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DTA HEATS UP Materials Analysis As interest in materials chemistry inAS quality creases, bench and research chemists are on a variety of tools to help charcontrol and product relying acterize a wide spectrum of samples ranging from glasses to ceramics to polydevelopment mers. One of these tools is thermal analy(TA), a family of techniques involving applications for DTA sis the measurement of specific physical as a function of temperature. become increasingly properties Some materials, particularly ceramics polymers, have properties that deimportant, instrument and pend on the method of preparation rather than chemical composition. Determinamanufacturers tion of percent crystallinity, glass transitemperature, thermal stability, and respond by designing tion mechanical properties are some of the capabilities of TA When informamodular systems that unique tion obtained from these techniques is combined with that obtained by other offer enhanced means (e.g., X-ray diffraction, analysis of residues and evolved gases by MS or IR flexibility spectroscopy), one can obtain both quantitative and qualitative measures of solid-state properties and reactions. TA has a variety of family members, most of whom are often referred to by their acronyms—TG (thermogravimetry), TMA (thermomechanical analysis), DMA (dynamic mechanical analysis), DSC (differential scanning calorimetry), and DTA (differential thermal analysis). Each technique uses different apparatus and
provides analysts with different, although often complementary, information. TG involves the measurement of the mass of a sample while its temperature is increased. From a plot of mass versus temperature, analysts can evaluate thermal stability, reaction rate, reaction processes, and sample composition. Many applications involve the study of polymers: their identification and decomposition mechanisms. TMA provides measurements of penetration, expansion, contraction, and extension of materials as a function of temperature. DMA is particularly useful for characterizing the modulus or stiffness of materials and for detecting transitions associated with polymer chain movements. It is based on the measurement of the resonant frequency and mechanical damping of a material forced to flex at a selected amplitude. The two differential techniques, DTA and DSC, involve measurements made on a sample and a thermally inert reference material. Any transitions within the sample result in the liberation or absorption of energy by the sample and a corresponding deviation of its temperature from the reference sample. In power-compensated DSC, heat is added to the sample or to the reference material as needed so that the two substances remain at the iden-
Analytical Chemistry, Vol. 66, No. 20, October 15, 1994 1035 A
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tical temperature. The added heat is re corded and compensates for the heat loss or gain that results from endothermic or exothermic changes in the sample. In heat-flux DSC, the sample and refer ence material are simultaneously heated at a constant rate. The temperature differ ence between them is proportional to the difference in heat flow (from the furnace) between the two materials. In DTA, one plots the temperature difference between the sample and reference material (AT) as a function of the programmed tempera ture at which the system is changing. From this plot, analysts can identify the temperature of the transition and deter
mine whether the transition is exothermic transfer and thermal conductivity consid erations, but advances in electronics and (i.e., adsorption, oxidation, or freezing) instrumentation have improved the accu or endothermic (i.e., fusion, vaporization, sublimation, absorption, desorption, de racy and precision of DTA results. Today, DSC provides better information (higher composition, or melting). Analytical Chemistry consulted Neil D. sensitivity) over a temperature range from Jespersen of St. John ' s University for help -180 °C to 725 °C, whereas DTA is most effectively used in evaluations between in understanding DTA and instrument options. Although Table 1 is not compre 600 °C and 1600 °C. Jespersen notes hensive, it includes representative prod that the ability to use ceramic materials as sample holders in DTA allows for higher ucts and common features of commer cially available DTA systems. DSC instru temperatures; operating temperatures are limited to 650 ° C if sample holders are ments will be the subject of a future made from materials such as aluminum. product review. Historically, DSC provided more accu Higher temperatures make it possible to characterize inorganic materials such as rate information because of better heat
Table 1 . Summary of representative products Labsys" Astra Scientific /Setaram 6900 Knoll Center Pkwy. Suite 417 Pleasanton, CA 94566 510-426-6900 ΙΝΑ 2 0 X 2 2 X 18 Metal resistor
Totalab Harrop Industries, Inc. 3470 East Fifth Ave. Columbus, OH 43219 614-231-3621
DTA 7 Perkin-Elmer Corporation 761 Main Ave. Norwalk, CT 06859 203-762-1000
$25,000-$40,000 11 Χ 17Χ 16 Wire-wound, SiC, and molybde num disilicide hairpin elements
$34,000 20 X 9.75 X 15 Pt and Pt/30% Rh wound elements
-150°Cto1700°C
Ambient to 1600°C
Temperature detector
Ambient to 1200 °C with platinel transducer; ambient to 1600 °C with Pt/Rh transducer Platinel or Type S thermocouples
Type Κ or R differential thermo couples
ΙΝΑ
Δ Τ sensitivity Temperature accuracy Temperature precision Programmable heating rate Atmosphere
ΙΝΑ ± 1 °C ±2°C 0.01-99.99 °C/min Static or controlled-flow gases
1 μν ± 1-2 °C ±2°C ΙΝΑ Static or controlled-flow gases
ΙΝΑ ΙΝΑ ± 0.2 °C 0.1-100 °C/min Static or dynamic with air, N2, 0 2 , Ar, and other inert/reactive gases
Test atmosphere pressure Sample Available crucibles
Ambient to 10 -3 torr
10-3 torr to 1500 psi
Atmospheric or vacuum
Alumina, Pt
Alumina, Pt, quartz, Pyrex
Alumina, magnesia, Pt
Volume/size Data system
100 μ ι
Up to 250 mm3
Up to 100 mm3
486-based PC/Windows Multitasking Windows-based soft ware; data export in ASCII or Lotus; operation fully controlled by software
486-based workstation/DOS Input and data analysis software; Lotus 123; can be customized
Designed for studies of chemi cals, polymers, and materials; can be converted to simultane ous TGA/DTA or TGA/DSC
Can perform analyses simultane ously with other Harrop thermal analyzers
DEC PC/UNIX 7 Series/UNIX thermal analysis application software provides instrument control, automatic cali bration, real-time data display, and automatic baseline correction Can perform an analysis simulta neously with any other PE 7 Series thermal analyzer
Product Company
List price (complete system) Dimensions (I X w X h, in.) Furnace
Temperature range
Platform Software features
Instrument scope/applications
ΙΝΑ = Information not available at press time Manufactured by Setaram (7 rue de l'Oratoire, 69300 Caluire, France; phor e 33 78 29 38 38) and distributed in tt e U.S. by Astra
a
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Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
metals, glasses, ceramics, and minerals. Specific measurements include glass tran sitions, melting and boiling points, phase transitions, and assessments of oxidative and thermal stability. Instrumentation
DTA system components include a circuit to measure temperature differences, a heating device (furnace) and temperature control unit, a means of amplifying and recording the observed signal, and a de vice to control the sample atmosphere. Temperatures are measured with thermocouples. The selection of which thermocouple to use is based on the opti
mum temperature and lifetime desired. Different thermocouples have different temperature response characteristics; the computerized instrument control pro grams compensate for these as needed. Because the thermocouples can be placed directly in the sample or attached to the sample container, DTA can provide high thermometric accuracy. This ar rangement, however, increases the impor tance of thermal equilibrium. To mini mize the effect of variations in tempera ture during heating, it is important to use the smallest sample size possible and to be certain that it is uniformly packed in the sample cup or tube.
The furnace in DTA instruments is gen erally designed to avoid electrical interfer ence with the thermocouples. It may in corporate an inner metallic or ceramic chamber that holds both the sample and reference material such as aluminum ox ide. Furnaces and sample block tempera tures are increased in a linear fashion, most often at a rate of 5° C/min to 12 ° C/ min. Precise control of the furnace temper ature is essential and is accomplished in today ' s thermal analysis systems by com puter control. Most instruments are con structed to permit the circulation of inert or reactive gases; some can also operate at high pressure or in a vacuum.
DTA 92 a Setaram/Astra Scientific 6900 Knoll Center Pkwy. Suite 417 Pleasanton, CA 94566 510-426-6900 $30,000-$50,000 ΙΝΑ Metal resistor, graphite furnace
DTA 50 Shimadzu Scientific Instruments 7102 Riverwood Dr. Columbia, MD 21046 410-381-1227
High-Temperature DTA TA Instruments 109 Lukens Dr. Newcastle, DE 19720 302-427-4000
Labtronic II SDP Theta Industries 26 Valley Rd. Port Washington, NY 11050 516-883-4088
$17,250-$30,000 21.7 X 6.8 X 19.7 Alumina furnace tube
$20,000-40,000 5 . 5 X 4 . 5 X 1 2 . 5 (cell only) Alumina furnace tube
$29,000 34 X 31 X 20 Wire-wound SiC tube furnace
Ambient to 1000 °C; ambient to 1600/1750 °C; ambient to 2400 °C
Ambient to 1600 °C
Ambient to 1600 °C
Ambient to 1600 °C
Platine); Pt/Pt-10% Rh; Pt6% Rh/Pt-30% Rh; W-Re ΙΝΑ ± 1 °C ±2°C 0.01-100 DC/min Inert, oxidizing, reducing
Pt/Pt-10% Rh alloy thermocouple (S-type) 0.1 °C ± 1 °C ±3°C 0.1-50.0 °C/min and 0.1-50.0 °C/h Air, N2, He, 0 2 , and other inert/reactive gases
Pt/Pt-18% Rh thermocouple pair
Platinel or Type S control thermocouples 0.1 °C ± 1 °C ±2°C 0.01-200 °C/min Air-controlled atmosphere
Atmosphere to vacuum
Vacuum to 10 torr
AI, alumina, Pt, W, graphite
Pt, AI, alumina, quartz, Ni, Cu, stainless steel 50 μί.
20-100 mm3
PC/Windows Instrument control; signal acquisi tion and processing
Choice of DTA and DSC plate probes; permits thermal analysis ol various types of materials from low to very high temperature
486-based PC/Windows General and extended analysis functions; real-time data display; resident ASCII conversion; "snap shot" feature allows real-time sam ple analysis Stand-alone operation; simultane ous operation with other Shimadzu Series 50 thermal analyzers
0.004 °C ± 1 °Cor 1% ±0.5°C 0.01-200 °C/min Static or controlled flow. Air, N2, 0 2 , Ar, or other inert/reactive gases Ambient
10-" torr
Up to 75 μΐ Thermal Analysts 2000, 2100, or 2200
200 μΙ_
486-, Pentium-based IBM PS/2 General analysis; instrument con trol; automatic calibration, real time data display
486-based PC Thetasoft 3 software provides instrument control, data acquisi tion, temperature inputs, instru ment calibration and verification tests; Lotus for Windows Available as dilatometer attach ment
Can be run simultaneously with up to three other TA techniques
Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
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The potential difference between the thermocouples is amplified and fed to a digital data acquisition and data handling system. This microprocessor-controlled system can also control the experiment, avoiding the need for operator intervention in operations such as baseline optimization and system calibration. These procedures are often tedious and contribute to experimental error. The changing marketplace Thermal analysis was commercialized in the early 1960s and was used primarily in industrial R&D labs for material characterization. Today, quality control and product development applications have become increasingly important, and instrument manufacturers have responded by designing modular thermal analysis systems that offer enhanced flexibility. Most DTA instruments can be expanded with the addition of modules that accommodate other thermal techniques such as DSC, TMA, and TG while allowing the analyst to use the same temperature controller and data system. Multitasking soft-
ware permits users to process data from previous runs while acquiring new data and to control the operation of several modules from a single computer. Tips for purchasers What should laboratory managers who are considering adding DTA to their repertoire of instruments keep in mind? Says Jespersen, "Potential purchasers must remember that thermal analysis is not a high-throughput technique. You need to count on allotting 10 min to 1 h per sample." This is where instrument design can be important. Manufacturers recognize that by increasing the efficiency of the cooling system in the internal furnace, they decrease the time required for the system temperature to return to a useable level. This decreased cycle time leads to higher productivity for the laboratory. Safety is a key issue, primarily because of the high temperatures at which these systems operate. Several manufacturers have incorporated locking devices that do not allow movement of the furnace assembly once the temperature has reached
a preset, user-selectable point. Others have designed their instruments so that the furnace assembly remains on the instrument during sample loading, eliminating the possibility of placing a hot furnace on a combustible surface. Finally, it's important not to forget the basics of any instrument purchase. Will the instrument company or supplier analyze a sample that you provide? How available and responsive is customer support? Many manufacturers have method development facilities as well as active applications laboratories that can help with the analysis of specific materials. Ask about the need for any special equipment or procedures. Sample preparation in DTA is normally nonexistent except for accurate weighing of milligram-sized samples. In terms of installation, it is important that the module be isolated from vibration sources and that an appropriate gas supply be available. Because a large number of TA instruments are run by technicians, the importance and availability of training cannot be overemphasized. Louise Voress
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