Figure 6 - 1 (a) The photograph and (b) transmittance of PLSMNTx ceramics (x = 2.5 – 5.5).
Figure 6 - 1 (a) shows PLSMNTx ceramics with various doping concentration. The high transmittance of the ceramics makes it easy to identify the letters behind the PLSMNTx ceramics. In the photograph, it seems that x = 2.8 to 3.5 have almost similar transmittance visually. In fact, PLSMNTx ceramics have almost the same transmittance from 2.8 to 3.1 in the wavelength of visible light. The transmittance of PLSMNTx ceramics increased with increasing doping concentration until x = 2.9. Then, as the doping
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concentration was further increased, the transmittance of PLSMNTx ceramics decreased as shown in Figure 6 - 1 (b). After x = 4.0, the graphs of transmittance show very irregular shapes depending on the wavelength, which has different tendency from those of PLSMNTx (x = 2.5 - 3.1) ceramics. The PLSMNTx ceramics have not only transparency, but also ferroelectricity and piezoelectricity as shown in Figure 6 - 2.
Figure 6 - 2 The (a) Hysteresis loop, (b) bi-polar strain curve and (c) uni-polar strain curve of PLSMNTx for various electric field.
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Figure 6 - 3 The piezoelectric coefficient and transmittance properties of PLSMNTx (x = 2.5 – 5.5)
The piezoelectric coefficient, d33 also increased with the increase of the value of x, and showed a maximum value when x = 3.1. After that, the d33 decreased with the increase of the x and almost lost the piezoelectricity after x = 5.0. The d33 in Figure 6 - 3 is the value when the thickness of the sample is 0.8 mm. The piezoelectric and dielectric properties of PLSMNTx ceramics are summarized in Table 6 - 1.
The increase in transmittance and piezoelectric properties with increasing doping concentration is related to the rare earth elements La3+ and Sm3+. It is well known that La3+ is not only effective to reduce the distortion of the oxygen octahedral (ABO3) unit cell in perovskite but also ability to enhance the densification of microstructure.10,12,17,23 Consequentially, it helps to enhance transparency of the ceramics by reducing light scattering resulting from the multiple refraction of light at the boundaries of the randomly oriented lattice and residual pore. The PLSMNTx ceramics show highly dense microstructure for all the composition as displayed in Figure 6 - 4.
The cause of the improvement of the piezoelectric properties of PLSMNTx ceramics is the enhanced local structural heterogeneity through Sm3+ doping. According to recent study, the improvement of local random field through rare-earth element doping in relaxor-PT systems is of great help in improving the piezoelectric properties by enhancing local structural heterogeneity at nanoscale.24-27 The advantage of the local structural heterogeneity improvement method is that it has little effect on the density and grain
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size of the ceramic. This is a positive phenomenon to improve the density by the La3+ substitution effect.
Consequentially, it is expected that the doping effects of La3+ and Sm3+ cause enhancement of transparency and ferroelectricity/piezoelectricity without interfering with each other in the ferroelectric ceramics.
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Table 6 - 1 The piezoelectric/dielectric properties of PLSMNTx ceramics.
d*g [10-15 ·m2 /N] 7796 12862 13938 12687 12633 14336 13648 8040 4454 372 22 12
g33 [mV·m/N] 15.17 17.89 16.83 17.26 15.52 15.88 15.6 10.51 7.75 2.26 0.52 0.37
d33 [pC/N] 514 719 828 735 814 903 875 765 575 165 42 32
εr 3828 4540 5556 4809 5924 6424 6336 8221 8384 8257 9083 9704
Qm 67.3 56.2 44.1 50.1 39.1 38.2 40.4 32.6 39.3 43 36 51.3
kp 0.4808 0.5729 0.5861 0.5861 0.6113 0.5926 0.5729 0.552 0.4339 0.1998 0.1734 0.1419
△F 23.1 33.25 34.91 35.2 38.5 35.6 33.12 30.86 19.04 3.93 3.1 2.03
Za [Ω] 3854.1 4183.5 3507.5 4140.5 3529.3 2852.2 2596.2 1806.7 906.3 157.1 83 79.8
fa [kHz] 241.37 240.4 240.4 242.35 242.35 239.44 239.44 241.37 246.28 246.28 258.46 253.31
Zr [Ω] 13 9.8 9.9 9.9 9.8 9.7 9.8 11.5 14 57 78.3 77.6
fr [kHz] 218.27 9.9 205.49 207.15 203.84 203.84 206.32 210.51 227.24 242.35 255.36 251.28
tanδ [m] 26.22 26.78 27.03 26.07 28.68 32.81 32.33 43.88 36.99 48.71 43.07 49.85
Cp [nF] 4.57 5.42 6.62 5.72 6.99 7.67 7.58 8.43 8.58 8.45 9.26 9.97
A [mm2 ] 80.91 80.91 80.75 80.6 79.96 80.91 81.07 81.07 80.91 80.91 80.6 81.23
d [mm] 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.7 0.7 0.7 0.7 0.7
Composition PLSMNT2.5 PLSMNT2.6 PLSMNT2.7 PLSMNT2.8 PLSMNT2.9 PLSMNT3.0 PLSMNT3.1 PLSMNT3.5 PLSMNT4.0 PLSMNT4.5 PLSMNT5.0 PLSMNT5.5
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Figure 6 - 5 The photograph of PLSMNT2.9 (1st row in (a)) and ITO coated PLSMNT2.9 (2nd row in (a).
(b) the transmittance of PLSMNT2.9 (solid line) and ITO coated PLSMNT2.9 (dash line).
Although the synthesized transparent piezoelectric ceramic shows high transmittance, it is necessary to achieve higher transmittance and lower thickness with transparent electrodes for device application. We selected Indium Tin Oxide (ITO) as the transparent electrode and checked the transmittance of PLSMNT2.9 ceramic coated with ITO by thickness. Figure 6 - 5 (a) shows transmittance and photograph of PLSMNT2.9 ceramics and ITO coated PLSMNT2.9 ceramics according to thickness.
The transmittance of PLSMNT2.9 ceramics increased with decreasing thickness and there is a clear visual difference. The transmittance is inversely proportional to the thickness of the sample because the thickness of the sample and the absorbance coefficient are inversely proportional. Unexpectedly, the transmittance of ITO coated PLSMNT2.9 ceramics improved compared to before ITO coating. It is presumably because the surface roughness was improved and the diffuse reflection of the surface decreased.
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Figure 6 - 6 The ferroelectricity and piezoelectricity graph of PLSMNT2.9 ceramics with various sample thickness: (a) hysteresis loop, (b) bi-polar strain curve, (c) Pmax/Pr/Ec (d) dielectric permittivity/loss, (e) piezoelectric charge coefficient/Smax and (f) kp/Qm.
It is necessary to find the optimal thickness for the device application because transparency and piezoelectricity have opposite thickness dependence. The ferroelectricity and piezoelectricity of PLSMNT2.9 ceramics were decreased gradually as decreasing sample thickness as displayed in Figure 6 - 6 (a-c). The piezoelectric properties, d33 and strain decreased as the thickness of the ceramic decreased as shown in Figure 6 - 6 (e). The piezoelectric charge coefficient is proportional to the
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thickness of the sample. This phenomenon has been reported by previous researchers, but unlike thin specimens (<100 um) whose causes have been identified28-29, the cause has not been investigated in the bulk scale of mm.30
Figure 6 - 7 The photograph of PLSMNT2.9 ceramics for various shape, dimension and thickness
Figure 6 - 7 shows a photograph of various sizes and shapes of transparent ceramics. One of the advantages of using pressureless sintering is that transparent piezoelectric ceramics of various shapes and sizes can be manufactured regardless of size and shape. It believes that the manufacturing process method regardless of the size or shape of the device will be a great advantage forindustry application.
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Figure 6 - 8 Comparison of optical and piezoelectric properties with other papers (graph for piezoelectric coefficient verse transmittance at 633 nm wavelength).
In recent studies, there are some outstanding values with hot-press sintering process. Kai Li et. al.
reported Eu3+ doped PMN-PT which have high piezoelectric coefficient of 850 pC/N with transmittance of 51 %.13 Yalin Qin et. al. prepared Pr3+ doped PMN-PT which have higher transmittance of 68 % at 800 nm wavelength and piezoelectric coefficient of 465 pC/N. The Eu3+ doped PMN-PT has a high piezoelectric coefficient, but the transmittance is not sufficient for device application. The high transmittance of 68% of Pr3+ doped PMN-PT is a value at a wavelength of 900 nm, not in the visible light area.16 These transmittance was measured at 800, 900 nm and that wavelength are outside the visible light. So, this graph needs to be modified with transmittance at visible light range.
The comparison for piezoelectric charge coefficient/transmittance (at 633 nm wavelength) of PLSMNTx ceramics with other research were shown in Figure 6 - 8. The PLSMNT2.9 ceramics show the highest transmittance in visible light area compared to the other research reported in academia so far and the piezoelectric properties are also considering to be high. The numerical values are displayed in Table 6 - 2. It is noteworthy that, despite using the pressureless sintering process, it has a much higher
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transmittance than the ceramic using the hot-press sintering process.
Table 6 - 2 The piezoelectric coefficient and transmittance at 633 nm wavelength by sintering process and thickness of materials
Materials Abb. [pC/N] d33 Transmittance [%]
Thickness
[mm] Sintering method
KSr2Nb5O15 KSN 13 50.8 0.5 2STEP sintering 1) 1250 ℃ (2 h)
2) 1350 ℃ (2 h) with O2 (K0.5Na0.5)0.9Sr0.1Nb0.9Ti0.1O3
–1%Bi2O3 KNSNT-10 26.1 50.6 0.5 Pressureless
sintering 1200 ℃ (2 h) 0.91(K0.37Na0.63)NbO3-
0.09Ca(Sc0.5Nb0.5)O3
KNN-
CSN9 35 51 0.5 Pressureless
sintering 1220 ℃ (4 h) (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)
O3 KNLNB 91 62.7 0.35 Pressureless
sintering 1060-1080 ℃ (4 h) 0.93K0.5Na0.5NbO3-
0.07SrZrO3-xLi2O KNN-SZ-
Li 149 48.9 0.3 Pressureless
sintering 1270-1280 ℃ (6-10 h) [(K0.5Na0.5)1-
2x(Sr0.75Ba0.25)x]0.93Li0.07Nb0.93
Bi0.07O3
KNNLB:Sr
/Ba 151 35.8 0.5 Hot-press
sintering 1070-1100 ℃ (4 h) with 12.7 MPa
BaTiO3 BT 420 32.6 0.5 Hot-press
sintering 950 ℃ (1 h) with 60 MPa
Pb0.97La0.02(Zn1/3Nb2/3)0.3(Zr0.
53Ti0.47)0.7O3 PLZnNZT 468 43.5 0.12 Hot-press
sintering 1200 ℃ (4 h) with O2, 4 ton
[001]-oriented 0.72Pb(Mg1/3Nb2/3)O3- 0.28PbTiO3
PMN-28PT 2190 65.2 0.5 modified
Bridgman method A.C. poling process
1.5 mol% Pr3+ doped 0.75Pb(Mg1/3Nb2/3)O3- 0.25PbTiO3
PMN-
PT:Pr 465 57.8 0.7 2STEP hot-press
sintering
1) 1230 ℃ with O2 2) 1230 ℃ with 100 MPa Eu3+-doped
Pb(Mg1/3Nb2/3)O3– 0.25PbTiO3
PMN-
PT:Eu 850 44.8 - 2STEP hot-press
sintering
1) 1200 ℃ with O2 2) 1200 ℃ with 30 MPa
Pb0.971La0.0145Sm0.0145(Mg1/3N b2/3)0.71Ti0.29O3
PLSMNT2.
9 with ITO coating (This work)
658 65.8 0.5
Pressureless
sintering 1160 ℃ (30 h) with O2
543 71.6 0.4
480 73.5 0.3
323 74.6 0.2
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Figure 6 - 9 (a) The photograph of PLSMNT2.9 ceramics placed on top of the devices for visual test with various color (red, blue, green, yellow, magenta, cyan and white), (b) The transmittance of PLSMNT2.9 ceramics with visible light spectrum from 380 to 750 nm.
For display application, visual characteristic is as important as numerical values because the transmittance is different from the actual visual characteristics seen with the naked eye. Figure 6 -9 (a) is the RGB test result of the specimen. According to transmittance graph as a function of wavelength in Figure 6 - 9 (a), the transmittance of the specimen is lower at blue color (465 nm) than at green (532 nm) or yellow color (580 nm). But, when the specimen is actually viewed with the naked eye, the transmittance of blue color is higher than those of yellow or green color. In addition, humans cannot accurately quantify and recognize the difference in transmittance seen with the naked eye, and there are also individual differences between people. This is why the actual visible characteristics are more important.
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Figure 6 - 10 The photograph for visual comparison, (a) reference photograph of devices
Figure 6 - 10 (a) is reference picture of smart devices (left: Galaxy S20 Ultra 5G, right: Galaxy Watch Active 2 and Apple Watch SE). Figure 6 - 10 (b) is comparative photographs of the specimen placed on the electronic devices. We can clearly recognize the screen under the device with no visual distortion after put the sample on devices. The video test also was confirmed, the screen attached transparent piezo-ceramics was recognized well regardless of the type of color, brightness, or contrast. (See supplementary video 1 at https://www.youtube.com/channel/UC1PGfghCl4b5hnd-YpXkfgg)
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Figure 6 - 11 (a) The photograph of simple speaker system for transparent piezo-speaker. (b) The photograph of transparent piezo-speaker and (c) their schematic illustration.
The simple speaker system was made with D.C. power source, audio amplifier, audio signal source (device: GalaxyS20 Ultra, music program: Melon) and transparent piezo-speaker to confirm the sound properties of transparent piezo-ceramics asshown in Figure 6 - 11 (a). The transparent piezo-speaker we made can play various kinds of music in spite of poor condition (poor diaphragm, no structural design). The sound quality of music played by the transparent piezo-speaker can be checked in the supplementary video 2 at https://www.youtube.com/channel/UC1PGfghCl4b5hnd-YpXkfgg.
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Figure 6 - 12 (a) The schematic diagram of measurement system for sound pressure level. (b) The frequency dependent sound pressure level of transparent piezo-speaker.
The sound pressure level of transparent piezo-speaker was measured with 200 – 400V (500 – 20000 Hz) in an anechoic chamber as drown in Figure 6 - 12 (a). Figure 6 - 12 (b) shows the frequency dependent sound pressure level graph of transparent piezo-speaker with various thickness of piezo-ceramics within audio frequency range. Unlike the piezoelectric characteristics, which decreased with thickness, the sound pressure characteristics did not show a significant difference according to the thickness of the piezoelectric element.
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Figure 6 - 13 The schematic illustration for principle of (a) display speaker and (b) piezo-speaker. The concept image of display applied transparent piezo-ceramics for various functional working such as speaker, haptic, fingerprint recognition.
The needs for a transparent piezoelectric material has emerged with the recent development of the display field. In order to understand the demand for transparent ceramics However, it is necessary to look at the development direction of the display industry. Until now, displays have developed in the direction of minimizing the thickness and bezel, but one of the obstacles to this is the speaker. Recently, ultra slim display technologies that can make sound without speaker part are on the market. That technologies use the panel itself as a diaphragm by a vibration extractor to make sound like a speaker.
The schematic diagram of principle of that technologies is displayed in Figure 6 - 13 (a). The principle of that technologies is similar to those of piezoelectric speaker displayed in Figure 6 - 13 (b).
If the piezoelectric material becomes transparent, it can be positioned under the glass on the top of the display to perform the speaker operation. In addition, since it has the characteristics of a piezoelectric material, it is expected that various functions such as a haptic, fingerprint sensor, and a touch sensor will be possible like Figure 6 - 13 (c). The more details of configuration for the display panel applied transparent piezo-ceramics are shown in Figure 6 - 14 (a, b).
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Figure 6 - 14 The schematic illustration of (a) device concept for speaker/haptic operation and (b) display panel structure applied transparent piezo-ceramics.
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