14 Jun 2019

Investigating adaptive camouflage | High Temperature High Pressure Cell

Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending objects naturally into the surrounding environment, has long been a great challenge. Now, the gap is narrowing, and new materials are being developed that could make cloaking devices for both visible and infra-red light a reality.

adaptive camouflage chameleon

Chameleons have long been known to have the ability to rapidly change their skin color, adapting to their environment to conceal themselves. There are now a number of examples of adaptive camouflage systems including microfluidic network soft machines, electro-mechano-chemically responsive elastomers, adaptive optoelectronic camouflage systems, chameleon-inspired stretchable electronic skin, and mechanical chameleons. 

However, these systems are not perfect and many challenges still remain. Firstly, current adaptive materials are primarily excited by electrical signals and often require additional sensors, which make them complex, with high energy consumption. Adaptive camouflage devices need to be simplified and use less energy. 

Secondly, current adaptive materials and devices largely operate in the visible region and not infra-red. IR camouflage is emerging as an important technology that could be used to protect military vehicles and personnel from detection by IR sensors. IR radiation intensity differences between military objects and background signals vary dynamically under different active factors, such as the heat they generate during working conditions, solar radiation and ground radiation.

Therefore, the IR emissivity of these objects must vary dynamically according to outside changes, so that these objects remain invisible.

IR and adaptive camouflage

Adaptive materials active in the thermal infrared (IR) region have recently been identified and studied. Vanadium dioxide (VO2) thin films have been seen to exhibit a reversible metal-insulator transition (MIT) from an IR transparent state to a reflecting state under external heating stress. 

The IR adaptive material is a thermochromic VO(M) thin film, which is formed by the transformation of metastable VO(B). The VO2(B) thin films are grown onto a 0.5mm thick quartz glass substrate in an RF magnetron sputtering chamber with a base pressure better than 1×10−3Pa using 99.99% vanadium metal as target. 

The VO2/quartz layers are then fixed to a silicone elastomer sheet with high absorption in the IR region and high thermal conductivity to improve thermochromic properties in the mid-wavelength and long-wavelength ranges.

How does thermochromic VO2work?

Thermochromic vanadium dioxide (VO2) is an excellent adaptive IR camouflage material because of its unique phase transition. VO2undergoes a first-order reversible metal-insulator transition (MIT) upon heating changing its IR signature. 
This means that if its temperature rises above 68 °C it has a metallic phase with the tetragonal rutile structure designated VO2(R). Conversely if its temperature falls below 68 °C an insulator phase with a monoclinic structure designated VO2(M) dominates. 

In addition to the metal-insulator transitions, the optical conductivity of VO2thin films changes significantly, from being transparent to IR below the MIT to a high-temperature IR reflecting state exhibiting spectral emissivity in the thermal IR spectral region. Therefore, because VO2thin films exhibit variable emissivity upon temperature changes they provide a very real approach to adaptive IR camouflage.  In effect VO2can regulate emitted thermal power as well as maintaining the minimum radiation energy difference between the object and background.

Monitoring using IR spectroscopy

In a recent study by Liu et al. (2018), transmittance and reflectance spectra were measured using a Fourier transform IR (FTIR) spectrometer of the different VO2/thermally active silicone layered materials at different temperatures. 

The FTIR spectrometer was equipped with a high temperature/high pressure cell from Specac (GS05850) along with a reflectance mode upgrade kit also from Specac (GS05860). The incident angle for reflectance measurement was 15°.

Medium wavelength IR (MWIR) and long wavelength IR (LWIR) thermal images were taken using IR cameras at various temperatures using the high temperature/high pressure cell from Specac and a customized heater to accurately control the sample temperature.

Results showed the emissivity of the VO2(M)/quartz/silicone structure changing by 0.21 and 0.49 across MIT in the MWIR and LWIR ranges, exhibiting IR chameleon-like behavior. In addition, if heated above the MIT temperature, the VO2autonomously lowered its IR radiation intensity to match the background and appear to be at the same or an even lower temperature than on the IR camera. For example, if the real temperature of the VO2was 95 °C, the apparent temperature (Ta) on the MWIR camera was 51.9 °C close to the background of 51.8 °C. These results provide new opportunities for cheating IR cameras.

High temperature, high pressure FTIR

The use of Specac’s High Temperature High Pressure Cell (HTHP) Accessory was essential in this research where temperature control of the sample was so crucial. This is a multipurpose cell designed specifically for the FTIR analysis of solid samples in transmission, decomposition, or specular reflectance modes. 

The sample temperature in the cell can be increased to 800ºC and it can operate at pressures from vacuum (0.1 torr) to 1000 psi for in-situ reaction studies. The standard HTHP Cell is multi-modal with the option of being used for transmission analysis of 13 mm diameter discs, or even for decomposition and spectral analysis of gases evolved from a solid sample held in a sample cup just below the beam height. 

In the case of the VO2analyses, spectral reflectance measurements were made easier by the use of an alternative baseplate configuration with transfer optics, and with an angled window housing assembly. The HTHP cell was invaluable in this study where the results were so reliant upon accurate IR spectra. 

The Specac HTHP cell is manufactured largely from corrosion resistant and vacuum compatible materials with ZnSe windows and silicone seals as standard. Temperature control of the accessory was provided by a dedicated power controller.

VO2adaptive camouflage materials

Compared to other chameleon-like adaptive camouflage technologies, VO2(M)/quartz/silicone structure is simple without complex sensors and circuitry. As well as this the system is thermo-driven and needs no external power. This emissivity engineering for thermal camouflage is now regarded as a more promising system compared to just temperature control that has to dissipate a large amount of excessive heat. 

It is believed that this device will find wide application not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators. The VO2(M)/quartz/silicone structure provides new opportunities for adaptive IR camouflage which can dynamically deceive IR cameras.

References

  1. Yablonovitch, Eli & Wudl, Fred & Dunn, Bruce & R. Reynolds, John & Tanner, David & Baughman, Ray & Zakhidov, Anvar. (2005). Electrochromic Adaptive Infrared Camouflage, California Univ Los Angeles, Defense Technical Information Center, 2005, 18 pages
  2. Sayan Chandra, Daniel Franklin, Jared Cozart, Alireza Safaei, Debashis Chanda, Adaptive Multispectral Infrared Camouflage, ACS Photonics 2018 Article ASAP
  3. Lin Xiao, He Ma, Junku Liu, et al., Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films, Nano Lett. 2015, 15, 12, 8365-8370
  4. Dongqing Liu, Haining Ji, et al., Infrared chameleon-like behavior from VO2(M) thin films prepared by transformation of metastable VO2(B) for adaptive camouflage in both thermal atmospheric windows, Solar Energy Materials and Solar Cells 185 (2018) 210–217

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