Pyrolytic carbon grown from gas via chemical vapor deposition

Pyrolytic carbon is a metastable substance—intermediate between graphite and amorphous carbon—that grows on a substrate surface through a chemical vapor deposition process involving carbon-containing gases (primarily hydrocarbons) under high-temperature conditions. (Thermodynamically, graphite is the solid carbon material with the lowest Gibbs free energy.)

Ⅰ. Pyrolytic Carbon and Chemical Vapor Deposition

The process by which precursor compounds undergo chemical reactions under specific process conditions and deposit onto the surface of a substrate to form a solid coating is called chemical vapor deposition. A schematic diagram of the main steps in the chemical vapor deposition process is shown in Figure 1 [2]:

(1) The precursor enters the reaction zone;

(2) The precursors undergo gas-phase reactions to form intermediate products;

a) When the temperature in the reaction zone is high or in regions far from the substrate, the intermediate products continue to react in the gas phase, forming solid powders and volatile byproducts. The solid powders settle onto the substrate surface, potentially serving as nucleation sites for surface deposition, while the volatile byproducts diffuse out of the reaction zone;

b) When the temperature in the reaction zone is lower or in regions close to the deposition substrate, the intermediate products diffuse onto the substrate surface, where the following steps occur:

(3) The intermediate products adsorb onto the substrate surface and undergo heterogeneous reactions, producing active deposition products and byproducts;

(4) The active deposition products diffuse across the substrate surface and accumulate to form a continuous coating;

(5) Gaseous byproducts leave the region near the deposition substrate;

(6) Unreacted feed gas and reaction byproducts leave the reaction zone.

Figure 1: Schematic diagram of the main steps of chemical vapor deposition process

The formation of pyrolytic carbon is an irreversible entropy-decreasing process in which carbon atoms transition from a disordered to an ordered state and from a gaseous to a solid state at high temperatures. It is governed by the ultimate direction of chemical thermodynamics while retaining a non-equilibrium microstructure through kinetic pathways. A deep understanding of this process not only lays the material foundation for carbon-carbon composites, high-temperature coatings, and nuclear fuel cladding but also provides a dual experimental and theoretical framework for the existence of carbon under extreme conditions.

Ⅱ. Preparation Process and Applications of Pyrolytic Carbon

Pyrolytic carbon is produced through the high-temperature pyrolysis of hydrocarbons at 1200–1400°C in an oxygen-free environment. Using a vertical fluidized bed reactor, the process is carried out through the following steps: an inert gas (nitrogen or helium) is introduced from the bottom of the reaction tube to fluidize the refractory particles; an external induction coil heats the contents of the reaction tube; and hydrocarbon gas is introduced to initiate the pyrolysis reaction.

Figure 2: Schematic diagram of fluidized bed for pyrolytic carbon preparation[3]

Ⅲ. Pyrolytic Carbon High-Temperature Protective Coatings

Figure 3: Thermal field structure in Czochralski crystal furnace[4]

Silicon, as the most common semiconductor material , is widely used in electronics and photovoltaics, particularly in the form of crystalline silicon in solar cells. Its high photovoltaic conversion efficiency makes it a core component in the photovoltaic industry. The preparation of monocrystalline silicon primarily employs the Czochralski method, which is widely used in monocrystalline silicon production both domestically and internationally due to its high production efficiency, controllable growth process, and precise doping control. However, as crystal sizes increase, the cost of graphite thermal fields also rises, particularly in silicon vapor environments where graphite components are prone to corrosion, leading to a shortened equipment lifespan. To address these issues, researchers have developed corrosion-resistant coatings for carbon material surfaces, such as pyrolytic carbon (PyC), silicon carbide (SiC), and tantalum carbide (TaC) coatings, to enhance the service life of carbon materials in high-temperature corrosive environments.

Cao et al. investigated the silicification corrosion of graphite (G330 graphite and AC200C/C composite materials) used in monocrystalline silicon furnaces. They proposed that open pores extending from the interior of the graphite to the surface provide pathways for the erosion and diffusion of molten silicon, thereby accelerating the silicification process of the carbon materials.

Zhao et al. deposited a pyrolytic carbon (PyC) coating on the graphite surface via chemical vapor deposition (CVD). The resulting PyC coating completely covered the graphite substrate and exhibited a dense structure with no obvious pores or penetrating microcracks. After treating the coated graphite material in a silicon vapor corrosion environment at 1600°C for 2 hours, the cross-sectional morphology of the coating and the EDX test results are shown in Figure 4. As shown in Figure 4a, even after silicon etching, the coating remained tightly adhered to the substrate surface without any visible cracks. Analysis of the elemental distribution of carbon and silicon revealed almost no silicon distribution beneath the pyrolytic carbon coating, indicating that it provides excellent protection for the graphite substrate [5].

Figure 4: Cross-sectional morphology of PyC coated graphite after silicide etching

Ⅳ. Pyrolytic Carbon-Coated Nuclear Fuel Seeds

Figure 5 shows a schematic diagram of the coating process for coated fuel pellets required for the 10 MW high-temperature gas-cooled reactor (HTR-10), developed by the Institute of Nuclear Energy Technology Design at Tsinghua University. As a core component of high-temperature gas-cooled reactors, the performance of coated fuel pellets directly affects the safe operation of the reactor. The Institute of Nuclear Energy Technology Design at Tsinghua University has successfully produced TRISO-type coated fuel pellets that meet design requirements using chemical vapor deposition (CVD) technology in a fluidized bed reactor [3].

TRISO-type coated fuel pellets consist of a four-layer structure, arranged from the innermost to the outermost layers as follows:

Fuel core: Uranium dioxide microspheres with a diameter of approximately 500 μm;

Loose pyrolytic carbon (PyC) layer: density ≤ 1.10 g/cm³, thickness 90 ± 18 μm;

Inner dense pyrolytic carbon layer: density 1.9 ± 0.1 g/cm³, thickness 40 ± 10 μm;

Silicon carbide (SiC) layer: density > 3.18 g/cm³, thickness 35 ± 5 μm;

Outer dense pyrolytic carbon layer: density 1.9 ± 0.1 g/cm³, thickness 40 ± 10 μm;

Each layer has a clearly defined function:

Loose pyrolytic carbon layer: provides storage space for fission products and accommodates fuel core expansion

Dense pyrolytic carbon layer: acts as a pressure shell to block the release of fission products

Silicon carbide layer: Possesses strong shielding capability, particularly against metallic fission products such as Cs, Sr, and Ba

Figure 5: Schematic diagram of coating encapsulated by nuclear fuel particles

This research not only resolved key technical challenges for fuel elements in China’s first high-temperature gas-cooled reactor but also laid a crucial foundation for the future development of high-temperature gas-cooled reactor technology. By precisely controlling the deposition process parameters of each layer, coated fuel pellets with ideal microstructures and properties were successfully produced, ensuring the safe and stable operation of the reactor.

A tiny nuclear fuel pellet embodies the tireless efforts of countless research teams, while a thin coating silently bears the weight of a nuclear power plant’s safe operation and the nation’s energy security—much like carbon atoms linking hand in hand to form a brilliant diamond, or the 1.4 billion descendants of the Dragon working together to support the bright future of the great rejuvenation of the Chinese nation.

Liufang Technology has long been dedicated to the research and development of protective coating products for high-temperature environments—such as silicon carbide and tantalum carbide—using chemical vapor deposition (CVD) methods, as well as the study of coating mechanisms. Recently, the company has been conducting research on pyrolytic carbon coating products to provide one-stop solutions for customers with protective coating needs across a wide range of operating conditions.

If the semiconductor equipment you're responsible for needs to operate in an inert, high-temperature environment, requiring extreme chemical inertness and particle control, but you're struggling because SiC coatings can't withstand the temperature or corrosion—consider pyrolytic carbon. In fact, pyrolytic carbon is an irreplaceable "pure carbon armor" in the field of high-temperature semiconductor manufacturing processes.

Want to learn more about pyrolytic carbon technology parameters, or obtain details about SiC/pyrolytic carbon composite solutions? Please contact us for further information.

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References:

1. Zhang, Weigang. Chemical Vapor Deposition - From Hydrocarbon Gases to Solid Carbon. Science Press, Chapter 1.

2. Hu, Chunxia. Methane Chemical Vapor Deposition Technology. Doctoral Dissertation, Northwestern Polytechnical University.

3. Zhu, Junguo. Yang, Bing. Zhang, Bingzhong. Huang, Jintao. Xu, Shijiang. Chemical Vapor Deposition of Fuel Particles Coated in High-Temperature Gas-Cooled Reactors. Journal of Tsinghua University (Natural Science Edition). Vol. 36, No. 11, 1996. 65-71.

4. Jianxin Tu, Kui Hao, Caixia Huo, Ziyuan Guo, Jianhao Wang, Aijun Li, Ruicheng Bai, Zhihao Ji. Research Progress on Corrosion-Resistant Coatings of Carbon-Based Materials for the Semiconductor Field. Progress in Materials Science. 1-13.

5. Wei Zhao, Bo Zhu, Weiwei Cao. Preparation and Properties of Pyrolytic Carbon Coating on Carbon Materials Used in Czochraski Single Crystal Silicon Furnace. Applied Mechanics and Materials, Vol. 597, 2014, 170-174;

6. Robert More, J. Sines, Ling Ma, Jack Bokros, Pyrolytic carbon.