179
Original scientific paper
 MIDEM Society
Processing and sintering of sodium-potasium 
niobate–based thick films
Hugo Mercier1,2,3, Barbara Malič1,2, Hana Uršič1,  Danjela Kuscer1, Franck Levassort3
1Jožef Stefan Institute, Electronic Ceramics Department, Ljubljana, Slovenia
2Jožef Stefan International Postgraduate School, Ljubljana, Slovenia
3GREMAN UMR-CNRS 7347, Université de Tours, INSA Centre Val de Loire, Tours, France
Abstract: The electrophoretic deposition (EPD) and sintering of (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) thick films on platinized alumina 
substrate is reported. We demonstrate that by a two-step deposition-sintering the KNNSr films thicker than 30 μm without defects 
can be prepared. The effect of the sintering time on structural, microstructural, dielectric and electromechanical characteristics of the 
KNNSr thick films is discussed. By increasing the sintering time from 2 to 4 hours, the density and the dielectric permittivity of the thick 
films increased. The unit cell parameters of the perovskite phase decreased which could be related to the formation of polyniobate 
and volatilization of akalies. Processed KNNSr exhibited promising electromechanical and piezoelectric properties, with a thickness 
coupling factor up to 35 % and piezoelectric coefficient d33 up to 80 pC/N. 
Keywords: sodium potassium niobate; electrophoretic deposition; thick films, piezoelectric properties; electromechanical properties 
Priprava in sintranje debelih plasti na osnovi 
natrijevega kalijevega niobata
Izvleček: Študirali smo sintranje debelih plasti (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr), ki smo jih na metalizirani  korundni podlagi pripravili 
z metodo elektroforetskega nanosa. Pokazali smo, da z dvostopenjskim nanosom in sintranjem lahko pripravimo plasti KNNSr 
debelejše od 30 µm in brez defektov. V prispevku poročamo o vplivu časa sintranja plasti KNNSr na njihovo strukturo, mikrostrukturo 
ter dielektrične in elektromehanske lastnosti. Gostota in dielektrična konstanta plasti KNNSr se povečata s podaljšanjem časa sintranja 
od 2 na 4 ure, parametri perovskitne osnovne celice pa se zmanjšajo, kar pripisujemo tvorbi sekundarne faze poliniobata in izhajanju 
alkalijskih oksidov med sintranjem. Plasti KNNSr imejo obetajoče elektromehanske in piezoelektrične lastnosti: povprečni sklopitveni 
faktor je 35 %, piezoelektrični koeficient d33 pa do 80 pC/N.
Ključne besede: natrijev kalijev niobat; elektroforetski nanos; debele plasti; piezoelektrične lastnosti; elektromehanske lastnosti
* Corresponding Author’s e-mail:  hugo.mercier@univ-tours.fr
Journal of Microelectronics, 
Electronic Components and Materials
Vol. 47, No. 3(2017), 179 – 185
1 Introduction
Piezoelectric energy harvester (PEH) allows the conver-
sion of mechanical energy into electrical energy which 
can power wireless, self-powered microsystems and 
macroscale devices [1, 2]. A typical PEH structure is a 
bimorph cantilever with two layers of piezoelectric ma-
terial on both sides of a flexible substrate. 
Piezoelectric layers of lead-based materials, typically 
lead zirconate titanate, have been mainly used due to 
their outstanding piezoelectric properties [3]. However 
due to lead toxicity and environmental problem there 
is a need to replace them with environment-benign 
material. Among the lead-free piezoelectric materials, 
potassium sodium niobate ((K0.5Na0.5)NbO3 - KNN) have 
been extensively studied. The processing of a dense 
single phase KNN ceramics is challenging, due to the 
hygroscopic nature of the raw material, the narrow 
temperature range of sintering, and the volatilization 
of alkaline species at the processing temperature [4]. 
Literature reports that addition of 0.5 % Sr, as A-site do-
nor dopant, ((K0.5Na0.5)0.99Sr0.005NbO3 - KNNSr) improves 
the density and dielectric/piezoelectric properties of 
the material (d33 ~ 80 pC/N, kt ~ 0.4, dielectric permit-
tivity ~ 500 and dielectric losses ~ 0.03) [4, 5]. 
180
Bimorph cantilever PEH have been often prepared by 
thinning piezoelectric bulk ceramics from a few hun-
dreds to tens of micrometers and consequent gluing of 
slices [7,8] onto a substrate. This process is complicated 
and time consuming. Another possibility is to process 
the thick piezoelectric films directly onto a conductive 
substrate using thick film technologies. Among them, 
electrophoretic deposition (EPD) [9] allows the depo-
sition of piezoelectric thick films within a few minutes 
onto complex-shape substrates. EPD is suitable meth-
od for processing a cantilever, since the deposition can 
be done simultaneously on the top and bottom side of 
the substrate.  
KNN-based thick films have been prepared by EPD us-
ing ethanol-, acetone- and water-based suspensions 
[10–12]. In a previous work, the processing of KNNSr 
thick films prepared from ethanol-based suspension, 
and sintered at 1100 °C for 2 hours in air, was reported. 
The processed thick films were around 25 μm thick and 
they exhibited a relative density of 75 %, a dielectric 
constant of 294, dielectric losses of 0.05 at 1 MHz at 20 °C 
and a d33 of 60 pC/N [12]. 
The lower density of thick films compared to bulk ce-
ramic can be related to the sintering of the thick film in 
constrained conditions. The clamping of the thick film 
on the substrate may result in a tensile stress which 
hindered the densification of the layers [13, 14]. The 
stresses relax through growth of defects and or/delam-
ination of the thick film from the substrate [15].
In order to improve the density and functional prop-
erties of KNN-based thick films, the effect of sintering 
conditions were studied. It was shown that density of 
KNN-based material increases with the sintering time 
[16], but larger time may lead to the formation of larger 
amount of secondary phases [17]. Sintering of KNN-
based ceramics in oxygen atmosphere showed several 
advantages compared to sintering in air: increased 
density [18], limited amount of secondary phases [17], 
reduced oxygen vacancy concentration and improved 
electromechanical properties [19, 20]. 
The aim of this work was to process KNNSr thick films 
on platinized alumina substrate by EPD with final thick-
ness greater than 30 μm. In this case the use of a two 
steps deposition-sintering process was developed. 
Moreover the effect of sintering time, i.e., 2 and 4 hours, 
on the structure, microstructure, dielectric and electro-
mechanical properties of the thick films was studied. 
2 Experimental
(K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) powder was prepared 
by solid-state synthesis as described elsewhere [21]. 
Bulk KNNSr ceramics was prepared by compacting 
KNNSr powder with a uniaxial press into cylindrical 
samples, followed by cold isostatically pressing at 200 
MPa and sintering at 1120 °C in air with heating and 
cooling rates of 5 K/min.
The suspension was prepared by mixing 1 vol % of the 
KNNSr powder in absolute ethanol (C2H5OH, anhydrous, 
Carlo Ebra, Italy) with 50 μmol/g poly(acrylic acid-co-
maleic acid) (Mw 3.000, 50 wt % water solution, Sigma 
Aldrich, Germany) and 100 μmol/g n-butylamine (Alfa 
Aesar, Germany), as detailed in [12]. 
The substrate acting as a working electrode was pre-
pared by screen-printing platinum paste (Ferro 1192, 
USA) onto an alumina substrate (A493, Kyocera, Japan). 
The paste was deposited on a region of 8 x 8 mm and 
fired at 1200 °C for 1 hour in air. The counter electrode 
was a platinum disc (thickness of 0.1 mm and diameter 
of 8 mm). 
EPD were performed in a custom-made setup at a 
constant current density of 1.56 A/cm2 provided by 
a Keithley 2400 source meter. The layers were depos-
ited either in a single step for 120 s (sample denoted 
KNNSr-R) or in two steps. In two steps deposition, the 
first step included the deposition of KNNSr for 60 s, and 
the presintering of as-deposited layers at 1000 °C for 2 
h in oxygen with heating and cooling rate of 2K/min. 
Samples were denoted KNNSr-L. In the second step, 
second layers were deposited onto KNNSr-L for 60 s 
and afterward sintered at 1100 °C in oxygen for 2 hours 
(sample denoted KNNSr-2h) and 4 hours (sample de-
noted KNNSr-4h).
The X-ray powder-diffraction data (XRD) were collected 
in the 2θ range from 20° to 60° in steps of 0.034°, by 
PANalytical diffractometer (X’Pert PRO MPD, The Neth-
erlands). The phases were identified using the soft-
ware X-Pert High Score and the PDF-2 database [22]. 
The unit cell parameters were refined using JANA2006 
software, by performing a full pattern matching, as-
suming a P1m1 space group. A zero-error shift correc-
tion was used to obtain the correct initial position, the 
background was calculated using a Legendre polyno-
mial and the peak profiles were refined using a pseudo-
Voigt function. 
The polished cross-sections of the sintered films were 
analyzed using scanning electron microscopes (SEM, 
JSM-5800 and JSM 7600F, both JEOL, Japan). 
H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185
181
Gold electrodes, with a diameter of 1.5 mm and a thick-
ness of ~100 nm, were sputtered (5 Pascal, Italy) on top 
of the sintered samples. The capacitance and dielectric 
losses (tan δ) were measured in the frequency range 10 
kHz-1 MHz at room temperature with an impedance 
spectroscopy analyzer (4192A Hewlett Packard, USA). 
Samples were poled for 40 min at 120 °C with a DC-
electric field of 3 kV/mm. The piezoelectric constants 
d33 were measured with a Berlincourt piezometer (Take 
Control PM10, Birmingham, UK).
The complex electrical impedance, around the fun-
damental thickness mode was measured using a vec-
tor analyzer (HP4395) and its impedance test kit. A 
theoretical model, based on the Krimholtz-Leedom-
Matthaei (KLM) equivalent electrical circuit [23], deliv-
ers the theoretical complex impedance of the thick-
ness mode as a function of the frequency. By fitting 
the experimental data with the theoretical model, the 
thickness mode parameters of the KNNSr sintered and 
poled thick films are deduced. The modeled structure 
was composed of 3 layers: the alumina substrate; the 
Pt bottom electrode and the thick piezoelectric film. 
The top gold electrode was neglected as it was too thin 
to have a significant effect. The parameters of the inert 
layers [24] and the thickness of the piezoelectric films 
are considered as fixed in the model. The following pa-
rameters were deduced: the thickness coupling factor 
kt, the longitudinal wave velocity vl, the dielectric con-
stant at constant strain ε33
S/ε0 and the tangent of the 
dielectric loss angle δe. 
3 Results and discussion
3.1 Processing of KNNSr thick films
Sample KNNSr-R, deposited in a single step, exhibited 
defects at its surface, resulting in shortcuts, preventing 
the electrical measurements, see Figure 1.a. The thick-
ness of the as-deposited layer KNNSr-R was around 60 
μm and around 40 μm after sintering. This thickness 
was presumably above the critical thickness thus de-
fects occurred during the drying and/or the sintering 
of the thick film [25,26].  In order to process defect-free 
thick films with final thicknesses above 30 μm, a two 
steps process was used. A first layer was deposited and 
pre-sintered (KNNSr-L). KNNSr-L had a deposited yield 
of 3.5 ± 0.5 mg/cm2 and a sintered thickness of 18 ± 5 
μm and did not exhibit large defects after its process-
ing (Figure 1.b).
The second layer was deposited on the top of the first 
one (60 s) and sintered at selected conditions given for 
KNNSr-2h and KNNSr-4h. The cumulative deposition 
time for the two layers was the same as for KNNSr-R, 
i.e., 120 s. The processed thick films prepared with the 
described procedure did not exhibit large defects at 
their surfaces (Figure 1.c and d). 
3.2 Sintered KNNSr thick films 
The XRD patterns of the thick films sintered for 2 (KNNSr-
2h) and 4 hours (KNNSr-4h) are presented in Figure 2. 
Both samples consisted of a main monoclinic perovs-
kite phase indexed as K0.65Na0.35NbO3 (PDF: 77-0038), 
and a polyniobate phase indexed as K5.75Nb10.85O30 (PDF: 
38-0297). In addition the diffraction peaks of the top 
gold electrode and of the platinum bottom electrode 
were also observed on the diffraction pattern. The for-
mation of the polyniobate phase, which was not identi-
fied in the initial powder, may be related to the volatili-
zation  and segregation of alkaline species [4].
Figure 2: XRD patterns of KNNSr thick films sintered at 
1100 °C in oxygen for 2 and 4 hours. x: K0.65Na0.35NbO3 
(PDF: 77-0038), o: K5.75Nb10.85O30 (PDF: 38-0297), Δ: Plati-
num (PDF:04-0802), □: Gold (PDF: 01-1172).
H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185
Figure 1: SEM image of the surface of sintered KNNSr 
thick films a) KNNSr-R b) KNNSr-L c) KNNSr-2h and d) 
KNNSr-4h.
182
The calculated cell parameters for the samples KNNSr-
2h and KNNSr-4h are presented in Table 1. The calcu-
lated unit cell parameters exhibited lower values than 
the one reported in the literature for bulk ceramics with 
the same composition [27]. The reduction in the unit 
cell parameters may be attributed to two phenomena. 
First phenomenon was the loss of alkali oxides during 
the sintering. Literature reports that potassium vapor 
pressure above KNN is greater than sodium one [28,29]. 
Since potassium ions have a larger radius (K+, 0.164 nm) 
than sodium ions (Na+, 0.139 nm) the perovskite with 
K/Na ratio smaller than 1 resulted in a decrease of the 
unit cell parameters of the perovskite phase [29,30]. In 
the case of a thick film this phenomenon is expected 
to be enhanced due the increased area to volume ratio 
in comparison to bulk ceramic. The second phenom-
enon was the formation of a polyniobate phase, clearly 
observed on the XRD spectra. The polyniobate phase 
had a potassium-rich composition, thus the perovskite 
phase could have lower potassium content and this 
again decreased the unit cell parameters of the per-
ovskite phase. On XRD patterns peaks attributed to the 
polyniobate phase had higher intensity in the samples 
KNNSr-4h in comparison to KNNSr-2h which indicated 
the presence of a larger amount of polyniobate phase 
and may explain the further reduction in the unit cell 
parameters of KNNSr-4h. 
The polished cross-section SEM images of the sintered 
thick films KNNSr-2h and KNNSr-4h, are presented in 
Figure 3.a and b, respectively. The microstructure of the 
samples consisted of micrometers sized pores randomly 
distributed in a KNNSr particles matrix, see insets Figure 
3. Phase composition by EDXS showed that the matrix is 
a perovskite with K/Na ratio of about 1. Larger grains (see 
black arrows on the SEM images) had a K/Na ratio great-
er than 1 and were presumably polyniobate phase. The 
relative density, estimated from the SEM images were 77 
± 4 % and 87 ± 4 % for KNNSr-2h and KNNSr-4h, respec-
tively. This showed that by increasing the sintering time 
from 2 to 4 hours, the density of the thick films increased 
by 10 %. The thickness of KNNSr-2h and KNNSr-4h were 
33 and 45 μm, respectively. The discrepancy in the thick-
ness was attributed to the variation in the as-deposited 
layers thickness, i.e. ± 5μm, for each deposited layer.
Table 1: Refined unit cell parameters of the KNNSr thick films sintered at 1100 °C in oxygen for 2 h (KNNsr-2h) and 4 h 
(KNNSr-4h) and unit cell parameters reported for the KNNSr ceramics.
Sample
a b c β V
[nm] [nm] [nm] [°] [nm3]
KNNSr-2h 0.40028(9) 0.39443(6) 0.39961(9) 89.70(2) 0.06309(4)
KNNSr-4h 0.40016(9) 0.39431(6) 0.39946(9) 89.69 0.06303(2)
KNNSr bulk [27] 0.400375 0.394596 0.399938 90.3228 0.06318
Estimated error ± 0.0001 ± 0.0001 ± 0.0001 ± 0.001 ± 0.00002
3.3 Functional characterizations
The dielectric measurements of the thick films are pre-
sented in Figure 4. The permittivity and losses decreased 
with increasing frequency. Values of KNNSr-4h permit-
tivity and losses are higher than those of KNNSr-2h in the 
same measured frequency range. The dielectric permit-
tivity and losses at 100 kHz were 230 and 0.04, respec-
Figure 3: Polished cross-section SEM images of KNNSr 
thick films sintered at 1100 °C in oxygen for a) 2 hours 
(KNNSr-2h) and b) 4 hours (KNNSr-4h). The insets show 
KNNSr thick films microstructures at larger magnifica-
tion; the black arrows show examples of larger grains 
(polyniobate). 
H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185
183
tively, for KNNSr-2h against 280 and 0.13, respectively 
for KNNSr-4h samples. The increased permittivity may 
be related to the increased density of the thick film. The 
increased losses at low frequencies are presumably re-
lated to the increased electrical conductivity in the sam-
ple. This may be due to the increased amount of second-
ary phases and/or to the humidity during the dielectric 
measurement which can strongly impact the dielectric 
permittivity and losses at low frequencies (under 10 kHz) 
in strontium doped KNN ceramics [31].
The piezoelectric and electromechanical properties of 
KNNSr-2h and KNNSr-4h are presented in Table 2. The 
experimental and theoretical complex impedances of 
the KNNSr-2h sample are presented in Figure 5, show-
ing the good agreement between the adjusted KLM 
scheme and the experimental data.
For both KNNSr-2h and KNNSr-4h samples, high d33 co-
efficients around 80 pC/N were obtained. Similar elec-
tromechanical properties for KNNSr-2h and KNNSr-4h 
samples were measured with kt up to 35 %. Dielectric 
constant, kt and d33 of the thick films were lower than 
the one of the processed KNNSr bulk ceramics, which 
is related to the lower density of the thick films, their 
deviation from the stoichiometry KNN, and the polyni-
obate phase. The thick films electromechanical prop-
erties were similar to values reported in the literature 
[32]. This demonstrates that the thick films prepared by 
EPD were at the state of the art in terms of electrome-
chanical properties. 
4 Summary
KNNSr thick films have been processed by EPD on plati-
nized alumina substrates using a two steps deposition 
method followed by sintering at 1100 °C in oxygen for 
2 and 4 hours. The structural, microstructural, dielectric 
and electromechanical properties have been reported. 
The increasing sintering time from 2 to 4 hours leads 
to higher density, and reduction in unit cell parameters 
which was related to the formation of a polyniobate 
phase and the volatilization of alkali oxides. Dielectric 
constant of the thick film sintered for 4 hours was 280 
at 100 kHz and room temperature which is higher value 
compared to the samples sintered for 2 hours and in 
agreement with its increased density. The electrome-
chanical performance stayed relatively stable with kt 
up to 35 % and d33 up to 80 pC/N.
Table 2: Properties of thick piezoelectric films KNNSr-2h, KNNSr-4h, KNNSr bulk ceramic and KNN-based screen print-
ed thick films.
Sample
Thickness kt ε33S/ ε0 δe d33
[μm] [%] [%] [pC/N]
KNNSr-2h 33 35 155 3 80
KNNSr-4h 45 32 165 5 80
KNNSr-bulk ceramic 1120 °C, 2 h, air / 40 300 5 90
KNN-based screen printed thick films [32] 43 30 140 / /
Figure 4: Dielectric permittivity (square) and dielectric 
losses (triangle) of the KNNSr thick films sintered at 
1100 °C in oxygen for a) 2 hours (□,△, KNNSr-2h) and b) 
4 hours (■,▲, KNNSr-4h).
H. Mercier et al; Informacije Midem, Vol. 47, No. 3(2017), 179 – 185
Figure 5: Complex electrical impedance (Re(Z): real 
part, Im(Z): imaginary part) of KNNSr-2h thick film on 
platinized alumina substrate as a function of the fre-
quency around fundamental resonance (■: experimen-
tal;  ̶̶̶̶̶ ̶̶ ̶  : theoretical).
184
5 Acknowledgment
This work was supported by the Slovenian Research 
Agency (P2-0105), the French Research Agency (ANR 
14-LAB5-004), the bilateral project BI-FR-16-17-PRO-
TEUS-009/PHC PROTEUS 2016 (No. 35246NJ) and the 
Erasmus+ programme. The authors would like to thank 
the Center of Excellence NAMASTE for the use of equip-
ment, Jena Cilenšek, Silvo Drnovšek, Maja Majcen and 
Hafsa Znibrat for their technical assistance.
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Accepted: 22. 11. 2017
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