Silicon Carbide (SiC): industrial production methods and applications

Abstract

Silicon Carbide (SiC) is a versatile material with a wide range of industrial applications. This article provides an overview of SiC, its production methods, and a specialized infrared pyrometer used in its manufacturing process. We also explore the main industrial applications of SiC.

Introduction

Silicon Carbide (SiC) is a compound of silicon and carbon with a unique combination of properties that make it highly desirable for various industrial applications. SiC exhibits exceptional hardness, high-temperature stability, chemical resistance, and excellent electrical and thermal conductivity. This article addresses the nature of SiC, its industrial production methods, and the importance of precise temperature measurement during its manufacture.

Silicon Carbide: Natural Occurrence and Industrial Form

In its natural state, silicon carbide occurs as the mineral moissanite. However, the SiC used in industry is synthetically produced in various forms. Industrial SiC is typically produced in two primary forms: abrasives and ceramics. These forms are tailored to meet different industrial needs, including cutting, grinding, refractories, and electronic components.

Production Methods

Two significant methods to produce SiC are today the Acheson process and Physical Vapor Transport (PVT) technique.

The Acheson Process

The Acheson process, developed by Edward Acheson in 1891, involves the high-temperature reaction of silica (SiO2) and carbon (C) in an electric resistance furnace. This process typically requires temperatures above 2000°C and results in the formation of SiC crystals. It is a cost-effective method due the quantity of electricity required.

The main steps of this process are:

·        Raw material, silica sand composed mainly of silicon dioxide (SiO2) and carbon from coke or anthracite coal are mixed together with additives or reagents to enhance the properties of the silicon carbide produced.

·        The materials are inserted inside an electric resistance furnace. The furnace consists of a cylindrical chamber. Graphite electrodes are used to generate the necessary heat for the reaction.

·        Inside the furnace, a high-temperature reaction zone is established. The temperature in this zone can reach as high as 2,500 to 3,000 degrees Celsius (4,500 to 5,400 degrees Fahrenheit). This extreme temperature is required to facilitate the reaction between the silica and carbon.

·        The main chemical reaction in the Acheson process can be summarized as follows:    SiO2 + 3C → SiC + 2CO    In this reaction, silica (SiO2) from the sand and carbon (C) from the coke combine to produce silicon carbide (SiC) and carbon monoxide (CO) gas. The silicon carbide crystals begin to form in the high-temperature environment and start to accumulate on the carbon lining of the furnace.

·        Cooling and Collection: as the reaction progresses, the silicon carbide crystals grow and adhere to the carbon lining. After a certain period of time, the furnace is cooled down, and the silicon carbide product is carefully removed. The material is typically crushed and sorted into different grades depending on the desired application.

·        The raw silicon carbide produced through the Acheson process may contain impurities. Additional steps, such as acid washing or other purification techniques, are often employed to remove these impurities and obtain high-purity silicon carbide for specific applications.

Physical Vapor Transport (PVT) Technique

The PVT technique, on the other hand, involves sublimating SiC material at high temperatures and then depositing it onto a cooler substrate where it condenses and crystallizes. This method offers greater control over the SiC crystal quality, making it a preferred choice for semiconductor-grade SiC.

Description of the process: 

·        The source material - often in the form of a polycrystalline powder or a small single crystal – is placed in a sealed ampoule or quartz tube. It is then heated to its sublimation temperature, where it transforms directly from a solid to vapor phase.

·        A small single crystal or seed crystal is placed in the vicinity of the source material. The new single crystal will grow from this seed crystal.

·        One end of the quartz tube is heated, and this region is referred to as the heating zone. The source material sublimes in this zone.

·        The opposite end of the quartz tube is kept cooler. It is in this region where the vapor from the source material condenses and crystallizes onto the seed crystal.

·        The system is maintained at a specific pressure (usually reduced pressure or controlled atmosphere) to optimize the growth conditions.

Importance of the temperature

The sublimation temperature must be reached to allow the solid material to transform directly in vapor. But more important is the control of the cooling. Over time, as the deposition continues, a single crystal with the same crystal structure as the seed crystal is grown. The temperature gradient between the heating and cooling zones plays a crucial role in controlling the growth rate and crystal quality.

For high standard quality of SiC, infrared pyrometer like the CellaCrystal PA 45 are widely used.

Advantages of the PVT technique

It guarantees:

·        a high purity. PVT is known for producing high-purity single crystals, which is essential for applications in semiconductors, photonics, and other high-tech fields.

·        A controlled growth: the growth conditions, such as temperature, pressure, and gas composition, can be precisely controlled, resulting in high-quality and well-oriented crystals.

·        Large Crystals: PVT can be used to produce relatively large single crystals, which are valuable for various scientific and industrial applications, such as laser optics, semiconductor substrates, and more.

Temperature Measurement in SiC Manufacturing

In the high-temperature manufacturing process of SiC, precise temperature measurement is crucial. For this purpose, the CellaCrystal PA 45 infrared pyrometer from the KELLER brand has proven to be an appropriate choice. This pyrometer is specifically designed for extreme temperature measurement requirements, offering a sight of only 8 mm over nearly 1.5 meters, making it ideal for measuring through sight tubes. Its measuring range extends up to 3500°C, and it features specific calibration over the entire range, ensuring high stability during the manufacturing process.

Main Industrial Applications

SiC's exceptional properties make it invaluable in various industries, including:

·        Abrasives: for abrasive applications such as grinding wheels, sandpapers, and cutting tools due to its hardness and durability.

·        Refractories: a refractory material in high-temperature applications such as furnace linings, kiln furniture, and crucibles.

·        Electronics: thanks to its excellent thermal conductivity and electronic properties, SiC is nowadays a key component in power electronics, high-frequency devices, and advanced semiconductor devices.

·        Metallurgy: SiC is utilized in the metallurgical industry for its ability to increase the strength and wear resistance of various metals and alloys.

·        Aerospace: SiC composites are employed in aerospace applications to reduce weight while maintaining high-temperature stability.

·        Energy: SiC is used in energy applications, including solar inverters, high-efficiency motors, and nuclear reactors.

Figure 1 : moissanite crystals Source Wikipedia

Figure 2 Basic furnace for Acheson process Source Wikipedia

Figure 3 Example of furnace for PVT Source KELLER

Figure 4 Pyrometer CellaCrystal PA 45 from KELLER Source KELLER

Figure 5 Spot size of less than 8 mm along 1.5 meters Source KELLER