Advanced Smart and Multifunctional Materials Volume 1: Fundamentals and Types

The advancement of high-performance materials in various technological applications, such as sensors/actuators, structural health reinforcement and monitoring, healthcare applications, energy harvesting and storage, and environmental applications relies on smart and (multi)functional materials. Smart materials, also identified as stimuli-responsive materials, respond to external stimuli with changes in their physical-chemical properties. External stimuli include thermal, magnetic, electrical, mechanical, or electromagnetic. This chapter describes the main types of smart materials, including the different stimuli that trigger responses, the primary smart materials, and their functional responses. Furthermore, the main conclusions and future perspectives on this class of materials are presented.

Advanced multifunctional materials are important for enabling technologies that require complex demands of modern applications, incorporating many properties for versatile, efficient, and sustainable performance. The precise control of material synthesis can improve understanding of the material and the relationship between microstructure and properties, enabling alteration of hydrophilic-hydrophobic interfaces. This chapter discusses the definition and main physical principles of advanced multifunctional materials. Advanced multifunctional materials are designed to demonstrate various electrical, mechanical, thermal, and optical functions inside a singular material or system. They improve performance and facilitate innovative applications in several sectors, such as energy, electronics, and biomedicine. This chapter examines the fundamental physical principles, encompassing interface engineering, phase transitions, nanostructure, and synergetic effects. The importance of composite design, synergistic effects, and stimulus-responsive features is emphasized. The discussion addresses key difficulties and future directions in developing multifunctional materials, emphasizing the integration of autonomy, personalization, and environmental sustainability. By tackling these difficulties, modern and advanced multifunctional materials possess the capacity to improve numerous industries and offer a sustainable future.

Piezoelectric materials convert mechanical solicitations into electrical signals, and vice versa. Through this property, they have become extremely versatile and useful in a wide range of applications areas, including automotive and aerospace, robotics, smart industry and biomedicine, among others. This chapter provides an overview for this class of materials, focusing on the piezoelectric principles, the main materials, figures of merit, (multi)functional characteristics and their applications. Regarding applications, recent and relevant advances of these materials in sensors and actuators, energy harvesting and storage, biomedical and environmental sensors are presented. Finally, the main conclusions and future trends for these materials are also provided.

Thermoelectric materials, a crucial class of multifunctional materials, have attracted significant attention due to their potential applications in energy conversion technologies. These materials offer the ability to directly convert heat into electricity, making them highly valuable for a wide range of applications, including power generation and refrigeration. Among them, bulk inorganic thermoelectric materials stand out due to their exceptional thermal stability, mechanical robustness, and reliable performance across a wide temperature range, making them particularly suitable for practical and extreme operating conditions. Recent advancements have focused on tailoring these materials to achieve optimal efficiency within specific temperature ranges. This chapter provides a comprehensive overview of the latest research and development in typical thermoelectric materials, systematically categorizing them according to their optimal operational temperature ranges. Through this structured approach, the chapter highlights the key advancements, challenges, and future prospects in the field, offering insights into the potential applications of these materials in diverse energy scenarios.

Electrochromic materials change their optical properties as a function of their oxidation state, which makes them suitable for smart windows, displays, sensors, or even energy storage devices. This chapter provides an overview of the most common electrochromic materials, the figures of merit used to describe them, and their most usual constructions. The chapter covers classic materials, such as transition metal oxides, viologens, and redox polymers, while also looking at the role of nanomaterials to overcome common problems like slow switching or poor durability. The chapter also discusses how these materials can be processed into inks and printed, which opens up possibilities for flexible or low-cost devices, although not without challenges. Towards the end, we look at some of the current limitations, such as colour range and long-term stability, and comment on where the field seems to be heading, such as multifunctional devices.

Electrocaloric effect (ECE) is the reversible change in the temperature of a dielectric due to an applied electric field under adiabatic conditions. It can provide substantial temperature change, which can be potentially exploited in solid-state cooling applications as an alternative to the conventional vapor compression-based cooling technology. This chapter aims to provide a concise overview of the latest developments in electrocaloric (EC) materials with a strong emphasis on the physical mechanisms that contribute to the EC response.

Self-healing materials are an emerging class of smart materials that exhibit excellent healing attributes and can restore damage without external intervention. These smart materials have gained much attention over the last few years because of their special characteristics, such as increased product service life, lower repair maintenance overheads, higher safety aspects, better durability, lower recycling frequency and enhanced sustainability practices. This chapter gives an overview of the history, generations, and fundamental aspects of self-healing, the types of mechanisms involved in the self-healing materials (intrinsic and extrinsic healing mechanisms with autonomous and non-autonomous approaches), major classifications of self-healing materials such as metallic, ceramic, cementitious, and polymeric materials with the detailed explanation of self-healing mechanisms applied in major technical and industrial sectors and potential applications in numerous engineering sectors like healthcare (medical/biomedical), infrastructure (concrete/cementitious), transportation (automobiles, airplanes, satellites), energy and environment (water purification membranes, supercapacitors, rechargeable batteries, E-skins, nanogenerators and solar cells). Moreover, this chapter also provides a brief overview of major challenges and future aspects that need serious consideration during the design and development of advanced and robust self-healing materials.

To enhance the durability of cement-based materials, crystalline admixtures (CA) and mineral capsules self-healing technologies were developed. The self-healing properties of Engineered Cementitious Composites and concrete members were also investigated. The results of these studies showed that CA and mineral capsules could significantly promote the self-healing efficiency of cracked cementitious materials. The main reasons are that the chelating agent in CA accelerated the production of calcium carbonate and that mineral capsules promoted the composition and content of calcite and magnesium-rich products. Additionally, the effect of cementitious capillary crystalline waterproof materials and calcium-containing environment jointly promoted the self-healing of cracks. CA heightened the crack closure and resistance to corrosion of the concrete members in marine exposure. These studies are expected to achieve the engineering application of concrete with self-healing capability, thereby achieving the sustainable development of the construction industry.

The discovery of rare-earth-based permanent magnets revolutionized modern technology, enabling advances in renewable energy, medical applications, defense, and consumer electronics. Since the 1960s, rare-earth-transition metal alloys, such as SmCo and NdFeB, have demonstrated exceptional magnetic properties, with NdFeB magnets now dominating the market due to their superior energy product ((BH)_max > 440 kJ·m^-3) and high coercivity (> 2 T). However, the reliance on critical raw materials, such as rare earth elements (REs) and cobalt, poses significant environmental and geopolitical challenges. The extraction and refinement of these materials are complex, energy-intensive, and generate hazardous waste. To address these concerns, research efforts have focused on alternative hard magnetic materials, including Fe-rich phases, Mn-based alloys and ThMn₁₂-type compounds, which reduce RE content while maintaining competitive magnetic performance. Despite advancements, these alternatives still have to improve the performance of NdFeB magnets. The growing demand for permanent magnets in wind energy and electric vehicles underscores the urgent need for sustainable, high-performance materials. This chapter explores the historical development, material properties, and future perspectives of permanent magnets, emphasizing strategies to mitigate supply chain

Magnetostriction and magnetoelasticity effects enable the development of a wide variety of sensors and transducers by exploiting the ability of certain magnetic materials to convert magnetic energy into mechanical energy and vice versa. Among these, amorphous ferromagnetic materials, also known as metallic glasses, are widely used, primarily in the form of long ribbons. When subjected to an alternating magnetic field, these materials experience magnetostrictive deformations that generate elastic waves and mechanical resonances at specific frequencies. These deformations, in turn, magnetoelastically induce changes in the magnetization, which can be remotely detected. The extreme sensitivity of the magnetoelastic resonance to external parameters as external pressure, temperature changes, mass deposited on the ribbon surface, and damping effects from surrounding fluids makes these devices highly effective for diverse sensing applications. With their inherently simple design and intrinsic wireless operation, these sensors offer a versatile and efficient solution for real-time monitoring. In this chapter and after a brief overview of the magnetoelastic phenomena and relevant materials, two recent applications of the magnetoelastic resonance will be reviewed, related to human health monitoring and the detection of gases for environmental air quality assessment.

This chapter focuses on the flexoelectric effect, an interesting electromechanical coupling phenomenon in dielectric materials. It begins with an overview of the development history and fundamental concepts of the flexoelectric effect. Following this, various experimental methods for characterizing the flexoelectric effect are summarized. Current research on the flexoelectric effect is primarily focused on ferroelectric materials and experimental measurements often yield flexoelectric coefficients that significantly exceed theoretical prediction. Understanding the underlying mechanisms behind this discrepancy is crucial to understand this effect. Thus, we further discuss the mechanisms causing the large flexoelectric response in ferroelectrics. Finally, the potential applications of the flexoelectric effect in sensors, actuators, and other fields are comprehensively examined. The primary goal of this chapter is to provide readers with an in-depth understanding of the flexoelectric effect, offer a theoretical foundation for the design of materials with enhanced flexoelectric response, and contribute to the development of advanced electromechanical coupling materials and devices.

Metal halides belong to a remarkable category of mechanically responsive smart materials with unique properties because of their both composition and dimensional engineering. These metal halides-based smart materials have earned significant attention because of their exceptional optical properties and advantages, making them versatile in a wide range of applications. In this regard, high pressure has recently been identified as a promising tool for regulating excitonic transitions in metal halides via continuous structural modulations. This technique offers a novel perspective for understanding their intriguing excitonic emissions. This chapter provides an in-depth and systematic analysis of these materials, with particular emphasis on organic-inorganic complex mechanofluorochromic materials. The band gaps and emission energy of metal halides, which are directly linked to the efficiency of excitonic formation, can be precisely modulated under compression through mechanisms such as contraction, distortion, and excitonic transitions. As a result, this chapter emphasizes a fundamental strategy for modulating and optimizing the optical properties of organic-inorganic hybrid metal halides through precise exciton regulation achieved via high-pressure methods.