Ternary molten salts are among the most important phase change materials (PCMs) for medium- and high-temperature thermal energy storage, offering tunable melting points, high energy density, and excellent thermal stability. Accurate latent heat testing is essential for quantifying the storage capacity of these salt systems and validating their performance in concentrated solar power, industrial waste heat recovery, and advanced reactor applications. This article provides a comprehensive guide to ternary molten salt phase change latent heat testing, covering representative salt systems, measurable parameters, DSC-based test methods, sample handling, and frequently asked questions.
Technical Overview
Ternary molten salts are three-component inorganic salt mixtures engineered to form eutectic or near-eutectic compositions. By introducing a third salt into a binary system, the liquidus temperature is depressed well below that of the individual constituents, while the mixture retains a large enthalpy of fusion. This combination of a sharp, predictable solid-liquid transition and high latent heat makes ternary salts highly attractive as phase change materials for thermal energy storage (TES).
Phase change latent heat testing quantifies the thermal energy absorbed or released when the salt melts or solidifies. The latent heat of fusion, typically expressed in J/g or kJ/kg, directly defines the gravimetric and volumetric energy storage density of the material and is therefore a critical specification for system designers. A complete characterization also reveals melting and freezing temperatures, supercooling behavior, phase separation tendencies, and the stability of the latent heat over repeated thermal cycles, all of which govern the long-term reliability of storage installations that may experience thousands of melting-freezing cycles.
Testing Samples
The most frequently tested ternary molten salt systems span a wide temperature range, from approximately 140°C to over 700°C, and each family presents distinct measurement challenges.
Nitrate-Nitrite Systems (Hitec Salt)
The sodium nitrate-sodium nitrite-potassium nitrate ternary, widely known as Hitec salt, is a benchmark medium-temperature PCM and heat transfer fluid. Its eutectic composition melts at approximately 142°C, with latent heat values reported in the range of roughly 80 to 120 J/g depending on purity and composition. Because sodium nitrite oxidizes and decomposes at elevated temperature, samples must be protected from air, and maximum service temperatures are generally kept below about 450°C.
Fluoride Systems (FLiNaK)
The LiF-NaF-KF ternary eutectic (approximately 46.5/11.5/42 mol%) melts near 454°C and is a leading candidate coolant and storage medium for advanced nuclear and next-generation CSP concepts. Its latent heat is on the order of 400 J/g, among the highest of practical molten salt systems. FLiNaK is strongly hygroscopic and can be corrosive, so strict drying and crucible material selection are essential.
Chloride Systems
Magnesium chloride-based ternaries such as MgCl2-NaCl-KCl eutectics melt in the vicinity of 380-400°C and offer high latent heat, low material cost, and potential for operation beyond 700°C. These salts are extremely hygroscopic, and residual moisture promotes hydrolysis and corrosion, making sample preparation the dominant source of measurement error.
Carbonate Systems
The Li2CO3-Na2CO3-K2CO3 ternary eutectic melts near 397°C and provides good thermal stability at high temperature, with latent heat values on the order of 200 J/g or higher depending on composition. Lithium-rich carbonates deliver superior storage density but at significantly higher cost, so latent heat testing is frequently used to optimize the lithium fraction for cost-performance trade-offs.
Testing Items
A standard ternary molten salt latent heat test campaign typically reports the following parameters:
- Melting onset temperature (extrapolated onset of the endothermic peak), commonly used as the characteristic melting point
- Melting peak temperature (temperature of maximum heat flow during fusion)
- Latent heat of fusion (integrated endothermic peak enthalpy, J/g)
- Freezing onset temperature and latent heat of solidification, measured on the cooling segment
- Supercooling degree, defined as the difference between melting and freezing onset temperatures
- Specific heat capacity of the solid and liquid phases, measured as supporting data for total storage capacity calculations
- Thermal cycling stability, tracking the retention of latent heat and transition temperature over tens to thousands of melt-freeze cycles
Testing Methods
Differential Scanning Calorimetry (DSC)
DSC is the primary method for latent heat measurement of ternary molten salts. A small sample, typically 5 to 20 mg, and an inert reference are heated at a controlled rate, commonly 5 to 10 K/min, under a nitrogen or argon purge of roughly 20 to 50 mL/min. The integrated area of the melting endotherm, corrected by instrument calibration, yields the enthalpy of fusion in J/g. Testing generally follows ASTM E793 for fusion enthalpy and ASTM E1269 for specific heat, with temperature and enthalpy calibration performed per ASTM E967 and ASTM E968, or the equivalent ISO 11357 series procedures.
T-History Method
For salt quantities too large for DSC, the T-history method measures the temperature response of a sealed tube containing grams to hundreds of grams of salt as it cools through the phase transition. Comparing the sample curve with that of a reference material allows the latent heat and transition temperature to be extracted. T-history testing is valuable for verifying that laboratory-scale DSC results are representative of bulk behavior, where segregation and nucleation effects can appear.
Sample Preparation and Handling
Correct preparation is critical because molten salts are hygroscopic and moisture causes spurious endotherms and mass loss. Recommended practice includes:
- Drying the salt under vacuum at an appropriate temperature, for example 105 to 200°C for nitrate and chloride salts, prior to weighing
- Grinding and homogenizing the sample to ensure the small DSC specimen represents the bulk composition
- Using hermetically sealed aluminum pans up to about 600°C, and stainless steel, graphite, or platinum crucibles for higher-temperature fluoride and chloride salts
- Weighing the pan before and after testing to confirm no leakage or significant mass loss occurred
- Running at least two heating-cooling cycles and reporting data from the second cycle, which reflects stabilized behavior
Calibration and Data Evaluation
Temperature and enthalpy calibration should bracket the expected transition range, using certified reference materials such as indium (156.6°C, 28.45 J/g), tin (231.9°C), zinc (419.5°C), and aluminum (660.3°C). Consistent integration limits, a sigmoidal baseline for broad peaks, and cross-checking against the cooling exotherm improve accuracy. With proper technique, latent heat reproducibility of better than ±5% is routinely achievable.
Applications
Latent heat test data for ternary molten salts underpin design decisions across the clean energy and process industries, including:
- Concentrated solar power (CSP) plants, where ternary nitrate and chloride PCMs buffer intermittent solar input and extend generation after sunset
- Grid-scale thermal batteries and Carnot battery concepts that store electricity as high-temperature heat
- Industrial waste heat recovery from steel, glass, cement, and chemical processes
- Advanced and next-generation nuclear systems using fluoride salt coolants and storage buffers
- High-temperature process heat supply, drying, and sterilization applications requiring stable heat delivery near a fixed temperature
- Materials research programs screening new eutectic compositions and assessing long-term cycling durability
FAQ
What sample mass is required for a DSC latent heat test?
Most laboratories use 5 to 20 mg. Smaller samples reduce thermal gradients within the melt and sharpen the transition, while larger samples improve the signal-to-noise ratio for low-enthalpy salts. Duplicate runs at two masses are recommended when accuracy is critical.
Why are sealed pans and inert atmospheres necessary?
Ternary molten salts absorb moisture readily, and nitrites, chlorides, and carbonates can oxidize, hydrolyze, or volatilize at high temperature. Sealed pans and a nitrogen or argon purge prevent these effects, which would otherwise distort the measured peak shape and enthalpy.
What is supercooling and why does it matter?
Supercooling occurs when a salt remains liquid below its equilibrium freezing point before nucleating. Excessive supercooling causes stored heat to be released at unpredictable temperatures, degrading system control. Comparing melting and freezing onsets during latent heat testing quantifies this effect for each salt and cycling condition.
How does thermal cycling affect latent heat?
Repeated melting and freezing can cause component segregation, slow decomposition (notably of sodium nitrite in Hitec salt), and corrosion products, all of which gradually reduce latent heat. Cycling tests that periodically re-measure the enthalpy of fusion allow degradation rates to be extrapolated over the plant lifetime.
Which standards govern the test?
Latent heat is commonly determined per ASTM E793, specific heat per ASTM E1269, and calibration per ASTM E967 and ASTM E968; ISO 11357 procedures are also widely applied. Thermal stability screening is often performed per ASTM E537 as a complementary test.