Nanopowder - Vapor Phase Preparation Method

Nano powders, also known as ultrafine powders or ultrafine particles, generally refer to powders or particles with a particle size of less than 100 nm. They are solid particle materials that exist in an intermediate state between atoms, molecules, and macroscopic objects, exhibiting unique surface effects, small size effects, volume effects, quantum size effects, and macroscopic quantum tunneling effects, with wide applications.

Preparation methods of nanomaterials

From small to large: atom → cluster → nanoparticle

Requirements for the preparation of nanomaterials

- Size and dimensions controllable (generally less than 100 nm)

- Compositional components controllable (elemental composition)

- Morphology controllable (shape)

- Crystal type controllable (crystal structure, superlattice)

- Surface physical and chemical properties controllable (surface state)

- (Surface modification and surface coating)

The preparation methods of nano powders are commonly classified as follows

Divided into physical methods, chemical methods, and physicochemical methods based on whether a chemical reaction occurs.

Physical methods involve physical changes such as evaporation, melting, solidification, deformation, and particle size changes.

Chemical methods include gas-phase deposition, precipitation, hydrothermal synthesis, solvothermal synthesis, sol-gel method, microemulsion method, vacuum condensation method, etc.

Comprehensive methods involve accompanying some chemical reactions during the preparation process while also involving the phase change process of particles. Certain physical means may be applied during the preparation process to ensure the smooth progress of chemical reactions.

Based on the material state during the reaction process, it can be categorized into three main types: solid-phase method, liquid-phase method, and gas-phase method. These are also commonly used methods for preparing nanoparticles.

Solid-phase methodThis method manufactures nano powders through changes from solid phase to solid phase without accompanying state (phase) changes from gas phase to solid phase or liquid phase to solid phase. The mechanism of material micronization can be roughly divided into two categories: one is a method that finely divides large bulk materials (size reduction) without changing the material itself; this includes mechanical grinding methods, explosive sintering methods, leaching methods (chemical treatment), etc.; the other is a method that combines the smallest units (molecules or atoms) (construction process), which involves changes in the material itself such as thermal decomposition methods and solid-phase reaction methods.

Liquid-phase methodThis is a widely adopted method for preparing nano powders in laboratories and industries. Based on chemical means and without the need for complex instruments, nano powders can be prepared through simple solution processes. This includes precipitation methods, hydrothermal methods, solution evaporation methods, sol-gel methods, radiation chemical synthesis methods, etc.

Gas-phase methodThis method directly utilizes gases or transforms substances into gases through various means to induce physical and chemical reactions in the gaseous state. Finally, during the cooling process, condensation occurs to form nano particles. The gas-phase method can be roughly divided into gas evaporation condensation methods, chemical vapor reaction methods, chemical vapor condensation methods, and sputtering methods. The electric explosion method is a special type of resistance heating method and is one of the gas-phase methods.

Gas-phase method

1. Evaporation-condensation method

Basic principle: Introduce low-pressure inert gas (He or Ar) into a vacuum evaporation chamber to heat and evaporate the evaporation material. The evaporated atomic substances collide with inert gas atoms to lose energy and condense into nano particles. Alloy nano particles can be obtained by simultaneously evaporating two or more metals.

Main characteristics: Since the formation of nano particles occurs under a high temperature gradient, the resulting nano particles have a narrow size range. Additionally, particle agglomeration and condensation characteristics can be controlled. Nano particles produced by inert gas evaporation-condensation have good crystallinity and clean surfaces with a dense oxide film that serves as a protective layer for easy handling and safe storage. However, this method has certain limitations; it is more suitable for preparing low-melting-point metal nano particles.

The heating methods used in IGC for preparing nano powders can be mainly divided into six types: resistance heating method, plasma spray heating method, induction heating method, electron beam heating method, laser heating method, and glow discharge plasma sputtering method.

1.1 Resistance heating method

The resistance heating method generates heat by passing current through resistive materials to heat the material.

1.2 High-frequency induction method

Using high-frequency induction coils as heat sources to heat conductive materials in crucibles through eddy currents under low-pressure inert gas conditions for evaporation; after evaporation, atoms collide with inert gas atoms to cool and condense into nano particles.

Wide heating temperature range: Temperatures can be selected within a wide range from above room temperature up to about 3000°C according to heating process requirements.

Precise heating control: Accurate temperature control can be achieved.

Uniform heating: Materials can be heated and processed in vacuum conditions or controlled atmospheres or liquid media.

High thermal efficiency: There is minimal heat loss during the heating process with high efficiency.

Less environmental pollution: Few pollutants are generated during the heating process with minimal environmental impact.

1.3 Plasma beam heating

This involves electric heating using high temperatures formed by ionizing working gases into plasma and energy released when free electrons in plasma recombine with positive ions. Working gases may include nitrogen, hydrogen, argon or mixtures such as nitrogen-argon or argon-hydrogen depending on usage requirements. In chemical industries, working gases may also include certain gases that participate in chemical reactions.

Plasma formed by gas ionization consists of un-ionized gas molecules, atoms, and positively charged ions, free electrons, and negative ions with equal total charge. It is referred to as the fourth state of matter, following solid, liquid, and gas. Plasma is overall neutral but has a high conductivity, and its motion is mainly governed by electromagnetic forces. Plasma has a very high temperature; the higher the degree of gas ionization, the higher the temperature of the plasma. Although natural plasma is rare on Earth, it is a major form of matter in outer space.

According to different degrees of ionization, plasma is divided into two main categories: fully ionized plasma with ultra-high temperature and ultra-high energy density (such as nuclear fusion) and weakly ionized plasma with less than 1% ionization (such as arc discharge). The plasma used in industry belongs to the latter category. This weakly ionized plasma is further divided into equilibrium plasma (high-temperature plasma) and non-equilibrium plasma (low-temperature plasma) based on whether the neutral particles, ions, and electrons are in thermal equilibrium.

1.4 Electron Beam Heating

Electron beam heating is a phase change treatment where the electron beam rapidly raises the surface temperature of metal materials to the austenite phase transformation temperature (below the melting point). After a period of time, the electron beam stops bombarding, and heat quickly diffuses to the cooler bulk metal, causing self-quenching of the heated surface, transforming its structure into martensite and significantly increasing surface hardness.

Electron beam furnaces are commonly used to manufacture or refine high-purity metals, especially titanium, tantalum, niobium, hafnium, and some exotic alloys. The electron beam furnace uses an electron gun (usually a thermionic electron gun is more common; if field emission type is used, one must also consider the interference of electromagnetic fields on the electron beam's flight path) to generate an electron beam. Typically, the electron gun applies high voltage to ensure that electrons in the beam have sufficient flight speed.

1.5 Laser Beam Heating

The principle of laser beam heating is to irradiate the workpiece surface with high-energy laser pulses, generating heat in the irradiated area. The heat is then transferred from the surface to the interior through thermal diffusion, convection, conduction, etc., achieving effects such as heating, melting, and quenching. The principle of laser heating is based on the high energy density and good directionality of laser beams, allowing materials to be heated to very high temperatures in a short time while precisely controlling the heating area and energy distribution.

Energy Absorption: When a laser beam irradiates the workpiece surface, light energy is absorbed by the material's surface and converted into thermal energy. Even if an object has a transmittance of 100% at a specific wavelength, it can still be heated if the laser power is sufficiently high. This is because some light will be scattered and reflected by the object; this scattered and reflected light can also be absorbed by the object.

Heat Transfer: The absorbed heat diffuses within the material's surface layer through thermal conduction mechanisms, creating a corresponding temperature field that causes changes in material properties within a certain range.‌‌

1.6 Sputtering Method

The sputtering method is a physical vapor deposition (PVD) technique mainly used to deposit thin films on substrates by bombarding a target material's surface with ions in a vacuum environment. The main types of sputtering methods include direct current sputtering, radio frequency sputtering, binary sputtering, and reactive sputtering.‌

Direct Current Sputtering: Uses direct current power supply suitable for conductive materials. In a vacuum chamber, voltage is applied between the cathode (target material) and anode (substrate), ionizing gas (such as argon) into plasma. Ions bombard the target material's surface under electric field action, causing target atoms to escape and deposit on the substrate.

Radio Frequency Sputtering: Suitable for non-conductive materials. Under radio frequency power supply action, gas is ionized into plasma; ions bombard the target material's surface in a radio frequency electric field causing target atoms to escape and deposit on the substrate. This method can process various materials including insulators and semiconductors.

Binary Sputtering: Combines features of both direct current and radio frequency sputtering methods by simultaneously using both power supplies suitable for depositing multiple materials or preparing multilayer films.

Reactive Sputtering: Introduces reactive gases (such as oxygen) during sputtering so that target atoms react with reactive gases to form compound films. This method is commonly used for preparing oxide or nitride compound films.‌

1.7 Flowing Liquid Surface Vacuum Deposition Method

Metal atoms evaporated in high vacuum form ultra-fine particles within a flowing oil surface; products are paste-like oils containing numerous ultra-fine particles.

Features: ① Can prepare ultra-fine particles with an average diameter of about 3nm; ② Uniform particle size with narrow distribution; ③ Nanoparticles evenly distributed in oil; ④ Particle size can be controlled.

1.8 Electric Heating Evaporation Method

By contacting carbon rods with metal and applying electric heating to melt the metal. The metal reacts with high-temperature carbon rods and evaporates to form carbide ultra-fine particles.

Device for preparing SiC ultra-fine particles

2. Chemical Vapor Deposition Method

The chemical vapor deposition method uses one or more reactive gases under heat, microwave, laser, or plasma action to initiate chemical reactions between reactive gases to generate desired compounds that rapidly condense in a gaseous environment to prepare various nanoparticles.

Main Features: When preparing nanoparticles using CVD methods, there are many controllable process parameters such as concentration, flow rate, temperature, composition, and ratio. Therefore, it is possible to achieve active control over nanoparticle composition, morphology, size, and crystal phase by controlling process parameters. In gaseous states, there is ample space for particle nucleation and growth which results in narrow particle size distribution with good monodispersity and uniform morphology.

3. Laser-Induced Chemical Vapor Deposition Method

LICVD utilizes the absorption of specific wavelength laser beams by reactive gas molecules, causing thermal decomposition or chemical reactions in the reactive gas molecules, leading to the formation and growth of nuclei, thereby preparing nanoparticles.

Principle of LICVD

Main characteristics: Due to the fast heating rate of the LICVD method, short high-temperature residence time (about 10^-3 s), and fast cooling rate, the prepared nanopowder has small particle size and uniform distribution. At the same time, because the reaction center area is isolated from the reactor by raw materials, contamination is low, resulting in higher purity of the prepared nanopowder. The disadvantage is that the preparation cost is high.^-34. Chemical Vapor Condensation Method (CVC)

Under high temperature and high pressure conditions, organic raw materials undergo thermal decomposition to form clusters that further condense into nanoscale particles. Characteristics: large yield, small size, narrow distribution.

CVC Schematic

5. Direct Current Arc Plasma Method

Plasma gas-phase synthesis is one of the main methods for preparing nanopowders. In low-temperature plasma methods, particle formation results from chemical reactions and nucleation growth, similar to the thermochemical reaction processes of high-temperature pyrolysis reactions and laser-induced reactions. In high-temperature plasma methods, particle formation results from the cooling and condensation of plasma-activated reactive gases. Plasma gas-phase synthesis methods are further divided into direct current arc plasma method (DC method), radio frequency plasma method (RF method), and composite plasma method.

The direct current arc plasma method can synthesize not only metal nanopowders but also metal-ceramic nanopowders, ceramic nanopowders, and carbon nanotubes. Its advantages include simple equipment usage, ease of operation, fast powder synthesis speed, high purity, a wide variety of products, and strong activity, making it suitable for industrial mass production.

直流电弧等离子体法不但可以合成金属纳米粉，也可以合成金属一陶瓷纳米粉、陶瓷纳米粉和碳纳米管。其优点是使用设备简单、易操作,并且粉体合成速度快、纯度高、种类多、活性强,适合于工业化批量生产。