Why are metals thermally conductive?

Thermal conductivity of steel and other alloys: copper, brass and aluminum, heat transfer

Before working with various metals and alloys, you should study all the information regarding their basic characteristics. Steel is the most common metal and is used in various industries. An important indicator is thermal conductivity, which varies over a wide range and depends on the chemical composition of the material and many other indicators.

This term means the ability of various materials to exchange energy , which in this case is represented by heat. In this case, energy transfer passes from the hotter part to the colder part and occurs due to:

1. Molecules
2. Atoms.
3. Electrons and other particles of the metal structure.

The thermal conductivity of stainless steel will differ significantly from that of another metal - for example, the thermal conductivity of copper will be different than that of steel.

To indicate this indicator, a special value is used, called the thermal conductivity coefficient. It is characterized by the amount of heat that can pass through a material in a certain unit of time.

Indicators for steel

Thermal conductivity can vary significantly depending on the chemical composition of the metal. The coefficient of this value will be different for steel and copper. In addition, with an increase or decrease in carbon concentration, the indicator under consideration also changes.

There are other features of thermal conductivity:

1. For steel that does not have impurities, the value is 70 W/(m* K).
2. Carbon and high-alloy steels have much lower conductivity. Due to an increase in the concentration of impurities, it is significantly reduced.
3. The thermal effect itself can also affect the structure of the metal. As a rule, after heating, the structure changes its conductivity value, which is associated with a change in the crystal lattice.

The thermal conductivity coefficient of aluminum is much higher, which is due to the lower density of this material. The thermal conductivity of brass also differs from that of steel.

Effect of carbon concentration

The carbon concentration in steel affects the amount of heat transfer:

1. Low carbon steels have a high conductivity index. That is why they are used in the manufacture of pipes, which are then used to create the heating system pipeline. The coefficient value varies from 54 to 47 W/(m* K).
2. The average coefficient for common carbon steels is a value from 50 to 90 W/(m* K). That is why such material is used in the manufacture of parts for various mechanisms.
3. For metals that do not contain various impurities, the coefficient is 64 W/(m* K). This value does not change significantly under thermal influence.

Thus, the considered indicator for alloyed alloys may vary depending on the operating temperature.

Why is it important to consider thermal conductivity? A similar value is indicated in various tables for each metal and is taken into account in the following cases:

1. In the manufacture of various heat exchangers. Heat is one of the important carriers of energy. It is used to provide comfortable living conditions in residential and other premises. When creating heating radiators and boilers, it is important to ensure rapid and complete heat transfer from the coolant to the end consumer.
2. In the manufacture of outlet elements. You can often encounter a situation where you need to remove heat rather than supply it. An example is the case of heat removal from the cutting edge of a tool or gear teeth. To ensure that the metal does not lose its basic performance qualities, rapid removal of thermal energy is ensured.
3. When creating insulating layers. In some cases, the material should not conduct thermal energy transfer. For such operating conditions, a metal is selected that has a low heat conductivity coefficient.

The indicator under consideration is determined when testing under various conditions. As previously noted, the thermal conductivity coefficient may depend on the operating temperature. Therefore, the tables indicate several of its values.

Source: https://tokar.guru/metally/stal/teploprovodnost-stali-alyuminiya-latuni-medi.html

The most heat-conducting metal - Metalist's Handbook

The high thermal conductivity of copper and its other useful characteristics were one of the reasons for the early development of this metal by humans. To this day, copper and copper alloys are used in almost all areas of our lives.

Copper plates

A little about thermal conductivity

In physics, thermal conductivity is understood as the movement of energy in an object from more heated small particles to less heated ones. Thanks to this process, the temperature of the object in question as a whole is equalized. The magnitude of the ability to conduct heat is characterized by the thermal conductivity coefficient. This parameter is equal to the amount of heat that a material 1 meter thick passes through a surface area of ​​1 m2 for one second at a unit temperature difference.

MaterialThermal conductivity coefficient, W/(m*K)

Silver	428
Copper	394
Aluminum	220
Iron	74
Steel	45
Lead	35
Brick	0,77

Copper has a thermal conductivity coefficient of 394 W/(m*K) at temperatures from 20 to 100 °C. Only silver can compete with it. And for steel and iron this figure is 9 and 6 times lower, respectively (see table). It is worth noting that the thermal conductivity of products made from copper largely depends on impurities (however, this also applies to other metals). For example, the rate of heat conduction decreases if substances such as:

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

Copper wire

If you add zinc to copper, you get brass, which has a much lower thermal conductivity coefficient. At the same time, adding other substances to copper can significantly reduce the cost of finished products and give them characteristics such as strength and wear resistance. For example, brass is characterized by higher technological, mechanical and anti-friction properties.

Since high thermal conductivity is characterized by rapid distribution of heating energy throughout the entire object, copper is widely used in heat exchange systems. At the moment, radiators and tubes for refrigerators, vacuum units and cars are made from it for rapid heat removal. Copper elements are also used in heating installations, but for heating.

Copper heating radiator

In order to maintain the thermal conductivity of the metal at a high level (and therefore make the operation of copper devices as efficient as possible), forced airflow by fans is used in all heat exchange systems. This decision is due to the fact that as the temperature of the environment increases, the thermal conductivity of any material decreases significantly, because heat transfer slows down.

Aluminum and copper - which is better?

Aluminum has one disadvantage compared to copper: its thermal conductivity is 1.5 times less, namely 201–235 W/(m*K). However, compared to other metals, these are quite high values. Aluminum, like copper, has high anti-corrosion properties. In addition, it has advantages such as:

• low density (specific gravity 3 times less than that of copper);
• low cost (3.5 times less than copper).

Aluminum heating radiator

Thanks to simple calculations, it turns out that an aluminum part can be almost 10 times cheaper than a copper part, because it weighs much less and is made of cheaper material. This fact, along with high thermal conductivity, allows the use of aluminum as a material for cookware and food foil for ovens. The main disadvantage of aluminum is that it is softer, so it can only be used in alloys (for example, duralumin).

For effective heat transfer, the rate of heat transfer to the environment plays an important role, and this is actively facilitated by the cooling of radiators. As a result, the lower thermal conductivity of aluminum (relative to copper) is leveled out, and the weight and cost of the equipment are reduced. These important advantages allow aluminum to gradually replace copper from use in air conditioning systems.

Use of copper in electronics

In some industries, for example, in the radio industry and electronics, copper is essential.

The fact is that this metal is very ductile in nature: it can be drawn into extremely thin wires (0.005 mm), and also can be used to create other specific conductive elements for electronic devices.

And high thermal conductivity allows copper to extremely effectively remove the heat that inevitably arises during the operation of electrical appliances, which is very important for modern high-precision, but at the same time compact equipment.

The use of copper is relevant in cases where it is necessary to deposit a certain shape on a steel part. In this case, a copper template is used, which is not connected to the element being welded. Using aluminum for these purposes is impossible, as it will melt or burn through. It is also worth mentioning that copper can act as a cathode when welding with a carbon arc.

1 - gear, 2 - template fastenings, 3 - welded gear tooth, 4 - copper templates

Disadvantages of the high thermal conductivity of copper and its alloys

Copper has a much higher cost than brass or aluminum. At the same time, this metal has its disadvantages, which are directly related to its advantages. High thermal conductivity leads to the need to create special conditions during cutting, welding and soldering of copper elements. Since copper elements need to be heated much more concentrated compared to steel. Also, preliminary and concomitant heating of the part is often required.

Don’t forget that copper pipes require careful insulation if they make up the main line or distribution of the heating system. Which leads to an increase in the cost of network installation compared to options when other materials are used.

Example of thermal insulation of copper pipes

Difficulties also arise with gas welding of copper: this process will require more powerful torches. When welding metal 8–10 mm thick, two or three torches will be required. While one torch is used for welding, the other is heating the part. In general, welding work with copper requires increased costs for consumables.

It should also be said about the need to use special tools. So, to cut brass and bronze up to 15 cm thick, you will need a cutter capable of working with high-chromium steel 30 cm thick. Moreover, the same tool is enough to work with pure copper only 5 cm thick.

Plasma cutting of copper

Is it possible to increase the thermal conductivity of copper?

Copper is widely used in the creation of microcircuits for electronic devices and is designed to remove heat from parts heated by electric current.

When trying to increase the speed of modern computers, developers were faced with the problem of cooling processors and other parts. One of the solutions was to split the processor into several cores.

However, this method of combating overheating has exhausted itself, and now it is necessary to look for new conductors with higher thermal and electrical conductivity.

One solution to this problem is the recently discovered element graphene. Thanks to graphene deposition, the thermal conductivity of the copper element increases by 25%. However, the invention is still at the development level.

Source: https://ssk2121.com/samyy-teploprovodnyy-metall/

Heat transfer coefficients of steels and other materials: factors affecting the thermal conductivity of alloys

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 quantity is measured in W/(K*m) (Watt 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 amount 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 one Joule of thermal energy is transferred through the material:

• in one second;
• across an area of ​​one meter square;
• at a distance of one meter;
• when the temperature difference on surfaces located one meter apart in a material is equal to one Kelvin.

It is clear that the higher the thermal conductivity value, 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 transfers heat 10,000 times better than polyurethane, whose thermal conductivity is 0.035 W/(m*K).

Heat transfer at the molecular level

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

This explanation of the mechanism of thermal conduction at the molecular level distinguishes it from the mechanism of thermal convection, in which heat transfer occurs due to the transfer of matter.

All solids 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, and 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 heat flow through space to exist, there must be some substance present in this space, therefore in open space (empty space) thermal conductivity is zero.

Each homogeneous (homogeneous) material is characterized by a thermal conductivity coefficient (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 thickness of one meter of material, 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 measured, as a rule, 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:

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

From the data under consideration it is clear 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 greater than 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 to transfer thermal energy well is used in microelectronics to remove heat from microcircuits. This property is also used in special devices that allow one to distinguish a real diamond from a fake.

Some industrial processes try to increase the ability to transfer heat, which is achieved either through 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 thermal conductivity, which is achieved through the use of heat insulators, voids in structures and reducing the contact area of ​​elements.

Heat transfer coefficients of steels

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

Simple carbon steels, with increasing carbon content, reduce their specific gravity, according to which their ability to transfer heat also decreases from 54 to 36 W/(m*K) when the percentage of carbon in the steel changes from 0.5 to 1.5%.

Stainless steels contain chromium (10% or more), which together with carbon forms complex carbides that prevent 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 is galvanized steel with a specific gravity of 7,850 kg/m3, which is obtained by applying coatings to the steel consisting of iron and zinc. Since zinc conducts heat more easily than iron, the thermal conductivity of galvanized steel will be relatively high compared to other classes of steel. It ranges from 47 to 58 W/(m*K).

The thermal conductivity of steel at different temperatures, as a rule, does not change much. For example, the thermal conductivity coefficient of steel 20 with an increase in temperature from room temperature to 1200 °C decreases from 86 to 30 W/(m*K), and for steel grade 08Х13, an increase in temperature from 100 to 900 °C does not change its thermal conductivity coefficient (27-28 W/(m*K).

Factors influencing 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 nonmetals. In metals, conductivity is mainly due to 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, electrical conductivity decreases with increasing temperature, so thermal conductivity remains approximately constant.

In the case of alloys, 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 nonmetals is mainly associated with lattice vibrations and the exchange of lattice phonons.

With the exception of high-quality crystals and low temperatures, the path of phonons in the lattice does not decrease significantly at high temperatures, and 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, decreases significantly.

Phase transitions and structure

When a material undergoes a first-order phase transition, for example 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 between 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 different scattering intensities of lattice phonons, the main heat carriers in nonmetals, and in different directions in the crystal. A striking example here is sapphire, whose conductivity 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, moving freely throughout the crystal lattice of the metal, transfer 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 mechanism of heat transfer).

Convection process

Air and other gases are, as a rule, good heat insulators in the absence of convection. This principle is the basis for the operation of many heat-insulating materials containing a large number of small voids and pores.

This structure does not allow convection to spread over long distances. Examples of such man-made materials are polystyrene and silicide airgel.

In nature, heat insulators such as animal skin and bird plumage work on the same principle.

https://www.youtube.com/watch?v=RlZiF-jLJOI

Light gases such as hydrogen and gel have high thermal conductivities, while heavy gases such as argon, xenon and radon are poor conductors of heat.

For example, argon, an inert gas that is heavier than air, is often used as an insulating gas filler in double-glazed windows and light bulbs.

An exception is sulfur hexafluoride (SF6 gas), which is a heavy gas and has a relatively high thermal conductivity due to its high heat capacity.

Source: https://obrabotkametalla.info/stal/koefficient-teploprovodnosti-i-teploperedachi-stali

10 interesting facts about metals and their amazing properties

Metals are a group of chemical elements in the form of simple substances. They all have their own properties, according to which they can be divided into different groups.

The word “metal” itself came into Russian from Germany. At first it meant the same thing as “mineral, ore.” They began to separate concepts only after Lomonosov’s works.

The word has entered the language remarkably well, now everyone knows it. The most famous metals are probably gold, silver, mercury, copper and iron. But knowledge even about them is very incomplete. Nature always finds something to surprise us with.

In this article we will look at 10 interesting facts about metals.

10. Titanium is used as an implant

Implantation is a way to restore lost teeth.
Now this method is very common due to its speed and accessibility. It consists of the following: a rod is implanted into the jaw, which becomes a support for the new tooth. This very rod is made of titanium.

As a metal, it has high strength, and its elasticity is similar to human bone, so implantation is easier. Titanium is the very basis of a dental implant, which reduces the risk of bone destruction .

9. Silver has bactericidal properties

Silver was known to people even before our era. For some time it was even valued higher than gold. However, people are still learning about its various properties.

For example, the direct effect of ionic silver on bacteria is still debated. It has been proven that when bacteria and ions come into contact, the former die as a result of exposure.

Many theories have already been put forward, but the exact reasons for the death of microorganisms under the influence of silver are still unknown.

The ions of this metal cope well with the pathogens of typhoid, protea, diphtheria and others . Where silver does not kill bacteria, it may slow the germination of new spores and the spread of microorganisms.

8. Tantalum is widely used in prosthetics

Tantalum is an unusual metal that is quite rarely found in its pure form. For this reason it became very expensive.

He is difficult to obtain, so he was named after the hero of Greek myths. There Tantalus constantly tried to get at least a little food and water, but he still failed.

A chemist trying to obtain this metal in its pure form compared his work to tantalum flour. Despite this, tantalum has now found application in many areas.

It is very widespread in medicine because the human body does not reject it . It is used to produce plates for skulls, paper clips for connecting blood vessels, threads for replacing tendons and stitching together fibers. Sometimes used to make eye prostheses.

7. Aluminum is part of the earth's crust

Immediately after its discovery, aluminum was highly valued due to its similarity to silver. And extracting it in its pure form was not easy.

Scientists have already proven that this metal is widespread everywhere. Almost 8% of the earth's crust consists of it .

If we compare metals by the amount of their content in the earth's crust, it is second only to oxygen and silicon. But here’s an interesting fact: despite its prevalence, aluminum cannot be found in nature in its free form.

6. Mercury evaporates into the air

When people first discovered mercury, it was given the name "living silver." This is a very accurate definition of what mercury looks like.

The rare metal is a liquid, but it is also very heavy. The most common item where you can see mercury is an old thermometer. All parents forbid their children to touch it. And all because of the properties of mercury, which can evaporate in the air .

The vapors generated during evaporation are very toxic and can harm the human body. They penetrate inside, disrupt the composition and structure of proteins, which is why some processes begin to flow in the opposite direction, which entails poisoning and death.

But only large amounts of this metal cause death, more than in a regular thermometer. However, measures to eliminate the problem must be taken immediately in any case.

5. Tin is the most fusible metal

One metal that has already revealed almost all its secrets to people is tin. It has been known to mankind for a long time.

Before the discovery of the properties of iron, almost everything was made from an alloy of tin and copper: from weapons to jewelry. And this is understandable.

Tin is one of the most fusible metals . Its temperature is 232 - 240 degrees Celsius. Thus, only one requirement must be met - for molds it should not melt at temperatures up to 250 degrees. That's all, this metal has no more restrictions for melting, casting, soldering and other uses.

4. Iridium is the densest metal

Iridium is an interesting metal. It is found in the earth's crust even less frequently than gold and platinum. There is an assumption that its quantity is much larger, but it is located closer to the Earth’s core, out of reach.

Relative to the earth's crust, iridium is often found in meteorites. It is the densest and most refractory metal .
Its melting point is 2466 degrees Celsius. In terms of density, it is comparable only to osmium. They are almost equal, and the difference in numbers can be attributed to an error.

3. The Valcambi company produces ingots from expensive metals in the form of chocolate bars

Valcambi is an organization in Switzerland that has taken a very creative approach to the concept of ingots.

When they say the phrase “gold bars,” people always imagine beautiful shiny bricks stacked on top of each other in a pyramid. But the company decided to destroy this idea.

They make gold, silver, platinum and palladium bars in the form of chocolate bars . This was a wonderful gift idea.

Such an ingot can be broken into several small pieces (about 1 gram) and given as a gift to loved ones. Another use case is to pay for purchases in stores that accept this type of payment, of course.

2. Olympic medals are not gold at all.

Olympic gold medals are silver . In fact, the International Olympic Committee has declared that gold sports awards must be plated with just 6 grams of gold.

The rest of the medals may be silver. So, for example, if you study a medal from the London 2012 Olympic Games, the research results will be quite surprising. gold in a gold medal is only 1%, although all conditions are met.

1. More than 50% of the world's gold is found in Africa

For as long as humanity has existed, people have been drawn to gold. Finding a vein meant untold riches. For his sake they lied, stole, killed. But using all the methods possible to us, about 161 thousand tons of it have been found throughout history.

Most of this expensive metal was discovered in South Africa . But in reality this is not as much as it might seem at the very beginning. It is easier to present this fact differently.

If you melt all the gold found in the world into one large cube, its side will be only 20 meters. Half of this cube was found in Africa. Approximately every hour people take out a cube of iron of the same size from the ground. And all the gold in the world is worth about 9 trillion dollars.

Source: https://top10a.ru/interesnye-fakty-o-metallax.html

Thermal conductivity of metals

From the data presented above, it is clear that the bimetallic heating device has the highest heat transfer rate. Structurally, such a device is presented by RIFAR in a ribbed aluminum housing. in which metal tubes are located, the entire structure is secured by a welded frame. This type of battery is installed in high-rise buildings, as well as in cottages and private houses. The disadvantage of this type of heating device is its high cost.

Important! When this type of battery is installed in houses with a large number of floors, it is recommended to have its own boiler station, which has a water treatment unit. This condition for the preliminary preparation of the coolant is associated with the properties of aluminum batteries

they can be subject to galvanic corrosion when it is supplied in poor quality through the central heating network. For this reason, it is recommended to install aluminum heating devices in separate heating systems.

Cast iron batteries are significantly inferior in this comparative system of parameters; they have low heat transfer and a large weight of the heating device. But, despite these indicators, MS-140 radiators are in demand among the population, the reason for which is the following factors:

Duration of trouble-free operation, which is important in heating systems. Resistance to the negative effects (corrosion) of the thermal fluid.

Thermal inertia of cast iron.

This type of heating device has been operating for more than 50 years; for it there is no difference in the quality of the preparation of the thermal fluid. They cannot be installed in houses where the operating pressure of the heating network may be high; cast iron is not a durable material.

Comparison by other characteristics

One feature of battery operation – inertia – has already been mentioned above. But in order for the comparison of heating radiators to be correct, it must be made not only by heat transfer, but also by other important parameters:

• working and maximum pressure;
• the amount of water held;
• mass.

The limitation on the operating pressure determines whether the heating device can be installed in multi-storey buildings, where the height of the water column can reach hundreds of meters. By the way, this restriction does not apply to private houses, where the pressure in the network is not high by definition. Comparing the capacity of radiators can give an idea of ​​the total amount of water in the system that will have to be heated. Well, the mass of the product is important when determining the place and method of its fastening.

As an example, below is a comparison table of the characteristics of various heating radiators of the same size:

Note. In the table, a heating device consisting of 5 sections is taken as 1 unit, except for the steel one, which is a single panel.

Thermal conductivity and density of aluminum

The table shows the thermophysical properties of aluminum Al depending on temperature. The properties of aluminum are given over a wide temperature range - from minus 223 to 1527 ° C (from 50 to 1800 K).

As can be seen from the table, the thermal conductivity of aluminum at room temperature is about 236 W/(m deg), which allows this material to be used for the manufacture of radiators and various heat sinks.

In addition to aluminum, copper also has high thermal conductivity. Which metal has the greater thermal conductivity? It is known that the thermal conductivity of aluminum at medium and high temperatures is still less than that of copper, however, when cooled to 50K, the thermal conductivity of aluminum increases significantly and reaches a value of 1350 W/(m deg). For copper, at such a low temperature, the thermal conductivity value becomes lower than for aluminum and amounts to 1250 W/(m deg).

Aluminum begins to melt at a temperature of 933.61 K (about 660 ° C), while some of its properties undergo significant changes. The values ​​of properties such as thermal diffusivity, aluminum density and thermal conductivity are significantly reduced.

The density of aluminum is mainly determined by its temperature and depends on the state of aggregation of this metal. For example, at a temperature of 27°C, the density of aluminum is 2697 kg/m3, and when this metal is heated to its melting point (660°C), its density becomes equal to 2368 kg/m3. The decrease in aluminum density with increasing temperature is due to its expansion when heated.

from here

The table shows the thermal conductivity values ​​of metals (non-ferrous), as well as the chemical composition of metals and technical alloys in the temperature range from 0 to 600°C.

Source: https://mr-build.ru/newteplo/teplootdaca-medi-i-aluminia.html

Thermally conductive metals examples

The thermal conductivity coefficient is a physical quantity that characterizes the ability of a substance to conduct heat.

The thermal conductivity coefficient is designated in different ways. There are designations: K, and some others.

Gas thermal conductivity coefficient

In accordance with the kinetic theory for gas, the thermal conductivity coefficient is equal to:

where is the average speed of thermal motion of molecules, is the average free path of a molecule, is the density of the gas, and is the specific heat of the gas in an isochoric process.

Thermal conductivity coefficient of metals

Metals are good conductors of heat. Thermal conductivity in metals occurs (mainly) through the fact that energy is transferred by free electrons. The coefficient of electronic thermal conductivity of metals is calculated using the formula:

where is the Boltzmann constant, is the electron concentration in the metal, is the mean free path, which corresponds to the Fermi energy limit () for the distribution of electrons over temperatures at T=0K, is the electron mass, is the average free path velocity for the same conditions as .

For an ideal electron gas, expression (2) is transformed to the form:

where is the mean free path and is the average speed of thermal motion of electrons.

It should be noted that the thermal conductivity, which is carried out by the crystal lattice of metals, is significantly less than the electronic one. It can be calculated for crystals by considering the movement of photons across the crystal using the formula:

where c is the heat capacity per unit volume, is the speed of sound, is the mean free path of the photon

Thermal conductivity coefficient and Fourier equation

The thermal conductivity coefficient is included in the basic equation that describes the phenomenon of heat transfer or the Fourier equation. The phenomenon of thermal conductivity appears if there is a temperature gradient. In the one-dimensional stationary case, the Fourier equation can be written as:

where, in addition to the thermal conductivity coefficient (), there are: - the amount of heat that is transferred through the area in the direction that coincides with the direction of the normal to, in the direction of decreasing temperature, - the temperature gradient. In our case

Units

The basic unit of measurement of thermal conductivity in the SI system is:

=W/m•K

Examples of problem solving

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Source: https://steelfactoryrus.com/teploprovodnye-metally-primery/

What metals have thermal conductivity

The table shows the thermal conductivity of metals depending on temperature at negative and positive temperatures (in the range from -200 to 2400°C).

The table of thermal conductivity of metals contains the thermal conductivity values ​​of the following pure metals: aluminum Al, cadmium Cd, sodium Na, silver Ag, potassium K, nickel Ni, lead Pb, cobalt Co, beryllium Be, lithium Li, antimony Sb, bismuth Bi, magnesium Mg, zinc Zn, tungsten W, tin Sn, uranium U, iron Fe, palladium Pd, zirconium Zr, manganese Mn, platinum Pt, gold Au, copper Cu, rhodium Rh, thallium Tl, molybdenum Mo, tantalum Ta, iridium Ir.

It should be noted that the thermal conductivity of metals varies over a wide range and can differ tens of times under the same conditions.

For example, of the metals listed in the table, the metal such as silver Ag has the greatest thermal conductivity - its thermal conductivity coefficient is 392 W/(m deg) at 100°C and this is the most thermally conductive metal.

The lowest value of thermal conductivity at the same temperature corresponds to the metal bismuth Bi with a thermal conductivity of only 7.7 W/(m deg).

The thermal conductivity of most metals decreases when heated. Their maximum thermal conductivity is achieved at low negative temperatures. For example, at a temperature of minus 100°C, silver has a thermal conductivity of 419.8, and bismuth - 11.9 W/(m deg).

Note: The table also shows the thermal conductivity values ​​of ultra-high purity metals (up to 99.999%). The value of the thermal conductivity coefficient in the table is indicated in the dimension W/(m deg).

• Thermophysical properties and freezing point of aqueous solutions of NaCl and CaCl2
• Thermophysical properties, composition and thermal conductivity of aluminum alloys

Thermal conductivity of building materials, their density and heat capacity

Density, thermal conductivity and specific heat capacity of building and other popular materials. More than 400 materials in the table!

Density of water, thermal conductivity and physical properties of H2O

Detailed tables of water density, its thermal conductivity and other thermophysical properties depending on temperature

Physical properties of air: density, viscosity, specific heat capacity

Tables of physical properties of air: air density, its specific heat capacity and viscosity depending on temperature

Thermal conductivity of steel and cast iron. Thermophysical properties of steel

Thermal conductivity of steel and cast iron, physical properties of steel in tables at different temperatures

Plexiglas: thermal and mechanical characteristics

Thermal, mechanical, optical and electrical characteristics of organic glass are considered

Source: https://ostwest.su/instrumenty/kakie-metally-imejut-teploprovodnost.php/

On the thermal conductivity of copper and its alloys

The high thermal conductivity of copper and its other useful characteristics were one of the reasons for the early development of this metal by humans. To this day, copper and copper alloys are used in almost all areas of our lives.

Types of heat transfer: thermal conductivity, convection, radiation

Heat transfer is a way of changing the internal energy of a body by transferring energy from one part of the body to another or from one body to another without doing work. types of heat transfer exist : conduction, convection and radiation.

Thermal conductivity

Thermal conduction is the process of transferring energy from one body to another or from one part of a body to another due to the thermal movement of particles. It is important that during thermal conduction there is no movement of matter; energy is transferred from one body to another or from one part of the body to another.

Different substances have different thermal conductivities. If you put a piece of ice at the bottom of a test tube filled with water and place its upper end over the flame of an alcohol lamp, then after a while the water in the upper part of the test tube will boil, but the ice will not melt. Consequently, water, like all liquids, has poor thermal conductivity.

Gases have even poorer thermal conductivity. Let's take a test tube containing nothing but air, and place it over the flame of an alcohol lamp. A finger placed in a test tube will not feel any heat. Consequently, air and other gases have poor thermal conductivity.

Metals are good conductors of heat, while highly rarefied gases are the worst. This is explained by the peculiarities of their structure. Molecules of gases are located at distances from each other that are greater than molecules of solids, and collide much less frequently. Therefore, the transfer of energy from one molecules to others in gases does not occur as intensely as in solids. The thermal conductivity of a liquid is intermediate between the thermal conductivity of gases and solids.

Convection

As is known, gases and liquids conduct heat poorly. At the same time, the air is heated from steam heating batteries. This occurs due to a type of thermal conductivity called convection.

If a pinwheel made of paper is placed over a heat source, the pinwheel will begin to rotate. This happens because the heated, less dense layers of air rise upward under the action of the buoyant force, and the colder ones move down and take their place, which leads to the rotation of the turntable.

Convection is a type of heat transfer in which energy is transferred through layers of liquid or gas. Convection is associated with the transfer of matter, so it can only occur in liquids and gases; Convection does not occur in solids.

Radiation

The third type of heat transfer is radiation . If you bring your hand to the coil of an electric stove plugged in, to a burning light bulb, to a heated iron, to a radiator, etc., you can clearly feel the heat.

Experiments also show that black bodies absorb and emit energy well, while white or shiny bodies emit and absorb it poorly. They reflect energy well. Therefore, it is understandable why people wear light-colored clothes in the summer, and why they prefer to paint houses in the south white.

By radiation, energy is transferred from the Sun to the Earth. Since the space between the Sun and the Earth is a vacuum (the height of the Earth’s atmosphere is much less than the distance from it to the Sun), energy cannot be transferred either by convection or by thermal conduction. Thus, the transfer of energy by radiation does not require the presence of any medium; this heat transfer can also be carried out in a vacuum.

Lesson summary “Types of heat transfer: thermal conductivity, convection, radiation.”

Next topic: “Amount of heat. Specific heat".

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