With the rapid development of military high technology, the detection, tracking, and attack capabilities of the defense systems of various countries have become increasingly strong. The survivability of military targets on the land, sea, and air, as well as the penetration capability of weapon systems are increasingly threatened. In order to improve the survivability of ground military targets in the national defense system and the penetration and in-depth strike capabilities of weapon systems, the development and application of stealth technology has become an important direction for the development of the national defense system. Stealth material is the most important link in stealth technology. Therefore, the research upsurge of stealth materials has been raised in recent years.The new types of stealth materials currently being used and under developed include broadband absorbers, polymer stealth materials, chiral stealth materials, and nano stealth materials. In recent years, more and more reports on the high electromagnetic wave absorption coefficient of nanomaterials have caused great interest to military science and technology personnel.

New Stealth Materials Applied to Military Fields

Nano zinc oxide is one of the research hotspots in nano stealth materials. Nano-zinc oxide is a very promising new type of military radar wave absorber with advantages of light weight, thin thickness, light color, and strong wave-absorption ability.

1.The preparation methory of nano zinc oxide

There are many kinds of chemical preparation methods for nano-zinc oxide, and new techniques never cease, such as the liquid-solid-solution phase transfer and separation method. However, mainly methods are precipitation method, sol-gel method, microemulsion method, and hydrothermal (solvent heat) method etc.

(1) precipitation method

The precipitation method is generally divided into direct precipitation method and homogeneous precipitation method. In the direct precipitation method, a zinc oxide precursor is prepared by adding a precipitant to a soluble zinc salt solution, and the nano zinc oxide is thermally decomposed at a certain temperature after being washed. The common precipitants are ammonia, ammonium bicarbonate, etc. The precursors are Zn(OH)2, Zn2(OH)2CO3, etc.

(2) sol-gel method

The sol-gel method is a wet chemical method for preparing ultrafine particles. The basic principle is to dissolve a metal inorganic salt or a metal alkoxide in a solvent to form a uniform solution, and the solute and the solvent are hydrolyzed or react with alcohol, and the reaction products generally form nanoparticles and sols after aggregation.

(3) microemulsion method

Thermodynamically stable, isotropic, apparently transparent or translucent dispersion systems with a particle size of 1-100 nm formed by two mutually-immiscible liquids under the action of surfactants are called microemulsions. Microemulsions are generally transparent, isotropic, thermodynamically stable systems composed of surfactants, co-surfactants, oils, and water.

(4) hydrothermal (solvent heat) method

Hydrothermal synthesis is a new method for preparing ultrafine particles in liquid phase. Generally, the construction of atoms and molecules at the molecular level and the crystal growth are realized at a temperature of 100-150°C and in a high-pressure environment. It is called water hydrothermal method when the solvent is water, it is called solvothermal method when solvents are others such as ethanol, isopropanol, etc.

SEM of nanometer ZnO

2 The basic principle and application of nano zinc oxide as stealth material

Stealth materials are materials used to reduce the detectability of military targets. The basic principle of material stealth is to reduce the signal intensity emitted by the target itself or reflect the outside; or reduce the signal contrast between the target and the environment to make it lower than the threshold of the detector; or to make the laws of the object and the environment contrast disorder, resulting in the target geometry Identification difficulties. Radar is still dominant in various types of detectors. Therefore, radar wave stealth material is the most important and fastest-developing stealth material in stealth technology. The basic performance requirement of radar wave stealth materials is to absorb radar waves, so this material is also called radar absorbing material.

The radar absorbing material absorbs the incident energy to reduce the reflected energy and reduces the detectability of military targets. Absorbing principle is usually the following three categories: First, when radar waves act on materials, the materials absorb the energy of radar waves, and through the generation of conductance loss, high-frequency dielectric loss, hysteresis loss, etc., the electromagnetic energy is converted into heat energy; The second is to spread the reflected wave energy of the radar wave on the surface of the material to each part of the target surface to reduce the electromagnetic energy scattered in the direction of the radar receiving antenna; third is to make the reflected wave of the radar wave on the surface of the material and the underlying material after entering the material. Reflected wave superposition produces interference cancellation.

3     waves and substances interaction

Zinc oxide is a multifunctional wide band gap inorganic semiconductor material with direct band gap. At room temperature, the band gap is 3.37 EV, the exciton binding energy is up to 60 MeV, and it has good piezoelectric properties, such as ultraviolet emission material, transparent conduction, field emission display device, solar cell and gas sensor. Ultraviolet semiconductor optoelectronic device materials have been widely used. Recently people have found zinc oxide is a good absorbing material, which has aroused great interest.
Nano zinc oxide is a new multifunctional inorganic material, which has many excellent properties in acoustic, magnetic, optical and electrical fields. Now, the industrial production of nano-zinc oxide has been realized. Therefore, with the development of research, Nanometer zinc oxide may become a kind of multifunctional composite wave absorbing stealth material.

Figure 4  Nano zinc oxide used in stealth materials