Types of Graphite

1. Classification and Characteristics of Graphite:

1.1 Natural Graphite

Natural graphite is formed from carbon-rich materials under high temperature and pressure over long geological periods, resulting in a crystalline structure. The industrial value and applications of natural graphite are primarily determined by its crystal morphology. Different crystal forms have different industrial values and uses. Natural graphite can be categorized into three types based on crystal morphology: dense crystalline graphite, flake graphite, and cryptocrystalline graphite. In China, the main types of natural graphite are flake graphite and cryptocrystalline graphite.

1.2 Synthetic Graphite

Synthetic graphite is similar to polycrystalline materials in crystallography. There are various types of synthetic graphite with diverse production processes. Broadly, any graphite material obtained through carbonization and subsequent high-temperature graphitization can be termed synthetic graphite, such as carbon (graphite) fibers, pyrolytic carbon (graphite), and foam graphite. Narrowly, synthetic graphite typically refers to block solid materials made from low-impurity carbonaceous raw materials (like petroleum coke and pitch coke) as aggregates and binders, undergoing processes like mixing, kneading, molding, carbonization (known industrially as baking), and graphitization, such as graphite electrodes and isostatic graphite.

2. Differences Between Natural and Synthetic Graphite

Since narrowly defined synthetic graphite is often prepared from natural graphite, the differences between natural graphite and synthetic graphite are analyzed and discussed here.

2.1 Crystal Structure

• Natural Graphite: Exhibits well-developed crystals, with flake graphite showing higher graphitization degrees, whereas natural microcrystalline graphite typically has lower graphitization degrees.
• Synthetic Graphite: The crystal development degree depends on raw materials and heat treatment temperature. Generally, higher heat treatment temperatures result in higher graphitization degrees. Industrially produced synthetic graphite usually has lower graphitization degrees.

2.2 Organizational Structure

• Natural Flake Graphite: A single crystal with a simple structure, exhibiting crystallographic defects (e.g., point defects, dislocations, stacking faults), and macroscopic anisotropy. Natural microcrystalline graphite has small grains arranged randomly with impurities removed, showing macroscopic isotropy.
• Synthetic Graphite: A multi-phase material, including graphite phases from carbonaceous particles (e.g., petroleum coke or pitch coke), graphite phases from binder transformation, and pores formed from particle accumulation or binder heat treatment.

2.3 Physical Form

• Natural Graphite: Usually exists as a powder, used alone or in composite with other materials.
• Synthetic Graphite: Comes in various forms, including powders, fibers, and blocks, with block form being typical for narrowly defined synthetic graphite, requiring processing into shapes for use.

2.4 Physical and Chemical Properties

Natural and synthetic graphite share common physical and chemical properties but also have distinct performance differences. Both are good conductors of heat and electricity, but for graphite powders of the same purity and particle size, natural flake graphite has better thermal and electrical conductivity, followed by natural microcrystalline graphite, and then synthetic graphite. Graphite’s lubrication and plasticity are generally better in natural flake graphite due to its well-developed crystals, followed by dense crystalline graphite and cryptocrystalline graphite, with synthetic graphite being less favorable.

3. Applications of Natural and Synthetic Graphite

Graphite’s diverse properties make it useful in metallurgy, machinery, electrical, chemical, textile, and national defense industries. Natural and synthetic graphite have overlapping and unique applications.

3.1 Metallurgical Industry

• Natural Flake Graphite: Used in producing refractory materials like carbon bricks and aluminum-carbon bricks due to its oxidation resistance.
• Synthetic Graphite: Used as electrodes in steelmaking. Electrodes made from natural graphite are used in more demanding conditions in steelmaking furnaces.

3.2 Mechanical Industry

• Graphite Materials: Commonly used as lubricants and sealing materials. Natural flake graphite, due to its good lubricity, is often added to lubricating oils. Synthetic graphite components like piston rings, seals, and bearings are used in equipment handling corrosive media, often with lubricating oil added during operation. Natural graphite-polymer composites also serve these purposes but with lower performance compared to synthetic graphite.

3.3 Chemical Industry

• Synthetic Graphite: Due to its corrosion resistance, good thermal conductivity, and low permeability, it is used in heat exchangers, reaction tanks, absorption towers, and filters. Natural graphite-polymer composites can also be used in these areas, but synthetic graphite offers superior thermal conductivity and corrosion resistance.