2 Materials and Functionality: A Historical Perspective

Functional

Introduction

Materials

Materials and Functionality: A Historical Perspective

Prof. Ashish Garg

Learning objectives

Importance of functional materials and their classification
A broad definition of functional materials
A historical perspective on evolution of materials
Application domains of Functional Materials
A brief review of structure and bonding in materials
Review of planes, directions and Miller indices

Introduction

Materials such as stone, metals, ceramics and polymers have played a very important role in the progress of mankind and their evolution has marked significant steps in the sustenance and transition of civilizations. This is because everything that we use for carrying out our tasks is made of some material. It is the properties of that material which define the utility of that particular material for intended applications. However, most materials that we see around us today were not available in their present. They were invented by the humans which were further improved by successive generations of scientists and engineers. While, today, we possibly stand at the pinnacle of technological prowess of mankind, this process of invention and improvement of materials for even futuristic as well as present applications is a continuous process and the search of mankind for ideal materials goes on.

So, let us now get back to understand what are functional materials? On a very casual basis, one can say that any material whose application is related to a particular utility is functional material. For example, steel can be termed as a functional material because it is used in utensils, automobiles, tools etc. or for that matter, a ceramic that is used in applications such as pottery, insulator, refractory applications etc. This is one way of classifying functional materials, which is quite simple and intuitive. However, this classification is quite vague as considers the application only without taking into account that which particular property of the material is being utilized.

From an engineering perspective, we define functional materials according to the specific functional property possessed by them and then design and choose appropriate materials which fit the criterion of that specific functionality. Although many of these functionalities are typically driven by applications, the criterion to choose a classification scheme based on specific charac teristics of materials is quite useful. A very useful aspect of many such materials is that materials can be tailored or engineered for these functional properties by a significant margin. For example, one can tailor steel for applications requiring high hardness and strength e.g. cutting tools as well as those that require moderate strength and high ductility such as structural parts. So, here mechanical properties of steel become a functionality of the material. It is also true for electrical, optical, magnetic, ferroelectric or piezoelectric behavior of materials, which makes them useful for certain types of applications. For example, piezoelectric effect in materials such as Pb(Zr,TiO3) or commonly known as PZT which is manifested by a two-way coupling between electrical and mechanical properties, is exploited for many modern devices including sesnors, actuators and transducers which are used in applications starting from defense to healthcare to electronic equipment.

Another way of classifying the materials is based on their type i.e. whether the material is a metal, characterized by metallic bonding, or a ceramic, typically characterized by high bond strength or polymers which have lower bond strength to metal or ceramics but are extremely light and are easily deformable. This classification can further be expanded based on the functional aspects as described in the preceding paragraph. As one can see, there can several aspects of materials functionalities and types of materials and the whole functional materials universe is quite overwhelming. The application include structural applications, electrical and electronic applications, automotive applications, and more specific devices such as sensors, actuators, transducers, devices for health care industries and so on and so forth. Sky is the only limit to the imagination of functionality of materials and their applications.

Before, we start talking of functional materials in detail; let us first look at how materials have evolved to understand a historical perspective.

A Historical Perspective of Materials

Materials have played a key role in the development of mankind. Early man lives its life as a hunter and to sustain itself, it was dependent on hunting of animals for which it needed tools. In the abse nce of modern weaponry to which we have access to, man had to depend on stone and wood. What was the reason behind it? Because, stone are naturally occurring, some of them are very hard and can be converted into pointed sharp objects, which can be useful as a weapon. Similarly, use of wood allowed early man to develop pointed but light objects such as spears as well as use the wood for burning as well as building shelter. As know from our history lessons, this early age of mankind was called as Stone Age and the materials in that age were primarily stone and wood based on availability.

Figure 1 shows a historical evolution of materials. Subsequent evolution after 20,000 BC or so paved the way for development of materials such as gold, copper, silver, tin and bronze marked by tremendous developments in the metallurgy. Humans found the ways to extract these metals from the ores and this development was very much dependent on the availability of raw material as well as any trade linkages with the neighbouring civilizations. For example, resident of Indus valley civilization developed various jewellery items made of bronze, copper and gold: the most famous of them being the statue of dancing girl made of bronze (see Fig. 2 (a)). At the same time, there were signif icant developments in the use of ceramic materials such as pottery, For example, many vessels discovered from the ruins Indus valley civilization between 6000 B.C. to 1500 B.C. prove the development of clay pottery (see Fig. 2 (b)). Similar objects have also been excavated from other parts of the world, such as middle east and Egypt (then part of Mesopotamian civilization) and Europe.

Figure 1 A schematic guide to the historical evolution of materials (Adapted from Michael F. Ashby, “Materials Selection in Mechanical Design”, Elsevier, 4thEdition)

Fig. 2 Objects from various historical periods in human civilization (a) statue of dancing girl made of bronze form Indus Valley Civilization (S ource: BBC, www.bbc.com), (b) a large storage vessel from Indus valley civilization (Source: Wikipedia), (c) a knife made of Damascus steel or Wootz Steel, commonly made in south India in 6th century B.C. (Source: Wikipedia)

This era further gave way to one of the most important points in the human civilization i.e. Iron Age, starting at around 1500 B.C. This era paved the way towards the development of most important engineer material mankind has seen, called as Steel. Earlier developments of Iron age took place sporadically in various parts of the world and India played a major role in those developments. One of the finest examples of Iron Age is “Iron Pillar” currently located in Delhi which has stood the test of time since ca. 4th Century C.E. Although earlier forms of iron were high in carbon and other impurities, the advances in iron metallurgy gave rise to a much more useful form called as “Steel”. Although steel making goes back to 1800 B.C. in Anatolia, high quality steel such as Wootz Steel and Damascus Steel began to be produced in South India at around 300-400 B.C.1 Steel has a played very important in livelihood of people across the world and was used quite heavily as war material in the form of swords, spears etc. The first step in modern steel making was taken in Europe in 17th century, when smelting of iron ore into pig iron was carried out in a blast furnace. Earlier methods primarily used charcoal to reduce the iron ore which was later replaced by coke in modern blast furnaces leading to production of higher quality and cheaper pig iron. The pig iron contained very high carbon content, often more than 4 wt%, and other impurities such as Si, P etc and hence was not a very useful material. The pig iron to steel conversion was first initiated by Henry Bessemer in England in 1855 where air was blown through molten iron to convert it into steel with much lower concentrations of Carbon. Air was further replaced by oxygen by Linz-Doawitz of Austria in 1950s leading to the production of steel with much improved quality and at a lower cost and hence the process is now called as L-D process. Steel has evolved as the most used material in the history of mankind which was synthesized by the humans and is used in a wide array of applications. Since 1700 onwards, after industrial revolution and renaissance in western science due to advances in chemistry, many more metals were synthesized from their ores such as nickel, platinum, manganese, aluminium, zinc,

1 Sharada Srinivasan; Srinivasa Ranganathan (2004). India’s Legendary Wootz Steel: An Advanced Material of the Ancient World. National Institute of Advanced Studies, Bangalore (India). titanium, magnesium etc. Among these, aluminium and titanium have paved the way for lighter metals with high enough strength to enable their use in variety of light weight applications ranging from aerospace industry, automotive industry etc.

Another important material which constitutes a very significant part of modern world is polymer which is a lightweight material, can be easily deformed and fabricated, and is an insulator. Although natural rubber has existed since long times, its scientific properties were first realized by Frenchman Charles Marie de La Condamine in 1736. Subsequently first synthetic rubber, a polymer of isoprene, was invented in 1879 by another Frenchman Gustave Bouchardat (1842-1918). Overtime, several forms of polymers have come into the existence with a wide range of properties such as polyethylene, polystyrene, poly vinyl chloride etc. Last century also saw development of other materials such as semiconductors with Silicon being the most important one, ceramics of various types such as glasses, oxides, carbides, nitrides which are hard and brittle materials but with exotic electronic, optical and magnetic properties. All of these materials have played vital role in development of technologies that we see around us today, whether it is in consumer electronics items such as television, phones, computers or transportation sector such as cars, aero-planes, ships or telecom sector or health care. Materials have played a key role in improving life standards, reducing mortality, improved health care etc. via development of technologies that employ them. However, a flip side has been severe environmental degradation by burning of unprecedented quantities of coal, mining for ores to extract metals, contamination of land and water by toxic elements, waste in the form of non-biodegradable materials such as plastic etc. Hence, today a huge challenge lies in front of us to innovate and explore methods and develop materials which do not cause harm of the environment and our future generations.

Factors behind development of materials

Now the question arises that what has driven the evolution of these materials? The most obvious answer seems to be that the evolution of materials has been need based. The needs defined the required functionalities which led to explorations. However, in the modern world, we also tend to explore materials based on specific functional aspects and then we design applications which are conceived later. It is a back and forth kind of approach which has worked well thus far. Based on improved understanding of materials, we are now able to classify them into categories such as metals, ceramics, glasses, polymers and composites. Fig. 3 aptly summarizes the evolution of materials in a different scheme of classification, i.e. by their characteristics, in the forms of metals, ceramics, polymers etc. versus their importance. In this figure, one can see that in the early part of human existence, the important materials were stone and wood. This was because these were naturally occurring and they provided the functionality than mankind needed. Wood is a naturally occurring fibrous material which is easy to cut, is light, can bend and flex, and can be easily burnt. So, it provided an ideal solution to make shelter, to generate energy in the form of heat for cooking or other purposes and to make weapons such as spears. Stones are hard and hence they acted as tools, primarily for hunting as well as protection. These were also useful for the purpose of making pottery. Then there was paper whose earlier forms were leafs or straw bricks.

Fig. 3 Evolution of different classes of materials (Adapted from Materials Selection in Mechanical Design, Michael F. Ashby (Elsevier), 2011)

Earliest examples of metals excavated are gold, copper, bronze and tin. Gold was primarily used for ornamental purpose due to its yellow luster which attracted humans. It was possibly used as currency too in later times. Subsequently, copper, bronze and tin also served the same purpose but were more extensively used, as these were probably more widely available than gold ore and were easier to melt, mix with other elements and fabricate. Presence of jewellery and other artifacts of these suggested that primary use of these metals was for decorative purposes. These metals could also have been used for making tools for agriculture in the Bronze Age. Development of fire from the previous era, possibly gave rise to rudimentary smelting techniques where humans could form these materials whose essential functions were their ability to be formed into various shapes by use of force. In this era, possibly metals provided the key attributes such as malleability, strength, environmental stability and their ability to be polished and formed which appears evident in the objects discovered from Bronze ages. Later, these attributes were also exploited in the fabrication of utensils, which could withstand high enough temperatures and were perfect for cooking as well as storage.

Subsequent evolution of Iron Age really opened the doors into new realms of technology as iron could be hard as well as soft, it could withstand higher temperatures and just like its predecessors, it could be converted into various shapes with the use of force. Iron’s exploits were built on its fantastic properties: it offers high mechanical strength which increases and ductility or malleability increases as Carbon content decreases, it can be extremely hard as well as moderately soft, it is extremely sensitive to the processing conditions such as heat treatment conditions of time and temperature allowing it to be tailored appropriately for required applications. Of courses, subsequent evolutions saw fabrication of steel, which has lower Carbon content and is a far more useful material than previously used high Carbon content iron as far as strength and ductility combination is concerned. Later the mankind learnt how to make it environmentally stable by fabricating stainless steel. Its combinations with various other elements allowed iron to be formed in the form

So one can see from Fig. 3, that the time progressed, utilization of metals and alloys increased with the advances in forming and smelting techniques aided with the improved knowledge of thermochemistry and thermodynamics of variety of metals. Advent of other metals such as Copper, Aluminum, Nickel, Titanium along with Iron led to a massive reliance of humans on the metals and this peaked at around 1950s due to large scale use in structural applications including automotive sector, bridges, rails, trains, electric cables, ships etc. Subsequently, it was also realized whilst metals were good for various structural applications, they had a few disadvantages, i.e. they were high conductive providing little insulations, they softened as temperatures increases, they were susceptible to corrosion and they were opaque as well as heavy. With the advances in electronic devices and semiconductor, there were large-scale efforts on developing other materials and the emphasis was on developing synthetic polymers, ceramics and glasses and composites. This was possible because the scientific understanding of materials physics and chemistry was much more advanced that it was before 1600-1700 AD and with that, the man was able to artificially tailor materials of desired properties in the laboratory. This is what possibly gave rise to the age of functional materials because now one could make fanciful materials with amazing properties and then conceive applications, which were never thought of before. Advances in the functional materials led to significant progress in electronic devices and hence consumer electronics, health care products, improved communication, faster transportation etc.

Now let us see the evolution of materials from the perspective of a few applications. Main function of a kettle is to heat the water, which requires the use of materials, which could conduct the heat well and withstand high temperatures of the flame during boiling. Earlier generation saw use of metals such as iron or copper or bronze. This changed dramatically in 1928 when an electrical kettle was introduced which consisted of a electric heating element metal sealed inside a would metal tube placed within a chamber made either of metal or plastic. The kettle body was no long exposed to high temperatures and hence from, the perspective of electric insulation, it made sense to use plastic which is an electric insulator.

Transport sector is again a marvelous example of materials evolution. A transport vehicle requires it to move at sufficiently high speed and hence it should be light as well as strong. Earlier generation transport vehicles were typically carts made of wood, which were carried by animals, and wood needed to be thick to bear the load. Advent of strong iron saw these vehicles getting stronger with much reduced material being used. Subsequently, invention of coal engines and steam engines saw arrival of steel bodied train carts, which ran on rails, made of steel. Similarly water transport required ships to be made lighter as they need to float (Fig. 4). Earlier generation ships used wood, which degraded quickly. Currently, the ships use several types of metals and plastics to give ship a shape so that it can float and yet being highly durable and strong.

Fig. 4 (a) A replica of the ship, Santa Maria, used Christopher Columbus in 15 th century, primarily made of wood and (b) Passenger ship of Köln-Düsseldorfer on the river Rhine in Germany (Source: Wikipedia)

Advent of electricity has been a major landmark in the human history. Electricity plays a very dominant role in our lives as most appliances that we use on routine basis use electricity. Early evolution of electricity applications required the development materials, which could transport electricity, i.e. a conductor. This led to the development of materials such as aluminium and copper. Further advances in physics led to conception of materials which were different than metals i.e. they were semiconductor and insulators which found applications in diodes which can acts as switches or capacitors which don’t conduct electricity. Post-second world war world has witnessed explosion of electronic devices, which use metals only for contact purposes. Most devices for their functionality employ either a semiconductor or an oxide ceramic or a polymer. Most of these materials are not found in nature and they have very specific functional properties required in the target application. Such as a solar cell required a P-N junction made of a semiconductor or a TV monitor requires a liquid crystal as its core element, which is a polymer. A ferroelectric material, which is an insulator, is used as a memory element as well as a piezoelectric or sensors, actuator, and transducers. Its properties are highly dependent on the size, composition, form and shape in which it is fabricated. Similar, magnetic materials such as Fe, Sm, Co, Ni are used a variety of magnetic applications such as transformer cores, frequency dependent applications, memories and the properties of these materials can be tailored by changing compositions, microstructure and the process. A key aspect of modern function materials is their ability of be tailored for intended applications by making modifications to their composition, structure, microstructure as well as size and shape which are affected by the process, made possible by the advances in manufacturing techniques.

Summary and Outline of the course

We have briefly learnt about the evolution of materials and their functional aspects, which are related to applications to give reader a glimpse into it. Whilst earlier development of materials was primarily need based, advances in science and technology in 1900s have expanded the materials universe several times as now one can design materials in the laboratory and tailor their properties for intended applications. In fact, modern approach also leads to applications which were not possible earlier as based on materials exotic behavior one can imagine fancy applications. The world of functional materials is extremely wide and it is impossible to cover every aspect in this course. From the perspective of an optimum combination of depth and the breadth, in this course we will limit the discussion to selected important topics. The emphasis is on providing a flavor of materials with three

different functionalities: mechanical, electrical/magnetic and optical and provide background knowledge as well as latest status on these materials and their applications.