Instrumentation thermal analysis

Perhaps the most important advance in commercial thermal analysis instrumentation during the past 10-12 years has been the use of microprocessors and/or dedicated microcomputers to control the operating parameters of the instrument and to process the collected experimental data. This innovation is by no means unique to thermal analysis instrumentation alone since these techniques have been applied to almost every type of analytical instrument. Unfortunately, the automation of thermal analysis has not become a commercial reality. Complete automation is defined here as automatic sample changing, control of the instrument, and data processing. Such instruments were first described by Wendlandt and co-workers in the early 1970s (See Chapters 3 and 5) although they lacked microprocessor control of the operating conditions. 

Due to the rapid changing technology in microcomputers and microprocessors, data and control systems have evolved rapidly a life time of 3-4 years is about the maximum for such a system. Thus, only the most current computer system will be described here for a particular type of thermal analysis system. No attempt will be made to give details on the software programs in use these can be obtained from the commercial vendor of the system, if desired. Almost all the commercially available thermal analysis instrumentation employs a microprocessor for operating system control or a microcomputer for data processing. Either a proprietary or a commercially available microcomputer is employed to process the experimental data into conventional thermal analysis plots or to perform more sophisticated kinetics or purity determination calculations. 

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A variety of techniques have been used to determine the extent of crystallinity in a polymer, including X-ray diffraction, density, IR, NMR, and heat of fusion [Sperling, 2001 Wunderlich, 1973], X-ray diffraction is the most direct method but requires the somewhat difficult separation of the crystalline and amorphous scattering envelops. The other methods are indirect methods but are easier to use since one need not be an expert in the field as with X-ray diffraction. Heat of fusion is probably the most often used method since reliable thermal analysis instruments are commercially available and easy to use [Bershtein and Egorov, 1994 Wendlandt, 1986], The difficulty in using thermal analysis (differential scanning calorimetry and differential thermal analysis) or any of the indirect methods is the uncertainty in the values of the quantity measured (e.g., the heat of fusion per gram of sample or density) for 0 and 100% crystalline samples since such samples seldom exist. The best technique is to calibrate the method with samples whose crystallinites have been determined by X-ray diffraction. 

Metal content was determined by a LABTAM 8401 inductively coupled plasma spectrometer. X-ray powder diffraction was carried out on a Rigaku 2304 diffractometer with CuK radition(Ni filtered). IR and UV-vis spectra of the solid samples were recorded on a PE FTIR 1760 spectrometer and a PE Lambda Bio 40 instrument respectively. TG-DTA was performed on a CN8076E(Rigaku) thermal analysis instrument. 

Thermal Analysis Instruments, http //www.tainstruments.com/product.aspx id=9 n=l siteid=ll (accessed June 16, 2008). 

Although there axe a myriad of devices used to measure the temperature of an object, thermal analysis instruments predominantly use thermocouples, platinum resistance thermometers, and thermistors. Thus, only these items, with special emphasis on thermocouples, will be discussed. 

Furnaces for thermal analysis instruments are nearly always electric resistance heated. Wound furnaces consist of a refractory metal wire wrapped around or within4 an alumina or other refractory tube. Nichrome (nickel/chromium alloy) or Kanthal (a trade name for an iron/chromium alloy 72% Fe, 5% Al, 22% Cr,. 5% Co) windings may be used inexpensively for heating to a maximum temperature of 1300°C. More expensive plat-... 

See appendix A for names and addresses of contemporary thermal analysis instrumentation manufacturers. 

A block diagram for a feedback control furnace system, used in thermal analysis instrumentation, is shown in Figure 2.10. The SCR receives a control instruction, and in turn permits a... 

As alluded to in the introduction, thermal analysis instruments must be calibrated using well-characterized materials. The melting of pure metals is the most common calibrant for DTA s and DSC s. Table 3.1 provides the melting temperature and latent heats of transformation of standard materials. The software in more contemporary instruments permit input of peak area values and onset temperatures determined by a test run, as well as values from the literature, into a program. It then automatically applies abscissa and ordinate corrections to all future data collected by the instrument. 

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. 

Table 10.2 lists the thermal events that may occur when a solid (A) is heated in an inert atmosphere. The heat flow between a solid and its surrounding atmosphere at a certain temperature is indicated as enthalpy change (AH) in Table 10.2. AH quantifies the heat flowing into or out of a solid under constant pressure. Commonly, thermal events occurring under a constant-pressure atmosphere are analyzed in thermal analysis instruments.

Thermal analysis instrumentation is complex but all thermal analysis instruments have common features, as shown schematically in Fig. 1. 

There is a need to evaluate, by newer thermal analysis instrumentation and techniques, polymers previously only briefly characterized, emphasizing those products which show potential industrial application or other meritorious property. Such products should be well characterized with regard to such factors as chain length, molecular weight distribution, endgroup, purity, nature and amount of impurities, and actual morphological structure of the polymer. 

Thermal Analysis Instrument Product Brochure, Shimadzu Scientific Instrument, Inc., Columbia, MD, 1997. 

Figure 1. Block layout of electret thermal analysis instrument. The sample is polarized at high temperature, cooled, and the thermal discharge current monitored at a programmed rate of heating.

One of the important trends in chemical analytical instrumentation during the past decade has been the use of digital computers as data processing aids. Raw experimental data from an instrument is manipulated, displayed, and printed by use of a microcomputer or minicomputer. This trend has become very apparent in thermal analysis instrumentation. A small dedicated microcomputer is used to set the instrument s operating parameters as well as to process and display the experimental data. Unfortunately, the T A instruments such as thermobalances, DTA, DSC, and TMA units are of designs that are at least 20 years or more old. New instrument designs have not been developed but, rather, the emphasis has been on computerization. 

This chapter attempts to summarize the important applications of digital, and, in certain cases, analog computers to thermal analysis instrumentation. No attempt has been made to make it comprehensive in scope, due to the voluminous literature on this subject. Instead, it is hoped that the discussion will provide a background on the general subject of computerization of TA techniques and an insight into what to expect from commercially available computer-assisted instruments. The latter subject changes at very short-time intervals due to the rapid advances in the technology of small computers. 

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