Power compensation sample holder

Figure 2.4. Typical power compensation sample holder with twin furnaces and sensors [from Wunderlich (1990) reprinted with permission of Elsevier, Perkin-Elmer and B. Wunderhch].

In a power compensation DSC (figure 12.3), the sample and the reference crucible holders consist of two small furnaces, As and Ar, each one equipped with a temperature sensor, Bs or Br, and a heat source, Cs or Cr. The furnaces... 

The power compensation DSC instrument was first described by Watson et al.3) and by O Neill4) and it was developed into a commercial instrument by the Perkin-Elmer Corporation. It utilises separate sample and reference holders of low thermal mass, with individual heaters and platinum thermometers, as shown schematically in Fig. 1. In addidion to controlling the average temperature the instrument employs a... 

Three kinds of sample holders are available for DTA and DSC (Fig. 4.8.6). Type 1 holders are commonly used for a DTA apparatus. In this type, the sample and reference holders are placed on the same metal block and heated by the same heater the temperature difference between the two holders is indicated by a thermocouple. Type 2 holders are generally used in a quantitative DTA (heat-flux DSC) apparatus. Both sample and reference holders are maintained at the same temperature by two individual internal heaters, which, in turn, are heated by the same main heater. The temperature difference between the sample and the reference material is measured by a thermocouple placed outside of the holders. Type 3 holders are customarily used for a power-compensated DSC apparatus. This apparatus has separate heaters for heating the sample and reference holders thus maintaining the sample and the reference... 

Fig. 4.8.6. Arrangements of sample holder (5) and reference holder (R) in the heater of a DTA or DSC apparatus 1 DTA 2 heat-flux DSC 3 power-compensated DSC...

Figure 10.4 Differential scanning calorimetry (DSC) instrumentation design (a) heat flux DSC and (b) power compensation DSC. A, furnace B, separate heaters and C, sample and reference holders. (Reproduced with permission from E.L. Charsley and S.B. Warrington, Thermal Analysis Techniques and Applications, Royal Society of Chemistry, Cambridge, UK. 1992 Royal Society of Chemistry.)...

In so-called power-compensation DSC, sample and reference are completely isolated from each other (Fig. 3.2). Both the sample and reference crucible have their own heating element and temperature sensing element. With the aid of a temperature programmer, both sample and reference are heated and always have the same temperature. As soon as changes in the sample occur, extra (or less) heat will be needed to maintain the set heating rate. With the aid of specialized electronic circuitry, extra (or less) power is now sent to the sample holder in order to keep the temperature difference zero. In this way, power and consequently heat flow and enthalpy changes are measured. 

The power-compensating DSC has two nearly identical (in terms of heat losses) measuring cells, one for the sample and one reference holder. Both cells are heated with separate heaters, their temperatures are measured with separate sensors. The temperature of both cells can be linearly varied as a function of time being controlled by an average-temperature control loop. A second-differential-control loop adjusts the power input as soon as a temperature difference starts to occur due to some exothermic or endothermic process in the sample. The differential power signal is recorded as a function of the actual sample temperature. 

The DSC measurements reported in this book are performed with the power-compensating DSC-2 and DSC-7 systems from Perkin Elmer. The block surrounding the DSC sample holders is kept at -150°C + 1°C with the aid of a controlled liquid nitrogen supply, both cells are purged with helium (60 ml/minute). 

FIGURE 31 -11 Power-compensated DSC sample and reference holders and heaters. A temperature program is generated by the computer system. Platinum resistance thermometers, in contact with the sample and reference holders, sense any difference between the programmed temperature and the temperatures of the sample and reference, The error signal ts used to adjust the power applied to the sample and the power applied to the reference platinum resistance heaters. The DSC output signal is the difference in the power required between the sample and the reference so that bolh equal the programmed temperature. 

A totally different DSC design is used in power-compensation DSC system. Here, the sample and the reference holders are insulated from each other and have their own individual heaters and sensors as shown in Fig. 7.19. During heating, the same heating power is supphed to both microfumaces via a control circuit and ideally the temperature of both microfurnaces is identical. When a reaction takes place (e.g. melting process, endotherm) the sample temperature becomes less than that of the reference, which is recorded by the temperature sensor and is immediately compensated by the sample heater. Thus, a power-compensated DSC measures the electrical power that is required to keep both... 

In power compensated DSC the small size of the individual sample and reference holders makes for rapid response. The temperature sensors are platinum (Pt) resistive elements. The individual furnaces are made of Pt/Rh alloy. It is important that the thermal characteristics of the sample and reference assemblies be matched precisely. The maximum operating temperature is limited to about 750 °C. High temperature DSC measurements (750-1600°C) are made by heat flux instruments using thermocouples of Pt and Pt/Rh alloys. The thermocouples often incorporate a plate to support the crucible. The use of precious metal thermocouples is at the expense of a small signal strength. Both chromel/alumel and chromel/constantan are used in heat flux DSC equipment for measurements at temperatures to about 750 °C. Multiple thermocouple assemblies offer the possibility of an increased sensitivity - recently a 20-junction Au/Au-Pd thermocouple assembly has been developed. Thermocouples of W and W/Re are used in DTA equipment for measurements above 1600°C. The operating temperature is the predominant feature which determines the design and the materials used in the con-... 

In a power compensation differential scanning calorimeter (DSC), the base of the sample holder unit is in direct contact with a reservoir of coolant (Figure 2.12). The sample and reference holders are indiv idually equipped with a resistance sensor, which measures the temperature of the base of the holder, and a resistance heater. Upon detection of a temperature difference between the sample and reference, due to a phase change in the sample, electrical power is supplied so that the temperature difference falls below a threshold value, typically <0.01 K. 

Like differential-temperature devices, power-compensated scanning calorimeters always feature duplex construction, with sample and reference holders heated in such a way that at every... 

Similar to the Mettler Toledo DSC, an advantage of the Perkin-Elmer power compensation DSC is the simplicity of its temperature calibration for any heating rate (Perkin-Elmer 1976). This was true for the sample holder of the Perkin-Ehner DSC-2, and it is true for the sample holder of the Diamond Pyris DSC, since the structure of the sample holder did not change. In general, the following equation describes the temperature calibration of a power compensation DSC on heating... 

The heart of the DSC is the sample holder or the DSC cell. This should always be kept clean, and from time to time it should be cleaned and conditioned. The power compensation DSCs should be regularly burned off that is, the sample holder should be opened in air (the purge gas should be shut off), and the temperature should be raised to 700-720°C for -10-15 min to burn off organic contaminations. The purge gas helps remove part of the contamination during the DSC measurements, but not all the contamination. 

Calibration of a DTA involves adjustment of instrumental electronics, handling and manipulation of the data in order to ensure the accuracy of the measured quantities temperature, heat capacity and enthalpy [614,615,621]. Temperature sensors such as thermocouples, resistivity thermometers or thermistors may experience drifts that affect the mathematical relationship between the voltage or resistance and the absolute temperature. Also, significant differences between the true internal temperature of a sample with poor thermal conductivity and the temperature recorded by a probe in contact with the sample cup can develop when the sample is subjected to faster temperature scans. The important quantity measured in DTA experiments is the AT output from which enthalpy or heat capacity information is extracted. The proportionality constant must thus be determined using a known enthalpy or heat capacity - the power-compensated DSC requires lower attentiveness as it works already in units of power. The factors such as mass of the specimen, its form and placement, interfaces and surface within the sample and at its contact to holder, atmosphere... 

FIGURE 18.4 Differential scanning calorimeter (power compensation type). The platinum resistance thermometer (PRT) sensor detects a small error signal between the programmed temperature and the sample temperature and calls for more or less power to each heater to keep both holders on program. The difference in power required between the sample and the reference is amplified and recorded. (Courtesy of PerkinElmer Corporation, Norwalk, CT.)... 

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