Warm- this is one of the forms of energy that is contained in the movement of atoms in matter. We measure the energy of this movement with a thermometer, although not directly.
Like all other types of energy, heat can be transferred from body to body. This always happens when there are bodies of different temperatures. Moreover, they do not even have to be in contact, as there are several ways to transfer heat. Namely:
Thermal conductivity. This is the transfer of heat through direct contact of two bodies. (There can be only one body if its parts are of different temperatures.) Moreover, the greater the temperature difference between the bodies and the larger the area of their contact, the more heat is transferred every second. In addition, the amount of heat transferred depends on the material - for example, most metals conduct heat well, but wood and plastic are much worse. The quantity characterizing this ability to transfer heat is also called thermal conductivity (more correctly, thermal conductivity coefficient), which can lead to some confusion.
If it is necessary to measure the thermal conductivity of a material, then this is usually carried out in the following experiment: a rod is made from the material of interest and one end is maintained at one temperature and the other at a different, for example lower, temperature. Let, for example, cold end will be placed in water with ice - in this way a constant temperature will be maintained, and by measuring the rate of ice melting one can judge the amount of heat received. Dividing the amount of heat (or rather, power) by the temperature difference and the cross-section of the rod and multiplying by its length, we obtain the thermal conductivity coefficient, measured, as follows from the above, in J * m / K * m 2 * s, that is, in W / K*m. Below you see a table of the thermal conductivity of some materials.
| Material | Thermal conductivity, W/(m K) |
| Diamond | 1001—2600 |
| Silver | 430 |
| Copper | 401 |
| Beryllium oxide | 370 |
| Gold | 320 |
| Aluminum | 202—236 |
| Silicon | 150 |
| Brass | 97—111 |
| Chromium | 107 |
| Iron | 92 |
| Platinum | 70 |
| Tin | 67 |
| Zinc oxide | 54 |
| Steel | 47 |
| Aluminum oxide | 40 |
| Quartz | 8 |
| Granite | 2,4 |
| Solid concrete | 1,75 |
| Basalt | 1,3 |
| Glass | 1-1,15 |
| Thermal paste KPT-8 | 0,7 |
| Water under normal conditions | 0,6 |
| Construction brick | 0,2—0,7 |
| Wood | 0,15 |
| Petroleum oils | 0,12 |
| Fresh snow | 0,10—0,15 |
| Glass wool | 0,032-0,041 |
| Stone wool | 0,034-0,039 |
| Air (300 K, 100 kPa) | 0,022 |
As can be seen, thermal conductivity differs by many orders of magnitude. Diamond and some metal oxides conduct heat surprisingly well (compared to other dielectrics); air, snow, and KPT-8 thermal paste conduct heat poorly.
But we are accustomed to thinking that air conducts heat well, but cotton wool does not, although it may consist of 99% air. The point is convection. Hot air is lighter than cold air and “floats” to the top, generating constant air circulation around a heated or very cooled body. Convection improves heat transfer by an order of magnitude: without it, it would be very difficult to boil a pan of water without constantly stirring it. And in the range from 0°C to 4°C water when heated shrinks, which leads to convection in the opposite direction from the usual one. This leads to the fact that, regardless of the air temperature, at the bottom of deep lakes the temperature is always set at 4°C
To reduce heat transfer, air is pumped out from the space between the walls of thermoses. But it should be noted that the thermal conductivity of air depends little on pressure up to 0.01 mm Hg, that is, the limit of deep vacuum. This phenomenon is explained by the theory of gases.
Another method of heat transfer is radiation. All bodies emit energy in the form electromagnetic waves, but only those that are sufficiently heated (~600°C) emit in the range visible to us. The radiation power, even at room temperature, is quite high - about 40 mW per 1 cm 2. In terms of the surface area of the human body (~1m2), this will be 400W. The only saving grace is that in our usual environment, all the bodies around us also emit with approximately the same power. The radiation power, by the way, strongly depends on temperature (as T 4), according to the law Stefan-Boltzmann. Calculations show that, for example, at 0°C the power of thermal radiation is approximately one and a half times weaker than at 27°C.
Unlike thermal conductivity, radiation can propagate in a complete vacuum - it is thanks to it that living organisms on Earth receive the energy of the Sun. If heat transfer by radiation is undesirable, then it is minimized by placing opaque partitions between cold and hot objects, or the absorption of radiation (and emission, by the way, to exactly the same extent) is reduced by covering the surface with a thin mirror layer of metal, for example, silver.
- Data on thermal conductivity were taken from Wikipedia, and they got there from reference books such as:
- "Physical quantities" ed. I. S. Grigorieva
- CRC Handbook of Chemistry and Physics
- A more rigorous description of thermal conductivity can be found in a physics textbook, for example, in “General Physics” by D.V. Sivukhin (Volume 2). In volume 4 there is a chapter devoted to thermal radiation (including the Stefan-Boltzmann law)
Page 3
The thermal conductivity of the enamel coating, even with ordinary enamel, is quite low, - 0 8 - 1 0 Watt per meter degree. For comparison: the thermal conductivity of iron is 65; steel - 70 - 80; copper - 330 watts per meter degree. If there are gas bubbles in the enamel, which leads to a decrease in its apparent density, thermal conductivity decreases. For example, with an apparent density of enamel of 2.48 grams per cubic centimeter, the thermal conductivity is equal to 1.18 Watts per meter degree, then with an apparent density of 2.20 grams per cubic centimeter, the thermal conductivity is already equal to 0.46 Watts per meter degree.
The crystal lattice of aluminum consists, like many other metals, of face-centered cubes (see page. The thermal conductivity of aluminum is twice the thermal conductivity of iron and equal to half the thermal conductivity of copper. Its electrical conductivity is much higher than the electrical conductivity 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 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 approximately six times greater than the thermal conductivity of iron. The strength of copper decreases so much that even with light impacts, cracks form. Copper melts at a temperature of 1083 C.
The modulus of elasticity of titanium is almost half that of iron, is on par 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 metal for pressure treatment and welding. The electrical resistance of titanium is approximately 6 times greater than that of iron and 20 times greater than that of aluminum.
The modulus of elasticity of titanium is almost half that of iron, is on par 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 chemical reagents, with the exception of strong oxidizing agents. 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 point of iron is 1535 C, combustion 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 -mol, 3Fe 2O2 Fe3O4 H - 266 9 kcal / g-mol, 2Fe 1 5O2 Fe2O3 198 5 kcal / g-mol, and the thermal conductivity of iron is limited.
Titanium and its alloys, due to their high physical properties, chemical properties It is increasingly used as a structural material for aviation and rocket technology, chemical engineering, instrument making, shipbuilding and mechanical engineering, in food and other industries. Titanium is almost two times lighter than steel, its density is 4 5 g/cm3, it has high mechanical properties, corrosion resistance at normal and high temperatures and in many active environments, the thermal conductivity of titanium is almost four times less than the thermal conductivity of iron.
One of these solutions is that a pipe wound onto a cooled surface is only welded to this surface, after which the joint between the pipe and the casing is coated with epoxy resin mixed with iron powder. The thermal conductivity of the mixture is close to the thermal conductivity of iron. As a result, good thermal contact is created between the casing and the pipe, which improves the cooling conditions of the casing.
All these conditions are satisfied by iron and carbon steels. The oxides FeO and Fe304 melt at temperatures of 1350 and 1400 C. The thermal conductivity of iron is not high compared to other structural materials.
For metals operating at low temperatures, it is also very important how their thermal conductivity changes with temperature changes. 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 Xc - 10 C - at -243 C .
The most widely used soldering is with a soldering iron, gas torches, immersion in molten solder and in ovens. The limitations in its use are caused only by the fact that a soldering iron can only solder thin-walled parts at a temperature of 350 C. Massive parts, due to their high thermal conductivity, which is 6 times greater than the thermal conductivity of iron, are soldered with gas torches. For tubular copper heat exchangers, soldering by immersion in molten salts and solders is used. When soldering by immersion in molten salts, salt bath furnaces are usually used. Salts usually serve as a source of heat and have a fluxing effect, so additional fluxing is not required during soldering. In dip soldering, pre-fluxed parts are heated in molten solder, which fills the joint gaps at soldering temperature. Solder mirror protect activated carbon or inert gas. The disadvantage of soldering in salt baths is that in some cases it is impossible to remove residual salts or flux.
Introduction
Determining the thermal conductivity of metals plays an important role in some areas, for example, in metallurgy, radio engineering, mechanical engineering, and construction. Currently, there are many different methods that can be used to determine the thermal conductivity of metals.
This work is devoted to the study of the main property of metals - thermal conductivity, as well as the study of methods for studying thermal conductivity.
The object of the study is the thermal conductivity of metals, as well as various methods of laboratory research.
The subject of the study is the thermal conductivity coefficients of metals.
Planned result - production laboratory work“Determination of the thermal conductivity coefficient of metals” based on the calorimetric method.
To achieve this goal, it is necessary to solve the following tasks:
Study of the theory of thermal conductivity of metals;
Study of methods for determining the thermal conductivity coefficient;
Selection of laboratory equipment;
Experimental determination of the thermal conductivity coefficient of metals;
Setting up laboratory work “Determination of the thermal conductivity coefficient of metals.”
The work consists of three chapters in which the assigned tasks are revealed.
Thermal conductivity of metals
Fourier's law
Thermal conductivity is the molecular transfer of heat between directly contacting bodies or particles of the same body with different temperatures, during which the energy of movement of structural particles (molecules, atoms, free electrons) is exchanged.
Thermal conductivity is determined by the thermal movement of microparticles of the body.
The basic law of heat transfer by thermal conductivity is Fourier's law. According to this law, the amount of heat dQ transferred by thermal conductivity through a surface element dF perpendicular to the heat flow during time df is directly proportional to the temperature gradient, surface dF and time df.
The proportionality coefficient l is called the thermal conductivity coefficient. The thermal conductivity coefficient is a thermophysical characteristic of a substance that characterizes the ability of a substance to conduct heat.
The minus sign in formula (1) indicates that heat is transferred in the direction of decreasing temperature.
The amount of heat passed per unit time through a unit of isothermal surface is called heat flux:
Fourier's law is applicable to describe the thermal conductivity of gases, liquids and solids; the difference will only be in the thermal conductivity coefficients.
Thermal conductivity coefficient of metals and its dependence on the parameters of the state of the substance
The thermal conductivity coefficient is a thermophysical characteristic of a substance that characterizes the ability of a substance to conduct heat.
Thermal conductivity coefficient is the amount of heat passing per unit time through a unit area, perpendicular to grad t.
For different substances, the thermal conductivity coefficient is different and depends on the structure, density, humidity, pressure and temperature. These circumstances must be taken into account when using lookup tables.
Highest value has the thermal conductivity coefficient of metals for which. The most thermally conductive metal is silver, followed by pure copper, gold, aluminum, etc. For most metals, an increase in temperature leads to a decrease in the thermal conductivity coefficient. This dependence can be approximately approximated by the straight line equation
here l, l0 are, respectively, the thermal conductivity coefficients at a given temperature t and at 00C, β is the temperature coefficient. The thermal conductivity coefficient of metals is very sensitive to impurities.
For example, when even traces of arsenic appear in copper, its thermal conductivity coefficient decreases from 395 to 142; for steel at 0.1% carbon l = 52, at 1.0% - l = 40, at 1.5% carbon l = 36.
The thermal conductivity coefficient is also affected by heat treatment. So, for hardened carbon steel, l is 10 - 25% lower than for soft steel. For these reasons, the thermal conductivity coefficients of commercial metal samples at the same temperatures can vary significantly. It should be noted that alloys, unlike pure metals, are characterized by an increase in thermal conductivity with increasing temperature. Unfortunately, it has not yet been possible to establish any general quantitative patterns that govern the thermal conductivity of alloys.
The thermal conductivity coefficient of building and heat-insulating materials - dielectrics is many times less than that of metals and amounts to 0.02 - 3.0. For the vast majority of them (the exception is magnesite brick), the thermal conductivity coefficient increases with increasing temperature. In this case, you can use equation (3), keeping in mind that for solids - dielectrics, β>0.
Many building and thermal insulation materials have a porous structure (brick, concrete, asbestos, slag, etc.). For them and powdered materials, the thermal conductivity coefficient depends significantly on the bulk density. This is due to the fact that with increasing porosity, most of the volume is filled with air, the coefficient of thermal conductivity of which is very low. At the same time, the higher the porosity, the lower the bulk density of the material. Thus, a decrease in the bulk density of a material, other things being equal, leads to a decrease in l.
For example, for asbestos, a decrease in bulk density from 800 kg/m to 400 kg/m results in a decrease from 0.248 to 0.105. The influence of humidity is very great. For example, for dry brick l = 0.35, for liquid 0.6, and for wet brick l = 1.0.
These phenomena must be paid attention to when determining and technical calculations of thermal conductivity. The thermal conductivity coefficient of droplet liquids lies in the range of 0.08 - 0.7. At the same time, for the vast majority of liquids, the thermal conductivity coefficient decreases with increasing temperature. The exceptions are water and glycerin.
The thermal conductivity of gases is even lower.
The thermal conductivity coefficient of gases increases with increasing temperature. Within the range of 20 mmHg. up to 2000 at (bar), i.e. in the area that is most often encountered in practice, l does not depend on pressure. It should be borne in mind that for a mixture of gases (flue gases, the atmosphere of thermal furnaces, etc.) it is impossible to determine the thermal conductivity coefficient by calculation. Therefore, in the absence of reference data, a reliable value of l can only be found experimentally.
At value l< 1 - вещество называют тепловым изолятором.
To solve problems of thermal conductivity, it is necessary to have information about some macroscopic properties (thermophysical parameters) of a substance: thermal conductivity coefficient, density, specific heat capacity.
Explanation of thermal conductivity of metals
The thermal conductivity of metals is very high. It is not reduced to the thermal conductivity of the lattice; therefore, another heat transfer mechanism must operate here. It turns out that in pure metals thermal conductivity is carried out almost entirely due to the electron gas, and only in heavily contaminated metals and alloys, where the conductivity is low, the contribution of lattice thermal conductivity turns out to be significant.
The numerical characteristic of the thermal conductivity of a material can be determined by the amount of heat passing through a material of a certain thickness in a certain time. The numerical characteristic is important when calculating the thermal conductivity of various profile products.
Thermal conductivity coefficients of various metals
For thermal conduction to occur, direct physical contact between two bodies is required. This means that heat transfer is only possible between solids and stationary liquids. Direct contact allows kinetic energy to move from the molecules of the warmest substance to the coldest one. Heat exchange occurs when bodies of different temperatures come into direct contact with each other.
Here you should pay attention to the fact that molecules of a warm body cannot penetrate into a cold body. Only kinetic energy is transferred, which gives uniform heat distribution. This transfer of energy will continue until the bodies in contact become uniformly warm. In this case, thermal equilibrium is achieved. Based on this knowledge, it is possible to calculate what insulating material will be required for thermal insulation of a particular building.
Among large quantity parameters characterizing metals, there is such a thing as thermal conductivity. Its importance is difficult to overestimate. This parameter is used when calculating parts and assemblies. For example, gear transmissions. 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 areas of the body to cold ones. Transfer is carried out by freely moving elementary particles, which include atoms, electrons, etc.
The heat transfer process itself occurs in any body, but the method of energy transfer largely depends on state of aggregation bodies.
In addition to this, thermal conductivity can be given another definition - it is a quantitative parameter of a body’s ability to conduct thermal energy. If we compare thermal and electrical networks, this concept is similar to electrical conductivity.
The ability of a physical body to prevent the propagation of thermal vibrations of molecules is called thermal resistance. By the way, some are sincerely mistaken, confusing this concept with thermal conductivity.
The concept of thermal conductivity coefficient
The thermal conductivity coefficient is a value that is equal to the amount of heat transferred through a unit surface in one second.
The thermal conductivity of metal was established back in 1863. It was then that it was proven that free electrons, of which there are a great many in the metal, are responsible for the transfer of heat. This is why the thermal conductivity of metals is significantly higher than that of dielectric materials.
What does thermal conductivity depend on?
Thermal conductivity is a physical quantity and largely 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 compiled into reference tables, which are then used in various scientific and engineering calculations.
Bodies have different temperatures and during heat exchange it (temperature) will be distributed unevenly. In other words, you need to know how the thermal conductivity coefficient 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 determined by the shape of its crystal lattice.
In many ways, the thermal conductivity coefficient depends on the structure of the material, the size of its pores and humidity.
When is the thermal conductivity coefficient taken into account?
Thermal conductivity parameters must be taken into account when choosing materials for enclosing structures - walls, ceilings, etc. In rooms where the walls are made of materials with high thermal conductivity, it will be quite cool in the cold season. Decorating the room won't help either. In order to avoid this, the walls must be made quite thick. This will certainly lead to increased costs for materials and labor.
That is why the construction of the walls requires the use of materials with low thermal conductivity (mineral wool, polystyrene foam, etc.).
Indicators for steel
- In reference materials on thermal conductivity various materials A special place is occupied by the data presented on steels of different grades.
Thus, the reference materials contain experimental and calculated data for the following types of steel alloys:
resistant to corrosion and elevated temperatures; - intended for the production of springs and 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 classified as martensitic - from 30 to 45 W/(m×deg);
- alloys classified as austenitic from 12 to 22 W/(m×deg).
These reference materials contain information about the properties of cast iron.
Thermal conductivity coefficients of aluminum, copper and nickel alloys
When carrying out calculations related to non-ferrous metals and alloys, designers use reference materials, placed in special tables.
They present materials on the thermal conductivity of non-ferrous metals and alloys; in addition to these data, information about chemical composition alloys The studies were carried out at temperatures from 0 to 600 °C.
According to the information collected in these tabular materials, it is clear that non-ferrous metals with high thermal conductivity include 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 of application. For example, high thermal conductivity is good for kitchen utensils. Materials with low thermal conductivity are used to create permanent connections of metal parts. There are entire families of tin-based alloys.
Disadvantages of the high thermal conductivity of copper and its alloys
Copper has a much higher value 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 forces the creation of special conditions for its processing. That is, copper billets must be heated more accurately than steel. In addition, there is often pre- or auxiliary heating before starting treatment.
We must not forget that pipes made of copper imply that careful thermal insulation will be carried out. This is especially true for those cases when the heating supply system is assembled from these pipes. This significantly increases the cost of installation work.
Certain difficulties arise when using gas welding. To get the job done, a more powerful tool is required. 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 welds the copper pipe, and the rest are busy heating it. In addition, working with copper requires more consumables.
Working with copper requires the use of specialized tools. For example, when cutting parts made of bronze or brass with a thickness of 150 mm, you will need a cutter that can work with steel with a large amount of chrome. If it is used for processing copper, then the maximum thickness will not exceed 50 mm.
Is it possible to increase the thermal conductivity of copper?
Not long ago, a group of Western scientists conducted a series of studies to increase 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. To apply it, gas deposition technology was used. During the research, many instruments were used that were designed to confirm the objectivity of the results obtained.
Research results have shown that graphene has one of the highest thermal conductivities. After it was applied to a copper substrate, the thermal conductivity dropped slightly. But, during this process, the copper heats up and the grains in it increase, 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 purposes of copper is to remove excess heat from electronic and electrical diagrams. Using graphene deposition, this problem will be solved much more efficiently.
Effect of carbon concentration
Steels with 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 lies in the range of 47-54 W/(m×K).
Importance 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 exchange between the external and internal environment. This means that a building constructed from a material with low thermal conductivity will be warm in winter and cool in summer.
When constructing various buildings, including residential buildings, it is impossible to do without knowledge about 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 maximum thermal conductivity belongs to concrete, while for wood it is 6 times less.
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 radiators are used. They are necessary to transfer thermal energy into rooms.
- A heating radiator is a structure inside that moves 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. The whole point is that it 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 reference tables that determine the materials that are optimal for use in heating systems. From the data contained in them, it becomes clear that the most effective material considered copper. 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 being 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 heat output power.
On the heating systems market you can purchase radiators made of cast iron, steel, aluminum and bimetal.
Methods for studying thermal conductivity parameters
When studying thermal conductivity parameters, 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 a metal produced by casting may differ significantly from the characteristics of a material manufactured using powder metallurgy methods. The properties of raw metal are fundamentally different from those that have undergone 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 able to reach different levels recrystallization, and this is reflected in the thermal conductivity parameters.
We can say the following: when conducting studies of thermal conductivity parameters, it is necessary to use samples of metals and their alloys in a standard and specific technological state, for example, after heat treatment.
For example, there are requirements for grinding metal to conduct research using thermal analysis methods. Indeed, such a requirement exists in a number of studies. There are also such requirements - such as the production of special plates and many others.
The non-thermal stability of metals poses 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 of 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 thermal power introduced into the test sample.
This method can be called relative. It uses test and comparison samples. The main thing is that the samples have the same emitting surface. When conducting research, the temperature affecting the samples must change in steps, and upon reaching the specified parameters, it is necessary to maintain a certain amount of time. The direction of temperature change and its step must be selected in such a way that the sample intended for testing is heated evenly.
At these moments, the heat flows will be equal and the heat transfer ratio will be determined as the difference in the rates of temperature fluctuations.
Sometimes during these studies, the source of indirect heating of the test and comparative sample.
Additional thermal loads may be created on one of the samples in comparison with the second sample.
Which thermal conductivity measurement method 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 the sizes 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 study 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 expediency of its use primarily in electrical engineering. Copper produces conductors with excellent performance characteristics. Of course, this metal is used not only in electrical engineering, but also in other industries. Its demand is explained, among other things, by its qualities such as resistance to corrosion damage in a number of aggressive environments, refractoriness, ductility, etc.
Historical background
Copper is a metal known person since ancient times. The early acquaintance of people with this material is explained primarily by its wide distribution in nature in the form of nuggets. Many scientists believe that copper was the first metal recovered by man from oxygen compounds. Once upon a time, rocks were simply heated over a fire and cooled sharply, causing them to crack. Later, the reduction of copper began to be carried out on fires with the addition of coal and blowing with bellows. The improvement of this method ultimately led to the creation. Later, this metal began to be produced by the method of oxidative smelting of ores.
Copper: electrical conductivity of the material
In a quiet 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 to which a metal can pass through itself is called electrical conductivity. Its unit of measurement in the International SI is Siemens, defined as 1 cm = 1 ohm -1.
The electrical conductivity of copper is very high. In this indicator, it surpasses all base metals known today. Only silver passes 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 at the moment is the most common conductor of all used for industrial and domestic purposes.
Copper withstands stress very well and is also 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 in a heated state for a long time. In terms of prevalence as a current conductor, only aluminum can compete with this metal.
The 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, even today it is almost impossible to obtain completely pure Cu. Copper always contains various types of impurities. This could be, for example, silicon, iron or beryllium. Meanwhile, the more impurities in copper, the lower its electrical conductivity. For the manufacture of wires, for example, only sufficiently pure metal is suitable. According to 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 ductility of the material. The electrical conductivity of copper and sulfur is very different. This impurity does not conduct current at all. That is, it is a good insulator. However, sulfur has virtually no effect on the electrical conductivity of copper. The same applies to thermal conductivity. With antimony and arsenic the opposite picture is observed. These elements can significantly reduce the electrical conductivity of copper.
Alloys
Various types of additives can be used specifically to increase the strength of such a ductile material as copper. They also reduce its electrical conductivity. But their use can significantly extend the service life of various types of products.
Most often, Cd (0.9%) is used as an additive to increase the strength of copper. The result is cadmium bronze. Its conductivity is 90% of that of copper. Sometimes aluminum is also used as an additive instead of cadmium. The conductivity of this metal is 65% of that of copper. To increase the strength of wires, other materials and substances can be used in the form of additives - tin, phosphorus, chromium, beryllium. The result is bronze of a certain grade. The combination of copper and zinc is called brass.
Alloy characteristics
It may depend not only on the amount of impurities present in them, but also on other indicators. For example, as the heating temperature increases, the ability of copper to pass current through itself decreases. Even the method of its manufacture affects the electrical conductivity of such 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, the additives used and their quantity have the greatest influence on 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% P) | ||
The electrical conductivity of brass and copper is comparable. However, for the first metal this figure is, of course, slightly lower. But at the same time it is higher than that of bronzes. Brass is used quite widely 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 from brass.
High resistance copper alloys
Such conductor materials are mainly used in the manufacture of resistors, rheostats, measuring instruments and electric heating devices. The most commonly used copper alloys for this purpose are constantan and manganin. The resistivity of the first (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 thermal conductivity coefficient
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 connection between the thermal conductivity coefficient 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 = π 2 / 3 (k/e) 2 T, where y is the specific conductivity, k is the Boltzmann constant, e is the elementary charge.
Of course, a similar connection exists for a metal such as copper. Its thermal conductivity and electrical conductivity are very high. It is in second place after silver in both of these indicators.
Connection of copper and aluminum wires
IN lately Electrical equipment of increasingly higher power began to be used in everyday life and industry. During Soviet times, wiring was made mainly 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 refractoriness, is that during the oxidation process their conductive properties do not decrease.
Often when modernizing electrical networks, aluminum and copper wires have to be connected. This cannot be done directly. Actually, the electrical conductivity of aluminum and copper does not differ too much. But only for 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 oxidation film of aluminum has much greater resistance than that of copper. Therefore, the connection of these two types of conductors must be made exclusively through special adapters. These could be, for example, clamps containing a paste that protects metals from the appearance of oxide. This adapter option is usually used outdoors. Branch compressors are more often used indoors. Their design includes a special plate that eliminates direct contact between aluminum and copper. If such conductors are not available at home, instead of twisting the wires directly, it is recommended to use a washer and nut as an intermediate “bridge”.
Physical properties
Thus, we found out what electrical conductivity copper has. This indicator may vary depending on the impurities contained in the metal. However, the demand for copper in industry is also determined by its other useful properties. physical properties, information about which can be obtained from the table below.
Parameter | Meaning |
Face-centered cubic, a=3.6074 Å |
|
Atomic radius | |
Specific heat | 385.48 J/(kg K) at +20 o C |
Thermal conductivity | 394.279 W/(m K) at +20 o C |
Electrical resistance | 1.68 10-8 Ohm m |
Linear expansion coefficient | |
Hardness | |
Tensile strength |
Chemical properties
According to these characteristics, copper, whose electrical and thermal conductivity is very high, occupies an intermediate position between the elements of the first triad of the eighth group and the alkali elements of the first group of the periodic table. Its main chemical properties include:
tendency to form complexes;
ability to produce colored compounds and insoluble sulfides.
The most characteristic of copper is the divalent state. It has practically no similarity with alkali metals. Its chemical activity is also low. In the presence of CO 2 or moisture, a green carbonate film forms on the surface of the copper. All copper salts are toxic substances. In the mono- and divalent state, this metal forms very stable ammonia compounds that are of greatest importance for industry.
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 its application. Among other things, copper can be used:
in jewelry;
in architecture;
when assembling plumbing and heating systems;
in gas pipelines.
For the production of various types jewelry An alloy of copper and gold is mainly 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 communications
The main advantages of copper water pipes are also durability and reliability. In addition, this metal is capable of imparting special unique properties to water, making it beneficial for the body. For the assembly of gas pipelines and heating systems copper pipes are also ideally suited - mainly due to their corrosion resistance and ductility. In the event of an emergency increase in pressure, such lines can withstand a much greater load than steel ones. The only disadvantage of copper pipelines is their high cost.