
Citation: | Haitao Li, Mingyang Li, Xiaoxue Jiang, Changda Zhu, Hongguang Wang, Aowei Deng, Guozhong Zang, Yongjun Gu. Enhanced piezoelectric properties for potassium sodium niobate lead-free piezoelectric ceramics prepared by microwave technology[J]. Materials Lab, 2024, 3(1): 230019. doi: 10.54227/mlab.20230019 |
Potassium-sodium niobate (Na, K)NbO3 (NKN) powder was synthesized at low temperature via microwave-assisted hydrothermal solovthermal method (MHSM). The resultant powder was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD results showed that pure (Na, K)NbO3 powder with a single perovskite structure was successfully synthesized when the concentration of mineralization exceeded 1 M. In order to reduce the volatilization of alkaline elements, NKN ceramics derived from 5M-powder were prepared in a microwave furnace. Microstructure, stoichiometry, and electrical properties of the obtained ceramics were investigated. The piezoelectric coefficient (
Although PbZr1-xTixO3 (PZT)-based piezoelectric ceramics have dominated the market of piezoelectric devices for more than half a century because of their superior piezoelectric properties, they will be replaced in the near future due to the toxicity of lead. Therefore, it is urgent to find environment-friendly alternative materials to meet the growing environmental awareness. Among lead-free piezoelectric compositions, potassium-sodium niobate ((Na, K)NbO3, NKN) is considered to be one of the promising candidate owing to its high curie temperature and good piezoelectricity[1-6]. However, the high-temperature sintering of traditional solid-state method makes alkali metal volatilize seriously, which causes the stoichiometric ratio deviation and affects the density of KNN ceramics, thereby damaging their piezoelectric properties[7-8]. Therefore, the piezoelectric coefficient of pure NKN ceramics fabricated by traditional solid-state method was no more than 80 pC/N[3, 9].
Special techniques like hot pressing and spark plasma sintering[10-11] are reported to be effective means for getting dense NKN ceramics, but the complex process and high fabricating cost make them unsuitable for industrial production. Therefore, sintering aids such as ZnO, CuO, and MgO have been applied to lower the sintering temperature and improve the density of NKN ceramics[5,12-15].
Another efficacious solution to obtain dense NKN ceramics is to manufacture them with ultrafine powders, based on the idea that the driving force of sintering is inversely proportional to the particle size of powders[16]. Several wet-chemical methods, such as sol-gel, hydrothermal method and pechini method, have been studied to synthesize ultrafine NKN powder[17-19]. Among them hydrothermal method is promising because it requires low temperature and delivers pure ultrafine powders without further heat treatment. Nevertheless, this method usually require dense mineralization and spend a long aging time[19]. Microwave-assisted hydrothermal solovthermal method (MHSM), as a modified hydrothermal method, has attracted more attention due to its thin mineralization concentration and short synthesis time. Bai et al. have successfully synthesized NKN powders through MHSM, reducing the concentration of mineralization from 10 M to 2 M[20]. In our previous study[21], ultrafine NKN powders have been fabricated using MHSM at 200 °C for only 30 min.
For NKN based ceramics, sintering temperature and holding time are two important factors that affect the volatilization of alkali metals. Due to the large particle size and uneven distribution of the powder synthesized by traditional solid-state method, ceramics often requires higher temperatures and longer holding times to density, which inevitably exacerbates the volatilization of alkali metals. On the contrary, the powder synthesized by wet chemical method can achieve densification by sintering at lower temperatures due to its fine particle size and uniform distribution. Wu et al. synthesized NKN ceramic powder using traditional solid-state method and sintered it at a temperature of
In the present study, microwave-assisted hydrothermal solovthermal method (MHSM) was utilized to synthesize NKN powder, and the influence of the mineralization concentration on the synthesized powder was characterized. Also, the electrical properties of NKN ceramics (from as-MHSMed powder) sintered in microwave furnace were investigated with special emphasis on sintering temperature. The d33 of 132 pC/N and Pr of 26.3 µC cm-2 can be achieved by microwave sintering at
Analytical-grade potassium hydroxide (KOH), sodium hydroxide (NaOH), niobiumpentoxide (Nb2O5) and isopropanol were used as starting materials. All of these chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd, China. First, a mixed solution of KOH and NaOH with a KOH/NaOH ratio of 3:1 was adjusted to 1.0-8.0 mol L−1. Then appropriate amounts of Nb2O5 and isopropanol were dispersed in the mixed hydroxide solution, and stirred for 30 min. The slurry was poured into a 100 ml teflon lined reactor with 60% filling ratio. The reactor was placed in a microwave synthesizer (XH-800G, BJXH, China) and the reaction was operated at 2.45 GHz. According to our previous study, MHSM temperature was set at 200 °C and hold time to 90 min. After the reaction was completed, the precipitates were filtered and washed several times with deionized water and dried at 100 °C for 6 h.
The as-MHSMed powder was mixed with 5 wt.% polyvinyl alcohol(PVA) solution, and then uniaxially pressed into pellets of 10 mm in diameter and 1.0 mm in thickness under a pressure of 80 MPa. After burning out PVA, the green disks were sintered in microwave furnace at selected temperatures between
Density of ceramics was determined by the Archimedes method. Phase structure was determined using x-ray powder diffraction with a Cu K radiation (λ=
Fig. 1 shows the XRD patterns of NKN powders synthesized by MHSM at 200 °C for 90 min with different concentration of mineralizer. It can be seen that the samples show a single phase perovskite structure without any detectable secondary phases when the concentration of mineralizer is greater than 1 M. For the sample with 1 M, a minute amount of second phase Nb2O5 (PDF32-0710) is formed due to too thin alkaline concentration. As the alkaline concentration increases from 3 M to 8 M, a splitting peak of (202)/(020) occurs at 2θ = 45-47°, which indicates the formation of a single phase orthorhombic perovskite structure. It is interesting that as the alkaline concentration increases from 1 M to 5 M, the 2θ of diffraction peak in (110) space shifts to a lower angle, and almost no changes as it further increases to 8 M. This may be attributed to the fact that NaOH is more reactive than KOH, and thus there are more Na+ ions react with Nb2O5 at low alkaline concentration, even though the K+/Na+ molar ratio is constant in the detected alkaline solution[24]. With increasing alkaline concentration, there are more K+ ions will enter into the lattice of NKN, which leads to the diffraction peak of (110) shift to a low angle owing to that K+ (1.64 Å) radius is larger than Na+ (1.39 Å) radius. However, when the alkaline concentration exceeds 5 M, the angle of diffraction peak in (110) remains nearly unchanged, indicating that the solid solubility of K+ in NKN lattice will no longer increase.
The SEM images of as-MHSMed powders with different concentration of mineralizer are shown in Fig. 2. It is observed that as-MHSMed powders exhibit a typical quadrate-like morphology, except that a few smaller crystals adhere to the larger ones for the sample with 3 M concentration, which leads to slight agglomeration, as shown in Fig. 2a. When the concentration of mineralizer exceeds 3 M, the crystal presents a stepwise growth mode, as shown by the arrows in Fig. 2b and Fig. 2c, in addition, obvious small holes appear on the crystal surface of the sample with 8 M concentration.
The corresponding particle size distribution of as-MHSMed powders are shown in Fig. 3. It can be observed that as the concentration of mineralizer increases from 3 M to 8 M, the average particle size (da) and cutting particle size (d50) increase from 2.56 μm and 2.12 μm to 5.92 μm and 3.62 μm, respectively. In addition, compared to the samples with 3M and 8M concentration, the powder with 5 M concentration has a fine morphology and a narrow distribution.
To evaluate the microwave sintering of NKN ceramics, the as-MHSMed powder with 5 M concentration was pelletized and sintered at
The EDS spectra of NKN ceramics sintered at different temperatures are shown in Fig. 5. It is observed that as the sintering temperature increases from
P-E hysteresis loops of NKN ceramics sintered at different temperatures are depicted in Fig. 6. The samples sintered at
For NKN-based piezoelectric ceramics, samples with good piezoelectric and ferroelectric properties should have high Pr and low Ec. Zuo et al. have reported a remnant polarization of 15 µC cm−2 in NKN[27]. Mahdi et al. obtained a Pr = 18 µC cm-2 in NKN prepared by microwave technology[23], and Gu et al. measured a Pr = 22.9 µC cm−2 in Ta doped NKN[19]. All these reported values are less than 26.3 µC cm−2 determined in this work, indicating that the sample sintered at
Density and electrical properties of NKN ceramics sintered at different temperatures are summarized in Table 1. It can be seen that as the sintering temperature increases from
ST (°C) | d33 (pC/N) | kp (% ) | ρ (g. cm−3 ) | pr (µC. cm−2 ) | tanδ (@10 KHz) | εr (@10 KHz) | εrpr (µC. cm−2 ) |
86 | 25 | 4.21 | 16.2 | 0.042 | 410 | ||
125 | 35 | 4.28 | 18.9 | 0.031 | 458 | ||
132 | 38 | 4.30 | 26.3 | 0.028 | 522 | ||
120 | 32 | 4.25 | 20.8 | 0.018 | 531 |
In addition, it can also be observed from Table 1 that the relative dielectric constant (εr) monotonically increase, while the dielectric loss (tanδ) monotonically decreases with the the increase of sintering temperature. The tanδ drops from 0.042 to 0.018, while εr raises from 410 to 531, as the sintering temperature increases from
A single phase orthorhombic perovskite NKN powder has been successfully synthesized using MHSM with a thin concentration of alkali liquor above 1 M. In order to avoid severe volatilization of alkali metals during high-temperature sintering process, as-MHSMed powder with 5 M concentration was pelleted and sintered in a microwave furnace. Enhanced electrical properties of d33 ~132 pC/N, kp ~38%, Pr ~ 26.3µC cm−2, and εr ~522 are obtained in the ceramic sintered at an optimal temperature of
Haitao Li conducted the research; Mingyang Li wrote the results and discussion; Xiaoxue Jiang wrote the introduction; Changda Zhu organized the data; Hongguang Wang wrote the experimental; Aowei Deng wrote the conclusion; Guozhong Zang reviewed & edited; Yongjun Gu draw charts.
This work was supported by the Higher Education Key Project of Henan Province (Grant No. 15A430005), the PhD Start-up Fund of Henan University of Science and Technology (Grant No. 09001542).
The authors declare no conflict of interest.
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ST (°C) | d33 (pC/N) | kp (% ) | ρ (g. cm−3 ) | pr (µC. cm−2 ) | tanδ (@10 KHz) | εr (@10 KHz) | εrpr (µC. cm−2 ) |
86 | 25 | 4.21 | 16.2 | 0.042 | 410 | ||
125 | 35 | 4.28 | 18.9 | 0.031 | 458 | ||
132 | 38 | 4.30 | 26.3 | 0.028 | 522 | ||
120 | 32 | 4.25 | 20.8 | 0.018 | 531 |
XRD patterns of NKN powders synthesized by MHSM with different mineralizer concentrations.
SEM images of NKN powders with different mineralizer concentrations: a 3 M; b 5 M; c 8 M.
Particle size distribution of NKN powders with different mineralizer concentrations: a 3 M; b 5 M; c 8 M.
SEM images of the fracture surface of NKN ceramics sintered at different temperatures: a
EDS spectra of NKN ceramics sintered at different temperatures: a
P-E hysteresis loops of NKN ceramics sintered at different temperatures.