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DSC- Basics PPT.ppt

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DSC- Basics PPT.ppt

  1. 1. 2/7/2023 Dr.Anand, MME, NITK 1 Thermal Analysis Differential Scanning Calorimetry (DSC)
  2. 2. Dr.Anand, MME, NITK 2 2/7/2023 Differential Scanning Calorimetry (DSC)  Principle DSC measures the differences in heat flow into a substance and a reference as a function of sample temperature while both are subjected to a controlled temperature program  DSC provides access to accurate thermodynamic data as well as information regarding reactivity and phase transformations
  3. 3. -0.4 -0.3 -0.2 -0.1 0.0 0.1 Heat Flow (W/g) 0 25 50 75 100 125 150 Temperature (°C) Exo Up Endothermic Heat Flow  Heat flows into the sample as a result of either Heat capacity (heating) Glass Transition (Tg) Melting Evaporation Other endothermic processes Endothermic
  4. 4. -0.1 0.0 0.1 Heat Flow (W/g) 0 20 40 60 80 100 120 140 160 Temperature (°C) Exo Up Exothermic Heat Flow  Heat flows out of the sample as a result of either  Heat capacity (cooling)  Crystallization  Curing  Oxidation  Other exothermic processes Exothermic
  5. 5. DSC Heat Flow t) (T, dt dT Cp dt dH f   signal flow heat DSC dt dH  Weight Sample Heat x Specific Sample Capacity Heat Sample Cp   Rate Heating dt dT  (kinetic) re temperatu absolute an at time of function is that flow Heat t) (T,  f
  6. 6. Dr.Anand, MME, NITK 6 2/7/2023  DSC is the most sophisticated and advanced of the thermal methods.  There are two principal types: power compensated DSC heat-flux DSC DSC - types
  7. 7. Dr.Anand, MME, NITK 7 2/7/2023 Power Compensated DSC- principles  Temperature difference is maintained zero, i.e., ΔT = 0, by supplying heat into the sample or reference according to heat emission or absorption  Electrical power is proportional to heat change in the sample i.e., P = I2.R
  8. 8. Dr.Anand, MME, NITK 8 2/7/2023 Power Compensated DSC- principles  Rate of change of power input is plotted against average S & R temperature  x-axis (abscissa) is temperature and the y- axis (ordinate) is difference in power input (which is proportional to the heat change i.e., enthalpy)
  9. 9. Dr.Anand, MME, NITK 9 2/7/2023 Instrumentation
  10. 10. Dr.Anand, MME, NITK 10 2/7/2023 Furnace and sample holder
  11. 11. Dr.Anand, MME, NITK 11 2/7/2023  Small, flat samples are contained in shallow pans, with the aim of making a good thermal contact between sample, pan and heat flux plate.  Symmetrical heating of the cell, and therefore S and R, is achieved by constructing the furnace from a metal of high thermal conductivity – for example, silver How does it work?
  12. 12. Dr.Anand, MME, NITK 12 2/7/2023 How does it work?  Sample and reference material are heated by separate heaters in two independent furnaces  The furnaces are imbedded in a large temperature-controlled heat sink  Sample holders are above the furnaces  Pt resistance thermometers are imbedded in the furnaces to monitor the temperatures of sample and reference continuously
  13. 13. Dr.Anand, MME, NITK 13 2/7/2023 How does it work?  There is provision for establishing gas flow through the cell, to sweep away volatiles, provide the required atmosphere, and to assist in heat transfer.  Control of the furnace, signal acquisition, and data storage and analysis are handled by a computer.
  14. 14. Dr.Anand, MME, NITK 14 2/7/2023 How does it work?  Two control circuits are used to obtain differential thermograms One for average temperature control One for ΔT control  S and R temp. signals are fed into a differential amplifier via a comparator circuit that determines which is greater  The amplifier output then adjusts the power input to the two furnaces in such a way that their temperatures are kept identical
  15. 15. Dr.Anand, MME, NITK 15 2/7/2023 How does it work?  Throughout the experiment, S and R are isothermal  A signal proportional to difference in power input to the S and R furnaces is transmitted to the data acquisition system  The power differences are plotted as a function of the sample temperature and its unit is milli Watts (mW)
  16. 16. Dr.Anand, MME, NITK 16 2/7/2023 Heat flux DSC Constantan disk Chromel disk Chromel wire Alumel wire
  17. 17. Dr.Anand, MME, NITK 17 2/7/2023 How does it work?  S and R are heated by a single heater  Differential heat flow into the S and R pans is monitored by chromel disk/constantan thermocouple  Differential heat flow in to S and R pans is directly proportional to the difference in output of the two thermocouple junctions  Sample temp. is estimated by chromel/alumel junction under the sample disk
  18. 18. Dr.Anand, MME, NITK 18 2/7/2023 Purge gases  Typical purge gases are air/N2  He is useful for efficient heat transfer and removal of volatiles.  Ar is preferred as an inert purge when examining samples that can react with nitrogen.  The experiment can also be carried out under a vacuum or under high pressure.
  19. 19. Dr.Anand, MME, NITK 19 2/7/2023 Crucibles Choice of crucible is critical. • Thermal properties of crucible. • Reactive properties with samples. • Catalytic behaviour with samples. Aluminum: inexpensive, low temp Copper: used as catalyst (testing polymers) Gold: higher temp, expensive Platinum: still higher temp, expensive. Alumina (Al2O3): very high temp Sapphire: crystalline alumina, more chemically resistant than amorphous Al2O3.
  20. 20. Dr.Anand, MME, NITK 20 2/7/2023 DSC thermograms S – glass transition Ex – exothermic reaction En – endothermic reaction Exo Endo DSC thermogram of poly(ethylene terephthalate)
  21. 21. Dr.Anand, MME, NITK 21 2/7/2023 Enthalpy changes  The DSC curve may show an exothermic or endothermic peak  The enthalpy changes associated with the events occurring are given by the area under the peaks.  Peaks may be characterized by:  Position (i.e., start, end, extrapolated onset and peak temperatures)  Size (related to the amount of material and energy of the reaction)  Shape (which can be related to the kinetics of the process)
  22. 22. Dr.Anand, MME, NITK 22 2/7/2023 Process Exotherm Endotherm Solid-solid transition * * Crystallization * Melting * Vaporisation * Sublimation * Adsorption * Desorption * Desolvation (drying) * Decomposition * * Solid-solid reaction * * Solid-liquid reaction * * Solid-gas reaction * * Curing * Polymerization * Catalytic reactions * Some possible processes giving enthalpic peaks
  23. 23. DSC Heat Flow t) (T, dt dT Cp dt dH f   signal flow heat DSC dt dH  Weight Sample Heat x Specific Sample Capacity Heat Sample Cp   Rate Heating dt dT  (kinetic) re temperatu absolute an at time of function is that flow Heat t) (T,  f
  24. 24. Dr.Anand, MME, NITK 24 2/7/2023 Exo Endo 7.96 Cal/g 8.04 Cal/g crystallization melting Measurement of ΔH of thermal transitions
  25. 25. Dr.Anand, MME, NITK 25 2/7/2023
  26. 26. Dr.Anand, MME, NITK 26 2/7/2023 Calibration of DSC  Temperature calibration is carried out by running standard materials, usually very pure metals with accurately known melting points.  Energy calibration may be carried out by using either known heats of fusion for metals, commonly indium, or known heat capacities.
  27. 27. Dr.Anand, MME, NITK 27 2/7/2023
  28. 28. -30 -25 -20 -15 -10 -5 0 Heat flow (mW) 140 145 150 155 160 165 170 175 180 Temperature (°C) Indium as a Measure of Sensitivity & Resolution Height Width at Half-Height Peak Height Increases Peak Width Decreases Height/Width Increases
  29. 29. Melting of Indium 157.01°C 156.60°C 28.50J/g Indium 5.7mg 10°C/min -25 -20 -15 -10 -5 0 Heat Flow (mW) 150 155 160 165 Temperature (°C) Exo Up Universal V4.0B TA Instruments Peak Temperature Extrapolated Onset Temperature Heat of Fusion For pure, low molecular weight materials (mw<500 g/mol) use Extrapolated Onset as Melting Temperature
  30. 30. Melting of PET 249.70°C 236.15°C 52.19J/g PET 6.79mg 10°C/min -7 -6 -5 -4 -3 -2 -1 Heat Flow (mW) 200 210 220 230 240 250 260 270 Temperature (°C) Exo Up Universal V4.0B TA Instruments Extrapolated Onset Temperature Peak Temperature Heat of Fusion For polymers, use Peak as Melting Temperature
  31. 31. Dr.Anand, MME, NITK 31 2/7/2023 Effect of heating rate Exo Endo Temperature (0 C) Heat flow (mW)
  32. 32. Dr.Anand, MME, NITK 32 2/7/2023 Effect of heating rate  Slower heating rates will more accurately depict the onset temperature of transformation  Two transformations which are very close in temperature range may be more distinctly seen as separate peaks, whereas they may be mistaken for a single transformation under a rapid heating rate
  33. 33. Comparison of First and Second Heating Runs 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 - 0 . 2 4 - 0 . 2 0 - 0 . 1 6 - 0 . 1 2 - 0 . 0 8 - 0 . 0 4 T e m p e r a t u r e ( ° C ) H e a t F l o w ( W / g ) T g T g 1 5 5 . 9 3 ° C 1 0 2 . 6 4 ° C 2 0 . 3 8 J / g R e s i d u a l C u r e F i r s t S e c o n d
  34. 34. Method Design Rules  Start Temperature  Generally, the baseline should have two (2) minutes to completely stabilize prior to the transition of interest. Therefore, at 10°C/min., start at least 20°C below the transition onset temperature  End Temperature  Allow a two (2) minute baseline after the transition of interest in order to correctly select integration or analysis limits  Don’t Decompose sample in DSC Cell
  35. 35. Selecting Optimum Experimental Conditions  "Always" run a TGA experiment before beginning DSC tests on new materials  Heat approximately 10mg sample in the TGA at 10°C/min to determine:  Volatile content Unbound water or solvent is usually lost over a broader temperature range and a lower temperature than a hydrate/solvate  Decomposition temperature DSC results are of little value once the sample has lost 5% weight due to decomposition (not desolvation) Decomposition is a kinetic process (time + temperature dependent). The measured decomposition temperature will shift to lower temperatures at lower heat rates
  36. 36. 2/7/2023 Dr.Anand, MME, NITK 37 Applications of DSC in materials analysis
  37. 37. Dr.Anand, MME, NITK 38 2/7/2023  Here is the DSC curve for a polymeric material such as high density polyethylene (HDPE). We see three phase transition temperatures: glass transition temperature (Tg), crystallization temperature (Tc), and the melting temperature (Tm) Tg Tc Tm Polymer characterization
  38. 38. Dr.Anand, MME, NITK 39 2/7/2023
  39. 39. Glass Transitions  The change in heat capacity at the glass transition is a measure of the amount of amorphous phase in the sample  Enthalpic recovery at the glass transition is a measure of order in the amorphous phase. Annealing or storage at temperatures just below Tg permit development of order as the sample moves towards equilibrium  The glass transition is a step change in molecular mobility (in the amorphous phase of a sample) that results in a step change in heat capacity  The material is rigid below the glass transition temperature and rubbery above it.  Amorphous materials flow, they do not melt (no DSC melt peak)
  40. 40. Measuring/Reporting Glass Transitions  The glass transition is always a temperature range  The molecular motion associated with the glass transition is time dependent. Therefore, Tg increases when heating rate increases or test frequency (MDSC®, DMA, DEA, etc.) increases.  When reporting Tg, it is necessary to state the test method (DSC, DMA, etc.), experimental conditions (heating rate, sample size, etc.) and how Tg was determined  Midpoint based on ½ Cp or inflection (peak in derivative)
  41. 41. Glass Transition Analysis Polystyrene 9.67mg 10°C/min
  42. 42. Step Change in Cp at the Glass Transition % Amorphous = 0.145/0.353= 41% PET 9.43mg
  43. 43. What Affects the Glass Transition? Heating Rate Heating & Cooling Aging Molecular Weight Plasticizer Filler Crystalline Content Copolymers Side Chains Polymer Backbone Hydrogen Bonding Anything that affects the mobility of the molecules, affects the Heat Capacity and, in turn, the Glass Transition
  44. 44. Dr.Anand, MME, NITK 45 2/7/2023 Polymer characterization  Tg may be used to identify polymers  The amount or effectiveness of a plasticizer may be judged by how much it reduces Tg or affects the shape of the transition.  Examination of the transitions in polymer blends gives information as to their compatibility.  Curing reactions result in an increase in Tg and measurements can be used to monitor the extent of cure.
  45. 45. Dr.Anand, MME, NITK 46 2/7/2023 It is well known that Tg increases with increasing molecular weight, M. This is expressed by the Fox and Flory equation: Tg = Tg() - Kg/M where Tg() is the limiting Tg at a very high molecular weight and Kg is a constant.
  46. 46. Dr.Anand, MME, NITK 47 2/7/2023
  47. 47. Dr.Anand, MME, NITK 48 2/7/2023 Polymer characterization  Thermogram of the blend shows two distinct Tg.  Therefore, the components of this blend are immiscible in each other exo endo Polymer A Polymer B Blend of A+B
  48. 48. Dr.Anand, MME, NITK 49 2/7/2023 Polymer characterization  Tg also varies with chain length for a related group of polymers.  Additional features occurring in the glass transition region, often a superimposed endothermic peak, are related to the aging undergone by the material in the glassy state, and can sometimes obscure the transition, making precise temperature measurement difficult.
  49. 49. Dr.Anand, MME, NITK 50 2/7/2023 This is due to aging
  50. 50. Dr.Anand, MME, NITK 51 2/7/2023 Analysis of explosives  Ammonium perchlorate is an important component of high explosives. The stability of this material is critical to their safe handling. Mechanism of decomposition was investigated.
  51. 51. Dr.Anand, MME, NITK 52 2/7/2023 solid-solid phase transition to the cubic phase decomposition Literature values for Ea 37 - 260 kJ/mol with different mechanisms proposed. This work clarified the mechanism and identified the activation energy as 115 kJ/mol.
  52. 52. Dr.Anand, MME, NITK 53 2/7/2023 Analysis of lubricants  An important test in the automotive industry is to determine the stability of lubricating oils at elevated temperatures and pressures. This will impact its utility as a lubricant in motors. In this case, the oil is brought to a high operating temperature and held there under an oxygen atmosphere.
  53. 53. Dr.Anand, MME, NITK 54 2/7/2023 At some point, the oil begins to oxidize and then quickly decomposes exothermically. Note how the synthetic oil has a much longer oxidation induction time (OIT) than does the mineral oil.
  54. 54. Guidelines for interpreting data Dr.Anand, MME, NITK 55 2/7/2023 It is better to have some idea of what transitions to look for and why. For example: 1. What type of sample is it? 2. What type of transitions can it undergo? 3. What would any changes appear as? 4. What is the temperature range of interest? 5. What data are available from complimentary techniques, e.g. TGA? 6. Has there been any previous analysis? Then examine the data: 1. Is the event an endotherm or exotherm? 2. Is the event repeatable on a fresh sample or on a reheat? 3. What happens on cooling? 4. Is the event the same in a sealed and unsealed pan? 5. Is the transition sharp or gradual, large or small? 6. Does the event look real? Thermal events are not normally excessively sharp.
  55. 55. Dr.Anand, MME, NITK 56 2/7/2023 Text book  D. A. Skoog et al., Principles of instrumental analysis, fifth edition, Harcourt Publishers, 2001.  R. F. Speyer, Thermal analysis of materials, Marcel Dekker, 1994. Reference