Diamonds are classified into single crystals, aggregated crystals, and polycrystals based on their internal structure. In the Chinese materials science community, the terms "polycrystals, aggregated crystals, and microcrystals" are used loosely. Polycrystals refer to single crystals. Although aggregated crystals are also a type of polycrystal, they primarily refer to several single crystals bonded together through chemical bonding, whereas polycrystals in the conventional sense refer to single crystals physically bonded together. Microcrystals refer to a special structure of aggregated crystals in which the geometric dimensions of the crystalline regions are extremely small, generally in the micrometer range.
Diamond single crystals are generally classified based on their optical properties. Diamonds are typically categorized into Type I and Type II. The presence of nitrogen and boron as impurities within the diamond crystals distinguishes between Type I and Type II. For example, Type I diamonds contain nitrogen impurities. Nitrogen impurities are widely present in natural and synthetic diamonds and significantly affect their physical and chemical properties. Diamonds containing nitrogen impurities are further divided into Type IaA, containing nitrogen pairs; Type IaB, containing four clustered nitrogen atoms; and Type IB, containing free nitrogen.
Diamonds synthesized under high pressure and high temperature are typically not subjected to denitrification measures during loading, resulting in a slightly green to yellowish hue. These diamonds are Type IB diamonds. Type IIa diamonds are nitrogen-free and contain relatively low levels of nitrogen and other impurities. Due to their excellent thermal conductivity, Type IIa diamonds are primarily used in solid-state microwave devices and heat sinks. Large-grained Type IIa diamonds can be artificially produced, and they offer an inexpensive method for bonding diamonds to other materials.
Type IIb diamonds have the same optical properties as Type IIa diamonds. Approximately one-thousandth of Type II diamonds are Type IIb, exhibiting semiconductor properties. Type IIb diamonds have a wide bandgap and are resistant to high temperatures. Their excellent heat dissipation, mechanical strength, and corrosion resistance allow them to operate under challenging conditions.