TECHNICAL NOTE

 

January 2009
 
 

 

 


Differential Scanning Calorimeter DAW 108

 

 

All materials change theirs physical properties or chemical characteristics subject to temperature variation. These changes most often can be studied by specific heat and enthalpy changes by application of programmed temperature variation for heating and cooling. The Differential Scanning Calorimetry (DSC) is one method that allows for precise determination the enthalpy changes from recorded temperature difference, DT, between reference and sample branch. There are several types of available commercial calorimetric equipments with different in design from instrument to instrument for example: TA Instruments, Mettler or Perkin-Elmer. This selection by no means covers all available commercial equipment. Beside of commercial lines there were developed in many laboratories specialized models for certain experiments.

The present DSC model was developed in our laboratory. The primary application field was biological systems that are in a form of low concentration aqueous solutions of proteins investigated in the temperature range between deeply freezing and above the boiling point of water.

 

The instrument is based on the classical design of heat flux differential calorimeters. The left figure displays a top view of the heat leak disk made of silver. The reference and sample crucibles are placed on it separated by constantan bridges that permit lag in temperature proportional to the heat capacity. The sample temperature and temperature difference are measured with thermocouples welded as close as possibly under sample and reference place ensuring the best thermal contact. The temperature of the heating block is measured by Pt-100 sensor. Each sensor can be alternatively used for control the temperature
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Short specification

Temperature range:                                         -150 - +200°C

Temperature sensors:                                     thermocouple K (DT)

                                                                          Pt-100 (block)

Control temperature by:                                 block Tb, reference Tr

or sample Ts

Heating rate:                                                    from isothermal up to 20 Kmin-1

Cooling:                                                           gaseous N2 from LN2

Sample mass:                                                   max. equivalent .to 10 mg of H2O

Sensitivity:                                                       0.2 mW

 

Application

The specified temperature range determines an application of the calorimeter that was originally designed for investigation the biological systems to separate different types of water: free bulk, protein surface bound or glassy. High performance with flexibility of program makes the calorimeter applicable in many other fields such as low temperature metal physics, polymer science, purity analysis, quality testing. The instrument is very suitable for students and young researchers interested in study the experimental thermal analysis and creating new experiments applying specialized temperature program (modulation, step wise) or external magnetic field, etc.

Electronics

The principle of measuring technique is based on the converting small DC signals into alternating (AC) using choppers and than applying the lock-in amplifier (phase sensitive detector) to get noise reduced information about DC input signals. The electrical scheme and its physical realization are shown in the figure and photo below. The sample temperature, Ts, and the differential temperature, DT, are measured by thermocouples welded under the sample (S) and reference (R) place. The block temperature is measured by Pt100 sensor. The thermocouple assemble in the heat flux cell <1> is electrically isolated from the rest of the electronics by optoisolated coupling.

 

 
 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

The sensitive electronics such as preamplifiers, together with choppers and optoisolated amplifiers are placed in a separate, carefully screened box <2>. The temperature profile is obtained by heating the block by current generated by heater driver <3> in a feedback with the temperature program. The heater driver box <3> contains the battery based power supply for optoisolated electronics.

The entire experimental procedure is controlled by software via digital phase sensitive detector (DPSD). There is applied the DPSD constructed as a card controlled by host computer via USB which is programmed as four channels and two frequencies receiver. Two channels at the frequency of the chopper (2 kHz) measure the sample and differential temperatures. Next two channels at frequency of 760 Hz measure the current and voltage to obtain a resistance of the Pt100 sensor and then the temperature of the block.

 

The electronics is applicable with different type of the calorimetric cells including for instance specialized arrangements for simultaneous detection thermal and structural changes of materials properties in vacuum chamber of the scanning electron microscope (SEM). The last approach is the unique solution of the microcalorimetric cell giving possibility to record simultaneously the thermal and magnetic properties of the ferromagnetic materials as a function of temperature or time

Information: Pawel Kamasa, e-mail: kamasa@szfki.hu

Metals Research Department

Research Institute for Solid State Physics and Optics

 Hungarian Academy of Sciences

H-1525 Budapest P.O.B. 49

Phone: (+361) 392-2222/Ext. 3029, Fax.: (+361) 392-2215