Method for preparing aluminum nitride whiskers in batches

Abstract

The invention discloses a method for preparing aluminum nitride whiskers in batches, and belongs to the field of inorganic non-metallic material preparation. The preparation method comprises the following specific steps: uniformly mixing aluminum particles, iron powder and carbon powder according to a certain mass ratio, putting the mixture into an alumina crucible, transferring the alumina crucible into a graphite sintering furnace, and carrying out a reaction in a nitrogen atmosphere at a nitrogen flow rate of 0.1-2 L / min at a reaction temperature of 1500-1700 DEG C for 0.5-6 h so as to obtain the white aluminum nitride whisker with the high length-diameter ratio after the reaction is finished. The prepared aluminum nitride whisker is high in purity and large in length-diameter ratio, the aluminum particles are used as the raw material, the carbon powder and the iron powder are added, the yield is remarkably increased, the technological method is simple, the raw material cost is low, the product can be used as a heat conduction filler for a thermal interface material and an aluminum nitride ceramic toughening material, and the method is suitable for industrial production. The method has wide application prospects in the fields of aviation, environmental protection, communication power and the like.

Description

Technical Field

[0001] The invention belongs to the field of inorganic non-metallic material preparation, and in particular relates to a method for batch preparing aluminum nitride whiskers. Background Art

[0002] Aluminum nitride has an ultra-high theoretical thermal conductivity of 320W / mK, and has excellent properties such as high resistivity, low dielectric constant, low thermal expansion coefficient and non-toxicity. It is an ideal substrate material and electronic device packaging material. Its one-dimensional whisker morphology is a single crystal with nearly perfect crystal properties, which can reduce phonon scattering at the grain boundary and exhibit an extremely high aspect ratio (5-1000). It has broad application prospects in the fields of ceramic toughening and thermal conductive fillers. However, the existing methods for preparing AlN whiskers often have problems such as low safety factor, low yield, low purity and high cost.

[0003] Invention patent CN103382113A discloses a method for preparing aluminum nitride fiber, which uses aluminum powder and alumina powder as raw materials, adopts a briquetting method, and sintering in a nitrogen atmosphere to obtain aluminum nitride fiber. However, the raw aluminum powder itself is flammable and highly active, and the fine powder and air easily form a flammable and explosive mixture, which poses a safety hazard and has relatively high requirements for equipment.

[0004] Invention patent CN102584244A discloses a method for preparing hexagonal aluminum nitride nanofibers by combining electrospinning with ammonia nitridation. The method uses aluminum nitrate as the main raw material, first prepares nano-alumina fibers, then mixes carbon powder and nitrides it into aluminum nitride fibers in an ammonia atmosphere. However, due to the large difference in density between alumina and carbon powder, the problem of uneven distribution of alumina fibers and carbon powder is prone to occur during the mixing process, and the ammonia used in the nitridation process is harmful to the human respiratory tract, which seriously limits its promotion and application in industry.

[0005] Invention patent CN98103408.X discloses a method for synthesizing aluminum nitride fiber, which uses microcrystalline aluminum silicate fiber with sillimanite structure and carbon black as starting materials, and performs heating and heat preservation treatment in a nitrogen atmosphere to finally form aluminum nitride fiber whiskers. However, since the aluminum silicate in the original material contains SiO2, this causes the reaction product to contain SiC, and the purity of the aluminum nitride fiber decreases.

[0006] Invention patent CN104211025A discloses a method for preparing cubic aluminum nitride fiber, which first prepares a mixed solution of anhydrous aluminum chloride, tetrabutylammonium azide and a template agent, then performs a solvent thermal reaction, obtains a reaction product after washing, centrifugation and drying, then calcines in a vacuum or inert atmosphere to crack the attached template agent, and then calcines in an air atmosphere to remove organic matter, and finally obtains cubic aluminum nitride. However, this organic precursor method is complex, costly and has a long cycle.

[0007] It can be seen that in order to further realize the large-scale preparation of aluminum nitride whiskers and expand its application scope, further improvements are needed based on the existing methods. Summary of the invention

[0008] The present invention aims at the problem of low yield of existing aluminum nitride whiskers, and proposes a method for batch preparation of aluminum nitride whiskers. In the method, the generation of aluminum nitride is mainly based on direct nitridation method, supplemented by carbon thermal reduction nitridation method, wherein the overall reaction formula of the direct nitridation method is:

[0009] 2Al(s)+N2(g)→2AlN(s)

[0010] The overall reaction formula of the carbothermal reduction nitridation method is:

[0011] Al2O3(s)+3C(s)+N2(g)→2AlN(s)+3CO(g)

[0012] The present invention prepares high-purity, high-aspect-ratio aluminum nitride whiskers by combining a direct nitridation method with a carbon thermal reduction nitridation method. The key points are as follows: (1) large-particle aluminum particles are used as raw materials. Generally, there is an oxide layer of a certain thickness on the surface of the aluminum particles. The addition of carbon powder can cause a carbon thermal reduction reaction with the oxide layer on the surface of the aluminum particles, thereby increasing the yield of aluminum nitride; (2) while the surface oxide layer is slowly consumed, the aluminum source inside the large-particle aluminum particles is slowly released to the outside in the form of vapor. After contacting with nitrogen, aluminum nitride whiskers are generated through a gas-solid (VS) reaction mechanism; (3) iron powder is added as a catalyst to further increase the concentration of Al vapor, thereby promoting the transformation of the Al-N reaction from a liquid-gas reaction to a gas-gas reaction, thereby avoiding the rapid nucleation of the AlN liquid phase. Under low supersaturation, AlN preferentially grows into a one-dimensional morphology.

[0013] Specifically, the present invention provides a method for batch preparation of aluminum nitride whiskers, comprising the following steps:

[0014] (1) Batching: Evenly mix aluminum particles, iron powder and carbon powder according to a certain mass ratio;

[0015] (2) Synthesis: The mixed material obtained above is placed in an alumina crucible, and the crucible is transferred to a graphite sintering furnace. After heating from room temperature to 1500-1700°C in a nitrogen atmosphere, the temperature is kept for 0.5-6 hours, and then naturally cooled to obtain white aluminum nitride whiskers with a high aspect ratio.

[0016] Furthermore, the aluminum particles in step (1) have a size of 0.5 mm to 1 mm. Too small aluminum particles will result in excessively high raw material reactivity, and the aluminum vapor will evaporate quickly, causing the reaction rate to be too fast, making it impossible to achieve low supersaturation and obtain a one-dimensional morphology; too large aluminum particles will result in incomplete reaction, and aluminum impurities will be present in the product. The mass ratio of aluminum particles to iron powder is 1:1 to 1:2.5.

[0017] Furthermore, the carbon powder used in step (1) is carbon black, graphite or activated carbon, and the mass ratio of the carbon powder to the aluminum particles is 1:100 to 5:100.

[0018] Furthermore, in step (2), nitrogen gas is introduced into the reaction furnace at a flow rate of 0.1-2 L / min.

[0019] Furthermore, in step (2), the heating rate of the atmosphere sintering furnace is 5-60°C / min.

[0020] The present invention also provides aluminum nitride whiskers prepared by the method, which have a one-dimensional morphology and an aspect ratio of 500:1 to 5000:1.

[0021] The present invention proposes a method for batch preparation of aluminum nitride whiskers, which uses aluminum particles as raw materials and adds carbon powder and iron powder to increase the yield of aluminum nitride whiskers. First, the aluminum oxide on the surface of the aluminum particles contacts with carbon powder to undergo a carbon thermal reduction reaction, generating a large amount of aluminum-containing gas phase compounds such as Al2O, which are further contacted with nitrogen to generate aluminum nitride. The consumption of the oxide layer on the surface of the aluminum particles can also promote the slow evaporation of aluminum vapor from the internal aluminum, which contacts with nitrogen to undergo direct nitridation, and generate aluminum nitride whiskers by a gas-solid (VS) mechanism at low supersaturation. In addition, the addition of iron powder can further increase the concentration of aluminum vapor and increase the yield of aluminum nitride whiskers.

[0022] The present invention not only has simple process, high production efficiency and low production cost, and can greatly increase the whisker output, but also can control the growth habits of the whiskers by changing the ratio of carbon powder and the nitrogen flow rate, and is expected to provide a new and efficient way to prepare aluminum nitride whiskers with controllable morphology.

[0023] The innovative idea of ​​the present invention is to use aluminum particles as raw materials and add iron powder and carbon powder to significantly increase the yield of aluminum nitride whiskers. Compared with the prior art, the advantages of the present invention are:

[0024] 1. The main raw materials used in the present invention are cheap aluminum particles, iron powder, carbon powder, nitrogen, etc., with low raw material cost, high feasibility, and easy industrialization.

[0025] 2. This method uses aluminum particles as the aluminum source, which has a higher safety factor than explosive aluminum powder.

[0026] 3. Large-particle aluminum particles are used as raw materials. There is an oxide layer of a certain thickness on the surface of the aluminum particles. The addition of carbon powder can cause a carbon thermal reduction reaction with the oxide layer on the surface of the aluminum particles, thereby increasing the yield of aluminum nitride. In addition, while the surface oxide layer is slowly consumed, the aluminum source inside the large-particle aluminum particles is slowly released to the outside in the form of vapor. After contacting with nitrogen, aluminum nitride whiskers are generated through a gas-solid (VS) reaction mechanism.

[0027] 4. Adding iron powder as a catalyst further increases the concentration of aluminum vapor, prompting the Al-N reaction to transform from a liquid-gas reaction to a gas-gas reaction, avoiding the rapid nucleation of AlN liquid phase, and preferentially growing AlN into a one-dimensional morphology under low supersaturation.

[0028] 5. The aluminum nitride whiskers prepared by the present invention have high purity and high aspect ratio, and have broad application prospects in the fields of ceramic toughening and thermal conductivity and heat dissipation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 The macroscopic morphology of the aluminum nitride whisker sample synthesized in Example 1;

[0031] Figure 2 is an optical microscope photograph of the aluminum nitride whiskers synthesized in Example 1;

[0032] Figure 3 is a scanning electron microscope (SEM) photograph of the aluminum nitride whisker synthesized in Example 1;

[0033] Figure 4 is an X-ray diffraction (XRD) analysis of the aluminum nitride whiskers synthesized in Example 1;

[0034] Figure 5 The macroscopic morphology of the aluminum nitride whisker sample synthesized in Example 2;

[0035] Figure 6 The macroscopic morphology of the aluminum nitride whisker sample synthesized in Example 3;

[0036] Figure 7 The macroscopic morphology of the aluminum nitride whisker sample synthesized in Comparative Example 1;

[0037] Figure 8 The macroscopic morphology of the aluminum nitride whisker sample synthesized in Comparative Example 2;

[0038] Fig. 9 This is the sample macroscopic morphology of the aluminum nitride whisker synthesized in Comparative Example 3. DETAILED DESCRIPTION

[0039] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0040] Example 1

[0041] 3 g of 0.5 mm aluminum particles, 5 g of iron powder and 0.1 g of activated carbon were uniformly mixed, and the mixture was placed in an alumina crucible, transferred to a graphite sintering furnace, introduced with 1 L / min of nitrogen, heated to 1700°C at 50°C / min, and kept warm for 1 hour.

[0042] The macroscopic morphology of the synthesized aluminum nitride whisker samples is shown in Figure 1 It can be seen that a large number of white aluminum nitride whiskers with high aspect ratio are obtained on the wall of the alumina crucible.

[0043] Optical microscope photos of aluminum nitride whiskers are shown in Figure 2 , Scanning electron microscope (SEM) photos are shown in Figure 3 It can be seen that the aluminum nitride whiskers present a clear one-dimensional morphology, are hexagonal, have a smooth surface, are uniform in thickness, and have an ultra-high aspect ratio, with an average aspect ratio of ≥2000:1.

[0044] The XRD pattern of aluminum nitride whiskers is shown in Figure 4 Its characteristic diffraction peaks are consistent with those of AlN (JCPDS No 25-1133), and it has a high phase purity.

[0045] Example 2

[0046] 3 g of 0.5 mm aluminum particles, 7.5 g of iron powder and 0.05 g of carbon black were uniformly mixed, and the mixture was placed in an alumina crucible, transferred to a graphite sintering furnace, introduced with 0.1 L / min of nitrogen, heated to 1600°C at 20°C / min, and kept warm for 2 h.

[0047] The macroscopic morphology of the aluminum nitride whisker sample synthesized in Example 2 is shown in Figure 5 , it can be seen that a large number of aluminum nitride whiskers grew on the alumina crucible.

[0048] Example 3

[0049] 3 g of 1 mm aluminum particles, 3 g of iron powder and 0.03 g of carbon black were evenly mixed, and the mixture was placed in an alumina crucible, transferred to a graphite sintering furnace, introduced with 1 L / min of nitrogen, heated to 1650°C at 20°C / min, and kept warm for 4 hours.

[0050] The macroscopic morphology of the aluminum nitride whisker sample synthesized in Example 3 is shown in Figure 6 , it can be seen that a large number of aluminum nitride whiskers have grown on the alumina crucible.

[0051] Comparative Example 1

[0052] Basically the same as Example 1, except that the activated carbon content is 0.

[0053] The product morphology in Comparative Example 1 is as follows Figure 7 As shown, compared with Example 1 in which activated carbon was added, the yield of aluminum nitride whiskers in Comparative Example 1 in which activated carbon was not added was significantly less.

[0054] Comparative Example 2

[0055] It is basically the same as Example 1, except that the aluminum particles are replaced with aluminum powder of the same mass (average particle size ≥ 200 μm).

[0056] The product morphology in Comparative Example 2 is as follows Figure 8 As shown, compared with Example 1 using aluminum particles as raw material, the yield of aluminum nitride whiskers in Comparative Example 2 using aluminum powder as raw material is significantly less.

[0057] Comparative Example 3

[0058] Basically the same as Example 1, except that the iron powder content is 0.

[0059] The product morphology in Comparative Example 3 is as follows Fig. 9 As shown, compared with Example 1 in which iron powder was added, the yield of aluminum nitride whiskers in Comparative Example 3 in which no iron powder was added was significantly less, and large agglomerates were generated at the bottom of the crucible.

[0060] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for batch preparation of aluminum nitride whiskers, characterized in that: The following steps are involved: (1) Batching: Evenly mix aluminum particles, iron powder and carbon powder according to a certain mass ratio; (2) Synthesis: The mixed material obtained above is placed in an alumina crucible, and the crucible is transferred to a graphite sintering furnace. After heating from room temperature to 1500-1700°C in a nitrogen atmosphere, the temperature is kept for 0.5-6 hours, and then naturally cooled to obtain white aluminum nitride whiskers with a high aspect ratio.

2. The method for batch preparing aluminum nitride whiskers according to claim 1, characterized in that: The aluminum particles described in step (1) have a size of 0.5 mm to 1 mm.

3. The method for batch preparing aluminum nitride whiskers according to claim 2, characterized in that: In step (1), the mass ratio of aluminum particles to iron powder is 1:1 to 1:2.

5.

4. The method for batch preparation of aluminum nitride whiskers according to claim 1, characterized in that: The carbon powder used in step (1) is carbon black, graphite or activated carbon.

5. The method for batch preparing aluminum nitride whiskers according to claim 4, characterized in that: The mass ratio of carbon powder to aluminum particles is 1:100~5:

100.

6. The method for batch preparation of aluminum nitride whiskers according to claim 1, characterized in that: In step (2), nitrogen gas is introduced into the reaction furnace at a flow rate of 0.1-2 L / min.

7. The method for batch preparing aluminum nitride whiskers according to claim 6, characterized in that: In step (2), the heating rate of the atmosphere sintering furnace is 5-60°C / min.

8. Aluminum nitride whiskers prepared by the method according to any one of claims 1 to 7.

Images