10.3 Thermal property analysis techniques

Thermal property analysis is crucial for evaluating thermoelectric materials. These techniques measure thermal conductivity, heat capacity, and thermal stability, which directly impact a material's performance in thermoelectric devices.

From laser flash to differential scanning calorimetry, these methods provide vital data on heat transfer and phase transitions. Understanding these techniques helps researchers optimize materials for better thermoelectric efficiency and durability.

Thermal Conductivity Measurement Techniques

Fundamentals of Thermal Conductivity Measurement

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• Thermal conductivity quantifies a material's ability to conduct heat
• Measured in watts per meter-kelvin (W/m·K)
• Crucial for evaluating thermoelectric material performance
• Influences the figure of merit (ZT) of thermoelectric devices
• Measurement techniques categorized as steady-state or transient methods

Laser Flash Method

• Non-contact transient technique for thermal diffusivity measurement
• Applies short laser pulse to one side of a thin sample
• Measures temperature rise on opposite side using infrared detector
• Calculates thermal diffusivity from temperature-time curve
• Thermal conductivity derived using equation: $k = α * ρ * c_p$
  • k: thermal conductivity
  • α: thermal diffusivity
  • ρ: density
  • c_p: specific heat capacity
• Suitable for various materials (metals, ceramics, polymers)
• Operates over wide temperature range (room temperature to 2800°C)

3-Omega Method

• AC technique for measuring thermal conductivity of thin films and bulk materials
• Uses metal line as both heater and thermometer
• Applies AC current at frequency ω to metal line
• Measures voltage component at 3ω frequency
• Thermal conductivity extracted from 3ω voltage amplitude and phase
• Advantages include high accuracy and ability to measure anisotropic thermal conductivity
• Suitable for materials with low to moderate thermal conductivity
• Requires careful sample preparation and electrode deposition

Thermal Analysis Techniques

Differential Scanning Calorimetry (DSC)

• Measures heat flow difference between sample and reference as function of temperature
• Provides information on phase transitions, melting points, and specific heat capacity
• Two main types: heat flux DSC and power compensation DSC
• Heat flux DSC measures temperature difference between sample and reference
• Power compensation DSC maintains zero temperature difference by adjusting heating power
• Applications include characterizing thermoelectric materials' phase transitions
• Determines Curie temperature in ferroelectric materials
• Measures specific heat capacity for ZT calculations

Thermogravimetric Analysis (TGA)

• Measures mass changes in materials as function of temperature or time
• Provides information on thermal stability, decomposition, and oxidation behavior
• Essential for evaluating thermoelectric materials' high-temperature stability
• Consists of precision balance, furnace, and temperature control system
• Operates in various atmospheres (air, inert gas, vacuum)
• Analyzes volatile content, decomposition temperatures, and oxidation kinetics
• Combined with mass spectrometry or FTIR for evolved gas analysis
• Helps optimize synthesis and processing conditions for thermoelectric materials

Specific Heat Capacity Measurement

• Quantifies amount of heat required to raise material's temperature by one degree
• Crucial parameter for calculating thermal conductivity and ZT
• Measured using various techniques (DSC, adiabatic calorimetry, drop calorimetry)
• DSC method involves comparing sample heat flow to reference material
• Adiabatic calorimetry minimizes heat exchange with surroundings
• Drop calorimetry suitable for high-temperature measurements
• Specific heat capacity varies with temperature and phase transitions
• Contributes to understanding lattice and electronic contributions to thermal properties
• Helps optimize thermoelectric material composition and doping strategies