Warmly- this is one of the forms of energy, which 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 forms of energy, heat can be transferred from body to body. This happens whenever there are bodies of different temperatures. At the same time, 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 between two bodies. (The body can be one if its parts are of different temperatures.) Moreover, the greater the temperature difference between the bodies and the greater 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, while wood and plastic are much worse. The value characterizing this ability to transfer heat is also called thermal conductivity (more correctly, the coefficient of thermal conductivity), which can lead to some confusion.
If it is necessary to measure the thermal conductivity of any 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, the cold end 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, which is 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 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 |
| Aluminium oxide | 40 |
| Quartz | 8 |
| Granite | 2,4 |
| solid concrete | 1,75 |
| Basalt | 1,3 |
| Glass | 1-1,15 |
| Thermal grease KPT-8 | 0,7 |
| Water under normal conditions | 0,6 |
| Building 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, the 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 do not conduct heat well.
But we are used to thinking that air conducts heat well, and cotton wool does not, although it can be 99% air. The thing is convection. Hot air is lighter than cold air and "floats" up, giving rise to a constant circulation of air around a hot or very cold body. Convection improves heat transfer by an order of magnitude: in its absence, it would be very difficult to boil a pot 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 to 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 boundaries of deep vacuum. This phenomenon is explained by the theory of gases.
Another method of heat transfer is radiation. All bodies radiate energy in the form of electromagnetic waves, but only sufficiently heated ones (~600°C) radiate in the visible range. The radiation power even at room temperature is quite large - about 40 mW s 1 cm 2 . In terms of the surface area of the human body (~ 1m 2), this will be 400W. The only thing that saves us is that in the environment we are used to, all the bodies around us also radiate with approximately the same power. The radiation power, by the way, strongly depends on temperature (like T 4), according to the law Stefan-Boltzmann. Calculations show that, for example, at 0°С, the power of thermal radiation is approximately one and a half times weaker than at 27°С.
Unlike heat conduction, 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 is reduced (and emission, by the way, to the same extent), covering the surface with a thin mirror layer of metal, for example, silver.
- The data on thermal conductivity are 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 textbook on physics, for example, in "General Physics" by D.V. Sivukhin (Volume 2). Volume 4 has a chapter on thermal radiation (including the Stefan-Boltzmann law)
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 application primarily in electrical engineering. Copper makes 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.
History reference
Copper is a metal known to man 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 it was copper that was the first metal recovered by man from oxygen compounds. Once upon a time, rocks were simply heated on a fire and cooled sharply, as a result of which they cracked. Later, the recovery of copper began to be carried out on fires with the addition of coal and blowing with bellows. The improvement of this method eventually led to the creation. Even later, this metal began to be obtained by the oxidative smelting of ores.
Copper: electrical conductivity of the material
At rest, 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 ability of a metal to pass the latter through itself is called electrical conductivity. The unit of its measurement in the International SI is siemens, defined as 1 cm = 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 current better than it. The electrical conductivity index 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 used for industrial and domestic purposes.
Copper perfectly withstands permanent and is also distinguished by reliability and durability. 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 antiquity. However, even today it is practically impossible to obtain completely pure Cu. There are always various kinds of impurities in copper. It can 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 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 is very different. This impurity does not conduct current at all. That is, it is a good insulator. However, sulfur has almost no effect on the electrical conductivity of copper. The same applies to thermal conductivity. With antimony and arsenic, the reverse picture is observed. These elements can significantly reduce the electrical conductivity of copper.
Alloys
Various additives can also be used specifically to increase the strength of such a plastic material 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.
Most often, Cd (0.9%) is used as an additive that increases the strength of copper. The result is cadmium bronze. Its conductivity is 90% 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 in the form of an additive, other materials and substances can be used - tin, phosphorus, chromium, beryllium. The result is bronze of a certain grade. The combination of copper and zinc is called brass.
Alloy characteristics
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 way it is made affects 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. In the first variety, the ability to pass current through itself is higher.
However, the most influence, of course, the additives used and their amount 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) | 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, 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 transmits 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 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 one (86% Cu, 12% Mn, 2% Ni) is 0.42-0.48 µOhm/m, and the second one (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, but only at a temperature of +20 o C. There is a certain relationship between the thermal conductivity of any metal and the specific conductivity. Establishes his 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, e is the elementary charge.
Of course, there is a similar connection with 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
Recently, electrical equipment of ever higher power has begun to be used in everyday life and industry. In Soviet times, wiring was made mainly from cheap aluminum. Unfortunately, its operational characteristics no longer correspond to 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 oxidative process.
Often, when modernizing electrical networks, 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 for these metals themselves. Oxidation films of aluminum and copper have different properties. Because of this, the conductivity at the junction is significantly reduced. The oxidation film of aluminum is much more resistant than that of copper. Therefore, the connection of these two types of conductors should be made exclusively through special adapters. These can be, for example, clamps containing a paste that protects metals from the appearance of oxide. This version of the adapters is usually used when outdoors. Branch clamps are more often used indoors. Their design includes a special plate that excludes direct contact between aluminum and copper. In the absence of such conductors in domestic conditions, 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 the electrical conductivity of copper is. This indicator may 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, 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 ° 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 alkaline elements of the first group of the periodic table. Its main chemical properties include:
tendency to complex formation;
the ability to give colored compounds and insoluble sulfides.
The most characteristic of copper is the divalent 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 divalent state, this metal forms very stable ones. Ammonia metals are of the greatest importance for industry.
Scope of use
The high thermal and electrical conductivity of copper determines its wide application in various industries. Of course, most often this metal is 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 manufacture of various kinds of 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 been reliably protecting its interior space for almost 700 years.
Engineering Communication
The main advantages of copper plumbing are also durability and reliability. In addition, this metal is able to give water special unique properties, making it useful for the body. For the assembly of gas pipelines and heating systems, copper pipes are also ideal - 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.
Page 3
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 is 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 of 2.48 grams per cubic centimeter, the thermal conductivity is 1.18 watts per meter degree, then with an apparent density of 2.20 grams per cubic centimeter, the thermal conductivity is already 0.46 watts per meter degree.
The crystal lattice of aluminum consists, like that of many other metals, of face-centered cubes (see p. 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 shrinkage cavities. The thermal conductivity of the alloy is about half the thermal conductivity of iron, which should be taken into account in the manufacture of 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. With the strength of copper is so reduced that cracks form even with light impacts. Copper melts at a temperature of 1083 C.
The modulus of elasticity of titanium is almost half that of iron, is on the same level with the modulus of copper alloys and is much higher than that of aluminum. The thermal conductivity of titanium is low: it is about 7% of the thermal conductivity of aluminum and 165% of the thermal conductivity of iron. This must be taken into account when heating metal for forming and welding. The electrical resistance of titanium is about 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 the same level with the modulus of copper alloys and is much 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, the combustion 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 -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 and chemical properties, are increasingly being used as a structural material for aviation and rocket technology, chemical engineering, instrumentation, shipbuilding and mechanical engineering, in 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 the thermal conductivity of iron.
One such solution is that the pipe wound on the cooled surface is only welded to this surface, after which the pipe-to-shell joint is coated with epoxy resin mixed with iron powder. The thermal conductivity of the mixture is close to that of iron. The result is a good thermal contact between the shell and the pipe, which improves the cooling conditions of the shell.
All these conditions are met by iron and carbon steels. FeO and Fe304 oxides 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. The thermal conductivity of steel increases with decreasing temperature. Pure iron is very sensitive to temperature change. 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 - 19C - at - 173 C and 0 85 cal cm - x Xs - 10C - at -243 C .
Soldering with a soldering iron, gas burners, immersion in molten solder and in furnaces is the most widely used soldering. Limitations in its use 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 the thermal conductivity of iron, are soldered with gas burners. For tubular copper heat exchangers, soldering by immersion in molten salts and solders is used. When soldering by immersion in salt melts, as a rule, salt bath furnaces are used. Salts are usually a source of heat and have a fluxing effect, so additional fluxing is not required when soldering. In bath brazing, pre-fluxed parts are heated in a solder melt that fills the joint gaps at the soldering temperature. The solder mirror is protected by activated carbon or inert gas. The disadvantage of soldering in salt baths is the impossibility in some cases of removing salt or flux residues.
The high thermal conductivity of copper and its other useful characteristics were one of the reasons for the early development of this metal by man. And to this day they find application in almost all areas of our life.
A little about thermal conductivity
In physics, thermal conductivity is understood as the movement of energy in an object from more heated particles to less heated ones. Thanks to this process, the temperature of the object in question as a whole is leveled. The value of the ability to conduct heat is characterized by the coefficient of thermal conductivity. This parameter is equal to the amount of heat that passes through a material 1 meter thick through a surface area of 1 m2 for one second at a unit temperature difference.
Copper has a thermal conductivity of 394 W / (m * K) at a temperature of 20 to 100 ° C. Only silver can compete with it. And for steel and iron, this figure is 9 and 6 times lower, respectively (see table). It should be noted that the thermal conductivity of products made of copper is largely dependent on impurities (however, this also applies to other metals). For example, the rate of heat conduction decreases if substances such as:
- iron;
- arsenic;
- oxygen;
- selenium;
- aluminum;
- antimony;
- phosphorus;
- sulfur.
If you add zinc to copper, you get brass, which has a much lower thermal conductivity. At the same time, the addition of other substances to copper can significantly reduce the cost of finished products and give them such characteristics as strength and wear resistance. For example, brass is characterized by higher technological, mechanical and anti-friction properties.
Since high thermal conductivity is characterized by the rapid distribution of heating energy throughout the object, copper has been widely used in heat transfer systems. At the moment, radiators and pipes for refrigerators, vacuum plants and cars are made from it to quickly remove heat. Also, copper elements are used in heating installations, but already for heating.
In order to maintain the thermal conductivity of the metal at a high level (and, therefore, to make the operation of copper devices as efficient as possible), forced airflow by fans is used in all heat exchange systems. This decision is due to the fact that with an increase in the temperature of the medium, the thermal conductivity of any material decreases significantly, because heat transfer slows down.
Aluminum and copper - which is better?
Aluminum has one disadvantage compared to copper: its thermal conductivity is 1.5 times less, namely 201–235 W / (m * K). However, compared to other metals, these values are quite high. Aluminum, like copper, has high anti-corrosion properties. In addition, it has advantages such as:
- low density (specific gravity is 3 times less than that of copper);
- low cost (3.5 times less than copper).
Thanks to simple calculations, it turns out that an aluminum part can be almost 10 times cheaper than a copper one, because it weighs much less and is made of a cheaper material. This fact, along with high thermal conductivity, allows the use of aluminum as a material for dishes and food foil for ovens. The main disadvantage of aluminum is that it is softer, so it can only be used in alloys (for example, duralumin).
For efficient heat transfer, the rate of heat transfer to the environment plays an important role, and this is actively promoted by blowing radiators. As a result, the lower thermal conductivity of aluminum (relative to copper) is leveled, and the weight and cost of equipment are reduced. These important advantages allow aluminum to gradually replace copper from use in air conditioning systems.
In some industries, such as radio and electronics, copper is indispensable. The fact is that this metal is inherently very plastic: it can be drawn into an extremely thin wire (0.005 mm), as well as create other specific conductive elements for electronic devices. And the high thermal conductivity allows copper to very effectively remove the heat that inevitably occurs during the operation of electrical appliances, which is very important for modern high-precision, but at the same time compact technology.
The use of copper is relevant in cases where it is required to make a surfacing of a certain shape on a steel part. In this case, a copper template is used, which is not connected to the element to be welded. The use of aluminum for these purposes is impossible, as it will be melted or burned through. It is also worth mentioning that copper is able to act as a cathode in carbon arc welding.
1 - gear, 2 - fastening templates, 3 - deposited gear tooth, 4 - copper templates
Disadvantages of high thermal conductivity of copper and its alloys
Copper is much more expensive than brass or aluminum. At the same time, this metal has its drawbacks, 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. Preheating and afterheating of the part is also often required.
Do not forget that copper pipes require careful insulation if they consist of a main or heating system wiring. Which leads to an increase in the cost of installing the network in comparison with options when other materials are used.
Difficulties also arise with copper: this process will require more powerful burners. When welding metal with a thickness of 8–10 mm, two or three torches will be required. While one torch is being 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, for cutting up to 15 cm thick, you will need a cutter that can work with high-chromium steel 30 cm thick. Moreover, the same tool is enough to work with a thickness of only 5 cm.
The table shows the density of iron d, as well as the values of its specific heat capacity Cp, thermal diffusivity a, thermal conductivity coefficient λ , electrical resistivity ρ , Lorentz functions L/L 0 at various temperatures - in the range from 100 to 2000 K.
The properties of iron significantly depend on temperature: when this metal is heated, its density, thermal conductivity and thermal diffusivity decrease, and the value of the specific heat capacity of iron increases.
The density of iron is 7870 kg / m 3 at room temperature. When iron is heated, its density decreases. Since iron is the main element in the composition of steel, the density of iron also determines the value. The dependence of the density of iron on temperature is weak - when it is heated, the density of the metal decreases and takes a minimum value of 7040 kg / m 3 at a melting point of 1810 K or 1537 ° C.
The specific heat capacity of iron, according to the table, is 450 J / (kg deg) at a temperature of 27°C. Depending on the structure, the specific heat capacity of solid iron changes differently with increasing temperature. The values in the table show a characteristic maximum of the heat capacity of iron near T c and jumps during structural transitions and during melting.
In the molten state, the properties of iron undergo changes. So, the density of liquid iron decreases and becomes equal to 7040 kg / m 3. The specific heat capacity of iron in the molten state is 835 J/(kg deg), while the thermal conductivity of iron decreases to 39 W/(m deg). In this case, the specific electrical resistance of this metal increases and at 2000 K takes the value of 138·10 -8 Ohm·m.
The thermal conductivity of iron at room temperature is 80 W / (m deg). With increasing temperature, the thermal conductivity of iron decreases - it has a negative temperature coefficient in the temperature range of 100-1042 K, and then begins to grow slightly. The minimum value of the thermal conductivity of iron is 25.4 W/(m deg) near the Curie point. During the β-γ transition, a slight change in thermal conductivity is observed, which also takes place during the γ-δ transition.
The thermal conductivity of iron drops sharply as the amount of impurities increases., especially and . Very pure electrolytic iron has the highest thermal conductivity - its thermal conductivity at 27 ° C is 95 W / (m deg).
The dependence of the thermal conductivity of iron on temperature is also determined by the degree of purity of this metal. The purer the iron, the higher its thermal conductivity and the more in absolute value it decreases with increasing temperature.
