Thermal conductivity of copper - how does it affect the properties of copper?

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In electrical engineering, it is sometimes necessary to calculate the parameters of a conductor depending on the material from which it is made, resistance, cross-section, length and temperature. The KIP and A program has a built-in module that allows you to calculate:

• Resistance of an electrical conductor , according to its length, cross-section, temperature and the substance from which it is made.
• The length of the electrical conductor , according to its cross-section, temperature and the substance from which it is made.
• Electrical conductor cross-section for a given current ⁄ power.

The electrical properties of a conductor largely depend on the material from which it is made. The most important are:

• Specific resistance of the conductor substance [ρ] , measured in Ohm m in the international system of units (SI). This means that the SI unit of resistivity is equal to the resistivity of a substance such that a homogeneous conductor 1 m long with a cross-sectional area of ​​1 m², made from that substance, has a resistance of 1 ohm. Ohm mm²/m is also quite often used . 1 Ohm mm²/m = 10−6 Ohm m
• Temperature coefficient of electrical resistance [α], characterizes the dependence of electrical resistance on temperature and is measured in Kelvin to the minus first power K−1. This is a value equal to the relative change in the specific ⁄ electrical resistance of a substance when the temperature changes by one. Resistivity calculation ρt at any temperature t produced according to the classical formula (1):
  ρt = ρ20[1 + α(t – 20)]

  ρt - resistivity at temperature t t - temperature ρ20 - resistivity at temperature 20°C α - temperature coefficient of resistance The formula is applicable in a small temperature range: from 0 to 100 °C. Outside this range or for accurate results, more complex calculations are used.

Below is a table of the most popular metals for making conductors, with their resistivities and temperature coefficients of electrical resistance. The table data is taken from various sources. It should be noted that both the resistivity of the conductor and its temperature coefficient of electrical resistance depend on the purity of the metal, and in the case of alloys (steel) they can differ significantly from brand to brand.

When calculating the properties of an electrical conductor, the Instrumentation and Control program

• The substance from which the conductor is made (See table 1)
• Conductor length. mm, cm, m, km, inches, feet, yards
• Conductor temperature. °C, °F
• Conductor diameter. mm
• Conductor cross section. mm², kcmil kcmil - thousand circular mils = 0.5067 mm²
• Conductor resistance. Ohm, kOhm, MOhm

Below, the figures show screenshots of the KIP and A program for calculating conductor parameters.

Picture 1

Figure 2

Figure 3

What is thermal conductivity

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.

What is the temperature coefficient on a gas receipt?

Temperature coefficient (TC) is a calculated value necessary to bring the volume of fuel passing through the meter to standard conditions.

Interesting materials:

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Properties of copper Cu: thermal conductivity and density of copper

The table shows the thermophysical properties of copper depending on temperature in the range from 50 to 1600 degrees Kelvin.

The density of copper is 8933 kg/m3 (or 8.93 g/cm3) at room temperature . Copper is almost four times heavier than aluminum and iron. These metals will float on the surface of the liquid copper. The copper density values ​​in the table are indicated in kg/m3.

The dependence of copper density on its temperature is presented in the table. It should be noted that the density of copper decreases when it is heated, both as a solid metal and as a liquid copper. The decrease in the density of this metal is due to its expansion when heated - the volume of copper increases. It should be noted that liquid copper has a density of about 8000 kg/m3 at temperatures up to 1300°C.

The thermal conductivity of copper is 401 W/(m deg) at room temperature, which is a fairly high value among metals, comparable to the thermal conductivity of silver.

At 1357K (1084°C) copper goes into a liquid state, which is reflected in the table by a sharp drop in the value of the thermal conductivity coefficient of copper. It can be seen that the thermal conductivity of liquid copper is almost two times lower than that of solid metal.

The thermal conductivity of copper tends to decrease when it is heated, but at temperatures above 1400 K, the thermal conductivity value begins to increase again.

The table discusses the following thermophysical properties of copper at various temperatures:

• copper density, kg/m3;
• specific heat capacity, J/(kg deg);
• thermal diffusivity, m2/s;
• thermal conductivity of copper, W/(m K);
• electrical resistivity, Ohm m;
• Lorentz function;
• heat capacity ratio.

Electrical resistivity

ρ is a value characterizing the ability of a material to resist electric current. Specific resistance is expressed by the formula:

For long conductors (wires, cords, cable cores, busbars), the conductor length l is usually expressed in meters, the cross-sectional area S is in mm², the conductor resistance r is in Ohms, then the dimension of resistivity

Data on the resistivities of various metal conductors are given in the article “Electrical Resistance and Conductivity.”

Pros and cons of aluminum radiators

By comparing the strengths and weaknesses of devices, you can understand their main differences. After all, the difference between copper and aluminum radiators lies in their main characteristics. What is considered an objective advantage for one person turns out to be a serious disadvantage for another. Just look at the pros and cons of aluminum products and you will understand the difference between them.

Let's start with the positive aspects of aluminum as a material for the manufacture of car heater radiators.

1. Price. While cost was considered a disadvantage of copper radiators, here it is a serious advantage. If we compare the price tags for both products, aluminum ones will win by about 2 times. Much depends on the manufacturer, but still the difference in cost remains significant. The buyer can save a lot. Because of this, aluminum units have such a large audience.
2. Heat dissipation. Provided that the number of plates is increased, that is, the cooling area becomes larger, aluminum will not be much inferior to copper in terms of heat transfer. Therefore, in this component they are practically the same. But remember that aluminum ones are cheaper.
3. Range. A huge proportion of modern cars that have been produced over the past few years are equipped with aluminum units from the factory. Because of this, the number of their analogues and original spare parts offered by different manufacturers is growing. The copper versions have a more modest choice.

We're done with the benefits. Let's move on to the other side of the coin. Aluminum is not doing so well. The stated advantages are beyond doubt. But still, motorists make a choice in favor of copper after they have studied the main disadvantages of the design option under consideration.

Therefore, the disadvantages should definitely be pointed out. This clearly shows the differences between the elements. The main disadvantages include:

1. Thermal conductivity indicators. This is a very important drawback that literally negates all the objective positive qualities of the devices. If the driver needs to get the most efficient radiator so that the heating system works efficiently and fully warms up the interior, he will not look in the direction of aluminum.
2. Repairability

Approximately the same conclusions can be drawn regarding these devices, which are made from two different materials.

Which conducts heat better: aluminum or copper?

Today, radiators are made from a variety of materials, the most common being steel, stainless steel and aluminum.

Always have doubts about which radiator to choose for installation in your home? Obviously, this depends on personal taste, as well as on the requirements that you have set for yourself regarding the quality of the heating of the room.

Aluminum is by far the most environmentally friendly material and has a huge number of advantages.

Differences between copper and aluminum

The main concerns regarding winding material selection reflect five characteristic differences between copper and aluminum:

Table : Five characteristic differences between copper and aluminum

How to choose a heating radiator: expert advice

In this article we will not consider cast iron radiators, because... they are losing popularity among buyers.

Let's focus on the most popular models.

The material will tell you in detail about the advantages of aluminum and steel batteries.

Aluminum radiators are lightweight

Aluminum radiators are lighter than traditional steel or cast iron radiators, this fact makes it possible to place such a radiator on any wall in the room.

Aluminum batteries can be hung on the wall, even in situations where the thickness does not allow for deep fastening.

This significantly saves the cost of paying for construction work, since they can be hung very quickly and reliably.

Aluminum is a corrosion resistant material

Aluminum is not subject to corrosion, which makes it an ideal material for the production of radiators that are intended to be installed in areas such as bathrooms and kitchens where there is high humidity.

Aluminum conducts heat well

Aluminum heats up quickly, making it an excellent heat conductor.

Aluminum radiators have a low water content, which means that once turned on, such devices give an intense burst of heat and heat up rooms quite quickly.

By installing aluminum radiators, you can quickly achieve the required temperature in the rooms, as they have the shortest response time.

The main advantage is a significant saving in energy costs during the heating season and, as a wonderful bonus, saving money, since aluminum radiators can be turned off while you are away from the house, and when you return home, turn them on and quickly get a warm home without spending a long time waiting.

Aluminum radiators come in a wide range of designs and colors

There is a common belief that efficient heat cannot be beautiful and original. Fortunately, the days when design must give way to superior performance are over.

Aluminum radiators have a diverse range of designs and offer even the most demanding buyer a decent choice.

You can choose your own finishing color that will perfectly match the style of your home, the shape of the radiator will be one hundred percent in harmony with your home or office atmosphere.

Stainless steel

The use of steel for the production of heat exchangers allows us to obtain durable products, which are mainly used for individual heating systems in houses and cottages.

Due to the ability to control the quality of the coolant and the pressure in the system, steel appliances will be an excellent choice for autonomous heating systems.

Provided that high-quality coolant is supplied and the working fluid pressure is moderate, such devices will last more than 30 years.

Connectivity

Oxides, chlorides, sulfides or base metals that are more conductive on copper than on aluminum. This fact makes cleaning and protecting aluminum connectors more important. Some consider copper and aluminum compounds to be incompatible. Also questionable is the mating of connections between the aluminum of the transformers and the copper connection wire.

Expansion coefficient

When temperature changes, aluminum expands almost a third more than copper. This expansion, along with the ductile nature of aluminum, causes some problems for improperly installed bolted connections.

To avoid loosening of the connection, it must be spring-loaded. By using either cup washers or pressure washers, the necessary elasticity during articulation can be achieved without compressing the aluminum.

When using proper fittings, aluminum connections can be equal in quality to copper ones.

Thermal conductivity

Some argue that since the thermal conductivity of copper is higher than aluminum, this has the effect of reducing the hot spot temperature of the transformer winding.

This is only true when the copper and aluminum winding conductors are of the same size, geometry and design.

Therefore, for any power transformer of a given size, the thermal conductivity characteristics of aluminum can be very similar to copper.

Temperature coefficient of resistivity

7.3 Temperature coefficient of resistivity

The number of charge carriers in a metal conductor n remains practically constant with increasing temperature. However, due to increased vibrations of crystal lattice nodes, as T increases, more and more obstacles appear in the path of the directional movement of electrons, i.e., the average electron path length λ decreases, electron mobility decreases and, as a consequence, specific conductivity (7.8) decreases.

The temperature coefficient of resistivity (°K-1) is determined by the formula

(7.14)

In practice, when changing temperature in narrow ranges, the expression is used

, (7.15)

where ρ1 and ρ2 are the resistivities of the conductor material at temperatures T1 and T2 (T2 > T1); average temperature coefficient of resistivity.

When transitioning from a solid to a liquid state, most metals experience an increase in c, as can be seen, for example, for copper in Figure 31. The resistivity increases for those metals whose volume increases during melting, i.e., density decreases, and, conversely, for metals (gallium, bismuth), which reduce their volume during melting, it decreases.

Figure 31 - Dependence for copper. The jump corresponds to a melting temperature of 1083°C

7.4 Effect of impurities on the resistivity of metals

Impurities and violations of the correct structure of metals increase their resistivity. A significant increase in c is observed when two metals are alloyed if they form a solid solution when the atoms of one metal enter the crystal lattice of the other.

To illustrate this phenomenon, Figure 32 shows the dependences of the parameters of copper-nickel alloys on the composition (in percent by weight, where a is the resistivity c; b is the temperature coefficient of resistivity bs. It can be seen that pure metals have relatively small values ​​of c and large values ​​of bs ( copper and nickel).

Figure 32 — Dependence of parameters of copper-nickel alloys on composition

If an alloy of two metals creates separate crystallization and the structure of the solidified alloy is a mixture of crystals of each of the components, i.e. if these crystals do not form a solid solution and distortion of the crystal lattice of each component is not observed, then the specific conductivity of the alloy changes with a change in composition approximately linearly, i.e., determined by the arithmetic mixing rule. For example, Figure 33 shows the dependence of the specific conductivity r of copper-tungsten alloys on the composition. It can be seen that the conductivity of copper is the maximum value, and that of tungsten is the minimum.

Figure 33 - Dependence of specific conductivity g of copper-tungsten alloys on composition

The influence of the metals included in the alloy on the resistivity of alloys is analytically expressed by the dependence (Nordheim's law)

, (7.10)

where rf is the resistance due to the scattering of electrons by thermal vibrations of the lattice; compost – residual resistance associated with electron scattering on inhomogeneities in the alloy structure; C is a constant depending on the nature of the alloy; Xa and Xb are the atomic fractions of components in the alloy.

7.5 Thermoelectromotive force

When two different metal conductors come into contact, a potential difference occurs between them. The reason for the appearance of this EMF lies in the difference in the values ​​of the work function of electrons from metals, and also in the fact that the concentration of electrons, and therefore the pressure of the electron gas, may not be the same for different metals and alloys. From the electronic theory of metals it follows that the contact potential difference between metals A and B is equal to

, (7.16)

where UA and UB are the potentials of contacting metals; nA and nB – electron concentration in metals A and B; k – Boltzmann constant; e is the absolute value of the electron charge.

If the temperatures of the “junctions” are the same, then the sum of the potential difference in a closed circuit is zero. The situation is different when one of the junctions has a temperature of T1, and the other has a temperature of T2 (Figure 34). In this case, a thermo-EMF arises between the junctions equal to

, (7.17)

which can be written in the form

, (7.18)

where Ш is the constant thermo-EMF coefficient for a given pair of conductors, i.e. thermo-EMF must be proportional to the difference in temperature of the junctions.

Figure 34 — Thermocouple diagram

7.6 Resistance of thin metal films.

Dimensional effects

Metal films of MP are widely used in microelectronics as interconnections, capacitor plates, magnetic and resistive elements of integrated circuits. The electrical properties of MP differ from the properties of bulk samples of the starting material. The main reasons are:

— a variety of structural characteristics of MFs obtained by the condensation method of molecular beams in a high vacuum from an extremely disordered finely dispersed state (amphora capacitor) to the structure of a very perfect single-crystal layer (epitaxial films);

- size effects, i.e. with an increasing role of surface processes compared to volumetric ones. In particular, in electrical conductivity, the size effect occurs in the case when the thickness of the film turns out to be commensurate with the mean free path of the electron. Figure 35 shows typical dependences of resistivity and temperature coefficient of resistivity on film thickness. The dotted line shows the values ​​for bulk samples.

Figure 35 — Dependencies logρд=ƒ(д) and αρд=ƒ(д)

For metal films, 3 regions are observed: I - MP thickness is about 0.1 μm; II - d=10-1-10-2 µm; III - d≈10-3 µm.

To explain these dependencies, it is necessary to take into account that MPs have an island structure, i.e., with a small amount of deposited metal, the particles are located on the dielectric substrate (base) in the form of individual scattered grains - islands. Electrical conductivity of a magnetic field occurs when there is a certain amount of deposited metal.

When an electric field is applied (in the plane of the film), electrons pass through narrow gaps between neighboring islands.

The mechanisms responsible for charge transfer are thermionic emission and tunneling; in particular, electrons located above the Fermi level can tunnel, since with increasing temperature the surface resistance of areas of the substrate without metal grains decreases (a property of dielectrics). These reasons determine the negative effect at low thickness (section III).

As the amount of deposited metal increases, the gap between the islands decreases, the conductivity of the films increases, the negative one decreases, and then changes sign. The value of the metal thickness at which the sign changes depends on the type of metal.

In the process of further condensation of the substance on the substrate, the islands merge and a conducting chain is formed, and then the same chain of a continuous homogeneous layer. However, even in a continuous film, the electrical resistivity is greater than the resistivity of the conductor. This indicates a high concentration of defects in the crystal lattice: dislocations, grain boundaries formed when islands grow together, etc.

An increase in the electrical resistivity of the film is facilitated by the size effect, i.e., a reduction in the mean free path of electrons due to their reflection from the surface of the sample.

The electronic theory of electrical conductivity of metals provides the following explanation for these phenomena. The nature of film nucleation and growth depends on many random factors, so it is difficult to obtain accurate information for films of the same thickness. In this regard, for a comparative assessment of the conductive properties of thin MFs, they use the square resistance parameter Ra, which is numerically equal to the resistance of a section of the film, the length of which is equal to the width when the current passes through its 2 opposite faces parallel to the surface of the substrate

(7.19)

Since it does not depend on the value of the square, the resistance of a thin-film resistor can be determined by the formula

(7.20)

where l is the length of the resistor in the direction of current flow; d is the width of the film.

For the manufacture of thin-film resistors, films with Rkv = 500 – 1000 Ohm/square are used. Refractory metals (tungsten, molybdenum, chromium, etc.) are used as resistive materials.

Characteristics of thermal conductivity of materials

The concept of thermal conductivity of materials is characterized by the ability to transfer thermal energy within a certain object from heated parts to cold ones. The process is carried out by atoms, molecules, electrons and occurs in any body with an uneven temperature distribution.

From the standpoint of kinetic physics, this process occurs as a result of the interaction of particles of molecules in hotter areas within the sample with other elements characterized by a lower temperature. The mechanism and rate of heat transfer depends on the state of aggregation of the substance.

The thermal conductivity category involves determining the heating rate of a material sample and the movement of a temperature wave in a certain direction. The indicator depends on physical parameters:

• density;
• temperature of phase transition to liquid state
• speed of sound propagation (for dielectrics).

Thermal conductivity of brass and bronze

The table shows the thermal conductivity values ​​of brass, bronze, as well as copper-nickel alloys (constantan, copel, manganin, etc.) depending on temperature - in the range from 4 to 1273 K.

The thermal conductivity of brass, bronze and other copper-based alloys increases when heated. According to the table, L96 brass has the highest thermal conductivity of the alloys considered at room temperature . Its thermal conductivity at a temperature of 300 K (27°C) is 244 W/(m deg).

Also copper alloys with high thermal conductivity include: brass LS59-1, tombac L96 and L90, tin tombac LTO90-1, rolled tombac RT-90. In addition, the thermal conductivity of brass is generally higher than that of bronze. It should be noted that bronzes with high thermal conductivity include: phosphorus, chromium and beryllium bronzes, as well as BrA5 bronze.

The copper alloy with the lowest thermal conductivity is manganese bronze - its thermal conductivity coefficient at a temperature of 27°C is 9.6 W/(m deg).

The thermal conductivity of copper alloys is always lower than the thermal conductivity of pure copper, all other things being equal. In addition, the thermal conductivity of copper-nickel alloys is particularly low. The most thermally conductive of them at room temperature is cupronickel MNZhMts 30-0.8-1 with a thermal conductivity of 30 W/(m deg).

Table of thermal conductivity of brass, bronze and copper-nickel alloys Alloy Temperature, KThermal conductivity, W/(m deg) Copper-nickel alloys Brass Bronze

Metal characteristics

The melting temperature of brass, depending on its composition, ranges from 880-950°C. Thus, with an increase in zinc impurity in the material under consideration, the melting point will decrease. It is worth noting that brass, due to its properties, can be welded well.

Brass is processed by resistance welding and can be rolled. The uncoated surfaces of the metal in question turn black when in contact with air. Brass has a yellow color and is highly polished. The non-ferrous metal in question can be melted within certain temperature limits, depending on the impurities in the composition of the material.

Metal specifications:

• Melting point – 880-950°C;
• Material density – 8,300-8,700 kg/cubic meter;
• Specific heat capacity - 0.377 kJ kg−1 K−1 at 20°C;
• Electrical resistivity - (0.07-0.08)·10−6 Ohm·m.

It is useful to know that bismuth, as well as lead, have a detrimental effect on brass, since they reduce the ability to deform when hot.

What are the advantages of non-ferrous metal, grade and application?

Brass belongs to the category of non-ferrous metals. It is useful to know about the chemical and physical benefits that brass has.

Advantages:

• Corrosion resistance;
• High degree of fluidity;
• Excellent anti-friction properties;
• Slight tendency to segregation;
• Excellent technological properties;
• Excellent mechanical properties.

The list presented above does not limit the advantages and beneficial properties of this metal. You should not ignore the most popular brands of material, as well as application.

Melting point of brass

The melting point of brass of the considered brands varies in the range from 865 to 1055 °C. The most fusible is manganese brass LMts58-2 with a melting point of 865°C. Low-melting brasses also include: L59, L62, LAN59-3-2, LKS65-1.5-3 and others.

L96 brass has the highest melting point (1055°C). Among the refractory brasses, according to the table, we can also distinguish: brass L90, LA85-0.5, tin tombak LTO90-1.

Melting temperature of brass Brass, °СBrass, °С

Melting point of bronze

The melting point of bronze ranges from 854 to 1135°C. Bronze AZHN11-6-6 has the highest melting point - it melts at a temperature of 1408 K (1135°C). The melting point of this bronze is even higher than the melting point of copper, which is 1084.6°C.

Bronzes with a low melting point include: BrOTs8-4, BrB2, BrMTsS8-20, BrSN60-2.5 and the like.

Melting point of bronze Bronzet, °СBronzet, °С

Note: The melting and boiling points of other common metals are given in this table.

Thermal conductivity of non-ferrous metals and technical alloys

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.

Non-ferrous metals and alloys: nickel Ni, monel, nichrome; nickel alloys (according to GOST 492-58): cupronickel NM81, NM70, constantan NMMts 58.5-1.54, copel NM 56.5, monel NMZhMts and K-monel, alumel, chromel, manganin NMMts 85-12, invar; magnesium alloys (according to GOST 2856-68), electron, platinum-rhodium; soft solders (according to GOST 1499-70): pure tin, lead, POS-90, POS-40, POS-30, Rose alloy, Wood alloy.

The table shows that magnesium alloys and nickel have high thermal conductivity (at room temperature). Low thermal conductivity is characteristic of nichrome, invar and Wood's alloy.

Thermal conductivity coefficients of alloys

The table shows the thermal conductivity values ​​of alloys in the temperature range from 20 to 200ºС. Alloys: aluminum bronze, bronze, phosphor bronze, invar, constantan, manganin, magnesium alloys, copper alloys, Rose alloy, Wood's alloy, nickel alloys, nickel silver, platinum-iridium, electron alloy, platinum-rhodium.

Thermal conductivity coefficient of other materials

MaterialHumidity mass fraction % W/(m•K)

Thermal conductivity coefficient of asbestos and foam concrete at different temperatures

(ρa=576kg/m3, ρп=400kg/m3,λ, W/(m•K))

Material-18oС0oС50oС100oС150oС

Thermal conductivity coefficient of liquid W/(m•K) at different temperatures

Material0oС50oС100oС

Specific resistance and temperature coefficient of expansion (CTE) of metal wire (at 18ºС)

The table shows the values ​​of electrical resistivity and CTE of metal wire made of various metals and alloys. Wire material: aluminum, tungsten, iron, gold, brass, manganin, copper, nickel, constantan, nichrome, tin, platinum, lead, silver, zinc. As can be seen from the table, nichrome wire has a high electrical resistivity and is successfully used as incandescent heating coils for many household and industrial devices.

Specific heat capacity of multicomponent special alloys

The specific (mass) heat capacity of multicomponent special alloys is given in the table at temperatures from 0 to 1300ºС. Heat capacity dimension cal/(g deg). Heat capacity of special alloys: alumel, bell metal, Wood's alloy, Invar, Lipowitz alloy, Manganin, Monel, Rose alloy, phosphorus bronze, chromel, Na-K alloy, Pb - Bi alloy, Pb - Bi - Sn, Zn - Sn - Ni - Fe - Mn.

Temperature coefficient of linear expansion of metals, solids, liquids (Table)

Temperature coefficient of linear expansion of metals and alloys

The table shows the average values ​​of the temperature coefficient of linear expansion ɑ of metals and alloys in the range from 0 to 100 °C (unless a different temperature is indicated).

Temperature coefficient of linear expansion of solids

The table shows the average values ​​of the temperature coefficient of linear expansion ɑ of solids in the range from 0 to 100 °C (unless a different temperature is indicated).

Temperature coefficient of volumetric expansion of liquids

The table shows the average values ​​of the temperature coefficient of volumetric expansion β of liquids at a temperature of 20 °C (unless otherwise indicated).

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

λ is a quantity characterizing the amount of heat passing per unit time through a layer of matter. Thermal conductivity dimension

Thermal conductivity is of great importance in thermal calculations of machines, apparatus, cables and other electrical devices.

Thermal conductivity value λ for some materials

From the data presented it is clear that metals have the greatest thermal conductivity. Non-metallic materials have significantly lower thermal conductivity. It reaches especially low values ​​for porous materials, which I use specifically for thermal insulation. According to the electronic theory, the high thermal conductivity of metals is due to the same conduction electrons as electrical conductivity.

What does thermal conductivity depend on?

Studying the ability of heat transfer by metal products, it was revealed that thermal conductivity depends on:

• type of metal;
• chemical composition;
• porosity;
• sizes.

Metals have different crystal lattice structures, and this can change the thermal conductivity of the material. For example, in steel and aluminum, the structural features of microparticles affect differently the rate of transfer of thermal energy through them.

The thermal conductivity coefficient can have different values ​​for the same metal when the exposure temperature changes. This is due to the fact that different metals have different melting degrees, which means that under other environmental parameters, the properties of the materials will also differ, and this will affect thermal conductivity.

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.

SCS design - basic requirements

The correct answer to such a question would be to quote from beginning to end all the texts of telecommunication standards :), since all of them are either directly or indirectly related to the design of SCS. Based on these standards, training manuals for SCS manufacturers on the design and installation of their proprietary systems have been written. There are a lot of different requirements and rules; such training courses take several full days. For narrowly formulated questions, we can give a quote from the standard as an answer, but your question is too broad. The only way we can help you is to list the main thematic sections of the most popular standards so that you can navigate which standard you need to find and read.

TIA/EIA-568-B.1 Commercial Building Telecommunications Cabling Standard, Part 1: General Requirements Standard for commercial building telecommunications cabling systems. Part 1: General requirements

• Horizontal subsystem
• Trunk subsystem
• Workplace
• Telecommunications
• Hardware
• City inputs
• Installation requirements
• Transmission performance and testing requirements

TIA/EIA-568-B.2 Commercial Building Telecommunications Cabling Standard, Part 2: Balanced Twisted-Pair Cabling Components Standard for commercial building telecommunications cabling systems. Part 2: Balanced Twisted Pair Cabling Components

• Cables balanced twisted pair 100 Ohm
• 100 Ohm Balanced Twisted Pair Switching Equipment
• Patch cords and cross-connect jumpers

The appendices to this standard provide important information about the parameters and methodology for testing the characteristics of copper cable systems, requirements for measuring equipment, etc.

TIA/EIA-568-B.3 Optical Fiber Cabling Components Standard

• Fiber Optic Cables
• Switching equipment
• Patch cords
• Field testers

TIA/EIA-569 Commercial Building Standard for Telecommunications Pathways and Spaces

• Horizontal routes and associated spaces
• Main routes inside the building and associated spaces
• Workplace
• Telecommunications cabinet
• Hardware
• City input

One of the annexes also provides requirements for main routes between buildings and associated spaces.

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.

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.

Importance in everyday life and production

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.

Contact potential difference and thermoelectromotive force

As stated in the article “Metal Conductors”, positive metal ions are located at the nodes of the crystal lattice, forming, as it were, its frame. Free electrons fill the lattice like a gas, sometimes called “electron gas.” The pressure of the electron gas in a metal is proportional to the absolute temperature and the number of free electrons per unit volume, which depends on the properties of the metal. When two dissimilar metals come into contact at the point of contact, the pressure of the electron gas equalizes. As a result of electron diffusion, a metal whose number of electrons decreases is charged positively, and a metal whose number of electrons increases is charged negatively. A potential difference occurs at the point of contact. This difference is proportional to the temperature difference between the metals and depends on their type. A thermoelectric current arises in a closed circuit. The electromotive force (EMF) that creates this current is called thermoelectromotive force (thermo-EMF).

The phenomenon of contact potential difference is used in technology to measure temperature using thermocouples. When measuring small currents and voltages in a circuit at the junction of different metals, a large potential difference may arise, which will distort the measurement results. In this case, it is necessary to select materials so that the measurement accuracy is high.

Application

The state of aggregation of materials has a distinctive structure of molecules and atoms. This is what has a great influence on metal products and their properties, depending on their purpose.

The different chemical composition of components and parts made of iron allows them to have different thermal conductivities. This is due to the structure of metals such as cast iron, steel, copper and aluminum. The porosity of cast iron products promotes slow heating, and the density of the copper structure, on the contrary, accelerates the heat transfer process. These properties are used for rapid heat removal or gradual heating of inert products. An example of using the properties of metal products is:

• kitchen utensils with various properties;
• pipe soldering equipment;
• irons;
• rolling and sliding bearings;
• plumbing equipment for heating water;
• heating devices.

Copper tubes are widely used in radiators of automobile cooling systems and air conditioners used in everyday life. Cast iron radiators retain heat in the apartment, even with an inconsistent supply of coolant at the required temperature. And radiators made of aluminum contribute to the rapid transfer of heat to the heated room.

When high temperatures occur as a result of friction of metal surfaces, it is also important to take into account the thermal conductivity of the product. In any gearbox or other mechanical equipment, the ability to remove heat will allow the mechanism parts to maintain strength and not be subject to destruction during operation. Knowledge of the heat transfer properties of various materials will allow you to competently use certain alloys of non-ferrous or ferrous metals.

Sources

• https://tokar.guru/metally/stal/teploprovodnost-stali-alyuminiya-latuni-medi.html
• https://thermalinfo.ru/svojstva-materialov/metally-i-splavy/svojstva-medi-plotnost-teploemkost-teploprovodnost
• https://smolgelios.ru/provodka/teplo-elektroprovodnost-medi.html
• https://ometalledo.ru/teploprovodnost-medi-i-alyuminiya-tablica.html
• https://vse-otoplenie.ru/teplootdaca-aluminia
• https://thermalinfo.ru/svojstva-materialov/metally-i-splavy/teploprovodnost-splavov-medi-temperatura-plavleniya-bronzy-i-latuni
• https://kangen.ru/raznoe/temperatura-plavleniya-latuni-v-domashnih-usloviyah.html
• https://thermalinfo.ru/svojstva-materialov/metally-i-splavy/teploprovodnost-metallov-teploemkost-i-plotnost-splavov
• https://zaozmi.ru/polezno/tablica_teploprovodimosti_metallov.html
• https://tpspribor.ru/vidy-metalla/teploprovodnost-medi-dve-storony-odnoy-medali.html
• https://ectrl.ru/provodka/elektro-i-teploprovodnost-medi.html
• https://met-all.org/cvetmet-splavy/med/teploprovodnost-medi-i-ee-splavov.html
• https://prompriem.ru/metally/teploprovodnost.html

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Temperature coefficient of resistance

α is a value characterizing the change in conductor resistance depending on temperature. The average value of the temperature coefficient of resistance in the temperature range t2° - t1° can be found by the formula:

The temperature coefficients of resistance of various conductor materials are given in the table below.

The value of temperature coefficients of resistance of metals