High purity grade alkali nitrates and alkali metal nitrites from Merck or Fluka were used (Table 3, right column). The mixtures were pre-melted in air atmosphere (typically at 400°C) and subsequently ground. The mixtures were kept dry in order to exclude effects from moisture.

For a preliminary examination of the salt systems, phase diagrams with the solidus and liquidus line were obtained by a melting point apparatus (Stanford Research Systems, OptiMelt MPA100). The system performance was verified and optimized by the measurement of the well known system KNO3-NaNO3. The impact of the heating rate was also examined and a rate of 1 K/min was selected. For each examined salt system several mixtures with different compositions were prepared and subsequently measured.

For some salt systems, more precise latent heat and melting temperature measurements were made with a heat flux type DSC (Netzsch DSC404) in argon flow (100 ml/min) with a heating rate of 10 K/min. As a crucible open platinum/rhodium pans with a lid were used. The first heating cycle was excluded from the analysis in order to exclude effects such as moisture.

Protsenko et al. reported a phase diagram of the Ba(NO3)2-KNO2 system [43]. The phase diagram by Protsenko and values obtained in this work are shown in Fig. 3 (left). The system has an eutectic point at about 50wt% Ba(NO3)2 and a promising melting temperature of around 170°C. DSC-Measurements of this eutectic composition gave a melting enthalpy of about 60 J/g. Besides this relatively low melting enthalpy, other disadvantages include the handling and the availability of KNO2, as well as the steep gradient in the liquidus line on the left and right hand side of the eutectic composition. For long-term and large scale use of the eutectic composition, it may be difficult to avoid the two phase area of the system due to the characteristic of the liquidus line.

Fig. 3. Phase diagram of Ba(NO3)2-KNO2 (left) and Ba(NO3)2-NaNO2 (right)

Literature data about the Ba(NO3)2-NaNO2 could not be identified. Fig. 3 (right) shows the phase diagram obtained by the melting point apparatus. The eutectic composition is around 60wt% NaNO2 with a minimum melting temperature of around 205°C. This melting temperature could be confirmed by DSC measurements with several melting cycles.

No phase diagram of the NaNO2-Sr(NO3)2 could be identified. The minimum melting temperature by DSC and melting point apparatus was between 190 and 195°C. The eutectic composition is estimated to be near 60wt% NaNO2 with a melting enthalpy of around 120 J/g.

For the system KNO2-Sr(NO3)2, also no literature data could be identified. The minimum melting temperature by DSC and melting point apparatus was in the range 170 to 175°C with a eutectic composition near 60wt% KNO2. The liquidus line was less steep near the minimum melting temperature compared to the Ba(NO3)2-KNO2 system. DSC-measurements of the 61wt%Sr(NO3)2- KNO2 resulted in an melting enthalpy of about 85 J/g (Fig. 4).