Design and Verification of High-Temperature Sintering Equipment for Aluminum Nitride Ceramics

Aluminum nitride ceramic, renowned for its high strength, hardness, Wear Resistance, and excellent high-temperature and corrosion resistance, is widely used as a catalyst and refractory material in precision casting processes. The unique properties of this ceramic impose higher requirements on sintering equipment. In traditional sintering processes, aluminum nitride ceramic is typically sintered using high-temperature resistance furnaces, but this method suffers from complex procedures, low efficiency, and high costs. To meet the sintering needs of aluminum nitride ceramic, this paper proposes a design for a high-temperature sintering equipment and validates it. The design features a detachable furnace door structure, allowing size adjustment according to actual needs, and can also achieve automated control based on user requirements.

1. Related Technologies and Equipment

1.1 Introduction to High-Temperature Sintering Process

1.1.1 Principles of High-Temperature Sintering

Sintering involves the thermal decomposition or oxidation of powders, inducing a phase transformation in powder materials or compounds at a certain temperature, converting them from a solid state to a liquid or gaseous state. The reaction equation is as follows:

During high-temperature sintering, ceramic powders undergo decomposition or oxidation reactions under high temperatures, exhibiting the following characteristics: (1) decomposition reactions occur under high-temperature conditions; (2) ceramic powders undergo oxidation reactions.

In actual production, to achieve good sintering results for aluminum nitride ceramic, it must undergo high-temperature sintering. After high-temperature sintering, aluminum nitride ceramic exhibits the following characteristics:

1.1.2 High-Temperature Sintering Process Parameters

In actual production, high-temperature sintering of aluminum nitride ceramic requires specific process conditions, including sintering temperature, sintering time, and holding time. Common high-temperature sintering equipment for aluminum nitride ceramic includes the following: (1) Vacuum sintering. Under vacuum conditions, aluminum nitride ceramic undergoes decomposition reactions, breaking down into α-Al₂O₃, γ-Al₂O₃, etc., at high temperatures; (2) Atmospheric pressure sintering. Under atmospheric pressure, aluminum nitride ceramic undergoes decomposition reactions at lower temperatures and oxidation reactions at high temperatures; (3) Hot-press sintering. Using liquid alumina as the raw material, chemical reactions with alumina occur under high pressure and temperature.

1.2 Introduction to Aluminum Nitride Ceramic Material

1.2.1 Characteristics of Aluminum Nitride Ceramic

Aluminum nitride (AlN) is an inorganic compound with a hexagonal crystal structure, a density of approximately 4.5 g/cm³, high hardness, and great flexural strength, earning it the title "King of Ceramics." It exhibits good chemical stability at high temperatures, remaining stable up to 1500°C. Additionally, aluminum nitride offers advantages such as high thermal stability, dielectric properties, and mechanical strength. Among ceramic materials, AlN is the only material with birefringence properties, hence it is also called "optical glass." AlN crystals have a reflectivity of over 90% for visible light (400–700 nm) and near-infrared light (850–1200 nm), making them suitable for manufacturing optical glass. In high-temperature environments, AlN boasts high thermal conductivity, low thermal expansion coefficient, and corrosion resistance, leading to its widespread use in the electronics industry.

1.2.2 Application Areas of Aluminum Nitride Ceramic

Due to its extremely high hardness and excellent machinability, aluminum nitride ceramic finds extensive applications in mechanical processing. As shown in Figure 1, during mechanical processing, aluminum nitride ceramic can be machined into various shapes such as drill bits, cutting tools, and molds. Owing to its high hardness, it is also used in grinding and polishing fields. Furthermore, it can be employed in manufacturing components like sensors and transducers.

1.2.3 Application Fields of Aluminum Nitride Ceramic

The main application fields of aluminum nitride ceramic include: electronic tubes, diodes, thyristors, and various electronic components in the electronics industry; thermal insulation materials, friction pair materials, and anti-corrosion materials in the aerospace field; machine tool guides, bearings, cutting tools, etc., in the mechanical industry; and fuel cells, electric heaters, thermocouples, etc., in the energy industry.

2. Design of High-Temperature Sintering Equipment for Aluminum Nitride Ceramic

2.1 Design Principles and Requirements

The sintering equipment for aluminum nitride ceramic uses aluminum nitride ceramic as the raw material and employs high-temperature processes for sintering. Its heating principle involves generating mixtures of compounds such as metallic aluminum, metallic iron, and aluminum nitride through oxidation-reduction reactions at specific temperatures, followed by mechanical extrusion molding, and finally sintering at high temperatures to produce high-strength aluminum nitride ceramic products. During sintering, reactions between metallic aluminum, metallic iron, aluminum nitride, and other compounds produce oxides or nitrides while forming various crystals.

2.2 Design Scheme

Based on the above requirements, this paper adopts a two-stage heating method: the first stage involves radiant heating, and the second stage involves conductive heating. The first stage of radiant heating heats the ceramic green body through high-temperature radiation, while the second stage of conductive heating heats the ceramic green body through conduction.

Considering the sintering temperature range of aluminum nitride ceramic, this paper employs a two-stage heating treatment technology, combining the first stage of high-temperature radiant heating with the second stage of conductive heating. The specific process flow is as follows:

The first stage of high-temperature radiant heating primarily uniformly heats the aluminum nitride ceramic green body to 1200°C–1900°C to enhance its strength and density. The second stage of conductive heating involves holding the green body at a constant temperature based on the first stage of low-temperature radiation to improve its sintering performance.

2.3 Design Validation Scheme

The equipment consists of a heating device, control system, and sintering device. Its structural diagram is as follows:

After the equipment design was completed, preliminary processing and assembly were carried out, followed by process parameter tests, including temperature curve tests, heating speed tests, pressure tests, and power tests. Based on this, the equipment's performance was validated. The results show that the equipment meets the requirements for the sintering temperature range of aluminum nitride ceramic (1200°C–1900°C) and exhibits good sealing and temperature uniformity. Additionally, the equipment's pressure control range during operation is 10 MPa–50 MPa, meeting users' pressure control requirements for aluminum nitride ceramic.

3. Experimental Validation and Result Analysis

3.1 Experimental Conditions and Methods

(1) Equipment dimensions: length × width × height = 600 mm × 300 mm × 500 mm; (2) Raw material formulation: Al₂O₃ mass fraction 35%, Zirconia mass fraction 35%, aluminum nitride mass fraction 5%; (3) Sintering temperature 1600°C, holding time 30 min; (4) Raw material particle size: maximum particle diameter 40 μm, particle size distribution range 0.5–1 μm. Al₂O₃, zirconia, and aluminum nitride were added to the molding mold and pressed into 200 mm × 100 mm green bodies. In the sintering furnace, the green bodies were heated to 1600°C and held for 30 min, then pressurized to 1 MPa in a vacuum press, and finally placed in a high-temperature furnace for sintering.

3.2 Experimental Result Analysis

Using the aforementioned sintering equipment, aluminum nitride ceramic was sintered at 1600°C. The sintered samples are shown in Figure 6. As seen in Figure 6, with increasing sintering temperature, the sample density significantly increases. When the sintering temperature reaches 1600°C, the sample density reaches its maximum value. Simultaneously, samples sintered at 1600°C exhibit noticeable cracking, indicating that 1600°C is the optimal sintering temperature for aluminum nitride ceramic. This cracking phenomenon is due to oxidation reactions occurring in aluminum nitride ceramic at high temperatures, leading to internal cracks in the samples. Additionally, during sintering at 1600°C, aluminum nitride ceramic also shows a significant volume shrinkage rate.

Conclusion

This paper proposes an overall structural design for high-temperature sintering equipment for aluminum nitride ceramic and validates its feasibility through experiments. The design employs three heating modules, achieving rapid heating of aluminum nitride ceramic materials through reasonable configuration. Simultaneously, a dual-channel feeding and dual-channel discharging structural design is adopted to increase material loading capacity. Furthermore, a high-temperature sintering furnace control system based on PLC control is designed, enabling precise control of temperature and pressure.