An article to understand the properties and applications of nano-ceramics

Nanoceramics are composite materials made by introducing nanoscale ceramic particles, whiskers, fibers, and other structures into a ceramic matrix to improve its properties. Nanoceramics enhance the room temperature mechanical properties of the matrix material, improve high-temperature performance, and have the ability to be cut and shaped with ultra-plasticity. This material is a new type of advanced structural ceramic developed in the past 20 years.

1. Types of Nanoceramics

Nanoceramics can be divided into two categories based on their properties: nanocrystalline ceramics and nanofunctional ceramics.

① Nanocrystalline ceramics:

Nanocrystalline ceramics are produced by adding nanoscale particles to traditional ceramic powders or by nano-sizing traditional ceramic powders, controlling the size and distribution of crystal phases during sintering to change the microstructure of ceramics, and improving their mechanical properties, including hardness, strength, plasticity, and toughness.

② Nanofunctional ceramics:

Nanofunctional ceramics are ceramics with unique functions due to the addition of nanophases or particles with special properties that cannot be expressed fully in conventional micron-sized states but can be exhibited after ultra-fine treatment. These special functions include acoustics, optics, electronics, magnetism, bioreactivity, and sensitivity to the environment.

2. Characteristics of Nanoceramics

(1) Surface effect: As the particle size decreases, the ratio of the number of surface atoms to the total number of atoms of nanoscale particles increases sharply, causing changes in their properties.

(2) Volume effect: Due to the small volume of nanoscale particles, they contain very few atoms, resulting in extremely low mass. Therefore, many phenomena cannot be explained by the properties of bulk materials composed of an infinite number of atoms. This special phenomenon is called the volume effect.

(3) Quantum size effect: When the size of nanoscale particles decreases to a certain value, the electron energy level near the metal particle interface changes from quasi-continuous to discrete energy levels, and the phenomenon of widening of the energy gap due to the existence of discontinuous molecular orbital energy levels in nanoscale semiconductor particles is called the quantum size effect of nanomaterials.

3. Preparation Methods of Nanoceramics

(1) Physical preparation methods: The two main methods are evaporation and condensation and high-energy mechanical ball milling.

① Evaporation and condensation: In a vacuum evaporation chamber filled with low-pressure inert gas, a metal or compound evaporator is heated to produce an atomic mist, which loses energy by colliding with inert gas atoms, and condenses into nanoscale clusters that gather on a liquid nitrogen-cooled rod to obtain nanoscale powder.

② High-energy mechanical ball milling: Powder is placed in a closed container, and ultra-fine particles are obtained by rotating, vibrating, or shaking the container.

Other physical preparation techniques include mechanical grinding and spark erosion.

(2) Chemical preparation methods: The main methods are gas-phase chemical and liquid-phase chemical methods.

① Gas-phase chemical method: The reaction product vapor forms a very high supersaturated vapor pressure under conditions far above the thermodynamic critical reaction temperature, causing it to automatically condense into a large number of crystal nuclei. These crystal nuclei continuously grow in the heating zone and aggregate into particles. As the airflow enters the low-temperature zone, the particle growth, aggregation, and crystallization processes stop, and nanoceramic powder is collected in the collection chamber.

② Liquid-phase chemical method: Powder is synthesized through liquid phase, including precipitation, sol-gel, spray pyrolysis, and hydrothermal synthesis.

· Precipitation method: Ceramic precursors are precipitated from metal salt solutions by adding appropriate precipitants, and then calcined to form nanoceramic powder. To avoid severe hard agglomeration, techniques such as freeze-drying, supercritical drying, and azeotropic distillation are usually introduced.

· Sol-gel method: This method dissolves metal salts in alcohol and then adds distilled water to hydrolyze and polymerize them to form a sol, which is then transformed into a gel with more water. The gel is dried at low temperature under vacuum to obtain a porous dry gel, which is then calcined at high temperature to form oxide nanoceramic powder.

· Spray pyrolysis method: Metal salt solution is sprayed into a high-temperature atmosphere in mist form, and the solvent evaporates and the metal salt decomposes immediately, leading to oversaturation and solid-phase precipitation, and oxide nanoscale powder is directly obtained. Alternatively, the solution can be sprayed into a high-temperature atmosphere and dried, followed by heat treatment to form powder.

· Hydrothermal synthesis method: Materials are prepared in a sealed reactor using an aqueous solution as the reaction system, and the solution is heated to (or near) the critical temperature.

Examples of Applications of 4-nanometer Ceramics

The above-mentioned properties of nanoceramics overcome the limitations of most engineering ceramics, and play an important role in extreme environments such as ultra-high temperatures and strong corrosion, with considerable market prospects. The applications of nanoceramics in fields such as protective materials, high-temperature materials, and the automotive industry are as follows:

① Protective Materials

Ordinary ceramics have poor toughness and are prone to a series of chain reactions such as micro-damage, transgranular fracture, interfacial damage, and crack propagation in the contact area when subjected to bullet impacts, greatly reducing the bullet-resistant performance of protective ceramic products.

Nanoceramic materials with high fracture strength and fracture toughness also exhibit strong hardness and elastic modulus, coupled with superior impact resistance, making nanoceramics play a powerful role in the production of military armor, tanks, warships, military aircraft, rockets, and military protective materials.

Specific applications are as follows:

(1) The excellent impact resistance of nanoceramics effectively improves the bullet-resistant ability of the main battle tank's composite armor, enhances the anti-corrosion and anti-impact properties of ceramic pipes in rapid-fire weapons.

Continuing the translation:

(2) By using bulletproof ceramic coatings and nanotube composite materials to make substrates, super-strong bulletproof vests resistant to impact can be produced.

(3) The high-temperature and anti-oxidation properties of nanoceramics can improve the impact resistance of weapons such as artillery and torpedoes, and extend their service life.

In the civilian field, the performance of nanoceramics in the field of protection can be applied to cars, cushioning the impact of accidents on passengers.

② High-Temperature Materials

The high heat resistance, excellent high-temperature oxidation resistance, low density, high fracture toughness, corrosion resistance, and wear resistance of nanoceramics can play an important role in increasing the turbine line temperature of aircraft engines, thereby increasing the thrust-to-weight ratio of the engine and reducing the energy consumption of combustion. Additionally, the low-temperature superplasticity of nanoceramics makes them easy to manufacture, making them an ideal material for ships, military turbines, and other high-temperature components, improving engine efficiency, reliability, and working life.

③ Automotive Industry

Nanoceramics can be used as materials for automotive connecting rods, push rods, bearings, cylinder liners, piston tops, etc. They can also be used as oxygen sensors to detect automobile exhaust, and in the manufacture of high-temperature fuel cells for fuel cell vehicles.

The electrical properties of nanoceramics make them suitable as piezoelectric ceramic materials, widely used in electronics, lasers, communications, biology, medicine, navigation, automatic control, precision machining, piezoelectric sensing, metrology, ultrasonic acoustics, ignition, and explosion in military, commercial and civilian fields.