SGTE strives to generate cooperation in a broad international effort to unify thermodynamic data and assessment methods.
The SGTE methodology gobally accepted include the following methods:
- Calphad (Calculation of Phase Diagrams),
- ab initio calculations (quantum mechanics),
- experimental methods.
The current challenge for computational thermochemistry lies in the combination and optimization of these three methods.
SGTE accepts this challenge and is contributing to both, fundamental research as well as standardizatin efforts.
Phase diagrams provide the graphical presentation of the equilibrium state of a material as a function of temperature, pressure, and composition of the components. This is why they are frequently used as roadmaps for alloy design or a better understanding of the processing of materials. The thermodynamic properties of materials, such as the heat of solidification or the chemical activities of components, are also frequently used to understand, for example, metallurgical reactions of materials. These two aspects, phase diagrams and thermodynamic properties, have been treated separately for a very long time despite the fact that their fundamental interrelations had been established more than a century ago by J. W. Gibbs.
Eventually, the mathematical calculation of phase diagrams arose, and in 1970, Larry Kaufman et al initiated the Calphad Method with a detailed description of procedures together with a listing of computer software. The subsequent meetings of the ever growing group, organized by Larry Kaufman, soon reached the level of annual international conferences, and the international Calphad journal was established as well.
The modern Calphad approach is characterised by the following points:
A predictive capability allows the extrapolation of thermodynamic descriptions and phase equilibrium calculation from assessed binary systems to ternary, quaternary and higher order systems.
Identification of key experiments drastically reduces the necessary experimental effort in multicomponent systems.
Stable and metastable phase equilibria can be calculated.
The driving forces for all phase transformations are available.
Local phase equilibria can be calculated, providing a numerical input to materials processing software, for example in solidification simulation or reactor modelling.
The reading of multicomponent phase diagrams is drastically simplified by the fact that all the interesting two-dimensional sections in temperature, composition or even chemical potential con be readily calculated, plotted and read directly.
This powerful tool in materials research goes beyond a mere “calculation of phase diagrams”. This is why the term “computational thermochemistry” is frequently used to describe the current state of the Calphad approach.
Reference: “Focused Development of Magnesium Alloys using the Calphad Approach”, Rainer Schmid-Fetzer and Joachim Gröbner, Advanced Engineering Materials, vol 3 No 12, pp. 947-961, 2001
Determination of phase diagrams
Metallography is a science, related to metallurgy that looks at the composition and structure of metals and alloys. It may involve techniques and tools such as visual inspection, low-powered magnification, optical microscopes, electron microscopes and X-ray crystallography.
X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. This generally leads to an understanding of the material and molecular structure of a substance. The spacings in the crystal lattice can be determined using Bragg's law. The electrons that surround the atoms, rather than the atomic nuclei themselves, are the entities which physically interact with the incoming X-ray photons. This technique is widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds, DNA and proteins. X-ray diffraction is commonly carried out using single crystals of a material, but if these are not available, microcrystalline powdered samples may also be used, although this requires different equipment, gives less information, and is much less straightforward.
X-ray powder diffraction finds frequent use in materials science because sample preparation is relatively easy, and the test itself is often rapid and non-destructive. The vast majority of engineering materials are crystalline, and even those which are not yield some useful information in diffraction experiments. The pattern of powder diffraction peaks can be used to quickly identify materials (thanks to the JCPDS pattern database), and changes in peak width or position can be used to determine crystal size, purity, and texture.
Differential Thermal Analysis (DTA)
Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. In Differential Thermal Analysis (DTA), the temperature difference that develops between a sample and an inert reference material is measured, when both are subjected to heat-treatments. This differential temperature is then plotted against time, or against temperature. Changes in the sample which lead to the absorption or evolution of heat can be detected relative to the inert reference.
Differential temperatures can also arise between two inert samples when their response to the applied heat-treatment is not identical. DTA can therefore be used to study thermal properties and phase changes which do not lead to a change in enthalpy. The baseline of the DTA curve should then exhibit discontinuities at the transition temperatures and the slope of the curve at any point will depend on the microstructural constitution at that temperature.
A DTA curve can be used as a finger print for identification purposes, for example, in the study of clays where the structural similarity of different forms renders diffraction experiments difficult to interpret.
The area under a DTA peak can be related to the enthalpy change and is not affected by the heat capacity of the sample.
Thermogravimetric Analysis (TGA) is a thermal analysis technique used to measure changes in the weight (mass) of a sample as a function of temperature and/or time. TGA is commonly used to determine polymer degradation temperatures, residual solvent levels, absorbed moisture content, and the amount of inorganic (noncombustible) filler in polymer or composite material compositions.
A simplified explanation of a TGA sample evaluation may be described as follows. A sample is placed into a tared TGA sample pan which is attached to a sensitive microbalance assembly. The sample holder portion of the TGA balance assembly is subsequently placed into a high temperature furnace. The balance assembly measures the initial sample weight at room temperature and then continuously monitors changes in sample weight (losses or gains) as heat is applied to the sample. TGA tests may be run in a heating mode at some controlled heating rate, or isothermally. Typical weight loss profiles are analyzed for the amount or percent of weight loss at any given temperature, the amount or percent of noncombusted residue at some final temperature, and the temperatures of various sample degradation processes.
Determination of thermochemical properties
Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry involves the use of a calorimeter. The word calorimetry is derived from the latin word calor, meaning heat.
Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) is a technique for measuring the energy necessary to establish a nearly zero temperature difference between a substance and an inert reference material, as the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate.
There are two types of DSC systems in common use. In power-compensation DSC, the temperatures of the sample and reference are controlled independently using separate, identical furnaces. The temperatures of the sample and reference are made identical by varying the power input to the two furnaces; the energy required to do this is a measure of the enthalpy or heat capacity changes in the sample relative to the reference.
In heat-flux DSC, the sample and reference are connected by a low-resistance heat-flow path (a metal disc). The assembly is enclosed in a single furnace. Enthalpy or heat capacity changes in the sample cause a difference in its temperature relative to the reference; the resulting heat flow is small compared with that in differential thermal analysis (DTA) because the sample and reference are in good thermal contact. The temperature difference is recorded and related to enthalpy change in the sample using calibration experiments.
Direct reaction calorimetry
The enthalpy of formation of a compound can be determined by this technique. Fine powders of the constituents are mixed together and pressed into pellets. These pellets are then dropped into a calorimeter which is held at a given temperature. The powder mixture reacts and the heat which evolves upon reaction is recorded by a thermopile. In order to get the enthalpy of formation at the calorimeter temperature, a second drop of the formed compound is necessary to determine the heat content of the reacted sample. This method is suitable for enthalpies of formation lower than -10 kJ/moles of constituents. The samples have to be checked afterwards to verify that the reaction is complete and that no contamination with the atmosphere or the crucible material has taken place.
Drop solution calorimetry
In order to determine the enthalpy of formation of a compound at any temperature and for low absolute values, drop dissolution calorimetry is the method of choice. A compound is dropped into a liquid bath which is placed inside the calorimeter where it dissolves completely. In the case of intermetallics, Al and Sn are often used as solvents. The generated heat is measured by a thermopile and the signal as a function of time is recorded. The total enthalpy of dissolution corresponds to the integral of the signal over time. To get an absolute value, a calibration prior to the measurement is necessary. This calibration is done by dropping pieces of the solvent material into the bath. In a second run a mechanical mixture of the constituents is dissolved. The enthalpy of formation at the drop temperature (usually room temperature) is then the difference between the two values.
Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio (m/z) of ions. It is most generally used to find the composition of a physical sample by generating a mass spectrum representing the masses of sample components.
A mass spectrometer is a device used for mass spectrometry, and produces a mass spectrum of a sample to find its composition. This is normally achieved by ionizing the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. A typical mass spectrometer comprises three parts: an ion source, a mass analyzer, and a detector.
Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. Most often the term is used to describe a liquid's tendency to evaporate. It is the tendency of molecules and atoms, which form when escaping from a liquid or a solid. At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean the saturation vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The higher the vapor pressure of a material at a given temperature, the lower the boiling point.
Equilibrium vapor pressure can be defined as the pressure reached when a condensed phase is in equilibrium with its own vapor. In the case of an solid, this can be defined as the pressure when the rate of sublimation of the solid matches the rate of deposition of its vapor phase. For most solids this pressure is very low, but some notable exceptions are naphthalene, dry ice and ice. Ice will still continue to disappear even though the ambient temperature is below the freezing point of water. All solid materials have a vapor pressure. However, due to their often extremely low values, measurement of vapor pressures can be rather difficult. Typical techniques include the use of thermogravimetry and gas transpiration.