Preparation method of ultra-long BNNT and low-temperature spray freeze-drying dispersion method of ultra-long BNNT

Abstract

The invention provides a preparation method and a low-temperature spray freeze-drying dispersion method of ultra-long BNNT, a boron source, metal nitrate and nano metal oxide are uniformly mixed, mixed gas is introduced, heating annealing is performed to prepare the ultra-long BNNT, the diameter of the prepared ultra-long BNNT is 50-150 nm, the tube length of the BNNT is 100-500 [mu] m, and the wall thickness of the BNNT is 10-30 nm; the mixed gas is nitrogen and ethanol. The agglomeration effect of the nanotube powder is not obvious, the freeze-drying efficiency of the nanotube powder is greatly improved, and the homogeneous dispersion process of the prepared BNNT is simple in process, high in efficiency and good in uniformity and has great industrialization advantages.

Description

Technical Field

[0001] The invention belongs to the technical field of nano ceramic materials, and in particular relates to a preparation method of an ultra-long BNNT and a low-temperature spray freeze-drying dispersion method thereof. Background Art

[0002] Boron Nitride Nanotube (BNNT) has extremely high thermal conductivity, strong mechanical properties, excellent dielectric properties, strong insulation properties and extremely high wave transmission performance, and is an ideal thermal conductive and wave-transmitting insulating film substrate. The thermal conductivity of BNNT is as high as 2400Wm -1 K -1 , Young's modulus is 0.8~1.3TPa, generation gap width is 5~6eV, and it remains stable at 900℃ in air. It is the flexible material with the highest thermal conductivity in the field of insulation and wave transmission.

[0003] In 1995, Zettle and Chopra N.G. et al. of the University of California, Berkeley prepared BNNT for the first time through the arc discharge method in the experiment. However, this method has extremely high requirements for the temperature resistance of the equipment. The temperature required for the preparation of BNNT is as high as 4000K, and the outer wall of the prepared BNNT has a large number of boron nitride particles, and the overall purity of the product is low. Chen Ying et al. used nano-scale high-activity B powder after ball milling for 150 hours as raw material and annealed it below 1500℃ to grow BNNT. This method requires long-term ball milling of boron powder to obtain a highly active B source. Yoshio Bando et al. used MgO and B powder as raw materials to use the B2O2 gas phase intermediate produced at high temperature to achieve gas phase growth of BNNT. However, most of the BNNTs prepared by the above methods are in a disordered and entangled state. As additives, they cannot be directly used in the fields of thermal conductivity, metal reinforcement and ceramic toughening. Ultrasonic dispersion is required to prepare BNNT to achieve full mixing of BNNT and matrix materials. At present, most ultrasonic homogenization methods involve dispersion at room temperature or a relatively high solute temperature, and drying at high temperature. The process is long, the cost is high, and it is difficult to achieve continuous dispersion. In addition, due to the small size of BNNT, the large aspect ratio, and the significant capillary effect in the solute, it is difficult for BNNT to achieve a good dispersion effect, and it is very easy to introduce impurities, which seriously affects the use effect of BNNT. Methods such as supercritical dispersion require extremely high pressure, have great safety hazards, and are difficult to popularize. Therefore, simple and efficient dispersion and homogenization technology is crucial for the large-scale use of BNNT. Summary of the invention

[0004] In view of this, the present invention aims to propose a method for preparing ultra-long BNNTs and a low-temperature spray freeze-drying dispersion method thereof to solve at least one technical problem in the background technology.

[0005] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0006] A method for preparing an ultra-long BNNT comprises the following steps: uniformly mixing a boron source, a metal nitrate and a nano-metal oxide, introducing a mixed gas, and heating and annealing to prepare an ultra-long BNNT;

[0007] The mixed gas is nitrogen and ethanol.

[0008] Further, the boron source includes one or both of boron powder and metal boride;

[0009] And / or, the metal boride includes one or more of iron boride, tungsten boride, nickel boride, molybdenum boride, magnesium boride, chromium boride, and aluminum boride;

[0010] And / or, the particle size of the boron source is 100 to 400 μm.

[0011] Further, the metal nitrate additive includes one or more of iron nitrate, cobalt nitrate, nickel nitrate, chromium nitrate, manganese nitrate, copper nitrate or zinc nitrate.

[0012] Further, the nano metal oxide includes one or more of magnesium oxide, aluminum oxide, iron oxide and zinc oxide;

[0013] And / or, the particle size of the nano metal oxide is 10-150 nm, the particle size of the nano metal oxide is 10-150 nm, and the added mass of the nano metal oxide is 1%-15% of the sum of the mass of the boron source, the metal nitrate and the nano metal oxide.

[0014] Further, the pressure range of the mixed gas is 0.005 to 0.02 MPa;

[0015] And / or, the volume ratio of ethanol to nitrogen in the mixed gas is 50% to 5%, preferably, the volume ratio is 10%;

[0016] And / or, the annealing temperature of the heating annealing is 900-1500° C., and the annealing time is 0.5 h-8 h.

[0017] Furthermore, the diameter of the prepared ultra-long BNNT is 50-150 nm, the tube length of the BNNT is 100-500 μm, and the wall thickness of the BNNT is 10-30 nm.

[0018] A low-temperature spray freeze-drying dispersion method for ultra-long BNNTs comprises the following steps:

[0019] S1: The ultra-long BNNT prepared by the above-mentioned method for preparing an ultra-long BNNT is stirred in hot water, and then the ultra-long BNNT is added to an organic solvent to obtain a suspension of ultra-long BNNT;

[0020] S2: ultrasonically stirring the suspension of ultra-long BNNT obtained in step S1, adding it to a high-speed centrifugal spray tower in low-temperature spray freeze-drying, centrifugally forming small isopropanol droplets containing BNNT, and rapidly dropping them into the liquid nitrogen in the spray tower to form BNNT microparticles;

[0021] S3: Filter the BNNT particles obtained in step S2 into a low-temperature buffer bin, remove excess nitrogen, evacuate, sublimate and freeze-dry to obtain uniformly dispersed BNNT powder.

[0022] Furthermore, the hot water temperature in step S1 is 98-100° C., and the stirring time is 1.8-2.4 h.

[0023] Furthermore, the organic solvent in step S1 includes one or more of methanol, ethanol, propanol and isopropanol.

[0024] And / or, the mass fraction of BNNT in the suspension of ultra-long BNNT in step S1 is 30-70%. Further, the ultrasonic power of the suspension of ultra-long BNNT in step S2 is 200-1000W, the duty cycle of the suspension of ultra-long BNNT is 10-80%, and the stirring speed is 200-2500rpm;

[0025] And / or, the solid content of the ultra-long BNNT suspension in step S2 after ultrasonic stirring is 10-80%.

[0026] And / or, in step S2, the BNNT suspension is centrifuged in the spray tower at a speed of 5000 to 30000 rpm.

[0027] Furthermore, the diameter of the BNNT particles obtained in step S2 is 1 to 200 micrometers.

[0028] And / or, in step S3, the freeze-drying temperature of the BNNT particles is -80 to -40°C, and the freeze-drying time is 24 hours to 120 hours.

[0029] Compared with the prior art, the preparation method of ultra-long BNNT and its low-temperature spray freeze-drying dispersion method described in the present invention have the following advantages:

[0030] The present invention utilizes an advanced low-temperature spray rapid freezing method to rapidly freeze the ultra-long nanotubes and isopropanol medium in liquid nitrogen, thereby avoiding the agglomeration of BNNT solutes due to capillary forces during high-temperature volatilization during traditional high-temperature spray drying. In the later stage, the organic solvent in the nanotubes is slowly freeze-dried by freeze-drying. In this process, no liquid phase solute volatilizes, and the freeze-dried BNNT is soft agglomerates. The entire process realizes continuous preparation, with a good homogenization effect, the agglomeration effect of the nanotube powder is not obvious, and the freeze-drying efficiency of the nanotube powder is greatly improved. The homogenization and dispersion process of the BNNT is simple, efficient, and uniform, and has great industrial advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 The SEM image and EDS spectrum of the combustion synthesized nano-scale amorphous boron and magnesium oxide composite powder described in Example 3 of the present invention (a is the SEM image of the amorphous boron and magnesium oxide composite powder; b is the B spectrum; c is the Mg spectrum; d is the O spectrum);

[0033] Figure 2 This is a SEM image of the ultra-long BNNT prepared after annealing described in Example 1 of the present invention (the electron microscope magnification of a is 2000 times, and the electron microscope magnification of b is 5000 times);

[0034] Figure 3 TEM images of the ultra-long BNNT described in Example 1 of the present invention (the electron microscope magnification of a is 800,000 times, the electron microscope magnification of b is 1.2 million, and c is a TEM image of a high-magnification electron microscope magnification);

[0035] Figure 4 This is the EDS spectrum of the ultra-long BNNT described in Example 1 of the present invention (a is a SEM image; b is a B element image, and c is a N element image);

[0036] Figure 5 This is the XRD diagram of the combustion synthesized nano-scale amorphous boron and magnesium oxide composite powder shown in Example 3 of the present invention (line a is the XRD diagram of the combustion synthesized nano-scale amorphous boron generating boron nitride and magnesium oxide composite powder, and line b is the XRD diagram of BNNT after removing magnesium oxide). DETAILED DESCRIPTION

[0037] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0038] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0039] Example 1

[0040] Take 10g of boron powder with a particle size of 150μm as raw material, take 1g of chromium nitrate and 1g of nano-alumina as additives, mix the boron source and the additives evenly, put them in a tube furnace, pass 0.01MPa of 80% nitrogen and 20% ethanol mixed gas, heat anneal at 1350℃ for 3h to synthesize ultra-long BNNTs, and the SEM, TEM, and EDS spectra of the ultra-long BNNTs prepared after annealing are as follows: Figure 2-4 shown.

[0041] The obtained 20g ultra-long BNNT was stirred in 100℃ hot water for 2h, filtered to remove the residual boron, and then the ultra-long BNNT was added to the isopropanol organic solvent to prepare a suspension with a solid content of 50%, and then ultrasonicated at 1500W power and stirred at 2000rpm. The uniformly mixed ultra-long BNNT suspension was pumped into the high-speed centrifugal spray tower in the low-temperature spray freeze-drying by peristaltic pump, centrifuged at 25000rpm, and the formed isopropanol droplets containing BNNT were quickly dripped into the liquid nitrogen in the spray tower to form 10-micron-sized microbeads. The 10-micron-sized microbeads formed by freezing were then filtered into the low-temperature buffer warehouse, vacuumed after removing excess nitrogen, transferred to the freeze-drying chamber, sublimated and freeze-dried at -80℃ for 72h, and 18g of uniformly dispersed BNNT powder was obtained.

[0042] Example 2

[0043] Take 22g of magnesium boride with a particle size of 75μm as raw material, 1g of cobalt nitrate and 1g of nano-alumina as additives, mix the boron source and additives evenly, place them in a tube furnace, pass 0.01MPa of 90% nitrogen and 10% ethanol mixed gas, and heat anneal at 1350℃ for 4h to synthesize ultra-long BNNT. Stir the obtained 18g ultra-long BNNT in 100℃ hot water for 2h, filter to remove residual boron, then add the ultra-long BNNT to the propanol organic solvent to prepare a suspension with a solid content of 60%, then ultrasonicate at 2000W power and stir at 2500rpm. Use a peristaltic pump to blast the uniformly mixed ultra-long BNNT suspension into the high-speed centrifugal spray tower in the low-temperature spray freeze-drying, centrifuge at 20000rpm, and the formed isopropanol droplets containing BNNT quickly drip into the liquid nitrogen in the spray tower to form 25 micron-level small micro beads. The 25-micron microbeads formed by freezing were then filtered into a low-temperature buffer chamber, evacuated after removing excess nitrogen, and transferred to a freeze-drying chamber. They were sublimated and freeze-dried at -80°C for 96 hours to obtain 16 g of uniformly dispersed BNNT powder.

[0044] Example 3

[0045] Take 45g of 100μm iron boride as raw material, 1g of cobalt nitrate and 1g of nano magnesium oxide as additives, mix the boron source and additives evenly, place in a tube furnace, pass 0.01MPa of 70% nitrogen and 30% ethanol mixed gas, and heat anneal at 1300℃ for 2h to synthesize ultra-long BNNT. Stir the obtained 17g ultra-long BNNT in 100℃ hot water for 2h, filter to remove residual boron, then add the ultra-long BNNT to isopropanol organic solvent to prepare a suspension with a solid content of 60%, then ultrasonicate at 1800W power and stir at 2500rpm. Use a peristaltic pump to blast the uniformly mixed ultra-long BNNT suspension into the high-speed centrifugal spray tower in the low-temperature spray freeze-drying, centrifuge at 15000rpm, and the formed isopropanol droplets containing BNNT quickly drip into the liquid nitrogen in the spray tower to form 45 micron-level small micro beads. Then the 45-micron microbeads formed by freezing were filtered into a low-temperature buffer chamber, evacuated after removing excess nitrogen, and transferred to a freeze-drying chamber. They were sublimated and freeze-dried at -60°C for 120 hours to obtain 15g of uniformly dispersed BNNT powder. The SEM image, EDS spectrum and XRD of the combustion-synthesized nano-scale amorphous boron and magnesium oxide composite powder are as follows: Figure 1 and Figure 5 shown.

[0046] Example 4

[0047] Take 47g of aluminum boride with a particle size of 150μm as raw material, 1g of ferric nitrate and 1g of nano-alumina as additives, mix the boron source and additives evenly, place them in a tube furnace, pass 0.01MPa of 90% nitrogen and 10% ethanol mixed gas, and heat anneal at 1400℃ for 3h to synthesize ultra-long BNNT. Stir the obtained 19g ultra-long BNNT in 100℃ hot water for 1.5h, filter to remove residual boron, and then add the ultra-long BNNT to the isopropanol organic solvent to prepare a suspension with a solid content of 70%, then ultrasonicate at 2500W power and stir at 2500rpm. Use a peristaltic pump to blast the uniformly mixed ultra-long BNNT suspension into the high-speed centrifugal spray tower in the low-temperature spray freeze-drying, centrifuge at 15000rpm, and the formed isopropanol droplets containing BNNT quickly drip into the liquid nitrogen in the spray tower to form 50-micron-level small micro beads. The 50-micron microbeads formed by freezing were then filtered into a low-temperature buffer chamber, evacuated after removing excess nitrogen, transferred to a freeze-drying chamber, and sublimated and freeze-dried at -70°C for 120 hours to obtain 15 g of uniformly dispersed BNNT powder.

[0048] This application uses boron powder or boride as raw materials, nitrates and nano-oxides as additives, and synthesizes low-cost, ultra-long BNNTs in situ by reacting in ethanol and nitrogen. Ultrasonic dispersion is used to homogenize BNNTs in an organic solvent, and ultra-long BNNTs are quickly frozen into microspheres by low-temperature spraying, which breaks through the problem of large-scale secondary agglomeration of BNNTs due to capillary phenomena when the liquid phase solute evaporates during traditional high-temperature spray drying. Then, freeze drying is used to slowly sublimate and freeze-dry the frozen medium in the nanotubes. During the entire drying process, there is no liquid phase medium, which maintains the good uniformity of the nanotubes after ultrasonic dispersion, and solves the shortcomings of poor uniformity and severe agglomeration of traditional BNNT high-temperature spray drying. Through this low-temperature freeze-spray drying method, high-uniformity BNNTs are achieved in large quantities, without agglomeration, and continuous freeze-drying and dispersion.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing an ultra-long BNNT, characterized in that: The method comprises the following steps: uniformly mixing a boron source, a metal nitrate and a nano-metal oxide, introducing a mixed gas, and heating and annealing to prepare an ultra-long BNNT, wherein the diameter of the prepared ultra-long BNNT is 50-150 nm, the tube length of the BNNT is 100-500 μm, and the wall thickness of the BNNT is 10-30 nm; The mixed gas is nitrogen and ethanol.

2. The method for preparing an ultra-long BNNT according to claim 1, characterized in that: The boron source includes one or two of boron powder and metal boride; And / or, the metal boride includes one or more of iron boride, tungsten boride, nickel boride, molybdenum boride, magnesium boride, chromium boride, and aluminum boride; And / or, the particle size of the boron source is 100-400 μm.

3. A method for preparing an ultra-long BNNT according to claim 1, characterized in that: The metal nitrate additive includes one or more of iron nitrate, cobalt nitrate, nickel nitrate, chromium nitrate, manganese nitrate, copper nitrate or zinc nitrate.

4. The method for preparing an ultra-long BNNT according to claim 1, characterized in that: The nano metal oxide includes one or more of magnesium oxide, aluminum oxide, iron oxide and zinc oxide; And / or, the particle size of the nano metal oxide is 10-150 nm, and the added mass of the nano metal oxide is 1%-15% of the sum of the mass of the boron source, the metal nitrate and the nano metal oxide.

5. The method for preparing an ultra-long BNNT according to claim 1, characterized in that: The pressure range of the mixed gas is 0.005~0.02 MPa.

6. The method for preparing an ultra-long BNNT according to claim 1, characterized in that: The volume ratio of ethanol to nitrogen in the mixed gas is 50% to 5%; And / or, the annealing temperature of the heating annealing is 900-1500° C., and the annealing time is 0.5 h-8 h.

7. A low-temperature spray freeze-drying dispersion method for ultra-long BNNTs, characterized in that: The method comprises the following steps: S1: stirring an ultra-long BNNT obtained by the method for preparing an ultra-long BNNT according to any one of claims 1 to 6 in hot water, and then adding the ultra-long BNNT into an organic solvent to obtain a suspension of the ultra-long BNNT; S2: subjecting the ultra-long BNNT suspension obtained in step S1 to ultrasonic stirring, and adding the suspension to a high-speed centrifugal spray tower in a low-temperature spray freeze-drying process to form BNNT microparticles; S3: Filter the BNNT particles obtained in step S2 into a low-temperature buffer bin, remove excess nitrogen, evacuate, sublimate and freeze-dry to obtain uniformly dispersed BNNT powder.

8. A low-temperature spray freeze-drying dispersion method for ultra-long BNNTs according to claim 7, characterized in that: The hot water temperature in step S1 is 98-100°C and the stirring time is 1.8-2.4h; And / or, the organic solvent in step S1 includes one or more of methanol, ethanol, propanol and isopropanol; And / or, the mass fraction of BNNT in the suspension of ultra-long BNNT in step S1 is 30-70%.

9. A low-temperature spray freeze-drying dispersion method for ultra-long BNNTs according to claim 7, characterized in that: The ultrasonic power of the ultra-long BNNT suspension in step S2 is 200 to 1000 W, the duty cycle of the ultra-long BNNT suspension is 10 to 80%, and the stirring speed is 200 to 2500 rpm; And / or, the solid content of the ultra-long BNNT suspension in step S2 after ultrasonic stirring is 10 to 80%; And / or, in step S2, the BNNT suspension is centrifuged in the high-speed centrifugal spray tower at a speed of 5000 to 30000 rpm.

10. A low-temperature spray freeze-drying dispersion method for ultra-long BNNTs according to claim 7, characterized in that: The diameter of the BNNT microparticles obtained in step S2 is 1 to 200 microns; And / or, in step S3, the freeze-drying temperature of the BNNT particles is -80 to -40°C, and the freeze-drying time is 24 hours to 120 hours.

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