THERMODYNAMIC STUDY OF SEMICONDUCTING RELATED MATERIALS BY USE OF EMF METHOD WITH SOLID ELECTROLYTE

Electromotive force method with solid electrolyte is briefly explained, and a thermodynamic study of semiconducting compound solid solution ZnTe-CdTe is picked up to show the way how thermodynamic functions of this system are obtained by several experimental methods based on our published papers and recently published data are added for comparison.


Introduction
Solids materials whose ionic conductivities approach those of molten salts and electrolyte solutions, and whose electronic and hole contribution to the electric conductivities are negligibly small can be used as solid electrolytes for thermodynamic study of inorganic materials at high temperature.They are used for emf measurement and for titration method: The former is to measure the chemical potential difference in both electrodes between which solid electrolyte is insertedm as electromotive force produced from the concentration cell.The latter is to control the chemical potential in one side of the electrodes by applying the voltage to the cell .In both cases reference electrode whose chemical potential is well established and known becomes important.
The chemical potential of the electrodes corresponds to the kind of the ion which moves predominantly in the solid electrolytes; In the thermodynamic study of inorganic materials at high temperature, stabilized zirconia(O 2-), oxide doped thoria(O 2-), β-alumina(Na + ) and CaF 2 (F -)have been used often, and recently perovskite type oxides are used as the proton conductor to measure the hydrogen or H 2 O in liquid alloys.
The principle of the experimental method or properties of the solid electrolytes were already shown in the classic text books [1] in detail.
In this paper emf method with stabilized zirconia is treated and materials studied are semiconducting alloys and compounds.
Since the original work by Kiukkola and Wagner [2] in 1957 to show that the thermodynamic properties of the inorganic materials could be obtained by emf method with solid electrolyte, so many data have been accumulated.Rapp and Maak [3] showed originally that the activity of the component of alloys could be derived from the oxygen potential measurements.After that so many alloy systems have been studied under the conditions that: 1) ionic transference number of oxygen ion in the solid electrolyte is nearly unity, that limits the utility of the solid electrolyte and electrode under suitable oxygen potential and temperature range, 2) phase relationship in the alloy system containing oxygen( or oxide) is clear, 3) the difference in the Gibbs energy of formation of the related oxide (which is added to keep or produce the oxygen potential at the electrode to be studied at constant values) is large enough not to cause the displacement reaction, 4) the oxygen solubility in the alloys is negligibly small.In a similar way, CaF 2 solid electrolyte which is the F -ion conductor has been used for the thermodynamic study of active metal-containing materials.
In this paper, the examples of the study on Zn-Te-Cd ternary system is picked up and: 1) activity measurements of related binary system, 2) determination of the phase relation between the ZnTe-CdTe compound solid solutions and liquid metal or alloys, 3) activity measurement of the ZnTe-CdTe compounds are explained mainly based on our experimental study [4], and they are compared to the calculated values obtained from the model calculation based on the data of the related 3 binary alloys(Kaufman′s treatment).

Experimental
The ternary aloys used in this study were prepared from high purity materials (ZnTe and CdTe: 99.9999 mass% purity, polycrystal, Furukawa Co.Ltd).They were ground to fine powder, weighed to give mixtures with desired compositions, mixed, pressed into a pellet and heat-treated in a sealed evacuated quartz ampoule at 913k over 1.7 10 6 s.After the heat-treatment the formation of the solid solution was confirmed with an X-ray diffractometer, and lattice parameter was measured.In order to make a constant oxygen potential of the alloy electrode, ZnO powder was mixed with the solid solution.And a very small amount of Te was added to the mixture to hold the alloy electrode composition within the two phase region (ZnTe-CdTe + Te), as the result the alloy electrode is composed from 3 phases including ZnO.On the other hand for the Te-poor side electrode, Zn or Cd was added to the mixture, pressed into a pellet and heat-treated at 843K for 1.2 10 6 s in a sealed evacuated quartz ampoule before emf measurement.The composition of the solid solution was determined from its lattice parameter.
In this study, air,Pt was used as a reference electrode.Re wires(0.254mmφ,5mm) were spot-welded to kanthal wires for the alloy electrode as electric lead materilas to avoid the reaction between the Ga-containing electrode and lead wire.The electrode was charged in the bottom of the zirconia tube with the lead wire.At the start of each experimental run the inside of the zirconia tube was evacuated and flushed with purified argon gas repeatedly, and the cell was heated in a kanthal-resistant furnace.The emf of the cell was measured with a digital voltmeter with a printer.
To determine the tie lines between the solid solutions of ZnTe-CdTe and liquid alloys in the Te-rich and Te-poor regions, the binary compound or solid solution was sealed in a small evacuated quartz ampoule with a appropriate amount of pure metal.The ampoule was held at 873K for more than 1.6 10 6 s for equilibration, and quenched into ice-water.The specimen showed two phases(compound solid solution + alloy).The composition of the solid solution was determined from the lattice parameter and that of the alloy by EPMA analysis.
They were used to check the composition change of the solid solution before and after emf measurements and to determine the composition of the solid solutions in the quenched specimen for the study of the phase relation.

Emf measurement of the pseudobinary system of ZnTe-CdTe
From the emf value E of the following cell(I), activity of zinc in the ZnTe-CdTe solid solution can be calculated by eq.( 1) (-) Zn,ZnOZrO 2 (+Y 2 O 3 )ZnTe-CdTe,M,ZnO (+) (I) -2EF=RTln a Zn (1) where F is the Faraday constant, R is the gas constant.M in cell(I) is Te or Zn or Cd dissolved in the alloy phase which coexists with the solid solution of ZnTe-CdTe, and very important because of holding the composition of the solid solution on the Te-rich or Te-poor side in the two phase region.

Derivation of the activity of ZnTe in ZnTe-CdTe compound solid solution coexisting with Te
When the following cell is constructed, emf gives the activity of ZnTe in the solid solutions by eq.( 2 (2) From the emf values in Te-rich region in this study, we can derive the activity of ZnTe indirecly.The activity of ZnTe shows small positive deviations from Raoults law.The results are shown in Fig. 2 with those by another researchers.Zabdyr [6]obtained the activity to obey Raoul′s law at 900K by fused salt emf method.Goncharuk et al. [7] and Gupta et al. [8] report positive deviation in the whole composition range from fused salt emf method.Alikhanian et al. [9] determined the vapor pressure above ZnTe-CdTe at 900 K by Knudsen cell mass spectrometry, and obtained larger positive deviation from ideality than the values by any other researchers.It seems very difficult to determine the accurate values without control the alloy composition within the line compound by the technique.

Binary system Zn-Cd(l)
The emfs of cell (IV) were measured.(-) W Zn,ZnO|ZrO 2 (+Y 2 O 3 )|Zn-Cd(l),ZnO|W (+) (V) The emf data are shown in Fig. 3, and temperature dependence of emf is shown in Table 1.Derived activity values at 800 and 900K are listed in Table 2. Fig. 4 shows the activity data obtained by several experimental Only present work is from the emf method with solid electrolyte.As a whole there seems very small scatter in the experimental results.This activity curve and phase relationship described above explain the composition dependence of activity of Zn in the Te-poor region of the ZnTe-CdTe solid solution, i.e. a Zn keep small value up to x ZnTe = 0.7, but increase sharply in the region x ZnTe =0.7 to 1.

Figure 1a
Figure 1a Activity of Zn in ZnTe-CdTe in Te-rich region.

Figure 1b
Figure 1b Activity of Zn in ZnTe-CdTe in Te-poor region.

Figure 3 .
Figure 3. E/mV:T/K plot of cell V

Figure 4 .
Figure 4. Activity of Zn in liquid Zn-Cd alloys