A solid is one of the four fundamental states of matter, besides liquid, gas and plasma. It is characterized by structural rigidity and resistance to changes in shape or volume. Unlike a liquid, a solid object does not flow or take the shape of the container in which it is placed. A solid does not expand to fill the entire available volume as a gas does.
Atoms in a solid are closely connected to each other, are in an ordered state at the nodes of the crystal lattice (these are metals, ordinary ice, sugar, salt, diamond), or are arranged irregularly, do not have strict repeatability in the structure of the crystal lattice (these are amorphous bodies, such How window glass, rosin, mica or plastic).
Crystalline solids or crystals have a distinctive internal feature- a structure in the form of a crystal lattice in which atoms, molecules or ions of a substance occupy a certain position.
The crystal lattice leads to the existence of special flat faces in crystals, which distinguish one substance from another. When exposed to X-rays, each crystal lattice emits a characteristic pattern that can be used to identify the substance. The edges of crystals intersect at certain angles that distinguish one substance from another. If the crystal is split, the new faces will intersect at the same angles as the original.
For example, galena - galena, pyrite - pyrite, quartz - quartz. The crystal faces intersect at right angles in galena (PbS) and pyrite (FeS 2), and at other angles in quartz.
Examples amorphous bodies that do not have a strict structure and repeatability of crystal lattice cells are: glass, resin, Teflon, polyurethane, naphthalene, polyvinyl chloride.
They have two characteristic properties: isotropy and lack of a specific melting point.
Isotropy of amorphous bodies is understood as the sameness physical properties substances in all directions.
In an amorphous solid, the distance to neighboring nodes of the crystal lattice and the number of neighboring nodes varies throughout the material. Therefore, different amounts of thermal energy are required to break intermolecular interactions. Hence, amorphous substances slowly soften in wide range temperatures and do not have a clear melting point.
A feature of amorphous solids is that at low temperatures they have the properties of solids, and when the temperature rises, they have the properties of liquids.
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In addition to solids that have a crystalline structure, which is characterized by a strict order in the arrangement of atoms, there are amorphous solids.
Amorphous bodies do not have a strict order in the arrangement of atoms. Only the nearest neighbor atoms are arranged in some order. But there is no strict repeatability in all directions of the same structural element, which is characteristic of crystals, in amorphous bodies. In terms of the arrangement of atoms and their behavior, amorphous bodies are similar to liquids. Often the same substance can be found in both crystalline and amorphous states.
Theoretical studies lead to the production of solids whose properties are completely unusual. It would be impossible to obtain such bodies by trial and error. The creation of transistors, which will be discussed later, is a striking example of how understanding the structure of solids led to a revolution in all radio engineering.
Obtaining materials with specified mechanical, magnetic, electrical and other properties is one of the main directions of modern solid state physics.
Amorphous bodies
Amorphous substances (bodies)(from ancient Greek. ἀ "not-" and μορφή "type, form") - a condensed state of a substance, the atomic structure of which has short-range order and does not have long-range order, characteristic of crystal structures. Unlike crystals, stable amorphous substances do not solidify with the formation of crystalline faces, and (unless they were under a strong anisotropic influence - compression or electric field, for example) have isotropic properties, that is, they do not exhibit different properties in different directions. And they do not have a specific melting point: with increasing temperature, stable amorphous substances gradually soften and above the glass transition temperature (T g) they turn into a liquid state. Substances with a high crystallization rate, usually having a (poly-)crystalline structure, but strongly supercooled during solidification into an amorphous state, upon subsequent heating shortly before melting, recrystallize (in the solid state with little heat release), and then melt as ordinary polycrystalline substances.
They are obtained at a high rate of solidification (cooling) of a liquid melt or by condensation of vapors onto a substrate (any object) cooled noticeably below the MELTING temperature (not boiling!). The ratio of the actual cooling rate (dT/dt) and the characteristic crystallization rate determines the proportion of polycrystals in the amorphous volume. The rate of crystallization is a parameter of a substance that weakly depends on pressure and temperature (strongly around the melting point). And it strongly depends on the complexity of the composition - for metals it is on the order of fractions to tens of milliseconds; and for glass at room temperature- hundreds and thousands of years (old glass and mirrors become cloudy).
Electrical and mechanical properties amorphous substances are closer to those for single crystals than for polycrystals due to the absence of sharp and heavily contaminated intercrystalline transitions (boundaries) with often a completely different chemical composition.
The non-mechanical properties of semi-amorphous states are usually intermediate between amorphous and crystalline and are isotropic. However, the absence of sharp intercrystalline transitions noticeably affects the electrical and mechanical properties, making them similar to amorphous ones.
At external influences amorphous substances exhibit both elastic properties, like crystalline solids, and fluidity, like liquids. Thus, under short-term impacts (impacts), they behave like solids and, with a strong impact, break into pieces. But with very prolonged exposure (for example, stretching), amorphous substances flow. For example, resin (or tar, bitumen) is also an amorphous substance. If you break it into small parts and fill the vessel with the resulting mass, then after some time the resin will merge into a single whole and take the shape of the vessel.
Depending on the electrical properties, separate amorphous metals, amorphous nonmetals, and amorphous semiconductors.
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Let's do an experiment. We will need a piece of plasticine, a stearin candle and an electric fireplace. Let's place plasticine and a candle at equal distances from the fireplace. After some time, part of the stearin will melt (become liquid), and part will remain in the form of a solid piece. During the same time, the plasticine will soften only a little. After some time, all the stearin will melt, and the plasticine will gradually “corrode” along the surface of the table, softening more and more.
So, there are bodies that do not soften when melted, but turn from a solid state immediately into a liquid. During the melting of such bodies, it is always possible to separate the liquid from the not yet melted (solid) part of the body. These bodies are crystalline. There are also solids that, when heated, gradually soften and become more and more fluid. For such bodies it is impossible to indicate the temperature at which they turn into liquid (melt). These bodies are called amorphous.
Let's do the following experiment. Throw a piece of resin or wax into a glass funnel and leave it in a warm room. After about a month, it will turn out that the wax has taken the shape of a funnel and even began to flow out of it in the form of a “stream” (Fig. 1). In contrast to crystals, which retain their own shape almost forever, amorphous bodies exhibit fluidity even at low temperatures. Therefore, they can be considered as very thick and viscous liquids.
The structure of amorphous bodies. Studies using an electron microscope, as well as using X-rays, indicate that in amorphous bodies there is no strict order in the arrangement of their particles. Take a look, figure 2 shows the arrangement of particles in crystalline quartz, and the one on the right shows the arrangement of particles in amorphous quartz. These substances consist of the same particles - molecules of silicon oxide SiO 2.
The crystalline state of quartz is obtained if molten quartz is cooled slowly. If the cooling of the melt is rapid, then the molecules will not have time to “line up” in orderly rows, and the result will be amorphous quartz.
Particles of amorphous bodies oscillate continuously and randomly. They can jump from place to place more often than crystal particles. This is also facilitated by the fact that the particles of amorphous bodies are located unequally densely: there are voids between them.
Crystallization of amorphous bodies. Over time (several months, years), amorphous substances spontaneously transform into a crystalline state. For example, sugar candies or fresh honey left alone in a warm place will become opaque after a few months. They say that honey and candy are “candied.” By breaking a candy cane or scooping up honey with a spoon, we will actually see the sugar crystals that have formed.
Spontaneous crystallization of amorphous bodies indicates that the crystalline state of a substance is more stable than the amorphous one. The intermolecular theory explains it this way. Intermolecular forces of attraction and repulsion cause particles of an amorphous body to jump preferentially to where there are voids. As a result, a more ordered arrangement of particles appears than before, that is, a polycrystal is formed.
Melting of amorphous bodies.
As the temperature increases, the energy of the vibrational motion of atoms in a solid increases and, finally, a moment comes when the bonds between atoms begin to break. In this case, the solid turns into a liquid state. This transition is called melting. At a fixed pressure, melting occurs at a strictly defined temperature.
The amount of heat required to convert a unit mass of a substance into a liquid at its melting point is called the specific heat of fusion λ .
To melt a substance of mass m it is necessary to expend an amount of heat equal to:
Q = λ m .
The process of melting amorphous bodies differs from melting crystalline bodies. As the temperature increases, amorphous bodies gradually soften and become viscous until they turn into liquid. Amorphous bodies, unlike crystals, do not have a specific melting point. The temperature of amorphous bodies changes continuously. This happens because in amorphous solids, as in liquids, molecules can move relative to each other. When heated, their speed increases, and the distance between them increases. As a result, the body becomes softer and softer until it turns into liquid. When amorphous bodies solidify, their temperature also decreases continuously.
