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What is Chemical Potential Diagram?

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This program is to construct the generalized chemical potential diagram.

  1. What is the chemical potential diagram?
  2. What is the generalization?
      The current CHD program constructs diagrams using the generalized construction method. In what follows, comparison will be made between the conventional and generalized method in constructing strategy and in utilization with an emphasis on the features of generalization.

    1. The comvensional chemical potential diagram:
      • Selection of specified element：
        This is based on the selection of one redox element, and then shows the chemical forms of this element in various chemical environment as functions of chemical potentials. This can be regarded as the stability diagram for the selected element.
        • For example in the Fe-O-H-e- system, the Pourbaix diagram can be set up by using pH and the electric potential as coordinates for presenting the stability fields of the Iron containing species/compounds.
        • For example in the Fe-S-O-H system, the high temperature chemical potential diagram can be set up by using log p(O2) vs. log p(S2) as the environmentally regulable chemical potential.
      • Coordinates as Environmentally controllable variable:
        Implicitly, the coordinate variable is selected in view of chemical potentials which can controll the chemical feature of the environmnet.
      • Meaning of Borderlines：
        Borderlines in the diagram show the change in the chemical form of the selected redox element. This borderline can be represented by one chemical reaction.
      • Chemical Reactions from the constructing strategy
        To construct one diagram, many chemical reactions are written down and the most important chemical reactions for one species are selected. It is thus common to write down the chemical reactions containing H+ or electrons to construct the Pourbaix diagram.
      • Redox element：
        It will be convenient to see systematically the redox eraction as well as the acid-base reaction concerning the selected element. Thus, this selected element is often called the redox element.
      • Limitation in applying to Multicomponent systems:
        The conventional approach has demerits in difficulties of representing the complicated equilibrium realation for alloys or double oxide systems.
      • Relation to Mass Transfer：
        The conventional approach is based on the idea that the mass transfer associated with changes in the environmental variables is quite fast and therefore the chemical reactions can be emphasized. For example, mass tranfer associated with the change in the chemical state from FeS to FeO will not be paied attentions.

    2. Generalization:
      • No selection of elements is made for particular purposes in the constructing strategy. In the n-dimension chemical potential space, the thermodynamic relations among compounds will be examined. This makes possible to treat alloy and non aolloy element without any difference. Furthermore, temperature and pressure are also included as one of the dimension consgtructing space. In a similar manner, the charge can be treated in a similar way to element. Thus, the characteristic features in comparison with the conventional one can be pointed out as follows: This means that some important differences will appear such that the borderlines in diagrams do not necessarily indicate chemical reactions but sometimes indicate phase boundaries like in the compositional phase diagram. In this sense, this new generalized chemical potential diagrams can be well compared with phase diagrams.
        1. No difference on treatment of elements. This makes it possible to treat alloys and double oxides easily.
        2. The generalized way of selecting coordinate variables. It is possible to adopt the environmentally related chemical potentials and also to adopt more generalized chemical potential variables. Thus, it is erasonable to adopt the diffusion potential defined as the differences between two elemental chemical potentials; this is very convenient to consider the diffusion properties across the interfaces bewteen disimilar materials.
        3. The same algorithm can be applied to the Pourbaix diagram and the high temperature chemical potential diagram so that it becomes easy to convert the phase relations in the Pourbaix diagram to the high temperature chemical potential diagram. Thus, it is possible to compare the phase relations in the aquous system with those in the wet system or the dry system.
        4. Temperature and pressure can be selected as coordinates variables. This makes it possible to construct the popular diagram in the high temperature chemistry such as Ellingham diagram or Alrenius plot.
      • There is no requirement in the constructing stargety to construct the three dimensional chemical potential diagram, and the calculation time is not long so that it becomes possible to construct more complicated diagrams than those which the convetional diagram can be set up. Even so, it is quite difficult to obtain the whole features of the three dimensional diagrams. This leads to new requirements in handling the diagrams with complicated but newly appearing features.
        1. Touch diagram mode：
          Since one point in the diagram always corresponding to one point in the original thermodyanmic chemical potential space. This informaion can be displyed in response to the movement of mouse.
        2. Dissections：
          It is not easy to see inside the three dimensional polyhedron. The dissection by a selected (arbitrary) chemical potential value can be set up as additional diagram.
        3. Profile diagram：
          The dissection on the two dimentional diagram will lead to the profile diagram which shows changes in the partial pressure of gaseous species or the activity of aqueous species along the dissected line.
        4. Transparent：
          The three dimensional chemical potential diagram consists of several polyhedrons corresponding to respective compounds. When some of compound polyhedrons can be tranparent to make other polyhefrons visible. This makes also possible to touch on one polygon of non-transparent compound polyhedron and show the thermodynamic information of that point.
        5. Swing：
          By swing the chemical potential value of the dissection, it is easily examined how the dissection diagram or the profile diagram will change in features on swing.
        6. Rotation：
          It is possible to rotate the three dimensional diagram with or without transparent polyhedrons.
      • The following treatment is adopted to make consistent treatment with the conventional diagram.
        1. Specification of elements in some aspects:
          As described above, it is possible to make transparent for the selected compounds. In addition to this function, a new treatment is introduce to make systematic treatment of transparency by specification of elements for several purposes such as transparency. In particular, the Pourbaix diagram for the multielemts sytems can be set up in a simmilar manner. For example, in the Fe-S-O-H-e- system, Fe is selcted as the redox element in the conventional one. In the present situation, this means exactly that S is selected to be transparent. When the transparency of S compounds is relaxed, the normal three dimensional Pourbaix diagram will be constructed.
      • New interesting features:
        1. Diffusion and Reactions:
          The chemical potential difference which controlls diffusion is called the diffusion potential. As described in the convetional diagram, the chemical potential is also the key property in understanding chemical reactions of the condensed materials in the various chemical media. By combining these ideas, it is reasonable to consider that the chemical potential diagram is very convenient tools for examing the diffusion and chemical reactions simultaneously. This is particularly important in the interface chemistry for solid-solid interfaces. Thus, it is highly suggested to draw the reaction diffusion path on the chemical potential diagram with the thermodynamic information along such path.
        2. Cooperation with Phase diagram calculation or chemcil equilibrium calculations
          The generalized chemical potential diagrams have essentially the same thermodynamic information with the phase diagram. Furthermore, it well corresponds to the chemical equilibrium calculations which are convenient for calculating the complicated equilibriums by the Gibbs minimization method. In MALT system, gem is such software to calculate the complicated chemical equilibria. It is therefor quite encouraging to be able to apply such major thermodyanmic software by using the same thermodynamic data. This will make the quality of the thermodynamic considerations quite high in many industrial fields.

    For further understanding the generalized chemical potential diagrams, see the followings