Thermal analysis is a widely used physico-chemical technique that studies how the physical and chemical properties of a material change as a function of temperature. In thermal analysis, a property of the sample—such as mass, enthalpy, dimension, or mechanical behavior—is monitored while the sample is subjected to controlled heating, cooling, or isothermal conditions. These methods provide valuable insights into melting, boiling, crystallization, decomposition, and phase transitions.
Thermal analysis includes a family of techniques such as Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC), Thermometric Titration, Dynamic Mechanical Analysis (DMA), Direct Injection Enthalpimetry, and Thermomechanical Analysis (TMA).
These methods are indispensable in studying pharmaceuticals, polymers, cosmetics, organic and inorganic compounds, ceramics, metals, alloys, glasses, mineral samples, and many industrial materials. Thermal behavior, stability, purity, and structural properties can all be understood through thermal analysis.
Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry is one of the most important thermal analysis techniques. DSC measures the difference in heat flow required to increase the temperature of a sample and a reference under controlled conditions. Both sample and reference are maintained at nearly the same temperature, and the difference in heat input indicates thermal events such as melting, crystallization, oxidation, or glass transition.
Calorimetry refers to the measurement of heat absorbed or released during physical or chemical processes. DSC was developed by Watson and O’Neill in 1960 and became commercially available in 1963.
Principle
DSC examines how a sample responds to heat. When a polymer or any other sample is heated, it may undergo phase transitions such as melting, crystallization, or glass transition. DSC measures the energy required to keep the sample and reference at the same temperature. Any difference in energy input reflects a thermal event in the sample.
Instrumentation
A typical DSC instrument includes:
- Sample and reference pans (usually aluminum or inert metals)
- Heaters for controlled temperature programming
- A furnace capable of heating from −180°C to 700°C (or up to 1600°C in special instruments)
- Cooling accessory such as liquid nitrogen unit
- Sensors for heat flow measurement
Types of DSC Instruments
1) Heat Flux DSC
In heat flux DSC, both the sample and reference receive heat from the same furnace. The pans sit on a thermoelectric disk made of Cu/Ni alloy. Thermocouples measure the temperature difference between sample and reference. The heat flow is proportional to the thermocouple output. Although the thermocouple is not inserted directly into the sample, it provides an accurate estimation of temperature.
2) Power Compensated DSC
In this type, separate heaters are used for the sample and reference. When a temperature difference arises, extra power is supplied to maintain equal temperatures. Power compensated DSC offers high accuracy, high precision, and excellent sensitivity. Temperature is measured using platinum resistance sensors.
Sample Preparation for DSC
- Sample mass is generally between 3–20 mg.
- Pans may be aluminum, platinum, nickel, or hermetically sealed depending on the sample.
- Sample should uniformly cover the bottom for good thermal contact.
- Overfilling must be avoided to prevent thermal lag.
Purge Gases
Inert gases such as nitrogen, helium, or argon are used to prevent oxidation. Nitrogen improves sensitivity, while helium improves peak resolution. Oxygen may be used intentionally for oxidative studies.
Heating Rate
Heating rate significantly affects accuracy and resolution. Faster heating increases sensitivity but reduces resolution. Slower heating improves resolution. A typical heating rate of 10°C/min provides balanced results.
DSC Curve
The DSC thermogram plots heat flow against temperature or time. Endothermic and exothermic peaks represent thermal events. Important features include:
- Tg: glass transition temperature
- Tc: crystallization temperature
- Tm: melting temperature
The enthalpy change (ΔH) associated with each transition is determined by integrating the peak area (ΔH = KA). DSC serves as a fingerprint method for identifying compounds, evaluating purity, and detecting mixed polymer waste.
Factors Affecting DSC Curves
a) Instrumental Factors
- Heating rate
- Chart speed
- Furnace atmosphere
- Material and design of sample holder
- Sensor position
- System sensitivity and accuracy
b) Sample Factors
- Sample mass (optimal: 3–15 mg)
- Sample nature and preparation
- Particle size and shape
- Thermal conductivity
- Heat of reaction
Applications of DSC
- Study of liquid crystals and phase transitions
- Oxidative stability studies for drugs
- Moisture content estimation and solid dispersion analysis
- Characterization of polymers and determination of impurities
- Food analysis such as water dynamics and thermal behavior
- Determination of heat capacity (Cp)
- Determination of Tg, Tc, and Tm
- Protein stability studies
- Enzyme kinetics and binding studies
- Qualitative and quantitative mineral analysis
Advantages of DSC
- Requires very small sample quantity
- Applicable to solids, liquids, and polymers
- Fast and highly sensitive
- Minimal calibration required
- Suitable for multiple types of thermal events
Disadvantages of DSC
- Not suitable for two-phase mixtures
- Cannot detect gas evolution directly
- Heat of fusion and transition temperatures may have uncertainty
- Sample preparation can be difficult for volatile samples
- Sensitivity and resolution cannot be optimized simultaneously
Differential Thermal Analysis (DTA)
DTA measures the temperature difference between the sample and a thermally inert reference as both are heated under identical conditions. A DTA curve plots differential temperature (ΔT) against reference temperature or time.
Principle
Sample and reference have different heat capacities. During thermal events such as melting, decomposition, or crystallization, heat is either absorbed or released, causing a shift in ΔT. This shift produces peaks that indicate endothermic or exothermic reactions.
Instrumentation of DTA
- Furnace: Heats samples up to 2000°C depending on furnace type.
- Sample and reference holders: Made of metals or ceramic materials.
- Temperature detectors: Thermocouples measure temperature difference.
- Amplifier: Converts heat signal to electrical signal.
- Readout device: Plots thermograms digitally or on paper.
- Insulator: Reduces heat loss.
DTA Curve and Interpretation
The DTA curve shows endothermic dips or exothermic peaks. Each peak corresponds to a thermal event such as melting, decomposition, or crystallization. Peak area is proportional to the quantity of reacting material. Peak temperature is characteristic of the compound.
Applications of DTA
- Identification of minerals
- Crystallinity and polymer characterization
- Determination of melting, boiling, and decomposition temperatures
- Quality control of cement, soil, glass, etc.
- Evaluation of thermal stability of inorganic compounds
Detailed Notes:
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