sensing skin - materials

7/31/2019 Sensing Skin - Materials

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Polymer based interfaces as bioinspired smart skinsAdvances in Colloid and Interface Science 116 (2005) 165 178

The piezoelectric effect is observable in a wide array of crystalline substances that have asymmetric unit cells.

When an external force mechanically strains a piezoelectric element, polarized unit cells shift and align in aregular pattern in the crystal lattice. Discrete dipole effects accumulate, developing an electrostatic potentialbetween opposing faces of the element. Relationships between the force applied and the subsequent response ofa piezoelectric element depend on three factors: structure dimensions and geometry, material piezoelectricproperties, and mechanical or electrical excitation vector.

The wide variety of configurations and design options adopted in tactile sensing technology originates from theexploitation of many different transduction effects and materials capable of mechano-electric, mechano-magneticand mechano-optic conversion.

Piezoelectricity exists in some naturally occurring crystals such as quartz and rochelle salt, and it can be inducedin polymers such as nylon and copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE), or withtetrafluoroethylene (TeFE).


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The thermal sensing functionality of skin thermoreceptors can be accomplished in artificial structures byexploiting basically two physical effects: thermoelectricity and pyroelectricity.

The pyroelectric effect consists of the manifestation of a temperature-dependent spontaneous polarisation of amaterial. This means that a pyroelectric material subject to a temperature change displays a modified amount ofelectric charges on opposite surfaces. Likewise to piezoelectric sensors, these charges can be harvested bymeans of two electrodes disposed on the material surfaces. If a material shows pyroelectric properties, it is alsopiezoelectric.

Polymer based interfaces as bioinspired smart skinsAdvances in Colloid and Interface Science 116 (2005) 165 178

Pyroelectric properties have been reported for many types of organics, whose use as thermal radiation sensors

has been well demonstrated today. PVDF and P(VDFTrFE)) are certainly the most performing representatives ofthis class. They typically exhibit pyroelectric coefficients of about 25 AC/m2 K and 40 AC/m2 K, respectively.However, these values are one order of magnitude lower than the coefficient (380 AC/m2 K) measured for one ofthe most relevant ceramics, PZT.


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Polymers with significant piezoelectric properties possess them because of favorable aspects of their structurethat are often enhanced by fabrication techniques. An important aspect is crystallinity, which can be altered

materially by the preparation and manipulation of the polymer. The ferroelectric polymers polyvinylidene fluoride(PVDF), polyvinyl fluoride (PVF), several co-polymers and blends, and composites with ceramics such as PZThave received the greatest attention

Piezoelectric PolymersIEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1989

Among polymeric materials exhibiting piezoelectric

properties, the most used is polyvinylidene fluoride(PVDF). Polyvinylidene fluoride is not piezoelectric inits raw state, but can be made piezoelectric byheating/stretching under an electric field. A thin layerof metallization is applied to both sides of apiezoelectric sheet to collect the charge and permitelectrical connections being made.


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Piezoelectric Sensors and Sensor MaterialsJournal of Electroceramics 2:4, 257272, 1998

With reference to the periodic table, there are two major groups of active ions, both of which are near electronic``crossover'' points where different types of atomic orbitals are comparable in energy and where hybrid bondformation is prevalent.The first group, typified by Ti4+, Nb5+, and W6+, consists of d0 ions octahedrally coordinated to oxygen. For Ti4+, the

electronic crossover involves the 3d, 4s, and 4p orbitals, which combine with the sigma and pi orbitals of its six O2-

neighbors to form the (TiO6)

8- complex. The bond energy of the complex can be lowered by distorting theoctahedron to a lower symmetry. This leads to molecular dipole moments, ferroelectricity, large dielectricconstants, and piezoelectricity.A second group of active elements contributing to polar distortions in ceramic dielectrics are the lone-pair ionshaving two electrons outside a closed shell in an asymmetric hybrid orbital. Among oxides, the most important ofthese lone-pair ions are Pb2+ and Bi3+, which are involved in a number of ferroelectrics (PbTiO3, Bi4Ti3O12,PbNb2O6) with high Curie temperatures


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Piezoelectric Sensors and Sensor MaterialsJournal of Electroceramics 2:4, 257272, 1998

The first polycrystalline ferroelectric ceramic was barium titanate (BaTiO3), which has the perovskite structure. itremained the primary electroceramic material until the discovery of lead zirconate titanate (PZT)


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If one considers to replace the Pb ion while retaining the good ferro- and piezoelectric properties, one shouldcontemplate ions that have both high polarizability, i.e. a large radius and a high effective number of electrons andpossess a lone electron pair in an outer shell. Sb3+ and Te4+ fulfill the second requirement; their polarizabilities,however, are significantly smaller than that of Pb2+. Only Tl+ and Bi3+ meet both the requirements.Of the two, Tl is quite expensive, prohibiting its use on an industrial scale and, even more importantly, its toxicity

dwarfs even that of lead. From the atomistic point of view, bismuth-containing compounds thus seem to be themost likely successors to lead-based piezoelectrics.

Perspective on the Development of Lead-free PiezoceramicsJ. Am. Ceram. Soc., 92 [6] 11531177 (2009)


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Lead-Free Piezoelectric Ceramics vs. PZT?IEEE 2006

Compositionally, PZT ceramics lie near a morphotropic phase boundary (MPB) separating tetragonal

and rhombohedral phases, at ~ x=0.48 PT. MPB compositions have anomalously high dielectric andpiezoelectric properties as a result of enhanced polarizability arising from the coupling between twoequivalent energy states, i.e., the tetragonal and rhombohedral phases, allowing optimum domainreorientation during the poling process.

Recent reports on lead-free piezoelectrics can becategorized into two main perovskite families:

(1)K0.5Na0.5NbO3 (KNN),

(2)(2) Na0.5Bi0.5TiO3 (NBT)

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In KNN phase diagram morphotropic phase boundary was reported atx~0.5 separating two orthorhombic ferroelectric phases.

The enhancement in piezoelectricity of current KNN ceramics has beenachieved by compositionally shifting the TO-T downward to near roomtemperature with additives such as LiTaO3, LiNbO3, LiSbO3 and SrTiO3


L d F Pi l i C i PZT?


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Analogous to PZT, a morphotropic phase boundaryseparating ferroelectric tetragonal, andrhombohedral phases exists in the NBT phasediagram

Unlike that of the PZT system, the MPB is stronglycurve, and prior to the prototypic cubic transformation,a phase transformation to an anti-ferroelectric phase

occurs. The consequence of this transformation is aloss of polarization and thus piezoelectric activity.


BaTiO3 (BT) has a low Curie temperature (Tc = 120C) causing the working temperature range of this ceramicnarrow for actual piezoelectric applications. To increase the Tc of BaTiO3-based ceramics, a binary system ofBaTiO3 (Bi0.5K0.5)TiO3 (BKT) was investigated. The Tc of BKT was reported to be about 380C.

BNT is one of the important lead-free piezoelectric materials withperovskite structure. The main drawback of this material is its highconductivity.For the pure BNT system, d33 lies in the range of 57 64 pC/N asdiscovered. It has been reported that BNT-based compositions

modified with BaTiO3, BiKTiO3, NaNbO3, BiFeO3, MnO2, Sc2O3,La2O3, CeO2, etc.

L d F Pi l t i C i PZT?


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L d F Pi l t i C i PZT?


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Perspective on the Development of Lead free Piezoceramics


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Perspective on the Development of Lead-free PiezoceramicsJ. Am. Ceram. Soc., 92 [6] 11531177 (2009)

Th ti l St d th St ti P f f Pi l t i C i P l C it ith 2 2


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An older and more time-tested approach has been the incorporation of piezoceramics into composites.

Composites provide the capability of utilizing the best aspects of each component in the composite whileminimizing the poorest features. Most composite hydrophones consist of two phases: a stiff piezoceramic and asoft polymer. Newnham, et al., established the notation for describing the number of dimensions each phase isphysically in contact with itself. There are only ten ways in which two distinct components can be incorporated intoa single composite. They are given the notation of 0-0, 1-0, 2-0, 3-0, 1-1, 2-1, 3-1, 2-2, 3-2, or 3-3 [26]. To date,eight different connectivity types of two-phase piezoelectric composites ( piezocomposites) have been studied: 0-3, 1-3, 2-2, 2-3, 3-0, 3-1, 3-2, and 3-3. In the case of piezocomposites, the first number in the notation denotes thephysical connectivity of the active phase and the second number refers to the physical connectivity of the passive

phase.

Theoretical Study on the Static Performance of Piezoelectric Ceramic-Polymer Composites with 2-2ConnectivityIEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 40, NO.2, MARCH 1993

Theoretical Study on the Static Performance of Piezoelectric Ceramic Polymer Composites with 2 2


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Composites with 0-3 connectivity consist of a random array of piezoelectric particlesdispersed in a 3-D polymer matrix. The primary advantage of these composites is

their ability to be formed into shapes while remaining piezoelectrically active.The composite consists of fine PbTiO3 particles of a very narrow size distribution(centered around 20 mm) embedded in a chloroprene polymer matrix. A typicalthickness of a 0-3 composite sheet is 500 mm. Good bonding between the particlesand the rubber is necessary to achieve successful poling and subsequent goodpiezoelectric properties. The use of a polymer with a high thermal coefficient ofresistivity makes it possible to pole the composite at elevated temperatures by usingthe improved ceramic/ polymer resistivity balance to give saturation poling while still

retaining the high resistance and low loss at typical operating temperatures


Theoretical Study on the Static Performance of Piezoelectric Ceramic Polymer Composites with 2 2


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The mechanism of the stress transfer in composite structures under a static elastic stress (electric field) can beexplained as follows. Due to the difference in their elastic stiffness (piezoelectric properties), the polymer andceramic phases should deform different amounts under a given stress (electric field). However, the nonslipinterface forces the displacements of the two phases to be the same at the interface so that a shear stress isgenerated through this nonslip bonding.This inhomogeneous shear stress in turn produces additional nonuniform deformations in both the ceramic andthe polymer phases, producing enhancement to the response of the active harder ceramic at the expense of theresponse of the passive softer phase (the effect is the opposite for the converse piezoelectric response).Anamplification factor y has been defined, which is the ratio of the effective total stress on the ceramic and theactually applied external stress. This amplification factor y characterizes the effectiveness of the stress transfer ina given composite structure.It is found that the effective piezoelectric constant of the composite not only depends on the volume percentage ofthe active component, but also strongly depends on the aspect ratio and the configurations.


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