Specific Heat Capacity Testing: Sample Preparation, Procedure Selection, and Data Analysis

Specific heat capacity is a core thermophysical property. It is the heat required to raise the temperature of a unit mass of a substance by 1 K. A specific heat capacity tester measures heat changes during heating and provides data for materials development, process optimization, and engineering applications. The principal stages are sample preparation, test-procedure selection, and data analysis.

Sample preparation

Sample quality directly affects the accuracy and repeatability of specific heat measurements. Sample dimensions and shape depend on the method: DSC specimens generally weigh 5–20 mg and may be flat or powdered; LFA specimens have a diameter of Ø12.7 mm and a thickness of 1–3 mm; and adiabatic calorimetry specimens should weigh at least 10 g to reduce heat-loss error. Specimen surfaces should be flat and parallel, with roughness no greater than 1.6 μm. Brittle materials such as ceramics can be cut with a diamond wire to avoid microcracks.

To remove thermal history from polymers or metal alloys, heat at 10 °C/min to the glass-transition temperature (Tg) or to 50 °C above the melting point, hold for 5 min, and cool rapidly. For hygroscopic materials such as wood and some polymers, condition the specimen in a desiccator to constant mass, with mass change below 0.1%, or record moisture content for data correction. For LFA, apply a graphite coating thinner than 5 μm to improve light absorption. For DSC, seal the sample pan to prevent volatilization.

Record mass with an analytical balance having an accuracy of at least 0.01 mg. If a coating is applied, measure mass before and after coating. Use a micrometer with an accuracy of ±1 μm to measure thickness and diameter, then calculate volume for density correction.

Selecting a test procedure

Choose the method according to material type, temperature range, and required accuracy.

DSC is suited to medium- and low-temperature testing from −150 to 600 °C. It measures the heat-flow difference between the specimen and reference under programmed temperature control. Set a heating rate of 1–10 °C/min; for low-temperature testing, use no more than 5 °C/min. This method applies to polymers, pharmaceuticals, foods, and low-melting-point metals.

LFA is suited to high-temperature testing from room temperature to 2,000 °C. It measures the temperature-rise curve on the rear face of a specimen after a laser pulse and combines thermal diffusivity with density to calculate specific heat capacity. LFA is suitable for high-thermal-conductivity materials such as metals, ceramics, and composites.

Adiabatic calorimetry is suited to high-accuracy reference testing from −260 to 100 °C. It measures input heat and specimen temperature rise under adiabatic conditions to calculate specific heat capacity. Accuracy can reach ±0.1%, but the test period is long, with a single point requiring several hours; the method is used mainly to assign values to reference materials.

Data analysis

For DSC baseline correction, run an empty crucible to obtain the baseline and subtract it from the specimen curve. For polymers, account for baseline shifts before and after the glass transition. For LFA heat-loss correction, fit the temperature-rise curve with the Cowan model or a least-squares method to correct radiative and convective heat loss.

For DSC, use Cp = (ΔQ/Δt)/(m·β), where ΔQ/Δt is the heat-flow difference, m is mass, and β is the heating rate. For LFA, use Cp = λ/(α·ρ), combining thermal diffusivity (α) and density (ρ); λ is thermal conductivity and must be measured separately.

Uncertainty sources include mass error (±0.1%), temperature error (±0.5 °C), and heat-flow calibration error (±1%). Cross-check the same material by two methods; the difference between DSC and LFA results should be below 5%. For pure aluminum at 100 °C, DSC gives 0.902 J/(g·K), LFA gives 0.895 J/(g·K), and the difference is 0.8%.

Examples include DSC specific-heat testing of NCM811 lithium-battery cathode material for thermal-management simulation, with a required accuracy of ±3%; measurement of the specific-heat-versus-temperature curve of paraffin/graphite phase-change energy-storage composites for thermal-storage assessment; and LFA testing of carbon/carbon composites at 1,000 °C for the thermal-protection design of re-entry vehicles. Reliable results depend on disciplined sample preparation, an appropriate procedure, and rigorous data analysis.