POWDER-ASSEMBLED COMPOSITE MICRO-NANO FIBER AND PREPARATION METHOD THEREOF

Abstract

The present invention provides a powder-assembled composite micro-nano fiber and a method for preparing the powder-assembled composite micro-nano fiber. The method includes the following steps: (1) preparing two-dimensional cellulose from a cellulose-raw-material; (2) dispersing the two-dimensional cellulose and a powder material in a solvent to form a mixed suspension; and (3) performing freeze drying on the mixed suspension to obtain the powder-assembled composite micro-nano fiber, wherein a temperature difference between two ends in a vertical direction of the mixed suspension is controlled to be 10-100? C. in a freezing process of the freeze drying. In the present invention, a new non-destructive processing path from primary particles to macro applications is constructed, and rich material platforms and infinite possibilities are provided for basic studies and technical applications. The technology will play a huge role in energy, medical material, environment, protection, catalysis, photoelectricity, food engineering, daily necessity and other vast fields.

Claims

1. A method for preparing a powder-assembled composite micro-nano fiber, comprising the steps of: (1) preparing a two-dimensional cellulose from a cellulose-raw-material; (2) dispersing the two-dimensional cellulose and a powder material in a solvent to form a mixed suspension; and (3) performing freeze drying on the mixed suspension to obtain the powder-assembled composite micro-nano fiber, wherein: the two-dimensional cellulose has a flake structure with a thickness of 0.1-20 nm; a size of the two-dimensional cellulose in a planar direction is greater than 1 micrometer; a ratio of the maximum size in the planar direction to the thickness is greater than 200; a temperature difference between two ends in a vertical direction of the mixed suspension is controlled to be 10-100? C. in a freezing process of the freeze drying in step (3); preparing the two-dimensional cellulose from the cellulose-raw-material in step (1) comprises the steps of: (1-1) partially dissociating the cellulose-raw-material in a first solvent with high-frequency ultrasound to form a suspension of cellulose nanofibers with continuous branched structures; and (1-2) replacing the first solvent in the suspension with a second solvent, and then performing freeze drying on the obtained cellulose nanofiber suspension to obtain a crude two-dimensional cellulose; the first solvent is selected from one or more of water, ethanol, and ethylene glycol; the second solvent is selected from one or more of tert-butanol, benzene, and methylbenzene; and a temperature difference between two ends in a vertical direction of the suspension is controlled to be 10-100? C. in a freezing process of the freeze drying in step (1-2).

2. (canceled)

3. The method in claim 1, wherein step (1) further comprises: repeating steps (1-1) and step (1-2) twice to five times for improving the purity of the two-dimensional cellulose to 95% or above.

4. The method in claim 1, wherein: the degree of crystallization of the two-dimensional cellulose is less than that of ligno-based cellulose.

5. The method in claim 1, wherein: forming the mixed suspension in step (2) includes dispersing the two-dimensional cellulose and the powder material in the solvent simultaneously or sequentially.

6. The method in claim 1, wherein step (2) comprises: dispersing the two-dimensional cellulose in the solvent to form a two-dimensional cellulose suspension; and dispersing the powder material in the two-dimensional cellulose suspension to form the mixed suspension.

7. The method in claim 1, wherein: the concentration of the two-dimensional cellulose in the mixed suspension is 0.01-10 g/L.

8. The method in claim 7, wherein: the concentration of the two-dimensional cellulose in the mixed suspension is 1-5 g/L.

9. The method in claim 1, wherein: the volume fraction f of the powder material in the mixed suspension is 0.001-10; $f=\frac{V1}{M2},$ where V1 is a bulk volume of the powder material; $V1=\frac{M1}{d1};$ M1 is a mass of the powder material; d1 is a tap density of the powder material; and M2 is a mass of the two-dimensional cellulose.

10. The method in claim 9, wherein: the volume fraction f of the powder material in the mixed suspension is 0.1-9.

11. The method in claim 1, wherein: step (2) comprises ultrasonically dispersing the mixed suspension containing the two-dimensional cellulose and the powder material for 1 second to 1 hour at -10 to 30? C.

12. The method in claim 1, wherein: the freezing process in step (3) is carried out with the assistance of an ultra-low temperature fluid fumigation device, so as to control the temperature difference.

13. The method in claim 12, wherein: the ultra-low temperature fluid is at least one of liquid helium, liquid nitrogen and liquid oxygen.

14. The method in claim 1, wherein: the drying process of the freeze drying in step (3) is carried out in a freeze dryer; and the duration of freeze drying is 2-80 hours.

15. The method in claim 1, wherein: the cellulose-raw-material in step (1) is plant cellulose fiber or a bacterial cellulose fiber; and the two-dimensional cellulose is self-assembled by plant cellulose fibers or bacterial cellulose fibers through hydrogen bonds.

16. The method in claim 15, wherein the plant cellulose is taken from at least one of moso bamboo, neosinocalamus affinis, spruce, fir, Korean pine, poplar, sorghum stalk, corn stalk, mulberry bark, bark of Wikstroemiapilosa Cheng, rice straw, wheat straw, reed, cotton, cotton linter, kenaf, jute, flax, banana leaf, agave hemp, Chinese alpine rush, Cyperus malaccensis and bagasse.

17. The method in claim 15, wherein: microorganisms for preparing the bacterial cellulose are selected from at least one of Acetobacter, Rhizobium, Gluconobacter, Acetobacter xyloides, Sarcina, Pseudomonas, Achromobacter, Alcaligenes, Aerobacter, azotobacter and agrobacterium; and a carbon source of a fermentation culture medium for preparing the bacterial cellulose is selected from at least one of glucose, sucrose, lactose, maltose, mannitol and arabinose.

18. The method in claim 1, wherein a thickness of the two-dimensional cellulose is 0.5-5 nanometers.

19. The method in claim 1, wherein the content of amorphous cellulose in the two-dimensional cellulose is 30-50 wt %.

20. The method in claim 1, wherein a particle size of the powder material is 1 nanometer-10 micrometers.

21. The method in claim 1, wherein: a diameter of the composite micro-nano fiber prepared with the method is 0.01-20 micrometers; and a length of the composite micro-nano fiber prepared with the method is 100 micrometers-50 millimeters.

22. The method in claim 1, wherein: the solvent is at least one of water, ethanol, ethylene glycol, tert-butanol, benzene and methylbenzene.

23. A powder-assembled composite micro-nano fiber prepared with the method in claim 1.

24. The powder-assembled composite micro-nano fiber in claim 23, wherein: a diameter of the composite micro-nano fiber is 0.01-20 micrometers; and a length of the composite micro-nano fiber is 100 micrometers-50 millimeters.

25. The powder-assembled composite micro-nano fiber in claim 23, wherein: the content of the powder material in the composite micro-nano fiber is 0.01 wt %-95 wt %.

26. The powder-assembled composite micro-nano fiber in claim 23, wherein: the powder material in the composite micro-nano fiber is wrapped in the two-dimensional cellulose in a layer-by-layer winding manner; and the powder material shows no structural and morphological damage.

Description

BRIEF DESCRIPTION OF FIGURES

[0047] FIG. 1 is taken from FIG. 4 in SHELDON et al., Vapor-phase Fabrication and Properties of Continuous-Filament Ceramic Composites, Science, Sep. 6, 1991, 1104-1109, Vol. 253;

[0048] FIG. 2 is taken from FIG. 1 in BAO et al., One-pot Synthesis of Pt-Co Alloy Nanowire Assemblies with Tunable Composition and Enhanced Electrocatalytic Properties, Angew. Chem. Int. Ed., 2015, 3797-3801, Vol. 54;

[0049] FIG. 3 is taken from FIG. 2 in WU et al., Stable Cycling of Double-walled Silicon Nanotube Battery Anodes through Solid-electrolyte Interphase Control, Nature Nanotechnology, May 2012, 310-315, Vol. 7;

[0050] FIG. 4 is taken from FIG. 2 in NIE et al., Properties and Emerging Applications of Self-assembled Structures Made from Inorganic Nanoparticles, Nature Nanotechnology, Jan. 2010, 15-25, Vol. 5;

[0051] FIG. 5 is taken from FIG. 1 in CHANG et al., Reversible Fusion and Fission of Graphene Oxide-based Fibers, Science, May 7, 2021, 614-617, Vol. 372;

[0052] FIG. 6 is taken from FIG. 2B in BAUMGARTEN, Electrostatic Spinning of Acrylic Microfibers, Journal of Colloid and Interface Science, May 1971, 71-79, Vol. 36;

[0053] FIG. 7 is taken from FIG. 4 in KAUFMAN et al., Structured Spheres Generated by an In-fibre Fluid Instability, NATURE, Jul. 18, 2012, 463-467, Vol. 487(7408);

[0054] FIG. 8 is taken from FIG. 4 in MURPHY et al., Microcrystalline Cellulose Reinforced Polylactic Acid Biocomposite Filaments for 3D Printing, Polymer Composites, 2018, 1311-1320, Vol. 39(4);

[0055] FIG. 9 is an exemplary thickness of two-dimensional cellulose prepared with the present invention;

[0056] FIG. 10 is a scanned electron microscopy (SEM) image of two-dimensional cellulose prepared with the present invention;

[0057] FIG. 11 is a comparative diagram of degrees of crystallization of two-dimensional cellulose prepared with the present invention and lignocellulose paper;

[0058] FIG. 12 is a comparative diagram of chemical structures of two-dimensional cellulose prepared with the present invention and traditional cellulose;

[0059] FIG. 13 is an atomic force microscope (AFM) image of the two-dimensional cellulose prepared in Embodiment 1 of the present invention;

[0060] FIG. 14 is a transmission electron microscope (TEM) image of cellulose nano fibers with continuous branched structures prepared in step 1 of Embodiment 2 of the present invention;

[0061] FIG. 15 is a SEM image of crude two-dimensional cellulose prepared in step 2 of Embodiment 2 of the present invention;

[0062] FIG. 16 is an SEM image and an AFM image of high-quality two-dimensional cellulose prepared in step 3 of Embodiment 2 of the present invention;

[0063] FIG. 17 is a TEM image of a manganese dioxide nanoflower powder-assembled composite micro-nano fiber prepared in Embodiment 3 of the present invention;

[0064] FIG. 18 is an SEM image of a manganese dioxide nanoflower powder-assembled composite micro-nano fiber prepared in Embodiment 3 of the present invention;

[0065] FIG. 19 is an SEM image of a nano iron oxide-assembled composite micro-nano fiber prepared in Embodiment 4 of the present invention;

[0066] FIG. 20 is an optical microscope image of a nano iron oxide thread prepared in Embodiment 4 of the present invention;

[0067] FIG. 21 is an SEM image of a ten-element mixed powder-assembled composite micro-nano fiber prepared in Embodiment 5 of the present invention;

[0068] FIG. 22 is an element distribution diagram corresponding to the SEM image of the ten-element mixed powder-assembled composite micro-nano fiber prepared in Embodiment 5 of the present invention;

[0069] FIG. 23, FIG. 24 and FIG. 25 are SEM images of 46-representative-powder-assembled composite micro-nano fibers prepared in Embodiment 6 of the present invention;

[0070] FIG. 26 is an SEM enlarged view of 16-representative-powder-assembled composite micro-nano fibers prepared in Embodiment 6;

[0071] FIG. 27, FIG. 28, FIG. 29, FIG. 30, FIG. 31 and FIG. 32 are X-Ray Diffraction (XRD) images of representative powder materials before being assembled and after being assembled into a composite micro-nano fiber in Embodiment 6 of the present invention;

[0072] FIG. 33 shows SEM images of a hydroxyapatite nanowire (a and b) and a hydroxyapatite nanowire/Zn-MOF fiber (c and d) prepared in Embodiment 7;

[0073] FIG. 34 shows an SEM image of a diameter distribution of a representative sample fiber prepared in Embodiment 8, and a composite SEM image formed by splicing high-resolution SEM images of ZnO-GAF along a fiber direction;

[0074] FIG. 35 is an SEM image of a composite micro-nano fiber prepared in Embodiment 9 of the present invention;

[0075] FIG. 36 is a steady-state thermal measurement result of a bulk structural body material prepared from a silicon carbide ceramic powder-assembled fiber in Embodiment 10 of the present invention;

[0076] FIG. 37 is an SEM image of a SnO.sub.2/CN-GAF film prepared in Embodiment 11 and a SnO2/CNT/PVDF film prepared in comparative example 2;

[0077] FIG. 38 shows a gas-sensitive property test result of the SnO.sub.2/CN-GAF film prepared in Embodiment 11 and a SnO.sub.2/CNT/PVDF film prepared in comparative example 2; and

[0078] FIG. 39 is an SEM image of a zinc oxide powder-assembled composite micro-nano fiber prepared in comparative example 1 where a temperature difference is only 1? C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0079] The present invention will be further described below in conjunction with the embodiments. The embodiments are only explanatory, and are not meant to limit the scope of the present invention in any way.

[0080] In the following embodiments, for the sake of brevity, the abbreviation GAF stands for micro-nano fiber.

Embodiment 1

[0081] (1) 10 grams of moso bamboo cellulose was put into 100 ml of a mixed solution of water and ethanol (3:1); 100 ml of hydrochloric acid aqueous solution (2 mol/L) was added; the mixture was heated at 80? C. for 5 hours.

[0082] (2) After filtration, a mixed solution of cellulose molecules/hydrochloric acid was obtained. The hydrochloric acid, water and ethanol in the suspension were then replaced with tert-butanol to obtain a high-purity cellulose molecule tert-butanol solution.

[0083] (3) 0.05 ml of supernatant obtained in step (2) was dropped to a surface of an ice cube at ?50? C.; after the supernatant was frozen, the frozen supernatant on the surface of the ice cube was scrapped off with a scraper.

[0084] (4) The frozen supernatant was placed in vacuum for drying at a low pressure to obtain two-dimensional cellulose.

[0085] FIG. 13 is an AFM image of the two-dimensional cellulose obtained in step 4 of this embodiment, which confirms that the two-dimensional cellulose can be obtained by this method and has a thickness of about 4 nanometers.

Embodiment 2

[0086] (1) 50 grams of poplar cellulose was put into 1 liter of a mixed solvent of ethanol and water (1:1); and after 50 ml of formic acid (99.9%, Aladdin reagent) was added, ultrasonic treatment was performed at power of 200 W for 10 minutes to obtain ethanol/water suspension of cellulose nano fibers with continuous branched structures (FIG. 14).

[0087] (2) The ethanol and the formic acid in the suspension were replaced with water to obtain cellulose nano fiber aqueous suspension with continuous branched structures. The suspension was put in a container, and the temperatures of upper and lower ends were controlled to be 0? C. and ?50? C. respectively under the assistance of ultra-low temperature liquid oxygen fluid fumigation, and a temperature difference was 50? C., so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?40? C. for 80 hours to obtain crude two-dimensional cellulose (FIG. 15).

[0088] (3) Steps (1) and (2) were repeated for three times on the obtained crude two-dimensional cellulose to obtain high-quality two-dimensional cellulose (a and b in FIG. 16).

[0089] FIG. 14 is a TEM image of the cellulose nano fibers with continuous branched structures prepared in step 1 of this embodiment, showing that the cellulose nano fibers have the continuous branched structures.

[0090] FIG. 15 is an SEM image of the crude two-dimensional cellulose prepared in step 2 of this embodiment, showing that only part of the crude two-dimensional cellulose is transformed into two-dimensional cellulose.

[0091] FIG. 16 shows an SEM image and an AFM image of the high-quality two-dimensional cellulose obtained in step 3 of this embodiment, showing that the cellulose has been completely transformed into a 2D structure with a thickness of 5 nanometers.

Embodiment 3

[0092] (1) Manganese dioxide nanoflower powder with a volume fraction of 1.0 [about 60 nanometers; prepared according to the method in LIU et al., One-step Synthesis of Single-Layer MnO.sub.2 Nanosheets with Multi-Role Sodium Dodecyl Sulfate for High-Performance Pseudocapacitors, small, 2015, Vol. 11(18), which is incorporated herein by reference in its entirety] was added to aqueous suspension of two-dimensional cellulose (which was extracted and processed from the moso bamboo, and had a thickness of 3-5 nanometers) with a concentration of 6 g/L; ultrasonic dispersion was performed for 30 minutes; and the temperature was controlled at 10? C.

[0093] (2) The manganese dioxide nanoflower/two-dimensional cellulose suspension was put in a container, and the temperatures of upper and lower ends were controlled to be 5? C. and ?45? C. respectively under the assistance of ultra-low temperature liquid oxygen fluid fumigation, and a temperature difference was 50? C., so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?50? C. for 64 hours to obtain a manganese dioxide nanoflower powder-assembled composite micro-nano fiber.

[0094] FIG. 17 is a TEM image of the manganese dioxide nanoflower powder-assembled composite micro-nano fiber prepared in this embodiment. This figure shows that the manganese dioxide nanoflower structure in the fiber is intact.

[0095] FIG. 18 is an SEM image of the manganese dioxide nanoflower powder-assembled composite micro-nano fiber prepared in this embodiment. This figure shows that the structural form of the manganese dioxide nanoflower powder-assembled composite micro-nano fiber is intact, and all the fibers are of a fibrous structure.

Embodiment 4

[0096] (1) Nano iron oxide powder (30 nanometers, Aladdin reagent) with a volume fraction of 2.0 was added into aqueous solution of two-dimensional cellulose (which was extracted from basswood and had a thickness of 3-5 nanometers) with a concentration of 5 g/L; ultrasonic dispersion was performed for 15 minutes, and the temperature was controlled at 15? C.

[0097] (2) The nano iron oxide/two-dimensional cellulose mixed suspension prepared in step (1) was put into a container, and the temperatures of upper and lower layers were controlled to be 20? C. and ?80? C. respectively under the assistance of ultra-low temperature liquid helium fluid fumigation, and a temperature difference was 100? C., so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?40? C. for 80 hours to obtain a nano iron oxide powder-assembled composite micro-nano fiber.

[0098] (3) The nano iron oxide powder-assembled composite micro-nano fiber prepared in step (2) was twisted into a thread to obtain a nano iron oxide thread with a diameter of hundreds of micrometers.

[0099] FIG. 19 is an SEM image of the nano iron oxide-assembled composite micro-nano fiber prepared in step (2) of this embodiment. This figure shows that the structural form of the nano iron oxide-assembled composite micro-nano fiber is intact, and all the fibers are of a fibrous structure.

[0100] FIG. 20 is an optical microscope image of the nano iron oxide thread prepared in step (3) of this embodiment. It is confirmed that the micro-nano fiber prepared in this embodiment can be further processed.

Embodiment 5

[0101] (1) Carbon nano tubes, titanium carbide, molybdenum sulfide, manganese dioxide, nickel, silicon dioxide, diamond, silicon nitride, zinc-MOF and gadolinium oxide ten-element nano mixed powder (in which powder of each component had a size distribution of 30 nanometers to 5 micrometers, and was purchased from Aladdin and McLean and prepared in a laboratory) with a total volume fraction of 1.0 was added into tert-butanol suspension of two-dimensional cellulose (which was extracted and processed from poplar and had a thickness of 3-5 nanometers) with a concentration of 2 g/L; ultrasonic dispersion was performed for 10 minutes, and the temperature was controlled at 10? C.

[0102] (2) The ten-element mixed powder/two-dimensional cellulose mixed suspension prepared in step (1) was put into a container, and the temperatures of upper and lower layers were controlled to be 0? C. and ?98? C. respectively under the assistance of ultra-low temperature liquid nitrogen fluid fumigation, and a temperature difference was 98? C., so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?40? C. for 64 hours to obtain a ten-element mixed powder-assembled composite micro-nano fiber.

[0103] FIG. 21 is an SEM image of the ten-element mixed powder-assembled composite micro-nano fiber prepared in this embodiment. This figure shows that the structural form of the composite micro-nano fiber is intact; there is no powder residue; and the forming effect is excellent.

[0104] FIG. 22 is an element distribution diagram corresponding to the SEM image of the ten-element mixed powder-assembled composite micro-nano fiber prepared in this embodiment. This figure shows that the various kinds of powder are distributed uniformly in the figure and are not agglomerated, which has confirmed that this method has no requirement for the physiochemical properties of the powder.

Embodiment 6

[0105] A 46-representative-powder-assembled composite micro-nano fiber was prepared according to the same method as that in Embodiment 1. 46 kinds of representative powder include a metal element, a non-metal element, an oxide, a carbide, a nitride, a sulfide, a phosphide and an organic matter. Specifically, the 46 kinds of representative powder include: tungsten powder, niobium powder, nickel powder, titanium powder, molybdenum powder, tantalum powder, carbon powder, silicon powder, diamond, silver powder, iron powder, chromium oxide, tungsten oxide, hydroxyapatite, holmium oxide, barium titanate, indium oxide, nickel oxide, zinc oxide, aluminum oxide, silicon dioxide, cerium oxide, magnesium oxide, strontium titanate, samarium oxide, gadolinium oxide, iron oxide, lithium iron phosphate, manganese dioxide, tin antimony oxide, zirconium oxide, zinc ferrite, copper oxide, zirconium carbide, silicon carbide, titanium carbide, tungsten carbide, silicon nitride, titanium nitride, molybdenum sulfide, tungsten sulfide, germanium phosphide, zinc -MOF, polypyrrole, iron-MOF and polystyrene.

[0106] FIG. 23, FIG. 24 and FIG. 25 show the SEM images of the 46-representative-powder-assembled composite micro-nano fiber. A result shows that these materials can well form fibrous structures.

[0107] FIG. 26 is an SEM enlarged view of a 16-representative-powder-assembled composite micro-nano fiber in this embodiment. This figure shows that the microstructures of the powder are well retained in the fiber.

[0108] FIGS. 27-32 are XRD images of the representative powder materials in this embodiment before being assembled and after being assembled into a composite micro-nano fiber. According to the XRD images, a crystal structure of the powder material does not change before and after fibering.

Comparative Example 1

[0109] A zinc oxide powder-assembled composite micro-nano fiber was prepared according to the same method as that in Embodiment 1, and a difference lied in that a temperature difference between the upper end and the lower end in the freezing process of freeze drying in the step (2) was only 1? C.

[0110] A result is shown in FIG. 39. It can be seen by comparison with the result of Embodiments 1-4 that when the temperature difference between the upper end and the lower end is extremely small or there is no temperature difference, only a small part in the material is formed into a fiber, and most parts are still kept in the original flake structure.

Embodiment 7

[0111] (1) Mixed powder of a hydroxyapatite nanowire [Feynman (Liaoning Province)Nanomaterials Technology Co., Ltd.] and zinc-MOF (AVCI et al., Self-assembly of Polyhedral Metal-organic Framework Particles into Three-dimensional Ordered Superstructures, Nature Chemistry, 2018, 78-84, Vol. 10, which is incorporated herein by reference in its entirety) with a volume fraction of 5.0 was added into aqueous suspension of two-dimensional cellulose (which was extracted and processed from Acetobacter bacteria and had a thickness of 3-5 nanometers) with a concentration of 5 g/L; ultrasonic dispersion was performed for 15 minutes, and the temperature was controlled at 15? C.

[0112] (2) The hydroxyapatite nanowire/zinc-MOF/two-dimensional cellulose mixed suspension prepared in step (1) was put into a container, and the temperatures of upper and lower layers were controlled to be 20? C. and ?80? C. respectively under the assistance of ultra-low temperature liquid helium fluid fumigation, and a temperature difference was 100? C., so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?40? C. for 80 hours to obtain a hydroxyapatite nanowire/zinc-MOF powder-assembled composite micro-nano fiber.

[0113] FIG. 33 shows SEM images of a commercially available hydroxyapatite nanowire raw material (a and b) and the hydroxyapatite nanowire/Zn-MOF fiber (c and d) prepared in this embodiment. It can be seen from FIG. 33 that the hydroxyapatite nanoparticles are arranged along a radial direction of the composite fiber, and zinc-MOF nano particles are inlaid in the hydroxyapatite nanowire.

Embodiment 8

[0114] (1) A zinc oxide powder-assembled micro-nano composite fiber was prepared according to the same method as that in Embodiment 1, and the volume fraction of zinc oxide powder was controlled (0.01-10) to obtain representative functional fibers with different powder contents and thicknesses, and images of single composite fiber samples were taken.

[0115] FIG. 34a shows samples with different powder contents prepared in this embodiment, with a thickness distribution range from 15 nanometers to 10 micrometers. FIG. 34b is an SEM image of a single zinc oxide composite fiber prepared in this embodiment, showing that a fiber length can reach a millimeter level.

Embodiment 9

[0116] (1) Silicon powder and tantalum powder were used as powder materials according to the same method as that in Embodiment 1; the volume fraction of the tantalum powder was controlled to be 3.46; and the volume fraction of silicon was controlled to be 0.0625.

[0117] FIG. 35a and FIG. 35b are SEM images of a tantalum composite nano fiber prepared in this embodiment. The mass content of the powder material is less than 1%; FIG. 35c and FIG. 35d are SEM images of a silicon composite fiber prepared in this embodiment. The mass content of the powder material is greater than 95%.

[0118] It can be seen from Embodiments 6 and 7 that the thickness of the composite micro-nano fiber prepared by the method of the present invention is related to the content of the powder material. As the powder content increases, the thickness of the fiber increases. The minimum thickness is dozens of nanometers, and the maximum thickness can be up to dozens of micrometers (FIG. 35). The content of the powder material in the composite micro-nano fiber of the present invention can be as small as less than 1% or as large as greater than 95%.

Embodiment 10

[0119] In this example, the thermal insulation performance of the silicon carbide ceramic powder-assembled composite micro-nano fiber prepared in Embodiment 6 of the present invention was tested. A specific test method was as follows:

[0120] The thermal conductivity of the silicon carbide composite micro-nano fiber and commercially available silicon carbide ceramics (from Shenzhen Kejing Star Technology Co., Ltd.) in air was measured with a thermal constant analyzer (TPS 2500S, Hot Disk): a heater was from 25? C. to 300? C., 600? C. and 1000? C., and cooling was from 25? C. to -40? C.

[0121] The steady-state thermal measurement in FIG. 36 shows that thermal conductivity coefficients of the material prepared in this embodiment in air is 0.013 W/(m.Math.K) at ?40? C. and is 0.067 W/(m.Math.K) at 1000? C., which are far less than 22.17 W/(m.Math.K) and 6.55 W/(m.Math.K) of pure SiC at ?40? C. and 1000? C., indicating that the material of the present invention exhibits more excellent performance. This may be related to complete retention and good exposure of the original micro-nano structure of the powder in the fiber.

Embodiment 11

[0122] In this embodiment, a SnO.sub.2/CNT mixed powder-assembled fiber film was prepared, and its gas-sensitive property was tested.

[0123] (1) Nano SnO.sub.2 powder (50-70 nanometers, Aladdin reagent) with a volume fraction of 3 and 0.5 g/LCNT (99.9%, Aladdin reagent) were added into aqueous solution of two-dimensional cellulose (which was extracted from fir and had a thickness of 3-5 nanometers) with a concentration of 2 g/L; ultrasonic dispersion was performed for 15 minutes, and the temperature was controlled at 0? C.

[0124] (2) The mixed suspension prepared in step (1) was put into a container, and a temperature difference between upper and lower layers was controlled to be 50? C. under the assistance of ultra-low temperature liquid helium fluid fumigation, so as to control the crystallization rate of the mixed suspension. After being completely frozen, the suspension was put into a freeze dryer for freeze drying at ?40? C. for 72 hours to obtain a SnO.sub.2/CNT mixed powder-assembled composite micro-nano fiber.

[0125] (3) The SnO.sub.2/CNT mixed powder-assembled composite micro-nano fiber obtained in step (2) was pressed into a fiber film (SnO.sub.2/CNT-GAF film) with a thickness of 1 millimeter.

Comparative Example 2

[0126] A SnO.sub.2/CNT/PVDF film prepared by the traditional molding method was compared with a SnO.sub.2/CNT-GAF film prepared in Embodiment 11.

[0127] SnO.sub.2 CNT and PVDF binders with a mass ratio of 8:1:2 were dispersed in an n-methyl-2-pyrrolidone solvent by ultrasound to obtain mixed slurry. The mixed slurry was dried at 80? C. for 2 hours to obtain slightly wet slurry. The slurry was rolled for multiple times to form the SnO.sub.2/CNT/PVDF film.

[0128] FIG. 37 is an SEM image of the SnO.sub.2/CN-GAF film (a) prepared in Embodiment 11 and the SnO.sub.2/CNT/PVDF film (b) prepared in comparative example 2. The SEM image of the SnO.sub.2/CNT-GAFs shows that SnO.sub.2 particles are inlaid in a uniform fiber, forming a bead-like structure (FIG. 37a). For a SnO.sub.2/CNT/PVDF composite film, most of the SnO.sub.2 nanoparticles were covered in the composite film, and a small number of exposed nanoparticles were severely agglomerated (FIG. 37b).

[0129] The gas-sensitive properties of the SnO.sub.2/CNT-GAF film obtained in Embodiment 11 and the SnO.sub.2/CNT/PVDF film obtained in comparative example 2 were tested according to the following method:

[0130] a gas sensor was measured: The SnO.sub.2/CNT-GAF film was cut into a specific flake, and was then fixed on a test device with water serving as a solvent. Similarly, the SnO.sub.2/CNT/PVDF slurry was also coated on the test device. The gas-sensitive properties of the two samples to methanol were tested with a Navigation 4000-NMDOG instrument at 120? C. after the two samples are completely dried.

[0131] The gas-sensitive response properties of the SnO.sub.2/CNT-GAF film sensor and the SnO.sub.2/CNT/PVDF film sensor were calculated, and a time curve of the concentration of methanol was drawn (as shown in FIG. 38a). The response of the SnO.sub.2/CNT-GAF film increased linearly with the increase of the concentration (5-300 ppm) of methanol, from 3.87 times to 17.78 times. However, the response of the SnO.sub.2/CNT/PVDF film sensor prepared by the traditional molding method was poor, which was 1.87-7.44 times. At the concentration of 5 ppm, the response time and recovery time of the SnO.sub.2/CNT-GAF film were 12 seconds and 18 seconds respectively, shorter than the response time and recovery time (22 seconds and 63 seconds) of the SnO.sub.2/CNT/PVD film. In addition, the sensitivity of the Sn.sub.2/CNT-GAF film was 0.071 ppm?1, which was better than that (0.029 ppm?1) of the Sn.sub.2/CNT/PVDF film (as shown in FIG. 38b). These results show that the composite micro-nano fiber prepared in the present invention has great application potential in high-performance gas-sensitive devices.