Solid amorphous

Solids contain large numbers of atoms. For example, a 1 -mm cube of NaCl contains over 2 X 10 atoms. How can we hope to describe such a large collection of atoms Fortunately, the structures of many solids have patterns that repeat over and over in three dimensions. We can visualize the solid as being formed by stacking a large number of small, identical structural units, much like a wall can be built by stacking identical bricks. 

Solids in which atoms are arranged in an orderly repeating pattern are called crystalline solids. These solids usually have flat surfaces, or faces, that make definite angles with one another. The orderly arrangements of atoms that produce these faces also cause the solids to have highly regular shapes ( FIGURE 12.2). Examples of crystalline solids include sodium chloride, quartz, and diamond. 

Amorphous solids (from the Greek words for without form ) lack the order found in crystalline solids. At the atomic level the structures of amorphous solids are similar to the structures of liquids, but the molecules, atoms, and/or ions lack the freedom of motion they have in liquids. Amorphous solids do not have the well-defined faces and shapes of a crystal. Familiar amorphous solids are rubber, glass, and obsidian (volcanic glass). 

In a crystalline solid there is a relatively small repeating unit, called a unit cell, that is made up of a unique arrangement of atoms and embodies the structure of the solid. The structure of the crystal can be built by stacking this unit over and over in all three dimensions. Thus, the structure of a crystalline solid is defined by (a) the size and shape of the unit cell and (b) the locations of atoms within the unit cell. 

Before describing the structures of solids, we need to understand the properties of crystal lattices. Ifs useful to begin with two-dimensional lattices because they are simpler to visualize than three-dimensional ones. FIGURE 12.3 shows a two-dimensional array of lattice points. Each lattice point has an identical environment. The positions of the lattice points are defined by the lattice vectors a and b. Beginning from any lattice point it is possible to move to any other lattice point by adding together whole-number multiples of the two lattice vectors.  

Solids are most stable in crystalline form. However, if a solid is formed rapidly (for example, when a liqnid is cooled qnickly), its atoms or molecules do not have time to align themselves and may become locked in positions other than those of a regular crystal. The resulting solid is said to be amorphous. Amorphous solids, such as glass, lack a regular three-dimensional arrangement of atoms. In this section, we will discuss briefly the properties of glass. 

Glass is one of civilization s most valnable and versatile materials. It is also one of the oldest—glass articles date back as far as 1000 b.c. Glass commonly refers to 

Despite the initial excitement, this class of high-temperature superconductors has not fully lived up to its promise. After more than 20 years of intense research and development, scientists still puzzle over how and why these compounds superconduct. It has also proved difficult to make wires of these compounds, and other technical problems have hmited their large-scale commercial applications thus far. 

Crystal structure of MgB. The Mg atoms plue) form a hexagonal layer, while the B atoms (gold) form a graphlte-like honeycomb layer. 

An experimental levitation train that operates on superconducting material at temperature of liquid helium. 

Amorphous solids differ from crystalline solids because no long-range order occurs. So, variable coupling exists between the vibrational modes of similar or equivalent structural units. Consequently, amorphous solids can be treated in the same way as liquids and gases. The vibrational spectra of amorphous materials can present a smaller number of broader features than those of corresponding crystalline materials, where crystal coupling effects can produce multiple sharp features. 

Amorphous solids are noncrystalline. Many have small, somewhat ordered regions connected by large disordered regions. Charcoal, rubber, and glass are some familiar examples of amorphous solids. 

CHAPTER 12 Intermolecular Forces Liquids, Solids, and Phase Changes 

CHAPTER 12 Intermolecubr Fcrces liquid Sotds, and Phase Changes 

The amorphous solids are obtained by sol-gel precipitation. Sol is a dispersed homogeneous phase. Colloidal solutions are constituted by micelles. The micelles are formed due to electric charges, whose repulsive force prevents coagulation. Micelles are formed by polycondensation [1]. 

The electrostatic potential varies with distance, which can be associated with van der Waals forces. The electrostatic potential is generated by the surface with ions concentrated near the wall and diffusion of ions in both directions of the liquid. The electrostatic potential can be represented by the following equation i/r = where yr is the electrostatic potential, 5 the layer thickness at the interface, and x the interface distance, k being a constant. The negative ions at the interface and the positive ions in the layer 5 near the liquid phase form the set of micelles as outlined in Fig. 7.7 [1]. Note that the maximum potential is at the interface y/Q. The ions are distributed in the liquid phase reducing its concentration with increasing x. 

An example is the preparation of silica, schematized in Fig. 7.8. The following steps occur  

The formation of hydrogel depends on the gelation time which comprises of 90 % water inside. 

There are clusters forming gel in the gelling process. This gel is formed by three-dimensional structures, namely [1] (Fig. 7.9), 

The final category in Table 1.1 is the amorphous solid, which includes, as a subset, the glassy or vitreous state that is further discussed in Section 1.6. These phases are totally 

Many real solids lack any type of long-range order in their structure, even the type of order we discussed for quasicrystals. We refer to these solids as amorphous. We can distinguish two general types of amorphous solids. 

Why is there a centered rectangular lattice but not a centered square lattice  

Tiling means covering a surface entirely, which is impossible for some geometric shapes, as shown here for pentagons. 

Both quartz and silica glass are primarily composed of silica, SiOj. 

The first silica glass was produced around 3500 B.c. in Mesopotamia (present day Syria and Iraq), although there is evidence of early production in Egypt and Phoenicia (Lebanon). The melting point of pure SiO is 1713°C, but the mixing of other 

Surface tension results from the fact that surface molecules are pulled toward the interior of the liquid as compared to interior molecules where the forces are balanced. 

In capillary action, the adhesion between the walls of the tube and the liquid is greater than the cohesion between liquid molecules. The liquid rises up the tube and forms a meniscus that is concave upward. The smaller the tube is, the greater the height the liquid rises. 

Certain chemicals have the ability to lower the surface tension of water. This allows water to spread out over a surface rather than bead up. Wetting agents decrease the cohesive forces between water molecules, and this helps water to spread over the surface of an object by adhesive force. 

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Defining order in an amorphous solid is problematic at best. There are several qualitative concepts that can be used to describe disorder [7]. In figure Al.3.28 a perfect crystal is illustrated. A simple fonn of disorder involves crystals containing more than one type of atom. Suppose one considers an alloy consisting of two different atoms (A and B). In an ordered crystal one might consider each A surrounded by B and vice versa. 

Unlike the solid state, the liquid state cannot be characterized by a static description. In a liquid, bonds break and refomi continuously as a fiinction of time. The quantum states in the liquid are similar to those in amorphous solids in the sense that the system is also disordered. The liquid state can be quantified only by considering some ensemble averaging and using statistical measures. For example, consider an elemental liquid. Just as for amorphous solids, one can ask what is the distribution of atoms at a given distance from a reference atom on average, i.e. the radial distribution function or the pair correlation function can also be defined for a liquid. In scattering experiments on liquids, a structure factor is measured. The radial distribution fiinction, g r), is related to the stnicture factor, S q), by... 

Zilker S J, Kador L, Friebel J, Vainer Y G, Kol chenko M A and Personov R I 1998 Comparison of photon echo, hole burning, and single molecule spectroscopy data on low-temperature dynamics of organic amorphous solids J. Phys. Chem 109 6780-90... 

Glass is the name given to any amorphous solid produced when a liquid solidifies. Glasses are non-crystalline and isotropic, i.e. their physical properties are independent of the direction in which they are measured. When a glass is heated, it does not melt at a fixed temperature but gradually softens until a liquid is obtained. 

Two point defects may aggregate to give a defect pair (such as when the two vacanc that constitute a Schottky defect come from neighbouring sites). Ousters of defects ( also form. These defect clusters may ultimately give rise to a new periodic structure oi an extended defect such as a dislocation. Increasing disorder may alternatively give j to a random, amorphous solid. As the properties of a material may be dramatically alte by the presence of defects it is obviously of great interest to be able to imderstand th relationships and ultimately predict them. However, we will restrict our discussion small concentrations of defects. 

Catlow C R A 1994. An Introduction to Disorder in Solids, hi NATO ASl Series C 418 [Defects and Disorder in Cnfstalline and Amorphous Solids), pp. 1-23. 

The modeling of amorphous solids is a more dilhcult problem. This is because there is no rigorous way to determine the structure of an amorphous compound or even dehne when it has been found. There are algorithms for building up a structure that has various hybridizations and size rings according to some statistical distribution. Such calculations cannot be made more efficient by the use of symmetry. 

The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

R. Zahen, Physics of Amorphous Solids ]ohxi Wiley Sons, Inc., New York, 1983. 

A high percentage 01 water remains after the sublimation process, present as adsorbed water, water of hydration or dissolved in the diy amorphous solid this is difficult to remove. Usually, shelf-temperature is increased to 25 to 40°C and chamber pressure is lowered as far as possible. This stiU does not result in complete diying, however, which can be achieved only by using even higher temperatures, at which point thermally induced product degradation can occur. 

Considerably complicated realizes ablation of water from Zn Co j P O -H O. Heating of it to 603 K is accompanied with practically full destaiction of diphosphate stmcture. In composition of X-ray amorphous solid phase take place the processes of anion condensation. On their realization indicates formation of triphosphate with lineai anion stmcture (5,6 mas.% in count on P,0 ) in composition of burning products. 

Dihydrostreptomycin sesquisulfate [5490-27-7] M 461.4, m 250 (dec), 255-265 (dec), [a]p -92.4 (c 1, H2O), pKgsJd)-- 9.5 (NMe), pKes,(2,3) 13.4 (guanidino). It crystallises from H2O with MeOH, -BuOH or methyl ethyl ketone. The crystals are not hygroscopic like the amorphous powder, however both forms are soluble in H2O but the amorphous solid is about 10 times more soluble than the crystals. The free base also crystallises from H20-Me2C0 and has [a]p -92° (aqueous solution pH 7.0). [Solomons and Regina Science 109 515 7949 Wolf et al. Science 109 515 7949 McGilveray and Rinehart J Am Chem Soc 87 4003 1956]. 

Ceramics are crystalline, inorganic, non-metals. Glasses are non-crystalline (or amorphous) solids. Most engineering glasses are non-metals, but a range of metallic glasses with useful properties is now available. 

In our discussion so far we have begged the question of just how the atoms in a solid move around when they diffuse. There are several ways in which this can happen. For simplicity, we shall talk only about crystalline solids, although diffusion occurs in amorphous solids as well, and in similar ways. 

When, instead, the polymer solidifies to a glass (an amorphous solid) the blurring is much greater, as we shall now see. 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

C fi3 diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film. 

Activated carbon is an amorphous solid with a large internal surface area/pore strucmre that adsorbs molecules from both the liquid and gas phase [11]. It has been manufactured from a number of raw materials mcluding wood, coconut shell, and coal [11,12]. Specific processes have been developed to produce activated carbon in powdered, granular, and specially shaped (pellet) forms. The key to development of activated carbon products has been the selection of the manufacturing process, raw material, and an understanding of the basic adsorption process to tailor the product to a specific adsorption application. 

The dipole-dipole (Keesom) interaetion eomes about from the faet that on the average, two freely rotating dipoles will align themselves so as to result in an attraetive foree, similar to that eommonly observed with bar magnets. In order to ealeulate the net dipole-dipole interaetion, it is neeessary to examine all the possible orientations of the dipoles with respeet to one another. It is also neeessary to determine any jr effeets due to the field assoeiated with a point eharge, in order to determine the net effeet when amorphous solids are plaeed side by side. We also need to eonsider what happens if the dipoles ean reorient in eaeh other s fields. 

Fig. 12. Schematic representation of solid-like (crystalline), amorphous solid, and liquid-like surface layers (reproduced from [87], copyright American Chemical Society).

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