Synthesis of Nano-aluminum Nitride powder at low temperature by Sol-gel method

In this paper, molecular-level mixed aluminum nitride has been prepared from aluminum sec-butyl alcohol, glucose, and urea by sol-gel processing. Then, the nano-aluminum nitride powder with uniform particle size distribution was obtained by carbothermal reduction nitridation reaction. The effects of different feed ratios, reaction temperatures, and holding times on the phase of AlN powder were systematically studied. The prepared nano-aluminum nitride powder has a uniform particle size and a near-spherical shape, and its grain size is approximately 20~30 nm. The preparation temperature in this method was significantly reduced from the same preparation method of aluminum nitride powder.


Introduction
Aluminum nitride (AlN) ceramic materials have high thermal conductivity and are matched with the thermal expansion coefficient of silicon to make substrates and packaging materials for large-scale integrated circuits.Transparent AlN ceramics have good transmittance and high-temperature thermal stability for visible light and infrared light.At the same time, these ceramics can maintain high hardness and mechanical strength before decomposition temperature (about 2, 300℃).Therefore, AlN ceramic materials also have high application value in transparent protective armors and high temperature-resistant windows [1][2] .
In general, the preparation of high-purity dense AlN ceramics is greatly affected by the purity, particle size, and particle uniformity of AlN ceramic powders.Therefore, the relatively mature preparation methods of AlN powder mainly include the direct nitriding method of aluminum powder, the thermal nitridation reduction method of alumina carbon, and the chemical vapor deposition method.Among them, the aluminum powder prepared by the direct nitriding method has a relatively high oxygen content and a large particle size, resulting in difficulty in preparing high-density AlN ceramics.In addition, high-purity AlN powder can be produced by the chemical vapor deposition method, but it is difficult to achieve mass production due to the constraints of its process conditions.In comparison, the thermal nitriding reduction method of carbon is a cost-effective way even though the reaction temperature is generally above 1, 500°C.We aim to reduce the reaction temperature and prepare uniform nano-sized AlN powder with good moldability and high sintering activity.In this paper, the precursor of AlN was prepared by sol-gel method using aluminum sec-butoxide and glucose as raw materials, and then the precursor was prepared by carbothermal reduction at a temperature lower than 1, 500℃ [3][4][5][6][7][8] .

Experimental procedure
First, Al 2 O 3 sol (ethanol solution of sec-butanol aluminum) with 59 mL and 0.04 mol, glucose (C 6 H 12 O 6 ), and urea were fully mixed evenly according to the ratio of the amount of the substance as [Al]/[C]/[U] = 4:1:4, 4:2:4, 4:2.5:4, and 4:3:4 at 60℃.Further, the mixed gel was obtained, which was continuously dried to obtain a precursor sample of pale yellow AlN.Then, the precursor was heattreated under a nitrogen atmosphere, which reached a certain temperature at a heating rate of 8℃/ min and kept for a certain period.After that, it cooled with the furnace to obtain AlN powder.The obtained powder samples were subjected to carbon removal in an air atmosphere at 700℃ for 1 h and then characterized by XRD and SEM.

Reactions and processes in the experiment
In the first step, the aluminum sec-butoxide solution reacted with water to produce Al (OH) 3 gel [4] .The Al (OH) 3 gel encapsulates the urea molecules and the glucose molecules to achieve molecular level mixing.Then, the Al (OH) 3 gel was heat-treated at a certain temperature to prepare γ-Al 2 O 3 .The processes are shown in Equation (1).
The reaction of Equation ( 2) occurred in the second step.Glucose was pyrolyzed to carbon black under anaerobic conditions, allowing carbon black and Al 2 O 3 to achieve molecular level horizontal mixing.During this process, the carbon atoms in glucose could not be completely cleaved into carbon black, and too much glucose may cause waste of raw materials and difficulty in removing carbon in subsequent experiments.Therefore, a suitable amount of glucose needs to be explored.

C 𝐻 𝑂 ⎯⎯⎯⎯⎯⎯ 6𝐶
(2) In the third step, γ-Al 2 O 3 was subjected to a thermal decomposition reaction of carbon in a nitrogen atmosphere, and γ-Al 2 O 3 was reduced to AlN by carbon black in this process, as shown in Equation (3).

𝐶 𝑂 ⎯⎯⎯⎯⎯⎯ 𝐶𝑂
(4)   have weak intensity and wide peak shape, indicating that the sample prepared under this reaction conditions has poor crystallinity and small grain size.Single-phase AlN with a six-direction nanocrystal was obtained when the reaction temperature reached 1, 350℃, and its unit cell parameters are a=b=3.099and c=4.997 [12][13] .As the reaction temperature increased, the diffraction intensity of the AlN sample was continuously enhanced and the peak shape was narrowed, indicating that the grains of AlN were continuously growing.Figure 3 indicates that the AlN powder has spherical particles of uniform size when the reaction temperature was 1, 350℃ and 1, 450℃.Among them, the particle size of AlN powder is about 20-30 nm at 1, 350℃.The sample still has a uniform particle size when the temperature reaches 1, 500℃, but the morphology has been significantly deformed to increase the particle size to 100 nm.

Effects of thermal reaction time on phase composition of products
Figure 4 represents the XRD patterns of the samples with [Al]/[C]=4:2.5 reacted at 1, 350℃ for 0.5-2 h in a nitrogen atmosphere.Figure 4 reveals that a bit γ-Al 2 O 3 did not react completely at 1, 350℃ for 0.5 h, but pure phase AlN was obtained as the reaction time extended to 1 h.When the reaction time was 1 h, the broad spectral peaks of the XRD line indicated that the AlN powder had poor crystallinity [14] .
According to the Sherrer Equation (where K is a constant, taken as K=0.89; B is the half-width of the diffraction peak), the XRD particle size analysis of the powder samples with different holding times is shown in Table 1.It can be seen from Table 1 that all four powder samples had nanoscale grain sizes, and the grain size was increasing as the nitriding heat reaction time increased.

Carbon removal of aluminum nitride powders
Figure 5 shows the DSC curve of the sample with [Al]/[C]=4:2.5 prepared by reacting at 1, 350℃ for 1 h in a nitrogen atmosphere.The DSC curve in Figure 5 indicates that the sample has a significant exothermic peak at about 630℃, which indicates that the residual carbon in the sample was oxidized to carbon dioxide gas.Further, the AlN powder began to be oxidized at 800℃, so the carbon removal temperature of the aluminum nitride powder was selected to be 700℃.Therefore, an off-white AlN powder sample was obtained by removing carbon for 1 h at 700℃ in the air atmosphere.

Conclusion
The glucose in the precursor had carbon loss during the cracking process, and the raw material ratio was nAl/nC 6 H 12 O 6 =4:2.5.The temperature of this synthesis method was about 200℃ lower than that of the mixed nitriding reduction method of alumina and carbon powder.The grain size of the AlN powder increased as the nitriding thermal reaction temperature or the thermal reaction time duration increased.The grain size of the AlN sample was approximately 20 nm with uniform spherical morphology.

Figure 1
represents the XRD patterns of the powder samples prepared by four precursors with different [Al]/[C] ratios which thermally reacted at 1, 350℃ for 2 h.It can be seen that when [Al]/[C] = 4:1, the phase composition of the product was γ-Al 2 O 3 and AlN.As the amount of glucose increased, the diffraction peak of γ-Al 2 O 3 gradually decreased, and a single-phase AlN powder was obtained when [Al]/[C] = 4:2.5.By analyzing the line of [Al]/[C] = 4:1 in Figure1, it can be known that the AlN powder is directly reacted by γ-Al 2 O 3 near its phase transition temperature point without the phase transition of γ-Al 2 O 3 →α-Al 2 O 3 .The lattice loosens and internal defects significantly increased due to the increase of the particle activity near the phase transition temperature, so γ-Al 2 O 3 has higher reactivity than α-Al 2 O 3 , which greatly reduced the preparation reaction temperature of AlN powder.

Figure 2
shows the XRD patterns of the samples obtained from the precursors with [Al]/[C]=4:2.5 which thermally reacted at different temperatures.It can be seen from Figure 2 that the products consist of two phases of γ-Al 2 O 3 and AlN when the precursors were reacted at 1, 250℃ for 2 h.This indicates that γ-Al 2 O 3 had undergone a carbothermal reduction reaction at 1, 250℃, which further verified that the reaction occurred near the γ-Al 2 O 3 phase transition temperature (the phase transition temperature of γ-Al 2 O 3 →α-Al 2 O 3 is about 1, 200℃).The diffraction peaks of the sample at 1, 250℃

Figure 2 .
Figure 2. XRD patterns of the precursors after thermal reaction at different temperatures for 2 h.

Table 1 .
The grain size of samples under different process conditions.