Patent Application: US-74484607-A

Abstract:
a composition of electrically conductive composites for temperature sensing comprises conductive particles . the composite forms from a suspension . the suspension comprises the particles and solvent , and the particles are conductive particles with aspect ratio larger than one . the conductive composite retains a negative temperature coefficient when in contact with certain specific surfaces . the particles within the composite self align .

Description:
the present invention makes use of new composition of conductive carbon particles for preparation of electric temperature sensors . the composition allows the manufacturing of conductive carbon composites suitable for temperature measurement . such composites are shown as a viable material for fabrication of resistive temperature sensors . the composite composition allows for scaling the size of cnt - graphite temperature sensors to that the size of other carbon nanotube ensemble bases sensors assuring compatibility of such temperature sensors with fabrication of nanosensor arrays . as temperature is a fundamental and variable environmental property , incorporation of temperature sensor in a sensor array increases usefulness of an array . this new composition leads to fabrication of temperature sensors in a simpler and lower cost procedure , as compared to other methods . in addition , the use of carbon - based coatings compositions with predictable thermal response of conductivity further allows prediction of change in emi shielding effectiveness of carbon based coatings , thus increasing value in use of conductive carbon based composites . as combustible , such materials represent much simpler disposal requirements . in order to obtain a composite , an intimate mixture of particles is formulated by mixing with any of the common mixing methods . the intimate mixture of particles is suspended in any of the common solvents or a common solvent mixture by any of common methods . the suspension of particles is applied by any means in the desired amount to a substrate and allowed to solidify into the composite by natural or forced solvent evaporation . to obtain the composite with useful response of conductivity to temperature , the intimately intermixed particles comprise carbon nanotube and graphite particles , carbon nanotube particles having aspect ratio of length to width larger than one and graphite particles having aspect ratio of diameter to thickness larger than one . in preferred embodiment of the present invention , the average length of carbon nanotubes is no smaller than 25 nm . the average diameter of conductive planar particles is of the same order of magnitude as the average length of the carbon nanotubes in a preferred embodiment the composite is prepared by use of a composite material containing suspension of a mixture of conductive graphite platelets and carbon nanotube particles in a solvent . the solvent may comprise a solvent mixture . the composite material may also contain binder and other optional ingredients , for example analytical chemical reagents , surfactants , viscosity modifiers , and dyes or pigments with the limitation to these that do not chemically interact with the mixture to the extent changing the temperature coefficient of the electrical resistance of the resulting material . if these components were to disturb the temperature response of the temperature sensor , such materials must be separated from the temperature sensor by a protective layer of the compatible material . the mixture of conductive graphite platelets and carbon nanotube particles may contain as little as , or less than 5 % graphite by weight , the conductive particle balance being nanotubes . wherein a nanotube is a hexagonal lattice of carbon rolled into a cylinder ( a nanotube is defined by its diameter , length , and chirality , or twist . besides having a single cylindrical wall ( single wall nanotubes , or swnts ), nanotubes can have multiple walls ( multiple wall nanotubes , or mwnts )— cylinders or scrolls inside the other cylinders or scrolls ). aggregates otherwise known as bundles of either single wall nanotubes or multiple wall nanotubes may also be used . it is not excluded that even less than 5 % of planar graphite particles in the particle composition will suffice for fabrication of adequate temperature sensors , however other than approximately cylindrical or planar carbon particle shapes are deemed detrimental to achieving temperature sensing conductive carbon composites . examples of low molecular weight , highly volatile solvents include : water , ethers , alcohols , ketones , hydrocarbons , halogenated hydrocarbons , preferably c 1 - c 16 , more preferably c 1 - c 10 , and mixtures thereof . examples of alcohols include methanol , ethanol , isopropanol , perfluoropropanol , 1 - butanol , 2 - butanol , 2 - butoxyethanol and octanol . examples of ketones are acetone , methylethylketone , diethyl ketone . examples of hydrocarbons include hexane , heptanes , octane , nonane , and decane , dichloromethane , chloroform , 1 , 1 , 1 - trichloroethane , trichloroethylene , tetrachloroethylene , benzene , toluene , xylene , 1 , 2 , 4 - trimethylbenzene , phenol and naphthalene . examples classes of binders include polyalkylenes , polyalkylene glycols , polyalkylene alcohols , polyalkylene glycols , polyalkylene esters , and copolymers or mixtures thereof . specific examples of binders include polyethylene , polypropylene , polyvinyl alcohol , cellulose and cellulose derivatives , polysaccharides , polystyrol , and mixtures or copolymers thereof . example supports include insulators such as : paper , glass , ceramics , polymers and plastics , polyethylene and other polyolephins , as well as wood , and knit , woven , and non - woven natural and synthetic fibrous materials . in a resistance thermometer embodiment the solid support has electrical connections placed before or after the deposition of the thermometric coating . the electrical connections can be of any conductive material providing electrical contact with the thermometric composite , such that the contacts &# 39 ; and the leads &# 39 ; resistance is smaller than that of the composite , preferably two or more orders of magnitude smaller than the resistance of the thermometric composite . in any of the above - enumerated and future embodiments , the sensing element , that is the thermometric composite on a support is enclosed or encapsulated in a protective enclosure or an inert layer . such enclosure must effectively eliminate other than temperature influences on electrical resistance of the sensing element . these influences typically include pressure , light , and most importantly humidity and chemical exposures . the enclosure protects against degradation of the active temperature sensing elements to insure reproducible sensing . in embodiments requiring electrical connections , such connections are incorporated and accessible to couple to a device for detecting a quantity indicative of electron transfer along the cnt - graphite based composite . it is envisioned that mixtures of approximately planar and approximately cylindrical particles of other conductive or semiconductive materials in varying ratios will exhibit useful thermometric properties . it is envisioned that orientation of the particles in the thermometric composite may affect the useful temperature range or sensitivity or temperature range and sensitivity of such composite . in the preferred embodiment the particles are randomly distributed within the composite . the composites described above may be used to manufacture , for example , smart coatings , electronic components , electrodes , displays , and electromagnetic interference ( emi ) protective and antistatic devices . moreover , these materials are particularly useful for the manufacture of nanothermometers , resistance thermometers , temperature sensors and temperature sensing components of sensor arrays . for example , a one to one by weight mixture of multi wall cnt ( e . g ., mwcnt o . d .× i . d . 40 - 70 nm × 5 - 40 nm × 0 . 5 - 2 μm ) and graphite is mixed into the composite material . such weight ratio of graphite particles of average diameter approximating the average length of the cnt particles assures that the total area of the planar particles is of the same order of magnitude as the product of the number of tubular particles present and of the area established by the mean square radius of gyration of the tubular particles . the resulting composite material is applied to an area of the substrate between and including terminals of electrical connectors already on the substrate . as shown in fig3 a , a resistive thermometer is designated generally by the reference number 10 and is hereinafter referred to as “ thermometer 10 .” thermometer 10 includes a substrate 12 and external leads comprising first and second electrical connectors shown at 14 and 16 disposed in electrical communication with the substrate . the substrate 12 may be glass or the like , and the first and second electrical connectors 14 , 16 may be copper wires . the conductive carbon composite comprising the cylindrical and planar forms of conductive carbon as described herein is randomly distributed on and cured on the substrate 12 with one or a combination of the existing methods [ kirkor ]. subsequently , the composite is encapsulated and sealed in a protective enclosure or inert layer 18 , which may comprise low - density polyethylene ( ldpe ). as shown in fig3 b , electron transport through the thermometer 10 comprises electron flow from the first electrical connector 14 , through the substrate 12 , and out the second electrical connector 16 . the thermal response of the resistance of the composite is calibrated against traceable temperature standard , in this case a k type thermocouple . using a dc voltmeter , the resistance can be measured and converted to temperature . the temperature calibration of a series of such devices is presented in fig2 . the temperature range of the device of the example 1 is limited by the thermal properties of the encapsulating material , not the carbon composite . graphite exhibits thermal stability to a very high temperature . ( at least up to 1600 ° c .) carbon nanotubes are thermally stable at least to 400 ° c ., the lowest threshold for cnt oxidation in air . purcell et al . [ purcell ] demonstrated that a mwcnt emitter could be heated by its field - emitted current up to 2000 k and remain stable . still resistive heating of individual mwcnt at or above 900k might cause shell breakdown and layer ablation at 200 - microampere currents [ j . y . huang 2005 & amp ; 2006 ]. it is demonstrated here that cnt - graphite mixtures can serve as temperature sensing materials at least up to 400 ° c . and envisioned that probably well above 400 ° c . ( fig4 b ) depending on the protective enclosure . certain continuous inkjet ( cij ) printers ( amir noy , sgia journal , first quarter 1999 , pp . 31 - 33 ) can handle inks with large particulates that would clog the nozzles of typical inkjet printers . thus , conductive composite materials containing carbon particles such as mixtures of graphite platelets and carbon nanotubes may also be used for printing of low cost , carbon based temperature - sensing devices . 1 ) formulation of composite material for manufacturing of a painted resistive thermometer a 10 mg of multiwall carbon nanotubes ( average length 0 . 5 to 2 micron ) were added to 50 mg of a composite material containing 20 % by weight of colloidal graphite platelets dissolved in isopropanol with small quantities of ketones and cellulosic binder . the weight ratio of graphite and cnt in the composite material was thus established as 1 : 1 . the modified composite material was painted in approximately 0 . 5 cm wide , 50 micron thick , and 0 . 5 cm to 4 cm long traces on a glass substrate . the composite was left to dry in ambient air for several hours . stable at room temperature conductivity of the composite samples indicated completion of the drying . copper wire electrical connectors were affixed to the opposite ends of each carbon composite with the conductive silver ( ag ) paint . the silver paint was allowed to fully cure according to its manufacturer &# 39 ; s recommendation . the carbon composite on supporting glass and the ag paint traces were sealed in ldpe leaving copper leads exposed . the device was placed in contact with a thermal bath of measured temperature and its resistance measured with a dc voltmeter . each device was tested in two cycles of increasing and decreasing temperature in range from − 80 to + 110 c . table 1 contains resistance values ( ohms ) measured for six thermometers . the resistance change is proportional to the original device resistance and the resistance decreases with increasing temperature mimicking semiconductor behavior . additionally , the devices demonstrate negative temperature coefficients . the graphical results plotting resistance versus temperature for 5 typical devices are shown in fig1 . 2 ) quantifying the temperature sensitivity of the painted graphite - cnt composites . the graphite - cnt 1 : 1 coatings were painted on polyethylene , equipped with metal connectors , and placed in a glove bag filled with dry air . radiant heat source was also placed in the same bag , while a k type thermocouple led to an outside fluke 52 temperature meter was attached to the polyethylene support under the carbon composite . the connectors to the painted resistors extended outside the bag to the fluke 75 dc voltmeter . the resistivity of the composites and the temperature registered with the thermocouple were simultaneously recorded with digital photography . the temperature range was from 20 ° to 60 ° c ., narrow enough for linear approximation of the resistance change with temperature . the average calculated resistance at 0 ° c ., r 0 , and the slope of the resistance are displayed in table 2 and plotted in fig2 . the temperature sensitivity of the composites was calculated according to the formula ( dr / r )( dt / t ). the sensitivity coefficient exhibited by the composites is comparable to the results obtained with graphite thermometers at cryogenic temperatures . composite material consisting of mwcnt and graphite in 1 : 1 and 5 : 1 ratio by weight was painted in approximately 0 . 5 cm wide , 50 micron thick , and 0 . 5 cm to 4 cm long traces on a glass substrate . the samples were subjected to high temperatures ranging from 360 - 580k for the 1 : 1 and 670 - 780k for the 5 : 1 ratios respective as seen in fig4 , a and b . the change in resistance with temperature is significantly less for the device with the larger mwcnt ratio ( fig4 , b ). this is a demonstration of temperature sensitivity tuning by variation in composite material composition . 3 ) composite material consisting of graphite and cnt as previously described was painted in approximately 0 . 5 cm wide , 50 micron thick , and 0 . 5 cm to 4 cm long traces on a polyethylene substrate and on glass . the g band of the raman spectra of the composite of composite material containing cnt and graphite on polyethylene are polarized fig5 . the d band (˜ 1320 cm − 1 ) is expected to be isotropic , so this is taken as an internal intensity standard and normalized to one . the intensities of the g bands (˜ 1580 cm − 1 ) of the mwcnt and of graphite excited by p vs . s polarized light reveals the presence or absence of orientation in the composite material with these interpenetrating components . more of the mwcnt and graphite respond to the s polarized light than the p at the g band indicating that the mwcnts and graphite are aligned lengthwise to the s - polarized light . thus , both mwcnt and graphite are oriented in the layer of the composite . this alignment occurs on oriented polymeric substrates without external influence or special processing . the conductive carbon particles in the thermometric composite on glass , that is , on an amorphous substrate , display no alignment in the raman spectra , fig6 . this mwcnt and graphite alignment is expected in the samples quantified for temperature sensitivity , as the same components , mixture ratios and deposition techniques were used . 4 ) prophetic example . preparation of a resistive temperature nanosensor . the thermometer from the example 1 ) is prepared by scaling down temperature sensing composite size by dispensing onto a prepared substrate about a picoliter of appropriately diluted composite material containing the preferred planar to tubular conductive particle ratio and by flash evaporation of the solvent . the surface of the support would be already equipped with pre - deposited electrical connectors to a readout device . such nanodevices could be conveniently mass - produced by ink - jet printing . it is envisioned that technologies similar to dip pen technique are capable to deposit nanosize composites suitable for the temperature sensing . the resulting composites are used individually or in multisensor arrays for determining the temperature . 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