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A Comparative study of KNN-based piezoceramic composite synthesized by two different sintering methods

This study demonstrates that the two-step sintering method significantly enhances the dielectric, ferroelectric, and piezoelectric properties of lead-free KNN-based piezoceramics compared to conventional sintering, resulting in superior performance metrics such as a higher piezoelectric coefficient (d33 = 288 pC/N) and stronger relaxor behavior.

Original authors: Saraswati Rawat, Chongtham Jiten, Radhapiyari Laishram, K. Chandramani Singh

Published 2026-07-23

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CC BY 4.0

Original authors: Saraswati Rawat, Chongtham Jiten, Radhapiyari Laishram, K. Chandramani Singh

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where your phone charger, your medical ultrasound machine, and even the speaker in your smart home could run without using lead—a toxic metal that's great at making things vibrate but terrible for the environment. Scientists are on a mission to find "lead-free" heroes to take over. One of the star candidates is a material called KNN (Sodium Potassium Niobate). Think of KNN as a tiny, energetic dance floor made of atoms. When you squeeze it, the atoms shuffle, creating electricity; when you zap it with electricity, they dance and move. This two-way magic is called the piezoelectric effect.

However, there's a catch. KNN is a bit of a diva. It contains ingredients like potassium and sodium that are very volatile, meaning they love to evaporate into the air if you get the heat just a little too high. If you try to bake these ceramic tiles like a normal cookie, they often end up full of holes (porous) or missing ingredients, which ruins their dance moves. To fix this, scientists have been trying different "baking recipes" (sintering methods) to get the atoms to pack together tightly without running away. The big question is: Can we tweak the baking process to make these lead-free materials dance better than ever before?

This research paper dives into that exact question by comparing two different ways of baking a specific lead-free ceramic recipe. The scientists took a mixture of KNN with a few extra ingredients (BNZ and SnO₂) and baked two batches: one using the standard, "one-and-done" oven method (Conventional Sintering), and another using a more complex "two-step" technique (Two-Step Sintering). Think of the standard method as baking a cake at a steady temperature until it's done. The two-step method is more like starting the oven hot to get the batter to set quickly, then turning the heat down significantly to let the cake finish cooking slowly without burning or drying out.

The results show that the "two-step" baker was the clear winner. The ceramic made with this method turned out to be denser and more uniform, like a perfectly packed suitcase with no empty air gaps. When the scientists tested how well these materials performed, the two-step sample was significantly stronger. It had a much higher ability to store electrical energy (a dielectric constant of 2496 compared to 1502 for the standard method) and could hold onto more electric charge (remanent polarization of 9.55 µC/cm² versus 5.53 µC/cm²). Most importantly, when it came to the actual "dance" of generating electricity from pressure, the two-step sample scored a piezoelectric coefficient of 288 pC/N, beating the standard sample's 221 pC/N.

The paper also looked at how the atoms behaved inside the material. They found that the two-step method created a "relaxor" behavior, which is a fancy way of saying the atoms are a bit more flexible and chaotic in a helpful way, allowing them to switch directions more easily. This flexibility, combined with the tighter packing, made the material much more efficient. The study concludes that by simply changing the heating schedule—using the two-step sintering method—we can significantly boost the performance of these eco-friendly materials, making them a much more promising choice for future electronics and medical devices.

Technical Summary: Comparative Study of KNN-Based Piezoceramic Composites Synthesized by Two Different Sintering Methods

Problem Statement
Lead-free piezoceramics based on Sodium Potassium Niobate (KNN) are recognized as superior alternatives to lead-based materials due to their environmental compatibility and excellent properties. However, the fabrication of high-density KNN ceramics via conventional air sintering is hindered by the high volatility of its constituent alkaline elements (Na and K). This volatility often leads to stoichiometric deviations and poor densification, limiting the material's dielectric and piezoelectric performance. While advanced techniques like hot pressing and spark plasma sintering exist, they are complex and costly. Consequently, there is a need to optimize simpler, industrially viable methods, such as air sintering, by refining parameters like temperature profiles to achieve dense, stoichiometric ceramics with enhanced electrical properties.

Methodology
The study investigates the electrical and microstructural properties of a lead-free piezoceramic composite with the composition 0.982(K0.485Na0.485Li0.03)(Nb0.96Sb0.04)O3-0.015Bi0.5Na0.5ZrO3-0.003SnO2 (KNN-BNZ-SnO2). The powders were synthesized using a solid-state reaction method involving high-purity raw materials, ball milling, and calcination at specific temperatures (800°C–850°C).

The core of the research involves a comparative analysis of two distinct sintering techniques applied to the same composite material:

Conventional Sintering (CS): A standard single-step air sintering process.
Two-Step Sintering (TSS): A method utilizing a high initial temperature followed by a lower holding temperature to control grain growth and densification.

The resulting ceramics were characterized using:

X-Ray Diffraction (XRD): To determine phase purity and crystal structure.
Scanning Electron Microscopy (SEM): To analyze grain morphology and calculate average grain size.
Archimedes Method: To measure bulk density.
Impedance Analysis: To evaluate temperature- and frequency-dependent dielectric properties, including the calculation of the diffusivity coefficient ( $\gamma$ ) using the modified Curie-Weiss law.
Ferroelectric Loop Tracing: To measure polarization hysteresis ( $P-E$ loops), remanent polarization ( $P_r$ ), spontaneous polarization ( $P_s$ ), and coercive field ( $E_c$ ).
Piezoelectric Testing: To determine the piezoelectric charge coefficient ( $d_{33}$ ) after poling.

Key Results
The comparative analysis revealed distinct differences in the performance of ceramics produced by the two methods:

Microstructure and Phase: Both methods yielded pure perovskite phases without secondary phases. However, XRD analysis indicated that the CS sample possessed an orthorhombic crystal structure, whereas the TSS sample exhibited a mixed orthorhombic-tetragonal phase. This mixed phase in the TSS sample suggests an increased number of poling directions, facilitating easier dipole switching. SEM analysis showed that the TSS sample achieved a higher bulk density (4.1607 g/cm³ vs. 4.0293 g/cm³ for CS) and a more uniform grain size distribution (average grain size of 3.12 µm vs. 3.29 µm for CS).
Dielectric Properties: The TSS sample demonstrated superior dielectric performance. At room temperature, the dielectric constant ( $\varepsilon_{rt}$ ) was 2496 for TSS compared to 1502 for CS. The TSS sample also exhibited a stronger relaxor ferroelectric character, evidenced by a higher diffusivity coefficient ( $\gamma = 1.70$ ) compared to the CS sample ( $\gamma = 1.33$ ). Furthermore, the TSS sample showed lower dielectric loss across the measured temperature range (dropping to 0.025 vs. 0.045 for CS).
Ferroelectric Properties: The TSS sample displayed enhanced ferroelectric behavior with a higher remanent polarization ( $P_r = 9.55$ µC/cm² vs. 5.53 µC/cm²) and spontaneous polarization ( $P_s = 11.7$ µC/cm² vs. 7.45 µC/cm²). Notably, the coercive field ( $E_c$ ) was lower for the TSS sample (5.61 kV/cm) compared to CS (9.86 kV/cm), indicating easier domain switching.
Piezoelectric Properties: Correlating with the improved dielectric and ferroelectric metrics, the TSS sample achieved a significantly higher piezoelectric coefficient ( $d_{33} = 288$ pC/N) compared to the CS sample ( $d_{33} = 221$ pC/N).

Significance and Claims
The authors conclude that the Two-Step Sintering (TSS) method is a superior approach compared to conventional sintering for enhancing the overall performance of KNN-BNZ-SnO2 piezoceramics. The study attributes the improvements in the TSS sample to better densification, reduced defect concentrations, and a more uniform microstructure, which collectively promote the formation of polar nano-regions and facilitate domain switching.

The paper claims that by simply varying sintering parameters—specifically adopting a two-step temperature profile—researchers can tailor the properties of ceramic samples to achieve desired high-performance metrics without resorting to complex, high-cost processing techniques. The results suggest a promising future for the TSS method in the development of high-performance, lead-free piezoelectric materials.

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