Thermo-electromagnetic conductivity of metals and alloys
Thermo-electromagnetic conductivity (TEMC) measurement is an effective method of express analysis of metals and alloys. Analyzing metals and alloys TEMC can be done in using two types of methods:
- Methods based on dependence of the alloy electromagnetic resistance Z (T, ξ) on the alloy temperature T and the alloy structure ξ.
- Differential thermodynamics methods.
Alloy electromagnetic resistance is usually analyzed as a function of temperature together with the first and second derivatives of this dependence. The goal here is to express the analysis of changes, both in chemical composition and in crystal structure of the alloy. Main advantages of this method are high sensitivity and considerable volume of the received information.
Differential thermodynamics methods analyze the dependence T (t, ξ): temperature T of alloy in time t during alloy cooling or heating. Also with this approach we often investigate the first and second derivative of temperature change. The goal of this method is to determine the exact temperature when the alloy crystal structure changes and analyze the influence of temperature change speed on this process.
Both of the above methods supplement each other and provide substantial scientific information. Let us have a more in depth looks at both methods.
Electromagnetic resistance of metals and alloys
Metals conduct an electric current being electrons conductors. It means that inside metals, both in a solid and in a liquid state, electronic gas electrons move, creating an electric current. Electromagnetic resistance of metal is caused by chaotic movement of atoms (in melted form) or fluctuations around their position in a crystal form. The amplitude and speed of these movements grow with the temperature. Therefore the growth of electromagnetic resistance with temperature is an essential property of every metal.
Resistance of pure metals, for instance, increases 0.4 % per degree C on the average. The total range of temperatures contains parts where this dependence is close to linear. Resistance growth with temperature significantly decreases at the edges of a temperature range. In high alloys the derivative dependence of resistance on temperature can decrease thousand times at some temperatures.
When little change of the alloy temperature has a significant effect on its electromagnetic resistance, it indicates that either the melt composition or the structure of the solid metal has changed. For example, when certain components of metal alloy burn, the electromagnetic resistance of this alloy changes dramatically with a little change of temperature. When the alloy crystal structure changes, its electromagnetic resistance change jumps with minor temperature change. Such changes can therefore indicate alterations of melt or a crystal structure of the alloy.
During cooling or heating of an alloy its crystal structure changes. This change of crystal structure also takes place during alloy machining or annealing. During recrystallization points the resistance – time dependence undergoes a break, changes jump. This means that the Z (T, ξ) function can offer significant information on metal condition and structure.
Differential thermodynamics of metals and alloys
Function T (t, ξ) (differential thermodynamics) is used to investigate melt crystallization and recrystallization process. It is well-known that melt crystallization process is accompanied by heat emission. This so called crystallization heat causes a reduction in the decrease of melt temperature and the melt temperature remains constant for some time. Some temperature standards are based on this phenomenon when the mix of the melt and crystal copper, aluminum, gold, zinc, tin maintains the same precision temperature for a long period of time.
The same phenomenon takes place during metal recrystallization. Only in this case the recrystallization heat is much lower. We have to analyze the derivative of the temperature-time T (t, ξ) dependence, which decreases sharply within these points. Analysis of the T (t, ξ) function allows us to determine crystallization and recrystallization points of metals and alloys and their dependence on the substance structure.
Measuring metals electromagnetic resistance in the melt condition during their crystallization and recrystallization presents considerable difficulties. Specific metal electromagnetic resistance is usually rather low and measurements have to be carried out under significant interference. Such measurements can therefore be performed only with specially designed measurement equipment.
The same applies to differential thermodynamics. Melt temperature measurements are usually performed with thermocouples, platinum / platinum-rhodium thermocouples (up to temperatures of 1300 degrees) or tungsten – rhenium thermocouples (up to temperatures of 2500 degrees). If the temperature range permits it, platinum / platinum-rhodium thermocouples are used despite their low measuring slope (about 10 microvolts on degree). This is caused by relatively high price of tungsten – rhenium thermocouples, even though their measuring slope is about three times higher.
We should also note that measurements in differential thermodynamics are performed in dynamics (Т (t)!), which doesn’t allow using signal accumulation methods to compensate for measurement interference.
Experimental measuring system
Taking into account all above a special sensor system has been designed for performing alloys TEMC measurements. This system includes four thermocouples built into a ceramic base. Two external thermocouples are used as current electrodes for alloy specific electromagnetic resistance measurements. Two internal thermocouples are used as potential electrodes. Measurements are carried out on certain frequencies chosen to fight interference from nearby sources.
Signals from thermocouples are summarized for average temperature definition in the sensor space. Collecting measurements from separate thermocouples allows asynchronous analysis of the process in different melt points. See sketch of the measuring system below.
DVM is actively looking for scientific and industrial partners for joint projects in this field.