Patent Application: US-201414287372-A

Abstract:
robust , flexible , lightweight , low profile enhanced performance dielectric barrier discharge actuators based on aerogels / nanofoams with controlled pore size and size distribution as well as pore shape . the plasma actuators offer high body force as well as high force to weight ratios . the flexibility and mechanical robustness of the actuators allows them to be shaped to conform to the surface to which they are applied . carbon nanotube based electrodes serve to further decrease the weight and profile of the actuators while maintaining flexibility while insulating nano - inclusions in the matrix enable tailoring of the mechanical properties . such actuators are required for flow control in aeronautics and moving machinery such as wind turbines , noise abatement in landing gear and rotary wing aircraft and other applications .

Description:
the following detailed description is of the best presently contemplated mode of carrying out the invention . this description is not to be taken in a limiting sense , but is made merely for the purpose of illustrating general principles of embodiments of the invention . the embodiments of the invention and the various features and advantageous details thereof are more fully explained with reference to the non - limiting embodiments and examples that are described and / or illustrated in the accompanying drawings and set forth in the following description . it should be noted that the features illustrated in the drawings are not necessarily drawn to scale , and the features of one embodiment may be employed with the other embodiments as the skilled artisan recognizes , even if not explicitly stated herein . descriptions of well - known components and techniques may be omitted to avoid obscuring the invention . the examples used herein are intended merely to facilitate an understanding of ways in which the invention may be practiced and to further enable those skilled in the art to practice the invention . accordingly , the examples and embodiments set forth herein should not be construed as limiting the scope of the invention , which is defined by the appended claims . moreover , it is noted that like reference numerals represent similar parts throughout the several views of the drawings . the present invention describes dbd actuators 12 where the dielectric materials are nanofoams / aerogels with controlled porosity to achieve normally mutually exclusive properties of ultra - low dielectric constant and high dielectric breakdown strength . for a material to function as a dbd actuator 12 , a very high electric field must be applied to ionize the air and accelerate the charged particles . this requires that the dielectric sustain an applied field of the order of many kv / mm . high porosity is required to attain low dielectric constants . the dielectric constant tends to one (∈→ 1 ) as the total volume of empty space increases . the present invention obtains ultra - low dielectric constants through the use of high porosity (& gt ; 80 %) nanofoams / aerogels , and high dielectric breakdown strength by ensuring that the empty volume is made up of pores with diameters in the nanometer range . such small diameters prevent the acceleration of charge carriers required for breakdown [ ref 8 ]. by specifically combining matrix material selection and the incorporation of porosity at the nanoscale with controlled pore size / shape and size distribution as well as the distribution of these pores within the matrix ( spherical , narrow size range and evenly dispersed to prevent electrical stress buildup ), these unique requirements for dbd actuators 12 can be attained . fig2 and 3 show the dielectric constants and loss tangents of polyimide ( pi ) aerogel , a polyetherimide ( pei ) microfoam and some state - of - the - art dbd materials near room temperature ( 30 ° c .) and at 120 ° c . fig2 shows the frequency dependence of the dielectric constant and loss tangent while fig3 shows them at 30 ° c . and an actuator test frequency ( 5 khz ). the aerogel and microfoam have the lowest dielectric constants , approaching 1 . 0 and these remain stable both as a function of the frequency and temperature , a desirable attribute for practical actuator performance . the foams also have very low losses with that of the aerogel being second only to that of ptfe ( teflon ®), a very low loss dielectric . note that in addition to the porosity , the loss can be controlled by selection of the aerogel / nanofoam matrix . the high temperature stability conferred to the aerogels by use of the polyimide matrix ( glass transition temperature ( t g )& gt ; 200 ° c .) leads to the very small changes in the dielectric properties between room temperature and 120 ° c . and ensures that these materials can function as plasma actuator dielectrics over a wide temperature range . by selection of a fluorinated polyimide matrix , it is expected that the loss of the aerogel will be lowered even further while maintaining a wide service temperature range . fig4 shows the leakage current ( an indicator of dielectric breakdown and energy loss ) as a function of the applied electric field in a test conducted according to astm d149 - 09 “ standard test method for dielectric breakdown voltage and dielectric strength of solid electrical insulating materials at commercial power frequencies ”). the choice of the dbd dielectric material affects the dielectric breakdown strength and hence the ability to act as an actuator 12 . the figure shows the properties of a polyetherimide ( pei ) microfoam , a highly flexible aerogel containing hydrophilic chemical groups ( 4 , 4 ′- oxidianiline ( oda ) and biphenyl - 3 , 3 ′, 4 , 4 ′- tetracarboxylic dianhydride ( bpda ) with octa ( aminophenyl ) silsesquioxane ( oaps ), a polyhedral oligomer silesquioxane ( poss )), a slightly more rigid but still flexible and hydrophobic formulation ( 50 % oda / 50 % 2 , 2 ′- dimethylbenzidine ( dmbz ) and bpda with oaps crosslinks ). breakdown within the pores of the pei microfoam leads to large leakage currents below the field required to produce a plasma . the hydrophilic ( oda and bpda with oaps crosslinks ) aerogel also breaks down ( rapid increase in the leakage current ) at a much lower field than the hydrophobic formulation (( 50 % oda / 50 % 2 , 2 ′- dimethylbenzidine ( dmbz ) and bpda with oaps crosslinks ). the microfoam , though having a low dielectric constant , has a high leakage current and low dielectric breakdown strength due to discharges within the pores . with suitable selection of the chemistry , a hydrophobic aerogel is formulated with pore sizes and a matrix that prevent breakdown and sustain the generation of a plasma ( fig5 ). a fluorinated aerogel 25 % 2 , 2 - bis ( 3 , 4 - dicarboxyphenyl ) hexafluoropropane dianhydride ( 6fda )/ 75 % oda and bpda with 1 , 3 , 5 - triaminophenoxybenzene ( tab ) crosslinks was also found to be suitable for plasma actuator applications . fig6 shows the thrust vs applied voltage for a low profile aerogel actuator compared to some state - of - the - art ( thick dielectric ) actuators at a common test frequency ( 5 khz ). it can be seen that not only can a plasma be sustained by the flexible aerogel , but the material generates a thrust that compares with and in cases exceeds the state - of - the - art . enhanced force can be obtained by tuning the test frequency to the device &# 39 ; s characteristic frequency and , as the actuator saturation voltage has not been reached ( instrument limit ), by increasing the applied voltage . fig7 shows the mechanical properties of the polymeric aerogel material . these materials are robust , far exceeding very brittle silica based aerogels , with tensile strengths of 4 . 6 mpa and strain at break in close to 10 %. this is in contrast to inorganic silica based aerogels that are highly brittle and friable . for applications requiring greater mechanical and thermal performance , such as stiffer or more damage tolerant actuators or higher temperature environments , the aerogel matrix is preferably reinforced with electrically insulating , low dielectric constant and loss fillers such as boron nitride nanotubes ( bnnts ) with electrical properties similar to hexagonal boron nitride ( hbn ) (∈→ 4 . 8 and tan δ ˜ 2 × 10 − 4 at 5 khz ). for high control authority , dbd actuators 12 may require application over relatively large areas and in strategic locations so the total force and effect on the flow is increased ( fig8 ). flexible , light weight , low profile dbd actuators 12 can be applied over large areas of airfoils 21 , providing increased forces and therefore control authority at a minimal weight penalty and invasive modification to the surface to which they are applied . dbd actuators 12 can be used to control flow separation , noise abatement and other aerodynamic functions . when applied over these large areas , the weight can become significant . to minimize the contribution of the electrodes 14 , 18 to the actuator weight , carbon nanomaterials are used as the electrodes 14 , 18 . aerogel / cnt sheet actuators are preferred in robust , flexible , low profile and lightweight actuators . the density of cnts is approximately 1 . 3 g / cm 3 while those of copper and gold are 8 . 96 g / cm 3 , and 19 . 30 g / cm 3 respectively . in some applications , cnt yarns are already replacing copper cabling because of the huge weight savings gained . carbon nanotube based materials have good high frequency conductivity that for doped , highly purified and defect free tubes is greater than aluminum and copper [ ref . 10 ]. the carbon nanotubes are preferably doped with copper , iodine , bromine , silver , gold or nickel to enhance the electrical conductivity while maintaining low weight . conductivities continue to improve as material preparation processes are being refined . fig9 shows the ac conductivity of a cnt tape electrode 14 , 18 and the current density for a cnt yarn . the conductivity of state - of - the - art , routinely produced , cnt material already exceeds 3000 s / cm and the current density sustained is higher than would be required for these voltage driven devices . carbon nanotube based electrodes 14 , 18 are very flexible / conformable to surfaces allowing them to be readily applied to curved surfaces . the superior mechanical properties of carbon nanomaterials may mean longer lifetimes for applications where the actuator , and thus electrode 14 , 18 , may suffer mechanical deformations or fatigue inducing vibrations . carbon nanotube derived electrodes 14 , 18 , including modified cnts have chemical characteristics and in particular secondary electron emission coefficients that are different from copper which changes the boundary conditions for the plasma generation in the dbd actuator 12 , an important parameter in their performance [ ref . 11 ]. numerous processes of modifying the cnt electrodes 14 , 18 , including doping , electrochemical means , supercritical fluid infusion and microwave assisted metal deposition are already described in the literature . for practical applications , a dbd actuator 12 needs to be robust as well as have temperature , plasma and environmental resistance . fig4 shows the leakage current as a function of the applied voltage , for two flexible aerogel materials . the more hydrophobic material allows a higher electric field to be generated without a catastrophic increase in the leakage current and therefore forms a basis for a dbd actuator 12 . control of the chemistry of the dielectric thus allows for high performance actuators that are able to withstand the application environment . furthermore , the chemistry and surface morphology of the actuator are most preferably such that it promotes adhesion of the electrodes 14 , 18 for long actuator lives . surface texturing can be used to promote adhesion of the electrodes 14 , 18 to the surface 19 . polyimides and fluorinated polymers are able to withstand a range of environments in which dbd actuators 12 may be used . thus , polyimides and fluorinated aerogels which include fluorinated polyimide aerogels , form a suitable material for high performance dbd actuators 12 . for high temperature applications actuators can be constructed out of inert materials such as boron nitride and aerogels with bn as a matrix . bulk hbn is known to be a high dielectric strength material and it is shown in fig2 and 3 that it has a relatively low dielectric constant and loss . fig1 shows the thrust generated by a bulk hbn actuator at test voltages well below the saturation voltage . a plasma is sustained and using bn nanofoams and aerogels as well as operation at higher voltages increases the thrust further while maintaining the environmental and chemical resistance . for harsh environments , robust lightweight actuators can be constructed from , and nanofoams / aerogels based on , hexagonal boron nitride whose bulk dielectric 20 properties are shown in fig2 and 3 and thrust vs the applied voltage is shown in fig1 . many of the physical characteristics of the aerogels / nanofoams , such as thermal , acoustic and mechanical properties , are well known and it has been demonstrated that they have very low and tailorable dielectric constants (∈≈ 1 ). the present invention relates to the development of robust , flexible , lightweight materials for dbd actuators 12 with enhanced performance by controlling the chemical nature of the matrix , pore size , shape and size distribution . the chemical properties of the nanofoam / aerogel matrix described above are preferably tailored to optimize dbd and mechanical performance , as well as endurance of the application environment . for applications requiring highly robust actuators , the aerogel matrix is preferably reinforced with insulating nanoinclusions such as boron nitride nanotubes . it is known that hbn , a chemical analogue of bnnts , has a low dielectric constant and loss , enabling the formation of a plasma . yet more robust actuators , for harsh environments , including high temperatures are preferably formulated from aerogels / nanofoams with hbn as the matrix material . an alternate embodiment is the use of carbon nanotube and graphene nanosheet based electrodes 14 , 18 for ultra - light weight actuators . carbon based nanomaterials ( carbon nanotubes and graphene sheets ) are excellent electrical conductors , particularly at high frequencies . macroscopic forms of these such as tapes and sheets preferably form the electrodes 14 , 18 for a lightweight dbd actuator 12 . the nanotube material is used as one or both the electrodes 14 , 18 , depending on the application and desired electrode lifetime . additives to enhance the conductivity or act as catalyst are infused into the carbon based electrode material as desired . the cnt ( and modified cnt ) electrode 14 , 18 provides a chemically different electrode from copper changing the boundary conditions ( secondary electron emission coefficient ) for the plasma generation , an important performance parameter , in potentially advantageous ways . similarly , the nanofoam / aerogel dielectric 16 and in particular the top surface 19 can be doped / infused with a catalytic material that enhances surface charge generation while the bulk 20 is unmodified to retain the low dielectric constant and high dielectric breakdown strength as shown in fig1 e . such catalyst include radioisotopes and materials with high secondary electron emission coefficients such as sodium chloride ( nacl ) and hydrogen terminated ( h - terminated ) diamond . obviously , many modifications may be made without departing from the basic spirit of the present invention . accordingly , it will be appreciated by those skilled in the art that within the scope of the appended claims , the invention may be practiced other than has been specifically described herein . many improvements , modifications , and additions will be apparent to the skilled artisan without departing from the spirit and scope of the present invention as described herein and defined in the following claims . 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