Three-step sintering method paper

Three-step sintering method

Abstract: The sintering aid, CuO was added to (K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃-CaZrO₃piezoceramics by three-step sintering method to industrialise the lead-free piezoelectric ceramic products. Through a three-step sintering method, the sintering temperature of the KNN based lead-free ceramics system was effectively reduced to 700^oC. At a sintering temperature of 700^oC, the good piezoelectric property of d₃₃ = 238pC/N was obtained.

Keywords: Three-step sintering, CuO, Sintering temperature, KNN

1. Introduction

Products based on potassium-sodium niobate (KNN) with better environmental stability were considered as a promising candidate to replace the commonly used lead-based products and used in actuators and sensors. However, the sintering temperature for KNN based ceramics should be lowered for application to multilayer devices. Most ternary KNN-based materials have been sintered at over 1100^oC, resulting in easy volatilisation of potassium and sodium elements.

Reducing the temperature of the sintering is an important volatilisation process. Because of its low melting point and liquid phase formation, CuO has documented being able to reduce the sintering temperature for KNN-based materials, similarly to its application for low-temperature sintering. However, CuO’s sintering temperature added KNN based ceramics remains around 950^oC, the temperature of which is too high for industrial application.

In our previous work, 0.96(K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃-0.04CaZrO₃ ceramics (KNLNS-CZ) were prepared using three-step sintering method; excellent piezoelectric properties were obtained, d₃₃ = 420pC/N but the sintering temperature was approximately 1050^oC for this system, and holding 5 hours. Therefore, CuO as sintering aid has been added to KNN-based ceramics in this work and prepared using three step sintering methods.

Experimental procedure

The raw materials powders CuO (99%), Li₂CO₃(99%), Na₂CO₃(99%),K₂CO₃(99%),Nb₂O₅(99%), ZrO₂(99%), Sb₂O₅ (99%) and CaCO₃(99%) were mixed according to a stoichiometric ratio, followed by planetary mill for 24h with ethanol. After 3 hours of drying at 120^oC, the mixture was calcined at 850^oC for 6 hours. Then pressed into 10 mm pellets in diameter after the dried powder has been mixed with a polyvinyl alcohol binder solution, and prepared by three step sintering method, the first step was sintered at 1000^oC, the second step was sintered at 1100^oC, and the third step was sintered at 700^oC for 5h. The phase structure was examined at room temperature by XRD (Bruker D8 Advance, Germany). The microstructures have been characterised by SEM (JSM, EMP-800, Tokyo, Japan). The temperature dependence of the dielectric constant and the loss of the samples was measured using a high-precision LCR meter (E4980A, Agilent, Palo Alto, CA). The FE hysteresis loop curves of the samples were measured using the FE test system (Precision Premier II; Radiant Technologies Inc., Albuquerque, NM). A quasi-static piezoelectric constant test meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Science) was used to verify the piezoelectric coefficient d₃₃ at room temperature.

2. Results and discussion

Fig.1 shows the KNN based ceramic XRD patterns, sintered for 10 hours at 600^oC. Typical perovskite structure with specific CuO content added was observed without the secondary phases. As can be seen from Fig.1, the splitting peaks change with the increased CuO content. It has been reported that Cu^{2 +} has the possibility of replacing A or B site ions in KNN-based ceramics. Low Cu^{2 +} could be incorporated into the A (K⁺, Na⁺) lattice, while higher Cu^{2 +} could be incorporated into the Nb^{5 +} lattice. (200) peaks gradually reduce with CuO content increases in the range of 0 ≤x≤0.2% because of partial Cu²⁺(R=0.73Å, CN=6) replace in K⁺(R=1.64 Å, CN=12) and Na⁺(R=1.39 Å,CN=12) site, while owing to Cu²⁺(R=0.73Å, CN=6) replace in Nb⁵⁺(R=0.64Å, CN=6) site, (002) peaks gradually increase with CuO content increases in the range of 0.5% ≤χ≤2%. The change in splitting peaks indicates that the CuO can induce the transition phase of KNN-based ceramics.

Fig.2 shows the XPS patterns of the KNN based ceramics. All Cu ions are +2, when the content of Cu²⁺ is less, Cu²⁺ will replace K⁺ and Na⁺ , which results in a vacancy at A site. Due to the formation of cation vacancy in the sintering process, the ion diffusion process is greatly accelerated and the sintering of piezoelectric ceramics is promoted. Cu^{2 +} will replace Nb^{5 +} at the B site with an increase in Cu content, which will cause a large number of oxygen vacancies, lead to crystal cell contraction and decrease the diffusion velocity. The results also show that the diffraction peak is moving in a high angle direction with the CuO content increasing from 0.05% mol to 2% mol in Figure 2.

Fig.3 shows the SEM of KNLNS-0.04CaZrO3 ceramic samples with different CuO content. Fig.3 shows that all ceramic samples have a uniform and compact microstructure, the addition of CuO facilitates the sintering of ceramic samples, the grain size increases as the CuO content increases.

Fig.4 shows dielectric constant against temperatures of KNLNS-0.04CaZrO₃ ceramic samples with different CuO content, measured at 10kHz and in the temperature range from 25^oC to 500^oC. The dielectric peak decreases first and then increases with increase of CuO content, there is no significant change in Curie temperature (T_c) as 0%mol ≤CuO≤0.5%mol, however, T_c increases as CuO≥1%mol, it is mainly because a small amount of CuO will replace A site, while a large amount of CuO will replace B site. There is no obvious change in the temperature of orthorhombic phase transformation to tetragonal phase (T_O-T)with increase CuO content .

Fig.5 shows P–E hysteresis loops of the KNLNS-0.04CaZrO₃ ceramics with different

CuO content, measured at a frequency of 10kHz. All the ceramic samples are well

Saturated hysteresis loops. Clearly, with the introduction of CuO, the ferroelectric properties of KNN-based ceramics are weakened, the remnant polarisations(P_r) decrease with the increase of CuO content. Previous reports found similar results.

Fig.6 shows piezoelectric constant d₃₃ and relative density with different CuO contents. It can be seen from Fig.6 that both piezoelectric constant d₃₃ and relative density are increase first and then decrease. The ceramic samples are obtained excellent performances when CuO=0.5%, where d₃₃ and relative density show peak values of 237pC/N and 98% respectively.

Conclusions

Cuo was used as a sintering aid for KNN-based ceramic material and a three-step sintering method for ceramics was introduced. The excellent properties were obtained when CuO=0.5% and sintered at 700°C, d₃₃=237pC/N, Tc=210°C. This ceramic system and three-step sintering method should be useful for the industrialisation of lead-free piezoelectric ceramics

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