Insulators

Type of physical science: Condensed matter physics

Field of study: Solids

All matter can be placed into one of three categories: conductors, semiconductors, or insulators. Each is distinguished from the others by its capacity for conducting electromagnetic energy. Insulators are materials, natural and man-made, through which an electromagnetic current will not pass efficiently.

Overview

The term "insulator" is used in two contexts in discussions regarding electromagnetic phenomena. The first refers to any form of matter that is by its nature a very poor conductor of electromagnetic energy. The second context refers to a man-made device designed to protect an electric conductor from its immediate environment and, conversely, to keep the nearby environment from coming in contact with the potentially harmful conducting medium. The distinction between these definitions is increasingly blurred in the modern high-technology environment. A good description of insulators can be made by contrasting them with their opposites: conductors.

All materials found in nature and those created by combining basic elements into compounds, alloys, conglomerates, and the like are made up of atoms that by their very nature are either good, mediocre, or poor conductors of electromagnetic energy. The determining factor is the number of electrons present in the atoms of each. Electrons are subatomic particles bonded to atoms that carry negative charges. When a significant positive electromagnetic force is applied to materials that serve as good conductors, electrons contained in them tend to rush to the region of the force to "equalize" it with their negative charges, in the process causing other electrons in the material to break their bonds and move into the vacated spaces they leave behind. Good conductors tend to have atomic structures that allow a positive force to excite and interact with their atoms, while poor conductors have atomic structures that are not significantly altered by the introduction of force fields. Generally speaking, matter composed of atoms with odd numbers of electrons makes a good conductor, and matter with even numbers of electrons makes a poor conductor and, consequently, a good insulator.

Good conductors are made up of atoms that contain free electrons. Scientists theorize that free electrons are similar to any other electron attached to an atom except that the bond holding them is weaker, making them particularly susceptible to external electromagnetic forces.

In some matter, this bond is so weak that free electrons wander throughout the mass regardless of whether there is an external electromagnetic force playing upon them. Insulators, on the other hand, contain atoms whose electrons are tightly bonded to them. Additionally, those atoms are tightly bound to one another within the mass and therefore are not easily reconfigured through electrochemical manipulation of one kind or another. The proper term for this characteristic is "high resistivity," which makes these materials useful as isolators, or separators, between and among conductors of lower and differing potentials.

Insulators include materials that exist in all states: solids, gases, and liquids. In all cases, the atoms that make up these materials contain a limited number of free electrons. The perfect insulator would be a material that conducts no electricity; however, such a material does not exist. All materials, natural or man-made, conduct electricity to some degree. It is the degree to which they act as conductors when a common value of electromagnetic force is applied that characterizes them as either conductors or insulators.

Stephen Gray (1666-1736), an English scientist, discovered as early as 1729 that some materials make good insulators and some do not. After a number of experiments, he began to use wire as conducting material even though manufacturers skilled in the process of drawing wire were still hard to find in Europe. Gray's experiments with very simple equipment led to an understanding of the nature of conductivity that has prevailed to the present day.

There are many materials that are neither good conductors nor good insulators. As a group, they are known as semiconductors and are often used to manufacture electronic components, for which they are ideal. Two such materials, silicon and germanium, are used extensively in the manufacture of transistors and microchips. In their natural states, these materials are insulators, but by adding small amounts of conductive impurities, they can be made to conduct electricity in exactly the right amounts under conditions specified in component designs.

Materials used as insulators in electrical systems are many and vary widely in their physical characteristics. Depending upon the application to which they are assigned, such materials may be hard and rigid, soft and flexible, liquid, gaseous, even fibrous. In some cases, the presence of insulators is extremely critical to the proper function of electrical systems, while in other cases it is not as critical. In some cases, electrical conductors must be extremely flexible, while in other cases flexibility is not required. Perhaps the most common challenge in the design of such systems is the requirement that they be small, compact, and portable. The development and ongoing evolution of microcircuitry have led to research and development efforts to identify new kinds of insulating materials that can function in these environments. These materials must also be manageable in the manufacturing process and must stand up to rigorous testing in the field. Many insulators are fabricated as chemical liquids so that components can be dipped into them to acquire coatings that conform to the physical dimensions of those components.

Applications

It has been understood since the early eighteenth century that some materials are conductors and others are nonconductors of electricity. When Stephen Gray made his discovery, he was in the midst of experiments designed to extend the distance over which an electrical charge could be transmitted. He used a variety of materials that exhibited a variety of conductive properties. His early use of wire led to the incorporation of those three concepts--electricity, conductivity, and distance--into a design for a means to transmit electricity from one location to another consistently and predictably. The result was the first electric circuit. Since then, increasingly sophisticated electrical systems have appeared, each designed to perform a specific task.

Some electrical systems must operate under extreme environmental conditions of one kind or another. Transmission wires that carry electric current over great distances, for example, are susceptible to extreme temperature swings. They are also subject to the effects of lightning, moisture, wind, and other atmospheric phenomena. Transmission wire insulators must serve a number of purposes. First, although copper wire is an efficient conductor of electric current, it is not a perfect one; a significant proportion of the electrical energy transmitted over copper wire is lost in the form of radiation. Insulators must act as a barrier to that radiation to ensure the efficiency of the transmission process. Additionally, insulators must act as barriers to prohibit the transmission circuit from finding a shortcut to the ground. Electricity will always seek the nearest path to ground whether in the form of man-made electrical transmission or lightning that is generated during an electrical storm. Finally, insulating materials must also protect the outside environment from coming in contact with the transmission medium. If, for example, that transmission medium is located such that a person or object coming in contact with it could become a short circuit to the ground, then the result might be injury or death to the person, damage to or destruction of the object, or, at minimum, a breakdown in the circuit.

Given all these requirements, an appropriate material must be found that can operate consistently and effectively over time to ensure that they are met. Often, a combination of materials is used. In the case of transmission media, the medium itself is often encased in insulating material that acts as a barrier to both the electromagnetic radiation of energy to the environment from the medium and interference with the transmission process from the outside environment. The insulating material is also designed to protect the medium from the ravages of weather.

Given that no insulator is a perfect nonconductor, other insulators protect against the possibility that conditions might converge to provide a short circuit to the ground in the form of an unintended circuit, for example, along a transmission support pole or tower. These materials separate the transmission medium from its support structure, which is installed upon the ground.

Support structures are made of materials that have varying conductive potentials. Wood poles, for example, are extremely poor conductors, although it is notable that during rainstorms they can provide an unintended path for electrical current that may pass along the film of water that covers the pole. Water, although an inefficient conductor relative to most metals, is a more efficient conductor than wood. Metal support structures offer even better potential for unintended short circuits, significantly better than, for example, structures constructed of concrete.

On the other end of the spectrum is the microchip containing thousands of miniature circuits embedded on a specially designed semiconductor platform. Insulating requirements for microchips are often substantially different from those of the large transmission media. Also, there are many applications that fall between the two.

The specific material used for insulating electrical circuitry depends on the application.

In the case of high-voltage power lines, the wire is often sheathed in a combination of plastic and fiber to protect against the weather and electromagnetic radiation from both the inside and the outside. This media is further insulated from support structures by devices made of ceramics, including porcelain or glass. Extremely thin wire is often encapsulated in enamel or specifically selected resins, polyurethanes, and epoxies. Circuits that must operate in very high temperatures are often insulated with Teflon, effective up to 230 degrees Celsius. Electronic components that must function under conditions of mechanical adversity are often insulated using abrasion-resistant nylon and other polymer-based substances. High-frequency applications require either polystyrene or polyethylene. Manufacturers of capacitors use Mylar. Electrical equipment is often insulated on the inside with fiberglass or mica.

Context

With Gray's discovery that every substance has its own unique propensity for conducting electric current, the stage was set for the emergence of the electronic era. Once that concept was understood, electricity could be transmitted efficiently and safely in a predictable manner through the appropriate integration of materials that were designed to act as conductors and insulators. The simplicity of the technology made it possible to create electrical circuits using naturally occurring and easily acquired materials. Later, scientists acquired an understanding of the way in which materials behave in the presence of electromagnetic fields, experiencing a major leap foward when the subatomic particle known as the electron was correctly identified as the catalyst in the process.

The difficult task of categorizing substances in accordance with the degree to which they are able, or unable, to conduct electricity was made much easier when it was eventually learned that such behavior could be predicted based upon the known atomic structure of the material. A significant practical breakthrough came in the mid-twentieth century with the invention of transistors and other semiconducting devices that integrated knowledge of the electromagnetic behavior of electrons in specific atomic configurations and the newly developed capability to construct miniature circuits. The resulting technological innovation helped manufacturers to meet a demand for extremely compact electronic circuitry for incorporation into the design of computers and other sophisticated electronic systems.

The evolution of electronic technology has resulted in the successful design and operation of microsystems that incorporate sophisticated electronics on microscopic platforms so small that they can be seen only through a microscope. Each of these systems must be insulated from others on the same platform and from those environmental conditions that might interfere with its function. The only way to accomplish this type of insulation is to identify naturally occurring substances or design new ones that exhibit the precise electromagnetic characteristics required for the task at hand. In this sense, the discovery process continues as engineering requirements become more demanding and as other technological innovations converge to make electricity increasingly preeminent as the energy source of choice for technical systems.

The search for ideal insulating materials for use in electrical systems was enhanced during the mid-nineteenth century, when telegraph cables were strung over long distances and atmospheric and physical obstacles to the transmission of electrical energy were encountered for the first time. Indeed, the attraction of telegraph technology was its ability to transmit information over long distances instantaneously through the simple process of opening and closing an electric circuit. Telegraph operators soon discovered, however, that certain metals and certain insulating materials were better than others in meeting the requirements for the successful operation of the telegraph. At about the same time, telegraph transmission cables were laid upon the ocean floor for the first time, and new environmental challenges to this technology were encountered. Eventually, the telegraph transmission medium was refined to a degree that allowed for the consistent coast-to-coast and continent-to-continent transmission of a quality telegraph signal by incorporating the right combination of appropriate conductors and insulators.

Before the end of the nineteenth century, further demands were made upon the system with the invention of the telephone, a technology that required the transmission of electrical signals capable of carrying an accurate representation of the human voice with consistency and quality over long distances. For the first time, electromagnetic interference from storms and other atmospheric phenomena could be heard as well as detected. In telegraph systems, the quality of the single frequency code over the wire was not as critical to the quality of the message, which was the case with voice transmission. This need resulted in a push to achieve a higher technical quality that could meet the demand for greater signal integrity, a capability that would result in higher message quality and more consistent service. The newly conceived concept of universal service that emerged at the beginning of the twentieth century drove the broad effort to deploy electric and telephone service across the nation and provided an incentive to increase the understanding of conductivity and nonconductivity. The first transcontinental telephone circuits were undependable and subject to annoying interruptions. By the 1920's, the necessary understanding had essentially been achieved. Over the years, the proliferation of telephone service and innovative electronic systems resulted in new challenges. Insulators had to be designed that would eliminate cross talk on heavily used telephone circuits and curtail other forms of electromagnetic interference that had begun to degrade the quality of over-the-air broadcast signals. Eventually, new substances were found to be suitable as state-of-the-art insulators.

Another challenge emerged during the 1960's. With the invention of the transistor and its incorporation into data-processing equipment, there came a need for electrical transmission circuitry over which computer data in the form of digital codes could be sent at extremely high speeds and with a high degree of signal integrity. New materials and new coding schemes were devised to accommodate the march of progress that still shows no signs of subsiding.

One area of exploration that will most certainly result in the need for new and increasingly exotic materials designed for use as insulators is that of superconductivity. During the late 1980's, scientists were able to create substances that conducted electric current with virtually no resistance under conditions never before achieved. Their demonstrations suggest that one day the widespread application of superconducting technology might revolutionize the way in which electrical technology is used. The thought that superconducting substances may be renderable conjures up visions of remarkable new products, processes, and services whose existence will not be possible without them. Ironically, the development of superconducting technology may also result in the formulation of materials that exhibit superinsulating behavior, or perfect nonconductivity. That prospect also conjures up visions of novel electrical concepts that would certainly impact significantly on the way electrical technology is applied.

Principal terms

CONDUCTOR: a material through which an electric current passes with a relatively high degree of efficiency

ELECTROMAGNETISM: a magnetic force produced by the presence of an electric current

ELECTRON: a subatomic, negatively charged particle bonded to an atom

INSULATOR: any material or device that by its nature or design is unable to conduct electricity with a significant degree of efficiency

RESISTANCE: the tendency of matter to interfere with the flow of electrons from one point to another within that matter when an electromagnetic force is present; the atomic structure of a given material is a partial determinant of the degree to which it resists electron flow

SEMICONDUCTOR: a material whose resistivity is between that of insulators and conductors

Bibliography

Albert, Arthur Lemuel. ELECTRONICS AND ELECTRON DEVICES. New York: Macmillan, 1956. Contains a lucid and comprehensible discussion of basic electronic theory that includes a particularly useful discussion of the atomic characteristics of conductors, insulators, and semiconductors. Other areas covered are amplifiers, rectifiers, oscillators, semiconductors, and photoelectric devices. Contains illustrations, diagrams, index.

Braun, Ernest, and Stuart MacDonald. REVOLUTION IN MINIATURE: THE HISTORY AND IMPACT OF SEMICONDUCTOR ELECTRONICS. Cambridge, England: Cambridge University Press, 1978. A nontechnical source that discusses the evolution of semiconductor electronics and how it affects daily life.

Leinwoll, Stanley. FROM SPARK TO SATELLITE: A HISTORY OF RADIO COMMUNICATION. New York: Charles Scribner's Sons, 1979. Over the course of the history of the evolution of electrical systems, perhaps no single development has had the impact caused by the discovery of wireless communication. Insulators are used to preserve the integrity of those transmissions, and this book contains an interesting discussion of that topic. Excellent pictures, graphs, charts. Index.

Lurch, E. Norman. FUNDAMENTALS OF ELECTRONICS. New York: John Wiley & Sons, 1981. In addition to a good discussion of fundamental electronic principles, this volume contains an excellent in-depth discussion of the theory incorporated in the principles of free-electron bonding applied in electronic circuitry. Contains diagrams, illustrations, index.

Meyer, Herbert. A HISTORY OF ELECTRICITY AND MAGNETISM. Norwalk, Conn.: Burndy Library, 1972. Contains an entertaining and informative chronology of the evolution of electrical technology including a discussion of Stephen Gray's discovery of the varying insulating characteristics of matter. The book is interesting in that it gives a unique perspective on many of the developments that preceded the scramble to apply electrical theory to everyday life. Perhaps most illuminating is the clear case it makes for the fact that much of the technology has been around for centuries. Index.

Charges and Currents

Electrons and Atoms

Forces on Charges and Currents

Insulators and Dielectrics