Differential Scanning Calorimetry By Dr Kundan Tayade Calorimeter

Differential Scanning Calorimetry By : Dr. Kundan Tayade.

Calorimeter A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. The world’s first ice-calorimeter, used in the winter of 1782 -83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. These experiments mark the foundation of thermochemistry. The name calorimeter was made up by Antoine Lavoisier. In 1780, he used a guinea pig in his experiments with this device to measure heat production. The heat from the guinea pig's respiration melted snow surrounding the calorimeter, showing that respiratory gas exchange is a combustion, similar to a candle burning.

DSC: The Technique • Differential Scanning Calorimetry (DSC) measures the differential heat flows associated with transitions in substance and reference materials linearly heated at a predetermined rate as a function of time and sample. 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.

• Both in DSC and DTA sample and reference both are heated (or cooled) at constant temperature linearly at predetermined rate. • During a thermal event in the sample, the system will transfer heat to or from the sample pan to maintain the same temperature in reference and a sample pans • There are two basic types of DSC instruments: power compensation DSC and heat-flux DSC • Both differ fundamentally in their instrumentation.

Power Compensation DSC individual heaters sample pan controller DP reference pan sample holder • Al or Pt pans sensors inert gas vacuum thermocouple DT = 0 • Pt resistance thermocouples • separate sensors and heaters for the sample and reference furnace • separate blocks for sample and reference cells temperature controller • differential thermal power is supplied to the heaters to maintain the temperature of the sample and reference at the program value

POWER COMPENSATED DSC: Two control circuits are employed. One for avg. -temp. control and other for differential-temp. control. power compensation DSC In average-temp. control circuit , electrical signal is proportional to desired avg. temp. of sample and ref. holders as a function of time. In differential –temp. circuit : sample and reference Signals from platinum resistance sensors are fed into differential amplifier. The amplifier output then adjusts power input into two furnaces in such a way that their temperature are kept identical.

heating coil Heat Flux DSC sample holder • sample and reference are connected by constantan a low-resistance heat flow path chromel/alumel wires • Al or Pt pans placed on constantan disc sensors inert gas vacuum sample reference pan thermocouples chromel wafer • chromel®-constantan (a copper-nickel alloy usually consisting of 55% copper and 45% nickel) area thermocouples (differential heat flow) • chromel®-alumel thermocouples (sample temperature) furnace • one block for both sample and reference cells temperature controller • the temperature difference between the sample and reference is converted to differential thermal power, d. Dq/dt, which is supplied to the heaters to maintain the temperature of the sample and reference at the programmed value

HEAT FLUX DSC Heat flows into both sample and reference material via electrically heated constantan thermoelectric disk. Sample and reference is placed on constantan disc. Heat is transferred to it. Heat flux DSC The differential heat flow into two pans is directly proportional to difference in output of two thermocouple junctions. Sample temperature is estimated by chromel/alumel junction under sample disk.

Modulated DSC • Heating and cell arrangement is same as in heat flux DSC. • With the help of Fourier Transform programmed, the overall signal is mathematically Deconvoluted into two parts, 1) a reversing heat flow signal 2) a non-reversing heat flow signal. • Usually step transitions such as glass transition appears only in the reversing heat flow signal and exothermic or endothermic events may appear in both signals.

Significance: What can DSC measure? • Glass transitions • Melting and boiling points • Crystallization time and temperature • Percent crystallinity • Heats of fusion and reactions • Specific heat capacity • Oxidative/thermal stability • Rate and degree of cure • Reaction kinetics • Purity

Heat Flow Endothermic< >Exothermic - DSC Thermogram Cross-Linking (Cure) Crystallisation Glass Transition Melting Temperature 6 Oxidation

WHY PURGE GAS? Purge gas should always be used during DSC experiments. Provides dry , inert atmosphere Ensures even heating Helps sweep away any off gases that might be released Nitrogen Most common Increases Sensitivity Typical flow rate of 50 ml/min

Keeping the DSC Cell Clean! One of the first steps to ensuring good data is to keep the DSC cell clean. How do DSC cells get dirty? Decomposing samples during DSC runs. Samples spilling out of the pan. Transfer from bottom of pan to sensor.

How do we keep DSC cells clean? DO NOT DECOMPOSE SAMPLES IN THE DSC CELL!!! Run TGA to determine the decomposition temperature Stay below that temperature! Make sure bottom of pans stay clean Use lids Use hermetic pans if necessary

CLEANING CELL: If the cell gets dirty. Clean w/ brush. Brush gently both sensors and cell if necessary. Be careful with thermocouple. Blow out any remaining particles.

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