Complex Composition - Nanomaterials Company

What are Nanomaterials?

Size as an Independent Degree of Freedom

More on Size

Perspective: Conventional, Ordinary or Non-nanomaterials

Crystalline Materials

Amorphous Materials

Composites and Nanocomposites

atoms per nanoparticle

Periodic Table of Elements

NANOFACTS

The purpose of this page is to give the reader a better understanding of what nanomaterials are and what they might do for your products.

Nanomaterials N. V. Coppa, Nanomaterials Company, 1995

Nanomaterials are materials possessing one or more dimensional features having a length on the order of a billionth of a meter to less than 100 billionths of a meter. They are important because 1) they exhibit unique properties which are derived from the size of these features and 2) we have fairly recently learned how to manipulate matter on these dimensions so as to understand and exploit their unique properties and understand the relationship of these properties to size.

The US Government supported National Nanotechnology Initiative NNI website which puts these ideas in a simple phrase, "At the nanoscale, the physical, chemical, and biological properties of materials differ in fundamental and valuable ways from the properties of individual atoms and molecules or bulk matter." This phrase can be further simplified to read: At the nanoscale, physical, chemical, and biological properties differ from the properties of individual atoms and molecules or bulk matter.

The properties exhibited by any material (see below) are largely a result of composition, and the conditions under which the material was produced (structure). The properties of most materials can be controlled through the variation of composition, temperature and pressure and the rates at which these independent parameters are changed during the material's synthesis or production. Control of structure at the nanometer length scale affords the nanotechnologist or nanomaterial scientist an additional independent degree of freedom, namely size.

Size as an Independent Degree of Freedom: The notion of size as an independent degree of freedom which can be manipulated independent of composition, temperature and pressure to yield materials that possess new properties not exhibited by their conventional counterparts, is only very recently being realized from a commercial perspective. When materials possesses size features that are on the order of a few billionths of a meter, those materials often exhibit new properties not found in their ordinary material counterparts and those properties can be modified independently of the materials composition. The trick is to produce nanomaterials with tightly controlled size and size distribution so that the size dependent properties emerge and are distinguishable. Nanomaterials that have a wide particle size distribution may exhibit unique properties, but those properties are a statistical result of the ensemble of individual sizes present in the distribution. To exploit the unique size-dependent properties of which nanomaterials are capable of exhibiting, the nanomaterial must be composed of monodisperse or nearly monodisperse nanoparticles. Size dependency is further complicated when specific surface features are responsible for the unique properties of the material. In that case processing must be controlled to yield both size and the particular surface features that are responsible for the materials unique characteristics.

In the study of elementary solid state physics, the idea of the (Born-von Karman) periodic boundary condition is introduced to derive from a structural model of a crystalline material physical properties such as the heat capacity and other properties. The periodic boundary condition is introduced to avoid edge or surface effects that complicate the calculations and add little to the outcome of the result, that is, sol long as the crystal or crystallite is large and such effects are insignificant. In the limit that the size of the crystal goes to zero, edge and surface effects become important and the periodic boundary condition can no longer be used to accurately predict the properties of such materials. Questions which remain are how the intrinsic properties of a material change as the particle size diminishes. Clearly in or at the zero limit the material exists as isolated molecules which we understand to not exhibit properties of the solid state. So it is easy to imagine that there exists linear or more likely, nonlinear relationships between the property of a material and the size of the crystallites or particles of which it is composed. The nanomaterials designer exploits the size emergent properties.

Nanomaterials include materials where the size of the particles, crystallites or grains of which the material is composed is on the order of nanometers. As a result, properties emerge that are not characteristic of their counterparts having conventional structural features. In nanometer sized grains or particles, a large proportion of the ions or molecules are located at the grain boundary or surface as compared to the interior of the grain. As the grain size decreases the proportion of molecules or ions at the grain boundary increases. The ratio of molecules or ion at the surface to the total in the grain is proportional to 1/r where r is the radius of the particle size or grain. The size dependent properties that emerge in the nanometer length domain are in part a result of this increased ratio. Accordingly, the properties of a large collection of nanomaterials are dominated by the properties of the grain boundary or surface.

Perspective on Conventional, Ordinary or Non-Nanomaterials Materials

To understand nanomaterials, conventional or ordinary materials (non-nanomaterials) are described. Materials are composed of atoms, molecules, ions and compositions thereof. Solid materials can be broadly classified into crystalline and amorphous. Glass and table salt are familiar examples of an amorphous and crystalline material, respectively. Materials of different types can be brought together to form composites. "Fiber glass" is a well known example of a composed of glass fibers which impart tensile strength and an polymeric binding material which provides cohesiveness.

Crystalline materials come in two general forms, single crystalline and polycrystalline. A crystalline material is composed of an orderly repeating array of atoms, molecules or ions. Crystalline materials generally have short and long range order. That simply means that the manner in which atoms are arranged at any one location within a crystal is identical to the arrangement of at any other location. An example of a crystalline material is ordinary table salt. Each grain of salt is a single crystal of sodium chloride, that is sodium and chlorine atoms are arranged periodically on a three dimensional lattice (in this case the cubic face centered lattice). A diamond a single crystal of carbon atoms. The semiconductor or chip inside a computer’s microprocessor is a single crystal of silicon, upon which the complex microcircuitry was built. A polycrystalline material is a consolidated assembly of small single crystals. An example of a polycrystalline material is a ceramic dish or coffee cup. If the broken edge of such item is examined with a magnifying glass one can easily see the individual crystals, referred to as crystallites or grains. Crystallites are randomly oriented within a polycrystalline material. Most metals are polycrystalline. When a metal breaks, the grains or crystallites of which it is composed can be observed. A metallograph of a polished metal surface is shown in FIG.1. (Try repetitive twisting a paperclip until it breaks; then examine the broken edge using a magnifying glass and observe the metal crystallites.) The surface between grains is called the “grain boundary,” and they appear as the line between grains in FIG. 1.

All materials scatter x-rays, but crystalline materials exhibit a special type of scattering called Bragg scattering. Scattering comes from the positive or negative interference that occurs when x-ray photons interact with the sets of individual atoms or ions within the material. Positive interference (Bragg scattering) only occurs at certain angles between the incident and reflected photons and is a result of long range order or periodicity within the material. Other angles yield negative interference. A diffraction pattern is the intensity of x-rays measured as a function of the scattering angles. Such patterns exhibit intense scattering where positive interference occurs and low intensity elsewhere. X-ray diffraction from crystalline materials yield relatively few "peaks" (an angle where intense positive interference occurs). The degree of order affects the intensity and sharpness of the peaks. Sharp intense peaks arise from very well ordered materials, that is, materials with long range order contain sets with very high numbers (population) of atoms or ions that contribute to the positive interference. Every material of unique composition and structure has its own diffraction pattern. Accordingly, x-ray diffraction is a powerful tool used to characterize the structure of a material.

Amorphous materials are non crystalline. They have short range order but lack long range order. Polymers such as polyethylene, polyester and polypropylene are amorphous. Ordinary glass is an amorphous solid and is composed of random meandering chains of silicon and oxygen atoms and other components. While the chains do not exhibit long range order, short range order exists. Amorphous materials are not composed of grains. Examination of the broken edge of an amorphous material will reveal a smooth surface devoid of any crystalline features.

The idea of short and long range order is easily understood by considering the following example. Imagine a large bucket filled with lengths of ordinary metal chains where each length is 6 feet (~2 meters). Suppose the chain is the type that might be used to lock a bicycle or suspend a swing, that is consisting of oval metal links each about 1 inch (2.54 cm). While there may be very little or no long range order with respect to the orientation of the links within the bucket, there is short range order. When the bucket is examined on the short length scale (1 inch) one finds a link attached to two other links. Examination of any such spot within the bucket will yield the same result, a link attached to two other links, that is, there is short range order. As the length scale over which the examination occurs is increased, the configuration (degree of folding, path, etc.) of any one chain or set of chains will differ, that is, a lack of long range order.

Crystalline and amorphous materials represent two extremes of the materials types. Nanomaterials represent materials that from a structural perspective are somewhere in between these two extremes.