting has capable in fabricating bone-like structures with ease and good control. This will open many new applications of porous scaffolds in biomedical, energy devices, chemical catalyst and many more.

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.

Nanomaterial Patterning in 3D Printing

The synergistic integration of nanomaterials with 3D printing technologies can enable the creation of architecture and devices with an unprecedented level of functional integration. In particular, a multiscale 3D printing approach can seamlessly interweave nanomaterials with diverse classes of materials to impart, program, or modulate a wide range of functional properties in an otherwise passive 3D printed object. However, achieving such multiscale integration is challenging as it requires the ability to pattern, organize, or assemble nanomaterials in a 3D printing process. This review highlights the latest advances in the integration of nanomaterials with 3D printing, achieved by leveraging mechanical, electrical, magnetic, optical, or thermal phenomena. Ultimately, it is envisioned that such approaches can enable the creation of multifunctional constructs and devices that cannot be fabricated with conventional manufacturing approaches.

Advances in Ice-Templated and Freeze-Casted Ceramics

Ice-templating, also known as freeze-casting, has become over the past 15 years a wellestablished materials processing route [...]