Chapter 5 Materials Characterization Lecture III

Transcription

1 Chapter 5 Materials Characterization Lecture III Dr. Alagiriswamy A A (PhD., PDF) Dept. of Physics and Nanotechnology SRM University Main Campus, Ktr., SRM Nagar, Chennai, Tamilnadu

2 5.0 Characterization Techniques 5.8 UV-Vis Spectroscopy (UV) 5.9 Thermogravimetric analysis (TGA) 5.10 Differential Thermogravimetric analysis (DTA) 5.11 Differential scanning calorimetric analysis (DSC)

3 Electronic Spectroscopy Ultraviolet and visible

4 Ultraviolet: 190~400nm Violet: nm Indigo: nm Blue: nm Green: nm Yellow: nm Orange: nm Red: nm

5 Internal Energy of Molecules E total =E trans +E elec +E vib +E rot +E nucl E elec : electronic transitions (UV, X-ray) E vib : vibrational transitions (Infrared) E rot : rotational transitions (Microwave) E nucl : nucleus spin (nuclear magnetic resonance) or (MRI: magnetic resonance imaging

6 Electronic Spectroscopy Ultraviolet (UV) and visible (VIS) spectroscopy This is the earliest method of molecular spectroscopy. A phenomenon of interaction of molecules with ultraviolet and visible lights. Absorption of photon results in electronic transition of a molecule, and electrons are promoted from ground state to higher electronic states.

7 Terms describing UV absorptions 1. Chromophores: functional groups that give electronic transitions. 2. Auxochromes: substituents with unshared pair e's like OH, NH, SH..., when attached to π chromophore they generally move the absorption max. to longer λ. 3. Bathochromic shift: shift to longer λ, also called red shift. 4. Hysochromic shift: shift to shorter λ, also called blue shift. 5. Hyperchromism: increase in ε of a band. 6. Hypochromism: decrease in ε of a band.

8 Ultraviolet-Visible Spectroscopy Introduction to UV-Visible Absorption spectroscopy from 160 nm to 780 nm Measurement of transmittance Conversion to absorbance A=-logT= bc Measurement of transmittance and absorbance Beer s law Noise Instrumentation

9 Beer s Law Based on absorption of light by a sample dp x /P x =ds/s ds/s=ratio of absorbance area to total area Proportional to number of absorbing particles ds=adn a is a constant, dn is number of particles n is total number of particles within a sample P P o dp P x Po ln P Po log P x n 0 adn S an S an 2.303S

10 Beer s Law Area S can be described by volume and length S=V/b (cm 2 ) Substitute for S anb log P o n/v = concentration P V Substitute concentration and collect constant into single term Beer s law can be applied to mixtures A tot =SA x

11 Instrumentation Light source Deuterium and hydrogen lamps W filament lamp Xe arc lamps Sample containers Cuvettes Plastic Glass Quartz

12 Deuterium Lamps-a truly continuous spectrum in the ultraviolet region is produced by electrical excitation of deuterium at low pressure. (160nm~375nm) Tungsten Filament Lamps-the most common source of visible and near infrared radiation. Used as a filter: the monochromator will select a narrow portion of the spectrum (the bandpass) of a given source Used in analysis: the monochromator will sequentially select for the detector to record the different components (spectrum) of any source or sample emitting light.

13 Two different Spectrophotometers

14 Principle of Photomultiplier Detector The impinging electrons strike with enough energy to eject two to five secondary electrons, which are accelerated to the second dynode to eject still more electrons. A photomultiplier may have 9 to 16 stages, and overall gain of 10 6 ~10 9 electrons per incident photon.

15 Single and Double Beam Spectrometer Single-Beam: There is only one light beam or optical path from the source through to the detector. Double-Beam: The light from the source, after passing through the monochromator, is split into two separate beams-one for the sample and the other for the reference.

16 Principle of Barrier Layer/Photovoltaic Detector This device measures the intensity of photons by means of the voltage developed across the semiconductor layer. The potential depends on the number of photons hitting the detector. Electrons, ejected by photons from the semiconductor, are collected by the silver layer.

17 Quantitative Analysis Beer s Law A= bc : the molar absorptivity (L mol -1 cm -1 ) b: the path length of the sample c :the concentration of the compound in solution, expressed in mol L -1

18 Transmittance I 0 I b log ) log( ) 2.303log( ) ln( k bc A T I I I I kbc I I db kc I di kcdb I di I I T I I b

19 Path length / cm %T Absorbance

20 Absorbance An Electronic Spectrum Each band can be a superposition of many transitions max with certain extinction UV Visible Wavelength,, generally in nanometers (nm) Make solution of concentration low enough that A 1 (Ensures Linear Beer s law behavior) Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent. Can extract the value if conc. (M) and b (cm) are known UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed on UV

21 A bc bv C bv C A x x s s kv C kv C x x s s V V t t y b ax (a kv, x C, b kv C ) s s x x V C A 0 kv C kv C C s x x s s x V C x : unknown concentration x s

22 Prepare the Standards The concentration and volume of the stock solution added should be chosen to increase the concentration of the unknown by about 30% in each succeeding flask.

23 Solvents for UV (showing high energy cutoffs in nm) Water 205 CH 3 C N 210 C 6 H Ether 210 EtOH 210 Hexane 210 MeOH 210 Dioxane 220 THF 220 CH 2 Cl CHCl CCl benzene 280 Acetone 300

24 Electron transitions Electronic transitions p, s, and n (non-bonding) electrons p, s, and n electrons d and f electrons Charge transfer reactions

25 Absorption and emission pathways

26 More Complex Electronic Processes Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity Singlet state: spins are paired, no net angular momentum (and no net magnetic field) Triplet state: spins are unpaired, net angular momentum (and net magnetic field)

27 The UV Absorption process * and * transitions: high-energy, accessible in vacuum UV ( max <150 nm). Not usually observed in molecular UV-Vis. n * and * transitions: non-bonding electrons (lone pairs), wavelength ( max ) in the nm region. Energies correspond to a 1-photon of 300 nm light are ca. 95 kcal/mol n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = nm. Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.

28 Nature of electronic Transitions sigma->sigma* UV photon required, high energy Methane at 125 nm Ethane at 135 nm n->pi*, pi->pi* Organic compounds, wavelengths 200 to 700 nm Requires unsaturated groups n->p* low e (10 to 100) Shorter wavelengths p->p* higher e (1000 to 10000) n-> sigma* Saturated compounds with unshared e - Absorption between 150 nm to 250 nm e between 100 and 3000 L cm -1 mol -1 Shifts to shorter wavelengths with polar solvents

29 Application of UV- Visible Spectroscopy Identification of inorganic and organic species Magnitude of molar absorptivities Absorbing species methods

30 Thermogravimetry

31 definitions. a technique in which the mass of a substance is measured as a function of temperature, while the substance is subjected to a controlled temperature programme. Controlled temperature programme can mean: heating and/or cooling at a linear rate (by far commonest) isothermal measurements combinations of heating, cooling and isothermal stages other, more modern approaches, in which the temperature is modified according to the behaviour of the sample. profile

32 PROCESS WEIGHT GAIN WEIGHT LOSS Ad- or absorption Desorption, drying Dehydration, desolvation Sublimation Vaporisation Decomposition Solid-solid reactions (some) Solid-gas reactions Magnetic transitions

33 instrumentation GAS-TIGHT GAS IN ENCLOSURE WEIGHT BALANCE CONTROLLER SAMPLE HEATER SAMPLE TEMP. POWER FURNACE TEMP. TEMPERATURE PROGRAMMER

34 balance/furnace configurations example curve Mass (%) in green, rate of mass loss (%/ C) in blue.

35 polymer stability studies a = PVC, b= nylon-6, c = LDPE, d= PTFE

36 Principle of the thermal analysis technique (1) Equipment Heating block Sample and inert reference material Thermocouple Measurement Heating 36

37 Principle of the thermal analysis technique (2) Heating of the sample and the inert reference material at a constant heating rate Record of the furnace, sample and inert reference material temperatures Différence between sample and inert reference material temperatures 37

38 Endothermic and exothermic effects Heat Flux EXO Which peak direction for the endothermic and exothermic effecst? Temperature ( C) heating cooling ENDO Time (sec) 38

39 Mathematical expression of the calorimetric signal Expression of the heatflux from the reference side Expression of the heatflux from the sample side 39

40 Mathematical expression of the calorimetric signal Expression of the heatflux from the reference side Expression of the heatflux from the sample side 40

41 Mathematical expression of the calorimetric signal Determination of the difference of thermal power exchanged through the thermal resistance (R) between the furnace and each container Final equation giving the mathematical expression of the calorimetric signal 41

42 The calorimetric curve Base line Heat Flux (mw) T f Temperature ( C) T i Area under the peak corresponding to the heat effect (expressed in mj) Time (sec) 42

43 (dq / dt) Specific heat determination Definition of the Specific heat capacity Cp P calibration run How to measure Cp from the DSC curve 3 m cal c p cal 2 P specimen run Cp experiments I m sp c p sp II 1 P blank run T 43

44 Differential Scanning Calorimetry

45 Definitions A calorimeter measures the heat into or out of a sample. A differential calorimeter measures the heat of a sample relative to a reference. A differential scanning calorimeter does all of the above and heats the sample with a linear temperature ramp. Endothermicheat flows into the sample. Exothermic heat flows out of the sample. Technical Group Talk

46 DSC: The Technique Differential Scanning Calorimetry (DSC) measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature in a controlled atmosphere. These measurements provide quantitative and qualitative information about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity. Technical Group Talk

47 Diagram of a DSC apparatus Gas control Furnace A DSC apparatus is built around : - a differential detector - a signal amplifier - a furnace - a temperature controller - a gas control device - a data acquisition device Detectors Sample Reference Microvolt amplifier Furnace controller four Data acquisition 47

48 Different calorimetric principles (DSC, calorimetry) Heat Flux Plate DSC Heat Flux Calvet DSC 48

49 What can DSC measure? Glass transitions Melting and boiling points Crystallisation time and temperature Percent crystallinity Heats of fusion and reactions Specific heat capacity Oxidative/thermal stability Rate and degree of cure Reaction kinetics Purity Technical Group Talk

50 Conventional DSC Sample Empty Metal 1 Sample Temperature Metal 2 Metal 1 Reference Temperature Metal 2 Temperature Difference = Heat Flow A linear heating profile even for isothermal methods Technical Group Talk

51 DSC Thermogram Heat Flow -> exothermic Glass Transition Crystallisation Melting Cross-Linking (Cure) Oxidation Temperature

52 physical limitations on the heating process EXCHANGE OF GASES: REACTING GASES IN, PRODUCTS OUT CONVECTION THROUGH SURROUNDING ATMOSPHERE RADIATION FROM FURNACE WALL CONDUCTION THROUGH SAMPLE PAN AND INSTRUMENT INDICATION OF SAMPLE TEMPERATURE

53 sources of error A) MASS Classical buoyancy Effect temp. on balance convection and/or turbulence viscous drag on suspension These are lumped together as the buoyancy correction, and if significant, can be allowed for by a blank run NOISY OR ERRATIC RECORDS CAN ARISE FROM: static vibration pressure pulses in lab. uneven gas flow B) TEMPERATURE Temperature calibration difficult to carry out accurately. Many methods exist, but none totally satisfactory. Best accuracy from simultaneous TG-DTA or TG-DSC instrument.

54 factors that affect the results A) INSTRUMENTAL heating rate furnace atmosphere and flow-rate geometry of pan and furnace material of pan Reversible Transitions Glass Transition Melting Non-reversible Crystallisation Curing Oxidation/degradation Evaporation B) SAMPLE-RELATED mass particle size sample history/pre-treatment packing thermal conductivity heat of reaction For a given instrument, careful standardisation of experimental procedures leads to highly reproducible results.

55 Thursday, April 03, 2014