Ceramic Nanocomposites

Rajat Banerjee, Indranil Manna

• Publisher: Woodhead Publishing
• Year: 2013
• Print ISBN: 9780857093387
• eBook ISBN: 9780857093493
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Ceramic nanocomposites have been found to have improved hardness, strength, toughness and creep resistance compared to conventional ceramic matrix composites. Ceramic nanocomposites reviews the structure and properties of these nanocomposites as well as manufacturing and applications.Part one looks at the properties of different ceramic nanocomposites, including thermal shock resistance, flame retardancy, magnetic and optical properties as well as failure mechanisms. Part two deals with the different types of ceramic nanocomposites, including the use of ceramic particles in metal matrix composites, carbon nanotube-reinforced glass-ceramic matrix composites, high temperature superconducting ceramic nanocomposites and ceramic particle nanofluids. Part three details the processing of nanocomposites, including the mechanochemical synthesis of metallic–ceramic composite powders, sintering of ultrafine and nanosized ceramic and metallic particles and the surface treatment of carbon nanotubes using plasma technology. Part four explores the applications of ceramic nanocomposites in such areas as energy production and the biomedical field.With its distinguished editors and international team of expert contributors, Ceramic nanocomposites is a technical guide for professionals requiring knowledge of ceramic nanocomposites, and will also offer a deeper understanding of the subject for researchers and engineers within any field dealing with these materials.- Reviews the structure and properties of ceramic nanocomposites as well as their manufacturing and applications- Examines properties of different ceramic nanocomposites, as well as failure mechanisms- Details the processing of nanocomposites and explores the applications of ceramic nanocomposites in areas such as energy production and the biomedical field

Part I

Properties

1

Thermal shock resistant and flame retardant ceramic nanocomposites

N.R. Bose, Sols 4 All Consultants, India

Abstract:

This chapter discusses the performance behaviour of ceramic nanocomposites under conditions of thermal shock, i.e. when they are subjected to sudden changes in temperature during either heating or cooling or may be in flame propagating zones. For example, during emergency shut-downs of gas turbines, cool air is drawn from the still spinning compressor and driven through the hot sections: the temperature at the turbine outlet decreases by more than 800 °C within one second and ceramic nanocomposite materials are an appropriate choice for such application. Furthermore, such a situation may arise about 100 times during the lifetime of a modern gas turbine engine. Similarly, in the nuclear industries, apart from the moderate shocks inflicted during start-up and shut-down of the system, the plasma-facing material can suffer rapid heating due to plasma discharge. Thus, when a body is subjected to a rapid temperature change such that non-linear temperature gradients appear, stresses arise due to the differential expansion of each volume element at a different temperature. The design principles for the fabrication of high-performance thermal shock resistant ceramic nanocomposites with improved mechanical properties are highlighted in this chapter. Moreover, the pertinent factors such as interface characteristics, densification methods, superplasticity and the role of nano-size particulate dispersion, which are responsible for the development of thermal shock resistant and flame retardant nanoceramic materials, are addressed and reviewed. Various test methods for the characterisation and evaluation of ceramic nanocomposites are described. Finally, the new concept of materials design for future structural ceramic nanocomposites is discussed for safe applications in high-temperature thermal shock zones.

Key words

thermal shocks

densification

superplasticity

interface

ceramic nanocomposites

1.1 Introduction

Dramatic improvements in toughness, strength, creep strength and thermoresistance of ceramic-matrix composites have been achieved by incorporating either nanocrystalline oxide/non-oxide ceramic particles or their hybrid combination in a microcrystalline matrix. The other group of nanophase ceramic composites is nanocrystalline matrix composites, also called nanoceramics, in which the matrix grain size is below 100 nm. The nano–nano type microstructure can also be formed when the second phase is also nanoscaled. The main objective of this chapter is to present the performance behaviour of ceramic nanocomposites under conditions of thermal shock, i.e. when they are subjected to sudden changes in temperature during either heating or cooling or may be in flame propagating zones. Such conditions are possible in the high-temperature applications for which ceramic nanocomposite materials can be selected (Ohnabe et al. 1991. The importance of the use of thermal shock resistant ceramic nanocomposite materials was reported by Baste in 1993 during steady-state operation of a gas turbine (Baste 1993). While thermal shock is not a concern during steady-state operation of a gas turbine, it becomes of great importance during emergency shut-downs, when cool air drawn from the still spinning compressor is driven through the hot sections and can result in a temperature decrease of more than 800 °C at the turbine inlet within one second. An additional factor is that such a situation may arise about 100 times during the lifetime of a modern gas turbine engine.

The use of thermal shock resistant ceramic nanocomposite materials was also illustrated by Jones et al. (2002) for fusion energy applications. In the nuclear industries, SiC reinforced with SiC fibres has been proposed as a structural material for the first wall and blanket in several conceptual design studies of future fusion power reactors. In this case, apart from the moderate shocks inflicted during start-up and shut-down of the system, the plasma-facing material can suffer rapid heating due to plasma discharge. When a body is subjected to a rapid temperature change such that a nonlinear temperature gradient appears, stresses arise due to the differential expansion of each volume element at a different temperature. The temperature at each point changes with time at a rate that depends on the surface heat transfer coefficient (HTC) between the media at different temperature and the body, the shape of the body and its thermal conductivity. High HTCs, large dimensions and low thermal conductivities result in large temperature gradients and, thus, large stresses. The dimensionless parameter, the Biot modulus (Bi), can be used to describe the heat transfer condition (Kastritseas et al. 2006)

[1.1]

where l is a characteristic material dimension (e.g. the half-thickness of a plate), h is the HTC between the body and the medium and k is the thermal conductivity of the body. The larger the value of Bi, the larger is the rate of heat transfer between a medium of different temperature and the body. The sudden temperature change (∆T) that generates non-linear temperature gradients in a body and, as a consequence, thermal stresses is termed ‘thermal shock’. If ∆T is positive (i.e. the temperature reduces) the material is subjected to a cold shock, whereas if ∆T is negative the material is subjected to a hot shock. The term refers to a single thermal cycle (N = 1) in contrast to terms such as thermal cycling, cyclic thermal shock and thermal fatigue, which apply to multiple thermal cycles (N > 1) (Kastritseas et al. 2006).

Although ceramic materials based on oxides, non-oxides, nitrides, carbides of silicon, aluminium, titanium and zirconium, alumina, etc. possess some very desirable characteristics (e.g. high strength and hardness, excellent high-temperature structural applications, chemical inertness, wear resistance and low density), they are not very robust under tensile and impact loading and, unlike metals, they do not show any plasticity and are prone to catastrophic failure under mechanical or thermal loading (thermal shock) (Banerjee and Bose 2006). Many researchers have focused attention on both oxide and non-oxide ceramic materials for the improvement of microstructure. However, many problems are not yet solved. For instance, non-oxide ceramic materials such as Si₃N₄ and SiC suffer from degradation of mechanical properties at high temperature due to slow crack growth caused by the softening of grain boundary impurity phases associated with sintering additives. Oxide ceramic materials such as Al₂O₃, MgO and ZrO₂ suffer from relatively low fracture toughness and strength, significant strength degradation at high temperatures and poor creep, fatigue and thermal shock resistance.

Attempts have been made by many researchers to solve these problems, as well as ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Composites Science and Engineering
• Part I: Properties
• Part II: Types
• Part III: Processing
• Part IV: Applications
• Index