The infrared film has a wide range of applications due to its wide wavelength range. With the development of science and technology this year, various coating materials and coating processes have gradually improved. Of course, the development of infrared films has also been greatly improved. Now infrared films have gradually developed in various fields such as national defense and military, biotechnology, teaching and research, and life applications, and they are becoming increasingly mature.
What is the infrared film?
The infrared film refers to a solid film that is transparent to infrared radiation. Its main transparent area is in the mid-wave range (0.9um~5um ) and long-wave range 8um~12um. Most materials in the mid-wave range are also transparent in the visible light range (0.3um~0.7um ).
Common infrared film materials and their applications
Germanium
Germanium advantage is that the mechanical properties of a good and a high refractive index, half-wave) germanium film often significantly reduces reflection losses. Under the condition of 300 ℃, 8~12 um is almost completely devitrified. In the multi-layer antireflection coating design, a single layer is used ( the transmission rate of germanium decreases with the increase of temperature, and the transmission rate is only 25 at 3~5 um. ℃ at 20% or so. therefore, germanium should not be used in high temperatures and high power environments.
Silicon
Si melting point of about 1414 degrees Celsius, in 1.1um and 8um having good spectral properties through the range, the refractive index in the near-infrared region can reach 3.4 or so. Since silicon has a high melting point, good thermal conductivity, high hardness, chemical stability and other properties, and thus is a very important semiconductor material, its excellent optical properties and physical and chemical properties make application prospects in the infrared band of the optical film is very broad.
Magnesium Fluoride
Magnesium fluoride (MgF2) is a widely used material, with high mechanical strength and thermal shock resistance. MgF2 is a widely used optical coating material with excellent performance. It has a wide transparent band (120nm, 8000 nm), low refractive index (n = 1.38 ), good thermal stability, the high mechanical strength of the film and laser damage higher threshold excellent performance, is prepared antireflection film, the high-reflection film, high damage threshold film of the film material of choice, therefore, MgF2 is widely used in the preparation of various optical months Mo layer.
The infrared transmittance of hot-pressed magnesium fluoride in the 3 ~ 5 um mid-wave infrared band can reach more than 90 %, which is the highest in the existing infrared materials. Pressing magnesium fluoride also has high mechanical strength, thermal shock resistance, chemical resistance and isotropic, and many other features, and its dielectric constant and dielectric loss is small, a good window MWIR, rectifying cover Materials, and medium-wave infrared millimeter-wave microwave combined radome materials are widely used in infrared guidance, infrared imaging guidance, infrared millimeter-wave microwave combined guidance, and infrared tracking, detection, and guidance systems such as aircraft infrared pods and photoelectric radars.
Oxide thin-film materials
Most oxide materials have many similar properties. Commonly used oxide materials such as Al2O3 and SiO2 are characterized by: high-temperature resistance, thermal shock resistance, high mechanical strength, rigidity, etc., which are very suitable for windows Material.
Al2O3 material as a very common high refractive index material is widely used in multilayer dielectric films. The optical properties of the film strongly depend on other factors such as coating process conditions and impurity contamination. This is often because of the stability of its own, as the last layer of the coating film in the process, does protection covering.
SiO2 film is an important nanofilm material, with a wide transparent area ( 0.15 ~ 8 um) , low refractive index, high hardness, and low thermal expansion. The advantages of coefficient, friction resistance, acid and alkali resistance, corrosion resistance, etc., are widely used in the fields of optical film components, semiconductor integrated circuits, electronic devices, sensors, laser devices, chemical catalysis, biomedicine, surface modification, and pharmaceutical packaging.
Zinc selenide and zinc sulfide
ZnS is a group II ~ VI compound semiconductor with high transparency in the visible light range. It is an excellent photoelectric functional crystal material with good optical uniformity and multi-band transmission performance and good chemical stability.
The properties of zinc selenide are basically the same as those of zinc sulfide. The difference is that the mechanical strength is lower, the melting point is not as high as that of zinc sulfide, and the absorption is very small at lower temperatures. According to different preparation processes, ZnS is usually divided into hot-pressed ZnS, CVD ZnS, and multispectral ZnS.
Hot-pressed ZnS is an infrared material developed and perfected in the 1950s and 1960s. It is prepared by using the hot-pressing process in powder metallurgy. However, it is difficult to demold, which can easily cause the product to break, and it cannot reach the theoretical density. , Contains a large number of micropores, forming a scattering center.
The optical properties of CVD zinc sulfide are significantly better than hot-pressed zinc sulfide. However, CVD zinc sulfide is opaque in the visible light range, and its appearance is orange. This is because the hydrogen atoms contained in ZnS combine with S vacancies and Zn to form a complex that becomes the scattering center, which results in orange-yellow color and decreased transmittance.
CVD zinc sulfide is called multispectral zinc sulfide after a hot isostatic pressing (HIP ) treatment; its transmission shortwave starts at 0.35 um. Compared with CVD zinc sulfide, the transmittance is improved in the entire transmission band.