Smart Materials. Progettazione Molecolare dei Materiali - Lezione 14

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1 Smart Materials

2 How can a material be smart?

3 How can a material be smart? A smart materials has the ability of responding to external stimuli and to adapt his properties to the new environment Temperature They can be seen as sensors and actuators at the same time, transductors, and each one of these stimuli can be the input or the output of the transduction: temperature light/colour electric/magnetic field mechanical stress/strain Light/Color Smart materials Electric Magnetic fields Stress/Strain Mimosa Pudica reacts to external input (touching)

4 Smart responses Mechanochromic Shape memory Piezoelectric Thermochromic Pyroelectric Electrochromic

5 Piezoelectricity

6 Historical background Piezoelectricity (piezo = pressure) was discovered in 1880 by Pierre and Jacques Curie who measured superficial electrical charge generated under meccanical stress by some crystals (i.e. tormaline, quartz and Rochelle salts) After some time also the opposite effect (i.e. Mechanical deformation under an applied electrical voltage) was found During Second World War Langevin applied piezoelectric material for an underwater ultrasound source (sonar = SOund NAvigation and Ranging) placing a quartz element between two steel plates When the astonishing properties of Barium Titanate and Lead Zirconate Titanate where discovered the research on piezoelectric materials also spred to USA, URSS and Japan between 1940 and 1950

7 Piezoelectric crystalline classes Crystals are divided into 32 point group (classes) in dependence of symmetries (e.g. rotational, reflections) 21 of these 32 classes are non-centrosymmetric (not having a center of symmetry) 20 of these 21 are exhibit direct piezoelectricity If the dipole moment can be reversed by the application of an electric field, the material is said to be ferroelectric

8 Direct and inverse piezoelectricity Direct piezoelectricity: deformation (input) induces an electrical voltage (output) -> sensors Inverse piezoelectricity: an electrical voltage (input) induces a deformation (output)-> sensors

9 Some applications Lighters generate an electrical spark under pressure of a button Piezoelectric sensors: force and acceleration sensors, acoustic sensors Piezo actuators: used to precisely adjust the position of optical lenses and mirrors on nanometers scale

10 Examples of piezoelectric Natural piezoelectrics; Quartz Tormaline Rochelle salts LiNbO3 LiTaO3 Langasite (La3Ga5SiO14) Li2B4O6 ZnO Piezoelectric materials after polarization: Piezoceramics polycristallines (BaTiO3, PbTiO3, PZT, PbNb2O6) Piezocomposites (polymer-piezoceramic) Piezopolymers (Polyvinylidene fluoride PVDF, TrFE and TeFE copolymers)

11 Piezoceramics Polycristalline materials characterized by randomly distributed directions of grains Piezoelectricity arises only after polarization: electrical dipoles are oriented due to a high voltage electric field Advantages high efficiency of electro-mechanical uptaking easy to process easy to shape feasible for mass-production

12 Perovskite piezoceramics Perovskite crystalline structure (PZT, BaTiO3, PbTiO3) 1) If T > T C the crystal cell is cubic and symmetric (the crystal is not piezoelectric) 2) If T < T C the crystal cell is tetragonal and non-symmetric (the crystal is piezoelectric) The critical temperature (T C ) is called Curie temperature At this temperature there is a transition from a non-symmetric to a symmetric crystalline structure and the piezoelectricity is switched off Struttura della Perovskite neutra: O, Pb, Zr

13 Lead Zirconate Titanate (PZT) - PbZrO3 PbTiO3 Solid solution of ortorombic PbZrO3 (52, 54%) and tetragonal PbTiO3 (48, 46%) perovskitic structure Its properties and behaviour are related to the percentage composition of the two components Vibrational modes change in function of the relative direction of mechanical deformation (or electric field) and the polarization vector of the crystal lattice

14 Limits Depolarization: intense electric fields directed against the crystal polarization vector intense alternative electric fields high mechnical stresses temperature above Curie critical temperature Aging: Loss of piezoelectric properties with time from the induced polarization of the crystal Piroelectricity: Change in polarization state due to temperature

15 Piezoelectric coefficient The third axis is conventionally parallel to polarization vector Conventional axes system used to describe piezoelectric properties d ij, g ij, k ij, i = electric direction j = mechanical direction

16 Piezoelectric constitutive equations Piezoelectric coefficient for charge (or deformation) d Is the mechanical deformation induced by an applied electric field d = deformation electric field d = m short circuit charge density stress V C/m 2 N/m 2 Elevated d ij are required for piezoelectric transducers d ij = ε i E j Piezoelectric coefficient for tension g Is the open circuit electrical field induced by a deformation g = open circuit electric field stress V/m N/m 2 Elevated g ij are required for piezoelectric sensors g = deformation applied charge density m 2 C g ij = E i σ j

17 Direct and transverse coefficients a b Depending on the direction of the deformation the piezoelectric will respond differently (i.e. different d coefficients) - d 33 (d direct) the force is applied along the direction 3 (polarization axis) on the same faces that will develop the electric potential difference; the stress is parallel to the dipole vector, generating an increment of polarization in the same direction - d 31 (d transverse) the force is applied perpendicularly to direction 3 and the potential difference will be developed on the same faces of the direct case (b)

18 Electromechanical coupling coefficient Electromechanc coupling coefficient k It descrive the energy conversion from mechanical to electrical representing the piezoelectric efficiency of the material k = electrical charge mechanical energy High values of k ij are required for piezoelectric actuators Dielectric constant K Is the open circuit electrical field induced by a deformation K = ε ε 0 ε=dielectric permittivity of the material ε 0 =dielectric permittivity of vacuum g ij d ij K 0 To overcome electrical losses related to cabling high dielectric constant are required, but the increasing of K results in the reduction of the sensibility of the sensor due to the inverse relation betweem g and K

19 Constitutive equations ε = S σ ε = d E ε 1 ε 2 ε 3 ε 4 ε 5 ε 6 = E 1 E 2 E 3 = S 11 S 12 S 13 S 12 S 11 S 13 S 13 S 13 S E = g σ g 31 g 31 g S S S 66 Mechanical deformation 0 g 15 0 g Piezoelectric direct effect σ 1 σ 2 σ 3 σ 4 σ 5 σ 6 σ 1 σ 2 σ 3 σ 4 σ 5 σ 6 ε S d σ = D d t ε e E Complete constitutive equation ε 1 ε 2 ε 3 ε 4 ε 5 ε 6 = 0 0 d d d 33 0 d 15 0 d E 1 E 2 E 3 Piezoelectric inverse effect D 1 D 2 D 3 = D = ε e E ε ε ε 33 Electric displacement E 1 E 2 E 3

20 Piezoelectric composites Advantages: Improved sensibility Increased frequency range width Improved efficiency Possible design for specific applications

21 Piezoelectric polymers Polyvinylidene fluoride (PVDF) copolymers: TrFE (trifluoroetilene), TeFE (tetrafluoroetilene) They are usede at high frequencies and in substitution of piezoceramics when brittleness is an issue Pros: large range of frequencies low acoustic impedance high elastic deformability high dielectric strength Cons: low working temperatures weak electro-mechanic transmission

22 Polyvinylidene fluoride PVDF and his copolymers are semicrystalline Tg 40 C Tm 180 C Tc 100 C PVDF piezoelectricity is due to the possibility of polarize the crystalline phase This is done through mechanical deformation in an electric field In PVDF d 33, d 31 and d 15 have opposite direction respect to PZT This means that along the direction of electric field PVDF shrinks insted of elongating Perpendicularly to electric field direction it extends instead of contracting (as PZT)

23 Applications Reversing sensors Distance sensors in air Level sensors Ultrasound cleaners Ultrasound nebulizers SONAR (sound navigation and ranging) Surgical blades

24 Electro/Magnetostrictivity

25 Electrostrictivity Those materials does not have a linear behaviour like piezoelectric but the strain and the electric field have a quadratic correlation

26 Electrostrictive materials Are a class of materials capable of changing shape when an external electric field is applied and, viceversa, producing an electric field when a mechanical stress is applied Many of these materials has a perovskite structure but they mantain the symmetry and are not ferroelectric (they cannot be polarized) Different type of electrostrictors exist: Lead (Pb), Magnesium (Mg) and Niobate (Nb) containg ceramics (PMN) Electrostrictive polymers: irradiated PBDF films (Penn state), G- elastomer actuators (NASA)

27 Magnetostrictivity In absence of an external magnetic field the material is divided into different magnetic domains (Weiss domains) randomly oriented Under an external magnetic field these domains follow the field lines leading to a change of shape The intesity of deformation is related to the intesity of the applied field Magnetic domains orientation under an applied magnetic field

28 Magnetostrictive materials Magnetostriction was first found in Nickel in 1842 by James Prescott Joule Subsequently also Cobalt, Iron and their alloys were found as magnetostrictivite (strain max = 0.005%) In 1970 the Naval Ordinance Lab (NOL) found that Terfenol-D (Tb x Dy 1-x Fe 2 ) had more intese magnetostrictive responses (strain max = %) Joule effect the material deforms under a magnetic field (actuators) Villari effect the material changes his magnetic state under deformation (sensors)

29 Pyroelectricity

30 Fundamentals Of the 20 piezoelectric crystal classes 10 are polar and exhibit pyroelectricity All pyroelectric materials are also piezoelectric (but not all the piezoelectric are pyroelectric) Pyroelectricity is the property of generating electricity due to a change of temperature with time Do not confuse with thermoelectric effect (i.e. Peltier and Seebeck effects)

31 Pyroelectricity VS Thermoelectricity In pyroelectricity a change in temperature with time, with respect to a reference temperature T 0, (i.e. the whole crystal is uniformly heated) leads to a potential difference between to opposite faces of the crystal In thermoelectricity a spatial temperature difference leads to a potential difference between to region of the material/device Pyroelectricity Seebeck effect Peltier effect

32 Pyroelectricity mechanism Two contributions make up pyroelectric effect In the primary pyroelectric effect the crystal is rigidly clamped to prevent expansion or contraction A change in temperature causes a change in electric displacement as shown by the green line (dashed green line) The second contribution the secondary pyroelectric effect is a result of crystal deformation Thermal expansion causes a strain that alters the electric displacement via a piezoelectric process (dashed red lines)

33 Chromic materials

34 Electrochromic materials Electrochromism was discovered in 1968 by S.K. Deb and J.A. Chopoorian Some are smart windows and mirrors (e.g. darkening a window to control the inlet of sun light), active optical filters (e.g. sunglasses), displays and computer data storage A material is able to reversibly change its color when it is placed in a different electronic state by absorbing an electron (the materials is reduced) or by ejecting one (the material is oxidized) O + ne - R One way of making a working cell is by placing the electrochromic material between two transparent electrodes (usually Indium Tin Oxide, ITO) changing the potential of the cell by charging the electrodes

35 Electrochromic materials An example of an electrochromic material that is able to turn blue is EV (Ethyl Viologen) It is originated from the bipyridinium group where R' is equal to R and R' is equal to the ethylene group The coloring of EV from completely transparent to intense blue happens by absorption of an electron (reversible) If the EV-molecule absorbs a second electron it turns pale blue (irreversible and not wanted) Is possible to obtain a complete set of tones of the same color just by varying the applied voltage bipyridinium ethylene

36 Mechanochromic materials These materials are produced by incorporating trace amounts of excimer-forming photoluminescent chromophores into ductile host polymers The approach relies on the initial formation of nanometer-scale aggregates of the sensor molecules in the polymer matrix and exploits that deformation transforms the nanophase-separated systems into molecular mixtures This leads to a pronounced shift from excimer- to monomerdominated emission

37 Mechanochromic materials C1-RG31 and C18-RG15 chromophores poly(vinylidene fluoride) (PVDF) poly[(vinylidene fluoride)-co-(hexafluoropropylene)] (PVDF-HFP) Blends comprised of one of the dyes and one of the polymers are prepared using a melt mixing technique C1-RG31 and C18-RG15 chromophores Joseph Lott and Christoph Weder,

38 Thermochromic materials leuco dyes A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy Temperature-sensitive glass, film and wallpaper results from the application of thermochromic pigments which change color based on ambient, body or water temperature Temperature-sensitive mugs Temperature-sensitive glass tile Leuco dye Thermochromic film Mood ring Shi Yuan s thermochromic wallpaper

39 Thermochromic materials liquid crystals Some liquid crystals are capable of displaying different colors at different temperatures This change is dependent on selective reflection of certain wavelengths by the crystalline structure of the material as it changes between the lowtemperature crystalline phase Only the nematic mesophase has thermochromic properties restricting the effective temperature range of the material Thermochromic liquid crystals Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation Thermochromic liquid crystals

40 Photostrictivity

41 Photostrictivity Light-matter interactions that result in non-thermal sample deformation is termed photostrictivity The photostriction differs in origin depending on the type of investigated material The photostriction in electrically polar solids can be defined as photoinduced deformation of the lattice associated with a change in the internal electric field leading to a converse piezoresponse in the photovoltaic compounds Photostriction of SbSI single crystal In nonpolar semiconductors (e.g. Si, Ge) usually larger than a band gap creates an excess of electron-hole pairs in the conduction band, leading to deformation of the sample directly or via a change in atomic bonds in covalently bonded amorphous semiconducting materials In organic polymers, light can trigger a transformation in molecular structure within the same chemical formula inducing large volumetric changes Azobenzene attached to a cantilever

42 Photostrictivity in polymers A photosensitive organic molecules resulting in the lightinduced reorientation or ionization reaction are generally responsible for the photostriction mechanism in these materials The response and recovery times were in the range of tens of minutes needed for chemical structure transition A heating contribution may be present but independent measurements as a function of temperature had shown that contraction induced by light is much larger than a thermal deformation

43 Walking graphene origami A graphene oxide ribbon is functionalized on one side with an hydrophilic polymer (polydopamine) At equilibrium only the functionalized side of the ribbon absorbs moisture from the ambient Illumination with IR light increases the temperature of the ribbon leading to the desorption of water molecules and to the subsequent local contraction of one side This contraction generates a twisting moment that bends the ribbon Special folding of the ribbon (origami inspired) allows to induce a unidirectional movement cycling the IR exposure

44 Walking graphene origami

45 Bibliography - Piezoelectricity: - «Piezoelectricity_1.pdf» (2.1, 2.2, 2.3, read 2.5, 2.6, 2.7) - «Piezoelectric_constitutive_equations.pdf» - Electro/magnetostrictivity: - «Electrostrictivity_wiki.pdf» - «Magnetostrictivity_wiki.pdf» - Pyroelectricity: - «Pyroelectricity.pdf» (Pages 1-3, Applications excluded, Box 1 is extremely important) - «Thermoelectric_wiki.pdf» (Pages 1-3, Thomson effect excluded) - «ThermoelectricDevices.pdf» (useful to understand thermoelectric principle) - Chromic materials: - «Thermochromic_wiki.pdf» - «Mechanochromism.pdf» ( Abstract, Sample preparation and Conclusion paragraphs)