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A review of smart materials – from shape memory alloy, piezoelectrics

and ZnO nanotubesPrincipal sources of information:

Smart Materials and Structures, IOP, Bristol, UKJ. Intelligent Materials, Systems and Structures. Technomic, PA.Sensors and Actuators. Elsevier, New YorkMicromechanics and Microengineering, Nanotechnology. IOP.

B. Culshaw. Smart Structures & Materials. Artech House, 1996.M. V. Gandhi & B. S. Thompson. Smart Materials and Structures. Chapman & Hall 1992.Worden, Bullough and Haywood. Smart Technologies. World Scientific, 2003.

Some definitionsSmart Structure: A structure capable of sensing and/or adapting to changes in its environmentSmart Material: A material which can sense a change in its environment, produce a change in response to an external stimulus or both, I.e. it can sense and actuate

Smart System: A system comprising a smart material, a smart structure and intelligent processing

Candidate Smart Materials:PIEZOCERAMICS (PZTs)PIEZOPOLYMERS (PVDFs)ELECTROSTRICTIVE CERAMICS (PMNs)MAGNETOSTRICTIVESSHAPE MEMORY ALLOYS (SMAs)ELECTRO RHEOLOGICAL FLUIDS (ERFs)MAGNETO RHEOLOGICAL FLUIDS (MRFs)POLYMER GELS

Smart Structures Technology Evolutionary Process

StructureExternal Disturbances

Response

Passive

Structure

Controller

Actuators Sensors

External Disturbances Response

Active: actuators and sensors monofunctional

Actuators Sensors Structure

Controller

Active, Smart: actuators and sensors multifunctional

Actuator/sensor capabilities - 1

Actuator Sensor

PZT X XPMN XMAGNETOSTRICTIVE XSMA X XERF XMRF XGELS X X

Actuator/sensor capabilities - 2

Characteristics PMN ERF Magnetostrictives SMA PZT

Cost M M M L M

Technical maturity F F F G G

Networkable Y Y Y Y YEmbed ability G F G Ex Ex

Linearity F F G G G

Response (Hz) 1-20 K 0-12 K 1-20 K 0-5 1-20 K

Max microstrain 200 --- 200 5000 200

Max Temp. (Celsius)

300 300 400 300 300

M – medium, L – Low, F – Fair, G – Good, Ex - Excellent

Piezoelectric Materials/Actuators

Piezoelectric and electrostrictive materials are both ferroelectrics which share many common features. If a piezoelectric material is deformed, electric dipoles are generated creating a potential difference. For this to occur, the crystal lattice should have no centre of symmetry, I.e., the material crystal structure is anisotropic. There is also the converse effect: if a potential difference is applied to the crystal structure, a deformation takes place.

Commercial PZTs are created by a “poling” process. This requires the material to be exposed to temperatures above the Curie point while imposing a high electric field in a desired direction (“poled”) and then cooled to “lock-in” the crystal structure.

Poling direction

Piezoelectric Materials/Actuators

(http://www.nec-tokin.com/english/product/piezode

vice2/ceramicgyro.html)

(http://www.dynasen.com/images/gauges/cnstraingauge.gif)

(http://www.p-wholesale.com/upimg/5/297a1/piezoelectric-accelerometers-ca-yd-

117-658.jpg)

Constitutive Relationsεσ c=

σε s=DE ⋅= β

ED ⋅= ε

{ } [ ] { } [ ]{ }σdEεD T += { } [ ] { } [ ]{ }σsEdε T +=coupled

33131333

13111231

13121131

3331313

00000000000000000000000000000000000000000000000000000000

σσσσσσ

εεεεεε

sdsssdsssdsssdddd

Piezoelectric actuationConsider a thin slab of ZnO, deposited (typically by sputtering) so that its 3 axis is normal to the plane of the slab. L is the length of the free film

Electric field:

tVE −=3

Film free Zero stresses From constitutive equations

LL ∆,

dEdtt 3

333333 ==∆=ε

31331 −=−=∆

This effect can be put to good use as an actuator since by extending or contracting a pair of ZnO or PZT parches attached to a structure one induces positive of negative bending moments

Constitutive data for selected piezoelectric materials

From: S. D. Senturia. Microsystem design.Kluwer Academic Publishers, 2001.

Basic surface actuation principles - 1

Green: PZT patch actuator (a)

Grey: beam structure (s)

Uniform Strain Approach: the strain is uniform along the actuator thickness and there is perfect bonding between structure and actuator. The model is valid when:

as tt > >

Equilibrium of forces and moments at the interface between actuator and beam:

sFtM = (1)

Assuming pure bending:

IMy s==σ (2)

surface 2

ssurface E

σε = (3)

Stress and strain in actuator are:

F=σ βσε −−=A

Piezoelectric induced strain

Displacement compatibility gives:

Equating (4) and (7) we obtain the induced strain on the interface between actuator and structure:

AS εε =

β−−=AAs

+−= βε

If the beam has rectangular cross-section and inserting the piezoelectric strain term:

The curvature induced in the structure is, by definition:

Equations (9) and (8) imply that for maximum control, the ratio of structure to actuator thickness should be small.

S tε2=Φ

Actuator locationFor maximum actuation performance, the PZT actuators should be located in zones having the highest curvature. For cantilever beams, curvature functions are easily obtained from second partial differentials of the mode shape functions. These identify nodal strain regions to be avoided if modal control with PZT is to be effective.

No strain nodes Strain node at 0.216 x/L

σλλ

ϕ sinsinhcoscosh Mode shape i

Definition of Shape Memory Alloy - 1

Shape Memory Alloys (SMAs) are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. The SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite. In addition, the martensite can be in one of two forms: twinned and detwinned. A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs. The key effects of SMAs associated with the phase transformation are pseudoelasticity and shape memory effect. (From: D. Lagoudas,)

Definition of Shape Memory Alloy - 2Upon cooling in the absence of applied load the material transforms from austenite into twinned (self-accommodated) martensite. As a result of this phase transformation no observable macroscopic shape change occurs. Upon heating the material in the martensitic phase, a reverse phase transformation takes place and as a result the material transforms to austenite. Four characteristic temperatures are defined in SMA: martensitic start temperature (M0s) which is the temperature at which the material starts transforming from austenite to martensite; martensitic finish temperature (M0f), at which the transformation is complete and the material is fully in the martensitic phase; austenite start temperature (Aos) at which the reverse transformation (austenite to martensite) initiates; and austenite finish temperature (Aof) at which the reverse phase transformation is completed and the material is the austenitic phase.

Thermomechanical-induced transformation -1 If a mechanical load is applied to the material in the state of twinned martensite (at low temperature) it is possible to detwin the martensite. Upon releasing of the load, the material remains deformed. A subsequent heating of the material to a temperature above A0f will result in reverse phase transformation (martensite to austenite) and will lead to complete shape recovery. The above described process results in manifestation of the Shape Memory Effect (SME).

Pseudoelastic behaviour It is also possible to induce a phase transformation by applying a pure mechanical load. The result of this load application is fully detwinned martensite and very large strains are observed. If the temperature of the material is above A0f, a complete shape recovery is observed upon unloading, thus, the material behavior resembles elasticity. Thus the above-described effect is known under the name of Pseudoelastic Effect.

Training SMA The superelastic behavior constitutes an approximation to the actual behaviour of SMAs under applied stress. only a partial recovery of the transformation strain induced by the applied stress is observed. A small residual strain remains after each unloading. Further cooling of the material, in the absence of applied stress, is now related to the occurrence of a macroscopic transformation strain contrary to what is observed in the SMA material before cycling. The thermomechanical cycling of the SMA material results in training process. Different training sequences can be used, i.e., by inducing a non-homogeneous plastic strain (torsion, flexion) at a martensitic or austenitic phase; by aging under applied stress, in the austenitic phase, in order to stabilize the parent phase, or in the martensitic phase, in order to create a precipitant phase (Ni-Ti alloys); by thermomechanical, either superelastic or thermal cycles. The main result of the training process is the development of Two-Way Shape Memory Effect (TWSME). In the case of TWSME, a shape change is obtained both during heating and cooling. The solid exhibits two stable shapes: a high-temperature shape in austenite and a low-temperature shape in martensite. Transition from the high-temperature shape to the low-temperature shape (and reverse) is obtained without any applied stress assistance.

(Courtesy of R. Gotthardt, EPFL, SUI)

Surface control actuation (DARPA and Boeing)

The flaps on a wing generally have a large hydraulic system attached at the point of the actuator connection. "Smart" wings, which incorporate shape memory alloys, involve a more compact and efficient layout, in that the shape memory wires only require an electric current for movement.

The shape memory wire is used to manipulate a flexible wing surface. The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current, which is easily supplied through electrical wiring, eliminating the need for large hydraulic lines. By removing the hydraulic system, aircraft weight, maintenance costs, and repair time are all reduced

Surgical toolsBone plates are surgical tools, which are used to assist in the healing of broken and fractured bones. The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets. Repairs like this fall into an area of medicine known as osteosynthesis.

Bone plates can be fabricated in titanium and steel, but also using shape memory alloys, in particular nickel titanium. Using a bone plate made out of NiTi, which has a transformation temperature of around Af much greater than 15 C surgeons follow the same procedure as is used with conventional bone plates. The NiTi plates are first cooled to well below their transformation temperature, then they are placed on the set break just like titanium plates. However, when the body heats the plate up to body temperature the NiTi attempts to contract applying sustained pressure on the break or fracture for far longer than stainless steel or titanium. This steady pressure assists the healing process and reduces recovery time. There are still some problems to consider before NiTi bone plates will become commonplace. Designing plates to apply the appropriate amount of pressure to breaks and fractures is the most important difficulty, which must be overcome

SMA honeycombsThe rationale behind the concept is to obtain a core material with the same structural properties of classical honeycombs (high shear stiffness per unit weight, compressive strength) but also with actuation authority, in order to be implemented in deployable structures.

Single ribbon stress-strain loading at austenite phase (96 C)

A 6 X 4 cell honeycomb has been manufactured using a Ni-Ti ribbon from Medical Technologies@ (SME_NTC01_OX_0.20x6.4 mm). Each ribbon was shaped in a steel mould in half unit cell honeycomb. A cold joining approach (cyanoacrilate) has been taken to maintain the ribbon temperature below transformation. The aspect ratio of the cells is one, with average internal cell angle of 40o (OX honeycomb)

Stress Strain For SMA Honeycomb

0 1 2 3 4 5

Strain (%)

Experimental

Analytical

Zinc Oxide nanostructures - 1

(Tu, C. Z. and Hu, X., 2006. Phys. Rev. B 74, 035434)

Zinc Oxide nanostructures - 2

(Tu, C. Z. and Hu, X., 2006. Phys. Rev. B 74, 035434)

Wurtzite nanocrystals

Zinc Oxide Nanotubes - 1

(Chowdhury, R. and Adhikari, S. and Scarpa, F., 2010. Physica E 42, 2036)

Buckingham potential:

(Chowdhury, R. and Adhikari, S. and Scarpa, F., 2010. Physica E 42, 2036)

(Chowdhury, R. and Adhikari, S. and Scarpa, F., 2011. App. Phys. A 102, 301)

Vibrational properties of ZnO NTs

Equivalent material for ZnO behaves anisotropically

MEMS based on ZnO nanowires - 1

(Desai, A. V. and Haque, M. A., 2007. Sens. Act. A 134, 169

MEMS based on ZnO nanowires - 2

(Desai, A. V. and Haque, M. A., 2007. Sens. Act. A 134, 169

Piezoelectric polymers with CNTs - 1

(Ramaratnam, A. and Jalili, N. 2006. J. Int. Mat. Syst. Struct. 17, 199

Poly(vinylidene fluoride) (PVDF) films doped with MWCNTs – SWCNTsUsed as strain gauge sensors over a cantilevered beam

Piezoelectric polymers with CNTs - 1

(Ramaratnam, A. and Jalili, N. 2006. J. Int. Mat. Syst. Struct. 17, 199

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