Zinc oxide: characteristics, applications and development trends of multifunctional inorganic materials

Zinc oxide (ZnO), commonly known as zinc white, is an inorganic compound composed of zinc and oxygen elements. The chemical formula is ZnO, CAS number 1314-13-2, and molar mass 81.4084 g·mol⁻¹. This substance, which appears as a white and odorless solid, is difficult to dissolve in water. However, it plays an important role in the process of human civilization due to its unique physical and chemical properties. From ancient coatings and medicinal uses to core materials in modern high-tech fields, its application scenarios continue to expand, becoming a multi-functional material spanning traditional industry and cutting-edge technology.
1. Core psychological properties: structure determines performance

The excellent properties of zinc oxide stem from its special crystal structure and electronic properties. At room temperature, it mainly exhibits a hexagonal wurtzite structure. Each zinc ion is surrounded by six oxygen ions, and the oxygen ions are surrounded by four zinc ions. This structure gives it a piezoelectric effect and a pyroelectric effect, laying the foundation for the application of electronic devices. Under high pressure environment, zinc oxide can be converted into a sodium chloride-type octahedral structure, showing structural plasticity.

In terms of physical properties, zinc oxide has a density of 5.606 g/cm³, a melting point of 1975°C (decomposition temperature), a boiling point of 2360°C, and a refractive index nD of 2.0041. It has high thermal conductivity and electrical conductivity. It is a direct bandgap wide-bandgap semiconductor with a band gap of about 3.37 eV at room temperature and an exciton binding energy as high as 60 meV, which gives it unique advantages in the field of optoelectronics. It is worth noting that zinc oxide has thermochromic properties. It will turn yellow when heated in the air and return to white after cooling. This is caused by the formation of a non-stoichiometric compound Zn₁₊ₓO at high temperatures.

In terms of chemical properties, zinc oxide, as a typical amphoteric oxide, can react with acids and bases to form corresponding salts, such as reacting with hydrochloric acid to form zinc chloride, and reacting with sodium hydroxide to form zincate. It is stable at room temperature and can react with most non-metallic elements at high temperatures. It can also be reduced to metallic zinc by carbon, aluminum powder, etc. In addition, it can react with hydrogen sulfide to produce zinc sulfide, which is used in commercial scenarios such as deodorants. The zinc phosphate produced by reacting with phosphoric acid can be used as a dental material.

2. Preparation process: technological evolution from traditional to high-end

The preparation process of zinc oxide continues to iterate with application needs, forming two major systems: industrial scale production and laboratory precision synthesis. Different processes determine product purity, particle size and performance differences, adapting to different application scenarios.

(1) Industrial preparation process

1. Indirect method (French method): Using high-purity metal zinc ingots as raw materials, the zinc is first melted at 600-700°C, then evaporated at 1250-1300°C to form zinc vapor, which is oxidized by air and then cooled and collected. The purity of the product can reach 99.5%-99.7%. Through multi-stage sedimentation and surface modification technology optimization, precise control of particle size distribution can be achieved, which is suitable for high-end rubber, electronic ceramics and other fields.

2. Direct method (U.S. method): Use zinc ore, metallurgical slag, etc. as raw materials, mix with anthracite and heat to 800-1000°C. The zinc ore is first reduced to generate zinc steam, and then oxidized in the same reactor to generate zinc oxide. In 2025, technological breakthroughs will be achieved to achieve integrated flue gas desulfurization (SO₂ recovery rate >99%). The total zinc recovery rate will be increased to more than 97%. The product purity is 90%-95%. The cost is only 45% of high-purity products. It is widely used in ceramic glazes, animal feed and other scenarios.

3. Chemical method: Using zinc salts such as zinc chloride and zinc sulfate as raw materials, it reacts with alkaline substances such as sodium hydroxide and sodium carbonate to form zinc hydroxide precipitate, which is converted into zinc oxide after washing, drying, and calcination, including ammonium bicarbonate method, nitrate method, etc. The acid process is optimized through crystal growth inhibitors and continuous fluidized bed calcination. The specific surface area of ​​active zinc oxide exceeds 80 m²/g, which is much higher than the 10-15 m²/g of the fire process. It is suitable for high-end needs such as photovoltaic encapsulants and battery separator coatings.

(2) Laboratory and new technology

Hydrothermal methods, vapor deposition methods, sol-gel methods, etc. are commonly used in laboratories to prepare special forms of zinc oxide. The hydrothermal method obtains zinc oxide crystals through high-pressure and high-temperature reactions; the vapor deposition method can prepare high-quality zinc oxide films; and the sol-gel method can accurately control the product structure. The microemulsion method uses cyclohexane organic phase and nonionic surfactant system to prepare zinc oxide with special particle structure.

New processes are promoting high-end zinc oxide materials. The radio frequency plasma-assisted gas phase method can prepare arrayed nanopillars, and the piezoelectric response efficiency is two orders of magnitude higher than that of thin film materials. Spray pyrolysis technology uses ultrasonic atomization and microwave thermal fields to produce monodispersed 10nm spherical particles, which have entered the quantum dot backlight film market. The zincate molten salt electrolysis method can prepare high crystallinity flakes with a thermal conductivity of 280 W/(m·K), which can be used in LED thermal management systems.

3. Diverse application areas: functional materials penetrating into various industries

The global annual output of zinc oxide is about 1.05 million tons, and its applications cover many fields such as rubber, medicine, electronics, and catalysis. Its core value comes from its multiple properties such as reinforcement, anti-ultraviolet, semiconductor, and antibacterial properties.

(1) Rubber industry: core reinforcement and activator

About 50% of zinc oxide is used in the rubber industry. When used in conjunction with stearic acid as a vulcanization activator, it can accelerate the rubber cross-linking reaction and improve the mechanical properties, wear resistance and aging resistance of the product. Adding zinc oxide thermally conductive powder to silicone rubber can significantly improve thermal conductivity and maintain high resistance. Nanoscale zinc oxide can achieve high thermal conductivity at low filling amounts, but the problem of particle agglomeration needs to be solved to avoid affecting mechanical properties. In addition, zinc oxide can protect rubber from fungi and UV rays, extending its service life.

(2) Medicines and cosmetics: safety protection and care

In the field of medicine, zinc oxide is an OTC Class B drug. Its ointment is suitable for acute/subacute dermatitis, eczema, prickly heat and mild skin ulcers. The oil can be used for small area burns and scald care. It is used externally twice a day. Adverse reactions such as burning sensation and rash are occasionally seen. Avoid contact with mucous membranes and oral administration when using. In dentistry, zinc oxide can be used in toothpaste ingredients, temporary fillings and tooth repair pastes. The cement-like product produced by reacting with phosphoric acid is a commonly used dental material.

In the field of cosmetics, zinc oxide has become the core ingredient of sunscreen because of its high efficiency in absorbing ultraviolet rays and high safety. Although traditional nano-zinc oxide preparations are viscous and have poor cosmetic properties, they have excellent UV protection effects. With the development of surface modification technology, the use experience has been gradually improved. In addition, zinc oxide can also be used as a dietary supplement to provide the human body with essential zinc.

(3) Electronics and optoelectronics: new forces in semiconductor materials

Zinc oxide is widely used in the electronic field due to its wide band gap and high exciton binding energy. It is used to produce varistors (varistors), which utilizes nonlinear current-voltage characteristics to achieve lightning protection for high-voltage lines and surge protection for equipment. High-density materials are required to ensure stable performance. In the field of optoelectronics, it can be used in ultraviolet lasers, solar cells, gas sensors and other devices. Its photoluminescence properties and piezoelectric properties can also be used to design MEMS sensors to identify bending radii and angles. In addition, zinc oxide is expected to replace GaN as the core material for UV LDs and LEDs, with huge potential.

(4) Other fields: function expansion and innovative applications

In the catalytic industry, zinc oxide can be used as an organic synthesis catalyst and desulfurization agent. It generates electron-hole pairs under ultraviolet light and degrades organic pollutants through direct or indirect oxidation. Adding other components can expand its visible spectrum response range. In the textile industry, nano-zinc oxide coating gives textiles waterproof, self-cleaning and anti-ultraviolet functions. Its breathability is better than that of the main material, and it is suitable for military and daily clothing.

In the construction field, nano-zinc oxide is added to cement to improve compressive strength and anti-fouling properties; in the food industry, it is used as a zinc source in breakfast cereals and feed additives, and can be used in combination with other technologies to improve sterilization efficiency. At the same time, zinc oxide also has important applications in batteries, ferrite materials, flame retardant materials and other fields.

4. Safety, environmental protection and development trends: greening and high-end in parallel

(1) Safety and environmental protection matters

Zinc oxide itself is relatively safe, but you need to avoid inhaling dust and direct skin contact. You should wear protective clothing, gloves and protective masks when operating, keep it well ventilated, and store it away from acids. The European Union lists it as an environmentally hazardous substance (R50/53) and needs to follow the S60 and S61 safety terms. During the production process, the direct method needs to control SO₂ emissions, and the wet method needs to treat wastewater and residue. New processes are reducing environmental impact through low energy consumption and closed loops. For example, the ammonia recovery rate in the ammonia process exceeds 99.8%, and the cost of regenerated zinc oxide is lower than that of primary mineral products.

In terms of recycling, zinc oxide can be recovered from scrap zinc products through pyrometallurgy and hydrometallurgy, with a recovery rate of 80%-90%. In the future, the closed-loop recycling system will combine nanotechnology and intelligent monitoring to improve resource utilization and reduce the risk of heavy metal pollution.

(2) Industry development trends

The zinc oxide industry is evolving towards greening and high-end development. In terms of energy efficiency, the wet process (power consumption per ton is 3800 kWh) has more advantages than the thermal process (6000 kWh). In the era of green power parity, marginal benefits continue to increase. Driven by environmental protection policies, there has been a wave of conversion to the third-generation wet process in the EU, and carbon capture technology is gradually being used to reduce emissions.

In terms of value, the price of ordinary zinc oxide is about 20,000 yuan/ton, while target-grade nanopowder has exceeded 1.2 million yuan/ton. High-end has become the key to the company's success. In the future, quantum computing and machine learning will assist in the design of high-performance zinc oxide materials, green processes such as bio-based synthesis and microwave synthesis will be further optimized, and the full life cycle assessment system will be gradually improved to promote the industry to achieve a balance between environmental and economic values.

From ancient natural mineral utilization to modern nano-precision preparation, zinc oxide continues to empower technological upgrades in various industries with its controllable physical and chemical properties. With the intersection of materials science and environmental engineering, this ordinary white powder will surely shine with more dazzling value in fields such as new energy, high-end manufacturing, and ecological environmental protection.