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DTA dan DCS teknik kimia
Differential Thermal Analysis
Differential Thermal AnalysisPrincipleDifferential thermal analysis (DTA) is a technique in which the temperature of the substance under investigation is compared with the temperature of a thermally inert material such as -alumina and is recorded with furnace temperature as the substance is heated or cooled at a predetermined uniform rate. When a physical change takes place in the sample, heat is absorbed or generated. For example, when a metal carbonate decomposes, CO2 is evolved. This is an endothermic reaction; heat is absorbed and the sample temperature decreases. The sample is now at a lower temperature than the reference. The temperature difference between the sample and reference generates a net signal, which is recordedA differential thermogram consists of a record of the difference in sample and reference temperature(T) plotted as a function of time t, sample temperature(Ts), reference temperature(Tr) or furnace temperature(Tf).
When no reaction occurs in the sample material, the temperature of the sampleremains similar to that of reference substance. This is because both are being heated exactly under identical condition i.e. temperature difference T (TsT) will be zero for no reaction. As reaction starts, the sample becomes either hot or cooldepending upon whether the reaction is exothermic or endothermic. A peak developson the curve for the temperature difference T against temperature of furnace or time. During TG, pure fusion reactions, crystalline transition, glass transition and crystallization and solid state reactions with no volatile product would not be indicated because they provide no change in mass of the specimen
Analysis of a polymer shows several features due to physical and chemical changes, including:glass transition: glassy, amorphous polymer becomes flexiblecrystallization of amorphous polymer into microcrystals is exothermic.oxidation peak would be absent in N2 atmosphere
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InstrumentationA typical DTA apparatus consists of the following components.1}Furnace2}Sample holder3}temperature controller and recorder4}thermocouple5}Cooling device
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DIFFERENTIAL SCANNING CALORIMETRYDSC is a thermal method in which the difference in the amount of heat required to increases or decreased the temperature of a sample and reference are measured as a function of temperature.The basic difference between DTA and DSC DSC- calorimetric method, energy differences measured.DTA- temperature differences measured.
When the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature, depends on whether the process is exothermic or endothermic.Eg! A solid sample melts to a liquid it will required more heat flowing to the sample to increases its temperature at the same rate as the reference.This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid.Eg!!Sample undergoes exothermic processes (such as crystalization) less heat is required to raise the sample temperature.
By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions.
11 May [email protected], there are two types of DSC;Power compensated DSC: Sample and reference heated by separate heaters to keep same temperature, as T is changed linearly
Heat flux DSC: Difference in heat flow into sample and and reference is measured with (linear) change in sample temperature.
Power Compensated DSCInstrument has 2 small, independent furnaces, one for sample and one for reference, embedded in a large temperature-controlled heat sink . The sample and reference holders have Pt resistance thermometers for T measurements
Platinum sensorsSample heaterReference heaterHeat Flux DSC
sample and reference are connected by a low-resistance heat flow pathAl or Pt pans placed on constantan disc
Sensors:chromel-constantan area thermocouples (differential heat flow)chromel-alumel thermocouples (sample temperature
the temperature difference between the sample and reference is converted to differential thermal power, dq/dt, which is supplied to the heaters to maintain the temperature of the sample and reference at the program value