On the figure on the left side, are given the different temperatures that are important during this test:
Tp is the controlled temperature delivered by the controller to heat the metallic block, at a constant scanning rate. A linear slope is obtained for Tp TR is the temperature recorded in the inert reference material. Compared to Tp ,there is a shift in the temperature curve corresponding to the thermal gradient of the material and also the time of response of the furnace. After a certain time, the TR curve becomes parallell to the Tp curve
All types of DSC apparatus are built around the following parts: A differential detector for the sample and the inert reference material. Different types of detectors are available according to the manufacturers. The main ones are described in a following slide As the electric signal measured by the detector is mostly weak, it is needed to amplify this signal and to convert the analog output in a digital output. The detector is located in a thermostated furnace. Different types of furnaces (mostly with a metallic heating element) are used according to the temperature range. For a DSC it is important to have a furnace that can be fastly heated and cooled A temperature programmer and controller is associated to the furnace to apply heating and cooling The atmosphere around the sample is controlled via a gas panel. The gas is selected according to the test to be run. It is possible to adapt an automatic switching device in order to change the gas during the test.
Without going too much in details in the mathematical expression of the DSC signal, it is important to understand how the DSC detector is working and what is measured by the system.
Firstly consider the reference side as seen on the picture. The inert reference material is contained in a pan and does not show any thermal transformation for the investigated temperature range. So the reference material detector alone measures a heatflux signal that corresponds to the thermal power needed to heat the sample at a defined scanning rate dT/dt. This power depends on the heat capacity Cr of the reference material (with the container). The reference heatflux signal is expressed as:
Different types of DSC principles are recognized/
Heat Flux plate DSC: A technique in which the difference in heat flow rate to a test sample and to a reference sample is analysed while they are subjected to the same temperature variation (heated or cooled).It is called a plate-DSC detector.
Power compensated DSC: A technique in which the temperature difference between a test specimen and a reference specimen occurring through subjecting both specimens to the same controlled temperature program is compensated by appropriately adjusting the difference of heating power to the test and reference specimen. The differential heating power is recorded against temperature or time
Calvet DSC: A technique in which the difference of heat flow rate to the sample and to the reference sample is analysed while they are subjected to a temperature variation (heated or cooled). The detector is a cylindrical DSC detector. The sample is fully surrounded by the detector, that provides a detection of the full exchange of heat.
Power-compensated DSC was introduced in the early 1960s.
Ag heating block dissipates heat to the sample and reference via the constantan disc.
Thermal Analysis …
A group of analytical techniques
Each technique defines a material property
Differential Scanning Calorimetry
Ti is the initial temperature(before reaction )
Tf is the final temperature(after reaction)
Ti <Tf, rxn is exothermic
Ti> Tf, rxn is endothermic
Heat flow is calculated as Q=M.Cp.T
It is a thermal technique 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
DIFFERENTIAL-measure difference in heat flow
from sample & reference.
SCANNING –scan over a range of temperature.
CALORIMETER-used to measure heat or heat
Principle of the thermal analysis :
Heating of the sample and the inert reference
material at a constant heating rate
Record of the furnace,
sample and inert
sample and inert
BASIC DSC APPARATUS
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
TYPICAL PURGE GASES ARE AIR/NITROGEN
HELIUM MAY BE USEFUL FOR VOLATILE
CAN BE CARRIED OUT UNDER HIGH
Power Compensated DSC
• Al or Pt pans
• Pt resistance thermocouples
• separate sensors and heaters for the sample and reference
• separate blocks for sample and reference cells
• differential thermal power is supplied to the heaters to maintain the temperature of
the sample and reference at the program value
HEAT COMPENSATED DSC
• sample and reference are connected by
a low-resistance heat flow path
• Al or Pt pans placed on constantan disc
• chromel®-constantan area thermocouples
(differential heat flow)
one block for both sample and reference
the temperature difference between the
sample and reference is converted to
heat flow in & out of the sample and is
recorded against time or temperature.
MATERIALS MELTING POINT(oC)
DSC: Main Sources of
•Sample preparation – how sample is loaded into a pan
•Residual solvents and moisture.
PROPER CALIBERATION OF
PROPER PLACING INSTRUMENT IN
AVOID EXCESSIVE HEATING RATE
PROPER GAS FLOW RATE
CONSTANT POWER SUPPLY
1) ENTHALPY OF TRANSITION:-Hd=KA
2) GLASS TRANSITIONS(Tg)
3) POLYMER DEGRADATION STUDIES
4) LIQUID CRYSTALS
5) OXIDATIVE STABILITY
6) DRUG PURITY ANALYSIS
A Thermogravimetric Analyzer (TGA)
measures the change in mass of a sample as
the sample is heated, cooled or held at a
constant (isothermal) temperature.
Isothermal or static
Quasistatic thermo gravimetry
Principle: In this technique the change in sample
weight is measured while the sample is heated at
a constant rate (or at constant temperature),
under air (oxidative) or nitrogen (inert)
This technique is effective for quantitative
analysis of thermal reactions that are
accompanied by mass changes, such as
evaporation, decomposition, gas absorption,
desorption and dehydration.
WORKING PRINCIPLE OF
Change in the sample mass causes a deflection of the beam.
The resulting imbalance in the photodiode current is amplified and fed
into coil E, which is situated between the poles of permanent magnet F.
The magnetic field generated by the current in the coil restores the
beam to its original position.
The amplified photodiode current is monitored and transformed into
mass or mass loss information by the data acquitting system.
In most cases mass vs temperature data can be either plotted in real
time or stored for further manipulation or display at a later time.
The various components of modern thermo balance are
Furnace temperature programmer or controller
It is the most important component of the
Commercially available balance can provide
quantitative information about 1 mg-100 gm
There are two types of balance.
2) Null-Point Balance
It is more commonly used. In this balance, a
sensor is employed to detect the deviation of
the beam from its null position.
A restoring force of either electrical or
mechanical weight loading is applied to the
beam to restore its null position from the
horizontal or vertical norm.
The restoring force is proportional to the
weight change and this force is recorded
directly or by transducer.
2) SAMPLE HOLDER
It is constructed from glass, quartz, alumina, stainless steel, platinum,
In practice 4 types of sample holders have been used.
Loosely covered crucibles
3) THE FURNACE
The choice of furnace heating element and type of
furnace depends upon the temperature range being
1100 0C Nichrome
1100-1500 0C Platinum or alloy of pt-rhodium
1100-1750 0C Pt-Rh in which (Rh-40)
>1750 0C tungsten or molybdenum
The most common method is thermocouple.
For 1100 0C: chromel or alumel thermocouples made up of
alloys of Pt and rhodium.
For higher temperature: tungsten or rhenium thermocouple.
The position of temperature measuring device relative to the
sample is very important.
1. Time-base potentiometric strip chart
2. X-Y recorders.
In some instruments, light-beam-galvanometer photographic paper recorders or one
recorder with two or more pens are used.
One can check the heating rate of the furnace for linearity.
In x-y recorders we get curves having plot of weights against temperature.
In most cases normal mode of recording data for Thermogravimetry is the weight
change vs temperature or time.
But % mass change vs time or temperature is more suitable.
Effect of furnace atmosphere
Characteristic s of the sample
Weight of sample
Sample particle size
Compactness of sample
Previous history of the sample
What is DTA ?
A technique in which the temperature
difference between a substance and a
reference(Al2o3) material is measured
as a function of temperature while the
substance and reference are
subjected to a controlled temperature
A differential thermogram
consists of a record of the
difference in sample and
plotted as a function of
time t, sample
temperature(Tr) or furnace
A differential thermal analyzer is composed
of five basic components, namely :
3}temperature controller and recorder
Tubular furnace is most commonly used because it
possess the desired characteristic for good
temperature regulation and programming.
Dimension of the furnace is depends upon the length
of the uniform temperature zone desired.
The choice of resistance material is depends on the
maximum temperature of the operation and gaseous
Grooved muffled cores preferred.
2} Sample holder
Should having low cost, ease of fabrication and inertness towards
Metallic material: nickel, stainless steel, platinum
Non-metallic material: glass, vitreous silica or sintered alumina.
Most commonly the shape of holder is cylindrical.
The nature of physical constant between the sample,
thermocouple junction and the specimen holder affect the DTA
signals. So to maintain it, a sample holder with dimples in which
thermocouple junctions are inserted (thermocouple wells) are
3} Temperature controller and
A] Temperature Controller
In order to control temperature, the three basic elements are
required. sensor, control element and heater
The control element governs the rate of heat-input required to
match the heat loss from the system.
The location of sensor with respect to the heater and mode of heat
transfer measure the time elapsed between sensing and variation
in heat input.
B] Temperature programming
It transmits a certain time-based instruction to the control unit.
By this device one can achieve linearity in the rate of heating or
cooling it is driven in a non-linear fashion using a special cam-
Heating rates of 10-20 o C / mints are employed.
The signals obtained from the sensors can be recorded in which
the signal trace is produced on paper or film, by ink, heating
stylus, electric writing or optical beam.
Thermocouples are the temperature sensors.
It is made up from chromel and alumel wires are
used to measure and control temperature up to
1100 0C in air.
For above 1100 0C one should use thermocouple
made from pure platinum & platinum-rhodium alloy
Used to analyze filler content in polymers; carbon black in oils; ash and carbon in
Automatic Thermogravimetric Analysis
Evaluation of gravimetric precipitates
Evaluation of suitable standards
Testing of purity of samples
Curie point determination