CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10202251O33G filed September 16, 2022, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a fiber. Various embodiments of this disclosure may relate to a method of forming a fiber.

BACKGROUND

[0003] Porous fibers are commonly used in applications such as energy harvesting, energy storage, health management, catalysis, human computer interaction, sensors, disease prevention, and more. Methods have been designed to increase the porosity of fibers, such as freeze drying, wet spinning, electrospinning, thermal drawing, and so on. The resulting fibers can partially realize desired properties such as electrochemical capacity, catalytic efficiency, adsorbing capacity, etc. due to their high specific surface area. On the contrary, some other performances, such as cycle life, electrical conductivity, and mechanical property, may be limited by the obstruction of transmission path and poor structures with the random pores. The poor performance of existing porous fibers may be due to the uncontrollability of the fabrication method.

[0004] In order to obtain anisotropic porous fibers with aligned structures, some efforts have been proposed to form the oriented pores using freeze- spinning. However, despite enhanced thermal insulation, a major challenge remains to achieve highly anisotropic pores with controllable aligned structures for realizing the full potential of active materials. Thus, in order to fundamentally address this challenge, a procedure that can achieve anisotropic porous fibers with aligned structures in a controllable and continuous manner is urgently required.

[0005] Further, after porous fibers are fabricated, functional textiles based on these fibers may be constructed for large-scale coverage of the human body. A common reason limiting long-term usage is the degradative performance of these fibers when fully exposed to the environment and skin, and also due to physical impact caused by body movements. In addition, the performance of these fibers is hugely weakened due to routine maintenance, including washing and drying. In order to address these problems, a protective layer may be formed on the outer surfaces of these fibers. However, the additional process steps required to form the protective layer may give rise to uncertainties in controlling the interfacial interactions between the inner fibers and the outer coatings.

SUMMARY

[0006] Various embodiments may relate to a method of forming a fiber. The method may include forming a tube using a thermal drawing process. The method may also include providing one or more mixtures into the tube. The method may further include freezing the one or more mixtures within the tube using a freeze-casting process such that a first plurality of walls formed by MXene sheets and a second plurality of walls formed by nanomaterials, the MXene sheets and the nanomaterials comprised in the one or more mixtures, define a plurality of spaces within the tube, thereby forming the fiber.

[0007] Various embodiments may relate to a fiber. The fiber may include a tube. The fiber may also include a first plurality of walls formed by MXene sheets. The fiber may also include a second plurality of walls formed by nanomaterials. The first plurality of walls and the second plurality of walls may define a plurality of spaces within the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

FIG. 1 is a general illustration of a method of forming a fiber according to various embodiments.

FIG. 2 is a general illustration of a fiber according to various embodiments.

FIG. 3A is a schematic illustrating fabrication of porous fibers via continuous thermal drawing and freeze-casting according to various embodiments.

FIG. 3B shows a plot of pressure as a function of temperature illustrating the freeze-casting process including slurry preparation, solidification and sublimation according to various embodiments, with the insets showing the scanning electron microscopy (SEM) images of the fibers at different stages.

FIG. 3C is a line drawing of a roll of anisotropic fibers prepared by the continuous and large- scale fabrication process according to various embodiments.

FIG. 4A is a plot of interlamellar spacing (in micrometers or pm) as a function of draw speed (in rounds or R) illustrating the variation of interlamellar spacing under various prepared conditions of draw speed according to various embodiments.

FIG. 4B is a plot of interlamellar spacing (in micrometers or pm) as a function of concentration (in micrograms per microliter or mg ml?) illustrating the variation of interlamellar spacing under various concentrations of the slurry according to various embodiments.

FIG. 4C is a plot of interlamellar spacing (in micrometers or pm) as a function of temperature (in negative degrees Celsius or -°C) illustrating the variation of interlamellar spacing under various freezing temperatures according to various embodiments.

FIG. 4D is a plot of interlamellar spacing (in micrometers or pm) as a function of inner diameter (in micrometers or pm) illustrating the variation of interlamellar spacing with various inner diameters of the hollow polymer tube according to various embodiments.

FIG. 5A shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on MXene nanosheets according to various embodiments.

FIG. 5B shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on multi-walled carbon nanotubes (MCNT)-based nanofibers according to various embodiments. FIG. 5C shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on titanium dioxide (TiO2) nanoparticles according to various embodiments.

FIG. 6 shows the scanning electron microscopy (SEM) images of anisotropic fibers with different traverse cross-sectional shapes prepared by freeze-casting according to various embodiments.

FIG. 7 is a schematic showing a setup to prepare a polycarbonate (PC)/ carbon-loaded polycarbonate (CPC) / barium titanate (IV) (Ba) / MXene (M) fiber with concentric circle structure by freeze-casting according to various embodiments. FIG. 8A shows a schematic of a traverse cross-sectional view of a polycarbonate (PC)/ carbon- loaded polycarbonate (CPC) / barium titanate (IV) (Ba) / MXene (M) fiber according to various embodiments.

FIG. 8B shows a line drawing of the fiber according to various embodiments.

FIG. 9A is a scanning electron microscopy (SEM) image showing the concentric circle structure of a fiber according to various embodiments.

FIG. 9B is a magnified scanning electron microscopy (SEM) image showing the concentric circle structure of a fiber according to various embodiments.

FIG. 9C shows further magnified scanning electron microscopy (SEM) images of portions of the fiber shown in FIG. 9B according to various embodiments.

FIG. 10A shows a schematic of a setup for testing the performance of acoustic energy harvesting of anisotropic porous fibers according to various embodiments.

FIG. 10B is a plot of voltage (in volts or V) as a function of time (in seconds or s) illustrating the output voltage of the fiber according to various embodiments in response to the loudspeaker after the signal has been amplified ~50 times and shown by the digital phosphor oscilloscope. FIG. IOC is a plot of voltage (in milli-Volts or mV) as a function of frequency (in hertz or Hz) illustrating the voltage recorded by the electrometer in response to the loudspeaker according to various embodiments.

DESCRIPTION

[0009] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0010] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments. [0011] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements. [0012] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g. within 10% of the specified value.

[0013] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0014] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0015] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0016] Embodiments described in the context of one of the fibers are analogously valid for the other fibers. Similarly, embodiments described in the context of a method are analogously valid for a fiber, and vice versa.

[0017] Various embodiments may help to address one or more issues facing existing fibers. [0018] FIG. 1 is a general illustration of a method of forming a fiber according to various embodiments. The method may include, in 102, forming a tube using a thermal drawing process. The method may also include, in 104, providing one or more mixtures into the tube. The method may further include, in 106, freezing the one or more mixtures within the tube using a freeze-casting process such that a first plurality of walls formed by MXene sheets and a second plurality of walls formed by nanomaterials, the MXene sheets and the nanomaterials included in the one or more mixtures, define a plurality of spaces within the tube, thereby forming the fiber.

[0019] In other words, the method may include using thermal drawing to form a tube, followed by providing one or more mixtures including MXene sheets and nanomaterials into the tube, and freezing the one or more mixtures using a freeze-casting process such that the MXene sheets and the nanomaterials form walls separated by internal spaces within the tube. [0020] The fiber may be an acoustic energy harvester. [0021] In various embodiments, step 102 may occur first, followed by step 104. Step 106 may occur after step 104.

[0022] MXene sheets may include transition metal carbides, nitrides or carbonitrides. In various embodiments, the MXene sheets may be TisC2T sheets, wherein Ti is titanium, C is carbon, T is a functional group (e.g. -OH, -O, or -F) and x is any number. In various other embodiments, the MXene sheets may be V2CT , wherein V is vanadium, C is carbon, T is a functional group and x is any number. In yet various other embodiments, the MXene sheets may be Nb2CT , wherein Nb is niobium, C is carbon, T is a functional group and x is any number. In various embodiments, the MXene sheets may be nanosheets (i.e. having a thickness of 100 nm or less). In various embodiments, the MXene sheets may be dispersed in deionized water (H2O). In other words, the one or more mixtures may include a dispersion of MXene sheets and deionized water (H2O).

[0023] The plurality of spaces may also be referred to as interlamellar spaces or spacings. [0024] In various embodiments, the tube may include any suitable polymer or polymers. The tube may alternatively be referred to as hollow tube or hollow polymer tube. In various embodiments, the tube may include an inner layer of carbon-loaded polycarbonate (CPC). The tube may further include an outer layer of polycarbonate (PC) surrounding a circumferential surface of the inner layer.

[0025] In various embodiments, the tube may include electrical conductors, such as metal (e.g. copper (Cu)) wires. The electrical conductors may serve as electrical interconnections for the acoustic energy harvester. For instance, the inner layer of carbon-loaded polycarbonate (CPC) may include two copper wires to provide electrical interconnection.

[0026] In various embodiments, the one or more mixtures may include a dispersion including the MXene sheets and deionized water. The one or more mixtures may include a nano-materials mixture or solution including the nanomaterials.

[0027] In various embodiments, the nanomaterials may include tetragonal barium titanate (IV) (TBaTiOs) nanoparticles, titanium (IV) oxide nanoparticles and/or carbon nanotubes. In other words, the nanomaterials may include tetragonal barium titanate (IV) (TBaTiOs) nanoparticles, titanium (IV) oxide nanoparticles, carbon nanotubes or any combination thereof. [0028] In various embodiments, the nano-materials mixture or solution may include a suitable medium or solvent, such as an alginic acid sodium (AG) salt solution. [0029] In various embodiments, the method may include placing a preform in a heating furnace before the thermal drawing process. The tube may be formed from the preform. The preform may include a polytetrafluoroethylene (PTFE) rod, with polycarbonate (PC) layers/films and carbon-loaded polycarbonate (CPC) layers/films successively wrapped around the PTFE rod. Rods with diameters smaller than the PTFE rod/inner diameter of the tube (e.g. smaller PTFE rods) may be inserted or provided during the wrapping of PC layers/films and/or CPC layers/films around the PTFE rod. Metal (e.g. Cu) wires may subsequently be inserted into the rods with the smaller diameters during the thermal drawing process to form the electrical conductors.

[0030] In various embodiments, the furnace may have two heating zones. A first heating zone of the two heating zones may have a temperature selected from a range from about 130 °C to about 170 °C, e.g. about 150 °C. A second heating zone of the two heating zones may have a temperature selected from a range from about 330 °C to about 370 °C, about 350 °C.

[0031] In various embodiments, freezing the one or more mixtures within the tube using the freeze-casting process may include providing the tube through a cold source such that the one or more mixtures provided into the tube freeze as heat is removed from the one or more mixtures via the cold source. The cold source may be a heat conductor ring in thermal connection to a low temperature circulation system. The low temperature circulation system may refer to a circulation system configured to reduce and/or maintain a temperature of the cold source below a predetermined temperature, e.g. -80 °C or -100 °C.

[0032] In various embodiments, the method may include freeze drying the fiber (e.g. in a freeze dryer) after freezing the one or more mixtures within the tube using the freeze-casting process.

[0033] In various embodiments, the one or more mixtures may undergo solidification during the freeze-casting process. Thereafter, the frozen one or more mixtures may undergo sublimation to form the first plurality of walls and the second plurality of walls defining the plurality of spaces within the tube.

[0034] In various embodiments, the fiber may have a cross-sectional shape selected from a group consisting of a circle, a triangle, a rhombus, a semicircle and a rectangle.

[0035] In various embodiments, a first group of the plurality of spaces defined by the first plurality of walls formed by the MXene sheets may be aligned along a predetermined direction. In other words, the first group of the plurality of spaces may be aligned in parallel along the predetermined direction. In various embodiments, a second group of the plurality of spaces defined by the second plurality of walls formed by the nanomaterials may not be aligned along the predetermined direction. The second group of the plurality of spaces may instead extend radially from a center of the fiber or tube (e.g. to the inner layer of the tube).

[0036] In various embodiments, the second group of the plurality of spaces defined by the second plurality of walls formed by the nanomaterials may surround the first plurality of spaces defined by the first plurality of walls formed by the MXene sheets across a traverse cross - section of the fiber.

[0037] In various embodiments, each of the plurality of spaces may have a dimension selected from a range from 10 micrometers to 80 micrometers.

[0038] FIG. 2 is a general illustration of a fiber according to various embodiments. The fiber may include a tube 202. The fiber may also include a first plurality of walls 204 formed by MXene sheets. The fiber may also include a second plurality of walls 206 formed by nanomaterials. The first plurality of walls 204 and the second plurality of walls 206 may define a plurality of spaces within the tube 202.

[0039] In other words, the fiber may include a tube 202 with a first plurality of internal walls 204 including MXene sheets as well as a second plurality of internal walls 206 including other nanomaterials.

[0040] For avoidance of doubt, FIG. 2 seeks to illustrate some feature of a fiber according to various embodiments, and is not intended to limit for instance, the arrangement, orientation, shape, size etc. of the various features.

[0041] In various embodiments, the tube 202 may include any suitable polymer or polymers. In various embodiments, the tube 202 may include an inner layer of carbon-loaded polycarbonate (CPC). The tube 202 may further include an outer layer of polycarbonate (PC) surrounding a circumferential surface of the inner layer.

[0042] In various embodiments, the tube 202 may include electrical conductors, such as copper wires.

[0043] In various embodiments, the nanomaterials may include tetragonal barium titanate (IV) (TBaTiOs) nanoparticles, titanium (IV) oxide nanoparticles and/or carbon nanotubes. In other words, the nanomaterials may include tetragonal barium titanate (IV) (TBaTiOs) nanoparticles, titanium (IV) oxide nanoparticles, carbon nanotubes or any combination thereof. [0044] In various embodiments, the fiber may have a cross-sectional shape selected from a group consisting of a circle, a triangle, a rhombus, a semicircle and a rectangle.

[0045] In various embodiments, a first group of the plurality of spaces defined by the first plurality of walls 204 formed by the MXene sheets may be aligned along a predetermined direction. In other words, the first group of the plurality of spaces may be aligned in parallel along the predetermined direction. In various embodiments, a second group of the plurality of spaces defined by the second plurality of walls 206 formed by the nanomaterials may not be aligned along the predetermined direction. The second group of the plurality of spaces may extend radially from a center of the fiber or tube 202 (e.g. to the inner layer of the tube 202).

[0046] In various embodiments, the second group of the plurality of spaces defined by the second plurality of walls 206 formed by the nanomaterials may surround the first plurality of spaces defined by the first plurality of walls 204 formed by the MXene sheets across a traverse cross-section of the fiber.

[0047] In various embodiments, the fiber may be an acoustic energy harvester.

[0048] In various embodiments, each of the plurality of spaces may have a dimension selected from a range from 10 micrometers to 80 micrometers.

[0049] Various embodiments may relate to a strategy to fabricate anisotropic porous fibers with controllable aligned structures via a combination of thermal drawing and freeze-casting. Firstly, various hollow tubes with different diameters and polymer materials may be prepared via thermal drawing. Then, interlamellar spacings may be formed via freeze-casting. As the ice crystals grow directionally in the process, controllable interlamellar spacings from ~10 pm to ~80 pm may be formed to prepare various anisotropic porous fibers. The dimensions of the interlamellar spacings may be controlled by changing the parameters of draw speed, solution concentration, freezing temperatures, inner diameters of the hollow polymer tubes etc.

[0050] Anisotropic porous fibers with highly aligned structures can be fabricated based on MXene nanosheets, carbon nanotubes, and titanium dioxide. Furthermore, anisotropic porous fibers with the concentric circle structures may also be successfully realized from tetragonal barium titanate nanoparticles and MXene nanosheets via this developed approach. Due to the active materials and the aligned structures, the obtained anisotropic porous fibers (as an acoustic energy harvesting device) may be configured to output high voltages of ~10 mV to ~24 mV at 100 Hz to 900 Hz. The demonstrated method combining thermal drawing with freeze-casting may also lead to fabrication of anisotropic porous fibers with other functions. [0051] Synthesis Of Anisotropic Porous Fibers Via Thermal Drawing And Freeze-Casting [0052] TisC2T _x (MXene) nanosheets were fabricated via etching and shaking from preliminary TisAlCg (MAX). The prepared MXene nanosheets have a lateral size of 10.6 ± 2.0 pm with a thickness of ~ 1.5 nm according to the scanning electron microscope (SEM) and atomic force microscopy (AFM) images. Moreover, X-ray diffraction (XRD) patterns do not show 104 and 105 peaks, illustrating the successful preparation of MXene nanosheets from MAX.

[0053] FIG. 3A is a schematic illustrating fabrication of porous fibers via continuous thermal drawing and freeze-casting according to various embodiments. As shown in FIG. 3A, anisotropic porous fibers are prepared via thermal drawing followed by freeze-casting. The process may be referred to as thermal drawing - freeze-casting (TDFC) process. Briefly, the hollow tubes (e.g. hollow polymer tubes) of different diameters and shapes may be fabricated via thermal drawing. As shown in FIG. 3 A, the hollow tubes may be fabricated from a preform placed in a drawing furnace. Then, the hollow tubes containing one or more well-dispersed viscous aqueous mixtures or solutions may pass through the cryogenic zone of a cryostat, and the tubes with the one or more mixtures or solutions may be drawn at a constant speed by the motor in the cryostat during the freeze-casting process. When the tubes pass through the cryogenic zone, ice crystals may grow directionally and rapidly from the one or more mixtures or solutions into a lamellar pattern in the tube. The ice crystals may be formed from liquid or solvents, e.g. deionized water, contained or included in the one or more mixtures. The MXene nanosheets and other nanomaterials (e.g. nanosheets, nanofibers, and nanoparticles) included in the one or more mixtures or solutions may be assembled using the ice morphology formed as a template.

[0054] A stable interface for solid/liquid is formed above the cryogenic zone, while the freezing speed is proportional to the draw speed. The anisotropic porous fibers are obtained with controllable aligned structures after freeze drying. The processing of freeze-casting may include three steps of slurry preparation, solidification, and sublimation to prepare anisotropic porous fibers. FIG. 3B shows a plot of pressure as a function of temperature illustrating the freeze-casting process including slurry preparation, solidification and sublimation according to various embodiments, with the insets showing the scanning electron microscopy (SEM) images of the fibers at different stages. FIG. 3C is a line drawing of a roll of anisotropic fibers prepared by the continuous and large-scale fabrication process according to various embodiments. [0055] Characterization Of Anisotropic Porous Fibers Via Thermal Drawing and Freeze- Casting

[0056] Like a typical freeze-casting procedure, the ice nucleation/growth as well as the microstructure of the obtained anisotropic porous fibers can be controlled by the preparation parameters, including draw speed, solution concentration/viscosity, freezing temperature, inner diameter of hollow tubes etc. Therefore, different anisotropic porous fibers with controlled interlamellar spacings (IS) have been prepared by changing these parameters in the process of TDFC with the same component weight percentage (MXene: deionized water = 95:5). FIG. 4A is a plot of interlamellar spacing (in micrometers or pm) as a function of draw speed (in rounds or R) illustrating the variation of interlamellar spacing under various prepared conditions of draw speed according to various embodiments. As shown in FIG. 4A, the interlamellar spacing (IS) is increased with the increasement of draw speed from 39.0 ± 1.0 pm to 56.0 ± 4.0 pm while keeping the other parameters constant.

[0057] FIG. 4B is a plot of interlamellar spacing (in micrometers or pm) as a function of concentration (in micrograms per microliter or mg mL ^-1 ) illustrating the variation of interlamellar spacing under various concentrations of the slurry according to various embodiments. When increasing the freezing concentration from 5 mg mL ¹ to 80 mg mL ¹ , the interlamellar spacing (IS) is sharply decreased from 79.2 ± 11.7 pm to 34.5 ± 1.8 pm.

[0058] Moreover, the temperature in TDFC also affects the IS, with the value decreasing to 17.0 ± 1.8 pm with a freezing temperature of -100 °C. FIG. 4C is a plot of interlamellar spacing (in micrometers or pm) as a function of temperature (in negative degrees Celsius or -°C) illustrating the variation of interlamellar spacing under various freezing temperatures according to various embodiments.

[0059] FIG. 4D is a plot of interlamellar spacing (in micrometers or pm) as a function of inner diameter (in micrometers or pm) illustrating the variation of interlamellar spacing with various inner diameters of the hollow polymer tube according to various embodiments. The decrease in inner diameter from 2000 pm to 200 pm of hollow tubes also reduces the IS from 18.0 ± 0.5 pm to 10.0 ± 0.2 pm. This could be mainly due to the freezing rate affecting the ice nucleation and growth. A smaller diameter of the hollow polymer tube increases the freezing rate. With the increase in the freezing rate, more nucleated ice crystals may result in smaller pores. [0060] Apart from the nanosheets for MXene/ deionized water dispersions, other nanomaterials (e.g. multi-walled carbon nanotubes (MCNT)-based nanofibers, titanium dioxide (TiCh) nanoparticles etc.) can also act as the freezing components, as illustrated in FIGS. 5A- C. FIG. 5A shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on MXene nanosheets according to various embodiments. FIG. 5B shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on multi-walled carbon nanotubes (MCNT)- based nanofibers according to various embodiments. FIG. 5C shows the scanning electron microscopy (SEM) images as well as a plot of count as a function of spacing (in micrometers or pm) of anisotropic porous fibers prepared based on titanium dioxide (TiO2) nanoparticles according to various embodiments.

[0061] With the same freezing parameters of a draw speed of 10 R, solution concentration of 80 mg mL ¹ , a freezing temperature of -100 °C, and an inner diameter of 2000 pm, anisotropic porous fibers with high aligned porous structure have successfully been fabricated by TDFC for MXene nanosheets, MCNT-based nanofibers, and TiO2 nanoparticles. According to the SEM images, the anisotropic porous fibers for MXene nanosheets, MCNT-based nanofibers, and TiO2 nanoparticles have IS of 16.2 ± 2.0 pm, 20.8 ± 2.6 pm, and 20.0 ± 2.4 pm, respectively. The differences in IS may possibly be attributed to the different dimensions and heat conductivity coefficients of the various nano-materials. FIG. 6 shows the scanning electron microscopy (SEM) images of anisotropic fibers with different traverse cross-sectional shapes prepared by freeze-casting according to various embodiments. FIG. 6 shows that various anisotropic porous fibers with different traverse cross-sectional shapes of the tubes can be ideally realized from the MXene/deionized water dispersions, e.g. triangle, rhombus, combinations of semicircle, rectangle, and so on. This shows that the TDFC as described herein may be used to fabricate various anisotropic porous fibers with controllable aligned structures. [0062] Fabrication And Characterization Of Anisotropic Porous Fibers With The Concentric Circle Structure Via Thermal Drawing And Freeze-Casting

[0063] Based on the developed TDFC process, the anisotropic porous fibers with the concentric circle structure are constructed using the improved set-up as shown in FIG. 7. FIG. 7 is a schematic showing a setup to prepare a polycarbonate (PC)/ carbon-loaded polycarbonate (CPC) / barium titanate (IV) (Ba) / MXene (M) fiber with concentric circle structure by freeze- casting according to various embodiments. The setup may include a cryostat. The cryostat may include a low temperature circulation system, and a heat conductor ring (e.g. copper ring) in thermal connection to the low temperature circulation system. The heat conductor ring that is in thermal connection to the low temperature circulation system may function as a cold source to freeze the one or more mixtures in the hollow tube. The setup may also include an injection pump configured to provide the one or more mixtures into the hollow tubes. The setup may further include a motor (not shown in FIG. 7) configured to control a draw speed of the tube with the one or more mixtures, i.e. a speed in which the tube with the one or more mixtures passes through the heat conductor ring.

[0064] FIG. 8A shows a schematic of a traverse cross-sectional view of a polycarbonate (PC)/ carbon-loaded polycarbonate (CPC) / barium titanate (IV) (Ba) / MXene (M) fiber according to various embodiments. The fiber may include an inner layer 802a of carbon-loaded polycarbonate (CPC), and an outer layer 802b of polycarbonate (PC) surrounding a circumferential surface of the inner layer 802a. The inner layer 802a may include two copper (Cu) wires 808. The wires 808 may be arranged on opposing portions of the inner layer 802a. The hollow tube with the double polymer layers 802a, 802b and the copper wires 808 may be formed using thermal drawing. The outer layer 802b of polycarbonate (PC) may have a thickness of about 150 pm, while the inner layer 802a of carbon-loaded polycarbonate (CPC) may have a thickness of about 300 pm. The two copper wires 808 may be located in the center of the inner layer 802a. In order to fabricate an anisotropic porous fiber as shown in FIG. 8A, a dispersion of MXene nanosheets in deionized water, as well as a mixture/solution of tetragonal barium titanate (IV) (TBaTiOs) and alginic acid sodium (AG) may be provided into the hollow PC/CPC tube. The dispersion of MXene nanosheets in deionized water and the mixture/solution of tetragonal barium titanate (IV) (TBaTiOs) and alginic acid sodium (AG) may be provided into the tube at the same time. The dispersion of MXene nanosheets in deionized water may form a first plurality of walls 804 in the inner core region, while the mixture/solution of tetragonal barium titanate (IV) (TBaTiOs) and alginic acid sodium (AG) may form a second plurality of walls 806 in the outer concentric ring region around the inner core region.

[0065] The fiber may include the first plurality of walls 804 including the MXene nanosheets and the second plurality of walls 806 including the tetragonal barium titanate (IV) (TBaTiOs). The walls 804, 806 may be formed due to stable interface between MXene/deionized water and TBaTiOs/AG. As shown in FIG. 8A, a first group of the plurality of spaces may be defined by the first plurality of walls 804 (in the inner core region). A second group of the plurality of spaces may be defined by the second plurality of walls 806 (in the outer concentric ring region). As also shown in FIG. 8 A, the first group of the plurality of spaces defined by the first plurality of walls 804 formed by the MXene nanosheets may be aligned along a predetermined direction (e.g. horizontal direction), while the second group of the plurality of spaces defined by the second plurality of walls 806 formed by the nanomaterials may not be aligned along the predetermined direction. Instead, the second group of the plurality of spaces defined by the second plurality of walls 806 may extend radially from a center of the fiber to the inner layer of the tube 802a. The ends of the first group of the plurality of spaces may be bound/defined by internal end walls which separate the first plurality of walls 804 and the second plurality of walls 806. On the other hand, one end of the second group of the plurality of spaces may be bound/defined by inner layer 802a, while another end of the second group of the plurality of spaces may be bound/defined by the internal end walls which separate the first plurality of walls 804 and the second plurality of walls 806.

[0066] The ice crystals may be grown directionally in the process of freeze-casting, and the fiber may be freeze-dried after the freeze-casting process. FIG. 8B shows a line drawing of the fiber according to various embodiments. The white and black rings indicate TBaTiOs/AG and MXene/deionized water, respectively.

[0067] FIG. 9A is a scanning electron microscopy (SEM) image showing the concentric circle structure of a fiber according to various embodiments. FIG. 9A shows a traverse crosssection of the fiber. The concentric circle structure may be formed due to freeze-casting. FIG. 9A shows that the obtained fibers include polymer layers and inner walls formed by MXene nanosheets and other nanomaterials. FIG. 9B is a magnified scanning electron microscopy (SEM) image showing the concentric circle structure of a fiber according to various embodiments. FIG. 9B also shows a traverse cross-section of the fiber. As shown in FIG. 9B, the anisotropic porous walls including BaTiOs are located around the anisotropic porous walls of MXene nanosheets. FIG. 9C shows further magnified scanning electron microscopy (SEM) images of portions of the fiber shown in FIG. 9B according to various embodiments. The further magnified images show the MXene nanosheets and the BaTiOs nanoparticles. The heterojunction between TBaTiOs/AG based walls and MXene/deionized water based walls may be formed due to the interfacial interactions of covalent bonds between the MXene nanosheets and the TBaTiOs nanoparticles. The anisotropic porous fibers with the concentric circle structure may be used for various applications.

[0068] Applications For Acoustic Energy Harvesting Of Anisotropic Porous Fibers

[0069] Acoustic waves may act as a sustainable, clean, and ubiquitous energy source from living activities, traffic, and construction sites etc. Acoustic energy is one of the energies abundant in daily life that is wasted. Until now, acoustic energy harvesting has not been as popular as other types of energy harvesting, such as harvesting of solar energy, thermoelectric energy, wind energy, and so on. This is mainly because in addition to acoustic waves being able to output only at a much lower power density, effective and relevant technology is also challenging to develop for harvesting of such energy from various sound noises. Moreover, although many approaches have been developed and used to harvest acoustic energy based on piezoelectric and electrostatic effects, there are limits on their performances, such as low power density, low energy conversion efficiency, high requirements of active materials, and high frequencies generated at the range of a few kilo-hertz (kHz) to mega-hertz (MHz) etc.

[0070] Piezoelectric TBaTiOs is a kind of active nanoparticle which can convert acoustic waves into electric energy based on its high piezoelectric coefficient. An acoustic energy harvester may be formed from various embodiments of the fiber as described herein. The TBaTiOs/AG may function as the negative active material, while the MXene/deionized water may function both as the positive active material and electrical conductor. The CPC inner layer with the two copper (Cu) wires may accelerate electron transport. The CPC inner layer is in contact with the walls including TBaTiCh/AG, and may act as an electrical conductor during acoustic energy harvesting. When acoustic waves are incident on the walls including TBaTiCh/AG, the TBaTiOs/AG may generate electrical energy due to the pressure of the acoustic waves.

[0071] FIG. 10A shows a schematic of a setup for testing the performance of acoustic energy harvesting of anisotropic porous fibers according to various embodiments. As shown in FIG. 10A, the fiber 1002 may be suspended on a metal holder 1004. A loudspeaker 1006 is electrically connected to a waveform generator 1007 and used to generate acoustic waves which are subsequently converted by the fiber 1002 to electrical energy. An electrometer 1008 is electrically connected to both ends of the fiber 1002. The output of the electrometer 1008 is shown by a single digital phosphor oscilloscope 1010 coupled to the electrometer 1008. [0072] According to the single digital phosphor oscilloscope 1010, the obtained anisotropic porous fiber 1002 (i.e. PC/CPC/Ba/M fiber) can produce a maximum voltage of ~24 mV within 0.15 s. FIG. 10B is a plot of voltage (in volts or V) as a function of time (in seconds or s) illustrating the output voltage of the fiber 1002 according to various embodiments in response to the loudspeaker 1006 after the signal has been amplified ~50 times and shown by the digital phosphor oscilloscope 1010. FIG. 10C is a plot of voltage (in milli-Volts or mV) as a function of frequency (in hertz or Hz) illustrating the voltage recorded by the electrometer 1008 in response to the loudspeaker 1006 according to various embodiments. For the frequency range from 100 Hz to 900 Hz, the anisotropic porous fibers generated output voltages of ~10 mV to ~24 mV. The high performance of the fibers for acoustic energy harvesting may be attributed to the active materials and the anisotropic porous structures. Therefore, the fibers prepared by TDFC may be an effective acoustic energy harvester of various sound noises.

[0073] Various embodiments may provide a large-scale, continuous and effective strategy to fabricate anisotropic porous fibers with controllable aligned structures via a combination of thermal drawing and freeze-casting. Due to directional growth of ice crystals, various anisotropic porous fibers may be prepared with controlled interlamellar spacings by changing the parameters of the TDFC process. Consequently, the resulting anisotropic porous fibers with highly aligned structures may be fabricated from nanosheets, nanofibers, and nanoparticles. An anisotropic porous PC/CPC/Ba/M fiber with the concentric circle structure may be fabricated using this approach. Meanwhile, due to the aligned structures including the active materials of TBaTiOs nanoparticles and MXene nanosheets, the obtained anisotropic porous fibers may be used for acoustic energy harvesting with a high performance of voltage output. The obtained results indicate that the strategy may provide an effective way to fabricate anisotropic porous fibers for intelligent textile materials with the function of acoustic energy harvesting. Various embodiments may relate to the preparation of other materials to construct anisotropic porous fibers.

[0074] Materials

[0075] MAX (TisAK/y powders (particle size of ~ 400 mesh) were purchased from Jilin 11 Technology Co., Ltd. Lithium fluoride, deionized water (H2O), cubic barium titanate (BaTiOs) (IV), titanium (IV) oxide, alginic acid sodium salt, and multi-walled carbon nanotube were purchased from Sigma-Aldrich Co., Ltd. Polycarbonate (PC) film was obtained from Goodfellow Co., Ltd. All materials were used as received. [0076] Preparation Of MXene Nanosheet Dispersion

[0077] MAX (TisAKZy powders (1.8 g) were added to a 9 M hydrochloric (HC1) solution (40 mL) that contained LiF (1.8 g) under stirring at 45 °C for 30 hours. When the solution reaction was completed, the accordion-like MXene bulk was respectively washed using hydrochloric (HC1) solution (9 M) (three cycles) and deionized water (about eight cycles), and each cycle involved 5 minutes of centrifugation at 3500 rpm. When a supernatant solution reached pH ~ 7, 200 mL of deionized water was added to the resulting sediments, followed by continuously vibrating of the resulting solution for 13 minutes. After centrifugation of the vibrated solution at 1,500 rpm for 0.5 hours, the sediments were obtained from the supernatant solution by centrifuging at 3,500 rpm for another 0.5 hours. Finally, various concentration mixtures/solutions of MXene nanosheets were prepared by dispersing the sediments in deionized water.

[0078] Preparation Of Tetragonal Barium Titanate (IV) Nanoparticles

[0079] Cubic barium titanate (IV) nanoparticles (10 g) were placed in an alumina crucible and heated at 1200 °C for 24 h with air in a tube furnace. After natural cooling, the annealed powder was grounded into a fine powder of tetragonal barium titanate (IV) nanoparticles.

[0080] Preparation Of Alginic Acid Solution Salt Solution

[0081] 3 g alginic acid sodium salt was added into 50 mL deionized water. Then, the solution was stirred at 50 °C for 12 hours to obtain a concentration of 60 mg mL ¹ .

[0082] Preparation Of MXene/Deionized water Dispersion

[0083] Different weight percentages of glutaraldehyde solution were added to solution/mixture of MXene nanosheets. Then, the resulting mixtures were each stirred for 12 hours to prepare various concentrations of MXene/deionized water dispersions.

[0084] Preparation Of Nano-materials Mixture/Solution

[0085] Various weight percentages of tetragonal barium titanate (IV), titanium (IV) oxide, and multi-walled carbon nanotubes were added into the alginic acid sodium salt solution and stirred for 12 hours to prepare nano-materials mixtures/solutions.

[0086] Preparation Of Hollow Polymer Tube Via Thermal Drawing

[0087] Polycarbonate (PC) film was wrapped in polytetrafluoroethylene (PTFE) and stored in the vacuum oven at 165 °C for 1.5 hours to prepare the PC rod to preform. In order to prepare the hollow PC/CPC (carbon-loaded polycarbonate) (Cu wire) tube, PC films and CPC films were successively wrapped on the polytetrafluoroethylene (PTFE) rod with a diameter of 30 mm with two smaller rods (i.e. rods with diameters smaller than 30 mm), e.g. PTFE rods, and heated in the vacuum oven at 220 °C for 1.5 hours. Then, hollow PC and PC/CPC (Cu wire) tubes with various inner diameters were fabricated via the thermal drawing process using various draw-down ratios. A copper wire may be inserted in each of the two smaller PTFE rods during the thermal drawing process. The preform was placed in a two-zone heating furnace with the top zone and the bottom zone heated to 150 °C and 350 °C, respectively.

[0088] Fabrication Of Anisotropic Porous Fibers Using Freeze-Casting

[0089] A mixture containing different nanomaterials was fed into various hollow polymer tubes in advance. A copper ring connected to the low-temperature circulation system was used as the cold source. When the hollow tube with the mixture was passed through the cold copper (Cu) ring, it gradually froze. By controlling the draw speed of the tubes using the motor and the temperature of the Cu ring cold source, the tubes could be frozen at a controllable speed. Finally, the anisotropic porous fibers were continuously prepared on a large scale by freeze- drying for more than 48 h at -50 °C via using the freeze dryer under ~5 Pa pressure.

[0090] Characterization

[0091] Scanning electron microscopy (SEM) images were conducted using a JEOL JSM- 7600F Schottky field emission scanning electron microscope at a voltage of 5 kV. Acoustic energy harvesting was tested using an electrometer (Keithley 6517B) and a digital phosphor oscilloscope (DPO 5104B). A function/arbitrary waveform generator (Agilent 33250A) was used as a signal generator to power a loudspeaker via outputting the desired frequency.

[0092] Measurement For Acoustic Energy Harvesting Of Anisotropic Porous Fibers

[0093] For the test, the samples with a length of 5 cm were fixed on a metal holder and placed 50 cm from the loudspeaker. Copper (Cu) wires were connected to both ends of fibers and the electrometer. When acoustic waves were generated by the loudspeaker, the output electrical signals generated by the fibers, including voltage and current signals, were recorded by the electrometer with software installed on a computer.