225
Original scientific paper
Journal of Microelectronics, 
Electronic Components and Materials
Vol. 54, No. 3(2024), 225 – 233
https://doi.org/10.33180/InfMIDEM2024.306
Cold Sintering of Perovskite–Perovskite Particulate 
Composite Based on K
0.5
Na
0.5
NbO
3
 and BiFeO
3
 
Samir Salmanov
1,2
, Danjela Kuščer
1,2
, Mojca Otoničar
1,2
1
Electronic Ceramics Department, Jožef Stefan Institute, Ljubljana, Slovenia
2
Jožef Stefan International Postgraduate School, Ljubljana, Slovenia
Abstract: The cosintering of particulate composites often presents a challenge due to the conventionally high processing 
temperatures used. Inter-diffusion of species between two phases and their volatilization, formation of secondary phases, and 
cracking or delamination of ceramics are effects that hinder the coupling of functional properties and reduce the final responses of 
such composites. This is particularly relevant when producing perovskite–perovskite composites from phases that are conventionally 
sintered at different temperatures (Ts), such as K
0.5
Na
0.5
NbO
3
 (KNN; Ts≈1100 °C) and BiFeO
3
 (BFO; Ts≈800 °C). Obtaining high-quality 
KNN–BFO multifunctional composite was the goal of this study. We demonstrate herein that the low-temperature sintering technique 
known as the Cold Sintering Process (CSP) can be utilized for producing particulate perovskite–perovskite composites, specifically 
KNN–BFO. We show that cold-sintered KNN–BFO composites have a dense microstructure, good phase-to-phase contact, are crack-
free and exhibit ferroelectric properties. Their dielectric and ferroelectric properties are strongly affected by the fraction of KNN, 
increasing polarization saturation, while BFO aids in increasing their dielectric breakdown strength.
Keywords: cold sintering process; ferroelectric; particulate composite; BiFeO
3
; K
0.5
Na
0.5
NbO
3
Hladno sintranje prahov perovskitov na osnovi 
K
0.5
Na
0.5
NbO
3
 in BiFeO
3
Izvleček: Sosintranje dveh različnih praškastih keramičnih materialov v kompozitne strukture predstavlja izziv, saj keramika zahteva 
pripravo pri visokih temperaturah. Pri teh pogojih, difuzija ionov med dvema fazama, sublimacija, tvorba sekundarnih faz ter 
nastanek razpok ali delaminacija keramike vplivajo na lastnosti kompozita in poslabšajo njegove funkcijske lastnosti. To je še posebej 
pomembno pri izdelavi kompozitov iz dveh perovskitnih faz, ki se običajno sintrajo pri različnih temperaturah (Ts). Tak primer so 
kompoziti na osnovi K
0.5
Na
0.5
NbO
3
 (KNN; Ts≈1100 °C) in BiFeO
3
 (BFO; Ts≈800 °C) feroelektričnih sestav, KNN-BFO, katerih pripravo in 
lastnosti obravnava ta študija. Dokazali smo, da je mogoče tehniko nizkotemperaturnega sintranja, znano kot postopek hladnega 
sintranja (CSP), uporabiti za izdelavo kompozitov iz prahov dveh različnih feroelektričnih perovskitnih materialov, KNN in BFO. Pokazali 
smo, da imajo hladno sintrani kompoziti gosto mikrostrukturo, dober stik med fazami, so brez razpok in izkazujejo feroelektrični odziv. 
Na povišanje dielektričnih in feroelektričnih lastnosti močno vpliva delež KNN v kompozitu, ki povečuje polarizacijsko nasičenost, 
medtem ko BFO pripomore k povečanju njihove dielektrične prebojne trdnosti.
Ključne besede: hladno sintranje; feroelektriki; kompozit; BiFeO
3
; K
0.5
Na
0.5
NbO
3
* Corresponding Author’s e-mail: samir.salmanov@ijs.si
How to cite:
S. Salmanov et al., “Cold Sintering of Perovskite–Perovskite Particulate Composite Based on K
0.5
Na
0.5
NbO
3
 and BiFeO
3
" , Inf. Midem-J. Micro-
electron. Electron. Compon. Mater., Vol. 54, No. 3(2024), pp. 225–233
1 Introduction
Ferroelectric perovskites are widely used in a variety 
of electronic devices such as capacitors, sensors, ac-
tuators, and transducers [1]. Among environmentally 
friendly lead-free materials, K
0.5
Na
0.5
NbO
3
 (KNN) is a 
suitable candidate to replace Pb(Zr
1-x
Tix)O
3
 (PZT) in 
piezoelectric applications, and BiFeO
3
 (BFO) [2]–[4] is 
widely investigated and used for multiferroic coupling 
or high-temperature piezoelectric applications. Com-
bination of the two functional materials in the form 
of a multiferroic composites with properties that can 
be fine-tuned by adjusting amounts of each individual 

226
S. Salmanov et al.; Informacije Midem, Vol. 54, No. 3(2024), 225 – 233
phase could be beneficial for use in electronic devices, 
integrated as a single multifunctional component [5].
However, processing of perovskite-perovskite particu-
late composites by a conventional cosintering route is 
challenging due to different sintering temperatures of 
materials of different composition, volatilization of el-
ements and their diffusion coefficients. In the case of 
KNN and BFO ceramics, KNN densifies in a narrow tem-
perature range at 
~
1100 °C, close to its melting point of 
1140 °C [2], while BFO densifies at ~300 °C lower tem-
perature of 800 °C. Above ~950 °C BFO decomposes 
into liquid phase and Fe-rich phase and is fully melted 
at 1100 °C [4], [6]–[8]. It was also reported that cosinter-
ing of KNN and BaFe
12
O
19
 at 1125 °C led to volatilization 
of alkalies, resulting in decreased relative density of the 
composite [9].  It was further demonstrated for CoFe
2
O
4
 
(CFO) and PZT composite that Fe
3+
 can diffuse into 
PZT, and Ti
4+
 can diffuse into CFO already at 900–1000 
°C [10]–[12]. This diffusion facilitates secondary phase 
formation, limiting functional-property coupling be-
tween the two phases, and can cause creation of oxy-
gen vacancies, leading to enhanced conductivity of the 
composite [10]. In an attempt to limit such diffusion, in 
the case of cosintering of BFO and BaTiO
3
 [13], as well 
as BFO and PZT [14], low sintering temperatures were 
used, but the microstructure was highly porous.
Beside the different sintering temperatures of the com-
posite phases and with that the possible interdiffusion 
of species between the two phases during cosinter-
ing, forming a third solid-solution phase, producing 
composites is challenging also due to different ther-
mal expansion coefficient (TEC) of individual phases. 
The latter may cause strains at the interfaces of differ-
ent phases and microcracks [15], [16] during cooling 
of the composite from the processing temperature 
due to different shrinkage parameters of the lattices. 
The cracking effect due to different TEC of two com-
posite phases is more evident in the case of a layered 
composite structure, e.g., cosintering CFO and PZT in 
a sandwiched structure resulted in cracks along the 
interface and delamination or the structure [17]. On 
the other hand, it was shown for CFO–PZT [18] and Co-
doped BFO–PZT composite [12], as well as in the co-
sintered BFO–Ni
0.5
Co
0.5−x
Zn
x
Fe
2
O
4
 composite [19] that 
interfacial strains influence the lattice distortions of in-
dividual phases, contributing to large magnetoelectric 
coupling. In the case of materials of our interest, BFO 
and KNN, BFO has TEC of 10.9 x10
-6
 K
-1
 and 12.2x10
-6
 
K
-1
 for the temperature range 25–325 °C and 344–838 
°C, respectively [20], while KNN has TEC of 2.96x10
-6
 
K
-1
, 4.35x10
-6
 K
-1
 and 7.52 x10
-6
 K
-1
 for the temperature 
range 30–195 °C, 208–364 °C and 434–790 °C, respec-
tively [21]. This implies that BFO will shrink almost twice 
more than KNN when cooling down from the sintering 
temperature, likely producing strains at the interfaces 
and/or cracks.
A way to severely limit thermal expansion/shrinkage, 
diffusion or volatilization of species during sintering 
of composites is to sinter at temperatures below 300 
°C. Cold sintering process (CSP) is a sintering technique 
that allows densification of materials at temperatures 
even below 300 °C, under applied uniaxial pressure of a 
few hundreds of MPa and in the presence of a transient 
liquid phase, which facilitates material densification 
through a pressure-dissolution process [22]. The low 
processing temperature of the CSP provides opportu-
nities for combining a variety of materials, i.e., ceramic 
with polymers or metals, as well as different types of 
oxides [23]–[27]. While different perovskites have been 
cold-sintered so far, including KNN-based composi-
tions and BFO ceramics [28], there are no reports yet 
on cold sintering of perovskite-perovskite composites, 
which is a viable way to produce multifunctional com-
ponents. 
In this contribution we studied the processing of KNN–
BFO perovskite-perovskite particulate composite with 
different ratios of phases, using the CSP . Based on our 
previous investigations on cold sintering of KNN [29], 
[30] and BFO [28], we utilized a eutectic mixture of 
NaOH and KOH as a transient liquid phase (TLP) to aid 
the sintering of the composite, which resulted in a ho-
mogeneous distribution of the two phases, no second-
ary phases formed at the grain boundaries, and a dense 
microstructure. The aim of our work is to illustrate that 
cold sintering can be successfully employed for com-
bining two ferroelectric perovskite compositions to 
produce high-quality composites as multifunctional 
components. 
2 Materials and methods
The initial powder compositions, BiFeO
3
 with added 
0.1 wt.% Co (named BFO hereafter), and K
0.5
Na
0.5
NbO
3
 
(KNN), were prepared via solid-state synthesis. Details 
on preparation can be found elsewhere (KNN powder 
was calcined at 850 ℃) [31], [32]. Both powders were 
separately milled at 200 rpm for 0.5h (KNN) and 2h 
(BFO) after calcination to break the agglomerates. The 
mean particle of KNN was d
50
 = 0.50 µm and for BFO 
d
50
 = 1.39 µm. Different ratios of KNN and BFO powders 
were then mixed together in an agate mortar to pro-
duce (1-x)KNN–xBFO compositions with x= 0, 0.33, 0.50 
and 0.66 and 1 (in weight fraction). The samples are de-
noted as: KNN, 33%BFO, 50%BFO, 66%BFO and BFO, 
respectively. Converting BFO weight % into volume 
%, the three compositions are 20%BFO, 35%BFO and 

227
50%BFO, respectively. For each composition, 75 µl of 
NaOH-KOH aqueous equimolar mixture was added to 
0.6 g of powders and manually mixed in an agate mor-
tar. The NaOH-KOH solution was prepared by dissolv-
ing NaOH (molarity is 4.26 M) and KOH (molarity is 4.26 
M) in water. The moist powders were put in the CSP in-
strument and pressed under 676 MPa uniaxial pressure 
(with a pressing rate of 13 MPa/minute) and heated to 
300 °C (with heating and cooling rates of 2 K/min) and 
cold-sintered for 2 hours. The slow rates have previous-
ly been shown to produce reliable crack-free samples 
[33] and were chosen here for two reasons, one being 
better liquid/particle distribution in the green compact 
and the other being the polymorphic phase transition 
at about 170 °C for KNN which might cause large strains 
due to lattice volume changes and cracking of grains/
pellet. After CSP, the obtained pellets with thickness 
~1.8 mm and diameter of 8 mm were dried in vacuum 
drier at 80 °C overnight. Microstructure analysis of cold-
sintered samples was performed on polished samples 
using a scanning electron microscope using backscat-
tered electrons (SEM; JSM 7600 F, Jeol Ltd., Tokyo, Ja-
pan). Relative density of cold-sintered samples was 
calculated based on measured porosity from several 
SEM images. For dielectric and ferroelectric measure-
ments, the samples were thinned to 0.15-0.25 mm, 
and gold-sputtered on both sides (using sputtering 
system 5Pascal, Italy). Dielectric properties were meas-
ured with an LCR meter (Hewlett Packard, Tokyo, Ja-
pan). Ferroelectric properties (polarization vs. electric 
field) were measured with a TF2000 analyzer (AixACCT 
Systems GmbH, Aachen, Germany) at a frequency of 
100 Hz with a sinusoidal voltage waveform until their 
breakdown. Selected samples were annealed in O
2
 flow 
at 500 °C for 2 hours, with 2 K/min heating and cooling 
rates for dielectric measurements.
Figure 1: SEM images of ceramic samples cold sintered at 300 °C for 2 hours at 635 MPa with NaOH–KOH as a transient 
liquid phase: (a) KNN; (b) BFO; (c) 33%BFO; (d) 50%BFO and (e) 66%BFO (percentage given in wt.%). Light grey color 
of grains in the composites is BFO, and dark grey color of grains is KNN for (c), (d) and (e). Black spots are pores. (f) 
Enlarged area from 33%BFO composite in (c), showing grain-boundary contacts between KNN grains (white arrow in 
the right image).
S. Salmanov et al.; Informacije Midem, Vol. 54, No. 3(2024), 225 – 233

228
3 Results and discussion
The microstructures of cold-sintered KNN–BFO com-
posites with compositions 33%BFO, 50%BFO and 
66%BFO, as well as single-phase KNN and BFO for 
comparison, are presented in Figure 1. The average 
grain size of KNN is about 1 µm or less, while BFO has 
larger grains, ranging up to 5 µm. No secondary phases 
are observed at the grain boundaries in KNN and BFO 
from the cold sintering procedure (Figure 1a-b). KNN 
shows higher relative density compared to BFO, with 
estimated 98 % compared to 94 %, respectively. Such a 
difference could be related to several reasons, one be-
ing better particle packing in KNN due to smaller initial 
particles. On the other hand, higher density of cold-
sintered KNN compared to BFO can also be explained 
by the different levels of solubility of each ceramic in 
the base solution and/or eutectic liquid phase. It was 
previously shown for KNN dipped in aqueous solution 
of NaOH that the concentration of Nb
5+
 ions starts to 
increase already at 50 °C [34], indicating its significant 
solubility. No studies were found for the solubility of 
BFO, however, it was shown that during its hydrother-
mal synthesis it was only possible to dissolve and pre-
cipitate BFO species at temperatures above 175-200 °C 
[35]. This may indicate that KNN is more soluble in the 
NaOH-KOH liquid phase compared to BFO, leading to 
higher densification levels of KNN during cold sinter-
ing. The third argument for better compaction of KNN 
could be its ability to significantly plastically deform 
with applied pressure that seems to exceed the yield 
strength of the material (e.g., for KNbO
3
 crystal yield 
strength of 40 to 120 MPa was reported [36], [37]. Final-
ly, the plastic deformation is realized through disloca-
tion formation and lattice gliding [29], [30], [38], which 
may have been more efficient in KNN due to its lower 
density and/or weaker chemical bonding, resulting in a 
denser microstructure of KNN compared to BFO. 
Microstructures of KNN–BFO composites (Figure 1c–e) 
show that KNN (dark grey phase) and BFO (bright grey 
phase) are homogeneously distributed, and the relative 
densities of 33%BFO, 50%BFO and 66%BFO, calculated 
from measured porosity from the microstructures, are 
∼96 %. Enlarged view of the 33%BFO composite in Fig-
ure 1f shows grain-boundary contacts, indicating good 
adhesion between the KNN grains, creating a matrix, 
with BFO grains well compacted between the sintered 
KNN grains.
These boundaries appear to be slightly curved, con-
firming the pressure-dissolution process [22] during 
Figure 2: Comparison of dielectric permittivity and dielectric losses of as-prepared cold-sintered samples and sam-
ples annealed at 500 °C for 2 hours in O
2
 (a) KNN and (b) BFO. (c) and (d) dielectric permittivity and losses of 33%BFO, 
50%BFO and 66%BFO composites after annealing
S. Salmanov et al.; Informacije Midem, Vol. 54, No. 3(2024), 225 – 233

229
cold sintering, but also appear lighter-grayish, which 
could indicate a Bi- or Nb-rich phase as a consequence 
of liquid-phase formation and precipitation of such a 
phase. The quantity of this phase, however, was too 
low to be identified by energy dispersive X-ray spec-
troscopy in SEM or X-ray diffraction. In 50%BFO, and 
especially in 66%BFO sample, contacts between BFO 
grains are observed that also show good contact and 
adhesion. No cracks are seen throughout the com-
posites, which reflects good sintering and adhesion of 
grains, and minimal thermal expansion of the phases in 
the temperature range from sintering (300 °C) to room 
temperature.
The dielectric properties, presented in Figure 2, were 
measured on the as-prepared and dried cold-sintered 
KNN, BFO and composite samples, and after O
2
 anneal-
ing. The as-prepared BFO exhibits the relative dielectric 
permittivity of ∼120 and dielectric losses below 3% in a 
frequency range from 1 kHz to 1 MHz, and after anneal-
ing in O
2
 no significant decrease in relative dielectric 
permittivity and dielectric losses is observed (Figure 
2a). The cold-sintered KNN is characterized by a relative 
dielectric permittivity of ∼800 and dielectric losses of 
4% in a frequency range from 1 kHz to 1 MHz. How-
ever, after annealing in O
2
, the dielectric permittivity in-
creased by more than 50 %, while the dielectric losses 
decreased (Figure 2b). These results are consistent with 
previous data [30], [39].  The changes in dielectric prop-
erties after annealing in O
2
 are believed to arise from 
the reoxidation of the perovskite and changes in the 
valence states of Nb atom, which was reduced from 
Nb
5+
 to Nb
4+
 during the CSP . For this reason, composites 
were also annealed in O
2
.
The relative dielectric permittivity and dielectric losses 
of 33%BFO, 50%BFO and 66%BFO composites after 
annealing at 500 °C in O
2
 are shown in Figure 2c and 
2d, respectively. The 66%BFO composite exhibited 
relative dielectric permittivity of ~200, slightly higher 
than that of BFO. We can further follow a trend with 
increasing amount of KNN in the composites, result-
ing in an increase in the relative dielectric permittivity, 
with the value for the 33%BFO composite still lower 
than that of KNN. Especially notable is the difference 
in relative dielectric permittivity between 33%BFO and 
50%BFO compared to the difference between 50%BFO 
and 33%BFO, which is minimal. This could be related 
to a critical amount of KNN in the composite. Since in 
33%BFO there is only 20 vol. % of BFO, this suggests 
that the contribution to the relative dielectric permit-
tivity of KNN dominates over BFO. On the other hand, 
the amount of BFO increases to 35 vol. % and 50 vol. 
% in 50%BFO and 66%BFO composites, respectively, 
increasing the contribution of BFO to the dielectric 
properties, decreasing the relative dielectric permittiv-
ity. Interestingly, the values of the dielectric losses of all 
the composites were similar in the studied frequency 
range, but slightly higher in the low-frequency range 
in 66%BFO, similar to pure BFO, which could indicate 
local conductivity due to defect states in the BFO phase 
(Figure 2d).
The P–E-loops at 150 kV/cm applied electric field of 
cold-sintered KNN and BFO and composites 33%BFO, 
50%BFO and 66%BFO are shown in Figure 3a. The re-
sults show that the coercive field (E
c
) and remanent 
polarization (P
r
) values of the composites are between 
the values for KNN and BFO. 33%BFO and 50%BFO 
samples show a similar P
r
 value, while P
r
 of 66%BFO is 
smaller, which is in line with the value of the major con-
stituent phase, BFO. The KNN loop shows polarization 
saturation reached at 150 kV/cm applied, with P
s
=30 
µC/cm
2
. The P–E loop of BFO, however, remains flat at 
this field and only starts to open at an applied electri-
cal field greater than 150 kV/cm (see inset in Figure 3a 
for the BFO loop at 200 kV/cm). This indicates defect 
states in the material that unpin the domains only at 
stronger fields applied. The effect of defects seems to 
be hindered in the case of composites with increas-
ing KNN phase, increasing also the P
r
 value. On the 
other hand, dielectric breakdown strength seems to be 
strongly affected by the amount of BFO in the compos-
ite, increasing the maximum possible applied electric 
field (E
max
) (Figure 3b). We note here that while we did 
not systematically measure the dielectric breakdown 
strength of these materials, we consistently observe 
for all cold-sintered perovskites that the samples with-
stand larger electrical fields applied without their di-
electric breakdown. The final reached value of P
r
 of the 
composite seems to be affected by the strength of the 
grain-boundary contacts between the KNN grains (see 
enlarged view in Figure 1f), which are more frequent 
in the composite with larger KNN amount. The analysis 
of the evolution of P–E loops with gradually increasing 
electric field for composites (Figure 3c–e) shows that in 
66%BFO at maximum applied field (see the loop at 170 
kV/cm) the P–E loop starts to open and becomes more 
rounded. This indicates a major contribution to electri-
cal properties from the BFO phase with unpinning of 
defects and increased conductivity [31]. Comparison 
of the functional properties of cold-sintered samples is 
summarized in Table 1.
4 Conclusions
We demonstrated that CSP can be effectively utilized 
for sintering of perovskite–perovskite particulate com-
posites based on different ratios of KNN and BFO. Micro-
structure analysis revealed that the composites have a 
S. Salmanov et al.; Informacije Midem, Vol. 54, No. 3(2024), 225 – 233

230
dense structure and homogeneously distributed KNN 
and BFO grains. Moreover, the contact regions between 
KNN and BFO grains did not exhibit any cracks or large 
pores, nor significant secondary phases. After anneal-
ing in O
2
, the dielectric losses were below 3% across the 
entire frequency range of 1 kHz to 1 MHz and the val-
ues of relative dielectric permittivity were in line with 
KNN amount of KNN, increasing with increasing KNN 
fraction. Nevertheless, the composite with 66 wt. % of 
BFO, where the contacts between KNN grains are rare, 
demonstrated approximately two times higher permit-
tivity that single-phase cold-sintered BFO, and similar 
dielectric loss values. The composites showed decent 
ferroelectric properties and increasing polarization sat-
uration with KNN content, while dielectric breakdown 
strength appeared higher with increasing BFO content. 
Remanent polarization is also enhanced by larger KNN 
fraction when up to 150 kV/cm field is applied, while 
Figure 3: Comparison of ferroelectric properties cold-sintered KNN, BFO and composite ceramics after annealing 500 
°C for 2 hours in O2. (a) Polarization–electric-field (P–E) loops at 150 kV/cm. Inset shows the P–E loop of BFO at 200 
kV/cm. (b) Maximum applied electric field during measurement of P–E loops for all samples before breakdown. (c)–(e) 
Evolution of P–E loops with increasing E for (c) 33%BFO, (d) 50%BFO and (e) 66%BFO composites. All measurements 
were done at 100 Hz
Table 1: Comparison of functional properties of cold-sintered samples
Permittivity at 1kHz Dielectric losses at 1kHz Pr at 150 kV/cm Pr at 200 kV/cm Emax, kV/cm
KNN ~1300 ~0.03 ~14 - 150
33%BFO ~800 ~0.02 ~8 - 150
50%BFO ~300 ~0.02 ~8 - 160
66%BFO ~200 ~0.02 ~6 - 170
BFO ~100 ~0.02 ~2 ~14 220
S. Salmanov et al.; Informacije Midem, Vol. 54, No. 3(2024), 225 – 233

231
larger fraction of BFO in the composite allows to ap-
ply fields above 150 kV/cm, apparently increasing the 
breakdown strength of the composite.
5 Acknowledgments
The authors acknowledge the help of Brigita Kmet for 
KNN powder synthesis, Electronic Ceramics Depart-
ment, Jožef Stefan Institute. The authors acknowledge 
the financial support of the Slovenian Research and In-
novation Agency (core funding J2-2508, J2-50077, pro-
gram P2-0105). 
6 Conflict of Interest
The authors declare no conflict of interest.
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Arrived: 29. 04. 2024
Accepted: 06. 08. 2024
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