Cnt flow-sensorss


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Cnt flow-sensorss

  1. 1. ABSTRACT Direct generation of measurable voltages and currents is possible when a fluids flows over a variety of solids even at the modest speed of a few meters per second. In case of gases underlying mechanism is an interesting interplay of Bernoullis principle and the Seebeck effect: Pressure differences along streamlines give rise to temperature differences across the sample; these in turn produce the measured voltage. The electrical signal is quadratically dependent on the Mach number M and proportional to the Seebeck coefficient of the solids. This discovery was made by professor Ajay sood and his student Shankar Gosh of IISC Bangalore, they had previously discovered that the flow of liquids, even at low speeds ranging from 10-1 metre/second to 10-7 m/s (that is, over six orders of magnitude), through bundles of atomic-scale straw-like tubes of carbon known as nanotubes, generated tens of micro volts across the tubes in the direction of the flow of the liquid. Results of experiment done by Professor Sood and Ghosh show that gas flaw sensors and energy conversion devices can be constructed based on direct generation of electrical signals. The experiment was done on single wall carbon naontubes (SWNT).These effect is not confined to naotubes alone these are also observed in doped semiconductors and metals. The observed effect immediately suggests the following technology application, namely gas flow sensors to measure gas velocities from the electrical signal generated. Unlike the existing gas flow sensors, which are based on heat transfer mechanisms from an electrically heated sensor to the fluid, a device based on this newly discovered effect would be an active gas flow sensor that gives a direct electrical response to the gas flow. One of the possible applications can be in the field of aerodynamics; several local sensors could be mounted on the aircraft body or aerofoil to measure streamline velocities and the effect of drag forces. Energy conversion devices can be constructed based on direct generation of electrical signals i.e. if one is able to cascade millions these tubes electric energy can be produced. 1. INTRODUCTION As the state of art moves towards the atomic scales, sensing presents a major hurdle. The discovery of carbon nanotubes by Sujio Iijima at NEC, Japan in 1991 has provided new channels towards this end. A carbon nanotube (CNT) is a sheet of graphene which has been rolled up and capped with fullerenes at the end. The nanotubes are exceptionally strong, have excellent thermal conductivity, are chemically inert and have interesting electronic properties which depend on its chirality. The main reason for the popularity of the CNTs is their unique properties. Nanotubes are very strong, mechanically robust, and have a high Young’s modulus and aspect ratio. These properties have been studied experimentally as well as
  2. 2. www.techalone.comnumerical tools. Bandgap of CNTs is in the range of 0~100 meV, and hence theycan behave as both metals and semiconductors. A lot of factors like the presence ofa chemical species, mechanical deformation and magnetic field can causesignificant changes in the band gap, which consequently affect the conductance ofthe CNTs. Its unique electronic properties coupled with its strong mechanicalstrength are exploited as various sensors. And now with the discovery of a newproperty of flow induced voltage exhibited by nanotubes discovered by two Indianscientists recently, has added another dimension to micro sensing devices. 2. CNT STRUCTURE AND GENERAL PROPERTIES There are two main types of nanotubes that can have high structural perfection.Single walled nanotubes (SWNTs) consist of a single graphite sheet seamlesslywrapped into a cylindrical tube (Fig. 1, A to D). Multiwalled nanotubes (MWNTs)comprise an array of such nanotubes that are concentrically nested like rings of atree trunk (Fig. 1, E).Fig.1 : Schematic representation of structures of carbon nanotubes (A, A1)armchair, (B, B1) Zigzag and (C, C1) Chiral SWNTs. Projections normal to thetube axis and perspective views along the tube axis are on the top and bottom,respectively. (D) Tunneling electron microscope image. (E) Transmission electronmicroscope (TEM) image of a MWNT containing a concentrically nested array ofnine SWNTs. The structure of a nanotube can be defined using a roll-up vector r and/or a chiralangle θ as shown in Fig. 2. The rollup vector can be defined as a linear combinationof base vectors a and b of the basic hexagon r = na + mb with m and n beingintegers. The roll-up vector is perpendicular to the axis of the nanotube. In Fig. 2,the shaded zone is the area which is rolled up along an axis perpendicular to theroll-up vector. Different types of nanotubes are defined by the values of m and nthus chosen. Three major categories of nanotube structures can be identified basedon the values of m and n: m=n “Armchair”
  3. 3. m = 0 or n=0 “Zigzag” m≠n “Chiral” C.Fig 2: Relation between the hexagonal carbon lattice and the chirality of CarbonNanotubes. (A) The construction of a carbon nanotube from a single graphene sheet. By rollingup the sheet along the wrapping vector C, that is, such that the origin (0, 0)coincides with point C, nanotube is formed. Wrapping vectors along the dotted lineslead to tubes that are zigzag or armchair. All other wrapping angles lead to chiraltubes whose wrapping angle is specified relative to either the zigzag direction (q )or to the armchair direction (F= 30 ْ - q ). Dashed lines are perpendicular to C andrun in the direction of the tube axis indicated by vector T. The solid vector H isperpendicular to the armchair direction and specifies the direction of nearest-neighbor hexagon rows indicated by the black dots. The angle between T and H isthe chiral angle F.( B ) Schematic of a two-dimensional graphene sheet illustrating lattice vectors a1and a2, and the roll-up vector Ch = na + mb. The limiting cases of (n, 0) zigzag and(n, n) armchair tubes are indicated with dashed lines. As represented here, the anglebetween the zigzag configuration and Ch is negative.(C) Chiral nanotube showing angle F. The electronic properties of perfect MWNTs are rather similar to those ofperfect SWNTs, because the coupling between the cylinders is weak in MWNTs.Because of the nearly one-dimensional electronic structure, electronic transport in
  4. 4. www.techalone.commetallic SWNTs and MWNTs occurs ballistically (i.e., without scattering) overlong nanotube lengths, enabling them to carry high currents with essentially noheating. Phonons also propagate easily along the nanotube: The measured roomtemperature thermal conductivity for an individual MWNT (3000 W/m K) isgreater than that of natural diamond and the basal plane of graphite (both 2000 W/mK). Superconductivity has also been observed, but only at low temperatures, withtransition temperatures of 0.55 K for 1.4-nm-diameter SWNTs and 5 K for 0.5-nm-diameter SWNTs grown in zeolites. Small-diameter SWNTs are quite stiff andexceptionally strong, meaning that they have a high Young’s modulus and hightensile strength. With the total area per nanotube in a nanotube bundle fornormalizing the applied force to obtain the applied stress, the calculated Young’smodulus for an individual nanotube is 0.64 TPa, which is consistent withmeasurements. Because small-diameter nanotube ropes have been extendedelastically by5.8% before breaking, the SWNT strength calculated from the productof this strain and modulus is 37 GPa, which is close to the maximum strength ofsilicon carbide nanorods (53 GPa) This modulus of 0.64 TPa is about the same asthat of silicon carbide nanofibers (0.66 TPa) but lower than that of highly orientedpyrolytic graphite (1.06 TPa). More impressive and important for applications needing light structuralmaterials, the density-normalized modulus and strength of this typical SWNT are,respectively,19 and 56 times that of steel wire, respectively, and 1.7 times that ofsilicon carbide nanorods. The challenge is to achieve these properties of individualSWNTs in nanotube assemblies found in sheets and continuous fibers. 3. CNT ELECTRONIC PROPERTIES Electrically CNTs are both semiconductor and metallic in nature which isdetermined by the type of nanotube, its chiral angle, diameter, relation between thetube indices etc. The electronic properties structure and properties is based on thetwo dimensional structure of Graphene. For instance if the tube indices, n and m,satisfies the condition n-m=3q where q is and integer it behaves as a metal. Metal,in the sense that it has zero band gap energy. But in case of armchair (where n=m)the Fermi level crosses i.e. the band gap energy merges. Otherwise it is expectedthe properties of tube will be that of semiconductor. The table below (Table 1) isthe observations of experiments done on nanotubes by Scanning tunnelingmicroscope (STM) and Scanning tunneling spectroscopes (STS). The dependencyof energy gap or band gap energy on chiral angle and diameter is clear from theobservation below.Table 1. Here d is the nanotube diameter; F is the chiral angle; E gap is the apparentband gap in the STS I–V spectra and dE is the shift of the Fermi energy due todoping of the tube by the substrate. Note that a chiral angle 0ْ denotes armchairnanotube and an angle of 30ْ a zigzag tube. The flat Au surface allowed the
  5. 5. www.techalone.comdiameter d of the nanotubes to be determined with an accuracy of 0.1nm bymeasuring the tube heights relative to the surface. A possible systematic uncertaintyin determining the diameter is due to a difference in barrier heights for the goldsubstrate and the tubes. The wrapping angle F can be determined with an accuracyof 1ْ. Accuracy in F is limited by the curvature of the tubes. A combination of highaccuracy in both F and d (0.05 nm) is required for an unambiguous identification ofthe n, m indices. Accuracy in Egap and dE is 0.05–0.1 eV. For this sample a shiftin the Fermi energy towards the conduction band was observed, instead of a shifttowards the valence band as observed in the other samples. From the table above, two categories can be distinguished: one with gapvalues around 0.5–0.6 eV, the other with significantly larger gap values. The firstcategory of tubes is identified as the semi conducting type, the second as metallictubes.Another noted property of nanotube is that oscillation of Band Gap energy whenmagnetic field is applied parallel to the tube axis. It has been showed theoreticallythat the band gap energy would oscillate with increasing magnetic field, so a metallictube is converted to a semiconductor and then metallic again, with a period dependingon the magnetic field strength. Fig 3 : Electronic properties of single- walled carbon nanotubes. (A). Current–voltage curves obtained by tunneling spectroscopy on various individual nanotubes is shown here. Tubes no. 1 to 6 are chiral, no. 7 is zigzag and no. 8 is armchair. The bias voltage is applied to the sample, which means that the sign of Vbias corresponds to that of the energy relative to the tube Fermi level. Curves no. 1–7 show a low conductance at low bias, followed by several kinks at larger bias voltages. The armchair tube does not show clear kinks in the range 1 to 1V (B) Energy gap Egap versus diameter d for semiconducting chiral tubes. The data points correspond quite well to the theoretical predicted values. The solid line denotes a fit of: Egap = 2g0aC–C /d with g0 = 2:7 eV. Where, g0 is the c-c tight binding energy. aC–C, the nearest neighbour distance (.142nm).
  6. 6. www.techalone.comTunnel currents I as a function of the bias voltage V applied to the sample wererecorded with a home-built STM while scanning and feedback were switched off.
  7. 7. 4. FLUID FLOW THROUGH CARBON NANOTUBE Recently there has been extensive study on the effect of fluid flow throughnanotubes,which is a part of an ongoing effort worldwide to have a representative inthe microscopic nano-world of all the sensing elements in our present macroscopicworld. Indian Institute of Science has a major contribution in this regard. It wastheorotically predicted that flow of liquid medium would lead to generation of flow-induced voltage. This was experimetally established by two Indian scientist at IISc.Only effect of liquid was theorotically investigated and established experimentally,but effect of gas flow over nanotubes were not investigated, until A.K Sood andShankar Ghosh of IISc investigated it experimentally and provided theoroticalexplanation for it. The same effect as in case of liquid was observed, but for entirelydifferent reason.These results have interesting application in biotechnology and can be used insensing application. Micro devices can be powered by exploiting these properties.4.1 Effect Of Liquid Flow Through Nanotube P král and M shapiro published a paper in Physical review letters [ Vol.86No.131 (2001)], that dealt with development of voltage / current when liquid flowsthrough CNTs. Generally an electric current in a material is produced when flow offree charge carriers is induced in the material. According to Král and Shapiro, thegeneration of an electric current in a nanotube is essentially due to transfer ofmomentum from the flowing liquid molecules to acoustic phonons in nanotube soas to have a dragging effect on the free charge carriers in the nanotube. Theoutcome, according to these workers, is a linear dependence of the induced electriccurrent on the flow velocity. Another mechanism involved, as per these authors involves a direct scatteringof the free carriers from the fluctuating Coulombic fields of the ions or polarmolecules in the flowing liquid. They argued, however, that the latter mechanismcreates a current that is five orders of magnitude smaller than the current that resultsfrom the phonon-induced electron drag. These predictions were experimentally verified by A K Sood and ShankarGhosh of IISc. In sharp contrast, Sood and coworkers have found that the behaviouris highly sub linear where the induced voltage fits a logarithmic velocitydependence over nearly six decades of velocity. Strong dependence of inducedvoltage on polarity and ionic nature of the liquid and relatively weak dependence onits viscosity was revealed from this experiment.4.1.1 Verification on Theorotical Prediction: Experimental setup, observed results and hence the inference gathered from theobservations, dependencies of voltage induced on various factors that has beenobserved are explained in following sections
  8. 8. www.techalone.com4.1.2 Specification of materials and instuments used: The SWNT bundles used for the experiment were prepared by electric arcmethod, followed by purification process. The dimensions and physical propertiesof samples are listed below:Diameter of nanotube 1.5nmResistivity 0.02Ω-mSensor using this tube was prepared by densly packing nanotubes between twometal electordes. The dimensions of Sensor:Along the flow 1mmThickness 0.2mmWidth 0.2mm The specimen was placed inside glass tube of innner dia 0.03 and length 0.9m ,kept vertically. Voltage mesurment were used by using KEITHLEY 2000multimeter. Hydrocloric and Glycerine to increase polarity and viscocity of fluid mediarespectively.4.1.3 The Experiment: The scematic representation of experiment is shown in figure 4.To avoid anyturbulent flow that may have been caused by the expansion of the flow at the inletof the flow chamber it was placed at the center of glass tube. The velocity u of theliquid flowing past the sample was measured from the bulk flow rate, which wasvaried by adjusting the height of the reservoir and the opening of the valve. Theliquid enters the flow chamber against gravity such that the formation of airpockets is avoided. The flow at the center of the cell is expected to be laminarbecause the Reynolds number (Re) (~300 for a velocity of 10-2 m/s) was much lessthan the critical value of 2000 for the onset of turbulent flow in a pipe. Themeasurements shown here were taken after the transients had subsided. Fig 4 : Experimental setup where R is the reservoir, L is the valve controlling the liquid flow, S is the cylindrical glass flow chamber, and G is the Voltmeter.
  9. 9. www.techalone.com4.1.4 Observations and Inferences: When the sample was immersed in water at rest, a voltage (~1 mV) developedalong the sample as an electrochemical potential difference at the interface of theSWNT bundle with the metal electrodes. This offset voltage was subtracted fromthe voltage measured at finite velocities of the liquid to obtain the flow-inducedvoltage. The voltage developed only along the flow direction; no voltage wasgenerated in the direction perpendicular to the flow (i.e., along the width).Figure 5 shows the observed dependence of the flow induced voltage on flowvelocity of water. Even at avery low speed of 5x10-6 m/s, avoltage of 0.65 mV wasgenerated. The solid line inFig5 is a fit to empiricalrelation: V= a log(ub+1)Where a and b are parameters,a= 0.6mV and b= 6.5 x 106 s/m. Figure 6 compares the voltage developed along nanotubes as a function of the flowvelocity for a series of fluid mixtures. A notable effect is the strong dependence ofthe flow-induced voltage on the ionic strength of the flowing liquid. It can be seen
  10. 10. www.techalone.comthat for u = 10-4 m/s, the voltage for 1.2 M HCl is about five times that for water.A comparison of expected results as per Král and shapirov and experimental resultsare compared in following paragraph, They considered metallic nanotubes that were densely packed in two layersseparated by a layer of water of thickness d (~10 nm). In their mechanism, theforce exerted by the liquid with a flow velocity u on the surface of the nanotubesis: F = (hu/d) X interface periphery X interface length. This force results in the steady-state quasi-momentum (p) transfer to thephonon bath was calculated using p ~ Ft where h is the shear viscosity of theliquid, t known as phonon umklapp time. Using this value of p, the resultingcurrent was calculated, the results showed that current depends on viscosity offluid and flow velocity Vs flow induced voltage would be linear in nature. Usingglycerol mixed with water at ratios 88:12 and 75:25 having h= 113 and h= 234mPa s respectively revealed the weak dependence of induced voltage on h. Theinduced voltage decreased with increase in shear viscosity yet the effect wasnegligible. The experiment also revealed the dependence of flow induced voltage onpolarity of liquid which is clear from figure 6, there is a steep increase in flowinduced voltage with increase in polarity of liquid. 0.6 M and 0.12 M HCL andmethanol were used for this purpose. Thus, for u =6 x10-4 m/s, Vwater = 2.1 mV,whereas Vmethanol = 0.2 mV. Thus, data set reveals three features: the sublinear(almost logarithmic) voltage response to flow velocity, its strong dependence onthe ionic and polar nature of the flowing liquid, and its relatively weakdependence on the liquid viscosity—the latter, in fact, acting to reduce the flow-induced voltage. The predominant mechanism here that results in the flow induced voltage isfluctuating columbic fields. As ions and polar molecules have fluctuating Coulomb fields, Sood andcoworkers have assigned the observed behaviour to scattering of free carriers bythese fluctuating fields. The exact model is termed as ‘the pulsating asymmetricalratchet model’. The terms describe the phenomenon itself, whereby ions flowingpast a certain point in the nanotube produce Coulomb pulses while the asymmetryis provided by the velocity gradient at the liquid–solid interface. An important point must be made regarding the direction (polarity) of thevoltage and current in relation to the fluid flow direction. In the experiments, itwas observed that the electric current and the fluid were flowing in the samedirection. In a linear response theory based on the viscous drag (the phonon wind),the particle current has the same direction as the fluid flow velocity, and,therefore, the sign of the electric voltage and current is determined by the sign ofthe charge carriers (electron or hole). Thus, for the hole like carriers, the inducedvoltage and current have the same direction as the flow velocity. This, however, isnot the case for a pulsating ratchet model in general. For a given direction of theasymmetry, the direction of the voltage is independent of the sign of the charge onthe carriers. It will, of course, depend on the sense of the bias (asymmetry) of theratchet potential. In a polar fluid of a given ionic strength, such as concentrationsof H+ and OH– ions (along with their hydration shells), it is expected to haveratchet potentials of either sign (bias). The net effect is then determined by the
  11. 11. www.techalone.comdominant ratchet. What is important here, however, is that there is no a priori(symmetry) reason to expect the oppositely biased ratchets to give an exactcancellation.4.2 Applications Flow sensor that is based on SWNT directly produces an electrical signal inresponse to a fluid flow. This sensor can be scaled down to length dimensions ofthe order of micrometers—i.e., the length of the individual nanotubes—making itusable in very small liquid volumes. The sensor also has high sensitivity at lowvelocities and a fast response time (better than 1 ms). The nanotubes also could beused to make a voltage and current source in a flowing liquid environment, whichmay have interesting biomedical applications. In bio medical applications it is speculated that carbon nanotubes can be usedto power coronary heart pace makers using flowing blood as source of power thiswould make the use of batteries in coronary heart pace makers obsolete. Theresults using blood have shown encouraging results. Another application would be as a chemical sensor which can be used to checkthe polarity, acidity and detecting any impurities (that are polar in nature) presentin the liquid media. If CNTs become cheaper and economically viable then it can be used in bulkto produce electricity in small scale using flowing water as source of active media.4.3 Effect of Gas Flow over Carbon Nanotubes The effects of Gas flow over CNTs were not studied both theoretically andexperimentally, until A K Sood and his Co workers investigated on it. Paperspertaining to these works were published in Physical Review letters on 20 Aug2004. Research works on these findings and possible applications are beinginvestigated by Sood and coworkers. The same effect of flow induced current/voltage observed with liquid mediawas observed when gas medium was used. Although the effect is same, yet thereason for it is completely different from that of liquid flow. Further this effect isnot confined to nanotubes, same effect is observed in wide range of solidsincluding semiconductors, metals for wide range of velocities. The underlying principle is an interesting interplay of Bernoulli’s Principleand Seebeck effect. Pressure differences along the streamlines give rise totemperature difference along the tube which, in turn produce the measuredvoltage. Unlike in case of liquid the flow induced voltage in gas, depends not onlyon flow velocity but also on the orientation of specimen. The experimental setup,observation, findings, inferences and theory behind this phenomenon is explainedin the following sections.4.3.1 Specifications of Specimen and Instrument: To establish the ubiquity of this phenomenon variety of solids were used, theyinclude nanotubes, semiconductors and metal which are described below:
  12. 12. www.techalone.comn-type Ge(Sb-doped) of s(conductivity)=100/Wcm, p-type Si s =100/Wcm, n-type Si s =100/Wcm, SWNTs, MWNTs graphite, poly-crystalline cooper andplatinum were used. The sizes of the semiconducting samples and copper areabout 3 mm along the flow and 1mm perpendicular to the flow. The electricalcontacts for the semiconducting materials (Si, Ge) were made with copper leads of1.3X10-4 m dia using silver emulsion [shown by shaded region in schematicrepresentation of experimental setup fig.7] The exposed (active) portion of thesample, that is not covered by silver emulsion is about 2mm [marked as d in Fig7b] along the flow and 1mm perpendicular to the flow. The samples of SWNTs,MWNTs and graphite, prepared by densely packing the powder between twometal electrodes, were about 1mm along the flow, 2mm wide and 0.2mm thick.Proper electromagnetic interference shielded cables were used to measure thesignals using a KEITHLEY 2000 multimeter. Active medium i.e. gases used were oxygen, argon, nitrogen and air. Thesewere contained in a gas chamber kept at a pressure of 150 bars. The flow velocitywas measured indirectly using a Rota meter.4.3.2 Experimental setup, Observations and Inferences: Figure 7, (next page) shows a schematic layout of experiment of theexperimental setup to achieve a calibrated gas flow velocity on the sample. Thegas from a compressed gas cylinder (maximum pressure of 150 bars) is let out at agiven pressure in a tube of diameter 7 X 10-3 m. The flow rate Q is measured usinga Rotameter as shown in Fig. 7(a). The average velocity u at the end of the tubewith cross-sectional area (Φ) is deduced from Q as u= Q ∕Φ. The sample is keptat an angle α=π∕4 with respect to the horizontal axis to achieve the optimal signal.At an angle α= 0ْ would produce no effect since the pressure gradient is zero, andα=π∕2 too would give no signal due to symmetry. The results are similar when thesample is kept inside the tube (at a distance of 2 X 10-2 m from the exit point) or1 X 10-2 m outside the tube.Fig 7 : (a) Schematic of the experimental setup. The flow rate at the exit point isdeduced from the measured flow rate at the side port using the rotameter. (b)Sample: shaded portions mark the electrodes. The positive terminal of thevoltmeter is connected to the right ® of the sample of active length d kept at andangle α=π∕4 with respect to the horizontal axis.
  13. 13. Figure 8 below shows the voltage across the n-Ge sample as a function of timewhen the gas flow (u= 7 ms) over the sample is switched on and off. The gas usedhere is argon. Data for the steady flow voltage V for a variety of samples and flowvelocities are shown in Figs. 9 and 10. It can be clearly seen that for nitrogen gasover p-Si, n-Si, n-Ge, p-Ge, SWNT, MWNT and graphite, the voltage V generatedvaries as u2 overa wide range of u( as does the 2current, not shown). Figure 9 shows the fit to V= Du with fit parameter D givenin Table2. This is even clearer in Fig 9 which shows the same data plotted versusM2, where M is the Mach number given by M=u / c, c being the sound velocity (=333 m/s for nitrogen and 323 m/s for argon at 300K). The solid lines fit to curve V=AM2, where A is a fitting parameter also given in Table 2. Experiments using polycrystalline copper sheet for which the slope A is very small. It can be seen that the sign of P-type Si and SWNT is opposite to that for n-type Si, n-type Ge, graphite andcopper. SWNT samplesare usuallyunintentionally p-doped,which can explain thesign of the flow-inducedvoltage which was foundto be the same for SWNTand p-Si. The inset in
  14. 14. www.techalone.comfig10 shows the slope A versus the known Seebeck coefficient S of the samples ofthe same d, as given in Table 2. The coefficient depends linearly on S, as shownby fitted line in the inset, with slope=60K.Figure 10 shows the generated voltage V over a large range of values of M2 for theflow of argon (solid squares) and nitrogen gases (open circles) over n-Ge. Achange in slope is clearly seen around M2≤.05 (Called regime 1) is higher forargon than for nitrogen. The ratio of slopes 2[A (argon) / A (nitrogen) = 1.2]. That there are actually two M regimes is clearfrom our theoretical analysis of the mechanism behind the generation of electricalsignal induced by the flow of gases over the solids, and is discussed below. Forthe adiabatic steady inviscid flow of a gas, Bernoulli’s equation gives the pressuredifference along a streamline in terms of Mach number M as: CP (CV) being the specific heat at a constant pressure (volume). The values ofg for argon and nitrogen are 1.667 and 1.404, respectively. In eq1 P0 is themaximum pressure at a point on the streamline where velocity is zero . Such apoint is the leading edge on the surface of the sample past which the gas is movingand is called the stagnation point. For the sample geometry shown in Fig. 7 (b),the pressure difference between the two ends of the active sample exposed to thegas flow (i.e., without the electrodes) is hence: The subscriptsL(R) denote the
  15. 15. www.techalone.comleft (right) of the active sample when the gas flows from left to right. From theideal gas law, the fractional temperature difference DT/T is related to the pressuredifference DP/P and the density difference Dr/r as DT/T=DP/P-Dr/r. When M<<1, the change in density of the gas isnegligible, i.e., the fluid is essentially incompressible and hence DT/T=DP/P.Therefore, the temperature difference along a streamline between two pointsseparated by distance d for M<<1 (called regime I) is: Where, DT= TL - TR > 0.The gas flowing past thesample kept at an angle a with respect to the horizontal axis corresponds to anaccelerating flow and hence MR>ML.The tangential component of the velocity u ofthe outer flow depends on the streamline distance x measured along the flatboundary as: For this geometrya= pi/4 and hence u(x) .The temperature difference along the streamline in the gasflow will induce a temperature difference in the solid along the flow direction.The temperature difference, in turn, will result in a voltage difference V, definedas VL – VR, due to the Seebeck effect. Thus: Where,S is the Seebeckcoefficient of the solids, positive for p-type and negative for n-type materials. Thefactor k depends on the specific interactions between the gas and the solid surfaceas well as on the boundary conditions of the temperature difference between thegas and the solid. In nonrarefied gases, the boundary conditions at the surface of asolid is that the temperatures of the gas and solid are equal, in which case k=1. Butthis boundary condition applies only if the mean free path of the gas molecules isvanishingly small. The factor k takes into account all the differences from thisideal boundary condition. The present phenomenon of voltage generation by flowof gases is not applicable to the flow of liquids where the viscous drag dominates.Beyond a certain value of M (~0.2), called regime II the density changes of thegas should be taken into account, which gives:And hence:On comparing the predictions of this model with the experiments:
  16. 16. 1. From equation 4 that it can be deduced that induced voltage V is directly proportional to M2 and the experimental results show that the curve fits to V=A M2 . 2. From eq 5 the slope A should depend linearly on S in regime 1 where : The inset in figure 10 agrees with the result obtained theoretically. 3. Another property can be deduced from the equations above that for substance that has S=0 will have no induced voltage. Results with platinum as specimen have showed no induced voltage (for Pt S~0). 4. From equation 5 in regime 1 ratio of voltage generated for argon and nitrogen with same velocities for same samples were 1.2 which is the ratio of g (argon) / g (nitrogen), hence consistent with theoretical predictions. Hence, the theoretical predictions deduced are consistent with the experiment.4.4 Applications The experiments clearly suggest that a sensor to measure the flow velocity ofthe gases can be made based on the generated electrical signal. It is an activesensor which gives direct electrical response to the gas flow. This should becompared with the widely used gas flow sensor based on thermal anemometry,wherein the fluid velocity is sensed by measuring changes in heat transfer from asmall, electrically heated sensor (wire or thin film) exposed to the fluid. Thermalanemometry works on heat balance equations and hence any small changes in thetemperature, pressure, or composition of the gas can cause erroneous readings.Such effects are minimum or can be easily taken into account in the sensors basedon the direct generation of flow-induced voltage or current in the sensor material.The magnitude of generated voltage can be easily scaled up by using series andparallel and connections of sensing elements. This suggests that flow energy canbe directly converted into electrical signal without any moving part, thus having apotential for application in generating electricity. Ajay Sood and his coworkers have shown that this property can beexploited for construction of nanotube based vibrational sensor in liquids and asaccelerometer. The SWNT based accelerometer has so far measured the frequencyrange 0.5 Hz to 1 kHz and the minimum detectable acceleration by theaccelerometer is 10-3 g. 5. CONCLUSION
  17. 17. www.techalone.comThe Past as Indication of the Future: The exponential increase in patentfilings and publications on carbon nanotubes indicates growing industrialinterest that parallels academic interest.Nanotubes electronic devices might be the most promising field. Impressiveadvances have been made in demonstrating nanotube electronic device concepts,but a decade or more of additional progress is likely required to reliably assess ifand when these breakthroughs will reach commercial application, So will thenanotube sensing applications as flow sensors / electrical signal generators. Asfor now control on their electronic properties which is not so impressive wherelarge scale production is considered, poses a major hurdle in this regard.Possible chemical sensing applications of nonmetallic nanotubes are interesting,because nanotube electronic transport and thermo power (voltages betweenjunctions caused by inter-junction temperature differences) are very sensitive tosubstances that affect the amount of injected charge. The main advantages arethe minute size of the nanotube sensing element and the correspondingly smallamount of material required for a response. The mechanical robustness of thenanotubes dramatically increase probe life and minimize sample damage duringrepeated hard crashes into. These uses may not have the business impact ofother applications, but they increase the value of measurement systems forcharacterization and manipulation on the nanometer scale. With the addition ofthis new phenomenon i.e. flow induced voltage nanotube seems to head forubiquity in nanoworld as sensor, actuator probes, energy storage and generatingdevices.Independent of the outcome of the ongoing races to exploit nanotubes inapplications, carbon nanotubes have provided possibilities in nanotechnologythat were not conceived in the past. Nanotechnologies of the future in manyareas will build on the advances that have been made for carbon nanotubes.