How to handle Piezotech products?

This section provides technical information about how to process Piezotech® P(VDF-TrFE) and P(VDF-TrFE-CTFE/CFE) products.

How to formulate?

Piezotech® P(VDF-TrFE) and P(VDF-TrFE-CTFE/CFE) polymers are soluble in different solvents. Polymer concentration must be tuned in order to obtain the appropriate viscosity corresponding to your printing process.

Polymer dissolution can be achived with very good mechanical stirring under moderate heanting (below 80 °C). A filtration step is required to achieved high purity inks.

 

Table of possible solvents for Piezotech® polymers.

 

Boiling Point (°C)

Flash Point (°C)

Acetone

56

- 18

Tetrahydrofuran

65

- 17

Methyl Ethyl Ketone

80

- 6

Methyl Isobutyl Ketone

118

23

Glycol Ethers

118

40

Glycol Ether Esters

120

30

N-Butyl Acetate

135

24

Dimethyl formamide

153

67

Cyclohexanone

157

54

Dimethyl acetamide

166

70

Diaceton Alcool

167

61

Diisobutyl Ketone

169

49

Tetramethyl urea

177

65

Ethyl Aceto Acetate

180

84

Dimethyl Sulfoxide

189

35

Trimethyl phosphate

195

107

N-Methyl-2-Pyrrolidone

202

95

Butyrolactone

204

98

Isophorone

215

96

Triethyl phosphate

215

116

Carbitol Acetate

217

110

Propylene Carbonate

242

132

Glyceryl triacetate

258

146

Dimethyl Phtalate

258

149

How to dry and anneal?

Drying - Solvent Evaporation 

 

In order to get rid of any residual solvent and enhance film properties, a solvent evaporation step under atmospheric pressure or under vacuum can be carried out below the solvent’s boiling-point temperature. Evaporation strongly depends on evaporation facilities and air flow. It can thus be performed from room temperature to 140 °C, depending on the product grade.

 

 

Annealing

 

Annealing is a critical step to obtain the films with the best properties. It will control crystallization of the material and enhance electrical as well as mechanical properties.

 

For P(VDF-TrFE) copolymers

Annealing should be performed between the Curie temperature and the melting point, in the paraelectric phase, for 2-15 min. This corresponds to a 135-145 °C range. For Piezotech® FC20, 80/20 P(VDF-TrFE), the 140-145 °C range is preferred.

 

For P(VDF-TrFE) copolymers, annealing should be performed in the paraelectric phase, as close as possible to the melting.

 

 

For P(VDF-TrFE-CTFE/CFE) terpolymers

Films may need to be annealed at an appropriate temperature during a variable time from 3 to 20 min, typically 5-15 °C below the melting point. A rapid annealing may be obtained on thin films using Infrared or Flash annealing.

How to pole?

In order to activate their piezoelectric and ferroelectric properties, P(VDF-TrFE) copolymers layers must be poled to orientate the C-F dipoles. Contact poling with top and bottom electrode, or non-contact Corona poling with only the bottom electrode can be carried out.

 

 

Typical process for contact poling

 

AC or DC volage can be applied, at room temperature, up to twice the coercive field: 100 V.µm-1.

For AC, frequencies in the 0.1-10 Hz are advised.

Example of DC (blue) and AC (red) voltage ramps to pole a P(VDF-TrFE) layer

 

Example

An electric field is applied according to the following cycle:

Voltage ramp: 0 V.µm-1 to 100 V.µm-1

Frequency: 0.05 Hz (higher is possible)

Number of cycles to reach 100 V.µm-1: 15

Signal wave form: Sinusoidal

 

Typical ferroelectric loop of a P(VDF-TrFE) copolymer poled at 100 V.µm-1

How to monitor poling?

Contact poling or Corona poling can be monitor by the current flowing through the P(VDF-TrFE) layer. 

 

 

Case of contact poling

 

For contact poling with AC voltage, the current between top and bottom electrodes is proportional to the sum of the resistive current, capacitive current and polarization current. After substracting the capacitive and resistive components of the raw current, polarization current can be extracted and after integration give the ferroelectric loop.

 

How to measure piezoelectric signal?

Charge sensors.

 

Concepts: A charge sensor generates on its surface equal electric charges of opposite sign when it is stimulated by a mechanical force (piezoelectric effect) or a variation of temperature (pyro electric effect).

 

Equivalent diagrams: A piezo- pyro- electric sensor can be modelled by one of the following equivalent diagrams:

 

Thevenin equivalent diagram: The electromotive force e = Q/C0 is in series with a capacitor C0.

 

Norton equivalent diagram: The current source i = dq/dt is in parallel with a capacitor C0.

 


Mode of operation: When using a piezo- pyro- electric sensor, the measured signal strongly depends on the conditions of experimentation. It is possible to measure a proportional signal, or to its derivate. The limit of these operation modes is given by the cut-off frequency of your instrumentation chain.

 

 

Signal conditioning

 

Direct connection: By connecting your sensor directly to your amplifier, oscilloscope, or DAC (data acquisition card) it must be kept in mind that external impedances may have a critical influence on the signal you will measure. In order to determine the said influences, the involved impedances (from the sensor, cable and measuring instrument input) are modelled as follows:

 

 

Equivalent diagram of a direct connection of a sensor.

 

Further analysis of the diagram shows a high-pass filter behaviour,for which the cut-off frequency is given by:

 

When f >> fc, the measured signal is directly proportional to the generated charges: When f << fc, the measured signal is proportional to the derivate:
 

 

 

Charge transducer

 

In order to drastically limit the influence of external impedances, the use of a charge transducer is recommended. The operating principle is based on the use of an external capacitor C to store the generated charges. A charge transducer may be also be called a charge-voltage converter.

 

Practical implementation of a charge-voltage
transducer

 

As shown in the diagram, we consider the operational amplificator as ideal. The differential voltage between its inputs is null, so the current i generated by the sensor is driven to the feedback capacitor C. In order to prevent saturation of the output by parasitic influences, a parallel resistor R must be added to the feedback capacitor.

 

Such a circuit has a high-pass filter behaviour, for which the cut-off frequency is given by:

 

When f >> fc, the measured signal is directly proportional to the generated charges: When f << fc, the measured signal is proportional to the derivate:
 

Piezoelectric basic calculations

Deformation under electrical field

 

The deformation caused by the applied electric field can be defined by the simple formula:

Si = d3i.E

Where:

  • Si = Δxi/xi = ΔL/L represents the strain (relative deformation) of the sample in the xi direction
  • d3i is the piezoelectric strain constant (in a perpendicular direction with respect to that of the applied electric field)

 

 

Example of an element with the following characteristics:

 Length: l = 1 cm

Width: w = 2 mm

Thickness: t = 9 µm

Applied voltage: V = 200 V            →        E (electric field) = V/t

(d31, d32, d33) = (11, 10, -30) pC.N-1

 

The piezo strain constant d is given by:

d = strain developped/applied field = S/E

 

 

 

 

Sensors: Orders of Magnitude, Voltage generation

 

The electric field output (E) caused by an applied force can be defined by:

 E = giji

Where:

  • σi is the applied mechanical stress (N.m-2)
  • gij is the piezoelectric voltage constant (Vm.Nm-2)

 

g = electrical field developed/applied mechanical stress = E/σ

with g = d/ε = d/εrε0

 

 

Example of an element with the following characteristics:

Length: l = 2 cm

Width: w =2 cm

Thickness: t = 100 µm

Compression 0.1 Bar           →        σ = -10000 N.m-2

(g31, g32, g33) = (216, 19, -339) *10-3 - Vm.Nm-2

 

E = V0/t = g333

 

V0 = g333.t = 339 mV

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