Fundamentals of Smart Materials

Mohsen Shahinpoor, Mohsen Shahinpoor

• Publisher: Royal Society of Chemistry
• Year: 2020
• ISBN:
  9781788019460
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Smart materials are of significant interest and this is the first textbook to provide a comprehensive graduate level view of topics that relate to this field. Fundamentals of Smart Materials consists of a workbook and solutions manual covering the basics of different functional material systems aimed at advanced undergraduate and postgraduate students.

Topics include piezoelectric materials, magnetostrictive materials, shape memory alloys, mechanochromic materials, thermochromic materials, chemomechanical polymers and self-healing materials. Each chapter provides an introduction to the material, its applications and uses with example problems, fabrication and manufacturing techniques, conclusions, homework problems and a bibliography.

Edited by a leading researcher in smart materials, the textbook can be adopted by teachers in materials science and engineering, chemistry, physics and chemical engineering.

1General Introduction to Smart Materials

Mohsen Shahinpoor

Mechanical Engineering Dept., University of Maine, USA

Email: [email protected]

1.1 Introduction

Before elaborating on the nature of this textbook, let us redefine smart materials as stimuli-responsive multifunctional materials with actuation, energy-harvesting, and sensing capabilities, along with companion thermal, electromagnetic, chemical, and physical functions. As mentioned in the preface, there are only a few smart materials textbooks available, which only cover a shortlist of stimuli-responsive smart materials. The current textbook reviews some 25 families of smart materials.

In this introductory chapter, the spirit and structure of this book are presented. In general, some fundamental aspects of various smart materials are described, and the stage is set for the coverage of the current family of smart materials as special stimuli-responsive smart materials capable of a variety of actual functions needed for a large family of engineering, scientific, industrial and medical applications. These functions include actuation, energy harvesting, and sensing, plus some other complimentary physical or chemical properties changed via external or internal stimuli such as electric or magnetic fields, fluid-thermal fields, strain and stress fields, plus others such as the ionic field within the materials.

The chapters are presented such that initially an Introduction is given followed by applications with example problems, followed by brief Modeling on Constitutive Equations, a brief discussion on Fabrication and Manufacturing followed by Conclusions and References where ‘properties’ is used in the broad sense of the word. ‘Stimuli-responsive’ or ‘Multi-functional’ could cover just about any material, but here these terms are meant as special functions, such as actuation, energy harvesting, and sensing, among others. The chapters are also equipped with exercises and solutions, as well as homework problems. The book also has a solutions manual for the homework problems, available as ESI.^† This book will have twenty four chapters, including this chapter, which is an general introductory chapter on the fundamentals of smart materials. Each chapter will describe the characteristics of a particular material and system that is currently available and can be manufactured or fabricated to act as stimuli-responsive multi-functional smart actuators, sensors, and energy harvesters, among other functions and properties.

Let us briefly present a summary of the book to prepare the students for more comprehensive coverage of the materials and topics. Because of their long history, a review on piezoelectric materials, such as piezoceramics like PZT and piezo polymers like PVDF is presented. The piezoelectric effect describes a reversible electrodynamic relationship that exists in some solid crystalline structures with embedded dipoles. These crystalline solids possess microscopic regions containing dipole charges such that when placed under an applied mechanical stress, they change their internal arrangements of embedded electrical dipoles, generating a voltage across the material boundaries. Conversely, an applied voltage to the solid crystalline changes the orientation of the embedded internal dipole charges and generates deformation or strain in the solid. The description of piezoelectric materials is then followed by a review on piezoresistive materials as smart sensors. Piezoresistivity is a property of certain materials such as semiconductors for which the materials electrical resistance changes purely due to mechanical pressure, force, acceleration, strain, and stress. It is the physical property of certain materials that have been widely used to convert a mechanical signal into an electrical signal, in smart sensors, accelerometers, tactile sensors, strain gauges, flow meters, and similar devices and microdevices. The piezoresistive effect is present in semiconductors such as germanium, amorphous silicon, polycrystalline silicon, silicon carbide, among other materials.

A concise review on the response of electrostrictive materials is then given. Electrostriction is the nonlinear electromechanical coupling in all electrical-nonconductors (dielectric materials). Under the application of an electric field, these materials show deformation, strain, and stress. Generally speaking, all electrostrictive materials exhibit second-order nonlinear coupling between the elastic strains or stresses and dielectric terms, such as the strain tensor. For a single uniaxial strain (deformation), the induced strain (deformation) is directly proportional to the square of the applied electric field (voltage).

Contractile ionic polyacrylonitrile (PAN) fibers are then introduced to mimic mammalian muscles. Polyacrylonitrile (PAN) fibers in an active form (PAN or PAN gel modified by annealing/cross-linking and partial hydrolysis) elongate and contract when immersed in pH solutions (caustic and acidic solutions, respectively). Activated polyacrylonitrile (PAN) fibers can also contract and expand in polyelectrolyte when electrically and ionically activated with cations and anions, respectively. The change in length for these pH-activated fibers is typically greater than 100%. However, more than 900% contraction/expansion of PAN nanofibers (less than 1 micron in diameter) have been observed in our laboratories. PAN muscles present great potential as artificial muscles for linear actuation, and PAN fibers can convert chemical energy directly into mechanical motion.

Magnetostrictive materials are then introduced, in which deformation is observed in ferromagnetic materials when they are subjected to a magnetic field. This effect was first identified in 1842 by James Joule when observing a sample of nickel (the Joule Effect). At the fundamental level, the change in dimensions results from the interactive coupling between an applied magnetic field and the magnetization and magnetic moments of the material's magnetic dipoles, for a material initially under some stress. A review of giant magnetoresistive (GMR) materials is then presented. Magnetoresistance is defined as the property of a material whereby it can change its electrical conductivity or inverse electrical resistance when an external magnetic field is applied. In 1851, William Thomson (Lord Kelvin) discovered that when pieces of iron or nickel are placed within an external magnetic field that the electrical resistance increases when the current is in the same direction as the magnetic force which is aligned with the magnetic N–S vector and decreases when the current is perpendicular to the direction of the magnetic force. Lord Kelvin was unable to reduce the electrical resistance of any metal by more than about 5%. This effect is commonly called the ordinary magnetoresistance (OMR) effect to differentiate it from the more recent discovery of GMR. GMR materials generally possess alternating layers of ferromagnetic and non-magnetic but conductive layers made up of iron–chromium and cobalt–copper. Following the information on GMR, a brief review of magnetic gels (ferrogels) is presented by Zrinyi and co-workers. A prelude to the development of ferrogels was a classic paper by Rosenzweig in 1985 on ferrohydrodynamics. A colloidal ferrofluid, or a magnetic fluid, is a colloidal dispersion of monodomain magnetic particles. Typically, monodomain magnetic particles have typical sizes of around 10–15 nm, and they are superparamagnetic, in which magnetization can randomly flip direction under the influence of temperature.

A review of electrorheological fluids (ERFs) is then presented. ERFs belong to a class of smart materials capable of changing from a liquid phase to a much more viscous liquid and then to an almost solid phase in the presence of a dynamic electric field. They are essentially colloidal suspensions of highly polarizable particles in a nonpolarizable solvent. The solid phase of an ERF typically has mechanical properties similar to a solid like a gel and can undergo a phase change from liquid to a thick liquid like honey and then solid or in reverse from a solid transform to a thick liquid and then a thin liquid in a matter of a few milliseconds. This effect is called the “Winslow effect” after its discoverer Willis M. Winslow, who obtained a US patent on the effect in 1947 and published an article on it in 1949. The effect is better described as electric field dependent shear yield stress. Magnetorheological fluids (MRFs) are then introduced, which are suspensions of micron-sized magnetic particles such as iron carbonyl powder in a host liquid, usually a type of oil with some additives, to minimize particle sedimentation and particle wear and tear. When the MRF suspension is placed in a magnetic field, the suspended colloidal particles reconfigure to form chains in the direction of the magnetic flux and make the solution more solid-like than liquid.

A review of dielectric elastomers (DEs) is then presented. If rubbery elastomers like a silicone rubber sheet are sandwiched between two compliant electrodes, then any imposed electric field induces electrostatic forces (attraction) between the electrodes. Thus, the rubber sheet in between them can be compressed by the electrostatic forces, which then causes the rubbery sheet to expand sideways due to the Poisson's ratio effect and actuation results. In 1880, Röntgen demonstrated this actuation using two glasses as dielectrics, and once the opposing surfaces of these glasses were charged, small thickness changes were observed. Later, electrostatically-induced pressures acting to compress dielectrics became known as the “Maxwell stress.” It was, however, Pelrine, Kornbluh, and Joseph in 1998 who introduced dielectric elastomer technology with compliant electrodes. They concluded that by deliberately choosing polymers with relatively low moduli of elasticity, the field-induced strain response due to Maxwell stress could be large.

Shape-memory alloys (SMAs) are then reviewed. The shape-memory effect (SME) is a property of materials that are capable of solid-phase transformation from a body-centered tetragonal form called thermoelastic martensite to a face-centered cubic superelastic called austenite. These materials are named shape-memory materials (SMMs) and the thermal versions are called SMAs. These martensitic crystalline structures are capable of returning to their original shape in the austenite phase, after a large plastic deformation in the martensitic phase and return to their original shape when heated towards austenitic transformation. These novel effects are called thermal shape-memory and superelasticity (elastic shape-memory), respectively. Magnetic shape-memory (MSM) alloys (materials) are then described. MSMs, often also referred to as ferromagnetic shape-memory alloys (FSMAs), have emerged as an interesting extension of the class of SMMs. FSMAs combine the attributes and properties of ferromagnetism with a reversible martensitic crystalline solid phase transformation. MSM phenomena were originally suggested by Ullakko, O'Handley, and Kantner and were demonstrated for a Ni–Mn–Ga alloy in as early as 1996. Naturally, the SME is now extended to polymers as shape-memory polymers (SMPs). SMPs belong to the family of SMMs, and are stimuli-sensitive polymers that can be deformed into a predetermined shape under some specific applied fields or parameters such as temperature, electric or magnetic field, as well as strain and stress. These shapes can be relaxed back to their original field-free shapes under thermal, electrical, magnetic, strain, stress, temperature, laser, or environmental stimuli. These transformations are essentially as a result of the elastic energy stored in SMMs during the initial deformation.

Smart materials for controlled drug release are then described. Systemically-administered controlled release systems allow fine-tuning of drug bioavailability, via regulation of the amount and rate at which the drug reaches the bloodstream, which is critical for the success of the therapy. Some drugs pose important efficacy and safety problems (e.g., antitumor drugs, antimicrobials) and suffer from instability problems in the biological environment (e.g., gene materials), and thus the therapeutic performance of these drugs is improved when they are selectively directed (targeted) from the bloodstream to the site of action (tissues, cells or cellular structures). Both macro-dosage forms and nano-delivery systems may notably benefit from stimuli-responsive materials. Differently, to pre-programmed drug release systems, formulations that provide discontinuous release as a function of specific signals (stimuli) are advantageous in many situations. The when, where and how drug release triggering occur require detailed knowledge of the changes that the illness causes, in terms of physiological parameter...