# Life Cycle Assessment of piezoelectric materials used for energy harvesting systems PZT versus KNN

Rabie Aloui, Raoudha Gaha, Berk Celik, Hélène Debéda, U-Chan Chung, Catherine Elissalde, Armaghan Salehian, Barbara Lafarge

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Rabie Aloui, Raoudha Gaha, Berk Celik, Hélène Debéda, U-Chan Chung, et al.. Life Cycle Assessment of piezoelectric materials used for energy harvesting systems PZT versus KNN. 8th International conference on Smart Materials and Nanotechnology - SMN 2024, American University of Sharjah, Nov 2024, Sharjah, United Arab Emirates. 10.5281/zenodo.14725810. hal-04891831

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LIFE CYCLE ASSESSMENT OF PIEZOELECTRIC MATERIALS USED FOR ENERGY HARVESTING SYSTEMS: PZT VERSUS KNN

Rabie Aloui<sup>1,2,*</sup>, Raoudha Gaha<sup>1</sup>, Berk Celik<sup>2</sup>, Hélène Debéda<sup>3</sup>,
U-Chan Chung<sup>4</sup>, Catherine Elissalde<sup>4</sup>, Armaghan Salehian<sup>5</sup>, and Barbara Lafarge<sup>1</sup>

ABSTRACT

Over the past decade, piezoelectric materials have been extensively used in various engineering areas, partic-
ularly for energy harvesting, due to their efficient electromechanical conversion. Several scientific researches
reported in the literature have performed modeling, prototyping, and experimentation to improve the per-
formance of harvesters. Piezoelectric transducers, especially those used for energy harvesting, could have
significant environmental impacts. Few studies have emphasized the environmental issue of harvesters in
their life cycle. This paper aims to study the environmental impacts of piezoelectric energy harvesters
obtained with screen-printing process by performing a life cycle assessment (LCA). Firstly, a comprehensive
overview of piezoelectric materials and their implementation in the context of micro electromechanical sys-
tems is first presented. Secondly, the environmental impacts of these materials are briefly discussed based
on previous studies. Here the aim is to present a comparative analysis of the environmental consequences
of energy harvesters based on different piezoelectric materials, from cradle to gate. The harvester based on
lead zirconate titanate Pb(Zr,Ti)O₃ (PZT) is taken as a reference. The impact assessment is conducted by
evaluating the selected impact categories using ILCD 2011 method. Results highlight the most impactful
components of energy harvesters referring to the unique scores calculated for different impact categories at
the Midpoint. They highlight also the importance of LCA and offer technical guidance and crucial recom-
mendations for eco-design.

**Keywords:** Life cycle assessment, ecodesign, sustainability, energy harvesting, piezoelectric materials

# 1 Introduction

Piezoelectric materials have been widely investigated across various engineering fields over the past years, especially for self-powered sensing systems through energy harvesting techniques [1, 2, 3]. Zhang et al. [4] developed an energy harvester that captures rotational energy from machinery using an arc-shaped piezoelectric sheet placed between an outer race and the pedestal of the bearing. In addition to powering sensors, it can detect bearing faults. Its performance was successfully tested on a rotor test rig, demonstrating both fault detection and self-powered wireless sensing capabilities. Khazaee et al. [5] developed a self-powered condition monitoring system for rotating machines that generates energy from machine vibrations. Using a voltage multiplier, the system captures and stores energy to power a microcontroller and radiofrequency transmitter for real-time monitoring. By estimating the piezoelectric harvester output and refining the system with an electromechanical model, it was successfully tested on a water pump, where it autonomously detected faults. Hazeri et al. [6] explored the use of piezoelectric materials in tires to capture energy from

<footnote n="1"><sup>1</sup>Roberval, Centre de recherche Royallieu, Université de Technologie de Compiègne, rue du docteur Schweitzer CS 60319 - 60203, France.</footnote>

<footnote n="2"><sup>2</sup>Avenues, Centre Pierre Guillaumat, Université de Technologie de Compiègne, 60203, France.</footnote>

<footnote n="3"><sup>3</sup>University of Bordeaux, IMS Laboratory, UMR 5218, F-33405 Talence, France.</footnote>

<footnote n="4"><sup>4</sup>CNRS, University of Bordeaux, ICMCB Laboratory, UMR 5026, 87 avenue du Dr A. Schweitzer, 33608 Pessac, France.</footnote>

<footnote n="5"><sup>5</sup>University of Waterloo, Energy Harvesting and Vibrations Lab, Mechanical and Mechatronics Engineering, Waterloo, ON, N2L 3G1, Canada.</footnote>

<footnote>*Corresponding author: rabie.aloui@utc.fr</footnote>
tire deformations, generating 2.31 W from 56 piezoelectric patches. Energy harvesting becomes more efficient as vehicle speed and weight increase, particularly when the elements are placed on the outer surface of the tire. However, the technology slightly increases the environmental impact, hence further research is required to assess its durability and longevity. Zheng et al. [7] reviewed advancements in piezoelectric wind energy harvesters, focusing on micro electromechanical systems (MEMS). They discussed piezoelectric material classifications and operational principles by exploring research from various perspectives. The review highlighted existing literature and future development directions by offering valuable insights for industry innovation.

The most commonly used piezoceramic for energy harvesting is lead zirconate titanate Pb(Zr,Ti)O₃ (PZT), due to its high electromechanical conversion efficiency [8, 9]. Fernandes et al. [10, 11] introduced a piezoelectromagnetic energy harvester, which is manufactured using PZT screen-printing technology and featuring a centrally-supported meandering structure. Experimental tests on a fabricated harvester, which had a thick PZT layer printed on a stainless steel substrate, confirmed the accuracy of frequency response functions predicted through a finite elements model. The proposed design demonstrates a higher normalized power density compared to other MEMS-based piezoelectromagnetic devices and exhibits significant potential when compared to larger devices that utilize bulk or thin-film piezoelectrics. Taborda et al. [11] have developed a printed mechanical energy harvester for powering autonomous systems, especially microelectromechanical systems. The resonant harvester consisted of a multilayer structure formed by a screen-printed PZT layer between two screen-printed gold electrodes and a stainless steel substrate. Santawitee et al. [12] introduced a cost-effective technology utilizing screen-printing to fabricate PZT-based microdisks with sacrificial layers. The study investigated the influence of different electrode materials, including Au and Ag/Pd, on the release behavior. A comparison of PZT particle sizes showed that microdisks made from nanometric PZT particles, combined with Ag/Pd electrodes, exhibited high electromechanical coupling efficiency. Despite the wide application of lead-based materials its use is limited by regulations aimed at environmental protection. Bell et al. [13] review the toxic effects of lead and outline existing regulations aimed at mitigating its environmental impact. Additionally, their research seeks to assess the risks linked to the production, utilization, and disposal of lead-based piezoelectric materials. The study highlights that the piezoelectric industry is contributing an increasing portion, around 0.1%, to electronic waste while intensifying the demand for lead-free alternatives. As a result, ongoing research has focused on developing lead-free piezoelectric materials that demonstrate promising performance.

Significant progress has been made over the last few years in developing lead-free piezoceramics to replace PZT in electromechanical devices. Rödel et al. [14] outlined guidelines for developing lead-free piezoelectric ceramics by focusing on selecting chemical elements based on cost, toxicity, and ionic polarizability. It discusses various crystal structures and phase diagrams with good piezoelectric properties and uses insights from density functional theory to refine these concepts. Moreover, several studies focus on the environmental assessment of lead- and lead-free piezoceramics based on their life cycle flowchart, as shown in Figure 1. Ibn-Mohammed et al. [15] highlighted the environmental advantages of potassium sodium niobate (K,Na)NbO₃ (KNN), which is considered a less hazardous piezoelectric material compared to PZT, primarily due to its ~60 wt% Nb₂O₅ content. This makes it significantly less toxic to humans than lead oxide in piezoelectric applications. As a result, KNN and sodium bismuth titanate (NBT) have been proposed as alternatives to replace PZT in various electronic components due to its toxicity. Da Silva Lima et al. [16] performed a life cycle assessment to quantify the environmental impacts at the production of both FeNb and Nb₂O₅, by using production data provided by the niobium manufacturer responsible for roughly 75% of the global market. Ibn-Mohammed et al. [17] conducted an environmental assessment comparing NBT, KNN and PZT for their potential transition from lab to market. The study found that NBT has a more favorable environmental profile than both KNN and PZT, primarily due to its lower energy consumption during synthesis and production. However, when comparing NBT to PZT specifically, Bi₂O₃ (used in NBT) has a greater environmental impact than PbO (used in PZT) in certain areas including climate change. Wu et al. [18] conducted a life cycle assessment comparing PZT and KNN-based ceramics with a volume of 0.001 m³ from cradle to gate. The study found that PZT has a greater environmental impact than KNN ceramics, mainly due to the extraction and processing of lead. KNN ceramics account for only 28% of PZT's overall environmental impact in areas such as toxicity and resource use, making them a more eco-friendly option. However, KNN production still requires improvements to reduce energy consumption and emissions, particularly during Nb₂O₅ extraction. The study offers important insights into material sustainability.

This paper concentrates on assessing the environmental impact of a piezoelectric energy harvester produced through the screen-printing process detailed in these works [10, 11]. It marks an initial step towards adopting an eco-design approach aimed at replacing PZT with KNN, while preserving similar performance levels. Cradle-to-gate life cycle assessment will be performed using an appropriate method to calculate the
<figure><img src="image_2.png" /><figcaption>Figure 1: The flowchart of life cycle assessment of piezoceramics [18].</figcaption></figure>

unique score for impact categories. This paper begins with a brief introduction highlighting advancements in energy harvesting using piezoelectric materials and the environmental need for lead-free alternatives in piezoceramics. The following sections cover specific topics: Section 2 introduces the methodology for conducting a life cycle assessment on a piezoelectric energy harvester, detailing the life cycle inventory (LCI), which includes identifying materials, estimating their weights, and assigning manufacturing processes to understand the sensor's structure. Section 3 presents the results and discussion, incorporating a life cycle impact assessment (LCIA) for selected environmental impact categories, along with initial interpretations. Section 4 concludes with technical recommendations for design improvements and sustainability.

## 2 Methodology

The case study involves conducting a LCA on a zigzag piezoelectric energy harvester, as described in the [10, 11] research study. The harvester is sized for a first resonance frequency of 60 Hz which corresponds to the resonance frequency of the power grid. The harvester is excited by an electromagnetic device with a permanent magnet mounted at its free end and a conductive wire is placed under the magnet with a well-defined spacing. The latter is supplied by an alternating current from the power supplier system. Consequently, a Lorentz-type excitation force is created which causes the harvester to vibrate. Figure 2 presents the real harvester along with a drawing illustrating its geometric dimensions. As mentioned in the introduction, the environmental impacts of this harvester will be assessed throughout its life cycle for both PZT and KNN piezoceramics.

<figure><img src="image_3.png" /><figcaption>Figure 2: The piezoelectric screen-printed energy harvester: (a) the physical harvester with an enlarged view, and (b) the design drawing with geometric dimensions [10, 11].</figcaption></figure>
The LCA study aligns with the ISO 14040 and ISO 14044 standards, which outline the principles and specific requirements as well as offer guidelines for conducting LCA. The overall methodological structure of LCA is divided into four key stages [19]: (1) goal and scope definition, (2) inventory analysis, (3) impact assessment, and (4) interpretation.

## 2.1 Scope and goal

Conducting a LCA for the zig-zag piezoelectric harvesters involves several key steps. The objectives of the study are outlined below:

* Identify the components and stages that contribute most significantly to the overall environmental impact.

* Assess the environmental impacts associated with the harvester from the production phase through to its use phase.

The harvesters adhere to the same functional unit, possess identical lifespans, and operate under similar assumptions regarding transportation and usage until the end of their service life. Table 1 outlines the key elements of the LCA, including the functional unit, lifespan, system boundaries, and evaluation method. It offers a clear and concise overview of the essential aspects of the LCA by ensuring a thorough understanding of the assessment's parameters and objectives.

<table><caption>Table 1: The scope of the LCA of the piezoelectric energy harvester.</caption><thead><tr><td>LCA Keys</td><td>Descriptions</td></tr></thead><tbody><tr><td>Functional unit</td><td>Piezoelectric energy harvester converts the vibration into electrical signal during its lifespan.</td></tr><tr><td>Lifespan</td><td>It is assumed to be 3 years.</td></tr><tr><td>System Boundaries</td><td>Cradle-to-gate: From raw material extraction to use stage</td></tr><tr><td>Method/Normalization/Ponderation</td><td>European, ILCD 2011 Midpoint+/EC-JRC Global, Equal weighting</td></tr><tr><td>Environmental impact categories</td><td>16 indicators of Midpoint</td></tr></tbody></table>

## 2.2 Life cycle inventory analysis

Life cycle inventory (LCI) involves the collection and compilation of data within a defined system boundary by leading to the subsequent phase of life cycle impact assessment (LCIA). Based on laboratory estimates and literature reviews, LCI gathers detailed information on raw materials, energy use, product outputs, and environmental emissions throughout the life cycle of piezoelectric harvesters. As the second phase of life cycle assessment, LCI focuses on collecting and organizing data on processes according to the established system boundaries.

The characteristics of the harvester under examination are detailed in Table 2, outlining key attributes that contribute to its operational efficiency and durability. These specific features are not only integral to the harvester's ability to carry out its functional unit but also play a crucial role in its long-term performance and sustainability.

<table><caption>Table 2: Characteristics of piezoelectric energy harvester.</caption><tr><td>PEH architecture</td><td>Zig-zag</td></tr><tr><td>Layer materials</td><td>Stainless Steel (SS)/Au/PZT/Au</td></tr><tr><td>Technology</td><td>All layers are screen-printed on Stainless Steel substrate and co-fired</td></tr><tr><td>Area dimensions</td><td>11.5 mmx12.7 mm</td></tr><tr><td>Au thickness</td><td>~ 10µm</td></tr><tr><td>PZT thickness</td><td>~ 50µm</td></tr><tr><td>Energy harvesting test</td><td>Piezoelectric-magnetic principle</td></tr></table>
Throughout its lifespan, these core attributes ensure the harvester remains reliable and capable of meeting the demands of various operational conditions. Therefore, any design modifications or optimizations must take these characteristics into careful consideration. Preserving or enhancing these features is essential to maintaining the harvester's functionality while adhering to the fundamental principles of ecodesign. This approach ensures that design improvements do not compromise environmental impact objectives, such as resource efficiency, reduced emissions, and minimal ecological footprint, which are central to sustainable product development.

In this study, lead zirconate titanate (PZT) and potassium sodium niobate (KNN) piezoceramics undergo similar preparation processes including weighing, ball milling, drying, calcination, re-milling, screen printing, layer pressing, and sintering. Electrical energy consumption for each of the manufacturing processes is calculated as follows:

$$E = \frac{P \times t}{1000} \qquad (1)$$

where E is the electrical energy required (kWh), P is the electrical power (W), and t denotes the process time (h).

The electrical energy consumption inputted into our LCA model is taken from the study of Wu et al. [18], which provides the data necessary for the production of 0.001m³ of PZT and KNN. To determine the weight percentage of PZT chemical compositions, we used energy-dispersive X-ray spectroscopy (EDXS), which provides detailed elemental and chemical analysis. Figure 3 shows the EDXS spectrum of the PZT powder with sintering aid, obtained under controlled experimental conditions with a 15 kV accelerating voltage and 4 µ-seconds sampling time. A preset time of 60 seconds was used to gather enough X-ray counts for accurate and reliable results.

<figure><img src="image_4.png" /><figcaption>Figure 3: The energy-dispersive X-ray spectroscopy spectra with microphotograph for PZT powder with sintering aid.</figcaption></figure>

Our LCA study focuses on evaluating the stratified piezoelectric harvester composed of a linear-elastic substrate, a piezoceramic layer, and high electrical conductivity electrodes, as shown in Figure 4. The thin substrate is fabricated from stainless steel SS301, and the electrodes are printed from the commercial gold ink ESL8836 developed by ElectroScience. The piezoceramic layer is composed of 97%wt of Pz 26 powder, produced by Ferroperm Piezoceramics [20], and 3%wt of a sintering aid LBCu (0.8% Li₂CO₃, 1.2% Bi₂O₃, and 1% CuO by weight). The percentage of the chemical composition measured by weight of the PZT powder is given in the pie chart in Figure 4. The study involves analyzing resource use and environmental impacts. This includes aspects such as materials, energy inputs, transportation, manufacturing processes, and other relevant factors. The collected data of these different processes are then inputted into the LCA model which is powered by the Ecoinvent 3 database.

The study incorporates several assumptions regarding transportation and manufacturing locations, as well as the use scenarios. It is assumed that all components of the harvester are produced in France, with the supply chain confined to French territories within a maximum path of 1074 km. Additionally, electrical energy consumption for certain manufacturing processes is estimated based on data from similar previous
<figure><img src="image_5.png" /><figcaption>Figure 4: Piezoelectric energy harvester breakdown structure with the weight percentage of the chemical composition of the piezoceramic powder obtained with EDXS.</figcaption></figure>

research studies. The comprehensive list of impact indicators based on elementary flows presents the output of LCI, which serves as input for the LCIA phase.

**3 Results and discussion**

Figure 5 presents a comparison of the environmental impact of PZT and KNN material extraction and processing across the 16 midpoint impact categories. The total impact score for PZT is 2.878, while for KNN it is 0.717, indicating that PZT's environmental impact is approximately four times greater than that of KNN. For PZT, the most significantly impacted environmental aspects are mineral, fossil, and renewable resource depletion, freshwater ecotoxicity, and human toxicity (both cancer and non-cancer effects). Impact categories with unique scores less than 0.0039 Pt are assumed to be less impacted, which are grayed-out in the legend: climate change, ozone depletion, ... etc. Furthermore, water resource depletion and land use have the least environmental impact. For KNN, the most significantly impacted environmental aspects are the depletion of mineral, fossil, and renewable resources, as well as freshwater ecotoxicity and human toxicity (both carcinogenic and non-carcinogenic effects). However, the overall impact scores for these aspects are lower than those of PZT. The least impacted aspects of KNN are ozone depletion and land use. Ionizing radiation E (intermediate) is assessed as unaffected, and its unique score is zero for both PZT and KNN.

<figure><img src="image_6.png" /><figcaption>Figure 5: The unique score versus the piezoceramic materials (PZT and KNN) of the harvester for the 16 impact categories of Midpoint: The grayed-out impact categories are assumed to be unimpacted.</figcaption></figure>

The environmental profiles of PZT and KNN are compared across the impact categories in Figure 6, providing a detailed assessment of their ecological footprints. These materials are evaluated based on their
respective ratios in the six impact categories mentioned above. KNN exhibits notable environmental burdens across all categories by reflecting its broad impact on the environment. In contrast, PZT shows a much higher ratio in certain categories, specifically in human toxicity, both carcinogenic and non-carcinogenic effects, ozone layer depletion, ionizing radiation, freshwater eutrophication, and freshwater ecotoxicity. These categories suggest that PZT's environmental cost is particularly severe in areas related to chemical toxicity and radiation exposure. Conversely, KNN's environmental impact is more concentrated in categories associated with atmospheric and land-related effects, such as particulate matter formation. The higher impacts in these areas are largely attributable to the initial stages of KNN's life cycle, notably during raw material extraction and purification processes. This highlights the significance of upstream activities in shaping KNN's environmental profile. While KNN's overall environmental burden may be lower than that of PZT in certain categories, its contributions to specific environmental stressors, particularly those linked to atmospheric and land-use impacts, remain significant. The comparison underscores the complex trade-offs involved in selecting materials, as neither PZT nor KNN can be considered universally superior in terms of environmental sustainability. Each material carries distinct ecological risks that must be weighed according to the application and environmental priorities.

<figure><img src="image_7.png" /><figcaption>Figure 6: The environmental profile of piezoceramic materials (PZT and KNN) of the harvester with the six considered impact categories.</figcaption></figure>

Figure 7 presents the unique environmental score in Points (Pt) for the primary components of the harvester analyzed in LCA study. The unique score is derived from 16 selected impact categories at the Midpoint level, as previously described. Impact categories with unique scores below 0.0039 Pt are considered less significant and are shown in gray in the legend. According to the results, the most environmentally impactful component is the electrodes, with a unique score of 7.57, followed by the piezoceramic layer (PZT), which has a score of 2.88, and the stainless steel substrate SS301, with a score of 0.77. For the electrodes, the dominant impact categories include human toxicity (non-cancer effects), freshwater ecotoxicity, and human toxicity (cancer effects), signifying the considerable contribution of the electrodes to toxicological impacts on both human health and ecosystems. Specifically, the electrodes and the PZT layer demonstrate notably higher levels of human toxicity without carcinogenic effects, indicating potential risks to human health due to their material composition and production processes. Meanwhile, the substrate is most affected by climate change and ionizing radiation, particularly highlighting its contribution to global warming potential and human health risks from radiation exposure. Notably, ionizing radiation emerges as a significant concern across all components of the harvester. This can be attributed to the energy mix utilized in the LCA, which reflects the characteristics of French high-voltage nuclear electricity production, which is known for its radiation-related impacts due to nuclear power dependency. These findings emphasize the importance of material selection and energy sourcing in minimizing environmental and health impacts in the production and operation of the harvester.
<figure><img src="image_8.png" /><figcaption>Figure 7: The unique score versus the different part of the harvester for the 16 impact categories of Midpoint: The grayed-out impact categories are assumed to be unimpacted.</figcaption></figure>

## 4 Conclusions

This research compares the environmental impacts of PZT and KNN materials by highlighting that PZT has a significantly higher total environmental impact, about four times greater than KNN, particularly in areas like resource depletion, toxicity, and ecotoxicity. However, KNN also has its environmental burdens, especially in categories related to atmospheric and land effects, such as climate change and acidification, largely driven by its early life cycle processes like raw material extraction. Both materials show trade-offs in their environmental profiles. While PZT is particularly harmful in toxicity and radiation-related categories, KNN has notable impacts on land use and atmospheric emissions. Neither material can be considered universally better in terms of sustainability; the selection between them should depend on the specific application and environmental priorities. Additionally, the study of the harvester's components shows that environmental impact of electrodes are determined the most significant, which is followed by the PZT layer with significant contributions to human toxicity and ecotoxicity. The precise chemical composition of Au ink, along with thorough environmental assessment, is necessary for optimization and could be a focus of our study. The stainless-steel substrate has a lesser but still notable impact, especially in climate change and ionizing radiation. The importance of energy sourcing, particularly from nuclear power in France, is emphasized due to its contribution to radiation-related environmental impacts.

## Acknowledgments

This work is co-funded by the multidisciplinary initiative "mastery of safe and sustainable technological systems" of the Sorbonne University Alliance.

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