Among the large number of parameters that characterize metals, there is also such a concept as thermal conductivity. Its importance can hardly be overestimated. This parameter is used when calculating parts and assemblies. For example, gear drives. In general, a whole branch of science called thermodynamics deals with thermal conductivity.
What is thermal conductivity and thermal resistance
The thermal conductivity of metals can be characterized as follows - this is the ability of materials (gas, liquid, etc.) to transfer excess thermal energy from heated parts of the body to cold ones. The transfer is carried out by freely moving elementary particles, which include atoms, electrons, etc.
The process of heat exchange itself occurs in any bodies, but the way of transferring energy largely depends on the state of aggregation of the body.
In addition to this thermal conductivity, one more definition can be given - it is a quantitative parameter of the body's ability to conduct thermal energy. If we compare heat and electrical networks, then this concept is similar to electrical conductivity.
The ability of a physical body to prevent the propagation of thermal vibration of molecules is called thermal resistance. By the way, some are sincerely mistaken, confusing this concept with thermal conductivity.
The concept of the coefficient of thermal conductivity
The coefficient of thermal conductivity is called a value that is equal to the amount of heat transferred through a unit of surface per second.
The thermal conductivity of the metal was established back in 1863. It was then that it was proved that free electrons are responsible for the transfer of heat, of which there are a great many in the metal. That is why the thermal conductivity of metals is much higher than that of dielectric materials.
What determines the thermal conductivity index
Thermal conductivity is a physical quantity and mostly depends on the parameters of temperature, pressure and type of substance. Most of the coefficients are determined empirically. Many methods have been developed for this. The results are summarized in reference tables, which are then used in various scientific and engineering calculations.
Bodies have different temperatures and during heat exchange it (temperature) will be unevenly distributed. In other words, you need to know how the thermal conductivity depends on temperature.
Numerous experiments show that for many materials the relationship between the coefficient and the thermal conductivity itself is linear.
The thermal conductivity of metals is due to the shape of its crystal lattice.
In many respects, the coefficient of thermal conductivity depends on the structure of the material, the size of its pores and moisture content.
When the thermal conductivity coefficient is taken into account
The parameters of thermal conductivity must be taken into account when choosing materials for enclosing structures - walls, ceilings, etc. In rooms where walls are made of materials with high thermal conductivity in the cold season it will be pretty cool. Decorating the room will not help either. In order to avoid this, the walls must be made quite thick. This will inevitably lead to an increase in the cost of materials and labor costs.
That is why the use of materials with low thermal conductivity (mineral wool, foam, etc.) is provided for in the construction of the walls.
Indicators for steel
- In reference materials on the thermal conductivity of various materials, a special place is occupied by data presented on steels of different grades.
So, in the reference materials, experimental and calculated data of the following types of steel alloys are collected:
resistant to corrosion, high temperature; - intended for the production of springs, cutting tools;
- saturated with alloying additives.
The tables summarize the indicators that were collected for steels in the temperature range from -263 to 1200 degrees.
The average indicators are for:
- carbon steels 50 - 90 W / (m × deg);
- corrosion-resistant, heat- and heat-resistant alloys related to martensitic - from 30 to 45 W / (m × deg);
- alloys related to austenitic from 12 to 22 W / (m × deg).
These reference materials contain information and properties of cast irons.
Coefficients of thermal conductivity of aluminum, copper and nickel alloys
During calculations related to non-ferrous metals and alloys, designers use reference materials placed in special tables.
They contain materials on the thermal conductivity of non-ferrous metals and alloys, in addition to these data, information on the chemical composition of alloys is indicated. The studies were carried out at temperatures from 0 to 600 ° C.
According to the information collected in these tabular materials, it can be seen that non-ferrous metals with high thermal conductivity are alloys based on magnesium and nickel. Metals with low thermal conductivity include nichrome, invar and some others.
Most metals have good thermal conductivity, some have more, others less. Metals with good thermal conductivity include gold, copper and some others. Materials with low thermal conductivity include tin, aluminum, etc.
High thermal conductivity can be both an advantage and a disadvantage. It all depends on the scope. For example, high thermal conductivity is good for kitchen utensils. Materials with low thermal conductivity are used to create permanent joints of metal parts. There are entire families of tin-based alloys.
Disadvantages of high thermal conductivity of copper and its alloys
Copper is much more valuable than aluminum or brass. But meanwhile, this material has a number of disadvantages that are associated with its positive aspects.
The high thermal conductivity of this metal makes it necessary to create special conditions for its processing. That is, copper billets need to be heated more accurately than steel. In addition, often, before starting treatment, pre-heating or concomitant heating.
We must not forget that pipes made of copper imply that thorough thermal insulation will be carried out. This is especially true for those cases when a heating supply system is assembled from these pipes. This significantly increases the cost of installation work.
Certain difficulties arise when using gas welding. A more powerful tool is required to get the job done. Sometimes, to process copper with a thickness of 8-10 mm, it may be necessary to use two or even three torches. In this case, one of them is welding a copper pipe, and the rest are busy with its heating. In addition, working with copper requires more consumables.
Working with copper requires the use of a specialized tool. For example, when cutting parts made of bronze or brass 150 mm thick, you will need a torch that can handle steel with a lot of chrome. If it is used for processing copper, then the limiting thickness will not exceed 50 mm.
Is it possible to increase the thermal conductivity of copper
Not so long ago, a group of Western scientists conducted a series of studies to improve the thermal conductivity of copper and its alloys. For their work, they used films made of copper with a thin layer of graphene deposited on its surface. For its application, the technology of its deposition from gas was used. During the research, many instruments were used, which were designed to confirm the objectivity of the results obtained.
Research results have shown that graphene has one of the highest thermal conductivity. After it was applied to a copper substrate, the thermal conductivity dropped somewhat. But, during this process, copper is heated and grains increase in it, and as a result, the permeability of electrons increases.
When copper was heated, but without applying this material, the grains retained their size.
One of the uses of copper is to remove excess heat from electronic and electrical circuits. The use of graphene sputtering will solve this problem much more efficiently.
Effect of carbon concentration
Steels with a low carbon content have high thermal conductivity. That is why materials of this class are used for the manufacture of pipes and fittings for it. The thermal conductivity of steels of this type is in the range of 47-54 W / (m × K).
Value in everyday life and production
Application of thermal conductivity in construction
Each material has its own thermal conductivity index. The lower its value, the correspondingly lower the level of heat transfer between the external and internal environment. This means that in a building constructed of a material with low thermal conductivity, it will be warm in winter and cool in summer.
When constructing various buildings, including residential buildings, one cannot do without knowledge of the thermal conductivity of building materials. When designing building structures, it is necessary to take into account data on the properties of materials such as concrete, glass, mineral wool and many others. Among them, the limiting thermal conductivity belongs to concrete, meanwhile, it is 6 times less for wood.
Heating systems
The key task of any heating system is the transfer of thermal energy from the coolant to the premises. For such heating, batteries or heating radiators are used. They are needed to transfer heat energy to the premises.
- A heating radiator is a structure inside that moves the coolant. The main characteristics of this product include:
the material from which it is made; - type of construction;
- dimensions, including the number of sections;
- heat transfer indicators.
It is heat transfer that is the key parameter. It's all about what determines the amount of energy that is transferred from the radiator to the room. The higher this indicator, the lower the heat loss will be.
There are look-up tables that determine which materials are optimal for use in heating systems. From the data they contain, it becomes clear that copper is considered the most effective material. But, due to its high price and certain technological difficulties associated with copper processing, their applicability is not so high.
That is why models made of steel or aluminum alloys are increasingly used. A combination of different materials, such as steel and aluminum, is often used.
Each manufacturer of radiators, when marking finished products, must indicate such a characteristic as the heat output.
On the heating system market, you can buy radiators made of cast iron, steel, aluminum and bimetal.
Methods for studying thermal conductivity parameters
When studying the parameters of thermal conductivity, one must remember that the characteristics of a particular metal or its alloys depend on the method of its production. For example, the parameters of the metal obtained by casting can differ significantly from the characteristics of the material manufactured by powder metallurgy methods. The properties of the raw metal are fundamentally different from those that went through heat treatment.
Thermal instability, that is, the transformation of individual properties of the metal after exposure to high temperatures is common to almost all materials. As an example, metals, after prolonged exposure to different temperatures, are capable of reaching different levels of recrystallization, and this is reflected in the thermal conductivity parameters.
We can say the following - when researching the parameters of thermal conductivity, it is necessary to use samples of metals and their alloys in a standard and certain technological state, for example, after heat treatment.
For example, there are requirements for metal grinding for its research using thermal analysis methods. Indeed, such a requirement exists in a number of studies. There is also such a requirement - as the manufacture of special plates and many others.
The non-thermostability of metals imposes a number of restrictions on the use of thermophysical research methods. The fact is that this method of conducting research requires heating the samples at least twice, in a certain temperature range.
One of the methods is called relaxation-dynamic. It is designed to perform mass measurements of heat capacity in metals. In this method, the transition curve of the sample temperature between its two stationary states is recorded. This process is a consequence of a jump in the thermal power introduced into the test sample.
This method can be called relative. It uses a test subject and a comparative sample. The main thing is that the samples have the same emitting surface. When carrying out research, the temperature acting on the samples should change stepwise, while upon reaching the specified parameters, it is necessary to withstand a certain amount of time. The direction of temperature change and its step should be selected in such a way that the sample intended for testing is heated evenly.
At these moments, the heat fluxes will equalize and the heat transfer ratio will be determined as the difference in the temperature fluctuation rates.
Sometimes, in the process of these studies, a source of indirect heating of the investigated and comparative sample.
Additional heat loads can be created on one of the samples in comparison with the second sample.
Which method of measuring thermal conductivity is best for your material?
There are methods for measuring thermal conductivity such as LFA, GHP, HFM and TCT. They differ from each other in size and geometric parameters of the samples used to test the thermal conductivity of metals.
These abbreviations can be deciphered as:
- GHP (hot guard zone method);
- HFM (heat flow method);
- TCT (hot wire method).
The above methods are used to determine the coefficients of various metals and their alloys. At the same time, using these methods, they are studying other materials, for example, mineral ceramics or refractory materials.
The metal samples on which the research is carried out have overall dimensions of 12.7 × 12.7 × 2.
In many branches of modern industry, a material such as copper is very widely used. The electrical conductivity of this metal is very high. This explains the feasibility of its application primarily in electrical engineering. Copper leads to excellent performance conductors. Of course, this metal is used not only in electrical engineering, but also in other industries. Its demand is explained, among other things, by such qualities as resistance to corrosive destruction in a number of corrosive environments, refractoriness, plasticity, etc.
History reference
Copper is a metal known to man since ancient times. This explains the early acquaintance of people with this material, first of all, by its widespread occurrence in nature in the form of nuggets. Many scientists believe that it was copper that was the first metal reduced by man from oxygen compounds. Once upon a time, rocks were simply heated over a fire and sharply cooled, as a result of which they cracked. Later, copper reduction began to be carried out on fires with the addition of coal and blowing with furs. The improvement of this method ultimately led to the creation. Even later, this metal began to be obtained by the method of oxidative smelting of ores.
Copper: material conductivity
In a quiescent state, all free electrons of any metal revolve around the nucleus. When an external source of influence is connected, they line up in a certain sequence and become current carriers. The degree of the metal's ability to pass the latter through itself is called electrical conductivity. Its unit of measurement in the International SI is Siemens, defined as 1 S \u003d 1 Ohm -1.
The electrical conductivity of copper is very high. According to this indicator, it surpasses all base metals known today. Only silver passes the current better than it. The electrical conductivity of copper is 57x104 cm -1 at a temperature of +20 ° C. Due to this property, this metal is currently the most common conductor of all those used for industrial and domestic purposes.
Copper perfectly withstands constant and, moreover, is reliable and durable. Among other things, this metal is also characterized by a high melting point (1083.4 ° C). And this, in turn, allows copper to work for a long time in a heated state. In terms of prevalence as a current conductor, only aluminum can compete with this metal.
Influence of impurities on the electrical conductivity of copper
Of course, in our time, much more advanced techniques are used to smelt this red metal than in ancient times. However, it is practically impossible to obtain completely pure Cu even today. Various kinds of impurities are always present in copper. This can be, for example, silicon, iron or beryllium. Meanwhile, the more impurities in copper, the lower the indicator of its electrical conductivity. For the manufacture of wires, for example, only sufficiently pure metal is suitable. According to the regulations, copper with an amount of impurities not exceeding 0.1% can be used for this purpose.
Very often this metal contains a certain percentage of sulfur, arsenic and antimony. The first substance significantly reduces the plasticity of the material. The electrical conductivity of copper and sulfur varies greatly. This impurity does not conduct current at all. That is, it is a good insulator. However, sulfur has practically no effect on the electrical conductivity of copper. The same goes for thermal conductivity. The opposite picture is observed with antimony and arsenic. These elements can significantly reduce the electrical conductivity of copper.
Alloys
Various kinds of additives can also be used specifically to increase the strength of a plastic material such as copper. They also reduce its electrical conductivity. But on the other hand, their use can significantly extend the service life of various kinds of products.
Cd (0.9%) is most often used as an additive to increase the strength of copper. The result is cadmium bronze. Its conductivity is 90% of that of copper. Sometimes, instead of cadmium, aluminum is also used as an additive. The conductivity of this metal is 65% of that of copper. To increase the strength of wires in the form of an additive, other materials and substances can also be used - tin, phosphorus, chromium, beryllium. The result is a certain grade of bronze. The combination of copper with zinc is called brass.
Characteristics of alloys
It can depend not only on the amount of impurities present in them, but also on other indicators. For example, as the heating temperature rises, the ability of copper to pass current through itself decreases. Even the method of its manufacture influences the electrical conductivity of such a wire. In everyday life and in production, both soft annealed copper conductors and hard-drawn ones can be used. The first variety has a higher ability to pass current through itself.
However, most of all, of course, the additives used and their amount affect the electrical conductivity of copper. The table below provides the reader with comprehensive information regarding the current carrying capacity of the most common alloys of this metal.
Alloy | Condition (O - annealed, T-hard drawn) | Electrical conductivity (%) |
Pure copper | ||
Tin Bronze (0.75%) | ||
Cadmium Bronze (0.9%) | ||
Aluminum bronze (2.5% A1, 2% Sn) | ||
Phosphor Bronze (7% Sn, 0.1% Ρ) | ||
The electrical conductivity of brass and copper is comparable. However, for the first metal, this indicator, of course, is slightly lower. But at the same time it is higher than that of bronzes. Brass is widely used as a conductor. It passes current worse than copper, but at the same time it costs less. Most often, contacts, clamps and various parts for radio equipment are made of brass.
High resistance copper alloys
Such conductive materials are mainly used in the manufacture of resistors, rheostats, measuring instruments, and electric heating devices. Copper alloys constantan and manganin are most often used for this purpose. The specific resistance of the former (86% Cu, 12% Mn, 2% Ni) is 0.42-0.48 μOhm / m, and the second (60% Cu, 40% Ni) is 0.48-0.52 μOhm / m.
Relationship with the coefficient of thermal conductivity
Copper - 59,500,000 S / m. This indicator, as already mentioned, is correct, however, only at a temperature of +20 o C. There is a certain relationship between the coefficient of thermal conductivity of any metal and specific conductivity. It is established by the Wiedemann-Franz law. It is performed for metals at high temperatures and is expressed in the following formula: K / γ \u003d π 2/3 (k / e) 2 T, where y is the specific conductivity, k is the Boltzmann constant, and e is the elementary charge.
Of course, there is a similar connection with a metal such as copper. Its thermal and electrical conductivity is very high. It is in second place after silver in both of these indicators.
Connecting copper and aluminum wires
Recently, electrical equipment of ever higher power has begun to be used in everyday life and industry. During Soviet times, wiring was mainly made of cheap aluminum. Unfortunately, its performance characteristics no longer meet the new requirements. Therefore, today in everyday life and in industry they very often change to copper. The main advantage of the latter, in addition to their refractoriness, is that their conductive properties do not decrease during the oxidation process.
Often, when upgrading power grids, aluminum and copper wires have to be connected. You cannot do this directly. Actually, the electrical conductivity of aluminum and copper does not differ too much. But only these metals themselves. The oxidizing films of aluminum and copper have different properties. Because of this, the conductivity at the junction is significantly reduced. The oxidizing film of aluminum is much more resistant than that of copper. Therefore, the connection of these two types of conductors must be made exclusively through special adapters. This can be, for example, clamps containing a paste that protects metals from oxide formation. This version of adapters is usually used outdoors. In premises, branch clamps are often used. Their design includes a special plate that excludes direct contact between aluminum and copper. In the absence of such conductors in a domestic environment, instead of twisting the wires directly, it is recommended to use a washer and a nut as an intermediate "bridge".
Physical properties
Thus, we found out what is the electrical conductivity of copper. This indicator can vary depending on the impurities that make up this metal. However, the demand for copper in industry is also determined by its other useful physical properties, information on which can be obtained from the table below.
Parameter | Value |
Face-centered cubic, а \u003d 3.6074 Å |
|
Atomic radius | |
Specific heat | 385.48 J / (kg K) at +20 о С |
Thermal conductivity | 394.279 W / (m K) at +20 о С |
Electrical resistance | 1.68 10-8 Ohm m |
Linear expansion coefficient | |
Hardness | |
Tensile strength |
Chemical properties
According to such characteristics, copper, the electrical conductivity and thermal conductivity of which are very high, occupies an intermediate position between the elements of the first triad of the eighth group and the alkaline elements of the first group of the periodic table. Its main chemical properties include:
tendency to complexation;
the ability to give colored compounds and insoluble sulfides.
The most characteristic of copper is the bivalent state. It has practically no similarities with alkali metals. Its chemical activity is also low. In the presence of CO 2 or moisture, a green carbonate film forms on the copper surface. All copper salts are poisonous. In the mono- and bivalent state, this metal forms very stable. The most important for the industry are ammonia.
Scope of use
The high thermal and electrical conductivity of copper determines its widespread use in a wide variety of industries. Of course, this metal is most often used in electrical engineering. However, this is far from the only area of \u200b\u200bits application. Among other things, copper can be used:
in jewelry;
in architecture;
when assembling plumbing and heating systems;
in gas pipelines.
For the manufacture of all kinds of jewelry, mainly copper-gold alloy is used. This allows you to increase the resistance of jewelry to deformation and abrasion. In architecture, copper can be used for cladding roofs and facades. The main advantage of this finish is durability. For example, the roof of a well-known architectural landmark - the Catholic cathedral in the German city of Hildesheim - is sheathed with sheets of this particular metal. The copper roof of this building has reliably protected its interior for almost 700 years.
Engineering Communication
The main advantages of copper water pipes are also durability and reliability. In addition, this metal is able to impart special unique properties to water, making it useful for the body. Copper pipes are also ideal for assembling gas pipelines and heating systems - mainly due to their corrosion resistance and ductility. In the event of an emergency increase in pressure, such lines are able to withstand a much greater load than steel ones. The only drawback of copper pipelines is their high cost.
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The thermal conductivity of the enamel coating, even with ordinary enamel, is quite low, - 0 8 - 1 0 watts per meter degree. For comparison: the thermal conductivity of iron - 65; steel - 70 - 80; copper - 330 watts per meter degree. In the presence of gas bubbles in the enamel, which leads to a decrease in its apparent density, the thermal conductivity decreases. For example, with an apparent density of enamel 2 48 grams per centimeter cubic, the thermal conductivity is 1 18 watts per meter degree, then with an apparent density of 2 20 grams per centimeter cubic thermal conductivity is already 0 46 watts per meter degree.
The crystal lattice of aluminum, like many other metals, consists of face-centered cubes (see p. The thermal conductivity of aluminum is twice the thermal conductivity of iron and is equal to half the thermal conductivity of copper. Its electrical conductivity is much higher than that of iron and reaches 60% of the electrical conductivity of copper).
| Composition and mechanical properties of some chromium cast irons. |
The alloy is very prone to the formation of shrinkage cavities. The thermal conductivity of the alloy is about half of the thermal conductivity of iron, which should be taken into account when manufacturing thermal equipment from chromium cast iron.
When arc welding copper, it should be taken into account that the thermal conductivity of copper is about six times that of iron. With the strength of copper decreases so much that cracks are formed even with light impacts. Copper melts at a temperature of 1083 C.
The modulus of elasticity of titanium is almost half the modulus of elasticity of iron, is on the same level with the modulus of copper alloys and is significantly higher than that of aluminum. The thermal conductivity of titanium is low: it is about 7% of the thermal conductivity of aluminum and 16 5% of the thermal conductivity of iron. This must be taken into account when heating the metal for pressure treatment and when welding. The electrical resistance of titanium is about 6 times that of iron and 20 times that of aluminum.
The modulus of elasticity of titanium is almost half the modulus of elasticity of iron, is on the same level with the modulus of copper alloys and is significantly higher than that of aluminum. The thermal conductivity of titanium is low: it is about 7% of the thermal conductivity of aluminum and 16 5% of the thermal conductivity of iron.
This material has satisfactory mechanical strength and exceptionally high chemical resistance to almost all, even the most aggressive, chemicals, with the exception of strong oxidants. In addition, it differs from all other non-metallic materials in its high thermal conductivity, more than twice the thermal conductivity of iron.
All these requirements are met by iron, carbon and low-alloy structural steels with a low carbon content: the melting temperature of iron is 1535 C, the burning temperature is 1200 C, the melting point of iron oxide is 1370 C. The thermal effect of oxidation reactions is quite high: Fe 0 5O2 FeO 64 3 kcal / g -mole, 3Fe 2О2 Fe3O4 Н - 266 9 kcal / g-mol, 2Fe 1 5О2 Fe2O3 198 5 kcal / g-mol, and the thermal conductivity of iron is limited.
Titanium and its alloys, due to their high physicochemical properties, are increasingly used as a structural material for aviation and rocket technology, chemical engineering, instrument making, shipbuilding and mechanical engineering, in the food and other industries. Titanium is almost two times lighter than steel, its density is 45 g / cm3, it has high mechanical properties, corrosion resistance at normal and high temperatures and in many active media, the thermal conductivity of titanium is almost four times less than that of iron.
One of these solutions is that the pipe wound on the cooled surface is only tacked to this surface by welding, after which the pipe joint with the casing is covered with epoxy resin mixed with iron powder. The thermal conductivity of the mixture is close to the thermal conductivity of iron. The result is a good thermal contact between the jacket and the pipe, which improves the cooling conditions for the jacket.
All these conditions are satisfied by iron and carbon steels. Oxides FeO and Fe304 melt at temperatures of 1350 and 1400 C. The thermal conductivity of iron is not great compared to other structural materials.
For metals operating at low temperatures, it is also very important how their thermal conductivity changes with temperature. The thermal conductivity of steel increases with decreasing temperature. Pure iron is very sensitive to temperature changes. Depending on the amount of impurities, the thermal conductivity of iron can change dramatically. Pure iron (99 7%), containing 0 01% C and 0 21% O2, has a thermal conductivity of 0 35 cal cm-1 s - 19 C - at - 173 C and 0 85 cal cm - x Xs - 10 C - at -243 C ...
The most widely used soldering iron, gas torches, immersion in molten solder and furnaces. Limitations in its application are caused only by the fact that only thin-walled parts can be soldered with a soldering iron at a temperature of 350 C. Massive parts, due to their high thermal conductivity, which is 6 times higher than the thermal conductivity of iron, are soldered with gas burners. For tubular copper heat exchangers, immersion brazing in molten salts and solders is used. When brazing by immersion in molten salts, salt baths are usually used. Salts usually serve as a heat source and have a fluxing effect, so additional fluxing is not required during brazing. When soldering by immersion in a solder bath, pre-fluxed parts are heated in the solder melt, which fills the connecting gaps at the soldering temperature. The mirror of the solder is protected with activated carbon or inert gas. The disadvantage of soldering in salt baths is the impossibility in some cases of removing the remnants of salts or flux.
Metals have a large number of characteristics that determine their performance and the ability to use in the manufacture of certain products. An important characteristic of all materials is thermal conductivity. This indicator determines the ability of a material body to transfer thermal energy. The table of thermal conductivity of metals is found in various reference books, it may depend on their various features. An example is the fact that the mechanism of thermal energy transfer largely depends on the state of aggregation of matter.
What determines the thermal conductivity index
Considering the thermal conductivity of metals and alloys (the table was created not only for metals, but also for other materials), it should be borne in mind that the most important indicator is the thermal conductivity coefficient. It depends on the points below:
In tables for some metals and alloys, the thermal conductivity coefficient is indicated already in the liquid phase.
Today, in practice, practically do not measure the indicator in question. This is due to the fact that the thermal conductivity coefficient remains practically unchanged with an insignificant change in the chemical composition. Tabular data is used in design and other calculations.
The concept of the coefficient of thermal conductivity
To designate the value under consideration, the symbol λ is used - the amount of heat that is transferred per unit of time through a unit of surface at the time the temperature rises. This value is used in various calculations.
The description of the thermal conductivity property of many metals is carried out according to the formula k \u003d 2.5 · 10−8σT. This formula takes into account:
- Temperature measured in Kelvin.
- Indicator of electrical conductivity.
This ratio is most suitable for determining the properties of conductors at the time of operation during heating, but recently it has also been used to measure the degree of conductivity of thermal energy.
Semiconductors and insulators have lower heat conductivity, which is due to the peculiarities the structure of their crystal lattice.
When taken into account
When considering the various properties of materials, attention is often paid to thermal conductivity. This indicator is important in the following cases:
In conclusion, we note that before the development of the molecular kinetic theory, it was accepted to consider the transfer of thermal energy as a sign of the overflow of hypothetical caloric. The advent of modern equipment has made it possible to study the structure of materials and study the behavior of particles when exposed to high temperatures. Energy transfer occurs due to the rapid movement of molecules that begin to collide, and sets in motion other molecules that are in a calm state.
Thermal conductivity is a physical quantity that determines the ability of materials to conduct heat. In other words, thermal conductivity is the ability of substances to transfer the kinetic energy of atoms and molecules to other substances that are in direct contact with them. In SI, this value is measured in W / (K * m) (Watt per Kelvin meter), which is equivalent to J / (s * m * K) (Joule per second-Kelvin meter).
Thermal conductivity
It is an intense physical quantity, that is, a quantity that describes the property of matter, independent of the amount of the latter. Temperature, pressure, electrical conductivity are also intense quantities, 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. So, a thermal conductivity of one watt per meter-Kelvin means that thermal energy of one Joule is transferred through the material:
- in one second;
- across an area of \u200b\u200bone square meter;
- at a distance of one meter;
- when the temperature difference on surfaces located at a distance of one meter from each other in the material is 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 is 10,000 times better at transferring heat than polyurethane, the thermal conductivity of which is 0.035 W / (m * K).
Molecular heat transfer
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 vibrate more intensively 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 an exchange of kinetic energy between particles (atoms and molecules) of a substance, without the transfer of the latter.
This explanation of the mechanism of thermal conductivity 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, while liquids and gases, if there are temperature gradients in them, transfer heat mainly due to convection processes.
Thermal conductivity of materials
Metals have a pronounced ability to conduct heat. Polymers are characterized by low thermal conductivity, and some of them practically do not conduct heat, for example, fiberglass, such materials are called heat insulators. For this or that heat flux through space to exist, there must be some substance in this space, therefore, in open space (empty space), thermal conductivity is zero.
Each homogeneous (homogeneous) material is characterized by a coefficient of thermal conductivity (denoted by the Greek letter lambda), that is, a value that determines how much heat needs to be transferred through an area of \u200b\u200b1 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 changes depending on its temperature, therefore this coefficient is measured, as a rule, at room temperature (300 K) to compare the characteristics of different substances.
If the material is inhomogeneous, for example, reinforced concrete, then the concept of a useful thermal conductivity coefficient is introduced, which is measured according to the coefficients of 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;
- wood 0.13;
- paraffin 0.21;
- brick 0.80;
- diamond 2300.
From the data under consideration, it can be seen that the thermal conductivity of metals is much higher than that of non-metals. The exception is diamond, which has a heat transfer coefficient five times that of copper. This property of diamond is associated with strong covalent bonds between 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. And also this property is used in special devices to distinguish a real diamond from a fake.
Some industrial processes try to increase the ability to transfer heat, which is achieved either by good conductors or by increasing the contact area between the components of the structure. Examples of such designs are heat exchangers and heat dissipators. In other cases, on the contrary, they try to reduce the thermal conductivity, which is achieved by using heat insulators, voids in structures and reducing the contact area of \u200b\u200bthe elements.
Heat transfer coefficients of steels
The ability to transfer heat to steels depends on two main factors: composition and temperature.
With an increase in carbon content, simple carbon steels reduce their specific gravity, in accordance with which their ability to transfer heat from 54 to 36 W / (m * K) also decreases when the percentage of carbon in steel changes from 0.5 to 1.5%.
Stainless steels contain chromium (10% or more), which together with carbon form complex carbides that prevent the oxidation of the material, and also increases the electrode potential of the metal. The thermal conductivity of stainless steel is low in comparison with other steels and ranges from 15 to 30 W / (m * K), depending on its composition. Heat-resistant chromium-nickel steels have even lower values \u200b\u200bof this coefficient (11-19 W / (m * K).
Another class is galvanized steels with a specific gravity of 7,850 kg / m3, which are obtained by coating steel with iron and zinc. Since zinc conducts heat more easily than iron, the thermal conductivity of galvanized steel will be relatively high compared to other grades 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 08X13, an increase in temperature from 100 to 900 ° C does not change its thermal conductivity coefficient (27-28 W / (m * K).
Factors affecting the physical quantity
The ability to conduct heat depends on a number of factors, including temperature, structure, and electrical properties of the substance.
Material temperature
The effect of temperature on the ability to conduct heat is different for metals and non-metals. In metals, conductivity is mainly associated with free electrons. According to the Wiedemann-Franz law, the thermal conductivity of a metal is proportional to the product of the absolute temperature, expressed in Kelvin, by its electrical conductivity. In pure metals, electrical conductivity decreases with increasing temperature, so the thermal conductivity remains approximately constant. In the case of alloys, the electrical conductivity changes little with increasing temperature, so the thermal conductivity of the alloys increases in proportion to the temperature.
On the other hand, heat transfer in non-metals is mainly associated with lattice vibrations and exchange of lattice phonons. With the exception of crystals of high quality and low temperatures, the path of phonons in the lattice does not decrease significantly at high temperatures, therefore the thermal conductivity remains constant over the entire temperature range, that is, it is insignificant. At temperatures below the Debye temperature, the ability of non-metals to conduct heat, along with their heat capacity, is significantly reduced.
Phase transitions and structure
When a material undergoes a first-order phase transition, for example, from a solid to a liquid state or from a liquid to a gas, then its thermal conductivity may change. A striking example of such a change is the difference in this physical quantity for ice (2.18 W / (m * K) and water (0.90 W / (m * K)).
Changes in the crystal structure of materials also affect thermal conductivity, which is explained by the anisotropic properties of various allotropic modifications of a substance of the same composition. Anisotropy affects the different scattering intensity of lattice phonons, the main heat carriers in non-metals, and in different directions in the crystal. Here a striking example is sapphire, the conductivity of which varies from 32 to 35 W / (m * K) depending on the direction.
Electrical conductivity
Thermal conductivity in metals changes with electrical conductivity according to the Wiedemann-Franz law. This is due to the fact that valence electrons, freely moving along the crystal lattice of a 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 non-metals, lattice phonons play the main role in the mechanism of heat transfer).
Convection process
Air and other gases are generally good heat insulators in the absence of convection. This is the principle behind the operation of many thermal insulation materials containing a large number of small voids and pores. This structure does not allow convection to propagate over long distances. Examples of such human-made materials are polystyrene and silicide airgel. In nature, heat insulators such as animal skins and bird plumage work on the same principle.
Light gases such as hydrogen and gel have high thermal conductivity values, while heavy gases such as argon, xenon and radon are poor heat conductors. For example, argon, an inert gas that is heavier than air, is often used as a heat insulating filler gas in double windows and light bulbs. The exception is sulfur hexafluoride (SF6), which is a heavy gas and has a relatively high thermal conductivity due to its high heat capacity.