Solids. Crystal bodies

MINISTRY OF EDUCATION

PHYSICS 8TH GRADE

Report on the topic:

“Amorphous bodies. Melting of amorphous bodies.”

8th grade student:

2009

Amorphous bodies.

Let's do an experiment. We will need a piece of plasticine, a stearine 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 the melting of 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.

Along with crystalline solids, amorphous solids are also found. Amorphous bodies, unlike crystals, do not have a strict order in the arrangement of atoms. Only the closest atoms - neighbors - 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.

Often the same substance can be found in both crystalline and amorphous states. For example, quartz can be in either crystalline or amorphous form (silica). The crystalline form of quartz can be schematically represented as a lattice of regular hexagons (Fig. 77, a). The amorphous structure of quartz also has the appearance of a lattice, but of irregular shape. Along with hexagons, it contains pentagons and heptagons (Fig. 77, b).

Properties of amorphous bodies. All amorphous bodies are isotropic: their physical properties are the same in all directions. Amorphous bodies include glass, many plastics, resin, rosin, sugar candy, etc.

Under external influences, amorphous bodies exhibit both elastic properties, like solids, and fluidity, like liquids. Under short-term impacts (impacts), they behave like a solid body and, with a strong impact, break into pieces. But with very long exposure, amorphous bodies flow. For example, a piece of resin gradually spreads over a solid surface. Atoms or molecules of amorphous bodies, like molecules of a liquid, have a certain “settled life” time, the time of oscillations around the equilibrium position. But unlike liquids, this time is very long. In this respect, amorphous bodies are close to crystalline ones, since jumps of atoms from one equilibrium position to another rarely occur.

At low temperatures, amorphous bodies resemble solids in their properties. They have almost no fluidity, but as the temperature rises they gradually soften and their properties become closer and closer to the properties of liquids. This happens because with increasing temperature, jumps of atoms from one position gradually become more frequent.

balance to another. There is no specific melting point for amorphous bodies, unlike crystalline ones.

Solid state physics. All properties of solids (crystalline and amorphous) can be explained on the basis of knowledge of their atomic-molecular structure and the laws of motion of molecules, atoms, ions and electrons that make up solids. Studies of the properties of solids are united in a large field of modern physics - solid state physics. The development of solid state physics is stimulated mainly by the needs of technology. Approximately half of the world's physicists work in the field of solid state physics. Of course, achievements in this area are unthinkable without deep knowledge of all other branches of physics.

1. How do crystalline bodies differ from amorphous ones? 2. What is anisotropy? 3. Give examples of monocrystalline, polycrystalline and amorphous bodies. 4. How do edge dislocations differ from screw dislocations?

Solids are divided into amorphous and crystalline, depending on their molecular structure and physical properties.

Unlike crystals, the molecules and atoms of amorphous solids do not form a lattice, and the distance between them fluctuates within a certain range of possible distances. In other words, in crystals, atoms or molecules are mutually arranged in such a way that the formed structure can be repeated throughout the entire volume of the body, which is called long-range order. In the case of amorphous bodies, the structure of molecules is preserved only relative to each one such molecule, a pattern is observed in the distribution of only neighboring molecules - short-range order. An illustrative example is presented below.

Amorphous bodies include glass and other substances in a glassy state, rosin, resins, amber, sealing wax, bitumen, wax, as well as organic substances: rubber, leather, cellulose, polyethylene, etc.

Properties of amorphous bodies

The structural features of amorphous solids give them individual properties:

1. Weak fluidity is one of the most well-known properties of such bodies. An example would be glass drips that have been sitting in a window frame for a long time.
2. Amorphous solids do not have a specific melting point, since the transition to a liquid state during heating occurs gradually, through softening of the body. For this reason, the so-called softening temperature range is applied to such bodies.

1. Due to their structure, such bodies are isotropic, that is, their physical properties do not depend on the choice of direction.
2. A substance in an amorphous state has greater internal energy than in a crystalline state. For this reason, amorphous bodies are able to independently transform into a crystalline state. This phenomenon can be observed as a result of glass becoming cloudy over time.

Glassy state

In nature, there are liquids that are practically impossible to transform into a crystalline state by cooling, since the complexity of the molecules of these substances does not allow them to form a regular crystal lattice. Such liquids include molecules of some organic polymers.

However, with the help of deep and rapid cooling, almost any substance can transform into a glassy state. This is an amorphous state that does not have a clear crystal lattice, but can partially crystallize on the scale of small clusters. This state of matter is metastable, that is, it persists under certain required thermodynamic conditions.

Using cooling technology at a certain speed, the substance will not have time to crystallize and will be converted into glass. That is, the higher the cooling rate of the material, the less likely it is to crystallize. For example, to produce metal glasses, a cooling rate of 100,000 - 1,000,000 Kelvin per second will be required.

In nature, the substance exists in a glassy state and arises from liquid volcanic magma, which, interacting with cold water or air, quickly cools. In this case, the substance is called volcanic glass. You can also observe glass formed as a result of the melting of a falling meteorite interacting with the atmosphere - meteorite glass or moldavite.

Solids are characterized by constant shape and volume and are divided into crystalline and amorphous.

Crystal bodies

Each chemical substance in a crystalline state corresponds to a specific crystal lattice, which determines the physical properties of the crystal.

Did you know?
Many years ago in St. Petersburg, in one of the unheated warehouses, there were large stocks of white tin shiny buttons. And suddenly they began to darken, lose their shine and crumble into powder. Within a few days, the mountains of buttons turned into a pile of gray powder. "Tin Plague"- this is how this “disease” of white tin was called.
And this was just a rearrangement of the order of atoms in tin crystals. Tin, passing from a white variety to a gray one, crumbles into powder.
Both white and gray tin are crystals of tin, but at low temperatures their crystal structure changes, and as a result the physical properties of the substance change.

Crystals can have different shapes and are limited to flat edges.

In nature there are:
A) single crystals- these are single homogeneous crystals that have the shape of regular polygons and have a continuous crystal lattice

Single crystals of table salt:

b) polycrystals- these are crystalline bodies fused from small, chaotically located crystals.
Most solids have a polycrystalline structure (metals, stones, sand, sugar).

Bismuth polycrystals:

Anisotropy of crystals

In crystals it is observed anisotropy- dependence of physical properties (mechanical strength, electrical conductivity, thermal conductivity, refraction and absorption of light, diffraction, etc.) on the direction inside the crystal.

Anisotropy is observed mainly in single crystals.

In polycrystals (for example, in a large piece of metal), anisotropy does not appear in the normal state.
Polycrystals consist of a large number of small crystal grains. Although each of them has anisotropy, due to the disorder of their arrangement, the polycrystalline body as a whole loses its anisotropy.

Any crystalline substance melts and crystallizes at a strictly defined melting point: iron - at 1530°, tin - at 232°, quartz - at 1713°, mercury - at minus 38°.

Particles can disrupt the order of arrangement in a crystal only if it begins to melt.

As long as there is an order of particles, there is a crystal lattice, a crystal exists. If the structure of the particles is disrupted, it means that the crystal has melted - turned into liquid, or evaporated - turned into steam.

Amorphous bodies

Amorphous bodies do not have a strict order in the arrangement of atoms and molecules (glass, resin, amber, rosin).

In amorphous bodies it is observed isotropy- their physical properties are the same in all directions.

Under external influences, amorphous bodies exhibit simultaneously elastic properties (when impacted, they break into pieces like solids) and fluidity (with prolonged exposure, they flow like liquids).

At low temperatures, amorphous bodies resemble solids in their properties, and at high temperatures they are similar to very viscous liquids.

Amorphous bodies do not have a specific melting point, and hence the crystallization temperature.
When heated, they gradually soften.

Amorphous bodies occupy intermediate position between crystalline solids and liquids.

Same substance can occur in both crystalline and non-crystalline forms.

In a liquid melt of a substance, particles move completely randomly.
If, for example, you melt sugar, then:

1. if the melt solidifies slowly, calmly, then the particles gather in even rows and crystals form. This is how granulated sugar or lump sugar is obtained;

2. if cooling occurs very quickly, then the particles do not have time to line up in regular rows and the melt solidifies non-crystalline. So, if you pour melted sugar into cold water or onto a very cold saucer, sugar candy, non-crystalline sugar, is formed.

Marvelous!

Over time, a non-crystalline substance can “degenerate”, or, more precisely, crystallize; the particles in them gather in regular rows.

Only the period is different for different substances: for sugar it is several months, and for stone it is millions of years.

Let the candy lie quietly for two or three months. It will become covered with a loose crust. Look at it with a magnifying glass: these are small crystals of sugar. Crystal growth has begun in non-crystalline sugar. Wait a few more months - and not only the crust, but the entire candy will crystallize.

Even our ordinary window glass can crystallize. Very old glass sometimes becomes completely cloudy because a mass of small opaque crystals forms in it.

In glass factories, sometimes a “goat” is formed in the furnace, that is, a block of crystalline glass. This crystal glass is very durable. It is easier to destroy a furnace than to knock out a stubborn “goat” from it.
Having studied it, scientists created a new, very durable glass material - ceramic glass. This is a glass-crystalline material obtained as a result of volumetric crystallization of glass.

Curious!

Different crystal forms may exist the same substance.
For example, carbon.

Graphite is crystalline carbon. Pencil leads are made from graphite, which leaves a mark on paper when pressed lightly. The structure of graphite is layered. The layers of graphite shift easily, so the graphite flakes stick to the paper when writing.

But there is another form of crystalline carbon - diamond.

Unlike crystalline solids, there is no strict order in the arrangement of particles in an amorphous solid.

Although amorphous solids are capable of maintaining their shape, they do not have a crystal lattice. A certain pattern is observed only for molecules and atoms located in the vicinity. This order is called close order . It is not repeated in all directions and does not persist over long distances, as with crystalline bodies.

Examples of amorphous bodies are glass, amber, artificial resins, wax, paraffin, plasticine, etc.

Features of amorphous bodies

Atoms in amorphous bodies vibrate around points that are randomly located. Therefore, the structure of these bodies resembles the structure of liquids. But the particles in them are less mobile. The time they oscillate around the equilibrium position is longer than in liquids. Jumps of atoms to another position also occur much less frequently.

How do crystalline solids behave when heated? They begin to melt at a certain melting point. And for some time they are simultaneously in a solid and liquid state, until the entire substance melts.

Amorphous solids do not have a specific melting point . When heated, they do not melt, but gradually soften.

Place a piece of plasticine near the heating device. After some time it will become soft. This does not happen instantly, but over a certain period of time.

Since the properties of amorphous bodies are similar to the properties of liquids, they are considered as supercooled liquids with very high viscosity (frozen liquids). Under normal conditions they cannot flow. But when heated, jumps of atoms in them occur more often, viscosity decreases, and amorphous bodies gradually soften. The higher the temperature, the lower the viscosity, and gradually the amorphous body becomes liquid.

Ordinary glass is a solid amorphous body. It is obtained by melting silicon oxide, soda and lime. By heating the mixture to 1400 o C, a liquid glassy mass is obtained. When cooled, liquid glass does not solidify like crystalline bodies, but remains a liquid, the viscosity of which increases and the fluidity decreases. Under normal conditions, it appears to us as a solid body. But in fact it is a liquid that has enormous viscosity and fluidity, so low that it can barely be distinguished by the most ultrasensitive instruments.

The amorphous state of a substance is unstable. Over time, it gradually turns from an amorphous state into a crystalline state. This process occurs at different rates in different substances. We see candy canes becoming covered in sugar crystals. This does not take very much time.

And for crystals to form in ordinary glass, a lot of time must pass. During crystallization, glass loses its strength, transparency, becomes cloudy, and becomes brittle.

Isotropy of amorphous bodies

In crystalline solids, physical properties vary in different directions. But in amorphous bodies they are the same in all directions. This phenomenon is called isotropy .

An amorphous body conducts electricity and heat equally in all directions and refracts light equally. Sound also travels equally in amorphous bodies in all directions.

The properties of amorphous substances are used in modern technologies. Of particular interest are metal alloys that do not have a crystalline structure and belong to amorphous solids. They are called metal glasses . Their physical, mechanical, electrical and other properties differ from those of ordinary metals for the better.

Thus, in medicine they use amorphous alloys whose strength exceeds that of titanium. They are used to make screws or plates that connect broken bones. Unlike titanium fasteners, this material gradually disintegrates and is replaced over time by bone material.

High-strength alloys are used in the manufacture of metal-cutting tools, fittings, springs, and mechanism parts.

An amorphous alloy with high magnetic permeability has been developed in Japan. By using it in transformer cores instead of textured transformer steel sheets, eddy current losses can be reduced by 20 times.

Amorphous metals have unique properties. They are called the material of the future.