What determines the heat and electrical conductivity of metals?

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properties of iron and sulfur (aggressive state, thermal conductivity, electrical conductivity, smell)
All products used by humans are capable of transmitting and maintaining the temperature of an object or the environment touched by them. The ability to transfer heat from one body to another depends on the type of material through which the process passes. Properties of metals allow heat to be transferred from one object to another, with certain changes, depending on the structure and size of the metal structure.The thermal conductivity of metals is one of the parameters that determine their operational capabilities.

What is thermal conductivity and why is it needed?

The process of transferring the energy of atoms and molecules from hot objects to products with a cold temperature is carried out during the chaotic movement of moving particles. Such heat exchange depends on the state of aggregation of the metal through which the transmission passes. Depending on the chemical composition of the material, thermal conductivity will have different characteristics. This process is called thermal conductivity, it consists in the transfer of kinetic energy by atoms and molecules, which determines the heating of a metal product during the interaction of these particles, or is transferred from a warmer part to one that is less heated.

The ability to transfer or store thermal energy makes it possible to use the properties of metals to achieve the necessary technical goals in the operation of various components and assemblies of equipment used in the national economy. An example of such an application would be a soldering iron that heats up in the middle part and transfers heat to the edge of the working rod, which is used to solder the necessary elements. Knowing the properties of thermal conductivity, metals are used in all industries, using the required parameter for its intended purpose.

What determines the heat and electrical conductivity of metals?

General properties of metals. Physico-mechanical and chemical properties of metals.

They are explained by the special structure of the crystal lattice - the presence of free electrons (“electron gas”).

– Plasticity – the ability to change shape upon impact, stretch into wire, roll into thin sheets. In the series Au, Ag, Cu, Sn, Pb, Zn, Fe decreases.

– Shiny, usually gray in color and opaque. This is due to the interaction of free electrons with light quanta incident on the metal.

– Electrical conductivity. Explained by the directional movement of free electrons from the negative pole to the positive one under the influence of a small potential difference. In the series Ag, Cu, Al, Fe decreases. When heated, electrical conductivity decreases, because As the temperature increases, vibrations of atoms and ions in the nodes of the crystal lattice intensify, which complicates the directional movement of the “electron gas”.

– Thermal conductivity. The pattern is the same. It is caused by the high mobility of free electrons and the vibrational movement of atoms, due to which the temperature quickly equalizes over the mass of the metal. The highest thermal conductivity is found in bismuth and mercury.

– Hardness. The hardest is chrome (cuts glass); the softest alkali metals - potassium, sodium, rubidium and cesium - are cut with a knife.

– Density. The smaller the atomic mass of the metal and the larger the radius of its atom (the lightest is lithium (r=0.53 g/cm3); the heaviest is osmium (r=22.6 g/cm3).

Metals having r

III. Interaction with water

Active (alkali and alkaline earth metals) form a soluble base and hydrogen:

2Na0 + 2H2O 2NaOH + H2

Ca0 + 2H2O Ca(OH)2 + H2

Metals of intermediate activity are oxidized by water when heated to an oxide:

Zn0 + H2O ZnO + H2

Inactive (Au, Ag, Pt) - do not react.

Displacement of less active metals by more active metals from solutions of their salts:

Fe+ CuSO4 Cu + FeSO4

Metallic bonding is a bond between positive ions in metal crystals, which occurs due to the attraction of electrons moving freely throughout the crystal. According to their position in the periodic table, metal atoms have a small number of valence electrons. These electrons are rather weakly bound to their nuclei and can easily break away from them. As a result, positively charged ions and free electrons appear in the crystal lattice of the metal. Therefore, in the crystal lattice of metals there is great freedom of movement of electrons: some of the atoms will lose their electrons, and the resulting ions can accept these electrons from the “electron gas”. As a consequence, the metal represents a number of positive ions localized in certain positions of the crystal lattice, and a large number of electrons moving relatively freely in the field of positive centers. This is an important difference between metallic bonds and covalent bonds, which have a strict orientation in space.

A metal bond also differs from a covalent bond in strength: its energy is 3-4 times less than the energy of a covalent bond.

Binding energy is the energy required to break a chemical bond in all molecules that make up one mole of a substance. The energies of covalent and ionic bonds are usually high and are on the order of 100-800 kJ/mol.

Thermal conductivity The ability of a body to transfer heat from its more heated parts to less heated parts Ag, Cu, Au, Al, W, Fe

There is a decrease in thermal conductivity in the series

Electrical conductivity The property of a substance to conduct electric current (due to the presence of free electrons in it) Ag, Cu, Au, Al, W, Fe

A decrease in electrical conductivity is observed in the series.

When heated, electrical conductivity decreases, since the vibrational motion of atoms and ions at lattice sites increases and the movement of electrons becomes more difficult -

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The concept of thermal resistance and thermal conductivity coefficient

If thermal conductivity characterizes the ability of metals to transfer the temperature of bodies from one surface to another, then thermal resistance shows an inverse relationship, i.e. the ability of metals to prevent such transfer, in other words, to resist. Air has high thermal resistance. It is he who, most of all, prevents the transfer of heat between bodies.

The quantitative characteristic of the change in temperature of a unit area per unit of time by one degree (K) is called the thermal conductivity coefficient. The international system of units usually measures this parameter in W/m*deg. This characteristic is very important when choosing metal products that must transfer heat from one body to another.

Thermal conductivity coefficient of metals at temperature, °C

Static electrical conductivity and thermal conductivity of metals according to Drude

Usually Ohm's law for conductors is written as U = IR

, where
R
is the resistance of the conductor, depending on the material and shape of the conductor (in most cases, and on temperature).
The dependence on the shape of the conductor can be eliminated by introducing a new quantity that depends only on the material of the conductor (for example, on the type of metal), namely, the resistivity
ρ, which has a dimension in the SI system in ohm-meters (Ohm∙m).

E = ρ ∙j

This is Ohm's law in differential form. In an isotropic conductor, the current density vector j is parallel to the electric field strength vector E and the flow of charged particles. In the case of electron movement, the electric current, as is known, is directed against their movement.

When studying physical processes, Ohm's differential law is usually written in the form j = E / ρ = σ∙ E , where σ is the conductivity of the material.

The reciprocal of the ohm is called the siemens (Sm). Accordingly, the unit of σ is siemens per meter (S/m).

In the absence of an electric field, electrons (valence) move chaotically at different speeds and with equal probability in directions. Therefore, the vector of their average thermal velocity is zero. But in the presence of an electric field, an additional velocity component appears along (but against) the electric field. Now the average speed v av

electrons along the field (sometimes called drift) is no longer zero, and the current density can be written in the form

j = — ne v avg

This speed v av

can be calculated as follows.
We will assume that per unit time an electron collides with an ion with a probability equal to 1/τ. This means that the probability for an electron to experience a collision within an infinitesimal time interval dt
is simply
dt/τ.
Time τ is called relaxation time or free travel time.
It plays a fundamental role in the theory of conductivity of metals. From this assumption it follows that an electron chosen at random at the present time will move on average
for a time τ before its next collision, and has already moved
on average
for a time τ since its previous collision.

Let's consider some electron at the zero moment of time. Let t –

the time that has passed since his last encounter.
The speed of this electron at zero time will be equal to its speed vо immediately after the collision plus the additional speed – e E t/m
, which the electron acquired after the collision.
Since we assume that after the collision the electron velocity can have any direction, the contribution from vo to the average electron velocity is zero, and therefore it is equal to the average value of the quantity - e E t/m.
However, the average value
t
is equal to the relaxation time τ. Therefore we have

v av= - e E τ/ m,
j
= E
= σ∙ E , σ =
Thus, we obtained a linear dependence of j on E and found an expression for conductivity σ, which includes only known quantities and relaxation time τ. This relaxation time can now be determined from the experimental values ​​of σ and it turns out to be on the order of τ ~ 10-14 – 10-15 s at room temperatures.

Note that in the Drude theory, the electron velocity distribution corresponds to the gas kinetic theory in the form of the so-called. Maxwellian distribution, i.e.

Estimates here of the average thermal velocity of electrons give values ​​of the order of vTe ~

105 m/s.
This speed determines the mean free path of the electron λ = vTe∙
τ ~ 10-10 – 10-9 m, which is comparable to the interatomic distances in the metal and thus, it would seem, confirms the reasonableness of all the premises of the Drude theory.

Another impressive success of the Drude model at the time it was proposed was the explanation of the empirical law of Wiedemann and Franz (1853) relating the thermal and electrical properties of metals. Let us recall that, in accordance with the well-known Fourier equation, the heat flux density j q is proportional to the temperature gradient T,

which is expressed, for example, in the one-dimensional case in the form

jq = —

χ
∙dT/dx
where χ is the thermal conductivity coefficient. The Wiedemann-Franz law states that the ratio of the coefficients of thermal conductivity and electrical conductivity χ/σ

for most metals it is directly proportional to temperature, and the proportionality coefficient
with
sufficient accuracy for all metals, i.e.

χ /σ = cT

To explain this pattern within the framework of the Drude model, it is assumed that the main part of the heat flow in the metal is transferred by conduction electrons. This assumption is based on the empirical observation that metals conduct heat much better than dielectrics. The corresponding calculations within the framework of the named model lead to the following value of the coefficient c

:

With

=

close to the experimental values ​​of this coefficient.

Despite these and some other successes, Drude's model, however, quickly revealed significant flaws. For example, in reality, as it turned out, the thermal velocities of electrons in metals are more than an order of magnitude higher than those calculated by the Drude theory. In addition, the Drude theory cannot explain the appearance at low temperatures

electron mean free path, 1000 or more times greater than the interatomic distance.
But the most striking drawback of the Drude model is the inability to explain the virtual absence of the influence of electrons on the heat capacity of the metal. But in accordance with the theory of free and independent
electrons, each electron should have an average energy, which would ultimately provide an additional contribution to the molar heat capacity of any metal. However, such a contribution was not found in any experiment!!

All of the above and many other shortcomings of the Drude model are associated with the lack of consideration of quantum mechanical phenomena, discussed below. To be fair, it should be pointed out that at the time of the formulation of the Drude model and the calculations based on it, detailed quantum concepts in relation to a solid body did not yet exist.

Measurement methods

To measure the thermal conductivity of metals, two methods are used: stationary and non-stationary. The first is characterized by the achievement of a constant value of the changed temperature on the controlled surface, and the second - by a partial change in it.

Stationary measurement is carried out experimentally, requires a lot of time, as well as the use of the metal under study in the form of blanks of the correct shape, with flat surfaces. The sample is placed between the heated and cooled surface, and after touching the planes, the time during which the workpiece can increase the temperature of the cool support by one degree Kelvin is measured. When calculating thermal conductivity, the dimensions of the sample being studied must be taken into account.

Non-stationary research methods are used in rare cases due to the fact that the result is often biased. Nowadays, no one except scientists is involved in measuring the coefficient; everyone uses long-established experimental values ​​for various materials. This is due to the constancy of this parameter while maintaining the chemical composition of the product.

Fundamentals of Heat and Mass Transfer for Cooling Systems

According to the second law of thermodynamics, heat spreads from one body to another (or from one part of a body to another part of the same body) if there is a temperature difference. In this case, the heat flow is directed from a point with a higher temperature to a point with a lower temperature. According to the first law of thermodynamics (conservation of energy), the flow of thermal energy is conserved in the absence of heat sources or sinks. Therefore, in a solid body there is a temperature distribution that depends on the spatial coordinates and observation time:

It can be assumed that in this case there is a surface in a solid body, upon observation of which at a certain time it turns out that all its points have the same temperature. Such a surface is called an isothermal surface.

It is possible to detect other isothermal surfaces inside this body, the temperatures of which differ from the temperature of the specified surface by +δt. These isothermal surfaces never intersect, since no point exists in this solid at two different temperatures at the same time. Thus, a solid body appears to us as if composed of a certain number of arbitrarily thin isothermal shells, which, of course, change with time.

Below, only isotropic solids are considered, i.e., solids whose properties and structure in the vicinity of any point do not depend on direction. In this case, due to symmetry, the heat flow at a point necessarily has a direction perpendicular to the isothermal surface through the point. This position will be discussed below,

The mathematical formulation of the law of thermal conductivity can be expressed as follows:

(1)

Equation (1) can be explained using Fig. 1. Heat flow Q/A flows along (perpendicular to

) to area
A
in the direction of decreasing temperature, i.e. in the direction of the negative temperature gradient. The minus sign in equation (2-1) indicates that the heat flow is in the direction of the negative gradient, and serves to make the heat flow positive in this sense.

Rice. 1 To the derivation of the law of thermal conductivity.

The proportionality coefficient λ expresses thermal conductivity and is a characteristic of the material through which the heat flow passes.

For an infinitesimal area selected from the area, equation (1) can be written as:

(2)

Equations (1) and (2) are usually attributed to the French mathematician Jean Baptiste Fourier and are called the Fourier heat equations in his honor.

The amount of heat passing per hour through a unit area of ​​any surface is called the specific heat flux q

and is measured in
kcal/m2
• h
or
W
/m2
.

Heat flow is a vector, in other words, it must be characterized by both magnitude and direction.

Heat flow can be defined along any direction through an area perpendicular to that direction.

In Fig. 2 shows body isotherms t

and
tdt .
The perpendicular to these isotherms is indicated by the ray n, which is also perpendicular to the area element
dA.
The heat flow along the perpendicular and in the direction S can be calculated as follows:

It is easy to show that n = s cosα. That's why

(3)

Or otherwise, qs is a component of the heat flow vector q p .

From equation (3) it follows that the largest heat flux will be the one calculated along the normal to the isothermal surfaces. In particular, if the component flows belong to planes having coordinate systems x, y, z,

then these will be streams

(4)

The flows expressed by equation (4) are components of the heat flow vector.

Thermal conductivity.

It should be noted that the thermal conductivity coefficient λ

does not have to be permanent. In reality, thermal conductivity is a function of temperature for all phases, and in liquids and gases it also depends on pressure, especially near the critical state. Thermal conductivity in wood and crystals also changes noticeably depending on direction. For example, the thermal conductivity in wood across the fiber, compared to the thermal conductivity of wood along the fiber, changes by a factor of 2 to 4.

The dependence of thermal conductivity on temperature for certain small temperature ranges can be reasonably expressed in linear form:

(5)

where: λ0 is the value of thermal conductivity under certain initial conditions, β is the temperature coefficient, it can be positive or negative depending on the material.

Rice. 3 shows the change in the temperature gradient in the body depending on whether β is positive or negative.

It is easy to understand that a linear temperature gradient exists only at constant thermal conductivity.

It is interesting to note that the Fourier equation for thermal conductivity is completely analogous to Ohm's law for an electrical conductor. Ohm's law for a conductor of any shape can be expressed as follows:

(6)

In equation (6), the electric current corresponds to the heat flow Q, the electric potential E

corresponds to temperature
I
and electrical conductivity
σ
(σ = I/р, where ρ is electrical resistance) corresponds to thermal conductivity.

Since equations (2) and (6) have the same form, the temperature field inside a heated body and the electric voltage field in bodies of the same shape are similar, provided that the temperature distribution on the surface corresponds to the surface distribution of electric voltage. This analogy contributes to a more detailed understanding of the problems of thermal conductivity using similar electrical circuits.

In inorganic, non-metallic, refractory materials (ceramics, natural stone materials, concrete, etc.), the number of free electrons that could move through the crystal lattice and carry out energy transfer is insufficient and heat is mainly transferred due to lattice vibrations.

The magnitude of thermal conductivity depends on the nature of lattice vibrations. With harmonic oscillations, there is no resistance to energy transfer and thermal conductivity can reach enormous values. However, in real crystals the vibrations are anharmonic in nature, which contributes to the partial damping of elastic thermal vibrations and a significant reduction in thermal conductivity.

In the theory of thermal conductivity, it is assumed that normal vibrations are quantized and, by analogy with photons in the theory of light, these quanta are called phonons

, and the mechanism of thermal energy transfer is
phonon thermal conductivity.
Thus, in solid non-metallic bodies the transfer of thermal energy is carried out due to the interaction of phonons, as a result of their movement, collision, dispersion, etc. By analogy with the kinetic theory of gases, the phonon thermal conductivity of solids can be represented as

λ = с
ν l
;

where: l
is
the mean free path of phonons, c is the specific heat capacity of the body,
ν
is the average speed of phonons;

In metals, the transfer of thermal energy is determined by the movement and interaction of conduction electrons, since the lattice phonon component of thermal conductivity is vanishingly small and

λ el >> λ solve

The phenomenon of heat transfer in semiconductors is more complex than in dielectrics and metals, since both the lattice and electronic components of thermal conductivity are significant for them. In addition, here the thermal conductivity depends on the thermal conductivity of impurities and many other factors.

Thermal conductivity

is the transfer of thermal energy by structural particles of a substance (molecules, atoms, ions) during their thermal motion. Such heat exchange can occur in any body with a non-uniform temperature distribution, but the mechanism of heat transfer will depend on the state of aggregation of the substance. The phenomenon of thermal conductivity is that the kinetic energy of atoms and molecules, which determines the temperature of a body, is transferred to another body when they interact or is transferred from more heated areas of the body to less heated areas. Sometimes thermal conductivity is also called a quantitative assessment of the ability of a particular substance to conduct heat.

Numerical characteristics

The thermal conductivity of a material is equal to the amount of heat passing through a material 1 m thick and 1 square meter in area. m per unit of time (second) with a temperature difference on two opposite surfaces of 1 K. This numerical characteristic is used to calculate thermal conductivity for calibrating and cooling profile products.

Table 1

Thermal conductivity Si (300 K) 149 W/(m K)

Thermal conductivity of metals and its relationship with electrical conductivity

Wiedemann-Franz law

is a physical law stating that for metals the ratio of the thermal conductivity coefficient (or thermal conductivity tensor) K to the electrical conductivity (or conductivity tensor) σ is proportional to temperature:

K/σ = LT

or

K/σ = π2/3*(k/e)2*T

where: k

— Boltzmann constant,
e
— electron charge, T — temperature.

Thermal conductivity of crystals.

Phonon

In crystals and at low temperatures, the quantum nature of thermal waves must be taken into account.

If ћω D
<< k B T
then during heat transfer any vibrations in the crystal are excited, all quantum transitions are possible, and therefore the quantum nature of the heat transfer phenomenon is not noticeable.

At low temperatures, when ћωD >> k

BT, only oscillations with low frequencies are excited in the crystal, and large energy steps cannot be overcome by the resulting thermal “shocks”. Let us consider the process of heat transfer based on the concept of phonons.

From Debye's theory it follows that the excited state of the lattice can be represented as an ideal gas of phonons moving freely in the volume of the crystal. A phonon gas in a certain temperature range behaves like an ideal gas, and since phonons are the main carriers of heat in a solid (this statement is true only for dielectrics), the thermal conductivity coefficient of a solid can be expressed by the same dependence as the thermal conductivity coefficient of an ideal gas

λ = 1/3 C' (l f )U m (7)

where: C' – heat capacity per unit volume of phonon gas, l f

is the mean free path of a phonon,
υ m
is the speed of sound propagation in a given body.

Calculating the mean free path of a phonon is a complex task, since it depends on where phonons are scattered: on other phonons, on structural defects, or on the external faces of the sample. However, theoretical analysis leads to the fact that at sufficiently high temperatures the mean free path of a phonon is inversely proportional to the absolute temperature. Therefore, the thermal conductivity coefficient of solids at temperatures above the characteristic temperature (T >ΘD) is inversely proportional to the absolute temperature.

In sufficiently pure and defect-free crystals at temperatures close to absolute zero, a dependence of the mean free path of phonons on the size of the sample arises. This is explained by the fact that at low temperatures the concentration of phonons is low, and therefore the probability of scattering of phonons by other phonons is low. An example of the dependence of the thermal conductivity coefficient on temperature for different sections of a LiF single crystal sample is shown in Fig. 6.10 [98]. It can be seen that the difference in thermal conductivity for samples of different cross sections appears only in the low temperature region.

Rice. 4 Lattice (phonon) thermal conductivity as a function of temperature for a LiF single crystal with its different sections: a - 1.33 x 0.91 mm; b - 7.55 x 6.97 mm

Assuming the mean free path of phonons to be approximately equal to the linear dimensions of the crystal ( (l f )~L

, where
L
is the linear size of the crystal), equation (7) can be rewritten as:

λ ~ 1/3 C' Lυ m
(

On the right side of equation (8), only the heat capacity per unit volume of the phonon gas C' depends on temperature. At temperatures close to absolute zero, the heat capacity is proportional to T3 (Debye's T3 law), therefore the thermal conductivity coefficient l

proportional to the cube of absolute temperature. This conclusion is confirmed by experimental data.

Anisotropy of binding forces in crystals leads to anisotropy of the thermal conductivity coefficient. This can be illustrated using the example of a quartz single crystal (Fig. 5). In table Figure 2 presents data on the thermal conductivity coefficient in the direction parallel c

, and in a direction perpendicular to this axis.

Rice. 5 Schematic representation of a quartz crystal and the direction of the axes in it

Thermal conductivity of a quartz crystal in various directions (units λ - W/(m * K) according to L.3, Chapter 6-5

table 2

From the data in Table 2 it is clear that the thermal conductivity coefficient along the hexagonal axis c

quartz is approximately twice as high as the corresponding values ​​in directions perpendicular
c
, i.e. in directions lying in the basal plane of the crystal. As the temperature decreases, the thermal conductivity increases, as predicted by quantum theory.

All of the above applies to the lattice (phonon) part of the heat capacity of a solid, characteristic of non-metallic crystals. In metals, in addition to the atoms of the crystal lattice, free electrons also participate in heat transfer, which at the same time are carriers of electric charge, providing high electrical conductivity of metals. Moreover, in pure metals the main heat carriers are free electrons, not phonons. At sufficiently high temperatures of metals, the lattice component of thermal conductivity is only 1–2% of the electronic thermal conductivity. This explains the high thermal conductivity of pure metals compared to dielectrics. For example, aluminum at room temperature has a thermal conductivity coefficient l

= 2.26x106 W/(cmxK), which is approximately two orders of magnitude greater than that of quartz (see Table 1). However, at very low temperatures in metals, the electronic part of the thermal conductivity is less than the lattice part. This is explained by the effects of electron-phonon scattering.

In Fig. Figure 6 shows the dependence of thermal conductivity on temperature for dielectrics and metals.

Rice. 6 Comparative temperature dependences of the thermal conductivity coefficient l: a - for dielectrics; b - for metals

In dielectrics, which have practically no free electrons, heat transfer is carried out only by phonons. It was said above that the average free path (l f )

depends on the processes of phonon scattering on various objects.
All this leads to the fact that the temperature dependence of the thermal conductivity coefficient l
for dielectrics has the form of a curve with a maximum (Fig. 6,
a
).
The left ascending branch of the dependence is due to an increase in the number of phonons with increasing temperature, and the right descending branch is associated with attenuating phonon-phonon and other types of scattering. The type of dependence l
(
T
) for metals (Fig. 6,
b
) is qualitatively similar to the curve for dielectrics. This is due to the predominance of the phonon heat transfer mechanism at very low temperatures. However, with increasing temperature, the contribution of the phonon component to this process becomes negligible and heat transfer is carried out mainly by free electrons.

At relatively high temperatures in metals, the thermal conductivity coefficient l

to change with increasing
T.

Rice. 7. Thermal conductivity of individual materials over a wide temperature range

In table 1 and in Fig. Figure 7 shows the values ​​of thermal conductivity coefficients for individual materials. The problem of heat removal for some microelectronic and optoelectronic products is one of the key ones that ensures their reliability. For example, for semiconductor lasers operating in continuous lasing mode, this problem is the main one. From the data in table. 1 and fig. 7 it follows that to improve heat dissipation, crystals can be mounted on diamond or silicon carbide substrates (as non-conducting) or on silver or copper substrates (as conductive).

Due to an increase in the degree of integration and a decrease in the geometric dimensions of elements, including conductive paths, from the values ​​of table. 1 it follows that the only alternative to aluminum metallization is the use of copper. Silver, although it has a higher thermal conductivity, cannot be used for metallization due to its high diffusion coefficient.

Literature:

1. E.Z. Eckert, R.M. Drake, Theory of Heat and Mass Transfer, trans. from English Edited by A.V. Lykova, SEI, Moscow
2. G.N. Dulnev, Heat and mass transfer in radio-electronic equipment, M, Higher School, 1984,
3. Gurtov V.A., Osaulenko R.N., Gushcheva K.V. Solid State Physics for Engineers, https://dssp.petrsu.ru/files/tutorial/ftt/giv.htm, Chapter 6.5 https://dssp.petrsu.ru/files/tutorial/ftt/Part6/part6_5.htm
4. Heat transfer, collection of formulas, https://edu.delfa.net/CONSP/mkt8.html

There are exceptions:

— The thermal conductivity of non-metallic crystals and metals decreases from a minimum value at very low temperatures to the melting point.

— The thermal conductivity of most liquids decreases with decreasing temperature. However, in some liquids, such as water, it increases with temperature in some temperature ranges and decreases in others.

Collected by A. Sorokin Oct. 2011

Thermal conductivity of steel, copper, aluminum, nickel and their alloys

Ordinary iron and non-ferrous metals have different structures of molecules and atoms. This allows them to differ from each other not only in mechanical properties, but also in thermal conductivity properties, which, in turn, affects the use of certain metals in various sectors of the economy.

Steel has a thermal conductivity coefficient at an ambient temperature of 0 degrees. (C) equal to 63, and when the degree increases to 600, it decreases to 21 W/m*degree. Aluminum, under the same conditions, on the contrary, will increase the value from 202 to 422 W/m*deg. Aluminum alloys will also increase thermal conductivity as the temperature increases. Only the value of the coefficient will be an order of magnitude lower, depending on the amount of impurities, and range from 100 to 180 units.

Composition and structure of iron

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

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

Iron is characterized by polymorphism: there are as many as 4 modifications, differing in structure and lattice parameters:

• α-Fe – exists from zero to +769 C. It has a body-centered cubic lattice and is ferromagnetic, that is, it retains magnetization in the absence of an external magnetic field. +769 C – Curie point for metal;
• from +769 to +917 C β-Fe appears. It differs from the α phase only in lattice parameters. Almost all physical properties are preserved with the exception of magnetic ones: iron becomes paramagnetic, that is, it loses the ability to magnetize and is drawn into a magnetic field. Metallurgy 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, the δ phase appears, which is characterized by a body-centered cubic lattice.

At high pressure, as well as when the metal is doped with certain additives, an ε-phase with a hexagonal close-packed lattice is formed.

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

Now it’s time for the properties of the metal iron.

This video tells about the structure of iron: