Aml series piezoelectric materials green energy and sensing - crossing point

Piezoelectric materials – a crossing
between the sensing electronics and
the “green” energy
Mariya Aleksandrova
Technical University of Sofia, Department of Microelectronics,
Kliment Ohridksi blvd, 8, bl. 1, Sofia 1000, Bulgaria
m_aleksandrova@tu-sofia.bg
1
WebSymposium and WebSession on Structural and Engineering Materials, 12.02.2021

Outline:
• Problem definition
• Current and future trends in the field of piezoelectric
converters
• Flexible piezoelectric energy harvesting and sensing elements –
materials selection
• Nanostructured piezoelectric energy harvesting and sensing
elements – technology aspect
• Perspectives and future work
2

• Problem definition
3
- Portable and smart electronic devices are powered by batteries whose life is limited.
The conversion of energy from various sources at the surrounding environment (energy
harvesting) solves the problem of battery-less power supply and satisfies the modern
concepts of "clean" energy. However, not all of them solve the problem with the
compactness.
The main kinds of renewable energy sources.
Microsoft Clip Art, Mark Fedkin, Faculty, Department of Mechanical and Nuclear Engineering / Dutton e-
Education Insitute. College of Earth and Mineral Sciences, The Pennsylvania State University
- Normally, the energy harvesting devices are bulk converting elements with relatively
large scale. Their dimensions should be decreased, but their efficiency should not.

4
One of the mechanisms for electrical energy generation is piezoelectric effect, which relies
on mechanical activation (vibrations, pressure, force). A major obstacle for
implementation of the thin film piezoelectric generators (PEG) is their low efficiency of
about 10-15%.
Yang et al., Joule, Volume 2, Issue 4, 18 April 2018, Pages 642-697
Common vibration sources
The piezoelectric direct and
converse effect
https://www.sintef.no/en/expertise/sintef-materials-
and-chemistry/materials-and
nanotechnology/piezoelectric-materials-for-sensors-
actuators-and-ultrasound-transducers/

5
More sophisticated applications of the
piezoelectric nanogenerators
• Current and future trends in the field of piezoelectric converters
These applications require to design compact thin-film piezoelectric nanogenerators,
lightweight, sensitive to weak activating stimuli and battery-less, if possible. They use
motion or small displacement in order to convert force or pressure change into electrical
energy due to the piezoelectric effect. Such elements use thin films grown on plastic
substrates. Because of the small thickness and activating volume these devices are
inefficient (~0.5-1 µW).

6
Most of the suitable piezoelectric materials are ferroelectric, which significantly broaden their
field of application, including ferroelectric memories (FeRAM), accelerometers and tire
pressure sensors in the automotive industry, acoustic measurements for estimation of noise
isolation, vibrational diagnostics for equipment condition monitoring, complex sensor
systems for measurement of the ambient temperature and atmospheric pressure, infrared
sensors for motion detecting, etc.
https://news.skhynix.com/tag/feram/
K. Hema, IJSR, ,Volume 2 Issue 5, 2013
https://acrobotic.com/products/sen-00020
https://ec.kemet.com/blog/pyroelectric-sensors-for-
motion-gas-food-or-flame/

7
More sophisticated applications
of the piezoelectric sensors
T. Trung, N. -E. Lee, Advanced Materials , Vol.28, 4338-4372, 2016
Important application, driving the development of this kind of material is the possibility to
integrate sensor and self-sustainable power supply in one device – biomedical implant or
outside body wearable element harvesting energy from natural motions of internal organs
and measuring the regularity of their motion as an indication for the health condition.

8
2016 2017 2018 2019 2020 2021
0
200
400
600
800
1000
1200
1400
already exists
Num
ber
of
papers
Year of publication
Distribution of the papers with keywords piezoelectric energy converters
over the last 5 years, including also the existing publishing activity in 2021:
a) distribution by year; b) distribution by paper type.
9%
12%
78%
book
chapters
review
papers
research papers
Mariya Aleksandrova, Advanced Materials Letters, 11 (10), 20101561, 2020
Interest to the topic

9
Basic requirements:
To the energy harvesting elements
1) Technological compatibility with the flexible (low temperature processes on plastic
substrates) with a suitable patterning of the piezoelectric areas (reverse lift-off
photolithography) so that they experience maximum mechanical load;
2) Application of piezocomposites, i.e. mixture of oxide piezoelectric materials with
appropriate polymeric additives, influencing its elemental composition and facilitating
the polarization effects;
3) Application of piezoelectric films with micrometric thickness by using thick-film
technology (such as screen printing) for higher throughput;

10
To the sensing elements
1) Technological compatibility with the conventional MEMS technology – sputtering and
conventional direct photolithography with possibility to control the formation of the
crystal lattice and the domain regions by tuning the microstructural and morphological
characteristics as well as the thicknesses of the layers, which increases the internal
potentials and the magnitude of the polarization.
2) Nanostructuriing of the piezoelectric materials to improve the sensor properties of the
device. Preparation of complex oxides and polymers with controllable morphology at
nanoscale level, in particular nanorods, nanotubes, nanodendrite formations, with
adjustable size, in order to increase the specific area and thus to improve its sensitivity by
achieving a high surface-to-volume ratio across the total material thickness.
3) Using the ferroelectric nature of these materials to fabricate multifunctional 2-in-1 sensor
– piezoelectric and pyroelectric.
In general
Insufficient theoretical and experimental data are yet available, demonstrating the
influence of the substrate type, electrode materials, composite or hybrid type piezoelectric
materials, deposition process and patterning of the coatings and the nanostructuring on the
efficient and stable work of the PEGs and pressure sensors at deformation.

11
Compared to the stationary PEGs, the flexible ones should fulfil specific requirements
such as great durability, ability to follow curves, being light weight and tiny and being
able to convert efficiently deformation into electricity at small activating vibrations or
forces. For the elements, serving as sensors, the most important is integration with the
rest components of the microelectromechanical system (MEMS), which are based on
the silicon technology. Therefore, the plastic polyethylene terephthalate (PET),
polyethylene naphthalate (PEN) and the deep etched silicon substrates are the basic
substrate materials.
• Flexible piezoelectric energy harvesting and sensing elements –
materials selection
Mariya Aleksandrova, Micro and nanoengineering conference (MNE’19), September
2019, Rhodes, Greece.
Mariyia Aleksandrova et al., 2019 IOP Conf. Ser.: Mater. Sci. Eng. 618 012014
Flexible PEG on plastic substrate
Flexible pressure sensor
on silicon membrane

12
Starting point – to replace lead-zirconium titanate (PZT) film with eco-friendly one
Potassium niobate KNbO3 – lead-free thin films grown by RF sputtering with stable
behavior at multiple bending for energy harvesting purposes
70 75 80 85 90 95 100
6.5
7.0
7.5
8.0
8.5
9.0
9.5
P(Ar)
= 10
-3
Torr
P(Ar)
= 10
-2
Torr
Deposition
rate,
nm/min
Plasma power, W
Deposition rate of KNbO3 films on
PEN as a function of the plasma
power and sputtering gas pressure
The average roughness of the film was 4.6 nm per total thickness of 901 nm, produced at 0.7 kV, 4.1
nm roughness per 870 nm film thickness , produced at 0.8 kV and 2.7 nm per 740 nm film thickness,
produced at 0.9 kV. The effective value of the generated voltage and current for the optimal case of
deposition (the smoothest film) were 431 mV and 5.394 µA, respectively. They were produced from
active area of 4 cm2 under stress application of up to 3 kg at frequency of 50 Hz.
AFM images, showing surface roughness of the
KNbO3 films prepared at different RF sputtering
voltages a) 0.7 kV; b) 0.8 kV and c) 0.9 kV.
M.P. Aleksandrova, et al., Energy 205 (2020) 118068

13
0.5 1.0 1.5 2.0 2.5 3.0
200
250
300
350
400
450
fresh
after 1000 cycles
generated
voltage,
mV
mass load, kg
Piezoelectric output voltage versus stress
applied by vertical compressive force for sample
produced at optimal sputtering conditions
2D AFM images and analysis report of the coating
surface before and after 1000 cycles of bending
Shape of the piezoelectric voltage generated
during vertical press-release vibrational motion
of the sample at 3 kg.
The number of cracks is less than 10 on a
scanned field of view 188 µm x 141 µm
(width ~ 1.2 µm, length between 1.7 and
3 µm, depth 190 nm).
No significant degradation of the output voltage
was recorded after 1000 cycles of movement, as
compared with the fresh samples. The decrease
of the voltage was approximately 2.2 %.

14
0.7 0.8 0.9
0.0
0.5
1.0
1.5
2.0
2.5
sputtering voltage, kV
output
power,

W
20
40
60
80
contact
resistance,
Ohm
0 10 20 30 40
50
100
150
200
250
d
33
pC/N
load, g
Power produced from the KNbO3 based thin film
flexible energy harvesting and contact resistance
as a function of the sputtering voltage.
Piezoelectric coefficient at small
mechanical load after the optimum
film deposition.
The achieved values of the piezoelectric coefficient of > 250 pC/N at 40 g are untypically
high for lead-free and non-nanostructured coatings. The reason could be the strong
asymmetry of the unit lattice of the material, leading to fluent charges separation even in
thin films. This is one of the main advantages of the proposed energy harvesting.
The contact resistance varied in the range 83-19 Ω, according to the sputtering voltage
and decreased with the sputtering voltage increase. This can be ascribed to the lowest
roughness, high density and regularity of the thin KNbO3 film.

15
Zinc oxide doped by gallium (GZO) - lead-free thin films grown by RF sputtering on
silicon for sensing purposes
The doping approach is applied to increase the conductivity of the piezoelectric films, and
thus enhance the power produced and the sensing ability. The most promising results for
achieving controllable and reproducible morphology of the films seem to exist for Ga-
doped ZnO. However, the focus of the known studies was on the optical characteristics in
solar cells on glass substrates, and less on the piezoelectric response of the films.
XRD patterns of Ga-doped ZnO thin films
on silicon as a function of the deposition
voltage and pressure.
0.7 kV, Ar=2.10-2
Torr 0.9 kV, Ar=2.10-2
Torr 1.1 kV, Ar=2.10-2
Torr
1.3 kV, Ar=2.10-2
Torr 1.1 kV, Ar =9.10-4
Torr 1.1 kV, Ar=6.10-1
Torr
97.49 nm
0.00 nm
94.03 nm
0.00 nm
103.37 nm
0.00 nm
194.04 nm
0.00 nm
192.21 nm
0.00 nm
215.98 nm
0.00 nm
AFM images of the Ga-doped ZnO
nanocoatings sputtered on silicon
wafers at different conditions.
M Aleksandrova et al 2019 IOP Conf. Ser.: Mater. Sci. Eng. 618 012014

16
0 20 40 60 80 100
20
40
60
80
100 U=1.1 kV
U=0.9 kV
d
33
,
pC/N
f, Hz
Piezoelectric coefficient d33 as a function of
the vibration frequency for GZO sputtered at
0.9 kV and 1.1 kV (constant pressure of
2.5x10-2 Torr) on Si (100) .
It was found that film with rms roughness ~6 nm (thickness ~103 nm) and dominant
orientation of crystallites (002) grown at sputtering voltage 1.1 kV and Ar=2.10-2 Torr,
exhibited the highest piezoelectric coefficient of 98.3 pC/N for frequency of 16 Hz at 800
g. The sensor is characterized by high sensitivity and broad operational range, as compared
to similar sensors using other lead-free piezoelectric materials, due to the optimized
sputtering conditions of GZO films.
0 200 400 600 800
20
40
60
80
100
Voltage,
mV
Load, g
Piezoelectric voltage versus load weight
in the range of 2-800 g, produced during
the membrane deflection.
M Aleksandrova et al 2019 IOP Conf. Ser.: Mater. Sci. Eng. 618 012014

17
Barium strontium titanate (BaSrTiO3 or BST)
Scanning electron microscopy (SEM) of
Ba0.5Sr0.5TiO3 film with high surface to
volume ratio grown at optimized sputtering.
Capacity variation against the number of
bending cycles for devices with conductive
polymer PEDOT:PSS as electrode coating.
• Successfully fabricated flexible thin-film energy harvesting element, containing lead- free
piezoelectric material, as well as polymer based electrode.
• The BST film’s surface is characterized with high surface to volume ratio, resulting in high
capacity of µF for film thickness of 420 nm, and area of 3 cm2 – suitable for sensor device.
• The device is characterized with improved mechanical durability due to the presence of
conductive polymer as bottom electrode - the capacity variation was not greater than 17%
up to 32 000 bending cycles, and the device resisted 50 000 cycles without degradation.
100 200 300 400 500
0
1
2
3
4
5
C,

F
Number of bending cycles x 100
Mariya Aleksandrova, et al. Proceeding of the IEEE Conference International Spring Seminar of Electronics 2018, Zlatibor, Serbia

18
• RF sputtering at plasma voltage 0.7 kV resulted in a formation of Ba0.5Sr0.5TiO3 thin film
with enhanced piezoelectric response in the low and middle-to-high frequencies.
• The device works efficiently in the low-frequency range of mechanical stimulation,
where it exhibits low dielectric losses of ~10-3 and generates average voltage of 800 mV,
thus providing a power density of 2.34 µW/cm2.
• Maximal piezoelectric coefficient d33 was found to be 10.8 pC/N and this value is
superior compared to the results available in the literature for similar devices, especially
considering the PEDOT:PSS contact, which is less conductive and non-Ohmic.
Dielectric losses for Ba0.5Sr0.5TiO3 device
demonstrated the highest performance.
Oscillograms of the voltage generated from
harvesting device with BST film produced at
sputtering voltage of 0.7 kV.

19
Piezoelectric polymer PVDF-TrFE
• Polyethylene naphthalate (PEN) plastic substrates
125 μm.
• Screen printing of PVDF-TrFE polymeric ink with
mesh count of 180 n/cm and wire diameter of 30
µm was used and the deposition parameters were
squeegee speed of 200 mm/s and pressure of 3
bars. The film thickness was approximately 3 μm.
• Gold films was DC sputtered.
• Aluminium and silver electrode films were vacuum
thermally evaporated.
poly[(vinylidenefluoride-co-
trifluoroethylene] - P(VDF-TrFE)
• low Young modulus of 0.61 GPa,
• favourable for durability at bending
and twisting substrates
Screen-printing of polymeric
P(VDF-TrFE) ink
a) bottom electrode patterns b) flexed PEN/Au c) completed structure
Photos of the prepared samples
M. Aleksandrova et al., 2020, IOP Conf. Ser. Mater. Sci. Eng. 876, 012006

20
1 2 3
0
200
400
600
800
Al
Au
Ag
piezoelectric
voltage,
mV
types of metal electrodes
rectangular
meander
side comb
Comparison of the metals and their pattern
suitable for PVDF-TrFe piezoelectric flexible
elements at mass load of 100 g and 50 Hz.
Piezoelectric voltage generated from
PEN/Ag/PVDF-TrFe/Ag with rectangular
electrodes at mass load of 100 g, 50 Hz.
0 200 400 600 800 1000 1200 1400
300
400
500
600
700
800
side comb
meander
rectangular
p
ie
zo
e
le
ctric
vo
lta
g
e
,
m
V
number of bends
0 200 400 600 800 100012001400
400
500
600
700
800
900
side comb
meander
rectangular
piezoelectric
voltage,
mV
number of bends
0 200 400 600 800 100012001400
200
250
300
350
400
450
500
550
600
side comb
meander
rectangular
pipezoelectric
voltage,
mV
number of bends
Stability of the piezoelectric voltage produced from PVDF-TrFe samples with different
metal electrodes and different patterns
silver gold
aluminum

21
• The speed of degradation is strongly dependent on the electrode shape.
• The side comb configuration exhibited the most stable behavior (less than 2%
degradation), because the ink adhere mostly to the PEN substrate.
• It was also found that there is relation with the wetting ability of the electrode
materials from the polymeric ink during printing.
Ag electrode Au electrode Al electrode
rectang. meand. s.comb rectang. meand. s.comb rectang. meand. s.comb
UPE, mV 916 840 446 800 768 362 528 400 258
ΔUPE @1500
bends, % 12.3 8.6 1.5 17.5 11.4 1.9 16.4 9.5 1.7
Polymer
microstru-
cture
Smooth, uniform and
dense coating
Relatively low
density coating
Fine granular, homogeneous
P, µW ~22.8 ~13 ~6.3
M. Aleksandrova et al., 2020, IOP Conf. Ser. Mater. Sci. Eng. 876, 012006

22
One of the approaches to increase the functional area by keeping the device small-sized is
to make film’s nanostructuring and increase its specific surface area per unit volume. This
is expected to enhance the piezoelectric device’s parameters, such as signal magnitude
and sensitivity. One possible approach is to use amorphous seed sub-layer,
• Nanostructured piezoelectric energy harvesting and sensing
elements – technology aspect
SEM images of the surface morphology of nanobranched ZnO and Ga-doped ZnO
grown on seed PEDOT:PSS conductive polymer , which is amorphous layer.
• The specific nanostructure of the ZnO, capturing variety of frequencies and broadening
the working frequency range of the harvesting device due to the nanobranches with
different lengths.
• The critical limits for the device functioning in term of mechanical loading are 11 000
bends at dynamic loading (350 gr mass loading and 10 Hz) and 3.6 kg at static loading.
M. Aleksandrova et al, Appl. Sci. 2017, vol. 7; M. Aleksandrova, Microelectron. Eng., 2020, vol. 233

23
5 10 15 20 25 30 35 40 45
0
50
100
150
200
250
300
350
piezoelectric
voltage,
mV
mass load, g
PEDOT: PSS
bottom electrode
Al bottom
electrode
10 20 30 40 50 60 70 80
0
2
4
6
8
10
strain
current,

A
current
0
1
2
3
4
5
6
7
power
density,

W/cm
2
power density
Generated piezoelectric voltage from PEN/bottom electrode/Ga-ZnO/top electrode
without (left) and with PEDOOT:PSS (right) film as electrode, i.e. nanostructured film.
Electrical parameters of nanostructured Ga-ZnO with PEDOT:PSS seed layer under cyclic
bending for harvesting purposes (left) and under strain (right) for sensing purposes.

24
Nanowires of KNbO3 were grown by filling nanoporous templates of both side opened anodic
aluminum oxide (AAO) through radiofrequency vacuum sputtering for multisensor
fabrication. The precise geometrical ordering of the AAO matrix led to well defined single axis
oriented wire-shaped material inside the pores.
Sensing ability of piezoelectric oxide nanowires grown in template of nanopores
Fabrication process of nanoporous anodic aluminum oxide
M. Aleksandrova, Materials 2020, 13, 1777; T. Tsanev, M. Aleksandrova, Adv. Mat. Lett., 2020, 11 (10)
SEM images of the top view of AAO pores with different filling degree

25
2 4 6 8 10
270
300
330
360
390
420
450
40 g
piezoelectric
voltage,
mV
nanowire length, m
Distribution of the elements inside the AAO nanotubes along their length and piezoelectric
response of elements with nanostructured materials with different nanowires length
Cross section of nanowires with different length sputtered in AAO template

26
The magnitude and shape of the voltage from the nanostructured KNbO3 film is strong
and repetitive without fluctuations. This approach of nanostructuring results in
voltage doubling (here shown for 10g/50Hz).
10 20 30 40 50 60
1
2
3
4
5
6.3 m
1.3 m
10 m
pyroelectric
voltage,

V/m
2
C
o
temperature, C
o
Pyroelectric coefficient of the KNbO3
nanowires as functions of their length –
sensor two-in-one.
M. Aleksandrova, Materials 2020, 13, 1777;
T. Tsanev, M. Aleksandrova, Adv. Mat. Lett., 2020, 11 (10)

27
• It was obtained higher generated voltage, due to the increased surface to volume
ratio of the piezoelectric material, for the nanostructured coating as compared to
the non-structured.
• The nanostructuring affects the piezoelectric response to a great extent, while the
pyroelectric response seemed less sensitive to the length of the nanowires.
• The prepared multi-sensor device is able to detect linearly mass loading ranging
from 10 to 50g with a frequency of 50 Hz. It was observed stable pyroelectric
voltage in the temperature range 10-40 oC.
• To the best of the authors’ knowledge this is the first time study of multisensing
device involving combination of AAO template and ferroelectric KNbO3 nanowire
material, combining piezoelectric and pyroelectric principle of operation.

28
Perspectives and future work
New piezoceramic materials will be developed for high-power applications, as the low-
power applications are well covered by the current state-of-the-art.
Considerable demand from the medical industry is expected to drive the market. The
development of energy autonomous biomedical devices will have important effect on the
healthcare industry.
The future prospective is for development and optimization of commercial wearable and
implantable components as the next-generation portable devices. Among them, it is
expected that the fiber-based self-powered systems will be dominant technology for the
wearable devices’ fabrication.
Some problems are still unsolved and great efforts are required to overcome the
technological challenges. Specific applications need properties like adaptability, durability,
shape and size that have to be customized and in some cases – personalized.
In the context of the present events and the COVID-19 pandemic, it seems that the
functionality offered by the piezoelectric converters will be transformed into sensing
ability for virus detection through advanced surface acoustic wave (SAW) devices.

29
THANK YOU FOR YOUR ATTENTION!
m_aleksandrova@tu-sofia.bg
The author acknowledges Bulgarian National Science Fund, grant number KP06-H27/1.

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