Thermal Analysis

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Intro to Thermal Analysis

Supply heat to Cause heating, then measure using Thermocouple

Design

  • Measurement cell: measure properties
  • Temperature control: accurately measure temperature
  • Cooling system: help maintain desired Temperature
  • Atmospheric control: inert gas environmengt to avoid degradation of sample e.g. in Oxygen
  • Data Capture and analyses
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Differential Thermal Analysis

Temperature difference between refference and sample in oven

  • Reference should be inert and has the same temperature ads the oven
  • Temperature of the sample may not be the temperature of the oven
  • Phase change  occurs: Temperature remains constant until all of the sample has changed. The Energy supplied to the sample is used to change phase
  • Temperature - time graph: area under the peak is propertional to the energy required for a phase change

Used to Identify the order of a transition

  • 1st order: melting, boiling, high peak
  • 2nf order: ferromagnetic , glass transition, shallow curve

It is used for high temperature measurements (T> 1600 degrees)

Disadv: It does not allow quantitative measurements of samples ( heat capacity etc.

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Differential Scanning Caliorimetry (DSC)

Heat transfer between sample relative to the reference. Temperature of the sample is controlled

  • Measures Temperature of the sample and the reference and the heat flow between them
  • Two designs

Power compensation

  • sample and reference in separate ovens 
  • Energy supplied measured (to keep both ovens at the programme temperature)
  • Direct measurement of heat; high heating rates and sesnsitivity

Heat flux

  • Same oven
  • sensitive thermocouple used
  • Difference in temperature is converted to energy by calibrating the instrument
  • cheap, robust
  • Quantitative measurenment of the heat capacity 
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1st and 2nd order transitions

1st order

  • involves latent heat
  • Heat capacity undefined
  • peak
  • equilibrium of systems
  • discontinuous volumes and entropy across transition

2nd order

  • no latent heat
  • heat capacity changes
  •  inflection point and change in slope 
  • continuous volume and entropy
  •  discontinuous heat capacity
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Calibration

Callibration: baseline/ heat capacity of oven, calibrant and sample measured

  • The reference should be inert so that q(t,T) is related to the sample thermal events only
  • Measure Tg of polymers using freshly melted sample
    • as get relaxation enthalpy if there is thermal history
  • Types of calibration: heat capacity, temperature, enthalpy
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Modulated DSC

  • Separate simultaneous events that occur at the same temeprature by adding modulation
  • Splits reversing and non-reversing heat flow (reversible and non-reversible
  • Adv: High sensitiity 
  • Disadv: Slow heating rate,More parameters
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Thermogravimetric analysis (TGA)

  • Meaurement of sample weight as a function of temperature
  • Sample attached to sensitive balance

High resolution TGA

  • control mass loss
  • Change temperature at different time intervals
  • more precise
  • Disadv: takes longer
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Thermomechanical Analysis and Dynamical Mechanical

TMA

Measurement of samples dimensions as a function of temperature

Dynamical Mechanical Thermal Analysis (DMTA)

  • Mechanical properties as a function of temperature
  • Sinusodial
  • Least amount of deformation desirable
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