Development status and industrialization prospect of functional diamond

Diamond has excellent electrical properties: ultra-wide band gap, ultra-high breakdown field strength, high electron and hole mobility, and is expected to become the "ultimate semiconductor"; Acoustically, diamond has the highest surface acoustic wave velocity and very high Young's modulus among all materials. Optically, diamond can transmit photons with less than bandgap energy from far infrared to ultraviolet. In thermal terms, its thermal conductivity exceeds that of copper, so diamond has the potential for cross-field applications

1. Synthesis of functional diamond

Compared with the traditional diamond high temperature and high pressure (HPHT) synthesis method, the functional diamond mainly adopts the (atmospheric pressure) chemical vapor deposition (CVD) method. CVD diamond is divided into CVD film (conventional diamond film, nano diamond film, thickness less than 50μm) and self-supporting thick film (single crystal diamond and polycrystalline diamond). According to synthesis technology, it can be divided into microwave assisted type (MPCVD), hot wire type and DC type. MPCVD technique is the main method of synthesizing high quality diamond.

2, the application of functional diamond

1), gem-grade artificial diamond
The high temperature and high pressure (HTHP) method is the main method for producing cultivated diamonds. The chemical vapor deposition (CVD) method of manufacturing cultivated diamonds has made rapid progress in the past three years, the cost has been greatly reduced, the process stability has made significant progress, and it has been mass-produced and put into the market in industrial batches.

2), diamond semiconductor
Diamond is considered to be the most promising material for the preparation of the next generation of high-power, high-frequency, high-temperature and low-power loss electronic devices, and is known as the "ultimate semiconductor" in the industry. Diamond is an indirect bandgap semiconductor material with a bandgap width of about 5.5eV and a thermal conductivity of up to 22W/(cm·K). Room temperature electron and hole mobility as high as 4500cm2/(v·S) and 3800cm2/(v·S) are much higher than third-generation semiconductor materials GaN and SiC. In addition, because diamond has a large exciton binding energy (about 80meV), so that it can achieve high intensity free exciton emission at room temperature (light wavelength of about 235nm), it has great potential in the preparation of high-power deep ultraviolet LED, and also plays an important role in the development of extreme ultraviolet deep ultraviolet and high-energy particle detectors. In addition, for the growth of single crystal diamond substrate material, but also have a high growth rate and large crystal size, while to achieve the semiconductor function of diamond requires effective doping, so that it has a good n type or p type conductive properties.

It can be expected that with the continuous development of diamond semiconductor technology, the future will break through the bottleneck problems such as N-type doping technology, large size and high-quality single crystal preparation and high smoothness and high uniformity material epitaxial technology, and achieve diamond electronic devices with higher power performance, thus creating faster, lighter and simpler devices for consumers. Diamond semiconductor devices are cheaper and thinner than silicon chips, and diamond-based electronic products are likely to become the industry standard for energy-efficient electronic products, which will have a significant impact on some high-tech industries and high-tech products. These include faster supercomputers, advanced radar and telecommunications systems, ultra-efficient hybrid vehicles, electronics in extreme environments, and the next generation of aerospace electronics.

3), the thermal application of diamond
Diamond has the highest thermal conductivity of known natural substances (2200W/(m·K)), 4 times larger than silicon carbide (SiC), 13 times larger than silicon (Si), 43 times larger than GaAs, 4 to 5 times larger than copper and silver, and low coefficient of thermal expansion (0.8×10) -6~1.5×10-6K-1) and high elastic modulus and other excellent properties. It is an excellent electronic packaging material with good prospect. Typical applications include diamond-reinforced metal packaging materials (Diamond/Cu, Diamond/Al) and heat sink - diamond substrate GaN devices. Diamond reinforced metal-based packaging materials have been commercial, its thermal conductivity has reached 350~600W/(m·K), with the price of artificial diamond significantly reduced, the price of diamond particles per unit volume has been close to or even lower than the price of W, Mo and other refractory metals, creating the necessary conditions for large-scale production.

The high thermal conductivity combined with electrical insulation makes diamond the heat sink of choice for many high power density devices. Laser diode junction heat dissipation is one of the earliest applications of CVD diamond. Recently, the integration of CVD diamonds with Gans has become possible on the order of wafers, allowing for higher power densities at radio frequencies.

4). Optical applications
The ideal power transmission window would have:
(a) Very low total absorption (window and coating absorption coefficient x window thickness.
(b) Low coefficient of thermal expansion (geometry changes less with temperature).
(c) High thermal conductivity (the ability to disperse heat from a heated object).
(d) High strength and Young's modulus (allowing for thinner Windows).
(e) Refractive index varies little with temperature.

Diamond almost all meet the above requirements, polycrystalline diamond in the optical window has many application scenarios, some have been industrialized.

In addition to the optical window, polycrystalline diamond also has a decorative role, the coating of diamond not only has a flash effect but also a variety of colors. It is used in the manufacture of high-end watches, decorative coating of luxury goods and directly as fashion products. Diamond is six times stronger and 10 times harder than Corning glass, so it is also used in mobile phone displays and camera lenses.

5), diamond acoustic devices
Diamond's low density and high Young's modulus and strength make it a high-performance high-frequency sound material.

Diamond, which can maintain perfect vibration motion without distortion at frequencies up to 70kHz, provides almost perfect sound reproduction. In addition, diamond also has the highest surface acoustic wave speed and is an excellent material for surface acoustic wave filters, which can improve the filtering frequency and power tolerance.

6), boron-doped diamond (BDD) electrode
Studies have shown that BDD is a new type of environmental protection electrode material, as the anode electrochemical method on dyes, pesticides, antibiotics, endocrine disruptors and other difficult to degrade organic pollutants, macromolecular substances have shown a good removal effect, almost can achieve complete mineralization, is an environmentally friendly pollution treatment method, so it has received more and more attention.

The use of boron-doped diamond (BDD) as an electrode electrochemical oxidation method, the use of water with strong oxidation performance of hydroxyl radical (·OH), can quickly and efficiently degrade organic matter in sewage, this type of electrode has the widest electrochemical window, extremely high.

Oxygen evolution potential, excellent chemical stability, can be in strong acid, strong alkali, high salt environment for a long time continuous operation, is expected to become the most potential organic wastewater treatment technology.

7). Application of diamond quantum technology
Diamond quantum technology offers potential solutions to two key problems of the 21st century: biomedicine and the ever-growing information economy. Diamond has the ability to turn a defect into a quantum resource, and this defect is specific, namely a nitrogen vacancy defect (NV), whose unique properties allow its quantum state to be manipulated and read at room temperature using light. In quantum-based applications, artificial diamonds act as the main body of impurities or defects, acting like solid atomic traps. The quantum properties of these impurities, such as nitrogen vacancy defects, can be manipulated individually and made to interact with each other, and the photons of light emitted from these impurities can be used to read out their properties quantum information.

Because of its broad application prospects, chemical inertness, mechanical hardness, surface modification ability, adjustable conductivity and biocompatibility, diamond has been widely studied as the interface between medical devices and the human body. Diamond coatings have been considered for a variety of medical device applications, including protective coatings, biological interfaces, and biosensors. Large size single crystal/polycrystalline diamonds can be made into sharp blades. Nanodiamonds can be used to transport drugs and target materials with labeling properties according to their properties.

8), nano diamond lubricating oil
Nano-diamond has a large surface area, and the surface has a rich functional group, these characteristics make its surface has a special adsorption, easy to chemical reaction, so the treated spherical nano-diamond has a good dispersion in the lubricating oil. In recent years, a large number of experiments have shown that adding one-thousandth of spherical nanodiamonds to lubricating oil can effectively improve the anti-wear and shock absorption effect of lubricating oil, and can be widely used in various mechanical structures

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