A boron nitride-based aerogel reinforced phase change energy storage composite material and a preparation method thereof

Abstract

The present invention discloses a boron nitride-based aerogel reinforced phase change energy storage composite material and a preparation method thereof. The preparation method uses boron nitride precursor whiskers and (BN or carbon) nanotubes as raw materials, and in-situ converts the boron nitride precursor to form boron nitride nanowhiskers, realizing the "soft-hard" reactive chemical bonding of (BN or carbon) nanotubes in the raw materials. The three-dimensional porous aerogel formed by the in-situ conversion of boron nitride nanowhiskers and the chemical bonding of the raw material nanotubes to each other realizes the formation of heat conduction channels, increases the shape stability and mechanical properties of the overall structure, and plays a role in shaping and preventing leakage of organic solid-liquid phase change materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite functional materials, and relates to a boron nitride-based aerogel enhanced phase change energy storage composite material and a preparation method thereof. Background Art

[0002] Energy storage technology is one of the most important options for developing new energy and renewable energy, and can store energy in the form of latent heat and sensible heat using materials. Compared with sensible heat energy storage, latent heat storage systems store or release a large amount of heat in the form of latent heat of material phase change. Materials that use latent heat for energy storage are called phase change materials, which are the main direction of current research on energy storage technology.

[0003] According to the phase states before and after phase change, they can be divided into solid-solid phase change materials, solid-liquid phase change materials, and liquid-gas phase change materials. Among them, solid-liquid phase change materials are widely used because of their good chemical stability and high energy storage density. Polyethylene glycol, organic acids, and paraffin are the most common solid-liquid phase change materials. Currently, the problems faced by organic solid-liquid phase change materials are: on the one hand, when they absorb heat and undergo a phase change from solid to liquid, their shape stability is poor and leakage occurs, which not only causes a decrease in the phase change heat storage capacity but also causes device contamination. On the other hand, the thermal conductivity of the phase change material itself is not high, so its heat storage efficiency still needs to be improved. To address the above problems, methods such as microencapsulation, porous adsorption, and polymer-based shaping are generally used to prevent leakage, and at the same time, high thermal conductivity fillers are added and three-dimensional skeletons with high thermal conductivity are used to improve the thermal conductivity of the phase change material.

[0004] Boron nitride has excellent physical properties such as high thermal conductivity, high resistivity, low thermal expansion coefficient, high electric breakdown strength, and low dielectric loss, and is an excellent thermal conductivity filler. Preparing boron nitride into an aerogel and then impregnating it with an organic solid-liquid phase change material can achieve the dual functions of high thermal conductivity and leakage prevention of the phase change composite material.

[0005] Currently, the main methods for preparing boron nitride aerogel are template method or freeze-drying method, etc. However, the template method has problems such as long cycle and high cost, and the freeze-drying method cannot form a chemically bonded three-dimensional network and cannot inhibit the leakage of phase change materials. In recent years, using melamine and boric acid to self-assemble into a boron nitride precursor aerogel and then finally obtaining boron nitride aerogel through high-temperature heat treatment can achieve shape stability of the phase change material and a certain thermal conductivity enhancement effect, so it has attracted people's attention.

[0006] However, when preparing boron nitride-based aerogels using traditional methods, the heat treatment temperature is generally below 1400°C. However, the boron nitride obtained at lower heat treatment temperatures has lower crystallinity and lower thermal conductivity. On the other hand, if the heat treatment temperature is further increased to above 1400°C, the BN aerogel will crystallize, harden, and collapse, and the aerogel structure will be destroyed. Therefore, when using boron nitride aerogel as a thermal conductivity enhancement and structural shaping material, how to improve the thermal conductivity and structural stability of boron nitride-based aerogels, and thereby solve the problems of low thermal conductivity and easy leakage of organic solid-liquid phase change composites, is a scientific problem that needs to be urgently solved in the application of organic solid-liquid phase change materials. Summary of the Invention

[0007] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide a boron nitride-based aerogel-reinforced phase change energy storage composite material and a preparation method thereof, so as to solve the problem in the prior art that boron nitride aerogel is easy to harden, collapse and structural damage, thereby causing the solid-liquid phase change material to have low thermal conductivity and easy leakage.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for preparing a boron nitride-based aerogel-reinforced phase-change energy storage composite material comprises the following steps:

[0010] Step 1: adding nanotubes to a solvent to obtain a nanotube mixed dispersion solution; the nanotubes are carbon nanotubes or nitrogen nanotubes;

[0011] Step 2: adding melamine and boric acid to the nanotube mixed dispersion solution to obtain a mixed solution; stirring the mixed solution and freeze-drying it to obtain a boron nitride precursor aerogel;

[0012] Step 3, subjecting the boron nitride precursor aerogel to a high-temperature ceramic heat treatment to obtain a boron nitride-based aerogel; the boron nitride-based aerogel is formed by combining nanotubes and boron nitride whiskers;

[0013] Step 4: Immerse the organic phase change material into the boron nitride-based aerogel by vacuum impregnation to obtain a boron nitride-based aerogel-reinforced phase change energy storage composite material.

[0014] A further improvement of the present invention is:

[0015] Preferably, in step 1, the concentration of the carbon nanotubes or nitrogen nanotubes in the nanotube mixed dispersion solution is 0.01-1.00M.

[0016] Preferably, in step 1, the solvent is water and tert-butanol, and the volume ratio of water to tert-butanol is 1:0-0.1:1.

[0017] Preferably, in step 2, the concentration of melamine in the nanotube mixture is 0.05 - 0.2 M.

[0018] Preferably, in step 2, the molar ratio of melamine to boric acid is 1:1 - 1:4.

[0019] Preferably, in step 2, the stirring temperature is 80 - 120 °C.

[0020] Preferably, in step 3, the heating rate of the high - temperature ceramization heat treatment is 2 - 10 °C / min, the final heat treatment temperature is 1400 - 1800 °C, and the heat preservation time is 0.5 - 6 h.

[0021] Preferably, in step 4, the organic solid - liquid phase change material is polyethylene glycol or stearic acid.

[0022] A boron nitride - based aerogel - enhanced phase - change energy - storage composite material prepared by the preparation method described in any one of the above, comprising a boron nitride - based aerogel and an organic phase - change material. The boron nitride - based aerogel is a three - dimensional porous material, and the organic phase - change material is immersed in the porous material; the boron nitride - based aerogel is composed of a combination of nanotubes and boron nitride whiskers.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention discloses a preparation method of a boron nitride - based aerogel - enhanced phase - change energy - storage composite material. This preparation method uses (BN or carbon) nanotubes, melamine, and boric acid as raw materials. Melamine and boric acid generate a boron nitride precursor in the form of whiskers. The boron nitride precursor is in - situ transformed into hard boron nitride nanowhiskers (with high crystallinity and higher thermal conductivity) at high temperature (1400 - 1800 °C), realizing the "hard - soft" reactive chemical bonding of the soft and highly thermally conductive (BN or carbon) nanotubes in the raw materials. On the one hand, by improving the crystallinity of boron nitride, the thermal conductivity of the boron nitride aerogel is increased. This high - thermal - conductivity material coats the organic solid - liquid phase - change material, replacing the low - thermal - conductivity air and increasing the overall heat transfer ability of the composite material. On the other hand, through the capillary action of the porous three - dimensional structure of the aerogel itself, the problem of easy leakage of the phase - change material is solved.

[0025] The present invention also discloses a boron nitride - based aerogel - enhanced phase - change energy - storage composite material. The advantages of this structure are: 1) Both the in - situ transformed boron nitride nanowhiskers and the raw material nanotubes have high intrinsic thermal conductivity; 2) The in - situ transformed boron nitride nanowhiskers and the raw material nanotubes are chemically bonded to each other, and the formed three - dimensional porous aerogel realizes the formation of heat - conduction channels, which helps to further improve the thermal conductivity; 3) The aerogel with interpenetrating whiskers - nanotubes and chemical bonding increases the shape stability and mechanical properties of the overall structure, playing a role in shaping and preventing leakage of the organic solid - liquid phase - change material. Brief Description of the Drawings

[0026] Figure 1 XRD pattern of the boron nitride-based aerogel prepared in Example 1.

[0027] Figure 2 SEM image of the boron nitride-based aerogel prepared in Example 1.

[0028] Figure 3 SEM image of the boron nitride-based aerogel reinforced phase change energy storage composite material prepared in Example 1, indicating that the organic phase change material has been completely immersed in the boron nitride-based aerogel. Detailed Description of the Invention

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] The present invention discloses a preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material. The preparation method includes the following steps: using melamine, boric acid, and boron nitride nanotubes (or carbon nanotubes) as raw materials, through ultrasonic dispersion, heating and stirring, freeze-drying, and high-temperature ceramization heat treatment to obtain a boron nitride-based aerogel; impregnating a phase change energy storage material in the boron nitride-based aerogel structure to obtain a boron nitride-based aerogel reinforced phase change energy storage composite material.

[0031] The matrix material of the composite material is a three-dimensional porous structure, which is composed of boron nitride nanowhiskers and raw material nanotubes connected by chemical bonds. The three-dimensional porous material is filled with an organic phase change material. The raw material nanotubes are carbon nanotubes or nitrogen nanotubes, and the organic phase change material is an organic material that can undergo a phase change. More specifically, it is polyethylene glycol or stearic acid. In this structure, the three-dimensional porous aerogel formed by the in-situ transformation of boron nitride nanowhiskers and raw material nanotubes connected by chemical bonds realizes the formation of heat conduction channels, increases the shape stability and mechanical properties of the overall structure, and plays a role in shaping and preventing leakage of the organic solid-liquid phase change material.

[0032] The present invention uses boron nitride to prepare a three-dimensional network structure of aerogel for preparing a phase change energy storage composite material. Its advantage lies in that boron nitride is a ceramic with high thermal conductivity (the thermal conductivity of the hexagonal phase is 400 W / m·K) and high resistance (>10 15 Ω·cm). At the same time, during the ceramization transformation process, the boron nitride nanomaterials and the original nanotubes are "soft-hard" reactively chemically combined, and a phase change energy storage shaping material with high thermal conductivity, excellent mechanical properties, and high resistivity can be obtained, providing technical support for the field of composite functional materials technology.

[0033] The present invention discloses a preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material, including the following steps:

[0034] Step 1: Add boron nitride nanotubes (or carbon nanotubes) into a mixed solvent of water and tert-butanol. After ultrasonic dispersion, a nanotube mixed dispersion solution is obtained. The concentration of carbon nanotubes or boron nitride nanotubes in the mixed dispersion solution is 0.01 - 1.00 M, and the volume ratio of water to tert-butanol is 1:0 - 0.1:1.

[0035] Step 2: Further dissolve melamine and boric acid in the above mixed solution, heat up and stir. After freeze-drying, a boron nitride precursor aerogel is obtained.

[0036] Specifically, the concentration of melamine used in the mixed solution is 0.05 - 0.2 M, and the molar ratio of melamine to boric acid is 1:1 - 1:4; the temperature during the stirring process is 80 - 120 °C. After stirring, whisker-like boron nitride precursors are formed by melamine and boric acid (boron nitride is not formed at this time).

[0037] Step 3: Perform high-temperature ceramization heat treatment on the boron nitride precursor aerogel to obtain a boron nitride-based aerogel; the heating rate of the high-temperature ceramization heat treatment of the boron nitride-based precursor aerogel is 2 - 10 °C / min, the final temperature is 1400 - 1800 °C, and the holding time is 0.5 - 6 h to obtain a boron nitride-based aerogel with a three-dimensional porous structure. During this process, the boron nitride nanotubes or carbon nanotubes form a three-dimensional porous structure. At the same time, the whisker-like boron nitride precursors will remove moisture through freeze-drying and be transformed into boron nitride whiskers through heat treatment, enabling the in-situ generation of boron nitride nanowhiskers on the nanotubes. Meanwhile, the boron nitride whiskers and the three-dimensional porous structure material are bonded together by chemical bonds.

[0038] Step 4: Through the vacuum impregnation method, pour the liquid organic phase change material onto the surface of the boron nitride-based aerogel precursor. Under the action of capillary force, it infiltrates into the three-dimensional porous structure, and finally a boron nitride-based aerogel enhanced phase change energy storage composite material is obtained; the organic solid-liquid phase change material is an organic material such as polyethylene glycol and stearic acid that can undergo phase changes.

[0039] Example 1

[0040] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0041] (1) Ultrasonically disperse carbon nanotubes in a mixed solvent of water and tert-butanol, where the concentration of carbon nanotubes is 0.42 M, and the volume ratio of water to tert-butanol is 1:1. Then dissolve melamine and boric acid in a molar ratio of 1:3, where the molar concentration of melamine is 0.10 M. Place it in an oil bath at 95 °C and stir. After stirring until it becomes clear, continue stirring for 30 min, then cool to room temperature and perform freeze-drying. Subsequently, place it in a tube furnace and heat it up to 1400 °C at a heating rate of 2.5 °C / min, and hold for 3 h to obtain a boron nitride-based thermally conductive three-dimensional network structure;

[0042] (2) By the vacuum impregnation method, the organic solid-liquid phase change material polyethylene glycol is immersed in the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0043] In this example, boron nitride was successfully generated by heat treatment (such as Figure 1 ), and this aerogel is a three-dimensional network porous structure (such as Figure 2 ). In the boron nitride-based aerogel enhanced phase change energy storage composite material obtained by the vacuum impregnation method, the phase change material has been completely immersed therein (such as Figure 3 ), and the thermal conductivity is 0.4650 W·m -1 K -1 , and the melting enthalpy and crystallization enthalpy are 176.0 and 151.5 J·g -1 respectively.

[0044] Example 2

[0045] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0046] (1) Carbon nanotubes are ultrasonically dispersed in a mixed solvent of water and tert-butanol, wherein the concentration of boron nitride nanotubes is 0.58 M, and the volume ratio of water to tert-butanol is 1:1. Then, melamine and boric acid are dissolved therein at a molar ratio of 1:3, wherein the molar concentration of melamine is 0.10 M. It is placed in an oil bath at 95 °C and stirred. After stirring until it becomes clear, stirring is continued for 30 min, then it is cooled to room temperature and freeze-dried, and then placed in a tubular furnace and heated to 1400 °C at a heating rate of 2.5 °C / min and held for 3 h to obtain a boron nitride-based aerogel;

[0047] (2) By the vacuum impregnation method, the organic solid-liquid phase change material polyethylene glycol is immersed in the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0048] In this example, a boron nitride-based aerogel enhanced phase change energy storage composite material is obtained, and the thermal conductivity is 0.4350 W·m -1 K -1 , and the melting enthalpy and crystallization enthalpy are 174.4 and 156.4 J·g -1 respectively.

[0049] Example 3

[0050] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0051] (1) Ultrasonically disperse carbon nanotubes in a mixed solvent of water and tert-butanol, where the concentration of boron nitride nanotubes is 0.75 M and the volume ratio of water to tert-butanol is 1:1. Then dissolve melamine and boric acid in it at a molar ratio of 1:3, where the molar concentration of melamine is 0.10 M. Place it in an oil bath at 95 °C and stir. After stirring until it becomes clear, continue stirring for 30 min, then cool to room temperature and perform freeze-drying. Subsequently, place it in a tube furnace and heat it to 1600 °C at a heating rate of 2.5 °C / min, and hold for 1 h to obtain boron nitride-based aerogel;

[0052] (2) By the vacuum impregnation method, immerse the organic solid-liquid phase change material polyethylene glycol into the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0053] In this example, a boron nitride-based aerogel enhanced phase change energy storage composite material is obtained, with a thermal conductivity of 0.4310 W·m -1 K -1 , and the melting enthalpy and crystallization enthalpy are 177.1 and 160.1 J·g -1 .

[0054] Example 4

[0055] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0056] (1) Ultrasonically disperse carbon nanotubes in a mixed solvent of water and tert-butanol, where the concentration of carbon nanotubes is 0.01 M and the volume ratio of water to tert-butanol is 1:0. Then dissolve melamine and boric acid in it at a molar ratio of 1:4, where the molar concentration of melamine is 0.15 M. Place it in an oil bath at 100 °C and stir. After stirring until it becomes clear, continue stirring for 30 min, then cool to room temperature and perform freeze-drying. Subsequently, place it in a tube furnace and heat it to 1500 °C at a heating rate of 5 °C / min, and hold for 6 h to obtain a boron nitride-based three-dimensional thermal conductive network structure;

[0057] (2) By the vacuum impregnation method, immerse the organic solid-liquid phase change material polyethylene glycol into the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0058] Example 5 ;

[0059] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0060] (1) Carbon nanotubes were ultrasonically dispersed in a mixed solvent of water and tert-butanol, wherein the concentration of carbon nanotubes was 1 M and the volume ratio of water to tert-butanol was 1:0.5. Melamine and boric acid were then dissolved in a molar ratio of 1:2, wherein the molar concentration of melamine was 0.2 M. The mixture was placed in an oil bath at 90°C and stirred. After stirring until clear, stirring was continued for 30 minutes. The mixture was cooled to room temperature and freeze-dried. The mixture was then placed in a tube furnace and heated to 1600°C at a heating rate of 7°C / min and kept warm for 4 hours to obtain a boron nitride-based thermally conductive three-dimensional network structure.

[0061] (2) The organic solid-liquid phase change material polyethylene glycol is immersed in the boron nitride-based aerogel obtained in step (1) by vacuum impregnation to obtain a boron nitride-based aerogel-reinforced phase change energy storage composite material.

[0062] Example 6

[0063] A boron nitride-based aerogel-reinforced phase-change energy storage composite material comprises the following steps:

[0064] (1) Carbon nanotubes were ultrasonically dispersed in a mixed solvent of water and tert-butanol, wherein the concentration of carbon nanotubes was 0.2 M and the volume ratio of water to tert-butanol was 0.5:1. Melamine and boric acid were then dissolved in a molar ratio of 1:1, wherein the molar concentration of melamine was 0.05 M. The mixture was placed in an oil bath at 80°C and stirred. After stirring until clear, stirring was continued for 30 minutes, the mixture was cooled to room temperature and freeze-dried. The mixture was then placed in a tube furnace and heated to 1700°C at a heating rate of 3°C / min and kept warm for 2 hours to obtain a boron nitride-based thermally conductive three-dimensional network structure.

[0065] (2) The organic solid-liquid phase change material polyethylene glycol is immersed in the boron nitride-based aerogel obtained in step (1) by vacuum impregnation to obtain a boron nitride-based aerogel-reinforced phase change energy storage composite material.

[0066] Example 7

[0067] A boron nitride-based aerogel-reinforced phase-change energy storage composite material comprises the following steps:

[0068] (1) Carbon nanotubes were ultrasonically dispersed in a mixed solvent of water and tert-butanol, wherein the concentration of carbon nanotubes was 0.8 M and the volume ratio of water to tert-butanol was 0.1:1. Melamine and boric acid were then dissolved in a molar ratio of 1:4, wherein the molar concentration of melamine was 0.05 M. The mixture was placed in an oil bath at 110°C and stirred. After stirring until clear, stirring was continued for 30 minutes, the mixture was cooled to room temperature and freeze-dried. The mixture was then placed in a tube furnace and heated to 1800°C at a heating rate of 2°C / min and kept warm for 0.5 hours to obtain a boron nitride-based thermally conductive three-dimensional network structure.

[0069] (2) By the vacuum impregnation method, the organic solid-liquid phase change material polyethylene glycol is immersed into the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0070] Example 8

[0071] A boron nitride-based aerogel enhanced phase change energy storage composite material, comprising the following steps:

[0072] (1) The carbon nanotubes are ultrasonically dispersed in a mixed solvent of water and tert-butanol, where the concentration of the carbon nanotubes is 0.5 M, and the volume ratio of water to tert-butanol is 0.7:0.8. Then, melamine and boric acid are dissolved therein at a molar ratio of 1:2, where the molar concentration of melamine is 0.80 M. It is placed in an oil bath at 120 °C and stirred. After stirring until it becomes clear, stirring is continued for 30 min, then it is cooled to room temperature and freeze-dried. Subsequently, it is placed in a tubular furnace and heated to 1500 °C at a heating rate of 10 °C / min and held for 5 h to obtain a boron nitride-based three-dimensional thermal conductive network structure;

[0073] (2) By the vacuum impregnation method, the organic solid-liquid phase change material polyethylene glycol is immersed into the boron nitride-based aerogel obtained in step (1) to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

[0074] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material, characterized in that, It includes the following steps: Step 1: Add carbon nanotubes to a solvent to obtain a carbon nanotube mixed dispersion solution; the carbon nanotubes are carbon nanotubes; Step 2: Add melamine and boric acid to the carbon nanotube mixed dispersion solution to obtain a mixed solution; after stirring and freeze-drying the mixed solution, obtain a boron nitride precursor aerogel; In Step 2, the stirring temperature is 80 - 120 °C; Step 3: After performing high-temperature ceramization heat treatment on the boron nitride precursor aerogel, obtain a boron nitride-based aerogel; the boron nitride-based aerogel is formed by the combination of carbon nanotubes and boron nitride whiskers; The heat treatment temperature is 1400 - 1800 °C, and the heat preservation time is 0.5 - 6 h; Step 4: Immerse an organic phase change material into the boron nitride-based aerogel by vacuum impregnation to obtain a boron nitride-based aerogel enhanced phase change energy storage composite material.

2. The preparation method of a boron nitride-based aerogel enhanced phase change energy storage composite material according to claim 1, wherein In Step 1, the concentration of carbon nanotubes in the carbon nanotube mixed dispersion solution is 0.01 - 1.00 M.

3. The preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material according to claim 1, wherein, In Step 1, the solvent is water and tert-butanol, and the volume ratio of water to tert-butanol is 1:0 - 0.1:

1.

4. The preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material according to claim 1, characterized in that, In Step 2, the concentration of melamine in the carbon nanotube mixed solution is 0.05 - 0.2 M.

5. The preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material according to claim 1, characterized in that, In Step 2, the molar ratio of melamine to boric acid is 1:1 - 1:

4.

6. The preparation method of a boron nitride-based aerogel reinforced phase change energy storage composite material according to claim 1, wherein In Step 3, the heating rate of the high-temperature ceramization heat treatment is 2 - 10 °C / min.

7. The preparation method of a boron nitride-based aerogel enhanced phase change energy storage composite material according to claim 1, characterized in that In Step 4, the organic solid-liquid phase change material is polyethylene glycol or stearic acid.

8. A boron nitride-based aerogel reinforced phase change energy storage composite material prepared by the preparation method according to any one of claims 1-7, characterized in that, It includes a boron nitride-based aerogel and an organic phase change material. The boron nitride-based aerogel is a three-dimensional porous material, and the organic phase change material is immersed in the porous material; the boron nitride-based aerogel is composed of the combination of carbon nanotubes and boron nitride whiskers.

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