U.S. patent application number 14/556559 was filed with the patent office on 2015-03-26 for spectrally selective coatings and associated methods for minimizing the effects of lightning strikes.
The applicant listed for this patent is The Boeing Company. Invention is credited to Michael M. Ladd.
Application Number | 20150086788 14/556559 |
Document ID | / |
Family ID | 42315601 |
Filed Date | 2015-03-26 |
United States Patent
Application |
20150086788 |
Kind Code |
A1 |
Ladd; Michael M. |
March 26, 2015 |
SPECTRALLY SELECTIVE COATINGS AND ASSOCIATED METHODS FOR MINIMIZING
THE EFFECTS OF LIGHTNING STRIKES
Abstract
A coating composition for reducing structural damage to a
substrate resulting from interaction between the substrate and
lightening plasma is disclosed. The coating composition includes a
binder having a plurality of pigment particles that have a size
distribution of a first portion of pigment particles that are
relatively large compared to ultraviolet (UV) wavelengths from
lightning plasma thereby backscattering UV energy, and a second
portion of pigment particles that are relatively small compared to
infrared (IR) wavelengths thereby forward scattering IR energy.
Inventors: |
Ladd; Michael M.; (Renton,
WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Boeing Company |
Chicago |
IL |
US |
|
|
Family ID: |
42315601 |
Appl. No.: |
14/556559 |
Filed: |
December 1, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12474965 |
May 29, 2009 |
8911836 |
|
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14556559 |
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Current U.S.
Class: |
428/408 ;
252/587; 428/425.9 |
Current CPC
Class: |
B64D 45/02 20130101;
Y10T 428/31609 20150401; Y10T 428/30 20150115; C09D 5/004
20130101 |
Class at
Publication: |
428/408 ;
252/587; 428/425.9 |
International
Class: |
C09D 5/33 20060101
C09D005/33 |
Claims
1. A coating composition for reducing structural damage to a
substrate resulting from interaction between the substrate and
lightening plasma comprising: a binder having a plurality of
pigment particles; wherein the plurality of pigment particles has a
size distribution comprising a first portion of pigment particles
that are relatively large compared to ultraviolet (UV) wavelengths
from lightning plasma thereby backscattering UV energy, and a
second portion of pigment particles that are relatively small
compared to infrared (IR) wavelengths thereby forward scattering IR
energy.
2. The coating composition of claim 1 wherein the first portion of
pigment particles backscatter wavelengths from about 10 nm to 200
nm.
3. The coating composition of claim 1 wherein the first portion of
pigment particles backscatter wavelengths below 500 nm.
4. The coating composition of claim 1 wherein the first portion of
pigment particles and the second portion of pigment particles
include substantially spherical pigment particles or substantially
flat-shaped pigment particles.
5. The coating composition of claim 1 wherein the pigment particles
of the plurality include a quarter wave stack including silicon
dioxide defining a first sub-layer thickness and titanium dioxide
defining a second sub-layer thickness.
6. The coating composition of claim 1 wherein the pigment particles
of the plurality include a quarter wave stack including a first
material defining a first sub-layer thickness and a second material
defining a second sub-layer thickness, wherein the first material
has a different index of refraction as a function of the wavelength
than the second material.
7. The coating composition of claim 6 wherein the quarter wave
stack is received over a layer of aluminum.
8. The coating composition of claim 7 wherein the layer of aluminum
is at most about 1 .mu.m thick and the total thickness of the
spectrally selective coating, including the layer of aluminum, is
less than about 1.4 .mu.m.
9. The coating composition of claim 1 wherein the binder comprises
one or more of a material that is transparent or semitransparent to
visible light, and a material sufficiently transparent in the
vacuum ultraviolet range.
10. A composite for reducing structural damage thereto resulting
from lightening plasma comprising: a substrate coated with a
coating composition comprising: a binder having a plurality of
pigment particles; wherein the plurality of pigment particles has a
size distribution comprising a first portion of pigment particles
that are relatively large compared to ultraviolet (UV) wavelengths
from lightning plasma thereby backscattering UV energy, and a
second portion of pigment particles that are relatively small
compared to infrared (IR) wavelengths thereby forward scattering IR
energy.
11. The composite of claim 10 wherein the substrate is not
inherently reflective of UV energy.
12. The composite of claim 10 wherein the substrate include carbon
fibers.
13. The composite of claim 10 wherein the substrate includes a
carbon nanotube reinforced polymer material.
14. The composite of claim 10 wherein the plurality of pigment
particles includes niobium particles, silicon dioxide particles,
titanium dioxide particles, or combinations thereof.
15. The composite of claim 10 wherein the binder includes
polyurethane.
16. The composite of claim 10 wherein the first portion of pigment
particles and the second portion of pigment particles include
substantially spherical pigment particles or substantially
flat-shaped pigment particles.
17. The composite of claim 10 wherein the first portion of pigment
particles and the second portion of pigment particles include
substantially spherical pigment particles.
18. The composite of claim 10 wherein the first portion of pigment
particles and the second portion of pigment particles include
substantially flat-shaped pigment particles.
19. The composite of claim 10 wherein the pigment particles of the
plurality include a quarter wave stack including silicon dioxide
defining a first sub-layer thickness and titanium dioxide defining
a second sub-layer thickness.
20. The composite of claim 10 wherein the pigment particles of the
plurality include a quarter wave stack including a first material
defining a first sub-layer thickness and a second material defining
a second sub-layer thickness, wherein the first material has a
different index of refraction as a function of the wavelength than
the second material.
21. The coating composition of claim 20 wherein the quarter wave
stack is received over a layer of aluminum.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/474,965 filed on May 29, 2009, the entirety
of which is hereby incorporated by reference.
FIELD
[0002] The present patent application relates to coatings and
methods for resisting plasma damage and degradation and, more
particularly, to coatings and associated methods for imparting
substrates with the ability to withstand lightning strikes and
exposure to similar plasmas.
BACKGROUND
[0003] Carbon fiber reinforced plastic ("CFRP") materials are
increasingly being used in place of aluminum to form the skin
panels and structural members of commercial airplanes. CFRP
materials are advantageous compared to aluminum due to the higher
strength-to-weight ratios provided by carbon composites. However,
CFRP materials appear to be more susceptible to catastrophic damage
from lightning strikes than aluminum materials.
[0004] Typical lightning strike related failures in CFRP materials
appear to be driven by high temperatures. The carbon fibers in the
composite materials become very hot when subjected to a lightning
strike. The temperature may exceed the pyrolization temperature of
the resin, turning the resin from a solid to a gas. The
corresponding increase in internal gas pressure contributes to the
damage to the structure by delaminating the layers and, possibly,
puncturing the underlying systems or structure. The high
temperatures can also cause permanent damage to the fibers,
resulting in expansion of the fiber diameter. Localized heating in
combination with material coefficient of thermal expansion also
influences damage. Impact forces associated with lightning strike
are acoustic, electromagnetic and pneumatic in origin. These forces
can exceed material tensile strength limits at corresponding
elevated temperatures.
[0005] Current solutions use electrical paths to effectively
conduct and distribute the electrical current away from the
lightning attachment zone in an effort to avoid sparking. These
electrical paths are integrated into the CFRP material design, such
as beneath an outer paint layer (e.g., a polyurethane layer).
However, despite the use of well-designed electrical paths to
conduct the electrical current, substantial structural damage is
still being observed.
[0006] Accordingly, those skilled in the art continue to seek new
techniques for avoiding structural damage resulting from lightning
strikes.
SUMMARY
[0007] In one aspect, the disclosed coating composition is for
reducing structural damage to a substrate resulting from
interaction between the substrate and lightening plasma. The
coating composition has a binder having a plurality of pigment
particles that have a size distribution of a first portion of
pigment particles that are relatively large compared to ultraviolet
(UV) wavelengths from lightning plasma thereby backscattering UV
energy, and a second portion of pigment particles that are
relatively small compared to infrared (IR) wavelengths thereby
forward scattering IR energy. In one embodiment, the first portion
of pigment particles backscatter wavelengths from about 10 nm to
200 nm. In another embodiment, the first portion of pigment
particles backscatter wavelengths below 500 nm.
[0008] In one aspect, the first portion of pigment particles and
the second portion of pigment particles include substantially
spherical pigment particles or substantially flat-shaped pigment
particles.
[0009] In another aspect, the pigment particles of the plurality
include a quarter wave stack including silicon dioxide defining a
first sub-layer thickness and titanium dioxide defining a second
sub-layer thickness. In one embodiment, the quarter wave stack
includes a first material defining a first sub-layer thickness and
a second material defining a second sub-layer thickness, where the
first material has a different index of refraction as a function of
the wavelength than the second material.
[0010] In another aspect, composites for reducing structural damage
thereto resulting from lightening plasma are disclosed. The
composite may have a substrate coated with a coating composition as
described herein. The substrate is not inherently reflective of UV
energy.
[0011] In one embodiment, the substrate includes carbon fibers. In
another embodiment, the substrate includes a carbon nanotube
reinforced polymer material.
[0012] The plurality of pigment particles may include niobium
particles, silicon dioxide particles, titanium dioxide particles,
or combinations thereof, and the binder may include polyurethane.
In one embodiment, the first portion of pigment particles and the
second portion of pigment particles include substantially spherical
pigment particles or substantially flat-shaped pigment
particles.
[0013] In another embodiment, the pigment particles of the
plurality include a quarter wave stack including silicon dioxide
defining a first sub-layer thickness and titanium dioxide defining
a second sub-layer thickness.
[0014] Other aspects of the disclosed spectrally selective coatings
and associated methods for minimizing the effects of lightning
strikes will become apparent from the following description, the
accompanying drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a graphical illustration of the spectral radiance
of a black body modeling radiant energy versus wavelength and
temperature;
[0016] FIG. 2A is a graphical illustration of the outer and inner
surface temperatures versus time for a substrate provided with a
radiative heating absorption coefficient of 0.2;
[0017] FIG. 2B is a graphical illustration of the outer and inner
surface temperatures versus time for a substrate provided with a
radiative heating absorption coefficient of 0.1;
[0018] FIG. 3A is a graphical illustration of effective ultraviolet
absorptivity versus wavelength for various lightning plasma
temperatures;
[0019] FIG. 3B is a graphical illustration of effective infrared
emissivity versus wavelength for various skin temperatures;
[0020] FIG. 4 is a graphical illustration of plasma wavelengths for
various metals;
[0021] FIG. 5A is a schematic cross-sectional view of a substrate
coated with a spectrally selective coating comprising spherical
pigments;
[0022] FIG. 5B is a schematic cross-sectional view of a substrate
coated with a spectrally selective coating comprising plate-like
pigments;
[0023] FIG. 6 is a graphical illustration of the spectral and net
reflectivity for a thick layer of aluminum adjacent to a
20,000.degree. K plasma;
[0024] FIG. 7 is a graphical illustration of net reflectivity
versus thickness for an aluminum layer positioned between a
20,000.degree. K plasma and a CFRP substrate; and
[0025] FIG. 8 is a block diagram of a CFRP substrate provided with
a spectrally selective coating in accordance with an aspect of the
present disclosure.
DETAILED DESCRIPTION
[0026] Lightning has both direct and indirect effects on
substrates. Specifically, it has now been discovered that radiative
heating has a significant direct effect, in addition to other
direct effects, such as electrical resistance heating,
electromagnetic and acoustic forces and electrical sparking.
Indirect effects include an intense electromagnetic pulse (EMP),
bound charges and induced voltage exposure to wire bundles and
avionics.
[0027] Typical lightning direct effects protection provides the
substrate (e.g., the exterior structure of an aircraft) with the
ability to withstand the initial and subsequent attachments of the
lightning plasma at currents of up to 200,000 amperes without
adverse consequence. Typical lightning direct effects protection
may also assure that no sparking occurs at structural joints or on
fuel and hydraulic couplings as the electrical current travels
between the lightning attachment and exit points.
[0028] Pursuant to the present disclosure, lightning direct effects
protection may also include protection from radiative heating
between the lightning plasma (or any other destructive high
temperature plasma) and the substrate. Specifically, in one aspect
of the present disclosure, a substrate may be provided with a
spectrally selective coating that is designed and fabricated to
provide a high level of reflectivity of the incident ultraviolet
("UV") energy radiated from the lightning plasma source. In another
aspect, the spectrally selective coating may also be emissive in
the infrared ("IR") bands, thereby helping to dissipate the energy
absorbed from the lightning plasma by re-radiating it back to the
environment to minimize heat soaking into the substrate, which, as
discussed above, may adversely affect the substrate.
[0029] As used herein, "substrate" broadly refers to any substrate
capable of being damaged by a lightning strike or other damaging
plasma. The substrate may be any substrate that is not inherently
reflective of UV energy or a substrate that is in need of
additional UV reflectivity. CFRP materials are only one example of
appropriate substrates.
[0030] Lightning is believed to have a plasma temperature of about
28,000.degree. K, perhaps ranging from about 25,000 to about
30,000.degree. K. Therefore, referring to FIG. 1, which is a
graphical illustration of Plank's law regarding the spectral
radiance of electromagnetic radiation emanating from a black body,
the peak wavelength of lightning occurs at about 100 nm. At higher
plasma temperatures, the peak wavelength shifts to the left (i.e.,
higher energy UV). At lower temperatures, the peak wavelength
shifts toward the right (i.e., lower energy UV).
[0031] Thus, in one aspect, the disclosed spectrally selective
coating may be configured to reflect or backscatter the shorter
wavelength (e.g., about 100 nm) incident UV energy from the
lightning plasma. In another aspect, the disclosed spectrally
selective coating may be configured to reflect or backscatter
incident UV energy in the vacuum ultraviolet ("VUV") regime (i.e.,
200 nm to 10 nm). In yet another aspect, the disclosed spectrally
selective coating may be configured to reflect or backscatter
incident UV energy in the other UV regimes that affect lightning
strike performance, including UVC (below 280 nm), UVB (320 nm-280
nm) and UVA (400 nm-320 nm). In yet another aspect, the disclosed
spectrally selective coating may provide high reflectivity below
500 nm.
[0032] FIGS. 2A and 2B illustrate how the outer and inner surface
temperatures of a substrate are affected by the absorption
coefficient to the UV energy. A first substrate is provided with a
20 mil coating of niobium (a high melting-point reflective metal)
to yield a plasma radiative heating absorption coefficient of 0.2
(20% of the incident energy is absorbed into the panel surface).
Niobium was selected as the outer layer in order to avoid energy
terms associated with material phase change and oxidation so that
only effects of absorptivity are illustrated in the parametric
trends. Upon exposure to a 10.28 kA simulated lightning strike
having a plasma temperature of 30,500.degree. K, the outer surface
of the first substrate reached a peak temperature of about
3,354.degree. F. and the inner surface reached a peak temperature
of about 500.degree. F. (see FIG. 2A). In contrast, a second
substrate having a plasma radiative heating absorption coefficient
of 0.1 reached a peak outer surface temperature of about
1,869.degree. F., with a corresponding lower peak inner surface
temperature (see FIG. 2B). Therefore, the reduction in the
absorption coefficient reduced the back side heat soak temperature
and, as such, reduced radiative heating related damage.
[0033] Similar analyses have shown the benefits of a higher IR
emissivity after the lightning strike where the higher emissivity
effectively re-radiates heat from the panel post-strike surface due
to elevated surface temperatures compared to the surrounding
environment, but at lower temperatures than the lightning
plasma.
[0034] The foregoing establishes the benefits of controlling the
substrate spectral absorptivity. In this case, if the UV
absorptivity can be reduced to 0.1, the peak outer surface
temperature is limited to 1,869.degree. F. If the IR emissivity is
maximized after the lightning strike the backside heat soak
temperature is limited. Low UV absorptivity and high IR emissivity
is possible by selectively tailoring the coating emissivity.
[0035] FIG. 3A illustrates the effective coating absorptivity of a
surface exposed to lightning temperatures of 10,000.degree. K,
15,000.degree. K and 20,000.degree. K. This analysis was performed
by integrating the product of the temperature dependent spectral
blackbody emission by the spectral absorptivity and dividing by the
total blackbody emissions. The parametrics include two absorptivity
values, one below the transition wavelength and one above the
transition wavelength shown on the x axis. Below the transition
wavelength the absorptivity is 0.3 and above the transition
wavelength the absorptivity is 0.8 (a step function). For the range
of higher plasma temperatures shown, the effective UV absorptivity
remains near the low value of 0.3 below and above the transition
wavelength of 0.5 microns. A high IR (wavelength greater than about
1.0 micron) absorptivity does not adversely impact the desired low
UV absorptivity. This is because only a small fraction of the
source energy at these temperatures exists in the IR.
[0036] FIG. 3B shows the effective IR emissivity for skin
temperatures of 1922.degree. K, 1366.degree. K and 811.degree. K at
parametric transition wavelengths along the x-axis. Below the
transition wavelength the absorptivity is 0.3 and above the
transition wavelength the absorptivity is 0.8. The environmental
temperature is 70 F. Here, the effective IR emissivity is little
changed for the higher skin temperatures by the transition
wavelength.
[0037] Thus, the coating spectral selectivity may be facilitated by
the temperature regimes of the lightning plasma temperatures and
subsequent surface temperatures which separate the peak energy
spectral regimes from about 100 nm UV peak (based upon a source
temperature of 2,897.degree. K) to about 1 .mu.m and above, where
IR surface emissions are significant.
[0038] In one aspect, the disclosed spectrally selective coating
may be designed and fabricated to be reflective in the UV
wavelengths and emissive in the IR wavelengths by depositing
particles upon the surface of the substrate which are relatively
large compared to the UV wavelengths and relatively small compared
to the IR wavelengths. UV reflectivity measurements, including VUV
reflectivity measurements, are possible with special instruments
available within the industry. In one example, these particles may
be metallic (e.g., niobium particles). As another example, these
particles may include SiO.sub.2 or TiO.sub.2, as well as
combinations thereof.
[0039] The particles may be contained in a binder which may be
transparent or semi-transparent to visual light. For example, the
binder may be polyurethane. The interactions of the pigment,
binder, environment and human visual response result in perceived
brightness and color. These characteristics can also be broken down
into directional and spectral quantification.
[0040] A phenomenon with metals exposed to the VUV spectral region
where the UV energy propagates through the metals is referred to as
the ultraviolet transparency of metals. FIG. 4 identifies candidate
metals (and non-metals), showing source blackbody temperatures and
wavelengths where propagation may occur within the materials. For
example, at 30,000.degree. K, the blackbody peak wavelength
corresponds to a number less than 1.0 E-7 m (100 nm). When exposed
to the 30,000.degree. K plasma, metals with plasma wavelengths
above this blackbody peak energy wavelength will propagate the
radiative energy. As the energy propagates through the metal, it is
also scattered and absorbed, thereby resulting in a net high
radiative absorptivity. Therefore, these metals may not be
considered appropriate candidates for UV reflectivity enhancement.
Instead, metals with metallic plasma wavelengths below the peak
energy blackbody wavelengths may be used to provide a coating
material that is reflective in such UV bands. Additionally, the
metal optical properties should remain functional at resulting
lightning strike temperatures.
[0041] Two types of materials which remain transmissive at
sub-micron wavelengths are silicone based polymers and
fluoropolymers, although currently these are not transmissive in
the VUV. Silicone based polymers have transparencies of 85 percent
at 250 nm for a 60 micron thick film. In lieu of available VUV
transmissive binder materials, the pigment may be mixed into a
standard binder such as polyurethane. With properly tuned
pigment/binder volumetric fractions the binder is expected to erode
only to the depth of underlying pigment particles (including binder
residue) with little to no degradation of the surface
reflectivity.
[0042] At this point, those skilled in the art will appreciate that
(1) the functional pigment particles may be bound to the upper
surface of a substrate and contained on the upper surface of the
substrate within a binder material, (2) the pigment particles may
be contained within a binder material that is sufficiently
transparent in the VUV, (3) the pigment particles may be contained
within a binder material that is not (or substantially not)
transparent in the VUV, and (4) combinations thereof.
[0043] In another aspect, the disclosed spectrally selective
coating may also limit reflectivity in the IR, thereby functioning
in a manner similar to a "cold mirror." A cold mirror is a
specialized dielectric mirror, a dichroic filter which reflects the
visible light spectrum while efficiently transmitting IR
wavelengths.
[0044] Typically, cold mirrors that are made for optical purposes
are relatively small and expensive compared to coverage required
for a composite panel coating and compared to the cost that would
be allowable for a composite panel coating. The cold mirrors are
produced by stacking layers of materials, which through optical
properties and layer thicknesses result in constructive and
destructive interference of the wavefront. Dichroic filters are not
typically available for the wavelength associated with peak
lightning strike radiant energy (i.e., about 100 nm).
[0045] One way of accomplishing the desired dichroic functionality
of the coating is through the use of a distribution of surface
particles that are relatively large compared to the UV wavelength
and relatively small compared to the IR wavelengths. As shown in
FIG. 5A, spherical coating pigment particles 10 may be suspended in
a binder 12 and layered over a substrate 14 (e.g., a CFRP
substrate) to form a spectrally selective coating. Similarly, as
shown in FIG. 5B, plate-like coating pigment particles 16 may be
suspended in a binder 18 and layered over a substrate 20.
[0046] According to MIE theory, depending upon particle optical
properties, small wavelengths will backscatter and larger
wavelengths will forward scatter. The particle materials may also
be fabricated from multi-layer stacks where the materials are
produced through various deposition processes, ground and sieved to
produce the appropriate size distributions.
[0047] Aluminum is a common reflective material choice for
space-based optical sensors designed to study solar VUV. FIG. 6
illustrates the total spectral reflectivity of non-oxidized
aluminum adjacent to a plasma. The total spectral reflectivity
includes both Transverse Electric (TE) and Transverse Magnetic (TM)
polarizations. If a lambertian extended source is placed in front
of the aluminum, photons will reflect from the aluminum as a
function of wavelength and incident angle. The surface is
interrogated using a Monte Carlo method to simulate the net
reflectivity of a thick layer of aluminum in a 20,000.degree. K
radiative environment. The net reflectivity is 0.87 based upon the
spectral/spatial reflectivity and probability function of the
incident angles of the incident photons.
[0048] FIG. 7 shows the net reflectivity of aluminum versus
thickness when sandwiched between 20,000.degree. K lightning plasma
and a CFRP substrate. One problem with aluminum is that it must be
about 10 .mu.m (i.e., about 0.5 mil) to reach a net reflectivity on
the order of 80 percent. Therefore, as a coating pigment, aluminium
may require a substantial coating thickness and weight. As such,
aluminum may be more acceptable as an applique film with the
appropriate passivation layer (aluminum quickly oxidizes), although
a metal such as niobium would better suit the thermal
environment.
[0049] FIG. 8 illustrates a pigment particle exposed to
20,000.degree. K lightning plasma, wherein the pigment particle
includes a 1 .mu.m thick layer of aluminum treated with an 8-layer
quarter wave stack of SiO.sub.2 and TiO.sub.2. The coating
sub-layer thickness T.sub.SiO2 of SiO.sub.2 may be determined using
Eq. 1 as follows:
T Si O 2 = .lamda. peak 4 n Si O 2 ( .lamda. peak ) ( Eq . 1 )
##EQU00001##
and the coating sub-layer thickness T.sub.TiO2 of TiO.sub.2 may be
determined using Eq. 2 as follows:
T Ti O 2 = .lamda. peak 4 n Ti O 2 ( .lamda. peak ) ( Eq . 2 )
##EQU00002##
wherein .lamda..sub.peak is the peak wavelength,
n.sub.SiO2(.lamda..sub.peak) is the index of refraction of
SiO.sub.2 as a function of the wavelength and
n.sub.TiO2(.lamda..sub.peak) is the index of refraction of
TiO.sub.2 as a function of the wavelength.
[0050] Those skilled in the art will appreciate that the aluminum
layer may alternatively be positioned between layers of TiO.sub.2
and SiO.sub.2. Also, those skilled in the art will appreciate that
metals other than aluminum may be used.
[0051] For example, when the peak wavelength is 100 nanometers, the
composition and thicknesses of the layers of the pigment particle
may be as shown in Table 1.
TABLE-US-00001 TABLE 1 Layer Composition Thickness (nm) 1A Titanium
dioxide 26 1B Silicon dioxide 15 2A Titanium dioxide 26 2B Silicon
dioxide 15 3A Titanium dioxide 26 3B Silicon dioxide 15 4A Titanium
dioxide 26 4B Silicon dioxide 15 5A Titanium dioxide 26 5B Silicon
dioxide 15 6A Titanium dioxide 26 6B Silicon dioxide 15 7A Titanium
dioxide 26 7B Silicon dioxide 15 8A Titanium dioxide 26 8B Silicon
dioxide 15 9 Aluminum 100
[0052] The total pigment thickness may be less than about 1.4 .mu.m
and may provide a net reflectivity of about 96 percent. The
thickness of the layers in the stack may shorten by the real index
of refraction component compared to in a vacuum.
[0053] Accordingly, the disclosed spectrally selective coatings
provide a means of eliminating or reducing material and structural
damage from extreme radiative heating due to lightning strike.
Subsequent to the lightning strike, heat soak may be minimized by
maximizing the thermal emissions in the IR band. Therefore, those
skilled in the art will appreciate that the disclosed spectrally
selective coatings may be used in conjunction with available
electrical conductivity designs, including conductive adhesive
binder materials, to minimize or prevent panel damage due to
lightning strike.
[0054] Furthermore, those skilled in the art will appreciate that
parametrics have shown that a spectral transition wavelength of
about 0.5 .mu.m with a high reflectivity, such as about 0.1 below
about 0.5 .mu.m, and a high emissivity, such as about 0.8 above
about 0.5 .mu.m, significantly reduces material temperatures during
and after a lightning strike.
[0055] Still furthermore, those skilled in the art will appreciate
that the selective reflectivity coating may be attained through the
development of a coating comprising a distribution of particles
sized to backscatter the UV energy, but to allow forward IR
scattering into the coating. The forward IR scattering results in
absorption. The coating will likewise provide high emissivity in
the IR bands which will effectively radiate post lightning strike
radiative energy to the surrounding environment. The high UV
reflectivity may be attained through the use of quarter wave stack
lay-ups over aluminum.
[0056] Although various aspects of the disclosed spectrally
selective coatings and associated methods for minimizing the
effects of lightning strikes have been shown and described,
modifications may occur to those skilled in the art upon reading
the specification. The present application includes such
modifications and is limited only by the scope of the claims.
* * * * *