PIEZOCERAMIC AND NANOTUBE MATERIALS FOR HEALTH MONITORING

Transcription

1 NDE PIEZOCERAMIC AND NANOTUBE MATERIALS FOR HEALTH MONITORING Mark J. Schulz, Goutham R. Kirikera, Saurabh Datta Department of Mechanical Engineering University of Cincinnati, Cincinnati, OH Mannur J. Sundaresan Department of Mechanical Engineering North Carolina A&T State University, Greensboro, NC Promod R. Pratap Department of Physics and Astronomy University of North Carolina at Greensboro, Greensboro, NC 27411

2 OUTLINE 1. INTRODUCTION 2. ACTIVE FIBER SENSORS 3. AN ARTIFICIAL NEURAL SYSTEM 4. NANOTUBE MATERIALS 5. CONCLUSIONS

3 1. INTRODUCTION Currently, Structural Health Monitoring (SHM) is not widely adopted This is because too many individual sensors are required, and sequential signal processing of sensor data is not efficient Highly-distributed interconnected sensors and parallel processing may simplify and make SHM more practical Miniature distributed sensor nodes can be formed using piezoceramic and possibly nanotube fibers These nodes (~10) can be connected in series to form a continuous sensor Continuous sensors (~20) in turn can form an Artificial Neural System (ANS) The ANS can measure low frequency dynamic strains and high frequency ency Acoustic Emission (AE) signals to identify damage growth

4 2. ACTIVE FIBER SENSORS The advantages are: Unidirectional sensing, mostly self-powered More rugged than monolithic PZT The electroding can be designed for spatial filtering of waves Fiber preforms can be cut to different shapes High bandwidth and high voltage coefficient for sensing acoustic waves Piezoceramic Fiber Preform and Active Fiber Sensor (preform from CeraNova Corp.)

5 Active Fiber Sensor Design Keep the capacitance low by series connectivity of nodes New concept of transverse poling and longitudinal sensing will be adopted Temporary electrodes will be used to pole the sensor and then a new electrode will be used for sensing purpose Two orthognal electrically independent unidirectional sensors Two electrically independent bidirectional sensors Series electroded ribbon sensor

6 3. AN ARTIFICIAL NEURAL SYSTEM Characteristics of the Biological Neural System Information flows in only one direction Inhibition and firing control information flow No receptors are exposed to the surface The skin is the interface between the biological system and its surroundings Skin an important component in the neural system and its functions are not fully understood, e.g. why is skin piezoelectric?

7 The Biological Neural System A typical neuron Neurons in the visual system Equivalent circuit of a dendrite or axon

8 The ANS with n nodes in each neuron, m neurons, and the PST/Computer system Biomimetic Strategy Develop the ANS using smart materials, microelectronics, and new signal processing The receptor and dendrite are modeled together using active fibers Programmable Signal Transducer (PST) a. Series/Parallel neurons, b. Channel switching, c. Signal acquisition, d. Low/high freq. filtering, e. Sensor self-check Computer Signal Processing Diagnostics Prognostics The biomimetic nerve will replicate the efficiency of the Biological Neural System to sense dynamic strain as pain Dual node bi-directional sensors are used for this example Signals from neurons

9 Channel 1 (columns output) Channel 2 (rows output) An ANS in a composite panel in which 100 bi-directional AFS nodes (N1-N100) form 20 neurons with outputs (V1-V20) where the final output of the neural system consists of 20 on/off neuron firing signals and only two time history signals

10 Simulation of Wave Propagation in a 4 by 4 x ¼ in fiberglass panel The 100 dual node neural system is modeled as being on the plate surface A A step excitation is put at the center of the plate The modeling is discussed in Artificial neural system for structural tural monitoring by W.N. Martin et al, NDE , 06, and Wave propagation sensing for damage detection in plates by A. Ghoshal et al, SS Wave motion in a plate

11 Low Frequency response, 3x3 neural system

12 Low Frequency response, 3x3 neural system, column switches

13 Low Frequency response, 3x3 neural system, column outputs

14 Low Frequency response, 3x3 neural system, row switches

15 Low Frequency response, 3x3 neural system, row outputs

16 Signal processing in the ANS The neuron switches close when the voltage level of the neuron exceeds 0.1 volt (switch 0v=open, 1v=closed) The voltage output of the amplifier and diodes in each neuron (only 5 curves are shown due to symmetry of the plate) (5,6 firing together)

17 The final time signal output is the sum of the voltages from all the column neurons In this example, because of symmetry, the row and column outputs are identical Damage is located at the intersection of the row and column neurons that are firing The logic to interpret the neural system response is being developed The experimental ANS is being built using electronic components

18 Crack propagation testing on an aluminum bar (NCA&T)

19 4. NANOTUBE MATERIALS The properties of single wall Carbon Nanotubes (CNT) include a high length/diameter aspect ratio of up to 10,000, an elastic modulus of 1TPa, a tensile strength of 50GPa, light weight with a density of 1.4g/cm^3, high temperature capability, and a predicted actuation energy density times greater than existing smart materials. When loaded in compression, the tubes will bend over to large angles then ripple and buckle. However, these deformations are elastic. Upon removal of the load, the nanotube will return to its original undeformed shape. With the property of superelasticity, if nanotubes could be put together to make macro-scale structures, these structures might become almost unbreakable. These amazing properties make CNT and Single Wall Boron Nitride Nanotubes (BNT) (which are structurally equivalent to carbon) potentially the materials of choice for reinforcing and actuating polymer composite materials. CNT/BNTs have a good potential to make sensors and actuators because of their nanoscale size and electromechanical transduction properties and high power density. A key advantage of using CNTs and BNTs for actuation is that they are also load-bearing. In this sense, the use of nanotubes provides a great potential for health monitoring of structures because the structure is also the sensor. In reality, salient problems including high cost must be solved before Smart Nanocomposites can become practical. The following sections discuss recent research in the area of nanotube actuation and sensing and the possibilities for using nanotubes for sensing and actuation for health monitoring of structures.

20 Carbon Nanotube Electrochemical Sensors/Actuators Actuation of CNT was developed in 1999 by Ray Baughman et al [8-10]; in CNT ropes and aligned membranes were developed [11-13]; in 2001 composite materials strengthened with CNT were studied using Raman scattering [14]; in 2001 the piezoelectric effect in SWCNT and BNT was modeled [15], and experiments to study the actuation properties of a single CNT were begun [16]. When positive charges are injected into the SWCNT graphite crystal, the graphite tends to shrink, and conversely, negative charges cause the graphite to expand. This structural deformation and conversion of electrical energy to mechanical energy through radial and longitudinal expansion or contraction is mainly caused by the change in electronic structures rather than coulomb interactions, and large actuation capabilities are predicted for a SWCNT. The mechanisms for actuation can be electrochemical reduction and oxidation reactions, capacitive injection of charge, a change in the electric or magnetic fields, or by changing the localized PH or ion or solvent concentration. The input/output of ions and the resulting increase/decrease of the double layer charge around the CNT causes the actuation effect and the CNT to behave as a large capacitor filled with an electrolyte. Development of these CNT actuators is in an early stage [8-10] and is mostly focused on artificial muscles, structural actuation/sensing is a more difficult problem. As produced, SWNTs form ropes that are diameters in size and the tube-tube interactions within the ropes are weak, similar to the coupling between adjacent graphene planes in 3-D crystalline graphite. This weak intertube coupling is dominated by the van der Waals interaction, but contains a nonzero covalent component that has a significant effect on the vibration and electronic states of the CNT.

21 As the rope diameter increases [14], shear deformation reduces the effective moduli of nanoropes by an order of magnitude with respect to that of a SWNT. Thus, transferring charge to the inner tubes in a rope, and transferring the shear load through the rope are current roadblocks to realizing the theoretically predicted performance of CNT actuators. Boron Nitride Nanotubes (BNT) in chiral or zigzag symmetric structures are intrinsically polar and therefore the piezoelectric effect can be used to develop structural actuators. Most nanotubes have the chiral or helical arrangement of hexagons around the tube axis. The armchair or zigzag structures have high symmetry and the BN tubes are insulating. No satisfactory methods exist for controlling the chirality and producing aligned BNTs for actuator applications. The research efforts discussed verify that CNT based materials have extraordinarily high strength, superelasticity and actuation capability at the same time. Therefore, CNTs are potentially a game changing material [8] that can improve, by one or two orders of magnitude, the actuation properties of AFC materials. Practically, however, the actual performance of nanophase composites and CNT actuators has been far below the predicted performance, and significant advances must be made before useful implementation of CNT materials can be realized. This is shown in Table 1, where the theoretical Energy Density of CNTs is over two orders of magnitude greater than AFCs, but the achieved Energy Density of CNT actuators is comparable to AFCs.

22 Piezoelectric constitutive equations: D S = T ε d t d E E s T D T = ε - e S t e E E c S Material Elastic Modulus Y (GPa) Preliminary Properties of Active Materials Bandwidth (Hz) Voltage Range Density 3 ( Kg / m ) Peak Strain ε m Strain-Volt. Coeff. (peak strain/volt) e (N/VM) (1) test data Q.M. Zhang/R. Baughman (2) Theory based on molecular modeling by N.G. Lebedev, I.V. Zaposotskova, Volgograd State University, and L. Chernozatonskii, of the Inst. of Biochemical Physics, Russia. The coefficients represent stretch of NTs along the axis. d (M/V) Strain Energy 2 3 Yε / 2 ( J / m ) m Energy Density Y ε 2 / 2ρ ( J / Kg) Active Fiber Composite Kv x 10 e = 33 3 d = , Bundled SWCNT / x 10 N/A N/A 11,000 electrochemical actuator (approx) (approx) (estimated) (1) Nonbundled SWCNT / x 10 N/A N/A 29x 10 2,400 electrochemical actuator (est) (theory) SWCNT Piezoelectric properties (1) SWBNT Piezoelectric properties (2) m 8.25 (estimated) _ CNT(5,5) 4 e xx = 3.3x10 BNT(6,0) BNT(6,6) e zz = 9 d zz = 0.16 BNT(6,6) e = 0.04 zz

23 Although intensive research on carbon nanotubes has been performed, controlling length, diameter, and preferable alignment of CNTs is still a big challenge [18, 19]. Uniform synthesis of well-aligned carbon nanotubes with high quality is a prerequisite for their applications in sophisticated nanoscale devices. In particular, the electrochemical behavior of CNT strongly depends on the density, structure and purity of the CNT. Purification and functionalization appear to be essential in defining active surface area, charge transfer rate, and adsorption/desorption at the electrode surface. The synthesis of SWCNTs requires a catalyst and as of now there is no method known to control the chirality of the nanotubes produced. This is a problem for electrical/actuation applications, but the elastic behavior of CNTs is not strongly dependent on the chirality. Possibilities for using nanotubes for sensors for SHM are given next.

24 CNT Sensors using Raman Spectroscopy Raman spectroscopy can be used to characterize single-wall and multi-wall nanotubes. The intensities of the low frequency radial ( cm-1) and high frequency tangential ( cm-1) modes of single wall carbon nanotubes exhibit strong resonance effects related to the tube diameter, and the frequency of the radial mode is inversely proportional to the radius of the tube. Thus, measurements of the frequency of this mode can be used to determine the radius of SWNTs. Raman spectroscopy can also be used to map the stress distribution in the vicinity of discontinuities in a polymer using single wall nanotubes seeded in a composite material. Direction sensitive strain mapping using 0.1 % weight of randomly dispersed single wall nanotubes is also possible because the strain transferred from the composite matrix to the nanotubes causes a large wavenumber shift in the Raman spectrum. The limitation of the spectroscopy technique is that is it a NDI technique and would be difficult to use for insitu monitoring of structural health.

25 CNT Sensors using Conductance Measurements The conductance in carbon nanotubes can be significantly altered by functionalization. The conductance change can possibly be used for sensing by creating artificial nerves and monitoring their conductance. Phase-coherent transport in ropes of single wall nanotubes with damage to the ropes is shown to be strongly affected by temperature. The nanotube ropes may be useful as sensors of temperature, strain and chemical species. The dynamic conductance of nanotubes differs significantly from the DC conductance, displaying both capacitive and inductive responses. It has been shown that the resistance of carbon nanotubes remains constant regardless of their length or width, for lengths up to 5 microns. This property might be used to design long neural systems with nanotubes or ropes that are sensitive to nanoscale damages. This could improve the sensitivity of damage detection while using a small number of monitoring signals.

26 BNT Sensors/Actuators Equally interesting are non-carbon nanotubes, which have received much less attention in recent years and remain relatively unexplored. Similar to carbon, III-V compounds found in the hexagonal graphite structure could also lead to a microscopic tubular structure. The III-V material most closely related to C is boron nitride, which like carbon, is found in both sp2- and sp3- bonded structures. Hexagonal BN nanotubes have been theoretically predicted [20, 21] and experimentally observed for the first time [22]. This was followed by findings of other groups [23]. Thus, BN nanotubes have exceptional thermal, electrical and mechanical properties and may serve as a perfect nano-insulating tubular shield for any conducting material encapsulated within, and, moreover, the B-N bonding also tends to be dipolar. These characteristics offer the possibility of piezoelectric actuation tailored over a wide bandwidth. The electronic structure of such tubes is controlled by their chemistry rather than their geometry (helicity and diameter) as for their carbon counterparts. Analysis in the literature predicts low piezoelectric strain coefficients for BNT. Future generations of actuators, sensors, devices and active structures may be based on carbon, boron nitride and other nanotubes that have extraordinary actuation and mechanical properties

27 The production of high quality and bulk amounts of the carbon and, especially of BN nanotubes, is an exceedingly challenging scientific and technological problem. Current processing techniques such as plasma arc discharge, substitution reaction, laser ablation, electron beam irradiation, reactive ball milling, and thermal CVD need improvements to make nanotubes of controlled quality. Up to now, there has been limited success on mass production of carbon and particularly of BN nanotubes [62]. Also, the product often contains metal or carbon impurities and structural defects like poorly crystalline or polycrystalline walls, which may affect the electrical and mechanical properties of the tubes. Thus, development of new approaches for synthesis of carbon and BN nanotubes is required. Although the synthesis of BN nanotubes by Laser Ablation is a challenge that offers better control over process variables, only a few reports have addressed this issue [23-25]. The target material is usually prepared by mixing h-bn with metal catalyst powder followed by hot pressing and sintering. In some cases, no catalysts are used. The target is ablated by an excimer laser at 1200 C in an inert atmosphere of Ar, or N2 or He. Another approach utilizes single crystal c-bn specimens heated above 5000K by CO2 laser under nitrogen pressure of 5-15 GPa [25].

28 CNT Electronics CNT s are molecular quantum wires that can have conducting or semiconductor properties depending upon the exact geometry of the tube. Perfect tubes can have ballistic conductance in which the resistance is independent of the length and large currents can be carried. In a magnetic field, the resistance of the nanotube can be modulated. If a cobalt material that is magnetic is attached to the ends of the tube, the resistance of the tube depends on the relative orientation of the magnetization, and the direction of the electron spins must be maintained as they move along the tube. Spintronic sensors that exploit the spin rather than the charge of electrons may be possible. Intermolecular junctions of nanotubes can form nanoscale electronic devices. A metal-semiconductor kink junction behaves like a molecular diode. Single-molecule transistors have been made that consist of a semiconducting nanotube on two metal nanoelectrodes with a substrate that is a gate electrode. A small quantum dot that is the active element of a transistor was formed by introducing sharp bends in a nanotube. Single molecule nanotube digital logic circuits have also been formed. Based on these recent advances, molecular electronics has the potential to miniaturize health monitoring systems such as the artificial neural system discussed. This could enable damage detection at the molecular level by replacing sensor elements, operational amplifiers, diodes, wires, switches and signal processing instrumentation with nanotube equivalents. Full coverage of structures with sensors might then be possible. A major challenge will be to communicate between the possible millions of nanolevel sensors/computers and humans. Biomimitecs and neuroscience may provide these solutions.

29 Nano-sensors-actuators-composites development Carbon Nanotubes (CNT) and Boron Nitride Nanotubes (BNT) can be used to develop Smart Nanocomposites for health monitoring This work is a collaboration between the Mechanical Engineering and Materials Science and Engineering Departments at the UC, and the Mechanical Engineering Department at NCA&TSU Advantages of Nanotechnology Nanotubes are load bearing and can strengthen the composite Nanotubes may be actuated using electrochemical, piezoelectric and other effects (the electrochemical response of BNT is unknown) Nanotubes may be used to develop artificial nerves for health monitoring using piezoelectric or conductivity effects

30 Developing Piezo-Nanocomposites Top electrode Field direction BNT fibers in a flexible matrix Bottom Electrode Single Wall Boron Nitride Nanotube Boron nitride nanotubes for piezoelectric actuation

31 Developing Electrochemical-Nanocomposites Single Wall Carbon Nanotube CNTs coated with polymer electrolyte (black) form unidirectional ropes and a fiber that is actuated by setting up an electric field using interdigitated electrodes (red/blue) Concept for using CNTs and electrochemical process for actuation and sensing

32 5. CONCLUSIONS The ANS offers: Distributed sensing Parallel processing Continuous monitoring of the condition of the structure to prevent damage Detect Propagating Damage Warn of overstress and anticipate failure Predict the remaining life of the structure Nanotubes offer: Structural strength and sensing together Sensing at the molecular level Miniaturization of electronics

33 A new technology offers the possibility for large advances in SHM. BIO-NANOTECHNOLOGY The New Frontier in Materials and Structures, Health Monitoring, and Smart Structures

34 Skin is a multifunctional and layered material that can improve the design of structures

35 ACKNOWLEDGMENT This work is supported by the U.S. Army Research Office under contract grant number G DAAD ; the NSF Center for Advanced Materials and Smart Structures at NCA&TSU; and the NASA Center for Aerospace Research at NCA&TSU. The wave propagation code used to perform the simulations was developed by Dr. William N. Martin and Dr. Anindya Ghoshal. Endevco Corporation and CeraNova Corporation provided advice on developing the neural system and the active fiber sensor. Carbon Nanotechnologies Inc. provided advice on using the CNTs. All of this support is gratefully acknowledged.