What is nanomaterial? Nanomaterial refers to a material with at least one dimension in the nanoscale (1-100nm) in three-dimensional space. It is a new generation of material composed of nanoparticles with sizes between atoms, molecules, and macroscopic systems.

The size of nanomaterials is approaching the coherence length of electrons, and their properties undergo significant changes due to the self-organization brought about by strong coherence. Furthermore, their scale is close to the wavelength of light, coupled with the special effects of their large surface area. Therefore, the characteristics they exhibit, such as melting point, magnetism, optics, thermal conductivity, electrical conductivity, and so on, often differ from those exhibited by the substance in its bulk state.

Nanomaterials possess certain unique characteristics. When the scale of matter is reduced to a certain extent, it becomes necessary to replace traditional mechanics with quantum mechanics to describe its behavior. When the size of powder particles decreases from 10 micrometers to 10 nanometers, although the particle diameter changes by a factor of 1000, when converted to volume, the change is as significant as a factor of 10^9. Therefore, there will be significant differences in the behavior of the two.

Characteristics of nanomaterials: Surface and interface effects refer to the changes in properties caused by the sharp increase in the ratio of the number of surface atoms to the total number of atoms as the particle size decreases. This is manifested as a reduction in diameter and an increase in the number of surface atoms.

The surface of ultrafine particles exhibits high reactivity, and metal particles in air will rapidly oxidize and ignite. To prevent spontaneous combustion, surface coating or intentional control of the oxidation rate can be employed, allowing for slow oxidation to form an extremely thin and dense oxide layer, ensuring surface stabilization. Leveraging surface activity, metal ultrafine particles are expected to become a new generation of high-efficiency catalysts, gas storage materials, and low-melting-point materials.

Small size effectWhen the size of nanoparticles is comparable to or smaller than the wavelength of light, the de Broglie wavelength of conduction electrons, and the physical characteristic dimensions such as the coherence length and transmission depth of superconducting states, its periodic boundary is destroyed, resulting in 'novel' phenomena in its acoustic, optical, electrical, magnetic, and thermodynamic properties. With the quantitative change of particle size, under certain conditions, it can cause a qualitative change in the properties of the particles. The change in macroscopic physical properties caused by the decrease in particle size is called the small size effect. For ultrafine particles, as the size decreases, their specific surface area also increases significantly, resulting in the following properties:

1. Special optical properties

All metals appear black in the ultrafine particle state. The smaller the size, the darker the color, with silver-white platinum (white gold) turning into platinum black and metallic chromium turning into chromium black.

Metals appear black in the state of ultrafine particles

It can be seen that metal ultrafine particles have a very low reflectivity to light, typically below 1%, and can be completely extinguished with a thickness of only a few microns. This characteristic can be utilized to manufacture high-efficiency photothermal and photovoltaic conversion materials, which can convert solar energy into thermal and electrical energy with high efficiency. Additionally, it may also be applied to infrared sensitive components, infrared stealth technology, and so on.

2. Special thermal properties

When solid substances exist in large-sized forms, their melting points are fixed. However, after being ultra-fine, it is found that their melting points will significantly decrease, especially when the particles are smaller than the order of magnitude of 10 nanometers. The property of ultra-fine particles having a reduced melting point holds certain appeal for the powder metallurgy industry.

3. Special magnetic properties

In the process of studying nanomaterials, scientists have discovered the presence of ultra-fine magnetic particles in organisms such as pigeons, dolphins, butterflies, bees, and magnetotactic bacteria living in water. These particles enable these organisms to discern direction under the guidance of the geomagnetic field, endowing them with the ability to navigate back to their home.

Small-sized magnetic ultrafine particles differ significantly from bulk materials. The coercivity of pure iron in bulk form is approximately 80 A/m, but when the particle size is reduced to below 2×10-2 micrometers, its coercivity can increase by a factor of 1000. If the size is further reduced, to approximately less than 6×10-3 micrometers, the coercivity decreases to zero, exhibiting superparamagnetism.

Utilizing the characteristic of high coercivity of magnetic ultrafine particles, magnetic recording powder with high storage density has been produced and widely applied in magnetic tapes, disks, cards, and magnetic keys. Leveraging superparamagnetism, magnetic ultrafine particles have been transformed into versatile magnetic liquids.

4. Special mechanical properties

American scholars have reported that calcium fluoride nanomaterials can bend significantly without breaking at room temperature. Research has shown that the high strength of human teeth is attributed to their composition of nanomaterials such as calcium phosphate. Metals in the form of nanocrystalline grains are 3 to 5 times harder than traditional coarse-grained metals. Metal-ceramic composite nanomaterials can alter the mechanical properties of materials over a wider range, presenting a vast application prospect.

The small-size effect of ultrafine particles is also manifested in aspects such as superconductivity, dielectric properties, acoustic characteristics, and chemical properties.

Carbon nanotubes are a new type of super material, whose strength can reach 300 times that of steel

Quantum Size EffectWhen the size of particles reaches the nanometer scale, the electronic energy levels near the Fermi level split from continuous states into discrete energy levels. When the energy level spacing exceeds thermal energy, magnetic energy, electrostatic energy, magnetostatic energy, photonic energy, or the condensation energy of superconducting states, quantum effects of nanomaterials emerge, leading to changes in their magnetic, optical, acoustic, thermal, electrical, and superconducting electrical properties.

Energy level structure Quantum size effect: When the size of particles decreases to a certain value, the electronic energy levels near the Fermi level of metals change from quasi-continuous to discrete energy levels, and the energy gap between the discontinuous highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of semiconductor particles widens.

In nano-powders, due to the small number of constituent atoms in each particle, the surface atoms are in an unstable state, resulting in a larger amplitude of surface lattice vibrations. This leads to higher surface energy, which gives rise to unique thermal properties of ultrafine particles, namely a decrease in melting point. At the same time, nano-powders are easier to sinter at lower temperatures than traditional powders, making them excellent sintering-promoting materials.

Macroscopic Quantum Tunneling EffectThe ability of microscopic particles to penetrate potential barriers is known as the tunneling effect. The magnetization of nanoparticles also exhibits tunneling effects, where they can change by passing through the potential barriers of macroscopic systems. This is referred to as the macroscopic quantum tunneling effect of nanoparticles.