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 Mater. Res. Express 5 (2018) 025404 https://doi.org/10.1088/2053-1591/aaabe2

PAPER

Fabrication and performance of porous lithium sodium potassium

RECEIVED
6 December 2017
niobate ceramic
REVISED
20 January 2018
ACCEPTED FOR PUBLICATION
Caifeng Chen1,4 , Yuan Zhu1, Jun Ji1, Feixiang Cai1, Youming Zhang2, Ningyi Zhang3 and Andong Wang1
31 January 2018 1
School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
2
PUBLISHED School of Mechanical Engineering , Jiangsu University, Zhenjiang 212013, People’s Republic of China
3
15 February 2018 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, United States of America
4
Author to whom any correspondence should be addressed
E-mail: chenjsust@163.com

Keywords: foaming method, piezoelectric properties, porous ceramic, biomedical applications

Abstract
Porous lithium sodium potassium niobate (LNK) ceramic has excellent piezoelectric properties,
chemical stability and great chemical compatibility. It has a good application potential in the field of
biological bone substitute. In the paper, porous LNK ceramic was fabricated with egg albumen
foaming agent by foaming method. Effects of preparation process of the porous LNK ceramic on
density, phase structure, hole size and piezoelectric properties were researched and characterized. The
results show that the influence factors of LNK solid content and foaming agent addition are closely
relevant to properties of the porous LNK ceramic. When solid content is 65% and foaming agent
addition is 30%, the porous LNK ceramic has uniform holes and the best piezoelectric properties.

1. Introduction

At present, biology and bioscience have developed rapidly. With the aging of the population and the increase of
traffic accidents occurrence, requirement on the performance of biomedical materials is increasing. Because of
their excellent chemical stability and great chemical compatibility, biological piezoelectric ceramics have been
widely studied [1, 2]. For the first time in the middle of the twentieth century, the potassium sodium niobate
system ceramics were reported to have piezoelectric property. It was found that this kind of ceramic had simple
preparation process, greatly reduced sintering temperature, excellent electrical property and non-toxic, so it
should have a good biocompatibility when entering the human body in medical applications [3–6].
According to previous studies many parts of human body were tested with bioelectricity. Weak bioelectricity
have a stimulating effect on the cells, promoting the proliferation and differentiation of human cells [7, 8]. In
medical therapy, it had applied the artificial electrical signals to stimulate biological tissue, and a better
therapeutic effect was obtained. As a bone substitute material, it can accelerate bone repair and promote bone
formation [9–11]. However, most of the piezoelectric materials are porous free or few porous materials, the
space for bone cell proliferating and growing is limited. As a result, the efficiency of bone repair is inhibited.
Compared with the traditional piezoelectric materials, porous piezoelectric ceramic shows a better repair effect.
Nowadays, 3-0 and 3-3 type of porous lead zirconate titanate (PZT) piezoelectric ceramics with high
porosity, low acoustic impedance were prepared using the particle-stabilized foams and the gel casting technique
[12, 13]. Researchers confirmed the possibility of using TBA (Tert-butyl Alcohol)-based gel-casting process in
preparing porous PZT ceramics. The porous PZT ceramics possess a homogeneous porous structure with a
porosity as high as 58.6% [14]. Burnable plastic spheres(BURPS) process and foaming method are also the
widely used fabrication technique [15]. However, most of the porous piezoelectric ceramics are not a lead-free
ceramic, and the pore former used in the fabrication is almost polymer compound, which has a pungent odor
and toxicity.
In this work, egg albumen was for the first time used for preparing porous LNK lead-free piezoelectric
ceramics as the foaming agent. The effects of slurry solid content and foaming agent dosage on the electrical

© 2018 IOP Publishing Ltd

Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 1. Effect of LNK solid content on porosity (a) and density (b) of the porous LNK ceramic.

property of the LNK ceramic were examined and compared to reveal the possibility of porous LNK ceramics
used in bone substitute materials.

2. Experimental methods

According to the chemical formula (Li0.06Na0.5K0.44) NbO3 ingredients, LNK powder was pre-sintered at 890 °C
using tantalum pentoxide (Nb2O5), potassium carbonate (K2CO3), sodium carbonate (Na2CO3), and lithium
carbonate (Li2CO3) as raw materials. We extracted the albumen from the egg, removed the upper floating foam
to obtain pure albumen liquid for standby application. LNK powder, deionized water and a small amount of
dispersant (ammonium polyacrylate) were added into the planetary ball mill, the speed of the revolution and
rotation is 300 r min−1, and the LNK slurry was obtained by ball milling for 12 h. After the ball milling, the
albumen with different mass fraction was added into the LNK slurry as the foaming agent, and the magnetic
stirring was carried out for 2 h to ensure the full foaming. The foaming slurry pressed prior to sintering of the
sample. The fully foaming slurry was injected into the mold and placed at room temperature for 24 h, then
removed from the mold and sintered at 1000 °C. After conductive tape with excellent electrical conductivity was
pasted on both sides of the ceramic, the electrical field of 3 KV mm−1 was applied to polarize the samples at
100 °C for 20 min.
The surface microstructure of the ceramic was characterized by JSM-7001F scanning electron microscopy
(SEM), and the phase composition was determined by D/max-RB x-ray Diffraction(XRD). The Archimedes
method was used to test the density. The piezoelectric coefficient and capacitance were measured by quasi-static
d33 and LCR meter.

3. Results and analysis

3.1. Effect of LNK solid content on porous LNK ceramic properties

Figure 1 shows the variation of porosity and density of the porous LNK ceramics with the increase of LNK solid
content in the slurry. The curves represent the foaming agent content of 20 wt%, 25 wt%, 30 wt%, 35 wt% in the
slurry, respectively. From the figure, we can see the porosity of the material decreases and the density increases
sharply with the LNK solid content increasing. When the slurry has less LNK solid content, the slurry has low
viscosity, high foaming and porous structure, and the porous ceramic shows high porosity and low density.
Figure 2 shows the XRD comparison of porous LNK ceramic with different amount of solid phase when the
amount of the foaming agent is 30 wt%. The porous LNK in the diffraction angle of 22.3°, 22.6°, 31.9°, 39.2°,
45.2°, 46.1°, 51.8° and 56.9° have obvious diffraction peak in (002) and (200) crystal face appeared peak
phenomenon, it shows perovskite structure. Compared with the standard card, they are all lithium sodium
potassium niobate based ceramic materials. We know that doping Li element will lead to the oxygen octahedron
distorted. The deformation in the grain direction reduces the symmetry of the structure and forms a quartet
structure gradually. Thus, the LNK ceramic will have piezoelectric or ferroelectric properties, and shows a
lagging relationship between the polarization strength and the external electric field. In addition, from the figure,
there is no obvious irrelevant peak in the XRD patters, it indicates that the albumen has been completely burned
in the sintering process.

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Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 2. XRD patterns of the porous ceramic with different LNK solid content in the slurry.

Figure 3. Surface morphology of LNK porous ceramic with different solid content in the slurry (a) 50 wt% (b) 55 wt% (c) 60 wt% (d)
65 wt%.

Figure 3 is the surface microstructure of the porous ceramic with foaming agent content of 30 wt%. LNK
powder slurry foaming induce a honeycomb hole structure. They interconnected each other, as shown in the
white part of figure 3. There are many cracks in the holes, as shown in the black part in the figure 3. The cracks are
caused by egg white bubble burst in the process of solidification. However, the holes with crack are helpful to the
growth of the cells [16]. When the solid content is 50 wt% and 55 wt% respectively, the hole size is about
300–500 μm. With the increase of LNK solid content, the hole size decreases gradually, which is about
100–250 μm, and the size distribution becomes uniform. In bone substitute materials, the hole size has a major
influence on the porosity of osteoblasts. White et al believe that the most suitable pore diameter for bone repair is
above 150 μm [17]. The hole size of porous LNK meets this requirements for cell growth.
Figure 4 is the hole images of higher magnification(×1000). It can be seen that the wall thickness of the hole
has also changed, the hole and the wall inside also contain a lot of small holes with a few microns. The wall

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Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 4. Wall thickness of LNK porous ceramic with different solid content in the slurry (a) 50 wt% (b) 55 wt% (c) 60 wt% (d)
65 wt%.

Figure 5. Effect of solid content on electrical properties of porous LNK ceramic (a) piezoelectric constant; (b) dielectric constant.

thickness of the hole is increasing with the improvement of LNK solid content. When the solid content of
50 wt%, the wall thickness is approximately 10 μm. The wall thickness of the hole can reach 15–20 μm with the
increase of solid content.
Figure 5 shows the relationship between the electrical property of porous LNK ceramic and the solid content
of slurry. Figure 5(a) shows the piezoelectric constant(d33) is only about 5–10 pC/N when the solid content of
the slurry is 50 wt%. With the increase of the LNK solid content, the piezoelectric constant(d33) shows an
upward trend. When the solid content reaches 65 wt%, the piezoelectric constant(d33) is about 25–50 pC/N.
Reports have demonstrated that weak bioelectricity is beneficial to cell reproduction and differentiation, which
as a result, can promote the bone repairing [18, 19]. Although the piezoelectricity of porous LNK is reduced
sharply, the human body is so sensitive that it can still be stimulated by this biological bone repair materials.

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Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 6. Effect of foaming agent addition on porosity (a) and density (b) of porous LNK ceramic.

The dielectric constant indicates the ability of the material to store charge. Figure 5(b) shows the relationship
between the relative dielectric constant of the material and the LNK solid content of the slurry. When the solid
content is low, the relative dielectric constant of porous LNK is 20–50. When the solid content of the slurry is
gradually increased, the dielectric properties of porous LNK increases continuously, and the maximum value
appears at about 100.
Comprehensive analysis of material density, porosity, morphology and other aspects, we can find that when
the LNK solid content in the slurry is low, the porous ceramic has the characteristics of low density, high
porosity, poor cell growth degree, large pore size and uneven distribution, these properties will greatly reduce
material piezoelectric and dielectric properties. Above all, low content of solid in slurry caused change in
dielectric constant, and this is one of the main influencing factors. However, there are many other factors will
also affect the dielectric constant, such as the distribution and size of the pore. Therefore, it can say the dielectric
constant also reflect porosity level, but it is not absolute. When the solid content is increased, the density of the
material is improved, the hole size distribution is uniform, the pore wall increases gradually, and the electrical
properties of the material are improved significantly.

3.2. Effect of foaming agent content on porous LNK ceramic properties

Egg albumen is a kind of pure natural foaming agent with non-toxic and nonirritating smell. Figure 6 shows the
relationship between porosity and density of the porous LNK ceramic with different amount of albumen
foaming agent in the slurry. The curves represent the solid content of 50 wt%, 55 wt%, 60 wt%, 65 wt% in the
slurry, respectively. We know that the density of dense LNK ceramics sintered at 1000 °C is about 3.45 g cm−3.
Compare with dense LNK ceramics, the density of the porous LNK ceramic is lower, and the density changes
with the change of the amount of foaming agent.
When the amount of foaming agent is 20 wt%, the foam is not enough, and the hole can not be expanded
outward in the foaming process. Thus, the porosity of the ceramic is low, and the density is high. When the
amount of foaming agent is increased to 25 wt%, the porosity is increased about 20%, and the density of the
material is also decreased significantly, which indicates that the foam is more abundant. And with the content of
foaming agent increasing gradually, porosity rate continues to improve and the density decreases. When the
foaming agent content reaches 35 wt%, the porosity is not significantly improved because of its low foaming rate
and the gap between the slurry foaming. In addition, due to the excessive amount of foaming agent, the protein
macromolecular chain will be intertwined with each other, reducing the fluidity of the slurry, limiting the
foaming. Therefore, the amount of foaming agent should not be too much, the best range is 25 wt%–30 wt%.
Taking the LNK solid content of slurry 65 wt% as an example, the relationship between the foaming agent
and the phase structure of porous ceramics with different amount of foaming agent is analyzed as shown in
figure 7. The porous LNK ceramics with different amount of foaming agent have obvious characteristic peaks in
the specific diffraction angle, and the contrast standard cards are sodium potassium niobate based ceramics,
showing typical perovskite structure.
Comparing with the XRD of different foaming agent, it can be found that the diffraction peak of (102),
((201), (112), (121) crystal surface is smooth when the amount of foaming agent is very low, which means that
the cell growth is not completed. When the foaming agent content is gradually increased, the diffraction peaks
become clear and sharp. The reason is that if the foaming agent content is too low, the pore structure is not
uniform because of shortage of foam. And it will have a negative impact on the development of the unit cell in the
sintering process. When the content of albumen is too much, it will increase the slurry viscosity, the foaming

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Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 7. XRD patterns of porous LNK ceramic with different amount of foaming agent addition.

Figure 8. Surface morphology of porous LNK ceramic with different amount of foaming agent addition (a) 20 wt% (b) 25 wt% (c)
30 wt% (d) 35 wt%.

process will also be affected. It can be concluded that the foaming process has a major influence on the grain
growth of porous materials.
With increasing amount of foaming agent, the surface morphology of porous LNK with 65% solid content is
shown in figure 8. After the foaming process, porous LNK ceramics exhibit a loose porous structure, and the
holes are interconnected with each other. As a bone substitute material, the structural characteristics are
beneficial to the repair of bone. Figure 8(a) shows that the foaming process is not sufficient when the foaming
agent content is 20%, and the hole diameter is about 300–500 μm. With the addition of foaming agent, the pore
size of the material is reduced to about 50–200 μm, and the distribution becomes uniform. However, when the
addition amount of foaming agent is too much, as 8 (d) shows, there are a large number of closed pores. Gas hole
will affect the bioactivity of the materials as bone substitute materials.

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Mater. Res. Express 5 (2018) 025404 C Chen et al

Figure 9. Wall thickness of porous LNK ceramic with different amount of foaming agent addition (a) 20 wt% (b) 25 wt% (c) 30 wt%
(d) 35 wt%.

Figure 10. Effect of foaming agent addition on electrical properties of porous LNK ceramic (a) piezoelectric constant; (b) dielectric
constant.

Figure 9 is the hole images of higher magnification of porous LNK ceramic with different amount of foaming
agent addition. When the amount of foaming agent is only 20 wt%, the foaming effect is not good, and the hole
wall is thicker, the value is about 25 μm, as shown in figure 9(a). Figure 9(d) shows the wall thickness is about
10 μm with the foaming agent content of 35 wt%. In generally, with the increasing of the amount of foaming
agent, the wall thickness decreases sharply.
Figure 10 shows the relationship between the amount of foaming agent in the slurry and the electrical
properties of porous LNK ceramic. In figure 10(a), when the amount of foaming agent is 20 wt%, the uneven
distribution of holes in the porous ceramic has a great influence on the piezoelectric properties. The piezoelectric
constant(d33) of the porous LNK ceramics is only about 10 pC/N. With the increase of foaming agent, the
piezoelectric properties improve significantly because of the holes become smaller and more evenly distributed.
The piezoelectric constant(d33) of porous LNK ceramics are up to 48 pC/N with foaming agent content of

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Mater. Res. Express 5 (2018) 025404 C Chen et al

30 wt%. But the dielectric constant of porous LNK ceramic decrease when the foaming agent of the slurry is
gradually increased, as shown in figure 10(b). The porosity of the ceramic has a great influence on the dielectric
constant, and the influence is greater than that of piezoelectric constant.

4. Conclusions

In summary, albumen is successfully used as foaming agent to prepare porous LNK ceramics in the work. The
LNK solid content of the slurry has a significant effect on the performance of porous LNK ceramic. With the
increase of the LNK solid content of the slurry, the porosity of the porous LNK ceramics decreases, the hole sizes
distribute more uniformly, the piezoelectric and dielectric properties of the materials are improved. The
properties of the porous ceramic are also closely related to the amount of foaming agent added. With the increase
of the foaming agent, the porosity of the ceramic increases, the hole structure becomes more uniform, but the
density and dielectric constant decreases gradually.
When the solid content of the slurry is 65 wt%, and the amount of the foaming agent is 30 wt%, the porous
ceramic obtains its optimized piezoelectric properties. The porous LNK ceramics have the piezoelectric constant
(d33) of 48 pC/N, the relative dielectric constant of about 67.29. The hole size of porous LNK ceramics is suitable
for cells to grow into holes and the sizes distribute uniformly. Excellent piezoelectric properties can promote cell
proliferation and differentiation, and can effectively repair bone. Therefore, the porous LNK ceramic will have
good application potential in the field of biological bone substitute.

Acknowledgments

This work was funded by Science and Technology Support Program of Jiangsu Province (BE2014007-4).

ORCID iDs

Caifeng Chen https://orcid.org/0000-0002-0093-604X

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