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Introduction to Ceramic Materials

Laura Treccani

CSMT Gestione S.c.a.r.l., Polo Tecnologico - Innovative Contamination HUB, Via Branze 45, 25123 Brescia, Italy

1.1 Introduction: Ceramics for Biotechnological and Environmental Applications

Ceramic materials have been a part of human life for close to 30 000 years, and because of their superior properties, functionalities, and aesthetic characteristics, have become indispensable for a wide range of technical applications, consumer products, and everyday items [1].

Ceramics were the first man-made materials [2], and they played a very important role until about 1500 C.E., when metal technology took over (Figure 1.1). A second ceramic age started in the mid-twentieth century that witnessed the development and use of "new advanced ceramics," overcoming age-old limitations, such as poor mechanical strength and brittleness [1, 4]. A fast technical evolution supported by innovative processing methods enabled the development of multifunctional and smart materials with outstanding characteristics and opened new unimaginable possibilities. This has gone along with new approaches and prospective in material design and, generally speaking, there has been a shift from "homogeneity to controlled inhomogeneity." As the environments where materials are used become more complex, demanding, and severe, there are frequent cases in which the conventional homogeneous ceramics are inadequate and defective. "Inhomogeneous materials," or more precisely functionally graded materials, are characterized by smooth varying structure or composition. This means that their properties change continuously, and different characteristics can be achieved on separate surfaces, layers or parts. A gradation of properties across the volume, performing a set of specific and diverse functions within the given material, overcomes the inherent limit of homogeneous ceramics and leads to highly customizable multifunctional materials (see, e.g. Chapter 16 "Functionally Graded Materials" in [5]).

Now, the importance of ceramics is stronger than ever, and global trends show that different ceramics with multiple functionalities, along with sustainable, long-term innovative technologies, are tackling some of the major global challenges such as healthcare, environmental pollution, and energy shortages among others. For the sake of completeness, it must be stated that the production volume of advanced ceramics falls strongly behind that of the classic ceramics (bricks, tiles, cement/concrete, etc.), but owing to their high value-added nature, their sales volume approaches those of the classic ceramics [1].

Figure 1.1 Historical timeline of development of materials.

Source: Adapted from Ashby et al. [3].

In comparison to other materials, ceramics feature unique structural and functional properties and are well suited to being combined with other materials. This makes them the ideal choice for long-life components for thermally, mechanically, and chemically demanding applications, such as chemical and pharmaceutical processes, biomedicine, environmental monitoring, remediation, and food processing.

Examples of the application of ceramics to relevant technical fields are here briefly introduced. Ceramic membranes were developed more than 30 years ago and are becoming an important technology in the emerging areas of separation, purification, and water management for the upstream oil and gas industry, as well as in refineries [6-13]. Ceramic membranes outdo their polymeric counterparts because of their capacity to withstand harsh operating conditions in terms of pH, temperature, pressure, and chemical stability. These peculiarities make ceramic membranes usable several times without any loss in performance. Therefore, they can be considered potential candidates for targeting worldwide goals aimed at improving environmental remediation, alternative energy, and raw material-saving. Ceramic membranes are easy to clean and sterilize, and they can contribute to the production of high-quality foods as well as chemical and pharmaceutical compounds, thus allowing this industrial sector to satisfy more demanding quality standards. This makes them suitable for filtration of biological fluids, foodstuff and water purification, recovery and recycling of organic solvents, dye separation, filtration of acids and caustics, and so on [8, 14]. Ceramic membranes represent an inexpensive option for building microbial fuel cells (MFCs), a highly promising technology to produce electricity, and these can be used to treat wastewater [15]. Ceramic membranes are proposed as suitable candidates for the purification and recycling of chemicals like boric acid from radioactive waste water, thus reducing the emission of radioactive wastewater as well as enabling the reuse of high-value 10B-rich nuclear-grade boric acid from nuclear power plants [16]. Ceramic filters can be effectively employed to reduce gaseous and volatile organic compound (VOC) emissions from industrial complexes and foster the Clean Air Act, implemented and enforced by the Environmental Protection Agency (EPA), to safeguard public from exposure to harmful toxic gases [17]. Different forms of ceramics are widely used for air quality, environmental protection and fight climate change [18], by e.g. capturing or decomposing particulate matter (PM), nitrogen oxides (NOX), and other VOC, which are mainly responsible for heavy pollution. A striking example is titania, which is largely used in photocatalytic, self-cleaning, and "antipollution" systems [19]. Continuous growth of the global ceramic filter market is expected because of more stringent environmental regulations in developed and developing countries with regards to water management (both drinking and disposal water) and air quality. However, some issues regarding performance and manufacturing costs still need to be overcome to meet the economic feasibility.

Another area where ceramic materials are widely used is in sensing and monitoring. Ceramics are considered a high-potential alternative to silicon for the fabrication of thick films for microelectromechanical systems (MEMS) and microdevices [20, 21]. Ceramic sensors feature mechanical strength and resistance to chemicals, and they are thermally and physically stable in hostile liquid or gaseous environments. MEMS devices are used in different sectors, such as automotive, aerospace, medical, industrial process control, instrumentation, and telecommunications for products such as airbag crash sensors, pressure sensors, biosensors and ink jet printer heads [20], humidity sensors [22], and high-temperature sensors [23]. For example, negative temperature coefficient of resistance (NTCR) ceramic sensors occupy a respected market position because they afford the best sensitivity and accuracy at the lowest price [24].

These examples illustrate, indeed, only some of the application potential of ceramics for biotechnological and environmental purposes. Due to the rapidly changing technological, environmental, social, and industrial trends and requirements, a constant increase in the demand for customized, unconventional materials exists. In response to the changing needs, several approaches have been pursued to create or add and combine new functionalities in materials. A particularly important approach is surface functionalization, which can be roughly defined as the modification of the surface by physical, chemical, or biological methods, or a combination of them. New characteristics can be given to the surface, which can largely differ from the original one. Surface functionalization is extremely versatile and can be applied to particles, thick and thin films, or bulky parts with varying compositions.

The choice of a particular material and a functionalization strategy is a rather delicate issue. Not only the properties of the single components must be considered but also their interaction, the whole manufacturing process, and the final application setting (e.g. chemical and environmental conditions). Functionalized oxide and nonoxide ceramics have been developed and contributed to revolutionizing many technical sectors as well as everyday life. For example, by wet chemistry approaches, also known as chimie douce, organic and inorganic components can be brought together and offer new opportunities for the scientist's imagination and the synthesis of innovative, multifunctional hybrid, or bioinspired materials. Surface functionalization can be achieved by other methods, for example, based on physical approaches, and these are not less important. Despite this, the scientific and industrial communities are still actively working to develop innovative and better performing materials and developing strategies for the design of new materials, characterization, and understanding of their structure and properties. Surface functionalization will be thoroughly treated in Chapter 3.

In this chapter, a general definition of ceramics is given, and their main features will be described. In addition, the main application fields of ceramic materials together with some examples, with a main focus on...