Unidirectional Freezing Research Articles

3D Printed MXene Aerogels with Truly 3D Macrostructure and Highly Engineered Microstructure for Enhanced Electrical and Electrochemical Performance.

Assembling 2D materials such as MXenes into functional 3D aerogels using 3D printing technologies gains attention due to simplicity of fabrication, customized geometry and physical properties, and improved performance. Also, the establishment of straightforward electrode fabrication methods with the aim to hinder the restack and/or aggregation of electrode materials, which limits the performance of the electrode, is of great significant. In this study, unidirectional freeze casting and inkjet-based 3D printing are combined to fabricate macroscopic porous aerogels with vertically aligned Ti3 C2 Tx sheets. The fabrication method is developed to easily control the aerogel microstructure and alignment of the MXene sheets. The aerogels show excellent electromechanical performance so that they can withstand almost 50% compression before recovering to the original shape and maintain their electrical conductivities during continuous compression cycles. To enhance the electrochemical performance, an inkjet-printed MXene current collector layer is added with horizontally aligned MXene sheets. This combines the superior electrical conductivity of the current collector layer with the improved ionic diffusion provided by the porous electrode. The cells fabricated with horizontal MXene sheets alignment as current collector with subsequent vertical MXene sheets alignment layers show the best electrochemical performance with thickness-independent capacitive behavior.

Crossover from lamellar to intersected ice morphologies within a single ice crystal during unidirectional freezing of an aqueous solution

Ice growth from liquid phase has been extensively investigated in various conditions, especially for ice freely grown in undercooled water and aqueous solutions. Although unidirectional ice growth plays a significant role in various domains, the detailed pattern formation of unidirectionally grown ice in an aqueous solution remains elusive. For the first time, we in situ proved a crossover from lamellar to intersected ice morphologies of a single ice crystal via unidirectional freezing of an aqueous solution. It is revealed that the tilted lamellar morphology occurs within a single ice crystal and consists of two groups of tilted lamellar ice with different tilting directions with respect to basal plane of ice. Further observations also confirm a competition relation between the two groups of ice lamellae which are either “divergent” or “convergent”. This intersected morphology which originates from peculiar growth habit of ice is crystallographically different from the competitive growth of bi-crystal composed of two differently orientated grains which usually appears in directional solidification. These results are suggested to provide a more complete physical picture of unidirectionally grown ice from aqueous solution and are believed to promote our understanding of various pattern of ice in many relevant domains where pattern formation of ice crystal is vital.

Experimental investigation on frost heave property of gravel mixed soil

Gravel mixed soil, a certain amount of gravel mixed into the clay, is often used as core material of earth-rockfill dams to improve the stress-deformation behavior of the core wall. In this paper, a series of unidirectional freezing tests were conducted to study effects of coarse grains content on the freezing process and reveal frost characteristics of gravel mixed soil. Experimental results show that the variation of coarse grains content can cause changes in the pore structure, hydraulic and thermal property of mixed soil, and then affect the temperature distribution, water intake and frost heave. It is also found that temperature gradient is the dominant factor affecting freezing front location in comparison with coarse grains content, whereas coarse grains content is the dominant factor affecting water intake in freezing process of gravel mixed soil.

Research on water migration and deformation characteristics of coastal composite strata under artificial ground freezing affected by seepage

When the artificial ground freezing (AGF) method is used to construct underground structures in the coastal composite strata, the differences between characteristics of soft clay and silty sand under seepage often increase the risk of freezing construction and the difficulty of deformation control. In order to find out water migration and deformation characteristics of costal composite strata under seepage condition in freezing process, a series of unidirectional freezing tests of silty sand and soft clay in Shanghai and AGF model experiments on composite strata of soft clay and underlying silty sand were carried out under seepage condition. The results of unidirectional freezing tests of single soil type showed that the layered deformation curve of silty sand and soft clay can be divided into two types that steep mode of silty sand and gradual mode of soft clay. It also indicates that the water migration is fully developed within soft clay during freezing; but most of water inside silty sand froze quickly, thus water migration was not developed as that much in soft clay. In AGF model experiments, the seepage contributed to the development of water migration inside composite strata. However, the deformation characteristics of composite strata under seepage condition were evidently different from that of unidirectional freezing tests. The frost heave force development of the lower silt sand in the area directly affected by seepage was gradual mode, while that of the upper soft clay in the area indirectly affected by seepage was steep mode. The above results demonstrated that influence on freezing process of seepage is greater than that of soil properties in freezing process. Through research results, some optimization strategies can be made for the relevant measures to deal with the adverse effects of seepage and frost heave in the practice of the freezing method for coastal composite strata.

Tilting Behavior of Lamellar Ice Tip during Unidirectional Freezing of Aqueous Solutions.

Freezing of ice has been largely reported from many aspects, especially its complex pattern formation. Ice grown from liquid phase is usually characteristic of lamellar morphology that plays a significant role in various domains. However, tilted growth of ice via transition from coplanar to noncoplanar in directional solidification has been paid little attention in previous studies and there was a misleading explanation of the formation of tilted lamellar ice. Here, we in situ investigated the variations of tilting behavior of lamellar ice tips under different conditions within a single ice crystal of manipulated orientation via unidirectional freezing of aqueous solutions. It is found that tilted growth of ice tips is sensitive to pulling velocity and solute type. These experimental results reveal intrinsic tilted growth behavior of lamellar ice, which is suggested to enrich our understanding of pattern formation of ice in relevant physical processes.

Poly(vinyl alcohol)/MXene biomimetic aerogels with tunable mechanical properties and electromagnetic interference shielding performance controlled by pore structure

Lightweight biomimetic aerogels with absorption-dominated electromagnetic interference (EMI) shielding have shown greater potential than traditional reflection-dominated metal shields in both civilian and military applications. However, the influence of morphology parameters on the performance of biomimetic aerogels remains significantly unexplored. In this work, Poly(vinyl alcohol) (PVA) assisted transition metal carbides (MXene) aerogels with cellular morphology were fabricated by unidirectional freezing method. The pore channel size/wall thickness was tailored via modulating freezing temperature. Interestingly, the EMI shielding ability improves and the compressible strength declines with increasing the pore channel size/wall thickness. For EMI shielding ability, there should be a saturation of pores-induced multiple reflections and scattering. Therefore, the wall thickness-dominant absorption turns into the decisive factor. In regard of mechanical properties, the deterioration in compressible strength can be attributed to the enhanced stress concentration and the looser stacking of the PVA and MXene. Ultimately, the lightweight PVA/MXene composite aerogels (~33 mg cm−3) can reach the compressible strength at 60% strain of 127.3 kPa and the EMI shielding effectiveness of 40.6 dB based on the freezing temperatures while the volume content of MXene is merely 0.58%.

A Multi-Scale Structural Engineering Strategy for High-Performance MXene Hydrogel Supercapacitor Electrode.

MXenes as an emerging two‐dimensional (2D) material have attracted tremendous interest in electrochemical energy‐storage systems such as supercapacitors. Nevertheless, 2D MXene flakes intrinsically tend to lie flat on the substrate when self‐assembling as electrodes, leading to the highly tortuous ion pathways orthogonal to the current collector and hindering ion accessibility. Herein, a facile strategy toward multi‐scale structural engineering is proposed to fabricate high‐performance MXene hydrogel supercapacitor electrodes. By unidirectional freezing of the MXene slurry followed by a designed thawing process in the sulfuric acid electrolyte, the hydrogel electrode is endowed with a three‐dimensional (3D) open macrostructure impregnated with sufficient electrolyte and H+‐intercalated microstructure, which provide abundant active sites for ion storage. Meanwhile, the ordered channels bring through‐electrode ion and electron transportation pathways that facilitate electrolyte infiltration and mass exchange between electrolyte and electrode. Furthermore, this strategy can also be extended to the fabrication of a 3D‐printed all‐MXene micro‐supercapacitor (MSC), delivering an ultrahigh areal capacitance of 2.0 F cm–2 at 1.2 mA cm–2 and retaining 1.2 F cm–2 at 60 mA cm–2 together with record‐high energy density (0.1 mWh cm–2 at 0.38 mW cm–2).

Highly aligned graphene/biomass composite aerogels with anisotropic properties for strain sensing

Flexible sensors are of great interests for future applications in the next-generation intelligent lifestyles. However, available fabrication methods exhibit shortages such as tedious preparation process and non-environmentally friendly matrix, while the obtained sensors usually possess poor ability to discriminate stress from different directions. Here, we propose a highly sensitive aligned graphene/sodium alginate (SA) aerogel sensor with anisotropic properties fabricated through a facile unidirectional freezing method. The resulted orientated nanostructure combined with designed SA/Ca2+ network endows the aerogel with desired anisotropic properties and enhanced mechanical properties. The anisotropic performance data of resistance, strain sensitivity and compressive strength between the two directions reach up to 57, 3 and 1.5 times respectively. Moreover, the obtained sensors can be used in expression recognition and real-time health monitoring owing to the excellent sensitivity and desired anisotropic conductive response ability. This work is a step toward bio-based high-performance anisotropic strain sensors, which opens up new opportunities for the design and fabrication of advanced intelligent electronic devices for future lifestyles.

Thermal insulating, light-weight and conductive cellulose/aramid nanofibers composite aerogel for pressure sensing

Conductive nanocellulose aerogels have attracted significant attention in pressure sensing for wearable devices owing to lightweight, sustainability and good chemical stability. Limited by its flammability and weak mechanical properties, aramid nanofiber (ANF) was designed as reinforcement to overcome the shortcoming mentioned above. Herein, the unidirectional freeze casting method was proposed to fabricate nanocellulose/aramid nanofiber (CA) aerogel. Then, the CA/PPy (CAP) aerogel was obtained by using the oriented structure of CA aerogel as a template for inducing conductive polypyrrole (PPy) in-situ formation inside the composite aerogel. The conductive aerogel with the ordered microstructure exhibited the anisotropic mechanical properties and thermal conductivity. And it could withstand high temperature without any destruction phenomenon. Moreover, the aerogel sensor revealed high strain sensitivity and satisfactory electrochemical performance. Lightweight CAP aerogel with controllable alignment, sensitive sensing property and thermal stability is very promising in pressure sensor under some extreme conditions.

Synthesis of Ethylenediaminetetraacetic Acid-Functionalized Chitosan Cryogels as Potential Sorbents of Heavy Metal Ions

An original functionalization strategy is proposed here to design chitosan (CS)-based cryogels with ethylenediaminetetraacetic acid (EDTA) moieties. Cryogels with aligned micro-sized tubular structures were further engineered through an unidirectional freezing approach. Attachment of EDTA groups onto CS chains was proved by 1H-RMN and FT-IR spectroscopy. The formation of EDTA-functionalized 3D porous CS-based cryogels was demonstrated by several methods of characterization (FTIR spectroscopy, optical microscopy, SEM, porosity measurements, swelling behavior, copper (II) retention capacity). The sorption tests pointed out the high potential of EDTA-functionalized CS-based cryogels for heavy metal ions retention.

Biomimetic 3D aligned conductive tubular cryogel scaffolds with mechanical anisotropy for 3D cell alignment, differentiation and in vivo skeletal muscle regeneration

Developing 3D conductive aligned cryogels has great potential for skeletal muscle trauma treatment because they can mimic anisotropic structure, conductivity, and recoverable cyclic compression of the microenvironment of native skeletal muscle. In this work, a series of cryogels possessing 3D aligned morphology, conductivity, and excellent anisotropic mechanical compression property based on gelatin (GT) and polydopamine coated carbon nanotubes (PCNTs) were fabricated as skeletal muscle tissue scaffolds by using unidirectional freeze casting technology. The aligned microstructure of cryogels depended on gelatin concentration, and GT7.5 (with the gelatin content of 7.5% w/v) showed excellent aligned structure. Interestingly, the mechanical property of the aligned cryogels was similar to that of native skeletal muscle in terms of the dynamic contraction behavior and the anisotropic compression property due to the internal anisotropy structure. The aligned cryogel GT7.5 with good biocompatibility significantly promoted the alignment and elongation of C2C12 myoblasts. Moreover, the introduction of PCNTs enhanced the mechanical properties of cryogel GT7.5 and had a positive effect on myogenic differentiation of C2C12 cells. The aligned conductive GT7.5C2 cryogel significantly promoted new born muscle tissue generation compared to non-aligned group (GT7.5C2N) and non-conductive group (GT7.5) in a rat tibialis anterior muscle defect model. These data suggested that the 3D aligned conductive cryogel with conductivity and anisotropic compression property is a promising scaffold candidate for skeletal muscle tissue engineering.

Porous Ti6Al4V alloys with high strength-to-modulus ratio fabricated by unidirectional freeze casting of SiC fiber-containing slurry

Porous titanium alloys with high strength-to-modulus ratio are one of the promising materials for bone implants. In the present work, porous Ti6Al4V alloys with SiC fiber bridges and fiber reinforced wall structure were obtained by unidirectional freeze casting of SiC fiber-containing slurry. The effect of SiC fiber content on the pore structure, microstructure, interface and compressive properties was systematically investigated. The formations of fiber bridges and fiber reinforced wall structure were elaborated through analyzing the interaction between fibers and ice front. Addition of SiC fibers has significant effects on the porosity, structural wavelength, and pore morphology of porous alloys. Porosity increases with the increase in fiber content, and pore morphology changes from lamellar to dendritic. Porous Ti6Al4V alloys mainly contain equiaxed α and β phase. Crystalline phases of Ti5Si3 and TiC are present at the interface. Micropores are also observed on the fiber side close to the interface due to the Kirkendall effect. Both Young's modulus and yield stress increase with the increase in SiC fiber content, while they dramatically decrease at higher fiber content of 10 vol%. SiC fiber bridges and fiber reinforced wall structure have synergetic strengthening effects for suppressing buckling deformation of lamellar walls and thus for improving the strength of porous titanium alloys. Porous Ti6Al4V alloys fabricated by 2 vol% SiC fibers possess high porosity of 67% and high strength-to-modulus ratio of 0.027.

Application potential of three-dimensional silk fibroin scaffold using mesenchymal stem cells for cardiac regeneration

Cardiac tissue engineering focusing on biomaterial scaffolds incorporating cells from different sources has been explored to regenerate or repair damaged area as a lifesaving approach.The aim of this study was to evaluate the cardiomyocyte differentiation potential of human adipose mesenchymal stem cells (hAD-MSCs) as an alternative cell source on silk fibroin (SF) scaffolds for cardiac tissue engineering. The change in surface morphology of SF scaffolds depending on SF concentration (1-6%, w/v) and increase in their porosity upon application of unidirectional freezing were visualized by scanning electron microscopy (SEM). Swelling ratio was found to increase 2.4 fold when SF amount was decreased from 4% to 2%. To avoid excessive swelling, 4% SF scaffold with swelling ratio of 10% (w/w) was chosen for further studies.Biodegradation rate of SF scaffolds depended on enzymatic activity was found to be 75% weight loss of SF scaffolds at the day 14. The phenotype of hAD-MSCs and their multi-linage potential into chondrocytes, osteocytes, and adipocytes were shown by flow cytometry and immunohistochemical staining, respectively.The viability of hAD-MSCs on 3D SF scaffolds was determined as 90%, 118%, and 138% after 1, 7, and 14 days, respectively. The use of 3D SF scaffolds was associated with increased production of cardiomyogenic biomarkers: α-actinin, troponin I, connexin 43, and myosin heavy chain. The fabricated 3D SF scaffolds were proved to sustain hAD-MSCs proliferation and cardiomyogenic differentiation therefore, hAD-MSCs on 3D SF scaffolds may useful tool to regenerate or repair damaged area using cardiac tissue engineering techniques.

A novel model of heat-water-air-stress coupling in unsaturated frozen soil

The accurate prediction of frost heave in unsaturated frozen soil is crucial for the construction and protection of projects in cold regions. The occurrence of frost heave in unsaturated frozen soil is accompanied by an extremely complicated process of heat and mass transfer, which is closely related to the temperature gradient, water migration, air transfer, latent heat, deformation, overburden pressure, and so on. Based on thermodynamics and continuum mechanics, a comprehensive coupling model is proposed to describe the temperature variation and liquid water/vapor redistribution in unsaturated frozen soil. The Clapeyron equation was used to establish the relationship between the ice pressure and the water pressure during phase equilibrium. The initial water content of the frost heave, frozen fringe, and consolidation compression deformation are also introduced into the model to calculate the frost heave in unsaturated frozen soil. The proposed model was validated through a comprehensive experiment wherein a unidirectional freezing test was performed on a soil column in a closed system in our lab. The well-known experiment conducted by Mizoguchi and the simulation of this test by other study were also considered to validate the feasibility and adaptability of the model. The results reveal that the temperature variation, total water content distribution, and frost heave obtained by the proposed model are consistent with the measurement results.

Theoretical and Numerical Analyses on Hydro–Thermal–Salt–Mechanical Interaction of Unsaturated Salinized Soil Subjected to Typical Unidirectional Freezing Process

Abstract The freezing process of salinized soil is a complex, dynamic, and interactive hydro–thermal–salt–mechanical (HTSM) coupled physical phenomenon. In many recent studies, soil was assumed to ...

Freezing Pressurized Water into a Standard Cylindrical Ice Sample in a Triaxial Cell

The mechanical characteristics of high-pressure frozen ice are a basis for the design of deep underground frozen walls, the drilling of thick permafrost and ice sheets, and the probing of extraterrestrial ice. The continuous control of the sample stress state from freezing to testing is essential for the experimental study of in situ mechanical response of high-pressure frozen ice. In the context, we developed a preparation technique for freezing pressurized water into a standard cylindrical ice sample in a triaxial cell. Through theoretical analysis, a cylindrical water sample with precise dimensions and strong sealing was fabricated using heat shrinkable tubing, sectional end caps, and an assembly cylinder. A mounting device was designed to insert the water sample into the triaxial cell without deformation. In order to deal with the lateral surface irregular of the resulting ice sample caused by freezing expansion, we proposed a pressurization method in which the volume of the confining medium is controlled to restrict the radial deformation of the sample, and the axial pressure on the sample is kept constant; thus, the freezing expansion will develop along the height direction through releasing the expansion pressure. Based on the analysis of sample deformation and finite element numerical simulations, the control method of the temperature fields of the sample and the confining medium was obtained, and the standard cylindrical ice sample which satisfies the geometric accuracy requirements was produced. The comparison of ice samples frozen by different freezing methods showed that the control of the confining medium mean temperature and the sample unidirectional freezing is necessary to improve the dimensional precision of the ice sample.

Insights into the kinetic processes of solute migration by unidirectional freezing in porous media with micromodel visualization at the pore-scale

Field investigations have proved that frozen soil does not act as a completely impermeable barrier for contaminants in cold regions. However, the subsurface behaviors of solutes in freezing and frozen porous media are still unclear. To unveil their nature, the pore-scale behavior of potassium permanganate in saturated porous media subjected to the unidirectional freezing was investigated using micromodel visualizations. An optical microscope was applied to obtain the pore-scale kinetics of solute redistribution in a two-dimensional micromodel using a calibration curve between the color intensity and concentration. We found that (1) the solute migration was not only limited to the ice-water interface but also occurred in the freezing area; (2) the redistribution of solutes had a significant hysteresis effect relative to the freezing front movement during the freezing of the porous media. By combining these results with the theory of sea ice, we suggested that the formation and re-motion of solute-rich inclusions in the mushy layer appear to be vital processes responsible for these phenomena. It was believed that the major mechanism for the re-motion of liquid inclusions was brine diffusion and expulsion in this experiment. The results of this study provide a better understanding of the physics of contaminant migration and their complex kinetics at the pore scale, which has important implications for the assessment and remediation of contaminated soils in seasonal frozen soils and permafrost.

Liquid water–vapour migration tracing and characteristics of coarse-grained soil under high-speed railway train loading in cold regions

Although high-speed railway (HSR) subgrades in cold regions are filled with coarse-grained soil (CGS), they still undergo frost-heaving displacement. The liquid water–vapour migration mechanism can explain the frost-heaving process; therefore, the characteristics of liquid water–vapour migration in CGS are investigated in this study. The results show that in CGS, vapour migration is dominant under unidirectional freezing and dynamic loading. The water redistribution in the frozen zone and freezing front is primarily caused by vapour migration, whereas that in the unfrozen zone is primarily caused by liquid water migration. The final migration height of liquid water and the amount of vapour migration are positively correlated with the amplitude and frequency of dynamic loading. Continuous pore-structure changes and soil-particle movement provide favourable conditions for vapour migration, which are amplified by increasing the dynamic loading amplitude or frequency. A pumping action produced by dynamic loading exists inside CGS. In the design and maintenance of HSR subgrades in cold regions, vapour migration merits careful consideration because it induces more than 50% of the frost-heaving displacement.

Highly anisotropic graphene aerogels fabricated by calcium ion-assisted unidirectional freezing for highly sensitive sensors and efficient cleanup of crude oil spills

Anisotropic graphene aerogels (GAs) are usually fabricated by unidirectional-freezing with graphene oxide (GO) dispersion as the precursor. However, polymer additives or pre-gelation are usually required to enhance the anisotropic architectures, which often cause attenuations in electrical conductivity and shape control accuracy. Herein, we demonstrate a calcium ion-assisted unidirectional-freezing approach for fabricating highly anisotropic GAs by unidirectionally freezing aqueous suspensions of GO with a trace amount of calcium ions, followed by freeze-drying and thermal reduction. The calcium ions could slightly crosslink GO sheets by coordination, and hence reduce the interaction between GO sheets and in situ grown ice pillars, benefiting vertical alignment of the GO sheets. After freeze-drying to sublime these ice pillars, high-temperature annealing is adopted to thermally reduce the anisotropic GO aerogels by removing their oxygen-containing groups, resulting in highly anisotropic GAs. The lightweight and anisotropic GA exhibits excellent sensitivity and durable reversibility as a piezoresistive sensor because of its highly aligned porous architecture. Furthermore, GA exhibits prominent adsorption capacity for many organic pollutants from wastewaters. Interestingly, the vertically aligned porous structure of GA enhances its solar-light absorption and solar-thermal energy conversion, making GA efficient in decreasing the viscosity of crude oil and thus enhancing its adsorption capacity.

Meshless Numerical Analysis of Phase Change Problems in Artificial Freezing Technology Applied in Geo-Media

Numerical solutions to heat conduction problems involving phase change have traditionally been obtained using the grid-based finite difference or finite element methods. However, due to the unique mathematical form of the equations and the limitations of the grid algorithm, difficulties may arise in solutions. In this study, smooth particle hydrodynamics (SPH) method is used to calculate heat conduction with phase change so as to obtain reasonable numerical solutions. An SPH-based program is presented, and its practicality and reliability are verified through a benchmark problem. Based on the unidirectional freezing test, the validity and accuracy of the soil freezing model considering the phase change effect is verified compared with the finite element results. Then, artificial freezing technology is applied in geo-media, where the heat conduction with phase change during soil freezing is simulated. The temperature distribution characteristics around freezing pipes and the development of thermal regime of characteristic points with time are obtained. The frozen front development is analyzed, considering different freezing-pipe temperatures and different thermal parameters for the media. The findings of this study may provide a reference for the design of future engineering applications using artificial freezing technology.