A review of materials for spectral design coatings in signature management applications
Summary (3 min read)
Introduction
- Spectral Design, Signature Management, Camouflage, Military Utility, Military-Technology, Coating 1, also known as Keywords.
- In Swedish defense policy1 focus has again widened from the focus in recent years on force protection in asymmetric expeditionary scenarios, to include national security.
- Both factors add to the utility of developing new more effective solutions for signature management.
- Thu ance to detec ptimized for esign leg of a nt system leve d structural p other design m r techniques an ctical utilizatio s spectral des tion.
- In order rements proc the freedom must also ba ction.
1.1 Terminology and Physical quantities
- Electromagnetic radiation of some wavelength, λ, incident on an optically thin coating (thin film) is reflected, absorbed or transmitted, hence I(λ)=R(λ)+.
- Light reflected from a rough surface is diffuse, i.e. scattered in all directions.
- If the x- and y- components of the electric field vector of a light wave propagating in z-direction are completely correlated the light is said to be totally polarized.
- If there is no correlation the light is said to be unpolarized, like from the sun or ordinary lamps.
- In general, man-made objects with smooth surfaces have more defined polarization signatures than natural objects and tend to take on a polarized component in reflected or emitted radiation.
1.2 Desired
- From the def justification f limit their inte to design coa emitted radia scattering of greatly affect optical prope MWIR-region T iagram togethe agnetic radiat t source.
- The right u ldier respective entified for det ectra of a coat nated by refle relevant to st in the detecte he reflectanc a potentially t of the backg accordance w -NIR region.
- The emissivity of naturally occurring backgrounds, such as vegetation, rock or sand is almost always higher than man-made objects, especially those made of metal.
- In the radar wavelength region a target is detected by illuminating the object and detecting the reflected energy.
2. METHOD AND LITERATURE
- The survey was conducted as a literature search followed by a qualitative assessment of utility, using the six characteristics of a coating for multispectral optical SMT identified above as indicators.
- The databases Web of Science, Scopus, CSA and SPIE were chosen and probed for articles.‡.
- Articles were excluded if their focus was clearly on other phenomena, properties or applications than those of interest to this study, or if the article focused on measurement techniques.
- In a first iteration the exclusion criteria were applied to titles and in a second to abstracts.
3.1 Paints an
- As an introdu 3, and the pig designing its size and shap design of VIS ratio between the paint prop the region of paints), prote In 1993 Wak challenges of and inorganic emissivity.
- Hence, it looks black in VIS and can be used to lower reflectivity.
- The conductivity of the Al-coated cenospheres was, however, found to be lower than that of corresponding metal pigments, which is why the authors expect the radar transmittance to be better.
- Metal flakes have a tendency to agglomerate and thus cause high reflectivity in RR.**.
- No other indicator characteristics than LWIR emissivity is reported.
3.2 One-dimensional structures
- Moving from paints with pigments to coatings with layers of deposited optical films, the authors find other techniques and applications.
- Nevertheless, pending the reasons for insufficient performance in the paint formulated by Hallberg et al.
- In a paper on recent advances in chromogenics in 200949 Granqvist et al report on EC-coatings for fenestration applications with considerable controllable transmittance intervals.
- The resulting emittance is reported to be 0.1.59 Högström et al.60,61 have also studied the coexistence of polaritonic and structure bandgaps in a multi-layer Si/SiO2 onedimensional photonic crystal produced using Chemical Vapor Deposition.
- A third direction of development is flexible and controllable electro-chromic devices utilizing conducting polymers, potentially applied as decals on military platforms.
3.3 Multidimensional structures
- In 1987 Yablonovitch and John published papers on how to control electromagnetic wave propagation in man-made periodic structures of dielectric media, later called Photonic Crystals (PhC).
- It was shown that, under certain structural symmetry conditions and a large enough ratio between refractive indices of the constituent dielectric materials, an energy band gap appears.
- If the lattice potential is strong enough, the gap can extend to cover all possible propagation directions, resulting in a complete band gap.
- 68 There is a continued interest in metallic PhCs and some reported applications in spectral design.
- Today, when doing literature search for PhCs, there is a vast amount of matching and very recent references, most with applications such as PhC-fibers, optical waveguides, components in efficient lasers and other.
3.4 Biomimic and Metamaterials
- Environmental friendly non-fading brilliant colors seen on the feathers of some birds, and on the wings of butterflies, are due to structures and are thus called Structural Colors.
- It has been suggested that the structures of cicada wings have a camouflage-like anti-reflection function93 and inorganic replicas of these structures, for use in solar cell applications, are now emerging.
- These coatings can be useful in improving conversion efficiency and reducing glare in optical devices.
- Applications range from perfect or super-focusing lenses, antennas, all-optical memories and gradient-index materials to “invisibility” cloaking.
- Great challenges remain to overcome problems regarding bandwidth and losses at short frequencies.
4. SUMMARY AND CONCLUSIONS
- An overview of materials, surface structures and coating principles of potential interest for signature management applications using spectral design is presented.
- Pigments and binders used for a narrower spectrum are absorbent in thermal infrared and hence cannot meet an optically multispectral threat.
- Here multilayered structures are regarded as the second order of complexity.
- There are, however, promising ideas about how to move forward, which is why spectral design is assessed to have continued great potential.
- The utility of spectral design is also assessed to benefit from research into other applications and from commercial interest in nano-technology, boosted, for example, by the electronics, construction and energy industries.
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Citations
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"A review of materials for spectral ..." refers methods in this paper
...5μm diameters, followed by melt infiltration with chalcogenide glass Ge33As12Se55 and removal of the SiO2 spheres by chemical etching.(74) The title of the paper indicates fabrication of pigments, but this part of the process is not described....
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"A review of materials for spectral ..." refers methods in this paper
...The resulting emittance is reported to be 0.1.59 Högström et al.60,61 have also studied the coexistence of polaritonic and structure bandgaps in a multi-layer Si/SiO2 onedimensional photonic crystal produced using Chemical Vapor Deposition....
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...Högström et al.(60,61) have also studied the coexistence of polaritonic and structure bandgaps in a multi-layer Si/SiO2 onedimensional photonic crystal produced using Chemical Vapor Deposition....
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22 citations
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Frequently Asked Questions (17)
Q2. What are the future works mentioned in the paper "A review of materials for spectral design coatings in signature management applications" ?
Therefore, in order to spend limited military R & D funding more effectively, and to meet more quickly the survivability requirements of signature management systems from an increased threat, there is a need to study the balancing process depicted in the introduction.
Q3. What are the optical consequences of a photonic crystal?
The optical consequences are suppression of emission in the crystal and total reflectance of light in a wavelength region determined by the lattice constant of the structure and with a broadening corresponding to the band gap.
Q4. What is the third direction of development?
A third direction of development is flexible and controllable electro-chromic devices utilizing conducting polymers, potentially applied as decals on military platforms.
Q5. What are some examples of interesting optical properties?
In biology there are examples of interesting optical properties, such as ageresistant and environmentally friendly colors, originating from complex structures made out of relatively simple materials.
Q6. What is the way to achieve controllability?
Controllability for near real-time applications may be obtained using electro-chromic devices based on conducting polymers, or further in the future using tunable 3D photonic crystal.
Q7. What is the main reason for the blending of artificial and natural fibers in uniforms?
The blending of artificial and natural fibers in fabric for modern uniforms normally gives NIR-reflectance that is too high when compared to natural background,33 which is why new pigments are being developed for lowering NIR-reflectivity, such as NIR-absorbing Vat dyes.34
Q8. What is the emissivity of a layer of BeO on a BN?
Ribbing showed in 1993 that a 2.5μm layer of BeO on BN could result in a coating with low emissivity, lower than 0.2 on average, covering the width of the LWIR atmospheric window58.
Q9. What is the nature of the coloration of mimicked structures?
The coloration of mimicked structures will, as in the natural case, arise through common physical mechanisms such as thin film or multilayer interference, diffraction gratings, scattering (coherent and incoherent) and photonic crystals etc., depending on the composition of the building blocks.
Q10. What is the average transmittance of a TC coating?
The mid-luminous transmittance, ΔT, is reported to be 55% with maximum transmittance around 70%, and the time from colored to bleached state is about 30s.
Q11. What is the way to reduce VIS-NIR?
Incorporating pigments in the polymer fiber at the fiber forming process, however, seems to be feasible since this can be combined with textile printing with reactive dyes for VIS-NIR performance.
Q12. What is the way to tailor the reflectivity of multilayers?
Breaking metal based multilayers into pigments is one approach to tailoring the reflectivity, but no paint coating system reported yet meets the potential of the respective components.
Q13. What is the challenge of combining pigments and binders in a system?
The challenge is to combine pigments and binders in a system where the properties of the components create a synergetic optical response throughout the spectrum from VIS to TIR.
Q14. What is the utility of spectral design?
The utility of spectral design is also assessed to benefit from research into other applications and from commercial interest in nano-technology, boosted, for example, by the electronics, construction and energy industries.
Q15. What is the likely explanation for the low emissivity of pure metal pigments?
Hallberg et al. conclude that among pure metal pigments, only Al, in combination with polymer-based binders, gives acceptably low emissivity.
Q16. What is the effect of the optical modeling of the polymerized colloidal crystalline array?
75,76 Kadiyala et al. demonstrate, through optical modeling of polymerized colloidal crystalline array (PCCA) structures, a novel concept that has the potential to enable the use of tunable 3D photonic crystals for adaptive camouflage.
Q17. What is the way to improve the bandwidth of a mantle cloak?
A low profile of an ultrathin mantle cloak improves bandwidth limitations; however, little or nothing is said about the robustness to losses.