Simultaneous thermal analysis instrumentation

Simultaneous Thermal Analysis (STA) A technique which combines DTA, TGA, DSC (cal.) and derivatives of DTA/TGA in a single instrument, offers advantages over the performance of each technique as a separate experiment on stand-alone instruments. By examining a single sample, one ensures no variation in sample homogeneity, atmosphere difference and other instrumental parameters. At the same time, there is a provision for detection and analysis of evolved gases (EGA) during thermal decomposition. Also, derivatives of TGA and DTA can be precisely measured (electronically). 

The compositions of the products were determined by inductively coupled plasma (ICP) with a Perkin-Elmer plasma 40 emission spectrometer. Simultaneous differential thermal analysis and thermogravimetric (DTA-TG) curves were carried out by using Perkin-Elmer DTA-7000, TGA-7 PC series thermal analysis instrument in air with a heating rate of 10 °C /min. The infrared (IR) spectra were recorded on an Impact 410 IR spectrometer on samples pelletized with KBr powder. Valence states were determined by X-ray photoelectron spectroscopy (XPS). The XPS for powder samples fixed on double sided tapes was measured on an ESCA-LAB MKII X-ray photoelectron spectrometer. The Cis signal was used to correct the charge effects. 

This chapter is concerned with the large family of Simultaneous Thermal Analysis (STA) techniques, in which two or more types of measurement are made at the same time on a single sample. This methodology, entailing a more complex instrument, often specially built, has been found to be essential in a variety of thermal studies, and instruments for simultaneous measurements have been constructed for more than fifty years - often as soon as it was technically possible, as the benefits of this approach were rapidly appreciated. 

Some commercial instruments have been developed which combine a thermobalance with a thermal analyzer in a single heating process, recording TG, DTG, and DTA curves simultaneously. Representatives of this kind of equipment are the De-rivatograph (Hungarian Optical Works) shown in Figure 3.7 and the STA system (simultaneous thermal analysis system) of Stanton Redcroft Ltd. (UK). 

While TGA provides useful data when a mass change is involved in a reaction, we have seen that many reactions do not have a change in mass associated with them. The use of both TGA and DTA or TGA and DSC provides much more information about a sample than either technique alone provides. There are several commercial thermal analysis instrument manufacturers who offer simultaneous combination systems, often called simultaneous thermal analysis (STA) systems. Simultaneous TGA-DTA and simultaneous TGA-DSC instruments are available. Instrument combinations cover a wide temperature range and come in both analytical sample size (1-20 mg)... 

DSC Modifications and Simuitaneous Techniques. DSC and other thermal analysis instrumentation have imdergone many modifications and developments in recent years. Among these innovations is the coupling of various methods. In this context the terms parallel, concurrent, and simultaneous should be defined. Parallel techniques use separate samples, each in its own imique thermal environment. Concurrent or combined techniques also use separate samples, but the experiment is carried out in a common atmosphere and thermal environment. Simultaneous techniques use the same sample in the same atmospheric and thermal environment. Such methods are increasing in popularity. While a detailed treatment of this topic is outside the scope of this discussion, a brief list will be presented in order to provide input for those new to the subject area. Interested readers should consult the chapter by Gallagher (5) and the additional references mentioned therein. 

Principles and Characteristics Simultaneous thermal analysis techniques, such as TG-DSC/DTA offer vital information on polymer structure based on heat flow behaviour and mass change [290], but little direct information on the composition of evolved gas products. A more complete thermal profile is provided when a thermal analyser is coupled to an identification tool. Henderson et al. [433] have recently described TG-DSC/DTA with evolved gas analysers (MS and FTIR). The skimmer coupling is the most advanced commercial way of combining a thermobalance or simultaneous TG-DSC/DTA instrument with a quadrupole mass spectrometer [338]. For descriptions of interface techniques in this coupled instrumentation, cfr. ref. [411]. Simultaneous TG-DSC-MS is capable of operation up to 2000°C [434]. 

Two different thermal methods were used for the analysis of the high-temperature behavior of the various ash san5>Ies, i.e., simultaneous DTA/TGA using a NETSCH STA 429 friermal analysis instrument, and sintering tests performed in a simple muffle furnace, both in combination with a SEM/EDX technique for the elemental analysis of the thermally treated ash samples, using a JEOL 6300 scaiming microscope. 

We shall describe the design and performance of multiple sample instruments for differential thermal analysis which do runs on several samples simultaneously The first of these is a five-sample DTA system that served as a basis for the DSC system. Following this is a description of the principles of operation of the isothermal boundaries that were formed as integral parts of the heating block and thermoelectric disk of a heat flow type multiple sample DSC cell. 

The approach that has been taken for the solution of these problems has been to develop an instrument that will run several samples simultaneously inside the same furnace with a single reference while storing the information in a data system. Following the run, the necessary computations and the generation of hard copies of the thermal analysis curves are done by the data system. 

The gaseous products evolved during a TG measurement are a rich source of information and these gases are readily analysed by coupling an appropriate instrument to the TG apparatus. This form of thermal analysis is often referred to as evolved gas analysis (EGA), and is discussed in Section 6.1. Mass spectrometers (TG-MS), Fourier transform infrared spectrometers (TG-FTIR) and gas chromatographs (TG-GC) may be coupled for simultaneous TG-EGA. 

Thermal gravimetric analysis and differential thermal analysis (TGA/DTA) can be performed by using a SDT 2960 Simultaneous Differential Thermal Analyzer (TA Instruments, Inc., New Castle, DE). The instrument was calibrated with gold supplied by Perkin-Ehner. Samples (70 mg) of as-prepared powders were hand-pressed in a 3 mm dual action die and placed inside Pt sample cups and heated at the rates of 10 K/ min from ambient temperature to 1400°C. The reference material was used as a pellet of a-alumina. A flow of synthetic air at 50mL/min was maintained during the experiments. 

Some of the simultaneous instruments available commercially are as follows TGA/DTA/DSC, TGADTA/FTIR, DTA(DSC)/EGA(EGD), and DSC/FTIR. Also, high pressure DSC and photo-DSC instruments are available commercially. In addition, individual researchers have used simultaneous DSC/XRD (X-ray diffraction), DSC/EGA/XRD, and DSC/TRXRD (time-resolved X-ray diffraction). Experiments with parallel, combined, and simultaneous techniques help to affirm the conclusions drawn from a single technique and very often offer definitive clues to the actual mechanisms taking place during thermal analysis. 

The discussion of DTA in the previous section describes the precautions and recommended practices for calculating the melting point from DTA or DSC curves. The advent of instrumentation for simultaneous DTA/TG or DTA/EGA allows the TG and EGA instruments to be calibrated directly as well. Many secondary standards for temperature measurements have also been developed for thermal analysis. These have been based on solidj—solid2 crystallographic transitions or magnetic transitions, that is, Curie or Neel temperatures. 

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