Metals are substances that have a crystalline structure. When heated, they are able to melt, that is, go into a fluid state. Some of them have a low melting point: they can be melted by placing them in an ordinary spoon and holding them over a candle flame. These are lead and tin. Others can only be melted in special furnaces. Copper and iron are high. To lower it, additives are introduced into the metal. The resulting alloys (steel, bronze, cast iron, brass) have a melting point lower than the original metal.

What does the melting point of metals depend on? All of them have certain characteristics - the heat capacity and thermal conductivity of metals. Heat capacity is the ability to absorb heat when heated. Its numerical indicator is the specific heat capacity. It refers to the amount of energy that a unit mass of metal, heated by 1 ° C, can absorb. The fuel consumption for heating the metal billet to desired temperature. The heat capacity of most metals is in the range of 300-400 J / (kg * K), metal alloys - 100-2000 J / (kg * K).

The thermal conductivity of metals is the transfer of heat from hotter particles to colder ones according to the Fourier law with their macroscopic immobility. It depends on the structure of the material, its chemical composition and the type of interatomic bond. In metals, heat transfer is carried out by electrons, in others hard materials- phonons. The thermal conductivity of metals is the higher, the more perfect the crystalline structure they have. The more impurities the metal has, the more distorted the crystal lattice, and the lower the thermal conductivity. Doping introduces such distortions into the structure of metals and lowers the thermal conductivity relative to the base metal.

All metals have good thermal conductivity, but some are higher than others. An example of such metals is gold, copper, silver. Lower thermal conductivity - in tin, aluminum, iron. The increased thermal conductivity of metals is an advantage or disadvantage, depending on the scope of their use. For example, it is necessary for metal utensils to quickly heat food. At the same time, the use of metals with high thermal conductivity for the manufacture of cookware handles makes it difficult to use - the handles heat up too quickly, and it is impossible to touch them. Therefore, heat-insulating materials are used here.

Another characteristic of a metal that affects its properties is thermal expansion. It looks like an increase in the volume of the metal when it is heated and a decrease - when it is cooled. This phenomenon must be taken into account in the manufacture of metal products. So, for example, pot lids are made overhead, kettles also have a gap between the lid and the body so that the lid does not jam when heated.

For each metal, the coefficient is calculated. It is determined by heating by 1 ° C prototype, having a length of 1 m. Lead, zinc, and tin have the largest coefficient. It is smaller in copper and silver. Even lower - iron and gold.

According to their chemical properties, metals are divided into several groups. There are active metals (for example, potassium or sodium) that can instantly react with air or water. The six most active metals, which make up the first group of the periodic table, are called alkaline. They have a low melting point and are so soft that they can be cut with a knife. When combined with water, they form alkaline solutions hence their name.

The second group consists of alkaline earth metals - calcium, magnesium, etc. They are part of many minerals, more solid and refractory. Examples of metals of the following, third and fourth groups, are lead and aluminum. These are fairly soft metals and are often used in alloys. Transition metals (iron, chromium, nickel, copper, gold, silver) are less active, more malleable and are often used in industry in the form of alloys.

The position of each metal in the activity series characterizes its ability to react. How more active metal the easier it takes oxygen. They are very difficult to isolate from compounds, while inactive ones can be found in their pure form. The most active of them - potassium and sodium - are stored in kerosene, outside of it they are immediately oxidized. Of the metals used in industry, copper is the least active. It is used to make tanks and pipes for hot water and electrical wires.

Introduction

Determining the thermal conductivity of metals plays an important role in some areas, such as metallurgy, radio engineering, mechanical engineering, and construction. Currently, there are many different methods by which it is possible to determine the thermal conductivity of metals.

This work is devoted to the study of the main property of metals - thermal conductivity, as well as the study of methods for studying thermal conductivity.

The object of study is the thermal conductivity of metals, as well as various methods laboratory research.

The subject of research is the coefficients of thermal conductivity of metals.

Planned result - staging laboratory work"Determination of the thermal conductivity of metals" based on the calorimetric method.

To achieve this goal, it is necessary to solve the following tasks:

Study of the theory of thermal conductivity of metals;

Study of methods for determining the coefficient of thermal conductivity;

Selection of laboratory equipment;

Experimental determination of the thermal conductivity of metals;

Statement of laboratory work "Determination of the thermal conductivity of metals."

The work consists of three chapters in which the assigned tasks are disclosed.

Thermal conductivity of metals

Fourier law

Thermal conductivity is the molecular transfer of heat between directly contacting bodies or particles of the same body with different temperatures, at which the energy exchange of the movement of structural particles (molecules, atoms, free electrons) occurs.

Thermal conductivity is determined by the thermal motion of the microparticles of the body.

The basic law of heat transfer by thermal conductivity is the Fourier law. According to this law, the amount of heat dQ transferred by means of heat conduction through the surface element dF, which is perpendicular to the heat flow, during the time df is directly proportional to the temperature gradient, the surface dF and the time df.

The coefficient of proportionality l is called the coefficient of thermal conductivity. The thermal conductivity coefficient is a thermophysical characteristic of a substance, it characterizes the ability of a substance to conduct heat.

The minus sign in formula (1) indicates that heat is transferred in the direction of decreasing temperature.

The amount of heat that has passed per unit of time through a unit of isothermal surface is called heat flux:

Fourier's law is applicable to describe the thermal conductivity of gases, liquids and solids, the difference will be only in the thermal conductivity coefficients.

The coefficient of thermal conductivity of metals and its dependence on the parameters of the state of matter

The thermal conductivity coefficient is a thermophysical characteristic of a substance, it characterizes the ability of a substance to conduct heat.

Thermal conductivity coefficient - the amount of heat passing per unit time through a single area, perpendicular to grad t.

For different substances, the thermal conductivity coefficient is different and depends on the structure, density, humidity, pressure and temperature. These circumstances must be taken into account when using lookup tables.

The greatest value is the coefficient of thermal conductivity of metals, for which. The most thermally conductive metal is silver, followed by pure copper, gold, aluminum, etc. For most metals, an increase in temperature leads to a decrease in the thermal conductivity. This dependence can be approximated by the straight line equation

here l, l0 - respectively, the thermal conductivity coefficients at a given temperature t and at 00C, in - temperature coefficient. The thermal conductivity of metals is very sensitive to impurities.

For example, when even traces of arsenic appear in copper, its thermal conductivity decreases from 395 to 142; for steel at 0.1% carbon l \u003d 52, at 1.0% - l \u003d 40, at 1.5% carbon l \u003d 36.

Heat treatment also affects the thermal conductivity. So, for hardened carbon steel, l is 10–25% lower than for soft steel. For these reasons, the thermal conductivity coefficients of commercial metal samples at the same temperatures can vary significantly. It should be noted that alloys, in contrast to pure metals, are characterized by an increase in the thermal conductivity coefficient with increasing temperature. Unfortunately, it has not yet been possible to establish any general quantitative patterns that govern the coefficient of thermal conductivity of alloys.

The value of the coefficient of thermal conductivity of building and heat-insulating materials - dielectrics is many times less than that of metals and is 0.02 - 3.0. For the vast majority of them (the exception is magnesite brick), the thermal conductivity coefficient increases with increasing temperature. In this case, equation (3) can be used, bearing in mind that for solids - dielectrics v>0.

Many building and heat-insulating materials have a porous structure (brick, concrete, asbestos, slag, etc.). For them and powdered materials, the thermal conductivity coefficient significantly depends on the bulk density. This is due to the fact that with increasing porosity, most of volume is filled with air, the coefficient of thermal conductivity of which is very low. However, the higher the porosity, the lower the bulk density of the material. Thus, a decrease in the bulk density of a material, ceteris paribus, leads to a decrease in l.

For example, for asbestos, a decrease in bulk density from 800 kg/m to 400 kg/m results in a decrease from 0.248 to 0.105. Humidity influence is very high. For example, for dry brick l \u003d 0.35, for liquid 0.6, and for wet brick l \u003d 1.0.

It is necessary to pay attention to these phenomena when determining and technical calculations of thermal conductivity. The coefficient of thermal conductivity of dropping liquids is in the range of 0.08 - 0.7. At the same time, for the vast majority of liquids, the thermal conductivity coefficient decreases with increasing temperature. The exceptions are water and glycerin.

The coefficient of thermal conductivity of gases is even lower.

The thermal conductivity of gases increases with increasing temperature. Within the range of 20 mm Hg. up to 2000 at (bar), i.e. in the area that is most often encountered in practice, l does not depend on pressure. It should be borne in mind that for a mixture of gases (flue gases, atmosphere of thermal furnaces, etc.) it is impossible to determine the coefficient of thermal conductivity by calculation. Therefore, in the absence of reference data, a reliable value of l can only be found empirically.

With a value of l< 1 - вещество называют тепловым изолятором.

To solve problems of thermal conductivity, it is necessary to have information about some macroscopic properties (thermophysical parameters) of a substance: the coefficient of thermal conductivity, density, specific heat.

Explanation of the thermal conductivity of metals

The thermal conductivity of metals is very high. It is not reduced to the thermal conductivity of the lattice, therefore, another heat transfer mechanism must operate here. It turns out that in pure metals the thermal conductivity is carried out almost entirely due to the electron gas, and only in highly contaminated metals and alloys, where the conductivity is low, does the contribution of the lattice thermal conductivity turn out to be significant.

The numerical characteristic of the thermal conductivity of a material can be determined by the amount of heat passing through a material of a certain thickness in certain time. The numerical characteristic is important when calculating the thermal conductivity of various profile products.

Thermal conductivity coefficients of various metals

Heat conduction requires direct physical contact carried out between two bodies. This means that heat transfer is feasible only between solids and motionless liquids. Direct contact enables kinetic energy to move from the molecules of the warmest substance to the coldest. Heat exchange occurs when bodies of different temperatures are in direct contact with each other.

Here we should pay attention to the fact that the molecules of a warm body cannot penetrate into a cold body. There is only a transfer of kinetic energy, which gives a uniform distribution of heat. This transfer of energy will continue until the contacting bodies become uniformly warm. In this case, thermal equilibrium is reached. Based on this knowledge, it is possible to calculate what kind of insulation material is required for the thermal insulation of a building.

The high thermal conductivity of copper, along with other remarkable properties, has given this metal a significant place in the history of the development of human civilization. Products made of copper and its alloys are used in almost all spheres of our life.

1

Thermal conductivity is the process of transferring the energy of particles (electrons, atoms, molecules) of more heated parts of the body to particles of its less heated parts. This heat exchange leads to temperature equalization. Only energy is transferred along the body, matter does not move. A characteristic of the ability to conduct heat is the coefficient of thermal conductivity, numerically equal to the amount of heat that passes through a material with an area of ​​1 m 2, a thickness of 1 m, in 1 second at a unit temperature gradient.

The coefficient of thermal conductivity of copper at a temperature of 20–100 °C is 394 W/(m * K) - only silver is higher. inferior to copper in this indicator by almost 9 times, and iron - by 6. Various impurities have different effects on physical properties metals. With copper, the rate of heat transfer is reduced when added to the material or ingested as a result technological process substances such as:

• aluminum;
• iron;
• oxygen;
• arsenic;
• antimony;
• sulfur;
• selenium;
• phosphorus.

High thermal conductivity is characterized by the rapid spread of heating energy throughout the volume of the object. This ability provided copper wide application in any heat exchange systems. It is used in the manufacture of tubes and radiators of refrigerators, air conditioners, vacuum units, cars to remove excess heat from the coolant. In heating appliances, such copper products are used for heating.

Copper's ability to conduct heat decreases as it heats up. The value of the coefficient of thermal conductivity of copper in air depends on the temperature of the latter, which affects the heat transfer (cooling). The higher the temperature environment, the slower the metal cools and the lower its thermal conductivity. Therefore, all heat exchangers use forced airflow with a fan - this increases the efficiency of the devices and at the same time maintains thermal conductivity at an optimal level.

2

The thermal conductivity of aluminum and copper is different - in the first it is less than in the second, by 1.5 times. For aluminum, this parameter is 202–236 W / (m * K) and is quite high compared to other metals, but lower than that of gold, copper, silver. The scope of aluminum and copper, where high thermal conductivity is required, depends on a number of other properties of these materials.

Aluminum is not inferior to copper in anti-corrosion properties and is superior in the following indicators:

• the density (specific gravity) of aluminum is 3 times less;
• the cost is 3.5 times lower.

A similar product, but made of aluminum, is much lighter than copper. Since the weight of the metal requires 3 times less, and its price is 3.5 times lower, the aluminum part can be about 10 times cheaper. Due to this and high thermal conductivity, aluminum has found wide application in the manufacture of dishes, food foil for ovens. Since this metal is soft, it is not used in its pure form - its alloys are mainly common (the most famous is duralumin).

In various heat exchangers, the main thing is the rate of return of excess energy to the environment. This problem is solved by intensive blowing of the radiator by means of a fan. At the same time, the lower thermal conductivity of aluminum practically does not affect the quality of cooling, and equipment and devices are much lighter and cheaper (for example, computer and Appliances). AT recent times in production, there has been a tendency to replace copper tubes in air conditioning systems with aluminum ones.

Copper is practically indispensable in the radio industry, electronics as a conductive material. Due to its high ductility, wires up to 0.005 mm in diameter can be drawn from it and other very thin conductive connections used for electronic devices can be made. Higher conductivity than aluminum provides minimal losses and less heating of radioelements. Thermal conductivity allows you to effectively remove the heat generated during operation to the external elements of devices - the case, the supply contacts (for example, microcircuits, modern microprocessors).

Copper templates are used in welding when it is necessary to make a surfacing of the desired shape on a steel part. High thermal conductivity will not allow the copper template to connect to the welded metal. Aluminum cannot be used in such cases, since it is likely to melt or burn through. Copper is also used in carbon arc welding - a rod of this material serves as a non-consumable cathode.

3

Low thermal conductivity in many cases is a desirable property - this is the basis of thermal insulation. The use of copper pipes in heating systems leads to much greater heat loss than when using pipelines and wiring from other materials. Copper pipelines require more thorough thermal insulation.

Copper has a high thermal conductivity, which causes sufficient difficult process installation and other works that have their own specifics. Welding, soldering, cutting copper requires more concentrated heating than for steel, and often pre-heating and concomitant heating of the metal.

When gas welding copper, it is necessary to use torches with a power of 1-2 numbers higher than for steel parts of the same thickness. If copper is thicker than 8-10 mm, it is recommended to work with two or even three burners (often welding is done with one, while the others are heated). Welding work on alternating current with electrodes is accompanied by increased spatter of the metal. A cutter capable of cutting 300mm of high chromium steel is suitable for cutting brass, bronze (copper alloys) up to 150mm and pure copper as little as 50mm. All work is associated with significantly higher costs for consumables.

4

Copper is one of the main components in electronics and is used in all microcircuits. It removes and dissipates the heat generated during the passage of current. The limitation of the speed of computers is due to an increase in the heating of the processor and other circuit elements with an increase in clock frequency. Splitting into several cores working simultaneously, and other ways to deal with overheating, have exhausted themselves. Currently, developments are underway aimed at obtaining conductors with higher electrical and thermal conductivity.

Recently discovered by scientists, graphene can significantly increase the thermal conductivity of copper conductors and their ability to dissipate heat. During the experiment, the copper layer was covered with graphene from all sides. This improved the heat transfer of the conductor by 25%. As the scientists explained, the new substance changes the structure of heat transfer and allows energy to move more freely in the metal. The invention is under development - the experiment used a copper conductor much large sizes than in the processor.

- the first in importance and prevalence structural material. It has been known since ancient times, and its properties are such that when iron was learned to be smelted in significant quantities, the metal replaced all other alloys. The age of iron has come and, judging by, this time will not end soon. This article will tell you what is the specific gravity of iron, what is its melting point in its pure form.

Iron is a typical metal, and chemically active. The substance reacts with normal temperature, and heating or increasing humidity significantly increase its reactivity. Iron corrodes in air, burns in an atmosphere of pure oxygen, and in the form of fine dust it can also ignite in air.

Pure iron is malleable, but in this form the metal is very rare. In fact, iron is an alloy with small proportions of impurities - up to 0.8%, which is characterized by the softness and malleability of a pure substance. Meaning for National economy has alloys with carbon - steel, cast iron, stainless steel.

Polymorphism is inherent in iron: there are as many as 4 modifications that differ in structure and lattice parameters:

• α-Fe - exists from zero to +769 C. It has a body-centered cubic lattice and is a ferromagnet, that is, it retains magnetization in the absence of an external magnetic field. +769 С – Curie points for metal;
• from +769 to +917 C, β-Fe appears. It differs from the α-phase only in the lattice parameters. In this case, almost all physical properties are preserved, with the exception of magnetic ones: iron becomes paramagnetic, that is, it loses its ability to magnetize and is drawn into a magnetic field. Metal science does not consider the β-phase as a separate modification. Since the transition does not affect significant physical characteristics;
• in the range from 917 to 1394 C, there is a γ-modification, which is characterized by a face-centered cubic lattice;
• at temperatures above +1394 C, a δ-phase appears, which is characterized by a body-centered cubic lattice.

At high pressure, and also when the metal is alloyed with some additives, an ε-phase with a hexagonal close-packed lattice is formed.

The temperature of phase transitions noticeably changes upon doping with the same carbon. Actually, the very ability of iron to form so many modifications serves as the basis for processing steel in various temperature conditions. Without such transitions, the metal would not have become so widespread.

Now it is the turn of the properties of iron metal.

This video tells about the structure of iron:

Metal properties and characteristics

Iron is a fairly light, moderately refractory metal, silver-gray in color. It reacts readily with dilute acids and is therefore considered an element of medium activity. In dry air, the metal is gradually covered with an oxide film, which prevents further reaction.

But at the slightest humidity, instead of a film, rust appears - loose and heterogeneous in composition. Rust does not prevent further corrosion of iron. However, the physical properties of the metal, and, most importantly, its alloys with carbon are such that, despite the low corrosion resistance, the use of iron is more than justified.

Mass and Density

The molecular weight of iron is 55.8, which indicates the relative lightness of the substance. What is the density of iron? This indicator is determined by the phase modification:

• α-Fe - 7.87 g / cu. cm at 20 C, and 7.67 g / cu. cm at 600 C;
• the γ-phase is distinguished by an even lower density - 7.59 g / cc at 1000C;
• the density of the δ-phase is 7.409 g/cm3.

As the temperature rises, the density of iron naturally decreases.

And now let's find out what is the melting point of iron in Celsius, comparing it, for example, with or cast iron.

Temperature Range

The metal is classified as moderately refractory, which means a relatively low temperature of the change in the state of aggregation:

• melting point - 1539 C;
• boiling point - 2862 C;
• Curie temperature, that is, the loss of the ability to magnetize - 719 C.

It should be borne in mind that when talking about the melting or boiling point, they are dealing with the δ-phase of a substance.

This video will tell you about the physical and chemical properties gland:

Mechanical characteristics

Iron and its alloys are so common that although they began to be used later than, for example, and, they became a kind of standard. When metals are compared, they point to iron: stronger than steel, 2 times softer than iron, and so on.

Characteristics are given for a metal containing small proportions of impurities:

• hardness on the Mohs scale - 4–5;
• Brinell hardness - 350-450 Mn / sq. m. Moreover, chemically pure iron has a higher hardness - 588–686;

Strength indicators are extremely dependent on the amount and nature of impurities. This value is regulated by GOST for each brand of alloy or pure metal. Thus, the ultimate compressive strength for unalloyed steel is 400–550 MPa. When hardening this grade, the tensile strength increases to 700 MPa.

• the impact strength of the metal is 300 MN/sq m;
• yield strength –100 MN/sq. m.

We will learn further about what is needed to determine the specific heat capacity of iron.

Heat capacity and thermal conductivity

Like any metal, iron conducts heat, although its performance in this area is low: in terms of thermal conductivity, the metal is inferior to aluminum - 2 times less, and - 5 times.

The thermal conductivity at 25°C is 74.04 W/(m·K). The value depends on the temperature;

• at 100 K the thermal conductivity is 132 [W/(m.K)];
• at 300 K - 80.3 [W / (m.K)];
• at 400 - 69.4 [W / (m.K)];
• and at 1500 - 31.8 [W / (m.K)].

• The coefficient of thermal expansion at 20 C is 11.7 10-6.
• The heat capacity of a metal is determined by its phase structure and depends rather intricately on temperature. With an increase to 250 C, the heat capacity slowly increases, then increases sharply until the Curie point is reached, and then begins to decrease.
• The specific heat capacity in the temperature range from 0 to 1000C is 640.57 J/(kg K).

Electrical conductivity

Iron conducts current, but not nearly as well as copper and silver. The specific electrical resistance of the metal at normal conditions– 9.7 10-8 ohm m.

Since iron is a ferromagnet, its performance in this area is more significant:

• saturation magnetic induction is 2.18 T;
• magnetic permeability - 1.45.106.

Toxicity

Metal does not pose a danger to the human body. steel and the manufacture of iron products can be dangerous, but only due to high temperatures and those additives that are used in the production of various alloys. Iron waste - scrap metal, poses a danger to the environment, but quite moderate, since the metal rusts in the air.

Iron does not have biological inertness, therefore it is not used as a material for prosthetics. However, in human body this element plays one of the most important roles: a violation in the absorption of iron or an insufficient amount of the latter in the diet guarantees anemia at best.

Iron is absorbed with great difficulty - 5-10% of the total amount supplied to the body, or 10-20% if there is a lack of it.

• Plain daily requirement in iron is 10 mg for men and 20 mg for women.
• The toxic dose is 200 mg/day.
• Lethal - 7-35 g. It is almost impossible to get such an amount of iron, so iron poisoning is extremely rare.

Iron is a metal whose physical characteristics, in particular strength, can be significantly changed by resorting to machining or the addition of a very small amount of alloying elements. This feature, combined with the availability and ease of extraction of metal, makes iron the most demanded structural material.

A specialist will tell you more about the properties of iron in the video below:

Thermal conductivity is a physical quantity that determines the ability of materials to conduct heat. In other words, thermal conductivity is the ability of substances to transfer the kinetic energy of atoms and molecules to other substances that are in direct contact with them. In SI, this value is measured in W/(K*m) (Watts per Kelvin meter), which is equivalent to J/(s*m*K) (Joule per second Kelvin meter).

The concept of thermal conductivity

It is an intensive physical quantity, that is, a quantity that describes a property of matter that does not depend on the quantity of the latter. Intensive quantities are also temperature, pressure, electrical conductivity, that is, these characteristics are the same at any point of the same substance. Another group of physical quantities are extensive, which are determined by the amount of matter, for example, mass, volume, energy, and others.

The opposite value for thermal conductivity is thermal resistance, which reflects the ability of a material to prevent the transfer of heat passing through it. For an isotropic material, that is, a material whose properties are the same in all spatial directions, thermal conductivity is a scalar quantity and is defined as the ratio of the heat flux through a unit area per unit time to the temperature gradient. Thus, a thermal conductivity of one watt per meter-Kelvin means that thermal energy of one Joule is transferred through the material:

• in one second;
• through an area of ​​one square meter;
• at a distance of one meter;
• when the temperature difference between surfaces that are one meter apart in a material is one Kelvin.

It is clear that what more value thermal conductivity, the better the material conducts heat, and vice versa. For example, the value of this value for copper is 380 W / (m * K), and this metal is 10,000 times better at transferring heat than polyurethane, whose thermal conductivity is 0.035 W / (m * K).

Heat transfer at the molecular level

When matter is heated, the average kinetic energy of its constituent particles increases, that is, the level of disorder increases, atoms and molecules begin to oscillate more intensively and with greater amplitude around their equilibrium positions in the material. Heat transfer, which at the macroscopic level can be described by the Fourier law, at the molecular level is an exchange kinetic energy between particles (atoms and molecules) of a substance, without transferring the latter.

This explanation of the mechanism of heat conduction at the molecular level distinguishes it from the mechanism of thermal convection, in which heat transfer takes place due to the transfer of matter. All solid bodies have the ability to conduct heat, while thermal convection is possible only in liquids and gases. Indeed, solids transfer heat mainly due to thermal conductivity, while liquids and gases, if there are temperature gradients in them, transfer heat mainly due to convection processes.

Thermal conductivity of materials

Metals have a pronounced ability to conduct heat. Polymers are characterized by low thermal conductivity, and some of them practically do not conduct heat, for example, fiberglass, such materials are called heat insulators. In order for this or that flow of heat through space to exist, the presence of some substance in this space is necessary, therefore, in open space(empty space) thermal conductivity is zero.

Each homogeneous (homogeneous) material is characterized by a coefficient of thermal conductivity (denoted by the Greek letter lambda), that is, a value that determines how much heat needs to be transferred through an area of ​​​​1 m², so that in one second, passing through a material thickness of one meter, the temperature at its ends changes per 1 K. This property is inherent in each material and varies depending on its temperature, so this coefficient is usually measured at room temperature (300 K) to compare the characteristics of different substances.

If the material is heterogeneous, for example, reinforced concrete, then the concept of a useful thermal conductivity coefficient is introduced, which is measured according to the coefficients of the homogeneous substances that make up this material.

The table below shows the thermal conductivity coefficients of some metals and alloys in W / (m * K) for a temperature of 300 K (27 ° C):

• steel 47-58;
• aluminum 237;
• copper 372.1-385.2;
• bronze 116-186;
• zinc 106-140;
• titanium 21.9;
• tin 64.0;
• lead 35.0;
• iron 80.2;
• brass 81-116;
• gold 308.2;
• silver 406.1-418.7.

The following table provides data for non-metallic solids:

• glass fiber 0.03-0.07;
• glass 0.6-1.0;
• asbestos 0.04;
• tree 0.13;
• paraffin 0.21;
• brick 0.80;
• diamond 2300.

It can be seen from the considered data that the thermal conductivity of metals is much higher than that of non-metals. The exception is diamond, which has a heat transfer coefficient five times that of copper. This property of diamond is due to the strong covalent bonds between the carbon atoms that form its crystal lattice. It is thanks to this property that a person feels cold when touching a diamond with his lips. The property of diamond is well tolerated thermal energy used in microelectronics to remove heat from microcircuits. And also this property is used in special devices that allow you to distinguish a real diamond from a fake.

In some industrial processes, an attempt is made to increase the ability to transfer heat, which is achieved either by good conductors or by increasing the contact area between the components of the structure. Examples of such structures are heat exchangers and heat dissipators. In other cases, on the contrary, they try to reduce the thermal conductivity, which is achieved through the use of heat insulators, voids in structures and a decrease in the contact area of ​​the elements.

Heat transfer coefficients of steels

The ability to transfer heat for steels depends on two main factors: composition and temperature.

With an increase in carbon content, simple carbon steels reduce their specific gravity, according to which their ability to transfer heat also decreases from 54 to 36 W / (m * K) with a change in the percentage of carbon in steel from 0.5 to 1.5%.

Stainless steels contain chromium (10% or more), which, together with carbon, forms complex carbides that prevent the oxidation of the material, and also increases the electrode potential of the metal. The thermal conductivity of stainless steel is low compared to other steels and ranges from 15 to 30 W / (m * K) depending on its composition. Heat-resistant chromium-nickel steels have even lower values ​​of this coefficient (11-19 W / (m * K).

Another class are galvanized steels with a specific gravity of 7,850 kg/m3, which are obtained by coating steel with iron and zinc. Since zinc conducts heat more easily than iron, the thermal conductivity of galvanized steel will be relatively high compared to other steel grades. It ranges from 47 to 58 W / (m * K).

Thermal conductivity of steel at various temperatures usually does not change much. For example, the thermal conductivity coefficient of steel 20 decreases from 86 to 30 W / (m * K) with an increase in temperature from room temperature to 1200 ° C, and for steel grade 08X13, an increase in temperature from 100 to 900 ° C does not change its thermal conductivity coefficient (27-28 W/(m*K).

Factors affecting the physical quantity

The ability to conduct heat depends on a number of factors, including the temperature, structure, and electrical properties of the substance.

Material temperature

The effect of temperature on the ability to conduct heat differs for metals and non-metals. In metals, conductivity is mainly associated with free electrons. According to the Wiedemann-Franz law, the thermal conductivity of a metal is proportional to the product of the absolute temperature, expressed in Kelvin, and its electrical conductivity. In pure metals, the electrical conductivity decreases with increasing temperature, so the thermal conductivity remains approximately constant. In the case of alloys, the electrical conductivity changes little with increasing temperature, so the thermal conductivity of alloys increases in proportion to temperature.

On the other hand, heat transfer in non-metals is mainly associated with lattice vibrations and the exchange of lattice phonons. Except for crystals High Quality and low temperatures, the phonon path in the lattice does not decrease significantly at high temperatures, therefore, the thermal conductivity remains constant over the entire temperature range, that is, it is insignificant. At temperatures below the Debye temperature, the ability of nonmetals to conduct heat, along with their heat capacity, is greatly reduced.

Phase transitions and structure

When a material experiences a first-order phase transition, such as from a solid to a liquid or from a liquid to a gas, its thermal conductivity may change. A striking example of such a change is the difference in this physical quantity for ice (2.18 W/(m*K) and water (0.90 W/(m*K).

Changes in the crystal structure of materials also affect thermal conductivity, which is explained by the anisotropic properties of various allotropic modifications of a substance of the same composition. Anisotropy affects the different intensity of scattering of lattice phonons, the main heat carriers in non-metals, and in different directions in the crystal. Here a prime example is sapphire, the conductivity of which varies from 32 to 35 W / (m * K) depending on the direction.

electrical conductivity

Thermal conductivity in metals changes along with electrical conductivity according to the Wiedemann-Franz law. This is due to the fact that valence electrons, freely moving along the crystal lattice of the metal, carry not only electrical, but also thermal energy. For other materials, the correlation between these types of conductivity is not pronounced, due to the insignificant contribution of the electronic component to thermal conductivity (in nonmetals, lattice phonons play the main role in the heat transfer mechanism).

convection process

Air and other gases are generally good thermal insulators in the absence of convection. This principle is based on the work of many heat-insulating materials containing a large number of small voids and pores. This structure does not allow convection to propagate over long distances. Examples of such man-made materials are polystyrene and silicide airgel. In nature, such heat insulators as the skin of animals and the plumage of birds work on the same principle.

Light gases, such as hydrogen and gel, have high thermal conductivity values, while heavy gases, such as argon, xenon, and radon, are poor heat conductors. For example, argon, an inert gas that is heavier than air, is often used as an insulating gas filler in double windows and light bulbs. The exception is sulfur hexafluoride (SF6), which is a heavy gas and has a relatively high thermal conductivity due to its high heat capacity.