Common Definition of Thermal Analysis

Transcription

1 Thermal Analysis

2 References Thermal Analysis, by Bernhard Wunderlich Academic Press Calorimetry and Thermal Analysis of Polymers, by V. B. F. Mathot, Hanser 1993.

3 Common Definition of Thermal Analysis A branch of materials science where the properties of materials are studied as they change with temperature. Techniques: Differential Scanning Calorimetry Dynamic Mechanical Analysis Thermomechanical Analysis Thermogravimetric Analysis Differential Thermal Analysis Dilatometry Optical Dilatometry Dielectric Thermal Analysis Evolved Gas Analysis Thermo-Optical Analysis Production Thermal Analysis of Metals Thermal Analysis of Foods

4 Concepts of Thermal Analysis Temperature A measure of kinetic energy of molecular motion Temperature Scales: 2 E mv 3 = = k 2 2 kt Newton (1701): freezing point of water 0, human body 12 Fahrenheit (1714): freezing point of water mixed with NaCl 0, human body 96, freezing point of water 32, boiling point of water 212 Celsius (1742): freezing point of water 0, boiling point of water 100 Kelvin (1848): absolute zero is the temperature at which molecular energy is a minimum and it corresponds to a temperature of C

5 Temperature Scales P. Atkins, Four Laws that drive the Universe, Oxford Univ. Press, 2007

6 Maxwell-Boltzmann Distribution P. Atkins, Four Laws that drive the Universe, Oxford Univ. Press, 2007

7 Some Important Temperatures Absolute zero (precisely by definition): 0 K or C Coldest measured temperature: 450 pk or C Water s triple point (precisely by definition): K or 0.01 C Water s boiling point: K or C Incandescent lamp: ~2500 K or ~2200 C Melting point of tungsten: 3695 K or 3422 C Melting point of carbon: K or 3500 C Sun s visible surface 5778 K or 5505 C Lightning bolt s channel 28,000 K or 28,000 C Sun s core 16 MK or 16M C Thermonuclear weapon (peak temperature) 350 MK or 350M C CERN s proton vs. nucleus collisions 10 TK or 10 trillion C Universe s after the Big Bang K C

8 Concepts of Thermal Analysis Heat A form of energy produced by the motion of atoms and molecules Heat Units: J (Joule) [m 2 kg s -2 ], Cal (Calorie) 1 cal = J Heat is related to internal energy of a system and work done on or by a system through the First Law of Thermodynamics: du = δq δa = δq pdv TdS pdv ( S, V ) U internal energy, Q heat, A work, T temperature, V volume, S - Entropy = U = f Enthalpy H = U + PV dh = δq + Vdp = TdS + VdP H = f ( S, p) Heat Capacity C p = dq dt = H T p

9 Thermal Analysis Instrument Manufacturers Perkin Elmer Thermal Analysis Systems TA Instruments Mettler Toledo Thermal Analysis Systems Rheometric Scientific Haake NETZSCH Instruments SETARAM Instruments Instrument Specialists, Inc.

10 Thermogravimetric Analysis (TGA) A technique that permits the continuous weighing of a sample as a function of temperature and/or as a function of time at a desired temperature

11 TGA Applications: Inorganics Hydrates decomposition, drying phenomena Carbonates and other salts decomposition Kinetics and mechanisms of oxidation, and other solid-gas reactions Analysis of magnetic materials Etc.

12 TGA Applications: Organics Identification of polymers and pharmaceutical agents Thermal stability of synthetic and natural polymers and other organics Analysis of polymer-matrix composites Kinetics and mechanism of solid organics gas reactions Residual solvent determinations

13 TGA Applications: Oxidation of SWCNT Oxidation of amorphous carbon C+O 2 =CO 2 Oxidation of catalyst

14 TGA+Spectroscopy/Chromatography Combination TGA Gases, vapors IR or MS or GC

15 Kinetic studies The kinetic reaction mechanism can be determined from the Arrhenius equation, K=A exp (-E a /RT), where Ea is the activation energy; R is the universal gas constant; A is the pre-exponential factor; T is the absolute temperature; and K is the reaction rate constant. The above equation upon log transformation can be rewritten as lnk= lna - E a /RT The activation energy can be determined from the slope of the above plot, and the intercept value would yield the preexponential factor.

16 Arrhenius plot Determination of kinetic mechanism for volatilization of triacetin, diethyl phthalate, and glycerin from Arrhenius plots. The E a values are 66.45, 65.12, and kj/mol

17 Differential Thermal Analysis (DTA) Can be conducted at the same time with TGA DTA measures temperature difference between a sample and an inert reference (usually Al 2 O 3 ) while heat flow to the reference and the sample remains the same

18 Differential Scanning Calorimetry (DSC) Exothermal dq/dt Temperature DSC measures differences in the amount of heat required to increase the temperature of a sample and a reference as a function of temperature

19 Differential Scanning Calorimeter

20 Differential Scanning Calorimetry (DSC) To heat a sample and a reference with the same heating rate requires different amount of heat for the sample and the reference. Why? On the X-axis we plot the temperature, on the Y-axis we plot difference in heat output of the two heaters at a given temperature. Heat flow Heat Time δq = t Heat flow Temperature rate δq = t t ΔT = δq ΔT = C p Heat Flow Temperature

21 Major difference between TGA and DTA (DSC) TGA reveals changes of a sample due to weight, whereas DTA and DSC reveal changes not related to the weight (mainly due to phase transitions)

22 Types of Phase Transitions First order transitions, where first and second derivatives of thermodynamic potentials by temperature are not 0 Examples: crystallization and melting Second order transitions where the first derivatives of thermodynamic potentials by temperature are 0 and the second derivatives are not 0 Examples: ferromagnetic diamagnetic transition 0, Δ = Δ Δ p p T G S T G 0, Δ = = Δ Δ p p T G S T G

23 Differential Scanning Calorimeter Parts: Isolated box with 2 pans Heating element and thermocouple Liquid nitrogen Nitrogen gas Aluminum pan

24 Differential Scanning Calorimeter

25 Differential Scanning Calorimeter Perkin Elmer DSC 7 Sample heater Platinum sensors Reference heater Temperature range K Heating rate K/min (normally K/min) Noise ± 4 μw Sample volume up to 75 mm 3

26 An Example of Phase Transitions Studied by DSC Melting and freezing of water in ordered mesoporous silica materials. Pore size increases from 4.4 to 9.4 nm in the series SBA-15/1 to SBA 15/8 A.Schreiber et al. Phys.Chem.Chem.Phys.,2001,3,

27 An Example of Phase Transition in DSC: Martensite/Austenite Transition in Cu-Al-Ni Alloy

28 DSC in Polymer Analysis Main transitions which can be studied by DSC: Melting Freezing Glass transition

29 Polymers in Condensed State Lamellar crystals and Clusters Crystallinity concept the molecules are much larger than the crystals Chain folded 1. Fold length 5-50 nm 2. Best grown from dilute solution 3. Metastable lamellae because of the large fold surface area Extended chain: presents equilibrium crystals. 1. Produced by annealing: e.g. polyethylene polytetrafluoroethylene polychlorotrifluoroethylene 2. Produced by crystallization during polymerization: e.g. polyoxymethylene polyphosphates, selenium Glassy amorphous 1. Random copolymers 2. Atatic stereoisomers e.g. PS, PMMA, PP 3.Quenched slow crystallizing molecules e.g. PET, PC and others.

30 Glass Transition The glass transition temperature, Tg, is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. More specifically, it defines a pseudo second order phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of cristalline materials e.g. of an isotropic solid material.

31 How to observe T g Exothermal Exothermal Temperature Experimental curves on heating after cooling at K/min (1), 0.2 K/min (2) 0.52 K/min (3), 1.1 K/min (4), 2.5 K/min (5), 5 K/min (6), and 30 K/min (7).

32 Typical DSC Curve of a Thermoplastic Polymer Sample: PET80PC20_MM1 1min Size: mg Method: standard dsc heat -cool-heat Comment: 5/4/ DSC File: C:...\DSC\Melt Mixed 1\PET80PC20_MM1.001 Operator: SAC Run Date: 05-Apr :34 Instrument: DSC Q1000 V9.4 Build 287 T m C 1.0 T c T g Heat Flow (W/g) C(I) C C C 20.30J/g C 22.48J/g Cycle C Exo Down Temperature ( C) Universal V4.2E TA Instruments

33 Typical DSC Curve of a Thermosetting Polymer Heat Flow -> exothermic Glass Transition Crystallisation Melting Cross-Linking (Cure) Oxidation Temperature

34 Differential Scanning Calorimetry Melting Glass Transition ENDOTHERMIC EXOTHERMIC Crystallization Sample: Polyethylene terephthalate (PET) Temperature increase rate: 20 C/min Temperature range: 30 C C

35 The First law (Conservation of Energy) We define Internal Energy, U, by: du = δq - δw Can we measure the absolute value of the Internal Energy? How is it stored? Specific heat - increased atomic vibration Making or breaking of atomic bonds Latent heat Chemical Reaction Heat - breaking and remaking chemical bonds 2Mg + O 2 -> 2 MgO Statement of First Law: Internal Energy is a State Function: U = f (T,P, ) The same amount of work, however it is performed (motion, electrical current, friction, etc.) brings about the same change of the system (means, change of state is path independent)